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A Brief Overview of Electrochromic Materials and Related Devices: A Nanostructured Materials Perspective

A Brief Overview of Electrochromic Materials and Related Devices: A Nanostructured Materials... nanomaterials Review A Brief Overview of Electrochromic Materials and Related Devices: A Nanostructured Materials Perspective 1 , 2 , 3 Aleksei Viktorovich Shchegolkov *, Sung-Hwan Jang * , Alexandr Viktorovich Shchegolkov , 4 5 1 Yuri Viktorovich Rodionov , Anna Olegovna Sukhova and Mikhail Semenovich Lipkin Department of Chemical Technologies, Platov South-Russian State Polytechnic University (NPI), 346428 Novocherkassk, Russia; lipkin@yandex.ru Department of Civil and Environmental Engineering, Hanyang University ERICA, Ansan 15588, Korea Department of Technology and Methods of Nanoproducts Manufacturing, Tambov State Technical University, 392000 Tambov, Russia; Energynano@yandex.ru Department of Mechanics and Engineering Graphics, Tambov State Technical University, 392000 Tambov, Russia; rodionow.u.w@rambler.ru Department of Nature Management and Environment Protection, Tambov State Technical University, 392000 Tambov, Russia; apil1@yandex.ru * Correspondence: alexxx5000@mail.ru (A.V.S.); sj2527@hanyang.ac.kr (S.-H.J.) Abstract: Exactly 50 years ago, the first article on electrochromism was published. Today elec- trochromic materials are highly popular in various devices. Interest in nanostructured electrochromic and nanocomposite organic/inorganic nanostructured electrochromic materials has increased in the last decade. These materials can enhance the electrochemical and electrochromic properties of devices related to them. This article describes electrochromic materials, proposes their classification and systematization for organic inorganic and nanostructured electrochromic materials, identifies Citation: Shchegolkov, A.V.; Jang, their advantages and shortcomings, analyzes current tendencies in the development of nanomaterials S.-H.; Shchegolkov, A.V.; Rodionov, used in electrochromic coatings (films) and their practical use in various optical devices for protection Y.V.; Sukhova, A.O.; Lipkin, M.S. A from light radiation, in particular, their use as light filters and light modulators for optoelectronic Brief Overview of Electrochromic devices, as well as methods for their preparation. The modern technologies of “Smart Windows”, Materials and Related Devices: A which are based on chromogenic materials and liquid crystals, are analyzed, and their advantages Nanostructured Materials Perspective. and disadvantages are also given. Various types of chromogenic materials are presented, examples Nanomaterials 2021, 11, 2376. https:// of which include photochromic, thermochromic and gasochromic materials, as well as the main doi.org/10.3390/nano11092376 physical effects affecting changes in their optical properties. Additionally, this study describes elec- trochromic technologies based on WO films prepared by different methods, such as electrochemical Academic Editor: Yi Long deposition, magnetron sputtering, spray pyrolysis, sol–gel, etc. An example of an electrochromic “Smart Window” based on WO is shown in the article. A modern analysis of electrochromic devices Received: 26 July 2021 3 based on nanostructured materials used in various applications is presented. The paper discusses Accepted: 3 September 2021 Published: 13 September 2021 the causes of internal and external size effects in the process of modifying WO electrochromic films using nanomaterials, in particular, GO/rGO nanomaterials. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: electrochromic materials; nanostructured electrochromic materials; electrochromism; published maps and institutional affil- color; “Smart Windows”; transition metal oxides (TMO); nanomaterials; graphene oxide (GO); iations. reduced graphene oxide (rGO) Copyright: © 2021 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. Modern technology has a number of negative effects, such as atmospheric pollution, This article is an open access article global warming, the reduction of fossil resources, etc. Therefore, one of the most important distributed under the terms and tasks in the world is to improve energy efficiency and energy savings. To this end, it conditions of the Creative Commons is necessary to create new materials in a variety of sectors, including engineering, agro- Attribution (CC BY) license (https:// industry, building construction, electronics manufacturing, etc., primarily with the aim of creativecommons.org/licenses/by/ using new technologies and “smart” materials. 4.0/). Nanomaterials 2021, 11, 2376. https://doi.org/10.3390/nano11092376 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 2376 2 of 32 Functional materials are dependent on their initial state and properties, as well as on the energy and external effects applied to the material. “Smart” materials have more than one functional state, depending on the impacting impulse, which can change over time [1]. Electrochromic materials (EC) are materials that are able to change color under the influence of an electric field. EC are of great interest, both from the scientific point of view and with respect to their application in various technical systems, including as the basis for the creation of electrochromic devices (ECD) with low power requirements, such as [2–4]: - “Smart Windows”; - Displays; - Reflective blinds; - Variable reflection mirrors; - Sensors. The main purpose of ECD is protection against light in the visible wavelength range (380–780 nm). ECD include an electrochromic coating in the form of the EC film and a counter electrode placed in an electrolyte (ionic conductor), which is located between transparent conductive electrodes—ITO (In O -SnO ) or FTO (SnO -SnF). The principle 2 3 2 2 of ECD operation is the transformation of optical light flux and the modulation of the coefficient of light reflection/transmission, resulting in an electrochemical reaction, i.e., the “Smart Window” effect. Thus, “Smart Window” technology allows savings due to use of smaller amounts of energy for air conditioning in summer, as well as for heating in winter; an average of more than 30% compared to conventional windows. The purpose of this review is to systematize and summarize the data on organic, inorganic and nanostructured electrochromic materials and related devices over the past 50 years. 2. “Smart Windows” There are chromogenic materials [3], better known as “smart” materials, that are currently experiencing great popularity. These materials modulate reflected or diffused light by means of physical effects of different types. The “Smart Window” based on chromogenic materials is widely used in architecture, cars (rear-view mirrors and intelligent window tinting), and aircraft illuminators (Boeing 787 Dreamliner) [3–5], while translucent structure technology [6] must also be mentioned. Chromogenic materials change color and transparency. The following types of chromogenic materials can be distinguished: electrochromic materials (EC) (external conditions—electric field); photochromic materials (PhC) (external conditions—light); thermochromic materials (ThC) (external conditions— heat); gasochromic materials (GhC) (external conditions—gas); polymer-dispersed liquid crystals (PDLC) and liquid crystal dispersions (LCD); SPD—suspended particles device are Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 32 placed between the two electroconductive coatings. These materials can serve as a basis for “Smart Window” technologies [7–11], which are shown in Figure 1. Figure 1. “Smart Windows” classification. Figure 1. “Smart Windows” classification. The structures of SPD and PDLC windows [6,10–12] are shown in Figure 2. SPD tech- nology (Figure 2a) uses suspended particles to modulate light transmission, arranging themselves in an alternating current field, and the film becomes transparent. In the ab- sence of the electric field, the SPD window acquires color and absorbs light. The SPD win- dow is similar in structure to the PDLC window (Figure 2b), apart from the fact that in the absence of an electric field, the film becomes semi-transparent. (а) (b) Figure 2. “Smart Windows” SPD and PDLC technology sandwich structures and operating principles: (а) SPD: off—light modulation mode, on—transparent mode; (b) PDLC: off—semi-transparent mode, on—transparent mode; 1—retaining film; 2—suspended particle; 3—adhesive layer; 4—glass; 5—conductor; 6—liquid crystal layer; 7—interlayer film; 8—liq- uid crystal. Electrochromic windows (ECW) control the transmission of light in the visible spec- trum and switches between tinted and transparent/semi-transparent states in response to low voltage signal (Figure 3). ECW create a comfortable indoor environment; moreover, they have lower power consumption in comparison with other chromogenic devices [13]: the ECW modulates reflected light under the control voltage, and in the absence of the control voltage, modulation of the transmitted light occurs [2–4,7]. Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 32 Nanomaterials 2021, 11, 2376 3 of 32 Figure 1. “Smart Windows” classification. The structures of SPD and PDLC windows [6,10–12] are shown in Figure 2. SPD tech- The structures of SPD and PDLC windows [6,10–12] are shown in Figure 2. SPD nology (Figure 2a) uses suspended particles to modulate light transmission, arranging technology (Figure 2a) uses suspended particles to modulate light transmission, arranging themselves in an alternating current field, and the film becomes transparent. In the ab- themselves in an alternating current field, and the film becomes transparent. In the absence sence of the electric field, the SPD window acquires color and absorbs light. The SPD win- of the electric field, the SPD window acquires color and absorbs light. The SPD window is similar in structure to the PDLC window (Figure 2b), apart from the fact that in the absence dow is similar in structure to the PDLC window (Figure 2b), apart from the fact that in of an electric field, the film becomes semi-transparent. the absence of an electric field, the film becomes semi-transparent. (а) (b) Figure 2. “Smart Windows” SPD and PDLC technology sandwich structures and operating principles: (а) SPD: off—light Figure 2. “Smart Windows” SPD and PDLC technology sandwich structures and operating principles: (a) SPD: off—light modulation mode, on—transparent mode; (b) PDLC: off—semi-transparent mode, on—transparent mode; 1—retaining modulation mode, on—transparent mode; (b) PDLC: off—semi-transparent mode, on—transparent mode; 1—retaining film; 2—suspended particle; 3—adhesive layer; 4—glass; 5—conductor; 6—liquid crystal layer; 7—interlayer film; 8—liq- film; 2—suspended particle; 3—adhesive layer; 4—glass; 5—conductor; 6—liquid crystal layer; 7—interlayer film; 8—liquid uid crystal. crystal. Electrochromic windows (ECW) control the transmission of light in the visible spec- Electrochromic windows (ECW) control the transmission of light in the visible spec- trum and switches between tinted and transparent/semi-transparent states in response to trum and switches between tinted and transparent/semi-transparent states in response to low voltage signal (Figure 3). ECW create a comfortable indoor environment; moreover, low voltage signal (Figure 3). ECW create a comfortable indoor environment; moreover, they have lower power consumption in comparison with other chromogenic devices [13]: they have lower power consumption in comparison with other chromogenic devices [13]: the ECW modulates reflected light under the control voltage, and in the absence of the the ECW modulates reflected light under the control voltage, and in the absence of the control voltage, modulation of the transmitted light occurs [2–4,7]. control voltage, modulation of the transmitted light occurs [2–4,7]. The advantages of electrochromic technologies are as follows [14]: - electric energy is required only during mode switching; - low activation voltage (1–5 V); - a wide variety of “Smart Window” tints (blue, grey, brown, etc.); - in the bleached state, electrochromic devices have a transparency level of 50–70%, in the colored state—10–25%. Table 1 shows the basic advantages and shortcomings of chromogenic materials used in “Smart Windows”. Nanomaterials 2021, 11, 2376 4 of 32 (a) (b) Figure 3. “Smart Window” electrochromic technology: (a) sandwich structure and operating principle (bleached state): transmitted and reflected light modulation: 1—electrochromic layer; 2—ion storage layer; 3—glass; 4—conductive layer; 5—ion conductor/electrolyte; (b) total view. Table 1. Comparison of “Smart Window” technologies. Energy Saving, Energy W/m (Energy Transparency, Modulation Cost, Technology Efficiency, Saving in % Time, s (c.u./m ) W/m Building) ECW + + + – – SPD – – + + + PDLC – – + + + LCD – – + + + Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 32 Table 1. Comparison of “Smart Window” technologies. Energy Energy Saving, W/m Modulation Cost, Technology Efficiency, (Energy Saving in Transparency, % Time, s (c.u./m ) W/m Building) ECW + + + – – SPD – – + + + Nanomaterials 2021, 11, 2376 5 of 32 PDLC – – + + + LCD – – + + + Electronic devices emit high levels of electromagnetic radiation (EMR) in a wide fre- Electronic devices emit high levels of electromagnetic radiation (EMR) in a wide fre- quency range, leading to electromagnetic pollution, which negatively influences biologi- quency range, leading to electromagnetic pollution, which negatively influences biological cal objects and causing electronic device dysfunction [15]. Considering the fact that elec- objects and causing electronic device dysfunction [15]. Considering the fact that electro- tromagnetic radiation basically penetrates through glass surfaces, the problem of creating magnetic radiation basically penetrates through glass surfaces, the problem of creating a a universal electrochromic film capable of absorbing or reflecting electromagnetic radia- universal electrochromic film capable of absorbing or reflecting electromagnetic radiation tion is becoming relevant. is becoming relevant. 3. Electrochromism and Electrochromic Materials: Classification and Applications 3. Electrochromism and Electrochromic Materials: Classification and Applications Chromism (from ancient Greek ῶ (“color”)) is a phenomenon of material color Chromism (from ancient Greek   (“color”)) is a phenomenon of material color change under the influence of physical factors, such as electric field, heat, light or pressure change under the influence of physical factors, such as electric field, heat, light or pres- [16]. sure [16]. At At the end of the 1960s, sc the end of the 1960s, scientist ientist S. K. S. K. Deb Deb discover discovered ed the phenomenon ca the phenomenon called lled elec elec-- tr trochromism ochromism [[17]. This ne 17]. This newly wly ddiscov iscover er ed phenomen ed phenomenon on belongs to the sphere o belongs to the spheref electro- of elec- tr chemistry ochemistry and physic and physics s [18]. S. K. Deb d [18]. S. K. Deb esc described ribed a new electropho a new electrophotographic tographic system con- system consisting sisting of WO of WO 3 thin film thin film and a th and in-film a thin-film photoconductive layer photoconductive layer placed between placed between two elec- two electr trodes. odes. When When this composite structur this composite structur e was subjec e was subjected ted to an electric to an electric field, an field, optic an al projec- optical projection appeared. After subsequent modulation in the photoconductive layer, the oxide tion appeared. After subsequent modulation in the photoconductive layer, the oxide layer layer acquires the same color, and a visible image appears [17–19]. acquires the same color, and a visible image appears [17–19]. Since the middle of the 1970s, electrochromism has been considered to be a physi- Since the middle of the 1970s, electrochromism has been considered to be a physical cal phenomenon associated with a reversible change in transparency or color under the phenomenon associated with a reversible change in transparency or color under the in- influence of an electric field or electric current [19,20]. Electrochromism is traditionally fluence of an electric field or electric current [19,20]. Electrochromism is traditionally de- defined as a reversible change in optical properties (transparency and/or reflectivity) fined as a reversible change in optical properties (transparency and/or reflectivity) during during the oxidation–reduction reaction [21–23]. In some cases, there are more than two the oxidation–reduction reaction [21–23]. In some cases, there are more than two degrees degrees of oxidation, and the material is capable of showing several colors depending on of oxidation, and the material is capable of showing several colors depending on the cur- the current degree of oxidation (polyelectrochromic materials) [21]. Modern science uses a rent degree of oxidation (polyelectrochromic materials) [21]. Modern science uses a broad broad definition of electrochromism, including materials and devices used for the optical definition of electrochromism, including materials and devices used for the optical mod- modulation of radiation in the visible and microwave ranges. Ref. [24] focuses on the ulation of radiation in the visible and microwave ranges. Ref. [24] focuses on the problem problem of developing electrochromic displays that should replace LED and liquid crystal of developing electrochromic displays that should replace LED and liquid crystal dis- displays. In 1985, Svensson and Granqvist proposed using electrochromic materials in plays. In 1985, Svensson and Granqvist proposed using electrochromic materials in “Smart Windows” [14], and thus the term “Smart Window” appeared. The electrochromic “Smart Windows” [14], and thus the term “Smart Window” appeared. The electrochromic reaction can be described by the electrochemical equation in oxidized form: reaction can be described by the electrochemical equation in oxidized form: O + xe + Cation $ Reduced form, R (1) О + xe + Cation ↔ Reduced form, R (1) Applications of electrochromism include: Applications of electrochromism include: − Control of energy transfer in different environments, for example, filtering solar ra- - Control of energy transfer in different environments, for example, filtering solar radia- diation using “Smart Window” devices [25–27]. Fast mode switching (col- tion using “Smart Window” devices [25–27]. Fast mode switching (colored/bleached) ored/bleached) is not required, but the device should be capable of filtering both vis- is not required, but the device should be capable of filtering both visible and near- ible and near-infrared radiation. Moreover, the transparency of the window packages infrared radiation. Moreover, the transparency of the window packages must be at must be at least 70%. least 70%. − Color displays [22,24], for example, advertising boards. The requirements are as fol- - Color displays [22,24], for example, advertising boards. The requirements are as lows: fast mode switching, color scheme varies only in the visible area. Moreover, follows: fast mode switching, color scheme varies only in the visible area. Moreover, color color contr contrast ast shou should ld be be hi high gh enou enough, gh, tr transpar ansparent mo ent mode de i iss not re not requir quired. ed. -− Mirr Mirror or light light modulators [7], for ex modulators [7], for example, ample, antiglar antiglare e mirr ors mirrors for cars. for cars. Fast mode Fast switch- mode ing swit and ching high and high transpar trency ansparency are not a rr equir e noted. required. 3.1. Classification of Electrochromic Materials There are several inorganic and organic EC that change their optical properties (trans- parency, color) during oxidation–reduction [28–32]. Switching between oxidation and reduction states leads to color formation, i.e., formation of new spectral peaks in the visible area. Inorganic EC include transition metal oxides (TMO) from groups IV-VI [32], and hexacyanometallates (Prussian blue). Organic EC include viologens, conjugated conductive polymers (polypyrrole, polythiophene, polyaniline and their derivatives, metal polymers, metal phthalocyanines) [33,34]. The viologen family (4,4 -dipyridinium compounds) has Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 32 3.1. Classification of Electrochromic Materials There are several inorganic and organic EC that change their optical properties (transparency, color) during oxidation–reduction [28–32]. Switching between oxidation and reduction states leads to color formation, i.e., formation of new spectral peaks in the visible area. Inorganic EC include transition metal oxides (TMO) from groups IV-VI [32], and hexacyanometallates (Prussian blue). Organic EC include viologens, conjugated con- Nanomaterials 2021, 11, 2376 6 of 32 ductive polymers (polypyrrole, polythiophene, polyaniline and their derivatives, metal polymers, metal phthalocyanines) [33,34]. The viologen family (4,4′-dipyridinium com- pounds) has a general chemical formula as shown in Figure 4, where R may be an alkyl, a general chemical formula as shown in Figure 4, where R may be an alkyl, cyclo-alkyl cyclo-alkyl or other substitute, and X corresponds to halogen 4,4′-dipyridium compounds, or other substitute, and X corresponds to halogen 4,4 -dipyridium compounds, because because they turn a deep blue-purple on reduction [30]. The viologen ion as shown in they turn a deep blue-purple on reduction [30]. The viologen ion as shown in Figure 4a Figure 4a can have a two-step reduction, i.e., a one-electron or a two-electron reduction. can have a two-step reduction, i.e., a one-electron or a two-electron reduction. The general The general structure structur for e viol forog viologens ens mo modifying difying th the e titania titania surface surface is shown is shown in Figur in eFigure 4c. Table 4c 2. presents a list of the most popular EC. Table 2 presents a list of the most popular EC. (a) (b) (c) Figure Figure 4. Viologen: 4. Violo (gen a) general : (a) gen chemical eral chemic formulae al of form viologen; ulae of (b)vviologen iologen; ion; (b)( c vi ) general ologens io tructur n; (c) e gen for viologens eral strumodifying cture the titania surface. for viologens modifying the titania surface. Table 2 shows a general classification of EC. Table 2 shows a general classification of EC. Table 2. General classification of EC. Table 2. General classification of EC. EC Class Chemical Name Application Ref. Organic EC Class Chemical Name Application Ref. PEDOT (where EDOT = C H O S), 6 6 2 Organic Conductive PPy (where Py = Pyrrole = C H N), 4 5 “Smart Windows”, displays [13,33] PEDOT (where EDOT = C6H6O2S), polymers PT (where T = thiophene = C H S), 4 4 “Smart PANI (where ANI = aniline = C H S) Conductive PPy (where Py = Pyrrole = C4H5N),6 4 Windows”, [13,33] 3-aryl-4,5-bis (pyridine-4-yl) isoxazole Antiglare mirrors and polymers PT (where T = thiophene = C4H4S), Viologens [21,28] displays derivatives displays PANI (where ANI = aniline = C6H4S) II Transition metals and poly [Ru (vbpy) (py) ]Cl (being py = 2 2 2 Ant Smart igmirr lareors [26,30] lanthanoids pyridine = C H N) 5 5 Viologens 3-aryl-4,5-bis (pyridine-4-yl) isoxazole derivatives mirrors and [21,28] Metal phthalocyanines (Pc) [Lu(Pc) ] being Pc = C H N et al. Displays [7,30] 2 32 18 8 displays Inorganic II Transition metals and poly [Ru (vbpy)2(py)2]Cl2 (being py = pyridine = Transition metal oxides WO , MoO , V O , TiO Nb O , Ir(OH) , “Smart Windows”, antiglare 3 3 2 5 2 2 5 3 Smart mirrors [26,30] [32,34] lanthanoids С5H5N) (TMOs) NiO et al. mirrors Metal phthalocyanines (Pc) [Lu(Pc)2] being P Prussian c = C blue 32H(C 18N8 Fe et a N l.), Displays [7,30] 18 7 18 Prussian brown (C Fe N ), Prussian 6 2 6 Inorganic Prussian blue (PB) “Smart Windows”, displays [7,29] green (C FeN ), Prussian white 3 3 “Smart (C Fe N ) 6 3 6 Transition metal oxides Windows”, WO3, MoO3, V2O5, TiO2 Nb2O5, Ir(OH)3, NiO et al. [32,34] (TMOs) antiglare In general, organic EC, possessing color-changing abilities, exhibit faster response times and higher staining efficiencies than inorganic ones, but have a low UV protection mirrors index and show lower electrochemical stability. Therefore, mainly organic EC materials Prussian blue (C18Fe7N18), “Smart are used in electronic non-emissive displays [24,28]. Inorganic EC materials show high Prussian blue (PB) Prussian brown (С6Fe2N6), Prussian green (C3FeN3), Windows”, [7,29] chemical stability and cyclicity, which makes them suitable for “Smart Windows” and Prussi large-scale an whdata ite (C displays 6Fe3N6) [35 ]. displays Electrochromic materials are classified according to their solubility and according to their redox states [29,30]. Classification of EC was introduced by I. F. Chang in 1975 [36]. In general, organic EC, possessing color-changing abilities, exhibit faster response According to this classification, there are three types of EC solubility in redox states [29]: times and higher staining efficiencies than inorganic ones, but have a low UV protection (1) Type I EC materials, such as viologen, heptyl, etc., are soluble in both their reduced index and show lower electrochemical stability. Therefore, mainly organic EC materials and oxidized states. are used in electronic non-emissive displays [24,28]. Inorganic EC materials show high (2) Type II EC materials are soluble in their colorless redox state but form a solid film on the electrode surface. Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 32 chemical stability and cyclicity, which makes them suitable for “Smart Windows” and large-scale data displays [35]. Electrochromic materials are classified according to their solubility and according to their redox states [29,30]. Classification of EC was introduced by I. F. Chang in 1975 [36]. According to this classification, there are three types of EC solubility in redox states [29]: (1) Type I EC materials, such as viologen, heptyl, etc., are soluble in both their reduced and oxidized states. Nanomaterials 2021, 11, 2376 7 of 32 (2) Type II EC materials are soluble in their colorless redox state but form a solid film on the electrode surface. (3) Type III EC materials are solids in both redox states, and they form an insoluble film (3) Type III EC materials are solids in both redox states, and they form an insoluble film on on the electrode surface. Type III materials include groups IV, V transition metal ox- the electrode surface. Type III materials include groups IV, V transition metal oxides ides (TMO), conductive polymers, Prussian blue and metal polymers. Three types of (TMO), conductive polymers, Prussian blue and metal polymers. Three types of mechanism for chan mechanism ging color/t forra changing nsparen color/transpar cy (according ency to (accor I. F. C ding han to g) I.ar F.e Chang) present ared e pr esented in Figure 5. in Figure 5. (a) (b) (c) Figure 5. Types of ECW: (a) type I (solution); (b) type II (hybrid); (c) type III (battery-powered); EC—electrochromic layer; Figure 5. Types of ECW: (а) type I (solution); (b) type II (hybrid); (c) type III (battery-powered); CE—counter-electrode layer; TCO—inorganic oxide. EC—electrochromic layer; CE—counter-electrode layer; TCO—inorganic oxide. The electrochromic reaction can be described by the following equation: The electrochromic reaction can be described by the following equation: n m na (m+a)/y EC + yCE $ EC + yCE (2) (bleached) (coloured) n m n−a (m+a)/y (2) EC + yCE (bleached) ↔ EC (coloured) + yCE Table 3 contains examples of each type of EC. Table 3 contains examples of each type of EC. Table 3. Classification of EC materials (according to I. F. Chang [36]). Table 3. Classification of EC materials (according to I. F. Chang [36]). EC Electrochromic Reaction EC Material Application Ref. Type Mechanism EC Electrochromic Reaction EC Material Application Ref. Type Mechanism (1) Methylviologene (MV, 1, 10-dimethyl-4,4 -bipyrindnium, (1) Methylviologene (MV, 1, 10-dimethyl- 2+ I 3-aryl-4,5-bis (pyridine-4-yl) MV + Night vision systems, [37,38] 4,4′-bipyrindnium, 3-aryl-4,5-bis (pyri- + (solution) isoxazole; e $MV mirrors (bleached) (colored) I Night vision systems, 2+ − +● (2) Phenothiazine (C H NS) in dine-4-yl) isoxazole; 12 MV 9 + e (bleached)↔MV (colored) [37,38] (solution) mirrors non-aqueous solution (2) Phenothiazine (C12H9NS) in non-aque- ous solution (1) Cyanophenylparaquate (CPQ, 1-1 cyanophenyl-4,4 -bipyridine, (1) Cyanophenylparaquate (CPQ, 1-1 cy- paraquat = C H Cl N , otherwise 12 14 2 2 anophenyl-4,4′-bipyridine, paraquat = known as viologen, due to the 2+ II CPQ + e + Electrochromic paper, herbicide name) in aqueous solution [39–41] C12H14Cl2N2, otherwise known as vio- (hybrid) X $[CPQ X ] “Smart Window” (2) Heptyl or benzylviologene (HV or Electrochromic logen, due to the herbicide name) in aque- BzV) or methoxyfluorene compounds II 2+ − − +● − CPQ + e + X ↔[ CPQ X ] paper, “Smart [39–41] ous solution C H Cl F O in acetonitrile solution 3 4 2 2 (hybrid) (C H N) 2 3 Window” (2) Heptyl or benzylviologene (HV or BzV) or methoxyfluorene compounds (1) Almost all inorganic EC materials, C3H4Cl2F2O in acetonitrile solution such as transition metal oxides: WO , MoO , V O , TiO Nb O , Ir(OH) , (C2H3N) 3 2 5 2 2 5 3 NiO; (1) Almost all inorganic EC materials, “Smart Window” III (2) Phthalocyanine (Pc = C H N ) “Smart Window” 32 18 8 + III MO + x(H + (battery- (3) Metal complexes and (Boeing 757), [32,42,43] such as transition metal oxides: WO3, MOy + x(H + (Boeing 757), e )$H MO x y(colored) (battery- [32,42,43] powered) hexacyanometallates, such as Prussian Electro-chromic paper MoO3, V2O5, TiO2 Nb2O5, Ir(OH)3, NiO ; e )↔HxMOy(colored) Electro-chromic pa- blue (PB = C Fe N ) 18 7 18 powered) (2) Phthalocyanine (Pc = C32H18N8) per (4) Conductive polymers: polypyrrole (PPy), polythiophene (PT), polyaniline (PANI) Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 32 (3) Metal complexes and hexacyanometal- lates, such as Prussian blue (PB = C18Fe7N18) (3) Metal complexes and hexacyanometal- (4) Conductive polymer lates, such s: p as olypyrro Prussian le b lue (PB = (PPy), polythiophene (PT), polyaniline C18Fe7N18) (PANI) (4) Conductive polymers: polypyrrole (PPy), polythiophene (PT), polyaniline Nanomaterials 2021, 11, 2376 8 of 32 Type I and Type II EC are self-erasing, since an electrical current is required to main- (PANI) tain the colored state, i.e., after the power is turned off, the ECW loses its color. Type III ECW (battery-powered) remain colored for some time after the voltage is removed. Elec- Type I and Type II EC are self-erasing, since an electrical current is required to main- Type I and Type II EC are self-erasing, since an electrical current is required to main- trochromic technologies make it possible to modulate the optical properties, such as color, tain the colored state, i.e., after the power is turned off, the ECW loses its color. Type III tain the colored state, i.e., after the power is turned off, the ECW loses its color. Type III light transmission coefficient T(λ), reflection coefficient R(λ), and absorption coefficient ECW (battery-powered) remain colored for some time after the voltage is removed. Elec- ECW (battery-powered) remain colored for some time after the voltage is removed. Elec- A(λ), of materials atr ccor ochr din omic g to Kirchho technologiesff’s law make it possible [44]: to modulate the optical properties, such as color, trochromic technologies make it possible to modulate the optical properties, such as color, light transmission coefficient T(l), reflection coefficient R(l), and absorption coefficient R(λ) + A(λ) + T(λ) = 1 (3) light transmission coefficient T(λ), reflection coefficient R(λ), and absorption coefficient A(l), of materials according to Kirchhoff’s law [44]: A(λ), of materials according to Kirchhoff’s law [44]: All of these optical processes (Figure 6) are characterized by the EC transmittance R(λ) + A(λ) + T(λ) = 1 (3) R(l) + A(l) + T(l) = 1 (3) T(λ), absorption A(λ) and reflectance R(λ), which indicates the proportion of the incident light intensity that passes through, is absorbed by, or is reflected by the EC. All of these optical processes (Figure 6) are characterized by the EC transmittance All of these optical processes (Figure 6) are characterized by the EC transmittance T(l), absorption A(l) and reflectance R(l), which indicates the proportion of the incident T(λ), absorption A(λ) and reflectance R(λ), which indicates the proportion of the incident light intensity that passes through, is absorbed by, or is reflected by the EC. light intensity that passes through, is absorbed by, or is reflected by the EC. (a) (b) (с) (d) Figure 6. Interaction of radiation with an EC: (a) reflection; (b) absorption; (c) dispersion; (d) transmittance. (a) (b) (с) (d) Electrochromic properties depend on the electrochromic film structure; thus, differ- Figure 6. Figure Intera 6.ct Interaction ion of radia of radiation tion with an with an EC EC: : (a() arefle ) reflection; ction; ((b b)) absorption; absorption; (c)(dispersion; c) dispersio (dn; ( ) transmittance. d) transmittance. ent EC have different absorption spectra, and, consequently, differ in color. Electrochromic properties depend on the electrochromic film structure; thus, different Electrochromic properties depend on the electrochromic film structure; thus, differ- EC have different absorption spectra, and, consequently, differ in color. 3.2. Organic EC ent EC have different absorption spectra, and, consequently, differ in color. 3.2. Organic EC Organic films, such as conductive polymers, have multiple colored states, possess 3.2. Organic EC high optical contrast, and e Organic xhibit f films, assuch t respo as nse time conductive an polymers, d high staini haveng effic multiple iency color [45 ed states, –49]. possess high optical contrast, and exhibit fast response time and high staining efficiency [45–49]. Electrochromic behavior is observed in conjugated pyridine derivatives such as vio- Organic films, such as conductive polymers, have multiple colored states, possess Electrochromic behavior is observed in conjugated pyridine derivatives such as violo- logens (Figure 7), which exhibit high cyclicity, low operating potential and other valuable high optical contrast, and exhibit fast response time and high staining efficiency [45–49]. gens (Figure 7), which exhibit high cyclicity, low operating potential and other valuable properties [50,51]. Viologens exist in solid crystalline form and in powder form. The name Electrochromic behavior is observed in conjugated pyridine derivatives such as vio- properties [50,51]. Viologens exist in solid crystalline form and in powder form. The name ‘viologen’ alludes to violet, one color it can exhibit (Figure 7). logens (Figure 7), which exhibit high cyclicity, low operating potential and other valuable ‘viologen’ alludes to violet, one color it can exhibit (Figure 7). properties [50,51]. Viologens exist in solid crystalline form and in powder form. The name ‘viologen’ alludes to violet, one color it can exhibit (Figure 7). (a) (b) (c) Figure 7. Three general viologen redox states (in terms of electron transfer): (a) dication; (b) radical cation; (c) neutral state. Figure 7. Three general viologen redox states (in terms of electron transfer): (а) dication; (b) radical (a) (b) (c) cation; (c) neutral state. Viologens are used in RGB (red, green, blue) devices (Figure 8), which reproduce three Figure 7. Three general viologen redox states (in terms of electron transfer): (а) dication; (b) radical main colors, red, green and blue, although research in this area is not yet well developed. Viologens are used in RGB (red, green, blue) devices (Figure 8), which reproduce cation; (c) neutral state. Modern technologies require the use of multicolor EC, which, in turn, necessitates the three main colors, red, green and blue, although research in this area is not yet well de- creation of new functional composites [50]. veloped. Modern technologies require the use of multicolor EC, which, in turn, necessi- Viologens are used in RGB (red, green, blue) devices (Figure 8), which reproduce tates the creation of new functional composites [50]. three main colors, red, green and blue, although research in this area is not yet well de- veloped. Modern technologies require the use of multicolor EC, which, in turn, necessi- tates the creation of new functional composites [50]. Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 32 Nanomaterials 2021, 11, 2376 9 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 32 E = 0 V Е = 1.5 V E = 0 V Е = 1.5 V E = 0 V Е = 1.5 V Е = 0 V Е = 1.5 V Е = 0 V Е = 1.5 V Figure 8. Electrochromic transition cycle. Е = 0 V Е = 1.5 V Figure 8. Electrochromic transition cycle. The advantages of organic EC include compatibility with flexible substrates, low pro- Figure 8. Electrochromic transition cycle. duction cost and the possibility of adjusting their synthetic material properties. The advantages of organic EC include compatibility with flexible substrates, low pro- Figure 8. Electrochromic tran Thesadvantages ition cycle. of organic EC include compatibility with flexible substrates, low Phenylenediamine (PD) derivatives are of interest due to their stable electrochemical duction cost and the possibility of adjusting their synthetic material properties. production cost and the possibility of adjusting their synthetic material properties. reactions at the anode [52]. It is interesting to note that neutral arylamine is often colorless Phenylenediamine (PD) derivatives are of interest due to their stable electrochemical Phenylenediamine (PD) derivatives are of interest due to their stable electrochemical The advantages of organic EC include compatibility with flexible substrates, low pro- (it mostly absorbs UV light), but in redox states, it exhibits vivid color (Figure 9). Phe- reactions at the anode [52]. It is interesting to note that neutral arylamine is often colorless reactions at the anode [52]. It is interesting to note that neutral arylamine is often colorless duction cost and the possibility of adjusting their synthetic material properties. nylenediamines exhibit modulated visible absorption properties and high redox stability, (it mostly absorbs (it mostly UV ligabsorbs ht), but UV in rlight), edox st but ates, in rit edox exhibits states, vivi it exhibits d color vivid (Figucolor re 9).(Figur Phe- e 9). Phenylene- Phenylenediamine (PD) derivatives are of interest due to their stable electrochemical nylenediamines exhibit modulated visible absorption properties and high redox stability, which makes them sui diamines table f exhibit or RGB modulated devices visible [53]. absorption properties and high redox stability, which reactions at the anode [52]. It is interesting to note that neutral arylamine is often colorless which makes them suitable for RGB devices [53]. makes them suitable for RGB devices [53]. (it mostly absorbs UV light), but in redox states, it exhibits vivid color (Figure 9). Phe- nylenediamines exhibit modulated visible absorption properties and high redox stability, which makes them suitable for RGB devices [53]. Neutra Neutra l: l: Ra Radical dical cat cat ion ion : : Dicatio Dicatio n: n: Non-colored Colored Colored Non-colored Colored Colored Absorbs UV radiation High absorption in the Weak absorption in the Absorbs UV radiation High absorption in the Weak absorption in the near-infrared range near-infrared range near-infrared range near-infrared range Neutral: Radical cation: Dication: Figure 9. Redox chemistry of Phenylenediamine (Wurster’s blue), description of optical behavior in Non-colored Colored Colored Figure 9. Redox chemistry of Phenylenediamine (Wurster’s blue), description of optical behavior in Figure 9. Redox chemistry of Phenylenediamine (Wurster ’s blue), description of optical behavior in redox states. redox states. redox states. Absorbs UV radiation High absorption in the Weak absorption in the The color-changing abilities of conductive PEDOT polymers [54] are useful in elec- The color-changing abilities of conductive PEDOT polymers [54] are useful in elec- near-infrared range near-infrared range trochromic non-emissive displays (Figure 10). trochromic non-emissive displays (Figure 10). The color-changing abilities of conductive PEDOT polymers [54] are useful in elec- Figure 9. Redox chemistry of Phenylenediamine (Wurster’s blue), description of optical behavior in trochromic non-emissive displays (Figure 10). redox states. The color-changing abilities of conductive PEDOT polymers [54] are useful in elec- trochromic non-emissive displays (Figure 10). (a) (b) (c) (d) (e) Figure 10. New electrochromic compounds obtained by reactions involving the cycloaddition of Figure 10. New electrochromic compounds obtained by reactions involving the cycloaddition of nitrile oxides to 1,2-bis nitrile oxides to 1,2-bis (4-pyridinyl) ethylene derivatives (electrochromic transition): (a)—neutral (4-pyridinyl) ethylene derivatives (electrochromic transition): (a)—neutral state; (b–e)—oxidized states. (a) (b) (c) (d) (e) state; (b–e)—oxidized states. Figure 10. New electrochromic compounds obtained by reactions involving the cycloaddition of nitrile oxides to 1,2-bis (4-pyridinyl) ethylene derivatives (electrochromic transition): (a)—neutral state; (b–e)—oxidized states. (a) (b) (c) (d) (e) Figure 10. New electrochromic compounds obtained by reactions involving the cycloaddition of nitrile oxides to 1,2-bis (4-pyridinyl) ethylene derivatives (electrochromic transition): (a)—neutral state; (b–e)—oxidized states. Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 32 Nanomaterials 2021, 11, 2376 10 of 32 Heterocyclic aromatic compounds (Figure 11), such as thiophene, aniline, furan, car- N N Nano ano anom m materia ateria aterials ls ls 2021 2021 2021, , , 11 11 11, , , x FO x FO x FOR P R P R PE E EER RE ER RE ER REVIEW VIEW VIEW 10 10 10 of of of 32 32 32 Nano Nano materia materia ls 2021 ls 2021 , 11, , 11 x FO , x FO R PR P EER RE EER RE VIEW VIEW 10 10 of 32 of 32 N N N N ano N ano ano ano ano Nm m ano m m ateria m ateria ateria ateria ateria materia ls ls ls ls ls 2021 2021 2021 2021 ls 2021 2021 , , , 11 , 11 , 11 11 11 , , , , x FO , 11 x FO , x FO x FO x FO , x FO R P R P R P R P R P E R P E E E ER RE ER RE E ER RE ER RE ER RE EER RE VIEW VIEW VIEW VIEW VIEW VIEW 10 10 10 10 10 of 10 of of of of 32 32 of 32 32 32 32 bazole, azulene and indole [55,56], can be oxidized chemically or electrochemically to Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 32 Heterocyclic aromatic compounds (Figure 11), such as thiophene, aniline, furan, form anion-doped polypyrrole (PPy), polythiophene (PT) or polyaniline (PANI), poly(3- carbazole, azulene and indole [55,56], can be oxidized chemically or electrochemically to methyla Hetero n Hetero Hetero Hetero ilinecycl ) cycl cycl cycl (M ic ic ic ic EPA ar ar ar ar om om om om ), at at at at ic c po ic c ic c ic c om ly om om om (po 3 po po po -m unds unds unds unds eth (Fig yl (Fig (Fig (Fig -tu hioph u u u re re re re 11), 11), 11), 11), ene) ss s uch such uch uch as (P3MTh as as as th th th th iophen iophen iophen iophen ), ean e e e , , , , an an an an d ilil il il in po in in in e, e, e, e, ly f ura f f f(ura ura ura 3-n met n n n , , , , car- car- car- car- hylpyrrole) Heterocyclic aromatic compounds (Figure 11), such as thiophene, aniline, furan, car- form anion-doped polypyrrole (PPy), polythiophene (PT) or polyaniline (PANI), poly(3- Hetero Hetero Hetero Hetero Hetero Hetero cycl cycl cycl cycl cycl cycl ic ic ic ic ic ar ar ic ar ar ar om om om ar om om om at at at at at ic c ic c ic c at ic c ic c ic c om om om om om om po po po po po unds unds po unds unds unds unds (Fig (Fig (Fig (Fig (Fig (Fig u u u u re u re re re re u11), 11), re 11), 11), 11), 11), s ss uch s uch s uch uch uch such as as as as as th th as th th th iophen iophen th iophen iophen iophen iophen e ee e , , e , , an an , e an an an , il il an il il in il in in in in il e, e, e, in e, e, f f e, ura f ura ff ura ura ura fura n n n n , n , , , car- car- , n car- car- car- , car- baz baz baz baz oo o o le, le, le, le, az az az az ul ul ul ul ee e e ne ne ne ne an an an an d d d d indo indo indo indo le l lle e e [55, [ [ [55, 55, 55, 56] 56] 56] 56] , ,,, can can can can b b b b e e e e ox ox ox ox idized idized idized idized chem chem chem chem ically ically ically ically or or or or electroch electroch electroch electroch emi emi emi emi cally cally cally cally to to to to bazole, azulene and indole [55,56], can be oxidized chemically or electrochemically to (P3MPy). A change in the redox state (oxidized conductive state, reduced non-conductive methylaniline) (MEPA), poly(3-methyl-thiophene) (P3MTh), and poly(3-methylpyrrole) bazoHetero le, azul cycl ene ic an arom d indo atic c le om [55, po56] unds , can (Fig b ue re ox 11), idized such chem as thiophen ically e or , an electroch iline, fura emi n, car- cally to baz baz baz baz baz o o o o le, le, o le, le, le, az az az az az ul ul ul ul ul e ee e ne ne e ne ne ne an an an an an d d d d d indo indo indo indo indo lle l e ll e e e [ [55, [ 55, [[ 55, 55, 55, 56] 56] 56] 56] 56] ,, ,, ,can can can can can b b b b e e b e e e ox ox ox ox ox idized idized idized idized idized chem chem chem chem chem ically ically ically ically ically or or or or or electroch electroch electroch electroch electroch emi emi emi emi emi cally cally cally cally cally to to to to to form form form form an an an an ion ion ion ion -d - - -d d d op op op op ed ed ed ed po po po po ly ly ly ly pyr pyr pyr pyr role role role role (PPy ( ( (PPy PPy PPy ),), ), ), po po po po ly ly ly ly th th th th iophen iophen iophen iophen e e e e (PT) ( ( (PT) PT) PT) or or or or po po po po lyan l llyan yan yan ilil il il in in in in e e e e (PANI ( ( (PANI PANI PANI ), ) ) ), , , po po po po ly ly ly ly (3 ( ( (3 3 3 -- - - form anion-doped polypyrrole (PPy), polythiophene (PT) or polyaniline (PANI), poly(3- (P3MPy). A change in the redox state (oxidized conductive state, reduced non-conductive state, n baz eutr ole, al azstate) ulene an led ads indo to le chan [55,56] ges , can in bcolor e oxidized caused chem by ically sigor ni fi electroch cant chan emically ges in to the visible form form anion anion -dop -ded oped poly po pyr lypyr role role (PPy (PPy ), po ), ly po th ly iophen thiophen e (PT) e (PT) or po or lpo yan lyan ilinil e in (PANI e (PANI ), po ), ly po (3 ly - (3- form form form form an an an an ion ion ion ion -- d -- d d d op op op op ed ed ed ed po po po po ly ly ly ly pyr pyr pyr pyr role role role role (( PPy (( PPy PPy PPy ), ), ), ), po po po po ly ly ly ly th th th th iophen iophen iophen iophen e e e e (( PT) (( PT) PT) PT) or or or or po po po po ll yan ll yan yan yan il il il il in in in in e e e e (( PANI (( PANI PANI PANI )) , )) , , po , po po po ly ly ly ly (( 3 (( 3 3 - 3 - -- methylaniline) (MEPA), poly(3-methyl-thiophene) (P3MTh), and poly(3-methylpyrrole) met met met hyla hyla hyla nn n iline iline iline ) ) ) (M (M (M EPA EPA EPA ),), ), po po po ly ly ly (3 ( (3 3 -m - -m m eth eth eth yl yl yl -t - -hioph t thioph hioph ene) ene) ene) (P3MTh (P3MTh (P3MTh ), ) ), , an an an d d d po po po ly ly ly (3 ( (3 3 -met - -met met hy hy hy lpyrrol lpyrrol lpyrrol e) e) e) methylaniline) (MEPA), poly(3-methyl-thiophene) (P3MTh), and poly(3-methylpyrrole) state, neutral state) leads to changes in color caused by significant changes in the visible and form anion-doped polypyrrole (PPy), polythiophene (PT) or polyaniline (PANI), poly(3- met met hyla hyla niline niline ) (M ) (M EPA EPA ), po ), ly po (3 ly -m (3eth -meth yl-t yl hioph -thioph ene) ene) (P3MTh (P3MTh ), an ), d an po d ly po (3 ly -met (3-met hylpyrrol hylpyrrol e) e) andmet met met nea met hyla hyla hyla hyla r-inf n n n n iline iline r iline iline ared ) ) ) ) (M (M (M (M absorpt EPA EPA EPA EPA ), ), ), ), po po po po ion ly ly ly ly (( 3 ((spect 3 3 - 3 - m -- m m m eth eth eth eth r yl yl a yl yl -- t - th - t hioph tt hioph hioph hioph at vene) ar ene) ene) ene) y depe (P3MTh (P3MTh (P3MTh (P3MTh nding )) , )) , , , an an an an d d on d d po po po po th ly ly ly ly (e ( 3 (( 3 3 - 3 degree - met -- met met met hy hy hy hy lpyrrol lpyrrol lpyrrol lpyrrol of ox e) e) e) ie) dation/re- (P3MP (P3MPy) y). . A A change change in in th the e r redox edox state state (ox (oxid idiize zed d con cond du uct ctive ive state, state, re reduced duced non non- -c co onduct nductive ive (P3MP (P3MP (P3MP y) y) . y) . A A . change A change change in in th in th e th e redox r e edox redox state state state (ox (ox (ox id id iid ze ize id ze d con d con con dd uu d ct ct u ive ct ive ive state, state, state, re re duced re duced duced non non non -c -c o- o nduct cnduct onduct ive ive ive near-infrared absorption spectra that vary depending on the degree of oxidation/reduction methylaniline) (MEPA), poly(3-methyl-thiophene) (P3MTh), and poly(3-methylpyrrole) (P3MP (P3MP (P3MP y) y) . . y) A A . change A change change in in th in th e th e rr edox e edox redox state state state (ox (ox (ox id id ize id ize i d ze d con con d con d d uu ct d ct u ive ive ctive state, state, state, re re duced duced reduced non non non -c -c oo - nduct c nduct onduct ive ive ive (P3MP (P3MP (P3MP y) y) y) . . A . A A change change change in in in th th th e e e rr edox redox edox state state state (ox (ox (ox id id id ize iize ze d d d con con con dd d uu u ct ct ct ive ive ive state, state, state, re re re duced duced duced non non non -c --c o co nduct onduct nduct ive ive ive duction Switching between polymer films in their colored (reduced) and uncolored (oxi- state, state, state, n n neutr eutr eutral al al state) state) state) le le leads ads ads t t to o o chan chan changes ges ges in in in color color color c c caused aused aused by by by s s sig ig igni ni nifi fi fic c cant ant ant chan chan changes ges ges in in in th th the e e vis vis visibl ibl ible e e state, state, neutr neutr al state) al state) leads leads to t chan o chan ges ges in in color color caused caused by by sigs ni ig fi ni cant ficant chan chan ges ges in in th e th vis e vis iblibl e e Switching between polymer films in their colored (reduced) and uncolored (oxidized) (P3MPy). A change in the redox state (oxidized conductive state, reduced non-conductive state, state, state, state, state, state, n n n n eutr n eutr eutr eutr eutr neutr al al al al al state) state) al state) state) state) state) le le le le le ads ads ads ads le ads ads t to t o tt o o o chan chan tchan o chan chan chan ges ges ges ges ges ges in in in in in color color in color color color color c cc c aused aused c aused aused aused caused by by by by by by s ss ig s ig s ig ig ig s ni ni ni ig ni ni fi fi fi ni fi fi c cc c ant ant c fi ant ant ant cant chan chan chan chan chan chan ges ges ges ges ges ges in in in in in th th in th th th e e th e e e vis vis vis vis e vis vis ibl ibl ibl ibl ibl e e ibl e e e e dized) states changes their color from yellow to orange, red, purpuric, dark blue, green, and and and and nea nea nea nea rr r - rinf - - -inf inf inf rar r r rar ar ar ed ed ed ed absorpt absorpt absorpt absorpt ion i iion on on spect spect spect spect ra r r ra a a th th th th at at at at vv v v ar ar ar ar yy y y depe depe depe depe nding nding nding nding on on on on th th th th e e e e degree degree degree degree of of of of ox ox ox ox idation/re- i iidation/re- dation/re- dation/re- states and nea changes r-infrtheir ared color absorpt from ion yellow spectra toth orange, at varyr ed, depe purpuric, nding on dark the blue, degree gr een, of ox light idation/re- blue, state, neutral state) leads to changes in color caused by significant changes in the visible and near-infrared absorption spectra that vary depending on the degree of oxidation/re- and and and and and nea nea nea nea nea r rr - r -r inf - inf -- inf inf inf r rr ar r ar r ar ar ar ed ed ed ed ed absorpt absorpt absorpt absorpt absorpt iion i on ii on on on spect spect spect spect spect r rr a r a r a a a th th th th th at at at at at v v v v ar ar v ar ar ar y y y y y depe depe depe depe depe nding nding nding nding nding on on on on on th th th th th e e e e e degree degree degree degree degree of of of of of ox ox ox ox ox iidation/re- i dation/re- ii dation/re- dation/re- dation/re- light b duction duction duction duction lue, an Sw Sw Sw Sw d bl itching itching itching itching ack bet bet bet bet [57 ween ween ween ween ]. po po po po ly ly ly ly m m m m er er er er films films films films in in in in th th th th ei ei ei ei r r r r co co co co lo lo lo lo red red red red (red (red (red (red uced uced uced uced ) ) ) )and and and and uncolore uncolore uncolore uncolore d d d d (ox (ox (ox (ox i-i- i- i- and duction and black nearSw - [inf 57itching ]. rared absorpt between ion po spect lym ra erth films at var in y th depe eir nding colored on (red the uced degree ) and of ox uncolore idation/re- d (oxi- duction duction SwSw itching itching betbet ween ween po ly po m ly er m films er films in th inei th r ei co r lo co red lored (red (red uced uced ) and ) and uncolore uncolore d (ox d (ox i- i- duction duction duction duction Sw Sw Sw Sw itching itching itching itching bet bet bet bet ween ween ween ween po po po po ly ly ly ly m m m m er er er er films films films films in in in in th th th th ei ei ei ei r r r r co co co co lo lo lo lo red red red red (red (red (red (red uced uced uced uced )) )) and and and and uncolore uncolore uncolore uncolore d d d d (ox (ox (ox (ox i- i- i- i- dized) states changes their color from yellow to orange, red, purpuric, dark blue, green, dized) dized) dized) duction states states states Swchange itching change change s bet s s th th th ween eir eir eir color color color poly fm rom f from rom er films y y y ell ell ell ow ow ow in th to to to ei or r or or an co an an lo ge, ge, ge, red red red red (red , , , purp purp purp uced uri ) uri uri and cc c , , , da da da uncolore rk rk rk bl bl bl uu u e, d e, e, green (ox green green i- , , , dized) states changes their color from yellow to orange, red, purpuric, dark blue, green, dized) dized) states states change change s th s eir th eir color color from from yell yow ellow to or toan orge, ange, redred , purp , purp uriuri c, da c, rk dabl rk ubl e, ugreen e, green , , dized) dized) dized) dized) states states states states change change change change s s s s th th th th eir eir eir eir color color color color ff rom ff rom rom rom y y y ell y ell ell ell ow ow ow ow to to to to or or or or an an an an ge, ge, ge, ge, red red red red , , , purp , purp purp purp uri uri uri uri cc c , c , , da , da da da rk rk rk rk bl bl bl bl u u u u e, e, e, e, green green green green , , , , light blue, and black [57]. lili li ght b ght b ght b dized) lu lu lu ee e , an states , an , an d bl d bl d bl change ack ack ack [57 [57 [57 s ] th .] ] ..eir color from yellow to orange, red, purpuric, dark blue, green, light blue, and black [57]. light b light b luelu , an e, an d bl d bl ack ack [57[57 ]. ]. li li li li ght b ght b ght b ght b lu lu lu lu ee e , an e , an , an , an d bl d bl d bl d bl ack ack ack ack [57 [57 [57 [57 ]] .]] .. . light blue, and black [57]. Pyrrole Thiophene Aniline Furan Carbazole Azolene Indole Py Py Py Py rr rr rr rr ole ole ole ole Thioph Thioph Thioph Thioph en en en en ee e e Anili Anili Anili Anili ne ne ne ne Furan Furan Furan Furan Ca Ca Ca Ca rbazo rbazo rbazo rbazo le le le le Azolen Azolen Azolen Azolen ee e e In In In In dole dole dole dole Pyrrole Thiophene Aniline Furan Carbazole Azolene Indole Pyrrole Thiophene Aniline Furan Carbazole Azolene Indole Py Py Py Py Py rr rr rr rr rr ole ole ole ole ole Thioph Thioph Thioph Thioph Thioph en en en en en e ee e e Anili Anili Anili Anili Anili ne ne ne ne ne Furan Furan Furan Furan Furan Ca Ca Ca Ca Ca rbazo rbazo rbazo rbazo rbazo le le le le le Azolen Azolen Azolen Azolen Azolen e ee e e In In In In In dole dole dole dole dole Pyrrole Thiophene Aniline Furan Carbazole Azolene Indole Figure Figure 11. 11. Mol Molecules ecules of ofheter heterocyclic ocyclic ararom omatic ati compounds. c compounds. Figure Figure Figure 11. 11. 11. Mol Mol Molec ec ecules ules ules of of of heter heter heteroc oc ocyclic yclic yclic arom arom aromati ati atic c c co co com m mpou pou pounds nds nds... Figure Figure 11. 11. Mol Mol ecules ecules of heter of heter ocyclic ocyclic arom arom atic ati co c m co pou mpou nds nds . . Figure Figure Figure Figure Figure 11. 11. 11. 11. 11. Mol Mol Mol Mol Mol ec ec ec ec ules ules ec ules ules ules of of of of heter of heter heter heter heter oc oc oc oc yclic yclic oc yclic yclic yclic arom arom arom arom arom ati ati ati ati c ati c c c co co co co c m m co m m pou pou pou m pou pou nds nds nds nds nds . .. . . Figure Figure 11.11. Mol Mol ecules ecules of of heter heter oc oc yclic yclic arom arom ati ati c c co com mpou pounds nds.. Table 4 shows conductive polymers obtained by the oxidation of monomeric aromatic Table 4 shows conductive polymers obtained by the oxidation of monomeric aro- Table Table Table Table 4 4 4 4 show show show show s s s s con con con con dd d d uctive uctive uctive uctive po po po po ly ly ly ly mers mers mers mers ob ob ob ob taine taine taine taine d d d d by by by by th th th th e e e e oxida oxida oxida oxida tion t t tion ion ion oo o o f f f f mo mo mo mo nom nom nom nom eric eric eric eric aro aro aro aro -- - - Table 4 shows conductive polymers obtained by the oxidation of monomeric aro- compounds Table Table Table 4 show 4 (neutral 4 show show s con s s and con con ductive doxidized d uctive uctive po po ly po states). ly mers ly mers mers ob ob taine ob taine taine d d by by d by th th e e th oxida oxida e oxida ttion ion tion o o f f mo mo of nom mo nom nom eric eric eric aro aro - aro - - Table Table Table Table 4 4 4 4 show show show show s s s s con con con con d d d d uctive uctive uctive uctive po po po po ly ly ly ly mers mers mers mers ob ob ob ob taine taine taine taine d d d d by by by by th th th th e e e e oxida oxida oxida oxida tt ion tt ion ion ion o o o f o f f f mo mo mo mo nom nom nom nom eric eric eric eric aro aro aro aro -- -- matic compounds (neutral and oxidized states). matic com matic com matic com po po po unds (ne unds (ne unds (ne utra utra utra l a l l a a nd nd nd oxid oxid oxid ized ized ized states states states ).). ). matic com matic com pounds (ne pounds (ne utra utra l a l a nd nd oxid oxidized ized states states ). ). matic compounds (neutral and oxidized states). matic com matic com pounds (ne pounds (ne utra utra l and l a oxid nd oxid ized ized states states ). ). matic com matic com matic com matic com po po po po unds (ne unds (ne unds (ne unds (ne utra utra utra utra ll a ll a a a nd nd nd nd oxid oxid oxid oxid ized ized ized ized states states states states ). ). ). ). Table 4. Conductive polymers obtained by the oxidation of monomeric aromatic compounds (neutral Table Table 4 4.. Con Cond ductiv uctive e po polym lymers ers obta obtained ined by t by the oxid he oxidati ation on of of m monom onomeric eric ar aromatic omatic co com mpou pounds nds (neu (neu- - Table Table Table 44 . .Con 4 Con . Con dd uctiv uctiv ductiv e e po po e lym po lym lym ers ers ers obta obta obta ined ined ined by t by t by t he oxid he oxid he oxid ati ati on ati on on of of m of m onom m onom onom eric eric eric ar ar omatic ar omatic omatic co co m co m pou m pou pou nds nds nds (neu (neu (neu - - - Table 4. Conductive polymers obtained by the oxidation of monomeric aromatic compounds (neu- Table Table Table and Table 4 Table . Con oxidized 44 .4 .Con .Con 4 Con d . Con uctiv dd uctiv d states). uctiv uctiv ductiv e e po e e po po e po lym lym po lym lym lym ers ers ers ers ers obta obta obta obta obta ined ined ined ined ined by t by t by t by t by t he oxid he oxid he oxid he oxid he oxid ati ati ati on ati on on ati of on of of on m of m m onom onom of m onom onom m eric onom eric eric eric ar ar ar omatic omatic ar eric omatic omatic ar co co omatic co m m co m pou pou m pou pou nds nds nds co nds (neu m (neu (neu pou (neu - -- nds - (neu- Table Table 44 . .Con Con dd uctiv uctiv e e po po lym lym ers ers obta obta ined ined by t by t he oxid he oxid ati ati on on of of m m onom onom eric eric ar ar omatic omatic co co m m pou pou nds nds (neu (neu - - tral tral tral tral and and and and oxi oxi oxi oxi di di di di zz z ed zed ed ed s tates s s states tates tates ).) ) ) ... tral and oxidized states). tral and oxidized states). tral and oxidized states). tral tral tral tral tral and and and and and oxi oxi oxi oxi oxi di di di di di z zz z ed ed z ed ed ed s s s tates s tates s tates tates tates ) ).) .) ) .. . tral and oxidized states). Organic EC Organic E Organic E Organic E Organic E C C C C Organic EC Organic EC Organic EC Organic E Organic E Organic E Organic E Organic E C C C C C State PANI P3MPy MEPA P3MT PPY PT Organic EC Stat Stat Stat Stat ee e e PANI PANI PANI PANI P3 P3 P3 P3 M M M M Py Py Py Py MEPA MEPA MEPA MEPA P3 P3 P3 P3 MT MT MT MT PPY PPY PPY PPY PT PT PT PT Stat State e PANI PANI P3 P3 M M Py Py MEPA MEPA P3P3 MT MT PPY PPY PT PT State PANI P3MPy MEPA P3MT PPY PT Stat Stat Stat Stat Stat e ee e e PANI PANI PANI PANI PANI P3 P3 P3 P3 P3 M M M M M Py Py Py Py Py MEPA MEPA MEPA MEPA MEPA P3 P3 P3 P3 P3 MT MT MT MT MT PPY PPY PPY PPY PPY PT PT PT PT PT State PANI P3MPy MEPA P3MT PPY PT Neutral Neu Neu Neu Neutral Neu tr tr tr al al al tral Neutral Neu Neu tral tr al Neu Neu Neu Neu tr tr tr tr al al al al Neutral Oxidized Oxidi Oxidi Oxidi Oxidi ze ze ze ze dd d d Oxidized Oxidi Oxidi Oxidi Oxidi Oxidi Oxidized Oxidi ze ze ze ze ze d d ze d d d d Oxidized The shortcomings of polymer films include their low electrochemical stability and, Th Th Th Th e e e e short short short short co co co co mings mings mings mings o o o o f f f f po po po po lymer l llymer ymer ymer fil f f fil il il m m m m s s s s in in in in clu clu clu clu de de de de th th th th eir eir eir eir lo lo lo lo w w w w electroch electroch electroch electroch ee e e mica mica mica mica l l l l st st st st abi abi abi abi lity l llity ity ity and and and and , , , , The shortcomings of polymer films include their low electrochemical stability and, The Th short e short comings comings of po of lpo ymer lymer film fil s m in s clu inclu de de their their low loelectroch w electroch emica emica l stl abi stabi lity lity and and , , Th Th Th Th e e e e short short short short co co co co mings mings mings mings o o o f o f f f po po po po ll ymer ll ymer ymer ymer ff il ff il il il m m m m s s s s in in in in clu clu clu clu de de de de th th th th eir eir eir eir lo lo lo lo w w w w electroch electroch electroch electroch ee e mica e mica mica mica l l l l st st st st abi abi abi abi ll ity ll ity ity ity and and and and , , , , consequently, their low oxidation number [28–30]. The addition of inorganic materials The shortcomings of polymer films include their low electrochemical stability and, consequently, their low oxidation number [28–30]. The addition of inorganic materials con con con sequent sequent sequent ly, ly, ly, th th th eir eir eir low low low ox ox ox ida ida ida tion t tion ion numb numb numb er er er [28 [ [28 28 –– – 30] 30] 30] . . . TT T he he he add add add ition ition ition oo o f f f inor inor inor ga ga ga nn n ic ic ic materi materi materi als als als consequently, their low oxidation number [28–30]. The addition of inorganic materials con con sequent sequent ly, ly, th eir their low low oxida oxida tion tion numb numb er [ er 28[ –28 30] –30] . The . The add add ition ition of inor of inor gan ga ic nmateri ic materi als als con con con con sequent sequent sequent sequent ly, ly, ly, ly, th th th th eir eir eir eir low low low low ox ox ox ox ida ida ida ida tt ion tt ion ion ion numb numb numb numb er er er er [[ 28 [[ 28 28 28 –– – 30] – 30] 30] 30] . . . . T T T T he he he he add add add add ition ition ition ition o o o f o f f f inor inor inor inor ga ga ga ga n n n n ic ic ic ic materi materi materi materi als als als als improves the properties of electrochromic conductive polymers [58]. Inorganic materials consequently, their low oxidation number [28–30]. The addition of inorganic materials impro improves ves th the e pro propert perties ies o of f electroch electrochrom romiic c con conductive ductive po poly lymers mers [ [58] 58]. . Ino Inorg rgani anic c m material aterials s impro impro impro ves ves ves th th eth e pro pro e pro pert pert pert ies ies ies o o f f o electroch electroch f electroch rom rom rom ic ic con icon c con ductive ductive ductive po po po ly ly mers ly mers mers [ 58] [58] [58] . . Ino Ino . Ino rg rg ani rg ani ani cc m m c aterial m aterial aterial s s s impro impro Th impro e ves short ves ves th th eco th e pro pro emings pro pert pert pert ies ies o ies o f o f f po electroch o electroch f l electroch ymer rom rom fil rom m ic ic s con icon c in con ductive clu ductive ductive de th po po eir ly po ly mers mers ly lo mers w [ 58] [electroch 58] [58] . . Ino Ino . Ino rg rg ani rg ani eani mica cc m m caterial aterial m l aterial stabi s s l s ity and, impro impro impro ves ves ves th th th ee e pro pro pro pert pert pert ies ies ies o o f of f electroch electroch electroch rom rom rom ic iic c con con con ductive ductive ductive po po po ly ly ly mers mers mers [ 58] [[58] 58] . . Ino . Ino Ino rg rg rg ani ani ani cc c m m m aterial aterial aterial s s s improve the staining efficiency and reduce the switching time, but do not affect the elec- improves the properties of electrochromic conductive polymers [58]. Inorganic materi- impro impro improve ve ve th th the e e stai stai stainin nin ning g g ef ef effi fi fic c cie ie iency ncy ncy and and and re re redu du duce ce ce th th the e e switchi switchi switching ng ng time, time, time, but but but do do do not not not af af aff f fect ect ect th th the e e elec- elec- elec- impro impro ve ve the th stai e stai nin nin g ef g fi ef cie fic ncy iency and and redu redu ce ce the th switchi e switchi ng ng time, time, but but do do not not aff af ect fect th e th elec- e elec- impro impro impro impro impro ve ve ve ve ve th th th th e th e e e stai stai stai e stai stai nin nin nin nin nin g g g g ef ef g ef ef fi fi fi ef fi cc c ie c fi ie ie ie ncy cncy ncy ie ncy ncy and and and and and re re re re du du re du du ce du ce ce ce th ce th th th e th e e e switchi switchi switchi e switchi switchi ng ng ng ng ng time, time, time, time, time, but but but but but do do do do do not not not not not af af af af f ect f af ff ect ect ect f ect th th th th e th e e e elec- elec- elec- e elec- elec- impro trochem ve thical e stai prop nin erties g effi of cie th ncy e po and lyme rer; du th ce erefore the switchi , the pro ng blem time, of but impdo rovnot ing po afflect ymer the elec- elec- consequently, their low oxidation number [28–30]. The addition of inorganic materials als improve the staining efficiency and reduce the switching time, but do not affect the tro tro tro tro chem chem chem chem ical ical ical ical pp p p rop rop rop rop erties erties erties erties of of of of th th th th e e e e po po po po ly ly ly ly me me me me r; r; r; r; th t t th h h erefore erefore erefore erefore , , , , th th th th ee e e pro pro pro pro blem blem blem blem of of of of i mp i iimp mp mp rov rov rov rov ing ing ing ing po po po po lymer l llymer ymer ymer ee e e lec- lec- lec- lec- trochemical properties of the polymer; therefore, the problem of improving polymer elec- tro tro tro tro tro tro chem tro chem chem chem chem chem chem ical ical ical ical ical ical ical p p p p rop p rop rop rop s rop p tabilit rop erties erties erties erties erties erties y is of of of of of sti th th of th th th ll e e th e e e re po po po po e po leva ly ly po ly ly ly me me me ly me nt me me r; .r; r; r; E r; t tle h t r; h tt h h erefore h erefore ct t erefore erefore erefore h rochro erefore , , , , mic th th , th th th , e eth e e e pro pro f pro pro il e pro ms pro blem blem blem blem blem , such blem of of of of of as ii of mp i mp ii mp mp mp WO imp rov rov rov rov rov 3rov , ing ing Nb ing ing ing ing 2po po O po po po 5ll ,po ymer l ymer lNi l ymer ymer ymer lymer O, e ar ee e lec- lec- e lec- e lec- lec- elec- impro electr ves ochemical the propert properties ies of electroch of the polymer; romic ther con efor ductive e, the pr po oblem lymers of impr [58] oving . Inopolymer rganic materials tro tro tro tro chem chem chem chem ical ical ical ical ss s tabilit stabilit tabilit tabilit y y y y is is is is sti sti sti sti ll ll ll ll re re re re leva leva leva leva nt nt nt nt . ...E E E E le le le le ct ct ct ct rochro rochro rochro rochro mic mic mic mic fil f f fil il il ms ms ms ms , , , , such such such such as as as as WO WO WO WO 3, 3 3 3, , , Nb Nb Nb Nb 2O 2 2 2O O O 5, 5 5 5 ,,,Ni Ni Ni Ni O, O, O, O, ar ar ar ar e e e e trochemical stability is still relevant. Electrochromic films, such as WO3, Nb2O5, NiO, are trochemical stability is still relevant. Electrochromic films, such as WO3, Nb2O5, NiO, are tro tro tro tro tro chem prefer chem chem chem chem ical ical abl ical ical ical e s ss du tabilit s tabilit s tabilit tabilit tabilit e to t y y y y heir h y is is is is is sti sti sti sti sti ll ll igh ll ll ll re re re re re stability leva leva leva leva leva nt nt nt nt nt .. ..E E . a E E le E le n le le le ct d durabi ct ct ct ct rochro rochro rochro rochro rochro li mic mic mic mic ty mic . f fil f il ff il il ms il ms ms ms ms , , , , such such , such such such as as as as as WO WO WO WO WO 3 3, 3 , 33 , , Nb Nb , Nb Nb Nb 2 2O 2 O 22 O O O 5 5, 5 , 5 5 ,,Ni Ni , Ni Ni Ni O, O, O, O, O, ar ar ar ar ar e e e e e electrochemical stability is still relevant. Electrochromic films, such as WO , Nb O , NiO, impro prefer ve th abl e e stai due to t ning heir h effic igh iency stability and are nd durabi duce th lie tyswitchi . ng time, but 3 do not 2 5affect the elec- prefer prefer prefer abl abl abl e e e du du du e to t e to t e to t heir h heir h heir h igh igh igh stability stability stability a a a nn n d durabi d durabi d durabi lili li ty ty ty . .. preferable due to their high stability and durability. prefer prefer ablabl e du e e to t due to t heir h heir h ighigh stability stability an a d durabi nd durabi lityli . ty. prefer prefer prefer prefer abl abl abl abl e e e e du du du du e to t e to t e to t e to t heir h heir h heir h heir h igh igh igh igh stability stability stability stability a a a a n n n n d durabi d durabi d durabi d durabi li li li li ty ty ty ty . .. . are preferable due to their high stability and durability. trochemical properties of the polymer; therefore, the problem of improving polymer elec- 3.3. Transition Metal Oxides 3.3. Transition Metal Oxides 3.3. 3.3. 3.3. TT T ransitio ransitio ransitio nn n M M M etal O e etal O tal O xid xid xid es es es 3.3. Transition Metal Oxides trochemical stability is still relevant. Electrochromic films, such as WO3, Nb2O5, NiO, are 3.3. 3.3. Transitio Transitio n M ne M tal O etal O xidxid es es 3.3. 3.3. 3.3. 3.3. T T T T ransitio ransitio ransitio ransitio Inorga n n n M n n M M ic M ee ma e tal O e tal O tal O tal O terials xid xid xid xid es es es es incl ude a large group of EC, mostly the TMO МеxOy (Figure 12). 3.3. Transition Metal Oxides Ino Inorga rgan nic ic ma materials terials incl includ ude a e a larg large g e grou roup of EC, most p of EC, mostly ly the TMO the TMO Ме Меx xO Oy y (Fig (Figur ure 12). e 12). Ino Ino Ino rga rga rga nn ic ic nma ic ma ma terials terials terials incl incl incl ud ud ud e a e a e a larg larg larg e g e g e g rou rou rou p of EC, most p of EC, most p of EC, most ly ly the TMO ly the TMO the TMO Ме Ме Ме xO xO y x O y(Fig (Fig y (Fig ur ur e 12). ur e 12). e 12). The most common TMO [32,59], such as molybdenum (VI) oxide, vanadium (V) oxide, preferable due to their high stability and durability. Ino Ino Ino Ino rga rga rga rga nn ic n ic ic n ma ic ma ma ma terials terials terials terials incl incl incl incl ud ud ud e a ud e a e a e a larg larg larg larg e g e g e g e g rou rou rou rou p of EC, most p of EC, most p of EC, most p of EC, most ly ly ly the TMO the TMO ly the TMO the TMO Ме Ме Ме Ме xO xx O O y x yO (Fig y (Fig (Fig y (Fig ur ur ur e 12). ur e 12). e 12). e 12). Ino Ino Inor rga rga ganic nn ic ic ma ma materials terials terials incl incl include ud ud e a e a a larg larg large e g e g gr rou rou oup p of EC, most p of EC, most of EC, mostly ly ly the TMO the TMO the TMO Ме Me Ме xO xO O y y(Fig (Fig (Figur ur ur e 12). e 12). e 12). x y Th Th Th Th e e e e mo mo mo mo st st st st co co co co mm mm mm mm on on on on TMO TMO TMO TMO [32,5 [32,5 [32,5 [32,5 9] 9] 9] 9] , ,,,such such such such as as as as mo mo mo mo lybden l llybden ybden ybden um um um um (V (V (V (V I) I I I) ) ) ox ox ox ox id id id id e, e, e, e, vv v v anadi anadi anadi anadi um um um um (V) (V) (V) (V) ox ox ox ox ide ide ide ide , ,,, The most common TMO [32,59], such as molybdenum (VI) oxide, vanadium (V) oxide, niobium (V) oxide, iridium (III) oxide, tungsten (VI) oxide, are in the form of an octahe- Th Th Th Th Th e Th e e e e mo mo mo mo e mo mo st st st st st co co st co co co mm mm co mm mm mm mm on on on on on on TMO TMO TMO TMO TMO TMO [32,5 [32,5 [32,5 [32,5 [32,5 [32,5 9] 9] 9] 9] 9] ,, ,9] ,such ,such such such such , such as as as as as as mo mo mo mo mo mo llybden l ybden ll ybden ybden ybden lybden um um um um um um (V (V (V (V (V I I(V ) I ) II ) ) ) ox ox Iox ox ) ox id id ox id id id e, e, e, id e, e, v v e, v v anadi anadi v anadi anadi anadi vanadi um um um um um um (V) (V) (V) (V) (V) (V) ox ox ox ox ox ide ide ox ide ide ide ide ,, ,, , , The most common TMO [32,59], such as molybdenum (VI) oxide, vanadium (V) oxide, niobium niobium niobium niobium ( V) ( ( (V) V) V) oxide, oxide, oxide, oxide, ir ir ir ir idiu idiu idiu idiu m m m m (III) (III) (III) (III) ox ox ox ox ide, ide, ide, ide, tungsten tungsten tungsten tungsten (V (V (V (V I) I I I) ) ) oxi oxi oxi oxi de, de, de, de, ar ar ar ar ee e e i n i iin n n th th th th e e e e form form form form oo o o f f f fan an an an oct oct oct oct ahe- ahe- ahe- ahe- niobium (V) oxide, iridium (III) oxide, tungsten (VI) oxide, are in the form of an octahe- dron MeO6 (Figure 13). The crystal structure of СWO3 perovskite shown in Figure 14. niobium (V) oxide, iridium (III) oxide, tungsten (VI) oxide, are in the form of an octahe- niobium niobium niobium niobium niobium ( ( V) ( V) (( V) V) V) oxide, oxide, oxide, oxide, oxide, ir ir ir ir ir idiu idiu idiu idiu idiu m m m m m (III) (III) (III) (III) (III) ox ox ox ox ox ide, ide, ide, ide, ide, tungsten tungsten tungsten tungsten tungsten (V (V (V (V (V I I) I ) II ) ) ) oxi oxi oxi oxi oxi de, de, de, de, de, ar ar ar ar ar e ee e e i i n i n ii n n n th th th th th e e e e e form form form form form o o o o f f o f ff an an an an an oct oct oct oct oct ahe- ahe- ahe- ahe- ahe- niobium (V) oxide, iridium (III) oxide, tungsten (VI) oxide, are in the form of an octahedron 3.3. Transition Metal Oxides dron MeO dron MeO dron MeO dron MeO 6 6 6 6(Figur (Figur (Figur (Figur e e e e 13). 13). 13). 13). Th Th Th Th ee e e cryst cryst cryst cryst al al al al struct struct struct struct ure ure ure ure oo o o f f f f СWO СWO СWO СWO 3 3 3 3pero pero pero pero vskite vskite vskite vskite sh sh sh sh own own own own i i i i n n n n Fi Fi Fi Fi gg g g ur ur ur ur e 14. e 14. e 14. e 14. dron MeO6 (Figure 13). The crystal structure of СWO3 perovskite shown in Figure 14. dron MeO dron MeO 6 (Figur 6 (Figur e 13). e 13). Th e Th cryst e cryst al struct al struct ure ure of СWO of СWO 3 pero 3 pero vskite vskite shown shown in i Fi n gFi ur g e 14. ure 14. dron MeO dron MeO dron MeO MeO dron MeO (Figur 66 6(Figur 6 (Figur (Figur (Figur e 13). e e e e 13). The 13). 13). 13). Th crystal Th Th Th ee e cryst e cryst cryst cryst str al al al uctur al struct struct struct struct e of ure ure ure ure CWO o o o f o f f f СWO СWO СWO СWO per 3ovskite 3 3pero 3 pero pero pero vskite vskite vskite vskite shown sh sh sh sh own own own in own Figur i i i n i n n n Fi Fi Fi Fi eg g g 14 ur g ur ur ur .e 14. e 14. e 14. e 14. 6 3 Inorganic materials include a large group of EC, mostly the TMO МеxOy (Figure 12). The most common TMO [32,59], such as molybdenum (VI) oxide, vanadium (V) oxide, niobium (V) oxide, iridium (III) oxide, tungsten (VI) oxide, are in the form of an octahe- dron MeO6 (Figure 13). The crystal structure of СWO3 perovskite shown in Figure 14. Nanomaterials 2021, 11, x FOR PEER REVIEW 11 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 11 of 32 Nanomaterials 2021, 11, 2376 11 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 11 of 32 Figure 12. Electrochromic transition metal oxides. *—lantanoids. Figure 12. Electrochromic transition metal oxides. *—lantanoids. In the mentioned structures, electrochromic effects occur due to electron–ion separa- In the mentioned structures, electrochromic effects occur due to electron–ion separa- tion. As a result, metal atoms are introduced into TMO, and the valence electrons move tion. As a result, metal atoms are introduced into TMO, and the valence electrons move to the d-levels of the transition metal ion, reducing it. Evidently, the injected ions should to the d-levels of the transition metal ion, reducing it. Evidently, the injected ions should possess Figure a hig12. h di El ffusi ectrochrom on coefficient ic transitio and n metal a hig oxides h sol.ubility i *—lantanoid n the l s. attice of TMO [18,20]. Figure 12. Electrochromic transition metal oxides. *—lantanoids. possess a high diffusion coefficient and a high solubility in the lattice of TMO [18,20]. In the mentioned structures, electrochromic effects occur due to electron–ion separa- tion. As a result, metal atoms are introduced into TMO, and the valence electrons move - anion; - anion; to the d-levels of the transition metal ion, reducing it. Evidently, the injected ions should possess a high diffusion coefficient and a high solubility in the lattice of TMO [18,20]. - c - ation; cation; - cation. - cation. - anion; - cation; - cation. Figure 13. Crystal structure of MeO perovskite. Figure 13. Crystal structure of MeO6 perovskite. Figure 13. Crystal structure of MeO6 perovskite. Figure 13. Crystal structure of MeO6 perovskite. (а) (b) (c) (а) (b) (c) Figure Figure 14. Cr 14.ystal Crystal stru str ctuctur ure of e of СWO CWO 3 perovs perovskite: kite: ((а a) ) general general vie view; w; (b ()bh-WO ) h-WO along 3 along plane plane; (c);h-WO (c) h-WO along 3 alplane. ong plane. 3 3 3 Figure 14. Crystal structure of СWO3 perovskite: (а) general view; (b) h-WO3 along plane; (c) h-WO3 along plane. In the mentioned structures, electrochromic effects occur due to electron–ion separa- There are several highly efficient TMO (IrO2 [60], MoO3 [61], NiO [62], TiO2 [63], WO3 tion. As a result, metal atoms are introduced into TMO, and the valence electrons move [42,5 There 9]) ar the at sev are eral color hig less hly in efth fici e ent oxid T ize MO d state (IrO2and [60] c , olored MoO3 [ in 61] th , e N reduced iO [62], T state iO2 [(c 63] athodi , WO c 3 (а) (b) (c) to the d-levels of the transition metal ion, reducing it. Evidently, the injected ions should [42,5 EC, 9]) co thlor at are chacolor nge is less induce in th d e by oxid ion ize injec d state tion). and Ino c rg olored anic com in th poe und reduced s that ar state e col (c orles athodi s in c possess a high diffusion coefficient and a high solubility in the lattice of TMO [18,20]. Figure 14. Crystal structure of СWO3 perovskite: (а) general view; (b) h-WO3 along plane; (c) h-WO3 along plane. their reduced state and colored in their oxidized state are called anodic EC (color change EC, color change is induced by ion injection). Inorganic compounds that are colorless in There are several highly efficient TMO (IrO [60], MoO [61], NiO [62], TiO [63], 2 3 2 is induced by ion extraction). their reduc WO ed [42 ,state 59]) that and arcol e colorless ored in in their the oxidi oxidized zed state state and are color called ed an inodic the reduced EC (colstate or change There are several highly efficient TMO (IrO2 [60], MoO3 [61], NiO [62], TiO2 [63], WO3 (cathodic Vanadiu EC, m color oxides change [64] exhi is induced bit hybrid by ion featinjection). ures, and Inor ECD ganic usually compounds contain tw that o ar EC e films is induced by ion extraction). [42,59]) that are colorless in the oxidized state and colored in the reduced state (cathodic colorless in their reduced state and colored in their oxidized state are called anodic EC [32,59]; therefore, it would be relevant to simultaneously use a cathodic oxide (for exam- Vanadium oxides [64] exhibit hybrid features, and ECD usually contain two EC films EC, color change is induced by ion injection). Inorganic compounds that are colorless in (color change is induced by ion extraction). ple, Mo or Nb) and an anodic oxide (for example, Ni or Ir) [61,63]. [32,59]; therefore, it would be relevant to simultaneously use a cathodic oxide (for exam- their reduced state and colored in their oxidized state are called anodic EC (color change EC exhibit polychromism [65], for example, amorphous Nb2O5 is brown in its colored ple, Mo or Nb) and an anodic oxide (for example, Ni or Ir) [61,63]. is induced by ion extraction). state, while crystalline Nb2O5 acquires a blue color; WO3 is blue in its colored state, while EC exhibit polychromism [65], for example, amorphous Nb2O5 is brown in its colored Vanadium oxides [64] exhibit hybrid features, and ECD usually contain two EC films + + TiO2 obtains its color (blue or grey) as a result of ion injection (H or Li , respectively). The state, while crystalline Nb2O5 acquires a blue color; WO3 is blue in its colored state, while [32,59]; therefore, it would be relevant to simultaneously use a cathodic oxide (for exam- + + TiO2 obtains its color (blue or grey) as a result of ion injection (H or Li , respectively). The ple, Mo or Nb) and an anodic oxide (for example, Ni or Ir) [61,63]. EC exhibit polychromism [65], for example, amorphous Nb2O5 is brown in its colored state, while crystalline Nb2O5 acquires a blue color; WO3 is blue in its colored state, while + + TiO2 obtains its color (blue or grey) as a result of ion injection (H or Li , respectively). The Nanomaterials 2021, 11, 2376 12 of 32 Vanadium oxides [64] exhibit hybrid features, and ECD usually contain two EC Nanomaterials 2021, 11, x FOR PEER REVIEW 12 of 32 films [32,59]; therefore, it would be relevant to simultaneously use a cathodic oxide (for example, Mo or Nb) and an anodic oxide (for example, Ni or Ir) [61,63]. EC exhibit polychromism [65], for example, amorphous Nb O is brown in its colored 2 5 state, while crystalline Nb O acquires a blue color; WO is blue in its colored state, while 2 5 3 most investigated cathodic EC is WO3 [66]. The color change mechanism has still not been + + TiO obtains its color (blue or grey) as a result of ion injection (H or Li , respectively). sufficiently investigated, but most scientists agree that the extraction and injection of elec- The most investigated cathodic EC is WO [66]. The color change mechanism has still not + + + + trons and metal cations (Li , H , Na , K , etc.) play a crucial role in color change. NiO and been sufficiently investigated, but most scientists agree that the extraction and injection of + + + + IrO2 are the most popular anodic EC. High concentrations of cations in the electrolyte, electrons and metal cations (Li , H , Na , K , etc.) play a crucial role in color change. NiO which and IrOis ar an e the ion most conpopular ductor, anodic signifi EC. cantly High affect concentrations the electof rochro cations mic in the pro electr pertolyte, ies of the TMO, which is an ion conductor, significantly affect the electrochromic properties of the TMO, such as switching time, cyclicity and staining efficiency. such as switching time, cyclicity and staining efficiency. The majority of TMO have a band gap of 1–5 eV (Figure 15), and therefore occupy an The majority of TMO have a band gap of 1–5 eV (Figure 15), and therefore occupy intermediate position between semiconductors and dielectrics [67]. EC behavior is de- an intermediate position between semiconductors and dielectrics [67]. EC behavior is pendent on TMO structure. It should be noted that structural and impurity defects directly dependent on TMO structure. It should be noted that structural and impurity defects affect the properties—particularly the physicochemical properties—of the EC under directly affect the properties—particularly the physicochemical properties—of the EC study. under study . Figure 15. Classification of materials by conductivity (according to zone theory). Figure 15. Classification of materials by conductivity (according to zone theory). The optical band gap can be calculated according to Equation (4) [37]: The optical band gap can be calculated according to Equation (4) [37]: hv = A hv E (4) αhv = A(hv−E ) (4) where is the absorption coefficient, which can be measured by the ultraviolet spectropho- tometer; h is the Planck constant; v is the light frequency; A is a proportionality constant; where α is the absorption coefficient, which can be measured by the ultraviolet spectro- E is the optical band gap; n is a number that is for the direct band gap semiconductor photometer; h is the Planck constant; v is the light frequency; A is a proportionality con- and 2 for the indirect band gap semiconductor. stant; Eg is the optical band gap; n is a number that is ½ for the direct band gap semicon- The E of the WO films decreased from 3.62 eV to 3.30 eV when the annealing g 3 temperature was increased. In addition, the E of the colored WO films was less than that ductor and 2 for the indirect band gap sgemiconductor. of the bleached WO films [38]. The different band gap demonstrates that the conductivity The Eg of the WO3 films decreased from 3.62 eV to 3.30 eV when the annealing tem- of the WO film is enhanced with decreasing E , while the high conductivity increased the perature was increased. In addition, the Eg of the colored WO3 films was less than that of electrochromic response time. the bleached WO3 films [38]. The different band gap demonstrates that the conductivity of The transparency of inorganic EC with high staining efficiency varies in response to 2 1 the WO3 film is enhanced with decreasing Eg, while the high conductivity increased the the low-voltage signal. WO and NiO (Table 5) have a staining efficiency of ~40 sm C , 2 1 electrochromic response time. while for organic EC films, such as PEDOT, this value is more than 100 sm C [32,39]. Actually, TMO have a high physical and chemical stability. The transparency of inorganic EC with high staining efficiency varies in response to 2 −1 the low-voltage signal. WO3 and NiO (Table 5) have a staining efficiency of ~40 sm ∙C , 2 −1 while for organic EC films, such as PEDOT, this value is more than 100 sm ∙C [32,39]. Actually, TMO have a high physical and chemical stability. Table 5. Color variation in WO3, NiO and WO3/NiO electrochromic films (colored and bleached states). Inorganic EC State WO3 NiO NiO/WO3 Oxidized Reduction Nano Nano m Nateria ano m Nateria ano mls ateria m 2021 ls ateria 2021 ls , 2021 11 ls , , 2021 11 x FO , , 11 x FO , , R P 11 x FO , R P x FO EN ER RE R P E ano ER RE R P E N m ER RE ano VIEW ateria EER RE m VIEW ateria VIEW ls 2021 VIEW ls 2021 , 11 , , x FO 11, x FO R PER P ER RE EER RE VIEW VIEW 12 12 of 12 of 32 12 of 32 of 32 32 12 of 1232 of 32 NanomateriaN ls ano 2021 materia , 11, x FO ls 2021 R P , 11 EER RE , x FOVIEW R PEER RE VIEW 12 of 32 12 of 32 Nano N m ano ateria Nm ano ateria N ls m ano 2021 ateria ls materia 2021 , ls 112021 , , ls x FO 11 2021 , , x FO 11 R P , , x FO 11 E R P ER RE , x FO E R P ER RE E R P VIEW ER RE E VIEW ER RE VIEW VIEW 12 of 12 32 of 12 32 of 12 32 of 32 Nano Nano materia materia ls 2021 ls 2021 , 11 , , 11 x FO , x FO R P R P EER RE EER RE VIEW VIEW 12 12 of of 32 32 Nano Nano materia Nm ano ateria N m ls ano ateria 2021 N ls m ano 2021 ateria N , ls m 11 ano 2021 ateria N , , ls 11 m x FO ano N 2021 ateria , , x FO ano ls 11 m N R P 2021 ateria , ano , m x FO ls 11 N R P ateria E 2021 m ano , ER RE , ls x FO 11 ateria E R P 2021 ER RE m , ls , x FO ateria 11 E 2021 R P VIEW ls ER RE , , x FO 11 2021 E R P VIEW ls , ER RE , 11 x FO 2021 E , VIEW R P , 11 ER RE x FO , E , R P VIEW x FO 11 ER RE R P , E VIEW x FO ER RE R P E VIEW ER RE E R P ER RE VIEW E ER RE VIEW VIEW VIEW 12 12 of 32 of 12 32 of 12 32 of 12 32 of 12 32 of 12 12 32 of 12 of 32 of 12 32 32 of 32 most investigated cathodic EC is WO3 [66]. The color change mechanism has still not been mo mo st mo st investig investig st investig ated ated ated cathod cathod cathod mo ic st ic mo EC investig EC ic st is EC investig is WO WO is 3ated WO [66] 3 [66] ated 3. cathod [66] Th . Th cathod e . color Th e ic color e EC color ic change EC is change WO cis hange WO mec 3 [66] mec 3h mec [66] . an h Th an is h . e m Th an is color m ha is e m ha color s sti c ha s hange sti ll s c not sti ll hange not ll mec bee not bee mec n h bee an n h is nan m is ha m s ha stis llsti not ll not bee n bee n most mo investig st investig ated most cathod ated investig cathod ic EC ated ic isEC cathod WO is3 WO [66] ic EC 3 . Th [66] is e . WO color The 3 [66] color change . Th change e mec color hmec an change ish m an ha ismec m s sti ha h lls an not sti isll m bee not ha n s bee stin ll not been momo st investig mo st investig st investig ated ated cathod ated cathod cathod ic EC ic EC is ic WO EC is WO is 3 [66] WO 3 [66] . 3Th [66] . e Th color . e Th color e c color hange change change mec mec han mec his an m his an ha m is s ha m sti s ha ll sti not s ll sti not bee ll not bee n bee n n mo mo st investig st investig ated ated cathod cathod ic EC ic EC is WO is WO 3 [66] 3 [66] . Th . Th e color e color change change mec mec han han ism ism haha s sti s sti ll not ll not bee bee n n mo mo st investig mo st investig mo st investig mo st ated investig mo st ated investig mo st cathod ated mo investig st cathod mo ated investig st cathod mo ated investig st ic cathod EC investig ic ated st cathod EC investig ic ated is cathod EC WO ated is ic ated WO cathod EC is ic 3 cathod ated [66] WO EC is 3 ic cathod [66] WO EC . is ic 3cathod Th [66] WO . EC ic is Th 3e [66] EC ic color WO . is e 3 Th EC [66] ic color WO is . e 3Th EC WO [66] is color c. hange 3 e Th WO c [66] is color hange . 3 e Th WO [66] c color 3. hange [66] e Th mec c . 3 color hange Th [66] e mec . c color h Th hange e an mec . color h c e Th hange is an mec color m h c e is hange an mec color c m ha h hange is an c s mec ha m hange h sti is an s c mec ha hange m ll sti h is mec an s not ha ll m h sti mec is not an s ha h ll bee m sti mec an is not s h bee ha m ll an n sti is not h s m ha bee is ll n an sti m not ha s bee is ll n sti ha s m not bee sti ll n s ha not sti ll bee n s not ll sti bee n not ll bee n not bee n bee n n sufficiently investigated, but most scientists agree that the extraction and injection of elec- suffi suffi suffi cientl cientl cientl y in y vest in y vest inigated, vest igated, igated, suffi but but suffi c mo ientl but mo cst ientl mo y st sc in ientists sc st y vest ientists sc inientists vest igated, agree igated, agree agree but th th at but mo at th th st mo at th e ext sc e th st ext ientists e ra sc ext ra ct ientists io ct ra n io ct agree an n ioan d n agree an inject d th inject d at th inject th io at n e io th ext of n io e of n e ra ext lec of e ct lec ra -io elec ct - n io an - n dan inject d inject ion io of n eof lece -lec- suffic suffi ientl suffi suffi cy ientl cientl inc vest suffi ientl y y in in igated, c vest y ientl vest invest igated, igated, y but in igated, vest mo but but igated, st mo but sc mo ientists st mo st but sc sc st ientists ientists mo sc agree ientists st sc agree agree th ientists at agree th th th e at at agree ext th th th at ra e e ext ct th ext th io e ra at n ext ra ct th an ct io ra e io d n ct ext n inject an io an ra n dd ct an inject io inject io d n n inject of an io io ed n lec n of inject io of -n ee lec of lec io - en - lec of - elec- sufficiently investigated, but most scientists agree that the extraction and injection of elec- suffi suffi cientl cientl suffi y in y suffi vest in cientl vest cigated, ientl y igated, in y vest in but suffi vest igated, but suffi mo igated, cmo ientl st c ientl sc st but y ientists sc in but mo ientists y vest in mo st vest igated, sc agree st ientists agree igated, scientists th but at th agree but at th mo e agree th mo st ext e th sc ext st ra at ientists th sc ct ra th at io ientists ct e n th io ext an n e agree ra ext an d ct agree inject d ra io inject ct th n io at an io th n n th io d at an of e n inject th d ext of e e inject lec e ext ra io lec -ct n ra - io io of ct n n io e an of lec n d e an -lec inject d- inject ion io of n e of lec e- lec- suffi suffi cientl cientl y suffi in y vest in suffi c vest ientl suffi igated, csuffi ientl igated, y cientl in c y ientl but vest in y but vest in mo igated, y vest in mo igated, st vest sc st igated, ientists but sc igated, ientists but mo but mo st agree but sc mo agree stientists mo sc st th ientists sc st at th ientists sc th at agree ientists e th ext agree e ext ra agree th ct at ra agree th io ct th at n th io e an th n at ext th e d an at th ra ext inject d e th ct ext inject ra e io ct ext n io ra io an n ct ra io n of io d n ct an n inject io e of d lec an n e inject lec an d - io inject d -n inject io of n io e of lec n io e of n -lec of e- lec elec - - + + ++ ++ ++ ++ ++ + + + + + + + + + + + + + + ++ + + + + trons and metal cations (Li , H , Na , +K , e ++ tc + +.) + + pl + ay +++ a + +cr+ ucial + + role in color change. NiO and tro tro ns tro ns and ns and and me me tal me tal cations tal cations cations tro (L (L ins tro , i(L H and , ns iH , , N and H , me a N ,, a N tal K me , aK , e c , tal ations , e K tc.) c tc , e ations pl .) tc pl ay (L .) ay pl a i (L , ay a c H r i ucial c a , ,r H ucial N cra ucial , role N , K role a , e , role in K tc in c , e .) olor in c tc pl olor .) c ay olor ch pl a ch ange. Ni ay cange. Ni ch r a ucial ange. Ni crucial O an role O an role O an in d d c in olor d color change. Ni change. Ni O an O an d d trons tro and tro ns ns tro me and and ns tal tro and me me cns ations tal me tal and ctal ations cations (L me ci ations tal , H (L (L ci,ations i N , (L , H a H i , ,, ,K N H (L N a , e ,i a , N , tc , K H a K .), e , ,pl , e K N tc ay tc a .) , e .) a , pl tc K pl c ay .)r ay , e ucial pl a a tc ay c .) r c a ucial rrole pl ucial cay rucial a in role role c cr olor ucial role in in cch olor c in olor role ange. Ni color ch in ch ange. Ni ange. Ni cch olor O an ange. Ni ch d O an ange. Ni O an O an d d d O and trons and metal + + cations + + + (L + i + , +H , Na , K , etc.) play a crucial role in color change. NiO and + + + ++ + ++ +++ + ++ ++ + + ++ ++++ + + + + + + ++ + + + + + ++ + tro tro ns ns and and me me tal tal cations cations (L i (L , iH , H , N , a N, aK , K , e, e tc.) tc pl .) ay play a a cr ucial crucial role role in in color color chch ange. Ni ange. Ni O an O an d d trons troand ns troand ns tro me and tro ns tal me tro ns and me tal c tro ns ations and me tal tro ns c and ations me ns and tal cations (L me and tal c iations me (L tal , cH ations ime tal (L c , ,ations H iN tal c (L , ations a ,H i N c (L , , ations ,K a H i(L N , , , e ,a i K (L H N , tc , i , e H ,K (L a .) , N tc H , ,i pl , e a K N .) , ay ,tc , H a , e pl N K .) a , ,ay a tc , e K pl N c , .) a r ay a tc K , e ucial pl c , .) a tc , e r ay K ucial pl .) c tc , e a r ay role pl .) ucial tc c ay a pl rrole .) ucial ay in c a pl rrole ucial a c c ay in r olor ucial role c a r c in ucial olor role cch r c in ucial olor role ange. Ni ch c in role olor ange. Ni in ch c role olor in ange. Ni cch olor c O an in ange. Ni olor chc O an ange. Ni ch olor d ch ange. Ni O an ange. Ni d ch O an ange. Ni d O an d O an O an d d O an d d trotro ns ns and and me me tal tal cations cations (L i(L , i H, H , N,a N , a K, , e K tc , e .)tc pl .) ay pl a ay c a rucial crucial role role in in color color change. Ni change. Ni O an O an d d IrO IrO 2 ar 2 e arth e e thmo e mo st st popo pula pula r IrO anodic r 2anodic are th EC e EC . mo Hig . Hig st h po con h pula con cent cent r ra anodic tions rations o EC f o cat . f Hig cat ions ions h con in in th cent e the e ra lect etions lect rolyte rolyte of , cat , ions in the electrolyte, IrO IrO 2 ar 2 e arth e IrO e thmo e 2 mo ar st e st po th IrO po pula e 2mo pula ar IrO rst e anodic rth 2 po anodic ar e pula e mo th EC st r e EC anodic . mo po Hig . pula st Hig h po r con EC h pula anodic con . cent Hig r cent anodic ra h EC tions ra con tions . Hig cent EC of h o . ra cat f Hig con tions cat ions cent h ions con o in f ra in cat cent th tions e th ions e ra e lect tions o ef lin ect rolyte cat th rolyte ions o e f e , cat lect in , ions rolyte the in el, ect the rolyte electrolyte , , IrO2 ar IrO e th IrO 2 ar e 2mo e ar th e st e th po mo e pula mo st po st r pula anodic popula r anodic rEC anodic . Hig EC h EC . Hig con . Hig cent h con h ra con cent tions cent rao tions ra f cat tions ions of cat of in ions cat th ions e in elect th in e rolyte th ee lect el, rolyte ectrolyte , , IrO2 are the most popular anodic EC. High concentrations of cations in the electrolyte, IrO IrO 2 ar 2 e arth e e thmo e mo st st popo pula pula r anodic r anodic EC EC . Hig . Hig h con h con cent cent rations rations of o cat f cat ions ions in in the the e lect elect rolyte rolyte , , IrOIrO 2 ar IrO 2e ar th IrO 2e ar e IrO th 2mo e ar e IrO th 2mo e st IrO ar e 2th po e mo ar st IrO 2 e th e pula ar po mo st 2 e th e ar pula po mo st e th re mo pula e anodic po st th rmo pula e st po anodic rmo st po pula anodic r po EC st pula anodic r pula po . EC anodic Hig rpula EC . anodic r Hig h anodic . EC r con Hig h anodic EC . Hig con cent EC h . Hig con EC cent . h ra Hig EC con . cent h tions Hig ra con . h cent tions Hig ra con h cent o tions con ra f h cent cat o tions con ra f cent ions cat o tions ra f cent tions ra ions cat of tions in ra ions cat of th tions in o cat ions f e o th in cat e ions f le ect cat o th in ions f ee lrolyte cat ions ect th in el e in ions rolyte ect th ein e lth , rolyte ect e e th in lect rolyte , e e l th ect rolyte e , e lect rolyte e , lrolyte ect , rolyte , , , IrO IrO 2 ar 2 e ar th e e th mo e mo st po st po pula pula r anodic r anodic ECEC . Hig . Hig h con h con cent cent rations rations of o cat f cat ions ions in in the th ee lect elect rolyte rolyte , , which which which which is is an is an i on an is i on an con ion con ion ducto con ducto con ducto rwhich ducto , signi r, which r signi , signi rfi , is c signi fi antly an c is fi antly c ian on fi antly c affect antly icon on affect affect con ducto th affect e th ducto ele th e r, ele e th ct signi ele r rochro e ct , signi ele rochro ct firochro c ct antly rochro mic ficmic antly mic pro affect pro mic pert affect pro pert pro th ies pert e ies pert th ele of ies e of ct th ele ies rochro of e th ct TMO of th e rochro TMO e th TMO mic e , TMO , mic pro , , pro pert pert ies of iesth of e th TMO e TMO , , which is an which ion is con an ducto ion con r, signi ducto ficr antly , signi affect ficantly the affect electrochro the ele mic ctrochro propert mic ies pro of pert the ies TMO of , the TMO, which which is which an iis on an is con an ion ducto ion con con r ducto , signi ducto r, fisigni crantly , signi fic affect antly ficantly affect the affect ele th cte rochro th ele e ct ele rochro mic ctrochro pro mic pert mic pro ies pro pert of pert ies the ies of TMO th of e th , TMO e TMO , , which is an ion conductor, significantly affect the electrochromic properties of the TMO, which which is is an an ion ion con con ducto ducto r, signi r, signi ficfi antly cantly affect affect the thele e ele ctrochro ctrochro mic mic pro pro pert pert iesies of of the thTMO e TMO , , which which which is which an is which i an is on which an iis on con which an iis on con which ducto an is ion con ducto is an ion con r an ducto is , ion signi con an r i ducto on , con signi r ducto ion , fi con signi ducto crantly , con fi ducto signi c rantly , fi ducto signi r c , affect antly fi signi r, c affect antly signi fi r, c fi affect antly th signi ce fi antly affect th c ele antly fi e affect th c ct ele antly affect e rochro th ct ele affect e rochro th ct ele affect e th rochro mic ele ct e th rochro ele mic ct e th pro rochro ele ct mic e pro rochro pert ct ele mic rochro pro ct pert mic ies rochro pro pert mic ies of pro mic pert ies th of pro mic pert e pro ies th of TMO pert e pro ies th pert of TMO e ies th , pert of TMO ies e of th , TMO ies of e th , TMO e th of TMO , e th TMO , e TMO , , , which which is an is an ion ion con con ducto ducto r, signi r, signi ficantly ficantly affect affect the th ele e ele ctrochro ctrochro mic mic pro pro pert pert iesies of of the th TMO e TMO , , such such such as switc such as switc as switc as switc hing time hing time hing time hing time , cyc , cyc such , cyc lic, cyc li such ity c as switc li ity c and ity lias switc and city and staining hing time and staining sthing time aining st effic aining effic , cyc effic ien effic , cyc ien cy. liien ccy. ity ien li cy. c and ity cy. and staining staining effic effic iency. ien cy. such as switc such hing time as switching time , cyclicity , cyc and list city aining and effic staining iency. effic iency. such such as switc such as switc such as switc hing time as switc hing time hing time hing time , cyc , cyc lic , cyc ity lic , cyc ity and licity and list city and aining stand aining staining effic staining effic ien effic cy. ien effic ien cy.ien cy. cy. such such as switc as switc hing time hing time , cyc , cyc licli ity city and and staining staining effic effic ien ien cy. cy. such such as switc such as switc such as switc such hing time as switc such hing time as switc such hing time such as switc hing time such as switc , cyc hing time such as switc , cyc as switc li hing time , cyc city li as switc hing time c , cyc ity li and hing time c , cyc ity hing time and lic st , cyc hing time ity and li aining st c , cyc ity and aining li , cyc st city and aining li , cyc effic st city li and aining , cyc effic cst li ity and ien aining c effic ity st li and ien cy. c aining effic ity st and ien cy. aining effic st and aining cy. ien st effic aining ien cy. st effic aining ien cy. effic effic ien cy. ien effic cy. ien cy. ien cy. cy. The majority of TMO have a band gap of 1–5 eV (Figure 15), and therefore occupy an Th Th e majority Th e majority e majority of of TMO of TMO TMO have have Th have e a Th majority band a e band a majority band gaga p of o ga p TMO fof o p 1f– o TMO 1 5 f – e 5 1 have V – e5 V ( have Fi e( V a g Fi ur band (g Fi ur a e g band 15), e ur ga 15), e and 15), p ga and ofp and th 1o –th e f 5 refore 1 e e th – refore V 5 e( refore e Fi V occ g(ur occ Fi upy g e occ upy ur 15), e upy an 15), and an an and therefore therefore occupy occupy an an The majority The majority of Th TMO e of majority TMO have have a of band TMO a band ga have p o ga f a 1 p – band 5 ofe 1 V –5 ( ga Fi ep g V ur o (f Fi e 1g 15), –ur 5 e e V and 15), (Fith g and ur erefore e th 15), erefore occ and upy th occ e refore an upy an occupy an The Th majority e Th majority e majority of TMO of TMO of TMO have have a have band a band a ga band p ga op fga 1 o– p f5 1 o e – fV 5 1e – (V Fi 5 e g (Fi V ur g (e Fi ur 15), g e ur 15), and e 15), and th and eth refore eth refore erefore occocc upy occ upy an upy an an ThTh e majority e majority The Th of majority e of TMO majority TMO have of have TMO Th of a TMO e band Th a majority band have e majority have gaga a p band o p of a f o 1 band TMO f of – 1 5 ga – TMO e5 V p ga e have o ( V Fi p f have ( 1 g o Fi – ur f5 a g 1ur e e band –V a 5 15), e band e (15), Fi V ga and g (Fi ur and p ga g e o th ur p 15), f e th 1 e o refore – f 15), e 5 and 1 refore e –V 5 and e th ( occ V Fi e occ g ( th refore upy Fi ur e upy g refore e ur an 15), e occ an 15), and occ upy and upy th an eth refore an erefore occocc upy upy an an ThTh e majority e majority Th of e Th majority of TMO e Th TMO majority Th e majority have e majority have of a TMO of band a TMO of band of TMO have ga TMO have p ga a o have p fband 1 o have a – f band 5 1a – ega 5 band V a ep ( band V ga Fi o( g p fFi ga ur 1 o g – ga p f e ur 5 1 o 15), p e – e fV 5 o 1 15), f e – (and Fi V 5 1– e g and (5 V Fi ur th eg ( V e Fi e ur th 15), refore (g Fi e e ur refore 15), g and e ur 15), e occ and 15), th occ upy and eth refore and upy e th an refore e th an refore occ erefore occ upy occ upy an occ upy an upy an an intermediate position between semiconductors and dielectrics [67]. EC behavior is de- interm interm interm edi edi ate edi ate po ate po sition po sition si tion bet interm bet ween bet interm ween ween edi semico ate edi semico sate emico po nduct si po nduct tion nduct siors tion bet ors and ors ween bet and ween and di di electr semico electr di selectr emico ics nduct ics [67] ics nduct [67] . ors [67] EC . EC ors and . beh EC beh and di avi beh electr avi or di avi or electr is or ics is de is de ics [ -67] de - [. 67] -EC . EC beh beh avior aviis or de is - de- interm interm interm edi interm ate edi edi po interm ate edi ate sition po ate po edi si si po bet tion tion ate si ween tion bet po bet si ween bet ween tion semico ween bet semico semico nduct ween semico nduct ors nduct semico nduct and ors ors nduct di and ors and electr and di ors di electr ics electr di and electr [67] ics ics di . [ electr EC ics 67] [67] . [beh 67] . EC ics EC . avi beh [ EC 67] beh or avi . beh avi is EC or de avi or beh is -or is de avi de is - - or de-is de- intermediate position between semiconductors and dielectrics [67]. EC behavior is de- interm interm edi edi interm ate ate interm po interm po edi sition interm si edi ate tion interm edi ate bet po bet edi ate si ween interm po tion edi ween ate interm si po tion ate si s po bet edi emico tion s si po emico ween bet edi ate tion si bet ween ate nduct tion po bet nduct ween ssi po emico tion bet ween s ors si emico tion ors ween s emico nduct bet and s and bet emico nduct ween s di emico ors nduct ween electr di ors nduct electr s and emico nduct ors s ics and emico di ics ors and [nduct electr 67] di ors [and 67] nduct electr . di EC and ics . ors electr di EC ors ics beh electr [and di 67] beh ics electr [ avi and . 67] EC di ics avi [or 67] . electr di ics EC or [ beh is 67] . electr EC is [ de beh 67] avi . ics de EC -beh . ics or avi [ -EC 67] beh avi is or [67] . beh de EC avi or is . -de EC avi or is beh -de or is beh avi -de is avi or -deor is - de is -de- interm interm edi edi ate ate popo sition si interm tion bet bet ween edi ween ate semico po semico sition nduct nduct bet ors ween ors and and semico di electr dielectr nduct icsics ors [67] [ 67] . and EC . EC di beh electr beh avi avi or icsor is [67] de is . - de EC - behavior is de- pendent on TMO structure. It should be noted that structural and impurity defects directly pen pen dent pen dent dent on on T pen on MO TMO T dent MO str str ucture on pen str ucture pen T ucture dent MO . pen dent It. sho It str on . dent sho It ucture on uld T sho MO uld T on MO be uld be . T str not It MO be str ucture not sho ed ucture not str ed uld th ed ucture th at . It be at th struct . sho It not struct at sho . struct uld It ed ur sho uld ur al th beal uld ur at an be not an al struct d not be ed i an d mp ed inot d mp th ur uri iat th mp ed al uri ty at struct an uri th ty struct defe at d ty defe istruct ur mp ct defe ur al s ct uri di al an s ct ur di rect ty an d s al rect di idefe d mp ly an rect imp ly d uri ct ly iuri s mp ty di ty defe uri rect defe ty ly ctdefe s ct di s rect di ctrect s di ly rect ly ly pendent pen on pen dent Tdent MO on str on TMO ucture TMO structure str . Itucture sho. uld It . sho It be sho uld not uld be ed not be that not ed struct th ed at th ur struct at al struct an ur d al iur mp an al uri d an imp ty d idefe mp uriuri ty ctdefe s ty di defe rect cts ct ly dis rect direct ly ly pendent on TMO structure. It should be noted that structural and impurity defects directly pen pen dent dent on on TMO TMO strstr ucture ucture . It. sho It sho uld uld bebe not not ed ed that that struct struct urur al al anan d id mp imp uri uri ty ty defe defe cts ct di s rect direct ly ly pen pen dent pen dent on pen dent T pen on dent MO pen on T dent MO pen str T on dent MO pen ucture dent on str TMO on dent ucture str T MO on T ucture str . MO It on T ucture str sho MO . It T str ucture MO . sho uld It str ucture sho . uld ucture It str be . sho uld ucture It not be . sho It uld not be ed . sho It uld not . th be sho ed It uld at not be sho ed th uld struct at be not th ed uld be struct not at ed th ur not be struct at ed th al not ur struct ed at th an al ur struct at ed th d an al at struct iur th mp d an struct at al ur imp uri d an struct al ur imp ty uri d an ur al idefe mp an al d uri ty ur ian d mp defe al ty uri ct id mp an defe s uri ty ict di mp d uri defe s ty rect ict mp di uri ty defe s rect ly ct di uri ty defe s rect ct ly defe di ty s ct rect ly defe di s ct rect di ly s ct rect di ly s rect di ly rect ly ly pen pen dent dent on on TMO TMO strstr ucture ucture . It. sho It sho uld uld be be not not ed ed that thstruct at struct ural uran al d an imp d imp uriuri ty defe ty defe cts ct di s rect direct ly ly affect affect affect the thth e pro e pro pert pro pert ies pert ies — ies — part — p affect art icu part icu larly icu th larly e larly th pro e thpert th e ph e ph ysi ies ph ysi coch — ysi coch pcoch art em em icu ical em ilcal arly ipro cal pro pert th pro pert e ie pert ph sie — ysi s ie — of s coch — of th of e th em th e EC ical e EC under EC pro under under perties—of the EC under affect th affect e propert the affect ies pro — pert affect p th art e ies icu pro — th larly e p pert art pro icu ies th pert — e larly p ph ies art ysi — th icu coch p e art larly ph icu em ysi lth iarly coch cal e ph pro em th ysi e ipert cal coch ph ie ysi pro s em — coch pert i of cal em ie th pro se i— cal EC of pert pro th under ie e pert s— EC of ie s under th —e of EC th e under EC under affect affect the affect pro thpert e th pro e ies pro pert —pert pies art— ies icu p — lart arly picu artth icu larly e larly ph th ysi e th coch ph e ysi ph em coch ysi ical coch em pro em ical pert i cal pro ie pro s pert —pert of iesth — iee s of — EC of th e under th EC e EC under under affect the properties—particularly the physicochemical properties—of the EC under affect affect the the pro pro pert pert iesies —— part part icu icu larly larly the the phph ysi ysi coch coch em em ical ical pro pro pert pert iesie — s— of of the the EC EC under under affect affect affect the affect th pro affect e th pro affect pert e th affect pro pert e th ies affect pro pert e th — ies pro e th pert p— ies art e pro th pert p — pro ies icu e art pert p pro — ies l icu pert art arly p ies — icu pert l art arly ies p — th icu l art arly — p ies e art icu th lp arly — ph art e icu th lp arly ysi ph icu art e larly th coch ysi ph icu le arly th ysi coch ph le arly th em coch ysi ph e th i em cal e ph ysi coch th em i ph ysi e cal coch pro ysi em i ph coch cal pro pert ysi em coch i cal pro em pert coch iie cal em pro s i pert cal — ie em i pro cal s pert of — ie pro i cal s pert of th — pro ie pert e s of th pro — ie pert EC e sof ie th — pert EC s e ie under of — th sEC — of ie e under th sof EC — e th under of EC e th under e EC th under EC e under EC under under affect affect the thpro e pro pert pert iesies —p — art part icuicu larly larly the thph e ph ysiysi coch coch em em ical ical pro pro pert pert iesie — sof —of the thEC e EC under under study. study. study. study. study. study. study. study. study.study. study. study. study. study. study. study. study. study. study. study. study. study. study. study. Figure 15. Classification of mFigure aterials15. by c Clas ond siuctiv ficatio ity (a n of cc m or aterial ding s to by c zone ond the uctiv oryit ).y (a ccording to zone the ory ). Figure Figure Figure 15. 15. Clas 15. Clas Figure si Clas fi si cat fisi cat io 15. fin o cat io Figure Clas n o io f m n o f si a m Figure fi terial f 15. a cat m terial a Clas io terial s n o by c 15. s si by c f s fi Clas m ond by c cat a ond terial io uctiv si ond n o fi uctiv cat sf uctiv it by c io m y (a it n o ay (a terial ond it cc f y (a or m cc uctiv di s a or cc terial by c ng di orng it di to ond y (a s ng to zone by c uctiv cc zone to or ond zone the di it the ng uctiv y (a ory the ory to ). cc it ory zone or ). y (a di ). ng cc the or to ory di zone ng ). to the zone ory the ). ory). Figure Figure 15.Figure Clas 15. sifi 15. Clas cat Clas io sin o ficat sif fi m io cat a n o terial iof n o ms f a terial by c material ond s by c uctiv s by c ond itond y (a uctiv uctiv ccit or y (a di ity (a ng ccor to cc di or zone ng di to ng the zone to ory zone ). the ory theory ). ). Figure 15. Classification of materials by conductivity (according to zone the ory). Figure Figure 15.15. Clas Clas sifi si cat ficat ion o ion o f m f a m terial aterial s by c s by c ond ond uctiv uctiv ity (a ity (a ccor ccdi orng ding to to zone zone the the ory ory ). ). Figure Figure Figure 15.Figure Clas 15.Figure 15. Clas si Figure fi 15. Clas cat Figure si fi 15. Clas io Figure cat sin o 15. fi Clas io cat si 15. f n o Clas fi m io cat si 15. Clas a f n o fi terial si m io cat Clas fi f a n o si cat terial m io fi s a f n o cat si io terial by c m fi n o s f io a cat terial m by c n o ond f s io am terial by c f n o ond a uctiv m sterial by c f a ond uctiv terial m s it by c a ond s y (a terial uctiv by c it sond y (a uctiv by c ccit ond sor uctiv y (a by c cc ond di it or uctiv ng y (a cc ond di it uctiv or to y (a ng cc it di uctiv zone y (a or to ng it cc di y (a or zone cc to ng it the di y (a or cc zone ng to di or ory the cc zone ng di to or ). ory the ng zone to di ory the ). to ng zone zone the ory ). to the zone ory ). the ory ). the ory ). ory ). ). Figure Figure 15.15. Clas Clas sifisi cat ficat ion o ion o f mf am terial aterial s by c s by c ond ond uctiv uctiv ity (a ity (a ccor cc di or ng di ng to zone to zone the the ory ory ). ). The optical band gap can be calculated according to Equation (4) [37]: Th Th e opt Th e opt e opt ical b ical b ical b and gap c and gap c and gap c an b Th an b e opt e calcu an b Th e calcu e opt e calcu ical b la ical b ted la and gap c ted la accord ted and gap c accord accord ing an b ing to an b ing e calcu to Equation e calcu to Equation Equation lated la (4) ted accord (4) [ (4) 37] accord [37] :[ ing 37] : ing :to Equation to Equation (4) (4) [37][:37] : The optical b The opt and gap c ical band gap c an be calcu an b lated e calcu accord lated ing accord to Equation ing to (4) Equation [37]: (4) [37]: The opt The opt Th ical b e opt Th ical b e opt and gap c ical b and gap c ical b and gap c and gap c an b an b e calcu an b e calcu an b e calcu lated e calcu lated accord lated accord lated accord ing accord ing to ing Equation toing Equation to Equation to Equation (4) (4) [37] (4) [:37] (4) [37] : [37] : : ThTh e opt e opt ical b ical b and gap c and gap c an b an b e calcu e calcu lated lated accord accord ing ing to to Equation Equation (4) (4) [37] [37] : : ThTh e opt e opt Thical b e opt Th ical b e opt Th and gap c ical b e opt Th and gap c ical b e opt Th and gap c ical b Th e opt and gap c Th ical b e opt an b and gap c e opt Th ical b an b e calcu ical b e opt and gap c an b e calcu ical b and gap c an b ical b e calcu and gap c la an b and gap c e calcu ted la an b and gap c ted e calcu la accord an b ted e calcu accord an b lated e calcu an b accord la ing e calcu ted an b accord la ing e calcu to ted accord la ing e calcu to Equation ted la accord ing Equation ted to la accord ted ing Equation to la accord ted ing Equation accord to (4) ing Equation (4) accord to [ing 37] Equation (4) to [ ing 37] : to Equation (4) ing [ 37] :to Equation (4) [Equation to :37] (4) [Equation 37] : (4) [37] : (4) [37] (4) : [37] (4) : [37] : [37] : : n n n n n n n n n n n αhv = A(hv−E ) αhvαhv=αhv=A(A=hv(Ahv−α(hvhv−E E−)=)E A( )hv α−hvEα=hv) A =(hvA(−hvE−)E ) (4)(4) (4) (4) αhv =gαAhv n(αhvnhv =−A=(αEhvhvA)(−hv= EnA−()Ehv )− E ) (4) (4) (4) (4) αhv =g Ag(nhv g n−En ) n n n ng n g n (4) (4) (4) g gg g g (4) αhvαhv==A(Ahv(hv−E−E) ) g αhvα=hvαAhv=(αhvAhv =(α−hvAhv=(Eαhv −hvA=α)(Ehv−hvA =α)(Ehv=Ahv − ()AEhv=− ()hvAE−( )hv E− E)− )E ) (4)(4) αhvαhv= A=(hvA(ghv−E−)E ) (4) (4) (4) (4) (4)(4) (4) (4) g g g g g g g g g (4)(4) g g where where where α is α α is th is e th absorpt th e absorpt e absorpt ion ion where coeff ion coeff coeff ici α ici ent is ici ent , th ent w , e hich w absorpt , hich which can can ion be can be meas coeff be meas meas ici ured ured ent ured by , w by hich th by e thul th e can trav ul e ul trav be iol trav iol meas et iol e spect t e spect ured t spect ro- ro- by ro- the ultraviolet spectro- where αwhere is the α absorpt is where the ion where absorpt α is coeff th α e ion ici is absorpt ent th coeff e , absorpt w ici ion hich ent coeff , can ion which ici be coeff ent meas can , ici w ent hich be ured , meas w can by hich ured th be can e meas ul by trav be th ured meas iol e ul et trav by ured spect th iol e by ro- e ul t th spect trav e ul iol ro- trav et spect iolet ro- spectro- where where α where is thα e is absorpt α th ise th absorpt e ion absorpt coeff ion ici ion coeff ent coeff , ici went hich icient , w can , hich which be can meas can be ured be meas meas by ured th ured e by ulby trav the th ul iol e trav e ul t trav spect iole iol t ro- spect et spect ro-ro- where α is the absorption coefficient, which can be measured by the ultraviolet spectro- where where α is α is th e thabsorpt e absorpt ion ion coeff coeff iciici ent ent , w , hich which can can be be meas meas ured ured by by the thul e trav ultrav ioliol et e spect t spect ro- ro- where where where α where is α th where is α e where th is absorpt α where e th is absorpt α where e th is α absorpt e ion th is α absorpt e is th ion α coeff absorpt e th is ion absorpt coeff e th ici absorpt ion e coeff ent absorpt ion ici coeff ion ent , ici w coeff ion ent , hich ici coeff w ion ent , coeff hich ici w can ici ent coeff , hich w ici ent can , hich be w ent ici can , hich meas be w ent , can hich w be meas , hich can w ured be meas can hich ured be meas can by be ured meas can be by th meas ured e be meas by th ured ul e meas ured by trav th ul ured e by trav th ul iol ured by e th trav e ul iol by t e th trav spect e ul iol by e th t trav ul spect e e iol th t trav ro- ul spect e e iol trav t ul ro- spect iol etrav t ro- iol e spect t e spect ro- iol t spect e ro- t spect ro- ro-ro- where where α is α th is e th absorpt e absorpt ion ion coeff coeff icient icient , w , hich which can can be be meas meas ured ured by by the th ul e trav ultrav ioliol et spect et spect ro-ro- phph otph om otph om ot eter; om ot eter; om eter; h eter; is h is th h is e th h Planc th e is Planc e th Planc e ph k Planc con k ot ph con k om stant; ot con k eter; stant; om con stant; eter; v stant; h is v is is th v h th is e th is v e light th e is th Planc light e e th light Planc e frk light equ fr con equ fr k en equ fr con stant; en cy; equ en cy; stant; A en cy; v A is cy; is A is a v th pro A is a is e pro a is th light po pro a e po rtion light pro po fr rtion equ po rtion ality fr rtion ality equ en ality con- cy; en ality con- A cy; con- is con- A a is pro a pro portion portion ality ality con- con- photometer; phot h om is eter; the Planc h is th k con e Planc stant; k con v isstant; the light v is fr thequ e light ency; frequ A is en a cy; pro A po is rtion a pro ality portion con-ality con- photom ph eter; ot ph om ot h om eter; is eter; thh e is Planc h th ise th k Planc e con Planc stant; k con k con stant; v is stant; thv e is light v th ise th fr light equ e light en frequ cy; frequ A enis cy; en a cy; A pro is A po a is pro rtion a pro po ality rtion portion con- ality ality con- con- photometer; h is the Planck constant; v is the light frequency; A is a proportionality con- phph otom otom eter; eter; h is h is th e thPlanc e Planc k con k con stant; stant; v is v is th e thlight e light fr equ frequ enen cy; cy; A A is is a pro a pro popo rtion rtion ality ality con- con- phph otom ot ph om eter; ot ph om eter; ot ph h om eter; ot ph is h om eter; th ot ph is h om e eter; th ph ot is Planc h e ph om ot eter; th Planc is om h ot e ph eter; th is k Planc om h ot eter; e con th k is om Planc eter; h e con th stant; k is Planc h eter; e con stant; th is h k Planc e is th con stant; h k v Planc e th is con is stant; v Planc k e th th is con Planc stant; v k e e th is con Planc light k stant; v e th con k light is stant; v e con th k fr is light stant; v e equ con th fr stant; is light v equ e en stant; th fr is light v equ e cy; en th is fr v light equ cy; e is th en A fr v light e th equ cy; is is A en light fr e th a equ is cy; light A en fr pro e a equ is cy; light fr A en pro po equ a fr is cy; en A pro equ po rtion a fr en cy; is A pro rtion equ po en a cy; is A ality pro rtion cy; po en a A is ality pro rtion cy; po A a is con- ality pro rtion a is po A con- ality pro a rtion is po con- pro ality a po rtion con- pro po ality rtion con- rtion po ality con- ality rtion ality con- ality con- con- con- stant; Eg is the optical band gap; n is a number that is ½ for the direct band gap semicon- stant; stant; stant; EgE ig s E ith s g th e is op e th op tic e op tic al tic al ban stant; al ban d ban stant; g d ap; E g d ap; g g in s E ap; g is th n i s is e a nth op numb a is e numb tic a op numb al tic er ban er al ther th at ban d at th g is ap; d at i½ s g ½ iap; for n s ½ for is th n a for th is e numb di e a th di rect numb e rect di er rect band th er band at band th iga s at ½ ga pi s ga sem p for ½ sem p for th icon- sem e icon- th di icon- e rect direct band band gap ga sem p sem icon- icon- stant; stant; Eg is E th ge istant; s op th tic e op al Egban tic is al th d ban e gop ap; d tic g nal ap; is ban a nnumb is d a gap; numb er n th is at er a ith s numb ½ at for is er ½ th th for e at di th rect is e ½ di band for rect th band ga e di p rect sem gap band icon- semicon- gap semicon- stant; stant; stant; Eg iE s gth iE s e gth iop s e th tic op e al tic op ban al ticban al d g ban d ap; gd ap; ng is ap; na is n numb a is numb a numb er th er at th er iat s th ½ iat s for ½is for ½ th e for th di e rect th di e rect di band rect band ga band p ga sem p ga sem p icon- sem icon- icon- stant; stant; Eg E is g i th s e thop e op tictic al al ban ban d g d ap; gap; n is n is a numb a numb er er that that is i ½ s ½ for for th e thdi e rect direct band band ga ga p sem p sem icon- icon- stant; stant; stant; Eg E stant; is g th istant; E s g e th istant; E op s e gth stant; op tic iE s e gstant; th al tic i op E s e stant; gban al th tic i E op s e stant; ban gal th E d tic iop s ge ban g E th al d itic s ap; op g e g th i E ban al s d ap; tic op g e n th g iban s al op d ap; tic is e n th g op ban a tic al is d ap; e n numb tic g al a ban op is d ap; numb n al ban tic g a d is ap; ban numb n al er g d a is ap; ban er th numb n g d a ap; is at th g n er numb d ap; a i at is s n g th numb er ap; ½ a i is n at s th numb er for ½ a is in at s numb th a for er is ½ th inumb at s a th for er e th ½inumb di at s er e th for th ½ rect di ier at th s e for rect ½ th at i th di er s band e for at rect ½ th i th s di band e ½ i for at rect s th di band ga ½ for e i s rect th di p ga for band ½ e th sem rect p di for ga band e th sem rect di p e icon- ga band th rect di sem e icon- p ga band rect di sem p band icon- ga rect sem band p icon- ga sem band ga p icon- sem ga picon- sem p ga icon- sem picon- sem icon- icon- ductor and 2 for the indirect band gap semiconductor. ducto ducto ducto r and 2 r and 2 r and 2 for the for the for the indire indire ducto indire ct ducto band gap ct r band gap ct and 2 band gap r and 2 for the semicon for the semicon semicon indire ducto indire ducto ctducto band gap r. ct r. band gap r. semicon semicon ducto ducto r. r. ducto ducto r and 2 ducto r and 2 for the ducto r and 2 for the r indire and 2 for the indire ct for the band gap indire ct band gap indire ct band gap semicon ct band gap semicon ducto semicon ducto r. semicon ducto r. ducto r. r. ducto ducto r and 2 r and 2 for the for the indire indire ct band gap ct band gap semicon semicon ducto ducto r. r. ducto ducto r and 2 r and 2 ducto ducto for the r for the and 2 r and 2 indire indire for the ducto for the ct band gap ducto ctindire band gap r and 2 indire r and 2 ct band gap s ct for the emicon band gap semicon for the indire ducto sindire emicon ducto semicon ct r. band gap r. ct ducto band gap ducto r. sr. emicon semicon ducto ducto r. r. ducto ducto r and 2 r and 2 ducto for the ducto for the r ducto and 2 ducto r indire and 2 indire r and 2 for the r ct and 2 for the band gap ctfor the band gap indire for the indire indire ct s band gap emicon indire ct semicon band gap ct band gap ct ducto band gap ducto semicon r. semicon r. s emicon s ducto emicon ducto r. ducto ducto r. r. r. The Eg of the WO3 films decreased from 3.62 eV to 3.30 eV when the annealing tem- Th Th e Th E e g E of e g E of th g of th e WO e th WO e 3WO f3 il f ms il 3 ms fTh il de ms de e creased Th E de creased g e of creased E g th of from e th from WO e from WO 3.62 3 f 3.62 ilms 33.62 eV fil eV ms de to eV to creased 3.3 de to 3.3 0 creased 3.3 e 0 V e0 from V when ewhen V from when 3.62 th th e 3.62 eV anne e th anne e to eV anne aling 3.3 to aling 0 aling 3.3 e tem V 0 tem when e -tem V -when - the th anne e anne aling aling tem tem - - The E Th g Th of e e Th E th E g e e g of of E WO Th th g th of e e 3e WO th E fWO il g e ms of WO 3 3th f il de fil e ms 3ms creased WO fil de ms de 3creased f creased de ilms from creased de from 3.62 creased from from eV 3.62 3.62 to from 3.62 eV eV 3.3 to 0 eV to 3.62 e 3.3 V 3.3 to0 when eV 0 3.3 ee V to V 0 when e when 3.3 th V e when 0 anne e th V th e when e aling anne th anne e anne aling th tem aling e aling anne - tem tem aling - tem - - tem- The Eg of the WO3 films decreased from 3.62 eV to 3.30 eV when the annealing tem- The Eg of the WO3 films decreased from 3.62 eV to 3.30 eV when the annealing tem- The Eg of Th th e Th e Ee g Th WO of Ee g Th th of 3E f e e Th g il th of WO E ms Th e e g th of E Th WO e de 3 g e E th f of e Th WO il creased g 3 e E ms of th f e g WO il of 3 e E th ms de f g WO il e th of 3ms creased WO de f from e il th 3WO ms creased de fe il 3 WO ms creased f3.62 de il 3 ms from f creased il de 3ms eV ffrom de creased ilms 3.62 de creased from to creased de 3.62 3.3 from eV creased 3.62 0 from eV e to from 3.62 V 3.3 eV from to when 3.62 0 eV 3.3 from to 3.62 e V 3.62 eV 0 3.3 to th e when eV 3.62 V 0 3.3 e to eV e anne when to V 3.3 0 eV e to when 3.3 th 0 V aling e 3.3 e to when 0 th V anne e 0 3.3 e when V th e anne tem 0 V when e aling th e anne when V - e aling th anne when e th tem aling anne e th tem aling anne -e th anne tem aling - e aling anne tem - aling tem - aling tem - tem - tem - - ThTh e E e g E of g of th e th WO e WO 3 fil 3 ms films de creased decreased from from 3.62 3.62 eVeV to to 3.3 3.3 0 e0 V ewhen V when th e th anne e anne aling aling tem tem - - perature perature perature wa wa s wa incre s incre s incre ased asas ed . In ed . perature In add . In add ition, add ition, wa ition, th s e th incre E th e g E e of g as E of th g ed of e th . colo th e In colo e add red colo red ition, WO red WO 3 WO f th il 3 e m fil 3 E s f m il g wa s of mwa s s th wa le s e ss le colo s ss le thss an th red th an th WO an at th th at of 3 f at of ilm of s was less than that of peratureperature was incre perature wa as s ed incre perature . In wa as add ed s incre ition, . wa In s add as incre th ed e ition, . Eas In g of ed add th th . e In e ition, Ecolo add g of red th ition, the e colo WO Eg th of red 3 e f th E ilm g e WO of colo s wa th 3 e f red s ilcolo m less WO s wa red th 3 an s fWO il le m th ss s at 3 th wa fil of an m s s le th wa ss at s th of le an ss th th at anof that of perature perature perature was incre wa wa s as incre ed s incre . In ased add as. ed In ition, . add In add th ition, e ition, Eg th of e th th Ee e g of E colo g th of red e th colo e WO colo red 3 red fil WO mWO s 3 wa fil3 m s fil s le m wa ss s wa th s le an s ss le th th ss at an th of an that thof at of perature was increased. In addition, the Eg of the colored WO3 films was less than that of perature perature wa wa s incre s incre ased ased . In . In add add ition, ition, the thE e g E of g of the thcolo e colo red red WO WO 3 fil 3 m fils m wa s wa s le s ss less than than th at that of of perature perature perature perature wa perature s wa incre perature wa s perature incre wa s as perature incre ed wa s as incre wa . ed s In as wa incre s . ed add In as incre wa s . ed incre add In as ition, s . ed as incre add In ition, ed . as add In th ition, ed . as In add e . th ed ition, E In add e g . ition, th of add In Ee g ition, th th of add Eition, e e g th th of colo E ition, e e th g th of E colo e th red e g E th of e colo th g red e E of th WO e g colo of e red E th WO g colo 3 e th of red fil WO colo e 3 m th red f colo il WO s e 3 red m wa colo fWO il s red 3 m s wa f WO il s red le 3 m WO wa s fss il s 3 le m WO fth wa s il ss 3 s m le fan il wa s th ss 3 s m f le th an wa il s s th ss m at wa le an th s s th ss of le at wa s an th ss th le of at s ss an th th le of at an th ss than of at th th an at of that of thof at of perature perature wa wa s incre s incre ased as. ed In . In add add ition, ition, the th E e g E of g of the th colo e colo red red WO WO 3 fil 3 m fils m wa s wa s le s ss less than th an th at thof at of the th blea th e blea e th che blea e che blea d che WO d che WO d 3 WO d films 3 WO films 3 films [ 3 38] films [ th 38] . [ e Th 38] th blea . [Th 38] e e . Th di blea e che . ff di Th ee di ff d che re ee WO ff di nt re e d ff nt re band WO e 3 nt re films band nt band 3 films g band ap [g 38] ap g dap [ emo . 38] d g Th ap emo d . e emo nstrates Th d di nstrates emo e ff nstrates di ere nstrates ffnt e that re band tnt hat tth hat band e t th hat g con th e ap con e th g du d con ap e emo du ct con d du ivity ct emo nstrates du ivity ctivity ct nstrates of ivity of of that oft hat the th con e con duct du ivity ctivity of of the bleache the d blea WOche 3 films d WO [38] 3 films . The [ di 38] ffe . re Th nt e di band ffere gnt ap band demo gnstrates ap demo that nstrates the con that du th cte ivity condu of ctivity of the blea thche e th blea e d blea WO cheche d 3 films WO d WO 3 [films 38] 3 films . Th [38] e [di 38] . Th ff. ee Th re di nt eff di band ere ffe nt re g band nt ap band demo gap gnstrates ap demo demo nstrates tnstrates hat thte hat con that th du e th con ct e ivity con duct du of ivity ctivity of of the bleached WO3 films [38]. The different band gap demonstrates that the conductivity of the th blea e blea che che d WO d WO 3 films 3 films [38] [38] . Th . Th e di e ff di eff re ent re nt band band gap gap demo demo nstrates nstrates that that the th con e con dudu ctivity ctivity of of the th blea e th blea che e th blea che e d th blea WO che e th d blea WO e th che 3 d blea films e WO th che blea d 3 films e che WO blea d 3 [films che 38] WO d 3 [che films WO . 38] d Th 3 WO [films 38] d . e 3 Th films WO [ di 38] . 3 e Th films ff [di 38] 3 e . e films re Th [ ff 38] di . nt e[ e Th re 38] ff . di band e nt Th [ e re 38] . ff di Th band e e nt re ff . di g e Th band e nt ap ff di re e e g band nt ff re d ap di e emo nt g band re ff ap d e nt band emo g re nstrates ap d band nt emo gap nstrates d band g emo ap nstrates d gemo ap t d nstrates hat g emo ap d tnstrates emo hat th d nstrates te emo hat th nstrates con te hat th nstrates con du te hat th con tct du hat e ivity th tcon hat ct du e th t ivity con hat ct e du th of ivity con e ct du th of con ivity e du ct of con ivity du ct of ivity ct du ivity of ct ivity of of of the th blea e blea che che d WO d WO 3 films 3 films [38] [38] . Th . Th e di e ff di eff reent re nt band band gap gap demo demo nstrates nstrates that that the th con e con dudu ctivity ctivity of of the thWO th e WO e th 3 WO e fil 3 WO m fil 3 f m is il 3 m fis enh ilm is enh anced is enh anced enh anced th anced with e with th WO with e decre with WO 3 decre f il decre m 3 asi f decre il asi is ng m asi enh ng is E asi ng genh ,E anced whi ng gE , whi ganced ,E le whi g ,with le twhi he le t he with high le tdecre he high the high decre con asi high con d ng con asi uctivity d con uctivity E ng d guctivity , d whi E uctivity g,incre whi le incre the incre le asincre ted as high he as ed th high ed as e th con ed th e con d e th uctivity ed uctivity incre incre ased as th ed e the the WO3 f th ilm e WO is enh 3 filanced m is enh with anced decre with asing decre Eg, asi whi ng le E the g, whi high le t con he d high uctivity cond incre uctivity asedincre the ased the the th WO e th WO 3 e th fWO ile 3 m fWO il 3 is m f il enh 3 is m f il enh is anced m enh is anced enh anced with anced with decre with decre with asi decre ng asi decre ng asi Eg,ng asi E whi g,ng E whi le g, E whi tle he g, whi tle he high tle he high tcon he high con high ductivity con ductivity con ductivity dincre uctivity incre as incre ed as incre ed th as e ed th ase ed th e th e the thWO e WO 3 fil 3 m film is is enh enh anced anced with with decre decre asiasi ng ng Eg,E whi g, whi le le the the high high con con ductivity ductivity incre incre ased ased th e th e the th WO e th WO e 3 th fWO il 3 e m th fil WO 3 e m is th fil WO enh is e 3 m th f WO il enh e is 3 th m anced f WO il e enh th 3 is anced m WO fe il th enh 3 is anced m WO fwith e il 3 enh is m anced WO fwith il 3 enh m fis anced il decre with 3 m is enh fanced decre il with is m enh anced asi decre with enh is anced asi ng decre enh with anced ng asi decre E with anced g ng , asi E with decre whi g ,with ng asi E whi decre g le ,with decre ng asi E whi t le ghe decre ,ng asi E whi t le he gdecre high asi ,ng E whi tle he asi g high ng , E whi tle ng asi g con he high ,E whi t gcon ng le he ,E high d whi g uctivity t ,con le E he d high whi g uctivity le t ,con he d whi high le uctivity t he con high d tle he incre uctivity high con d tincre he uctivity high con d as incre high uctivity con ed d as con incre uctivity ed d th as uctivity con incre d e ed th uctivity as incre e d ed th uctivity as incre e ed th as incre e ed th incre ase ed as incre th ed e as th ed e as th ed e th e th e electrochromic response time. electroch electroch electroch rom rom irom c response ic response ic response electroch time. ti electroch me. ti me. rom rom ic response ic response time. time. electroch electroch romic response rom electroch ic response rom time. ic response ti me. time. electroch electroch electroch rom rom ic response rom ic response ic response time. time. time. electroch electroch rom electroch rom electroch ic response ic response rom rom ic response tii electroch me. c response time. electroch ti rom me. ti rom me. i c response ic response time. time. electroch electroch rom electroch rom ic response electroch ic response electroch electroch rom rom ti ic response me. rom ti ic response rom me. ic response ic response time. time. time. ti me. The transparency of inorganic EC with high staining efficiency varies in response to Th Th e Th transpa e transpa e transpa rency rency reof ncy of ino of ino Th rg ino rg an e Th an transpa rg ic e an ic EC transpa EC ic with EC re with ncy with re hig ncy h of h ig h ino st h ig of ai st h rg ino n ai st ing an n ai rg ing ic n an e ing fficienc EC e ic fficienc e EC with fficienc with y h var y ig var y h h ies var ig st ies h ai in ies st n in re ing ai in s re n po ing s re epo fficienc ns spo ns e e fficienc te ns o te o y tvar o y ivar es in ies re in spo res ns po e ns toe to The transpa Th Th e e Th transpa transpa e re transpa ncy The re re of transpa ncy ncy ino rency of rg of ino re an ino of ncy ic rg ino rg EC an an of rg ic with ic ino an EC EC ic rg with h EC an with igic h with h st EC h ig ai ig h n with h h ing st ig st ai h ai n e st h n ing fficienc ig ai ing h ne ing st e fficienc fficienc aiy e nfficienc ing vary ie es y var fficienc var in y ies var re ies s in y i po in es var re ns re in se po s ies re po to ns sin ns po e e re t ns o ts o e po to ns e to The transparency of inorganic EC with high staining efficiency varies in response to ThTh e transpa e transpa Thre e Th transpa ncy re e Th ncy transpa of e Th transpa of ino e Th retranspa ino ncy rg e re transpa Th an rg ncy re of an ic e Th ncy ino transpa re of ic EC e ncy EC transpa re ino rg of with ncy an ino rg of with re ic an ino rg of h ncy EC re ig ic an h ino rg ncy h ig EC with of ic an st h rg EC ai ino of st with ic an n ai h EC ino ing rg ic with ig n ing an EC h h rg with e ig st ic fficienc an h h e with ai ig EC fficienc ic st n h h ai ing ig EC st with n h h y ai ing ig e with st var n y fficienc h ai ing h var e st n ig ifficienc es ai ing h h e in ig es fficienc in st ing y h e ai in re fficienc var st n y e s re ai ing fficienc po var in s es y po ing ns var e iin es y fficienc ns e var e re t iin e y es o fficienc s tvar po o i re in es s y ns i re po in es var e s y ns po re in tvar o ie s es ns re po ti o s in e es ns po to re in e ns s tre o po e s tns o po e ns to e to ThTh e transpa e transpa rency rency of Th of ino e ino rg transpa an rgic anEC ic reEC ncy with with of h ino ig h h ig rg st h an ai st ic n ai ing EC ning ewith fficienc efficienc hig y h var y stai var ies ning ies in in re efficienc sre po spo nse ns y to e var toi es in response to 2 −1 2 2 −1 2−1 −1 2 −1 2 −1 2 −1 2 −1 2 −1 the low-voltage signal. Wth O3e and low - N vo iO lta (ge Table sign 5) al. have WO 3 a and staini Nn iO g e (Table fficienc 5) y h of ave ~40 a sm staini ∙Cng , efficienc 2 y 2 −1 of −1 2~40 −1 sm ∙C , thth e low e thlow e -low vo -vo lta -th vo lta ge e lta ge low sig ge sn ig -al. vo sn th ig al. lta e n W al. low W ge Oth 3O W s - and e 3 ig vo O low and n 3lta al. and N - ge iO vo N W iO s N lta O (ig Table iO 3ge ( n Table and al. (s Table ig W 5) N nO 5) al. iO h3ave 5) h and W (ave Table h O a ave 3 N sta a and iO sta 5) a ini sta ( ini h N Table nave g ini iO ne g n fficienc (a e Table g 5) fficienc sta efficienc hini ave y 5) n of y g a h of e ave y sta ~40 fficienc of ~40 ini a sm ~40 n sta sm g y ∙C ini sm efficienc of ∙C n , ∙C g ~40 , efficienc , sm y of ∙C ~40 y , of sm ~40 ∙Csm , ∙C , the low th-e vo th low lta e low ge -vos -lta vo ign ge lta al. ge sig W n sO ig al. 3n and al. WO W N 3 O and iO 3 and ( Table NiO NiO (Table 5) (h Table ave 5) a h 5) ave sta hini ave a n sta g a e ini sta fficienc n ini g n eg fficienc y efficienc of ~40 y of sm y ~40 of∙C ~40 sm , sm ∙C ∙C , , the low-voltage signal. WO3 and NiO (Table 5) have a staining efficienc 2 y 2−1 of −1 ~40 sm ∙C , 2 2−1 −1 2 −1 2 −1 2 −1 2 2 −1 2−1 2−1 2 −1 −1 the thlow e low -vo -vo ltalta ge ge sig sn ig al. nal. WW O3O and 3 and N iO NiO (Table (Table 5) 5) have have a sta a sta iniini ng ne g fficienc efficienc y of y of ~40 ~40 sm sm ∙C∙C , , the th low e th low -e vo th low - lta e vo th low ge -e lta th vo low e th s ge -lta vo ig low e th n ge - s lta vo low ig al. e -ge s n vo lta low ig W al. -vo lta ge n sO ig al. W -lta ge vo 3s n O and ig al. W ge lta s 3n ig O and al. ge W s n N 3 ig al. O and W iO s n ig 3 N al. O W and n iO ( Table 3 N al. W O and iO ( 3 O Table N W and 3iO ( O Table N and 5) 3 iO ( N h and Table 5) ave iO N (Table h 5) iO ave ( N Table a h 5) iO (ave sta Table a h 5) (ini ave Table sta a 5) hn ave ini sta 5) g h a ave n e ini sta h 5) fficienc g a ave n sta ini e h a g fficienc ave sta n a ini eg fficienc sta ini y n a eg fficienc of ini sta n y e g fficienc ~40 n ini of e y g fficienc n ~40 of esm g y fficienc ~40 e of y sm fficienc ∙C ~40 of y sm ∙C , of y ~40 sm of ∙C ~40 , y sm ~40 of ∙C , sm ~40 ∙C , sm ∙C , sm ∙C , ∙C , , the th low e low -vo -vo ltalta ge ge sigs n ig al. nal. WO W 3 O and 3 and N iO NiO (Table (Table 5) 5) have have a sta a sta iniini ng neg fficienc efficienc y of y of ~40 ~40 smsm ∙C ∙C , , 2 −1 2 2 −1 2−1 −1 2 −1 2 −1 Nanomaterials 2021, 11, 2376 2 −1 2 −1 2 −1 13 of 32 while while while for for org for org ani org ani c ani EC c EC c fEC ilm filwhile s f m il , s m s, uch s s, uch for such as as org PE as PE DO ani PE DO c T, DO EC T, thT, is th fil th is v m alue is s v, alue v such alue is is mo as is mo re PE mo re th DO re an th T, th an 100 an th 100 is sm 100 v sm alue ∙C sm ∙C ∙C [is 32,39] [mo 32,39] [32,39] re . 2th . an 2 −1 . −1 2100 −1 sm ∙C [32,39]. while for while organi for cwhile org EC ani fil while for m c s EC , org s for uch fani ilm org c as s , EC ani s PE uch f DO cil EC m as T, s , PE fs th iluch m DO iss , vT, as s alue uch th PE is is DO as v mo alue PE T, re DO th is th is T, an mo value th 100 re is th v is sm alue an mo ∙C 100 re is mo th [ sm 32,39] an re ∙C 100 th . an [32,39] sm 100 ∙C. sm [32,39] ∙C [32,39] . . while while forwhile org for ani for org c EC org anifani c il m EC cs , EC fs il uch m fil sm , as ss uch , PE such DO as PE as T, DO PE this DO T, value th T, is th v is is alue mo value re is mo th isan mo re 100 th rean th sm an 100 ∙C 100 sm [32,39] sm ∙C ∙C [. 32,39] [32,39] . . while for organic EC films, such as PEDOT, this value is more 2th2 an −1 −1 100 2sm−1 2∙C−1 [32,39]. 2 2 −1 −1 2 2 −1 −1 2 −1 2 2 −1 2−1 −1 while while for for org org ani ani c EC c EC film fils m , s s, uch such as as PE PE DO DO T, T, this th is value value is is mo mo re re th an than 100 100 sm sm ∙C∙C [32,39] [32,39] . . while while while for while for org while for org ani while for org while ani c for EC org while ani c for EC org fani c for il m org EC f ani c for il s org , m EC ani fc s il s org uch ani EC m , c fs il s EC uch ani c , m as f s il EC s uch c m , fPE as il s EC s m uch f, il DO PE as s s m uch , f il DO s s PE as T, m , uch s s DO as PE th uch T, , s is as DO PE uch th T, v as PE is DO alue th T, PE as v DO is alue th T, DO PE v is is T, alue th DO mo v T, is is th alue mo is v re th T, is alue is v th mo th re is alue v an is mo alue th re is v an 100 is mo alue th re an is mo 100 th re sm mo is an 100 re th sm ∙C mo re an th 100 sm an ∙C th re 100 [32,39] sm an ∙C th 100 [sm 32,39] an 100 ∙C [ sm . 32,39] 100 ∙C sm [. 32,39] ∙C sm [∙C 32,39] . [32,39] ∙C . [32,39] . [32,39] . . . while while for for org org ani ani c EC c EC film filsm , s such , such as as PE PE DO DO T, T, this th v isalue value is mo is mo re re th an than 100 100 smsm ∙C ∙C [32,39] [32,39] . . Act Act ually, TM Act ually, TM Act ually, TM ually, TM O have O have O have O have a h a igh h a Act igh h physic a igh h Act ually, TM physic igh physic ually, TM physic al al an al an O have d chem al an d chem O have an d chem d chem ic a al st h icigh a al st ich abi al st ic physic igh abi al st lity. abi physic lity. abi lity. al lity. an al d chem and chem ical st ical st abilabi ity.l ity. Actually, TM Actually, TM O have a O have high physic a high al physic and chem al an ical st d chem abilic ity. al st ability. Actually, TM Act Act ually, TM ually, TM O have O have a O have higha physic ha igh high physic al physic and chem al an al d chem an icd chem al stabi ical st l ic ity. al st abi abi lity. lity. Actually, TMO have a high physical and chemical stability. Act Act ually, TM ually, TM O have O have a h a igh high physic physic al al anan d chem d chem ical st ical st abi abi lity. lity. Act Act ually, TM Act ually, TM Act ually, TM Act ually, TM O have Act ually, TM O have Act ually, TM O have Act ually, TM a O have Act h ually, TM a igh Act O have h ually, TM a igh physic O have ually, TM high a physic O have ha igh physic O have h al O have a igh physic an al hO have a igh physic d chem an al h a igh d chem physic an h a al igh physic h d chem an a al igh ic physic h d chem an al st igh al ic physic d chem al st an al physic ic abi d chem al st al an abi ic lity. al an d chem al st abi lic ity. an d chem al al st l abi ic d chem an ity. al st abi ic d chem lity. al st ic abi lity. al st icabi lal st ity. ic abi lal st ity. abi lity. abi lity. lity. Table 5. Color variation in WO3, NiO and WO3/NiO electrochromic films (colored and bleached Table Table Table 5. 5. Co Co 5. lo r Co lo variati rlo variati r variati onon in Table on in WO Table in WO 35. ,WO Ni 3 ,Co Ni 5. O 3,lo O Ni Co and r and O variati lo r WO and variati WO 3on /NiO WO 3 /NiO in on 3/NiO WO e in le ecle WO trochrom 3,e c le Ni trochrom c 3,O trochrom Ni and O ic and ic fi WO l m fi ic lWO s 3 m fi /NiO (co l s m 3 (co /NiO l s ored (co e lored lec lored trochrom e an lean d ctrochrom ble d anble d ached ic ble a ched fia ic lm ched fi s l(co m s lored (colored and an ble d able ched ached Table Table 5. Colo 5. rCo Table variati lor variati on 5. Co in on lo WO r in variati 3, WO NiO on 3, and Ni in O WO WO and 3 3, /NiO WO NiO 3 /NiO e and lectrochrom WO elec3trochrom /NiO ic e file lm c ic trochrom s fi (co lm ls ored (co icl an ored fild mble s an (co a d ched lble ored a ched and bleached Table Table Table 5. Co 5. lo Co 5. r lo Co variati r lo variati r variati on on in WO on in WO in 3, Ni WO 3, O Ni 3,and O Ni and O WO and WO 3/NiO WO 3/NiO 3e/NiO lec etrochrom lec etrochrom lectrochrom ic fi ic lm fi ic s lm (co fi s lm l(co ored s l(co ored an lored d an ble d an a ble ched d a ble ched ached Table Table 5. 5. Co Co loTable rlo variati r variati 5.on Co on in lo r in WO variati WO 3, Ni 3, on Ni O O in and and WO WO 3WO , 3 Ni /NiO 3O /NiO and elee cWO le trochrom ctrochrom 3/NiO ic e le ic fic l m trochrom fils m (co s (co lored lic ored fian lm d an s ble (co d ble a lored ched ached an d bleached Table Table Table 5. Co 5. Co lo Table 5.r lo Co variati Table r lo variati Table 5. r Table variati Co on 5.Table lo on Co in 5. rTable lo on WO in variati Co 5.r 5. Co WO lo in variati 3 ,rCo lo 5. WO Ni variati on 3r ,lo Co O Ni variati on r in 3 ,lo and O variati Ni WO on r in and O variati on WO WO in 3and , on WO Ni in WO 3 3/NiO , on O in WO WO Ni 3 3/NiO ,and WO in O Ni 3 3 e ,/NiO and WO le O Ni 3WO ,e c trochrom le Ni and O 3WO c , e 3 O and trochrom /NiO Ni le WO c and 3 O trochrom /NiO WO and 3 e ic /NiO WO le 3c fi /NiO ic e trochrom WO l le m 3/NiO fi c e ic s l trochrom le m 3 (co /NiO fi e cs le trochrom lm (co e lcored le trochrom s icc le (co ored trochrom fi le ic lc m an l ored trochrom fi ic s d lan m (co fi ic ble s d l an m lfi ic (co ored ble al s d ched m fi l ic (co a ored ble ls m ched fi (co an ls a ored lm ched d (co lan ored s ble l(co d ored an a ble lched d an ored a ble d an ched ble d aan ched ble a d ched a ble ched a ched Table 5. Color variation in WO , NiO and WO /NiO electrochromic films (colored and bleached states). 3 3 states). states). states states st ).ates ). ). states). states)st . ates). states).st ates states ). ). states). states states ). ). states states ).st ates ).st ates ).st ates ) st . ates st ).ates ) st . ates ). ). states states ). ). Inorganic EC Inorganic EC Inorg Inorg Inorg anan ic E an ic E C Inorg ic E C C an ic E Inorg Inorg C an Inorg an ic E ic E an C ic E C C Inorgan Inorg ic E Inorg an C ic E an ic E C C Inorganic EC Inorg Inorg anan ic E ic E C C Inorg Inorg an Inorg ic E an Inorg ic E an Inorg C Inorg ic E an CInorg ic E an CInorg an ic E Can ic E C ic E an C ic E C C Inorg Inorg anan ic E ic E C C State WO NiO NiO/WO State WO3 NiO NiO/WO3 Stat Stat e Stat e e State WO Stat WO Stat 3WO 3e 3 Stat e 3 eWO 3 WO WO NiO 3 NiO WO 3 NiO 3 NiO NiO NiO/WO NiO NiO/WO NiO/WO NiO 3 3 NiO/WO 3 NiO/WO 3 NiO/WO NiO/WO 3 3 3 3 State Stat Stat e e WO3 WO WO 3 3 NiO NiO NiO NiO/WO NiO/WO NiO/WO 3 3 3 State WO3 NiO NiO/WO3 Stat Stat e e WO WO 3 3 NiO NiO NiO/WO NiO/WO 3 3 Stat Stat e Stat e Stat e Stat eStat e Stat eStat WO e WO e 3 WO 3 WO 3 WO 3 WO 3WO 3 WO 3 3 NiO NiO NiO NiO NiO NiO NiO NiO NiO/WO NiO/WO NiO/WO NiO/WO 3 NiO/WO 3 NiO/WO 3 NiO/WO 3 NiO/WO 3 3 3 3 Stat Stat e e WO WO 3 3 NiO NiO NiO/WO NiO/WO 3 3 Oxidi Oxidi Oxidi zeze d ze d d Oxidized Oxidized Oxidi zedOxidi ze Oxidi d zed Oxidized Oxidize Oxidi dOxidi zed ze d Oxidized Oxidi Oxidi zeze d d Oxidi Oxidi ze Oxidi dze Oxidi d ze Oxidi d Oxidi ze Oxidi d ze Oxidi d ze d ze d ze d Oxidi Oxidi zed ze d Reduction Red Red uction Red uction uction RedRed uction uction Reduction Red uction Red Red uction Red uction Red uction uction Red Red uction uction Reduction Red Red uction Red uction Red uction Red uction Red uction Red uction Red uction Red uction Red uction uction TMO belong to type III materials, according to I. F. Chang’s classification. Both anodic (A) and cathodic (C) reactions are possible, depending on the redox state of the electrochromic film. Table 6 describes the electrochemical anodic and cathodic reactions of certain oxides. Table 6. Electrochemical reactions of certain oxides. Metal Oxide Electrochemical Reaction Color Change Reaction Type Yellow $ Manganese oxide (II) MnO + ze + zH , MnO OH A ( ) (2z) brown Green $ Cobalt oxide (II) 3CoO + 2OH , Co O + H O + 2e A 3 4 2 light blue h i Colorless $ II III + Nickel oxide (II) A NiO H , Ni (1 z)Ni O H + zH + ze x y z x (yz) brown + 1 VI V Molybdenum oxide (VI) Colorless $ blue C MoO + x Li + e , Li Mo Mo O 3 x x 3 (1x) Blue $ brown (A) Li V O , V O + x Li + e (A) x 2 5 2 5 Vanadium oxide (V) C/A V O + x M + e , M V O (C) Yellow $ light blue (C) 2 5 x 2 5 Cerium oxide (IV) CeO + x Li + e , Li CeO Yellow $ transparent C 2 x 2 Niobium oxide (V) Nb O + x Li + e , Li Nb O Colorless $ light blue C 2 5 x 2 5 Ruthenium oxide (IV) RuO  2H O + H O + e , 0, 5(Ru O  5H O) + OH Blue $ brown/yellow C 2 2 2 2 3 2 + III I Indium oxide (ITO) In O + 2x Li + e , Li In In O Colorless $ light blue C 2 3 2x x 3 (1x) Iridium oxide (III) Ir(OH) , IrO  H O + H + e Colorless $ blue/grey C 2 2 V + VI V Tungsten oxide (VI) W O + x Li + e , Li W W O Colorless $ blue/black C 3 x 3 (1x) V + VI V W O + x H + e , H W W O 3 x 3 (1x) 3.4. WO Electrochromic Films Tungsten (VI) oxide (WO ) is the most universal EC, and its electrochromic properties were first described by S. K. Deb in 1969 [17]. This oxide is still widely investigated [32,40]. High functionality, high staining efficiency, high contrast, high chemical stability, and long life cycle are all features that make tungsten (VI) oxide useful in practice [41,43]. WO electrochromic films exhibit a deep blue color, preserve their color for some hours after the voltage is removed (electrochromic memory), and demonstrate high cyclic stability in comparison to other TMO [32]. The electrochromic mechanism of WO film is shown in Figure 16. Nanomaterials 2021, 11, x FOR PEER REVIEW 14 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 14 of 32 Nanomaterials 2021, 11, 2376 14 of 32 Figure 16. Electrochromic mechanism of WO3 film. Figure 16. Electrochromic mechanism of WO film. Figure 16. Electrochromic mechanism of WO3 film. WO3 films have different colors depending on x. At low values of x, the film is colored WO films have different colors depending on x. At low values of x, the film is colored blue, and WOat 3 films high hva avlu e different es of x, it c has olors eithe depen r a red ding or on gox. lden At lo tint w . val Thu es es e of phx enom , the fi ena lm ar is e col asso- ored blue, and at high values of x, it has either a red or golden tint. These phenomena are cia blue, ted and withat the high fact vath luat, es of firs x, tly it , has WO e3ithe is parti r a red ally or red goluced den tint to . th Th e es oxida e ph ti enom on stena ate V+ are , asso- and associated with the fact that, firstly + , WO is partially reduced to the oxidation state V+, and secondl ciated y, with the t he add fact ition thof at, th firs e Li tly, c WO ation 3 is occurs partially ; all red this uced leadto s to th ch e o ange xida s tiin on th st e ate band V+,g an ap d secondly, the addition of the Li cation occurs; all this leads to changes in the band gap and, a secondl s a y, con ths eequ add en ition ce, in t of he l the ig Li ht tran cation smitt occurs ance of t ; all he TMO this lead.s to changes in the band gap and, as a consequence, in the light transmittance of the TMO. At the same time, the molecular reaction in WO3 films can be described as follows and, as a consequence, in the light transmittance of the TMO. At the same time, the molecular reaction in WO films can be described as follows [68]: [68]: At the same time, the molecular reaction in WO3 films can be described as follows [68]: (5)(5) (5) In [69], it was shown that the electrochemical reaction at the WO3/electrolyte interface In [69], it was shown that the electrochemical reaction at the WO /electrolyte interface plays In an [69] imp , it era wtive as shown role i n thth at e th electroc e electroch hrom em ic ical perfo reaction rmance at th of e W WO O3 3/elect electrodes rolyte , and interth face e plays an imperative role in the electrochromic performance of WO electrodes, and the lipla thium ys an -ion imp trans erative format role ion in mec the hanis electroc m hrom at thic e perfo WO3/elect rmance rolyte of W interf O3 electrodes ace was , de and moth n-e lithium-ion transformation mechanism at the WO /electrolyte interface was demonstrated, strated, lithium wher -ion ein the transformat states ioar n e mec replhanis aced f m rom at one pha the WO se to anoth 3/electrolyte er. interface was demon- wherein the states are replaced from one phase to another. The high efficiency of amorphous WO3 films [40,70] manifests in a reversible switch strated, wherein the states are replaced from one phase to another. The high efficiency of amorphous WO films [40,70] manifests in a reversible switch from tran The spar high ent effic to iency dark b of lue amor duri ph ng ous elect WO rochem 3 films ical [40,7 redox 0] m re anifests actions in (Fig a r ure eversib 16). le Electro- switch from transparent to dark blue during electrochemical redox reactions (Figure 16). Elec- chrom from tran ic pro spar perties, ent to s dark uch as blue stai d nuri ing n g efel ficiency, ectrochem and ical switch redox ing re tim actio e ns are (Fig depen ure d 16) ent . Electro- on the trochromic properties, such as staining efficiency, and switching time are dependent on atom chrom ic ic struct prop ure, erties, nanop such art as icl st e ai size, ning po efficiency, re size and and ab switch sorpting ion tim prop e are erties depen [71,7 dent 2]. on O. th F. e the atomic structure, nanoparticle size, pore size and absorption properties [71,72]. O. F. Schirmer atomic struct suggested ure, nanop that t art he icl op e tica size, l absorpt pore siz ion e and phenom absorpt enon ion in pr WO op3erties films [71,7 was2] due . O.to F. Schirmer suggested that the optical absorption phenomenon in WO films was due to small (V) (V) (VI) (VI) small polaron (SP, charged and polarized quasiparticles) transitions from W ions to W Schirmer suggested that the optical absorption phenomenon in WO3 films was due to polaron (SP, charged and polarized quasiparticles) transitions from W ions to W ones. (V) (VI) ones. small In po [42,69,73] laron (SP, , th che ar lig geh d t and absorpt polariz ion ed mec quas hanism ipart icl in eamorph s) transious tions WO from 3 was W is ion describe s to Wd In [42,69,73], the light absorption mechanism in amorphous WO was is described as the (V) (V) as ones. the interv In [42,69,73] al optic, ally the ind ligh uced t absorpt transion fer of mec 5d1 hanism -electro in n amorph of the W ous i on WO (A 3 ) was to th is e describe adjacentd interval optically induced transfer of 5d1-electron of the W ion (A) to the adjacent empty (VI) (VI) (V) emp as th ty 5d e interv 0-oral bital op o tic f ally the ind W uced ion ( tr B ans ): fer of 5d1-electron of the W ion (A) to the adjacent 5d0-orbital of the W ion (B): (VI) empty 5d0-orbital of the W ion (B): ( V) ( VI) ( VI) ( V) hν (V) (VI) (VI) (V) (6) W A + W B ⎯⎯→ W A + W B ( ) ( ) ( ) ( ) W (A) + W (B) ! W (A) + W (B) (6) ( V) ( VI) ( VI) ( V) hν (6) W A + W B ⎯⎯→ W A + W B ( ) ( ) ( ) ( ) where А and В represent tungsten sublattice knots. where A and B represent tungsten sublattice knots. where А This a ph nd enom В repres enonent was tungsten studied sublatt using ice X-ray knotph s. otoelectron spectroscopy (XPS) and This phenomenon was studied using X-ray photoelectron spectroscopy (XPS) and electron Th is spin phresonanc enomenon e (E was SR) st spect udied rosc usi op ny g [74 X-ray ,75]. ph The otoele WO ct 3 ron films spect showed roscop high y (XP absorp- S) and electron spin resonance (ESR) spectroscopy [74,75]. The WO films showed high absorption tion in the near-infrared region due to polaron absorption [76]. Activated WO3 films are electron spin resonance (ESR) spectroscopy [74,75]. The WO3 films showed high absorp- in the near-infrared region due to polaron absorption [76]. Activated WO films are char tion acteri in thze e d ne by ar-a infrar wided e ab re sorpt gion ion due ban to d pola with ron a max absor imum ption of [76] 0.9 . – Ac 1.46 tivated eV, depe WOnding 3 films on are characterized by a wide absorption band with a maximum of 0.9–1.46 eV, depending on the th char e film acteri pro ze pd erties by a [wid 73]. e Figure absorpt 17 ion show ban s d thwith e op tic a max al tra imum nsmisof sion 0.9s –pec 1.46 tra eV, of depe WO3nding (Figuron e film properties [73]. Figure 17 shows the optical transmission spectra of WO (Figure 17a) 17a) the and film WO prop 3/G erties O (F[igure 73]. Figure 17b) fi17 lms show upos n color the op ing an tical td ble ransm achi issn ion g. spectra of WO3 (Figure and WO /GO (Figure 17b) films upon coloring and bleaching. 17a) and WO3/GO (Figure 17b) films upon coloring and bleaching. Nanomaterials 2021, 11, 2376 15 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 15 of 32 0.0 V −1.0 V 0.0 V −1.1 V −1.0 V −1.2 V −1.1 V 60 −1.3 V −1.2 V −1.4 V −1.3 V −1.5 V −1.4 V −1.6 V −1.5 V −1.7 V −1.6 V −1.8 V −1.7 V −1.9 V −1.8 V −2.0 V −1.9 V −2.1 V −2.0 V −2.2 V −2.1 V −2.3 V −2.2 V −2.3 V 400 600 800 1000 400 600 800 1000 , nm , nm (a) (b) Figure 17. Optical transmission spectra of WO3 film during electrochromic response obtain by electrochemistry (cathodic) Figure 17. Optical transmission spectra of WO film during electrochromic response obtain by electrochemistry (cathodic) deposition: (a) WO3 at constant potential; (b) WO3/GO deposition at AC potential [77]. deposition: (a) WO at constant potential; (b) WO /GO deposition at AC potential [77]. 3 3 ThThe e opoptical tical pro pr pert operties ies of of WO WO 3 thin thin film films s depen depend d on on their st theirrstr ucture uctur (crys e (crystalline, talline, popoly- ly- crysta crystalline, lline, amamorphous orphous or hybrid) or hybrid). . ColoColor red and ed color and les colorless s states states of WOof 3 fil WO ms are films not ar sym- e not met symmetric. ric. Switching Switching from fr trom ansparent transpar to ent colored to color states ed states, , poly polycrystalline crystalline WO WO 3 films films exhibit exhibit reflect reflective ive pr pr op operties, erties, and and amorphous amorphousWO WO3 films films exhibit exhibit absorption absorption properties. propert The ies. switch- The switching ing time tim depends e depen on dWO s on WO film 3 density film density and on and electr on olyte electrol concentration. yte concentration. Low-density Low-den films - with high-concentration electrolytes demonstrate the fastest switching speed [78]. sity films with high-concentration electrolytes demonstrate the fastest switching speed Nowadays, the importance of WO films has grown [79,80] due to their use in “Smart [78]. 3 Windows”, which smartly regulate indoor solar radiation by changing their optical trans- Nowadays, the importance of WO3 films has grown [79,80] due to their use in “Smart mittance, contributing to a significant reduction in a building’s energy consumption (as a Windows”, which smartly regulate indoor solar radiation by changing their optical trans- result of the optimization of air conditioning consumption) and helping to create comfort- mittance, contributing to a significant reduction in a building’s energy consumption (as a able indoor environments [81]. However, despite all the advantages of WO films, their life result of the optimization of air conditioning consumption) and helping to crea 3 te comfort- cycle is not very long: continuous switching between colored and colorless states causes able indoor environments [81]. However, despite all the advantages of WO3 films, their irreversible structural changes that affect their optical and electrical properties, ultimately life cycle is not very long: continuous switching between colored and colorless states leading to material degradation, the so-called “aging” effect [82]. Therefore, the task of causes irreversible structural changes that affect their optical and electrical properties, ul- increasing the life cycle of WO films involves the development of new nanomaterials timately leading to material degradation, the so-called “aging” effect [82]. Therefore, the and/or the improvement of existing materials through the use of modificatory additives, task of increasing the life cycle of WO3 films involves the development of new nanomateri- as well as the obtained improvement of WO film technologies [83–86]. als and/or the improvement of existing materials through the use of modificatory addi- tives, as well as the obtained improvement of WO3 film technologies [83–86]. 4. ECD (Electrochromic Device) Structure EC are able to reversibly change their optical properties through the application of 4. ECD (Electrochromic Device) Structure an electrical voltage, making them suitable for ECD, such as displays [30], electrochromic Nanomaterials 2021, 11, x FOR PEER REVIEW 16 of 32 EC are able to reversibly change their optical properties through the application of “Smart Windows” [16], anti-glare rear mirrors [19], and sensors [87]. an electrical voltage, making them suitable for ECD, such as displays [30], electrochromic ECD structures usually include transparent conductors, electrochromic layers, and “Smart Windows” [16], anti-glare rear mirrors [19], and sensors [87]. ion conductors (Figure 18). ECD structures usually include transparent conductors, electrochromic layers, and ion conductors (Figure 18). Substrate In O (ITO) 2 3 Ion conductor (electrolyte) U EC In O (ITO) 2 3 Substrate Figure 18. ECD structure. Figure 18. ECD structure. ECD for architectural applications include thin EC films placed between two glass panels, as shown in Figures 19 and 20. ECD for architectural applications include thin EC films placed between two glass panels, as shown in Figures 19 and 20. Figure 19. ECW scheme showing voltage-induced transfer of positive ions and electrons to trans- parent conductive layers. Cycle stability is an extremely important aspect in the performance of electrochromic devices. In a recent study [88], ECD were reported to have obtained a superior long-term cycling stability of over 10,000 cycles. This manuscript is recommended for its review of some reports of devices with high long-term stability. In [89], a strategy was presented involving an all-in-one self-healing electrochromic material, TAFPy-MA, which was used for the fabrication of a high-reliability, large-scale and easy-to-assemble smart electro- chromic window. The all-in-one self-healing electrochromic material was able to carry out in situ redox reactions with the Li ions. The Diels-Alder cross-linking network structure was able to heal the cracks, improving the reliability of the electrochromic layer. Great ion 2 −1 diffusivity (1.13 × 10–5 cm s ), rapid color switching (3.9/3.7 s), high coloration efficiency 2 −1 (413 cm C ), excellent stability (sustain 88.7% after 1000 cycles) and reliability (crack can be healed in 110 s), large-scale “Smart Windows” (30 × 35 cm ) were achieved using this all-in-one electrochromic material, and these exhibited fascinating and promising features for a wide range of applications in buildings, airplanes, etc. Figure 20. ECW color cycle (colored ↔ semitransparent ↔ transparent state). T, % T, % Nanomaterials 2021, 11, x FOR PEER REVIEW 16 of 32 Substrate In O (ITO) 2 3 Ion conductor (electrolyte) EC In O (ITO) 2 3 Substrate Figure 18. ECD structure. Nanomaterials 2021, 11, x FOR PEER REVIEW 16 of 32 ECD for architectural applications include thin EC films placed between two glass panels, as shown in Figures 19 and 20. Substrate In O (ITO) 2 3 Ion conductor (electrolyte) EC In O (ITO) 2 3 Substrate Figure 18. ECD structure. Nanomaterials 2021, 11, 2376 16 of 32 ECD for architectural applications include thin EC films placed between two glass panels, as shown in Figures 19 and 20. Figure 19. ECW scheme showing voltage-induced transfer of positive ions and electrons to trans- parent conductive layers. Cycle stability is an extremely important aspect in the performance of electrochromic devices. In a recent study [88], ECD were reported to have obtained a superior long-term cycling stability of over 10,000 cycles. This manuscript is recommended for its review of some reports of devices with high long-term stability. In [89], a strategy was presented involving an all-in-one self-healing electrochromic material, TAFPy-MA, which was used for the fabrication of a high-reliability, large-scale and easy-to-assemble smart electro- chromic window. The all-in-one self-healing electrochromic material was able to carry out in situ redox reactions with the Li ions. The Diels-Alder cross-linking network structure was able to heal the cracks, improving the reliability of the electrochromic layer. Great ion 2 −1 diffusivity (1.13 × 10–5 cm s ), rapid color switching (3.9/3.7 s), high coloration efficiency 2 −1 (413 cm C ), excellent stability (sustain 88.7% after 1000 cycles) and reliability (crack can be healed in 110 s), large-scale “Smart Windows” (30 × 35 cm ) were achieved using this all-in-one electrochromic material, and these exhibited fascinating and promising features Figure 19. ECW scheme showing voltage-induced transfer of positive ions and electrons to trans- Figure 19. ECW scheme showing voltage-induced transfer of positive ions and electrons to transpar- for a wide range of applications in buildings, airplanes, etc. parent conductive layers. ent conductive layers. Cycle stability is an extremely important aspect in the performance of electrochromic devices. In a recent study [88], ECD were reported to have obtained a superior long-term cycling stability of over 10,000 cycles. This manuscript is recommended for its review of some reports of devices with high long-term stability. In [89], a strategy was presented involving an all-in-one self-healing electrochromic material, TAFPy-MA, which was used for the fabrication of a high-reliability, large-scale and easy-to-assemble smart electro- chromic window. The all-in-one self-healing electrochromic material was able to carry out Figure Figure20. 20. EC ECW W co color lor ccycle ycle ((color colored ed ↔ $ semitranspar semitransparent ent $↔transpar transparent s ent state). tate). in situ redox reactions with the Li ions. The Diels-Alder cross-linking network structure was able to heal the cracks, improving the reliability of the electrochromic layer. Great ion Cycle stability is an extremely important aspect in the performance of electrochromic 2 −1 diffusivity (1.13 × 10–5 cm s ), rapid color switching (3.9/3.7 s), high coloration efficiency devices. In a recent study [88], ECD were reported to have obtained a superior long- 2 −1 (413 cm C ), excellent stability (sustain 88.7% after 1000 cycles) and reliability (crack can term cycling stability of over 10,000 cycles. This manuscript is recommended for its review of some reports of devices with high long-term stability. In [89], a strategy was be healed in 110 s), large-scale “Smart Windows” (30 × 35 cm ) were achieved using this presented involving an all-in-one self-healing electrochromic material, TAFPy-MA, which all-in-one electrochromic material, and these exhibited fascinating and promising features was used for the fabrication of a high-reliability, large-scale and easy-to-assemble smart for a wide range of applications in buildings, airplanes, etc. electrochromic window. The all-in-one self-healing electrochromic material was able to carry out in situ redox reactions with the Li ions. The Diels-Alder cross-linking network structure was able to heal the cracks, improving the reliability of the electrochromic layer. 2 1 Great ion diffusivity (1.13 10–5 cm s ), rapid color switching (3.9/3.7 s), high coloration 2 1 efficiency (413 cm C ), excellent stability (sustain 88.7% after 1000 cycles) and reliability (crack can be healed in 110 s), large-scale “Smart Windows” (30  35 cm ) were achieved using this all-in-one electrochromic material, and these exhibited fascinating and promising features for a wide range of applications in buildings, airplanes, etc. Electrochromic films change their color as a result of electrochemical oxidation/reduction reaction associated with ion transfer, which involves the use of an additional coating for the storage and transport of ions. Many companies offer “Smart Window” solutions; Figure 20. ECW color cycle (colored ↔ semitransparent ↔ transparent state). energy-saving “Smart Window” technology is available on the market [4]. Depending on the purpose, ECD may contain materials with different characteristics and properties. Figure 21 presents the classifications of ECD. Nanomaterials 2021, 11, x FOR PEER REVIEW 17 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 17 of 32 Electrochromic films change their color as a result of electrochemical oxidation/re- duction reaction associated with ion transfer, which involves the use of an additional coat- ing for the storage and transport of ions. Many companies offer “Smart Window” solu- Electrochromic films change their color as a result of electrochemical oxidation/re- tions; energy-saving “Smart Window” technology is available on the market [4]. duction reaction associated with ion transfer, which involves the use of an additional coat- Depending on the purpose, ECD may contain materials with different characteristics ing for the storage and transport of ions. Many companies offer “Smart Window” solu- and properties. Figure 21 presents the classifications of ECD. tions; energy-saving “Smart Window” technology is available on the market [4]. Nanomaterials 2021, 11, 2376 17 of 32 Depending on the purpose, ECD may contain materials with different characteristics and properties. Figure 21 presents the classifications of ECD. Figure 21. ECD classification. 4.1. Substrate Figure Figure 21. 21. ECD ECD classification. classification. Transparent ECW substrates include glassy (Figure 22) or transparent polymers (Fig- 4.1. Substrate ure 22), such as polyethylene terephthalate (PET), polyvinyl butyral (PVB) and polyeth- 4.1. Substrate Transparent ECW substrates include glassy (Figure 22) or transparent polymers (Fig- ylene naphthalate (PEN). Glass substrates are more common due to their greater trans- Transparent ECW substrates include glassy (Figure 22) or transparent polymers (Fig- ure 22), such as polyethylene terephthalate (PET), polyvinyl butyral (PVB) and polyethylene parency and their chemical stability, which makes them suitable for the production of ure 22), such as polyethylene terephthalate (PET), polyvinyl butyral (PVB) and polyeth- naphthalate (PEN). Glass substrates are more common due to their greater transparency and “Smart Windows”. In turn, polymer substrates allow the production costs of ECD to be ylene naphthalate (PEN). Glass substrates are more common due to their greater trans- their chemical stability, which makes them suitable for the production of “Smart Windows”. reduced [90–93]. parency and their chemical stability, which makes them suitable for the production of In turn, polymer substrates allow the production costs of ECD to be reduced [90–93]. “Smart Windows”. In turn, polymer substrates allow the production costs of ECD to be reduced [90–93]. Light (T=100%) Dark (T=0%) ITO-Glass/0.01M H SO 2 4 Light (T=100%) Dark (T=0%) ITO-Glass/0.01M H SO 2 4 200 400 600 800 1000 1200 , nm 200 400 600 800 1000 1200 Figure Figure22. 22. Vis Visible ible and andnear near -infrar -infrared ed transmission transmissispectra on spec of tra o WO f WO -ITO-glass. 3-ITO-glass. , nm 4.2. Transparent Conductive Electrode Figure 22. Visible and near-infrared transmission spectra of WO3-ITO-glass. The electrical resistivity and the light transmission coefficient are the most important properties of transparent conductive electrodes (layers). An electrode should possess high electrical conductivity in order to form the electric field required for ECD. Transparent conductive electrodes include metal-based and oxide-based electrodes, but the electrode properties should not affect the transmission properties of the electrochromic windows. Indium-tin oxide (ITO) electrodes (indium (III) oxide and tin (IV) oxide) are among the best T, % T, % Nanomaterials 2021, 11, 2376 18 of 32 transparent electrodes to have been investigated ((In O )0.9-(SnO )0.1: 90% and 10%) [90], 2 3 2 possessing high electrical conductivity (~104 Ssm ) and low optical absorption (band gap ~4 eV, refractive index 1.9), making it preferable to fluorine-doped tin oxide (FTO). Transparent ITO electrode contains different numbers of doped Sn atoms, and consequently, free electron density varies [94]. 4.3. Electrochromic Layer EC films reversibly change their optical properties, switching between transparent, semi-transparent and colored states, modeling solar radiation and thus ensuring reliable ECW operation. EC films (layers) can be divided into three different types according to their color schemes [32]: - EC film exhibiting one color, for example, transition metal oxides, Prussian blue [31]; - EC film exhibiting two colors, for example, polythiophene [28]; - EC film exhibiting multiple colors, for example, poly (3,4-propylenedioxypyrrole) [29]. 4.4. Electrolyte (Ion Conductor) Electrolytes can be classified into liquid, gel and solid electrolytes [32]. Liquid elec- trolytes are dissolved ions. Such electrolytes provide high ionic mobility. Polymer elec- trolytes are the most suitable for EC devices, as they provide a long circuit break and uniformity of coloration [95]. Electrochromic device electrolytes are ionic materials that possess ionic conductivity. Electrochromic device electrolytes should satisfy the following requirements [77]: - compatibility with anodic and cathodic materials; - high ionic conductivity; - no electron transfer between electrochromic layers; - high transparency without scattering effect. In [96], a novel Zn–Prussian blue (PB) system was reported for aqueous electrochromic 2+ + batteries. By using different dual-ion electrolytes with various cations (e.g., Zn –K and 2+ 3+ Zn –Al ), the Zn–PB electrochromic batteries demonstrated excellent performance. We + 2+ showed that the K –Zn dual-ion electrolyte in the Zn–PB configuration endowed a rapid self-bleaching time (2.8 s), high optical contrast (83% at 632.8 nm), and fast switching times (8.4 s/3 s for the bleaching/coloration processes). Remarkably, the aqueous elec- trochromic battery exhibited a compelling energy retrieval of 35.7 mWhm , where only 47.5 mWhm was consumed during the round-trip coloration–bleaching process. These findings may open up new directions for the development of advanced net-zero-energy- consumption ECD. In [4,34,58,97], a hybrid electrolyte was developed based on aluminum trifluoromethane- sulphonate (Al(TOF)3) and H PO that could effectively alleviate the passivation, and 3 4 which exhibited superior stability. Additionally, an ex situ study revealed that the PANI cathode undergoes a process of cointercalation/deintercalation of Al(H PO )x(TOF )y 2 4 +(H O)n, TOF , and H during the charging/discharging process, with high reversibility and stability. As a proof of concept, an electrochromic Al//PANI battery was fabricated that combined both electrochromism and energy storage and delivered a higher coloration 2 1 efficiency of 84 cm C at a wavelength of 630 nm. 4.5. Counter Electrode The counter electrode provides ions, which, depending on the polarity of the applied voltage, are injected into or extracted from the electrochromic coating. The counter electrode should be transparent, with high conductivity, in order to reduce the voltage drop and Nanomaterials 2021, 11, x FOR PEER REVIEW 19 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 19 of 32 4.5. Counter Electrode The counter electrode provides ions, which, depending on the polarity of the applied Nanomaterials 2021, 11, 2376 4.5. Counter Electrode 19 of 32 voltage, are injected into or extracted from the electrochromic coating. The counter elec- The counter electrode provides ions, which, depending on the polarity of the applied trode should be transparent, with high conductivity, in order to reduce the voltage drop voltage, are injected into or extracted from the electrochromic coating. The counter elec- and prevent side reactions. Counter electrodes may include EC films, such as WO3/PANI trode should be transparent, with high conductivity, in order to reduce the voltage drop prevent side reactions. Counter electrodes may include EC films, such as WO /PANI films [98], switching from transparent to blue. and prevent side reactions. Counter electrodes may include EC films, such as WO3/PANI films [98], switching from transparent to blue. films [98], switching from transparent to blue. + − WO + PANI + xM A MxWO + PANI A ( ) 3 3 x + − (7)  WO + PANI + xM A MxWO + PANI A ()  33 x  ( tra nsparent)  (colored) (7 (7) ) (transparent) (colored) + + ‒ ‒ where x is the number of cations (М , H ) and anions (A , SO4 ). + + ‒ ‒ where x is the number of cations (М , H ) and anions (A , SO4 ). Thus, thin-film electrodes broaden + + the ECD color pa lette and strengthen the electro- where x is the number of cations (M , H ) and anions (A , SO4 ). Thus, thin-film electrodes broaden the ECD color palette and strengthen the electro- chromic effect. Thus, thin-film electrodes broaden the ECD color palette and strengthen the elec- chromic effect. trochromic effect. 5. WO3 Film Fabrication 5. WO3 Film Fabrication 5. WO Film Fabrication The EC WO3 layer is obtained as a thin film on a conductive substrate with an FTO The EC WO3 layer is obtained as a thin film on a conductive substrate with an FTO The EC WO layer is obtained as a thin film on a conductive substrate with an FTO or ITO electro or ITO electro de. There de. Th are ereseveral are severa WO3l fWO abric 3 at fabri ion t ce ation chniqtec ueshniq [99] ues (Fig ure [99]2 ( 3Fi ), in gure clud 23), ing including or ITO electrode. There are several WO fabrication techniques [99] (Figure 23), including magnetron sputtering [100], electrochemical deposition [101–106], spray pyrolysis [107], magnetron sputtering [100], electrochemical deposition [101–106], spray pyrolysis [107], magnetron sputtering [100], electrochemical deposition [101–106], spray pyrolysis [107], sol–gel [108,109], mechanical sputtering [110,111], etc. These technologies are based on sol–gel [108,109], mechanical sputtering [110,111], etc. These technologies are based on sol–gel [108,109], mechanical sputtering [110,111], etc. These technologies are based on electrochemical, chemical and physical principles. C. G. Granqvist [32] provided a com- electr electroch ochemical, emical, chemical chemic and al an physical d physi principles. cal princip C. G. les Granqvist . C. G. Granqv [32] pris ovided t [32]a pro compr vided e- a com- prehensive survey of WO3 fabrication technologies. hensive survey of WO fabrication technologies. prehensive survey of WO3 fabrication technologies. Figure 23. Classification of WO3 fabrication technologies. Table 7 shows a comparative analysis of WO3 fabrication technologies. Figure Figure 23. 23. Classification Classificatio of n o WO f Wfabrication O3 fabricat technologies. ion technologies. Table 7. Comparison of three basic approaches to films WO3 fabrication. Table 7 shows a comparative analysis of WO fabrication technologies. Table 7 shows a comparative analysis of WO3 fabrication technologies. Technology Types Scalability Equipment Cost Process Costs Coating Uniformity Table 7. Comparison of three basic approaches to films WO fabrication. Table 7. Comparison of three basic approaches to films WO3 fabrication. Electrochemical +/− + + +/− Technology Types Scalability Chemical Equipment +/− + Cost Process Costs+ Coating Uniformity − Technology Types Scalability Equipment Cost Process Costs Coating Uniformity Physical + − − + Electrochemical +/ + + +/ Electrochemical +/− + + +/− Chemical +/ + + Chemical +/− + + − The majority of technologies shown in Figure 24 are currently in use at the time of Physical + + writing. Optical contrast is Physical a ke + y parameter for − ev aluating EC dev −i ce quality. Howeve+ r, nowadays, there is no universal method that would satisfy all modern requirements. Each The majority of technologies shown in Figure 24 are currently in use at the time of method has its own advantages and shortcomings. The majority of technologies shown in Figure 24 are currently in use at the time of writing. Optical contrast is a key parameter for evaluating EC device quality. However, writing. Optical contrast is a key parameter for evaluating EC device quality. However, nowadays, there is no universal method that would satisfy all modern requirements. Each nowadays, there is no universal method that would satisfy all modern requirements. Each method has its own advantages and shortcomings. method has its own advantages and shortcomings. Nanomaterials 2021, 11, x FOR PEER REVIEW 20 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 20 of 32 Nanomaterials 2021, 11, 2376 20 of 32 Spray pyrolysis CVD Sputering Sol-gel Electrodeposition Spray pyrolysis Thermal evaporation CVD Sputering Years Sol-gel Electrodeposition Thermal evaporation Figure 24. Contrast response curves for WO3 films obtained by different processes during the re- Years porting period. WO3 film characteristics include porosity, crystallinity and crystal size; these proper- Figure 24. Contrast response curves for WO films obtained by different processes during the Figure 24. Contrast response curves for WO3 films obtained by different processes during the re- ties are highly dependent on manufacturing conditions and on production technology. reporting period. porting period. The requirements for WO3 thin films include uniformity, low production cost, and long life cycle. Unf WO ortfilm unatel characteri y, the pro stics duction include of a pun orosity iform , crystallinity WO3 film wand ith good crystal adhes size; ion these still prop- WO3 film characteristics include porosity, crystallinity and crystal size; these proper- remain erties s a pro arblem. e highly dependent on manufacturing conditions and on production technology. ties are highly dependent on manufacturing conditions and on production technology. Th The e vacuu requir m ements deposifor tion WO meth thin od makes films include it possibl uniformity e to obtain , low high pr -d oduction ensity WO cost, 3 films and long life cycle. Unfortunately, the production of a uniform WO film with good adhesion still on a Th large e requir flat sur emen face, and ts for the WO thicknes 3 thin s and films com inc po lu side tion un can iform be con ity, troll low ed pro during duction the cost, and long remains a problem. deposition process [100]. Vacuum-deposited WO3 films have an amorphous structure, and life cycle. Unfortunately, the production of a uniform WO3 film with good adhesion still The vacuum deposition method makes it possible to obtain high-density WO films annealed WO3 films have a crystalline structure. However, these technologies are highly remains a problem. on a large flat surface, and the thickness and composition can be controlled during the expensive due to the expensive equipment. Many glass manufacturing companies still The vacuum deposition method makes it possible to obtain high-density WO3 films deposition process [100]. Vacuum-deposited WO films have an amorphous structure, and prefer vacuum deposition technologies, regardless of 3 the cost, because WO3 films ob- on a large flat surface, and the thickness and composition can be controlled during the annealed WO films have a crystalline structure. However, these technologies are highly tained by vacuum d 3eposition are stable, reliable and adjustable. deposition process [100]. Vacuum-deposited WO3 films have an amorphous structure, and expensive due to the expensive equipment. Many glass manufacturing companies still Chemical vapor deposition (CVD) is used for depositing WO3 films on a substrate prefer vacuum deposition technologies, regardless of the cost, because WO films obtained annealed WO3 films have a crystalline structure. However, these technologies are highly [112]. However, during the deposition process substrates are heated to a high tempera- by vacuum deposition are stable, reliable and adjustable. ture, which can lead to structural changes in the conductive layer. Electron beam evapo- expensive due to the expensive equipment. Many glass manufacturing companies still Chemical vapor deposition (CVD) is used for depositing WO films on a substrate [112]. ration technology is a well-known method for preparing electrochromic WO3 films prefer vacuum deposition technologies, regardless of the cost, because WO3 films ob- However, during the deposition process substrates are heated to a high temperature, which [113,114]. tained by vacuum deposition are stable, reliable and adjustable. can lead to structural changes in the conductive layer. Electron beam evaporation technol- Chemical vapor deposition (CVD) is used for depositing WO3 films on a substrate ogy is a well-known method for preparing electrochromic WO films [113,114]. 5.1. Electrochemical Deposition [112]. However, during the deposition process substrates are heated to a high tempera- Electrochemical deposition (electrodeposition) is a method of low-temperature syn- 5.1. Electrochemical Deposition ture, which can lead to structural changes in the conductive layer. Electron beam evapo- thesis of WO3 films. Figures 25 and 26 show a three-electrode system in which conductive Electrochemical deposition (electrodeposition) is a method of low-temperature syn- ration technology is a well-known method for preparing electrochromic WO3 films FTO or ITO electrodes serve as a working electrode and a platinum electrode is used as a thesis of WO films. Figures 25 and 26 show a three-electrode system in which conductive [113,114]. counter electrode. FTO or ITO electrodes serve as a working electrode and a platinum electrode is used as a counter electrode. 5.1. Electrochemical Deposition Electrochemical deposition (electrodeposition) is a method of low-temperature syn- thesis of WO3 films. Figures 25 and 26 show a three-electrode system in which conductive FTO or ITO electrodes serve as a working electrode and a platinum electrode is used as a counter electrode. (a) (b) Figure 25. Two types of electrodeposition processes: (а) electroplating; (b) electrophoretic deposi- Figure 25. Two types of electrodeposition processes: (a) electroplating; (b) electrophoretic deposition. tion. (a) (b) Figure 25. Two types of electrodeposition processes: (а) electroplating; (b) electrophoretic deposi- tion. 2001-2003 2004-2006 2007-2009 2010-2012 2001-2003 2013-2015 2004-2006 2016-2018 2007-2009 2019-2020 2010-2012 2013-2015 2016-2018 2019-2020 Production, % Production, % Nanomaterials 2021, 11, x FOR PEER REVIEW 21 of 32 Nanomaterials 2021, 11, 2376 21 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 21 of 32 Figure 26. Growth mechanism of electrodeposited WO3 film. Figure 26. Growth mechanism of electrodeposited WO film. Figure 26. Growth mechanism of electrodeposited WO3 film. The applied potential is shown relative to the reference electrode. The most common The applied potential is shown relative to the reference electrode. The most common The applied potential is shown relative to the reference electrode. The most common reference electrode is the silver/silver chloride (Ag/AgCl) electrode (Figure 26), due to the reference electrode is the silver/silver chloride (Ag/AgCl) electrode (Figure 26), due to the reference electrode is the silver/silver chloride (Ag/AgCl) electrode (Figure 26), due to the stability of the electrode potential. stability of the electrode potential. stability of the electrode potential. The mechanism of electrochemical deposition of electrochromic WO3 films has been The mechanism of electrochemical deposition of electrochromic WO films has been The mechanism of electrochemical deposition of electrochromic WO3 films has been well investigated [106]; metal or precursor ions are transferred to the working electrode well investigated [106]; metal or precursor ions are transferred to the working electrode well investigated [106]; metal or precursor ions are transferred to the working electrode (cathode) under the influence of an applied electrical field. In this case, the metal deposi- (cathode) under the influence of an applied electrical field. In this case, the metal deposition (cathode) under the influence of an applied electrical field. In this case, the metal deposi- tion process can be described by the reaction: process can be described by the reaction: tion process can be described by the reaction: + − (8) M + e → M + − M + e ! M (8) M + e → M (8) As already mentioned [115,116], the electrochemical deposition method makes it pos- As already mentioned [115,116], the electrochemical deposition method makes it pos- As already mentioned [115,116], the electrochemical deposition method makes it sible to deposit WO3 films on large-area conductive substrates. However, special equip- sible to deposit WO3 films on large-area conductive substrates. However, special equip- possible to deposit WO films on large-area conductive substrates. However, special ment is required for the deposition process. The main advantages of this method include: ment is required for the deposition process. The main advantages of this method include: equipment is required for the deposition process. The main advantages of this method low cost and fast deposition, while not requiring high-temperature heating and deep vac- low include: cost and low fcost ast depo and s fast itiodeposition, n, while notwhile requiri not ng r hi equiring gh-temperature high-temperatur heating and e heating deep vac- and uum. uu deep m. vacuum. 5.2. Sol–Gel 5.2. Sol–Gel 5.2. Sol–Gel Colloidal oxide can be synthesized by polycondensation, by acidification of aqueous Colloidal oxide can be synthesized by polycondensation, by acidification of aqueous Colloidal oxide can be synthesized by polycondensation, by acidification of aqueous salt solution, or by hydrolysis of organometallic compounds (Figure 27). Recently, there salt solution, or by hydrolysis of organometallic compounds (Figure 27). Recently, there has salt solution, or by hydrolysis of organometallic compounds (Figure 27). Recently, there has been growing interest in the use of the sol–gel process to produce multilayer electro- been growing interest in the use of the sol–gel process to produce multilayer electrochromic has been growing interest in the use of the sol–gel process to produce multilayer electro- chromic coatings based on non-organic compounds. The main advantage of this reaction coatings based on non-organic compounds. The main advantage of this reaction is that chromic coatings based on non-organic compounds. The main advantage of this reaction is that liquid compounds are converted into solid compounds [117]. liquid compounds are converted into solid compounds [117]. is that liquid compounds are converted into solid compounds [117]. Figure 27. Sol–gel process scheme. Nanomaterials 2021, 11, x FOR PEER REVIEW 22 of 32 Figure 27. Sol–gel process scheme. Nanomaterials 2021, 11, 2376 22 of 32 Most alkoxides used for electrochromic materials can be produced in several stages [91]: (1) hydrolysis with the formation of reactive M-OH groups: Most alkoxides used for electrochromic materials can be produced in several stages [91]: (1) hydrolysis with the formation of reactive M-OH groups: M-OR + H2O → M-OH + ROH (9) M OR + H O ! M OH + ROH (9) (2) condensation resulting in bridge oxygen formation: (2) condensation resulting in bridge oxygen formation: M-OH + RO-M → M-O-M + ROH (10) M OH + RO M ! M M + ROH (10) M-OH + HO-M → M-O-M + H2O (11) M OH + HO M ! M M + H O (11) There are different types of sol–gel processes, such as c2entrifugation, immersion coat- ing and spraying (Figure 28). The sol–gel method, widely applied in material synthesis, is There are different types of sol–gel processes, such as centrifugation, immersion also u coating sed to and mo spraying dify the e (Figur lect erode s 28). The urfa sol–gel ce [118] method, . widely applied in material synthesis, is also used to modify the electrode surface [118]. (a) (b) (c) Figure 28. Types of sol–gel processes: (a) immersion coating; (b) centrifugation; (c) spraying. Figure 28. Types of sol–gel processes: (а) immersion coating; (b) centrifugation; (c) spraying. Sol–gel methods make it possible to produce large-area WO films at lower cost Sol–gel methods make it possible to produce large-area WO3 films at lower cost in in comparison with traditional vacuum methods [119]. The advantages of this method comparison with traditional vacuum methods [119]. The advantages of this method in- include: universality of sol–gel processes, easy control of microstructure and composition under low-temperature conditions, relatively simple and inexpensive equipment, control clude: universality of sol–gel processes, easy control of microstructure and composition of microstructure, crystal size, porosity and composition of the deposited films, which under low-temperature conditions, relatively simple and inexpensive equipment, control is important, since these characteristics affect thin film kinetics, durability and staining of microstructure, crystal size, porosity and composition of the deposited films, which is efficiency [120]. However, many problems still remain to be solved, among them solution important, since these characteristics affect thin film kinetics, durability and staining effi- stability, large-area uniformity, insufficient adhesion, insufficient film thickness, and low ciency [120]. However, many problems still remain to be solved, among them solution repeatability. stability, large-area uniformity, insufficient adhesion, insufficient film thickness, and low 5.3. Spray Pyrolysis repeatability. The main principle of spray pyrolysis is the pyrolytic decomposition of salt solution sprayed on substrate consisting of deposition target material (Figure 29). The sprayed 5.3. Spray Pyrolysis solution undergoes pyrolytic decomposition and forms a crystallite or a crystallite cluster when The ma thein dr pri op comes nciple into of spr contact ay pyro with lysis the hot is substrate the pyrol surface. ytic decomposition of salt solution sprayed on substrate consisting of deposition target material (Figure 29). The sprayed so- lution undergoes pyrolytic decomposition and forms a crystallite or a crystallite cluster when the drop comes into contact with the hot substrate surface. By-products and solvents evaporate during spraying. The hot substrate provides thermal energy for thermal decomposition. After thermal decomposition, sintering and crystallization of the crystallite clusters occur, ultimately leading to film formation. The technique is used for the deposition of dense and porous films on different substrates, such as glass, ceramics and metal. Nanomaterials 2021, 11, 2376 23 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 23 of 32 Figure 29. Pyrolytic deposition of EC films. Figure 29. Pyrolytic deposition of EC films. By-products and solvents evaporate during spraying. The hot substrate provides Spray pyrolysis is a simple and relatively inexpensive method that does not require thermal energy for thermal decomposition. After thermal decomposition, sintering and a vacuum. This method allows large-area uniform films with good adhesion to be pro- crystallization of the crystallite clusters occur, ultimately leading to film formation. The duced. Moreover, film properties can be easily modified by changing the spray parame- technique is used for the deposition of dense and porous films on different substrates, such ters, such as substrate temperature, flow pressure and the molarity of the precursor solu- as glass, ceramics and metal. tion. Th Spray e ma pyr in olysis advanta is age simple of thand is met relatively hod is inexpensive that it works method at mo that derdoes ate tem notper requir atures e a (100– vacuum. This method allows large-area uniform films with good adhesion to be produced. 500 °C) and allows films to be obtained even on low-quality substrates. It offers an easy Moreover, film properties can be easily modified by changing the spray parameters, such way of doping films with any elements in any proportion by adding them in some form as substrate temperature, flow pressure and the molarity of the precursor solution. The to the spray solution [121,122]. In [123], V2O5-WO3 composite films were reported to ex- main advantage of this method is that it works at moderate temperatures (100–500 C) 2 −1 hibit high coloration efficiency (49 cm ∙C ). Ref. [124], a fibrous reticulated WO3 film ob- and allows films to be obtained even on low-quality substrates. It offers an easy way of tained by pulsed spray pyrolysis was reported to have a coloration efficiency of 34 cm doping films with any elements in any proportion by adding them in some form to the −1 ∙C at λ = 630 nm. spray solution [121,122]. In [123], V O -WO composite films were reported to exhibit high 2 5 3 2 1 Spray pyrolysis is a cost-effective method for obtaining highly adhesive homogene- coloration efficiency (49 cm C ). Ref. [124], a fibrous reticulated WO film obtained by 2 1 ous pulsed WO3spray films pyr with olysis differe wasnt reported microst to ruc have tures. a coloration The techno efficiency logy can of 34 also cm be use C d at to pro = duce 630 nm. multilayer films, which is achieved by varying the spray composition. However, this Spray pyrolysis is a cost-effective method for obtaining highly adhesive homogeneous method also has disadvantages, such as the non-uniformity of films, large grain size due WO films with different microstructures. The technology can also be used to produce to uncontrolled sputtering, solvent loss, and low deposition rate. The mentioned ad- multilayer films, which is achieved by varying the spray composition. However, this vantages of the spray pyrolysis method make it suitable for industrial applications. method also has disadvantages, such as the non-uniformity of films, large grain size due to uncontrolled sputtering, solvent loss, and low deposition rate. The mentioned advantages 5.4. Magnetron Sputtering of the spray pyrolysis method make it suitable for industrial applications. Magnetron sputtering is a deposition technology defined as “cathodic sputtering of 5.4. Magnetron Sputtering target material in magnetron discharge plasma (crossed field discharge)”, and is shown Magnetron sputtering is a deposition technology defined as “cathodic sputtering of in Figure 30. target material in magnetron discharge plasma (crossed field discharge)”, and is shown in In this process, permanent magnets are arranged below the target plate so as to pro- Figure 30. duce a magnetic field close to the target material. This concentrates the electrons and causes them to travel in a spiral fashion along the magnetic flux lines of the target instead of wandering around the target material [100]. Nanomaterials 2021, 11, 2376 24 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 24 of 32 Figure 30. Magnetron sputtering apparatus (working principal). Figure 30. Magnetron sputtering apparatus (working principal). In this process, permanent magnets are arranged below the target plate so as to Magnetron sputtering is the most up-to-date deposition technology [99,100], and is produce a magnetic field close to the target material. This concentrates the electrons and widely used in the industrial and scientific spheres. The frequency of the applied positive causes them to travel in a spiral fashion along the magnetic flux lines of the target instead DC voltage varies from 20 to 350 kHz, while reversed pulse duration is dependent on of wandering around the target material [100]. dielectric surface discharge [125]. Negative voltage usually varies by an amount equiva- Magnetron sputtering is the most up-to-date deposition technology [99,100], and is lent to 10% of the average positive voltage. When the duration and number of positive widely used in the industrial and scientific spheres. The frequency of the applied positive voltage pulses are sufficient to create electric current, the target surface is bombarded with DC voltage varies from 20 to 350 kHz, while reversed pulse duration is dependent on ions, and when the voltage becomes negative, the incoming ions are repelled. H.-C. Chen dielectric surface discharge [125]. Negative voltage usually varies by an amount equivalent [126,127] investigated WO3 films deposited by pulsed magnetron sputtering at a constant to 10% of the average positive voltage. When the duration and number of positive voltage frequency of 70 kHz; the O2/Ar ratio was reported to vary from 0.2 to 1.0. pulses are sufficient to create electric current, the target surface is bombarded with ions, and The disadvantages of this method include the expensive equipment required and the when the voltage becomes negative, the incoming ions are repelled. H.-C. Chen [126,127] high energy intensity, which significantly increases ECD cost. The magnetron sputtering investigated WO films deposited by pulsed magnetron sputtering at a constant frequency technique is used to produce FTO or ITO electrodes on transparent surfaces. of 70 kHz; the O /Ar ratio was reported to vary from 0.2 to 1.0. The disadvantages of this method include the expensive equipment required and the 6. Nanomaterials for Electrochromic Devices high energy intensity, which significantly increases ECD cost. The magnetron sputtering ECW control the transmittance of light and solar radiation by changing their optical technique is used to produce FTO or ITO electrodes on transparent surfaces. transmittance (transparent, semitransparent and colored states), which ensures comforta- ble 6. Nanomaterials indoor environm for ent Electrochromic s and makes it Devices possible to achieve energy savings in buildings. Recent advances in ECD technology emerging in the 1970s led to the creation of different ECW control the transmittance of light and solar radiation by changing their optical types of ECD. However, there are still problems with respect to the commercialization of transmittance (transparent, semitransparent and colored states), which ensures comfortable EC devices, including aspects such as their high production cost [99], the stability of their indoor environments and makes it possible to achieve energy savings in buildings. Recent long-term operation, and the production of uniform electrochromic films to provide uni- advances in ECD technology emerging in the 1970s led to the creation of different types of formity of coloration in large-area ECW [28–32]. Nanotechnologies can be efficiently used ECD. However, there are still problems with respect to the commercialization of EC devices, to produce low-cost high-performance ECD [128]. including aspects such as their high production cost [99], the stability of their long-term In [129], an experiment was described in which reduced graphene oxide (rGO) films operation, and the production of uniform electrochromic films to provide uniformity of were electrodeposited on indium tin-oxide-coated polyethylene terephthalate substrates coloration in large-area ECW [28–32]. Nanotechnologies can be efficiently used to produce (I low-cost TO-PET) high-performance from graphene oxide ECD (GO) [128]., and the resulting flexible transparent electrodes were used in ethyl viologen (EtV2 ) electrochromic devices. During continuous testing, In [129], an experiment was described in which reduced graphene oxide (rGO) films th wer e resu e electr lting odeposited devices, whic on indium h contained tin-oxide-coated GO/rGO in tpolyethylene he electrochrom terephthalate ic mixture, substrates exhibited a (IT lower O-PET) switc frhing om graphene voltage bet oxide ween (GO), the colored and the and resulting bleached flexible states. transpar Graphen ent e ox electr ide (GO) odes were used in ethyl viologen (EtV2 ) electrochromic devices. During continuous testing, and reduced graphene oxide (rGO) enabled devices with higher optical contrast than the resulting devices, which contained GO/rGO in the electrochromic mixture, exhibited a those free of GO at the same applied voltage. lower switching voltage between the colored and bleached states. Graphene oxide (GO) In [130], WO3/rGO nanocomposite film was fabricated by sol–gel centrifugation us- ing a mixed colloidal dispersion of WO3 precursor and GO. It was reported that the Nanomaterials 2021, 11, 2376 25 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 25 of 32 and reduced graphene oxide (rGO) enabled devices with higher optical contrast than those Nanomaterials 2021, 11, x FOR PEER REVIEW 25 of 32 free of GO at the same applied voltage. In [130], WO /rGO nanocomposite film was fabricated by sol–gel centrifugation WO3/rGO nanocomposite film exhibited shorter coloration and bleaching times (Tc = 9.5 s using a mixed colloidal dispersion of WO precursor and GO. It was reported that the WO3/rGO nanocomposite film exhibited shorter coloration and bleaching times (Tc = 9.5 s 2 −1 and Tb = 7.6 s), hWO igher /rGO colora nanocomposite tion efficiency film (75.3 exhibited cm ∙C at shorter 633 nm coloration ), larger op and ticbleaching al modu- times (T = 9.5 s 3 c 2 −1 and Tb = 7.6 s), higher coloration efficiency (75.3 cm ∙C at 633 nm), larger optical modu- 2 1 latory range (59.6 and % aT t 633 = nm 7.6 ) s), and bet higher ter cyc coloration lic stabief lifi ty com ciency par (75.3 ed with WO cm C 3 fiat lms; 633 thnm), ese larger optical latory range (59.6% at 633 nm) and better cyclic stability compared with WO3 films; these modulatory range (59.6% at 633 nm) and better cyclic stability compared with WO films; advantages were attributed to faster Li ion diffusion and electron transfer rate. advantages were attributed to faster Li ion diffusion and electron transfer rate. Optically adj these ustaadvantages ble electrochro wermi e attributed c films arto e bas faster ic and Li ion impo dif rta fusion nt com and po electr nents on of transfer rate. Optically adjustable electrochromic films are basic and important components of Optically adjustable electrochromic films are basic and important components of electrochromic devices; therefore, the performance of EC devices is strongly dependent electrochromic devices; therefore, the performance of EC devices is strongly dependent electrochromic devices; therefore, the performance of EC devices is strongly dependent on on EC film structure, morphology and fabrication method [131]. on EC film structure, morphology and fabrication method [131]. EC film structure, morphology and fabrication method [131]. Amorphous WO3 films have a porous structure. Crystalline WO3 exhibits better du- Amorphous WO3 films have a porous structure. Crystalline WO3 exhibits better du- Amorphous WO films have a porous structure. Crystalline WO exhibits better rability compared to amorphous WO3, 3 due to its denser structure and low dissolution rate 3 rability compared to amorphous WO3, due to its denser structure and low dissolution rate durability compared to amorphous WO , due to its denser structure and low dissolution (stability in acidic solution is less than 4 pH) [93,94,132]3 . However, crystalline WO3 pos- (stability in acidic solution is less than 4 pH) [93,94,132]. However, crystalline WO3 pos- rate (stability in acidic solution is less than 4 pH) [93,94,132]. However, crystalline WO sesses high bulk density, which increases switching time and reduces coloring efficiency, 3 sesses high bulk density, which increases switching time and reduces coloring efficiency, possesses high bulk density, which increases switching time and reduces coloring efficiency, so nanostructured WO3 with a large specific surface area is expected to have a faster re- so nanostructured WO3 with a large specific surface area is expected to have a faster re- so nanostructured WO with a large specific surface area is expected to have a faster sponse time and a good durability. Recently, publications have appeared [105,133] on the sponse time and a good durability. Recently, publications have appeared [105,133] on the response time and a good durability. Recently, publications have appeared [105,133] on the use of nanoscale or nanoporous WO3 (Figure 31) that exhibit fast switching speed and use of nanoscale or nanoporous WO3 (Figure 31) that exhibit fast switching speed and use of nanoscale or nanoporous WO (Figure 31) that exhibit fast switching speed and high high coloration efficiency due to possessing a good and suitable band gap (~2.6 eV). In high coloration efficiency due to possessing a good and suitable band gap (~2.6 eV). In coloration efficiency due to possessing a good and suitable band gap (~2.6 eV). In [134–136] [134–136] the technologies for producing nanostructured WO3 films are discussed (Figure [134–136] the technologies for producing nanostructured WO3 films are discussed (Figure the technologies for producing nanostructured WO films are discussed (Figure 32). 32). 32). Figure 31. FE-SEM micrographs for nc-TiO2 nanoparticles film: (a) before deposition; (b) deposited Figure 31. FE-SEM micrographs for nc-TiO nanoparticles film: (a) before deposition; (b) deposited on H W O electrolyte 2 2 2 11 Figure 31. FE-SEM micrographs for nc-TiO2 nanoparticles film: (a) before deposition; (b) deposited on H2W2O11 electrolyte surface. surface. on H2W2O11 electrolyte surface. (a) (b) (a) (b) Figure 32. Nanostructured films obtained by electrochemical deposition: (a) WO3(GO2ml); (b) Figure 32. Nanostructured films obtained by electrochemical deposition: (a) WO3(GO2ml); (b) Figure 32. Nanostructured films obtained by electrochemical deposition: (a) WO (GO ); 2ml WO3(GO1ml) [135]. WO3(GO1ml) [135]. (b) WO (GO ) [135]. 3 1ml Nanocrystal-in-glass WO3 thin films are considered to be the most promising ca- Nanocrystal-in-glass WO3 thin films are considered to be the most promising ca- thodic electrochromic material [113]. In [137], an all-solution technology was developed thodic electrochromic material [113]. In [137], an all-solution technology was developed for large-area low-cost preparation of electrochromic films. A WO3/ITO dispersion was for large-area low-cost preparation of electrochromic films. A WO3/ITO dispersion was successfully developed; high-electrical-conductivity ITO nanoparticle networks along successfully developed; high-electrical-conductivity ITO nanoparticle networks along Nanomaterials 2021, 11, 2376 26 of 32 Nanocrystal-in-glass WO thin films are considered to be the most promising cathodic electrochromic material [113]. In [137], an all-solution technology was developed for large- area low-cost preparation of electrochromic films. A WO /ITO dispersion was successfully developed; high-electrical-conductivity ITO nanoparticle networks along with ITO coating on glass were able to serve as extended 3-dimensional electrodes, forming a microelectrical field and acting as the pathways for electron diffusion to WO nanorods. In [138], h-WO 3 3 QDs with an average size of 1.2 nm were successfully prepared by a simple decomposition process of tungsten acid in ethylene glycol. At present, various interactions have been introduced at the interface between the organic and inorganic phases. The expected improved electrochemical and electrochromic performances of the nanocomposites have been obtained. Among of these interactions, covalent bonds have the strongest interaction, although their preparation is relatively complicated [131,138]. Thus, it is an important first step for the fabrication of inexpensive EC “Smart Win- dows”, and should shape the future research on solution-based processes. 7. Conclusions It was possible, within the scope of this article, to provide a comprehensive review of the large area of new electrochromic materials, and the authors had to use their discretion in choosing up-to-date findings to illustrate this exciting area. In summarizing this review of the literature on electrochromism in electrochromic materials, and in WO films in particular, the following conclusions can be drawn: (1) There are several hypotheses concerning the mechanism of electrochromism in WO . Generally, the electrochromic effect in WO films can be described as an electrochemi- cal cathodic polarization during which H ions are transferred from the electrolyte and an electron is transferred from the ITO electrode. As a result, WO film switches from a bleached to a colored state; its color varies from pale blue to dark blue and black. The conductivity of WO films is determined by the presence of cations (H , Li , etc.) and electrons. As already mentioned, the coloration mechanism in WO films has still been insufficiently investigated. (2) Despite a large number of works devoted to the study of electrochromic WO films, the influence of the structural state on optical properties during the electrochemical reaction has not been fully investigated. Different film deposition techniques have been proposed. Film morphology is dependent on deposition technique and can be amorphous, crystalline, nanocrystalline or hybrid. Additionally, there is still a constant need for new technologies to produce WO films, and nanostructured WO films in particular. Therefore, there is a necessity to study the fabrication of amorphous, crystalline and nanocrystalline WO films, including their GO/rGO modification. Analysis of literary sources makes it possible to identify prospects for the development of WO /rGO fabrication technologies. The obtained data will be useful in the development of WO fabrication technologies. Today, the development of the energy-efficient glazing sector is impossible without EC Modern nanomaterials make ECD an interesting commercial product that has obvious advantages over its competitors, such as PDLC, LCD and SPD. In this regard, according to some forecasts, the market for electrochromic “Smart Window” will expand in the next 5–7 years. First of all, thanks to the development of modern technologies and nanomaterials, as well as intensive research into EC by companies and scientific laboratories around the world. Author Contributions: Conceptualization, A.V.S. (Aleksei Viktorovich Shchegolkov), A.V.S. (Alexandr Viktorovich Shchegolkov) and S.-H.J.; methodology, M.S.L. and A.V.S. (Aleksei Viktorovich Shchegolkov); validation, Y.V.R. and A.O.S.; formal analysis, S.-H.J.; investigation, M.S.L. and A.V.S. (Aleksei Vik- torovich Shchegolkov); resources, S.-H.J.; writing—original draft preparation, Y.V.R. and A.O.S.; writing—review and editing, A.V.S. (Aleksei Viktorovich Shchegolkov) and A.V.S. (Alexandr Vik- Nanomaterials 2021, 11, 2376 27 of 32 torovich Shchegolkov); visualization, A.V.S. (Aleksei Viktorovich Shchegolkov); supervision, A.V.S. (Aleksei Viktorovich Shchegolkov), A.V.S. (Alexandr Viktorovich Shchegolkov) and S.-H.J.; project administration, A.V.S. (Aleksei Viktorovich Shchegolkov); funding acquisition, S.-H.J. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Research Foundation (NRF) grant funded by the Korea government (MSIT) (No. 2020R1C1C1005273, 2021M3H4A02056037). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations EC electrochromic materials; PhC photochromic materials; ThC thermochromic materials; GhC gasochromic materials; PDLC polymer-dispersed liquid crystals; LDC liquid crystal dispersions; ECD electrochromic devices; ECW electrochromic windows; TMO transition metal oxides; GO graphene oxide; rGO reduced graphene oxide; SPD suspended particles; EMR electromagnetic radiation. References 1. Addington, D.M.; Schodek, D.L. Smart Materials and New Technologies for the Architecture and Design Professions; Elsevier Science: Oxford, UK, 2005; p. 241. 2. Granqvist, C.G.; Green, S.; Niklasson, G.A.; Mlyuka, N.R.; Kraemer, S.; Georén, P. Advance in chromogenic materials and devices. Thin Solid Film. 2010, 518, 3046–3053. [CrossRef] 3. Bamfield, P. Chromic Phenomena the Technological Applications of Colour Chemistry; Royal society of Chemistry (RSC): Cambridge, UK, 2001; p. 374. 4. Baetens, R.; Jelle, B.P.; Gustavsen, A. 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A Brief Overview of Electrochromic Materials and Related Devices: A Nanostructured Materials Perspective

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nanomaterials Review A Brief Overview of Electrochromic Materials and Related Devices: A Nanostructured Materials Perspective 1 , 2 , 3 Aleksei Viktorovich Shchegolkov *, Sung-Hwan Jang * , Alexandr Viktorovich Shchegolkov , 4 5 1 Yuri Viktorovich Rodionov , Anna Olegovna Sukhova and Mikhail Semenovich Lipkin Department of Chemical Technologies, Platov South-Russian State Polytechnic University (NPI), 346428 Novocherkassk, Russia; lipkin@yandex.ru Department of Civil and Environmental Engineering, Hanyang University ERICA, Ansan 15588, Korea Department of Technology and Methods of Nanoproducts Manufacturing, Tambov State Technical University, 392000 Tambov, Russia; Energynano@yandex.ru Department of Mechanics and Engineering Graphics, Tambov State Technical University, 392000 Tambov, Russia; rodionow.u.w@rambler.ru Department of Nature Management and Environment Protection, Tambov State Technical University, 392000 Tambov, Russia; apil1@yandex.ru * Correspondence: alexxx5000@mail.ru (A.V.S.); sj2527@hanyang.ac.kr (S.-H.J.) Abstract: Exactly 50 years ago, the first article on electrochromism was published. Today elec- trochromic materials are highly popular in various devices. Interest in nanostructured electrochromic and nanocomposite organic/inorganic nanostructured electrochromic materials has increased in the last decade. These materials can enhance the electrochemical and electrochromic properties of devices related to them. This article describes electrochromic materials, proposes their classification and systematization for organic inorganic and nanostructured electrochromic materials, identifies Citation: Shchegolkov, A.V.; Jang, their advantages and shortcomings, analyzes current tendencies in the development of nanomaterials S.-H.; Shchegolkov, A.V.; Rodionov, used in electrochromic coatings (films) and their practical use in various optical devices for protection Y.V.; Sukhova, A.O.; Lipkin, M.S. A from light radiation, in particular, their use as light filters and light modulators for optoelectronic Brief Overview of Electrochromic devices, as well as methods for their preparation. The modern technologies of “Smart Windows”, Materials and Related Devices: A which are based on chromogenic materials and liquid crystals, are analyzed, and their advantages Nanostructured Materials Perspective. and disadvantages are also given. Various types of chromogenic materials are presented, examples Nanomaterials 2021, 11, 2376. https:// of which include photochromic, thermochromic and gasochromic materials, as well as the main doi.org/10.3390/nano11092376 physical effects affecting changes in their optical properties. Additionally, this study describes elec- trochromic technologies based on WO films prepared by different methods, such as electrochemical Academic Editor: Yi Long deposition, magnetron sputtering, spray pyrolysis, sol–gel, etc. An example of an electrochromic “Smart Window” based on WO is shown in the article. A modern analysis of electrochromic devices Received: 26 July 2021 3 based on nanostructured materials used in various applications is presented. The paper discusses Accepted: 3 September 2021 Published: 13 September 2021 the causes of internal and external size effects in the process of modifying WO electrochromic films using nanomaterials, in particular, GO/rGO nanomaterials. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: electrochromic materials; nanostructured electrochromic materials; electrochromism; published maps and institutional affil- color; “Smart Windows”; transition metal oxides (TMO); nanomaterials; graphene oxide (GO); iations. reduced graphene oxide (rGO) Copyright: © 2021 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. Modern technology has a number of negative effects, such as atmospheric pollution, This article is an open access article global warming, the reduction of fossil resources, etc. Therefore, one of the most important distributed under the terms and tasks in the world is to improve energy efficiency and energy savings. To this end, it conditions of the Creative Commons is necessary to create new materials in a variety of sectors, including engineering, agro- Attribution (CC BY) license (https:// industry, building construction, electronics manufacturing, etc., primarily with the aim of creativecommons.org/licenses/by/ using new technologies and “smart” materials. 4.0/). Nanomaterials 2021, 11, 2376. https://doi.org/10.3390/nano11092376 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 2376 2 of 32 Functional materials are dependent on their initial state and properties, as well as on the energy and external effects applied to the material. “Smart” materials have more than one functional state, depending on the impacting impulse, which can change over time [1]. Electrochromic materials (EC) are materials that are able to change color under the influence of an electric field. EC are of great interest, both from the scientific point of view and with respect to their application in various technical systems, including as the basis for the creation of electrochromic devices (ECD) with low power requirements, such as [2–4]: - “Smart Windows”; - Displays; - Reflective blinds; - Variable reflection mirrors; - Sensors. The main purpose of ECD is protection against light in the visible wavelength range (380–780 nm). ECD include an electrochromic coating in the form of the EC film and a counter electrode placed in an electrolyte (ionic conductor), which is located between transparent conductive electrodes—ITO (In O -SnO ) or FTO (SnO -SnF). The principle 2 3 2 2 of ECD operation is the transformation of optical light flux and the modulation of the coefficient of light reflection/transmission, resulting in an electrochemical reaction, i.e., the “Smart Window” effect. Thus, “Smart Window” technology allows savings due to use of smaller amounts of energy for air conditioning in summer, as well as for heating in winter; an average of more than 30% compared to conventional windows. The purpose of this review is to systematize and summarize the data on organic, inorganic and nanostructured electrochromic materials and related devices over the past 50 years. 2. “Smart Windows” There are chromogenic materials [3], better known as “smart” materials, that are currently experiencing great popularity. These materials modulate reflected or diffused light by means of physical effects of different types. The “Smart Window” based on chromogenic materials is widely used in architecture, cars (rear-view mirrors and intelligent window tinting), and aircraft illuminators (Boeing 787 Dreamliner) [3–5], while translucent structure technology [6] must also be mentioned. Chromogenic materials change color and transparency. The following types of chromogenic materials can be distinguished: electrochromic materials (EC) (external conditions—electric field); photochromic materials (PhC) (external conditions—light); thermochromic materials (ThC) (external conditions— heat); gasochromic materials (GhC) (external conditions—gas); polymer-dispersed liquid crystals (PDLC) and liquid crystal dispersions (LCD); SPD—suspended particles device are Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 32 placed between the two electroconductive coatings. These materials can serve as a basis for “Smart Window” technologies [7–11], which are shown in Figure 1. Figure 1. “Smart Windows” classification. Figure 1. “Smart Windows” classification. The structures of SPD and PDLC windows [6,10–12] are shown in Figure 2. SPD tech- nology (Figure 2a) uses suspended particles to modulate light transmission, arranging themselves in an alternating current field, and the film becomes transparent. In the ab- sence of the electric field, the SPD window acquires color and absorbs light. The SPD win- dow is similar in structure to the PDLC window (Figure 2b), apart from the fact that in the absence of an electric field, the film becomes semi-transparent. (а) (b) Figure 2. “Smart Windows” SPD and PDLC technology sandwich structures and operating principles: (а) SPD: off—light modulation mode, on—transparent mode; (b) PDLC: off—semi-transparent mode, on—transparent mode; 1—retaining film; 2—suspended particle; 3—adhesive layer; 4—glass; 5—conductor; 6—liquid crystal layer; 7—interlayer film; 8—liq- uid crystal. Electrochromic windows (ECW) control the transmission of light in the visible spec- trum and switches between tinted and transparent/semi-transparent states in response to low voltage signal (Figure 3). ECW create a comfortable indoor environment; moreover, they have lower power consumption in comparison with other chromogenic devices [13]: the ECW modulates reflected light under the control voltage, and in the absence of the control voltage, modulation of the transmitted light occurs [2–4,7]. Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 32 Nanomaterials 2021, 11, 2376 3 of 32 Figure 1. “Smart Windows” classification. The structures of SPD and PDLC windows [6,10–12] are shown in Figure 2. SPD tech- The structures of SPD and PDLC windows [6,10–12] are shown in Figure 2. SPD nology (Figure 2a) uses suspended particles to modulate light transmission, arranging technology (Figure 2a) uses suspended particles to modulate light transmission, arranging themselves in an alternating current field, and the film becomes transparent. In the ab- themselves in an alternating current field, and the film becomes transparent. In the absence sence of the electric field, the SPD window acquires color and absorbs light. The SPD win- of the electric field, the SPD window acquires color and absorbs light. The SPD window is similar in structure to the PDLC window (Figure 2b), apart from the fact that in the absence dow is similar in structure to the PDLC window (Figure 2b), apart from the fact that in of an electric field, the film becomes semi-transparent. the absence of an electric field, the film becomes semi-transparent. (а) (b) Figure 2. “Smart Windows” SPD and PDLC technology sandwich structures and operating principles: (а) SPD: off—light Figure 2. “Smart Windows” SPD and PDLC technology sandwich structures and operating principles: (a) SPD: off—light modulation mode, on—transparent mode; (b) PDLC: off—semi-transparent mode, on—transparent mode; 1—retaining modulation mode, on—transparent mode; (b) PDLC: off—semi-transparent mode, on—transparent mode; 1—retaining film; 2—suspended particle; 3—adhesive layer; 4—glass; 5—conductor; 6—liquid crystal layer; 7—interlayer film; 8—liq- film; 2—suspended particle; 3—adhesive layer; 4—glass; 5—conductor; 6—liquid crystal layer; 7—interlayer film; 8—liquid uid crystal. crystal. Electrochromic windows (ECW) control the transmission of light in the visible spec- Electrochromic windows (ECW) control the transmission of light in the visible spec- trum and switches between tinted and transparent/semi-transparent states in response to trum and switches between tinted and transparent/semi-transparent states in response to low voltage signal (Figure 3). ECW create a comfortable indoor environment; moreover, low voltage signal (Figure 3). ECW create a comfortable indoor environment; moreover, they have lower power consumption in comparison with other chromogenic devices [13]: they have lower power consumption in comparison with other chromogenic devices [13]: the ECW modulates reflected light under the control voltage, and in the absence of the the ECW modulates reflected light under the control voltage, and in the absence of the control voltage, modulation of the transmitted light occurs [2–4,7]. control voltage, modulation of the transmitted light occurs [2–4,7]. The advantages of electrochromic technologies are as follows [14]: - electric energy is required only during mode switching; - low activation voltage (1–5 V); - a wide variety of “Smart Window” tints (blue, grey, brown, etc.); - in the bleached state, electrochromic devices have a transparency level of 50–70%, in the colored state—10–25%. Table 1 shows the basic advantages and shortcomings of chromogenic materials used in “Smart Windows”. Nanomaterials 2021, 11, 2376 4 of 32 (a) (b) Figure 3. “Smart Window” electrochromic technology: (a) sandwich structure and operating principle (bleached state): transmitted and reflected light modulation: 1—electrochromic layer; 2—ion storage layer; 3—glass; 4—conductive layer; 5—ion conductor/electrolyte; (b) total view. Table 1. Comparison of “Smart Window” technologies. Energy Saving, Energy W/m (Energy Transparency, Modulation Cost, Technology Efficiency, Saving in % Time, s (c.u./m ) W/m Building) ECW + + + – – SPD – – + + + PDLC – – + + + LCD – – + + + Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 32 Table 1. Comparison of “Smart Window” technologies. Energy Energy Saving, W/m Modulation Cost, Technology Efficiency, (Energy Saving in Transparency, % Time, s (c.u./m ) W/m Building) ECW + + + – – SPD – – + + + Nanomaterials 2021, 11, 2376 5 of 32 PDLC – – + + + LCD – – + + + Electronic devices emit high levels of electromagnetic radiation (EMR) in a wide fre- Electronic devices emit high levels of electromagnetic radiation (EMR) in a wide fre- quency range, leading to electromagnetic pollution, which negatively influences biologi- quency range, leading to electromagnetic pollution, which negatively influences biological cal objects and causing electronic device dysfunction [15]. Considering the fact that elec- objects and causing electronic device dysfunction [15]. Considering the fact that electro- tromagnetic radiation basically penetrates through glass surfaces, the problem of creating magnetic radiation basically penetrates through glass surfaces, the problem of creating a a universal electrochromic film capable of absorbing or reflecting electromagnetic radia- universal electrochromic film capable of absorbing or reflecting electromagnetic radiation tion is becoming relevant. is becoming relevant. 3. Electrochromism and Electrochromic Materials: Classification and Applications 3. Electrochromism and Electrochromic Materials: Classification and Applications Chromism (from ancient Greek ῶ (“color”)) is a phenomenon of material color Chromism (from ancient Greek   (“color”)) is a phenomenon of material color change under the influence of physical factors, such as electric field, heat, light or pressure change under the influence of physical factors, such as electric field, heat, light or pres- [16]. sure [16]. At At the end of the 1960s, sc the end of the 1960s, scientist ientist S. K. S. K. Deb Deb discover discovered ed the phenomenon ca the phenomenon called lled elec elec-- tr trochromism ochromism [[17]. This ne 17]. This newly wly ddiscov iscover er ed phenomen ed phenomenon on belongs to the sphere o belongs to the spheref electro- of elec- tr chemistry ochemistry and physic and physics s [18]. S. K. Deb d [18]. S. K. Deb esc described ribed a new electropho a new electrophotographic tographic system con- system consisting sisting of WO of WO 3 thin film thin film and a th and in-film a thin-film photoconductive layer photoconductive layer placed between placed between two elec- two electr trodes. odes. When When this composite structur this composite structur e was subjec e was subjected ted to an electric to an electric field, an field, optic an al projec- optical projection appeared. After subsequent modulation in the photoconductive layer, the oxide tion appeared. After subsequent modulation in the photoconductive layer, the oxide layer layer acquires the same color, and a visible image appears [17–19]. acquires the same color, and a visible image appears [17–19]. Since the middle of the 1970s, electrochromism has been considered to be a physi- Since the middle of the 1970s, electrochromism has been considered to be a physical cal phenomenon associated with a reversible change in transparency or color under the phenomenon associated with a reversible change in transparency or color under the in- influence of an electric field or electric current [19,20]. Electrochromism is traditionally fluence of an electric field or electric current [19,20]. Electrochromism is traditionally de- defined as a reversible change in optical properties (transparency and/or reflectivity) fined as a reversible change in optical properties (transparency and/or reflectivity) during during the oxidation–reduction reaction [21–23]. In some cases, there are more than two the oxidation–reduction reaction [21–23]. In some cases, there are more than two degrees degrees of oxidation, and the material is capable of showing several colors depending on of oxidation, and the material is capable of showing several colors depending on the cur- the current degree of oxidation (polyelectrochromic materials) [21]. Modern science uses a rent degree of oxidation (polyelectrochromic materials) [21]. Modern science uses a broad broad definition of electrochromism, including materials and devices used for the optical definition of electrochromism, including materials and devices used for the optical mod- modulation of radiation in the visible and microwave ranges. Ref. [24] focuses on the ulation of radiation in the visible and microwave ranges. Ref. [24] focuses on the problem problem of developing electrochromic displays that should replace LED and liquid crystal of developing electrochromic displays that should replace LED and liquid crystal dis- displays. In 1985, Svensson and Granqvist proposed using electrochromic materials in plays. In 1985, Svensson and Granqvist proposed using electrochromic materials in “Smart Windows” [14], and thus the term “Smart Window” appeared. The electrochromic “Smart Windows” [14], and thus the term “Smart Window” appeared. The electrochromic reaction can be described by the electrochemical equation in oxidized form: reaction can be described by the electrochemical equation in oxidized form: O + xe + Cation $ Reduced form, R (1) О + xe + Cation ↔ Reduced form, R (1) Applications of electrochromism include: Applications of electrochromism include: − Control of energy transfer in different environments, for example, filtering solar ra- - Control of energy transfer in different environments, for example, filtering solar radia- diation using “Smart Window” devices [25–27]. Fast mode switching (col- tion using “Smart Window” devices [25–27]. Fast mode switching (colored/bleached) ored/bleached) is not required, but the device should be capable of filtering both vis- is not required, but the device should be capable of filtering both visible and near- ible and near-infrared radiation. Moreover, the transparency of the window packages infrared radiation. Moreover, the transparency of the window packages must be at must be at least 70%. least 70%. − Color displays [22,24], for example, advertising boards. The requirements are as fol- - Color displays [22,24], for example, advertising boards. The requirements are as lows: fast mode switching, color scheme varies only in the visible area. Moreover, follows: fast mode switching, color scheme varies only in the visible area. Moreover, color color contr contrast ast shou should ld be be hi high gh enou enough, gh, tr transpar ansparent mo ent mode de i iss not re not requir quired. ed. -− Mirr Mirror or light light modulators [7], for ex modulators [7], for example, ample, antiglar antiglare e mirr ors mirrors for cars. for cars. Fast mode Fast switch- mode ing swit and ching high and high transpar trency ansparency are not a rr equir e noted. required. 3.1. Classification of Electrochromic Materials There are several inorganic and organic EC that change their optical properties (trans- parency, color) during oxidation–reduction [28–32]. Switching between oxidation and reduction states leads to color formation, i.e., formation of new spectral peaks in the visible area. Inorganic EC include transition metal oxides (TMO) from groups IV-VI [32], and hexacyanometallates (Prussian blue). Organic EC include viologens, conjugated conductive polymers (polypyrrole, polythiophene, polyaniline and their derivatives, metal polymers, metal phthalocyanines) [33,34]. The viologen family (4,4 -dipyridinium compounds) has Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 32 3.1. Classification of Electrochromic Materials There are several inorganic and organic EC that change their optical properties (transparency, color) during oxidation–reduction [28–32]. Switching between oxidation and reduction states leads to color formation, i.e., formation of new spectral peaks in the visible area. Inorganic EC include transition metal oxides (TMO) from groups IV-VI [32], and hexacyanometallates (Prussian blue). Organic EC include viologens, conjugated con- Nanomaterials 2021, 11, 2376 6 of 32 ductive polymers (polypyrrole, polythiophene, polyaniline and their derivatives, metal polymers, metal phthalocyanines) [33,34]. The viologen family (4,4′-dipyridinium com- pounds) has a general chemical formula as shown in Figure 4, where R may be an alkyl, a general chemical formula as shown in Figure 4, where R may be an alkyl, cyclo-alkyl cyclo-alkyl or other substitute, and X corresponds to halogen 4,4′-dipyridium compounds, or other substitute, and X corresponds to halogen 4,4 -dipyridium compounds, because because they turn a deep blue-purple on reduction [30]. The viologen ion as shown in they turn a deep blue-purple on reduction [30]. The viologen ion as shown in Figure 4a Figure 4a can have a two-step reduction, i.e., a one-electron or a two-electron reduction. can have a two-step reduction, i.e., a one-electron or a two-electron reduction. The general The general structure structur for e viol forog viologens ens mo modifying difying th the e titania titania surface surface is shown is shown in Figur in eFigure 4c. Table 4c 2. presents a list of the most popular EC. Table 2 presents a list of the most popular EC. (a) (b) (c) Figure Figure 4. Viologen: 4. Violo (gen a) general : (a) gen chemical eral chemic formulae al of form viologen; ulae of (b)vviologen iologen; ion; (b)( c vi ) general ologens io tructur n; (c) e gen for viologens eral strumodifying cture the titania surface. for viologens modifying the titania surface. Table 2 shows a general classification of EC. Table 2 shows a general classification of EC. Table 2. General classification of EC. Table 2. General classification of EC. EC Class Chemical Name Application Ref. Organic EC Class Chemical Name Application Ref. PEDOT (where EDOT = C H O S), 6 6 2 Organic Conductive PPy (where Py = Pyrrole = C H N), 4 5 “Smart Windows”, displays [13,33] PEDOT (where EDOT = C6H6O2S), polymers PT (where T = thiophene = C H S), 4 4 “Smart PANI (where ANI = aniline = C H S) Conductive PPy (where Py = Pyrrole = C4H5N),6 4 Windows”, [13,33] 3-aryl-4,5-bis (pyridine-4-yl) isoxazole Antiglare mirrors and polymers PT (where T = thiophene = C4H4S), Viologens [21,28] displays derivatives displays PANI (where ANI = aniline = C6H4S) II Transition metals and poly [Ru (vbpy) (py) ]Cl (being py = 2 2 2 Ant Smart igmirr lareors [26,30] lanthanoids pyridine = C H N) 5 5 Viologens 3-aryl-4,5-bis (pyridine-4-yl) isoxazole derivatives mirrors and [21,28] Metal phthalocyanines (Pc) [Lu(Pc) ] being Pc = C H N et al. Displays [7,30] 2 32 18 8 displays Inorganic II Transition metals and poly [Ru (vbpy)2(py)2]Cl2 (being py = pyridine = Transition metal oxides WO , MoO , V O , TiO Nb O , Ir(OH) , “Smart Windows”, antiglare 3 3 2 5 2 2 5 3 Smart mirrors [26,30] [32,34] lanthanoids С5H5N) (TMOs) NiO et al. mirrors Metal phthalocyanines (Pc) [Lu(Pc)2] being P Prussian c = C blue 32H(C 18N8 Fe et a N l.), Displays [7,30] 18 7 18 Prussian brown (C Fe N ), Prussian 6 2 6 Inorganic Prussian blue (PB) “Smart Windows”, displays [7,29] green (C FeN ), Prussian white 3 3 “Smart (C Fe N ) 6 3 6 Transition metal oxides Windows”, WO3, MoO3, V2O5, TiO2 Nb2O5, Ir(OH)3, NiO et al. [32,34] (TMOs) antiglare In general, organic EC, possessing color-changing abilities, exhibit faster response times and higher staining efficiencies than inorganic ones, but have a low UV protection mirrors index and show lower electrochemical stability. Therefore, mainly organic EC materials Prussian blue (C18Fe7N18), “Smart are used in electronic non-emissive displays [24,28]. Inorganic EC materials show high Prussian blue (PB) Prussian brown (С6Fe2N6), Prussian green (C3FeN3), Windows”, [7,29] chemical stability and cyclicity, which makes them suitable for “Smart Windows” and Prussi large-scale an whdata ite (C displays 6Fe3N6) [35 ]. displays Electrochromic materials are classified according to their solubility and according to their redox states [29,30]. Classification of EC was introduced by I. F. Chang in 1975 [36]. In general, organic EC, possessing color-changing abilities, exhibit faster response According to this classification, there are three types of EC solubility in redox states [29]: times and higher staining efficiencies than inorganic ones, but have a low UV protection (1) Type I EC materials, such as viologen, heptyl, etc., are soluble in both their reduced index and show lower electrochemical stability. Therefore, mainly organic EC materials and oxidized states. are used in electronic non-emissive displays [24,28]. Inorganic EC materials show high (2) Type II EC materials are soluble in their colorless redox state but form a solid film on the electrode surface. Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 32 chemical stability and cyclicity, which makes them suitable for “Smart Windows” and large-scale data displays [35]. Electrochromic materials are classified according to their solubility and according to their redox states [29,30]. Classification of EC was introduced by I. F. Chang in 1975 [36]. According to this classification, there are three types of EC solubility in redox states [29]: (1) Type I EC materials, such as viologen, heptyl, etc., are soluble in both their reduced and oxidized states. Nanomaterials 2021, 11, 2376 7 of 32 (2) Type II EC materials are soluble in their colorless redox state but form a solid film on the electrode surface. (3) Type III EC materials are solids in both redox states, and they form an insoluble film (3) Type III EC materials are solids in both redox states, and they form an insoluble film on on the electrode surface. Type III materials include groups IV, V transition metal ox- the electrode surface. Type III materials include groups IV, V transition metal oxides ides (TMO), conductive polymers, Prussian blue and metal polymers. Three types of (TMO), conductive polymers, Prussian blue and metal polymers. Three types of mechanism for chan mechanism ging color/t forra changing nsparen color/transpar cy (according ency to (accor I. F. C ding han to g) I.ar F.e Chang) present ared e pr esented in Figure 5. in Figure 5. (a) (b) (c) Figure 5. Types of ECW: (a) type I (solution); (b) type II (hybrid); (c) type III (battery-powered); EC—electrochromic layer; Figure 5. Types of ECW: (а) type I (solution); (b) type II (hybrid); (c) type III (battery-powered); CE—counter-electrode layer; TCO—inorganic oxide. EC—electrochromic layer; CE—counter-electrode layer; TCO—inorganic oxide. The electrochromic reaction can be described by the following equation: The electrochromic reaction can be described by the following equation: n m na (m+a)/y EC + yCE $ EC + yCE (2) (bleached) (coloured) n m n−a (m+a)/y (2) EC + yCE (bleached) ↔ EC (coloured) + yCE Table 3 contains examples of each type of EC. Table 3 contains examples of each type of EC. Table 3. Classification of EC materials (according to I. F. Chang [36]). Table 3. Classification of EC materials (according to I. F. Chang [36]). EC Electrochromic Reaction EC Material Application Ref. Type Mechanism EC Electrochromic Reaction EC Material Application Ref. Type Mechanism (1) Methylviologene (MV, 1, 10-dimethyl-4,4 -bipyrindnium, (1) Methylviologene (MV, 1, 10-dimethyl- 2+ I 3-aryl-4,5-bis (pyridine-4-yl) MV + Night vision systems, [37,38] 4,4′-bipyrindnium, 3-aryl-4,5-bis (pyri- + (solution) isoxazole; e $MV mirrors (bleached) (colored) I Night vision systems, 2+ − +● (2) Phenothiazine (C H NS) in dine-4-yl) isoxazole; 12 MV 9 + e (bleached)↔MV (colored) [37,38] (solution) mirrors non-aqueous solution (2) Phenothiazine (C12H9NS) in non-aque- ous solution (1) Cyanophenylparaquate (CPQ, 1-1 cyanophenyl-4,4 -bipyridine, (1) Cyanophenylparaquate (CPQ, 1-1 cy- paraquat = C H Cl N , otherwise 12 14 2 2 anophenyl-4,4′-bipyridine, paraquat = known as viologen, due to the 2+ II CPQ + e + Electrochromic paper, herbicide name) in aqueous solution [39–41] C12H14Cl2N2, otherwise known as vio- (hybrid) X $[CPQ X ] “Smart Window” (2) Heptyl or benzylviologene (HV or Electrochromic logen, due to the herbicide name) in aque- BzV) or methoxyfluorene compounds II 2+ − − +● − CPQ + e + X ↔[ CPQ X ] paper, “Smart [39–41] ous solution C H Cl F O in acetonitrile solution 3 4 2 2 (hybrid) (C H N) 2 3 Window” (2) Heptyl or benzylviologene (HV or BzV) or methoxyfluorene compounds (1) Almost all inorganic EC materials, C3H4Cl2F2O in acetonitrile solution such as transition metal oxides: WO , MoO , V O , TiO Nb O , Ir(OH) , (C2H3N) 3 2 5 2 2 5 3 NiO; (1) Almost all inorganic EC materials, “Smart Window” III (2) Phthalocyanine (Pc = C H N ) “Smart Window” 32 18 8 + III MO + x(H + (battery- (3) Metal complexes and (Boeing 757), [32,42,43] such as transition metal oxides: WO3, MOy + x(H + (Boeing 757), e )$H MO x y(colored) (battery- [32,42,43] powered) hexacyanometallates, such as Prussian Electro-chromic paper MoO3, V2O5, TiO2 Nb2O5, Ir(OH)3, NiO ; e )↔HxMOy(colored) Electro-chromic pa- blue (PB = C Fe N ) 18 7 18 powered) (2) Phthalocyanine (Pc = C32H18N8) per (4) Conductive polymers: polypyrrole (PPy), polythiophene (PT), polyaniline (PANI) Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 32 (3) Metal complexes and hexacyanometal- lates, such as Prussian blue (PB = C18Fe7N18) (3) Metal complexes and hexacyanometal- (4) Conductive polymer lates, such s: p as olypyrro Prussian le b lue (PB = (PPy), polythiophene (PT), polyaniline C18Fe7N18) (PANI) (4) Conductive polymers: polypyrrole (PPy), polythiophene (PT), polyaniline Nanomaterials 2021, 11, 2376 8 of 32 Type I and Type II EC are self-erasing, since an electrical current is required to main- (PANI) tain the colored state, i.e., after the power is turned off, the ECW loses its color. Type III ECW (battery-powered) remain colored for some time after the voltage is removed. Elec- Type I and Type II EC are self-erasing, since an electrical current is required to main- Type I and Type II EC are self-erasing, since an electrical current is required to main- trochromic technologies make it possible to modulate the optical properties, such as color, tain the colored state, i.e., after the power is turned off, the ECW loses its color. Type III tain the colored state, i.e., after the power is turned off, the ECW loses its color. Type III light transmission coefficient T(λ), reflection coefficient R(λ), and absorption coefficient ECW (battery-powered) remain colored for some time after the voltage is removed. Elec- ECW (battery-powered) remain colored for some time after the voltage is removed. Elec- A(λ), of materials atr ccor ochr din omic g to Kirchho technologiesff’s law make it possible [44]: to modulate the optical properties, such as color, trochromic technologies make it possible to modulate the optical properties, such as color, light transmission coefficient T(l), reflection coefficient R(l), and absorption coefficient R(λ) + A(λ) + T(λ) = 1 (3) light transmission coefficient T(λ), reflection coefficient R(λ), and absorption coefficient A(l), of materials according to Kirchhoff’s law [44]: A(λ), of materials according to Kirchhoff’s law [44]: All of these optical processes (Figure 6) are characterized by the EC transmittance R(λ) + A(λ) + T(λ) = 1 (3) R(l) + A(l) + T(l) = 1 (3) T(λ), absorption A(λ) and reflectance R(λ), which indicates the proportion of the incident light intensity that passes through, is absorbed by, or is reflected by the EC. All of these optical processes (Figure 6) are characterized by the EC transmittance All of these optical processes (Figure 6) are characterized by the EC transmittance T(l), absorption A(l) and reflectance R(l), which indicates the proportion of the incident T(λ), absorption A(λ) and reflectance R(λ), which indicates the proportion of the incident light intensity that passes through, is absorbed by, or is reflected by the EC. light intensity that passes through, is absorbed by, or is reflected by the EC. (a) (b) (с) (d) Figure 6. Interaction of radiation with an EC: (a) reflection; (b) absorption; (c) dispersion; (d) transmittance. (a) (b) (с) (d) Electrochromic properties depend on the electrochromic film structure; thus, differ- Figure 6. Figure Intera 6.ct Interaction ion of radia of radiation tion with an with an EC EC: : (a() arefle ) reflection; ction; ((b b)) absorption; absorption; (c)(dispersion; c) dispersio (dn; ( ) transmittance. d) transmittance. ent EC have different absorption spectra, and, consequently, differ in color. Electrochromic properties depend on the electrochromic film structure; thus, different Electrochromic properties depend on the electrochromic film structure; thus, differ- EC have different absorption spectra, and, consequently, differ in color. 3.2. Organic EC ent EC have different absorption spectra, and, consequently, differ in color. 3.2. Organic EC Organic films, such as conductive polymers, have multiple colored states, possess 3.2. Organic EC high optical contrast, and e Organic xhibit f films, assuch t respo as nse time conductive an polymers, d high staini haveng effic multiple iency color [45 ed states, –49]. possess high optical contrast, and exhibit fast response time and high staining efficiency [45–49]. Electrochromic behavior is observed in conjugated pyridine derivatives such as vio- Organic films, such as conductive polymers, have multiple colored states, possess Electrochromic behavior is observed in conjugated pyridine derivatives such as violo- logens (Figure 7), which exhibit high cyclicity, low operating potential and other valuable high optical contrast, and exhibit fast response time and high staining efficiency [45–49]. gens (Figure 7), which exhibit high cyclicity, low operating potential and other valuable properties [50,51]. Viologens exist in solid crystalline form and in powder form. The name Electrochromic behavior is observed in conjugated pyridine derivatives such as vio- properties [50,51]. Viologens exist in solid crystalline form and in powder form. The name ‘viologen’ alludes to violet, one color it can exhibit (Figure 7). logens (Figure 7), which exhibit high cyclicity, low operating potential and other valuable ‘viologen’ alludes to violet, one color it can exhibit (Figure 7). properties [50,51]. Viologens exist in solid crystalline form and in powder form. The name ‘viologen’ alludes to violet, one color it can exhibit (Figure 7). (a) (b) (c) Figure 7. Three general viologen redox states (in terms of electron transfer): (a) dication; (b) radical cation; (c) neutral state. Figure 7. Three general viologen redox states (in terms of electron transfer): (а) dication; (b) radical (a) (b) (c) cation; (c) neutral state. Viologens are used in RGB (red, green, blue) devices (Figure 8), which reproduce three Figure 7. Three general viologen redox states (in terms of electron transfer): (а) dication; (b) radical main colors, red, green and blue, although research in this area is not yet well developed. Viologens are used in RGB (red, green, blue) devices (Figure 8), which reproduce cation; (c) neutral state. Modern technologies require the use of multicolor EC, which, in turn, necessitates the three main colors, red, green and blue, although research in this area is not yet well de- creation of new functional composites [50]. veloped. Modern technologies require the use of multicolor EC, which, in turn, necessi- Viologens are used in RGB (red, green, blue) devices (Figure 8), which reproduce tates the creation of new functional composites [50]. three main colors, red, green and blue, although research in this area is not yet well de- veloped. Modern technologies require the use of multicolor EC, which, in turn, necessi- tates the creation of new functional composites [50]. Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 32 Nanomaterials 2021, 11, 2376 9 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 32 E = 0 V Е = 1.5 V E = 0 V Е = 1.5 V E = 0 V Е = 1.5 V Е = 0 V Е = 1.5 V Е = 0 V Е = 1.5 V Figure 8. Electrochromic transition cycle. Е = 0 V Е = 1.5 V Figure 8. Electrochromic transition cycle. The advantages of organic EC include compatibility with flexible substrates, low pro- Figure 8. Electrochromic transition cycle. duction cost and the possibility of adjusting their synthetic material properties. The advantages of organic EC include compatibility with flexible substrates, low pro- Figure 8. Electrochromic tran Thesadvantages ition cycle. of organic EC include compatibility with flexible substrates, low Phenylenediamine (PD) derivatives are of interest due to their stable electrochemical duction cost and the possibility of adjusting their synthetic material properties. production cost and the possibility of adjusting their synthetic material properties. reactions at the anode [52]. It is interesting to note that neutral arylamine is often colorless Phenylenediamine (PD) derivatives are of interest due to their stable electrochemical Phenylenediamine (PD) derivatives are of interest due to their stable electrochemical The advantages of organic EC include compatibility with flexible substrates, low pro- (it mostly absorbs UV light), but in redox states, it exhibits vivid color (Figure 9). Phe- reactions at the anode [52]. It is interesting to note that neutral arylamine is often colorless reactions at the anode [52]. It is interesting to note that neutral arylamine is often colorless duction cost and the possibility of adjusting their synthetic material properties. nylenediamines exhibit modulated visible absorption properties and high redox stability, (it mostly absorbs (it mostly UV ligabsorbs ht), but UV in rlight), edox st but ates, in rit edox exhibits states, vivi it exhibits d color vivid (Figucolor re 9).(Figur Phe- e 9). Phenylene- Phenylenediamine (PD) derivatives are of interest due to their stable electrochemical nylenediamines exhibit modulated visible absorption properties and high redox stability, which makes them sui diamines table f exhibit or RGB modulated devices visible [53]. absorption properties and high redox stability, which reactions at the anode [52]. It is interesting to note that neutral arylamine is often colorless which makes them suitable for RGB devices [53]. makes them suitable for RGB devices [53]. (it mostly absorbs UV light), but in redox states, it exhibits vivid color (Figure 9). Phe- nylenediamines exhibit modulated visible absorption properties and high redox stability, which makes them suitable for RGB devices [53]. Neutra Neutra l: l: Ra Radical dical cat cat ion ion : : Dicatio Dicatio n: n: Non-colored Colored Colored Non-colored Colored Colored Absorbs UV radiation High absorption in the Weak absorption in the Absorbs UV radiation High absorption in the Weak absorption in the near-infrared range near-infrared range near-infrared range near-infrared range Neutral: Radical cation: Dication: Figure 9. Redox chemistry of Phenylenediamine (Wurster’s blue), description of optical behavior in Non-colored Colored Colored Figure 9. Redox chemistry of Phenylenediamine (Wurster’s blue), description of optical behavior in Figure 9. Redox chemistry of Phenylenediamine (Wurster ’s blue), description of optical behavior in redox states. redox states. redox states. Absorbs UV radiation High absorption in the Weak absorption in the The color-changing abilities of conductive PEDOT polymers [54] are useful in elec- The color-changing abilities of conductive PEDOT polymers [54] are useful in elec- near-infrared range near-infrared range trochromic non-emissive displays (Figure 10). trochromic non-emissive displays (Figure 10). The color-changing abilities of conductive PEDOT polymers [54] are useful in elec- Figure 9. Redox chemistry of Phenylenediamine (Wurster’s blue), description of optical behavior in trochromic non-emissive displays (Figure 10). redox states. The color-changing abilities of conductive PEDOT polymers [54] are useful in elec- trochromic non-emissive displays (Figure 10). (a) (b) (c) (d) (e) Figure 10. New electrochromic compounds obtained by reactions involving the cycloaddition of Figure 10. New electrochromic compounds obtained by reactions involving the cycloaddition of nitrile oxides to 1,2-bis nitrile oxides to 1,2-bis (4-pyridinyl) ethylene derivatives (electrochromic transition): (a)—neutral (4-pyridinyl) ethylene derivatives (electrochromic transition): (a)—neutral state; (b–e)—oxidized states. (a) (b) (c) (d) (e) state; (b–e)—oxidized states. Figure 10. New electrochromic compounds obtained by reactions involving the cycloaddition of nitrile oxides to 1,2-bis (4-pyridinyl) ethylene derivatives (electrochromic transition): (a)—neutral state; (b–e)—oxidized states. (a) (b) (c) (d) (e) Figure 10. New electrochromic compounds obtained by reactions involving the cycloaddition of nitrile oxides to 1,2-bis (4-pyridinyl) ethylene derivatives (electrochromic transition): (a)—neutral state; (b–e)—oxidized states. Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 32 Nanomaterials 2021, 11, 2376 10 of 32 Heterocyclic aromatic compounds (Figure 11), such as thiophene, aniline, furan, car- N N Nano ano anom m materia ateria aterials ls ls 2021 2021 2021, , , 11 11 11, , , x FO x FO x FOR P R P R PE E EER RE ER RE ER REVIEW VIEW VIEW 10 10 10 of of of 32 32 32 Nano Nano materia materia ls 2021 ls 2021 , 11, , 11 x FO , x FO R PR P EER RE EER RE VIEW VIEW 10 10 of 32 of 32 N N N N ano N ano ano ano ano Nm m ano m m ateria m ateria ateria ateria ateria materia ls ls ls ls ls 2021 2021 2021 2021 ls 2021 2021 , , , 11 , 11 , 11 11 11 , , , , x FO , 11 x FO , x FO x FO x FO , x FO R P R P R P R P R P E R P E E E ER RE ER RE E ER RE ER RE ER RE EER RE VIEW VIEW VIEW VIEW VIEW VIEW 10 10 10 10 10 of 10 of of of of 32 32 of 32 32 32 32 bazole, azulene and indole [55,56], can be oxidized chemically or electrochemically to Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 32 Heterocyclic aromatic compounds (Figure 11), such as thiophene, aniline, furan, form anion-doped polypyrrole (PPy), polythiophene (PT) or polyaniline (PANI), poly(3- carbazole, azulene and indole [55,56], can be oxidized chemically or electrochemically to methyla Hetero n Hetero Hetero Hetero ilinecycl ) cycl cycl cycl (M ic ic ic ic EPA ar ar ar ar om om om om ), at at at at ic c po ic c ic c ic c om ly om om om (po 3 po po po -m unds unds unds unds eth (Fig yl (Fig (Fig (Fig -tu hioph u u u re re re re 11), 11), 11), 11), ene) ss s uch such uch uch as (P3MTh as as as th th th th iophen iophen iophen iophen ), ean e e e , , , , an an an an d ilil il il in po in in in e, e, e, e, ly f ura f f f(ura ura ura 3-n met n n n , , , , car- car- car- car- hylpyrrole) Heterocyclic aromatic compounds (Figure 11), such as thiophene, aniline, furan, car- form anion-doped polypyrrole (PPy), polythiophene (PT) or polyaniline (PANI), poly(3- Hetero Hetero Hetero Hetero Hetero Hetero cycl cycl cycl cycl cycl cycl ic ic ic ic ic ar ar ic ar ar ar om om om ar om om om at at at at at ic c ic c ic c at ic c ic c ic c om om om om om om po po po po po unds unds po unds unds unds unds (Fig (Fig (Fig (Fig (Fig (Fig u u u u re u re re re re u11), 11), re 11), 11), 11), 11), s ss uch s uch s uch uch uch such as as as as as th th as th th th iophen iophen th iophen iophen iophen iophen e ee e , , e , , an an , e an an an , il il an il il in il in in in in il e, e, e, in e, e, f f e, ura f ura ff ura ura ura fura n n n n , n , , , car- car- , n car- car- car- , car- baz baz baz baz oo o o le, le, le, le, az az az az ul ul ul ul ee e e ne ne ne ne an an an an d d d d indo indo indo indo le l lle e e [55, [ [ [55, 55, 55, 56] 56] 56] 56] , ,,, can can can can b b b b e e e e ox ox ox ox idized idized idized idized chem chem chem chem ically ically ically ically or or or or electroch electroch electroch electroch emi emi emi emi cally cally cally cally to to to to bazole, azulene and indole [55,56], can be oxidized chemically or electrochemically to (P3MPy). A change in the redox state (oxidized conductive state, reduced non-conductive methylaniline) (MEPA), poly(3-methyl-thiophene) (P3MTh), and poly(3-methylpyrrole) bazoHetero le, azul cycl ene ic an arom d indo atic c le om [55, po56] unds , can (Fig b ue re ox 11), idized such chem as thiophen ically e or , an electroch iline, fura emi n, car- cally to baz baz baz baz baz o o o o le, le, o le, le, le, az az az az az ul ul ul ul ul e ee e ne ne e ne ne ne an an an an an d d d d d indo indo indo indo indo lle l e ll e e e [ [55, [ 55, [[ 55, 55, 55, 56] 56] 56] 56] 56] ,, ,, ,can can can can can b b b b e e b e e e ox ox ox ox ox idized idized idized idized idized chem chem chem chem chem ically ically ically ically ically or or or or or electroch electroch electroch electroch electroch emi emi emi emi emi cally cally cally cally cally to to to to to form form form form an an an an ion ion ion ion -d - - -d d d op op op op ed ed ed ed po po po po ly ly ly ly pyr pyr pyr pyr role role role role (PPy ( ( (PPy PPy PPy ),), ), ), po po po po ly ly ly ly th th th th iophen iophen iophen iophen e e e e (PT) ( ( (PT) PT) PT) or or or or po po po po lyan l llyan yan yan ilil il il in in in in e e e e (PANI ( ( (PANI PANI PANI ), ) ) ), , , po po po po ly ly ly ly (3 ( ( (3 3 3 -- - - form anion-doped polypyrrole (PPy), polythiophene (PT) or polyaniline (PANI), poly(3- (P3MPy). A change in the redox state (oxidized conductive state, reduced non-conductive state, n baz eutr ole, al azstate) ulene an led ads indo to le chan [55,56] ges , can in bcolor e oxidized caused chem by ically sigor ni fi electroch cant chan emically ges in to the visible form form anion anion -dop -ded oped poly po pyr lypyr role role (PPy (PPy ), po ), ly po th ly iophen thiophen e (PT) e (PT) or po or lpo yan lyan ilinil e in (PANI e (PANI ), po ), ly po (3 ly - (3- form form form form an an an an ion ion ion ion -- d -- d d d op op op op ed ed ed ed po po po po ly ly ly ly pyr pyr pyr pyr role role role role (( PPy (( PPy PPy PPy ), ), ), ), po po po po ly ly ly ly th th th th iophen iophen iophen iophen e e e e (( PT) (( PT) PT) PT) or or or or po po po po ll yan ll yan yan yan il il il il in in in in e e e e (( PANI (( PANI PANI PANI )) , )) , , po , po po po ly ly ly ly (( 3 (( 3 3 - 3 - -- methylaniline) (MEPA), poly(3-methyl-thiophene) (P3MTh), and poly(3-methylpyrrole) met met met hyla hyla hyla nn n iline iline iline ) ) ) (M (M (M EPA EPA EPA ),), ), po po po ly ly ly (3 ( (3 3 -m - -m m eth eth eth yl yl yl -t - -hioph t thioph hioph ene) ene) ene) (P3MTh (P3MTh (P3MTh ), ) ), , an an an d d d po po po ly ly ly (3 ( (3 3 -met - -met met hy hy hy lpyrrol lpyrrol lpyrrol e) e) e) methylaniline) (MEPA), poly(3-methyl-thiophene) (P3MTh), and poly(3-methylpyrrole) state, neutral state) leads to changes in color caused by significant changes in the visible and form anion-doped polypyrrole (PPy), polythiophene (PT) or polyaniline (PANI), poly(3- met met hyla hyla niline niline ) (M ) (M EPA EPA ), po ), ly po (3 ly -m (3eth -meth yl-t yl hioph -thioph ene) ene) (P3MTh (P3MTh ), an ), d an po d ly po (3 ly -met (3-met hylpyrrol hylpyrrol e) e) andmet met met nea met hyla hyla hyla hyla r-inf n n n n iline iline r iline iline ared ) ) ) ) (M (M (M (M absorpt EPA EPA EPA EPA ), ), ), ), po po po po ion ly ly ly ly (( 3 ((spect 3 3 - 3 - m -- m m m eth eth eth eth r yl yl a yl yl -- t - th - t hioph tt hioph hioph hioph at vene) ar ene) ene) ene) y depe (P3MTh (P3MTh (P3MTh (P3MTh nding )) , )) , , , an an an an d d on d d po po po po th ly ly ly ly (e ( 3 (( 3 3 - 3 degree - met -- met met met hy hy hy hy lpyrrol lpyrrol lpyrrol lpyrrol of ox e) e) e) ie) dation/re- (P3MP (P3MPy) y). . A A change change in in th the e r redox edox state state (ox (oxid idiize zed d con cond du uct ctive ive state, state, re reduced duced non non- -c co onduct nductive ive (P3MP (P3MP (P3MP y) y) . y) . A A . change A change change in in th in th e th e redox r e edox redox state state state (ox (ox (ox id id iid ze ize id ze d con d con con dd uu d ct ct u ive ct ive ive state, state, state, re re duced re duced duced non non non -c -c o- o nduct cnduct onduct ive ive ive near-infrared absorption spectra that vary depending on the degree of oxidation/reduction methylaniline) (MEPA), poly(3-methyl-thiophene) (P3MTh), and poly(3-methylpyrrole) (P3MP (P3MP (P3MP y) y) . . y) A A . change A change change in in th in th e th e rr edox e edox redox state state state (ox (ox (ox id id ize id ize i d ze d con con d con d d uu ct d ct u ive ive ctive state, state, state, re re duced duced reduced non non non -c -c oo - nduct c nduct onduct ive ive ive (P3MP (P3MP (P3MP y) y) y) . . A . A A change change change in in in th th th e e e rr edox redox edox state state state (ox (ox (ox id id id ize iize ze d d d con con con dd d uu u ct ct ct ive ive ive state, state, state, re re re duced duced duced non non non -c --c o co nduct onduct nduct ive ive ive duction Switching between polymer films in their colored (reduced) and uncolored (oxi- state, state, state, n n neutr eutr eutral al al state) state) state) le le leads ads ads t t to o o chan chan changes ges ges in in in color color color c c caused aused aused by by by s s sig ig igni ni nifi fi fic c cant ant ant chan chan changes ges ges in in in th th the e e vis vis visibl ibl ible e e state, state, neutr neutr al state) al state) leads leads to t chan o chan ges ges in in color color caused caused by by sigs ni ig fi ni cant ficant chan chan ges ges in in th e th vis e vis iblibl e e Switching between polymer films in their colored (reduced) and uncolored (oxidized) (P3MPy). A change in the redox state (oxidized conductive state, reduced non-conductive state, state, state, state, state, state, n n n n eutr n eutr eutr eutr eutr neutr al al al al al state) state) al state) state) state) state) le le le le le ads ads ads ads le ads ads t to t o tt o o o chan chan tchan o chan chan chan ges ges ges ges ges ges in in in in in color color in color color color color c cc c aused aused c aused aused aused caused by by by by by by s ss ig s ig s ig ig ig s ni ni ni ig ni ni fi fi fi ni fi fi c cc c ant ant c fi ant ant ant cant chan chan chan chan chan chan ges ges ges ges ges ges in in in in in th th in th th th e e th e e e vis vis vis vis e vis vis ibl ibl ibl ibl ibl e e ibl e e e e dized) states changes their color from yellow to orange, red, purpuric, dark blue, green, and and and and nea nea nea nea rr r - rinf - - -inf inf inf rar r r rar ar ar ed ed ed ed absorpt absorpt absorpt absorpt ion i iion on on spect spect spect spect ra r r ra a a th th th th at at at at vv v v ar ar ar ar yy y y depe depe depe depe nding nding nding nding on on on on th th th th e e e e degree degree degree degree of of of of ox ox ox ox idation/re- i iidation/re- dation/re- dation/re- states and nea changes r-infrtheir ared color absorpt from ion yellow spectra toth orange, at varyr ed, depe purpuric, nding on dark the blue, degree gr een, of ox light idation/re- blue, state, neutral state) leads to changes in color caused by significant changes in the visible and near-infrared absorption spectra that vary depending on the degree of oxidation/re- and and and and and nea nea nea nea nea r rr - r -r inf - inf -- inf inf inf r rr ar r ar r ar ar ar ed ed ed ed ed absorpt absorpt absorpt absorpt absorpt iion i on ii on on on spect spect spect spect spect r rr a r a r a a a th th th th th at at at at at v v v v ar ar v ar ar ar y y y y y depe depe depe depe depe nding nding nding nding nding on on on on on th th th th th e e e e e degree degree degree degree degree of of of of of ox ox ox ox ox iidation/re- i dation/re- ii dation/re- dation/re- dation/re- light b duction duction duction duction lue, an Sw Sw Sw Sw d bl itching itching itching itching ack bet bet bet bet [57 ween ween ween ween ]. po po po po ly ly ly ly m m m m er er er er films films films films in in in in th th th th ei ei ei ei r r r r co co co co lo lo lo lo red red red red (red (red (red (red uced uced uced uced ) ) ) )and and and and uncolore uncolore uncolore uncolore d d d d (ox (ox (ox (ox i-i- i- i- and duction and black nearSw - [inf 57itching ]. rared absorpt between ion po spect lym ra erth films at var in y th depe eir nding colored on (red the uced degree ) and of ox uncolore idation/re- d (oxi- duction duction SwSw itching itching betbet ween ween po ly po m ly er m films er films in th inei th r ei co r lo co red lored (red (red uced uced ) and ) and uncolore uncolore d (ox d (ox i- i- duction duction duction duction Sw Sw Sw Sw itching itching itching itching bet bet bet bet ween ween ween ween po po po po ly ly ly ly m m m m er er er er films films films films in in in in th th th th ei ei ei ei r r r r co co co co lo lo lo lo red red red red (red (red (red (red uced uced uced uced )) )) and and and and uncolore uncolore uncolore uncolore d d d d (ox (ox (ox (ox i- i- i- i- dized) states changes their color from yellow to orange, red, purpuric, dark blue, green, dized) dized) dized) duction states states states Swchange itching change change s bet s s th th th ween eir eir eir color color color poly fm rom f from rom er films y y y ell ell ell ow ow ow in th to to to ei or r or or an co an an lo ge, ge, ge, red red red red (red , , , purp purp purp uced uri ) uri uri and cc c , , , da da da uncolore rk rk rk bl bl bl uu u e, d e, e, green (ox green green i- , , , dized) states changes their color from yellow to orange, red, purpuric, dark blue, green, dized) dized) states states change change s th s eir th eir color color from from yell yow ellow to or toan orge, ange, redred , purp , purp uriuri c, da c, rk dabl rk ubl e, ugreen e, green , , dized) dized) dized) dized) states states states states change change change change s s s s th th th th eir eir eir eir color color color color ff rom ff rom rom rom y y y ell y ell ell ell ow ow ow ow to to to to or or or or an an an an ge, ge, ge, ge, red red red red , , , purp , purp purp purp uri uri uri uri cc c , c , , da , da da da rk rk rk rk bl bl bl bl u u u u e, e, e, e, green green green green , , , , light blue, and black [57]. lili li ght b ght b ght b dized) lu lu lu ee e , an states , an , an d bl d bl d bl change ack ack ack [57 [57 [57 s ] th .] ] ..eir color from yellow to orange, red, purpuric, dark blue, green, light blue, and black [57]. light b light b luelu , an e, an d bl d bl ack ack [57[57 ]. ]. li li li li ght b ght b ght b ght b lu lu lu lu ee e , an e , an , an , an d bl d bl d bl d bl ack ack ack ack [57 [57 [57 [57 ]] .]] .. . light blue, and black [57]. Pyrrole Thiophene Aniline Furan Carbazole Azolene Indole Py Py Py Py rr rr rr rr ole ole ole ole Thioph Thioph Thioph Thioph en en en en ee e e Anili Anili Anili Anili ne ne ne ne Furan Furan Furan Furan Ca Ca Ca Ca rbazo rbazo rbazo rbazo le le le le Azolen Azolen Azolen Azolen ee e e In In In In dole dole dole dole Pyrrole Thiophene Aniline Furan Carbazole Azolene Indole Pyrrole Thiophene Aniline Furan Carbazole Azolene Indole Py Py Py Py Py rr rr rr rr rr ole ole ole ole ole Thioph Thioph Thioph Thioph Thioph en en en en en e ee e e Anili Anili Anili Anili Anili ne ne ne ne ne Furan Furan Furan Furan Furan Ca Ca Ca Ca Ca rbazo rbazo rbazo rbazo rbazo le le le le le Azolen Azolen Azolen Azolen Azolen e ee e e In In In In In dole dole dole dole dole Pyrrole Thiophene Aniline Furan Carbazole Azolene Indole Figure Figure 11. 11. Mol Molecules ecules of ofheter heterocyclic ocyclic ararom omatic ati compounds. c compounds. Figure Figure Figure 11. 11. 11. Mol Mol Molec ec ecules ules ules of of of heter heter heteroc oc ocyclic yclic yclic arom arom aromati ati atic c c co co com m mpou pou pounds nds nds... Figure Figure 11. 11. Mol Mol ecules ecules of heter of heter ocyclic ocyclic arom arom atic ati co c m co pou mpou nds nds . . Figure Figure Figure Figure Figure 11. 11. 11. 11. 11. Mol Mol Mol Mol Mol ec ec ec ec ules ules ec ules ules ules of of of of heter of heter heter heter heter oc oc oc oc yclic yclic oc yclic yclic yclic arom arom arom arom arom ati ati ati ati c ati c c c co co co co c m m co m m pou pou pou m pou pou nds nds nds nds nds . .. . . Figure Figure 11.11. Mol Mol ecules ecules of of heter heter oc oc yclic yclic arom arom ati ati c c co com mpou pounds nds.. Table 4 shows conductive polymers obtained by the oxidation of monomeric aromatic Table 4 shows conductive polymers obtained by the oxidation of monomeric aro- Table Table Table Table 4 4 4 4 show show show show s s s s con con con con dd d d uctive uctive uctive uctive po po po po ly ly ly ly mers mers mers mers ob ob ob ob taine taine taine taine d d d d by by by by th th th th e e e e oxida oxida oxida oxida tion t t tion ion ion oo o o f f f f mo mo mo mo nom nom nom nom eric eric eric eric aro aro aro aro -- - - Table 4 shows conductive polymers obtained by the oxidation of monomeric aro- compounds Table Table Table 4 show 4 (neutral 4 show show s con s s and con con ductive doxidized d uctive uctive po po ly po states). ly mers ly mers mers ob ob taine ob taine taine d d by by d by th th e e th oxida oxida e oxida ttion ion tion o o f f mo mo of nom mo nom nom eric eric eric aro aro - aro - - Table Table Table Table 4 4 4 4 show show show show s s s s con con con con d d d d uctive uctive uctive uctive po po po po ly ly ly ly mers mers mers mers ob ob ob ob taine taine taine taine d d d d by by by by th th th th e e e e oxida oxida oxida oxida tt ion tt ion ion ion o o o f o f f f mo mo mo mo nom nom nom nom eric eric eric eric aro aro aro aro -- -- matic compounds (neutral and oxidized states). matic com matic com matic com po po po unds (ne unds (ne unds (ne utra utra utra l a l l a a nd nd nd oxid oxid oxid ized ized ized states states states ).). ). matic com matic com pounds (ne pounds (ne utra utra l a l a nd nd oxid oxidized ized states states ). ). matic compounds (neutral and oxidized states). matic com matic com pounds (ne pounds (ne utra utra l and l a oxid nd oxid ized ized states states ). ). matic com matic com matic com matic com po po po po unds (ne unds (ne unds (ne unds (ne utra utra utra utra ll a ll a a a nd nd nd nd oxid oxid oxid oxid ized ized ized ized states states states states ). ). ). ). Table 4. Conductive polymers obtained by the oxidation of monomeric aromatic compounds (neutral Table Table 4 4.. Con Cond ductiv uctive e po polym lymers ers obta obtained ined by t by the oxid he oxidati ation on of of m monom onomeric eric ar aromatic omatic co com mpou pounds nds (neu (neu- - Table Table Table 44 . .Con 4 Con . Con dd uctiv uctiv ductiv e e po po e lym po lym lym ers ers ers obta obta obta ined ined ined by t by t by t he oxid he oxid he oxid ati ati on ati on on of of m of m onom m onom onom eric eric eric ar ar omatic ar omatic omatic co co m co m pou m pou pou nds nds nds (neu (neu (neu - - - Table 4. Conductive polymers obtained by the oxidation of monomeric aromatic compounds (neu- Table Table Table and Table 4 Table . Con oxidized 44 .4 .Con .Con 4 Con d . Con uctiv dd uctiv d states). uctiv uctiv ductiv e e po e e po po e po lym lym po lym lym lym ers ers ers ers ers obta obta obta obta obta ined ined ined ined ined by t by t by t by t by t he oxid he oxid he oxid he oxid he oxid ati ati ati on ati on on ati of on of of on m of m m onom onom of m onom onom m eric onom eric eric eric ar ar ar omatic omatic ar eric omatic omatic ar co co omatic co m m co m pou pou m pou pou nds nds nds co nds (neu m (neu (neu pou (neu - -- nds - (neu- Table Table 44 . .Con Con dd uctiv uctiv e e po po lym lym ers ers obta obta ined ined by t by t he oxid he oxid ati ati on on of of m m onom onom eric eric ar ar omatic omatic co co m m pou pou nds nds (neu (neu - - tral tral tral tral and and and and oxi oxi oxi oxi di di di di zz z ed zed ed ed s tates s s states tates tates ).) ) ) ... tral and oxidized states). tral and oxidized states). tral and oxidized states). tral tral tral tral tral and and and and and oxi oxi oxi oxi oxi di di di di di z zz z ed ed z ed ed ed s s s tates s tates s tates tates tates ) ).) .) ) .. . tral and oxidized states). Organic EC Organic E Organic E Organic E Organic E C C C C Organic EC Organic EC Organic EC Organic E Organic E Organic E Organic E Organic E C C C C C State PANI P3MPy MEPA P3MT PPY PT Organic EC Stat Stat Stat Stat ee e e PANI PANI PANI PANI P3 P3 P3 P3 M M M M Py Py Py Py MEPA MEPA MEPA MEPA P3 P3 P3 P3 MT MT MT MT PPY PPY PPY PPY PT PT PT PT Stat State e PANI PANI P3 P3 M M Py Py MEPA MEPA P3P3 MT MT PPY PPY PT PT State PANI P3MPy MEPA P3MT PPY PT Stat Stat Stat Stat Stat e ee e e PANI PANI PANI PANI PANI P3 P3 P3 P3 P3 M M M M M Py Py Py Py Py MEPA MEPA MEPA MEPA MEPA P3 P3 P3 P3 P3 MT MT MT MT MT PPY PPY PPY PPY PPY PT PT PT PT PT State PANI P3MPy MEPA P3MT PPY PT Neutral Neu Neu Neu Neutral Neu tr tr tr al al al tral Neutral Neu Neu tral tr al Neu Neu Neu Neu tr tr tr tr al al al al Neutral Oxidized Oxidi Oxidi Oxidi Oxidi ze ze ze ze dd d d Oxidized Oxidi Oxidi Oxidi Oxidi Oxidi Oxidized Oxidi ze ze ze ze ze d d ze d d d d Oxidized The shortcomings of polymer films include their low electrochemical stability and, Th Th Th Th e e e e short short short short co co co co mings mings mings mings o o o o f f f f po po po po lymer l llymer ymer ymer fil f f fil il il m m m m s s s s in in in in clu clu clu clu de de de de th th th th eir eir eir eir lo lo lo lo w w w w electroch electroch electroch electroch ee e e mica mica mica mica l l l l st st st st abi abi abi abi lity l llity ity ity and and and and , , , , The shortcomings of polymer films include their low electrochemical stability and, The Th short e short comings comings of po of lpo ymer lymer film fil s m in s clu inclu de de their their low loelectroch w electroch emica emica l stl abi stabi lity lity and and , , Th Th Th Th e e e e short short short short co co co co mings mings mings mings o o o f o f f f po po po po ll ymer ll ymer ymer ymer ff il ff il il il m m m m s s s s in in in in clu clu clu clu de de de de th th th th eir eir eir eir lo lo lo lo w w w w electroch electroch electroch electroch ee e mica e mica mica mica l l l l st st st st abi abi abi abi ll ity ll ity ity ity and and and and , , , , consequently, their low oxidation number [28–30]. The addition of inorganic materials The shortcomings of polymer films include their low electrochemical stability and, consequently, their low oxidation number [28–30]. The addition of inorganic materials con con con sequent sequent sequent ly, ly, ly, th th th eir eir eir low low low ox ox ox ida ida ida tion t tion ion numb numb numb er er er [28 [ [28 28 –– – 30] 30] 30] . . . TT T he he he add add add ition ition ition oo o f f f inor inor inor ga ga ga nn n ic ic ic materi materi materi als als als consequently, their low oxidation number [28–30]. The addition of inorganic materials con con sequent sequent ly, ly, th eir their low low oxida oxida tion tion numb numb er [ er 28[ –28 30] –30] . The . The add add ition ition of inor of inor gan ga ic nmateri ic materi als als con con con con sequent sequent sequent sequent ly, ly, ly, ly, th th th th eir eir eir eir low low low low ox ox ox ox ida ida ida ida tt ion tt ion ion ion numb numb numb numb er er er er [[ 28 [[ 28 28 28 –– – 30] – 30] 30] 30] . . . . T T T T he he he he add add add add ition ition ition ition o o o f o f f f inor inor inor inor ga ga ga ga n n n n ic ic ic ic materi materi materi materi als als als als improves the properties of electrochromic conductive polymers [58]. Inorganic materials consequently, their low oxidation number [28–30]. The addition of inorganic materials impro improves ves th the e pro propert perties ies o of f electroch electrochrom romiic c con conductive ductive po poly lymers mers [ [58] 58]. . Ino Inorg rgani anic c m material aterials s impro impro impro ves ves ves th th eth e pro pro e pro pert pert pert ies ies ies o o f f o electroch electroch f electroch rom rom rom ic ic con icon c con ductive ductive ductive po po po ly ly mers ly mers mers [ 58] [58] [58] . . Ino Ino . Ino rg rg ani rg ani ani cc m m c aterial m aterial aterial s s s impro impro Th impro e ves short ves ves th th eco th e pro pro emings pro pert pert pert ies ies o ies o f o f f po electroch o electroch f l electroch ymer rom rom fil rom m ic ic s con icon c in con ductive clu ductive ductive de th po po eir ly po ly mers mers ly lo mers w [ 58] [electroch 58] [58] . . Ino Ino . Ino rg rg ani rg ani eani mica cc m m caterial aterial m l aterial stabi s s l s ity and, impro impro impro ves ves ves th th th ee e pro pro pro pert pert pert ies ies ies o o f of f electroch electroch electroch rom rom rom ic iic c con con con ductive ductive ductive po po po ly ly ly mers mers mers [ 58] [[58] 58] . . Ino . Ino Ino rg rg rg ani ani ani cc c m m m aterial aterial aterial s s s improve the staining efficiency and reduce the switching time, but do not affect the elec- improves the properties of electrochromic conductive polymers [58]. Inorganic materi- impro impro improve ve ve th th the e e stai stai stainin nin ning g g ef ef effi fi fic c cie ie iency ncy ncy and and and re re redu du duce ce ce th th the e e switchi switchi switching ng ng time, time, time, but but but do do do not not not af af aff f fect ect ect th th the e e elec- elec- elec- impro impro ve ve the th stai e stai nin nin g ef g fi ef cie fic ncy iency and and redu redu ce ce the th switchi e switchi ng ng time, time, but but do do not not aff af ect fect th e th elec- e elec- impro impro impro impro impro ve ve ve ve ve th th th th e th e e e stai stai stai e stai stai nin nin nin nin nin g g g g ef ef g ef ef fi fi fi ef fi cc c ie c fi ie ie ie ncy cncy ncy ie ncy ncy and and and and and re re re re du du re du du ce du ce ce ce th ce th th th e th e e e switchi switchi switchi e switchi switchi ng ng ng ng ng time, time, time, time, time, but but but but but do do do do do not not not not not af af af af f ect f af ff ect ect ect f ect th th th th e th e e e elec- elec- elec- e elec- elec- impro trochem ve thical e stai prop nin erties g effi of cie th ncy e po and lyme rer; du th ce erefore the switchi , the pro ng blem time, of but impdo rovnot ing po afflect ymer the elec- elec- consequently, their low oxidation number [28–30]. The addition of inorganic materials als improve the staining efficiency and reduce the switching time, but do not affect the tro tro tro tro chem chem chem chem ical ical ical ical pp p p rop rop rop rop erties erties erties erties of of of of th th th th e e e e po po po po ly ly ly ly me me me me r; r; r; r; th t t th h h erefore erefore erefore erefore , , , , th th th th ee e e pro pro pro pro blem blem blem blem of of of of i mp i iimp mp mp rov rov rov rov ing ing ing ing po po po po lymer l llymer ymer ymer ee e e lec- lec- lec- lec- trochemical properties of the polymer; therefore, the problem of improving polymer elec- tro tro tro tro tro tro chem tro chem chem chem chem chem chem ical ical ical ical ical ical ical p p p p rop p rop rop rop s rop p tabilit rop erties erties erties erties erties erties y is of of of of of sti th th of th th th ll e e th e e e re po po po po e po leva ly ly po ly ly ly me me me ly me nt me me r; .r; r; r; E r; t tle h t r; h tt h h erefore h erefore ct t erefore erefore erefore h rochro erefore , , , , mic th th , th th th , e eth e e e pro pro f pro pro il e pro ms pro blem blem blem blem blem , such blem of of of of of as ii of mp i mp ii mp mp mp WO imp rov rov rov rov rov 3rov , ing ing Nb ing ing ing ing 2po po O po po po 5ll ,po ymer l ymer lNi l ymer ymer ymer lymer O, e ar ee e lec- lec- e lec- e lec- lec- elec- impro electr ves ochemical the propert properties ies of electroch of the polymer; romic ther con efor ductive e, the pr po oblem lymers of impr [58] oving . Inopolymer rganic materials tro tro tro tro chem chem chem chem ical ical ical ical ss s tabilit stabilit tabilit tabilit y y y y is is is is sti sti sti sti ll ll ll ll re re re re leva leva leva leva nt nt nt nt . ...E E E E le le le le ct ct ct ct rochro rochro rochro rochro mic mic mic mic fil f f fil il il ms ms ms ms , , , , such such such such as as as as WO WO WO WO 3, 3 3 3, , , Nb Nb Nb Nb 2O 2 2 2O O O 5, 5 5 5 ,,,Ni Ni Ni Ni O, O, O, O, ar ar ar ar e e e e trochemical stability is still relevant. Electrochromic films, such as WO3, Nb2O5, NiO, are trochemical stability is still relevant. Electrochromic films, such as WO3, Nb2O5, NiO, are tro tro tro tro tro chem prefer chem chem chem chem ical ical abl ical ical ical e s ss du tabilit s tabilit s tabilit tabilit tabilit e to t y y y y heir h y is is is is is sti sti sti sti sti ll ll igh ll ll ll re re re re re stability leva leva leva leva leva nt nt nt nt nt .. ..E E . a E E le E le n le le le ct d durabi ct ct ct ct rochro rochro rochro rochro rochro li mic mic mic mic ty mic . f fil f il ff il il ms il ms ms ms ms , , , , such such , such such such as as as as as WO WO WO WO WO 3 3, 3 , 33 , , Nb Nb , Nb Nb Nb 2 2O 2 O 22 O O O 5 5, 5 , 5 5 ,,Ni Ni , Ni Ni Ni O, O, O, O, O, ar ar ar ar ar e e e e e electrochemical stability is still relevant. Electrochromic films, such as WO , Nb O , NiO, impro prefer ve th abl e e stai due to t ning heir h effic igh iency stability and are nd durabi duce th lie tyswitchi . ng time, but 3 do not 2 5affect the elec- prefer prefer prefer abl abl abl e e e du du du e to t e to t e to t heir h heir h heir h igh igh igh stability stability stability a a a nn n d durabi d durabi d durabi lili li ty ty ty . .. preferable due to their high stability and durability. prefer prefer ablabl e du e e to t due to t heir h heir h ighigh stability stability an a d durabi nd durabi lityli . ty. prefer prefer prefer prefer abl abl abl abl e e e e du du du du e to t e to t e to t e to t heir h heir h heir h heir h igh igh igh igh stability stability stability stability a a a a n n n n d durabi d durabi d durabi d durabi li li li li ty ty ty ty . .. . are preferable due to their high stability and durability. trochemical properties of the polymer; therefore, the problem of improving polymer elec- 3.3. Transition Metal Oxides 3.3. Transition Metal Oxides 3.3. 3.3. 3.3. TT T ransitio ransitio ransitio nn n M M M etal O e etal O tal O xid xid xid es es es 3.3. Transition Metal Oxides trochemical stability is still relevant. Electrochromic films, such as WO3, Nb2O5, NiO, are 3.3. 3.3. Transitio Transitio n M ne M tal O etal O xidxid es es 3.3. 3.3. 3.3. 3.3. T T T T ransitio ransitio ransitio ransitio Inorga n n n M n n M M ic M ee ma e tal O e tal O tal O tal O terials xid xid xid xid es es es es incl ude a large group of EC, mostly the TMO МеxOy (Figure 12). 3.3. Transition Metal Oxides Ino Inorga rgan nic ic ma materials terials incl includ ude a e a larg large g e grou roup of EC, most p of EC, mostly ly the TMO the TMO Ме Меx xO Oy y (Fig (Figur ure 12). e 12). Ino Ino Ino rga rga rga nn ic ic nma ic ma ma terials terials terials incl incl incl ud ud ud e a e a e a larg larg larg e g e g e g rou rou rou p of EC, most p of EC, most p of EC, most ly ly the TMO ly the TMO the TMO Ме Ме Ме xO xO y x O y(Fig (Fig y (Fig ur ur e 12). ur e 12). e 12). The most common TMO [32,59], such as molybdenum (VI) oxide, vanadium (V) oxide, preferable due to their high stability and durability. Ino Ino Ino Ino rga rga rga rga nn ic n ic ic n ma ic ma ma ma terials terials terials terials incl incl incl incl ud ud ud e a ud e a e a e a larg larg larg larg e g e g e g e g rou rou rou rou p of EC, most p of EC, most p of EC, most p of EC, most ly ly ly the TMO the TMO ly the TMO the TMO Ме Ме Ме Ме xO xx O O y x yO (Fig y (Fig (Fig y (Fig ur ur ur e 12). ur e 12). e 12). e 12). Ino Ino Inor rga rga ganic nn ic ic ma ma materials terials terials incl incl include ud ud e a e a a larg larg large e g e g gr rou rou oup p of EC, most p of EC, most of EC, mostly ly ly the TMO the TMO the TMO Ме Me Ме xO xO O y y(Fig (Fig (Figur ur ur e 12). e 12). e 12). x y Th Th Th Th e e e e mo mo mo mo st st st st co co co co mm mm mm mm on on on on TMO TMO TMO TMO [32,5 [32,5 [32,5 [32,5 9] 9] 9] 9] , ,,,such such such such as as as as mo mo mo mo lybden l llybden ybden ybden um um um um (V (V (V (V I) I I I) ) ) ox ox ox ox id id id id e, e, e, e, vv v v anadi anadi anadi anadi um um um um (V) (V) (V) (V) ox ox ox ox ide ide ide ide , ,,, The most common TMO [32,59], such as molybdenum (VI) oxide, vanadium (V) oxide, niobium (V) oxide, iridium (III) oxide, tungsten (VI) oxide, are in the form of an octahe- Th Th Th Th Th e Th e e e e mo mo mo mo e mo mo st st st st st co co st co co co mm mm co mm mm mm mm on on on on on on TMO TMO TMO TMO TMO TMO [32,5 [32,5 [32,5 [32,5 [32,5 [32,5 9] 9] 9] 9] 9] ,, ,9] ,such ,such such such such , such as as as as as as mo mo mo mo mo mo llybden l ybden ll ybden ybden ybden lybden um um um um um um (V (V (V (V (V I I(V ) I ) II ) ) ) ox ox Iox ox ) ox id id ox id id id e, e, e, id e, e, v v e, v v anadi anadi v anadi anadi anadi vanadi um um um um um um (V) (V) (V) (V) (V) (V) ox ox ox ox ox ide ide ox ide ide ide ide ,, ,, , , The most common TMO [32,59], such as molybdenum (VI) oxide, vanadium (V) oxide, niobium niobium niobium niobium ( V) ( ( (V) V) V) oxide, oxide, oxide, oxide, ir ir ir ir idiu idiu idiu idiu m m m m (III) (III) (III) (III) ox ox ox ox ide, ide, ide, ide, tungsten tungsten tungsten tungsten (V (V (V (V I) I I I) ) ) oxi oxi oxi oxi de, de, de, de, ar ar ar ar ee e e i n i iin n n th th th th e e e e form form form form oo o o f f f fan an an an oct oct oct oct ahe- ahe- ahe- ahe- niobium (V) oxide, iridium (III) oxide, tungsten (VI) oxide, are in the form of an octahe- dron MeO6 (Figure 13). The crystal structure of СWO3 perovskite shown in Figure 14. niobium (V) oxide, iridium (III) oxide, tungsten (VI) oxide, are in the form of an octahe- niobium niobium niobium niobium niobium ( ( V) ( V) (( V) V) V) oxide, oxide, oxide, oxide, oxide, ir ir ir ir ir idiu idiu idiu idiu idiu m m m m m (III) (III) (III) (III) (III) ox ox ox ox ox ide, ide, ide, ide, ide, tungsten tungsten tungsten tungsten tungsten (V (V (V (V (V I I) I ) II ) ) ) oxi oxi oxi oxi oxi de, de, de, de, de, ar ar ar ar ar e ee e e i i n i n ii n n n th th th th th e e e e e form form form form form o o o o f f o f ff an an an an an oct oct oct oct oct ahe- ahe- ahe- ahe- ahe- niobium (V) oxide, iridium (III) oxide, tungsten (VI) oxide, are in the form of an octahedron 3.3. Transition Metal Oxides dron MeO dron MeO dron MeO dron MeO 6 6 6 6(Figur (Figur (Figur (Figur e e e e 13). 13). 13). 13). Th Th Th Th ee e e cryst cryst cryst cryst al al al al struct struct struct struct ure ure ure ure oo o o f f f f СWO СWO СWO СWO 3 3 3 3pero pero pero pero vskite vskite vskite vskite sh sh sh sh own own own own i i i i n n n n Fi Fi Fi Fi gg g g ur ur ur ur e 14. e 14. e 14. e 14. dron MeO6 (Figure 13). The crystal structure of СWO3 perovskite shown in Figure 14. dron MeO dron MeO 6 (Figur 6 (Figur e 13). e 13). Th e Th cryst e cryst al struct al struct ure ure of СWO of СWO 3 pero 3 pero vskite vskite shown shown in i Fi n gFi ur g e 14. ure 14. dron MeO dron MeO dron MeO MeO dron MeO (Figur 66 6(Figur 6 (Figur (Figur (Figur e 13). e e e e 13). The 13). 13). 13). Th crystal Th Th Th ee e cryst e cryst cryst cryst str al al al uctur al struct struct struct struct e of ure ure ure ure CWO o o o f o f f f СWO СWO СWO СWO per 3ovskite 3 3pero 3 pero pero pero vskite vskite vskite vskite shown sh sh sh sh own own own in own Figur i i i n i n n n Fi Fi Fi Fi eg g g 14 ur g ur ur ur .e 14. e 14. e 14. e 14. 6 3 Inorganic materials include a large group of EC, mostly the TMO МеxOy (Figure 12). The most common TMO [32,59], such as molybdenum (VI) oxide, vanadium (V) oxide, niobium (V) oxide, iridium (III) oxide, tungsten (VI) oxide, are in the form of an octahe- dron MeO6 (Figure 13). The crystal structure of СWO3 perovskite shown in Figure 14. Nanomaterials 2021, 11, x FOR PEER REVIEW 11 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 11 of 32 Nanomaterials 2021, 11, 2376 11 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 11 of 32 Figure 12. Electrochromic transition metal oxides. *—lantanoids. Figure 12. Electrochromic transition metal oxides. *—lantanoids. In the mentioned structures, electrochromic effects occur due to electron–ion separa- In the mentioned structures, electrochromic effects occur due to electron–ion separa- tion. As a result, metal atoms are introduced into TMO, and the valence electrons move tion. As a result, metal atoms are introduced into TMO, and the valence electrons move to the d-levels of the transition metal ion, reducing it. Evidently, the injected ions should to the d-levels of the transition metal ion, reducing it. Evidently, the injected ions should possess Figure a hig12. h di El ffusi ectrochrom on coefficient ic transitio and n metal a hig oxides h sol.ubility i *—lantanoid n the l s. attice of TMO [18,20]. Figure 12. Electrochromic transition metal oxides. *—lantanoids. possess a high diffusion coefficient and a high solubility in the lattice of TMO [18,20]. In the mentioned structures, electrochromic effects occur due to electron–ion separa- tion. As a result, metal atoms are introduced into TMO, and the valence electrons move - anion; - anion; to the d-levels of the transition metal ion, reducing it. Evidently, the injected ions should possess a high diffusion coefficient and a high solubility in the lattice of TMO [18,20]. - c - ation; cation; - cation. - cation. - anion; - cation; - cation. Figure 13. Crystal structure of MeO perovskite. Figure 13. Crystal structure of MeO6 perovskite. Figure 13. Crystal structure of MeO6 perovskite. Figure 13. Crystal structure of MeO6 perovskite. (а) (b) (c) (а) (b) (c) Figure Figure 14. Cr 14.ystal Crystal stru str ctuctur ure of e of СWO CWO 3 perovs perovskite: kite: ((а a) ) general general vie view; w; (b ()bh-WO ) h-WO along 3 along plane plane; (c);h-WO (c) h-WO along 3 alplane. ong plane. 3 3 3 Figure 14. Crystal structure of СWO3 perovskite: (а) general view; (b) h-WO3 along plane; (c) h-WO3 along plane. In the mentioned structures, electrochromic effects occur due to electron–ion separa- There are several highly efficient TMO (IrO2 [60], MoO3 [61], NiO [62], TiO2 [63], WO3 tion. As a result, metal atoms are introduced into TMO, and the valence electrons move [42,5 There 9]) ar the at sev are eral color hig less hly in efth fici e ent oxid T ize MO d state (IrO2and [60] c , olored MoO3 [ in 61] th , e N reduced iO [62], T state iO2 [(c 63] athodi , WO c 3 (а) (b) (c) to the d-levels of the transition metal ion, reducing it. Evidently, the injected ions should [42,5 EC, 9]) co thlor at are chacolor nge is less induce in th d e by oxid ion ize injec d state tion). and Ino c rg olored anic com in th poe und reduced s that ar state e col (c orles athodi s in c possess a high diffusion coefficient and a high solubility in the lattice of TMO [18,20]. Figure 14. Crystal structure of СWO3 perovskite: (а) general view; (b) h-WO3 along plane; (c) h-WO3 along plane. their reduced state and colored in their oxidized state are called anodic EC (color change EC, color change is induced by ion injection). Inorganic compounds that are colorless in There are several highly efficient TMO (IrO [60], MoO [61], NiO [62], TiO [63], 2 3 2 is induced by ion extraction). their reduc WO ed [42 ,state 59]) that and arcol e colorless ored in in their the oxidi oxidized zed state state and are color called ed an inodic the reduced EC (colstate or change There are several highly efficient TMO (IrO2 [60], MoO3 [61], NiO [62], TiO2 [63], WO3 (cathodic Vanadiu EC, m color oxides change [64] exhi is induced bit hybrid by ion featinjection). ures, and Inor ECD ganic usually compounds contain tw that o ar EC e films is induced by ion extraction). [42,59]) that are colorless in the oxidized state and colored in the reduced state (cathodic colorless in their reduced state and colored in their oxidized state are called anodic EC [32,59]; therefore, it would be relevant to simultaneously use a cathodic oxide (for exam- Vanadium oxides [64] exhibit hybrid features, and ECD usually contain two EC films EC, color change is induced by ion injection). Inorganic compounds that are colorless in (color change is induced by ion extraction). ple, Mo or Nb) and an anodic oxide (for example, Ni or Ir) [61,63]. [32,59]; therefore, it would be relevant to simultaneously use a cathodic oxide (for exam- their reduced state and colored in their oxidized state are called anodic EC (color change EC exhibit polychromism [65], for example, amorphous Nb2O5 is brown in its colored ple, Mo or Nb) and an anodic oxide (for example, Ni or Ir) [61,63]. is induced by ion extraction). state, while crystalline Nb2O5 acquires a blue color; WO3 is blue in its colored state, while EC exhibit polychromism [65], for example, amorphous Nb2O5 is brown in its colored Vanadium oxides [64] exhibit hybrid features, and ECD usually contain two EC films + + TiO2 obtains its color (blue or grey) as a result of ion injection (H or Li , respectively). The state, while crystalline Nb2O5 acquires a blue color; WO3 is blue in its colored state, while [32,59]; therefore, it would be relevant to simultaneously use a cathodic oxide (for exam- + + TiO2 obtains its color (blue or grey) as a result of ion injection (H or Li , respectively). The ple, Mo or Nb) and an anodic oxide (for example, Ni or Ir) [61,63]. EC exhibit polychromism [65], for example, amorphous Nb2O5 is brown in its colored state, while crystalline Nb2O5 acquires a blue color; WO3 is blue in its colored state, while + + TiO2 obtains its color (blue or grey) as a result of ion injection (H or Li , respectively). The Nanomaterials 2021, 11, 2376 12 of 32 Vanadium oxides [64] exhibit hybrid features, and ECD usually contain two EC Nanomaterials 2021, 11, x FOR PEER REVIEW 12 of 32 films [32,59]; therefore, it would be relevant to simultaneously use a cathodic oxide (for example, Mo or Nb) and an anodic oxide (for example, Ni or Ir) [61,63]. EC exhibit polychromism [65], for example, amorphous Nb O is brown in its colored 2 5 state, while crystalline Nb O acquires a blue color; WO is blue in its colored state, while 2 5 3 most investigated cathodic EC is WO3 [66]. The color change mechanism has still not been + + TiO obtains its color (blue or grey) as a result of ion injection (H or Li , respectively). sufficiently investigated, but most scientists agree that the extraction and injection of elec- The most investigated cathodic EC is WO [66]. The color change mechanism has still not + + + + trons and metal cations (Li , H , Na , K , etc.) play a crucial role in color change. NiO and been sufficiently investigated, but most scientists agree that the extraction and injection of + + + + IrO2 are the most popular anodic EC. High concentrations of cations in the electrolyte, electrons and metal cations (Li , H , Na , K , etc.) play a crucial role in color change. NiO which and IrOis ar an e the ion most conpopular ductor, anodic signifi EC. cantly High affect concentrations the electof rochro cations mic in the pro electr pertolyte, ies of the TMO, which is an ion conductor, significantly affect the electrochromic properties of the TMO, such as switching time, cyclicity and staining efficiency. such as switching time, cyclicity and staining efficiency. The majority of TMO have a band gap of 1–5 eV (Figure 15), and therefore occupy an The majority of TMO have a band gap of 1–5 eV (Figure 15), and therefore occupy intermediate position between semiconductors and dielectrics [67]. EC behavior is de- an intermediate position between semiconductors and dielectrics [67]. EC behavior is pendent on TMO structure. It should be noted that structural and impurity defects directly dependent on TMO structure. It should be noted that structural and impurity defects affect the properties—particularly the physicochemical properties—of the EC under directly affect the properties—particularly the physicochemical properties—of the EC study. under study . Figure 15. Classification of materials by conductivity (according to zone theory). Figure 15. Classification of materials by conductivity (according to zone theory). The optical band gap can be calculated according to Equation (4) [37]: The optical band gap can be calculated according to Equation (4) [37]: hv = A hv E (4) αhv = A(hv−E ) (4) where is the absorption coefficient, which can be measured by the ultraviolet spectropho- tometer; h is the Planck constant; v is the light frequency; A is a proportionality constant; where α is the absorption coefficient, which can be measured by the ultraviolet spectro- E is the optical band gap; n is a number that is for the direct band gap semiconductor photometer; h is the Planck constant; v is the light frequency; A is a proportionality con- and 2 for the indirect band gap semiconductor. stant; Eg is the optical band gap; n is a number that is ½ for the direct band gap semicon- The E of the WO films decreased from 3.62 eV to 3.30 eV when the annealing g 3 temperature was increased. In addition, the E of the colored WO films was less than that ductor and 2 for the indirect band gap sgemiconductor. of the bleached WO films [38]. The different band gap demonstrates that the conductivity The Eg of the WO3 films decreased from 3.62 eV to 3.30 eV when the annealing tem- of the WO film is enhanced with decreasing E , while the high conductivity increased the perature was increased. In addition, the Eg of the colored WO3 films was less than that of electrochromic response time. the bleached WO3 films [38]. The different band gap demonstrates that the conductivity of The transparency of inorganic EC with high staining efficiency varies in response to 2 1 the WO3 film is enhanced with decreasing Eg, while the high conductivity increased the the low-voltage signal. WO and NiO (Table 5) have a staining efficiency of ~40 sm C , 2 1 electrochromic response time. while for organic EC films, such as PEDOT, this value is more than 100 sm C [32,39]. Actually, TMO have a high physical and chemical stability. The transparency of inorganic EC with high staining efficiency varies in response to 2 −1 the low-voltage signal. WO3 and NiO (Table 5) have a staining efficiency of ~40 sm ∙C , 2 −1 while for organic EC films, such as PEDOT, this value is more than 100 sm ∙C [32,39]. Actually, TMO have a high physical and chemical stability. Table 5. Color variation in WO3, NiO and WO3/NiO electrochromic films (colored and bleached states). Inorganic EC State WO3 NiO NiO/WO3 Oxidized Reduction Nano Nano m Nateria ano m Nateria ano mls ateria m 2021 ls ateria 2021 ls , 2021 11 ls , , 2021 11 x FO , , 11 x FO , , R P 11 x FO , R P x FO EN ER RE R P E ano ER RE R P E N m ER RE ano VIEW ateria EER RE m VIEW ateria VIEW ls 2021 VIEW ls 2021 , 11 , , x FO 11, x FO R PER P ER RE EER RE VIEW VIEW 12 12 of 12 of 32 12 of 32 of 32 32 12 of 1232 of 32 NanomateriaN ls ano 2021 materia , 11, x FO ls 2021 R P , 11 EER RE , x FOVIEW R PEER RE VIEW 12 of 32 12 of 32 Nano N m ano ateria Nm ano ateria N ls m ano 2021 ateria ls materia 2021 , ls 112021 , , ls x FO 11 2021 , , x FO 11 R P , , x FO 11 E R P ER RE , x FO E R P ER RE E R P VIEW ER RE E VIEW ER RE VIEW VIEW 12 of 12 32 of 12 32 of 12 32 of 32 Nano Nano materia materia ls 2021 ls 2021 , 11 , , 11 x FO , x FO R P R P EER RE EER RE VIEW VIEW 12 12 of of 32 32 Nano Nano materia Nm ano ateria N m ls ano ateria 2021 N ls m ano 2021 ateria N , ls m 11 ano 2021 ateria N , , ls 11 m x FO ano N 2021 ateria , , x FO ano ls 11 m N R P 2021 ateria , ano , m x FO ls 11 N R P ateria E 2021 m ano , ER RE , ls x FO 11 ateria E R P 2021 ER RE m , ls , x FO ateria 11 E 2021 R P VIEW ls ER RE , , x FO 11 2021 E R P VIEW ls , ER RE , 11 x FO 2021 E , VIEW R P , 11 ER RE x FO , E , R P VIEW x FO 11 ER RE R P , E VIEW x FO ER RE R P E VIEW ER RE E R P ER RE VIEW E ER RE VIEW VIEW VIEW 12 12 of 32 of 12 32 of 12 32 of 12 32 of 12 32 of 12 12 32 of 12 of 32 of 12 32 32 of 32 most investigated cathodic EC is WO3 [66]. The color change mechanism has still not been mo mo st mo st investig investig st investig ated ated ated cathod cathod cathod mo ic st ic mo EC investig EC ic st is EC investig is WO WO is 3ated WO [66] 3 [66] ated 3. cathod [66] Th . Th cathod e . color Th e ic color e EC color ic change EC is change WO cis hange WO mec 3 [66] mec 3h mec [66] . an h Th an is h . e m Th an is color m ha is e m ha color s sti c ha s hange sti ll s c not sti ll hange not ll mec bee not bee mec n h bee an n h is nan m is ha m s ha stis llsti not ll not bee n bee n most mo investig st investig ated most cathod ated investig cathod ic EC ated ic isEC cathod WO is3 WO [66] ic EC 3 . Th [66] is e . WO color The 3 [66] color change . Th change e mec color hmec an change ish m an ha ismec m s sti ha h lls an not sti isll m bee not ha n s bee stin ll not been momo st investig mo st investig st investig ated ated cathod ated cathod cathod ic EC ic EC is ic WO EC is WO is 3 [66] WO 3 [66] . 3Th [66] . e Th color . e Th color e c color hange change change mec mec han mec his an m his an ha m is s ha m sti s ha ll sti not s ll sti not bee ll not bee n bee n n mo mo st investig st investig ated ated cathod cathod ic EC ic EC is WO is WO 3 [66] 3 [66] . Th . Th e color e color change change mec mec han han ism ism haha s sti s sti ll not ll not bee bee n n mo mo st investig mo st investig mo st investig mo st ated investig mo st ated investig mo st cathod ated mo investig st cathod mo ated investig st cathod mo ated investig st ic cathod EC investig ic ated st cathod EC investig ic ated is cathod EC WO ated is ic ated WO cathod EC is ic 3 cathod ated [66] WO EC is 3 ic cathod [66] WO EC . is ic 3cathod Th [66] WO . EC ic is Th 3e [66] EC ic color WO . is e 3 Th EC [66] ic color WO is . e 3Th EC WO [66] is color c. hange 3 e Th WO c [66] is color hange . 3 e Th WO [66] c color 3. hange [66] e Th mec c . 3 color hange Th [66] e mec . c color h Th hange e an mec . color h c e Th hange is an mec color m h c e is hange an mec color c m ha h hange is an c s mec ha m hange h sti is an s c mec ha hange m ll sti h is mec an s not ha ll m h sti mec is not an s ha h ll bee m sti mec an is not s h bee ha m ll an n sti is not h s m ha bee is ll n an sti m not ha s bee is ll n sti ha s m not bee sti ll n s ha not sti ll bee n s not ll sti bee n not ll bee n not bee n bee n n sufficiently investigated, but most scientists agree that the extraction and injection of elec- suffi suffi suffi cientl cientl cientl y in y vest in y vest inigated, vest igated, igated, suffi but but suffi c mo ientl but mo cst ientl mo y st sc in ientists sc st y vest ientists sc inientists vest igated, agree igated, agree agree but th th at but mo at th th st mo at th e ext sc e th st ext ientists e ra sc ext ra ct ientists io ct ra n io ct agree an n ioan d n agree an inject d th inject d at th inject th io at n e io th ext of n io e of n e ra ext lec of e ct lec ra -io elec ct - n io an - n dan inject d inject ion io of n eof lece -lec- suffic suffi ientl suffi suffi cy ientl cientl inc vest suffi ientl y y in in igated, c vest y ientl vest invest igated, igated, y but in igated, vest mo but but igated, st mo but sc mo ientists st mo st but sc sc st ientists ientists mo sc agree ientists st sc agree agree th ientists at agree th th th e at at agree ext th th th at ra e e ext ct th ext th io e ra at n ext ra ct th an ct io ra e io d n ct ext n inject an io an ra n dd ct an inject io inject io d n n inject of an io io ed n lec n of inject io of -n ee lec of lec io - en - lec of - elec- sufficiently investigated, but most scientists agree that the extraction and injection of elec- suffi suffi cientl cientl suffi y in y suffi vest in cientl vest cigated, ientl y igated, in y vest in but suffi vest igated, but suffi mo igated, cmo ientl st c ientl sc st but y ientists sc in but mo ientists y vest in mo st vest igated, sc agree st ientists agree igated, scientists th but at th agree but at th mo e agree th mo st ext e th sc ext st ra at ientists th sc ct ra th at io ientists ct e n th io ext an n e agree ra ext an d ct agree inject d ra io inject ct th n io at an io th n n th io d at an of e n inject th d ext of e e inject lec e ext ra io lec -ct n ra - io io of ct n n io e an of lec n d e an -lec inject d- inject ion io of n e of lec e- lec- suffi suffi cientl cientl y suffi in y vest in suffi c vest ientl suffi igated, csuffi ientl igated, y cientl in c y ientl but vest in y but vest in mo igated, y vest in mo igated, st vest sc st igated, ientists but sc igated, ientists but mo but mo st agree but sc mo agree stientists mo sc st th ientists sc st at th ientists sc th at agree ientists e th ext agree e ext ra agree th ct at ra agree th io ct th at n th io e an th n at ext th e d an at th ra ext inject d e th ct ext inject ra e io ct ext n io ra io an n ct ra io n of io d n ct an n inject io e of d lec an n e inject lec an d - io inject d -n inject io of n io e of lec n io e of n -lec of e- lec elec - - + + ++ ++ ++ ++ ++ + + + + + + + + + + + + + + ++ + + + + trons and metal cations (Li , H , Na , +K , e ++ tc + +.) + + pl + ay +++ a + +cr+ ucial + + role in color change. NiO and tro tro ns tro ns and ns and and me me tal me tal cations tal cations cations tro (L (L ins tro , i(L H and , ns iH , , N and H , me a N ,, a N tal K me , aK , e c , tal ations , e K tc.) c tc , e ations pl .) tc pl ay (L .) ay pl a i (L , ay a c H r i ucial c a , ,r H ucial N cra ucial , role N , K role a , e , role in K tc in c , e .) olor in c tc pl olor .) c ay olor ch pl a ch ange. Ni ay cange. Ni ch r a ucial ange. Ni crucial O an role O an role O an in d d c in olor d color change. Ni change. Ni O an O an d d trons tro and tro ns ns tro me and and ns tal tro and me me cns ations tal me tal and ctal ations cations (L me ci ations tal , H (L (L ci,ations i N , (L , H a H i , ,, ,K N H (L N a , e ,i a , N , tc , K H a K .), e , ,pl , e K N tc ay tc a .) , e .) a , pl tc K pl c ay .)r ay , e ucial pl a a tc ay c .) r c a ucial rrole pl ucial cay rucial a in role role c cr olor ucial role in in cch olor c in olor role ange. Ni color ch in ch ange. Ni ange. Ni cch olor O an ange. Ni ch d O an ange. Ni O an O an d d d O and trons and metal + + cations + + + (L + i + , +H , Na , K , etc.) play a crucial role in color change. NiO and + + + ++ + ++ +++ + ++ ++ + + ++ ++++ + + + + + + ++ + + + + + ++ + tro tro ns ns and and me me tal tal cations cations (L i (L , iH , H , N , a N, aK , K , e, e tc.) tc pl .) ay play a a cr ucial crucial role role in in color color chch ange. Ni ange. Ni O an O an d d trons troand ns troand ns tro me and tro ns tal me tro ns and me tal c tro ns ations and me tal tro ns c and ations me ns and tal cations (L me and tal c iations me (L tal , cH ations ime tal (L c , ,ations H iN tal c (L , ations a ,H i N c (L , , ations ,K a H i(L N , , , e ,a i K (L H N , tc , i , e H ,K (L a .) , N tc H , ,i pl , e a K N .) , ay ,tc , H a , e pl N K .) a , ,ay a tc , e K pl N c , .) a r ay a tc K , e ucial pl c , .) a tc , e r ay K ucial pl .) c tc , e a r ay role pl .) ucial tc c ay a pl rrole .) ucial ay in c a pl rrole ucial a c c ay in r olor ucial role c a r c in ucial olor role cch r c in ucial olor role ange. Ni ch c in role olor ange. Ni in ch c role olor in ange. Ni cch olor c O an in ange. Ni olor chc O an ange. Ni ch olor d ch ange. Ni O an ange. Ni d ch O an ange. Ni d O an d O an O an d d O an d d trotro ns ns and and me me tal tal cations cations (L i(L , i H, H , N,a N , a K, , e K tc , e .)tc pl .) ay pl a ay c a rucial crucial role role in in color color change. Ni change. Ni O an O an d d IrO IrO 2 ar 2 e arth e e thmo e mo st st popo pula pula r IrO anodic r 2anodic are th EC e EC . mo Hig . Hig st h po con h pula con cent cent r ra anodic tions rations o EC f o cat . f Hig cat ions ions h con in in th cent e the e ra lect etions lect rolyte rolyte of , cat , ions in the electrolyte, IrO IrO 2 ar 2 e arth e IrO e thmo e 2 mo ar st e st po th IrO po pula e 2mo pula ar IrO rst e anodic rth 2 po anodic ar e pula e mo th EC st r e EC anodic . mo po Hig . pula st Hig h po r con EC h pula anodic con . cent Hig r cent anodic ra h EC tions ra con tions . Hig cent EC of h o . ra cat f Hig con tions cat ions cent h ions con o in f ra in cat cent th tions e th ions e ra e lect tions o ef lin ect rolyte cat th rolyte ions o e f e , cat lect in , ions rolyte the in el, ect the rolyte electrolyte , , IrO2 ar IrO e th IrO 2 ar e 2mo e ar th e st e th po mo e pula mo st po st r pula anodic popula r anodic rEC anodic . Hig EC h EC . Hig con . Hig cent h con h ra con cent tions cent rao tions ra f cat tions ions of cat of in ions cat th ions e in elect th in e rolyte th ee lect el, rolyte ectrolyte , , IrO2 are the most popular anodic EC. High concentrations of cations in the electrolyte, IrO IrO 2 ar 2 e arth e e thmo e mo st st popo pula pula r anodic r anodic EC EC . Hig . Hig h con h con cent cent rations rations of o cat f cat ions ions in in the the e lect elect rolyte rolyte , , IrOIrO 2 ar IrO 2e ar th IrO 2e ar e IrO th 2mo e ar e IrO th 2mo e st IrO ar e 2th po e mo ar st IrO 2 e th e pula ar po mo st 2 e th e ar pula po mo st e th re mo pula e anodic po st th rmo pula e st po anodic rmo st po pula anodic r po EC st pula anodic r pula po . EC anodic Hig rpula EC . anodic r Hig h anodic . EC r con Hig h anodic EC . Hig con cent EC h . Hig con EC cent . h ra Hig EC con . cent h tions Hig ra con . h cent tions Hig ra con h cent o tions con ra f h cent cat o tions con ra f cent ions cat o tions ra f cent tions ra ions cat of tions in ra ions cat of th tions in o cat ions f e o th in cat e ions f le ect cat o th in ions f ee lrolyte cat ions ect th in el e in ions rolyte ect th ein e lth , rolyte ect e e th in lect rolyte , e e l th ect rolyte e , e lect rolyte e , lrolyte ect , rolyte , , , IrO IrO 2 ar 2 e ar th e e th mo e mo st po st po pula pula r anodic r anodic ECEC . Hig . Hig h con h con cent cent rations rations of o cat f cat ions ions in in the th ee lect elect rolyte rolyte , , which which which which is is an is an i on an is i on an con ion con ion ducto con ducto con ducto rwhich ducto , signi r, which r signi , signi rfi , is c signi fi antly an c is fi antly c ian on fi antly c affect antly icon on affect affect con ducto th affect e th ducto ele th e r, ele e th ct signi ele r rochro e ct , signi ele rochro ct firochro c ct antly rochro mic ficmic antly mic pro affect pro mic pert affect pro pert pro th ies pert e ies pert th ele of ies e of ct th ele ies rochro of e th ct TMO of th e rochro TMO e th TMO mic e , TMO , mic pro , , pro pert pert ies of iesth of e th TMO e TMO , , which is an which ion is con an ducto ion con r, signi ducto ficr antly , signi affect ficantly the affect electrochro the ele mic ctrochro propert mic ies pro of pert the ies TMO of , the TMO, which which is which an iis on an is con an ion ducto ion con con r ducto , signi ducto r, fisigni crantly , signi fic affect antly ficantly affect the affect ele th cte rochro th ele e ct ele rochro mic ctrochro pro mic pert mic pro ies pro pert of pert ies the ies of TMO th of e th , TMO e TMO , , which is an ion conductor, significantly affect the electrochromic properties of the TMO, which which is is an an ion ion con con ducto ducto r, signi r, signi ficfi antly cantly affect affect the thele e ele ctrochro ctrochro mic mic pro pro pert pert iesies of of the thTMO e TMO , , which which which is which an is which i an is on which an iis on con which an iis on con which ducto an is ion con ducto is an ion con r an ducto is , ion signi con an r i ducto on , con signi r ducto ion , fi con signi ducto crantly , con fi ducto signi c rantly , fi ducto signi r c , affect antly fi signi r, c affect antly signi fi r, c fi affect antly th signi ce fi antly affect th c ele antly fi e affect th c ct ele antly affect e rochro th ct ele affect e rochro th ct ele affect e th rochro mic ele ct e th rochro ele mic ct e th pro rochro ele ct mic e pro rochro pert ct ele mic rochro pro ct pert mic ies rochro pro pert mic ies of pro mic pert ies th of pro mic pert e pro ies th of TMO pert e pro ies th pert of TMO e ies th , pert of TMO ies e of th , TMO ies of e th , TMO e th of TMO , e th TMO , e TMO , , , which which is an is an ion ion con con ducto ducto r, signi r, signi ficantly ficantly affect affect the th ele e ele ctrochro ctrochro mic mic pro pro pert pert iesies of of the th TMO e TMO , , such such such as switc such as switc as switc as switc hing time hing time hing time hing time , cyc , cyc such , cyc lic, cyc li such ity c as switc li ity c and ity lias switc and city and staining hing time and staining sthing time aining st effic aining effic , cyc effic ien effic , cyc ien cy. liien ccy. ity ien li cy. c and ity cy. and staining staining effic effic iency. ien cy. such as switc such hing time as switching time , cyclicity , cyc and list city aining and effic staining iency. effic iency. such such as switc such as switc such as switc hing time as switc hing time hing time hing time , cyc , cyc lic , cyc ity lic , cyc ity and licity and list city and aining stand aining staining effic staining effic ien effic cy. ien effic ien cy.ien cy. cy. such such as switc as switc hing time hing time , cyc , cyc licli ity city and and staining staining effic effic ien ien cy. cy. such such as switc such as switc such as switc such hing time as switc such hing time as switc such hing time such as switc hing time such as switc , cyc hing time such as switc , cyc as switc li hing time , cyc city li as switc hing time c , cyc ity li and hing time c , cyc ity hing time and lic st , cyc hing time ity and li aining st c , cyc ity and aining li , cyc st city and aining li , cyc effic st city li and aining , cyc effic cst li ity and ien aining c effic ity st li and ien cy. c aining effic ity st and ien cy. aining effic st and aining cy. ien st effic aining ien cy. st effic aining ien cy. effic effic ien cy. ien effic cy. ien cy. ien cy. cy. The majority of TMO have a band gap of 1–5 eV (Figure 15), and therefore occupy an Th Th e majority Th e majority e majority of of TMO of TMO TMO have have Th have e a Th majority band a e band a majority band gaga p of o ga p TMO fof o p 1f– o TMO 1 5 f – e 5 1 have V – e5 V ( have Fi e( V a g Fi ur band (g Fi ur a e g band 15), e ur ga 15), e and 15), p ga and ofp and th 1o –th e f 5 refore 1 e e th – refore V 5 e( refore e Fi V occ g(ur occ Fi upy g e occ upy ur 15), e upy an 15), and an an and therefore therefore occupy occupy an an The majority The majority of Th TMO e of majority TMO have have a of band TMO a band ga have p o ga f a 1 p – band 5 ofe 1 V –5 ( ga Fi ep g V ur o (f Fi e 1g 15), –ur 5 e e V and 15), (Fith g and ur erefore e th 15), erefore occ and upy th occ e refore an upy an occupy an The Th majority e Th majority e majority of TMO of TMO of TMO have have a have band a band a ga band p ga op fga 1 o– p f5 1 o e – fV 5 1e – (V Fi 5 e g (Fi V ur g (e Fi ur 15), g e ur 15), and e 15), and th and eth refore eth refore erefore occocc upy occ upy an upy an an ThTh e majority e majority The Th of majority e of TMO majority TMO have of have TMO Th of a TMO e band Th a majority band have e majority have gaga a p band o p of a f o 1 band TMO f of – 1 5 ga – TMO e5 V p ga e have o ( V Fi p f have ( 1 g o Fi – ur f5 a g 1ur e e band –V a 5 15), e band e (15), Fi V ga and g (Fi ur and p ga g e o th ur p 15), f e th 1 e o refore – f 15), e 5 and 1 refore e –V 5 and e th ( occ V Fi e occ g ( th refore upy Fi ur e upy g refore e ur an 15), e occ an 15), and occ upy and upy th an eth refore an erefore occocc upy upy an an ThTh e majority e majority Th of e Th majority of TMO e Th TMO majority Th e majority have e majority have of a TMO of band a TMO of band of TMO have ga TMO have p ga a o have p fband 1 o have a – f band 5 1a – ega 5 band V a ep ( band V ga Fi o( g p fFi ga ur 1 o g – ga p f e ur 5 1 o 15), p e – e fV 5 o 1 15), f e – (and Fi V 5 1– e g and (5 V Fi ur th eg ( V e Fi e ur th 15), refore (g Fi e e ur refore 15), g and e ur 15), e occ and 15), th occ upy and eth refore and upy e th an refore e th an refore occ erefore occ upy occ upy an occ upy an upy an an intermediate position between semiconductors and dielectrics [67]. EC behavior is de- interm interm interm edi edi ate edi ate po ate po sition po sition si tion bet interm bet ween bet interm ween ween edi semico ate edi semico sate emico po nduct si po nduct tion nduct siors tion bet ors and ors ween bet and ween and di di electr semico electr di selectr emico ics nduct ics [67] ics nduct [67] . ors [67] EC . EC ors and . beh EC beh and di avi beh electr avi or di avi or electr is or ics is de is de ics [ -67] de - [. 67] -EC . EC beh beh avior aviis or de is - de- interm interm interm edi interm ate edi edi po interm ate edi ate sition po ate po edi si si po bet tion tion ate si ween tion bet po bet si ween bet ween tion semico ween bet semico semico nduct ween semico nduct ors nduct semico nduct and ors ors nduct di and ors and electr and di ors di electr ics electr di and electr [67] ics ics di . [ electr EC ics 67] [67] . [beh 67] . EC ics EC . avi beh [ EC 67] beh or avi . beh avi is EC or de avi or beh is -or is de avi de is - - or de-is de- intermediate position between semiconductors and dielectrics [67]. EC behavior is de- interm interm edi edi interm ate ate interm po interm po edi sition interm si edi ate tion interm edi ate bet po bet edi ate si ween interm po tion edi ween ate interm si po tion ate si s po bet edi emico tion s si po emico ween bet edi ate tion si bet ween ate nduct tion po bet nduct ween ssi po emico tion bet ween s ors si emico tion ors ween s emico nduct bet and s and bet emico nduct ween s di emico ors nduct ween electr di ors nduct electr s and emico nduct ors s ics and emico di ics ors and [nduct electr 67] di ors [and 67] nduct electr . di EC and ics . ors electr di EC ors ics beh electr [and di 67] beh ics electr [ avi and . 67] EC di ics avi [or 67] . electr di ics EC or [ beh is 67] . electr EC is [ de beh 67] avi . ics de EC -beh . ics or avi [ -EC 67] beh avi is or [67] . beh de EC avi or is . -de EC avi or is beh -de or is beh avi -de is avi or -deor is - de is -de- interm interm edi edi ate ate popo sition si interm tion bet bet ween edi ween ate semico po semico sition nduct nduct bet ors ween ors and and semico di electr dielectr nduct icsics ors [67] [ 67] . and EC . EC di beh electr beh avi avi or icsor is [67] de is . - de EC - behavior is de- pendent on TMO structure. It should be noted that structural and impurity defects directly pen pen dent pen dent dent on on T pen on MO TMO T dent MO str str ucture on pen str ucture pen T ucture dent MO . pen dent It. sho It str on . dent sho It ucture on uld T sho MO uld T on MO be uld be . T str not It MO be str ucture not sho ed ucture not str ed uld th ed ucture th at . It be at th struct . sho It not struct at sho . struct uld It ed ur sho uld ur al th beal uld ur at an be not an al struct d not be ed i an d mp ed inot d mp th ur uri iat th mp ed al uri ty at struct an uri th ty struct defe at d ty defe istruct ur mp ct defe ur al s ct uri di al an s ct ur di rect ty an d s al rect di idefe d mp ly an rect imp ly d uri ct ly iuri s mp ty di ty defe uri rect defe ty ly ctdefe s ct di s rect di ctrect s di ly rect ly ly pendent pen on pen dent Tdent MO on str on TMO ucture TMO structure str . Itucture sho. uld It . sho It be sho uld not uld be ed not be that not ed struct th ed at th ur struct at al struct an ur d al iur mp an al uri d an imp ty d idefe mp uriuri ty ctdefe s ty di defe rect cts ct ly dis rect direct ly ly pendent on TMO structure. It should be noted that structural and impurity defects directly pen pen dent dent on on TMO TMO strstr ucture ucture . It. sho It sho uld uld bebe not not ed ed that that struct struct urur al al anan d id mp imp uri uri ty ty defe defe cts ct di s rect direct ly ly pen pen dent pen dent on pen dent T pen on dent MO pen on T dent MO pen str T on dent MO pen ucture dent on str TMO on dent ucture str T MO on T ucture str . MO It on T ucture str sho MO . It T str ucture MO . sho uld It str ucture sho . uld ucture It str be . sho uld ucture It not be . sho It uld not be ed . sho It uld not . th be sho ed It uld at not be sho ed th uld struct at be not th ed uld be struct not at ed th ur not be struct at ed th al not ur struct ed at th an al ur struct at ed th d an al at struct iur th mp d an struct at al ur imp uri d an struct al ur imp ty uri d an ur al idefe mp an al d uri ty ur ian d mp defe al ty uri ct id mp an defe s uri ty ict di mp d uri defe s ty rect ict mp di uri ty defe s rect ly ct di uri ty defe s rect ct ly defe di ty s ct rect ly defe di s ct rect di ly s ct rect di ly s rect di ly rect ly ly pen pen dent dent on on TMO TMO strstr ucture ucture . It. sho It sho uld uld be be not not ed ed that thstruct at struct ural uran al d an imp d imp uriuri ty defe ty defe cts ct di s rect direct ly ly affect affect affect the thth e pro e pro pert pro pert ies pert ies — ies — part — p affect art icu part icu larly icu th larly e larly th pro e thpert th e ph e ph ysi ies ph ysi coch — ysi coch pcoch art em em icu ical em ilcal arly ipro cal pro pert th pro pert e ie pert ph sie — ysi s ie — of s coch — of th of e th em th e EC ical e EC under EC pro under under perties—of the EC under affect th affect e propert the affect ies pro — pert affect p th art e ies icu pro — th larly e p pert art pro icu ies th pert — e larly p ph ies art ysi — th icu coch p e art larly ph icu em ysi lth iarly coch cal e ph pro em th ysi e ipert cal coch ph ie ysi pro s em — coch pert i of cal em ie th pro se i— cal EC of pert pro th under ie e pert s— EC of ie s under th —e of EC th e under EC under affect affect the affect pro thpert e th pro e ies pro pert —pert pies art— ies icu p — lart arly picu artth icu larly e larly ph th ysi e th coch ph e ysi ph em coch ysi ical coch em pro em ical pert i cal pro ie pro s pert —pert of iesth — iee s of — EC of th e under th EC e EC under under affect the properties—particularly the physicochemical properties—of the EC under affect affect the the pro pro pert pert iesies —— part part icu icu larly larly the the phph ysi ysi coch coch em em ical ical pro pro pert pert iesie — s— of of the the EC EC under under affect affect affect the affect th pro affect e th pro affect pert e th affect pro pert e th ies affect pro pert e th — ies pro e th pert p— ies art e pro th pert p — pro ies icu e art pert p pro — ies l icu pert art arly p ies — icu pert l art arly ies p — th icu l art arly — p ies e art icu th lp arly — ph art e icu th lp arly ysi ph icu art e larly th coch ysi ph icu le arly th ysi coch ph le arly th em coch ysi ph e th i em cal e ph ysi coch th em i ph ysi e cal coch pro ysi em i ph coch cal pro pert ysi em coch i cal pro em pert coch iie cal em pro s i pert cal — ie em i pro cal s pert of — ie pro i cal s pert of th — pro ie pert e s of th pro — ie pert EC e sof ie th — pert EC s e ie under of — th sEC — of ie e under th sof EC — e th under of EC e th under e EC th under EC e under EC under under affect affect the thpro e pro pert pert iesies —p — art part icuicu larly larly the thph e ph ysiysi coch coch em em ical ical pro pro pert pert iesie — sof —of the thEC e EC under under study. study. study. study. study. study. study. study. study.study. study. study. study. study. study. study. study. study. study. study. study. study. study. study. Figure 15. Classification of mFigure aterials15. by c Clas ond siuctiv ficatio ity (a n of cc m or aterial ding s to by c zone ond the uctiv oryit ).y (a ccording to zone the ory ). Figure Figure Figure 15. 15. Clas 15. Clas Figure si Clas fi si cat fisi cat io 15. fin o cat io Figure Clas n o io f m n o f si a m Figure fi terial f 15. a cat m terial a Clas io terial s n o by c 15. s si by c f s fi Clas m ond by c cat a ond terial io uctiv si ond n o fi uctiv cat sf uctiv it by c io m y (a it n o ay (a terial ond it cc f y (a or m cc uctiv di s a or cc terial by c ng di orng it di to ond y (a s ng to zone by c uctiv cc zone to or ond zone the di it the ng uctiv y (a ory the ory to ). cc it ory zone or ). y (a di ). ng cc the or to ory di zone ng ). to the zone ory the ). ory). Figure Figure 15.Figure Clas 15. sifi 15. Clas cat Clas io sin o ficat sif fi m io cat a n o terial iof n o ms f a terial by c material ond s by c uctiv s by c ond itond y (a uctiv uctiv ccit or y (a di ity (a ng ccor to cc di or zone ng di to ng the zone to ory zone ). the ory theory ). ). Figure 15. Classification of materials by conductivity (according to zone the ory). Figure Figure 15.15. Clas Clas sifi si cat ficat ion o ion o f m f a m terial aterial s by c s by c ond ond uctiv uctiv ity (a ity (a ccor ccdi orng ding to to zone zone the the ory ory ). ). Figure Figure Figure 15.Figure Clas 15.Figure 15. Clas si Figure fi 15. Clas cat Figure si fi 15. Clas io Figure cat sin o 15. fi Clas io cat si 15. f n o Clas fi m io cat si 15. Clas a f n o fi terial si m io cat Clas fi f a n o si cat terial m io fi s a f n o cat si io terial by c m fi n o s f io a cat terial m by c n o ond f s io am terial by c f n o ond a uctiv m sterial by c f a ond uctiv terial m s it by c a ond s y (a terial uctiv by c it sond y (a uctiv by c ccit ond sor uctiv y (a by c cc ond di it or uctiv ng y (a cc ond di it uctiv or to y (a ng cc it di uctiv zone y (a or to ng it cc di y (a or zone cc to ng it the di y (a or cc zone ng to di or ory the cc zone ng di to or ). ory the ng zone to di ory the ). to ng zone zone the ory ). to the zone ory ). the ory ). the ory ). ory ). ). Figure Figure 15.15. Clas Clas sifisi cat ficat ion o ion o f mf am terial aterial s by c s by c ond ond uctiv uctiv ity (a ity (a ccor cc di or ng di ng to zone to zone the the ory ory ). ). The optical band gap can be calculated according to Equation (4) [37]: Th Th e opt Th e opt e opt ical b ical b ical b and gap c and gap c and gap c an b Th an b e opt e calcu an b Th e calcu e opt e calcu ical b la ical b ted la and gap c ted la accord ted and gap c accord accord ing an b ing to an b ing e calcu to Equation e calcu to Equation Equation lated la (4) ted accord (4) [ (4) 37] accord [37] :[ ing 37] : ing :to Equation to Equation (4) (4) [37][:37] : The optical b The opt and gap c ical band gap c an be calcu an b lated e calcu accord lated ing accord to Equation ing to (4) Equation [37]: (4) [37]: The opt The opt Th ical b e opt Th ical b e opt and gap c ical b and gap c ical b and gap c and gap c an b an b e calcu an b e calcu an b e calcu lated e calcu lated accord lated accord lated accord ing accord ing to ing Equation toing Equation to Equation to Equation (4) (4) [37] (4) [:37] (4) [37] : [37] : : ThTh e opt e opt ical b ical b and gap c and gap c an b an b e calcu e calcu lated lated accord accord ing ing to to Equation Equation (4) (4) [37] [37] : : ThTh e opt e opt Thical b e opt Th ical b e opt Th and gap c ical b e opt Th and gap c ical b e opt Th and gap c ical b Th e opt and gap c Th ical b e opt an b and gap c e opt Th ical b an b e calcu ical b e opt and gap c an b e calcu ical b and gap c an b ical b e calcu and gap c la an b and gap c e calcu ted la an b and gap c ted e calcu la accord an b ted e calcu accord an b lated e calcu an b accord la ing e calcu ted an b accord la ing e calcu to ted accord la ing e calcu to Equation ted la accord ing Equation ted to la accord ted ing Equation to la accord ted ing Equation accord to (4) ing Equation (4) accord to [ing 37] Equation (4) to [ ing 37] : to Equation (4) ing [ 37] :to Equation (4) [Equation to :37] (4) [Equation 37] : (4) [37] : (4) [37] (4) : [37] (4) : [37] : [37] : : n n n n n n n n n n n αhv = A(hv−E ) αhvαhv=αhv=A(A=hv(Ahv−α(hvhv−E E−)=)E A( )hv α−hvEα=hv) A =(hvA(−hvE−)E ) (4)(4) (4) (4) αhv =gαAhv n(αhvnhv =−A=(αEhvhvA)(−hv= EnA−()Ehv )− E ) (4) (4) (4) (4) αhv =g Ag(nhv g n−En ) n n n ng n g n (4) (4) (4) g gg g g (4) αhvαhv==A(Ahv(hv−E−E) ) g αhvα=hvαAhv=(αhvAhv =(α−hvAhv=(Eαhv −hvA=α)(Ehv−hvA =α)(Ehv=Ahv − ()AEhv=− ()hvAE−( )hv E− E)− )E ) (4)(4) αhvαhv= A=(hvA(ghv−E−)E ) (4) (4) (4) (4) (4)(4) (4) (4) g g g g g g g g g (4)(4) g g where where where α is α α is th is e th absorpt th e absorpt e absorpt ion ion where coeff ion coeff coeff ici α ici ent is ici ent , th ent w , e hich w absorpt , hich which can can ion be can be meas coeff be meas meas ici ured ured ent ured by , w by hich th by e thul th e can trav ul e ul trav be iol trav iol meas et iol e spect t e spect ured t spect ro- ro- by ro- the ultraviolet spectro- where αwhere is the α absorpt is where the ion where absorpt α is coeff th α e ion ici is absorpt ent th coeff e , absorpt w ici ion hich ent coeff , can ion which ici be coeff ent meas can , ici w ent hich be ured , meas w can by hich ured th be can e meas ul by trav be th ured meas iol e ul et trav by ured spect th iol e by ro- e ul t th spect trav e ul iol ro- trav et spect iolet ro- spectro- where where α where is thα e is absorpt α th ise th absorpt e ion absorpt coeff ion ici ion coeff ent coeff , ici went hich icient , w can , hich which be can meas can be ured be meas meas by ured th ured e by ulby trav the th ul iol e trav e ul t trav spect iole iol t ro- spect et spect ro-ro- where α is the absorption coefficient, which can be measured by the ultraviolet spectro- where where α is α is th e thabsorpt e absorpt ion ion coeff coeff iciici ent ent , w , hich which can can be be meas meas ured ured by by the thul e trav ultrav ioliol et e spect t spect ro- ro- where where where α where is α th where is α e where th is absorpt α where e th is absorpt α where e th is α absorpt e ion th is α absorpt e is th ion α coeff absorpt e th is ion absorpt coeff e th ici absorpt ion e coeff ent absorpt ion ici coeff ion ent , ici w coeff ion ent , hich ici coeff w ion ent , coeff hich ici w can ici ent coeff , hich w ici ent can , hich be w ent ici can , hich meas be w ent , can hich w be meas , hich can w ured be meas can hich ured be meas can by be ured meas can be by th meas ured e be meas by th ured ul e meas ured by trav th ul ured e by trav th ul iol ured by e th trav e ul iol by t e th trav spect e ul iol by e th t trav ul spect e e iol th t trav ro- ul spect e e iol trav t ul ro- spect iol etrav t ro- iol e spect t e spect ro- iol t spect e ro- t spect ro- ro-ro- where where α is α th is e th absorpt e absorpt ion ion coeff coeff icient icient , w , hich which can can be be meas meas ured ured by by the th ul e trav ultrav ioliol et spect et spect ro-ro- phph otph om otph om ot eter; om ot eter; om eter; h eter; is h is th h is e th h Planc th e is Planc e th Planc e ph k Planc con k ot ph con k om stant; ot con k eter; stant; om con stant; eter; v stant; h is v is is th v h th is e th is v e light th e is th Planc light e e th light Planc e frk light equ fr con equ fr k en equ fr con stant; en cy; equ en cy; stant; A en cy; v A is cy; is A is a v th pro A is a is e pro a is th light po pro a e po rtion light pro po fr rtion equ po rtion ality fr rtion ality equ en ality con- cy; en ality con- A cy; con- is con- A a is pro a pro portion portion ality ality con- con- photometer; phot h om is eter; the Planc h is th k con e Planc stant; k con v isstant; the light v is fr thequ e light ency; frequ A is en a cy; pro A po is rtion a pro ality portion con-ality con- photom ph eter; ot ph om ot h om eter; is eter; thh e is Planc h th ise th k Planc e con Planc stant; k con k con stant; v is stant; thv e is light v th ise th fr light equ e light en frequ cy; frequ A enis cy; en a cy; A pro is A po a is pro rtion a pro po ality rtion portion con- ality ality con- con- photometer; h is the Planck constant; v is the light frequency; A is a proportionality con- phph otom otom eter; eter; h is h is th e thPlanc e Planc k con k con stant; stant; v is v is th e thlight e light fr equ frequ enen cy; cy; A A is is a pro a pro popo rtion rtion ality ality con- con- phph otom ot ph om eter; ot ph om eter; ot ph h om eter; ot ph is h om eter; th ot ph is h om e eter; th ph ot is Planc h e ph om ot eter; th Planc is om h ot e ph eter; th is k Planc om h ot eter; e con th k is om Planc eter; h e con th stant; k is Planc h eter; e con stant; th is h k Planc e is th con stant; h k v Planc e th is con is stant; v Planc k e th th is con Planc stant; v k e e th is con Planc light k stant; v e th con k light is stant; v e con th k fr is light stant; v e equ con th fr stant; is light v equ e en stant; th fr is light v equ e cy; en th is fr v light equ cy; e is th en A fr v light e th equ cy; is is A en light fr e th a equ is cy; light A en fr pro e a equ is cy; light fr A en pro po equ a fr is cy; en A pro equ po rtion a fr en cy; is A pro rtion equ po en a cy; is A ality pro rtion cy; po en a A is ality pro rtion cy; po A a is con- ality pro rtion a is po A con- ality pro a rtion is po con- pro ality a po rtion con- pro po ality rtion con- rtion po ality con- ality rtion ality con- ality con- con- con- stant; Eg is the optical band gap; n is a number that is ½ for the direct band gap semicon- stant; stant; stant; EgE ig s E ith s g th e is op e th op tic e op tic al tic al ban stant; al ban d ban stant; g d ap; E g d ap; g g in s E ap; g is th n i s is e a nth op numb a is e numb tic a op numb al tic er ban er al ther th at ban d at th g is ap; d at i½ s g ½ iap; for n s ½ for is th n a for th is e numb di e a th di rect numb e rect di er rect band th er band at band th iga s at ½ ga pi s ga sem p for ½ sem p for th icon- sem e icon- th di icon- e rect direct band band gap ga sem p sem icon- icon- stant; stant; Eg is E th ge istant; s op th tic e op al Egban tic is al th d ban e gop ap; d tic g nal ap; is ban a nnumb is d a gap; numb er n th is at er a ith s numb ½ at for is er ½ th th for e at di th rect is e ½ di band for rect th band ga e di p rect sem gap band icon- semicon- gap semicon- stant; stant; stant; Eg iE s gth iE s e gth iop s e th tic op e al tic op ban al ticban al d g ban d ap; gd ap; ng is ap; na is n numb a is numb a numb er th er at th er iat s th ½ iat s for ½is for ½ th e for th di e rect th di e rect di band rect band ga band p ga sem p ga sem p icon- sem icon- icon- stant; stant; Eg E is g i th s e thop e op tictic al al ban ban d g d ap; gap; n is n is a numb a numb er er that that is i ½ s ½ for for th e thdi e rect direct band band ga ga p sem p sem icon- icon- stant; stant; stant; Eg E stant; is g th istant; E s g e th istant; E op s e gth stant; op tic iE s e gstant; th al tic i op E s e stant; gban al th tic i E op s e stant; ban gal th E d tic iop s ge ban g E th al d itic s ap; op g e g th i E ban al s d ap; tic op g e n th g iban s al op d ap; tic is e n th g op ban a tic al is d ap; e n numb tic g al a ban op is d ap; numb n al ban tic g a d is ap; ban numb n al er g d a is ap; ban er th numb n g d a ap; is at th g n er numb d ap; a i at is s n g th numb er ap; ½ a i is n at s th numb er for ½ a is in at s numb th a for er is ½ th inumb at s a th for er e th ½inumb di at s er e th for th ½ rect di ier at th s e for rect ½ th at i th di er s band e for at rect ½ th i th s di band e ½ i for at rect s th di band ga ½ for e i s rect th di p ga for band ½ e th sem rect p di for ga band e th sem rect di p e icon- ga band th rect di sem e icon- p ga band rect di sem p band icon- ga rect sem band p icon- ga sem band ga p icon- sem ga picon- sem p ga icon- sem picon- sem icon- icon- ductor and 2 for the indirect band gap semiconductor. ducto ducto ducto r and 2 r and 2 r and 2 for the for the for the indire indire ducto indire ct ducto band gap ct r band gap ct and 2 band gap r and 2 for the semicon for the semicon semicon indire ducto indire ducto ctducto band gap r. ct r. band gap r. semicon semicon ducto ducto r. r. ducto ducto r and 2 ducto r and 2 for the ducto r and 2 for the r indire and 2 for the indire ct for the band gap indire ct band gap indire ct band gap semicon ct band gap semicon ducto semicon ducto r. semicon ducto r. ducto r. r. ducto ducto r and 2 r and 2 for the for the indire indire ct band gap ct band gap semicon semicon ducto ducto r. r. ducto ducto r and 2 r and 2 ducto ducto for the r for the and 2 r and 2 indire indire for the ducto for the ct band gap ducto ctindire band gap r and 2 indire r and 2 ct band gap s ct for the emicon band gap semicon for the indire ducto sindire emicon ducto semicon ct r. band gap r. ct ducto band gap ducto r. sr. emicon semicon ducto ducto r. r. ducto ducto r and 2 r and 2 ducto for the ducto for the r ducto and 2 ducto r indire and 2 indire r and 2 for the r ct and 2 for the band gap ctfor the band gap indire for the indire indire ct s band gap emicon indire ct semicon band gap ct band gap ct ducto band gap ducto semicon r. semicon r. s emicon s ducto emicon ducto r. ducto ducto r. r. r. The Eg of the WO3 films decreased from 3.62 eV to 3.30 eV when the annealing tem- Th Th e Th E e g E of e g E of th g of th e WO e th WO e 3WO f3 il f ms il 3 ms fTh il de ms de e creased Th E de creased g e of creased E g th of from e th from WO e from WO 3.62 3 f 3.62 ilms 33.62 eV fil eV ms de to eV to creased 3.3 de to 3.3 0 creased 3.3 e 0 V e0 from V when ewhen V from when 3.62 th th e 3.62 eV anne e th anne e to eV anne aling 3.3 to aling 0 aling 3.3 e tem V 0 tem when e -tem V -when - the th anne e anne aling aling tem tem - - The E Th g Th of e e Th E th E g e e g of of E WO Th th g th of e e 3e WO th E fWO il g e ms of WO 3 3th f il de fil e ms 3ms creased WO fil de ms de 3creased f creased de ilms from creased de from 3.62 creased from from eV 3.62 3.62 to from 3.62 eV eV 3.3 to 0 eV to 3.62 e 3.3 V 3.3 to0 when eV 0 3.3 ee V to V 0 when e when 3.3 th V e when 0 anne e th V th e when e aling anne th anne e anne aling th tem aling e aling anne - tem tem aling - tem - - tem- The Eg of the WO3 films decreased from 3.62 eV to 3.30 eV when the annealing tem- The Eg of the WO3 films decreased from 3.62 eV to 3.30 eV when the annealing tem- The Eg of Th th e Th e Ee g Th WO of Ee g Th th of 3E f e e Th g il th of WO E ms Th e e g th of E Th WO e de 3 g e E th f of e Th WO il creased g 3 e E ms of th f e g WO il of 3 e E th ms de f g WO il e th of 3ms creased WO de f from e il th 3WO ms creased de fe il 3 WO ms creased f3.62 de il 3 ms from f creased il de 3ms eV ffrom de creased ilms 3.62 de creased from to creased de 3.62 3.3 from eV creased 3.62 0 from eV e to from 3.62 V 3.3 eV from to when 3.62 0 eV 3.3 from to 3.62 e V 3.62 eV 0 3.3 to th e when eV 3.62 V 0 3.3 e to eV e anne when to V 3.3 0 eV e to when 3.3 th 0 V aling e 3.3 e to when 0 th V anne e 0 3.3 e when V th e anne tem 0 V when e aling th e anne when V - e aling th anne when e th tem aling anne e th tem aling anne -e th anne tem aling - e aling anne tem - aling tem - aling tem - tem - tem - - ThTh e E e g E of g of th e th WO e WO 3 fil 3 ms films de creased decreased from from 3.62 3.62 eVeV to to 3.3 3.3 0 e0 V ewhen V when th e th anne e anne aling aling tem tem - - perature perature perature wa wa s wa incre s incre s incre ased asas ed . In ed . perature In add . In add ition, add ition, wa ition, th s e th incre E th e g E e of g as E of th g ed of e th . colo th e In colo e add red colo red ition, WO red WO 3 WO f th il 3 e m fil 3 E s f m il g wa s of mwa s s th wa le s e ss le colo s ss le thss an th red th an th WO an at th th at of 3 f at of ilm of s was less than that of peratureperature was incre perature wa as s ed incre perature . In wa as add ed s incre ition, . wa In s add as incre th ed e ition, . Eas In g of ed add th th . e In e ition, Ecolo add g of red th ition, the e colo WO Eg th of red 3 e f th E ilm g e WO of colo s wa th 3 e f red s ilcolo m less WO s wa red th 3 an s fWO il le m th ss s at 3 th wa fil of an m s s le th wa ss at s th of le an ss th th at anof that of perature perature perature was incre wa wa s as incre ed s incre . In ased add as. ed In ition, . add In add th ition, e ition, Eg th of e th th Ee e g of E colo g th of red e th colo e WO colo red 3 red fil WO mWO s 3 wa fil3 m s fil s le m wa ss s wa th s le an s ss le th th ss at an th of an that thof at of perature was increased. In addition, the Eg of the colored WO3 films was less than that of perature perature wa wa s incre s incre ased ased . In . In add add ition, ition, the thE e g E of g of the thcolo e colo red red WO WO 3 fil 3 m fils m wa s wa s le s ss less than than th at that of of perature perature perature perature wa perature s wa incre perature wa s perature incre wa s as perature incre ed wa s as incre wa . ed s In as wa incre s . ed add In as incre wa s . ed incre add In as ition, s . ed as incre add In ition, ed . as add In th ition, ed . as In add e . th ed ition, E In add e g . ition, th of add In Ee g ition, th th of add Eition, e e g th th of colo E ition, e e th g th of E colo e th red e g E th of e colo th g red e E of th WO e g colo of e red E th WO g colo 3 e th of red fil WO colo e 3 m th red f colo il WO s e 3 red m wa colo fWO il s red 3 m s wa f WO il s red le 3 m WO wa s fss il s 3 le m WO fth wa s il ss 3 s m le fan il wa s th ss 3 s m f le th an wa il s s th ss m at wa le an th s s th ss of le at wa s an th ss th le of at s ss an th th le of at an th ss than of at th th an at of that of thof at of perature perature wa wa s incre s incre ased as. ed In . In add add ition, ition, the th E e g E of g of the th colo e colo red red WO WO 3 fil 3 m fils m wa s wa s le s ss less than th an th at thof at of the th blea th e blea e th che blea e che blea d che WO d che WO d 3 WO d films 3 WO films 3 films [ 3 38] films [ th 38] . [ e Th 38] th blea . [Th 38] e e . Th di blea e che . ff di Th ee di ff d che re ee WO ff di nt re e d ff nt re band WO e 3 nt re films band nt band 3 films g band ap [g 38] ap g dap [ emo . 38] d g Th ap emo d . e emo nstrates Th d di nstrates emo e ff nstrates di ere nstrates ffnt e that re band tnt hat tth hat band e t th hat g con th e ap con e th g du d con ap e emo du ct con d du ivity ct emo nstrates du ivity ctivity ct nstrates of ivity of of that oft hat the th con e con duct du ivity ctivity of of the bleache the d blea WOche 3 films d WO [38] 3 films . The [ di 38] ffe . re Th nt e di band ffere gnt ap band demo gnstrates ap demo that nstrates the con that du th cte ivity condu of ctivity of the blea thche e th blea e d blea WO cheche d 3 films WO d WO 3 [films 38] 3 films . Th [38] e [di 38] . Th ff. ee Th re di nt eff di band ere ffe nt re g band nt ap band demo gap gnstrates ap demo demo nstrates tnstrates hat thte hat con that th du e th con ct e ivity con duct du of ivity ctivity of of the bleached WO3 films [38]. The different band gap demonstrates that the conductivity of the th blea e blea che che d WO d WO 3 films 3 films [38] [38] . Th . Th e di e ff di eff re ent re nt band band gap gap demo demo nstrates nstrates that that the th con e con dudu ctivity ctivity of of the th blea e th blea che e th blea che e d th blea WO che e th d blea WO e th che 3 d blea films e WO th che blea d 3 films e che WO blea d 3 [films che 38] WO d 3 [che films WO . 38] d Th 3 WO [films 38] d . e 3 Th films WO [ di 38] . 3 e Th films ff [di 38] 3 e . e films re Th [ ff 38] di . nt e[ e Th re 38] ff . di band e nt Th [ e re 38] . ff di Th band e e nt re ff . di g e Th band e nt ap ff di re e e g band nt ff re d ap di e emo nt g band re ff ap d e nt band emo g re nstrates ap d band nt emo gap nstrates d band g emo ap nstrates d gemo ap t d nstrates hat g emo ap d tnstrates emo hat th d nstrates te emo hat th nstrates con te hat th nstrates con du te hat th con tct du hat e ivity th tcon hat ct du e th t ivity con hat ct e du th of ivity con e ct du th of con ivity e du ct of con ivity du ct of ivity ct du ivity of ct ivity of of of the th blea e blea che che d WO d WO 3 films 3 films [38] [38] . Th . Th e di e ff di eff reent re nt band band gap gap demo demo nstrates nstrates that that the th con e con dudu ctivity ctivity of of the thWO th e WO e th 3 WO e fil 3 WO m fil 3 f m is il 3 m fis enh ilm is enh anced is enh anced enh anced th anced with e with th WO with e decre with WO 3 decre f il decre m 3 asi f decre il asi is ng m asi enh ng is E asi ng genh ,E anced whi ng gE , whi ganced ,E le whi g ,with le twhi he le t he with high le tdecre he high the high decre con asi high con d ng con asi uctivity d con uctivity E ng d guctivity , d whi E uctivity g,incre whi le incre the incre le asincre ted as high he as ed th high ed as e th con ed th e con d e th uctivity ed uctivity incre incre ased as th ed e the the WO3 f th ilm e WO is enh 3 filanced m is enh with anced decre with asing decre Eg, asi whi ng le E the g, whi high le t con he d high uctivity cond incre uctivity asedincre the ased the the th WO e th WO 3 e th fWO ile 3 m fWO il 3 is m f il enh 3 is m f il enh is anced m enh is anced enh anced with anced with decre with decre with asi decre ng asi decre ng asi Eg,ng asi E whi g,ng E whi le g, E whi tle he g, whi tle he high tle he high tcon he high con high ductivity con ductivity con ductivity dincre uctivity incre as incre ed as incre ed th as e ed th ase ed th e th e the thWO e WO 3 fil 3 m film is is enh enh anced anced with with decre decre asiasi ng ng Eg,E whi g, whi le le the the high high con con ductivity ductivity incre incre ased ased th e th e the th WO e th WO e 3 th fWO il 3 e m th fil WO 3 e m is th fil WO enh is e 3 m th f WO il enh e is 3 th m anced f WO il e enh th 3 is anced m WO fe il th enh 3 is anced m WO fwith e il 3 enh is m anced WO fwith il 3 enh m fis anced il decre with 3 m is enh fanced decre il with is m enh anced asi decre with enh is anced asi ng decre enh with anced ng asi decre E with anced g ng , asi E with decre whi g ,with ng asi E whi decre g le ,with decre ng asi E whi t le ghe decre ,ng asi E whi t le he gdecre high asi ,ng E whi tle he asi g high ng , E whi tle ng asi g con he high ,E whi t gcon ng le he ,E high d whi g uctivity t ,con le E he d high whi g uctivity le t ,con he d whi high le uctivity t he con high d tle he incre uctivity high con d tincre he uctivity high con d as incre high uctivity con ed d as con incre uctivity ed d th as uctivity con incre d e ed th uctivity as incre e d ed th uctivity as incre e ed th as incre e ed th incre ase ed as incre th ed e as th ed e as th ed e th e th e electrochromic response time. electroch electroch electroch rom rom irom c response ic response ic response electroch time. ti electroch me. ti me. rom rom ic response ic response time. time. electroch electroch romic response rom electroch ic response rom time. ic response ti me. time. electroch electroch electroch rom rom ic response rom ic response ic response time. time. time. electroch electroch rom electroch rom electroch ic response ic response rom rom ic response tii electroch me. c response time. electroch ti rom me. ti rom me. i c response ic response time. time. electroch electroch rom electroch rom ic response electroch ic response electroch electroch rom rom ti ic response me. rom ti ic response rom me. ic response ic response time. time. time. ti me. The transparency of inorganic EC with high staining efficiency varies in response to Th Th e Th transpa e transpa e transpa rency rency reof ncy of ino of ino Th rg ino rg an e Th an transpa rg ic e an ic EC transpa EC ic with EC re with ncy with re hig ncy h of h ig h ino st h ig of ai st h rg ino n ai st ing an n ai rg ing ic n an e ing fficienc EC e ic fficienc e EC with fficienc with y h var y ig var y h h ies var ig st ies h ai in ies st n in re ing ai in s re n po ing s re epo fficienc ns spo ns e e fficienc te ns o te o y tvar o y ivar es in ies re in spo res ns po e ns toe to The transpa Th Th e e Th transpa transpa e re transpa ncy The re re of transpa ncy ncy ino rency of rg of ino re an ino of ncy ic rg ino rg EC an an of rg ic with ic ino an EC EC ic rg with h EC an with igic h with h st EC h ig ai ig h n with h h ing st ig st ai h ai n e st h n ing fficienc ig ai ing h ne ing st e fficienc fficienc aiy e nfficienc ing vary ie es y var fficienc var in y ies var re ies s in y i po in es var re ns re in se po s ies re po to ns sin ns po e e re t ns o ts o e po to ns e to The transparency of inorganic EC with high staining efficiency varies in response to ThTh e transpa e transpa Thre e Th transpa ncy re e Th ncy transpa of e Th transpa of ino e Th retranspa ino ncy rg e re transpa Th an rg ncy re of an ic e Th ncy ino transpa re of ic EC e ncy EC transpa re ino rg of with ncy an ino rg of with re ic an ino rg of h ncy EC re ig ic an h ino rg ncy h ig EC with of ic an st h rg EC ai ino of st with ic an n ai h EC ino ing rg ic with ig n ing an EC h h rg with e ig st ic fficienc an h h e with ai ig EC fficienc ic st n h h ai ing ig EC st with n h h y ai ing ig e with st var n y fficienc h ai ing h var e st n ig ifficienc es ai ing h h e in ig es fficienc in st ing y h e ai in re fficienc var st n y e s re ai ing fficienc po var in s es y po ing ns var e iin es y fficienc ns e var e re t iin e y es o fficienc s tvar po o i re in es s y ns i re po in es var e s y ns po re in tvar o ie s es ns re po ti o s in e es ns po to re in e ns s tre o po e s tns o po e ns to e to ThTh e transpa e transpa rency rency of Th of ino e ino rg transpa an rgic anEC ic reEC ncy with with of h ino ig h h ig rg st h an ai st ic n ai ing EC ning ewith fficienc efficienc hig y h var y stai var ies ning ies in in re efficienc sre po spo nse ns y to e var toi es in response to 2 −1 2 2 −1 2−1 −1 2 −1 2 −1 2 −1 2 −1 2 −1 the low-voltage signal. Wth O3e and low - N vo iO lta (ge Table sign 5) al. have WO 3 a and staini Nn iO g e (Table fficienc 5) y h of ave ~40 a sm staini ∙Cng , efficienc 2 y 2 −1 of −1 2~40 −1 sm ∙C , thth e low e thlow e -low vo -vo lta -th vo lta ge e lta ge low sig ge sn ig -al. vo sn th ig al. lta e n W al. low W ge Oth 3O W s - and e 3 ig vo O low and n 3lta al. and N - ge iO vo N W iO s N lta O (ig Table iO 3ge ( n Table and al. (s Table ig W 5) N nO 5) al. iO h3ave 5) h and W (ave Table h O a ave 3 N sta a and iO sta 5) a ini sta ( ini h N Table nave g ini iO ne g n fficienc (a e Table g 5) fficienc sta efficienc hini ave y 5) n of y g a h of e ave y sta ~40 fficienc of ~40 ini a sm ~40 n sta sm g y ∙C ini sm efficienc of ∙C n , ∙C g ~40 , efficienc , sm y of ∙C ~40 y , of sm ~40 ∙Csm , ∙C , the low th-e vo th low lta e low ge -vos -lta vo ign ge lta al. ge sig W n sO ig al. 3n and al. WO W N 3 O and iO 3 and ( Table NiO NiO (Table 5) (h Table ave 5) a h 5) ave sta hini ave a n sta g a e ini sta fficienc n ini g n eg fficienc y efficienc of ~40 y of sm y ~40 of∙C ~40 sm , sm ∙C ∙C , , the low-voltage signal. WO3 and NiO (Table 5) have a staining efficienc 2 y 2−1 of −1 ~40 sm ∙C , 2 2−1 −1 2 −1 2 −1 2 −1 2 2 −1 2−1 2−1 2 −1 −1 the thlow e low -vo -vo ltalta ge ge sig sn ig al. nal. WW O3O and 3 and N iO NiO (Table (Table 5) 5) have have a sta a sta iniini ng ne g fficienc efficienc y of y of ~40 ~40 sm sm ∙C∙C , , the th low e th low -e vo th low - lta e vo th low ge -e lta th vo low e th s ge -lta vo ig low e th n ge - s lta vo low ig al. e -ge s n vo lta low ig W al. -vo lta ge n sO ig al. W -lta ge vo 3s n O and ig al. W ge lta s 3n ig O and al. ge W s n N 3 ig al. O and W iO s n ig 3 N al. O W and n iO ( Table 3 N al. W O and iO ( 3 O Table N W and 3iO ( O Table N and 5) 3 iO ( N h and Table 5) ave iO N (Table h 5) iO ave ( N Table a h 5) iO (ave sta Table a h 5) (ini ave Table sta a 5) hn ave ini sta 5) g h a ave n e ini sta h 5) fficienc g a ave n sta ini e h a g fficienc ave sta n a ini eg fficienc sta ini y n a eg fficienc of ini sta n y e g fficienc ~40 n ini of e y g fficienc n ~40 of esm g y fficienc ~40 e of y sm fficienc ∙C ~40 of y sm ∙C , of y ~40 sm of ∙C ~40 , y sm ~40 of ∙C , sm ~40 ∙C , sm ∙C , sm ∙C , ∙C , , the th low e low -vo -vo ltalta ge ge sigs n ig al. nal. WO W 3 O and 3 and N iO NiO (Table (Table 5) 5) have have a sta a sta iniini ng neg fficienc efficienc y of y of ~40 ~40 smsm ∙C ∙C , , 2 −1 2 2 −1 2−1 −1 2 −1 2 −1 Nanomaterials 2021, 11, 2376 2 −1 2 −1 2 −1 13 of 32 while while while for for org for org ani org ani c ani EC c EC c fEC ilm filwhile s f m il , s m s, uch s s, uch for such as as org PE as PE DO ani PE DO c T, DO EC T, thT, is th fil th is v m alue is s v, alue v such alue is is mo as is mo re PE mo re th DO re an th T, th an 100 an th 100 is sm 100 v sm alue ∙C sm ∙C ∙C [is 32,39] [mo 32,39] [32,39] re . 2th . an 2 −1 . −1 2100 −1 sm ∙C [32,39]. while for while organi for cwhile org EC ani fil while for m c s EC , org s for uch fani ilm org c as s , EC ani s PE uch f DO cil EC m as T, s , PE fs th iluch m DO iss , vT, as s alue uch th PE is is DO as v mo alue PE T, re DO th is th is T, an mo value th 100 re is th v is sm alue an mo ∙C 100 re is mo th [ sm 32,39] an re ∙C 100 th . an [32,39] sm 100 ∙C. sm [32,39] ∙C [32,39] . . while while forwhile org for ani for org c EC org anifani c il m EC cs , EC fs il uch m fil sm , as ss uch , PE such DO as PE as T, DO PE this DO T, value th T, is th v is is alue mo value re is mo th isan mo re 100 th rean th sm an 100 ∙C 100 sm [32,39] sm ∙C ∙C [. 32,39] [32,39] . . while for organic EC films, such as PEDOT, this value is more 2th2 an −1 −1 100 2sm−1 2∙C−1 [32,39]. 2 2 −1 −1 2 2 −1 −1 2 −1 2 2 −1 2−1 −1 while while for for org org ani ani c EC c EC film fils m , s s, uch such as as PE PE DO DO T, T, this th is value value is is mo mo re re th an than 100 100 sm sm ∙C∙C [32,39] [32,39] . . while while while for while for org while for org ani while for org while ani c for EC org while ani c for EC org fani c for il m org EC f ani c for il s org , m EC ani fc s il s org uch ani EC m , c fs il s EC uch ani c , m as f s il EC s uch c m , fPE as il s EC s m uch f, il DO PE as s s m uch , f il DO s s PE as T, m , uch s s DO as PE th uch T, , s is as DO PE uch th T, v as PE is DO alue th T, PE as v DO is alue th T, DO PE v is is T, alue th DO mo v T, is is th alue mo is v re th T, is alue is v th mo th re is alue v an is mo alue th re is v an 100 is mo alue th re an is mo 100 th re sm mo is an 100 re th sm ∙C mo re an th 100 sm an ∙C th re 100 [32,39] sm an ∙C th 100 [sm 32,39] an 100 ∙C [ sm . 32,39] 100 ∙C sm [. 32,39] ∙C sm [∙C 32,39] . [32,39] ∙C . [32,39] . [32,39] . . . while while for for org org ani ani c EC c EC film filsm , s such , such as as PE PE DO DO T, T, this th v isalue value is mo is mo re re th an than 100 100 smsm ∙C ∙C [32,39] [32,39] . . Act Act ually, TM Act ually, TM Act ually, TM ually, TM O have O have O have O have a h a igh h a Act igh h physic a igh h Act ually, TM physic igh physic ually, TM physic al al an al an O have d chem al an d chem O have an d chem d chem ic a al st h icigh a al st ich abi al st ic physic igh abi al st lity. abi physic lity. abi lity. al lity. an al d chem and chem ical st ical st abilabi ity.l ity. Actually, TM Actually, TM O have a O have high physic a high al physic and chem al an ical st d chem abilic ity. al st ability. Actually, TM Act Act ually, TM ually, TM O have O have a O have higha physic ha igh high physic al physic and chem al an al d chem an icd chem al stabi ical st l ic ity. al st abi abi lity. lity. Actually, TMO have a high physical and chemical stability. Act Act ually, TM ually, TM O have O have a h a igh high physic physic al al anan d chem d chem ical st ical st abi abi lity. lity. Act Act ually, TM Act ually, TM Act ually, TM Act ually, TM O have Act ually, TM O have Act ually, TM O have Act ually, TM a O have Act h ually, TM a igh Act O have h ually, TM a igh physic O have ually, TM high a physic O have ha igh physic O have h al O have a igh physic an al hO have a igh physic d chem an al h a igh d chem physic an h a al igh physic h d chem an a al igh ic physic h d chem an al st igh al ic physic d chem al st an al physic ic abi d chem al st al an abi ic lity. al an d chem al st abi lic ity. an d chem al al st l abi ic d chem an ity. al st abi ic d chem lity. al st ic abi lity. al st icabi lal st ity. ic abi lal st ity. abi lity. abi lity. lity. Table 5. Color variation in WO3, NiO and WO3/NiO electrochromic films (colored and bleached Table Table Table 5. 5. Co Co 5. lo r Co lo variati rlo variati r variati onon in Table on in WO Table in WO 35. ,WO Ni 3 ,Co Ni 5. O 3,lo O Ni Co and r and O variati lo r WO and variati WO 3on /NiO WO 3 /NiO in on 3/NiO WO e in le ecle WO trochrom 3,e c le Ni trochrom c 3,O trochrom Ni and O ic and ic fi WO l m fi ic lWO s 3 m fi /NiO (co l s m 3 (co /NiO l s ored (co e lored lec lored trochrom e an lean d ctrochrom ble d anble d ached ic ble a ched fia ic lm ched fi s l(co m s lored (colored and an ble d able ched ached Table Table 5. Colo 5. rCo Table variati lor variati on 5. Co in on lo WO r in variati 3, WO NiO on 3, and Ni in O WO WO and 3 3, /NiO WO NiO 3 /NiO e and lectrochrom WO elec3trochrom /NiO ic e file lm c ic trochrom s fi (co lm ls ored (co icl an ored fild mble s an (co a d ched lble ored a ched and bleached Table Table Table 5. Co 5. lo Co 5. r lo Co variati r lo variati r variati on on in WO on in WO in 3, Ni WO 3, O Ni 3,and O Ni and O WO and WO 3/NiO WO 3/NiO 3e/NiO lec etrochrom lec etrochrom lectrochrom ic fi ic lm fi ic s lm (co fi s lm l(co ored s l(co ored an lored d an ble d an a ble ched d a ble ched ached Table Table 5. 5. Co Co loTable rlo variati r variati 5.on Co on in lo r in WO variati WO 3, Ni 3, on Ni O O in and and WO WO 3WO , 3 Ni /NiO 3O /NiO and elee cWO le trochrom ctrochrom 3/NiO ic e le ic fic l m trochrom fils m (co s (co lored lic ored fian lm d an s ble (co d ble a lored ched ached an d bleached Table Table Table 5. Co 5. Co lo Table 5.r lo Co variati Table r lo variati Table 5. r Table variati Co on 5.Table lo on Co in 5. rTable lo on WO in variati Co 5.r 5. Co WO lo in variati 3 ,rCo lo 5. WO Ni variati on 3r ,lo Co O Ni variati on r in 3 ,lo and O variati Ni WO on r in and O variati on WO WO in 3and , on WO Ni in WO 3 3/NiO , on O in WO WO Ni 3 3/NiO ,and WO in O Ni 3 3 e ,/NiO and WO le O Ni 3WO ,e c trochrom le Ni and O 3WO c , e 3 O and trochrom /NiO Ni le WO c and 3 O trochrom /NiO WO and 3 e ic /NiO WO le 3c fi /NiO ic e trochrom WO l le m 3/NiO fi c e ic s l trochrom le m 3 (co /NiO fi e cs le trochrom lm (co e lcored le trochrom s icc le (co ored trochrom fi le ic lc m an l ored trochrom fi ic s d lan m (co fi ic ble s d l an m lfi ic (co ored ble al s d ched m fi l ic (co a ored ble ls m ched fi (co an ls a ored lm ched d (co lan ored s ble l(co d ored an a ble lched d an ored a ble d an ched ble d aan ched ble a d ched a ble ched a ched Table 5. Color variation in WO , NiO and WO /NiO electrochromic films (colored and bleached states). 3 3 states). states). states states st ).ates ). ). states). states)st . ates). states).st ates states ). ). states). states states ). ). states states ).st ates ).st ates ).st ates ) st . ates st ).ates ) st . ates ). ). states states ). ). Inorganic EC Inorganic EC Inorg Inorg Inorg anan ic E an ic E C Inorg ic E C C an ic E Inorg Inorg C an Inorg an ic E ic E an C ic E C C Inorgan Inorg ic E Inorg an C ic E an ic E C C Inorganic EC Inorg Inorg anan ic E ic E C C Inorg Inorg an Inorg ic E an Inorg ic E an Inorg C Inorg ic E an CInorg ic E an CInorg an ic E Can ic E C ic E an C ic E C C Inorg Inorg anan ic E ic E C C State WO NiO NiO/WO State WO3 NiO NiO/WO3 Stat Stat e Stat e e State WO Stat WO Stat 3WO 3e 3 Stat e 3 eWO 3 WO WO NiO 3 NiO WO 3 NiO 3 NiO NiO NiO/WO NiO NiO/WO NiO/WO NiO 3 3 NiO/WO 3 NiO/WO 3 NiO/WO NiO/WO 3 3 3 3 State Stat Stat e e WO3 WO WO 3 3 NiO NiO NiO NiO/WO NiO/WO NiO/WO 3 3 3 State WO3 NiO NiO/WO3 Stat Stat e e WO WO 3 3 NiO NiO NiO/WO NiO/WO 3 3 Stat Stat e Stat e Stat e Stat eStat e Stat eStat WO e WO e 3 WO 3 WO 3 WO 3 WO 3WO 3 WO 3 3 NiO NiO NiO NiO NiO NiO NiO NiO NiO/WO NiO/WO NiO/WO NiO/WO 3 NiO/WO 3 NiO/WO 3 NiO/WO 3 NiO/WO 3 3 3 3 Stat Stat e e WO WO 3 3 NiO NiO NiO/WO NiO/WO 3 3 Oxidi Oxidi Oxidi zeze d ze d d Oxidized Oxidized Oxidi zedOxidi ze Oxidi d zed Oxidized Oxidize Oxidi dOxidi zed ze d Oxidized Oxidi Oxidi zeze d d Oxidi Oxidi ze Oxidi dze Oxidi d ze Oxidi d Oxidi ze Oxidi d ze Oxidi d ze d ze d ze d Oxidi Oxidi zed ze d Reduction Red Red uction Red uction uction RedRed uction uction Reduction Red uction Red Red uction Red uction Red uction uction Red Red uction uction Reduction Red Red uction Red uction Red uction Red uction Red uction Red uction Red uction Red uction Red uction uction TMO belong to type III materials, according to I. F. Chang’s classification. Both anodic (A) and cathodic (C) reactions are possible, depending on the redox state of the electrochromic film. Table 6 describes the electrochemical anodic and cathodic reactions of certain oxides. Table 6. Electrochemical reactions of certain oxides. Metal Oxide Electrochemical Reaction Color Change Reaction Type Yellow $ Manganese oxide (II) MnO + ze + zH , MnO OH A ( ) (2z) brown Green $ Cobalt oxide (II) 3CoO + 2OH , Co O + H O + 2e A 3 4 2 light blue h i Colorless $ II III + Nickel oxide (II) A NiO H , Ni (1 z)Ni O H + zH + ze x y z x (yz) brown + 1 VI V Molybdenum oxide (VI) Colorless $ blue C MoO + x Li + e , Li Mo Mo O 3 x x 3 (1x) Blue $ brown (A) Li V O , V O + x Li + e (A) x 2 5 2 5 Vanadium oxide (V) C/A V O + x M + e , M V O (C) Yellow $ light blue (C) 2 5 x 2 5 Cerium oxide (IV) CeO + x Li + e , Li CeO Yellow $ transparent C 2 x 2 Niobium oxide (V) Nb O + x Li + e , Li Nb O Colorless $ light blue C 2 5 x 2 5 Ruthenium oxide (IV) RuO  2H O + H O + e , 0, 5(Ru O  5H O) + OH Blue $ brown/yellow C 2 2 2 2 3 2 + III I Indium oxide (ITO) In O + 2x Li + e , Li In In O Colorless $ light blue C 2 3 2x x 3 (1x) Iridium oxide (III) Ir(OH) , IrO  H O + H + e Colorless $ blue/grey C 2 2 V + VI V Tungsten oxide (VI) W O + x Li + e , Li W W O Colorless $ blue/black C 3 x 3 (1x) V + VI V W O + x H + e , H W W O 3 x 3 (1x) 3.4. WO Electrochromic Films Tungsten (VI) oxide (WO ) is the most universal EC, and its electrochromic properties were first described by S. K. Deb in 1969 [17]. This oxide is still widely investigated [32,40]. High functionality, high staining efficiency, high contrast, high chemical stability, and long life cycle are all features that make tungsten (VI) oxide useful in practice [41,43]. WO electrochromic films exhibit a deep blue color, preserve their color for some hours after the voltage is removed (electrochromic memory), and demonstrate high cyclic stability in comparison to other TMO [32]. The electrochromic mechanism of WO film is shown in Figure 16. Nanomaterials 2021, 11, x FOR PEER REVIEW 14 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 14 of 32 Nanomaterials 2021, 11, 2376 14 of 32 Figure 16. Electrochromic mechanism of WO3 film. Figure 16. Electrochromic mechanism of WO film. Figure 16. Electrochromic mechanism of WO3 film. WO3 films have different colors depending on x. At low values of x, the film is colored WO films have different colors depending on x. At low values of x, the film is colored blue, and WOat 3 films high hva avlu e different es of x, it c has olors eithe depen r a red ding or on gox. lden At lo tint w . val Thu es es e of phx enom , the fi ena lm ar is e col asso- ored blue, and at high values of x, it has either a red or golden tint. These phenomena are cia blue, ted and withat the high fact vath luat, es of firs x, tly it , has WO e3ithe is parti r a red ally or red goluced den tint to . th Th e es oxida e ph ti enom on stena ate V+ are , asso- and associated with the fact that, firstly + , WO is partially reduced to the oxidation state V+, and secondl ciated y, with the t he add fact ition thof at, th firs e Li tly, c WO ation 3 is occurs partially ; all red this uced leadto s to th ch e o ange xida s tiin on th st e ate band V+,g an ap d secondly, the addition of the Li cation occurs; all this leads to changes in the band gap and, a secondl s a y, con ths eequ add en ition ce, in t of he l the ig Li ht tran cation smitt occurs ance of t ; all he TMO this lead.s to changes in the band gap and, as a consequence, in the light transmittance of the TMO. At the same time, the molecular reaction in WO3 films can be described as follows and, as a consequence, in the light transmittance of the TMO. At the same time, the molecular reaction in WO films can be described as follows [68]: [68]: At the same time, the molecular reaction in WO3 films can be described as follows [68]: (5)(5) (5) In [69], it was shown that the electrochemical reaction at the WO3/electrolyte interface In [69], it was shown that the electrochemical reaction at the WO /electrolyte interface plays In an [69] imp , it era wtive as shown role i n thth at e th electroc e electroch hrom em ic ical perfo reaction rmance at th of e W WO O3 3/elect electrodes rolyte , and interth face e plays an imperative role in the electrochromic performance of WO electrodes, and the lipla thium ys an -ion imp trans erative format role ion in mec the hanis electroc m hrom at thic e perfo WO3/elect rmance rolyte of W interf O3 electrodes ace was , de and moth n-e lithium-ion transformation mechanism at the WO /electrolyte interface was demonstrated, strated, lithium wher -ion ein the transformat states ioar n e mec replhanis aced f m rom at one pha the WO se to anoth 3/electrolyte er. interface was demon- wherein the states are replaced from one phase to another. The high efficiency of amorphous WO3 films [40,70] manifests in a reversible switch strated, wherein the states are replaced from one phase to another. The high efficiency of amorphous WO films [40,70] manifests in a reversible switch from tran The spar high ent effic to iency dark b of lue amor duri ph ng ous elect WO rochem 3 films ical [40,7 redox 0] m re anifests actions in (Fig a r ure eversib 16). le Electro- switch from transparent to dark blue during electrochemical redox reactions (Figure 16). Elec- chrom from tran ic pro spar perties, ent to s dark uch as blue stai d nuri ing n g efel ficiency, ectrochem and ical switch redox ing re tim actio e ns are (Fig depen ure d 16) ent . Electro- on the trochromic properties, such as staining efficiency, and switching time are dependent on atom chrom ic ic struct prop ure, erties, nanop such art as icl st e ai size, ning po efficiency, re size and and ab switch sorpting ion tim prop e are erties depen [71,7 dent 2]. on O. th F. e the atomic structure, nanoparticle size, pore size and absorption properties [71,72]. O. F. Schirmer atomic struct suggested ure, nanop that t art he icl op e tica size, l absorpt pore siz ion e and phenom absorpt enon ion in pr WO op3erties films [71,7 was2] due . O.to F. Schirmer suggested that the optical absorption phenomenon in WO films was due to small (V) (V) (VI) (VI) small polaron (SP, charged and polarized quasiparticles) transitions from W ions to W Schirmer suggested that the optical absorption phenomenon in WO3 films was due to polaron (SP, charged and polarized quasiparticles) transitions from W ions to W ones. (V) (VI) ones. small In po [42,69,73] laron (SP, , th che ar lig geh d t and absorpt polariz ion ed mec quas hanism ipart icl in eamorph s) transious tions WO from 3 was W is ion describe s to Wd In [42,69,73], the light absorption mechanism in amorphous WO was is described as the (V) (V) as ones. the interv In [42,69,73] al optic, ally the ind ligh uced t absorpt transion fer of mec 5d1 hanism -electro in n amorph of the W ous i on WO (A 3 ) was to th is e describe adjacentd interval optically induced transfer of 5d1-electron of the W ion (A) to the adjacent empty (VI) (VI) (V) emp as th ty 5d e interv 0-oral bital op o tic f ally the ind W uced ion ( tr B ans ): fer of 5d1-electron of the W ion (A) to the adjacent 5d0-orbital of the W ion (B): (VI) empty 5d0-orbital of the W ion (B): ( V) ( VI) ( VI) ( V) hν (V) (VI) (VI) (V) (6) W A + W B ⎯⎯→ W A + W B ( ) ( ) ( ) ( ) W (A) + W (B) ! W (A) + W (B) (6) ( V) ( VI) ( VI) ( V) hν (6) W A + W B ⎯⎯→ W A + W B ( ) ( ) ( ) ( ) where А and В represent tungsten sublattice knots. where A and B represent tungsten sublattice knots. where А This a ph nd enom В repres enonent was tungsten studied sublatt using ice X-ray knotph s. otoelectron spectroscopy (XPS) and This phenomenon was studied using X-ray photoelectron spectroscopy (XPS) and electron Th is spin phresonanc enomenon e (E was SR) st spect udied rosc usi op ny g [74 X-ray ,75]. ph The otoele WO ct 3 ron films spect showed roscop high y (XP absorp- S) and electron spin resonance (ESR) spectroscopy [74,75]. The WO films showed high absorption tion in the near-infrared region due to polaron absorption [76]. Activated WO3 films are electron spin resonance (ESR) spectroscopy [74,75]. The WO3 films showed high absorp- in the near-infrared region due to polaron absorption [76]. Activated WO films are char tion acteri in thze e d ne by ar-a infrar wided e ab re sorpt gion ion due ban to d pola with ron a max absor imum ption of [76] 0.9 . – Ac 1.46 tivated eV, depe WOnding 3 films on are characterized by a wide absorption band with a maximum of 0.9–1.46 eV, depending on the th char e film acteri pro ze pd erties by a [wid 73]. e Figure absorpt 17 ion show ban s d thwith e op tic a max al tra imum nsmisof sion 0.9s –pec 1.46 tra eV, of depe WO3nding (Figuron e film properties [73]. Figure 17 shows the optical transmission spectra of WO (Figure 17a) 17a) the and film WO prop 3/G erties O (F[igure 73]. Figure 17b) fi17 lms show upos n color the op ing an tical td ble ransm achi issn ion g. spectra of WO3 (Figure and WO /GO (Figure 17b) films upon coloring and bleaching. 17a) and WO3/GO (Figure 17b) films upon coloring and bleaching. Nanomaterials 2021, 11, 2376 15 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 15 of 32 0.0 V −1.0 V 0.0 V −1.1 V −1.0 V −1.2 V −1.1 V 60 −1.3 V −1.2 V −1.4 V −1.3 V −1.5 V −1.4 V −1.6 V −1.5 V −1.7 V −1.6 V −1.8 V −1.7 V −1.9 V −1.8 V −2.0 V −1.9 V −2.1 V −2.0 V −2.2 V −2.1 V −2.3 V −2.2 V −2.3 V 400 600 800 1000 400 600 800 1000 , nm , nm (a) (b) Figure 17. Optical transmission spectra of WO3 film during electrochromic response obtain by electrochemistry (cathodic) Figure 17. Optical transmission spectra of WO film during electrochromic response obtain by electrochemistry (cathodic) deposition: (a) WO3 at constant potential; (b) WO3/GO deposition at AC potential [77]. deposition: (a) WO at constant potential; (b) WO /GO deposition at AC potential [77]. 3 3 ThThe e opoptical tical pro pr pert operties ies of of WO WO 3 thin thin film films s depen depend d on on their st theirrstr ucture uctur (crys e (crystalline, talline, popoly- ly- crysta crystalline, lline, amamorphous orphous or hybrid) or hybrid). . ColoColor red and ed color and les colorless s states states of WOof 3 fil WO ms are films not ar sym- e not met symmetric. ric. Switching Switching from fr trom ansparent transpar to ent colored to color states ed states, , poly polycrystalline crystalline WO WO 3 films films exhibit exhibit reflect reflective ive pr pr op operties, erties, and and amorphous amorphousWO WO3 films films exhibit exhibit absorption absorption properties. propert The ies. switch- The switching ing time tim depends e depen on dWO s on WO film 3 density film density and on and electr on olyte electrol concentration. yte concentration. Low-density Low-den films - with high-concentration electrolytes demonstrate the fastest switching speed [78]. sity films with high-concentration electrolytes demonstrate the fastest switching speed Nowadays, the importance of WO films has grown [79,80] due to their use in “Smart [78]. 3 Windows”, which smartly regulate indoor solar radiation by changing their optical trans- Nowadays, the importance of WO3 films has grown [79,80] due to their use in “Smart mittance, contributing to a significant reduction in a building’s energy consumption (as a Windows”, which smartly regulate indoor solar radiation by changing their optical trans- result of the optimization of air conditioning consumption) and helping to create comfort- mittance, contributing to a significant reduction in a building’s energy consumption (as a able indoor environments [81]. However, despite all the advantages of WO films, their life result of the optimization of air conditioning consumption) and helping to crea 3 te comfort- cycle is not very long: continuous switching between colored and colorless states causes able indoor environments [81]. However, despite all the advantages of WO3 films, their irreversible structural changes that affect their optical and electrical properties, ultimately life cycle is not very long: continuous switching between colored and colorless states leading to material degradation, the so-called “aging” effect [82]. Therefore, the task of causes irreversible structural changes that affect their optical and electrical properties, ul- increasing the life cycle of WO films involves the development of new nanomaterials timately leading to material degradation, the so-called “aging” effect [82]. Therefore, the and/or the improvement of existing materials through the use of modificatory additives, task of increasing the life cycle of WO3 films involves the development of new nanomateri- as well as the obtained improvement of WO film technologies [83–86]. als and/or the improvement of existing materials through the use of modificatory addi- tives, as well as the obtained improvement of WO3 film technologies [83–86]. 4. ECD (Electrochromic Device) Structure EC are able to reversibly change their optical properties through the application of 4. ECD (Electrochromic Device) Structure an electrical voltage, making them suitable for ECD, such as displays [30], electrochromic Nanomaterials 2021, 11, x FOR PEER REVIEW 16 of 32 EC are able to reversibly change their optical properties through the application of “Smart Windows” [16], anti-glare rear mirrors [19], and sensors [87]. an electrical voltage, making them suitable for ECD, such as displays [30], electrochromic ECD structures usually include transparent conductors, electrochromic layers, and “Smart Windows” [16], anti-glare rear mirrors [19], and sensors [87]. ion conductors (Figure 18). ECD structures usually include transparent conductors, electrochromic layers, and ion conductors (Figure 18). Substrate In O (ITO) 2 3 Ion conductor (electrolyte) U EC In O (ITO) 2 3 Substrate Figure 18. ECD structure. Figure 18. ECD structure. ECD for architectural applications include thin EC films placed between two glass panels, as shown in Figures 19 and 20. ECD for architectural applications include thin EC films placed between two glass panels, as shown in Figures 19 and 20. Figure 19. ECW scheme showing voltage-induced transfer of positive ions and electrons to trans- parent conductive layers. Cycle stability is an extremely important aspect in the performance of electrochromic devices. In a recent study [88], ECD were reported to have obtained a superior long-term cycling stability of over 10,000 cycles. This manuscript is recommended for its review of some reports of devices with high long-term stability. In [89], a strategy was presented involving an all-in-one self-healing electrochromic material, TAFPy-MA, which was used for the fabrication of a high-reliability, large-scale and easy-to-assemble smart electro- chromic window. The all-in-one self-healing electrochromic material was able to carry out in situ redox reactions with the Li ions. The Diels-Alder cross-linking network structure was able to heal the cracks, improving the reliability of the electrochromic layer. Great ion 2 −1 diffusivity (1.13 × 10–5 cm s ), rapid color switching (3.9/3.7 s), high coloration efficiency 2 −1 (413 cm C ), excellent stability (sustain 88.7% after 1000 cycles) and reliability (crack can be healed in 110 s), large-scale “Smart Windows” (30 × 35 cm ) were achieved using this all-in-one electrochromic material, and these exhibited fascinating and promising features for a wide range of applications in buildings, airplanes, etc. Figure 20. ECW color cycle (colored ↔ semitransparent ↔ transparent state). T, % T, % Nanomaterials 2021, 11, x FOR PEER REVIEW 16 of 32 Substrate In O (ITO) 2 3 Ion conductor (electrolyte) EC In O (ITO) 2 3 Substrate Figure 18. ECD structure. Nanomaterials 2021, 11, x FOR PEER REVIEW 16 of 32 ECD for architectural applications include thin EC films placed between two glass panels, as shown in Figures 19 and 20. Substrate In O (ITO) 2 3 Ion conductor (electrolyte) EC In O (ITO) 2 3 Substrate Figure 18. ECD structure. Nanomaterials 2021, 11, 2376 16 of 32 ECD for architectural applications include thin EC films placed between two glass panels, as shown in Figures 19 and 20. Figure 19. ECW scheme showing voltage-induced transfer of positive ions and electrons to trans- parent conductive layers. Cycle stability is an extremely important aspect in the performance of electrochromic devices. In a recent study [88], ECD were reported to have obtained a superior long-term cycling stability of over 10,000 cycles. This manuscript is recommended for its review of some reports of devices with high long-term stability. In [89], a strategy was presented involving an all-in-one self-healing electrochromic material, TAFPy-MA, which was used for the fabrication of a high-reliability, large-scale and easy-to-assemble smart electro- chromic window. The all-in-one self-healing electrochromic material was able to carry out in situ redox reactions with the Li ions. The Diels-Alder cross-linking network structure was able to heal the cracks, improving the reliability of the electrochromic layer. Great ion 2 −1 diffusivity (1.13 × 10–5 cm s ), rapid color switching (3.9/3.7 s), high coloration efficiency 2 −1 (413 cm C ), excellent stability (sustain 88.7% after 1000 cycles) and reliability (crack can be healed in 110 s), large-scale “Smart Windows” (30 × 35 cm ) were achieved using this all-in-one electrochromic material, and these exhibited fascinating and promising features Figure 19. ECW scheme showing voltage-induced transfer of positive ions and electrons to trans- Figure 19. ECW scheme showing voltage-induced transfer of positive ions and electrons to transpar- for a wide range of applications in buildings, airplanes, etc. parent conductive layers. ent conductive layers. Cycle stability is an extremely important aspect in the performance of electrochromic devices. In a recent study [88], ECD were reported to have obtained a superior long-term cycling stability of over 10,000 cycles. This manuscript is recommended for its review of some reports of devices with high long-term stability. In [89], a strategy was presented involving an all-in-one self-healing electrochromic material, TAFPy-MA, which was used for the fabrication of a high-reliability, large-scale and easy-to-assemble smart electro- chromic window. The all-in-one self-healing electrochromic material was able to carry out Figure Figure20. 20. EC ECW W co color lor ccycle ycle ((color colored ed ↔ $ semitranspar semitransparent ent $↔transpar transparent s ent state). tate). in situ redox reactions with the Li ions. The Diels-Alder cross-linking network structure was able to heal the cracks, improving the reliability of the electrochromic layer. Great ion Cycle stability is an extremely important aspect in the performance of electrochromic 2 −1 diffusivity (1.13 × 10–5 cm s ), rapid color switching (3.9/3.7 s), high coloration efficiency devices. In a recent study [88], ECD were reported to have obtained a superior long- 2 −1 (413 cm C ), excellent stability (sustain 88.7% after 1000 cycles) and reliability (crack can term cycling stability of over 10,000 cycles. This manuscript is recommended for its review of some reports of devices with high long-term stability. In [89], a strategy was be healed in 110 s), large-scale “Smart Windows” (30 × 35 cm ) were achieved using this presented involving an all-in-one self-healing electrochromic material, TAFPy-MA, which all-in-one electrochromic material, and these exhibited fascinating and promising features was used for the fabrication of a high-reliability, large-scale and easy-to-assemble smart for a wide range of applications in buildings, airplanes, etc. electrochromic window. The all-in-one self-healing electrochromic material was able to carry out in situ redox reactions with the Li ions. The Diels-Alder cross-linking network structure was able to heal the cracks, improving the reliability of the electrochromic layer. 2 1 Great ion diffusivity (1.13 10–5 cm s ), rapid color switching (3.9/3.7 s), high coloration 2 1 efficiency (413 cm C ), excellent stability (sustain 88.7% after 1000 cycles) and reliability (crack can be healed in 110 s), large-scale “Smart Windows” (30  35 cm ) were achieved using this all-in-one electrochromic material, and these exhibited fascinating and promising features for a wide range of applications in buildings, airplanes, etc. Electrochromic films change their color as a result of electrochemical oxidation/reduction reaction associated with ion transfer, which involves the use of an additional coating for the storage and transport of ions. Many companies offer “Smart Window” solutions; Figure 20. ECW color cycle (colored ↔ semitransparent ↔ transparent state). energy-saving “Smart Window” technology is available on the market [4]. Depending on the purpose, ECD may contain materials with different characteristics and properties. Figure 21 presents the classifications of ECD. Nanomaterials 2021, 11, x FOR PEER REVIEW 17 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 17 of 32 Electrochromic films change their color as a result of electrochemical oxidation/re- duction reaction associated with ion transfer, which involves the use of an additional coat- ing for the storage and transport of ions. Many companies offer “Smart Window” solu- Electrochromic films change their color as a result of electrochemical oxidation/re- tions; energy-saving “Smart Window” technology is available on the market [4]. duction reaction associated with ion transfer, which involves the use of an additional coat- Depending on the purpose, ECD may contain materials with different characteristics ing for the storage and transport of ions. Many companies offer “Smart Window” solu- and properties. Figure 21 presents the classifications of ECD. tions; energy-saving “Smart Window” technology is available on the market [4]. Nanomaterials 2021, 11, 2376 17 of 32 Depending on the purpose, ECD may contain materials with different characteristics and properties. Figure 21 presents the classifications of ECD. Figure 21. ECD classification. 4.1. Substrate Figure Figure 21. 21. ECD ECD classification. classification. Transparent ECW substrates include glassy (Figure 22) or transparent polymers (Fig- 4.1. Substrate ure 22), such as polyethylene terephthalate (PET), polyvinyl butyral (PVB) and polyeth- 4.1. Substrate Transparent ECW substrates include glassy (Figure 22) or transparent polymers (Fig- ylene naphthalate (PEN). Glass substrates are more common due to their greater trans- Transparent ECW substrates include glassy (Figure 22) or transparent polymers (Fig- ure 22), such as polyethylene terephthalate (PET), polyvinyl butyral (PVB) and polyethylene parency and their chemical stability, which makes them suitable for the production of ure 22), such as polyethylene terephthalate (PET), polyvinyl butyral (PVB) and polyeth- naphthalate (PEN). Glass substrates are more common due to their greater transparency and “Smart Windows”. In turn, polymer substrates allow the production costs of ECD to be ylene naphthalate (PEN). Glass substrates are more common due to their greater trans- their chemical stability, which makes them suitable for the production of “Smart Windows”. reduced [90–93]. parency and their chemical stability, which makes them suitable for the production of In turn, polymer substrates allow the production costs of ECD to be reduced [90–93]. “Smart Windows”. In turn, polymer substrates allow the production costs of ECD to be reduced [90–93]. Light (T=100%) Dark (T=0%) ITO-Glass/0.01M H SO 2 4 Light (T=100%) Dark (T=0%) ITO-Glass/0.01M H SO 2 4 200 400 600 800 1000 1200 , nm 200 400 600 800 1000 1200 Figure Figure22. 22. Vis Visible ible and andnear near -infrar -infrared ed transmission transmissispectra on spec of tra o WO f WO -ITO-glass. 3-ITO-glass. , nm 4.2. Transparent Conductive Electrode Figure 22. Visible and near-infrared transmission spectra of WO3-ITO-glass. The electrical resistivity and the light transmission coefficient are the most important properties of transparent conductive electrodes (layers). An electrode should possess high electrical conductivity in order to form the electric field required for ECD. Transparent conductive electrodes include metal-based and oxide-based electrodes, but the electrode properties should not affect the transmission properties of the electrochromic windows. Indium-tin oxide (ITO) electrodes (indium (III) oxide and tin (IV) oxide) are among the best T, % T, % Nanomaterials 2021, 11, 2376 18 of 32 transparent electrodes to have been investigated ((In O )0.9-(SnO )0.1: 90% and 10%) [90], 2 3 2 possessing high electrical conductivity (~104 Ssm ) and low optical absorption (band gap ~4 eV, refractive index 1.9), making it preferable to fluorine-doped tin oxide (FTO). Transparent ITO electrode contains different numbers of doped Sn atoms, and consequently, free electron density varies [94]. 4.3. Electrochromic Layer EC films reversibly change their optical properties, switching between transparent, semi-transparent and colored states, modeling solar radiation and thus ensuring reliable ECW operation. EC films (layers) can be divided into three different types according to their color schemes [32]: - EC film exhibiting one color, for example, transition metal oxides, Prussian blue [31]; - EC film exhibiting two colors, for example, polythiophene [28]; - EC film exhibiting multiple colors, for example, poly (3,4-propylenedioxypyrrole) [29]. 4.4. Electrolyte (Ion Conductor) Electrolytes can be classified into liquid, gel and solid electrolytes [32]. Liquid elec- trolytes are dissolved ions. Such electrolytes provide high ionic mobility. Polymer elec- trolytes are the most suitable for EC devices, as they provide a long circuit break and uniformity of coloration [95]. Electrochromic device electrolytes are ionic materials that possess ionic conductivity. Electrochromic device electrolytes should satisfy the following requirements [77]: - compatibility with anodic and cathodic materials; - high ionic conductivity; - no electron transfer between electrochromic layers; - high transparency without scattering effect. In [96], a novel Zn–Prussian blue (PB) system was reported for aqueous electrochromic 2+ + batteries. By using different dual-ion electrolytes with various cations (e.g., Zn –K and 2+ 3+ Zn –Al ), the Zn–PB electrochromic batteries demonstrated excellent performance. We + 2+ showed that the K –Zn dual-ion electrolyte in the Zn–PB configuration endowed a rapid self-bleaching time (2.8 s), high optical contrast (83% at 632.8 nm), and fast switching times (8.4 s/3 s for the bleaching/coloration processes). Remarkably, the aqueous elec- trochromic battery exhibited a compelling energy retrieval of 35.7 mWhm , where only 47.5 mWhm was consumed during the round-trip coloration–bleaching process. These findings may open up new directions for the development of advanced net-zero-energy- consumption ECD. In [4,34,58,97], a hybrid electrolyte was developed based on aluminum trifluoromethane- sulphonate (Al(TOF)3) and H PO that could effectively alleviate the passivation, and 3 4 which exhibited superior stability. Additionally, an ex situ study revealed that the PANI cathode undergoes a process of cointercalation/deintercalation of Al(H PO )x(TOF )y 2 4 +(H O)n, TOF , and H during the charging/discharging process, with high reversibility and stability. As a proof of concept, an electrochromic Al//PANI battery was fabricated that combined both electrochromism and energy storage and delivered a higher coloration 2 1 efficiency of 84 cm C at a wavelength of 630 nm. 4.5. Counter Electrode The counter electrode provides ions, which, depending on the polarity of the applied voltage, are injected into or extracted from the electrochromic coating. The counter electrode should be transparent, with high conductivity, in order to reduce the voltage drop and Nanomaterials 2021, 11, x FOR PEER REVIEW 19 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 19 of 32 4.5. Counter Electrode The counter electrode provides ions, which, depending on the polarity of the applied Nanomaterials 2021, 11, 2376 4.5. Counter Electrode 19 of 32 voltage, are injected into or extracted from the electrochromic coating. The counter elec- The counter electrode provides ions, which, depending on the polarity of the applied trode should be transparent, with high conductivity, in order to reduce the voltage drop voltage, are injected into or extracted from the electrochromic coating. The counter elec- and prevent side reactions. Counter electrodes may include EC films, such as WO3/PANI trode should be transparent, with high conductivity, in order to reduce the voltage drop prevent side reactions. Counter electrodes may include EC films, such as WO /PANI films [98], switching from transparent to blue. and prevent side reactions. Counter electrodes may include EC films, such as WO3/PANI films [98], switching from transparent to blue. films [98], switching from transparent to blue. + − WO + PANI + xM A MxWO + PANI A ( ) 3 3 x + − (7)  WO + PANI + xM A MxWO + PANI A ()  33 x  ( tra nsparent)  (colored) (7 (7) ) (transparent) (colored) + + ‒ ‒ where x is the number of cations (М , H ) and anions (A , SO4 ). + + ‒ ‒ where x is the number of cations (М , H ) and anions (A , SO4 ). Thus, thin-film electrodes broaden + + the ECD color pa lette and strengthen the electro- where x is the number of cations (M , H ) and anions (A , SO4 ). Thus, thin-film electrodes broaden the ECD color palette and strengthen the electro- chromic effect. Thus, thin-film electrodes broaden the ECD color palette and strengthen the elec- chromic effect. trochromic effect. 5. WO3 Film Fabrication 5. WO3 Film Fabrication 5. WO Film Fabrication The EC WO3 layer is obtained as a thin film on a conductive substrate with an FTO The EC WO3 layer is obtained as a thin film on a conductive substrate with an FTO The EC WO layer is obtained as a thin film on a conductive substrate with an FTO or ITO electro or ITO electro de. There de. Th are ereseveral are severa WO3l fWO abric 3 at fabri ion t ce ation chniqtec ueshniq [99] ues (Fig ure [99]2 ( 3Fi ), in gure clud 23), ing including or ITO electrode. There are several WO fabrication techniques [99] (Figure 23), including magnetron sputtering [100], electrochemical deposition [101–106], spray pyrolysis [107], magnetron sputtering [100], electrochemical deposition [101–106], spray pyrolysis [107], magnetron sputtering [100], electrochemical deposition [101–106], spray pyrolysis [107], sol–gel [108,109], mechanical sputtering [110,111], etc. These technologies are based on sol–gel [108,109], mechanical sputtering [110,111], etc. These technologies are based on sol–gel [108,109], mechanical sputtering [110,111], etc. These technologies are based on electrochemical, chemical and physical principles. C. G. Granqvist [32] provided a com- electr electroch ochemical, emical, chemical chemic and al an physical d physi principles. cal princip C. G. les Granqvist . C. G. Granqv [32] pris ovided t [32]a pro compr vided e- a com- prehensive survey of WO3 fabrication technologies. hensive survey of WO fabrication technologies. prehensive survey of WO3 fabrication technologies. Figure 23. Classification of WO3 fabrication technologies. Table 7 shows a comparative analysis of WO3 fabrication technologies. Figure Figure 23. 23. Classification Classificatio of n o WO f Wfabrication O3 fabricat technologies. ion technologies. Table 7. Comparison of three basic approaches to films WO3 fabrication. Table 7 shows a comparative analysis of WO fabrication technologies. Table 7 shows a comparative analysis of WO3 fabrication technologies. Technology Types Scalability Equipment Cost Process Costs Coating Uniformity Table 7. Comparison of three basic approaches to films WO fabrication. Table 7. Comparison of three basic approaches to films WO3 fabrication. Electrochemical +/− + + +/− Technology Types Scalability Chemical Equipment +/− + Cost Process Costs+ Coating Uniformity − Technology Types Scalability Equipment Cost Process Costs Coating Uniformity Physical + − − + Electrochemical +/ + + +/ Electrochemical +/− + + +/− Chemical +/ + + Chemical +/− + + − The majority of technologies shown in Figure 24 are currently in use at the time of Physical + + writing. Optical contrast is Physical a ke + y parameter for − ev aluating EC dev −i ce quality. Howeve+ r, nowadays, there is no universal method that would satisfy all modern requirements. Each The majority of technologies shown in Figure 24 are currently in use at the time of method has its own advantages and shortcomings. The majority of technologies shown in Figure 24 are currently in use at the time of writing. Optical contrast is a key parameter for evaluating EC device quality. However, writing. Optical contrast is a key parameter for evaluating EC device quality. However, nowadays, there is no universal method that would satisfy all modern requirements. Each nowadays, there is no universal method that would satisfy all modern requirements. Each method has its own advantages and shortcomings. method has its own advantages and shortcomings. Nanomaterials 2021, 11, x FOR PEER REVIEW 20 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 20 of 32 Nanomaterials 2021, 11, 2376 20 of 32 Spray pyrolysis CVD Sputering Sol-gel Electrodeposition Spray pyrolysis Thermal evaporation CVD Sputering Years Sol-gel Electrodeposition Thermal evaporation Figure 24. Contrast response curves for WO3 films obtained by different processes during the re- Years porting period. WO3 film characteristics include porosity, crystallinity and crystal size; these proper- Figure 24. Contrast response curves for WO films obtained by different processes during the Figure 24. Contrast response curves for WO3 films obtained by different processes during the re- ties are highly dependent on manufacturing conditions and on production technology. reporting period. porting period. The requirements for WO3 thin films include uniformity, low production cost, and long life cycle. Unf WO ortfilm unatel characteri y, the pro stics duction include of a pun orosity iform , crystallinity WO3 film wand ith good crystal adhes size; ion these still prop- WO3 film characteristics include porosity, crystallinity and crystal size; these proper- remain erties s a pro arblem. e highly dependent on manufacturing conditions and on production technology. ties are highly dependent on manufacturing conditions and on production technology. Th The e vacuu requir m ements deposifor tion WO meth thin od makes films include it possibl uniformity e to obtain , low high pr -d oduction ensity WO cost, 3 films and long life cycle. Unfortunately, the production of a uniform WO film with good adhesion still on a Th large e requir flat sur emen face, and ts for the WO thicknes 3 thin s and films com inc po lu side tion un can iform be con ity, troll low ed pro during duction the cost, and long remains a problem. deposition process [100]. Vacuum-deposited WO3 films have an amorphous structure, and life cycle. Unfortunately, the production of a uniform WO3 film with good adhesion still The vacuum deposition method makes it possible to obtain high-density WO films annealed WO3 films have a crystalline structure. However, these technologies are highly remains a problem. on a large flat surface, and the thickness and composition can be controlled during the expensive due to the expensive equipment. Many glass manufacturing companies still The vacuum deposition method makes it possible to obtain high-density WO3 films deposition process [100]. Vacuum-deposited WO films have an amorphous structure, and prefer vacuum deposition technologies, regardless of 3 the cost, because WO3 films ob- on a large flat surface, and the thickness and composition can be controlled during the annealed WO films have a crystalline structure. However, these technologies are highly tained by vacuum d 3eposition are stable, reliable and adjustable. deposition process [100]. Vacuum-deposited WO3 films have an amorphous structure, and expensive due to the expensive equipment. Many glass manufacturing companies still Chemical vapor deposition (CVD) is used for depositing WO3 films on a substrate prefer vacuum deposition technologies, regardless of the cost, because WO films obtained annealed WO3 films have a crystalline structure. However, these technologies are highly [112]. However, during the deposition process substrates are heated to a high tempera- by vacuum deposition are stable, reliable and adjustable. ture, which can lead to structural changes in the conductive layer. Electron beam evapo- expensive due to the expensive equipment. Many glass manufacturing companies still Chemical vapor deposition (CVD) is used for depositing WO films on a substrate [112]. ration technology is a well-known method for preparing electrochromic WO3 films prefer vacuum deposition technologies, regardless of the cost, because WO3 films ob- However, during the deposition process substrates are heated to a high temperature, which [113,114]. tained by vacuum deposition are stable, reliable and adjustable. can lead to structural changes in the conductive layer. Electron beam evaporation technol- Chemical vapor deposition (CVD) is used for depositing WO3 films on a substrate ogy is a well-known method for preparing electrochromic WO films [113,114]. 5.1. Electrochemical Deposition [112]. However, during the deposition process substrates are heated to a high tempera- Electrochemical deposition (electrodeposition) is a method of low-temperature syn- 5.1. Electrochemical Deposition ture, which can lead to structural changes in the conductive layer. Electron beam evapo- thesis of WO3 films. Figures 25 and 26 show a three-electrode system in which conductive Electrochemical deposition (electrodeposition) is a method of low-temperature syn- ration technology is a well-known method for preparing electrochromic WO3 films FTO or ITO electrodes serve as a working electrode and a platinum electrode is used as a thesis of WO films. Figures 25 and 26 show a three-electrode system in which conductive [113,114]. counter electrode. FTO or ITO electrodes serve as a working electrode and a platinum electrode is used as a counter electrode. 5.1. Electrochemical Deposition Electrochemical deposition (electrodeposition) is a method of low-temperature syn- thesis of WO3 films. Figures 25 and 26 show a three-electrode system in which conductive FTO or ITO electrodes serve as a working electrode and a platinum electrode is used as a counter electrode. (a) (b) Figure 25. Two types of electrodeposition processes: (а) electroplating; (b) electrophoretic deposi- Figure 25. Two types of electrodeposition processes: (a) electroplating; (b) electrophoretic deposition. tion. (a) (b) Figure 25. Two types of electrodeposition processes: (а) electroplating; (b) electrophoretic deposi- tion. 2001-2003 2004-2006 2007-2009 2010-2012 2001-2003 2013-2015 2004-2006 2016-2018 2007-2009 2019-2020 2010-2012 2013-2015 2016-2018 2019-2020 Production, % Production, % Nanomaterials 2021, 11, x FOR PEER REVIEW 21 of 32 Nanomaterials 2021, 11, 2376 21 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 21 of 32 Figure 26. Growth mechanism of electrodeposited WO3 film. Figure 26. Growth mechanism of electrodeposited WO film. Figure 26. Growth mechanism of electrodeposited WO3 film. The applied potential is shown relative to the reference electrode. The most common The applied potential is shown relative to the reference electrode. The most common The applied potential is shown relative to the reference electrode. The most common reference electrode is the silver/silver chloride (Ag/AgCl) electrode (Figure 26), due to the reference electrode is the silver/silver chloride (Ag/AgCl) electrode (Figure 26), due to the reference electrode is the silver/silver chloride (Ag/AgCl) electrode (Figure 26), due to the stability of the electrode potential. stability of the electrode potential. stability of the electrode potential. The mechanism of electrochemical deposition of electrochromic WO3 films has been The mechanism of electrochemical deposition of electrochromic WO films has been The mechanism of electrochemical deposition of electrochromic WO3 films has been well investigated [106]; metal or precursor ions are transferred to the working electrode well investigated [106]; metal or precursor ions are transferred to the working electrode well investigated [106]; metal or precursor ions are transferred to the working electrode (cathode) under the influence of an applied electrical field. In this case, the metal deposi- (cathode) under the influence of an applied electrical field. In this case, the metal deposition (cathode) under the influence of an applied electrical field. In this case, the metal deposi- tion process can be described by the reaction: process can be described by the reaction: tion process can be described by the reaction: + − (8) M + e → M + − M + e ! M (8) M + e → M (8) As already mentioned [115,116], the electrochemical deposition method makes it pos- As already mentioned [115,116], the electrochemical deposition method makes it pos- As already mentioned [115,116], the electrochemical deposition method makes it sible to deposit WO3 films on large-area conductive substrates. However, special equip- sible to deposit WO3 films on large-area conductive substrates. However, special equip- possible to deposit WO films on large-area conductive substrates. However, special ment is required for the deposition process. The main advantages of this method include: ment is required for the deposition process. The main advantages of this method include: equipment is required for the deposition process. The main advantages of this method low cost and fast deposition, while not requiring high-temperature heating and deep vac- low include: cost and low fcost ast depo and s fast itiodeposition, n, while notwhile requiri not ng r hi equiring gh-temperature high-temperatur heating and e heating deep vac- and uum. uu deep m. vacuum. 5.2. Sol–Gel 5.2. Sol–Gel 5.2. Sol–Gel Colloidal oxide can be synthesized by polycondensation, by acidification of aqueous Colloidal oxide can be synthesized by polycondensation, by acidification of aqueous Colloidal oxide can be synthesized by polycondensation, by acidification of aqueous salt solution, or by hydrolysis of organometallic compounds (Figure 27). Recently, there salt solution, or by hydrolysis of organometallic compounds (Figure 27). Recently, there has salt solution, or by hydrolysis of organometallic compounds (Figure 27). Recently, there has been growing interest in the use of the sol–gel process to produce multilayer electro- been growing interest in the use of the sol–gel process to produce multilayer electrochromic has been growing interest in the use of the sol–gel process to produce multilayer electro- chromic coatings based on non-organic compounds. The main advantage of this reaction coatings based on non-organic compounds. The main advantage of this reaction is that chromic coatings based on non-organic compounds. The main advantage of this reaction is that liquid compounds are converted into solid compounds [117]. liquid compounds are converted into solid compounds [117]. is that liquid compounds are converted into solid compounds [117]. Figure 27. Sol–gel process scheme. Nanomaterials 2021, 11, x FOR PEER REVIEW 22 of 32 Figure 27. Sol–gel process scheme. Nanomaterials 2021, 11, 2376 22 of 32 Most alkoxides used for electrochromic materials can be produced in several stages [91]: (1) hydrolysis with the formation of reactive M-OH groups: Most alkoxides used for electrochromic materials can be produced in several stages [91]: (1) hydrolysis with the formation of reactive M-OH groups: M-OR + H2O → M-OH + ROH (9) M OR + H O ! M OH + ROH (9) (2) condensation resulting in bridge oxygen formation: (2) condensation resulting in bridge oxygen formation: M-OH + RO-M → M-O-M + ROH (10) M OH + RO M ! M M + ROH (10) M-OH + HO-M → M-O-M + H2O (11) M OH + HO M ! M M + H O (11) There are different types of sol–gel processes, such as c2entrifugation, immersion coat- ing and spraying (Figure 28). The sol–gel method, widely applied in material synthesis, is There are different types of sol–gel processes, such as centrifugation, immersion also u coating sed to and mo spraying dify the e (Figur lect erode s 28). The urfa sol–gel ce [118] method, . widely applied in material synthesis, is also used to modify the electrode surface [118]. (a) (b) (c) Figure 28. Types of sol–gel processes: (a) immersion coating; (b) centrifugation; (c) spraying. Figure 28. Types of sol–gel processes: (а) immersion coating; (b) centrifugation; (c) spraying. Sol–gel methods make it possible to produce large-area WO films at lower cost Sol–gel methods make it possible to produce large-area WO3 films at lower cost in in comparison with traditional vacuum methods [119]. The advantages of this method comparison with traditional vacuum methods [119]. The advantages of this method in- include: universality of sol–gel processes, easy control of microstructure and composition under low-temperature conditions, relatively simple and inexpensive equipment, control clude: universality of sol–gel processes, easy control of microstructure and composition of microstructure, crystal size, porosity and composition of the deposited films, which under low-temperature conditions, relatively simple and inexpensive equipment, control is important, since these characteristics affect thin film kinetics, durability and staining of microstructure, crystal size, porosity and composition of the deposited films, which is efficiency [120]. However, many problems still remain to be solved, among them solution important, since these characteristics affect thin film kinetics, durability and staining effi- stability, large-area uniformity, insufficient adhesion, insufficient film thickness, and low ciency [120]. However, many problems still remain to be solved, among them solution repeatability. stability, large-area uniformity, insufficient adhesion, insufficient film thickness, and low 5.3. Spray Pyrolysis repeatability. The main principle of spray pyrolysis is the pyrolytic decomposition of salt solution sprayed on substrate consisting of deposition target material (Figure 29). The sprayed 5.3. Spray Pyrolysis solution undergoes pyrolytic decomposition and forms a crystallite or a crystallite cluster when The ma thein dr pri op comes nciple into of spr contact ay pyro with lysis the hot is substrate the pyrol surface. ytic decomposition of salt solution sprayed on substrate consisting of deposition target material (Figure 29). The sprayed so- lution undergoes pyrolytic decomposition and forms a crystallite or a crystallite cluster when the drop comes into contact with the hot substrate surface. By-products and solvents evaporate during spraying. The hot substrate provides thermal energy for thermal decomposition. After thermal decomposition, sintering and crystallization of the crystallite clusters occur, ultimately leading to film formation. The technique is used for the deposition of dense and porous films on different substrates, such as glass, ceramics and metal. Nanomaterials 2021, 11, 2376 23 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 23 of 32 Figure 29. Pyrolytic deposition of EC films. Figure 29. Pyrolytic deposition of EC films. By-products and solvents evaporate during spraying. The hot substrate provides Spray pyrolysis is a simple and relatively inexpensive method that does not require thermal energy for thermal decomposition. After thermal decomposition, sintering and a vacuum. This method allows large-area uniform films with good adhesion to be pro- crystallization of the crystallite clusters occur, ultimately leading to film formation. The duced. Moreover, film properties can be easily modified by changing the spray parame- technique is used for the deposition of dense and porous films on different substrates, such ters, such as substrate temperature, flow pressure and the molarity of the precursor solu- as glass, ceramics and metal. tion. Th Spray e ma pyr in olysis advanta is age simple of thand is met relatively hod is inexpensive that it works method at mo that derdoes ate tem notper requir atures e a (100– vacuum. This method allows large-area uniform films with good adhesion to be produced. 500 °C) and allows films to be obtained even on low-quality substrates. It offers an easy Moreover, film properties can be easily modified by changing the spray parameters, such way of doping films with any elements in any proportion by adding them in some form as substrate temperature, flow pressure and the molarity of the precursor solution. The to the spray solution [121,122]. In [123], V2O5-WO3 composite films were reported to ex- main advantage of this method is that it works at moderate temperatures (100–500 C) 2 −1 hibit high coloration efficiency (49 cm ∙C ). Ref. [124], a fibrous reticulated WO3 film ob- and allows films to be obtained even on low-quality substrates. It offers an easy way of tained by pulsed spray pyrolysis was reported to have a coloration efficiency of 34 cm doping films with any elements in any proportion by adding them in some form to the −1 ∙C at λ = 630 nm. spray solution [121,122]. In [123], V O -WO composite films were reported to exhibit high 2 5 3 2 1 Spray pyrolysis is a cost-effective method for obtaining highly adhesive homogene- coloration efficiency (49 cm C ). Ref. [124], a fibrous reticulated WO film obtained by 2 1 ous pulsed WO3spray films pyr with olysis differe wasnt reported microst to ruc have tures. a coloration The techno efficiency logy can of 34 also cm be use C d at to pro = duce 630 nm. multilayer films, which is achieved by varying the spray composition. However, this Spray pyrolysis is a cost-effective method for obtaining highly adhesive homogeneous method also has disadvantages, such as the non-uniformity of films, large grain size due WO films with different microstructures. The technology can also be used to produce to uncontrolled sputtering, solvent loss, and low deposition rate. The mentioned ad- multilayer films, which is achieved by varying the spray composition. However, this vantages of the spray pyrolysis method make it suitable for industrial applications. method also has disadvantages, such as the non-uniformity of films, large grain size due to uncontrolled sputtering, solvent loss, and low deposition rate. The mentioned advantages 5.4. Magnetron Sputtering of the spray pyrolysis method make it suitable for industrial applications. Magnetron sputtering is a deposition technology defined as “cathodic sputtering of 5.4. Magnetron Sputtering target material in magnetron discharge plasma (crossed field discharge)”, and is shown Magnetron sputtering is a deposition technology defined as “cathodic sputtering of in Figure 30. target material in magnetron discharge plasma (crossed field discharge)”, and is shown in In this process, permanent magnets are arranged below the target plate so as to pro- Figure 30. duce a magnetic field close to the target material. This concentrates the electrons and causes them to travel in a spiral fashion along the magnetic flux lines of the target instead of wandering around the target material [100]. Nanomaterials 2021, 11, 2376 24 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 24 of 32 Figure 30. Magnetron sputtering apparatus (working principal). Figure 30. Magnetron sputtering apparatus (working principal). In this process, permanent magnets are arranged below the target plate so as to Magnetron sputtering is the most up-to-date deposition technology [99,100], and is produce a magnetic field close to the target material. This concentrates the electrons and widely used in the industrial and scientific spheres. The frequency of the applied positive causes them to travel in a spiral fashion along the magnetic flux lines of the target instead DC voltage varies from 20 to 350 kHz, while reversed pulse duration is dependent on of wandering around the target material [100]. dielectric surface discharge [125]. Negative voltage usually varies by an amount equiva- Magnetron sputtering is the most up-to-date deposition technology [99,100], and is lent to 10% of the average positive voltage. When the duration and number of positive widely used in the industrial and scientific spheres. The frequency of the applied positive voltage pulses are sufficient to create electric current, the target surface is bombarded with DC voltage varies from 20 to 350 kHz, while reversed pulse duration is dependent on ions, and when the voltage becomes negative, the incoming ions are repelled. H.-C. Chen dielectric surface discharge [125]. Negative voltage usually varies by an amount equivalent [126,127] investigated WO3 films deposited by pulsed magnetron sputtering at a constant to 10% of the average positive voltage. When the duration and number of positive voltage frequency of 70 kHz; the O2/Ar ratio was reported to vary from 0.2 to 1.0. pulses are sufficient to create electric current, the target surface is bombarded with ions, and The disadvantages of this method include the expensive equipment required and the when the voltage becomes negative, the incoming ions are repelled. H.-C. Chen [126,127] high energy intensity, which significantly increases ECD cost. The magnetron sputtering investigated WO films deposited by pulsed magnetron sputtering at a constant frequency technique is used to produce FTO or ITO electrodes on transparent surfaces. of 70 kHz; the O /Ar ratio was reported to vary from 0.2 to 1.0. The disadvantages of this method include the expensive equipment required and the 6. Nanomaterials for Electrochromic Devices high energy intensity, which significantly increases ECD cost. The magnetron sputtering ECW control the transmittance of light and solar radiation by changing their optical technique is used to produce FTO or ITO electrodes on transparent surfaces. transmittance (transparent, semitransparent and colored states), which ensures comforta- ble 6. Nanomaterials indoor environm for ent Electrochromic s and makes it Devices possible to achieve energy savings in buildings. Recent advances in ECD technology emerging in the 1970s led to the creation of different ECW control the transmittance of light and solar radiation by changing their optical types of ECD. However, there are still problems with respect to the commercialization of transmittance (transparent, semitransparent and colored states), which ensures comfortable EC devices, including aspects such as their high production cost [99], the stability of their indoor environments and makes it possible to achieve energy savings in buildings. Recent long-term operation, and the production of uniform electrochromic films to provide uni- advances in ECD technology emerging in the 1970s led to the creation of different types of formity of coloration in large-area ECW [28–32]. Nanotechnologies can be efficiently used ECD. However, there are still problems with respect to the commercialization of EC devices, to produce low-cost high-performance ECD [128]. including aspects such as their high production cost [99], the stability of their long-term In [129], an experiment was described in which reduced graphene oxide (rGO) films operation, and the production of uniform electrochromic films to provide uniformity of were electrodeposited on indium tin-oxide-coated polyethylene terephthalate substrates coloration in large-area ECW [28–32]. Nanotechnologies can be efficiently used to produce (I low-cost TO-PET) high-performance from graphene oxide ECD (GO) [128]., and the resulting flexible transparent electrodes were used in ethyl viologen (EtV2 ) electrochromic devices. During continuous testing, In [129], an experiment was described in which reduced graphene oxide (rGO) films th wer e resu e electr lting odeposited devices, whic on indium h contained tin-oxide-coated GO/rGO in tpolyethylene he electrochrom terephthalate ic mixture, substrates exhibited a (IT lower O-PET) switc frhing om graphene voltage bet oxide ween (GO), the colored and the and resulting bleached flexible states. transpar Graphen ent e ox electr ide (GO) odes were used in ethyl viologen (EtV2 ) electrochromic devices. During continuous testing, and reduced graphene oxide (rGO) enabled devices with higher optical contrast than the resulting devices, which contained GO/rGO in the electrochromic mixture, exhibited a those free of GO at the same applied voltage. lower switching voltage between the colored and bleached states. Graphene oxide (GO) In [130], WO3/rGO nanocomposite film was fabricated by sol–gel centrifugation us- ing a mixed colloidal dispersion of WO3 precursor and GO. It was reported that the Nanomaterials 2021, 11, 2376 25 of 32 Nanomaterials 2021, 11, x FOR PEER REVIEW 25 of 32 and reduced graphene oxide (rGO) enabled devices with higher optical contrast than those Nanomaterials 2021, 11, x FOR PEER REVIEW 25 of 32 free of GO at the same applied voltage. In [130], WO /rGO nanocomposite film was fabricated by sol–gel centrifugation WO3/rGO nanocomposite film exhibited shorter coloration and bleaching times (Tc = 9.5 s using a mixed colloidal dispersion of WO precursor and GO. It was reported that the WO3/rGO nanocomposite film exhibited shorter coloration and bleaching times (Tc = 9.5 s 2 −1 and Tb = 7.6 s), hWO igher /rGO colora nanocomposite tion efficiency film (75.3 exhibited cm ∙C at shorter 633 nm coloration ), larger op and ticbleaching al modu- times (T = 9.5 s 3 c 2 −1 and Tb = 7.6 s), higher coloration efficiency (75.3 cm ∙C at 633 nm), larger optical modu- 2 1 latory range (59.6 and % aT t 633 = nm 7.6 ) s), and bet higher ter cyc coloration lic stabief lifi ty com ciency par (75.3 ed with WO cm C 3 fiat lms; 633 thnm), ese larger optical latory range (59.6% at 633 nm) and better cyclic stability compared with WO3 films; these modulatory range (59.6% at 633 nm) and better cyclic stability compared with WO films; advantages were attributed to faster Li ion diffusion and electron transfer rate. advantages were attributed to faster Li ion diffusion and electron transfer rate. Optically adj these ustaadvantages ble electrochro wermi e attributed c films arto e bas faster ic and Li ion impo dif rta fusion nt com and po electr nents on of transfer rate. Optically adjustable electrochromic films are basic and important components of Optically adjustable electrochromic films are basic and important components of electrochromic devices; therefore, the performance of EC devices is strongly dependent electrochromic devices; therefore, the performance of EC devices is strongly dependent electrochromic devices; therefore, the performance of EC devices is strongly dependent on on EC film structure, morphology and fabrication method [131]. on EC film structure, morphology and fabrication method [131]. EC film structure, morphology and fabrication method [131]. Amorphous WO3 films have a porous structure. Crystalline WO3 exhibits better du- Amorphous WO3 films have a porous structure. Crystalline WO3 exhibits better du- Amorphous WO films have a porous structure. Crystalline WO exhibits better rability compared to amorphous WO3, 3 due to its denser structure and low dissolution rate 3 rability compared to amorphous WO3, due to its denser structure and low dissolution rate durability compared to amorphous WO , due to its denser structure and low dissolution (stability in acidic solution is less than 4 pH) [93,94,132]3 . However, crystalline WO3 pos- (stability in acidic solution is less than 4 pH) [93,94,132]. However, crystalline WO3 pos- rate (stability in acidic solution is less than 4 pH) [93,94,132]. However, crystalline WO sesses high bulk density, which increases switching time and reduces coloring efficiency, 3 sesses high bulk density, which increases switching time and reduces coloring efficiency, possesses high bulk density, which increases switching time and reduces coloring efficiency, so nanostructured WO3 with a large specific surface area is expected to have a faster re- so nanostructured WO3 with a large specific surface area is expected to have a faster re- so nanostructured WO with a large specific surface area is expected to have a faster sponse time and a good durability. Recently, publications have appeared [105,133] on the sponse time and a good durability. Recently, publications have appeared [105,133] on the response time and a good durability. Recently, publications have appeared [105,133] on the use of nanoscale or nanoporous WO3 (Figure 31) that exhibit fast switching speed and use of nanoscale or nanoporous WO3 (Figure 31) that exhibit fast switching speed and use of nanoscale or nanoporous WO (Figure 31) that exhibit fast switching speed and high high coloration efficiency due to possessing a good and suitable band gap (~2.6 eV). In high coloration efficiency due to possessing a good and suitable band gap (~2.6 eV). In coloration efficiency due to possessing a good and suitable band gap (~2.6 eV). In [134–136] [134–136] the technologies for producing nanostructured WO3 films are discussed (Figure [134–136] the technologies for producing nanostructured WO3 films are discussed (Figure the technologies for producing nanostructured WO films are discussed (Figure 32). 32). 32). Figure 31. FE-SEM micrographs for nc-TiO2 nanoparticles film: (a) before deposition; (b) deposited Figure 31. FE-SEM micrographs for nc-TiO nanoparticles film: (a) before deposition; (b) deposited on H W O electrolyte 2 2 2 11 Figure 31. FE-SEM micrographs for nc-TiO2 nanoparticles film: (a) before deposition; (b) deposited on H2W2O11 electrolyte surface. surface. on H2W2O11 electrolyte surface. (a) (b) (a) (b) Figure 32. Nanostructured films obtained by electrochemical deposition: (a) WO3(GO2ml); (b) Figure 32. Nanostructured films obtained by electrochemical deposition: (a) WO3(GO2ml); (b) Figure 32. Nanostructured films obtained by electrochemical deposition: (a) WO (GO ); 2ml WO3(GO1ml) [135]. WO3(GO1ml) [135]. (b) WO (GO ) [135]. 3 1ml Nanocrystal-in-glass WO3 thin films are considered to be the most promising ca- Nanocrystal-in-glass WO3 thin films are considered to be the most promising ca- thodic electrochromic material [113]. In [137], an all-solution technology was developed thodic electrochromic material [113]. In [137], an all-solution technology was developed for large-area low-cost preparation of electrochromic films. A WO3/ITO dispersion was for large-area low-cost preparation of electrochromic films. A WO3/ITO dispersion was successfully developed; high-electrical-conductivity ITO nanoparticle networks along successfully developed; high-electrical-conductivity ITO nanoparticle networks along Nanomaterials 2021, 11, 2376 26 of 32 Nanocrystal-in-glass WO thin films are considered to be the most promising cathodic electrochromic material [113]. In [137], an all-solution technology was developed for large- area low-cost preparation of electrochromic films. A WO /ITO dispersion was successfully developed; high-electrical-conductivity ITO nanoparticle networks along with ITO coating on glass were able to serve as extended 3-dimensional electrodes, forming a microelectrical field and acting as the pathways for electron diffusion to WO nanorods. In [138], h-WO 3 3 QDs with an average size of 1.2 nm were successfully prepared by a simple decomposition process of tungsten acid in ethylene glycol. At present, various interactions have been introduced at the interface between the organic and inorganic phases. The expected improved electrochemical and electrochromic performances of the nanocomposites have been obtained. Among of these interactions, covalent bonds have the strongest interaction, although their preparation is relatively complicated [131,138]. Thus, it is an important first step for the fabrication of inexpensive EC “Smart Win- dows”, and should shape the future research on solution-based processes. 7. Conclusions It was possible, within the scope of this article, to provide a comprehensive review of the large area of new electrochromic materials, and the authors had to use their discretion in choosing up-to-date findings to illustrate this exciting area. In summarizing this review of the literature on electrochromism in electrochromic materials, and in WO films in particular, the following conclusions can be drawn: (1) There are several hypotheses concerning the mechanism of electrochromism in WO . Generally, the electrochromic effect in WO films can be described as an electrochemi- cal cathodic polarization during which H ions are transferred from the electrolyte and an electron is transferred from the ITO electrode. As a result, WO film switches from a bleached to a colored state; its color varies from pale blue to dark blue and black. The conductivity of WO films is determined by the presence of cations (H , Li , etc.) and electrons. As already mentioned, the coloration mechanism in WO films has still been insufficiently investigated. (2) Despite a large number of works devoted to the study of electrochromic WO films, the influence of the structural state on optical properties during the electrochemical reaction has not been fully investigated. Different film deposition techniques have been proposed. Film morphology is dependent on deposition technique and can be amorphous, crystalline, nanocrystalline or hybrid. Additionally, there is still a constant need for new technologies to produce WO films, and nanostructured WO films in particular. Therefore, there is a necessity to study the fabrication of amorphous, crystalline and nanocrystalline WO films, including their GO/rGO modification. Analysis of literary sources makes it possible to identify prospects for the development of WO /rGO fabrication technologies. The obtained data will be useful in the development of WO fabrication technologies. Today, the development of the energy-efficient glazing sector is impossible without EC Modern nanomaterials make ECD an interesting commercial product that has obvious advantages over its competitors, such as PDLC, LCD and SPD. In this regard, according to some forecasts, the market for electrochromic “Smart Window” will expand in the next 5–7 years. First of all, thanks to the development of modern technologies and nanomaterials, as well as intensive research into EC by companies and scientific laboratories around the world. Author Contributions: Conceptualization, A.V.S. (Aleksei Viktorovich Shchegolkov), A.V.S. (Alexandr Viktorovich Shchegolkov) and S.-H.J.; methodology, M.S.L. and A.V.S. (Aleksei Viktorovich Shchegolkov); validation, Y.V.R. and A.O.S.; formal analysis, S.-H.J.; investigation, M.S.L. and A.V.S. (Aleksei Vik- torovich Shchegolkov); resources, S.-H.J.; writing—original draft preparation, Y.V.R. and A.O.S.; writing—review and editing, A.V.S. (Aleksei Viktorovich Shchegolkov) and A.V.S. (Alexandr Vik- Nanomaterials 2021, 11, 2376 27 of 32 torovich Shchegolkov); visualization, A.V.S. (Aleksei Viktorovich Shchegolkov); supervision, A.V.S. (Aleksei Viktorovich Shchegolkov), A.V.S. (Alexandr Viktorovich Shchegolkov) and S.-H.J.; project administration, A.V.S. (Aleksei Viktorovich Shchegolkov); funding acquisition, S.-H.J. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Research Foundation (NRF) grant funded by the Korea government (MSIT) (No. 2020R1C1C1005273, 2021M3H4A02056037). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations EC electrochromic materials; PhC photochromic materials; ThC thermochromic materials; GhC gasochromic materials; PDLC polymer-dispersed liquid crystals; LDC liquid crystal dispersions; ECD electrochromic devices; ECW electrochromic windows; TMO transition metal oxides; GO graphene oxide; rGO reduced graphene oxide; SPD suspended particles; EMR electromagnetic radiation. References 1. Addington, D.M.; Schodek, D.L. Smart Materials and New Technologies for the Architecture and Design Professions; Elsevier Science: Oxford, UK, 2005; p. 241. 2. Granqvist, C.G.; Green, S.; Niklasson, G.A.; Mlyuka, N.R.; Kraemer, S.; Georén, P. Advance in chromogenic materials and devices. Thin Solid Film. 2010, 518, 3046–3053. [CrossRef] 3. Bamfield, P. Chromic Phenomena the Technological Applications of Colour Chemistry; Royal society of Chemistry (RSC): Cambridge, UK, 2001; p. 374. 4. Baetens, R.; Jelle, B.P.; Gustavsen, A. 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Journal

NanomaterialsMultidisciplinary Digital Publishing Institute

Published: Sep 13, 2021

Keywords: electrochromic materials; nanostructured electrochromic materials; electrochromism; color; “Smart Windows”; transition metal oxides (TMO); nanomaterials; graphene oxide (GO); reduced graphene oxide (rGO)

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