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Optically Transparent Wood Substrate for Perovskite Solar Cells

Optically Transparent Wood Substrate for Perovskite Solar Cells This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes. Research Article pubs.acs.org/journal/ascecg Cite This: ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ,†,# ∥,# † ‡ ,‡ † Yuanyuan Li,* Ming Cheng, Erik Jungstedt, Bo Xu, Licheng Sun,* and Lars Berglund Wallenberg Wood Science Center, Department of Fibre and Polymer Technology, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), KTH Royal Institute of Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, P. R. China Organic Chemistry, Centre of Molecular Devices, Department of Chemistry, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), KTH Royal Institute of Technology, Teknikringen 42, SE-100 44 Stockholm, Sweden * Supporting Information ABSTRACT: Transparent wood is a candidate for use as an energy-saving building material due to its low density (ca. 1.2 g/ 3 −1 −1 cm ), high optical transmittance (over 85% at 1 mm thickness), low thermal conductivity (0.23 W m K ), and good load- bearing performance with tough failure behavior (no shattering). High optical transmittance also makes transparent wood a candidate for optoelectronic devices. In this work, for the first time, perovskite solar cells processed at low temperature (<150 °C) were successfully assembled directly on transparent wood substrates. A power conversion efficiency up to 16.8% was obtained. The technologies demonstrated may pave the way for integration of solar cells with light transmitting wood building structures for energy-saving purposes. KEYWORDS: Biocomposite, Perovskite solar cell, Energy-Efficient, Building material, Transparent wood, Mechanical properties INTRODUCTION The substrate is a key component for solar cells, which determines the end-use of the products and influences the It has been predicted that the energy consumption in the world sustainability of final solar cell products. Glass and plastics are will increase by 48% and carbon dioxide emissions by 34% commonly used substrates for solar cells. However, using low from 2012 to 2040. The building sector accounts for over cost materials from renewable resources as the substrates is of 30% of the total energy consumption and carbon dioxide great interest due to the goal of sustainability. In addition, as a emissions, leading to an urgent need for more energy-efficient building material, glass shows limitations because of the high buildings. Integration of clean energy technologies with brittleness and high thermal conductivity. conventional building structures is a promising development. Wood is by far the most important structural material from Photovoltaics, which convert solar energy into direct current renewable resources, and it is to a large extent used in electricity through semiconducting materials, are becoming construction for load-bearing applications. The potential of increasingly attractive. Commercial technologies are domi- using wood as a substrate for functional materials has been 7−9 nated by crystalline silicon solar cells and thin film solar cells. discussed in the literature. One limitation for application of They are based on high-purity, single-crystalline semi- wood-based materials in photovoltaics is that wood is not conductors and therefore rely on high-temperature manufac- transparent, although wood-based cellulose paper or nano- turing processes. Although the price of solar cell-based cellulose paper/films have been studied as substrates or 10,11 electricity is dropping, even lower cost and higher power functional light management layers in solar cell structures. conversion efficiency (PCE) are desirable. Perovskite solar One reason for low optical transmittance of wood is the light scattering at the interfaces between the cell wall tissue and the cells (PSCs) have attracted great attention since the first work in 2009 due to the high PCE, easy processability, possible low 4,5 processing cost, and so on. Even though with the rapid Received: November 29, 2018 development of PSCs, some challenges still remain such as Revised: February 11, 2019 stability, toxicity, scale-up technologies, and sustainability. Published: February 18, 2019 © 2019 American Chemical Society 6061 DOI: 10.1021/acssuschemeng.8b06248 ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ACS Sustainable Chemistry & Engineering Research Article Figure 1. Schematic sketch showing the process of transparent wood preparation and assembling of a solar cell on the transparent wood substrate. The solar cell structure is transparent wood substrate/ITO layer/compact TiO /perovskite/spiro-OMeTAD/Au. The yellow arrows represent light. PMMA refers to poly(methyl methacrylate). Figure 2. (a) SEM image of transparent wood cross section. (b) Transmittance and haze spectra of transparent wood, specimen thickness 1 mm; inset image is the photo of a green laser beam (diameter of 4 mm) scatters as it passes through transparent wood. The dashed line marks the geometry of the transparent wood sample. The size of unit grid is 5 mm × 5 mm. (c) Scheme of transparent wood samples for mechanical test. The orange arrows show the direction of loading force, and the green arrows show the direction of fibers in the transparent wood, F means force/ loading. (d) Stress−strain curves of PMMA, transparent wood TL, and transparent wood LT respectively. (e) SEM image of transparent wood cross section, showing the interface between PMMA and wood. The red arrows point at the interface between wood cell wall and PMMA. (f) Photos of transparent wood after the mechanical test; the left one shows the crack propagation pattern in transparent wood LT, and the right one shows the crack propagation pattern in transparent wood TL. empty pore space (“lumen”) in wood cells (e.g., cells such as In the present study, transparent wood was for the first time tracheids, wood fibers, and vessels). In addition, the presence used as the substrate for solar cells. PSCs with PCE up to 16.8% were prepared directly on transparent wood substrates of strongly light absorbing polymers (mainly lignin) in the cell using a low temperature process (<150 °C). Figure 1 shows a wall is a problem. Transparent wood was originally prepared sketch of the transparent wood preparation procedure and the in 1992 for wood morphology studies. Recently, efforts were PSC assembly on a transparent wood substrate. developed to combine optical transparency with mechanical performance for light-transmitting, energy-efficient building 14−18 RESULTS AND DISCUSSION applications. In recent reviews, the progress of transparent 7,9,19 wood technology was discussed in detail. Transparent The lack of transparency in wood is mainly due to the porous wood exhibits high optical transmittance and haze (over 70%). lumen space at the center of fibers, tracheids, and vessel cells, The high haze is interesting to be used for light diffusing for with diameters in the order of tens of micrometers. In addition, solar cells, which is demonstrated by the Hu group. lignin, tannins, and other phenolic compounds absorb light However, studies of using transparent wood directly as the through chromophoric groups. Lignin is the main component substrate for solar cell are rare. contributing to the brownish wood color. To make wood 6062 DOI: 10.1021/acssuschemeng.8b06248 ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ACS Sustainable Chemistry & Engineering Research Article Figure 3. (a) Optical transmittance and haze spectra of transparent wood after ITO coating. Inset image is the transparent without and with ITO coating. (b) AFM image of transparent wood, showing the surface roughness. SEM images of (c) compact TiO layer, (d) perovskite layer, and (e) Spiro-OMeTAD coating during solar cell assemble. transparent, the wood was first delignified with NaClO and from balsa. The fracture toughness has not been studied. The then infiltrated with a refractive index matched polymer, critical stress intensity factor K (a measure of initiation of poly(methyl methacrylate) (PMMA). Figure 2a shows the crack propagation) at the peak load is a measure of the fracture transparent wood microstructure in cross section, and the toughness of the composite. The critical stress intensity factor K for DEN specimens was lumen pore space is filled by the polymer. In Figure 1, the estimated according to eq 1: photo of transparent wood specimens on top of a leaf is shown, demonstrating the optical transparency. i ay j z To validate that transparent wood is a suitable substrate for Ka =σπf j z c0 j z w (1) k { solar cell applications, materials and device characterization was performed. Optical properties were first studied. Figure 2b a 21 where f is a geometry dependent function. σ is shows the optical transmittance and haze spectra of transparent () 0 4 w wood. A high optical transmittance of 86% was demonstrated determined at peak load. Since transparent wood is at a wavelength of 550 nm and a thickness of 1.0 mm, which anisotropic, two different cases are studied as shown in Figure met the requirements for a substrate for solar cells. At the same 2c. Loading is applied either parallel to the fiber direction time, transparent wood shows a haze of around 70% in the (longitudinal tangential, LT) or perpendicular to fiber visible light range. Haze is the ratio between diffused light direction (tangential longitudinal, TL). Tensile stress−strain transmittance to total transmittance (diffused + direct). The curves of notched transparent wood DEN specimens and haze of 70% means that diffused transmittance dominates PMMA are shown in Figure 2d. The K value for PMMA is 1/2 despite high optical transmittance. The inset image in Figure 1.48 MPa m , which is comparable with the literature 1/2 2b demonstrates the light diffusion pattern after the beam has (around 0.9−1.70 MPa m ). Transparent wood LT shows a 1/2 passed through transparent wood. High haze should be higher K value of 3.2 MPa m . This is due to the orientation favorable for solar cells since the light path in the active of the reinforcing wood template skeleton in the composite. layer is increased. This was demonstrated by attaching Fibers are oriented perpendicular to the plane of the initial transparent wood on top of a solar cell, with an improved crack notch. The wood-PMMA bond integrity appears energy conversion efficiency of 18%. favorable at the sub-micrometer scale (Figure 2e), which Mechanical property is important for solar cell substrates, leads to good microscale load transfer in the composites. In since it influences the end performance of the device. addition, the softer biocomposite structure with nanocellulosic Mechanical tests were performed in uniaxial tension by control cell walls and a polymer matrix phase leads to tougher failure of the displacement rate. Each specimens had 3 mm deep mode compared with glass, which may show brittle fracture notches on each side of the specimen edge, double edge (shattering) leading to potential safety problems. Transparent notched (DEN). The initial crack was generated by a sharp wood TL demonstrates lower initiation K value of 0.67 MPa 1/2 steel blade. Earlier work on elastic property characterization, m , even lower than that of the neat polymer phase. Figure 2f rather than toughness, was carried out on transparent wood shows the crack propagation pattern in transparent wood 6063 DOI: 10.1021/acssuschemeng.8b06248 ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ACS Sustainable Chemistry & Engineering Research Article Figure 4. (a) Current density−voltage properties of PSCs (scan rate: 20 mV/s). (b) IPCE spectra of PSC. (c) Steady-state current density and PCE at max power output points (0.82 V). (d) The PCE histogram chart of the devices (a batch of 20 cells). samples. In transparent wood TL, the crack is progressing pulsed laser deposition. Optical transmittance data showed a straight between the notches following the weakest plane in the decrease due to the deposition of the ITO layer, although there biocomposite. For transparent wood LT, the crack most likely was little in optical haze before and after ITO deposition started at the notch on the right-hand side and then deviated (Figure 3a). Figure 3a inset image shows that a light green from the plane perpendicular to the loading direction. The color appears after ITO deposition on transparent wood. The reason is the lower toughness for cracks growing along the surface roughness of the substrate is important for solar cell fiber direction. Although the fracture toughness K is lower in assembly. Low surface roughness will increase the conductivity the TL direction, the problem can be addressed by lamination of the coated ITO layer and decrease the risk for pin-holes. of transparent wood layers as in a plywood structure. In The transparent wood substrate described here demonstrated a 1/2 surface roughness of 30 nm within the scanning area of 5 μm × comparison with glass (K is 0.7−0.85 MPa m for soda-lime 24,25 glass), transparent wood shows higher fracture toughness. 5 μm. After ITO deposition, the surface roughness changed to Even in the weakest direction, the fracture toughness of a nominal value of 9 nm (Figure 3b), which is comparable with fluorine doped tin oxide (FIO)-glass that is commonly used for transparent wood is comparable to that of glass. The toughness criterion provides an argument for trans- solar cell assembly. A perovskite solar cell was then successfully parent wood as a replacement for glass as solar cell substrate. assembled on the ITO-coated transparent wood. The detailed In addition, transparent wood shows much better thermal device structure is transparent wood substrate/ITO/compact insulation properties than glass. Transparent wood has a lower TiO /(FAPbI ) (MAPbBr ) /Spiro-OMeTAD/Au as 2 3 0.85 3 0.15 −1 −1 −1 thermal conductivity (0.23 W m K ) than glass (1.0 W m shown in Figure 1. In fabricated devices, the compact TiO −1 K ). Low thermal conductivity contributes to the energy layer (40−50 nm thick) functions as electron transport requirements reduction for air-conditioning systems and lower material (ETM), and Spiro-OMeTAD (around 150 nm thermal energy exchange between indoor and outdoor thick) functions as hole transport material (HTM). A mixed environments. Another strong argument is that wood is from perovskite (FAPbI ) (MAPbBr ) (around 450 nm thick) 3 0.85 3 0.15 renewable resources and may substantially reduce the carbon was used as the light harvesting material. The morphologies of footprint associated with building structures. Overall, trans- the compact TiO , perovskite layer, and Spiro-OMeTAD are parent wood is potentially suitable as a load-bearing substrate shown in Figure 3c−e. A flat dense perovskite film is observed for solar cells and shows advantages over glass in energy- from Figure 3d. The perovskite film is fully covered by a uniform Spiro-OMeTAD layer (Figure 3e), which is very efficient buildings. Transparent wood is nonconductive. Therefore, in order to important for restricting the charge recombination in PSCs. It assemble a solar cell on transparent wood, a transparent should be noted that low temperature processing was adapted 27,28 conductive layer with sufficient conductivity is required. In in order to avoid thermal degradation of the transparent wood this work, an indium tin oxide (ITO) film was deposited by substrate. 6064 DOI: 10.1021/acssuschemeng.8b06248 ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ACS Sustainable Chemistry & Engineering Research Article Table 1. Photovoltaic Performance of PSCs Based on Transparent Wood Substrates −2 2 2 scan direction V /V J /mA·cm FF/% PCE/% hysteresis index/% R /Ω·cm R /Ω·cm oc sc sr sh from OC to SC 1.09 21.9 70.2 16.8 0.09 11.6 4628 from SC to OC 1.09 21.9 66.4 15.9 Figure 5. Aging test results: (a) V , (b) J , (c) FF, and (d) PCE of transparent wood substrate based PSCs. oc sc The photovoltaic performance of transparent wood (a batch of 20 cells). Over 50% of the manufactured devices substrate-based PSC is presented in Figure 4, and the relevant obtained a PCE exceeding 15.5%. data are collected in Table 1. The pervoskite-based solar cells Long-term stability is a crucial concern for practical on transparent wood substrates exhibited the highest PCE of applications of perovskite solar cells. Figure 5 shows the J , sc 16.8% at 100 mW/cm AM 1.5 G simulated irradiation with a V , FF, and PCE as a function of time for the transparent oc −2 short current density (J ) of 21.9 mA·cm , an open circuit sc wood-based PSCs, in which the devices were kept under air voltage (V ) of 1.09 V, and a fill factor (FF) of 70.2% (Figure oc conditions in the dark. It was found that the devices could 4a), which are slightly lower than that of FTO-glass based retain 77% of its initial performance after 720 h of aging, −2 PSCs (PCE of 18.9%, J of 24.2 mA·cm , V of 1.10 V, FF of sc oc showing a good long-term stability. 71.1%) (Figure S1 in Supporting Information). The lower J sc of transparent wood substrate-based PSC can be mainly CONCLUSION ascribed to the lower transmittance of conductive transparent wood substrate than that of FTO-glass. The above results Transparent wood shows high optical transmittance and haze, indicate that transparent wood could be an interesting good mechanical properties, a smooth surface, and a low candidate for ecofriendly solar cell substrates with a reduced thermal conductivity. This makes it suitable as a substrate for carbon footprint. From the incident-photon-to-current con- solar cell assembly with potential in energy-efficient building version efficiency (IPCE) spectrum (Figure 4b), it can be applications. For the first time, perovskite solar cells with a concluded that PSCs display a very wide photoelectric power conversion efficiency up to 16.8% were successfully response to the solar spectrum with a long wavelength limit assembled on optically transparent wood substrates, using a at around 800 nm, consistent with the band gap of the 30 low temperature process below 150 °C. The devices also (FAPbI ) (MAPbBr ) well. The steady-state power 3 0.85 3 0.15 showed good long-term stability. Our results suggest that output characteristic at the maximum power point was further transparent wood is a substrate candidate for assembly of investigated, and the results are shown in Figure 4c. The sustainable solar cells to replace glass and lower the carbon transparent wood substrate-based PSC showed a steady-state −2 footprint for the device. Through molecular and nanoscale current density of 20.3 mA·cm and a PCE of 16.6% under materials design of the transparent wood substrate, trans- 0.82 V bias, respectively, matching well with the photo- mittance and haze can be optimized, so that higher solar cell current−voltage (J−V) measurement. The histogram chart (Figure 4d) demonstrates a high reproducibility of the devices efficiency can be anticipated. 6065 DOI: 10.1021/acssuschemeng.8b06248 ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ACS Sustainable Chemistry & Engineering Research Article (9) Jiang, F.; Li, T.; Li, Y.; Zhang, Y.; Gong, A.; Dai, J.; Hitz, E.; Luo, EXPERIMENTAL SECTION W.; Hu, L. Wood-Based Nanotechnologies toward Sustainability. Adv. All the experimental information is present in the Supporting Mater. 2018, 30 (1), 1703453. Information. (10) Fang, Z.; Zhu, H.; Yuan, Y.; Ha, D.; Zhu, S.; Preston, C.; Chen, Q.; Li, Y.; Han, X.; Lee, S.; et al. Novel Nanostructured Paper with ASSOCIATED CONTENT Ultrahigh Transparency and Ultrahigh Haze for Solar Cells. Nano Lett. 2014, 14 (2), 765−773. * Supporting Information (11) Yao, Y.; Tao, J.; Zou, J.; Zhang, B.; Li, T.; Dai, J.; Zhu, M.; The Supporting Information is available free of charge on the Wang, S.; Fu, K. K.; Henderson, D.; et al. Light Management in ACS Publications website at DOI: 10.1021/acssusche- Plastic-Paper Hybrid Substrate towards High-Performance Optoelec- meng.8b06248. tronics. Energy Environ. Sci. 2016, 9 (7), 2278−2285. (PDF) (12) Li, Y.; Fu, Q.; Yang, X.; Berglund, L. A. Transparent Wood for Functional and Structural Applications. Philos. Trans. R. Soc., A 2018, 376 (2112), 20170182. AUTHOR INFORMATION (13) Fink, S. Transparent Wood − A New Approach in the Functional Study of Wood Structure. Holzforschung 1992, 46 (5), Corresponding Authors 403−408. *(L.S.) E-mail: lichengs@kth.se. (14) Li, Y.; Fu, Q.; Yu, S.; Yan, M.; Berglund, L. Optically *(Y.L.) E-mail: yua@kth.se. Transparent Wood from a Nanoporous Cellulosic Template: ORCID Combining Functional and Structural Performance. Biomacromolecules 2016, 17 (4), 1358−1364. Yuanyuan Li: 0000-0002-1591-5815 (15) Zhu, M.; Song, J.; Li, T.; Gong, A.; Wang, Y.; Dai, J.; Yao, Y.; Ming Cheng: 0000-0003-0793-0326 Luo, W.;Henderson,D.; Hu,L.HighlyAnisotropic,Highly Licheng Sun: 0000-0002-4521-2870 Transparent Wood Composites. Adv. Mater. 2016, 28 (26), 5181− Lars Berglund: 0000-0001-5818-2378 Author Contributions (16) Li, T.; Zhu, M.; Yang, Z.; Song, J.; Dai, J.; Yao, Y.; Luo, W.; Pastel, G.; Yang, B.; Hu, L. Wood Composite as an Energy Efficient Y.L. and M.C. contributed equally to this work. Building Material: Guided Sunlight Transmittance and Effective Notes Thermal Insulation. Adv. Energy Mater. 2016, 6 (22), 1601122. The authors declare no competing financial interest. (17) Li, Y.; Yang, X.; Fu, Q.; Rojas, R.; Yan, M.; Berglund, L. A. Towards Centimeter Thick Transparent Wood through Interface ACKNOWLEDGMENTS Manipulation. J. Mater. Chem. A 2018, 6 (3), 1094−1101. (18) Yu, Z.; Yao, Y.; Yao, J.; Zhang, L.; Chen, Z.; Gao, Y.; Luo, H. We acknowledge funding from KTH and European Research Transparent Wood Containing Cs WO Nanoparticles for Heat- x 3 Council Advanced Grant (No. 742733) Wood NanoTech, the Shielding Window Applications. J. Mater. Chem. A 2017, 5 (13), funding from Knut and Alice Wallenberg foundation through 6019−6024. the Wallenberg Wood Science Center at KTH Royal Institute (19) Li, Y.; Vasileva, E.; Sychugov, I.; Popov, S.; Berglund, L. of Technology, and The Swedish Energy Agency and the Optically Transparent Wood: Recent Progress, Opportunities, and Swedish Strategic Research Foundation (SSF). Alireza Hajian Challenges. Adv. Opt. Mater. 2018, 6 (14), 1800059. is acknowledged for the help of taking the AFM image. Min (20) Zhu, M.; Li, T.; Davis, C. 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Optically Transparent Wood Substrate for Perovskite Solar Cells

ACS Sustainable Chemistry & Engineering , Volume 7 (6) – Feb 18, 2019

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Abstract

This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes. Research Article pubs.acs.org/journal/ascecg Cite This: ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ,†,# ∥,# † ‡ ,‡ † Yuanyuan Li,* Ming Cheng, Erik Jungstedt, Bo Xu, Licheng Sun,* and Lars Berglund Wallenberg Wood Science Center, Department of Fibre and Polymer Technology, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), KTH Royal Institute of Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, P. R. China Organic Chemistry, Centre of Molecular Devices, Department of Chemistry, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), KTH Royal Institute of Technology, Teknikringen 42, SE-100 44 Stockholm, Sweden * Supporting Information ABSTRACT: Transparent wood is a candidate for use as an energy-saving building material due to its low density (ca. 1.2 g/ 3 −1 −1 cm ), high optical transmittance (over 85% at 1 mm thickness), low thermal conductivity (0.23 W m K ), and good load- bearing performance with tough failure behavior (no shattering). High optical transmittance also makes transparent wood a candidate for optoelectronic devices. In this work, for the first time, perovskite solar cells processed at low temperature (<150 °C) were successfully assembled directly on transparent wood substrates. A power conversion efficiency up to 16.8% was obtained. The technologies demonstrated may pave the way for integration of solar cells with light transmitting wood building structures for energy-saving purposes. KEYWORDS: Biocomposite, Perovskite solar cell, Energy-Efficient, Building material, Transparent wood, Mechanical properties INTRODUCTION The substrate is a key component for solar cells, which determines the end-use of the products and influences the It has been predicted that the energy consumption in the world sustainability of final solar cell products. Glass and plastics are will increase by 48% and carbon dioxide emissions by 34% commonly used substrates for solar cells. However, using low from 2012 to 2040. The building sector accounts for over cost materials from renewable resources as the substrates is of 30% of the total energy consumption and carbon dioxide great interest due to the goal of sustainability. In addition, as a emissions, leading to an urgent need for more energy-efficient building material, glass shows limitations because of the high buildings. Integration of clean energy technologies with brittleness and high thermal conductivity. conventional building structures is a promising development. Wood is by far the most important structural material from Photovoltaics, which convert solar energy into direct current renewable resources, and it is to a large extent used in electricity through semiconducting materials, are becoming construction for load-bearing applications. The potential of increasingly attractive. Commercial technologies are domi- using wood as a substrate for functional materials has been 7−9 nated by crystalline silicon solar cells and thin film solar cells. discussed in the literature. One limitation for application of They are based on high-purity, single-crystalline semi- wood-based materials in photovoltaics is that wood is not conductors and therefore rely on high-temperature manufac- transparent, although wood-based cellulose paper or nano- turing processes. Although the price of solar cell-based cellulose paper/films have been studied as substrates or 10,11 electricity is dropping, even lower cost and higher power functional light management layers in solar cell structures. conversion efficiency (PCE) are desirable. Perovskite solar One reason for low optical transmittance of wood is the light scattering at the interfaces between the cell wall tissue and the cells (PSCs) have attracted great attention since the first work in 2009 due to the high PCE, easy processability, possible low 4,5 processing cost, and so on. Even though with the rapid Received: November 29, 2018 development of PSCs, some challenges still remain such as Revised: February 11, 2019 stability, toxicity, scale-up technologies, and sustainability. Published: February 18, 2019 © 2019 American Chemical Society 6061 DOI: 10.1021/acssuschemeng.8b06248 ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ACS Sustainable Chemistry & Engineering Research Article Figure 1. Schematic sketch showing the process of transparent wood preparation and assembling of a solar cell on the transparent wood substrate. The solar cell structure is transparent wood substrate/ITO layer/compact TiO /perovskite/spiro-OMeTAD/Au. The yellow arrows represent light. PMMA refers to poly(methyl methacrylate). Figure 2. (a) SEM image of transparent wood cross section. (b) Transmittance and haze spectra of transparent wood, specimen thickness 1 mm; inset image is the photo of a green laser beam (diameter of 4 mm) scatters as it passes through transparent wood. The dashed line marks the geometry of the transparent wood sample. The size of unit grid is 5 mm × 5 mm. (c) Scheme of transparent wood samples for mechanical test. The orange arrows show the direction of loading force, and the green arrows show the direction of fibers in the transparent wood, F means force/ loading. (d) Stress−strain curves of PMMA, transparent wood TL, and transparent wood LT respectively. (e) SEM image of transparent wood cross section, showing the interface between PMMA and wood. The red arrows point at the interface between wood cell wall and PMMA. (f) Photos of transparent wood after the mechanical test; the left one shows the crack propagation pattern in transparent wood LT, and the right one shows the crack propagation pattern in transparent wood TL. empty pore space (“lumen”) in wood cells (e.g., cells such as In the present study, transparent wood was for the first time tracheids, wood fibers, and vessels). In addition, the presence used as the substrate for solar cells. PSCs with PCE up to 16.8% were prepared directly on transparent wood substrates of strongly light absorbing polymers (mainly lignin) in the cell using a low temperature process (<150 °C). Figure 1 shows a wall is a problem. Transparent wood was originally prepared sketch of the transparent wood preparation procedure and the in 1992 for wood morphology studies. Recently, efforts were PSC assembly on a transparent wood substrate. developed to combine optical transparency with mechanical performance for light-transmitting, energy-efficient building 14−18 RESULTS AND DISCUSSION applications. In recent reviews, the progress of transparent 7,9,19 wood technology was discussed in detail. Transparent The lack of transparency in wood is mainly due to the porous wood exhibits high optical transmittance and haze (over 70%). lumen space at the center of fibers, tracheids, and vessel cells, The high haze is interesting to be used for light diffusing for with diameters in the order of tens of micrometers. In addition, solar cells, which is demonstrated by the Hu group. lignin, tannins, and other phenolic compounds absorb light However, studies of using transparent wood directly as the through chromophoric groups. Lignin is the main component substrate for solar cell are rare. contributing to the brownish wood color. To make wood 6062 DOI: 10.1021/acssuschemeng.8b06248 ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ACS Sustainable Chemistry & Engineering Research Article Figure 3. (a) Optical transmittance and haze spectra of transparent wood after ITO coating. Inset image is the transparent without and with ITO coating. (b) AFM image of transparent wood, showing the surface roughness. SEM images of (c) compact TiO layer, (d) perovskite layer, and (e) Spiro-OMeTAD coating during solar cell assemble. transparent, the wood was first delignified with NaClO and from balsa. The fracture toughness has not been studied. The then infiltrated with a refractive index matched polymer, critical stress intensity factor K (a measure of initiation of poly(methyl methacrylate) (PMMA). Figure 2a shows the crack propagation) at the peak load is a measure of the fracture transparent wood microstructure in cross section, and the toughness of the composite. The critical stress intensity factor K for DEN specimens was lumen pore space is filled by the polymer. In Figure 1, the estimated according to eq 1: photo of transparent wood specimens on top of a leaf is shown, demonstrating the optical transparency. i ay j z To validate that transparent wood is a suitable substrate for Ka =σπf j z c0 j z w (1) k { solar cell applications, materials and device characterization was performed. Optical properties were first studied. Figure 2b a 21 where f is a geometry dependent function. σ is shows the optical transmittance and haze spectra of transparent () 0 4 w wood. A high optical transmittance of 86% was demonstrated determined at peak load. Since transparent wood is at a wavelength of 550 nm and a thickness of 1.0 mm, which anisotropic, two different cases are studied as shown in Figure met the requirements for a substrate for solar cells. At the same 2c. Loading is applied either parallel to the fiber direction time, transparent wood shows a haze of around 70% in the (longitudinal tangential, LT) or perpendicular to fiber visible light range. Haze is the ratio between diffused light direction (tangential longitudinal, TL). Tensile stress−strain transmittance to total transmittance (diffused + direct). The curves of notched transparent wood DEN specimens and haze of 70% means that diffused transmittance dominates PMMA are shown in Figure 2d. The K value for PMMA is 1/2 despite high optical transmittance. The inset image in Figure 1.48 MPa m , which is comparable with the literature 1/2 2b demonstrates the light diffusion pattern after the beam has (around 0.9−1.70 MPa m ). Transparent wood LT shows a 1/2 passed through transparent wood. High haze should be higher K value of 3.2 MPa m . This is due to the orientation favorable for solar cells since the light path in the active of the reinforcing wood template skeleton in the composite. layer is increased. This was demonstrated by attaching Fibers are oriented perpendicular to the plane of the initial transparent wood on top of a solar cell, with an improved crack notch. The wood-PMMA bond integrity appears energy conversion efficiency of 18%. favorable at the sub-micrometer scale (Figure 2e), which Mechanical property is important for solar cell substrates, leads to good microscale load transfer in the composites. In since it influences the end performance of the device. addition, the softer biocomposite structure with nanocellulosic Mechanical tests were performed in uniaxial tension by control cell walls and a polymer matrix phase leads to tougher failure of the displacement rate. Each specimens had 3 mm deep mode compared with glass, which may show brittle fracture notches on each side of the specimen edge, double edge (shattering) leading to potential safety problems. Transparent notched (DEN). The initial crack was generated by a sharp wood TL demonstrates lower initiation K value of 0.67 MPa 1/2 steel blade. Earlier work on elastic property characterization, m , even lower than that of the neat polymer phase. Figure 2f rather than toughness, was carried out on transparent wood shows the crack propagation pattern in transparent wood 6063 DOI: 10.1021/acssuschemeng.8b06248 ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ACS Sustainable Chemistry & Engineering Research Article Figure 4. (a) Current density−voltage properties of PSCs (scan rate: 20 mV/s). (b) IPCE spectra of PSC. (c) Steady-state current density and PCE at max power output points (0.82 V). (d) The PCE histogram chart of the devices (a batch of 20 cells). samples. In transparent wood TL, the crack is progressing pulsed laser deposition. Optical transmittance data showed a straight between the notches following the weakest plane in the decrease due to the deposition of the ITO layer, although there biocomposite. For transparent wood LT, the crack most likely was little in optical haze before and after ITO deposition started at the notch on the right-hand side and then deviated (Figure 3a). Figure 3a inset image shows that a light green from the plane perpendicular to the loading direction. The color appears after ITO deposition on transparent wood. The reason is the lower toughness for cracks growing along the surface roughness of the substrate is important for solar cell fiber direction. Although the fracture toughness K is lower in assembly. Low surface roughness will increase the conductivity the TL direction, the problem can be addressed by lamination of the coated ITO layer and decrease the risk for pin-holes. of transparent wood layers as in a plywood structure. In The transparent wood substrate described here demonstrated a 1/2 surface roughness of 30 nm within the scanning area of 5 μm × comparison with glass (K is 0.7−0.85 MPa m for soda-lime 24,25 glass), transparent wood shows higher fracture toughness. 5 μm. After ITO deposition, the surface roughness changed to Even in the weakest direction, the fracture toughness of a nominal value of 9 nm (Figure 3b), which is comparable with fluorine doped tin oxide (FIO)-glass that is commonly used for transparent wood is comparable to that of glass. The toughness criterion provides an argument for trans- solar cell assembly. A perovskite solar cell was then successfully parent wood as a replacement for glass as solar cell substrate. assembled on the ITO-coated transparent wood. The detailed In addition, transparent wood shows much better thermal device structure is transparent wood substrate/ITO/compact insulation properties than glass. Transparent wood has a lower TiO /(FAPbI ) (MAPbBr ) /Spiro-OMeTAD/Au as 2 3 0.85 3 0.15 −1 −1 −1 thermal conductivity (0.23 W m K ) than glass (1.0 W m shown in Figure 1. In fabricated devices, the compact TiO −1 K ). Low thermal conductivity contributes to the energy layer (40−50 nm thick) functions as electron transport requirements reduction for air-conditioning systems and lower material (ETM), and Spiro-OMeTAD (around 150 nm thermal energy exchange between indoor and outdoor thick) functions as hole transport material (HTM). A mixed environments. Another strong argument is that wood is from perovskite (FAPbI ) (MAPbBr ) (around 450 nm thick) 3 0.85 3 0.15 renewable resources and may substantially reduce the carbon was used as the light harvesting material. The morphologies of footprint associated with building structures. Overall, trans- the compact TiO , perovskite layer, and Spiro-OMeTAD are parent wood is potentially suitable as a load-bearing substrate shown in Figure 3c−e. A flat dense perovskite film is observed for solar cells and shows advantages over glass in energy- from Figure 3d. The perovskite film is fully covered by a uniform Spiro-OMeTAD layer (Figure 3e), which is very efficient buildings. Transparent wood is nonconductive. Therefore, in order to important for restricting the charge recombination in PSCs. It assemble a solar cell on transparent wood, a transparent should be noted that low temperature processing was adapted 27,28 conductive layer with sufficient conductivity is required. In in order to avoid thermal degradation of the transparent wood this work, an indium tin oxide (ITO) film was deposited by substrate. 6064 DOI: 10.1021/acssuschemeng.8b06248 ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ACS Sustainable Chemistry & Engineering Research Article Table 1. Photovoltaic Performance of PSCs Based on Transparent Wood Substrates −2 2 2 scan direction V /V J /mA·cm FF/% PCE/% hysteresis index/% R /Ω·cm R /Ω·cm oc sc sr sh from OC to SC 1.09 21.9 70.2 16.8 0.09 11.6 4628 from SC to OC 1.09 21.9 66.4 15.9 Figure 5. Aging test results: (a) V , (b) J , (c) FF, and (d) PCE of transparent wood substrate based PSCs. oc sc The photovoltaic performance of transparent wood (a batch of 20 cells). Over 50% of the manufactured devices substrate-based PSC is presented in Figure 4, and the relevant obtained a PCE exceeding 15.5%. data are collected in Table 1. The pervoskite-based solar cells Long-term stability is a crucial concern for practical on transparent wood substrates exhibited the highest PCE of applications of perovskite solar cells. Figure 5 shows the J , sc 16.8% at 100 mW/cm AM 1.5 G simulated irradiation with a V , FF, and PCE as a function of time for the transparent oc −2 short current density (J ) of 21.9 mA·cm , an open circuit sc wood-based PSCs, in which the devices were kept under air voltage (V ) of 1.09 V, and a fill factor (FF) of 70.2% (Figure oc conditions in the dark. It was found that the devices could 4a), which are slightly lower than that of FTO-glass based retain 77% of its initial performance after 720 h of aging, −2 PSCs (PCE of 18.9%, J of 24.2 mA·cm , V of 1.10 V, FF of sc oc showing a good long-term stability. 71.1%) (Figure S1 in Supporting Information). The lower J sc of transparent wood substrate-based PSC can be mainly CONCLUSION ascribed to the lower transmittance of conductive transparent wood substrate than that of FTO-glass. The above results Transparent wood shows high optical transmittance and haze, indicate that transparent wood could be an interesting good mechanical properties, a smooth surface, and a low candidate for ecofriendly solar cell substrates with a reduced thermal conductivity. This makes it suitable as a substrate for carbon footprint. From the incident-photon-to-current con- solar cell assembly with potential in energy-efficient building version efficiency (IPCE) spectrum (Figure 4b), it can be applications. For the first time, perovskite solar cells with a concluded that PSCs display a very wide photoelectric power conversion efficiency up to 16.8% were successfully response to the solar spectrum with a long wavelength limit assembled on optically transparent wood substrates, using a at around 800 nm, consistent with the band gap of the 30 low temperature process below 150 °C. The devices also (FAPbI ) (MAPbBr ) well. The steady-state power 3 0.85 3 0.15 showed good long-term stability. Our results suggest that output characteristic at the maximum power point was further transparent wood is a substrate candidate for assembly of investigated, and the results are shown in Figure 4c. The sustainable solar cells to replace glass and lower the carbon transparent wood substrate-based PSC showed a steady-state −2 footprint for the device. Through molecular and nanoscale current density of 20.3 mA·cm and a PCE of 16.6% under materials design of the transparent wood substrate, trans- 0.82 V bias, respectively, matching well with the photo- mittance and haze can be optimized, so that higher solar cell current−voltage (J−V) measurement. The histogram chart (Figure 4d) demonstrates a high reproducibility of the devices efficiency can be anticipated. 6065 DOI: 10.1021/acssuschemeng.8b06248 ACS Sustainable Chem. Eng. 2019, 7, 6061−6067 ACS Sustainable Chemistry & Engineering Research Article (9) Jiang, F.; Li, T.; Li, Y.; Zhang, Y.; Gong, A.; Dai, J.; Hitz, E.; Luo, EXPERIMENTAL SECTION W.; Hu, L. Wood-Based Nanotechnologies toward Sustainability. Adv. All the experimental information is present in the Supporting Mater. 2018, 30 (1), 1703453. Information. (10) Fang, Z.; Zhu, H.; Yuan, Y.; Ha, D.; Zhu, S.; Preston, C.; Chen, Q.; Li, Y.; Han, X.; Lee, S.; et al. Novel Nanostructured Paper with ASSOCIATED CONTENT Ultrahigh Transparency and Ultrahigh Haze for Solar Cells. Nano Lett. 2014, 14 (2), 765−773. * Supporting Information (11) Yao, Y.; Tao, J.; Zou, J.; Zhang, B.; Li, T.; Dai, J.; Zhu, M.; The Supporting Information is available free of charge on the Wang, S.; Fu, K. K.; Henderson, D.; et al. Light Management in ACS Publications website at DOI: 10.1021/acssusche- Plastic-Paper Hybrid Substrate towards High-Performance Optoelec- meng.8b06248. tronics. Energy Environ. Sci. 2016, 9 (7), 2278−2285. (PDF) (12) Li, Y.; Fu, Q.; Yang, X.; Berglund, L. A. Transparent Wood for Functional and Structural Applications. Philos. Trans. R. Soc., A 2018, 376 (2112), 20170182. AUTHOR INFORMATION (13) Fink, S. Transparent Wood − A New Approach in the Functional Study of Wood Structure. Holzforschung 1992, 46 (5), Corresponding Authors 403−408. *(L.S.) E-mail: lichengs@kth.se. (14) Li, Y.; Fu, Q.; Yu, S.; Yan, M.; Berglund, L. Optically *(Y.L.) E-mail: yua@kth.se. Transparent Wood from a Nanoporous Cellulosic Template: ORCID Combining Functional and Structural Performance. Biomacromolecules 2016, 17 (4), 1358−1364. Yuanyuan Li: 0000-0002-1591-5815 (15) Zhu, M.; Song, J.; Li, T.; Gong, A.; Wang, Y.; Dai, J.; Yao, Y.; Ming Cheng: 0000-0003-0793-0326 Luo, W.;Henderson,D.; Hu,L.HighlyAnisotropic,Highly Licheng Sun: 0000-0002-4521-2870 Transparent Wood Composites. Adv. Mater. 2016, 28 (26), 5181− Lars Berglund: 0000-0001-5818-2378 Author Contributions (16) Li, T.; Zhu, M.; Yang, Z.; Song, J.; Dai, J.; Yao, Y.; Luo, W.; Pastel, G.; Yang, B.; Hu, L. Wood Composite as an Energy Efficient Y.L. and M.C. contributed equally to this work. Building Material: Guided Sunlight Transmittance and Effective Notes Thermal Insulation. Adv. Energy Mater. 2016, 6 (22), 1601122. The authors declare no competing financial interest. (17) Li, Y.; Yang, X.; Fu, Q.; Rojas, R.; Yan, M.; Berglund, L. A. Towards Centimeter Thick Transparent Wood through Interface ACKNOWLEDGMENTS Manipulation. J. Mater. Chem. A 2018, 6 (3), 1094−1101. (18) Yu, Z.; Yao, Y.; Yao, J.; Zhang, L.; Chen, Z.; Gao, Y.; Luo, H. We acknowledge funding from KTH and European Research Transparent Wood Containing Cs WO Nanoparticles for Heat- x 3 Council Advanced Grant (No. 742733) Wood NanoTech, the Shielding Window Applications. J. Mater. Chem. A 2017, 5 (13), funding from Knut and Alice Wallenberg foundation through 6019−6024. the Wallenberg Wood Science Center at KTH Royal Institute (19) Li, Y.; Vasileva, E.; Sychugov, I.; Popov, S.; Berglund, L. of Technology, and The Swedish Energy Agency and the Optically Transparent Wood: Recent Progress, Opportunities, and Swedish Strategic Research Foundation (SSF). Alireza Hajian Challenges. Adv. Opt. Mater. 2018, 6 (14), 1800059. is acknowledged for the help of taking the AFM image. Min (20) Zhu, M.; Li, T.; Davis, C. 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