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Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells

Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells ARTICLE Received 24 Jul 2014 | Accepted 17 Sep 2014 | Published 10 Nov 2014 DOI: 10.1038/ncomms6293 OPEN Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells 1, 1, 1, 1 2,w 1 1 1 Yuhang Liu *, Jingbo Zhao *, Zhengke Li *, Cheng Mu , Wei Ma , Huawei Hu , Kui Jiang , Haoran Lin , 2 1,3 Harald Ade &HeYan Although the field of polymer solar cell has seen much progress in device performance in the past few years, several limitations are holding back its further development. For instance, current high-efficiency (49.0%) cells are restricted to material combinations that are based on limited donor polymers and only one specific fullerene acceptor. Here we report the achievement of high-performance (efficiencies up to 10.8%, fill factors up to 77%) thick-film polymer solar cells for multiple polymer:fullerene combinations via the formation of a near- ideal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains. This morphology is controlled by the temperature-dependent aggregation behaviour of the donor polymers and is insensitive to the choice of fullerenes. The uncovered aggregation and design rules yield three high-efficiency (410%) donor polymers and will allow further synthetic advances and matching of both the polymer and fullerene materials, potentially leading to significantly improved performance and increased design flexibility. 1 2 Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. Department of Physics and ORaCEL, North Carolina State University, Raleigh, North Carolina 27695, USA. HKUST-Shenzhen Research Institute, No. 9 Yuexing 1st RD, Hi-tech Park, Nanshan, Shenzhen 518057, China. * These authors contributed equally to this work. w Current Address: XJTU-HKUST Joint School of Sustainable Development, Xi’an Jiaotong University, Xi’an, P.R. China. Correspondence and requests for materials should be addressed to W.M. (email: wma5@ncsu.edu) or to H.A. (email: harald_ade@ncsu.edu) or to H.Y. (email: hyan@ust.hk). NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 onventional inorganic solar cells can achieve high optimum’ PSC morphology in order to achieve thick-film PSCs efficiencies but are produced through complicated, costly that have comparable or higher efficiencies than state-of-the-art Cprocesses. The desirability of lower costs is driving the PTB7 materials systems. development of several third-generation solar technologies. In the following, we report the achievement of high- 1–6 Among these, polymer solar cell (PSC) technology is an performance (efficiencies up to 10.8% and fill factors (FFs) up excellent example of low-cost production because PSCs can be to 77%) thick-film PSCs based on three different donor polymers produced using extremely high-throughput roll-to-roll printing and 10 polymer:fullerene combinations, all of which exhibit methods similar to those used to print newspapers . PSCs also efficiencies higher than the previous state of the art. In contrast to offer several other advantages: vacuum processing and high- state-of-the-art PTB7-based materials systems, the high PSC temperature sintering are not needed, and no toxic materials are performances in this report are achieved via the formation of an used in the end product. Most importantly, a tandem cell ‘optimum PSC morphology’ that contains highly crystalline, 6,8–10 architecture can be easily implemented with PSCs and has sufficiently pure, yet reasonably small polymer domains. The high proven to improve PSC efficiency by B40–50% (refs 6,8). As polymer crystallinity and thus excellent hole transport ability, PSCs are two-component, donor–acceptor material systems, it is combined with sufficiently pure polymer domains, are the main generally important to control the morphology of the reasons why the PSCs exhibit high FFs and efficiency even when donor:acceptor blends and to find an optimal materials the active layer is 300 nm thick. Importantly, this ipso facto near- combination with excellent optical and electronic properties. In perfect morphology is controlled by the temperature-dependent the last few years, record-efficiency PSCs were achieved with only aggregation behaviour of the donor polymers during casting and three donor polymers (which all belong to a specific polymer is insensitive to the choice of fullerenes. Taking advantage of the family based on fluorinated thieno[3,4-b]thiophene, for example, robust polymer:fullerene morphology enabled by the three donor PTB7) that are, furthermore, constrained to be used with a polymers, many non-traditional fullerenes are also used. Tradi- 11–13 specific fullerene, PC BM, to achieve their best performance . tional PCBMs, the most dominant fullerenes in PSCs, are out- 14–16 In general, the morphologies and thus performance of state- performed by several other non-traditional fullerenes, clearly of-the-art donor polymers (for example, PTB7 (refs 11,17) and indicating the benefits of exploring different fullerenes and the PBDT-DTNT ) are sensitive to the choice of fullerene and robust morphology formation. Comparative studies on several replacing PC BM with another C -based or non-PCBM structurally similar polymers reveal that the 2-octyldodecyl 71 60 fullerene decreases PSC efficiency to 6-7% (refs 11,18–20) The (2OD) alkyl chains sitting on quaterthiophene is the key dominant role of PC BM places serious constraints on PSC structural feature that causes the polymers’ highly temperature- material development, because the properties of the polymers dependent aggregation behaviour that allows for the processing of must be precisely matched with fixed targets set by PC BM. the polymer solutions at elevated temperature, and, more As tandem PSCs require two sets of perfectly matching polymer/ importantly, controlled aggregation and strong crystallization of fullerene materials, the constraint on their development is the polymer during the film cooling and drying process. The compounded. It has thus been pointed out that it is crucial branching position and size of the branched alkyl chains are to have the flexibility of being able to use different fullerenes and critically important in enabling an optimal aggregation beha- more generally to remove material constraints to achieve viour. With our approach, PSC production is no longer tandem PSCs with 15–20% efficiency envisioned by Brebac constrained by the use of a single fullerene or by a very thin 9,10,21 and colleagues . The development of polymer:fullerene active layer. Our aggregation and morphology control approach material systems that are morphologically insensitive to and polymer design rules can be applied to multiple polymer: fullerene choice will remove these material constraints, and fullerene materials systems and will allow the PSC community to greatly accelerate material development for single-junction and explore many more polymers and fullerene materials and to 10,22 tandem PSCs . optimize their combinations (energy offsets, bandgap and so on) Another important fundamental issue for the PSC field is how under a well-controlled morphological landscape, which would to control the morphology of polymer:fullerene blends to achieve greatly accelerate the materials and process development towards the best PSC performance. There is likely more than one near- improved PSCs. optimum PSC morphology. The famous PTB7 family donor polymers enabled one type of the near-optimum PSC morphol- ogy, as high external quantum efficiencies (EQEsB80%) have Results been reported for PTB7-based cells . However, the PTB7-based PSC device performance. Among the three donor polymers, we PSC materials and devices have certain limitations. Besides the developed that achieved power conversion efficiency410%, we sensitivity of the choice of fullerenes, another important first focus on poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)- 000 0 0 00 00 000 limitation for PTB7 family polymers is that they cannot alt-(3,3 -di(2-octyldodecyl)-2,2 ;5 ,2 ;5 ,2 -quaterthiophen- perform well when relatively thick active layers (B300 nm) are 5,5 -diyl)], PffBT4T-2OD (Fig. 1a). PffBT4T-2OD is a material used in the PSC device. Thick-film PSCs are important for the that enables six cases of high-efficiency (9.6–10.8%), high FF industrial application of PSCs, and thick films should also further (73–77%) and thick-film (250–300 nm) PSCs (Table 1) when increase the absorption strength of the solar cell and thus cell combined with traditional PCBM and many non-traditional efficiency. The reason why PTB7 does not perform well in fullerenes (Fig. 1b). A typical J–V plot of a PffBT4T-2OD:fuller- thick-film PSCs is partially owing to the relatively low hole ene PSC is shown in Fig. 1c, with EQE spectra shown in the inset. transport ability (space charge limited current (SCLC) mobility The benefits of thick-film PSCs are obvious. The thick cell 4 2  1  1 B6  10 cm V s ; ref. 17) related to the low crystallinity exhibits 10–20% higher EQE values, and the effective absorption of the PTB7 polymer. There has been also evidence that high bandwidth of a thick PSC can be increased as the result of a purity of the polymer domain may be an important factor to B20 nm red-shift of the ‘leading, low energy edge’ of a PSC’s 14,23,24 achieve efficient thick-film PSCs . The PTB7-based EQE spectrum. Combined, these account for a B30% increase in materials systems are characterized by relatively impure short circuit current (J ). Taking advantage of PffBT4T-2OD’s SC polymer domains , which could be a reason why these excellent aggregation properties (as delineated further below), polymers do not perform well in thick-film PSCs. Clearly, there we synthesized more than a dozen known or new fullerene is a need for new materials systems that explore a different ‘near- derivatives (Fig. 1b) to find the best acceptor match for PffBT4T- 2 NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 ARTICLE C H C H C H 10 21 C H C H C H 10 21 C H C H 10 21 8 17 S 10 21 C H S 8 17 10 21 8 17 S C H 8 17 8 17 N N C H N C H 10 21 N N N 8 17 S S S S SS S S S F F n N N PffBT4T-2OD PBTff4T-2OD PNT4T-2OD Ar COOMe Ar 1 2 COOMe COOR n TFP: Ar = PC MM: m = 0, R = Me; 61 1 TC BM, Ar = 71 1 TC PM: n = 2; 61 ICMA: R = H; PC PM: m = 2, R = Me; 2 61 1 S NCMA: R = H; TC BM: n = 3. PFP: Ar = 61 2 ICMM: R = COOMe; PC BM: m = 3, R = Me; NCMM: R = COOMe. 61 1 3 PC BM, Ar = 71 1 ICEM: R = CH COOMe; PC BE: m = 3, R = Et. 2 2 61 1 MOPFP: Ar = OMe 1.0 Temperature (°C) 85 0.8 –5 60 45 0.6 ‘Red-edge’ shift –10 Film 0.4 300 400 500 600 700 800 –15 0.2 Wavelength (nm) 0.0 –20 0.0 0.2 0.4 0.6 0.8 400 500 600 700 800 V (V) Wavelength (nm) Figure 1 | Chemical structures and optical and photovoltaic properties. (a,b) Chemical structures of donor polymers and fullerenes; (c) J–V curve of a PffBT4T-2OD:PC BM cell under AM1.5G illumination with an irradiation intensity of 100 mWcm (one Sun). Inset: representative EQE spectra of PSCs with a thick (300 nm) and thin (150 nm) active layer. The arrow indicates the shift of the ‘low energy edge’ of the PSCs. (d) Ultraviolet–visible (UV-Vis) absorption spectra of a PffBT4T-2OD film and a PffBT4T-2OD solution (0.02 mg ml in DCB) at temperatures as indicated. PffBT4T-2OD:fullerene blend films. Both exhibit a high Table 1 | PSC performance of 10 high-efficiency degree of molecular order, as evidenced by strong lamellar polymer:fullerene material combinations. (100), (200) and even (300) reflection peaks and, more importantly, a large (010) coherence length (GIWAXS 2D Active layer V (V) J (mA cm ) FF PCE (%) patterns shown in Fig. 2a,b and Supplementary Fig. 1). OC SC The (010) coherence length (that is, extent of ordering) of PffBT4T-2OD:TC BM 0.77 18.8 0.75 10.8 (10.3)* PffBT4T-2OD:PC BM 0.77 18.4 0.74 10.5 (10.2) PffBT4T-2OD:PC PM blend films was calculated using PffBT4T-2OD:PC PM 0.77 17.7 0.76 10.4 (10.1) 61 Scherrer analysis to be B8.5 nm, which corresponds to B24 PffBT4T-2OD:ICMA 0.78 16.4 0.77 9.8 (9.4) p-stacked copolymers. In contrast, the (010) coherence length of PffBT4T-2OD:TC PM 0.75 17.4 0.74 9.7 (9.3) PTB7:PC BM, for example, is only B2nm (ref. 16). Owing to PffBT4T-2OD:PC BM 0.77 17.1 0.73 9.6 (9.3) the high crystallinity and preferential face-on orientation of PBTff4T-2OD:PC BM 0.77 18.2 0.74 10.4 (10.0) polymer domains, relatively high SCLC hole mobility of PBTff4T-2OD:TC BM 0.76 18.7 0.68 9.7 (9.4) 2 2  1  1 1.5–3.0  10 cm V s were obtained for various PBTff4T-2OD:PC PM 0.76 18.6 0.69 9.6 (9.2) PffBT4T-2OD:fullerene blend films in a hole-only diode device PNT4T-2OD:PC BM 0.76 19.8 0.68 10.1 (9.7) configuration (Supplementary Fig. 2). The importance of FF, fill factor; PCE, power conversion efficiency; PSC, polymer solar cell. mobility for good FF was recently illustrated . *The values in parentheses stand for the average PCEs from over 20 devices for PffBT4T-2OD and from over 10 devices for PBTff4T-2OD and PNT4T-2OD. Polymer:fullerene domain size and average domain purity.In 14,15,25,28–30 addition, resonant soft X-ray scattering (R-SoXS; 2OD. All of these fullerenes form similar morphologies with Fig. 2c) and atomic force microscopy (AFM; Supplementary PffBT4T-2OD and can produce PSCs with high efficiencies in Fig. 3) analysis reveals that the various PffBT4T-2OD:fullerene the range of 8.6–10.8% (Table 1 and Supplementary Table 1). films all exhibit multi-length scale morphologies with reasonably The best efficiency (10.4%) in the C family was achieved by small median domain sizes of B30–40 nm, which is similar to PC PM (Fig. 1b), and the most commonly used C -based full- 61 60 16,25 previous cases of high-performance polymers . R-SoXS can erene, PC BM, is not the best match for PffBT4T-2OD. also reveal the average composition variations, which are indicative of the average purity of the polymer and fullerene Polymer crystallinity and hole mobility. Grazing incident wide- regions as well as a possible third phase of polymer- 26 26,31 angle X-ray diffraction (GIWAXS) reveals the molecular rich domains . An annealing sequence on PffBT4T- packing and orientational texture of pure PffBT4T-2OD and 2OD:fullerene blends revealed that the non-annealed devices NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications 3 & 2014 Macmillan Publishers Limited. All rights reserved. –2 J (mA cm ) EQE (%) Normalized absorption SiO background PC PM ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 –1 2 q (nm) 64 2 86 4 2 2.6 3.6 –6 2.0 10 3.2 2.8 2.0 (010) (010) 2.4 Log PffBT4T-2OD:PC BM –7 1.0 10 PffBT4T-2OD:ICMA 1.0 (300) (300) PC PM PffBT4T-2OD:PC PM 61 61 (200) (200) Aggregation PffBT4T-2OD:PC BM (100) (100) –8 1.0 0.0 2 46 8 2 0.0 –1.0 –2.0 0.0 –1.0 –2.0 –2 –1 10 10 –1 –1 q (Å ) q (Å ) –1 xy xy q (nm ) 1.5 1,800 5,000 r.p.m. Spinrate CL 3,000 r.p.m. 600 r.p.m. (r.p.m.) (nm) 1,000 r.p.m. high crystallinity 1,500 800 r.p.m. 700 ; 5.7 600 r.p.m. 1,000 ; 5.2 1.0 2,000 ; 3.5 1,200 3,000 ; 1.0 5,000 ; 0.9 900 5,000 r.p.m. 0.5 poor crystallinity 300 0.0 1.6 1.7 1.8 1.9 500 600 700 800 –1 q (nm ) Wavelength (nm) Figure 2 | Morphological characterization data. (a) Two-dimensional (2D) GIWAXS pattern of a pure PffBT4T-2OD film. (b) 2D GIWAXS pattern of a PffBT4T-2OD:PC PM. (c) R-SoXS profiles in log scale for four samples of PffBT4T-2OD:fullerene blends. Blue line: PC BM; red line: PC BM; 61 61 71 black line: ICMA; green line: PC PM. (d) (010) diffraction peak (obtained from XRD) of PffBT4T-2OD pure films spun at different rates, the inset indicates the (010) coherence length (CL) of the films. (e) UV-Vis absorption spectra of PffBT4T-2OD:PC PM blend films obtained with different spin rates and substrate temperatures. Blue line, 5,000 r.p.m./110 C; dark green line, 3,000 r.p.m./100 C; red line, 1,000 r.p.m./90 C; pink line, 800 r.p.m./80 C; emerald line, 600 r.p.m./70 C. All spectra are normalized based on the intensity of their 0-1 transition peak (at B640 nm) to highlight the change of the intensity of 0-0 transition peaks. presented here exhibited almost 90% average purity compared low concentration PffBT4T-2OD solution in 1,2-dichlorobenzene with the asymptotic limit (Fig. 3a,b), which corresponds to an (DCB) is lowered from 85 to 25 C (Fig. 1d). At elevated unusually low residual concentration of 3.2% fullerene averaged temperature, PffBT4T-2OD is well dissolved and disaggregated. over all PffBT4T-2OD domains in the film as measured by X-ray At progressively lower temperatures, a strong 0-0 transition peak 25,32,33 microscopy (Fig. 3c) . In general, PSCs with significantly atB700 nm emerges with significant strength at 25 C, indicating impure polymer phases exhibit detrimental bimolecular charge strong aggregation of the polymer chains in solution at that recombination when the polymer film is too thick, whereas temperature. Note that the absorption spectrum of the 25 C pure phases can help to minimize recombination . These solution of PffBT4T-2OD is almost identical to that of the opti- morphological data show that PffBT4T-2OD can form a mized PffBT4T-2OD solid film (Fig. 1d), which is observed to be polymer:fullerene morphology containing highly crystalline and highly crystalline by GIWAXS. Consequently, devices are always sufficiently pure yet reasonably small polymer domains. Note that cast from warm solutions (60–80 C) of PffBT4T-2OD, which PTB7-type polymers have been the best donor polymer in PSCs then aggregates during the cooling and film-forming process. for the past few years. By its very nature of high performance in To understand details of PffBT4T-2OD’s aggregation beha- thin films, PTB7 can form a ‘near-optimum’ PSC morphology viour during the film-forming process, the critical p–p molecular characterized by relatively low molecular ordering, relatively low ordering ((010) coherence length and intensity of the (010) peak) hole mobilities and impure polymer domains . PffBT4T-2OD is determined with X-ray diffraction (XRD) for a series of exhibits high molecular ordering (‘crystallinity’), high hole PffBT4T-2OD films spun at different rates. As shown in Fig. 2d mobilities and purer polymer domains, which appears to be a and Supplementary Table 2, the p–p ordering decreases markedly different ipso facto ‘near-optimum’ PSC morphology. Although with increasing spin rates. As PffBT4T-2OD exhibits a strong yet PTB7 enabled great thin-film PSC performance, PffBT4T-2OD progressively evolving aggregation property, the extent of offers high performance even in thick-film PSCs owing to the PffBT4T-2OD’s aggregation depends upon temperature and high mobility of the highly ordered and sufficiently pure polymer concentration changes, the film drying time and the kinetics of domains it forms. aggregation. During a slow spin process (for example, 700 r.p.m.), it takes a relatively long time for the solution and film to dry, during which the temperature of the substrate and the wet film Morphology control via temperature-dependent aggregation. We attribute PffBT4T-2OD’s excellent performance and robust also decreases significantly. When using an ultra-fast rate (for morphology to its significant temperature-dependent aggregation example, 5,000 r.p.m.), however, the solvent evaporates more behaviour that can be exploited during device fabrication. quickly, which results in kinetically quenched, poorly ordered The UV-Vis absorption spectra exhibit a marked red-shift when a films, whereas slow spin rates provides PffBT4T-2OD sufficient 4 NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. –1 q (Å ) Intensity –1 q (Å ) Absorbance 2 –2 Intensity q (nm ) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 ARTICLE from the common processing protocol of PTB7 family polymers 1.00 1.00 that are typically processed at room temperature. 0.98 0.98 0.96 0.96 Discussion 0.94 0.94 The key structural feature of PffBT4T-2OD that enables its 0.92 0.92 pronounced, yet gradual temperature-dependent aggregation 0.90 0.90 (Fig. 1d) is the second-position branched alkyl chains (2OD) 0 40 80 120 on a quaterthiophene (4T-2OD). To elucidate this aspect, we 040 80 120 Annealing time (min) Annealing time (min) contrast PffBT4T-2OD with two structurally very similar polymers. These two polymers have the same backbone but their alkyl chains are branched at the first or third side-chain carbon 1.4 atom (for ease of comparison, these two polymers are named as PffBT4T-1ON and PffBT4T-3OT; Fig. 4a). In contrast to 1.2 PffBT4T-2OD, PffBT4T-1ON is disaggregated at 85 C, and more importantly, also disaggregated at 25 C (Fig. 4b). As a 1.0 result, PffBT4T-1ON cannot aggregate easily during the film- forming process, leading to films with poor crystallinity 0.8 (GIWAXS pattern shown in Fig. 4c) and thus PSC devices of 3.2% wt 0.6 only B0.6% efficiency. PffBT4T-3OT exhibits the other extreme, showing excessive aggregation at both 25 and 85 C. During our 0.4 attempt to process PffBT4T-3OT, the PffBT4T-3OT solution quickly becomes a gel (Fig. 4d) even before the start of spin 0.2 casting. These comparisons indicate that PffBT4T-1ON’s alkyl 285 290 295 300 350 400 chains cause too much steric hindrance, which results in poor Energy (eV) aggregation and crystallinity. PffBT4T-3OT’s alkyl chains provide too little steric hindrance that makes aggregation of PffBT4T- Figure 3 | Domain purity data and analysis. (a) R-SoXS results revealing 3OT too strong even at 85 C and makes it difficult to process. relative purity of PffBT4T-2OD:ICMA and (b) PffBT4T-2OD:PC BM blends PffBT4T-2OD’s second-position branched alkyl chains offer an annealed at 130 C. (c) Following methodology developed by Collins 40 optimal tradeoff that allows for controllable aggregation of et al., residual ICMA in PffBT4T-2OD is only 3.2% after a PffBT4T- PffBT4T-2OD during the film-forming process. More discussions 2OD:ICMA blend has been annealed extensively until all excess ICMA has on the impact of alkyl chain branching positions are provided in agglomerated into macro-phase domains or crystals. Red lines/symbols are Supplementary Note 1. near edge X-ray absorption fine structure data/uncertainty and black lines Several structurally similar donor polymers containing are fits. Yellow and blue lines are PffBT4T-2OD and fullerene reference quaterthiophene substituted with second-position branched alkyl spectra used in the fit. Error bars represent the average of three different 34–37 chains were reported in the literature , including a recent spots on the sample. A value of 3.2% indicates that there are 3.2% (w.t) report in which a polymer with longer alkyl chains, FBT-Th (1,4) fullerene in the average polymer domains that includes polymer crystals (named as PffBT4T-2DT for simplicity in this paper, structure and the mixed, amorphous phase. shown in Fig. 4a), achieved 7.64% efficiency . The difference of PffBT4T-2OD and PffBT4T-2DT devices can be understood from time to aggregate and to form crystalline polymer domains with the following results. R-SoXS (Fig. 4e) and prior AFM studies large coherence lengths. Importantly, studies for PffBT4T-2OD show that the average domain size of PffBT4T-2DT:fullerene pure films and PffBT4T-2OD blend films with two different films is too large (B100 nm). The R-SoXS data furthermore show fullerenes yield similar trends (Supplementary Table 2 and that the average purity of the polymer/polymer-rich domains in Supplementary Fig. 4), demonstrating that the aggregation of an PffBT4T-2DT:fullerene film is B87% of that in PffBT4T- PffBT4T-2OD is insensitive to the presence of fullerenes. Not 2OD:fullerene film. Lower purity of average polymer/polymer- surprisingly, high substrate temperatures were found to have a rich domains can result in significant recombination for 14,23,24 similar effect to fast spin rates. PffBT4T-2OD:fullerene films thick-film PSC devices and thus lower performance . prepared with fast spin rates/high substrate temperatures show a Regarding molecular ordering, the two-dimensional GIWAXS decrease in the 0-0 transition peaks and a pronounced shift in the mapping of PffBT4T-2DT:PC BM films shows that PffBT4T- 0-0 transition energy in their UV-Vis absorption spectra, 2DT:fullerene films exhibit weak laminar packing peaks, which indicative of significant disorder (Fig. 2e). The corresponding are significantly weaker than those of PffBT4T-2OD:fullerene hole-only and PSC devices fabricated using high films. The smaller degree of laminar packing of PffBT4T-2DT is spin rates and high substrate temperature also exhibit consistent with the lower average purity of PffBT4T-2DT 3 2  1  1 markedly decreased hole mobilities (3.1  10 cm V s ; polymer domains compared with that of PffBT4T-2OD’s, as Supplementary Table 3) and PSC efficiencies (3.6%; more fullerene is expelled by the crystalline polymer domains of Supplementary Table 4). These morphological, spectroscopic PffBT4T-2OD. Lastly, the absorption coefficient of PffBT4T-2DT and electric data demonstrate that PffBT4T-2OD’s morphology is is lower than that of PffBT4T-2OD owing to longer alkyl chains mainly controlled by the progress of its aggregation during the that does not contribute to light absorption (Supplementary film-casting process until the film is dry, which locks-in the Fig. 5). These studies show that the branching position and the length scale of the morphology. PffBT4T-2OD’s strong yet well- size of the alkyl chains are critically important in obtaining the controllable aggregation property allows for convenient optimi- optimal aggregation properties of PffBT4T-2OD. Insufficient zation of processing conditions that led to a near-ideal aggregation (for example, PffBT4T-1ON) and unnecessarily polymer:fullerene morphology that is insensitive to the choices long alkyl chains (for example, PffBT4T-2DT) resulted in of fullerene. This approach of controlling the extent of polymer low crystallinity and/or impure polymer domains. Excessive aggregation during a warm solution casting process is different aggregation (for example, PffBT4T-3OT) makes the processing NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications 5 & 2014 Macmillan Publishers Limited. All rights reserved. Relative purity Absorbance (OD) Relative purity ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 C H C H 8 17 8 17 N N 1 1.0 C H C H 8 17 8 17 PffBT4T-1ON S 0.8 S S F F 0.6 C H C H 10 21 10 21 C H 2 S C H 8 17 8 17 N 0.4 PffBT4T-1ON, 85 °C S 0.2 PffBT4T-1ON, 25 °C PffBT4T-3OT S S PffBT4T-3OT, 85 °C F F n PffBT4T-3OT, 25 °C 0.0 C H C H 12 25 12 25 400 500 600 700 800 C H S C H 10 21 10 21 N N Wavelengh (nm) PffBT4T-2DT (FBT-Th [1,4]) 4 S S S –1 2 q (nm) F F 6 4 22 8 6 4 2.6 –6 2.0 –7 1.0 Domain size: Purity: PffBT4T-2OD 40 nm 100% 0.0 PffBT4T-2DT 100 nm 87% 0.0 –1.0 –2.0 –8 –1 2 2 46 8 4 q (Å ) xy –2 –1 10 10 –1 q (nm ) Figure 4 | Comparative studies on structurally similar polymers. (a) Chemical structures of PffBT4T-1ON, PffBT4T-3OT and PffBT4T-2DT. (b) Normalized UV-Vis absorption spectra of PffBT4T-1ON and PffBT4T-3OT in a 85 C DCB solution and in a DCB solution at room temperature as indicated on the plot. (c) Two-dimensional GIWAXS pattern of a pure PffBT4T-1ON film. (d) Uneven film formed by the PffBT4T-3OT solution forming a gel before being able to be spun. (e) R-SoXS profiles in log scale for PffBT4T-2OD and PffBT4T-2DT, the inset indicates the domain size and relative purity. and aggregation difficult to control. Similar to recent detail in the Supplementary Information (Supplementary Fig. 7, 32,36,38 observations , the molecular weight of PffBT4T-2OD has Table 1 and Supplementary Table 1). a significant impact on its aggregation property and performance. Although second-position branched alkyl chains are a well- Lower molecular weight (M ¼ 16.6 kDa, M ¼ 29.5 kDa) batches known structural motif and have been previously used on n w of PffBT4T-2OD exhibit weaker aggregation and thus lower quaterthiophene-based polymers, previous work did not utilize a efficiency (7.7%) than the high molecular weight (M ¼ 47.5 kDa, polymer with the most suitable alkyl chains nor were warm- M ¼ 93.7 kDa) PffBT4T-2OD batches (Supplementary Table 5 casting methods used that optimally harnessed aggregation. They and Supplementary Fig. 6). The impacts of polymer molecular thus failed to reveal the connections between chemical structure, weight on PSC performances are discussed in details in the polymer aggregation during warm processing, morphology Supplementary Note 2. formation, polymer crystallinity and consequently PSC perfor- Following the rationale described above, we synthesized two mance. Our study uncovered a new approach of aggregation and 0 00 other polymers (poly[(2,1,3-benzothiadiazol-4,7-diyl)-alt-(4 ,3 - morphology control enabled by a structural feature (2OD alkyl 000 0 0 00 00 000 000 difluoro-3,3 -di(2-octyldodecyl)-2,2 ;5 ,2 ;5 ,2 -quaterthiophen-5,5 chain) that is seemingly simple and commonly known, yet has -diyl)] (PBTff4T-2OD) and poly[(naphtho[1,2-c:5,6-c ]bis[1,2,5] surprisingly profound impact on PSC performances. The wide 000 0 0 00 00 000 thiadiazol-5,10-diyl)-alt-(3,3 -di(2-octyldodecyl)-2,2 ;5 ,2 ;5 ,2 - ranging applicability of our morphology control approach is quaterthiophen-5,5 -diyl)] (PNT4T-2OD); Fig. 1a) with supported by the three polymers and over 10 polymer:fullerene significantly different polymer backbones but with similar combinations that all yielded similar blend morphology and arrangements of 2OD alkyl chains. Both PBTff4T-2OD and high-efficiency thick-film PSCs. Furthermore, the aggregation PNT4T-2OD exhibit significant temperature-dependent aggrega- behaviour as observed by UV-Vis might serve as a useful tion behaviour that leads to processing and morphology control screening tool to identify materials that yield good devices when and thus efficiency (including 410% for thick-film PSCs; Table 1, cast from warm solutions. Supplementary Table 1 and Supplementary Fig. 7) comparable to Note that the chemical structures of the three donor polymers those achieved by PffBT4T-2OD-based PSCs. R-SoXS and AFM presented in the paper are distinctively different from previous 12,13,17 studies confirmed that the polymer domain size of these two new state-of-the-art PTB7 family of polymers . The PTB7 family polymers are similar to that of PffBT4T-2OD (30–40 nm). XRD polymers consist of an electron deficient fluorinated thieno characterization of PBTff4T-2OD:fullerene and PNT4T-2OD: [3,4-b]thiophene unit and a benzodithiophene unit with alkoxy, fullerene films also showed strong (010) p–p stacking peaks that alkylthienyl or alkylthiothienyl substitution groups. The three are similar to those observed for PffBT4T-2OD. Note that polymers in this paper consist of an electron deficient unit PNT4T-2OD also significantly outperforms its analogue polymer (either difluorobenzothiadiazole or benzothiadiazole or with 2-decyltetradecyl (2DT) alkyl chains , providing another naphthobisthiadiazole) combined with a quaterthiophene unit example that supports the critical importance of the size of with two 2OD alkyl chains sitting on the first and fourth the alkyl chains. The synthesis, characterization and device thiophenes. The difference in the chemical structures caused performance of PBTff4T-2OD and PNT4T-2OD are described in different aggregation properties, based on which different 6 NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. –1 q (Å ) 2 –2 Intensity q (nm ) Normalized absorption NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 ARTICLE processing protocols are used. While PTB7 family polymers Where e is the permittivity of free space, e is the dielectric constant of the 0 r polymer, m is the hole mobility, V is the voltage drop across the device and L is the do not exhibit a strong temperature-dependent aggregation thickness of the polymer. The dielectric constant e is assumed to be B3, which is a property and are often processed from room temperature typical value for conjugated polymers. solutions, the 4T-2OD based polymers are processed from warm solutions to utilize their temperature-dependent aggregation GIWAXS characterization. GIWAXS measurements were performed at beamline property so that the morphology and extent of molecular 7.3.3 at the Advanced Light Source (ALS) . Samples were prepared on Si substrates using identical blend solutions as those used in devices. The 10 keV ordering can be explicitly controlled during casting. X-ray beam was incident at a grazing angle of 0.11–0.15, which maximized the To summarize, we report that exquisite control of aggregation scattering intensity from the samples. The scattered X-rays were detected using a results in high-performance thick-film PSCs for three different Dectris Pilatus 1 M photon counting detector. donor polymers and 10 polymer:fullerene combinations, all of which yielded efficiencies higher than the previous state of the art Resonant soft X-ray scattering. R-SoXS transmission measurements were per- (9.5%). The common structural feature of the three donor 30 formed at beamline 11.0.1.2 at the ALS . Samples for R-SoXS measurements were polymers, the 2OD alkyl chains on quaterthiophene, causes a prepared on a PSS modified Si substrate under the same conditions as those used for device fabrication, and then transferred by floating in water to a 1.5  1.5 mm, temperature-dependent aggregation behaviour that allows for the 100-nm thick Si N membrane supported by a 5  5 mm, 200mm thick Si frame 3 4 processing of the polymer solutions at moderately elevated (Norcada Inc.). Two dimensional scattering patterns were collected on an temperature, and more importantly, controlled aggregation and in-vacuum CCD camera (Princeton Instrument PI-MTE). The beam size at the strong crystallization of the polymer during the film cooling and sample is B100mm by 200mm. The composition variation (or relative domain purity) over the length scales probed can be extracted by integrating scattering drying process. This results in a near-ideal polymer:fullerene profiles to yield the total scattering intensity. The purer the average domains morphology (containing highly crystalline, preferentially orien- are, the higher the total scattering intensity. Owing to a lack of absolute flux tated, yet small polymer domains) that is controlled by polymer normalization, the absolute composition cannot be obtained by only R-SoXS. aggregation during casting and thus insensitive to the choice of In order to get a sense of how pure the domains are, we annealed the PffBT4T- 2OD/fullerene blend at 130 C for different length of time, 0, 10, 20, 40 and fullerenes. The branching position and size of the branched alkyl 120 min. The unannealed sample exhibits very pure domains, that is, almost 90% chains are critically important in enabling a well-controllable of the saturated value. aggregation behaviour. Unnecessarily long alkyl chains (for example, 2DT) cause several detrimental effects including weaker AFM characterization. AFM measurements were performed by using a Scanning laminar stacking, poorer absorption properties and less pure Probe Microscope-Dimension 3100 in tapping mode. All film samples were spin polymer domains. Our structural design rationales and aggrega- casted on ITO/ZnO substrates. tion and morphology control approach offer a new route to achieve high-performance thick-film PSCs that cannot be Photoluminescence quenching measurements. Photoluminescence spectra were obtained from previous state-of-the-art material systems. Given measured on samples on ITO/ZnO substrates upon excitation of a 671-nm laser beam. The PL quenching efficiency of PffBT4T-2OD was estimated from the ratio that the field and record performance in the last few years has of the PL intensity of a PffBT4T-2OD:fullerene film sample to that of the PffBT4T- been mostly dominated by a single system (PTB7 family with 2OD control sample. (Supplementary Fig. 10) PC BM), the 10 material systems and three polymers based on a single and simple design feature presented here point to a Solar cell fabrication and testing. Pre-patterned ITO-coated glass with a sheet plethora of possible materials combinations that should further resistance of B15O per square was used as the substrate. It was cleaned by improve the performance. Our approach will allow the PSC sequential sonications in soap DI water, DI water, acetone and isopropanol for community to explore many more polymers and fullerene 15 min at each step. After ultraviolet/ozone treatment for 60 min, a ZnO electron transport layer was prepared by spin coating at 5,000 r.p.m. from a ZnO precursor materials and to optimize their combinations (energy offsets, solution (diethyl zinc). Active layer solutions (D/A ratio 1:1.2) were prepared in bandgap and so on) under a well-controlled morphological CB/DCB (1:1 volume ratio) with or without 3% of DIO (polymer concentration: landscape that would greatly accelerate the materials and process  1 9mgml ). To completely dissolve the polymer, the active layer solution should development towards improved PSCs. be stirred on a hot plate at 110 C for at least 3 h. Before spin coating, both the polymer solution and ITO substrate are preheated on a hot plate at B110 C. Active layers were spin coated from the warm polymer solution on the preheated Methods substrate in a N glovebox at 800 r.p.m. to obtain thicknesses of B300 nm. X-ray diffraction. XRD data were obtained from a PANanalytical XRD instrument (The spin casting of high-performance PSC films is described in Supplementary (model name: Empyrean) using the parallel beam mode that is recommended by the Note 4 in details. The processing of PNT4T-2OD also requires the use of a metal instrument manufacturer to characterize thin-film samples. All XRD samples were chuck as described in Supplementary Note 4. High-temperature and high spin rate spin cast on Si substrates to avoid strong scattering background of glass substrates. samples are described in Supplementary Note 5). The polymer/fullerene films were To rule out the effect of substrate properties on the crystallinity of polymer film then annealed at 80 C for 5 min before being transferred to the vacuum chamber samples, we also investigated polymer films on Si/ZnO substrates and found that the of a thermal evaporator inside the same glovebox. At a vacuum level of 3  10 polymer films have similar scattering profiles (Supplementary Fig. 8) on these two Torr, a thin layer (20 nm) of MoO or V O was deposited as the anode interlayer, 3 2 5 types of substrates (Si/ZnO and Si). The polymer crystallinity is thus rather insen- followed by deposition of 100 nm of Al as the top electrode. All cells were sitive to the surface properties of the substrates. More details of XRD characteriza- encapsulated using epoxy inside the glovebox. Device J–V characteristics was tions are provided in Supplementary Note 3. measured under AM1.5G (100 mW cm ) using a Newport solar simulator. The light intensity was calibrated using a standard Si diode (with KG5 filter, purchased from PV Measurement) to bring spectral mismatch to unity. J–V Cyclic voltammetry. Cyclic voltammetry was performed in an electrolyte solution characteristics were recorded using a Keithley 236 source meter unit. Typical cells of 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile, both working have devices area of B5.9 mm , which is defined by a metal mask with an aperture and counter electrodes were platinum electrode. Ag/AgCl electrode was used as the aligned with the device area. EQEs were characterized using a Newport EQE system reference electrode; the Fc/Fc redox couple was used as an external standard equipped with a standard Si diode. Monochromatic light was generated from a (Supplementary Fig. 9 and Supplementary Table 6). Newport 300 W lamp source. One of our best cells was sent to an accredited solar cell calibration laboratory (Newport Corporation) for certification, confirming an UV-Vis absorption. UV-Vis absorption spectra were acquired on a Gary ± ± ± efficiency of 10.36 0.22%, with V ¼ 0.7743 0.0077 V, I ¼ 0.00079 0.00001 OC SC 50 UV-Vis spectrometer. All film samples were spin cast on ITO/ZnO substrates. 2 ± ± A, area ¼ 0.0425 0.0001 cm ,FF ¼ 72.0 1.5 (Supplementary Fig. 11). Hole-only device. The hole mobility were measured using the SCLC method by References using a device architecture of ITO/V O /PffBT4T-2OD (300 nm)/V O /Al by 2 5 2 5 1. Yu, G., Gao, J., Hummelen, J. C., Wudl, F. & Heeger, A. J. Polymer photovoltaic taking current–voltage curves and fitting the results to a space charge limited form, cells-enhanced efficiencies via a network of internal donor-acceptor where the SCLC is described by: heterojunctions. Science 270, 1789–1791 (1995). 2. Halls, J. J. M. et al. 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Donor-acceptor conjugated polymer based on naphtho[1,2- maintenance. The work described in this paper was partially supported by a grant from c:5,6-c]bis[1,2,5]thiadiazole for high-performance polymer solar cells. J. Am. the Research Grants Council of the Hong Kong Special Administrative Region, China, Chem. Soc. 133, 9638–9641 (2011). under Theme-based Research Scheme through project no. T23-407/13-N. 20. Morinaka, Y. et al. Synthesis and photovoltaic properties of acceptor materials based on the dimerization of fullerene C60 for use in efficient polymer solar Author contributions cells. Chem. Commun. 49, 3670–3672 (2013). Y.L. synthesized PffBT4T-2OD; J.Z. designed PNT4T-2OD, synthesized 5,10-Dibromo- 21. Li, N. et al. Towards 15% energy conversion efficiency: a systematic study of the naphtho[1,2-c:5,6-c’]bis[1,2,5]thiadiazole and carried out AFM measurements; solution-processed organic tandem solar cells based on commercially available Z.L. synthesized PBTff4T-2OD; J.Z., H.L., Y.L., H.H. and Z.L. synthesized fullerenes; materials. Energy Environ. Sci. 6, 3407–3413 (2013). C.M., H.H., K.J. and H.Y. fabricated and tested PSC devices; W.M. carried out GIWAXS 22. Scharber, M. C. & Sariciftci, N. S. Efficiency of bulk-heterojunction organic and R-SoXS measurements and analysis; K.J. carried out XRD analysis; H.L. synthesized solar cells. Prog. Polym. Sci. 38, 1929–1940 (2013). PNT4T-2OD; H.A. supervised GIWAXS and R-SoXS work, and helped design experi- 23. Albrecht, S. et al. Quantifying charge extraction in organic solar cells: the case mental protocols; H.Y. conceived and directed the project; H.A. and H.Y. wrote the paper of fluorinated PCPDTBT. J. Phys. Chem. Lett. 5, 1131–1138 (2014). with input from all authors who reviewed the final paper. 24. Stuart, A. C. et al. Fluorine substituents reduce charge recombination and drive structure and morphology development in polymer solar cells. J. Am. Chem. Soc. 135, 1806–1815 (2013). Additional information 25. Collins, B. A. et al. Absolute measurement of domain composition and Supplementary Information accompanies this paper at http://www.nature.com/ nanoscale size distribution explains performance in PTB7:PC71BM solar cells. naturecommunications Adv. Energy Mater. 3, 65–74 (2013). Competing financial interests: The authors declare no competing financial interests. 26. Mu¨ller-Buschbaum, P. The active layer morphology of organic solar cells probed with grazing incidence scattering techniques. Adv. Mater. doi:10.1002/ Reprints and permission information is available online at http://npg.nature.com/ adma.201304187 (2014). reprintsandpermissions/ 27. Smilgies, D. M. Scherrer grain-size analysis adapted to grazing-incidence scattering with area detectors. J. Appl. Crystallogr. 42, 1030–1034 (2009). How to cite this article: Liu, Y. et al. Aggregation and morphology control enables 28. Ma, W., Ye, L., Zhang, S. Q., Hou, J. H. & Ade, H. Competition between multiple cases of high-efficiency polymer solar cells. Nat. Commun. 5:5293 morphological attributes in the thermal annealing and additive processing of doi: 10.1038/ncomms6293 (2014). polymer solar cells. J. Mater. Chem. C 1, 5023–5030 (2013). This work is licensed under a Creative Commons Attribution 4.0 29. Swaraj, S. et al. Nanomorphology of bulk heterojunction photovoltaic thin films probed with resonant soft X-ray scattering. Nano Lett. 10, 2863–2869 International License. The images or other third party material in this (2010). article are included in the article’s Creative Commons license, unless indicated otherwise 30. Gann, E. et al. Soft x-ray scattering facility at the Advanced light source in the credit line; if the material is not included under the Creative Commons license, with real-time data processing and analysis. Rev. Sci. Instrum. 83, 045110 users will need to obtain permission from the license holder to reproduce the material. (2012). To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ 8 NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nature Communications Springer Journals

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Abstract

ARTICLE Received 24 Jul 2014 | Accepted 17 Sep 2014 | Published 10 Nov 2014 DOI: 10.1038/ncomms6293 OPEN Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells 1, 1, 1, 1 2,w 1 1 1 Yuhang Liu *, Jingbo Zhao *, Zhengke Li *, Cheng Mu , Wei Ma , Huawei Hu , Kui Jiang , Haoran Lin , 2 1,3 Harald Ade &HeYan Although the field of polymer solar cell has seen much progress in device performance in the past few years, several limitations are holding back its further development. For instance, current high-efficiency (49.0%) cells are restricted to material combinations that are based on limited donor polymers and only one specific fullerene acceptor. Here we report the achievement of high-performance (efficiencies up to 10.8%, fill factors up to 77%) thick-film polymer solar cells for multiple polymer:fullerene combinations via the formation of a near- ideal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains. This morphology is controlled by the temperature-dependent aggregation behaviour of the donor polymers and is insensitive to the choice of fullerenes. The uncovered aggregation and design rules yield three high-efficiency (410%) donor polymers and will allow further synthetic advances and matching of both the polymer and fullerene materials, potentially leading to significantly improved performance and increased design flexibility. 1 2 Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. Department of Physics and ORaCEL, North Carolina State University, Raleigh, North Carolina 27695, USA. HKUST-Shenzhen Research Institute, No. 9 Yuexing 1st RD, Hi-tech Park, Nanshan, Shenzhen 518057, China. * These authors contributed equally to this work. w Current Address: XJTU-HKUST Joint School of Sustainable Development, Xi’an Jiaotong University, Xi’an, P.R. China. Correspondence and requests for materials should be addressed to W.M. (email: wma5@ncsu.edu) or to H.A. (email: harald_ade@ncsu.edu) or to H.Y. (email: hyan@ust.hk). NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 onventional inorganic solar cells can achieve high optimum’ PSC morphology in order to achieve thick-film PSCs efficiencies but are produced through complicated, costly that have comparable or higher efficiencies than state-of-the-art Cprocesses. The desirability of lower costs is driving the PTB7 materials systems. development of several third-generation solar technologies. In the following, we report the achievement of high- 1–6 Among these, polymer solar cell (PSC) technology is an performance (efficiencies up to 10.8% and fill factors (FFs) up excellent example of low-cost production because PSCs can be to 77%) thick-film PSCs based on three different donor polymers produced using extremely high-throughput roll-to-roll printing and 10 polymer:fullerene combinations, all of which exhibit methods similar to those used to print newspapers . PSCs also efficiencies higher than the previous state of the art. In contrast to offer several other advantages: vacuum processing and high- state-of-the-art PTB7-based materials systems, the high PSC temperature sintering are not needed, and no toxic materials are performances in this report are achieved via the formation of an used in the end product. Most importantly, a tandem cell ‘optimum PSC morphology’ that contains highly crystalline, 6,8–10 architecture can be easily implemented with PSCs and has sufficiently pure, yet reasonably small polymer domains. The high proven to improve PSC efficiency by B40–50% (refs 6,8). As polymer crystallinity and thus excellent hole transport ability, PSCs are two-component, donor–acceptor material systems, it is combined with sufficiently pure polymer domains, are the main generally important to control the morphology of the reasons why the PSCs exhibit high FFs and efficiency even when donor:acceptor blends and to find an optimal materials the active layer is 300 nm thick. Importantly, this ipso facto near- combination with excellent optical and electronic properties. In perfect morphology is controlled by the temperature-dependent the last few years, record-efficiency PSCs were achieved with only aggregation behaviour of the donor polymers during casting and three donor polymers (which all belong to a specific polymer is insensitive to the choice of fullerenes. Taking advantage of the family based on fluorinated thieno[3,4-b]thiophene, for example, robust polymer:fullerene morphology enabled by the three donor PTB7) that are, furthermore, constrained to be used with a polymers, many non-traditional fullerenes are also used. Tradi- 11–13 specific fullerene, PC BM, to achieve their best performance . tional PCBMs, the most dominant fullerenes in PSCs, are out- 14–16 In general, the morphologies and thus performance of state- performed by several other non-traditional fullerenes, clearly of-the-art donor polymers (for example, PTB7 (refs 11,17) and indicating the benefits of exploring different fullerenes and the PBDT-DTNT ) are sensitive to the choice of fullerene and robust morphology formation. Comparative studies on several replacing PC BM with another C -based or non-PCBM structurally similar polymers reveal that the 2-octyldodecyl 71 60 fullerene decreases PSC efficiency to 6-7% (refs 11,18–20) The (2OD) alkyl chains sitting on quaterthiophene is the key dominant role of PC BM places serious constraints on PSC structural feature that causes the polymers’ highly temperature- material development, because the properties of the polymers dependent aggregation behaviour that allows for the processing of must be precisely matched with fixed targets set by PC BM. the polymer solutions at elevated temperature, and, more As tandem PSCs require two sets of perfectly matching polymer/ importantly, controlled aggregation and strong crystallization of fullerene materials, the constraint on their development is the polymer during the film cooling and drying process. The compounded. It has thus been pointed out that it is crucial branching position and size of the branched alkyl chains are to have the flexibility of being able to use different fullerenes and critically important in enabling an optimal aggregation beha- more generally to remove material constraints to achieve viour. With our approach, PSC production is no longer tandem PSCs with 15–20% efficiency envisioned by Brebac constrained by the use of a single fullerene or by a very thin 9,10,21 and colleagues . The development of polymer:fullerene active layer. Our aggregation and morphology control approach material systems that are morphologically insensitive to and polymer design rules can be applied to multiple polymer: fullerene choice will remove these material constraints, and fullerene materials systems and will allow the PSC community to greatly accelerate material development for single-junction and explore many more polymers and fullerene materials and to 10,22 tandem PSCs . optimize their combinations (energy offsets, bandgap and so on) Another important fundamental issue for the PSC field is how under a well-controlled morphological landscape, which would to control the morphology of polymer:fullerene blends to achieve greatly accelerate the materials and process development towards the best PSC performance. There is likely more than one near- improved PSCs. optimum PSC morphology. The famous PTB7 family donor polymers enabled one type of the near-optimum PSC morphol- ogy, as high external quantum efficiencies (EQEsB80%) have Results been reported for PTB7-based cells . However, the PTB7-based PSC device performance. Among the three donor polymers, we PSC materials and devices have certain limitations. Besides the developed that achieved power conversion efficiency410%, we sensitivity of the choice of fullerenes, another important first focus on poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)- 000 0 0 00 00 000 limitation for PTB7 family polymers is that they cannot alt-(3,3 -di(2-octyldodecyl)-2,2 ;5 ,2 ;5 ,2 -quaterthiophen- perform well when relatively thick active layers (B300 nm) are 5,5 -diyl)], PffBT4T-2OD (Fig. 1a). PffBT4T-2OD is a material used in the PSC device. Thick-film PSCs are important for the that enables six cases of high-efficiency (9.6–10.8%), high FF industrial application of PSCs, and thick films should also further (73–77%) and thick-film (250–300 nm) PSCs (Table 1) when increase the absorption strength of the solar cell and thus cell combined with traditional PCBM and many non-traditional efficiency. The reason why PTB7 does not perform well in fullerenes (Fig. 1b). A typical J–V plot of a PffBT4T-2OD:fuller- thick-film PSCs is partially owing to the relatively low hole ene PSC is shown in Fig. 1c, with EQE spectra shown in the inset. transport ability (space charge limited current (SCLC) mobility The benefits of thick-film PSCs are obvious. The thick cell 4 2  1  1 B6  10 cm V s ; ref. 17) related to the low crystallinity exhibits 10–20% higher EQE values, and the effective absorption of the PTB7 polymer. There has been also evidence that high bandwidth of a thick PSC can be increased as the result of a purity of the polymer domain may be an important factor to B20 nm red-shift of the ‘leading, low energy edge’ of a PSC’s 14,23,24 achieve efficient thick-film PSCs . The PTB7-based EQE spectrum. Combined, these account for a B30% increase in materials systems are characterized by relatively impure short circuit current (J ). Taking advantage of PffBT4T-2OD’s SC polymer domains , which could be a reason why these excellent aggregation properties (as delineated further below), polymers do not perform well in thick-film PSCs. Clearly, there we synthesized more than a dozen known or new fullerene is a need for new materials systems that explore a different ‘near- derivatives (Fig. 1b) to find the best acceptor match for PffBT4T- 2 NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 ARTICLE C H C H C H 10 21 C H C H C H 10 21 C H C H 10 21 8 17 S 10 21 C H S 8 17 10 21 8 17 S C H 8 17 8 17 N N C H N C H 10 21 N N N 8 17 S S S S SS S S S F F n N N PffBT4T-2OD PBTff4T-2OD PNT4T-2OD Ar COOMe Ar 1 2 COOMe COOR n TFP: Ar = PC MM: m = 0, R = Me; 61 1 TC BM, Ar = 71 1 TC PM: n = 2; 61 ICMA: R = H; PC PM: m = 2, R = Me; 2 61 1 S NCMA: R = H; TC BM: n = 3. PFP: Ar = 61 2 ICMM: R = COOMe; PC BM: m = 3, R = Me; NCMM: R = COOMe. 61 1 3 PC BM, Ar = 71 1 ICEM: R = CH COOMe; PC BE: m = 3, R = Et. 2 2 61 1 MOPFP: Ar = OMe 1.0 Temperature (°C) 85 0.8 –5 60 45 0.6 ‘Red-edge’ shift –10 Film 0.4 300 400 500 600 700 800 –15 0.2 Wavelength (nm) 0.0 –20 0.0 0.2 0.4 0.6 0.8 400 500 600 700 800 V (V) Wavelength (nm) Figure 1 | Chemical structures and optical and photovoltaic properties. (a,b) Chemical structures of donor polymers and fullerenes; (c) J–V curve of a PffBT4T-2OD:PC BM cell under AM1.5G illumination with an irradiation intensity of 100 mWcm (one Sun). Inset: representative EQE spectra of PSCs with a thick (300 nm) and thin (150 nm) active layer. The arrow indicates the shift of the ‘low energy edge’ of the PSCs. (d) Ultraviolet–visible (UV-Vis) absorption spectra of a PffBT4T-2OD film and a PffBT4T-2OD solution (0.02 mg ml in DCB) at temperatures as indicated. PffBT4T-2OD:fullerene blend films. Both exhibit a high Table 1 | PSC performance of 10 high-efficiency degree of molecular order, as evidenced by strong lamellar polymer:fullerene material combinations. (100), (200) and even (300) reflection peaks and, more importantly, a large (010) coherence length (GIWAXS 2D Active layer V (V) J (mA cm ) FF PCE (%) patterns shown in Fig. 2a,b and Supplementary Fig. 1). OC SC The (010) coherence length (that is, extent of ordering) of PffBT4T-2OD:TC BM 0.77 18.8 0.75 10.8 (10.3)* PffBT4T-2OD:PC BM 0.77 18.4 0.74 10.5 (10.2) PffBT4T-2OD:PC PM blend films was calculated using PffBT4T-2OD:PC PM 0.77 17.7 0.76 10.4 (10.1) 61 Scherrer analysis to be B8.5 nm, which corresponds to B24 PffBT4T-2OD:ICMA 0.78 16.4 0.77 9.8 (9.4) p-stacked copolymers. In contrast, the (010) coherence length of PffBT4T-2OD:TC PM 0.75 17.4 0.74 9.7 (9.3) PTB7:PC BM, for example, is only B2nm (ref. 16). Owing to PffBT4T-2OD:PC BM 0.77 17.1 0.73 9.6 (9.3) the high crystallinity and preferential face-on orientation of PBTff4T-2OD:PC BM 0.77 18.2 0.74 10.4 (10.0) polymer domains, relatively high SCLC hole mobility of PBTff4T-2OD:TC BM 0.76 18.7 0.68 9.7 (9.4) 2 2  1  1 1.5–3.0  10 cm V s were obtained for various PBTff4T-2OD:PC PM 0.76 18.6 0.69 9.6 (9.2) PffBT4T-2OD:fullerene blend films in a hole-only diode device PNT4T-2OD:PC BM 0.76 19.8 0.68 10.1 (9.7) configuration (Supplementary Fig. 2). The importance of FF, fill factor; PCE, power conversion efficiency; PSC, polymer solar cell. mobility for good FF was recently illustrated . *The values in parentheses stand for the average PCEs from over 20 devices for PffBT4T-2OD and from over 10 devices for PBTff4T-2OD and PNT4T-2OD. Polymer:fullerene domain size and average domain purity.In 14,15,25,28–30 addition, resonant soft X-ray scattering (R-SoXS; 2OD. All of these fullerenes form similar morphologies with Fig. 2c) and atomic force microscopy (AFM; Supplementary PffBT4T-2OD and can produce PSCs with high efficiencies in Fig. 3) analysis reveals that the various PffBT4T-2OD:fullerene the range of 8.6–10.8% (Table 1 and Supplementary Table 1). films all exhibit multi-length scale morphologies with reasonably The best efficiency (10.4%) in the C family was achieved by small median domain sizes of B30–40 nm, which is similar to PC PM (Fig. 1b), and the most commonly used C -based full- 61 60 16,25 previous cases of high-performance polymers . R-SoXS can erene, PC BM, is not the best match for PffBT4T-2OD. also reveal the average composition variations, which are indicative of the average purity of the polymer and fullerene Polymer crystallinity and hole mobility. Grazing incident wide- regions as well as a possible third phase of polymer- 26 26,31 angle X-ray diffraction (GIWAXS) reveals the molecular rich domains . An annealing sequence on PffBT4T- packing and orientational texture of pure PffBT4T-2OD and 2OD:fullerene blends revealed that the non-annealed devices NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications 3 & 2014 Macmillan Publishers Limited. All rights reserved. –2 J (mA cm ) EQE (%) Normalized absorption SiO background PC PM ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 –1 2 q (nm) 64 2 86 4 2 2.6 3.6 –6 2.0 10 3.2 2.8 2.0 (010) (010) 2.4 Log PffBT4T-2OD:PC BM –7 1.0 10 PffBT4T-2OD:ICMA 1.0 (300) (300) PC PM PffBT4T-2OD:PC PM 61 61 (200) (200) Aggregation PffBT4T-2OD:PC BM (100) (100) –8 1.0 0.0 2 46 8 2 0.0 –1.0 –2.0 0.0 –1.0 –2.0 –2 –1 10 10 –1 –1 q (Å ) q (Å ) –1 xy xy q (nm ) 1.5 1,800 5,000 r.p.m. Spinrate CL 3,000 r.p.m. 600 r.p.m. (r.p.m.) (nm) 1,000 r.p.m. high crystallinity 1,500 800 r.p.m. 700 ; 5.7 600 r.p.m. 1,000 ; 5.2 1.0 2,000 ; 3.5 1,200 3,000 ; 1.0 5,000 ; 0.9 900 5,000 r.p.m. 0.5 poor crystallinity 300 0.0 1.6 1.7 1.8 1.9 500 600 700 800 –1 q (nm ) Wavelength (nm) Figure 2 | Morphological characterization data. (a) Two-dimensional (2D) GIWAXS pattern of a pure PffBT4T-2OD film. (b) 2D GIWAXS pattern of a PffBT4T-2OD:PC PM. (c) R-SoXS profiles in log scale for four samples of PffBT4T-2OD:fullerene blends. Blue line: PC BM; red line: PC BM; 61 61 71 black line: ICMA; green line: PC PM. (d) (010) diffraction peak (obtained from XRD) of PffBT4T-2OD pure films spun at different rates, the inset indicates the (010) coherence length (CL) of the films. (e) UV-Vis absorption spectra of PffBT4T-2OD:PC PM blend films obtained with different spin rates and substrate temperatures. Blue line, 5,000 r.p.m./110 C; dark green line, 3,000 r.p.m./100 C; red line, 1,000 r.p.m./90 C; pink line, 800 r.p.m./80 C; emerald line, 600 r.p.m./70 C. All spectra are normalized based on the intensity of their 0-1 transition peak (at B640 nm) to highlight the change of the intensity of 0-0 transition peaks. presented here exhibited almost 90% average purity compared low concentration PffBT4T-2OD solution in 1,2-dichlorobenzene with the asymptotic limit (Fig. 3a,b), which corresponds to an (DCB) is lowered from 85 to 25 C (Fig. 1d). At elevated unusually low residual concentration of 3.2% fullerene averaged temperature, PffBT4T-2OD is well dissolved and disaggregated. over all PffBT4T-2OD domains in the film as measured by X-ray At progressively lower temperatures, a strong 0-0 transition peak 25,32,33 microscopy (Fig. 3c) . In general, PSCs with significantly atB700 nm emerges with significant strength at 25 C, indicating impure polymer phases exhibit detrimental bimolecular charge strong aggregation of the polymer chains in solution at that recombination when the polymer film is too thick, whereas temperature. Note that the absorption spectrum of the 25 C pure phases can help to minimize recombination . These solution of PffBT4T-2OD is almost identical to that of the opti- morphological data show that PffBT4T-2OD can form a mized PffBT4T-2OD solid film (Fig. 1d), which is observed to be polymer:fullerene morphology containing highly crystalline and highly crystalline by GIWAXS. Consequently, devices are always sufficiently pure yet reasonably small polymer domains. Note that cast from warm solutions (60–80 C) of PffBT4T-2OD, which PTB7-type polymers have been the best donor polymer in PSCs then aggregates during the cooling and film-forming process. for the past few years. By its very nature of high performance in To understand details of PffBT4T-2OD’s aggregation beha- thin films, PTB7 can form a ‘near-optimum’ PSC morphology viour during the film-forming process, the critical p–p molecular characterized by relatively low molecular ordering, relatively low ordering ((010) coherence length and intensity of the (010) peak) hole mobilities and impure polymer domains . PffBT4T-2OD is determined with X-ray diffraction (XRD) for a series of exhibits high molecular ordering (‘crystallinity’), high hole PffBT4T-2OD films spun at different rates. As shown in Fig. 2d mobilities and purer polymer domains, which appears to be a and Supplementary Table 2, the p–p ordering decreases markedly different ipso facto ‘near-optimum’ PSC morphology. Although with increasing spin rates. As PffBT4T-2OD exhibits a strong yet PTB7 enabled great thin-film PSC performance, PffBT4T-2OD progressively evolving aggregation property, the extent of offers high performance even in thick-film PSCs owing to the PffBT4T-2OD’s aggregation depends upon temperature and high mobility of the highly ordered and sufficiently pure polymer concentration changes, the film drying time and the kinetics of domains it forms. aggregation. During a slow spin process (for example, 700 r.p.m.), it takes a relatively long time for the solution and film to dry, during which the temperature of the substrate and the wet film Morphology control via temperature-dependent aggregation. We attribute PffBT4T-2OD’s excellent performance and robust also decreases significantly. When using an ultra-fast rate (for morphology to its significant temperature-dependent aggregation example, 5,000 r.p.m.), however, the solvent evaporates more behaviour that can be exploited during device fabrication. quickly, which results in kinetically quenched, poorly ordered The UV-Vis absorption spectra exhibit a marked red-shift when a films, whereas slow spin rates provides PffBT4T-2OD sufficient 4 NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. –1 q (Å ) Intensity –1 q (Å ) Absorbance 2 –2 Intensity q (nm ) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 ARTICLE from the common processing protocol of PTB7 family polymers 1.00 1.00 that are typically processed at room temperature. 0.98 0.98 0.96 0.96 Discussion 0.94 0.94 The key structural feature of PffBT4T-2OD that enables its 0.92 0.92 pronounced, yet gradual temperature-dependent aggregation 0.90 0.90 (Fig. 1d) is the second-position branched alkyl chains (2OD) 0 40 80 120 on a quaterthiophene (4T-2OD). To elucidate this aspect, we 040 80 120 Annealing time (min) Annealing time (min) contrast PffBT4T-2OD with two structurally very similar polymers. These two polymers have the same backbone but their alkyl chains are branched at the first or third side-chain carbon 1.4 atom (for ease of comparison, these two polymers are named as PffBT4T-1ON and PffBT4T-3OT; Fig. 4a). In contrast to 1.2 PffBT4T-2OD, PffBT4T-1ON is disaggregated at 85 C, and more importantly, also disaggregated at 25 C (Fig. 4b). As a 1.0 result, PffBT4T-1ON cannot aggregate easily during the film- forming process, leading to films with poor crystallinity 0.8 (GIWAXS pattern shown in Fig. 4c) and thus PSC devices of 3.2% wt 0.6 only B0.6% efficiency. PffBT4T-3OT exhibits the other extreme, showing excessive aggregation at both 25 and 85 C. During our 0.4 attempt to process PffBT4T-3OT, the PffBT4T-3OT solution quickly becomes a gel (Fig. 4d) even before the start of spin 0.2 casting. These comparisons indicate that PffBT4T-1ON’s alkyl 285 290 295 300 350 400 chains cause too much steric hindrance, which results in poor Energy (eV) aggregation and crystallinity. PffBT4T-3OT’s alkyl chains provide too little steric hindrance that makes aggregation of PffBT4T- Figure 3 | Domain purity data and analysis. (a) R-SoXS results revealing 3OT too strong even at 85 C and makes it difficult to process. relative purity of PffBT4T-2OD:ICMA and (b) PffBT4T-2OD:PC BM blends PffBT4T-2OD’s second-position branched alkyl chains offer an annealed at 130 C. (c) Following methodology developed by Collins 40 optimal tradeoff that allows for controllable aggregation of et al., residual ICMA in PffBT4T-2OD is only 3.2% after a PffBT4T- PffBT4T-2OD during the film-forming process. More discussions 2OD:ICMA blend has been annealed extensively until all excess ICMA has on the impact of alkyl chain branching positions are provided in agglomerated into macro-phase domains or crystals. Red lines/symbols are Supplementary Note 1. near edge X-ray absorption fine structure data/uncertainty and black lines Several structurally similar donor polymers containing are fits. Yellow and blue lines are PffBT4T-2OD and fullerene reference quaterthiophene substituted with second-position branched alkyl spectra used in the fit. Error bars represent the average of three different 34–37 chains were reported in the literature , including a recent spots on the sample. A value of 3.2% indicates that there are 3.2% (w.t) report in which a polymer with longer alkyl chains, FBT-Th (1,4) fullerene in the average polymer domains that includes polymer crystals (named as PffBT4T-2DT for simplicity in this paper, structure and the mixed, amorphous phase. shown in Fig. 4a), achieved 7.64% efficiency . The difference of PffBT4T-2OD and PffBT4T-2DT devices can be understood from time to aggregate and to form crystalline polymer domains with the following results. R-SoXS (Fig. 4e) and prior AFM studies large coherence lengths. Importantly, studies for PffBT4T-2OD show that the average domain size of PffBT4T-2DT:fullerene pure films and PffBT4T-2OD blend films with two different films is too large (B100 nm). The R-SoXS data furthermore show fullerenes yield similar trends (Supplementary Table 2 and that the average purity of the polymer/polymer-rich domains in Supplementary Fig. 4), demonstrating that the aggregation of an PffBT4T-2DT:fullerene film is B87% of that in PffBT4T- PffBT4T-2OD is insensitive to the presence of fullerenes. Not 2OD:fullerene film. Lower purity of average polymer/polymer- surprisingly, high substrate temperatures were found to have a rich domains can result in significant recombination for 14,23,24 similar effect to fast spin rates. PffBT4T-2OD:fullerene films thick-film PSC devices and thus lower performance . prepared with fast spin rates/high substrate temperatures show a Regarding molecular ordering, the two-dimensional GIWAXS decrease in the 0-0 transition peaks and a pronounced shift in the mapping of PffBT4T-2DT:PC BM films shows that PffBT4T- 0-0 transition energy in their UV-Vis absorption spectra, 2DT:fullerene films exhibit weak laminar packing peaks, which indicative of significant disorder (Fig. 2e). The corresponding are significantly weaker than those of PffBT4T-2OD:fullerene hole-only and PSC devices fabricated using high films. The smaller degree of laminar packing of PffBT4T-2DT is spin rates and high substrate temperature also exhibit consistent with the lower average purity of PffBT4T-2DT 3 2  1  1 markedly decreased hole mobilities (3.1  10 cm V s ; polymer domains compared with that of PffBT4T-2OD’s, as Supplementary Table 3) and PSC efficiencies (3.6%; more fullerene is expelled by the crystalline polymer domains of Supplementary Table 4). These morphological, spectroscopic PffBT4T-2OD. Lastly, the absorption coefficient of PffBT4T-2DT and electric data demonstrate that PffBT4T-2OD’s morphology is is lower than that of PffBT4T-2OD owing to longer alkyl chains mainly controlled by the progress of its aggregation during the that does not contribute to light absorption (Supplementary film-casting process until the film is dry, which locks-in the Fig. 5). These studies show that the branching position and the length scale of the morphology. PffBT4T-2OD’s strong yet well- size of the alkyl chains are critically important in obtaining the controllable aggregation property allows for convenient optimi- optimal aggregation properties of PffBT4T-2OD. Insufficient zation of processing conditions that led to a near-ideal aggregation (for example, PffBT4T-1ON) and unnecessarily polymer:fullerene morphology that is insensitive to the choices long alkyl chains (for example, PffBT4T-2DT) resulted in of fullerene. This approach of controlling the extent of polymer low crystallinity and/or impure polymer domains. Excessive aggregation during a warm solution casting process is different aggregation (for example, PffBT4T-3OT) makes the processing NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications 5 & 2014 Macmillan Publishers Limited. All rights reserved. Relative purity Absorbance (OD) Relative purity ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 C H C H 8 17 8 17 N N 1 1.0 C H C H 8 17 8 17 PffBT4T-1ON S 0.8 S S F F 0.6 C H C H 10 21 10 21 C H 2 S C H 8 17 8 17 N 0.4 PffBT4T-1ON, 85 °C S 0.2 PffBT4T-1ON, 25 °C PffBT4T-3OT S S PffBT4T-3OT, 85 °C F F n PffBT4T-3OT, 25 °C 0.0 C H C H 12 25 12 25 400 500 600 700 800 C H S C H 10 21 10 21 N N Wavelengh (nm) PffBT4T-2DT (FBT-Th [1,4]) 4 S S S –1 2 q (nm) F F 6 4 22 8 6 4 2.6 –6 2.0 –7 1.0 Domain size: Purity: PffBT4T-2OD 40 nm 100% 0.0 PffBT4T-2DT 100 nm 87% 0.0 –1.0 –2.0 –8 –1 2 2 46 8 4 q (Å ) xy –2 –1 10 10 –1 q (nm ) Figure 4 | Comparative studies on structurally similar polymers. (a) Chemical structures of PffBT4T-1ON, PffBT4T-3OT and PffBT4T-2DT. (b) Normalized UV-Vis absorption spectra of PffBT4T-1ON and PffBT4T-3OT in a 85 C DCB solution and in a DCB solution at room temperature as indicated on the plot. (c) Two-dimensional GIWAXS pattern of a pure PffBT4T-1ON film. (d) Uneven film formed by the PffBT4T-3OT solution forming a gel before being able to be spun. (e) R-SoXS profiles in log scale for PffBT4T-2OD and PffBT4T-2DT, the inset indicates the domain size and relative purity. and aggregation difficult to control. Similar to recent detail in the Supplementary Information (Supplementary Fig. 7, 32,36,38 observations , the molecular weight of PffBT4T-2OD has Table 1 and Supplementary Table 1). a significant impact on its aggregation property and performance. Although second-position branched alkyl chains are a well- Lower molecular weight (M ¼ 16.6 kDa, M ¼ 29.5 kDa) batches known structural motif and have been previously used on n w of PffBT4T-2OD exhibit weaker aggregation and thus lower quaterthiophene-based polymers, previous work did not utilize a efficiency (7.7%) than the high molecular weight (M ¼ 47.5 kDa, polymer with the most suitable alkyl chains nor were warm- M ¼ 93.7 kDa) PffBT4T-2OD batches (Supplementary Table 5 casting methods used that optimally harnessed aggregation. They and Supplementary Fig. 6). The impacts of polymer molecular thus failed to reveal the connections between chemical structure, weight on PSC performances are discussed in details in the polymer aggregation during warm processing, morphology Supplementary Note 2. formation, polymer crystallinity and consequently PSC perfor- Following the rationale described above, we synthesized two mance. Our study uncovered a new approach of aggregation and 0 00 other polymers (poly[(2,1,3-benzothiadiazol-4,7-diyl)-alt-(4 ,3 - morphology control enabled by a structural feature (2OD alkyl 000 0 0 00 00 000 000 difluoro-3,3 -di(2-octyldodecyl)-2,2 ;5 ,2 ;5 ,2 -quaterthiophen-5,5 chain) that is seemingly simple and commonly known, yet has -diyl)] (PBTff4T-2OD) and poly[(naphtho[1,2-c:5,6-c ]bis[1,2,5] surprisingly profound impact on PSC performances. The wide 000 0 0 00 00 000 thiadiazol-5,10-diyl)-alt-(3,3 -di(2-octyldodecyl)-2,2 ;5 ,2 ;5 ,2 - ranging applicability of our morphology control approach is quaterthiophen-5,5 -diyl)] (PNT4T-2OD); Fig. 1a) with supported by the three polymers and over 10 polymer:fullerene significantly different polymer backbones but with similar combinations that all yielded similar blend morphology and arrangements of 2OD alkyl chains. Both PBTff4T-2OD and high-efficiency thick-film PSCs. Furthermore, the aggregation PNT4T-2OD exhibit significant temperature-dependent aggrega- behaviour as observed by UV-Vis might serve as a useful tion behaviour that leads to processing and morphology control screening tool to identify materials that yield good devices when and thus efficiency (including 410% for thick-film PSCs; Table 1, cast from warm solutions. Supplementary Table 1 and Supplementary Fig. 7) comparable to Note that the chemical structures of the three donor polymers those achieved by PffBT4T-2OD-based PSCs. R-SoXS and AFM presented in the paper are distinctively different from previous 12,13,17 studies confirmed that the polymer domain size of these two new state-of-the-art PTB7 family of polymers . The PTB7 family polymers are similar to that of PffBT4T-2OD (30–40 nm). XRD polymers consist of an electron deficient fluorinated thieno characterization of PBTff4T-2OD:fullerene and PNT4T-2OD: [3,4-b]thiophene unit and a benzodithiophene unit with alkoxy, fullerene films also showed strong (010) p–p stacking peaks that alkylthienyl or alkylthiothienyl substitution groups. The three are similar to those observed for PffBT4T-2OD. Note that polymers in this paper consist of an electron deficient unit PNT4T-2OD also significantly outperforms its analogue polymer (either difluorobenzothiadiazole or benzothiadiazole or with 2-decyltetradecyl (2DT) alkyl chains , providing another naphthobisthiadiazole) combined with a quaterthiophene unit example that supports the critical importance of the size of with two 2OD alkyl chains sitting on the first and fourth the alkyl chains. The synthesis, characterization and device thiophenes. The difference in the chemical structures caused performance of PBTff4T-2OD and PNT4T-2OD are described in different aggregation properties, based on which different 6 NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. –1 q (Å ) 2 –2 Intensity q (nm ) Normalized absorption NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6293 ARTICLE processing protocols are used. While PTB7 family polymers Where e is the permittivity of free space, e is the dielectric constant of the 0 r polymer, m is the hole mobility, V is the voltage drop across the device and L is the do not exhibit a strong temperature-dependent aggregation thickness of the polymer. The dielectric constant e is assumed to be B3, which is a property and are often processed from room temperature typical value for conjugated polymers. solutions, the 4T-2OD based polymers are processed from warm solutions to utilize their temperature-dependent aggregation GIWAXS characterization. GIWAXS measurements were performed at beamline property so that the morphology and extent of molecular 7.3.3 at the Advanced Light Source (ALS) . Samples were prepared on Si substrates using identical blend solutions as those used in devices. The 10 keV ordering can be explicitly controlled during casting. X-ray beam was incident at a grazing angle of 0.11–0.15, which maximized the To summarize, we report that exquisite control of aggregation scattering intensity from the samples. The scattered X-rays were detected using a results in high-performance thick-film PSCs for three different Dectris Pilatus 1 M photon counting detector. donor polymers and 10 polymer:fullerene combinations, all of which yielded efficiencies higher than the previous state of the art Resonant soft X-ray scattering. R-SoXS transmission measurements were per- (9.5%). The common structural feature of the three donor 30 formed at beamline 11.0.1.2 at the ALS . Samples for R-SoXS measurements were polymers, the 2OD alkyl chains on quaterthiophene, causes a prepared on a PSS modified Si substrate under the same conditions as those used for device fabrication, and then transferred by floating in water to a 1.5  1.5 mm, temperature-dependent aggregation behaviour that allows for the 100-nm thick Si N membrane supported by a 5  5 mm, 200mm thick Si frame 3 4 processing of the polymer solutions at moderately elevated (Norcada Inc.). Two dimensional scattering patterns were collected on an temperature, and more importantly, controlled aggregation and in-vacuum CCD camera (Princeton Instrument PI-MTE). The beam size at the strong crystallization of the polymer during the film cooling and sample is B100mm by 200mm. The composition variation (or relative domain purity) over the length scales probed can be extracted by integrating scattering drying process. This results in a near-ideal polymer:fullerene profiles to yield the total scattering intensity. The purer the average domains morphology (containing highly crystalline, preferentially orien- are, the higher the total scattering intensity. Owing to a lack of absolute flux tated, yet small polymer domains) that is controlled by polymer normalization, the absolute composition cannot be obtained by only R-SoXS. aggregation during casting and thus insensitive to the choice of In order to get a sense of how pure the domains are, we annealed the PffBT4T- 2OD/fullerene blend at 130 C for different length of time, 0, 10, 20, 40 and fullerenes. The branching position and size of the branched alkyl 120 min. The unannealed sample exhibits very pure domains, that is, almost 90% chains are critically important in enabling a well-controllable of the saturated value. aggregation behaviour. Unnecessarily long alkyl chains (for example, 2DT) cause several detrimental effects including weaker AFM characterization. AFM measurements were performed by using a Scanning laminar stacking, poorer absorption properties and less pure Probe Microscope-Dimension 3100 in tapping mode. All film samples were spin polymer domains. Our structural design rationales and aggrega- casted on ITO/ZnO substrates. tion and morphology control approach offer a new route to achieve high-performance thick-film PSCs that cannot be Photoluminescence quenching measurements. Photoluminescence spectra were obtained from previous state-of-the-art material systems. Given measured on samples on ITO/ZnO substrates upon excitation of a 671-nm laser beam. The PL quenching efficiency of PffBT4T-2OD was estimated from the ratio that the field and record performance in the last few years has of the PL intensity of a PffBT4T-2OD:fullerene film sample to that of the PffBT4T- been mostly dominated by a single system (PTB7 family with 2OD control sample. (Supplementary Fig. 10) PC BM), the 10 material systems and three polymers based on a single and simple design feature presented here point to a Solar cell fabrication and testing. Pre-patterned ITO-coated glass with a sheet plethora of possible materials combinations that should further resistance of B15O per square was used as the substrate. It was cleaned by improve the performance. Our approach will allow the PSC sequential sonications in soap DI water, DI water, acetone and isopropanol for community to explore many more polymers and fullerene 15 min at each step. After ultraviolet/ozone treatment for 60 min, a ZnO electron transport layer was prepared by spin coating at 5,000 r.p.m. from a ZnO precursor materials and to optimize their combinations (energy offsets, solution (diethyl zinc). Active layer solutions (D/A ratio 1:1.2) were prepared in bandgap and so on) under a well-controlled morphological CB/DCB (1:1 volume ratio) with or without 3% of DIO (polymer concentration: landscape that would greatly accelerate the materials and process  1 9mgml ). To completely dissolve the polymer, the active layer solution should development towards improved PSCs. be stirred on a hot plate at 110 C for at least 3 h. Before spin coating, both the polymer solution and ITO substrate are preheated on a hot plate at B110 C. Active layers were spin coated from the warm polymer solution on the preheated Methods substrate in a N glovebox at 800 r.p.m. to obtain thicknesses of B300 nm. X-ray diffraction. XRD data were obtained from a PANanalytical XRD instrument (The spin casting of high-performance PSC films is described in Supplementary (model name: Empyrean) using the parallel beam mode that is recommended by the Note 4 in details. The processing of PNT4T-2OD also requires the use of a metal instrument manufacturer to characterize thin-film samples. All XRD samples were chuck as described in Supplementary Note 4. High-temperature and high spin rate spin cast on Si substrates to avoid strong scattering background of glass substrates. samples are described in Supplementary Note 5). The polymer/fullerene films were To rule out the effect of substrate properties on the crystallinity of polymer film then annealed at 80 C for 5 min before being transferred to the vacuum chamber samples, we also investigated polymer films on Si/ZnO substrates and found that the of a thermal evaporator inside the same glovebox. At a vacuum level of 3  10 polymer films have similar scattering profiles (Supplementary Fig. 8) on these two Torr, a thin layer (20 nm) of MoO or V O was deposited as the anode interlayer, 3 2 5 types of substrates (Si/ZnO and Si). The polymer crystallinity is thus rather insen- followed by deposition of 100 nm of Al as the top electrode. All cells were sitive to the surface properties of the substrates. More details of XRD characteriza- encapsulated using epoxy inside the glovebox. Device J–V characteristics was tions are provided in Supplementary Note 3. measured under AM1.5G (100 mW cm ) using a Newport solar simulator. The light intensity was calibrated using a standard Si diode (with KG5 filter, purchased from PV Measurement) to bring spectral mismatch to unity. J–V Cyclic voltammetry. Cyclic voltammetry was performed in an electrolyte solution characteristics were recorded using a Keithley 236 source meter unit. Typical cells of 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile, both working have devices area of B5.9 mm , which is defined by a metal mask with an aperture and counter electrodes were platinum electrode. Ag/AgCl electrode was used as the aligned with the device area. EQEs were characterized using a Newport EQE system reference electrode; the Fc/Fc redox couple was used as an external standard equipped with a standard Si diode. Monochromatic light was generated from a (Supplementary Fig. 9 and Supplementary Table 6). 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Donor-acceptor conjugated polymer based on naphtho[1,2- maintenance. The work described in this paper was partially supported by a grant from c:5,6-c]bis[1,2,5]thiadiazole for high-performance polymer solar cells. J. Am. the Research Grants Council of the Hong Kong Special Administrative Region, China, Chem. Soc. 133, 9638–9641 (2011). under Theme-based Research Scheme through project no. T23-407/13-N. 20. Morinaka, Y. et al. Synthesis and photovoltaic properties of acceptor materials based on the dimerization of fullerene C60 for use in efficient polymer solar Author contributions cells. Chem. Commun. 49, 3670–3672 (2013). Y.L. synthesized PffBT4T-2OD; J.Z. designed PNT4T-2OD, synthesized 5,10-Dibromo- 21. Li, N. et al. Towards 15% energy conversion efficiency: a systematic study of the naphtho[1,2-c:5,6-c’]bis[1,2,5]thiadiazole and carried out AFM measurements; solution-processed organic tandem solar cells based on commercially available Z.L. synthesized PBTff4T-2OD; J.Z., H.L., Y.L., H.H. and Z.L. synthesized fullerenes; materials. Energy Environ. Sci. 6, 3407–3413 (2013). C.M., H.H., K.J. and H.Y. fabricated and tested PSC devices; W.M. carried out GIWAXS 22. Scharber, M. C. & Sariciftci, N. S. Efficiency of bulk-heterojunction organic and R-SoXS measurements and analysis; K.J. carried out XRD analysis; H.L. synthesized solar cells. Prog. Polym. Sci. 38, 1929–1940 (2013). PNT4T-2OD; H.A. supervised GIWAXS and R-SoXS work, and helped design experi- 23. Albrecht, S. et al. Quantifying charge extraction in organic solar cells: the case mental protocols; H.Y. conceived and directed the project; H.A. and H.Y. wrote the paper of fluorinated PCPDTBT. J. Phys. Chem. 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Scherrer grain-size analysis adapted to grazing-incidence scattering with area detectors. J. Appl. Crystallogr. 42, 1030–1034 (2009). How to cite this article: Liu, Y. et al. Aggregation and morphology control enables 28. Ma, W., Ye, L., Zhang, S. Q., Hou, J. H. & Ade, H. Competition between multiple cases of high-efficiency polymer solar cells. Nat. Commun. 5:5293 morphological attributes in the thermal annealing and additive processing of doi: 10.1038/ncomms6293 (2014). polymer solar cells. J. Mater. Chem. C 1, 5023–5030 (2013). This work is licensed under a Creative Commons Attribution 4.0 29. Swaraj, S. et al. Nanomorphology of bulk heterojunction photovoltaic thin films probed with resonant soft X-ray scattering. Nano Lett. 10, 2863–2869 International License. The images or other third party material in this (2010). article are included in the article’s Creative Commons license, unless indicated otherwise 30. Gann, E. et al. Soft x-ray scattering facility at the Advanced light source in the credit line; if the material is not included under the Creative Commons license, with real-time data processing and analysis. Rev. Sci. Instrum. 83, 045110 users will need to obtain permission from the license holder to reproduce the material. (2012). To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ 8 NATURE COMMUNICATIONS | 5:5293 | DOI: 10.1038/ncomms6293 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved.

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