Boosting the Performance of Non-Fullerene Organic Solar Cells via

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Boosting the Performance of Non-Fullerene Organic Solar Cells via Cross-Linked Donor Polymers Design Fan Yang,†,‡ Wenchao Zhao,‡ Qinglian Zhu,§ Cheng Li,*,†,‡ Wei Ma,§ Jianhui Hou,‡ and Weiwei Li*,†,‡ †

State Key Laboratory of Organic−Inorganic Composites, University of Chemical Technology, Beijing 100029, P. R. China Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China § State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, P. R. China Macromolecules Downloaded from pubs.acs.org by MACQUARIE UNIV on 02/19/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: In general, cross-link is applied into conjugated polymers for improving stability of organic solar cells, but its effect on the photovoltaic performance received little attention. Particularly, cross-linked conjugated polymers with poor solubility show strong aggregation in bulk-heterojunction thin films, resulting in low charge generation efficiencies and hence poor performance in solar cells. Herein, we were able to develop a series of cross-linked conjugated polymers as electron donor for application in organic solar cells with a non-fullerene acceptor IT-4F, in which the photovoltaic performance and morphological stability in a nitrogen environment could be significantly improved. The polymers contain an electron-deficient thieno[3,4-c]pyrrole-4,6-dione (TPD) unit alternated with two-dimensional benzodithiophene, in which a four brominated-TPD monomer was applied into polymerization to obtain cross-linked polymers. All these cross-linked polymers perform identical absorption spectra, energy levels, and hole mobilities with the non-cross-linked polymer, but their photovoltaic performance was different. The cross-linked polymer containing 3% cross-linker as electron donor realized a high power conversion efficiency of 12.18% in non-fullerene organic solar cells, while the polymer donor without cross-linker only provided a low PCE of 7.56%. The greatly enhanced performance in cross-linked polymer solar cells was due to optimized nanophase separation with crystalline and small domain in blended thin films, resulting in efficient charge generation. Our results demonstrate that by rationally designing cross-linked conjugated polymers, it is possible to simultaneously obtain high performance and morphological stability in a nitrogen environment in organic solar cells, enabling their great potential application in large-area devices.

1. INTRODUCTION

the stability in these high performance NFOSCs, from new materials design and new devices configuration. Cross-link has been recognized as one of the most effective strategies to improve the stability of OSCs because cross-linked materials can “freeze” the initial morphology, prevent the phase separation between physically-blended donor and acceptor, and preserve long-term performance.32−35 For example, Hashimoto et al. have demonstrated that cross-linked regioregular poly(3-(5-hexenyl)thiophene) (P3HNT) could stabilize the film morphology in polymer photovoltaic cells.36 Meanwhile, some other research focused on the cross-linked electron acceptors, in which the heat-promoted aggregation in the acceptors can be dramatically inhibited and lead to obviously enhanced thermal stability.37−41 Despite these fruitful advances in cross-link engineering, the performance

Since the donor−acceptor (D:A) bulk-heterojunction (BHJ) concept was applied into organic solar cells (OSCs),1,2 tremendous progress has been made in developing both novel materials and photovoltaic devices.3−6 Initially, fullerene derivatives are the dominant electron acceptors in OSCs,7−10 but their narrow absorption bands and deep energy levels are difficult to be modified, resulting in the limited application in OSCs. Recently, non-fullerene electron acceptors with tunable chemical structures have made rapid progress,11−20 in which the power conversion efficiencies (PCEs) have exceeded 14% in single-junction cells and 17% in tandem cells.21−24 The PCEs in non-fullerene organic solar cells (NFOSCs) are much better than those of fullerene-based solar cells,24 also indicating their great potential application in large-area devices.25−27 Efficiency, stability, and cost are the three key issues for OSCs’ industry application, in which it is still a big challenge to simultaneously realize high efficiency and excellent stability in one cell.20,28−31 Therefore, it is an important task to improve © XXXX American Chemical Society

Received: November 26, 2018 Revised: January 16, 2019

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DOI: 10.1021/acs.macromol.8b02526 Macromolecules XXXX, XXX, XXX−XXX

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resulting in high PCEs up to 12.18% in NFOSCs, which is much higher than its non-cross-linked donor polymer with a low PCE of 7.56%. We also provided detailed study about nanophase separation in these systems to reveal the origination of improved PCEs in these cross-linked polymers.

of these cross-linked materials is still far below expectations (Figure 1). It remains a great challenge to realize high performance solar cells based on the cross-linked polymers and meanwhile maintain good stability.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization. The synthetic procedures of the monomers and the polymers are shown in Figure 2. The compounds 1,3-dibromo-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (1)53 and 1,3-dibromo-5-octyl-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione (M2)54 were synthesized according to the literature. The monomer (4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-2,6diyl)bis(trimethylstannane) (M3) was purchased from Solarmer Materials Inc. The compounds 1,3-dibromo-5-(8bromooctyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (2) and 5,5′-(ethane-1,2-diyl)bis(1,3-dibromo-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione) (M1) were synthesized similar to our previous procedures.55 The key monomer M1 containing four bromine atoms was used as the cross-link agent for polymerization. The polymers were prepared via Stille polymerization by using the catalyst Pd2(dba)3/P(o-tolyl)3 in toluene and purified via extraction by acetone, n-hexane, and dichloromethane in turns. In this work, we change the content of M1 from 0.01 equiv to 0.03 and 0.05 equiv during polymerization, resulting in three cross-linked polymers PTPDBDT1−5. PTPDBDT, PTPDBDT1, and PTPDBDT3 can be totally extracted by dichloromethane, and the yields are >85%. However, PTPDBDT5 with high cross-linked content failed to be exacted by dichloromethane, so chlorobenzene (CB) was used to dissolve this polymer but with a low yield of 32%. The low yield was due to the insoluble part of PTPDBDT5 with a high content of cross-linked agent. The cross-linked polymers show similar number-average molecular weight (Mn) of 12.8−14.8 kDa with their parent polymer PTPDBDT, as determined by high temperature gelpermeation chromography measurement (GPC, Figures S1 and S2 and Table S1). But their weight-average molecular weight (Mw) (from 28.3 to 62.7 kDa) and polydispersity (PDI,

Figure 1. PCEs vs FF for published cross-linked conjugated polymer solar cells. The number in the circles indicates the cited references.34,35,38−51 The cross-linked acceptors are fullerene-based acceptors.38,40,41,50,51 The red-stars show the results in this work.

To maximize the photovoltaic performance, three mutually restrictive parameters, open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF), need simultaneous optimization.25 In our previous work, we reported an effective approach to obtain a high Voc with a small energy loss of 0.54 eV in the donor polymer PTPDBDT (Figure 2b) based solar cell due to the less nonradiative recombination.52 However, the Jsc and FF in this cell are relatively low due to the unfavorable microphase separation. Inspired by this, herein, we report our study of the cross-linked PTPDBDTs, in which a crosslinker M1 with different contents is incorporated into the polymers (Figure 2a). We propose that the cross-linkers can increase the steric hindrance between polymer backbones without destroying the topology of assemble structures. Thus, the crystal size and phase separation can be flexibly controlled, and charge recombination can be significantly reduced. Therefore, we found that cross-linked conjugated polymers as donor show simultaneously enhanced Voc, Jsc, and FF,

Figure 2. (a) Chemical structures of the cross-linked conjugated polymers and their synthetic routes, (b) its analogy polymer PTPDBDT, and (c) the non-fullerene acceptor IT-4F used in this work. Reagents and conditions: (i) K2CO3, DMF, rt; (ii) K2CO3, DMF, 80 °C; (iii) Stille polymerization using Pd2(dba)3/P(o-tolyl)3, toluene, 115 °C. B

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Macromolecules from 2.28 to 5.03) were gradually increased following the enhancing cross-linked agent. Because we use the same polymerization condition, we propose that the repeating units in the four polymers are similar, but the cross-linked agent can enhance the physical size of the polymers, resulting in high molecular weight. The influence of cross-linker on the thermal stability was studied by thermogravimetric analysis (TGA, Figure S3). PTPDBDT without cross-linker has a decomposition temperature (5% weight loss) of 415 °C, and the decomposition temperatures of the three cross-linked polymers with 1%, 3%, and 5% linker are enhanced to 444, 438, and 442 °C, indicating that the introduction of cross-linker can improve the thermal stability of these polymers. 2.2. Absorption and Energy Levels. The absorption spectra of these polymers in chloroform are almost the same as in films with a bandgap of 1.85 eV (Figure 3 and Figure S4). As

Figure 4. (a) J−V characteristics under simulated AM1.5G conditions of optimized solar cells of PTPDBDTswith IT-4F. (b) EQE spectra of the corresponding devices.

that the Vocs are also slightly improved to 0.90, 0.90, and 0.91 V. As a result, the PCEs can increase to 11.57%, 12.18%, and 11.79% with EQEmax exceeding 0.75 in these cross-linked polymer solar cells. The stability of the optimized PTPDBDT3 cells in a nitrogen atmosphere was also investigated, which showed no significant reduction of performance in 200 h (Figure S6). Such remarkable improvement of photovoltaic properties via cross-link was very rare in previous reports. We further perform systematical studies about the morphology of photoactive layers. 2.4. Charge Transport Properties. To probe the charge transport process of these cross-linked systems, space charge limit current (SCLC) measurements were performed under the optimized conditions with devices structure of ITO/ PEDOT:PSS/PPTPDBDTs:IT-4F/Au for hole mobility and ITO/ZnO/PPTPDBDTs:IT-4F/Ca/Al for electron mobility. The hole and electron mobilities are presented in Table 2 and Figure S7. The hole mobilities of the cross-linked polymers are slightly higher than that of PTPDBDT, and the electron mobilities are almost the same as that of PTPDBDT, which proves both polymers and IT-4F in the cross-linked systems form an excellent bicontinuous and interpenetrating network. In addition, the hole and electron mobilities are of the same order of magnitude, resulting in balanced mobilities that are beneficial for the charge transport to electrodes. 2.5. Crystallinity Properties and Morphology. The crystallinity properties were studied by grazing-incidence wideangle X-ray scattering (GIWAXS).57 The diffraction patterns and the corresponding crystallographic parameters are shown in Figure 5 and Table 3. The four pure polymers show preferential face-on packing modes with alkyl stacking (100) peaks at a value of d ≈ 2.36 nm in the in-plane directions and (010) signals at d ≈ 0.38 nm corresponding to the π−π stacking in the out-of-plane directions. Interestingly, the coherence length of the pure cross-linked polymers is greatly reduced with the increase of cross-linker content, while the dspacings of (100) and (010) peaks in both pure polymers and blended thin films remain almost the same as those of PTPDBDT (Table 3). These values indicate that the introduction of cross-linker could effectively adjust the crystal size without obviously changing the crystal lattice parameters, resulting in small domain size (≈20 nm) in these crystalline polymers. Therefore, efficient exciton diffusion and charge transport can be simultaneously realized toward high performance solar cells. Resonant soft X-ray scattering (R-SoXS) was also used to investigate the phase separation of the blend films (Figure 6 and Table 3).58,59 Both the statistics average domain size and domain purity gradually decrease with the increase of crosslinker content. These results agreed quite well with the

Figure 3. (a) Optical absorption spectra of PTPDBDT, PTPDBDT1, PTPDBDT3, and PTPDBDT5 in chloroform and (b) in thin films.

shown in Figure S4, the intensity of the absorption peaks at 606 nm of the cross-linked polymers in solutions is slightly lower than that of PTPDBDT, which proves that the aggregation of these polymers could be inhibited by the introduction of cross-linker. A similar effect can also be observed in thin films (Figure 3b). The frontier energy levels of the four polymers were determined by cyclic voltammetry (CV) measurement. As shown in Figure S5, the four polymers show identical highest occupied molecular orbital (HOMO) levels with the same oxidation potentials. From these results, we can conclude that the cross-linkers in this work have negligible effect on their absorption spectra and energy levels, enabling us to focus on their influence on nanophase separation in BHJ thin films. 2.3. Photovoltaic Properties. The photovoltaic properties of the four polymers blended with an electron acceptor IT4F56 (Figure 2c) were studied with an inverted device configuration of ITO/ZnO/active layer/MoOx/Al. The active layers were spin-coated from chloroform solutions with 1,8diiodooctane (DIO) as additive. The solution processing parameters, including the solvent, the amount of additive, and the ratio of donor to acceptor (Tables S2−S6), were carefully optimized. The optimized performances of these cells are shown in Figure 4 and Table 1. The PTPDBDT:IT-4F system shows a relatively low PCE of 7.56% with a Jsc of 13.37 mA/ cm2, a Voc of 0.88 V, and a FF of 0.64 with the maximum EQE (EQEmax) of 0.57 (Figure 4b). Interestingly, the photovoltaic parameters of cross-linked polymers have gained significant improvement. The Jscs of PTPDBDT1, PTPDBDT3, and PTPDBDT5 based cells are enhanced to 18.07, 18.37, and 18.50 mA/cm2 individually. Meanwhile, the FFs remarkably rise to 0.71, 0.73, and 0.70. Moreover, it is worth mentioning C

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Macromolecules Table 1. Characteristics of Optimized Solar Cells in CF:0.5% DIO with a D:A Weight Ratio of 1:1 donor PTPDBDT PTPDBDT1 PTPDBDT3 PTPDBDT5

Jsc(mA/cm2) 13.37 18.07 18.37 18.50

(13.47 (18.01 (18.27 (18.52

± ± ± ±

Voc (V)

0.34) 0.25) 0.17) 0.23)

0.88 0.90 0.90 0.91

(0.86 (0.90 (0.90 (0.90

± ± ± ±

PCEa (%)

FF

0.012) 0.004) 0.006) 0.007)

0.64 0.71 0.73 0.70

(0.61 (0.70 (0.73 (0.70

± ± ± ±

0.03) 0.02) 0.008) 0.009)

7.56 11.57 12.18 11.79

(7.06 ± 0.41) (11.37 ± 0.19) (11.93 ± 0.17) (11.60 ± 0.12)

a

Statistics in parentheses from six independent cells.

We further use atomic force microscopy (AFM) to study the morphology in these blended thin films, as shown in Figure 7. PTPDBDT shows large-sized domain in blend films, which also confirmed by the R-SoXS scattering profiles (Figure 6). The surface roughnesses are reduced with the increase of cross-linker content. Phase images of PTPDBDT3 and PTPDBDT5 show fibril-like structures, which would be beneficial for charge transport. AFM results clearly show that when adding cross-linker into conjugated polymer, the domain size of polymers in blended thin films can be reduced and meanwhile the crystallinity can be improved, which is consistent with GIWAXS and R-SoXS measurement.

Table 2. Hole and Electron Mobilities of Optimized Solar Cells of PTPDBDTs:IT-4F from SCLC Measurements active layer PTPDBDT:IT-4F PTPDBDT1:IT-4F PTPDBDT3:IT-4F PTPDBDT5:IT-4F

μh (cm2 V−1 s−1) 4.55 6.96 7.34 6.16

× × × ×

−4

10 10−4 10−4 10−4

μe (cm2 V−1 s−1) 2.66 1.04 1.15 1.12

× × × ×

10−4 10−4 10−4 10−4

photovoltaic properties. PTPDBDT with the largest scale of phase separation displays low Jsc, FF, and PCE, while the moderate domain size and domain purity of the cross-linker systems contribute to overall enhancement in PCEs.

Figure 5. (a−h) GIWAXS images of the pure polymers and blended thin films. (i) Out-of-plane and in-plane cuts of the corresponding GIWAXS patterns. (a) PTPDBDT, (b) PTPDBDT1, (c) PTPDBDT3,(d) PTPDBDT5, (e) PTPDBDT:IT-4F, (f) PTPDBDT1:IT-4F, (g) PTPDBDT3:IT4F, and (h) PTPDBDT5:IT-4F. D

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and crystal lattice parameters of these polymers as demonstrated in absorption, CV, GIWAXS, and SCLC measurements. However, the crystal size and phase separation of the blend films could be effectively adjusted in these cross-linked polymer based BHJ films. Fine-tuning the content of crosslinker could provide an ideal phase separation and then ensure the efficient exciton diffusion and charge transport at the same time. Solar cells based on PTPDBDT3:IT-4F exhibit a decent PCE of 12.18%, which is much higher than PTPDBDT based cells with a PCE of 7.56%. In addition, the devices also showed good morphological stability in a nitrogen environment. These results highlight that by rationally designing conjugated polymers containing cross-linked agent, the performance of NFOSCs can be significantly enhanced.

Table 3. Crystallographic Parameters of the Pure Polymer and Blend Thin Filmsa IP (100) dspacing (nm) PTPDBDT PTPDBDT1 PTPDBDT3 PTPDBDT5 PTPDBDT:IT-4F PTPDBDT1:IT-4F PTPDBDT3:IT-4F PTPDBDT5:IT-4F

2.34 2.36 2.36 2.39 2.41 2.41 2.38 2.32

CLb (nm) 9.6 8.4 6.9 6.4

OOP (010) dspacing (nm)

domain size (nm)

domain purity

0.38 0.38 0.38 0.38 0.37 0.37 0.37 0.37

38 35 22 18

1 0.87 0.81 0.74

a

OOP: out-of-plane; IP: in-plane; CL: coherence length. bCL = 2πk/ fwhm, where k is a shape factor (here uses 0.9).

4. EXPERIMENTAL SECTION 4.1. Measurements. The detailed information about measurement, characterization, and device fabrication is present in the Supporting Information. 4.2. Synthesis. 2. To a solution of 1 (128 mg, 0.4 mmol) and 1,8dibromooctane (336 mg, 1.2 mmol) in anhydrous N,N-dimethylformamide (20 mL) was added potassium carbonate (114 mg, 0.8 mmol), and the mixture was stirred at 0 °C for 4 h and at room temperature for 8 h, after which it was poured into 100 mL of water and extracted with ethyl acetate. Then the organic phased was dried by anhydrous sodium sulfate and evaporated, and the crude product was purified by silica gel chromatography (dichloromethane:petroleum ether, v/v = 1:2 as eluent) to obtain 2 as a white solid. 1H NMR (CDCl3, 300 MHz): δ (ppm) 3.57−3.61 (t, 2H), 3.37−3.42 (t, 2H), 1.82−1.86 (m, 2H), 1.58−1.65 (m, 2H), 1.41 (s, 2H), 1.32−1.37 (m, 6H). 13C NMR (300 MHz, CDCl3): δ (ppm) 160.5, 134.9, 113.1, 38.9, 34.1, 32.9, 29.0, 28.7, 28.3, 28.2, 26.8. EI: 503 (calcd for C14H16Br3NO2S: 502.8). M1. To a solution of 1 (81 mg, 0.3 mmol) and 2 (93 mg, 0.2 mmol) in anhydrous N,N-dimethylformamide (20 mL) was added potassium carbonate (38 mg, 0.3 mmol), and the mixture was stirred at 0 °C for 4 h and at room temperature for 8 h, after which it was poured into 100 mL of water and extracted with ethyl acetate. Then the organic phased was dried by anhydrous sodium sulfate and evaporated, and the crude product was purified by silica gel chromatography (dichloromethane:petroleum ether, v/v = 1:1 as eluent) to obtain M1 as a white solid. 1H NMR (CDCl3, 300 MHz): δ (ppm) 3.56−3.60 (t, 4H), 1.61 (s, 4H), 1.30 (s, 8H). 13C NMR (300 MHz, CDCl3): δ (ppm) 160.5, 134.9, 113.1, 38.9, 29.1, 28.3, 26.8. EI: 731 (calcd for C20H16Br4N2O4S2: 731.7). PTPDBDT. The polymer PTPDBDT was prepared according to the literature.52 GPC (o-DCB, 140 °C): Mn = 12.5 kDa, Mw = 28.3 kDa, and PDI = 2.28.

Figure 6. R-SoXS scattering profiles at 285.8 eV of the polymers blended with IT-4F.

From GIWAXS, GISAXS, and AFM, we can conclude that when introducing cross-linker into the polymer PTPDBDT, the size of polymer domain in blended thin films can be reduced, and additionally the well-ordered fibrillar structures can also be generated. These changes can improve the efficiency of both exciton diffusion and charge transport. Therefore, the PCEs based on these cross-linked polymers desire significant enhancement.

3. CONCLUSION We successfully demonstrate an effective method to significantly enhance the performance of NFOSCs via the cross-link strategy. The introduction of cross-linker has negligible effect on the optical properties, electrical properties, hole mobilities,

Figure 7. AFM height and phase images (3 × 3 μm2) of the photoactive layers: (a, e) PTPDBDT:IT-4F, (b, f) PTPDBDT1:IT-4F, (c, g) PTPDBDT3:IT-4F, and (d, h) PTPDBDT5:IT-4F. The root-mean-square (rms) roughness is also included. E

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Macromolecules PTPDBDT1. To a degassed solution of monomer M1 (0.97 mg, 1.3 μmol), M2 (54.79 mg, 129.5 μmol), and M3 (119.51 mg, 132.1 μmol) in toluene (2.5 mL) were added Pd2(dba)3 (3.63 mg, 4.0 μmol) and tri(o-tolyl)phosphine (9.65 mg, 31.7 μmol) . The mixture was stirred at 115 °C for 36 h in a nitrogen atmosphere, after which it was precipitated in methanol and filtered through a Soxhlet thimble. The polymer was extracted with acetone, hexane, and dichloromethane. Then the solvent was evaporated, and the polymer was precipitated in acetone. The polymer was collected by filtering over a 0.45 μm PTFE membrane filter and dried in a vacuum oven to yield PTPDPBI1 (100.00 mg, 90.4%) as a dark solid. GPC (o-DCB, 140 °C): Mn = 14.0 kDa, Mw = 38.8 kDa, and PDI = 2.78. PTPDBDT3. The same procedure as for PTPDBDT1 was used, but now monomer M1 (3.28 mg, 4.5 μmol), M2 (59.33 mg, 140.2 μmol), and M3 (134.92 mg, 149.2 μmol), Pd2(dba)3 (4.10 mg, 4.5 μmol), and tri(o-tolyl)phosphine (10.90 mg, 35.8 μmol) were added. Yield: 105.00 mg, 85.2%. GPC (o-DCB, 140 °C): Mn = 14.8 kDa, Mw = 51.4 kDa, and PDI = 3.46. PTPDBDT5. The same procedure as for PTPDBDT1 was used, but now monomer M1 (3.37 mg, 4.6 μmol), M2 (35.08 mg, 82.9 μmol), and M3 (83.32 mg, 92.1 μmol), Pd2(dba)3 (2.53 mg, 2.8 μmol), and tri(o-tolyl)phosphine (6.73 mg, 22.4 μmol) were added. And after being extracted by hexane, the polymer was dissolved in chlorobenzene (110 °C), and the polymer cannot be dissolved completely. The solution was filtered to remove the insoluble part. Yield: 24.00 mg, 32.0%. GPC (o-DCB, 140 °C): Mn = 12.5 kDa, Mw = 62.7 kDa, and PDI = 5.03

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Contract No. DE-AC02-05CH11231. The authors thank Chenhui Zhu at beamline 7.3.3, and Cheng Wang at beamline 11.0.1.2 for assistance with data acquisition.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02526. Materials and measurements, GPC, TGA, and CV of the polymers and solar cells (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Cheng Li: 0000-0001-9377-9049 Wei Ma: 0000-0002-7239-2010 Jianhui Hou: 0000-0002-2105-6922 Weiwei Li: 0000-0002-7329-4236 Author Contributions

F.Y. and W.Z. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Jianqi Zhang from National Center for Nanoscience and Technology, CAS, for GIWAXS measurement, Dr. Wei Wang and Prof. Junfeng Xiang from Institute of Chemistry, Chinese Academy of Sciences, for NMR measurement. This study is jointly supported by MOST (2018YFA0208504, 2017YFA0204702) and NSFC (51773207, 21574138, 51603209, and 91633301, 21875182 and 21534003) of China. This work was further supported by the Strategic Priority Research Program (XDB12030200) of the Chinese Academy of Sciences. X-ray data was acquired at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under F

DOI: 10.1021/acs.macromol.8b02526 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02526 Macromolecules XXXX, XXX, XXX−XXX