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Effect of Side-Chain Engineering of Bithienylbenzodithiophene-altfluorobenzotriazole-Based Copolymers on the Thermal Stability and Photovoltaic Performance of Polymer Solar Cells He Huang,†,‡ Haijun Bin,*,‡ Zhengxing Peng,§ Beibei Qiu,†,‡ Chenkai Sun,†,‡ Alex Liebman-Pelaez,∥ Zhi-Guo Zhang,‡ Chenhui Zhu,∥ Harald Ade,*,§ Zhanjun Zhang,*,† and Yongfang Li*,†,‡,⊥ †

School of Chemical Science, University of Chinese Academy of Sciences, Beijing 100049, China CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § Department of Physics and Organic and Carbon Electronics Lab (ORaCEL), North Carolina State University, Raleigh, North Carolina 27695, United States ∥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China

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‡

S Supporting Information *

ABSTRACT: Side-chain engineering of conjugated polymer donor materials is an important way for improving photovoltaic performances of polymer solar cells (PSCs). On the basis of the polymer J61 synthesized in our group, here, we design and synthesize three new 2Dconjugated polymers J62, J63, and J64 with different types of side chains to further investigate the effect of side chain on their physicochemical and photovoltaic properties. With the narrow bandgap n-type organic semiconductor (n-OS) ITIC as acceptor, the optimized PSCs based on polymer donor of J62 with linear octyl, J63 with linear unsaturated hexylene, and J64 with cyclohexane side chains display power conversion efficiency (PCE) of 10.81%, 8.13%, and 8.59%, respectively. After thermal treatment at 200 °C for 2 h on the active layer,the PCE of the PSC based on J63 still keeps 92% of the original value, which verifies that the crosslinking of the polymer can improve the thermal stability of PSCs. Morphological studies show that the active layer based on J63 displays strong lamellar packing with RMS 1.26, and the active layer based on J64 shows little phase separation with RMS 0.65. The RMS of the active layer based on J62 is 0.900, and the size of phase separation is between that of J63 and J64, which indicates the excessive high lamellar packing or low phase separation is harmful to the performance of PSCs. These results indicate that the side-chain engineering is an effective way to adjust the aggregation of polymers and the morphology of blend films, which are key factors to influence the performance of PSCs.

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INTRODUCTION Over the past decades, polymer solar cells (PSCs) based on the blended active layer of p-type conjugated polymers as the donor and fullerene derivatives or n-type organic semiconductors (n-OS) as the acceptor have attracted extensive studies due to their unique advantages, including mechanical flexibility, light weight, and low-cost fabrications.1−10 To improve the photovoltaic performance of PSCs, a variety of polymer donors with different backbone structures have been designed and synthesized.11−16 In general, the backbone structures have a big influence on the optical and electrical properties of polymers and thus influence the photovoltaic performance of the PSCs with the polymers as the donor.17−20 Importantly, as the efficiency of PSCs has now improved to over 14% in single-junction devices,21 thermal and operational © XXXX American Chemical Society

stability is becoming an increasingly important aspect of PSC technology. Side-chain engineering is another effective way to tune the energy levels of the polymers and the morphology of their blend active layers for improving their photovoltaic performance.22−34 For example, our group developed a series of 2D conjugated D−A copolymers based on bithienylbenzodithiophene (BDTT)-alt-fluorobenzotriazole (FBTA).22−26 Through changing the side chain of the thienyl substituents with branched alkyl (J52), branched alkylthio (J60), and linear alkylthio (J61), the power conversion Received: May 15, 2018 Revised: July 20, 2018

A

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Figure 1. (a) Chemical structures of the polymers and ITIC. (b) Absorption spectra of the polymers and ITIC films. (c) Diagram of the HOMO and LUMO energy levels of the polymer donors and ITIC acceptor.

efficiency (PCE) of the PSCs with the polymer as donor and a narrow bandgap n-OS ITIC as acceptor was improved from 5.51% for J52 to 8.91% for J60 and 10.57% for J61.5,22 To further investigate the effect of side-chain engineering of the conjugated polymer donor on their photovoltaic properties and thermal stability, herein, we synthesized three new 2Dconjugated BDTT-alt-FBTA-based D−A copolymers J62, J63, and J64 with different types of alkylthio side chains on the thienyl substituents of the polymers. We synthesize J62 by changing the linear alkyl chain length of J61 from 12 carbons to 8 carbons for studying the impact of the linear alkyl sidechain length on their self-assembly and photovoltaic performance, J63 with linear unsaturated hexylene side-chain27 that can cross-link at a higher temperature for improving the thermal stability of PSCs, and J64 with saturated cyclohexane side chain to investigate the effect of side-chain steric hindrance28

on morphology of the blend active layer of the PSCs. Among the three polymers, the PSCs based on J62 as the donor and ITIC as the acceptor exhibits a maximum PCE of 10.81% with an open-circuit voltage (Voc) of 0.915 V, a short-circuit current density (Jsc) of 17.43 mA cm−2, and a high fill factor (FF) of 70.09%, which is slightly better than that of the PSC based on J61/ITIC. The J63-based PSCs show the best thermal stability with 92% of its initial PCE retained after thermal treatment at 200 °C for 2 h while the J64-based PSCs show a lower Jsc and poorer photovoltaic performance due to the poor morphology of the J64-based blend active layer because of the steric hindrance of its cyclohexane side chain. These results indicate that side-chain engineering is an effective way to improve the photovoltaic performance and, more importantly, the thermal stability of the PSCs. B

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polymer films. The absorption spectra of the three polymer films all show a maximum absorption at 548 nm and a shoulder peak at 596 nm, which indicates the existence of strong π−π stacking interactions and the ordered aggregations in the polymer films. In comparison with the polymer solutions, the absorption spectra of J62, J63, and J64 films are red-shifted by 54, 46, and 54 nm, respectively. The absorption spectrum of the n-OS acceptor ITIC film is also shown in Figure 1b for comparison. It can be seen that the three polymer donors possess complementary absorption with the acceptor ITIC in the wavelength range of 350−800 nm, which is also confirmed from the absorption spectra of the blend films of the polymers and ITIC as shown in Figure S2b. Hole mobilities of the three polymer films were measured by the space-charge-limited current (SCLC) method,29 and the experimental results are shown in Figure S3. The calculated hole mobility values are 3.46 × 10−4 cm2 V−1 s−1 for J62, 2.69 × 10−4 cm2 V−1 s−1 for J63, and 1.86 × 10−4 cm2 V−1 s−1 for J64 (see Table S2). The higher hole mobilities of J62 and J63 could be ascribed to the ordered aggregation and stronger π−π interactions of the two polymers with the linear alkylthio side chains. Electronic energy levels of the conjugated polymers are also key parameters for their application as the donor in PSCs. We measured the HOMO energy level (EHOMO) and LUMO energy level (ELUMO) of the polymers by cyclic voltammetry (CV) with Ag/AgCl as the reference electrode. Figure S4 shows the cyclic voltammograms of the polymers. We calculated the HOMO/LUMO energy levels (EHOMO/LUMO) of the polymers from their onset oxidation/reduction potentials according to the equation EHOMO/LUMO = −e(Eox/red + 4.34) eV. (The redox potential of ferrocene/ferrocenium (Fc/Fc+) couple is 0.46 V vs Ag/AgCl (see Figure S4), and we take the energy level of Fc/Fc+ as 4.8 eV under vacuum.) The calculated EHOMO/LUMO levels of J62, J63, and J64 are −5.31/− 3.08, −5.28/−3.04, and −5.33/−3.09 eV, respectively. As

RESULTS AND DISCUSSION Materials Synthesis and Optoelectronic Properties. The synthetic routes of the three D−A copolymers J62, J63, and J64 are depicted in Scheme 1, and their chemical structures are shown in Figure 1a. The detailed synthetic procedures are described in the Supporting Information. The polymers were synthesized with high yields, and the numberaverage molecular weight (Mn) of J62, J63, and J64 is 8.5, 9.9, and 11.1 kDa with corresponding polydispersity index (PDI) of 1.69, 2.33, and 2.34, respectively, as listed in Table 1. Table 1. Molecular Weights and Thermal and Physicochemical Properties of the Copolymers Mn polymers (g mol−1) J62 J63 J64

8.5K 9.9K 11.1K

PDI (Mw/Mn)

Td (°C)

HOMO (eV)

LUMO (eV)

Egopt (eV)

1.69 2.33 2.34

376 369 371

−5.31 −5.28 −5.33

−3.08 −3.04 −3.09

1.92 1.92 1.92

Thermogravimetric analysis (TGA) plots of the polymers J62, J63, and J64 are shown in Figure S1 of the Supporting Information, and their thermal decomposition temperatures (Td) at 5% weight loss are 376, 369, and 371 °C, respectively (see Table 1), which is high enough for their application in PSCs. The ultraviolet−visible−near-infrared (UV−vis−NIR) absorption spectra of J62, J63, and J64 solutions are shown in Figure S2a. The absorption maximum of the polymer solutions is at 542 nm for J62, 550 nm for J63, and 542 nm for J64. The difference of the absorption peaks should result from the different alkylthio substituent of the three polymers. Figure 1b shows the absorption spectra of J62, J63, and J64 films. It can be seen that the three polymer films have nearly the same absorption spectra, which indicates that the different side chains have no big influence on the optical aggregation of the

Table 2. Photovoltaic Performance Parameters of the PSCs Based on Polymer Donor:ITIC (1.5:1, w/w) with Different Thermal Annealing Conditions under the Illumination of AM1.5G, 100 mW/cm2 device J62/ITIC

TA as cast 160 °C/5 min 200 °C/5 min

J63/ITIC

as cast 160 °C/5 min 200 °C/5 min 200 °C/2 h

J64/ITIC

as cast 160 °C/5 min 200 °C/5 min

Voc (V) 0.904 (0.905 0.915 (0.914 0.905 (0.903 0.882 (0.884 0.868 (0.867 0.861 (0.861 0.849 (0.848 0.908 (0.908 0.889 (0.888 0.856 (0.856

± 0.003)a ± 0.004) ± 0.003) ± 0.003) ± 0.002) ± 0.003) ± 0.002) ± 0.002) ± 0.003) ± 0.002)

Jsc (mA cm−2) 16.12 (15.88 16.85 (16.57 12.98 (12.65 15.72 (15.49 15.72 (15.34 15.46 (15.19 15.33 (15.20 15.22 (14.98 15.40 (15.12 13.84 (13.55

Jcal (mA cm−2) 15.51

± 0.42) 16.12 ± 0.39) 12.80 ± 0.47) 15.21 ± 0.38) 15.36 ± 0.46) 15.04 ± 0.43) 15.22 ± 0.34) 14.94 ± 0.44) 15.16 ± 0.36) 13.74 ± 0.48)

FF (%) 62.36 (61.86 70.09 (69.45 65.62 (65.38 53.11 (52.09 59.58 (58.39 59.76 (59.34 57.23 (56.38 54.32 (53.55 62.71 (61.05 58.77 (58.24

PCE (%)

± 1.52) ± 2.08) ± 0.97) ± 1.28) ± 1.74) ± 1.07) ± 1.06) ± 1.49) ± 1.66) ± 0.86)

9.09 (8.87 ± 0.19) 10.81 (10.68 ± 0.11) 7.71 (7.63 ± 0.08) 7.38 (7.24 ± 0.13) 8.13 (8.02 ± 0.09) 7.96 (7.73 ± 0.21) 7.45 (7.29 ± 0.14) 7.51 (7.23 ± 0.19) 8.59 (8.36 ± 0.18) 6.97 (6.58 ± 0.32)

a

Average values with standard deviation are obtained from 20 devices. C

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Figure 2. (a) J−V characteristics of the optimized PSCs based on the polymer donor:ITIC (1.5:1, w/w) with thermal annealing at 160 °C for 5 min, under the illumination of AM1.5G, 100 mW/cm2. (b) EQE spectra of the corresponding devices. (c) Light intensity dependence of Jsc. (d) Light intensity dependence of Voc of the corresponding devices.

annealing treatment of 100 °C/10 min),22 the change of the optimized D/A weight ratio and thermal treatment condition may come from the decreased Mn and the changed crystallinity of the polymers. The characteristic current density−voltage (J−V) curves of the optimized PSCs are presented in Figure 2a, and the detailed photovoltaic performance data are listed in Table 2 for a clear comparison. For the optimized PSC based on polymer J62 with linear octyl side chain, a PCE of 10.81% with a Voc of 0.915 V, a Jsc of 16.85 mA cm−2, and a FF of 70.09% is obtained. Compared with the polymer J61 with 12 carbon linear alkyl chain in our earlier report,5,22 the improved PCE mainly comes from the larger FF, which could be ascribed to the better molecular selfassembly and higher hole mobility of J62 because of the shorter alkyl side chain. The PSC based on J63 with linear hexylene side chain delivers a PCE of 8.13%, with a Voc of 0.868 V, Jsc of 15.72 mA cm−2, and FF of 59.58%. For the saturated cyclohexane-substituted J64-based device, a PCE 8.59% with a Voc of 0.889 V, a Jsc of 15.40 mA cm−2, and FF of 62.71% is obtained. The results indicate that the change of side chain does make a big influence on the photovoltaic performance of the polymer donor materials, and the polymer with linear alkyl side chain shows better photovoltaic performance than that of the other two polymers with unsaturated or cyclohexane side chains. Compared with photovoltaic performance of the devices without thermal annealing, the increased PCE of the devices based on J63/ITIC and J64/ITIC after thermal annealing mainly comes from the increased FF (see Figure S5 and Table 2). After thermal annealing, their FF values are increased from

shown in Figure 1c, the EHOMO/LUMO levels of J62 are almost the same with that of J6122 because of their similar molecular structure. The slightly upshifted EHOMO/LUMO levels of J63 could be ascribed to the stronger molecular stacking17 of the polymer with hexylene side chains, which could upshift the energy level. As for the slightly downshifted EHOMO/LUMO levels of J64, we believe that the steric hindrance of cyclohexane side chain could influence the molecular stacking of polymer,24 which may decrease the intramolecular force of polymer and downshift EHOMO/LUMO levels of J64. The EHOMO/LUMO levels of the ITIC acceptor are located at −5.48/−3.83 eV, respectively.30 The HOMO energy offset (ΔEHOMO) between the polymer donors and ITIC acceptor is 0.17, 0.20, and 0.15 eV for J62, J63, and J64, respectively, which are enough for the exciton dissociation and hole transfer from the acceptor to donor in the PSCs with the n-OS ITIC as acceptor.24 Photovoltaic Performance. The photovoltaic performance of the polymer donors was investigated by fabricating the PSCs with ITIC as acceptor and using the device structure of ITO/PEDOT:PSS/polymer:ITIC/PDINO/Al. At first, we optimized the donor:acceptor weight ratios by testing D/A weight ratios of 1:1, 1.5:1, and 2:1 with the total concentration of 18 mg/mL in chloroform solution and the spin-coating speed of 3000 rpm, and the optimized D/A weight ratio is 1.5:1 (see Table 2 and Table S2). We also performed the thermal annealing treatment at 100 °C for 10 min (100 °C/10 min), 130 °C/5 min, 160 °C/5 min, and 180 °C/5 min, and the optimized thermal annealing condition is at 160 °C for 5 min (see Table S3). Compared with the optimized preparation condition of J61-based PSC (D/A weight ratio 1:1 and thermal D

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photocurrent density. The Jph/Jsat ratio for the PSCs based on J62/ITIC, J63/ITIC, and J64/ITIC are calculated to be 0.938, 0.939, and 0.928 under short-circuit conditions. The nearly same and high Jph/Jsat ratio indicates that the degree of exciton dissociation and charge collection in the three devices based on different polymer donors are similar and effective. For the three devices without thermal annealing treatment, the relationship curves between Jsc and Plight are shown in Figure S7, and the curves of Jph vs Veff are shown in Figure S8. The α value in Jsc ∝ Plightα and the Jph/Jsat ratio get a significant increase after thermal treatment, which is beneficial for improving the photovoltaic performance of PSCs. The result indicates that the thermal annealing treatment could improve the morphology of the active layer, which contributes to the reduced charge recombination, increased exciton dissociation, and charge collection. The SCLC method is used to measure the charge carrier mobilities of the thermal annealed PSCs. As shown in Figure S9 and Table S2, the electron mobility μe and hole mobility μh of the device based on J62 are estimated to be 4.95 × 10−4 cm2 V−1 s−1 and 1.23 × 10−4 cm2 V−1 s−1, respectively, with a μe/μh ratio of 4.02. However, the μe and μh of device based on J63 are estimated to be 3.37 × 10−4 cm2 V−1 s−1 and 8.18 × 10−5 cm2 V−1 s−1 with a μe/μh ratio of 4.12, and the ue and uh of device based on J64 are estimated to be 3.55 × 10−4 cm2 V−1 s−1 and 6.54 × 10−5 cm2 V−1 s−1 with a μe/μh ratio of 5.43. The higher charge carrier mobilities and more balanced μe/μh value of the device based on J62 should be one of the reasons why the J62-based PSC has the superior photovoltaic performance. In addition, the device based on J61 also owns high ue and uh in our earlier report,22 which illustrate that the higher charge carrier mobilities and more balanced μe/μh value of the device based on linear alkyl-substituted polymer (J61 and J62) should be one of the reasons for their superior photovoltaic performance.38 To investigate the effect of the cross-linkable unsaturated side chains in J63 on the thermal stability of the PSCs, we test the thermal stability of the three PSCs through hightemperature treatment at 200 °C. As shown in Table 2, after thermal treatment at 200 °C for 5 min, the PCE of the PSC based on J62 reduced from 10.81% to 7.71% and the PCE of the PSC based on J64 reduced from 8.59% to 6.97%, degraded to 71% and 81% of their original values. Interestingly, for the PSC based on J63, after the thermal treatment at 200 °C for 5 min and 2 h, the PCE still keeps the values of 7.96% and 7.45%, which is 98% and 92% of its initial value. The better stability of the device based on J63 should be ascribed to the improved morphology stability of the blend of J63:ITIC due to the cross-linking of the hexylene side chains at higher temperatures. To testify the cross-linking of the hexylene side chains of J63 at higher temperature, we measured the infrared (IR) spectrum of J63 film with or without thermal treatment. As shown in Figure S10, after thermal treatment at 160 °C for 5 min, the vibrational stretching of the vinyl groups between 1600 and 1650 cm−1 weakened obviously, which proves the occurrence of cross-linking in the polymer film. Morphological Characterization. To further understand the effect of side chain on the aggregation properties of polymers, we measured the X-ray diffraction (XRD) of J62, J63, and J64 films; the XRD patterns are shown in Figure S11a. It can be seen that the diffraction peak intensity of J63 film is stronger than J62 film, and the J64 film shows nearly no diffraction peak in the XRD pattern. This result illustrates the

53.11% to 59.58% for the J63-based PSC and from 54.32% to 62.71% for the J64-based PSC, which could be explained as an improved crystallinity after thermal annealing. Interestingly, for the J62-based PSC, the improved PCE after thermal annealing comes from the increase of both FF and Voc, the FF increased from 62.36% to 70.09%, and the Voc increased from 0.904 to 0.915 V. The increased FF may result from the better morphology of the active layer, and the increased Voc could be attributed to the reduced charge recombination in the thermal annealed blend film with their morphology improvement. Figure 2b compares the EQE spectra of the PSCs based on J62/ITIC, J63/ITIC, and J64/ITIC. The similarity of the EQE spectra of the three devices provides evidence that both the absorption of the polymer donor and the ITIC acceptor contribute to the photocurrent. Compared with the EQE spectra of the J63- and J64-based devices, the EQE spectrum of the PSC based on J62 shows a high photoresponse between 620 and 750 nm with EQE peak values of 72%, so as to get larger Jsc value in its photovoltaic performance. The Jsc values calculated from the EQE spectra are 16.12, 15.36, and 15.16 mA cm−2 for the devices based on J62/ITIC, J63/ITIC, and J64/ITIC, respectively, which are in good agreement with the Jsc values obtained from the J−V curves within a 5% mismatch. To better understand the effect of polymer donors on the photovoltaic performance, here we measured the Jsc versus light intensity (Plight) curves to investigate the charge recombination behavior, as displayed in Figure 2c. Charge recombination behavior is a critical factor that could have great influence on the Jsc and FF.10,31,32 In general, the relationship between Jsc and light intensity should follow the formula Jsc ∝ Plightα;35 if all the free carriers are swept out and collected at the electrode with negligible bimolecular recombination, the value of α would be 1.36 In our experiment, for the devices based on J62/ITIC, J63/ITIC, and J64/ITIC with thermal annealing at 160 °C for 5 min, the α values are calculated to be 0.997, 0.972, and 0.934, respectively. The larger α value close to 1 for device based on J62/ITIC indicates weaker bimolecular recombination inside its active layer. In addition, we further investigated the monomolecular recombination properties of the PSC through measuring the dependence of Voc on Plight curves. In general, if bimolecular recombination is the exclusive recombination form in PSCs, the relationship between Voc and Plight could be described as Voc = kT/q ln Plight (where q is the elementary charge, k is the Boltzmann constant, and T is the Kelvin temperature).37 As shown in Figure 2d, for devices based on J62/ITIC, J63/ITIC, and J64/ITIC, the slopes of the fitted line of Voc vs ln Plight are 1.081kT/q, 1.198kT/q, and 1.189kT/q, respectively. Interestingly, the device based on J63/ ITIC displays a larger α in Jsc ∝ Plightα which indicates weak bimolecular recombination and also displays a larger slope in Voc vs ln Pligh which indicates strong monomolecular recombination. The weak bimolecular but strong monomolecular recombination of the device based on J63/ITIC should be responsible for the lower Jsc and FF value. In contrary, the larger Jsc and FF value of the device based on J62/ITIC result from the weak bimolecular and monomolecular recombination simultaneously. Furthermore, to investigate the exciton dissociation and charge collection properties of the devices, we measured the dependence of the photocurrent density (Jph) on the effective voltage (Veff) of the devices, as shown in Figure S6. Here, the Jph/Jsat ratio shows positive correlation with the charge dissociation probability (P(E,T)), where Jsat is the saturated E

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the root-mean-square (RMS) roughness is 0.900, 1.26, and 0.637 nm for the thermal annealed active layer of the PSCs based on J62/ITIC, J63/ITIC, and J64/ITIC, respectively. The oversize and undersize RMS should be responsible for the lower performance of PSCs based on J63/ITIC and J64/ITIC, and the corresponding TEM images (see Figure S12) are in agreement with the morphologies from the AFM images. The AFM images of the blend film based on J62, J63, and J64 without thermal treatment show the RMS of 0.970, 0.973, and 0.564 nm. Compared with the as-cast blend film, the slightly reduced RMS of the thermal annealed blend film based on J62 indicates better miscibility between the linear alkyl substituted J62 polymer donor and ITIC acceptor, which should be responsible for the increased Voc. However, after thermal annealing treatment, the stronger interchain lamellar packing of the blend film based on J63 with increased RMS value results in stronger phase separation, which shows increased FF but reduced Voc in its photovoltaic performance. We hypothesize that the oversize phase separation of the blend film based on J63 should come from the molecular crosslinking of J63, and the undersize phase separation for active layer of the PSCs based on J64/ITIC come from the strong steric hindrance, which result in the weak stacking of the polymer. To explore more details in morphology, grazing incident wide-angle X-ray scattering (GIWAXS) and resonant soft X-ray scattering (RSoXS) are applied to reveal the influence of side chain on phase separation and molecular packing and consequently the photovoltaic performance of the PSCs. As shown in Figure S13 and Table S4, for the neat polymer films, J62 and J63 both show lamellar packing peaks of (100), (200), and (300) while J63 also shows an additional (400) peak, but J64 shows only a (100) peak, which indicates the weaker lamellar packing of polymer J64 and the stronger lamellar packing of polymer J63. For the blend film, as shown in Figure 4, J62:ITIC and J63:ITIC systems show the lamellar peak of (100) and the

existence of linear unsaturated hexylene side chain enhances the interchain lamellar packing of the polymer indeed, and the steric effect of cyclohexane side chain reduces the interchain lamellar packing of the polymer. To further testify for the aggregation behavior of the polymers, differential scanning calorimetry (DSC) was measured, as shown in Figure S11b. There are no endothermic/exothermic peaks on the DSC curves, indicating amorphous characteristics of the three polymers. The X-ray diffraction peak of J63 could come from the stacking of its side chains. In addition, the DSC thermogram of J63 also indicates that the energy change of the cross-linking reaction of J63 is quite small. In addition, the active layer morphologies of the PSCs were measured by atomic force microscopy (AFM) and transmission electron microscopy (TEM). As shown in Figure 3,

Figure 3. AFM images of as-cast blend films based on (a) J62, (b) J63, and (c) J64 and the thermally annealed blend films based on (d) J62, (e) J63, and (f) J64.

Figure 4. Plots and 2D patterns of the GIWAXS measurements. (a) Line cuts of the GIWAXS 2D patterns for different devices, dashed lines are for in-plane direction, and solid lines are for out-of-plane direction. (b−d) 2D patterns of GIWAXS for the as-cast polymer/ITIC films and (e−g) 2D patterns of GIWAXS for the thermally annealed polymer/ITIC blend films. F

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π−π stacking peak of (010) in both in-plane direction and outof-plane direction, suggesting that the coexistence of both faceon and edge-on orientated aggregations, while J64:ITIC system tends to preferentially adopt the face-on orientation relative to the substrate. It has been reported that the mixed face-on and edge-on orientations might help to improve the efficiency of PSCs since three-dimensional (3-D) charge pathways can be formed.39 Upon thermal annealing, all the blend films show sharper lamellar stacking peaks and π−π stacking peaks than the as-cast films, suggesting improved molecular packings. The π−π stacking distance of all the samples are similar (Table S4), but the coherence length of π−π stacking is improved after thermal annealing, which is beneficial for charge transport. Our RSoXS data (Figure S14 and Table S5) also show that after thermal annealing the rootmean-square composition variations (referring to average domain purity) are increased, which should be responsible for the improved FF values of the three devices.40 The domain spacing is reduced for the J62/ITIC blend film after thermal annealing, which is beneficial for the improvement of Jsc. It should be noted that the annealed J63:ITIC also shows an additional, very large domain of 177 nm, which might come from the strong lamellar packing of J63.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected] (H.B.). [email protected] (H.A.). [email protected] (Z.Z.). [email protected] (Y.L.).

ORCID

Zhi-Guo Zhang: 0000-0003-4341-7773 Harald Ade: 0000-0002-7871-1158 Yongfang Li: 0000-0002-2565-2748 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (Nos. 91633301, 21734008, and 91433117), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB12030200), and Molecular Science and Science Education Fusion Innovation project of Chinese Academy of Sciences (No. 2016). GIWAXS/R-SoXS measurements and analysis by Z.P. and H.A. are supported by NCSU and ONR Grant N000141712204. X-ray data were acquired at beamline 7.3.341 and 11.0.1.242 at the Advanced Light Source, which are supported by the Director of the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231. H. Hu, S. Stuard, and I. Angunawelaare are acknowledged for assisting with the measurements and data acquisition. C. Wang is acknowledged for help with X-ray experimental setup and maintenance of the beamlines.

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CONCLUSION Based on the 2D conjugated bithienyl-benzodithiophene-altfluorobenzotriazole D−A copolymers, the polymers J62 with linear octyl side chain, J63 with linear unsaturated hexylene side chain, and J64 with cyclohexane side chain are designed and synthesized. Among these three polymers, J63-based blend film shows stronger lamellar packing because of the stronger molecular stacking of the linear hexylene side chains of the polymer. The J64-based blend film shows weak lamellar packing because of the influence of steric hindrance, which root in the cyclohexane side chain of the polymer. The PSC based on J62 shows superior morphology and photovoltaic performance when ITIC is used as acceptor, a high PCE of 10.81% with a Voc of 0.915 V, a Jsc of 17.43 mA/cm2, and a high FF of 70.09% is obtained. Notably, the PCE of PSC based on J63 still keeps 92% of its initial value after thermal treatment at 200 °C for 2 h. The result highlights that the sidechain engineering is an effective way to adjust the lamellar packing of the polymers and the phase separation of the blend films and thus to influence the photovoltaic performance and stability of the PSCs.

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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.8b01036. Detailed experimental procedures including synthesis, characterization, and device optimization, and the results of TGA, absorption spectra, SCLC mobility measurements, cyclic voltammograms, photovoltaic performance of the as-cast PSCs, dependence of Jph on the effective voltage, light intensity dependence of Jsc, IR, XRD, DSC, TEM, GIWAXS, RSoXS, NMR spectra of the synthetic intermediates and polymers (PDF) G

DOI: 10.1021/acs.macromol.8b01036 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b01036 Macromolecules XXXX, XXX, XXX−XXX