A Simple Approach to Prepare Chlorinated Polymer Donors with Low

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A Simple Approach to Prepare Chlorinated Polymer Donors with Low-lying HOMO Level for High Performance Polymer Solar Cells Beibei Qiu, Shanshan Chen, Hongneng Li, Zhenghui Luo, Jia Yao, Chenkai Sun, Xiaojun Li, Lingwei Xue, Zhi-Guo Zhang, Changduk Yang, and Yongfang Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05352 • Publication Date (Web): 31 Mar 2019 Downloaded from http://pubs.acs.org on March 31, 2019

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Chemistry of Materials

A Simple Approach to Prepare Chlorinated Polymer Donors with Low-lying HOMO Level for High Performance Polymer Solar Cells Beibei Qiu,a b Shanshan Chen, c Hongneng Li, a Zhenghui Luo, a Jia Yao, a Chenkai Sun,a b Xiaojun Li,a b Lingwei Xue,a Zhi-Guo Zhang,a Changduk Yang, c Yongfang Li a b d * a

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids,

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b

School of Chemical Science, University of Chinese Academy of Sciences, Beijing 100049, China

c

Department of Energy Engineering, School of Energy and Chemical Engineering, Low Dimensional

Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689798, South Korea d

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and

Materials Science, Soochow University, Suzhou, Jiangsu 215123, China * Corresponding authors: [email protected] (Y. Li)

Abstract Both photovoltaic performance and cost of materials are the key factors for commercial application of polymer solar cells (PSCs). In this study, we designed and synthesized two new conjugated polymers with chlorine-substituent at -position and flexible substituent on -position of thiophene conjugated side chain, J11 (with alkyl flexible substituent) and J12 (with alkoxy flexible substituent), through a relatively simple synthetic method. The two polymers displayed similar UV-vis absorption profile with deep-lying HOMO (the highest occupied molecular orbital) energy levels. Compared with the alkyl substituted polymer J11, alkoxy substituted polymer J12 displayed slightly weaker aggregation and excessive miscibility with the n-type organic semiconductor (n-OS), m-ITTC (which is an isomeric counterpart of ITTC). As a result, the device based on J11: m-ITTC demonstrated a higher PCE of 12.32% with a high fill factor (FF) of 0.73, while the J12: m-ITTC based device displayed a lower PCE of 8.74%, with a slightly higher Voc of 0.943 V but a low FF of 0.56, which 1

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should be ascribed to the relatively low hole mobility, resulting from its small phase separation. These results indicate that altering the position of chlorine atom (Cl) from - to -position of thiophene unit is a simple but effective method to prepare efficient chlorinated polymers for further development of PSCs.

Introduction During the past several decades, polymer solar cells (PSCs) have attracted considerable interests as one of the potential alternative technologies for utilizing the renewable solar energy, because of their opportunities in fabricating large-area ﬂexible light harvesting devices using simple solution processing techniques.1-5 Commonly, PSCs are fabricated with a bulk-heterojunction (BHJ) architecture with p-type polymers as donors and n-type semiconductors (n-OS) as acceptors.6-8 At the early stage, fullerene derivatives have acted as the most common acceptor materials and successfully promote the progress of PSCs, because of their excellent electron mobility.9-12 Recently, possessing the advantages of strong absorption and adjustable energy levels, acceptor-donor-acceptor (A-D-A) type n-OS acceptors have been developed to further boost the performance of PSCs.13-17 Most recently, benefiting from the material innovation and device optimization, power conversion efficiencies (PCEs) for single-junction PSCs up to 15% have been achieved, 18, 19 highlighting great potential of the PSCs for commercial applications. Open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) are the critical device photovoltaic parameters of the PSCs.1,

7, 20

To obtain efficient Jsc, the design concept of

complementary absorptions of donor and acceptor materials is a vital way to enhance the absorption ability of photoactive layer.21-24 In order to better match with the absorption of polymer donors, several methods have been applied to boost the absorption spectra and coefficient of A-D-A type nOS acceptors, including extending the conjugation and introducing end groups with strong electronwithdrawing properties.25-28 However, these molecular engineering strategies usually lead to lowlying the lowest unoccupied molecular orbital (LUMO) levels of these n-OS acceptors, result in 2


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relatively low Voc. Therefore, to maintain the high Jsc and improve the Voc, donor polymers with deeplying highest occupied molecular orbital (HOMO) levels are needed to better match with these n-OS acceptors. Besides, polymer donors with relatively enhanced aggregation characteristics usually possess relatively higher hole mobility, which is beneficial for charge separation and collection to achieve higher FF and better photovoltaic performance.29-32 As for the polymer donors, the fused ring structure of benzo[1,2-b:4,5-b']dithiophene (BDT) unit appears to be a versatile constructing block for efficient polymer donors.33 Besides, side chain engineering on BDT block is effective method to fine-tune the energy levels and aggregation properties.5,

34-36

To date, many efforts have been devoted to developing bithienyl-BDT-based

conjugated polymers with low-lying HOMO energy levels.37-41 Scheme 1 displays several widely used BDT units for constructing polymer donor materials. Among them, an efficient method to downshift HOMO level is introducing fluorine substituents on the thiophene conjugated side chain of the BDT unit. However, the relatively higher synthetic cost of the ﬂuorinated organic photovoltaic materials would impede the future application of PSCs.42, 43 Therefore, highly efficient photovoltaic materials that are easy and inexpensive to synthesize will be favorable for the commercialization of PSCs.44-46

Scheme 1. Chemical structures of widely used BDT units in photovoltaic materials 3



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Based on the analysis mentioned above, herein two chlorinated conjugated polymers J11 and J12 (Cl-substituted at -position of the thiophene conjugated side chain, as shown in Figure 1) were designed and synthesized through simple chlorination reaction, for the application as donor in PSCs. Compared with the corresponding fluorinated polymers, the synthetic routes of the two polymers are much simple. Besides, the side chain at -position of the thiophene conjugated side chains (which is an alkyl substituent for J11 and alkoxy substituent for J12) could be used to adjust molecular aggregation behavior of the polymer donors. The two polymers showed similar UV-vis absorption characteristics and deep-lying HOMO levels. Photovoltaic performance of the two polymer donors were studied by using an n-OS, m-ITTC (Figure 1) which is a derivative of m-ITIC,47, 48 as acceptor. The power conversion efficiency (PCE) of the PSC based on J11: m-ITTC reached 12.32% with an open circuit voltage (Voc) of 0.935 V, a short circuit current (Jsc) of 18.05 mA cm-2, and a fill factor (FF) of 0.73, while the J12-based PSC displayed a lower PCE of 8.74%. The results indicate that chlorine-substitution is an effective way to improve photovoltaic performance of the polymer donor for PSCs.

Figure 1. Molecular structures of polymers J11, J12 and acceptor m-ITTC, and device structure of PSCs.

Results and discussion 4


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Materials and Synthesis Synthetic routes of the two polymers J11 and J12 are shown in Scheme 2. Compound 4,8Dihydrobenzo[1,2-b:4,5-b']dithiophen-4,8-dione (BDT) was bought from Derthon Optoelectronic Materials Science Technology Co LTD. Pd(PPh3)4 was obtained from J＆K chemical Co. Compounds 1a, 1b, 2a and 2b were synthesized according to previous literatures.50 Compounds 2a and 2b were synthesized with high yields of ~80% via simple chlorination reaction using N-Chlorosuccinimide (NCS), demonstrating lower synthetic cost compared with fluorine substitution.38, 43 Besides, the two chlorinated BDT units displayed relatively low HOMO energy levels similar as fluorinated BDT units, estimated by DFT calculations, as displayed in Figure S6 in Supporting Information. The detailed synthetic procedures of polymers J11 and J12 are provided in Supporting Information (SI). In comparison with polymers with Cl-substituted at -position of the thiophene conjugated side chain, polymers with Cl-substituted at -position possess some advantages in molecular modification. By shifting the chlorine-substituent from - to -position, a variety of flexible substituents could be more easily introduced into thiophene conjugated side chain to adjust molecular aggregation behavior, demonstrating the role of the feasible and simple approach in preparing chlorinated polymers for PSCs. The number average molecular weight (Mn) of J11 and J12 are estimated to be 18.8 and 31.9 KDa with polydispersity index of 1.81 and 1.28 respectively (Figure S22 and S23 in SI). Figure 1 shows the chemical structures of the polymers, J11 and J12, and the n-OS m-ITTC. The thermogravimetric analysis (TGA) plots in Figure S1 in SI indicate that both polymers, J11 and J12, and n-OS acceptor m-ITTC possess excellent thermal stability with 5% weight loss at 385, 344 and 335 oC, respectively.

5



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Scheme 2. Synthetic routes of polymers J11 and J12

Optical Properties and Electronic Energy Levels Figure 2a and Figure 2b displayed the normalized UV-vis absorption spectra of the two polymer donors J11 and J12 and n-OS acceptor m-ITTC in dilute CHCl3 solution and thin film state. Table 1 summarized the corresponding absorption characteristics of the two polymers and n-OS acceptor. The two polymer solutions present similar maximum absorption peaks (543 nm for J11 and 548 nm for J12). Compared to J12 solution, J11 solution displayed slightly enhanced shoulder peak, indicating the stronger aggregation characteristic for J11 in solution state. To better illustrate the correlation between polymer aggregation ability in solution, temperature-dependent absorption of polymer J11 6


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and J12 (in CB) have been measured and reported in Supporting Information (Figure S2). As temperature rises, the shoulder peaks of both J11 and J12 polymer films became weaker, and their absorption peaks were slightly blue-shifted. In film state, the two polymer films exhibit almost coincident absorption profile, with similar maximum absorption peak (543 nm for J11 and 545 nm for J12) and absorption edge (634 nm for J11 and 638 nm for J12). As shown in Figure S3 in SI, the absorption coefficients of J11 and J12 films are estimated to be 8.26×104 cm-1 and 8.08×104 cm-1 respectively. It should be mentioned that due to the similar chemical structure of the two polymers, similar absorption profile could be expected, as confirmed by the almost coincidence absorption profile in film state. Therefore, we could draw the conclusion that the shoulder peak in solution could reflect the aggregation ability49, 50 and J11 in solution state possess slightly stronger aggregation than J12.

Figure 2. (a) Absorption spectra of J11, J12 and m-ITTC in dilute chloroform solution. (b) Absorption spectra of J11, J12 and m-ITTC in film state. (c) Cyclic voltammograms of polymer donors J11 and J12 with Ag/AgCl as reference electrode. (d) Energy level diagram of the materials 7



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involved in the PSCs.

Electrochemical cyclic voltammetry (CV) was carried out to measure the HOMO and LUMO energy levels of the two polymers, J11 and J12, and the n-OS acceptor, m-ITTC. From the cyclic voltammograms in Figure 2(c), the ELUMO/EHOMO of J11 and J12 were calculated from the onset oxidation potentials (φox) / onset reduction potentials (φred) to be -3.23 eV/-5.48 eV and -3.26 eV/5.51 eV respectively, according to the equations of EHOMO/ELUMO = -e(φox/φred+4.8-φFc/Fc+) (eV) where φFc/Fc+ was measured to be 0.44 V vs Ag/AgCl in this measurement system (Figure S4 in SI) and then the calculation equations could be expressed as EHOMO/ELUMO=-e(φox/φred+4.36) (eV).51 Compared with J11, polymer J12 with alkoxy side chain possesses a slightly down-shifted HOMO energy level, which is beneficial for obtaining higher Voc. For comparison, the theoretical ELUMO/EHOMO of three benzotriazole-based polymers (J52, J11 and J12) were calculated to be -2.26 eV/-4.95 eV, -2.37/5.12 eV and -2.41/-5.13 eV, respectively by DFT calculations, as shown in Figure S5. The calculated EHOMO of J12 is slightly lower than that of J11, which is in agreement with the results obtained by the CV measurement. It should be noted that polymers with ortho alkoxy chain substituted thiophene unit always possess relatively higher HOMO level, thus leading to lower Voc.37 The slightly lower HOMO level of J12 might be ascribed to the stronger inductive effect than conjugative effect of the meta alkoxy chain of thiophene unit on BDT block of J12. Figure S5 displayed the CV curves of mITTC. The ELUMO offsets and EHOMO offsets of J11/ m-ITTC or J12/ m-ITTC are calculated to be 0.66 eV/0.14 eV or 0.63 eV/0.11 eV, respectively, which is sufficient for charge separation for this nOS acceptor-based PSCs system, as confirmed by the photoluminescence quenching experiment (Figure S8 in SI).

Table 1. Absorption properties and electronic energy levels of J11, J12 and m-ITTC λmax Photoactive Material

solution (nm)

λmax film

λonset film

Egopt

(nm)

(nm)

(eV)

φox

φred

(V vs.

(V vs.

Ag/AgCl)

Ag/AgCl)

8


EHOMO

ELUMO

(eV)

(eV)

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J11

543

543

634

1.96

1.12

-1.13

-5.48

-3.23

J12

548

545

638

1.94

1.15

-1.10

-5.51

-3.26

m-ITTC

688

711

769

1.61

1.26

-0.47

-5.62

-3.89

Photovoltaic performance PSCs were fabricated with a conventional device architecture of ITO/PEDOT: PSS/J11 or J12: m-ITTC/PDINO/Al to investigate the photovoltaic performance of the two chlorinated polymer donors (J11 and J12), and to deeply understand the effect of alkyl and alkoxy side chain on their photovoltaic properties. The photovoltaic performance of the polymers was optimized by changing donor/acceptor (D/A) weight ratios and with different thermal annealing (TA) treatment conditions. Table S3 and S4 in SI summarized the photovoltaic performance of the PSCs based on polymer: mITTC with different D/A ratios and with different TA treatment conditions. Figure 3(a) illustrates the typical current density-voltage (J-V) curves of the optimized PSCs based on J11: m-ITTC (1:1, w/w) and J12: m-ITTC (1:1, w/w), and the corresponding device performance parameters deduced from the J-V curves are summarized in Table 2. For PSC devices without thermal annealing treatment, J11based device displayed a PCE of 10.16%, with a high Voc of 0.944 V, a Jsc of 15.74 mA cm-2 and a FF of 0.68. In contrast, the as-cast J12-based device showed an inferior PCE of 8.31%, with a slightly higher Voc of 0.955 V, a similar Jsc of 15.56 mA cm-2 but an inferior FF of 0.55. After thermal annealing treatment at 150 oC for 1 min, the J11-based device demonstrated a boosted PCE of 12.32%, which is mainly ascribed to the enhanced Jsc of 18.05 mA cm-2 and higher FF of 73.0%. In contrast, the J12: m-ITTC based device with thermal annealing treatment at 150 oC for 1 min delivered a PCE of 8.74% similar with its as-cast device, with a Voc of 0.943 V, a slightly enhanced Jsc of 16.64 mA cm-2 and a similar low FF of 55.7%. It should be noted that, both J11 and J12 based devices displayed comparable Voc with their corresponding fluorinated polymer J91, as shown in Table S3 in SI, demonstrating the deep-lying HOMO energy levels of the two chlorinated polymers. Besides, because of the impact of molecular weight on photovoltaic performance52, to clearly illustrate the performance difference between the two polymer donors of J11 and J12, polymer J11 with high molecular weight 9



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(Mn of 34.8 KDa, Mw of 60.6 Kda (Figure S24 in SI), similar with the Mn of 31.9 KDa for J12) was synthesized, and the photovoltaic performance of the high molecular weight J11-based PSCs was characterized as listed in Table S5. The device based on the high molecular weight J11 showed a PCE of 12.15% efficiency, with slightly lower Voc and FF, but a higher Jsc, indicating that the performance difference between the two polymer donors of J11 and J12 should be ascribed to their different side chain structures. The thermal annealing treatment slightly enhanced the absorption properties of both polymer blend films (as shown in Figure S9 in SI), which is beneficial for obtaining higher Jsc. The higher Jsc and better FF of the J11-based devices could be ascribed to the higher charge mobility and better phase separation.

Figure 3. (a) J-V curves of the best PSCs based on J11: m-ITTC(1:1, w/w) and J12: m-ITTC (1:1, w/w) under the illumination of AM1.5G, 100 mW cm-2. (b) IPCE curves of the corresponding PSCs. (c) Jph versus Veff of the best devices. (d) Dependence of light intensity on Jsc of the best devices.

The input photon to converted current efficiency (IPCE) spectra of the best PSC devices were 10


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measured to elucidate the photoresponse of the PSCs based on the two chlorinated polymers, as depicted in Figure 3(b). Clearly, both the J11: m-ITTC and J12: m-ITTC based devices display broad photoresponse covering the entire visible spectral region of 300~800 nm, confirming the contributions of polymer donors and the m-ITTC acceptor in the blends to the photocurrent generation. In comparison with J12, the J11-based devices present higher photoresponse range from 350 to 750 nm, with a broad plateau IPCE values of around 70-80% (in the wavelength range of ca. 460~730 nm) and a maximum IPCE value approaching 78%, resulting in the fairly higher Jsc. The Jsc values calculated from the integration of IPCE spectra are 17.72 mA cm-2 and 16.43 mA cm-2, respectively, which are consistent with those obtained from corresponding J-V curves (within 2% mismatch).

Table 2. Photovoltaic parameters of the PSCs based on J11: m-ITTC (1:1, w/w) and J12: m-ITTC (1:1, w/w) under the illumination of AM1.5G, 100 mW cm-2 Active layer

Condition As Cast

J11: m-ITTC 150 oC, 1 min As Cast J12: m-ITTC 150 oC, 1 min

Voc

Jsc

FF

PCE

Slope

Slope

(V)

(mA cm-2)

(%)

(%)

a

b

(Voc)c

0.944

15.74

68.4

10.16

(0.943±0.003)

(16.12±0.45)

(65.9±1.4)

(10.01±0.24)

0.935

18.05

73.0

12.32

(0.933±0.003)

(17.92±0.42)

(70.7±1.1)

(11.81±0.21)

0.955

15.56

55.9

8.31

(0.958±0.003)

(15.42±0.40)

(54.1±1.3)

(7.99±0.13)

0.943

16.64

55.7

8.74

(0.939±0.004)

(16.27±0.38)

(54.4±1.4)

(8.31±0.25)

(Jsc)

0.972 0.978 0.939 0.948

1.12 kT/q 1.07 kT/q 1.17 kT/q 1.12 kT/q

a Average

values with standard deviations were obtained from more than 10 devices. b The slope of the dependence of logJ on logP(P represents light intensity). sc c The slope of the dependence of V on lnP. oc

To deeply investigate the effect of conjugated side chains of BDT unit on charge transport properties, space-charge-limited-current (SCLC) measurements were applied to examine the hole (μh) and electron (μe) mobilities (Figure S14-S17 in SI). Table 3 summarized the corresponding charge carrier mobilities. It should be noted that the μe values of the two polymer blend films with m-ITTC 11



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are similar. Therefore, the differences in μh values would be the key factor that influence the device photovoltaic performance. The neat J11 film possesses a μh of 0.956×10-4 cm2 V-1 s-1, while the μh of J12 (0.338×10-4 cm2 V-1 s-1) is much lower, demonstrating significant effect of the side chain type on hole mobility of the polymers. For the blend films, in comparison with as cast blends, thermalannealing (TA) treated blend films displayed enhanced μh and μe. The μh of J11: m-ITTC blend films were calculated to be 0.78×10-4 and 1.06×10-4 cm2 V-1 s-1 for the device before and after the TA treatment, respectively. However, the μh of the J12: m-ITTC blend films were quite low whether before or after the TA treatment, resulting in relatively poor FF and photovoltaic performance. For the TA treated J11-based devices, the ratio of μh/μe was slightly balanced, which is beneficial for achieving higher FF.52-54

Table 3. Charge transport properties of the J11: m-ITTC and J12: m-ITTC blend films. Active layer

Treatment As cast

J11: m-ITTC

150 oC, 1 min As cast

J12: m-ITTC

a Average

150 oC, 1 min

Hole mobility (μh) (cm2

V-1 s-1) a

Electron mobility (μe) (cm2 V-1 s-1)

a

0.78×10-4

2.43×10-4

(0.60±0.11)×10-4

(1.71±0.42)×10-4

1.06×10-4

2.84×10-4

(0.87±0.19)×10-4

(2.27±0.46)×10-4

0.15×10-4

1.93×10-4

(0.12±0.03)×10-4

(1.48±0.38)×10-4

0.22×10-4

2.54×10-4

(0.17±0.04)×10-4

(2.08±0.39)×10-4

μh/μe 0.32 0.37 0.08 0.09

values with standard deviations were obtained from 5 devices.

Considering that the charge dissociation characteristics could deeply influence the device photovoltaic performance, the dependence of photocurrent density on effective applied voltage (Veff) of the optimal devices was characterized.55 The charge dissociation probability (P(E, T)) could be obtained by calculating the value of Jph/Jsat. It should be noted that, from Figure 3(c), when Veff reaches ~3 V, Jph values of the J11 and J12 based devices reached saturation with similar Jsat, indicating similar amount of excitons were generated. Under short circuit and maximal power output conditions, 12


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the P(E, T) values of the J11-based devices are estimated to be 95% and 81%, suggesting a high charge collection efficiency. In contrast, the P(E, T) values of the J12-based devices are 91% and 69%, indicating relatively serious charge recombination, which should be ascribed to its poorer hole mobility. These results imply that the lower Jsc and FF of the J12: m-ITTC based device should be ascribed to the relatively low exciton dissociation efficiency. The higher exciton dissociation efficiency in the J11: m-ITTC device results in higher photovoltaic performance. To probe the charge recombination behavior in the PSCs, the dependence of Jsc on the illumination intensity (P) was further studied. In principle, Jsc follows a power-law dependence with the relation of Jsc ∝Pα. The exponent α values should be close to 1 if there is no bimolecular recombination in the PSCs.56

As shown in Figure 3(d) and Figure S12 in SI, the TA treatment could

slightly increase α values towards 1 for both J11 and J12-based devices. The α values are calculated to be 0.978 and 0.948 for the TA treated devices based on J11: m-ITTC and J12: m-ITTC, respectively. The higher α value (more close to 1) of the J11-based devices suggests the relatively weak bimolecular recombination at the short circuit condition, agreeing with its higher Jsc and FF values. While the lower α of the J12-based device suggests the existence of some bimolecular recombination. Additionally, the dependence of Voc on light intensity (P) was measured to investigate the recombination mechanism. For the J11-based devices, the slopes of the fitted line of Voc vs. lnPlight are calculated to be 1.12 (as cast) and 1.07 kT/q (with TA treatment) respectively, as shown in Figure S13 in SI. Compared with the J11-based devices, the corresponding J12-based devices displayed higher values of 1.17 (as cast) and 1.12 kT/q (with TA treatment) respectively. Generally, the slope close to kT/q for the TA treated J11-device suggests the recombination at the open circuit condition is a bimolecular dominated process (without monomolecular recombination). In combining with the results of the dependence of Jsc on Plight, the J11: m-ITCC based devices possess less bimolecular and monomolecular charge recombination, suggesting more efficient charge collection, which is beneficial for obtaining higher FF and PCE values for the PSCs based on J11: m-ITTC.

Morphology analysis 13



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Figure 4. (a) GIWAXS patterns of the polymer: m-ITTC blend films: (i) J11: m-ITTC as cast, (ii) J11: m-ITTC with TA treatment, (iii) J12: m-ITTC as cast, (iv) J11: m-ITTC with TA treatment. (b) Corresponding line-cut profiles of the GIWAXS patterns of the polymer: m-ITTC blend films. (c) dspacing and coherence length estimated from the out-of-plane (010) diffraction of the polymer: mITTC blend films. (d). Lamellar d-spacing and coherence length estimated from the in-plane (100) diffraction of the polymer: m-ITTC blend films.

To further understand the structure-function relationship of the two chlorinated polymers and the effect of alkyl and alkoxy side chain on the morphological characteristics, detailed investigation on the neat films and blend films were performed by applying grazing incidence wide-angle X-ray scattering (GIWAXS) measurements.57, 58 Figure S10 in SI displays the GIWAXS patterns of neat m14


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ITTC film, the intense (010) π-π diffraction in the out-of-plane direction means its predominant faceon orientation, with a (010) π-π diffraction at 1.74 Å-1 (d-spacing: 3.61 Å). The neat J11 film showed a face-on orientation with a (010) π-π diffraction at 1.69 Å-1 (d-spacing: 3.73 Å) and a (100) diffraction at 0.28 Å-1 (d-spacing: 22.37 Å). The neat J12 film displayed a slightly weaker (010) diffraction at 1.70 Å-1 (d-spacing: 3.70 Å) with a (100) diffraction at 0.26 Å-1 (d-spacing: 24.21 Å), as shown in Figure S11. The stronger (010) diffraction intensity of neat J11 film should be ascribed to the slightly stronger aggregation, as confirmed by the corresponding peak deconvolution. Besides, the larger lamellar d-spacing of J12 should be ascribed to the slightly longer alkoxy chain compared to alkyl chain. When blending the polymer donors with n-OS acceptor m-ITTC, the (100) lamellar diffraction of both blend films without TA treatment appeared along both the qxy and qz axes, indicating the coexistence of the face-on and edge-on orientation (Figure 4(a): (i) and (iii)). Besides, the d-spacings of (010) and lamellar (100) diffractions, and their corresponding coherence lengths (CCLs) in the blend films were calculated, as shown in Figure 4(c) and 4(d). It should be mentioned that, for the out-of-plane (010) π-π stacking diffraction, the diffraction peaks of two neat polymer films were quite close to m-ITTC, therefore it is hard to clearly separate them out in the BHJ blends. While for in-plane (100) lamellar diffraction, the diffraction signals of two blend films were much closer to the corresponding neat polymer films, therefore it might be reasonable to attribute them to the donor polymers. For both blend films, TA treated blends exhibited narrowed peaks clearly, including (010) π-π stacking diffraction and (100) lamellar diffraction, with obviously enhanced coherence lengths, which is beneficial for better charge transport, indicating that TA treatment could effectively promote the polymers packing. As shown in Figure S20, the pole figure extraction from the (100) diffraction was carried out to deeply investigate the molecular orientation behavior. Within the azimuthal angle, the relatively larger population of the face-on crystallites of the TA treated films for both polymer blends could be clearly observed, demonstrating the effect of the TA treatment on the molecular orientation. Besides, by calculating the Axy/ (Axy + Az) values for the two TA-treated 15



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blend films (0.467 for J11: m-ITTC and 0.369 for J12: m-ITTC), we noticed that the J11: m-ITTC film more prefers the face-on orientation packing, which is beneficial for charge transportation. Moreover, it is interesting that though the hole mobilities of J12: m-ITTC blend films were much lower than those of J11: m-ITTC blend films, the coherence lengths of J12: m-ITTC blends were quite similar as that of the J11: m-ITTC blends.

Figure 5. (a) AFM height images (size: 5×5 μm2) and (b) TEM images (scale bar=200 nm) of polymer: m-ITTC blend films: (i) J11: m-ITTC as cast, (ii) J11: m-ITTC with TA treatment, (iii) J12: mITTC as cast, (iv) J11: m-ITTC with TA treatment.

Considering that the device performance is closely related to the degree of phase separation in blend film, the morphological characteristics of both polymer: m-ITTC blend films were deeply investigated to further understand the differences in device performance. Atomic force microscopy (AFM) was applied to investigate the surface topography, and transmission electron microscopy (TEM) was utilized to illustrate the phase separation of blend films before and after thermal TA treatment. As shown in Figure 5 (a), both BHJ films displayed quite smooth surface. Compared with J11: m-ITTC blend films, J12: m-ITTC blend films displayed slightly smaller root-mean-square (RMS) values. Furthermore, the TEM images reveal clear phase separation feature for both polymer: m-ITTC blend films. For blend films before TA treatment, the TEM image of J12: m-ITTC shows 16


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relatively smaller aggregation than J11: m-ITTC blend films, confirming the excessive miscibility of J12 and m-ITTC. After thermal annealing, the J11: m-ITTC blend film displayed slightly enhanced phase separation, which is beneficial for charge transportation, therefore, better device performance could be achieved. While for the TA treated J12: m-ITTC blend, although the coherence lengths of (010) and (100) diffractions were significantly improved as discussed in GIWAXS part, the phase separation situation was still similar with the as-cast blend film. The more intermixed morphology with smaller phase-separated domains of the J12: m-ITTC blend would lead to higher bimolecular recombination, which might be ascribed to the development of excessively impure domains, thus resulting in relatively lower charge mobility and charge separation efficiency, as well as inferior photovoltaic performance.59, 60 Collectively, the favorable face-on orientation stacking and the moderate-scale phase separation are achieved in the J11: m-ITTC blend, which results in efficient charge transport and exciton dissociation characteristics, thus better photovoltaic performance with improved FF and Jsc.

Conclusions In conclusion, we report a feasible way to simply prepare chlorinated BDT-based polymers with high photovoltaic performance. The theoretical calculation and photovoltaic results clearly show that the two new chlorinated BDT units and their corresponding polymers, J11 and J12, possess low-lying HOMO energy levels similar as their corresponding fluorinated polymers. Besides, the adjacent side chain of the chlorine substituents could effectively adjust molecular aggregation behavior and the blend films morphology. The PSCs based on J11: m-ITTC showed a high PCE of 12.32%, with a relatively high FF of 0.73, while the J12: m-ITTC-based device gave an inferior PCE of 8.74%. The relatively lower performance of the J12-based device could be attributed to the more intermixed morphology with smaller phase-separated domains in its blend film with m-ITTC. It should be mentioned that although the substitution of alkoxy chain in J12 leads to relatively inferior photovoltaic performance, the better intermixed morphology characteristics might be useful to fine 17



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tune other polymer systems that possess too strong aggregation ability for obtaining suitable phase separation. Overall, our results indicate that altering the position of chlorine atom (Cl) from meta- to ortho-position of thiophene unit is a simple but effective method to prepare efficient chlorinated polymer donors with low-lying HOMO energy level, and the side chain type could effectively affect aggregation morphology, charge carrier mobility, and photovoltaic performance of the conjugated polymers.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details of materials (J11, J12 and m-ITTC) synthesis, TGA thermograms of neat J11, J12 and m-ITTC, solution UVvis absorption spectra of J11 and J12, thin film absorption spectra of blend films, DFT calculation of BDT units and polymers, photoluminescence spectra of blend films, J1/2-V characteristics of the devices, GIWAXS images and corresponding in-plane and out-of-plane line cuts of neat J11, J12 and m-ITTC films, light intensity dependence of Jsc of the as cast devices, light intensity dependence of Voc, pole figure plots from the (100) lamellar diffraction of polymer: m-ITTC films at out-of-plane direction.

Acknowledgements This work was supported by NSFC (Nos. 91633301, 21734008 and 51673200) and the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB12030200.

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A Simple Approach to Prepare Chlorinated Polymer Donors with Low

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