Phenanthrodithiophene–Isoindigo Copolymers: Effect of Side Chains

Apr 29, 2015 - ... developed semiconducting polymers containing phenanthro[1,2-b:8,7-b′]dithiophene (PDT) combined with a bis(thienyl)isoindigo (IID...
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Phenanthrodithiophene−Isoindigo Copolymers: Effect of Side Chains on Their Molecular Order and Solar Cell Performance Shuhei Nishinaga,† Hiroki Mori,† and Yasushi Nishihara*,†,‡ †

Division of Earth, Life, and Molecular Sciences, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan ‡ ACT-C, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: The synthesis, characterization, and solar cell application of newly developed semiconducting polymers containing phenanthro[1,2-b:8,7-b′]dithiophene (PDT) combined with a bis(thienyl)isoindigo (IID) unit are described. The polymers with longer alkyl chains are sufficiently soluble to be compatible with the processes required to manufacture solar cells. In conventional solar cell devices, polymers with all branched alkyl chains tend to form a higher proportion of a well-ordered face-on crystallite in the π-stack direction than those with both linear and branched alkyl chains, which significantly improves the fill factor (FF), resulting in higher power conversion efficiency (PCE). In terms of optimizing the alkyl chain lengths, the installation of longer alkyl side chains on the polymer backbone leads to low molecular weight polymer, which may promote a large phase separation. As a result, the polymers 12OD and BOBO, bearing shorter alkyl groups, performed better, and a BOBO polymer-based solar cell (PSC) showed the best PCE value up to 3.83%. In the inverted PSCs, the polymers with all branched alkyl chains have a higher face-on ratio than those with both linear and branched alkyl chains. Because of their improved Jsc, inverted PSCs with BOBO/PC71BM gave the best performance, with a PCE up to 5.28%. Although an obvious dependence of photovoltaic properties on molecular order was observed in conventional solar cell devices, no trend was observed in inverted cells, possibly attributable to their amorphous nature, which arises from the axisymmetrical structure of PDT, leading to less effective π−π overlap and low crystallinity.

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for efficient carrier transport.24−27 However, these strong intermolecular interactions significantly reduce the solubility of the polymers. For this reason, D−A polymers with highly πextended cores require many long and/or bulky solubilizing groups to ensure sufficient solubility for efficient device fabrication. From the viewpoint of material design, another important concern is the choice of appropriate side chains. Installation of optimal side chains can enhance both intermolecular interactions and crystallinity, which lead to suitable morphology and effective carrier transport.28−33 In addition, the topology, length, and number of side chains dramatically affect the molecular orientation and morphology of the polymer backbone.34−39 Since PSCs possess a vertically arranged electrode, polymers should ideally adhere in a face-on orientation, which is known to facilitate carrier transport even in films over 300 nm thick, resulting in high Jsc and no loss of FF. 34,40,41 Thus, optimization of the side chains and investigation of the relationship between the thin film’s structure and its photovoltaic properties are essential to the development of high-performance PSCs.

INTRODUCTION PSCs are state-of-the-art renewable energy sources that can replace conventional silicon technology because of their lightweight, flexibility, and easy fabrication as large-area devices using low-energy processes such as printing technology.1−4 In recent years, various donor−acceptor (D−A)-type semiconducting polymers, processing methods, and device architectures have been developed by many chemists and physicists.5−17 As a result of these efforts, power conversion efficiencies (PCEs) have exceeded 10% for single-junction cells18−20 and 11−12% for tandem and triple-junction systems.21−23 Since many of these high-performance PSCs could become practical with somewhat improved materials and the development of new building units, the elucidation of structure−property relationships has become very important for the realization and general adoption of these very promising devices. One of the most effective approaches to achieving the best performance in PSCs is the incorporation of extended πelectron cores in the polymer backbone, which can provide strong intermolecular interactions and effective π−π overlaps between neighboring polymers. In addition, the electrostatic interaction of D−A-type semiconducting polymers enhances their intermolecular interactions, which can facilitate the formation of the long-range-ordered structures so important © XXXX American Chemical Society

Received: March 25, 2015 Revised: April 18, 2015

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DOI: 10.1021/acs.macromol.5b00622 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Route to Polymers

chose alkyl chains with similar lengths to facilitate the desired face-on orientation. Herein, we report the synthesis and characterization of the new PDT-based polymers containing the bisthienyl IID unit and evaluate the relationship between particular molecular orientations in the solid state and solar cell performance as part of the discussion of this structure− property relationship.

Recently, we reported the synthesis, characterization, and electronic device application of newly developed phenanthro[1,2-b:8,7-b′]dithiophene (PDT)42,43-based semiconducting polymers in combination with a dialkylated diketopyrrolopyrrole (DPP) known as a common acceptor unit.44 These PDTbased polymers have some superior features, including strong aggregation behavior, high thermal stability, extended absorption up to 1000 nm with a small band gap of ∼1.2 eV, and short π−π stacking distance (3.5−3.6 Å), qualities due to the large and rigid π-electron systems of PDT. However, such strong intermolecular interaction limits molecular weights and also leads to their low solubility. Strong aggregation and oligomeric features promote a large phase separation, which prevents efficient photocurrent generation, resulting in limited Jsc (∼5 mA cm−2). In addition, since PDT−DPP polymers with some alkyl side chains have a high-lying HOMO energy level (−5.2 eV) due to localized HOMO coefficients on the DPP core, their solar cells show a low Voc of 0.65 V and PCE of 2.0%. In this study, in order to improve solar cell performance, we designed five new D−A copolymers based on PDT as donor unit and bis(thienyl)isoindigo (IID) as acceptor unit (Scheme 1) for the following reasons. First, IID is a well-known electronacceptor unit in high-performance organic electronics, which has strong electron affinity and a deep HOMO energy level.45 In combination with weak electron-donor units, such features may allow the delocalization of the HOMO over the whole polymer molecule. Actually, HOMO coefficients delocalize over the end of a PDT moiety, as is evident from DFT calculations on the model compound (Figure S1). Second, a bisthienyl IID motif can easily introduce various alkyl chains at the N-position of IID and the β-position of a thiophene spacer, which can improve the solubility and molecular weight of the polymer. As a result, the easily controlled morphology may lead to a high Jsc. Third, the installation of alkyl chains with different length and topology on the polymer backbone, for instance, the combination of linear−branched and branched−branched alkyl chains, can change their molecular orientation, which can realize a well-defined structure−property relationship to improve PSC performance. In view of published reports,34 we

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EXPERIMENTAL SECTION

General. All the reactions were carried out under an Ar atmosphere using standard Schlenk techniques. Glassware was dried in an oven (130 °C) and heated under reduced pressure prior to use. Dehydrated tetrahydrofuran (THF), N,N-dimethylformamide (DMF), and toluene were purchased from Kanto Chemicals Co., Ltd. For thin layer chromatography (TLC) analyses throughout this work, Merck precoated TLC plates (silica gel 60 GF254, 0.25 mm) were used. Silica gel column chromatography was carried out using silica gel 60 N (spherical, neutral, 40−100 μm) from Kanto Chemicals Co., Ltd. The 1 H and 13C{1H} NMR spectra were recorded on a Varian 400-MR (400 MHz) spectrometer. Infrared spectra were recorded on a Shimadzu IRPrestige-21 spectrophotometer. Elemental analyses were carried out with a PerkinElmer 2400 CHN elemental analyzer at Okayama University. Polymerizations were performed with a Biotage initiator microwave reactor. Molecular weights of polymers were determined by gel-permeation chromatography (GPC) with a Senshu Scientific SSC-7120 using a polystyrene standard and o-dichlorobenzene (o-DCB) as the eluent at 140 °C. 4-Alkyl-2-trimethylstannylthiophenes (1a−1d),46 (E)-6,6′-dibromo1,1′-bis(alkyl)-[3,3′-biindolinylidene]-2,2′-diones (2a−2d),47 and 2,9bis(trimethylstannyl)phenanthro[1,2-b:8,7-b′]dithiophene (5)44 were synthesized according to the reported procedures. All other chemicals were used without further purification unless otherwise indicated. Typical Procedure for the Synthesis of (E)-1,1′-Bis(2decyltetradecyl)-6,6′-bis(4-methylthiophen-2-yl)-[3,3′-biindolinylidene]-2,2′-dione (3a). To a solution of (E)-6,6′-dibromo-1,1′bis(2-decyltetradecyl)-[3,3′-biindolinylidene]-2,2′-dione (2a) (2.5 g, 2.3 mmol) and 4-methyl-2-trimethylstannylthiophene (1a) (1.5 g, 5.7 mmol) in anhydrous toluene (100 mL) was added Pd(PPh3)4 (53 mg, 2 mol %). The mixture was stirred at 100 °C for 24 h. Then the mixture was cooled to room temperature, poured into water (20 mL), and extracted with diethyl ether (50 mL × 3). The combined organic layers were washed with brine and dried over MgSO4. After the B

DOI: 10.1021/acs.macromol.5b00622 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules removal of the volatiles under reduced pressure, the obtained solid was purified by silica gel column chromatography using dichloromethane/ hexane (1/2) as the eluents to afford 3a (Rf = 0.44) as a dark purple solid (2.4 g, 93%); mp 95−97 °C. FT-IR (KBr, cm−1): 2922 (m), 1690 (s), 1610 (s), 1454 (s), 1365 (m), 1108 (m), 855 (m), 812 (s). 1H NMR (400 MHz, CDCl3, rt): δ 0.86 (m, 12H), 1.15−1.44 (m, 68H), 1.87 (br, 2H), 2.31 (s, 6H), 3.61 (d, J = 7.2 Hz, 4H), 6.88 (s, 2H), 6.93 (s, 2H), 7.21 (s, 2H), 7.25 (d, J = 8.4 Hz, 2H), 9.12 (d, J = 8.4 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3, rt): δ 14.1, 15.8, 22.7, 26.6, 29.4, 29.7, 30.0, 31.8, 36.3, 44.4, 104.8, 119.0, 120.9, 121.5, 126.4, 130.1, 131.8, 137.8, 138.9, 143.8, 145.6, 168.5. Anal. Calcd for C74H114N2O2S2: C, 78.80; H, 10.19; N, 2.48. Found: C, 78.85; H, 10.14; N, 2.47. (E)-1,1′-Bis(2-octyldodecyl)-6,6′-bis(4-dodecylthiophen-2-yl)[3,3′-biindolinylidene]-2,2′-dione (3b). Dark purple solid. Yield: 95% (dichloromethane/hexane (1/2) as the eluents: Rf = 0.63); mp 59−61 °C. FT-IR (KBr, cm−1): 2920 (m), 1695 (s), 1610 (s), 1466 (s), 1360 (m), 1114 (m), 860 (m), 823 (m), 717 (s). 1H NMR (400 MHz, CDCl3, rt): δ 0.86 (m, 18H), 1.10−1.50 (m, 100H), 1.65 (m, 4H), 1.98 (br, 2H), 2.63 (t, J = 7.6 Hz, 4H), 3.70 (d, J = 7.2 Hz, 4H), 6.95 (m, 4H), 7.25 (m, 4H), 9.15 (d, J = 7.6 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3, rt): δ 14.1, 22.7, 29.3, 29.6, 30.5, 31.9, 36.4, 44.4, 104.9, 119.0, 120.9, 125.6, 130.1, 131.9, 138.0, 143.7, 144.7, 145.7, 168.7. Anal. Calcd for C88H142N2O2S2: C, 79.82; H, 10.81; N, 2.12. Found: C, 79.75; H, 10.89; N, 2.06. (E)-1,1′-Bis(2-decyltetradecyl)-6,6′-bis(4-dodecylthiophen-2-yl)[3,3′-biindolinylidene]-2,2′-dione (3c). Dark purple solid. Yield: 96% (dichloromethane/hexane (1/2): Rf = 0.62); mp 57−58 °C. FT-IR (KBr, cm−1): 2920 (m), 1687 (s), 1614 (s), 1456 (s), 1361 (m), 1112 (m), 868 (s), 840 (s), 812 (s), 721 (s). 1H NMR (400 MHz, CDCl3, rt): δ 0.87 (m, 18H), 1.20−1.50 (m, 116H), 1.66 (m, 4H), 1.98 (br, 2H), 2.63 (t, J = 7.6 Hz, 4H), 3.69 (d, J = 7.2 Hz, 4H), 6.95 (m, 4H), 7.26 (m, 4H), 9.15 (d, J = 8.4 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3, rt): δ 14.1, 22.7, 26.6, 29.4, 29.7, 30.5, 31.8, 36.4, 44.4, 104.9, 119.0, 120.8, 125.5, 130.1, 131.9, 138.0, 143.7, 144.7, 145.6, 168.7. Anal. Calcd for C96H158N2O2S2: C, 80.27; H, 11.09; N, 1.95. Found: C, 80.27; H, 10.94; N, 1.95. (E)-1,1′-Bis(2-butyloctyl)-6,6′-bis(4-(2-butyloctyl)thiophen-2-yl)[3,3′-biindolinylidene]-2,2′-dione (3d). Dark purple solid. Yield: 85% (dichloromethane/hexane (1/2): Rf = 0.65); mp 65−66 °C. FT-IR (KBr, cm−1): 2926 (m), 1693 (s), 1609 (s), 1454 (s), 1348 (m), 1111 (s), 845 (s), 820 (s), 725 (s). 1H NMR (400 MHz, CDCl3, rt): δ 0.86 (m, 24H), 1.20−1.48 (m, 64H), 1.63 (br, 2H), 1.95 (br, 2H), 2.57 (d, J = 6.8 Hz, 4H), 3.69 (d, J = 7.2 Hz, 4H), 6.92 (s, 2H), 6.95 (s, 2H), 7.21 (s, 2H), 7.27 (d, J = 8.4 Hz, 2H), 9.14 (d, J = 8.4 Hz, 2H). 13 C{1H} NMR (100 MHz, CDCl3, rt): δ 14.1, 22.6, 26.6, 28.8, 29.6, 31.5, 31.9, 33.0, 33.3, 35.0, 36.4, 38.8, 44.4, 104.9, 119.0, 120.9, 121.7, 126.0, 130.0, 131.9, 138.0, 143.4, 145.6, 168.7. Anal. Calcd for C72H110N2O2S2: C, 78.63; H, 10.08; N, 2.55. Found: C, 78.58 H, 9.83; N, 2.54. (E)-1,1′-Bis(2-hexyldecyl)-6,6′-bis(4-(2-hexyldecyl)thiophen-2-yl)[3,3′-biindolinylidene]-2,2′-dione (3e). Dark purple solid. Yield: 96% (dichloromethane/hexane (1/2): Rf = 0.59); mp 42−43 °C. FT-IR (KBr, cm−1): 2924 (m), 1693 (s), 1614 (s), 1458 (s), 1361 (m), 1112 (s), 867 (s), 816 (m), 721 (s). 1H NMR (400 MHz, CDCl3, rt): δ 0.86 (m, 24H), 1.20−1.50 (m, 96H), 1.65 (br, 2H), 1.93 (br, 2H), 2.56 (d, J = 6.8 Hz, 4H), 3.70 (d, J = 7.2 Hz, 4H), 6.91 (s, 2H), 6.96 (s, 2H), 7.20 (s, 2H), 7.27 (d, J = 8.4 Hz, 2H), 9.16 (d, J = 8.4 Hz, 2H). 13 C{1H} NMR (100 MHz, CDCl3, rt): δ 14.1, 22.6, 26.6, 29.3, 29.6, 30.0, 31.8, 33.3, 35.1, 36.4, 38.9, 44.4, 104.9, 119.0, 120.9, 121.7, 126.0, 130.1, 131.9, 138.0, 143.4, 145.7, 168.7. Anal. Calcd for C88H142N2O2S2: C, 79.82; H, 10.81; N, 2.12. Found: C, 80.18; H, 11.07; N, 2.21. Typical Procedure for the Synthesis of (E)-1,1′-Bis(2decyltetradecyl)-6,6′-bis(5-bromo-4-methylthiophen-2-yl)[3,3′-biindolinylidene]-2,2′-dione (4a). To a solution of 3a (230 mg, 0.2 mmol) in freshly distilled THF (5 mL), was slowly added Nbromosuccinimide (NBS) (35.6 mg, 0.2 mmol) in THF (2.5 mL) over 10 min at room temperature, and the mixture was stirred for 4 h in the dark. The mixture was cooled down to 0 °C, and another portion of

NBS (35.6 mg, 0.2 mmol) in THF (2.5 mL) was slowly added; the resulting mixture was stirred for an additional 1 h. Then the reaction mixture was quenched by water and extracted with diethyl ether (15 mL × 2). The combined organic layers were washed with brine and dried over MgSO4. After the removal of the solvent by a rotary evaporator, the resulting solid was purified by column chromatography on silica gel eluted with hexane/dichloromethane (2/1) to give 4a (Rf = 0.51) as dark purple solid (220 mg, 86%); mp 115−117 °C. FT-IR (KBr, cm−1): 2922 (m), 1693 (s), 1613 (s), 1460 (s), 1113 (s), 1030 (s), 868 (s), 812 (s). 1H NMR (400 MHz, CDCl3, rt): δ 0.84−0.87 (m, 12H), 1.22−1.45 (m, 64H), 1.56 (m, 4H), 1.91 (s, 2H), 2.24 (s, 6H), 3.68 (d, J = 7.2 Hz, 4H), 6.85 (s, 2H), 7.08 (s, 2H), 7.17 (d, J = 8.8 Hz, 2H), 9.14 (d, J = 8.4 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3, rt): δ 14.1, 15.3, 22.7, 26.6, 29.4, 29.7, 30.0, 31.7, 32.0, 36.3, 44.5, 104.5, 110.3, 118.7, 121.2, 126.0, 130.3, 132.0, 137.0, 142.8, 143.1, 145.8, 168.6. Anal. Calcd for C74H112Br2N2O2S2: C, 69.13; H, 8.78; N, 2.18. Found: C, 69.24; H, 8.40; N, 2.12. Synthesis of (E)-1,1′-Bis(2-octyldodecyl)-6,6′-bis(5-bromo-4dodecylthiophen-2-yl)-[3,3′-biindolinylidene]-2,2′-dione (4b). To a solution of 3b (286 mg, 0.2 mmol) in freshly distilled THF (5 mL) was slowly added a solution of N-bromosuccinimide (NBS) (71.2 mg, 0.4 mmol) in THF (5 mL) at room temperature in the dark. After 24 h, the mixture was quenched with water, and the organic phase was extracted with diethyl ether (15 mL × 2). The combined organic layers were washed with brine and dried over MgSO4. After the removal of solvent by a rotary evaporator, the resulting solid was purified by column chromatography on silica gel eluted with hexane/dichloromethane (2/1) to give 4b (Rf = 0.65) as a purple solid (273 mg, 86%); mp 85−86 °C. FT-IR (KBr, cm−1): 2920 (m), 1691 (s), 1614 (s), 1460 (s), 1351 (m), 1112 (m), 868 (s), 816 (m), 721 (s). 1H NMR (400 MHz, CDCl3, rt): δ 0.86 (m, 18H), 1.10−1.50 (m, 100H), 1.61 (m, 4H), 1.96 (br, 2H), 2.58 (t, J = 7.6 Hz, 4H), 3.69 (d, J = 7.2 Hz, 4H), 6.86 (s, 2H), 7.09 (s, 2H), 7.17 (d, J = 8.4 Hz, 2H), 9.15 (d, J = 8.4 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3, rt): δ 14.1, 22.7, 26.6, 29.3, 29.6, 30.0, 31.8, 36.4, 44.4, 104.5, 109.9, 118.6, 121.1, 124.9, 130.3, 131.9, 137.0, 143.2, 143.6, 145.7, 168.5. Anal. Calcd for C88H140Br2N2O2S2: C, 71.32; H, 9.52; N, 1.89. Found: C, 71.34; H, 9.76; N, 1.86. Isoindigo monomers 4c−4e were synthesized by the same procedure as 4b. (E)-1,1′-Bis(2-decyltetradecyl)-6,6′-bis(5-bromo-4-dodecylthiophen-2-yl)-[3,3′-biindolinylidene]-2,2′-dione (4c). Dark purple solid. Yield: 86% (hexane/dichloromethane (2/1): Rf = 0.73); mp 85−86 °C. FT-IR (KBr, cm−1): 2920 (m), 1693 (s), 1614 (s), 1466 (s), 1103 (s), 867 (s), 815 (m), 721 (s), 619 (s). 1H NMR (400 MHz, CDCl3, rt): δ 0.85 (m, 18H), 1.20−1.50 (m, 116H), 1.63 (m, 4H), 1.96 (br, 2H), 2.58 (t, J = 7.6 Hz, 4H), 3.68 (d, J = 6.8 Hz, 4H), 6.85 (s, 2H), 7.08 (s, 2H), 7.18 (d, J = 8.4 Hz, 2H), 9.14 (d, J = 8.4 Hz, 2H). 13 C{1H} NMR (100 MHz, CDCl3, rt): δ 14.1, 22.7, 26.7, 29.3, 29.6, 30.0, 31.8, 36.4, 44.4, 104.5, 109.9, 118.7, 121.2, 125.0, 130.3, 132.0, 137.1, 143.3, 143.6, 145.8, 168.6. Anal. Calcd for C96H156Br2N2O2S2: C, 72.33; H, 9.86; N, 1.76. Found: C, 72.36; H, 9.83; N, 1.77. (E)-1,1′-Bis(2-butyloctyl)-6,6′-bis(5-bromo-4-(2-butyloctyl)thiophen-2-yl)-[3,3′-biindolinylidene]-2,2′-dione (4d). Dark purple solid. Yield: 85% (hexane/dichloromethane (2/1): Rf = 0.60); mp 121−122 °C. FT-IR (KBr, cm−1): 2926 (m), 1692 (s), 1614 (s), 1458 (s), 1109 (m), 869 (s), 810 (m). 1H NMR (400 MHz, CDCl3, rt): δ 0.87 (m, 24H), 1.20−1.50 (m, 64H), 1.70 (br, 2H), 1.90 (br, 2H), 2.52 (d, J = 7.2 Hz, 4H), 3.68 (d, J = 7.2 Hz, 4H), 6.84 (s, 2H), 7.04 (s, 2H), 7.18 (d, J = 8.4 Hz, 2H), 9.15 (d, J = 8.4 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3, rt): δ 14.1, 22.7, 23.1, 26.5, 29.7, 31.5, 31.9, 33.0, 33.3, 34.3, 36.4, 38.5, 44.4, 104.5, 110.6, 118.6, 121.2, 125.5, 130.3, 131.9, 137.1, 142.8, 143.0, 145.8, 168.6. Anal. Calcd for C72H108Br2N2O2S2: C, 68.76; H, 8.66; N, 2.23. Found: C, 68.89; H, 8.46; N, 2.31. (E)-1,1′-Bis(2-hexyldecyl)-6,6′-bis(5-bromo-4-(2-hexyldecyl)thiophen-2-yl)-[3,3′-biindolinylidene]-2,2′-dione (4e). Dark purple solid. Isolated yield: 88% (hexane/dichloromethane (2/1): Rf = 0.63); mp 93−94 °C. FT-IR (KBr, cm−1): 2924 (m), 1689 (s), 1614 (s), 1458 (s), 1112 (m), 870 (s), 810 (m), 721 (s). 1H NMR (400 MHz, C

DOI: 10.1021/acs.macromol.5b00622 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules CDCl3, rt): δ 0.86 (m, 24H), 1.20−1.50 (m, 96H), 1.70 (br, 2H), 1.90 (br, 2H), 2.51 (d, J = 7.2 Hz, 4H), 3.68 (d, J = 7.2 Hz, 4H), 6.84 (s, 2H), 7.04 (s, 2H), 7.18 (d, J = 8.4 Hz, 2H), 9.15 (d, J = 8.4 Hz, 2H). 13 C{1H} NMR (100 MHz, CDCl3, rt): δ 14.1, 22.7, 26.6, 29.3, 29.7, 30.0, 31.8, 33.4, 34.6, 36.4, 38.6, 44.4, 104.5, 110.5, 118.6, 121.7, 125.5, 130.3, 132.0, 137.1, 142.8, 143.0, 145.8, 168.5. Anal. Calcd for C88H140Br2N2O2S2: C, 71.32; H, 9.52; N, 1.89. Found: C, 71.08; H, 9.56; N, 1.82. Typical Procedure for the Synthesis of Polymers: Poly[phenanthro[1,2-b:8,7-b′]dithiophene-2,9-diyl-alt-(E)-1,1′-bis(2-decyltetradecyl)-bis(4-methylthiophe-2-yl)-(3,3′-biindolinylidene)-2,2′-dione-6,6′-diyl] (1DT). Monomers 4a (129 mg, 0.1 mmol) and 5 (61.6 mg, 0.1 mmol), Pd(PPh3)4 (2.4 mg, 2 μmol), and toluene (5 mL) were added to a reaction vessel, which was sealed and refilled with argon, and was heated at 180 °C for 40 min in a microwave reactor. After being cooled to room temperature, the reaction mixture was poured into 100 mL of methanol containing 5 mL of concentrated hydrochloric acid and stirred for 3 h. The precipitate was then subjected to sequential Soxhlet extraction with methanol, hexane, and chloroform to remove low-molecular-weight fractions. The residue was extracted with chlorobenzene, and concentrated solution was poured into 100 mL of methanol. The formed precipitates were collected by filtration and dried in vacuo to afford the title polymer (6 mg, 9%) as a pale purple solid. GPC (oDCB, 140 °C): Mn = 9.4 kDa, Mw = 11.9 kDa, PDI = 1.27. Anal. Calcd for C92H122N2O2S4: C, 78.02; H, 8.68; N, 1.98. Found: C, 79.21; H, 9.42; N, 2.29. Poly[phenanthro[1,2-b:8,7-b′]dithiophen-2,9-diyl-alt-(E)-1,1′-bis(2-decyltetradecyl)-bis(4-dodecylthiophen-2-yl)-(3,3′-biindolinylidene)-2,2′-dione-6,6′-diyl] (12OD). Monomers 4b (148 mg, 0.1 mmol) and 5 (61.6 mg, 0.1 mmol), Pd(PPh3)4 (2.4 mg, 2 μmol), and toluene (5 mL) were subjected to the polymerization procedure, and sequential Soxhlet extraction with the same solvents used for 1DT was employed to obtain the title polymer (90 mg, 54%) as a metallic purple solid. GPC (o-DCB, 140 °C): Mn = 26.8 kDa, Mw = 37.3 kDa, PDI = 1.39. Anal. Calcd for C106H150N2O2S4: C, 78.95; H, 9.38; N, 1.74. Found: C, 78.56; H, 9.01; N, 1.74. Poly[phenanthro[1,2-b:8,7-b′]dithiophen-2,9-diyl-alt-(E)-1,1′-bis(2-decyltetradecyl)-bis(4-dodecylthiophen-2-yl)-(3,3′-biindolinylidene)-2,2′-dione-6,6′-diyl] (12DT). Monomers 4c (159 mg, 0.1 mmol) and 5 (61.6 mg, 0.1 mmol), Pd(PPh3)4 (2.4 mg, 2 μmol), and toluene (5 mL) were subjected to the polymerization procedure, and sequential Soxhlet extraction with methanol, hexane, and then chloroform was employed to obtain the title polymer (160 mg, 96%) as a bright purple solid. GPC (o-DCB, 140 °C): Mn = 13.5 kDa, Mw = 20.5 kDa, PDI = 1.52. Anal. Calcd for C114H166N2O2S4: C, 79.38; H, 9.70; N, 1.62. Found: C, 78.43; H, 9.59; N, 1.59. Poly[phenanthro[1,2-b:8,7-b′]dithiophen-2,9-diyl-alt-(E)-1,1′-bis(2-butyloctyl)-bis(4-(2-butyloctyl)thiophen-2-yl)-(3,3′-biindolinylidene)-2,2′-dione-6,6′-diyl] (BOBO). Monomers 4d (126 mg, 0.1 mmol) and 5 (61.6 mg, 0.1 mmol), Pd(PPh3)4 (2.4 mg, 2 μmol), and toluene (5 mL) were subjected to the polymerization procedure, and sequential Soxhlet extraction with the same solvents used for 1DT was employed to obtain the title polymer (108 mg, 78%) as a metallic purple solid. GPC (o-DCB, 140 °C): Mn = 56.9 kDa, Mw = 106.1 kDa, PDI = 1.87. Anal. Calcd for C90H118N2O2S4: C, 77.87; H, 8.57; N, 2.02. Found: C, 77.23; H, 7.76; N, 1.93. Poly[phenanthro[1,2-b:8,7-b′]dithiophen-2,9-diyl-alt-(E)-1,1′-bis(2-hexyldecyl)-bis(4-(2-hexyldecyl)thiophen-2-yl)-(3,3′-biindolinylidene)-2,2′-dione-6,6′-diyl] (HDHD). Monomers 4e (148 mg, 0.1 mmol) and 5 (61.6 mg, 0.1 mmol), Pd(PPh3)4 (2.4 mg, 2 μmol), and toluene (5 mL) were subjected to the polymerization procedure, and sequential Soxhlet extraction with methanol, hexane, and then chloroform was employed to obtain the title polymer (144 mg, 90%) as a bright purple solid. GPC (o-DCB, 140 °C): Mn = 19.0 kDa, Mw = 28.7 kDa, PDI = 1.51. Anal. Calcd for C106H154N2O2S4: C, 78.75; H, 9.60; N, 1.73. Found: C, 78.75; H, 9.46; N, 1.69. Instrumentation and Theoretical Calculations. Differential scanning calorimetry (DSC) was performed on a Mettler Toledos DSC-1 at 10 °C/min for both heating and cooling steps. UV−vis absorption spectra were measured using a Shimadzu UV-2450 UV−vis

spectrometer. The PYS spectra of thin films were measured using a Bunkokeiki BIP-KV201AD photoelectron yield measurement system. Cyclic voltammograms (CVs) were recorded on an electrochemical analyzer (CHI-600B) in acetonitrile containing tetrabutylammonium hexafluorophosphate (TBAP, 0.1 M) as supporting electrolyte at a scan rate of 100 mV/s. A Pt electrode (surface area: A = 0.071 cm2, BAS), an Ag/Ag+ (Ag wire in 0.01 M AgNO3/0.1 M TBAP/CH3CN), and a Pt wire electrode were used as working, reference, and counter electrodes, respectively. Samples of the polymer films were prepared by drop-casting on a working electrode from their chloroform solutions. All the potentials were calibrated with the standard ferrocene/ferrocenium redox couple (Fc/Fc+: E1/2 = +0.08 V measured under identical conditions). Dynamic force-mode atomic force microscopy was carried out using an SPA 400-DFM (SII Nano Technologies). Two-dimensional grazing incidence X-ray diffraction (2D-GIXD) analyses were carried out at SPring-8 on beamlines BL19B2 and BL46XU. The samples were irradiated at a fixed angle on the order of 0.12° through a Huber diffractometer with an X-ray energy of 12.39 keV (λ = 1 Å), and the GIXD patterns were recorded on a 2D image detector (Pilatus 300 K). Films of the polymers with or without PC61BM were fabricated by spin-coating on the (PEDOT:PSS) or ZnO-treated ITO substrate. Geometry optimizations and normal-mode calculations were performed at the B3LYP/631G(d) level using the Gaussian 09, Revision A. 02, program package.48 Fabrication and Characterization of Hole-Only Devices. The ITO substrates (ITO, Geomatec Co. Ltd., thickness = 150 nm, sheet resistance