Phenanthrodithiophene (PDT)–Difluorobenzothiadiazole (DFBT

Feb 9, 2018 - Comparison of alkyl side chain length at the 4,7-positions of PDT ... to have low surface roughness and small domain size of active laye...
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Article Cite This: Macromolecules 2018, 51, 1357−1369

Phenanthrodithiophene (PDT)−Difluorobenzothiadiazole (DFBT) Copolymers: Effect on Molecular Orientation and Solar Cell Performance of Alkyl Substitution onto a PDT Core Hiroki Mori,† Ryosuke Takahashi,‡ Keita Hyodo,‡ Shuhei Nishinaga,‡ Yuta Sawanaka,‡ and Yasushi Nishihara*,† †

Research Institute for Interdisciplinary Science and ‡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 S Supporting Information *

ABSTRACT: Synthesis, characterization, and solar cell application of three 4,7-dialkylated phenanthro[1,2-b:8,7-b′]dithiophene (PDT)−difluorobenzothiadiazole (DFBT copolymers (P1−P3) with different linear alkyl side chains to improve solubility, molecular weight, and molecular orientation are described. The utilization of Ir-catalyzed direct borylation and sequential functionalization can selectively afford the target 4,7dialkylated PDT as the monomers. Migita−Kosugi−Stille coupling in the presence of CuI can accelerate polymerization to afford high-molecular-weight polymers along with their improved solubility. The effect of alkyl substitution at the 4,7-positions on the electronic structure of PDT−DFBT copolymers is negligible. By installation of additional alkyl chains at the 4,7-positions of PDT, the synthesized polymers P1−P3 have lower intermolecular interaction than that of nonalkylated P0, but they still maintained aggregation behavior in solution. In addition, they formed a favorable face-on orientation with a short π-stacking distance of 3.6 Å, which can enhance their carrier transport ability, resulting in high Jsc and FF. As a result, their fabricated solar cells reached a PCE exceeding 6%, which are about 1.7-fold higher than that of P0. Comparison of alkyl side chain length at the 4,7-positions of PDT revealed that all polymers formed a predominantly face-on orientation and have a similar face-on ratio in blended films, but their crystallinity was decreased as the carbon chains at the 4,7-positions of PDT became shorter. On the other hand, the polymers with short alkyl side chains tended to have low surface roughness and small domain size of active layers, which is an ideal phase separation structure for highperformance PSCs. From these results, it could be seen that the polymers have a trade-off relationship between their domain size and crystallinity, but the impact of alkyl side chain length on their photovoltaic properties is rather small. Thus, the construction of face-on orientation is highly important to achieve a high PCE. Among three polymers, the P3/PC61BM-based solar cell with an optimal nanoscale phase separation structure with bicontinuous domain showed the highest PCE of up to 6.6%.

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INTRODUCTION In semiconducting polymer/C60-based solar cells (PSCs), the control of molecular orientation is the most crucial issue to achieving high performance.1−3 In general, typical semiconducting polymers exhibit one-dimensional (1D) anisotropic carrier transport, which basically occurs through a π−π stacking direction.4,5 Hence, the construction of the optimal arrangement in thin films often achieves efficient carrier transport, leading to high short-circuit current density (Jsc) and fill factor (FF). Since typical PSCs have their anode and cathode electrodes arranged in an interlaced pattern, polymers should form a face-on orientation in an out-of-plane π-stacking manner, where the polymers form a horizontally aligned structure relative to the substrate. Indeed, fabricated solar cells based on polymers having a face-on arrangement often exhibit significantly high FF, resulting in high PCE.6−15 Moreover, another important factor is the construction of a highcrystalline face-on arrangement with a long-range ordered © 2018 American Chemical Society

structure in the solid state. This is important for the following reasons. In general, when the thickness of the fabricated PSCs is around 300 nm, the solar cell devices exhibit enhanced light absorption, leading to higher EQE values than that of thin-film PSCs (∼100 nm), resulting in high Jsc and PCE. In addition, such a well-ordered face-on structure can transport the carrier effectively, even in thick-film PSCs.7−11,13,14,16,17 In particular, FF of the solar cells with a low-crystalline face-on structure heavily depends on the active layer thickness, but solar cells based on well-ordered polymers with thick-film PSCs (∼300 nm) exhibit a high Jsc without any loss of FF. Thus, the construction of a fully face-on orientation with a long-range ordered structure is highly desired for high-performance PSCs. Received: December 25, 2017 Revised: January 23, 2018 Published: February 9, 2018 1357

DOI: 10.1021/acs.macromol.7b02734 Macromolecules 2018, 51, 1357−1369

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Figure 1. Chemical structures of PDT−DFBT copolymers.

leading to limited molecular weight (∼15.9 kDa).36 In addition, although their solar cells formed an optimal phase separation structure, the construction of an entirely edge-on orientation prevents efficient carrier transport, resulting in limited Jsc (∼8 mA cm−2) and FF.36 Thus, we have investigated how to improve their solubility, molecular weight, and molecular orientation while retaining the high crystalline and long-range ordered structure to realize high-performance PSCs. In this study, in order to overcome the above problems, we designed 4,7-dialkylated PDT-DFBT copolymers (Figure 1). Introduction of additional alkyl side chains into a polymer backbone can increase solubility, leading to high molecular weight. In addition, an increase in side chain attachment density by installation of alkyl chains can improve its molecular orientation similar to reports in the published literature.1,23−25,27 Moreover, 4,7-positions of PDT are a sterically less congested position, which may help to maintain their high crystalline and long-range ordered structure in the solid state. Herein, we report the synthesis, characterizations, and application of 4,7-dialkylated PDT-DFBT copolymers to PSCs to achieve high-performance PSCs. In addition, to establish the optimal side chain, three 4,7-dialkylated PDTDFBT copolymers with different linear side chains at the 4,7positions were synthesized and characterized. Finally, we evaluated the relationship between their molecular orientations in the solid state and the obtained solar cell performance.

Several researchers have demonstrated the construction of a favorable face-on orientation by utilizing side-chain engineering.1,18−20 Although there is not a systematic understanding of their precise driving force, the effects of alkyl chain installation have become clear. One of the most effective methods to facilitate face-on orientation is to control side-chain attachment density.1,21−27 For instance, DeLongchamp and McCulloch reported diketopyrrolopyrrole (DPP)-based copolymers with different side-chain attachment density to control their molecular orientation.21 By changing the length of the oligothiophene unit of DPP-based polymers, their molecular orientation can be changed from a preferentially edge-on to a predominantly face-on orientation. According to the literature, the edge-on orientation is basically favorable from a thermodynamic standpoint, but polymers with high side-chain attachment density may prevent effective π−π stacking and a well-ordered lamellar structure. Consequently, the face-on orientation may be more energetically favorable, as the polymers form a metastable face-on structure. In 2013, Osaka and Takimiya reported the control of molecular orientation in the naphthodithiophene (NDT)−naphthobisthiadiazole (NTz) copolymer by installation of additional alkyl side chains to increase side-chain attachment density.24 By installation of dodecyl chains into the 5,10-positions of an NDT core, the NDT−NTz copolymers predominantly formed a face-on orientation, whereas a nonsubstituted NDT−NTz copolymer exhibited an edge-on orientation. The construction of a favorable face-on orientation provided 1 order of magnitude higher hole mobility, leading to high Jsc without any loss of FF under 300 nm thick conditions, resulting in high PCE. The introduction of linear side chains onto an NDT core may also prevent an interdigitated structure between neighboring polymer backbones, leading to slow formation of a wellordered lamellar structure, resulting in an ordered face-on orientation. Thus, the control of side-chain attachment density is a powerful method to afford a face-on orientation. Recently, we reported the synthesis, characterization, and solar cell and transistor applications of newly developed phenanthro[1,2-b:8,7-b′]dithiophene (PDT)28−35-based semiconducting polymers combined with a dialkylated dithienyldifluorobenzothiadiazole (DFBT) known as a common acceptor unit.36 The synthesized PDT−DFBT polymers (P0, Figure 1) have some outstanding features: strong aggregation behavior, high thermal stability, and short π−π stacking distance (3.5−3.6 Å) that originates from the extended π-electron system and rigid structure of PDT.36 In addition, they formed a high crystalline and densely packed thin film with a long-range ordered structure in the solid state, which is favorable for high-performance PSCs. However, such strong intermolecular interaction drastically decreased their solubility,

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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), N-methylpyrrolidone (NMP), cyclohexane, methanol, 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, 13C{1H}, 11B{1H}, and 29Si{1H} NMR spectra were recorded on a Varian 400-MR (400 MHz) and Varian INOVA-600 (600 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 TOSOH HLC8321GPC/HT and TSKgel GMHHR-H HT using a polystyrene standard and o-dichlorobenzene (o-DCB) as the eluent at 140 °C. Phenanthro[1,2-b:8,7-b′]dithiophene (1),29 4,7-bis(5-bromo-4-(2hexyldecyl)thiophen-2-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole (8),37 and CuI nanoparticle38 were synthesized according to the 1358

DOI: 10.1021/acs.macromol.7b02734 Macromolecules 2018, 51, 1357−1369

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reaction mixture was refluxed for 7 h, quenched with water, and extracted with chloroform. The combined organic layers were washed with brine and dried over MgSO4. After removal of the solvent by a rotary evaporator, the resulting product was purified by column chromatography on silica gel eluted with hexane to give 5a (Rf = 0.69) as a colorless liquid (755 mg, 96%). FT-IR (KBr, cm−1): 2924 (w), 2854 (w), 1573 (s), 1454 (w), 1382 (s), 999 (m), 883 (m), 650 (m), 500 (s). 1H NMR (600 MHz, CDCl3, rt): δ 0.92 (t, J = 7.2 Hz, 6H), 1.26 (d, J = 7.8 Hz, 36H), 1.30−1.37 (m, 28H), 1.45 (quin, J = 7.8 Hz, 4H), 1.51−1.57 (m, 10H), 1.93 (quin, J = 7.8 Hz, 4H), 3.22 (t, J = 7.8 Hz, 4H), 7.77 (s, 2H), 8.17 (s, 2H), 8.42 (s, 2H). 13C{1H} NMR (150 MHz, CDCl3, rt): δ 11.9, 14.1, 18.7, 22.7, 29.4, 29.6, 29.66, 29.69, 29.73, 29.75, 29.8, 31.1, 31.9, 35.0, 119.1, 122.8, 125.5, 127.5, 131.5, 134.8, 136.2, 138.6, 143.2. 29Si{1H} NMR (119 MHz, CDCl3, rt): δ 2.16. Anal. Calcd for C60H98S2Si2: C, 76.69; H, 10.51%. Found: C, 76.72; H, 10.69%. 4,7-Didecyl-2,9-bis(triisopropylsilyl)phenanthro[1,2-b:8,7-b′]dithiophene (5b). Colorless liquid. Yield: 72% (hexane: Rf = 0.67). FT-IR (KBr, cm−1): 2930 (w), 2862 (w), 1572 (s), 1454 (w), 1368 (s), 999 (m), 883 (m), 650 (m), 501 (s). 1H NMR (600 MHz, CDCl3, rt): δ 0.88 (t, J = 7.2 Hz, 6H), 1.21 (d, J = 7.2 Hz, 36H), 1.26−1.33 (m, 20H), 1.41 (quin, J = 7.8 Hz, 4H), 1.46−1.55 (m, 10H), 1.89 (quin, J = 7.8 Hz, 4H), 3.20 (t, J = 7.8 Hz, 4H), 7.73 (s, 2H), 8.16 (s, 2H), 8.41 (s, 2H). 13C{1H} NMR (150 MHz, CDCl3, rt): δ 12.0, 14.3, 18.8, 22.8, 29.5, 29.75, 29.78, 29.84, 29.92, 31.3, 32.1, 35.1, 119.3, 123.0, 125.7, 127.7, 131.7, 135.1, 136.4, 138.7, 143.3. 29Si{1H} NMR (119 MHz, CDCl3, rt): δ 1.94. Anal. Calcd for C56H90S2Si2: C, 76.12; H, 10.27%. Found: C, 76.24; H, 10.31%. 4,7-Dioctyl-2,9-bis(triisopropylsilyl)phenanthro[1,2-b:8,7-b′]dithiophene (5c). Colorless liquid. Yield: 78% (hexane: Rf = 0.72). FT-IR (KBr, cm−1): 2922 (w), 2864 (w), 1570 (s), 1460 (m), 1381 (s), 999 (m), 841 (m), 648 (m), 500 (s). 1H NMR (400 MHz, CDCl3, rt): δ 0.90 (t, J = 7.2 Hz, 6H), 1.22 (d, J = 7.2 Hz, 36H), 1.28−1.34 (m, 12H), 1.40−1.55 (m, 20H), 1.90 (quin, J = 7.6 Hz, 4H), 3.20 (t, J = 7.6 Hz, 4H), 7.74 (s, 2H), 8.16 (s, 2H), 8.41 (s, 2H). 13C{1H} NMR (100 MHz, CDCl3, rt): δ 12.1, 14.3, 18.8, 22.8, 29.5, 29.7, 29.9, 31.3, 32.1, 35.1, 119.2, 123.0, 125.7, 127.7, 131.7, 135.1, 136.4, 138.7, 143.3. 29 Si{1H} NMR (119 MHz, CDCl3, rt): δ 2.51. Anal. Calcd for C52H82S2Si2: C, 75.47; H, 9.99%. Found: C, 75.29; H, 9.84%. A Representative Synthetic Procedure for 6a−6c. Synthesis of 4,7-Didodecylphenanthro[1,2-b:8,7-b′]dithiophene (6a). A solution of 5a (507 mg, 0.54 mmol) and TBAF (1 M in THF, 5.4 mL, 5.4 mmol) in freshly distilled THF (22 mL) was stirred for 15 h. After the reaction, the mixture was quenched with water and extracted with chloroform. The combined organic layers were washed with brine and dried over MgSO4. After removal of solvent by a rotary evaporator, the resulting solid was purified by column chromatography on silica gel eluted with hexane to give 6a (Rf = 0.52) as a white solid (315 mg, 93%); mp: 84−86 °C. FT-IR (KBr, cm−1): 3039 (m), 3072 (s), 3043 (s), 2954 (w), 2914 (w), 2848 (w), 1577 (s), 1469 (w), 1344 (s), 1153 (m), 854 (w), 802 (w), 694 (w). 1H NMR (600 MHz, CDCl3, rt): δ 0.89 (t, J = 7.8 Hz, 6H), 1.22−1.35 (m, 28H), 1.41 (quin, J = 7.8 Hz, 4H), 1.51 (quin, J = 7.8 Hz, 4H), 1.87 (quin, J = 7.8 Hz, 4H), 3.17 (t, J = 7.8 Hz, 4H), 7.56 (d, J = 5.4 Hz, 2H), 7.60 (d, J = 5.4 Hz, 2H), 8.11 (s, 2H), 8.43 (s, 2H). 13C{1H} NMR (150 MHz, CDCl3, rt): δ 14.1, 22.7, 29.4, 29.6, 29.65, 29.69, 29.71, 29.8, 31.1, 31.9, 35.0, 119.2, 122.6, 123.0, 125.1, 125.7, 127.7, 136.6, 137.3, 139.0. Anal. Calcd for C42H58S2: C, 80.45; H, 9.32%. Found: C, 80.51; H, 9.17%. 4,7-Didecylphenanthro[1,2-b:8,7-b′]dithiophene (6b). White solid. Yield: 87% (hexane: Rf = 0.36); mp: 87−89 °C. FT-IR (KBr, cm−1): 3075 (s), 2951 (m), 2918 (w), 2849 (w), 1467 (m), 1344 (s), 1260 (s), 868 (m), 808 (w), 692 (w). 1H NMR (600 MHz, CDCl3, rt): δ 0.92 (t, J = 7.2 Hz, 6H), 1.31−1.36 (m, 20H), 1.43 (quin, J = 7.8 Hz, 4H), 1.52 (quin, J = 7.8 Hz, 4H), 1.88 (quin, J = 7.8 Hz, 4H), 3.15 (t, J = 7.8 Hz, 4H), 7.55 (d, J = 5.4 Hz, 2H), 7.59 (d, J = 5.4 Hz, 2H), 8.09 (s, 2H), 8.41 (s, 2H). 13C{1H} NMR (150 MHz, CDCl3, rt): δ 14.3, 22.85, 29.52, 29.78, 29.84, 29.85, 30.0, 31.2, 32.1, 35.1, 119.3, 122.7, 123.1, 125.2, 125.8, 127.8, 136.6, 137.4, 139.1. Anal. Calcd for C38H50S2: C, 79.94; H, 8.83%. Found: C, 80.03; H, 9.17%.

reported procedures. All other chemicals were used without further purification unless otherwise indicated. Synthesis of 2,9-Bis(triisopropylsilyl)phenanthro[1,2-b:8,7b′]dithiophene (2). To a solution of phenanthro[1,2-b:8,7-b′]dithiophene (1) (581 mg, 2.0 mmol) in anhydrous THF (36 mL) was added n-BuLi (1.6 M hexane solution, 2.75 mL, 4.4 mmol) at −78 °C. The mixture was stirred at room temperature for 1 h. After cooling to −78 °C, triisopropylsilyl chloride (1.03 mL, 4.8 mmol) was slowly added, and the mixture was refluxed for 24 h. Then the mixture was treated with water and 1 M HCl(aq) and extracted with chloroform. The combined organic layers were washed with brine and dried over MgSO4. After removal of the solvent under reduced pressure, the obtained solid was purified by silica gel column chromatography using hexane as eluent to afford 2 (Rf = 0.38) as a white solid (1.10 g, 92%); mp: 98−100 °C. FT-IR (KBr, cm−1): 2941 (w), 2989 (m), 2864 (w), 1564 (s), 1460 (m), 1284 (m), 1072 (m), 949 (w), 883 (m), 842 (m), 686 (m), 592 (m). 1H NMR (600 MHz, CDCl3, rt): δ 1.20 (d, J = 7.2 Hz, 36H), 1.55 (sept, J = 7.2 Hz, 6H), 7.68 (s, 2H), 8.04 (d, J = 8.4 Hz, 2H), 8.26 (s, 2H), 8.65 (d, J = 9.0 Hz, 2H). 13C{1H} NMR (150 MHz, CDCl3, rt): δ 11.9, 18.7, 120.2, 122.2, 123.8, 126.8, 127.4, 133.5, 135.8, 138.9, 142.9. 29Si{1H} NMR (119 MHz, CDCl3, rt): δ 2.16. Anal. Calcd for C36H50S2Si2: C, 71.70; H, 8.36%. Found: C, 71.72; H, 8.36%. Synthesis of 4,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan2-yl)-2,9-bis(triisopropylsilyl)phenanthro[1,2-b:8,7-b′]dithiophene (3). A solution of [Ir(OMe)(cod)]2 (55 mg, 0.083 mmol, 5 mol %), 4,4′-di-tert-butyl-2,2′-bipyridine (dtbpy) (45 mg, 0.17 mmol, 10 mol %), and bis(pinacolato)diboron (B2pin2) (843 mg, 3.32 mmol) in anhydrous cyclohexane (33 mL) was stirred at room temperature for 10 min. Then, compound 2 (1.00 g, 1.66 mmol) was added, and the reaction mixture was heated at 80 °C for 10 h. After cooling to room temperature, the mixture was treated with water and extracted with chloroform. The combined organic layers were washed with brine and dried over MgSO4. After removal of the solvent under reduced pressure, the obtained solid was purified by silica gel column chromatography using hexane/ethyl acetate (5/1) as eluents to afford 3 (Rf = 0.54) as a white solid (1.18 g, 87%); mp: 174−175 °C. FT-IR (KBr, cm−1): 2943 (w), 2891 (m), 2866 (w), 1587 (m), 1463 (m), 1317 (w), 1303 (w), 1143 (w), 1099 (m), 974 (w), 846 (m), 680 (m), 605 (s). 1H NMR (600 MHz, CDCl3, rt): δ 1.25 (d, J = 7.8 Hz, 36H), 1.48−1.53 (m, 30H), 8.33 (s, 2H), 8.48 (s, 2H), 9.32 (s, 2H). 13C{1H} NMR (150 MHz, CDCl3, rt): δ 12.0, 18.7, 25.1, 83.9, 125.1, 126.9, 128.6, 129.2, 135.1, 136.1, 142.5, 142.7. The carbon signal adjacent to B was not observed due to low intensity. 11B{1H} NMR (192 MHz, CDCl3, rt): δ 31.6. 29Si{1H} NMR (119 MHz, CDCl3, rt): δ 1.96. Anal. Calcd for C48H72B2O4S2Si2: C, 67.43; H, 8.49%. Found: C, 67.14; H, 8.52%. Synthesis of 4,7-Dibromo-2,9-bis(triisopropylsilyl)phenanthro[1,2-b:8,7-b′]dithiophene (4). A solution of 3 (676 mg, 0.79 mmol) and copper(II) bromide (1.06 g, 4.74 mmol) in NMP/MeOH/H2O (15 mL/6 mL/3 mL) was refluxed for 15 h. After cooling to room temperature, the mixture was treated with 1 M HCl(aq). The precipitate was collected by filtration and washed with hexane. After drying in a vacuum, compound 4 was obtained as a white solid (568 mg, 95%); mp: 184−185 °C. FT-IR (KBr, cm−1): 2943 (w), 2889 (m), 2864 (w), 1548 (s), 1460 (m), 1087 (m), 954 (w), 881 (m), 648 (m), 599 (m). 1H NMR (600 MHz, CDCl3, rt): δ 1.22 (d, J = 7.2 Hz, 36H), 1.51 (sept, J = 7.2 Hz, 6H), 7.79 (s, 2H), 8.09 (s, 2H), 8.66 (s, 2H). 13C{1H} NMR (150 MHz, CDCl3, rt): δ 11.9, 18.6, 116.8, 123.0, 123.6, 125.9, 127.3, 133.6, 137.3, 138.7, 143.3. 29Si{1H} NMR (119 MHz, CDCl3, rt): δ 2.55. Anal. Calcd for C36H48Br2S2Si2: C, 56.83; H, 6.36%. Found: C, 56.83; H, 6.30%. A Representative Synthetic Procedure for 5a−5c. Synthesis of 4,7-Didodecyl-2,9-bis(triisopropylsilyl)phenanthro[1,2b:8,7-b′]dithiophene (5a). A solution of 1-dodecene (558 μL, 2.52 mmol) and 9-BBN dimer (314 mg, 1.29 mmol) in THF (17 mL) was heated at 60 °C for 1 h. After cooling to room temperature, to the resultant solution of alkylboron reagent was added 4a (639 mg, 0.84 mmol), Pd(dba)2 (48 mg, 0.08 mmol, 10 mol %), [HPtBu3]BF4 (49 mg, 0.17 mmol, 20 mol %), and KOH (283 mg, 5.04 mmol). The 1359

DOI: 10.1021/acs.macromol.7b02734 Macromolecules 2018, 51, 1357−1369

Article

Macromolecules 4,7-Dioctylphenanthro[1,2-b:8,7-b′]dithiophene (6c). White solid. Yield: 95% (hexane: Rf = 0.23); mp: 92−93 °C. FT-IR (KBr, cm−1): 3097 (m), 3074 (s), 3043 (s), 2954 (m), 2920 (w), 2851 (w), 1576 (s), 1466 (m), 1379 (s), 1153 (m), 804 (w), 698 (w), 588 (s). 1H NMR (400 MHz, CDCl3, rt): δ 0.91 (t, J = 7.2 Hz, 6H), 1.25−1.45 (m, 16H), 1.52 (quin, J = 7.6 Hz, 4H), 1.88 (quin, J = 7.6 Hz, 4H), 3.16 (t, J = 7.6 Hz, 4H), 7.56 (d, J = 5.6 Hz, 2H), 7.60 (d, J = 5.6 Hz, 2H), 8.10 (s, 2H), 8.42 (s, 2H). 13C{1H} NMR (100 MHz, CDCl3, rt): δ 14.3, 22.8, 29.5, 29.7, 30.0, 31.3, 32.1, 35.1, 119.3, 122.7, 123.2, 125.3, 125.8, 127.8, 136.7, 137.5, 139.1. Anal. Calcd for C34H42S2: C, 79.32; H, 8.22%. Found: C, 78.93; H, 8.22%. A Representative Synthetic Procedure for 7a−7c. Synthesis of 4,7-Didodecyl-2,9-bis(trimethylstannyl)phenanthro[1,2b:8,7-b′]dithiophene (7a). To a solution of 6a (376 mg, 0.6 mmol) in THF (24 mL) was slowly added n-BuLi (1.6 M hexane solution, 1.13 mL, 1.8 mmol) at 0 °C. The reaction mixture was stirred and refluxed for 2 h. After cooling at 0 °C, trimethyltin chloride (478 mg, 2.4 mmol) was added, and the mixture was stirred at room temperature for 12 h. The reaction mixture was quenched with water and extracted with dichloromethane. The combined organic layers were washed with brine and dried over MgSO4. After removal of the solvent under reduced pressure, the obtained product was purified by high performance liquid chromatography (HPLC) using chloroform as eluent to afford 7a as a colorless liquid (572 mg, 76%). FT-IR (KBr, cm−1): 2924 (w), 2852 (w), 1571 (s), 1465 (s), 1377 (s), 950 (s), 771 (m), 532 (m). 1H NMR (600 MHz, CDCl3, rt): δ 0.46 (s, JSn−H satellite = 28.8 Hz, 18H), 0.88 (t, J = 7.2 Hz, 6H), 1.22−1.36 (m, 28H), 1.43 (quin, J = 7.8 Hz, 4H), 1.53 (quin, J = 7.8 Hz, 4H), 1.90 (quin, J = 7.8 Hz, 4H), 3.20 (t, J = 7.8 Hz, 4H), 7.65 (s, 2H), 8.15 (s, 2H), 8.40 (s, 2H). 13C{1H} NMR (150 MHz, CDCl3, rt): δ −8.16 (t, JC−Sn = 177 Hz), 14.1, 22.7, 29.4, 29.6, 29.67, 29.69, 29.72, 29.8, 31.0, 31.9, 35.0, 119.1, 122.8, 125.4, 127.2, 130.8, 136.0, 138.6, 138.8, 143.9. Anal. Calcd for C48H74S2Sn2: C, 60.52; H, 7.83%. Found: C, 60.74; H, 7.96%. 4,7-Didecyl-2,9-bis(trimethylstannyl)phenanthro[1,2-b:8,7-b′]dithiophene (7b). Pale yellow oil. Yield: 69%. FT-IR (KBr, cm−1): 2924 (w), 2853 (w), 1572 (s), 1464 (s), 1377 (s), 953 (s), 764 (m), 532 (m). 1H NMR (600 MHz, CDCl3, rt): δ 0.50 (s, JSn−H satellite = 28.8 Hz, 18H), 0.88 (t, J = 7.2 Hz, 6H), 1.27−1.35 (m, 20H), 1.43 (quin, J = 7.8 Hz, 4H), 1.53 (quin, J = 7.8 Hz, 4H), 1.90 (quin, J = 7.8 Hz, 4H), 3.20 (t, J = 7.8 Hz, 4H), 7.65 (s, 2H), 8.15 (s, 2H), 8.40 (s, 2H). 13 C{1H} NMR (150 MHz, CDCl3, rt): δ −8.00 (t, JC−Sn = 177 Hz), 14.3, 22.9, 29.5, 29.75, 29.84, 30.0, 31.2, 32.1, 35.1, 119.2, 123.0, 125.6, 127.4, 131.0, 136.1, 138.8, 138.9, 144.0. Anal. Calcd for C44H66S2Sn2: C, 58.95; H, 7.42%. Found: C, 59.20; H, 7.13%. 4,7-Dioctyl-2,9-bis(trimethylstannyl)phenanthro[1,2-b:8,7-b′]dithiophene (7c). White solid. Yield: 77%; mp: 54−56 °C. FT-IR (KBr, cm−1): 2922 (w), 2850 (w), 1572 (s), 1464 (s), 1306 (s), 945 (s), 770 (m), 531 (m). 1H NMR (400 MHz, CDCl3, rt): δ 0.50 (s, JSn−H satellite = 28.8 Hz, 18H), 0.89 (t, J = 6.8 Hz, 6H), 1.25−1.55 (m, 20H), 1.90 (quin, J = 7.6 Hz, 4H), 3.19 (t, J = 7.6 Hz, 4H), 7.65 (s, 2H), 8.14 (s, 2H), 8.40 (s, 2H). 13C{1H} NMR (150 MHz, CDCl3, rt): δ −8.01 (t, JC−Sn = 177 Hz), 14.3, 22.8, 29.5, 29.7, 29.9, 31.1, 32.1, 35.1, 119.2, 123.0, 125.6, 127.4, 131.0, 136.1, 138.8, 138.9, 144.0. Anal. Calcd for C40H58S2Sn2: C, 57.16; H, 6.96%. Found: C, 57.12; H, 6.93%. A Representative Polymerization Procedure for P1−P3. Synthesis of Polymers P-PDT-DFBT-12HD, P1. Monomers 7a (47.8 mg, 0.05 mmol) and 8 (47.3 mg, 0.05 mmol), CuI (1.0 mg, 5.0 μmol), Pd(PPh3)4 (1.2 mg, 1.0 μmol), and toluene (2.5 mL) were added to a reaction vessel, which was sealed and refilled with argon. The mixture 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 subsequently subjected to sequential Soxhlet extraction with methanol, hexane, and chloroform to remove low-molecular-weight fractions. Finally, the residue was extracted with chlorobenzene, and the concentrated solution was poured into 100 mL of methanol. The obtained precipitates were collected by filtration and dried in vacuo to afford

polymer P1 (66.8 mg, 95%) as a metallic purple solid. GPC (o-DCB, 140 °C): Mn = 47.1 kDa, Mw = 85.7 kDa, PDI = 1.82. Anal. Calcd for C88H126F2N2S5: C, 74.95; H, 9.01; N, 1.99. Found: C, 74.34; H, 8.73; N, 1.85. P2 (P-PDT-DFBT-10HD). Monomers 7b (45.2 mg, 0.05 mmol) and 8 (47.6 mg, 0.05 mmol), CuI (1.0 mg, 5.0 μmol), Pd(PPh3)4 (1.2 mg, 1.0 μmol), and toluene (2.5 mL) were subjected to the polymerization procedure described above. Sequential Soxhlet extraction with methanol, hexane, and chloroform was employed to obtain the polymer P2 (54.4 mg, 80%) as a metallic purple solid. GPC (o-DCB, 140 °C): Mn = 39.2 kDa, Mw = 67.8 kDa, PDI = 1.73. Anal. Calcd for C84H118F2N2S5: C, 74.50; H, 8.78; N, 2.07. Found: C, 74.79; H, 8.56; N, 2.03. P3 (P-PDT-DFBT-8HD). Monomers 7c (42.6 mg, 0.05 mmol) and 8 (47.9 mg, 0.05 mmol), CuI (1.0 mg, 5.0 μmol), Pd(PPh3)4 (1.2 mg, 1.0 μmol), and toluene (2.5 mL) were subjected to the polymerization procedure. Sequential Soxhlet extraction with methanol, hexane, chloroform, and chlorobenzene was employed to obtain the polymer P3 (63.7 mg, 97%) as a metallic purple solid. GPC (o-DCB, 140 °C): Mn = 46.1 kDa, Mw = 81.0 kDa, PDI = 1.76. Anal. Calcd for C80H110F2N2S5: C, 74.02; H, 8.54; N, 2.16. Found: C, 72.67; H, 8.38; N, 2.01. Instrumentation and Theoretical Calculations. UV−vis absorption spectra were measured using a Shimadzu UV-2450 UV− vis spectrometer. Cyclic voltammograms (CVs) were recorded on 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+: measured under identical conditions). Dynamic force-mode atomic force microscopy was carried out using an SPA 400-DFM (SII Nano Technologies). Grazing incidence wide-angle X-ray diffraction (GIWAXS) analyses were carried out at SPring-8 on beamline 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 GIWAXS patterns were recorded on a 2D image detector (Pilatus 300 K). Films of the polymers with PC61BM were fabricated by spin-coating on a ZnO-coated ITO substrate. Geometry optimizations and normal-mode calculations were performed at the B3LYP/6-31G(d) level using the Gaussian 09, Revision D.01, program package.39 Fabrication of Inverted Bulk-Heterojunction Solar Cells. The inverted bulk-heterojunction solar cells were fabricated as follows. ZnO precursor solution was prepared by hydrolysis of Zn(OAc)2.40 The ITO substrates (ITO, Geomatec Co. Ltd., thickness = 150 nm, sheet resistance