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Dec 15, 2016 - Polycondensation for Producing High Performance OPV Materials ...... Pouliot, J. R.; Veilleux, J.; Leclerc, M. Synthesis of 5-alkyl[3,4-.

Article pubs.acs.org/Macromolecules

Suppression of Homocoupling Side Reactions in Direct Arylation Polycondensation for Producing High Performance OPV Materials Junpei Kuwabara,*,† Yohei Fujie,† Keisuke Maruyama,† Takeshi Yasuda,‡ and Takaki Kanbara*,† †

Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan ‡ Research Center for Functional Materials, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan S Supporting Information *

ABSTRACT: Suppression of side reactions in C−H direct arylation polycondensation is important for developing this method as a reliable synthetic tool for conjugated polymer materials. To find appropriate reaction conditions for avoiding homocoupling side reactions, two types of reaction conditions were investigated: the direct arylation of electron-rich C−H monomers in N,N-dimethylacetamide (DMAc system) and the direct arylation of electron-poor C−H monomers in toluene (toluene system). The investigation reveals that homocoupling side reactions are suppressed under the toluene system. Because the combination of electron-poor C−H monomer (acceptor) and electron-rich C−Br monomer (donor) is applicable to the toluene system, a donor−acceptor polymer without a defect structure can be synthesized under the toluene system. The obtained polymer shows almost same power conversion efficiency (PCE) in bulk-heterojunction OPVs as the same polymer prepared by a conventional method and purified by Soxhlet extraction. These results show that the established direct arylation polycondensation affords a high-quality material in terms of both structural integrity and purity. OPV cells with an optimized device structure result in a maximum PCE of 6.8%.



INTRODUCTION π-Conjugated polymers are considered promising materials for organic photovoltaics (OPVs).1−3 Recent advances in the development of polymer materials and device structures have given rise to high power conversion efficiencies (PCEs) in excess of 10% in single-junction cells.4−8 In addition to improved polymer materials and device structures, the development of an efficient production method for the preparation of conjugated polymer materials is an important consideration with regards to developing practical applications of OPVs because the key materials must be produced on a large scale with low costs and low environmental load.9,10 So-called direct arylation polycondensations, involving dehydrohalogenative cross-coupling reactions,11−15 are seen as a promising candidate for addressing this problem. In contrast to conventional methods that utilize cross-coupling reactions, such as the Suzuki−Miyaura16 and Migita−Kosugi−Stille17 reactions, direct arylation polycondensations do not require organometallic monomers;18−23 therefore, the metalation step can be omitted from the monomer synthesis, which lowers the cost of material production.24,25 This also means that stoichiometric amount of metal-containing waste is not produced by the polycondensation reactions, which lowers the environmental load. This reduction in the amount of generated waste gave rise to highpurity polymers,26,27 which is expected to demonstrate excellent performance as an OPV material.28−30 Indeed, direct © 2016 American Chemical Society

arylation polycondensation afforded a highly pure form of the polymer, which showed higher PCE (4%) than when the same polymer was synthesized by a conventional method (0.5%).27,31 However, there are a limited number of successful examples for direct arylation polycondensations for the preparation of highperformance OPV materials,32−34 presumably due to their limited application range in terms of available monomers and the fact that some structural defects arise in the polymer synthesis.35,36 In particular, defects that result from homocoupling reactions reduce the performance of the polymers.37,38 Several research groups have suppressed C−H/C−H and C− Br/C−Br homocoupling reactions by lowering the reaction temperature,36 choosing appropriate monomer combinations,39−43 using phosphine ligands,36,44−46 and using aminebased compounds.47 In this work, we mainly focused on changing the reaction solvent in order to avoid homocoupling side-reactions. Previously, we developed two different types of reaction conditions: a phosphine-free Pd catalyst in N,Ndimethylacetamide (DMAc system) or a Pd−PCy3 catalyst in toluene (toluene system) (Scheme 1a).48 In terms of the degree of polymerization, the DMAc system was suitable for direct C−H arylation of electron-rich thiophene monomers, Received: November 3, 2016 Revised: November 30, 2016 Published: December 15, 2016 9388

DOI: 10.1021/acs.macromol.6b02380 Macromolecules 2016, 49, 9388−9395

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Macromolecules

Scheme 1. (a) Previously Developed Two Reaction Conditions, DMAc System and Toluene System48 (1-AdCOOH: 1Adamantanecarboxylic Acid); (b) Schematic Image of This Work

while the toluene system was suitable when electron-poor thiophene monomers were used.48 Since homocoupling side reactions have not been evaluated previously, we assessed which conditions were effective for suppressing homocoupling reactions in this work. It was found that the toluene system afforded a polymer without homocoupling defects, while the DMAc system afforded a polymer with defects due to homocoupling (Scheme 1b). These observations were taken into consideration for the preparation of a donor−acceptor polymer without structural defects. The OPV device with the defect-free polymer achieved a maximum PCE of 6.8%, which indicates that the material is of high quality.

Scheme 2. Direct Arylation Polycondensation of 3,4Dioctylthiophene Using the DMAc System



RESULTS AND DISCUSSION Reaction of Electron-Rich Monomer Using the DMAc System. 2,7-Bis(5-bromothien-2-yl)-9,9-dioctylfluorene (2) was selected as the dibromoaryl monomer instead of the dibromofluorene monomer (1) (Scheme 1) because suppression of homocoupling side reactions between 2,7-bis(5bromothien-2-yl)-9,9-dioctylfluorene was predicted to be more challenging than between the dibromofluorene monomer according to previously reported trends.20,46 If homocoupling side reactions could be suppressed with a challenging monomer, this could be useful for producing a wide range of polymers in the future. The direct arylation polycondensation of the electron-rich monomer, 3,4-dioctylthiophene (3), with 2,7-bis(5-bromothien-2-yl)-9,9-dioctylfluorene was conducted under the DMAc system (Scheme 2).48−50 The reaction afforded polymer (P1) with a molecular weight of 14 900 in 85% yield. P1 was evaluated by 1H NMR spectroscopy in order to ascertain the consistency of the repeating structure (Figure 1). In the aromatic region, signals indicating the presence of repeating units were observed (a−e). Signals indicating the presence of terminal units could be assigned (a′−e′ and g) by comparing the 1H NMR spectrum with spectra of the corresponding monomers. The signals for the debrominated terminal unit were also observed (Figure S1, Supporting Information). The additional signals at 7.19 and 7.28 ppm were

assigned to structural defects (i, h), which were caused by homocoupling reactions between bromothienyl moieties; this was confirmed by comparison with the corresponding signals of poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(2,2′-bithiophene-5,5′diyl)].42 In the aliphatic region, the signal at 2.52 ppm corresponds to the methylene protons (j) of the product of the homocoupling reaction between dioctylthiophene moieties. In effect, P1 contains a certain quantity of C−H/C−H and C− Br/C−Br homocoupling structures. The MALDI-TOF-MS spectrum also indicates the presence of these homocoupling structures in the polymer (Figure S2). Homocoupling side reactions are thought to result from the Pd(II) intermediate and occur before the desired C−H bond cleavage process (Scheme S1, Supporting Information). Acceleration of the C−H bond cleavage process is thus expected to result in suppression of the homocoupling reaction. Therefore, a reaction was conducted with a monomer possessing highly reactive C−H bonds, 3,3-dioctyl-3,4dihydro-2H-thieno[3,4-b][1,4]dioxepine (4)51 under the same reaction conditions (Scheme 3), because alkoxythiophenes show higher reactivity than alkylthiophenes under the DMAc system.26,48 The reaction afforded P2 with a molecular weight 9389

DOI: 10.1021/acs.macromol.6b02380 Macromolecules 2016, 49, 9388−9395

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Macromolecules

Figure 1. 1H NMR spectrum of P1 (600 MHz, C2D2Cl4, 353 K).

Scheme 3. Direct Arylation Polycondensation of 3,4Dialkoxythiophene Using the DMAc System

Scheme 4. Direct Arylation Polycondensation of Thienopyrroledione Using the Toluene System

of 19 400 in 83% yield. During the reaction, precipitation of the polymer was observed within 10 min, indicating effective polymerization caused by the high reactivity of the monomer. Contrary to expectations, the 1H NMR spectrum and MALDITOF-MS of P2 indicated the presence of homocoupling structures (Figures S3 and S4). The signals corresponding to homocoupling structure in the 1H NMR spectrum of P2 were relatively less intense than in the spectrum of P1; therefore, a highly reactive monomer partly suppresses homocoupling reactions. However, complete suppression of side-reactions under the DMAc system might be difficult. Reaction of Electron-Poor Monomer Using the Toluene System. The reaction under the toluene system was conducted using the electron-poor thiophene monomer 5(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (5, TPD).23,44,48,52 In this reaction, Pd(PCy3)2 was selected as the catalyst on the basis of previous investigations.53,54 The polycondensation reaction afforded the corresponding polymer (P3), with a molecular weight of 40 100, in 89% yield (Scheme 4). The 1H NMR spectrum of P3 showed signals which could be attributed to the repeating unit (Figure 2). Peaks at around 7.19 and 7.28 ppm corresponding to defects arising from C− Br/C−Br homocoupling reactions were not observed. The MALDI-TOF-MS spectrum of P3 also indicated that no

Figure 2. 1H NMR spectrum of P3 (600 MHz, C2D2Cl4, 353 K).

homocoupling defects were present (Figure 3). An alternating structure with a hydrogenated terminal, arising from a debromination reaction during the polycondensation, is the dominant structure. Previously, Ozawa reported that a debromination reaction induced a subsequent homocoupling reaction.47 Leclerc suggested homocoupling defects were caused by direct arylation of the C−H bond formed from the debromination reaction.20,55 However, observations in this study were not consistent with these reports. One of the reasons is the relatively low frequency of the debromination reaction, which can be inferred from the high molecular weight of the resultant polymer (40 100). Another possible explanation is that the C−H bond formed from the debromination reaction 9390

DOI: 10.1021/acs.macromol.6b02380 Macromolecules 2016, 49, 9388−9395

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Macromolecules Scheme 5. Synthesis of P4 by Direct Arylation Polycondensation Using the Toluene System

Figure 3. MALDI-TOF-MS spectrum of P3.

is less reactive in toluene because corresponding C−H moiety is not electron poor.48 The result of a control experiment demonstrated the low reactivity of the C−H bond (Scheme S2, Figure S7). In addition, the polymer has no β-defect caused by undesired C−H direct arylation reactions at the β-position of the thiophene moiety, which is another major side reaction in direct arylation polycondensations.49 These results show that only the C−H bond of the TPD unit is reactive in this direct arylation polycondensation under the toluene system. This high degree of selectivity is regarded as an advantage of performing these types of reactions in toluene. As described above, the homocoupling side reactions are thought to result from disproportionation of the Pd(II) intermediate (Scheme S1). DMAc is a highly polar and strongly coordinating solvent that accelerates disproportionation reactions.56−58 On the other hand, toluene is nonpolar and weakly coordinating; therefore, toluene efficiently suppresses the homocoupling reactions. Thus, several defect-free polymers have been successfully synthesized in toluene.41,47 Based on these results, an appropriate strategy for the synthesis of polymers without homocoupling defects involves C−H direct arylation of electron-poor thiophene monomers with dibromoaryl monomers composed of electron-rich thiophene derivatives in toluene. This combination of the monomers is appropriate for preparation of donor−acceptor (D−A) polymers that could be used as OPV materials. Preparation of an OPV Material. The established reaction conditions using an electron-poor C−H monomer in toluene were applied to the preparation of an OPV material. P4 was selected as the target polymer because it has previously been prepared by a conventional method using cross-coupling reactions, and a relatively high PCE of 5.2% was reported.59 Horie reported that direct arylation polycondensation in DMAc afforded P4 (Mn = 2500) with homocoupling defects observed at the early stage of development of this method.60 In this study, the reaction of TPD bearing an octyl side chain (6), with dibromocyclopentadithiophene (7), afforded P4 with a molecular weight of 25 000 in 82% yield (Scheme 5). This was considered a satisfying outcome in comparison with synthesis of the same polymer by polycondensation using the Migita−Kosugi−Stille coupling reaction (Mn = 15 000, 52% yield).59 No homocoupling defect was detected by 1H NMR and MALDI-TOF-MS (Figures S8 and S10). To prove the effectiveness of direct arylation polycondensation, the purity of the obtained polymer was evaluated by elemental analysis. The result showed that the polymer was highly pure, despite the fact that no special purification steps, such as a treatment of a terminal unit, Soxhlet extraction, or HPLC purification were

undertaken. In addition to producing less metal-containing waste, the debromination reaction also contributes to the high purity because of the elimination of the Br moiety in the terminal.27,47 Characteristics of OPV Materials. The OPV characteristics of the highly pure polymer, which did not contain any structural defects, were evaluated for bulk heterojunction (BHJ) OPVs with PC70BM. Previously, the same polymer, which was prepared by a conventional method using cross-coupling reactions, was evaluated for BHJ OPVs;59 therefore, the same device configuration obtained using the same fabrication method was adopted to compare the quality of the polymer materials. The atomic force microscopy (AFM) image of the fabricated BHJ layer shows smooth surface without large aggregates and other film defects (Figure S11). The root-meansquare (RMS) roughness of 2.84 nm indicates that P4 and PC70BM are well-mixed, which is similar to the reported observation.59 The OPV cell showed PCE of 5.1 ± 0.3%, which is essentially the same as the previously reported PCE of 5.2%59 (entry 1 in Table 1 and Figure 4). Even without special purification steps, the established synthetic protocol afforded the OPV material showing high performance comparable to the reported polymer which was purified by Soxhlet extraction.59 To improve the electron extraction efficiency, LiF was inserted between the BHJ layer and Al electrode as a cathode buffer layer.61 The presence of LiF improved the PCE to 6.4 ± 0.3% on average (with a maximum PCE of 6.8%) (entry 2 in Table 1 and Figure 4). Since the improved fill factor (FF) was the main reason for the improved PCE, the LiF layer was likely to contribute efficient electron extraction at the cathode as expected.59



CONCLUSIONS Two reaction systems (DMAc and toluene system) were assessed in terms of suppression of structural defects arising from homocoupling side reactions in direct arylation polycondensation. The results showed that an appropriate strategy for synthesis of polymers without homocoupling defects is C−H direct arylation of electron-poor thiophene monomers with dibromoaryl monomers composed of electronrich thiophene derivatives under the toluene system. The combination of an electron-poor thiophene (C−H monomer) and an electron-rich thiophene (C−Br monomer) is suitable for the preparation of D−A polymers which could be used as OPV materials. Indeed, the established strategy was used to 9391

DOI: 10.1021/acs.macromol.6b02380 Macromolecules 2016, 49, 9388−9395

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Macromolecules Table 1. OPV Characteristicsa entry 1 2

interlayer

Jsc (mA cm−2)

Voc (V)

FF

PCE (%)

LiF

13.3 ± 0.2 13.4 ± 0.4

0.86 ± 0.02 0.90 ± 0.01

0.45 ± 0.01 0.527 ± 0.008

5.1 ± 0.3 (5.5) 6.4 ± 0.3 (6.8)

a

The average values with standard deviations were calculated from the results of six OPV samples. OPV configuration: ITO/PEDOT:PSS (40 nm)/ P4:PC70BM (1:2)/interlayer (1 nm)/Al (80 nm). bThe best values obtained are written in parentheses. degassing and filling with N2, DMAc (0.7 mL) and pivalic acid (7.0 μL. 0.063 mmol) were added under a N2 stream. The mixture was stirred at 100 °C for 24 h under a N2 atmosphere. After cooling to room temperature, an aqueous solution of sodium diethyldithiocarbamate (0.01 g/mL) was added. The suspension was stirred for 1 h at room temperature. The precipitates were collected by filtration and washed with a 1 M HCl solution, distilled water, methanol, and hexane. The remaining solid was extracted with CHCl3. After filtration through Celite, the solution was concentrated and reprecipitated into methanol. The precipitates were collected by filtration and dried under reduced pressure. P1 was isolated as a yellow solid in 85% yield. Mn = 14 900, Mw/Mn = 2.14. 1H NMR signals of the repeating unit (600 MHz, C2D2Cl4, 353 K): δ 7.66 (d, J = 8.4 Hz), 7.58−7.54 (4H, m), 7.31 (br), 7.13 (2H, br), 2.77 (br), 2.01 (Br), 1.63 (br), 1.45−1.24 (m), 1.18−1.08 (m), 0.87 (m), 0.78 (m). Synthesis of P2. P2 was synthesized by the same procedure for P1 in a 0.30 mmol scale with 3,3-dioctyl-3,4-dihydro-2H-thieno[3,4b][1,4]dioxepine (4) (114.2 mg, 0.30 mmol) instead of 3,4dioctylthiophene. P2 was isolated as an orange solid in 83% yield. Mn = 19 400, Mw/Mn = 1.51. 1H NMR signals of the repeating unit (600 MHz, C2D2Cl4, 353 K): δ 7.65 (d, J = 7.8 Hz), 7.57 (d, J = 7.8 Hz), 7.55 (s), 7.27 (d, J = 3.6 Hz), 7.22 (br), 4.07 (s), 2.01 (br), 1.51 (br), 1.33−1.28 (m), 1.18 (m), 1.08 (m), 0.87 (t, J = 7.2 Hz), 0.78 (m). Synthesis of P3. To a 25 mL Schlenk tube were added 5-(2ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (5) (64.5 mg, 0.24 mmol), 2,7-bis(5-bromothien-2-yl)-9,9-dioctyllfluorene (2) (173.3 mg, 0.24 mmol), and Cs2CO3 (198.1 mg, 0.61 mmol). After degassing and filling with N2, Pd(PCy3)2 (3.2 mg, 4.8 μmol), toluene (1.2 mL), and pivalic acid (8.2 μL. 0.072 mmol) were added under a N2 atmosphere. The mixture was stirred at 100 °C for 24 h. After volatiles were evaporated under reduced pressure, an aqueous solution of sodium diethyldithiocarbamate (0.01 g/mL) was added. The suspension was stirred for 1 h at room temperature. The precipitates were collected by filtration and washed with a 1 M HCl solution, distilled water, methanol, and hexane. The remaining solid was extracted with CHCl3. After filtration through Celite, the solution was concentrated and reprecipitated into methanol. The precipitates were collected by filtration and dried under reduced pressure. P3 was isolated as a red solid in 89% yield. Mn = 40 100, Mw/Mn = 3.39. 1H NMR (600 MHz, C2D2Cl4, 353 K): δ 8.06 (2H, br), 7.68 (2H, br), 7.62 (2H, br), 7.59 (2H, s), 7.37 (2H, br), 3.58 (2H, br), 2.05 (4H, br), 1.89 (1H, m), 1.45−1.32 (6H, m), 1.18 (4H, m), 1.10 (18H, m), 0.94 (3H, t, J = 7.2 Hz), 0.90 (3H, t, J = 7.2 Hz), 0.78 (10H, m). 13C{1H} NMR (150 MHz, C2D2Cl4, 353 K): δ 162.66, 152.09, 148.39, 140.85, 136.06, 132.40, 131.34, 131.20, 128.54, 125.17, 124.30, 120.31, 120.31, 55.40, 42.71, 39.94, 38.23, 31.54, 30.74, 29.77, 28.96, 28.92, 28.53, 24.10, 23.76, 22.85, 22.35, 13.78, 13.78, 10.48. Synthesis of P4. P4 was synthesized by the same procedure for P3 in a 0.25 mmol scale with 5-octylthieno[3,4-c]pyrrole-4,6-dione (6) (66.3 mg, 0.25 mmol) and 2,6-dibromo-4,4-bis(2-ethylhexyl)-4Hcyclopenta[2,1-b:3,4-b′] dithiophene (7) (140.1 mg, 0.25 mmol). P4 was isolated as a deep blue solid in 82% yield. Mn = 25 000, Mw/Mn = 1.92. 1H NMR (600 MHz, C2D2Cl4, 353 K): δ 7.89 (m, 2H), 3.66 (br, 2H), 2.00 (br, 4H), 1.70 (br, 1H), 1.35−1.28 (m, 10H), 1.04−0.98 (m, 16H), 0.87 (t, J = 7.8 Hz, 3H), 0.74 (br, 8H), 0.65 (t, J = 6.9 Hz, 6H). 13C{1H} NMR (150 MHz, C2D2Cl4, 353 K): δ 162.23, 160.44, 140.20, 136.16, 133.93, 128.19, 124.53, 54.44, 43.18, 38.53, 35.62, 34.29, 31.56, 28.99, 28.89, 28.58, 28.37, 27.48, 26.83, 22.64, 22.35. 13.76, 13.73, 10.51. Anal. Calcd for C39H53NO2S3: C, 70.54; H, 8.05;

Figure 4. (a) J−V curves and (b) IPCE spectra of the OPV device without LiF (Table 1, entry 1, dashed line) and the device with LiF (Table 1, entry 2, solid line).

synthesize an OPV material without structural defects. The advantage of using the described approach was that the resultant polymer was highly pure despite the fact that only simple purification steps were carried out. The PCE of the OPV device containing the polymer reached 6.4% on average (with a maximum PCE of 6.8%). This work has demonstrated that a direct arylation polycondensation is a step-economical and practical protocol to prepare OPV materials, with high structural integrity and purity.



EXPERIMENTAL SECTION

Materials. Pd(OAc)2, chlorobenzene (CB), 1,8-diiodooctane (DIO), and other chemicals were received from commercial suppliers and used without further purification. Pd(PCy3)2 was purchased from Aldrich and stored under a N2 atmosphere at 0 °C. Anhydrous DMAc and toluene were purchased from Kanto Chemical and used as a dry solvent. 5-Octylthieno[3,4-c]pyrrole-4,6-dione (6) and 2,6-dibromo4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (7) was purchased from Wako Chemical. 2,7-Bis(5-bromothien-2-yl)-9,9dioctylfluorene (2),62 3,4-dioctylthiophene (3),63 3,3-dioctyl-3,4dihydro-2H-thieno[3,4-b][1,4]dioxepine (4),64 and 5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (5)65 were prepared according to previously reported methods. Poly(3,4-ethylenedioxythiophene)− poly(styrenesulfonate) (PEDOT:PSS, CLEVIOS P VP AI 4083) was purchased from Heraeus. PC70BM (purity 99%) was purchased from Solenne. General Methods. The NMR spectra were recorded on AVANCE-400 and AVANCE-600 NMR spectrometers (Bruker). Elemental analyses were carried out with a Yanaco MT-5 CHN autorecorder. Gel permeation chromatography (GPC) measurements were conducted using a prominence GPC system (Shimadzu) equipped with polystyrene gel columns, using CHCl3 as the eluent after calibration with polystyrene standards (40 °C). The matrixassisted laser desorption/ionization mass (MALDI-TOF-MS) spectra were recorded on a MALDI TOF/TOF 5800 (AB SCIEX) in linear mode using dithranol as the matrix. The atomic force microscopy (AFM) images were obtained using AFM5100N (Hitachi High-Tech Science). All of the manipulations for the reactions performed under a nitrogen atmosphere using a glovebox or standard Schlenk techniques. Synthesis of P1. To a 25 mL Schlenk tube were added Pd(OAc)2 (0.92 mg, 4.0 μmol), K 2 CO3 (71.0 mg, 0.53 mmol), 3,4dioctylthiophene (3) (63.4 mg, 0.21 mmol), and 2,7-bis(5-bromothien-2-yl)-9,9-dioctylfluorene (2) (146.4 mg, 0.21 mmol). After 9392

DOI: 10.1021/acs.macromol.6b02380 Macromolecules 2016, 49, 9388−9395

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Macromolecules N, 2.11; S, 14.48; Br, 0.00. Found: C, 70.28; H, 7.81; N, 2.14; S, 14.77; Br, below detection limit. Fabrication and Characterization of OPV Cells. The OPV cells were fabricated in the following configuration: ITO/PEDOT:PSS/ BHJ layer/LiF/Al. The patterned ITO (conductivity: 10 Ω/square) glass was precleaned in an ultrasonic bath containing acetone and ethanol and then treated in an ultraviolet-ozone chamber. A thin layer (40 nm) of PEDOT:PSS was spin-coated onto the ITO at 3000 rpm and air-dried at 110 °C for 10 min on a hot plate. Then, the substrate was transferred to a N2-filled glovebox where it was redried at 110 °C for 10 min on a hot plate. A CB solution (4% DIO) consisting of P4 and PC70BM blended in a 1:2 ratio was subsequently spin-coated onto the PEDOT:PSS surface to form the BHJ layer. The substrates with the BHJ layers were dried for 10 min at 70 °C. Then, LiF (1 nm) and Al (80 nm) were deposited onto the active layer by conventional thermal evaporation at a chamber pressure lower than 5 × 10−4 Pa, which provided the devices with an active area of 5 × 2 mm2. The thicknesses of the BHJ and PEDOT:PSS layers were measured using an automatic microfigure measuring instrument (SURFCORDER ET200, Kosaka Laboratory, Ltd.). The current density−voltage (J−V) curves were measured using an ADCMT 6244 dc voltage current source/monitor under AM 1.5 solar-simulated light irradiation (100 mW cm−2) (OTENTO-SUN III, Bunkoh-Keiki Co., Ltd.). The incident-photon-to-current conversion efficiency (IPCE) was measured using an SM-250 system (Bunkoh-Keiki Co., Ltd.).



(4) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5 (9), 5293. (5) Chen, J. De; Cui, C.; Li, Y. Q.; Zhou, L.; Ou, Q. D.; Li, C.; Li, Y.; Tang, J. X. Single-Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Adv. Mater. 2015, 27 (6), 1035−1041. (6) Vohra, V.; Kawashima, K.; Kakara, T.; Koganezawa, T.; Osaka, I.; Takimiya, K.; Murata, H. Efficient Inverted Polymer Solar Cells Employing Favourable Molecular Orientation. Nat. Photonics 2015, 9 (6), 403−408. (7) Kawashima, K.; Fukuhara, T.; Suda, Y.; Suzuki, Y.; Koganezawa, T.; Yoshida, H.; Ohkita, H.; Osaka, I.; Takimiya, K. Implication of Fluorine Atom on Electronic Properties, Ordering Structures, and Photovoltaic Performance in Naphthobisthiadiazole-Based Semiconducting Polymers. J. Am. Chem. Soc. 2016, 138 (32), 10265− 10275. (8) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1 (2), 15027. (9) Osedach, T. P.; Andrew, T. L.; Bulović, V. Effect of Synthetic Accessibility on the Commercial Viability of Organic Photovoltaics. Energy Environ. Sci. 2013, 6 (3), 711−718. (10) Burke, D. J.; Lipomi, D. J. Green Chemistry for Organic Solar Cells. Energy Environ. Sci. 2013, 6 (7), 2053−2066. (11) Satoh, T.; Miura, M. Catalytic Direct Arylation of Heteroaromatic Compounds. Chem. Lett. 2007, 36 (2), 200−205. (12) Ackermann, L.; Vicente, R.; Kapdi, A. R. Transition-MetalCatalyzed Direct Arylation of (Hetero)arenes by C-H Bond Cleavage. Angew. Chem., Int. Ed. 2009, 48 (52), 9792−9827. (13) Liégault, B.; Lapointe, D.; Caron, L.; Vlassova, A.; Fagnou, K. Establishment of Broadly Applicable Reaction Conditions for the Palladium-Catalyzed Direct Arylation of Heteroatom-Containing Aromatic Compounds. J. Org. Chem. 2009, 74 (5), 1826−1834. (14) Rossi, R.; Bellina, F.; Lessi, M.; Manzini, C. Cross-Coupling of Heteroarenes by C-H Functionalization: Recent Progress towards Direct Arylation and Heteroarylation Reactions Involving Heteroarenes Containing One Heteroatom. Adv. Synth. Catal. 2014, 356 (1), 17−117. (15) Segawa, Y.; Maekawa, T.; Itami, K. Synthesis of Extended πSystems through C-H Activation. Angew. Chem., Int. Ed. 2015, 54 (1), 66−81. (16) Sakamoto, J.; Rehahn, M.; Wegner, G.; Schlüter, A. D. Suzuki Polycondensation: Polyarylenes à la Carte. Macromol. Rapid Commun. 2009, 30 (9−10), 653−687. (17) Carsten, B.; He, F.; Son, H. J.; Xu, T.; Yu, L. Stille Polycondensation for Synthesis of Functional Materials. Chem. Rev. 2011, 111 (3), 1493−1528. (18) Kowalski, S.; Allard, S.; Zilberberg, K.; Riedl, T.; Scherf, U. Direct Arylation Polycondensation as Simplified Alternative for the Synthesis of Conjugated (Co)polymers. Prog. Polym. Sci. 2013, 38 (12), 1805−1814. (19) Okamoto, K.; Zhang, J.; Housekeeper, J. B.; Marder, S. R.; Luscombe, C. K. C-H Arylation Reaction: Atom Efficient and Greener Syntheses of π-Conjugated Small Molecules and Macromolecules for Organic Electronic Materials. Macromolecules 2013, 46 (20), 8059− 8078. (20) Bura, T.; Blaskovits, J. T.; Leclerc, M. Direct (Hetero)arylation Polymerization: Trends and Perspectives. J. Am. Chem. Soc. 2016, 138 (32), 10056−10071. (21) Wang, Q.; Takita, R.; Kikuzaki, Y.; Ozawa, F. PalladiumCatalyzed Dehydrohalogenative Polycondensation of 2-Bromo-3hexylthiophene: An Efficient Approach to Head-to-Tail poly(3Hexylthiophene). J. Am. Chem. Soc. 2010, 132 (33), 11420−11421. (22) Lu, W.; Kuwabara, J.; Kanbara, T. Polycondensation of Dibromofluorene Analogues with Tetrafluorobenzene via Direct Arylation. Macromolecules 2011, 44 (6), 1252−1255. (23) Berrouard, P.; Najari, A.; Pron, A.; Gendron, D.; Morin, P. O.; Pouliot, J. R.; Veilleux, J.; Leclerc, M. Synthesis of 5-alkyl[3,4-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02380. Characterization data for polymers and OPV device characteristics (PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.K.). *E-mail [email protected] (T.K.). ORCID

Junpei Kuwabara: 0000-0002-9032-5655 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors thank the Chemical Analysis Center of University of Tsukuba for the measurements of NMR and MALDI-TOFMS spectra. The authors thank Prof. K. Osakada, D. Takeuchi, and the Center for Advanced Materials Analysis, Technical Department, Tokyo Institute of Technology for the elemental analysis. This work was supported by Industrial Technology Research Grant Program in 2011 from New Energy and Industrial Technology Development Organization (NEDO) of Japan, and Grant-in-Aid for Young Scientists (B) (15K17922).



REFERENCES

(1) Boudreault, P.-L. T.; Najari, A.; Leclerc, M. Processable LowBandgap Polymers for Photovoltaic Applications. Chem. Mater. 2011, 23 (3), 456−469. (2) Zhou, H.; Yang, L.; You, W. Rational Design of High Performance Conjugated Polymers for Organic Solar Cells. Macromolecules 2012, 45 (3), 607−632. (3) Zhang, S.; Ye, L.; Hou, J. Breaking the 10% Efficiency Barrier in Organic Photovoltaics: Morphology and Device Optimization of WellKnown PBDTTT Polymers. Adv. Energy Mater. 2016, 6 (11), 1502529. 9393

DOI: 10.1021/acs.macromol.6b02380 Macromolecules 2016, 49, 9388−9395

Article

Macromolecules c]thienopyrrole-4,6-Dione-Based Polymers by Direct Heteroarylation. Angew. Chem., Int. Ed. 2012, 51 (9), 2068−2071. (24) Po, R.; Bernardi, A.; Calabrese, A.; Carbonera, C.; Corso, G.; Pellegrino, A. From Lab to Fab: How Must the Polymer Solar Cell Materials Design Change? − an Industrial Perspective. Energy Environ. Sci. 2014, 7 (3), 925−943. (25) Po, R.; Bianchi, G.; Carbonera, C.; Pellegrino, A. “All That Glisters Is Not Gold”: An Analysis of the Synthetic Complexity of Efficient Polymer Donors for Polymer Solar Cells. Macromolecules 2015, 48 (3), 453−461. (26) Estrada, L. A.; Deininger, J. J.; Kamenov, G. D.; Reynolds, J. R. Direct (Hetero)arylation Polymerization: An Effective Route to 3,4Propylenedioxythiophene-Based Polymers with Low Residual Metal Content. ACS Macro Lett. 2013, 2 (10), 869−873. (27) Kuwabara, J.; Yasuda, T.; Choi, S. J.; Lu, W.; Yamazaki, K.; Kagaya, S.; Han, L.; Kanbara, T. Direct Arylation Polycondensation: A Promising Method for the Synthesis of Highly Pure, High-MolecularWeight Conjugated Polymers Needed for Improving the Performance of Organic Photovoltaics. Adv. Funct. Mater. 2014, 24 (21), 3226− 3233. (28) Henson, Z. B.; Müllen, K.; Bazan, G. C. Design Strategies for Organic Semiconductors beyond the Molecular Formula. Nat. Chem. 2012, 4 (9), 699−704. (29) Krebs, F. C.; Nyberg, R. B.; Jørgensen, M. Influence of Residual Catalyst on the Properties of Conjugated Polyphenylenevinylene Materials: Palladium Nanoparticles and Poor Electrical Performance. Chem. Mater. 2004, 16 (7), 1313−1318. (30) Kettle, J.; Horie, M.; Majewski, L. A.; Saunders, B. R.; Tuladhar, S.; Nelson, J.; Turner, M. L. Optimisation of PCPDTBT Solar Cells Using Polymer Synthesis with Suzuki Coupling. Sol. Energy Mater. Sol. Cells 2011, 95 (8), 2186−2193. (31) Kuwabara, J.; Yasuda, T.; Takase, N.; Kanbara, T. Effects of the Terminal Structure, Purity, and Molecular Weight of an Amorphous Conjugated Polymer on Its Photovoltaic Characteristics. ACS Appl. Mater. Interfaces 2016, 8 (3), 1752−1758. (32) Marrocchi, A.; Facchetti, A.; Lanari, D.; Petrucci, C.; Vaccaro, L. Current Methodologies for a Sustainable Approach to Π-Conjugated Organic Semiconductors. Energy Environ. Sci. 2016, 9 (3), 763−786. (33) Guérette, M.; Najari, A.; Maltais, J.; Pouliot, J.-R.; Dufresne, S.; Simoneau, M.; Besner, S.; Charest, P.; Leclerc, M. New Processable Phenanthridinone-Based Polymers for Organic Solar Cell Applications. Adv. Energy Mater. 2016, 6 (9), 1502094. (34) Dudnik, A. S.; Aldrich, T. J.; Eastham, N. D.; Chang, R. P. H.; Facchetti, A.; Marks, T. J. Tin-Free Direct C−H Arylation Polymerization for High Photovoltaic Efficiency Conjugated Copolymers. J. Am. Chem. Soc. 2016, 138, 15699−15709. (35) Kowalski, S.; Allard, S.; Scherf, U. Scope and Limitations of a Direct Arylation Polycondensation Scheme in the Synthesis of PCPDTBT-Type Copolymers. Macromol. Rapid Commun. 2015, 36 (11), 1061−1068. (36) Lombeck, F.; Komber, H.; Gorelsky, S. I.; Sommer, M. Identifying Homocouplings as Critical Side Reactions in Direct Arylation Polycondensation. ACS Macro Lett. 2014, 3 (8), 819−823. (37) Hendriks, K. H.; Li, W.; Heintges, G. H. L.; Van Pruissen, G. W. P.; Wienk, M. M.; Janssen, R. A. J. Homocoupling Defects in Diketopyrrolopyrrole-Based Copolymers and Their Effect on Photovoltaic Performance. J. Am. Chem. Soc. 2014, 136 (31), 11128−11133. (38) Lombeck, F.; Komber, H.; Fazzi, D.; Nava, D.; Kuhlmann, J.; Stegerer, D.; Strassel, K.; Brandt, J.; Diaz, A.; Mendaza, D. Z.; Müller, C.; Thiel, W.; Caironi, M.; Friend, R.; Sommer, M. On the Effect of Prevalent Carbazole Homocoupling Defects on the Photovoltaic Performance of PCDTBT:PC71BM Solar Cells. Adv. Energy Mater. 2016, 6 (21), 1601232. (39) Pouliot, J.-R.; Sun, B.; Leduc, M.; Najari, A.; Li, Y.; Leclerc, M. A High Mobility DPP-Based Polymer Obtained via Direct (Hetero)arylation Polymerization. Polym. Chem. 2015, 6 (2), 278−282. (40) Broll, S.; Nübling, F.; Luzio, A.; Lentzas, D.; Komber, H.; Caironi, M.; Sommer, M. Defect Analysis of High Electron Mobility

Diketopyrrolopyrrole Copolymers Made by Direct Arylation Polycondensation. Macromolecules 2015, 48 (20), 7481−7488. (41) Wang, K.; Wang, G.; Wang, M. Balanced Ambipolar Poly(diketopyrrolopyrrole-alt-Tetrafluorobenzene) Semiconducting Polymers Synthesized via Direct Arylation Polymerization. Macromol. Rapid Commun. 2015, 36 (24), 2162−2170. (42) Morin, P. O.; Bura, T.; Sun, B.; Gorelsky, S. I.; Li, Y.; Leclerc, M. Conjugated Polymers à La Carte from Time-Controlled Direct (Hetero)arylation Polymerization. ACS Macro Lett. 2015, 4 (1), 21− 24. (43) Livi, F.; Gobalasingham, N. S.; Bundgaard, E.; Thompson, B. C. Influence of Functionality on Direct Arylation of Model Systems as a Route toward Fluorinated Copolymers via Direct Arylation Polymerization (DArP). J. Polym. Sci., Part A: Polym. Chem. 2015, 53 (22), 2598−2605. (44) Wakioka, M.; Nakamura, Y.; Hihara, Y.; Ozawa, F.; Sakaki, S. A Highly Efficient Catalyst for the Synthesis of Alternating Copolymers with Thieno[3,4-c]pyrrole-4,6-dione Units via Direct Arylation Polymerization. Macromolecules 2014, 47 (2), 626−631. (45) Iizuka, E.; Wakioka, M.; Ozawa, F. Mixed-Ligand Approach to Palladium-Catalyzed Direct Arylation Polymerization: Synthesis of Donor-Acceptor Polymers with Dithienosilole (DTS) and Thienopyrroledione (TPD) Units. Macromolecules 2015, 48 (9), 2989−2993. (46) Bura, T.; Morin, P. O.; Leclerc, M. En Route to Defect-Free Polythiophene Derivatives by Direct Heteroarylation Polymerization. Macromolecules 2015, 48 (16), 5614−5620. (47) Iizuka, E.; Wakioka, M.; Ozawa, F. Mixed-Ligand Approach to Palladium-Catalyzed Direct Arylation Polymerization: Effective Prevention of Structural Defects Using Diamines. Macromolecules 2016, 49 (9), 3310−3317. (48) Kuwabara, J.; Yamazaki, K.; Yamagata, T.; Tsuchida, W.; Kanbara, T. The Effect of a Solvent on Direct Arylation Polycondensation of Substituted Thiophenes. Polym. Chem. 2015, 6 (6), 891−895. (49) Fujinami, Y.; Kuwabara, J.; Lu, W.; Hayashi, H.; Kanbara, T. Synthesis of Thiophene- and Bithiophene-Based Alternating Copolymers via Pd-Catalyzed Direct C-H Arylation. ACS Macro Lett. 2012, 1 (1), 67−70. (50) Kowalski, S.; Allard, S.; Scherf, U. Synthesis of Poly(4,4-Dialkylcyclopenta[2,1-b:3,4-b′]dithiophene-Alt-2,1,3-Benzothiadiazole) (PCPDTBT) in a Direct Arylation Scheme. ACS Macro Lett. 2012, 1 (4), 465−468. (51) Wakioka, M.; Ishiki, S.; Ozawa, F. Synthesis of Donor-Acceptor Polymers Containing thiazolo[5,4-d]thiazole Units via PalladiumCatalyzed Direct Arylation Polymerization. Macromolecules 2015, 48 (22), 8382−8388. (52) Jo, J.; Pron, A.; Berrouard, P.; Leong, W. L.; Yuen, J. D.; Moon, J. S.; Leclerc, M.; Heeger, A. J. A New Terthiophene-Thienopyrrolodione Copolymer-Based Bulk Heterojunction Solar Cell with High Open-Circuit Voltage. Adv. Energy Mater. 2012, 2 (11), 1397−1403. (53) Kuwabara, J.; Sakai, M.; Zhang, Q.; Kanbara, T. Mechanistic Studies and Optimisation of a Pd-Catalysed Direct Arylation Reaction Using Phosphine-Free Systems. Org. Chem. Front. 2015, 2 (5), 520− 525. (54) Kuwabara, J.; Kuramochi, M.; Liu, S.; Yasuda, T.; Kanbara, T. Direct Arylation Polycondensation for the Synthesis of BithiazoleBased Conjugated Polymers and Their Physical Properties. Polym. J. 2017, DOI: 10.1038/pj.2016.75. (55) Morin, P.-O.; Bura, T.; Leclerc, M. Realizing the Full Potential of Conjugated Polymers: Innovation in Polymer Synthesis. Mater. Horiz. 2016, 3 (1), 11−20. (56) Yamamoto, T.; Wakabayashi, S.; Osakada, K. Mechanism of CC Coupling Reactions of Aromatic Halides, Promoted by Ni (COD)2 in the Presence of 2,2′-Bipyridine and PPh3 to Give Biaryls. J. Organomet. Chem. 1992, 428 (1−2), 223−237. (57) Osakada, K.; Yamamoto, T. Transmetallation of Alkynyl and Aryl Complexes of Group 10 Transition Metals. Coord. Chem. Rev. 2000, 198 (1), 379−399. 9394

DOI: 10.1021/acs.macromol.6b02380 Macromolecules 2016, 49, 9388−9395

Article

Macromolecules (58) Suzaki, Y.; Yagyu, T.; Osakada, K. Transmetalation of Arylpalladium and Platinum Complexes. Mechanism and Factors to Control the Reaction. J. Organomet. Chem. 2007, 692 (1−3), 326−342. (59) Kim, H.; Lee, B. H.; Lee, K. C.; Kim, G.; Yu, J. Y.; Kim, N.; Lee, S. H.; Lee, K. Role of the Side Chain in the Phase Segregation of Polymer:fullerene Bulk Heterojunction Composites. Adv. Energy Mater. 2013, 3 (12), 1575−1580. (60) Chang, S. W.; Waters, H.; Kettle, J.; Kuo, Z. R.; Li, C. H.; Yu, C. Y.; Horie, M. Pd-Catalysed Direct Arylation Polymerisation for Synthesis of Low-Bandgap Conjugated Polymers and Photovoltaic Performance. Macromol. Rapid Commun. 2012, 33 (22), 1927−1932. (61) Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariciftci, N. S.; Denk, P. Effect of LiF/metal Electrodes on the Performance of Plastic Solar Cells. Appl. Phys. Lett. 2002, 80 (7), 1288−1290. (62) Woon, K. L.; Aldred, M. P.; Vlachos, P.; Mehl, G. H.; Stirner, T.; Kelly, S. M.; O’Neill, M. Electronic Charge Transport in Extended Nematic Liquid Crystals. Chem. Mater. 2006, 18 (9), 2311−2317. (63) Fujisawa, K.; Satake, A.; Hirota, S.; Kobuke, Y. Construction of Giant Porphyrin Macrorings Self-Assembled from ThiophenyleneLinked Bisporphyrins for Light-Harvesting Antennae. Chem. - Eur. J. 2008, 14 (34), 10735−10744. (64) Liang, Y.; Peng, B.; Liang, J.; Tao, Z.; Chen, J. TriphenylamineBased Dyes Bearing Functionalized 3,4-Propylenedioxythiophene Linkers with Enhanced Performance for Dye-Sensitized Solar Cells. Org. Lett. 2010, 12 (6), 1204−1207. (65) Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. M.; Fréchet, J. M. J. Synthetic Control of Structural Order in N-Alkylthieno[3,4-c]pyrrole-4,6-Dione-Based Polymers for Efficient Solar Cells. J. Am. Chem. Soc. 2010, 132 (22), 7595−7597.

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DOI: 10.1021/acs.macromol.6b02380 Macromolecules 2016, 49, 9388−9395

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