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Regioregular Low Bandgap Polymer with Controlled Thieno[3,4b]thiophene Orientation for High-Efficiency Polymer Solar Cells Honggi Kim, Hyungjin Lee, Donghyun Seo, Youngjun Jeong, Keun Cho, Jaechol Lee, and Youngu Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00632 • Publication Date (Web): 07 Apr 2015 Downloaded from http://pubs.acs.org on April 11, 2015
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Chemistry of Materials
Regioregular Low Bandgap Polymer with Controlled Thieno[3,4-b]thiophene Orientation for High-Efficiency Polymer Solar Cells Honggi Kim,† Hyungjin Lee,† Donghyun Seo,† Youngjun Jeong,† Keun Cho,§ Jaechol Lee,§ and Youngu Lee*, † †
Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 50-1 Sang-Ri, Hyeonpung-Myeon, Dalseong-Gun, Daegu, 711-873, Republic of Korea. §
Future Technology Center, Corporate R&D, LG Chem Research Park, 104-1, Moonji-dong, Yuseong-gu, Daejeon, 305-380, Republic of Korea. ABSTRACT: The regioregular p-type copolymer PBDTTT-C-T comprised of TT-BDT-TT-BDT repeating units (TT = thieno[3,4-b]thiophene, BDT = benzo[1,2-b:4,5-b’]dithoiphene) and perfectly controlled TT orientation was synthesized. The optical, thermal, and electrochemical properties of the reigoreular PBDTTT-C-T were characterized and compared with the random PBDTTT-C-T without structural regioreguarity. The regioregular PBDTTT-C-T showed lower optical bandgap (1.55 eV) and higher degree of crystallinity compared to the random PBDTTT-C-T. The inverted bulk heterojunction PSCs based on the regioregular PBDTTT-C-T exhibited a power conversion efficiency as high as 7.79%, which is 19% higher than the random PBDTTT-C-T based PSCs. It was found that the improved photoabsorption and increase in charge carrier mobility due to high regioegulartity of conjugated polymer backbones and effective ordering between polymer chains are the most likely reasons for enhancement of power conversion efficiency in PSCs.
1. INTRODUCTION Polymer solar cells (PSCs) have attracted great attention due to their advantages of flexibility, lightweight, low costs of materials, and low fabrication costs.1-6 The bulk heterojuction (BHJ) consisting of electron donor polymer blended with electron acceptor material such as fullerenes is used as a typical active layer in PSCs.7-10 The electron donor polymers for polymer solar cells (PSCs) are required to possess several characteristics which are efficient light harvesting ability, high hole mobility, proper HOMO and LUMO energy levels, and good miscibility with the acceptor materials.11-14 The electronic and optical properties of electron donor polymers are basically determined by the molecular structures of monomers. Moreover, they depend strongly on the structural regularity. For example, regioregular poly(3-hexylthiophenes) (P3HT) based PSCs exhibited higher power conversion efficiency due to improved light absorption and charge carrier mobility compared to P3HT with lower head-tail content.15-17 Additionally, the regioregular P3HT could provide inherent advantages such as well-defined molecular structures with defined molecular weight and little batch-to-batch variations. Thus, they can be prepared with high purity and reproducibility.18,19 Recently, the series of poly(thieno[3,4-b]thiophene)benzothiophene (PTB) copolymers20-22 comprised of thieno[3,4b]thiophene (TT)23 and benzo[1,2-b:4,5-b']dithiophene (BDT)24-26 segments such as PTB727-30 PBDTTT-C,31,32 and PBDTTT-C-T33,34 have been developed for high-efficiency
PSCs. Generally, the PTB copolymers have been synthesized by using the palladium catalyzed Stille or Suzuki polycondensation reaction with a dibrominated TT segment and bis-stannylated or bis-boronated BDT segment. Although the PTB copolymers possess various merits mentioned above, they possess intrinsically a structural drawback, negatively influencing the photovoltaic performance of solar cell devices. Since the TT segment has an asymmetric molecular structure, it is impossible to control the orientation of the TT segment in conjugated polymer backbone during the polycondensation reaction. Accordingly, the PTB copolymers basically possess random structural regioregularity. However, in spite of intensive research for the PTB copolymers, little is known about the effect of the structural regioregularity of the PTB copolymers on the physical properties and photovoltaic performance because of synthetic difficulty of the regioregular PTB copolymers. In this Article, we report synthesis of a regioregular ptype copolymer PBDTTT-C-T comprised of TT-BDT-TTBDT repeating units and perfectly controlled TT orientation using kinetically controlled mono-bromination on the 6-position of TT segment and the Stille coupling reaction. The regioregular PBDTTT-C-T exhibited lower optical bandgap (1.55 eV) and higher degree of crystallinity compared to the random PBDTTT-C-T without structural regioreguarity. The inverted bulk heterojunction PSCs based on the regioregular PBDTTT-C-T showed a power
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Chemistry of Materials
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conversion efficiency as high as 7.79%, which is 19% higher than the random PBDTTT-C-T based PSCs. 2. EXPERIMENTAL SECTION The synthetic route of the regioregular PBDTTT-C-T is shown in Scheme 1. In order to synthesize the regioregular PBDTTT-C-T, it is required to synthesize a monohalogenated TT segment to control the orientation of the TT segment. However, it is very difficult to selectively introduce a bromine atom on 4- or 6-position of the TT segment (1).35 Therefore, we carried out kinetically controlled mono-bromination on 6-position of the TT segment to obtain a mono-brominated TT segment (2). It is noteworthy that the mono-brominated TT segment was successfully isolated in pure form for the first time. Two mono-brominated TT segments were then coupled with a bis-trimethylstannylated BDT to afford TT-BDT-TT (3) using the palladium catalyzed Stille coupling reaction. Therefore, sulfur atoms in both TT segments of TT-BDTTT (3) could be positioned facing each other with the BDT segment between them, leading to perfectly controlled structural regioregularity. Then, TT-BDT-TT (3) was dibrominated to afford monomer 1. Finally, monomer 1 was copolymerized with a bis-trimethylstannylated BDT to afford the regioregular PBDTTT-C-T using the palladium-catalyzed Stille polycondensation reaction. The random PBDTTT-C-T without structural regioreguarity was also synthesized to compare with the regioregular PBDTTT-C-T. 2-Ethyl-1-thieno[3,4-b]thiophene-2-ylhexan-1-one was purchased from 1-Materials and 1,1'-[4,8bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5b']dithiophene-2,6-diyl]bis[1,1,1-trimethylstannane] was synthesized according to the literature procedures. All reactions were carried out under nitrogen. Random PBDTTT-C-T was purchased from 1-Materials and also synthesized. 1-(6-Bromo-thieno[3,4-b]thiophene-2-yl)-2-ethylhexan-1-one (2). 2-ethyl-1-thieno[3,4-b]thiophene-2ylhexan-1-one (1) (2.00 g, 7.50 mmol) was added into a round flask with DMF (10 mL). A solution of Nbromosuccinimide (NBS) (1.33 g, 7.50 mmol) in DMF (10 mL) was added dropwise to the reaction mixture at a speed of 0.1 mL/min and stirred for 30 min at room temperature. The reaction mixture was poured into deionized (DI) water and extracted with ethyl acetate several times. The organic phase was dried over anhydrous sodium sulfate. The solvent was removed under vacuum. The residue was purified by column chromatography on silica gel using methylene chloride and hexane (1:2) as an eluent to obtain pure (2) (0.95 g, 36.6%) as orange oil. 1H NMR (400 MHz, CDCl3): δ 7.45 (s, 1H), 7.24 (s, 1H), 3.23-3.16 (m, 1H), 1.84-1.74 (m, 2H), 1.66-1.50 (m, 2H), 1.34-1.22 (m 4H), 0.980.78 (m, 6H). 13C NMR (100 MHz, CDCl3): δ 199.80, 152.13, 146.53, 138.86, 120.64, 122.70, 103.90, 49.44, 32.19, 29.81, 25.96, 22.88, 13.91, 12.03. HRMS (m/z): calcd for C14H17BrOS2, m/z = 345.99; found 346.00 [M]+. 2-Ethyl-1-(6-{6-[2-(2-ethyl-hexanoyl)-thieno[3,4b]thiophen-6-yl]-4,8-bis-[5-(2-ethyl-hexyl)-thiophen-
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2-yl]-1,5-dithia-s-indacen-2-yl}-thieno[3,4-b]thiophen2-yl)-hexan-1-one. (3). 1,1'-[4,8-bis[5-(2-ethylhexyl)-2thienyl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]bis[1,1,1trimethylstannane] (0.99 g, 1.10 mmol), tetrakis (triphenylphosphine)palladium(0) (0.06 g, 0.06 mmol)
Scheme 1 The synthetic route for regioregular PBDTTT-C-T and random PBDTTT-C-T. Condition: a) NBS, DMF, rt. 30 min, b) bis-trimethylstannylated BDT, Pd(PPh3)4, toluene, 110˚C; c) NBS, CHCl3, rt, 10 min. and compound 2 (0.95 g, 2.75 mmol) were dissolved in toluene (20 mL) The mixture was refluxed and stirred overnight at 110 °C under nitrogen. The mixture was cooled to room temperature, and then the organic phase was dried over anhydrous sodium sulfate, solvent was removed under vacuum. The residue was purified by column chromatography on silica gel using methylene chloride and hexane (1:1.5) as an eluent to obtain pure compound 3 (0.28 g, 22.9%) as red solid. 1H NMR (400 MHz, CDCl3,): δ 7.93 (s, 2H), 7.83 (s, 2H), 7.40-7.39 (d, 2H), 7.247.23 (d, 2H), 6.95-6.96 (d, 2H), 3.20-3.16 (m, 2H), 2.91-2.89 (m, 4H), 1.88-1.80 (m, 4H), 1.76-1.70 (m, 2H), 1.68-1.55 (m, 4H), 1.50-1.30 (m, 24H), 0.99-0.92 (m, 24H). 13C NMR (100 MHz, CDCl3): δ 199.58, 152.07, 146.36, 143.33, 140.86, 139.09, 137.20, 136.55, 129.72, 128.04, 125.62, 123.77, 121.96, 120.66, 111.60, 49.71, 41.45, 34.30, 32.58, 32.34, 29.90, 28.93, 26.09, 25.72, 23.04, 22.89, 14.18, 13.96, 12.21, 10.89. HRMS
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Chemistry of Materials
(m/z): calcd for C62H74O2S8, m/z = 1106.35; found, 1106.27 [M]+. Monomer 1. Compound 1 (0.28 g, 0.25 mmol) was added into a round flask with chloroform (10 mL). A solution of NBS (0.10 g, 0.55 mmol) in chloroform (10 mL) was added dropwise to the reaction mixture and stirred for 10 min at room temperature. The reaction mixture was poured into DI water and extracted with ethyl acetate several times. The organic phase was dried over anhydrous sodium sulfate and solvent was removed under vacuum. The residue was purified by column chromatography on silica gel using methylene chloride and hexane (1:1) as an eluent to obtain pure monomer 1 (0.22 g, 69.5%) as deep red solid. 1H NMR (400 MHz, CDCl3): δ 7.88 (s, 2H), 7.75 (s, 2H), 7.37-7.36 (d, 2H), 6.96-6.95 (d, 2H), 3.173.11 (m, 2H), 2.91-2.89 (m, 4H), 1.88-1.81 (m, 4H), 1.73-1.70 (m, 2H), 1.68-1.55 (m, 4H), 1.50-1.27 (m, 24H), 0.99-0.85 (m, 24H). 13C NMR (100 MHz, CDCl3): δ 199.23, 152.29, 146.56, 142.42, 142.38, 139.17, 137.19, 136.30, 135.76, 130.97, 128.10, 125.69, 123.92, 122.48, 120.98, 98.49, 49.71, 41.46, 34.30, 32.59, 32.26, 29.86, 28.94, 26.00, 25.73, 23.06, 22.87, 14.20, 13.95, 12.16, 10.90. HRMS (m/z): calcd for C62H74O2S8, m/z = 1264.16; found, 1264.23 [M]+. Polymerization : Regioregular PBDTTT-C-T was prepared by the Stille polycondensation reaction with monomer 1 (0.156 g, 0.123 mmol) and same equivalent of 1,1'[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5b']dithiophene-2,6-diyl]bis[1,1,1-trimethylstannane] (0.111 g, 0.123 mmol), and tetrakis(triphenylphosphine)palladium(0) (0.005 g, 4 mol%) in toluene (8 mL) and DMF (1.6 mL), respectively. The reaction mixture was stirred and refluxed for 24 h at 110 °C. The mixture was cooled to room temperature. Then, toluene was removed under vacuum. To remove byproducts and oligomers, soxhlet extractions were used with n-hexane and methanol and pure polymers were obtained by drying extracted solution from chloroform under vacuum. The elemental analytical and IR results of the regioregular and random PBDTTT-C-T are as follows. Regioregular PBDTTT-C-T. Yield: 88%. 1H NMR (400 MHz, CDCl3): δ 8.17-6.46 (br, 14H), 3.40-2.54 (br, 10H), .2.14-1.20 (br, 16H), 1.19-0.54 (br, 72H). Elemental analysis calcd (%) for (C96H112O2S12)n: C, 68.52; H, 6.71; S, 22.87 Found: C, 68.73; H, 6.75; S, 22.60% FT-IR (ATR): (cm-1) 2919.3 (-CH3, -CH2-), 2853.8 (-CH3, -CH2-), 1657.6 (C=O), 1454.1 (-CH3, -CH2-), 1174.5, 799.0 (=C-H) Random PBDTTT-C-T. 1H NMR (400 MHz, CDCl3): δ 8.02-6.64 (br, 7H), 3.49-2.54 (br, 5H), 2.12-1.20 (br, 8H), 1.19-0.49 (br, 36H). Elemental analysis calcd (%) for (C48H56OS6)n: C, 68.52; H, 6.71; S, 22.87 Found: C, 68.52; H, 6.67; S, 22.83% FT-IR (ATR): (cm-1) 2918.6 (-CH3, -CH2-), 2853.3 (-CH3, -CH2-), 1657.4 (C=O), 1454.0 (-CH3, -CH2-), 1173.4, 797.5 (=C-H) Device Fabrication and Characterization BHJ polymer solar cell devices are prepared as follow. An Indium Tin Oxide (ITO) coated glass was cleaned by ultrasonic treatment in acetone, DI water, and isopropyl alcohol and dried by using nitrogen gas. The cleaned ITO
coated glass was treated in an UV-ozone chamber for 20 min and immediately spin-coated with a ZnO solution. The ZnO layer was formed by sol-gel method. The sol-gel derived ZnO was prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 1.0 g) and ethanolamine (NH2CH2CH2OH, 0.28 g) in 2-methoxyethanol (CH3OCH2CH2OH, 10 mL) under vigorous stirring for 24 h in the air. The ZnO coated ITO glass was annealed on a hot plate for 1 h at 200 °C in air. The thickness of the ZnO layer was approximately 40 nm. The polymer and [6,6]phenyl-C71-butyric acid methyl ester (PC71BM), which were blended in different weight ratio, were dissolved in chlorobenzene with 3 vol% DIO. Then, the polymer:PC71BM solution was spin-coated on top of the ZnO layer and dried for 60 min at room temperature. Finally, an anode layer composed of a MoO3 layer (10 nm) and an Ag layer (100 nm) was deposited by thermal evaporation with the shadow mask in a high vacuum thermal evaporator (
