New Conjugated Copolymers Based on Benzo[1,2-b; 3,4-b 0 ]dithiophene and Derivatives of Benzo[g]quinoxaline for Bulk Heterojunction Solar Cells PO-I LEE, STEVE LIEN-CHUNG HSU, JUNG FENG LEE, HUNG-YI CHUANG, PIYUN LIN Department of Materials Science and Engineering, National Cheng-Kung University, Tainan, Taiwan 701-01, Republic of China Received 14 September 2010; accepted 31 October 2010 DOI: 10.1002/pola.24477 Published online 2 December 2010 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: A series of new low-band gap copolymers based on dioctyloxybenzo[1,2-b;3,4-b 0 ] dithiophene and bis(2-thienyl)-2,3- diphenylbenzo[g]quinoxaline monomers have been synthesized via a Stille reaction. The effect of different functional groups attached to bis(2-thienyl)-2,3-diphenylbenzo[g]quinoxaline was investigated and compared with their optical, electrochemical, hole mobility, and photovoltaic properties. Polymer solar cell (PSC) devices of the copolymers were fabricated with a configuration of ITO/ PEDOT: PSS/copolymers: PCBM (1:4 wt ratio)/Ca/Al. The best performance of the PSC device was obtained by using PbttpmobQ as the active layer. A power conversion efficiency of 1.42% with an open-circuit voltage of 0.8 V, a short-circuit current (JSC) of 5.73 mA cm 2 , and a fill factor of 30.9% was achieved under the illumi- nation of AM 1.5, 100 mW cm 2 . V C 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 662–670, 2011 KEYWORDS: benzodithiophene; benzoquinoxaline; conjugated poly- mers; copolymerization; monomer; polymer; solar cell; synthesis INTRODUCTION In recent years, polymer solar cells (PSCs) have attracted great attention as a new generation of renew- able energy sources due to flexibility, low cost, light weight, and easy manufacturing. 1–4 The bulk heterojunction solar cells that are composed of an electron-donating conjugated polymer blended with an electron acceptor have played a major role in reaching high efficiencies. 5–7 At present, the most widely investigated PSCs are based on blends of regioregular poly(3-hexylthiophene) (P3HT) with 6.6-phenyl C 61 -butyric acid methyl ester (PC 61 BM), which have achieved a power conversion efficiency (PCE) of 4 to 6%. 8–10 How- ever, the PCE of a P3HT/PC 61 BM blended system is limiting, because its absorption wavelength is less than 650 nm. To improve device performance, many researches have devel- oped conjugated polymers with low-band gap, high mobility, and broader absorption of the solar spectrum. 11–18 Internal charge transfer (ICT) from an electron donor (D) to electron acceptor (A) has been used to synthesize low-band gap conjugated polymers. 19,20 Several new D–A polymers were developed, which exhibited good performance. Conju- gated copolymers containing 3,6-dithiophen-2-yl-2,5-dihydro- pyrrolo[3,4-c]pyrrole-1,4-dione were applied to PSCs, and a high PCE of 4.45% was achieved. 21 Biniek et al. 22 reported [3,2-b]thienothiophene-alt-benzothiadiazole copolymer for photovoltaic applications with PCE up to 5.2%. In this work, we developed a series of copolymers based on dioctyloxybenzo [1,2-b;3,4-b 0 ]dithiophene and bis(2-thienyl)- 2,3-diphenylbenzo[g]quinoxaline. Benzo[1,2-b;3,4-b 0 ]dithiophene exhibits a large planar conjugated structure, and its copolymers have a high hole mobility. 23 Bis(2-thienyl)-2,3-bis-(4-phenyl)- benzo[g]quinoxaline is a monomer with a D–A molecular struc- ture, and can be used to lower the band gap of copolymers. We will discuss the properties of the copolymers with different electron-withdrawing and electron-donating functional groups. EXPERIMENTAL Materials 8-Dihydrobenzo[1,2-b:4,5-b 0 ]dithiophen-4,8-dione (1) was prepared according to the published method. 24 Trimethyltin chloride, and 2-(tributylstannyl)-thiophene (8) were obtained from Aldrich Chemicals. 1-Bromooctane, bromine, 2,3-diamino- naphthalene (4), 4,4 0 -dimethoxybenzil (6a), benzil (6b), 4,4 0 - difluorobenzil (6c), tetrakis(triphenylphosphine) palladium [Pd(PPh 3 ) 4 ], N-bromosuccinimide (NBS), and bis(triphenyl- phosphine)palladium(II) dichloride [PdCl 2 (PPh 3 ) 2 ] were pur- chased from Acros Organics. Tetrabutylammonium bromide (TBAB) was obtained from TCI. n-Butyllithium was obtained from Strem Chemicals. Toluene, dichlorobenzene, and N,N- dimethylformamide (DMF) were obtained from TEDIA. All reagents were used as received. Measurements and Characterization 1 H and 13 C NMR spectra were recorded on a Brucker Advance 600 spectrometer, and the deuterated CDCl 3 , and DMSO were used as solvents, and the chemical shifts were reported in ppm. The molecular weights and distributions of the polymers were obtained by using a Waters GPC 2414 in Correspondence to: S. L.-C. Hsu (E-mail: lchsu@mail.ncku.edu.tw) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 662–670 (2011) V C 2010 Wiley Periodicals, Inc. 662 WILEYONLINELIBRARY.COM/JOURNAL/JPOLA