Synthesis and Photovoltaic Properties of Low Band Gap Polymers Containing Benzo[1,2-b:4,5-c′]dithiophene-4,8-dione Jiamin Cao, †,‡ Wei Zhang, ‡ Zuo Xiao, ‡ Lingyan Liao, ‡ Weiguo Zhu,* ,† Qiqun Zuo, § and Liming Ding* ,‡ † Key Lab of Environment-Friendly Chemistry and Application of the Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China ‡ National Center for Nanoscience and Technology, No. 11 Beiyitiao, Zhongguancun, Beijing 100190, China § Jiahong Optoelectronics, Suzhou 215151, China * S Supporting Information A s the energy crisis is becoming a serious problem all over the world, cheap production of electricity from solar energy has attracted more and more attention. Among all the photovoltaic devices, polymer bulk heterojunction solar cells (PSCs) have shown many advantages, such as low fabrication cost, excellent mechanical flexibility and lightweight. The active layer of PSCs is based on the blend of a conjugated polymer as the electron donor and a fullerene derivative as the electron acceptor. 1 Up to now, poly(3-hexylthiophene) (P3HT) is one of the best commercialized donor materials. P3HT/PC 61 BM- based PSCs have achieved power conversion efficiencies (PCE) up to ∼4-5%. 2 However, P3HT only absorbs part of the solar emission (300-600 nm) and its high-lying HOMO energy level (-4.76 eV) 3 limits the open circuit voltage (V oc ) of the solar cells. Therefore, design and synthesis of new conjugated polymers with deep HOMO levels for high V oc and low bandgaps for broad absorption, as well as high mobilities and good solubilities, are essential for obtaining high performance PSCs. 4 A recent strategy to satisfy these requirements above is to design conjugated copolymer containing alternating donor and acceptor units in the polymer backbone. Yu et al. reported the D-A polymers based on poly(benzo-[1,2- b :4,5- b ′ ]- dithiophene)-alt-(thieno[3,4-b]thiophene) (PBDTTT) with efficiency up to 7.73%. 5 However, the synthesis of the monomer thieno[3,4-b]thiophene in PBDTTT is very tedious and with low yield. Li et al. reported a new strong electron- withdrawing unit naphtho[2,3-c]thiophene-4,9-dione (NTDO), which was synthesized in short route and with high yield. The copolymer with NTDO as acceptor unit has relatively lower HOMO (-5.52 eV) and gives a high PCE of 5.21%. 6 Inspired by Li’s work, we designed a new acceptor unit benzo[1,2-b:4,5-c′ ]- dithiophene-4,8-dione (BDTD) by replacing the benzene ring of NTDO with thiophene ring because the fused thiophene ring systems are well-known to stabilize the quinoidal structure and can efficiently reduce the bandgap (Scheme 1). 7 Furthermore, BDTD unit possesses the similar structure as thieno[3,4-b]thiophene but more electron deficient. In this work, we synthesized two low-bandgap polymers P1 and P2 by copolymerization of BDTD with benzo-[1,2-b:4,5-b′ ]dithiophene (BDT) and dithieno[3,2- b:2′,3′-d]silole (DTS), respectively. Both P1 and P2 exhibit low-lying HOMO and good light-absorption properties. We further demonstrated that P1 and P2 are good donor materials in PSCs. Solar cells based on P2/PC 61 BM blend afforded a PCE of 4.33% with an impressively high V oc , 0.93 V. The synthetic routes for monomers and polymers were illustrated in Scheme 1. Thiophene-3,4-dicarboxylic was brominated by bromine in acetic acid to give 2,5-dibromothio- phene-3,4-dicarboxylic acid in 50% yield. Treating 2,5- dibromothiophene-3,4-dicarboxylic acid with oxalyl chloride afforded the intermediate 2,5-dibromothiophene-3,4-dicarbonyl dichloride. The 2,5-dibromothiophene-3,4-dicarbonyl dichlor- ide was then treated with 2-hexylthiophene in presence of AlCl 3 to obtain monomer BDTD in 12% yield. Monomers BDT and DTS were prepared according to the literature methods. 8,9 P1 and P2 were both synthesized by the Stille reaction using Pd(PPh 3 ) 4 as the catalyst. The crude products were precipitated in methanol. The impurities in the products were successively removed by methanol and hexane in a Soxhlet extractor 24 h each. Finally, the polymers were extracted with chloroform and reprecipitated in methanol and dried under vacuum. P1 and P2 have good solubilities in common organic solvents such as chloroform and toluene. The molecular weights of P1 and P2 were determined by gel permeation chromatography (GPC) against polystyrene standards in THF eluent, and the data are listed in Table 1. Thermogravimetric analyses of P1 and P2 are shown in Figure 1. The temperature of 5% weight-loss was selected as the onset point of decomposition (T d ). The T d values of P1 and P2 are 329 and 346 °C, respectively, indicating that they are thermally stable for organic photovoltaic applications. The absorption spectra of the polymers in chloroform and films are shown in Figure 2. The optical properties of P1 and P2 are summarized in Table 2. Both polymers show strong absorption in the range of 500-650 nm. The absorption maxima of the polymers in solution are located at 551 and 586 nm for P1 and P2, respectively. P2 shows stronger absorp- tion at the long wavelength region than P1, while P1 exhibits a broader spectrum, especially in the 400-500 nm region. In solid films, both P1 and P2 show red shifts at the absorption maxima (576 nm for P1 and 626 nm for P2), indicating the enhanced interchain π-π stacking in the solid state. The optical band gaps were calculated to be 1.78 eV for P1 and 1.63 eV for P2. Received: November 28, 2011 Revised: January 11, 2012 Published: January 23, 2012 Note pubs.acs.org/Macromolecules © 2012 American Chemical Society 1710 dx.doi.org/10.1021/ma202578y | Macromolecules 2012, 45, 1710-1714