Synthesis and characterization of dithienothiophene/benzothiadiazole based low band gap donor–acceptor copolymers for bulk hetero junction photovoltaic cells

Two newly designed low band gap copolymers P2TDTT-BT and P2TDTT-DTBT, comprised of a substituted dithienyl-dithieno[3,2-b:2′,3′-d]thiophene derivative as a donor and a benzothiadiazole or dithienyl benzothiadiazole based acceptor, respectively, were synthesized by Stille polymerization. The UV–visible absorption of P2TDTT-BT and P2TDTT-DTBT films ranged from the UV to NIR regions. The HOMO/LUMO energy levels of P2TDTT-BT and P2TDTT-DTBT were estimated to be −5.41/−3.74 eV and −5.34/-3.65 eV, corresponding to energy band gaps of 1.67 and 1.69 eV, respectively. Bulk heterojunction photovoltaic cells were fabricated using a blend of P2TDTT-BT or P2TDTT-DTBT and PC61BM in a 1:1 ratio and delivered power conversion efficiencies of 0.31% and 0.92%, respectively.