Synthesis of Thieno[3,4-b]pyrazine-Based and 2,1,3-Benzothiadiazole-Based Donor−Acceptor Copolymers and their Application in Photovoltaic Devices

Two kinds of thieno[3,4- b ]pyrazine-based monomers, 2,3-dimethyl-5,7-di(2-bromothien-5-yl)-thieno[3,4- b ]pyrazine and 2,3-diphenyl-5,7-di(2-bromothien-5-yl)-thieno[3,4- b ]pyrazine, were synthesized via an improved synthetic route. These two monomers and 4,7-di(2-bromothien-5-yl)-2,1,3-benzothiadiazole were copolymerized with three donor segments (fluorene, carbazole, and indolocarbazole) separately by a Suzuki cross-coupling reaction to give six types of 5,7-dithien-2-yl-thieno[3,4- b ]pyrazine (DTTP)-based donor−acceptor (D−A) copolymers (TP1-6) and three types of 5,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based D−A copolymers (PF-DTBT, PC-DTBT, and PIC-DTBT). The optical properties, electrochemical behavior, and energy levels of these nine copolymers were investigated. The photovoltaic performance of the copolymers was compared and discussed considering their energy levels.