Synthesis and photovoltaic performances of conjugated copolymers with 4,7-dithien-5-yl-2,1,3-benzothiadiazole and di(p-tolyl)phenylamine side groups

Abstract

Three new copolymers (PT-TPA, PT-DTBT and PT-DTBTTPA) based on benzo[1,2-b:4,5-b]dithiophene (BDT) and thiophene with different conjugated side chains (di(p-tolyl)phenylamine (TPA), 4,7-dithien-5-yl-2,1,3-benzothiadiazole (DTBT) and DTBT-TPA) were synthesized via Stille coupling polymerization. The TPA and the DTBT were introduced to improve the hole-transport ability and broaden the absorption spectrum. The effects of different conjugated side groups on thermal, optical, electrochemical, hole-transporting and photovoltaic properties of these copolymers were investigated. Field effect results show that the copolymer PT-DTBTTPA containing TPA and DTBT in the side chain showed the highest hole mobility. The three copolymers exhibit deep-lying HOMO energy levels, which were effectively tuned by changing the side groups. Photovoltaic cells were fabricated with the synthesized copolymers as electron donors and [6,6]-phenyl-C-butyric acid methyl ester (PCBM) as the electron acceptor. Bulk heterojunction polymer solar cells based on PT-DTBT and PT-DTBTTPA showed promising power conversion efficiencies of 5.50% and 5.16%, respectively.