Synthesis and photovoltaic properties of copolymers based on benzo[1,2-b:4,5-b′]dithiophene and thiazole with different conjugated side groups

Two new copolymers (PT-Tz-DTBT and PT-Tz-DTffBT) of benzo[1,2-b:4,5-b′]dithiophene and thiazole (Tz) with different conjugated side groups 4,7-dithien-5-yl-2,1,3-benzodiathiazole (DTBT) and 5,6-difluoro-4,7-bis(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTffBT) were synthesized via the Stille coupling polymerization and developed for polymer solar cell applications. The thermal, photophysical, electrochemical and photovoltaic properties of the copolymers were investigated. The two copolymers exhibited relatively low HOMO energy levels of −5.58 eV for PT-Tz-DTBT and −5.59 eV for PT-Tz-DTffBT, respectively. Bulk heterojunction (BHJ) solar cells with two as-synthesized copolymers as electron donors and (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor exhibit power conversion efficiencies (PCEs) of 2.63% and 3.75% for PT-Tz-DTBT and PT-Tz-DTffBT, respectively. When PC71BM was used as an electron acceptor, a promising PCE of 3.65% and 4.42% for PT-Tz-DTffBT was achieved by using a conventional and inverted device structure, respectively.