Synthesis and photovoltaic properties of low-bandgap alternating copolymers consisting of 3-hexylthiophene and [1,2,5]thiadiazolo[3,4-g]quinoxaline derivatives

Two different kinds of low-bandgap alternating copolymers consisting of 3-hexylthiophene and [1,2,5]thiadiazolo[3,4-g]quinoxaline were synthesized via the Stille coupling reaction. The copolymers show broad absorption band from visible to infrared region. The bandgaps (1.02–1.35 eV) of the copolymers are successfully tuned by changing the substituent on the acceptor unit. The copolymers are highly soluble in common organic solvents due to three 3-hexylthiophenes in the repeating unit. By optimizing the device parameters such as the blend ratio of copolymer to PCBM and the choice of processing solvent, the power conversion efficiency reaches as high as 1.27% under the standard solar radiation condition (AM 1.5G, 100 mW/cm2).