Synthesis of novel narrow-band-gap copolymers based on [1,2,5]thiadiazolo[3,4-f]benzotriazole and their application in bulk-heterojunction photovoltaic devices

Two novel conjugated alternating copolymers with [1,2,5]thiadiazolo[3,4-f]benzotriazole as acceptor and 9,9-dioctylfluorene or N-9’-heptadecanyl-carbazole as donors respectively, were synthesized by Suzuki polycondensation. Both of the two copolymers have nearly ideal band gaps and show excellent absorption spectra in near infrared region. Polymer solar cells based on the blends of them and [6,6]-phenyl-C71 butyric acid methyl ester show excellent performance when using a water/alcohol soluble conjugated polymer as cathode interlayer, which exhibit a maximum power conversion efficiency of 3.17% with the short-circuit current density of 8.50 mA/cm2, the open-circuit voltage of 0.70 V and the fill factor of 41%. Our results demonstrate that [1,2,5]thiadiazolo[3,4-f]benzotriazole is a promising acceptor unit for low band gap polymer donor materials design.