Two copolymers comprised of different fused thiadiazole electron withdrawing units and benzo[1,2-b:4,5-b'] dithiophene electron donating unit have been synthesized by Stille reaction. The structural, optical, electrochemical and photovoltaic properties of the copolymers were investigated. Both copolymers in the film state exhibited abroad absorption spectra with an extremely low bandgap of similar to 1.2 eV. Bulk heterojunction solar cells were further fabricated and the best device delivered a power conversion efficiency of 0.52% when thermal annealed at 90 degrees C. (C) 2014 Elsevier B.V. All rights reserved.