New low-band-gap copolymers, including thieno[3,2-b] thiophene and 2,1,3-benzothiadiazole, were synthesized as photovoltaic materials. Thiophene was introduced to provide extended p-conjugation length and charge transfer properties. A band gap(E(g)(op) = 1.62 eV, E(g)(ec) = 1.51 eV) of this polymer was investigated through UV-vis spectroscopy and cyclic voltammetry. A bulk heterojunction structure of glass/indium tin oxide(ITO)/PEDOT: PSS/polymer-PCBM(1:3)/LiF/Al was fabricated for investigating photovoltaic properties. PC(71)BM was used as an acceptor material, due to its increased absorption in the visible region, incomparison with PC(61)BM. In this polymer, incident photon-to-current conversion efficiency(IPCE) was as high as 50%. Moreover, maximum power conversion efficiency(PCE) of upto 1.72% was achieved under AM 1.5G conditions. It demonstrated relatively high V(OC) (0.67 V) and J(SC) (6.86 mA/cm(2)), while a low fill factor value (0.37) was obtained. (C) 2009 Elsevier B.V. All rights reserved.