Stile-coupling of carboxylate-substituted dibrominated benzodithiophene (BDTC) with 2,5-distannylthieno-[3,4-b]thiophene gave novel donor acceptor type alternating copolymers, PBDTC-TT, where BDTC works as an electron-accepting unit in the polymers. They showed broad absorption bands from 500 nm to the near-infrared region, optical band gap (E-g) about 1.5 eV, small pi-stacking distance (3.6 angstrom), and good thermal stability. The hole mobilities of PBDTC-TT determined from performance of their organic field-effect transistors were 3.1-6.9 x 10(-4) (cm(2) V-1 s(-1)). The bulk heterojunction (BHJ) solar cells were fabricated with configuration of ITO/PEDOT:PSS/polymer:PC70BM/LiF/Al. A PBDTC-TT device exhibited photocurrent response upon exposure to light with wavelength of 300-900 nm and incident photon to current conversion efficiency over 40% in the range of 400-750 nm. The power conversion efficiency of the best-performed device reached 3.03% with short-circuit current density of 12.54 mA cm(-2), fill factor of 0.48, and open circuit voltage of 0.51 V under illumination of AM 1.5 G/100 mW cm(-2). These results show that the BDTC unit can behave as an electron accepting building block for donor acceptor type narrow band gap polymers, and these types of polymers can be used as a donor material in the active layer for BHJ photovoltaic cells.