The benzo[c][1,2,5] thiadiazole (BT) based copolymer, poly[(4,8-bis(5-(2-ethylhexyl) thiophen-2-yl) benzo[1,2-b: 4,5-b'] dithiophene)-alt-(diethyl 4,7-di(thiophen-2-yl) benzo[c][1,2,5] thiadiazole-5,6-dicarboxylate)] (PBDTBT-COOR), was synthesized by Stille polycondensation. The BT unit with carboxylate ester groups was designed and synthesized as an electron-withdrawing building block for donor-acceptor type copolymers with the aim of achieving good solubility and low-lying highest occupied molecular orbital energy levels of the synthesized polymer. Consequently, the synthesized PBDTBT-COOR copolymer showed excellent solubility in common organic solvents and a relatively deep HOMO energy level. An inverted organic photovoltaic device was fabricated using PBDTBT-COOR and [6,6]-phenyl C-71 butyric acid methyl ester as the electron donor and the acceptor, respectively, with the device structure of indium tin oxide/ZnO/active layer/MoO3/Ag. Under the illumination condition of AM 1.5 G, i.e., 100 mW/cm(2), the maximum power conversion efficiency of the device was 3.15% with an open-circuit voltage of 0.87 V, short circuit current of 9.68 mA/cm(2), and a fill factor of 37.6%.