A new fused building block benzothiadiazole[1,2-b:4,3-b'] dithiophene (BTDT) was prepared by covalently locking thiophene unit on both sides of benzothiadiazole (BT). On the basis of this building block, a series of conjugated copolymers containing homopolymer (P1) or electron-rich comonomers such as carbazole (P2), benzodithiophene (P3 and P4) and thiophene (P5) were obtained. All polymers have good solubility in common organic solvents. The thermal, optical, electrochemical and photovoltaic properties of the polymers were investigated systematically. The thiophene units, which were covalently fastened to the BT moiety, enlarged the planarization of the polymer backbone and thus induced stronger intermolecular pi-pi interaction, meanwhile, decreased the electron-withdrawing ability of the BT unit. The device based on P3:PC71BM exhibited a high open-circuit voltage (V-OC) of 0.96 V and moderate power conversion efficiency (PCE) of 2.16%. (C) 2014 Elsevier B.V. All rights reserved.