A series of benzo.,[1,2-b:4,5-b']dithiophene an thieno[3,4-c]pyrrole-4,6-dione (BDT-TPD) based copolymers (P1-P4) with D-pi-A structures are designed and synthesized: When the pi bridges change from 2,2'-bithiophene to thieno[3,2-b]thiophene and then to 3-hexylthieno[3,2-b)]-thiophene, the performance of the polymer photovoltaic devices Shows Significant improvement Especially, the device based On P4 with alkylthienyl-substituted Bar and pi bridge of 3-hexylthieno[3,2-b]thiophene exhibits power conversion efficiency as high as 7.71%, and the short-circuit current of the cells:is up to 13.70 mA cm(-2). The results indicate that the Modulation of pi bridges is a feasible way to achieve highly efficient polymer solar cells.