Semiconducting copolymers based on benzo[1,2-b:4,5-b']dithiophene and thieno[3,4-c]pyrole-4,6-dione containing oligo(thienylenevinylene) side chains with different lengths. were synthesized to examine the effect of the pi-conjugated side chains on the performance of polymer solar cells (PSCs). Using the copolymers that had pi-conjugated side chains as the PSC electron donor resulted in a higher short circuit current and fill factor compared with the reference copolymers, which had no side chain or an analogous side chain with no pi-conjugation, resulting in an increase of power conversion efficiency of 10-22%. Measurements of hole mobility by space-charge-limited current and internal quantum efficiency indicated that introducing the pi-conjugated side-chain units can facilitate both charge transport and charge separation in the polymer:PC71BM blend films.