A series of semiconducting copolymers (PTAT-x) containing extended pi-conjugated tetrathienoanthracene units have been synthesized. It was shown that the extended conjugation system enhanced the pi-pi stacking in the polymer/PC61 BM blend films and facilitated the charge transport in heterojunction solar cell devices. After structural fine-tuning, the polymer with bulky 2-butyloctyl side chains (PTAT-3) exhibited a PCE of 5.6% when it was blended with PC61BM.