A low band-gap conjugated polymer, PBDTDPP, composed of unsubstituted benzo[1,2-b:4,5-b']dithiophene and diketopyrrolo[3,4-c]pyrrole was synthesized. The deep HOMO level of PBDTDPP enhances the V(OC) of a PSC up to 0.82 V and exhibits a PCE of 5.16%, while alkoxy substituted PBDTDPP-OR yields a PCE of 2.24% with a V(OC) of 0.61 V.