A series of donor–acceptor (D–A) conjugated polymers P1–P4 was synthesized by copolymerization of a novel pentacyclic aromatic lactam acceptor unit, thieno[20,30:5,6]pyrido[3,4-g]thieno[3,2-c]-isoquinoline-5,11(4H,10H)-dione (TPTI), with a donor unit, benzo-[1,2-b:4,5-b0]dithiophene (BDT) or dithieno[3,2-b:20,30-d]silole (DTS). The effect of the donor units and the side chains on TPTI on polymer properties and solar cell performance was investigated. Bulk heterojunction solar cells based on P1 and PC71BM afforded the highest power conversion efficiency (PCE) of 5.30%.