We have developed a ladder-type multifused thienyl-phenylene-thienylene- phenylene-thienyl (TPTPT) unit where each thiophene ring is covalently fastened with the adjacent benzene rings by a carbon bridge, forming four cyclopentadiene rings embedded in a nonacyclic structure. This rigid and coplanar TPTPT building block was copolymerized with electron-deficient acceptors, dibromobenzothiadiazole (BT) or dibromodithienyldiketopyrrolopyrrole (DPP), via Stille polymerization. By varying the feed ratio of the monomers, a new series of random copolymers PTPTPTBT11, PTPTPTBT12, PTPTPTDPP11, PTPTPTDPP12, and PTPTPTDPP13 with tunable optical and electronic properties were prepared. The PTPTPTDPP12/PC71BM (1:4, w/w) based device exhibited the highest short circuit current (Jsc) of 10.78 mA/cm2 with a good power conversion efficiency (PCE) of 4.3% due to the much boarder absorption ability and the highest hole mobility of PTPTPTDPP12. The devices based on PTPTPTDPP13, PTPTPTDPP11, PTPTPTBT12, and PTPTPTBT11 polymers also displayed promising efficiencies of 4.1%, 3.6%, 3.1%, and 2.8%, respectively. Most importantly, PTPTPTDPP12 has been demonstrated as a superior low-band-gap material for polymer solar cell with inverted architecture, achieving a high PCE of 5.1%.