A novel electron-accepting unit cyclopenta[2,1-b:3,4-c']dithiophene-4-one (CPDTO-c'), which is an isomer of CPDTO-b' was developed. CPDTO-c' can be incorporated into the DA backbone through 5, 7 positions. The 2 position of CPDTO-c' can be easily functionalized with an electron-withdrawing chain. By copolymerizing CPDTO-c' with four different donor units: benzo[1,2-b:4,5-b']dithiophene (BDT), dithieno[3,2-b:2',3'-d]silole (DTS), carbazole, and fluorene, four new conjugated copolymers P1P4 were obtained. All these polymers have good solubility and low-lying HOMO energy levels (-5.41 similar to -5.92 eV). Among them, P1 and P2 exhibit broad absorption and narrow optical bandgaps of 1.91 and 1.72 eV, respectively. Solar cells based on P1/PC71BM afforded a PCE up to 2.72% and a high Voc up to similar to 0.9 V.