A new donor-acceptor copolymer, containing benzodithiophene (BDT) and methyl thiophene-3-carboxylate (3MT) units, is designed and synthesized for polymer solar cells (PSCs). The 3MT unit is used as an electron acceptor unit in this copolymer to provide a lower highest occupied molecular orbital (HOMO) level for obtaining polymer solar cells with a higher open-circuit voltage (VOC). The resulting bulk heterojunction PSC made of the copolymer and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibits a power conversion efficiency (PCE) up to 4.52%, a short circuit current (JSC) of 10.5 mA center dot cm-2, and a VOC of 0.86 V.