To increase the open circuit voltage (V-OC) of polymer solar cells (PSCs) based on polythiophene, two new ester group functionalized polythiophene derivatives, PCTDT and PCTBDT, were designed and synthesized via alternating copolymerization of thiophene-3-carboxylate (CT) with the 2,2'-bithiophene (DT) and benzodithiophene (BDT) units, respectively. The resulting copolymers exhibited broad and strong absorptions in the visible region, which was similar to that of the commonly used poly(3-hexylthiophene) (P3HT). Through cyclic voltammetry measurements, it was found that both copolymers showed lower HOMO energy levels (-5.27 eV for PCTDT and -5.36 eV for PCTBDT) than that of P3HT (-5.03 eV), indicating that the HOMO energy level could be efficiently reduced by introducing the ester group into the polymer side chain. Photovoltaic properties of the copolymers blended with [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) as electron acceptor were investigated. The obtained two devices possessed both relatively large short circuit current (I-SC) and higher V-OC than that of P3HT:PCBM blend. For PCTBDT: PCBM blend, a power conversion efficiency (PCE) up to 2.32%, an I-SC of 6.94 mA.cm(-2), and a V-OC of 0.80V were observed while PCTDT:PCBM system demonstrated a PCE of 1.75% with a V-OC of 0.68 V.