To increase the open circuit voltage (VOC) 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‐C61‐butyric acid methyl ester (PCBM) as electron acceptor were investigated. The obtained two devices possessed both relatively large short circuit current (ISC) and higher VOC than that of P3HT:PCBM blend. For PCTBDT:PCBM blend, a power conversion efficiency (PCE) up to 2.32%, an ISC of 6.94 mA · cm?2, and a VOC of 0.80 V were observed while PCTDT:PCBM system demonstrated a PCE of 1.75% with a VOC of 0.68 V.
