Alternating copolymers based on cyclopentadithiophene (CPDT) and five different electron-deficient aromatic units with reduced optical band gaps are synthesized via Suzuki coupling. All polymers show a significant photovoltaic response when mixed with a fullerene acceptor. The frontier orbital levels of the new polymers are designed to minimize energy losses by increasing the open-circuit voltage with respect to the optical band gap, while maintaining a high coverage of the absorption with the solar spectrum. The best cells are obtained for a copolymer of CPDT and benzooxadiazole (BO) with a band gap of 1.47 eV. This cell gives a short-circuit current of 5.4 mA cm-2, an opencircuit voltage of 0.78 V, and a fill factor of 0.6, resulting in a power conversion efficiency of about 2.5%.