Five novel conjugated copolymers (P1-P5) containing coplanar cyclopentadithiophene (CPDT) units (incorporated with arylcyanovinyl and keto groups in different molar ratios) were synthesized and developed for the applications of polymer solar cells (PSCs). Polymers P1-P5 covered broad absorption ranges from U-V to near infrared (400-900 nm) with narrow optical band gaps of 1.38-1.70 eV, which are compatible with the maximum solar photon reflux. Partially reversible p- and n-doping processes of P1-P5 in electrochemical experiments were observed, and the proper molecular design for highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels of P1-P5 induced the highest photovoltaic open-circuit voltage in the PSC devices, compared with those previously reported CPDT-based narrow-band-gap polymers. Powder X-ray diffraction (XRD) analyses suggested that these copolymers formed self-assembled pi-pi stacking and pseudobilayered structures. Under 100 mW/cm(2) of AM 1.5 white-light illumination, bulk hetero-junction PSC devices containing an active layer of electron donor polymers P1-P5 mixed with electron acceptor [6,6]-phenyl C-61 butyric acid methyl ester (PCBM) in the weight ratio of 1:4 were investigated. The PSC device containing P1 gave the best preliminary result with an open-circuit voltage of 0.84 V, a short-circuit current of 2.36 mA/cm(2), and a fill factor of 0.38, offering an overall power conversion efficiency (PCE) of 0.77% as well as a maximal quantum efficiency of 23% from the external quantum efficiency (EQE) measurements. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2073-2092, 2009