A series of novel narrow-band-gap copolymers (P1-P12) composed of alkyl-substituted fluorene (FO) units and six analogous mono- and bis(2-aryl-2-cyano-vinyl)- 10-hexylphenothiazine monomers (M1-M6) were synthesized by a palladium-catalyzed Suzuki coupling reaction with two different feed in ratios of FO to M1-M6 (molar ratio = 3:1 and 1:1). The absorption spectra of polymers P1-P12 exhibited broad peaks located in the UV and visible regions from 400 to 800 nm with optical band gaps at 1.55-2.10 eV, which fit near the wavelength of the maximum solar photon reflux. Electrochemical experiments displayed that the reversible p- and n-doping processes of copolymers were partially reversible, and the proper HOMO/LUMO levels enabled a high photovoltaic open-circuit voltage. As blended with [6,6]-phenyl C-61 butyric acid methyl ester (PCBM) as an electron acceptor in bulk heterojunction photovoltaic devices, narrow-band-gap polymers P1-P12 as electron donors showed significant photovoltaic performance which varied with the intramolecular donor-acceptor interaction and their mixing ratios to PCBM. Under 100 mW/cm(2) of AM 1.5 white-light illumination, the device of copolymer P12 produced the highest preliminary result having an open-circuit voltage of 0.64 V, a short-circuit current of 2.70 mA/cm(2), a fill factor of 0.29, and an energy conversion efficiency of 0.51%. (c) 2008 Wiley Periodicals, Inc.