Two new low-bandgap alternating copolymers (CEHTF and CEHTP) consisting of 4,6-bis(3'-(2-ethylhexyl)thien-2'-yl)thieno[3,4-c][1,2,5] thiadiazole and 9,9-bis(2-ethylhexyl)fluorene or 2,5-bis(isopentyloxy)benzene were synthesized by Suzuki coupling reaction of corresponding comonomers. Their optical, electrochemical, and photovoltaic (PV) properties were studied and are reported. Both the copolymers exhibited long-wavelength absorption covering the whole visible spectral region, which is in CEHTP thin films extended up to near infrared region, ambipolar redox properties, and electrochromism. High-electron affinities and low-optical bandgap values, 1.37 and 1.15 eV, were determined for CEHTF and CEHTP, respectively. PV devices with bulk heterojunction made of blends of copolymers and fullerene derivative [6,6]-phenyl-C-61-butyric acid methyl ester ([60]PCBM) were prepared and characterized. Effects of intramolecular charge transfer strength and side-chain nature and length on photophysical properties are discussed. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 3426-3436, 2011