An alternating copolymer composed of heal-to-tail-structured 3,4'-dihexyl-2,2'-bithiophene (DHBT) and pyrene units [poly(DHBT-alt-PYR)] was synthesized using a Stille coupling reaction for use in photovoltaic devices as a p-type donor. For the reduction of the bandgap energy of poly(DHBT-alt-PYR), 4,7-bis(3'-hexyl-2,2'-bithiophen-5-yl)benzo[c][1,2,5]thiadiazole (BHBTBT) units were introduced into the polymer. Poly(DHBT-co-PYR-co-BHBTBT)s were synthesized using the same polymerization reaction. The synthesized polymers were soluble in common organic solvents and formed smooth thin films after spin casting. The optical bandgap energies of the polymers were obtained from the onset absorption wavelengths. The measured optical bandgap energy of poly(DHBT-alt-PYR) was 2.47 eV. As the BHBTBT content in the ter-polymers increased, the optical bandgap energies of the resulting polymers decreased. The bandgap energies of poly(50DHBT-co-40PYR-co-10BHBTBT) and poly(50DHBT-co-20PYR-co-30BHBTBT) were 1.84 and 1.73 eV, respectively. Photovoltaic devices were fabricated with a typical sandwich structure of ITO/PEDOT:PSS/active layer/LiF/Al using the polymers as electron donors and [6,6]-phenyl C71-butyric acid methyl ester as the electron acceptor. The device using poly(50DHBT-co-20PYR-co-30BHBTBT) showed the best performance among the fabricated devices, with an open-circuit voltage, short-circuit current, fill factor, and maximum power conversion efficiency of 0.68 V, 5.54 mA/cm2, 0.35, and 1.31%, respectively. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012