A series of low band gap polymers composed of benzotriazole and benzo[1,2-b:4,5-b']dithiophene derivatives were synthesized using a Stille cross-coupling reaction for use in organic photovoltaic devices. Linear or branched alkyl groups were incorporated into the benzothiazole-based accepting monomer part, and alkoxy or alkylthiophene groups were introduced to benzo[1,2-b:4,5-b'] dithiophene-based donating monomer part. Changes in photo-physical properties of the polymers by the structural modification of the donor-acceptor type low band gap polymers were investigated. The synthesized polymers were soluble in common organic solvents, and the resulting polymer solutions could be used to form smooth and uniform thin films by spin-casting. The synthesized polymers were found to exhibit good thermal stability, losing less than 5% of their weight upon heating to approximately 300 degrees C. The intra-molecular charge transfer interaction between the electron donating and electron accepting blocks in the polymeric backbone induced a broad absorption from 300 to 650 nm. The optical band gap energies of the polymers were measured to be 2.03-1.90 eV depending on the polymer structure. Solution-processed field-effect transistors were fabricated and characterized using the polymers as p-type channel materials. Bulk hetero-junction photovoltaic devices were fabricated using the polymers with [6,6]-phenyl C-71-butyric acid methyl ester (PC71BM) as the electron acceptor. One of the fabricated device showed the maximum power conversion efficiency of 3.20% under AM 1.5 G (100 mW/cm(2)) conditions. (C) 2012 Elsevier B.V. All rights reserved.