The advantage of dimethyl-2H-benzimidazole compared to the benzothiadiazole moiety of PCDTBT is to improve the solubility of the polymer while keeping the 1,2-quinoid form to lead coplanarity of the backbone. New random copolymers, to broaden the absorption range for the wider coverage of the solar spectrum, were synthesized by Stille coupling reactions to generate PFDTMBIs (or PFBBTMBIs). The PFDTMBIs in solid films show two maxima peaks at about 420-427 and 626-641 nm. The solid films of PFBBTMBIs show absorption bands with two maximum peaks at about 443-452 and 643-652 am which are red-shifted about 20 am as compared to PFDTMBIs, which is caused by the introduction of more rigid bithiophene unit in their polymer backbones to improve the photocurrents. The device with PFBBTMBI5:PC61BM blend demonstrated a V-OC value of 0.68 V, a J(SC) value of 3.10 mA/cm(2), and an FF of 0.35, leading to the efficiency of 0.74%. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.