A novel low band gap alternating phenylenevinylene copolymer, P. with dithenyl (thienothiadiazole) segments was synthesized by Heck coupling. It was soluble in common organic solvents, showed broad absorption curve with long-wavelength absorption maximum at 580-598 nm and optical band gap of 1.74 eV, which is comparable with the electrochemical band gap of about 1.80 eV. P (electron donor) was blended with PCBM or modified PCBM i.e. F (electron acceptor) to fabricate bulk heterojunction (BHJ) solar cells. The power conversion efficiency (PCE) of the devices based on P:PCBM and P:F cast from 1.2-dichlorobenzene (DCB) was found to be 1.40% and 2.32%, respectively. The higher value of PCE for the device with P:F as compared to P:PCBM is attributed to the increase in both short circuit current (J(sc)) and open circuit voltage (V(oc)). The increase in the J(sc) is due to the stronger light absorption of F in visible region as compared to PCBM, i.e. more exciton generation in the blend. On the other hand, the higher difference between the LUMO levels of P and F, as compared to P and PCBM is responsible for the enhancement in the V(oc). A maximum overall PCE of 4.20% was obtained for the BHJ polymer cell based on the active layer (P:F) deposited from mixed solvents 1-chloronapthalene/1,2-dichlorobenzene (CN/DCB) and subsequent thermal annealing at 120 degrees C. This improvement in the PCE has been attributed to the enhanced crystallinity of the blend and more balanced charge transport in the device due to the thermal treatment. (C) 2011 Elsevier B.V. All rights reserved.