We report on the incorporation of a low band gap copolymer based on thiophene and benzothiadiazole with a band gap of 1.65 eV into a bulk heterojunction device with the structure ITO/PEDOT:PSS/polymer:PCBM/Al. We have investigated the effects of process variable, including choice of solvent, polymer concentration and annealing temperature, on the photovoltaic device performance. The devices show spectral response down to the absorption edge of 1.65 eV and exhibit an efficiency of 1% under AM 1.5 illumination and a peak external quantum efficiency of 18 % at 600 nm. (c) 2007 Elsevier B.V. All rights reserved.