Characteristics of the organic photovoltaic devices prepared by in-situ electrochemical copolymerization of substituted fullerene with alkylthiophene were studied. The active layer was formed by direct electrochemical copolymerization between thiophene moiety substituted fullerene and 3-octylthiophene on PEDOT-coated indium tin oxide (ITO) glass, where fullerene is anchored within conducting polymer chain through covalent bonding. Since two monomers utilized in this study had an opposite oxidation potential, the step potential bias method was used to copolymerize these two monomers. Bias potential and concentration of monomers were carefully controlled so that blend ratio of composite film was exactly same as monomer feed. Power conversion efficiency of electrochemically copolymerized photovoltaic device changed within experimental error ranges for before and after heat treatment. Meanwhile, composite films prepared by spin coating of blend of poly(3-octylthiophene) and PCBM showed morphological changes after heat treatment at 8 C for 1 h according to field emission scanning electron microscopy (FE-SEM) Study. This study suggests a new approach for preventing migration of fullerene from its optimized morphology. (c) 2008 Elsevier B.V. All rights reserved.