The series of copolymers based on terthiophene 3′-substituted with C60-pyrrolidine or styryl group, were electrochemically synthesized and characterized. The dense distribution of C60 side group caused the steric effects between them and the perturbation in the extent of π-electron delocalization of the poly(terthiophene) backbone. The ESR spectroscopy showed the presence of close-packed clusters especially in homopolymer: poly(terthiophene-C60). The oxidative electrochemical copolymerization of terthiophene units was performed to avoid this steric interactions and in results to extend the effective π-conjugation length in the polymer. The use of different ratio of the co-monomers contributed to the variable distribution of C60 unit and different effective π-conjugation of the resulting polymers. Those changes affected electrochemical and photocurrent-generating properties of these materials.