A spectroelectrochemistry study was carried out for thin films of a series of copolymers with repeat units consisting of monosilylene and oligothienylene (PSnT, n＝5, 7, 8, 10 and 14, n denotes the ring number of an oligothienylene unit) over a wide window of doping potentials. It was found that the films of these PSnT samples showed reversible electrochromic change in a proper range of doping potentials. Both the spectroelectrochemistry and cyclic voltammetry studies of PSnT film indicate that during electrochemical doping the oligothienylene unit of each PSnT is oxidized in the first oxidation step to polaron in equilibrium with p-dimers, and in the second step to bipolaron. Bipolarons are not stable in PS5T and PS7T films, but can exist safely in other PSnT films with longer oligothienylene unit. The formation of p-dimers was found to be favored by extension of the oligothienylene unit in PSnT. Apparent mobilities for these copolymers with longer p-conjugation lengths started to rise at lower doping potentials and the mobility enhancement also followed an increasing order of the π-conjugation length.

Key words: oligothiophene, copolymer, spectroelectrochemistry, doping, charge carrier