EIS evidence for charge trapping in n-doped poly-3-(3,4,5-trifluorophenyl) thiophene

CV characterizations of poly-3-(3,4,5-trifluorophenyl)thiophene (PTFPT) film electrodes in contact with acetonitrile (AN), propylene carbonate (PC) and sulfolane (SF) showed a pronounced, solvent-dependent slow-down of the p- and n-doping/undoping kinetics in the sequence AN > PC > SF. Further characterizations using electrochemical impedance spectroscopy (EIS) revealed two specific features: (i) in the high-frequency region, Nyquist plots for n-doping in the SF solution, were drastically different from those for PC and AN solutions, which was ascribed to the low bulk conductivity of the doped film in the SF solution; (ii) in the low-frequency region, Nyquist plots for n-doping in an AN solution, showed a distinct semicircle with a relatively high resistance, in contrast to a well-known, sloping capacitive line, typical of a low-frequency behavior of the same film in contact with PC and SF solutions. This semicircle was ascribed to a charge trapping effect occurring during consecutive n-doping/undoping. As was shown by the related structural impedance modeling, the effect depends strongly on the solvent nature, decreasing in the sequence: SF ≫ PC > AN. We proposed herein a model describing the different impedance behavior of the polymer electrode in the above three solvents.