Impedance spectroscopy of undoped, doped and overoxidized polypyrrole films

The frequency response of polypyrrole thin films was studied in the presence of NC3-, F-, Cl- and Br- in the potential region of interest for the study of the insulator-conductor transition. The impedance spectra have been interpreted using the Randles equivalent circuit modified with a constant phase element (CPE), using nonlinear least-squares fitting of the parameters of the circuit to experimental data. The results show that the distribution of relaxation times widens with oxidation of the polymer, indicating the presence of domains with different electrical properties, consistent with hopping of charge carriers in the oxidized state. It is also shown that in F- solution and at moderate potentials, overoxidation leads to irreversible degradation of the electroactive properties, affecting the electronic conductivity and diffusion coefficients of ionic species within the film.