Studies on the redox behaviour of some polythiophene derivatives by impedance spectroscopy in symmetrical and asymmetrical configurations

The generation and transport of ionic and electronic charges in three polythiophene derivatives, namely poly(3-methyl thiophene), poly(thiophene-3-methanol) and poly(3-thienyl methacrylate), have been studied using impedance spectroscopy in both asymmetrical (Pt ∣ polymer film ∣ electrolyte) and symmetrical (Pt ∣ polymer film ∣ metal contact) configurations. The symmetrical measurements are to our knowledge reported for the first time in the literature for these polymer films. The homogeneous film model proposed by Vorotyntsev was found to describe adequately the impedance spectra for poly(3-methyl thiophene) in the asymmetrical configuration. For the two other polymers, the use of the ordinary Nernst–Planck equations to describe the charge transport seems to be an oversimplification. The impedance spectra of the systems studied in the symmetrical configuration were found to be described adequately by an extension of the homogeneous film model for the metal ∣ film ∣ metal contact. On the basis of the calculated impedance parameters, estimates for the diffusion coefficient and concentration of ionic and electronic charge carriers in the film as a function of potential are obtained. The results are discussed in connection with previous dc resistance and FTIR/UV–vis spectroscopic measurements of the same materials.