ELECTROCHEMICAL STUDY OF POLY(3-OCTYLTHIOPHENE) FILM ELECTRODES - IMPEDANCE OF THE POLYMER FILM SEMICONDUCTOR ELECTROLYTE INTERFACE

The small amplitude ac data of the poly(3-octylthiophene) electrode-electrolyte system are analysed. The measurements are done at several dc potentials for different thicknesses of the polymer film and for different electrolyte concentrations (lithium tetrafluoroborate in propylene carbonate). The Ladder type electrical circuit model is found to give best fit of the data. The high frequency double layer capacitance is shown to contain contributions from both sides of of the interface, ie the space charge layer of the polymer semiconductor and the Helmholtz layer of the electrolyte. The experimental data are compared with calculated data by using a simple model of two capacitors in series, for a potential range including that where the reversible oxidation of the polymer takes place. The adsorption/surface state capacitance is shown to pass through a maximum and the corresponding coverage of surface species is determined. The structure of the double layer, the physical interpretation of the model parameters, and the energy level diagram for the polymer semiconductor electrode are proposed.