Impedance Spectroscopy of Electrochromic Hydrous Tungsten Oxide Films

Abstract

Tungsten oxide is a widely used electrochromic material with important applications in variable-transmittance smart windows as well as in other optoelectronic devices. Here we report on electrochemical impedance spectroscopy applied to hydrous electrochromic tungsten oxide films in a wide range of applied potentials. The films were able to reversibly bleach and color upon electrochemical cycling. Interestingly, the bleaching potential was found to be significantly higher than in conventional non-hydrous tungsten oxide films. Impedance spectra at low potentials showed good agreement with anomalous diffusion models for ion transport in the films. At high potentials, where little ion intercalation takes place, it seems that parasitic side reactions influence the spectra. The potential dependence of the chemical capacitance, as well as the ion diffusion coefficient, were analyzed. The chemical capacitance is discussed in terms of the electron density of states in the films and evidence was found for a band tail extending below the conduction band edge.