Heat treatment of amorphous electrochromic WO3 thin films deposited onto indium-tin oxide substrates

Electrochromic tungsten oxide thin films, obtained by vacuum evaporation, were studied before and after heat treatment between 25 and 250°C for 2 h in air. Electrochromic properties were investigated in acid electrolyte by simultaneous measurements of the electrical and optical parameters. A.c. complex impedance techniques and voltammetry were used to characterize the films from an electrical point of view. We observed an enhancement of the electrochromic response times during both coloration and bleaching after heat treatment carried out between 150 and 220°C. This phenomenon was associated with a decrease of the ohmic drop in the electrode and a continuous variation of the impedance diagrams of these electrochromic electrodes. Moreover, we observed that the diffusion coefficient of H⁺ ions into WO₃, obtained on colored thin films, increased as the electrochromic kinetics increased.