Relaxation processes in the coloration of amorphous WO3 thin films studied by combined impedance and electro-optical measurements

We introduce in this paper a phenomenological model (Havriliak-Negami function) to describe the relaxation of the intercalated Li ions into amorphous ${\mathrm{WO}}_3$ electrochromic films. Kinetic behavior of intercalation was analyzed by a combined measurement of electrochemical impedance and modulated transmittance. The frequency-dependent capacitance and modulated transmittance show the processes of ion diffusion and rearrangement towards equilibrium in the stationary sites of the structure. In case of Li-doped samples, capacitance and transmittance spectra overlap along a wide interval of frequencies, thus indicating that ion diffusion does not play a central role within the measuring frequency range employed, so that the kinetics of coloring mechanism can be wholly described by HN relaxation. The rates of the coloring process are located near 100 mHz, although their probability decays slowly towards very low frequencies.