The influence of microstructure on the electrochromic properties of Li_xWO₃ thin films: Part I. Ion diffusion and electrochromic properties

The chemical diffusion coefficients of lithium ions in Li_xWO₃ films were investigated as a function of lithium concentration and film porosity. Thin films were deposited with different porosities by thermal evaporation of WO₃ powder in various partial water pressures. Our results indicate that diffusion coefficients increase with film porosity and decrease with increasing lithium concentration. Large diffusion coefficients that were found for small lithium concentrations appear to be due to the contribution of protons generated from ion exchange reactions between lithium and water incorporated in the film. Simultaneous electrical and in situ optical measurements were carried out to study the effect of porosity on the electrochromic properties of Li_xWO₃. The coloring efficiency of porous WO₃ films increases by approximately 70% when deposited in partial water pressure of 10⁻⁴ Torr, but decreases with further increments in water pressure.