In Situ Monitoring of Lateral Hydrogen Diffusion in Amorphous and Polycrystalline WO3 Thin Films

Abstract

AbstractThin films of electrochromic materials such as tungsten trioxide (WO3) are essential components of electrochromic devices such as “Smart Windows.” The transport of charge carriers in WO3 thin films has been investigated intensively over the past decades, yielding a wide spread of diffusion coefficients of hydrogen in WO3. Most of those analyses are based on measurements by electrochemical methods. However, such analyses are only applicable and the results reliable in cases where diffusive transport is the rate‐determining process. Here, WO3 thin films of different morphology are prepared by electron‐beam evaporation of WO3 powder. The combined lateral diffusion of protons and electrons is investigated inside amorphous and polycrystalline WO3 thin films by in situ transmission spectroscopy during electrochemical hydrogen insertion into the film. The insertion of the protons is carried out locally and lateral diffusion of H takes place in the film plane. The induced change of the film absorbance at a wavelength of (637 ± 15) nm is monitored and spatially and temporally resolved. The results reveal a concentration‐dependent diffusion process of hydrogen in both, amorphous and polycrystalline WO3, which are critically discussed in the context of existing sata from literature.