In Situ Monitoring of Lateral Hydrogen Diffusion in Amorphous and Polycrystalline WO 3 Thin Films

Thin films of electrochromic materials such as tungsten trioxide (WO 3 ) are essential components of electrochromic devices such as “Smart Windows.” The transport of charge carriers in WO 3 thin films has been investigated intensively over the past decades, yielding a wide spread of diffusion coefficients of hydrogen in WO 3 . 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, WO 3 thin films of different morphology are prepared by electron‐beam evaporation of WO 3 powder. The combined lateral diffusion of protons and electrons is investigated inside amorphous and polycrystalline WO 3 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 WO 3 , which are critically discussed in the context of existing sata from literature.