Physical Structure and the Electrochromic effect in Tungsten Oxide Films

ABSTRACTIn all previous studies of tungsten oxide films it has been implicitly assumed that the film is uniform except for grain boundaries in polycrystalline films. In this study we show that amorphous tungsten oxide thin films contain highly anisotropic void networks which not only dominate their physical structure but also control their electrochromic behaviour. The void network structure is controlled primarily through ion bombardment of the growing film during deposition while the film stoichiometry HyWO3−x is controlled by the reactive sputtering processes of tungsten in Ar/O2/H2 atmospheres. The evolutionary growth model of physical structure was studied in detail by both transmission and scanning electron microscopy. On the basis of physical structure-property correlations we are able to consistently explain the basic electrochromic characteristics and chemical stability of the films. Such information leads to better understanding of the problems and limitations inherent in thin film electrochromic devices.