The influence of metal overlayers on electrochromic behavior of Wo3 films
Metallic overlayers have been deposited on vapor-deposited WO3 to control the kinetics of coloration and bleaching in a WO3-liquid electrolyte electrochromic cell. These over-layers, when porous, have been found to increase the rate of coloration/bleaching and the proton transfer rate at the WO3-electrolyte interface, when appropriate voltages were applied. This effect has been characterized with potentio-static and galvanostatic measurements and compared to the coloration and bleaching of WO3 (without metallic overlayers). Coloration of WO3 films is influenced at short times by resistance and electric field effects, in disagreement with the general assumption of high conductivity in vapor-deposited WO3. The rate of coloration was also influenced by mixed barriers at the WO3-electrolyte interface. The barriers consist of a Schottky barrier at the WO3 surface and a Helm-holtz double layer. The coloration and bleaching mechanisms for WO3 with overlayers have been discussed based upon dif-fusion of electrons through the WO3, WO3-overlayer interface, or the SnO2-WO3 interface, and proton diffusion at the WO3-electrolyte interface. Resistance effects in the WO3 were found to be reduced by the overlayer. Electron injection in WO3 occurred both at the SnO2 and the overlayer inter-faces, thus increasing the rate of proton transfer, and the rate of coloration/bleaching.