Preparation and aging of sputtered tungstic oxide films

Conditions for the preparation of electrochromic tungstic oxide films with high efficiency for coloration have been investigated. Tungstic oxide films were deposited on glass substrates by rf sputtering in Ar-O2 mixture from a compressed powder WO3 target. As-deposited films require an ‘‘aging process,’’ in which the density of charges extracted in bleaching is smaller than that injected in coloring. It is considered that a part of the protons introduced during the aging combine with bonds of unstable oxygen contained in the as-deposited films. After the aging period, sputtered films have reversible coloration and bleaching cycles. Maximum electrochromic coloration efficiencies are 0.09 and 0.04 cm2/mC at wavelengths of 800 and 500 nm, respectively. They are obtained when the sputtering atmosphere is 0.04–0.05-Torr total pressure and 5% oxygen content. At any rate, substrate temperature should be kept at 100 °C or less.