Physical and electrochromic properties of the amorphous and crystalline tungsten oxide thick films prepared under reducing atmosphere

The physical and electrochemichromic properties of the amorphous and crystalline tungsten oxide films prepared by thermal evaporation of WO3 powder under a reducing atmosphere have been investigated. The oxide films with 850–24 500 Å thick were deposited on the substrates maintained at a temperature ranging from 50 to 500 °C under a vacuum below 1×10−5 Torr. Properties of the oxide films formed depend considerably on the deposition temperature. The as-prepared oxide films formed at a temperature higher than 400 °C have a resistivity of 10−3−10 Ω cm, and are blue colored, and mainly composed of crystalline WO3. The oxide films formed at a temperature lower than 350 °C have a resistivity of 105–109 Ω cm, and are transparent and amorphous. Both of the amorphous and crystalline oxide films have good electrochemichromic properties. From x-ray analysis, the crystalline WO3 thick films were found to change to hydrogen tungsten bronze films with a chemical composition of H0.33WO3 during electrochemical coloration, and the H0.33WO3 films were found to be reversed to the WO3 films during bleaching. In the electrochemical coloration using the cells composed of a crystalline or amorphous oxide film, the transmittance (λ=0.5 μm) and current were found to reach simultaneously an electrochromic steady state when a sufficient charge was injected. The density of the tungsten oxide films with 5000–24 500-Å thickness was also measured, and found to be 5.3–6.6 g/cm3 depending on the deposition temperature. The optical properties and the results of observations by scanning electron microscopy are also described.