Influence of deposition temperature on electrochromic properties of sputtered WO3 thin films

The influence of deposition temperature on the electrochromic properties of reactively sputtered WO, films has been investigated. A set of WO3 films produced in the temperature range of 47-400 degrees C is electrochemically cycled with a constant current supply, which provides an easy control of the charge injected into the films. Optical absorption after coloration, and, hence, the coloration efficiency of the films, significantly increases with increasing deposition temperature (up to 400 degrees C) in the wavelength range of similar to 1000-2500 nm. Both optical absorption and coloration efficiency decrease at the highest temperatures studied (350-400 degrees C) over most of the visible spectrum (similar to 500-800 nm). Optical modulation across the solar spectrum remains fairly constant for charge injection of 10 mC/cm(2), and the modulation is guile satisfactory for all films for electrochromic device applications. The films produced at higher substrate temperatures show smaller modulation of the visible spectrum, compared with the films sputtered at lower substrate temperatures. This could be more suitable for some applications of the solar control systems, The resistance of the films during the electrochemical process is found to increase with increasing substrate temperature. This causes difficulty in charge injection, requiring significantly larger voltages in techniques such as a cyclic voltammetry for the films deposited at higher temperatures. As a result the good electrochromic performance of the films deposited at higher temperatures can be masked by experimental conditions.