Optimization of mixed conductivity through nanostructure control in WO3 thin films

Abstract

Tungsten trioxide (WO3) thin films have become increasingly important in recent years both from the fundamental research point of view as well as for their application potential as smart windows for energy efficiency. The electrochromic coloration efficiency as well as the kinetics of coloration are very important for these applications. Although much emphasis is placed on rendering the films optically efficient by working with nanostructured WO3 films, not much work is done to optimize their electrical properties, especially their mixed conductivity (ionic and electronic) which is equally important. In this work, we have attempted to optimize the mixed conductivity of the thermally evaporated WO3 thin films by systematically varying their nanostructure. Based on our previous work, working pressure (between 1.3×10−3 and 0.13Pa) has been used as the control parameter to induce varying nanostructure in the films. The grain size and the film roughness are found to increase systematically with the working pressure. The electronic and ionic conductivities have been deduced from the electrochemical measurements. With increasing working pressure in the chamber the ionic conductivity is found to increase systematically while the electronic conductivity decreased. The lithium ion diffusion is also found to increase with increasing working pressure. The optimum values are found to be around a working pressure of 7.3×10−2Pa, yielding values of around 5.5×10−7 (Ω−1cm−1) and 5.5×10−10 (Ω−1cm−1) for ionic and electronic conductivities, respectively.