Electrochromic Stability of WO3 Thin Films with Nanometer-Scale Periodicity and Varying Degrees of Crystallinity

Abstract

Electrochromic tungsten oxide (WO3) thin films with nanometer-scale porosity have been synthesized via a sol−gel procedure making use of evaporation-induced self-assembly. According to wide-angle X-ray scattering data combined with electrochemical analyses, the degree of crystallinity ranging from fully amorphous to 100% crystalline can be adjusted by straightforward annealing. The three-dimensional cubic pore structure is thereby almost not affected. Aside from the material characterization, in this work we specifically focus on the overall electrochemical and electrochromic behavior (coloration efficiency, charge capacity, etc.) upon changes in the operating temperature. As a main result, only the mesoporous highly crystalline WO3 films display long-term cycling stability under realistic environmental conditions. We further demonstrate that sufficient crystallinity is needed to ensure stability of the inherent electrochemical properties at high operating temperatures (up to 70 °C). Thus, only the WO3 fi...