Ultrathin Al2O3 Protective Layer to Stabilize the Electrochromic Switching Performance of Amorphous WOx Thin Films

Abstract Electrochromic materials play a key role in smart windows, displays or anti‐glare rear‐view mirrors. Tungsten oxide is an intensely studied representative due to its extraordinary coloring performance. For commercial use, however, further optimization of the general cycle stability as well as the protection against external factors, such as moisture, is a still ongoing focus of research. In this study, the stabilization of the electrochromic switching performance of tungsten oxide is investigated using an ultrathin optimized Al 2 O 3 protective coating, grown by atomic layer deposition. Amorphous tungsten oxide (a‐WO x ) thin films are prepared by reactive radio‐frequency sputtering. The composition as well as electronic structure of the pure a‐WO x films is studied by X‐ray photoelectron spectroscopy. The electrochromic properties of the multilayer system are investigated in a nonaqueous electrolyte as well as in an electrolyte containing 10 vol% water. On the basis of these results, the stabilizing effect on the electrochromic switching characteristics of a‐WO x by the use of the additional thin Al 2 O 3 protective layer is evident. It is shown that degradation and ageing of a‐WO x due to moisture can be prevented and coloration efficiencies of 50.4 cm 2 C −1 can be achieved at 630 nm.