Chromic Mechanism in Amorphous  WO 3 Films

We propose a new model for the chromic mechanism in amorphous tungsten oxide films ( WO 3 − y ⋅ n H 2 O ) . This model not only explains a variety of seemingly conflicting experimental results reported in the literature that cannot be explained by existing models, it also has practical implications with respect to improving the coloring efficiency and durability of electrochromic devices. According to this model, a typical as‐deposited tungsten oxide film has tungsten mainly in W 6 + and W 4 + states and can be represented as W 1 − y 6 + W y 4 + O 3 − y ⋅ n H 2 O . The proposed chromic mechanism is based on the small polaron transition between the charge‐induced W 5 + state and the original W 4 + state instead of the W 5 + and W 6 + states as suggested in previous models. The correlation between the electrochromic and photochromic behavior in amorphous tungsten oxide films is also discussed.