The discoloration process of tungsten oxide for preparing smart heat insulation film is more complicated, and the mechanism is still controversial. Currently, the widely accepted and applied discoloration model is the dual injection model, namely the Fanghnan model.
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The Fanghnan model states that when ions are injected, the tungsten trioxide electrochromic film exhibits a blue color, and at this time, the ion injection is equivalent to the doping of the semiconductor. When the ions are extracted, the film can be changed back to the transparent state. Fanghnan also pointed out that when cations and electrons are double injected into the WO3 film to form tungsten bronze, W5+ and W6+ are simultaneously present. The change in color of the tungsten trioxide film is due to the change in light absorption caused by the transition of electrons between two different energy levels of W5+ and W6+. In addition, models for the discoloration mechanism of WO3 electrochromic thin films include color center models, small polaron models, electrochemical reaction models, and the like.
