Tungsten Trioxide Charge Transfer Model

In 1969, for the first time, some scholars found the electrochromism of amorphous tungsten trioxide. Due to its high electrochromic color contrast, good cycle stability and good thermal stability, tungsten trioxide has become the most studied and commercially available inorganic electrochromic material in recent years.

Due to the extremely complex electrochromic mechanism of tungsten trioxide, the true cause of discoloration is mainly related to the charge transfer process. Many scholars have proposed various models to explain the color change mechanism of tungsten trioxide, including the color center model, the small polaron model and the charge transfer model. The electrowetting phenomenon by tungsten trioxide researchers generally accepted is the charge transfer model.

The trioxide charge transfer model is as follows:

The charge transfer model has different properties in different crystalline tungsten trioxide. Tungsten trioxide are amorphous and crystalline two types. Amorphous tungsten trioxide films show better electrochromic properties due to the elimination of the blocking effect of the grain boundaries on charge transport. However, the amorphous tungsten trioxide film is less stable in cycling because of its inability to withstand the structural changes caused by the insertion and extraction of charges during the electrochromic process, and the ions easily fall into the material to lower the light modulation range of the film.

Ideally, crystalline tungsten trioxide can be considered as a deformed perovskite crystal structure. The chemical bond is an ionic bond between tungsten and oxygen, but both also have significant covalent components. This distorted perovskite structure consists of a tungsten oxide octahedron, which is arranged in a diagonal stack. Tungsten atoms are in the center of the octahedron and each oxygen atom is used as bridging oxygen for the two-tungsten trioxide octahedron common. Like other metal oxides, the crystal phase of tungsten trioxide undergoes many changes with temperature. In many tungsten trioxide phases, the one-dimensional ion channels in the hexagonal tungsten trioxide lattice contribute to charge transport and diffusion. So, its electrochromic performance is relatively good.

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