Tungsten Trioxide Electrochromic Thin Film

Tungsten Trioxide Electrochromic Thin Film

Electrochromism, which is short for EC, is a phenomenon that the material itself undergoes a reversible color change under external electric field. Study has found that many transition metal oxides have the property of electrochromic; these metal oxides can be divided into cathodic coloring material reduction process (e.g., oxides of W, Mo, V, Nb, and Ti) and anodic oxidation coloring material (e.g., oxides of Ir, Rh, Ni, Co oxide) by coloring mode. Wherein, WO₃ is the most studied cathodically coloring electrochromic material.

About the electrochromic mechanism of WO₃ films, theory of transition between valences proposed by Faughnan is the most widely cited currently, which the electron and cation ore are injected simultaneously from both sides of the WO₃ film due to the applied electric field, the electrons are trapped by atoms W to form the localized states, the metal ions M+ resides form a deep blue tungsten bronze compound-M_xWO₃ exist in different valence states M_xWO₃ in W ions in this area, electronic transitions between neighboring atoms of different valence W led WO₃ film color change. The electrochemical process is the following formula:
WO₃ (colorless) + xM⁺+ xe^- → M_xWO₃ (Dark Blue)
Where M is typically on behave H, Li, Na, Ag, etc., 0 xWO₃ formed after the H⁺ implantation.

Using the effect of WO₃ electrochromic, the researchers have prepared a variety of electrochromic device and applied to real life. Electrochromic device has many excellent properties, such as transmittance can be continuously changed over a wide range (the practical need to achieve 4: 1 or more) with hand adjustable; a low driving voltage (1 ~ 2V), and the simple power supply, low power consumption; viewing angle restrictions on the display, a memory function without consuming electrical energy. These properties make the tungsten oxide electrochromic material has a wide range of applications in optical information storage and display, military camouflage, special smart windows and so on. Furthermore, WO₃ can be changed reversibly between a colorless and dark blue under the applied voltage pulse, which makes it can be used as a photo-switch.