Synthesis of tungsten oxide films at low processing temperature and their electrochromic properties

Tungsten oxide films with electrochromic properties were prepared by spin coating WCl6 ethanol solution on indium tin oxide (ITO) glass and polyethylene terephthalate (PET) substrate, and followed heat treatment at 60-400 degrees C in air for 1 h. The films were characterized by X-ray powder diffraction, scanning electron microscopy, energy spectrum analysis, electrochemistry and ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrophotometer. Tungsten oxide films with different components can be obtained at different heat treatment temperatures. With the increase of heat treatment temperature, the products gradually lose the crystal water from WO3 center dot 2H(2)O to WO3 center dot H2O and finally to WO3. The as-prepared film has many tiny cracks, which is beneficial for effective electrolyte penetration. The tungsten oxide PET films also show good electrochromic properties, and the coloring and bleaching processes are consistent with that of tungsten oxide device. The treatment at low temperature is no damage to the substrate and maintains the flexibility of the films. The method provides a simple, low-cost and large-scale practical scheme for the flexible electrochromic films.