Structural and optical characterization of amorphous and crystalline evaporated WO3 layers

Amorphous and crystalline WO3 films show electrochromic properties by means of double injection of cations and electrons. At low substrate temperature thermal evaporation technique produces an amorphous material mixed to a nanocrystalline phase. When this material is annealed at temperatures higher than 400°C a fully crystalline structure is obtained. Different absorption mechanisms are responsible for colored amorphous and crystalline WO3 layers, small polaron transitions and Drude-like free electron scattering, respectively. Furthermore, both O/W ratio and crystalline structure affect electrochromic efficiency of a WO3 film. It has been confirmed that a hydrogen tungsten bronze structure appears when the material is colored. Finally, a self-bleaching process occurs when a colored WO3 film is exposed to an oxidative environment.