Influence of proton insertion on the conductivity, structural and optical properties of amorphous and crystalline electrochromic WO3 films

Amorphous and crystalline tungsten oxide films were RF-sputtered from a metallic target in oxygen/argon atmosphere onto un-heated and heated substrates. Upon cyclic electrochemical treatment in 0.1 M H2SO4 aqueous solution, both types of films showed good electrochromic reversibility beyond 1000 cycles. The crystallinity changes of both types of films were studied by XRD. For c-WO3, the results showed that hydrogen insertion enhance the degree of crystallinity with the clear appearance of tungsten bronze diffraction peaks due to the formation of H0.1WO3, with tetragonal crystalline structure. For a-WO3, coloration weakens the amorphous features and increases the crystallinity properties. The DC electrical conductivity increases, in a reversible way, with four order of magnitudes namely from 10(-7) to 10(-3) (Omega cm)(-1) for bleached and colored films, respectively. Both types of films exhibit remarkable solar transmission modulation, about 51%, which is adequate for smart windows applications. In contrast to amorphous films, crystalline tungsten oxide films show reflection modulation in the NIR reaching 25%. upon charge insertion. The Drude reflectance edge arises for colored films suggesting that coloration in c-WO3 is attributed to a scattering mechanism of Drude-like free electron, while coloration of a-WO3 is attributed to a hopping mechanism of small polaron. The visible, infrared and solar modulations are given for both types of films. Coloration/bleaching were found to be faster for amorphous films. Self-bleaching process in different oxidative environments involves a redox cycle aided by water polarity. Crystalline WO3 showed pronounced coloring persistence relative to amorphous films which is attributed to different binding energy related to large and small polarons. (C) 2004 Elsevier B.V. All rights reserved.