Influence of proton insertion on the conductivity, structural and optical properties of amorphous and crystalline electrochromic WO 3 films

Abstract

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 H 2SO 4 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-WO 3, 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 H 0.1WO 3, with tetragonal crystalline structure. For a-WO 3, 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 (Ω 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-WO 3 is attributed to a scattering mechanism of Drude-like free electron, while coloration of a-WO 3 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 WO 3 showed pronounced coloring persistence relative to amorphous films which is attributed to different binding energy related to large and small polarons.