The influence of microstructure on the electrochromic properties of Li x WO 3 thin films: Part I. Ion diffusion and elec
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The chemical diffusion coefficients of lithium ions in Li^W0 3 films were investigated as a function of lithium concentration and film porosity. Thin films were deposited with different porosities by thermal evaporation of WO 3 powder in various partial water pressures. Our results indicate that diffusion coefficients increase with film porosity and decrease with increasing lithium concentration. Large diffusion coefficients that were found for small lithium concentrations appear to be due to the contribution of protons generated from ion exchange reactions between lithium and water incorporated in the film. Simultaneous electrical and in situ optical measurements were carried out to study the effect of porosity on the electrochromic properties of L^WC^. The coloring efficiency of porous WO 3 films increases by approximately 70% when deposited in partial water pressure of 1CT4 Torr, but decreases with further increments in water pressure.
I. INTRODUCTION The electrochromic properties of WO 3 have been studied extensively during the last two decades since Deb's initial paper.1 The application of WO 3 and other transition metal oxides to electrochromic windows has attracted worldwide interest in recent years, as indicated in several recent reviews.2"7 The electrochromic properties of these materials are based on the double injection/extraction of ions (usually H + or Li + ) and electrons under a low voltage. Lithium ions have been shown to be more stable than protons (H + ) in an ambient environment, especially in all solid-state devices that have been proposed for electrochromic windows.2"7 Consequently, lithium has attracted more and more attention among research groups as an active insertion ion. The mechanism of ion insertion into WO 3 is one of the key issues that remains to be resolved to better understand the coloring/bleaching (C/B) process in these devices because it determines the response time and the repeatable longevity of the device. Unfortunately, only a few data are available on the chemical diffusion coefficient (£)) of lithium ions in L^WO^, and most of these data are only estimates of the average diffusion coefficients8"10 rather than detailed measurements as a function of lithium concentration. Another important issue in the development of electrochromic devices based on LijWO3 is the effect of water on device performance. Bohnke et al.u studied the effect of water added to a liquid electrolyte (up to 10% in weight), but did not identify the role of water chemisorbed in the WO 3 films during sample preparation, which would be more relevant for a solid-state device. J. Mater. Res., Vol. 8, No. 10, Oct 1993 http://journals.cambridge.org
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In this paper, we report on a systematic investigation of the influence of preparation conditions and corresponding film porosity on the lithium ion diffusion in Li^W0 3 . The chemical diffusion coefficient and other electro-optical properties are described as a function of lithium concentration (x) and correlated with the partial water pre
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