The influence of microstructure on the electrochromic properties of Li x WO 3 films: Part II. Limiting mechanisms in col
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Various factors affecting the coloring and bleaching processes of Li^WO3 films have been studied. The rate of the coloring process is limited by the decreasing electromotive force in the Li x WO 3 film and by the components of the series circuit resistance, including the electrolyte resistance and the diffusion impedance within the film. The bleaching process in a thick film is limited by either the space charge or by the diffusion impedance, depending on the experimental conditions. A more complete and quantitative model of the coloring/bleaching process has been developed and shows good agreement with experimental results. Our analysis also indicates that the lithium concentration value of x near the Li^WOs/electrolyte interface can greatly exceed its reversible limit during the coloring process, even though the average x value within the film remains much lower than the reversible limit. This phenomenon may introduce some irreversible structural changes in the film, which in turn may constitute one of the film's degradation mechanisms.
I. INTRODUCTION Since Deb's first report on the electrochromic properties of H^WC^,1 many device designs and possible applications have been proposed.2 In many of these applications, the response speed is critical. This response speed is restricted by the rate-limiting mechanisms of the coloring/bleaching (C/B) processes, and their identification and quantitative understanding have remained a crucial problem that has hindered the design of practical electrochromic devices. In the early studies of the C/B process in electrochromic WO3 films, display devices were the primary and targeted application.3"5 These types of display devices required a very fast switching time, and H^WC^ was promoted and studied as the most promising, highspeed electrochromic material. Crandall and Faughnan3 observed that the coloration current in H^WO3 decreased with time as t~m. They suggested that the low-voltage coloring process was controlled by a barrier for current flow at the HxWO3/electrolyte interface. Reichman and Bard6 also observed at~m dependence for the coloration of HjWO 3 , but they attributed this behavior to proton diffusion processes in the films. Similar discrepancies also existed about the rate-limiting step in the bleaching mechanism. Faughnan et al.4 proposed that the bleaching process was controlled by a space-charge-limited current. Results from several other groups supported this model,7'8 but Haranahalli and Holloway9 reported a much different behavior in the bleaching process of H^WO 3 . J. Mater. Res., Vol. 8, No. 10, Oct 1993 http://journals.cambridge.org
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Today, liquid-crystal display technology has matured, and interest in electrochromic materials for displays has correspondingly declined. During the last decade the focal point for the application of electrochromic materials has shifted to daylighting management, glare control, and solar heat gain control in windows for buildings and vehicles. Consequently, the response speed for these applications
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