THE INFLUENCE OF MICROSTRUCTURE ON THE ELECTROCHROMIC PROPERTIES OF LIXWO3 FILMS .2. LIMITING MECHANISMS IN COLORING AND BLEACHING PROCESSES

Various factors affecting the coloring and bleaching processes of LixW O3 films have been studied. The rate of the coloring process is limite d by the decreasing electromotive force in the LixWO3 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 t he diffusion impedance, depending on the experimental conditions. A mo re complete and quantitative model of the coloring/bleaching process h as been developed and shows good agreement with experimental results. Our analysis also indicates that the lithium concentration value of x near the LixWO3/electrolyte interface can greatly exceed its reversibl e limit during the coloring process, even though the average x value w ithin the film remains much lower than the reversible limit. This phen omenon may introduce some irreversible structural changes in the film, which in turn may constitute one of the film's degradation mechanisms .