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The Mechanism of Trapped Ions Eroding the Electrochromic Performances of WO3 Thin Films

The electrochromic WO 3 devices normally suffer from the degradation of long-term performances since the accumulation of trapped ions in the host structure. In this study, we notice that the degradation of WO 3 film consists of the decrease of bleaching state and the decay of coloration state, and the latter is the main factor of affecting the cyclic stability of the WO 3 film. Here, it is discovered that the degradation of coloration state is due to the decrease of the charge density of inserted ions in the film during cycling process. By employing the open circuit potential (OCP) measurement, we find that the accumulation of trapped ions in WO 3 film will cause the decay of OCP between the work electrode of WO 3 film and counter electrode, which is the main reason of the decrease of inserted ions. This is because that the decay of OCP will weaken the electric field force used to drive ions into the film and shorten the duration time of ion insertion process. Consequently, the decline of charge density of the inserted ions will cause the decay of coloration state, which is the real mechanism of trapped ions eroding the cyclic life of the WO 3 film.