Electrochromism in a Thin-Film Device Using Li2WO4 as an Li-Electrolyte

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Abstract
A thin-film electrochromic device using Li2WO4, which is shown to be an Li electrolyte by the fact that the coloration response is independent of the atmosphere, is found to exhibit quick response (coloration: 50 ms, bleaching: 200 ms). The spontaneous coloration is suppressed by the deposition of a WO3 film at low pressure (10-6 Torr), and by using a metal with a small work function as the counter-electrode. The threshold voltage for coloration increases as the work function of the counter-electrode material decreases. Supressions of the spontaneous coloration and the threshold voltage shift are induced by the potential difference between the WO3 and the counter-electrode introduced by the difference in the Fermi levels. Complete bleaching of the device is achieved in a device with a WO3 film deposited at 10-6 Torr at a substrate temperature of 160°C, while it is not achieved in a device with a WO3 film deposited at 10-4 Torr. This suggests that Li2WO4 is colored by proton injection from WO3 into Li2WO4, which arises from the electrolysis of water in the WO3 film deposited at high pressure.