Degradation caused by substrate glass in WO3 electrochromic devices

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Abstract
The causes of degradation resulting from alkaline elemental diffusion from various glass substrates into WO3 films is clarified. Cells with a soda‐glass substrate covered with a transparent conductive layer that was heat treated (250 °C, 1 h, in vacuum) before WO3 evaporation show poor initial coloring‐erasing properties. The life of such cells is found to be short. Secondary ion mass spectroscopy analysis shows that the poor cell properties are due to Na diffusion from the soda‐glass substrate into the WO3 films through the transparent conductive layer. On the other hand, cells with a 7059 glass or a spin‐on‐glass‐coated soda‐glass substrate showed good initial coloring‐erasing properties, even if these substrates had been heat treated before WO3 evaporation. These cells demonstrate a high reliability. The results open the way for expanded use of WO3 film in realizing more advanced electrochromic devices.