Electrochromic switchable mirror foil with tantalum oxide thin film prepared by reactive DC magnetron sputtering in hydrogen-containing gas

We have developed an electrochromic switchable mirror foil with a structure of Mg₄Ni/Pd/Al/Ta₂O₅/H_XWO₃/indium tin oxide on a polyethylene terephthalate (PET) substrate. When a voltage is applied to the device, the protons in the WO₃ layer move to the Mg₄Ni layer, which is then hydrogenated to form MgH₂ and Mg₂NiH₄. The hydrides have higher transparency, and thus the device can be effectively switched to a transparent state. We have previously reported a process where the device was fabricated by using a sulfuric acid solution to introduce protons to the WO₃ layer. However, the solution-based process appeared to have low adaptability to commercial processes. In the present work, we focused on fabricating the device by only solid-state processing. Specifically, a hydrogen-containing solid electrolyte layer of Ta₂O₅ thin film was fabricated by reactive dc magnetron sputtering in a mixture gas of argon, oxygen and hydrogen. We optimized the fabrication conditions of the hydrogen-containing Ta₂O₅ thin film for the device. In comparison with the device fabricated by the solution-based process, the device fabricated by only the solid-state process was more durable, retaining its optical switching properties for 7 days at a temperature of 40 °C and relative humidity of 80%. This result was related to the prevention of degradation on the surface of the Mg₄Ni thin film.