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/HXWO3/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.

Research Highlights
► An electrochromic switchable mirror foil with a structure of Mg4Ni/Pd/Al/Ta2O5/HXWO3/indium tin oxide on a polyethylene terephthalate (PET) substrate was fabricated by only dry process in room temperature.
► 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.
► The durability of the device in the environment was also evaluated by using a thermostat/humidistat bath.