Thermochromic multilayer films of WO3/VO2/WO3 sandwich structure with enhanced luminous transmittance and durability

A novel thermochromic WO3/VO2/WO3 sandwich structure was deliberately designed and deposited by a reactive magnetron sputtering technique. The double layer of WO3 not only functions as an antireflection (AR) layer to enhance the luminous transmittance (T-lum) of VO2, but also performs as a good protective layer for thermochromic VO2. Basically, the bottom WO3 layer functions as a buffer beneficial for the formation of the intermediate VO2 layer and serves as an AR layer while the intermediate VO2 layer with primary monoclinic phase acts as an automatic solar/heat control for energy saving. The top WO3 layer acts as another AR layer, and provides protection in a complex environment. An obvious increase in Tlum by 49% (from 37.2% to 55.4%) is found for VO2 films after introducing double-layer WO3 AR coating. The VO2 deposited on glass exhibits good thermochromism with an optical transition at 54.5 degrees C, which decreases to 52 degrees C in WO3/VO2/WO3 sandwich structure, and the hysteresis loop is sharper around the transition temperature, which may be ascribed to the strain and interfacial diffusion. In comparison with single-layer VO2, the durability in automatic solar/heat control of sandwich-structure VO2 films is improved nearly 4 times in high temperature and humidity conditions. This multilayer will open a new avenue for the design and integration of advanced thermochromic heterostructures with controllable functionalities for intelligent window and sensing system applications.