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 T_lum 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 °C, which decreases to 52 °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.