Smart windows based on VO2 and WS2 monolayers

Abstract

Smart windows automatically adjust their properties to control infrared (IR) radiation which helps with saving energy by reducing the need for heating and cooling. In the current study, we use vanadium dioxide (VO₂), a phase change material that in temperatures above 68 °C behaves as a metal, to design smart windows. In its metallic phase, VO₂ transmits less IR than would be expected from a smart window. Visible light transmission through the VO₂ window in both its insulator and metallic phase is low which causes low indoor lighting. To solve this problem, we propose structuring the VO₂ as a grating on a silica substrate. A thin film of VO₂ blocks 62% of IR and transmits 46% of visible light, while a grating with an 800 nm period and 700 nm ribbon width improves IR blockage to 67%/63% and increases the visible light transmission to 53%/47% under transverse magnetic (TM)/electric (TE) radiation. Another issue of VO₂ windows is the unpleasant yellow-brownish color of them. To solve this problem, we inserted atomic layers of Tungsten disulfide (WS₂) in the window structure. Adding 5 layers of WS₂ keeps IR blockage and visible light transmission almost the same, while reducing transmission at the wavelength of 620 nm from 49%/41% in the case of VO₂ grating, to 34%/30% under TM/TE radiation, which changes the window color. These window properties are consistent under different light angles. Notably, in the proposed VO₂-based smart windows, all three critical factors of visible light transmission, IR blockage, and pleasant window color are simultaneously optimized for an unpolarized incident light in a wide range of radiation angles. The simulated reported results of this paper pave a new way in the world of smart windows.