Improved thermochromic properties in bilayer films of VO2 with ZnO, SnO2 and WO3 coatings for energy efficient glazing

Abstract

The thermochromic properties of bilayer films consisting of a nanostructured VO₂ under-layer and top-layers of ZnO, SnO₂ or WO₃ are presented. In each case, the VO₂ under-layer was deposited by electric field assisted AACVD on a fluorine doped tin oxide coated glass substrate, with the metal oxide top-layer deposited using sol–gel spin coating. The deposited films were analysed and characterised using scanning electron microscopy, X-ray powder diffraction, variable temperature UV/Vis/NIR spectroscopy, X-ray photoelectron spectroscopy, glow discharge photoelectron emission spectroscopy and contact angle measurements. Compared to bare VO₂ films, the bilayer films exhibit superior thermochromic properties, with dramatic increases in the visible light transmittance (T_vis), integrated luminous transmittance (T_lum) and solar modulation (ΔT_sol). The thermochromic efficiency was also improved in some cases, with a reduction in the hysteresis loop width to ca. 10 °C, and a decrease in the transition temperature (T_c), which was as low as 44 °C and 43 °C for VO₂/ZnO and VO₂/SnO₂ films, respectively. The best balance of thermochromic, thermal protection and aesthetic properties was achieved for ZnO coated films, with the VO₂ under-layer deposited at a field strength of 1500 V m⁻¹ under a negative DC bias (T_c = 47 °C, T_lum = 61.6%, T_vis = 68% and ΔT_sol = 6%).