Improved thermochromic properties in bilayer films of VO2 with ZnO, SnO2 and WO3 coatings for energy efficient glazing†
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 (Tvis), integrated luminous transmittance (Tlum) and solar modulation (ΔTsol). 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 (Tc), 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 (Tc = 47 °C, Tlum = 61.6%, Tvis = 68% and ΔTsol = 6%).