Design, formation and characterization of a novel multifunctional window with VO₂ and TiO₂ coatings

Abstract

A novel multifunctional window was demonstrated using VO₂-based thermochromic films with a TiO₂ antireflection coating. A strong enhancement in luminous transmittance of VO₂ was calculated using either a single layer or a double layer of TiO₂ for antireflection, with the double layer giving the better performance. A TiO₂ (25 nm)/VO₂ (50 nm)/TiO₂ (25 nm) structure, of which the film thickness has been optimized to a maximum integrated luminous transmittance (T_lum) by calculation, was formed on SiO₂ glass by sputtering in turn a V target in Ar+O₂ for VO₂ and a TiO₂ target in Ar for TiO₂. Optical properties were measured with a spectrophotometer, and the integrated solar and luminous properties were calculated based on the measured spectra. A maximum increase in T_lum by 86% (from 30.9% to 57.6%) was obtained for VO₂ after double-layer TiO₂ antireflection coating. The deposited VO₂ film on SiO₂ exhibits good thermochromism with a sharp optical transition at 58.5 °C, which decreased slightly to 57.5 °C after TiO₂ coating. The proposed window is the most advanced among those similarly reported in being multifunctional with automatic solar/heat control, ultraviolet stopping and possibly a wide range of photocatalytic functions in addition to a high value of the luminous transmittance.