Sputtered VO2 coatings on commercial glass substrates for smart glazing applications

Abstract

VO2 is a well-known thermochromic material that can be potentially applied as a coating on “smart” glazing systems. In this work VO2 thermochromic films, were grown by the rf sputtering technique on two different types of commercial glass substrates, namely fused silica and float glass, at the low deposition temperature of 300 °C. The optimum deposition conditions, in order to succeed the thermochromic monoclinic phase of VO2(M), were investigated and found to be subject to substrate and thickness. It was observed that on the float glass substrate, only films with thicknesses below 50 nm grown with 3% O2 in plasma exhibited a thermochromic behavior, displaying a critical transition temperature of TC = 46.1 °C, width of transmittance hysteresis loop at 2000 nm ΔTC = 9.1 °C, IR-switching ΔTrIR = 14.6%, integrated solar transmittance modulation ΔTrsol = 1.3% and integrated luminous transmittance at 25 °C equal to Trlum = 36.6%. In contrast, films deposited on fused silica glass demonstrated a thermochromic phase, for all thicknesses, when grown with 1% O2 in the plasma. These thermochromic films were characterized by much lower TC(<40 °C) and ΔTC<10 °C, along with significant thickness variations of the ΔTrIR, ΔTrsol and Trlum parameters. Finally, rapid thermal annealing (RTA) was used and found to enhance the thermochromic performance of films deposited on float glass, with thickness greater than 50 nm. These differences might be attributed to the different composition of each kind of glass, as well as to diffusion phenomena between film and substrate.