Porous vanadium dioxide thin film-based Fabry−Perot cavity system for radiative cooling regulating thermochromic windows: experimental and simulation studies

Abstract

Radiative cooling in smart windows using VO2 – a dynamic thermal management material, is of potential interest for enhancing energy savings in buildings due to its both solar and emittance tuneability in response to changing temperatures. However, studies related to the effects of VO2 thin film microstructure in a multilayer system on emissivity regulation are currently lacking. The present study addresses the thermochromic and emissivity performance of VO2/ZnSe/ITO/Glass Fabry−Perot (F–P) cavity thin film system, by manipulating the porosity in VO2 thin film. The device is fabricated by commercially feasible physical vapor deposition methods such as sputtering and thermal evaporation, most suitable for mass production. The optimized sample with porous VO2 delivers an enhanced long-wave infrared (LWIR) emissivity contrast of ΔεLWIR ≥ 0.4 preserving a high visible transparency Tlum(avg) of ∼41 % compared to dense VO2. Then finite difference time domain (FDTD) simulation is performed to further understand the effects of varying VO2 porosity and ZnSe thickness on the F–P cavity properties. The reduced low-temperature εLWIR (0.1–0.2) gives this film better energy saving in regions where warming demand is dominant as simulated by EnergyPlus.