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

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 Δɛ _LWIR ≥ 0.4 preserving a high visible transparency T _lum(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 ɛ _LWIR (0.1-0.2) gives this film better energy saving in regions where warming demand is dominant as simulated by EnergyPlus.