Index-tunable anti-reflection coatings: Maximizing solar modulation ability for vanadium dioxide-based smart thermochromic glazing

Vanadium dioxide (VO₂) nanoparticles with reversible semiconductor-metal phase transition holds the tremendous potential as a thermochromic material for the energy-saving smart glazing. However, the trade-off between improving the luminous transmittance (T_lum) while sacrificing the solar modulation ability (ΔT_sol) hampers its bench-to-market translation. Previous studies of anti-reflection coatings (ARCs) focused primarily on increasing T_lum while neglecting ΔT_sol, which is a key energy-saving determinant. The intrinsically low ΔT_sol (<16%) is due to the fact that VO₂ has a higher refractive index (RI) from 500 nm to 2200 nm wavelength (λ) below its critical transition temperature (τ_c), which causes excessive reflection at a lower temperature. This study aims to investigate ARCs with tunable RI (1.47–1.92 at λ = 550 nm) to improve the antireflection effect at a lower temperature, thereby maximizing ΔT_sol for various VO₂ nanosubstrates, e.g. continuous thin films, nanocomposites, and periodic micro-patterning films. We showed that the best performing coatings could maximize ΔT_sol (from 15.7% to 18.9%) and increase T_lum(avg) (from 39% to 44%) simultaneously, which surpasses the current bench-mark specifications ever reported for ARC-coated VO₂ smart glazing. In addition, the cytotoxicity analyses evidence that ARCs are feasible to improve the cyto-compatibility of VO₂ nanoparticles-based nanocomposites. The presented RI-tunable ARC, which circumvents the complex materials selection and optical design, not only paves the way for practical applications of VO₂-based smart windows but also has extensive applications in the field of solar cells, optical lenses, smart display, etc.