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

Abstract

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 (Tlum) while sacrificing the solar modulation ability (ΔTsol) hampers its bench-to-market translation. Previous studies of anti-reflection coatings (ARCs) focused primarily on increasing Tlum while neglecting ΔTsol, which is a key energy-saving determinant. The intrinsically low ΔTsol (<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 (τc), 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 ΔTsol for various VO2 nanosubstrates, e.g. continuous thin films, nanocomposites, and periodic micro-patterning films. We showed that the best performing coatings could maximize ΔTsol (from 15.7% to 18.9%) and increase Tlum(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.