High luminous transmittance and solar modulation of VO2-based smart windows with SiO2 anti-reflection coatings

We report the effect of anti-reflection SiO2 coatings on the performance of luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of VO2-based thermochromic smart windows. VO2 films with different thicknesses of 60, 90, and 120 nm were deposited on ZnO-buffered glasses by an RF magnetron sputtering with substrate biasing. Through the simulations, we found that SiO2 film with a low refractive index (n ∼ 1.5) would be effective for anti-reflection on VO2/ZnO/glass layered structure. As the results, the highest ΔTsol of 17.7% was achieved in the 120 nm-VO2 with 174 nm-SiO2 coating. On the other hand, the highest Tlum of 52.5% was achieved in the 60 nm-VO2 with 85 nm-SiO2 coating. The 90 nm-VO2 with 85 nm-SiO2 coating realized balanced values of Tlum and ΔTsol of 44.2 and 11.6%, simultaneously. The achieved Tlum and ΔTsol are comparable to the highest values reported for VO2-based smart windows suggesting that the SiO2 coatings are quite effective for anti-reflection on VO2/ZnO/glass structure. Depth profiles of atomic concentrations by X-ray photoelectron spectroscopy (XPS), proved sharp SiO2/VO2 interface without inter-diffusion, while certain diffusion of Zn into VO2 layer, suggesting the necessity of further low-temperature processing for VO2 deposition. This study suggests a route for realizing the further improvement of VO2-based smart windows as practical applications.