Optimization of antireflection coating for VO2-based energy efficient window

Vanadium dioxide (VO2) films have been proposed as energy efficient window coatings for their thermochromism, with which the solar energy transmission in the IR region may be controlled passively. These coatings suffer from low visible (or luminous) transmission (380–760 nm in wavelength), which hinders their practical uses. We here consider an antireflection (AR) coating for the VO2-based window. Optical calculation was first performed upon a basic structure for thermochromic window composed of a VO2 layer on glass with an AR layer of refractive index n and thickness d. Optimization was carried out on n and d for a maximum integrated luminous transmittance (Tlum). The calculation demonstrates that the optimal n value changes with thickness of VO2, and at n≈2.2 it gives the highest Tlum enhancement from 32% (without AR coating) to 55% for 50-nm VO2. Experiment was done on a structure of 50-nm VO2 on quartz glass using ZrO2, of which n≈2.2 matching the best n value, as AR coating. Formation of an optimized structure, ZrO2 (56 nm)/VO2 (50 nm)/quartz, was done by sputtering, and its optical properties were characterized with spectrophotometry. An improvement of Tlum from 32.3% to 50.5% was confirmed for the semiconductor phase with similarity also for the metallic one. The IR switching properties were not much deteriorated.