Tailoring of luminous transmittance upon switching for thermochromic VO2 films by thickness control

The difference in luminous transmittance (DeltaT(lum)) upon switching of VO2 films strongly affects its solar controllability when used as a thermochromic window. It was found that DeltaT(lum) is controllable by film thickness. Optical calculation for a VO2 film on quartz glass revealed that the low-temperature semiconductor phase exhibits lower T-lum than the high-temperature metallic phase for thickness below 50 nm, while the relationship is reversed above 50 nm. The calculation was confirmed by film deposition and measurement. Maximum DeltaT(lum) is located near 80 nm. An enhanced DeltaT(lum) contributes largely to solar efficiency.