Optical design of antireflective on thermochromic coatings

Abstract

Energy regulating VO2 thermochromic films has been strongly recommended to be applied on the smart building windows. However, it has yet to be widely commercialized in large scale since VO2 films commonly undergo low luminous transmittance measured from 380 nm to 780 nm. In the recent two decades, many researchers are dedicating to solve this issue. Depositing a single layer ARC on the top of a well-fabricated VO2 film is recommended due to its relatively simple and cheap fabrication processes. The thickness and refractive index are more flexible to control during preparation. In this thesis, TEOS and Si-Al gel are nominated with great potential as anti-reflective coating (ARC) and they are capable to provide an overall improvement of solar transmittance without degrading the VO2 intrinsic IR modulating ability.

The AR properties of thin films are closely associated with the variances of the refractive indexes of the films and their substrates, as well as the thickness of ARC. If the film has a refractive index lower than its attached substrate and its thickness is controlled around a quarter of incident visible wavelength, reflective losses can be avoided. The determination of the optical constants of thin films is of great importance for the design of any optical and optoelectronic device. Thermal annealing is an effective method to improve optical properties of AR films. In this project, variation is performed to the dip-coating withdrawing rate to control the ARC thickness and the sintering temperature to vary refractive indexes. The optimal ARC fabrication condition is specified at a dip-coating speed of 300mm/min, followed by a post heat treatment at 100°C for both Si-Al and TEOS ARC. Si-Al ARC gel is generally found to possess lower refractive indexes than TEOS ARC and also acquire higher visible transmittance increments (32.91% to 44.9% at 20°C and 37.77% to 48.95% at 90°C) compared to TEOS ARC (35.59% to 44.99% at 20°C and 39.59% to 49.53% at 90°C).