The Effect of CeO_2 Antireflection Layer on the Optical Properties of Thermochromic VO_2 Film for Smart Window System

摘要

CeO_2-VO_2 bilayer structure was fabricated to investigate the effect of depositing CeO_2 film on the optical properties of VO_2 film for smart window application. CeO_2 was employed as an antireflection (AR) layer material of VO_2 film because of its advantages which include high transparency in the visible-near infrared range and high refractive index. All the films were deposited on soda-lime glass substrate by pulsed laser deposition method. Optical calculations were carried out using transfer-matrix method for the purpose of designing CeO_2-VO_2 bilayer structure with enhanced integrated luminous transmittance (T_(lum)) and switching efficiency (ΔT_(sol)). The optical constants of VO_2 and CeO_2 films needed for the optical calculation were measured by spectroscopic ellipsometer. The curve of T_(lum) the shape of which depends on the thickness of CeO_2 layer, was calculated in each VO_2 sample, which showed two maxima. The samples were divided into two groups; one for the highest enhancement of T_(lum) and the other for balanced enhancement between T_(lum) and ΔT_(sol). The sample with the structure of ～60 nm CeO_2 AR layer on 39-nm thick VO_2 film showed large increase of T_(lum) (～27%) with ΔT_(sol) of ～5%, which is the largest increase in T_(lum) reported so far. Two samples in the other group showed the balanced enhancement in T_(lum) (～57, ～50%) and ΔT_(sol) (～9, ～10.5%). The effect of CeO_2 AR layer on the optical properties of VO_2 film was confirmed with the optical calculation and the experimental results. CeO_2-VO_2 bilayer structure showed notable improvement of optical properties compared to the single VO_2 film, indicating that CeO_2 layer can be effectively used as the antireflection layer while working as a protective layer that can prevent the oxidation of VO_2 layer as well.