Enhancing thermochromic performance of VO2 films via increased microroughness by phase separation

Application of VO 2 to smart windows calls for an enhancement of both luminous transmittance ( T lum ) and solar transmittance modulation (Δ T sol ). These two properties are related to optical constants , microstructure and film thickness. In this article, VO 2 -based films with increased microroughness are prepared by polymer assisted deposition , and ZnCl 2 is added to the precursor solution to control the microstructure of the films. Addition of ZnCl 2 increases the microroughness of films through phase separation, which largely decreases the refraction index and the reflectivity of the films, resulting in enhanced luminous transmittance and solar transmittance modulation; two films with balanced optical performance show T lum =50.5% and Δ T sol =11.3%, and T lum =59.3% and Δ T sol =10.5%, respectively. Moreover, this kind of microstructure control has no influence on doping VO 2 , which is proven by the deposition and characterization of W-doped VO 2 -based films. By the formation of a W-doped VO 2 /F:SnO 2 /glass multilayer stack, a film shows excellent thermochromic properties and low emissivities; typically, a film with a phase transition temperature, T lum , Δ T sol of 43.4   °C, 30.0%, 8.82%, respectively, and emissivity of 0.20 for monoclinic and 0.33 for rutile phase is successfully prepared.