Solution-based fabrication of vanadium dioxide on F:SnO2 substrates with largely enhanced thermochromism and low-emissivity for energy-saving applications

Vanadium dioxide is a key material for thermochromic smart windows that can respond to environmental temperatures to modulate near infrared irradiation from a transparent state at low-temperature to an opaque state at high-temperature while maintaining the visible transmittance. This paper reports a novel VO2/FTO/glass multi-layered structure, which shows promising optical properties for application to energy-efficient smart windows. VO2 thin films are deposited on F-doped SnO2 (FTO) glasses by annealing a precursor film that is obtained via a solution-based process. The rutile-structured FTO substrate enhances the crystallinity of the VO2 films and lowers the synthesis temperature to similar to 390 degrees C. The VO2/FTO/substrate double-layered films show both improved low-emissivity performance and distinct thermochromic properties. For a 65 nm thick VO2/FTO substrate double-layered film, low emissivities of 0.19 and 0.27 before and after the metal-insulator phase transition (MIPT) are obtained, while a solar transmittance modulation efficiency (eta, in the wavelength range of 280-2600 nm) of 4.9% is achieved. A TiO2 anti-reflective coating (ARC) is incorporated to form a three-layered TiO2/VO2/FTO/substrate structure to boost the integrated visible transmittance (T-vis) while maintaining the low-emissivity performance. A 29.4% improvement for T-vis from 34.0% to 44.0% at room temperature is achieved for a 55 nm thick VO2 film coated with a TiO2 layer while emissivities of 0.13 and 0.24 before and after MIPT are maintained. Moreover, h is also increased significantly, from 4.3% for the VO2/FTO/substrate structure to 8.8% for the TiO2/VO2/FTO/substrate structure. Our results demonstrate a newapproach of combining both thermochromism and low-emissivity performance for applications such as VO2-based energy-saving windows.