Enhancing luminous transmittance and hysteresis width of VO2-based thermochromic coatings by combining GLAD and RGPP approaches

Abstract

This work reports on alternative strategies for improving the performance and thickness uniformity of VO2-based thermochromic coatings by atmospheric oxidation of zigzag vanadium films sputtered at glancing angles in presence of reactive oxygen. With the aim of scrutinizing the effect of deposition parameters, VO2–x films of 25, 50 and 100 nm nominal thickness were deposited on glass substrates without reactive oxygen and by injecting oxygen pulses for 4 or 8 s every 16 s of deposition. Comprehensive structural, compositional and functional characterizations of samples annealed at reaction temperatures between 475–550◦C for times ≤ 60 s confirmed the synthesis of the VO2(M) phase and the development of singular microstructural and thermochromic features depending on the duration of the oxygen injection pulses. Here, it was evidenced that longer oxygen pulses lead to significantly higher luminous transmittances and broader hysteresis loops thanks to the formation of generally larger grains, although with heterogeneous morphology and size distribution. These same events also caused unwanted increases in the metal-to-insulator transition temperature and the occurrence of asymmetric hysteresis loops. The best performances for smart glazing applications were eventually achieved by instantaneous oxidation of 25 nm thick films deposited with 8 s oxygen pulses, exhibiting remarkable luminous transmittances (>55%), solar modulation abilities (>5%), beneficial drops in critical temperature without doping (up to 7◦C below the standard value for pure VO2) and an extraordinarily wide variable hysteresis between 18–26◦C. These results represent a substantial enhancement, mainly regarding visible transmittance and hysteresis width, over those attained through similar two-step approaches (atmospheric annealing of vanadium films sputtered at glancing angles), paving the way for further doping processes.