Sn−W co-doping improves thermochromic performance of VO₂ films for smart windows

VO₂-based thermochromic films are promising candidates for new-generation energy-saving smart windows. However, the overly high transition temperature of metal−insulator transition (T_MIT), low visible transmittance (T_lum), and unsatisfying solar modulation (ΔT_sol) limit their application. W doping is an effective way to lower the T_MIT but simultaneously deteriorates the transmittance and optical modulation of VO₂. In this work, a Sn−W co-doping approach is proposed to effectively lower the T_MIT and meanwhile acquire an improved T_lum to satisfy the application. In our results, adding Sn in W-doped VO₂ significantly enhances the T_lum by ∼1.8 times, while it does not obviously change the T_MIT reduced by W dopants. As a result, the obtained Sn−W co-doped VO₂ films exhibit excellent thermochromic performance, e.g., ΔT_sol of ∼13.4% and T_lum of ∼41.1% with a low T_MIT of 26 °C for a 1.9 at. % Sn + 1.6 at. % W co-doped VO₂ film. Ellipsometry and optical absorption measurements indicate that the Sn−W co-doped VO₂ films present a widened optical band gap (E_g), lower refractive index (n), and extinction coefficient (k) in the visible light range, which favors the improvement of the visible transmittance. This work demonstrates the Sn−W co-doping strategy is promising to realize VO₂-based smart-window applications.