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 (TMIT), low visible transmittance (Tlum), and unsatisfying solar modulation (ΔTsol) limit their application. W doping is an effective way to lower the TMIT 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 TMIT and meanwhile acquire an improved Tlum to satisfy the application. In our results, adding Sn in W-doped VO2 significantly enhances the Tlum by ∼1.8 times, while it does not obviously change the TMIT reduced by W dopants. As a result, the obtained Sn–W co-doped VO2 films exhibit excellent thermochromic performance, e.g., ΔTsol of ∼13.4% and Tlum of ∼41.1% with a low TMIT 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 (Eg), 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.