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 (Delta T-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 similar to 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., Delta T-sol, 01 of similar to 13.4% and T(lum )of similar to 41.1% with a low T-MIT of 26 degrees 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.