Sn dopants improve the visible transmittance of VO₂ films achieving excellent thermochromic performance for smart window

How to achieve high visible-light transmission meanwhile maintain satisfying solar modulation (ΔT_sol) remains a challenge for VO₂-based thermochromic smart windows. In this study, Sn dopants exhibit the ability of enhancing the visible transmittance (T_lum) of VO₂ films, as a result, endowing the films with excellent thermochromic properties. Sn-doped VO₂ films with different Sn contents (0at%, 0.43at%, 0.74at%, 1.73at%, 1.86at%) are prepared via hydrothermal method. The 0.74at% Sn-doped film has a T_lum value about twice that of the undoped film. The improvement of the visible transmittance is rationalized by ellipsometry and optical absorption measurements, in which it is revealed that the Sn dopants reduce the refractive index (n) and extinction coefficient (k) of VO₂ in the VIS-NIR wavelength range, and widen the band gap (E_g). The 0.74at% Sn-doped VO₂ film realizes excellent thermochromic performance with the ΔT_sol up to ~15.4% and the T_lum reaching ~51.8%. This work demonstrates the unique characteristic of Sn dopants to improve the visible transmittance of VO₂ films, suggesting a rational way in optimizing the thermochromic property of VO₂.