Enhancing Thermochromic Properties for Thermal Regulation: A Synergistic Study of Er-W-Codoped Vanadium Dioxide (VO₂)

A vanadium dioxide (VO₂)-based thermochromic passive smart window is an ideal energy-efficient solution to maintain the subambient temperature of a building in all seasons for energy savings. A high metal-insulator transition temperature (T_MIT), low visible transmittance (Tr_lum), and poor solar modulation (ΔTr_sol) of VO₂ hinder its widespread adoption. W doping effectively lowers the T_MIT of VO₂ but adversely affects Tr_lum and infrared (IR) modulation. To lower the T_MIT and enhance Tr_lum along with better IR modulation, an Er-W codoping approach is proposed in this work by producing synergy effects. Incorporating Er ions in W-doped VO₂ significantly lowers the T_MIT further and improves the Tr_lum and ΔTr_sol by ∼1.9 and ∼1.8 times, respectively. In the Er-W-codoped VO₂ M-phase, W and Er dopants lead to the creation of localized electrons and holes near neighboring vanadium atoms, resulting in an uncompensated charge within the host lattice. Additionally, the Er-W codoping destabilizes M-phase by softening V-V dimers and introducing indirect 3d-4f hybridization, resulting in a reduction in the T_MIT. The combined effect of Er-W codoping in VO₂ has been elucidated through both experimental and density functional theory investigations. This study demonstrates that Er-W codoping could serve as a promising approach for developing VO₂-based passive smart windows.