Phase Transition Hysteresis of Tungsten Doped VO₂ Synergistically Boosts the Function of Smart Windows in Ambient Conditions

The critical challenge for the practical applications of VO₂-based smart windows is to nucleate and propagate the phase transformation in the ambient condition while maintaining a balance between solar energy regulation and visible light transmittance. Doping proves effective in the modification of the thermochromic performance in VO₂ but always causes unfavorable degradation accompanied with the significant reduction of phase transition temperature. Joule heating has been introduced in VO₂-based devices to activate the metal–insulator transition (MIT) at ambient temperature. A synergy between these two strategies is necessary to achieve a well-balanced performance of VO₂ films at the appropriate temperature. Here, we systematically investigate the thermochromic properties of tungsten (W)-doped VO₂ composite films with Joule heating triggered MIT. The phase transition temperature of VO₂ is effectively decreased by 21.6 °C per at. % W, and the balanced luminous transmittance (T_lum = 50.8%) and solar energy modulation ability (ΔT_sol = 11.4%) are achieved at 0.6 at. % W doping. Importantly, as a synergetic result of W doping and hysteresis behavior of MIT in VO₂, the optimal infrared blocking performance can be retained at a reduced cost of energy consumption and at the ambient temperature down to 47 °C. This is comparable to the glass window temperature in the summer in subtropical and tropical regions. This result suggests quite low and even zero energy consumption to maintain the optimal infrared blocking performance of VO₂ films. This study provides a practical and advanced setup for operable and efficient smart windows in ambient conditions.