  VO2 material is promising for developing energy-saving "smart window", owing
to its thermochromic property induced by metal-insulator transition (MIT).
However, its practical application is greatly limited by the relatively high
critical transition temperature (~68oC), low luminous transmittance (<60%) and
poor solar energy regulation ability (<15%). Here we developed a reversible and
non-volatile electric-field control on the MIT of monoclinic VO2 film. With a
solid electrolyte layer assisted gating treatment, we modulated the
insertion/extraction of hydrogens into/from VO2 lattice at room temperature,
causing tri-state phase transitions accompanied with controllable transmission
adjustment. The dramatic increase of visible/infrared transmittance during the
phase transition from the metallic (lightly H-doping) to insulating (heavily
H-doping) phase leads to an increased solar energy regulation ability up to
26.5%, while keep 70.8% visible-luminous transmittance. These results beat all
previous records and even exceeded the theoretical limit for traditional VO2
smart window, removing intrinsic disadvantages of VO2 for energy-saving
utilizations. Our findings not only demonstrated an electric-field controlled
phase modulation strategy, but also open the door for high-performance
VO2-based smart window applications.
