A low-temperature synthesis of strongly thermochromic W and Sr co-doped VO2 films with a low transition temperature

The reversible semiconductor-to-metal transition of vanadium dioxide (VO2 ) makes VO2 -based coatings a promising candidate for thermochromic smart windows, reducing the energy consumption of buildings. We report low-temperature (320 degrees C) depositions of thermochromic V(1-x-y)W(x)SryO(2 )films with a thickness of 71-73 nm onto 170-175 nm thick Y-stabilized ZrO2 layers on a 1 mm thick conventional soda-lime glass. The developed deposition technique is based on reactive high-power impulse magnetron sputtering with a pulsed O(2 )flow feedback control allowing us to prepare crystalline W and Sr co-doped VO2 films of the required stoichiometry without any substrate bias or post-deposition annealing. The W doping of VO2 decreases the transition temperature below 25 degrees C, while the Sr doping of VO2 increases the integral luminous transmittance, T-lum, significantly due to widening of the visible-range optical bandgap, which is consistent with lowering of the absorption coefficient of films. We present the discussion of the effect of the Sr content in the metal sublattice of VO2 on the electronic and crystal structure of V(1-x-y)W(x)SryO(2) films, and on their temperature-dependent optical and electrical properties. An optimized V(0.855)W(0.0)18Sr(0.127)O(2) film exhibits a high T-lum = 56.8% and modulation of the solar energy transmittance Delta T-sol = 8.3%, which are 1.50 times and 1.28 times, respectively, higher compared with those of the V(0.984)W(0.0)16O(2) film. The achieved results constitute an important step toward a low-temperature synthesis of large-area thermochromic VO2 -based coatings for future smart-window applications, as it is easy to further increase the T-lum and Delta T-sol by >6% and >3%, respectively, using a 280 nm thick top SiO(2 )antireflection layer.