Thermochromic Ta Doped VO2 Films: Enhanced Luminous Transmittance, Significantly Depressed Phase Transition Temperature and Hysteresis Width

VO2 based films are considered as a promising thermochromic material for smart windows but not widely used due to their unbalanced thermochromic performance, in particular the relatively high phase transition temperature (T-c , similar to 68 degrees C). Herein, tantalum (Ta) doped VO2 films were fabricated by sol-gel method and the effects of Ta dopant on the thermochromic properties including metal-to-insulator transition (MIT) temperature, hysteresis width (Delta T), luminous transmittance (T-lum), and solar modulation efficiency (Delta T-sol) were investigated for the first time. It is demonstrated that T-c and Delta T were decreased by approximately 8.0 degrees C and 2.4 degrees C/at.% Ta, respectively, which was attributed to the raised defects concentration derived from Ta doping. And the 4at.% Ta doped VO2 film exhibited T-c of 24.8 degrees C and an extremely narrow Delta T of 3.5 degrees C. Also, the film showed an enhanced Tlum of 47.1% compared with pure VO2 film (40.1%) while keeping Delta Tsol beyond being impaired. The enhancement in optical properties is probably ascribed to the reduced particle size and widened optical bandgap. Therefore, this work could open opportunity to improve thermochromic performance of VO2 film in general, and provide insights into the relationship between the overall performance improvement and the structure variation induced by element doping.