Stability of Thermochromic Vanadium Dioxide Thin Films in Harsh Environments of Temperature and Humidity

Stability of thermochromic VO2 thin films against harsh environmental conditions, which is important if the films are to be used in practical smart windows, was investigated. The films were prepared on normal soda-lime glass substrates by reactive radio frequency magnetron sputtering of metallic vanadium target (99.95% purity) at a working pressure of mbar, argon and oxygen flow rates of 76 and 1.4 – 3.0 ml/min, respectively, substrate temperature of 400 °C and sputtering power of 150 watts. After deposition, the films were exposed to extreme temperatures and relative humidity at 100 °C, 400 °C and 89%, respectively, for different time durations. Upon exposure, the films structural properties were investigated using the transmission electron microscope and the atomic force microscope, whereas the UV/VIS/NIR spectrophotometer was used to investigate the films’ optical properties. The results showed some degradation in the first high-temperature exposure cycle (at t = ~ 100 °C) but were quite stable, and with sufficient thermochromism, after the subsequent cycles. Exposure of the films to extreme temperatures at 400 °C, resulted in complete loss of thermochromism. Exposure of the films to extreme humidity (RH ~ 89%) displayed a gradual increase in degradation with exposure time; however, the films retained sufficient thermochromism even after 3 weeks of exposure. Possible explanations for the degradation mechanism are discussed by correlating the observed effects in the structural and optical properties of the films.