Optimizing thermochromic VO2 by co-doping with W and Sr for smart window applications

Smart windows possess the potential to contribute significantly to reducing the world-wide energy consumption in the future. The properties of the thermochromic metal oxide VO2 are closest to the material requirements set by economic considerations for the use as an active layer in thermochromic glazings. We show that the required parameters can be achieved by modifying VO2 on the atomic level, i.e., by simultaneous co-doping with Sr and W on the cation site. In particular, the transition temperature ϑc can be adjusted in the range between 15 and 68 °C by varying W contents, whereas the incorporation of Sr mainly increases the band gap yielding a greyish color of the films. Interestingly, the simultaneous presence of W and Sr enhances both effects. The co-doping leads to values of the luminous transmittance Tlum and the solar transmittance Tsol fulfilling the requirements set by application. The variation of the solar transmittance ΔTsol of the plain thin films on a glass substrate already is larger than 5% for all samples promising that the required value of ΔTsol=10% is achievable by using such thin films as part of an optically engineered multilayer system.