CuxTi1-xO2 buffer layers in VO2–based smart windows – a viable compromise towards large-scale industrial production

The use of rutile copper titanium oxide (Cu_xTi_1-xO₂) as a buffer layer for the low-temperature growth of thermochromic vanadium dioxide is investigated. Specifically, this study examines how incorporating copper influences the structural and optical properties of titanium dioxide. On the one hand, its incorporation yields alloy formation and lowers the transition temperature of the anatase-to-rutile phase transition, enabling rutile Cu_xTi_1-xO₂ to be formed in the sputtering deposition process at temperatures as low as 200 °C, compared to the minimum of 600 °C required for rutile TiO₂. However, on the other hand, the optical transparency of Cu_xTi_1-xO₂ in the visible range of the electromagnetic spectrum decreases with increasing Cu content. A tri-layer structure consisting of a Cu_xTi_1-xO₂ buffer layer, a thermochromic VO₂ layer, and an anatase TiO₂ antireflection coating is designed and grown, and its thermochromic key parameters are studied. The performance is almost as good as that of a tri-layer structure where rutile TiO₂ is used as the buffer layer. Therefore, using a rutile Cu_xTi_1-xO₂ buffer layer allows VO₂-based multilayer structures for advanced thermochromic applications to be grown at deposition temperatures approaching those compatible with industrial sputtering apparatuses.