Thermochromic properties of double-doped VO2 thin films prepared by a wet coating method using polyvanadate-based sols containing W and Mo or W and Ti

A mixture of metallic vanadium, tungsten and molybdenum powder was dissolved in a hydrogen peroxide solution, to yield a polyvanadate sol containing W and Mo. A thin film of V1-x-yWxMoyO2 was fabricated on a fused silica substrate by spin-coating using such a sol followed by reduction in hydrogen and successive annealing in a nitrogen atmosphere. Thin films in the system V1-x-zWxTizO2 were similarly prepared using a sol containing W and Ti. All the films obtained in this study showed a clear thermochromic switching performance with a large transmittance change in the infrared region. For V1-x-yWxMoyO2, the switching temperature (Tt) was lowered by doping at approximately Tt = Tt0 (= 67°C)-ax-by, where a and b are the decrease rates of each individual dopant (about 1920 and 870°C, respectively, for 0 ≤ x, y ≤ 0.03). Doping of Ti had little effect on Tt, but markedly reduced hysteresis of the transition. We demonstrated that a double-doped film of V1-x-zWxTizO2 (x = 0.018 and y = 0.10) showed thermochromic switching almost without any hysteresis at a temperature as low as 45°C.