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 0x, y0.03). 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.