Properties of V–Ce mixed-oxide thin films deposited by RF magnetron sputtering

Thin films of vanadium cerium mixed oxides are good counter-electrodes for electrochromic devices because of their passive optical behavior and very good charge capacity. We deposited thin films of V–Ce mixed oxides on glass substrates by RF magnetron sputtering under argon at room temperature using different power settings. The targets were pressed into pellets of a powder mixture of V₂O₅ and CeO₂ at molar ratios of 2:1, 1:1, and 1:2. For a molar ratio of 2:1, the resulting crystalline film comprised an orthorhombic CeVO₃ phase and the average grain size was 89 nm. For molar ratios of 1:1 and 1:2, the resulting films were completely amorphous in nature. Scanning electron microscopy images and energy-dispersive X-ray spectroscopy data confirmed these results. The optical properties of the films were studied using UV-Vis-NIR spectrophotometry. The transmittance and indirect allowed bandgap for the films increased with the RF power, corresponding to a blue shift of the UV cutoff. The average transmittance increased from 60.9% to 85.3% as the amount of CeO₂ in the target material increased. The optical bandgap also increased from 1.94 to 2.34 eV with increasing CeO₂ content for films prepared at 200 W. Photoacoustic amplitude (PA) spectra were recorded in the range 300–1000 nm. The optical bandgap was calculated from wavelength-dependent normalized PA data and values were in good agreement with those obtained from UV-Vis-NIR data. The thermal diffusivity calculated for the films increased with deposition power. For thin films deposited at 200 W, values of 53.556×10⁻⁸, 1.069×10⁻⁸, and 0.2198×10⁻⁸ m²/s were obtained for 2:1, 1:1, and 1:2 V₂O₅/CeO₂, respectively.