Indium-vanadium oxides deposited by radio frequency sputtering: New thin film transparent materials for Li-insertion electrochemical devices

Thin films of mixed In/V oxides have been obtained by reactive RF sputtering. Their optical and electrochemical performances have been investigated in order to determine their possible applications in electrochromic devices as optically passive ion-storage layers. The targets used have been made of In2O3 and V2O5 powders mixed in different In/V ratios in a reactive sputtering atmosphere. Cyclic voltammetry and chronopotentiometry studies of these films have been performed. The films demonstrate high ion-storage capacity retained even after 1000 cycles without any significant degradation. Lithium diffusion coefficients, calculated by the potentiostatic intermittent titration technique (PITT), range around 10(-13) cm(-1) s(-1). The optical measurements, taken in the UV-vis-NIR transmittance and reflectance modes, have demonstrated that films are electrochromic, but the presence of In enhances their transparency and optical passiveness. The photometric spectra evaluation by a computer fitting shows that the refractive index is lower for films with higher In content. This result is in good agreement with Rutherford backscattering spectroscopy (RBS) measurements. The material structure has been discussed on the basis of the above results, as a function of the measured thin film composition, and is corroborated by preliminary results of XPS surface analysis.