Optical constants and electrochromic properties of sol-gel V$_{2}$O$_{5}$ thin films

Vanadium pentoxide thin films are prepared by the sol-gel route by dissolving V$_{2}$O$_{5}$ powder (99.5% purity) in H$_{2}$O$_{2}$ solution. The solution is spin - coated on glass substrates for optical (UV-VIS-NIR) analysis, and on ITO-coated glass substrates for electrochromic measurements. The samples are then annealed at 150 $^{\circ}$C for 1 hour. The resulting films have a yellow-orange color, typical of polycrystalline V$_{2}$O$_{5}$. The optical and physical constants (n, $\alpha $, E$_{\rm g}$, the thickness d and the mean thickness inhomogeneity $\sigma )$ of the films are calculated using a simple and accurate method based on the transmission spectrum alone. Electrochromism of the films is studied using cyclic voltammetry (CV) in propylene carbonate solution containing 1 mol/1LiClO$_{4}$. The films show reversible multichromism (yellow-green-blue) upon Li$^{+}$ ion insertion/extraction. The absorbance of films colored at three different potentials is measured in the UV-VIS wavelength range, and this study shows that the changes in the optical absorption are consistent with the film color changes. Finally, the optical and electrochromic properties of the films prepared by this method are compared with those of the sputtered films already studied in our previous works