Sol-Gel Deposited Amorphous Tantalum Oxide and Niobium Oxide Films as Protonic Conductors

In this work we report on the preparation of tantalum oxide and niobium oxide films by the sol-gel method for use as proton ion conductors in electrochromic devices. Measurement of the proton conductivity was derived from impedence spectroscopy measurements. The proton conductivity for Ta₂O₅ and NbO₅ films was 4.6 x 10^-6 Scm^-1 and 3.2 x 10^-7 Scm^-1 respectively. The structural and chemical properties of the films were investigated by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. These films were found to be amorphous and close to the stoichometry of Ta₂O₅ and NbO₅ respectively. In order to estimate the compatibility of these sol-gel deposited layers as ion conductors for EC devices, we investigated tantalum oxide and niobium oxide films deposited on sputtered WO₃ films. Charge balance and coloration characteristics were investigated during voltammetric cycling in a pH 2 liquid electrolyte. Spectral transmittance was measured for colored and bleached conditions. The photopic weighted transmittance change and solar weighted transmittance change were T_p=35.2% - 21.8% and T_s=75.7% - 14.2% for tantala films on tungsten oxide. For niobia on tungsten oxide the values were T_p=85.3% - 35.2% and T_s=75.8% - 28.1%.