Structural and optical properties of sol-gel deposited proton conducting Ta2O5 films

Proton conducting tantalum oxide films were deposited on ITO (Indium Tin Oxide) coated glass, fused silica and soda-lime glass substrates by spin coating using a sol-gel process. The coating solutions were prepared using Ta(OC2H5)(5) as a precursor. X-ray diffraction studies determined that the sol-gel films, heat treated at temperatures below 400 degrees C, were amorphous. Films heat treated at higher temperatures were crystalline with the hexagonal delta-Ta2O5 structure. The solar transmission values (T-s) of tantala films on glass generally range from 0.8-0.9, depending on thickness. The refractive index and the extinction coefficient were evaluated from transmittance characteristics in the UV-VIS-NIR regions. The refractive index values calculated at lambda = 550 nm increased from n = 1.78 to 1.97 with increasing heat treatment from 150 to 450 degrees C. The films heat treated at different temperatures showed low absorption, with extinction coefficients of smaller than k = 1 x 10(-3) in the visible range. Impedance spectroscopic investigations performed on Ta2O5 films revealed that these films have a protonic conductivity of 3.2 x 10(-4) S/m. The films are suitable for proton conducting layers in electrochromic (EC) devices.