Optical properties of sol–gel dip-coated Ta 2O 5 films for electrochromic applications

Abstract

Amorphous Ta 2O 5 films were prepared by sol–gel dip process on different substrates. The dip-coating technique was used to prepare amorphous Ta 2O 5 films by hydrolysis and condensation of tantalum ethoxide, Ta(OC 2H 5) 5, precursor. Stable coating solutions were prepared using acetic acid as a chelating ligand and catalyzer. Single layer and multi-layered Ta 2O 5 films were fabricated at a dipping rate of 107 mm/min. The microstructure, stoichiometry and optical properties of these films were investigated as a function of the film thickness. Room temperature CV measurements clearly revealed a protonic conductor behavior for Ta 2O 5 films. Optical properties such as refractive index, extinction coefficient and optical band gap value of the Ta 2O 5 films were calculated from optical transmittance measurements. It was found that the refractive index and extinction coefficient values were affected by the thickness of the coatings. The refractive index at a wavelength of 550 nm increased from 1.70 to 1.72 with increasing film thickness. The optical band gap value (3.75±0.12 eV) of the coating was unaffected by the film thickness. These results indicate that sol–gel-deposited Ta 2O 5 films have a promising application as proton conductors in electrochromic devices.