Influence of the porosity of RF sputtered Ta2O5 thin films on their optical properties for electrochromic applications

Thin films of Ta2O5 were deposited at room temperature by RF-magnetron reactive sputtering from a tantalum oxide target. Fused silica, aluminised glasses and bilayered WO3/ITO structures on glass were used as substrates. The deposition parameters considered were RF power, oxygen ratio and the total pressure inside the reactor. The samples with fused silica were optically characterized with a UV–visible spectrophotometer (190–900 nm). From the transmittance spectra, exceeding 90%, we obtained the refractive index, the optical gap, the absorption and the film thickness. Electro-optical measurements were performed by applying cyclic voltammetries to the samples, including the electrochromic layer of WO3, in an electrochemical cell compatible with light transmission and therefore for in-situ optical analysis. The transferred charge, which reached up to 70 mC/cm2, provides an approach for quantifying the ionic conductivity of the deposited material, and its probable correlation with porosity. Comparing these structural characteristics to the related optics, a relationship between ion conduction and porosity was found. Variation of process parameters to achieve optimal performance offers the industry new possibilities to use these films as solid electrolytes for electrochromic devices built on glass substrates or polymers.