Thickness dependent surface roughness of sputtered Li_2.5TaO_x ion conductor and its effect on electro-optical performance of inorganic monolithic electrochromic device

Transparent amorphous Li_2.5TaO_x thin films with different thicknesses were deposited by RF magnetron sputtering. The ionic conductivities of as-deposited films determined by ac impedance spectroscopy were of the order of 10⁻⁸ S/cm at room temperature. The surface roughness increased rapidly with the film thickness increasing as confirmed by scanning electron microscopy and atomic force microscopy. Inorganic monolithic electrochromic devices (ECDs) based on WO₃//NiO complementary structure were fabricated using the Li_2.5TaO_x with different thicknesses as the ion conductor layers. The electro-optical performance of the ECDs was characterized by cyclic voltammetry and chronoamperometry. The ECDs showed a highest optical modulation of 40% at 550 nm driven by − 1.5 V (coloration) and 1 V (bleaching) with switching time of 30 s. Both the amount of transferred charge and coloration efficiency were found to be highly dependent on the film thickness of Li_2.5TaO_x layer. The high charge transfer resistance at the Li_2.5TaO_x/WO₃ interface and poor growth of WO₃ layer due to the high surface roughness of Li_2.5TaO_x may well account for the decreased device performance.