Effects of Substrate Temperature on Electrochromic Properties of Cobalt Oxide and Oxyhydroxide Thin Films Prepared by Reactive Sputtering Using O2and H2O Gases

Abstract

Cobalt oxide and oxyhydroxide thin films were prepared by reactive sputtering of a Co target in O2 and H2O gases at substrate temperatures from -20 to +200 °C. Co3O4 films were formed at all the substrate temperatures in O2 gas and at 200 °C in H2O gas, and amorphous CoOOH films were formed in the range from 10 to -20 °C in H2O gas. A large transmittance change of approximately 26% and high electrochromic (EC) coloration efficiency of 20 cm2/C at a wavelength of 600 nm were obtained for the CoOOH thin films deposited at -20 °C in 0.1 M KOH aqueous solution. The good EC performance of the CoOOH film is attributed to the low film density and amorphous structure.