Effects of Substrate Temperature on Electrochromic Properties of Cobalt Oxide and Oxyhydroxide Thin Films Prepared by Reactive Sputtering Using O₂ and H₂O Gases

Cobalt oxide and oxyhydroxide thin films were prepared by reactive sputtering of a Co target in O 2 and H 2 O gases at substrate temperatures from -20 to +200 °C. Co 3 O 4 films were formed at all the substrate temperatures in O 2 gas and at 200 °C in H 2 O gas, and amorphous CoOOH films were formed in the range from 10 to -20 °C in H 2 O gas. A large transmittance change of approximately 26% and high electrochromic (EC) coloration efficiency of 20 cm 2 /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.