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

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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.