Effect of preparation conditions on the morphology and electrochromic properties of amorphous tungsten oxide films

Abstract

Electrochromic tungsten oxide thin films have been prepared by reactive dc-magnetron sputtering under different deposition conditions. Through the use of electrochromic coloration and spectroscopic ellipsometry experiments, the effect of preparation conditions on film morphology and electrochromic properties has been studied. The results of this study are consistent with previous reports finding that amorphous tungsten oxide thin films are dominated by a columnar morphology and an intercolumnar void network that strongly influence the electrochromic properties. In addition real-time spectroscopic ellipsometry (SE) experiments on the tungsten oxide electrochromic coloration process have been performed for the first time. In studies of very thin (∼15 nm) films by real-time SE, the formation of tungsten bronze exhibits reaction-limited kinetic behavior, and thus we are able to extract accurate dielectric function spectra for the film in different stages of the coloration process. These spectra can be understood in terms of a two-phase physical mixture of tungsten oxide and tungsten bronze components with relative volume fractions determined in a regression analysis of the spectra.