Optical Constants of Electrochromic WO3 Determined by Variable Angle Spectroscopic Ellipsometry

Abstract

Variable angle spectroscopic ellipsometry (VASE) was used to measure the optical constants and thickness of electrochromic WO3 films with various intercalation levels. The films were deposited by RF magnetron sputtering to thickness between 800nm and 2000nm. The substrates were quartz with electrode layers of optically thin Al or ITO. Li+ ions were inserted into the films by applying a negative voltage to the substrate electrode (Al or ITO) relative to a counter electrode, a gold wire, in a Li+ containing electrolyte. This electrolyte was a 0.5 molar solution of LiClo4 in propylene carbonate. Additionally, in-situ experiments were done in a electrochemical chamber mounted on a VASE. Ellipsometry and transmission data were taken at several angles of incidence with several different amounts of Li+ inserted and the optical constants and thickness were solved for at each level of intercalation. Additional characterization included AFM, interference microscopy, and x-ray diffraction. Figure 1 and 2 attached show the ellipsometric and Ψ and Δ before and after intercalation of the WO3 with Li+. Figure 3 shows the transmissional data. By combining these data simultaneously in the regression fit, we are able to unambiguously determine the index of refraction and extinction coefficient during all stage of “coloring”.