Ellipsometry Measurements as Direct Evidence of the Drude Model for Polycrystalline Electrochromic  WO ₃ Films

The optical constants of polycrystalline electrochromic (EC) tungsten trioxide films, prepared by rf sputtering, were measured by ellipsometry at three wavelengths: 940, 633, and 546 nm. For each wavelength, the optical constants were measured at various states of coloration. It is shown that these measurements provide direct evidence for the validity of the free‐electron Drude model (i.e., the reflectivity modulation that is observed in polycrystalline EC films of is primarily determined by the density and scattering of the free electrons).