Optical Indices of Electrochromic Tungsten Oxide

Tungsten trioxide (WO₃) is the most widely used material for the active layer of electrochromic devices. Knowledge of the complex refractive index over the range of coloration states is required for device design. Optical constants of WO₃ over the whole solar spectrum were determined as a function of injected charge. Films of WO₃ were prepared by electron-beam evaporation, then colored in several steps by reduction with lithium (Li) up to 68 mC cm^-2 µm^-1 injected charge. Measurements included variable-angle spectroscopic ellipsometry and spectroscopic transmittance and reflectance at normal incidence. Analysis was complicated by the fact that a transparent-conducting layer of indium tin oxide (ITO) was required to perform lithiation. Optical indices of the glass substrate and ITO transparent conductor were determined separately and then fixed in the model. The indices of WO₃ could then be extracted from measurements on the complete structure. A parametric dispersion model corresponding to Gaussian broadening of the oscillators was used to represent the dielectric response of WO₃.