Variable Angle Spectroscopic Ellipsometry Study of Poly(3,4-ethylenedioxythiophene):Polystyrene Sulfonate Thin Films in Contact with Air

Thin films of conducting polymer complex poly(3,4-ethylenedioxythiophene) : polystyrene sulfonate (PEDOT:PSS) are studied by means of varied angle spectroscopic ellipsometry (VASE) in the wavelength range 370-880 nm, at five different angles of incidence (AOI). The goal is to simultaneously determine the film thicknesses and its dielectric function. Samples of varied film thickness are prepared by spin coating the aqueous suspension onto Si/SiO2 wafers at different spin rates, and subsequent annealing at 170 degrees C. The measured spectra at constant angles of incidence are fitted by applying Fresnel's equations for a stratified optical layers model (SLM) which postulates several optical layers with well defined optical interfaces, in combination with the dielectric function (epsilon) over tilde(lambda). Different possibilities for data fitting are critically assessed, pointing to some pitfalls in the VASE data analysis and interpretation. Two methods which involve the assessment of a theoretical Lorentz-Drude (LD) dielectric function were compared: one in which a unique function is assumed for all of the prepared samples (i.e. film thicknesses), while in the other, a variation of the dielectric function was permitted. The validation criteria include comparisons with the experimental film thickness measured by means of the atomic force microscope, the quasi-experimental dielectric function fitted independently for each wavelength, and the literature value of the main optical transition energy calculated by quantum mechanical modelling. In the limit of the studied system and model, all of these comparisons point to the necessity of simultaneously fitting the VASE data measured for films of varied thickness, in which case both the thickness and the dielectric function may be determined.