Spectroscopic Ellipsometry of Conducting Anisotropic Pedot thin Films

We report a comprehensive spectroscopic investigation of poly[3,4-ethylenedioxythiophene] (PEDOT) thin films, including both those synthesized from solution in the doped state and from commercial PEDOT:PSS (poly(styrene sulfonate)) formulations. Variable-angle spectroscopic ellipsometry is combined with transmittance and reflectance measurements spanning the UV to mid-infrared spectral range. Data from the different techniques are consistently analyzed within a unified framework based on the complex dielectric function of each film. Uniaxial optical modeling assuming an edge-on molecular orientation reveals a markedly anisotropic response, with the ordinary (in-plane) component exhibiting significantly higher conductivity than the out-of-plane dielectric component. Notably, the synthesized PEDOT films show enhanced in-plane optical conductivity, in agreement with sheet resistance measurements. The molecular structure and anisotropy of PEDOT are further corroborated by grazing-incidence wide-angle X-ray scattering (GIWAXS), which reveals an intra-lamellar structure characterized by π-π stacking of PEDOT chains at a distance of approximately 3.9 Å and an interlamellar periodicity of about 15.7 Å. Overall, these results represent a significant step toward a deeper understanding of the electronic and optical properties of this distinctive class of macromolecules and their potential use in photonics and optical metamaterials.