Advanced Optical Characterization of PEDOT:PSS by Combining Spectroscopic Ellipsometry and Raman Scattering

Abstract

The optical properties of various PEDOT:PSS films obtained by drop casting and blade coating are analyzed by variable-angle spectroscopic ellipsometry in the visible-UV spectral range. We discuss observed differences in the optical spectra due to PSS content and DMSO treatment and correlate them to structural changes extracted from Raman measurements. In particular, we investigate the optical anisotropy of the complex refractive indices which arises from the in-plane arrangement of the PEDOT backbones, giving rise to optically uniaxial behavior with the optic axis perpendicular to the film plane. Although this is widely accepted, most investigations disregard the anisotropy for simplicity, which sometimes leads to inaccurate conclusions. In this work, we compare the results of isotropic and anisotropic analyses to clarify which kind of errors we can expect if anisotropy is not considered. Finally, the correlation between Raman scattering and ellipsometric analyses shows that not only local structural changes of the chain conformation but also the overall morphology of the composite films are significant in the interpretation of Raman spectra.

Figure 1

(a) Complex refractive index components for different films of Sigma-Aldrich 483095 PEDOT:PSS. Data extracted from ref (3) are plotted for comparison. (b) Experimental spectra and fits using both isotropic and anisotropic models for the thinner (d = 930 nm) dropcast film.

Figure 2

Raman spectrum measured in a dropcast Sigma-Aldrich 483095 PEDOT:PSS film. The fitted components are shown and the obtained Raman shifts are compared to calculated frequencies for PEDOT, given in parentheses according to values tabulated in ref (18).

Figure 3

Graphical representation of the main modes of PEDOT according to calculated values tabulated in ref (18). Main bond contributions are represented here by thicker lines, and angles are also proportionally drawn to their variations.

Figure 4

Complex refractive index components for different films of Heraeus Clevios AI4083 PEDOT:PSS. One of the blade coated films, the 87 nm thick one, is almost isotropic, like that from ref (10) plotted for comparison. Results obtained here using both isotropic and anisotropic models for both blade-coated films are shown.

Figure 5

This figure highlights the importance of morphology in the average conjugation length. Dedoping can lead either to red or blue shift of the main Raman band depending on the actual phase separation in the films.