Doping of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) films studied by means of electrochemical variable angle spectroscopic ellipsometry

Dielectric function plays central role in understanding the opto-electronic properties of thin conducting polymer films, and is pertinent to the charge transfer mechanisms in organic electronic devices. In the present study, we demonstrate the applicability of variable angle spectroscopic ellipsometry (VASE) in combination with an anisotropic Tauc-Lorentz-Drude model, for ex- and in-situ determination of the complex dielectric function of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) thin film, spin-coated from the aqueous suspension containing dimethyl sulfoxide as co-solvent and 3-glycidoxypropyltrimethoxysilane as cross-linker. For in-situ VASE measurements, the Kretschmann cell configuration is introduced, which enables extension of the experimentally accessible spectral range into the near infrared region, thus making distinguishable free charge carrier from valence band absorptions. We applied this technique for studying the interplay between those two contributions when the equilibrium doping state of the film, deposited at the working electrode in a three-electrode cell, is varied by applying a positive cell potential (“electrochemical doping”). The latter method is designated as electrochemical VASE. As results, we report substantial dichroism in the complex dielectric function with similar magnitudes for both ex- and in-situ films, as well as trends of the DC conductivity and the main optical transition versus the applied cell potential. These trends are pertinent to the two electrochemical regimes of doping, which can be identified with the field effect and redox doping operation mechanisms of organic thin film transistors.