Electrochemical and Solvent-Driven Swelling in a Conducting Polymer Film

Poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is likely the most widely used conducting polymer and a champion material for emerging electrochemical technologies. However, the fundamental polymer science describing this material in an electrochemical context remains an active area of investigation. We employ in situ UV–vis absorption and in situ spectroscopic ellipsometry to measure changes to a cross-linked PEDOT:PSS film under an applied electrochemical potential. Using multiple component regression data analyses, we find that two polaronic charge carriers and two neutral forms of PEDOT are necessary to adequately describe the data. Systematic studies of swelling of PEDOT:PSS films in a range of non-aqueous solvents demonstrate that a traditional Flory–Huggins model is suitable to describe film thickness changes. Electrochemical oxidation and reduction also cause dramatic thickness changes to PEDOT:PSS films. In-operando spectroscopic ellipsometry shows that film thickness increases upon reduction and decreases during oxidation, indicating that cation transport is of primary importance in the electrochemistry of PEDOT:PSS. The changes in film thickness in response to electrochemically driven swelling can also be approached using a Flory–Huggins approach when a voltage dependence is included in the enthalpy of mixing.