Conductive polymer PEDOT:Tos (3,4-ethylenedioxythiophene doped with molecular tosylate) gained considerable attention in various devices for bioelectronic applications, such as organic transistors and sensors. Many of these devices function upon oxidation/reduction processes in contact with aqueous electrolytes. So far, theoretical insight into morphological changes, ion injection, and water intake during these processes was rather limited. In the present work, we combined experiments and molecular dynamics simulations to study the water intake, swelling, and exchange of ions in the PEDOT:Tos film during cyclic voltammetry. We showed that the film underwent significant changes in morphology and mass during the redox processes. We observed both experimentally and in simulations that the film lost its mass during reduction, as tosylate and Na were expelled and gained mass during oxidation mainly due to the uptake of anions, i.e., tosylate and Cl. The results were in line with the UV–VIS–NIR absorption measurements and X-ray photoelectron spectroscopy (XPS) measurements, which revealed that during the redox process a portion of Tos was replaced by Cl– as the counterion for PEDOT. Also, the relative mass change between the most oxidized and reduced states was ∼10 to 14% according to both experiments and simulations. We detected an overall material loss of the film during voltammetry cycles indicating that a portion of the material leaving the film during reduction did not return to the film during the consecutive oxidation. Our combined experimental/simulation study unraveled the underlying molecular processes in the PEDOT:Tos film upon the redox process, providing the essential understanding needed to improve and assess the performance of bioelectronic devices.

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