What Can We Learn about PEDOT:PSS Morphology from Molecular Dynamics Simulations of Ionic Diffusion?

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is one of the most important mixed electron-ion conducting polymers, where the efficiency of the ion transport is crucial for many of its applications. Despite the impressive experimental progress in the determination of ionic mobilities in PEDOT:PSS, the fundamentals of ion transport in this material remain poorly understood, and the theoretical insight into the ion diffusion on the microscopical level is completely missing. In the present paper, a Martini 3 coarse-grained molecular dynamics (MD) model for PEDOT:PSS is developed and applied to calculate the ion diffusion coefficients and ion distribution in the film. We find that the ion diffusion coefficients for Na+ ions are practically the same in the PEDOT-rich and PSS-rich regions and do not show sensitivity to the oxidation level. We compare the calculated diffusion coefficients with available experimental results. Based on this comparison and based on the MD morphology simulation of PEDOT:PSS revealing the formation of pores inside the film, we revise a commonly accepted granular morphological model of PEDOT:PSS. Namely, we argue that PEDOT:PSS films, in addition to PEDOT-rich and PSS-rich regions, must contain a network of pores where the ion diffusion takes place.