(618d) Heterogeneous Three-Dimensional Morphology of Mixed Ionic-Electronic Conducting Polymers Revealed By Electron Tomography

Organic mixed ionic-electronic conducting (OMIEC) polymers can be considered as the intersection of conjugated polymers, and polymer electrolytes/polyelectrolytes. Among them, poly(3,4‑ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) is one of the most widely studied and utilized OMIECs, with applications spanning electrochemical transistors to stretchable electronics. Electron microscopy has shown significant promise in characterizing polymeric materials at the nanometer scale. While high-resolution electron microscopy has proven successful in mapping in-plane phase distributions, conventional techniques like TEM and X-ray scattering inherently reduce 3D structures to 2D projections, thereby obscuring critical out-of-plane information. This lack of three-dimensional understanding is particularly limiting given that out-of-plane morphology plays a crucial role in governing water, ion, and charge transport in key applications such as organic electrochemical transistors and organic photovoltaics. A central, unanswered question is whether in-plane structural insights can be extrapolated to out-of-plane organization, or whether OMIECs require entirely new morphological models.

To address this gap, we turn to electron tomography, a transformative technique that enables nanoscale 3D reconstruction of polymer morphology and functional domains. Using electron tomography, we captured the 3D morphology of PEDOT:PSS thin films for the first time, to our knowledge. We further extended our study to various PEDOT:PSS formulations, including PEDOT:PSS with crosslinked structures and incorporation of molecular and ionic secondary dopants. Our analysis revealed a vertically heterogeneous structure in neat, crosslinked, and doped PEDOT:PSS films. Specifically, three distinct layers were observed: fibrillar PEDOT:PSS domains at the top and bottom surfaces sandwiching a middle layer associated with amorphous PEDOT:PSS. Out-of-plane conductivity measurements supported this structural model. Notably, the removal of the intermediate amorphous layer resulted in a more than tenfold increase in out-of-plane conductivity. We further utilized electron tomography to assess the porosity in these films. Formulations processed with ionic dopants exhibited porosity levels of 2–3%, whereas cross-linked PEDOT:PSS showed dramatically reduced porosity, below 0.3%. Such significant differences are expected to directly impact water and salt transport properties, key factors for performance in OECTs and related devices. Our work lays the foundation for advancing OMIEC performance across a range of technologies, including electrochemical transistor channels, thermal detectors, and flexible electronic electrodes.