Cryogenic transmission electron microscopy reveals assembly and nanostructure of PEDOT:PSS

Soft and conducting organic materials are ideal candidates for stretchable bioelectronics and wearable devices. Despite recent advances, our understanding of conducting polymer nanostructures and how they arise remains incomplete, given the limited high-resolution studies and molecular-level descriptions of these systems. Here, we employ cryogenic transmission electron microscopy (cryo-EM) to investigate the evolution of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) morphology in solution and the resulting solid state structure in the presence of ionic and molecular additives. Our results reveal the formation of heterostructural elongated fibers consisting of PEDOT:PSS micelles in solution. Cryo-EM further reveals that additives increase the number of fibrils, in addition to inducing the formation of crystalline domains. We observe that fibril and crystalline phases in solutions act as a template for the growth of these nanostructures in the solid state. Furthermore, exploiting cryo-EM reveals the role of solid-liquid interactions in PEDOT:PSS through the imaging of PEDOT:PSS nanostructures after the hydration of thin films. Hydration leads to the swelling of heterostructural fibers while reducing the crystalline domain size. Such behavior explains the mechanical robustness of PEDOT:PSS thin films processed with various additives as well as the high electrical conductivity of PEDOT:PSS in applications such as organic electorchemical transistors.