Mixed Ionic-Electronic Transport for PEDOT:PSS-Based Zero-Gated Organic Electrochemical Transistors Using Impedance Spectroscopy and Micro-Raman Imaging

Recent studies are being focused on establishing a link between structural and mixed ionic-electronic transport properties based on the characterization of organic mixed ionic-electronic conductors (OMIECs) under operation conditions in organic electrochemical transistor (OECT) devices. Here, we report the simultaneous access to microstructural, chemical, and physical information using impedance spectroscopy and micro-Raman imaging techniques to address the mixed ionic-electronic transport mechanisms in PEDOT:PSS-based OMIECs working as OECT channel materials. We focused on the out-of-plane and in-plane mixed ionic-electronic transport mechanisms and complement our experimental studies with computational simulations using molecular dynamics (MD) and density functional theory (DFT) methodologies. Our studies revealed that the out-of-plane transport is based on ionic injection and diffusion mainly associated with the formation of nanopore channels in PEDOT:PSS exhibiting sulfonic groups toward the aqueous media. Thus, we also evidenced that the dedoping is not only due to the direct interaction of positive ions with PEDOT:PSS bulk region but also indirectly by the rearrangements of sulfonic groups from PSS toward the aqueous media. The in-plane transport in the OECT channel was properly described as a mixed ionic-electronic transport, with not only ionic transport of sodium (Na⁺) but also hydronium (H₃O⁺) species mainly through nanopores and PEDOT:PSS bulk regions and with electronic transport mainly through the PEDOT:PSS bulk region. Although it is shown that the electronic transport monotonically declines with increasing ionic concentration, the Na⁺ diluted regime (1 mM) exhibits weak selectivity compared to H₃O⁺ species, and the Na⁺ concentrated regime (100 mM) induces extreme porosity in PEDOT:PSS yielding to the destruction of the device. These results introduce some particular features for PEDOT:PSS-based OECT channel materials that need to be considered not only for ionic concentration and selectivity but also other related stability issues.