Quantitative composition and mesoscale ion distribution in p-type organic mixed ionic-electronic conductors

Understanding the ionic composition and distribution in organic mixed ionic-electronic conductors (OMIECs) is crucial for understanding their structure-property relationships. However, direct measurement of OMIEC ionic composition and distribution is not common. In this work, we investigate the ionic composition and mesoscopic structure of three typical p-type OMIEC materials: an ethylene glycol treated crosslinked OMIEC with large excess fixed anionic charge (EG/GOPS-PEDOT:PSS), an acid treated OMIEC with tunable fixed anionic charge (crys-PEDOT:PSS), and a single component OMIEC absent any fixed anionic charge (pg2T-TT). A combination of X-ray fluorescence (XRF) and photoelectron spectroscopies (XPS), gravimetry, coulometry, and grazing incidence small angle X-ray scattering (GISAXS) techniques were employed to characterize these OMIECs following electrolyte exposure and electrochemical cycling. In particular, XRF provided quantitative ion-to-monomer compositions for these OMIECs from passive ion uptake following aqueous electrolyte exposure, and potential driven ion uptake/expulsion following electrochemical doping and dedoping. Single ion (cation) transport in EG/GOPS-PEDOT:PSS due to Donnan exclusion was directly confirmed, while despite significant fixed anion concentration in crys-PEDOT:PSS doping and dedoping was shown to occur through mixed anion and cation transport. Controlling the fixed anionic (PSS-) charge density in crys-PEDOT:PSS mapped the strength of Donnan exclusion in OMIEC systems following a Donnan-Gibbs model. Anion transport dominated pg2T-TT doping and dedoping, but a surprising degree of anionic charge trapping (~1020 cm-3) was observed. GISAXS revealed minimal ion segregation both between PEDOT- and PSS-rich domains in EG/GOPS-PEDOT:PSS, and between amorphous and semicrystalline domains in pg2T-TT, but showed significant ion segregation in crys-PEDOT:PSS at length scales of tens of nm, ascribed to inter-nanofibril void space. These results bring new clarity to the ionic composition and distribution of OMIECs that are crucial for accurately connecting structure and properties in these materials.