Quantitative Composition and Mesoscale Ion Distribution in p-Type Organic Mixed Ionic-Electronic Conductors

Understanding the ionic composition and distributionin organicmixed ionic-electronic conductors (OMIECs) is crucial for understandingtheir structure-property relationships. Despite this, directmeasurements of OMIEC ionic composition and distribution are not common.In this work, we investigated the ionic composition and mesoscopicstructure of three typical p-type OMIEC materials: an ethylene glycol-treatedcrosslinked OMIEC with a large excess fixed anionic charge (EG/GOPS-PEDOT:PSS),an acid-treated OMIEC with a tunable fixed anionic charge (crys-PEDOT:PSS),and a single-component OMIEC without any fixed anionic charge (pg2T-TT).A combination of X-ray fluorescence (XRF) and X-ray photoelectronspectroscopies, gravimetry, coulometry, and grazing incidence small-angleX-ray scattering (GISAXS) techniques was employed to characterizethese OMIECs following electrolyte exposure and electrochemical cycling.In particular, XRF provided quantitative ion-to-monomer compositionsfor these OMIECs from passive ion uptake following aqueous electrolyteexposure and potential-driven ion uptake/expulsion following electrochemicaldoping and dedoping. Single-ion (cation) transport in EG/GOPS-PEDOT:PSSdue to Donnan exclusion was directly confirmed, while significantfixed anion concentrations in crys-PEDOT:PSS doping and dedoping wereshown to occur through mixed anion and cation transport. Controllingthe fixed anionic (PSS-) charge density in crys-PEDOT:PSSmapped the strength of Donnan exclusion in OMIEC systems followinga Donnan-Gibbs model. Anion transport dominated pg2T-TT dopingand dedoping, but a surprising degree of anionic charge trapping (& SIM;10(20) cm(-3)) was observed. GISAXS revealed minimalion segregation both between PEDOT- and PSS-rich domains in EG/GOPS-PEDOT:PSSand between amorphous and semicrystalline domains in pg2T-TT but showedsignificant ion segregation in crys-PEDOT:PSS at length scales oftens of nm, ascribed to inter-nanofibril void space. These resultsbring new clarity to the ionic composition and distribution of OMIECswhich are crucial for accurately connecting the structure and propertiesof these materials.

Automated summary

This study investigated the ionic composition and mesoscopic structure of three typical p-type organic mixed ionic-electronic conductors (OMIECs). The results revealed different ion compositions and distributions in different materials, which is crucial for accurately connecting the structure and properties of these materials.