Counterion Condensation Near Conjugated Polymer Backbones at High Doping Potentials

Organic mixed ionic/electronic conductors (ΟΜΙΕCs) are a versatile class of materials that have shown promise for a wide range of applications, from neuromorphic computing to bioelectronic sensors and interfaces.¹ Application of an external bias enables charge coupling between ions and electrons (or holes), leading to large transconductances at gating voltages <1 Volt. The development of a thorough understanding of mixed conduction in OMIECs is complicated by nano- and mesoscale strains, the mobile nature of compensating charges (counterions), and the semi-crystallinity of these polymers.²

Herein, we employ in operando grazing-incidence resonant X-ray diffraction (GIRXRD) in a standard KCl electrolyte to elucidate spatial locations of counterions within crystalline domains of a model OMIEC as a function of charge state. By scanning the incident energy of an X-ray beam across the counterion absorption edge, lamellar diffraction features are uniquely modified to reveal the counterion position with respect to the backbone.³ In contrast with ex-situ experiments, where counterions always reside near the lamellar half-plane, we find that counterions condense near conjugated polymer backbones at high doping potentials, implying strong coupling between holes and counterions.

We also utilize molecular dynamics (MD) simulations to gain deeper insights into counterion movement during doping. MD data suggest that at low doping counterions are evenly distributed in the crystal, with slight preference for ordering near the half-plane (~1 nm away from the electronic charge carriers). However, at high charge states (>0.83 electronic charges per monomer) counterions localize near the backbone (~0.4 nm away from the electronic charge carriers). Diffraction simulations on the MD crystals agree with experimental data, confirming our interpretation of counterion condensation at high doping potentials. It has been previously shown that OMIECs exhibit a drop in crystalline conductivity at high doping;^4,5 we hypothesize that in addition to hole-hole repulsion, strong coupling between holes and counterions leads to the measured drop in hole mobility.

1. Tropp, J., Meli, D. & Rivnay, J. Organic mixed conductors for electrochemical transistors. Matter S2590238523002199 (2023) doi:10.1016/j.matt.2023.05.001.
2. Wu, R., Matta, M., Paulsen, B. D. & Rivnay, J. Operando Characterization of Organic Mixed Ionic/Electronic Conducting Materials. Chem. Rev. 122, 4493–4551 (2022).
3. Flagg, L. Q. et al. Resonant X-ray Diffraction Reveals the Location of Counterions in Doped Organic Mixed Ionic Conductors. Chem. Mater. 35, 3960–3967 (2023).
4. Cavassin, P. et al. Electrochemical Doping in Ordered and Disordered Domains of Organic Mixed Ionic-Electronic Conductors. Adv. Mater. 35, 2300308 (2023).
5. Cho, K. G., Adrahtas, D. Z., Lee, K. H. & Frisbie, C. D. Sub-Band Filling and Hole Transport in Polythiophene-Based Electrolyte-Gated Transistors: Effect of Side-Chain Length and Density. Adv Funct Mater (2023).