Publication date: 21st July 2025
Organic electrochemical transistors (OECTs) rely on the use of organic mixed ionic-electronic conductors (OMIECs) as active channel materials [1]. These materials must simultaneously support electronic and ionic transport throughout the bulk of the film, enabling dynamic modulation of their redox states and conductivity via interactions with electrolyte ions and solvent molecules. Conjugated polymers have emerged as promising OMIEC platforms: their π-conjugated backbones facilitate electronic conduction, while their bulk structure allows for ion penetration and transport. In this context, poly(benzimidazobenzophenanthroline) (BBL), a ladder-type conjugated polymer with a rigid, planar backbone and high density of redox-active sites, provides a compelling model system [2]. Despite extensive experimental investigations, the atomistic-level understanding of ion and solvent interactions within BBL remains limited.
A combined classical molecular dynamics and quantum chemical (DFT) approach was employed to investigate the interaction mechanisms between BBL and two electrolytes (NaCl and NH₄Cl) in order to rationalize extensive experimental studies (including operando GIWAXS and 2H-NMR, E-QCMD, IR, CV and THz conductivity measurements) performed on various BBL:cation systems.
Our DFT calculations reveal distinct interaction modes between NH₄⁺ and BBL, ranging from hydrogen bonding to proton transfer, depending on the redox state and specific binding site within the polymer. To further mimic the OECT working environment, we performed molecular dynamics simulations of BBL crystallites immersed in electrolyte solutions at different doping levels. These simulations allowed us to quantify the swelling of the crystallites upon doping and to characterize the nature and spatial distribution of intercalated species. Notably, we observed differences in the behavior of cations based on their ability to form hydrogen bonds.
Altogether, this multi-scale approach sheds light on the fundamental ion-polymer interactions in BBL systems, and more broadly contributes to the molecular-level understanding of OMIEC operation, offering guidelines for the design of future materials with optimized mixed conduction properties.
The computational resources in Mons are supported by the FNRS “Consortium des Equipements de Calcul Intensif-CECI” program Grant No. 2.5020.11. This work was also financially supported by the European Commission through the European Commission through the FET-OPEN project MITICS (964677), the Knut and Alice Wallenberg Foundation (2021.0058 and the Wallenberg Initiative Materials Science for Sustainability WISE), the Swedish Research Council (2020-03243, 2022-04053, 2022-04553), the European Research Council through the ERC Consolidator Grant project INFER (101125879), and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU 2009-00971). T.v.d.P. is grateful for the financial support of the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 101148701 (project IONIC). Z.T. is FNRS research fellow. D.B. is FNRS Research Director.
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