Electrochemical Impedance Spectroscopy (EIS) is a noninvasive technique widely used for understanding charge transfer and charge transport processes in electrochemical systems and devices. Standard approaches for the interpretation of EIS data involve starting with a hypothetical circuit model for the physical processes in the device based on experience/intuition and then fitting the EIS data to this circuit model. This work explores a mathematical approach for extracting key characteristic features from EIS data by relying on fundamental principles of complex analysis. These characteristic features can suggest the presence of inductors and constant phase elements (nonideal capacitors) from impedance data and enable us to answer questions about the identifiability and nonuniqueness of equivalent circuit models. In certain scenarios such as models with only resistors and capacitors, we are able to enumerate all possible families of circuit models. Finally, we apply the mathematical framework presented here to real-world electrochemical systems and highlight results using impedance measurements from a lithium-ion battery coin cell.

Despite the technological appeal of polymeric organic mixed ionic/electronic conductors (OMIECs) for diverse applications, a deep understanding of the fundamentals of mixed charge transport in these materials, especially regarding the complex interplay between polymer, ion and solvent structure in determining transport, is lacking. Herein, extensive molecular dynamics (MD) simulations of a model OMIEC representing various electrochemically gated states are reported that reveal charge state-dependent counterion condensation. X-ray diffraction simulations based on the MD data predict a measurable change in the scattering intensity at the counterion absorption edge, indicative of counterion repositioning with charging. We leverage an operando resonant X-ray scattering technique to experimentally corroborate the simulated scattering and report excellent agreement between predicted and experimental data, confirming that counterions preferentially reside in the lamellar mid-plane of crystallites at low doping, and near the polymer backbone at higher doping. Driving forces for ion type-dependent spatial repositioning and implications thereof are discussed.

ABSTRACT Organic mixed ionic‐electronic conductors (OMIECs) are key components for the next‐generation bioelectronics, albeit with n ‐type materials remaining significantly underrepresented compared to their p ‐type counterparts. Furthermore, reaching low operating voltages alongside high ambient and operational stability remains challenging when employing n ‐type OMIECs in organic electrochemical transistor (OECT)‐based applications. Herein, we report two novel n ‐type polymeric OMIECs, based on an azaisatin and benzodifurandione‐based backbone, incorporating either non‐fluorinated or fluorinated bithiophene donor units. The resulting polymers, obtained via Aldol polymerization, have high electron affinities of 4.5–4.7 eV. Both polymers function in n ‐type accumulation mode OECTs with indications of tunable antiambipolar behavior. The fluorinated polymer displays a low threshold voltage of 0.14 V, while the non‐fluorinated analogue offers excellent operational stability, with the OECT retaining 96% of its initial current after 1800 s of cycling.

Polymeric organic mixed ionic-electronic conductors have shown remarkable potential as ideal semiconductor materials for organic electrochemical transistors (OECTs), which are emerging as a key technology in next-generation bioelectronics. While numerous conjugated donor-acceptor (D-A) materials based on thiophene, naphthalenediimide, and diketopyrrolopyrrole have been developed, isoindigo counterparts remain comparatively less explored. Herein, we report three D-A polymers based on azaisoindigo (AIG) as the acceptor unit, each containing different donor moieties: bithiophene (2T), difluorinated bithiophene (ff2T), and ethylenedioxy-substituted bithiophene (bisEDOT). By systematically varying the donor strength along the polymer backbone, we switch the dominant charge carrier from p-type with AIG-bisEDOT containing the most electron-rich donor moiety to n-type with AIG-ff2T incorporating the least electron-rich donor when tested in an OECT device configuration. The underlying structure-property relationships dictating this behaviour are elucidated by means of a joint experimental and computational approach.

The mechanical mismatch between semiconductors and biological tissues can be a challenge for the development of conformal bioelectronics. Organic mixed ionic-electronic conductors (OMIECs) such as conjugated polymers with oligoether side chains are promising materials due to their low stiffness, which may minimize adverse immune reactions and thus promote biocompatibility. However, significant volume changes during electrochemical cycling-driven by ion and water ingression and expulsion-can lead to drastic changes in stiffness, complicating device-tissue mechanical matching across redox states. Here, the electromechanical response of a thienothiophene-based conjugated polymer with triethylene glycol side chains is investigated. Electrochemical nanoindentation and atomic force microscopy reveal a modest and reversible increase in elastic modulus at room temperature from approximate to 70 to more than 120 MPa upon electrochemical oxidation. This unusual mechanical stability is attributed to a reversible increase in pi-stacking that compensates for swelling-induced softening. These findings demonstrate that it is feasible to design OMIEC materials with stable mechanical properties across redox states, opening new possibilities for compliant and tissue-matched bioelectronic interfaces that remain mechanically invariant during operation.

Organic electrochemical transistors (OECTs) are electrolyte-gated devices that permit large signal amplification, ionic-to-electronic signal transduction, and controlled variable resistance. OECTs offer increased detection sensitivity compared to other electrolyte-gated devices because their electroactive channel materials are mixed ionic-electronic conductors (MIEC) that enable volumetric faradaic interactions with the adjacent electrolyte. As a device class, OECTs are still in an early stage of development, but have demonstrated utility in applications such as neuromorphic computing platforms and bioelectronic devices. However, the vast majority of OECT studies have employed spin-coated polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS) as the active material. While PEDOT:PSS exhibits many desirable qualities for an OECT channel material ( e.g. , biocompatibility, high transconductance, relatively low mechanical modulus, and responsivity to aqueous ions and bioanalytes), the commercially available formulation is deposited via spin-coating, operates in depletion mode, and has pre-determined physical characteristics ( e.g. , molecular weight, counterion type, dopant-to-monomer ratio) that preclude investigation of varied material configurations. In addition, the material has general susceptibility to changes in ionic strength of the solution it is measuring, which limits its utility for ion-specific detection and systems for which changes in ionic strength accompany the events/changes that the OECT seeks to detect or influence ( i.e. neurotransmitter detection). As the magnitude of signal amplification in OECTs is directly proportional to the channel transconductance, strategies are needed to increase control over material properties that influence both electronic and ionic conductivity. Electrodeposition, in conjunction with concurrent gravimetric and spectroelectrochemical analyses, provides a convenient method to evaluate the origin and development of electronic/ionic transport properties during materials synthesis. Although electropolymerization of MIEC polymers like polypyrrole, polythiophene, and polyaniline is well-studied, only a few isolated reports 1,2 have evaluated the benefits of electropolymerization for OECT channel fabrication and performance ( i.e. , compared to spin-coating or vapor-phase polymerization). In this report, we characterize the effect of electrosynthesis parameters on channel material characteristics and device performance ( i.e. , total channel capacity, transconductance, ionic/electronic conductivity, and overall electrochemical impedance). We examine the influence of two electrodeposition methods (cyclic voltammetry versus differential pulse voltammetry) on the total degree of polymerization, and the dopant used during electrosynthesis (comparing small, labile dopants such as chloride or tosylate, versus larger semi-labile dopants such as dodecyl benzene sulfonate, versus large immobile polymer dopants such as PSS). Our results suggest that electropolymerization provides several advantages for OECT channel fabrication: (1) permit more tailored fabrication of channels with diverse characteristics and dopants; (2) increase control over material properties that influence both electronic and ionic conductivity; (3) produce channels that operate in either accumulation or depletion mode, depending on the counterion used during synthesis. References S. Wustoni, T.C. Hidalgo, A. Hama, D. Ohayon, A. Savva, N. Wei, N. Wehbe, S. Inal. In Situ Electrochemical Synthesis of a Conducting Polymer Composite for Multimetabolite Sensing Adv. Mater. Technol . 2020 , 5 , 1900943. L. Zhang, T.L. Andrew. Deposition Dependent Ion Transport in Doped Conjugated Polymer Films: Insights for Creating High-Performance Electrochemical Devices. Adv. Mater. Interfaces 2017 , 4 , 1700873.

Organic electrochemical transistors (OECTs) can be used to create biosensors, wearable devices and neuromorphic systems. However, restrictions in the micro- and nanopatterning of organic semiconductors, as well as topological irregularities, often limit their use in monolithically integrated circuits. Here we show that the micropatterning of organic semiconductors by electron-beam exposure can be used to create high-density (up to around 7.2 million OECTs per cm2) and mechanically flexible vertical OECT arrays and circuits. The energetic electrons convert the semiconductor exposed area to an electronic insulator while retaining ionic conductivity and topological continuity with the redox-active unexposed areas essential for monolithic integration. The resulting p- and n-type vertical OECT active-matrix arrays exhibit transconductances of 0.08-1.7 S, transient times of less than 100 μs and stable switching properties of more than 100,000 cycles. We also fabricate vertically stacked complementary logic circuits, including NOT, NAND and NOR gates.

Heteroatom substitution is a powerful tool to tune the intra- and intermolecular structure of conjugated polymers as well as their resulting optoelectronic and electrochemical properties. A series of oligoethylene glycol bithiophene chalcogenophene polymers (p(g3T2-X)) with systematically varied furan, thiophene, selenophene, and tellurophene comonomers have been synthesized for mixed ionic-electronic conducting applications. Their microstructures have been thoroughly characterized ex situ and in situ with X-ray scattering, and their mixed conducting properties have been probed in electrochemical transistor testbeds. Chalcogenophene heteroatom choice was found to clearly dictate the polymer microstructure (crystallite dimensionality and orientation) and tune mixed conducting properties. Proceeding down Group 16, from O to Se systematically directed the molecular ordering of 2D polymer crystallites from face-on (O) to mixed (S) to edge-on (Se) orientations, with Te driving the polymer to form well-oriented edge-on 3D crystallites. Heteroatom dictated crystallite quality, and orientation tuned relative ionic transport by 2 orders of magnitude. Hole mobility (mu(hole)) and mixed conducting figure of merit (mu C*) were each tuned over an order of magnitude depending on heteroatom choice, with the Te-containing polymer reaching mu(hole) = 3.60 cm(2) V-1 s(-1) and mu C* = 483 F cm(-1) V-1 s(-1), due to improved molecular ordering. Insights from this polymer series highlight target microstructures for enhanced mixed conduction in future conjugated polymers.

Organic semiconductors are increasingly being decorated with hydrophilic solubilising chains to create materials that can function as mixed ionic-electronic conductors, which are promising candidates for interfacing biological systems with organic electronics. While numerous organic semiconductors, including p- and n-type materials, small molecules and polymers, have been successfully tailored to encompass mixed conduction properties, common to all these systems is that they have been semicrystalline materials. Here, we explore how side chain engineering in the nano-crystalline indacenodithiophene-co-benzothiadiazole (IDTBT) polymer can be used to instil ionic transport properties and how this in turn influences the electronic transport properties. This allows us to ultimately assess the mixed ionic-electronic transport properties of these new IDTBT polymers using the organic electrochemical transistor as the testing platform. Using a complementary experimental and computational approach, we find that polar IDTBT derivatives can be infiltrated by water and solvated ions, they can be electrochemically doped efficiently in aqueous electrolyte with fast doping kinetics, and upon aqueous swelling there is no deterioration of the close interchain contacts that are vital for efficient charge transport in the IDTBT system. Despite these promising attributes, mixed ionic-electronic charge transport properties are surprisingly poor in all the polar IDTBT derivatives. Albeit a "negative" result, this finding clearly contradicts established side chain engineering rules for mixed ionic-electronic conductors, which motivated our continued investigation of this system. We eventually find this anomalous behaviour to be caused by increasing energetic disorder in the polymers with increasing polar side chain content. We have investigated computationally how the polar side chain motifs contribute to this detrimental energetic inhomogeneity and ultimately use the learnings to propose new molecular design criteria for side chains that can facilitate ion transport without impeding electronic transport.

Understanding the structural and dynamic properties of disordered systems at the mesoscale is crucial. This is particularly important in organic mixed ionic–electronic conductors (OMIECs), which undergo significant and complex structural changes when operated in an electrolyte. In this study, we investigate the mesoscale strain, reversibility and dynamics of a model OMIEC material under external electrochemical potential using operando X-ray photon correlation spectroscopy. Our results reveal that strain and structural hysteresis depend on the sample’s cycling history, establishing a comprehensive kinetic sequence bridging the macroscopic and microscopic behaviours of OMIECs. Furthermore, we uncover the equilibrium and non-equilibrium dynamics of charge carriers and material-doping states, highlighting the unexpected coupling between charge carrier dynamics and mesoscale order. These findings advance our understanding of the structure–dynamics–function relationships in OMIECs, opening pathways for designing and engineering materials with improved performance and functionality in non-equilibrium states during device operation.

A key challenge in the development of organic mixed ionic-electronic conducting materials (OMIEC) for high performance electrochemical transistors is their stable performance in ambient. When operating in aqueous electrolyte, potential reactions of the electrochemically injected electrons with air and water could hinder their persistence, leading to a reduction in charge transport. Here, the impact of deepening the LUMO energy level of a series of electron-transporting semiconducting polymers is evaluated, and subsequently rendering the most common oxidation processes of electron polarons thermodynamically unfavorable, on organic electrochemical transistors (OECTs) performance. Employing time resolved spectroelectrochemistry with three analogous polymers having varying electron affinities (EA), it is found that an EA below the thermodynamic threshold for oxidation of its electron polarons by oxygen significantly improves electron transport and lifetime in air. A polymer with a sufficiently large EA and subsequent thermodynamically unfavorable oxidation of electron polarons is reported, which is used as the semiconducting layer in an OECT, in its neutral and N-DMBI doped form, resulting in an excellent and air-stable OECT performance. These results show a general design methodology to avoid detrimental parasitic reactions under ambient conditions, and the benefits that arise in electrical performance.

Developing material design guidelines for organic mixed ionic–electronic conductors (OMIECs) is critical to enable high efficacy mixed transport within bioelectronics. One important feature which has yet to be thoroughly explored is the role of molecular weight on OMIEC performance. In this work, we examined a series of prototypical glycolated polythiophene materials (P3MEEET) with systematically increasing molecular weights within organic electrochemical transistors (OECTs) – a common testbed for investigating mixed transport. We find that there is improved performance beyond an intermediate molecular weight, however, this relationship is electrolyte dependent. Operando analysis suggests that the enhanced mobility at higher molecular weights may be negated by significant swelling when operated in NaCl due to disruption of intercrystallite charge percolation. The role of molecular weight is revealed through operation in KTFSI, as doping occurs through cation expulsion, preventing detrimental swelling and maintaining percolative pathways. These findings demonstrate the importance of both molecular weight and electrolyte composition to enhance the performance of OMIECs.

By tracking the electrochromic doping front, a hole-limited electrochemical doping mechanism is discovered in organic mixed ionic–electronic conductors.

Organic electrochemical transistors (OECTs) have emerged as a powerful platform for bioelectronic communication, enabling various technologies including neuromorphic devices, stimulation elements, and biosensors. These devices leverage the ionic-electronic coupling of organic semiconductors, known as organic mixed ionic-electronic conductors (OMIECs), to transduce signals across biotic and abiotic interfaces or mimic biological functions. The efficiency and behavior of this ionic-electronic communication are material- and electrolyte-dependent; therefore, the utility of OECTs depends on our control over OMIECs within a particular environment. Here we critically review material design considerations for the next generation of mixed conductors for OECT applications. Recent advances and strategies toward high-performance p- and n-type OMIECs are summarized. Important topics, such as batch-to-batch variability, assessing stability, processing methodologies, and alternative material platforms, are also covered—areas rarely discussed within the OMIEC community. Challenges and opportunities related to these topics are discussed, offering a practical guide to designing the next generation of OMIECs for bioelectronic applications.

Operational stability is essential for the success of organic electrochemical transistors (OECTs) in bioelectronics. The oxygen reduction reaction (ORR) is a common electrochemical side reaction that can compromise the stability of OECTs, but the relationship between ORR and materials degradation is poorly understood. In this study, the impact of ORR on the stability and degradation mechanisms of thiophene‐based OECTs is investigated. The findings show that an increase in pH during ORR leads to the degradation of the polymer backbone. By using a protective polymer glue layer between the semiconductor channel and the aqueous electrolyte, ORR is effectively suppressed and the stability of the OECTs is significantly improved, resulting in current retention of nearly 90% for ≈2 h cycling in the saturation regime.

The introduction of oligoether side chains onto a polymer backbone can help to stabilise polymeric dispersions in water without the necessity of surfactants or additives when conjugated polymer nanoparticles are prepared. A series of poly(3-hexylthiophene) (P3HT) derivatives with different content of a polar thiophene derivative 3-((2-methoxyethoxy)methyl)thiophene was interrogated to find the effect of the polar chains on the stability of the formed nanoparticles, as well as their structural, optical, electrochemical, and electrical properties. Findings indicated that incorporation of 10-20 percent of the polar side chain led to particles that are stable over a period of 42 days, with constant particle size and polydispersity, however the particles from the polymer with 30 percent polar side chain showed aggregation effects. The polymer dispersions showed a stronger solid-like behaviour in water with decreasing polar side chain content, while thin film deposition from water was found to afford globular morphologies and crystallites with more isotropic orientation compared to conventional solution-processed films. As a proof-of-principle, field-effect transistors were fabricated directly from the aqueous dispersions demonstrating that polymers with hydrophilic moieties can be processed in water without the requirement of surfactants.

This perspective offers insights from discussions conducted during the Telluride Science meeting on organic mixed ionic and electronic conductors, outlining the challenges associated with understanding the behavior of this intriguing materials class.

A series of fully fused n-type mixed conduction lactam polymers p(g7NCnN), systematically increasing the alkyl side chain content, are synthesized via an inexpensive, nontoxic, precious-metal-free aldol polycondensation. Employing these polymers as channel materials in organic electrochemical transistors (OECTs) affords state-of-the-art n-type performance with p(g7NC10N) recording an OECT electron mobility of 1.20 × 10-2 cm2 V-1 s-1 and a μC* figure of merit of 1.83 F cm-1 V-1 s-1. In parallel to high OECT performance, upon solution doping with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), the highest thermoelectric performance is observed for p(g7NC4N), with a maximum electrical conductivity of 7.67 S cm-1 and a power factor of 10.4 μW m-1 K-2. These results are among the highest reported for n-type polymers. Importantly, while this series of fused polylactam organic mixed ionic-electronic conductors (OMIECs) highlights that synthetic molecular design strategies to bolster OECT performance can be translated to also achieve high organic thermoelectric (OTE) performance, a nuanced synthetic approach must be used to optimize performance. Herein, we outline the performance metrics and provide new insights into the molecular design guidelines for the next generation of high-performance n-type materials for mixed conduction applications, presenting for the first time the results of a single polymer series within both OECT and OTE applications.

Achieving high stability is critical for the implementation of organic electrochemical transistors (OECTs) in more diverse and demanding applications. However, the sources and mechanisms of OECT degradation have not been rigorously explored. Here, we employ a variety of biasing schemes to separate the relative effects of oxidative bias stress, reductive bias stress, and current stress on degradation of thiophene-based, p-type OECTs. We find that accelerated degradation arises from the compounding effects of simultaneous oxidative and reductive bias stress and is common across several thiophene-based channel materials. To understand the underlying mechanism of OECT channel degradation, we explore the individual contributions of dissolved oxygen and source-drain electrode materials. We determine that the reaction of dissolved oxygen at the buried Au/OMIEC interface of the drain electrode experiencing reductive potentials produces a mobile reactive species that aggressively degrades the oxidized OMIEC throughout the device, destroying its conjugation and disrupting electronic charge transport. Importantly, we find that this mechanism can be disrupted by alternatively removing oxygen, avoiding reductive potentials in the device biasing scheme, replacing Au electrodes with a noncatalytic alternative, or passivating Au electrodes with self-assembled monolayers. These conclusions can inform both future standards of stability testing in the field as well as design considerations of OECT implementation in long-term applications.