Organic electrochemical transistors (OECTs) have recently been investigated intensively due to their switching and sensing functionality in an electrolytic environment which is compatible with biological systems. The working principle of OECTs differs significantly from the classical electrostatically gated field-effect transistors (FET) due the volumetric capacity control of charge carrier concentration in the channel by ionic motion. The mixed electron-ion transport of the devices has consequence for many parameters of the devices including dynamics and hysteresis. In this talk, our recent work on a better understanding of OECT operation is reviewed. We discuss the properties of a wide variety of devices including solid state electric devices and vacuum-processed devices. It turns out that entropic terms play a major role in the operation of the devices due to the entirely different statistical properties of the ions in the channel. The results show that the often observed hysteresis of OECT can be separated in an intrinsic bistable behavior and a conventional hysteresis behavior related to ion dynamics and other effects. The bistability of the devices offers interesting device applications.

Doping of organic semiconductor films enhances their conductivity for applications in organic electronics, thermoelectrics and bioelectronics. However, much remains to be learnt about the properties of the conductive charges in order to optimize the design of the materials. Electrochemical doping is important for organic electrochemical transistors (OECTs) used in biomedical sensors. Benefits of doping via electrochemistry include controllable doping levels, reversibility and high achievable carrier densities. We introduce a new technique, applying in-situ terahertz (THz) spectroscopy directly to electrochemically doped polymers in combination with time-resolved spectro-electrochemistry, chronoamperometry and OECT device measurements. We evaluate the intrinsic short-range transport properties of the polymers (without the effects of long-range disorder, grain boundaries and contacts), while precisely tuning the doping level via the applied oxidation voltage. Moreover, temperature-dependent measurements allow to extract the thermodynamic and activation parameters of the electrochemical processes. Results will be shown for a variety of polymers based on polythiophene backbones with different sidechains, aligned polymer chains and novel 2D organic films.

Organic mixed ionic-electronic conductors (OMIECs) have recently risen as a promising material choice for bioelectronic devices due to their low impedance, soft mechanical properties, and ability to transduce ionic signals to electronic currents. These properties have enabled the development of high-performance devices for electrophysiological recordings, chemical sensing, cell monitoring, and neuromorphic devices. The unique behaviour of OMIECs arises from “electrochemical doping” where ion intercalation through the bulk of the material modifies the oxidation state, and therefore charge carrier concentration, of the conjugated polymer. However, the current understanding of electrochemical gating remains limited. To address this knowledge gap, we use a newly developed hyperspectral differential transmission microscopy technique to probe electrochemical gating in operando.

First, I will present a comprehensive drift-diffusion model to describe mixed ionic-electronic transport in OMIECs using the data collected from in operando microscopy which captures the complex behaviours of electrochemical doping. The proposed model captures several features observed for electrochemical (de)doping of OMIECs including diffusion-like rather than drift-like ion transport and the voltage-dependence of ion kinetics. The results suggest that diffusion of electronic carriers (holes) down electrochemical potential gradients plays a significant role in operation of OMIEC-based devices. In the second part of the presentation, I will show that electrochemical doping consists of two distinct kinetic regimes. Generally, electrochemical doping is assumed to be limited by motion of ions due to their large mass compared to electrons/holes. However, in a state-of-the-art polythiophene, electrochemical doping speeds can be limited by poor hole transport at low doping levels, leading to substantially slower switching speeds than expected. We show that the timescale of hole-limited doping can be controlled by the degree of microstructural heterogeneity, enabling the design of OMIECs with improved electrochemical performance. The framework for understanding the driving forces and kinetics of mixed ionic-electronic transport provides guidance for device design as well as optimization of new materials for improved performance in bioelectronic devices.

Organic semiconductors with polar side groups are capable of mixed electronic and ionic conduction, making them candidates for electrodes in electrochemical devices. In the case of conjugated polymers, electrochemical and redox properties can be tuned through choice of polymer backbone and side chain [1, 2].  Conjugated polymer electrodes can show excellent charging and discharging rates, high coulometric efficiency, efficient charge transport and reversible charging in simple aqueous electrolytes, making them interesting for different applications including energy storage. However, the specific capacity and operational stability need to be improved [3]. We use molecular modelling together with electrochemical measurements to investigate the relationship between polymer chemical structure and the redox properties, charging behaviour and cycling stability of the polymer electrodes. We use molecular dynamics simulations to explore how the side chain structure influences polymer packing, film swelling and stability [4]; how structure influences polymer-ion interactions; and how these interactions influence electronic transport properties. Finally, we use simulations to explore how chemical structure influences specific capacity. The findings can help to develop chemical designs for improved conjugated polymer electrodes for aqueous environments.

The charge-transport properties of conjugated polymers have been studied intensively for both electronic and optoelectronic device applications. Some polymer semiconductors not only support the ambipolar transport of electrons and holes, but do so with comparable carrier mobilities. This opens the possibility of gaining deeper insight into the charge-transport physics of these complex materials via comparison between electron and hole dynamics while keeping other factors, such as polymer microstructure or vibrational fluctuations, equal. Field-induced electron spin resonance spectroscopy has been recently used to compare the spin relaxation behavior of electron and hole polarons in three ambipolar conjugated polymers, showing three spin relaxation regimes as a function of temperature. We interpret these findings on the basis of calculated electronic structure calculations and state-of-the-art numerical propagation of the hole and electron carrier wavefunctions coupled to nuclear motion in these polymers.

In the second part of the talk, we will report on recent simulations of ion intake and diffusion in crystalline microstructures of conjugated polymers equipped with glycolated side chains for organic electrochemical transistor applications. All-atom simulations support a crystal phase transition upon doping that occurs via a zipping mechanism. We will also describe preliminary phenomenological calculations aiming at describing the changes in electrical conductivity with increasing carrier concentration and that take into account the electrostatic interactions with the ions. 

Engineering conjugated polymers with glycol-based side chains has been the main strategy used to improve the performance of mixed ionic-electronic conductors (OMIECs), a promising material class for interfacing biological systems with electronics. Organic electrochemical transistors (OECTs) are a solution for this task and are often used to benchmark OMIECs performance. Different studies have showed that polymers designed with hydrophilic solubilizing chains show better OECT performance, mostly due to their higher ionic uptake and stability in aqueous environments. [1]

In this work, we explore how side chain engineering in poly(3-hexylthiophene) (P3HT) and in indacenodithiophene-co-benzothiadiazole (IDTBT) impact their ionic transport properties. We used time-resolved spectroelectrochemistry to measure the kinetics of doping in these materials with high time resolution. We observe that for thicker films (around 100-200 nm), the films with more glycol content are doped faster, confirming that the ion transport is better in more polar polymers. However, for thinner films (around 20nm), we observe that the kinetics becomes independent of glycol content. We hypothesize that the kinetics in thin films are less dependent on the ion transport and closer to the electrochemical limit. Therefore, the polar content does not affect the intrinsic electrochemical reactions rate and the doping favorability, which remain controlled by the polymer backbone.

Organic electrochemical transistors (OECTs) are a rapidly advancing technology that plays a crucial role in the development of next-generation bioelectronic devices. Recent advances in p-type/n-type organic mixed ionic-electronic conductors (OMIECs) have enabled power-efficient complementary OECT technologies for various applications, such as chemical/biological sensing, large-scale logic gates, and neuromorphic computing. However, ensuring both high performance and long-term operational stability remains a significant challenge that hinders their widespread adoption. Ladder-type conjugated polymers, with their rigid backbone structure composed of double-strand chains linked by condensed π-conjugated units, can sustain high electrochemical doping levels without any conformational disorder, which leads to exceptional operational stability, high charge carrier mobility, and large volumetric capacitance. In this report, we demonstrate our recent development on ladder-type conjugated polymers, with p-type/n-type high mixed ionic-electronic conducting performance and high stability in long-term OECT operation. Reasonable molecular design and improvement of synthesis methods have enabled the synthesis of high-performance p-type/n-type ladder polymers with mobility increased by more than 10 times. Their unique ladder-type structure enables them to have long-term stability within >90% current remain after 6 hours continues operation, and enables them to accommodate more carrier injections with of up to two charges per repeat unit. Density of states filling and opening of a hard Coulomb gap around the Fermi energy at high electrochemical doping levels enable the ion-tunable antiambipolarity in these ladder polymers. The development of ladder-based polymer ink formulations has also enabled printed electronics. With these ladder-type polymers, we developed complementary inverters with a record-high DC gain of 194 V/V and excellent stability. We report the first organic electrochemical neurons (OECNs) with ion-modulated spiking, based on all-printed complementary organic electrochemical transistors. We demonstrate facile bio-integration of OECNs with Venus Flytrap (Dionaea muscipula) to induce lobe closure upon input stimuli. We report a biorealistic conductance-based organic electrochemical neuron (c-OECN) using a mixed ion–electron conducting ladder-type polymer with stable ion-tunable antiambipolarity. The latter is used to emulate the activation/inactivation of sodium channels and delayed activation of potassium channels of biological neurons. These c-OECNs can spike at bioplausible frequencies nearing 100 Hz, emulate most critical biological neural features, demonstrate stochastic spiking and enable neurotransmitter-/amino acid-/ion-based spiking modulation, which is then used to stimulate biological nerves in vivo.

Conductive polymers combine high ionic and electronic conductivities with soft mechanical properties, and achieve the conversion of electrochemical processes in liquid environment to mechanical formalization.^[1] The resulting material electroactivity has been exploited in electrochemical actuators with low-voltage drive and nanoscale precision, leading to the development of bioelectronics soft actuators.^[2] On the other hand, the electrically-induced swelling of conductive polymer layers can be detrimental for applications such as thin film neuromodulation devices, where shear stresses with the substrate can lead to delamination and device failure.^[3] During volume change, electroactive polymers translate an external stimulus to a change of the physical properties on a nanometer scale, but the intrinsic mechanism of actuation is not completely understood. In this work, we address this gap of knowledge introducing quantitative measurements of electroswelling using atomic force microscopy (AFM). We exploit AFM as a local probe for volume changes and interface forces that provides transient data on dynamical effects related to electroactuation. We combine electroswelling measurements in the frequency domain with electrochemical impedance spectroscopy on a model conductive polymer, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) to correlate morphological changes with information on ionic uptake and local electrochemical potential. Through the development of a mathematical theory, we describe electroswelling in terms of transport and accumulation of hydrated ions, and we achieve multimodal AFM experiments mapping electroswelling amplitude and phase on soft polymer thin films. Our findings highlight the physicochemical mechanisms limiting actuation width and timescales and can be crucial to develop functional materials with enhanced electroactive properties.

The polyelectrolyte complex of poly(3,4-ethylene dioxythiophene) with poly(styrene sulfonate), PEDOT:PSS, is ubiquitous in the field of organic electronics. Yet, its complex structure in water and in the solid state make it challenging to understand structure-property relationships for optimization or functionalization. Our group focuses on developing a better understanding of PEDOT:PSS, particularly for using it as a mixed ionic-electronic conductor for organic electrochemical transistors (OECTs). In this talk, I will share two recent projects. The first project will describe how to increase the OECT performance of common formulations of PEDOT:PSS without sacrificing stability in water by using a surface functionalization method instead of blending with a crosslinker. For a simple formulation of PEDOT:PSS with ethylene glycol and a dodecylbenzene sulfonic acid (DBSA) surfactant, we found that surface functionalization with (3-glycidyloxypropyl)trimethoxysilane (GOPS) led to a [μC*] value of 376 ± 74 F cm^‒1 V^‒1 s^‒1 instead of 104 ± 20 F cm^‒1 V^‒1 s⁻¹ when blended with GOPS (the most commonly used approach to maintain water stability). In the second part, I will describe the use of polyethylene oxide (PEO) as either a polymeric additive to commercial formulations or as a block copolymer with PSS to increase the volumetric capacitance of PEDOT:PSS, up to 200 F cm⁻³ (after washing the films in water) and 120 F cm⁻³ respectively. When using a relatively low molecular weight PEO, 400 Da, the PEDOT:PSS:PEO blend led to a [μC*] value up to  632 F cm^‒1 V^‒1 s^‒1 after washing with water, close to that obtained for crystalline PEDOT but without having to use sulfuric acid. For both projects, I will describe our approach for the preparation of different formulations, how the additives or block copolymers affect the chemical composition, and detailed microstructure characterization to tease out structure-property relationships in PEDOT:PSS-based mixed conductors.

Organic mixed ionic/electronic conductors (OMIECs) have gained considerable interest in bioelectronics, power electronics, circuits and neuromorphic computing. These organic, often polymer-based, semiconductors rely on a combination of ionic transport, electronic transport, and high volumetric charge storage capacity. Despite recent progress and a rapidly expanding library of new materials, a full understanding of fundamental processes of OMIECs remains largely unexplored. Critically, useful studies require us to probe these systems in device-relevant conditions, fully considering the effects of ions and solvent on microstructure and transport. To this end, we report on recent efforts towards structure/composition-property relations in high performance organic mixed conductors using ex-situ, in-situ, and operando scattering and spectroscopic techniques. We use a combination of GIWAXS, GISAXS, XPCS, E-QCMd, UV-Vis, XRF, as well as device measurements to explore kinetics, composition, and structural evolution during electrochemical operation. Importantly, these findings provide insights into materials design for enhancing device performance and stability.

Conjugated polymers (CPs) with hydrophilic side chains are capable of simultaneously conducting electronic and ionic charges, making them interesting materials for applications in bioelectronics, electrochromics, electrocatalysis and energy storage. Whilst numerous p-type mixed-conducting polymers have shown excellent stability and performance, n-type mixed-conducting polymers usually suffer from poor stability in aqueous environments and in air. To date, most reported solution processable n-type mixed conducting polymers that show good stability, high electron mobility and high specific capacity, involve the naphthalenediimide (NDI) unit. [1-3] However, the electrochemical stability of NDI-based mixed-conducting polymers varies with the specific chemical structure and choice of electrolyte. The underlying mechanisms by which ionic charging/discharging deteriorates the electrochemical stability are not properly understood. In this work, we study several NDI-based n-type mixed conducting polymers with different side chain and backbone structures operating in aqueous electrolytes of systematically varying concentrations to control the electrochemical stabilities. We observe that increasing electrolyte concentration and replacing a small fraction of hydrophilic side chains with hydrophobic ones largely improve the electrochemical stability of the NDI-based n-type mixed conducting polymers. To understand the phenomena, we use spectroelectrochemical and electrogravimetric characterisation tools and molecular dynamic (MD) simulations to explore the impact of side chains and electrolytes on the electrochemical stabilities of the polymers. We show that the improvement in stability can be tentatively attributed to a reduction in swelling, which avoids excessive electrolyte uptake, thereby improving the environment for ion transport and maintaining the intermolecular connections. We propose design rules for both material and electrolyte to maximise reversible multi-electron charging of n-type mixed conducting polymers.

The development of organic electrochemical transistors (OECTs) capable of maintaining their high amplification, fast transient speed, and operational stability in harsh environments will advance the growth of next‐generation wearable electronics, robotics and biological electronics. In this study, a high-performance solid-state OECT (SSOECT) is successfully demonstrated, showing a recorded high transconductance of 220 ± 59 S cm−1, ultrafast device speed of ≈10 kHz with excellent operational stability over 10 000 switching cycles, and thermally stable under a wide temperature range from −50 to 110 °C. The developed SSOECTs are successfully used to detect low-amplitude physiological signals, showing a high signal-to-noise-ratio of 32.5 ± 2.1 dB. For the first time, the amplifying power of these SSOECTs is also retained and reliably shown to collect high-quality electrophysiological signals even under harsh temperatures. The demonstration of high-performing SSOECTs and its application in harsh environment are core steps toward their implementation in next-generation wearable electronics and bioelectronics. We also will present our recent work on developing fibrous electrolyte containing polymer matrix and ionic liquid, which is highly robust, breathable, waterproof, and conformal with human skin is reported. Serving as fibrous substrate and electrolyte of OECTs, a high transconductance of ≈0.8 mS, stability over pulsing and time (≈1000 cycles and 30 days) are achieved. The softness of fibrous OECTs enables a comfortable contact after attaching to human skin, which can reduce the interfacial impedance to achieve a high-quality local amplification of the electrocardiography signals (signal-to-noise ratio of 21.7 dB) even in skin squeezed state or after one week. These results indicated that our fibrous OECTs have huge potential for versatile on-skin electronics such as non-invasive medical monitoring, soft sensors, and textile electronics.

The presentation will give an overview over recent studies in my group about design and processing of polymeric mixed-ionic-electronic conductors for opto-electronic and switchable technologies including actuators, electrochromic displays and electrically switchable metasurfaces.

In the first part, electrochemistry will be shown as versatile tool to control the doping level of semiconducting polymer films. Data on tunable absorption properties, electrical conductivities and wetting behavior will be presented for p-type materials.[1] Both, in-situ as well as ex-situ electrochemical techniques will be presented and correlations to the morphology of the films will be drawn. For energy level determination within fully organic solar cells particularly in-situ spectroelectrochemistry of blend films proved to be highly useful.[2]

In the second part, poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) will be presented as multifunctional material. Electrochemical switching in organic and in aqueous electrolytes results in different degrees of volume swelling as characterized by in-situ spectroscopic ellipsometry.[3] The humidity dependence of the PSS polyelectrolyte phase further allows to prepare volume changes by water uptake. The bending behavior of “intelligent” humidity-triggered bilayer actuators with poly(dimethylsiloxane) (PDMS) passive layers can be explained by the humidity-dependent mechanical behavior of PEDOT:PSS.[4]

Chemical and electrochemical doping are widely used to modulate the electrical properties of conjugated polymers. Doping of thin films facilitates fundamental spectroscopic studies and, ultimately, is important for the operation of devices from transistors to solar cells. However, some applications such as thermoelectrics and wearable electronics require bulk materials, which complicates doping processes and mass transport, limiting the performance of devices. This talk will explore some of the fundamentals of doping of both thin films and bulk materials. The impact of doping on the electrical as well as mechanical properties will be discussed, and it is shown how doping can be used to tune both the conductivity and stiffness of conjugated polymers. Finally, strategies are explored that permit to decouple the effect of doping on the electrical and mechanical properties, which can be used to prepare a variety conducting bulk materials, from stiff composites and foams to hydrogels and stretchable fibers.

Electrolyte-Gated Transistors (EGTs) have emerged as an integral part of numerous applications in biosensing and bioelectronics, owing to their remarkable ability to efficiently transduce biological events into amplified electronic signals while stably operating in aqueous electrolytes. EGT are three-terminal devices consisting of a semiconducting channel between source and drain electrodes capacitively coupled with the gate electrode through ions in the electrolyte. Understanding these devices at the nanoscale is paramount in order to leverage their respective, or combined, functionality for various applications. An optimized level of crystallinity or a balance between ionic and electronic conduction within the semiconductor might be desired, which directly relates to the physical and chemical nature of the semiconducting material and its response to applied electric fields. However, probing the nanoscale properties under operating conditions has been challenging due to the complications arising from the electrolyte environment. In this communication, I will review the progress made in our research group towards developing an advanced scanning probe microscopy technique able to probe different functional properties of the semiconductor materials (morphological, electrical and mechanical) at the nanoscale in operating electrolyte-gated transistors (EGTs). The technique is based on in-Liquid Scanning Dielectric Microscopy (in-Liquid SDM) to which we added automated functionalities and multiparametric characterization capabilities for comprehensive and simultaneous probing of the nanoscale electrical, mechanical and morphological properties in operating EGTs. Examples of applications to Electrolyte Gated Organic Field Effect Transistors (EGOFETs) [1] and Organic Electrochemical Transistors (OECTs) will be presented.

We explore the concept of constructing microscopic electronic neurons that mimic natural brain systems in order to create brain-inspired computational artificial systems. We discuss the essential material and device properties required for these spiking neurons, which can be characterized using impedance spectroscopy and small perturbation equivalent circuit elements. We identify the structural conditions necessary for smooth oscillations, which depend on the dynamics of a conducting system with internal state variables. We emphasize the significance of detecting the Hopf bifurcation, a critical point for achieving spiking behavior, through spectral features of the impedance. We focus on the mixed ionic-electronic conductors in which the slow ionic motion introduces a internal state variable that controls the electronic conductivity. Our findings reveal that the minimal neuron system requires a capacitor, a chemical inductor, and a negative resistance, which can be integrated into the physical response of the device rather than being built from separate circuit elements. Thus, we propose a method to quantify the physical and material properties of devices in order to produce the dynamic properties of neurons that are necessary for a specific computational scheme. We present a novel approach towards building brain-inspired artificial systems and provides insights into the important material and device considerations for achieving spiking behavior in electronic neurons [1].

In recent years, there has been continuous progress in the development of n-type organic thermoelectrics (OTE) due to the synthesis of new conjugated polymers and dopants. The primary focus of most designs has been to enhance doping efficiency and increase electrical conductivity, resulting in values surpassing 102 S cm^-1. Although effective, doping encounters limitations because of the inherent inverse relationship between the properties that determine the figures of merit in thermoelectric materials. To advance the field further, alternative approaches are needed beyond optimizing doping efficiency and the host:dopant ratio. In this study, we propose the concept of self-induced anisotropy as a solution to overcome the inverse coupling between electrical conductivity and Seebeck coefficient in n-type OTEs. Previous research has suggested methods like rubbing and drawing to induce anisotropy in p-type OTEs based on thiophenes. In our work, we exploit the interactions between solvent, host, and dopant to induce a preferential orientation. This orientation increases the in-plane delocalization length, thereby improving electrical conductivity whitout hindering the Seebeck coefficient. By adopting this approach, we have achieved promising results in the 2DPP-2CNTVT:N-DMBI system. At room temperature, we obtained a Power Factor of 115 μW m^-1 K^-2 and a figure of merit of 0.17, which further increased to 188 μW m^-1 K^-2 and 0.36, respectively, at 105 ºC. To explain the origin of self-induced anisotropy, we utilized Hansen Solubility Parameters and established guidelines for formulating solvent-host-dopant systems that can achieve a similar effect. This discovery opens new possibilities for advancing n-type OTEs.

Conjugated porous polymers (CPPs) are solely organic members of porous materials family. They are very promising compounds for photocatalytic and electronic applications thanks to their the electron rich and porous backbone offering high accessible surface areas and advanced photophysical properties.[1,2] Moreover, their high dimensionality make them particularly interesting for organic electronic devices due to the possibility of orientation independent electron/hole mobility through their 2D/3D skeleton.[3,4] Additionally, the open voids along the porous backbone are perfectly separated phases to introduce guest molecules to form charge-transfer complexes (i.e. doping), which leads improved electrical conductivities.[4]

We have previously reported formation of charge-transfer complexes of electron rich CPPs after iodine treatment.[5–7] In a particular example, we showed that a thiophene based CPP became more conductive after iodine exposure when compared to its linear analogue whereas the latter outperformed the former before the doping.[7]

In this communication, chemical doping an electron rich CPP by using different techniques (i.e. mixing dopant+CPP in solution or vacuum impregnation) will be presented. The spectral (e.g. UV, IR, NMR, EDX, XPS) and physical characterizations (thermal elemental analysis, TGA, and porosity measurements) of the pristine and doped forms indicated the successful incorporation of the guest molecules in the pores. Furthermore, effect of the doping on the electrical impedance and conductivity will also be discussed.