Balancing Ionic and Electronic Conduction for High‐Performance Organic Electrochemical Transistors

Conjugated polymers that support mixed (electronic and ionic) conduction are in demand for applications spanning from bioelectronics to energy harvesting and storage. To design polymer mixed conductors for high‐performance electrochemical devices, relationships between the chemical structure, charge transport, and morphology must be established. A polymer series bearing the same p‐type conjugated backbone with increasing percentage of hydrophilic, ethylene glycol side chains is synthesized, and their performance in aqueous electrolyte gated organic electrochemical transistors (OECTs) is studied. By using device physics principles and electrochemical analyses, a direct relationship is found between the OECT performance and the balanced mixed conduction. While hydrophilic side chains are required to facilitate ion transport—thus enabling OECT operation—swelling of the polymer is not de facto beneficial for balancing mixed conduction. It is shown that heterogeneous water uptake disrupts the electronic conductivity of the film, leading to OECTs with lower transconductance and slower response times. The combination of in situ electrochemical and structural techniques shown here contributes to the establishment of the structure–property relations necessary to improve the performance of polymer mixed conductors and subsequently of OECTs.     

Mixed Conduction in an N-Type Organic Semiconductor in the Absence of Hydrophilic Side-Chains

Organic electrochemical transistors (OECTs) are the building blocks of biosensors, neuromorphic devices, and complementary circuits. One rule in the materials design for OECTs is the inclusion of a hydrophilic component in the chemical structure to enable ion transport in the film. Here, it is shown that the ladder-type, side-chain free polymer poly(benzimidazobenzophenanthroline) (BBL) performs significantly better in OECTs than the donor–acceptor type copolymer bearing hydrophilic ethylene glycol side chains (P-90). A combination of electrochemical techniques reveals that BBL exhibits a more efficient ion-to-electron coupling and higher OECT mobility than P-90. In situ atomic force microscopy scans evidence that BBL, which swells negligibly in electrolytes, undergoes a drastic and permanent change in morphology upon electrochemical doping. In contrast, P-90 substantially swells when immersed in electrolytes and shows moderate morphology changes induced by dopant ions. Ex situ grazing incidence wide-angle X-ray scattering suggests that the particular packing of BBL crystallites is minimally affected after doping, in contrast to P-90. BBL's ability to show exceptional mixed transport is due to the crystallites’ connectivity, which resists water uptake. This side chain-free route for the design of mixed conductors could bring the n-type OECT performance closer to the bar set by their p-type counterparts.     

Measurement of Electrophysiological Signals In Vitro Using High-Performance Organic Electrochemical Transistors

Biological environments use ions in charge transport for information transmission. The properties of mixed electronic and ionic conductivity in organic materials make them ideal candidates to transduce physiological information into electronically processable signals. A device proven to be highly successful in measuring such information is the organic electrochemical transistor (OECT). Previous electrophysiological measurements performed using OECTs show superior signal-to-noise ratios than electrodes at low frequencies. Subsequent development has significantly improved critical performance parameters such as transconductance and response time. Here, interdigitated-electrode OECTs are fabricated on flexible substrates, with one such state-of-the-art device achieving a peak transconductance of 139 mS with a 138 µs response time. The devices are implemented into an array with interconnects suitable for micro-electrocorticographic application and eight architecture variations are compared. The two best-performing arrays are subject to the full electrophysiological spectrum using prerecorded signals. With frequency filtering, kHz-scale frequencies with 10 µV-scale voltages are resolved. This is supported by a novel quantification of the noise, which compares the gate voltage input and drain current output. These results demonstrate that high-performance OECTs can resolve the full electrophysiological spectrum and suggest that superior signal-to-noise ratios could be achieved in high frequency measurements of multiunit activity.     

n-Type Glycolated Imide-Fused Polycyclic Aromatic Hydrocarbons with High Capacity for Liquid/Solid-Electrolyte-based Electrochemical Devices

Currently, n-type small-molecule mixed ionic-electronic conductors remain less explored and their molecular design rules are not mature enough. Herein, two n-type glycolated imide-fused polycyclic aromatic hydrocarbons (IPAHs), d-gdiPDI and t-gdiPDI, are developed to probe the effects of molecular conformation on the electronic, electrochemical, morphological, and coupled ionic-electronic transport properties. It is found that the highly twisted scaffold in d-gdiPDI, compared to the nearly planar one of t-gdiPDI, has a strong positive effect on the charge storage properties and thus the performance of organic electrochemical transistors (OECTs). d-gdiPDI exhibits a volumetric capacitance of 657 F cm⁻³, obviously outperforming that of t-gdiPDI (261 F cm⁻³), which is the highest value reported to date for small-molecule OECT materials. Moreover, a high charge-storage capacity of up to 479 F g⁻¹ is observed for d-gdiPDI. Arising from such high ionic-electronic coupling characteristic, d-gdiPDI-based OECTs present a ≈2 × times higher geometry-normalized transconductance (g_m,norm) of 105.3 mS cm⁻¹ relative to that of t-gdiPDI counterparts. Significantly, further application of d-gdiPDI in solid-electrolyte OECTs delivers a g_m,norm of 142.4 mS cm⁻¹. These findings indicate that IPAHs are very promising candidates for n-type small-molecule OECTs and highlight the superiority of twisting conformation manipulation in materials design toward high-performance electrochemical devices.     

Fast-switching all-printed organic electrochemical transistors

Symmetric and fast (～5 ms) on-to-off and off-to-on drain current switching characteristics have been obtained in screen printed organic electrochemical transistors (OECTs) including PEDOT:PSS (poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid)) as the active transistor channel material. Improvement of the drain current switching characteristics is made possible by including a carbon conductor layer on top of PEDOT:PSS at the drain electrode that is in direct contact with both the channel and the electrolyte of the OECT. This carbon conductor layer suppresses the effects from a reduction front that is generated in these PEDOT:PSS-based OECTs. In the off-state of these devices this reduction front slowly migrate laterally into the PEDOT:PSS drain electrode, which make off-to-on switching slow. The OECT including carbon electrodes was manufactured using only standard printing process steps and may pave the way for fully integrated organic electronic systems that operate at low voltages for applications such as logic circuits, sensors and active matrix addressed displays.     

Single-Material OECT-Based Flexible Complementary Circuits Featuring Polyaniline in Both Conducting Channels

The organic electrochemical transistor (OECT) with a conjugated polymer as the active material is the elementary unit of organic bioelectronic devices. Improved functionalities, such as low power consumption, can be achieved by building complementary circuits featuring two or more OECTs. Complementary circuits commonly combine both p- and n-type transistors to reduce power draw. While p-type OECTs are readily available, n-type OECTs are less common mainly due to poor stability of the n-type active channel material in aqueous electrolyte. Here, a complementary circuit is made using a pair of OECTs having polyaniline (PANI) as the channel material in both transistors. PANI, with a finite electrochemical window accessible at voltages lower than 1 V, exhibits a peak in current versus gate voltage when used as an active channel in an OECT. The current peak has two slopes, one n-like and one p-like, which correspond to different electrochemical regimes of the same underlying conjugated polymer. The electrochemistry enables the design of a complementary circuit using only PANI as the channel material. The PANI-based circuit is shown to have excellent performance with gain of ≈7 and is transferred on a flexible biocompatible chitosan substrate with demonstrated operation in aqueous electrolyte.     

Tuning the Transconductance of Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) operate at very low voltages, transduce ions into electronic signals, and reach extremely large transconductance values, making them ideally suited for bio-sensing applications. However, despite their promising performance, the dependence of their maximum transconductance on device geometry and applied voltages are not correctly captured by current capacitive device models. Here, current scaling laws are revised in the light of a recently developed 2D device model that adequately accounts for drift and diffusion of ions inside the polymer channel. It is shown that the maximum transconductance of the devices is found at the transition between the depletion and accumulation region of the transistors, which as well provides an explanation for the observed shift of the transconductance peak with geometric dimensions and the drain potential. Overall, the results provide a better understanding of the working mechanisms of OECTs, and facilitate design rules to optimize OECT performance further.     

Solid-State Homojunction Electrochemical Transistors and Logic Gates on Plastic

Organic electrochemical transistors (OECTs) have attracted significant attention due to their unique ionic–electronic charge coupling, which holds promise for use in a variety of bioelectronics. However, the typical electronic components of OECTs, such as the rigid metal electrodes and aqueous electrolytes, have limited their application in solid-state bioelectronics that requires design flexibility and a variety of form factors. Here, the fabrication of a solid-state homojunction OECT consisting of a pristine polymer semiconductor channel, doped polymer semiconductor electrodes, and a solid electrolyte is demonstrated. This structure combines the photo-crosslinking of all of the electronic OECT components with the selective doping of the polymer semiconductor. Three Lewis acids (gold (III) chloride (AuCl₃), iron (III) chloride (FeCl₃), and copper (II) chloride (CuCl₂) ) are utilized as dopants for the metallization of the polymer semiconductor. The AuCl₃-doped polymer semiconductor with an electrical conductivity of ≈100 S cm⁻¹ is successfully employed as the source, drain, and gate electrodes for the OECT, which exhibited a high carrier mobility of 3.4 cm² V⁻¹ s⁻¹ and excellent mechanical stability, with negligible degradation in device performance after 5000 cycles of folding at a radius of 0.1 mm. Homojunction OECTs are then successfully assembled to produce NOT, NAND, and NOR logic gates.     

Organic Electrochemical Transistor Common-Source Amplifier for Electrophysiological Measurements

The portability of physiological monitoring has necessitated the biocompatibility of components used in circuitry local to biological environments. A key component in processing circuitry is the linear amplifier. Amplifier circuit topologies utilize transistors, and recent advances in bioelectronics have focused on organic electrochemical transistors (OECTs). OECTs have shown the capability to transduce physiological signals at high signal-to-noise ratios. In this study high-performance interdigitated electrode OECTs are implemented in a common source linear amplifier topology. Under the constraints of OECT operation, stable circuit component parameters are found, and OECT geometries are varied to determine the best amplifier performance. An equation is formulated which approximates transistor behavior in the linear, nonlinear, and saturation regimes. This equation is used to simulate the amplifier response of the circuits with the best performing OECT geometries. The amplifier figures of merit, including distortion characterizations, are then calculated using physical and simulation measurements. Based on the figures of merit, prerecorded electrophysiological signals from spreading depolarizations, electrocorticography, and electromyography fasciculations are inputted into an OECT linear amplifier. Using frequency filtering, the primary features of events in the bioelectric signals are resolved and amplified, demonstrating the capability of OECT amplifiers in bioelectronics.     

Synergistic Effect of Multi-Walled Carbon Nanotubes and Ladder-Type Conjugated Polymers on the Performance of N-Type Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) have the potential to revolutionize the field of organic bioelectronics. To date, most of the reported OECTs include p-type (semi-)conducting polymers as the channel material, while n-type OECTs are yet at an early stage of development, with the best performing electron-transporting materials still suffering from low transconductance, low electron mobility, and slow response time. Here, the high electrical conductivity of multi-walled carbon nanotubes (MWCNTs) and the large volumetric capacitance of the ladder-type π-conjugated redox polymer poly(benzimidazobenzophenanthroline) (BBL) are leveraged to develop n-type OECTs with record-high performance. It is demonstrated that the use of MWCNTs enhances the electron mobility by more than one order of magnitude, yielding fast transistor transient response (down to 15 ms) and high μC* (electron mobility × volumetric capacitance) of about 1 F cm^?1 V^?1 s^?1. This enables the development of complementary inverters with a voltage gain of >16 and a large worst-case noise margin at a supply voltage of <0.6 V, while consuming less than 1 µW of power.     

Ultimately Sensitive Organic Bioelectronic Transistor Sensors by Materials and Device Structure Design

Organic bioelectronic sensors are gaining momentum as they can combine high‐performance sensing level with flexible large‐area processable materials. This opens to potentially highly powerful sensing systems for point‐of‐care health monitoring and diagnostics at low cost. Prominent to detect biochemical recognition events, are electrolyte‐gated organic field‐effect transistors (EGOFETs) and organic electrochemical transistors (OECTs) as they are easily fabricated and operated. EGOFETs are recently shown to be capable of label‐free single‐molecule detections, even in serum. This progress report aims to provide a critical perspective through a selected overview of the literature on both EGOFET and OECT biosensors. Attention is paid to correctly attribute them to the potentiometric and amperometric biosensor categories, which is important to set the right conditions for quantification purposes. Moreover, to deepen the understanding of the sensing mechanisms, with the support of unpublished data, focus is put on two among the most critical aspects, namely, the capacitance interplay and the role of Faradaic currents. The final aim is to provide a rationale of the functional mechanisms encompassing both EGOFET and OECT sensors, to improve materials and devices' designs taking advantage of the processes that enhance the sensing response enabling the extremely high‐performance level resulting in ultimate sensitivity, selectivity, and fast response.     

Tuning Channel Architecture of Interdigitated Organic Electrochemical Transistors for Recording the Action Potentials of Electrogenic Cells

Organic electrochemical transistors (OECTs) have emerged as versatile electrophysiological sensors due to their high transconductance, biocompatibility, and transparent channel material. High maximum transconductances are demonstrated facilitating the extracellular recording of signals from electrogenic cells. However, this requires large channel dimensions and thick polymer films. These large channel dimensions lead to low transistor densities. Here, interdigitated OECTs (iOECTs) are introduced, which feature high transconductances at small device areas. A superior device performance is achieved by systematically optimizing the electrode layout regarding channel length, number of electrode fingers and electrode width. Interestingly, the maximum transconductance (g_max) does not straightforwardly scale with the channel width‐to‐length ratio, which is different from planar OECTs. This deviation is caused by the dominating influence of the source–drain series resistance R_sd for short channel devices. Of note, there is a critical channel length (15 µm) above which the channel resistance R_ch becomes dominant and the device characteristics converge toward those of planar OECTs. Design rules for engineering the performance of iOECTs are proposed and tested by recording action potentials of cardiomyocyte‐like HL‐1 cells with high signal‐to‐noise ratios. These results demonstrate that interdigitated OECTs meet two requirements of bioelectronic applications, namely, high device performance and small channel dimensions.     

The Effect of Alkyl Spacers on the Mixed Ionic-Electronic Conduction Properties of N-Type Polymers

Conjugated polymers with mixed ionic and electronic transport are essential for developing the complexity and function of electrochemical devices. Current n-type materials have a narrow scope and low performance compared with their p-type counterparts, requiring new molecular design strategies. This work presents two naphthalene diimide-bithiophene (NDI-T2) copolymers functionalized with hybrid alkyl-glycol side chains, where the naphthalene diimide unit is segregated from the ethylene glycol (EG) units within the side chain by an alkyl spacer. Introduction of hydrophobic propyl and hexyl spacers is investigated as a strategy to minimize detrimental swelling close to the conjugated backbone and balance the mixed conduction properties of n-type materials in aqueous electrolytes. It is found that both polymers functionalized with alkyl spacers outperform their analogue bearing EG-only side chains in organic electrochemical transistors (OECTs). The presence of the alkyl spacers also leads to remarkable stability in OECTs, with no decrease in the ON current after 2 h of operation. Through this versatile side chain modification, this work provides a greater understanding of the structure-property relationships required for n-type OECT materials operating in aqueous media.     

All-Polymer Fiber Organic Electrochemical Transistor for Chronic Chemical Detection in the Brain

Continuous and precise monitoring of chemicals in the brain can assist in understanding the working mechanism of the brain and exploring therapeutics for nerve disorders. Organic electrochemical transistors (OECTs) are employed for this purpose due to their high sensitivity from the in situ amplification effect. However, the chronic and stable detection of chemicals in the brain is rarely reported for OECTs. It is possibly due to the chronic inflammation from mechanical mismatch between the device and soft brain tissue as well as the biofouling that hinder the diffusion of chemicals to decrease the sensitivity similar to other implanted devices. Therefore, an all-polymer fiber OECT (PF-OECT) is designed, composed solely of conductive polymers and fluorine rubber. The PF-OECT shows matching modulus with the soft brain tissue and good anti-biofouling performance. It also demonstrates both high sensitivity and electrochemical stability under dynamic deformations and in complex protein solutions. Finally, the PF-OECT is implanted into the mouse brain, achieving a stable 14-day ascorbic acid monitoring. The design strategy of PF-OECT presents a potential avenue for developing more biomedical devices.     

Polydiketopyrrolopyrroles Carrying Ethylene Glycol Substituents as Efficient Mixed Ion-Electron Conductors for Biocompatible Organic Electrochemical Transistors

A comprehensive investigation of four polydiketopyrrolopyrroles (PDPPs) with increasing ethylene glycol (EG) content and varying nature of comonomer is presented, and guidelines for the design of efficient mixed ion-electron conductors (MIECs) are deduced. The studies in NaCl electrolyte-gated organic electrochemical transistors (OECTs) reveal that a high amount of EG on the DPP moiety is essential for MIEC. The PDPP containing 52 wt% EG exhibits a high volumetric capacitance of 338 F cm⁻³ (at 0.8 V), a high hole mobility in aqueous medium (0.13 cm² V⁻¹ s⁻¹), and a μC* product of 45 F cm⁻¹ V⁻¹ s⁻¹. OECTs using this polymer retain 97% of the initial drain-current after 1200 cycles (90 min of continuous operation). In a cell growth medium, the OECT-performance is fully maintained as in the NaCl electrolyte. In vitro cytotoxicity and cell viability assays reveal the excellent cell compatibility of these novel systems, showing no toxicity after 24 h of culture. Due to the excellent OECT performance with a considerable cycling stability for 1200 cycles and an outstanding cell compatibility, these PDPPs render themselves viable for in vitro and in vivo bioelectronics.     

Supported Lipid Bilayer Assembly on PEDOT:PSS Films and Transistors

Lipid bilayers are widely employed as a model system to investigate interactions between cells and their environment. Supported lipid bilayers (SLB) with integrated transmembrane proteins are emerging as a preferred platform for sensing applications. Challenges lie in the generation of SLB on surfaces which allow transduction of signals for characterization of lipid bilayer and incorporated transmembrane proteins. For the first time, the formation of SLBs is shown on films of the conducting polymer, poly(3,4‐ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS), using traditional methods for characterizing lipid bilayer quality and function (QCM‐D, FRAP) combined with impedance spectroscopy. Further, partial formation of SLBs on PEDOT:PSS based organic electrochemical transistors (OECTs) is successfully demonstrated, as well as the ability to integrate and sense the ion pore α‐hemolysin, confirming the sensitivity of the OECT as a transducer of biological membrane function. This work represents a highly promising first step toward the use of such OECTs for functional readout of transmembrane proteins in their native environment.     

On the transient response of organic electrochemical transistors

We present a universal model for the transient drain current response in organic electrochemical transistors (OECTs). Using equivalent circuits and charge injection physics, we are able to predict the drain current in OECT devices upon application of a gate voltage input. The model is applicable to both plain and membrane-functionalized devices, and allows us to extract useful physical quantities such as resistances and capacitances, which are related to functional properties of the system. We are also able to use the model to reconstruct the magnitude and shape in time of an applied voltage source based on the observed drain current response. This was experimentally demonstrated for drain current measurements under an applied action potential.     

Manipulating the Sensitivity and Selectivity of OECT‐Based Biosensors via the Surface Engineering of Carbon Cloth Gate Electrodes

Organic electrochemical transistors (OECTs) provide the opportunity to fabricate flexible biosensors with high sensitivity. However, there are currently very few methods to improve the selectivity of OECT sensors. In this work, nitrogen/oxygen‐codoped carbon cloths (NOCCs) are prepared by the carbonization of polyaniline‐wrapped carbon cloths at 750 °C under different atmospheres. The resulting NOCC electrodes exhibit different electrochemical sensing behaviors toward ascorbic acid (AA) and dopamine (DA), enabling the fabrication of OECT sensors with high sensitivity and selectivity that are comparable to the state‐of‐the‐art OECT sensors for AA and DA. The structural characterization and theoretical calculation reveal that the electrochemical sensing behaviors of the NOCC electrodes are closely related to their surface compositions, providing an unprecedented strategy for the design of flexible OECT sensors with high sensitivity and selectivity.     

Steady‐State and Transient Behavior of Organic Electrochemical Transistors

In recent years, organic electrochemical transistors (OECTs) have emerged as attractive devices for a variety of applications, particularly in the area of sensing. While the electrical characteristics of OECTs are analogous to those of conventional organic field effect transistors, appropriate models for OECTs have not yet been developed. In particular, little is known about the transient characteristics of OECTs, which are determined by a complex interplay between ionic and electronic motion. In this paper a simple model is presented that reproduces the steady‐state and transient response of OECTs by considering these devices in terms of an ionic and an electronic circuit. A simple analytical expression is derived that can be used to fit steady‐state OECT characteristics. For the transient regime, comparison with experimental data allowed an estimation of the hole mobility in poly(3,4‐ethylenedioxythiophene) doped with poly(styrene sulfonate). This work paves the way for rational optimization of OECTs.     

Branched Oligo(ether) Side Chains: A Path to Enhanced Processability and Elevated Conductivity for Polymeric Semiconductors

Commercialization of stable conjugated polymers (CPs) with tunable electronic properties will remain a challenge without adequate solution processability due to the importance of techniques such as roll-to-roll manufacturing. Consequently, modifying CP backbones with polar side chains has recently resurged as an attractive structural design approach to improve polymer solubility and to provide CPs with the capability of transporting both electrons and ions, which is crucial for applications such as organic electrochemical transistors (OECTs). Here, a new dioxythiophene copolymer comprised of 2,​2'-bis-(3,4-ethylenedioxy)thiophene (biEDOT) and 3,4-propylenedioxythiophene (ProDOT) substituted with branched oligo(ether) side chains (PE₂-biOE2OE3) is synthesized using two direct hereto(arylation) polymerization (DHAP) techniques. The typical DHAP technique results in a lower molecular weight polymer (PE₂-biOE2OE3(L)), which is soluble in acetone and demonstrated a solid-state conductivity after oxidative doping of 55 ± 3 S cm⁻¹. Alternatively, a unique temperature ramp DHAP methodology results in a higher molecular weight polymer (PE₂-biOE2OE3(H)) with an especially high solid-state conductivity of 430 ± 60 S cm⁻¹. Notably, the first OECT fabricated from an acetone-processed polymer is reported, which is stable up to 500 cycles and can provide a pathway for future material design aimed at eliminating the use of toxic chlorinated solvents in OECT active layer processing.