Complementary Logic Circuits Based on High-Performance n-Type Organic Electrochemical Transistors

Organic Mixed Conductors for Neural Biomimicry and Biointerfacing

Organic mixed ionic-electronic conductors (OMIECs) could revolutionize bioelectronics by enabling seamless integration with biological systems. This review explores their role in neural biomimicry and biointerfacing, with a focus on how backbone design, sidechain optimization, and antiambipolarity impact performance. Recent advances highlight OMIECs' biocompatibility and mechanical compliance, making them ideal for bioelectronic applications. However, challenges such as mechanical mismatch and electrical impedance remain. We discuss innovative solutions to these issues to enhance OMIEC functionality. In neuromorphic bioelectronics, OMIECs show promise for bridging artificial and biological neural systems, though further improvements in conductivity and resolution are needed. Continued innovation in materials and design is crucial to unlocking their full potential, driving advancements in both technology and medicine.

A Novel Solubilizing Fully π-Conjugated Ladder Polymer and Its Thermoelectric Properties

Fully conjugated ladder polymers (CLPs) have emerged as promising candidates for next-generation organic electronics due to their extended π-conjugation pathways, intrinsic charge transport anisotropy, and exceptional chemical and thermal stability. However, insolubility remains a critical bottleneck, primarily stemming from side reactions that disrupt the linear structure. Herein, we report a soluble CLP, benzo[a]benzo[5,6][1,4]thiazino[3,2-c]phenothiazine ladder (NBBTL), via a stepwise strategy to preserve the linear structure of the polymer. The resulting polymer exhibits unprecedented solution processability while maintaining structural integrity, enabling the first reported thermoelectric generator fabrication. Thermoelectric performance evaluation revealed that FeCl3-doped NBBTL achieved a conductivity of 3.18 ± 0.33 S cm-1 and a power factor of 0.94 ± 0.09 μW m-1 K-2. Further analysis of the polymer chain orientation in films by grazing-incidence wide-angle X-ray scattering revealed that NBBTL exhibits a bimodal orientation with both face-on and edge-on. This study demonstrates that strategic molecular engineering can overcome long-standing insolubility issues in CLPs, potentially paving the way for their wider application in organic electronics and thermoelectric energy conversion.

Regiochemistry and Side-Chain Engineering Enable Efficient N-Type Mixed Conducting Polymers

Developing high-performance n-type organic mixed ionic-electronic conducting (OMIEC) polymers with simple structural motifs is still challenging. We show that high-performance, low-threshold-voltage n-type OMIEC polymers can be achieved using a simple diketopyrrolopyrrole unit flanked by thiazole groups, which is functionalized with glycolated side chains. Interestingly, the regiospecific sp2-N position in the repeating unit's thiazole governs the polymer chains' solvation and molecular packing. This specific backbone chemistry enhances conjugation efficiency, reduces trap density, and improves electrochemical doping efficiency. Moreover, systematic variation of glycolated side-chain lengths induces a sequential shift in molecular orientation-from edge-on through bimodal to face-on preferential alignment. This structural evolution achieves optimized ionic-electronic transport balance, resulting in exceptional device metrics: a geometrically normalized transconductance of 31.9 S cm-1, a figure-of-merit µC* of 96.3 F cm-1 V-1 s-1, and a threshold voltage of 0.31 V, positioning these materials among the highest-performing n-type OMIECs. An organic complementary inverter made from the optimized n-type OMIEC polymer and a reported p-type polymer exhibits a voltage gain of 198 V V-1, effectively amplifying the ECG signal and enhancing signal quality. This work establishes structure-property guidelines for designing bioelectronic n-type OMIECs.

Flexible End-Group Triggered Temporal Supramolecular Reordering on Substrate Surfaces and Its Impact on Electron Transport

The temporal stability of domain microstructure and morphologies is critical in regulating charge transport in organic field effect transistors (OFETs), ensuring consistent device performance and long-term reliability. Overcoming stability challenges in small molecular weight n-type materials, particularly in solution-processed active layers, necessitates careful molecular design. While flexible end groups or side chains enhance solution processability, they also introduce temporal disorder, or temporal reordering compromising device endurance. This study examines the trade-offs between rigid and flexible end groups in n-type molecular semiconductors, emphasizing their influence on morphological reordering in active layer films. As representative materials, n-Hex2NDI, with flexible end groups, is compared to tBCyH2NDI, which features rigid end groups. Films containing flexible side chains exhibit pronounced sensitivity to thermal annealing, significantly modifying the molecular and supramolecular arrangement and the crystallinity of n-Hex2NDI. Moreover, these flexible chains reduce current-voltage hysteresis, indicating enhanced structural order at elevated temperatures. Conversely, tBCyH2NDI demonstrates minimal temporal disorder at the molecular level organization, maintaining stable optical properties and negligible changes in current-voltage hysteresis upon annealing.

Organic electrochemical transistors based on a conjugated diketopyrrolopyrrole-dialkoxybithiazole copolymer

Organic electrochemical transistors (OECTs) are promising for bioelectronics due to their ability to amplify signals by converting ionic signals into electronic signals. The performance of OECTs relies heavily on the interaction between electrolyte ions and organic mixed ionic-electronic conductors (OMIECs). We examined how different aqueous electrolytes affect OECTs based on an ethylene glycol-substituted diketopyrrolopyrrole-dialkoxybithiazole copolymer (PDPP-TEG-2Tz), which is primarily p-type and electrochemically doped with anions. Our findings show that compared to the small, highly hydrated chloride anion (Cl-), the larger hexafluorophosphate (PF6-) and bis(trifluoromethanesulfonyl)imide (TFSI-) anions result in a lower threshold voltage and a faster transient response. Cations like Li+, Na+, and K+ have little impact on OECT performance. Additionally, we created a complementary inverter using p-type PDPP-TEG-2Tz with an n-type naphthalene diimide-bithiophene copolymer (PNDI2C8TEG-2T), achieving a maximum voltage gain of 22.6 at a supply voltage of 0.7 V.

Thermal Processing Creates Water-Stable PEDOT:PSS Films for Bioelectronics

Organic mixed ionic-electronic conductors have emerged as a key material for the development of bioelectronic devices due to their soft mechanical properties, biocompatibility, and high volumetric capacitance. In particular, PEDOT:PSS has become a choice material because it is highly conductive, easily processible, and commercially available. However, PEDOT:PSS is dispersible in water, leading to delamination of films when exposed to biological environments. For this reason, chemical cross-linking agents such as (3-glycidyloxypropyl)trimethoxysilane (GOPS) are used to stabilize PEDOT:PSS films in water, but at the cost of decreased electrical performance. Here, it is shown that PEDOT:PSS thin films become water-stable by simply baking at high temperatures (>150 °C) for a short time (≈ 2 min). It is shown that heat-treated PEDOT:PSS films are as stable as their chemically-cross-linked counterparts, with their performance maintained for >20 days both in vitro and in vivo. The heat-treated films eliminate electrically insulating cross-linkers, resulting in a 3× increase in volumetric capacitance. Applying thermal energy using a focused femtosecond laser enables direct patterning of 3D PEDOT:PSS microstructures. The thermal treatment method is compatible with a wide range of substrates and is readily substituted into existing workflows for manufacturing devices, enabling its rapid adoption in the field of bioelectronics.

Band Filling, Electrochemical Reaction, and Re-Entrant Insulating Behavior in Electrolyte-Gated BBL Polymer Semiconductor Films

Electrochemical doping of the n-type polymer poly(benzimidazobenzophenanthroline) (BBL) in contact with ionic liquids reveals a peak in the drain current (ID) vs gate voltage (VG) behavior, i.e., conductivity versus electron density. The conductivity peak is related to simultaneously acquired gate current-gate voltage (IG-VG) charging curves that are integrated to yield total charge accumulation. The IG-VG traces reveal three separate redox events upon charging BBL that can be correlated with the ID-VG behavior. We assign the first broad IG-VG peak to accumulation of mobile electrons in the BBL LUMO and an increase in ID. Two subsequent sharp IG-VG peaks correspond to electrochemical transformation of BBL to a polymer salt with 2:1 and 1:1 repeat unit-to-cation stoichiometries, respectively. Salt formation correlates with conductivity collapse at high VG; the 1:1 salt phase is insulating. Ex situ grazing incidence wide-angle X-ray scattering (GIWAXS) indicates an initial 5% contraction of the lamellar spacing upon doping with subsequent retention of lamellar (para)crystalline order for repeated doping cycles. Overall, our results reveal a complex interplay between band filling and electrochemical reaction in the transport behavior of electrochemically doped BBL films and provide additional evidence of nonmonotonic conductivity versus charge behavior that appears to be general in polymer semiconductor films.

Gel-Based Electrolytes for Organic Electrochemical Transistors: Mechanisms, Applications, and Perspectives

Organic electrochemical transistors (OECTs) have emerged as the core component of specialized bioelectronic technologies due to their high signal amplification capability, low operating voltage (<1 V), and biocompatibility. Under a gate bias, OECTs modulate device operation via ionic drift between the electrolyte and the channel. Compared to common electrolytes with a fluid nature (including salt aqueous solutions and ion liquids), gel electrolytes, with an intriguing structure consisting of a physically and/or chemically crosslinked polymer network where the interstitial spaces between polymers are filled with liquid electrolytes or mobile ion species, are promising candidates for quasi-solid electrolytes. Due to relatively high ionic conductivity, the potential for large-scale integration, and the capability to suppress channel swelling, gel electrolytes have been a research highlight in OECTs in recent years. This review summarizes recent progress on OECTs with gel electrolytes that demonstrate good mechanical as well as physical and chemical stabilities. Moreover, various components in forming gel electrolytes, including different mobile liquid phases and polymer components, are introduced. Furthermore, applications of these OECTs in the areas of sensors, neuromorphics, and organic circuits, are discussed. Last, future perspectives of OECTs based on gel electrolytes are discussed along with possible solutions for existing challenges.

An optoelectrochemical synapse based on a single-component n-type mixed conductor

Organic mixed ionic-electronic conductors (OMIECs) are materials that can be used to build bio-inspired electronic devices as they emulate ion-based cellular communication through doping with aqueous ionic charges. The integration of charge photogeneration and electrochemical doping processes in the polymer film enables optoelectronic applications that involve synaptic transistors. However, no OMIEC has yet been implemented to create a miniaturized photoactive platform capable of perceiving and processing multi-spectral visual information. Here, we present a materials and device design concept in which an n-type OMIEC film is incorporated into the micron-scale channel of an electrochemical transistor operating directly in an aqueous electrolyte under ambient conditions. The conjugated polymer channel, consisting of a fluorinated bisistain-lactone-bithiazole acceptor, has a current modulated in response to both electrical and optical stimuli, emulating the multimodal function of the visual nervous system. Our optoelectrochemical synapse achieves multilevel conductance states as well as transduction of visual information covering ultraviolet, visible, and near-infrared regions of the spectrum - a range beyond that of the human visual system's perception. The resulting transistor active-matrix array is capable of adaptive sensing, memory, and pre-processing of visual information, demonstrating an efficient optoelectronic neuromorphic system with multi-task learning capability.

π-Conjugated Polymers for High-Performance Organic Electrochemical Transistors: Molecular Design Strategies, Applications and Perspectives

The last decade has witnessed significant progress in organic electrochemical transistors (OECTs) due to their enormous potential applications in various bioelectronic devices, such as artificial synapses, biological interfaces, and biosensors. The remarkable advance in this field is highly powered by the development of novel organic mixed ionic/electronic conductors (OMIECs). π-Conjugated polymers (CPs), which are widely used in various optoelectronics, are emerging as key channel materials for OECTs. In this review, after briefly introducing OECT, we then mainly focus on the latest progress in CPs for high-performance OECTs. The correlations of their structure, basic physicochemical properties, and device performance are elucidated by evaluating their electronic characters, optoelectronic properties, and OECT performance. Then, the applications of CP-based OECTs are briefly presented. Finally, we discuss several remaining issues or challenges in this field and give our insights into advancing CPs for enhanced OECT performance.

Matching P- and N-type Organic Electrochemical Transistor Performance Enables a Record High-gain Complementary Inverter

The charge transport of channel materials in n-type organic electrochemical transistors (OECTs) is greatly limited by the adverse effects of electrochemical doping, posing a long-standing puzzle for the community. Herein, an n-type conjugated polymer with glycolated side chains (n-PT3) is introduced. This polymer can adapt to electrochemical doping and create more organized nanostructures, mitigating the adverse effects of electrochemical doping. This unique characteristic gives n-PT3 excellent charge transport in the doped state and reversible ion storage, making it highly suitable as an n-type organic mixed ionic-electronic conducting (OMIEC) material. n-PT3 exhibits a high electron mobility of µ ≈ 1.0 cm2 V-1 s-1 and a figure of merit value of µC* ≈ 100 F cm-1 V-1 s-1, representing one of the best results for n-type OMIEC materials. A new p-type OMIEC polymer has been synthesized as the channel material for constructing a complementary inverter to match the n-type OECT channel layer based on n-PT3. As a result, a voltage gain value of up to 307 VV-1 has been achieved, which is a record value for sub-1 V complementary inverters based on OECTs. This work offers valuable insights into designing electrochemical doping adaptive n-type OMIEC materials and fabricating high-gain organic complementary inverters.

Controlling Ion Uptake in Carboxylated Mixed Conductors

Organic mixed ionic-electronic conductors (OMIECs) have garnered significant attention due to their capacity to transport both ions and electrons, making them ideal for applications in energy storage, neuromorphics, and bioelectronics. However, charge compensation mechanisms during the polymer redox process remain poorly understood, and are often oversimplified as single-ion injection with little attention to counterion effects. To advance understanding and design strategies toward next-generation OMIEC systems, a series of p-channel carboxylated mixed conductors is investigated. Varying side-chain functionality, distinctive swelling character is uncovered during electrochemical doping/dedoping with model chao-/kosmotropic electrolytes. Carboxylic acid functionalized polymers demonstrate strong deswelling and mass reduction during doping, indicating cation expulsion, while ethoxycarbonyl counterparts exhibit prominent mass increase, pointing to an anion-driven doping mechanism. By employing operando grazing incidence X-ray fluorescence (GIXRF), it is revealed that the carboxyl functionalized polymer engages in robust cation interaction, whereas ester functionalization shifts the mechanism towards no cation involvement. It is demonstrated that cations are pivotal in mitigating swelling by counterbalancing anions, enabling efficient anion uptake without compromising performance. These findings underscore the transformative influence of functionality-driven factors and side-chain chemistry in governing ion dynamics and conduction, providing new frameworks for designing OMIECs with enhanced performance and reduced swelling.

Backbone Engineering of Bithiophene Imide Dimer-Based Polymeric Mixed Ionic-Electronic Conductors for High-Performance n-Type Organic Electrochemical Transistors

Polymeric mixed ionic-electronic conductors (PMIECs) are gaining significant attention due to their potential applications in organic electrochemical transistors (OECTs). However, the performance of n-type OECTs still lags behind that of their p-type counterparts. Here, the structure-performance correlation of fused bithiophene imide dimer (BTI2)-based PMIECs is systematically investigated with the backbone evaluation from acceptor-strong donor (A-SD) to acceptor-donor (A-D), to acceptor-weak donor (A-WD), to acceptor-weak acceptor (A-WA), and finally to A-A structures. Compared to other PMIECs, the A-A backbone-based PMIEC PBTI2g-BTI features the most suppressed lowest unoccupied molecular orbital energy level, a highly planar backbone, and superior n-type electrochemical dopability. Consequently, polymer PBTI2g-BTI delivers an exceptional unipolar n-type OECT performance with a high electron mobility of 0.25 cm2 V⁻¹ s⁻¹ and a remarkable µC* value of 63.79 F cm⁻¹ V⁻¹ s⁻¹, both of which are significantly higher than those of other types of PMIECs. This study demonstrates that the A-A backbone strategy is an effective approach to developing high-performance n-type PMIECs for applications in OECTs.

Small-Molecule Mixed Ionic-Electronic Conductors for Efficient N-Type Electrochemical Transistors: Structure-Function Correlations

The fundamental challenge in electron-transporting organic mixed ionic-electronic conductors (OMIECs) is simultaneous optimization of electron and ion transport. Beginning from Y6-type/U-shaped non-fullerene solar cell acceptors, we systematically synthesize and characterize molecular structures that address the aforementioned challenge, progressively introducing increasing numbers of oligoethyleneglycol (OEG; g) sidechains from 1 g to 3 g, affording OMIECs 1gY, 2gY, and 3gY, respectively. The crystal structure of 1gY preserves key structural features of the Yn series: a U-shaped/planar core, close π-π molecular stacking, and interlocked acceptor groups. Versus inactive Y6 and Y11, all of the new glycolated compounds exhibit mixed ion-electron transport in both conventional organic electrochemical transistor (cOECT) and vertical OECT (vOECT) architectures. Notably, 3gY with the highest OEG density achieves a high transconductance of 16.5 mS, an on/off current ratio of ~106, and a turn-on/off response time of 94.7/5.7 ms in vOECTs. Systematic optoelectronic, electrochemical, architectural, and crystallographic analysis explains the superior 3gY-based OECT performance in terms of denser ngY OEG content, increased crystallite dimensions with decreased long-range crystalline order, and enhanced film hydrophilicity which facilitates ion transport and efficient redox processes. Finally, we demonstrate an efficient small-molecule-based complementary inverter using 3gY vOECTs, showcasing the bioelectronic applicability of these new small-molecule OMIECs.

Spatial control of doping in conducting polymers enables complementary, conformable, implantable internal ion-gated organic electrochemical transistors

Complementary transistors are critical for circuits with compatible input/output signal dynamic range and polarity. Organic electronics offer biocompatibility and conformability; however, generation of complementary organic transistors requires introduction of separate materials with inadequate stability and potential for tissue toxicity, limiting their use in biomedical applications. Here, we discovered that introduction of source/drain contact asymmetry enables spatial control of de/doping and creation of single-material complementary organic transistors from a variety of conducting polymers of both carrier types. When integrated with the vertical channel design and internal ion reservoirs of internal ion-gated organic electrochemical transistors, we produced matched complementary IGTs (cIGTs) that formed high-performance conformable amplifiers with 200 V/V uniform gain and 2 MHz bandwidth. These amplifiers showed long-term in vivo stability, and their miniaturized biocompatible design allowed implantation in developing rodents to monitor network maturation. cIGTs expand the use of organic electronics in standard circuit designs and enhance their biomedical potential.

The Effect of Organic Semiconductor Electron Affinity on Preventing Parasitic Oxidation Reactions Limiting Performance of n-Type Organic Electrochemical Transistors

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.

Exploiting Spatial Ionic Dynamics in Solid-State Organic Electrochemical Transistors for Multi-Tactile Sensing and Processing

The human nervous system inspires the next generation of sensory and communication systems for robotics, human-machine interfaces (HMIs), biomedical applications, and artificial intelligence. Neuromorphic approaches address processing challenges; however, the vast number of sensors and their large-scale distribution complicate analog data manipulation. Conventional digital multiplexers are limited by complex circuit architecture and high supply voltage. Large sensory arrays further complicate wiring. An 'in-electrolyte computing' platform is presented by integrating organic electrochemical transistors (OECTs) with a solid-state polymer electrolyte. These devices use synapse-like signal transport and spatially dependent bulk ionic doping, achieving over 400 times modulation in channel conductance, allowing discrimination of locally random-access events without peripheral circuitry or address assignment. It demonstrates information processing from 12 tactile sensors with a single OECT output, showing clear advantages in circuit simplicity over existing all-electronic, all-digital implementations. This self-multiplexer platform offers exciting prospects for circuit-free integration with sensory arrays for high-quality, large-volume analog signal processing.

Polyethylene glycol-decorated n-type conducting polymers with improved ion accessibility for high-performance organic electrochemical transistors

High-performance n-type organic mixed ionic-electronic conductors (OMIECs) are essential for advancing complementary circuits based on organic electrochemical transistors (OECTs). Despite significant progress, current n-type OMIECs often exhibit lower transconductance and slower response times compared to their p-type counterparts, limiting the development of OECT-based complementary circuits. Optimizing the conjugated backbone and side chain structures of OMIECs is critical for enhancing both ion and electron transport efficiencies while maintaining a delicate balance between the two. In this study, hydrophilic polyethylene glycol (PEG) side chains were incorporated into the highly conductive n-type polymer poly(3,7-dihydrobenzo[1,2-b:4,5-b']difuran-2,6-dione) (PBFDO) backbone to achieve this goal. The incorporation of PEG chains improved ion accessibility, and by adjusting the PEG content, the electronic and ionic transport properties were fine-tuned, ultimately enhancing the performance of OECTs and related p-n complementary circuits. The n-type OECTs based on PBFDO-PEG50wt% demonstrated exceptional transfer characteristics, including a transient response time (τON) as low as 72 μs, a high geometry-normalized transconductance exceeding 400 S cm-1, and an impressive μC* value surpassing 720 F cm-1 V-1 s-1. Notably, the use of PBFDO-PEG50wt% in a complementary inverter resulted in a voltage gain of 20 V/V, more than five times higher than that achieved with unmodified PBFDO (<4 V/V). These findings highlight the importance of balancing electron and ion transport characteristics in OMIECs to achieve high performance in OECTs and their associated circuits, and they validate PEG decoration as an effective approach.

Improving both performance and stability of n-type organic semiconductors by vitamin C

Organic semiconductors (OSCs) are one of the most promising candidates for flexible, wearable and large-area electronics. However, the development of n-type OSCs has been severely held back due to the poor stability of their most candidates, that is, the intrinsically high reactivity of negatively charged polarons to oxygen and water. Here we demonstrate a general strategy based on vitamin C to stabilize n-type OSCs, remarkably improving the performance and stability of their device, for example, organic field-effect transistors. Vitamin C scavenges reactive oxygen species and inhibits their generation by sacrificial oxidation and non-sacrificial triplet quenching in a cascade process, which not only lastingly prevents molecular structure from oxidation damage but also passivates the latent electron traps to stabilize electron transport. This study presents a way to overcome the long-standing stability problem of n-type OSCs and devices.

Stable n-Type Perylene Derivative Ladder Polymer with Antiambipolarity for Electrically Reconfigurable Organic Logic Gates

Organic electrochemical transistors (OECTs) are one of the promising building blocks to realize next-generation bioelectronics. To date, however, the performance and signal processing capabilities of these devices remain limited by their stability and speed. Herein, the authors demonstrate stable and fast n-type organic electrochemical transistors based on a side-chain-free ladder polymer, poly(benzimidazoanthradiisoquinolinedione). The device demonstrated fast normalized transient speed of 0.56 ± 0.17 ms um-2 and excellent long-term stability in aqueous electrolytes, with no significant drop in its doping current after 50 000 successive doping/dedoping cycles and 2-month storage at ambient conditions. These unique characteristics make this polymer especially suitable for bioelectronics, such as being used as a pull-down channel in a complementary inverter for long-term stable detection of electrophysiological signals. Moreover, the developed device shows a reversible anti-ambipolar behavior, enabling reconfigurable electronics to be realized using a single material. These results go beyond the conventional OECT and demonstrate the potential of OECTs to exhibit dynamically configurable functionalities for next-generation reconfigurable electronics.

Reconfiguration of organic electrochemical transistors for high-accuracy potentiometric sensing

Organic electrochemical transistors have emerged as a promising alternative to traditional 2/3 electrode setups for sensing applications, offering in-situ transduction, electrochemical amplification, and noise reduction. Several of these devices are designed to detect potentiometric-derived signals. However, potentiometric sensing should be performed under open circuit potential conditions, allowing the system to reach thermodynamic equilibrium. This criterion is not met by conventional organic electrochemical transistors, where voltages or currents are directly applied to the sensing interface, that is, the gate electrode. In this work, we introduce an organic electrochemical transistor sensing configuration called the potentiometric‑OECT (pOECT), which maintains the sensing electrode under open circuit potential conditions. The pOECT exhibits a higher response than the 2-electrode setup and offers greater accuracy, response, and stability compared to conventional organic electrochemical transistors. Additionally, it allows for the implementation of high-impedance electrodes as gate/sensing surfaces, all without compromising the overall device size.

Over 60 h of Stable Water-Operation for N-Type Organic Electrochemical Transistors with Fast Response and Ambipolarity

Organic electrochemical transistors (OECTs) are of great interest in low-power bioelectronics and neuromorphic computing, as they utilize organic mixed ionic-electronic conductors (OMIECs) to transduce ionic signals into electrical signals. However, the poor environmental stability of OMIEC materials significantly restricts the practical application of OECTs. Therefore, the non-fused planar naphthalenediimide (NDI)-dialkoxybithiazole (2Tz) copolymers are fine-tuned through varying ethylene glycol (EG) side chain lengths from tri(ethylene glycol) to hexa(ethylene glycol) (namely P-XO, X = 3-6) to achieve OECTs with high-stability and low threshold voltage. As a result, the NDI-2Tz copolymers exhibit ambipolarity, rapid response (<10 ms), and ultra-high n-type stability. Notably, the P-6O copolymers display a threshold voltage as low as 0.27 V. They can operate in n-type mode in an aqueous solution for over 60 h, maintaining an on-off ratio of over 105. This work sheds light on the design of exceptional n-type/ambipolar materials for OECTs. It demonstrates the potential of incorporating these ambipolar polymers into water-operational integrated circuits for long-term biosensing systems and energy-efficient brain-inspired computing.

Advanced Neuromorphic Applications Enabled by Synaptic Ion-Gating Vertical Transistors

Bioinspired synaptic devices have shown great potential in artificial intelligence and neuromorphic electronics. Low energy consumption, multi-modal sensing and recording, and multifunctional integration are critical aspects limiting their applications. Recently, a new synaptic device architecture, the ion-gating vertical transistor (IGVT), has been successfully realized and timely applied to perform brain-like perception, such as artificial vision, touch, taste, and hearing. In this short time, IGVTs have already achieved faster data processing speeds and more promising memory capabilities than many conventional neuromorphic devices, even while operating at lower voltages and consuming less power. This work focuses on the cutting-edge progress of IGVT technology, from outstanding fabrication strategies to the design and realization of low-voltage multi-sensing IGVTs for artificial-synapse applications. The fundamental concepts of artificial synaptic IGVTs, such as signal processing, transduction, plasticity, and multi-stimulus perception are discussed comprehensively. The contribution draws special attention to the development and optimization of multi-modal flexible sensor technologies and presents a roadmap for future high-end theoretical and experimental advancements in neuromorphic research that are mostly achievable by the synaptic IGVTs.

Large-Area Deposition of Highly Crystalline F4-Tetracyanoquinodimethane Thin Films by Molecular Step Templates

Theoretical studies have unequivocally determined the exceptional electron transport properties of the fluorinated tetracyanoquinodimethane (Fx-TCNQ) family, presenting a promising avenue for the realization of high-performance n-channel organic thin-film transistors (OTFTs). However, owing to the intrinsic low crystallinity of this class of materials, Fx-TCNQ-based n-channel OTFTs have not been experimentally achieved so far. Herein, a molecular step template (MST)-assisted method that dramatically improves the crystallinity of F4-TCNQ thin films is reported. The MST not only lowers the nucleation barrier of F4-TCNQ molecules along the in-plane direction but also reduces the nucleation density. This approach facilitates the realization of compact, oriented, and highly crystalline F4-TCNQ thin films, resulting in impressive electron mobility of up to 2.58 cm2 V-1 s-1. Notably, this achievement surpasses the electron mobility of F4-TCNQ thin films fabricated without the MST by a factor of 107. Furthermore, the incorporation of the p-type MST provides a novel pathway for constructing complementary inverters, showcasing a high voltage gain of 112.6 V V-1 and a substantial noise margin of 89.3% with exceptional uniformity. In this work, a general and efficient route is paved to produce high-performance n-channel OTFTs toward organic complementary circuits.

Air-Stable Perylene Diimide Trimer Material for N-Type Organic Electrochemical Transistors

A new method is reported to make air-stable n-type organic mixed ionic-electronic conductor (OMIEC) films for organic electrochemical transistors (OECTs) using a solution-processable small molecule helical perylene diimide trimer, hPDI[3]-C11. Alkyl side chains are attached to the conjugated core for processability and film making, which are then cleaved via thermal annealing. After the sidechains are removed, the hPDI[3] film becomes less hydrophobic, more ordered, and has a deeper lowest unoccupied molecular orbital (LUMO). These features provide improved ionic transport, greater electronic mobility, and increased stability in air and in aqueous solution. Subsequently, hPDI[3]-H is used as the active material in OECTs and a device with a transconductance of 44 mS, volumetric capacitance of ≈250 F cm-3, µC* value of 1 F cm-1 V-1 s-1, and excellent stability (> 5 weeks) is demonstrated. As proof of their practical applications, a hPDI[3]-H-based OECTs as a glucose sensor and electrochemical inverter is utilized. The approach of side chain removal after film formation charts a path to a wide range of molecular semiconductors to be used as stable, mixed ionic-electronic conductors.

Ultrastable N-Type Semiconducting Fiber Organic Electrochemical Transistors for Highly Sensitive Biosensors

Organic electrochemical transistors (OECTs) have attracted increasing attention due to their merits of high transconductance, low operating voltage, and good biocompatibility, ideal for biosensors. However, further advances in their practical applications face challenges of low n-type performance and poor stability. Here, it is demonstrated that wet-spinning the commercially available n-type conjugated polymer poly(benzimidazobenzophenanthroline) (BBL) into highly aligned and crystalline fibers enhances both OECT performance and stability. Although BBL is only soluble in high-boiling-point strong acids, it can be wet-spun into high-quality fibers with adjustable diameters. The BBL fiber OECTs exhibit a record-high area-normalized transconductance (gm,A) of 2.40 µS µm-2 and over 10 times higher figure-of-merit (µC*) than its thin-film counterparts. More importantly, these fiber OECTs exhibit remarkable stability with no noticeable performance attenuation after 1500 cycles over 4 h operation, outperforming all previously reported n-type OECTs. The superior performance and stability can be attributed to shorter π-π stacking distance and ordered molecular arrangement in the fibers, endowing the BBL fiber OECT-based biosensors with outstanding sensitivity while keeping a miniaturized form factor. This work demonstrates that, beyond new material development, developing new fabrication technology is also crucial for addressing the performance and stability issues in n-type OECTs.

High-mobility organic mixed conductors with a low synthetic complexity index via direct arylation polymerization

Through direct arylation polymerization, a series of mixed ion-electron conducting polymers with a low synthetic complexity index is synthesized. A thieno[3,2-b]thiophene monomer with oligoether side chains is used in direct arylation polymerization together with a wide range of aryl bromides with varying electronic character from electron-donating thiophene to electron-accepting benzothiadiazole. The obtained polymers are less synthetically complex than other mixed ion-electron conducting polymers due to higher yield, fewer synthetic steps and less toxic reagents. Organic electrochemical transistors (OECTs) based on a newly synthesized copolymer comprising thieno[3,2-b]thiophene with oligoether side chains and bithiophene exhibit excellent device performance. A high charge-carrier mobility of up to μ = 1.8 cm2 V-1 s-1 was observed, obtained by dividing the figure of merit [μC*] from OECT measurements by the volumetric capacitance C* from electrochemical impedance spectroscopy, which reached a value of more than 215 F cm-3.

Flexible Organic Transistors for Biosensing: Devices and Applications

Flexible and stretchable biosensors can offer seamless and conformable biological-electronic interfaces for continuously acquiring high-fidelity signals, permitting numerous emerging applications. Organic thin film transistors (OTFTs) are ideal transducers for flexible and stretchable biosensing due to their soft nature, inherent amplification function, biocompatibility, ease of functionalization, low cost, and device diversity. In consideration of the rapid advances in flexible-OTFT-based biosensors and their broad applications, herein, a timely and comprehensive review is provided. It starts with a detailed introduction to the features of various OTFTs including organic field-effect transistors and organic electrochemical transistors, and the functionalization strategies for biosensing, with a highlight on the seminal work and up-to-date achievements. Then, the applications of flexible-OTFT-based biosensors in wearable, implantable, and portable electronics, as well as neuromorphic biointerfaces are detailed. Subsequently, special attention is paid to emerging stretchable organic transistors including planar and fibrous devices. The routes to impart stretchability, including structural engineering and material engineering, are discussed, and the implementations of stretchable organic transistors in e-skin and smart textiles are included. Finally, the remaining challenges and the future opportunities in this field are summarized.

Vertical organic electrochemical transistor platforms for efficient electropolymerization of thiophene based oligomers

Organic electrochemical transistors (OECTs) have emerged as promising candidates for various fields, including bioelectronics, neuromorphic computing, biosensors, and wearable electronics. OECTs operate in aqueous solutions, exhibit high amplification properties, and offer ion-to-electron signal transduction. The OECT channel consists of a conducting polymer, with PEDOT:PSS receiving the most attention to date. While PEDOT:PSS is highly conductive, and benefits from optimized protocols using secondary dopants and detergents, new p-type and n-type polymers are emerging with desirable material properties. Among these, low-oxidation potential oligomers are highly enabling for bioelectronics applications, however the polymers resulting from their polymerization lag far behind in conductivity compared with the established PEDOT:PSS. In this work we show that by careful design of the OECT geometrical characteristics, we can overcome this limitation and achieve devices that are on-par with transistors employing PEDOT:PSS. We demonstrate that the vertical architecture allows for facile electropolymerization of a family of trimers that are polymerized in very low oxidation potentials, without the need for harsh chemicals or secondary dopants. Vertical and planar OECTs are compared using various characterization methods. We show that vOECTs are superior platforms in general and propose that the vertical architecture can be expanded for the realization of OECTs for various applications.

2D MXene electrochemical transistors

The solid-state field-effect transistor, FET, and its theories were paramount in the discovery and studies of graphene. In the past two decades another transistor based on conducting polymers, called organic electrochemical transistor (ECT), has been developed and largely studied. The main difference between organic ECTs and FETs is the mode and extent of channel doping; while in FETs the channel only has surface doping through dipoles, the mixed ionic-electronic conductivity of the channel material in organic ECTs enables bulk electrochemical doping. As a result, organic ECTs maximize conductance modulation at the expense of speed. To date ECTs have been based on conducting polymers, but here we show that MXenes, a class of 2D materials beyond graphene, enable the realization of electrochemical transistors (ECTs). We show that the formulas for organic ECTs can be applied to these 2D ECTs and used to extract parameters like mobility. These MXene ECTs have high transconductance values but low on-off ratios. We further show that conductance switching data measured using ECT, in combination with other in situ-ex situ electrochemical measurements, is a powerful tool for correlating the change in conductance to that of the redox state, to our knowledge, this is the first report of this important correlation for MXene films. 2D ECTs can draw great inspiration and theoretical tools from the field of organic ECTs and have the potential to considerably extend the capabilities of transistors beyond those of conducting polymer ECTs, with added properties such as extreme heat resistance, tolerance for solvents, and higher conductivity for both electrons and ions than conducting polymers.

Azonia-Naphthalene: A Cationic Hydrophilic Building Block for Stable N-Type Organic Mixed Ionic-Electronic Conductors

Conjugated polymers that can efficiently transport both ionic and electronic charges have broad applications in next-generation optoelectronic, bioelectronic, and energy storage devices. To date, almost all the conjugated polymers have hydrophobic backbones, which impedes efficient ion diffusion/transport in aqueous media. Here, we design and synthesize a novel hydrophilic polymer building block, 4a-azonia-naphthalene (AN), drawing inspiration from biological systems. Because of the strong electron-withdrawing ability of AN, the AN-based polymers show typical n-type charge transport behaviors. We find that cationic aromatics exhibit strong cation-π interactions, leading to smaller π-π stacking distance, interesting ion diffusion behavior, and good morphology stability. Additionally, AN enhances the hydrophilicity and ionic-electronic coupling of the polymer, which can help to improve ion diffusion/injection speed, and operational stability of organic electrochemical transistors (OECTs). The integration of cationic building blocks will undoubtedly enrich the material library for high-performance n-type conjugated polymers.

High-Performance n-Type Polymeric Mixed Ionic-Electronic Conductors: The Impacts of Halogen Functionalization

Developing high-performance n-type polymer mixed ionic-electronic conductors (PMIECs) is a grand challenge, which largely determines their applications in vaious organic electronic devices, such as organic electrochemical transistors (OECTs) and organic thermoelectrics (OTEs). Herein, two halogen-functionalized PMIECs f-BTI2g-TVTF and f-BTI2g-TVTCl built from fused bithiophene imide dimer (f-BTI2) as the acceptor unit and halogenated thienylene-vinylene-thienylene (TVT) as the donor co-unit are reported. Compared to the control polymer f-BTI2g-TVT, the fluorinated f-BTI2g-TVTF shows lower-positioned lowest unoccupied molecular orbital (LUMO), improved charge transport property, and greater ion uptake capacity. Consequently, f-BTI2g-TVTF delivers a state-of-the-art µC* of 90.2 F cm-1 V-1 s-1 with a remarkable electron mobility of 0.41 cm2 V-1 s-1 in OECTs and an excellent power factor of 64.2 µW m-1 K-2 in OTEs. An OECT-based inverter amplifier is further demonstrated with voltage gain up to 148 V V-1 , which is among the highest values for OECT inverters. Such results shed light on the impacts of halogen atoms on developing high-performing n-type PMIECs.

Selenophene Substitution Enabled High-Performance n-Type Polymeric Mixed Ionic-Electronic Conductors for Organic Electrochemical Transistors and Glucose Sensors

High-performance n-type polymeric mixed ionic-electronic conductors (PMIECs) are essential for realizing organic electrochemical transistors (OECTs)-based low-power complementary circuits and biosensors, but their development still remains a great challenge. Herein, by devising two novel n-type polymers (f-BTI2g-SVSCN and f-BSeI2g-SVSCN) containing varying selenophene contents together with their thiophene-based counterpart as the control, it is demonstrated that gradually increasing selenophene loading in polymer backbones can simultaneously yield lowered lowest unoccupied molecular orbital levels, boosted charge-transport properties, and improved ion-uptake capabilities. Therefore, a remarkable volumetric capacitance (C*) of 387.2 F cm-3 and a state-of-the-art OECT electron mobility (µe,OECT ) of 0.48 cm2 V-1 s-1 are synchronously achieved for f-BSeI2g-SVSCN having the highest selenophene content, yielding an unprecedented geometry-normalized transconductance (gm,norm ) of 71.4 S cm-1 and record figure of merit (µC*) value of 191.2 F cm-1 V-1 s-1 for n-type OECTs. Thanks to such excellent performance of f-BSeI2g-SVSCN-based OECTs, a glucose sensor with a remarkably low detection limit of 10 nMm and decent selectivity is further implemented, demonstrating the power of selenophene substitution strategy in enabling high-performance n-type PMIECs for biosensing applications.

Demonstration of the threshold-switching memory devices using EMIm(AlCl3)Cl and ZnO for neuromorphic applications

The threshold-switching behaviors of the synapses lead to energy-efficient operation in the neural computing system. Here, we demonstrated the threshold-switching memory devices by inserting the ZnO layer into the ionic synaptic devices. The EMIm(AlCl3)Cl is utilized as the electrolyte because its conductance can be tuned by the charge states of the Al-based ions. The redox reactions of the Al ions in the electrolyte can lead to the analog resistive switching characteristics, such as excitatory postsynaptic current, paired-pulse facilitation, potentiation, and depression. By inserting the ZnO layer into the EMIm(AlCl3)-based ionic synaptic devices, the threshold switching behaviors are demonstrated. Using the resistivity difference between ZnO and EMIm(AlCl3)Cl, the analog resistive switching behaviors are tunned as the threshold-switching behaviors. The threshold-switching behaviors are achieved by applying the spike stimuli to the device. Demonstration of the threshold-switching behaviors of the ionic synaptic devices has a possibility to achieve high energy-efficiency for the ion-based artificial synapses.

Stable organic electrochemical neurons based on p-type and n-type ladder polymers

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 long-term operational stability remains a significant challenge that hinders their widespread adoption. While p-type OMIECs are generally more stable than n-type OMIECs, they still face limitations, especially during prolonged operations. Here, we demonstrate that simple methylation of the pyrrole-benzothiazine-based (PBBT) ladder polymer backbone results in stable and high-performance p-type OECTs. The methylated PBBT (PBBT-Me) exhibits a 25-fold increase in OECT mobility and an impressive 36-fold increase in μC* (mobility × volumetric capacitance) compared to the non-methylated PBBT-H polymer. Combining the newly developed PBBT-Me with the ladder n-type poly(benzimidazobenzophenanthroline) (BBL), we developed complementary inverters with a record-high DC gain of 194 V V-1 and excellent stability. These state-of-the-art complementary inverters were used to demonstrate leaky integrate-and-fire type organic electrochemical neurons (LIF-OECNs) capable of biologically relevant firing frequencies of about 2 Hz and of operating continuously for up to 6.5 h. This achievement represents a significant improvement over previous results and holds great potential for developing stable bioelectronic circuits capable of in-sensor computing.

Efficient N-Type Organic Electrochemical Transistors and Field-Effect Transistors Based on PNDI-Copolymers Bearing Fluorinated Selenophene-Vinylene-Selenophenes

n-Type organic electrochemical transistors (OECTs) and organic field-effect transistors (OFETs) are less developed than their p-type counterparts. Herein, polynaphthalenediimide (PNDI)-based copolymers bearing novel fluorinated selenophene-vinylene-selenophene (FSVS) units as efficient materials for both n-type OECTs and n-type OFETs are reported. The PNDI polymers with oligo(ethylene glycol) (EG7) side chains P(NDIEG7-FSVS), affords a high µC* of > 0.2 F cm-1  V-1  s-1 , outperforming the benchmark n-type Pg4NDI-T2 and Pg4NDI-gT2 by two orders of magnitude. The deep-lying LUMO of -4.63 eV endows P(NDIEG7-FSVS) with an ultra-low threshold voltage of 0.16 V. Moreover, the conjugated polymer with octyldodecyl (OD) side chains P(NDIOD-FSVS) exhibits a surprisingly low energetic disorder with an Urbach energy of 36 meV and an ultra-low activation energy of 39 meV, resulting in high electron mobility of up to 0.32 cm2  V-1  s-1 in n-type OFETs. These results demonstrate the great potential for simultaneously achieving a lower LUMO and a tighter intermolecular packing for the next-generation efficient n-type organic electronics.

Flexible and Stretchable Organic Electrochemical Transistors for Physiological Sensing Devices

Flexible and stretchable bioelectronics provides a biocompatible interface between electronics and biological systems and has received tremendous attention for in situ monitoring of various biological systems. Considerable progress in organic electronics has made organic semiconductors, as well as other organic electronic materials, ideal candidates for developing wearable, implantable, and biocompatible electronic circuits due to their potential mechanical compliance and biocompatibility. Organic electrochemical transistors (OECTs), as an emerging class of organic electronic building blocks, exhibit significant advantages in biological sensing due to the ionic nature at the basis of the switching behavior, low driving voltage (<1 V), and high transconductance (in millisiemens range). During the past few years, significant progress in constructing flexible/stretchable OECTs (FSOECTs) for both biochemical and bioelectrical sensors has been reported. In this regard, to summarize major research accomplishments in this emerging field, this review first discusses structure and critical features of FSOECTs, including working principles, materials, and architectural engineering. Next, a wide spectrum of relevant physiological sensing applications, where FSOECTs are the key components, are summarized. Last, major challenges and opportunities for further advancing FSOECT physiological sensors are discussed.

Hole-limited electrochemical doping in conjugated polymers

Simultaneous transport and coupling of ionic and electronic charges is fundamental to electrochemical devices used in energy storage and conversion, neuromorphic computing and bioelectronics. While the mixed conductors enabling these technologies are widely used, the dynamic relationship between ionic and electronic transport is generally poorly understood, hindering the rational design of new materials. In semiconducting electrodes, electrochemical doping is assumed to be limited by motion of ions due to their large mass compared to electrons and/or holes. Here, we show that this basic assumption does not hold for conjugated polymer electrodes. Using operando optical microscopy, we reveal that electrochemical doping speeds in a state-of-the-art polythiophene 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 conjugated polymers with improved electrochemical performance.

New Chemical Dopant and Counterion Mechanism for Organic Electrochemical Transistors and Organic Mixed Ionic-Electronic Conductors

Organic mixed ionic-electronic conductors (OMIECs) have varied performance requirements across a diverse application space. Chemically doping the OMIEC can be a simple, low-cost approach for adapting performance metrics. However, complex challenges, such as identifying new dopant materials and elucidating design rules, inhibit its realization. Here, these challenges are approached by introducing a new n-dopant, tetrabutylammonium hydroxide (TBA-OH), and identifying a new design consideration underpinning its success. TBA-OH behaves as both a chemical n-dopant and morphology additive in donor acceptor co-polymer naphthodithiophene diimide-based polymer, which serves as an electron transporting material in organic electrochemical transistors (OECTs). The combined effects enhance OECT transconductance, charge carrier mobility, and volumetric capacitance, representative of the key metrics underpinning all OMIEC applications. Additionally, when the TBA+ counterion adopts an "edge-on" location relative to the polymer backbone, Coulombic interaction between the counterion and polaron is reduced, and polaron delocalization increases. This is the first time such mechanisms are identified in doped-OECTs and doped-OMIECs. The work herein therefore takes the first steps toward developing the design guidelines needed to realize chemical doping as a generic strategy for tailoring performance metrics in OECTs and OMIECs.

Bioadhesive polymer semiconductors and transistors for intimate biointerfaces

The use of bioelectronic devices relies on direct contact with soft biotissues. For transistor-type bioelectronic devices, the semiconductors that need to have direct interfacing with biotissues for effective signal transduction do not adhere well with wet tissues, thereby limiting the stability and conformability at the interface. We report a bioadhesive polymer semiconductor through a double-network structure formed by a bioadhesive brush polymer and a redox-active semiconducting polymer. The resulting semiconducting film can form rapid and strong adhesion with wet tissue surfaces together with high charge-carrier mobility of ~1 square centimeter per volt per second, high stretchability, and good biocompatibility. Further fabrication of a fully bioadhesive transistor sensor enabled us to produce high-quality and stable electrophysiological recordings on an isolated rat heart and in vivo rat muscles.

Dual-Analyte Sensing with a Molecularly Imprinted Polymer Based on Enhancement-Mode Organic Electrochemical Transistors

Novel enhancement-mode organic electrochemical transistors (OECTs) have been prepared by poly(3, 4-ethylenedioxythiophene)-poly(styrenesulfonate) de-doped polyethylenimine on the multi-walled carbon nanotube-modified viscose yarn. The fabricated devices exhibit low power consumption with a high transconductance of 6.7 mS, rapid response time < 2 s, and excellent cyclic stability. In addition, the device has washing durability and bending and long-term stability suitable for wearable applications. Biosensors based on enhancement-mode OECTs for the selective detection of adrenaline and uric acid (UA) are developed by using molecularly imprinted polymer (MIP)-functionalized gate electrodes. The detection limits of adrenaline and UA analysis are as low as 1 pM, with the linear ranges of 0.5 pM to 10 μM and 1 pM to 1 mM, respectively. Moreover, the sensor based on enhancement-mode transistors can efficiently amplify the current signals according to the modulation of the gate voltage. The MIP-modified biosensor has high selectivity in the presence of interferents and desirable reproducibility. Additionally, due to the wearable nature of the developed biosensor, this sensing tool has the capability of being integrated with fabrics. Therefore, it has successfully been applied in textiles for the determination of adrenaline and UA in artificial urine samples. The excellent recoveries and rsds are 90.22-109.05% and 3.97-6.94%, respectively. Ultimately, these sensitive, low-power, wearable, and dual-analyte sensors help to develop non-laboratory tools for early disease diagnosis and clinical research.

Downsizing the Channel Length of Vertical Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are promising building blocks for bioelectronic devices such as sensors and neural interfaces. While the majority of OECTs use simple planar geometry, there is interest in exploring how these devices operate with much shorter channels on the submicron scale. Here, we show a practical route toward the minimization of the channel length of the transistor using traditional photolithography, enabling large-scale utilization. We describe the fabrication of such transistors using two types of conducting polymers. First, commercial solution-processed poly(dioxyethylenethiophene):poly(styrene sulfonate), PEDOT:PSS. Next, we also exploit the short channel length to support easy in situ electropolymerization of poly(dioxyethylenethiophene):tetrabutyl ammonium hexafluorophosphate, PEDOT:PF₆. Both variants show different promising features, leading the way in terms of transconductance (g_m), with the measured peak g_m up to 68 mS for relatively thin (280 nm) channel layers on devices with the channel length of 350 nm and with widths of 50, 100, and 200 μm. This result suggests that the use of electropolymerized semiconductors, which can be easily customized, is viable with vertical geometry, as uniform and thin layers can be created. Spin-coated PEDOT:PSS lags behind with the lower values of g_m; however, it excels in terms of the speed of the device and also has a comparably lower off current (300 nA), leading to unusually high on/off ratio, with values up to 8.6 × 10⁴. Our approach to vertical gap devices is simple, scalable, and can be extended to other applications where small electrochemical channels are desired.

Soft Fiber Electronics Based on Semiconducting Polymer

Fibers, originating from nature and mastered by human, have woven their way throughout the entire history of human civilization. Recent developments in semiconducting polymer materials have further endowed fibers and textiles with various electronic functions, which are attractive in applications such as information interfacing, personalized medicine, and clean energy. Owing to their ability to be easily integrated into daily life, soft fiber electronics based on semiconducting polymers have gained popularity recently for wearable and implantable applications. Herein, we present a review of the previous and current progress in semiconducting polymer-based fiber electronics, particularly focusing on smart-wearable and implantable areas. First, we provide a brief overview of semiconducting polymers from the viewpoint of materials based on the basic concepts and functionality requirements of different devices. Then we analyze the existing applications and associated devices such as information interfaces, healthcare and medicine, and energy conversion and storage. The working principle and performance of semiconducting polymer-based fiber devices are summarized. Furthermore, we focus on the fabrication techniques of fiber devices. Based on the continuous fabrication of one-dimensional fiber and yarn, we introduce two- and three-dimensional fabric fabricating methods. Finally, we review challenges and relevant perspectives and potential solutions to address the related problems.

A guide for the characterization of organic electrochemical transistors and channel materials

The organic electrochemical transistor (OECT) is one of the most versatile devices within the bioelectronics toolbox, with its compatibility with aqueous media and the ability to transduce and amplify ionic and biological signals into an electronic output. The OECT operation relies on the mixed (ionic and electronic charge) conduction properties of the material in its channel. With the increased popularity of OECTs in bioelectronics applications and to benchmark mixed conduction properties of channel materials, the characterization methods have broadened somewhat heterogeneously. We intend this review to be a guide for the characterization methods of the OECT and the channel materials used. Our review is composed of two main sections. First, we review techniques to fabricate the OECT, introduce different form factors and configurations, and describe the device operation principle. We then discuss the OECT performance figures of merit and detail the experimental procedures to obtain these characteristics. In the second section, we shed light on the characterization of mixed transport properties of channel materials and describe how to assess films' interactions with aqueous electrolytes. In particular, we introduce experimental methods to monitor ion motion and diffusion, charge carrier mobility, and water uptake in the films. We also discuss a few theoretical models describing ion-polymer interactions. We hope that the guidelines we bring together in this review will help researchers perform a more comprehensive and consistent comparison of new materials and device designs, and they will be used to identify advances and opportunities to improve the device performance, progressing the field of organic bioelectronics.

Backbone coplanarity manipulation via hydrogen bonding to boost the n-type performance of polymeric mixed conductors operating in aqueous electrolyte

The development of high-performance n-type semiconducting polymers remains a significant challenge. Reported here is the construction of a coplanar backbone via intramolecular hydrogen bonds to dramatically enhance the performance of n-type polymeric mixed conductors operating in aqueous electrolyte. Specifically, glycolated naphthalene tetracarboxylicdiimide (gNDI) couples with vinylene and thiophene to give gNDI-V and gNDI-T, respectively. The hydrogen bonding functionalities are fused to the backbone to ensure a more coplanar backbone and much tighter π-π stacking of gNDI-V than gNDI-T, which is evidenced by density functional theory simulations and grazing-incidence wide-angle X-ray scattering. Importantly, these copolymers are fabricated as the active layer of the aqueous-based electrochromic devices and organic electrochemical transistors (OECTs). gNDI-V exhibits a larger electrochromic contrast (ΔT = 30%) and a higher coloration efficiency (1988 cm² C^-1) than gNDI-T owing to its more efficient ionic-electronic coupling. Moreover, gNDI-V gives the highest electron mobility (0.014 cm² V^-1 s^-1) and μC* (2.31 FV^-1 cm^-1 s^-1) reported to date for NDI-based copolymers in OECTs, attributed to the improved thin-film crystallinity and molecular packing promoted by hydrogen bonds. Overall, this work marks a remarkable advance in the n-type polymeric mixed conductors and the hydrogen bond functionalization strategy opens up an avenue to access desirable performance metrics for aqueous-based electrochemical devices.

Naphthalene diimide-based n-type small molecule organic mixed conductors for accumulation mode organic electrochemical transistors

Organic mixed ionic-electronic conductors (OMIECs), which transport both ionic and electronic charges, development are important for progressing bioelectronic and energy storage devices. The p-type OMIECs are extensively investigated and used in various applications, whereas the n-type ones lag far behind due to their moisture and air instability. Here, we report the synthesis of the novel n-type naphthalene diimide (NDI)-based small-molecule OMIECs for organic electrochemical transistors (OECTs). The electro-active NDI molecule with the linear ethylene glycol side chains is a promising candidate for n-type channel material to obtain accumulation mode OECTs. This NDI-based small-molecule OMIEC, gNDI-Br2, demonstrates ion permeability due to the attachment of the glycol side chains with optimized ionic-electronic conductions. OECT devices with gNDI-Br2 channel material displays excellent performance in water and ambient stability. OECTs fabricated with two different concentrations, 50 mg mL-1 and 100 mg mL-1 of gNDI-Br2 demonstrate a transconductance value of 344 ± 19.7 μS and 814 ± 124.2 μS with the mobility capacitance product (μC*) of 0.13 ± 0.03 F cm-1 V-1 s-1 and 0.23 ± 0.04 F cm-1 V-1 s-1, respectively. These results demonstrate the n-type OMIEC behaviour of the NDI-based small-molecule and its applicability as an OECT channel material.

Perovskite Solar Cell-Gated Organic Electrochemical Transistors for Flexible Photodetectors with Ultrahigh Sensitivity and Fast Response

Photodetectors (PDs) are the building block of various imaging and sensing applications. However, commercially available PDs based on crystalline inorganic semiconductors cannot meet the requirements of emerging wearable/implantable applications due to their rigidity and fragility, which creates the need for flexible devices. Here, a high-performance flexible PD is presented by gating an organic electrochemical transistor (OECT) with a perovskite solar cell. Due to the ultrahigh transconductance of the OECT, the device demonstrates a high gain of ≈10⁶ , a fast response time of 67 µs and an ultrahigh detectivity of 6.7 × 10¹⁷ Jones to light signals under a low working voltage (≤0.6 V). Thanks to the ultrahigh sensitivity and fast response, the device can track photoplethysmogram signals and peripheral oxygen saturation under ambient light and even provide contactless remote sensing, offering a low-power and convenient way for continuous vital signs monitoring. This work offers a novel strategy for realizing high-performance flexible PDs that are promising for low-power, user-friendly and wearable optoelectronics.

Hydration of a Side-Chain-Free n-Type Semiconducting Ladder Polymer Driven by Electrochemical Doping

We study the organic electrochemical transistor (OECT) performance of the ladder polymer poly(benzimidazobenzophenanthroline) (BBL) in an attempt to better understand how an apparently hydrophobic side-chain-free polymer is able to operate as an OECT with favorable redox kinetics in an aqueous environment. We examine two BBLs of different molecular masses from different sources. Regardless of molecular mass, both BBLs show significant film swelling during the initial reduction step. By combining electrochemical quartz crystal microbalance gravimetry, in-operando atomic force microscopy, and both ex-situ and in-operando grazing incidence wide-angle X-ray scattering (GIWAXS), we provide a detailed structural picture of the electrochemical charge injection process in BBL in the absence of any hydrophilic side-chains. Compared with ex-situ measurements, in-operando GIWAXS shows both more swelling upon electrochemical doping than has previously been recognized and less contraction upon dedoping. The data show that BBL films undergo an irreversible hydration driven by the initial electrochemical doping cycle with significant water retention and lamellar expansion that persists across subsequent oxidation/reduction cycles. This swelling creates a hydrophilic environment that facilitates the subsequent fast hydrated ion transport in the absence of the hydrophilic side-chains used in many other polymer systems. Due to its rigid ladder backbone and absence of hydrophilic side-chains, the primary BBL water uptake does not significantly degrade the crystalline order, and the original dehydrated, unswelled state can be recovered after drying. The combination of doping induced hydrophilicity and robust crystalline order leads to efficient ionic transport and good stability.