Vertical, electrolyte-gated organic transistors show continuous operation in the MA cm-2 regime and artificial synaptic behaviour

A Biomimetic Nociceptor Based on a Vertical Multigate, Multichannel Neuromorphic Transistor

Nociceptors, crucial sensory receptors within biological systems, are essential for survival in diverse and potentially hazardous environments. Efforts to replicate nociceptors through advanced electronic devices, such as memristors and neuromorphic transistors, have achieved limited success, capturing basic nociceptive functions while more advanced characteristics like various forms of central sensitization and analgesic effect remain out of reach. Here, we introduce a vertical multigate, multichannel electrolyte-gated transistor (Vm-EGT), designed to mimic nociceptors. Utilizing the hybrid mechanism combining electric-double-layer (EDL) with ion intercalation/deintercalation in EGTs, our approach successfully replicates peripheral sensitization and desensitization characteristics of nociceptors. The intricate multigate and multichannel design of the Vm-EGT enables the emulation of more advanced nociceptive functionalities, including central sensitization and analgesic effect. Furthermore, we demonstrate that by exploiting the inherent current-voltage relationship, the Vm-EGT can simulate these advanced nociceptive features and seamlessly transition between them. Integrating a Vm-EGT with a thermistor and a heating plate, we have developed an artificial thermal nociceptor that closely mirrors the sensory attributes of its biological counterpart. Our approach significantly advances the emulation of nociceptors, providing a basis for the development of sophisticated artificial sensory systems and intelligent robotics.

Crucial role of interfacial thermal dissipation in the operational stability of organic field-effect transistors

The operational stability becomes a key issue affecting the commercialization for organic field-effect transistors (OFETs). It is widely recognized to be closely related to the defects and traps at the interface between dielectric and organic semiconductors, but this understanding does not always effectively address operational instability, implying that the factors influencing the operational stability have not been fully understood. Here, we reveal that the self-heating effect is another crucial factor in operational stability. By using hexagonal boron nitride (hBN) to assist interfacial thermal dissipation, the dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) FETs exhibit high mobility of 14.18 cm2 V-1 s-1 and saturated power density up to 1.8 × 104 W cm-2. The OFET can operate at a power density of 1.06 × 104 W cm-2 for 30,000 s with negligible performance degradation, showing excellent operational stability under high power density. This work deepens the understanding on operational stability and develops an effective way for ultrahigh stable devices.

Electric-Field-Driven Localization of Molecular Nanowires in Wafer-Scale Nanogap Electrodes

As integrated circuits continue to scale toward the atomic limit, bottom-up processes, such as epitaxial growth, have come to feature prominently in their fabrication. At the same time, chemistry has developed highly tunable molecular semiconductors that can perform the functions of ultimately scaled circuit components. Hybrid techniques that integrate programmable structures comprising molecular components into devices however are sorely lacking. Here we demonstrate a wafer-scale process that directs the localization of a conductive polymer, Mw = 20 kg mol-1 polyaniline, from dilute solutions into 50 nm vertical nanogap device architectures using electric-field-driven self-assembly. The resulting metal-polymer-metal junctions were characterized by electron microscopy, Raman spectroscopy and transport measurements demonstrating that our technique is highly selective, assembling conductive polymers only in electrically activated nanogaps. Our results represent a step toward scalable hybrid nanoelectronics that seamlessly integrate established lithographic top-down fabrication with bottom-up synthesized molecular functional circuit components.

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.

Bio-Inspired Organic Synaptor with In Situ Ion-Doped Ultrathin Polyelectrolyte Containing Acetylcholine-Like Cation

An ion-based synaptic transistor (synaptor) is designed to emulate a biological synapse using controlled ion movements. However, developing a solid-state electrolyte that can facilitate ion movement while achieving large-scale integration remains challenging. Here, a bio-inspired organic synaptor (BioSyn) with an in situ ion-doped polyelectrolyte (i-IDOPE) is demonstrated. At the molecular scale, a polyelectrolyte containing the tert-amine cation, inspired by the neurotransmitter acetylcholine is synthesized using initiated chemical vapor deposition (iCVD) with in situ doping, a one-step vapor-phase deposition used to fabricate solid-state electrolytes. This method results in an ultrathin, but highly uniform and conformal solid-state electrolyte layer compatible with large-scale integration, a form that is not previously attainable. At a synapse scale, synapse functionality is replicated, including short-term and long-term synaptic plasticity (STSP and LTSP), along with a transformation from STSP to LTSP regulated by pre-synaptic voltage spikes. On a system scale, a reflex in a peripheral nervous system is mimicked by mounting the BioSyns on various substrates such as rigid glass, flexible polyethylene naphthalate, and stretchable poly(styrene-ethylene-butylene-styrene) for a decentralized processing unit. Finally, a classification accuracy of 90.6% is achieved through semi-empirical simulations of MNIST pattern recognition, incorporating the measured LTSP characteristics from the BioSyns.

First Demonstration of Yttria-Stabilized Hafnia-Based Long-Retention Solid-State Electrolyte-Gated Transistor for Human-Like Neuromorphic Computing

Electrolyte-gated transistors have strong potential for high-performance artificial synapses in neuromorphic bio-interfaces owing to their outstanding synaptic characteristics, low power consumption, and human-like mechanisms. However, the short retention time is a hurdle to overcome owing to the natural diffusion of protons. Here, a novel modulation technique of ionic conductivity is proposed with yttria-stabilized hafnia for the first time to enhance the retention characteristic of a solid-state electrolyte-gated transistor-based artificial synapse. With the optimization of the ionic conductivity in yttria-stabilized hafnia, a high retention time of over 300 s and remarkable synaptic characteristics are accomplished by regulating channel conductance with precise modulation of the strength of the proton-electron coupling intensity along the input signals. Furthermore, pattern recognition simulation is conducted based on the measured synaptic characteristics, exhibiting 94.41% of operation accuracy, which implies a promising solution for neuromorphic in-memory computing systems with a high operation accuracy and low power consumption.

All-Photolithography Fabrication of Ion-Gated Flexible Organic Transistor Array for Multimode Neuromorphic Computing

Organic ion-gated transistors (OIGTs) demonstrate commendable performance for versatile neuromorphic systems. However, due to the fragility of organic materials to organic solvents, efficient and reliable all-photolithography methods for scalable manufacturing of high-density OIGT arrays with multimode neuromorphic functions are still missing, especially when all active layers are patterned in high-density. Here, a flexible high-density (9662 devices per cm2) OIGT array with high yield and minimal device-to-device variation is fabricated by a modified all-photolithography method. The unencapsulated flexible array can withstand 1000 times' bending at a radius of 1 mm, and 3 months' storage test in air, without obvious performance degradation. More interesting, the OIGTs can be configured between volatile and nonvolatile modes, suitable for constructing reservoir computing systems to achieve high accuracy in classifying handwritten digits with low training costs. This work proposes a promising design of organic and flexible electronics for affordable neuromorphic systems, encompassing both array and algorithm aspects.

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.

Spiers Memorial Lecture: Challenges and prospects in organic photonics and electronics

While a substantial amount of research activity has been conducted in fields related to organic photonics and electronics, including the development of devices such as organic field-effect transistors, organic photovoltaics, and organic light-emitting diodes for applications encompassing organic thermoelectrics, organic batteries, excitonic organic materials for photochemical and optoelectronic applications, and organic thermoelectrics, this perspective review will primarily concentrate on the emerging and rapidly expanding domain of organic bioelectronics and neuromorphics. Here we present the most recent research findings on organic transistors capable of sensing biological biomarkers down at the single-molecule level (i.e., oncoproteins, genomes, etc.) for the early diagnosis of pathological states and to mimic biological synapses, paving the way to neuromorphic applications that surpass the limitations of the traditional von Neumann computing architecture. Both organic bioelectronics and neuromorphics exhibit several challenges but will revolutionize human life, considering the development of artificial synapses to counteract neurodegenerative disorders and the development of ultrasensitive biosensors for the early diagnosis of cancer to prevent its development. Moreover, organic bioelectronics for sensing applications have also triggered the development of several wearable, flexible and stretchable biodevices for continuous biomarker monitoring.

Monolithically integrated high-density vertical organic electrochemical transistor arrays and complementary circuits

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

An Amorphous Native Oxide Shell for High Bias-Stress Stability Nanowire Synaptic Transistor

The inhomogeneous native oxide shells on the surfaces of III-V group semiconductors typically yield inferior and unstable electrical properties metrics, challenging the development of next-generation integrated circuits. Herein, the native GaOx shells are profitably utilized by a simple in-situ thermal annealing process to achieve high-performance GaSb nanowires (NWs) field-effect-transistors (FETs) with excellent bias-stress stability and synaptic behaviors. By an optimal annealing time of 5 min, the as-constructed GaSb NW FET demonstrates excellent stability with a minimal shift of transfer curve (ΔVth ≈ 0.54 V) under a 60 min gate bias, which is far more stable than that of pristine GaSb NW FET (ΔVth ≈ 8.2 V). When the high bias-stress stability NW FET is used as the chargeable-dielectric free synaptic transistor, the typical synaptic behaviors, such as short-term plasticity, long-term plasticity, spike-time-dependent plasticity, and reliable learning stability are demonstrated successfully through the voltage tests. The mobile oxygen ion in the native GaOx shell strongly offsets the trapping states and leads to enhanced bias-stress stability and charge retention capability for synaptic behaviors. This work provides a new way of utilizing the native oxide shell to realize stable FET for chargeable-dielectric free neuromorphic computing systems.

Stochastic Electrical Detection of Single Ion-Gated Semiconducting Polymers

Semiconducting polymer chains constitute the building blocks for a wide range of electronic materials and devices. However, most of their electrical characteristics at the single-molecule level have received little attention. Elucidating these properties can help understanding performance limits and enable new applications. Here, coupled ionic-electronic charge transport is exploited to measure the quasi-1D electrical current through long single conjugated polymer chains as they form transient contacts with electrodes separated by ≈10 nm. Fluctuations between internal conformations of the individual polymers are resolved as abrupt, multilevel switches in the electrical current. This behavior is consistent with the theoretical simulations based on the worm-like-chain (WLC) model for semiflexible polymers. In addition to probing the intrinsic properties of single semiconducting polymer chains, the results provide an unprecedented window into the dynamics of random-coil polymers and enable the use of semiconducting polymers as electrical labels for single-molecule (bio)sensing assays.

Effect of ionic conductivity of electrolyte on printed planar and vertical organic electrochemical transistors

Conducting polymers with mixed electronic/ionic transport are attracting a great deal of interest for applications in organic electrochemical transistors (OECTs). Ions play a crucial role in OECT performance. The concentration and mobility of ions in the electrolyte influence the current flow in the OECT and its transconductance. This study examines the electrochemical properties and ionic conductivity of two semi-solid electrolytes, iongels, and organogels, with diverse ionic species and properties. Our results indicate that the organogels exhibited higher ionic conductivities than the iongels. Furthermore, the geometry of OECTs plays an important role in determining their transconductance. Thus, this study employs a novel approach for fabricating vertical-configuration OECTs with significantly shorter channel lengths planar devices. This is achieved through a printing method that offers advantages, such as design versatility, scalability, expedited production time, and reduced cost relative to traditional microfabrication methods. The transconductance values obtained for the vertical OECTs were significantly (approximately 50 times) higher than those of the planar devices because of their shorter channel lengths. Finally, the impact of different gating media on the performance of both planar and vertical OECTs was studied, and devices gated by organogels demonstrated improved transconductance and switching speed (almost two times higher) than those gated by iongels.

A low-power vertical dual-gate neurotransistor with short-term memory for high energy-efficient neuromorphic computing

Neuromorphic computing aims to emulate the computing processes of the brain by replicating the functions of biological neural networks using electronic counterparts. One promising approach is dendritic computing, which takes inspiration from the multi-dendritic branch structure of neurons to enhance the processing capability of artificial neural networks. While there has been a recent surge of interest in implementing dendritic computing using emerging devices, achieving artificial dendrites with throughputs and energy efficiency comparable to those of the human brain has proven challenging. In this study, we report on the development of a compact and low-power neurotransistor based on a vertical dual-gate electrolyte-gated transistor (EGT) with short-term memory characteristics, a 30 nm channel length, a record-low read power of ~3.16 fW and a biology-comparable read energy of ~30 fJ. Leveraging this neurotransistor, we demonstrate dendrite integration as well as digital and analog dendritic computing for coincidence detection. We also showcase the potential of neurotransistors in realizing advanced brain-like functions by developing a hardware neural network and demonstrating bio-inspired sound localization. Our results suggest that the neurotransistor-based approach may pave the way for next-generation neuromorphic computing with energy efficiency on par with those of the brain.

Organic Electrochemical Transistor Based on Hydrophobic Polymer Tuned by Ionic Gels

Hydrophobic conjugated polymers have poor ionic transport property, so hydrophilic side chains are often grafted for their application as organic electrochemical transistors (OECTs). However, this modification lowers their charge transport ability. Here, an ionic gel interfacial layer is applied to improve the ionic transport while retaining the charge transport ability of the polymers. By using the ionic gels comprising gel matrix and ionic liquids as the interfacial layers, the hydrophobic polymer achieves the OECT feature with high transconductance, low threshold voltage, high current on/off ratio, short switching time, and high operational stability. The working mechanism is also revealed. Moreover, the OECT performance can be tuned by varying the types and ratios of ionic gels. With the proposed ionic gel strategy, OECTs can be effectively realized with hydrophobic conjugated polymers.

Emerging Iontronic Neural Devices for Neuromorphic Sensory Computing

Living organisms have a very mysterious and powerful sensory computing system based on ion activity. Interestingly, studies on iontronic devices in the past few years have proposed a promising platform for simulating the sensing and computing functions of living organisms, because: 1) iontronic devices can generate, store, and transmit a variety of signals by adjusting the concentration and spatiotemporal distribution of ions, which analogs to how the brain performs intelligent functions by alternating ion flux and polarization; 2) through ionic-electronic coupling, iontronic devices can bridge the biosystem with electronics and offer profound implications for soft electronics; 3) with the diversity of ions, iontronic devices can be designed to recognize specific ions or molecules by customizing the charge selectivity, and the ionic conductivity and capacitance can be adjusted to respond to external stimuli for a variety of sensing schemes, which can be more difficult for electron-based devices. This review provides a comprehensive overview of emerging neuromorphic sensory computing by iontronic devices, highlighting representative concepts of both low-level and high-level sensory computing and introducing important material and device breakthroughs. Moreover, iontronic devices as a means of neuromorphic sensing and computing are discussed regarding the pending challenges and future directions.

Molecular Electronics: Creating and Bridging Molecular Junctions and Promoting Its Commercialization

Molecular electronics is driven by the dream of expanding Moore's law to the molecular level for next-generation electronics through incorporating individual or ensemble molecules into electronic circuits. For nearly 50 years, numerous efforts have been made to explore the intrinsic properties of molecules and develop diverse fascinating molecular electronic devices with the desired functionalities. The flourishing of molecular electronics is inseparable from the development of various elegant methodologies for creating nanogap electrodes and bridging the nanogap with molecules. This review first focuses on the techniques for making lateral and vertical nanogap electrodes by breaking, narrowing, and fixed modes, and highlights their capabilities, applications, merits, and shortcomings. After summarizing the approaches of growing single molecules or molecular layers on the electrodes, the methods of constructing a complete molecular circuit are comprehensively grouped into three categories: 1) directly bridging one-molecule-electrode component with another electrode, 2) physically bridging two-molecule-electrode components, and 3) chemically bridging two-molecule-electrode components. Finally, the current state of molecular circuit integration and commercialization is discussed and perspectives are provided, hoping to encourage the community to accelerate the realization of fully scalable molecular electronics for a new era of integrated microsystems and applications.

In Liquido Computation with Electrochemical Transistors and Mixed Conductors for Intelligent Bioelectronics

Next-generation implantable computational devices require long-term-stable electronic components capable of operating in, and interacting with, electrolytic surroundings without being damaged. Organic electrochemical transistors (OECTs) emerged as fitting candidates. However, while single devices feature impressive figures of merit, integrated circuits (ICs) immersed in common electrolytes are hard to realize using electrochemical transistors, and there is no clear path forward for optimal top-down circuit design and high-density integration. The simple observation that two OECTs immersed in the same electrolytic medium will inevitably interact hampers their implementation in complex circuitry. The electrolyte's ionic conductivity connects all the devices in the liquid, producing unwanted and often unforeseeable dynamics. Minimizing or harnessing this crosstalk has been the focus of very recent studies. Herein, the main challenges, trends, and opportunities for realizing OECT-based circuitry in a liquid environment that could circumnavigate the hard limits of engineering and human physiology, are discussed. The most successful approaches in autonomous bioelectronics and information processing are analyzed. Elaborating on the strategies to circumvent and harness device crosstalk proves that platforms capable of complex computation and even machine learning (ML) can be realized in liquido using mixed ionic-electronic conductors (OMIECs).

Biological function simulation in neuromorphic devices: from synapse and neuron to behavior

As the boom of data storage and processing, brain-inspired computing provides an effective approach to solve the current problem. Various emerging materials and devices have been reported to promote the development of neuromorphic computing. Thereinto, the neuromorphic device represented by memristor has attracted extensive research due to its outstanding property to emulate the brain's functions from synaptic plasticity, sensory-memory neurons to some intelligent behaviors of living creatures. Herein, we mainly review the progress of these brain functions mimicked by neuromorphic devices, concentrating on synapse (i.e. various synaptic plasticity trigger by electricity and/or light), neurons (including the various sensory nervous system) and intelligent behaviors (such as conditioned reflex represented by Pavlov's dog experiment). Finally, some challenges and prospects related to neuromorphic devices are presented.

A multilevel electrolyte-gated artificial synapse based on ruthenium-doped cobalt ferrite

Synaptic devices that emulate synchronized memory and processing are considered the core components of neuromorphic computing systems for the low-power implementation of artificial intelligence. In this regard, electrolyte-gated transistors (EGTs) have gained much scientific attention, having a similar working mechanism as the biological synapses. Moreover, compared to a traditional solid-state gate dielectric, the liquid dielectric has the key advantage of inducing extremely large modulation of carrier density while overcoming the problem of electric pinholes, that typically occurs when using large-area films gated through ultra-thin solid dielectrics. Herein we demonstrate a three-terminal synaptic transistor based on ruthenium-doped cobalt ferrite (CRFO) thin films by electrolyte gating. In the CRFO-based EGT, we have obtained multilevel non-volatile conductance states for analog computing and high-density storage. Furthermore, the proposed synaptic transistor exhibited essential synaptic behavior, including spike amplitude-dependent plasticity, spike duration-dependent plasticity, long-term potentiation, and long-term depression successfully by applying electrical pulses. This study can motivate the development of advanced neuromorphic devices that leverage simultaneous modulation of electrical and magnetic properties in the same device and show a new direction to synaptic electronics.

2D metal-organic frameworks for ultraflexible electrochemical transistors with high transconductance and fast response speeds

Electrochemical transistors (ECTs) have shown broad applications in bioelectronics and neuromorphic devices due to their high transconductance, low working voltage, and versatile device design. To further improve the device performance, semiconductor materials with both high carrier mobilities and large capacitances in electrolytes are needed. Here, we demonstrate ECTs based on highly oriented two-dimensional conjugated metal-organic frameworks (2D c-MOFs). The ion-conductive vertical nanopores formed within the 2D c-MOFs films lead to the most convenient ion transfer in the bulk and high volumetric capacitance, endowing the devices with fast speeds and ultrahigh transconductance. Ultraflexible device arrays are successfully used for wearable on-skin recording of electrocardiogram (ECG) signals along different directions, which can provide various waveforms comparable with those of multilead ECG measurement systems for monitoring heart conditions. These results indicate that 2D c-MOFs are excellent semiconductor materials for high-performance ECTs with promising applications in flexible and wearable electronics.

Vertical organic electrochemical transistors for complementary circuits

Organic electrochemical transistors (OECTs) and OECT-based circuitry offer great potential in bioelectronics, wearable electronics and artificial neuromorphic electronics because of their exceptionally low driving voltages (<1 V), low power consumption (<1 µW), high transconductances (>10 mS) and biocompatibility^1-5. However, the successful realization of critical complementary logic OECTs is currently limited by temporal and/or operational instability, slow redox processes and/or switching, incompatibility with high-density monolithic integration and inferior n-type OECT performance^6-8. Here we demonstrate p- and n-type vertical OECTs with balanced and ultra-high performance by blending redox-active semiconducting polymers with a redox-inactive photocurable and/or photopatternable polymer to form an ion-permeable semiconducting channel, implemented in a simple, scalable vertical architecture that has a dense, impermeable top contact. Footprint current densities exceeding 1 kA cm^-2 at less than ±0.7 V, transconductances of 0.2-0.4 S, short transient times of less than 1 ms and ultra-stable switching (>50,000 cycles) are achieved in, to our knowledge, the first vertically stacked complementary vertical OECT logic circuits. This architecture opens many possibilities for fundamental studies of organic semiconductor redox chemistry and physics in nanoscopically confined spaces, without macroscopic electrolyte contact, as well as wearable and implantable device applications.

An Electrochemical-Electret Coupled Organic Synapse with Single-Polarity Driven Reversible Facilitation-to-Depression Switching

Neuromorphic engineering and artificial intelligence demands hardware elements that emulates synapse algorithms. During the last decade electrolyte-gated organic conjugated materials have been explored as a platform for artificial synapses for neuromorphic computing. Unlike biological synapses, in current devices the synaptic facilitation and depression are triggered by voltages with opposite polarity. To enhance the reliability and simplify the operation of the synapse without lowering its sophisticated functionality, here, an electrochemical-electret coupled organic synapse (EECS) possessing a reversible facilitation-to-depression switch, is devised. Electret charging counterbalances channel conductance changes due to electrochemical doping, inducing depression without inverting the gate polarity. Overall, EECS functions as a threshold-controlled synaptic switch ruled by its amplitude-dependent, dual-modal operation, which can well emulate information storage and erase as in real synapses. By varying the energy level offset between the channel material and the electret, the EECS's transition threshold can be adjusted for specific applications, e.g., imparting additional light responsiveness to the device operation. The novel device architecture represents a major step forward in the development of artificial organic synapses with increased functional complexity and it opens new perspectives toward the fabrication of abiotic neural networks with higher reliability, efficiency, and endurance.

Triboelectric Potential Powered High-Performance Organic Transistor Array

Triboelectric potential gated transistors have inspired various applications toward mechanical behavior controlled logic circuits, multifunctional sensors, artificial sensory neurons, etc. Their rapid development urgently calls for high-performance devices and corresponding figure of merits to standardize the tribotronic gating properties. Organic semiconductors paired with solution processability promise low-cost manufacture of high-performance tribotronic transistor devices/arrays. Here, we demonstrate a record high-performance tribotronic transistor array composed of an integrated triboelectric nanogenerator (TENG) and a large-area device array of C8-BTBT-PS transistors. The working mechanism of effective triboelectric potential gating is elaborately explained from the aspect of conjugated energy bands of the contact-electrification mediums and organic semiconductors. Driven by the triboelectric potential, the tribotronic transistor shows superior properties of record high current on/off ratios (>10⁸), a steep subthreshold swing (29.89 μm/dec), high stability, and excellent reproducibility. Moreover, tribotronic logic devices modulated by mechanical displacement have also been demonstrated with good stability and a high gain of 1260 V/mm. The demonstrated large-area tribotronic transistor array of organic semiconductor exhibits record high performance and offers an effective R&D platform for mechano-driven electronic terminals, interactive intelligent system, artificial robotic skin, etc.

Magnetic-field-controlled counterion migration within polyionic liquid micropores enables nano-energy harvest

Efficient separation of positive and negative charges is essential for developing high-performance nanogenerators. In this article, we describe a method that was not previously demonstrated to separate charges which enables us to fabricate a magnetic energy harvesting device. The magnetic field induces the migration of the mobile magnetic counterions (Dy(NO₃)₄^-) which establishes anion gradients within a layer of polyionic liquid micropores (PLM). The PLM is covalently cross-linked on which the positive charges are fixed on the matrix, that is, immobile. In a device with a structure of Au/dielectric//mag-PLM//dielectric/Au, the charge gradient is subsequently transformed into the output voltage through electrostatic induction. Removing the magnetic field leads to the backflow of magnetic anions which produces a voltage with a similar magnitude but reversed polarity. The parameters in fabricating the magnetic PLM such as photoinitiator concentration, UV irradiation time, water treatment time, and temperature are found to dramatically influence the size of micropores and the effective concentration of magnetic anions. Under optimized conditions, an output voltage with an amplitude of approximately 4 V is finally achieved. We expect this new method could find practical applications in further improving the output performance.

Organic Neuroelectronics: From Neural Interfaces to Neuroprosthetics

Requirements and recent advances in research on organic neuroelectronics are outlined herein. Neuroelectronics such as neural interfaces and neuroprosthetics provide a promising approach to diagnose and treat neurological diseases. However, the current neural interfaces are rigid and not biocompatible, so they induce an immune response and deterioration of neural signal transmission. Organic materials are promising candidates for neural interfaces, due to their mechanical softness, excellent electrochemical properties, and biocompatibility. Also, organic nervetronics, which mimics functional properties of the biological nerve system, is being developed to overcome the limitations of the complex and energy-consuming conventional neuroprosthetics that limit long-term implantation and daily-life usage. Examples of organic materials for neural interfaces and neural signal recordings are reviewed, recent advances of organic nervetronics that use organic artificial synapses are highlighted, and then further requirements for neuroprosthetics are discussed. Finally, the future challenges that must be overcome to achieve ideal organic neuroelectronics for next-generation neuroprosthetics are discussed.

Non-von Neumann multi-input spike signal processing enabled by an artificial synaptic multiplexer

Multiplexing is essential for technologies that require processing of a large amount of information in real time. Here, we present an artificial synaptic multiplexing unit capable of realizing parallel multi-input control system. Ion gel was used as a dielectric layer of the artificial synaptic multiplexing unit because of its ionic property, allowing multigating for parallel input. A closed-loop control system that enables multi-input-based feedback for actuator bending control was realized by incorporating an ion gel-based artificial synaptic multiplexing unit, an actuator, and a bending angle sensor. The proposed multi-input control system could simultaneously process input and feedback signals, offering a breakthrough in industries in which the processing of vast amounts of streaming data is essential.

MXene based saturation organic vertical photoelectric transistors with low subthreshold swing

Vertical transistors have attracted enormous attention in the next-generation electronic devices due to their high working frequency, low operation voltage and large current density, while a major scientific and technological challenge for high performance vertical transistor is to find suitable source electrode. Herein, an MXene material, Ti₃C₂T_x, is introduced as source electrode of organic vertical transistors. The porous MXene films take the advantage of both partially shielding effect of graphene and the direct modulation of the Schottky barrier at the mesh electrode, which significantly enhances the ability of gate modulation and reduces the subthreshold swing to 73 mV/dec. More importantly, the saturation of output current which is essential for all transistor-based applications but remains a great challenge for vertical transistors, is easily achieved in our device due to the ultra-thin thickness and native oxidation of MXene, as verified by finite-element simulations. Finally, our device also possesses great potential for being used as wide-spectrum photodetector with fast response speed without complex material and structure design. This work demonstrates that MXene as source electrode offers plenty of opportunities for high performance vertical transistors and photoelectric devices.

The modulation of Schottky barrier, which dominates the carrier injection in vertical organic field-effect transistors, strongly depends on the source electrode. Here, Chen et al. utilize MXene as a source electrode, achieving a subthreshold swing down to 73 mv/dec and a large gate control ability.

Vertically stacked, low-voltage organic ternary logic circuits including nonvolatile floating-gate memory transistors

Multi-valued logic (MVL) circuits based on heterojunction transistor (HTR) have emerged as an effective strategy for high-density information processing without increasing the circuit complexity. Herein, an organic ternary logic inverter (T-inverter) is demonstrated, where a nonvolatile floating-gate flash memory is employed to control the channel conductance systematically, thus realizing the stabilized T-inverter operation. The 3-dimensional (3D) T-inverter is fabricated in a vertically stacked form based on all-dry processes, which enables the high-density integration with high device uniformity. In the flash memory, ultrathin polymer dielectrics are utilized to reduce the programming/erasing voltage as well as operating voltage. With the optimum programming state, the 3D T-inverter fulfills all the important requirements such as full-swing operation, optimum intermediate logic value (~V_DD/2), high DC gain exceeding 20 V/V as well as low-voltage operation (< 5 V). The organic flash memory exhibits long retention characteristics (current change less than 10% after 10⁴s), leading to the long-term stability of the 3D T-inverter. We believe the 3D T-inverter employing flash memory developed in this study can provide a useful insight to achieve high-performance MVL circuits.

High-density information processing without increasing the circuit complexity is highly desired in electronics. Here, Im et al. demonstrate a low-voltage organic ternary logic circuit vertically integrated with the nonvolatile flash memory, increasing the information density by a factor of 3.

A robust vertical nanoscaffold for recyclable, paintable, and flexible light-emitting devices

Organic light-emitting devices are key components for emerging opto- and nanoelectronics applications including health monitoring and smart displays. Here, we report a foldable inverted polymer light-emitting diode (iPLED) based on a self-suspended asymmetrical vertical nanoscaffold replacing the conventional sandwich-like structured LEDs. Our empty vertical-yet-open nanoscaffold exhibits excellent mechanical robustness, proven by unaltered leakage current when applying 1000 cycles of 40-kilopascal pressure loading/unloading, sonication, and folding, with the corresponding iPLEDs displaying a brightness as high as 2300 candela per square meter. By using photolithography and brush painting, arbitrary emitting patterns can be generated via a noninvasive and mask-free process with individual pixel resolution of 10 μm. Our vertical nanoscaffold iPLED can be supported on flexible polyimide foils and be recycled multiple times by washing and refilling with a different conjugated polymer capable of emitting light of different color. This technology combines the traits required for the next generation of high-resolution flexible displays and multifunctional optoelectronics.

Nanoscopic Electrolyte-Gated Vertical Organic Transistors with Low Operation Voltage and Five Orders of Magnitude Switching Range for Neuromorphic Systems

Electrolyte-gated organic transistors (EGOTs) are promising candidates as a new class of neuromorphic devices in hardware-based artificial neural networks that can outperform their complementary metal oxide semiconductor (CMOS) counterparts regarding processing speed and energy consumption. Several ways in which to implement such networks exist, two prominent methods of which can be implemented by nanoscopic vertical EGOTs, as we show here. First, nanoscopic vertical electrolyte-gated transistors with a donor-acceptor diketopyrrolopyrrole-terthiophene polymer as an active material can be used to reversibly switch the channel conductivity over five orders of magnitude (3.8 nS to 392 μS) and perform switching at low operation voltages down to -1 mV. Second, nanoscopic EGOTs can also mimic fundamental synaptic functions, and we show an interconnection of up to three transistors, highlighting the possibility to emulate biological nerve cells.

Electrolyte-Gated Vertical Synapse Array based on Van Der Waals Heterostructure for Parallel Computing

Recently, three-terminal synaptic devices, which separate read and write terminals, have attracted significant attention because they enable nondestructive read-out and parallel-access for updating synaptic weights. However, owing to their structural features, it is difficult to address the relatively high device density compared with two-terminal synaptic devices. In this study, a vertical synaptic device featuring remotely controllable weight updates via e-field-dependent movement of mobile ions in the ion-gel layer is developed. This synaptic device successfully demonstrates all essential synaptic characteristics, such as excitatory/inhibitory postsynaptic current (E/IPSC), paired-pulse facilitation (PPF), and long-term potentiation/depression (LTP/D) by electrical measurements, and exhibits competitive LTP/D characteristics with a dynamic range (G_max /G_min ) of 31.3, and asymmetry (AS) of 8.56. The stability of the LTP/D characteristics is also verified through repeated measurements over 50 cycles; the relative standard deviations (RSDs) of G_max /G_min and AS are calculated as 1.65% and 0.25%, respectively. These excellent synaptic properties enable a recognition rate of ≈99% in the training and inference tasks for acoustic and emotional information patterns. This study is expected to be an important foundation for the realization of future parallel computing networks for energy-efficient and high-speed data processing.

A Critical Outlook for the Pursuit of Lower Contact Resistance in Organic Transistors

To take full advantage of recent and anticipated improvements in the performance of organic semiconductors employed in organic transistors, the high contact resistance arising at the interfaces between the organic semiconductor and the source and drain contacts must be reduced significantly. To date, only a small portion of the accumulated research on organic thin-film transistors (TFTs) has reported channel-width-normalized contact resistances below 100 Ωcm, well above what is regularly demonstrated in transistors based on inorganic semiconductors. A closer look at these cases and the relevant literature strongly suggests that the most significant factor leading to the lowest contact resistances in organic TFTs so far has been the control of the thin-film morphology of the organic semiconductor. By contrast, approaches aimed at increasing the charge-carrier density and/or reducing the intrinsic Schottky barrier height have so far played a relatively minor role in achieving the lowest contact resistances. Herein, the possible explanations for these observations are explored, including the prevalence of Fermi-level pinning and the difficulties in forming optimized interfaces with organic semiconductors. An overview of the research on these topics is provided, and potential device-engineering solutions are discussed based on recent advancements in the theoretical and experimental work on both organic and inorganic semiconductors.

Stimuli-Responsive Memristive Materials for Artificial Synapses and Neuromorphic Computing

Neuromorphic computing holds promise for building next-generation intelligent systems in a more energy-efficient way than the conventional von Neumann computing architecture. Memristive hardware, which mimics biological neurons and synapses, offers high-speed operation and low power consumption, enabling energy- and area-efficient, brain-inspired computing. Here, recent advances in memristive materials and strategies that emulate synaptic functions for neuromorphic computing are highlighted. The working principles and characteristics of biological neurons and synapses, which can be mimicked by memristive devices, are presented. Besides device structures and operation with different external stimuli such as electric, magnetic, and optical fields, how memristive materials with a rich variety of underlying physical mechanisms can allow fast, reliable, and low-power neuromorphic applications is also discussed. Finally, device requirements are examined and a perspective on challenges in developing memristive materials for device engineering and computing science is given.

Nanomaterials and their applications on bio-inspired wearable electronics

Wearable electronics featuring conformal attachment, sensitive perception and intellectual signal processing have made significant progress in recent years. However, when compared with living organisms, artificial sensory devices showed undeniable bulky shape, poor adaptability, and large energy consumption. To make up for the deficiencies, biological examples provide inspirations of novel designs and practical applications. In the field of biomimetics, nanomaterials from nanoparticles to layered two-dimensional materials are actively involved due to their outstanding physicochemical properties and nanoscale configurability. This review focuses on nanomaterials related to wearable electronics through bioinspired approaches on three different levels, interfacial packaging, sensory structure, and signal processing, which comprehensively guided recent progress of wearable devices in leveraging both nanomaterial superiorities and biorealistic functionalities. In addition, opinions on potential development trend are proposed aiming at implementing bioinspired electronics in multifunctional portable sensors, health monitoring, and intelligent prosthetics.

Flexible Artificial Synapses with a Biocompatible Maltose-Ascorbic Acid Electrolyte Gate for Neuromorphic Computing

As constructing hardware technology is widely regarded as an important step toward realizing brain-like computers and artificial intelligence systems, the development of artificial synaptic electronics that can simulate biological synaptic functions is an emerging research field. Among the various types of artificial synapses, synaptic transistors using an electrolyte as the gate electrode have been implemented as the high capacitance of the electrolyte increases the driving current and lowers operating voltages. Here, transistors using maltose-ascorbic acid as the proton-conducting electrolyte are proposed. A novel electrolyte composed of maltose and ascorbic acid, both of which are biocompatible, enables the migration of protons. This allows the channel conductance of the transistors to be modulated with the gate input pulse voltage, and fundamental synaptic functions including excitatory postsynaptic current, paired-pulse facilitation, long-term potentiation, and long-term depression can be successfully emulated. Furthermore, the maltose-ascorbic acid electrolyte (MAE)-gated synaptic transistors exhibit high mechanical endurance, with near-linear conductivity modulation and repeatability after 1000 bending cycles under a curvature radius of 5 mm. Benefitting from its excellent biodegradability and biocompatibility, the proposed MAE has potential applications in environmentally friendly, economical, and high-performance neuromorphic electronics, which can be further applied to dermal electronics and implantable electronics in the future.

Ultrahigh-Gain Organic Electrochemical Transistor Chemosensors Based on Self-Curled Nanomembranes

Organic electrochemical transistors (OECTs) are technologically relevant devices presenting high susceptibility to physical stimulus, chemical functionalization, and shape changes-jointly to versatility and low production costs. The OECT capability of liquid-gating addresses both electrochemical sensing and signal amplification within a single integrated device unit. However, given the organic semiconductor time-consuming doping process and their usual low field-effect mobility, OECTs are frequently considered low-end category devices. Toward high-performance OECTs, microtubular electrochemical devices based on strain-engineering are presented here by taking advantage of the exclusive shape features of self-curled nanomembranes. Such novel OECTs outperform the state-of-the-art organic liquid-gated transistors, reaching lower operating voltage, improved ion doping, and a signal amplification with a >10⁴  intrinsic gain. The multipurpose OECT concept is validated with different electrolytes and distinct nanometer-thick molecular films, namely, phthalocyanine and thiophene derivatives. The OECTs are also applied as transducers to detect a biomarker related to neurological diseases, the neurotransmitter dopamine. The self-curled OECTs update the premises of electrochemical energy conversion in liquid-gated transistors, yielding a substantial performance improvement and new chemical sensing capabilities within picoliter sampling volumes.

High-Performance Vertical Organic Transistors of Sub-5 nm Channel Length

Miniaturization of electronic circuits increases their overall performance. So far, electronics based on organic semiconductors has not played an important role in the miniaturization race. Here, we show the fabrication of liquid electrolyte gated vertical organic field effect transistors with channel lengths down to 2.4 nm. These ultrashort channel lengths are enabled by using insulating hexagonal boron nitride with atomically precise thickness and flatness as a spacer separating the vertically aligned source and drain electrodes. The transistors reveal promising electrical characteristics with output current densities of up to 2.95 MA cm^-2 at -0.4 V bias, on-off ratios of up to 10⁶, a steep subthreshold swing of down to 65 mV dec^-1 and a transconductance of up to 714 S m^-1. Realizing channel lengths in the sub-5 nm regime and operation voltages down to 100 μV proves the potential of organic semiconductors for future highly integrated or low power electronics.

Emerging Nanopixel Light-Emitting Displays: Significance, Challenges, and Prospects

The requirement for increased resolution has created the concept of displays with nanoscale pixels; that is, each subpixel consists of multiple or even a single nanolight source, which is considered the ultimate light source for light field, near-eye, and implantable displays. However, related research is still at an early stage, and further insights into this future display concept should be provided. In this Perspective, we provide our proposed term for this future display, namely, nanopixel light-emitting display (NLED). We present an overview of nanolight-emitting diodes, which are considered the core component of NLEDs. Then, a roadmap to realize NLEDs from the view of material design is provided. Finally, we introduce our proposed operation mode (nonelectrical contact and noncarrier injection mode) for NLEDs and recommend possible nanopixel-level drive approaches. We hope that this Perspective will be helpful in designing innovative display technologies.

Organic synaptic devices based on ionic gel with reduced leakage current

In this work, we presented a solid-state hybrid electrolyte dielectric film fabricated by a facile solution process, composed of ionic liquid and high-k polymers for leakage current reduction. With ions involved in the dielectric, the organic transistor can be operated under low voltage, and some essential synaptic behaviors were successfully simulated by the electrostatic coupling effect for building neuromorphic computing systems.

Solution-processed electronics for artificial synapses

Artificial synaptic devices and systems have become hot topics due to parallel computing, high plasticity, integration of storage, and processing to meet the challenges of the traditional Von Neumann computers. Currently, two-terminal memristors and three-terminal transistors have been mainly developed for high-density storage with high switching speed and high reliability because of the adjustable resistivity, controllable ion migration, and abundant choices of functional materials and fabrication processes. To achieve the low-cost, large-scale, and easy-process fabrication, solution-processed techniques have been extensively employed to develop synaptic electronics towards flexible and highly integrated three-dimensional (3D) neural networks. Herein, we have summarized and discussed solution-processed techniques in the fabrication of two-terminal memristors and three-terminal transistors for the application of artificial synaptic electronics mainly reported in the recent five years from the view of fabrication processes, functional materials, electronic operating mechanisms, and system applications. Furthermore, the challenges and prospects were discussed in depth to promote solution-processed techniques in the future development of artificial synapse with high performance and high integration.

Modern bio and chemical sensors and neuromorphic devices based on organic semiconductors

This review summarizes and highlights the current state-of-the-art of research on chemical sensors and biosensors in liquid environment and neuromorphic devices based on electrolyte-gated organic transistors with the active semiconductor layer of organic π-conjugated materials (small molecules, oligomers and polymers). The architecture and principles of operation of electrolyte-gated organic transistors and the main advantages and drawbacks of these devices are considered in detail. The criteria for the selection of organic semiconductors for these devices are presented. The causes of degradation of semiconductor layers and ways of their elimination are discussed. Examples of the use of electrolyte-gated organic transistors as bio and chemical sensors, artificial synapses and computing devices are given.

The bibliography includes 132 references.

Vertical organic permeable dual-base transistors for logic circuits

The main advantage of organic transistors with dual gates/bases is that the threshold voltages can be set as a function of the applied second gate/base bias, which is crucial for the application in logic gates and integrated circuits. However, incorporating a dual gate/base structure into an ultra-short channel vertical architecture represents a substantial challenge. Here, we realize a device concept of vertical organic permeable dual-base transistors, where the dual base electrodes can be used to tune the threshold voltages and change the on-currents. The detailed operation mechanisms are investigated by calibrated TCAD simulations. Finally, power-efficient logic circuits, e.g. inverter, NAND/AND computation functions are demonstrated with one single device operating at supply voltages of <2.0 V. We believe that this work offers a compact and technologically simple hardware platform with excellent application potential for vertical-channel organic transistors in complex logic circuits.

The development of vertical organic transistors with controllable threshold voltage is highly desirable for integrated circuit-based displays and sensors. Here, the authors report vertical organic permeable dual-based transistors with independently tunable on-currents and threshold voltages.

Vertical organic synapse expandable to 3D crossbar array

Recently, three-terminal synaptic devices have attracted considerable attention owing to their nondestructive weight-update behavior, which is attributed to the completely separated terminals for reading and writing. However, the structural limitations of these devices, such as a low array density and complex line design, are predicted to result in low processing speeds and high energy consumption of the entire system. Here, we propose a vertical three-terminal synapse featuring a remote weight update via ion gel, which is also extendable to a crossbar array structure. This synaptic device exhibits excellent synaptic characteristics, which are achieved via precise control of ion penetration onto the vertical channel through the weight-control terminal. Especially, the applicability of the developed vertical organic synapse array to neuromorphic computing is demonstrated using a simple crossbar synapse array. The proposed synaptic device technology is expected to be an important steppingstone to the development of high-performance and high-density neural networks.

The Design of 3D-Interface Architecture in an Ultralow-Power, Electrospun Single-Fiber Synaptic Transistor for Neuromorphic Computing

Synaptic electronics is a new technology for developing functional electronic devices that can mimic the structure and functions of biological counterparts. It has broad application prospects in wearable computing chips, human-machine interfaces, and neuron prostheses. These types of applications require synaptic devices with ultralow energy consumption as the effective energy supply for wearable electronics, which is still very difficult. Here, artificial synapse emulation is demonstrated by solid-ion gated organic field-effect transistors (OFETs) with a 3D-interface conducting channel for ultralow-power synaptic simulation. The basic features of the artificial synapse, excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), and high-pass filtering, are successfully realized. Furthermore, the single-fiber based artificial synapse can be operated by an ultralow presynaptic spike down to -0.5 mV with an ultralow reading voltage at -0.1 mV due to the large contact surface between the ionic electrolyte and fiber-like semiconducting channel. Therefore, the ultralow energy consumption at one spike of the artificial synapse can be realized as low as ≈3.9 fJ, which provides great potential in a low-power integrated synaptic circuit.

Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?

The field of organic electronics has been prolific in the last couple of years, leading to the design and synthesis of several molecular semiconductors presenting a mobility in excess of 10 cm² V^-1 s^-1 . However, it is also started to recently falter, as a result of doubtful mobility extractions and reduced industrial interest. This critical review addresses the community of chemists and materials scientists to share with it a critical analysis of the best performing molecular semiconductors and of the inherent charge transport physics that takes place in them. The goal is to inspire chemists and materials scientists and to give them hope that the field of molecular semiconductors for logic operations is not engaged into a dead end. To the contrary, it offers plenty of research opportunities in materials chemistry.