Biological nervous systems possess versatile attributes, serving diverse functions; for instance, the central nervous system (CNS) governs learning and memory, while the peripheral nervous system (PNS) is responsible for sensory perception. Consequently, there is a need to engineer artificial synapses tailored to adapt to performance requirements in various applications. Brain-inspired neuromorphic computing aims to emulate the learning and memory capabilities of the CNS, being inspired from the long-term potentiation (LTP) observed in biological synapses. The application of artificial synapses in the fields of nervetronics and neuroprosthetics requires the emulation of the short-term plasticity (STP), enabling the rapid signal transmission and fast responses akin to those in the biological PNS. To demonstrate the broad applicability, spanning areas including neuromorphic computing and bio-inspired nervetronics, our study has explored the modulation of STP and LTP using ion-gel gated polymer synaptic transistors (IGPSTs). We have modulated the polymer semiconductor (PSC) film’s crystallinity through post-deposition film annealing, self-assembly monolayer treatment, and the introduction of various sidechain length, leading to the conversion between STP and LTP properties in IGPSTs. Moreover, we have utilized a straightforward yet effective approach by blending two PSCs with the same backbone but different sidechains. This blend strategy has led to a substantial improvement in LTP characteristics, whereas IGPSTs employing each PSC individually show only STP properties. IGPSTs with enhanced LTP properties demonstrate their potential as neuromorphic computing devices, effectively simulating learning processes in artificial neural networks. Concurrently, IGPSTs featuring STP capabilities are used to demonstrate various artificial nervous systems, such as artificial reflex arcs, neuromuscular systems, and neuro-prosthetic nerves incorporating artificial proprioceptors. These pioneering studies on neuromorphic devices have expanded the scope of applications for artificial synapses and validate the feasibility of these innovative technologies.

Electrochemical random access memory (ECRAM) and organic electrochemical transistors (OECT) are gaining significant attention due to the unique properties introduced via the mobile ions. Despite the progress in device fabrication, there is very little in terms of device models.[1, 2] Here, we will describe results obtained using a 2D semiconductor device model that incorporates ions and reactions in a self-consistent manner.

For example, although OECTs electrolytes contain both cations and anions, it is common to consider only the primary ion (as a cation in a normally-off p-type transistor). We simulate normally-on and normally-off transistors showing that the normally-on is highly affected by the counter ions (anions) and field-screening at the contacts.

Inorganic ECRAM transistor devices have a structure similar to OECT, where the memory (ion retention) is provided by the diffusion's very high electric field activation. Comparing measured multi-level potentiation [3] to detailed device simulation, we reveal the role of electrolyte polarisation. We also show that sublinear potentiation response can be explained by an electrochemical reaction similar to that of lithium plating in batteries. Namely, electrochemical reactions must be considered when dealing with electrochemical devices.

Neuromorphic sensors, designed to emulate natural sensory systems, hold the promise of revolutionizing data extraction by facilitating rapid and energy-efficient analysis of extensive datasets. However, a significant challenge lies in accurately distinguishing specific analytes within mixtures of chemically similar compounds using existing neuromorphic chemical sensors. In this study, we present an artificial olfactory system (AOS), which is developed through the integration of human olfactory receptors (hORs) and artificial synapses, for the first time. This AOS is sophisticatedly engineered by interfacing an hOR-functionalized extended-gate with an organic synaptic device. The AOS generates clearly distinct patterns for odorants and mixtures thereof, at the molecular chain length level, attributed to specific hOR-odorant binding affinities. This approach enables precise pattern recognition via training and inference simulations. These findings establish a solid foundation for the development of high-performance sensor platforms and artificial sensory systems, which are ideal for applications in wearable and implantable devices.

Developing high-mobility p-type semiconductors that can be grown using cost-effective scalable methods at low temperatures, has remained challenging in the electronics community for the integration of complementary electronics with the well-developed n-type metal oxide counterparts. Tin (Sn²⁺) halide perovskites emerge as promising p-type candidates but suffer from low crystallisation controllability and high film defect density, which result in uncompetitive device performance (1). In this talk, I would like to introduce a general overview and recent progress of our group of p-type Sn-based metal halide perovskites for the application of field-effect transistors (FETs). In the first part of the talk, I will mainly address inorganic perovskite thin-film transistors with exceptional performance using high-crystallinity and uniform cesium-tin-triiodide-based semiconducting layers with moderate hole concentrations and superior Hall mobilities, which are enabled by the judicious engineering of film composition and crystallization. The optimized devices exhibit high field-effect hole mobilities of over 50 cm² V⁻¹ s⁻¹, large current modulation greater than 108, and high operational stability and reproducibility (2). In the second part of the talk, I will introduce A-site cation engineering method to achieve high-performance pure-Sn perovskite thin-film transistors (TFTs). We explore triple A-cations of caesium-formamidinium-phenethylammonium to create high-quality cascaded Sn perovskite channel films, especially with low-defect phase-pure perovskite/dielectric interface. As such, the optimized TFTs show record hole mobilities of over 70 cm² V⁻¹ s⁻¹ and on/off current ratios of over 10⁸, comparable to the commercial low-temperature polysilicon technique level (3). The p-channel perovskite TFTs also show high processability and compatibility with the n-type metal oxides, enabling the integration of high-gain complementary inverters and rail-to-rail logic gates.

Halide perovskites have been attracted as potential candidates in various electronic devices such as memristors and neuromorphic devices with superior resistive switching properties such as low power consumption, scalability, and compositional and mechanical flexibility as well as photovoltaics, LED, and so on. Among them, we report here low-dimensional lead-free halide perovskites for the resistive switching memory devices and propose their potential applicability in multi-functional electronic devices using the resistive switching characteristics. Although most studies have been focused on the lead-based halide perovskites, it is necessary to focus the research on lead-free halide perovskites without toxicity of lead for various viable electronic applications such as tin, copper, antimony, or bismuth-based materials. Here we found that copper (Cu²⁺)- and bismuth (Bi³⁺)-based halide perovskites can be suitable candidates for a variety of applications, especially memristor devices due to their visible characteristic electrical traits and improved stability. With these reports, we also suggest materials and devices design perspectives for multi-functional electronic devices including next generation memristors. For copper(II)-based halide perovskites, 2-dimensional (2D) (BzA)₂CuBr₄ (BzA = benzylammonium) perovskite was synthesized on Pt substrate, which retained a higher ON/OFF ratio over 10⁸ even at lower operational voltage around +0.2 - -0.3 V. Longer endurance over 2,000 cycles was observed with Schottky conduction at HRS. [1] For bismuth (III)-based halide perovskites, layered double perovskite with chemical formula of BA₂CsAgBiBr₇ was synthesized on Pt substrate by phase conversion of 3D Cs₂AgBiBr₆, which showed endurance of 1,000 cycles and retention time over 2×10⁴ s at operational voltage as low as 0.5 V, along with ON/OFF ratio as high as ~10⁷. [2] Both the previously reported 3D counterparts on ITO substrate [3] and 2D Ag-Bi system [2] here showed excellent memristor properties, but especially the ON/OFF ratio of 2D Ag-Bi system was five orders of magnitude larger than those of 3D counterparts. Modification of Schottky conduction at high resistance state (HRS) is found to be responsible for high ON/OFF ratio in low-dimensional structure. In addition, as another study of bismuth-based materials, we fabricated a low-dimensional Cs₃Bi₂Br₉-based memristor with a thin film less than 1 micrometer, where synthetic Cs₃Bi₂Br₉ powder precursor enabled uniform thin film formation.[4] Thin film also exhibited low operating voltages as +0.44 V with gradual current increase at the SET process with reliable cell-to-cell and device-to-device resistive switching behavior. It was also found that the 2D low-dimensional copper and bismuth-based halide perovskites studied here showed much better stability under high humidity and at elevated temperature than the 3D and/or lead-based counterparts, which is beneficial for practical applications. Therefore, it was confirmed that low-dimensional lead-free halide perovskite materials are promising candidates for resistive switching memory devices, and we also expect that various halide perovskites can be derived and applied to multi-functional electronic devices including memristors, neuronic or synaptic devices, and so on.

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

Organic electrochemical transistors (OECTs) prove to be effective devices in various applications such as neuromorphic functionalities, bioelectronics, and sensors. Analyzing these mixed ionic-electronic devices is often complex due to the coupling of hole transport along the channel with ion insertion from the electrolyte. Numerous literature reports highlight persistent dynamical hysteresis effects in current-voltage curves, attributed to the gradual ionic charging of the channel under applied gate voltage.

We introduce a model that takes into account the primary electrical and electrochemical operational aspects of the device. This model is based on a thermodynamic function of ion insertion, allowing for the convenient classification of hysteresis effects. Such hysteresis can be categorized as capacitive or inductive. We specifically identify the volume capacitance as the derivative of the thermodynamic function, linked to the chemical capacitance of the ionic-electronic film.

Our findings reveal that the inductor response observed in impedance spectroscopy is associated with ionic diffusion from the surface, filling the channel up to the equilibrium value. The model uncovers multiple dynamical features tied to specific kinetic relaxations that govern the transient and impedance response of the OECT.

In biological tactile somatosensory system, the cooperation of mechanoreceptors, neurons and synapses allows human to efficiently detect, transmit and process the tactile information.  Emulation of the tactile sensory nerve to achieve advanced sensory functions in robotics with artificial intelligence is of great interest. Here, we report an artificial organic afferent nerve (AOAN) by integrating novel pressure-activated organic electrochemical synaptic transistor (OEST) and artificial mechanoreceptors. Owing to the effect of electrochemical ion doping and ion trapping in bulk conjugated semiconductor, external mechanical stimulation enables activation and modulation of OEST, endowing the system with the recognition/sensation of spatiotemporal tactile information and a low retention loss during signal transmission. Dendritic integration function for neurorobotics is achieved to perceive directional movement of object. An intelligent robot with our system, coupling with a closed-loop feedback program is demonstrated for slip detection and prevent slippage of objects. This work provides a promising approach towards next-generation intelligent neurorobotics and low power biomimetic electronics.

Halide perovskites have been widely explored for numerous optoelectronic applications among which phototransistors appeared as one of the most promising light signal detectors. However, it is still a great challenge to endow halide perovskites with both mobility and high photosensitivity because of their high sensitivity to moisture in ambient atmosphere, which limits the efficiency of transporting and collecting charge carrier. Here, we explore FAPbBr₃ perovskite quantum dots (QDs) phototransistor with band-like charge transport and measure a dark hole mobility of 14.2 cm²V^-1s^-1 at ambient atmosphere which is about an order of magnitude higher than solution processed perovskite QDs devices. Attaining both high mobility and good optical figures of merit, including photoresponsivity and detectivity, a detectivity of ~10¹⁶ Jones is achieved, which is a record for halide perovskite nanocrystals. Simple A-site salt (FABr) treatments offer a mechanism for connecting between perovskite QDs for better charge transfer in high-quality devices. All these important properties are superior to most advanced inorganic semiconductor phototransistor, indicating a promising future in optoelectronic applications.

Future brain-computer interfaces will necessitate electronic circuits capable of processing signals in a localized and highly individualized manner within the nervous system and other living tissues. However, traditional neuromorphic implementations based on silicon have limitations in bio-integration due to poor biocompatibility, circuit complexity, and low energy efficiency. Organic mixed ionic-electronic conductors (OMIECs) are an emerging technology that has the potential to overcome these limitations. OMIECs enable efficient signal transduction by tightly coupling ions and electrons, making them ideal for interfacing electronics with biological systems. Here, we explore the use of OMIECs to develop organic electrochemical neurons and synapses that can be modulated using different types of chemical signals. These soft and flexible organic electrochemical neurons and synapses operate at low voltage and respond to multiple stimuli, signaling a new era for printed organic electronics [1]. We will discuss their ease of integration with biological nerves [2] and demonstrate the potential for OMIECs to enable highly localized and precise signal processing in brain-computer interfaces.

An iontronic-based artificial tactile nerve is a promising technology for emulating the tactile recognition and learning of human skin with low power consumption. However, its weak tactile memory and complex integration structure remain challenging. We present an ion trap and release dynamics (iTRD)-driven, neuro-inspired monolithic artificial tactile neuron (NeuroMAT) that can achieve tactile perception and memory consolidation in a single device. Through tactile-driven release of ions initially trapped within iTRD-iongel, NeuroMAT only generates non-intrusive synaptic memory signals when mechanical stress is applied under voltage stimulation. The induced tactile memory is augmented by auxiliary voltage pulses independent of tactile sensing signals. We integrate NeuroMAT with an anthropomorphic robotic hand system to imitate memory-based human motion; the robust tactile memory of NeuroMAT enables the hand to consistently perform reliable gripping motion. We utilized the augmented tactile memory of NeuroMAT to develop a NeuroMATICS-based robotic hand system, which consistently and reliably emulated tactile-memory-driven human motion. Our theoretical study and practical demonstration of the iTRD behavior can provide an inspiration with artificial nerve disciplines and can be extended to in the field of neuromorphic electronic skin and machine learning.

Halide perovskite materials are mixed electronic and ionic conductors that find use in several applications. The ionic conductivity is responsible for a memory effect that leads to undesirable hysteresis in the solar cell configuration but it is a requirement in their use as a resistive memory. In this presentation, it is shown how conductive and insulating states are formed via migration of halide vacancy and electrochemically active metals halide perovskite useful as memristors. We show that the working mechanism and performance of the memory devices can be tuned and improved by a careful selection of each structural layer. Several configurations are  evaluated in which structural layers are modified systematically: formulation of the perovskite,¹ the nature of the buffer layer^2,3  and the nature of the metal contact⁴. We show that in order to efficiently promote migration of metal contact the use of pre-oxidized metals greatly enhance the performance of the memristor and reduces the energy requirements. Overall, we provide solid understanding on the operational mechanism of halide perovskite memristors that has enabled increased stabilities approaching 10⁵ cycles with well separated states of current and further improvements expected.⁵