Flexible Memristor Constructed by 2D Cadmium Phosphorus Trichalcogenide for Artificial Synapse and Logic Operation

The development of advanced microelectronics requires new device architecture and multi-functionality. Low-dimensional material is considered as a powerful candidate to construct new devices. In this work, a flexible memristor is fabricated utilizing 2D cadmium phosphorus trichalcogenide nanosheets as the functional layer. The memristor exhibits excellent resistive switching performance under different radius and over 10³ bending times. The device mechanism is systematically investigated, and the synaptic plasticity including paired-pulse facilitation and spiking timing-dependent plasticity are further observed. Furthermore, based on the linearly conductance modulation capacity of the flexible memristor, the applications on decimal operation are explored, that the addition, subtraction, multiplication, and division of decimal calculation are successfully achieved. These results demonstrate the potential of metal phosphorus trichalcogenide in novel flexible neuromorphic devices, which accelerate the application process of neuromorphic computing.     

Dual‐Phase All‐Inorganic Cesium Halide Perovskites for Conducting‐Bridge Memory‐Based Artificial Synapses

Neuromorphic computing, which mimics biological neural networks, can overcome the high‐power and large‐throughput problems of current von Neumann computing. Two‐terminal memristors are regarded as promising candidates for artificial synapses, which are the fundamental functional units of neuromorphic computing systems. All‐inorganic CsPbI₃ perovskite‐based memristors are feasible to use in resistive switching memory and artificial synapses due to their fast ion migration. However, the ideal perovskite phase α‐CsPbI₃ is structurally unstable at ambient temperature and rapidly degrades to a non‐perovskite δ‐CsPbI₃ phase. Here, dual‐phase (Cs₃Bi₂I₉)_0.4?(CsPbI₃)_0.6 is successfully fabricated to achieve improved air stability and surface morphology compared to each single phase. Notably, the Ag/polymethylmethacrylate/(Cs₃Bi₂I₉)_0.4?(CsPbI₃)_0.6/Pt device exhibits non‐volatile memory functions with an endurance of ≈10³ cycles and retention of ≈10⁴ s with low operation voltages. Moreover, the device successfully emulates synaptic behavior such as long‐term potentiation/depression and spike timing/width‐dependent plasticity. This study will contribute to improving the structural and mechanical stability of all‐inorganic halide perovskites (IHPs) via the formation of dual phase. In addition, it proves the great potential of IHPs for use in low‐power non‐volatile memory devices and electronic synapses.     

Flexible Electro-Optical Perovskite/Electrolyte Synaptic Transistor to Emulate Photoelectric-Synergistic Neural Learning Rules and Reflex-Arc Behavior

The design and fabrication of a flexible electric-optical perovskite/electrolyte synaptic transistor are demonstrated for the first time, which emulates important neuromorphic functions under dual-mode modulation. Benefiting from the bipolar charge transport properties of light-harvesting perovskite and the high specific capacitance and mechanically robust multi-ion electrolyte, the device exhibits bidirectional plasticity, better reliability, retaining >70% of the initial current level after 2500 flex per flat laps, and a very wide operating voltage window from 0.04 to 10 V, and responsive to ultralow stimuli down to tens of millivolt level with femtojoule-level energy consumption. The synergistic high response under dual-mode modulation enables the device to emulate complex neural learning rules and achieve neuromorphic applications, including classical conditioning and spatiotemporal learning, and image recognition tasks with higher accuracy of 81%. Moreover, an enhanced artificial reflex-arc behavior is emulated by employing the flexible electro-optical artificial synapses that serve as key information-receiving-processing units to manipulate the actions of electrochemical artificial muscles to a larger extent. These properties show great potential in soft neurorobotic systems and prostheses.     

Recent Advances in Transistor‐Based Artificial Synapses

Simulating biological synapses with electronic devices is a re‐emerging field of research. It is widely recognized as the first step in hardware building brain‐like computers and artificial intelligent systems. Thus far, different types of electronic devices have been proposed to mimic synaptic functions. Among them, transistor‐based artificial synapses have the advantages of good stability, relatively controllable testing parameters, clear operation mechanism, and can be constructed from a variety of materials. In addition, they can perform concurrent learning, in which synaptic weight update can be performed without interrupting the signal transmission process. Synergistic control of one device can also be implemented in a transistor‐based artificial synapse, which opens up the possibility of developing robust neuron networks with significantly fewer neural elements. These unique features of transistor‐based artificial synapses make them more suitable for emulating synaptic functions than other types of devices. However, the development of transistor‐based artificial synapses is still in its very early stages. Herein, this article presents a review of recent advances in transistor‐based artificial synapses in order to give a guideline for future implementation of synaptic functions with transistors. The main challenges and research directions of transistor‐based artificial synapses are also presented.     

All‐Inorganic Ionic Polymer‐Based Memristor for High‐Performance and Flexible Artificial Synapse

The implementation of memristors that are wearable and transparent has attracted significant attention. However, the development of high‐performance memristors that simultaneously possess high flexibility and environmental stability has remained a tremendous challenge suffering from limited choice of materials with both good ion‐electron mobility and structural flexibility. Inspired by the unique poly‐ionic nature of ammonium polyphosphate (APP), a novel Au/APP/ITO memristor with favorable flexibility and stability is prepared. Synaptic behaviors can be stimulated by voltage pulses that are 20 ns in width, 0.1 V in amplitude, and repeatable under 10⁴ pulse cycles, thereby outperforming several other benchmark memristors. Further, the device, prepared on conductive silicone, can sustain its synaptic performance even under 360° bending. Furthermore, the device can sustain its synaptic behaviors even after exposure to fire for 60 s and 5.6 kGy of ionic irradiation. Additionally, APP is determined to be nontoxic, biodegradable, and transparent when compared with all the organics and inorganics used in previous memristors. The results of this study will inspire the development of more inorganic polymers for their utilization in future environmentally stable and flexible electronics.     

Efficient Transparent Thin Dye Solar Cells Based on Highly Porous 1D Photonic Crystals

A working electrode design based on a highly porous 1D photonic crystal structure that opens the path towards high photocurrents in thin, transparent, dye‐sensitized solar cells is presented. By enlarging the average pore size with respect to previous photonic crystal designs, the new working electrode not only increases the device photocurrent, as predicted by theoretical models, but also allows the observation of an unprecedented boost of the cell photovoltage, which can be attributed to structural modifications caused during the integration of the photonic crystal. These synergic effects yield conversion efficiencies of around 3.5% by using just 2 μm thick electrodes, with enhancements between 100% and 150% with respect to reference cells of the same thickness.     

Graphene Oxide Quantum Dots Based Memristors with Progressive Conduction Tuning for Artificial Synaptic Learning

Memristors as electronic artificial synapses have attracted increasing attention in neuromorphic computing. Emulation of both “learning” and “forgetting” processes requires a bidirectional progressive adjustment of memristor conductance, which is a challenge for cutting‐edge artificial intelligence. In this work, a memristor device with a structure of Ag/Zr_0.5Hf_0.5O₂:graphene oxide quantum dots/Ag is presented with the feature of bidirectional progressive conductance tuning. The conductance of proposed memristor is adjusted through voltage pulse number, amplitude, and width. A series of voltage pulses with an amplitude of 0.6 V and a width of 30 ns is enough to modulate conductance. The impacts of pulses with different parameters on conductance modulation are investigated, and the potential relationship between pulse amplitude and energy is revealed. Furthermore, it is proved that the pulse with low energy can realize the almost linear conductance regulation, which is beneficial to improve the accuracy of pattern recognition. The bidirectional progressive conduction modulation mimics various plastic synapses, such as spike‐timing‐dependent plasticity and paired‐pulse facilitation. This progressive conduction tuning mechanism might be attributed to the coexistence of tunneling effect and extrinsic electrochemical metallization effect. This work provides one way for memristor to attain attractive features such as bidirectional tuning, low‐power consumption, and fast speed switching that is in urgent demand for further evolution of neuromorphic chips.     

Organic Field Effect Transistor-Based Photonic Synapses: Materials,Devices, and Applications

Photonic artificial synapses-based neuromorphic computing systems have been regarded as promising candidates for replacing von Neumann-based computing systems due to the high bandwidth, ultrafast signal transmission, low energy consumption, and wireless communication. Although significant progress has been made in developing varied device structures for synaptic emulation, organic field-effect transistors (OFETs) hold the compelling advantages of facile preparation, liable integration, and versatile structures. As a powerful and effective platform for photonic synapses, OFETs can fulfill not only the simulation of simple synaptic functions, but also complex photoelectric dual modulation and simulation of the visual system. Herein, an overview of OFET-based photonic synapses, including functional materials, device configurations, and innovative applications is provided. Meanwhile, rules for selecting materials, mechanism of photoelectric conversion, and fabrication techniques of devices are also highlighted. Finally, challenges and opportunities are all discussed, providing solid guidance for multilevel memory, multi-functional tandem artificial neural system, and artificial intelligence.     

Flexible Artificial Synaptic Devices Based on Collagen from Fish Protein with Spike‐Timing‐Dependent Plasticity

Neuromorphic and cognitive computing with a capability of analyzing complicated information is explored as a new paradigm of intelligent systems. An implementation of a renewable material as an essential building block of an artificial synaptic device is suggested and a flexible and transparent synaptic device based on collagen extracted from fish skin is demonstrated. This device exhibits essential synaptic behaviors including analog memory characteristics, excitatory postsynaptic current, and paired‐pulse facilitation as short‐term plasticity. The brain‐inspired electronic synapse undergoes incremental potentiation and depression when flat or bent. The device emulates spike‐timing‐dependent plasticity when stimulated by engineered pre‐ and post‐neuron spikes with the appropriate time difference between the imposed pulses. The proposed synaptic device has the advantage of being biocompatible owing to use of Mg electrodes and collagen as a naturally abundant protein. This device has a potential to be used in flexible and implantable neuromorphic systems in the future.     

Photonic Organolead Halide Perovskite Artificial Synapse Capable of Accelerated Learning at Low Power Inspired by Dopamine‐Facilitated Synaptic Activity

The ability of high‐order tuning of the synaptic plasticity in an artificial synapse can offer significant improvement in the processing time, low‐power recognition, and learning capability in a neuro‐inspired computing system. Inspired by light‐assisted dopamine‐facilitated synaptic activity, which achieves rapid learning and adaptation by lowering the threshold of the synaptic plasticity, a two‐terminal organolead halide perovskite (OHP)‐based photonic synapse is fabricated and designed in which the synaptic plasticity is modified by both electrical pulses and light illumination. Owing to the accelerated migration of the iodine vacancy inherently existing in the coated OHP film under light illumination, the OHP synaptic device exhibits light‐tunable synaptic functionalities with very low programming inputs (≈0.1 V). It is also demonstrated that the threshold of the long‐term potentiation decreases and synaptic weight further modulates when light illuminates the device, which is phenomenologically analogous to the dopamine‐assisted synaptic process. Notably, under light exposure, the OHP synaptic device achieves rapid pattern recognition with ≈81.8% accuracy after only 2000 learning phases (60 000 learning phases = one epoch) with a low‐power consumption (4.82 nW/the initial update for potentiation), which is ≈2.6 × 10³ times lower than when the synaptic weights are updated by only high electrical pulses.     

Ferroelectrics-Electret Synergetic Organic Artificial Synapses with Single-Polarity Driven Dynamic Reconfigurable Modulation

Most current studies of artificial synapses only mimic the static plasticity, which is far from achieving the complex behaviors of the human brain. The few reported dynamic reconfigurable synapses based on ambipolar transistors switch the operating states by voltages with opposite polarity, which impedes the development of highly efficient synaptic readout circuits. To improve the efficiency, flexibility, and biocompatibility of dynamic reconfigurable synapses, here a ferroelectrics-electret synergetic organic synaptic p-type transistor (FESOST) is devised. Owing to the synergetic action of ferroelectric polarization switching and charge capture, FESOST exhibits single-polarity driven dynamic reconfigurable operating states with different synaptic behaviors (potentiation and depression) in response to the same gate pulse in different modes (excitatory and inhibitory). In addition, various single-polarity driven synaptic behaviors including short-term/long-term plasticity, paired-pulse facilitation/depression, spike-rate-dependent plasticity, and spike-number-dependent plasticity are also simulated. Finally, the reconfigurable artificial temperature perception system is simulated for the complex emotions of humans in response to different weather stimuli for people of different constitutions. The novel device architecture represents a major step forward in the development of dynamic, reconfigurable, high-efficiency, organic synapses.     

Flexible and Self‐Powered Lateral Photodetector Based on Inorganic Perovskite CsPbI3–CsPbBr3 Heterojunction Nanowire Array

Self‐powered perovskite photodetectors mainly adopt the vertical heterojunction structure composed of active layer, electron–hole transfer layers, and electrodes, which results in the loss of incident light and interfacial accumulation of defects. To address these issues, a self‐powered lateral photodetector based on CsPbI₃–CsPbBr₃ heterojunction nanowire arrays is designed on both a rigid glass and a flexible polyethylene naphthalate substrate using an in situ conversion and mask‐assisted electrode fabrication method. Through adding the polyvinyl pyrrolidone and optimizing the concentration of precursors under the pressure‐assisted moulding process, both the crystallinity and stability of perovskite nanowire array are improved. The nanowire array–based lateral device shows a high responsivity of 125 mA W^?1 and a fast rise and decay time of 0.7 and 0.8 ms under a self‐powered operation condition. This work provides a new strategy to fabricate perovskite heterojunction nanoarrays towards self‐powered photodetection.     

Memristor with Ag‐Cluster‐Doped TiO2 Films as Artificial Synapse for Neuroinspired Computing

Memristor, based on the principle of biological synapse, is recognized as one of the key devices in confronting the bottleneck of classical von Neumann computers. However, conventional memristors are difficult to continuously adjust the conduction and dutifully mimic the biosynapse function. Here, TiO₂ films with self‐assembled Ag nanoclusters implemented by gradient Ag dopant are employed to achieve enhanced memristor performance. The memristors exhibit gradual both potentiating and depressing conduction under positive and negative pulse trains, which can fully emulate excitation and inhibition of biosynapse. Moreover, comprehensive biosynaptic functions and plasticity, including the transition from short‐term memory to long‐term memory, long‐term potentiation and depression, spike‐timing‐dependent plasticity, and paired‐pulse facilitation, are implemented with the fabricated memristors in this work. The applied pulses with a width of hundreds of nanoseconds timescale are beneficial to realize fast learning and computing. High‐resolution transmission electron microscopy observations clearly demonstrate that Ag clusters redistribute to form Ag conductive filaments between Ag and Pt electrode under electrical field at ON‐state device. The experimental data confirm that the oxides doped with Ag clusters have the potential for mimicking biosynaptic behavior, which is essential for the further creation of artificial neural systems.     

Reconfigurable Logic‐in‐Memory and Multilingual Artificial Synapses Based on 2D Heterostructures

Nonvolatile logic devices have attracted intensive research attentions recently for energy efficiency computing, where data computing and storage can be realized in the same device structure. Various approaches have been adopted to build such devices; however, the functionality and versatility are still very limited. Here, 2D van der Waals heterostructures based on direct bandgap materials black phosphorus and rhenium disulfide for the nonvolatile ternary logic operations is demonstrated for the first time with the ultrathin oxide layer from the black phosphorus serving as the charge trapping as well as band‐to‐band tunneling layer. Furthermore, an artificial electronic synapse based on this heterostructure is demonstrated to mimic trilingual synaptic response by changing the input base voltage. Besides, artificial neural network simulation based on the electronic synaptic arrays using the handwritten digits data sets demonstrates a high recognition accuracy of 91.3%. This work provides a path toward realizing multifunctional nonvolatile logic‐in‐memory applications based on novel 2D heterostructures.     

Inorganic and Hybrid Organo‐Metal Perovskite Nanostructures: Synthesis,Properties, and Applications

Hybrid perovskite and all‐inorganic perovskite have attracted much attention in recent years owing to their successful use in the photovoltaic field. Usually the perovskite is used in its bulk form, although recently, perovskites' nanocrystalline form has received increased attention. Recent developments in the evolving research field of nanomaterial‐based perovskite are reviewed. Both hybrid organic‐inorganic and all‐inorganic perovskite nanostructures are discussed, as well as approaches to tune the optical properties by controlling the size and shape of perovskite nanostructures. In addition, chemical modifications can change the perovskite nanostructures' band‐gap, similar to their bulk counterpart. Several applications, including light‐emitting diodes, lasers, and detectors, demonstrate the latent potential of perovskite nanostructures.     

Ultrafast Responsive and Low-Energy-Consumption Poly(3-hexylthiophene)/Perovskite Quantum Dots Composite Film-Based Photonic Synapse

Emulation of photonic synapses through photo-recordable devices has aroused tremendous discussion owing to the low energy consumption, high parallel, and fault-tolerance in artificial neuromorphic networks. Nonvolatile flash-type photomemory with short photo-programming time, long-term storage, and linear plasticity becomes the most promising candidate. Nevertheless, the systematic studies of mechanism behind the charge transfer process in photomemory are limited. Herein, the physical properties of APbBr₃ perovskite quantum dots (PQDs) on the photoresponsive characteristics of derived poly(3-hexylthiophene-2,5-diyl) (P3HT)/PQDs-based photomemory t hrough facile A-site substitution approach are explored. Benefitting from the lowest valance band maximum and longest exciton lifetime of FAPbBr₃ quantum dot (FA-QDs), P3HT/FA-QDs-derived photomemory not only exhibits shortest photoresponsive characteristic time compared to FA_0.5Cs_0.5PbBr₃ quantum dots (Mix-QDs) and CsPbBr₃ quantum dots (Cs-QDs) but also displays excellent ON/OFF current ratio of 2.2 upon an extremely short illumination duration of 1 ms. Moreover, the device not only achieves linear plasticity of synapses by optical potentiation and electric depression, but also successfully emulates the features of photon synaptic such as pair-pulse facilitation, long-term plasticity, and multiple spike-dependent plasticity and exhibits extremely low energy consumption of 3 × 10⁻¹⁷ J per synaptic event.     

Optoelectronic Synapses and Photodetectors Based on Organic Semiconductor/Halide Perovskite Heterojunctions: Materials,Devices, and Applications

Both photodetectors (PDs) and optoelectronic synaptic devices (OSDs) are optoelectronic devices converting light signals into electrical responses. Optoelectronic devices based on organic semiconductors and halide perovskites have aroused tremendous research interest owing to their exceptional optical/electrical characteristics and low-cost processability. The heterojunction formed between organic semiconductors and halide perovskites can modify the exciton dissociation/recombination efficiency and modulate the charge-trapping effect. Consequently, organic semiconductor/halide perovskite heterojunctions can endow PDs and OSDs with high photo responsivity and the ability to simulate synaptic functions respectively, making them appropriate for the development of energy-efficient artificial visual systems with sensory and recognition functions. This article summarizes the recent advances in this research field. The physical/chemical properties and preparation methods of organic semiconductor/halide perovskite heterojunctions are briefly introduced. Then the development of PDs and OSDs based on organic semiconductor/halide perovskite heterojunctions, as well as their innovative applications, are systematically presented. Finally, some prospective challenges and probable strategies for the future development of optoelectronic devices based on organic semiconductor/halide perovskite heterojunctions are discussed.     

Undoped Hole Transport Layer Toward Efficient and Stable Inorganic Perovskite Solar Cells

Numerous strategies have been practiced to improve the power conversion efficiency of CsPbI₂Br-based perovskite solar cells (PSCs), which definitely makes efficiency gradually approach the theoretical efficiency limit. However, sufficient device stability is still in urgent demand for commercialization, pushing to overcome some instability sources induced by hygroscopicity of spiro-OMeTAD and residual strain of perovskite layer. To address these issues, p-type semiconductor of PCPDTBT is used to replace spiro-OMeTAD, enabling dual functions of hole transport and strain regulation. On the one hand, undoped PCPDTBT performs excellent hole extraction and transport, while avoiding the perovskite degradation caused by the hygroscopicity of common additives. On the other hand, PCPDTBT assisted by a thermally spin-coating method compensates for the thermally-induced residual strain in perovskite layer owing to its high thermal expansion coefficient. Consequently, CsPbI₂Br-based PSCs with PCPDTBT layer achieve improved efficiency of 16.5% as well as enhanced stability. This study provides a simple and facile strategy to achieve efficient and stable CsPbI₂Br-based PSCs.