Ultra-flexible nonvolatile memory based on donor-acceptor diketopyrrolopyrrole polymer blends

An ultrasmall organic synapse for neuromorphic computing

High-performance organic neuromorphic devices with miniaturized device size and computing capability are essential elements for developing brain-inspired humanoid intelligence technique. However, due to the structural inhomogeneity of most organic materials, downscaling of such devices to nanoscale and their high-density integration into compact matrices with reliable device performance remain challenging at the moment. Herein, based on the design of a semicrystalline polymer PBFCL10 with ordered structure to regulate dense and uniform formation of conductive nanofilaments, we realize an organic synapse with the smallest device dimension of 50 nm and highest integration size of 1 Kb reported thus far. The as-fabricated PBFCL10 synapses can switch between 32 conductance states linearly with a high cycle-to-cycle uniformity of 98.89% and device-to-device uniformity of 99.71%, which are the best results of organic devices. A mixed-signal neuromorphic hardware system based on the organic neuromatrix and FPGA controller is implemented to execute spiking-plasticity-related algorithm for decision-making tasks.

Nonvolatile resistive switching and synaptic characteristics of lead-free all-inorganic perovskite-based flexible memristive devices for neuromorphic systems

Recently, several types of lead halide perovskites have been actively researched for resistive switching (RS) memory or artificial synaptic devices due to their current-voltage hysteresis along with the feasibility of fabrication, low-temperature processability and superior charge mobility. However, the toxicity and environmental pollution potential of lead halide perovskites severely restrict their large-scale commercial prospects. In the present work, the environmentally friendly and uniform CsSnCl₃ perovskite films are introduced to act as an active layer in the flexible memristors. Ag/CsSnCl₃/ITO devices demonstrate bipolar RS with excellent electrical properties such as forming free characteristics, good uniformity, low operating voltages, a high ON/OFF ratio (10²) and a long retention time (>10⁴ s). The RS mechanism has been well explained in the outline of electric field-induced formation and rupture of Ag filaments in the CsSnCl₃ layer. The metallic nature of the conducting filament has been further confirmed by temperature-dependent variation of low and high resistance states. Additionally, various pulse measurements have been carried out to mimic some of the basic synaptic functions including postsynaptic current, paired-pulse facilitation, long-term potentiation and long-term depression under normal as well as bending conditions. Our work provides the opportunity for exploring artificial synapses based on lead-free halide perovskites for the development of next-generation flexible electronics.

Boron(iii) β-diketonate-based small molecules for functional non-fullerene polymer solar cells and organic resistive memory devices

A class of acceptor-donor-acceptor chromophoric small-molecule non-fullerene acceptors, 1-4, with difluoroboron(iii) β-diketonate (BF2bdk) as the electron-accepting moiety has been developed. Through the variation of the central donor unit and the modification on the peripheral substituents of the terminal BF2bdk acceptor unit, their photophysical and electrochemical properties have been systematically studied. Taking advantage of their low-lying lowest unoccupied molecular orbital energy levels (from -3.65 to -3.72 eV) and relatively high electron mobility (7.49 × 10-4 cm2 V-1 s-1), these BF2bdk-based compounds have been employed as non-fullerene acceptors in organic solar cells with maximum power conversion efficiencies of up to 4.31%. Moreover, bistable resistive memory characteristics with charge-trapping mechanisms have been demonstrated in these BF2bdk-based compounds. This work not only demonstrates for the first time the use of a boron(iii) β-diketonate unit in constructing non-fullerene acceptors, but also provides more insights into designing organic materials with multi-functional properties.

Building memory devices from biocomposite electronic materials

Natural biomaterials are potential candidates for the next generation of green electronics due to their biocompatibility and biodegradability. On the other hand, the application of biocomposite systems in information storage, photoelectrochemical sensing, and biomedicine has further promoted the progress of environmentally benign bioelectronics. Here, we mainly review recent progress in the development of biocomposites in data storage, focusing on the application of biocomposites in resistive random-access memory (RRAM) and field effect transistors (FET) with their device structure, working mechanism, flexibility, transient characteristics. Specifically, we discuss the application of biocomposite-based non-volatile memories for simulating biological synapse. Finally, the application prospect and development potential of biocomposites are presented.

Developments of Diketopyrrolopyrrole-Dye-Based Organic Semiconductors for a Wide Range of Applications in Electronics

In recent times, fused aromatic diketopyrrolopyrrole (DPP)-based functional semiconductors have attracted considerable attention in the developing field of organic electronics. Over the past few years, DPP-based semiconductors have demonstrated remarkable improvements in the performance of both organic field-effect transistor (OFET) and organic photovoltaic (OPV) devices due to the favorable features of the DPP unit, such as excellent planarity and better electron-withdrawing ability. Driven by this success, DPP-based materials are now being exploited in various other electronic devices including complementary circuits, memory devices, chemical sensors, photodetectors, perovskite solar cells, organic light-emitting diodes, and more. Recent developments in the use of DPP-based materials for a wide range of electronic devices are summarized, focusing on OFET, OPV, and newly developed devices with a discussion of device performance in terms of molecular engineering. Useful guidance for the design of future DPP-based materials and the exploration of more advanced applications is provided.

Flexible memristive devices based on polyimide:mica nanosheet nanocomposites with an embedded PEDOT:PSS layer

Flexible memristive devices with a structure of Al/polyimide:mica/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/indium-tin-oxide/polyethylene glycol naphthalate showed electrical bistability characteristics. The maximum current margin of the devices with mica nanosheets was much larger than that of the devices without mica nanosheets. For these devices, the current vs. time curves showed nonvolatile characteristics with a retention time of more than 1 × 10⁴ s, and the current vs. number-of-cycles curves demonstrated an endurance for high resistance state/low resistance state switchings of 1 × 10² cycles. As to the operation performance, the “reset” voltage was distributed between 2.5 and 3 V, and the “set” voltage was distributed between −0.7 and −0.5 V, indicative of high uniformity. The electrical characteristics of the devices after full bendings with various radii of curvature were similar to those before bending, which was indicative of devices having ultra-flexibility. The carrier transport and the operation mechanisms of the devices were explained based on the current vs. voltage curves and the energy band diagrams.

Effects of a Nb nanopin electrode on the resistive random-access memory switching characteristics of NiO thin films

We report the effects of bottom electrode shapes on resistive random-access memory (RRAM) devices composed of Nb (bottom electrode)/NiO (dielectric)/Nb (top electrode) structures. By adopting a nano-fabrication process using an anodic aluminum oxide (AAO) nanotemplate, a well-aligned Nb nanopin array bottom electrode was formed on the surface of a Si substrate. For comparison, a Nb thin film was employed as a different type of bottom electrode. Then, a NiO thin film dielectric was prepared on both the Nb bottom electrodes via a spin coating method, followed by Nb sputtering for the Nb top electrode. Both the RRAM devices with Nb nanopin and thin film bottom electrodes exhibited typical unipolar resistive switching behavior. However, a lower SET/RESET voltage was observed for the Nb nanopin electrode compared to the Nb thin film electrode by virtue of an enhanced electric field induced by the nanopin-shaped electrode. More significantly, on the basis of endurance and retention characteristics, the Nb nanopin electrode played a key role in minimizing the dispersion of the low- and high-resistance state currents and the variation in the SET/RESET voltage by developing more-concise conducting filaments in the conducting path. Therefore, we foresee that this approach can provide an insight into the optimal design of RRAM devices.

Biological Spiking Synapse Constructed from Solution Processed Bimetal Core-Shell Nanoparticle Based Composites

Inspired by the highly parallel processing power and low energy consumption of the biological nervous system, the development of a neuromorphic computing paradigm to mimic brain-like behaviors with electronic components based artificial synapses may play key roles to eliminate the von Neumann bottleneck. Random resistive access memory (RRAM) is suitable for artificial synapse due to its tunable bidirectional switching behavior. In this work, a biological spiking synapse is developed with solution processed Au@Ag core-shell nanoparticle (NP)-based RRAM. The device shows highly controllable bistable resistive switching behavior due to the favorable Ag ions migration and filament formation in the composite film, and the good charge trapping and transport property of Au@Ag NPs. Moreover, comprehensive synaptic functions of biosynapse including paired-pulse depression, paired-pulse facilitation, post-tetanic potentiation, spike-time-dependent plasticity, and the transformation from short-term plasticity to long-term plasticity are emulated. This work demonstrates that the solution processed bimetal core-shell nanoparticle-based biological spiking synapse provides great potential for the further creation of a neuromorphic computing system.

Diketopyrrolopyrrole based organic semiconductors with different numbers of thiophene units: symmetry tuning effect on electronic devices

Three new DPP small molecules were synthesized and used them in OFET devices.

Naphthalene flanked diketopyrrolopyrrole based organic semiconductors for high performance organic field effect transistors

A newly reported alkylated naphthalene flanked DPP monomers without any further functionalization show high hole mobility in OFET devices.

Self-rectifying bipolar resistive switching memory based on an iron oxide and graphene oxide hybrid

A resistive random access memory (RRAM) device with self-rectifying I-V characteristics was fabricated by inserting a silicon nitride (Si₃N₄) layer between the bottom electrode and solution-processed active material of an iron oxide-graphene oxide (FeO_x-GO) hybrid. The fabricated Au/Ni/FeO_x-GO/Si₃N₄/n⁺-Si memory device exhibited an excellent resistive switching ratio and a rectification ratio higher than 10⁴. In the Au/Ni/FeO_x-GO/Si₃N₄/n⁺-Si device, resistive switching occurs in both the FeO_x-GO and Si₃N₄ layers separately, resulting in a highly uniform and stable switching performance. The resistive switching from a high resistance state to a low resistance state in the Au/Ni/FeO_x-GO/Si₃N₄/n⁺-Si device occurs through a trap-assisted tunneling process in the Si₃N₄ layer, enabled by the FeO_x-GO layer which prevents diffusion of the migrating Ni metal into the switching nitride layer. The intrinsic self-rectifying characteristics of our memory devices arise from the asymmetric barriers for electrons tunneling into the traps of the Si₃N₄ layer which is sandwiched between the top and bottom electrodes having dissimilar work functions. Our study confirmed that integrating a suitable dielectric layer into the conventional RRAM cell is an innovative strategy to simplify the architecture and fabrication process to realize self-rectifying crossbar arrays.

Resistive switching of Pt/TiO x /Pt devices fabricated on flexible Parylene-C substrates

Pt/TiO _x /Pt resistive switching (RS) devices are considered to be amongst the most promising candidates in memristor family and the technology transfer to flexible substrates could open the way to new opportunities for flexible memory implementations. Hence, an important goal is to achieve a fully flexible RS memory technology. Nonetheless, several fabrication challenges are present and must be solved prior to achieving reliable device fabrication and good electronic performances. Here, we propose a fabrication method for the successful transfer of Pt/TiO _x /Pt stack onto flexible Parylene-C substrates. The devices were electrically characterised, exhibiting both digital and analogue memory characteristics, which are obtained by proper adjustment of pulsing schemes during tests. This approach could open new application possibilities of these devices in neuromorphic computing, data processing, implantable sensors and bio-compatible neural interfaces.

Development of Conjugated Polymers for Memory Device Applications

This review summarizes the most widely used mechanisms in memory devices based on conjugated polymers, such as charge transfer, space charge traps, and filament conduction. In addition, recent studies of conjugated polymers for memory device applications are also reviewed, discussed, and differentiated based on the mechanisms and structural design. Moreover, the electrical conditions of conjugated polymers can be further fine-tuned by careful design and synthesis based on the switching mechanisms. The review also emphasizes and demonstrates the structure-memory properties relationship of donor-acceptor conjugated polymers for advanced memory device applications.

Synthesis and optical and electrochemical memory properties of fluorene–triphenylamine alternating copolymer

A fluorene–triphenylamine copolymer (PF–TPA) was designed and synthesized under Suzuki coupling reaction conditions in this work. It exhibited a typical electrical conductance switching behavior and non-volatile flash memory effects.

Controlling the Resistive Switching Behavior in Starch-Based Flexible Biomemristors

Implementation of biocompatible materials in resistive switching memory (ReRAM) devices provides opportunities to use them in biomedical applications. We demonstrate a robust, nonvolatile, flexible, and transparent ReRAM based on potato starch. We also introduce a biomolecular memory device that has a starch-chitosan composite layer. The ReRAM behavior can be controlled by mixing starch with chitosan in the resistive switching layer. Whereas starch-based biomemory devices which show abrupt changes in current level; the memory device with mixed biopolymers undergoes gradual changes. Both devices exhibit uniform and robust programmable memory properties for nonvolatile memory applications. The explicated source of the bipolar resistive switching behavior is assigned to formation and rupture of carbon-rich filaments. The gradual set/reset behavior in the memory device based on a starch-chitosan mixture makes it suitable for use in neuromorphic devices.

Incorporation of SnO2 nanoparticles in PMMA for performance enhancement of a transparent organic resistive memory device

Various sizes of SnO₂ NPs have been successfully synthesized and embedded into the insulating PMMA layer sandwiched between ITO and Al electrodes.