Organic Resistive Memory Research Articles

Enhanced resistive switching behaviors of organic resistive random access memory devices by adding polyethyleneimine interlayer

Organic nonvolatile memory devices have garnered significant attention as next-generation electrical memory units owing to their potential for low-cost and straightforward fabrication through a solution process. In this study, we successfully fabricated fully solution-processed organic resistive random access memory (RRAM) devices using poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the resistive switching (RS) layer, with poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) employed as the top electrode. Additionally, to enhance performance further, a polyethyleneimine (PEIE) interlayer was introduced between the bottom electrode and the P3HT:PCBM RS layer. The resulting organic RRAM devices with the PEIE interlayer exhibited bipolar resistive switching, with improved endurance increasing from 50 to 100 cycles, a retention time of 103 s, and a low SET voltage of 0.7 V. The organic RRAM devices featuring the PEIE interlayer demonstrated superior RS performance attributed to the higher Schottky barrier at the interface between the bottom electrode and the active switching layer, creating an asymmetric structure. I–V curve fitting confirmed that the potential switching mechanism involved Schottky emission in the high-resistance state and Ohmic conduction in the low-resistance state. Our findings suggest that organic RRAM devices with a PEIE interlayer hold promise for stable nonvolatile memory applications.

SIMULATING MOLECULAR OPTICAL PROPERTIES OF POLYIMIDE FOR NONVOLATILE RESISTIVE PHOTOMEMORY DEVICES

In this study, the focus was on utilizing the polyimide (PI) thin film as a resistive conversion layer for organic polyimide-based resistive random-access memory (ReRAM) applications. We utilized the 3D molecular editor WebMO to manipulate and analyze the structures of Aromatic-PI and Quinoid-PI molecules. The molecular chain length, molecular weight, and HOMO/LUMO molecular orbitals were investigated to gain insights into the fundamental properties of PI molecular structures. The analysis revealed that the Quinoid-PI molecule exhibited a shorter molecular chain compared to Aromatic-PI, attributed to the inability of the cyclic structure in Quinoid-PI to maintain a planar conjugated structure, resulting in an uneven overall structure and reduced molecular chain length (Norcorss, 1988). The research results demonstrate that the optical energy gap of Aromatic-PI remains constant regardless of the number of molecular bonds, with a value of approximately 3.2661 eV. In contrast, for Quinoid-PI, the optical energy gap undergoes a significant decrease as the number of molecular bonds ranges from 1 to 5. As the bond count increases to 20~23, the change in the optical energy gap gradually diminishes, ultimately resulting in an optical energy gap of 0.6932 eV. This observation indicates that longer molecular chains composed of Aromatic-PI act as insulators, while Quinoid-PI transforms into a low-energy gap conductor.

Synthesis, Characterization, Supramolecular Self-Assembly, and Organic Resistive Memory Applications of BODIPY Derivatives

A new class of BODIPY conjugated molecules modified with hydrophobic alkoxy chains or hydrophilic triethylene glycol chains has been designed, synthesized, and characterized. Supramolecular self-assembly behaviors of the compounds investigated by temperature-dependent ultraviolet–visible (UV–vis) absorption studies are found to display a cooperative growth mechanism with the formation of uniform and well-defined morphology of the aggregates in THF and water mixtures, which have been studied by transmission electron microscopy (TEM). Their self-assembly processes are found to be modulated by π–π stacking as well as hydrophobic interactions. The resistive memory devices fabricated with compounds 3 and 6 exhibit encouraging binary memory characteristics with low threshold voltages, high ON/OFF ratios, long retention times, and favorable stability.

Novel ternary organic resistive switching memory doped with bipolar materials

Organic resistive switching memory (ORSM) shows great potential for neotype memory devices due to the preponderances of simple architecture, low power consumption, high switching speed and feasibility of large-area fabrication. Herein, solution-processed ternary ORSM devices doped with bipolar materials were achieved with high ON/OFF ratio and outstanding device stability. The resistive switching performance was effectively ameliorated by doping two bipolar materials (DpAn-InAc and DpAn-5BzAc) in different blending concentration into the PVK:OXD-7 donor-accepter system. Compared with the binary system (PVK: 30 wt% OXD-7), the ON/OFF ratios of the ternary devices doped with 6 wt% DpAn-5BzAc were greatly increased from 7.91 × 102 to 4.98 × 104, with the operating voltage (∣V set–V reset∣) declined from 4.90 V to 2.25 V, respectively. Additionally, the stability of resistance state and uniformity of operating voltage were also significantly optimized for the ternary devices. For comparison, ternary devices doped with DpAn-InAc have been explored, which also achieved improved resistive switching behavior. A detailed analysis of electrical characteristics and the internal charge transfer properties of ORSM was performed to unveil the performance enhancement in ternary devices. Results indicate that the use of bipolar materials favors the efficient operation of OSRMs with proper energy level alignment and effective charge transfer.

High endurance organic resistive switching memory based on 1, 2-dicyanobenzene and polymer composites

As the emerging data storage technology, organic resistive switching memory (ORSM) possesses numerous superiorities as the substitution for or the complementation of the traditional Si-based semiconductor memory. Poly(3-hexylthiophene) (P3HT) has been widely used as a polymer donor component of ORSMs due to its advantages of high mobility and high chemical stability. Up to now, ORSM based on P3HT has achieved high on/off current ratio (<i>I</i><sub>on/off</sub>), but the endurance still needs to be improved. Herein, high endurance ORSMs based on 1,2-dicyanobenzene (O-DCB) and P3HT composite are fabricated by spin coating and thermally evaporating, and exhibit non-volatile and bipolar memory characteristics. The ORSMs based on P3HT:15 wt.% O-DCB and P3HT:30 wt.% O-DCB exhibit the values of <i>I</i><sub>on/off</sub> exceeding 10<sup>4</sup> and 10<sup>3</sup> respectively, and both of them exert excellent endurance of 400 times, retention time of more than 10<sup>5</sup> s. The mechanism of the switching is explored by linear fitting of <i>I-V</i> curve and electrochemical impedance spectrum . The results indicate that the filling and vacant process of the charge traps induced by O-DCB and the inherent traps in P3HT bulk lead to a resistive switching effect. The negative or positive bias triggers off trapping and detrapping process, which leads the conductive way of charges to change, resulting in the resistive switching effect. The excellent endurance of ORSM is attributed to the uniform distribution of O-DCB in P3HT bulk because of the small molecular size and high solubility of O-DCB, resulting in well-distributed and stable charge traps. On the other hand, the out-bound planarity of O-DCB molecular promotes the close interaction with the conjugated chains of P3HT. This study enlightens an effective strategy to carry out high-endurance ORSM and facilitates their electronic applications in future.

Rational design of a Ru(II) complex with a donor-acceptor-donor structure for organic resistive memory devices.

A novel Ru(II) complex with a donor-acceptor-donor (D-A-D) ligand was designed and synthesized to prepare organic memory devices. The fabricated Ru(II) complex-based devices exhibited obvious bipolar resistance switching behavior with a low switching voltage (∼1.13 V) and a large ON/OFF ratio (105). The dominant switching mechanism can be explained by the distinct charge-transfer states endowed by the interaction between metals and ligands, which is verified by density functional theory (DFT) calculations. Excitingly, the device displays a much lower switching voltage than most of the previously reported metal complex based memory devices due to the intense intramolecular charge transfer caused by the strong built-in electric field in D-A systems. This work not only reveals the potential of the Ru(II) complex in resistive switching devices, but also provides new inspiration to manipulate the switching voltage at the molecular level.

Regulating WORM/Flash electrical memory behavior of metallopolymers through varying metal centers

To study the influence of metal atoms on the performance of organic polymer memory devices, four new metallopolymers P0, P1, P2 and P3 were designed and synthesized for organic resistive random-access memory (RRAM) applications. Among them, the device based on P1 shows nonvolatile binary write-once-read-many-times (WORM) memory behavior, while the devices based on P2 and P3 show vastly different nonvolatile binary Flash-type memory behavior. The charge transfer (CT) effect between the platinum (Pt) ligand and porphyrin moieties of metallopolymers is determined to be the origin of the electrical switching behavior of P1, which is proved by the electrochemical and DFT calculations. While the reason of the Flash type memory characteristic of P2 and P3 is ascribed to the formation of metal filaments because of the introduction of metal ions (Co and Ni) into the porphyrin ring. It is suggested that the metal ions can form an internal electrode and act as a bridge during the CT process, which contributes to both the CT and back CT, consequently regulating the WORM/Flash conversion. The results indicate the feasibility of modulating the memory behavior of metallopolymers by varying the central metal ions, manifesting the significance of metal complexes in the application of organic memory devices and will attract wide research interest of scientists. The as-fabricated device based on P3 was also successfully applied to memory logic gates and display function, which offers great application prospect as smart sensor in artificial intelligence (AI) network.

The facile implementing ternary resistive memory in graphite-like melamine-cyanuric acid hydrogen-bonded organic framework with high ternary yield and environmental tolerance

The low ternary yield and expensive organic active materials hinder the potential industrialization of organic multi-level resistive memories. Here, facile melamine (M)-cyanuric acid (C) hydrogen-bonded organic framework (M−C HOF) was firstly fabricated as a new type of ternary memorizer. In the M−C HOF, the robust graphite-like planar 2D sheet along ab-plane containing a rosette motif is constructed from nine hydrogen bonds among M and C, and the 2D sheets are stacked into a 3D network through strong π–π stacking interactions. The FTO/M−C/Ag device exhibits a typical ternary memory performance with considerable OFF/ON1/ON2 current ratio of 1:103.1:104.6, relatively low set voltages (1.14 and 1.70 V), good reliability, and high ternary device yield of 74 %. The ternary resistive switching behavior could be ascribed to the filled traps in 2D layer (for ON1 state) and the carriers flowing in 3D network (for ON2 state), which have been verified by quenched fluorescence of isolated M and C, increased (201) lattice plane distance upon external voltages. Besides, this ternary memorizer has superior thermal stability (300 °C) and irradiative tolerance. This attempt of HOF-based ternary memorizer presents a new choice for efficient and facial multi-level resistive material.

Molecular-Shape-Controlled Binary to Ternary Resistive Random-Access Memory Switching of N-Containing Heteroaromatic Semiconductors.

In organic resistive random-access memory (ReRAM) devices, deeply understanding how to control the performance of π-conjugated semiconductors through molecular-shape-engineering is important and highly desirable. Herein, we design a family of N-containing heteroaromatic semiconductors with molecular shapes moving from mono-branched 1Q to di-branched 2Q and tri-branched 3Q. We find that this molecular-shape engineering can induce reliable binary to ternary ReRAM switching, affording a highly enhanced device yield that satisfies the practical requirement. The density functional theory calculation and experimental evidence suggest that the increased multiple paired electroactive nitrogen sites from mono-branched 1Q to tri-branched 3Q are responsible for the multilevel resistance switching, offering stable bidentate coordination with the active metal atoms. This study sheds light on the prospect of N-containing heteroaromatic semiconductors for promising ultrahigh-density data-storage ReRAM application.

Enhancement of resistive switching behavior of organic resistive random access memory devices through UV-Ozone treatment

We fabricated organic resistive random-access memory (RRAM) devices using a low-cost solution-process method. All the processes were performed at temperatures below 135 °C under ambient atmospheric conditions. The RRAM resistive switching layer was formed from a polymer-fullerene bulk heterojunction using poly(3-hexylthiophene-2,5-diyl) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM). The fabricated organic RRAM device exhibited typical nonvolatile bipolar resistive switching behavior with an ON/OFF ratio of ∼40, but it provided a low endurance of 27 cycles. Therefore, for enhanced stability, simple UV–Ozone (UVO) treatment was applied to the P3HT:PCBM organic bulk heterojunction layer. The organic RRAM device with UVO treatment exhibited an enhanced performance with an ON/OFF ratio of ∼400 and an endurance of 47 cycles. In addition, complementary resistive switching behavior was observed. The conduction mechanisms of the organic RRAM device were investigated by fitting the measured I–V data to numerical equations, and Schottky emission and Ohmic conduction were the main conduction mechanisms for the high-resistance and low-resistance states for the RRAM device with or without UVO treatment.

Unraveling the Role of Polydopamines in Resistive Switching in Al/Polydopamine/Al Structure for Organic Resistive Random-Access Memory.

In an era of rapidly evolving artificial intelligence and 5G communications technologies, massive data storage and processing are required for the real-time operation of digital processors in conventional wearable devices. However, classical von-Neumann architecture computers are limited by bottleneck-related issues. As a solution, resistive random-access memory (RRAM) devices are being considered as next generation in-memory computing devices. Among various materials, a polydopamine (PDA) is an attractive candidate for the fabrication of wearable and flexible RRAM devices. Herein, an aluminum/PDA/aluminum structure is proposed to investigate the influence of the PDA layer on resistive switching. The resistance-switching characteristics of an Al/PDA/Al structure are investigated by changing the PDA’s coating time and an on/off ratio of 2.48 × 103 is recorded. X-ray photoelectron spectroscopy reveals the presence of an Al2O3 layer in Al/PDA/Al structure, and the contents of oxygen vacancies are changed according to PDA coating time. Conductive filaments in the PDA/Al structure are confirmed by conductive atomic-force microscopy. As an application, a flexible Al/PDA/Al structure is fabricated using polyethylene terephthalate substrate and its operation is successfully confirmed. These results describe the resistive-switching characteristics, including oxygen vacancies, of Al/PDA/Al structures and provide new ways of understanding the resistive-switching mechanism of PDA-based RRAM devices.

High On/Off Ratio Organic Resistive Switching Memory Based on Carbazolyl Dicyanobenzene and a Polymer Composite

High On/Off Ratio Organic Resistive Switching Memory Based on Carbazolyl Dicyanobenzene and a Polymer Composite

Enhanced intra/intermolecular charge transfer for efficient multilevel resistive memory

Adjustment of molecular structure can effectively improve the organic memory device behaviors due to the optimization of intra/intermolecular interaction and molecular packing motif. However, the intrinsic mechanism of how intermolecular interaction and molecular packing motif affect the device performance needs to be further explored. Herein, we synthesized four new small molecules (X-TEBT) by introducing the alkynyl π bridges and different substituents (X = H, CN, tBu, OMe) into the backbone of conjugated donor–acceptor molecules, namely, TEBT, CN-TEBT, tBu-TEBT and OMe-TEBT, for solution-processed organic resistive memory. All memory devices displayed nonvolatile write-once-read-many-times (WORM) behaviors with low threshold voltage (Vth), high production yield, and good thermal stability. The champion device (tBu-TEBT) exhibits ternary WORM property with the highest ON2/ON1/OFF ratio and the lowest Vth1 (1.6 V) and Vth2 (2.4 V). More importantly, the single crystal structure of tBu-TEBT was successfully obtained which helped us analyze the origin of the physical mechanism of resistive switching behavior and establish a common structure–property relationship for high performance organic memory. Its excellent properties enable us to perform logic gates and information display functions which offer possibilities as smart sensor in Internet of Things (IoT) application.

Polyelectrolyte Bilayer-Based Transparent and Flexible Memristor for Emulating Synapses.

Memristors will be critical components in the next generation of digital technology and artificial synapses. Researchers are investigating innovative mechanistic understanding of the memristor devices based on low-cost, solution-processable, and organic materials as promising candidates. Here, we demonstrate a novel polyelectrolyte-based memristor device, which is simply prepared by spin-coating poly(acrylic acid) (PAA) and polyethylenimine (PEI) on an indium tin oxide (ITO) substrate followed by a magnetron sputtering of the ITO as the top electrode. The device has a potential to achieve excellent resistive switching (RS) performance and synapse functionality as well as greater flexibility and transmittance when compared to the oxide-based memories. An on/off resistance ratio of 50 can be maintained without degradation for up to 20 000 cycles (flat state) and over 4000 cycles (bending to a 2 mm radius 10 000 times) in the DC sweep mode. Moreover, the device performs various synaptic functions, including spike-timing-dependent plasticity, pulse pair plasticity, and short-term and long-term plasticity in the potentiation and depression processes. The counterions and two oppositely charged polyelectrolyte chains can move in and out of each other depending on the applied electrical potential (pulse), resulting in a change in the potential drop at the interface of the polyelectrolyte bilayer and its electrodes, which can be attributed to the RS mechanism and various synaptic functions. This insight may accelerate the technological deployment of the organic resistive memories.

The effect of the PVA hydrolysis degree on the electrical properties of organic resistive memories based on PVA + CNT composites

This work studies the interaction effect between functionalized multiwall carbon nanotubes (f-CNTs) and a polymeric matrix in the electrical behavior of resistive memories. f-CNTs were synthesized by spray pyrolysis using xylene-ethanol (1:0.8 v/v) as the reaction mixture, allowing the in-situ functionalization (OH, COOH, CH, C–OH groups) on the f-CNT surface and, then, improving their solubility in polar solvents as well as their dispersion in a polymeric matrix. Two types of polyvinyl alcohol (PVA) with different hydrolysis degrees were used, PVA with high hydrolysis (HD) degree (Mw 89,000-98,000, 99 + % hydrolyzed) and PVA with low hydrolysis (LD) degree (Mw 9000-10,000, 80% hydrolyzed), for the fabrication of the organic resistive memories. The PVA + f-CNT composites show significant differences in the morphology, thickness and current-voltage (I-V) electrical behavior as a function of the hydrolysis degree of the PVA matrix. In the case of IV measurements, our results are broadly different from those reported in the literature, PVA-LD + f-CNT composites at 1, 3 and 5 wt% f-CNTs show rewritable behavior, whereas PVA-HD + f-CNT composites the rewritable behavior started at 5 wt%. From here, we conclude that the polymeric matrix can play an essential role in the resistive memory behavior.

Cyclometalated Platinum(II) Complexes with Donor-Acceptor-Containing Bidentate Ligands and Their Application Studies as Organic Resistive Memories.

A series of heteroleptic cyclometalated platinum(II) complexes, [Pt(C^N)(O^O)], (1-10) with various donors and acceptors has been synthesized and characterized by 1 H NMR spectroscopy, elemental analyses, infrared spectroscopy and mass spectrometry. The X-ray structure of 2 has also been determined. The electrochemical and photophysical properties of the platinum(II) complexes were studied. These experimental results have been supported by computational studies. Furthermore, two of the complexes have been employed as the active material in the fabrication of resistive memory devices, exhibiting stable binary memory performance with low operating voltage, high ON/OFF ratio and long retention time.

Toward Highly Robust Nonvolatile Multilevel Memory by Fine Tuning of the Nanostructural Crystalline Solid-State Order.

Organic resistive memory (ORM) offers great promise for next-generation high-density multilevel-cell (MLC) data storage. However, the fine tuning of crystalline order among its active layer still remains challenging, which largely restricts ORM behavior. Here, an exceptional solid-state transition from disordered orientations to highly-uniform orientation within the ORM layer is facilely triggered via molecular strategic tailoring. Two diketopyrrolopyrrole-based small molecular analogues (NI1 TDPP and NI2 TDPP) are demonstrated to display different symmetry. The asymmetric NI1 TDPP shows an irregular solid-state texture, while the centro-symmetric NI2 TDPP conforms to an ordered out-of-plane single-crystalline pattern that aligns with the foremost charge transportation along the substrate normal, and exhibits excellent MLC memory characteristics. Moreover, this highly oriented pattern guarantees the large-area film uniformity, leading to the twofold increase in the yield of as-fabricated ORM devices. This study reveals that the solid-state crystalline nanostructural order of organic materials can be controlled by reasonable molecular design to actuate high-performance organic electronic circuits.

Carbon-based Multi-layered Films for Electronic Application: A Review

The current article reviews the construction of thin films comprising carbon-based nanomaterials for application in electrical and electronic fields. Carbon-based materials such as carbon nanotubes, graphene, and fullerene are known to possess excellent electrical and electronic properties, which makes them desirable materials for the fabrication of micro- and nano-electromechanical devices. The fabrication process of thin films, including the deposition of several layers, removal of layers, solution processing methods, inkjet printing, micro-emulsion polymerization methods, deposition through filtration processes, spin coating, dip coating, pen lithography, vacuum-assisted flocculation, vacuum-assisted layer-by-layer assembly laser writing, etc., has been extensively reviewed. It is evident from the past findings that the fabricated thin films constituting carbon nanomaterials shows predominant alteration in electrical and electronic properties like sheet resistance, ionic transport, potential difference, conductivity, electro-rheological, transparency, trans-conductance, transmittance, bending stability, etc. In view of the referred properties, the developed materials find wide applications in charge-trap flash memories, flexible organic resistive memory devices, photovoltaic devices, flexible and transparent electronics, heat sinks in electronic materials, liquid crystal displays (LCDs), thin-film solar cells, flexible touch-screen panels, electronic papers, micro-batteries, electrochemical micro-capacitors, humidity sensors, optoelectronic devices, etc. We expect that the current review article will be a valuable asset for the researchers working in the field of carbon nanomaterials.

Switching the resistive memory behavior from binary to ternary logicviasubtle polymer donor and molecular acceptor design

Subtle polymer donor and molecular acceptor design enables efficient solution-processed organic resistive switching memory

A benzothiadiazole-containing π-conjugated small molecule as promising element for nonvolatile multilevel resistive memory device

A novel 2,1,3-benzothiadiazole (BT)-based donor−acceptor (D−A) type small molecule (NBTC) has been demonstrated for promising application in organic multilevel resistive memory devices. NBTC exhibits excellent nonvolatile ternary memory effects with high ON2/ON1/OFF current ratios of 107.5/103.8/1 and threshold voltages of −2.70 and −4.80 ​V, attributing to the different electron-withdrawing abilities of acceptors that can induce stepwise D−A charge transfer and charge trapping processes. This result suggests that BT-based D−A type materials can hold great promise for achieving ultrahigh-density data-storage devices through the rational D−A modulation, which provides guidelines for designing more proof-of-concept organic memory electronics for big data storage application.