Organic Resistive Memory Devices 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.

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.

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.

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.

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.

Charge-transfer processes in metal complexes enable luminescence and memory functions

Molecular transition-metal–ligand complexes are emerging as useful paradigms in materials science. Transition metal complexes have diverse metal d electron configurations, oxidation states, coordination numbers and geometries such that they can undergo a diverse array of electronic transitions. Metal-to-ligand charge-transfer transitions and their associated excited states are especially attractive given their rich redox properties and robustness. This chemistry is accessible by appropriate choice of low-valence metal centres and strong π-acceptor ligands. An in-depth fundamental understanding of their charge-transfer, assembly and structure–property relationships is important to allow us to rationally design complexes and tune their characteristics for an intended application. In addition to their attractive light-harvesting and photocatalytic applications, this Perspective describes recent developments in the use of transition metal complexes as materials in phosphorescent organic light-emitting diodes and resistive memory devices. Metal–ligand complexes undergo diverse charge-transfer processes when stimulated by light or electric fields. This Perspective describes how these processes can be exploited in photosensitizers, luminophores and resistive memory materials.

Photoresponsive Dithienylethene-Containing Tris(8-hydroxyquinolinato)aluminum(III) Complexes with Photocontrollable Electron-Transporting Properties for Solution-Processable Optical and Organic Resistive Memory Devices.

The rational design of a new class of photoresponsive tris(8-hydroxyquinolinato)aluminum(III) (Alq3) complexes has been developed. By incorporating the photochromic dithienylethene units with different peripheral heterocycles into the Alq3 framework, the photochromic properties as well as photoswitching efficiency can be readily modulated, through effective photocyclization of the Al(III) complex. Such intrinsic photochromic behavior leads to the unprecedented enhancement in the electron-transporting properties as demonstrated by the as-fabricated electron-only device, rendering the realization of photoswitchable electron mobility. In addition, one of these complexes is capable of serving as an active layer for solution-processable resistive memory devices. Photocontrollable memory performance has been achieved with a binary memory behavior, with high ON/OFF ratio and long retention time. This work represents not only the first example of photoresponsive Alq3-based electron-transporting materials but also the solution-processable Alq3-based optical and resistive memory devices with photocontrollable performance.

Recent advances in organic‐based materials for resistive memory applications

AbstractWith the rapid development of data‐driven human interaction, advanced data‐storage technologies with lower power consumption, larger storage capacity, faster switching speed, and higher integration density have become the goals of future memory electronics. Nevertheless, the physical limitations of conventional Si‐based binary storage systems lag far behind the ultrahigh‐density requirements of post‐Moore information storage. In this regard, the pursuit of alternatives and/or supplements to the existing storage technology has come to the forefront. Recently, organic‐based resistive memory materials have emerged as promising candidates for next‐generation information storage applications, which provide new possibilities of realizing high‐performance organic electronics. Herein, the memory device structure, switching types, mechanisms, and recent advances in organic resistive memory materials are reviewed. In particular, their potential of fulfilling multilevel storage is summarized. Besides, the present challenges and future prospects confronted by organic resistive memory materials and devices are discussed.image

Benzoselenadiazole-Based Conjugated Molecules: Active Switching Layers with Nanofibrous Morphology for Nonvolatile Organic Resistive Memory Devices.

In this work, two symmetrical donor-acceptor-donor (D-A-D) type benzoselenadiazole (BSeD)-based π-conjugated molecules were synthesized and employed as an active switching layer for non-volatile data storage applications. BSeD-based derivatives with different donor units attached through common vinylene linkers showed different electrical and optical properties. 4,7-Di((E)-styryl)benzo[c][2,1,3]selenadiazole (DSBSeD) and 4,7-bis((E)-4-methoxystyryl)benzo[c][2,1,3]selenadiazole (DMBSeD) are sandwiched between gallium-doped ZnO (GZO) and metal aluminum electrodes respectively through solution-processed spin-coating method. The solution-processed nanofibrous switching layer containing the DMBSeD-based memory device showed reliable memory characteristics in terms of write and erase operations with low SET voltage than the random-aggregated DSBSeD-based device. The nanofibrous molecular morphology of switching layer overcomes the interfacial hole transport energy barrier at the interface of the DMBSeD thin-film and the bottom GZO electrode. The memory device GZO/DMBSeD/Al based on nanofibrous switching layers shows switching characteristics at compliance current of 10 mA with Vset =0.79 V and Vreset =-0.55 V. This work will be beneficial for the rational design of advanced next-generation organic memory devices by controlling the nanostructured morphology of active organic switching layer for enhanced charge-transfer phenomenon.

Three-Dimensional Spirothienoquinoline-Based Small Molecules for Organic Photovoltaic and Organic Resistive Memory Applications.

A new electron-rich spirothienoquinoline unit, tBuSAF-Th, has been developed via incorporation of a thienyl unit instead of a phenyl unit into the six-membered ring of the spiroacridine (SAF) and utilized for the first time as a building block for constructing small-molecule electron donors in organic solar cells (OSCs) and as active layers in organic resistive memory devices. The resulting three-dimensional spirothienoquinoline-containing 1-4 exhibit high-lying highest occupied molecular orbital (HOMO) energy levels. By the introduction of electron-deficient benzochalcogenodiazole linkers, with the chalcogen atoms being varied from O to S and Se, a progressively lower lowest unoccupied molecular orbital (LUMO) energy level has been achieved while keeping the HOMO energy levels similar. This strategy has allowed an enhanced light-harvesting ability without compromising open-circuit voltage (Voc) in vacuum-deposited bulk heterojunction OSCs using 1-4 as donors and C70 as the acceptor. Good photovoltaic performances with power conversion efficiencies (PCEs) of up to 3.86% and high short-circuit current densities (Jsc) of up to 10.84 mA cm-2 have been achieved. In addition, organic resistive memory devices fabricated with these donor-acceptor small molecules exhibit binary logic memory behaviors with long retention times and high on/off current ratios. This work indicates that the spirothienoquinoline moiety is a potential building block for constructing multifunctional organic electronic materials.

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.

Effects of the Molecular Chain Length of Polyimide on the Characteristics of Organic Resistive Random Access Memories

We developed organic resistive random access memory (ReRAM) devices using spin-coated polyimide (PI) as the resistive layer. In this article, the effect of the chain length of the PI macromolecules on the electrical performance of ReRAM devices was studied. The chain length of the PI macromolecule was controlled by repeating the addition step of 4, $4^\prime $ -diaminodioxydianiline in a polyamic acid precursor. A long molecular chain of the PI film could be obtained by increasing the addition times. The electrical properties of the PI-based ReRAM devices revealed that the leakage current in the high-resistance state and memory window can be improved by increasing the molecular chain length of the PI film. A model is proposed to explain the effects of the chain length on the conduction mechanism in the PI film. Moreover, PI films with long molecular chains exhibited an increased retention time.

Physical mechanisms involved in the formation and operation of memory devices based on a monolayer of gold nanoparticle-polythiophene hybrid materials.

Understanding the physical and chemical mechanisms occurring during the forming process and operation of an organic resistive memory device is a requisite for better performance. Various mechanisms were suggested in vertically stacked memory structures, but the analysis remains indirect and needs destructive characterization (e.g. analysis of the cross-section to access the organic layers sandwiched between electrodes). Here, we report a study on a planar, monolayer thick, hybrid nanoparticle/molecule device (10 nm gold nanoparticles embedded in an electro-generated poly(2-thienyl-3,4-(ethylenedioxy)thiophene) layer), combining in situ physical (scanning electron microscopy, physicochemical thermogravimetry and mass spectroscopy, and Raman spectroscopy) and electrical (temperature dependent current–voltage) characterization on the same device. We demonstrate that the forming process causes an increase in the gold particle size, almost 4 times larger than the starting nanoparticles, and that the organic layer undergoes a significant chemical rearrangement from an sp3 to sp2 amorphous carbon material. Temperature dependent electrical characterization of this nonvolatile memory confirms that the charge transport mechanism in the device is consistent with a trap-filled space charge limited current in the off state, with the sp2 amorphous carbon material containing many electrically active defects.

Miniaturization and Integration of Organic Resistive Memory Devices

Recently, organic resistive memory devices have attracted great interest because they can be fabricated to low cost, flexible, and printable memory cells. Here, we reviewed recent advances of miniaturization and integration of organic resistive memory devices. We introduced research background of fabrication of organic resistive memory devices. Then we arranged achievements on miniaturization and integration of organic resistive memory devices in chronological order. Finally, we summarized research outlook of miniaturization and integration of organic resistive memory devices.

Nanostructured Fused Pyrrole Thin Films: Encoding Nano “Bits” with Temporary Remanence

AbstractA comprehensive understanding of resistive switching phenomenon and its dependence on molecular structure is imperative for enhancing the consistency and reliability of organic resistive memory (ORM) devices. Here, the efforts are directed to establish a premise for providing detailed insights into the molecular property, thin film assembly, and digital memory performance of a 1,4‐dihydropyrrolo[3,2‐b]pyrrole (DHPP) derivative. The fabricated devices display switching characteristics with an ION/OFF ratio of ≈105, howbeit, with a “temporary remanence” of ≈2 min. The ON state can be sustained under a constant electrical duress of −1 V and can be repeatedly reprogrammed for >110 cycles. Conductive atomic force microscope (C‐AFM) studies demonstrate that the thin film can be electrically written to a “0” or “1” state under extremely low compliance currents of ±250 pA with an appreciable ON/OFF ratio of 102. Conservative estimates for the switching area of ≈150 nm2 with energy as low as 15 fJ to induce a switching event underscore the possibility of nanoscale data storage with high areal density. The role of charge transfer interactions during the OFF to ON transitions and the origin of volatile memory behaviour are further elucidated in conjunction with electrochemical impedance studies (EIS) and theoretical simulations.

Thiadizoloquinoxaline-Based N-Heteroacenes as Active Elements for High-Density Data-Storage Device.

A novel thiadiazoloquinoxaline (TQ)-based donor-acceptor (D-A)-type N-heteroacene (Py-1-TQ) has been demonstrated for promising applications in organic multilevel resistive memory devices. Compared with its counterparts (Py-0-TQ and Py-2-TQ), which show flash-type binary memory behaviors, Py-1-TQ exhibits excellent nonvolatile write-once-read-many-times-type ternary memory effects with high ON2/ON1/OFF current ratios (105.8:103.4:1), which can be attributed to the different electron-withdrawing abilities between the pyrazine unit and TQ species that can induce stepwise D-A charge-transfer processes. These results suggest that TQ-based N-heteroacenes can be potentially useful in ultrahigh-density data-storage devices through the rational D-A tuning.

A Flexible High-Performance Photoimaging Device Based on Bioinspired Hierarchical Multiple-Patterned Plasmonic Nanostructures.

In insect eyes, ommatidia with hierarchical structured cornea play a critical role in amplifying and transferring visual signals to the brain through optic nerves, enabling the perception of various visual signals. Here, inspired by the structure and functions of insect ommatidia, a flexible photoimaging device is reported that can simultaneously detect and record incoming photonic signals by vertically stacking an organic photodiode and resistive memory device. A single-layered, hierarchical multiple-patterned back reflector that can exhibit various plasmonic effects is incorporated into the organic photodiode. The multiple-patterned flexible organic photodiodes exhibit greatly enhanced photoresponsivity due to the increased light absorption in comparison with the flat systems. Moreover, the flexible photoimaging device shows a well-resolved spatiotemporal mapping of optical signals with excellent operational and mechanical stabilities at low driving voltages below half of the flat systems. Theoretical calculation and scanning near-field optical microscopy analyses clearly reveal that multiple-patterned electrodes have much stronger surface plasmon coupling than flat and single-patterned systems. The developed methodology provides a versatile and effective route for realizing high-performance optoelectronic and photonic systems.

Ferroelectric-mediated filamentary resistive switching in P(VDF-TrFE)/ZnO nanocomposite films.

In ferroelectric (FE) polymer-semiconducting polymer blend based organic resistive random access memory devices (ReRAM), the carriers are injected into the semiconductor region of the blend because of the polarization originated internal electric field in the FE polymer. A higher concentration of semiconducting polymer in the FE polymer-semiconducting polymer blends usually generate a high leakage current and degrades the FE characteristics of the FE polymer resulting in a high OFF current and consequently a low ON/OFF ratio. In order to achieve a high ON/OFF ratio in the FE polymer/semiconducting polymer blends, the FE properties of the FE polymer should be preserved. In this study, organic ReRAMs based on ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and ZnO nanoparticle (NPs) blends exhibiting bipolar resistive switching and a high ON/OFF ratio were realized using a low-cost solution process. Unlike conventional ferroelectric polymer-semiconducting polymer blend systems where FE characteristics are suppressed in ReRAMs, our Au/P(VDF-TrFE)_ZnO NPs/n++Si devices retain the FE characteristics of the P(VDF-TrFE) polymers. Our devices switch between bi-stable resistance states via the ferroelectric-assisted filamentary conduction mechanism. Based on ex situ transmission electron microscopy and elemental mapping analyses, we found that the resistive switching occurs through the formation of conduction paths consisting of Zn-rich/F-deficient regions. The device fabricated at a blend ratio of 20 wt% ZnO NPs in P(VDF-TrFE) matrix exhibited optimal stable resistive switching behavior with an ON/OFF ratio of up to 2 × 107 and a retention time of 104 s.

Racemic Effect on the Performance of Organic Multilevel Memory: Beyond Molecular Design

AbstractImproving organic devices via molecular design is challenging and difficult to rationalize because individual molecular properties always convolute with intermolecular interaction and crystallinity to contribute the device performance. In this work, it is demonstrated for the first time that racemic effect can be utilized to effectively improve memory device performance. A pair of enantiomers and their mixture are used to prepare organic multilevel resistive random access memory (RRAM) devices. The RRAM devices fabricated from the equimolar mixture of enantiomers are of the lowest onset voltages and highest ternary device yield, independent of the preparation methods. Structural analysis of the powder, film, and single crystal of the equimolar mixture reveals formation of a real racemic compound, where π–π stacking in contrast to the only C–H–π interaction in the film of pure enantiomers appears. Since individual enantiomer has identical electronic properties, the discrepancy in device performance is attributed to the different intermolecular interactions.