Organic Nonvolatile Memory Devices Research Articles

Inherent D-A Architecture in Indoloquinoxalines with an Array of Substituents for Non-Volatile Memory Device Applications.

Donor-acceptor (D-A)-based architecture has been the key to increase storage capability efficiency through the enhanced charge transportation in the fabricated device. We have designed and synthesized a series of functionalized indoloquinoxalines (IQ) for non-volatile organic memory devices. The investigation on UV-visible spectra reveals the absorption maxima of the compounds around 420 nm, attributed to the intramolecular charge transfer between indole and quinoxaline moiety. The irreversible anodic peak in the 1.0 to 1.5 V range indicates the indole moiety's oxidation ability. Besides, the cathodic peak in the range of -0.5 to -1.0 V, contributed to the stability of the reduced quinoxaline unit. All the compounds exhibited uniformly covered thin film in SEM analysis, potentially facilitating the seamless charge carrier migration between adjacent molecules. The methoxyphenyl substituted compound exhibited the binary write-once read-many (WORM) memory behavior with the lowest threshold voltage of -0.81 V. The molecular simulations displayed the efficient intramolecular charge transfer, providing the fabricated device's distinctive conductive states. Except for the tert-butylphenyl compound, which showed volatile dynamic random-access memory (DRAM) behavior, all the other compounds exhibited non-volatile WORM memory behavior, suggesting IQs potential as an intrinsic D-A molecule in organic memory devices on further structural refinement.

A study on memory characteristics of hybrid-based charge trap-type organic non-volatile memory device according to gate stack thickness

Continuous research on charge trap-type organic non-volatile memory (CT-ONVM) devices aims to achieve highly reliable large memory window characteristics, comparable to inorganic-based poly-silicon/oxide/nitride/oxide/silicon (SONOS) devices. This study introduces hybrid-based ultra-thin films via the initiated chemical vapor deposition (iCVD) process as the gate-stack of highly reliable CT-ONVM devices. One method of achieving a wide memory window is by increasing the gate-stack thickness of hybrid-based gate stacks of CT-ONVM devices. However, thick gate-stacks (>50 nm) do not dramatically change the memory window and deteriorate the subthreshold swing (S.S.) compared to thin gate-stacks (<30 nm). Electrons passing through the thickened tunneling dielectric layer (TDL) generate a large number of interface traps, which degrade the S.S. and hinder the charge injection into the charge trapping layer (CTL). This contributes to the non-change of the memory window to increase along with the leakage through the blocking dielectric (BDL) to the gate electrode. Based on electrical characterization, our group proposes directions and follow-up plans to effectively improve the memory window and reliability of CT-ONVM devices.

Biomaterial Inks from Peptide-Functionalized Silk Fibers for 3D Printing of Futuristic Wound-Healing and Sensing Materials.

This study illustrates the sensing and wound healing properties of silk fibroin in combination with peptide patterns, with an emphasis on the printability of multilayered grids, and envisions possible applications of these next-generation silk-based materials. Functionalized silk fibers covalently linked to an arginine-glycine-aspartic acid (RGD) peptide create a platform for preparing a biomaterial ink for 3D printing of grid-like piezoresistors with wound-healing and sensing properties. The culture medium obtained from 3D-printed silk fibroin enriched with RGD peptide improves cell adhesion, accelerating skin repair. Specifically, RGD peptide-modified silk fibroin demonstrated biocompatibility, enhanced cell adhesion, and higher wound closure rates at lower concentration than the neat peptide. It was also shown that the printing of peptide-modified silk fibroin produces a piezoresistive transducer that is the active component of a sensor based on a Schottky diode harmonic transponder encoding information about pressure. We discovered that such biomaterial ink printed in a multilayered grid can be used as a humidity sensor. Furthermore, humidity activates a transition between low and high conductivity states in this medium that is retained unless a negative voltage is applied, paving the way for utilization in non-volatile organic memory devices. Globally, these results pave the way for promising applications, such as monitoring parameters such as human wound care and being integrated in bio-implantable processors.

An organic ambipolar charge trapping non-volatile memory device based on double heterojunctions

An organic ambipolar charge trapping non-volatile memory device is proposed based on tris(8-hydroxy quinoline) aluminum(Alq3)/ fullerene(C60)/pentacene double heterojunctions. The double heterojunctions were analyzed and compared with the sole heterojunction. Even though the Alq3/pentacene sole heterojunction shows a negative threshold voltage shift for programming under negative biasing, light illumination is required to efficiently erase. In contrast, for C60/pentacene sole heterojunction, the negative shift of threshold voltage is observed even with a positive erasing voltage. By combining the two trapping layers, a large memory window with fully-electrical programming and erasing is obtained. The efficient programming and erasing of the double heterojunctions structure are analyzed, sufficient electrons can be injected that enables the electrical erasing, and the negative shift of the threshold voltage under positive erasing voltage for the C60/pentacene structure is suppressed due to the suppression of the large built-in potential for the sole heterojunction, which can be verified by a large positive shift for the pentacene/C60/pentacene double heterojunctions.

Single-Crystalline Thin-Film Memory Arrays of Molecular Ferroelectrics with Ultralow Operation Voltages

Though most of the recently developed molecular ferroelectrics (MFs) exhibit excellent ferroelectric properties, the practical applications are still impeded by the limited polarization axes, poor processing capability for a high-quality thin film, and the incompatibility with matured lithography techniques for microelectronics. Here, we successfully demonstrated MF-based single-crystalline microdevice arrays using a lithography-compatible, solution-processed strategy that can avoid the above-mentioned obstacles at one time, i.e., dewetting-assisted patterning crystallization strategy. As a protype, uniform and well-aligned single-crystalline thin-film arrays of multiaxial MF [3-oxoquinuclidinium]ClO4 ([3-O-Q]ClO4) are prepared. Owing to the well-aligned crystallographic orientation and polarization direction, the obtained single-crystalline nonvolatile memory (NVM) arrays can exhibit an ultralow operating voltage of ∼1.6 V and long endurance cycles of 106, which are superior to other organic NVM devices. This work implies a promising route to high-density single-crystalline memory arrays for data storage, especially for low-cost flexible electronics.

Stable and Multilevel Data Storage Resistive Switching of Organic Bulk Heterojunction.

Organic nonvolatile memory devices have a vital role for the next generation of electrical memory units, due to their large scalability and low-cost fabrication techniques. Here, we show bipolar resistive switching based on an Ag/ZnO/P3HT-PCBM/ITO device in which P3HT-PCBM acts as an organic heterojunction with inorganic ZnO protective layer. The prepared memory device has consistent DC endurance (500 cycles), retention properties (104 s), high ON/OFF ratio (105), and environmental stability. The observation of bipolar resistive switching is attributed to creation and rupture of the Ag filament. In addition, our conductive bridge random access memory (CBRAM) device has adequate regulation of the current compliance leads to multilevel resistive switching of a high data density storage.

The Effect of Shallow Trap Density on the Electrical Characteristics of an Organic Nonvolatile Memory Device Based on Eight-Hydroxyquinoline

An organic nonvolatile memory is proposed based on the synergistic effect of the (three-aminopropyl) triethoxysilane (APTES) self-assembled monolayer (SAM) and the polystyrene (PS):eight-hydroxyquinoline (8-HQ) blend layer, where 8-HQ is used to provide the trapping sites for the nonvolatile memory, and PS is used to form the matrix for 8-HQ in order to improve the memory property. In addition, the introduced APTES SAM aims to further increase the memory window. Two kinds of PS:8-HQ blend films are adopted, with lower 8-HQ concentration of 50 mg/mL and high spin-coating speed of 5000 rpm to form smooth thin film, while with high 8-HQ concentration of 100 mg/mL and low spin-coating speed of 1000 rpm to form rough thick film. The roughness and the surface modification property of the PS:8-HQ blend film greatly affect the morphology of the active layer, and thus the shallow trap density is also influenced, which further affects the electrical characteristics and memory properties of the nonvolatile memory device. The better characteristics of the memory device with thin 8-HQ layer indicate the potential applications in many domains.

High‐Performance Flexible Organic Nonvolatile Memories with Outstanding Stability Using Nickel Oxide Nanofloating Gate and Polymer Electret

AbstractOrganic nonvolatile memory devices based on organic field‐effect transistors (OFETs) have attracted attention as promising memory components in flexible organic integrated circuits. OFET‐based organic memories can be fabricated using solution processes with high portability and flexibility. However, many challenges still remain, such as achieving high levels of transparency and data storage while maintaining device reliability. In this study, the high‐performance of flexible organic nonvolatile memory devices is demonstrated through the synergistic effect of using a combination of polymer electret (poly(2‐vinyl naphthalene), PVN) and nanofloating gates (nickel oxide nanoparticles (NiOx NPs)). The PVN/NiOx NPs bilayer successfully acts as a charge storage medium with high transparency in the visible range. This system greatly improves memory performance including high data storage capacity, stable cyclic endurance, and quasi‐permanent retention characteristics (&gt;10 years). Furthermore, these devices also show excellent charge storage characteristics after 1000 bending cycles and even under the sharply bent state (bending radius 3 mm). This enhanced mechanical stability of flexible synergistic memory devices demonstrates that NiOx NPs can play an important role as a reliable data storage material for next‐generation wearable electronic devices.

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.

Memristive response and electrochemical processes in polyaniline based organic devices

We investigated in details the redox mechanism of an electrochemical two-terminal solid-state device with memory, otherwise named organic memristive device (OMD), by varying the composition of solid polyelectrolytes, the doping agents of the conducting polymer, and the material of the gate electrode. The switching mechanism resulted dependent on the presence of a redox active polymer with high conducting and low conducting states, the presence of a redox counter-reaction, and the ability to transport ions between the polymer and electrolyte phases. The memristive response was observed in hydrochloric acid doped and silver gated devices. The crucial role of the chloride ions and of the silver wire as gate electrode for the promotion of electrochemical reactions is demonstrated. The redox reactions occurring in OMD, the silver dissolution process and the redox reactions of PANI result to be tuned by silver chloride precipitation and ion migration into/out of the polymer matrix to compensate the excess of charges inside the film. The dopant anions are released from polymer network during its oxidation and deprotonation process. Our results, giving rise to a more detailed understanding of the mechanisms involved in redox gated memory devices, provide also a better identification and important details about the requirements for the conducting polymer, the redox counter reaction, and the polyelectrolyte for optimizing the development and fabrication of organic solid-state nonvolatile memory devices. • The redox mechanism of a conductive polymer in a solid-state device was investigated. • Different formulations of polymeric blends as well as the electrode materials were considered. • The current rectification was observed without the presence of any cation in the polyelectrolyte. • The memristive response was due to chloride acid in silver gated devices.

Pentafluorosulfanylated polymers as electrets in nonvolatile organic field-effect transistor memory devices

Styrenic polymer electrets with pentafluorosulfanylated sidechains demonstrate their charge-storage capabilities in organic nonvolatile memory devices.

Structural parameters affecting the performance of non-volatile memory based on organic field-effect transistors

To obtain better memory performance, we have varied different structural parameters such as thickness of the gold electrodes dAu and thickness of the electret de for organic non-volatile memory (ONVM) devices based on pentacene field-effect transistors. It was found that the memory devices gained larger memory windows by increasing dAu. This effect is attributed to the improved electron injection at the Au/pentacene contact. On the other hand, decreasing de facilitated the transport of the injected electrons, leading to a highly efficient programming. However, both strategies in excessive cases would unfortunately suffer from some limitations, which should be considered for practical applications. Additionally, with help of the results, we achieved an overall understanding about the operating mechanism of this memory type in aspects of charge injection and charge trapping.

Organic non-volatile memory device based on cellulose fibers

The present manuscript reports the development of Ag/cellulose fibers/Al memory device using the electrospinning technique. The morphological characterization suggested that the active layer is composed of micro-fibers. The developed device shows fingerprint pinched hysteresis loop of the memristive device in I–V plane without any additional electroforming step. An excellent endurance for 6 × 103 resistive switching cycles with 10x memory window is achieved. Furthermore, the data retention capability of the developed device is extended for the 3 × 102 seconds without any observable degradation in the resistance states. The statistical results suggested that the high resistance state loosely distributed whereas tight distribution is observed for low resistance state. The electrical characterization results suggested that the formation and breaking of Ag conductive filament in the active cellulose fiber layer are responsible for the bipolar resistive switching effect. Our results suggested that the cellulose fibers based memristive device is a potential candidate for the organic non-volatile memory application.

Resistive‐Switching Memory: Investigation of Time–Dependent Resistive Switching Behaviors of Unipolar Nonvolatile Organic Memory Devices (Adv. Funct. Mater. 35/2018)

Resistive‐Switching Memory: Investigation of Time–Dependent Resistive Switching Behaviors of Unipolar Nonvolatile Organic Memory Devices (Adv. Funct. Mater. 35/2018)

Reversible Charge Trapping in Bis-Carbazole-Diimide Redox Polymers with Complete Luminescence Quenching Enabling Nondestructive Read-Out by Resonance Raman Spectroscopy.

The coupling of substituted carbazole compounds through carbon–carbon bond formation upon one-electron oxidation is shown to be a highly versatile approach to the formation of redox polymer films. Although the polymerization of single carbazole units has been proposed earlier, we show that by tethering pairs of carbazoles double sequential dimerization allows for facile formation of redox polymer films with fine control over film thickness. We show that the design of the monomers and in particular the bridging units is key to polymer formation, with the diaminobenzene motif proving advantageous, in terms of the matching to the redox potentials of the monomer and polymer film and thereby avoiding limitations in film thickness (autoinsulation), but introduces unacceptable instability due to the intrinsic redox activity of this moiety. The use of a diimide protecting group both avoids complications due to p-diamino-benzene redox chemistry and provides for a redox polymer in which the photoluminescence of the bis-carbazole moiety can be switched reversibly (on/off) with redox control. The monomer design approach is versatile enabling facile incorporation of additional functional units, such as naphthalene. Here we show that a multicomponent carbazole/naphthalene containing monomer (APCNDI) can form redox polymer films showing both p- and n- conductivity under ambient conditions and allows access to five distinct redox states, and a complex electrochromic response covering the whole of the UV/vis–NIR spectral region. The highly effective quenching of the photoluminescence of both components in poly-APCNDI enables detailed characterization of the redox polymer films. The poly-APCNDI films show extensive charge trapping, which can be read out spectroscopically in the case of films and is characterized as kinetic rather than chemical in origin on the basis of UV/vis–NIR absorption and resonance Raman spectroscopic analyses. The strong resonantly enhanced Raman scattering for the various oxidized and reduced states of APCNDI enables nondestructive “read-out” of the state of the polymer, including that in which charges are trapped kinetically at the surface, making poly-APCNDI highly suitable for application as a component in organic nonvolatile memory devices.

Flexible rewritable organic memory devices using nitrogen-doped CNTs/PEDOT:PSS composites

Nonvolatile rewritable organic memory devices based on poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and nitrogen doped multi-walled carbon nanotube (NCNT) nanocomposites were fabricated on glass and PET substrates.Organic memory devices with bistable resistive switching were obtained using very low NCTN concentration (∼0.002 wt%) in the polymeric matrix. The memory devices exhibited a good ON/OFF ratio of approximately three orders of magnitude, a good retention time of 104 s under operating voltages ≤ |4V| and a few hundredths of write-read-erase-read cycles. The bistable resistive switching is mainly attributed to the creation of oxygen vacancies. These defects are introduced into the thin native Al oxide (AlOx) layer on the bottom electrode during the first voltage sweep. The well-dispersed NCNTs immersed in PEDOT:PSS play a key role as conductive channels for the electronic transport, hindering the electron trapping at the AlOx-polymer interface and inducing a soft dielectric breakdown of the AlOx layer. These PEDOT:PSS + NCNTs memory devices are to easy to apply in flexible low-cost technology and provide the possibility of large-scale integration.

Effect of device structure on the resistive switching characteristics of organic polymers fabricated through all printed technology

Developments in organic bi-stable non-volatile memory devices have shown that organic materials are essential for the next generation of electrical memory unit owing to their low cost, high flexibility and large scalability. This study depicts an important aspect of organic memory devices by observing the effect of changing device structure on its switching characteristics. Memory devices with a bilayer and bulk-heterojunction structure were fabricated through an all printed technology by utilizing two organic polymers such as MEH: PPV and PMMA. Silver (Ag) was selected as the top and bottom electrode due to its high conductivity and easy processing. Though identical polymers were used in both device structures, but interestingly change in structure caused change in properties. It was observed that bilayer structure had much higher switching ratio and stability against various biasing cycles as compared to its bulk-heterojunction counterpart. Superior switching characteristics of bilayer structure were due to the presence of a well-defined interface between both polymers. Bulk-heterojunction device suffers the drawback of phase separation in a single organic layer between the two polymers.

Fluorinated p-n type copolyfluorene as polymer electret for stable nonvolatile organic transistor memory device

In this study, a kind of fluorinated copolyfluorene, named poly[(4-(octyloxy)-9,9-diphenylfluorene-2,7-diyl)-alt-(2,3,5,6-tetrafluoro-1,4-phenylene)] (PODPF-TFP), is synthesized by facile palladium-based direct aromatization. Compared to the non-fluorinated counterpart, poly[(4-(octyloxy)-9,9-diphenylfluorene-2,7-diyl)-alt-(p-phenylene)] (PODPF-P), deeper HOMO/LUMO energy level combined with steric hindrance effect endow PODPF-TFP with excellent spectra and morphology stability. Finally, organic field-effect transistor (OFET) memory devices are fabricated with PODPF-P/PODPFTFP as the dielectric layers, and they both exhibit flash type storage characteristic. Owing to the electronegativity of fluorine atom, the device based on PODPF-TFP exhibits larger memory window and more stable I on/I off ratio during a retention time of 104 s as well as a better aging stability. The present study suggests that fluorinated p-n copolyfluorene electrets could enhance the capabilities of charge trapping and storage, which are promising for OFET memory devices.

A complementary switching mechanism for organic memory devices to regulate the conductance of binary states

We have fabricated an organic non-volatile memory device wherein the ON/OFF current ratio has been controlled by varying the concentration of a small organic molecule, 2,3-Dichloro-5,6-dicyano-p-benzoquinone (DDQ), in an insulating matrix of a polymer Poly(4-vinylphenol) (PVP). A maximum ON-OFF ratio of 106 is obtained when the concentration of DDQ is half or 10 wt. % of PVP. In this process, the switching direction for the devices has also been altered, indicating the disparity in conduction mechanism. Conduction due to metal filament formation through the active material and the voltage dependent conformational change of the organic molecule seem to be the motivation behind the gradual change in the switching direction.

Integration of Flexible and Microscale Organic Nonvolatile Resistive Memory Devices Using Orthogonal Photolithography.

We present the integration of flexible and microscale organic nonvolatile resistive memory devices fabricated in a cross-bar array structure on plastic substrates. This microscale integration was made via orthogonal photolithography method using fluorinated photoresist and solvents and was achieved without causing damage to the underlying organic memory materials. Our flexible microscale organic devices exhibited high ON/OFF ratio (I(ON/I(OFF) > 10(4)) under bending conditions. In addition, the ON and OFF states of our flexible and microscale memory devices were maintained for 10,000 seconds without any serious degradation.