Nonvolatile Organic Transistor Memory Research Articles

Solution-Processable Anion-doped Conjugated Polymer for Nonvolatile Organic Transistor Memory with Synaptic Behaviors

Brain-inspired synaptic transistors have been considered as a promising device for next-generation electronics. To mimic the behavior of a biological synapse, both data processing and nonvolatile memory capability are simultaneously required for a single electronic device. In this work, a simple approach to realize a synaptic transistor with improved memory characteristics is demonstrated by doping an ionic additive, tetrabutylammonium perchlorate (TBAP), into an active polymer semiconductor without using any extra charge storage layer. TBAP doping is first revealed to improve the memory window of a derived transistor memory device from 19 to 32 V (∼68% enhancement) with an on/off current ratio over 103 at VG = -10 V. Through morphological analysis and theoretical calculations, it is revealed that the association of anion with polymers enhances the charge retention capability of the polymer and facilitates the interchain interactions to result in improved memory characteristics. More critically, the doped device is shown to successfully mimic the synaptic behaviors, such as paired-pulse facilitation (PPF), excitatory and inhibitory postsynaptic currents, and spike-rate dependent plasticity. Notably, the TBAP-doped device is shown to deliver a PPF index of up to 204% in contrast to the negligible value of an undoped device. This study describes a novel approach to prepare a synaptic transistor by doping conjugated polymers, which can promote the future development of artificial neuromorphic systems.

Non-Volatile Organic Transistor Memory Based on Black Phosphorus Quantum Dots as Charge Trapping Layer

High performance organic nano-floating gate transistor memory (NFGTM) has important prerequisites of low processing temperature, solution–processable layers and charge trapping medium with high storage capacity. We demonstrate organic NFGTM using black phosphorus quantum dots (BPQDs) as a charge trapping medium by simple spin-coating and low processing temperature ( 104 over 10,000 sec by introducing PMMA as the tunneling layer.

Low-Voltage Organic Nonvolatile Memory Transistors with Water-Soluble Polymers Containing Thermally Induced Radical Dipoles.

A water-soluble acidic polymer, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA), was applied as a gate-insulating layer for organic field-effect transistors (OFETs). Before depositing the poly(3-hexylthiophene) (P3HT) channel layers, the PAMPSA layers were subjected to thermal treatment at various temperatures from 140 to 230 °C. The OFET performance was greatly enhanced by thermal treatment between 140 and 170 °C, whereas it became very poor at higher temperatures (200-230 °C). In particular, the transfer curves showed pronounced hysteresis phenomena at 170 °C. Various measurements including thermogravimetric analysis and X-ray photoelectron spectroscopy disclosed that the PAMPSA chains underwent thermal degradation from ca. 160 °C and could generate carbon radicals leading to the formation of dipoles with the nitrogen lone pair electrons. The carbon-nitrogen dipoles delivered hysteresis phenomena to the OFETs with the PAMPSA layers treated at 170 °C, which exhibited excellent memory retention characteristics up to 10 000 cycles even at -1 V. Hence, it is expected that the thermally treated PAMPSA layers can be used as one of the viable gate-insulating memory materials for low-voltage transistor-type organic memory devices (TOMDs).

Solution-Processed Nonvolatile Organic Transistor Memory Based on Semiconductor Blends.

Solution-processed nonvolatile organic transistor memory devices are fabricated by employing semiconductor blends of p-channel 6,13-bis(triisopropylsilylethynyl)pentacene and n-channel poly{[ N, N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-2T); N2200) on polystyrene-brush as a polymer electret. Electret-based memory characteristics are significantly changed depending on the frontier molecular orbitals of the active semiconductors because the charge-trapping efficiency is mainly determined by the energy barrier to transfer electrons and holes from the active channel to the electret layer. A semiconductor mixture with an optimized blending ratio results in an efficient programming and erasing process. Thus, we obtained a remarkably high ratio of ON/OFF current (memory ratio) about 107 and a large amount of shifts in the threshold voltage (memory window) between the programmed and erased states of 55 V, while single-component N2200 showed only writing-once-read-many (WORM)-type memory. Especially, the programmed data can be stably retained more than 10 years with a sufficient memory ratio of 103. Furthermore, our semiconductor blend system leads to preferable vertical phase separation, which affords good reliability under a sequential memory operation condition as well as stability in ambient air. It is expected that our memory devices can be applied for versatile data storage in printed and flexible electronic applications.

Surface engineering of ferroelectric polymer for the enhanced electrical performance of organic transistor memory

We suggest a viable surface control method to improve the electrical properties of organic nonvolatile memory transistors. For viable surface control, the surface of the ferroelectric insulator in the memory field-effect transistors was modified using a smooth-contact-curing process. For the modification of the ferroelectric polymer, during the curing of the ferroelectric insulators, the smooth surface of a soft elastomer contacts intimately with the ferroelectric surface. This smooth-contact-curing process reduced the surface roughness of the ferroelectric insulator without degrading its ferroelectric properties. The reduced roughness of the ferroelectric insulator increases the mobility of the organic field-effect transistor by approximately eight times, which results in a high memory on–off ratio and a low-voltage reading operation.

High-Performance Nonvolatile Organic Transistor Memory Using Quantum Dots-Based Floating Gate

A novel nonvolatile floating-gate transistor memory device using CdSe@ZnS quantum dots (QDs) embedded the insulating polymer as a charge-storage layer along with the rational design of device structure is presented. The core-shell structure CdSe@ZnS QDs can efficiently trap both holes and electrons under the applied writing/erasing operations, resulting in a considerable threshold voltage shifts (ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> ) over 50 V and forming high-conductance (ON) and low-conductance (OFF) states at a gate voltage of 0 V. The value of threshold voltage shift is controlled by writing and erasing voltages, regardless with source-drain voltages. Furthermore, it exhibits a long retention time (the ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> can maintain 76% at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> s) and outstanding endurance characteristics (>500 cycles), demonstrating extraordinary stable and reliable memory property. Moreover, a thin layer of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> was introduced as tunneling dielectric layer which is essential for the high-performance floating-gate transistor memory device. The nonvolatile organic transistor memory devices using QDs-based floating gate show great potential application for high-performance organic memory devices.

High performance organic nonvolatile memory transistors based on HfO2 and poly(α-methylstyrene) electret hybrid charge-trapping layers

In this work, we fabricated a high performance flash-type organic nonvolatile memory transistor, which adopted polymer-electret poly(α-methylstyrene) (PαMS) and HfO2 films as hybrid charge trapping layer (CTL). Compared with a single HfO2 or PαMS CTL structure, the hybrid HfO2/PαMS CTL structure can provide enhanced charge trapping efficiency to increase the device operation speed and reduce the leakage current to boost the device reliability. The fabricated nonvolatile organic memory transistors with the hybrid CTL shows excellent electrical properties, including low operation voltage (8 V), high speed (&amp;lt;10 ms), excellent data retention (on-off current ratio of 2.6 × 104 after 104 s), and good endurance (more than 2000 program/erase cycles). The present work provides useful idea for the design of future low-power consumption and highly reliable organic nonvolatile memories.

Floating-gate nanofibrous electret arrays for high performance nonvolatile organic transistor memory devices

Nanofibrous electret arrays based organic field-effect floating-gate transistor memory was firstly developed by electrospinning. The nanofiber arrays are composed of a novel porphyrin molecule of [5,15-bis[4-(pyridyl)ethynyl]-10,20-diphenyl]-21H,23H-porphyrin (DPP) as charge-trapping elements and polystyrene (PS) as the tunneling layer. The floating-gate transistor memory based on electrospinning nanofibrous electret arrays exhibited a reliable controllable threshold voltage shift and effective charge-trapping ability which was obviously superior to the counterparts fabricated with widely employed spin-coating technique. The result shows that electrospinning can be used as an effective artificial strategy to produce predesigned microstructure for the electrets, optimize the electrical memory characteristics, and may be applied in future nonwoven electronic memory devices.

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.

Strong molecular weight effects of gate-insulating memory polymers in low-voltage organic nonvolatile memory transistors with outstanding retention characteristics

Organic nonvolatile memory transistors, featuring low-voltage operation (⩽5 V) and high retention characteristics (>10 000 cycles), are demonstrated by introducing high molecular weight poly(vinyl alcohol) (PVA) as a gate insulating layer. PVA polymers with four different molecular weights (9.5–166 kDa) are examined for organic memory devices with poly(3-hexylthiophene) channel layers. All devices show excellent p-type transistor behavior and strong hysteresis in the transfer curves, but the lower molecular weight PVA delivers the higher hole mobility and the wider memory window. This has been attributed to the higher ratio of hydroxyl group dipoles that align in the out-of-plane direction of the PVA layers, as supported by impedance spectroscopy (dielectric constants), polarized Fourier transform-infrared spectroscopy and synchrotron radiation grazing incidence X-ray diffraction measurements. However, outstanding retention characteristics (<4% current variation after 10 000 cycles) have been achieved with the higher molecular weight PVA (166 kDa) rather than the lower molecular weight PVA (9.5 kDa). Researchers have created an organic transistor that retains memory states for over 10,000 cycles with minimal, sub-5-volt power requirements. Flexible memory devices made from semiconducting plastics are significantly easier to fabricate than silicon-based materials. However, the high voltages usually needed to switch the on/off state of organic transistors are detrimental to device longevity. An international team led by Youngkyoo Kim from Kyungpook National University in Korea used insulating poly(vinyl alcohol) (PVA) films with high molecular weights to reduce the voltage demands of plastic circuits. The researchers inserted PVA films between a transparent electrode and a semiconducting polymer, and then fabricated the stack into a source/drain/gate transistor setup. The most stable data retention characteristics were achieved with the heaviest PVA films — a relationship the researchers attribute to hydroxyl dipole orientations at interfacial boundaries. Low-voltage (<5 V) organic memory transistors with outstanding retention stability (>10 000 cycles), which were achieved by employing both the P3HT channel layer and the PVA gate-insulating memory layer, were strongly influenced by the PVA molecular weights due to the different orientation of hydroxyl dipoles.

Nanostructured materials for non-volatile organic transistor memory applications

Over the past decades, the demand for organic memory has rapidly increased due to the development of flexible electronics.

Phthalocyanine‐Cored Star‐Shaped Polystyrene for Nano Floating Gate in Nonvolatile Organic Transistor Memory Device

A novel non‐volatile organic memory device is developed based on an organic field effect transistor (OFET) with a dielectric layer of a four‐armed, star‐shaped polymer featuring a copper phthalocyanine (CuPc) core. The CuPc core unit of the star‐shaped polystyrene (CuPc‐PS4) self‐assembles to form aggregates via π–π interactions, which are isolated and dispersed in the polymer thin film. The OFET memory device employing CuPc‐PS4 shows significant hole‐trapping characteristics with a high memory ON/OFF current ratio of 107, long retention ability of over 104 s, and good endurance of more than 100 cycles, which is attributed to a flash‐type memory. The novel polymer design with CuPc core assemblies inside the polymer matrix is a promising candidate for nano floating gates in non‐volatile OFET memory.

Controlled growth of a graphene charge-floating gate for organic non-volatile memory transistors

We report memory application for graphene as a floating gate in organic thin-film transistor (OTFT) structure. For graphene floating gate, we demonstrate a simpler synthesis method to form a discrete graphene layer by controlling the growth time during a conventional CVD process. The resulting organic memory transistor with the discrete graphene charge-storage layer is evaluated. The device was demonstrated based on solution-processed tunneling dielectric layers and evaporated pentacene organic semiconductor. The resulting devices exhibited programmable memory characteristics, including threshold voltage shifts (∼28 V) in the programmed/erased states when an appropriate gate voltage was applied. They also showed an estimated long data retention ability and program/erase cycles endurance more than 100 times with reliable non-volatile memory properties although operated without encapsulation and in an ambient condition.

5 V driving organic non-volatile memory transistors with poly(vinyl alcohol) gate insulator and poly(3-hexylthiophene) channel layers

Organic non-volatile memory devices were fabricated by employing organic field-effect transistors (OFETs) with poly(vinyl alcohol) (PVA) and poly(3-hexylthiophene) as a gate insulating layer and a channel layer, respectively. The 10-nm-thick nickel layers were inserted for better charge injection between the channel layer and the top source/drain electrodes. The fabricated PVA-OFET memory devices could be operated at low voltages (≤5 V) and showed pronounced hysteresis characteristics in the transfer curves, even though very small hysteresis was measured from the output curves. The degree of hysteresis was considerably dependent on the ratio of channel width (W) to channel length (L). The PVA-OFET memory device with the smaller W/L ratio (25) exhibited better retention characteristics upon 700 cycles of writing-reading-erasing-reading operations, which was assigned to the stability of charged states in devices.

Organic Electronics: Conjugated Polymer Nanoparticles as Nano Floating Gate Electrets for High Performance Nonvolatile Organic Transistor Memory Devices (Adv. Funct. Mater. 10/2015)

Organic Electronics: Conjugated Polymer Nanoparticles as Nano Floating Gate Electrets for High Performance Nonvolatile Organic Transistor Memory Devices (Adv. Funct. Mater. 10/2015)

Quasi-unipolar pentacene films embedded with fullerene for non-volatile organic transistor memories

Quasi-unipolar non-volatile organic transistor memory (NOTM) can combine the best characteristics of conventional unipolar and ambipolar NOTMs and, as a result, exhibit improved device performance. Unipolar NOTMs typically exhibit a large signal ratio between the programmed and erased current signals but also require a large voltage to program and erase the memory cells. Meanwhile, an ambipolar NOTM can be programmed and erased at lower voltages, but the resulting signal ratio is small. By embedding a discontinuous n-type fullerene layer within a p-type pentacene film, quasi-unipolar NOTMs are fabricated, of which the signal storage utilizes both electrons and holes while the electrical signal relies on only hole conduction. These devices exhibit superior memory performance relative to both pristine unipolar pentacene devices and ambipolar fullerene/pentacene bilayer devices. The quasi-unipolar NOTM exhibited a larger signal ratio between the programmed and erased states while also reducing the voltage required to program and erase a memory cell. This simple approach should be readily applicable for various combinations of advanced organic semiconductors that have been recently developed and thereby should make a significant impact on organic memory research.

Conjugated Polymer Nanoparticles as Nano Floating Gate Electrets for High Performance Nonvolatile Organic Transistor Memory Devices

A molecular nano‐floating gate (NFG) of pentacene‐based transistor memory devices is developed using conjugated polymer nanoparticles (CPN) as the discrete trapping sites embedded in an insulating polymer, poly (methacrylic acid) (PMAA). The nanoparticles of polyfluorene (PF) and poly(fluorene‐alt‐benzo[2,1,3]thiadiazole (PFBT) with average diameters of around 50–70 nm are used as charge‐trapping sites, while hydrophilic PMAA serves as a matrix and a tunneling layer. By inserting PF nanoparticles as the floating gate, the transistor memory device reveals a controllable threshold voltage shift, indicating effectively electron‐trapping by the PF CPN. The electron‐storage capability can be further improved using the PFBT‐based NFG since their lower unoccupied molecular orbital level is beneficial for stabilization of the trapped charges, leading a large memory window (35 V), retention time longer than 104 s with a high ON/OFF ratio of &gt;104. In addition, the memory device performance using conjugated polymer nanoparticle NFG is much higher than that of the corresponding polymer blend thin films of PF/polystyrene. It suggests that the discrete polymer nanoparticles can be effectively covered by the tunneling layer, PMAA, to achieve the superior memory characteristics.

Fluorene based donor-acceptor polymer electrets for nonvolatile organic transistor memory device applications

Fluorene based donor-acceptor polyimides, including poly[bis-(4-aminophenyl)fluorene-hexanediamide] [PA(BAP F-AC)], poly[bis-(4-aminophenyl)fluorene-hexafluoroisopropylidenediphthalimide] [PI(BAPF-6FDA)], poly[bis-(4-aminophenyl)fluorene-oxydiphthalimide] [PI(BAPF-ODPA)], and poly[bis-(4-aminophenyl)fluorene-1,2,4,5-cyclohexanetetracarboxylic diimide] [PI(BAPF-HPMDA)], as charge storage layer (electret) are employed for nonvolatile memory device applications. The polyimides are consisted of electron-donating fluorene diamine moiety (BAPF) and neutral (AC and HPMDA) or electron-accepting (6FDA and ODPA) moieties, respectively. The memory characteristics of these devices can be tuned from the EORM (erase once and read many times) behavior [PA(BAPF-AC)], semi-flash [PI(BAPF-ODPA)], [PI(BAPF-HPMDA)], to a flash type memory [PI(BAPF-6FDA)]. The PI(BAPF-6FDA) devices show the largest memory window of 77 V and a long retention time over 104 s with a high Ion/Ioff current ratio of 108. This is attributed to the largest torsion angle of PI(BAPF-6FDA) stabilizing charge transfer (CT) complexes. The write-read-erase-read cycles were stably operated over 100 cycles. This work provides a new insight into the relationship between the CT effect and the nonvolatile memory behavior. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 602–614

High-performance nonvolatile organic transistor memory devices using the electrets of semiconducting blends.

Organic nonvolatile transistor memory devices of the n-type semiconductor N,N'-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI) were prepared using various electrets (i.e., three-armed star-shaped poly[4-(diphenylamino)benzyl methacrylate] (N(PTPMA)3) and its blends with 6,6-phenyl-C61-butyric acid methyl ester (PCBM), 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pen) or ferrocene). In the device using the PCBM:N(PTPMA)3 blend electret, it changed its memory feature from a write-once-read-many (WORM) type to a flash type as the PCBM content increased and could be operated repeatedly based on a tunneling process. The large shifts on the reversible transfer curves and the hysteresis after implementing a gate bias indicated the considerable charge storage in the electret layer. On the other hand, the memory characteristics showed a flash type and a WORM characteristic, respectively, using the donor/donor electrets TIPS-pen:N(PTPMA)3 and ferrocene:N(PTPMA)3. The variation on the memory characteristics was attributed to the difference of energy barrier at the interface when different types of electret materials were employed. All the studied memory devices exhibited a long retention over 10(4) s with a highly stable read-out current. In addition, the afore-discussed memory devices by inserting another electret layer of poly(methacrylic acid) (PMAA) between the BPE-PTCDI layer and the semiconducting blend layer enhanced the write-read-erase-read (WRER) operation cycle as high as 200 times. This study suggested that the energy level and charge transfer in the blend electret had a significant effect on tuning the characteristics of nonvolatile transistor memory devices.

High performance organic nonvolatile flash memory transistors with high-resolution reduced graphene oxide patterns as a floating gate.

High-performance organic nonvolatile memory transistors (ONVMTs) are demonstrated, the construction of which is based on novel integration of a highly conductive polymer as a semiconductor layer, hydroxyl-free polymer as a tunneling dielectric layer, and high-resolution reduced graphene oxide (rGO) patterns as a floating gate. Finely patterned rGO, with a line width of 20-120 μm, was embedded between SiO2 and the polymer dielectric layer, which functions as a nearly isolated charge-trapping center. The resulting ONVMTs demonstrated ideal memory behavior, and the transfer characteristics promptly responded to writing and erasing the gate bias. In particular, the retention time of written/erased states tended to increase as the rGO line width was reduced, implying that the line width is a critical factor in suppressing charge release from rGO. Using a 20-μm-wide rGO pattern, a nonvolatile large memory window (>20 V) was retained for more than 5 × 10(5) s, which is 50 times longer than non-patterned rGO films.