Organic Field-effect Transistor Memory Research Articles

Graphdiyne particles as nano-floating gates for high-performance nonvolatile organic field-effect transistor memory

Graphdiyne particles (GDYP), newcomers of carbon family, were employed as nano-floating gates in organic field-effect transistor (OFET) memories. The charge-trapping layer was prepared by blending GDYP into a polystyrene (PS) matrix. The effect of GDYP doping ratio on memory performances was investigated, and the OFET memory with the optimal doping concentration of the GDYP (0.3 mg/mL) exhibits ambipolar memory behavior with a wide memory window as large as 86 V, excellent retention time over 104 s with a high ON/OFF current ratio of 7.5 × 105. This work demonstrates that graphdiyne is a promising charge-trapping material for high-performance nonvolatile OFET memory devices.

Ambipolar charge-trapping in self-assembled nanostructures of a supramolecular miktoarm star-shaped copolymer with a zinc phthalocyanine core

Supramolecular miktoarm star-shaped copolymers with a zinc phthalocyanine core exhibit phase-separated nanostructures that enhance ambipolar charge-trapping capabilities in the organic field-effect transistor memory devices.

Exploring the Charge-Trapping Behavior of Self-Assembled Sugar-based Block Copolymers with a Pendent Design in Photoassisted Memory

With unique advantages and features, photonic organic field-effect transistor memory has gained widespread attention over extensive research fields in recent years. In this work, we report the synthesis and device characteristics of three different sugar-based block copolymers, where each contains a pendent maltose (Mal) block and a pendent photoluminescent arene (anthracene (AN), carbazole (CZ), and fluorene (FL)) on a norbornene backbone polymerized through metathesis ring opening polymerization. The corresponding block copolymers are introduced as a polymer electret layer in the photoassisted memory. The hydrophilic maltose block paves the way for self-assembled morphology (ordered vertical cylinder and horizontal cylinder nanostructures) via solvent annealing, while the hydrophobic and photoluminescent arenes act as the charge-trapping site. The optical, electrochemical, and morphological properties were investigated to understand the impact on device performance. The photoassisted memory devices were electrically written by gate bias of −60 V for 1 s and recovered with three kinds of light sources (254-, 365-, and 530 nm lights). Among the studied devices, Mal-b-AN exhibited excellent memory performance with rapid photoresponse owing to the favorable energy levels’ alignment and morphological orientation. Accordingly, the device produced a high memory ratio (>106) over 104 s, a stable performance over 45 endurance cycles, and a wide memory window (∼35 V). This study not only established a detailed explanation of the nanostructures and energy level effect on the memory behaviors of sugar-based block copolymers with a pendent design but also provides an approach for the next-generation optoelectronic devices using green materials.

A BN-Doped U-Shaped Heteroacene as a Molecular Floating Gate for Ambipolar Charge Trapping Memory.

Two wide-bandgap U-shaped polycyclic aromatic hydrocarbons with/without boron and nitrogen (BN-) doping (BN-1 and C-1) were synthesized to tune the electronic features to suit the performance requirements in organic field-effect transistor memory (OFET-NVM). The chemical structures were characterized by scanning tunneling microscopy and single crystal diffraction. Owing to the electron donor effect of N and the high electron affinity of B, the BN-1-based OFET-NVM displays large ambipolar memory windows and an enhanced charge storage density compared to C-1 and most of the reported small molecules. A novel supramolecular system formed via BN-1 and PMMA contributes to fabricating uniform films with homogeneous microstructures, which serve as a two-in-one tunnelling dielectric and charge trapping layer to realize long-term charge retention and reliable endurance. Our results demonstrate that both BN doping and supramolecular engineering are crucial for the charge trapping of OFET-NVM.

Deep-trap dominated degradation of the endurance characteristics in OFET memory with polymer charge-trapping layer

Organic field-effect transistors (OFETs) with polymer charge-trapping dielectric, which exhibit many advantages over Si-based memory devices such as low cost, light weight, and flexibility, still suffer challenges in practical application due to the unsatisfied endurance characteristics and even the lack of fundamental of behind mechanism. Here, we revealed that the degradation of endurance characteristics of pentacene OFET with poly(2-vinyl naphthalene) (PVN) as charge-storage layer is dominated by the deep hole-traps in PVN by using the photo-stimulated charge de-trapping technique with the fiber-coupled monochromatic-light probes. The depth distribution of hole-traps in PVN film of pentacene OFET is also provided.

Rubrene/CuInP2S6 organic/inorganic heterojunctions for ferroelectric organic transistor memory devices

Ferroelectric organic field-effect transistor (Fe-OFET) memory devices exhibit low power consumption, low cost and good flexibility, which are indispensable to next generation flexible and wearable memory devices. However, up until the present, the ferroelectric materials used in Fe-OFETs have been limited to ferroelectric polymers, i.e. poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], and suffer from large surface roughness and high-risk of depolarization. This work integrates a new two-dimensional CuInP2S6 crystal into Fe-OFET, which can introduce excellent ferroelectric property due to its single crystalline smoothness and noncentrosymmetric crystal structure. Meanwhile, the high-mobility rubrene crystal is selected as the channel material to construct rubrene/CuInP2S6 organic/inorganic van der Waals heterojunctions. Associated with synergetic effect of single-crystalline nature of the heterojunction and atomically flat/defect-free interface, the memory devices possess a high programming speed of 0.5 s and long retention time of 104 s with a current ratio larger than 103. Furthermore, the performance of flexible devices is also demonstrated, including a programming/erase cycle of 1000 with a current on/off ratio approaching 103, data retention time of 104 s with a current on/off ratio of 103 and a current on/off ratio maintained at 102 even after 100 bending cycles under a bending radius of 25 mm, which are qualified for memory buffer usage or short-term data storage especially when they are integrated with other flexible electronics. The proposed configuration of memory transistors using organic/inorganic heterojunctions provides a rational design of device structure toward flexible Fe-OFET non-volatile memory applications.

Influence of Molecular Weight of Polymer Electret on the Synaptic Organic Field‐Effect Transistor Performance

AbstractChargeable polymer electret‐based organic field‐effect transistor (OFET) memory devices are attractive for intrinsically flexible artificial synapse. However, the effects of molecular weight (Mw) of polymer electret on the charge trapping properties, especially the implementation of synaptic functions, are not yet well‐understood. In this work, the authors study pentacene‐based synaptic OFET memory made from two different molecular weight polymer electrets, with poly(N‐vinylcarbazole) (PVK) as the case study. Utilizing the synergistic effect of smooth surface morphology, higher polymer chain‐end density and lower dielectric constant, OFET memory device with lower Mw PVK electret showed a larger memory window (28.2 V) and faster write speed (1 ms) compared to that with higher Mw PVK. Benefiting from its charge‐trapping ability, the synaptic OFETs with lower Mw PVK delivered a better modulation of synaptic weight. The co‐modulation of photonic and electric operations enabled the reconfigurable short‐term plasticity and long‐term plasticity (LTP) in lower Mw PVK devices, further leading to the STP‐based emulation of visible color recognition and LTP‐based neural network simulation. This work enlightens a detailed understanding of the molecular weight‐dependent charge trapping behavior for the future integration of visible information sensing‐memory‐processing in optoelectronic OFET memory.

Biomass-Derived Degradable Poly(azomethine)s for Flexible Bistable Photonic Transistor Memories

We report on the synthesis and photonic memory applications of poly(azomethine)s (PAMs) synthesized from the condensation reaction of biomass-derived diamine, (3R,6S)-hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-aminobenzoate) (ISBA) with photoactive 9,9-di-n-octylfluorene-2,7-dicarboxaldehyde (FLDA) and biomass-based 2,5-furandicarboxaldehyde (FCA). All the prepared PAMs exhibited a higher thermal stability (Tg: 118–123 °C) and lower band gaps (Eg: 2.83–2.86 eV) compared with polymer P4 (Tg: 102 °C, Eg: 3.11 eV) without the imine linkage due to chain rigidity and structural coplanarity. The organic field effect transistor (OFET) memories using P1–P4 as the electrets showed a memory window of 17–25 V and a memory ratio of up to 2.1 × 104 with photowriting-electrical-erasing characteristics. The discrepancy in the energy level between the transporting layer and chargeable electret enabled the negative photoinduced excitons stored in the electret. On the other hand, the increased ratio of the furan in the prepared PAM resulted in a higher highest occupied molecular orbital level, resulting in a larger memory window of 25 V. The imine linkages along the polymer backbone endowed a higher lowest unoccupied molecular orbital level and provided a charge stabilizer to consolidate the trapped charges, which was superior to the polymer without the imine linkage. We further demonstrated flexible memory using biomass-based polymers as the substrate and the electret, and the memory characteristics were retained even after 1000 cycles of bending under a 5 mm bending radius. By virtue of the degradable imine and ester linkages in the polymer chain, the PAM degraded into ISB biosegments under acidic conditions after 1 week. This result suggested that the prepared photoresponsive polyazomentines have the potential to be used for green electronics.

Steric Hindrance Functionalized Porphyrins as Charge Trapping Elements for Organic Field Effect Transistor Memory

Steric Hindrance Functionalized Porphyrins as Charge Trapping Elements for Organic Field Effect Transistor Memory

Nanostructured interfacial dipole layers for high-performance and highly stable nonvolatile organic field-effect transistor memory

An interfacial dipole layer formed between the solution-processed charge trapping nanostructures and the active layer enables significant improvement of the charge storage capability and stability.

Polymer electret-based organic field-effect transistor memory with a solution-processable bilayer (PαMS/ cross-linked PVP) gate dielectric

Pentacene based organic field-effect transistors (OFETs) were fabricated, with both cross-linked poly vinyl phenol (CL-PVP) and a bilayer of poly(α-methylstyrene) (PαMS)/ CL-PVP as gate dielectric. The PαMS layer decreases the surface energy of the gate dielectric and increases the hydrophobic nature, which leads to favorable growth of pentacene and the corresponding field-effect mobility, though at a higher gate voltage span, increases three times compared to that of the device with only CL-PVP as the gate dielectric. OFET with bilayer polymer gate dielectric exhibited non-volatile memory behavior with an on-off ratio 103, retention time >103 s and a large memory window of −25 V. The memory effect observed in the device was due to the charge trapping in the PαMS layer, with CL-PVP acting as a blocking dielectric. Our studies indicate that the bilayer dielectric, comprising of solution-processable PαMS/CL-PVP is a good choice for obtaining non-volatile electret memory on an OFET platform.

Solution-processed flexible nonvolatile organic field-effect transistor memory using polymer electret

Flexible organic field-effect-transistor (OFET) memory is one of the promising candidates for next-generation wearable nonvolatile data storage due to its low price, solution-processability, light-weight, mechanically flexibility, and tunable energy level via molecular tailoring. In this paper, we report flexible nonvolatile OFET memory devices fabricated with solution-processed polystyrene-brush electret and organic semiconductor blends of p-channel 6, 13-bis-(triisopropylsilylethynyl)pentacene (TIPS-PEN) 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). Fabricated flexible OFET memory devices exhibited high memory window (30 V) and ON/OFF current ratio (memory ratio) over 103. Furthermore, we obtained reliable memory ratio (~103) over retention time of 108 s, 100 times of repeated programming/erasing cycles, and 1000 times of bending tests at a radius of 3 mm.

Unveiling the Photoinduced Recovery Mystery in Conjugated Polymer-Based Transistor Memory.

A straightforward mechanism for the photorecovery behavior of photoresponsive nonvolatile organic field-effect transistor (OFET) memories is proposed by employing a commercially available conjugated polymer, the poly(9,9-dioctylfluorene) (PFO), the conjugated monomer fluorene (FO), and the nonconjugated poly(vinyl alcohol) (PVA), as charge storage layers beneath the semiconducting pentacene layer. As photoexcitons are generated upon light exposure, the respective charges recombine with the trapped charges in electrets and neutralize the memory device. However, whether the excitons are generated in the semiconducting layer or the electret part, the origin that mainly governs the photorecovery behavior remains unclear. In this study, we show that when PVA, a nonphotoactive electret, replaces PFO the photorecovery behavior is totally absent, and it confirms the photorecovery behavior dominated by the excitons in situ generated in a charged electret. Moreover, PFO as a photoactive electret, exhibiting an excellent hole-trapping ability over 24 h in the dark and high Ion/Ioff current ratio of 108, has successfully demonstrated rapid photoinduced recovery under UV light. The devices also display a reliable switching ability between electrical charge trapping and optical recovery cycles for optical-recording application. This report presents a clear understanding behind photorecovery phenomena that demonstrates useful guidance to boost the development of photoactive OFET memories based on conjugated polymer electrets.

High performance nonvolatile organic field‐effect transistor memory devices based on pyrene diimide derivative

AbstractDeveloping high‐quality electret layer is important for the fabrication of high‐performance nonvolatile organic field effect transistor memory devices (OFET‐NVMs). In this work, three representative aromatic diimide frameworks are employed for comparative studies as n‐type doping materials for the electret layers in OFET‐NVMs, which are naphthalene diimide (NDI), perylene diimide (PDI), and pyrene diimide (PyDI). When blended with polystyrene (PS) to prepare the electret layers, all the memory devices containing aromatic diimide dopants exhibited significantly improved performances compared with the undoped counterparts, indicating that low‐lying LUMO energy levels of these dopants are beneficial for charge injection. All the devices with n‐type dopants exhibited long retention times (more than 104 s) and good switching reliability in more than 400 continuous write‐read‐erase‐read cycles. Among them, the PyDI‐based memory device exhibited superior performance compared with other aromatic diimides, which achieved a memory window of 34.0 V, a trapping charge density of 1.98 × 1012 cm−2 along with an on/off ratio higher than 104. This work indicates that PyDI framework could be a new platform for the future design of n‐type dopant for high‐performance nonvolatile organic field‐effect transistor memory devices.image

Electrically programmable multilevel nonvolatile memories based on solution-processed organic floating-gate transistors

The development of nonvolatile organic field-effect transistor (OFET) memories with a satisfactory solution processability is highly desirable to fabricate the data storage media for flexible and printed electronic devices. In this study, we fabricate top-gate/bottom-contact OFET memories having an organic floating-gate structure by a spin-coating process and investigate their memory characteristics. An ambipolar polymer semiconductor of poly(N-alkyldiketopyrrolo-pyrrole-dithienylthieno[3,2-b]thiophene) (DPP-DTT) was used to fabricate an organic semiconductor layer, on which an organic composite of polystyrene and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) was deposited to form an organic floating-gate structure through vertical phase separation. The existence of a deep lowest unoccupied molecular orbital (LUMO) level and the excellent electron transport property of the DPP-DTT enables the injection of electrons from the Au source-drain electrodes to the DPP-DTT semiconductor layer and the storage of electrons in the LUMO level of the TIPS-pentacene floating gates by programming under dark conditions. A high work function metal oxide layer of MoO3 was inserted at the Al gate electrode/CYTOP gate insulator interface to tune the energy level difference between the Au source-drain and Al gate electrodes. The DPP-DTT FET memories with MoO3/Al gate electrodes exhibit satisfactory retention characteristics and, because of the ambipolar trapping characteristics, allow the storing of holes in the highest occupied molecular orbital level of the TIPS-pentacene floating gates in the erasing process. Furthermore, the molecular floating-gate OFET memories exhibit a high storage capacity for multi-level data, and four state levels can be recorded with stable retention characteristics.

Study of the pyridyl-containing charge-trapping functional materials in the organic field effect transistor memory devices

Interface materials can effectively improve the storage characteristics of the memory devices. In this paper, a series of functional materials with propyl chain bridging pyridyl ring and triethoxysilyl group, 3-(3-(triethoxysilyl)propyl)pyridine (Pyridyl-C), N-(3-(triethoxysilyl)propyl)pyridin-3-amine (Pyridyl-N) and 3-(3-(triethoxysilyl)propoxy)pyridine (Pyridyl-O), were selected as the charge-storage interface materials to study the storage characteristics of nonvolatile organic field-effect transistor (OFET) memory devices. The results demonstrated that the memory performance could be enhanced by adding lone-pair-electron atoms (N, O) between pyridyl group and propyl chain. With the programming voltage of −30 V staying in dark condition for 1 s, the memory windows were increased from 12.48 V to 14.95 V, then to 21.99 V when the –O and N atoms were introduced, respectively. All memory devices could remain stable for more than 104 s. The morphology images indicated that there were aggregates on the interfacial layers, and the size was increased when the atoms (O, N) were added because of the hydrogen-bonding interaction. The aggregates can increase the contact area between pentacene and interface materials and further enlarge the effective tunneling area. In addition, the variation of self-assembling immersing time for preparing Pyridyl-N layers was studied, indicating that with the extension of the immersing time, the size of aggregates was increasingly enlarged and memory performance was improved correspondingly.

Stretchable OFET Memories: Tuning the Morphology and the Charge-Trapping Ability of Conjugated Block Copolymers through Soft Segment Branching

The mechanical properties and structural design flexibility of charge-trapping polymer electrets have led to their widespread use in organic field-effect transistor (OFET) memories. For example, in the electrets of polyfluorene-based conjugated/insulating block copolymers (BCPs), the confined fiberlike polyfluorene nanostructures in the insulating polymer matrix act as effective hole-trapping sites, leading to controllable memory performance through the design of BCPs. However, few studies have reported intrinsically stretchable charge-trapping materials and their memory device applications, and a practical method to correlate the thin-film morphology of BCP electrets with their charge-trapping ability has not yet been developed. In this study, a series of new conjugated/insulating BCPs, poly(9,9-di-n-hexyl-2,7-fluorene)-block-poly(δ-decanolactone)s (PF-b-PDLx, x = 1-3), as stretchable hole-trapping materials are reported. The linear and branched PDL blocks with comparable molecular weights were used to investigate the effect of polymer architecture on morphology and device performance. Moreover, the coverage area of the polyfluorene nanofibers on the BCP films was extracted from atomic force microscopy images, which can be correlated with the trapping density of the polymer electrets. The branched PDL segments not only improve stretchability but also tailor crystallinity and phase separation of the BCPs, thus increasing their charge-trapping ability. The OFET memory device with PF-b-PDL3 as the electret layer exhibited the largest memory window (102 V) and could retain its performance at up to 100% strain. This research highlights the importance of the BCP design for developing stretchable charge-trapping materials.

An organic field effect transistor memory adopting octadecyltrichlorosilane self-assembled monolayer

An organic nonvolatile memory is proposed based on octadecyltrichlorosilane self-assembled monolayer (ODTS SAM) and copper hexadecafluorophthalocyanine (F16CuPc) heterostructure, where F16CuPc acts as the active layer of the organic thin film transistor memory device, and ODTS SAM functions as the charge trapping layer. By adopting N-type active layer, sufficient electrons ensure the erasing process that is independent of the light illumination, indicating a fully electrically driving memory device, where large memory window is obtained. AFM images demonstrates that the monolayer provides the trapping sites. The trapping mechanism is analyzed based on the electric dipoles theory, the dipoles orientation in the monolayer is critical for providing trapping sites, which fixes the mobile carriers at the ODTS SAM/F16CuPc interface. The writing–reading–erasing–reading (WRER) tests and the retention time are shown, and they can be further optimized by involving a thin polystyrene tunneling layer between active layer and charge trapping layer, with an retention time of approximately 125 d, and a sufficient WRER reading current ratio of more than 103 for practical fully electrically driving binary nonvolatile memory device applications.

Recent advance in renewable materials and green processes for optoelectronic applications

With the growing global environmental awareness, the development of renewable and green materials has gained increased worldwide interest. Being at the heart of current scientific research studies, renewable polymers, include natural, semisynthetic, and microbial polymers, have a scope in vast diverse applications in packaging, agriculture, medicine, and optoelectronic technological fields. In recent years, research interest has been increasingly focused on integrating biodegradable polymers into optoelectronic devices for the sake of reducing non-degradable waste and conserving our environment. Numerous biobased materials have been successfully incorporated in organic field-effect transistor memory, resistor memory, and organic photovoltaic (OPV) devices. Besides natural biomaterials, there are also various biobased synthetic polymers used in electronic devices; for example, sugar-based block copolymers have been synthesized to fabricate stretchable memory devices. Several recent studies have also been devoted to integrating green materials into OPV devices in hope of fulfilling sustainable development. In this mini-review, science development of exploiting renewable materials, introducing environmentally friendly processing solvents, and their corresponding applications toward an environmental sustainable life will be briefly narrated.

High-performance Nonvolatile Organic Photoelectronic Transistor Memory Based on Bulk Heterojunction Structure.

Depending on the storage mechanisms, organic field-effect transistor (OFET) memory is usually divided into floating gate memory, ferroelectric memory, and polymer-electret-based memory. In this work, a new type of nonvolatile OFET memory is proposed by simply blending a p-type semiconductor and a n-type semiconductor without using an extra trapping layer. The results show that the memory window can be effectively modulated by the dopant concentration of the n-type semiconductor. With the addition of a 5% n-type semiconductor, blending devices exhibit a large memory window up to 57.7 V, an ON/OFF current ratio (ION/IOFF) ≈ 105, and a charge retention time of over 10 years, which is comparable or even better than those of most of the traditional OFET memories. The discontinuous n-type semiconductor is set as a charge-trapping center for charge storage due to the quantum well-like organic heterojunctions. The generalization of this method is also investigated in other organic systems. Moreover, the blend devices are also applied to optical memory and show multilevel optical storage, which are further scaled up to 8 × 8 array to map up two-dimensional (2D) optical images with long-term retention and reprogramming characteristic. The results reveal that the novel system design has great potential application in the field of digital image memory and photoelectronic system.