Pentacene-based Organic Field-effect Transistor Research Articles

Nature of Ohmic and Schottky contacts on pentacene-based organic field-effect transistor

Organic field-effect transistors (OFETs) have become promising components for a wide range of applications in the fast-growing field of organic electronics. Nevertheless, with the increasing need for high-performance OFETs, it is crucial to understand and challenge the contact resistance in these devices, because it represents a critical bottleneck that can significantly suppress the overall performance. Herein, we report the evaluation of the voltage dependence of the injection barrier in pentacene-based OFET devices with Au or Ag electrodes. The injection barrier of the device with Au electrodes showed an Ohmic nature with a barrier of approximately 170 ∼ 200 meV, whereas the barrier in the device with Ag electrodes behaved as a Schottky contact and the zero-field injection barrier reached 552 ± 2 meV.

High Sensitivity of HCl Gas Sensor Based on Pentacene Organic Field-Effect Transistor

The gas sensing properties were investigated for detecting various hydrochloric acid gas concentrations by fabricating the pentacene-based organic field-effect transistor (FET). The pentacene thin film was simply deposited by thermal evaporation process using a shadow mask. The source/drain electrode was formed on heavily boron doped silicon substrate in the form of interdigitated electrode pattern, which showed high gas reactivity by dense patterns between electrodes. The field-effect mobility, subthreshold slope, threshold voltage, on/off current ratio have been observed as 1.8 cm2/Vs, 0.64 V/dec, 5.6 V, 106, respectively. We measured the change in the amount of drain current depending on the concentration of hydrochloric acid gas from 3 to 20 ppm. As a result, the amount of drain current increased as the concentration of hydrochloric acid gas increased. Additionally, at room temperature, we were able to observe the recovery time to its initial state about 200 s at hydrochloric acid gas concentration of 20 ppm. These pentacene-based organic FETs are expected to be able to detect various hazardous acid gases which are hardly detected by inorganic sensors due to corrosion problem in the form of arrays in the future.

Organic Field-Effect Transistor Memory Device Based on an Integrated Carbon Quantum Dots/Polyvinyl Pyrrolidone Hybrid Nanolayer

In this work, we present a pentacene-based organic field-effect transistor memory (OFETM) device, which employs one-step microwave-assisted hydrothermal carbon quantum dots (CQDs) embedded in a polyvinyl pyrrolidone (PVP) matrix, to form an integrated hybrid nanolayer as the charge trapping layer. The as-prepared CQDs are quasi-spherical amorphous C, with sizes ranging from 5 to 20 nm, with a number of oxygen-containing groups and likely some graphite-like domains that produce CQDs with excellent electron-withdrawing characteristics. The incorporation of CQDs into PVP dielectric materials results in a bidirectional storage property. By optimizing the concentration of CQDs embedded into the PVP matrix, the OFETM shows excellent memory characteristics with a large memory window of 8.41 V under a programming/erasing (P/E) voltage of ± 60 V and a retention time of up to 104 s.

Pentacene-based depletion load pMOS inverter realized by threshold voltage control utilizing nitrogen-doped LaB6 interfacial layer

In this study, a depletion load pMOS inverter, which is called an enhancement/depletion (E/D) inverter, with a single organic semiconductor was demonstrated utilizing threshold voltage controlled pentacene-based organic field-effect transistor (OFET) realized by a nitrogen-doped (N-doped) LaB6 interfacial layer (IL). It was found that a N-doped LaB6 IL introduced in drive OFET realized the inverter characteristic with logic swing of 4.3 V at an operation voltage of −5 V. In addition, a common-gate electrode structure for drive and load OFETs was demonstrated, which would contribute to further scaling and high integration of organic devices.

Linkage effects of triphenylamine-based aromatic polymer electrets on electrical memory performance

Here, we use polyimide (PI), polyether (PE) and polyester (PET) with diphenyl sulfonyl (S) moieties combined with triphenylamine (TPA) units to obtain three polymer electrets, TPA-PIS, TPA-PES, and TPA-PETS, respectively. The pentacene-based organic field effect transistor (OFET) memories with the charge-trapping polymer electrets layer were examined. The variation in linkages in the polymer backbone noticeably influences the energy structures and molecular polarity, and thus their memory properties were systematically characterized and compared. Volatile memory characteristics can be observed for all three polymer electrets-based memories. Among these, the TPA-PIS and TPA-PETS electret OFET memory devices exhibit ambipolar trapping behavior capable of storing holes and electrons, with a memory window (MW) of 32.4 ± 1.2 V and 21.7 ± 1.0 V, respectively, and a memory ratio of 104∼106. Importantly, programming/erasing switching endurance for at least 100 cycles is achieved, allowing for tri-state memory storage. However, hole-only unipolar trapping of OFETs memory with the TPA-PES electret presents relatively slower charge detrapping phenomena and longer retention time ability due to the higher molecular dipole moment. Our comparative study demonstrates the important role of the linkage effect in aromatic polymers for developing polymer electret-based OFET memories.

Top-gate pentacene-based organic field-effect transistor with amorphous rubrene gate insulator

The scaling of organic field-effect transistors (OFETs) is necessary for high-density integration and for this, OFETs with a top-gate configuration are required. There have been several reports of damageless lithography processes for organic semiconductor or insulator layers. However, it is still difficult to fabricate scaled OFETs with a top-gate configuration. In this study, the lift-off process and the device characteristics of the OFETs with a top-gate configuration utilizing an amorphous (α) rubrene gate insulator were investigated. We have confirmed that α-rubrene shows an insulating property, and its extracted linear mobility was 2.5 × 10−2 cm2/(V·s). The gate length and width were 10 and 60 µm, respectively. From these results, the OFET with a top-gate configuration utilizing an α-rubrene gate insulator is promising for the high-density integration of scaled OFETs.

Steep subthreshold swing of pentacene-based organic field-effect transistor with nitrogen-doped LaB6 interfacial layer

A pentacene-based organic field-effect transistor (OFET) is necessary to work at a low operation voltage and a steep subthreshold swing. The subthreshold swing of pentacene-based OFET was markedly improved by introducing a nitrogen-doped LaB6 interfacial layer (N-doped LaB6 IL) although charge-injection-type hysteresis was observed in ID–VG characteristics. In this study, the thickness dependence of N-doped LaB6 IL for p-type pentacene-based OFET was investigated. A 1.2–2.7-nm-thick N-doped LaB6 IL was deposited on an SiO2 gate insulator by RF sputtering at an RF power of 20–30 W. It was found that a 1.2-nm-thick N-doped LaB6 IL realized a steep subthreshold swing of 75 mV/dec with a mobility of 0.26 cm2/(V·s) for p-type pentacene-based OFET.

Effect of thickness of polymer electret on charge trapping properties of pentacene-based nonvolatile field-effect transistor memory

In this report, a set of pentacene-based organic field-effect transistor (OFET) memory devices using different thicknesses (ranged from 17.8 to 100.4 nm) of Poly (N-vinylcarbazole) (PVK) as charge trapping layers were fabricated, and the dependences of thickness on charge trapping behaviors were systematically investigated. As the thickness increased, the charge trapping capacity shows a Gaussian distributed growth behavior while the surface tunneling distance demonstrates the property of exponential decrease, which is ascribed to the synergistic effects of potential redistribution of trapped charge carriers and the co-existence of direct tunneling and Fowler–Nordheim (FN) tunneling. The optimum thickness (dot) to possess the most efficient charge trapping properties, which means a reasonably low programming voltage and high charge trapping capacity with good bias stress stability, is approximately 40 ± 5 nm. By calculating the threshold thickness (dth) of PVK for an ultrathin memory, we proposed a model of superficial tunneling distance to deconstruct the continuous chargeable polymer electret-based OFET memory. Our work provided a quantitative evaluation method and can improve the understanding of charge trapping process from the aspect of electret thickness.

Synergistic Effects of Self-Doped Nanostructures as Charge Trapping Elements in Organic Field Effect Transistor Memory.

Despite remarkable advances in the development of organic field-effect transistor (OFET) memories over recent years, the charge trapping elements remain confined to the critical electrets of polymers, nanoparticles, or ferroelectrics. Nevertheless, rare reports are available on the complementary advantages of different types of trapping elements integrated in one single OFET memory. To address this issue, we fabricated two kinds of pentacene-based OFET memories with solution-processed amorphous and β-phase poly(9,9-dioctylfluorene) (PFO) films as charge trapping layers, respectively. Compared to the amorphous film, the β-PFO film has self-doped nanostructures (20-120 nm) and could act as natural charge trapping elements, demonstrating the synergistic effects of combining both merits of polymer and nanoparticles into one electret. Consequently, the OFET memory with β-PFO showed nearly 26% increment in the storage capacity and a pronounced memory window of ∼45 V in 20 ms programming time. Besides, the retention time of β-PFO device extended 2 times to maintain an ON/OFF current ratio of 10(3), indicating high bias-stress reliability. Furthermore, the β-PFO device demonstrated good photosensitivity in the 430-700 nm range, which was attributed to the additive effect of smaller bandgap and self-doped nanostructures of β-phase. In this regard, the tuning of molecular conformation and aggregation in a polymer electret is an effective strategy to obtain a high performance OFET memory.

Physical implication of transition voltage in organic nano-floating-gate nonvolatile memories

High-performance pentacene-based organic field-effect transistor nonvolatile memories, using polystyrene as a tunneling dielectric and Au nanoparticles as a nano-floating-gate, show parallelogram-like transfer characteristics with a featured transition point. The transition voltage at the transition point corresponds to a threshold electric field in the tunneling dielectric, over which stored electrons in the nano-floating-gate will start to leak out. The transition voltage can be modulated depending on the bias configuration and device structure. For p-type active layers, optimized transition voltage should be on the negative side of but close to the reading voltage, which can simultaneously achieve a high ON/OFF ratio and good memory retention.

High Performance Nonvolatile Transistor Memories Utilizing Functional Polyimide-Based Supramolecular Electrets.

We report pentacene-based organic field-effect transistor memory devices utilizing supramolecular electrets, consisting of a polyimide, PI(6FOH-ODPA), containing hydroxyl groups for hydrogen bonding with amine functionalized aromatic rings (AM) of 1-aniline (AM1), 2-naphthylamine (AM2), 2-aminoanthracene (AM3), and 1-aminopyrene (AM4). The effect of the phenyl ring size and composition of AM1-AM4 on the hole-trapping capability of the fabricated devices was investigated systematically. Under an operating voltage under ±40 V, the prepared devices using the electrets of 100 % AM1-AM4/PI ratios exhibited a memory window of 0, 8.59, 25.97, and 29.95 V, respectively, suggesting that the hole-trapping capability increased with enhancing phenyl ring size. The memory window was enhanced as the amount of AM in PI increased. Furthermore, the devices showed a long charge-retention time of 10(4) s with an ON/OFF current ratio of around 10(3) -10(4) and multiple switching stability over 100 cycles. This study demonstrated that the electrical characteristics of the OFET memory devices could be manipulated through the chemical compositions of the supramolecular electrets.

Tracking speed bumps in organic field-effect transistors via pump-probe Kelvin-probe force microscopy

One of the great challenges in designing modern organic field-effect transistors is lowering the injection barriers that arise at the interfaces between the metallic electrodes and the semiconducting transport channel. Currently, these barriers are quantified mostly by time-independent and external inspection, techniques lacking temporal insight into the most relevant switching dynamics. We address this problem here by pump-probe Kelvin-probe force microscopy, which combines the high spatial resolution of standard Kelvin-probe force microscopy with a pump-probe, enabling time resolution down to nanoseconds. When investigating a dynamically operated pentacene-based organic field-effect transistor, pump-probe Kelvin-probe force microscopy is capable of in-situ probing the temporal charge evolution at any sample spot within the device. Thus, Schottky-barriers arising at the boundaries between electrodes and transport channel are identified as speed bumps for high-speed organic field-effect transistor operation, manifested by residual charges that are retained within the organic film upon switching the device.

Direct probing of electron and hole trapping into nano-floating-gate in organic field-effect transistor nonvolatile memories

Electron and hole trapping into the nano-floating-gate of a pentacene-based organic field-effect transistor nonvolatile memory is directly probed by Kelvin probe force microscopy. The probing is straightforward and non-destructive. The measured surface potential change can quantitatively profile the charge trapping, and the surface characterization results are in good accord with the corresponding device behavior. Both electrons and holes can be trapped into the nano-floating-gate, with a preference of electron trapping than hole trapping. The trapped charge quantity has an approximately linear relation with the programming/erasing gate bias, indicating that the charge trapping in the device is a field-controlled process.

Semi-conjugated acceptor-based polyimides as electrets for nonvolatile transistor memory devices

We report new polyimides as electrets for pentacene-based organic field-effect transistor (OFET) memory devices, consisting of different electron-accepting dianhydride moieties (PI(BPDA-DAP), PI(PMDA-DAP), and PI(ODPA-DAP)), and varied aliphatic spacer lengths (PI(PMDA-DAH) and PI(PMDA-DAD)). The effects of the molecular structures on the charge mobility and OFET memory characteristics were systematically investigated in terms of the related frontier energy levels and morphologies. All the devices showed bistable nonvolatile memory characteristics but with a distinct charge storage behavior. The OFET memory devices using the PI(BPDA-DAP) electret exhibited a larger memory window of 81 V compared to those of PI(PMDA-DAP) and PI(ODPA-DAP), mainly attributed to its low-lying LUMO energy level that results from its more extended conjugation. Both the high retention time (around 104 s for the ON/OFF states) and steady write-read-erase-read (WRER) cycles suggested good stability and read/write properties of the studied devices. As the aliphatic spacer length increased from PI(PMDA-DAP) to PI(PMDA-DAH) and PI(PMDA-DAD), the memory characteristics changed from flash to WORM (write once read many times) behaviour. Furthermore, the memory window was reduced with the increased aliphatic spacer length, due to the reduced density of the conjugated electron-trapping sites. This study demonstrated that the electrical characteristics and charge mobility of transistor memory devices can be effectively modulated through the extent of conjugation of the electron accepting moiety and spacer length in polymer electrets.

Photon-energy-dependent light effects in organic nano-floating-gate nonvolatile memories

A pentacene-based organic field-effect transistor nonvolatile memory, in which polystyrene covered Au nanoparticles act as the nano-floating-gate, is probed under different illumination conditions. The memory window can be greatly enlarged upon illumination depending on incident photon energy and intensity, and two light effects are proposed and discussed. The minority multiplication effect enhances the minority carrier tunneling into the nano-floating-gate, resulting in the remarkable positive VT shift. The excitation-induced injection effect is strongly photon energy dependent, and it is responsible for the significant negative VT shift. Appropriate illumination is favorable for reducing the programming/erasing voltage of organic nano-floating-gate nonvolatile memories.

Performance enhancement of pentacene-based organic field-effect transistor by inserting a WO3 buffer layer

The pentacene-based organic field effect transistor (OFET) with a thin transition metal oxide (WO3) layer between pentacene and metal (Al) source/drain electrodes was fabricated. Compared with conventional OFET with only metal Al source/drain electrodes, the introduction of the WO3 buffer layer leads to the device performance enhancement. The effective field-effect mobility and threshold voltage are improved to 1.90 cm2/(V·s) and 13 V, respectively. The performance improvements are attributed to the decrease of the interface energy barrier and the contact resistance. The results indicate that it is an effective approach to improve the OFET performance by using a WO3 buffer layer.

Elucidation of ambient gas effects in organic nano-floating-gate nonvolatile memory

Pentacene-based organic field-effect transistor nonvolatile memories employing nano-floating-gate show high performance in vacuum, typically with field-effect mobility of 0.6 cm2/Vs, memory window of 45 V, reading ON/OFF ratio over 106, and excellent retention ability and programming/erasing endurance. The memory performance is unstable in air, which is demonstrated to result mainly from the device operation instability in O2. The O2-induced acceptor-like trap states reduce the electron supply in pentacene during programming, limiting the electron trapping into the nano-floating-gate and thus suppressing the positive threshold voltage shift. The corresponding hole trapping during erasing is not effectively influenced by the ambient gas effects.

Croconic Acid Thin Film Formation for Ferroelectric Gate OFETs

ABSTRACTPentacene-based ferroelectric gate transistors with croconic acid (CrA) thin film was fabricated for the first time. The memory window (MW) of 1.9 V was obtained from the capacitance-voltage (C-V) characteristics of Al/CrA(50 nm)/SiO2/Si(100) metal-ferroelectric-insulator-semiconductor (MFIS) diode, where the deposition temperature of CrA was room temperature (RT). Butterfly type C-V characteristics was observed for Al/CrA(50 nm)/Al/SiO2/ Si(100) metal-ferroelectric-metal (MFM) diode. Furthermore, a pentacene-based p-type organic field-effect transistor (OFET) with CrA gate insulator was fabricated, and clockwise hysteresis loop was observed in ID-VG characteristic, which is attributed to the ferroelectric properties of CrA gate insulator.

Strain induced anisotropic effect on electron mobility in C60 based organic field effect transistors

The electron mobility was found to increase (decrease) upon applied compressive (tensile) strain, respectively, when a high-performance flexible C60-based organic field-effect transistor (OFET) was subjected to different bending radii. The observed almost twofold relative change in the electron mobility is considerably larger than that reported before for pentacene-based OFETs. Moreover, the strain dependency of electron mobility in C60 films is strongly anisotropic with respect to the strain direction measured relative to the current flow. Analysis within a hopping-transport model for OFET mobility suggests that the observed strain dependency on electron transport is dominated mostly by the change of inter-grain coupling in polycrystalline C60 films.

Ultra-thin films of polysilsesquioxanes possessing 3-methacryloxypropyl groups as gate insulator for organic field-effect transistors

Polysilsesquioxanes (PSQs) possessing 3-methacryloxypropyl groups as an organic moiety of the side chain were synthesized by sol–gel condensation copolymerization of the corresponding trialkoxysilanes. The ultra-thin PSQ film with a radical initiator and a cross-linking agent was prepared by a spin-coating method, and the film was cured integrally at low temperatures of less than 120°C through two different kinds of polymeric reactions, which were radical polymerization of vinyl groups and sol–gel condensation polymerization of terminated silanol and alkoxy groups. The obtained PSQ film showed the almost perfect solubilization resistance to acetone, which is a good solvent of PSQ before polymerization. It became clear by atomic force microscopy observation that the surface of the PSQ film was very smooth at a nano-meter level. Furthermore, pentacene-based organic field-effect transistor (OFET) with the PSQ film as a gate insulator showed typical p-channel enhancement mode operation characteristics and therefore the ultra-thin PSQ film has the potential to be applicable for solution-processed OFET systems.