Pentacene Field-effect Transistors Research Articles

Monolayer field effect transistor as a probe of electronic defects in organic semiconducting layers at organic/inorganic hetero-junction interface

The origin of a large negative threshold voltage observed in monolayer (ML) field effect transistors (FETs) is explored using in-situ electrical measurements through confining the thickness of an active layer to the accumulation layer thickness. Using ML pentacene FETs combined with gated multiple-terminal devices and atomic force microscopy, the effect of electronic and structural evolution of a ML pentacene film on the threshold voltage in an FET, proportional to the density of deep traps, was probed, revealing that a large negative threshold voltage found in ML FETs results from the pentacene/SiO2 and pentacene/metal interfaces. More importantly, the origin of the threshold voltage difference between ML and thick FETs is addressed through a model in which the effective charge transport layer is transitioned from the pentacene layer interfacing with the SiO2 gate dielectric to the upper layers with pentacene thickness increasing evidenced by pentacene coverage dependent threshold voltage measurements.

Value-added Synthesis of Graphene: Recycling Industrial Carbon Waste into Electrodes for High-Performance Electronic Devices.

We have developed a simple, scalable, transfer-free, ecologically sustainable, value-added method to convert inexpensive coal tar pitch to patterned graphene films directly on device substrates. The method, which does not require an additional transfer process, enables direct growth of graphene films on device substrates in large area. To demonstrate the practical applications of the graphene films, we used the patterned graphene grown on a dielectric substrate directly as electrodes of bottom-contact pentacene field-effect transistors (max. field effect mobility ~0.36 cm2·V−1·s−1), without using any physical transfer process. This use of a chemical waste product as a solid carbon source instead of commonly used explosive hydrocarbon gas sources for graphene synthesis has the dual benefits of converting the waste to a valuable product, and reducing pollution.

Interface effect in pentacene field-effect transistors from high energy proton beam irradiation

We report the effect of irradiation using 10 MeV high energy proton beams on pentacene organic field-effect transistors (OFETs). The electrical characteristics of the pentacene OFETs were measured before and after proton beam irradiation with fluence (dose) conditions of 1012, 1013, and 1014 cm−2. After proton beam irradiation with fluences of 1012 or 1013 cm−2, the threshold voltage of the OFET devices shifted to the positive gate voltage direction with an increase in the current level and mobility. In contrast, for a high proton beam fluence condition of 1014 cm−2, the threshold voltage shifted to the negative gate voltage direction with a decrease in the current level and mobility. It is evident from the electrical characteristics of the pentacene OFETs treated with a self-assembled monolayer that these experimental observations can be attributed to the trapped charges in the dielectric layer and pentacene/SiO2 interface. Our study will enhance the understanding of the influence of high energy particles on organic field-effect transistors.

First principle study of the effect of defects on performance of single-molecule pentacene field effect transistors

In this work, we have performed first principle study on a single-molecule pentacene field effect transistor and studied various oxygen- and hydrogen-induced defects in the same device configuration. Further, we have investigated the effect of these defects on the various electronic transport properties of the device and compared them with those of the original device along with reporting the negative differential region window and the peak-to-valley ratio in different cases. For this purpose, we have applied the density functional theory in conjugation with non-equilibrium green’s function (NEGF) formalism on a 14.11 Å pentacene device to obtain the I–V characteristics, conductance curves and transmission spectra in various device scenarios.

Surface-Order Mediated Assembly of π-Conjugated Molecules on Self-Assembled Monolayers with Controlled Grain Structures

This study systematically demonstrates the effects of the grain structure of crystalline self-assembled monolayers (SAMs) on the growth of organic semiconductor thin films on such monolayers, as well as the electrical characteristics of the resulting semiconductor films. The grain structure of the octadecyltrichlorosilane (OTS) monolayers could be tailored by constructing the monolayers at three different temperatures: −30 °C (−30 °C OTS), −5 °C (−5 °C OTS), and 20 °C (20 °C OTS). Among the three layers, −30 °C OTS exhibited the largest crystalline grains and longest-range homogeneity of alkyl chain arrays. We found that pentacene films deposited on −30 °C OTS monolayers show larger crystalline grains with higher degrees of crystallinity and lateral alignment compared to that of films deposited on −5 °C OTS or 20 °C OTS monolayers, following the surface characteristics of the underlying OTS monolayers. Furthermore, pentacene field-effect transistors fabricated with −30 °C OTS monolayers showed lower charg...

Response enhancement mechanism of NO2 gas sensing in ultrathin pentacene field-effect transistors

By optimization of structural and physical–chemical properties at a precision level of molecules, organic sensing devices seek to realize the state of the art in monolayer based device applications. In our work, pentacene ultrathin film transistor has been integrated into the implement of a gas sensor based on an increase in its mobility and a shift of its threshold voltage. A limit of detection of pentacene monolayer field-effect transistor was found to be at sub ppm level when it is applied for detection of NO2. Compared with a thick layer sensor device, the pentacene monolayer NO2 sensor has boosted up sensing response with three orders of magnitude. An enhancement of high selectivity and response mechanism can be understood with a reasonable description emphasizing on the 2D transport characters and a series of gauzy interplay for monolayer pentacene film with NO2 analyte.

Stencil nano lithography based on a nanoscale polymer shadow mask: towards organic nanoelectronics.

A stencil lithography technique has been developed to fabricate organic-material-based electronic devices with sub-micron resolution. Suspended polymethylmethacrylate (PMMA) membranes were used as shadow masks for defining organic channels and top electrodes. Arrays of pentacene field effect transistors (FETs) with various channel lengths from 50 μm down to 500 nm were successfully produced from the same batch using this technique. Electrical transport measurements showed that the electrical contacts of all devices were stable and the normalized contact resistances were much lower than previously studied organic FETs. Scaling effects, originating from the bulk space charge current, were investigated by analyzing the channel-length-dependent mobility and hysteresis behaviors. This novel lithography method provides a reliable means for studying the fundamental transport properties of organic materials at the nanoscale as well as enabling potential applications requiring the fabrication of integrated organic nanoelectronic devices.

Electron and hole transport in ambipolar, thin film pentacene transistors

Solution-processed, ambipolar, thin-film pentacene field-effect transistors were employed to study both electron and hole transport simultaneously in a single, organic solid-state device. Electron and hole mobilities were extracted from the respective unipolar saturation regimes and show thermally activated behavior and gate voltage dependence. We fit the gate voltage dependent saturation mobility to a power law to extract the characteristic Meyer-Neldel (MN) energy, a measure of the width of the exponential distribution of localized states extending into the energy gap of the organic semiconductor. The MN energy is ∼78 and ∼28 meV for electrons and holes, respectively, which reflects a greater density of localized tail states for electrons than holes. This is consistent with the lower measured electron than hole mobility. For holes, the well-behaved linear regime allows for four-point probe measurement of the contact resistance independent mobility and separate characterization of the width of the localized density of states, yielding a consistent MN energy of 28 meV.

Pn-Heterojunction effects of perylene tetracarboxylic diimide derivatives on pentacene field-effect transistor.

We investigated the heterojunction effects of perylene tetracarboxylic diimide (PTCDI) derivatives on the pentacene-based field-effect transistors (FETs). Three PTCDI derivatives with different substituents were deposited onto pentacene layers and served as charge transfer dopants. The deposited PTCDI layer, which had a nominal thickness of a few layers, formed discontinuous patches on the pentacene layers and dramatically enhanced the hole mobility in the pentacene FET. Among the three PTCDI molecules tested, the octyl-substituted PTCDI, PTCDI-C8, provided the most efficient hole-doping characteristics (p-type) relative to the fluorophenyl-substituted PTCDIs, 4-FPEPTC and 2,4-FPEPTC. The organic heterojunction and doping characteristics were systematically investigated using atomic force microscopy, 2D grazing incidence X-ray diffraction studies, and ultraviolet photoelectron spectroscopy. PTCDI-C8, bearing octyl substituents, grew laterally on the pentacene layer (2D growth), whereas 2,4-FPEPTC, with fluorophenyl substituents, underwent 3D growth. The different growth modes resulted in different contact areas and relative orientations between the pentacene and PTCDI molecules, which significantly affected the doping efficiency of the deposited adlayer. The differences between the growth modes and the thin-film microstructures in the different PTCDI patches were attributed to a mismatch between the surface energies of the patches and the underlying pentacene layer. The film-morphology-dependent doping effects observed here offer practical guidelines for achieving more effective charge transfer doping in thin-film transistors.

Bioinspired peptide nanostructures for organic field-effect transistors.

Peptide-based nanostructures derived from natural amino acids are superior building blocks for biocompatible devices as they can be used in a bottom-up process without the need for expensive lithography. A dense nanostructured network of l,l-diphenylalanine (FF) was synthesized using the solid-vapor-phase technique. Formation of the nanostructures and structure-phase relationship were investigated by electron microscopy and Raman scattering. Thin films of l,l-diphenylalanine micro/nanostructures (FF-MNSs) were used as the dielectric layer in pentacene-based field-effect transistors (FETs) and metal-insulator-semiconductor diodes both in bottom-gate and in top-gate structures. Bias stress studies show that FF-MNS-based pentacene FETs are more resistant to degradation than pentacene FETs using FF thin film (without any nanostructures) as the dielectric layer when both are subjected to sustained electric fields. Furthermore, it is demonstrated that the FF-MNSs can be functionalized for detection of enzyme-analyte interactions. This work opens up a novel and facile route toward scalable organic electronics using peptide nanostructures as scaffolding and as a platform for biosensing.

Charge mobility anisotropy of functionalized pentacenes in organic field effect transistors fabricated by solution processing

To understand and optimize the performance of thin-film electronic devices incorporating crystalline organic semiconductors, it is important to consider the impact of their structural anisotropy on the charge transport. Here we report on the charge mobility anisotropy in 6,13-bis(triisopropylsilylethynyl) (TIPS) and 6,13-bis(triethylsilylethynyl) (TES) pentacene field effect transistors, in which microstructure is controlled by solution processing conditions. Thin-film structures that range from millimetre size, crystalline domains to macroscopic, high-aspect-ratio (∼1 μm wide and >1 cm long) needles are systematically produced by controlling the substrate displacement rate during zone-cast deposition. Through precise control of the microstructure we experimentally explore the differences in charge transport anisotropy between TIPS- and TES-pentacene molecules. Aligned needles of TIPS- pentacene result in a mobility anisotropy (μ∥/μ⊥) of ∼20 (mobility of ∼0.7 cm2 V−1 s−1) whereas TES-pentacene produce an order of magnitude lower mobility (∼0.06 cm2 V−1 s−1) but much higher mobility anisotropy (>45). Such significant changes in absolute mobility and mobility anisotropy are attributed to their different packing structures, which permit 2D charge transport in TIPS-pentacene and 1D transport in TES-pentacene. Bulky TIPS- side groups (diameter ∼7.5 A) force a brick-wall type packing structure, whereas TES- side groups (diameter ∼6.6 A) pack in a 1D slipped-stack. Furthermore, through precise control of the molecular alignment, the impact of crystal orientation on charge transport is investigated. TIPS-pentacene achieves the highest mobility when the angle between the needle long-axis and charge transport directions is ∼35°, whereas in TES-pentacene it is much closer to 0°. These results are supported by theoretical simulations.

Low-voltage organic field-effect transistors based on novel high-κ organometallic lanthanide complex for gate insulating materials

A novel high-κ organometallic lanthanide complex, Eu(tta)3L (tta=2-thenoyltrifluoroacetonate, L = 4,5-pinene bipyridine), is used as gate insulating material to fabricate low-voltage pentacene field-effect transistors (FETs). The optimized gate insulator exhibits the excellent properties such as low leakage current density, low surface roughness, and high dielectric constant. When operated under a low voltage of −5 V, the pentacene FET devices show the attractive electrical performance, e.g. carrier mobility (μFET) of 0.17 cm2 V−1 s−1, threshold voltage (Vth) of −0.9 V, on/off current ratio of 5 × 103, and subthreshold slope (SS) of 1.0 V dec−1, which is much better than that of devices obtained on conventional 300 nm SiO2 substrate (0.13 cm2 V−1 s−1, −7.3 V and 3.1 V dec−1 for μFET, Vth and SS value when operated at −30 V). These results indicate that this kind of high-κ organometallic lanthanide complex becomes a promising candidate as gate insulator for low-voltage organic FETs.

Inverse transfer method using polymers with various functional groups for controllable graphene doping.

The polymer-supported transfer of chemical vapor deposition (CVD)-grown graphene provides large-area and high-quality graphene on a target substrate; however, the polymer and organic solvent residues left by the transfer process hinder the application of CVD-grown graphene in electronic and photonic devices. Here, we describe an inverse transfer method (ITM) that permits the simultaneous transfer and doping of graphene without generating undesirable residues by using polymers with different functional groups. Unlike conventional wet transfer methods, the polymer supporting layer used in the ITM serves as a graphene doping layer placed at the interface between the graphene and the substrate. Polymers bearing functional groups can induce n-doping or p-doping into the graphene depending on the electron-donating or -withdrawing characteristics of functional groups. Theoretical models of dipole layer-induced graphene doping offered insights into the experimentally measured change in the work function and the Dirac point of the graphene. Finally, the electrical properties of pentacene field effect transistors prepared using graphene electrodes could be enhanced by employing the ITM to introduce a polymer layer that tuned the work function of graphene. The versatility of polymer functional groups suggests that the method developed here will provide valuable routes to the development of applications of CVD-grown graphene in organic electronic devices.

Spectroscopic analysis of electron trapping levels in pentacene field-effect transistors

Electron trapping phenomena have been investigated with respect to the energy levels of localized trap states and bias-induced device instability effects in pentacene field-effect transistors. The mechanism of the photoinduced threshold voltage shift (ΔVT) is presented by providing a ΔVT model governed by the electron trapping. The trap-and-release behaviour functionalized by photo-irradiation also shows that the trap state for electrons is associated with the energy levels in different positions in the forbidden gap of pentacene. Spectroscopic analysis identifies two kinds of electron trap states distributed above and below the energy of 2.5 eV in the band gap of the pentacene crystal. The study of photocurrent spectra shows the specific trap levels of electrons in energy space that play a substantial role in causing device instability. The shallow and deep trapping states are distributed at two centroidal energy levels of ∼1.8 and ∼2.67 eV in the pentacene band gap. Moreover, we present a systematic energy profile of electron trap states in the pentacene crystal for the first time.

Dependence of interface charge trapping on channel engineering in pentacene field effect transistors.

We investigate the dependence of charge carrier mobility by trap states at various interface regions through channel engineering. Prior to evaluation of interface trap density, the electrical performance in pentaene field effect transistors (FET) with high-k gate oxide are also investigated depending on four channel engineering. As a channel engineering, gas treatment, coatings of thin polymer layer, and chemical surface modification using small molecules were carried out. After channel engineering, the performance of device as well as interface trap density calculated by conductance method are remarkably improved. It is found that the reduced interface trap density is closely related to decreasing the sub-threshold swing and improving the mobility. Particularly, we also found that performance of device such as mobility, subthreshold swing, and interface trap density after gas same is comparable to those of OTS.

Formation of solution-processed multistacked ferroelectric layers for performance improvement of ferroelectric-gated pentacene field-effect transistors

In solution-processes, the formation of multistacked layers (MSLs) is difficult because of natural dissolution and/or damage to the underlying film by the solvent used for the upper film. To form MSLs in solution-processes, using orthogonal solvents for each layer is a representative remedy. As the number of MSLs increases, the selection of appropriate solvents for every layer becomes harder. In this work, we report a viable method to form MSLs, even when using one material for the entire solution-process. We present that solution-processed MSLs can be formed by tuning the solubility by mixing an orthogonal solvent. We fabricated ferroelectric MSLs using a ferroelectric polymer and applied the MSLs into a ferroelectric-gated field-effect transistor. We obtained improvements in electrical properties such as current on/off ratio, subthreshold swing, and mobility in transistors with MSLs, in comparison to the single layer case.

Lower activation energy in organic field effect transistors with carbon nanotube contacts

We study temperature dependent charge transport properties of pentacene field effect transistors (FET) with carbon nanotube (CNT) electrodes. The field effect mobilities at different temperatures and gate voltages follow activated hopping behavior from which we calculate an activation energy of 48meV for the CNT contacted pentacene FET. This value is lower than that of our control Pd contacted pentacene FET device (83meV). The lower activation energy of the CNT contacted devices is attributed to improved CNT/pentacene interface.

Ultra-thin and graded sliver electrodes for use in transparent pentacene field-effect transistors

The authors report the fabrication of efficient and transparent pentacene field-effect transistors (FETs) using a graded structure of ultra-thin silver (Ag) source and drain (S–D) electrodes. The S–D electrodes were prepared by thermal evaporation with a controlled deposition rate to form Ag layer with a graded structure, leading to a reduced injection barrier and smoothing the contact surface between the electrode and the pentacene channel. The sheet resistance of such Ag electrode was found to be as low as 9Ω/sq. In addition, a hole-only behavior of device with Ag electrode characterized by current–voltage measurement and conductive atomic-force microscopy shows the injection property of high current flowing as compared with device using Au electrode, resulting in an efficient injection condition existing at the interface of the graded Ag/pentacene. Device characterization indicates the transparent pentacene FET with a graded ultra-thin Ag electrode and organic capping layer of N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine exhibits a high transmission rate of ∼75% in the range of visible light from 400 to 550nm, a threshold voltage of −6.0V, an on–off drain current ratio of 8.4×105, and a field-effect mobility of 1.71cm2/Vs, thus significantly outperforming pentacene FETs with multilayer oxide electrodes or other transparent thin metal layers.

Micropatterned single-walled carbon nanotube electrodes for use in high-performance transistors and inverters.

We demonstrated the solution-processed single-walled carbon nanotube (SWNT) source-drain electrodes patterned using a plasma-enhanced detachment patterning method for high-performance organic transistors and inverters. The high-resolution SWNT electrode patterning began with the formation of highly uniform SWNT thin films on a hydrophobic silanized substrate. The SWNT source-drain patterns were then formed by modulating the interfacial energies of the prepatterned elastomeric mold and the SWNT thin film using oxygen plasma. The SWNT films were subsequently selectively delaminated using a rubber mold. The patterned SWNTs could be used as the source-drain electrodes for both n-type PTCDI-C8 and p-type pentacene field-effect transistors (FETs). The n- and p-type devices exhibited good and exactly matched electrical performances, with a field-effect mobility of around 0.15 cm(2) V(-1) s(-1) and an ON/OFF current ratio exceeding 10(6). The single electrode material was used for both the n and p channels, permitting the successful fabrication of a high-performance complementary inverter by connecting a p-type pentacene FET to an n-type PTCDI-C8 FET. This patterning technique was simple, inexpensive, and easily scaled for the preparation of large-area electrode micropatterns for flexible microelectronic device fabrication.

Small device-to-device variation of 6,13-bis(triisopropylsilylethynyl)pentacene field-effect transistor arrays fabricated by a flow-coating method

Bottom-contact/bottom-gate-type organic field-effect transistor (OFET) arrays have been fabricated using a flow-coating method, and the device-to-device variation has been examined. The flow-coated active layer of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) was composed of arrays of needle-shaped crystals whose long axes were aligned along the flow-coating direction. By measuring the electrical characteristics of one hundred twenty OFETs with the channel current direction parallel or perpendicular to the flow-coating direction, we evaluated the device-to-device variation in the device properties. The field-effect hole mobilites (average ± standard deviation) for the parallel and perpendicular OFET sets were 0.51 ± 0.03 and 0.12 ± 0.03 cm2·V−1·s−1, respectively. The small standard deviations clearly show the high spatial uniformity of the TIPS-PEN active layer. The much smaller relative standard deviation, a measure of device-to-device variation, for the parallel OFET set (6%) can be attributed to a high degree of alignment of needle-shaped TIPS-PEN crystals along the flow-coating direction.