Behavior of pentacene initial nucleation on various dielectrics and its effect on carrier transport in organic field-effect transistor

The influence of dielectric surface energy on the initial nucleation and the growth of pentacene films as well as the electrical properties of the pentacene-based field-effect transistors are investigated. We have examined a range of organic and inorganic dielectrics with different surface energies, such as polycarbonate/SiO2, polystyrene/SiO2, and PMMA/SiO2 bi-layered dielectrics and also the bare SiO2 dielectric. Atomic force microscopy measurements of sub-monolayer and thick pentacene films indicated that the growth of pentacene film was in Stranski-Kranstanow growth mode on all the dielectrics. However, the initial nucleation density and the size of the first-layered pentacene islands deposited on different dielectrics are drastically influenced by the dielectric surface energy. With the increasing of the surface energy, the nucleation density increased and thus the average size of pentacene islands for the first mono-layer deposition decreased. The performance of fabricated pentacene-based thin film transistors was found to be highly related to nucleation density and the island size of deposited Pentacene film, and it had no relationship to the final particle size of the thick pentacene film. The field effect mobility of the thin film transistor could be achieved as high as 1.38 cm2Ns with on/off ratio over 3 x 10(7) on the PS/SiO2 where the lowest surface energy existed among all the dielectrics. For comparison, the values of mobility and on/off ratio were 0.42 cm2Ns and 1 x 10(6) for thin film transistor deposited directly on bare SiO2 having the highest surface energy.     

Synthesis and characterization of polystyrene brushes for organic thin film transistors

We synthesized and characterized polystyrene brushes on a silicon wafer using surface-initiated atom transfer radical polymerization. The thickness of the polymer brush was controlled by adjusting the reaction time. We investigated monomer conversion as well as the molecular weight and density of the polymer brushes. When the monomer conversion reached 100%, the number-average molecular weight and film thickness reached 135,000 and 113 nm, respectively. The estimated densities of the synthesized polystyrene brushes were in the range 0.34-0.54 chains/nm2, high enough to be categorized in the "concentrated brush" regime. The synthesized polymer brush was used as an insulating layer in an organic thin-film transistor. Organic thin-film transistors were fabricated using pentacene as an active p-type organic semiconductor and a polystyrene brush on a SiO2 layer as a gate dielectric. The pentacene based organic thin-film transistor with the polystyrene brush exhibited a field-effect mobility microFET of 0.099 cm2/(V x s).     

Fabrication of a submicron-channel organic field-effect transistor using a controllable electrospun single fibre as a shadow mask

We demonstrate a simple and versatile method for the fabrication of a submicron channel for an organic field-effect transistor (OFET) using a single electrospun fibre as a shadow mask. A single electrospun fibre is produced by an alternative switching electrospinning method and is stretched 2.5-fold. The average diameter of the stretched fibres is 302 nm. The stretched fibre is placed on ultrathin dielectric layers of aluminium oxide and a self-assembled monolayer (SAM). During electrode deposition the fibre acts as a very small shadow mask. After removing the fibre, electrodes with very narrow gaps of around 350 nm and with high uniformity are easily obtained. We fabricate an OFET by depositing pentacene as an active layer onto the electrodes. The OFET is operable at low voltages, with a threshold voltage of - 1.1 V and a subthreshold swing of 0.27 V decade(-1), values which are one order of magnitude lower than those obtained with a channel length of 75 μm.     

Low-voltage-driven pentacene thin-film transistors with cross-linked poly(4-vinylphenol)/high-k Bi55b3O15 hybrid dielectric for phototransistor

This paper describes the fabrication of pentacene thin-film transistors (TFTs) with an organic/inorganic hybrid gate dielectric, consisting of cross-linked poly(4-vinylphenol) (PVP) and Bi5Nb3O15. A 300-nm-thick Bi5Nb3O15 dielectric film, grown at room temperature, exhibits a high dielectric constant (high-k) value of 40 but has an undesirable interface with organic semiconductors (OSC). To form better interfaces with OSC, a cross-linked PVP dielectric was stacked on the Bi5Nb3O15 dielectric. It is shown that, with the introduction of a hybrid dielectric, our devices not only can be operated at a low voltage (- -5 V) but also have improved electrical characteristics and photoresponse, including a field-effect mobility of 0.72 cm2/V x s, current sub-threshold slopes of 0.29 V/decade, and a photoresponse of 4.84 at a gate bias V(G) = 0 V under 100 mW/cm2 AM 1.5 illumination.     

Low operating voltage single ZnO nanowire field-effect transistors enabled by self-assembled organic gate nanodielectrics

The development of nanowire transistors enabled by appropriate dielectrics is of great interest for flexible electronic and display applications. In this study, nanowire field-effect transistors (NW-FETs) composed of individual ZnO nanowires are fabricated using a self-assembled superlattice (SAS) as the gate insulator. The 15-nm SAS film used in this study consists of four interlinked layer-by-layer self-assembled organic monolayers and exhibits excellent insulating properties with a large specific capacitance, 180 nF/cm2, and a low leakage current density, 1 x 10(-8) A/cm2. SAS-based ZnO NW-FETs display excellent drain current saturation at Vds = 0.5 V, a threshold voltage (Vth) of -0.4 V, a channel mobility of approximately 196 cm2/V s, an on-off current ratio of approximately 10(4), and a subthreshold slope of 400 mV/dec. For comparison, ZnO NW-FETs are also fabricated using 70-nm SiO2 as the gate insulator. Implementation of the SAS gate dielectric reduces the NW-FET operating voltage dramatically with more than 1 order of magnitude enhancement of the on-current. These results strongly indicate that SAS-based ZnO NW-FETs are promising candidates for future flexible display and logic technologies.     

Bootstrapped inverter using a pentacene thin-film transistor with a poly(methyl methacrylate) gate dielectric

A bootstrapped inverter incorporating pentacene organic thin-film transistors (OTFTs), with poly(methyl methacrylate) as the gate dielectric, has been designed, fabricated and tested. The inverter uses capacitive coupling and bootstrapping effects, and exhibits superior performance to the normal diode-connected load inverter. The pentacene OTFTs used for the inverter possess a field-effect mobility of 0.32 cm²/V/s, a threshold voltage of -10.0 V, a subthreshold slope of 1.5 V per decade and an on/off current ratio of 2.2 times 10⁶. The inverter has a 30 mus rise time and a 450 mus fall time, at an operating frequency of 1 kHz and 30 V drive voltage.     

Solution-processable organic dielectrics for graphene electronics

We report the fabrication, at low-temperature, of solution processed graphene transistors based on carefully engineered graphene/organic dielectric interfaces. Graphene transistors based on these interfaces show improved performance and reliability when compared with traditional SiO(2) based devices. The dielectric materials investigated include Hyflon AD (Solvay), a low-k fluoropolymer, and various organic self-assembled monolayer (SAM) nanodielectrics. Both types of dielectric are solution processed and yield graphene transistors with similar operating characteristics, namely high charge carrier mobility, hysteresis free operation, negligible doping effect and improved operating stability as compared to bare SiO(2) based devices. Importantly, the use of SAM nanodielectrics enables the demonstration of low operating voltage (?相似文献   

High performance flexible organic thin film transistors (OTFTs) with octadecyltrichlorsilane/ Al2O3/poly(4-vinylphenol) multilayer insulators

The incorporation of a thin, atomic layer deposited Al2O3 layer in between a spin-coated poly-4-vinyl phenol (PVP) organic layer and octadecyltrichlorsilane (OTS) in the multilayer gate dielectric for pentacene organic thin film transistors on a n(+)-Si substrate reduced the gate leakage current and thereby significantly enhanced the current on/off ratio up to 2.8 x 10(6). Addition of the OTS monolayer on the UV-treated Al2O3 improved the crystallinity of the pentacene layer, where the OTS/UV-treated Al2O3 surfaces increased their contact angles to 100 degrees. X-ray diffraction (XRD) analysis revealed a more intense (001) crystal reflectance of pentacene deposited on OTS/UV-treated Al2O3 surface than that on OTS/Al2O3 surface. Moreover, the improved pentacene layer contributed to the field effect mobility (0.4 cm2/Vs) and subsequently improved the electrical performances of organic thin film transistor (OTFT) devices. This PVP/UV treated Al2O3/OTS multilayer gate dielectric stack was superior to those of the device with the single PVP gate dielectrics due to the improved crystallinity of pentacene.     

Control of carrier density by self-assembled monolayers in organic field-effect transistors

Organic thin-film transistors are attracting a great deal of attention due to the relatively high field-effect mobility in several organic materials. In these organic semiconductors, however, researchers have not established a reliable method of doping at a very low density level, although this has been crucial for the technological development of inorganic semiconductors. In the field-effect device structures, the conduction channel exists at the interface between organic thin films and SiO(2) gate insulators. Here, we discuss a new technique that enables us to control the charge density in the channel by using organosilane self-assembled monolayers (SAMs) on SiO(2) gate insulators. SAMs with fluorine and amino groups have been shown to accumulate holes and electrons, respectively, in the transistor channel: these properties are understood in terms of the effects of electric dipoles of the SAMs molecules, and weak charge transfer between organic films and SAMs.     

Processing and performance of organic insulators as a gate layer in organic thin film transistors fabricated on polyethylene terephthalate substrate

Fabrication of organic thin film transistor (OTFT) on flexible substrates is a challenge, because of its low softening temperature, high roughness and flexible nature. Although several organic dielectrics have been used as gate insulator, it is difficult to choose one in absence of a comparative study covering processing of dielectric layer on polyethylene terephthalate (PET), characterization of dielectric property, pentacene film morphology and OTFT characterization. Here, we present the processing and performance of three organic dielectrics, poly(4-vinylphenol) (PVPh), polyvinyl alcohol (PVA) and poly(methyl methacrylate) (PMMA), as a gate layer in pentacene-based organic thin film transistor on PET substrate. We have used thermogravimetric analysis of organic dielectric solution to determine annealing temperature for spin-coated films of these dielectrics. Comparison of the leakage currents for the three dielectrics shows PVA exhibiting lowest leakage (in the voltage range of ?30 to +30 V). This is partly because solvent is completely eliminated in the case of PVA as observed by differential thermogravimetric analysis (DTGA). We propose that DTGA can be a useful tool to optimize processing of dielectric layers. From organic thin film transistor point of view, crystal structure, morphology and surface roughness of pentacene film on all the dielectric layers were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). We observe pyramidal pentacene on PVPh whereas commonly observed dendritic pentacene on PMMA and PVA surface. Pentacene morphology development is discussed in terms of surface roughness, surface energy and molecular nature of the dielectric layer.     

Low-voltage p- and n-type organic self-assembled monolayer field effect transistors

We report on p- and n-type organic self-assembled monolayer field effect transistors. On the base of quaterthiophene and fullerene units, multifunctional molecules were synthesized, which have the ability to self-assemble and provide multifunctional monolayers. The self-assembly approach, based on phosphonic acids, is very robust and allows the fabrication of functional devices even on larger areas. The p- and n-type transistor devices with only one molecular active layer were demonstrated for transistor channel lengths up to 10 μm. The monolayer composition is proven by electrical experiments and by high-resolution transmission electron microscopy, electron energy loss spectroscopy, XPS, and AFM experiments. Because of the molecular design and the contribution of isolating alkyl chains to the hybrid dielectric, our devices operate at low supply voltages (-4 V to +4 V), which is a key requirement for practical use and simplifies the integration in standard applications. The monolayer devices operate in ambient air and show hole and electron mobilities of 10(-5) cm(2)/(V s) and 10(-4) cm(2)/(V s) respectively. In particular the n-type operation of self-assembled monolayer transistors has not been reported before. Hereby, structure-property relations of the SAMs have been studied. Furthermore an approach to protect the sensitive C(60) from immediate degradation within the molecular design is provided.     

Structures and electronic properties of thin-films of polycyclic aromatic hydrocarbons

We report the fabrication and characterization of organic thin-film transistors (TFTs) using several polycyclic aromatic hydrocarbons (PAHs). Pentacene, ovalene, dibenzocoronene and hexabenzocoronene were deposited as organic semiconductors on silicon wafers with gold electrodes as the bottom-contact configuration of the TFTs. The pentacene TFT showed the highest field-effect mobility of more than 0.1 cm²/Vs in comparison with the other PAHs. The results clarified that the high field-effect mobility of the pentacene thin film is due to large grain size and intrinsic electronic properties.     

Gate insulator effects on the electrical performance of ZnO thin film transistor on a polyethersulphone substrate

Low temperature processing for fabrication of transistor backplane is a cost effective solution while fabrication on a flexible substrate offers a new opportunity in display business. Combination of both merits is evaluated in this investigation. In this study, the ZnO thin film transistor on a flexible Polyethersulphone (PES) substrate is fabricated using RF magnetron sputtering. Since the selection and design of compatible gate insulator is another important issue to improve the electrical properties of ZnO TFT, we have evaluated three gate insulator candidates; SiO2, SiNx and SiO2/SiNx. The SiO2 passivation on both sides of PES substrate prior to the deposition of ZnO layer was effective to enhance the mechanical and thermal stability. Among the fabricated devices, ZnO TFT employing SiNx/SiO2 stacked gate exhibited the best performance. The device parameters of interest are extracted and the on/off current ratio, field effect mobility, threshold voltage and subthreshold swing are 10(7), 22 cm2/Vs, 1.7 V and 0.4 V/decade, respectively.     

Effects of the surface treatment with monolayers of twisted biaryls on the efficiency of pentacene thin-film transistors

A pentacene thin-film transistor (TFT) was fabricated on a SiO2 gate insulator modified with twisted biaryls. The biaryl monolayer, in particular a binaphthyl (BN) monolayer, is amorphous surface where the naphthalene rings are randomly oriented with no lateral order because of their rigid, twisted, and asymmetric shape. When the BN monolayer was used for the surface treatment of SiO2, large grains were obtained in the early stages of the pentacene crystal growth. The pentacene TFT had a field effect mobility (microm) in excess of 0.4 cm2/Vs and an on/off ratio greater than 10(5). The surface treatment improved the mobility of the pentacene TFT by a factor of 50% compared with non-treated devices. The morphology of the semiconductor layer was investigated using atomic force microscopy (AFM) and X-ray diffraction (XRD).     

Soluble fullerene-based n-channel organic thin-film transistors printed by using a polydimethylsiloxane stamp

A polydimethylsiloxane stamp was applied for the first time to the fabrication of n-channel thin-film transistors based on soluble small molecule organic semiconducting materials. The stamping method was found to facilitate film transfer onto a gate insulator surface irrespective of its surface free energy. We used [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) and C(60)-fused N-methylpyrrolidine-meta-dodecyl phenyl (C60MC12) as n-channel materials. The stamped thin-film transistors of C60MC12 achieved a high electron mobility of 0.39 cm(2)/(V s) and a current on-off ratio of 1 × 10(7). The mobility of the stamped C60MC12 thin-film transistors did not depend much on the surface free energy of the SiO(2) gate insulator with and without surface treatment using a silane-coupling reagent. In particular, the stamped C60MC12 thin-film transistor exhibited a relatively high mobility of 0.1 cm(2)/(V s) on a high energy surface of untreated SiO(2). In addition, a complementary inverter composed of an n-channel and a p-channel stamped thin-film transistor was demonstrated for the first time, which exhibits a maximum gain of 63 at a supply voltage of 50 V.     

High-performance transparent inorganic-organic hybrid thin-film n-type transistors

High-performance thin-film transistors (TFTs) that can be fabricated at low temperature and are mechanically flexible, optically transparent and compatible with diverse substrate materials are of great current interest. To function at low biases to minimize power consumption, such devices must also contain a high-mobility semiconductor and/or a high-capacitance gate dielectric. Here we report transparent inorganic-organic hybrid n-type TFTs fabricated at room temperature by combining In2O3 thin films grown by ion-assisted deposition, with nanoscale organic dielectrics self-assembled in a solution-phase process. Such TFTs combine the advantages of a high-mobility transparent inorganic semiconductor with an ultrathin high-capacitance/low-leakage organic gate dielectric. The resulting, completely transparent TFTs exhibit excellent operating characteristics near 1.0 V with large field-effect mobilities of >120 cm2 V(-1) s(-1), drain-source current on/off modulation ratio (I(on)/I(off)) approximately 10(5), near-zero threshold voltages and sub-threshold gate voltage swings of 90 mV per decade. The results suggest new strategies for achieving 'invisible' optoelectronics.     

Electric characteristics of organic thin-film transistors and logic circuits with a ferroelectric gate insulator

Organic electronic devices using a pentacene have improved importantly in the last several years. We fabricated pentacene organic thin-film transistors (OTFTs) with dielectric SiO₂ and ferroelectric Pb(Zr_0.3,Ti_0.7)O₃ (PZT) gate insulators. The organic devices using SiO₂ and PZT films had the field-effect mobility of approximately 0.1 and 0.004 cm²/V s, respectively. The drain current in the transfer curve of pentacene/PZT transistors showed a hysteresis behavior originated in a ferroelectric polarization switching. In order to investigate the polarization effect of PZT gate dielectrics in a logic circuit, the simple voltage inverter using SiO₂ and PZT films was fabricated and measured by an output-input measurement. The gain of inverter at the poling-down state was approximately 7.2 and it was three times larger than the value measured at the poling-up state.     

Carrier injection and transport in organic field-effect transistor investigated by impedance spectroscopy

In this research, impedance spectroscopy (IS) has been applied on top-contact pentacene organic field-effect transistors (OFETs) for characterization of the contact resistance and carrier transport properties. The various relaxation processes were analyzed based on the Maxwell–Wagner model, assuming pentacene as a dielectric material. The mobilities obtained from the IS analysis correspond well with that obtained from the current–voltage (I–V) analysis. This alternative method by IS provides an additional advantage of isolating the effect of carrier concentration when evaluating the mobility of OFETs.     

High-mobility solution-processed copper phthalocyanine-based organic field-effect transistors

Abstract

Solution-processed films of 1,4,8,11,15,18,22,25-octakis(hexyl) copper phthalocyanine (CuPc₆) were utilized as an active semiconducting layer in the fabrication of organic field-effect transistors (OFETs) in the bottom-gate configurations using chemical vapour deposited silicon dioxide (SiO₂) as gate dielectrics. The surface treatment of the gate dielectric with a self-assembled monolayer of octadecyltrichlorosilane (OTS) resulted in values of 4×10^?2 cm² V^?1 s^?1 and 10⁶ for saturation mobility and on/off current ratio, respectively. This improvement was accompanied by a shift in the threshold voltage from 3 V for untreated devices to -2 V for OTS treated devices. The trap density at the interface between the gate dielectric and semiconductor decreased by about one order of magnitude after the surface treatment. The transistors with the OTS treated gate dielectrics were more stable over a 30-day period in air than untreated ones.     

Effect of organic buffer layer in the electrical properties of amorphous-indium gallium zinc oxide thin film transistor

In this research, we reported on the fabrication of top-contact amorphous-indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with an organic buffer layer between inorganic gate dielectric and active layer in order to improve the electrical properties of devices. By inserting an organic buffer layer, it was possible to make an affirmation of the improvements in the electrical characteristics of a-IGZO TFTs such as subthreshold slope (SS), on/off current ratio (I(ON/OFF)), off-state current, and saturation field-effect mobility (muFE). The a-IGZO TFTs with the cross-linked polyvinyl alcohol (c-PVA) buffer layer exhibited the pronounced improvements of the muFE (17.4 cm2/Vs), SS (0.9 V/decade), and I(ON/OFF) (8.9 x 10(6)).