Showing papers on "Organic semiconductor published in 2001"

Functionalized Pentacene: Improved Electronic Properties from Control of Solid-State Order

Abstract: Molecular order has proven to be a significant factor in the performance of devices based on organic semiconductors. Recent studies involving solubilized versus unsubstituted thiophene oligomers have demonstrated that modifications which increase orbital overlap in the solid state can improve device performance by more than an order of magnitude. 1 Similar studies on pentacene, a compound which has already demonstrated remarkable potential for device applications, 2 have also focused on maximizing orbital overlap by inducing order in films. 3 However, these pentacene studies have thus far relied on substrate modification, rather than on pentacene functionalization, 4 to achieve the desired goals. We report here the preparation of two functionalized pentacene derivatives, and the effect of this functionalization on both the solid-state ordering and the electronic properties of the resulting crystals. Our goal for a functionalized pentacene was two-fold: First, the substituents should impart solubility to the acene, to simplify purification and processing. Second, the substituents should induce some capability for self-assembly of the aromatic moieties into ﷿-stacked arrays to enhance intermolecular orbital overlap. We anticipated that both of these goals could be accomplished by exploiting a rigid spacer to hold the necessarily bulky solubilizing groups well away from the aromatic core, allowing the closest possible contact between the aromatic rings. 5 Our initial targets were the bis(triisopropylsilylethynyl)pentacenes 1 and 2. Both of these compounds are easily prepared in near quantitative yield in a one-pot reaction from 6,13-pentacenequinone and 5,14pentacenequinone, respectively. 6

Highly efficient phosphorescence from organic light-emitting devices with an exciton-block layer

Abstract: One of the keys to highly efficient phosphorescent emission in organic light-emitting devices is to confine triplet excitons generated within the emitting layer. We employ “starburst” perfluorinated phenylenes (C60F42) as a both hole- and exciton-block layer, and a hole-transport material 4,4′,4″-tri(N-carbazolyl) triphenylamine as a host for the phosphorescent dopant dye in the emitting layer. A maximum external quantum efficiency reaches to 19.2%, and keeps over 15% even at high current densities of 10–20 mA/cm2, providing several times the brightness of fluorescent tubes for lighting. The onset voltage of the electroluminescence is as low as 2.4 V and the peak power efficiency is 70–72 lm/W, promising for low-power display devices.

Synthetic chemistry for ultrapure, processable, and high-mobility organic transistor semiconductors.

Abstract: An essential aspect of the development of organic-based electronics is the synthetic chemistry devised for the preparation of the semiconductor materials responsible for the activity of organic field-effect transistors. Access to organic semiconductors in sufficient purity and variety has led to breakthroughs in solid-state physics and circuit realization. In this Account, we review the synthetic methods that have been most useful for preparing a range of semiconductors, including thiophene-based oligomers, several kinds of fused rings, and polymers. The tradeoff between process efficiency and target purity is emphasized.

Fabrication of bulk heterojunction plastic solar cells by screen printing

Abstract: We demonstrate the use of screen printing in the fabrication of ultrasmooth organic-based solar cells. Organic films on the order of several tens of nanometers in thickness and 2.6 nm surface roughness were made. The first-generation screen-printed plastic solar cells demonstrated 4.3% in power conversion efficiency when using an aluminum electrode and 488 nm illumination.

Interface-limited injection in amorphous organic semiconductors

Abstract: We examine electron transport in the archetype amorphous organic material tris(8-hydroxyquinoline) aluminum $({\mathrm{Alq}}_{3}).$ It is established that for Al, LiF/Al, and Mg:Ag cathodes, injection processes at the metal/organic contact dominate the current-voltage characteristics. We find that transport is also injection-limited at low temperatures, but that the cathode dependence of current-voltage characteristics at $T=30$ K is substantially reduced, raising doubts over metal-to-organic injection models that depend on the cathode work function. Given that ultraviolet photoelectron spectroscopy measurements show a shift in the vacuum potential at the metal/${\mathrm{Alq}}_{3}$ interface of \ensuremath{\sim}1 eV, we investigate the impact of interfacial dipoles on adjacent molecules in the organic film. Consequently, we propose that injection is limited by charge hopping out of interfacial molecular sites whose energy distribution is broadened by local disorder in the interfacial dipole field. We derive a general analytic model of injection from interfacial states and find that it accurately predicts the current-voltage characteristics of transport in ${\mathrm{Alq}}_{3}$ over many orders of magnitude in current and over a wide range of temperatures. The model is extended to other amorphous organic semiconductors and is found to be applicable to both polymers and small molecular weight organic compounds.

Lithium doping of semiconducting organic charge transport materials

Abstract: We study the effects of lithium (Li) incorporation in the cathodes of organic light-emitting devices. A thermally evaporated surface layer of metallic Li is found to diffuse through, and subsequently dope, the electron transporting organic semiconducting thin films immediately below the cathode, forming an Ohmic contact. A diffusion length of ∼700 A is inferred from analyses of the current–voltage and secondary ion mass spectrometry data. The conductivity of the Li-doped organic films is ∼3×10−5 S/cm. Photoemission spectroscopy suggests that Li lowers the barrier to injection at the organic/cathode interface, introduces gap states in the bulk of the organic semiconductor, and dopes the bulk to facilitate efficient charge transport.

Synthesis and device characterisation of side-chain polymer electron transport materials for organic semiconductor applications

Abstract: Improved syntheses and polymerisations are reported of monomers bearing electron transporting substituents based on 2,5-diphenyloxadiazole and 2,3-diphenylquinoxaline attached directly to a vinyl group. By copolymerisation and by use of mixtures of homopolymers, these materials have been incorporated into light emitting polymer devices in which hole conduction properties are provided by 4-vinyltriphenylamine groups. High luminescence efficiency is achieved by use of a fluorescent additive. The resulting devices show narrow emission bands and high brightnesses, except in the case of those based on a diphenyloxadiazole–triphenylamine polymer blend. Thermal analysis data are equivocal but we present evidence that in this system, but not the quinoxaline blend, phase separation occurs. The minority charge carrying capacity of the homopolymers is probed: it is shown that the quinoxaline derivative has hole blocking properties superior to those of the oxadiazole polymer and is a good candidate for use in optimised devices.

Contact resistance in organic thin film transistors

Abstract: We present an analysis of the electrical characteristics of pentacene OTFTs fabricated on flexible polyethylene naphthalate (PEN) film. Nickel, silicon dioxide, and palladium were deposited by ion-beam sputtering and patterned by photolithography and lift-off to form the gate electrodes, the gate dielectric layer, and the source and drain contacts, respectively. An octadecyltrichlorosilane vapor prime was used to prepare the SiO/sub 2/ gate dielectric surface for the deposition of the pentacene layer, which was deposited by thermal evaporation and patterned using a water-soluble, photo-patterned polyvinyl alcohol layer.

Organic Field Effect Transistors

Abstract: The remarkable development of organic thin film transistors (OTFTs) has led to their emerging use in active matrix flat-panel displays, radio frequency identification cards, and sensors. Exploring one class of OTFTs, Organic Field-Effect Transistors provides a comprehensive, multidisciplinary survey of the present theory, charge transport studies, synthetic methodology, materials characterization, and current applications of organic field-effect transistors (OFETs). Covering various aspects of OFETs, the book begins with a theoretical description of charge transport in organic semiconductors at the molecular level. It then discusses the current understanding of charge transport in single-crystal devices, small molecules and oligomers, conjugated polymer devices, and charge injection issues in organic transistors. After describing the design rationales and synthetic methodologies used for organic semiconductors and dielectric materials, the book provides an overview of a variety of characterization techniques used to probe interfacial ordering, microstructure, molecular packing, and orientation crucial to device performance. It also describes the different processing techniques for molecules deposited by vacuum and solution, followed by current technological examples that employ OTFTs in their operation. Featuring respected contributors from around the world, this thorough, up-to-date volume presents both the theory behind OFETs and the latest applications of this promising technology.

High-performance bottom electrode organic thin-film transistors

Abstract: Pentacene-based organic field effect transistors (FETs) exhibit enormous potential as active elements in a number of applications. One significant obstacle to commercial application remains: no completely lithographic process exists for forming high-performance devices. Processing constraints prevent electrodes from being lithographically patterned once the semiconductor is deposited, but depositing the electrodes before the semiconductor leads to low-performance transistors. By using self-assembled monolayers (SAMs) to change the surface energy of the metal electrodes and morphology of the pentacene subsequently grown on the electrodes, high-performance transistors may be formed using a process compatible with lithographic definition of the source and drain electrodes.

Monolithically integrated, flexible display of polymer-dispersed liquid crystal driven by rubber-stamped organic thin-film transistors

Abstract: This letter describes the monolithic integration of rubber-stamped thin-film organic transistors with polymer-dispersed liquid crystals (PDLCs) to create a multipixel, flexible display with plastic substrates. We report the electro-optic switching behavior of the PDLCs as driven by the organic transistors, and we show that our displays operate robustly under flexing and have a contrast comparable to that of newsprint.

Electron mobility in tris(8-hydroxy-quinoline)aluminum thin films determined via transient electroluminescence from single- and multilayer organic light-emitting diodes

Abstract: Transient electroluminescence (EL) from single- and multilayer organic light-emitting diodes (OLEDs) was investigated by driving the devices with short, rectangular voltage pulses. The single-layer devices consist of indium-tin oxide (ITO)/tris(8-hydroxy-quinoline)aluminum (Alq3)/magnesium (Mg):silver (Ag), whereas the structure of the multilayer OLEDs are ITO/copper phthalocyanine (CuPc)/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB)/Alq3/Mg:Ag. Apparent model-dependent values of the electron mobility (μe) in Alq3 have been calculated from the onset of EL for both device structures upon invoking different internal electric field distributions. For the single-layer OLEDs, transient experiments with different dc bias voltages indicated that the EL delay time is determined by the accumulation of charge carriers inside the device rather than by transport of the latter. This interpretation is supported by the observation of delayed EL after the voltage pulse is turned off. In the multilayer OLED the ...

Pentacene TFT with improved linear region characteristics using chemically modified source and drain electrodes

Abstract: We have fabricated pentacene active layer organic thin film transistors (OTFTs) using chemically-modified source and drain contacts with improved contact and linear region characteristics. OTFTs fabricated on heavily doped, thermally oxidized single-crystal silicon substrates have linear field-effect mobility greater than 0.5 cm/sup 2//V-s at a drain-source voltage of -0.1 V, on/off current ratio greater than 10/sup 7/, and subthreshold slope as low as 0.7 V/decade.

Mechanisms of injection enhancement in organic light-emitting diodes through an Al/LiF electrode

Abstract: The mechanisms of enhanced electron injection into the electron transport layer of Alq3 [tris(8-hydroxyquinoline)-aluminum] via LiF interlayers are studied by means of I–V characteristics, secondary ion mass spectroscopy (SIMS), and Kelvin probe measurements Devices for single carrier injection were prepared using aluminum electrodes, Alq3 electron transport layers, and thin intermediate layers of LiF It was found that devices deposited in the order Alq3-LiF-aluminum have a much higher electron injection capability than structures deposited in the order aluminum-LiF-Alq3 SIMS depth profile analysis reveals that the evaporation of Al on LiF leads to a spatial separation of Li and F probably induced by a chemical reaction of Al with LiF Simple thermodynamic calculations support the energetic feasibility of such a reaction Titanium cathodes in the same layer sequence also exhibit electron injection enhancement, probably due to their similar chemical reactivity However, electron injection from Ag electr

Order on disorder: Copper phthalocyanine thin films on technical substrates

Abstract: We have studied the molecular orientation of the commonly used organic semiconductor copper phthalocyanine (CuPC) grown as thin films on the technically relevant substrates indium tin oxide, oxidized Si, and polycrystalline gold using polarization-dependent x-ray absorption spectroscopy, and compare the results with those obtained from single crystalline substrates [Au(110) and GeS(001)]. Surprisingly, the 20–50 nm thick CuPC films on the technical substrates are as highly ordered as on the single crystals. Importantly, however, the molecular orientation in the two cases is radically different: the CuPC molecules stand on the technical substrates and lie on the single crystalline substrates. The reasons for this and its consequences for our understanding of the behavior of CuPC films in devices are discussed.

Flexible electronic futures.

Abstract: Microelectronic devices incorporating organic materials could find a host of applications. That prospect inches nearer with the development of a strategy for growing thin films of the organic semiconductor pentacene.

Method for improving performance of organic semiconductors in bottom electrode structure

Abstract: A method for improving the performance of an organic thin film field effect transistor comprising the steps of: (a) forming a transistor structure having patterned source and drain electrodes; and (b) treating the patterned source and drain electrodes with a thiol compound having the formula, RSH, wherein R is a linear or branched, substituted or unsubstituted, alkyl, alkenyl, cycloalkyl or aromatic containing from about 6 to about 25 carbon atoms under conditions that are effective in forming a self-assembled monolayer of said thiol compound on said electrodes. Organic thin film transistor structures containing the self-assembled monolayer of the present invention are also disclosed.

Mobility-Dependent Charge Injection into an Organic Semiconductor

Abstract: Measurements of charge injection from indium tin oxide (ITO) into the organic semiconductor, tetraphenyl diamine doped polycarbonate (PC:TPD), were carried out. The current injected at the contact was measured as a function of the hole mobility in the organic semiconductor, which was varied from 10(-6) to 10(-3) cm (2)/V x s by adjusting the concentration of the hole transport agent, TPD, in the PC host. These experiments reveal that the current injected at the contact is proportional to the hole mobility in the bulk. As a result, the ITO/PC:TPD contact is found to limit current flow in all samples, regardless of the hole mobility in PC:TPD.

Supramolecular photosensitive and electroactive materials

Abstract: Phthalocyanines: Synthesis, Supramolecular Organization, and Physical Properties Sandwich-Type Phthalocyaninato and Porphyrinato Metal Complexes Electronic Properties of Molecular Organic Semiconductor Thin Films Polydiacetylenes Structural and Optical Properties of Conjugated Molecules in Perhydrotriphenylene (PHTP) and in Other Channel-Forming Inclusion Compounds Charge Transfer Properties of Photosynthetic and Respiratory Proteins Optical and Electronic Properties of Carbon Nitride Polyimides for Microelectronics and Tribology Applications Anomalous Charge Transport and Polarization in Semiconductors Oxides and Porous Film Electrodes Electroactive and Photoactive Dendrimers Electrical Properties of Organic Monolayer Films

Gate voltage dependent resistance of a single organic semiconductor grain boundary

Abstract: Conducting probe atomic force microscopy (CP-AFM) was used to examine electrical transport through an individual grain boundary (GB) in the organic semiconductor sexithiophene (6T, Egap ∼ 2.3 eV). The sample consisted of a pair of grains grown by vapor deposition onto an SiO2/Si substrate. A variable channel length transistor was constructed using a microfabricated Au electrode contacting one grain, a Au-coated AFM tip as a positionable electrode, and the doped Si substrate as a gate. The GB resistance was found to be gate voltage dependent and large, on the order of 109−1010 Ω for a 1 μm boundary length. Resistances across single 6T grains were an order of magnitude lower. The results indicate that GBs can be the principal bottleneck to charge transport in polycrystalline organic semiconductor films, particularly at low gate fields, consistent with a recent model that proposes potential barriers exist between grains. We estimate the GB barrier height to be on the order of 100 meV.

Patterning organic–inorganic thin-film transistors using microcontact printed templates

Abstract: We report the simple, low-cost, and parallel fabrication of patterned organic–inorganic thin-film transistors (TFTs) by microcontact printing a molecular template on the substrate surface prior to film deposition. We printed molecules with hydrophobic tail groups on the gate oxide surfaces of TFTs to chemically, differentiate the substrate surface and confine the self-assembly of thin films, deposited from solutions flooded across the entire surface, to the transistor channels. TFTs are fabricated with good device characteristics and no current leakage. This process is more general to the patterning of other solution-deposited thin-film materials.

Morphological and Transistor Studies of Organic Molecular Semiconductors with Anisotropic Electrical Characteristics

Abstract: Oriented thin films of organic semiconducting small molecules were prepared by crystallization on rubbed alignment layers. Polarized absorption spectra showed that the long axis of the conjugated backbones was highly oriented along the rubbing direction and parallel to the substrates. Transmission electron microscopy and diffraction confirmed that the molecules and in many cases the resulting crystals are aligned. Using the above aligned films as semiconducting layers, we fabricated field-effect transistors having anisotropic mobilities with ratios greater than 15. Several common organic semiconductors have been investigated, and the results indicate that this growth method is generally successful for achieving macroscopic alignment of these semiconducting molecules (and frequently their crystals, as well).

Orientation of pentacene films using surface alignment layers and its influence on thin-film transistor characteristics

Abstract: We have investigated the effect of surface order on the orientation and mobility of pentacene. The surface order was created using monolayers and polymers that are normally used to align liquid crystals. Rubbed polyvinylalcohol layers were found to align approximately 27% of the pentacene grains within a 30° range. When introduced in a thin-film transistor, they were found to enhance the saturation current by a factor of 2.5. A mechanism for this enhancement is proposed.

Modification of Indium Tin Oxide for Improved Hole Injection in Organic Light Emitting Diodes

Abstract: Modification of indium tin oxide (ITO) electrode interface for improved hole injection in organic light emitting diodes (OLED) was investigated The injection efficiency measurements were carried out to characterize contact between ITO and the organic semiconductor triphenyldiamine (TDP) layer Coating of ITO with self-assembled ultrathin platinum (Pt) films as interface modification layers, dramatically enhances OLED efficiency and contact with TDP becomes nearly ohmic The surface morphology of ITO electrodes was investigated by atomic force microscopy (AFM)

Field effect transistors and materials and methods for their manufacture

Abstract: A field effect transistor in which a continuous semiconductor layer comprises: a) an organic semiconductor; and, b) an organic binder which has an inherent conductivity of less than 10-6Scm-1 and a permittivity at 1,000 Hz of less than 3.3 and a process for its production comprising: coating a substrate with a liquid layer which comprises the organic semiconductor and a material capable of reacting to form the binder, and, converting the liquid layer to a solid layer comprising the semiconductor and the binder by reacting the material to form the binder.

Low driving voltages and memory effect in organic thin-film transistors with a ferroelectric gate insulator

Abstract: In this letter, two organic thin-film transistors with SiO2 and ferroelectric PbZrTiO3 (PZT) gate insulator are compared. The fabrication of the devices is described and their electrical properties estimated. The PZT-based devices show better performance: Low driving voltage, high Ion/Ioff ratio, etc. Moreover, a memory effect is reported in correlation with ferroelectric properties of PZT thin films.

N-type thiophene semiconductors

Abstract: The new fluorocarbon-functionalized and/or heterocycle-modified polythiophenes, in particular, α,ω-diperfluorohexylsexithiophene DFH-6T can be straightforwardly prepared in high yield and purity. Introduction of such modifications to a thiophene core affords enhanced thermal stability and volatility, and increased electron affinity versus the unmodified compositions of the prior art. Evaporated films behave as n-type semiconductors, and can be used to fabricate thin film transistors with FET mobilities ˜0.01 cm 2 Vs—some of the highest reported to date for n-type organic semiconductors.