Showing papers on "Organic semiconductor published in 1998"

Strong exciton–photon coupling in an organic semiconductor microcavity

Abstract: The modification and control of exciton–photon interactions in semiconductors is of both fundamental1,2,3,4 and practical interest, being of direct relevance to the design of improved light-emitting diodes, photodetectors and lasers5,6,7. In a semiconductor microcavity, the confined electromagnetic field modifies the optical transitions of the material. Two distinct types of interaction are possible: weak and strong coupling1,2,3,4. In the former perturbative regime, the spectral and spatial distribution of the emission is modified but exciton dynamics are little altered. In the latter case, however, mixing of exciton and photon states occurs leading to strongly modified dynamics. Both types of effect have been observed in planar microcavity structures in inorganic semiconductor quantum wells and bulk layers1,2,3,4,5,6,7,8. But organic semiconductor microcavities have been studied only in the weak-coupling regime9,10,11,12,13,14,15,16,17,18. Here we report an organic semiconductor microcavity that operates in the strong-coupling regime. We see characteristic mixing of the exciton and photon modes (anti-crossing), and a room-temperature vacuum Rabi splitting (an indicator of interaction strength) that is an order of magnitude larger than the previously reported highest values for inorganic semiconductors. Our results may lead to new structures and device concepts incorporating hybrid states of organic and inorganic excitons19, and suggest that polariton lasing20,21,22 may be possible.

Temperature-independent transport in high-mobility pentacene transistors

Abstract: The charge-carrier transport mechanism in the organic semiconductor pentacene is explored using thin-film transistor structures. The variation of the field-effect mobility with temperature differs from sample to sample, ranging from thermally activated to temperature-independent behavior. This result excludes thermally activated hopping as the fundamental transport mechanism in pentacene thin films, and suggests that traps and/or contact effects may strongly influence the observed characteristics. These results also indicate that field-effect transistors may not be appropriate vehicles for illuminating basic transport mechanisms in organic materials.

Molecular level alignment at organic semiconductor-metal interfaces

Abstract: In order to clarify the electronic structure of metal-molecular semiconductor contacts, we use photoemission spectroscopy to investigate the energetics of interfaces formed by vacuum deposition of four different molecular thin films on various metals. We find that the interface electron and hole barriers are not simply defined by the difference between the work functions of the metals and organic solids. The range of interface Fermi level positions is material dependent and dipole barriers are present at all these interfaces. The results demonstrate the breakdown of the vacuum level alignment rule at interfaces between these organic molecular solids and metals.

A metal-free cathode for organic semiconductor devices

Abstract: We introduce a class of low-reflectivity, high-transparency, nonmetallic cathodes useful for a wide range of electrically active, transparent organic devices. The metal-free cathode employs a thin film of copper phthalocyanine (CuPc) capped with a film of low-power, radio-frequency sputtered indium tin oxide (ITO). The CuPc prevents damage to the underlying organic layers during the ITO sputtering process. We present a model suggesting that damage-induced states at the cathode/organic film interface are responsible for the electron injection properties of the contact. Due to the low contact reflectivity, a non-antireflection-coated, metal-free transparent organic light-emitting device (MF-TOLED) is demonstrated with 85% transmission in the visible, emitting nearly identical amounts of light in the forward and backscattered directions. The MF-TOLED performance is found to be comparable to that of conventional TOLEDs employing a more reflective and absorptive cathode consisting of a semitransparent thin fil...

Controlled doping of phthalocyanine layers by cosublimation with acceptor molecules: A systematic Seebeck and conductivity study

Abstract: We investigate the doping of vanadyl–phthalocyanine by a fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic dye films by cosublimation of matrix and dopant. The electrical parameters of the films derived from conductivity and Seebeck measurements show that the results largely follow standard models used to describe the doping of crystalline semiconductors; e.g., a smooth shift of the Fermi level towards the valence states with increasing doping is observed. Other effects, like the superlinear increase of conductivity with the molar doping ratio, need the inclusion of additional effects like percolation.

Humidity-induced crystallization of tris (8-hydroxyquinoline) aluminum layers in organic light-emitting devices

Abstract: We report electroluminescence degradation studies of tris (8-hydroxyquinoline) aluminum (Alq3) organic light-emitting devices (OLEDs) under ambient conditions. Alq3 films and organic bilayer anode/naphthyl-substituted benzidine derivative/Alq3/cathode devices are studied via electroluminescence, photoluminescence, polarization microscopy and atomic force microscopy, and via microscopic infrared spectroscopy. Results reveal that humidity induces the formation of crystalline Alq3 structures in originally amorphous films. The same phenomenon is found to occur in OLEDs and causes cathode delamination at the Alq3/cathode interface that results in the formation of black (nonemissive) spots in the devices.

Photovoltaic measurement of the built-in potential in organic light emitting diodes and photodiodes

Abstract: We measure the voltage at which the current under illumination in poly[2-methoxy, 5-(2-ethylhexoxy)-1,4-phenylene vinylene] based light emitting diodes is equal to the dark current. At low temperatures, this voltage, which we term the “compensation” voltage, is found to be equal to the built-in potential, as measured with electroabsorption on the same diode. Diffusion of thermally injected charges at room temperature, however, shifts the compensation voltage to lower values. A model explaining this behavior is developed and its implications for the operation of organic light emitting diodes and photovoltaic cells are briefly discussed.

Dihexylquaterthiophene, A Two-Dimensional Liquid Crystal-like Organic Semiconductor with High Transport Properties†

Abstract: End-substitution of quaterthiophene with hexyl groups leads to a highly soluble conjugated oligomer, α,ω-dihexylquaterthiophene (DH4T), which has been characterized for its thermal, structural, and...

Exciplex formation at the organic solid-state interface: Yellow emission in organic light-emitting diodes using green-fluorescent tris(8-quinolinolato)aluminum and hole-transporting molecular materials with low ionization potentials

Abstract: The bilayer organic light-emitting diodes using green-fluorescent tris(8-quinolinolato)aluminum (Alq3) as an emitting material and hole-transport materials with low ionization potentials, 1,3,5-tris(3-methylphenylphenylamino)triphenylamine and 4,4′,4″-tris[bis(4-tert-buthylbiphenyl4-yl)amino]triphenylamine, emitted bright yellow light instead of green light. The yellow emission is attributed to exciplex formation at the solid interface between Alq3 and the hole-transport material. The exciplex formation was evidenced by the measurement of the photoluminescence spectra and lifetimes of the mixture of an equimolar amount of Alq3 and each of the hole-transport materials. The emission color can be tuned by varying the applied voltage.

Energy level offset at organic semiconductor heterojunctions

Abstract: We present an investigation via ultraviolet photoemission spectroscopy of the electronic structure of three organic-organic heterojunctions formed between the standard electron-transport emissive material tris(8-hydroxy-quinoline)aluminum (Alq3) and two hole-transport materials, i.e., 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA), and N,N′-diphenyl-N,N′-bis(l-naphthyl)-1-1′biphenyl-4,4″diamine (α-NPD). We measure directly the energy offsets between highest occupied molecular orbitals during the formation of the interfaces. We show that the relative positions of the highest occupied and lowest unoccupied molecular orbitals across the Alq3/PTCDA and Alq3/α-NPD interfaces are qualitatively different and explain, in part, the difference in the performance of electroluminescent devices based on these heterojunctions. We demonstrate the existence of charge transfer-induced dipoles which shift the molecular levels of one organic with respect to the other and invalidate the usual assumption of vacuum level...

Organic semiconductor lasers

Abstract: Lasers comprising a substrate and a layer of organic material over the substrate. The organic material includes host and dopant materials that result in the laser emission of a desired color when pumped by optical pump energy. Host materials include CBP and tris-(8-hydroxyquinoline) aluminum, which when combined with dopant materials such as coumarin-47, coumarin-30, perylene, rhodamine-6G, DCM, DCM2, and pyrromethane-546 result in the efficient lasing of colors such as blue, green and yellow.

Energy level alignment at interfaces of organic semiconductor heterostructures

Abstract: We have used ultraviolet photoelectron spectroscopy (UPS) to investigate the interfaces of two organic semiconductor heterostructures. UPS was used to determine the relative energies of the highest occupied molecular orbitals of the organic semiconductors, and to measure the interface dipoles at each interface. The two systems studied were the 4-4′-N,N′-dicarbazolyl-biphenyl(CBP)/tris(8-hydroxy-quinoline)aluminum (Alq3) interface, and the copper phthalocyanine (CuPc)/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′biphenyl-4,4″diamine (α-NPD) interface. The assumption of a common vacuum level was found to be valid, within experimental uncertainty, for both interfaces.

Thin-film transistors based on organic conjugated semiconductors

Abstract: The use of organic semiconductors as active layers in thin-film transistors has raised in the recent years a large interest, both for the fundamental understanding of the charge transport processes in organic materials, and also for the potential applications of these devices in the new field of flexible electronics. Short conjugated oligomers have been shown to possess much higher field-effect mobilities than their parent conjugated polymers. The origin of such increase in the efficiency of charge transport is mainly attributed to the close-packing and long-range structural organization displayed in thin films of conjugated oligomers. The various routes for controlling this organization are described, which allow to realize liquid crystal-like two-dimensional structures for these semiconductors, whose carrier mobility has now become equivalent to that of amorphous silicon. It is also shown that the effect of conjugation length on carrier mobility is not as critical as previously thought, but the associated increase of the band gap energy effects the efficiency of charge injection at the metal/semiconductor interface. This problem can be answered by realizing a local doping of the semiconductor, which allows the injection of charge to operate through an efficient tunneling mechanism. Organic-based thin-film transistors have now become viable devices.

α,ω-Dihexylquaterthiophene: A Second Thin Film Single-Crystal Organic Semiconductor

Abstract: The thin film structure and hole mobility of 5,5‘‘‘-dihexylquaterthiophene are reported. Films sublimed onto Si/SiO2 or carbon grids held at 30−100 °C are single-crystal-like over tens of micrometers as shown by electron diffraction. The orientation has the long axis of the quaterthiophene core nearly perpendicular to the substrate. The mobility ranges from 0.05 to 0.23 cm2/Vs, depending on the deposition temperature, much higher than would be expected based on earlier thiophene oligomer data.

Energy-level alignment at interfaces between metals and the organic semiconductor 4,4′-N,N′-dicarbazolyl-biphenyl

Abstract: We have used ultraviolet photoemission spectroscopy to study the formation of interfaces between the organic semiconductor, 4,4′-N,N′-dicarbazolyl-biphenyl (CBP), and the metals Au, Ag, and Mg. Each interface was studied by depositing the organic on the metal, and by depositing the metal on the organic. The two methods produced inequivalent interfaces, except in the case of Au/CBP. The position of the highest occupied molecular orbital relative to the Fermi level and the magnitude of the interface dipole were measured for each interface. The barrier to electron injection from each metal was estimated using the magnitude of the measured optical gap. An interface dipole, of magnitude nearly independent of the metal work function, was formed when CBP was deposited on a metal surface. The position of the Fermi level within the CBP gap was found to vary strongly with the metal work function.

Polymer/metal interfaces and the performance of polymer light-emitting diodes

Abstract: Conjugated polymers are often treated as semiconductors with low doping concentrations. Unlike the traditional semiconductors which have a high density of surface states (mainly due to the dangling bonds), the nature of the metal/polymer interface, including barrier height and charge injection efficiency, is quite sensitive to the work function of the contact metal. In this article, we present evidence to show that the pinning of the surface Fermi level effect commonly observed at the silicon/metal interface can also be observed at the metal/polymer interface. It is achieved by controlling the doping level at the metal/polymer [poly(2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) or MEH-PPV] interface. ITO/MEH-PPV/Al devices doped with 2 A of calcium on the cathode side of the interfacial layer have the same device performance as the ITO/MEH-PPV/Ca devices. The heavily n-doped region pins the surface energy level, hence the polymer interface at the cathode side is no longer sensitive to the work function of the overcoated metal. It is believed that either the midgap bipolaron energy states created by the dopants or the sharp band bending at the interface is responsible for facilitating the electron injection. On the other hand, a p-doped region at the anode side, obtained by using a thin layer of an acid at the interface, pins the surface energy level and makes the contact insensitive to the work function of the anode. Therefore, an efficient polymer light-emitting diode with the p-i-n structure has been demonstrated without the matching of the work function of the metal electrodes.

Naphthalimide side-chain polymers for organic light-emitting diodes : band-offset engineering and role of polymer thickness

Abstract: We report the fabrication of efficient green light-emitting diodes using a side-chain random polymer based on a high electron affinity (EA) naphthalimide moiety (PNI) The chromophore is attached to a polymethacrylate backbone through a spacer, and emits in the green with high efficiency (30% photoluminescence quantum yield) In single-layer light-emitting diodes (LEDs), we find that the electroluminescence quantum efficiency is not limited by Al cathodes as for poly(p-phenylene vinylene), PPV, and we attribute this to the increased EA We also report maximum internal quantum efficiencies of about 17% for Ca and 09% for Al in double-layer devices where PPV serves as both hole injector and emitter Compared to some oxadiazole based electron injection/transport layers, PNI gives higher efficiencies at high currents, and longer lifetimes Tuning of emission in the red is possible by dye doping (at high concentration) the PNI and causing the emission to happen in this layer We discuss the properties of the

Optically pumped blue organic semiconductor lasers

Abstract: Lasing at 460, 485, and 510 nm is demonstrated in optically pumped, vacuum-deposited amorphous thin films of a carbazole derivative doped with Coumarin 47, perylene, and Coumarin 30, respectively. Efficient, nonradiative Forster energy transfer between host and dopant organic molecules results in low lasing thresholds (5 μJ/cm2), high differential quantum efficiencies (15%), high peak output powers (20 W), and long operational lifetimes (>105 pulses at 100 times the threshold power).

Fabrication method of an organic electroluminescent devices

Abstract: A method for manufacturing an organic EL display including an organic EL elements and an organic field effect transistors being integrated on a same substrate is disclosed. A semitransparent electrode layer of the organic EL element and a gate electrode of the organic transistor are formed on a same transparent plastic substrate. An organic gate insulating layer is deposited on the gate electrode and an organic semiconductor layer is formed on the organic gate insulating layer. A source and drain electrodes is formed on the organic semiconductor layer. An organic EL layer is formed on the semitransparent electrode layer and a part of the drain electrode. The organic semiconductor layer can be made of a charge transfer complex or a thiophene derivative polymer. The resultant EL device is capable of mechanically bent, and then is readily adaptable for use in flexible displays.

Device model investigation of single layer organic light emitting diodes

Abstract: We present calculations of single layer organic light emitting diode (LED) characteristics using a device model which includes charge injection, transport, recombination, and space charge effects in the organic material. Contact limited and ohmic contacts, high and low carrier mobilities, and device thicknesses from 5 to 200 nm are considered. The scaling of device current with applied voltage bias and organic film thickness is described for contact limited and ohmic contacts. Calculated device current, light output, and quantum and power efficiency are presented for representative cases of material and device parameters. These results are interpreted using the calculated spatial variation of the electric field, charge density, and recombination rate density in the devices. We find that efficient single layer organic LEDs are possible for a wide range of organic material and contact parameters.

Process for fabricating organic semiconductor devices using ink-jet printing technology and device and system employing same

Abstract: An emission system for presenting visual image is disclosed. The emissive system typically contains first electrodes (90) deposited over and in contact with a substrate. One or more conjugated organic buffer layers (40) are then deposited over and in contact with the first electrodes, and second electrodes (22) are subsequently deposited over the conjugated organic buffer layers. The conjugated organic buffer layers (40) regulate current flow between the first electrodes (90) and the second electrodes (22). Either before or after the deposition of each conjugated organic buffer layer (40), but before the deposition of the second electrodes (22), conjugated organic deposits (34, 36, 38) are ink-jet printed such that they are in contact with at least one conjugated organic buffer layer. The conjugated organic deposits (34, 36, 38) help to generate an indicator when a voltage stimulus is applied across the first electrodes (90) and the second electrodes (22). Depending on the material of the conjugated organic deposits (34, 36, 38), the indicator may be luminescence, fluorescence, conductivity, or the like. A voltage source is used for selectively applying the voltage stimulus across the first electrodes (90) and the second electrodes (22).

Investigation of Charge Transport in Thin, Doped Sexithiophene Crystals by Conducting Probe Atomic Force Microscopy

Abstract: Conducting probe atomic force microscopy (CPAFM) was used to measure the electrical transport characteristics of 2−14 nm thick doped crystallites of the organic semiconductor sexithiophene (6T) grown on Au and SiO2 substrates by vacuum sublimation. To make the measurements, an AFM was modified to allow in situ switching from tapping mode imaging to point contact electrical characterization with an Au-coated tip. The crystals were characterized structurally by molecular contrast AFM imaging and consist of layers of 6T molecules oriented with their long axes nearly perpendicular to the substrate. For crystals grown on Au substrates, transport is probed through the thickness of the crystals (i.e., the vertical direction) using a CPAFM tip and the substrate as electrical contacts. On SiO2 substrates, transport is measured parallel to the substrate between the CPAFM tip and a nanofabricated Au electrode in contact with the crystallite. The measurements on Au reveal an unexpected dependence of the conductance o...

Current–voltage characteristic of organic light emitting diodes

Abstract: It has been claimed that the variation of current I with voltage V in an organic light emitting diode (LED), based on either metal chelate complexes or conducting polymers, is explained by shallow traps that trap carriers propagating in the conduction or valence band. However, because these are disordered materials all states are localized. We show that it is possible to fit the dependence of I on V and on film thickness without explicitly introducing traps, but taking their effect into account by including the mobility variation with the electric field that arises from the distribution in energy of the localized levels.

Effect of well number on organic multiple-quantum-well electroluminescent device characteristics

Abstract: A doping technique for fabricating organic multiple-quantum-well electroluminescent (EL) devices is demonstrated. This device consists of N,N′-Bis(3-methyphenyl)-N,N′-diphenylbenzidine used as a hole transporter, undoped tris(8-quinolinolato) aluminum (Alq) as a barrier potential or electron transporter, and Alq doped with 5,6,11,12-tetraphenylnaphthacene as a potential well and an emitter. Our experimental results suggest that the double-quantum-well EL devices show the optimum emission characteristics. The efficiency and the luminance of the device achieve 15.7 lm/W and 7500 cd/m2, respectively.

Display device with thin film transistor (TFT) and organic semiconductor film in a luminescent element

Abstract: The invention provides a display device in which parasitic capacitance associated with data lines and driving circuits is prevented using a bank layer whose primary purpose is to define areas on a substrate in which an organic semiconductor film is formed. When the organic semiconductor film for forming a luminescent element such as an electroluminescent element or an LED is formed is formed in pixel regions ( 7 ), the organic semiconductor film is formed in the areas surrounded by the bank layer (bank) formed of a black resist. The bank layer (bank) is also formed between an opposite electrode (op) and data lines (sig) for supplying an image signal to first TFTs ( 20 ) and holding capacitors (cap) in the pixel regions ( 7 ) thereby preventing parasitic capacitance associated with the data lines (sig).

Doping effects in organic electroluminescent devices

Abstract: Several possible doping effects in organic electroluminescent devices are studied within the framework of the trap-charge limited conduction models. A significant increase in the trapped charges in the doped layer can cause an overall trap charge redistribution and a shift of the location of the recombination zone. Some experimental data are analyzed and their possible relevance to our predictions is discussed. An experimental verification scheme is also proposed.

Organic heterostructures for electronic and photonic devices

Abstract: The successful fabrication of organic semiconductor devices for both electronic and photonic applications is discussed. Complex layer sequences of various organic semiconductor thin films, different metallizations, and indium tin oxide layers can be grown by means of the organic molecular beam deposition (OMBD) technique. Organic-on-inorganic heterostructure diodes based on crystalline thin PTCDA (3,4,9,10,-perylenetetracarboxylic dianhydride) films on III–V-semiconductors are investigated with regard to microwave applications. The optimization of the device structure for reduced forward voltages and high cutoff frequencies in the GHz regime is discussed, and a single balanced mixer with improved frequency conversion at low power levels is shown. Secondly, organic light emitting diodes (OLED) with bright emission in the blue, green, and red spectral region and with low operation voltages are presented. Embedding emissive organic thin films into planar Fabry-Perot microcavities light intensity enhancement, spectral narrowing, and spatial redistribution of the emission is achieved. Finally a 5×7 pixel organic matrix display is introduced.