Surface compositions and work functions (Φ) of commercially available indium tin oxide (ITO) substrates were measured by photoelectron spectroscopy (UPS/XPS). Whereas substrates cleaned by organic solvents are significantly contaminated and have low Φ values (3.9–4.2±0.1 eV), substrates cleaned by Ar+ sputtering typically have values of Φ=4.3±0.1 eV. Even higher Φ values (up to 4.7±0.1 eV) are obtained by reactive ion etching with oxygen, likely related to oxygen-containing surface impurities. Evaporated TPD is physisorbed on ITO, but causes a drop of the vacuum potential by 0.2–0.4 eV (depending on the ITO pretreatment) directly at the TPD/ITO interface, in contradiction to the common-vacuum level rule. The TPD highest occupied molecular orbital (HOMO) is found 1.1–1.3 eV below the Fermi level of the ITO, which indicates the presence of a significant barrier for hole injection.

The largest density variations in the solar wind occur near the streamer belt, where frequently they are associated with interplanetary coronal mass ejections (ICMEs). They tend to be embedded in larger‐scale regions of high pressure: high densities in ICME sheaths and corotating interaction regions and low densities in structures with distinctive, high‐magnetic‐pressure profiles, sometimes within ICMEs. On average, however, ICME densities are similar to ambient‐wind densities. For a set of 34 ICMEs identified in Wind data as magnetic clouds, the average density was 11 cm−3 in both the clouds and all slow wind during the same period. A set of low‐density structures observed earlier by ISEE 3 recurred for three solar rotations, possibly owing to recurrence of the streamer belt itself, with its frequent transient outflows. Density averages less than 1 cm−3 show a possible solar cycle variation which peaks 1–2 years prior to the peak of ICME signatures.

We have measured the current-voltage characteristics and device efficiency of organic Light emitting diodes (OLEDs) based on 8-hydroxyquinoline aluminum (Alq(3)) in combination with several cathode layer setups. The electron injection properties of cathode metals evaporated under high vacuum (HV) and ultra-high vacuum (UHV) conditions are compared. Further, cathodes incorporating a thin layer of lithium fluoride, which is covered with a metal capping layer, are investigated. It will be shown that aluminum is an outstanding capping metal and significantly improves both electron injection and device efficiency. Quasi-static and transient current-voltage measurements on single-layer devices will be presented. It will be demonstrated that cathodes, comprising 0.2 nm LiF and aluminum, are able to sustain space charge limited currents in Alq(3). Additionally, the efficiency and lifetime data of multi-layer devices using this cathode layer setup are discussed. (C) 2000 Elsevier Science S.A. All rights reserved.

We investigate electron injection and transport in single-layer devices of 8-hydroxyquinoline aluminum sandwiched between two electrodes. Electrodes comprising a thin lithium fluoride layer are compared with co-evaporated magnesium–silver cathodes and with pure aluminum cathodes. By employing both transient and quasistatic current measurements, the impact of the LiF-layer thickness on electron injection is investigated. It is demonstrated that contacts comprising 0.1–0.2 nm LiF and an aluminum capping layer are able to sustain space-charge-limited currents in 8-hydroxyquinoline aluminum. Further, steady-state current–voltage measurements as a function of temperature are discussed with respect to trap distributions in 8-hydroxyquinoline aluminum.

Bright white light emission in rubrene-doped organic LEDs has been demonstrated using spiro compounds—such as spiro-TAD, shown in the Figure—with high glass transition temperatures. These materials show high morphological stability for hole transport and emission of blue light, and thus still operate at temperatures well above 100 °C. The devices give a maximum luminance of 11800 cd/m2.

The lowest obtainable operating voltage for organic light emitting diodes (OLEDs) utilising a predefined organic layer setup can only be achieved with ohmic contacts both for electron and hole injection. We have investigated dark current transients of unipolar single-layer samples, and we have found ohmic contacts both for hole injection at indium tin oxide (ITO)/4,4′,4″-tris{N-(1-naphtyl)-N-phenylamino}-triphenylamine (1-Naphdata) interfaces and for electron injection at 8-hydroxyquinoline aluminum (Alq3)/LiF/Al interfaces. Therefore, the properties of OLEDs comprising these two interfaces are governed only by bulk material properties and internal organic/organic interfaces. In order to identify the dominating mechanisms concerning the temperature-dependent behaviour of prototypical double layer OLEDs, we have measured (with respect to the applied electric field) the activation energies of the charge carrier mobility and of the steady state current density in 1-Naphdata (holes) and Alq3 (electrons), the activation energies of the steady state current density and of the luminance in OLEDs comprising an 1-Naphdata/Alq3 heterojunction, plus the activation energy of the luminance onset. These experimentally activation energies are discussed with respect to device performance in the typical operating temperature range of flat panel displays including implications for further device optimisation.

We investigate the impact of the deposition of low work function metals such as calcium on thin layers of fluorene-type polymers by time-of-flight secondary ion mass spectroscopy. An implantation process rather than a slow metal diffusion is found to be the most probable source of metal contamination within the polymer layers. This contamination extends to a range of several tens of nanometers in the organic layers. Photoluminescence and electroluminescence measurements are performed with varying calcium layer thicknesses. The luminescence efficiency exhibits a strong correlation with the depth profile of the calcium present within the polymer. The results are discussed with respect to the exciton diffusion length in the fluorene polymer. A numerical model including exciton formation, migration, and quenching is proposed in order to describe the observed phenomena.

Balanced injection of positive and negative charge carriers is a key issue for the operation of highly efficient organic light emitting devices at low operating voltage. In this article, we will give an overview of our investigations on the optimization of charge carrier injection from the anode and the cathode into organic semiconductors. These investigations include proper pretreatment of the indium-tin oxide substrate, and stacking of several organic hole transporting layers to increase the hole injection current from the transparent anode into the emissive layer. On the cathode side, binary metal alloys, and the effect of an insulating layer between the respective metal cathode and the first electron transporting layer are investigated.

Using two complementary methods, we have investigated the individual contribution of the space charge-limited hole transport in vapor-deposited films of 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (m-MTDATA) to the temperature behavior of organic light-emitting diodes. In single-layer indium tin oxide/m-MTDATA/Ag structures, we have measured the activation energies of the current density and of the hole mobility as a function of the applied electric field. Both activation energies obtained under steady-state and pulsed conditions are comparable, which confirms that the temperature behavior of the current density is predominantly governed by the hole mobility.

A one-dimensional numerical model for the quantitative simulation of multilayer organic light emitting diodes (OLEDs) is presented. It encompasses bipolar charge carrier drift with field-dependent mobilities and space charge effects, charge carrier diffusion, trapping, bulk and interface recombination, singlet exciton diffusion and quenching effects. Using field-dependent mobility data measured on unipolar single layer devices, reported energetic levels of highest occupied and lowest unoccupied molecular orbitals, and realistic assumptions for experimentally not direct accessible parameters, current density and luminance of state-of-the-art undoped vapor-deposited two- and three-layer OLEDs with maximum luminance exceeding 10000 cd/m2 were successfully simulated over 4 orders of magnitude. For an adequate description of these multilayer OLEDs with energetic barriers at interfaces between two adjacent organic layers, the model also includes a simple theory of charge carrier barrier crossing and recombination at organic–organic interfaces. The discrete nature of amorphous molecular organic solids is reflected in the model by a spatial discretization according to actual molecule monolayers, with hopping processes for charge carrier and energy transport between neighboring monolayers.

The impact of oxygen plasma treatment of indium tin oxide anodes on performance and durability of vapor-deposited organic electroluminescent devices is shown. Investigations focused on the long-term stability using driving conditions suitable for passive matrix driven displays. Reliability studies of solvent only cleaned samples indicate the presence of a predominating degradation process at the interface between indium tin oxide and the hole injection layer which results in a drastic rise of the operating voltage. This voltage increase could be reduced to 0.31 mV/h by oxygen plasma treatment. As hole injection layer copper phthalocyanine is compared with a star-shaped amine derivative.

The efficiency of organic light-emitting devices is significantly influenced by the performance of the electron-injecting contact. Lowering the energetic barrier between the metal contact and the lowest unoccupied molecular orbital of the adjacent organic electron transport layer should facilitate the injection of negative charge carriers, and, thus, improve the electroluminescence yield by increasing the electron density in the emitting zone. Therefore, it is widely believed that lowering the work function of the cathode metal will improve the quantum efficiency of the devices and, concomitantly, reduce the operating voltage. Here, we report on measurements of devices with tris(8-hydroxyquinolinolato)aluminum-(III) as electron transport and emissive layer. The latter layer is contacted with a variety of chemically very different cathode metals (including some lanthanides), which cover a range from 2.63 eV up to 4.70 eV on the work function axis. We demonstrate the existence of an efficiency maximum at a work function of about 3.7 eV which, to the best of our knowledge, has not been reported yet. These results are of practical importance with respect to the choice of pure cathode metals for organic electroluminescent display applications.

Charge carrier transport in vapor-deposited films of 1,6,7,12-tetraphenoxy-N,N′-bis-(2,6-diisopropylphenyl)-perylene-3,4,9,10-bis(dicarboximide) was investigated using two different methods, the time-of-flight (TOF) technique and time-resolved electroluminescence. Electron mobilities of 10−5 cm2/V s were measured in the bulk using a time-of-flight technique. Hole transport was found to be dispersive and, thus, a transit time for holes could not be obtained. The above dye was also used to fabricate single layer light emitting diodes showing clearly visible red electroluminescence under ambient conditions. Our experiments on transit electroluminescence confirmed the measured electron mobility and ruled out the possibility that the transit time of holes is shorter than the time range investigated in our time-of-flight experiments.

We have made extensive studies of the charge-carrier mobilities in the discotic hexagonal mesophases of triphenylene-based discotic liquid crystals. Using the time-of-flight technique, transient photocurrents were measured yielding charge-carrier mobilities for various electric fields and temperatures. Starting from promising results obtained with the monomeric discotic liquid crystalline model compound hexapentyloxytriphenylene (H5T), we synthesized a ''discotic twin'' with two triphenylene units linked together by a suitable spacer. Additionally, we synthesized a discotic liquid crystalline oligomer consisting of four triphenylene units which are bound to a flexible cyclosiloxane ring. In the dimer and in the oligomer the discotic mesophase can be maintained in a discotic glass. Our data show that in both H5T as well as in the dimer and in the oligomer hole mobilities on the order of magnitude of 10(-3)cm(2)/Vs can be reached. Since the mobilities obtained in the oligomeric compounds are comparable to those found in the monomeric model compound, it becomes clear that the excellent charge-carrier transport properties in the discotic hexagonal mesophase can be preserved in easily processible and anisotropic organic thin films. Therefore, our results are an important step towards the applicability of highly ordered discotic liquid crystalline materials in the field of electronic applications.

Charge‐carrier transport in organic materials is the fundamental physical process behind devices such as laser printers. Charge‐carrier mobility data are presented for hexabutyloxytriphenylene (H4T), which exhibits an unusually high charge‐carrier mobility, which can be traced to the formation of a plastic discotic phase. The Figure shows the normal discotic hexagonal texture with six‐fold symmetry observed for H4T at 144°C; this symmetry disappears at lower temperatures (see also the cover). magnified image

Peripherally alkyl‐substituted aromatic molecules are of interest because they are predicted to promote the rapid vectorial conduction of electronic charge. It is shown—using the example of HHTT, see Figure—that the results obtained by flash‐photolysis time‐of‐flight conductivity and pulse‐radiolysis time‐resolved microwave conductivity measurements are complementary, allowing the charge‐carrier mobility to be determined in all four phases of HHTT. magnified image

Processible organic materials exhibiting high charge carrier mobilities are a step closer with the development of liquid crystalline photoconducting materials. The mobilities have been measured in various phase regions over the temperature range from – 100 to 165°C and the influence of different annealing conditions assessed, indicating that these materials could soon compete with amorphous semiconductors in electronics applications.

SIMS depth profiling experiments have been used to elucidate the layered structure, the impurity distribution, and current induced changes in polymeric light emitting diodes (LEDs). In the first investigated system (ITO/PPV/Al), a poly-p-phenylene-vinylene (PPV) layer has been deposited onto an indium/tin oxide (ITO) glass support, and covered by an aluminium top electrode. A well defined aluminium oxide interlayer has been found in between the polymer and the Al overlayer. Furthermore, an enrichment of chlorine has been detected at both electrode-polymer interfaces, a residue from the polymer preparation process. This observation points to a chemical reaction between the electrodes and elimination products that are liberated during the thermal decomposition of the polymer precursor. In the second system, three different polymeric layers have been spin-coated onto an ITO substrate, i.e. a pure poly-methylphenylsilane (PMPS) layer, a second PMPS layer doped with an organic dye, and finally a polystyrene (PS) layer containing an oxadiazole derivative. By the addition of a bromine containing label into the first layer, it can be shown that the two PMPS layers have been diffusing into each other, whereas the PMPS and the PS regions have remained well separated. As found with the single layer devices, the formation of an interfacial oxide layer between the PS layer and the Al top electrode has been observed. Investigations of driven multilayer LEDs have provided evidence for drastic current-induced degradation effects.

The novel conjugated polymer poly(9-hexyl-3,6-carbazoylyleneethylene) (16) was prepared by palladium-catalyzed polycondensation of 3,6-diiodo-9-hexylcarbazole and 3,6-diethynyl-9-hexylcarbazole. The polymer has a number-average molecular weight (M(n)BAR) of 3 100. By fractionation a polymer with M(n)BAR of 6400 was obtained. Besides polycondensation, a palladium-catalyzed bond-opening polymerization of the triple bonds occurs as a side reaction which can be partially suppressed by lowering the reaction temperature to 60-degrees-C. Two well-defined model compounds of the title polymer, a dimer and a trimer, were synthesized by stepwise reaction. The mass spectra of both model compounds demonstrate the remarkably high stability of the radical cations of the dimer and trimer. Since the trimer forms a stable glass it is ideally suited for the investigation of its photoconductive properties. First experiments carried out by the Time-of-Flight technique showed remarkably high carrier mobilities up to 2 . 10(-4) cm2/(V . s) for the trimer.

The fabrication of high‐efficiency light‐emitting diodes (LEDs) based on sublimed molecular films has attracted much attention in the search for materials for application in large‐area flat‐panel displays. Here, multilayered LEDs based on poly(methylphenylsilane), PMPS, as the hole transporting material are reported (see Figure). In contrast to polyphenylenevinylene, PMPS films exhibit a high effective mobility of holes, making the material suitable for fast switching applications. magnified image