Due to their outstanding properties organic light-emitting displays based on conjugated polymers are on the verge of commercialization. Two major disadvantages of the current processing technique for polymers, spin-coating of polymer solutions, are the material waste and the difficulties involved in patterning the polymers. Therefore we investigate the screen-printing for the production of polymer displays. Here we present performance data of screen-printed light-emitting diodes of different colors. In the production process of these diodes we printed two layers successively one over the other. Furthermore, we show images of printed multichrome demonstrators and passive matrix displays. Our data indicate that the screen-printing technique has the potential to replace the "classical" spin-coat process. We observe luminance of 10,000 cd/m(2) at 8 V and peak efficiencies exceeding 10 cd/A for green diodes and half lifetime of 170 hours at 80 degreesC and 100 cd/m(2) for red diodes which corresponds to about 7,000 hours at room temperature. These values of printed devices are comparable to those of spin-coated ones.

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 electrodes is not significantly improved by the introduction of a LiF interlayer.

Due to their outstanding properties, e.g., good contrast, wide viewing angle, low power consumption, and self-emission organic light-emitting (OLE) displays on the basis of conjugated polymers are on the verge of commercialization. Two major disadvantages of the current processing technique for the polymers—spin coating—are the material waste and the difficulties involved in patterning multichrome or even full-color displays. Therefore, we investigated the screen-printing technique for the production of OLE displays. In this letter, we present performance data and images of screen-printed OLE diodes. They are already comparable to spin-coated ones. We observed luminance of 10 000 cd/m2 at 8 V and peak efficiencies exceeding 10 cd/A for green diodes. These data indicate that printed organic displays have the potential to replace “classical” spin-coated devices.

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.

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.

An appropriate choice of the cathode material and the process of cathode deposition is a key issue in the development of polymer light emitting devices. In this paper, we report on the impact of low work function metals on the luminescence efficiency of thin films of polyfluorene type polymers. Photoluminescence as well as electroluminescence experiments are presented, and in both cases, a strong correlation between the metal layer thickness and the luminescence efficiency is demonstrated. By means of time-of-flight secondary ion mass spectroscopy (TOF-SIMS), the distribution of the metal contamination within the polymer layers is determined. The results strongly suggest that impurity quenching of excitons by metal atoms inside the polymer layer takes place and strongly affects luminescence and device efficiency.

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.

We report the experimental demonstration of "white-light" cooling of a high-velocity Li-7(+) ion beam stored at 6.4% of the speed of light in a storage ring. In a direct comparison with single-mode laser cooling, we show that white-light cooling is much more efficient to counteract strong intrabeam heating and leads to lower longitudinal beam temperatures at higher ion densities, i.e., much higher densities in longitudinal phase space.

We report on the observation of the indirect transverse laser cooling effect in a radio-frequency bunched beam of 7.3 MeV 9 Be + ions, stored in the Heidelberg Test Storage Ring and subject to direct longitudinal laser cooling. This bunched scheme offers particular advantages for producing ultracold beams with unprecedented phase-space densities.

Thematic mapper and ship data has been used to study small scale features in coastal waters of the North Sea. Three independent pieces of information from all 7 TM channesl were found with factor analysis: suspended matter concentration, atmospheric scattering and sea surface temperature.

Thematic Mapper data were analyzed with respect to its capability for mapping the complex structure and dynamics of suspeded matter distribution in the coastal area of the German Bight (North Sea). Three independent pieces of information were found by factor analysis of all seven TM channels: suspended matter concentration, atmospheric scattering, and sea surface temperature. For the required atmospheric correction the signal to noise ratios of Channels 5 and 7 have to be improved by averaging over 25×25 pixels, which allows us also to monitor aerosol optical depth and aerosol type over cloudfree water surfaces. Near surface suspended matter concentrations may be detected with an accuracy of factor <2 by using an algorithm derived from radiative transfer model calculation. The patchiness of suspended matter and its relation to underwater topography was analyzed with auto- and cross-correlation: Horizontal lengths, where the suspended matter concentration of single pixels, are significantly correlated either with each other or with water depth are in the order of 1 km.