A. Winnacker

Permeation rate measurements by electrical analysis of calcium corrosion

Highly sensitive permeation measurements are crucial for the characterization and development of polymeric substrates for flexible display applications. In particular, organic light-emitting devices require substrates with extremely low permeation rates for water and oxygen. Here we demonstrate a concept for measuring ultralow permeation rates. The amount of oxidative degradation in a thin Ca sensor is monitored by in situ resistance measurements. The benefits of this technique are demonstrated for polyester foils with single- and double-sided barrier coatings. A sensitivity limit is imposed by the quality of the encapsulation. The resulting base line contribution to the water vapor transmission rate of a glass reference is below 10−6 g/m2 day at accelerated test conditions.

Photon-gated hole burning: a new mechanism using two-step photoionization

We have observed photon-gated spectral hole burning, i.e., hole burning that occurs only in the presence of an additional gating-light source. Gating enhancement factors of 10(4) were observed. In BaClF:Sm(2+) this involves two step photoionization of Sm(2+) and leads to persistent holes in the (4)F(0) --> (5)D(0) (687.9-nm) and (7)F(0) --> (5)D(1) (629.7-nm) absorption lines. The hole widths of 25 MHz at 2 K are much narrower than the inhomogeneous broadening of 16 GHz. The action spectrum of the gating shows a threshold behavior around 2.5 eV. Erasing studies show that Sm(3)+ acts as a trap for the released electrons. A remarkable and novel feature is that the holes can be recovered after temperature cycling to 300 K.

Performance of flexible polymeric light-emitting diodes under bending conditions

Organic light-emitting diodes were fabricated on a 125-μm-thick polyethylene terephthalate substrate covered with 100 nm indium tin oxide. The luminance–current–voltage performance and the emission spectrum of the devices are investigated in the bent state under mechanical stress at different bending radii. Down to a curvature of 15 mm, no significant decrease in the device performance is found compared to the relaxed state, as well as to conventional devices on glass substrates.

Screen-printed passive matrix displays based on light-emitting polymers

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.

Space-charge-limited electron currents in 8-hydroxyquinoline aluminum

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.

Laser-enhanced electron-ion capture and antihydrogen formation

The electron-ion capture rate for low electron energies is calculated for various electron velocity distributions. Capture rates for electron-ion recombination stimulated by irradiation with light are evaluated. The results are applied to electron cooling and to positron-antiproton recombination to form antihydrogen. It is shown that laser-induced capture is a powerful method to study the electron cooling process and to maximize the antihydrogen rate. With this technique a pulsed antihydrogen beam of selectable energy and well collimated with an intensity of a few atoms per second can be anticipated.

Electron injection and transport in 8-hydroxyquinoline aluminum

Abstract 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.

Cathode-induced luminescence quenching in polyfluorenes

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.

On the sublimation growth of SiC bulk crystals: development of a numerical process model

Abstract The development of a numerical process model to simulate the sublimation growth of SiC bulk crystals is discussed. Radiation, conduction and convection are considered as heat transfer mechanisms. Mass transport by diffusion and convection is taken into account. First results on the simulation of heat and mass transfer in a 2 inch SiC growth set-up show a negligible effect of convection on process conditions. Chemical reactions in the SiC-graphite system have also been implemented into our model. Preliminary analysis on the dependence of concentration fields and growth velocity on possible chemical reaction mechanisms, e.g. graphitization, reveal that the incorporation of chemical processes into modelling is very important for an accurate description of SiC sublimation growth.

SiC-bulk growth by physical-vapor transport and its global modelling

4H- and 6H-SiC bulk crystals have been prepared by physical vapor transport (PVT) both in resistively and inductively heated growth reactors. Epitaxial SiC layers were grown on the wafers by chemical vapor deposition. Structural and electrical material properties of the 1–1.4 inch boules and epitaxial layers were investigated by defect etching and optical microscopy, stress birefringence and Hall effect. Single crystalline material exhibits a low micropipe density MPD ≈ 70 cm−2 and stress level. Blocking characteristics of the epitaxial layers have been determined electrically revealing high breakdown fields of 1.8–1.9 MV/cm. Finally simulation results applying a process model of SiC PVT crystallization including heat and mass transfer and chemical reactions are presented.