F.J.J. Janssen

Degradation effects in poly para-phenylene vinylene derivatives due to controlled oxygen exposure

Abstract The influence of oxygen exposure on three chemically different poly para -phenylene vinylene (PPV) derivatives used in polymeric light emitting devices (PLEDs) has been investigated. During device preparation, PPV layers have been exposed to oxygen either in the dark or during the irradiation with visible light, before the cathode was applied. Device efficiency was studied by electrical and optical characterisation and correlated to oxygen depth profiles measured with Elastic Recoil Detection Analysis (ERDA). Treatment with oxygen during light exposure leads to a decrease in current, light output and efficiency. It was found that two different PPV derivatives show the same current and light output reduction with different oxygen uptake. This behaviour is explained in terms of a different chemical structure and the number of structural defects (tolane-bisbenzyl moieties (TBBs)) incorporated. For two PPV derivatives treated with oxygen in the dark a reduction in current and light output was found, while the efficiency was unchanged. Another PPV derivative, however, shows a shift in the on-set voltage of the light output accompanied by an increased oxygen level at the PPV/Ca interface. It is concluded that after exposure in the dark, oxygen is incorporated in the PPV by Van der Waals interaction. During evaporation of the cathodes, oxygen will diffuse to the cathode and will be gettered by the calcium, which results in the formation of an electron injection barrier.

Influence of a partially oxidized calcium cathode on the performance of polymeric light emitting diodes

We investigated the influence of the presence of oxygen during the deposition of the calcium cathode on the structure and on the performance of polymeric light emitting diodes (pLEDs). The oxygen background pressure during deposition of the calcium cathode of polymeric LEDs was varied. Subsequently, the oxygen depth distribution was measured and correlated with the performance of the pLEDs. The devices have been fabricated in a recently built ultraclean setup. The polymer layers of the pLEDs have been spincoated in a dry nitrogen atmosphere and transported directly into an ultrahigh vacuum chamber where the metal electrodes have been deposited by evaporation. We used indium–tin–oxide as anode, OC1C10 PPV as electroluminescent polymer, calcium as cathode, and aluminum as protecting layer. We achieved reproducibility of about 15% in current and brightness for devices fabricated in an oxygen atmosphere of ≪10−9 mbar. For further investigations the calcium deposition was carried out in an oxygen atmosphere fr...

Interface instabilities in polymer light emitting diodes due to annealing

In polymer light emitting diodes (PLEDS) with an (ITO/PPV/Ca) structure we observed a significant reduction of both the current and the light output at constant voltage after heat treatment for onl ...

Doped polymeric cathodes for PPV/Al based LEDs

The effect of Li-doping in poly(para-phenylenevinylene) (PPV) based light emitting devices has been studied. In a standard structure with an indium tin oxide (ITO) anode, poly(3,4-ethylenedioxythio ...

Strain measurements in ultra-thin buried films (<50 Å) with RBS ion channeling

A method has been developed to measure strain in ultra-thin (<50 A) buried films. The presence of the film leads to a step in the yield of the host crystal in a channeled RBS spectrum for off-normal crystal axes. The size of this step depends on the flux distributions in the channel, which in turn depend on the angle ψ between the incoming beam and the crystal axis. Two maxima in the step size appear as a function of the angle ψ. Monte Carlo (MC) simulations have been used to interpret the experiments.

Planar ion-channeling measurements on buried nano-films

Planar MeV ion-channeling measurements on 2.2 nm thick Si1-xGex nano-films buried in Si are presented. The presence of the nano-film leads to a step in the yield of the host crystal in a {0 1 1} planar channeled RBS spectrum. In previous work we showed that with the help of Monte Carlo (MC) simulations the step height in axial channeling measurements provides useful information on the tetragonal distortion in buried nano-films. Since the step height is larger for planar channeling measurements, we extend this method to planar channeling in the present work. The measurements show that the flux distribution of channeled ions is not in statistical equilibrium, but still oscillates at the depth of the buried films (about 280 nm), This is confirmed by simulations performed with the MC simulation code FLUX and a qualitative understanding is obtained for the observed phenomena. The accuracy of the calculated step height curves appears much more sensitive to the choice of ion-atom potential than those from axial channeling, which can be attributed to the non-equilibrium distribution of channeled ions between the planes. Consequently, from planar channeling measurements, the tetragonal distortion can be determined with about the same accuracy as from axial channeling measurements in spite of the higher steps

Ion beam analysis at cryogenic temperatures for polymer light emitting diodes

Abstract The application of ion beam analysis on polymer light emitting diodes became possible by the development of a set-up which allows measurements at cryogenic temperatures (10–30 K). The set-up is equipped with a sample holder which combines compatibility with a load-lock, necessary for introduction of samples under controlled conditions, and a good thermal contact. A number of examples are reviewed in which interface stability and oxidation effects in poly-LEDs are studied with Rutherford backscattering spectrometry and elastic recoil detection.

Ion-channeling analysis of boron clusters in silicon

We have measured axially channeled Rutherford backscattering spectra of Si1−xGex nanofilms in silicon(001). A step in the yield of the host crystal was found for off-normal axes at the depth of the nanofilm. The step was measured as a function of the angle between the incoming beam and the [011] axis and shows two maxima. It is found that Monte Carlo simulations assuming tetragonal distortion reproduce the experimental results. A universal curve was derived which enables determination of the tetragonal distortion from ion-channeling experiments, for a given film thickness. The results are compared with XRD measurements.