Luc De Schepper

Effect of oxygen on the electrical characteristics of PPV-LEDs

The presence of oxygen in the surrounding atmosphere exerts an important influence on the electro-optical characteristics and the degradation behaviour of organic electroluminescent Al-PPV-ITO-diodes. Experiments in various gas atmospheres show that the devices possess gas sensing properties. For particular gas compositions the current-voltage (IV)-characteristics feature negative resistance sections in the form of anomalous bumps superimposed on Shockley-like diode characteristics. A new interpretation is provided for the memory switch phenomena related to these anomalous electrical characteristics. The gas sensor like behaviour and the increase in conductivity in an oxygen free ambient is believed to be related to the formation of localized Ohmic paths. The growth of these Ohmic paths is inhibited by oxidation and the paths can melt due to excessive joule heating.

Imaging of the ageing on organic electroluminescent diodes, under different atmospheres by impedance spectroscopy, scanning electron microscopy and SIMS depth profiling analysis

Abstract We have fabricated single-layer electroluminescent diodes with poly(p-phenylene vinylene) (PPV), using indium—tin oxide (ITO) as hole-injecting anodes and aluminium as electron-injecting cathodes. We continuously monitored the impedance of the diodes, both in inert atmosphere (Ar, vacuum, N2) and under air. The impedance results reveal a binary role of oxygen. Firstly, the equivalent resistance of the device in air is one order of magnitude larger with respect to that of a device in an inert atmosphere. Secondly, measurements in air under constant d.c.-bias stress reveal a rapid ageing of the device. In an inert atmosphere no change in performance is seen in the same period of time. Optical and scanning electron microscopy (SEM) inspections of devices stressed in air reveal the presence of damage in the electrode region. Device failure is observed in the form of inhomogeneously distributed tight packing of bubbles with blisters randomly dispersed over the electrode surface. For devices run until complete failure, tiny dark spots can be observed, due to local melting. Depth profiling of the structures by secondary ion mass spectrometry (SIMS) confirms the inhomogeneous ageing. Depth profiling shows not only an important change in the electrode structures but also reveals complete oxidation of the polymer layer in the aged parts of the device.

Morphology of MDMO-PPV:PCBM bulk heterojunction organic solar cells studied by AFM, KFM, and TEM

The microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells was studied by Atomic Force Microscopy (AFM) and Kelvin Force Microscopy (KFM) to image the surface morphology and by means of Transmission Electron Microscopy (TEM) to reveal images of the films interior. By introducing KFM, it was possible to demonstrate that phase separated domains have different local electrical properties than the surrounding matrix. Since blend morphology clearly influences global electrical properties and photovoltaic performance, an attempt to control the morphology by means of casting conditions was undertaken. By using AFM, it has been proven that not only the choice of solvent, but also drying conditions dramatically influence the blend structure. Therefore, the possibility of discovering the blend morphology by AFM, KFM and TEM makes them powerful tools for understanding todays organic photovoltaic performances and for screening new sets of materials.

Low-level optical absorption phenomena in organic thin films for solar cell applications investigated by high sensitive photocurrent and photothermal techniques

Optical absorption phenomena and in particular sub band gap absorption features are of great importance in the understanding of processes of charge generation and transport in organic pure and composite semiconductor films. To come towards this objective, an alternative and high sensitive spectroscopic approach is introduced to examine the absorption of light in pure and compound organic semiconductors. Because sub band gap absorption features are typically characterized by very low absorption coefficients, it is not possible to resolve them using common transmission and reflection measurements and high sensitive alternatives are needed. Therefore, a combination of photocurrent (Constant Photocurrent Method CPM/Fourier Transform Photocurrent Spectroscopy FT-PS) and photothermal techniques (Photothermal Deflection Spectroscopy PDS) has been used, increasing sensitivity by a factor of thousand, reaching detectable absorption coefficients ((E) down to 0.1 cm-1. In this way, the dynamic range of measurable absorption coefficients is increased by several orders of magnitude compared to transmission/reflection measurements. These techniques have been used here to characterize ground state absorption of thin films of MDMO-PPV, PCBM and a mixture of both materials in a 1:4 ratio, as typically used in a standard active layer in a fully organic solar cell. The spectra reveal defect related absorption phenomena and significant indication of existing interaction in the ground state between both materials, contrary to the widely spread conviction that this is not the case. Experimental details of the techniques and measurement procedures are explained.

Investigation of the formation of M 2 + -molecular ions in sputtering processes

The formation process of M2+ molecular ions sputtered from elementary target materials is investigated. In a previous article it was shown that these molecules can be used to quantitate major elements [1]. The quantitation method was based on the assumption that the M2+ molecular ions are formed by the atomic combination of independently sputtered M and M+ particles above the surface. In this paper this assumption will be investigated using a Monte Carlo model to simulate the formation mechanism. The model is used to calculate the velocity distribution of the M2+ dimers sputtered from three different elementary target materials (Fe, Ge, and Ni). The results are compared with experimental data. Good agreement exists between theory and experiment that supports the Monte Carlo model and hence also the assumed formation mechanism.

Quantitation of Major Elements With Secondary Ion Mass Spectrometry by Using M2+-Molecular Ions

A new quantitation method, based on the detection of M2+ molecular ions, is presented. It has been shown that M2+ molecular ions are formed by a recombination process between independently sputtered M and M+ particles. Based on this formation mechanism, it will be demonstrated that M2+ molecular ions can be used to quantitate major elements. The method will be used for quantitation of an AlxGa1−xAs multilayer. Furthermore, it will be shown that some matrix effects can be explained by the energy dependence of instrument transmission.

Investigation of the formation process of MCs+-molecular ions during sputtering

In secondary ion mass spectrometry, the detection of MCs+ clusters (with M an element of the specimen) under a Cs bombardment is frequently used for the quantification of major elements. Despite some very good results obtained by this method, some problems still remain. In order to gain some more insight into these problems, the formation mechanism of the MCs+ clusters is investigated using a Monte Carlo model. It is shown that the majority of the constituent particles of the formed clusters are initially first or second neighbor atoms at the surface and that the velocity distribution of the MCs+ clusters becomes broader and peaked at higher velocities with increasing surface binding energy of the M atom. In addition, it is demonstrated that the interaction potential between the M and Cs+ particle has no influence on the velocity distribution of the MCs+ clusters. On the other hand, the cluster formation probability, defined as the probability that a sputtered M and Cs+ particle will form a MCs+ cluster, is extremely sensitive to this interaction potential. It is also shown that the cluster formation probability decreases with increasing surface binding energy. Finally, a good correspondence is obtained between the calculated and experimental velocity distributions of MCs+ clusters sputtered from different monoatomic materials. As a consequence, the Monte Carlo model and the discussed results can be validated.

In-situ electrical and spectroscopical techniques for the study of degradation mechanisms and life time prediction of organic based electronic material systems

In order to tailor the synthesis of new robust organic materials for electronic applications and to guarantee the required life time for the emerging commercial plastic electronic applications it is of key importance to understand the underlying degradation mechanisms. Since plastic electronics is a rather young technology introducing new material systems, its reliability is characterized by new failure and degradation mechanisms, a relatively high amount of early failures and multi-modal failure distributions. To understand the mechanism responsible for a given failure or degradation mode, it is essential to study it separately, through appropriate test structures and test techniques. Powerful techniques for this purpose are a.o. analytical techniques (SEM, TEM, SPM,…), in-situ electrical measurement techniques and spectroscopical techniques ( in-situ FTIR, in-situ UV-Vis, PDS). The benefits of these in-situ techniques in the reliability study of organic based electronics will be illustrated in this contribution.

QUANTITATIVE ANALYSIS OF THE COMPOUND LAYER OF PLASMA NITRIDED PURE IRON

Pure iron is plasma nitrided in a low pressure triode ion plating equipment. During this process a compound layer, containing different Fe-N phases depending on the plasma process parameters used, is formed at the surface.