Elektronische Vielteilcheneffekte in dünnen organischen Filmen und an organischen Grenzflächen

The results of this thesis contribute to the understanding of the electronic properties of organic thin-films and interfaces. It is demonstrated that photoemission spectroscopy is very useful for studying surfaces and interfaces. Additionally it is shown, that many-body effects can be relevant for organic thin films, in particular at interfaces with strong interaction. These effects can have general implications for the material properties. In the first part of this thesis a systematic series of polyacene molecules is investigated with NEXAFS spectroscopy. The comparison of the data with core level and IPES data indicates that core excitations and core excitons need to be understood as many-body excitations. This finding implies for example that a high exciton binding energy is not necessarily associated with strong localization of the excited electron at the hole. As these effects apply also for valence excitons they can be relevant for the separation of charges and for the electron-hole recombination at interfaces. In the next chapter some fundamental effects in organic multilayer films and at organic-metal interfaces are studied with core level and NEXAFS spectroscopy. In this context a series of selected molecules is investigated, namely BTCDA, BTCDI, PTCDA and PTCDI. It is shown that in case of strong interface interaction a density of adsorbate-substrate states is formed which can lead to significant charge transfer satellites in the PES and NEXAFS spectra, similar to what is known for transition metal compounds. Moreover, it is demonstrated that the data can be modeled qualitatively by a basic approach which fuses the single impurity Anderson model with the description of charge transfer satellites by Sawatzky et al. This approach, which is equivalent to that of Gunnarsson and Schönhammer, allows even a relatively simple semi-quantitative analysis of the experimental data. The comparison of different adsorbate layers indicates that these many-body effects are particularly strong in case of partial occupation of the LUMO derived DOS. In the third part an organic multilayer film (SnPc), an organic-metal interface with strong coupling (SnPc/Ag) and an organic-organic interface (SnPc/PTCDA/Ag) are studied exemplarily with resonant Auger spectroscopy. The comparison of the data gives evidence for the contribution of many-body effects to the autoionization spectra. Furthermore, it is found that the electron-vibration coupling and the substrate-adsorbate charge transfer occurs on the time scale of the core hole life time. Moreover, the interaction at the organic-organic interface is weak, comparable to the intermolecular interaction in the multilayer films, despite a considerable rigid level shift for the SnPc layer. Furthermore, weak but significant electron-electron correlation is found for the molecular frontier orbitals, which are important for the substrate-adsorbate charge transfer. Therefore, these strongly coupled adsorbate films are briefly discussed within the context of the Hubbard model in the last part of this thesis. From the data derived in this work it can be estimated that such monolayer films are in the regime of medium correlations. Consequently one can expect for these adsorbate films properties which are related to the extraordinary behavior of strongly correlated materials, for which Mott metal-insulator transitions, sophisticated magnetic properties and superconductivity can be observed. Additionally some results from the investigation of alkyl/Si self-assembled monolayers are briefly discussed in the appendix. It is demonstrated exemplarily for the alkyl chains that the electronic band structure of short, finitely repeating units can be well modeled by a comparatively simple quantum well approach. In principle this approach can also be applied to higher dimensional systems, which makes it very useful for the description of E(k) relations in the regime of repeating units of intermediate length. Furthermore, the photoelectron and NEXAFS spectra indicate strong interaction at the alkyl/Si interface. It was found that the interface states can be modified by moderate x-ray irradiation, which changes the properties for charge transport through the SAM.Die Ergebnisse dieser Arbeit tragen zum generellen Verständnis der elektronischen Struktur von dünnen organischen Filmen und Grenzflächen bei. Es wird gezeigt, dass verschiedene Spektroskopieformen der Photoemission sehr hilfreich sind, um Oberflächen und Grenzflächen zu untersuchen. Die Daten in dieser Arbeit weisen darauf hin, dass Vielteilchen Effekte in organischen Dünnschichten eine wichtige Rolle spielen, besonders an Grenzflächen mit starker Wechselwirkung. Diese Effekte können für unterschiedliche Materialeigenschaften von Bedeutung sein. Im ersten Teil dieser Dissertation wird eine systematische Serie von Polyacen Molekülen mit NEXAFS Spektroskopie untersucht. Der Vergleich mit Rumpfniveau und IPES Daten zeigt, dass Rumpfanregungen und Rumpfexzitonen als Vielteilchenanregungen verstanden werden müssen. Dieser Befund impliziert zum Beispiel, dass eine große Exzitonenbindungsenergie nicht automatisch bedeutet, dass das angeregte Elektron nahe am Rumpfloch lokalisiert sein muss. Da diese Effekte auch für Valenzexzitonen auftreten, spielen sie auch bei der Separation von Ladungsträgern oder Rekombination von Elektronen und Löchern eine Rolle. Im nächsten Kapitel werden fundamentale Effekte in organischen Multilagenfilmen und Metall-Organik Grenzflächen mit Rumpfniveau- und NEXAFS Spektroskopie untersucht. Dies wird anhand der systematisch ausgewählten Molekülserie BTCDA, BTCDI, PTCDA, PTCDI durchgeführt. Es wird gezeigt, dass sich im Falle von starker Wechselwirkung an den Grenzflächen eine Substrat-Adsorbat-Zustandsdichte bildet, die zu starken Ladungstransfersatelliten führen kann, ähnlich wie sie für Übergangsmetallkomplexe bekannt sind. Die experimentellen Daten können mit einem Model verstanden werden, das das Single Impurity Anderson Modell mit dem Ansatz von Sawatzky et al. zur Beschreibung von Ladungstransfersatelliten in Übergangsmetallkomplexen vereint. Diese Herangehensweise ist equivalent zum Ansatz von Gunnarsson und Schönhammer für Adsorbate. Sie erlaubt jedoch eine relativ einfache semiquantitative Auswertung der experimentellen Daten. Ein Vergleich der Spektren für verschiedene Adsorbatschichten weist darauf hin, dass Vielteilcheneffekte besonders dann stark sind, wenn die vom LUMO abgeleitete Zustandsdichte teilweise gefüllt ist. Im dritten Teil dieser Arbeit wird exemplarisch jeweils ein organischer Multilagenfilm (SnPc), eine Organik-Metall Grenzfläche mit starker Wechselwirkung (SnPc/Ag) sowie eine Organik-Organik Grenzfläche (SnPc/PTCDA/Ag) mit resonanter Auger Spektroskopie untersucht. Durch den Vergleich der Daten wird der Beitrag der Vielteilcheneffekte zu den Autoionisationsspektren klar. Demnach laufen die Elektron-Vibrations-Kopplung und der Adsorbat-Substrat Ladungstransfer auf der Zeitskala der Rumpflochlebensdauer ab. Außerdem ist die Wechselwirkung an der Organik-Organik Grenzfläche zwischen SnPc und PTCDA sehr schwach, vergleichbar mit der intermolekularen Wechselwirkung in Multilagenschichten trotz einer parallelen Verschiebung aller elektronischen Niveaus in der SnPc Schicht. Desweiteren wird eine relativ schwache aber dennoch signifikante Elektron-Elektron Korrelation in den oberen Valenzorbitalen gefunden, die eine wichtige Rolle für den Ladungstransfer zwischen Adsorbat und Substrat spielt. Daher werden im letzten Teil dieser Dissertation die stark gekoppelten Adsorbat Filme kurz im Kontext des Hubbard Modells diskutiert. Mit den Daten aus dieser Arbeit können solche Monolagenfilme in den Bereich für mittlere Korrelationsstärke eingeordnet werden. Folglich kann man für solche Adsorbatfilme Eigenschaften erwarten, die dem außergewöhnlichen Verhalten stark korrelierter Systeme ähneln, für die z. B. Mott Metall-Isolator Übergänge, interessante magnetische Eigenschaften und Supraleitung beobachtet wurden. Zusätzlich werden im Anhang kurz einige Ergebnisse aus den Untersuchungen an einem Schichtsystem diskutiert, das aus einer Monolage Alkylketten auf dem anorganischen Halbleiter Silizium besteht und auch als self-assembled monolayer (SAM) bekannt ist. An den Alkylketten wird exemplarisch gezeigt, dass die elektronische Bandstruktur von kurzen, sich endlich wiederholenden Einheiten sehr gut durch einen relativ einfachen Quantentrog Ansatz wiedergegeben werden kann. Im Prinzip kann dieser Ansatz auch auf mehrdimensionale Systeme angewendet werden. Daher ist er für die Beschreibung von E(k) Relationen in intermediären Systemen mit endlichen Wiederholeinheiten sehr nützlich. Desweiteren wird in den Photoelektronen- und NEXAFS Spektren eine starke Wechselwirkung an der alkyl/Si Grenzfläche beobachtet. Es wird gezeigt, dass die Grenzflächenzustände durch moderate Röntgenstrahlung modifiziert werden können, was wiederum die Eigenschaften für Ladungstransport durch die Alkylschicht beeinflusst

Manufacturing Dense Thick Films of Lunar Regolith Simulant EAC-1 at Room Temperature

The Aerosol Deposition (AD, also known as gas kinetic spraying or vacuum deposition) method is a rather novel coating process to produce dense thick films directly from dry ceramic (or metal) powders on a variety of substrates without any heat treatment. Because of the similarity of the up to now used powders and lunar regolith, it is imaginable to use AD systems for future in situ resource utilization missions on the Moon planned by several space agencies. To test the feasibility of such an endeavor, the processability of lunar mare simulant EAC-1 by the AD method has been examined in this study. Three regolith films with an area of 25 × 10 mm2, and thicknesses between 2.50 µm and 5.36 µm have been deposited on steel substrates using a standard AD setup. Deposited films have been investigated by Laser Scanning Microscopy (LSM) and Scanning Electron Microscopy (SEM). Moreover, the roughness and Vickers hardness of the deposited films and the underlying substrates have been measured. It has been shown that dense consolidated films of regolith simulant can be produced within minutes by AD. The deposited films show a higher roughness and, on average, a higher hardness than the steel substrates. Since on the Moon, naturally available regolith powders are abundant and very dry, and since the required process vacuum is available, AD appears to be a very promising method for producing dense coatings in future Moon exploration and utilization missions

Characterization of Sulfur Bonding in CdS:O Buffer Layers for CdTe-based thin-film Solar Cells

On the basis of a combination of X-ray photoelectron spectroscopy and synchrotron-based X-ray emission spectroscopy, we present a detailed characterization of the chemical structure of CdS:O thin films that can be employed as a substitute for CdS layers in thin-film solar cells. It is possible to analyze the local chemical environment of the probed elements, in particular sulfur, hence allowing insights into the species-specific composition of the films and their surfaces. A detailed quantification of the observed sulfur environments (i.e., sulfide, sulfate, and an intermediate oxide) as a function of oxygen content is presented, allowing a deliberate optimization of CdS:O thin films for their use as alternative buffer layers in thin-film photovoltaic devices

Interrelation between Chemical, Electronic, and Charge Transport Properties of Solution-Processed Indium–Zinc Oxide Semiconductor Thin Films

Solution-processed metal oxide semiconductors are of high interest for the preparation of high-mobility transparent metal oxide (TMO) semiconductor thin films and thin film transistors (TFTs). It has been shown that the charge transport properties of indium-zinc oxide (IZO) thin films from molecular precursor solutions depend strongly on the preparation conditions, in particular on the precursor conversion temperature T-pc and, to some surprise, also on the concentration of the precursor solution. Therefore, the chemical and the electronic structure of solution-processed IZO thin films have been studied in detail with Xray photoelectron spectroscopy (XPS) under systematic variation of Tpc and the concentration of the precursor solution. A distinct spectral feature is observed in the valence band spectra close to the Fermi level at E-B = 0.45 eV binding energy which correlates with the trends in the sheet resistivity, the field effect mobility mu(FE), and the optical gap E-g(opt) from four-point-probe (4PP), TFT, and UV-vis measurements, respectively. A comprehensive model of the interrelation between the conditions during solution-processing, the chemical and electronic structure, and the charge transport properties is developed

A General Route toward Complete Room Temperature Processing of Printed and High Performance Oxide Electronics

Critical prerequisites for solution-processed/printed field-effect transistors (FETs) and logics are excellent electrical performance including high charge carrier mobility, reliability, high environmental stability and low/preferably room temperature processing. Oxide semiconductors can often fulfill all the above criteria, sometimes even with better promise than their organic counterparts, except for their high process temperature requirement. The need for high annealing/curing temperatures renders oxide FETs rather incompatible to inexpensive, flexible substrates, which are commonly used for high-throughput and roll-to-roll additive manufacturing techniques, such as printing. To overcome this serious limitation, here we demonstrate an alternative approach that enables completely room-temperature processing of printed oxide FETs with device mobility as large as 12.5 cm(2)/(V s). The key aspect of the present concept is a chemically controlled curing process of the printed nanoparticle ink that provides surprisingly dense thin films and excellent interparticle electrical contacts. In order to demonstrate the versatility of this approach, both n-type (In2O3) and p-type (Cu2O) oxide semiconductor nanoparticle dispersions are prepared to fabricate, inkjet printed and completely room temperature processed, all-oxide complementary metal oxide semiconductor (CMOS) invertors that can display significant signal gain (similar to 18) at a supply voltage of only 1.5 V