Organic Molecules on Transparent Conductive Oxides: Fundamental studies

Transparent conductive oxides (TCOs) are common contact materials in optoelectronic devices such as organic light emitting diodes, photovoltaic cells and liquid crystal displays. The interface between TCO and an organic film largely defines charge transport and light emission, and thus it is important to determine the interfacial structure and properties, and understand how it forms. Studies of adsorption on metal surfaces have revealed much diversity in molecular orientation and alignment in the first monolayer resulting from a competition between intermolecular and molecule-substrate interactions. Studies of relevant molecules on TCO surfaces are scarce, partly because obtaining a clean and sufficiently well- ordered TCO surface for surface science investigations is challenging, and partly because the communities studying organic and metal oxide surfaces are largely distinct, with little overlap. In this project, I aim to combine my experiences with organic overlayers and metal oxides surfaces and begin to bridge this divide. The project will involve some of the first studies of the adsorption of prototypical, industrially relevant conjugated organic molecules on model TCO surfaces. The key to this work is the use of rare In2O3 single crystals, which I have learned to prepare reliably and reproducibly during my postdoctoral studies. The well-defined substrate is crucial if we are to understand the atomic-scale structure of the TCO/organic interface, and is a vital prerequisite for theoretical calculations aimed at elucidating the atomic scale mechanisms in play. It will be fascinating to discover the extent to which the highly corrugated surface potential of the metal oxide influences the geometric and electronic properties. State-of-the-art surface science techniques such as low temperature scanning tunneling microscopy, low energy electron diffraction and photoemission electron microscopy will be used to understand the morphological properties, and photoelectron spectroscopies will be used to elucidate the molecular frontier orbitals and the band alignment, distinguish inter- and intra-molecular interactions, and determine the character of the interaction with the substrate. Organic films ranging from sub monolayer to several nanometers thickness will be compared to investigate the difference between the interface and the organic bulk. Granting this proposal would provide the springboard I need to launch my own independent research program that combines the skills acquired during my career to date. The research will be conducted in groups specializing in metal-oxide and organic surface science, in collaboration with my mentor Prof. Ulrike Diebold, and Prof. Georg Koller and Prof. Hans- Peter Steinrück, respectively. Theoretical calculations will be provided through the collaboration with Prof. Bernd Meyer. With the advice and support of four leading players in their respective fields, I am confident I can make a real and lasting contribution in an important technological area.

The project Organic Molecules on Transparent Conductive Oxides: Fundamental Studies pioneered the adsorption of prototypic organic molecules on the In2O3(111) surface. In the investigated systems, the coverages of the organic materials ranged from individual molecules on the surface to the growth of the first molecular layer and even beyond, to the onset of the second layer. Geometric and electronic information was gained for the individual molecules and coverages. Overall, this project led to a deeper understanding of the interaction of planar organic molecules with the In2O3(111) surface. The experimental research employed state-of-the-art surface-science techniques in an ultra-high vacuum environment, with a strong focus on imaging techniques (scanning tunneling microscopy and atomic force microscopy). Three device-relevant molecules were explored: (1) para-Sexiphenyl, a rod-like molecule consisting of six phenyl rings; this molecule was used as one of the first organic semiconductors in blue organic light-emitting diodes; (2) Cu-Phthalocyanine, a cross shaped and planar molecule with a Cu atom in its center; it is, e.g., utilized in organic solar cells for its efficient adsorption of light; (3) PTCDA, a planar, rectangular shaped molecule that is utilized, e.g., as a photocatalyst for water splitting. Most experiments were conducted on the clean In2O3(111) surface as well as on the less reactive hydroxylated (i.e., covered by OH groups) In2O3(111) surface, i.e., the surface after exposure to water molecules. Therefore, as a subtopic, water dissociation on In2O3(111) was fully characterized. The surface saturates with a well-defined hydroxyl layer that contains three water molecules per unit cell. By exploring the individual OH groups we also gained a deeper understanding of the reactivity of the individual surface oxygen atoms. This is of great importance for further research of adsorption processes on the In2O3(111) surface. The organic molecules adsorb in flat-lying geometries on both surface terminations (clean and hydroxylated) and in well-defined adsorption sites. The large size and inhomogeneity of the surface unit cell (the size is comparable to the size of the molecules) play an important role in the choice of adsorption configuration. On the hydroxylated surface, vacancies in the hydroxyl layer were found next to the organic molecules; they are assumed to trap the molecules. For the clean In2O3(111) surface, the a certain region of the unit cell (with topmost Indium atoms) is assumed to trap planar phenyl rings and stabilize the molecules. Most molecules seem to be mobile at room temperature and repulsive at very low coverages, i.e., islands or ordered structures were not exclusively observed but always coexisting with a significant density of single molecules. In conclusion, this project facilitated a deeper understanding of atomic processes on In2O3(111) surface, with focus on the adsorption of device-relevant organic molecules. I am very grateful that this chance has been given to me by the FWF.