Real-Time In-Situ Investigations of [6]Phenacene Organic-Inorganic Structures with Organic Molecular Beam Deposition

Abstract

Organic semiconductors (OSCs) that consist of fused benzene rings are promising candidates for high-performance electronic devices like organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaics (OPV). Compared to their inorganic counterparts, OSCs have a different charge transfer mechanism that depends on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). OSCs offer mechanical flexibility, stretchability as well as optical transparency and are low-cost alternatives for their inorganic counterpart that are suitable for mass production. A vast amount of different OSCs are available, which are under constant development and research. Especially the study of growth, structure and molecular packing is important due to its influence on the electronic properties of prepared devices. Devices made of OSCs consist of several layers where multiple materials are combined into a complex organic-inorganic interface or so-called hybrid system. Two parts of this interface were under investigation in this work. Alkali metal doped layers are used to manipulate the electronic properties of the original material and gold-organic interfaces that are used for electrical contacting. Both methods can lead to changes in the growth, structure and morphology of the original material and were therefore investigated in this thesis. This was achieved by utilizing preliminary work in the home laboratory as well as real-time in-situ measurements performed at state-of-the-art synchrotrons. Thin films were characterized by using various X-ray scattering methods combined with optical methods. The evaluation was done by examining the in-plane and out-of-plane structure during growth and calculating the unit cell structure, orientation and molecular packing. The molecule used for this study was [6]Phenacene deposited as a thin film by using organic molecular beam deposition (OMBD) under ultra-high vacuum (UHV) conditions. These results were also compared to other phenacenes. [6]Phenacene shows a Stranski-Krastanov growth behaviour which leads to wedding cake islands with a well-defined transition between the terraces. The unit cell parameters and molecular packing was calculated by assuming four molecules inside a unit cell. The molecular stacking is influenced by the parity of used molecules, which depends on the number of benzene rings of the used phenacene. Organic-inorganic interfaces of [6]Phenacene were investigated by using the alkali metals: potassium, rubidium and caesium. They were either deposited on top of a [6]Phenacene thin film or co-evaporated during growth leading to various changes in crystallinity and intercalation into the thin film. Further experiments with gold as a common contacting material were performed by depositing the metal on top of [6]Phenacene thin films. The results of this thesis showed various changes in structure and morphology for organic-inorganic systems which can lead to a further understanding of growth processes in thin films. This is especially important for the further development of OSCs.

Die Dissertation ist gesperrt bis zum 07. Februar 2026 !