Electronic and structural properties of conjugated molecules at molecular hetero-interfaces and on metal surfaces
Jens Niederhausen
- 22 May 2015
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TL;DR: In this paper, the electronic and structural properties of thin films of conjugated organic molecules (COMs) vacuum-deposited on metal surfaces are studied, and two approaches for engineering the energy-level alignment (ELA), and, thereby, optimizing hole injection barriers (HIBs), at organic/metal interfaces via (over)compensation of the detrimental "push-back"-Effekts are discussed.
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Abstract: Diese Arbeit behandelt elektronische und strukturelle Eigenschaften dunner Schichten aus konjugierten organischen Molekulen (COMs), aufgebracht auf Metalloberflachen per Vakuum-Sublimation. Diese Eigenschaften sind essenziell fur Funktionsrealisierung und -optimierung organischer Elektronikbauteile. Teil 1 diskutiert zwei Ansatze zur Energieniveauanpassung (ELA) an Organik-Metall-Grenzflachen zur Einstellung der dortigen Locherinjektionsbarrieren (HIBs) durch (Uber-)Kompensation des abtraglichen "Push-back"-Effekts: - Ausnutzung der besonderen ELA bei Chalkogen-Metall-Bindungen, hier gezeigt mit Hilfe von Rontgen- und Ultraviolettphotoelektronenspektroskopie (UPS/XPS) fur ein Seleno-funktionalisiertes COM - Einfugen von COMs mit ausgepragtem Elektronen-Akzeptorcharakter vor dem Aufbringen der aktiven Schicht. UPS-Messungen zeigen, dass beide Ansatze HIBs von ca. 0.3 eV ermoglichen. Teil 2 untersucht ausgewahlte organische Heterostrukturen auf Metallen. Die Untersuchungen identifizieren einen Ladungstransfer vom Metall zur Uberschicht (MOCT) als verantwortlich dafur, das System bei Ferminiveau-Pinning in den Gleichgewichtszustand zu uberfuhren. Detaillierte Untersuchungen gestatten die Identifikaton von ganzzahligem Ladungstransfer zu einem Teil der Molekule in der ersten Uberschichtlage und den Einfluss der Dipol-Abstosung in der Uberschicht. In Teil 3 dienen Metalloberflachen als Auflage fur supramolekulare Architekturen mit dipolaren Bausteinen. Rastertunnelmikroskopie (STM) an einer Serie von teils partiell fluorierten, stabchenformigen COMs mit unterschiedlich grosen Dipolmomenten ermoglicht die Entflechtung von Dipol-Dipol- und konkurierenden Wechselwirkungen physisorbierter Submonolagen auf Ag(111). Ein anderes, stark dipolares COM bildet bei Monolagenbedeckung auf Au(111) sechs Phasen, alle mit antiferroelektrischer Einheitszellen. UPS-Messungen ergeben eine bevorzugte Ausrichtung der Molekule in Multilagen.%%%%In this thesis, the electronic and structural properties of thin films of conjugated organic molecules (COMs) vacuum-deposited on metal surfaces are studied. These properties are essential for realization and optimization of device functionalities in the field of organic electronics. Part 1 discusses two approaches for engineering the energy-level alignment (ELA), and, thereby, optimizing hole injection barriers (HIBs), at organic/metal interfaces via (over)compensation of the detrimental "push-back": - Exploiting the peculiar ELA at chalcogen-metal bonds, shown here (with X-ray and ultraviolet photoelectron spectroscopy, UPS/XPS) for a seleno-functionalized COM - inserting electron-accepting COMs prior to deposition of active layers. UPS shows that both approaches realize HIBs into the active COM as low as 0.3 eV. Part 2 studies selected organic/organic heterostructures on metal surfaces. These studies allow to propose that metal to overlayer charge transfer (MOCT), is responsible for achieving electronic equilibrium when such systems are Fermi-level pinned. Detailed investigations allowed…
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