Präparation und Charakterisierung molekularer Kontaktschichten zur Modifikation der Austrittsarbeit von Edelmetalloberflächen

Als zentrale Herausforderung für effiziente Ladungsträgerinjektion und damit zur Verbesserung bestehender und Realisierung neuer Anwendungen der organischen Elektronik hat sich die Kontrolle der Grenzfläche zwischen Elektroden und organischen Halbleitern herausgestellt. Die Anpassung der Austrittsar...

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Bibliographic Details
Main Author: Widdascheck, Felix
Contributors: Witte, Gregor (Prof. Dr.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2021
Online Access:PDF Full Text
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Contact engineering has been shown to be crucial for efficient charge carrier injection at metal-organic contacts, which thereby improves the performance of organic electronics and allows new applications. Tuning the work function of the contact electrodes is a common method for energy-level alignment resulting in improved charge carrier injection and reduced contact resistance. This thesis summarizes several studies regarding the work function modification for Ag and Au surfaces through monolayer contact primers using a variety of different molecules. It further investigates the possible implementation in real devices, such as OFETs. By combining several experimental techniques, the structure of the contact primers of F4TCNQ, F6TCNNQ, F16CuPc, CuPc and TiOPc as well as their impact on the metal work function were analyzed on single crystalline as well as device relevant polycrystalline metal surfaces. For all studied molecules without an axial dipole moment, no significant differences were found comparing the effect of monolayer contact primers on the work function between single-crystalline and polycrystalline surfaces and a lying molecular orientation was found on both substrate types. It could be shown, that the resulting work function can be modified in a targeted manner using appropriate molecules. By means of a detailed analysis of the coverage-dependent work function the interface dipole could be determined arising during adsorption of the phthalocyanines (Pc) and the contribution of the axial dipole moment of the non-planar TiOPc. In addition to the modification of the work function, the targeted production of contact layers of precisely one monolayer could be achieved through the temperature-controlled desorption of multilayers. This method turned out to be a reliable and reproducible method on single crystalline as well as polycrystalline substrates – provided the chemical stability of the molecule is sufficient. While the thermal stabilization of the bilayer makes the production of the TiOPc monolayer more difficult, it enables the targeted manufacture of the bilayer itself. During the adsorption of F4TCNQ and F6TCNNQ on Ag, it was observed that these form extensive metal-organic complexes. A detailed analysis showed a thermally activated formation of the complex, which mainly occurs at the step edges and therefore has different dynamics between monocrystalline and polycrystalline surfaces. Compared to unmixed molecular films, the complex exhibited increased thermal stability and optical and electronic changes could be identified. An additional charge transfer into the molecules of the monolayer leads to a further stabilization of the monolayer and allows their targeted production through controlled desorption of the metal-organic complex. This could also be demonstrated for F6TCNNQ on Ag surfaces applied by means of silver ink, which allows the use of F6TCNNQ contact layers on printed silver electrodes. For the use of the temperature-controlled desorption of the multilayer for the production of defined monolayers on gold electrodes of structured components – such as OFETs – clean gold electrodes have to be produced. For this purpose, different cleaning methods of the electrodes were investigated on the basis of the molecular growth and the contact resistance in OFETs. The gold oxide produced by simple O2-plasma cleaning leads to an upright molecular orientation. In a subsequent heating step, clean, oxide-free gold surfaces can be produced and therefore the knowledge gained for polycrystalline surfaces can be transferred to so cleaned electrodes. In addition to the metal-organic interface with the contact layers, the interface between the promising F6TCNNQ contact layers and the on top deposited prototypical p-type semiconductor pentacene was investigated. An intermixing between pentacene and the underlying contact layers on Au was found, but no intermixing with F6TCNNQ on Ag. For further pentacene growth, dendritic islands with upright molecular orientation could be observed on the mixed bilayer on Au and directly on the F6TCNNQ contact layer on Ag. This showes signs of successful electronic decoupling of the pentacene layer from the underlying metal substrate, whereas no dependence on the crystallinity of the metal surfaces was found. On the basis of the systematic comparison between monocrystalline and polycrystalline surfaces, the work showed overall that in most of the investigated cases the results of the single crystalline model systems can be transferred to the polycrystalline surfaces. To expand the understanding of the film growth of molecular thin films, structural characterizations were carried out on the model system of TiOPc on Ag(111) and Au(111). Substantial indications were found that the covering capacity of organic thin films is ultimately limited by the crystal structure and the domain boundaries that occur. On the other hand, it could be shown that TiOPc multilayers form crystalline and molecularly smooth terraces, which are stabilized by a bilayer-wise growth and are therefore ideal for sharp interfaces with other OSCs. The presented research shows the possible modification of the work function of metal electrodes and also a general way of producing and characterizing well-defined contact primers to improve the energy-level alignment in real devices. This also enables the use of these devices for further analysis of the charge transport through metal-organic interfaces on well-characterized and well-defined structures, which enables a microscopic understanding of charge carrier injection and could further advance progress in organic electronics.