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New Concepts in Interfacial Dipole Engineering by Self-Assembled Monolayers

Sauter, Eric

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Abstract

Self-assembled monolayers (SAMs) are frequently used for interfacial engineering in organic electronics and photovoltaics. The manipulation of injection barriers by introduction of a specific dipole moment at the interfaces between the electrodes and adjacent organic layers (e.g., an organic semiconductor (OSC)) is of a particular interest. This manipulation is usually achieved by selection of a suitable dipolar terminal tail group comprising the SAM-ambient interface, which however has several essential drawbacks. This approach has been recently complemented by embedding dipolar groups into the molecular backbone of the SAMs, with both aliphatic and aromatic SAMs being engineered and mixed aromatic SAMs comprised of the molecules with the oppositely oriented dipolar groups being studied. The major goal of this work is extension and optimization of the embedding dipole approach, along with several other concepts in general context of interfacial dipole engineering. At first, I studied the mixed aliphatic SAMs comprised of molecules which were modified by a dipolar ester group embedded into the alkyl backbone at two different orientations, viz. with the dipole directed upwards and downwards from the substrate. Applying X-ray photoelectron spectroscopy (XPS) as a morphology tool, I could estimate that the mixed SAMs represent homogeneous intermolecular mixtures of both components, down to the molecular level, excluding existence of "hot spots" for charge injection. The composition of the mixed SAMs was found to mimic fully the mixing ratio of both components in solutions from which these SAMs were prepared, which suggests a minor role of the dipole-dipole interaction in the overall balance of the structure-building forces. Varying this composition, work function of the gold substrate could be tuned linearly and in controlled fashion within a ~1.1 eV range, between the ultimate values for the single-component monolayers. As the next task, I studied the applicability of the embedded dipole concept to the different substrates, taking Ag(111) as a representative example. The aromatic SAMs with the embedded pyrimidine group were found to be much more robust in this context as compared to the aliphatic ones (with the embedded ester group), which makes the former systems especially useful in context of the electrostatic interface engineering. In view of these favorable properties, the next task was optimization of the aromatic SAMs with the embedded pyrimidine group. This was achieved by shortening the molecular backbone and excluding aliphatic building blocks. The resulting, optimized monolayers preserved all useful properties of their prototypes in context of dipole engineering but exhibited much better electrical transport properties, which allowed our partners to fabricate organic thin film transistors with high performance and extremely low contact resistance. Another promising tool for tuning the dipole attributes and the respective work function was found to be electron irradiation. This was demonstrated by the example of aromatic SAMs with the embedded pyrimidine group and terminal pyridine group. The observed behavior is presumably related to specific chemical transformations involving the nitrogen atom in these moieties. It leads to several practical implications, including work function lithography, which could be demonstrated by representative patterns. Alternatively, to the embedding of a dipolar group, the selection of a specific anchoring motif was tried in context of interfacial dipole engineering, taking dithiocarbamate-based SAMs as a representative example. The combination of the spectroscopic and work function data with the results of theoretical simulations performed by our partners allowed understanding the structure and electrostatic properties of these monolayers in very detail, paving the way for their applications.

Item Type: Dissertation
Supervisor: Zharnikov, Prof. (apl.) Dr. Michael
Date of thesis defense: 16 November 2018
Date Deposited: 03 Dec 2018 12:46
Date: 2018
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institute of Physical Chemistry
Subjects: 540 Chemistry and allied sciences
Controlled Keywords: Dipole Engineering, Self-Assembled Monolayers, Work Function, Work Function Modification, New Concepts, Irradaition, Embedded Dipoles, Embedded Ester, Embedded Pyrimidines, Dithiocarbamate, Work Function Lithography
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