Vorschau

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Abstract

N-heteropolycycles are a class of aromatic compounds, in which C–H units of polycyclic aromatic hydrocarbons (PAHs) are substituted with N-atoms. These compounds are considered to be promising organic semiconductors with high potentials for the use as functional materials in (opto)electronic applications such as field-effect transistors and photovoltaic cells. For these applications, performance of a device is particularly dependent on the properties of the organic semiconductor at its interface with the metal electrodes. Therefore, gaining insights into the adsorption geometry and the electronic structure of N-heteropolycycles at the organic/metal interface and within their thin films are essential in improving and optimizing the device performance. In this thesis, this goal is achieved for various N-heteropolycyclic compounds by using two characterisation methods, high-resolution electron energy-loss spectroscopy (HREELS) and temperature-programmed desorption (TPD), in combination with density functional theory (DFT) calculations.

In the presented studies, all of the investigated molecules, at all coverages adopt a planar adsorption geometry relative to the substrate surface. By assigning the energies of the lowest excited singlet states (S) as well as the first triplet states (T1), it is found that N-introduction can effect the electronic structure of N-heteropolycycles in three ways, namely narrowing the optical gap (S0 → S1 transition), shifting the S0 → T1 transition to a higher energy and inducing a pronounced rise in the intensity of the α - band (S0 → S2 transition), in comparison to parent PAHs. Next, it is shown that the electronic structure of N-heteropolycycles can be fine-tuned by core substitution with halogens and aromatic groups, which results in a reduction of the transition energies of singlet and first triplet states. Subsequently, it is demonstrated that structural variation via connecting different moieties of N-heteropolycycles also leads to the narrowing the optical gap. Finally, it is determined that electron donating N-heteropolycycles in combination with well-known electron accepting molecules in donor/acceptor (D/A)-systems form charge transfer (CT)-complexes.