Defects in Organic Monolayers

Organic electronic devices, based on combinations of inorganic and organic matter, are becoming increasingly important for high-tech products. Displays are already commonly found in mobile phones and some TVs. Smart clothing products are frequently introduced at fares. Other, more exotic applications, are still at the conceptual state. The common bottleneck for all these applications is the interface at the hybrid inorganic/organic material, over which charge or energy has to be transported. So far, most theoretical studies that consider these interfaces from an atomistic perspective have mainly focused on idealized, perfectly well ordered interfaces. However, in reality, even if every effort is made to keep the interface well-defined, temperature and entropy will cause the formation of defects in the organic material. These defects can take several guises, from vacancies (i.e., the absence of single, individual molecules) to patches with a completely different orientation of the molecules. Such defects can and do significantly affect how charge and energy is transported across the interfaces. The aim for the present project is to incorporate such defects into atomistic, so-called first principle (quantum-mechanical) calculations. Here, the main challenge is to determine the surface morphology, i.e., the form and the concentration of defects, since the number of possibilities to (mis)align several components in a layer is huge. Therefore, a custom-tailored method will be employed to tackle this problem. The basic ansatz is to neglect intermolecular interactions at first and determine any possible geometry that an isolated molecule can assume on the surface. Then, the extended organic film is modelled as a combination of these local adsorption structures, arranged on a grid. Starting from a layer where all molecules are in their most favorable geometry, individual molecules will then be perturbed into a less favorable state. This procedure will be repeated until all structures within a given energy range are found. A particular advantage of this approach is that each structure can be uniquely identified by the state of all molecules, and therefore a large number of arrangements can be quickly calculated without needing to re-calculated already known structures. Finding out how this computationally expensive approach can be exploited such that the number of calculations are reduced to a minimum is a core task of this project. For each of the geometries found, we compute the properties of the defect and its interaction with the environment using quantum-mechanical calculation. In particular, we will investigate whether those defects are themselves charged or not, how the presence of defects or complete disorder changes the relative position of the quantum-levels of inorganic and organic material, and to which extend this may (positively or negatively) effect charge and energy transport across inorganic/organic interfaces.

Organic electronic devices, based on combinations of inorganic and organic matter, are becoming increasingly important for high-tech products. So far, most theoretical studies that consider these interfaces from an atomistic perspective have mainly focused on idealized, perfectly well-ordered interfaces. However, in reality, even if every effort is made to keep the interface well-defined, temperature and entropy will cause the formation of defects in the organic material. In the present project, we explored what kind of defects are commonly present and what impact they have on the electronic structure of interfaces. Already determining the nature of the defects is highly challenging, because the number of possibilities how molecules could misalign, rotate, desorb, etc., is enormous. Therefore, our first step was to find a way to explore the structural composition of organic monolayers. With the aid of machine-learning, we set up a method accurately determine the energies of millions of different structures by performing only about 200 quantum-mechanical calculations. The corresponding software, called Surface Adsorbate polyMoprh Prediction with Little Effort (SAMPLE), is now publicly available via the applicant's homepage and can be readily applied to a variety of different surface science problems, including determining polymorphs of various organic materials that would show improved properties. A graphical user interface, which is currently under development, ensures that the program is easy to learn and apply. With the help of this new algorithm and in close collaboration with experimentalists, we published 9 papers within this project. The most important results are worth highlighting: We example, we could show that irregular, defective structures are very common even at very low temperatures. Furthermore, we also found that the adsorption of organic molecules on metal frequently produces a counterintuitive type of defect, so called ad-atoms, where individual atoms are pulled out of the substrate the organic molecules are adsorbed on. These ad-atoms fundamentally change the bonding situation at the interface between organic adsorbate and its inorganic support, leading to fundamentally different structural motifs with fundamentally different surface dipoles, and, hence, electronic properties. Conversely, we also looked into the impact that defects of the inorganic support have on the electronic structure of organic materials. Interestingly, we found that surface defects of usually inert materials lead to an interaction mechanism that is fundamentally different from both that of defect-free semiconductors and from more reactive metals. This interaction mechanism, that has so far not been considered when designing functional interfaces, may provide a novel pathway to tune and to improve charge- and energy-transfer within organic electronic devices.