Tuning of the Interaction Strength at Inorganic/Organic Interfaces

Interfaces between inorganic materials and organic molecules are highly interesting the viewpoint of fundamental science, interesting since the flexibility of organic chemistry allows systematically tuning the strength of the interaction between the two components. At the same time, the properties of organic and inorganic materials often complement each other, which led to many practical applications, such as organic light-emitting or harvesting devices. For these applications and many other, emerging technologies to finally succeed, an in-depth atomistic insight into the quantum processes at the relevant interfaces is absolutely crucial. For instance, two different phenotypes of charge-transfer between inorganic and organic can be observed for physisorbed and weakly chemisorbed systems. While for unreactive, semiconducting substrates, the charge in the organic material is found to be strongly localized, for weakly reactive, metallic substrates, the charge is found to be completely delocalized. At present unclear, however, is how, e.g., degenerately doped semiconductors, which show quasi-metallic conductivity, fit into this classification. In this project, we will study by means of first principle calculations (density functional theory and beyond) (a) how the localization of charge is affected for a given interface as the nature and strength of the substrate/adsorbate interaction is gradually modified, and (b) how this affects observables at the interface. To this aim, we will investigate the adsorption of small, conjugated organic molecules on semiconductors with different doping concentrations and metals which reactivity will be modified through alloying. As a third approach, bulky spacer groups will be introduced into molecules, which mitigating the wave-function overlap and, thus, the interaction strength between inorganic substrate and organic adsorbate. Throughout the project, the choice of the density functional will be validated by many-body perturbation theory thereby giving an impetus to method development. In addition to the added value for the fundamental understanding of surfaces and interfaces, the project will provide in-depth atomistic insight into the quantum processes at interfaces relevant for nascent technologies, such as organic thermoelectric materials or spintronic devices.

The main question of the recently concluded project was to find out how the charge transfer between two materials can be fundamentally altered by tuning the interaction between them by employing suitable chemical modifications to the substrate. The starting point of this project was the experimental observation that the nature of the charge transfer to organic material, which are frequently used in modern-day nanotechnology applications, depends sensitively on the nature of the substrate: On a clean, reactive metal substrate, typically little charge is transferred, and that charge is smeared out over the whole interface. Conversely, on chemically less active semiconducting substrates, only a few molecules become strongly charged, while the rest of the material remains unaffected by the contact. One of the most interesting results of this project originated from investigating the gradual transition of a metallic substrate into a semiconductor. This can occur, e.g., by slow oxidation (i.e., the equivalent to rusting). Surprisingly, during this transition a completely new charge transfer mechanism occurs, that is not described in the pertinent literature. Of particular interest here is that particularly large charge transfer happens in a case where the established models would predict that none occurs at all. A further important and unexpected result is the observation that the aforementioned different charge transfer mechanisms are not mutually exclusive, as originally thought. Rather, designing an appropriate interface via a suitable choice of both substrate and organic material, it is possible to trigger both mechanism simultaneously. This is of particular relevance when both positive and negative charges are transferred. Then, it is possible enforce, e.g., the posite charge to be smeared out, while the negative charge must remain localized. This results in a much large charge contrast at the interface than conventional interfaces. Both of these results are not only of high relevance from a fundamental point of view, but have direct implications for technological application. Partly oxidized substrate materials occur during the processing or the ageing of organic devices, which are needed, e.g., for OLED-TVs. Similarily, the aforementioned large charge contrast allows to design interfaces with particularly beneficial charge-transfer properties, which may, in the future, allow to create less energy-intensive devices. Therefore, we expect that this project will not only benefit the general understanding in surface science, but also provide a tangible impetus for the organic electronics industry.