Combined Experimental and Theoretical Approach to Grain Boundary Engineering in Organic Semiconducting Crystals

• Applicants: Professor Dr. Konstantin Amsharov; Professor Dr. Harald Oberhofer; Professor Dr. R. Thomas Weitz
• Subject Area: Experimental Condensed Matter Physics ; Organic Molecular Chemistry - Synthesis and Characterisation ; Theoretical Condensed Matter Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 536757824

Organic semiconductor materials (OSC) have the potential to disruptively transform a number of industries—such as the IT sector or the field of photovoltaics—and are indeed already being used in some applications. They offer a number of improvements over traditional inorganic semiconductors such as low energetic production costs, novel materials properties and composition of only earth-abundant elements. Experimental and theoretical design efforts of new materials for OSCs for a long time mostly focused on incremental improvements of known materials, with modern data-driven efforts only very recently finding their use. Yet, all of these efforts to date only considered the properties of the bulk material, often in an idealized setting. Realistic materials, on the other hand, will always show imperfections such as boundaries between different grains and the recent discovery of barriers and traps for charge carriers demonstrated that any comprehensive design effort will have to include the influence of barriers. In the proposed work, we will therefore employ a combined experimental and theoretical approach for the optimization of organic molecular materials including the transport inefficiencies due to grain boundaries. On the theory side, we will explicitly compute the energetic barriers to charge transport at grain boundaries. Experimentally, real-space imaging by scanning probe microscopy, charge carrier transport as well as (photocurrent-)spectroscopy will experimentally identify grain boundary heights and compare this to theory computations based on a combination of density functional theory and effective modelling. While the scanning probe microscopy method and transport measurements are established to identify grain boundary heights, it is a serial and comparably slow technique to map grain boundaries. In contrast, the photocurrent spectroscopy method has up to now only been used for inorganic quantum wells, and will be her firstly adopted for organic materials and allow a fast screening of grain boundary heights of various materials. Initially, we will restrict ourselves to chemical modifications of perylene-based molecules as they have shown to generate thin films of reproducibly high quality. Theory will thereby not only elucidate the atomistic details of charge transport across the interfaces but also show the correlation of barriers with various properties of the employed molecules and the interfaces. It will also lay the foundation for a further, expanded investigation of other molecules beyond the initial perylene structures. Finally, we will adapt the machine learning approach developed by one of us to include cross-grain transport parameters. With this scheme, which will also be modified to incorporate experimental data, we will be able to efficiently search the molecular design space to identify promising molecules for use in organic semiconductors.

• DFG Programme: Research Grants