Record-High Organic Device Performance enabled by Polymorphism in Organic Semiconductors

• Applicants: Professor Dr. Stefan Mannsfeld; Professor Dr. Frank Ortmann
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Chemistry: Molecules, Materials, Surfaces
• Term: from 2017 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 368686449

Recent developments in thin film processing methods for organic semiconductors have allowed the stabilization of non-equilibrium polymorphs with drastically better electrical device performance. These advances highlight the importance of controlling polymorphism to maximize the performance of a given material. However, so far there is no way to predict for which materials polymorphism can be expected and what electrical performance the different polymorphs can deliver. As a likely result, many materials, both existing and yet to be synthesized, have been or will be declared poorly performing in devices simply because they are not processed in a way that produces films of the (unknown) high-performance polymorph(s). In order to remedy this situation, we propose to develop a theoretical framework to predict polymorphism in small molecule organic semiconductors and to rank potential polymorphs based on their predicted electrical properties. In a complementary effort, we will employ a special printing method to detect and stabilize polymorphic forms of soluble organic materials in printed thin films, which will provide the very necessary experimental benchmarks and calibration references to the to-be-developed theory. Various experimental tools will be used to precisely determine the molecular packing in the printed films thus providing valuable feedback to theory.The theoretical approach will establish a direct link between molecular structure and electrical performance by considering many different polymorphs beyond the one in thermodynamic equilibrium that is usually encountered when employing conventional deposition techniques. The approach creates a whole set of possible structural realizations with expectedly higher carrier mobilities, possibly by orders of magnitude for some of the molecules. In the experimental part, we will test the predictions made by this new theory, especially those that it suggests to be of high technological relevance (high charge carrier mobility). We will use modern deposition techniques to introduce the variability in deposition conditions which is necessary to realize such polymorphs. Materials will be characterized and electronic devices will be analyzed and compared to theory.The proposed device-oriented simulation framework will be developed based on a restricted set of structures but is generally applicable and is a strategic tool that could spark a vast amount of novel high-mobility materials that would otherwise remain undiscovered. This could significantly transform the way of performing material analysis and design and should catalyze the research along the whole chain from synthesis to device measurements.

• DFG Programme: Research Grants