(654e) Nucleation and Growth of Organic Semiconductor Polymorphs

Molecular packing has a profound impact on the solid-state properties of materials. Since many organic compounds can pack into distinct crystal structures (polymorphism), controlled polymorph formation is vital to a wide range of areas, e.g., pharmaceuticals, pigments, food, explosives, etc. Recently, crystal polymorphism is emerging as an important subject in the area of organic semiconductors (OSC), given that substantial change in the electronic properties can be induced by even small changes in the crystal packing. Many of the benchmark OSCs are found to be polymorphic. However, the discovery of polymorphs in this field has been mostly by serendipity. With higher performing molecules being designed at an ever-faster pace, controlled nucleation and growth of polymorphs is becoming more and more important not only for tuning the charge transport properties, but also for producing ‘organic’ circuits with consistent performance and high stability.

In this study, we take a comprehensive approach to unravel the nucleation and growth of organic semiconductor polymorphs, combining advanced synchrotron X-ray techniques with molecular simulations, and charge transport characterizations with quantum chemical calculations. Using in situ X-ray diffraction techniques, new polymorphs are identified for even extensively studied molecules. The relative stability and unit cell parameters of polymorphs predicted independently by molecular simulations closely match with the experimental observations. Using quantum chemical calculations, we found significant change in the charge transfer integral across different polymorphs, which is supported by orders of magnitude difference in the measured charge carrier mobility. In addition, we infer the molecular origin of polymorphism based on structural refinement and molecular simulations.