Understanding the Mechanism behind Polymorphism and Phase Transitions in p-Type Small Organic Semiconductor

Small organic semiconductors (OSs) have a predilection to pack into multiple, structurally distinct, crystal structures (polymorphism) with differing ability to transport charge. Controlling polymorphism is critically important since any slight variation in π-orbital overlap can lead to orders of magnitude difference in charge carrier mobility. In order to understand the origin behind polymorphism in single crystals of a high performing p-type OS, di-tertiary butyl benzothiophene (ditBu-BTBT), we conducted a detailed Molecular Dynamics (MD) study to predict the structures of stable and metastable polymorphs, and find ways to control inter-polymorph phase transitions in ditBu-BTBT systems. We used a combination of MD and advanced Bayesian optimization to accelerate the pace of finding the best combinations of candidate polymorphs (optimizing six dimensions of unit cell lengths and angles) and temperature that leads to the lowest energy structure. Next, we calculated the free energy barrier associated with the rotation of side-chains around the central core of ditBu-BTBT. We hypothesize that these calculations explain what triggers, or offsets, phase transitions in molecular crystals. The insight gained by this novel approach can be used to design next-generation materials.