Molecular crystals are bound by dispersion interactions, whose weak nature generates potential energy landscapes with many local minima that may be extremely close in energy. This often results in polymorphism, the crystallization of the same molecule in several different structures. Crystal structure may profoundly influence the physical and chemical properties, including the electronic and optical properties relevant for device applications. To predict the structure of molecular crystals and discover molecular crystals with enhanced electronic and optical properties we combine quantum mechanical simulations with machine learning and optimization algorithms. To perform structure search, we develop the genetic algorithm (GA) code, GAtor, and its associated random structure generator, Genarris. To predict the morphology of molecular crystals, we develop the Ogre code. All three are open-source Python packages designed for parallel execution on supercomputing platforms. To predict the electronic and optical properties of molecular crystals we use many-body perturbation theory within the GW approximation and the Bethe-Salpeter equation (BSE). An emerging application of molecular crystals is singlet fission (SF), the conversion of one photogenerated singlet exciton into two triplet excitons. SF has the potential to significantly increase the efficiency of organic photovoltaics by harvesting two charge carriers from one photon. To assess the likelihood of new materials to exhibit SF, we have proposed a two-dimensional descriptor based on the thermodynamic driving force for SF and the degree of singlet exciton charge transfer character. To evaluate the latter, we have developed the double-Bader analysis method for exciton wave-functions from BSE calculations. We have identified several promising candidates for intermolecular SF in the solid state.

Structure prediction and discovery of molecular crystals with enhanced electronic properties

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.