Computational discovery of new materials for intermolecular singlet fission in the solid state

Intermolecular singlet fission (SF) is the conversion of one photogenerated singlet exciton into two triplet excitons localized on different molecules. SF has the potential to significantly enhance the conversion efficiency of organic solar cells by harvesting two charge carriers from one photon. However, few materials are presently known to exhibit intermolecular SF in the solid state. Using many-body perturbation theory in the GW approximation and Bethe-Salpeter equation (BSE), we have elucidated the effect of crystal packing on the excitonic properties of molecular crystals. 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 including monoclinic rubrene, quaterrylene, and phenylated pentacene derivatives.