Optimal polymorphs of tetracene for singlet fission.

The efficiency of solar cells may be improved using singlet fission (SF), where one singlet exciton splits into two triplet excitons. A molecule may crystallize in more than one form. This phenomenon, known as polymorphism, may affect SF. Molecular crystal structure prediction (CSP) can identify potential crystal forms of a molecule. Here we search for polymorphs of tetracene using Genarris, a random structure generator, and GAtor, a genetic algorithm-based CSP code. Potential polymorphs of tetracene, in addition to the two experimentally known polymorphs, were identified. Using many-body perturbation theory in the GW approximation and Bethe-Salpeter equation, the SF performance of tetracene polymorphs was assessed. The excitation energies and the charge transfer character of the exciton wave function were computed. We observe that crystal packing can significantly affect excitonic properties in tetracene.