Ultrafast generation of the triplet-triplet spin biexciton with weak binding energy in singlet fission*

Singlet Fission (SF) is a spin-allowed multichromophore process in which a singlet exciton reached optically undergoes internal conversion to a bound triplet-triplet spin biexciton, which may further dissociate into two free triplets, each of which may contribute to the photoconductivity of an organic solar cell. Research on SF until recently focused on designing materials that give rapid singlet to triplet-triplet conversion. Rapid triplet-triplet generation, however, is a consequence of strong interchromophore coupling that also leads to fast triplet recombination instead of generating free triplets. Overcoming this impasse requires clear understanding of the relationship between bonding motifs and the generation of triplet-triplet with weak binding energy. Accordingly, we have investigated the SF candidate material PTP (P = pentacene, T = tetracene) which has been shown to display both fast triplet-triplet generation as well as slow triplet recombination¹. Our many-body calculations on this system allow us to describe its low-lying electronic states with very high accuracy and thus describe the nature of free triplet generation therein. This enables us to further pin down the fundamental rules for the best iSF candidate materials.

¹Pun, A. B. et al. Ultra-fast intramolecular singlet fission to persistent multiexcitons by molecular design. Nat. Chem. 11, 821-828 (2019)