Molecular Packing Determines Singlet Exciton Fission in Organic Semiconductors

Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states allows the extraction of up to two electrons per harvested photon and, hence, promises to overcome the Shockley-Queisser limit. However, the microscopic mechanism of singlet exciton fission, especially the relation between molecular packing and electronic response, remains unclear, which therefore hampers the systematic improvement of organic photovoltaic device. For the model system perfluoropentacene, we experimentally show that singlet exciton fission Is greatly enhanced for a slip-stacked molecular arrangement by addressing different crystal axis featuring different packing scheme. This reveals that the fission process strongly depends on the intermolecular coupling: slip-stacking favors delocalization of excitations and allows for efficient exciton fission, while face-to-edge molecular orientations commonly found in the prevailing herringbone molecular stacking patterns even suppress it. Furthermore, we clarify the controversially debated role of excimer states as intermediary rather than competitive or precursory. Our detailed findings serve as a guideline for the deign of next-generation molecular materials for application in future organic light-harvesting device exploiting singlet exciton fission.