Maximizing singlet fission in organic dimers: Theoretical investigation of triplet yield in the regime of localized excitation and fast coherent electron transfer

In traditional solar cells one photon absorbed can lead to at most one electron of current. Singlet fission, a process in which one singlet exciton is converted to two triplet excitons, provides a potential improvement by producing two electrons from each photon of sufficient energy. The literature contains several reports of singlet fission in various systems, but the mechanism of this process is poorly understood. In this paper we examine a two-step mechanism with a charge transfer state intermediate, applicable when the initial excited state is localized. Density matrix theory is used to examine how various molecular properties such as orbital energies and electronic couplings affect singlet fission yield in the regime of fast, coherent electron transfer. Several promising chromophores are discussed and density functional theory is used to predict fission yield for each in the context of this mechanism. Finally, implications for chromophore design are discussed, and future experiments are suggested.