Repackaging photon energy using exciton fission and fusion in molecular crystals (Conference Presentation)

Abstract

The ability to downconvert (1 photon to 2 photons) and upconvert (2 photons to 1 photon) energy can boost solar energy conversion efficiencies by 30% or more. Downconversion can be accomplished through exciton fission, in which an initially created high energy singlet exciton spontaneously splits into a pair of lower energy triplet excitons. In organic semiconductors like tetracene and rubrene, the Frenkel character of the excitons leads to energetically separate singlet and triplet bands, providing an ideal set of energy levels for both processes to take place. In this talk, our efforts to understand the basic photophysics of singlet fission using time-resolved transient absorption, photoluminescence and magnetic field effects will be described. The role of molecular packing in controlling the fission rate will be emphasized. Upconversion occurs via the reverse process, where a pair of triplet excitons fuse into a high-energy singlet state. While most approaches to upconversion require a sensitizer to populate the dark triplet states, an alternate approach is to take advantage of low-energy intermolecular states in organic crystals to sensitize triplet states. We show that this process can be surprisingly efficient in certain molecular crystals, even in the absence of sensitizers. The exciton interactions responsible for this process are investigated using steady-state and time-resolved spectroscopy.