Singlet exciton fission is a spin-allowed process whereby an initially created singlet state spontaneously splits into a pair of triplet excitons. This process is of practical interest as a way to make higher efficiency solar cells. We use time-resolved spectroscopy to determine the effects of quantum coherence, sample morphology, temperature, and magnetic field on this process in molecular semiconductors like tetracene and rubrene. Our results are consistent with the “direct” mechanism for exciton fission, whereby the triplet pair is created in a concerted two-electron transition that creates a long-lived spin-entangled triplet pair. Triplet exciton dynamics and triplet-singlet interconversion can be described using kinetic equations based on the time-dependent Liouville equation. The long-lived dynamics of the triplet pairs depend both on spatial diffusion of the excitons and spin relaxation rates. Preliminary attempts to interface molecular singlet fission materials with inorganic semiconductors to create hybrid organic-inorganic photovoltaic materials will be described.
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