A Study of Singlet Fission and its Mechanism Within Crystalline Tetracene

Singlet fission (SF) is a process in organic molecules that is able to create two triplet excitons from one initially excited singlet exciton. Beyond its interest as a unique photophysical process, SF is also worth studying due to its capability to enhance the overall efficiencies of photovoltaic cells beyond the Shockley-Queisser limit. In order to take advantage of this process, a more thorough understanding of how it occurs and how it can be enhanced is needed. Specifically, this work begins to answer some of the larger questions about SF within tetracene. SF was first observed in anthracene in 1965, and has since been found to occur in larger polyacenes and other conjugated organic molecules. This study focuses on crystalline tetracene for several reasons, most notably for its favorable energy levels for the study of the interaction between the singlet and triplet excitons, greater chemical stability, and large body of previous work. In this work, the electronic states of tetracene are characterized with time resolved photoluminescence and transient absorption spectroscopies in order to determine the rate and mechanism of SF within crystalline tetracene, which have not been conclusively shown in prior studies. Using spectroscopy measurements, this work has determined that SF is occurring within crystalline tetracene with a formation time of 80±3 ps, which leads to a triplet yield of 200%. Additionally, the study of the temperature dependence of crystalline tetracene photoluminescence has shown that the fission of a singlet into two triplets may not be thermally activated, a finding which is unexpected based on previous work. Furthermore, observation of quantum beats within the delayed fluorescence of single crystals of tetracene strongly suggests that the singlet fissions directly into two triplets rather than proceeding through a charge-transfer intermediate. Overall, this work has shown that SF occurs with a high efficiency within tetracene, and that tetracene is a viable candidate for use as a SF sensitizer within a photovoltaic cell.