2, Princeton University, Princeton, New Jersey, United States
Singlet fission is an exciton splitting process that occurs in selected molecular systems that can boost solar energy conversion efficiencies by generating two electron-hole pairs from one high-energy photon. Molecular materials are fundamentally capable of supporting quantitative (i.e., lossless) carrier multiplication, and significant efforts have been made to better understand the process so as to ensure that quantitative yields, a prerequisite for the practical implementation of singlet fission, are actually achieved. A critical intermediate, known colloquially as the triplet pair, enables the process and precedes the formation of the two independent triplet excitations required for carrier multiplication. While a detailed understanding of the first step of singlet fission, i.e., triplet pair formation, has emerged, there are a number of details regarding the dissociation of the triplet pair into independent triplet excitations that remain to be clarified.
In this talk, I show how side chains can be used to effectively balance intermolecular coupling in amorphous solids of several pentacene derivatives and achieve overall highly-efficient singlet fission. I show how side chain sterics sensitively govern local packing in these amorphous solids and how this is in turn governs both triplet pair formation and decay rates. While triplet pairs form in quantitative yields in all cases, it is found that compact side chains promote stronger couplings that cause triplet pairs to effectively couple to the ground state, resulting in detrimental losses. In contrast, bulkier side chains cause triplet pairs to appear more like two independent and long-lived triplet excitations, and thus promote quantitative yields. Our results clarify many outstanding aspects of the triplet pair especially relevant to losses—we find that the triplet pair is non-emissive, that it is not bound, and that the constituent triplets cannot be considered in an independent nature. This work represents an important step toward better understanding intermediates in singlet fission and how molecular packing and couplings govern overall triplet yields.