Ultrafast Spectroscopy of Organic Semiconductors: Singlet Fission and Nonfullerene Acceptors for Organic Photovoltaics
In this dissertation, we investigate two emerging strategies for enhancing the performance of organic photovoltaics. The first takes advantage of a process called singlet exciton fission, and the second embodies an exodus from the fullerene electron acceptors prominent in organic solar cells. Indeed, this versatile class of tunable small molecules are aptly termed nonfullerene acceptors. However, both strategies would benefit from a greater understanding of underlying principles. Singlet exciton fission is a photon-multiplying process in which a singlet exciton from a high-energy absorbed photon splits into two triplet excitons. The process could significantly reduce energy lost to heat in photovoltaic devices, but its mechanisms are still misunderstood. One model involves direct coupling between the singlet and triplet states, and another model involves an intermediate charge transfer state. Transient absorption spectroscopy allowed us to examine singlet fission in films of pentacene, fluorinated pentacene, and coevaporated blends of various mixing ratios. We directly observe an intermolecular charge transfer state during singlet fission in solid films of coevaporated pentacene and peruoropentacene, which supports the model of charge transfer state-mediated singlet fission. Furthermore, we successfully induced singlet fission in one blend by directly exciting the charge transfer state below the bandgap.
We use various types of steady state and time-resolved spectroscopy to characterize two types of nonfullerene electron acceptors. The first type is a group of tetraazabenzodiuoranthene diimide (BFI) dimers and a BFI monomer. The BFI dimers were designed to have twisted, nonplanar 3-dimensional structures and have helped achieve power conversion efficiencies of over 8% in organic solar cells. The other type of nonfullerene acceptor is a calamitic small molecule, and we consider the BAF-4CN electron acceptor, which has also been used in a solar cell whose efficiency exceeded 8%. Spectroscopic studies give insight into the performances of these nonfullerene devices in relation to fullerene-derivative counterparts. We find that the nonfullerene blends suffer from more geminate charge recombination. However, despite this drawback, in some cases, slower rates of nongeminate recombination may lead to successful power conversion efficiencies in nonfullerene solar cells.