Chris Bardeen, University of California Riverside
Repackaging photon energy through downconversion and/or upconversion can boost solar energy conversion efficiencies by 30% or more. In organic semiconductors, the Frenkel character of the excitons leads to energetically separate singlet and triplet bands, providing an ideal set of energy levels for exciton fission (leading to energy downconversion) and exciton fusion (leading to energy upconversion). Singlet exciton fission in organic molecular crystals is studied using time-resolved transient absorption, photoluminescence and magnetic field effects. Non-equilibrium spin state distributions can play an important role in both the ultimate triplet yield and the observation of experimental quantities like delayed fluorescence. Upconversion, where a pair of triplet excitons fuse into a high-energy singlet state, presents the inverse problem of populating the triplet states to initiate the process.To improve upconversion in at long wavelengths, triplet state sensitization using semiconductor nanocrystal sensitizers and conjugated organic ligands has been demonstrated to be an effective strategy. In the solid state, we find that upconversion in certain molecular crystals can occur even in the absence of sensitizers, possibly due to the presence of low-energy intermolecular states.