Excitonic Processes, Energy Transport, and Excited States in Organic Materials

Abstract This dissertation has roots in the distinctly human endeavor to harness energy. We study singlet exciton fission, which has the remarkable ability to double the number of energy carriers (excitons) through singlet fission, and its reverse process triplet fusion, which can combine triplet excitons. Understanding the fundamental mechanisms that enable singlet fission may allow for it to be engineered for use with other materials for solar cell applications.We experimentally investigate the creation of singlet and triplet excitons in rubrene single crystals, how one species can convert into the other by excitonic fission and fusion processes, and how triplet excitons can travel comparatively long distances.Using steady-state excitation, we determine that the efficiency of singlet exciton fission and triplet fusion are both large, likely exceeding 90%, and are only weakly magnetic field dependent in pristine rubrene single crystals. We find a decrease in fission efficiency by 20% when applying a magnetic field of 1 T, which is visible as an increase of 20% of the photoluminescence quantum yield in the limit of low excitation density; Further, we also report an increase in quantum yield of only about 5% in the limit of high excitation densities, where triplet fusion dominates the emitted PL. These observations are consistent with a magnetic field-induced reduction of both singlet fission efficiency and triplet fusion efficiency.We also investigate the PL quantum yield as a function of temperature and find an increase in PL quantum yield by about a factor of almost an order of magnitude between 295 K and temperatures of the order of 120–150 K; which is likely due to a temperature dependence of the fission processes through an activation energy barrier. We find no changes to the PL spectrum's intensity distribution in pristine rubrene crystals when changing either the applied magnetic field strength, or the sample's temperature.A high fission and fusion efficiency in rubrene single crystals means that it is possible to determine the triplet exciton diffusion length by directly imaging the photoluminescence emitted by a diffusing triplet population. We study how exci- ton diffusion depends on temperature and magnetic field and find that the diffusion length remains large at all investigated temperatures, keeping a value of 4.0 ± 0.5 μm with no observable temperature variation down to a temperature on the order of 225 K. For the magnetic field dependence of diffusion, we find that an applied mag- netic field of 1T increases the diffusion length from 4.0 microns at 0T field to 5.4 ± 0.4 μm.Finally, we investigate the singlet fission process by an extensive study of PL dy- namics after pulsed excitation. We determine the PL time dynamics as a function of excitation density and observe the appearance of a component of the photolumines- cence that follows an exponential decay with a decay time constant of 4.3 ± 0.5 ns. By studying how this component varies with excitation power and the level of im- purities in rubrene crystals, we show that this decay is likely related to the presence of a quantum superposition of singlet state and triplet-pair state which acts as the intermediate state of singlet fission. We then tentatively assign the 4.3 ns decay to the lifetime of this intermediate state, which is essentially given by the triplet component's dissociation time into two independent triplet excitons. We also show that the feature associated with this intermediate state is the only feature of the photoluminescence decay between 0.1 and 100 ns that shows any dependence on an applied magnetic field. This supports the interpretation that the 4ns transient is related to a quantum superposition of singlet and triplet-pair states.