Fluctuations and exciton dynamics in molecular semiconductors
This work describes experimental studies of the fluctuation-dominated electronic structure of molecular semiconductors and of the exciton dynamics in the same materials. The method employed in this thesis is time- and angle-resolved photoemission spectroscopy. In the first part, the electronic structure of molecular semiconductors at equilibrium is considered. We show that the experimental band structure of several representative molecular crystals can be described with a common tight-binding model and analyze how both the crystal structure and the π-topology impact the electronic structure. With this consistent description of the electronic states, it is discussed how fluctuations act on different materials and how they impact charge transport properties. These deliberations lead to non-trivial design rules for future materials with high charge mobilities that have been independently verified in charge transport experiments. The subsequent chapter concerns the spatial structure of the electronic states captured in momentum space. By applying the previously established tight-binding model, a strong connection between the anatomy of momentum maps, i.e., constant-energy cuts through the photoemission intensity, the electronic structure, and the real space description of electrons as Bloch states is built up. It is also discussed whether properties arising from fluctuations are visible in these maps. Most importantly, this part provides the basis for interpreting momentum maps of ground and excited states. In the second part, we analyzed the exciton dynamics in some of the same compounds that staged in the previous part. For pentacene single crystals, we obtained the first momentum maps of singlet and triplet excitons. These demonstrate the predicted, but never observed, similar orbital character and different localization properties of the two states and the images allow to study singlet exciton fission in unprecedented detail. Previous experimental results pointed to an instantaneous, coherent excitation of an intermediate bitriplet state. Using the exciton momentum maps, the dynamics could be projected to the orbital character of the underlying states and which proves that bitriplet states are not instantaneously excited, but rather created by a charge-transfer mediated transition from the photoexcited singlet exciton. That establishes the charge-transfer mediated mechanism of the primary step of singlet exciton fission. The last chapter describes the observed exciton dynamics in rubrene and tetracene single crystals, where singlet exciton fission is endothermic. We find that the momentum maps in tetracene are highly similar to those of the structural cousin pentacene. However, the dynamics differ substantially. We demonstrate that one key to understanding singlet exciton fission lies in the relative energy and the coupling strength of the charge-transfer states. In tetracene, the charge-transfer states stabilize the bitriplet state; that facilitates the ultrafast <200 fs creation of the bitriplet exciton but slows down its separation into two independent triplet excitons which occurs in 6 ps. Lastly, it is shown that singlet exciton fission in rubrene takes a different path due to the highly symmetric crystal structure which prohibits a coupling between bitriplet and charge-transfer states. Hence, the separated bitriplet exciton is directly created from the singlet without the involvement of charge-transfer states. We conclude by discussing the possible mechanisms of an ultrafast, but energetically steeply uphill, process, which highlights the need for theories that are able to treat both fluctuations and exciton dynamics.
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