Exciton Dynamics in Rubrene Single Crystals

Abstract

In this work, transient grating pump and probe experiments are used to investigate excitonic processes in the rubrene single crystal. Rubrene is a high quality organic crystal with particularly intriguing material properties that make it interesting for the study of various physical processes of importance for optoelectronics.On the nanosecond time scale we find that bimolecular interactions cause a photoinduced excited state density on the order of approximately 0.5 x 10^20 cm^-3 - corresponding to an average distance of approximately 3 nm between individual states - to decrease by a factor of two after 2 ns, following a typical power-law decay. We assign the observed power-law decays to high-density interactions between excited states. Because of the high efficiency singlet exciton fission observed in rubrene, these bimolecular interactions are likely those between triplet excitons or a singlet exciton and a pair of triplet excitons in a coherent quantum superposition.Transient grating experiments performed on the picosecond time scale are then used to track the build-up of the photoinduced excited state. The build up to a quasi steady-state amplitude of the grating within 10 ps is consistent with the affirmation that the build-up tracks the creation of triplet states by singlet fission, which then implies the bimolecular interactions seen on the nanosecond time scale are indeed those of triplets or the coherent quantum superposition of a singlet and a pair or triplet excitons.Lastly we use the transient grating techniques in a spectroscopic capacity in order to track how the observed grating dynamics and the exciton dynamics that can be inferred from it depend on the energy of the photons used for excitation, which determine the initial excitation energy deposited in the material and the vibrational mode of the excited state. We find that the time it takes to create triplet excitons by fission increases with increasing excitation energy, and once an excitation threshold of approximately 2.7 eV is passed, corresponding to approximately the third vibronic resonance of the singlet state, we loose the ability to track the triplet population in our experiment