Triplet-pair states in exciton fission and fusion

In this thesis, we present our investigations into the intermediate triplet-pair states that mediate the twin photophysical processes of singlet fission and triplet fusion. Unique to certain conjugated organic materials, singlet fission and triplet fusion refer to the splitting of bright, spin-0 singlet excitons into pairs of dark, spin-1 triplet excitons, and vice versa. First invoked more than 50 years ago, the triplet-pair states that mediate these processes have recently enjoyed a resurgence of interest, largely due to the realisation that exciton fission and fusion could be harnessed in photovoltaic devices to exceed the theoretical power conversion efficiency limit. Despite great progress in our understanding of these intriguing states, many aspects of their nature and behaviour remain unclear.

We begin by probing the photoluminescence signatures of the strongly exchange-coupled spin-0 triplet-pair state, 1(TT), in polycrystalline films of diF-TES-ADT and single crystals of pentacene, demonstrating that it is a real, observable intermediate state. We then present brief investigations of 1(TT) photoluminescence, or lack thereof, and 1(TT) formation in rubrene single crystals. Our results here cast doubt on the vibronic coherent mechanism of singlet fission and highlight the role of defect sites and disorder.

Next, we turn to the spin statistical factor governing the probability of obtaining a spin-0 state from the annihilation of two spin-1 triplets and explain why it has been incorrectly described in much of the recent literature. Using rubrene as a model system, we investigate the key parameters affecting this important quantity and uncover previously overlooked strategies for engineering materials with favourable spin statistics.

Finally, we investigate the photophysics of rubrene nanoparticles for photon upconversion. We show that the addition of a singlet energy collector, used to improve the upconversion quantum yield, does not suppress singlet fission as previously assumed but instead competes with triplet-pair separation.