The search for singlet fission: non-radiative loss pathways in organic semiconductors

In this thesis, we present in-depth investigations into three materials which have been
predicted to undergo singlet fission via density functional theory calculations. The first
is Pigment Red 254, presented by Padula et al., a diketopyrrolopyrrole derivative
which shows great stability in the solid state and is also very cheap to produce, both
of which are useful for device fabrication. The second and third are BoDiPy 6 and 7,
derivatives of boron dipyrromethane.
Singlet fission is a phenomenon which occurs in a small number of organic semiconductor
materials that permits the generation of two triplet excitons from one singlet exciton,
which promises to greatly increase the efficiency of solar cell devices. In the search for
more singlet fission materials, being able to predict the occurrence of singlet fission com-
putationally rather than relying on high-cost, time consuming experimental procedures,
it would greatly expedite the process of improving the efficiency of photovoltaics.
However, in this work we demonstrate that none of these materials unequivocally show
singlet fission, finding in the case of Pigment Red 254 evidence for rapid non-radiative
decay to the ground state which out-competes this process. We perform in-depth infra-
red investigations of this material, and demonstrate that this decay takes place via
a hydrogen bond mediated proton transfer process, resulting in its non-radiative loss
mechanism. In the case of the BoDiPy dyes, we show that intersystem crossing takes
place preferentially to singlet fission, taking place in the solution phase and showing a
strong dependence on the heavy atom substituents in the BoDiPy dyes.
We highlight here the need to account for well known structural markers of singlet fission
loss mechanisms when attempting to predict efficient singlet fission materials, including
hydrogen bonding and the heavy atom effect.