My research focuses on the modelling of excited states in organic semiconductors applied to
three different areas, namely, optical spin injection singlet fission, and optical detection of
polarons.
One of the challenges in the field of spintronics is spin injection, which has been achieved
optically in inorganic crystalline semiconductors, but not yet in organic semiconductors.
Here, we apply group theory and computational methods to design molecular materials in
which spin can be injected optically via circularly polarized light (CPL).
When designing molecules for their optical excitation properties, additional design rules
can be defined by considering the relationship between the symmetry of the molecule and
its excited state properties using group theory. The theory reveals that molecules with
C3h symmetry are good candidates, because they support circularly polarized transitions.
Among the molecules with the correct symmetry using TDDFT calculation we are looking
for candidates with low lying circularly polarized triplets corresponding to ms = +/-1 . To
preserve the spin the triplet must be excited directly from the ground state, thus it is required
to have a relatively high oscillator strength.
We designed a series of molecules with C3h symmetry which were synthesized, some for
the first time, by our collaborator. We present preliminary experimental validation of our
approach with spectroscopic studies of the new family of molecules. The research shows
how symmetry can be used in molecular design for spintronics applications.
My second study concerns understanding the dynamics of singlet fission in organic semiconductors.
Here I present a study on dynamics of singlet fission in diluted pentacene films
and calculations which can explain the differences in quintet rates of dissociation between
molecular pairs. We found out that the rate of the dissociation can be influenced by the
degree of delocalization and charge-transfer character of the orbitals and excited states in
the two different packing structures considered.
My last study concerns understanding the behaviour of conjugated polymer electrodes
operating in aqueous solution. We used calculations of absorption of polarons to quantify
the depth of charge in polymer electrode. We used calculations of reaction potentials to
identify likely side reactions that could limit the efficiency in the presence of oxygen.