In this thesis, we model electronic states, charge transfer and charge transport in organic semiconductors
(OSCs). We are interested in the effect of chemical structure, molecular packing and different
types of disorder on the charge transport in these materials, which is an important factor determining
the efficiencies of devices made with OSCs. This is an intrinsically multiscale problem, and we
use computational methods than span length and time-scales.
We investigate the effect of molecular packing and crystal structure on charge carrier mobility in
molecular crystals. We also model the electron mobility in disordered small molecules containing
different defects to find the effect of these defects on the performance of devices. We perform
electronic structure calculations to investigate the HOMO and LUMO levels in the presence of
different defects, as well as to generate absorption, photoluminescence and infrared spectra to
compare to experimental spectra.
We develop a computationally efficient method for modelling polarons in organic semiconductors
within a tight-binding framework. We apply the method to large assemblies of fullerene molecules
generated with coarse-grained molecular dynamics. The method allows us to explore the effect
of energetic and configurational disorder, as well as polaron formation, on the density of states
and size of charge states in these systems.
We further develop a method for calculating transfer integrals between molecules or molecular
fragments, based on performing calculations of molecular orbitals in a counterpoise basis set.
We apply it to calculating intramolecular transfer integrals and transfer integrals between donor
and acceptor molecules at interfaces in organic photovoltaics, and between dopant and host
molecules.