  A simple model is constructed to describe dissociation of charge transfer
excitons in bulk heterojunction solar cells, and its dependence on the physical
parameters of the system. In bulk heterojunction organic photovoltaics (OPVs),
exciton dissociation occurs almost exclusively at the interface between the
donor and acceptor, following one-electron initial excitation from the HOMO to
the LUMO levels of the donor, and charge transfer to the acceptor to make a
charge-transfer exciton. After exciton breakup, and neglecting the trapping of
individual carriers, the electron may undergo two processes for decay: one
process involves the electron and/or hole leaving the interface, and migrating
to the electrode. This is treated here as the electron moving on a set of
acceptor sites. The second loss process is radiationless decay following
recombination of the acceptor electron with the donor cation; this is treated
by adding a relaxation term. These two processes compete with one another. We
model both the exciton breakup and the subsequent electron motion. Results
depend on tunneling amplitude, energetics, disorder, Coulomb barriers, and
energy level matchups, particularly the so-called LUMO/LUMO offset.
