  Charge separation is a critical process for achieving high efficiencies in
organic photovoltaic cells. The initial tightly bound excitonic electron-hole
pair has to dissociate fast enough in order to avoid photocurrent generation
and thus power conversion efficiency loss via geminate recombination. Such
process takes place assisted by transitional states that lie between the
initial exciton and the free charge state. Due to spin conservation rules these
intermediate charge transfer states typically have singlet character. Here we
propose a donor-acceptor model for a generic organic photovoltaic cell in which
the process of charge separation is modulated by a magnetic field which tunes
the energy levels. The impact of a magnetic field is to intensify the
generation of charge transfer states with triplet character via inter-system
crossing. As the ground state of the system has singlet character, triplet
states are recombination-protected, thus leading to a higher probability of
successful charge separation. Using the open quantum systems formalism we
demonstrate that not only the population of triplet charge transfer states
grows in the presence of a magnetic field, but also how the power outcome of an
organic photovoltaic cell is in that way increased.
