  We investigate the electronic dynamics of model organic photovoltaic (OPV)
system consisting of polyphenylene vinylene (PPV) oligomers and [6,6]-phenyl
C61-butyric acid methylester (PCBM) blend using a mixed molecular
mechanics/quantum mechanics (MM/QM) approach. Using a heuristic model that
connects energy gap fluctuations to the average electronic couplings and
decoherence times, we provide an estimate of the state-to-state internal
conversion rates within the manifold of the lowest few electronic excitations.
We find that the lowest few excited states of a model interface are rapidly
mixed by C=C bond fluctuations such that the system can sample both
intermolecular charge-transfer and charge-separated electronic configurations
on a time scale of 20fs. Our simulations support an emerging picture of carrier
generation in OPV systems in which interfacial electronic states can rapidly
decay into charge-separated and current producing states via coupling to
vibronic degrees of freedom.
