  We present an exciton/lattice model of the electronic dynamics of primary
photoexcitations in a polymeric semiconductor heterojunction which includes
both polymer pi-stacking, energetic disorder, and phonon relaxation. Results
from our model are consistent with a wide range of recent experimental evidence
that excitons decay directly to well-defined polarons on a sub-100 fs
timescale, which is substantially faster than exciton relaxation processes.
Averaging over multiple samples, we find that as the interfacial offset is
increased, a substantial fraction of the density of electronic states in the
energy region about the initial exciton carries significant charge-transfer
character with two charges separated in the outer regions of the model lattice.
The results indicate a slight increase in the density of such current-producing
states if the region close to the interface is more disordered. However, since
their density of states overlaps the excitation line-shape of the primary
exciton, we show that it is possible that the exciton can decay directly into
current-producing states via tunneling on an ultrafast time-scale. We find this
process to be independent of the location of energetic disorder in the system,
and hence we expect exciton fission via resonant tunnelling to be a ubiquitous
feature of these systems.
