  All-organic-based photovoltaic solar cells have attracted considerable
attention because of their low-cost processing and short energy payback time.
In such systems the primary dissociation of an optical excitation into a pair
of photocarriers has been recently shown to be extremely rapid and efficient,
but the physical reason for this remains unclear. Here, two-dimensional
photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy,
is used to probe the ultrafast coherent decay of photoexcitations into
charge-producing states in a polymer:fullerene based solar cell. The
two-dimensional photocurrent spectra are interpreted by introducing a
theoretical model for the description of the coupling of the electronic states
of the system to an external environment and to the applied laser fields. The
experimental data show no cross-peaks in the two-dimensional photocurrent
spectra, as predicted by the model for coherence times between the exciton and
the photocurrent producing states of 20\,fs or less.
