  In photovoltaics (PVs) and artificial photosynthetic systems based on
excitons, the question of how energy must be lost in overcoming the Coulomb
barrier, converting excitons to free charges, is fundamental, as it determines
the achievable open-circuit voltage or over-potential1-3. Here, using transient
and steady-state optical spectroscopy we study a model system, pentacene/C60,
where pentacene triplet excitons are dissociated to form charge transfer
states, which we show to be degenerate in energy with the triplet excitons. We
directly track these charge transfer states undergoing efficient long-range
separation to free charges within 50 ps, despite a significant Coulomb barrier
that we measure to be 220 meV. We model this endothermic charge separation to
be driven by entropic gain. Our results demonstrate that endothermic charge
separation can proceed rapidly with near unity efficiency, overcoming Coulomb
barriers greater than 200meV via a gain in entropy, thus minimising loss of
open-circuit voltage or over-potential. In the pentacene/C60 system this leads
here to an effective VOC loss of only 400meV with respect to the energy of the
triplet exciton. This suggests that excitonic photoconversion systems have no
fundamental disadvantage compared to systems which generate free changes
directly upon photoexcitation.
