  Hot-carrier solar cells use the photon excess energy, that is, the energy
exceeding the absorber bandgap, to do additional work. These devices have the
potential to beat the upper limit for the photovoltaic power conversion
efficiency set by near-equilibrium thermodynamics. However, since their
conceptual inception in 1982, making this concept work under practical
conditions has proven a tremendous hurdle, mostly due to the fast
thermalization of photo-generated charges in typical semiconductor materials
like silicon. Here, we use noise spectroscopy in combination with numerical
modelling to show that common bulk heterojunction organic solar cells actually
work as hot-carrier devices. Due to static energetic disorder, thermalization
of photo-generated electrons and holes in the global density of states is slow
compared to the charge carrier lifetime, leading to thermal populations of
localized charge carriers that have an electronic temperature exceeding the
lattice temperature. Since charge extraction takes place in a high-lying,
narrow energy window around the transport energy, the latter takes the role of
an energy filter. For common disorder values, this leads to substantial
enhancements in open circuit voltage. We expect these results to inspire new
strategies to more efficiently convert solar energy into electricity.
