Hot carrier organic solar cells

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, no experimental realization that works under normal operational conditions has been demonstrated, mostly due to the fast thermalization of photo-generated charges in typical semiconductor materials. 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 enhancements in open circuit voltage of up to similar to 0.2 V. We show that this enhancement can be understood as a thermovoltage that is proportional to the temperature difference between the lattice and the charge populations and that comes on top of the near-equilibrium quasi-Fermi level splitting. Slow thermalization of photogenerated charge carriers in organic solar cells leads to an electronic temperature that is much larger than that of the lattice and to significantly enhanced open-circuit voltages.