The efficiency of a nano sized solar cell is investigated within the
framework of irreversible thermodynamics. The solar cell, being a heat engine, converts
part of the heat (energy) flow coming from the sun into work, delivered in
the form an electrical (particle) current. The conversion efficiency of such a
device is strongly determined by the degree of coupling between the two
flows. For an ideal nano sized solar cell, i.e. one for which non-radiative
recombination processes are ignored, the fluxes are perfectly coupled and the
efficiency at maximum power approaches the Curzon-Ahlborn result. When
non-radiative recombination processes are taken into account, the perfect
coupling condition is lost, and the efficiency is strongly reduced.
