  The thermodynamic limit of photovoltaic efficiency for a single-junction
solar cell can be readily predicted using the bandgap of the active light
absorbing material. Such an approach overlooks the energy loss due to
non-radiative electron-hole processes. We propose a practical ab initio
procedure to determine the maximum efficiency of a thin-film solar cell that
takes into account both radiative and non-radiative recombination. The required
input includes the frequency-dependent optical absorption coefficient, as well
as the capture cross-sections and equilibrium populations of point defects. For
kesterite-structured Cu$_2$ZnSnS$_4$, the radiative limit is reached for a film
thickness of around 2.6 micrometer, where the efficiency gain due to light
absorption is counterbalanced by losses due to the increase in recombination
current.
