The photovoltaic reciprocity theory relates the electroluminescence spectrum
of a solar cell under applied bias to the external photovoltaic quantum
efficiency of the device as measured at short circuit conditions. Its
derivation is based on detailed balance relations between local absorption and
emission rates in optically isotropic media with non-degenerate
quasi-equilibrium carrier distributions. In many cases, the dependence of
density and spatial variation of electronic and optical device states on the
point of operation is modest and the reciprocity relation holds. In
nanostructure-based photovoltaic devices exploiting confined modes, however,
the underlying assumptions are no longer justifiable. In the case of ultrathin
absorber solar cells, the modification of the electronic structure with applied
bias is significant due to the large variation of the built-in field.
Straightforward use of the external quantum efficiency as measured at short
circuit conditions in the photovoltaic reciprocity theory thus fails to
reproduce the electroluminescence spectrum at large forward bias voltage. This
failure is demonstrated here by numerical simulation of both spectral
quantities at normal incidence and emission for an ultrathin GaAs p-i-n solar
cell using an advanced quantum kinetic formalism based on non-equilibrium
Green's functions of coupled photons and charge carriers. While coinciding with
the semiclassical relations under the conditions of their validity, the theory
provides a consistent microscopic relationship between absorption, emission and
charge carrier transport in photovoltaic devices at arbitrary operating
conditions and for any shape of optical and electronic density of states.Comment: 5 pages, 4 figures, all figures replaced, minor changes and additions
to the tex
