  Emitted photons stemming from the radiative recombination of electron-hole
pairs carry chemical potential in radiative energy converters. This luminescent
effect can substantially alter the local net photogeneration in near-field
thermophotovoltaic cells. Several assumptions involving the luminescent effect
are commonly made in modeling photovoltaic devices; in particular, the photon
chemical potential is assumed to be zero or a constant prescribed by the bias
voltage. The significance of photon chemical potential depends upon the emitter
temperature, the semiconductor properties, and the injection level. Hence,
these assumptions are questionable in thermophotovoltaic devices operating in
the near-field regime. In the present work, an iterative solver that combines
fluctuational electrodynamics with the drift-diffusion model is developed to
tackle the coupled photon and charge transport problem, enabling the
determination of the spatial profile of photon chemical potential beyond the
detailed balance approach. The difference between the results obtained by
allowing the photon chemical potential to vary spatially and by assuming a
constant value demonstrates the limitations of the conventional approaches.
This study is critically important for performance evaluation of near-field
thermophotovoltaic systems.
