  The importance of considering near-field effects on photon recycling and
spontaneous emission in a thermophotovoltaic device is investigated.
Fluctuational electrodynamics is used to calculate external luminescence from a
photovoltaic cell as a function of emitter type, vacuum gap thickness between
emitter and cell, and cell thickness. The observed changes in external
luminescence suggest strong modifications of photon recycling caused by the
presence of the emitter. Photon recycling for propagating modes is affected by
reflection at the vacuum-emitter interface and is substantially decreased by
the leakage towards the emitter through tunneling of frustrated modes. In
addition, spontaneous emission by the cell can be strongly enhanced by the
presence of an emitter supporting surface polariton modes. It follows that
using a radiative recombination model with a spatially uniform radiative
lifetime, even corrected by a photon recycling factor, is inappropriate.
Applying the principles of detailed balance, and accounting for non-radiative
recombination mechanisms, the impact of external luminescence enhancement in
the near field on thermophotovoltaic performance is investigated. It is shown
that unlike isolated cells, the external luminescence efficiency is not solely
dependent on cell quality, but significantly increases as the vacuum gap
thickness decreases below 400 nm for the case of an intrinsic silicon emitter.
In turn, the open-circuit voltage and power density benefit from this enhanced
external luminescence toward the emitter. This benefit is larger as cell
quality, characterized by the contribution of non-radiative recombination,
decreases.
