  Various spectral control techniques can be applied to improve the performance
of a thermophotovoltaic (TPV) system. For example, a back surface reflector
(BSR) can improve the performance of TPV systems. A conventional metal BSR
structure enhances the photogeneration rate by increasing the absorption
probability of photons via back surface reflection, affording a second chance
for absorption. However, surface passivation and external luminescence effects
introduced by BSR structures have been previously ignored, which potentially
decreases the performance of TPV systems. Recently, a back gapped reflector
(BGR) structure was proposed to greatly improve the performance of far-field
TPV systems by reducing reflection loss at the semiconductor-metal interface.
In the present work, the performance improvement on a thin-film, near-field
InAs TPV system with a BGR is investigated, comparing its performance to that
with a conventional metal BSR. Surface passivation conditions are also
investigated to further improve the performance of TPV systems with back
reflectors. The output power and efficiency are calculated using an iterative
model combining fluctuational electrodynamics and the full drift-diffusion
model. For the well-passivated condition, when the BSR is replaced by the BGR,
the calculated conversion efficiency was improved from 16.4% to 21% and the
output power was increased by 10% for the near-field regime. Finally, the
reflection loss and external luminescence loss are analyzed to explain the
performance improvement.
