  An indium arsenide photovoltaic cell with gold front contacts is designed for
use in a near-field thermophotovoltaic (NF-TPV) device consisting of
millimeter-size surfaces separated by a nanosize vacuum gap. The device
operates with a doped silicon radiator maintained at a temperature of 800 K.
The architecture of the photovoltaic cell, including the emitter and base
thicknesses, the doping level of the base, and the front contact grid
parameters, are optimized for maximizing NF-TPV power output. This is
accomplished by solving radiation and charge transport in the cell via
fluctuational electrodynamics and the minority charge carrier continuity
equations, in addition to accounting for the shading losses due to the front
contacts and additional series resistance losses introduced by the front
contacts and the substrate. The results reveal that these additional loss
mechanisms negatively affect NF-TPV performance in a non-negligible manner, and
that the maximum power output is a trade-off between shading losses and series
resistance losses introduced by the front contacts. For instance, when the cell
is optimized for a 1 x 1 mm2 device operating at a vacuum gap of 100 nm, the
losses introduced by the front contacts reduce the maximum power output by a
factor of ~ 2.5 compared to the idealized case when no front contact grid is
present. If the optimized grid for the 1 x 1 mm2 device is scaled up for a 5 x
5 mm2 device, the maximum power output is only increased by a factor of ~ 1.08
with respect to the 1 x 1 mm2 case despite an increase of the surface area by a
factor of 25. This work demonstrates that the photovoltaic cell in a NF-TPV
device must be designed not only for a specific radiator temperature, but also
for specific gap thickness and device surface area.
