  It is well known that performance of a thermophotovoltaic (TPV) device can be
enhanced if the vacuum gap between the thermal emitter and the TPV cell becomes
nanoscale due to the photon tunneling of evanescent waves. Having multiple
bandgaps, multi-junction TPV cells have received attention as an alternative
way to improve its performance by selectively absorbing the spectral radiation
in each subcell. In this work, we comprehensively analyze the optimized
near-field tandem TPV system consisting of the thin-ITO-covered tungsten
emitter (at 1500 K) and GaInAsSb/InAs monolithic interconnected tandem TPV cell
(at 300 K). We develop a simulation model by coupling the near-field radiation
solved by fluctuational electrodynamics and the diffusion-recombination-based
charge transport equations. The optimal configuration of the near-field tandem
TPV system obtained by the genetic algorithm achieves the electrical power
output of 8.41 W/cm$^2$ and the conversion efficiency of 35.6\% at the vacuum
gap of 100 nm. We show that two resonance modes (i.e., surface plasmon
polaritons supported by the ITO-vacuum interface and the confined waveguide
mode in the tandem TPV cell) greatly contribute to the enhanced performance of
the optimized system. We also show that the near-field tandem TPV system is
superior to the single-cell-based near-field TPV system in both power output
and conversion efficiency through loss analysis. Interestingly, the
optimization performed with the objective function of the conversion efficiency
leads to the current matching condition for the tandem TPV system regardless of
the vacuum gap distances.
