  The performance of a near-field thermophotovoltaic system with a tandem-cell
structure composed of thin-film p-doped GaSb and n-doped InAs sub-cells on a
gold backside reflector is theoretically investigated. The temperatures of the
Ga-doped ZnO emitter and the tandem cells are set as 1800 K and 300 K, while
the thicknesses of GaSb and InAs sub-cells are considered as 1.5um and 0.5um,
respectively. Fluctuational electrodynamics along with the multilayer dyadic
Green's function is used to study near-field radiative heat transfer with the
consideration of photon chemical potential, whereas radiative recombination and
nonradiative Auger recombination are taken into account for evaluating the
electrical performance of the tandem cells. At a vacuum gap of 50 nm, it is
found that the tandem cells with independent charge collections achieve
electrical power output 468.8 kW/m2 at a conversion efficiency of 41%,
generating relatively 87% (or 21%) more power with about absolute 5% (or 10%)
higher efficiency than the single GaSb (or InAs) cell of the same 2-um
thickness. The physical mechanism of near-field spectral heat transfer is
elucidated with energy transmission coefficient, while the current-voltage
characteristics of sub-cells are discussed in detail. This work will pave the
way to enhance near-field thermophotovoltaic energy conversion performance with
tandem or multi-junction cells.
