  We evaluate near-field thermophotovoltaic (TPV) energy conversion systems
focusing in particular on their open-circuit voltage (Voc). Unlike previous
analyses based largely on numerical simulations with fluctuational
electrodynamics, here, we develop an analytic model that captures the physics
of near-field TPV systems and can predict their performance metrics. Using our
model, we identify two important opportunities of TPV systems operating in the
near-field. First, we show analytically that enhancement of radiative
recombination is a natural consequence of operating in the near-field. Second,
we note that, owing to photon recycling and minimal radiation leakage in
near-field operation, the PV cell used in near-field TPV systems can be much
thinner compared to those used in solar PV systems. Since non-radiative
recombination is a volumetric effect, use of a thinner cell reduces
non-radiative losses per unit area. The combination of these two opportunities
leads to increasingly large values of Voc as the TPV vacuum gap decreases.
Hence, although operation in the near-field was previously perceived to be
beneficial for electrical power density enhancement, here, we emphasize that
thin-film near-field TPVs are also significantly advantageous in terms of Voc
and consequently conversion efficiency as well as power density. We provide
numerical results for an InAs-based thin-film TPV that exhibits efficiency >
50% at an emitter temperature as low as 1100 K.
