  The conventional notion for achieving high efficiency in thermophotovoltaics
(TPVs) is to use a monochromatic emission at a photon energy corresponding to
the band gap of the cell. Here, we prove theoretically that such a notion is
only accurate under idealized conditions, and further show that when
non-radiative recombination is taken into account, efficiency improvement can
be achieved by broadening the emission spectrum, due to an enhancement in the
open-circuit voltage. Broadening the emission spectrum also improves the
electrical power density, by increasing the short-circuit current. To
practically illustrate these findings, we focus on surface polariton-mediated
near-field TPVs. We propose a versatile design strategy for broadening the
emission spectrum via stacking of multiple plasmonic thin film layers. As an
example, we consider a realistic ITO/InAs TPV, and predict a conversion
efficiency of $50\%$ simultaneously with a power density of nearly $80$
W$/\mathrm{cm}^2$ at a $1300$ K emitter temperature. The performance of our
proposed system far exceeds previous works in similar systems using a single
plasmonic layer emitter.
