Broadening near-field emission for performance enhancement in thermophotovoltaics

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.