Near-field thermophotovoltaic energy conversion by excitation of magnetic polariton inside nanometric vacuum gaps with nanostructured Drude emitter and backside reflector

In this work, we study the effect of magnetic polariton (MP) on the conversion performance of a near-field thermophotovoltaic (TPV) system made of a nanostructured Drude grating emitter and a nanometer-thick photovoltaic (PV) cell with a lossless metal as the backside reflector. Near-field radiative heat flux is calculated through scattering matrix theory coupled with rigorous coupled-wave analysis based on fluctuational electrodynamics. It is theoretically shown that MP can be excited inside the nanometric vacuum gap and the 100-nm PV cell to spectrally enhance the heat flux above the cell bandgap. The lossless metal backing, which could practically serve as electrode for charge collection, is employed to reflect photons back into the cell for recycling. Contour plots of energy transmission coefficient and an inductor-capacitor circuit model along with magnetic field distribution are used to verify and understand the MP excitation in the near-field. Effects of PV cell thickness and vacuum gap distances, are systematically investigated on the near-field radiative heat flux, electrical power output, and energy conversion efficiency. It is found that the conversion efficiency increases from 0.7% to 12.2% when the semi-infinite In_0.18Ga_0.82Sb cell is replaced by an ultra-thin PV cell supported by a backside reflector with the emitter at 1000 K and the cell at 300 K separated by 200 nm gap. The conversion efficiency can be also further improved to 17.6% using a nanostructured Drude grating emitter over a planar emitter. This work would open up a new way to improve the performance of TPV devices with nanostructures with near-field thermal radiation.