Four-layer metallodielectric emitter for spectrally selective near-field radiative transfer in nano-gap thermophotovoltaics

A four-layer metallodielectric planar thermal emitter is designed and optimized to tailor the spectral near-field radiative flux in a nano-gap thermophotovoltaic (TPV) system. The structure contains an ultra-thin refractory metallic layer (tungsten) sandwiched by two dielectric (hafnia) layers sitting on a metallic substrate. The theoretical calculation showed that the lossy metallic thin-layer acted as a perfect absorber that stimulated strong thermal emission, with its frequency and amplitude further modulated by the Fabry-Perot resonances in the dielectric layer. The spectral peak of near-field radiative flux can be manipulated by layer thickness to suit different cell bandgaps and emitter temperatures. The combination of optimized emitter and cell layer thickness results in selectively high near-field radiative flux above the bandgap, reaching a conversion efficiency of 50.2% (at radiative limit) with emitter temperature at 1600 K and 200 nm vacuum gap, surpassing or comparing with the efficiency and output power of state-of-the-art plasmonic and hyperbolic emitter designs. By tailoring the radiative heat flux mainly in the propagating and frustrated components, the design also circumvents possible electrical losses due to enhanced surface recombination when the radiative transfer is dominated by surface modes. This study offers a straightforward strategy to manipulate the near-field radiative transfer with great flexibility by a few-layer planar emitter and emphasizes the different consideration for designing a nano-gap TPV emitter from far-field PV/TPV emitter designs.