Metal-insulator-metal selective emitter design with an emissivity matching with gasb thermophotovolatic cell

A thermophotovoltaic (TPV) system converts infrared radiation to electrical energy by means of the PV effect. For a TPV system to work efficiently, the emission spectrum of its emitter must match the energy bandgap (Eg) of its TPV cell. An ideal selective emitter emits in-band photons, i.e., photons with energy, E > Eg (thus λ < λg). The ways of obtaining spectral selectivity-as proposed in the literature-include employing optical filters, surface nanostructures, and plasmonic metamaterials. Recent studies showed that plasmonic resonances of metal-insulator-metal structures can be exploited to obtain very high emissivity at resonant wavelengths. GaSb with its low bandgap (Eg = 0.72eV, λg = 1.72 µm) is an important receiver for modern TPV generators. Previous studies on developing selective emitters to optimally function with GaSb focused on having high emissivity at the λg of GaSb. On the other hand, GaSb has its highest quantum efficiency at lower wavelengths (within 1.2-1.6 µm) followed by a sharp decrease at wavelengths λ > 1.6 µm. Tungsten (W), with high emissivity in the visible and near infrared region is a suitable TPV emitter. Metamaterial structures consisting of W (base) and SiO2 (spacer) were also shown to be stable at high temperatures. In this work, a W-SiO2-W nano-patterned selective emitter is proposed by performing a finitedifference time-domain analysis. Grid search optimization is used to with an objective function constructed to maximize the average emissivity up to the cut-off wavelength of quantum efficiency of GaSb, and to minimize the emissivity after the bandgap of GaSb. The designed emitter has an average emissivity of 98.2% between 1.2-1.6 µm. At 1700 K, the ratio of in-band radiation to the total radiation emitted is calculated to be 72.3%. At the same temperature, this ratio is 27.9% for a standard SiC emitter.