Figure 1. Selective thermal emitters for thermophotovoltaics.

(a) The blackbody emission at 1,000 °C (1,273 K) normalized to its maximum and the emission of a selective TPV emitter at the same temperature normalized to the maximum of the blackbody emitter are presented. The selective emitter suppresses thermal emission throughout the infrared and simultaneously provides near blackbody emission at energies above the bandgap of the PV cell. The band-edge of a PV cell with 0.55 eV is presented by the dashed line indicating λ_PV. (b) Overlap of the spectral irradiance of a blackbody half-space with natural optical resonances. The blackbody spectrum for increasing temperatures between 750 and 1,500 K shows the peak lying in the near-infrared region for high temperatures (1,500 K) which is the spectral range for contemporary low-bandgap photovoltaics (blue shaded area). Note, these temperatures are necessary for high-efficiency energy conversion but are beyond the reach of conventional plasmonic building blocks for metamaterials because of their low melting point. On the other hand, thermal engineering approaches based on optical phonons are restricted to the mid-infrared spectrum and are difficult to move to the near-infrared range. Metamaterial principles extend the spectral range of bulk optical material resonances throughout the infrared.