Study of W/HfO2 grating selective thermal emitters for thermophotovoltaic applications.

This paper explores the performance potential of gratings based on tungsten/hafnia (W/HfO2) stacks for thermophotovoltaic thermal emitters via numerical simulations. Structures consisting of a W grating over a HfO2 spacer layer and a W substrate are analyzed over a range of geometries. For shallow gratings (W grating thickness much smaller than the grating pitch), an emittance of 99.9% can be achieved for transverse magnetic (TM) polarization, but the transverse electric (TE) performance is appreciably lower. For deep gratings (W grating thickness on the order of the grating pitch), peak emittances of 97.8% and 99.7% for TE and TM polarizations, respectively, are achieved. We find that both surface plasmon polaritons and magnetic polaritons play a crucial role in shaping the emittance for TM radiation. On the other hand, cavity resonances are responsible for the almost perfect emittance in the case of TE polarization. These results suggest that by introducing an HfO2 layer it is possible to reach high emittance for operating temperatures that match the absorption characteristics of GaSb and InGaAs photovoltaic cells.