Revisiting tantalum based nanostructures for efficient harvesting of solar radiation in STPV systems

The Shockley-Queisser limit places an upper bound on the efficiency of conventional semiconductor photovoltaic (PV) devices. A solar thermophotovoltaic (STPV) system (consisting of an ultra-broadband absorber, spectrally selective emitter, and low bandgap PV cell) can provide an efficient alternative to overcome this theoretical limit over efficiency. However, the stringent design requirements of absorber and emitter (like thermal stability and angular and polarization insensitivity) require a careful selection of appropriate material, meticulous engineering, and optimization of nanostructures to ensure high performance of the overall STPV system. In this work, we present a Tantalum (Ta) based nanostructured ultra-broadband absorber and thermal emitter for an SPTV system. Tantalum, being a refractory metal and having high thermal stability due to its high melting point, is an ideal candidate for the design of both absorber and the spectrally selective emitter. Both the presented absorber and emitter employ cross-shaped Ta nanostructures (as a top layer) due to their high spectral selectivity and polarization insensitivity. Our numerical investigations (well supported by a description of underlying mechanism) reveal that our carefully designed and optimized devices (both the absorber and emitter) outperform previously reported ones. The proposed absorber exhibits high absorptance for blackbody radiation at 5778 K and air mass (AM) 1.5, while the proposed emitter, due to its high spectral selectivity, outperforms previously reported demonstrations in terms of PV cell efficiency (41.8%) for Indium Gallium Arsenide Antimonide (InGaAsSb with 0.55 eV bandgap). Moreover, it is shown that by simple manipulation of length, we can steer the maxima of the emittance curve for different wavelengths as well, which is helpful in designing the emitter for other PV cells with different bandgaps. Furthermore, it is shown that dielectric coatings can be employed on designs to achieve thermal stability without compromising efficiency.