Opportunities and Limitations for Nanophotonic Structures To Exceed the Shockley-Queisser Limit.

Nanophotonic engineering holds great promise for photovoltaics, with several recently proposed approaches that have enabled efficiencies close to the Shockley-Queisser limit. Here, we theoretically demonstrate that suitably designed nanophotonic structures may be able to surpass the 1 sun Shockley-Queisser limit by utilizing tailored directivity of the scattering response of nanoparticles. We show that large absorption cross sections do not play a significant role in the efficiency enhancement, and on the contrary, directivity enhancement constitutes the nanoscale equivalent to concentration in macroscopic photovoltaic systems. Based on this principle, we discuss fundamental limits to the efficiency based on directivity bounds and a number of approaches to get close to these limits. We also highlight that, in practice, achieving efficiencies above the Shockley-Queisser limit is strongly hindered by whether high short-circuit currents can be maintained. Finally, we discuss how our results are affected by the presence of significant nonradiative recombination, in which case both directivity and photon escape probability should be increased to achieve voltage enhancement.