Shockley and Queisser showed that the limiting power conversion efficiency of a solar cell can be calculated using the principle of detailed balance. For a single junction solar cell this limit strongly depends on the semiconductor band gap and, under terrestrial conditions, it peaks at 33.6%. The detailed balance limit is entirely concerned with absorption and emission of photons, both of which can be controlled through nanophotonic engineering. As a result, there has been significant interest in applying nanophotonic concepts to approach or even exceed the Shockley-Queisser limit. In particular, there is significant interest in using nanophotonic concentration of light as a means to achieve efficiencies above the Shockley-Queisser limit, analogous to concentrator photovoltaics.<br/>Here we theoretically demonstrate that large absorption cross sections do not play a role in the efficiency enhancement, and on the contrary directivity in 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 non-radiative recombination, in which case both directivity and photon escape probability should be increased to achieve voltage enhancement.

Opportunities and limitations for nanoscale photovoltaics to surpass the Shockley-Queisser limit

253rd National Meeting (2017)

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