In thermophotovoltaic energy conversion, photovoltaic cells convert thermal radiation from a local emitter to electricity. The key is to find a way to exploit the great majority of low-energy thermal photons that would otherwise be unusable in a photovoltaic system. A highly reflective rear mirror can serendipitously boost the voltage, and regenerate the low-energy photons. Based on this concept, we demonstrate a 29.1% heat-to-electricity power conversion efficiency for thermophotovoltaics. We identify broadband mirrors as a major parameter to achieve thermophotovoltaic efficiency>50%. We show the challenges towards designing broadband mirrors and demonstrate an electromagnetic inverse-design approach based on Fresnel propagation. Finally, we demonstrate mirrors with >99% reflectivity over 3-octaves of frequency bandwidth. This new class of mirrors can make photonic heat engines based on thermophotovoltaics competitive with internal combustion engines.

Additionally, for record-breaking solar cells the external luminescence efficiency is decisive in determining the open-circuit voltage, Voc=Voc-ideal -kT|ln{ƞext}|. External luminescence efficiency, ƞext, has produced all the record solar cells in the past 10 years. On the other hand, for poor luminescent materials, the external luminescence is far less decisive than the internal luminescence efficiency, ƞint.. Most solar cells, including the important case of Silicon, luminesce very poorly. We express the open circuit voltage in terms of ƞint., that provides the correct Voc for poor luminescent materials. We show how the internal luminescence efficiency affects the open-circuit voltage. We identify the minimum internal luminescence needed to further improve open-circuit voltage through good external luminescence.




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