Optical Refrigeration for Ultra-Efficient Photovoltaics
  Improving the conversion efficiency of solar energy to electricity is most important to mankind. For single-junction photovoltaic solar-cells, the Shockley-Queisser thermodynamic efficiency limit is extensively due to the heat dissipation, inherently accompanying the quantum process of electro-chemical potential generation. Concepts such as solar thermo-photovoltaics and thermo-photonics, have been suggested to harness this wasted heat, yet efficiencies exceeding the Shockley-Queisser limit have not been demonstrated due to the challenge of operating at high temperatures. Here, we present a highly efficient converter based on endothermic photoluminescence, which operates at relative low temperatures. The thermally induced blue-shifted photoluminescence of a low-bandgap absorber is coupled to a high-bandgap photovoltaic cell. The high absorber's photo-current and the high cell's voltage results in 69% maximal theoretical conversion efficiencies. We experimentally demonstrate tenfold thermal-enhancement of useful radiation for the high-bandgap cell and 107% enhancement in average photon energy. This paves the way for introducing disruptive-innovation in photovoltaics. 
Assaf Manor, Leopoldo L. Martin and Carmel Rotschild
physics.optics