In 1961, the Nobel Prize-winning co-inventor of the transistor, William Shockley, and his colleague, Hans Quisser, identified the theoretical maximum conversion efficiency of a solar cell. They called this the “detailed balance limit,” because maximum efficiency would occur when photons absorbed from and emitted to the sun were in the proper balance. This theoretical limit has been beyond the limits of practicality until recently. Advances in fabrication and computer simulation now provide the tools for Shockley and Quisser’s predictions (the “SQ Limit”) to become relevant in practice.
New research from the California Institute of Technology and the Foundation for Fundamental Research on Matter shows how the highest efficiency cells can now reach their ultimate limit. Gallium-Arsenide (GaAs) solar cells currently hold the record for the most efficient energy conversion for any single material. How it does so is no mystery – the fundamental energy transition (the “bandgap”) of this material is close to the ideal value for our sun’s spectrum and the GaAs material contains very few defects. While new solar cells produced by the startup company Alta Devices have increased the conversion efficiency of GaAs to 29.1%, this is still far from the predicted maximum of >33%. This improvement would provide a 10 % boost to total system power for the same solar panel area, reducing costs and increasing effectiveness of solar concentrators. The key may be using highly reflective flat and parabolic mirrors to properly guide light into and out of the solar cells. By restricting light’s entry and exit to a small angle, researchers predict that efficiency should approach its theoretical limit. While their first attempt at fabricating the required structures confirmed the principle, they have not yet produced complete cells by this method. Future work by their or other groups may now perfect design and implementation of these angle-restricted cells, approaching finally the limit predicted over 50 years ago.
Article Title: Highly efficient GaAs solar cells by limiting light emission angle
Authors: Emily D Kosten, Jackson H Atwater, James Parsons, Albert Polman, and Harry A Atwater
Citation: Emily D Kosten, Jackson H Atwater, James Parsons, Albert Polman, and Harry A Atwater, Light: Science & Applications (2013) 2, e45