The opto-electronic physics that broke the efficiency limit in solar cells

The internal physics of a solar cell changes as it approaches the fundamental Shockley-Queisser limit. Photonic considerations overtake electronic ones, as an intense internal and external luminescence requires careful photon management. Counter-intuitively, maximizing light extraction increases voltage and therefore efficiency. Until 2010 the one-sun, single-junction efficiency record was set by a GaAs solar cell with an efficiency of 26.4% and an open-circuit voltage V_OC= 1.03 V. Alta Devices recently improved the record with a GaAs cell that achieved 28.8% efficiency and V_OC=1.12V, demonstrating the importance of photon management. Even with the best materials, the highest efficiencies cannot be achieved unless the solar cell is also designed to also be a good light emitting diode (LED). The physics of light extraction will be necessary in the next generation of high-efficiency solar cells.