Photon extraction: the key physics for approaching solar cell efficiency limits

Theoretical efficiency limits are useful primarily because they provide a means for selecting which technologies to pursue, and they are a driving force for further progress. Yet implicit in such a process is the assumption that the upper limit provides a realistic estimate of potential performance. Real systems will never be perfect, but small deviations in material quality or optical design should yield only small deviations in performance. Shockley-Queisser efficiencies are not robust to small deviations. Although they provide a simple calculational tool, they obscure important internal dynamics. We examine these dynamics, resulting in a surprising conclusion: instead of considering external emission as a loss mechanism, it should actually be designed for. A solar cell must have almost perfect photon extraction, or it will fall far short of the Shockley-Queisser efficiency limit.