Detailed balance efficiency limits with quasi-Fermi level variations

A central assumption in detailed balance efficiency limit calculations has been that the light generated carriers are collected by drift transport processes and have an infinite mobility, giving rise to constant quasi-Fermi levels (QFL's) across the solar cell. However, recent experimental and theoretical results for quantum well (QW) devices indicate that the QFL's need not be constant across the device. It is shown in this paper that transport mechanisms which cause a variation in the difference between the electron and hole QFL's give an increase in the limiting efficiency compared to previous detailed balance calculations. Further, QW solar cells which employ hot carrier transport across a well will have an efficiency limit in excess of a tandem solar cell while using the same number of semiconductor materials.