Voltage Matching, Étendue, and Ratchet Steps in Advanced-Concept Solar Cells

Solar cells with parallel optical transitions promise a 10% boost in conversion efficiency over the Shockley-Queisser single-junction limit. This study classifies various concepts according to their internal physical processes, and shows that voltage matching through ``ratchet steps'' of carrier relaxation may be crucial to realizing efficiency benefits. The ideal exothermic ratchet-step size is shown to depend logarithmically on the etendue ratio for emission and absorption, in addition to the luminescence extraction efficiency. While for some cells infinite Auger interaction rates lead to endothermic optimal ratchet configurations, more realistic rates lead to an exothermic arrangement.