  Absorbed sunlight in a solar cell produces electrons and holes. But, at the
open circuit condition, the carriers have no place to go. They build up in
density and, ideally, they emit external fluorescence that exactly balances the
incoming sunlight. Any additional non-radiative recombination impairs the
carrier density buildup, limiting the open-circuit voltage. At open-circuit,
efficient external fluorescence is an indicator of low internal optical losses.
Thus efficient external fluorescence is, counter-intuitively, a necessity for
approaching the Shockley-Queisser efficiency limit. A great Solar Cell also
needs to be a great Light Emitting Diode. Owing to the narrow escape cone for
light, efficient external emission requires repeated attempts, and demands an
internal luminescence efficiency >>90%.
