Strong Internal and External Luminescence as Solar Cells Approach the Shockley–Queisser Limit

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