Effect of luminescence coupling between InGaP and GaAs subcells to external quantum efficiency in triple-junction solar cells

Luminescence coupling, which is reabsorption of recombination radiation in multi-junction solar cell, induces artifact response on external quantum efficiency (EQE). We have been investigated the relation between luminescence coupling and EQE in triple-junction solar cells (3J) for space applications. In this study, we focused the luminescence coupling induced by the emission from InGaP-top cells. LED light sources were introduced as bias light. The strength of the luminescence coupling between InGaP-top and GaAs-middle cells in newer-design 3J cells is higher than that of older-design 3J cells. The strength of "series coupling", which is induced in Ge-bottom cells by the coupling between InGaP-top and GaAs-middle cells, is about an order of magnitude lower than the strength of coupling between InGaP-top and GaAs-middle cells. In 3J cells that have the high intensity of emission from InGaP-top cell, the EQE of Ge-bottom cells only with the activation of InGaP-top cells does not need artifact correction.