Characterizations of Radiation Damage in Multijunction Solar Cells Focused on Subcell Internal Luminescence Quantum Yields via Absolute Electroluminescence Measurements

Via absolute electroluminescence measurements, we characterized degradations of internal luminescence quantum yield or internal radiative efficiency ( $\eta _{{\rm int}i}$ ) in respective subcells in GaInP/GaAs/Ge triple-junction and GaInP/GaAs double-junction solar cells after irradiations of protons and electrons with different energy and fluence ( $\phi$ ). Compared with typical open-circuit-voltage characterizations, $\eta _{{\rm int}i}$ turned out to be a sensitive, high-dynamic-range, quantitative, and fair indicator of radiation damage, since it purely represents material-quality change due to radiation damage, independently from small differences in bandgap energy due to alloy composition fluctuations and in other cell structures. A detailed fluence-dependence study has shown that the data of $\eta _{{\rm int}i}$ versus $\phi$ in moderate and high $\phi$ regions are very similar and almost independent of subcell materials, while the difference in beginning-of-life qualities of GaInP and GaAs materials causes dominant difference in subcell sensitivity to the low-radiation damages.