Evaluation of subcell power conversion efficiencies of radiation-damaged triple-junction solar cells using photoluminescence decays

We characterize the radiation-induced damage of InGaP/GaAs/Ge solar cells for various proton irradiation energies and fluences using conventional current-voltage (I-V) measurements, external quantum efficiency, and a noncontact time-resolved photoluminescence (PL) technique. From the I-V curves, we obtain the conversion efficiency of the entire device. The external quantum efficiency showed that the short-circuit current is only determined by the top InGaP subcell. To obtain accurate information about the point of maximum power, a new PL technique is introduced. The PL time decays of the InGaP and GaAs subcells are measured to obtain the characteristic decay time constants of carrier separation and recombination. We empirically verify that the time-resolved PL method can be used to predict the electrical conversion efficiency of the subcells. We find that the limiting subcell at the point of maximum power is different from that for short-circuit current. Radiation damage in unexpected regions of the device is revealed using this optical method.