Effects of neutron radiation on the key component cells of upright metamorphic four-junction (UMM4J) solar cells

In this study, the bandgap of the top three junctions of the UMM4J solar cell was optimized based on the spectral matching principle. Three materials, namely AlGaInP (1.957 eV), AlInGaAs (1.495 eV), and InGaAs (1.171 eV), were selected as the materials for the top three sub-cells. The component cells were fabricated using metal organic chemical vapor deposition (MOCVD) to verify their radiation resistance performance. Neutron irradiation with an equivalent energy of 1 MeV and a fluence of 6 × 10¹³ cm⁻² was used to evaluate the end-of-life performance. The light current-voltage (LIV) and external quantum efficiency (EQE) results reveal that the AlGaInP (1.957 eV) component cells demonstrate superior radiation resistance performance, while the InGaAs (1.171 eV) component cells exhibit the poorest radiation resistance performance. After ensuring a sufficient margin of degradation for the InGaAs (1.171 eV) sub-cells, the AlInGaAs (1.495 eV) component cells serve as the current-limiting junction before and after radiation, resulting in an enhanced radiation hardness of the UMM4J solar cell. Furthermore, deep level transient spectroscopy (DLTS) was employed to directly assess the radiation-induced defects of the three-component cells. The degradation of solar cell performance primarily stems from the defects that induce Shockley-Read-Hall (SRH) recombination.