Electron Irradiation Effects and Defects Analysis of the Inverted Metamorphic Four-Junction Solar Cells

The degradation of inverted metamorphic fourjunction (GaInP/GaAs/In _0.3 Ga _0.7 As/In _0.58 Ga _0.42 As, IMM4J) solar cells irradiated by 1-MeV electrons was investigated via their spectral responses and the characterization of their electrical properties. As in the case of traditional three-junction (TJ) GaInP/GaAs/Ge solar cells, the electrical properties (I _sc , V _oc , and P _max ) decrease with the logarithmic change of the electron fluence. The degradation of open-circuit voltage (V _oc ) is slightly more pronounced than that of Isc in IMM4J solar cells because of the sum rule for V _oc and the limit rule for I _sc . The spectral response analysis showed that an In _0.3 Ga _0.7 As subcell was the most damaged subcell in the irradiated IMM4J solar cell, but an In _0.58 Ga _0.42 As subcell had the lowest initial short-circuit current density (J _sc ),which was maintained throughout the irradiation test. We therefore focused on the In _0.58 Ga _0.42 As subcell. Deep-level transient spectroscopy (DLTS) experiments were realized to study emission and capture processes in two special full configurations of In _0.58 Ga _0.42 As and In _0.3 Ga _0.7 As subcells of the IMM4J solar cell. DLTS measurements reveal a dominant electron trap at 0.52 eV below the conduction band (E _c ) of In _0.58 Ga _0.42 As, and the electron trap gradually evolved into E _c -0.46eV and E _c -0.58eV after 1-MeV electron irradiation. Based on the first-principles calculation, E _c -0.46 eV and E _c -0.58 eV can be assigned as V _Ga ⁰ /V _Ga ^-1 and V _In ⁰ /V _In ^-1 , respectively. However, only one shallow level E _c -0.03eV was observed within the bandgap of In _0.3 Ga _0.7 As after irradiation with DLTS detection. We summarize the issues faced for the space application of IMM4J solar cells by comparing the spectral responses of IMM3J, IMM4J, and TJ solar cells. Finally, the optimization of the design and fabrication of IMM solar cells are proposed.