Effects of proton radiation on the InGaAs component cells of inverted metamorphic four-junction solar cells

In this study, 5-MeV proton radiation was incident on the InGaAs (1.0 eV) and InGaAs (0.7 eV) component cells of inverted metamorphic four-junction (IMM4J) solar cells. Light current–voltage (LIV), external quantum efficiency (EQE), and deep level transient spectroscopy (DLTS) were used to examine the performance degradation and the radiation defects. For the two InGaAs component cells, in case of LIV, open circuit voltage (V_oc), short-circuit current (I_sc), and the maximum output power (P_max) decreased linearly with the function of logarithmic change of the proton fluence. Compared to InGaAs (1.0 eV) component cells, for the InGaAs (0.7 eV) component cells, V_oc and P_max decreased considerably and I_sc decreased slightly. In EQE, an evident measurement artifact related to the luminescence coupling effect was detected in InGaAs (1.0 eV) before proton radiation, and it disappeared after radiation. This phenomenon was not observed in InGaAs (0.7 eV). In DLTS, a hole trap, H₀, was detected at E_v + 0.09 eV in InGaAs (1.0 eV), and three hole traps H₁, H₂, and H₃, were detected at E_v + 0.07 eV, E_v + 0.30 eV, and E_v + 0.45 eV, respectively, in InGaAs (0.7 eV). H₀ and H₁ were attributed to shallow traps introduced during radiation, and H₂ and H₃ were the initial defects due to the growth process; moreover, H₂ and H₃ were determined to be V_{Ga (0/-1)} and V_{Ga (-1/-2)} or probably V_{As (+3/+1)}.