Abstract：Metamorphic In$_{0.55}$Ga$_{0.45}$P/In$_{0.06}$Ga$_{0.94}$As/Ge triple-junction (3J-MM) solar cells are grown on Ge (100) substrates via metal organic chemical vapor deposition. Epi-structural analyses such as high resolution x-ray diffraction, photoluminence, cathodoluminescence and HRTEM are employed and the results show that the high crystal quality of 3J-MM solar cells is obtained with low threading dislocation density of graded buffer (an average value of 6.8$\times$10$^{4}$/cm$^{2})$. Benefitting from the optimized bandgap combination, under one sun, AM0 spectrum, 25$^{\circ}\!$C conditions, the conversion efficiency is achieved about 32%, 5% higher compared with the lattice-matched In$_{0.49}$Ga$_{0.51}$P/In$_{0.01}$Ga$_{0.99}$As/Ge triple junction (3J-LM) solar cell. Under 1-MeV electron irradiation test, the degradation of the EQE and $I$–$V$ characteristics of 3J-MM solar cells is at the same level as the 3J-LM solar cell. The end-of-life efficiency is $\sim$27.1%. Therefore, the metamorphic triple-junction solar cell may be a promising candidate for next-generation space multi-junction solar cells.
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