Enhanced Efficiency of Metamorphic Triple Junction Solar Cells for Space Applications

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. (paper)