Investigation of Radiation Hardness of Germanium Photovoltaic Cells

This contribution discusses the radiation hardness of germanium (Ge) photovoltaic cells under space conditions corresponding to an irradiation dose of 1-MeV 1 × 10¹⁵ cm^-2 electrons. For this purpose, different germanium photovoltaic cell technologies based on p-type substrates are analyzed. The investigation comprises standard Ge photovoltaic cells with a substrate doping concentration of N_A = 1 × 10¹⁷ cm^-3, as well as recently developed Ge photovoltaic devices with extended quantum efficiency in the IR (λ > nm), which are based on low-doped substrates with N_A = 1-4 × 10¹⁶ cm^-3 in conjunction with an electrical rear-side passivation layer. In comparison to the standard Ge photovoltaic cell design, higher germanium subcell current densities of up to 14% at begin of life and up to 13% after electron exposure are achieved. This is particularly advantageous for high-efficiency metamorphic GaInP/GaInAs/Ge triple-junction solar cell structures, where the Ge subcell may become current limiting. Measurement results are presented, showing that the quantum efficiency related to the direct and indirect absorption transitions is differently affected by the cosmic electron particle irradiation. Furthermore, the impact of irradiation on free carrier absorption processes is discussed.