Analysis of radiation hardness of rear-surface passivated germanium photovoltaic cells

In this study the radiation hardness of germanium solar cells is examined. The end-of-life (EOL) irradiation dose corresponds to 1 MeV electrons at a fluence of 1 × 10¹⁵ cm^-2. Different solar cell technologies are analyzed comprising state-of-the-art Ge solar cells with a highly doped p = 1 × 10¹⁷ cm^-3 base layer. Additionally, a set of rear-side passivated Ge solar cells with different base layer doping concentrations in the range of p = 1-4 × 10¹⁶ cm^-3 are investigated. These structures benefit from an increased current-density which results from significantly larger diffusions lengths. Thus, carriers which are generated through the indirect absorption transition deep in the bulk material are able to diffuse to the front-side pn-junction. Measurements are presented showing that the quantum efficiency related to the direct and indirect absorption transition of the Ge solar cell is differently affected by electron irradiation.