Paper presented at the 45th IEEE Photovoltaic Specialists Conference, June 10th - 15th, 2018, Waikoloa, Hawaii, USA

Abstract
For future 4-junction Ge based multi-junction solar cells, the current generated in the Ge sub-cell gets very important. We developed an efficient rear-side passivation stack, which results in minority carrier lifetimes (τeff) ≈ 200 μs and investigated its performance in an accelerated aging experiment (1 MeV electron irradiation). The aging caused a strong lifetime decrease down to τeff = 4 μs, whereas the carrier mobility stayed constant. These experimental values provide the basis for Beginning-of-Life and End-of-Life solar cell simulations, which show that the potential of the rear-side passivation for 3-junction solar-cell performance is limited, but very promising for 4-junction solar cells.