Space Degradation of Triple Junction Solar Cells at Low Temperatures: II ‐electron Irradiation

We investigated the behavior of electron-irradiated 3G28 InGaP/GaAs/Ge triple junction solar cells and their component (top, middle and bottom) cells at low temperatures from 100 to 300 K and low illumination intensity. Significant degradation of their performances has been observed. We found that it is induced by an excess current associated with tunneling due to the presence of the radiation-induced defects introduced in the junctions of each sub-cell. The amount of tunneling current, hence of the effect of degradation, is significantly higher in the bottom sub-cell than in the top and middle sub-cells. This particular cell is formed by diffusion of group V atoms into p-Ge substrate, which leads to relatively inhomogeneous doping profile and thus tunneling current distribution compared to well controlled top and middle sub-cells deposited by epitaxy. The degradation associated with tunneling recovers significantly at 300 K in the bottom cell.