Analysis of Ge Junctions for GaInP/GaAs/Ge Three-Junction Solar Cells

We study Ge solar cells with epitaxial GaInP windows for application as the third junction of GaInP/GaAs/Ge three-junction solar cells. We demonstrate Ge junctions with open-circuit voltages above 230 mV, fill factors above 65%, and internal quantum efficiencies of ∼ 90%. By varying separately the base and emitter contributions to the junction dark current, we deduce the factors limiting the performance of this device, and we project the improvement to the device performance that may be obtainable if key limiting factors such as the emitter surface-recombination velocity can be mitigated.