26.1% solar cell efficiency for Ge mechanically stacked under GaAs

We have processed a diffused Ge wafer into a Ge concentrator solar cell and mechanically stacked it under a GaAs cell fabricated by Varian. We measured this stack’s efficiency to be 26.1% for terrestrial air mass 1.5 direct (AM1.5D) conditions at a 285× concentration ratio. We showed that this efficiency is limited by optical absorption in the Varian GaAs cell caused by high 2–4 (1018) cm−3 substrate doping. We used a 2×1017 cm−3 doped GaAs filter to estimate the stack efficiency as 27.4%, which would be achieved with the same Varian GaAs cell formed on a lower doped substrate. We project efficiencies assuming the best properties reported for a GaAs device. This gives a 29.6% efficiency for an improved, planar Ge cell and 31.6% efficiency for a proposed point contact geometry for the Ge cell. The corresponding space (AM0) efficiencies at a 159× concentration ratio range from the 23.4% value we measured on the stack up to 28.4% projected for the point contact Ge place under the best GaAs cell. We showed that Ge cells give higher efficiencies than Si when stacked under GaAs.