15.8%-efficient (1-sun, AM1.5G) GaAs solar cell on optical-grade polycrystalline Ge substrate

The authors present the first demonstration of a high efficiency GaAs solar cell on a commercially available, cast, optical-grade, polycrystalline Ge substrate. Under AM1.5G simulation, NREL-measured cell parameters include a V_oc of 0.9323 V, a J_sc of 24.87 mA/cm², a fill-factor of 0.6822, and an efficiency of 15.8%. This represents the highest 1-sun total-area efficiency for a polycrystalline solar cell in any compound semiconductor even though it is only an initial attempt at the demonstration of a poly GaAs cell with efficiencies comparable to those obtained in state-of-the-art CdTe and CIS material systems. The cell area is about 1 cm² and was grown by organometallic vapor phase epitaxy. The poly GaAs p⁺-n junction has been characterized by dark I-V measurements. The choice of junction polarity (p⁺-n vs. n⁺-p) for the growth of GaAs solar cell on poly-Ge substrates is discussed based on the reduction of dark currents originating from the grain boundaries. Introduction of an undoped spacer in the p⁺ -n junctions is shown to significantly improve the V_oc and fill-factor values in small-area (0.174 cm²) cells where series resistance is not limiting the fill factor, Spectral-response measurements indicate further improvement possible in the red response of the poly GaAs solar cells, I-V data indicate series resistance is presently limiting the efficiency of 1-cm²-area cells. Further development is likely to lead to 1-sun efficiency approaching over 21 percent and concentrator (500 suns) efficiencies approaching 24 percent in the near-term