18.2% (AM1.5) efficient GaAs solar cell on optical-grade polycrystalline Ge substrate

In this work, the authors present GaAs material and device-structure optimization studies that have led to achieve a open-circuit voltage of ~1 volt and a best solar cell efficiency of 18.2% under AM1.5G illumination, for a 4 cm² area GaAs cell on commercially-available, cast, optical-grade polycrystalline Ge substrate. This V_∞ is almost 70 mV higher than on their previously-reported best GaAs cell on similar substrates. They discuss the growth of high-quality GaAs-AlGaAs layers, across the various crystalline orientations of a polycrystalline Ge substrate, important for obtaining good device performance. Optimization studies of the minority-carrier properties of GaAs layers on poly-Ge substrates have revealed that lifetime-spread across various grains can be reduced through the use of lower doping for the Al_0.8Ga_0.2As confinement layers. The cell-structure optimization procedures for improved V_∞ and cell efficiency, include the use of thinner emitters, a spacer layer near the p⁺-n junction and an improved window layer. An experimental study of dark currents in these junctions, with and without the spacer, as a function of temperature (77 K to 288 K) is presented indicating that the spacer reduces the tunneling contribution to dark current