18.2% (AM1.5) Efficient GaAs Solar Cell on Optical-Grade Polycrystalline Ge Substrate

In this work, we present GaAs material and device-structure optimization studies that have led to achieve a open-circuit voltage of approx.1 Volt and a best 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 our previously-reported best GaAs cell on similar substrates. We 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 (77K to 288K) is presented indicating that the spacer reduces the tunneling contribution to dark current.