High-Efficiency GaAs Solar Cells on mm and sub-mm Grain-Size Polycrystalline Ge Substrates

GaAs material and device structure optimization studies on optical-grade, millimeter-and-less grain-size polycrystalline Ge substrates are presented. We discuss the growth of high-quality epitaxial layers across various crystalline orientations of a polycrystalline substrate; this is important for obtaining high-performance solar cells. The GaAs solar cell on n-type poly-Ge substrate is a p-on-ntype, with an undoped spacer between the p-emitter and the n-base. An experimental study of dark currents in these junctions, with and without the spacer, as a function of temperatuer (77K to 288K) is presented; this study suggests that the spacer reduces the tunneling contribution to dark current. In addition, we describe device-structure optimization studies that have led us to achieve anopen-circuit voltage (Voc) exceeding 1 Volt and an AM 1.5 efficiency of .apprx. 19% for a 4-cm2-area GaAs cell on sub-mm grain-size poly-Ge. We have also observed an efficiency .apprx. 21% for a 0.25-cm2-area cell on similar small-grain poly-Ge substrates.