High-Efficiency GaAs Solar Cells on Polycrystalline Ge Substrates: Final Report

This report describes work performed by Research Triangle Institute (RTI) under this subcontract. The objective of this program was to further improve on the performance of GaAs solar cells on poly-Ge substrates. Material and device issues related to the development of GaAs solar cells on poly-Ge substrates were addressed. Key material physics issues include the use of Group-VI dopant,specifically Se, to passivate the grain boundaries in the n-GaAs base to achieve large Jsc values. Device-structure optimization studies led RTI researchers to achieve an AM1.5 efficiency of~20% for a 4-cm2 GaAs cell and an efficiency of~21% for a 0.25-cm2 cell on sub-mm grain-size poly-Ge substrates. Key device physics issues include understanding of the dark-current reduction mechanism withan undoped spacer at the p+-n depletion layer. The successful demonstration of high-efficiency GaAs cells on sub-mm grain-size poly-Ge substrates motivated researchers to develop high-quality GaAs materials on lower-cost substrates such as glass and moly foils. They achieved a best minority-carrier lifetime of 0.41 ns in an n-GaAs thin-film on moly. The role of Group VI dopant in the possiblepasssivation of grain boundaries has been studied in GaAs films on moly foils.