20% (AM1.5) efficiency GaAs solar cells on sub-mm grain-size poly-Ge and its transition to low-cost substrates

Some of the key material and device issues related to the development of GaAs solar cells on poly-Ge substrates, including the dark-current reduction mechanism with an undoped spacer at the p/sup +/-n depletion layer, are discussed. Device-structure optimization studies that have led the authors to achieve an AM1.5 efficiency of /spl sim/20% for a 4 cm/sup 2/ area GaAs cell on sub-mm grain-size poly-Ge and an efficiency of /spl sim/21% for a 0.25 cm/sup 2/ area cell are presented. This successful demonstration of high-efficiency GaAs cells on sub-mm grain-size poly-Ge substrates have motivated them to consider the development of high-quality GaAs materials on significantly lower-cost substrates such as glass and moly foils. To date, they have achieved a best minority-carrier lifetime of 0.41 nsec in an n-GaAs thin-film on moly. The role of Group-VI dopant in the possible passivation of grain-boundaries in poly-GaAs is discussed. Development of PV-quality GaAs material, with minority-carrier lifetime of 1 to 2 nsec, on low-cost moly foils can significantly impact both the terrestrial and the space PV applications.