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⁺-n depletion layer, are discussed. Device-structure optimization studies that have led us to achieve an AM1.5 efficiency of approximately 20% for a 4-cm²-area GaAs cell on sub-mm grain-size poly-Ge and an efficiency of approximately 21% for a 0.25-cm²-area cell are presented. This successful demonstration of high-efficiency GaAs cells on sub-mm grain-size poly-Ge substrates have motivated us to consider the development of high-quality GaAs materials on significantly lower-cost substrates such as glass and moly foils. To date, we 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.