Triple-junction solar cell efficiencies above 32%: the promise and challenges of their application in high-conceniration-ratio PV systems

Results from Spectrolab-grown Ga_0.5In_0.5P/GaAs/Ge structures optimized for the AM1.5D spectrum are described along with progress toward developing next generation multijunction solar cells for high concentration ratios (X). The epitaxially-grown layers were processed into triple junction cells both at Spectrolab and NREL, and I-V tested vs. X. Cells were tested with efficiencies as high as 32.4% near 372 suns. The FF limited the performance with increasing X as a result of the increased role of the series resistance. The V_oc vs. X showed its log-linear dependence on I_sc over 1000 suns. Based on cell improvements for space applications, multijunction cells appear to be ideal candidates for high efficiency, cost effective, PV concentrator systems. Future development of new 1 eV materials for space cells, and further reduction in Ge wafer costs, promises to achieve cells with efficiencies >40% that cost $0.3/W or less at concentration levels between 300 to 500 suns