Efficient Multijunction Monolithic Cascade Solar Cells

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Abstract
Efficient two-junction monolithic cascade solar cells have been fabricated in two dif-ferent III-V materials systems. The cells are grown by metalorganic chemical vapor deposi-tion (MOCVD). Two-junction efficiencies as high as 25% (1-sun, AM2) are projected from the individual GaInAs (1.15 eV) and A1GaAs (1.72 to 1.75 eV) subcell efficiencies obtained in a cascade configuration. Factors influencing actual cascade efficiency and reproducibility are discussed. Metal interconnection of the AlGaAs and GaInAs subcells during post-growth processing yields two-terminal cascades with power conversion efficiencies (n) as high as 13.6% at 1-sun, airmass two (AM2) , and 11.1% at 1-sun, airmass zero (AMO). Three-terminal AlGaAs (1.72 eV)/GaInAs (1.15 eV) cascades have shown 11 of 12.6% (1-sun, AMO). Alternatively, insertion of a novel high-conductance junction between the cascade subcells during the MOCVD growth of the structure reduces the complexity of processing to that required of a single-junction cell. Voltage addition has been demonstrated, with n of 10.0% (1-sun, AM2) in the AlGaAs (1.72 eV)/GaInAs (1.15 eV) system. The A1GaAs (1.72 eV)/GaAs lattice-matched pair allows demonstration of proof-of-concept structures; tunnel-junction-connected A1GaAs (1.72 eV)/ GaAs cascades show n as high as 13.9% at 1-sun, AM2, and three-terminal, two-junction A1GaAs (1.72 eV)/GaAs (1.42 eV) systems as high as 23.0% at 1-sun, AM2.

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