Metamorphic GayIn1−yP/Ga1−xInxAs tandem solar cells for space and for terrestrial concentrator applications at C > 1000 suns

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Abstract
The use of Ga1−xInxAs instead of GaAs as a bottom solar cell in a GayIn1−yP/Ga1−xInxAs tandem structure increases the flexibility of choosing the optimum bandgap combination of materials for a multijunction solar cell. Higher theoretical efficiencies are calculated and different cell concepts are suggested for space and terrestrial concentrator applications. Various GayIn1−yP/Ga1−xInxAs material combinations have been investigated for the first time and efficiencies up to 24·1% (AM0) and 27·0% (AM1·5 direct) have been reached under one‐sun conditions. An efficiency of 30·0–31·3% was measured for a Ga0·35In0·65P/Ga0·83In0·17As tandem concentrator cell with prismatic cover at 300 suns. The top and bottom cell layers of this structure are grown lattice‐matched to each other, but a large mismatch is introduced at the interface to the GaAs substrate. This cell structure is well suited for the use in next‐generation terrestrial concentrators working at high concentration ratios. For the first time a cell efficiency up to 29–30% has been measured at concentration levels up to 1300 suns. A small prototype concentrator with Fresnel lenses and four tandem solar cells working at C = 120 has been constructed, with an outdoor efficiency of 23%. Copyright © 2001 John Wiley & Sons, Ltd.