Metamorphic Ga_yIn_(1-y)P/Ga_(1-x)In_xAs Tandem Solar Cells for Space and for Terrestrial Concentrator Applications at C>1000 Suns

摘要

The use of Ga_(1-x)In_xAs instead of GaAs as a bottom solar cell in a Ga_yIn_(1-y)P/Ga_(1-x)In_xAs tandem structure increases the flexibility of choosing the optimum band-gap 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 Ga_yIn_(1-y)P/Ga_(1-x)In_xAs material combinations have been investigated for the first time and efficiencies up to 24.1 percent (AM0) and 27.0 percent (AM1.5 direct) have been reached under one-sun conditions. An efficiency of 30.0-31.3 percent was measured for a Ga_(0.35)In_(0.65)P/Ga_(0.83)In_(0.17)As 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 percent 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 percent.