  Multijunction solar cell design is guided by both the theoretical optimal
bandgap combination as well as the realistic limitations to materials with
these bandgaps. For instance, triple-junction III-V multijunction solar cells
commonly use GaAs as a middle cell because of its near-perfect material
quality, despite its bandgap being higher than optimal for the global spectrum.
Here, we modify the GaAs bandgap using thick GaInAs/GaAsP strain-balanced
quantum well (QW) solar cells with excellent voltage and absorption. These
high-performance QWs are incorporated into a triple-junction inverted
metamorphic multijunction device consisting of a GaInP top cell, GaInAs/GaAsP
QW middle cell, and lattice-mismatched GaInAs bottom cell, each of which has
been highly optimized. We demonstrate triple-junction efficiencies of 39.5% and
34.2% under the global and space spectra, respectively, which are higher than
previous record six-junction devices.
