Metamorphic GaAsP and InGaP Solar Cells on GaAs

We have investigated wide-bandgap, metamorphic GaAs
_1-
x
P
x
and In
y
Ga
_1-
y
P solar cells on GaAs as potential subcell materials for future 4-6 junction devices. We identified and characterized morphological defects in tensile GaAs
_1-
x
P
x
graded buffers that lead to a local reduction in carrier collection and a global increase in threading dislocation density (TDD). Through adjustments to the graded buffer structure, we minimized the formation of morphological defects and, hence, obtained TDDs ≈ 10
⁶
cm
^-2
for films with lattice mismatch ≤1.2%. Metamorphic In
y
Ga
_1-
y
P solar cells were grown on these optimized GaAs
_1-
x
P
x
graded buffers with bandgaps (
Eg
) as high as 2.07 eV and open-circuit voltages (
Voc
) as large as 1.49 V. Such high bandgap materials will be necessary to serve as the top subcell in future 4-6 junction devices. We have also shown that the relaxed GaAs
_1-
x
P
x
itself could act as an efficient lower subcell in a multijunction device. GaAs
_0.66
P
_0.34
single-junction solar cells with
Eg
= 1.83 eV were fabricated with
V
_oc
= 1.28 V. Taken together, we have demonstrated that GaAs
_1-
x
P
x
graded buffers are an appropriate platform for low-TDD, metamorphic GaAs
_1-
x
P
x
and In
y
Ga
_1-
y
P solar cells, covering a wide bandgap range.