Comparative Study of ∼2.05 eV Lattice-Matched and Metamorphic (Al)GaInP Solar Cells Grown by MOCVD

This work investigates two conceptually different approaches, both based on the AlGaInP alloy family, to produce metal-organic chemical vapor deposition grown 2.05 eV direct bandgap materials for use in the top cell of AM0 multijunction solar cells. The first approach achieves the target bandgap via (Al
_0.32
Ga
_0.68
)
_0.52
In
_0.48
P lattice-matched to GaAs. The second approach achieves the target bandgap via relaxed, lattice-mismatched Ga
_0.66
In
_0.34
P. Solar cell characterization and analytical device modeling reveal merits and challenges for both approaches. Both devices are found to possess comparable minority carrier diffusion lengths within the p-type base and an equal bandgap-voltage offset of 0.54 V, suggesting effectively similar base material quality. However, the metamorphic Ga
_0.66
In
_0.34
P cell exhibits a superior quantum efficiency across all wavelengths, particularly at short wavelengths. Modeling results attribute the enhanced short wavelength response to the wider direct bandgap of the internally lattice-matched Al
_0.69
In
_0.31
P window and a longer emitter diffusion length. As such, despite the non-negligible lattice mismatch, the enhanced short wavelength response for the metamorphic Ga
_0.66
In
_0.34
P approach proved significantly advantageous, at least with respect to the growth conditions used in this study. Assuming the use of a perfect antireflection coating, the lower parasitic optical absorption from the wider direct bandgap window alone provides as much as ~0.72 mA/cm
²
additional photocurrent. Combined with the higher quality emitter layer observed in this study, ~1.1 mA/cm
²
additional photocurrent was achieved for the metamorphic Ga
_0.66
In
_0.34
P approach. These values correspond to ~3.9% and ~6.0% of the total available photocurrent for a 2.05 eV bandgap solar cell.