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

Abstract

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 (Al0.32Ga0.68) 0.52In0.48P lattice-matched to GaAs. The second approach achieves the target bandgap via relaxed, lattice-mismatched Ga0.66In0.34P. 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 Ga0.66In0.34P 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 Al0.69In0.31P window and a longer emitter diffusion length. As such, despite the non-negligible lattice mismatch, the enhanced short wavelength response for the metamorphic Ga0.66In0.34P 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/cm2 additional photocurrent. Combined with the higher quality emitter layer observed in this study, ∼1.1 mA/cm2 additional photocurrent was achieved for the metamorphic Ga0.66In 0.34P approach. These values correspond to ∼3.9% and ∼6.0% of the total available photocurrent for a 2.05 eV bandgap solar cell.