Highly Transparent Compositionally Graded Buffers for New Metamorphic Multijunction Solar Cell Designs

Abstract

The development of compositionally graded buffer layers (CGBs) with enhanced transparency would enable novel five and six junction solar cells, with efficiencies approaching 50% under high concentration. We demonstrate highly transparent grades between the GaAs and InP lattice constants on both A- and B-miscut GaAs substrates, employing Al x Ga y In1- x - y As and highly Se-doped Burstein–Moss (BM) shifted Ga x In 1- x P. Transparency to >810 and >890 nm wavelengths is demonstrated with BM-shifted Ga x In1- x P on B-miscut substrates and Al x Ga y In1- x - y As/Ga x In1- x P(Se) combined grades on A-miscut substrates, respectively. 0.74 eV GaInAs solar cells grown on these transparent CGBs exhibit ${{W}}_{{\rm{OC}}}= {\text{0.41}}\, {\text{V}}$ at ${\text{15}} {\text{mA/ cm}}^{2}$ , performance comparable with the state-of-the-art Ga x In1- x P grade employed in the four-junction-inverted metamorphic multijunction (IMM) cell. A ${\text{GaAs/ 0.74 eV}}$ GaInAs tandem cell was grown with a transparent BM-shifted Ga x In1- x P CGB to verify the CGB performance in a multijunction device structure. Quantum efficiency measurements indicate that the CGB is completely transparent to photons below the GaAs bandedge, validating its use in 4–6 junction IMM devices with a single-graded buffer. This tandem represents a highly efficient two-junction band gap combination, achieving 29.6% ± 1.2% efficiency under the AM1.5 global spectrum, demonstrating how the additional transparency enables new device structures.