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

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 _xGa_yIn_{1-x- y}As and highly Se-doped Burstein-Moss (BM) shifted Ga_xIn _1-xP. Transparency to >810 and >890 nm wavelengths is demonstrated with BM-shifted Ga_xIn_1-xP on B-miscut substrates and Al _xGa_yIn_{1-x- y}As/Ga_xIn_1-xP(Se) combined grades on A-miscut substrates, respectively. 0.74 eV GaInAs solar cells grown on these transparent CGBs exhibit W_OC= 0.41, V15mA/cm², performance comparable with the state-of-the-art Ga_xIn_{1- x}P grade employed in the four-junction-inverted metamorphic multijunction (IMM) cell. A GaAs/0.74 eV GaInAs tandem cell was grown with a transparent BM-shifted Ga_xIn_1-xP 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.