2.0–2.1 eV GaxIn1−xP solar cells grown on relaxed GaAsP step grades

A high quality solar cell with a bandgap in the range of 2.0-2.1 eV may enable the development of four- and five-junction solar cells for terrestrial and space applications. In this paper we describe a set of 2.0-2.1 eV nVp solar cells fabricated from Ga_xln_1-xP and grown on compositional step-grades of GaAs_1-yP_y, on GaAs substrates. Cells were grown by atmospheric pressure organometallic vapor phase epitaxy. The tensile grades were designed to achieve nearly complete relaxation of the active layers, and the in-situ stress as monitored during growth showed a residual tensile stress of <;10 MPa in the best samples. We have fabricated 1.98 eV cells with 1-sun and 70-sun efficiencies of 14.4% and 15.9%, respectively, under the direct spectrum, and 2.07 eV cells with 1-sun efficiencies of 10.7%. Improvements in the grade design that reduce the threading dislocation density below 106 cm^-2 are expected to lead to efficiency increases. Matching the lattice constants of the confinement and contact layers to the junction layers is critical to achieving low interface recombination velocities, and can be a challenge in lattice-mismatched structures if the graded layers are not sufficiently relaxed.