Direct-Gap 2.1-2.2 eV AlInP Solar Cells on GaInAs/GaAs Metamorphic Buffers

AlInP offers the highest direct bandgap (E_g) among nonnitride III-V materials, making it attractive for top cell applications in five-to-six-junction solar cells. We present novel 2.05-2.15 eV direct-gap AlInP solar cells, grown on GaInAs/GaAs-graded buffers by metal-organic chemical vapor deposition (MOCVD). Despite the high Al content of 36-39% in the active regions, secondary ion mass spectrometry results indicate oxygen concentrations less than 3.5 × 10¹⁶ cm^-3. The AlInP devices we present here exhibit superior photovoltaic performance to GaP and similar performance to metamorphic GaInP solar cells, reaching a E_g-voltage offset of 0.57 V. Design enhancements based on device and material characterization led to improvements of over ∼ 4 × in short-circuit current density from our first-generation AlInP devices. Our results indicate that a p-i-n device design is necessary to account for low minority carrier diffusion lengths in AlInP solar cells; additionally, diffusion of Zn dopant atoms poses another challenge that must be accounted for in cell design. The effect of offcut on cell performance was also investigated, with improved solar cells on samples offcut toward the A plane. The promising results in this work provide an alternative path toward realizing high-E_g top cells with possible applications in upright metamorphic multijunction solar cells.