0.6-eV bandgap In0.69Ga0.31As thermophotovoltaic devices with compositionally undulating step-graded InAsyP1-y buffers

Single-junction, lattice-mismatched In_0.69Ga_0.31As thermophotovoltaic (TPV) devices each with a bandgap of 0.6 eV are grown on InP substrate by metal-organic chemical vapour deposition (MOCVD). Compositionally undulating step-graded InAs_yP_1-y buffer layers with a lattice mismatch of ～1.2%, are used to mitigate the effect of lattice mismatch between the device layers and the InP substrate. With an optimized buffer thickness, the In_0.69Ga_0.31As active layers grown on the buffer displayed a high crystal quality with no measurable tetragonal distortion. High-performance single-junction devices are demonstrated, with an open-circuit voltage of 0.215 V and a photovoltaic conversion efficiency of 6.9% at a short-circuit current density of 47.6 mA/cm², which are measured under the standard solar simulator of air mass 1.5-global (AM 1.5 G).