Sub-0.6 eV Inverted Metamorphic GaInAs Cells Grown on Inp and GaAs Substrates for Thermophotovoltaics and Laser Power Conversion

Inverted metamorphic Ga_0.3In_0.7As photovoltaic converters with sub-0.60 eV bandgaps grown on InP and GaAs are presented. Threading dislocation densities are 1.3 ± 0.6 × 10⁶ and 8.9 ± 1.7 × 10⁶ cm⁻² on InP and GaAs, respectively. The devices generate open-circuit voltages of 0.386 and 0.383 V, respectively, under irradiance producing a short-circuit current density of ≈10 A cm⁻², yielding bandgap-voltage offsets of 0.20 and 0.21 V. Power and broadband reflectance measurements are used  to estimate thermophotovoltaic (TPV) efficiency. The InP-based cell is estimated to yield 1.09 W cm⁻² at 1100 °C versus 0.92 W cm⁻² for the GaAs-based cell, with efficiencies of 16.8 versus 9.2%. The efficiencies of both devices are limited by sub-bandgap absorption, with power weighted sub-bandgap reflectances of 81% and 58%, respectively, the majority of which is assumed to occur in the graded buffers. The 1100 °C TPV efficiencies are estimated to increase to 24.0% and 20.7% in structures with the graded buffer removed, if previously demonstrated reflectance is achieved. These devices also have application to laser power conversion in the 2.0–2.3 µm atmospheric window. Peak laser power converter efficiencies of 36.8% and 32.5% are estimated under 2.0 µm irradiances of 1.86 and 2.81 W cm⁻², respectively.