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

Inverted metamorphic Ga0.3In0.7As photovoltaic converters with sub-0.60 eV bandgaps grown on InP and GaAs are presented. Threading dislocation densities are 1.3 +/- 0.6 x 10(6) and 8.9 +/- 1.7 x 10(6) 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 approximate to 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 degrees 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 degrees 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 mu m atmospheric window. Peak laser power converter efficiencies of 36.8% and 32.5% are estimated under 2.0 mu m irradiances of 1.86 and 2.81 W cm(-2), respectively.