  We present inverted metamorphic Ga0.3In0.7As photovoltaic converters with
sub-0.60 eV bandgaps grown on InP and GaAs substrates. The compositionally
graded buffers in these devices have threading dislocation densities of
1.3x10^6 cm^-2 and 8.9x10^6 cm^-2 on InP and GaAs, respectively. The devices
generate open-circuit voltages of 0.386 V and 0.383 V, respectively, at a
current density of ~10 A/cm^2, yielding bandgap-voltage offsets of 0.20 and
0.21 V. We measured their broadband reflectance and used it to estimate
thermophotovoltaic efficiency. The InP-based cell is estimated to yield 1.09
W/cm^2 at 1100 degrees C vs. 0.92 W/cm^2 for the GaAs-based cell, with
efficiencies of 16.8 vs. 9.2%. The efficiencies of both devices are limited by
sub-bandgap absorption, with power weighted sub-bandgap reflectances of 81% and
58%, respectively, which we assess largely occurs in the graded buffers. We
estimate that the thermophotovoltaic efficiencies would peak at ~1100 degrees C
at 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 micron atmospheric window. We estimate
peak LPC efficiencies of 36.8% and 32.5% under 2.0 micron irradiances of 1.86
W/cm^2 and 2.81 W/cm^2, respectively.
