Ge concentrator cells for III-V multijunction devices

We identify a failure mode due to a photoactive back contact for Ge concentrator solar cells. This problem manifests itself as a leveling off and subsequent decrease of open-circuit voltage (V/sub oc/) as the concentration increases above /spl sim/20 suns. Correction of this problem yields a much improved Ge cell for which V/sub oc/ increases in an almost ideal n=1 manner from 0.2 volts at one sun to 0.4 volts at 1400 suns. This cell's fill factor remains at or above its one-sun value up to 500 suns, confirming that this cell is fully suitable for high-concentration use. We show that solving the back-contact problem can significantly improve the high-concentration performance of GaInP/GaAs/Ge three-junction solar cells.