Single-junction InGaP/GaAs solar cells grown on Si substrates with SiGe buffer layers

Single junction InGaP/GaAs solar cells displaying high efficiency and record high open-circuit voltage values have been grown by metal-organic chemical vapor deposition on Ge/graded SiGe/Si substrates. Open-circuit voltages of 980 in 17 under AM0 conditions have been verified to result from a single GaAs junction, with no evidence of Ge-related sub-cell photoresponse. AM0 efficiencies close to 16% have been measured for a large number of small-area cells, the performance of which is limited by non-fundamental current losses due to significant surface reflection resulting from >10% front-surface metal coverage and wafer handling during the growth sequence for these prototype cells. It is shown that at the material quality currently achieved for GaAs grown on Ge/SiGe/Si substrates, namely a 10 ns minority-carrier lifetime that results from complete elimination of anti-phase domains, and maintaining a threading dislocation density of -8 x 10(5) cm(-2), 19-20% AM0 single-junction GaAs cells are imminent. Experiments show that the high performance is not degraded for larger-area cells, with identical open-circuit voltages and higher short-circuit current (due to reduced front metal coverage) values being demonstrated, indicating that large-area scaling is possible in the near term. Comparison with a simple model indicates that the voltage output of these GaAs-on-Si cells follows the ideal behavior expected for lattice-mismatched devices, demonstrating that unaccounted-for defects and issues that have plagued other methods to epitaxially integrate III-V cells with Si are resolved by using SiGe buffers and proper GaAs nucleation methods. These early results already show the enormous and realistic potential of the virtual SiGe substrate approach for generating high-efficiency, lightweight and strong 111-V solar cells. Copyright (C) 2002 John Wiley Sons, Ltd.