3J solar cells comprising a lattice matched epitaxial SiGeSn subcell

The performance of quad-junction Ge based III-V multi-junction solar cells depends upon successful integration of a 1.0eV sub-cell into the existing InGaP/In
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GaAs/Ge stack. The SiGeSn ternary alloy offers a means to fabricate a lattice-matched, 1.0eV sub-cell with the advantage that SiGeSn has both a tuneable band-gap and variable lattice parameter, enabling the material to be integrated into lattice-matched multi-junction architectures. A 1eV SiGeSn junction has been grown as both a single junction device and as the third junction in a InGaP/In
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GaAs/SiGeSn triple junction device. The SiGeSn junction produces sufficient current to almost current match the InGaP and In
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GaAs junctions and achieves a peak external quantum efficiency > 80% is demonstrated suggesting a lower limit on the base minority hole diffusion length of 5m. However, a degradation in open-circuit voltage and fill factor are observed with the SiGeSn sub-cell. This low voltage from the SiGeSn subcell is is attributed to recombination from defect levels within the SiGeSn band gap.