NOVELTY - A multi-junction solar cell device (1500) comprises a substrate (300); a first photovoltaic sub-cell formed above the substrate and including a first semiconductor material having a first lattice constant; a second photovoltaic sub-cell formed below the first sub-cell and including a second semiconductor material having a second lattice constant; and a third photovoltaic sub-cell formed below the second sub-cell and below the substrate, and including a third semiconductor material having a third lattice constant.    USE - A multi-junction solar cell device for converting sunlight to electricity.    ADVANTAGE - The multi-junction solar cell device has increased device performance without being constrained by the need for lattice-matching, and has increased efficiency without regard for lattice matching.    DETAILED DESCRIPTION - A multi-junction solar cell device (1500) comprises:    (1) a substrate (300);    (2) a first photovoltaic sub-cell formed above the substrate and including a first semiconductor material having a first lattice constant;    (3) a second photovoltaic sub-cell formed below the first photovoltaic sub-cell, and including a second semiconductor material having a second lattice constant different from the first lattice constant; and    (4) a third photovoltaic sub-cell formed below the second photovoltaic sub-cell and below the substrate, and including a third semiconductor material having a third lattice constant different from the second lattice constant.    The first photovoltaic sub-cell comprises a first photovoltaic junction. The second photovoltaic sub-cell comprises a second photovoltaic junction defined in the substrate. A compositionally graded buffer layer is disposed between the first and second photovoltaic sub-cells. A defect-trapping layer is disposed between the first and second photovoltaic sub-cells.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic diagram illustrating the multi-junction solar cell device.    Substrate (300)    Base layer (1145)    Emitter layer (1150)    Photovoltaic junction (1152)    Multi-junction solar cell device (1500)