NOVELTY - The silicon germanium solar cell structure (100) comprises a silicon substrate (106), a silicon germanium graded buffer layer (108) grown on the substrate, an absorber layer, a silicon emitter layer provided on the absorber layer, a first contact on a top surface of the emitter layer, a second contact on a bottom surface of the substrate, and a middle contact on a top surface of the substrate. A first junction (102) is provided between the emitter layer and the absorber layer. The graded buffer layer has a grade rate of 10-50% germanium per micron of graded buffer layer thickness.    USE - The silicon germanium solar cell structure is useful for space-based and terrestrial applications.    ADVANTAGE - The high efficiency silicon germanium solar cell structure enables efficient collection of broad solar spectrum and high-energy photons, and has reduced junction area.    DETAILED DESCRIPTION - The silicon germanium solar cell structure (100) comprises a silicon substrate (106), a silicon germanium graded buffer layer (108) grown on the substrate, an absorber layer, a silicon emitter layer provided on the absorber layer, a first contact on a top surface of the emitter layer, a second contact on a bottom surface of the substrate, and a middle contact on a top surface of the substrate. A first junction (102) is provided between the emitter layer and the absorber layer. The graded buffer layer has a grade rate of 10-50% germanium per micron of graded buffer layer thickness, and an initial P-type doped region. The substrate has a first P-type doped surface, and a second N-type doped surface. The P-type and N-type surfaces and the second and middle contacts provide a top solar cell. The initial P-type doped region, the emitter layer and the first and middle contacts provide a bottom solar cell. An INDEPENDENT CLAIM is included for a method of making a silicon germanium solar cell.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a silicon germanium solar cell structure.    Silicon germanium junction solar cell structure (100)    First junction (102)    Substrate (106)    Graded buffer layer (108)    P-doped silicon germanium absorber layer (110)    N-doped silicon emitter layer. (112)