NOVELTY - The multi-solar cell structure comprises a monocrystalline silicon substrate (402) with n-type doping material, a graded buffer layer (404) grown on the substrate, a first solar subcell (410) within or on top of the graded buffer layer, a second solar subcell (406) grown on top of the first solar subcell, and top contacts on a top of the second solar subcell. The graded buffer layer has a final graded silicon germanium layer of 70-85% germanium composition. The multi-solar cell structure further comprises a back surface field layer that interfaces with the graded buffer layer.    USE - The multi-solar cell structure is useful for space-based and terrestrial applications.    ADVANTAGE - The multi-solar cell structure exhibits high power output and photons collection efficiency, and is economical and easy to install.    DETAILED DESCRIPTION - The multi-solar cell structure comprises a monocrystalline silicon substrate (402) with n-type doping material, a graded buffer layer (404) grown on the substrate, a first solar subcell (410) within or on top of the graded buffer layer, a second solar subcell (406) grown on top of the first solar subcell, and top contacts on a top of the second solar subcell. The graded buffer layer has a final graded silicon germanium layer of 70-85% germanium composition. The multi-solar cell structure further comprises a back surface field layer that interfaces with the graded buffer layer and that is matched to a final germanium composition of the graded buffer layer, a tunnel junction (408) that is positioned between the first and second solar subcells, and a transition layer that is disposed between the tunnel junction and the second solar subcell. An INDEPENDENT CLAIM is included for a method for making a multi-junction solar cell.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a multi-junction cell.    Monocrystalline silicon substrate (402)    Graded buffer layer (404)    Second subcell (406)    Tunnel junction (408)    First subcell. (410)