PROBLEM TO BE SOLVED: To produce a commercially viable and highly energy-efficient solar cell using commercially established fabrication processes for producing an inverted metamorphic multi-junction cell structure.SOLUTION: A multi-junction solar cell includes: an upper first solar subcell having a first band gap; a second solar subcell adjacent to the first solar subcell and having a second band gap smaller than the first band gap; a first graded interlayer adjacent to the second solar subcell and having a third band gap greater than the second band gap; and a third solar subcell adjacent to the first graded interlayer, having a fourth band gap smaller than the second band gap, and lattice mismatched with respect to the second subcell. A second graded interlayer is provided adjacent to the third solar subcell and having a fifth band gap greater than the fourth band gap. A lower fourth solar subcell is provided adjacent to the second graded interlayer, having a sixth band gap smaller than the fourth band gap, and lattice mismatched with respect to the third subcell.
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