NOVELTY - Forming multi-junction solar cell involves forming first solar subcell (A) on a substrate (101), with a first band gap; forming second solar subcell (B) on first subcell, with a second band gap smaller than first band gap; forming a barrier layer (116a) on second subcell, to reduce threading dislocations; forming a grading interlayer (116) on barrier layer, with a third band gap greater than second band gap; and forming third solar subcell (C) on interlayer, with a fourth band gap smaller than second band gap, such that third subcell is lattice mismatched with respect to second subcell.    USE - For forming a multi-junction solar cell (claimed) e.g. inverted metamorphic multi-junction solar cell, useful as power conversion device in power systems e.g. satellite power system.    ADVANTAGE - The barrier layer reduces propagation of threading dislocations from metamorphic layer (i.e. grading interlayer) of the solar cell. Thus, photogenerated current density in each solar subcell can be used effectively, leading to high efficiency for the multi-junction solar cell.    DETAILED DESCRIPTION - Forming a multi-junction solar cell involves providing a substrate (101) for epitaxial growth of semiconductor material; forming a first solar subcell (A) on the substrate, with a first band gap; forming a second solar subcell (B) on the first subcell, with a second band gap smaller than the first band gap; forming a barrier layer (116a) on the second subcell, to reduce threading dislocations; forming a grading interlayer (116) on the barrier layer, with a third band gap greater than the second band gap; and forming a third solar subcell (C) on the grading interlayer, with a fourth band gap smaller than the second band gap, such that the third subcell is lattice mismatched with respect to the second subcell. The method further involves forming a second barrier layer (116b) on the grading interlayer, prior to the formation of third solar subcell. The barrier layers are made of arsenic, phosphorus, nitrogen or antimony based III-V compound semiconductors, which have a bandgap energy greater than or equal to that of the grading interlayer. An INDEPENDENT CLAIM is included for a multi-junction solar cell comprising a substrate; a first solar subcell disposed on the substrate, and having a first band gap; a second solar subcell disposed on the first subcell, and having a second band gap smaller than the first band gap; a barrier layer disposed on the second subcell, for reducing propagation of threading dislocations; a grading interlayer disposed on the barrier layer, and having a third band gap greater than the second band gap; and a third solar subcell disposed on the grading interlayer that is lattice mis-matched with respect to the second subcell, and having a fourth band gap smaller than the second band gap.    DESCRIPTION OF DRAWING(S) - The figure shows an enlarged cross-sectional view of a multi-junction solar cell.    Substrate (101)    Grading interlayer (116)    Barrier layers (116a, 116b)    First solar subcell (A)    Second solar subcell (B)    Third solar subcell (C)