NOVELTY - The method comprises depositing a semiconductor material on a first substrate for the epitaxial growth of semiconductor material, forming a first solar subcell on the substrate (101) having a first band gap, forming a second subcell over the first subcell having a second band gap smaller than the first band gap, forming a grading interlayer over the second solar subcell, and forming a third subcell over the grading interlayer having a fourth band gap smaller than the second band gap such that the third subcell is lattice mismatched with respect to the second subcell.    USE - The method is useful for forming a multijunction solar cell, which is useful in applications such as concentrator terrestrial power systems and satellites used in data communications and as the power conversion devices for the on-board power systems.    ADVANTAGE - The method is capable of energy efficiently and reliably forming a multijunction solar cell with improved power and energy conversion characteristics thus improving the size, mass and cost of the satellite power systems.    DETAILED DESCRIPTION - The method comprises depositing a semiconductor material on a first substrate for the epitaxial growth of semiconductor material, forming a first solar subcell on the substrate (101) having a first band gap, forming a second subcell over the first subcell having a second band gap smaller than the first band gap, forming a grading interlayer over the second solar subcell, and forming a third subcell over the grading interlayer having 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 solar cell comprises an upper subcell, a middle subcell, a lower subcell, a base layer and an emitter layer, and a pseduomorphic window layer adjacent to the emitter layer. The grading interlayer has a third band gap greater than the second band gap. The window layer has a lattice constant, which differs from the lattice constant of the emitter layer by less than 0.9%. The window layer is strained so that dislocations do not propagate into the cell structure. The second solar cell is composed of a gallium indium phosphate-, gallium indium arsenide-, gallium arsenide antimonide-, gallium indium-arsenic nitride- or indium gallium phosphide emitter region, and a gallium indium arsenide-, gallium arsenide antimonide-, gallium indium arsenide nitride- or gallium arsenide base region. The grading interlayer is composed of arsenide, phosphorus, nitride, antimony based III-V compound semiconductors subjected to the constraints and having in-plane lattice parameter greater or equal to that of the second solar cell and less than or equal to that of the third solar cell, and has a band gap energy greater than that of the second solar cell. A surrogate second substrate is attached over the third solar cell while removing the first substrate. A surrogate second substrate is mounted on top of the sequence of layers, and then the first substrate is removed, after deposition of semiconductor material on the first substrate. The solar cell is bonded to the surrogate substrate by an adhesive. The surrogate substrate is thinned to a predetermined thickness. The support is a rigid coverglass. The middle and bottom subcells are lattice mismatched. A graded interlayer is present between the middle and bottom subcells, has a band gap greater than the band gap of the middle subcell. The graded interlayer is composed of (InxGa1-x)yAl1-yAs, where x and y are selected such that the band gap of the interlayer remains constant at 1.50 eV. INDEPENDENT CLAIMS are included for:    (1) a method for increasing current generation in a photovoltaic cell or other optoelectronic device; and    (2) a multijunction solar cell.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of the solar cell.    Substrate (101)    Buffer layer (102)    Etch stop layer (103)    Contact layer (104)    Window. (105)