NOVELTY - Formation of solar cell comprises: forming upper subcell (18) over growth substrate (14) so that upper subcell is lattice-matched to substrate; forming middle subcell (22) over upper subcell so that middle subcell is lattice-matched to substrate; and forming lower subcell(s) (26) over middle subcell so that lower subcell is lattice-mismatched with respect to substrate and has third lower band gap.    USE - For forming a multijunction solar cell used for converting solar energy into electrical power (claimed).    ADVANTAGE - The upper subcells(s) can be fabricated to have high performance by having atomic lattice spacing closely matched to that of the growth substrate. Lower subcells can be included with appropriate band gap, but with lattice spacing mismatched to the other subcells. The upper cell(s) can have performance that is adjusted or optimized, while the lattice-mismatch in the lower subcell(s) can be addressed separately. The lower subcell(s), growth later than the upper two cells, can be separately tailored or optimized to provide the desired electrical output. The internal design of the lower subcells(s) can also be appropriately selected or optimized, in layer thickness, and impurity concentrations and gradients.    DETAILED DESCRIPTION - Formation of a multijunction solar cell, comprises:    (a) providing a growth semiconductor substrate for the epitaxial growth of semiconductor material;    (b) forming upper subcell over the growth semiconductor substrate such that the upper subcell has a first, upper band gap and is lattice-matched to the growth substrate;    (c) forming middle subcell over the upper subcell such that the middle subcell is lattice-matched to the growth substrate and the middle subcell has a second middle band gap, the first upper band gap being larger than the second middle band gap;    (d) forming at least one lower subcell over the middle subcell such that the lower subcell is lattice-mismatched with respect to the growth substrate and the lower subcell has a third lower band gap, the third lower band gap being smaller than the second middle band gap;    (e) providing a support structure over the lower subcell; and    (f) removing the growth semiconductor substrate.    An INDEPENDENT CLAIM is also included for a multijunction solar cell used for converting solar energy into electrical power, comprising:    (a) a support structure having lateral spatial extent and rigidity for providing structural support for layers of material forming the solar cell, the support structure including a metal surface across the lateral spatial extent for reflecting light that passes through the layers of material and reaches the supports structure;    (b) at least one lower subcell comprising semiconductor material disposed over the metal surface of the support structure, the lower subcell having a first lower band gap;    (c) a middle subcell comprising semiconductor material disposed over the lower subcell, the middle subcell having a second middle band gap, the first lower band gap being smaller than the second middle band gap; and    (d) an upper subcell comprising semiconductor material disposed over the middle subcell, the upper subcell having a third, upper band gap, the third upper band gap being larger than the second middle band gap, where the upper subcell and the lower subcell are lattice-mismatched and the lower subcell is more strained than the upper subcell.    DESCRIPTION OF DRAWING(S) - The figure illustrates an epitaxial growth layer sequence to form a multijunction solar cell.    Semiconductor substrate (14)    Top subcell (18)    Middle subcell (22)    Lower subcell (26)