TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Solar Cell: The multijunction solar cell further comprises a set of metal grid lines disposed on the top surface, and at least one metal contact pad disposed along a first peripheral edge of the solar cell and electrically connected to the grid lines. The cut-out is arranged along a second peripheral edge of the solar cell opposite from the first edge. The number of cut-outs is equal to the number of contact pads along the top surface, and the spacing of such cut-outs is substantially similar to the spacing of the contact pads along the respective peripheral edges to facilitate the electrical interconnection of contact pads on one cell with the respective metal contact layers through the cut-outs on an adjacent solar cell. The graded interlayer is compositionally graded to lattice match the middle subcell on one side and the bottom subcell on the other side. The graded interlayer is composed of any of arsenic (As), phosphorus (P), nitrogen (N), tin (Sb) based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater or equal to that of the middle subcell and less than or equal to that of the bottom subcell, and having a band gap energy greater than that of the middle subcell. The graded interlayer is composed of (InxGa1-x)y Al1-yAs, with x and y selected such that the band gap of the interlayer material remains constant throughout its thickness. The upper subcell is composed of an gallium indium phosphide (GaInP), gallium arsenide (GaAs), gallim indium arsenide (GaInAs), gallium arsenic antimonide (GaAsSb), or gallium indium arsenic nitride (GaInAsN) emitter region and an GaInP, GaAs, GaInAs, GaAsSb, or GaInAsN base region. The middle subcell is composed of an GaAs base and emitter regions. The bottom solar subcell is composed of an InGaAs base layer and an InGaP emitter layer that is lattice matched to the base layer. Preferred String: In the multijunction solar cell string, the number of cut-outs of the second multijunction solar cell is equal to the number of contact pads of the first multijunction solar cell, and the spacing of such cut-outs is substantially similar to the spacing of the contact pads along the respective second and first peripheral edges to facilitate the electrical interconnection of the contact pads on the first multijunction solar cell by a set of electrical interconnects extending substantially orthogonal to the peripheral edges and through the respective cut-outs on the second peripheral edge along the top surface of the second multijunction solar cell to the respective metal contact layer of the second multijunction solar cell. The electrical interconnects are discrete planar metal strips welded to the contact pad on the first multijunction solar cell and to the metal layer on the second multijunction solar cell. The first and second multijunction solar cells comprise an upper first solar subcell having a first band gap disposed adjacent the top surface of the multijunction solar cell; a middle second solar subcell adjacent to the first solar subcell and having a second band gap smaller than the first band gap; a graded interlayer adjacent to the second solar subcell; the graded interlayer having a third band gap greater than the second band gap; and a lower solar subcell adjacent to the interlayer, the lower subcell 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 graded interlayer is compositionally graded to lattice match the middle subcell on one side and the bottom subcell on the other side.