TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Method: In forming the multi-junction solar cell string, the portion of the first electrical interconnect extending through the perforated carrier is connected to the respective portion of the second electrical interconnect extending through the perforated carrier by welding. The number of cut-outs of the second multi-junction solar cell is equal to the number of contact pads of the first multi-junction 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 multi-junction solar cell by a set of electrical interconnects. The perforated carrier is composed of a flexible composite material, where the perforated carrier is composed of an UltraFlex mesh. The perforated carrier has perforations that are approximately square in shape and approximately 0.25 cm in width. The method further comprises attaching a discrete bypass diode to the second side of the perforated carrier, and electrically connecting the respective terminals of the bypass diode to the terminals of the corresponding solar cell on the first side of the perforated carrier. The electrical interconnects are discrete planar metal strips welded to the contact pad on the first multi- junction solar cell and to the metal layer on the second multi-junction solar cell. Moreover, providing the first multi-junction solar cell comprises: providing a first substrate for the epitaxial growth of semiconductor material; forming an upper first solar subcell on the first substrate having a first band gap; forming a middle second solar subcell over the first solar subcell having a second band gap smaller than the first band gap; forming a graded interlayer over the second solar cell; forming a lower third solar subcell over the graded inter-layer 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, and including a metal contact layer; attaching a surrogate second substrate over the third solar subcell and removing the first substrate; and etching a first trough around the periphery of the solar cell to the metal contact layer so as to form a mesa structure on the surrogate second substrate and at least one bottom contact pad on the metal layer. The contact metal layer is a sequence of metal layers including titanium (Ti)/gold (Au)/silver (Ag)/Au. Also, providing the first multi-junction solar cell comprises: forming a first subcell comprising a first semiconductor material with a first band gap and a first lattice constant; forming a second subcell comprising a second semiconductor material with a second band gap and a second lattice constant, where the second band gap is less than the first band gap and the second lattice constant is greater than the first lattice constant to the second lattice constant; and forming a lattice constant transition material positioned between the first subcell and the second subcell, the lattice constant transition material having a lattice constant that changes gradually from the first lattice constant to the second lattice constant. The transition material is composed of any of the arsenic (As), phosphorus (P), nitrogen (N), antimony (Sb) based II-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater or equal to that of the first subcell and less than or equal to that of the second subcell, and having a band gap energy greater than that of the second subcell, and the band gap of the transition material remains constant at approximately 1.50 eV throughout its thickness. The transition material is composed of a sequence of (InxGa1-x)yAl1-xAs layers, with x and y selected such that the band gap of each layer remains constant throughout the thickness of the transition material. The first subcell is composed of an gallium indium phosphide (GaInP), gallium arsenide (GaAs), gallium indium arsenide (GaInAs), gallium arsenide antimonide (GaAsSb), or gallium indium arsenic nitride (GaInAsN) emitter region and an GaAs, GaInAs, GaAsSb, or GaInAsN base region, and the second subcell is composed of InGaAs base and emitter regions. Preferred Solar Cell: In the 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 and is composed of any of the As, P, N, 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 multi-junction solar cell further comprises a discrete bypass diode mounted on the second side of the perforated carrier, with the respective terminals of the bypass diode being connected to the terminals of the corresponding solar cell on the first side of the perforated carrier.