A method of forming a multijunction solar cell including an upper subcell, a middle subcell, and a lower subcell by providing a substrate for the epitaxial growth of semiconductor material; forming a first solar subcell on the substrate having a first band gap; forming a 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 subcell, the graded interlayer having a third band gap greater than the second band gap; forming a third solar subcell over the graded 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; and forming a contact composed of a sequence of layers over the first subcell at a temperature of 280° C. or less and having a contact resistance of less than 5×10−4 ohms-cm2.