NOVELTY - The method comprises forming a first junction over a top surface of a silicon substrate (110), forming a second junction comprising a III-V material over a top surface of the first junction, forming a cap layer (140) comprising first and second layers over a top surface of the second junction, forming a metal over a portion of a top surface of the cap layer, reacting the metal with the cap layer to form a contact layer disposed over the second junction, removing a portion of the substrate by thinning and/or waffling, and forming a third junction over the top surface of the second junction.    USE - The method is useful for forming a multi-junction solar cell.    ADVANTAGE - The method is capable of efficiently and economically forming the solar cell with high power-conversion efficiency and flexibility, superior performance, increased spectral sensitivity and lightweight without any dislocation density.    DETAILED DESCRIPTION - The method comprises forming a first junction over a top surface of a silicon substrate (110), forming a second junction comprising a III-V material over a top surface of the first junction, forming a cap layer (140) comprising first and second layers over a top surface of the second junction, forming a metal over a portion of a top surface of the cap layer, reacting the metal with the cap layer to form a contact layer disposed over the second junction, removing a portion of the substrate by thinning and/or waffling, forming a third junction over the top surface of the second junction, removing an unreacted portion of the cap layer, and forming a tunnel junction (135) between the first junction and the second junction. The formation of the first junction and the second junction comprises epitaxial deposition in a single reactor with no exposure of the substrate to oxygen between the first junction and the second junction. The third junction comprises a III-V material different from the III-V material of the second junction, and has a band gap different from the band gaps of the first and second junctions. The unreacted portion of the cap layer remains disposed between the second junction and the contact after reacting the metal layer. The formation of the tunnel junction comprises intentional introduction of a first dopant species during epitaxial growth, autodoping of a second dopant species having a polarity opposite that of the first dopant species, and mutual autodoping of first and second dopant species having opposite polarities. The entire cap layer consists of doped or undoped silicon. The cap layer is partially lattice-relaxed, partially polycrystalline and partially amorphous. The cap layer is formed such that a resulting density of defects exceeds a defect density of the second junction, where the defects comprise threading dislocations, misfit dislocations, and/or stacking faults. The portion of the cap layer disposed beneath the contact in the second region comprises a portion of the first layer. The first junction and the second junction have a lattice mismatch to germanium of greater than 1%.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a solar cell.    Substrate (110)    Template layer (120)    Junctions (125, 130)    Tunnel junction (135)    Cap layer. (140)