A solar cell comprising an epitaxial sequence of layers of semiconductor material thrilling at least a first and second solar subcells; a semiconductor contact layer disposed on the bottom surface of the second solar subcell; a reflective metal layer disposed below the semiconductor contact layer such that the reflectivity of the reflective metal layer is greater than 80% in the wavelength range 850 to 2000 nm, for reflecting light back into the second solar subcell.

A solar cell comprising an epitaxial sequence of layers of semiconductor material thrilling at least a first and second solar subcells; a semiconductor contact layer disposed on the bottom surface of the second solar subcell; a reflective metal layer disposed below the semiconductor contact layer such that the reflectivity of the reflective metal layer is greater than 80% in the wavelength range 850 to 2000 nm, for reflecting light back into the second solar subcell.

A multijunction solar cell including an upper first solar subcell having a first band gap and positioned for receiving an incoming light beam; and a second solar subcell disposed below and adjacent to and lattice matched with said upper first solar subcell, and having a second band gap smaller than said first band gap; wherein at least one of the solar subcells has a graded band gap throughout the thickness of at least a portion of its emitter layer and base layer.

A multijunction solar cell including an upper first solar subcell having a first band gap and positioned for receiving an incoming light beam; and a second solar subcell disposed below and adjacent to and lattice matched with said upper first solar subcell, and having a second band gap smaller than said first band gap; wherein at least one of the solar subcells has a graded band gap throughout the thickness of at least a portion of its emitter layer and base layer.

A multijunction solar cell including an upper first solar subcell having a first band gap and positioned for receiving an incoming light beam; a second solar subcell disposed below and adjacent to and lattice matched with said upper first solar subcell, and having a second band gap smaller than said first band gap; wherein at least one of the solar subcells has a graded band gap throughout the thickness of at least a portion of the active layer.

A multijunction solar cell including an upper first solar subcell having a first band gap and positioned for receiving an incoming light beam; a second solar subcell disposed below and adjacent to and lattice matched with said upper first solar subcell, and having a second band gap smaller than said first band gap; wherein the ratio of the thickness of the emitter layer to the thickness of the base layer in at least one of the solar subcells is between 5:1 and 1:1.

A multijunction solar cell including an upper first solar subcell having a first band gap and positioned for receiving an incoming light beam; a second solar subcell disposed below and adjacent to and lattice matched with said upper first solar subcell, and having a second band gap smaller than said first band gap; wherein the ratio of the thickness of the emitter layer to the thickness of the base layer in at least one of the solar subcells is between 5:1 and 1:1.

A multijunction solar cell including an upper first solar subcell having a first band gap and positioned for receiving an incoming light beam; a second solar subcell disposed below and adjacent to and lattice matched with said upper first solar subcell, and having a second band gap smaller than said first band gap; wherein at least one of the solar subcells has a graded band gap throughout the thickness of at least a portion of the active layer of the one solar subcell.

A multijunction solar cell including an upper first solar subcell having a first band gap and positioned for receiving an incoming light beam; a second solar subcell disposed below and adjacent to and lattice matched with said upper first solar subcell, and having a second band gap smaller than said first band gap; wherein at least one of the solar subcells has a graded band gap throughout the thickness of at least a portion of the active layer of the one solar subcell.

A multijunction solar cell assembly and its method of manufacture including interconnected first and second discreate semiconductor body subassemblies disposed adjacent and parallel to each other, in the sense of the incoming illumination, each semiconductor body subassembly including first top subcell, and possibly third middle subcells and a bottom solar subcell; wherein the interconnected subassemblies form at least a Three junction solar cell by a series connection being formed between the bottom solar subcell in the first semiconductor body with its at least least two junctions and the bottom solar subcell in the second semiconductor body representing the additional junction.