Multijunction solar cells having a graded-index structure

A multijunction solar cell comprising:

• an upper solar subcell having an emitter layer and a base layer forming a photoelectric junction;
• a first graded-index reflector structure disposed beneath the base layer of the upper solar subcell, wherein the first graded-index reflector structure comprises:
• a first plurality of pairs of alternating layers of InyAlxGa(1-x-y)As and a first different semiconductor material, wherein 0<x<1, 0≤y≤0.3, and a mole fraction of aluminum is increased for each of the InyAlxGa(1-x-y)As layers in the first plurality of pairs of alternating layers as the distance between the alternating layers and the upper solar subcell increases,
• a second plurality of pairs of alternating layers of InyAlxGa(1-x-y)As and a second different semiconductor material, wherein 0<x<1, 0≤y≤0.3, and a mole fraction of aluminum repeats for each of the InyAlxGa(1-x-y)As layers in the second plurality of pairs of alternating layers as the distance between the alternating layers and the upper solar subcell increases;
• and
• a third plurality of pairs of alternating layers of InyAlxGa(1-x-y)As and a third different semiconductor material, wherein 0<x<1, 0≤y≤0.3, and a mole fraction of aluminum is decreased for each of the InyAlxGa(1-x-y)As layers in the third plurality of pairs of alternating layers as the distance between the alternating layers and the upper solar subcell increases;
• a lower solar subcell disposed beneath the first graded-index reflector structure, wherein the lower solar subcell has an emitter layer and a base layer forming a photoelectric junction.

A method of fabricating a multijunction solar cell comprising:

• forming an upper solar subcell having an emitter layer and a base layer that form a photoelectric junction, the base layer having a bottom surface;
• forming a lower solar subcell having an emitter layer and a base layer that form a photoelectric junction;
• and
• forming a reflector structure using successive layers with a changing or graded index of refraction disposed above the emitter layer of the lower solar subcell and beneath the base layer of the upper solar subcell, wherein forming the reflector structure includes:
• forming a first plurality of pairs of alternating layers of InyAlxGa(1-x-y)As and a first different semiconductor material, wherein 0<x<1, and 0<y≤0.3 and a mole fraction of aluminum is increased for each of the InyAlxGa(1-x-y)As layers in the first plurality of pairs of alternating layers as the distance between the surface of the alternating layers and the bottom surface of the upper solar subcell increases;
• and
• forming a second plurality of pairs of alternating layers of InyAlxGa(1-x-y)As, and a second different semiconductor material, 0<x<1, and 0<y≤0.3 wherein a mole fraction of aluminum is decreased, for each of the plurality of pairs of InyAlxGa(1-x-y)As layers in the second plurality of pairs of alternating layers, as the distance between the surface of the alternating layers and the bottom surface of the upper solar subcell increases.