Inverted metamorphic multijunction solar cells including metamorphic layers

A multijunction solar cell comprising:

• an upper first solar subcell composed of an InGaP emitter layer and an InGaAlP base layer and having a first band gap in the range of 1.8 to 2.1 eV, and the base-emitter junction of the upper first solar subcell being a heterojunction;
• a second solar subcell composed of InGaP emitter layer and a GaAs base layer adjacent to said first solar subcell and having a second band gap smaller than said first band gap and in the range of approximately 1.35 to 1.45 eV;
• a first graded interlayer composed of (InxGa1-x)yAl1-yAs adjacent to said second solar subcell, said first graded interlayer having a third band gap greater than said second band gap subject to the constraints of having the in-plane lattice parameter greater or equal to that of the second subcell and less than or equal to that of the third subcell, wherein 0<x<1 and 0<y<1, and x and y are selected such that the band gap of the first graded interlayer remains constant throughout its thickness at 1.5 eV;
• a third solar subcell composed of an InGaP emitter layer and an InGaAs base layer adjacent to said first graded interlayer, said third subcell having a fourth band gap smaller than said second band gap and in the range of approximately 0.9 to 1.1 eV such that said third subcell is lattice mismatched with respect to said second subcell;
• a second graded interlayer composed of (InxGa1-x)yAl1-yAs adjacent to said third solar subcell, said second graded interlayer having a fifth band gap greater than said fourth band gap subject to the constraints of having the in-plane lattice parameter greater or equal to that of the third subcell and less than or equal to that of the bottom fourth subcell, wherein 0<x<1 and 0<y<1, and x and y are selected such that the band gap of the second graded interlayer remains constant throughout its thickness at 1.1 eV;
• and
• a lower fourth solar subcell composed of an InGaAs base layer and an InGaAs emitter layer adjacent to said second graded interlayer, said lower subcell having a sixth band gap smaller than said fourth band gap and in the range of approximately 0.6 to 0.8 eV such that said fourth subcell is lattice mismatched with respect to said third subcell.