Inverted metamorphic multijunction solar cell with a single metamorphic layer

1. A multijunction solar cell comprising:

a first sequence of layers of semiconductor material forming a first set of one or more solar subcells;

a first threading dislocation inhibition layer adjacent to said first sequence of layers;

a graded interlayer directly adjacent to said first threading dislocation inhibition layer, said graded interlayer being composed of (In x Ga 1-x ) y Al 1-y As, wherein 0<x<1 and 0<y<1 with x and y selected such that said graded interlayer has a band gap of 1.6 eV±3% that remains constant throughout its thickness, said graded interlayer having a composition that differs from that of the first threading dislocation inhibition layer;

a second sequence of layers of semiconductor material forming a second set of one or more solar subcells, the second sequence of layers being disposed on a side of the graded interlayer opposite a side on which the first sequence of layers is disposed;

wherein the graded interlayer is compositionally graded to lattice match a closest solar subcell of the first set on one side of the graded interlayer and to lattice match a closest solar subcell of the second set on a second opposite side of the graded interlayer;

the multijunction solar cell further comprising a high band gap contact layer adjacent said second sequence of layers, wherein the high band gap contact layer is composed of p++ type InGaAlAs.

2. The multijunction solar cell as defined in claim 1 , wherein the first set of one or more solar cells comprises an upper first solar subcell having a first band gap in the range of approximately 1.8 to 2.1 eV and a second solar subcell 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; and wherein the second set of one or more solar cells comprises a third solar subcell to said 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; and a lower fourth solar subcell adjacent to said third solar subcell, said lower fourth subcell having a fifth band gap smaller than said fourth band gap and in the range of approximately 0.6 to 0.8 eV.

3. The multijunction solar cell as defined in claim 2 , wherein the upper first subcell is composed of an InGaP emitter layer and an InGaP base layer.

4. The multijunction solar cell as defined in claim 2 , wherein the second subcell is composed of InGaP emitter layer and a GaAs base layer.

5. The multijunction solar cell as defined in claim 2 , wherein the third subcell is composed of an InGaP emitter layer and an InGaAs base layer.

6. The multijunction solar cell as defined in claim 2 , wherein the lower fourth subcell is composed of an InGaAs base layer and an InGaAs emitter layer.

7. The multijunction solar cell as defined in claim 2 , wherein the upper first subcell is composed of an InGaP emitter layer and an InGaP base layer, the second subcell is composed of InGaP emitter layer and a GaAs base layer, the third subcell is composed of an InGaP emitter layer and an InGaAs base layer, and the lower fourth subcell is composed of an InGaAs base layer and an InGaAs emitter layer.

8. The multijunction solar cell as defined in claim 1 wherein the graded interlayer is composed of a plurality of layers of InGaAlAs having monotonically changing lattice constant, each layer having a band gap of 1.6 eV.

9. The multijunction solar cell as defined in claim 1 further including a second threading dislocation inhibition layer disposed between the graded interlayer and the second sequence of layers, the second threading dislocation inhibition layer being disposed directly adjacent to the graded interlayer and having a composition different from that of the graded interlayer.

10. The multijunction solar cell as defined in claim 9 wherein the composition of the second threading dislocation inhibition layer differs from the composition of the second threading dislocation inhibition layer.

11. The multijunction solar cell as defined in claim 10 wherein the first threading dislocation inhibition layer is composed of InGa(Al)P and has a thickness of 0.25-1.0 micron.

12. The multijunction solar cell as defined in claim 11 wherein the second threading dislocation inhibition layer is composed of GaInP.