Metamorphic layers in multijunction solar cells

1. A multijunction solar cell comprising:

a first solar subcell having a first band gap;

a second solar subcell disposed below the first solar subcell and having a second band gap smaller than the first band gap;

an InGaAs buffer layer disposed below the second solar subcell, wherein the InGaAs buffer layer is composed of a crystalline structure, the crystalline structure of the buffer layer consisting of indium, gallium and arsenic;

an InGaAlAs grading interlayer disposed below, and adjacent to, the InGaAs buffer layer, wherein the InGaAlAs grading interlayer is composed of multiple layers each of which has a crystalline structure, wherein the crystalline structure of each of the multiple layers of the InGaAlAs grading interlayer consists of indium, gallium, arsenic and aluminum, and wherein the InGaAlAs grading interlayer has a constant third band gap throughout its thickness greater than the second band gap; and

a third solar subcell disposed below the InGaAlAs interlayer that is lattice mismatched with respect to second solar subcell and having a fourth band gap smaller than the third band gap,

wherein the InGaAlAs grading interlayer achieves a transition in lattice constant from the second subcell to the third subcell, and

wherein each of the first solar subcell, the second solar subcell, the InGaAs buffer layer, the InGaAlAs grading interlayer and the third solar subcell comprises one or more epitaxial layers of a same integrated semiconductor structure.

2. A multijunction solar cell as defined in claim 1 , wherein the constant band gap of the InGaAlAs grading interlayer is 1.5 eV.

3. A multijunction solar cell as defined in claim 1 wherein the InGaAlAs grading interlayer has a monotonically changing lattice constant.

4. A multijunction solar cell as defined in claim 1 wherein the InGaAs buffer layer has a thickness on the order of 1 μm.

5. A multijunction solar cell as defined in claim 1 , wherein the first solar subcell is composed of InGaP.

6. A multijunction solar cell as defined in claim 1 , wherein the first solar subcell includes an InGa(Al)P emitter region and an InGa(Al)P base region, and the second solar subcell includes an InGaP emitter layer.

7. A multijunction solar cell as defined in claim 1 further including a bottom contact layer below the third solar subcell and making electrical contact therewith.

8. A multijunction solar cell as defined in claim 1 further including a tunnel diode disposed below the second solar subcell and over the InGaAs buffer layer.

9. A multijunction solar cell as defined in claim 8 wherein the tunnel diode is a p++/n++ tunnel diode.

10. A multijunction solar cell as defined in claim 1 wherein the InGaAlAs grading interlayer includes a compositionally step-graded InGaAlAs series of layers.

11. A multijunction solar cell assembly comprising:

a cover glass;

a multjunction solar cell below the cover glass, the multijunction solar cell including:

one or more grid lines;

a first contact layer below the one or more grid lines;

a window layer below the first contact layer;

a first solar subcell disposed below the window layer and having a first band gap;

a second solar subcell disposed below the first solar subcell and having a second band gap smaller than the first band gap;

an InGaAs buffer layer disposed below the second solar subcell, wherein the InGaAs buffer layer is composed of a crystalline structure, the crystalline structure of the buffer layer consisting of indium, gallium and arsenic;

an InGaAlAs grading interlayer disposed below, and adjacent to, the InGaAs buffer layer, wherein the InGaAlAs grading interlayer is composed of multiple layers each of which has a crystalline structure, wherein the crystalline structure of each of the multiple layers of the InGaAlAs grading interlayer consists of indium, gallium, arsenic and aluminum, and wherein the InGaAlAs grading interlayer has a constant third band gap throughout its thickness greater than the second band gap;

a third solar subcell disposed below the InGaAlAs interlayer that is lattice mismatched with respect to second solar subcell and having a fourth band gap smaller than the third band gap, wherein the InGaAlAs grading interlayer achieves a transition in lattice constant from the second subcell to the third subcell; and

a second contact layer below the third solar subcell and making electrical contact therewith.

12. A multijunction solar cell assembly as defined in claim 11 including an antireflective coating over the one or more grid lines.

13. A multijunction solar cell assembly as defined in claim 11 , wherein the constant band gap of the InGaAlAs grading interlayer is 1.5 eV.

14. A multijunction solar cell assembly as defined in claim 11 wherein the InGaAlAs grading interlayer has a monotonically changing lattice constant.

15. A multijunction solar cell assembly as defined in claim 11 wherein the InGaAs buffer layer has a thickness on the order of 1 μm.

16. A multijunction solar cell assembly as defined in claim 11 , wherein the first solar subcell is composed of InGaP.

17. A multijunction solar cell assembly as defined in claim 11 , wherein the first solar subcell includes an InGa(Al)P emitter region and an InGa(Al)P base region, and the second solar subcell includes an InGaP emitter layer.

18. A multijunction solar cell assembly as defined in claim 11 further including a bottom contact layer below the third solar subcell and making electrical contact therewith.

19. A multijunction solar cell assembly as defined in claim 11 further including a tunnel diode disposed below the second solar subcell and over the InGaAs buffer layer.

20. A multijunction solar cell assembly as defined in claim 11 wherein the InGaAlAs grading interlayer includes a compositionally step-graded InGaAlAs series of layers.