Inverted metamorphic multijunction solar cells
View Patent ↗Multijunction solar cells that may include a first solar subcell with a first band gap, and a second solar subcell disposed over the first solar subcell and having a second band gap smaller than said first band gap. The solar cells may also include a grading interlayer disposed over the second solar subcell that may include a third band gap greater than the second band gap. The grading interlayer may not include phosphorus. The solar cells may also include a third solar subcell disposed over the interlayer that is lattice mismatched with respect to the second solar subcell. The third solar subcell may have a fourth band gap smaller than the third band gap.
1. A multijunction solar cell comprising:
a first solar subcell composed of InGa(Al)P and having a first band gap;
a second solar subcell disposed over said first solar subcell and having a second band gap smaller than said first band gap, the second solar subcell being composed of In 0.015 GaAs;
a grading interlayer composed of InGaAlAs disposed over said second solar subcell and having a third band gap that is constant throughout its thickness and greater than said second band gap; and
a third solar subcell disposed over said interlayer that is lattice mismatched with respect to second solar subcell and having a fourth band gap smaller than said third band gap, the third solar subcell being composed of In 0.30 GaAs,
wherein the grading interlayer achieves a transition in lattice constant from the second solar subcell to the third solar subcell.
2. A multijunction solar cell as defined in claim 1 , wherein the grading interlayer includes layers composed of In x Ga 1-x AlAs with 0<x<1 such that the band gap remains constant at 1.50 eV throughout the layers.
3. A multijunction solar cell as defined in claim 1 , wherein said grading interlayer is a compositionally step-graded metamorphic buffer layer composed of a series of InGaAlAs layers with a monotonically changing lattice constant with a lower layer having a lattice constant substantially the same as the second solar subcell and an upper layer having a lattice constant substantially the same as the third solar subcell, the grading interlayer having a band gap value slightly greater than the band gap of the second solar subcell.
4. A multijunction solar cell as defined in claim 1 , wherein the grading interlayer is composed of a plurality of layers with monotonically changing lattice constant.
5. A multijunction solar cell as defined in claim 1 , wherein the grading interlayer includes at least five stepped layers.
6. A multijunction solar cell as defined in claim 5 , wherein the band gap of each of the stepped layers is greater than the second band gap.
7. A multijunction solar cell as defined in claim 1 , wherein the third band gap is slightly greater than the second band gap.
8. A multijunction solar cell as defined in claim 1 further comprising a buffer between the second solar subcell and the grading interlayer, wherein the composition of the buffer layer is different from the composition of the grading interlayer.
9. A multijunction solar cell as defined in claim 8 further comprising a tunnel diode layer between the second solar subcell and the buffer layer.
10. A multijunction solar cell as defined in claim 9 wherein the buffer layer is composed of InGaAs.
11. A multijunction solar cell as defined in claim 1 wherein the buffer layer has a thickness of about one micron.
12. A multijunction solar cell as defined in claim 9 wherein the tunnel diode is a p++/n++ tunnel diode.