Multijunction solar cell with strained-balanced quantum well middle cell

1 . A multijunction photovoltaic cell, comprising:

a top subcell;

a second subcell disposed immediately adjacent to said top subcell, for producing a first photo-generated current; and including a sequence of first and second different semiconductor layers with different lattice constant; and

a lower subcell disposed immediately adjacent to said second subcell, for producing a second photo-generated current substantially equal in amount to the first photo-generated current.

2 . A multijunction photovoltaic cell as defined in claim 1 , wherein the sequence of first and second different semiconductor layers forms the base layer of the second subcell.

3 . A multijunction photovoltaic cell as defined in claim 1 , wherein the sequence of first and second different semiconductor layers comprises compressively strained and tensionally strained layers, respectively.

4 . A multifunction photovoltaic cell as defined in claim 1 , wherein an average strain of the sequence of first and second different semiconductor layers is approximately equal to zero.

5 . A multijunction photovoltaic cell as defined in claim 1 , wherein the first and second semiconductor layers are III-V semiconductor compounds, and the lower subcell is composed of germanium.

6 . A multijunction photovoltaic cell as defined in claim 1 , wherein each of the first and second semiconductor layers is approximately 100 nm to 300 angstroms thick.

7 . A multijunction photovoltaic cell as defined in claim 1 , wherein the first semiconductor layer comprises InGaAs and the second semiconductor layer comprises GaAsP.

8 . A multijunction photovoltaic cell as defined in claim 7 , wherein a percentage of indium in each InGaAs layer is in the range of 10 to 30%.

9 . A multifunction photovoltaic cell as defined in claim 1 , wherein the top subcell comprises InGaP and has a thickness so that it generates approximately 4-5% less current than said first current.

10 . A multijunction solar cell comprising:

a semiconductor substrate; and

a sequence of semiconductor layers disposed over the substrate and adapted to form a stack of subcells with the substrate, wherein a middle subcell of the stack comprises a sequence of alternating first and second different semiconductor layers, and wherein an average lattice constant of the sequence of alternating first and second semiconductor layers is approximately equal to a lattice constant of the substrate.

11 . The multijunction solar cell of claim 10 , wherein the sequence of first and second semiconductor layers form a base layer of the middle subcell.

12 . The multijunction solar cell of claim 10 , wherein the sequence of first and second semiconductor layers is disposed between a base layer and emitter layer of the middle subcell.

13 . The multijunction solar cell of claim 10 , wherein the total thickness of the sequence of first and second semiconductor layers is approximately 3 microns.

14 . The multifunction solar cell of claim 10 , wherein the thickness of each of the first and second semiconductor layers is in the range of 100 to 300 angstroms.

15 . A multijunction solar cell comprising:

a semiconductor structure including a sequence of semiconductor layers disposed and adapted to form a vertical stack of solar subcells; and

a plurality of semiconductor layers disposed in a middle subcell in the stack, wherein each alternating layer comprises a first compressively strained semiconductor layer and a second tensionally strained semiconductor layer.

16 . The multijunction solar cell of claim 15 , wherein the thickness of the layers of the middle solar cell is selected so that photo-generated current of the middle subcell is substantially equal to the photo-generated current density of the lower subcell adjacent to the middle subcell.

17 . The multijunction solar cell of claim 15 , wherein the semiconductor structure is grown on a semiconductor substrate and the average lattice constant of the plurality of semiconductor layers in the middle cell is approximately equal to the lattice constant of the semiconductor substrate.

18 . The multijunction solar cell of claim 15 , wherein the average of the strains of the plurality of strained layers is approximately zero.

19 . The multifunction solar cell of claim 15 , wherein each strained semiconductor layer comprises a III-V semiconductor compound and forms a quantum well.

20 . The multifunction solar cell of claim 15 , wherein the plurality of semiconductor layers have a total thickness of approximately 3 microns.