Solar cells and methods of making the same

1. A solar cell comprising:

a first p-type semiconductor layer on an n-type layer;

an electron reflector layer on the first p-type semiconductor layer forming a first interface therebetween, the electron reflector layer having a thickness and comprising a CdMTe layer, wherein M is selected from the group consisting of Mg, Mn, and Zn; and

a second p-type semiconductor layer on the electron reflector layer forming a second interface therebetween, wherein the second interface is an interface of the second p-type semiconductor layer and the electron reflector layer;

wherein the electron reflector layer has a graded concentration of M such that a concentration of M increases from substantially zero at the first interface to a peak concentration at a position in the electron reflector layer between the first interface and the second interface, and decreases from the peak concentration to substantially zero at the second interface.

2. The solar cell of claim 1 , wherein the concentration of M in the CdMTe layer is sufficient at the peak concentration of M in the electron reflector layer to create a conduction band barrier height of at least about 120 meV at the position corresponding to the peak concentration of M, wherein the conduction band barrier height is an amount of energy relative to a conduction band energy of the first p-type semiconductor layer.

3. The solar cell of claim 2 , wherein the conduction band barrier height is between about 120 meV and about 200 meV at the position corresponding to the peak concentration of M.

4. The solar cell of claim 1 , wherein the position corresponding to the peak concentration of M in the electron reflector layer intermediate between the first interface and the second interface is approximately equidistant between the first interface and the second interface.

5. The solar cell of claim 1 , wherein the graded concentration has a triangular profile.

6. The solar cell of claim 2 , wherein M is Mg present at a mole fraction of at least about 17% at the position corresponding to the peak concentration of M.

7. The solar cell of claim 2 , wherein M is Mn present at a mole fraction of at least about 14% at the position corresponding to the peak concentration of M.

8. The solar cell of claim 2 , wherein M is Zn present at a mole fraction of at least about 31% at the position corresponding to the peak concentration of M.

9. The solar cell of claim 1 , wherein the CdMTe layer is doped with a dopant selected from the group consisting of nitrogen, phosphorus, arsenic, antimony, bismuth, copper, silver, and gold.

10. The solar cell of claim 1 , wherein the first p-type semiconductor layer is doped with a dopant selected from the group consisting of nitrogen, phosphorus, arsenic, antimony, bismuth, copper, silver, and gold.

11. The solar cell of claim 1 , wherein the second p-type semiconductor layer is doped with a dopant selected from the group consisting of nitrogen, phosphorus, arsenic, antimony, bismuth, copper, silver, and gold.

12. The solar cell of claim 1 , wherein the electron reflector layer is doped with a dopant selected from the group consisting of nitrogen, phosphorus, arsenic, antimony, bismuth, copper, silver, and gold.

13. The solar cell of claim 1 , wherein the electron reflector layer has a thickness ranging from about 10 nm to about 100 nm.

14. The solar cell of claim 1 , further comprising a second electron reflector layer on the second p-type semiconductor layer, and a third p-type semiconductor layer on the second electron reflector layer.

15. The solar cell of claim 14 , wherein the second electron reflector layer is doped with a dopant selected from the group consisting of nitrogen, phosphorus, arsenic, antimony, bismuth, copper, silver, and gold.

16. The solar cell of claim 1 , wherein:

the first p-type semiconductor layer comprises cadmium and tellurium;

the second p-type semiconductor layer comprises cadmium and tellurium; and

the electron reflector layer has a thickness ranging from about 10 nm to about 100 nm.

17. The solar cell of claim 1 , wherein the thickness of the electron reflector layer is below a critical thickness defined by an intrinsic lattice parameter of the first p-type semiconductor layer.

18. A solar cell comprising:

a first p-type semiconductor layer on an n-type layer, wherein the first p-type semiconductor layer comprises CdTe;

an electron reflector layer on the first p-type semiconductor layer forming a first interface therebetween, the electron reflector layer having a thickness in a range from about 10 nm to about 100 nm;

a second p-type semiconductor layer on the electron reflector layer forming a second interface therebetween, wherein the second interface is an interface of the second p-type semiconductor layer and the electron reflector layer, and wherein the second p-type semiconductor layer comprises an alloy including cadmium and tellurium; and

a back contact on the second p-type semiconductor layer, wherein the back contact comprises a ZnTe p+ layer; wherein:

the electron reflector layer comprises CdZnTe, where Zn is present at a mole fraction ranging from about 0.31 to about 0.51 at a peak concentration,

the peak concentration corresponds to a position within the electron reflector layer between the first interface and the second interface, and

a conduction band barrier height is in a range from about 120 meV to about 200 meV at the peak position in the electron reflector layer relative to a conduction band energy of the first p-type semiconductor layer.

19. A solar cell comprising:

a first p-type semiconductor layer on an n-type layer, the first p-type semiconductor layer comprising cadmium and tellurium;

a second p-type semiconductor layer, the second p-type semiconductor layer comprising cadmium and tellurium; and

an electron reflector layer between the first p-type semiconductor layer and the second p-type semiconductor layer, the electron reflector layer comprising a CdMTe layer, where M is selected from the group consisting of Zn, Mg, and Mn;

wherein the electron reflector layer comprises a graded concentration of M such that the electron reflector layer forms a first pseudo-interface with the first p-type semiconductor layer and a second pseudo-interface with the second p-type semiconductor layer;

wherein a concentration of M in the CdMTe layer is sufficient at a position in the electron reflector layer to create a conduction band barrier height of at least about 120 meV; and

wherein the graded concentration of M is not abrupt at either the first pseudo-interface or the second pseudo-interface.

20. The solar cell of claim 19 , wherein M is Zn, and the CdMTe layer comprises Cd 1-x Zn x Te, where x has a maximum value in a range from about 0.31 to about 0.51 in the electron reflector layer.