Hexagonal phase epitaxial cadmium sulfide on copper indium gallium selenide for a photovoltaic junction

1. A method of manufacturing a photovoltaic structure, comprising:

forming a p-type semiconductor absorber layer comprising a copper indium gallium selenide based material over a first electrode;

forming a single crystal n-type cadmium sulfide layer having a predominant hexagonal crystal structure over the p-type semiconductor absorber layer by sputtering in an ambient including hydrogen gas and oxygen gas, the ambient having a partial pressure of the hydrogen gas that is greater than a partial pressure of the oxygen gas by 1.25 mTorr to 1.875 mTorr, wherein during the sputtering cadmium atoms from the n-type cadmium sulfide layer diffuse into an upper portion of the p-type semiconductor absorber layer to form a n-type cadmium doped copper indium gallium selenide semiconductor upper portion of the semiconductor absorber layer and thereby create a p-n junction between a p-type copper indium gallium selenide semiconductor lower portion of the absorber layer and the n-type cadmium doped copper indium gallium selenide semiconductor upper portion of the absorber layer; and

forming a second electrode over the cadmium sulfide layer,

wherein the ambient consists essentially of an inert sputter gas having a partial pressure in a range from 2.5 mTorr to 7.5 mTorr; the hydrogen gas having a partial pressure in a range from 1.25 mTorr to 3.75 mTorr; and the oxygen gas having a partial pressure in a range from 0.875 mTorr to 1.5 mTorr; and

wherein the step of forming the n-type cadmium sulfide layer by sputtering comprises sputtering the n-type cadmium sulfide layer from a CdS target in the ambient.

2. The method of claim 1 , wherein:

the ambient includes an inert sputtering gas; and

a total pressure of the ambient is in a range from 5 mTorr to 10 mTorr.

3. The method of claim 1 , wherein:

hydrogen gas is flowed into a sputtering chamber at a first flow rate;

oxygen gas is flowed into the sputtering chamber at a second flow rate; and

a difference between the first flow rate and the second flow rate is in a range from 30 sccm to 80 sccm.

4. The method of claim 1 , wherein forming the n-type cadmium sulfide layer comprises forming a single crystal intrinsically doped or zinc doped cadmium sulfide layer having the predominant hexagonal crystal structure by the sputtering in the ambient.

5. The method of claim 1 , wherein forming the n-type cadmium sulfide layer comprises forming a single crystal copper cadmium sulfide layer having the predominant hexagonal crystal structure by the sputtering in the ambient.

6. The method of claim 5 , wherein the single crystal copper cadmium sulfide layer is composed predominantly of one or more hexagonal phase cadmium sulfide grains.

7. The method of claim 1 , wherein forming the n-type cadmium sulfide layer comprises forming a first single crystal copper cadmium sulfide layer having the predominant hexagonal crystal structure by the sputtering in the ambient and forming a second single crystal cadmium sulfide layer having the predominant hexagonal crystal structure over the first single crystal copper cadmium sulfide layer by the sputtering in the ambient.

8. The method of claim 1 , wherein forming the second electrode comprises forming a zinc containing oxide layer such that zinc diffuses into the n-type cadmium sulfide layer to form a zinc doped cadmium sulfide layer.

9. The method of claim 8 , wherein oxygen and the zinc diffuse into the n-type cadmium sulfide layer to form a zinc and oxygen doped cadmium sulfide layer.

10. The method of claim 1 , wherein the n-type cadmium sulfide layer is doped with one or more of copper, zinc, oxygen or hydrogen.

11. The method of claim 1 , wherein the p-n junction is a homojunction.

12. The method of claim 1 , wherein the p-n junction is a p-n heterojunction.