Photovoltaic cell with copper poor CIGS absorber layer and method of making thereof

1. A method of making a photovoltaic cell comprising:

depositing a first electrode layer over a substrate;

depositing a first p-type copper-indium-gallium-selenide absorber layer over the first electrode;

depositing a second copper-indium-gallium-selenide absorber layer over the first copper-indium-gallium-selenide absorber layer, wherein the second absorber layer has less copper than the first p-type copper-indium-gallium-selenide absorber layer

depositing an n-type semiconductor layer over the second copper-indium-gallium-selenide absorber layer; and

depositing a second electrode layer over the n-type semiconductor layer,

wherein a content of the Cu, In, and Ga in the first copper-indium-gallium-selenide absorber layer satisfies the equation Cu/(In+Ga)≧0.6,

the first copper-indium-gallium-selenide absorber layer is deposited using a first copper-indium-gallium sputtering target, and

the second copper-indium-gallium-selenide absorber layer is deposited using a second copper-indium-gallium sputtering target in which a content of the Cu, In, and Ga in the second copper-indium-gallium-selenide sputtering target satisfies the equation Cu/(In+Ga)≦0.3,

wherein the content is measured in atomic percent.

2. The method of claim 1 , wherein the first and second copper-indium-gallium-selenide absorber layers are deposited by sputtering in a selenium containing ambient.

3. The method of claim 2 , wherein the sputtering of the second absorber layer is performed at a substrate temperature≧600° C.

4. The method claim 3 , wherein the sputtering of the second absorber layer is performed at a substrate temperature of 600-700° C.

5. The method of claim 2 , wherein depositing is performed with a series of sputtering targets and a last sputtering target in the series of sputtering targets includes a sulfur containing compound.

6. The method of claim 5 , wherein the sulfur compound comprises one or more of copper sulfide or sodium sulfide.

7. The method of claim 5 , wherein at least one sputtering target in a series of sputtering targets includes a sodium containing compound.

8. The method of claim 7 , wherein the sodium containing compound comprises one or more of sodium fluoride, sodium molybdate, sodium selenide, sodium hydroxide, sodium oxide, sodium sulfate, sodium selenate, sodium sulfide, sodium sulfite, sodium titanate, sodium metavanadate, or sodium orthovanadate.

9. The method of claim 2 , wherein the last sputtering target in a series of sputtering targets includes a sulfur containing compound and a sodium containing compound.

10. The method of claim 1 , wherein the Cu content is graded along a thickness of the compound semiconductor layer.

11. The method of claim 1 further comprising:

moving a web substrate through a plurality of sputtering chambers; and

forming the first and second copper-indium-gallium-selenide absorber layers in the same or different sputtering chambers.

12. The method of claim 11 , wherein:

depositing the first copper-indium-gallium-selenide absorber layer comprises sputtering from a plurality of first sputtering targets each comprising copper, indium and gallium; and

depositing the second copper-indium-gallium-selenide absorber layer comprises sputtering from a plurality of second sputtering targets each comprising copper, indium and gallium,

wherein the at least one second target contains less copper than the at least one first target.

13. The method of claim 12 , wherein the at least one second target contains a sodium salt or a sulfur compound.

14. The method of claim 11 , wherein the first copper-indium-gallium-selenide absorber layer is deposited at a lower temperature than the second copper-indium-gallium-selenide absorber layer.

15. The method of claim 1 , wherein the first copper-indium-gallium sputtering target contains about 29-41 wt % copper, about 36-62 wt % indium, and about 8-25 wt % gallium, and the second copper-indium-gallium sputtering target contains about 8-15 wt % copper, about 55-80 wt % indium, and about 10-25 wt % gallium.