Photovoltaic cell with high efficiency cigs absorber layer with low minority carrier lifetime and method of making thereof

1. A solar cell comprising:

a substrate;

a first electrode located over the substrate;

at least one copper indium gallium selenide p-type semiconductor absorber layer containing sodium;

an n-type semiconductor layer located over the p-type semiconductor layer; and

a second electrode located over the n-type semiconductor layer, wherein the solar cell has a conversion efficiency that is at least 13.4 percent average active area with a minority carrier lifetime below 2 nanoseconds,

wherein:

the p-type semiconductor absorber layer is formed with a graded composition employing steps of forming a first copper indium gallium selenide sublayer adjacent to the first electrode, forming a second copper indium gallium selenide sublayer over the first sublayer, and forming a third copper indium gallium selenide sublayer over the second sublayer and adjacent to the n-type semiconductor layer;

the first sublayer is formed with a higher ratio of Ga/(Ga+In) than the second sublayer;

the first sublayer is formed with a ratio of Ga/(Ga+In) of 0.4 to 0.6, and a ratio of Cu/(Ga+In) of 0.75 to 0.9;

the second sublayer is formed with a ratio of Ga/(Ga+In) of 0.2 to 0.3, and a ratio of Cu/(Ga+In) of 0.85 to 0.99; and

the third sublayer is formed with a ratio of Ga/(Ga+In) of 0.25 to 0.35, and a ratio of Cu/(Ga+In) between 0.05 and 0.3.

2. The solar cell of claim 1 , wherein:

the solar cell is located in a module; and

the module has an active area greater than or equal to 1 m 2 and a conversion efficiency greater than or equal to 15.7 percent average active area.

3. The solar cell of claim 2 , wherein the module has a conversion efficiency of 15.7 to 16.1 percent average active area and the minority carrier lifetime of 1.6 nanoseconds or less.

4. The solar cell of claim 1 , wherein the solar cell has an active area of ¼ cm 2 to ½ cm 2 and a conversion efficiency greater than or equal to 14.4 percent average active area.

5. The solar cell of claim 4 , wherein the solar cell has the conversion efficiency of 16.6 to 18.1 percent average active area.

6. The solar cell of claim 1 , wherein the solar cell has an open circuit voltage that is greater than 550 mV.

7. The solar cell of claim 6 , wherein the solar cell has an open circuit voltage of at least 650 mV.

8. The solar cell of claim 1 , wherein the solar cell has a minority carrier lifetime of less than 1 nanosecond.

9. The solar cell of claim 8 , wherein the solar cell has a minority carrier lifetime of 0.3 to 0.8 nanoseconds.

10. The solar cell of claim 1 , wherein the first electrode comprises a first transition metal layer which comprises a sodium and oxygen containing molybdenum layer.

11. The solar cell of claim 10 , wherein sodium diffuses from the first transition metal layer into the p-type semiconductor absorber layer during the step of depositing the p-type semiconductor absorber layer, and wherein the first transition metal layer comprises at least 59 atomic percent molybdenum, 5 to 40 atomic percent oxygen and 0.01 to 1.5 atomic percent sodium.

12. The solar cell of claim 11 , wherein:

the first electrode further comprises a molybdenum barrier layer located over the first transition metal layer;

the molybdenum barrier layer is substantially free of oxygen and has a higher density than the first transition metal layer; and

sodium diffuses from the first transition metal layer through the molybdenum barrier layer into the p-type semiconductor absorber layer during the step of reactively sputtering the p-type semiconductor absorber layer.

13. The solar cell of claim 10 , wherein the p-type semiconductor absorber layer has a band gap of 1.14 eV or higher.

14. The solar cell of claim 13 , wherein the p-type semiconductor absorber layer has a band gap in a range of 1.3 to 1.5 eV, and wherein the p-type semiconductor absorber layer has a majority carrier concentration of about 1×10 13 to about 1×10 15 cm −3 .

15. The solar cell of claim 1 , wherein the solar cell comprises a flexible solar cell on a flexible substrate and the solar cell is formed in a shape of roofing shingle.

16. A solar cell comprising:

a substrate;

a first electrode located over the substrate;

at least one copper indium gallium selenide p-type semiconductor absorber layer containing sodium;

an n-type semiconductor layer located over the p-type semiconductor layer; and

a second electrode located over the n-type semiconductor layer;

wherein:

the solar cell has a conversion efficiency that is at least 13.4 percent average active area with a minority carrier lifetime below 2 nanoseconds;

the solar cell has an open circuit voltage that is greater than 550 mV;

the first electrode comprises a first transition metal layer which comprises a sodium and oxygen containing molybdenum layer;

sodium diffuses from the first transition metal layer into the p-type semiconductor absorber layer during the step of depositing the p-type semiconductor absorber layer;

the p-type semiconductor absorber layer has a band gap of 1.14 eV or higher;

the p-type semiconductor absorber layer has a graded composition as a function of thickness and a majority carrier concentration of about 1×10 13 to about 1×10 15 cm −3 ;

the graded composition p-type semiconductor absorber layer comprises a first copper indium gallium selenide sublayer adjacent to the first electrode, a second copper indium gallium selenide sublayer over the first sublayer, and a third copper indium gallium selenide sublayer over the second sublayer and adjacent to the n-type semiconductor layer;

the first sublayer is formed with a higher ratio of Ga/(Ga+In) than the second sublayer; and

the third sublayer is formed with a lower ratio of Cu/(Ga+In) than the first and the second sublayers;

the first sublayer is formed with a ratio of Ga/(Ga+In) of 0.4 to 0.6, and a ratio of Cu/(Ga+In) of 0.75 to 0.9;

the second sublayer is formed with a ratio of Ga/(Ga+In) of 0.2 to 0.3, and a ratio of Cu/(Ga+In) of 0.85 to 0.99; and

the third sublayer is formed with a ratio of Ga/(Ga+In) of 0.25 to 0.35, and a ratio of Cu/(Ga+In) between 0.05 and 0.3.

17. The solar cell of claim 16 , wherein:

the solar cell is located in a module;

the module has an active area greater than or equal to 1 m 2 ;

the module has a conversion efficiency of 15.7 to 16.1 percent average active area;

the solar cell has an open circuit voltage of 650-660 mV;

the first transition metal layer comprises at least 59 atomic percent molybdenum, 5 to 40 atomic percent oxygen and 0.01 to 1.5 atomic percent sodium;

the first electrode further comprises a molybdenum barrier layer located over the first transition metal layer;

the molybdenum barrier layer is substantially free of oxygen and has a higher density than the first transition metal layer; and

sodium diffuses from the first transition metal layer through the molybdenum barrier layer into the p-type semiconductor absorber layer during the step of reactively sputtering the p-type semiconductor absorber layer; and

the p-type semiconductor absorber layer has a band gap in a range of 1.3 to 1.5 eV.