Tandem solar cell and method of manufacturing same

1. A tandem solar cell comprising:

a substrate;

a front electrode disposed on the substrate;

a back electrode disposed opposite to the front electrode on the substrate;

a first cell disposed below the front electrode and including a first buffer layer and a first light absorption layer; and

a second cell disposed above the back electrode and including a second light absorption layer and a second buffer layer, wherein

the first light absorption layer consists of a CuGaSeS layer and a CuGaSe layer,

the second light absorption layer includes a semiconductor compound selected from the group consisting of CuInSe 2 , CuInGaSe 2 , CuInSeS, CuInGaSeS and a combination thereof, and

the CuGaSeS layer of the first light absorption layer is disposed closer than the CuGaSe layer of the first light absorption layer to the front electrode.

2. The tandem solar cell of claim 1 , wherein

the CuGaSeS layer includes sulfur, and

a content of the sulfur is not more than about 0.3 parts by weight when a content of selenium in the CuGaSeS layer is about 100 parts by weight.

3. The tandem solar cell of claim 1 , wherein a concentration gradient of the sulfur in the CuGaSeS layer decreases along a direction from the front electrode toward the CuGaSe layer.

4. The tandem solar cell of claim 3 , wherein a concentration of sulfur on a first surface of the CuGaSeS layer, the first surface being closer to the front electrode than the CuGaSe layer, and a concentration of sulfur on a second surface of the CuGaSeS layer, the second surface being closer to the CuGaSe layer than the front electrode, have a concentration gradient of about 1 atomic percent to about 0.01 atomic percent through about 1 atomic percent to about 0.3 atomic percent.

5. The tandem solar cell of claim 1 , wherein a ratio of a thickness of the CuGaSeS layer to a thickness of the CuGaSe layer is from about 1:10 to about 1:2.

6. The tandem solar cell of claim 1 , wherein a bandgap energy of the CuGaSeS layer is from about 1.6 electron volts to about 1.9 electron volts.

7. The tandem solar cell of claim 1 , wherein a bandgap energy of the CuGaSe layer is from about 1.5 electron volts to about 1.8 electron volts.

8. The tandem solar cell of claim 1 , wherein a bandgap energy of the second light absorption layer is from about 1.0 electron volt to about 1.4 electron volts.

9. The tandem solar cell of claim 1 , wherein a bandgap energy difference between the first light absorption layer and the second light absorption layer is from about 0.1 electron volts to about 1.0 electron volt.

10. The tandem solar cell of claim 1 , wherein a bandgap energy difference between the first light absorption layer and the second light absorption layer is from about 0.3 electron volts to about 0.7 electron volts.

11. The tandem solar cell of claim 1 , wherein the first cell and the second cell are connected by a tunnel diode.

12. A method of manufacturing a tandem cell, comprising:

disposing a back electrode on a substrate;

forming a first cell by disposing a first light absorption layer and a first buffer layer on the substrate;

forming a second cell by disposing a second light absorption layer and a second buffer layer on the substrate; and

disposing a front electrode on the substrate, wherein

the first light absorption layer consists of a CuGaSeS layer and a CuGaSe layer,

the second light absorption layer includes a semiconductor compound selected from the group consisting of CuInSe 2 , CuInGaSe 2 , CuInSeS, CuInGaSeS and any combinations thereof, and

the CuGaSeS layer is disposed closer than the CuGaSe layer to the front electrode.

13. The method of claim 12 , wherein

the CuGaSeS layer includes sulfur, and

a content of the sulfur is not more than about 0.3 parts by weight when a content of selenium in the CuGaSeS layer is about 100 parts by weight.

14. The method of claim 12 , wherein a concentration gradient of the sulfur in the CuGaSeS layer decreases along a direction from the front electrode toward the CuGaSe layer.

15. The method of claim 14 , wherein a concentration of sulfur on a first surface of the CuGaSeS layer, the first surface being closer to the front electrode than the CuGaSe layer, and a concentration of sulfur on a second surface of the CuGaSeS layer, the second surface being closer to the CuGaSe layer than the front electrode, have a concentration gradient of about 1 atomic percent to about 0.01 atomic percent through about 1 atomic percent to about 0.3 atomic percent.

16. The method of claim 12 , wherein a ratio of a thickness of the CuGaSeS layer to a thickness of the CuGaSe layer is from about 1:10 to about 1:2.

17. The method of claim 12 , wherein a bandgap energy of the second light absorption layer is from about 1.0 electron volt to about 1.4 electron volts.

18. The method of claim 12 , wherein a bandgap energy difference between the first light absorption layer and the second light absorption layer is from about 0.1 electron volts to about 1.0 electron volt.

19. The method of claim 12 , wherein a bandgap energy difference between the first light absorption layer and the second light absorption layer is from about 0.3 electron volts to about 0.7 electron volts.

20. The method of claim 12 , wherein the first cell and the second cell are connected by a tunnel diode.