Back contact buffer layer for thin-film solar cells

1. A photovoltaic cell comprising:

a substrate layer;

a transparent conductive contact layer;

a semiconductor comprising:

a cadmium sulfide (CdS) layer having an n-type conductivity characteristic; and

a cadmium telluride (CdTe) layer having a p-type conductivity characteristic;

a buffer layer comprised of cadmium sulfide (CdS); and,

a back contact layer in direct contact with the buffer layer, the back contact layer comprising a conductive material containing an elemental constituent having a high work function.

2. The photovoltaic cell of claim 1 wherein the buffer layer is disposed between the CdTe layer and the back contact layer, the buffer layer further configured to have one of an intrinsic and slightly p-type conductivity characteristic.

3. The photovoltaic cell of claim 1 wherein the elemental constituent at least partially diffuses into the buffer layer, the elemental constituent having a concentration gradient through the buffer layer such that the concentration of the elemental constituent becomes less as the distance through the buffer layer from the back contact layer increases.

4. The photovoltaic cell of claim 3 wherein the elemental constituent diffuses from the back contact layer into the buffer layer, the diffusion being enhanced by way of thermal energy input.

5. The photovoltaic cell of claim 3 wherein the elemental constituent is copper and the concentration gradient is generally isolated to the CdS buffer layer.

6. The photovoltaic cell of claim 1 wherein the high work function is in the range of approximately 4.5 to 5.5 eV.

7. The photovoltaic cell of claim 1 wherein the elemental constituent comprises at least one of copper, silver, gold, titanium, carbon, and molybdenum.

8. The photovoltaic cell of claim 1 wherein the elemental constituent comprises a coating, a dopant, or a thin elemental layer, wherein the elemental constituent diffuses into the buffer layer to establish an elemental constituent gradient through at least the buffer layer.

9. The photovoltaic cell of claim 8 wherein the elemental constituent is one of copper, silver, gold, titanium, carbon, and molybdenum that at least partially diffuses through the CdS buffer layer and further at least partially diffuses through a portion of the CdTe p-type layer.

10. The photovoltaic cell of claim 1 wherein the CdS buffer layer has a thickness in a range of about 30 nanometers to about 50 nanometers.

11. The photovoltaic cell of claim 10 wherein the CdTe layer has a thickness in a range of about 300 nanometers to about 500 nanometers.

12. The photovoltaic cell of claim 10 wherein the CdS buffer layer has been exposed to vapors of cadmium chloride (CdCl 2 ).

13. The photovoltaic cell of claim 11 wherein the CdTe layer has been exposed to vapors of cadmium chloride (CdCl 2 ) before the CdS buffer layer is applied.

14. The photovoltaic cell of claim 1 wherein the semiconductor is a plurality of interconnected p-n or p-i-n semiconductor layers formed in a multi-junction cell configuration.

15. The photovoltaic cell of claim 1 wherein the CdS buffer layer improves at least one photovoltaic cell characteristic of efficiency, open circuit voltage, and fill-factor.

16. A photovoltaic cell comprising:

a flexible, transparent substrate layer;

a transparent conductive oxide layer;

a cadmium sulfide layer having an n-type conductivity characteristic;

a cadmium telluride layer having a p-type conductivity characteristic;

an intrinsic layer disposed between the cadmium sulfide layer and the cadmium telluride layer;

a buffer layer comprised of cadmium sulfide on the cadmium telluride layer; and,

a back contact layer in direct contact with the buffer layer, the back contact layer comprising a conductive material containing an elemental constituent having a high work function.

17. A photovoltaic cell comprising:

a flexible, transparent substrate layer;

a transparent conductive oxide layer;

a cadmium sulfide layer having an n-type conductivity characteristic;

a cadmium telluride layer having an intrinsic (i-type) characteristic;

a zinc telluride layer having a p-type conductivity characteristic;

a buffer layer comprised of cadmium sulfide; and,

a back contact layer in direct contact with the buffer layer, the back contact layer comprising a conductive material containing an elemental constituent having a high work function.

18. A method of forming a photovoltaic cell comprising the steps of:

providing a substrate layer;

applying a transparent conductive oxide (TCO) layer onto the substrate layer;

providing a deposition chamber having at least two target sources that include a cadmium sulfide (CdS) target source and a cadmium telluride (CdTe) target source;

depositing a CdS layer onto the TCO layer;

depositing a CdTe layer onto the CdS layer;

depositing a CdS buffer layer onto the CdTe layer within the same deposition chamber, the CdS buffer layer being formed from the CdS target source; and

applying a back contact layer directly onto the CdS buffer layer, the back contact layer comprising a conductive material containing an elemental constituent having a high work function.

19. The method of claim 18 including a cadmium chloride (CdCl 2 ) annealing step prior to the step of applying the back contact layer.

20. The method of claim 18 including a cadmium chloride (CdCl 2 ) annealing step after the step of applying the back contact layer.

21. A method of forming a photovoltaic cell comprising the steps of:

providing a substrate layer;

applying a transparent conductive oxide (TCO) layer onto the substrate layer;

providing a deposition chamber having at least two target sources that include a cadmium sulfide (CdS) target source and a cadmium telluride (CdTe) target source;

depositing a CdS layer onto the TCO layer;

depositing a CdTe layer onto the CdS layer;

depositing a CdS buffer layer onto the CdTe layer, the CdS buffer layer being formed from the CdS target source; and

applying a back contact layer that includes at least one of copper, silver, and gold directly onto the CdS buffer layer.

22. A method of forming a photovoltaic cell comprising the steps of:

providing a substrate layer;

applying a transparent conductive oxide (TCO) layer onto the substrate layer;

providing a deposition chamber having at least two target sources that include a cadmium sulfide (CdS) target source and a cadmium telluride (CdTe) target source;

depositing a CdS layer onto the TCO layer;

depositing a CdTe layer onto the CdS layer;

depositing a CdS buffer layer onto the CdTe layer within the same deposition chamber, the CdS buffer layer being formed from the CdS target source;

depositing a dopant of at least one of copper, silver, gold, molybdenum, titanium, and carbon directly onto the CdS buffer layer; and

applying a back contact layer onto the CdS buffer layer.

23. The method of claim 22 wherein the dopant is deposited within the same deposition chamber as the CdS buffer layer.

24. The method of claim 22 wherein a target source is provided for depositing the dopant, the target source including the at least one of copper, silver, gold, molybdenum, titanium, and carbon.

25. The method of claim 22 further comprising a step of treating the CdS and CdTe layers with an exposure to cadmium chloride (CdCl 2 ) conducted after the steps of depositing the CdS layer onto the TCO layer and depositing the CdTe layer onto the CdS layer.

26. The method of claim 22 further comprising a step of treating the CdS and CdTe layers with an exposure to cadmium chloride (CdCl 2 ) conducted after the step of forming the CdS buffer layer.

27. The method of claim 22 further comprising a step of treating the CdS and CdTe layers with an exposure to cadmium chloride (CdCl 2 ) conducted after the step of applying a back contact layer onto the CdS buffer layer.

28. The method of claim 22 wherein the dopant of at least one of copper, silver, gold, molybdenum, titanium, and carbon diffuses into the CdS buffer layer.