Broad spectrum solar cell

• US 20040118451A1
• Filed: 05/27/2003
• Published: 06/24/2004
• Est. Priority Date: 05/24/2002
• Status: Active Grant

• Current Assignee: Cornell Research Foundation Incorporated (Cornell University), Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Walukiewicz, Wladyslaw, Schaff, William J., Yu, Kin Man, Wu, Junqiao
• Granted Patent: US 7,217,882 B2
• Time in Patent Office: Days
• US Class Current: 136/262
• CPC Class Codes: H01L 31/0687 Multiple junction or tandem solar cells
• CPC Class Codes: H01L 31/0735 comprising only AIIIBV compound semiconductors, e.g. GaAs/AlGaAs or InP/GaInAs solar cells
• CPC Class Codes: Y02E 10/544 Solar cells from Group III-V materials

Abstract

An alloy having a large band gap range is used in a multijunction solar cell to enhance utilization of the solar energy spectrum. In one embodiment, the alloy is In_1-xGa_xN having an energy bandgap range of approximately 0.7 eV to 3.4 eV, providing a good match to the solar energy spectrum. Multiple junctions having different bandgaps are stacked to form a solar cell. Each junction may have different bandgaps (realized by varying the alloy composition), and therefore be responsive to different parts of the spectrum. The junctions are stacked in such a manner that some bands of light pass through upper junctions to lower junctions that are responsive to such bands.

Claims

1. A solar cell comprising:

a first junction based on a ternary alloy having a high bandgap; and

a second junction, based on the ternary alloy having different composition, electrically coupled to the first junction, the second junction having a low bandgap, where the relative bandgaps are adjusted to a desired range of the solar spectrum.

2. The solar cell of claim 1 wherein the ternary alloy is In_1-xGa_xN.

3. The solar cell of claim 2 wherein the In_1-xGa_xN alloy has an energy bandgap range of approximately 0.7 eV to 3.4 eV.

4. The solar cell of claim 1 wherein the first junction is closer to a source of solar energy and has a higher bandgap than the second junction.

5. The solar cell of claim 4 wherein a surface of the first junction closer to a source of solar energy is coated with an antireflective layer.

6. The solar cell of claim 1 wherein the first junction comprises n-type and p-type doped GaInN.

7. The solar cell of claim 5 wherein the second junction comprises n-type and p-type doped InN.

8. The solar cell of claim 1 wherein the bandgap of the junctions is adjusted between approximately 0.7 eV to 3.4 eV by varying the composition of the ternary alloy.

9. The solar cell of claim 1 and further comprising electrical contacts formed on opposite sides of the first and second junctions.

10. A solar cell comprising:

a first junction having a n-type and a p-type doped GaInN layer having a high bandgap;

a second junction having a n-type and a p-type doped InN layer having a low bandgap; and

a tunnel junction sandwiched between the first and second junctions.

11. The solar cell of claim 10 wherein the first junction has an energy bandgap of approximately 1.4 eV and the second junction has an energy bandgap of approximately 0.7.

12. The solar cell of claim 10 wherein the tunnel junction comprises an n-type and a p-type layer.

13. The solar cell of claim 12 wherein the tunnel junction layers are formed of InN.

14. The solar cell of claim 13 wherein the n-type layer is heavily doped.

15. The solar cell of claim 10 wherein doping levels for the n-type and p-type layers of the junctions range from approximately 10¹⁷ cm⁻

3 to 10¹⁸ cm<sup>−

3</sup>.

16. The solar cell of claim 11 wherein the first junction comprises In_1-xGa_xN, where x is approximately 0.39.

17. A solar cell comprising:

a first junction having a n and a p doped GaInN layer having a high bandgap;

a second junction having a n and a p doped InN layer having a low bandgap;

a front contact coupled to the first junction;

a back contact coupled to the second junction;

a set of interior contacts, wherein the interior contacts are coupled to respective first and second junctions, and aligned to provide electrical contact there between when the first and second junctions are in a stacked relationship.

18. The solar cell of claim 17 wherein the first junction has an energy bandgap of approximately 1.4 eV and the second junction has an energy bandgap of approximately 0.7 eV.

19. The solar cell of claim 17 wherein the first junction comprises In_1-xGa_xN, where x is approximately 0.39.

20. A multijunction solar cell comprising:

multiple electrically coupled junctions in stacked relationship, the junctions having a n-type and a p-type doped GaInN layer;

a bottom junction positioned beneath, and electrically coupled to the stack of multiple junctions, the bottom junction having a n-type and a p-type doped InN layer, wherein the bottom junction has a low energy bandgap, and the energy bandgap of each successive junction of the multiple stacked junctions from the bottom layer increases such that a top layer junction has the highest energy bandgap.

21. The solar cell of claim 20 wherein the junctions are formed of In_1-xGa_xN, wherein x is varied to adjust the energy bandgap.

22. The solar cell of claim 21 wherein the energy bandgap of the bottom and multiple layers has a range of approximately 0.7 eV to 3.4 eV.

23. The solar cell of claim 21 wherein x is approximately zero for the bottom junction.

24. The solar cell of claim 21 comprising three junctions having have x of approximately 0.55, 0.27 and 0.0.

25. The solar cell of claim 20 wherein the junctions are electrically coupled by a tunnel junction formed between each adjacent junction.

26. The solar cell of claim 20 wherein the junctions are electrically coupled by ohmic contacts formed on adjacent sides of successive junctions.

27. A method of forming a multijunction solar cell using a single alloy system, the method comprising:

forming a first junction on top of a buffer layer supported by a substrate, wherein the first junction comprises In_1-xGa_xN;

forming a tunnel junction on top of the first junction;

forming a second junction on top of the tunnel junction, wherein the second junction comprises In_1-xGa_xN, wherein x for the second junction is larger than x for the first junction;

removing the buffer layer and the substrate layer; and

forming contacts on the first junction and the second junction to form the solar cell.

28. The method of claim 27 and further comprising forming an antireflection coating on the second junction.

29. The method of claim 27 and further comprising forming further layers of tunnel junctions and junctions comprising In_1-xGa_xN between the first and second junctions, where x increases in successive junctions above the first junction.

30. The method of claim 27 wherein the first and second junctions are grown using molecular beam epitaxy.

31. A method of forming a multijunction solar cell, the method comprising:

forming multiple junctions on top of buffer layers supported by a substrate, wherein the multiple junctions comprises In_1-xGa_xN;

removing the buffer layers and the substrate layers from each of the junctions;

mechanically stacking the junctions such that a top of the stack is closest to a solar energy source, and wherein x decreases in each successive layer from the top; and

forming contacts on junctions to electrically connect each successive junction to adjacent junctions.

32. The method of claim 31 and further comprising forming an antireflection coating on each junction.