THIN FILM SOLAR CELL WITH CERAMIC HANDLING LAYER

• US 20110186117A1
• Filed: 04/23/2010
• Published: 08/04/2011
• Est. Priority Date: 03/08/2008
• Status: Active Grant

• Current Assignee: Svagos Technick, Inc.
• Original Assignee: Crystal Solar Inc.
• Inventors: Gopal, Vidyut, Ravi, Tirunelveli S., Kumar, Ananda H.
• Granted Patent: US 8,481,357 B2
• Time in Patent Office: Days
• US Class Current: 136/256
• CPC Class Codes: C03C 3/091 containing aluminium
• CPC Class Codes: C03C 3/097 containing phosphorus, niobium or tantalum
• CPC Class Codes: C03C 8/08 containing phosphorus
• CPC Class Codes: C03C 8/14 Glass frit mixtures having non-frit additions, e.g. opacifiers, colorants, mill-additions
• CPC Class Codes: H01L 31/02167 for solar cells
• CPC Class Codes: H01L 31/02168 the coatings being antireflective or having enhancing optical properties for the solar cells
• CPC Class Codes: H01L 31/022425 for solar cells
• CPC Class Codes: H01L 31/02363 of the semiconductor body itself, e.g. textured active layers
• CPC Class Codes: H01L 31/028 including, apart from doping material or other impurities, only elements of Group IV of the Periodic Table
• CPC Class Codes: H01L 31/048 Encapsulation of modules
• CPC Class Codes: H01L 31/0682 back-junction, i.e. rearside emitter, solar cells, e.g. interdigitated base-emitter regions back-junction cells
• CPC Class Codes: H01L 31/072 the potential barriers being only of the PN heterojunction type
• CPC Class Codes: H01L 31/0725 Multiple junction or tandem solar cells
• CPC Class Codes: H01L 31/0747 comprising a heterojunction of crystalline and amorphous materials, e.g. heterojunction with intrinsic thin layer
• CPC Class Codes: H01L 31/1804 comprising only elements of Group IV of the Periodic Table
• CPC Class Codes: H01L 31/1896 for thin-film semiconductors
• CPC Class Codes: Y02E 10/546 Polycrystalline silicon PV cells
• CPC Class Codes: Y02E 10/547 Monocrystalline silicon PV cells
• CPC Class Codes: Y02P 70/50 Manufacturing or production processes characterised by the final manufactured product

Abstract

A method for fabricating a photovoltaic (PV) cell panel wherein all PV cells are formed simultaneously on a two-dimensional array of monocrystalline silicon mother wafers affixed to a susceptor is disclosed. Porous silicon separation layers are anodized in the surfaces of the mother wafers. The porous film is then smoothed to form a suitable surface for epitaxial film growth. An epitaxial reactor is used to grow n- and p-type films forming the PV cell structures. A glass/ceramic handling layer is then formed on the PV cell structures. The PV cell structures with handling layers are then exfoliated from the mother wafer. The array of mother wafers may be reused multiple times, thereby reducing materials costs for the completed solar panels. The glass/ceramic handling layers provide structural integrity to the thin epitaxial solar cells during the separation process and subsequent handling.

Claims

1. A method of fabricating a solar cell comprising:

forming a stack of thin continuous epitaxial solar cell layers on a silicon wafer;

forming a handling layer on said stack, wherein said handling layer includes electrical contacts to said stack; and

separating said stack from said silicon wafer, wherein said stack remains attached to said handling layer.

2. A method as in claim 1, wherein said stack has a thickness in the range of 1 to 100 microns.

3. A method as in claim 2, wherein said stack has a thickness in the range of 25 to 50 microns.

4. A method as in claim 1, wherein said stack comprises a p-type layer.

5. A method as in claim 1, wherein said stack comprises a n-type layer.

6. A method as in claim 1, wherein said silicon wafer includes a porous silicon surface layer and said separating includes applying a separating force to fracture said porous silicon layer.

7. A method as in claim 1, wherein said silicon wafer is selected from the group consisting of a polycrystalline silicon wafer and a single crystal silicon wafer.

8. A method as in claim 1, wherein said thin continuous epitaxial solar cell layers include a layer selected from the group consisting of a silicon-germanium layer, a silicon carbide layer, a gallium nitride layer and a silicon layer.

9. A method as in claim 1, wherein said handling layer comprises a material selected from the group consisting of glass, glass-bonded ceramic and glass-ceramic.

10. A method as in claim 9, wherein said handling layer comprises cordierite glass-ceramic.

11. A method as in claim 9, wherein said handling layer has a thickness in the range of 5 to 1000 microns.

12. A method as in claim 9, wherein said handling layer has a thickness in the range of 250 to 500 microns.

13. A method as in claim 1, wherein said handling layer has a coefficient of thermal expansion greater than or equal to the coefficient of thermal expansion of said stack over a temperature range from ambient temperatures to a maximum temperature, said maximum temperature being the highest temperature reached during said forming said handling layer.

14. A method as in claim 1, wherein said handling layer has a coefficient of thermal expansion in the range from 5% less than to 25% greater than the coefficient of thermal expansion of said stack over a temperature range from ambient temperatures to a maximum temperature, said maximum temperature being the highest temperature reached during said forming said handling layer.

15. A method as in claim 1, wherein said forming said handling layer includes:

depositing a powder; and

sintering said powder.

16. A method as in claim 15, wherein said powder comprises ceramic powder and glass powder.

17. A method as in claim 15, wherein said powder is a powder paste.

18. A method as in claim 15, wherein said depositing is nozzle dispensing.

19. A method as in claim 18, wherein said nozzle dispensing is dispensing from multiple nozzles simultaneously.

20. A method as in claim 15, wherein said powder is green tape.

21. A method as in claim 20, wherein said green tape has preformed openings.

22. A method as in claim 21, further comprising, before said sintering, filling said preformed openings with a metal paste.

23. A method as in claim 15, wherein said sintering is firing in a temperature controlled furnace.

24. A method as in claim 15, further comprising, after said sintering, forming vias in said handling layer.

25. A method as in claim 24, wherein said forming said vias includes laser drilling.

26. A method as in claim 24, further comprising, after forming said vias, filling said vias with electrically conductive material.

27. A method as in claim 1, further comprising, before said forming said handling layer on said stack, forming a boundary layer on said stack.

28. A method as in claim 27, wherein said boundary layer is selected from the group consisting of a silicon oxide layer, a silicon nitride layer and an alumina layer.

29. A method as in claim 28, wherein said boundary layer has a thickness greater than 20 nanometers.

30. A method as in claim 28, wherein said boundary layer has a thickness in the range of 30 to 100 nanometers.

31. A method as in claim 1, further comprising, after said separating:

etching the surface of said stack opposite to said handling layer to form a textured surface; and

forming an antireflection coating over said textured surface.

32. A method as in claim 1, further comprising, before said forming said handling layer on said stack, forming electrical contacts on the surface of said stack.

33. A solar cell comprising:

a stack of thin single crystal solar cell layers;

a handling layer; and

a boundary layer between said stack and said handling layer, said boundary layer being attached to both said stack and said handling layer, said boundary layer being greater than 10 nanometers thick and parallel to the layers in said stack.

34. A solar cell as in claim 33, wherein said handling layer has a coefficient of thermal expansion greater than or equal to the coefficient of thermal expansion of said stack over a temperature range from ambient temperatures to a maximum temperature, said maximum temperature being the highest temperature reached during the forming of said handling layer.

35. A solar cell as in claim 33, wherein said handling layer has a coefficient of thermal expansion in the range from 5% less than to 25% greater than the coefficient of thermal expansion of said stack over a temperature range from ambient temperatures to a maximum temperature, said maximum temperature being the highest temperature reached during the forming of said handling layer.

36. A solar cell as in claim 33, wherein said handling layer reflects back into said stack visible light passing through said stack towards said handling layer.

37. A solar cell as in claim 33, wherein the interface between said handling layer and said boundary layer reflects visible light passing through said stack back into said stack.

38. A solar cell as in claim 33, further comprising electrical contacts at the interface between said stack and said handling layer.

40. A solar cell as in claim 33, wherein said handling layer has a waffle-shaped structure with a first set of ridges and a second set of ridges, said first set of ridges being perpendicular to said second sets of ridges.

41. A solar cell as in claim 40, wherein at least one of said first and second sets of ridges are aligned at a small angle to a cleavage plane of said stack of thin single crystal solar cell layers.

42. A solar cell as in claim 41, wherein said cleavage plane is the 100 family of planes.

43. A solar cell as in claim 41, wherein said small angle is in the range between 3 and 10 degrees.

44. A solar cell as in claim 33, wherein said handling layer has a waffle-shaped structure with an array of roughly square apertures.

45. A solar cell as in claim 44, wherein the square apertures are roughly less than 500 microns on a side.

46. A solar cell as in claim 33, wherein said handling layer has a waffle-shaped structure with an array of circular apertures.

47. A solar cell as in claim 33, wherein said boundary layer is selected from the group consisting of a silicon oxide layer, a silicon nitride layer and an alumina layer.

39. A solar cell as in claim 48, wherein said handling layer comprises metallic vias configured to provide electrical connections to said electrical contacts.