Photovoltaic devices comprising zinc stannate buffer layer and method for making

• US 6,169,246 B1
• Filed: 09/08/1998
• Issued: 01/02/2001
• Est. Priority Date: 09/08/1998
• Status: Expired due to Term

• Current Assignee: Alliance For Sustainable Energy LLC
• Original Assignee: Midwest Research Institute
• Inventors: Sheldon, Peter, Coutts, Timothy J., Wu, Xuanzhi
• Primary Examiner(s): CHAPMAN, MARK A
• Application Number:
  US09/149,430
• Time in Patent Office: 847 Days
• Field of Search: 136/265, 438/487
• US Class Current: 136/265
• CPC Class Codes: H01L 31/022466 made of transparent conductive layers, e.g. TCO, ITO layers
• CPC Class Codes: H01L 31/0392 including thin films deposited on metallic or insulating substrates ; characterised by specific substrate materials or substrate features or by the presence of intermediate layers, e.g. barrier layers, on the substrate textured substrates H01L31/02366
• CPC Class Codes: H01L 31/03925 including AIIBVI compound materials, e.g. CdTe, CdS
• CPC Class Codes: H01L 31/073 comprising only AIIBVI compound semiconductors, e.g. CdS/CdTe solar cells
• CPC Class Codes: H01L 31/1884 Manufacture of transparent electrodes, e.g. TCO, ITO
• CPC Class Codes: Y02E 10/543 Solar cells from Group II-VI materials

Abstract

A photovoltaic device has a buffer layer zinc stannate Zn₂SnO₄ disposed between the semiconductor junction structure and the transparent conducting oxide (TCO) layer to prevent formation of localized junctions with the TCO through a thin window semiconductor layer, to prevent shunting through etched grain boundaries of semiconductors, and to relieve stresses and improve adhesion between these layers.

Claims

1. A thin-film Cds/CdTe heterojunction photovoltaic device that has front contact layer of transparent conductive oxide (TCO) and further comprises:

a layer of zinc stannate interposed between the transparent conducting oxide (TCO) layer and the thin-film CdS/CdTe.

2. The thin-film CdS/CdTe heterojunction photovoltaic device of claim 1, wherein the zinc stannate layer is spinel crystalline Zn₂SnO₄.

3. A method of preventing localized TCO/CdTe junctions in a thin-film CdS/CdTe heterjunction photovoltaic device that has a CdS layer and aCdTe layer and in which the CdS layer is pouitioned between a front contact layer of transparent conductive oxide (TCO) and the CdTe layer, wherein the method comprises:

interposing a layer of zinc stannate between the layer of transparent conductive oxide (TCO) and the CdS layer.

4. The method of claim 3, wherein said zinc stannate layer is spinel crystalline Zn₂SnO₄.

5. The method of claim 4, comprising:

heat treating the CdS layer and the CdTe layer together with the layer of zinc stannate and the TCO front contact layer at a temperature of about 600°

C.

7. The method of claim 5, wherein said zinc stannate is spinel crystalline Zn₂SnO₄.

6. A method of preventing etched grain boundary shunts between a metallic back contact layer and a TCO front contact layer in a thin-film CdS/CdTe heterojunction photovoltaic device that has a CdS layer and a CdTe layer positioned between the transparent conductive oxide (TCO) front contact layer and the metallic back contact layer, wherein the method comprises:

interposing a layer of zinc stannate between the TCO front contact layer and the CdS layer.

8. A method of relieving interlayer stresses and preventing blistering and peeling from heat treating a CdS layer and a CdTe layer in a thin-film CdS/CdTe heterojunction photovoltaic device that has a CdS layer and a CdTe layer with a transparent conductive oxide (TCO) front contact layer positioned between a substrate and the CdS layer, wherein the method comprises:

interposing a layer of zinc stannate between the TCO front contact layer and the CdS layer prior to the heat treating.

9. The method of claim 8, wherein said zinc stannate is spinel crystalline Zn₂SnO₄.

10. The method of claim 8, comprising:

heat treating the CdS layer and the CdTe layer together with the layer of zinc stannate and the TCO front contact layer at a temperature in a range of about 400 to 700°

C.

11. A method of fabricating a thin-film CdS/CdTe heterojunction device, comprising:

depositing a TCO layer on a transparent substrate;

depositing a zinc stannate layer on the TCO layer;

depositing a CdS/CdTe heterojunction structure on the zinc stannate layer;

annealing the CdS/CdTe heterojunction structure;

surface etching the CdS/CdTe heterojunction structure to obtain a p⁺ surface region; and

depositing a back contact on the CdTe layer with a p⁺ surface region.

12. The method of claim 11, wherein the zinc stannate is Zn₂SnO₄ in a spinel crystal structure after said annealing.

13. A thin-film photovoltaic device comprising:

a substrate;

a film of transparent conductive oxide disposed on said substrate as a front contact;

a film of Zn₂SnO₄ disposed on said film of transparent conductive oxide;

a thin film comprising two or more layers of semiconductor materials forming a p/n or a p/i/n junction disposed on said film of Zn₂SnO₄; and

an electrically conductive film disposed on said thin film of semiconductor materials to form a rear electrical contact to said thin film.

14. The photovoltaic device of claim 13, wherein said substrate is transparent.

15. The photovoltaic device of claim 14, wherein said substrate is selected from the group consisting of silica and glass.

16. The photovoltaic device of claim 13, wherein said film of transparent conductive oxide comprises SnO₂.

17. The photovoltaic device of claim 13, wherein said film of transparent conductive oxide comprises Cd₂SnO₄.

18. The photovoltaic device of claim 13, wherein said thin film of semiconductor materials comprises cadmium sulfide.

19. The photovoltaic device of claim 13, wherein said thin film of semiconductor materials comprises cadmium telluride.

20. The photovoltaic device of claim 13, wherein said substrate, said film of transparent conductive oxide, and said film of Zn₂SnO₄ are substantially transparent to light between about 400 nm and 1000 nm.

21. The photovoltaic device of claim 13, wherein said film of Zn₂SnO₄ is in a range of about 200 to 300 Å

A thick.

22. The photovoltaic device of claim 13, wherein said film of Zn₂SnO₄ is about 1000 Å

thick.

23. The photovoltaic device of claim 13, wherein said film of Zn₂SnO₄ has a sheet resistivity greater than about 1000 ohm-cm.

24. A method for fabricating a photovoltaic device, comprising:

depositing a film of transparent conductive oxide onto a substrate;

depositing a film of Zn₂SnO₄ onto said film of transparent conductive oxide;

depositing onto said film of Zn₂SnO₄ a thin film comprising dissimilar semiconductor materials that together form a p/n or a p/i/n junction; and

depositing an electrically conductive film onto said thin film of semiconductor materials to provide an electrical contact therewith.

25. The method of claim 24, wherein depositing the film of Zn₂SnO₄ onto the substrate comprises:

(a) RF sputter coating a substantially amorphous Zn₂SnO₄ layer onto said film of transparent conductive oxide; and

(b) heating the Zn₂SnO₄ layer and semiconductor materials to a treatment temperature sufficient to induce crystallization of the Zn₂SnO₄ layer into a uniform single-phase spinel structure and for a time sufficient to allow full crystallization of the Zn₂SnO₄ layer at the treatment temperature to produce a poly-crystal Zn₂SnO₄ film.

26. The method of claim 25, wherein said substantially amorphous Zn₂SnO₄ layer is sputter coated onto said film of transparent conductive oxide at room temperature.

27. The method of claim 25, wherein said substantially amorphous Zn₂SnO₄ layer is RF sputtered onto said film of transparent conductive oxide in an atmosphere consisting essentially of oxygen.

28. The method of claim 25, wherein said treatment temperature is less than about 650°

C.

29. The method of claim 24, wherein said step of depositing a thin film of semiconductor materials onto said layer of Zn₂SnO₄ comprises a chemical bath deposition, or an RF sputtering deposition, or a close-space sublimation deposition.