Solar cell having doped semiconductor heterojunction contacts

Abstract

A silicon solar cell has doped amorphous silicon contacts formed on a tunnel silicon oxide layer on a surface of a silicon substrate. High temperature processing is unnecessary in fabricating the solar cell.

Claims

1. A solar cell comprising: a silicon substrate having a front surface and a back surface; a first doped silicon contact over the back surface of the silicon substrate; a second doped silicon contact over the first doped silicon contact and in an opening through the first doped silicon contact, the first and second doped silicon contacts having opposite conductivity types; and a first oxide layer under the first doped silicon contact, under the second doped silicon contact, and in the opening through the first doped silicon contact between the first and second doped silicon contacts.

2. The solar cell of claim 1, wherein the first doped silicon contact has P-type conductivity and the second doped silicon contact has N-type conductivity.

3. The solar cell of claim 1, wherein the first and second doped silicon contacts comprise doped amorphous silicon.

4. The solar cell of claim 1, wherein the first and second doped silicon contacts comprise doped poly crystalline silicon.

5. The solar cell of claim 1, further comprising: a second oxide layer between the first and second doped silicon contacts over the back surface of the silicon substrate.

6. The solar cell of claim 1, wherein the front surface of the silicon substrate is textured.

7. The solar cell of claim 6, further comprising: a third oxide layer over the textured front surface of the silicon substrate.

8. The solar cell of claim 1, further comprising: a first metal contact that is connected to the first silicon contact; and a second metal contact that is connected to the second silicon contact.

9. A method of fabricating a solar cell, the method comprising: providing a silicon substrate; forming an oxide layer on a surface of the silicon substrate; forming a first silicon contact of a first conductivity type over the oxide layer; forming an opening through the first silicon contact; forming another oxide layer that is continuous with the oxide layer on sidewalls of the opening; forming a second silicon contact of a second conductivity type that is opposite to the first conductivity type over the first silicon contact and in the opening; forming a first metal contact that is connected to the first silicon contact; and forming a second metal contact that is connected to the second silicon contact.

10. The method of claim 9, wherein the first and second silicon contacts comprise amorphous silicon.

11. The method of claim 9, wherein the first and second silicon contacts comprise poly crystalline silicon.

12. The method of claim 9, wherein the surface of the silicon substrate is opposite a light-receiving surface of the silicon substrate.

13. The method of claim 9, wherein forming the other oxide layer that is continuous with the oxide layer on the sidewalls of the opening comprises: etching the oxide layer exposed by the opening; and growing the other oxide layer on the sidewalls of the opening and on an exposed surface of the silicon substrate.

14. The method of claim 9, wherein the first silicon contact has P-type conductivity and the second silicon contact has N-type conductivity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a side view in section of an interdigitated back contact solar cell including doped amorphous contact in accordance with one embodiment of the invention.

(2) FIGS. 2A-2D are side views in section illustrating the solar cell of FIG. 1 during fabrication.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) In accordance with one embodiment of the invention, an interdigitated back contact (IBC) solar cell comprising a silicon semiconductor body having first and second opposing major surfaces receives radiation through the first surface and has first and second patterns of acceptor doped amorphous silicon and donor doped amorphous silicon, respectively, on the second or back surface for receiving electron and hole carriers created in the silicon substrate by radiated photons. The structure is similar to prior art back contact solar cells which utilize doped P and N conductivity contacts formed in the substrates for receiving the holes and electrons created by radiation. However, the use doped P and N contacts in the substrate requires photoresist masking, etching, dopant diffusion, and high temperature processing in the fabrication of the solar cell. The use of acceptor and donor amorphous silicon contacts on the structure, in accordance with an embodiment of the invention, obviates the need for photoresist masking and dopant diffusion and the high temperature processing required in annealing the diffused dopants. A tunnel silicon oxide can be placed between the contacts and the substrate to prevent epitaxial growth of the amorphous silicon on the substrate.

(4) Consider now FIG. 1 which is a side view in section of an interdigitated back contact (IBC) solar cell in accordance with one embodiment of the invention. The cell includes a light n-type monocrystalline or polycrystalline substrate 10 having a front surface which receives radiation and a textured surface on which is formed a thin (e.g. 10-150 angstrom) tunnel silicon oxide layer 12 with a passivation coating 14 over tunnel oxide 12 which can comprise silicon nitride, doped silicon carbide, or a doped amorphous silicon layer.

(5) On the back surface of substrate 10 is a second tunnel oxide layer 16 over which is formed P+ amorphous silicon contacts 18. A dielectric such as silicon oxide 20 separates P+ amorphous silicon 18 from N+ amorphous silicon 22 which is formed in openings through P+ amorphous silicon layer 18 and in contact with tunnel oxide 16. While the amorphous silicon layers 18, 22 are formed by low temperature vapor deposition, tunnel oxide 16 prevents any re-crystallization of the amorphous silicon by epitaxial growth from silicon substrate 10. Metal contact 24 engages P+ amorphous silicon layer 18, and metal contacts 26 engage N+ amorphous silicon layers 22.

(6) The back contact heterojunction enhances the rear passivation of the solar cell by the inclusion of tunnel oxide 16, heterojunction field provided by the amorphous silicon contacts, and contact passivation. As will be described further herein below, a process benefit in making the device is that high temperature dopant drive is not required.

(7) FIGS. 2A-2D are section views illustrating the solar cell of FIG. 1 during fabrication. Initially, as shown in FIG. 2A, silicon substrate 10, which can be either intrinsic or light doped, has a thin tunnel oxide 16 grown thereon which can have a thickness of from 10-20 angstroms, for example. Amorphous silicon layer 18 is then deposited with a boron dopant and a dopant concentration of 10.sup.20-10.sup.21 or 10E20-10E21 atoms per cubic centimeter and to a thickness of 500 to 2000 angstroms. The growth of a doped silicon layer by vapor deposition is a known silicon process. Inclusion of an intrinsic layer under the p-type doped silicon layer can be made if a PIN structure is desired.

(8) Thereafter, an insulating layer of silicon oxide 20 is deposited by low pressure chemical vapor deposition (LPCVD, PECVD, APCVD), or by a spin on glass process. Silicon oxide layer 16 is 500 to 1000 angstroms in this illustrative embodiment.

(9) Thereafter, as illustrated in FIG. 213, the front surface of substrate 10 is textured by chemical or mechanical abrasion. This process step can precede the process steps of FIG. 2A, if desired. A photoresist mask is then formed on the back surface of the substrate 10 and etched to form openings through silicon oxide layer 20 and amorphous silicon 18 to substrate 10. The thin tunnel oxide is removed in the etching process also, and a new layer of tunnel silicon oxide is then applied on the exposed surface of substrate 10 through the etched openings by chemical growth. In forming tunnel oxide 16 in FIG. 213, tunnel oxide layer 12 can be simultaneously formed on the front surface. Following the growth of the thin tunnel oxide in the etched openings, again to a thickness of 10-20 angstroms, an N+ doped amorphous silicon layer 22 is deposited over the back surface, as shown in FIG. 2C.

(10) The layer 22 is doped with an N dopant such as phosphorus with a concentration of 10.sup.20-10.sup.21 or 10E20-10E21 atoms per cubic centimeter. This can be deposited using plasma enhanced chemical vapor deposition (PECVD, LPCVD, APCVD). N+ amorphous silicon 22 is then masked and selectively etched to expose the underlying P+ amorphous silicon 18 for reception of metal contacts. In FIG. 2D, metal contacts 24 and 26 are made to P+ amorphous silicon 18 and N+ amorphous silicon 22 by metal deposition and photoresist masking and etching. The contacts can be formed by first scattering a seed layer of a conductive metal such as aluminum or copper and then pattern plating the seed metal to increase thickness. The cell is then completed by depositing a passivating layer 14 on tunnel oxide 12 on the front surface of substrate 10 using silicon nitride, doped silicon carbide, or N+ doped amorphous silicon.

(11) A heterojunction solar cell in accordance with the invention, using doped amorphous silicon contacts is readily fabricated using conventional semiconductor processing techniques without the for high temperature processing. While the invention has been described with reference to an interdigitated back contact solar cell in which both P+ and N+ contacts are employed, the invention can be applied to solar cells having a single doped amorphous silicon on the back surface. Further, while the heterojunction is provided by amorphous silicon, other high band gap material such as germanium-silicon alloy, doped silicon carbide, or other III-V compound material can be employed in the contact structures. Thus, while the invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.