FRONT CONTACT SOLAR CELL WITH FORMED EMITTER
A bipolar solar cell includes a backside junction formed by an N-type silicon substrate and a P-type polysilicon emitter formed on the backside of the solar cell. An antireflection layer may be formed on a textured front surface of the silicon substrate. A negative polarity metal contact on the front side of the solar cell makes an electrical connection to the substrate, while a positive polarity metal contact on the backside of the solar cell makes an electrical connection to the polysilicon emitter. An external electrical circuit may be connected to the negative and positive metal contacts to be powered by the solar cell. The positive polarity metal contact may form an infrared reflecting layer with an underlying dielectric layer for increased solar radiation collection.
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This application is a continuation of U.S. application Ser. No. 13/495,577, filed on Jun. 13, 2012, which is a divisional of U.S. application Ser. No. 12/070,742, filed on Feb. 20, 2008, now U.S. Pat. No. 8,222,516. The just-mentioned disclosures are incorporated herein by reference in their entirety.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates generally to solar cells, and more particularly but not exclusively to solar cell fabrication processes and structures.
2. Description of the Background Art
Solar cells are well known devices for converting solar radiation to electrical energy. They may be fabricated on a semiconductor wafer using semiconductor processing technology. A solar cell includes P-type and N-type diffusion regions that form a junction. Solar radiation impinging on the solar cell creates electrons and holes that migrate to the diffusion regions, thereby creating voltage differentials between the diffusion regions. In a backside contact solar cell, both the diffusion regions and the metal contacts coupled to them are on the backside of the solar cell. The metal contacts allow an external electrical circuit to be coupled to and be powered by the solar cell.
In a front contact solar cell, at least one of the metal contacts making an electrical connection to a diffusion region is on the front side of the solar cell. The front side of the solar cell, which is opposite the backside, faces the sun during normal operation to collect solar radiation. While backside contact solar cells have an aesthetic advantage over front contact solar cells due to the absence of metal contacts on the front side, and are thus preferred for residential applications, aesthetics is not a major requirement for power plants and other applications where power generation is the main concern. Disclosed herein are structures for a relatively efficient and cost-effective front contact solar cell and processes for manufacturing same.
SUMMARYA bipolar solar cell includes a backside junction formed by an N-type silicon substrate and a P-type polysilicon emitter formed on the backside of the solar cell. An antireflection layer may be formed on a textured front surface of the silicon substrate. A negative polarity metal contact on the front side of the solar cell makes an electrical connection to the substrate, while a positive polarity metal contact on the backside of the solar cell makes an electrical connection to the polysilicon emitter. An external electrical circuit may be connected to the negative and positive metal contacts to be powered by the solar cell. The positive polarity metal contact may form an infrared reflecting layer with an underlying dielectric layer for increased solar radiation collection.
These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims.
The use of the same reference label in different figures indicates the same or like components. The figures are not drawn to scale.
DETAILED DESCRIPTIONIn the present disclosure, numerous specific details are provided, such as examples of apparatus, process parameters, materials, process steps, and structures, to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention.
In the example of
An antireflective coating (ARC) of silicon nitride layer 103 is formed on the textured front side surface of the substrate 101. The texture front side surface and the silicon nitride layer 103 help improve solar radiation collection efficiency. A passivating oxide 124 may comprise silicon dioxide thermally grown to a thickness of about 10 to 250 Angstroms on the front side surface of the substrate 101.
In one embodiment, the polysilicon emitter 108 is formed on a tunnel oxide layer 107. The polysilicon emitter 108 may be formed by forming a layer of polysilicon using Chemical Vapor Deposition (CVD), such as Low Pressure CVD (LPCVD) or Plasma Enhanced CVD (PECVD), and thermal anneal. The polysilicon emitter 108 may have a sheet resistance of 100 Ω/sq, and a thickness of 1000 to 2000 Angstroms. The tunnel oxide layer 107 may comprise silicon dioxide thermally grown to a thickness of about 10 to 50 Angstroms on the backside surface of the substrate 101. A metal contact 110 electrically connects to the polysilicon emitter 108 through contact holes 123 formed through a dielectric comprising a silicon dioxide layer 109. The metal contact 110 provides a positive polarity terminal to allow an external electrical circuit to be coupled to and be powered by the solar cell 100. The silicon dioxide layer 109 provides electrical isolation and allows the metal contact 110 to serve as an infrared reflecting layer for increased solar radiation collection. In one embodiment, the metal contact 110 comprises silver having a conductance of about 5-25 mΩ·cm and a thickness of about 15-35 μm.
On the front side of the solar cell 100, the metal contact 102 electrically connects to the region 106 through a contact hole 120 formed through the silicon nitride layer 103. The metal contact 102 provides a negative polarity terminal to allow an external electrical circuit to be coupled to and be powered by the solar cell 100. In one embodiment, the metal contact 102 comprises silver having a sheet resistance of about 5 mΩ·cm and a thickness of about 15 μm. The pitch between adjacent metal contacts 102 may be about 1 to 4 mm. In one embodiment, the metal contacts 102 are spaced at 400 to 1000 μm along each metal contact 102 (see
In the example of
Solar cells have gained wide acceptance among energy consumers as a viable renewable energy source. Still, to be competitive with other energy sources, a solar cell manufacturer must be able to fabricate an efficient solar cell at relatively low cost. With this goal in mind, a process for manufacturing the solar cell 100 is now discussed with reference to
In
In
In
In
In
In
In one embodiment, the N-type dopant source 412 comprises silicon dioxide doped with phosphorus. Only one N-type dopant source 412 is shown in
In
The drive-in step to dope the polysilicon emitter 108 on the backside and to form the N-type doped regions 105 and 106 on the front side may be formed in-situ, which in the context of the present disclosure means a single manual (i.e., by fabrication personnel) loading of the substrate 101 in a furnace or other single chamber or multi-chamber processing tool. In one embodiment, the drive-in step is performed in a diffusion furnace. The preceding sequence of steps leading to the drive-in step allows for in-situ diffusion, which advantageously helps in lowering fabrication cost.
It is to be noted that the step of using an N-type dopant source 412 to diffuse dopants into the N-type doped region 106 may be omitted in some applications. That is, in an alternative process, the formation of the N-type dopant source 412 in
In
In
In
In
In
In
Formation of the metal contacts 110 and 102 may be followed by a firing step. The firing step is applicable when using screen printed silver paste as metal contacts, but not when using other processes or metals. The solar cell 100 may then be visually inspected and tested.
While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.
Claims
1. A solar cell, comprising:
- a substrate;
- a first tunnel dielectric disposed over the substrate;
- an emitter disposed over the first tunnel dielectric;
- a front electrode disposed over a front surface of the substrate; and
- a back electrode disposed over a back surface of the substrate.
2. The solar cell of claim 1, wherein the emitter is a polysilicon emitter.
3. The solar cell of claim 1, wherein the substrate is a monocrystalline silicon substrate.
4. The solar cell of claim 1, wherein the first tunnel dielectric is a tunnel oxide.
5. The solar cell of claim 1, wherein the emitter is disposed over the back surface of the substrate.
6. The solar cell of claim 1, further comprising a second tunnel dielectric disposed on an opposite side of the substrate than the first tunnel dielectric.
7. The solar cell of claim 1, wherein the back electrode includes silver.
8. The solar cell of claim 1, further comprising an antireflective layer disposed over the front surface of the substrate.
9. A solar cell, comprising:
- a substrate;
- an emitter over a back surface of the substrate;
- a first dielectric between the emitter and back surface;
- a front electrode disposed on a front surface of the solar cell; and
- a back electrode disposed on a back surface of the solar cell.
10. The solar cell of claim 9, wherein the emitter is a polysilicon emitter.
11. The solar cell of claim 9, wherein the substrate is a monocrystalline silicon substrate.
12. The solar cell of claim 9, wherein the first dielectric is an oxide.
13. The solar cell of claim 9, further comprising a second dielectric disposed on an opposite side of the substrate than the first dielectric.
14. The solar cell of claim 9, wherein the back electrode includes silver.
15. A method of fabricating a solar cell, the method comprising:
- forming a first tunnel dielectric on a front surface of a substrate;
- forming a second tunnel dielectric on a back surface of the substrate;
- forming an emitter over the first or second tunnel dielectric;
- forming a front side electrode on a front surface of the solar cell; and
- forming a back side electrode on a back surface of the solar cell.
16. The method of claim 15, wherein said forming the emitter comprises forming a polysilicon emitter.
17. The method of claim 15, wherein said forming the emitter comprises forming a first silicon layer on the second tunnel dielectric.
18. The method of claim 17, further comprising forming a second silicon layer on the first tunnel dielectric.
19. The method of claim 18, wherein forming the first and second silicon layers comprises forming first and second polysilicon layers, respectively.
20. The method of claim 15, wherein said forming the second tunnel dielectric comprises forming an oxide.
Type: Application
Filed: Oct 2, 2014
Publication Date: Mar 26, 2015
Applicant:
Inventor: Peter John COUSINS (Menlo Park, CA)
Application Number: 14/504,771
International Classification: H01L 31/0224 (20060101); H01L 31/18 (20060101); H01L 31/068 (20060101);