Passivated Emitter Rear Locally Patterned Epitaxial Solar Cell
Passivated emitter rear local epitaxy (PERL-e) thin Si solar cells may be formed with a heavily doped epitaxial back surface field (BSF) layer, which is patterned to form well spaced point contacts to the silicon base on the rear of the solar cell. The back side of the cell may be finished with a dielectric passivation layer and a metallization layer for making electrical contact to the cell. PERL-e thick Si solar cells may be formed with heavily doped epitaxial films as the back point contacts, where the point contacts are defined by the provision of a selectively patterned thermal oxide on the rear wafer surface. Furthermore, absorption of longer wavelength, infrared (IR), light in thin silicon solar cells may be improved by the addition of a dielectric stack on the rear surface of the solar cell (a back reflector), the stack acting to reflect the longer wavelength light back through the active layers of the solar cell.
This application is a continuation of U.S. patent application Ser. No. 13/241,112 filed Sep. 22, 2011, which claims the benefit of U.S. Provisional Application Ser. No. 61/403,962 filed Sep. 22, 2010, incorporated by reference in its entirety herein.
FIELD OF THE INVENTIONThe present invention relates generally to solar cells, and more particularly to solar cells with high efficiency.
BACKGROUNDThe highest efficiency (24.7%) crystalline silicon solar cell produced to date is the PERL (Passivated Emitter Rear Locally diffused) cell first developed by Martin Green et al. See IEEE Transactions on Electron Devices Vol. 46, No. 10, October 1999. However, these cells are costly to fabricate since expensive float zone silicon was utilized for the device fabrication and the use of photolithography for metal contact formation is not conducive to low manufacturing cost. There is a need for lower cost solar cells with comparable efficiency to the PERL cell, and improved methods of fabricating these cells that does not involve the high temperature diffusion of boron for the formation of back contacts.
As silicon wafer thickness is reduced to reduce silicon consumption and hence manufacturing costs of photovoltaic products, more of the long wavelength infrared radiation will pass through the thin wafer without being absorbed, due to the relatively poor absorption coefficient of silicon. Consequently cell efficiency is reduced for thin (less than roughly 50 microns) silicon cells. There is a need for solar cell structures which can compensate for the poor absorption of longer wavelength light.
SUMMARY OF THE INVENTIONSome embodiments of the present invention are PERL-e (Passivated Emitter Rear Local epitaxy) solar cells, which are fabricated with epitaxial deposition processes. The PERL-e solar cells may have some or all of the following advantages. First, a majority of the back side of the PERL-e cell is not heavily doped, unlike either Al fired p+ back surface, boron diffused p+ region or epitaxially grown p+ region solar cells. This significantly reduces free carrier absorption of light in the p+ region as well as reducing minority carrier lifetime in this region due to heavy doping. Second, a high quality lightly doped back surface can be passivated with various dielectric films (such as silicon dioxide, silicon nitride and amorphous silicon) to minimize the back surface recombination velocity. Third, with an epitaxially produced back surface field (BSF) followed by selective etch back of this region, the process of creating heavily boron doped regions can be lower in cost as compared to boron diffusion and better in quality as compared to boron diffused regions or to the use of laser fired contacts. Furthermore, the use of epitaxial back surface fields provides greater freedom in terms of junction depth (p+ layer thickness), junction profiles and dopant concentrations. PERL-e solar cells may be fabricated with either thin or thick silicon.
According to embodiments of the present invention, PERL-e thin Si solar cells may be formed with a heavily doped epitaxial BSF layer, which is patterned to form well spaced point contacts to the silicon base on the rear of the solar cell. The back side of the PERL-e cell may be finished with a dielectric passivation layer and a metallization layer for making electrical contact to the cell, Other embodiments of the present invention are PERL-e thick Si solar cells, which utilize heavily doped epitaxial films as the back point contacts, where the local heavily doped (e.g. p+) regions are defined by the provision of a selectively patterned thermal oxide on the rear wafer surface. The thermal oxide may be patterned by lithography and etching, the use of screen printed etch resists or by local laser ablation of the oxide.
According to further embodiments of the present invention, absorption of longer wavelength, infrared (IR), light in thin silicon solar cells is improved by the addition of a dielectric stack on the rear surface of the solar cell (a back reflector), said stack acting to reflect the longer wavelength light back through the active layers of the solar cell. A back reflector comprises a dielectric stack, including a dielectric film with a low refractive index as compared to silicon, and a conducting and reflecting metal layer on top of the dielectric. The dielectric stack may comprise one or a multiplicity of dielectric layers. Local point contacts are made between the metal (typically aluminum) and the rear surface of the silicon. Aspects of the present invention relate to the choice of materials, the method for material deposition and the fabrication sequence for providing IR reflective dielectric stacks on thin silicon solar cells.
These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein:
Embodiments of the present invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the present invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the invention is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration.
Commonly, prior art thick Si solar cell fabrication processes include metallization of the rear surface by screen printing aluminum and firing the metal into the silicon to form a p+ p junction. An issue with this approach is that the back p+ p junction and the aluminum-silicon interface is of poor electrical quality, precluding the achievement of high conversion efficiency in the solar cell. There is a need for improved solar cell structures and fabrication processes. Thick silicon PERL-e solar cell designs may provide the desired high performance with a relatively efficient fabrication process.
The process flows illustrated in
Furthermore, the PERL-e thick silicon process is applicable to both single crystal and multicrystalline silicon substrates.
In
Table 1 provides a list of some dielectric materials that may be used as a dielectric layer in the dielectric stack on the back side of the solar cell structure. For reference it is noted that the refractive indices (RI) of air and silicon are 1 and 3.5, respectively.
The dielectric stack is already integrated into the PERL-e solar cells, as will be apparent to those skilled in the art after reading the description of the present invention. Compare
Furthermore, dielectric stacks with multiple layers can be formed to provide improved reflection of IR. For example, the following multiple layers of dielectric can be effective: (1) combinations of amorphous Si and SiO2; and (2) layers of continually decreasing dielectric constant, viz. AlN followed by SiO2. For example,
Although the solar cells with dielectric stacks described herein are silicon-based solar cells, the teaching and principles of the present invention are also applicable to solar cells comprising single crystal silicon, multicrystalline silicon, polycrystalline silicon, microcrystalline silicon, nanocrystalline silicon, amorphous silicon, and various compound semiconductors, including cadmium sulphide, cadmium telluride, and CIGS (copper indium gallium selenium) materials.
Although the present invention has been particularly described with reference to certain embodiments thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the invention.
Claims
1. A silicon solar cell, said solar cell having a front surface and a back surface, comprising:
- a base layer;
- a back surface field structure contacting said base layer at a multiplicity of contact areas on the back surface of said base layer, said back surface field structure being epitaxial to said base layer at said contact areas, and
- a passivation layer covering the portion of the back surface of said base layer which is not said multiplicity of contact areas.
2. The solar cell as in claim 1, wherein said base layer is single crystal silicon.
3. The solar cell as in claim 1, wherein said base layer is multicrystalline silicon.
4. The solar cell as in claim 1, wherein said base layer is polycrystalline silicon.
5. The solar cell as in claim 1, wherein said back surface field structure includes a multiplicity of silicon mesas corresponding to said multiplicity of contact areas.
6. The solar cell as in claim 1, wherein said back surface field structure is a continuous layer of silicon.
7. The solar cell as in claim 6, wherein said continuous layer of silicon is polycrystalline.
8. The solar cell as in claim 1, further comprising a dielectric layer covering said passivation layer, wherein said dielectric layer and said passivation layer are formed of different materials, said different materials and the thicknesses of said dielectric layer and said passivation layer being chosen to improve the reflection of infrared light back into the active layers of said solar cell.
9. The solar cell as in claim 1, wherein said contact areas are less than 1% of the area of the back surface of said base.
10. The solar cell as in claim 1, wherein said passivation layer is a layer of thermal silicon oxide formed on said base layer.
11. The solar cell as in claim 1, wherein said multiplicity of contact areas are in an evenly spaced array.
12. A method of fabricating a silicon solar cell, said solar cell having front and back surfaces, said solar cell including a base layer, said method comprising:
- forming a passivation layer on the back surface of said base layer;
- patterning said passivation layer to form a multiplicity of apertures in said passivation layer corresponding to contact areas on the back surface of said base layer; and
- epitaxially depositing a back surface field structure on said contact areas on the back surface of said base layer.
13. The method as in claim 12, further comprising depositing a metal layer on the back surface of said back surface field structure.
14. The method as in claim 12, wherein said passivation layer is a layer of thermal silicon oxide.
15. The method as in claim 12, further comprising, after said forming and before said patterning, forming a dielectric layer over said passivation layer, wherein said patterning includes patterning said passivation layer and said dielectric layer to form a multiplicity of apertures in said passivation layer and said dielectric layer corresponding to contact areas on the back surface of said base layer.
16. The method as in claim 12, wherein said back surface field structure is continuous and polycrystalline.
17. The method as in claim 12, wherein said base layer is a silicon wafer.
18. A method of fabricating a silicon solar cell, said solar cell having front and back surfaces, said solar cell including a base layer, said method comprising:
- providing a back surface field layer and a base layer, said back surface field layer being on the back surface of said base layer, the interface between said back surface field layer and said base layer being an epitaxial crystalline interface;
- patterning said back surface field layer to form a multiplicity of mesas on the back surface of said base layer;
- depositing a passivation layer over said multiplicity of mesas and the back surface of said base layer; and
- patterning said passivation layer to form a multiplicity of apertures corresponding to said multiplicity of mesas.
19. The method as in claim 18, further comprising depositing a metal layer over said passivation layer and the surfaces of said multiplicity of mesas exposed by said multiplicity of apertures.
20. The method as in claim 18, further comprising, after said depositing and before said patterning said passivation layer, forming a dielectric layer over said passivation layer, wherein said patterning includes patterning said passivation layer and said dielectric layer to form a multiplicity of apertures in said passivation layer and said dielectric layer corresponding to said multiplicity of mesas.
21. The method as in claim 18, wherein said providing includes:
- providing a silicon substrate with a porous silicon separation layer on the surface thereof;
- epitaxially depositing said back surface field layer on said porous silicon separation layer;
- epitaxially depositing said base layer on said back surface field layer; and
- separating said base layer on said back surface field layer from said silicon substrate.
Type: Application
Filed: Aug 15, 2014
Publication Date: Mar 5, 2015
Inventors: Kramadhati V. Ravi (Atherton, CA), Tirunelveli S. Ravi (San Jose, CA)
Application Number: 14/461,250
International Classification: H01L 31/0224 (20060101); H01L 31/18 (20060101); H01L 31/0368 (20060101); H01L 31/028 (20060101); H01L 31/0216 (20060101);