LOW-COST SOLAR CELL METALLIZATION OVER TCO AND METHODS OF THEIR FABRICATION
Methods for fabricating busbar and finger metallization over TCO are disclosed. Rather than using expensive and relatively resistive silver paste, a high conductivity and relatively low cost copper is used. Methods for enabling the use of copper as busbar and fingers over a TCO are disclosed, providing good adhesion while preventing migration of the copper into the TCO. Also, provisions are made for easy soldering contacts to the copper busbars.
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1. Field of the Invention
The subject invention relates to solar photovoltaic cells and, more specifically, to method for manufacturing low cost metallization layers for such cells and the resulting cell device structure.
2. Related Art
The silkscreen, silver-paste metallization technology has been developed mainly by the “traditional” diffused junction solar cell manufacturers. In such cells, a top layer of silicon nitride is used and then the metallization is formed on top of the silicon nitride. However, electrical contact must be made between the metallization and the emitter—through the silicon nitride layer. Therefore, silkscreen is used to deposit the silver paste and then the cell is annealed at high temperature (e.g., 950° C.) so that the silver paste diffuses through the silicon nitride layer and makes contact to the emitter. Since diffused-junction solar cells make the bulk of the solar market (by a very large margin), silver-paste technology became a de facto standard in the solar cell industry and much of the manufacturing and development efforts are directed at improving the conductivity of the resulting metallization using silver paste.
A specific metallization layer that is particularly relevant to the subject invention is busbar and fingers over a transparent conductive oxide (TCO). One solar cell architecture that incorporates a silver busbar and fingers over TCO is known as the HIT cell, available from Sanyo® of Japan.
The SmartSilicon solar cell is formed using a p-type MG silicon substrate 200, forming an amorphous intrinsic layer 205 on the top surface, forming an amorphous n-type layer 210 over the intrinsic layer 205, forming a TCO 215 over the n-layer. A back metallization layer is formed by depositing a titanium layer 230 over the entire back surface of the substrate 200, and depositing a layer of aluminum 235 over the titanium layer 230. The busbar 220 and fingers 225 are formed of silver, using the silk screen method. The SmartSilicon cell's attractiveness is in its conversion efficiency being competitive with that of pure silicon solar cells, while using an extremely cheap MG silicon substrate. Consequently, the relative cost contribution of the silver metallization process to the cost of the entire cell increases.
While the solar industry embraces the silkscreen silver paste metallization process, the subject inventors have determined that there is a need to provide a cost-effective solution for busbar and finger metallization over a TCO, that is cheaper and more reliable than silkscreen silver paste, and that has lower resistivity than silver paste.
SUMMARYThe following summary of the invention is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below.
Various embodiments of the subject invention provide methods for fabricating busbar and finger metallization over or together with a TCO. Rather than using expensive and relatively resistive silver paste, consisting of pure silver and glass frit with binders, with a resistivity of many times more than that for bulk Silver, embodiments of the invention utilize the high conductivity and relatively low cost copper (or alloys containing copper) deposited at room temperature, with a resistivity of around 2 μOhmcm. Various embodiments provide methods for enabling the use of copper as busbar and fingers over a TCO, providing good adhesion while preventing migration of the copper into the TCO. Also, provisions are made for easy soldering contacts to the copper busbars. The various embodiments of the invention can be applied to any solar cell structure that utilizes TCO, such as the HIT or the SmartSilicon. While embodiments of the invention are particularly beneficial for solar cells made of MG silicon, they can be applied to solar cells made on any substrate.
According to a further aspect of the invention, a solar cell is provided, comprising: a substrate having a back surface and a front surface, the front surface designed for facing the sun; a photovoltaic structure formed on the front surface; an ITO formed over the photovoltaic structure; and, front contacts formed over the ITO, the front contacts comprising busbars and fingers comprising copper. Instead of copper, an alloy comprising copper and other materials, such as, e.g., nickel and/or tin can be substituted.
Other aspects and features of the invention would be apparent from the detailed description, which is made with reference to the following drawings. It should be appreciated that the detailed description and the drawings provides various non-limiting examples of various embodiments of the invention, which is defined by the appended claims.
The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale.
Embodiments of the subject invention provide methods for manufacturing solar cells at reduced costs. Embodiments of the invention provide a lower cost alternative to silver metallization, which provide low resistivity—nearly ten times lower resistivity than silver paste based metallization—thereby enhancing current collection from the photovoltaic cell. Also, lower deposition temperatures of essentially room temperature in order to achieve the lower resistivities. Various methods are provided to increase adhesion and enable soldering to the busbars.
Then, in step 305 a barrier/adhesion layer 415 is deposited. This layer is needed for two reasons. First, it is difficult to have copper adhere to TCO, especially to ITO. Second, copper tends to migrate and a study already showed that ITO is not a very good diffusion barrier to copper. The adhesion/barrier layer may be of a transition metal such as, e.g., chromium, nickel, titanium, etc. It may be deposited by, e.g., electroplating, electroless plating, PVD sputtering, etc. In step 310 copper layer 420 is plated over the barrier/adhesion layer 415. In this example the copper is plated using electroplating. In step 315 a cap layer 425 is formed over the copper 420. The cap layer 425 may be electroplated tin layer, which enables easy soldering onto the metallization layer, so as to connect a plurality of solar cells together, normally in a series connection. Alternatively, the barrier layer 415 may be sensitized by dipping in a liquid solution containing Pd++ (e.g. PdC12) and then electrolessly plated with Copper. The final Ni layer is also electroless plated on top of the electroless Copper. Both electroless plating steps do not require an external field to be applied during the plating process.
Step 610 is optional, but is shown in
In step 615 a mask 720 is formed, e.g., using wax inkjet printing or photoresist silkscreen, so as to delineate the metallization design. The PV cell is then transferred to an electroplating system to electroplate copper layer 725. In step 625 a cap layer 730 is also electroplated over the copper layer 725. Here, the thickness of the cap layer is deposited thicker than the final desired thickness since, as will be shown later, part of this cap layer 730 will be removed during further processing. As before, the cap layer may be nickel, chromium, tin, etc. In step 630 the mask 720 is removed using proper solvent, depending on the type of mask material used. For example, a diluted mixture of KOH (less than 10%) can be used to remove wax or resist mask at room or elevated temperature (e.g., 50° C.). Then, a mixture of sodium persulfate or ammonium persulfate is used to remove the copper that was exposed when the mask was removed. Thereafter, a mixture of potassium permanganate is used to remove the part of the barrier/adhesion layer that was exposed by the removal of the copper. In this step, part of the cap layer may also be removed, which is why it is suggested to make the cap layer thicker than the desired final thickness. Also, in this step the potassium permanganate does not etch the TCO, so that in effect there is a natural etch stop when the barrier/adhesion layer is fully removed. In order to prevent any lateral etching of the barrier and seed layers, especially undercutting of the barrier layer underneath the copper fingers, the permanganate etching may be done with a jet spray to impart directionality to the etch, minimize isotropic etching resulting from immersing the wafer in a stationary liquid bath.
For all of the above embodiments, when using electroplating, it is beneficial to prepare the surface of the TCO so that it is hydrophilic. This can be done by any of the following exemplary methods, or any combination thereof. According to one embodiment, the wafer with the ITO is rinsed in a “soap-like” solution to clean the surface of the TCO from any organic material. An example of such a solution may be the Micro-90, commercially available from Cole-Parmer of Vernon Hills, Ill. According to another embodiment, the surface of the TCO is treated with a surfactant. The surfactant treatment may be instead or in addition to the cleaning step. An example of surfactant may be a solution of sodium alkyl sulfates, mainly the lauryl, such as sodium dodecyl sulfate. According to another embodiment, the TCO is treated with UV light in an ozone atmosphere. This can be done at room or elevated temperature, e.g., 100°-200° C.
It should be understood that processes and techniques described herein are not inherently related to any particular apparatus and may be implemented by any suitable combination of components. Further, various types of general purpose devices may be used in accordance with the teachings described herein. It may also prove advantageous to construct specialized apparatus to perform the method steps described herein.
The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. For example, the disclosure relates to using copper; however, it should be appreciated that an alloy comprising copper and other materials, such as, e.g., nickel and/or tin can be substituted.
Claims
1. A solar cell, comprising:
- a substrate having a back surface and a front surface, the front surface designed for facing the sun;
- a photovoltaic structure formed on the front surface;
- a transparent conductive oxide (TCO) layer formed over the photovoltaic structure; and, front metallization layer formed over the TCO, the front metallization layer comprising a copper layer.
2. The solar cell of claim 1, wherein the TCO comprises an indium tin oxide (ITO).
3. The solar cell of claim 1, wherein the front metallization layer further comprises an adhesion/barrier layer formed between the TCO and the copper layer.
4. The solar cell of claim 3, wherein the front metallization layer further comprises a cap layer formed over the copper layer.
5. The solar cell of claim 3, wherein the front metallization layer further comprises a seed copper layer formed between the adhesion/barrier layer and the copper layer.
6. The solar cell of claim 3, wherein the adhesion/barrier layer comprises one of nickel, chromium, or titanium or alloy thereof.
7. The solar cell of claim 6, wherein the adhesion/barrier layer comprises one of electroplated nickel or electroplated chromium.
8. The solar cell of claim 6, wherein the adhesion/barrier layer comprises one of sputtered nickel or sputtered titanium.
9. The solar cell of claim 1, wherein the copper layer comprises electroplated copper or an alloy comprising copper.
10. The solar cell of claim 4, wherein the cap layer comprises tin.
11. The solar cell of claim 1, wherein the photovoltaic structure comprises a doped amorphous silicon layer.
12. The solar cell of claim 11, wherein the photovoltaic structure further comprises an intrinsic amorphous silicon layer between the substrate and the doped amorphous silicon.
13. The solar cell of claim 11, wherein the substrate comprises a doped silicon, and wherein the doped amorphous silicon layer is doped opposite the doping of the substrate.
14. A method for fabricating a solar cell, comprising:
- fabricating a photovoltaic structure over a substrate;
- fabricating a transparent conductive oxide (TCO) layer over the photovoltaic structure; and,
- fabricating front metallization layer in electrical contact with the TCO, the front metallization layer comprising a copper layer or an alloy with 50% or more of copper.
15. The method of claim 14, wherein fabricating front metallization layer comprises electroplating the copper layer.
16. The method of claim 15, further comprising treating the TCO to become hydrophilic prior to fabricating the metallization layer.
17. The method of claim 16, wherein fabricating front metallization layer further comprises fabricating a barrier/adhesion layer prior to electroplating the copper layer.
18. The method of claim 17, wherein fabricating front metallization layer further comprises fabricating a cap layer over the copper layer.
19. The method of claim 17, wherein fabricating a barrier/adhesion layer comprises sputtering a metal layer.
20. The method of claim 19, wherein fabricating front metallization layer further comprises forming a mask to delineate the metallization design.
21. The method of claim 14, wherein fabricating front metallization layer comprises electroplating the metallization layer directly over the photovoltaic structure.
Type: Application
Filed: Nov 18, 2011
Publication Date: May 23, 2013
Applicant: SUNPREME, LTD. (Sunnyvale, CA)
Inventors: Ashok Kumar Sinha (Los Altos Hill, CA), Roman Milter (Alviso, CA), Robert Broesler (San Francisco, CA)
Application Number: 13/299,752
International Classification: H01L 31/0216 (20060101); H01L 31/0224 (20060101);