Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2000-03-17
2001-01-30
Goodrow, John (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C264S212000
Reexamination Certificate
active
06180872
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to crystalline silicon plates, a process for growing crystalline silicon plates, a crystalline silicon plate-growth apparatus, and solar cell elements using crystalline silicon plates as substrates.
(2) Related Art Statement
The crystalline silicon solar cell is produced by the four steps of preparing a raw silicon material, producing a substrate from the raw silicon material, forming a cell structure on the substrate, and converting the cell structure into a module. The silicon substrate is ordinarily produced by the Czochralski process (CZ process: pulling-up process) (See “Cell Manual” published by Maruzen Co., Ltd.). Further, a lateral type crystal continuous growth apparatus has been known for growing silicon crystal plates for solar cells.
However, in such a lateral type crystal continuous growth apparatus, it was impossible to uniformly and continuously grow a crystalline silicon plate having an extremely small thickness. For example, a thickness of less than 200 &mgr;m. On the other hand, according to the Czochralski process, a cylindrical silicon crystal may be grown, but then must be cut. Furthermore, it is known that a crystalline silicon plates can be made by growing a polycrystal body according to the casting process and cutting it. However, according to such growing methods, a large percentage the silicon crystal which cut is thrown away. As a result, a lot of the material is wasted, resulting in a high crystal-growing production cost. Therefore, it is difficult to reduce production costs the solar cell element. Since silicon is scarce as a natural source and its material cost is high, this leaded to increase in the production cost of silicon based solar cell elements.
In order to produce the solar cell element, a P-type or N-type diffusion area is formed at a surface of a crystalline silicone plate by diffusing a dopant element thereinto to obtain a P-N junction. However, such a diffusing process is complicated, which is a large factor to raise the production cost.
SUMMARY OF THE INVENTION
It is an object of the present invention to uniformly and continuously grow a crystalline silicon plate having an extremely small thickness of less than 200 &mgr;m.
A first aspect of the present invention relates to a process for growing a crystalline silicon plate, comprising the steps of arranging a planar growth member and a growth crucible in which a melt of silicon is placed and which is provided with a melt draw-out opening at a lower side thereof, while at least a tip portion of the growth member is located under the draw-out opening, drawing out the melt from the crucible through the draw-out opening, bringing the drawn out melt into contact with the tip portion of the growth member, and growing the crystalline silicon plate by further pulling down the melt through the tip portion of the growth member.
The present invention also relates to a crystalline silicon plate-growth apparatus including a growth crucible in which a melt of silicon is placed and which is provided with a melt draw-out opening at a lower side thereof, a planar growth member and a driving mechanism, at least a tip portion of the growth member being located under the draw-out opening of the growth crucible, and said driving mechanism being adapted to bring the melt drawn out through the draw-out opening into contact with the tip portion of the growth member and further pull down the crystalline silicon plate through the tip portion of the growth member.
The present inventors came to a technical idea that a growth crucible in which a melt of silicon is placed and which is provided with a melt draw-out opening at a lower side thereof as well as a planar growth member are arranged such that at least a tip portion of the growth member is located under the draw-out opening of the growth crucible, and the melt is drawn out from the crucible through the draw-out opening, and brought into contact with the drawn out melt into contact with the tip portion of the growth member, and then the resulting crystalline silicon plate is further pulled down through the tip portion of the growth member. At that time, the melt is gradually drawn through the draw-out opening, moves along the surface of the tip portion of the growth member, while a very small melt reservoir is formed above the tip portion of the growth plate, and the melt is pulled down during which the melt is being crystallized. As a result, the inventors discovered that a crystalline silicon plate having a thickness of 5 to 200 &mgr;m could be grown at a speed of 100 to 10000 mm/hr.
In the present invention, the thickness of the crystalline silicon plate is more preferably 10-150 &mgr;m. Particularly, if such a crystalline silicon plate is used for a solar cell element, the thickness of the crystalline silicon plate is preferably 20-100 &mgr;m.
In the production of the crystalline silicon based solar power generating element, an important problem was in reducing the amount of silicon used to produce these elements. To accomplish this the present inventors' invented a process which allows a crystalline silicon plate having an extremely small thickness of not more than 200 &mgr;m to be continuously pulled down. This crystalline silicon plate can be immediately used as a solar cell element after electrodes are formed thereon, while the surface of the plate is left as it is or the surface is slightly polished. That is, after the crystalline silicon plate is pulled down, it can be used in a given length as it is and no cutting is necessary. Therefore, the silicon material is not wasted. Further, the growing speed is high.
At least the tip portion of the growth member may be located under the draw-out opening of the growth crucible in any of the following ways, for example.
(1) The growth member is arranged on one side of the crucible or on one side of the nozzle extending from the crucible in the state that the tip portion of the growth member is located immediately under the draw-out opening of the crucible. The melt flows in a melt reservoir defined between the draw-out opening and the growth member.
(2) The growth member is inserted into the crucible, while the tip portion of the growth member is partially projected through and out of the draw-out opening.
At least the tip portion of the growth member is preferably made of a material stable at the melting temperature of the melt of silicon, for example, at 1400° C. For instance, at least the tip portion of the growth member is made of silicon carbide, boron nitride, silicon oxide or carbon. Further, although the tip portion of the growth member may be integrally formed with the remainder, the tip portion of the growth member is preferably made of a bundle of numerous fibers. In this case, the configuration of the growth member may be easily adjusted according to the dimension of the crystalline silicon plate grown, the draw-down speed, a temperature surrounding near a draw-down point, etc.
Preferably, the growth member is made of a plurality of carbon fibers or silicon carbide fibers. If carbon fibers are used, the carbon fibers react with a melt of silicon, so that the carbon fibers may be converted to silicon carbide from the surface to the interior.
The fibers, particularly, the carbon fibers and silicon carbide fibers, which constitutes the growth member, preferably have the diameter of 5 to 20 &mgr;m.
These and other features, objects and advantages of the invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes could easily be made by the skilled person in the art to which the invention pertains.
REFERENCES:
patent: 4233338 (1980-11-01), Ricard et al.
patent: 4363769 (1982-12-01), Tsuya et al.
patent: 4481235 (1984-11-01), Foell et al.
patent: 4565600 (1986-01-01), Ricard
patent: 1 490 114 (1977-10-01), None
Fukuda Tsuguo
Imaeda Minoru
Imanishi Yuichiro
Burr & Brown
Goodrow John
NGK Insulators Ltd.
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