JPH10194718A - Production of polycrystalline silicon ingot for solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a polycrystalline silicon ingot for a solar cell, having orientation of crystal particles in one direction uniformly and excellent in photoelectric conversion efficiency by pouring a molten silicon of a high purity into a mold arranged with a single crystalline silicon base plate and coagulating by one way coagulation from the bottom part of the mold toward upper direction. SOLUTION: This method for producing a polycrystalline silicon ingot for a solar cell is to arrange a mold 5 made of a water cooled copper or a graphite applied with a releasing agent at the inside thereof in a small room blocked from an atmosphere, also arrange a seed crystal (a single crystalline silicon base plate 6) so as to become the seed crystal on coagulation at the bottom surface in the mold 5, then pre-heat the mold up to a temperature just below the melting point of silicon, pour a molten silicon housed in a ladle into the mold 5, set the out put of a heat source arranged at the upper direction of the mold 5 and simultaneously pass a cooling water into a water cooling jacket 9 arranged at the bottom of the mold 5, and coagulate the molten silicon 7 from the bottom part of the mold 5 toward the upper direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a polycrystalline silicon ingot for a solar cell. This is a technique for injecting silicon into a mold.

[0002]

2. Description of the Related Art At present, due to the demand for diversification of energy sources,
Solar power is in the limelight, but because of its high cost,
It is not widely used for electric power. Further, most of the substrate material for solar cells is silicon, but since there is no manufacturing process dedicated to the silicon, the manufacturing of the silicon occurred in the manufacturing process of silicon for a semiconductor as shown in FIG. It relies on scrap of high-purity silicon or scrap generated during single crystal pulling. The high-purity silicon shown in FIG. 3 is obtained by reacting metallic silicon with hydrochloric acid to gasify it as trichlorosilane, rectifying the gas to remove impurity elements, and depositing it from the gas by a so-called CVD method of reacting with hydrogen gas. It is a thing.

In the method shown in FIG. 3, the cost and time are required for the purification and deposition steps of silicon in a chemical process, and a single crystal must be pulled or cast.
In addition to being troublesome, there is a problem that the yield is low, re-melting equipment and energy are separately required, and the production cost increases. Therefore, currently available solar cells are expensive and hinder their general spread. Further, in the purification of metallic silicon mainly based on the above-described chemical process, there is a problem that a large amount of pollutants such as silane and chloride is inevitably generated, which hinders mass production. Furthermore, with the booming semiconductor industry, the amount of high-purity silicon used for semiconductors is becoming insufficient, and the amount of silicon used for solar cells is expected to decrease further in the future. Under such circumstances, it is necessary to obtain a silicon source that can be used for a solar cell at a lower cost than before by mainly using metallic silicon located further upstream than high-purity silicon.

Therefore, the present applicant has revised the purification of metallic silicon by the above chemical process, and
CT / JP96 / 02965), a method is proposed in which a large amount of silicon having a purity suitable for a solar cell is produced only by a metallurgical process as shown in FIG. 4 and then cast into a silicon substrate at a stretch. . It uses metallic silicon (purity 98-99% by weight Si) obtained by reducing silica stone as a starting material, removes easily volatile impurity elements such as P, Al and Ca by vacuum refining and removes molten metal. By coagulation and purification, impurity metal elements (Fe, Ti, Al, Ca) are roughly refined. Then, the obtained ingot is again melted, B and C are removed by oxidative refining, deoxidized, and then subjected to one-way solidification to finish solidification and purification of the impurity metal element. Then, a part of the ingot is cut off, The remaining portion is sliced to produce a silicon substrate for a solar cell as a continuous flow operation. According to such a manufacturing method, there is a prospect that silicon for solar cells can be mass-produced at a considerably lower cost than before.

In the above-mentioned chemical and metallurgical processes, molten silicon is cast into a mold in the final step to form an ingot, which is sliced into a substrate. In order to make this substrate a solar cell, its surface is usually treated and etched with an aqueous solution of hydrofluoric acid or nitric acid. That is,
The surface has a so-called pyramid structure 1 as shown in FIG. 5 to improve the photoelectric efficiency of incident sunlight. The pyramid structure 1 can be easily obtained in a substrate 2 sliced from a single crystal ingot having a uniform crystal orientation in one direction.

However, in the case of the substrate 3 made of a polycrystalline ingot, since the orientation of the crystal grains 4 on the surface is different, irregularities are formed in multiple directions by etching, and it is difficult to form the pyramid structure 1. In particular, the tendency is more likely to occur in the first solidified substrate having small crystal grains 4 obtained from the lower part of the ingot. Therefore, in the case of a polycrystalline silicon ingot, the yield as an effective substrate is lower than that of a single crystal ingot, which causes an increase in cost. This does not meet the applicant's development goal of mass-producing silicon for solar cells and further reducing the production cost.

[0007]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention is directed to a polycrystalline silicon for a solar cell, in which the orientation of crystal grains is aligned in one direction and the photoelectric efficiency is excellent even in a silicon substrate for a solar cell. It is an object of the present invention to provide a method for manufacturing a silicon ingot.

[0008]

Means for Solving the Problems In order to achieve the above object, the inventor has intensively studied and focused on using a seed substrate in casting molten silicon, and has embodied it. That is, the present invention is to inject high-purity molten silicon into a mold and, when unidirectionally solidifying upward from the bottom of the mold, a single-crystal silicon substrate is formed on the bottom surface in the mold to become a seed crystal at the time of solidification. And pouring molten silicon thereon. 2. A method for producing a polycrystalline silicon ingot for solar cells, comprising:

Further, the present invention is a method for manufacturing a polycrystalline silicon ingot for a solar cell, wherein the (100) plane of the single crystal silicon substrate is oriented upward. Further, the present invention is a method for producing a polycrystalline silicon ingot for a solar cell, wherein the single crystal substrate has the same shape and the same area as the bottom of the mold.

In the present invention, since the polycrystalline silicon ingot for a solar cell is manufactured with the above configuration, the orientation of the crystal grains forming the ingot becomes constant. As a result, when the surface of the substrate obtained by slicing the ingot is etched with hydrofluoric acid or nitric acid, the surface of the substrate becomes an appropriate pyramid structure, and the photoelectric efficiency of the solar cell manufactured on the substrate is reduced by the conventional polycrystalline silicon substrate. It was better than the one made in. Further, since the number of effective substrates from silicon ingots has increased, polycrystalline silicon substrates for solar cells can be manufactured at lower cost than before.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a situation in which a method for manufacturing a polycrystalline silicon ingot for a solar cell according to the present invention is performed. First, a mold 5 made of water-cooled copper or graphite coated with a release agent for preventing the ingot from adhering to the inner surface is arranged in a small room (not shown) in which the atmosphere is blocked. Then, a single crystal serving as a seed substrate 6 during solidification is placed therein, the mold is preheated to just below the melting point of silicon, and molten silicon (also referred to as molten metal) that has been purified to approximately the purity of silicon for solar cells is placed thereon. Pour 7 through ladle 8; The refining of the molten metal 7 is carried out by the metallurgical process shown in FIG. 4, and deboron, dephosphorization, deoxidation, and coarse metal elements such as Fe and Ti are coarsely coagulated and refined. The interior of the small room is preferably set to an argon gas atmosphere so that the ingot is not contaminated with air.

After injecting the molten metal 7 in such an amount as to form an ingot of the target size, the amount of cooling water in the water cooling jacket 9 arranged at the bottom of the mold 5 and the heating source arranged above the mold 5 (not shown, usually, (Using an electric heater) and start solidification so that the solid-liquid interface slowly rises at a constant speed from the bottom upward. In particular, this solidification not only produces an ingot, but also plays a role in finishing and refining the molten metal 7 to a purity acceptable as silicon for solar cells. Therefore, the solidification is unidirectional solidification, and the solidification speed (moving speed of the solid-liquid interface) is predetermined from the viewpoint of efficient purification. FIG. 2 schematically shows the state of crystal growth of the ingot 10 during solidification. According to the present invention, the crystal orientation substantially matching the seed crystal 6 grows upward, so that the crystal orientation is almost 100 even if cut at any height. This fact has been confirmed by observing the longitudinal section of the ingot 10 after the completion of the solidification.

The single crystal silicon plate used as the seed crystal preferably has a (100) plane. The reason is that when a silicon substrate obtained by slicing an ingot is used as a solar cell, the photoelectric efficiency becomes highest. Further, the single crystal silicon substrate used as the seed crystal preferably has the same shape and the same area as the bottom of the mold. that is,
This is because the start of crystal growth is all generated from the single crystal base material, and the crystal orientation in the ingot is aligned at the highest ratio.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 50 kg of molten silicon, which has been pretreated to approximately the purity of silicon for solar cells, is held in a ladle 8 and has an inner size of 3 kg.
It was poured into a mold 5 of 20 × 320 × H400 mm, and was unidirectionally solidified to produce an ingot. At this time, the single crystal silicon plate as the seed crystal 6 previously arranged in the mold 5 has various plane orientations, and a single crystal silicon substrate having a total of 320 × 320 mm with four 160 × 160 mm square single crystal substrates. The ingots were prepared and the production of the ingot 10 was tried for each. In addition, conventional production without using the seed crystal 6 is also possible.
Performed under almost the same conditions. Table 1 shows the amount of impurities in the molten metal 7 and Table 2
Shows conditions such as the melt temperature and the solidification rate.

[0015]

[Table 1]

[0016]

[Table 2]

After completion of solidification, each ingot 10 was cut at a position 20% from its upper end, and the portion was removed as scrap. Then, the remaining portion was sliced at a thickness of 450 μm using a wire saw to obtain a polycrystalline silicon substrate for a solar cell, and a part thereof was used as an analysis sample and a sample for a solar cell conversion test. Here, the solar cell conversion test means that the substrate is immersed in a mixed solution of hydrofluoric acid and nitric acid to remove the surface by about 50 μm, to form a pyramid structure, and then to form a pn junction to form the solar cell. It is to manufacture and measure the photoelectric conversion efficiency. Table 3 summarizes the results of the analysis and the solar cell production / test.

[0018]

[Table 3]

As can be seen from Table 3, when the method according to the present invention is employed, the substrate surface has an appropriate pyramid structure 1. Therefore, all of them show higher photoelectric conversion efficiency than the conventional method. Among them, the best result is obtained when the single crystal silicon plate having the (100) plane crystal orientation is used as the seed crystal 6. Further, when the entire bottom surface of the mold was formed as a seed crystal, the conversion efficiency was further improved. In addition, regarding the impurity elements, as is clear from comparison with Table 1, the finish purification was well performed in each case.

[0020]

As described above, according to the present invention, a polycrystalline silicon substrate for a solar cell having excellent photoelectric conversion efficiency can be stably manufactured. As a result, the amount of rejects at the stage of the substrate is reduced, the productivity of the silicon substrate for solar cells is improved, and the manufacturing cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing a state in which a method of manufacturing a polycrystalline silicon ingot for a solar cell according to the present invention is being performed.

FIG. 2 is a schematic diagram showing crystal growth of an ingot during solidification.

FIG. 3 is a manufacturing process diagram of a silicon substrate for a solar cell mainly using a conventional chemical process.

FIG. 4 is a manufacturing process diagram of a silicon substrate for a solar cell by a metallurgical process proposed by the present applicant.

FIG. 5 is a diagram illustrating a surface treatment of a silicon substrate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Pyramid structure 2 Substrate sliced from single crystal ingot 3 Substrate sliced from polycrystalline ingot 4 Crystal grain 5 Template 6 Seed crystal 7 Molten silicon (molten metal) 8 Ladle 9 Water cooling jacket 10 Ingot 11 Crystal growth direction (solidification direction) 12 Grain boundaries

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masamichi Abe 1 Kawasaki-cho, Chuo-ku, Chiba City Inside Kawasaki Steel Research & Technology Company (72) Inventor Yasuhiko Sakaguchi 1 Kawasaki-cho, Chuo-ku Chiba City Kawasaki Steel Technical Research Company In-house (72) Inventor Yoshihide Kato 1 Kawasaki-cho, Chuo-ku, Chiba City Kawasaki Steel Corp.

Claims (3)

[Claims] 1. Injecting high purity molten silicon into a mold,
When unidirectionally solidifying upward from the bottom of the mold, a single crystal silicon substrate is arranged on the bottom surface in the mold so as to be a seed crystal at the time of solidification, and molten silicon is poured thereon. Manufacturing method of polycrystalline silicon ingot for battery. 2. The method according to claim 1, wherein the single crystal silicon substrate is
2. The method for producing a polycrystalline silicon ingot for a solar cell according to claim 1, wherein the (0) face is directed upward. 3. The single crystal silicon substrate has the same shape and the same area as the bottom surface of the mold.
Or a method for producing a polycrystalline silicon ingot for a solar cell according to 2 above.