JPH11189408A - Production of silicon for solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simply obtain high grade silicon in good productivity by hydrolyzing a pure tetraalkoxysilane having the contents of B, P and impurities controlled to specific values or less, respectively, and subsequently reducing the obtained highly pure silica. SOLUTION: This method for producing silicon for solar batteries comprises hydrolyzing a pure tetraalkoxysilane having a B content of <=0.1 ppm, a P content of <=0.1 ppm and a total metal impurity content of <=20 ppm preferably in the presence of a solvent (for example, methanol) and a catalyst (for example, carbonic acid) with water in an amount of 3-12 moles per mole of the tetraalkoxysilane, reducing the obtained highly pure silica preferably having a purity of >=99.99% preferably by an arc thermal reduction method and subsequently purifying the reduction product, for example, by a coagulation purification method. The pure tetraalkoxysilane is obtained, for example, by the superfractionation of a crude tetraalkoxysilane obtained from powdery metal silicon and methanol as raw materials in the presence of sodium methylate catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing silicon for solar cells.

[0002]

2. Description of the Related Art Solar power generation has attracted attention as clean energy in the 21st century. Silicon-based and compound semiconductor-based materials have been developed for solar cells used for this solar power generation. In 1995, silicon-based solar cells accounted for 95% or more of the total solar cell production in the world. I have. Among silicon-based materials, there are crystalline and amorphous materials, but the ratio of crystalline silicon solar cells in the market has reached about 80%. With the spread of solar cells toward the 21st century, A shortage of high-purity silicon for solar cells as the raw material is expected. For this reason, low cost silicon manufacturing technology for solar cells has been actively developed in various countries around the world.

In silicon-based solar cells, silicon raw materials (high-purity silicon wafers) account for the module price.
Cost is about 60% for single crystal type and 4% for polycrystalline type.
Since it is 0% or more, development of a low-cost method for producing silicon raw materials has become a major issue. Silicon for solar cells does not require as high a purity as Eleven Nine, which is the purity of silicon for semiconductor devices. The purity of silicon for a solar cell may be about 99.9999% to 99.9999%, and there is an increasing social demand for the development of a manufacturing technique for silicon for a solar cell with an emphasis on cost reduction.

The techniques for producing silicon for solar cells are roughly classified into (A) a gasification purification method using a low-boiling intermediate compound,
(B) There is a method in which a reduction furnace and a metallurgical solid purification method are combined. The former is suitable for high purification, but involves a two-stage reduction process and has a limit in reducing costs, and it is considered difficult to achieve a low cost level. On the other hand, in the latter, most of the impurities in the silica raw material migrate into the silicon in the reduction furnace, and it is difficult to remove the problematic impurity elements such as boron in the metallurgical purification method of solidification purification. Thus, a high-purity silica raw material in which impurity elements such as boron and phosphorus are reduced in advance has been required.

Based on the above technical background, as a low-cost method for manufacturing silicon for solar cells, high-purity silica from which boron and phosphorus as impurity elements, which are particularly problematic in quality, have been removed. By carbothermal reduction of
A promising process is to produce crude silicon and then to solidify and refine it.

JP-A-58-151318 discloses that a synthetic siloxane is obtained by heating and oxidizing a polysiloxane obtained by hydrolyzing a silane having a hydrolyzable group. JP-A-62-59515 describes a method for synthesizing high-purity silica by hydrolyzing tetramethoxysilane in the presence of carbon dioxide gas, and also introduces some other techniques. Have been. However, these methods have been refined with a focus on reducing the concentration of uranium and thorium, which are α-ray decay substances, and have not been able to reduce boron and phosphorus.

There is also a method of producing high-purity silica from a natural silicate mineral by a wet method. However, in this method, it is necessary to dissolve the raw material mineral in a solution. Therefore, the silicate mineral is reacted with an alkali to form an alkali silicate. Requires a pre-process for manufacturing This not only results in increased costs,
Alkali is the most unfavorable impurity in products for electronic materials, so there is a problem in that a thorough cleaning process for dealkalization must be performed in a later step.

[0008]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an efficient method for producing silicon for solar cells using purified tetraalkoxysilane as a raw material.

[0009]

That is, the present invention provides a method for hydrolyzing a purified tetraalkoxysilane in which the contents of boron and phosphorus are controlled to 0.1 ppm or less, respectively, and the total content of metal impurities is controlled to 20 ppm or less. A method for producing silicon for solar cells, characterized in that the obtained high-purity silica is reduced.

The tetraalkoxysilane used as a raw material in the present invention is not particularly limited. Examples thereof include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and the like. Tetraethoxysilane is preferred. Such a tetraalkoxysilane is produced, for example, by a method of directly reacting silicon and an alcohol using sodium alcoholate or the like as a catalyst, a method of synthesizing by reacting a silicon chloride with an alcohol, a method of an alcohol exchange reaction, and the like. can do.

As the tetraalkoxysilane used as a raw material in the present invention, a purified tetraalkoxysilane prepared by a conventionally known method is not used as it is, but is purified so that a specific impurity concentration becomes a certain value or less. Use By a known purification method, for example, a purification method such as a precision distillation method, the contents of boron and phosphorus are each 0.1 pp.
m or less, and it is necessary to use those purified so that the total content of metal impurities is 20 ppm or less. On the other hand, if attention is paid only to conventionally known α-ray decay substances and the concentration is refined to the level of 0.1 ppb, it is insufficient as a silicon raw material for solar cells. This is because the volatility of α-ray decay substances uranium and thorium is extremely low compared to the volatility of tetraalkoxysilane, and can be easily separated, whereas the volatility of boron and phosphorus is close to that of tetraalkoxysilane, 50 ppm in metal silicon which is the raw material of alkoxysilane
This is because boron and phosphorus contained to a certain extent can be removed only by precision distillation or the like.
In the present invention, the content of boron and phosphorus is the weight concentration when the compound of boron and phosphorus present is converted to elemental boron and phosphorus, and the metal impurity content is the amount of the metal impurity compound existing. It is a weight concentration when converted to a metal element.

In the present invention, the amount of water used in the hydrolysis reaction may theoretically be 2 mol per 1 mol of tetraalkoxysilane, but if the amount of water added is small, the hydrolysis reaction may take a long time. Conversely, if the amount of water added is large, the hydrolysis reaction proceeds quickly, but a large amount of water must be removed in a subsequent step, which is not industrially advantageous. Therefore, it is preferable to add 3 to 12 mol of water to 1 mol of tetraalkoxysilane.

From the viewpoint of obtaining high-purity silica, so-called pure water or ultrapure water containing no compounds or ions that do not decompose into volatile substances in the heat treatment step is used as the water used in the hydrolysis reaction. preferable. When such water is used, undesirable impurities do not remain in the silica during the heat treatment step.

In the present invention, it is preferable to use a catalyst in order to promote the hydrolysis of the tetraalkoxysilane. Examples of such a catalyst include inorganic acids such as carbonic acid, hydrochloric acid, and nitric acid, organic acids such as acetic acid, and organic bases such as ammonia and amines from the viewpoint of easy removal from silica after the reaction. Can be.

In the present invention, no solvent may be added to the hydrolysis reaction, but it is preferable to add an appropriate amount of alcohols such as methanol and ethanol in order to carry out the reaction efficiently. The water, the catalyst, the solvent, and the like used for the hydrolysis are those that contain as little boron, phosphorus and metal impurities as possible.

The reaction temperature for hydrolyzing the tetraalkoxysilane is not particularly limited, but is usually from 0 ° C. to the boiling point of the alcohol produced by hydrolysis. For example, when performing hydrolysis under atmospheric pressure, when tetraalkoxysilane is tetramethoxysilane, it is 0 to 64 ° C, preferably 30 to 60 ° C, and when tetraalkoxysilane is tetraethoxysilane, it is 0 to 64 ° C. The temperature is 78 ° C, preferably 30 to 70 ° C.

Further, the hydrolysis reaction of the present invention may be performed either in a batch system or a continuous system. When the reaction is carried out in a batch system, for example, tetraalkoxysilane is charged as it is or as an alcohol solution in a reaction vessel, and water is added while blowing carbon dioxide gas into the reaction vessel to produce a silica sol. And drying. When the reaction is continuously performed, for example, a predetermined ammonia water or an alcohol solution thereof and tetraalkoxysilane are continuously fed into a reaction vessel by a metering pump, and the generated slurry is continuously taken out from the reaction vessel. It can be performed by a method of filtering and drying. The reaction vessel used for this hydrolysis reaction may be any of glass, stainless steel, resin-lined such as Teflon, etc., and impurities from the reaction vessel under the silica production conditions to be applied. It is preferable to select an appropriate material according to the amount of eluted.

The silica thus obtained has a total content of impurities other than water at a level of 100 ppm or less, and is dried and calcined at about 1000 ° C. or more, so that the contents of boron and phosphorus are each 0.1 ppm. And high-purity silica having a purity of 99.99% or more.

In order to produce silicon for solar cells from the high-purity silica thus produced, a conventionally known reduction technique may be used. For example, high-purity silica is converted into crude silicon by a carbon arc thermal reduction method. By purifying this by a solid purification method such as coagulation purification, the desired high-purity silicon can be obtained.

[0020]

EXAMPLES The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to the following examples.

Example 1 Metallic silicon powder (purity: 98% by weight, particle size: 20)
-100 μm) and methanol (purity 99.9% by weight) as raw materials, using sodium methylate as a catalyst,
The reaction was carried out under the conditions of a molar ratio of metallic silicon / methanol: 4, and a molar ratio of catalyst / methanol: 6 × 10 ^-3 to synthesize crude tetramethoxysilane. This crude tetramethoxysilane was charged into a precision distillation column and purified by distillation to obtain purified tetramethoxysilane. Table 1 shows the analysis results of impurities of the purified tetramethoxysilane. The unit of the content in Table 1 is ppm.

Next, 100 parts by weight of purified tetramethoxysilane and 12 parts by weight of methanol are charged into a reaction vessel equipped with a stirrer. Was charged with 60 parts by weight of water to carry out a hydrolysis reaction. After completion of the reaction, the gel was dried at 200 ° C. for 5 hours and calcined at 1100 ° C. for 4 hours to obtain 35 parts by weight of amorphous high-purity silica. Table 2 shows the analysis results of impurities of the high-purity silica. In addition, the unit of the content in Table 2 is ppm.

When the high-purity silica is purified by carbothermal reduction, decarburization and directional solidification by a conventional method, boron is reduced to 0.1%.
Silicon for solar cells having a total of 02 ppm, phosphorus 0.02 ppm, and other metal impurities of 1 ppm or less can be obtained.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

According to the present invention, highly pure silica having a low boron content and a low phosphorus content, which is very suitable as a silicon raw material for solar cells, can be efficiently produced by a very simple and highly productive process. By reducing this, silicon for solar cells can be advantageously produced.

Claims (2)

[Claims] 2. The method according to claim 1, wherein the contents of boron and phosphorus are each set at 0.1.
A method for producing silicon for solar cells, comprising reducing high-purity silica obtained by hydrolyzing purified tetraalkoxysilane in which the total content of metal impurities is controlled to 1 ppm or less and the total content of metal impurities to 20 ppm or less. 2. The method for producing silicon for solar cells according to claim 1, wherein the purified tetraalkoxysilane is obtained by precision distillation of crude tetraalkoxysilane.