JPH10251009A - Method for solidifying and refining silicon for solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for solidifying and refining silicon for solar batteries capable of removing metallic impurity elements from metal silicon with the higher efficiency than heretofore. SOLUTION: At the time of casting the molten silicon 4 obtd. by dephosphorizing, deboronizing, decarburizing and deoxidizing from the metal silicon, subjecting the molten silicon to unidirectional solidification and removing the metallic impurity elements contained in the silicon; the solidification rate R meeting the initial concn. of the metallic impurity elements contained in the molten silicon 4 is first determined. The molten silicon is then solidified by setting the solidification rate higher than its in the beginning of the start of the solidification and gradually lowering the rate as time passes by until the average solidification rate during the entire operation attains R.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for coagulating and refining silicon for solar cells, and more particularly, to a method for decomposing, decomposing and deboring metallic silicon from relatively pure molten silicon. The present invention relates to a technique for removing a metal element as an impurity by a unidirectional solidification method.

[0002]

2. Description of the Related Art One of the techniques for increasing the purity of a metal is a solidification refining method. It consists of a metal element to be purified (here, silicon) and an impurity element to be removed (for example, A
1, P, Ca, Fe, Ti, etc.). That is, when the solidus line and the liquidus line of the metal to be purified (A) containing a certain concentration (e wt%) of the impurity element (B) have a relationship as shown in FIG. When the target metal A solidifies, it is discharged from the solid phase to the liquid phase, and is concentrated in the liquid phase (B in the liquid phase).
(The concentration of B moves from point e to point f, and the concentration of B in the solid phase moves from point d to point e). Specifically, when the metal to be purified held in a casting vessel (hereinafter, referred to as a mold) is cooled and solidified, for example, in one direction upward from the bottom, the impurity concentration decreases below the ingot, and finally, It is concentrated to a coagulating liquid. Therefore, if the upper part (concentrated part) of the ingot is cut and discarded,
A high-purity metal to be purified can be obtained.

On the other hand, in recent years, photovoltaic power generation has been in the spotlight due to the demand for diversification of energy sources, and the production of silicon for solar cells required for power generation has been prosperous. Impurity elements must be reduced below acceptable values. Therefore, conventionally, as shown in FIG.
Metallic silicon is reacted with hydrochloric acid to gasify it as trichlorosilane, the gas is rectified to remove impurity elements, and silicon precipitated from the gas by a so-called CVD method of reacting with hydrogen gas has been used. The silicon deposited at this stage has a very high purity of so-called Eleven Nine,
Usually it can be used for semiconductor manufacturing. Therefore, in the conventional manufacturing method shown in FIG. 3, it is necessary to adjust the components of silicon, which has been highly purified even for semiconductors, so as to be suitable for solar cells, or to purify and cast again. In addition to this, there is a problem that the yield is poor, re-melting equipment and energy are separately required, and the production cost increases.
As a result, currently available solar cells are expensive and are an obstacle to 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.

Therefore, the present applicant has improved the purification of metallic silicon by the above-described chemical process, and manufactured silicon having a purity suitable for solar cells only by a metallurgical process.
We are studying a method of casting it at once to make it into a silicon substrate (see FIG. 4). As a part thereof, the improvement of the purity of metallic silicon using the above-described solidification refining method is adopted in two places (coarse solidification purification step and finish solidification purification step) in the step shown in FIG. So-called metal impurity elements such as phosphorus, calcium, aluminum, iron, and titanium are removed to a target value as silicon for solar cells.

[0005] To actually perform this solidification purification, the position of the solid-liquid interface is measured by a sensor such as a thermometer, and the moving speed of the solid-liquid interface upward, that is, the solidification speed is made constant. The power of the heater installed above or below the mold or the amount of water in the cooling water cooling jacket is adjusted. The present inventor has previously proposed in Japanese Patent Application No. 8-270814 that the solidification rate is predetermined before the molten metal is cast into a mold, and that the solidification rate is maintained constant.

However, according to subsequent studies, it has been found that, when a constant coagulation rate is maintained, the concentration of metal impurity elements in the residual liquid is slowed down as coagulation proceeds, resulting in a reduction in purification efficiency or poor purification. understood. The inventor
The goal is to produce silicon for solar cells at less than half the cost of conventional solar cells, but this goal will not be achieved without improving such inefficient coagulation purification.

[0007]

SUMMARY OF THE INVENTION In view of the foregoing, an object of the present invention is to provide a method for coagulating and refining silicon for solar cells, which is capable of removing metal impurity elements from metal silicon more efficiently than in the past.

[0008]

Means for Solving the Problems In order to achieve the above object, the present inventors return to the basis of the unidirectional solidification of molten metal,
The following conclusions were obtained. Generally, the distribution of the impurity element contained in the molten metal and the solidified metal at the time of the unidirectional solidification is expressed by the Schild equation. C _S = k · C ₀ · (1-f) ^k−1 ··· (1) Equation k = k ₀ / {k ₀ + (1-k ₀ ) · exp (−R _i · δ / D)} · Formula (2) where C _S : concentration of a certain impurity element in the solidified metal (% by weight) C ₀ : initial concentration of a certain impurity element in the molten metal (% by weight) k: effective distribution coefficient (−) k _0: equilibrium partitioning coefficient (-) f: fraction solid (-) R _i: solidification rate (cm / sec) δ: diffusion layer thickness (cm) D: diffusion coefficient (cm ² / sec) efficiency unidirectional solidification To do well is to bring the effective partition coefficient closer to the equilibrium partition coefficient. Therefore,
In the formula (2), R _i , that is, the solidification rate is to be reduced. However, reducing the R _i, productivity becomes longer clotting time is reduced. The inventor of the present invention thought that it would be better to reduce the solidification speed R _i while keeping the extension of the solidification time as short as possible, and completed the present invention.

That is, according to the present invention, molten silicon obtained by dephosphorization, deboronation, decarburization and deoxidation from metallic silicon is cast and unidirectionally solidified to remove metal impurity elements contained in the silicon. First, the solidification rate R according to the initial concentration of the metal impurity element contained in the molten silicon
At the beginning of solidification, the solidification speed is greater than its R,
A method for solidifying and refining silicon for solar cells, characterized in that the molten silicon is solidified such that the molten silicon is gradually reduced with time and the average solidification rate during the entire operation is R.

The present invention also provides a method for coagulating and refining silicon for solar cells, wherein the solidification rate R is set to the minimum value of the following three equations. When the metal impurity element is Al: R ₁ = 7.0 × (Al
(Initial concentration) ^{-0.21 When} metal impurity element is Fe: R ₂ = 6.0 × (Fe
(Initial concentration) ^{-0.18 When} metal impurity element is Ti: R ₃ = 6.6 × (Ti
(Initial concentration) ^-0.18 Further, according to the present invention, the solidification purification of solar cell silicon is characterized in that the solidification speed is reduced stepwise according to the height of the formed ingot, instead of the lapse of time. It is also a method.

In the present invention, when a molten silicon obtained by dephosphorizing, deboroning, decarburizing and deoxidizing metallic silicon is cast to produce an ingot for a solar cell by unidirectional solidification, the metal containing molten silicon is used. The solidification rate R was determined according to the initial concentration of the impurity element, and the solidification rate was determined to be early at the beginning of solidification and then gradually reduced so that the average solidification rate of the entire operation would be at that R. The portion to be coagulated and refined is higher than before. As a result, the yield of silicon is improved, the purification cost is reduced, and an inexpensive silicon substrate for a solar cell having a half value or less as compared with the conventional one can be manufactured.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, FIG. 1 shows an example of an apparatus for implementing a method for solidifying silicon for a solar cell according to the present invention. It is formed by disposing a graphite mold 1, a water cooling jacket 2 on the bottom, a heat insulating material 3 on the side wall, and an electric heater 5 for heating the upper part of the molten silicon 4 cast above. As the sensor 7 for detecting the movement of the solidification interface 6, a plurality of ultrasonic distance meters or thermometers are used. Also,
There is also provided an arithmetic and control unit 8 for comparing the solidification speed with a target value and adjusting the amount of water in the cooling jacket 2 and the amount of heat of the electric heater 5 so as to match the target value.

According to the present invention, the molten silicon 4 dephosphorized, deboroned, decarburized and deoxidized in the previous step is injected into the mold 1 so that the solidification interface 6 proceeds upward from the bottom. It solidifies. In this case, it is important that the solidification is performed at a solidification rate such that the impurity concentration at the solid-liquid coexistence region at the solidification interface 6 is as uniform as possible. Specifically, the amount of cooling water in the water cooling jacket 2 and the amount of heat of the electric heater 5 are appropriately set at the start of casting.
Note that, as described above, the present invention may be performed in any of the coarse solidification purification and the finish solidification purification in the step shown in FIG. The mold 1 may be a commonly used refining vessel other than that shown in FIG. 1, such as a ladle.

Next, the inventor repeated many experiments,
An appropriate solidification rate was determined according to the initial concentration of the metal impurity element in the molten silicon 4, and three stable solidification regions as shown in FIGS. That is, for Al, Fe, and Ti, the solidification rate at which the concentration in the melt can be performed smoothly was determined. By the way, since the actual molten silicon 4 contains all of these three components, it must be solidified at a solidification rate that simultaneously satisfies these three regions (the above three equations). Specifically, for example, in FIG.
When the initial concentration is a, the solidification rate is preferably b. Similarly, when the initial Fe concentration is c in FIG. 6, the solidification rate is preferably symbol d, and when the initial Ti concentration is e in FIG. 7, the solidification rate is preferably f. Therefore, in such a case, it is preferable to solidify at the minimum f among these solidification speeds b, d, and f. In addition, in the molten silicon 4, the above-described Al. There are various types other than Fe and Ti, but it has been confirmed that if these three are suppressed, no problem occurs in actual casting.

Previously, the solidification rate described above (eg, f)
Is constant throughout the operation,

[0016] It was solidified. However,

2. Description of the Related Art There is a problem as described in the section of Related Art. Therefore, the inventor considered that the coagulation speed should be faster than the above-mentioned f at the beginning of the coagulation, and then gradually decreased as time elapses. When the average solidification rate (arithmetic mean) during operation is set to the appropriate solidification rate f, the operation time (total time required for solidification) is almost the same as when the solidification rate is kept constant. The present invention was found to increase the high-purity portion (height) of the metal (ingot) 9 and made the present invention. In other words, the solidification rate in the early solidification phase where the concentration of impurities into the residual liquid is fast is increased,
The coagulation is performed so that their coagulation speed does not greatly deviate from the optimum coagulation speed value.

With respect to the change of the solidification rate, there are a method of gradually decreasing the solidification rate as described above and a method of decreasing the solidification rate in a stepwise manner. Good results were also obtained with the method. It was also tried to change the solidification speed in accordance with the passage of time and in accordance with the height (position) of the solidified metal 9, and confirmed that both were effective.

[0018]

EXAMPLE The molten silicon 6 was unidirectionally solidified and refined in a finish solidification purification step shown in FIG. 4 using a mold 1 of the same type as that shown in FIG. The height of the mold 1 is 40 cm and the cross-sectional area is 5
30 cm ² . At that time, two types of casting were performed, one to which the present invention was applied (Examples 1 and 2) and one to which the conventional constant solidification rate method was applied (Comparative Example 1). In addition,
The solidification rate was changed when the present invention was applied by operating the amount of cooling water in the water cooling jacket 2 and the amount of electric power in the electric heater 5. On the other hand, also in the case of the conventional method, the amount of cooling water and the amount of electric power were similarly operated so as to keep the optimum solidification rate obtained in advance, that is, the values determined in FIGS. Table 1 shows the initial concentration of the metal impurity element in the molten silicon 4 used in each case, and FIG. 8 shows the change from the average solidification rate of 2 mm / min.

[0019]

[Table 1]

Table 2 summarizes the results of coagulation purification. In each of the two cases to which the present invention was applied, impurities were reduced to the specifications of the solar cell in the obtained ingot 9, and a portion excellent in crystallinity was located at a height of 32 cm from the bottom of the ingot. on the other hand,
When the conventional method was applied, the height was 30 cm. That is, when the present invention is employed, the purification efficiency is improved by 2 cm compared to the conventional art. Then, the obtained ingot 9 was sliced to a thickness of 250 μm to obtain a silicon substrate for solar cells. When the photoelectric conversion efficiency of a solar cell manufactured using the substrate was measured, the value was 12 to 14 for both the substrate obtained by applying the present invention and the substrate obtained by the conventional method.
%, Which is a good result.

[0021]

[Table 2]

[0022]

As described above, according to the present invention, the removal of metal impurity elements from metal silicon can be performed more efficiently than in the prior art. As a result, improvement in silicon yield and reduction in purification cost have been achieved, and an inexpensive silicon substrate for solar cells can be manufactured.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing an example of an apparatus for performing a method for coagulating and refining silicon for a solar cell according to the present invention.

FIG. 2 is a diagram illustrating the principle of metal purification by the directional solidification method.

FIG. 3 is a flowchart showing a process for industrially producing a conventional silicon substrate for a solar cell.

FIG. 4 is a flowchart showing a process of industrially producing a silicon substrate for a solar cell proposed by the present applicant.

FIG. 5: Solidification rate of molten silicon and metallic impurity element Al
FIG. 6 is a diagram showing a relationship between the initial density and the initial density.

FIG. 6: Solidification rate of molten silicon and metallic impurity element Fe
FIG. 6 is a diagram showing a relationship between the initial density and the initial density.

FIG. 7: Solidification rate of molten silicon and metal impurity element Ti
FIG. 6 is a diagram showing a relationship between the initial density and the initial density.

FIG. 8 is a diagram showing a situation where the solidification rate changes during the implementation of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Water-cooling jacket 3 Insulation material 4 Molten silicon 5 Electric heater 6 Solidification interface 7 Sensor 8 Operation controller 9 Solidification metal (ingot)

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Masamichi Abe 1 Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Engineering Co., Ltd. (72) Inventor Kenkichi Yushita 1 Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Co., Ltd. In the laboratory (72) Inventor Tetsuya Fujii 1 Kawasaki-cho, Chuo-ku, Chiba City Kawasaki Steel Corporation Technical Research Laboratory

Claims (3)

[Claims] 1. Casting molten silicon obtained by dephosphorization, deboronation, decarburization and deoxidation from metallic silicon and solidifying the molten silicon in one direction to remove metal impurity elements contained in the silicon, The solidification rate R according to the initial concentration of the metal impurity element contained in the molten silicon is determined. At the beginning of solidification, the solidification rate is greater than R, and gradually decreases over time. A method for solidifying and refining silicon for a solar cell, wherein the molten silicon is solidified. 2. The method for solidifying and refining silicon for solar cells according to claim 1, wherein said solidification rate R is a minimum value of the following three equations. When the metal impurity element is Al: R ₁ = 7.0 × (Al
(Initial concentration) ^{-0.21 When} metal impurity element is Fe: R ₂ = 6.0 × (Fe
(Initial concentration) ^{-0.18 When} metal impurity element is Ti: R ₃ = 6.6 × (Ti
Initial concentration) ^-0.18 3. The solidification of silicon for a solar cell according to claim 1, wherein the solidification speed is reduced stepwise according to the height of the formed ingot, instead of passing the time. Purification method.