WO2009130786A1 - Process for producing silicon material for solar cell - Google Patents

Abstract

This invention provides a process for producing a silicon material for a solar cell, comprising the steps of mixing a silicon oxide material with sodium hydroxide and water in a vessel, raising the temperature of the mixture in a vessel to allow a reaction to proceed to prepare an aqueous alkali silicate solution, adding hydrochloric acid to the aqueous alkali silicate solution to precipitate silicic acid, and separating and drying high-purity silicon oxide. The separated and dried silicon oxide is reduced with a carbon material at an elevated temperature in an electric furnace to produce silicon. The silicon is then withdrawn from the electric furnace and is unidirectionally solidified while induction melting in a vertically electrically insulated crucible to cast a vertically long ingot. A part of the head part in the silicon ingot is cut and separated, and the remaining major part is used as a silicon material for a solar cell. According to the above constitution, a silicon material for a solar cell can be produced without the need to use a complicated apparatus and with low-energy consumption and in a short refining time.

Description

Method for producing silicon raw material for solar cell

The present invention relates to a method for producing a silicon raw material of solar cell grade purity used for a silicon substrate for a solar cell.

The present silicon raw material for solar cells is manufactured by the method shown in FIG. Among these production methods, the trichlorosilane hydrogen reduction method is the most important among the current production methods, and supplies most of the current semiconductor grade and solar cell grade silicon raw materials. Silane pyrolysis can produce the most pure silicon, but the production cost is high. The silicon tetrachloride zinc reduction method is the first industrialized method for semiconductor silicon production, but the purity of the produced silicon is insufficient, and it is difficult to continue the production equipment. Not used.

The above three production methods are called gas production methods because silicon is reacted with chlorine, hydrogen, or both to produce a gaseous silicon compound. In general, however, the semiconductor silicon gas production method has a drawback that the volume of the apparatus is large and the production cost is small because the production volume is small compared to the size of the apparatus.

With this technical background, the metallurgical refining method similarly shown in FIG. 1 is a silicon manufacturing method proposed to solve the manufacturing cost limitation of the gas manufacturing method. Silicon for solar cells usually does not require higher purity than silicon for semiconductor devices. The purity of silicon raw materials for semiconductor devices usually needs to be 9N (99.9999999%) or higher, but silicon raw materials for solar cells are used if they have a purity of 7N (99.99999%) excluding carbon, oxygen and boron. can do.

The metallurgical refining method was devised to produce such a medium-purity silicon raw material for solar cells, and impurities were removed to the purity of solar cell grade silicon by melting and refining metal grade silicon in a high purity vessel. (Refer nonpatent literature 1).

That is, first, metal grade silicon (about 99% purity) is melted in a high vacuum by electron beam melting to preferentially evaporate and remove vaporizable elements such as phosphorus, and then to silicon dissolved in an electromagnetic induction furnace. Irradiation with a plasma jet of argon containing a small amount of oxygen gas oxidizes boron, carbon, etc. to generate volatile oxide compounds, evaporates and removes them, and further dissolves silicon to unidirectionally solidify metal impurities. The solidification segregation accumulates the final solidified portion of the ingot, and the final solidified portion of the ingot is cut and discarded to reduce the impurity concentration of the remaining ingot portion.

Thus, in the metallurgical refining method, silicon is melted and refined, thereby reducing the size of the manufacturing apparatus and improving the productivity of the apparatus as compared with the gas manufacturing method.
Kazuhiro Hanazawa et al. “Purification of silicon using steel manufacturing technology” Materia Japan Vol. 45, No. 10 (2006), page 712

However, the above metallurgical refining method has the following drawbacks. That is, among impurities contained in metal silicon, an electron beam melting and refining process is mainly used to remove phosphorus by vaporization refining, and further, plasma jet irradiation irradiation is mainly performed to remove boron by oxidative vaporization refining. Two steps are used.

In general, metal refining by electron beam melting requires the installation of a complex apparatus consisting of a vacuum exhaust system in order to maintain a high vacuum. In addition, in order to dissolve the metal in the water-cooled copper crucible, a large amount of heat is taken from the dissolved metal to the water-cooled crucible, resulting in a refining method with high energy consumption. Also, metal refining by plasma jet irradiation generates a plasma gas with a temperature of 10,000 degrees and irradiates the metal, so that if the refining time is long, it becomes inevitable that it becomes a refining method with high energy consumption. In the above electron beam refining and plasma jet irradiation refining of silicon, the removal reaction rate is low and the refining time is long because the concentration of removed phosphorus and boron is in a low region of 1 ppm or less.

Therefore, it is inevitable to become a manufacturing method with high energy consumption and long refining time. For these reasons, the metallurgical refining method has a problem that although the productivity is improved by downsizing the manufacturing apparatus as compared with the gas manufacturing method, the improvement in the productivity is extremely insufficient.

The present invention has been made in view of the above-described problems, and provides a silicon raw material manufacturing method capable of manufacturing a silicon raw material for solar cells with low energy consumption and a short refining time without requiring a complicated apparatus. Is an issue.

In order to solve the above-mentioned problem, the present invention includes a first step in which a silicon oxide raw material, an alkali metal compound, and water are mixed and heated and reacted in a container to prepare an alkali silicate aqueous solution; A second step in which hydrochloric acid or sulfuric acid is added to the alkali silicate aqueous solution to precipitate silicic acid to separate and dry high-purity silicon oxide, and the separated and dried silicon oxide is reduced at a high temperature in an electric furnace with a carbon material. A third step of producing silicon and removing it from the electric furnace, and a fourth step of casting the elongated silicon ingot by inductively melting the extracted silicon in a crucible that is electrically insulated in the longitudinal direction and unidirectionally solidifying it. And a fifth step of partially cutting and separating the head portion of the cast silicon ingot and using the remaining most as the silicon raw material for the solar cell.

<Production of high-purity silicon oxide by chemical treatment of silica>
The rock resource quartzite is mainly composed of silicon oxide, but it contains oxides such as calcium, magnesium, aluminum, iron, boron and phosphorus as gangue. Rock quartzite cannot be easily dissolved with acids such as hydrochloric acid or sulfuric acid, but can be melted by mixing with alkali oxides such as sodium or potassium. In addition, the molten alkali silicate salt is water-soluble and can be combined with water to produce an aqueous alkali silicate salt solution. Further, when an acid such as hydrochloric acid or sulfuric acid is added to the aqueous alkali silicate solution, only the silicic acid component is insoluble in the product, and only the silicic acid component is precipitated in a solid state. The precipitated silicic acid can be filtered and further dehydrated to produce high-purity silicon oxide. This high-purity silicon oxide production method is used as a method for analyzing silicon oxide in rocks.

In the present invention, in consideration of the similarity of the above analysis method, the silica and the alkali metal compound are melted at a high temperature (usually 800 to 1200 ° C.) and cooled, and then combined with water to form an alkali silicate aqueous solution. The alkali silicate aqueous solution is produced by the production method or by the method of simultaneously mixing silica stone, an alkali metal compound, and water and reacting them at a low temperature (usually 80 to 180 ° C.) to produce the alkali silicate aqueous solution. Further, hydrochloric acid or sulfuric acid is added to the manufactured aqueous alkali silicate salt solution to precipitate a high-purity silicic acid component, which is separated by filtration and dehydrated to produce high-purity silicon oxide.

<Types of sodium compounds producing sodium silicate aqueous solution>
Among alkali metals, sodium is the most widely used industrially. FIG. 2 shows the change in Gibbs free energy as a function of temperature when a sodium compound reacts with silicon oxide to produce an alkali silicate salt. Under normal conditions, the reaction proceeds when the Gibbs free energy is negative. As shown in the figure, alkali silicate is produced by the reaction of silicon oxide and sodium hydroxide or the reaction of silicon oxide and sodium carbonate. The reaction between silicon oxide and sodium hydroxide proceeds from room temperature, but the reaction between silicon oxide and sodium carbonate proceeds only at about 700K or higher. Therefore, the production of sodium silicate by melting of sodium carbonate and quartzite proceeds efficiently at a temperature of about 800 ° C. or higher, which is a temperature at which sodium silicate softens and has fluidity.

<Types of alkali metal compounds that produce an aqueous silicate alkali salt solution>
Fig. 3 shows the change in Gibbs free energy related to the production of alkali silicate by the reaction of industrially available alkali metal (lithium, potassium) compounds and silicon oxide. The reaction between lithium hydroxide and potassium hydroxide and silicon oxide proceeds at room temperature or higher. The reaction between lithium carbonate and silicon oxide proceeds at 500K or more, and the reaction between potassium carbonate and silicon oxide proceeds at about 700K or more.

<Alkali metal compound and reaction temperature for producing alkali silicate aqueous solution>
As described above, it has been found that industrially available alkali metal compounds capable of producing an aqueous alkali silicate solution are preferably sodium, potassium and lithium hydroxides and carbonates.

However, when carrying out an aqueous solution reaction in which silica stone, an alkali metal compound and water are mixed, it is necessary to carry out the reaction in a pressure vessel in consideration of the vapor pressure of water. FIG. 4 shows the equilibrium vapor pressure with respect to the temperature change of water. Water has a vapor pressure of 1 atm at 100 ° C. and reaches a seaside pressure with a vapor pressure of 218 at 374 ° C. An industrial pressure vessel usually has a simple structure when 10 atmospheres or less is used. In order to carry out an aqueous solution reaction at 10 atm or less in a pressure vessel having a simple structure, it is necessary to carry out the reaction at a temperature of 180 ° C. or less at which the equilibrium vapor pressure of water reaches 10 atm. Therefore, among the reactions shown in FIG. 2 and FIG. 3, the reactions that proceed at 180 ° C. (453 K) or less are lithium, sodium, and potassium hydroxides having a negative Gibbs free energy change. Was found to be preferred.

<Production of high-purity silicon oxide by reaction of alkali silicate aqueous solution with acid>
In FIG. 5, as an example of the reaction between an alkali silicate aqueous solution and an acid, the reaction of an aqueous sodium silicate salt solution with hydrochloric acid and sulfuric acid is illustrated together with a change in Gibbs free energy with respect to temperature. When sodium silicate aqueous solution and hydrochloric acid or sulfuric acid are mixed, the free energy change of Gibbs becomes negative, so sodium chloride and hydrated silicic acid are generated. Sodium chloride is contained in the liquid as an aqueous solution, but silicic acid is not dissolved in an acidic aqueous solution and thus precipitates as hydrated silicic acid. In this case, a small amount of elements other than silicon present in the aqueous solution are contained in the liquid as an aqueous chloride solution or an aqueous sulfide solution. The precipitated hydrated silicic acid is separated from the aqueous solution by filtration, and the hydrated silicic acid separated by filtration is further thermally decomposed to separate moisture, whereby high-purity silicon oxide is produced.

<Removal of silicon impurities by applying unidirectional solidification segregation>
One of the means of the present invention, a method of removing impurities in silicon by unidirectional solidification while induction melting in a longitudinally electrically insulated crucible, is performed in a metal by a “zone melt method”. This is the same as the method for removing impurities.

This method is based on the thermodynamic principle that when the metal solidifies, the chemical potential of the solute element between the solid phase and the liquid phase is equal. If the segregation coefficient of solute elements in silicon is K, the solute concentration C in the solidified phase is given by C / C0 = 1-(1-K) exp (-K X / L). Here, C0 is the initial concentration of the solute, X is the length of the solidification phase, and L is the length of the dissolution zone. FIG. 6 shows segregation coefficients of solute elements in silicon. In FIG. 7, the concentration of the solute relating to each segregation coefficient due to the unidirectional solidification segregation of silicon is shown as a ratio to the initial concentration.

For example, for nickel with a segregation coefficient of 0.00013, if the initial concentration is 10 ppm, the concentration in silicon after solidification segregation is about 0.01 ppm. Further, with boron having a segregation coefficient of 0.80, if the initial concentration is 5 ppm, the concentration of boron after solidification segregation is about 4 ppm. As described above, if unidirectional solidification segregation of silicon is used, a solute element having a small segregation coefficient can be efficiently removed.

<Induction melting and casting of silicon with water-cooled copper crucible>
A method used as one of the combination techniques in the present invention, that is, a method of casting a vertically long ingot by solidifying in one direction while inducing and melting silicon in a crucible electrically insulated in a longitudinal direction, This is a silicon casting method using the “crucible induction melting method”.

Fig. 8 shows the configuration of the cooling crucible induction melting apparatus. As shown in the figure, the cooling crucible (cold crucible) induction melting method is a container formed by arranging vertically divided conductive segments electrically insulated from each other and cooled by cooling water in the circumferential direction. The copper cooling crucible (50) and an induction coil (51) arranged around the copper cooling crucible (50).

In the above-described cooling crucible induction melting method, when an alternating induction coil current (52) flows through the induction coil (51), the cooling crucible current (53) flows into each conductive segment of the cooling crucible (50) according to the principle of electromagnetic induction. Is induced as a skin current to form a loop current in each segment according to the principle of current conservation.

The cooling crucible current (53) further induces a skin current of the molten metal current (55) in the conductive molten metal (54) inside the copper cooled crucible (50). The molten metal current (55) heats the molten metal (54) and acts on the induced magnetic field to cause a force (Lorentz force) that pushes the surface of the molten metal (54) inward. By applying electromagnetic force acting on the metal surface, the molten metal can be held in the air in a non-contact manner. By this method, it is possible to dissolve and hold the material while eliminating the contamination of the impurities from the crucible, so that the high-purity metal can be melted and continuously cast.

According to the present invention, a solar cell grade silicon raw material can be manufactured by a combination of the first to fifth simple processes. For this reason, the complicated apparatus in the conventional metallurgical refining method is unnecessary. In addition, since the two processes of dephosphorization using an electron beam that consumes a lot of energy and boron refining using a plasma jet are not used, low energy consumption and a short refining time can be realized.

It is a figure which shows the manufacturing method of the conventional semiconductor grade and solar cell grade silicon raw material. It is a figure which shows the Gibbs free energy change regarding reaction of a sodium compound and silicon oxide. It is a figure which shows the Gibbs free energy change regarding reaction of an alkali metal compound and silicon oxide. It is a figure which shows the equilibrium vapor pressure of water vapor | steam with respect to a temperature change. It is a figure which shows the free energy change of Gibbs regarding the reaction of sulfuric acid and hydrochloric acid, and sodium silicate. It is a figure which shows the segregation coefficient of each solute element in silicon. It is a figure which shows the change of the density | concentration in a solid phase accompanying the solidification progress of the solute element in silicon by the zone melt method. It is a figure which shows the principle of the metal heating and levitation in the cooling crucible induction melting method. It is the schematic of a pressure reaction container. It is the schematic of a resin-made reaction container. It is a figure which shows the impurity analysis value of a silica raw material and the refined silicon oxide. It is the schematic of a stirring type pressure reaction container. It is the schematic of a direct-current arc electric reduction furnace. It is the schematic of the cooling crucible induction melting furnace in an airtight container. It is a figure which shows the impurity analysis result in the silicon | silicone after solidification segregation. It is the schematic of the cooling crucible induction melting furnace which added plasma heating in air | atmosphere. It is a figure which shows the impurity analysis result in the silicon | silicone after the solidification segregation by the crucible induction melting furnace cooled in air | atmosphere.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Pressure reaction vessel 101 ... Raw material inlet 102 ... Pressure gauge 103 ... Thermocouple thermometer 104 ... Gas bar bar 2 ... Resin reaction vessel 201 ... Stirrer 202 ... Hydrochloric acid feeder 203 ... Raw material inlet 3 ... DC arc electric reduction Furnace 301 ... Graphite brick 302 ... Alumina brick 303 ... Furnace body 304 ... Graphite electrode 305 ... Furnace lid 306 ... Raw material addition port 307 ... Metal fitting 4 ... Cooling crucible induction melting furnace 401 ... Cooling crucible 402 ... Inductive coil 403 ... Ingot Vertical movement device 404 ... Raw material supply device 405 ... Graphite lump 406 ... Graphite lump vertical movement device 407, 408 ... Gas port

Next, Examples 1 to 3 of the present invention will be described in detail.

<First step: Formation of aqueous sodium silicate solution by reaction of silica, sodium hydroxide and water>
As an example of preparing an alkali silicate aqueous solution by mixing a silicon oxide raw material, an alkali metal hydroxide, and water and raising the temperature and reacting in a vessel, pressure is obtained by mixing silica stone, sodium hydroxide, and water. The reaction was carried out by raising the temperature in the reaction vessel (1).

First, a pressure reaction vessel (1) having a volume of about 15 liters shown in FIG. 9 was prepared. The material of the pressure reaction vessel (1) is stainless steel, the body is 8 mm thick, and the thickness of the disk-shaped material that seals the raw material inlet (101) is 10 mm. In addition, a pressure gauge (102) and a thermocouple thermometer (103) are attached to the pressure reaction vessel (1), and a gas burner (104) is installed at the bottom.

The reaction procedure is as follows. 1.32 kg of silicic acid, 0.88 kg of sodium hydroxide and 8 liters of water were weighed and charged into the pressure reaction vessel (1). After charging the reactants, the raw material inlet (101) was sealed and the gas burner (104) installed at the bottom of the vessel was ignited. About 20 minutes after heating by the gas burner (104) started, the thermocouple thermometer (103) indicated 170 ° C. The pressure gauge (102) simultaneously showed 8.4 atmospheres. Once the temperature reached 170 ° C, the temperature was adjusted to indicate between 170 ° C and 180 ° C. The pressure gauge (102) exhibited between about 8.4 atmospheres and 10.5 atmospheres. The reaction in this state was maintained for 6 hours. After 6 hours of reaction, the gas burner (104) was turned off and the pressure reaction vessel (1) was cooled to room temperature. After cooling, the raw material inlet (101) was opened, and the reaction product in the pressure reaction vessel (1) was taken out. The reaction product was an aqueous solution with slight turbidity. When this aqueous solution was filtered, a colorless and transparent aqueous solution (sodium silicate aqueous solution) was obtained as the filtrate.

<Second step: Production of high-purity silicon oxide by reaction of aqueous sodium silicate solution and hydrochloric acid>
Examples for producing high-purity silicon oxide by depositing and drying silicic acid by adding hydrochloric acid or sulfuric acid to an aqueous solution of alkali silicate, and adding silica to an aqueous solution of sodium silicate, separating and drying, This was carried out in the example of producing pure silicon oxide.

First, a resin reaction vessel (2) shown in FIG. 10 was prepared. The container (2) was made of metal, but the inner surface was lined with polypropylene, and a rotary stirrer (201) coated with polypropylene was installed, and the internal volume was about 100 liters. A hydrochloric acid feeder (202) made of polypropylene material was installed on the upper part of the container (2), and a stopcock for adjusting the amount of hydrochloric acid to be supplied was installed.

The reaction was performed according to the following procedure. That is, 30 liters of the sodium silicate aqueous solution produced and filtered in the first step was charged into the container (2).

Next, after stirring the aqueous solution with a stirrer to make the flow of the aqueous solution constant, hydrochloric acid was started to be added from the hydrochloric acid feeder (202). After a while from the start of the supply of hydrochloric acid, a cloudy precipitate appeared, but the hydrochloric acid was still added for a while. Meanwhile, the ph test paper was immersed in the aqueous solution from the raw material inlet (203) of the container (2) as needed to estimate the hydrogen ion concentration. The supply of hydrochloric acid was stopped when the hydrogen ion concentration reached ph = 3.

Next, the aqueous solution in which precipitation of silicic acid was completed by the reaction in the container (2) was taken out, and the silicic acid was separated from the aqueous solution by a centrifugal filter. Further, the operation of adding water to the silicic acid separated in the centrifugal filter and washing it with water and further filtering it by centrifugation was repeated four times. Thereafter, silicic acid washed and separated with a centrifugal filter was placed in a thermostatic bath and dried at about 180 ° C. to produce high purity silicon oxide.

The chemical concentration of the raw silica and the high-purity silicon oxide produced by the above procedure was performed to determine the impurity concentration. The result was shown in FIG. Boron in the raw silica was 1.7 ppmw, phosphorus was 3.0 ppmw, aluminum, iron, and titanium were 14 ppmw to 136 ppmw. However, in the highly purified silicon oxide, boron was 0.5 ppmw or less, phosphorus was 0.5 ppmw or less and other metal impurities were reduced from 0.8 ppmw to 4.0 ppmw.

<Third step: carbon reduction of silicon oxide by electric furnace>
An example in which the separated and dried silicon oxide was reduced with a carbon material at a high temperature in an electric furnace to produce silicon and taken out from the electric furnace was performed by a DC arc electric furnace (3) as follows.

An outline of the DC arc electric furnace (3) in which this embodiment is performed will be described with reference to FIG. The maximum output of the DC power supply is 160 kW, the diameter in the furnace is 40 cm, the height is 50 cm, the furnace volume is 0.06 cubic meters, the furnace bottom is made of graphite brick (301), and the furnace wall is made of alumina brick (302). . The furnace body (303) has a structure in which bricks (301) and (302) are wrapped with an iron plate having a thickness of 3 mm. The graphite electrode (304) was a graphite rod having a diameter of 12 cm, and could move up and down to adjust the voltage during energization. Two raw material addition ports (306) having a diameter of 10 cm were provided in the furnace lid portion (305). A metal fitting (307) which was water-cooled was used for the conductive wire portion electrically connected to the graphite electrode (304) and the graphite brick (301).

The electric furnace reduction procedure for silicon is as follows. First, 85 kg of silicon oxide prepared in the second step, 38 kg of carbon powder prepared by pyrolysis of natural gas, and a small amount of sugar are mixed well with a small amount of water, and this mixed silicon oxide-carbon mixture is a rotating drum type. About 120 kg of silicon oxide-carbon mixed pellets having a diameter of about 2 cm. The mixed pellets were thoroughly dried with an incubator and then used as a reducing material for the DC arc electric furnace (3).

Next, among the dried mixed pellets, first, 30 kg of the mixed pellets are charged into the bottom of the DC arc electric furnace (3), and the graphite electrode (304) is lowered to contact the mixed pellets. A voltage was applied between (304) and the graphite brick (301) at the furnace bottom. When a voltage was applied, a current flowed through the graphite electrode (304), an electric arc was generated between the graphite electrode (304) and the mixed pellet, and heating was started. When the heating starts, the graphite electrode (304) and the mixed pellet under the graphite electrode (304) are heated to become white, and the graphite electrode (304) is gently raised to generate a stable arc. The graphite electrode (304) was stopped. At that time, the voltage was about 50 V and the current was 1500 A.

In this state, 50 kg of mixed pellets were newly charged evenly into the furnace from the raw material addition port (306) in the furnace lid (305). The charged mixed pellets were laminated so as to fill the red-hot graphite electrode (304). Further, the current to the graphite electrode (304) was increased to 2500 A.

After creating the conditions for silicon reduction in this way, as the amount of mixed pellets stacked as the silicon reduction progressed, the mixed pellets were additionally charged from the raw material addition port (306) to proceed the reduction. Such an operation was continued for about 5 hours. The total amount of the mixed pellets charged was 120 kg. In the power supply, the voltage gradually increased slightly to about 60 V, and the current was maintained at 2500A. After the supply of the mixed pellets was completed, the upper layer portion of the stacked mixed pellets was heated red, and the supply of power was stopped after the ejection of the gas generated by the reaction almost disappeared.

After the silicon reduction was completed by the above procedure and the furnace body was cooled, the furnace body (303) was disassembled and the silicon in the furnace was taken out. About 29 kg of silicon was produced. The results of impurity analysis of the produced silicon were 0.2 ppmw or less for both boron and phosphorus, 75 ppmw for aluminum, 164 ppmw for iron, 2 ppmw for titanium, 1 ppmw for chromium, and 1100 ppmw for carbon. In this example, the boron and phosphorus concentrations in the reduced silicon were lowered, and it became clear that the conditions that the silicon raw material for solar cells should satisfy with respect to the boron and phosphorus concentrations were achieved by the present invention.

In this example, high-temperature reduction using a carbon material of silicon oxide used in the DC arc electric furnace (3) was performed. However, in order to perform large-scale electric furnace reduction according to the present invention, an AC three-phase electric furnace is used. It does not exclude use.

Furthermore, it does not exclude the use of an electric furnace that uses a plasma jet of argon gas or the like as a heat source in a method of generating silicon from high temperature reduction in an electric furnace and taking it out of the electric furnace. If an electric furnace using a plasma jet as a heat source is used, there is no fear that impurities in the graphite material are mixed due to exhaustion of the graphite material as in the case of using an electric furnace of a graphite electrode. Therefore, an electric furnace using a plasma jet as a heat source can be used to manufacture silicon containing even lower impurities.

<4th step: Solidification segregation of silicon by cooling crucible induction melting method>
An example of solidification segregation in which a vertically long ingot was cast by inducing and melting silicon in a water-cooled copper crucible electrically insulated in the vertical direction was performed as follows.

The cooling crucible induction melting furnace (4) of this example is shown in FIG. That is, a cooling crucible (401) and an induction coil (402) surrounding the cooling crucible (401) are installed in a closed furnace in which the internal pressure can be controlled, and the ingot up-and-down moving device is directly below the cooling crucible (401). (403) was installed to continuously pull down silicon (S). Furthermore, a raw material supply device (404), a graphite lump (405), and a graphite lump vertical movement device (406) were installed above the furnace body. The graphite mass (405) is inserted into the height level of the induction coil (402) in the cooling crucible (401) from above to induce heat generation at the time of initial melting of silicon to assist the silicon (S). It is for heating. Reference numerals (407) and (408) denote gas ports.

In this example, the cross section of silicon (S) in the casting direction is circular and its diameter is 15 cm. Therefore, the inner diameter of the cooling crucible (401) was 15 cm, the outer diameter was 19 cm, and the number of divisions for electrically insulating the cooling crucible (401) in the vertical direction was 20. Each segment of the copper crucible divided into 20 was processed so that cooling water circulated inside, and an electrically insulating mica was inserted between the segments. The cooling water in the cooling crucible (401) has a total flow rate of 150 liters per minute. The induction power supply used a maximum output of 200 kW and a frequency of about 20 kHz. The induction coil (402) has three turns with an inner diameter of 21 cm and a coil height of 15 cm.

The procedure of this example is as follows. First, a graphite pedestal having a diameter of 15 cm in cross section with respect to the pulling-down direction is placed on the ingot vertical movement device (403) so that the upper surface of the pedestal is the same as the lower end position of the induction coil (402), and the crucible ( 401) was inserted from below, and 4.5 kg of silicon (S) was placed on the pedestal. A graphite block (405) having a circular cross section with respect to the pulling direction, a diameter of 14 cm, and a height of 5 cm is inserted 2 cm above the top surface of the inserted silicon (S) from above.

After the pressure in the melting furnace (4) was reduced to 0.1 Torr with a vacuum pump, argon gas was sent to atmospheric pressure, and an alternating current with a frequency of 20 kHz was applied to the induction coil (402) to an output of 200 kW. When the induction coil (402) is energized, the graphite mass (405) inserted above the silicon (S) is first heated by induction heat to turn red, and then the charged silicon (S) The temperature was raised by the radiant heat of the red graphite block (405). When the temperature of the silicon (S) reached about 500 ° C., the electric resistance value of the silicon (S) decreased, the induced current in the silicon (S) increased, and self-heating started.

At the same time as the silicon (S) started to self-heat, the graphite mass (405) was pulled upward from the cooling crucible (401). Silicon (S), which started self-heating, was further heated and soon dissolved completely. The side surface of the silicon (S) melt facing the inner wall of the cooling crucible (401) received electromagnetic force and was separated from the cooling crucible (401) without contact.

After the initially charged silicon (S) is completely dissolved and stably held, the silicon (S) produced in the third step and crushed and washed is supplied from the raw material supply device (404) located above. While continuously charging into the cooling crucible, the ingot up-and-down moving device (403) holding the dissolved silicon (S) was lowered to start casting. Since the descent of the ingot moving device (403) starts and the molten silicon (S) descends below the position of the lower end of the induction coil (402), the electromagnetic force received by the molten silicon (S) decreases rapidly. The melted silicon (S) has a small amount of heat generation and is cooled and solidified.

Thus, continuous casting was carried out by simultaneously supplying the raw material and continuously solidifying the silicon (S) ingot. In this example, the casting speed was 2.0 mm / min, and the induction power output during steady casting was about 130 kW. Casting was stopped when the total length of the ingot reached 60 cm.

<Fifth step: a part of the head of the cast silicon ingot is cut and separated>
After cooling the ingot of silicon (S) cast in the fourth step, a part of the head of the cast ingot is cut and separated, and the remaining most of the ingot is used as a silicon raw material for solar cells. .

In order to analyze impurities in each part of the ingot, samples were taken by cutting with a diamond cutting machine. The results of impurity analysis of the ingot are shown in FIG. That is, the impurity concentration in silicon produced by electric furnace reduction was 0.2 ppmw or less for both boron and phosphorus, 11 ppmw for aluminum, 167 ppmw for iron, 2 ppmw for titanium, 0.5 ppmw for chromium, and 770 ppmw for carbon. The results of solidification segregation by the melting method show that the average concentration of impurities is 0.2 ppmw or less for both boron and phosphorus, 0.1 ppmw for aluminum and 0.1 ppmw for iron in the lower 50 cm ingot portion of the ingot 60 cm. 1 ppmw or less, titanium 0.1 ppmw or less, chromium 0.1 ppmw or less, and carbon 7 ppmw.

The results of the removal of metal impurities from silicon are in good agreement with the liquid-phase and solid-phase distribution theory of impurities in <Impuration of silicon impurities applying unidirectional solidification segregation> in [Means for Solving the Problems]. Showed that. This purified ingot portion was used as a solar cell grade silicon for the trial production of the following solar cell.

That is, the production of a silicon (S) ingot to be segregated and purified by the cooling crucible induction melting method of the above-described embodiment was repeated to produce a total of 51 kg of solar cell grade silicon raw material. The 51 kg of silicon (S) was used for production of a polycrystalline ingot for solar cells by a normal mold unidirectional solidification method and production of a polycrystalline substrate for solar cells by a wire saw slice method.

That is, the silicon (S) is placed in a high-purity silica square mold, melted and unidirectionally solidified, and formed into an ingot having a width of 33 cm × 33 cm and a height of 20 cm. After the silicon (S) ingot was divided by a diamond cutting machine, it was processed into a silicon polycrystalline substrate for solar cells having a width of 15 cm × 15 cm and a thickness of 250 μm by a wire saw slicing machine.

Furthermore, 100 pieces of the silicon polycrystalline substrate were extracted and prototyped as solar cells. In the solar cell prototype process, hydrogen passivation technology was used, and the average value of solar cell conversion efficiency was 15.1% with an average value of 100 sheets. This example confirmed that the silicon raw material produced according to the present invention satisfies the solar cell quality.

<Second Step (Modification): Formation of Sodium Silicate Aqueous Solution in Pressure Vessel with Agitation>
As another example of preparing an alkali silicate aqueous solution by mixing a silicon oxide raw material, an alkali metal hydroxide, and water, and raising the temperature and reacting in a container, agitation of silica, sodium hydroxide and water The mixture was heated and reacted with stirring in a pressure vessel.

FIG. 12 shows the stirring pressure reaction vessel (1). The stirrer (105) was charged into the pressure reaction vessel (1) used in “Example 1” from the raw material charging port (101) at the top of the vessel and fixed. The material of the stirrer (105) was stainless steel.

The reaction procedure was the same as “Example 1” except that the reaction was carried out with stirring. That is, 1.32 kg of silicic acid, 0.88 kg of sodium hydroxide and 8 liters of water were charged into the container (1), and stirring was started. The stirring speed was kept constant at 20 rpm. After the gas burner (104) was ignited and the thermocouple thermometer indicated 170 ° C, the temperature was adjusted between 170 ° C and 180 ° C. This stirred reaction was held for 1 hour. After 1 hour of reaction, the reaction product was taken out. The reaction product was an aqueous solution with slight turbidity. When this aqueous solution was filtered, the filtrate showed a colorless and transparent aqueous solution (sodium silicate aqueous solution).

The colorless and transparent aqueous solution produced by the above procedure was reacted with hydrochloric acid in the same procedure as in “Example 1” to precipitate silicic acid, and further dehydrated to produce silicon oxide. As shown in FIG. 11, the results of chemical analysis of the produced silicon oxide were the same as those of “Example 1” for the concentrations of boron and phosphorus, but the concentrations of the other metal elements were subject to variations in analysis. Matched within range.

<Fourth Step (Modification): Solidification and Segregation of Silicon by Cooling Crucible Induction Melting Method with Plasma Jet Heating in Air>
Plasma jet from above the molten metal surface in the atmosphere when casting a vertically long ingot by solidifying in one direction while inductively melting silicon taken out of the electric furnace in a water-cooled copper crucible electrically insulated in the vertical direction. An example of casting with heating was performed as follows.

FIG. 16 shows a cooled crucible induction melting furnace (5) to which plasma jet heating is applied in the atmosphere for the present embodiment. That is, in order to install a cooling crucible (501) and an induction coil (502) surrounding the cooling crucible (501) in the atmosphere, and to pull down the silicon (S) ingot directly below the cooling crucible (501). The lowering shaft (503) and the vertical movement drive device (504) were installed so that the silicon (S) ingot was continuously pulled down. Further, a plasma jet device (505) and a raw material inlet (506) capable of moving up and down and rotating were installed above the furnace body.

In this example, the cross section of the silicon (S) ingot with respect to the casting direction was square, and one side thereof was 20 cm. Therefore, the inner diameter of the cooling crucible (501) was a square with a side of 20 cm, the outer diameter was also square with a side of 25 cm, and the number of divisions for electrically insulating the cooling crucible (501) in the vertical direction was 32. Each segment of the cooling crucible (501) divided into 32 was processed so that cooling water circulated therein, and mica of an electrically insulating material was inserted between the segments.

The cooling water in the cooling crucible (501) flowed at a flow rate of 200 liters per minute as a whole. The induction power supply used a maximum output of 300 kW and a frequency of about 12 kHz. The induction coil (502) was a square with an inner diameter of 28 cm, the number of turns was 3 turns, and the coil height was 20 cm.

The plasma jet device (505) for heating the molten silicon (S ′) from the top is a water-cooled plasma torch having a diameter of 9.8 cm and a length of 1.3 m, a DC power supply with a maximum output of 150 kW, and plasma gas ignition. It consisted of a high-frequency transmission device and an argon gas flow rate controller. The diameter of the plasma injection port of the plasma jet apparatus (505) was 18 mm.

The procedure of this example was performed in the air as follows. First, a graphite pedestal (507) having a square cross section with respect to the pull-down direction and a side of 20 cm is placed on the vertical movement drive device (504) so that the upper surface of the pedestal is the same as the lower end position of the induction coil (502). The cooling crucible (501) was inserted from below, and about 8 kg of silicon (S) was placed on the pedestal.

Next, the plasma jet device (505) is lowered so that the tip thereof approaches the silicon (S) on the pedestal (507), and argon is further introduced into the plasma torch at 150 liters per minute so that DC plasma is supplied to the cathode of the plasma torch. And ignited between silicon (S). After confirming the ignition of the plasma, an induction power source having a frequency of about 12 kHz was transmitted to apply power to the silicon (S).

】 When ignition of argon plasma and application of induction power were started and the input power was gradually increased, the temperature rise of silicon (S) was accelerated, and dissolution of silicon (S) started soon. After the dissolution of the silicon began, the silicon raw material was further charged from the raw material inlet (506) and the charging was continued until the amount of silicon (S) reached 18 kg. The state of the silicon melt (S ′) that has been irradiated with the plasma jet and is induction-melted in the cooling crucible (501) is stable, and the silicon melt (S ′) faces the inner wall of the cooling crucible (501). The side surface to be subjected to electromagnetic force was separated from the cooling crucible (501) in a non-contact manner.

After the initial dissolution has been made stable, the silicon prepared and crushed and washed in the third step is continuously charged into the cooling crucible (501) from the raw material charging port (506) located above, Casting was started by lowering the vertical movement driving device (504) holding (S). When the vertical drive unit (504) began to descend, continuous raw material supply and continuous ingot solidification were simultaneously performed, and continuous casting was performed. In this embodiment, the casting speed is 3.0 mm / min, the induction power output during steady casting is about 150 kW, the output for plasma generation is about 65 kW with a voltage of about 130 V and a current of about 500 A. It was. Casting was stopped when the total length of the silicon (S) ingot reached 60 cm.

<Fifth Step (Modification): Partially cutting and separating the head of the cast ingot>
In order to analyze impurities from the ingot of silicon (S) cast in the fourth step, an analysis sample was collected by cutting with a diamond cutter. The impurity analysis result of the silicon (S) ingot is shown in FIG.

That is, impurity concentrations in silicon (S) produced by electric furnace reduction were 0.2 ppmw or less for both boron and phosphorus, aluminum 8 ppmw, iron 108 ppmw, titanium 2 ppmw, chromium 0.8 ppmw, and carbon 920 ppmw. As a result of solidification segregation by the cooling crucible induction melting method with the addition of plasma jet heating under the atmosphere, the average concentration of impurities in the ingot portion having a length of 54 cm in the lower ingot portion of 60 cm of silicon (S), Both boron and phosphorus were 0.2 ppmw or less, aluminum 0.1 ppmw or less, iron 0.1 ppmw or less, titanium 0.1 ppmw or less, chromium 0.1 ppmw or less, carbon 5 ppmw, and oxygen 16 ppmw.

It was shown that the result of removing metal impurities from silicon to which this plasma jet heating was applied was in good agreement with the impurity liquid-phase and solid-phase partitioning theories as in Example 1. This purified ingot portion was used as a solar cell grade silicon in the following solar cell trial manufacture.

In this embodiment, silicon (S) was dissolved in the atmosphere so that oxygen in the air was mixed in the silicon. However, if oxygen in the silicon was dissolved in an inert gas such as argon, silicon monoxide gas Is removed from the silicon (S). That is, in the process of producing a polycrystalline ingot for a solar cell using the silicon raw material produced according to the present embodiment, the process is usually performed by melting and solidifying in an inert gas of argon. Since oxygen generates silicon monoxide gas and is removed from the silicon, there is no problem even if the initial oxygen concentration in the raw material is high.

The process of making a prototype of the silicon raw material produced in this example into a solar cell was performed as follows. The silicon raw material 51 kg produced in this example was used for the production of a polycrystalline ingot for a solar cell by a normal mold unidirectional solidification method. That is, the silicon was charged into a high-purity silica square mold and melted and unidirectionally solidified under an inert gas of argon, and produced into an ingot having a width of 33 cm × 33 cm and a height of 20 cm. . Next, this ingot was divided by a diamond cutting machine, and then processed into a silicon polycrystalline substrate for solar cells having a width of 15 cm × 15 cm and a thickness of 250 μm by a wire saw slicing machine. Furthermore, 100 pieces of the silicon polycrystalline substrate were extracted and prototyped as solar cells. In the solar cell prototype process, hydrogen passivation technology was used, and the average value of solar cell conversion efficiency was 14.9% with an average value of 100 sheets. This example confirmed that the silicon raw material produced according to the present invention satisfies the solar cell quality.

The silicon raw material for use in a crystalline silicon solar cell having sufficient quality can be produced at low cost by the method for producing silicon for solar cells of the present invention. Moreover, the manufacturing method of this invention can manufacture the silicon | silicone for solar cells with few energy consumption compared with the conventional method, since there are few processes until it manufactures a solar cell grade silicon | silicone from the silica stone raw material.

Claims (7)

A first step of preparing an alkali silicate aqueous solution by mixing an oxide raw material of silicon, an alkali metal compound, and water, and heating and reacting in a container;
A second step in which hydrochloric acid or sulfuric acid is added to the aqueous alkali silicate salt solution to precipitate silicic acid to separate and dry high-purity silicon oxide;
A third step of high-temperature reduction of the separated and dried silicon oxide with a carbon material in an electric furnace to generate silicon and take it out of the electric furnace;
A fourth step of casting the vertically elongated ingot by unidirectionally solidifying the taken-out silicon in a crucible that is electrically insulated in the vertical direction while induction melting;
A fifth step of producing a silicon raw material for a solar cell, comprising a fifth step of partially cutting and separating a head portion of the cast silicon ingot and using the remaining most as a silicon raw material for a solar cell. . The method for producing a silicon raw material for a solar cell according to claim 1, wherein the alkali metal compound is an alkali metal hydroxide. The method for producing a silicon raw material for a solar cell according to claim 2, wherein the alkali metal is lithium, sodium or potassium. In the first step, a silicon oxide raw material, an alkali metal compound, and water are mixed and heated and reacted while stirring in a container to produce an alkali silicate aqueous solution. The method for producing a silicon raw material for a solar cell according to any one of items 3 to 4. The said 1st process WHEREIN: After melt | dissolving and cooling the mixture of the oxide raw material of silicon and an alkali metal carbonate at high temperature, it combines with water and produces silicate alkali salt aqueous solution. A method for producing silicon raw materials for solar cells. The method for producing a silicon raw material for a solar cell according to claim 5, wherein the alkali metal carbonate is a carbonate of lithium, sodium or potassium. The method for producing a silicon raw material for a solar cell according to any one of claims 1 to 6, wherein in the fourth step, casting is performed by applying plasma jet heating from above silicon in the atmosphere.

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