JP2008001592A - Manufacturing process for silicon - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new and inexpensive manufacturing process for high purity silicon and high purity silicon prepared by the manufacturing process. <P>SOLUTION: In manufacturing silicon by molten salt electrolysis of silica in an electrolytic cell, the manufacturing process of this invention comprises a step (1) in which the cathode is a silicon-containing alloy being liquid at an electrolytic reaction temperature and the silicon content of the alloy increases as the electrolysis advances, a step (2) in which the silicon-containing alloy is taken out from the electrolytic cell before reaching the concentration at which silicon start depositing at the electrolytic reaction temperature, a step (3) in which the silicon is coagulated by cooling the silicon-containing alloy at a temperature higher than the eutectic point of the taken-out silicon-containing alloy and lower than the electrolytic temperature and a step (4) to recover the coagulated silicon. The steps are arranged in the order above. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing silicon, particularly to silicon for solar cell raw materials.

Metallurgical grade silicon is produced by reduction using an arc furnace by mixing carbon and silica. Trichlorosilane is synthesized by the reaction of the metallurgical grade silicon and HCl, and purified by rectification, and then reduced at a high temperature using hydrogen to produce semiconductor grade silicon. The raw material silicon for solar cells is mainly made of non-standard products generated when semiconductor grade silicon is manufactured.
The semiconductor-grade silicon manufacturing method can produce extremely high-purity silicon, but the conversion rate to silicon is low, and a large amount of hydrogen is required to make this equilibrium advantageous to silicon, and the conversion rate is still low. It is necessary to circulate a lot of unreacted gas again, and various halogenated silanes are generated in the unreacted gas. Therefore, separation by distillation is necessary, and it cannot be finally reduced with hydrogen. High cost due to the large amount of silicon tetrachloride produced.

  On the other hand, solar cells are attracting attention as an effective solution to environmental problems such as carbon dioxide gas in recent years, and the demand is also increasing significantly. However, current solar cells are still expensive, and the price of the electric power thus obtained is several times that of commercial electricity. Currently, the demand for solar cells is expanding in response to environmental problems and increasing energy demand, and it is becoming a situation where raw materials are insufficient with conventional non-semiconductor silicon alone. It is rare.

To meet this demand, a method has been proposed in which high-purity carbon and high-purity silica are synthesized and reduced in a reduction furnace using high-purity furnace materials to synthesize high-purity silicon. However, it was difficult to reduce the cost because the yield was not increased. In addition, a method of reducing silicon tetrachloride with aluminum has been proposed (Non-patent Document 1, Patent Document 1, and Patent Document 2), but it is difficult for phosphorus in aluminum to remain in silicon and to achieve a desired purity. Met.
In addition, various proposals such as a zinc reduction method of silicon tetrachloride (Non-Patent Document 2) and a fluidized bed reduction of trichlorosilane (Non-Patent Document 3) have been made, but none has been put into practical use yet.
In addition, a method for producing silicon by electrolysis of silica has also been studied.

Shiro Yoshizawa, Tomoyasu Tanno, Arata Sakaguchi, Aluminum reduction of silicon tetrachloride, Industrial Chemical Journal 64 (8) 1347-50 (1961) Japanese Patent Laid-Open No. 2-64006 JP 59-182221 A Evaluation of selecdted chemical processes for production of low-cost silicon, J. M. Blocher, et.al.Jet propulsion laboratory final report (1981) 1985-62 New Energy Development Organization commissioned work result report “Development of practical technology for photovoltaic power generation system low-cost silicon experimental refining verification”, 1988, Shin-Etsu Chemical Co., Ltd.

However, in the method for recovering silicon by molten salt electrolysis of silica, when electrolysis is performed at a temperature lower than the melting point of silicon (1410 ° C.), the silicon deposited on the cathode grows in a dendritic shape and causes a short circuit between the electrodes. If the electrolysis temperature is raised beyond the melting point of silicon, the current efficiency is low due to the reverse reaction of the reduced metal, and there are difficulties such as the lack of suitable furnace materials, making industrialization difficult. It was.
An object of the present invention is to provide a novel and inexpensive production method of high-purity silicon that can solve the above-mentioned problems, and high-purity silicon obtained by the production method, and particularly suitable as a raw material for solar cells. The object is to provide a new and inexpensive method for producing high-purity silicon used.

That is, the present invention provides [1] a method for producing silicon by subjecting silica to molten salt electrolysis in an electrolytic cell, wherein a silicon-containing alloy that is in a liquid phase at an electrolysis temperature is used as a cathode, and electrolysis is advanced to The step (1) of increasing the silicon content, the step (2) of taking out the silicon-containing alloy at the cathode outside the electrolytic cell before reaching the concentration at which silicon precipitates at the electrolysis temperature, and the silicon-containing alloy taken out are combined. A step (3) of solidifying the silicon-containing alloy by cooling the silicon-containing alloy in a temperature range higher than the melting point and lower than the electrolysis temperature, and a step (4) of recovering the solidified silicon in this order. This is a method for producing the silicon.
The present invention further includes [2] a step (5) of returning the alloy having a reduced silicon concentration to the cathode of the electrolytic cell by recovering silicon in the step (4). Production method,
[3] The method for producing silicon according to [1] or [2], including one or more selected from aluminum, copper, and tin as an alloy,
[4] The method for producing silicon according to any one of [1] to [3], wherein the electrolytic bath in molten salt electrolysis includes cryolite (3NaF · AlF ₃ ),
[5] The method for producing silicon according to any one of [1] to [4], wherein the specific gravity of the silicon-containing alloy serving as a liquid phase is larger than that of the electrolytic bath,
[6] The method for producing silicon according to any one of [1] to [4], wherein the specific gravity of the silicon-containing alloy serving as a liquid phase is smaller than that of the electrolytic bath,
[7] The method for producing silicon according to any one of [1] to [6], wherein the silica has a purity of 99.9% or more,
[8] The method for producing silicon according to any one of [1] to [7], wherein the electrolysis temperature exceeds the eutectic point of the silicon-containing alloy and is lower than the melting point of silicon.
[9] The method for producing silicon according to any one of [1] to [8], wherein the electrolysis temperature is 700 ° C. or higher and 1300 ° C. or lower,
[10] The method for producing silicon according to any one of [1] to [9], wherein the anode current density is 0.01-3 A / cm ² .
[11] The method for producing silicon according to any one of [1] to [10], wherein the cathode current density is 0.01-3 A / cm ² .
[12] Silicon obtained by the silicon production method according to any one of [1] to [11],
[13] The present invention relates to a solar cell having a silicon obtained by the method for producing silicon according to any one of [1] to [11].

According to the production method of the present invention, silicon once dissolved in the financial solution is taken out of the electrolytic cell, cooled at a temperature equal to or higher than the eutectic point of the alloy, and only silicon is precipitated, Reduce the silicon concentration. By returning the low-silicon concentration combined liquid to the electrolytic cell again, the silicon concentration can be increased by electrolysis up to the saturated silicon concentration at the electrolysis temperature. By this operation, the electrolytic reduction reaction can proceed continuously.
Therefore, a short circuit due to electrodeposition of solid silicon is prevented, and a stable electrode interface is obtained. Therefore, according to the manufacturing method of the present invention, silicon can be electrolyzed stably and continuously at a temperature lower than the melting point of silicon. . For this reason, the manufacturing method of this invention is very important industrially.

  When the silicon is recovered by electrolyzing silica, the cathode is made of a liquid silicon alloy at the electrolysis temperature, so that dendritic precipitation of silicon can be prevented and the melting point can be lowered as compared with silicon alone. For this reason, electrolysis at a lower temperature becomes possible. Moreover, the silicon concentration of the liquid silicon alloy can be increased by continuing the electrolysis. By taking out the alloy having such a high silicon concentration out of the electrolytic cell and cooling it in a temperature range equal to or higher than the eutectic point of the alloy, silicon can be deposited and recovered. An alloy in which the silicon concentration is lowered by depositing and recovering silicon is returned to the cathode of the electrolytic layer and used as a liquid cathode for electrolysis, thereby enabling a process that enables continuous electrolysis.

The method for producing silicon according to the present invention is a method for producing silicon by subjecting silica to molten salt electrolysis in an electrolytic cell, and a silicon-containing alloy that is in a liquid phase at an electrolysis temperature is used as a cathode (hereinafter referred to as “cathode alloy”). .
As the cathode in the present invention, an alloy of silicon and a metal forming a low-temperature eutectic point that is liquid under electrolysis conditions is used, and the alloy preferably has a low vapor pressure and is stable.
Examples of the metal forming such an alloy include aluminum, copper, tin, gallium, indium, silver, mercury, lead, and the like. However, in consideration of cost and environmental impact, the metal is selected from aluminum, copper, and tin. Metal is preferred, and the alloy may contain two or more of the metals.
The purity of the metal used in the present invention is preferably 4N or more, more preferably 6N or more, and particularly preferably 7N or more. In particular, the P and B contents are each preferably 0.5 ppm or less, more preferably 0.3 ppm or less, and particularly preferably 0.1 ppm or less.

  The silica which is a raw material in the present invention is preferably highly pure. The purity of the silica is preferably 4N or higher, more preferably 6N or higher, and particularly preferably 7N or higher. In particular, the contents of P and B are each preferably 0.5 ppm or less, more preferably 0.3 ppm or less, and particularly preferably 0.1 ppm or less.

In the present invention, the electrolytic bath used is preferably a metal halide. Examples of the metal halide include one or more selected from alkali metal, alkaline earth metal, aluminum, zinc, copper fluoride, chloride and bromide. Specific examples include cryolite (3NaF · AlF ₃ ), calcium chloride and the like because of industrial availability.
The purity of the electrolytic bath constituent material used in the present invention is preferably 4N or more, more preferably 6N or more, and particularly preferably 7N or more. In particular, the content of P and B is preferably 0.5 ppm or less, more preferably 0.3 ppm or less, and particularly preferably 0.1 ppm or less.
In the present invention, alkali metals and alkaline earth metals do not need to be considered as impurities because they are hardly mixed into silicon under electrolysis conditions. Moreover, it is not necessary to consider the constituent elements of the cathode alloy as impurities

In the present invention, the higher the electrolytic current density is, the more efficient the amount of silicon recovered per hour. However, if the density of the electrolysis current becomes too high, the overvoltage rises and the electrolysis voltage increases, resulting in energy loss. The current density depends on the electrolytic bath composition, temperature, dissolved silica concentration, etc., but is generally about 0.01-3 A / cm ² , more preferably 0.05-1 A / cm ² , most preferably as the cathode current density. Preferably it is 0.1-0.7 A / cm < ² >.

  In the present invention, the electrolysis temperature is appropriately selected according to the composition of the cathode alloy. That is, the electrolysis temperature is appropriately selected in the range exceeding the temperature (eutectic point) at which the cathode alloy becomes liquid and less than the melting point of silicon. If the electrolysis temperature is lower than the melting point of silicon, the current efficiency is further improved, and the selection of the electrolytic cell material becomes easier. Further, if the eutectic point is exceeded, electrolysis is more likely to proceed. The higher the electrolysis temperature, the higher the solubility of silicon in the cathode alloy, so that more silicon can be recovered. For example, when aluminum-silicon is used as the cathode alloy, the eutectic point is 577 ° C., so the electrolysis temperature is preferably set to exceed 577 ° C. and less than 1410 ° C. This temperature is preferably 700 ° C. or higher and 1300 ° C. or lower, more preferably 800 ° C. or higher and 1200 ° C. or lower, and most preferably 900 ° C. or higher and 1100 ° C. or lower. It is possible to operate at an economical optimum temperature within this range.

In the present invention, the cooling temperature of the cathode alloy is higher than the eutectic point of the cathode alloy. If the eutectic point is not reached, only the silicon cannot be recovered because the alloy is solidified. However, the amount of silicon that can be recovered corresponds to the difference in composition of the liquidus line of the alloy corresponding to the difference between the electrolysis temperature and the cooling temperature. Therefore, if the difference from the electrolysis temperature is small, the amount of silicon that can be recovered is small and economical. Therefore, generally, a temperature slightly higher than the eutectic point is preferable.
For example, in the case of an aluminum-silicon alloy, when the electrolysis temperature is 1100 ° C., the silicon concentration is maintained in a molten state up to 55%. When this alloy is taken out of the electrolytic cell and cooled to 600 ° C., the silicon concentration must be reduced to 15%, so that silicon corresponding to 40% of this difference can be recovered as a solid.

A known method can be used as a method for cooling silicon. That is, a method of holding in a container kept at a cooling temperature, holding the financial liquid in a container slightly higher than the cooling temperature, immersing a cooling body at the cooling temperature in the melt, and silicon on the cooling body And the like.
In this manner, the silicon in the metal melt containing the silicon that has been electrolytically reduced at high temperature near saturation is recovered in an amount corresponding to the difference from the solubility corresponding to the cooling temperature.
As described above, a series of electrolytic reduction reactions, extraction of the cathode melt outside the electrolytic cell, cooling precipitation reaction, and return of the melt with reduced silicon concentration to the electrolytic cell occur continuously. Thus, the reaction proceeds as long as the liquidity of the combined financial liquid can be maintained without the silicon reaching saturation and the reaction stagnating.

Here, the temperature of the low-temperature cooling part of the metal melt can be lowered to the eutectic point (eutectic point) of the metal and silicon, but as described below, it is actually higher than the melting point of the metal. Is preferable because the operation is simple.
For example, in the case of aluminum, the melting point is 660 ° C., but since the eutectic point of Al—Si is 580 ° C. or higher, the reaction is first started in a molten state of 660 ° C. or higher. When produced, it becomes a melt above the eutectic point, so the liquidus temperature can be lowered to 580 ° C. Below this temperature, only silicon cannot be recovered. There is no particular upper limit on the temperature of the low-temperature cooling section, as long as the temperature difference described below and the temperature requirement of the high-temperature electrolysis section are satisfied.
A larger temperature difference between the high-temperature electrolysis part and the low-temperature cooling part of the metal melt is generally preferable, and is 100 ° C. or higher, preferably 200 ° C. or higher, more preferably 300 ° C. or higher.
From the viewpoint of the yield of the reaction, the temperature of the high temperature electrolysis part is preferably as high as possible, 700 ° C. or higher, more preferably 900 ° C. or higher, particularly preferably 1100 ° C. or higher. However, the temperature of the high-temperature electrolysis part is preferably 1300 ° C. or lower due to restrictions such as electrolytic cell materials.

  According to the present invention, it is possible to obtain more silicon than 40% of the weight of the aluminum used, and even more than 45% silicon, so that the yield of silicon obtained is high, which is economically advantageous. In the method of the present invention, the production volume is controlled by current.

  The electrolytic cell material must be one that does not react with the metal used. Examples of the oxide include silica, alumina, zirconia, titania, zinc oxide, magnesia, and tin oxide. Examples of the nitride include silicon nitride and nitride. Examples thereof include aluminum, and those in which these constituent elements are partially substituted with other elements are also included. For example, a compound such as sialon composed of silicon, aluminum, oxygen, and nitrogen can also be used. Examples of the carbide include SiC, graphite, and the like, and those obtained by partially replacing these constituent elements with other elements can also be used. Further, a method of holding the bath with a solidified electrolyte (for example, cryolite) may be used in the same manner as aluminum electrolysis.

  In the method of the present invention, the reaction atmosphere is air or an inert gas, but it is preferable that water, oxygen and the like do not exist for the progress of the reaction.

The polycrystalline silicon obtained as described above has high purity and is suitably used as a raw material for solar cell silicon.
If necessary, the obtained polycrystalline silicon is subjected to treatment with acid or alkali, segregation such as directional solidification, dissolution at high vacuum, etc. in order to remove the residue of unattached metal components and unreacted metal components. Thus, the impurity element contained in the silicon can be further reduced. In particular, it is preferable to highly purify the obtained polycrystalline silicon by directional solidification.

A solar cell using polycrystalline silicon obtained by the present invention will be described.
Using the silicon obtained by this method, an ingot is produced by a casting method or an electromagnetic casting method. The conductivity type of the substrate of the solar cell is generally p-type, and the introduction of the dopant can be achieved, for example, by adding boron or leaving aluminum. The ingot is sliced by an inner peripheral cutting or a multi-wire saw. After slicing, both sides are lapped with free abrasive grains as necessary, and a polycrystalline substrate is obtained by immersing in an etching solution such as hydrofluoric acid to remove the damaged layer. In order to reduce the light reflection loss on the surface, a V-groove is mechanically formed by using a dicing machine, or a texture structure is formed by reactive ion etching, etching using acid, alkali, or the like. . Subsequently, a pn junction is obtained by forming a diffusion layer of n-type dopant such as phosphorus or arsenic on the light receiving surface. Furthermore, after forming an oxide film layer such as TiO ₂ on the surface, an electrode is attached to each surface, and an antireflection film such as MgF ₂ is attached to reduce the loss of light energy due to reflection. it can.

  While the embodiments of the present invention have been described above, the embodiments of the present invention disclosed above are merely examples, and the scope of the present invention is not limited to these embodiments. The scope of the present invention is defined by the terms of the claims, and further includes meanings equivalent to the description of the claims and all modifications within the scope.

Examples are provided to describe the present invention in more detail, but the present invention is not limited thereto.
Example 1
A graphite crucible is charged with aluminum, cryolite, and silica and set in an electric furnace having a mullite core tube. Next, these melts are electrolyzed at 1100 ° C.

After electrolysis, the alloy is recovered by cooling. The resulting alloy can be dissolved with hydrochloric acid to obtain silicon. This alloy is once melted at 1100 ° C. and held at 700 ° C. for 3 hours, and then the molten metal is subjected to solid-liquid separation to obtain a solid having a silicon concentration higher than the silicon concentration in the melt and lower than the silicon concentration in the solid. A melt having a silicon concentration can be obtained.
The melt having a silicon concentration lower than the silicon concentration in the solid is returned to the electrolytic furnace again to perform electrolysis of silica.

Example 2
With a quartz tube inserted into the inner wall of a graphite crucible having a bottom area of 22 cm ² and lined, 132 g of cryolite (95%; manufactured by Central Glass), 7 g of silica powder (99.5%; manufactured by Alfa), 43 g of aluminum particles (Made by Wako Pure Chemical Industries) was mixed, dried at 190 ° C for 2 days, kept at 1100 ° C for 3 hours in an electric furnace while blowing argon gas, then held at about 1000 ° C, and a carbon rod of φ12 mm was set as the anode Then, electrolysis was started after confirming a certain temperature. After electrolysis at an electrolysis current of 2.2 A for 1 hour, the current was kept for 2 hours without flowing. Thereafter, the melt was poured out and the metal part was recovered. As a result, 36.2 g could be recovered. When this was dissolved by concentrated hydrochloric acid and analyzed by ICP, an Al—Si alloy was produced, and the Si content was 22 wt% and 7.96 g.
30 g of this alloy was again charged into a graphite crucible and held at 1000 ° C., melted, gradually cooled, and held at 590 ° C. A graphite rod cooled to 550 ° C. with nitrogen was immersed in the melt and pulled up after 10 minutes. As a result, 2.7 g of silicon adhered to the graphite rod.
For reference, the same sample was charged with exactly the same setup and held for 3 hours without any current flowing. The recovered alloy was 37.2 g, and the Si content was 19 wt%, 7.07 g. The reason why silicon is obtained here is that aluminum directly reduces silica. Using this value as a reference value, the substantial increase in Si due to energization is calculated to be 7.96 g−7.07 g = 0.89 g. From the amount of energization, it was calculated that 0.57 g of Si was deposited, so although there was some error, deposition of Si corresponding to the amount of energization was recognized.

Example 3
In the same manner as in Example 2, cryolite, silica, and aluminum were charged, and only electrolysis current was set to 1.1 A. After electrolysis for 2 hours and 15 minutes, the current was maintained for 1 hour without flowing. The recovered alloy was 37.1 g, and the Si content was 21 wt%, 7.79 g.
30 g of this alloy was again charged into a graphite crucible and held at 1000 ° C., melted, gradually cooled, and held at 590 ° C. A graphite rod cooled to 550 ° C. with nitrogen was immersed in the melt and pulled up after 10 minutes. As a result, 2.7 g of silicon adhered to the graphite rod.
The alloy recovered in the reference experiment was 37.2 g, and the Si content was 19 wt% and 7.07 g. Therefore, when a substantial increase in Si due to energization was calculated, 7.79 g−7.07 g = 0.72 g. From the amount of energization, it was calculated that 0.65 g of Si was deposited, so although there was some error, deposition of Si corresponding to the amount of energization was recognized.

Claims (13)

  In the method for producing silicon by subjecting silica to molten salt electrolysis in an electrolytic bath, a step (1) in which a silicon-containing alloy that is in a liquid phase at an electrolysis temperature is used as a cathode and electrolysis proceeds to increase the silicon content in the alloy And step (2) of taking out the silicon-containing alloy from the cathode outside the electrolytic cell before reaching the concentration at which silicon is deposited at the electrolysis temperature, and higher than the eutectic point of the taken-out silicon-containing alloy and lower than the electrolysis temperature. The method for producing silicon, comprising: a step (3) for solidifying the silicon by cooling the silicon-containing alloy within a temperature range; and a step (4) for recovering the solidified silicon in this order.   The method for producing silicon according to claim 1, further comprising a step (5) of recovering the silicon in the step (4) to return the alloy having a lowered silicon concentration to the cathode of the electrolytic cell.   The manufacturing method of the silicon | silicone of Claim 1 or 2 containing 1 type, or 2 or more types chosen from aluminum, copper, and tin as an alloy. The method of producing silicon according to claim 1 electrolytic bath, containing cryolite (3NaF · AlF ₃₎ in the molten salt electrolysis.   The method for producing silicon according to any one of claims 1 to 4, wherein the specific gravity of the silicon-containing alloy serving as a liquid phase is larger than that of the electrolytic bath.   The method for producing silicon according to any one of claims 1 to 4, wherein the specific gravity of the silicon-containing alloy serving as a liquid phase is smaller than that of the electrolytic bath.   The method for producing silicon according to claim 1, wherein the silica has a purity of 99.9% or more.   The method for producing silicon according to claim 1, wherein the electrolysis temperature exceeds the eutectic point of the silicon-containing alloy and is lower than the melting point of silicon.   Electrolysis temperature is 700 degreeC or more and 1300 degrees C or less, The manufacturing method of the silicon | silicone in any one of Claims 1-8. The method for producing silicon according to claim 1, wherein the anode current density is 0.01-3 A / cm ² . The method for producing silicon according to claim 1, wherein the cathode current density is 0.01-3 A / cm ² .   Silicon obtained by the method for producing silicon according to any one of claims 1 to 11.   The solar cell which has a silicon | silicone obtained by the manufacturing method of the silicon | silicone in any one of Claims 1-11.