JP3247842B2 - Method for casting silicon for solar cells - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for casting silicon for a solar cell, and more particularly to a method for casting high purity molten metal silicon obtained by dephosphorization, decarburization and deboronation from metal silicon. The present invention relates to a technique for simultaneously removing a metal element and casting to form an ingot before forming a substrate.

[0002]

2. Description of the Related Art Research on solar cells has been carried out for quite a long time, and recently, those having a photoelectric conversion efficiency of about 13 to 15% under terrestrial sunlight have appeared, and have been practically used for various purposes. Is being transformed. However, the spread of electricity for general household use or as an energy source for automobiles, ships, machine tools, etc., at least in Japan, is not yet sufficient. The reason is that there is no established technology for mass production of silicon substrates required for manufacturing solar cells at low cost.

At present, in order to manufacture a silicon substrate for a solar cell on an industrial production scale, as shown in FIG. 9, metal silicon as a starting material is first reacted with hydrochloric acid to gasify it as trichlorosilane, and the gas is converted into trichlorosilane. After rectification to remove impurity elements, the gas is reacted with hydrogen gas to deposit high-purity silicon from the gas by a so-called CVD method. Therefore, the obtained high-purity silicon is simply an aggregate of silicon particles without controlling the crystallinity. In addition, boron contained in the high-purity silicon mass forming the aggregate is 0.001 pp.
m, which is not in the range of 0.1 to 0.3 ppm required to satisfy the specification of the specific resistance of the substrate for a P-type semiconductor of 0.5 to 1.5 ohm-cm. In order to make the massive high-purity metallic silicon into a silicon substrate for a solar cell, it is essential to adjust the specific resistance and control the crystallinity. As shown on the right side of FIG. In addition to the adjustment (addition of boron), it is necessary to use a pulling method in the case of a single crystal and an ingot using a unidirectional solidification method in the case of a polycrystal.

However, such a conventional manufacturing method is
Since the ingot, which has been highly purified for semiconductors, must be adjusted and refined again so as to be suitable for solar cells, it takes time and effort, and the yield is poor. Is also required separately, which increases the manufacturing cost. For this reason, as described above, currently available solar cells are expensive and hinder general use. In addition, in purifying metallic silicon in a chemical process, there is a problem that a large amount of pollutants such as silane and chloride is inevitably generated, which hinders mass production.

Accordingly, 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 researching and developing a method to cast it into a silicon substrate. Incidentally, with respect to the production of ingots using the unidirectional solidification method, see, for example, Japanese Patent Application Laid-Open No. 61-36113.
Japanese Patent Application Laid-Open Publication No. H11-15083 proposed a casting apparatus including a supply unit 2, a casting unit 3, a crystallization unit 4, and a cooling unit 5 of a mold 1 as shown in FIG. Then, the mold is successively transported from the supply section to the casting section, filled with molten silicon, transferred to the cooling section after the directional solidification of silicon occurs, and finally the mold is taken out of the apparatus, and the column-shaped silicon molding is performed. In the method for manufacturing a body, each mold entering the casting section from the supply section is subjected to 2
After the mold is filled with silicon and before the silicon is completely solidified, the mold is moved to a crystallization section where the silicon is directionally solidified due to the release of directional energy. Until the solidification step is completed at the time of solidification, the exposed surface of silicon is maintained at least partially in a molten state by energy supply, and after the silicon is completely solidified in the crystallization part, the mold is transported to the cooling part. And disclosed technology. The crystallization rate at that time is set to 0.1 to 5 mm, or the solidified silicon is tempered to 900 to 1300 ° C. at a cooling rate of 0.5 to 30 ° C./min. They are also quenched to temperature and then taken out.

In conducting the research mainly on the above-mentioned metallurgical process, the inventor disclosed a molten silicon having a purity of 5 nine obtained by dephosphorizing, deboronating, decarburizing and deoxidizing metallic silicon, as disclosed in An attempt was made to solidify an ingot using the casting method described in US Pat. However, when solidification was performed at a solidification speed of 3 mm / min or more, growth of columnar crystals did not occur, and an ingot could not be formed. That is, it was a mere aggregate of so-called silicon crystals that collapsed when removed from the mold. In this case, the substrate cannot be sliced immediately after casting.

On the other hand, the molten silicon having the purity of five nines still contains about 10 ppm of metal elements such as Al, Fe, and Ti. In this state, the composition of the silicon substrate for solar cells is low, and the purity is low. There was also a desire to remove more of these elements using inexpensive methods.

[0008]

SUMMARY OF THE INVENTION In view of such circumstances, the present invention provides a method for casting solar cell silicon capable of simultaneously removing a metal element from molten silicon having relatively low purity and manufacturing an ingot for a substrate. It is intended to provide a way.

[0009]

Means for Solving the Problems In order to achieve the above object, the inventor reviewed the casting method described in the above-mentioned JP-A-61-36113. As a result, the casting method uses CVD.
It is suitable for re-melting and casting ingots of highly purified silicon to the so-called eleven nine by the method and making it into an ingot, but it has been found that in low-purity silicon, metal impurity elements inhibit the growth of columnar crystals Was. It is also known that silicon is purified by unidirectional solidification, utilizing the small solid-liquid partition coefficient of metal impurity elements such as Al, Fe, and Ti, and finally solidifying and cutting off that portion. However, it was also found that it was not possible to purify to a desired value depending on the coagulation conditions (initial concentration, coagulation speed, etc.). Therefore, the inventor has worked diligently to find out casting conditions that allow metal impurities to be removed from silicon of this purity to an extent that the silicon substrate for solar cells can tolerate, and at the same time enable casting into an ingot for substrate. The result was embodied as the present invention. In addition, Al, Fe, Ti, and the like precipitate at the crystal grain boundaries during solidification, generate internal stress during the cooling period of the ingot, and reduce the photoelectric conversion efficiency of the solar cell. Then, the inventor adjusted the cooling rate of the ingot after solidification was completed in the present invention in order to suppress the internal stress.

That is, according to the present invention, when manufacturing molten silicon obtained by casting molten silicon obtained by dephosphorization, deboronation, decarburization and deoxidation from metallic silicon and then unidirectionally solidifying the same, the following 3 is required. Solidification speed of molten silicon obtained by the formula
Set the minimum value of the
The solidification rate of the concrete was measured and the measured value was less than the minimum value.
A method for casting silicon for solar cells, characterized by solidifying molten silicon while cooling or heating it . When the metal impurity element is Al: R ₁ = 7.0 × (Al initial concentration) ^{-0.21 When the} metal impurity element is Fe: R ₂ = 6.0 × (Fe initial concentration) ^{-0.18 When the} metal impurity element is Ti Case: R ₃ = 6.6 × (Ti initial concentration) ^-0.18

[0011] Metal silicon dephosphorization, upon de-boron, to produce an ingot for a solar cell by unidirectional solidification by casting the molten silicon obtained by decarburization, determined in advance following three equations coagulation
Target the minimum of the ingot cooling rate after completion
At the same time, the cooling rate of the ingot was measured and measured.
So that the measured value of the ingot is below the minimum value
A method for casting silicon for solar cells, characterized by cooling . When the metal impurity element is Al: C ₁ = 42 × (Al initial concentration) ^{-0.57 When the} metal impurity element is Fe: C ₂ = 41 × (Initial Fe concentration) ^{-0.51 When the} metal impurity element is Ti: C ₃ = 42 × (Ti initial concentration) ^{-0.43 In the} present invention, the molten silicon obtained by dephosphorization, deboronation, decarburization and deoxidation of metallic silicon is cast to produce a solar cell ingot by unidirectional solidification. Since the solidification rate or cooling rate is changed and set according to the initial concentration of the metal impurity element contained in the molten silicon, solidification is performed, so the concentration gradient in the solid-liquid coexistence region at the solidification interface varies depending on the location.
Becomes Razz uniform, so the growth of the thickening and the columnar crystals of the metal impurity elements in the ingot top is sufficiently performed.
As a result, the removal of the metal impurity element from the metal silicon and the production of the solar cell ingot, which have been conventionally performed separately, can be performed by one apparatus, and the improvement of the silicon yield and the reduction of the cost can be achieved. Become like In addition, an inexpensive silicon substrate for a solar cell having a half value or less as compared with the related art can be manufactured.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of an apparatus for carrying out a method for casting silicon for solar cells according to the present invention. that is,
A water-cooled mold 1 made of copper, a water-cooling jacket 8 on the bottom, a heat insulating material 9 on the side walls, and a heater 10 for heating the upper part of the molten silicon 6 cast above are provided. Further, a plurality of ultrasonic rangefinders are used as the sensor 11 for detecting the movement of the solidification interface 7. Further, in some cases, FIG.
As shown in the figure, the solidification rate is compared with a target set value, and the water amount of the cooling jacket 8 and the heater 1 are adjusted so as to match the target value.
An arithmetic controller 12 for adjusting the heat quantity of zero is also provided.

[0013] The present invention provides the above-mentioned mold,
Inject molten silicon that has been deboroned, decarburized and deoxidized,
The solidification is performed so that the solidification interface proceeds upward from the bottom. 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 is as uniform as possible. Specifically, this is performed by initially setting the amount of cooling water in the water cooling jacket and the amount of heat of the heater at the start of casting.

For this reason, the inventor has repeated many experiments and found the solidification rate necessary for maintaining the plane according to the initial concentration of the metal impurity element in the molten silicon, and obtained a stable solidification region as shown in FIGS. It was decided. That is, Al,
With respect to Fe and Ti, it is possible to select a solidification rate at which the concentration in the melt can be performed smoothly. Since the actual molten silicon 6 contains all of these components, a solidification rate that satisfies these three regions (the above three equations) at the same time, and a value as large as possible is set in consideration of productivity. Will be done. 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. 4, the solidification speed is preferably symbol d, and when the initial Ti concentration is e in FIG. 5, the solidification speed is preferably f. Therefore, the solidification is performed at a minimum f or less among the solidification speeds b, d, and f. In addition, in the molten silicon, 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.

Further, the inventor obtained a cooling rate such that no residual stress occurs in the ingot 13 after the solidification was completed, with respect to the initial concentration of the molten silicon 6 in the same manner as described above. It is shown in FIGS. Also in this case, as in the case of the solidification rate, a cooling rate that simultaneously satisfies the three regions is selected. In addition, the specific operation is that the solidification is completed by setting the upper temperature of the ingot in the mold to the melting point of silicon 1
The determination is made when the temperature reaches 410 ° C., and thereafter, the initial setting of the amount of cooling water and the amount of heat is performed so as to achieve the selected cooling rate.

In the present invention, the actual operation is performed by initially setting the amount of cooling water and the amount of heat in order to obtain the desired solidification rate and cooling rate. However, the present invention is not limited to this, and also detects fluctuations in coagulation speed using ultrasonic rangefinders and thermometers arranged at each part as sensors to adjust the fluctuation over time of coagulation speed and obtain a better plane. In addition, adjustment of cooling water and heat quantity was included to correct the target value.

[0017]

EXAMPLE A molten silicon 6 was cast into a mold 1 having a height of 30 cm and a cross-sectional area of 400 cm ² as shown in FIG.
At that time, the casting was divided into two types of casting, one to which the present invention was applied (Examples 1 to 3) and one to which the conventional method was applied (Comparative Examples 1 to 3). Shows the impurity concentration. When the present invention was applied, the solidification rate was read in accordance with the three impurity concentrations and determined to be the slowest as described above. Then, the amount of water and the amount of heat that maintain the solidification rate are determined by the water cooling jacket 8.
And the heater 10 were set. Further, after the solidification was completed, the cooling rate was similarly determined from FIGS. 6 to 8, and the amount of water and the amount of heat were changed so as to maintain the cooling rate. In addition,
The coagulation time was 5 hours and the cooling time was 2 hours.

Table 1 summarizes the casting results. When the present invention was applied, impurities were reduced to the specifications of the solar cell, and an ingot 13 having excellent crystallinity was obtained. Then, the obtained ingot 13 was sliced into a thickness of 250 μm to obtain a silicon substrate for solar cells. When the photoelectric conversion efficiency of the solar cell manufactured on the substrate was measured,
The substrate to which the present invention was applied was 12 to 14%. In the comparative example, the substrate could not be formed, or only a conversion rate of about 8% was obtained.

[0019]

[Table 1]

[0020]

As described above, according to the present invention, removal of metal impurity elements from metal silicon and manufacture of a solar cell ingot, which have been conventionally performed separately, can be stably performed by one apparatus. became. As a result, improvement in silicon yield and reduction in 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 implementing a method for casting silicon for solar cells according to the present invention.

FIG. 2 is a diagram showing another embodiment of FIG. 1;

FIG. 3 Solidification rate of molten silicon and metal impurity element Al
FIG. 6 is a diagram showing a relationship between the initial density and the initial density.

FIG. 4 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. 5: 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. 6 is a diagram showing a relationship between a cooling rate of a silicon ingot and an initial concentration of a metal impurity element Al.

FIG. 7 is a diagram showing a relationship between a cooling rate of a silicon ingot and an initial concentration of a metal impurity element Fe.

FIG. 8 is a diagram showing a relationship between a cooling rate of a silicon ingot and an initial concentration of a metal impurity element Ti.

FIG. 9 is a flowchart showing a process for industrially producing a conventional silicon substrate for a solar cell.

FIG. 10 is a plan view showing an example of a conventional molten silicon casting apparatus.

[Description of Signs] 1 mold 2 supply unit 3 casting unit 4 crystallization unit 5 cooling unit 6 molten silicon 7 solidification interface 8 water cooling jacket 9 heat insulating material 10 heater 11 sensor 12 arithmetic controller 13 ingot (solidified body)

Continuation of the front page (56) References JP-A-4-342496 (JP, A) JP-A-61-36113 (JP, A) JP-A-5-270814 (JP, A) JP-A 8-73297 (JP) , A) JP-A-4-240192 (JP, A) WO 93/12272 (WO, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) C30B 1/00-35/00 C01B 33 / 00 H01L 31/04

Claims (2)

(57) [Claims] 1. A molten silicon obtained by dephosphorization, deboronation, decarburization and deoxidation from metallic silicon is cast and unidirectionally solidified to produce an ingot for a solar cell . Of the solidification rate of molten silicon
Set the minimum value as the target and solidify the molten silicon
Measure the speed so that the measured value is less than the minimum value
And a method for casting silicon for solar cells, characterized by solidifying molten silicon while cooling or heating . When the metal impurity element is Al: R ₁ = 7.0 × (Al initial concentration) ^{-0.21 When the} metal impurity element is Fe: R ₂ = 6.0 × (Fe initial concentration) ^-0.18 When the metal impurity element is Ti: R ₃ = 6.6 × (Ti initial concentration) ^−0.18 2. Dephosphorization, deboronation, and decarburization of metal silicon.
Casting of molten silicon obtained by using directional solidification for solar cells
When manufacturing an ingot, the ingot was determined in advance using the following three equations.
Target the minimum ingot cooling rate after solidification is completed
While setting the cooling rate of the ingot,
The ingot is measured so that the measured value is less than the minimum value.
Of silicon for solar cell, characterized by cooling
Method. When the metal impurity element is Al: C ₁ = 42 × (Al initial concentration) ^{-0.57 When the} metal impurity element is Fe: C ₂ = 41 × (Initial Fe concentration) ^{-0.51 When the} metal impurity element is Ti: C ₃ = 42 × (Ti initial concentration) ^-0.43