JP2013220951A - Polycrystalline silicon ingot, and method of manufacturing the same, and use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing polycrystalline silicon ingot that is easy and inexpensive, and can reduce the impurity density of an ingot upper portion with good controllability, a polycrystalline silicon ingot obtained by using the method, and uses thereof.SOLUTION: When a polycrystalline silicon ingot is manufactured by solidifying molten silicon in a crucible upward in one direction from the bottom portion of the crucible, the molten silicon is solidified in the one direction while being discharged from the crucible by a rise in the liquid level of the molten silicon due to solidification expansion of the silicon.

Description

  The present invention relates to a polycrystalline silicon ingot, a method for producing the same, and use thereof.

The use of natural energy is attracting attention as an alternative to oil, which is causing various problems in the global environment. Among them, the solar cell does not require a large facility and does not generate noise during operation, and thus has been particularly actively introduced in Japan and Europe.
Solar cells using compound semiconductors such as cadmium tellurium have also been put into practical use, but solar cells using crystalline silicon substrates are large in terms of the safety of the materials themselves, past achievements, and cost performance. Solar cells using a polycrystalline silicon substrate (polycrystalline silicon solar cells) occupy a large share.

A polycrystalline silicon wafer (hereinafter also referred to as “wafer”), which is generally widely used as a substrate for polycrystalline silicon solar cells (hereinafter also referred to as “solar cells”), is obtained by unidirectionally solidifying molten silicon in a crucible. An ingot manufactured by a method called a casting method for obtaining a large polycrystalline silicon ingot (hereinafter also referred to as “ingot”) is cut into polycrystalline silicon blocks (hereinafter also referred to as “blocks”) and formed into wafers by slicing.
The wafer manufactured by the casting method generally has a distribution in the output characteristics of the solar cell as shown in FIG. 4 depending on the position in the height direction in the ingot or block.

The cause of the characteristic distribution of FIG. 4 is generally explained as follows.
First, in the region I at the initial stage of unidirectional solidification, the characteristics deteriorate due to the influence of impurities diffused from the crucible. In the region II on the upper side, since the incorporation of impurities in the raw material due to segregation and the occurrence of crystal defects are few, the characteristics are the best in the block. Further, in the upper region III, the amount of impurities taken into the crystal gradually increases, the generation of crystal defects increases, and the characteristics are deteriorated as compared with the region II. Furthermore, in the upper region IV, similarly to the region III, in addition to further increasing the amount of impurities incorporated into the crystal and the generation of crystal defects, the upper surface portion was formed after the ingot solidified to the end. Impurity reverse diffusion occurs from the high concentration portion of the impurity, and the amount of the impurity further increases, so that the characteristic deterioration becomes more remarkable than in the region III.
In the above description, the influence of impurities in the raw material and impurities eluted from the crucible is considered, but even if there is no such influence, in regions III and IV, crystals that become minority carrier traps toward the top Since defects increase gradually, the characteristics of solar cells tend to deteriorate.

Usually, since the foreign substance such as silicon carbide (SiC) or silicon nitride (SiN) is contained in the uppermost and lowermost regions of the ingot, the lower region of the ingot on the region I side and the upper portion of the ingot on the region IV side. The region is cut when the ingot is processed into a block shape and is not used as a solar cell wafer.
However, the characteristics of solar cells manufactured using wafers processed from the portion of region IV remaining on the block side tend to be extremely low compared to solar cells manufactured from wafers in regions II and III. There is.
As a market trend in this field in recent years, there is a high demand for high-power solar cells, and low-power solar cells manufactured from wafers in the above-mentioned region IV may become defective in extreme cases. This is one of the factors that increase the cost of solar cells.
Then, the following two methods are proposed as a method of reducing such a low output solar cell.

  The first method is to increase the cutting amount on the top side of the ingot when processing from the ingot to the block, so that the whole area IV or a part of the top part of the ingot is not used as a wafer, or the block is used as usual. This is a method of slicing and not using the wafer on the uppermost side of the ingot for solar cells.

The second method is a method for removing unsolidified molten silicon before completion of silicon solidification, that is, molten silicon containing a high concentration of impurities, when manufacturing an ingot.
Specifically, as disclosed in, for example, Japanese Patent Laid-Open No. 11-240710 (Patent Document 1), a sealing member is fitted into the upper end of the side wall of an ingot casting mold (hereinafter also referred to as “crucible”). A through hole provided with a stopper is provided in a part of the notch or the upper side wall of the mold, and before the completion of the solidification of the silicon, the sealing member or the plug is removed and the unsolidified molten silicon is discharged out of the mold.
By this method, unsolidified molten silicon enriched by segregation can be discharged out of the mold, and although the height of the resulting ingot is reduced, a high concentration impurity region is formed on the top surface of the ingot. It is considered that the back diffusion of impurities from here can be suppressed, the impurity concentration in the region IV can be reduced, and the output decrease of the solar cell manufactured from the wafer in the region IV can be reduced.

JP-A-11-240710

  The first method can obviously reduce the number of low-power solar cells, but reduces the number of usable wafers processed from the same size ingot or block. There is a problem that the cost becomes high.

  The second method requires a movable part for removing a partially-notched sealing member and a through-hole plug in a high-temperature ingot manufacturing apparatus, which increases the cost of the ingot manufacturing apparatus. Further, if the timing of discharging unsolidified molten silicon out of the mold is shifted, the amount of molten silicon discharged varies. If the timing is delayed, molten silicon is not discharged at all, and the desired effect may not be obtained. On the other hand, if the timing is too early, the molten silicon will be discharged out of the mold before the impurity concentration due to segregation occurs. A region corresponding to the region IV is formed.

  In order to control the discharge of molten silicon from the mold with good timing, it is necessary to detect a solidified (solidified) portion with high accuracy. For this purpose, an additional temperature measuring device such as a thermocouple or a radiation thermometer, or a solid-liquid interface that detects the position (height) at which the rod inserted into the molten silicon from the upper part of the mold contacts the solidified part, for example. The installation of a detection mechanism or the like is required, which increases the cost of the ingot manufacturing apparatus, that is, increases the cost per wafer.

  Therefore, the present invention provides a method for producing a polycrystalline silicon ingot that is simple, inexpensive, and capable of reducing the impurity concentration at the top of the ingot with good controllability, a polycrystalline silicon ingot obtained by the method, and uses thereof. Let it be an issue.

  As a result of extensive research, the present inventor has actively removed the molten silicon containing impurities at a high concentration by actively utilizing the liquid level rise due to solidification expansion of silicon in the production of a polycrystalline silicon ingot. The present inventors have found that the above problems can be solved, and have reached the present invention.

  Thus, according to the present invention, when the polycrystalline silicon ingot is produced by unidirectionally solidifying the molten silicon in the crucible from the bottom of the crucible, the liquid level of the molten silicon is increased by the solidification expansion of the silicon. A method for producing a polycrystalline silicon ingot is provided, wherein molten silicon is solidified in one direction while being discharged out of the crucible.

  Moreover, according to the present invention, a polycrystalline silicon ingot produced by the method for producing a polycrystalline silicon ingot, a polycrystalline silicon workpiece obtained by processing the polycrystalline silicon ingot, and the workpiece are used. A manufactured polycrystalline silicon solar cell is provided.

According to the present invention, it is possible to provide a method for producing a polycrystalline silicon ingot that is simple, inexpensive, and capable of reducing the impurity concentration at the top of the ingot with good controllability, a polycrystalline silicon ingot obtained by the method, and use thereof. it can.
That is, according to the present invention, the number of low-output solar cells can be reduced, and a polycrystalline silicon ingot and a polycrystalline silicon workpiece that can produce a polycrystalline silicon solar cell can be easily and well-controlled. A polycrystalline silicon solar cell having good solar cell characteristics can be supplied to the market at a lower cost.

In the method for producing a polycrystalline silicon ingot according to the present invention, when the internal volume of the crucible is X and the volume of molten silicon before the start of solidification is Y, the relationship 1 ≦ X / Y ≦ 1.07 is satisfied. In addition, the above effects are further exhibited.
In addition, the method for producing a polycrystalline silicon ingot according to the present invention is such that when the crucible has a notch at the upper end of the crucible side wall or a through hole in the crucible side wall in order to discharge the molten silicon out of the crucible, In the case of having a rectangular parallelepiped outer shape, and when the notch or the through hole is present at the upper end or upper corner of the crucible wall surface, the molten silicon is heated by the heater provided on the side of the crucible The above effects are further exhibited.

In the present invention, “polycrystalline silicon workpiece” means a polycrystalline silicon block, a polycrystalline silicon wafer, and the like.
In addition, “polycrystalline silicon solar cell” manufactured using a processed polycrystalline silicon product means “polycrystalline silicon solar cell” constituting the smallest unit and “polycrystalline silicon” in which a plurality of them are electrically connected. It means “solar cell module”.

In the present invention, a “silicon refined product” is a polycrystalline silicon ingot (lumps) obtained by unidirectionally solidifying molten silicon and efficiently removing metal impurities having a small segregation coefficient in a metal purification process using a metallurgical method. Yes, including both intermediate and final products of the metallurgical silicon purification process.
That is, the method for producing a polycrystalline silicon ingot according to the present invention can also be applied to a unidirectional solidification process in a silicon refining process by a metallurgical method. Can be used as a low-cost silicon purified product.

It is the schematic which shows the example of the crucible for polycrystalline-silicon ingot manufacture applicable to this invention, (A), (B), and (C). It is a figure which shows the relationship between the silicon raw material loading amount to a crucible, the low output cell generation rate, a cost increase rate, and the loading weight of the silicon raw material required in order to manufacture one high output cell (Test Example 1). It is a schematic sectional drawing which shows an example of the ingot manufacturing apparatus applicable to the manufacturing method of the polycrystalline silicon ingot of this invention. It is a conceptual diagram which shows the relationship between the position of the height direction of a common polycrystalline silicon ingot, and the output of the produced solar cell.

  In the method for producing a polycrystalline silicon ingot according to the present invention, the molten silicon in the crucible is unidirectionally solidified upward from the bottom of the crucible to produce a polycrystalline silicon ingot. By rising, the molten silicon is solidified in one direction while being discharged out of the crucible.

  Since the crystal structure of silicon is a diamond structure with a low filling rate, a volume expansion of about 7% occurs when the molten silicon solidifies. The manufacturing method of the polycrystalline silicon ingot of the present invention makes use of such a property to cause molten silicon having a high impurity concentration present in the upper part of the crucible to be discharged out of the crucible due to solidification expansion of the molten silicon, and the impurity concentration Low polycrystalline silicon ingot.

In carrying out the method for producing a polycrystalline silicon ingot of the present invention, as will be described later, for example, an ordinary polycrystalline silicon ingot producing apparatus capable of unidirectional solidification from below to above as shown in FIG. 3 is used. be able to.
When this molten silicon solidifies, a segregation phenomenon occurs, and only a part of the impurities in the original melt is taken into the solid side, so that solidification proceeds and the impurity concentration on the melt side increases. As described above, when all of the molten silicon in the crucible is solidified, the final solidified portion has the highest impurity concentration, and the impurity in that portion is solid-phase diffused below the polycrystalline silicon ingot while the temperature is high. In particular, iron that adversely affects the characteristics of crystalline silicon solar cells has a wide range of effects because of its rapid diffusion in silicon crystals. In the method for producing a polycrystalline silicon ingot of the present invention, such impurity concentration can be reduced.

In order to discharge the molten silicon out of the crucible, when the internal volume of the crucible is X and the volume of the molten silicon before the start of solidification is Y, the relationship 1 ≦ X / Y ≦ 1.07 is satisfied. preferable.
If the relationship between X and Y is within the above range, the molten silicon is discharged out of the crucible before the completion of solidification due to the solidification expansion of silicon. The smaller Y in the above range, the smaller the amount of molten silicon discharged, and the greater the amount of impurities above the polycrystalline silicon ingot. Conversely, as Y in the above range increases, the amount of molten silicon discharged increases and the raw material utilization rate decreases, but the amount of impurities in the upper portion of the polycrystalline silicon ingot decreases. Therefore, the optimum value of Y may be determined in consideration of cost and effect.
The relationship between X and Y is preferably 1.02 ≦ X / Y ≦ 1.05, and more preferably 1.03 ≦ X / Y ≦ 1.04.

The crucible is not particularly limited as long as the crucible has a dimension and shape that can satisfy the above-described relationship. For example, a crucible as shown in FIG.
(A) is a cuboid-shaped crucible conventionally used widely, (B) is a crucible having a notch at the upper end of the crucible side wall to discharge molten silicon to the outside of the crucible, and (C) is molten silicon. Is a crucible having a through-hole on the side wall of the crucible in order to discharge the gas from the crucible.
Here, the internal volume of the crucible means a volume of a region where liquid can be constantly held in the crucible in a state where the crucible is set in the polycrystalline silicon ingot manufacturing apparatus, It means the internal volume up to the lowest point (the lowest point in the case where the crucible side wall has a notch or a through hole). By setting the volume Y of molten silicon before the start of solidification within the above range, unsolidified (high impurity concentration) molten silicon is automatically discharged out of the crucible at the final solidification stage.
Therefore, as in the above (B) and (C), a crucible having a notch and a through hole on the crucible side wall is preferable.
Solidification of the top surface of the polycrystalline silicon ingot starts from the center, concentrically expands, and the vicinity of the corner solidifies last, so that the crucible has a rectangular parallelepiped outer shape with a square bottom surface, and a notch and a through hole are formed. A crucible present at the upper end or upper corner of the crucible wall is particularly preferred.

(Production method of polycrystalline silicon ingot)
The method for producing a polycrystalline silicon ingot of the present invention will be described below with reference to the drawings, but the present invention is not limited to this embodiment.
The method for producing a polycrystalline silicon ingot of the present invention can also be carried out using a known apparatus as shown in FIG.
FIG. 3 is a schematic cross-sectional view showing an example of an ingot manufacturing apparatus applicable to the method for manufacturing a polycrystalline silicon ingot of the present invention.

This apparatus is generally used for producing a polycrystalline silicon ingot, and has a chamber (sealed container) 7 constituting a resistance heating furnace.
A crucible 1 made of graphite, quartz (SiO ₂ ) or the like is disposed inside the chamber 7 so that the atmosphere inside the chamber 7 can be maintained in a sealed state.
In the chamber 7 in which the crucible 1 is accommodated, a graphite crucible base 3 that supports the crucible 1 is disposed. The crucible base 3 can be moved up and down by a lift drive mechanism 12, and the refrigerant (cooling water) in the cooling tank 11 is circulated therein.
An outer crucible 2 made of graphite or the like is disposed on the upper portion of the crucible base 3, and the crucible 1 is disposed therein. Instead of the outer crucible 2, a cover made of graphite or the like surrounding the crucible 1 may be disposed.

A heater 10 is disposed so as to surround the outer crucible 2, and a heat insulating material 8 is disposed so as to cover these from above.
The heater 10 can be heated from the periphery of the crucible 1 to melt (melt) the raw material silicon 4 in the crucible 1.
If heating by the heater 10, cooling from the lower side of the crucible 1 by the cooling tank 11 and raising and lowering of the crucible 1 by the lifting drive mechanism 12 are possible, a heating mechanism such as a heating element is possible. There are no particular restrictions on the form or arrangement of the above.

Here, the molten silicon is preferably heated by a heater provided on the side surface of the crucible.
In the polycrystalline silicon ingot manufacturing apparatus configured only with the upper heater of the crucible or only with the upper and lower heaters of the crucible, the shape of the solidification interface may be concave upward, and the final solidification part is located close to the center part of the upper surface of the ingot. Therefore, it is not preferable. On the other hand, in a polycrystalline silicon ingot manufacturing apparatus having a heater on the side of the crucible, the shape of the solidification interface is convex upward, the solidification of the top surface of the ingot spreads concentrically from the center, and the vicinity of the corner is preferably solidified at the end. .
For example, as shown in FIG. 3, the bottom surface of the crucible 1 is cooled by a cooling mechanism (including the cooling tank 11) such as a refrigerant circulation provided on the pedestal side of the crucible 1, and the elevating drive mechanism 12 A manufacturing apparatus of a system in which the molten silicon in the crucible 1 is gradually solidified from the vicinity of the bottom of the crucible by being used together with being away from the heating mechanism (heater 10).

  In order to detect the temperature of the bottom surface of the crucible 1, the outer crucible lower thermocouple 6 is disposed near the center of the lower surface of the outer crucible, and this output is input to the control device 9 to control the heating state by the heater 10. In addition to the thermocouple described above, a thermocouple or a radiation thermometer for detecting temperature may be arranged.

  The inside of the chamber 7 can be kept sealed so that oxygen gas or the like does not flow from the outside, and the inside of the chamber 7 is usually evacuated after the silicon raw material is charged and before melting, and then argon gas or the like is used. An inert gas is introduced and maintained in an inert atmosphere.

With the apparatus having such a configuration, basically, the silicon raw material 4 is filled into the crucible 1, degassing (evacuation), gas replacement in the chamber 7 by introducing an inert gas, melting of the silicon raw material 4 by heating, A polycrystalline silicon ingot is manufactured by melting confirmation and holding, temperature control and solidification start by operation of the lifting drive mechanism 12, molten silicon discharge, solidification completion confirmation and annealing, and ingot removal.
The amount of raw material silicon charged into the crucible 1 is preferably determined so that the inner volume X of the crucible and the volume Y of molten silicon before the start of solidification satisfy the aforementioned relationship.

Here, an apparatus in which a solid silicon raw material is loaded into the crucible as the raw material silicon has been shown, but, for example, a system in which molten silicon is prepared in another crucible and poured into the crucible 1 may be used.
In addition, it is more preferable to install something like a tray that receives molten silicon discharged out of the crucible from the viewpoint of suppressing damage to the apparatus.

  In addition, for example, when a solid silicon raw material is loaded in the crucible shown in FIG. 1A, it may be difficult to increase the filling rate depending on the shape of the raw material, and a predetermined amount may not be loaded. In such a case, a jig capable of loading a larger amount of solid silicon raw material is appropriately installed above the crucible opening, or an additional mechanism capable of adding additional solid silicon raw material to the crucible after starting melting of the raw material silicon is installed. It is desirable to do.

(Polycrystalline silicon ingot)
The polycrystalline silicon ingot of the present invention is manufactured by the polycrystalline silicon ingot manufacturing method of the present invention.

(Processed polycrystalline silicon)
The polycrystalline silicon processed product of the present invention can be obtained by processing the polycrystalline silicon ingot of the present invention.
As described above, the polycrystalline silicon workpiece means a polycrystalline silicon block, a polycrystalline silicon wafer, and the like.
The polycrystalline silicon block can be obtained by, for example, cutting the polycrystalline silicon ingot of the present invention into a columnar shape or a prismatic shape having a desired size using a known apparatus such as a band saw.
Moreover, you may grind | polish the surface of a polycrystalline silicon block as needed.
A polycrystalline silicon wafer can be obtained, for example, by slicing the polycrystalline silicon block to a desired thickness using a known apparatus such as a multi-wire saw. At present, a thickness of about 170 to 200 μm is generally used, but the tendency is to reduce the thickness for cost reduction.
Further, if necessary, the surface of the polycrystalline silicon wafer may be polished.

(Polycrystalline silicon solar cell)
The polycrystalline silicon solar cell of the present invention is manufactured using a polycrystalline silicon workpiece.
A polycrystalline silicon solar cell can be manufactured by a well-known solar cell process using the above-mentioned polycrystalline silicon wafer, for example. That is, in the case of a silicon wafer doped with a p-type impurity by a known method using a known material, an n-type impurity is doped to form an n-type layer to form a pn junction, and the surface electrode And a back surface electrode is formed and a polycrystalline silicon solar cell is obtained.
Similarly, in the case of a silicon wafer doped with n-type impurities, a p-type impurity is doped to form a p-type layer to form a pn junction, and a surface electrode and a back electrode are formed to form a polycrystalline silicon solar A battery cell is obtained.

In addition to those using pn junctions between silicon, MIS type solar cells in which a metal is deposited with a thin insulating layer interposed therebetween, for example, an amorphous silicon thin film having a conductivity type opposite to that of a polycrystalline wafer, etc. For example, a heterojunction (pn junction, pin junction, etc.) between a polycrystalline silicon wafer and amorphous silicon is used.
Furthermore, a polycrystalline silicon solar cell module can be obtained by electrically connecting a plurality of them.

  As described above, in this specification, the concept including “solar battery cell” and “solar battery module” is simply referred to as “solar battery”. Therefore, for example, what is described as “polycrystalline silicon solar cell” is meant to include “polycrystalline silicon solar cell” and “polycrystalline silicon solar cell module”.

(Purified silicon)
The method for producing a polycrystalline silicon ingot of the present invention can also be applied to a unidirectional solidification process in a silicon refining process by a metallurgical method, and the polycrystalline silicon ingot obtained in the process can be used as a purified silicon product. It can be used by crushing in advance at the time of use. Specifically, it will be described in detail in Test Example 2.

  The present invention will be specifically described below with reference to test examples, but the present invention is not limited to these test examples.

(Test Example 1) Study on loading amount of silicon raw material On a graphite crucible base 3 (88 cm × 88 cm × thickness 20 cm) in the polycrystalline silicon ingot manufacturing apparatus shown in FIG. 3, a graphite outer crucible 2 ( Inner dimensions: 90 cm × 90 cm × height 46 cm, bottom plate thickness and side wall thickness 2 cm), in which a quartz crucible 1 (inner dimensions: 83 cm × 83 cm × as shown in FIG. 1 (B)) 42 cm, bottom plate thickness and side wall thickness 2.2 cm, and four crucible corners having notches with a height of 12 cm and a width of 2.5 cm from the upper end of the side wall. In addition, a thermocouple 6 for temperature measurement was installed near the center of the lower surface of the outer crucible 2.
The crucible internal volume X of the crucible 1 is calculated as follows.
X = 83 cm × 83 cm × 30 cm = 206,670 cm ³
Although not shown, a graphite member was installed between the crucible 1 and the outer crucible 2 as a tray for the molten silicon discharged outside the crucible.

  Next, after a predetermined amount of the solid silicon raw material 4 whose boron dopant concentration was adjusted so that the specific resistance of the ingot was about 2 Ωcm was loaded into the crucible 1, the inside of the apparatus was evacuated and replaced with argon gas. Thereafter, the silicon raw material is melted by using a heating mechanism (heater 10) arranged beside the crucible as a heating means of the apparatus, and after confirming the melting of all raw materials, the silicon is unidirectionally solidified under a predetermined condition to be 1200 ° C. And annealed for 2 hours at a cooling rate of 100 ° C./hour, and the polycrystalline silicon ingot was taken out of the apparatus.

  As shown in Table 1, the volume Y of molten silicon before the start of solidification was changed within the range of 0.97 ≦ X / Y ≦ 1.10. The range of X / Y in the range of 1.00 to 1.07 is within the scope of the present invention, and from 1.07 to 1.10, molten silicon is not discharged out of the crucible. The position of the conventional example).

The obtained polycrystalline silicon ingot was cut using a band saw.
First, each 10 mm thickness of the bottom and top of the polycrystalline silicon ingot is cut, then processed into 25 blocks (15.6 cm × 15.6 cm × 28 cm), further sliced with a wire saw, About 18,000 crystal silicon wafers (15.6 cm × 15.6 cm × thickness 0.018 cm) were obtained.

  The obtained polycrystalline silicon wafer was put into a normal polycrystalline silicon solar cell process and about 18,000 solar cells (15.6 cm × 15.6 cm × thickness 0.018 cm) per ingot. The output (W) was measured.

Table 1 and FIG. 2 show the results obtained, the amount of silicon raw material loaded into the crucible, the low output cell generation rate, the cost increase rate (wafer cost increase rate), and the silicon required to manufacture one high output cell. The raw material loading weight (loading weight / number of high output cells) is shown.
Regarding the wafer cost increase rate and the load weight / the number of high output cells, the result for the polycrystalline silicon ingot produced under the condition of X / Y = 1.10 is used as a reference, the former is “0”, and the latter is “100”. As calculated.

As is apparent from the results of Table 1 and FIG. 2, it can be seen that the smaller the X / Y value, the lower the cell output rate, although the wafer cost increases.
Due to these trade-offs, when a low-power cell is defective, that is, when only a high-power cell having a higher output than a low-power cell is accepted, the silicon raw material necessary for manufacturing one high-power cell is loaded. The weight has a minimum value in the range of 1.00 ≦ X / Y ≦ 1.07, more preferably in the range of 1.02 ≦ X / Y ≦ 1.05, and among these, 1.03 ≦ X / Y It can be seen that the range of ≦ 1.04 is optimal.

  Note that X / Y = 1.07 is considered to be a boundary value for whether or not molten silicon is discharged out of the crucible in the calculation, but as a result of the experiment, some molten silicon is discharged out of the crucible. It is confirmed that the low power cell generation rate is 2.6% lower than the case of X / Y = 1.10. Possible reasons for this are low solidification expansion accuracy and deformation of the entire crucible.

  In Test Example 1, a crucible having a notch at the corner of the crucible was used, but a crucible having a through hole or a normal crucible can be used as long as it is within the range of 1.00 ≦ X / Y ≦ 1.07. is there. Further, as described above, the manufacturing method of the polycrystalline silicon ingot according to the present invention can reduce the loading weight necessary for manufacturing one high-power cell and can reduce the cost. The cost of the solar cell module thus made can also be reduced.

(Test Example 2) Examination of utilization as one step of silicon refining process by metallurgical method In Test Example 2, when metal silicon containing boron concentration of about 10 ppmw and phosphorus concentration of about 10 ppmw as impurities is purified by metallurgical method. An example of use as an impurity removal step will be given.
In the following, unidirectional solidification after the phosphorus removal step will be given as an example, but the method for producing a polycrystalline silicon ingot of the present invention is not concerned with other phosphorus removal methods, after the boron removal step, after the leaching step, etc. Is applicable.
First, metallic silicon was melted in a graphite crucible and held for 12 hours under conditions of 1700 ° C. and a pressure of 1 Pa or less to remove phosphorus by evaporation. Thereafter, molten silicon is poured into a graphite crucible 1 (inner dimensions: 83 cm × 83 cm × 42 cm, bottom plate thickness and side wall thickness 2 cm) as shown in FIG. did.
The crucible internal volume X of the crucible 1 is calculated as follows.
X = 83 cm × 83 cm × 42 cm = 289,338 cm ³
Although not shown, a graphite member was installed between the crucible 1 and the outer crucible 2 as a tray for the molten silicon discharged outside the crucible.

For the purpose of investigating the use of silicon as a step in the metallurgical purification process, the amount of molten metal (volume of molten silicon) Y is set to 0.97 ≦ X / Y ≦ 1.10. Varyed within range.
In the same manner as in Test Example 1, unidirectional solidification was performed from the bottom of the crucible 1, and the obtained polycrystalline silicon ingot was cooled and taken out from the crucible.
In order to evaluate the average iron concentration in the obtained polycrystalline silicon ingot (silicon refined product), it was split using a tungsten carbide hammer while paying attention to impurity contamination, and after melting again, the molten silicon had an outer diameter of 10 mm. A high-purity quartz tube having an inner diameter of 6 mm was inserted, and molten silicon was sampled.
Impurity (iron and phosphorus) concentrations were measured by ICP emission analysis of the obtained samples.
The obtained results are shown in Table 2 together with the loss rate (% by weight) to the outside of the crucible.

  As is clear from the results in Table 2, in the range of 0.97 ≦ X / Y ≦ 1.07, a significant reduction in iron concentration was confirmed compared to the case of X / Y = 1.10. When X / Y = 0.97, compared to the case of X / Y = 1.00, the yield decreases by 3% or more, but the iron concentration is the same, and X / Y <1. 00 is not preferable, and it is understood that a range of 1.00 ≦ X / Y ≦ 1.07 is preferable. However, as expected, a slight effect was observed for phosphorus with a large segregation coefficient of 0.35, but no significant effect was observed.

Similar to Test Example 1, X / Y = 1.07 is considered to be a boundary value for whether molten silicon is discharged or not from the crucible in the calculation. It is considered that the iron concentration was reduced to 20% of the case of X / Y = 1.10. The reason for this is that the accuracy of the solidification expansion coefficient is low and the entire crucible is deformed.
Of course, when X / Y = 1.10, the discharge loss ratio to the outside of the crucible is 0. However, from the viewpoint of impurity removal, it is necessary to remove the impurity concentration portion in accordance with the required impurity concentration. Yield decreases.
The silicon purified product thus obtained could be used as a raw material for a boron removal process, which is the next process.

DESCRIPTION OF SYMBOLS 1 Crucible 2 Outer crucible 3 Crucible stand 4 Raw material silicon 6 Outer crucible lower thermocouple 7 Chamber 8 Heat insulating material 9 Control device 10 Heater 11 Cooling tank 12 Lifting drive mechanism

Claims (9)

  When producing a polycrystalline silicon ingot by unidirectionally solidifying the molten silicon in the crucible upward from the bottom of the crucible, the molten silicon is discharged out of the crucible due to a rise in the liquid level of the molten silicon due to solidification expansion of silicon. A method for producing a polycrystalline silicon ingot, wherein the solidification is performed in one direction.   2. The method for producing a polycrystalline silicon ingot satisfies a relationship of 1 ≦ X / Y ≦ 1.07, where X is an inner volume of the crucible and Y is a volume of molten silicon before solidification starts. A method for producing a polycrystalline silicon ingot according to 1.   3. The method for producing a polycrystalline silicon ingot according to claim 1, wherein the crucible has a notch in the upper end of the crucible side wall or a through hole in the crucible side wall in order to discharge the molten silicon out of the crucible. .   The method for producing a polycrystalline silicon ingot according to claim 3, wherein the crucible has a rectangular parallelepiped outer shape with a square bottom surface, and the notch or the through hole is present at an upper end or an upper corner of the crucible wall. .   The method for producing a polycrystalline silicon ingot according to any one of claims 1 to 4, wherein the molten silicon is heated by a heater provided on a side surface of the crucible.   The polycrystalline silicon ingot manufactured by the manufacturing method of the polycrystalline silicon ingot as described in any one of Claims 1-5.   The polycrystalline silicon ingot according to claim 6, wherein the polycrystalline silicon ingot is a purified silicon product for producing a polycrystalline silicon ingot.   A processed polycrystalline silicon product obtained by processing the polycrystalline silicon ingot according to claim 6.   A polycrystalline silicon solar cell manufactured using the polycrystalline silicon workpiece according to claim 7.