JP2006083024A - Casting method of polycrystalline silicon ingot, polycrystalline silicon ingot using the same, polycrystalline silicon substrate and solar cell element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a casting method of a polycrystalline silicon ingot by which the loss of a molten metal is reduced when an assemble system casting mold exhibiting high cost merit is used, a polycrystalline silicon ingot attaining both of high quality and low cost, a polycrystalline silicon substrate and a solar cell element using the same. <P>SOLUTION: The casting method of the polycrystalline silicon ingot includes a pouring step for pouring a molten silicon 4 obtained by melting silicon to the casting mold 5 formed by assembling a plurality of members to be partially opened at one part from the opening part of the casting mold 5 and a solidifying step for solidifying the molten silicon 4 while keeping it in the casting mold 5. In the pouring step, gaps 10 among the plurality of the members constituting the casting mold 5 are sealed by an initial solidified layer 11 formed by solidifying the molten silicon 4 being brought into contact with the inner surface of the casting mold 5. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for casting a polycrystalline silicon ingot particularly suitable for casting polycrystalline silicon for solar cells, and a polycrystalline silicon ingot, a polycrystalline silicon substrate, and a solar cell element using the same.

  A solar cell converts incident light energy into electrical energy. Major solar cells are classified into crystalline, amorphous, and compound types depending on the type of materials used. Of these, most of the crystalline silicon solar cells currently on the market are in the market. This crystalline silicon solar cell is further classified into a single crystal type and a polycrystalline type. Single-crystal silicon solar cells have the advantage of high efficiency because the quality of the substrate is good, but have the disadvantage of high substrate manufacturing costs. On the other hand, the polycrystalline silicon solar cell has the advantage that it can be manufactured at a low cost although it has the disadvantage that it is difficult to increase the efficiency because the quality of the substrate is inferior. In recent years, conversion efficiency of about 18% has been achieved at the research level due to the improvement of the quality of the polycrystalline silicon substrate and the advancement of cell technology.

  On the other hand, since mass-produced polycrystalline silicon solar cells are low in cost, they have been distributed in the market. However, in recent years, demands are increasing as environmental problems are addressed.

  A polycrystalline silicon substrate used for a polycrystalline silicon solar cell is generally manufactured by a method called a casting method. This casting method is a method of forming a silicon ingot by cooling and solidifying a silicon melt in a mold made of quartz or the like coated with a release material. The ends of the silicon ingot are removed, cut into a desired size, and the cut out ingot is sliced to a desired thickness to obtain a polycrystalline silicon substrate for forming a solar cell.

  A general casting apparatus for casting silicon or the like disclosed in Patent Document 1 is shown in FIG. In FIG. 2, 1a is a melting crucible, 1b is a holding crucible, 2 is a pouring port, 3 is a heating means, 4 is a silicon melt, and 5 is a mold.

  A melting crucible 1a for melting the silicon raw material is disposed at the upper part of the casting apparatus while being held by the holding crucible 1b, and the molten crucible 1a is inclined at the upper edge of the melting crucible 1a to pour silicon melt. A pouring gate 2 is provided. A heating means 3 is disposed around the melting crucible 1a and the holding crucible 1b, and a mold 5 into which a silicon melt is poured is disposed below the melting crucible 1a and the holding crucible 1b. For example, high-purity quartz is used for the melting crucible 1a in consideration of heat resistance and the fact that impurities do not diffuse into the silicon melt. The holding crucible 1b is used for holding the melting crucible 1a made of quartz or the like because the melting crucible 1a is softened at a high temperature in the vicinity of the silicon melt and cannot keep its shape, and the material is graphite or the like. As the heating means 3, for example, a resistance heating type heater or an induction heating type coil is used.

  A schematic cross-sectional view of the solidified portion in the casting apparatus is shown in FIG. The mold 5 arranged below the melting crucible 1a and the holding crucible 1b is made of quartz, graphite or the like, and is used by applying a release material 6 containing silicon nitride, silicon oxide or the like as a main component on the inside thereof. A mold heat insulating material 7 is installed around the mold 5 in order to suppress heat removal. As the mold heat insulating material, a carbon-based material is generally used in consideration of heat resistance, heat insulating properties, and the like. Further, a cooling plate 8 for cooling and solidifying the poured silicon melt may be provided below the mold 5. These are all arranged in a vacuum vessel (not shown).

  In the casting apparatus shown in FIG. 2, the silicon raw material is put into the melting crucible 1a, the silicon raw material in the melting crucible 1a is melted by the heating means 3, and then all the silicon raw material in the melting crucible 1a is dissolved. Then, the crucible is tilted and the silicon melt is poured from the pouring port 2 at the upper edge of the melting crucible 1a to the mold 5 installed at the lower part.

  After pouring, the silicon in the mold is cooled from the bottom and solidified in one direction, and then slowly cooled while controlling the temperature to a temperature at which it can be taken out of the furnace. Finally, the silicon is taken out of the furnace and casting is completed.

Here, as the mold 5, a mold formed of silicon dioxide (SiO ₂ ) such as quartz or fused silica, graphite, or the like and having a bottom surface portion and a side surface portion integrated is used (for example, a patent). Reference 2). In order to form such an integral mold, a draft (taper) is required on the inner surface of the mold to remove the molded body from the mold in order to form the raw material into a mold shape by casting or press molding. Thus, if the taper angle of about 2 to 5 degrees is not provided on the inner surface, the cast block cannot be smoothly removed. As a result, the silicon ingot that solidifies in the mold has a shape that inherits the taper shape of the inner surface of the mold as it is. Will increase.

In order to avoid such problems, for example, in Patent Document 3, as shown in FIG. 4, plate-like bottom member 21a and side member 21b are produced using high-purity graphite and the like, and these are assembled to form a mold 21. The contents that form are described. After casting, the mold is disassembled and the block is taken out, so that it is not necessary to form an extra taper area in the block, and the mold material can be reused, reducing the running cost of the mold. It becomes possible.
Japanese Patent Laid-Open No. 11-180711 Japanese Patent Laid-Open No. 10-190025 JP 62-108515 A

  In general, in order to improve the quality of polycrystalline silicon substrates for solar cells, it is ideal to solidify completely in one direction in order to prevent structural defects such as thermal stress induced dislocations occurring in the mold. Has been. In order to perform unidirectional solidification in this way, a method is used in which the mold side surface is thermally insulated and the upper part is kept at a high temperature and the bottom part is kept at a low temperature.

  At this time, if the temperature of the mold is not raised to the vicinity of the melting point of silicon in advance, the melt is cooled along the low temperature mold inner surface (side surface and bottom surface) at the same time as pouring the silicon melt into the mold. An initial solidified layer is rapidly formed, and unidirectional solidification is inhibited. However, when the assembly type mold 21 shown in Patent Document 3 is adopted as the mold, the formation of the initial solidified layer along the inner surface of the mold can be prevented when the mold temperature is raised to near the melting point of silicon. There is a problem in that the melt flows out from the gap between the members constituting the mold, and a lot of the melt is lost, and further, the parts around the mold are damaged.

  The present invention provides a casting method for a polycrystalline silicon ingot that reduces the loss of melt when a cost-effective assembly type is used as a mold, and achieves both high quality and low cost. An object is to provide a polycrystalline silicon ingot, a polycrystalline silicon substrate, and a solar cell element using the same.

  A casting method for a polycrystalline silicon ingot according to the present invention includes a pouring step of pouring a silicon melt obtained by melting silicon into an open mold part of a mold formed by combining a plurality of members and partially opened. And a solidification step of solidifying the silicon melt while holding the inside of the mold, wherein in the pouring step, the silicon melt is in contact with the inner surface of the mold. Since the gap between the plurality of members constituting the mold is sealed by the initial solidified layer formed by solidification when contacted, leakage of silicon melt can be reduced with a simple apparatus configuration. Can do.

  And it is desirable to make the thickness of the said initial solidification layer formed in the said pouring process into 0.5 mm or more and 5 mm or less. By setting it as this range, the leakage of the silicon melt from the gap between the members constituting the mold can be more reliably reduced, and the unidirectional solidification of the formed polycrystalline silicon ingot can be kept good.

  In the pouring step, the initial solidified layer is preferably formed at a speed of 0.5 mm / min to 5 mm / min. In this range, the gap between the members constituting the mold can be reliably sealed by the initial solidified layer before the silicon melt leaks, and the leakage of the silicon melt is further reduced, and the formed polycrystalline silicon The adverse effect on the quality of the ingot can be reduced.

Furthermore, it is desirable that the pouring step is performed in a range where the maximum temperature of the inner surface of the mold is 900 ° C. or higher and 1000 ° C. or lower. By setting the temperature of the inner surface of the mold within this range, the formation rate of the initial solidified layer can be kept appropriate. Further, at this time, it is preferable that the pouring step is performed in a range where the temperature of the silicon melt is 1490 ° C. or more and less than 1620 ° C. In this way, by keeping the melt temperature of the silicon melt higher in the range of 70 ° C. to 200 ° C. than the melting point of the silicon melt (about 1420 ° C.), the silicon melt taken by the mold through the initial solidified layer. The polycrystalline silicon ingot of the present invention can maintain a proper heat balance, suppress the solidification rate after the initial solidified layer is formed, and allow the unidirectional solidification to proceed well inside the mold. The polycrystalline silicon ingot was produced by using this casting method, so that the leakage of the silicon melt was small, so that the cost was low and the unidirectional solidification was excellent. Moreover, the polycrystalline silicon substrate of the present invention is obtained by slicing a portion obtained by removing the initial solidified layer from the polycrystalline silicon ingot of the present invention, and has a high quality. Furthermore, since the solar cell element of the present invention is formed using the polycrystalline silicon substrate of the present invention, it has excellent electrical characteristics.

  In the present invention, the main part of the polycrystalline silicon ingot (the region where the polycrystalline silicon substrate of the present invention can be cut out) and the region of the initial solidified layer can be distinguished by the crystal grain size. Specifically, when the polycrystalline silicon ingot taken out from the mold is cut in a cross section with respect to the growth direction, the region of the main part of the polycrystalline silicon ingot has a crystal grain size of less than 1 mm, In the region of the initial solidified layer, the crystal grain size is 1 mm or more, so that both can be distinguished.

  As described above, the method for casting a polycrystalline silicon ingot according to the present invention includes a silicon melt obtained by melting silicon from an open portion of a mold into a mold in which a plurality of members are combined and partially opened. And a solidification step of solidifying the silicon melt while holding the silicon melt inside the mold, and in the pouring step, the silicon melt As a result of sealing the gaps between a plurality of members constituting the mold by an initial solidified layer formed by solidification when the inner surface of the mold contacts with the inner surface of the mold, Therefore, it is possible to prevent the apparatus from being damaged and to prevent damage to the apparatus due to leakage of the high-temperature silicon melt.

  In addition, since the polycrystalline silicon ingot of the present invention was produced by using the casting method of the polycrystalline silicon ingot of the present invention, the silicon melt was less leaked, the cost was low, and the high-quality excellent in unidirectional solidification. It will be a thing. In addition, since the polycrystalline silicon substrate of the present invention obtained by cutting the polycrystalline silicon ingot of the present invention is of low cost and high quality, the solar of the present invention formed using this polycrystalline silicon substrate. The battery element has good electrical characteristics.

  Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

  FIG. 2 is a longitudinal sectional view of a casting apparatus used in the method for casting a polycrystalline silicon ingot according to the present invention. The example shown here is an example, and the casting apparatus used in the present invention is not limited to this example. In the figure, 1a is a melting crucible, 1b is a holding crucible, 2 pouring ports, 3 is a heating means, 4 is a silicon melt, and 5 is a mold.

  The melting crucible 1a holds the charged silicon raw material inside, heats and melts it, and pours the silicon melt 4 into the mold 5. The polycrystalline silicon ingot obtained by cooling and solidifying the silicon melt 4 melted in the melting crucible 1a and poured into the mold 5 is used as, for example, a polycrystalline silicon substrate material for solar cells.

  The melting crucible 1a is usually made of high-purity quartz or the like, but at a temperature higher than the melting temperature of the silicon raw material, it is difficult to cause melting, evaporation, softening, deformation, decomposition, etc. It is not particularly limited as long as it is used as a battery element substrate. Further, since the melting crucible 1a is softened at a high temperature and cannot keep its shape, it is held by a holding crucible 1b made of graphite or the like. Further, the dimensions of the melting crucible 1a and the holding crucible 1b are set to dimensions that can enclose silicon raw materials according to the amount of dissolution that is dissolved at a time. The amount of silicon raw material dissolved is in the range of approximately 1 kg to 150 kg.

  A heating means 3 is disposed around the melting crucible 1a and the holding crucible 1b. By these heating means 3, the silicon raw material inside the melting crucible 1 a is heated and melted to obtain a silicon melt 4. As these heating means, for example, a resistance heating type heater or an induction heating type coil can be used.

  A pouring port 2 for pouring a silicon melt is provided at the upper edge of the melting crucible 1a, and after melting the silicon raw material and completely forming the melt, the crucible is tilted and the upper edge of the melting crucible 1a. The silicon melt is poured from the pouring port 2 at the bottom to the mold 5 installed at the bottom. Further, the shape of the main body of the melting crucible 1a is not particularly limited to the figure.

  A schematic cross-sectional view of the solidified portion in the casting apparatus is shown in FIG. The mold 5 disposed at the lower part of the melting crucible 1a and the holding crucible 1b has an open part opened upward, and receives the molten silicon melt 4 by the open part, and in the inside thereof, the silicon While holding the melt, it has a role of solidifying in one direction from below to above. The mold 5 is made of, for example, a carbon material such as graphite, and includes a single bottom member 5a constituting the bottom and a plurality of (for example, four) side members 5b constituting the side, and can be divided and assembled. It is configured as an assembling mold 5. These side members 5b may be erected so as to surround the outer periphery of the bottom member 5a as shown in FIG. 3, or may be placed on the bottom member as shown in FIG. Moreover, in order to couple | bond each member more firmly, the fitting structure fitted to the side part of the side member 5b corresponding to the side part of the adjacent side member 5b or the outer peripheral part of the bottom face member 5a is provided. It is good to provide and to mutually fit the uneven | corrugated | grooved part.

Further, it is desirable to provide a release material 6 on the inner surface portion of the mold 5. The release material 6 is made of, for example, each powder such as silicon nitride (Si ₃ N ₄ ), silicon oxide (SiO ₂ ), silicon carbide (SiC), or mixed powder, an organic binder such as PVA (polyvinyl alcohol), and a solvent. Can be formed by mixing in a solution composed of It is preferable to mix each powder or mixed powder with an aqueous solution to form a slurry because it can be easily applied with a spatula, a brush, a dispenser, or the like. Alternatively, the mold 5 may be formed by assembling materials previously applied by screen printing or the like on the inner surface side of each member constituting the mold 5. By providing such a release material 6, the inner wall of the mold 5 and the silicon ingot are less likely to be fused after the silicon melt is solidified, and each member constituting the assembly-type mold 5 can be used repeatedly. It becomes like this.

  A mold heat insulating material 7 is installed around the mold 5 in order to suppress heat removal from the mold side surface. The mold heat insulating material 7 is generally made of a material such as carbon felt in consideration of heat resistance, heat insulating properties, and the like. In addition, a cooling plate 8 made of a metal plate or the like for extracting and cooling the molten silicon melt from below to cool and solidify may be installed below the mold 5. Further, a mold heating means 9 composed of a carbon heater or the like capable of heating the mold 5 from above may be arranged, whereby the surface of the silicon melt 4 discharged to the mold 5 is heated appropriately and from below to above. The directed temperature gradient can be controlled more accurately.

  These casting apparatuses are preferably placed in a vacuum vessel (not shown) and performed in a reducing atmosphere such as an inert gas from the viewpoint of preventing contamination and oxidation of impurities.

  Next, a method for carrying out the method for casting a polycrystalline silicon ingot of the present invention using the above-described casting apparatus shown in FIG. 2 will be described. In addition, the figure for demonstrating the pouring process which concerns on FIG. 1 at this invention is shown, and it demonstrates collectively. Fig.1 (a) is typical sectional drawing which shows a mode that a silicon melt is poured into the inside of a casting_mold | template, FIG.1 (b) is the elements on larger scale of the A section of Fig.1 (a).

  The pouring process according to the method for casting a polycrystalline silicon ingot of the present invention comprises a combination of a plurality of members, and melts silicon from an open portion of an assembled mold 5 that is partially open. The silicon melt 4 is poured and is performed in the following procedure.

[Pouring process]
(1) A predetermined amount of silicon raw material is charged into the melting crucible 1a.

(2) The silicon raw material inside the melting crucible 1 a is heated and melted by the heating means 3 to obtain a silicon melt 4.

(3) After all the silicon raw material in the melting crucible 1a is melted, the crucible is tilted at a predetermined timing, and the silicon melt 4 is poured into the mold 5 from the pouring port provided at the upper edge of the melting crucible 1a. The The predetermined timing refers to, for example, when the temperature of the inner surface of the mold 5 or the temperature of the silicon melt 4 in the melting crucible 1a is in an appropriate range. The optimum range of these temperatures will be described later.

(4) When the poured silicon melt 4 comes into contact with the inner surface of the mold 5 having a temperature lower than the melting point of silicon, the heat is taken away and solidifies, and the initial melt along the inner surface of the mold 5 A solidified layer 11 is formed. The initial solidified layer 11 seals the gap 10 between a plurality of members (the bottom surface member 5a and the side surface member 5b) constituting the mold 5 (see FIG. 1B).

  And the solidification process which concerns on the casting method of the polycrystalline silicon ingot of this invention is performed after the above-mentioned pouring process, and solidifies it, hold | maintaining the silicon melt 4 poured into the casting_mold | template 5 inside, The following The procedure is as follows.

[Coagulation process]
(5) The silicon melt 4 is solidified in one direction while being held in the mold 5 to form a polycrystalline silicon ingot. At this time, it is performed while applying a predetermined temperature gradient from below to above the mold 5 by the cooling plate 8 disposed below the mold 5 or the mold heating means 9 for heating the mold 5 from above. .

  As described above, the method for casting a polycrystalline silicon ingot of the present invention can be carried out. As described above, in the pouring step, the gap between the assembling mold 5 is sealed by the initial solidified layer 11 formed when the poured silicon melt comes into contact with the mold 5. Without requiring a simple mechanism, leakage of the silicon melt poured into the mold 5 can be reduced with a simple apparatus configuration. Since leakage of high-temperature silicon melt can be reduced, there is little adverse effect on the devices and parts around the mold 5.

  Next, a further preferred embodiment of the method for casting a polycrystalline silicon ingot according to the present invention will be described.

  The inventor has found that the quality of the formed polycrystalline silicon ingot is greatly influenced by the initial solidified layer 11 serving as a base, and as a result of the examination, an initial stage in which a polycrystalline silicon ingot having a good quality can be obtained. The formation conditions of the solidified layer were found. Specifically, it is desirable to form the initial solidified layer 11 under the following conditions.

  First, it is desirable to form the initial solidified layer 11 at a speed of 0.5 mm / min to 5 mm / min. If it is smaller than this range, the silicon melt 4 poured into the mold 5 may leak out before the gap 10 between the mold members is sealed. On the other hand, if it is larger than this range, the quality of the initial solidified layer 11 is deteriorated, which may adversely affect the quality of the polycrystalline silicon ingot formed as a base. Note that this upper limit value substantially coincides with the maximum growth rate of a single crystal silicon ingot by the CZ method used as a general semiconductor silicon substrate. The formation speed of the initial solidified layer 11 can be adjusted by the temperature of the inner surface of the mold 5. When the temperature of the inner surface of the mold 5 is low, the formation speed of the initial solidified layer 11 increases. Conversely, when the temperature of the inner surface of the mold 5 is high, the formation speed of the initial solidified layer 11 tends to decrease. The temperature of the inner surface of the mold 5 can be adjusted by, for example, appropriately heating the mold 5 in advance by the mold heating means 9.

  Further, it is desirable that the thickness of the initial solidified layer 11 formed in the pouring step is 0.5 mm or more and 5 mm or less. If it is smaller than this range, the silicon melt 4 poured into the mold 5 may leak from the gap 10 between the mold members. On the contrary, if it is larger than this range, the quality of the initial solidified layer 11 is deteriorated, which may adversely affect the quality of the polycrystalline silicon ingot formed as a base. The thickness of the initial solidified layer 11 is closely related to the formation rate of the above-mentioned initial solidified layer 11 and the temperature of the molten silicon melt. Specifically, the initial solidified layer 11 is thick when the formation speed is high, and thin when it is small. Moreover, when the temperature of the molten silicon melt is high, it tends to be thin, and when it is low, it tends to be thick.

  Furthermore, it is desirable that the pouring step is performed in a range where the maximum temperature of the inner surface of the mold 5 is 900 ° C. or higher and 1000 ° C. or lower. By setting the temperature of the inner surface of the mold 5 within this range, the formation speed of the initial solidified layer 11 can be kept appropriate. Further, at this time, it is desirable that the pouring step is performed in a range where the temperature of the silicon melt 4 is 1490 ° C. or more and less than 1620 ° C. In this way, by keeping the melt temperature of the silicon melt 4 higher in the range of 70 ° C. to 200 ° C. than the melting point of the silicon melt 4 (about 1420 ° C.), the mold 5 is taken through the initial solidified layer 11. Therefore, the heat balance of the silicon melt 4 can be kept appropriate. When the temperature of the silicon melt 4 is lower than 1490 ° C., the degree of supercooling of the silicon melt 4 is increased, the formation speed of the initial solidified layer 11 is increased and the thickness becomes too large. The quality of the crystalline silicon ingot may be adversely affected. Conversely, when the temperature of the silicon melt 4 is 1620 ° C. or higher, the formation speed of the initial solidified layer 11 becomes too low, and the effect of preventing the silicon melt from leaking from the gap 10 between the members constituting the mold 5 is reduced. there's a possibility that. When the temperature of the silicon melt 4 to be poured is within this range, the initial solidified layer 11 is appropriately formed, so that a high-quality polycrystalline silicon ingot can be solidified in one direction in the solidification process.

  As described above, in the method for casting a polycrystalline silicon ingot according to the present invention, when an attempt is made to obtain a high quality product by improving the unidirectional solidification of the polycrystalline silicon ingot, the inner surface temperature of the mold 5 and the mold 5 are injected. It is desirable to use a casting apparatus having a function of adjusting the temperature of the silicon melt 4 to be heated. For example, the casting apparatus described in FIG. 2 meets this purpose. In this casting apparatus, the inner surface temperature of the mold 5 can be adjusted by the mold heating means 9, and the temperature of the silicon melt 4 poured into the mold 5 can be adjusted by the heating means 3.

  If a polycrystalline silicon ingot is produced by using the method for casting a polycrystalline silicon ingot according to the present invention as described above, when a separately assembled mold having cost merit is used, the gap between members constituting the mold 5 is reduced. Since the silicon melt can be prevented from leaking out, the cost can be further reduced. In addition, by optimizing the thickness and forming speed of the initial solidified layer 11, the unidirectional solidification of the polycrystalline silicon ingot formed using the initial solidified layer 11 as a base can be improved. Silicon ingots are of high quality.

  Further, in the polycrystalline silicon substrate of the present invention, a portion obtained by removing the region of the initial solidified layer 11 from the polycrystalline silicon ingot of the present invention (hereinafter, referred to as a main portion of the polycrystalline silicon ingot of the present invention) with respect to the solidification direction. And sliced in a substantially orthogonal direction. Since the main part of the polycrystalline silicon ingot of the present invention is excellent in unidirectional solidification, most of the crystal grains of the polycrystalline silicon substrate of the present invention obtained by slicing it grow vertically from the bottom. The columnar crystal has a shape in which the columnar crystal is cut in a direction normal to the crystal growth direction, and has uniform electrical characteristics in the plane of the substrate.

  Further, the polycrystalline silicon substrate of the present invention has a shape in which columnar crystals in which most crystal grains are grown perpendicularly from the bottom as described above are rounded in a direction normal to the crystal growth direction. Since the thermal stress induced dislocation entering the silicon solidified layer is greatly suppressed, the solar cell element of the present invention formed using this substrate was generated inside the substrate when an electric field was formed in the thickness direction of the substrate. The traveling direction of the carrier and the crystal grain boundary are positioned substantially in parallel. As a result, the solar cell element of the present invention is excellent in the effect of preventing carrier recombination, and thus has high characteristics as a solar cell and can obtain uniform quality characteristics.

  It should be noted that the embodiment of the present invention is not limited to the above-described example, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

  For example, when a silicon melt is poured from a melting crucible into a mold, the molten metal may be poured by a method other than the above method. For example, a pouring port may be provided at the bottom of the melting crucible, and the silicon melt may be poured from the bottom into a mold installed at the bottom. In this case, in order to hold the silicon melt in the melting crucible so that the silicon raw material before melting and the partially melted silicon melt do not leak from the pouring port before the silicon raw material is completely dissolved, It is possible to adjust the temperature of the silicon melt 4 to be poured by providing a pouring control means that can control pouring, such as installing a silicon raw material that closes a mechanical stopper or pouring gate. It becomes possible.

  Hereinafter, the Example of this invention implemented using the casting apparatus shown in FIG. 2 demonstrated above is described.

  As the mold 5, an assembling mold made of graphite opened upward was used. Then, the melting crucible 1a made of quartz was held by a holding crucible 1b made of graphite, and 100 kg of silicon raw material was put into the melting crucible 1a. The heating means 3 was provided around the melting crucible 1a, and the silicon raw material in the melting crucible 1a was dissolved by the heating means.

  Prior to pouring, the inner surface temperature of the mold 5 was maintained at a predetermined temperature. After the temperature of the silicon melt 4 in the melting crucible 1a was raised and the melt temperature was maintained at a predetermined temperature, the silicon melt 4 was poured into the mold 5 disposed below the melting crucible 1a. The inner surface temperature of the mold 5 and the temperature of the silicon melt 4 were both monitored by measuring the surface temperature with an infrared radiation thermometer.

  Thereafter, the silicon melt 4 is applied while applying a predetermined temperature gradient from below to above the mold 5 by the cooling plate 8 disposed below the mold 5 and the mold heating means 9 for heating the mold 5 from above. Was solidified in one direction while being held inside the mold 5. The polycrystalline silicon ingot obtained after the end of casting was cut in the center height direction, the thickness of the initial solidified layer 11 was determined from the crystal grain size, and the formation rate of the initial solidified layer 11 was determined.

  The inner surface temperature of the mold 5 was set to two conditions of 900 ° C. and 1000 ° C., and the molten silicon melt temperature was set to four conditions of 1427 ° C., 1477 ° C., 1527 ° C., and 1577 ° C.

  FIG. 5 shows the relationship between the temperature of the silicon melt when the inner surface temperature of the mold is 1000 ° C. and the thickness of the initial solidified layer. The horizontal axis represents the silicon melt temperature (° C.), and the vertical axis (left) represents the thickness (mm) of the initial solidified layer. In addition, the unidirectional solidification degree which evaluated numerically the degree of unidirectional solidification on the vertical axis | shaft (right) of FIG. 5 was also plotted. As shown in the lower diagram of FIG. 5, the degree of unidirectional solidification is determined from the crystal shape of the longitudinal section of the polycrystalline silicon ingot, the thickness a of the initial solidified layer, the thickness b of the unidirectional solidified region, and the initial solidification from the entire longitudinal section. When the thickness c of the portion excluding the layer (unidirectional solidification region + transition region) is defined as b / c × 100, it can be said that the larger the unidirectional solidification degree, the better the unidirectional solidification property. .

  As indicated by the hatched portion in FIG. 5, when the thickness of the initial solidified layer was 5 mm or less, the unidirectional solidification degree was 97% or more and good unidirectional solidification was obtained. In addition, in order to make the thickness of an initial stage solidified layer into 5 mm or less, it turns out that what is necessary is just to make silicon melt temperature 1477 degreeC or more.

  Next, FIG. 6 shows the relationship between the solidification rate (formation rate) of the initial solidified layer and the silicon melt temperature just before the hot water. The horizontal axis represents the silicon melt temperature (° C.), and the vertical axis represents the solidification rate (mm / min) of the initial solidified layer. The hatched portion in FIG. 6 is a portion in which good unidirectional solidification with a unidirectional solidification degree of 97% or more defined in FIG. 5 was obtained at both the inner surface temperature of the mold of 900 ° C. and 1000 ° C. Thus, good unidirectional solidification is obtained when the formation rate of the initial solidified layer is 5 mm / min. Therefore, it was found that when the surface temperature in the mold immediately before pouring is maintained at 900 ° C. to 1000 ° C., the silicon melt temperature should be 1490 ° C. or higher, which is 70 ° C. higher than the melting point 1420 ° C. of silicon.

  Although not plotted in FIG. 6, when the silicon melt temperature is higher than 200 ° C. (1620 ° C. or higher), the formation rate of the initial solidified layer is close to 0 mm / min, and the members constituting the mold 5 The silicon melt leaked from the gap, and the object of the present invention could not be achieved.

  A region of the initial solidified layer was removed from the polycrystalline silicon ingot produced by the method described in Example 1 and sliced to a thickness of 250 μm to obtain a polycrystalline silicon substrate. When this polycrystalline silicon substrate was observed, it showed a shape in which almost all of the crystal grains grew vertically from the bottom in a circular direction in the direction normal to the crystal growth direction, and solidified well in one direction. It was.

  Next, two types prepared from the above-described polycrystalline silicon substrate according to the present invention under the following conditions were selected.

Sample 1 ... Mold surface temperature 900 ° C, silicon melt temperature 1490 ° C
Sample 2 ... mold surface temperature 900 ° C, silicon melt temperature 1577 ° C
Using these substrates, general bulk-type solar cell elements were produced, and the conversion efficiency of the solar cell elements was characterized. For comparison, a substrate manufactured by a conventional method was similarly evaluated as a reference sample.

  As a result, the conversion efficiency of sample 1 was 15.7% and the conversion efficiency of sample 2 was 16.1%, but the reference sample was 15.4%.

  As described above, the solar cell element formed using the polycrystalline silicon substrates of Samples 1 and 2 has better characteristics than those of the reference sample. This is because the unidirectional solidification property of the portion of the silicon ingot formed by using the silicon casting method of the present invention that is substantially used as a substrate for a solar cell is remarkably improved, so that the polycrystalline material is solidified during solidification and cooling. It is presumed that thermal stress-induced dislocations contained in the silicon ingot were greatly suppressed.

  As described above, the effects of the present invention could be confirmed by the examples.

It is a figure for demonstrating the pouring process which concerns on this invention, a) is typical sectional drawing which shows a mode that a silicon melt is poured into the inside of a casting_mold | template, (b) is A part of (a). FIG. It is a schematic sectional structure figure of a general casting apparatus. It is a schematic cross-section figure of the solidification part in a general casting apparatus. It is a schematic perspective view which shows an example of embodiment of an assembly-type casting_mold | template. It is a figure which shows the relationship between the silicon melt temperature just before pouring, the thickness of an initial stage solidified layer, and a unidirectional solidification degree. It is a figure which shows the relationship between the inner surface temperature and melt temperature of a casting_mold | template just before pouring, and an initial stage solidified layer formation rate (solidification rate).

Explanation of symbols

1a: Melting crucible 1b: Holding crucible 2: Pouring port 3: Heating means 4: Silicon melt 5: Mold 5a: Bottom member 5b: Side member 6: Mold release material 7: Mold heat insulating material 8: Cooling plate 9: Mold heating means 10: gap 11: initial solidified layer 21: mold 21a: bottom member 21b: side member

Claims (8)

A pouring process of pouring a silicon melt in which silicon is melted into a mold in which a plurality of members are combined and partially opened, and the silicon melt is held inside the mold. A solidification step of solidifying while solidifying, wherein the initial solidification formed by solidification when the silicon melt comes into contact with the inner surface of the mold in the pouring step A method for casting a polycrystalline silicon ingot, wherein a gap between a plurality of members constituting the mold is sealed with a layer. The method for casting a polycrystalline silicon ingot according to claim 1, wherein the initial solidified layer formed in the pouring step has a thickness of 0.5 mm or more and 5 mm or less. The method for casting a polycrystalline silicon ingot according to claim 1 or 2, wherein, in the pouring step, the initial solidified layer is formed at a speed of 0.5 mm / min or more and 5 mm / min or less. The polycrystal according to any one of claims 1 to 3, wherein the pouring step is performed in a range where the maximum temperature of the inner surface of the mold is 900 ° C or higher and 1000 ° C or lower. Silicon ingot casting method. 5. The method for casting a polycrystalline silicon ingot according to claim 4, wherein the pouring step is performed in a temperature range of 1490 ° C. or more and less than 1620 ° C. of the silicon melt. A polycrystalline silicon ingot produced using the method for casting a polycrystalline silicon ingot according to any one of claims 1 to 5. A polycrystalline silicon substrate obtained by slicing a portion obtained by removing the initial solidified layer from the polycrystalline silicon ingot according to claim 6. A solar cell element formed using the polycrystalline silicon substrate according to claim 7.

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