JP2006151708A - Method for casting polycrystalline silicon, polycrystalline silicon ingot obtained by using the same, polycrystalline silicon substrate, and solar battery element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for casting polycrystalline silicon, by which a polycrystalline silicon ingot having large crystal grain diameters can be easily obtained at a low cost, and to provide a polycrystalline silicon ingot having good characteristics, obtained by using the same, a polycrystalline silicon substrate, and a solar battery element. <P>SOLUTION: The method for casting the polycrystalline silicon comprises uniaxially solidifying a silicon melt held in a casting mold 3 having a bottom part 11 toward an upper part. The inner surface of the bottom part 11 of the casting mold 3 is composed of first areas 8 where the contact angle with the silicon melt at a temperature range of 1,420-1,600°C is <40° and second areas 9 where the contact angle with the silicon melt at the same temperature range is ≥40°. When the surface area of the first areas 8 is defined as A and the surface area of the second areas 9 is defined as B, A and B are set to satisfy following formula: 0.1≤A/(A+B)≤0.75. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for casting polycrystalline silicon in which silicon is melted and solidified, in particular, a method for casting polycrystalline silicon excellent for use in solar cells, a polycrystalline silicon ingot obtained by this method, a polycrystalline silicon substrate, In addition, the present invention relates to a solar cell element.

  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. A single-crystal silicon solar cell has the advantage that it is easy to increase the efficiency because the quality of the substrate is good, but has the disadvantage that the manufacturing cost of the substrate is high. 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 polycrystalline 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.

  Further, the following two types of methods are generally employed as casting methods.

  As a first method, a silicon casting apparatus 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 mold, 4 is a heating means, and 5 is a silicon melt.

  A melting crucible 1a for melting the silicon raw material is arranged to be held by the holding crucible 1b, and a pouring port 2 for pouring the silicon melt by inclining the melting crucible 1a is provided at the upper edge of the melting crucible 1a. It is done. A heating means 4 is disposed around the melting crucible 1a and the holding crucible 1b, and a mold 3 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 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 4, for example, a resistance heating type heater or an induction heating type coil is used.

  The mold 3 disposed below the melting crucible 1a and the holding crucible 1b is made of quartz or the like, and is used by applying a release material (not shown) whose main component is silicon nitride or the like inside. A mold heat insulating material (not shown) is installed around the mold 3 in order to suppress heat removal. As the mold heat insulating material, a carbon-based material is generally used in consideration of heat resistance and heat insulating properties. Further, a cooling plate (not shown) for cooling and solidifying the poured silicon melt may be provided below the mold 3. These are all arranged in a vacuum vessel (not shown).

  A method for producing polycrystalline silicon using the silicon casting apparatus shown in FIG. 2 is as follows. First, the silicon raw material is charged into the melting crucible 1a, the silicon raw material in the melting crucible 1a is melted by the heating means 4, and after the melt is completely melted, the crucible is tilted and the melting crucible 1a is placed at the upper edge. The silicon melt is poured from the gate 2 into the mold installed at the bottom. 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.

  As a second method, a semiconductor material casting apparatus disclosed in Patent Document 2 is shown in FIG. This device is provided with a sealed container (not shown) that is shielded from the atmosphere. The hermetically sealed container may be connected to an external vacuum pump through a vacuum shut-off door (not shown) and the inside of the container may be kept in a vacuum. Alternatively, a configuration in which an inert gas such as argon circulates while maintaining the inside at normal pressure or slightly positive pressure may be employed. If the atmosphere in the sealed container is maintained in a non-oxidizing environment, even if the semiconductor material is heated and dissolved in the sealed container, it is not adversely affected by oxidation. A cylindrical heating furnace comprising a thermal insulator and heating means 24 is provided in the sealed container apart from the container side wall. The thermal insulator and the heating means 24 are both made of carbon fiber and graphite, for example. The heating means 24 may be made of a conductor such as metal. An induction heating coil (not shown) is wound around the heating furnace, particularly the outer periphery corresponding to the portion where the heating means 24 is located. The induction heating coil (not shown) is energized with a high-frequency current having a frequency of about 10 kHz, for example, and heats the heating means 24 by induction.

  Inside the sealed container (not shown), a mold 23 into which a semiconductor material and, if necessary, a seed crystal are charged is arranged. The mold 23 is disposed so as to be separated from the side wall and the upper portion of the heating means 24 in the internal space formed by the heating means 24. The mold 23 may be made of, for example, silica material, graphite material, or other materials such as tantalum, molybdenum, tungsten, silicon nitride, or boron nitride. The shape of the mold 23 may be adapted to the shape of the internal space of the heating means 24, and is selected as a cylindrical shape or a quadrangular prism shape. The bottom of the mold 23 is mounted on and supported by a support base in a sealed container (not shown). The support is rotatable around the central long axis. The rotation of the cylinder is transmitted from the pedestal to the mold 23 via the support base, and the mold 23 also rotates according to the rotation of the cylinder. If the semiconductor material is charged in the mold 23 and the mold 23 is heated in the heating means 24, if the rotation is also performed, the temperature distribution of the semiconductor material in the mold 23 becomes uniform.

In such a casting apparatus shown in FIG. 7, the material is uniformly melted in the mold 23 to form the material melt 25, and then a cooling medium (water, refrigerant gas, etc.) is passed through the base of the mold 23. Thus, heat is removed from the bottom of the mold 23, and the melt 25 can be solidified in one direction from the bottom.
Japanese Patent Laid-Open No. 11-180711 Japanese Patent Laid-Open No. 11-021120 JP 09-071497 A "Polycrystalline heterogeneity (Relationship between grain size and properties)" Tokyo University of Agriculture and Technology Saito, Applied Physics, Vol. 50, No. 4 (1981) Fine ceramics evaluation technology assembly (Realize), P110, Table-1

  When manufacturing a solar cell element using a polycrystalline silicon substrate, the grain boundary part has extremely inferior electrical characteristics, but the width at which the photocurrent decreases corresponds to the diffusion distance of minority carriers, so the grain size is the diffusion distance. There is a report that the grain boundary loss is negligible if it is sufficiently larger than the above, and in general, if the crystal grain size is several mm or more, it is theoretically possible to obtain the characteristics of a single crystal (for example, non-patent literature) 1).

  However, from the results of the inventor's research, thermal stress-induced dislocations caused by thermal stress generated during casting of the silicon ingot run toward the grain boundary within the crystal grain and are blocked by the grain boundary. It was found to proliferate. In addition, dislocation growth does not stop in the vicinity of the grain boundary, but the dislocation network that has stopped and proliferated becomes the next new grain boundary, blocking dislocations that run later, and dislocation growth / networking again at that part. When the stress progresses and the degree of stress is high, the entire crystal grains with a grain size of several millimeters are covered with dislocation networks, resulting in a significant decrease in lifetime due to a marked increase in the number of minority carrier recombination centers. In addition, it has been found that there are many cases where the crystal grains can be obtained only with low solar cell characteristics.

  This indicates that there is a need to reduce the thermal stress during casting of silicon ingots, but an ideal casting method that completely eliminates thermal stress significantly reduces productivity and reduces costs. Industrially, there is a limit. In Non-Patent Document 1, since these dislocation growth parameters are ignored, it is desirable that the crystal grain size of the polycrystalline silicon substrate is substantially increased as close to a single crystal as possible.

  FIG. 6 shows a schematic diagram of crystal growth by a general casting method. When the casting apparatus shown in FIG. 2 or FIG. 7 is used, the raw material is melted in different places. As shown in FIG. 6 (a), after the molten silicon is once held in the mold, This is common in that unidirectional solidification is achieved by cooling the slag. However, according to the research of the present inventor, generally, when solidification is started by such a method, as shown in FIG. 6B, the wettability between the mold bottom surface and the melt and the release material applied to the bottom surface It was found that the bottom surface of the mold was not cooled uniformly due to variations in smoothness. Actually, crystal nuclei are randomly generated at the most cooled portion at the bottom and where the melt is in contact with each other, and a large number of crystal growth proceeds from that. As shown in FIG. In fact, the substrate cut out from the obtained ingot becomes an aggregate of many columnar crystals.

  On the other hand, in the method disclosed in Patent Document 3 shown in FIG. 8, the bottom of the mold has an inverted pyramid shape, a seed crystal having an arbitrary plane orientation is placed on the bottom of the mold, and the seed crystal is seeded. By making it the main bottom cooling point, an attempt is made to obtain a crystal having a large grain size and having an arbitrary plane orientation. However, in order to effectively control the surface orientation by this method, there is a big problem that the angle of the inverted pyramid portion at the bottom of the mold must be made as small as possible. This is because, if the angle of this portion is large, crystal growth nuclei are formed at locations reaching the tapered portion, and the original purpose cannot be achieved. In addition, reducing the taper angle means that an ingot containing a large number of pyramid-shaped ingot regions that are not used as a silicon substrate is produced, resulting in a substantial increase in the cost of raw materials, which increases the price of solar cells. Not right. In general, such seeding technique requires very precise temperature control of the seed surface temperature. If the temperature is too high, the seed crystal dissolves, and if the temperature is too low, seeding from the entire surface of the seed crystal occurs. There are many problems that should be technically solved.

  The present invention has been made in view of the above circumstances, and provides a method for casting polycrystalline silicon that can obtain a polycrystalline silicon ingot having a large crystal grain size at a simple and low cost. It is an object of the present invention to obtain a polycrystalline silicon ingot, a polycrystalline silicon substrate, and a solar cell element having excellent characteristics.

The present inventor has paid attention to the fact that the contact angle of the silicon melt with respect to various materials differs depending on the material, and as a result of intensive research, it has been found that the silicon melt is melted in a temperature range above the melting point of silicon (especially, 1420 ° C to 1600 ° C). A material that has a large contact angle with respect to the liquid and is difficult to wet (for example, BN, Al ₂ O ₃ , or a mixture of silicon nitride and silicon dioxide at a predetermined ratio) has a small contact area with the silicon melt. Therefore, solidification is difficult to proceed, and conversely, it is understood that the degree of supercooling required for solidification is larger than that of easily wettable materials having a small contact angle (for example, graphite, silicon nitride, etc.) with respect to silicon melt. It was. The present inventor pays attention to this experimental fact, and disperses a material having good wettability with the silicon melt and a material having poor wettability at a predetermined ratio, thereby crystal growth from a material portion requiring a large degree of supercooling. By suppressing crystal growth and promoting crystal growth from a material portion that starts solidification with a small degree of supercooling, the crystal grain size was easily increased, and the following configuration of the present invention was found.

In view of the above, the method for casting polycrystalline silicon according to the present invention is a method for casting polycrystalline silicon in which a silicon melt held inside a mold having a bottom is solidified in one direction upward, and the bottom of the mold The inner surface includes a first region having a contact angle with the silicon melt in a temperature range of 1420 ° C. to 1600 ° C. of less than 40 ° and a second region having the contact angle of 40 ° or more. When the area of the region is A and the area of the second region is B, the following equation is established.
0.1 ≦ A / (A + B) ≦ 0.75
In this way, on the inner surface of the bottom of the mold that performs unidirectional solidification, the first region with good wettability with a small contact angle with respect to the silicon melt and the second region with large contact angle and poor wettability are expressed by the above equation. By presenting at the ratio shown, crystal growth from the second region that requires a large degree of supercooling can be suppressed, and crystal growth from the first region where solidification can be initiated with a small degree of supercooling can be promoted. The particle size can be increased. The temperature range of the silicon melt is limited to 1420 ° C. to 1600 ° C. The lower limit of 1420 ° C. is the melting point of silicon, and the upper limit of 1600 ° C. is used at a temperature higher than that of the mold material. This is because it is difficult to think about.

  Furthermore, in the method for casting polycrystalline silicon according to the present invention, the first region is provided in a dispersed state on the inner surface of the bottom of the mold, and each of the dispersed first regions is within a 10 mm square square frame. The shape fits. By dispersing the first region having a small contact angle and good wettability with respect to the silicon melt on the inner surface of the bottom of the mold, a crystal having a large particle size can be grown uniformly. Furthermore, since each dispersed first region has a shape that fits within a 10 mm square square frame, the nucleation density for crystal growth in each first region can be kept below a certain level. , The particle size is difficult to decrease.

  In the method for casting polycrystalline silicon according to the present invention, the first region and / or the second region are formed by applying a slurry containing an inorganic powder and a binder. The contact angle of the first region and the second region with respect to the silicon melt can be adjusted by appropriately adjusting the blending ratio of the plurality of inorganic powders having different wettability with respect to the silicon melt. Can also be performed.

  In the method for casting polycrystalline silicon according to the present invention, since at least one of the first region and the second region uses the member that forms the bottom of the mold as it is, the first region and the second region are efficiently used. Can be formed. A method of applying and forming a slurry containing an inorganic powder and a binder may be used in combination.

  In the method for casting polycrystalline silicon according to the present invention, the first region and the second region are formed so as to have a regular pattern adjacent to each other. A crystal having a small particle size grown from the second region having poor wettability is blocked by the growth of a crystal having a large particle size grown from the adjacent first region, so that the crystal particle size can be kept large. . Specifically, the regular pattern may be a checkered pattern.

  Since the polycrystalline silicon ingot of the present invention is produced using the polycrystalline silicon casting method of the present invention, the growth of crystals having a large grain size grown from the first region is promoted. Therefore, thermal stress-induced dislocation is suppressed and the quality is improved.

  Since the polycrystalline silicon substrate of the present invention is obtained by cutting the polycrystalline silicon ingot of the present invention, it becomes a high-quality polycrystalline silicon substrate in which thermal stress-induced dislocations are suppressed.

  Since the solar cell element of the present invention is formed using the polycrystalline silicon substrate of the present invention, thermal stress-induced dislocation that may be a recombination center of minority carriers is suppressed, and it has good characteristics. It becomes a solar cell element.

  As described above, the method for casting polycrystalline silicon according to the present invention has a large contact angle and a first region with good wettability with a small contact angle with respect to the silicon melt on the inner surface of the bottom of the mold that performs unidirectional solidification. By causing the second region having poor wettability to exist at a predetermined ratio shown in the above-described formula, the crystal growth from the second region requiring a large degree of supercooling is suppressed, and solidification starts with a small degree of supercooling. Crystal growth from the region can be promoted, and the crystal grain size can be easily increased. As described above, polycrystalline silicon having a large crystal grain size has excellent electrical characteristics because it has few thermal stress-induced dislocations serving as minority carrier recombination centers.

  In addition, the polycrystalline silicon ingot of the present invention and the polycrystalline silicon substrate obtained by cutting the polycrystalline silicon ingot were produced using the method for casting polycrystalline silicon of the present invention, so that they grew from the first region. Growth of crystals with a large particle size is promoted. Therefore, thermal stress-induced dislocation is suppressed, and the quality is excellent with excellent electrical characteristics.

  And since the solar cell element of this invention is formed using the above-mentioned polycrystalline silicon substrate of this invention, the thermal-stress induced dislocation which may become a recombination center of a minority carrier is suppressed. Therefore, the lifetime of minority carriers is improved and a solar cell element having good characteristics is obtained.

  The present invention will be described below with reference to the accompanying drawings. FIG. 1 is a schematic diagram for explaining a method for casting polycrystalline silicon according to the present invention, in which (a) shows a part of the inner surface of the bottom of a mold, and (b) shows the state of crystal growth. Show. FIG. 2 is a longitudinal sectional view showing an embodiment of a casting apparatus used in the present invention.

  In the figure, 1a is a melting crucible, 1b is a holding crucible, 2 is a pouring spout, 3 is a mold, 4 is a heating means, 5 is a silicon melt, 6 is a silicon columnar crystal controlled by the present invention, and 7 is a competition. Growing soot region, 8 is a first region according to the present invention (contact angle with silicon melt is less than 40 ° and wettability is large), 9 is a second region according to the present invention (contact angle with silicon melt is (Wettability is low at 40 ° or more) 10 is a region that has been crushed by competitive growth, 11 is a bottom member of the mold, and 12 is a dimension of one side of the first region.

  First, the overall apparatus configuration and the outline of the casting method will be described with reference to FIG. 2, and the features of the present invention will be described in more detail with reference to FIG.

  As shown in FIG. 2, the melting crucible 1 a is for pouring the silicon melt into the mold 3 by holding the charged silicon raw material inside and heating and melting it. The polycrystalline silicon ingot obtained by cooling and solidifying the silicon melt melted in the melting crucible 1a and poured into the mold 3 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. However, the melting crucible 1a has a purity that does not easily cause melting, evaporation, softening, deformation, decomposition, etc. at a temperature higher than the melting temperature of the silicon raw material and does not deteriorate the solar cell characteristics. If it does not specifically limit. Further, since the melting crucible 1a softens at a high temperature and cannot keep its shape, the melting crucible 1a is held by a holding crucible 1b made of graphite or the like. In addition, the dimensions of the melting crucible 1a and the holding crucible 1b need to be a dimension that can enclose silicon raw materials according to the amount of melting that is melted at once. The amount of silicon raw material dissolved is in the range of approximately 1 kg to 150 kg.

  A heating means 4 is arranged around the melting crucible 1a and the holding crucible 1b. By this heating means 4, the silicon raw material inside the melting crucible 1a is heated and melted to obtain a silicon melt. 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 3 installed at the bottom.

  The casting mold 3 disposed below the melting crucible 1a and the holding crucible 1b is made of silicon dioxide such as quartz, carbon material such as graphite, or ceramic material. For example, as a split / assembly mold that can be used repeatedly. It is configured. The mold 3 has a role of solidifying in one direction from the bottom to the top while holding the silicon melt 5 poured from the melting crucible 1a inside.

Further, it is desirable to provide a release material on the inner surface portion of the mold 3. This mold release material can be formed, for example, by mixing silicon nitride (Si ₃ N ₄ ) powder with a PVA (polyvinyl alcohol) aqueous solution and applying it to the inner surface of the mold 3. When silicon nitride is used, it is preferable to mix silicon nitride powder with a PVA aqueous solution having an appropriate concentration to form a slurry because it can be easily applied with a brush, spatula, brush, dispenser, or the like. By providing such a release material, the inner wall of the mold 3 and the silicon ingot are less likely to be fused after the silicon melt is solidified, so that each member constituting the assembly-type mold 3 can be used repeatedly. become. In addition, although mentioned later for details, as an example of embodiment of this invention, the effect of invention can be acquired by apply | coating a mold release material with the predetermined shape with respect to the inner surface of the bottom part of this casting_mold | template 3.

  A mold heat insulating material (not shown) is installed around the mold 3 to suppress heat removal from the side surface and promote unidirectional solidification. As the mold heat insulating material, a carbon-based material is generally used in consideration of heat resistance, heat insulating properties, and the like. In addition, a cooling plate (not shown) made of a metal plate or the like for removing the poured silicon melt from below and cooling and solidifying it may be installed below the mold 3. In addition, it is desirable that these casting apparatuses are placed in a vacuum vessel (not shown) and performed in a reducing atmosphere from the viewpoint of preventing impurities from being mixed and oxidized.

  While holding the silicon melt 5 with respect to the mold 3 and suppressing heat removal from the side surface by the mold heat insulating material as described above, a cooling plate or the like that cools from the bottom is used to lower the mold 3 from below. If an appropriate temperature gradient is applied upward, the silicon melt discharged in the mold 3 can be solidified in one direction from below to upward to produce a polycrystalline silicon ingot.

  Next, the features of the present invention will be described in more detail with reference to FIG. FIG. 1 is a schematic diagram for explaining a method for casting polycrystalline silicon according to the present invention, in which (a) shows a configuration example of the inner surface of the bottom of the mold, and (b) shows the inner surface of the bottom of the mold. The state of crystal growth of a polycrystalline silicon ingot grown from is shown.

In the present invention, as shown in FIG. 1A, the inner surface of the bottom member 11 of the mold 3 is a first region in which the contact angle with the silicon melt in the temperature range of 1420 ° C. to 1600 ° C. is less than 40 °. 8 and a second region 9 having a contact angle of 40 ° or more. When the area of the first region 8 is A and the area of the second region 9 is B, the following equation is established.
0.1 ≦ A / (A + B) ≦ 0.75
This expression represents the ratio of the area A of the first region 8 to the total area (A + B) of the inner surface of the mold, and this ratio is set to a predetermined range (0.1 to 0.75). Thus, it is possible to suppress crystal growth from the second region 9 that requires a large degree of supercooling, and to promote crystal growth from the first region 8 that starts solidification with a small degree of supercooling. It becomes possible to enlarge. The reason is presumed as follows.

  FIG. 1B shows a competitive growth soot region 7 where crystal growth from the first region 8 and the second region 9 converges. The second region 9 which has a larger contact angle with the silicon melt than the first region 8 and is less likely to get wet has a small contact area with the melt, so that solidification does not proceed easily and requires a higher degree of supercooling. Accordingly, when the silicon melt is solidified, crystal growth from the first region 8 becomes preferential. Thereafter, when crystal growth from the second region 9 occurs, the crystal already grown from the first region 8 grows large, and the crystal growth 10 from the second region 9 is blocked and hesitated, so that the crystal grain size is large. Silicon columnar crystals 6 grow.

  The crystal formed when the ratio of the area A of the first region 8 to the total area (A + B) of the inner surface of the bottom of the mold deviates from the above range (0.1 to 0.75). This is not desirable because the particle size of the material becomes small. If it is larger than 0.75, the area occupied by the first area 8 becomes too large, and as a result, the probability of nucleation from the first area 8 almost simultaneously increases with a low degree of supercooling. The crystals will survive. On the other hand, if it is smaller than 0.1, crystals nucleated from the first region 8 with a low degree of supercooling grow early, but the second region 9 occupies the majority of the bottom region. The probability of nucleation from the second region 9 almost simultaneously increases with a large degree of supercooling, and a large number of crystals survive from the second region 9. For this reason, it is estimated that the crystal grain size becomes small.

  Further, the first region 8 is provided in a dispersed state on the inner surface of the bottom of the mold 3 so that each of the first regions 8 in the dispersed state can be accommodated in a 10 mm square frame. It is desirable to do. By disposing the first region 8 on the inner surface of the bottom of the mold 3, it is possible to uniformly disperse crystals having a large particle size grown from the first region 8. Then, as shown in FIG. 1A, each of the dispersed first regions 8 has a shape in which one side dimension 12 can be accommodated in a square frame of 10 mm square, thereby reducing the grain size of the formed crystal. Hard to become. If the dispersed first region 8 is larger than this range, a large number of nuclei are generated in the region at the same time, even within the first region 8 having good wettability with the silicon melt. This seems to be because the increase in diameter is suppressed.

  Note that the fact that the first regions 8 are provided in a dispersed state means that each of the first regions 8 is dispersed, but each of the first regions 8 is not necessarily separated completely. For example, a part of the adjacent first regions 8 may be connected to each other. However, the thickness of the connected portion is preferably 5 mm or less. This is because if the thickness of the connected portion exceeds 5 mm, nucleation in this portion cannot be ignored and the crystal grain size may be reduced.

  The first region 8 and the second region 9 are desirably provided in a regular pattern adjacent to each other. Thus, by forming both the first region 8 and the second region 9 in a regular pattern adjacent to each other, a crystal having a small grain size grown from the second region 9 is adjacent to the adjacent first region 8. Since it is shielded by the crystal having a large particle diameter grown from the above, it can be surely kept large. Specifically, for example, as shown in FIG. 1A, if the first region 8 and the second region 9 are formed in a checkered pattern, a simple effect can be obtained. In addition to the checkered pattern, the first region 8a and the second region 9a constituted by substantially equilateral triangles may be combined as shown in FIG. 5A, or the substantially right triangle as shown in FIG. 5B. The first region 8b and the second region 9b configured by the above may be combined.

  Such a pattern may be modified within a range that does not change the regularity. For example, in the case of a checkered pattern, it is not necessary that the squares are completely continuous with each other, and the parallelogram may be a continuous pattern.

  The first region 8 and / or the second region 9 described above is preferably formed by applying a slurry containing an inorganic powder and a binder. Alternatively, at least one of the first region 8 and the second region 9 may use the bottom member 11 of the mold 3 as it is. According to the method of applying the inorganic powder as a slurry, the present invention can be easily implemented, and the mixing ratio of the plurality of inorganic powders having different wettability with respect to the silicon melt is appropriately adjusted. Accordingly, the contact angle of the first region 8 and the second region 9 with respect to the silicon melt can be adjusted. In addition, according to the method using the contact angle of the bottom member 11 of the mold 3 as it is, the first region 8 and the second region 9 can be efficiently formed.

Specific examples of the material exhibiting a predetermined contact angle with respect to the silicon melt as described above include alumina (Al ₂ O ₃ ; contact angle = 94 °), boron nitride (BN; contact angle = 105 °), silicon nitride. (Si ₃ N ₄ ; contact angle <40 °), silicon carbide (SiC; contact angle <40 °), magnesium oxide (MgO; contact angle = 95 °), graphite (C; contact angle = 0 °), etc. It is done. In addition, the contact angle described in parentheses is a contact angle with the silicon melt in the region above the silicon melting point described in Non-Patent Document 2.

  If an appropriate material is selected from the above materials to form an inorganic powder, and PVA (polyvinyl alcohol) or the like as a binder is mixed with pure water so as to have a concentration suitable for coating, It can be applied directly with a brush or the like.

  For example, with respect to the bottom member 11 of the mold 3, a low contact angle silicon nitride powder slurry is used as the first region 8, and a higher contact angle boron nitride powder slurry is used as the second region 9. What is necessary is just to apply | coat so that it may become a ratio. Further, graphite having a low contact angle is selected as the bottom member 11, and a slurry of high contact angle alumina powder is applied to a necessary portion so that the member becomes the first region 8 to form the second region 9. May be.

  As a method for obtaining the contact angle of the material with respect to the silicon melt, as described in Non-Patent Document 2, a test piece (a plate-like body of a desired material or a heat-resistant plate-like body (for example, graphite) In a state where the Si raw material is placed on a plate etc.) and heated in a furnace and melted, the silicon melt is observed from the side to determine the contact angle. Find it.

Moreover, as an inorganic powder, a powder containing silicon nitride known to have good releasability (for example, silicon nitride powder, a mixture of silicon nitride powder and silicon dioxide powder in a predetermined ratio, If these are used in combination to form a two-layer structure, etc.) and applied to the inner surface of the bottom of the mold 3 at a predetermined ratio defined in the present invention, only the effects of the present invention are exhibited. Rather, releasability can be imparted simultaneously. The inventor confirmed that the contact angle of quartz / silica (SiO ₂ ) is about 85 °, and silicon nitride powder (contact angle <40 °) and silicon can be obtained by appropriately adjusting the combination ratio. The contact angle with respect to the melt can be adjusted. For example, when the weight ratio of silicon nitride powder: silica powder = 9: 1 is used, the contact angle is less than about 40 °.

  The application shape of the inorganic powder is not limited to the shape shown in FIG. 1 (a). A material with poor wettability is used on the entire bottom surface of the mold, and an inorganic powder with good wettability is formed on the material. It may be applied in the form of dots. Also, the application method can be spray application or thermal spraying in addition to direct application by brush or brush. Furthermore, screen printing using various patterns of screens is a desirable implementation method in terms of cost.

  The polycrystalline silicon ingot of the present invention produced by using the method for casting polycrystalline silicon of the present invention as described above has a contact angle with the silicon melt at a predetermined range on the inner surface of the bottom of the mold 3. Since it is only necessary to adjust the crystal, it is extremely inexpensive and a crystal having a large grain size in which thermal stress-induced dislocation is suppressed is growing. Therefore, it can be expected to produce a polycrystalline silicon ingot for solar cells that is inexpensive and of high quality.

  The polycrystalline silicon substrate of the present invention is obtained by slicing the polycrystalline silicon ingot of the present invention in a direction substantially perpendicular to the solidification direction. As described above, in the polycrystalline silicon ingot of the present invention, the growth of crystals having a large particle diameter grown from the first region 8 is promoted. Therefore, the polycrystalline silicon substrate of the present invention obtained by slicing this is a high-quality polycrystalline silicon substrate having a large crystal grain size, suppressed thermal stress-induced dislocations, and high electrical characteristics.

  As described above, the polycrystalline silicon substrate of the present invention suppresses thermal stress-induced dislocation that may become a recombination center of minority carriers. Therefore, the solar cell element of the present invention formed using this substrate. Is excellent in the effect of preventing recombination of carriers generated inside the substrate when an electric field is formed in the thickness direction of the substrate, 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 are made without departing from the gist of the present invention.

  For example, in the description given above with reference to FIG. 1 (a), the mold 3 has been described as the divided bottom member 11, but even if it is an integral mold, the bottom thereof is configured similarly. The effects of the present invention are achieved.

  In addition, when the silicon melt is poured from the melting crucible into the mold, a method other than the method shown in FIG. 2 may be used. Specifically, 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, before the silicon raw material is completely dissolved, silicon that closes the mechanical plug or the pouring port near the pouring port so that the silicon raw material before melting or the partially melted silicon melt does not leak from the pouring port. It is preferable to provide a pouring control means capable of controlling pouring, such as installing raw materials. Further, a melting part for dissolving the silicon raw material may not be provided separately from the mold, and the silicon raw material may be melted in the mold to cool and solidify the silicon melt as shown in FIG. In any case, if the bottom surface portion in the mold has the above-described configuration according to the present invention, the effects of the present invention can be obtained and the method does not depend on the silicon raw material melting method.

  Examples of the present invention will be described below. The following experiment was performed using the casting apparatus shown in FIG. 2 described above.

  A 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 4 was installed around the melting crucible 1a, and the silicon raw material in the melting crucible 1a was melted by the heating means. Prior to pouring, the inner surface temperature of the graphite mold 3 was maintained at 1000 ° C.

  A slurry prepared by mixing this powder, pure water, and PVA (polyvinyl alcohol) is applied to the bottom of the mold with respect to an inorganic powder having a different contact angle with the silicon melt at the silicon melting point. And the second region 9 was formed. In addition, the application area of the 1st area | region 8 was changed between 0%-100% with respect to the total area of the bottom inner surface as follows. First, the size of the coating area of the inorganic powder at one point in the first area 8 was set to 10 mm × 10 mm, and the coating was regularly arranged with a predetermined interval. At this time, the space between the first regions 8 is defined as the second region 9, and the ratio between the first regions 8 and the second regions 9 is changed by adjusting the interval between the adjacent first regions 8.

The following combinations of inorganic powders were used.
(1) Silicon nitride (contact angle <40 °) is applied as the first region 8, and boron nitride (contact angle = 105 °) is applied as the second region 9, respectively.
(2) As the first region 8, the bottom of the graphite mold (contact angle = 0 °) is used as it is, and alumina (contact angle = 94 °) is applied as the second region 9. Next, the silicon melt in the melting crucible 1 a is applied. The temperature of the liquid 5 is raised, and after the melt temperature (measured by the infrared radiation thermometer, the surface temperature of the melt) reaches a predetermined temperature (changed from 1420 ° C. to 1600 ° C.), the melting crucible 1a is tilted and the pouring port The silicon melt 5 was poured from 2 into the mold 3. After casting a polycrystalline silicon ingot inside the mold 3 with a predetermined temperature gradient, the bottom of the ingot (10 mm above the inner surface of the bottom of the mold 3) is cut horizontally, and the average of the obtained crystal grains The particle size was calculated using image analysis software. The average grain size is obtained by taking a cross-sectional image of the bottom of the ingot with a scanner or digital camera and performing image processing such as contrast enhancement, and then using the analysis software to detect boundaries (grain boundaries), determine crystal grains, and measure the average grain size. It is calculated after passing.

  FIG. 3 shows the result of the experiment. FIG. 3A is a graph showing the result of the combination experiment of boron nitride and silicon nitride in (1), and FIG. 3B is the graph showing the result of the combination experiment of alumina and graphite in (2). FIG. The horizontal axis of each graph represents the area ratio (coating ratio) of silicon nitride (1) and graphite (2) corresponding to the first region 8. The vertical axis of the graph is the average grain size of the crystal grains at the bottom of the polycrystalline silicon ingot.

  As can be seen from FIG. 3, in any case of the combination of boron nitride and silicon nitride of (1) and the combination of alumina and graphite of (2), the ratio of the first region 8 that is the scope of the present invention is It can be seen that in the region of 0.1 to 0.75 (the portion surrounded by the dotted line), the bottom of the polycrystalline silicon ingot is a large crystal grain having an average grain size of 10 μm or more. In addition, it progresses in the direction which a crystal grain diameter increases as temperature rises from 1420 degreeC. This is probably because the nucleation density of the initial solidified layer formed by the solidification of the melt in the mold decreases as the temperature when the silicon melt is poured into the mold increases. Further, when the area ratio of silicon nitride or graphite having high wettability to the silicon melt is large, the crystal grain size is small. This shows that the nucleation density changes due to the difference in wettability, but if there are many of the same material, there is no significant difference in the number of nuclei generated at the same time. Both FIG. 3 (a) and FIG. 3 (b) As a result, it has a peak at the center and the particle size becomes smaller at both ends.

  The experiment was performed in the same manner as in Example 1 except that the following conditions were changed.

That is, as described in the above (1), silicon nitride (contact angle <40 °) is applied as the first region 8, and boron nitride (contact angle = 105 °) is applied as the second region 9, respectively. In the checkered pattern of 1 (a), the size of one point in the first region 8 was changed in the range of 2 mm square to 30 mm square.

  FIG. 4 is a histogram showing the relationship between the size of the dispersed first region (2 mm square to 30 mm square) and the bottom average particle diameter. FIG. 4 shows the histogram of the frequency values by sampling the crystal grain size for each point size of the first region 8. The frequency value was standardized so that the maximum value was 1.

  As can be seen from FIG. 4, when the size of one dispersed first region 8 is 15 mm square or more, the crystal grains are finer and the maximum value of the histogram is smaller than the particle diameter of 5 mm. On the other hand, when the size of the first region 8 is 10 mm square or less, the crystal particle has the maximum value of the histogram in the region of 5 mm to 10 mm. In particular, when the size of the first region 8 is 5 mm square to 10 mm square, the crystal grain size becomes an average of 10 mm or more, and good results are obtained. The maximum value of the histogram is about 8 mm. However, since the base is drawn toward the larger crystal grain size, the average grain size is larger than the maximum value of the histogram.

  Experiments were performed in the same manner as in Example 1 except that the following conditions were changed, and Samples 1 and 2 and a conventional sample according to the present invention were produced. The material having a contact angle of 85 ° when the silicon melt described below is 1420 ° C. is a powder of silicon dioxide, thereby forming the second region 9. The material of less than 40 ° is a powder obtained by mixing a silicon nitride powder and a silicon dioxide powder in a ratio of 9: 1, thereby forming the first region 8.

  (Sample 1) A material having a contact angle of slightly less than 40 ° and a material having a contact angle of 85 ° when the silicon melt is 1420 ° C. is applied to the bottom of the mold in the pattern shown in FIG. Application was performed so that the ratio of the total application area was 50:50.

  (Sample 2) A material with a contact angle of 85 ° when the silicon melt is 1420 ° C. is applied to the entire surface of the mold and dried, and then a material with a contact angle of less than 40 ° is evenly with a dimension of φ10 mm. It applied so that it might be scattered, and it applied so that the ratio of the total application area might be set to 60:40.

  (Conventional sample) As a release material, a silicon nitride powder (contact angle at 1420 ° C. of less than 40 °) mixed with a binder as slurry was applied on the entire surface of the mold bottom.

  The end portion is removed from the polycrystalline silicon ingot produced by the above method, cut into a desired size, and the cut polycrystalline silicon ingot is sliced to a predetermined thickness in a direction substantially perpendicular to the growth direction. In any of the samples 1 and 2 according to the present invention, the obtained polycrystalline silicon substrate had most of the crystal grains having an average grain size of 10 mm or more. Therefore, since each crystal grain has a clean area apart from the dislocation area that is densely packed near the crystal grain boundary, it becomes possible to prevent carrier recombination, resulting in improved solar cell characteristics and uniform quality It becomes possible to bring

  On the other hand, the polycrystalline silicon substrate obtained by slicing from the polycrystalline silicon ingot of the conventional sample has a small particle size and an average of about 3 mm as compared with the polycrystalline silicon substrate according to the present invention. The reason for this is that a silicon nitride release material layer having high wettability with respect to the silicon melt is provided on the entire inner surface of the bottom of the mold when producing a conventional sample. It is assumed that the particle size has been reduced.

  Using these substrates, general bulk-type solar cell elements were produced, and the conversion efficiency of the solar cell elements was characterized. As a result, Sample 1 had a conversion efficiency of 15.7% and Sample 2 had a conversion efficiency of 16.1%, whereas the conventional sample had a conversion efficiency of 15.4%.

  As described above, the solar cell elements of Sample 1 and Sample 2 formed using the polycrystalline silicon substrate according to the present invention obtained better characteristics than the conventional solar cell elements. This is because the crystal grain size of the portion of the polycrystalline silicon ingot formed by using the method for casting polycrystalline silicon according to the present invention that is substantially used as a substrate for solar cells is increased to an average grain size of 10 mm or more. For this reason, it is presumed that the electrical characteristics were improved because the influence of thermal stress-induced dislocations that became the recombination center of minority carriers in the silicon solidified layer during the solidification / cooling was reduced.

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

It is a figure for demonstrating the casting method of the polycrystalline silicon of this invention, (a) is the figure which showed the structural example of the inner surface of the bottom part of a casting_mold | template, (b) is the polycrystalline growing from this casting_mold | template. It is a schematic diagram which shows the mode of the crystal growth of a silicon ingot. It is a schematic sectional structure figure of a general casting apparatus. It is a figure which shows the result when apply | coating the inorganic powder which changed the kind as an Example of this invention, (a) is a combination of alumina / graphite, (b) is a combination of boron nitride / silicon nitride. It is a histogram figure showing the relationship between the size of the distributed 1st field, and the bottom average particle size. (A), (b) is a schematic diagram which shows the example of the formation method of a 1st area | region and a 2nd area | region based on this invention. (A) is a schematic cross-sectional structure diagram of a general mold, (b) is a diagram showing the boundary between the silicon melt and the mold bottom at the start of unidirectional solidification in the mold by a conventional method, (C) is a schematic diagram which shows the mode of crystal growth of the silicon ingot obtained by the unidirectional solidification in the conventional general casting_mold | template. It is a schematic sectional structure figure of a general casting apparatus. It is a schematic sectional structure figure of a general casting apparatus.

Explanation of symbols

1a: melting crucible 1b: holding crucible 2: pouring spout 3: mold 4, 4a: heating means 5: silicon melt 6: silicon columnar crystal 7: competitive growth trough regions 8, 8a, 8b: first regions 9, 9a, 9b: second region 10: crystal growth from the second region 11: bottom member 12: one side dimension of the first region 23: mold 24: heating means 25: melt

Claims (9)

A method for casting polycrystalline silicon in which a silicon melt held in a mold having a bottom is solidified in one direction upward,
The inner surface of the bottom of the mold consists of a first region having a contact angle with the silicon melt in the temperature range of 1420 ° C. to 1600 ° C. of less than 40 ° and a second region having the contact angle of 40 ° or more. ,
A polycrystalline silicon casting method in which the following equation is established, where A is the area of the first region and B is the area of the second region.
0.1 ≦ A / (A + B) ≦ 0.75 2. The polycrystalline silicon according to claim 1, wherein the first region is provided in a dispersed state on the inner surface of the bottom of the mold, and each of the dispersed first regions has a shape that fits within a 10 mm square frame. Casting method. 3. The method for casting polycrystalline silicon according to claim 1, wherein the first region and / or the second region is formed by applying a slurry containing an inorganic powder and a binder. 3. The method for casting polycrystalline silicon according to claim 1, wherein at least one of the first region and the second region is formed using a member that forms the bottom of the mold as it is. 5. The method for casting polycrystalline silicon according to claim 1, wherein the first region and the second region are formed in a regular pattern adjacent to each other. 6. The method for casting polycrystalline silicon according to claim 5, wherein the regular pattern is a checkered pattern. The polycrystalline silicon ingot produced using the casting method of the polycrystalline silicon as described in any one of Claims 1 thru | or 6. A polycrystalline silicon substrate obtained by cutting the polycrystalline silicon ingot according to claim 7. A solar cell element formed using the polycrystalline silicon substrate according to claim 8.

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