TW201335443A - Mold for silicon casting, method for silicon casting, silicon material, and manufacturing method for solar cell - Google Patents

Abstract

The present invention discloses a mold for silicon casting, characterized by solidifying a silicon melt and disposing a mold release layer containing an additive having an average particle diameter of 0.1 to 3.0 mm at least on the top surface of the bottom plate of the inner walls of the mold for silicon casting.

Description

Casting mold for tantalum casting, tantalum casting method, tantalum material, and method for manufacturing solar battery

The present invention relates to a casting mold for casting, a casting method using the same, a crucible material, and a method for producing a solar battery.

As an alternative to petroleum and other problems that cause various problems in the global environment, the use of natural energy is attracting attention. Among them, solar cells are particularly actively introduced in Japan or Europe because they do not require a large device and do not generate noise during operation.

Solar cells using compound semiconductors such as cadmium telluride are also practical, but solar cells using crystalline germanium substrates account for a large proportion of the safety of the materials themselves or the performance so far, and cost efficiency. Solar cells (polycrystalline solar cells) using polycrystalline germanium substrates account for a large proportion.

A polycrystalline silicon wafer which is widely used as a substrate of a polycrystalline silicon solar cell is obtained by cutting a crystal ingot which is obtained by a method called casting by a unidirectional solidification of a molten crucible in a mold to obtain a large polycrystalline germanium ingot. A block that is waferized by slicing.

The polycrystalline silicon wafer produced by the casting method generally has a distribution of output characteristics of the solar cell as shown in FIG. 3 depending on the position of the height direction in the ingot or the block.

The reason why the characteristic distribution of Fig. 3 is generated is generally explained as follows.

First, in the region I at the initial stage of unidirectional solidification, the characteristics are degraded due to the influence of impurities diffused from the mold. In the area II on the upper side, The impurities in the raw material caused by the segregation are less likely to be generated in the crystal or the crystal defects are generated, so that the characteristics in the block are the best. Further, in the region III on the upper side, in addition to the gradual increase in the amount of impurities absorbed in the crystallization, the generation of crystal defects increases, and the characteristics of the region II decrease. Further, in the region IV on the upper side, in addition to the generation of the impurity or crystal defects which are absorbed in the crystal in the same manner as in the region III, the impurity generated from the uppermost surface portion after the ingot is finally solidified The concentration portion causes a reverse diffusion of impurities, and the amount of impurities increases, so that further characteristics are significantly lower than those of the region III.

In the above description, the influence of the impurities in the raw material or the impurities eluted from the mold is considered, but even in the case where it is assumed that there is no such influence, in the regions III and IV, the minority carriers are formed toward the upper portion. The crystal defects caught are gradually increased, so there is a tendency that the characteristics of the solar cell are lowered.

The reason for the occurrence of crystal defects is the stress due to the temperature distribution in the ingot, and the following two methods have been proposed from the viewpoint of suppressing the viewpoint.

In Japanese Patent Laid-Open Publication No. 2005-152985 (Patent Document 1), it is proposed that in the case of unidirectional solidification (casting), the heat flux at the center portion is larger than that of the periphery as the casting at the lower portion of the mold. The method of the type of holder.

In the second embodiment, for example, the method of controlling the heat flow in the middle of the growth of the ingot by the structure in which the heat-receiving (heat exchange) area is variable is proposed in International Publication No. 2005/092791 (Patent Document 2).

Moreover, as a measure for improving the quality of the polycrystalline germanium ingot different from the above method, a method for increasing the particle size has been proposed.

For example, Japanese Patent No. 4,204, 363 (Patent Document 3) and Japanese Patent Laid-Open Publication No. 2005-132671 (Patent Document 4) The bottom of the mold is rapidly cooled to form a dendrite as a crystal nucleus at the bottom of the ingot (in the initial stage of solidification) to coarsen the crystal grains.

Japanese Patent No. 4,054,873 (Patent Document 5) proposes a method of obtaining a pseudo single crystal by growing a crystal piece (melting residue) remaining in the melting step of the ruthenium raw material and increasing the crystal grain.

Furthermore, Japanese Patent No. 4569957 (Patent Document 6) proposes a method in which a seed crystal of SiC or the like which is arranged in a uniform shape of a mold bottom is used to cause epitaxial growth of germanium in a heterogeneous manner to obtain a pseudo single crystal.

Prior technical literature Patent literature

Patent Document 1: Japanese Patent Laid-Open Publication No. 2005-152985

Patent Document 2: International Publication No. 2005/092791

Patent Document 3: Japanese Patent Laid-Open No. 4203603

Patent Document 4: Japanese Patent Laid-Open Publication No. 2005-132671

Patent Document 5: Japanese Patent No. 4054883

Patent Document 6: Japanese Patent No. 4569957

In the method of Patent Document 1, in particular, when the heater is placed beside the mold, there is a problem that the shape of the solid-liquid interface is further deteriorated, and the effect of reducing the crystal defect density or preventing cracking cannot be obtained.

In the method of Patent Document 2, although the controllability of cooling from the side wall of the mold can be improved, there is a problem that the device configuration is very complicated, and the movable portion at a high temperature is large, and the cost of the device is increased or the failure is increased.

In the methods of Patent Documents 3 to 5, the deterioration of the characteristics caused by the grain boundaries can be suppressed by the coarsening of the crystal grains, and particularly when the size of the ingot is small, the stress caused by the temperature distribution is small. The crystal defects introduced on the upper side of the ingot are also suppressed to some extent. However, since the crystal defects on the upper side increase as the size of the ingot increases, the characteristics of the bottom side are improved, but the characteristics of the solar cell produced on the upper side are still reduced.

In the method of Patent Document 6, it is considered that the germanium crystal grown from the seed crystal of the adjacent SiC or the like forms a defect at the boundary portion of the contact, and the ingot system electrically contains a large number of defects even if it is a single crystal. By. In addition, as for the upper side, the crystal defect density of the upper side is increased while the size of the ingot is increased, and the problem of lowering the characteristics of the solar cell produced on the upper side remains.

An object of the present invention is to provide a mold for casting a tantalum, a method for producing a tantalum material, a method for producing a tantalum material, and a solar cell, which are capable of suppressing a crystal defect density on an upper portion of an ingot, and which are capable of producing a solar cell having high cost efficiency. method.

In order to solve the above problems, the inventors of the present invention have conducted intensive studies. As a result, it has been found that at least the upper surface portion of the bottom plate portion of the mold is provided by the unidirectional solidification of the ruthenium melt from the bottom toward the upper portion in the mold. The above-mentioned problem can be solved by the release material layer of the additive having a particle diameter of 0.1 to 3.0 mm, and the present invention has been completed.

Thus, according to the present invention, there is provided a casting mold for casting, which is made by The melt is solidified, and at least the upper surface of the bottom plate portion of the inner wall of the casting mold is provided with a release material layer containing an additive having an average particle diameter of 0.1 to 3.0 mm.

Moreover, according to the present invention, there is provided a method of casting a tantalum, characterized in that: a tantalum melt is solidified in the above-mentioned cast casting mold; and a niobium material is produced by using the niobium casting method. A germanium material is obtained; and a method of manufacturing a solar cell, which uses the germanium material as a substrate to obtain a solar cell.

According to the present invention, it is possible to provide a mold for casting, a tantalum casting method, a method for producing a tantalum material, and a solar cell for manufacturing a solar cell capable of suppressing a crystal defect density on an upper portion of an ingot. method.

In the case where the mold for casting is provided only on the upper surface of the bottom plate portion, the mold for casting in the case of casting contains a material mainly composed of graphite or quartz (cerium oxide). When the additive is at least one selected from the group consisting of tantalum nitride, tantalum carbide, niobium oxide, and graphite as the main component, and the release material layer contains the additive at an areal density of 0.2 to 8.5/cm ² on the surface thereof. In the case of the above, the above effects are further exerted.

In the present specification, the term "upper part of the ingot" means the upper part of the twin ingot which is produced by solidifying the tantalum melt in the casting mold from the bottom to the upper side, that is, the side of the solidification step. . On the other hand, the lower portion of the twin ingot produced in the same manner, that is, the side of the solidification step, is referred to as "the bottom of the ingot".

In addition, the term "矽" means "矽", "矽" which is processed into a columnar shape from a tantalum ingot, and "矽 wafer" which is obtained by slicing a block.

Further, the term "a solar cell" means a "tank solar cell" which constitutes a minimum component, and a "solar cell module" which electrically connects the plurality of cells.

The casting mold for casting of the present invention is characterized in that the crucible melt is solidified, and at least the upper surface of the bottom plate portion of the inner wall of the casting mold is provided with an addition of an average particle diameter of 0.1 to 3.0 mm. a layer of release material.

The present inventors have evaluated, analyzed, and studied the crystal defects of a plurality of twin ingots which are unidirectionally solidified in a mold, and as a result, have found that the method of lowering the density of crystal defects on the upper side of the ingot has been considered as There are other methods that are effective and commonly used to reduce stress by suppression of temperature distribution.

Specifically, the inventors of the present invention found that the crystal grain size is small in the contrary to the technique described in Patent Documents 3 to 6 which suppresses the deterioration of the properties caused by the grain boundary by the coarsening of crystal grains. The polycrystalline germanium ingot is more stressful than the crystal grain size, and crystal defects are hard to occur.

According to the findings of the present inventors, (1) even in the vicinity of the polycrystalline germanium ingot, the crystal defect density of the particles having a larger crystal grain size and the smaller particles being introduced into the interior is largely different; (2) There is a correlation between the crystal grain size of the ingot and the crystal defect density on the upper side; (3) Although there are exceptions, the smaller the crystal grain size, the lower the crystal defect density on the upper side of the ingot. due to It is difficult to think that in the adjacent portion of the ingot, the thermal stress received by the ingot grows greatly, and it is considered that the portion having a smaller crystal grain size is crystallized in the grain by sliding of the grain boundary portion or the like. The stress is alleviated, and as a result, the introduction of crystal defects into the crystal grains is suppressed.

Therefore, when the tantalum melt is unidirectionally solidified in the mold to cast the twin ingot, the crystal grain size is reduced by promoting the generation of the crystal nucleus at the bottom of the mold, and the crystal on the upper side of the twin ingot can be reduced. defect.

In order to reduce the crystal defects on the upper side of the twin ingot, the thermal stress applied to the ingot is reduced in consideration of the flattening of the solid-liquid interface or the like, and in the present invention, only by crystallization The control of the crystal grain size in which the particle diameter becomes small can reduce the crystal defects on the upper side of the polycrystalline twin ingot. By using the mold of the present invention, the crystal grain size can be controlled to be small.

(矽 casting mold)

In the casting mold for casting of the present invention, at least the upper surface of the bottom plate portion of the inner wall of the casting mold is provided with a release material layer containing an additive having an average particle diameter of 0.1 to 3.0 mm.

In the mold for casting of the present invention, the material of the present invention is not particularly limited as long as the effect of the present invention is obtained, and a mold of the material used in the prior art can be used.

The casting mold for casting of the present invention preferably contains graphite or quartz (cerium oxide) as a main component in consideration of the influence of the impurities, the cost, the heat resistance, and the like of the mold-incorporated melt and the twin ingot. Material.

The shape and size of the mold are not particularly limited as long as the effect of the present invention can be obtained. For example, any of a rectangular shape and a cylindrical shape may be used as long as It is sufficient to have a size and shape of a twin ingot which can maintain the mash melt and solidify. Specifically, the mold used in the test examples can be cited.

In the mold for casting, generally, a mold release material layer is provided for taking out the twin ingot from the mold after casting, and in the present invention, the mold release material layer contains an additive having a specific average particle diameter.

According to the study by the inventors of the present invention, it has been found that the position where a specific additive exists in the release material layer is higher than the other portions when the ruthenium melt solidifies, and the position of the core (crystal nucleus) is higher. The bismuth melt in the mold is solidified unidirectionally from the bottom plate portion toward the upper portion to obtain a majority of the twin crystal system. Therefore, by introducing a specific additive into the release material layer of the upper surface portion of the bottom plate portion of the mold in which the initial nuclear generation is started, the probability of occurrence of nuclei can be increased, and the crystal grain size can be made small, and as a result It can suppress crystal defects on the upper side of the twin ingot.

In order to obtain the above effects effectively, the release material layer of the casting mold of the present invention is preferably provided only on the upper surface of the bottom plate portion of the inner wall of the casting mold. That is, it is preferable to provide a release material layer containing no additive on the side surface portion of the inner wall of the mold.

When the side wall portion of the inner wall of the mold is also provided with a layer of the release material containing a specific additive, the probability of occurrence of the side core is also high, and the crystal generated at the side core is separated from the side. Growing in the direction, the crystal grain boundary is inclined with respect to the upper and lower directions of the ingot. The peripheral portion of the crystal ingot in such a crystalline state is not preferably a cylindrical crystal for solar cells.

The material to be added is not particularly limited as long as it is in the range in which the effect of the present invention can be obtained, but it is considered that the self-additive is mixed into the bismuth melt and the bismuth ingot. The influence of impurities, cost, heat resistance, and the like is preferably at least one selected from the group consisting of tantalum nitride, tantalum carbide, niobium oxide, and graphite.

The average particle size of the additive is 0.1 to 3.0 mm.

When the average particle diameter of the additive is within the above range, it is expected that the output of the tantalum solar cell produced by using the twin ingot produced by the casting mold of the present invention is improved. This case means that the state of the crystals in the ingot is good, meaning that a twin ingot which can be used not only for a solar cell but also for other purposes is provided. A more preferred range is from 0.3 to 2.8 mm, and a preferred range is from 0.8 to 2.2 mm.

The specific particle diameter (mm) of the specific additive is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.5, 2.0, 2.5, 3.0.

The release material layer preferably contains an additive at a surface density of 0.2 to 8.5 /cm ² on the surface thereof.

If the areal density is less than 0.2/cm ² , the effect of controlling the crystal grain size of the crucible is lowered. On the other hand, when the areal density exceeds 8.5/cm ² , the crystal grain size of the crucible becomes too small, and although the crystal defect density in the upper portion of the ingot can be suppressed to be low, there is a re-bonding of the electron hole pair. The crystal grain boundary of the center becomes excessive. Such a crystal ingot is inferior in terms of conversion efficiency of the solar cell, and the surface density of the additive is in the range of an optimum value, more preferably in the range of 2.0 to 6.5 / cm ² , and further preferably The range is 2.5~6.0/cm ² .

The specific areal density (pieces/cm ² ) is ^{2.0, 2.5} , 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0.

The cast mold for casting of the present invention can be produced by a known method, and is not particularly limited. For example, it can be produced in the following manner.

In other words, the main material of the release material layer and the additive are dispersed in a solvent to prepare a slurry having a solid concentration of about 1 to 5%, and the obtained slurry is applied onto the inner surface of the mold to obtain the obtained slurry. The film is dry. Calcination to obtain a layer of release material.

The powder of the main material and the amount of the powder of the additive may be appropriately set so that the surface density of the additive in the surface of the release material layer is within the above range.

As the main material of the release material layer, a material of a known release material can be used, and examples thereof include a tantalum nitride having an average particle diameter of 0.1 to 60 μm, ruthenium oxide, and a powder of the mixture.

The specific average particle diameter (μm) is 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10 15, 20, 25, 30, 35, 40, 45, 50, 55, 60.

The solvent is not particularly limited as long as it disappears in the baking step and does not adversely affect the release material layer. For example, polyvinyl alcohol is exemplified.

The coating, drying and calcining steps can be carried out by appropriately setting conditions by a known method, and in order to obtain a release layer of a specific film thickness, a plurality of coating and drying steps can be repeated.

The calcination temperature and the time are also determined depending on the material to be used or the film thickness of the layer of the release material to be formed, and are about 800 to 1,100 ° C and about 1 to 8 hours.

The specific calcination temperature (°C) is 800, 825, 850, 875, 900, 925, 950, 975, 1000, 1025, 1050, 1075, 1100, and the specific calcination time (time) is 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8.

The film thickness of the release material layer is about 150 to 600 μm.

When the film thickness of the release material layer is thinner than the particle diameter of the additive, the effects of the present invention are remarkably obtained. On the contrary, when the film thickness of the release material layer is thicker than the particle diameter of the additive, the additive is buried inside the release material layer, and the surface shape of the release material layer is not reflected by the shape of the additive, and becomes a nuclear generating site. The density of the convex portion of the additive is reduced, so that it is remarkably difficult to obtain the effects of the present invention.

The specific film thickness (μm) is 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475, 500, 525, 550, 575, 600.

Therefore, in order to obtain the effect of the present invention while ensuring the film thickness of the release material layer, it is also possible to apply only the slurry containing the additive to form a release material layer having a specific film thickness, but it may also be coated by coating. After the slurry containing only the main material is not contained, the slurry containing the additive is applied to form a release material layer containing a plurality of layers.

As described above, in the tantalum ingot for solar cells, since the side core of the mold is poor in terms of battery characteristics, the release material layer is preferably provided only on the bottom plate portion of the mold. surface. In this case, a release material layer containing an additive may be formed only on the upper surface of the bottom plate portion of the mold, and a release material layer containing no additive may be formed on the side surface of the mold.

(矽 casting method)

The tantalum casting method of the present invention is characterized in that the tantalum melt is solidified in the tantalum casting mold of the present invention. According to this method, a twin ingot suitable for a cost-effective solar cell can be manufactured.

The crucible melt in the casting mold can melt the solid crucible material in the mold or can flow the crucible melt into the mold.

Further, the solidification of the cerium melt is not particularly limited as long as it is within the range in which the effects of the present invention can be obtained, and appropriate conditions may be set depending on the apparatus to be used.

The tantalum casting method of the present invention can be carried out, for example, by using a known apparatus as shown in Fig. 4, but the present invention is not limited to the embodiment.

Fig. 4 is a schematic cross-sectional view showing an example of a casting apparatus to which the crucible casting method of the present invention is applicable.

This apparatus is generally used for casting a polycrystalline germanium ingot, and has a chamber (closed container) 7 constituting a resistance heating furnace.

A mold 1 made of graphite or quartz (SiO ₂ ) or the like is disposed inside the chamber 7, and the environment inside the chamber 7 can be kept in a sealed state.

A mold base 3 made of graphite supporting the mold 1 is disposed in the chamber 7 in which the mold 1 is housed. The casting table 3 is lifted and lowered by the elevation drive mechanism 12, and the refrigerant (cooling water) in the cooling tank 11 is circulated inside.

A mold 2 other than graphite or the like is disposed on the upper portion of the mold table 3, and a mold 1 is disposed therein. Instead of the outer mold 2, an outer cover such as a graphite made of a mold 1 may be disposed.

Arranging resistance heating such as graphite heater in a manner of surrounding the outer mold 2 The body 10 is further provided with the heat insulating material 8 so as to cover the above.

The electric resistance heating body 10 can be heated from the periphery of the mold 1 to melt the crucible material 4 in the mold 1. Here, the heating element is a resistance heating method, but may be an induction heating method.

In order to detect the temperature of the bottom surface of the mold 1, the mold lower thermocouple 5 is placed near the center of the lower surface of the mold 1, and the outer mold type lower thermocouple 6 is placed near the center of the lower surface of the outer mold. The output is input to the control device 9, and the heating state of the heating body 10 is controlled by the electric resistance. A thermocouple or a radiation thermometer for detecting temperature may be disposed in addition to the thermocouple, and the thermocouple installation position is not particularly limited.

The chamber 7 can maintain the inside of the chamber 7 in a sealed state without flowing external oxygen, nitrogen, or the like. Usually, after the raw material such as polycrystalline germanium is introduced and before it is melted, the inside of the chamber 7 is evacuated, and then argon is introduced. An inert gas such as gas remains in an inert environment.

With such a device, the polycrystalline germanium ingot is basically cast by the following steps: filling of the raw material 4 of the mold 1, filling by degassing (vacuumization), and introduction of an inert gas. The internal gas replacement, the melting of the raw material 4 by heating, the confirmation of the melting, the holding, the solidification start by the temperature control and the operation of the elevation drive mechanism 12, the completion of the curing, the annealing, and the ingot removal. In addition, although an example of a device in which a raw material of a tantalum is heated and melted in a mold is exemplified here, the niobium raw material may be heated and melted in a crucible or the like different from the mold for casting, and the niobium is heated and melted. The melt is injected into a device of the type used in the casting mold, and the like.

(矽 material)

The tantalum material of the present invention is produced by the tantalum casting method of the present invention.

In other words, the crucible material of the present invention is, for example, a "polycrystalline ingot" produced by solidifying a crucible melt in a casting mold of the present invention, and cutting the "polycrystalline ingot" into a prismatic shape. "Polycrystalline germanium block" and "polycrystalline germanium wafer" obtained by processing the "polycrystalline germanium block" into a slice. These materials are suitable for materials of solar cells with higher cost effectiveness.

The "polycrystalline germanium block" can be obtained, for example, by processing the above-mentioned "polycrystalline germanium ingot" into a prismatic shape and a desired size by using a known device such as a band saw.

During processing, the surface portion of the polycrystalline germanium ingot in which the impurities such as the mold material are diffused may be cut, and the surface of the polycrystalline germanium block may be polished as needed.

The "polycrystalline germanium wafer" can be obtained, for example, by slicing the above-mentioned "polycrystalline germanium block" into a desired thickness by using a known device such as a multi-wire cutting machine.

In the current situation, the thickness is usually about 170 to 200 μm, but there is a tendency to reduce the thickness in order to reduce the cost. Moreover, the surface of the polysilicon wafer can also be polished as needed.

(矽 solar battery)

The tantalum solar cell of the present invention is produced by using the tantalum material (polycrystalline silicon wafer) of the present invention as a substrate.

There are various constructs in the solar cell, which can be fabricated by the known solar cell process using the germanium wafer of the present invention. For example, in the case of a germanium wafer doped with a p-type impurity, an n-type impurity (for example, phosphorus) is diffused (doped) from the surface to form an n-type layer to form a pn junction, and a surface electrode and a back electrode are formed. A polycrystalline solar cell unit is obtained. Similarly, in the case of a germanium wafer doped with an n-type impurity, a p-type impurity (for example, boron) is self-surfaced. Diffusion (doping) forms a p-type layer to form a pn junction, and a surface electrode and a back surface electrode are formed to obtain a polycrystalline silicon solar cell. Alternatively, in addition to the pn junctions of the ruthenium, there is also a MIS (Metal Insulator Semiconductor) type solar cell which is formed by sandwiching a thin insulating layer and vapor-deposited metal, for example, manufacturing and twinning. A thin film of amorphous or the like having the opposite conductivity type is a p-type or n-type germanium heterojunction having a different structure. Moreover, a polycrystalline germanium solar cell module is obtained by electrically connecting a plurality of them.

As described above, in the present specification, the concept of "solar battery unit" and "solar battery module" is simply referred to as "solar battery". Therefore, for example, if it is described as "polycrystalline solar cell", it means "polycrystalline solar cell" and "polycrystalline solar cell module".

[Examples]

Hereinafter, the present invention will be specifically described based on the test examples, but the present invention is not limited by the test examples.

(Test Example 1) Study on the average particle diameter of the additive contained in the release material layer

A polycrystalline germanium ingot is produced by coating a casting mold 1 made of quartz, which is formed of a release material layer containing an additive of various materials having a specific average particle diameter, on the upper surface of the mold base portion. The ingot is processed into a polycrystalline silicon wafer, and a solar cell is fabricated using the obtained wafer, and the relationship between the output of the obtained solar cell and the average particle diameter of the additive is evaluated.

Niobium nitride having a particle diameter of 1 to 60 μm as a main material of the release material layer The powder was mixed with a 3% polyvinyl alcohol aqueous solution in a weight ratio of 1:1, and the powder of the additive shown in Table 1 was added and dispersed to obtain a slurry A.

In addition, the amount of the powder of the additive is appropriately set so that the surface density of the additive becomes about 3.0 / cm ² as follows.

Similarly, the tantalum nitride powder and the 3% polyvinyl alcohol aqueous solution were mixed at a weight ratio of 1:1, and dispersed to obtain a slurry B.

The side surface of the mold was coated with only the slurry B and dried at about 50 ° C, and the obtained coating film was baked at 900 ° C for 2 hours to form a release film having a film thickness of about 250 μm on the side surface portion of the mold. Material layer.

The release material layer in which the coating method is referred to as "surface layer" in Table 1 includes a layer containing no additive and a layer containing an additive, and is formed in the following manner.

The obtained slurry B is applied only to the upper surface of the bottom plate portion of the mold and dried at about 50 ° C, and then the obtained slurry A is applied only to the upper surface of the bottom plate portion of the mold and is about The film was dried at 50 ° C, and the obtained coating film was baked at 900 ° C for 2 hours to form a release material layer having an areal density of about 3.0 /cm ² and a film thickness of about 250 μm.

On the other hand, the release material layer in which the coating method is shown as "mixing" in Table 1 is composed of a layer containing an additive, and is formed in the following manner.

The obtained slurry A was applied only to the upper surface of the bottom plate portion of the mold and dried at about 50 ° C, and the obtained coating film was baked at 900 ° C for 2 hours to form an areal density of about 3.0. A release material layer of /cm ² and a film thickness of about 250 μm.

Further, in order to obtain a specific film thickness, the slurry is applied and dried in an appropriate manner.

Further, the coating film on the side surface of the baking mold and the upper surface of the bottom plate portion are simultaneously performed.

Further, in the test example, the slurry A containing the tantalum nitride powder as the main material and the additive was used, but a slurry containing only the additive may be used in the same manner.

Further, although an example in which tantalum nitride is used as a main material of a mold release material is shown here, the present invention is not limited to tantalum nitride, and even if it is another material or a layer structure of tantalum nitride and tantalum oxide, It suffices that the effect of the present invention can be obtained.

On the graphite casting table 3 (880 mm × 880 mm × thickness 200 mm) surrounded by the heat insulating material 8 in the casting apparatus shown in Fig. 4, a cast outer mold 2 is provided (internal size: 900) Mm × 900 mm × height 460 mm, wall thickness of the bottom plate and side wall thickness 20 mm), in which the mold 1 made of quartz is provided (internal size: 830 mm × 830 mm × 420 mm, floor thickness and side wall thickness 22 Mm). Moreover, the thermocouples 5 and 6 for temperature measurement were respectively provided in the vicinity of the center of the lower surface of the mold 1 and the vicinity of the center of the lower surface of the outer mold 2, respectively.

Then, the concentration of boron is adjusted so that the specific resistance of the ingot is about 1.4 Ωcm, and 420 kg of the raw material 4 is placed in the mold 1, and the chamber 7 of the apparatus is vacuum-treated with argon gas. Perform the replacement. Thereafter, as a heating member of the apparatus, the crucible raw material 4 is melted by using a resistance heating body (graphite heater) 10 and a control device 9 located beside the mold, and after confirming melting of all the raw materials, the control device 9 and the cooling tank 11 are used. Lifting drive mechanism 12. The crucible is solidified in one direction under specific conditions (solidification speed of about 8 mm/hour).

Each of the obtained polycrystalline germanium ingots was processed into a polycrystalline germanium block (156 mm × 156 mm × 200 mm) using a band saw, and then sliced using a wire saw to obtain a polycrystalline germanium wafer (156 mm × 156 mm × thickness 0.18). Mm) About 12,000 pieces.

The obtained polycrystalline silicon wafer was put into a usual solar cell process, and about 12,000 solar cells (outer shape: 156 mm × 156 mm × thickness 0.18 mm) were produced for each ingot, and the output (W) thereof was measured. The average value of the output was calculated in units of each ingot, and the results of the standardization are shown in Table 1.

The standardization system uses the average output of the solar battery cells obtained from the ingot obtained by forming the mold having the release material layer containing no additive, and calculates the ingot unit in the case where it is set to 100. The average of the calculated outputs.

It is considered that most of the causes of the decrease in the output of the solar cell in one ingot are in the case where the raw material of the crucible is sufficiently high-purity for the solar cell, the crystal defect on the upper side of the ingot, and the casting on the bottom side of the ingot. Any of the types and diffusion of impurities of the release material. Therefore, in the ingot which solidifies unidirectionally under specific conditions, since the solidification time is substantially fixed, there is no significant difference in impurity diffusion on the bottom side of the ingot, and the output of the solar battery cell obtained by comparing each ingot can be compared. The crystal defects on the upper side of the ingot were evaluated on average.

Further, when the particle diameter of the additive contained in the release material layer is large, when the block of the polycrystalline germanium ingot is processed, the stress is considered to be on the bottom side. Caused by the breakdown. The presence or absence of this rupture is also shown in Table 1.

Further, Fig. 1 shows the relationship between the particle diameter of the additive contained in the release material layer of the casting mold for casting and the average output of the solar battery cells.

From the results of Table 1 and Figure 1, it can be seen that the average output of the solar cell is independent of whether the material of the additive is tantalum carbide, tantalum oxide, tantalum nitride or graphite, depending on the average particle diameter of the additive. That is, in the case where the average particle diameter of the additive is in the range of 0.1 to 3.0 mm, the average output is improved as compared with the case where the release material layer containing no additive is formed. Especially for the average of additives When the particle size is in the range of 0.3 to 2.8 mm, the relative average output is increased by 0.5% or more, and when the average particle diameter of the additive is in the range of 0.8 to 2.2 mm, it becomes a relative average output. The improvement rate is preferably 1.0% or more. Therefore, it is understood that the output of the solar cell module fabricated using the solar cell having such an average output is improved.

Further, in the case where the average particle diameter of the additive is in the range of 0.1 to 3.0 mm, the crack on the bottom side of the flawless block is good.

(Test Example 2) Study on the areal density of the additive contained in the release material layer

A casting mold 1 made of quartz, which is formed of a release material layer having an additive surface material having an additive surface density of a specific additive and having an average particle diameter of 0.8 mm, is applied only to the upper surface of the mold base portion. In the same manner as in Test Example 1, a polycrystalline germanium ingot was produced, a polycrystalline silicon wafer was processed from the obtained ingot, and a solar cell was fabricated using the obtained wafer, and the surface density of the output of the obtained solar cell and the additive was evaluated. Relationship.

The results obtained are shown in Table 2 and Figure 2. Fig. 2 shows the relationship between the areal density of the additive contained in the release material layer of the casting mold for casting and the average output of the solar battery cells.

From the results of Table 2 and Figure 2, it is known that the average output of the solar cell unit is independent of the material of the additive and the coating method of the release material layer, depending on the areal density of the additive. That is, in the case where the surface density of the additive is in the range of 0.2 to 8.5 /cm ^{2 ,} the average output is improved as compared with the case where the release material layer containing no additive is formed. In particular, when the surface density of the additive is in the range of 2.0 to 6.5/cm ² , the increase rate of the average output is 0.5% or more, and the surface density of the additive is 2.5 to 6.0 pieces/cm ² . In the case of the range, it is a preferable result that the increase rate of the average output is 1.0% or more. Therefore, it is understood that the output of the solar cell module fabricated using the solar cell having such an average output is improved.

Further, in Test Example 2, since a relatively small additive having an average particle diameter of 0.8 mm was used, the crack on the bottom side of the non-twisted block was excellent.

1‧‧‧ casting

2‧‧‧Outer mold

3‧‧‧ cast table

4‧‧‧矽 Raw materials

5‧‧‧Moulding thermocouple

6‧‧‧Outer mold thermocouple

7‧‧‧ chamber

8‧‧‧Insulation materials

9‧‧‧Control device

10‧‧‧Resistive heating body (graphite heater)

11‧‧‧Cooling trough

12‧‧‧ Lifting drive mechanism

Figure 1 is a view showing an additive contained in a release material layer of a casting mold for casting. Graph of the relationship between the particle diameter and the average output of the solar cell (Test Example 1).

Fig. 2 is a graph showing the relationship between the areal density of the additive contained in the release material layer of the casting mold for casting and the average output of the solar battery cells (Test Example 2).

Fig. 3 is a conceptual diagram showing the relationship between the position of the height direction of a usual polycrystalline germanium ingot produced by unidirectional solidification of a crucible melt and the average output of the produced solar cell.

Fig. 4 is a schematic cross-sectional view showing an example of a casting apparatus to which the crucible casting method of the present invention is applicable.

Claims (8)

A casting mold for casting a crucible, wherein a mold release material containing an additive having an average particle diameter of 0.1 to 3.0 mm is provided on at least an upper surface of an inner wall of the inner wall of the casting mold. Floor. The casting mold according to claim 1, wherein the release material layer is provided only on the upper surface of the bottom plate portion. The mold for casting according to claim 1, wherein the mold for casting the tantalum includes a material mainly composed of graphite or quartz (cerium oxide). The casting mold according to claim 1, wherein the additive is mainly composed of at least one selected from the group consisting of tantalum nitride, tantalum carbide, niobium oxide, and graphite. The mold for casting according to claim 1, wherein the surface of the release material layer contains the above additive at an areal density of 0.2 to 8.5 /cm ² . A tantalum casting method for solidifying a tantalum melt in a casting mold as claimed in claim 1. A method of producing a tantalum material obtained by using a tantalum cast by a tantalum casting method as claimed in claim 6. A method of manufacturing a solar cell, which is obtained by using a tantalum material manufactured by the manufacturing method of claim 7 as a substrate.