JP2005200279A - Method for manufacturing silicon ingot and solar battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a silicon ingot containing oxygen in a low concentration by using a reducing gas except hydrogen. <P>SOLUTION: The method for manufacturing the silicon ingot has a process (1) for melting a silicon material and a process (2) for crystallizing the molten silicon material. In either of the processes mentioned above, the molten silicon material is exposed to a reducing gas (e.g., a mixed gas of ammonia and an inert gas) not substantially containing hydrogen. The concentration of oxygen contained in the molten silicon material can be reduced by exposing the molten silicon material to the reducing gas. Consequently, the silicon ingot in which the concentration of oxygen is low can be manufactured. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a silicon ingot and a solar cell formed using a wafer obtained by slicing the ingot.

  First, an example of a conventional method for producing a silicon polycrystalline ingot will be described. First, a crucible filled with a silicon lump is placed in a metal sealed container having a heater for heating and melting the silicon lump as a raw material, and the temperature is raised to a temperature at which the silicon lump is melted. Next, by gradually cooling the melted silicon, the silicon begins to crystallize in the crucible, and the silicon crystallization ingot is formed by completing the entire crystallization.

  At this time, the sealed container is filled with argon gas which is an inert gas. However, since impurities are mixed in from the raw material silicon lump, crucible material, mold release agent, ingot production equipment, work environment, etc., various impurity elements such as silicon oxide are in high concentration in the molten silicon that has been heated and melted. Exists. In the cooling and crystallization process of the molten silicon, the molten silicon is generally cooled from the lower part of the crucible, so that crystallization starts from the lower part of the molten silicon and a silicon polycrystal grows toward the upper part. In the crystal growth process, the silicon purity is improved by the segregation phenomenon, and oxides and impurities are segregated at a high concentration at the top of the ingot to be crystallized at the end. Oxide and impurities of a concentration remain.

  These oxides and impurities become crystal defects in the silicon crystal, and in solar cells fabricated using wafers sliced from the ingot, carriers generated by light irradiation disappear at the defect, so excellent output characteristics Cannot be obtained.

In order to reduce oxides and impurities in the silicon polycrystalline ingot, it is necessary to reduce the oxygen concentration and impurities in the molten silicon. As this method, a method for producing a polycrystalline silicon ingot by cooling and crystallizing a silicon melt in a hydrogen gas atmosphere or an inert gas atmosphere containing hydrogen is known (for example, see Patent Document 1).
JP 58-99115 A

  However, in general, hydrogen is very flammable, and there is a risk of explosion in a wide range of hydrogen gas concentration in the air of 4% to 75%. As an atmospheric gas, hydrogen gas or inert gas and hydrogen gas It is not preferable to use a mixed gas.

  This invention is made | formed in view of the situation which concerns, and provides the manufacturing method of a silicon ingot with low content oxygen concentration using reducing gas other than hydrogen gas.

  The method for producing a silicon ingot according to the present invention includes the steps of (1) melting a silicon material and (2) crystallizing the molten silicon material. In any of the above steps, the molten silicon material substantially contains hydrogen gas. It is characterized by being exposed to a reducing gas not contained in the above.

According to the present invention, when the molten silicon material is exposed to a reducing gas, the oxygen concentration in the molten silicon material can be reduced, and as a result, a silicon ingot having a low oxygen concentration can be manufactured.
Moreover, when the ingot manufactured by the method of the present invention is used, a solar cell with few crystal defects and excellent output characteristics can be manufactured.

  The method for producing a silicon ingot according to the present invention includes the steps of (1) melting a silicon material and (2) crystallizing the molten silicon material. In any of the above steps, the molten silicon material substantially contains hydrogen gas. It is characterized by being exposed to a reducing gas not contained in the above.

Examples of the silicon material include granular and lump silicon.
The melting of the silicon material can be performed, for example, by heating a crucible containing the silicon material to a temperature not lower than the melting point of silicon.
Crystallization of the molten silicon material can be performed, for example, by cooling the lower part of the crucible containing the molten silicon material. According to this method, a polycrystalline silicon ingot is usually obtained. The crystallization of the silicon material can be performed, for example, by immersing a seed crystal made of single crystal silicon in the molten silicon material in the crucible and gradually pulling up the seed crystal. According to this method, a single crystal silicon ingot is usually obtained. That is, “crystallization” includes both single crystallization and polycrystallization.

“Substantially free of hydrogen gas” means that it does not contain an amount of hydrogen gas that substantially contributes to reducing the concentration of impurities contained.
The reducing gas is composed of a gas that has a property of reducing the concentration of oxygen and various impurities in the molten silicon material. Specifically, for example, ammonia, hydrazine, carbon monoxide, silane compounds ( For example, it contains at least one of dichlorosilane, trichlorosilane), hexamethyldisilazane, methylamine, ethylamine, and hydrogen sulfide. The reducing gas may consist of only these gases, but may also consist of a mixed gas of these gases and an inert gas.

  From another viewpoint, the present invention provides a solar cell formed by using a silicon wafer obtained by slicing a silicon ingot manufactured by the above-described method.

Since the silicon ingot produced by the method of the present invention has a low concentration of impurities such as oxygen, when a wafer obtained by slicing the silicon ingot of the present invention is used, a solar cell with good photoelectric conversion efficiency is produced. be able to.
Before slicing the silicon ingot, the ingot may be divided into, for example, square columnar blocks each having a side of 100 mm or more and 200 mm or less. The thickness of the silicon wafer can be set to 500 μm or less, for example.

  As Example 1, a method for producing a polycrystalline silicon ingot will be described. FIG. 1 shows a manufacturing apparatus for carrying out the manufacturing method of this embodiment.

  The external appearance of this manufacturing apparatus is such that the entire melted part is covered with a cylindrical sealed container 1 made of stainless steel. Inside, there are a crucible 2 as a container for melting and crystallizing a silicon lump as a raw material, a support base 3 and a pedestal 4 for installing it. The pedestal 4 can be rotated by a pedestal driving device 5 in order to improve temperature uniformity during heating and cooling of the crucible 2. The pedestal driving device 5 is also provided with a mechanism for moving the pedestal 4 up and down. At the time of cooling, the lower part of the crucible 2 is cooled by moving the crucible 2 downward from the heating region by the heater 10, and the molten silicon 6 Crystallization proceeds from the bottom.

  In order to cool the lower portion of the crucible 2 and advance crystallization of the molten silicon 6, it is necessary to move the crucible 2 away from the heater 10 by lowering the crucible 2 and to remove heat from the lower portion of the crucible 2. Therefore, this manufacturing apparatus is provided with a mechanism for cooling the pedestal 4. The cooling mechanism is configured to circulate a temperature-controlled cooling medium inside a cylindrical support rod 7 that supports the pedestal 4, and a cooling medium tank 8 that supplies the cooling medium to the outside of the sealed container 1. Is provided.

  In order to melt the silicon lump, a crucible 2 and a heating element 9 made of graphite for uniformly heating the silicon lump are provided so as to cover the crucible 2. An induction heating coil 10 for heating is attached. The temperature control of the heating body 9 is performed by feeding back the temperature measured by the thermocouple 11 embedded in the heating body 9. A cylindrical sealed container 1 made of SUS surrounds the crucible 2, the pedestal 4, the heating body 9, and the like, and a lid provided on the upper part of the sealed container 1 can be opened and closed. By opening the lid and raising and moving the heating element 9 in the lateral direction, it is possible to install the crucible 2 containing the silicon lump or take out the melted and crystallized polycrystalline silicon ingot. .

  Further, the lid portion of the sealed container 1 is provided with a gas inlet 12 for introducing various gases, and the bottom of the sealed container 1 is provided with a gas exhaust port 13 for discharging the gas. The atmosphere inside 1 is controlled. Atmospheric gas cylinders 15, 16, and 17 are connected to the gas inlet 12, and a regulator 14 that controls the gas flow rate, and an exhaust pump for reducing the pressure in the sealed container is connected to the gas outlet 13. Yes.

  Silica or graphite can be used as the material of the crucible 2, but in this embodiment, a crucible made of graphite is used. Its dimensions are the base of a substantially square with one side of about 60 cm, the height is about 50 cm, and the thickness is about 2 cm. In order to prevent the crucible 2 main body and the molten silicon 6 from reacting when the temperature is raised, silicon nitride powder is applied to the inner surface of the crucible 2, dried and sintered.

  The crucible 2 is placed on a support 3 and about 150 kg of silicon mass is filled therein. By preliminarily treating the silicon mass with acid, washing with water, and drying to remove contaminants on the surface and keeping it in a clean state, the contamination of the silicon crystals can be reduced as much as possible. The support 3 is lifted by a crane, the support 3 and the crucible 2 are installed at the center of the pedestal 4 in the sealed container, and then the heating body 9 is lowered and installed so as to surround the crucible 2.

  Next, the lid of the sealed container 1 is closed, the inside is decompressed by an exhaust pump, and the atmospheric gas is introduced into the sealed container 1 from the gas inlet 12. As the atmospheric gas, a mixed gas in which ammonia gas 15 and argon gas 16 are mixed at a ratio of 1: 4 is used, the gas flow rate is 10 to 20 liters / minute in total, and the atmospheric pressure is 0.5 to 0.95. Adjust the displacement so that it becomes atmospheric pressure.

  Next, the induction heating coil 10 is heated, and the crucible 2 and the silicon lump are heated and heated through the heating body 9. In order to obtain the temperature uniformity of the crucible 2, the silicon lump, and the silicon melt, the pedestal 4 is continuously rotated by the pedestal driving device 5 from heating to crystallization and cooling at a speed of 80 seconds / rotation. . Since the melting point of silicon is 1410 ° C., the temperature of the heating body 9 is raised to 1540 ° C., and the silicon lump is completely melted. Note that the temperature of the heating body 9 is controlled by feeding back the measured temperature from the thermocouple 11 embedded in the heating body 9 so that the rate of temperature rise is 400 ° C./hour. After holding the temperature of the heating element 9 at 1540 ° C. for 1 to 2 hours and the silicon melt temperature is homogenized, the silicon melt temperature is lowered to near its melting point by reducing the amount of current supplied to the induction heating coil 10. Thereafter, it is gradually cooled at a rate of 0.1 ° C./hour.

  At the same time, the pedestal 4 is moved downward from the heating body 9 by the pedestal driving device 5 at a speed of 7 mm / hour. When the pedestal is moved in this manner, the temperature at the bottom of the crucible 2 is lowered the fastest, so that silicon crystals are generated from the bottom of the crucible 2, and then the pedestal 4 is further lowered and the pedestal 4 is cooled to Polycrystals grow from the bottom to the top of the crucible 2. The crystallization time is approximately 20-25 hours.

  When the temperature of the silicon polycrystalline ingot is formed in the above process and the temperature is lowered to 900 ° C., the atmospheric gas in the sealed container 1 is switched from the mixed gas of the ammonia gas 15 and the argon gas 16 to the helium gas 17 and left at room temperature. Allow to cool. After cooling is completed, the hermetic container 1 is opened, and the crucible 2 and the polycrystalline silicon ingot 6 are taken out. Each end face of the silicon polycrystalline ingot thus produced can be cut and removed with a cut-and-sew, so that a high-quality ingot with less impurities can be produced.

  In the above-described embodiment, a mixed gas of ammonia gas 15 and argon gas 16 is used. Even when each gas and a mixed gas of them and an inert gas such as argon are used, it is possible to produce a high-quality silicon polycrystalline ingot with few impurities.

  A silicon polycrystalline ingot was manufactured by the method of Example 1, and a solar cell was manufactured using a wafer sliced from the ingot.

  Compared to a solar cell manufactured using a wafer sliced from an ingot manufactured by a conventional method, the carrier lifetime of this solar cell is improved by 57% from an average of 70 μs to 110 μs, and its photoelectric conversion efficiency is Under the condition of AM (Air Mass) 1.5, the average was improved from 14.1% to 15.2%.

The reason why the characteristics of the solar cell are improved in this way is explained as follows.
In general, contained impurities such as oxygen contained in the ingot become crystal defects in the silicon crystal. Therefore, in a solar cell manufactured using a wafer sliced from the ingot, carriers generated by light irradiation on silicon are trapped by the crystal defects, recombined, and disappear. Therefore, the carrier lifetime of this solar cell is shortened, and the photoelectric conversion efficiency is lowered.

  On the other hand, since the silicon polycrystalline ingot of the present invention is melted and crystallized in a reducing gas, the concentration of impurities such as oxygen contained in the molten silicon is lowered. Therefore, it is considered that the number of crystal defects that trap carriers generated by light irradiation is reduced, leading to improvement in carrier lifetime and photoelectric conversion efficiency.

  In addition, hydrogen atoms contained in ammonia in the atmospheric gas diffuse into the molten silicon melt and bond to crystal grain boundaries and crystal defect parts in the polycrystalline ingot, thereby inactivating the crystal defect parts and It is thought that the disappearance could be reduced.

Manufacturing apparatus for carrying out the method for manufacturing a silicon ingot according to the present invention Atmospheric gas piping system of manufacturing apparatus for carrying out the method for manufacturing a silicon ingot of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Airtight container 2 Crucible 3 Support base 4 Base 5 Base drive device 6 Molten silicon 7 Cylindrical support rod 8 Cooling medium tank 9 Heating body 10 Induction heating coil 11 Thermocouple 12 Gas inlet 13 Gas exhaust 14 Regulator 15 Ammonia gas cylinder 16 Argon gas cylinder 17 Helium gas cylinder

Claims (3)

(1) melting the silicon material; (2) crystallizing the molten silicon material, and in any of the above steps, the molten silicon material is exposed to a reducing gas substantially free of hydrogen gas. A method for producing a silicon ingot characterized by the above. The manufacturing method according to claim 1, wherein the reducing gas contains at least one of ammonia, hydrazine, carbon monoxide, a silane compound, hexamethyldisilazane, methylamine, ethylamine, and hydrogen sulfide. The solar cell formed using the silicon wafer obtained by slicing the silicon ingot manufactured by the manufacturing method of Claim 1 or 2.

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