JP2000327487A - Method and apparatus for producing crystalline silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce crystalline silicon having large crystal grain sizes. SOLUTION: In a method for producing crystalline silicon in which silicon molten liquid 10 in an ingot mold 1 is cooled from the bottom part 1a of the ingot mold 1 and gradually solidified toward the upper direction, the silicon molten liquid 10 at the bottom part is first solidified by cooling the bottom part 1a of the ingot mold 1 in the state that a positive temp. gradient is imparted in the horizontal direction. Then the molten liquid is gradually solidified toward the upper direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing crystalline silicon which cools a silicon melt and gradually solidifies it in one direction, and to an apparatus for producing crystalline silicon.

[0002]

2. Description of the Related Art A silicon-based solar cell, which occupies most of practically used solar cells, has a single crystal,
It is classified into polycrystalline and amorphous. Among them, polycrystalline silicon solar cells were initially researched and developed with the primary goal of cost reduction, and since then, much effort has been made toward higher efficiency, and as a result, the most produced today It is a solar cell. Single crystal silicon solar cells are superior in terms of high efficiency, but polycrystalline silicon solar cells are now able to achieve conversion efficiencies comparable to single crystal silicon solar cells due to improvements in various technologies. .

[0003] Since the power generation element (solar cell) of a polycrystalline silicon solar cell is made using a polycrystalline silicon substrate, its quality greatly affects the performance of the polycrystalline silicon solar cell. Therefore, various improvements have been made in the production of polycrystalline silicon.

[0004] The grain boundaries of the polycrystalline silicon substrate lead to shortening of the lifetime and mobility of the carriers in the solar cell, thereby lowering the energy conversion efficiency of the solar cell.
Therefore, in order to obtain a large energy conversion efficiency in a polycrystalline silicon solar cell, it is important in the production of polycrystalline silicon to reduce the crystal grain boundaries as much as possible, in other words, to increase the crystal grain size as much as possible. . The crystal grain size greatly depends on the method of manufacturing polycrystalline silicon.

A typical method for producing polycrystalline silicon is a method in which a silicon melt contained in a mold is gradually cooled from one direction of the mold and solidified. According to this method, a unidirectionally solidified polycrystalline silicon ingot having a crystal grain size of several mm or more sufficient for a solar cell wafer can be obtained. In some cases, a seed crystal is previously provided in a mold, and a crystal is grown using the seed crystal as a nucleus, thereby producing a silicon ingot having a larger crystal grain size.

[0006]

However, in order to achieve higher efficiency, polycrystalline silicon having a larger crystal grain size is desired, and for that purpose, some improvement is required in the above-mentioned method. It is. In this case, it is conceivable to use a seed crystal, but it is difficult to dissolve the raw material so as not to dissolve the seed crystal, and there is a disadvantage that the cost is increased.

The present invention has been made in view of the above circumstances, and provides a method of manufacturing crystalline silicon having a larger crystal grain size than the conventional method without using a seed crystal, and an apparatus for manufacturing crystalline silicon used in the method. The purpose is to provide.

[0008]

According to the present invention, there is provided a method for producing crystalline silicon in which a silicon melt contained in a mold is cooled from the bottom of the mold and gradually solidified upward. At the start of cooling, the bottom of the mold is cooled in a state where a positive temperature gradient is given in a horizontal direction from a part of the bottom of the mold to solidify the entire surface of the silicon melt at the bottom, and then gradually solidify in the upper direction. It is characterized.

That is, at the start of cooling, a positive temperature gradient from the bottom to the top of the mold by the cooling plate (hereinafter, referred to as a positive temperature gradient).
It is called the vertical temperature gradient. In addition, a positive horizontal temperature gradient (hereinafter referred to as a lateral temperature gradient) is provided from a part of the bottom of the mold to the bottom. It is a feature of the present invention to provide this lateral temperature gradient.

[0010] Crystal nuclei tend to be generated at the lowest temperature in the silicon melt. Then, the crystals in the silicon melt grow along the temperature gradient. In the case of the present invention, in the conventional method, crystal nuclei are dispersed and generated on the entire bottom surface of the mold, whereas crystal nuclei are generated on a part of the bottom of the mold, and the nuclei are generated in a horizontal direction along a lateral temperature gradient. By growing
Large crystal nuclei are generated at the bottom of the mold. Then, when the application of the horizontal temperature gradient is stopped and the temperature gradient is applied in the vertical direction, the crystal at the bottom of the silicon melt grows along the vertical temperature gradient to produce crystalline silicon.

The crystal grains of the crystalline silicon thus manufactured have a large diameter in the horizontal direction due to the growth along the lateral temperature gradient. That is, according to the method for manufacturing crystalline silicon according to the present invention, crystalline silicon having crystal grains having a large diameter in the horizontal direction can be manufactured.

It is preferable that the lateral temperature gradient is directed from the center of the bottom of the mold to the periphery. Such a lateral temperature gradient can be easily imparted by heating with an auxiliary heater provided on the outer periphery of the bottom of the mold.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a method for producing crystalline silicon and an apparatus for producing crystalline silicon according to the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a schematic front view of the periphery of a mold of a crystalline silicon manufacturing apparatus. In the drawing, reference numeral 1 denotes a mold placed inside a chamber (not shown), 2 denotes a main heater disposed above the mold, 3 denotes a cooling plate disposed below the mold, and 4 denotes a plate near the bottom of the mold. It is an auxiliary heater provided so as to surround the outer periphery in a ring shape. The presence of the auxiliary heater 4 is a feature of the apparatus for producing crystalline silicon of the present invention.

When producing crystalline silicon using the apparatus having such a configuration, first, solid silicon as a raw material is accommodated in the mold 1. Then, an inert gas flows into the chamber from above the chamber (not shown),
The solid silicon in the mold 1 is heated by the main heater 2 to make the solid silicon in the mold 1 a silicon melt 10. next,
After turning on the auxiliary heater 4, as shown in FIG. 1, the cooling plate 3 is pushed up by the shaft 5 attached to the cooling plate 3 to bring the cooling plate 3 into contact with the bottom 1 a of the mold 1. The cooling of the silicon melt 10 in the mold 1 is started.

At this time, since the silicon melt 10 is cooled from the bottom 1a of the mold 1 by the cooling plate 3 and is heated from the periphery by the auxiliary heater 4, the bottom 1a of the mold 1 There is a positive temperature gradient (the above-mentioned lateral temperature gradient) as schematically indicated by a broken-line arrow P in FIG. 2B. That is, since the center of the bottom 1a of the mold 1 has a lower temperature than the periphery thereof, FIG.
As shown schematically in FIG.
Occurs. At this time, it is more desirable that the portion to be cooled by the cooling plate 3 is only the central portion of the mold bottom 1a.

Since the generated crystal nucleus A is given the lateral temperature gradient P by heating by the auxiliary heater 4, the crystal nucleus A grows at a high growth rate along the temperature gradient by controlling the output of the auxiliary heater 4. . That is, as shown in FIG. 3, the silicon melt 10 at the bottom 1a solidifies to form the crystal B. Reference numeral 11 schematically shows a part of a crystal grain boundary of the crystal.

Thereafter, the auxiliary heater 4 is turned off to stop the heating to eliminate the horizontal temperature gradient, and the main heater 2 imparts a vertical temperature gradient, so that the crystal B is formed at the bottom 1.
As shown in FIG. 4, a crystal silicon ingot having columnar crystal grains C is grown along a positive temperature gradient from above a to the upper direction (the vertical temperature gradient).

As described above, in the method for producing crystalline silicon of the present invention in which a lateral temperature gradient is given at the start of cooling, first, the crystal is grown largely in the horizontal direction. It is a major feature that the crystalline silicon has columnar crystal grains having a large diameter.

In this case, the auxiliary heater is provided in a ring shape so as to surround the outer periphery near the bottom of the mold.
A positive temperature gradient was created radially from the center of the mold as the coolest part toward the periphery from there, but it was not necessarily surrounded by a ring, but a part of the outer periphery was opened to form a C-shape etc. May be provided with an auxiliary heater. In this case, a crystal nucleus is generated and grown at a position shifted in the radial direction from the center of the mold.

[0021]

EXAMPLE Crystal silicon was actually manufactured by the above-described crystal silicon manufacturing apparatus according to the present invention. A mold made of silica having a shape of 330 mm on a side and a height of 300 mm was used as a mold. The maximum output is 1 for the main heater.
The auxiliary heater used was a type of 00 kW, and the auxiliary heater used was of a type surrounding the entire periphery of the mold and having a maximum output of 10 kW. The crystal silicon obtained at this time had crystal grains having a maximum diameter of about 100 mm in the horizontal direction, and it was possible to produce crystal silicon having a larger crystal grain size than in the conventional method. Thus, the effect of heating the auxiliary heater at the start of cooling the silicon melt was confirmed.

[0022]

As described in detail above, according to the method for manufacturing crystalline silicon and the apparatus for manufacturing crystalline silicon according to the present invention, the following effects can be obtained.

In the method for producing crystalline silicon according to the present invention, crystal grains having a large diameter grow in the horizontal direction due to the horizontal temperature gradient applied to the bottom of the mold at the start of cooling. That is, crystalline silicon having a large crystal grain size can be manufactured. In addition, since crystal nuclei are easily generated at a part of the bottom of the mold, it is convenient to increase the crystal grain size and reduce the crystal grain boundaries.

In this case, if an auxiliary heater is provided in the vicinity of the bottom of the mold containing the silicon melt, a horizontal temperature gradient can be easily applied to the bottom of the mold at the start of cooling, and the crystal grain size can be reduced. Large crystalline silicon can be manufactured.

[Brief description of the drawings]

FIG. 1 is a schematic front view of the vicinity of a mold of an embodiment of a crystalline silicon manufacturing apparatus according to the present invention.

FIG. 2A is a schematic diagram when a crystal nucleus of silicon is generated. (B) It is a top view which shows typically the temperature gradient of the bottom part of the casting_mold | template at the time of FIG. 2 (a).

FIG. 3 is a schematic diagram in which the crystal nuclei in FIG. 2 have grown.

FIG. 4 is a schematic view showing crystalline silicon formed by growing the crystal in FIG.

[Explanation of symbols]

 Reference Signs List 1 mold 1a bottom 2 main heater 3 cooling plate 4 auxiliary heater 5 axis 10 silicon melt 11 crystal grain boundary A crystal nucleus B crystal C crystal silicon P temperature gradient

Claims (3)

[Claims] 1. A method for producing crystalline silicon in which a silicon melt contained in a mold is cooled from the bottom of the mold and gradually solidified in an upward direction, wherein at the start of the cooling, a part of the bottom of the mold is horizontal. A method for producing crystalline silicon, comprising: cooling a bottom portion of a mold in a state where a positive temperature gradient is applied in a direction to solidify a silicon melt at the bottom portion, and then gradually solidifying the silicon melt upward. 2. The method for producing crystalline silicon according to claim 1, wherein the positive temperature gradient is applied from the center to the periphery of the mold. 3. An apparatus for manufacturing crystalline silicon, comprising: a mold for containing a silicon melt; and a cooling plate below the mold for cooling the silicon melt, wherein an auxiliary heater is provided at an outer peripheral portion of a bottom of the mold. An apparatus for producing crystalline silicon, comprising: