JP5846437B2 - Method for producing silicon ingot - Google Patents

Description

  The present invention relates to a method for producing a silicon ingot for a solar cell, in which the quality in the vicinity of the final part of solidification does not deteriorate.

  Expectations for solar power generation are increasing to solve energy and environmental problems. Silicon is used as a main raw material for the solar cell which is the basic element. Silicon is a safe material that is not likely to be depleted, and is suitable as a material for expanding solar power generation on a global scale. The solar cell wafer can be obtained by cutting a silicon ingot made of single crystal silicon or polycrystalline silicon with a multi-wire saw or the like. Among them, a silicon ingot made of polycrystalline silicon can be manufactured by unidirectional solidification of a silicon melt in a large-capacity crucible, and therefore, the manufacturing cost is lower than that of single crystal silicon and has become the mainstream as a practical technique.

  However, polycrystalline silicon usually consists of many crystal grains having random crystal orientations, and crystal defects such as crystal grain boundaries and dislocations exist. Crystal grains having random crystal orientation are generated by heterogeneous nucleation at the bottom of the crucible. In addition, it is known that a crystal grain boundary between crystal grains having a random relative orientation relationship becomes a generation source of dislocations (for example, see Non-Patent Document 1). Since these crystal defects are recombination centers of carriers such as electrons and holes generated in silicon by light, the energy conversion efficiency of polycrystalline silicon solar cells is higher than that of single crystal silicon solar cells. Is also low.

  Therefore, research and development of high-quality silicon ingots with low crystal defect density and controlled crystal grain orientation and grain boundaries while taking advantage of the ability to manufacture large-capacity ingots has been conducted. ing. As a known technique for aligning the crystal grain orientation, there is a method of manufacturing a single crystal-like silicon ingot having a uniform growth orientation by arranging a seed crystal of single crystal silicon on the bottom of a crucible containing a silicon melt (for example, a patent) Reference 1).

  However, this method cannot suppress the generation of crystal grains having random orientations due to heterogeneous nucleation at the bottom of the crucible unless the entire bottom surface of the crucible is covered with one single crystal. Since there is no technique for manufacturing such a large single crystal, it is conceivable to arrange a plurality of single crystals having the same top surface orientation. However, it is impossible to perfectly align the relative orientations at the atomic level, and it is impossible to prevent the formation of small-angle grain boundaries that are the source of dislocations.

  In addition, there is a method of manufacturing a silicon polycrystal ingot having a uniform crystal grain orientation by expressing a dendrite crystal at the bottom of a crucible in the initial stage of crystal growth and growing it in one direction on the upper surface of the dendrite crystal ( For example, see Patent Document 2). However, with this method, it is difficult to cover the entire bottom of the crucible with only dendrite crystals, and it is impossible to prevent heterogeneous nucleation between adjacent dendrite crystals.

  It is possible to form the polycrystalline structure formed in the initial process of crystal growth with only a plurality of dendrite crystals by using the melt surface with a large interface energy and a low nucleation frequency as a nucleation site. It is known (see, for example, Non-Patent Document 2). In addition, it is also possible to control the structure using seed crystals by growing from the melt surface, and it is known that the crystal defect density can be reduced by floating the ingot in the melt during the growth process. (For example, refer to Patent Document 3). However, if the melt is completely solidified from the top to the bottom inside the crucible, the ingot and the crucible come into strong contact with each other due to expansion in the closed space in the final solidification process, and crystals are generated by applying stress near the bottom of the ingot. The occurrence of defects or leakage of the melt due to the crucible breakage.

I. Takahashi, N. Usami, K. Kutsukake, G. Stokkan, K. Morishita, and K. Nakajima, “Generation mechanism of dislocations during directional solidification of multicrystalline silicon using artificially designed seed”, J. Cryst. Growth, 2010, 312 , p.897-901 N. Usami, I. Takahashi, K. Kutsukake, K. Fujiwara, and K. Nakajima, “Implementation of faceted dendrite growth on floating cast method to realize high-quality multicrystalline Si ingot for solar cells”, J. Appl. Phys., 2011 Year, 109, p.083527

Japanese Patent Laid-Open No. 10-194718 Japanese Patent No. 4203603 Japanese Patent No. 4528995

  As described above, when the silicon melt in the crucible is grown from the top to the bottom, the crystal structure can be obtained by using seed crystals on the melt surface or covering the melt surface only with dendrite crystals. However, if the melt is completely solidified in the crucible, there is a problem that the crystal quality at the lower part of the ingot is lowered and the yield of the ingot is lowered.

  The present invention has been made paying attention to such problems, and in the method of crystal growth from the top to the bottom of the silicon melt, the application of stress to the ingot in the final process of solidification is suppressed, and the crystal orientation is controlled. Another object of the present invention is to provide a method for producing a silicon ingot having excellent crystal quality and high yield.

According to the present invention, a cylinder composed of a cylinder or a square cylinder is allowed to stand on the bottom plate or in the crucible,
The cylindrical body is filled with a silicon raw material, the cylindrical body is heated to a melting point of silicon or more to form a silicon melt in the cylindrical body, and then the entire cylindrical body is cooled to obtain the silicon melt. A silicon ingot manufacturing method is characterized in that the silicon melt is solidified from the top to the bottom of the liquid.

  Further, according to the present invention, there is obtained a method for producing a silicon ingot, wherein the silicon melt slightly oozes out from the lowermost gap of the cylindrical body in the final process of solidifying the silicon melt. It is done.

  Further, according to the present invention, a seed crystal is arranged on the upper part of the silicon raw material, and the silicon melt is solidified from the upper part to the lower part of the silicon melt from a state where the seed crystal is suspended in the silicon melt. Thus, a method for producing a silicon ingot can be obtained.

  Furthermore, according to the present invention, there is obtained a method for producing a silicon ingot, wherein the cylindrical body, the bottom plate, and the crucible are made of quartz or graphite.

  In the method for producing a silicon ingot according to the present invention, in order to solve the lowering of the crystal quality at the bottom of the silicon ingot, a double structure in which a cylinder without a bottom surface is arranged inside a normal crucible, or a cylinder or The structure is such that a square tube is left stationary. The silicon raw material is filled in the inner cylinder or the square cylinder. Further, a seed crystal for controlling the structure can be disposed on the silicon raw material. By setting the cylinder to a temperature equal to or higher than the melting point of silicon, all of the silicon raw material can be melted inside the cylinder. In addition, by arranging in a temperature distribution where the temperature is lower in the vertical direction in the vertical direction, all of the silicon raw material arranged in the lower part can be melted while keeping a part of the seed crystal arranged in the upper part in a solid state. . After that, when the silicon ingot is grown by lowering the temperature in a state where the temperature distribution is lower at the upper part in the vertical direction, the gap between the cylinder and the crucible or the bottom plate is caused by volume expansion in the final process of solidification. And the residual melt leaks to the outer crucible or the like. For this reason, strong contact between the ingot and the outer crucible or the like is suppressed, and the quality of the crystal does not deteriorate even at the lower part of the ingot. Further, as the material of the cylinder, the square cylinder, the bottom plate, and the crucible, those having a melting point equal to or higher than that of silicon and having little reaction with silicon are preferable. Specifically, quartz and graphite are preferable.

  According to the present invention, in a method of growing a silicon melt from the upper part to the lower part, the stress application to the ingot in the final process of solidification is suppressed, the crystal orientation is controlled, and the crystal quality is excellent and the yield is high. A method for producing a silicon ingot can be provided.

It is a model longitudinal cross-sectional view which shows the basic concept of the manufacturing method of the silicon ingot of embodiment of this invention. (A) It is a side view which shows the external shape of the ingot grown by the manufacturing method (this invention) of the conventional method (conventional method) and (b) silicon ingot of embodiment of this invention. (A) Conventional method (conventional method), (b) Plane showing the surface state of a wafer obtained by cutting the vicinity of the bottom of the ingot horizontally grown by the method for manufacturing a silicon ingot according to the embodiment of the present invention (the present invention) FIG. (A) Current voltage when a solar cell is produced using a wafer cut out from an ingot grown by a conventional method (conventional method), (b) method for producing a silicon ingot according to an embodiment of the present invention (the present invention) It is a graph which shows a curve. It is a perspective view which shows (a) cylinder and (b) square cylinder which can be used with the manufacturing method of the silicon ingot of embodiment of this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram showing a basic concept of a method for manufacturing a silicon ingot according to an embodiment of the present invention. In the method for producing a silicon ingot according to the embodiment of the present invention, as shown in FIG. 1A, an inner surface is placed in a crucible 1 having an inner diameter of 150 mm in which a release agent is applied to the inner bottom surface in an argon flow melting apparatus. A cylinder 2 having an inner diameter of 100 mm coated with a release agent was placed, and 500 g of silicon raw material was filled in the cylinder 2. The heater of the melting device is composed of three zones, upper, middle and lower. First, by setting all heater temperatures to 1490 ° C, the temperature of the silicon raw material is raised to 1450 ° C and the silicon raw material is completely melted. Thus, a silicon melt 3a was obtained.

  Next, as shown in FIG. 1 (b), the temperature gradient is set so that the upper part of the melt is lowered by lowering the upper heater to 1400 ° C., and then the entire temperature is set once per minute. By lowering at a rate, the silicon crystal 3b was grown from the upper part to the lower part. In the final process of solidifying the silicon melt 3a, a gap 4 is generated between the cylinder 2 and the crucible 1 due to volume expansion, and the residual melt 3c leaks to the outer crucible 1 as shown in FIG. And solidified. For comparison, as a conventional method, a silicon ingot was grown with the same temperature history even when a crucible with an inner diameter of 100 mm coated with a release agent was placed in a crucible with an inner diameter of 150 mm coated with a release agent.

  In FIG. 2, the external shape of the ingot grown by the manufacturing method (this invention) and the conventional method (conventional method) of the silicon ingot of embodiment of this invention is each shown. As shown in FIG. 2, it can be seen that in the conventional method, the bottom portion of the ingot is greatly deformed and a protrusion having a height of about 1 cm is formed in the lower portion. This indicates that in the final process of solidification, the silicon melt was solidified and expanded in a closed space, so that a large stress was generated to deform the crucible bottom. On the other hand, according to the present invention, it can be seen that the protrusion on the bottom of the ingot is 1 mm or less and is kept almost flat.

  FIG. 3 shows the state of crystal grains on the surface of a wafer obtained by cutting the vicinity of the bottom of the ingot shown in FIG. 2 in the horizontal direction. As shown in FIG. 3, it can be seen that in the conventional method, the crystal grains are refined at the bottom of the ingot, and there are many crystal grains having a grain size of about several mm. On the other hand, according to the present invention, it can be seen that crystals having a large grain size exceeding a few centimeters are obtained even at the bottom of the ingot.

  FIG. 4 shows a current-voltage curve when a solar cell is produced using a wafer cut out from the ingot shown in FIG. Five solar cells are produced for each ingot and all the characteristics are shown. As shown in FIG. 4, in the conventional method, the curve factor is reduced due to crystal defects, and the value of the curve factor shows a large variation of 0.56-0.74. On the other hand, in the present invention, the value of the fill factor is 0.68-0.74, which indicates that more uniform characteristics are obtained.

  FIG. 5 shows a cylinder and a square cylinder that can be used in the present invention. In the above-described embodiment, a cylinder as shown in FIG. 5A is used. However, the present invention is not limited to a cylinder, and a square cylinder as shown in FIG. 5B can also be used.

1 crucible 2 cylinder 3a silicon melt 3b silicon crystal 3c residual melt 4 gap

Claims (4)

Place a cylinder consisting of a cylinder or a square cylinder on the bottom plate or in a crucible,
Filling the cylinder with a silicon raw material,
Heating the cylinder above the melting point of silicon to form a silicon melt in the cylinder;
Thereafter, the entire cylindrical body is cooled to solidify the silicon melt from the upper part to the lower part of the silicon melt.   2. The method for producing a silicon ingot according to claim 1, wherein the silicon melt slightly oozes out from a gap at a lowermost portion of the cylindrical body in a final process of solidifying the silicon melt.   The seed crystal is disposed on the upper part of the silicon raw material, and the silicon melt is solidified from the upper part to the lower part of the silicon melt from a state where the seed crystal is suspended in the silicon melt. 3. A method for producing a silicon ingot according to 1 or 2. 4. The method for manufacturing a silicon ingot according to claim 1, wherein the cylindrical body, the bottom plate, and the crucible are made of quartz or graphite.