WO2013094245A1

WO2013094245A1 - Polycrystalline silicon ingot, process for producing same, and uses thereof

Info

Publication number: WO2013094245A1
Application number: PCT/JP2012/070001
Authority: WO
Inventors: 大石　隆一; 和也上野; 梶本　公彦
Original assignee: シャープ株式会社
Priority date: 2011-12-22
Filing date: 2012-08-06
Publication date: 2013-06-27
Also published as: TWI468563B; CN104010968A; CN104010968B; JP5135467B1; TW201326475A; JP2013129580A

Abstract

A process for producing a polycrystalline silicon ingot by unidirectionally solidifying molten silicon in a crucible upward from the bottom of the crucible, wherein the molten silicon is unidirectionally solidified under such conditions that when the temperature measured at a position in the vicinity of the center of the undersurface of the crucible falls from (Tm-20)°C to (Tm-60)°C, where Tm is the temperature measured at the position when the silicon temperature is the melting point of the silicon, then there is a period during which that temperature falls at a rate of 1-10 °C/hr, thereby obtaining the polycrystalline silicon ingot.

Description

Polycrystalline silicon ingot, method for producing the same, and use thereof

The present invention relates to a polycrystalline silicon ingot, its manufacturing method, and its use.

The use of natural energy is attracting attention as an alternative to oil, which is causing various problems in the global environment. Among them, the solar cell does not require a large facility and does not generate noise during operation, and thus has been particularly actively introduced in Japan and Europe.
Solar cells using compound semiconductors such as cadmium tellurium have also been put into practical use, but solar cells using crystalline silicon substrates are large in terms of the safety of the materials themselves, past achievements, and cost performance. Solar cells using a polycrystalline silicon substrate (polycrystalline silicon solar cells) occupy a large share.

A polycrystalline silicon wafer, which is generally widely used as a substrate for polycrystalline silicon solar cells, is an ingot manufactured by a method called a casting method in which molten silicon is unidirectionally solidified in a crucible to obtain a large polycrystalline silicon ingot. It is cut into blocks and made into wafers by slicing.
The polycrystalline silicon wafer manufactured by the casting method generally has a distribution in the output characteristics of the solar cell as shown in FIG. 5 depending on the position in the height direction in the ingot or block.

The cause of the characteristic distribution of FIG. 5 is generally explained as follows.
First, in the region I at the initial stage of unidirectional solidification, the characteristics deteriorate due to the influence of impurities diffused from the crucible. In the region II on the upper side, since the incorporation of impurities in the raw material due to segregation and the occurrence of crystal defects are few, the characteristics are the best in the block. Further, in the upper region III, the amount of impurities taken into the crystal gradually increases, the generation of crystal defects increases, and the characteristics are deteriorated as compared with the region II. Furthermore, in the upper region IV, similarly to the region III, in addition to further increasing the amount of impurities incorporated into the crystal and the generation of crystal defects, the upper surface portion was formed after the ingot solidified to the end. Impurity reverse diffusion occurs from the high concentration portion of the impurity, and the amount of the impurity further increases, so that the characteristic deterioration becomes more remarkable than in the region III.
In the above description, the influence of impurities in the raw material and impurities eluted from the crucible is considered, but even if there is no such influence, in regions III and IV, crystals that become minority carrier traps toward the top Since defects increase gradually, the characteristics of solar cells tend to deteriorate.

The cause of the occurrence of crystal defects is considered to be the stress caused by the temperature distribution in the ingot. From the viewpoint of suppressing this, the following two methods have been proposed.
First, for example, in Japanese Patent Application Laid-Open No. 2005-152985 (Patent Document 1), a mold holder that is installed at the lower part of a crucible during unidirectional solidification (casting) has a larger heat flux at the center than at the periphery. A method to use is proposed.
Secondly, for example, International Publication No. 2005/092791 (Patent Document 2) proposes a method of performing heat flow control during ingot growth with a structure that can change the heat receiving (heat exchange) area.

Further, as a measure for improving the quality of a polycrystalline silicon ingot different from the above method, a method aimed at increasing the particle size has been proposed.
For example, Japanese Patent No. 4203603 (Patent Document 3) and Japanese Patent Application Laid-Open No. 2005-132671 (Patent Document 4) disclose a dendrite as a crystal nucleus at the bottom of an ingot (at the beginning of solidification) by quenching the bottom of the crucible. There has been proposed a method of generating crystals and coarsening crystal grains.
Japanese Patent No. 4054873 (Patent Document 5) proposes a method of obtaining a pseudo single crystal by growing crystal pieces (undissolved) left in the melting step of silicon raw material and enlarging crystal grains. ing.
Furthermore, Japanese Patent No. 4569957 (Patent Document 6) proposes a method of obtaining a pseudo single crystal by heteroepitaxially growing silicon from a seed crystal such as SiC arranged with the crystal orientation aligned on the crucible bottom. .

Japanese Unexamined Patent Publication No. 2005-152985 International Publication No. 2005/092791 Japanese Patent No. 4203603 Japanese Unexamined Patent Publication No. 2005-132671 Japanese Patent No. 4054873 Japanese Patent No. 4569957

In the method of Patent Document 1, particularly when the heater is next to the crucible, the shape of the solid-liquid interface is further deteriorated, and there is a problem that effects such as reduction of crystal defect density and prevention of cracking cannot be obtained.
Although the method of Patent Document 2 can improve the controllability of heat removal from the crucible side wall, there is a problem that the device configuration is very complicated, there are many high-temperature movable parts, and the cost of the device increases and the failure increases. is there.

In the methods of Patent Documents 3 to 5, deterioration of characteristics due to grain boundaries can be suppressed by coarsening of crystal grains, and particularly when the ingot size is small, the stress due to temperature distribution is relatively small and is introduced on the top side of the ingot. There is an advantage that crystal defects can be suppressed to some extent. However, as the ingot size increases and crystal defects on the top side increase, the characteristics on the bottom side are improved, but the problem still remains that the characteristics of the solar cell fabricated on the top side deteriorate.

In the method of Patent Document 6, defects are formed at a boundary portion where silicon crystals grown from adjacent seed crystals such as SiC collide, and an ingot has many defects electrically even though it looks macroscopically. It is thought to include. On the top side, the ingot size increases and the crystal defect density on the top side increases, and the problem remains that the characteristics of the solar cell fabricated on the top side also deteriorate.

On the other hand, the larger the size of the polycrystalline silicon ingot, the lower the price per polycrystalline silicon wafer can be, so the size of the ingot tends to increase.
Therefore, in order to improve the performance and cost of the final polycrystalline silicon solar cell module, it is easy and low-cost to reduce the crystal defect density and prevent cracking in the production of large-sized polycrystalline silicon ingots. There is a need for a method that can be performed.

The present invention provides a method for producing a large-sized polycrystalline silicon ingot capable of reducing the crystal defect density and preventing cracks easily and at low cost, and the polycrystalline silicon ingot obtained thereby and use thereof. Is an issue.

As a result of intensive research, the inventors of the present invention have started solidification (crystal growth) when producing a polycrystalline silicon ingot by unidirectionally solidifying molten silicon in a crucible from the bottom to the top of the crucible. The present inventors have found that the above-mentioned problems can be solved by controlling the temperature at the bottom to conditions that promote the generation of crystal nuclei.

Thus, according to the present invention, there is a method for producing a polycrystalline silicon ingot by unidirectionally solidifying molten silicon in a crucible from the bottom of the crucible, and the center of the bottom surface of the crucible when the silicon temperature becomes the melting point of silicon. Conditions in which there is a time for the temperature to fall at a temperature change rate of 1 to 10 ° C./hour while the detected temperature is lowered from (Tm−20) ° C. to (Tm−60) ° C. with the detection temperature in the vicinity as Tm. Thus, there is provided a polycrystalline silicon ingot manufacturing method for obtaining a polycrystalline silicon ingot by unidirectionally solidifying the molten silicon.

Further, according to the present invention, there is provided a method for producing a polycrystalline silicon ingot by unidirectionally solidifying molten silicon in a crucible upward from the bottom of the crucible, and the center of the bottom surface of the crucible when the silicon temperature becomes the melting point of silicon. The temperature decreases at a temperature change rate of 1 to 10 ° C./hour while the detected temperature is lowered from (Tm′−20) ° C. to (Tm′−60) ° C. with the detected temperature at the lower 20 mm position in the vicinity as Tm ′. There is provided a method for producing a polycrystalline silicon ingot that obtains a polycrystalline silicon ingot by unidirectionally solidifying the molten silicon under conditions where there is time to perform.

Further, according to the present invention, a polycrystalline silicon ingot produced by the above-described polycrystalline silicon ingot producing method, a polycrystalline silicon block obtained by processing the polycrystalline silicon ingot, and processing the polycrystalline silicon block A polycrystalline silicon wafer obtained by the above and a polycrystalline silicon solar cell manufactured using the polycrystalline silicon wafer are provided.
In the present invention, the “solar battery” means a “solar battery module” in which a “solar battery cell” constituting a minimum unit and a plurality thereof are electrically connected.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the large-sized polycrystalline silicon ingot which can perform the reduction | decrease of a crystal defect density and prevention of a crack simply and at low cost, the polycrystalline silicon ingot obtained by that, and its use are provided. can do.
That is, according to the present invention, high-quality polycrystalline silicon ingots, blocks and wafers can be manufactured at low prices, and high-performance polycrystalline silicon solar cells with high cost performance can be supplied to the market. .

In the method for producing a polycrystalline silicon ingot according to the present invention, when the temperature change rate is 2 to 7 ° C./hour, the time during which the temperature decreases at the temperature change rate of 1 to 10 ° C./hour is 20% or more. When present, the above effects are particularly exerted.

It is a figure which shows the temperature change of the crucible lower surface center vicinity at the time of polycrystalline silicon ingot manufacture (Test Example 2). It is a figure which shows the relationship between the output of a solar cell, and the temperature change rate at the time of polycrystalline silicon ingot manufacture (Test Example 1). It is a figure which shows the relationship between the output of a solar cell, and the occupation rate of the temperature change rate at the time of polycrystalline silicon ingot manufacture (Test Example 2). It is a schematic sectional drawing which shows an example of the apparatus used for the manufacturing method of the polycrystalline semiconductor ingot of this invention. It is a conceptual diagram which shows the relationship between the position of the height direction of a common polycrystalline silicon ingot, and the output of the produced solar cell.

The method for producing a polycrystalline silicon ingot according to the present invention is a method for producing a polycrystalline silicon ingot by unidirectionally solidifying molten silicon in a crucible from the bottom of the crucible, and when the silicon temperature becomes the melting point of silicon. When the detected temperature near the center of the bottom of the crucible is Tm, the time during which the temperature decreases at a temperature change rate of 1 to 10 ° C./hour while the detected temperature decreases from (Tm-20) ° C. to (Tm-60) ° C. A polycrystalline silicon ingot is obtained by unidirectionally solidifying the molten silicon under existing conditions.

When the raw material silicon is melted in the crucible, the detected temperature “Tm” is equal to the silicon melt temperature immediately before the completion of silicon melting, and the detected temperature near the bottom of the crucible is almost the same. It can be determined as the detected temperature when it becomes constant. In this state, the silicon melt temperature is the melting point of silicon, and Tm is a detected temperature near the center of the bottom surface of the crucible at that time. Since the crucible base is always cooled, it is considered that Tm is several degrees lower than the melting point of silicon. The actual measured absolute value of Tm varies slightly due to the thermocouple calibration method, the degree of deterioration, individual differences, variation in installation in the apparatus, etc., and the error in the actual measured absolute value is large. However, by correcting the detected temperature in the vicinity of the center of the bottom surface of the crucible with reference to Tm, the above-mentioned variation factor can be eliminated, and the reproducibility of the crystal growth conditions can be ensured. The measured absolute value of Tm in this example was also found to be higher than the melting point of silicon for the reasons described above, and was in the range of 1407 ° C. to 1418 ° C.
In the case of a polycrystalline silicon ingot manufacturing system that pours silicon melt into a crucible, for example, by measuring the temperature of the melt with a radiation thermometer and correlating it with the detected temperature near the center of the bottom of the crucible It is possible to determine the “Tm”.

The method for producing a polycrystalline silicon ingot according to the present invention is a method for producing a polycrystalline silicon ingot by unidirectionally solidifying molten silicon in a crucible upward from the bottom of the crucible, and the silicon temperature becomes the melting point of silicon. The detected temperature at the lower 20 mm position near the center of the bottom surface of the crucible at the time is Tm ′, and the detected temperature decreases from (Tm′−20) ° C. to (Tm′−60) ° C. A polycrystalline silicon ingot is obtained by unidirectionally solidifying the molten silicon under a condition in which time for temperature decrease at a rate of change exists.

The detected temperature “Tm ′” can be determined in the same manner as “Tm”, except that the temperature measurement point is different. Here, the position 20 mm below the lower surface of the crucible is selected as the temperature measurement point. However, the temperature measurement point can be a temperature measurement point as long as it is thermally connected to the crucible and correlates with the temperature of silicon in the crucible.

As a result of evaluating, analyzing and examining crystal defects for a large number of polycrystalline silicon ingots, the present inventors have been considered to be effective as a method for reducing the crystal defect density on the top side of the ingot, and It has been found that there is a completely different method other than the stress reduction by suppressing the temperature distribution that is commonly used.
Specifically, the present inventors have developed a polycrystal having a small crystal grain size, which is completely opposite to the techniques described in Patent Documents 3 to 6 in which deterioration of characteristics due to grain boundaries is suppressed by coarsening of crystal grains. It has been found that a silicon ingot is more resistant to stress and less prone to crystal defects than a silicon ingot having a large crystal grain size.

According to the knowledge of the present inventors, (1) the density of crystal defects introduced into the inside is large between the grains having a large crystal grain size and the grains having a small crystal grain size even in the portion immediately adjacent to each other in the polycrystalline silicon ingot. Unlikely, (2) there is a correlation between the crystal grain size of the ingot and the crystal defect density on the top side. (3) Although there are exceptions, the smaller the crystal grain size, the lower the crystal defect density on the top side of the ingot. Since it is difficult to think that there is a large difference in thermal stress applied during ingot growth between adjacent parts in the ingot, the stress received in the crystal grains is mitigated due to slipping of the grain boundary part, etc. As a result, it is considered that the introduction of crystal defects into the crystal grains can be suppressed.

Therefore, when producing a polycrystalline silicon ingot by unidirectionally solidifying molten silicon in the crucible, the top of the polycrystalline silicon ingot is reduced by promoting the generation of crystal nuclei at the bottom of the crucible and reducing the crystal grain size. The side characteristic deterioration can be relieved.
In order to reduce crystal defects on the top side of a polycrystalline silicon ingot, it has been thought that it is necessary to reduce the thermal stress applied to the ingot, such as flattening of the solid-liquid interface. The crystal defects on the top side of the polycrystalline silicon ingot can be reduced only by controlling the crystal grain size to reduce the crystal grain size.

The polycrystalline silicon ingot production apparatus that can be used in the method for producing a polycrystalline silicon ingot of the present invention is not particularly limited, and can be implemented using a known production apparatus.
As an example, for example, by combining the cooling of the bottom of the crucible with a cooling mechanism such as a refrigerant circulation provided on the base side of the crucible and the distance from the heating mechanism with a lifting drive mechanism, It can be implemented by a manufacturing apparatus or the like that gradually melts molten silicon from molten silicon near the bottom of the crucible. At that time, the heater temperature is controlled by a known method, specifically, a thermocouple or a radiation thermometer, and the temperature change rate of melting and solidification of silicon and cooling is monitored.

(Production method of polycrystalline silicon ingot)
The method for producing a polycrystalline silicon ingot of the present invention will be described below with reference to the drawings, but the present invention is not limited to this embodiment.
The method for producing a polycrystalline semiconductor ingot of the present invention can also be carried out using a known apparatus as shown in FIG.
FIG. 4 is a schematic cross-sectional view showing an example of an apparatus used in the method for producing a polycrystalline semiconductor ingot according to the present invention.

This apparatus is generally used for producing a polycrystalline silicon ingot, and has a chamber (sealed container) 7 constituting a resistance heating furnace.
A crucible 1 made of graphite, quartz (SiO ₂ ) or the like is disposed inside the chamber 7 so that the atmosphere inside the chamber 7 can be maintained in a sealed state.
In the chamber 7 in which the crucible 1 is accommodated, a graphite crucible base 3 that supports the crucible 1 is disposed. The crucible base 3 can be moved up and down by a lift drive mechanism 12, and the refrigerant (cooling water) in the cooling tank 11 is circulated therein.
An outer crucible 2 made of graphite or the like is disposed on the upper portion of the crucible base 3, and the crucible 1 is disposed therein. Instead of the outer crucible 2, a cover made of graphite or the like surrounding the crucible 1 may be disposed.

A resistance heating body 10 such as a graphite heater is disposed so as to surround the outer crucible 2, and a heat insulating material 8 is disposed so as to cover these from above.
The resistance heating body 10 can be heated from the periphery of the crucible 1 to melt the raw material silicon 4 in the crucible 1.
If the temperature control of the present invention is possible by heating by the resistance heating body 10, cooling from the lower side of the crucible 1 by the cooling tank 11, and raising and lowering of the crucible 1 by the lifting drive mechanism 12, a heating mechanism such as a heating element can be used. The arrangement is not particularly limited.

In order to detect the temperature of the bottom surface of the crucible 1, a crucible lower thermocouple 5 is disposed near the center of the lower surface of the crucible 1, and an outer crucible lower thermocouple 6 is disposed near the center of the lower surface of the outer crucible. And the heating state by the resistance heater 10 is controlled. In addition to the thermocouple described above, a thermocouple or a radiation thermometer for detecting temperature may be arranged.

The inside of the chamber 7 can be kept in a sealed state so that external oxygen gas, nitrogen gas, etc. do not flow in. Normally, after the silicon raw material such as polycrystalline silicon is charged and before the melting, A vacuum is applied, and then an inert gas such as argon gas is introduced to maintain an inert atmosphere.

With the apparatus having such a configuration, basically, the silicon raw material 4 is filled into the crucible 1, degassing (evacuation), gas replacement in the chamber 7 by introducing an inert gas, melting of the silicon raw material 4 by heating, A polycrystalline silicon ingot is manufactured through the steps of melting confirmation and holding, temperature control and solidification start by operation of the lifting drive mechanism 12, solidification completion confirmation and annealing, and ingot removal.

In the manufacturing method of the present invention, while the detection temperature near the center of the bottom surface of the crucible when the silicon temperature becomes the melting point of silicon is Tm, the detection temperature decreases from (Tm−20) ° C. to (Tm−60) ° C. The conditions are such that there is a time for the temperature to drop at a temperature change rate of ˜10 ° C./hour.

Further, in the manufacturing method of the present invention, the detection temperature at a position 20 mm below the center of the bottom surface of the crucible when the silicon temperature becomes the melting point of silicon is Tm ′, and the detection temperature is from (Tm′−20) ° C. to (Tm′−60). While the temperature is lowered to 0 ° C., the temperature is changed at a rate of 1 to 10 ° C./hour so that the time for temperature reduction exists.

The present inventors have confirmed the following.
When the silicon raw material is melted in the crucible, the temperature in the vicinity of the center of the bottom surface of the crucible and the temperature in the lower 20 mm position near the center of the bottom surface of the crucible show a substantially constant value immediately before the completion of melting. By using this constant value as the reference temperature (Tm and Tm ′, respectively), for example, even when a thermocouple is replaced, individual differences (variations) can be eliminated, and the temperature condition can be stabilized. . When melting is completed, the temperature under the crucible rises as the liquid temperature rises, but usually the temperature is gradually lowered after that in order to enter the unidirectional solidification process.

The temperature measurement point does not necessarily have to be in the vicinity of the crucible bottom surface center or the lower 20 mm position near the crucible bottom surface center, and can be a temperature range where nucleation occurs at the crucible bottom surface, as long as the temperature can be correlated with the temperature near the crucible bottom surface center. A position convenient for thermocouple installation can be selected as appropriate. However, it is preferable that the temperature of the temperature measurement point selected in this way changes while maintaining a substantially constant difference from the temperature in the vicinity of the center of the bottom of the crucible. It is desirable that the central portion be less susceptible to changes in the heater output within the surface.

For example, in the apparatus of FIG. 4, when the detected temperature of the thermocouple installed near the center of the lower surface of the outer crucible 2 always has a certain temperature difference from the detected temperature near the center of the lower surface of the crucible 1, The detected temperature near the center of the bottom surface of the outer crucible 2 when T is constant (the melting point of silicon) is Tm ″, and the detected temperature near the center of the bottom surface of the outer crucible is from (Tm ″ −20) ° C. to (Tm ″ −60). The temperature can be controlled by assuming that the detection temperature in the vicinity of the center of the bottom surface of the crucible is equivalent to the decrease in temperature from (Tm-20) ° C to (Tm-60) ° C.
Specifically, as described in the examples, the detected temperature of the thermocouple installed near the center of the lower surface of the outer crucible 2 (downward 20 mm position) is always a difference of −10 ° C. from the detected temperature near the center of the lower surface of the crucible 1. The temperature can be controlled in the production method of the present invention while the temperature decreases from (Tm′−20) ° C. to (Tm′−60) ° C.

According to the knowledge of the present inventors, in the above temperature range, the generation of crystal nuclei starts from the molten silicon at the bottom of the crucible, and the crystal grain size is determined based on the probability of crystal nucleation at the bottom of the crucible and the growth rate of the crystal nuclei. It is thought that it depends on.
And since there is time to cool at the above temperature change rate, the horizontal growth rate of crystal nuclei at the crucible bottom is suppressed, and as a result, the generation density of crystal nuclei is increased and the crystal grain size is controlled to be small It is possible to reduce the crystal defects on the top side of the polycrystalline silicon ingot and relieve the deterioration of the characteristics.

When the temperature change rate of cooling is more than 0 ° C./hour and less than 1 ° C./hour, it is good in terms of controlling the crystal grain size to be small, but it takes too much time for crystal growth, and as a result, the crucible is melted into molten silicon. Alternatively, the diffusion (elution) of metal impurities into the solidified silicon is facilitated, and the merit may be offset. If the rate of temperature change of cooling exceeds 10 ° C./hour, the horizontal growth rate of crystal nuclei generated at the bottom of the crucible cannot be suppressed, and as a result, the generation density of crystal nuclei cannot be increased. Therefore, the temperature change rate of cooling is preferably between 1 and 10 ° C./hour. A more preferable temperature change rate is 2 to 7 ° C./hour.

In the production method of the present invention, the rate of time during which the temperature lowers at a temperature change rate of 1 to 10 ° C./hour during the above temperature decrease is preferably higher, for example, preferably 20% or more. % Or more is more preferable. When this ratio is high, the generation density of crystal nuclei at the bottom of the crucible is high, and the ratio of the region where the crystal grain size is small is increased.

(Polycrystalline silicon ingot)
The polycrystalline silicon ingot of the present invention is manufactured by the polycrystalline silicon ingot manufacturing method of the present invention.

(Polycrystalline silicon block)
The polycrystalline silicon block of the present invention can be obtained by processing the polycrystalline silicon ingot of the present invention.
The polycrystalline silicon block can be obtained, for example, by cutting a surface portion where impurities such as a crucible material may be diffused in the polycrystalline silicon ingot of the present invention using a known apparatus such as a band saw. it can.
Moreover, you may grind | polish the surface of a polycrystalline silicon block as needed.

(Polycrystalline silicon wafer)
The polycrystalline silicon wafer of the present invention can be obtained by processing the polycrystalline silicon block of the present invention.
The polycrystalline silicon wafer can be obtained, for example, by slicing the polycrystalline silicon block of the present invention to a desired thickness using a known apparatus such as a multi-wire saw. At present, a thickness of about 170 to 200 μm is generally used, but the trend is to reduce the thickness for cost reduction.
Further, if necessary, the surface of the polycrystalline silicon wafer may be polished.

(Polycrystalline silicon solar cell)
The polycrystalline silicon solar cell of the present invention is manufactured using the crystalline silicon wafer of the present invention.
A polycrystalline silicon solar cell can be manufactured, for example, by a known solar cell process using the crystalline silicon wafer of the present invention. That is, in the case of a silicon wafer doped with a p-type impurity by a known method using a known material, an n-type impurity is doped to form an n-type layer to form a pn junction, and the surface electrode And a back surface electrode is formed and a polycrystalline silicon solar cell is obtained.

Similarly, in the case of a silicon wafer doped with n-type impurities, a p-type impurity is doped to form a p-type layer to form a pn junction, and a surface electrode and a back electrode are formed to form a polycrystalline silicon solar A battery cell is obtained. Alternatively, in addition to those using a pn junction between silicon, MIS type solar cells in which a metal is deposited with a thin insulating layer interposed therebetween, for example, a silicon thin film such as a conductive amorphous type opposite to a polycrystalline wafer is used. There are films formed and utilizing p-type and n-type silicon heterojunctions having different structures. Further, a plurality of them are electrically connected to obtain a polycrystalline silicon solar cell module.

As described above, in this specification, the concept including “solar battery cell” and “solar battery module” is simply referred to as “solar battery”. Therefore, for example, what is described as “polycrystalline silicon solar cell” is meant to include “polycrystalline silicon solar cell” and “polycrystalline silicon solar cell module”.

Hereinafter, the present invention will be described in detail by test examples, but the present invention is not limited by these test examples.

(Test Example 1) Study on dependency of temperature change rate On the graphite crucible base 3 (880 mm × 880 mm × thickness 200 mm) in the polycrystalline silicon ingot manufacturing apparatus shown in FIG. Dimensions: 900 mm x 900 mm x 460 mm height, bottom plate thickness and side wall thickness 20 mm), quartz crucible 1 (inside dimensions: 830 mm x 830 mm x 420 mm, bottom plate wall thickness and side wall thickness 22 mm) in it installed. In addition, thermocouples for temperature measurement were installed at two locations near the center of the bottom surface of the crucible 1 and near the center of the bottom surface of the outer crucible 2.

Next, after charging 420 kg of raw silicon 4 with the boron dopant concentration adjusted so that the specific resistance of the ingot was about 1.5 Ωcm, the inside of the apparatus was evacuated and replaced with argon gas. Thereafter, the silicon raw material is melted using a heating mechanism (graphite heater 10) disposed beside the crucible as a heating means of the apparatus, and after confirming melting of all raw materials, the silicon is unidirectionally solidified under the following conditions. Annealing was performed at a temperature of 2 ° C. for 2 hours, the temperature was lowered at a cooling rate of 100 ° C./hour, and the polycrystalline silicon ingot was taken out from the apparatus.

In the solidification process, by controlling the heater temperature and the crucible lowering speed, the temperature change rate of the thermocouple becomes 0. 0% while the detected temperature near the center of the bottom of the crucible decreases from Tm-20 ° C to Tm-60 ° C. 5 ° C./hour, 1 ° C./hour, 2 ° C./hour, 5 ° C./hour, 7 ° C./hour, 10 ° C./hour, 15 ° C./hour, and 20 ° C./hour were almost constant. “Temperature change rate” indicates a negative slope in cooling. In this test, the measured value of Tm was in the range of 1410 ° C to 1418 ° C.
The temperature conditions other than the temperature change rate of the thermocouple were made almost the same, and in particular, only the influence of nucleation at the bottom of the ingot could be evaluated.
When the correlation between the detected temperatures in the vicinity of the bottom center of the crucible 1 and the vicinity of the bottom center of the outer crucible 2 (downward 20 mm position near the center of the bottom surface of the crucible) was confirmed, the difference was constantly maintained with a difference of about 10 ° C. It was confirmed that the temperature range of the detected temperature near the center from Tm-20 ° C to Tm-60 ° C corresponds to the detected temperature Tm'-20 ° C to Tm'-60 ° C near the bottom center of the outer crucible 2.

The obtained polycrystalline silicon ingot is processed into a block (156 mm × 156 mm × 200 mm) using a band saw, further sliced using a wire saw, and a polycrystalline silicon wafer (156 mm × 156 mm × thickness 0.18 mm) About 12,000 sheets were obtained.
The obtained polycrystalline silicon wafer was put into a normal solar cell process to produce 12,000 solar cells (outer dimensions 156 mm × 156 mm × thickness 0.18 mm) per ingot, and the output (W ) Was measured.

In general, most of the causes of low solar cell output are crystal defects on the top side of the ingot, especially when the crystal growth time, that is, the production time of the ingot is extremely long, impurities on the bottom side of the ingot. I know it is in diffusion.
Therefore, the quality of the ingot can be determined by evaluating the output distribution of the solar cell.
The output of each solar cell was normalized with the lower limit output of rank 1 as 100, classified into ranks 1 to 3 from the high output side, and the existence ratio (%) was calculated for each ingot.
Rank 1: Output 100 or more Rank 2: Output 93 or more and less than 100 Rank 3: Output less than 93 Table 1 and FIG. 2 show the obtained results.

As apparent from the results of Table 1 and FIG. 2, when the temperature change rate is 1 to 10 ° C./hour, particularly when it is 2 to 7 ° C./hour or less, the occurrence rate of high-rank products is high, and the ingot It turns out that the quality is good.
On the other hand, it can be seen that when the rate of temperature change is less than 1 ° C./hour and when it exceeds 10 ° C./hour, the occurrence rate of low-rank products is high and the ingot quality is not good.
Further, when the crystal grain size of the obtained polycrystalline silicon ingot was visually observed, it was found that the larger the temperature change rate, the larger the crystal grain size.

(Test Example 2) Examination of Occupancy Rate of Temperature Change Rate In the solidification process, by controlling the heater temperature and the crucible lowering speed, the ratio of time that the temperature change rate of the thermocouple becomes 1 to 10 ° C./hour (occupation Rate) is 0%, 20%, 40%, 60%, 80%, and 100%, respectively, except that a polycrystalline silicon ingot is manufactured and a solar cell is manufactured in the same manner as in Test Example 1. The output distribution was evaluated. Except for the range in which the temperature change rate is 1 to 10 ° C./hour, the average temperature change rate was adjusted to 25 ° C./hour.
In this test, the measured value of Tm was in the range of 1407 ° C to 1415 ° C.
FIG. 1 shows the relationship between the elapsed time (hour) when the occupation ratio is 60% and the detected temperature (° C.) near the center of the bottom surface of the crucible. In the figure, TG1, TG10, and TG25 indicate lines with temperature change rates of 1 ° C./hour, 10 ° C./hour, and 25 ° C./hour, respectively.
That is, in Test Example 2, as in Test Example 1, only the influence of nucleation at the bottom of the ingot can be evaluated.
The obtained results are shown in Table 2 and FIG.

As is clear from the results of Table 2 and FIG. 3, when the occupation rate of the specific temperature change rate is 20% or more, particularly when it is 40% or more, the occurrence rate of low-rank products is reduced and good. It can be seen that a quality ingot is obtained.

1 crucible 2 outer crucible 3 crucible stand 4 raw material silicon 5 thermocouple under crucible 6 thermocouple under outer crucible (thermocouple under crucible 20 mm)
7 Chamber 8 Heat insulating material 9 Control device 10 Resistance heater (graphite heater)
11 Cooling tank 12 Lifting drive mechanism TG1 Temperature change rate 1 ° C / hour line TG10 Temperature change rate 10 ° C / hour line TG25 Temperature change rate 25 ° C / hour line

Claims

A method for producing a polycrystalline silicon ingot by unidirectionally solidifying molten silicon in a crucible upward from the bottom of the crucible, wherein the detected temperature near the center of the lower surface of the crucible when the silicon temperature becomes the melting point of silicon is Tm. While the detection temperature decreases from (Tm−20) ° C. to (Tm−60) ° C., the molten silicon is unidirectionally moved under the condition that the temperature decreases at a temperature change rate of 1 to 10 ° C./hour. A method for producing a polycrystalline silicon ingot which is solidified to obtain a polycrystalline silicon ingot.
The method for producing a polycrystalline silicon ingot according to claim 1, wherein the rate of temperature change is 2 to 7 ° C / hour.
2. The method for producing a polycrystalline silicon ingot according to claim 1, wherein the time for the temperature to drop at a temperature change rate of 1 to 10 ° C./hour is 20% or more.
A polycrystalline silicon ingot produced by the polycrystalline silicon ingot producing method according to claim 1.
A polycrystalline silicon block obtained by processing the polycrystalline silicon ingot according to claim 4.
A polycrystalline silicon wafer obtained by processing the polycrystalline silicon block according to claim 5.
A polycrystalline silicon solar cell manufactured using the polycrystalline silicon wafer according to claim 6.
This is a method for producing a polycrystalline silicon ingot by unidirectionally solidifying molten silicon in a crucible from the bottom of the crucible, and a detection temperature at a lower 20 mm position near the center of the lower surface of the crucible when the silicon temperature becomes the melting point of silicon. And Tm ′, while the detected temperature falls from (Tm′−20) ° C. to (Tm′−60) ° C., the temperature falls at a rate of 1 to 10 ° C./hour. A method for producing a polycrystalline silicon ingot, wherein the molten silicon is solidified in one direction to obtain a polycrystalline silicon ingot.
The method for producing a polycrystalline silicon ingot according to claim 8, wherein the temperature change rate is 2 to 7 ° C / hour.
The method for producing a polycrystalline silicon ingot according to claim 8, wherein the time for temperature decrease at a temperature change rate of 1 to 10 ° C / hour is 20% or more.
A polycrystalline silicon ingot produced by the polycrystalline silicon ingot producing method according to claim 8.
A polycrystalline silicon block obtained by processing the polycrystalline silicon ingot according to claim 11.
A polycrystalline silicon wafer obtained by processing the polycrystalline silicon block according to claim 12.
A polycrystalline silicon solar cell manufactured using the polycrystalline silicon wafer according to claim 13.