JP2003243675A - Polycrystalline silicon wafer for solar cell and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polycrystalline silicon wafer for a solar cell which has superior photoelectric conversion efficiency and its manufacturing method. <P>SOLUTION: The polycrystalline silicon wafer for the solar cell which has many sliced crystal grains is characterized in that crystal grains of ≥3 in the ratio of the maximum value of the long size and the maximum value of the short side of rectangles circumscribed with the crystal grains are included in one wafer by ≥75% and, preferably, a standard deviation of a distribution of angles between the long-axis direction of the rectangle having the maximum long-side value among the rectangles circumscribed with the crystal grains and the reference line of the wafer is ≤25°. Such a polycrystalline silicon wafer for a solar cell can be formed by slicing a polycrystalline ingot for solar cells in parallel to the growing direction of the crystal grains. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polycrystalline silicon wafer for solar cells having excellent photoelectric conversion efficiency and a method for manufacturing the same.

[0002]

2. Description of the Related Art In recent years, demands for higher integration, finer integration of semiconductor devices and larger diameters of substrates have become more severe.
Along with this, it is required to improve the quality of silicon substrates used as semiconductor substrates, that is, to reduce crystal defects. Among the crystal defects, the crystal grain boundaries have high density of dangling bonds and impurity atoms, so that minority carriers are easily recombined.
This is a major factor in reducing the conversion efficiency of solar cells.

Therefore, as a manufacturing method in which the total grain boundary length occupying one wafer is shortened by increasing the crystal grain size, the HEM method of Crystal Systems, Inc. (US Pat.
No. 8-54115) has been proposed.
That is, a method of laying a seed crystal on the bottom of the crucible is disclosed in the HEM method in which the side wall of the crucible is maintained at a temperature equal to or higher than the melting point of the semiconductor material and heat is taken from the center of the bottom of the crucible to solidify.

[0004]

However, in order to obtain photoelectric conversion efficiency comparable to that of a single crystal, it is reported that the particle size should be several mm or more (Current status of polycrystalline Si solar cell: Applied Physics Vol. 50). No. 4, 1981), it is not possible to realize a high-efficiency solar cell simply by enlarging the crystal grains. Therefore, knowledge on the nature and shape of the grain boundaries has been required.

Regarding the properties of the grain boundaries, much research has been conducted on the passivation technology for inactivating dangling bonds by introducing hydrogen atoms into the wafer, but there are few studies on the grain shape. Regarding the grain shape required for a polycrystalline silicon wafer for solar cells with excellent conversion efficiency, it is described that most of the crystal grains are formed so as to have a major axis oriented in a certain direction (Actual Publication No. H5-55).
No. 564) and no clear grain shape was found. Therefore, conventionally, the cast polycrystalline silicon ingot for solar cells is sliced perpendicularly to the solidification direction, and such a wafer is not a wafer having a shape in which most of the crystal grains are oriented in a certain direction. The wafer had irregularly shaped grains distributed. That is, the ratio of the maximum value of the long side to the minimum value of the short side of the rectangle circumscribing the crystal grain was 3 or less, or 3 or more was 75% or less of the wafer.

In a general solar cell, carriers generated in crystal grains are collected by a comb-shaped electrode. However, if there is a grain boundary on the way of the carrier reaching the comb-shaped electrode, the grain boundary becomes a resistance. This will reduce the characteristics of the solar cell. Therefore, it is necessary to effectively arrange the comb-shaped electrodes to enhance the carrier collection efficiency, but in the wafer in which the grains having the irregular shape are distributed as described above, the crystal grain boundaries are also irregularly distributed. The comb-shaped electrode of the solar cell cannot be effectively arranged, which has been a factor of deteriorating the characteristics of the solar cell.

The present invention has been invented in view of the above problems, and clarifies the grain shape required for a polycrystalline silicon wafer for a solar cell having excellent conversion efficiency, and a solar cell having excellent photoelectric conversion efficiency. An object is to provide a polycrystalline silicon wafer for a battery and a method for manufacturing the same.

[0008]

In order to achieve the above object, in a polycrystalline silicon wafer for solar cells according to a first aspect, the maximum value of the long side and the short side of the rectangular side circumscribing the crystal grains in the silicon wafer are defined. One of the wafers is characterized by containing 75% or more of crystal grains having a minimum value ratio of 3 or more.

In the above-mentioned polycrystalline silicon wafer for solar cells, a crystal grain in which the ratio of the maximum value of the long side to the minimum value of the short side of the rectangle circumscribing the crystal grain in the silicon wafer is 3 or more is within one wafer. It is desirable that the standard deviation of the distribution of the angle between the long axis direction of the rectangle having the maximum value of the long sides among the rectangles circumscribing the crystal grains of 75% or more and the reference line of the wafer is 25 ° or less. .

According to a third aspect of the present invention, there is provided a method of manufacturing a polycrystalline silicon wafer for a solar cell, wherein the unidirectionally solidified silicon ingot is sliced to form a silicon wafer. It is characterized in that the silicon ingot is sliced horizontally with respect to the crystal grain growth direction.

[0011]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described below in detail with reference to the accompanying drawings. FIG. 1A is a diagram showing a polycrystalline silicon wafer for solar cells, and FIG. 1B is a diagram showing a circumscribing rectangle.

The silicon wafer 1 is made of a polycrystalline silicon wafer having a large number of crystal grains 2, and is formed by cutting a silicon ingot formed by a casting method into a prismatic shape and slicing it into a large number. The silicon wafer 1 has a large number of crystal grains that are sliced and defined by grain boundaries having a polygonal shape in a plan view. Infinite rectangles circumscribing the crystal grains 2 can be drawn,
Among them, in the present invention, a wafer containing 75% or more of crystal grains in which the ratio of the maximum value of the long side to the minimum value of the short side of the rectangle 3 circumscribing the crystal grain 2 is 3 or more, particularly 8
A wafer containing 5% or more is used.

The ratio of the maximum value of the long side of the rectangle 3 circumscribing the crystal grain 2 to the minimum value of the short side thereof is as shown in FIG. After drawing the circumscribing rectangle 3, find the maximum value of the long sides of the rectangle and the minimum value of the short sides of the rectangle, and calculate the (maximum value of the long side) (the minimum value of the short side). The value divided by.

In producing a solar cell, a wafer having a ratio of the maximum value of the long sides of the rectangle circumscribing the crystal grains to the minimum value of the short sides of 3 or less or 75% or less of 3 or more is irregular. Since the crystal grains 2 having a shape are distributed, the grain boundaries also have an irregular distribution. Ideally, the carriers generated in the grains are collected at the electrodes without straddling the grain boundaries, but with a wafer in which such grain boundaries are irregularly distributed, even if an ordinary comb-shaped electrode is provided, it is efficient. Unable to collect carriers.
Therefore, in order to manufacture a highly efficient solar cell with such a wafer, a complicated electrode structure is required.

However, the crystal grain 2 in which the ratio of the maximum value of the long side to the minimum value of the short side of the rectangle circumscribing the crystal grain 2 is 3 or more.
When a wafer containing 75% or more is used, the ratio of the maximum value of the long side of the rectangle circumscribing the crystal grain 2 to the minimum value of the short side thereof is 3 or less, or 3 or more is compared with the wafer of 75% or less. Many slender particles will be distributed. As a result, since the crystal grain boundaries are regularly distributed, it is possible to improve the carrier collection efficiency even with a normal comb-shaped electrode, and a complicated electrode structure is not required.

75% of the crystal grains have a ratio of the maximum value of the long side of the rectangle circumscribing the crystal grain 2 to the minimum value of the short side thereof is 3 or more.
In the wafer No. 2, it cannot be said that the crystal grain boundaries are more than perpendicular to the comb-shaped electrodes, and therefore, it is more preferable that the grain boundaries are contained in an amount of 85% or more in consideration of the stability of quality.

In providing the comb-shaped electrode, the more uniform the orientation of the crystal grains 2, the more effectively the carriers can be collected. Therefore, the maximum and short sides of the long side of the rectangle 4 circumscribing the crystal grains 2 are short. The long axis direction of the rectangle having the maximum value of the long sides among the rectangles that include 75% or more of the crystal grains 2 in which the ratio of the minimum values of the sides is 3 or more in the wafer and the reference line of the wafer. It is possible to further improve the photoelectric conversion efficiency of the solar cell by using a wafer having a standard deviation of the angle distribution of 25 ° or less. That is, of the rectangles 3 circumscribing the crystal grains 2, the rectangle 4 having the longest maximum value is extracted, and the standard deviation of the angle distribution between the long-axis direction 5 and the reference line 6 of the wafer is 25 ° or less. Further improvement in photoelectric conversion efficiency can be expected by using a certain wafer.

A wafer containing 75% or more of the crystal grains 2 in which the ratio of the maximum value of the long side to the minimum value of the short side of the rectangle 4 circumscribing the crystal grain 2 is 3 or more is a solidified silicon ingot for a solar cell. After being cut to a size, this semiconductor block is sliced by a multi-wire saw device (not shown) that supplies slurry containing abrasive grains and cuts the wire by rotating the wire at high speed. At this time, the semiconductor block can be easily manufactured by adhering the semiconductor block to a base material (not shown) made of carbon or the like and slicing it so that the wire can be inserted horizontally with respect to the crystal grain growth direction.

By simply slicing the crystal grains 2 horizontally with respect to the growth direction, crystal grains having a ratio of the maximum value of the long sides of the rectangle circumscribing the crystal grains 2 to the minimum value of the short sides of 3 or more are within the wafer. To produce a wafer whose standard deviation of the angle distribution between the long axis direction of the rectangle having the maximum value of the long side among the rectangles circumscribing the crystal grains and the reference line of the wafer is 25 ° or less. It is difficult to do so, but by slicing horizontally from the silicon ingot for solar cells with excellent unidirectional solidification with sufficient heat removal from the bottom while insulating the side surface of the mold with a multi-wire saw device in the growth direction of crystal grains. Can be manufactured.

Such a silicon ingot for a solar cell having excellent unidirectional solidification can be manufactured by a casting apparatus as shown in FIG. 2 as follows. In FIG. 2, 7 is a heat insulating material such as graphite, 8 is a heating body, 9 is a mold such as graphite, 10 is a cooling plate, and 11 is molten silicon. Molten silicon melted in a quartz crucible (not shown) is poured into a mold 9 whose inner wall surface is coated with a release material containing silicon nitride, silicon carbide or the like as a main component, and then the molten silicon 11 is solidified on the liquid surface. While heating the heating body 8 to a certain extent, the cooling plate 10 is brought into contact with the lower portion of the mold 9 and solidified from the lower portion to the upper portion to manufacture a silicon ingot.

At this time, while insulating the side surface of the mold 9,
Method to deteriorate the heat insulation of the mold release material (not shown) at the bottom of the mold,
A silicon ingot for a solar cell that is solidified in one direction by increasing the heat removal amount from the bottom by using a method of reducing the thickness of the bottom of the mold to deteriorate the heat insulating property, a method of increasing the heat removal amount of the cooling plate 10, and the like. Can be manufactured.

[0022]

EXAMPLES Next, examples of the present invention will be described. As shown in FIG. 3, one wafer contains 65% of crystal grains in which the ratio of the maximum value of the long side of the rectangle circumscribing the crystal grain to the minimum value of the short side thereof is 3 or more. )), Those containing 75% ((b) in the same figure), those containing 85% ((c) in the same figure)
Was prepared, and the ratio of the maximum value of the long side of the rectangle circumscribing the crystal grain to the minimum value of the short side of the rectangle occupying one wafer was examined. As a result, the distribution within one wafer was as shown in FIG. Here, of the polycrystalline silicon wafers used for comparison, (a) and (b) are wafers sliced perpendicularly to the crystal grain growth direction, and (c) are sliced horizontally to the crystal grain growth direction. It is a wafer manufactured by Table 1 shows the photoelectric conversion efficiency when these wafers are made into elements.

[0023]

[Table 1]

From Table 1, a wafer containing 75% or more of crystal grains in which the ratio of the maximum value of the long side to the minimum value of the short side of the circumscribed rectangle is 3 or more is 15% or more, which is extremely excellent. It can be seen that the photoelectric conversion efficiency of the wafer containing 85% or more is particularly excellent.

FIG. 5 shows a wafer in which the standard deviation of the angle distribution between the long axis direction of the rectangle having the maximum value of the long sides among the rectangles circumscribing the crystal grains and the reference line of the wafer is 30 ° (see FIG. )) And a 25 ° wafer ((b) in the same figure). 5A (the same wafer as FIG. 3C) and FIG. 5B are wafers produced by slicing horizontally with respect to the crystal grain growth direction. It is a polycrystalline silicon wafer containing 85% or more of crystal grains in which the ratio of the maximum value to the minimum value of the short side is 3 or more in one wafer. Further, the wafer (b) is a wafer sliced from an ingot in which the heat removal amount at the bottom is increased by 1.5 times by changing the heat insulating property of the release material to that in (a) to improve the unidirectional solidification property. Table 2 shows a comparison of photoelectric conversion efficiencies when the wafer is made into an element.

[0026]

[Table 2]

From Tables 1 and 2, 75% or more of crystal grains having a ratio of the maximum value of the long sides of the rectangle circumscribing the crystal grains to the minimum value of the short sides of 3 or more are contained in one wafer, and Of the rectangles circumscribing the crystal grains, the standard deviation of the distribution of the angle between the long axis direction of the rectangle having the longest maximum value and the wafer reference line is 25.
It can be seen that the photoelectric conversion efficiency of FIG.

[0028]

As described above, a crystal grain having a ratio of the maximum value of the long side to the minimum value of the short side of the rectangle circumscribing the crystal grain in the polycrystalline silicon wafer for solar cells of the present invention is 3 or more. By containing 75% or more in one wafer, many elongated particles are distributed. As a result, the regular crystal grain boundaries are distributed, so that the carrier collection efficiency can be increased even with a normal comb-shaped electrode, the photoelectric conversion efficiency is improved, and a complicated electrode structure is not required. .

Further, in providing the comb-shaped electrode, the more uniform the orientation of the crystal grains, the more effectively the carriers can be collected, so that the longest side of the rectangle circumscribing the crystal grains has the maximum value. The photoelectric conversion efficiency of the solar cell can be further improved by using a wafer having a standard deviation of 25 ° or less in the distribution of the angle between the long axis direction of the rectangle and the reference line of the wafer.

Further, by slicing the polycrystalline silicon ingot for solar cells horizontally with respect to the growth direction of crystal grains, the maximum value of the long side and the minimum value of the short side of the rectangle circumscribing the crystal grain in the silicon wafer are obtained. It becomes possible to easily manufacture a wafer in which a crystal grain having a ratio of 3 or more is 75% or more in one wafer. Furthermore, by slicing the solar cell silicon ingot solidified in one direction by increasing the amount of heat removed from the bottom horizontally than the normal direction, the longest side of the rectangle circumscribing the crystal grain is the largest. It is possible to manufacture a wafer having a standard deviation of 25 ° or less in the distribution of the angle between the long axis direction of a rectangle having a value and the reference line of the wafer.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a rectangle circumscribing crystal grains in a polycrystalline silicon wafer for solar cells according to the present invention.

FIG. 2 is a view showing a solidification part of a casting apparatus used for manufacturing a polycrystalline silicon wafer for solar cells according to the present invention.

FIG. 3 is a diagram showing a polycrystalline silicon wafer for solar cells, in which (a) shows 65% of crystal grains in which the ratio of the maximum value of the long side of the rectangle circumscribing the crystal grain to the minimum value of the short side is 3 or more. Including figure,
(B) is a diagram including 75%, and (c) is a diagram including 85%.

4 is a diagram showing a distribution of a ratio of a maximum value of a long side and a minimum value of a short side of a rectangle circumscribing a crystal grain in the wafer of FIG.

FIG. 5 is a diagram showing a polycrystalline silicon wafer for solar cells, in which (a) shows the angle between the long axis direction of the rectangle having the longest maximum value among the rectangles circumscribing the crystal grains and the reference line of the wafer. A diagram showing a wafer having a standard deviation of distribution of 30 °, (b) is 2
It is a figure which shows a 5 degree wafer.

[Explanation of symbols]

1: wafer, 2: crystal grain, 3: rectangle circumscribing the crystal grain, 4: rectangle having the longest maximum value among rectangles circumscribing, 5: major axis direction, 6: reference line, 7: graphite, etc. Insulating material consisting of: 8: Heating body, 9: Mold made of graphite, 10: Cooling plate, 11: Molten silicon

Claims (3)

[Claims] 1. In a polycrystalline silicon wafer for solar cells having a large number of sliced crystal grains, the ratio of the maximum value of the long sides of the rectangle circumscribing the crystal grains to the minimum value of the short sides thereof is 3.
A polycrystalline silicon wafer for a solar cell, characterized in that the above-mentioned crystal grains are contained in 75% or more in one wafer. 2. The polycrystalline silicon wafer for a solar cell according to claim 1, wherein among the rectangles circumscribing the crystal grains, the angle between the long axis direction of the rectangle having the longest maximum value and the reference line of the wafer. A polycrystalline silicon wafer for a solar cell, having a standard deviation of distribution of 25 ° or less. 3. A method of forming a polycrystalline silicon wafer for a solar cell, comprising slicing a unidirectionally solidified silicon ingot to form a silicon wafer, comprising slicing the silicon ingot horizontally with respect to a crystal grain growth direction. A method for producing a polycrystalline silicon wafer for a solar cell, which is characterized.