JP2003095630A - Silicon sheet and solar battery including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon sheet capable of standing both rapid growth and good semiconductor character and solar batteries using the same. SOLUTION: The silicon sheet is directly formed by solidifying from a liquid phase silicon by bringing a cooling substrate into contact with a silicon melt, the average crystal grain sizes appearing on a first main surface (1) on which the sheet contacted with the silicon melt and a second main surface (2) which contacted with the cooling substrate are less than 10 mm on both the main surfaces, and the average crystal grain size appearing on the first main surface (1) is greater than the average crystal grain size appearing on the second main surface (2).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to cost reduction of a silicon sheet, and more particularly to a low cost silicon sheet having sufficient semiconductor characteristics for solar cells.

[0002]

2. Description of the Related Art As a silicon substrate mainly for producing a solar cell, for example, polycrystalline silicon manufactured by using a casting method as disclosed in JP-A-11-21120 is often used. There is. The casting method is
A method for producing an ingot (solidified mass) mainly composed of long columnar crystal structures grown upward from the crucible bottom by solidifying the silicon melt by gradually cooling the silicon melted in the crucible from the bottom of the crucible. is there. At the beginning of cooling, the solid-liquid interface of silicon is close to the cooling surface at the bottom of the crucible, but the solid-liquid interface gradually moves away from the cooling surface due to the progress of solidification. Further, the thermal conductivity of solid phase silicon is smaller than that of the liquid phase, which also makes it difficult to carry out solid phase growth at a constant rate, which is desirable for homogenizing semiconductor characteristics.

As a means for improving this, Japanese Patent Laid-Open No. 11-1999
According to Japanese Patent Laid-Open No. 92284, the relationship between the ascending movement speed of a silicon solid-liquid interface and the amount of heat released from the bottom surface of the crucible is obtained in advance, and the solidification rate is stabilized by controlling the amount of heat released to obtain good semiconductor characteristics. I got a silicon ingot. The average crystal grain size appearing on the horizontal cross section of the ingot is larger than 10 mm. This technique enables stable unidirectional pseudo-solid growth from the bottom of the crucible upward. As shown in FIG. 9, the thickness direction cross section of the substrate horizontally cut out from the ingot produced by this casting method includes crystal grain boundaries substantially parallel to the thickness direction. That is, a silicon substrate obtained by using the casting method is manufactured by slicing in the horizontal direction parallel to the crucible bottom, and the semiconductor characteristics on both main surfaces of this substrate are almost the same.

However, in the casting method, a slicing step is required to obtain a polycrystalline silicon substrate from an ingot, and therefore, there is a limit in reducing the cost of the silicon substrate. On the other hand, since about 20 years ago, there has been no need for slicing, the web method or EFG (edge-defined film-fed).
The growth of silicon ribbons by the growth method has also been studied. Also, in recent years, aiming at faster growth, a thin plate-shaped silicon ribbon is directly produced from a silicon melt.
The GS (ribbon growth on substrate) method has been attracting attention (26th PVSC, 1997, pp. 9).
1-93).

The principle of the RGS method is to perform high-speed growth of a silicon ribbon by high-speed heat transfer (heat removal) from a surface close to a solidification growth surface. Specifically, by moving the lower surface supporting flat plate supporting the open lower surface relative to the side supporting frame supporting the side peripheral portion of the molten silicon in the lateral direction while cooling, the silicon is placed on the lower surface supporting flat plate. Grow the ribbon fast.

Immediately after laterally drawing the lower surface supporting flat plate portion in contact with the bottom surface of the molten silicon, the silicon on the flat plate is in a liquid phase, and the lower surface of the supporting flat plate and the silicon surface are simultaneously drawn from both sides. It will be cooled. It has been shown that in the silicon ribbon produced by this method, the solid-liquid interface becomes oblique to the surface of the supporting plate in a vertical section parallel to the moving direction of the supporting plate, and the silicon crystal extends obliquely and solidifies and grows. . That is, the shape of the grown crystal grains becomes a columnar crystal in the oblique direction with respect to the supporting flat plate surface.

Recently, a method of directly contacting a silicon melt with a substrate to obtain a silicon sheet by solidifying from a liquid phase has been disclosed, for example, in Japanese Patent Laid-Open No. 2001-223172.

[0008]

The casting method requires a long time of several tens of hours for producing one silicon ingot in order to grow the ingot without causing cracks and from the viewpoint of ensuring semiconductor quality. It costs. And when cutting a silicon substrate from an ingot,
Dozens of hours are required even if the slicing technique using a multi-wire saw is used. Therefore, it is difficult to reduce the cost of producing a silicon substrate by using the casting method.

In the ribbon manufacturing method such as the RGS method, there are many problems in the stable growth of the solidified phase itself, including the problem of controlling the crystallization state of the silicon ribbon, and stable semiconductor characteristics that can be put to practical use in solar cells and the like are obtained. It is not at the stage of obtaining the silicon ribbon that it has.

Further, also in the technique disclosed in Japanese Patent Laid-Open No. 2001-223172, further improvement is desired in order to obtain a silicon sheet having a preferable crystal structure and semiconductor characteristics.

Therefore, an object of the present invention is to provide a silicon sheet which can achieve both high-speed growth and good semiconductor characteristics, and a solar cell using the same.

[0012]

A silicon sheet according to the present invention is a silicon sheet directly formed by solidification from liquid phase silicon by bringing a substrate into contact with a silicon melt. The average crystal grain size appearing on the first main surface which was in contact with the liquid and the second main surface which was in contact with the cooling substrate was 10 m on both main surfaces.
It is less than m, and the average crystal grain size appearing on the first main surface is larger than the average crystal grain size appearing on the second main surface.

A silicon sheet having an average crystal grain size of 3 mm or less appearing on the first main surface and the second main surface can be produced relatively easily and at low cost. Further, the difference in the average crystal grain size appearing on the first main surface and the second main surface is 10 μm or more 5
It is possible to obtain a silicon sheet having a size of mm or less. The average crystal grain size can be defined as an average interval between intersections of an arbitrary straight line on the first main surface or the second main surface and a crystal grain boundary.

The silicon sheet may have a periodic and gradual thickness change. Crystal grain boundaries that are substantially parallel to the thickness direction are formed in the minimum value region of the thickness that periodically appears in this thickness change. It is appropriate that the thickness change cycle is 10 mm or less. The height difference of the unevenness due to the change in thickness is larger on the first main surface side where the silicon sheet was in contact with the silicon melt than in the second main surface side where the silicon sheet was in contact with the base body.

The silicon sheet is 100 μm to 1 mm
It is preferable to have an average thickness within the range of. The silicon sheet preferably has a purity of 5 nines or higher. Further, it is preferable that the height difference of the surface irregularities included in the silicon sheet is 200 μm or less. The silicon sheet may have a carrier diffusion length of 30 μm or more.

The silicon sheet as described above can be preferably used for solar cells. Light to be photoelectrically converted is preferably incident from the first main surface side of the silicon sheet having a relatively large crystal grain size.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION First, a basic procedure for producing a silicon sheet according to the present invention will be described. Figure 1
As shown in the schematic sectional view of (a), the surface of the heat-resistant substrate 3 whose temperature is controlled by the temperature control means 6 capable of heating and cooling to a temperature lower than 1415 ° C. which is the melting point of silicon is a crucible. The silicon sheet 7 grows on the surface of the substrate 3 by contacting (or immersing) the silicon melt 5 in 4 therein. After the silicon sheet 7 having a required thickness is grown, the substrate 3 to which the sheet is attached is taken out from the crucible 4. At the stage where the base body 3 integrated with the sheet 7 is cooled from a high temperature, the base body 3 and the sheet 7 are naturally separated due to the difference in their thermal expansion coefficients, as shown in FIG. Alternatively, a small impact is applied to the substrate 3 to obtain a silicon sheet 7 which is separated and directly formed by solidification from the liquid phase.

In the silicon sheet according to the present invention, the initial temperature of the substrate 3 is 120 ° C. higher than the melting point of silicon (1415 ° C.).
By controlling the temperature range from 1 to 1000 ° C. lower, and by using a graphite material with an appropriate thickness.
So that the heat capacity of the substrate 3 is appropriate, the gas is used as a coolant in the temperature control means 6 for heating and cooling the substrate 3, and the time for immersing the substrate 3 in the silicon melt 5 is such that a silicon sheet having an optimum thickness can be obtained. A polycrystalline silicon sheet is formed on the surface of the substrate 3 at high speed and stably by controlling and further by setting basic conditions such as accelerating the solidification of the silicon solution due to the fine irregularities on the surface of the substrate 3. be able to.

That is, since the substrate 3 is controlled at a temperature lower than the temperature of the silicon melt 5, silicon crystal nuclei are generated everywhere on the surface of the substrate. Then, these crystal nuclei grow in one direction toward the direction in which they are in contact with the silicon melt to form a polycrystalline silicon sheet. Base 3
In the silicon sheet separated from, the average crystal grain size appearing on one main surface is different from the average crystal grain size appearing on the other main surface.

More specifically, as shown in FIG. 2, which is a schematic sectional view parallel to the thickness direction of the silicon sheet, the crystal grains appearing on one main surface 1 of the sheet and the other main surface The average crystal grain size is different from that of the crystal grains appearing on the surface 2. That is, a plurality of crystal nuclei generated on the main surface 2 expand and grow in various directions toward the main surface 1 side, and the crystal grains collide with each other during the growth, whereby new small crystal grains are generated or grown. Suppressed. As a result, the average lengths between the intersections of an arbitrary straight line and the crystal grain boundaries on the main surface 1 and the main surface 2 are different from each other. More specifically, the average length between the intersections of an arbitrary straight line and a crystal grain boundary is small on the main surface 2 that was in contact with the base body 3 during sheet production, and the main surface that was in contact with the silicon melt 5 was small. 1
It gets bigger above.

If the average crystal grain size is large, the crystal grain boundary density, which causes deterioration of semiconductor characteristics, is reduced, the carrier diffusion length is extended, and the semiconductor characteristics of the silicon sheet can be improved. Due to this improvement effect, the silicon sheet directly formed by solidification from the liquid phase can be used for devices such as solar cells.

Since the average cross-sectional area of the crystal grains appearing on one main surface of the silicon sheet is larger than the average cross-sectional area of the crystal grains appearing on the other main surface, more specifically, in the cross section in the thickness direction. An average value length (average grain size) between the intersections of one main surface 1 of the sheet and the crystal grain boundaries, and an average length (average grain size) between the intersections of the other main surface 2 and the crystal grain boundaries When the absolute value of the difference is 10 μm or more and 5 mm or less, it becomes possible to manufacture a semiconductor element such as a solar cell using this sheet. From the viewpoint of semiconductor characteristics, it is more preferable that the average crystal grain size difference between the main surface 1 and the main surface 2 of the sheet is 50 μm or more and 1 mm or less.

By setting the thickness of the silicon sheet to 100 μm or more, a high handling property can be obtained in the manufacturing process of the solar cell using the sheet. Further, by setting the sheet thickness to 1 mm or less, the sheet manufacturing time can be shortened and a low-cost silicon substrate can be provided. By setting the average thickness of the sheet within the range of 100 μm to 1 mm, the slicing step as in the case of the casting method becomes unnecessary and good semiconductor characteristics can be obtained. From the viewpoint of easy sheet production, it is more preferable that the average thickness is in the range of 200 to 600 μm.

Since the purity of the silicon sheet is 5 nines or more, good device characteristic values can be obtained even when used in solar cells and the like. From the viewpoint of solar cell characteristics, a purity of 7 nines or more is more preferable. Since the maximum value of the height difference in the surface unevenness of the silicon sheet is 200 μm or less, the sheet can be used for a solar cell or the like without undergoing a process such as slicing or polishing, and the surface etching time can be shortened or the surface etching It can be omitted. When the diffusion length of the carrier in the silicon sheet is 30 μm or more, a solar cell having relatively good conversion efficiency can be obtained.

With respect to the crystal grains appearing on each main surface of the silicon sheet, a specific method for measuring and evaluating the above-mentioned average grain size will be described. First, the silicon sheet is cut along the thickness direction including the substantially central portion thereof. After finishing the cut with a grindstone of 2000 or more, 10
When etching is performed at 80 ° C. for 10 minutes using a mass% NaOH aqueous solution, crystal grains appear clearly due to the crystal orientation dependence of the etching rate. Next, an enlarged cross-sectional image as shown in FIG. 2 is obtained by using an image enlargement device using a CCD element or the like. Using a 20x magnified sectional image,
The number of intersections of the surface and the grain boundaries per distance 10 mm (200 mm in the expanded state) on one main surface of the sheet is counted. Next, the number of intersections between the surface and the crystal grain boundaries per 10 mm distance of the other main surface is counted.

In the example of FIG. 2, the number of intersections between the surface and the crystal grain boundaries is 7 on the main surface 1 and 13 on the main surface 2. In this case, the average grain size, which is the average value length between the intersections of the surface and the grain boundaries, is 1 on the main surface 1.
0 mm ÷ (7 + 1) = 1.25 mm is obtained, and 10 mm ÷ (13 + 1) = 0.714 mm is obtained on the main surface 2.

To obtain the average length between the intersections of the principal planes and the grain boundaries in the cross section along the thickness direction of a solar cell using a silicon sheet, an acid generated by heating concentrated nitric acid or aqua regia is used. After the electrode metal or the like is removed with a solution to extract the silicon sheet, it is cut along the thickness direction including the substantially central portion of the sheet as described above so that the crystal grains in the cross section can clearly appear. However, even after removing the electrode metal and the like, the outermost surface of the silicon sheet may be unclear in the alloy layer forming portion. In that case, the interface between the alloy layer and the semiconductor layer serves as the surface of the silicon sheet.

(Embodiment 1) An apparatus structure and method for manufacturing a silicon sheet will be described below. However, the apparatus for obtaining the silicon sheet according to the present invention is not limited to this. Of course, it goes without saying that the single-wafer type sheet manufacturing apparatus shown in FIG. 1 and other apparatuses can also be used.

FIG. 3 shows a schematic vertical sectional view of a sheet manufacturing apparatus capable of obtaining a silicon sheet according to the present invention. In this apparatus, a stainless steel chamber 70 is provided with a crucible 71, a heater 72, a silicon melt 73,
A base 74 and a rotary shaft 75 of the base are provided, and a winding mechanism 76 for winding the sheet outward from the sheet take-out hole in the upper portion of the chamber is provided. further,
A silicon material charging mechanism 77 is attached, and details thereof are omitted in the drawing. Although the resistance heating method is used for the heater 72, a high frequency heating method or the like having equivalent ability may be used.

The additional substrate 78 may be attached so as to be in contact with the cylindrical surface of the substrate 74, and a silicon sheet may be grown on the surface of the additional substrate 78. Although graphite is basically used as the material of the base 74 or the additional base 78, a base having silicon carbide formed on its surface by a thermal CVD method may be used. As the material of the additional base 78, ceramics such as silicon nitride or heat resistant metal that can withstand high temperature may be used, or carbon, ceramics, or heat resistant metal partially or entirely coated with ceramics may be used. is there.

The surface of the substrate 74 or the additional substrate 78 may be a flat surface, or may have grooves along the rotation direction of the substrate 74 or fine irregular surfaces arranged regularly or irregularly. . The grooves and fine irregularities formed on the surface of the substrate have the function of speeding up the growth of the silicon sheet.

As the temperature control means for the base body 74, a gas refrigerant system is employed in which a cavity is provided near the inner surface of the cylindrical base body 74 and either nitrogen, argon or air is introduced under pressure. A liquid refrigerant system in which a pipe made of metal such as stainless steel or copper is embedded in the substrate to control the temperature may be adopted.

Next, a procedure for manufacturing a silicon sheet in the apparatus shown in FIG. 3 will be described. First, raw silicon having a purity of about 6 nines was charged into the crucible 71, and then the chamber 70 was evacuated by a vacuum pump and replaced with argon gas. While heating the silicon raw material, A in the chamber 70
The r pressure was kept at about 10 Torr. However, the degree of vacuum may be further increased to promote degassing from the silicon raw material. After the silicon is melted, granular silicon is additionally charged from the silicon raw material charging mechanism 77, and the crucible 71
The height of the molten silicon surface inside was adjusted.

Although the temperature of the molten silicon during the production of the silicon sheet was 1450 ° C., it can be set within the range from the supercooling temperature of 1380 ° C. or higher to the higher temperature of 1600 ° C., depending on the growth conditions of the sheet. . After the silicon melt surface reaches a prescribed height, the temperature of the substrate 74 is controlled by passing the refrigerant gas through the substrate 74 so that the surface temperature of the substrate 74 becomes 1 or less.
The surface thereof was immersed in the silicon melt while being stabilized at 200 ° C. The temperature of the substrate 74 is preferably in the range of 1000 ° C. to 120 ° C. lower than the melting point of silicon.

When the base 74 was rotationally driven by the rotary shaft 75 in the above-described state, the silicon polycrystal grew on the base 74 stably at high speed and with good controllability. In the silicon sheet formed in this way,
By controlling the temperature of the substrate 74 to be equal to or lower than the melting point temperature of silicon, the polycrystal was grown to have a uniform thickness from the surface side of the substrate toward the silicon melt side. In the process in which the substrate 74 and the silicon sheet are cooled to room temperature, a natural impact or a small impact is applied due to the difference in thermal expansion coefficient between them,
The silicon sheet was easily peeled off from the substrate. The average thickness of the obtained sheet was about 500 μm, and the average crystal grain size appearing on the main surface on the side in contact with the silicon melt was the base 74.
It is larger than the main surface on the side that was in contact with.

FIG. 4 shows a photograph of a cross section of an example of the silicon sheet produced as described above. In this cross-sectional photograph, the lower main surface of the silicon sheet was the surface in contact with the substrate, and the number of intersections between the lower main surface and the grain boundaries was large, that is, the average length between the intersections (average grain size). Is small. On the other hand, the upper main surface of the silicon sheet is the surface that was in contact with the silicon melt, the number of intersections between the upper main surface and the grain boundaries is small, that is, the average length between the intersections (average grain size) is It turns out to be big. The average crystal grain size on the lower main surface of this silicon sheet was about 0.22 mm, and the average grain size on the upper main surface was about 0.38 mm. The absolute value of the difference in average particle diameter between the lower main surface and the upper main surface is 0.16 mm.

The number of intersections between the main surface and the crystal grain boundaries in the photograph cross section of FIG. 4 is 45 on the lower main surface in contact with the substrate 74, and on the upper main surface in contact with the silicon melt. Was 26. Of the angles formed by the main surface of the sheet and the crystal grain boundaries in the cross section of the photograph, 94% of the total angles were from 80 degrees to 90 degrees on the acute side. When the carrier diffusion length was measured, a value of 45 μm was obtained on the lower main surface side in contact with the substrate, and a value of 60 μm was obtained on the upper main surface side in contact with the silicon melt. When the unevenness of the surface was measured using a step gauge, the maximum value was 120μ on the lower main surface side of the sheet.
m and 150 μm on the upper main surface side. When the impurity concentration in the silicon sheet was measured, a silicon purity of 7 nines was obtained. It is considered that this improvement in purity was obtained by the difference in the solid-liquid distribution coefficient of impurities contained in the molten silicon.

A method for producing a solar cell using the silicon sheet thus produced will be described. The method can follow the procedure shown in the flow diagram of FIG. In this embodiment, the silicon sheet is a p-type semiconductor, but it may be an n-type semiconductor. When forming a p- or n-conductivity type silicon sheet, it is desirable to mix a dopant such as boron (B) or phosphorus (P) when the raw material silicon is melted. In the obtained silicon sheet, the average crystal grain sizes appearing on both main surfaces are different from each other, and therefore it is necessary to first decide which main surface is to be the light receiving surface of the solar cell to be produced. This is determined in consideration of the surface state of the silicon sheet and the semiconductor characteristics and compatibility with the solar cell process. In this Example 1,
The major surface having the larger average crystal grain size appeared on the surface was selected as the light receiving surface.

In the flow chart of FIG. 5, first, in steps Sl and S2, cleaning and surface etching of the silicon sheet were performed using a mixed solution of nitric acid and hydrofluoric acid. In the subsequent step S3, texture-etching was performed on the light incident side main surface of the sheet using sodium hydroxide. As this etching, a dry etching method using plasma discharge or the like is also possible, but by using the wet etching method, the surface texture can be formed at a lower cost. In step S4, the n-type diffusion layer was formed by PSG diffusion (diffusion method using a phosphosilicate glass film). In step S5, after removing the PSG film formed on the surface with hydrofluoric acid, a silicon nitride film was formed as an antireflection film on the main surface on the light receiving surface side. Next, in step S6, the diffusion layer formed on the back surface side was removed using a mixed solution of nitric acid and hydrofluoric acid. In step S7, A
An alloy layer and a back surface electrode were simultaneously formed on the back surface side using 1 paste. Finally, in step S8, the electrodes on the light-receiving surface side were formed by screen printing of a silver paste material.

In this way, five solar cells as shown in the schematic sectional view of FIG. 6 were produced. Figure 6
The solar cell of is a silicon sheet 50, a diffusion layer 51,
The photoelectric conversion layer 52, the alloy layer 53, the front surface electrode 54, and the back surface electrode 55 are included.

Table 1 shows the structure of the silicon sheet produced in Example 1 and the photoelectric conversion efficiency of the solar cell produced using this sheet.

(Example 2) In Example 2, the substrate 7 was used.
A silicon sheet was manufactured with the entire surface of 4 being coated with a 100 μm thick SiC layer. Other conditions regarding the manufacturing apparatus and the manufacturing method used in the second embodiment are the same as those in the first embodiment. Therefore, the temperatures of the silicon melt and the substrate in Example 2 are 1450 ° C. and 1200 ° C., respectively, as in Example 1.

When the silicon sheet is solidified and grown, the surface condition of the substrate 74 has a great influence on the crystal growth. SiC, which has higher wettability than graphite for silicon melt
By coating the surface of the substrate with a layer, the heat flow from the melt to the substrate is increased, and the degree of supercooling of the silicon melt is further reduced, resulting in faster crystal nucleation while suppressing giant dendrite growth. Crystal growth becomes possible. Further, in the second embodiment, in order to improve the wettability with respect to the silicon melt, a dense hydrocarbon may be coated instead of SiC.

In Example 2, as compared with Example 1, the crystal grain size of the silicon sheet was made more uniform over the entire surface, and the surface smoothness was improved on the main surface in contact with the silicon melt. The maximum value of the height difference of the surface irregularities was 30 μm on the main surface side in contact with the substrate, and 50 μm on the main surface side in contact with the silicon melt. The surface coating in Example 2 facilitated the peeling of the substrate from the silicon sheet as in Example 1. When a solar cell was manufactured using a silicon sheet having a higher surface smoothness, the electrode formation was facilitated.

The structure of the silicon sheet produced in Example 2 and the photoelectric conversion efficiency of the solar cell produced using this sheet are also shown in Table 1 as in Example 1.

(Embodiment 3) In Embodiment 3, FIG.
As shown in (a), an additional substrate 78 having a grooved surface
Was used. Both the width and the step of the groove were set to about 1 mm, and the additional base 78 was attached to the cooling rotator 74 so that the rotation direction of the rotating shaft 75 and the groove direction coincided with each other. Also,
As shown in FIG. 7 (b), an additional substrate 78 having an uneven surface so that pyramidal small projections are formed on the entire surface at regular intervals was also used. Both the interval of the unevenness and the step were set to about 1 mm. FIG. 7A in Example 3
Except for using the additional substrate shown in (b) and (b), the other conditions regarding the manufacturing apparatus and manufacturing method used are the same as in the case of Example 1.

Grooves and irregularities of the additional substrate as shown in FIGS. 7A and 7B are likely to be the starting points of silicon crystal growth. Therefore, the starting point of crystal growth can be determined by determining the distribution of the grooves and the irregularities. That is, by regularly forming the groove intervals and the uneven intervals, it is possible to improve the size and uniformity of the silicon crystal grains and also improve the uniformity of the sheet thickness over a wide area. Even if the arrangement of the individual grooves or irregularities is irregular, a slight change in the size of the crystal grains or the sheet thickness is recognized, but the sheet can be obtained.

The maximum value of the height difference of surface irregularities in the silicon sheet obtained in Example 3 is 40 μm on the main surface side in contact with the additional substrate and 80 μm on the main surface side in contact with the silicon melt. is there. During the growth of the silicon sheet, the silicon melt contacts only the apexes of the grooves or the irregularities (steps of about 1 mm) of the additional substrate, and does not contact the bottom. In addition, in Example 3, the silicon crystal grains could be increased by setting the groove interval or the uneven interval to be large.

FIG. 8 shows a photograph of a cross section of an example of a silicon sheet produced by using an additional substrate having a groove as shown in FIG. 7 (a). In this photograph of the cross section, the lower main surface of the silicon sheet is the surface in contact with the additional substrate, and the upper main surface is the surface in contact with the silicon melt.

As can be seen from the photograph of the cross section of FIG. 8, the thickness of the sheet periodically fluctuates corresponding to the period of the groove width. That is, the maximum value and the minimum value of the sheet thickness appear periodically. Then, in each of the minimum value regions of the thickness, crystal grain boundaries that are substantially parallel to the thickness direction are formed.

7A and 7B in the third embodiment.
The structure of the silicon sheet produced by using the additional substrate of and the photoelectric conversion efficiency of the solar cell produced by using this sheet,
These are shown in Table 1 as Examples 3 (a) and (b), respectively.

[0052]

[Table 1]

[0053]

As described above, the silicon sheet of the present invention is capable of high-speed growth and has good semiconductor characteristics in a polycrystalline state. It is also possible to reduce the cost of other various semiconductor devices.

Further, by adapting the structure and manufacturing process of the solar cell using the silicon sheet according to the present invention to the characteristics of the crystal grain structure of the sheet and the characteristics of the semiconductor, it is possible to inexpensively mass-produce the solar cell having good characteristics. You can

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view schematically showing a method for manufacturing a silicon sheet according to the present invention, in which (a) shows a growth stage of the silicon sheet and (b) shows a state where the silicon sheet is peeled from a substrate. Shows.

FIG. 2 is a schematic cross-sectional view parallel to the thickness direction of an example of the silicon sheet according to the present invention.

FIG. 3 is a schematic sectional view showing an example of an apparatus for producing a silicon sheet according to the present invention.

FIG. 4 is a cross-sectional photographic view showing an example of a silicon sheet according to the present invention.

FIG. 5 is a flow diagram showing an example of a process for producing a solar cell using the silicon sheet according to the present invention.

FIG. 6 is a cross-sectional view schematically showing an example of a solar cell manufactured using the silicon sheet according to the present invention.

FIG. 7 is a schematic perspective view showing a surface shape of an additional substrate that can be used for producing a silicon sheet according to the present invention, and (a) shows the additional substrate having periodic grooves formed on its surface, (B) shows an additional substrate having periodic pyramid-shaped irregularities formed on its surface.

FIG. 8 is a cross-sectional photographic view showing an example of a silicon sheet produced using the additional base body of FIG. 7 (a).

FIG. 9 is a schematic sectional view parallel to the thickness direction showing an example of a silicon substrate cut out from an ingot by a conventional casting method.

[Explanation of symbols]

1 main surface that was in contact with the substrate, 2 main surface that was in contact with the silicon melt, 3 substrate, 4 crucible, 5 silicon melt, 6
Cooling means.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Captain Nagano             22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka             Inside the company F term (reference) 4G072 AA01 BB09 BB12 DD04 GG01                       HH01 NN05 NN21 TT01 UU02                 5F051 AA03 CB04 CB30 DA03 DA20

Claims (14)

[Claims] 1. A silicon sheet directly formed by solidifying from a liquid phase silicon by bringing a substrate into contact with a silicon melt, the sheet having a first main surface in contact with the melt and the silicon sheet. The average crystal grain size appearing on the second main surface in contact with the substrate was 10 on both sides.
A silicon sheet having a thickness of less than mm and an average crystal grain size appearing on the first main surface is larger than an average crystal grain size appearing on the second main surface. 2. The silicon sheet according to claim 1, wherein the average crystal grain size appearing on the first main surface and the second main surface is 3 mm or less on both surfaces. 3. The silicon sheet according to claim 1, wherein the difference in the average crystal grain size between the first main surface and the second main surface is 10 μm or more and 5 mm or less. 4. The average crystal grain size is defined as an average interval between intersections of an arbitrary straight line on the first main surface or the second main surface and a crystal grain boundary. The silicon sheet according to any one of claims 1 to 3. 5. The silicon sheet according to claim 1, wherein the sheet has a periodic and gradual thickness change. 6. The crystal grain boundary that is substantially parallel to the thickness direction is formed in the minimum value region of the thickness that appears periodically in the thickness change. Silicone sheet. 7. The silicon sheet according to claim 5, wherein the cycle of the thickness change is 10 mm or less. 8. The height difference of the unevenness due to the change in the thickness is larger on the first main surface side than on the second main surface side, according to any one of claims 5 to 7. Silicon sheet. 9. The silicon sheet according to claim 1, having an average thickness in the range of 100 μm to 1 mm. 10. The silicon sheet according to claim 1, which has a purity of 5 nines or more. 11. The silicon sheet according to claim 1, wherein the height difference of surface irregularities included in the sheet is 200 μm or less. 12. The silicon sheet according to claim 1, which has a carrier diffusion length of 30 μm or more. 13. A solar cell comprising the silicon sheet according to any one of claims 1 to 12. 14. The solar cell according to claim 13, wherein light to be photoelectrically converted is incident from the first main surface side of the silicon sheet.