WO2003029143A1 - Silicon sheet and solar cell including the same - Google Patents

Abstract

A silicon sheet directly formed by solidification of a liquid phase silicon by bringing a silicon melt into contact with a cooling base is characterized in that the average silicon crystal particle sizes of the silicon crystals formed on a first major surface (1) that has been in contact with the silicon melt and on a second major surface (2) that has been in contact with the cooling base are both below 10 mm, and the average crystal particle size on the first major surface (1) is larger than that on the second major surface (2). By such a characteristic, a silicon sheet enabling both high-rate growth and good semiconductor characters and a solar cell using the sheet are provided.

Description

 Description Silicon sheet and solar cell containing it

 The present invention relates to reducing the cost of a silicon sheet, and more particularly to a low-cost silicon sheet having sufficient semiconductor characteristics, for example, for a solar cell. Background art

 As a silicon substrate mainly used for manufacturing a solar cell, for example, polycrystalline silicon manufactured using a casting method as disclosed in Japanese Patent Application Laid-Open No. H11-111210 is widely used. I have. In the casting method, silicon melted in a crucible is gradually cooled from the bottom of the crucible to solidify the silicon melt, and an ingot mainly composed of a long columnar crystal structure grown upward from the bottom of the crucible (solidification). Lump). At the beginning of cooling, the solid-liquid interface of silicon is close to the cooling surface at the bottom of the crucible, but as the solidification progresses, the solid-liquid interface gradually moves away from the cooling surface. In addition, the thermal conductivity of solid-phase silicon is lower than that of the liquid phase, which also makes it difficult to achieve a desired rate of solid-phase growth for homogenizing semiconductor properties.

 As a device that can improve this, Japanese Patent Application Laid-Open No. 11-92284 discloses a method in which the relationship between the ascending movement speed of the silicon-solid interface and the amount of heat released from the lower surface of the crucible is determined in advance. By controlling the amount of release, the solidification rate is stabilized and a silicon ingot with good semiconductor properties is obtained. The average grain size appearing on the horizontal cross section of the ingot is greater than 10 mm. According to this technology, stable one-way pseudo-solid growth upward from the bottom of the crucible becomes possible. The cross section in the thickness direction of the substrate cut out horizontally from the ingot produced by this casting method includes a grain boundary almost parallel to the thickness direction, as shown in FIG. That is, a silicon substrate obtained by the casting method is manufactured by slicing in a horizontal direction parallel to the bottom of the crucible, and the semiconductor characteristics on both main surfaces of the substrate are almost the same.

However, in the casting method, it is necessary to obtain a polycrystalline silicon substrate from the ingot. The need for a slicing process has limited the cost of silicon substrates. On the other hand, about 20 years ago, the growth of silicon ribbons by the slice-free web method and the EFG (edge-defined film-fed growth) method has been studied. In recent years, the RGS (ribbon growth on substrate) method, which produces thin silicon ribbons directly from a silicon melt, has been attracting attention in order to achieve faster growth ("MICROSTRUCTURAL ANALYSIS OF THE CRYSTALLIZATION"). OF SILICON RIBBONS PRODUCED BY THE RGS PROCESS ", I. Steinbach et al., 26th PVSC, 1997, pp. 91-93).

 The principle of the RGS method is to perform high-speed silicon ribbon growth by high-speed heat transfer (heat removal) from a surface close to the solidification growth surface. Specifically, the lower support plate that supports the open lower surface of the molten silicon is moved laterally relative to the side support frame that supports the side periphery of the molten silicon while cooling the lower support plate. Silicon ribbon is grown at high speed on top.

 Immediately after the lower supporting plate in contact with the bottom surface of the molten silicon is drawn out in the horizontal direction, the silicon on the flat plate is in a liquid phase, and is cooled simultaneously from both the lower surface of the drawn out supporting plate and the silicon surface. The Rukoto. In the silicon ribbon produced by this method, the solid-liquid interface is inclined with respect to the plane of the supporting plate in a vertical section parallel to the direction of movement of the supporting plate, and it is shown that the silicon crystal grows obliquely and solidifies. ing. That is, the shape of the grown crystal grains becomes columnar crystals oblique to the plane of the supporting plate.

 Recently, a method for directly obtaining a silicon sheet by solidification from a liquid phase by bringing a substrate into contact with a silicon melt has been disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-223172. .

However, the casting method requires several tens of hours to produce one silicon ingot in order to grow the ingot without cracking and to ensure semiconductor quality. In addition, it takes several tens of hours to cut a silicon substrate from an ingot, even if a slicing technique using a multi-wire source is used. Therefore, it is difficult to reduce the cost of manufacturing a silicon substrate using the casting method. Also, in the ribbon manufacturing method such as the RGS method, there are many problems in the stable growth of the solidification phase itself, including the problem of controlling the crystallization state of the silicon ribbon, and the stable semiconductor characteristics that can be put to practical use in solar cells etc. It is not at the stage where a silicon ribbon having this is obtained. Further, in the technology disclosed in Japanese Patent Application Laid-Open No. 2000-223172, further improvement is desired in order to obtain a silicon sheet having a favorable crystal structure and semiconductor characteristics.

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

 A silicon sheet according to an aspect of the present invention is a silicon sheet directly formed by solidification from liquid silicon by bringing a substrate into contact with a silicon melt, and the sheet is in contact with the silicon melt. The average crystal grain size that appeared on the first main surface and the second main surface that was in contact with the substrate was less than 1 O mm in both of the main surfaces, and the average crystal grain size that appeared on the first main surface was It is characterized by being larger than the average crystal grain size that appears on the two main surfaces.

 A silicon sheet having an average crystal grain size of 3 mm or less that appears on the first main surface and the second main surface can be produced relatively easily at low cost. However, when the difference of the average crystal grain size is more than 5 mm, the thickness of the silicon sheet becomes thicker than 1 mm, and the production cost increases. Further, it is possible to obtain a silicon sheet in which the difference between the average crystal grain sizes appearing on the first main surface and the second main surface is not less than 10 μπα and not more than 5 mm. 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. In the region of the minimum value of the thickness that periodically appears in the thickness change, a grain boundary substantially parallel to the thickness direction is formed. It is appropriate that the period of the thickness change is 10 mm or less. When the cycle of the thickness change is larger than 1 O mm, the height difference of the unevenness of the silicon sheet becomes larger than 300 m, and it becomes difficult to obtain a flat silicon sheet. The silicon sheet was in contact with the substrate due to the height difference of the unevenness in the thickness change It is larger on the first principal surface side that was in contact with the silicon melt than on the second principal surface side.

 Preferably, the silicon sheet has an average thickness in the range from 100 / xm to 1 mm. The silicon sheet preferably has a purity of 5 nines or more. Further, the height difference of the surface irregularities included in the silicon sheet is preferably 200 jum or less. The silicon sheet may have a carrier diffusion length of 30 μιη or more.

 The silicon sheet as described above can be preferably used for a solar cell. The 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. BRIEF DESCRIPTION OF THE FIGURES

 FIG. 1 is a schematic cross-sectional view schematically showing a growth stage of a silicon sheet in a method for manufacturing a silicon sheet according to the present invention. FIG. 1B is a schematic cross-sectional view schematically showing a state in which the silicon sheet is peeled off from the substrate in the method for producing a silicon sheet according to the present invention.

 FIG. 2 is a schematic 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 manufacturing a silicon sheet according to the present invention.

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

 FIG. 5 is a flowchart 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 illustrating an example of a solar cell manufactured using the silicon sheet according to the present invention.

7A and 7B are schematic perspective views showing the surface shape of an additional substrate that can be used for producing a silicon sheet according to the present invention. FIG. 7A shows an additional substrate having periodic grooves formed on the surface, and FIG. 7B shows an additional substrate having periodic viramid-like irregularities formed on the surface. FIG. 8 is a cross-sectional photograph showing an example of a silicon sheet produced using the additional substrate of FIG. 7A.

 FIG. 9 is a schematic cross-sectional view parallel to the thickness direction showing an example of a silicon substrate cut out from an ingot by a conventional casting method. BEST MODE FOR CARRYING OUT THE INVENTION

 First, a basic procedure for producing a silicon sheet according to the present invention will be described. As shown in the schematic cross-sectional view of FIG. 1A, the heat resistance is controlled by temperature control means 6 capable of heating and cooling to a temperature lower than the melting point of silicon, that is, 1415 ° C. The silicon sheet 7 grows on the surface of the base 3 by contacting or dipping the surface of the base 3 with the silicon melt 5 in the crucible 4. After the silicon sheet 7 having the required thickness is grown, the substrate 3 on which the sheet is adhered is taken out of the crucible 4. At the stage when the substrate 3 integral with the sheet 7 is cooled from a high temperature, as shown in FIG. 1B, the substrate 3 and the sheet 7 are naturally separated due to their difference in thermal expansion coefficient, and Alternatively, a silicon sheet 7 separated by applying a small impact to the substrate 3 and directly formed by solidification from the liquid phase is obtained.

 In the silicon sheet according to the present invention, the initial temperature of the substrate 3 is controlled in a temperature range from 120 ° C. to 100 ° C. lower than the silicon melting point (141 ° C.), and an appropriate thickness. Optimizing the heat capacity of the substrate 3 by using the graphite material described above, using a gas as a refrigerant in the temperature control means 6 for heating and cooling the substrate 3, and immersing the substrate 3 in the silicon melt 5 To obtain a silicon sheet with the optimum thickness, and by setting basic conditions such as promoting the solidification of the silicon solution by the fine irregularities on the surface of the substrate 3, the surface of the substrate 3 Thus, a polycrystalline silicon sheet can be formed quickly and stably.

That is, since the temperature of the substrate 3 is controlled to be lower than the temperature of the silicon melt 5, silicon crystal nuclei are generated everywhere on the surface of the substrate. Then, the crystal nuclei grow in one direction toward the direction in contact with the silicon melt to form a polycrystalline silicon sheet. In the silicon sheet separated from the base 3, the average crystal grain size appearing on one main surface is different from the average crystal grain appearing on the other main surface. More specifically, as shown in FIG. 2 which is a schematic cross-sectional view parallel to the thickness direction of the silicon sheet, the crystal grains appearing on one first main surface 1 of the sheet and the other (2) The size of the average crystal grain size differs from that of the crystal grains appearing on the main surface (2). In other words, a plurality of crystal nuclei generated on the second principal surface 2 grow and grow in various directions toward the first principal surface 1 side. The generation and growth of small crystal grains is suppressed. As a result, on each of the first main surface 1 and the second main surface 2, the average length between intersections of an arbitrary straight line and a crystal grain boundary is different from each other. More specifically, the average length of the intersection between an arbitrary straight line and the crystal grain boundary on the second principal surface 2 that was in contact with the base 3 during the sheet production was small, and was in contact with the silicon melt 5. It becomes larger on the first main surface 1.

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

 More specifically, 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. The average length (average grain size) between the intersections of the first main surface 1 and the grain boundary with the average length (the average grain size) between the intersections of the other second main surface 2 and the grain boundary When the absolute value of the difference from the above is not less than 10 μπι and not more than 5 mm, a semiconductor element such as a solar cell using this sheet can be manufactured. 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 / xm or more and lmm or less.

By setting the thickness of the silicon sheet to 100 μππ or more, high handling properties can be obtained in a solar cell manufacturing process using the sheet. Also, by reducing the sheet thickness to lmm 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 μπι to 1 mm, the slice process as in the case of the casting method becomes unnecessary, and good semiconductor characteristics can be obtained. From the viewpoint of sheet manufacturing easiness, it is preferable that the average thickness be in the range of 200 μππ to 600 zm. Is more preferable.

 When the silicon sheet has a purity of 5 nines or more, excellent device characteristic values can be obtained even when used in solar cells and the like. From the viewpoint of the characteristics of the solar cell, it is more preferable that the purity is 7 nines or more. Since the maximum value of the height difference in the unevenness of the surface of the silicon sheet is 200 / im or less, the sheet can be used for solar cells without going through a process such as slicing or polishing, and the surface etching time is reduced. It is possible to shorten or omit the surface etching. When the diffusion length of the carrier in the silicon sheet is 3Ομπι or more, a solar cell having relatively good conversion efficiency can be obtained.

 Regarding the crystal grains that appear on each main surface of the silicon sheet, the specific measurement and evaluation methods for the average particle size described above are described. First, the silicon sheet is cut along the thickness direction including the substantially central portion thereof. If the cut end is polished with a grinding stone of 2000 or more, etching is performed at 80 ° C for 10 minutes using a 10% by mass aqueous Na a solution, and the etching rate depends on the crystal orientation. The crystal grains clearly appear due to the nature. Next, an enlarged cross-sectional image as shown in FIG. 2 is obtained using an image magnifying device using a CCD element or the like. The number of intersections between the surface and the grain boundary per 10 mm distance (20 O mm in the enlarged state) on one main surface of the sheet is counted using a 20-fold enlarged cross-sectional image. Next, the number of intersections between the surface and the grain boundary per 1 O mm of the other main surface is counted.

 Taking FIG. 2 as an example, the number of intersections between the surface and the grain boundaries is seven on the first main surface 1 and 13 on the second main surface 2. In this case, the average grain size, which is the average length between the intersections of the surface and the grain boundaries, is 1 O mm ÷ (7 + 1) = 1.25 mm on the first main surface 1. On the second main surface 2, 1 O mm mm (13 + 1) = 0.714 mm is obtained.

When calculating the average length between the intersections of each major surface and the grain boundary in a cross section along the thickness direction from a solar cell using a silicon sheet, the electrode must be heated with concentrated nitric acid or an acid solution heated with aqua regia. After the metal sheet is removed and the silicon sheet is extracted, the sheet 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 thereof appear clearly. Even after removing the electrode metal etc., the alloy layer type Regarding the formed part, the outermost surface of the silicon sheet may be unclear. In that case, the interface between the alloy layer and the semiconductor layer is used as the surface of the silicon sheet.

 (Example 1)

 An apparatus configuration and a 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. It goes without saying that the sheet-fed sheet manufacturing apparatus shown in FIG. 1A and other apparatuses can also be used.

 FIG. 3 is a schematic longitudinal sectional view of a sheet manufacturing apparatus capable of obtaining a silicon sheet according to the present invention. In this apparatus, a crucible 71, a heater 72, a silicon melt 73, a substrate 74, and a rotation shaft 75 of the substrate are provided in a stainless steel chamber 70, and a sheet take-out hole at the top of the chamber is provided. A take-up mechanism 76 for taking up the sheet from the outside is provided. Further, a silicon raw material charging mechanism 77 is attached, and details thereof are omitted in the drawings. Although the heater 72 uses a resistance heating method, a high-frequency heating method or the like having equivalent performance may be used.

 The additional substrate 78 can be attached to the cylindrical surface of the substrate 74 so that a silicon sheet can be grown on the surface of the additional substrate 78. The material of the base 74 or the additional base 78 is basically graphite, but a base having silicon carbide formed on its surface by a thermal CVD method may be used. As the material of the additional substrate 78, ceramics such as silicon nitride, heat-resistant metals that can withstand high temperatures, and carbon or ceramics partially or entirely coated with ceramics can be used. Metals are also possible.

 The surface of the base 74 or the additional base 78 may be a flat surface, a groove along the rotation direction of the base 74, or a fine or irregular surface arranged regularly or irregularly. Good. Grooves and fine irregularities formed on the surface of the substrate have a function of accelerating the growth of silicon sheets.

As a means for controlling the temperature of the base 74, a gas refrigerant system is adopted in which a cavity is provided near the inner surface of the cylindrical base 74 and either nitrogen, argon or air is introduced under pressure. A metal pipe made of stainless steel, copper, etc. A liquid refrigerant system for controlling 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 the inside of the chamber 70 was evacuated with a vacuum pump and replaced with argon gas. While heating the silicon raw material, the Ar pressure in the chamber 70 was maintained at about 10 Torr (133 Pa). However, it is possible to further increase the degree of vacuum to promote outgassing from silicon raw materials. After silicon was melted, granular silicon was additionally charged from the silicon raw material charging mechanism 77 to adjust the height of the molten silicon surface of the crucible 71 7.

 The temperature of the silicon melt during the production of the silicon sheet was set at 1450 ° C, but depending on the balance with the growth conditions of the sheet, the temperature from the supercooling temperature of 1380 ° C or higher to the higher temperature of 1650 ° C It can be set in the range up to ° C. After the silicon melt surface reaches the specified height, temperature control is performed by passing refrigerant gas into the base 74, and the surface of the base 74 is stabilized with the surface temperature at 1200 ° C. It was immersed in the melt. It is desirable that the temperature of the base 74 be in the range of 100 ° C. to 120 ° C. lower than the silicon melting point.

 When the substrate 74 was driven to rotate by the rotation shaft 75 in the above state, the silicon polycrystal grew on the substrate 74 stably at high speed and with good controllability. In the silicon sheet thus formed, the polycrystal was grown to a uniform thickness from the substrate surface side to the silicon melt side because the base 74 was controlled to the melting point temperature of silicon or lower. In the process of cooling the base 74 and the silicon sheet to room temperature, the silicon sheet was easily peeled from the base by applying a natural or small impact due to the difference in thermal expansion coefficient between them. The average thickness of the obtained sheet was about 500 μm, and the average crystal grain size that appeared on the main surface on the side in contact with the silicon melt was smaller than that on the side in contact with the substrate 74. large.

FIG. 4 shows a cross-sectional photograph of an example of the silicon sheet produced as described above. In this cross-sectional photograph, the lower main surface of the silicon sheet is the second main surface in contact with the substrate, and the number of intersections between the lower main surface and the crystal grain boundaries is large, that is, the average length between the intersections ( (Average particle size) is small. On the other hand, the upper main surface of the silicon sheet is the first main surface that was in contact with the silicon melt, and the number of intersections between the upper main surface and the crystal grain boundaries was large. Is small, that is, the average length (average particle size) between the intersections is large. The average crystal grain size at the lower main surface of the silicon sheet was about 0.22 mm, and the average grain size at the upper main surface was about 0.38 mm. The absolute value of the difference between the average particle diameters of 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 cross section of the photograph in FIG. 4 is 45 at the lower main surface in contact with the base 74, and at the upper main surface in contact with the silicon melt. Number was 26. Of the angles formed by the main surface of the sheet and the crystal grain boundaries in the photograph cross section, those with an acute angle of 80 ° or more and 90 ° or less accounted for 94% of the whole. According to the measurement of the carrier diffusion length, it was found that a value of 45 μm was obtained on the lower main surface in contact with the substrate, and a value of 60 μm was obtained on the upper main surface in contact with the silicon melt. did it. When the unevenness of the surface was measured using a step gauge, the maximum value was 120 μm on the lower main surface side of the sheet 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 due to the difference in the solid-liquid partition coefficient of the impurities contained in the molten silicon.

 A method for manufacturing a solar cell using the silicon sheet thus manufactured will be described. This method can follow the procedure shown in the flowchart of FIG. In this embodiment, the silicon sheet is a p-type semiconductor, but may be an n-type semiconductor. When forming a p-type or n-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 size appearing on both main surfaces is different from each other, so it is necessary to first determine which main surface is to be the light receiving surface of the solar cell to be manufactured. This is determined in consideration of the surface condition of the silicon sheet and the compatibility between the semiconductor characteristics and the solar cell process. In Example 1, the main surface having the larger average crystal grain size that appeared on the surface was selected as the light receiving surface.

In the flow chart of FIG. 5, first, in steps S 1 and S 2, 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. This etching includes dry etching by plasma discharge. Although a tuning method or the like can be used, the use of the wet etching method enables formation of a surface texture at lower cost. In step S4, an n-type diffusion layer was formed by PSG diffusion (a diffusion method using a phosphor silicate glass film). In step S5, after removing the PSG film formed on the surface with hydrofluoric acid, a silicon nitride film was formed on the main surface on the light receiving surface side as an antireflection film. Next, in step S6, the diffusion layer formed on the back side was removed using a mixed solution of nitric acid and hydrofluoric acid. In step S7, an alloy layer and a back surface electrode were simultaneously formed on the back surface side using the A1 paste. Finally, in step S8, the electrode on the light receiving surface side was 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. The solar cell shown in FIG. 6 includes a silicon sheet 50, a diffusion layer 51, a photoelectric conversion layer 52, an alloy layer 53, a front electrode 54, and a back electrode 55. Table 1 shows the structure of the silicon sheet manufactured in Example 1 and the photoelectric conversion efficiency of the solar cell manufactured using the sheet.

 (Example 2)

 In Example 2, a silicon sheet was manufactured with a SiC layer having a thickness of 100 μππ coated on the entire surface of the base 74. Other conditions regarding the manufacturing apparatus and the manufacturing method used in the second embodiment are the same as those in the first embodiment. Accordingly, the temperatures of the silicon melt and the substrate in Example 2 were 1450 ° C. and 1200 ° C., respectively, as in Example 1.

 When the silicon sheet undergoes solidification growth, the surface condition of the substrate 74 has a great effect on crystal growth. By coating the surface of the substrate with a SiC layer that is more wettable than the graphite with respect to the silicon melt, the heat flow from the melt to the substrate increases and the degree of supercooling of the silicon melt decreases. Thus, crystal nucleation and crystal growth can be performed more quickly while suppressing huge dendrite growth. Further, in Example 2, in order to improve the wettability with respect to the silicon melt, a dense hydrocarbon may be coated instead of SiC.

In Example 2, the size of the crystal grains of the silicon sheet became more uniform over the entire surface compared to Example 1, and the surface smoothness was improved on the main surface on the side in contact with the silicon melt. Was. The maximum value of the difference in height of the surface irregularities was 30 m on the second principal surface side in contact with the substrate, and 50 m on the first principal surface side in contact with the silicon melt. The surface coating in Example 2 facilitated peeling of the substrate and the silicon sheet as in Example 1. If a solar cell was manufactured using a silicon sheet having a higher surface smoothness, the electrode formation became easier.

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

 (Example 3)

 In Example 3, as shown in FIG. 7A, 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 substrate 78 was attached to the cooling rotator 74 so that the direction of rotation of the rotating shaft 75 and the direction of the groove matched. Further, as shown in FIG. 7B, an additional substrate 78 was used which had been subjected to unevenness processing so as to form small viramid projections at regular intervals on the entire surface. Both the distance between these irregularities and the steps were reduced to about l mm. Except for using the additional substrate shown in FIGS. 7A and 7B in Example 3, other conditions regarding the used manufacturing apparatus and manufacturing method are the same as those in Example 1.

 The grooves and irregularities of the additional substrate as shown in FIGS. 7A and 7B tend 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 irregularities. That is, by regularly forming the groove interval and the irregularity interval, the size and uniformity of silicon crystal grains can be improved, and the uniformity of the sheet thickness over a wide area can be improved. It should be noted that even when the arrangement of individual grooves and irregularities is irregular, a slight change in crystal grain size / sheet thickness is recognized, but a sheet can be obtained.

The maximum value of the difference in height of the surface irregularities in the silicon sheet obtained in Example 3 was 40 / zm on the second main surface side in contact with the additional substrate, and the first main surface in contact with the silicon melt was It is 80 on the surface side. During the growth of the silicon sheet, the silicon melt contacts only the apexes of the grooves or irregularities (both about 1 mm steps) of the additional substrate, but not the bottom. Further, in Example 3, the silicon crystal grains could be increased by setting the groove interval or the unevenness interval to be relatively large. FIG. 8 shows a cross-sectional photograph of an example of a silicon sheet produced using the grooved additional substrate as shown in FIG. 7A. In this cross-sectional photograph, the lower main surface of the silicon sheet was the surface that was in contact with the additional substrate, and the upper main surface was the surface that was in contact with the silicon melt.

 As can be seen from the cross-sectional photograph of FIG. 8, the thickness of the sheet periodically fluctuates in accordance with the period of the groove width. That is, the maximum value and the minimum value of the sheet thickness appear periodically. In each of the minimum value regions of the thickness, a crystal grain boundary substantially parallel to the thickness direction is formed.

 The structure of the silicon sheet manufactured using the additional substrate shown in FIGS. 7A and 7B in Example 3 and the photoelectric conversion efficiency of the solar cell manufactured using this sheet are also shown in Table 1 in Example 3 (a ) And (b).

 【table 1】

産業上の利用可能性

Industrial applicability

 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, so that if it is used as a substrate for a solar cell, the cost can be significantly reduced. The cost of other various semiconductor devices can be reduced.

Claims

The scope of the claims 1. A silicon sheet directly formed by solidification from liquid silicon by bringing a substrate into contact with a silicon melt, the first principal surface of which the silicon sheet is in contact with the melt The average crystal grain size appearing on (1) and the second principal surface (2) in contact with the base was less than 1 Omm on both of the two surfaces, and the average grain diameter appeared on the first principal surface (1). A silicon sheet characterized in that the crystal grain size is larger than the average crystal grain size appearing on the second main surface (2).  2. The average crystal grain size appearing on the first main surface (1) and the second main surface (2) is 3 mm or less on any of both surfaces. Silicon sheet.  3. The method according to claim 1, wherein the difference between the average crystal grain sizes appearing on the first main surface (1) and the second main surface (2) is 10 μm or more and 5 mm or less. Silicon sheet as described. 4. The average crystal grain size is defined as an average interval between intersections of arbitrary straight lines on the first main surface (1) or the second main surface (2) and crystal grain boundaries. The silicon sheet according to claim 1, wherein the silicon sheet is characterized in that:  5. The silicon sheet according to claim 1, having an average thickness in a range of 100 // m to 1 mm. 6. The silicon sheet according to claim 1, having a purity of not less than 5 nines.  7. The silicon sheet according to claim 1, wherein a height difference of surface irregularities included in the silicon sheet is 200 μΐη or less.  8. The silicon sheet according to claim 1, having a carrier diffusion length of 30 m or more.  9. The silicon sheet according to claim 1, wherein the silicon sheet has a periodic and gentle change in thickness. 10. The grain boundary according to claim 9, wherein a grain boundary that is substantially parallel to the thickness direction is formed in a region where the thickness of the thickness changes periodically in the thickness change. Silicon sheet.  11. The silicon sheet according to claim 9, wherein a cycle of the thickness change is 1 Omm or less.  12. The silicon sheet according to claim 9, wherein a height difference of the unevenness in the thickness change is larger on the first main surface (1) side than on the second main surface (2) side. .  1 3. A solar cell comprising the silicon sheet according to claim 1. 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.