JP2002094091A - Method and device for manufacturing metal or semiconductor sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the manufacturing method of a substrate that can simply form unevenness which is effective for optical confinement on a substrate for a solar cell. SOLUTION: In this manufacturing method of metal or a semiconductor sheet, the metal or a semiconductor material is fused, and the hot-water surface is cooled and at the same time is pulled up as a sheet for obtaining the metal or a semiconductor sheet. In this case, when the hot-water surface is pulled up as the sheet while being cooled, the hot-water surface is vibrated, thus forming the unevenness on the surface of the metal or semiconductor sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for producing a metal or semiconductor sheet.
A metal or semiconductor in a molten state is cooled and pulled up as a sheet to produce a metal or semiconductor (elemental) sheet having irregularities on the surface. Specifically, a surface suitable for confining light on the surface of a solar cell substrate The present invention relates to a method for producing a crystal sheet for producing a crystal sheet having a shape formed simultaneously, and an apparatus for producing the crystal sheet, and is particularly useful for producing a low-cost silicon substrate for solar cells.

[0002]

2. Description of the Related Art Generally, a rolling method is used for industrially producing a metal sheet or plate. When a sheet having a non-flat surface of a specific shape is obtained, a jig used for rolling is processed into a corresponding shape. That is,
A metal sheet having a desired shape surface is obtained by subjecting the surface of the rolling jig to unevenness processing in advance.

On the other hand, a method for obtaining a crystalline silicon thin plate for a solar cell is to slice a single crystal ingot obtained by a CZ method or the like or a polycrystalline ingot obtained by a casting method to obtain a substrate having a thin smooth surface. Method and directly,
There are a ribbon method and a sheet method for producing a silicon thin plate from a silicon melt. Due to the rapid increase in demand for solar cells in recent years and concerns about a shortage of silicon raw materials, attention has been paid again to a growth method using a ribbon method without slicing a material (for example, Japanese Patent Application Laid-Open Nos. 61-275119 and 9-1995). No. 12394). A crystal plate formed by the ribbon method is generally polycrystalline silicon.

When a crystalline silicon substrate obtained by the ribbon method as described above is used for a solar cell, the surface thereof has a fine uneven shape capable of reducing reflection in order to make light efficiently enter the substrate. Is formed. In the case of single-crystal silicon, a substrate having a (100) plane orientation and a smooth surface is processed into a pyramid shape of about several to 10 μm by using anisotropic etching with an alkaline aqueous solution (texture etching). On the other hand, in the case of polycrystalline silicon, since crystal grains having various orientations are present on the substrate surface, a mechanical groove processing using a dicer or the like (for example, Japanese Patent Application Laid-Open No. 03-071677) rather than an anisotropic etching method, Formation of random irregularities by reactive ion etching [for example, Japanese Patent Publication No. 60-027195 (Patent No. 13)
No. 01206)] is being studied.

[0005]

Rolling can be easily performed with metals such as iron and aluminum, but not with non-metallic elements such as silicon which have no malleability. It cannot be used as a method for producing a sheet having an uneven shape. On the other hand, a substrate sliced from an ingot produced by the CZ method or the casting method had a smooth surface, and when it was formed into a cell as it was, the cell surface remained flat and surface reflection increased. To obtain high conversion efficiency with a practical solar cell, a high-cost additional process such as etching is required.
In particular, in polycrystalline cells, advanced anti-reflection techniques such as groove processing and plasma etching are under development, but in texture etch using anisotropic etching, reflection is sufficiently reduced due to plane orientation dependence. There is no situation. In short, at present, at the time of manufacturing a silicon substrate for a solar cell, unevenness cannot be simultaneously formed on the surface of the silicon substrate, and high conversion efficiency and low cost cannot be simultaneously promoted.

[0006]

SUMMARY OF THE INVENTION In view of the above-mentioned problems, the present invention comprises melting a metal or semiconductor material and pulling it up as a sheet while cooling the material to obtain a metal or semiconductor sheet. Provided is a method for manufacturing a metal or semiconductor sheet, characterized in that unevenness is formed on the surface of a metal or semiconductor sheet by vibrating a molten metal surface when the sheet is pulled up while cooling. That is, according to the present invention, when the molten metal or semiconductor material is pulled up as a sheet, the surface of the sheet can be easily formed with irregularities simply by vibrating the surface of the melt to make it undulate.

Here, the formation of irregularities on the sheet surface due to vibration of the melt surface will be described with reference to the drawings. FIG. 1 is a schematic explanatory view for explaining a step of forming irregularities on a sheet surface.

In FIG. 1, the frequency f (Hz) generated by the vibrator 1 is preferably 2 to 5000 Hz, and the wave 2 of the molten metal surface proceeds toward the rotary cooling body 3. The top of the wave 2 has a rotating cooling body 3 every 1 / f (s).
, And as shown in FIG. 1, the wave 2 of the melt contacts the rotary cooling body 3, and crystallization proceeds at the contacted portion. By continuously solidifying the portion where the top of the wave 2 comes into contact with a lapse of time, continuous irregularities (substantially parallel waves) are formed on the surface of the rotary cooling body 3. When the surface of the rotary cooling body 3 rotates at a peripheral speed v (m / s), the convex portion is formed on the substrate by v / f.
(M). The height of the unevenness was determined to be about one half of the period, that is, 1 /
It was found that this corresponds to the crystal growth thickness in the time of 2f (s). That is, the speed of crystal growth is a (m / s)
In this case, the projection has a height of about a / 2f (m). For example, when a is set to 160 × 10 ⁻⁶ m / s and f = 10, the height of the projection is about 8 × 10 ⁻³ mm.

FIG. 2 is a schematic sectional view showing the vicinity of the surface of the substrate thus manufactured. When a substrate having such an uneven surface is used for a solar cell, a part of the light 5 incident on the convex slope of the substrate surface 4 is reflected as shown in FIG. 2 and re-enters the substrate to contribute to photoelectric conversion. However, the conversion efficiency is improved as compared with the case where the substrate surface is flat. As described above, unevenness effective for improving the efficiency can be formed simultaneously with the production of the substrate without a post-process, and the cost can be reduced. In addition, since the unevenness can be formed without cutting the substrate, the utilization efficiency of the material can be improved. Furthermore, by forming the unevenness in a parallel wave shape, that is, a groove shape, the front electrode can be arranged in series with the groove at the time of manufacturing the solar cell, and the series resistance can be reduced and high efficiency can be achieved.

According to another aspect of the present invention, there is provided a crucible for melting and containing a metal or semiconductor material, a vibrator for vibrating the molten metal or semiconductor material in the crucible, and a driving means therefor. In addition, it is possible to provide an apparatus for manufacturing a metal or semiconductor sheet provided with a cooling pulling body for obtaining a metal or semiconductor sheet having irregularities on the surface by pulling up the sheet while cooling the surface of the vibrating metal or semiconductor material.

Here, as the cooling pulling body, a rotating cooling body in which a part (lower part) is immersed in a melting metal or semiconductor material, specifically, a disk-shaped or cylindrical rotating cooling body,
Alternatively, a moving cooling body (a flat sheet is obtained) in which a plurality of flat cooling plates are connected in a loop or chain shape and moved in the connecting direction is preferable.

According to still another aspect of the present invention, there is provided a silicon sheet having an uneven surface formed by the method for producing a metal or semiconductor sheet according to claim 3, and a silicon sheet formed in the silicon sheet. A solar cell comprising a PN junction, a light receiving surface electrode formed on the surface of the silicon sheet having irregularities, and a back electrode formed on the back surface of the silicon sheet.

Here, in order to form an N-type diffusion layer on the front side and a P-type layer on the back side of a silicon sheet (substrate) having irregularities on the surface, a method usually employed in this field is employed. For example, the entire silicon sheet can be formed of P-type silicon, and the surface side thereof can be partially made into an N-type diffusion layer by vapor-phase diffusion of POCl ₃ .

An anti-reflection film is formed on the front surface of the obtained silicon substrate via a passivation film of SiO 2 if necessary, and electrodes (light receiving surface or light-receiving surface) are formed on the reflection and the rear surface of the silicon substrate, respectively. A front electrode and a back electrode are formed, and these films and a pair of electrodes can be formed by a method generally used in this field.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method and an apparatus for producing a metal or semiconductor sheet according to the present invention will be described mainly based on an embodiment of the apparatus. Note that the present invention is not limited by this. First, the invention of the present application is characterized in that a metal surface vibration mechanism is provided in an apparatus for producing a metal or semiconductor (element) sheet. Specifically, a vibrator made of a refractory is placed at an arbitrary position in the melt and the molten metal surface is vibrated. While cooling from the hot surface of the vibrating state,
By pulling up the metal or semiconductor element sheet, a metal or semiconductor element sheet having an uneven surface can be manufactured.

Now, an apparatus for manufacturing a metal or semiconductor sheet according to the present invention will be described in detail below. FIG. 3 is a schematic explanatory sectional view (chamber wall 13) of a chamber of the semiconductor sheet manufacturing apparatus. In FIG. 3, 6 is a crucible made of high-purity graphite, and 7 is a vibrator made of a refractory (for example, carbon coated on its surface with silicon nitride) connected to a vibrating device 12 outside the chamber by vibrator support 11. Reference numeral 8 denotes a rotary cooling body through which a cooling gas passes, 9 denotes a resistance heater of the crucible 6, and 10 denotes a melt.

The material of the vibrator 7 may be any material that is durable even at high temperatures. For example, a molded body of high-purity carbon, refractory ceramics, heat-resistant metal, silicon nitride, and silicon carbide can be considered. Carbon materials are more preferable because they are inexpensive and rich in processability. Since the vibrator 7 is inserted into the metal melt, a material having low reactivity with the melt is more preferable, so that unnecessary impurities are not mixed into the seed to be taken out. The surface of the vibrator and the vibrator support may be coated with silicon nitride, silicon oxide, silicon carbide, boron nitride, or the like having low reactivity.

The height of the vibrator 7 in the crucible must be such that the lower end of the vibrator touches the molten metal surface when vibrating. As for the shape and position of the vibrator 7, it is desirable that the vibrator 7 has a side parallel to the sheet substrate surface and the length of the side is longer than the width of the rotary cooling body. In addition, it is desirable that the generated parallel waves are arranged sufficiently close so as to reach the rotary cooling body in parallel. This vibrates the molten metal surface in contact with the rotating cooling body uniformly,
This is for forming a linear wave (ridge or groove). As the vibration device 12, that is, the driving means of the vibrator 7, a piezoelectric vibration source (vibrator), a combination of a deformed cam and a motor, or the like can be adopted.

Embodiment 1 A case of manufacturing a low-cost silicon solar cell by applying the present invention will be described below. Prepare a master alloy containing boron as a high-purity silicon and its dopant, and obtain a desired boron concentration (2Ω · cm in this example).
Was adjusted and placed in a quartz crucible protected by a high-purity carbon crucible. The crucible is fixed in the chamber, the chamber is evacuated, and 2 × 1
Reduce the pressure to 0 ^-5 torr or less. Thereafter, Ar gas is introduced into the chamber, and the pressure is returned to normal pressure. Thereafter, the Ar gas is always kept flowing from the upper portion of the chamber. Next, the temperature of the silicon melting heater was set to 1500 ° C.
The silicon is completely melted. At this time, since the silicon material is melted and the molten metal surface (liquid level) is considerably lowered, the molten metal surface is adjusted to a predetermined position by adding a new silicon material. Here, a vibrator (length 200 mm × width 10 m) connected to a vibration device (not shown)
m) is opposed to where the surface of the rotary cooling body rises from the silicon melt (that is, the vibrator has a flat portion facing the tangent line between the rotary cooling body and the melt at the tip portion), At a position 5 cm away from the melt, it was inserted to a depth of 2 mm from the surface of the molten metal and vibrated at a frequency of 10 Hz. The vibrator is connected to the support at both ends and does not prevent the silicon sheet from being pulled out. Next, a rotating cooling body (diameter 10 mm, width 150 mm) was rotated at a peripheral speed of 1 cm / m.
While rotating at "in", it was immersed in molten silicon, and the silicon sheet was pulled out.

FIG. 4 is a schematic cross-sectional view of the surface of the silicon sheet thus obtained. In FIG. 4, the surface has a waveform having a peak pitch of about 17 μm and a height of 8 μm. When viewed from the top, fine waves (ridges or grooves)
Were formed in parallel. The surface reflectance of the obtained silicon sheet is about 15% in a medium wavelength range of 500 to 1000 nm.
And about 30 of the silicon sheet made without the vibration mechanism
% Had been greatly reduced. The cross-sectional shape is not limited to this embodiment, but can be adjusted in any way according to the vibration frequency of the vibrator and the rotation speed of the rotary cooling body.

Next, a solar cell was manufactured using the thus obtained silicon sheet. That is, although not shown, after cleaning the substrate, POCl ₃ gas phase diffusion is performed.
An N-type diffusion layer is formed, and then SiO 2 is formed on the surface by thermal oxidation.
_{2 A} passivation film was formed, and TiO ₂ was further deposited on the surface as an antireflection film. Next, after the back surface was etched to remove the N-type diffusion layer on the back surface, an aluminum paste was printed and baked on the back surface to form a back surface electric field layer and a back surface electrode. Next, a silver paste was printed and fired on the surface side to form a light receiving surface (surface) electrode, and solder coating was performed to complete the cell. The following table compares the characteristics of the cell according to the present invention with the characteristics of a cell manufactured using a silicon sheet having a smooth surface not according to the present invention. The cell according to the present invention has improved short circuit current and higher efficiency due to reduced surface reflection.

[0022]

[Table 1]

[0023]

According to the present invention, irregularities effective for improving the efficiency can be formed simultaneously with the production of the substrate without a post-process, and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view illustrating a step of forming irregularities on a substrate (sheet) surface according to the present invention.

FIG. 2 is a schematic explanatory cross-sectional view showing a light confinement effect by forming irregularities on a substrate surface according to the present invention.

FIG. 3 is a schematic view of an apparatus for producing a semiconductor sheet according to the present invention.

FIG. 4 is a schematic explanatory sectional view of a solar cell using a substrate produced according to the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 vibrator 2 wave 3 rotary cooling body 4 substrate surface 5 incident light 6 crucible 7 vibrator 8 rotary cooling body 9 heater 10 melt 11 vibrator support 12 vibrator 13 chamber wall

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Taniguchi F-term (reference) in Sharp Co., Ltd. 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka 4G072 AA01 BB02 GG03 GG04 HH01 NN21 UU01 5F051 AA03 CB06 CB29 DA03 GA04 GA14

Claims (9)

[Claims] 1. A method of melting a metal or semiconductor material and pulling it up as a sheet while cooling the material to obtain a metal or semiconductor sheet, and vibrating a molten metal surface when pulling up the material as a sheet while cooling the material. A method for producing a metal or semiconductor sheet, comprising forming irregularities on the surface of a metal or semiconductor sheet. 2. The method for producing a metal or semiconductor sheet according to claim 1, wherein the unevenness is substantially parallel wavy. 3. The method for producing a metal or semiconductor sheet according to claim 1, wherein the metal or semiconductor material is silicon. 4. The vibration of the molten metal surface has a frequency of 2 to 5000H.
The method for producing a metal or semiconductor sheet according to claim 1, wherein the vibration is z. 5. A crucible for melting and containing a metal or semiconductor material, a vibrator for vibrating a surface of the molten metal or semiconductor material in the crucible and a driving means therefor, and a vibrating metal or semiconductor material An apparatus for producing a metal or semiconductor sheet, comprising: a cooling and pulling body for pulling up a sheet while cooling a surface to obtain a metal or semiconductor sheet having an uneven surface. 6. The apparatus for producing a metal or semiconductor sheet according to claim 5, wherein the cooling pulling body is a rotary cooling body partially immersed in a melting metal or semiconductor material. 7. The apparatus for producing a metal or semiconductor sheet according to claim 6, wherein the vibrator has, at its tip, a flat portion facing a tangent between the cooling pulling body and the melt. 8. A silicon sheet obtained by the method for producing a metal or semiconductor sheet according to claim 3, wherein the silicon sheet has irregularities on its surface, and a P formed in the silicon sheet.
A solar cell comprising: an N-junction; a light-receiving surface electrode formed on the surface of the silicon sheet having irregularities; and a back electrode formed on the back surface of the silicon sheet. 9. A PN junction comprising: a P-type silicon base layer;
The solar cell according to claim 8, wherein the solar cell is formed from an N-type diffusion layer.