JP2002076388A - Manufacturing method of solar battery cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the manufacturing method of a solar battery cell which removes heavy metal impurities generated in the manufacturing processes of the solar battery cell without increasing the manufacturing cost of the solar battery cell and can prevent a reduction in the lifetime of a silicon wafer. SOLUTION: The manufacturing method of a solar battery cell conducts at least a P-N junction forming process and an electrode forming process on a silicon wafer formed by slicing a silicon single crystal to manufacture the solar battery cell. A work strain 1a on the side of the surface of the sliced silicon wafer 2 is removed. After at least a P-N junction forming treatment is performed on the side of the surface of the wafer 2, a work strain 1b on the side of the rear of the wafer 2 is removed. A silicon single crystal which is formed by a CZ method and contains Ga as a dopant is desirably used as the silicon single crystal, and the removal of the work strain on the side of the surface is performed by a spin etching.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a solar cell using a silicon single crystal wafer useful as a material for a solar cell, and more particularly to a method for gettering contaminants such as heavy metals generated in a manufacturing process of a solar cell. The present invention relates to a method for manufacturing a solar cell capable of removing a ring.

[0002]

2. Description of the Related Art When a solar cell is classified based on its substrate material, it can be roughly classified into three types: a silicon crystalline solar cell, an amorphous silicon solar cell, and a compound semiconductor solar cell. Further, the silicon crystal solar cells include a single crystal solar cell and a polycrystalline solar cell.
Among these, a solar cell having a high conversion efficiency, which is the most important characteristic as a solar cell, is a compound semiconductor solar cell, and its conversion efficiency reaches nearly 25%. The “conversion efficiency” is a value indicating a ratio of energy that can be converted into electric energy and extracted by the solar cell with respect to the energy of light incident on the solar cell, and is expressed as a percentage (%). Say value.

However, compound semiconductor-based solar cells have a problem in widespread use, such as making it very difficult to produce a compound semiconductor as a material, and increasing the manufacturing cost of a solar cell substrate. It has been done.

On the other hand, a silicon single crystal type solar cell has the second highest conversion efficiency after a compound semiconductor type solar cell, exhibits a conversion efficiency of about 20%, and relatively easily procures a silicon wafer as a solar cell substrate. Because it can be done, it is the mainstay of the popular solar cell.

[0005] A process for manufacturing a general solar cell using a silicon single crystal wafer will be described with reference to FIG. First,
A silicon wafer is manufactured by slicing a silicon single crystal ingot manufactured by the CZ method (Czochralski method) or the FZ method (floating zone melting method) or the like (FIG. 2).
(A)). This slicing is performed by a method in which a disk-shaped blade having diamond fine particles attached thereto is rotated at a high speed and cut, or a method in which a large number of sheets are cut at a time using a wire saw. Work distortions 1a and 1b due to mechanical shock at the time of cutting are generated on the front and back surfaces of the wafer 2 obtained by slicing the ingot in this manner.

The processing strains 1a and 1b lower the electrical characteristics such as the lifetime of the wafer and adversely affect the conversion efficiency of the solar cell, and are removed by chemical etching or polishing (FIG. 2 (b)). )). Thereafter, if necessary, one surface serving as a light receiving surface of the solar cell is subjected to a process called texturing process (usually alkaline etching) for reducing light reflection, and further a pn junction forming step and an electrode forming step Then, a solar cell is formed by performing an antireflection film forming step and the like.

FIG. 2C shows a pn junction forming step, in which a diffusion layer 3 having a conductivity type opposite to the conductivity type of the wafer is formed. Usually, a pn junction is formed on one side (light receiving surface side) by introducing an n-type impurity into the surface of a p-type silicon single crystal wafer. At this time, a gas diffusion method, a solid-phase diffusion method, an ion implantation method, or the like is used for introducing impurities, and a heat treatment at a temperature of several hundred degrees Celsius to 1000 degrees Celsius or more is performed.

FIG. 2D shows a step of forming an electrode on the light receiving surface side, after forming an ultra-thin (for example, several nm to several tens nm) oxide film 5 for passivation on the surface of the diffusion layer 3. This is a step of forming an electrode 4 made of a metal by a vapor deposition method, a plating method, a printing method, or the like. Usually, a heat treatment at about several hundred degrees Celsius is applied. Further, as shown in FIG. 2E, an electrode 7 is also formed on the back side of the wafer.

FIG. 2 (f) shows an anti-reflection film forming step, in which SiO, CeO is formed on the light-receiving surface side by CVD (chemical vapor deposition), PVD (physical vapor deposition), or the like. ₂
And the like, and a few hundred degrees C. to 80 degrees
A heat treatment at about 0 ° C. is applied.

[0010]

The solar cell needs to be manufactured at low cost, and the silicon single crystal wafer to be used is necessarily higher in purity than the silicon wafer used for manufacturing a semiconductor device such as an integrated circuit. Not something. This is because an attempt to use a high-purity, high-precision one inevitably increases the cost. However, when a silicon wafer containing impurities is used or when a solar cell is manufactured by the above-described process, the lifetime of the wafer is reduced due to heavy metal impurities generated during the process, and as a result, the solar cell There is a problem that conversion efficiency is reduced.

The present invention has been made in view of the above problems, and removes heavy metal impurities generated in a solar cell manufacturing process without increasing manufacturing costs.
It is an object of the present invention to provide a method for manufacturing a solar cell capable of preventing a reduction in the lifetime of a silicon wafer.

[0012]

In order to achieve the above object, the present invention provides a method for forming a pn junction at least on a silicon wafer prepared by slicing a silicon single crystal.
Performing a process of forming a solar cell by performing an electrode forming step, removing processing strain on the front side of the sliced silicon wafer, performing at least a pn junction forming process on the front side, and then processing the rear side. A method for manufacturing a solar battery cell, comprising removing distortion (claim 1).

As described above, if a process involving a heat treatment such as pn junction formation is performed in a state where the processing strain only on the front surface side of the wafer in which the processing strain due to the slice remains, heavy metal impurities and the like generated in the process are eliminated. The gettering is performed on the processing strain layer on the back side, and thereafter, the processing strain on the back side is removed, so that heavy metal impurities and the like of the silicon wafer can be removed.

Further, if the processing distortion on the front side is removed by spin etching (claim 2), the processing distortion only on the front side can be easily removed without protecting the back side with a protective film or the like. Can be.

Further, as a silicon single crystal for producing the silicon wafer, G single crystal produced by the CZ method is used.
It is preferable to use a silicon single crystal using a as a dopant (Claim 3). In particular, the concentration of Ga in the silicon single crystal is 3 × 10 ¹⁵ atoms / cm ^{3 to} 5 × 10 5
It is preferably ¹⁷ atoms / cm ³ (claim 4). If a crystal using Ga as a dopant by the CZ method is used as described above, it is easy to produce a large-diameter silicon single crystal ingot, and if a silicon single crystal wafer obtained by slicing the silicon ingot is used as a solar cell substrate, It can effectively prevent the deterioration of the lifetime due to light deterioration,
A solar cell having high conversion efficiency can be manufactured.

The inventor of the present invention has found that in the manufacturing process of a solar cell, heavy metal impurities are generated during the heat treatment, thereby contaminating the silicon wafer and shortening the lifetime, and consequently lowering the conversion efficiency of the solar cell. Thought it was possible. This is especially true in the manufacture of solar cells, because low-quality wafers may be used as raw material wafers. Accordingly, the present inventors have conceived that if the silicon wafer is provided with gettering ability during the manufacturing process of the solar cell, it is possible to prevent a reduction in the lifetime due to heavy metal impurities.

It is known that there are IG (intrinsic gettering) and EG (extrinsic gettering) as methods for giving a gettering ability to a silicon wafer. However, since IG is a method of forming an oxygen precipitate in the bulk of a silicon wafer, the IG may cause a reduction in the lifetime and is not appropriate. Further, as EG, a method of forming a distortion by forming a polysilicon film on the back surface of a wafer or damaging by sandblasting or the like is known, but any of these requires an additional step. Therefore, it is not preferable in the manufacturing process of the solar cell, in which the cost reduction is particularly required.

The present inventor has conducted intensive studies and, as a result, focused on the processing strain generated on the surface of a wafer sliced from a silicon single crystal, and formed at least a pn junction on a silicon wafer manufactured by slicing this silicon single crystal. When manufacturing the solar cell by performing the step and the electrode forming step, the processing strain on only the front side of the sliced silicon wafer is removed, and at least the pn junction forming process is performed on the front side, and then the back side. It has been found that heavy metal impurities can be removed by removing the processing strain of the present invention, and the present invention has been completed.

That is, according to the above-described method, heavy metal impurities are gettered by utilizing the processing distortion on the back surface side of the wafer caused by the slicing, and the silicon wafer from which the processing distortion has been removed constitutes a solar cell. . Therefore, substantially no additional processing is required, and a reduction in the lifetime can be prevented. As a result, a low-cost, high-performance solar cell can be manufactured.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described more specifically with reference to the drawings, but the present invention is not limited thereto. In the present invention, first, a silicon single crystal ingot for preparing a silicon wafer is prepared. The silicon single crystal to be used can be manufactured by the CZ method (including the MCZ method (pulling method under a magnetic field)) or the FZ method.

At present, the conductivity type of a wafer used as a solar cell is mainly p-type, and usually boron is added to this p-type wafer as a dopant. CZ
According to the method, a large-diameter single crystal doped with boron can be produced at a relatively low cost. But,
A high concentration of oxygen exists in a crystal manufactured by the CZ method. When strong light is applied to a solar cell made by using such a p-type CZ silicon single crystal, boron and oxygen in the crystal are removed. It may affect the lifetime characteristics and cause light degradation.

The silicon single crystal prepared in the present invention may be a commonly used boron-doped p-type silicon single crystal. However, in order to effectively prevent the photovoltaic cells from being deteriorated by light, the CZ method is used. It is preferable to use the produced silicon single crystal using Ga as a dopant.

As described above, when Ga is used as a p-type dopant,
When a silicon single crystal is produced by the Z method, it is not necessary to consider the adverse effect of boron and interstitial oxygen on the lifetime characteristics, so that it is not necessary to use a silicon single crystal having a low oxygen concentration, and a large diameter of 8 inches or more is required. It becomes easy to produce a silicon single crystal. In addition, when such a Ga-doped silicon single crystal is used, a photovoltaic cell which hardly causes light degradation and has high conversion efficiency can be manufactured.

The concentration of Ga to be doped is 3 × 10
^Fifteen~ 5 × 10¹⁷atoms / cm³(5 to 5 in resistivity
0.1 Ω · cm). Ga concentration is 3 ×
10^Fifteenatoms / cm³If smaller than
The resistivity of the manufactured silicon wafer is higher than necessary
Power is consumed by the internal resistance of the solar cell,
The rate may decrease. When the Ga concentration is 5 × 10
¹⁷atoms / cm³If it is larger than
Since the resistivity of c becomes extremely low,
Recombination reduces minority carrier lifetime
The concentration of Ga in the silicon single crystal is
It is preferably within the above range.

In order to produce a silicon single crystal to which Ga is added by the CZ method, G is added to the silicon melt in the crucible.
After adding a, the seed crystal may be brought into contact with the silicon melt and pulled up while rotating. In this case, the addition of Ga to the melt in the crucible is performed by growing a silicon crystal rod to which high-concentration Ga has been added in advance and crushing the high-concentration Ga-doped silicon crystal rod to obtain a doping agent that is appropriately calculated. It is preferable to add a small amount to the silicon melt.

FIG. 1 shows an example of a manufacturing process of a solar cell according to the present invention. A silicon wafer is manufactured by slicing a silicon single crystal ingot manufactured by the CZ method or the like as described above by an ordinary method using a disk-shaped blade or a wire saw. FIG. 1A shows a silicon wafer 2 immediately after slicing, and processing distortions 1a and 1b due to mechanical shock at the time of cutting are present on both front and back surfaces of the wafer 2.

FIG. 1B shows a step of removing only the processing strain 1a on the front surface side of the wafer 2 which is to be the light receiving surface of the solar cell, while leaving the processing strain 1b on the rear surface side. The wafer immediately after slicing generally has no distinction between the front side and the back side. However, in the present invention, the side serving as the light receiving surface is referred to as the front side, and the opposite side is referred to as the back side.

The method of removing the processing strain only on the front side is not particularly limited. For example, the back side is protected by an etching-resistant protective film or a protective tape, and mixed acid (for example, hydrofluoric acid, nitric acid, acetic acid, water) Immersion in an etching solution such as an alkaline aqueous solution containing KOH, NaOH, or the like, or a method in which only one surface is ground or mechanochemical polished. Preferably, only the processing distortion on the surface side can be removed by so-called spin etching.

In this spin etching, for example, as shown in FIG. 3, a wafer 11 is fixed on a wafer support table 10 so that the front side thereof is directed to an etching solution supply nozzle 12, and the wafer is rotated as described above. Supply an appropriate etchant. According to such spin etching, since the etching solution comes into contact with only the front side and is shaken off, only the processing distortion on the front side can be easily removed without the need to protect the back side. It should be noted that the etching margin for removing the processing strain cannot be unconditionally determined because the processing strain depth varies depending on the slice conditions.
Usually, it is about 10 μm to several tens μm.

The surface subjected to the etching is subjected to a process called a texturing process (usually alkali etching) for reducing the reflection of light as necessary. At this time, the etching may be performed while protecting the back surface side with a protective film or the like as in the etching of FIG. 1B, or the spin etching may be performed. In addition, it is also possible to perform both processing distortion removal and texture processing on the surface side.

FIG. 1C shows a pn junction formation step, in which a conductivity type (eg, p-type) opposite to the conductivity type (for example, p-type) of the wafer 2 is provided on the surface side of the wafer 2 from which the processing strain has been removed. Is formed. Gas diffusion, solid phase diffusion, ion implantation, or the like is used for impurity introduction at this time, and heat treatment at a temperature of several hundred degrees Celsius to 1000 degrees Celsius or more is performed, as in the past. Usually, this step is the highest temperature processing in the manufacturing process of the solar cell, and in the present invention, the heavy metal impurities generated are gettered to the work strained layer 1b on the back surface side of the wafer.

In the present invention, at least after the pn junction forming step, a process for removing the processing strain 1b on the back surface may be performed. Even in the heat treatment, heavy metal impurities can be gettered. Therefore, it is preferable to perform a process of removing the processing distortion 1b on the rear surface side after performing other steps as much as possible as in the following example.

FIG. 1D shows a step of forming an electrode on the front surface side (light receiving surface side). After forming an extremely thin oxide film 5 for passivation on the surface of the diffusion layer 3, a vapor deposition method and a plating method are performed. Forming the electrode 4 made of metal by a printing method or the like. Ni, Au, Ag, T
i, Pd, Al or the like can be used. After that, the processing distortion 1b on the back side may be removed, but it is preferable to perform the post-process as much as possible as described above.

FIG. 1E shows an antireflection film forming step, in which a deposited film 6 as an antireflection film is formed by a CVD method, a PVD method, or the like.
A heat treatment of about ° C is applied. As the antireflection film 6,
For example, SiO ₂ having a refractive index of 1.8 to 1.9 can be used, and in addition, CeO ₂ , Al ₂ O ₃ , and Si ₃ N
₄ , SnO ₂ , SiO ₂ —TiO _{2 and the} like are used.
When the anti-reflection film 6 has two layers, it is usually Ti
A material having a large refractive index such as O ₂ or Ta ₂ O ₅ is used.

FIG. 1F shows a step of removing the processing distortion 1b left on the back side of the wafer. As a method of removing only the processing distortion 1b on the back side, the same method as that in the step of FIG. 1B can be used. The anti-reflection film is protected by a protective tape or the like, and the mixed acid or alkaline aqueous solution is etched. It is possible to easily remove only the processing distortion 1b on the back side by a method of dipping in a liquid, a method of mechanochemical polishing only the back side, or spin etching.

FIG. 1 (G) shows a step of forming an electrode on the back side, in which the electrode metal 7 is formed on the back side of the wafer from which the processing strain has been removed together with the gettered heavy metal impurities. The specific types of the electrode metal and the forming method are the same as those for forming the electrode on the front surface side (light receiving surface side) in FIG. 1D.

As described above, in the manufacturing process of the solar cell, only the processing strain on the surface side of the silicon wafer obtained by slicing the silicon single crystal ingot is removed, and after performing at least the pn junction process, preferably If the processing strain on the back side is removed after performing many other steps involving heat treatment as much as possible, heavy metal impurities generated during these steps are gettered by the processing strain layer on the back side of the wafer. Can be removed. If the gettering is performed using the processing strain generated by the slicing as described above, there is no need for a substantially additional step such as sandblasting, and it is not necessary to form oxygen precipitates for gettering. Without increasing manufacturing costs
It is possible to prevent a decrease in the lifetime of the solar cell.

The present invention is not limited to the above embodiment. The embodiment described above is merely an example, and has substantially the same configuration as the technical idea described in the claims of the present invention, and has the same function and effect,
Anything is included in the technical scope of the present invention.

For example, in the above-described embodiment, the case where the processing distortion on the back surface side is removed after the anti-reflection film forming step (FIG. 1E) has been described. The process is an example, and a pn junction forming process (FIG. 1)
(C)) or a surface-side electrode forming step (FIG. 1 (D))
After that, the processing distortion on the back side can be removed. Further, for example, when the processing strain on the back side is removed after the electrode forming step (FIG. 1D), the anti-reflection film forming step can be performed after the electrodes are formed on the back side. Further, in the manufacturing process of the solar cell, the process may be replaced, added, or omitted, for example, by adding a texture process or omitting the formation of an antireflection film.

[0040]

According to the method for manufacturing a solar cell of the present invention, the processing strain of the sliced silicon wafer is left only on the back side, and is generated in the pn junction forming step, the electrode forming step, or the antireflection film forming step. After gettering heavy metal impurities, to remove backside processing distortion,
An additional step is substantially unnecessary, and a reduction in the lifetime of the silicon wafer can be prevented. Therefore, it is possible to suppress an increase in the manufacturing cost and to improve the performance of the solar cell, and to provide a solar cell with high conversion efficiency at low cost.

[Brief description of the drawings]

FIG. 1 is a process chart showing an example of a manufacturing process of a solar cell according to the present invention.

FIG. 2 is a process diagram showing a conventional solar cell manufacturing process.

FIG. 3 is a schematic view showing spin etching for removing a processing strain on only one surface of a wafer.

[Explanation of symbols]

1a, 1b: processing distortion, 2: silicon wafer, 3
... Diffusion layer, 4 ... Surface side electrode, 5 ... Oxide film, 6 ...
Deposition film (anti-reflection film), 7: back electrode, 10: wafer support, 11: silicon wafer, 12: etching liquid supply nozzle.

Claims (4)

[Claims] At least a pn junction forming step is performed on a silicon wafer produced by slicing a silicon single crystal,
Performing a process of forming a solar cell by performing an electrode forming step, removing processing strain on the front side of the sliced silicon wafer, performing at least a pn junction forming process on the front side, and then processing the rear side. A method for manufacturing a solar cell, comprising removing distortion. 2. The method for manufacturing a solar cell according to claim 1, wherein the processing strain on the surface side is removed by spin etching. 3. The method for manufacturing a solar cell according to claim 1, wherein a silicon single crystal using Ga as a dopant and manufactured by a CZ method is used as the silicon single crystal. . 4. The concentration of Ga in the silicon single crystal is:
3 × 10 ¹⁵ atoms / cm ^{3 to} 5 × 10 ¹⁷ atoms
The method of claim 3, wherein the rate is ms / cm ³ .