JP4434455B2 - Method for manufacturing solar battery cell - Google Patents

Description

[0001]
BACKGROUND 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 in particular, it is possible to getter and remove contaminants such as heavy metals generated in the manufacturing process of a solar cell. The present invention relates to a method for manufacturing a solar battery cell.
[0002]
[Prior art]
When solar cells are classified based on their substrate materials, there are roughly three types: silicon crystal solar cells, amorphous (amorphous) silicon solar cells, and compound semiconductor solar cells. The battery includes a single crystal solar cell and a polycrystalline solar cell. Among these, a solar cell with high conversion efficiency, which is the most important characteristic as a solar cell, is a compound semiconductor solar cell, and the conversion efficiency reaches nearly 25%. The “conversion efficiency” is a value indicating the ratio of energy that can be extracted by converting into electric energy by the solar battery with respect to the energy of light incident on the solar battery cell, and expressed as a percentage (%). Say the value was.
[0003]
However, it is very difficult to make a compound semiconductor as a material for the compound semiconductor solar cell, and there are problems in widespread use such as an increase in the manufacturing cost of the solar cell substrate, and its use is limited. It has become.
[0004]
On the other hand, the silicon single crystal solar cell has the next highest conversion efficiency after the compound semiconductor solar cell, exhibits a conversion efficiency of around 20%, and can easily procure a silicon wafer as a solar cell substrate. It has become the mainstay of popular solar cells.
[0005]
A general manufacturing process of a solar battery 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 FZ method (floating zone melting method) (FIG. 2A). This slicing is performed by a method of cutting a disk-shaped blade having diamond fine particles attached thereto at a high speed and a method of cutting a large number of sheets at once using a wire saw. Processing distortions 1a and 1b due to mechanical impact at the time of cutting are generated on both front and back surfaces of the wafer 2 obtained by slicing the ingot in this way.
[0006]
The processing strains 1a and 1b deteriorate electrical characteristics such as the lifetime of the wafer and adversely affect the conversion efficiency of the solar battery cell, and therefore are removed by chemical etching, polishing, or the like (FIG. 2B).
Thereafter, if necessary, one surface serving as the light receiving surface of the solar battery cell is subjected to a process (usually alkaline etching) for reducing light reflection called texture processing, and further a pn junction forming process and an electrode forming process Then, the solar cell is formed by applying an antireflection film forming step or the like.
[0007]
FIG. 2C shows a pn junction forming step, in which a diffusion layer 3 having a conductivity type opposite to that of the wafer is formed. Normally, 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. In this case, a gas diffusion method, a solid phase diffusion method, an ion implantation method, or the like is used for introducing impurities, and heat treatment is performed at a temperature of several hundred to 1000 ° C. or higher.
[0008]
FIG. 2D shows an electrode forming process on the light-receiving surface side. After forming an extremely thin oxide film 5 for passivation (for example, about several nm to several tens of nm) on the surface of the diffusion layer 3, a vapor deposition method is performed. In this step, the electrode 4 made of metal is formed by a plating method, a printing method, or the like, and usually a heat treatment of about several hundred degrees C is applied.
Furthermore, as shown in FIG. 2E, the electrode 7 is also formed on the back side of the wafer.
[0009]
FIG. 2 (f) shows an anti-reflection film forming step, and SiO, CeO ₂ or the like is formed on the light receiving surface side by a CVD method (chemical vapor deposition method), a PVD method (physical vapor deposition method), or the like. A deposited film 6 is formed, and a heat treatment of about several hundred to 800 ° C. is applied at that time.
[0010]
[Problems to be solved by the invention]
Solar cells need to be manufactured at low cost, and the silicon single crystal wafers used are not necessarily as pure as silicon wafers used in the manufacture of semiconductor devices such as integrated circuits. This is because the use of high purity and high accuracy inevitably increases the cost. However, when using a silicon wafer mixed with impurities or manufacturing a solar cell by the process as described above, the lifetime of the wafer is reduced due to heavy metal impurities generated during the process. There was a problem of reducing the conversion efficiency.
[0011]
The present invention has been made in view of such problems, and it is possible to remove heavy metal impurities generated in the manufacturing process of solar cells without increasing the manufacturing cost and prevent the lifetime of the silicon wafer from being lowered. It aims at providing the manufacturing method of the photovoltaic cell which can be performed.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a method for producing a solar cell by subjecting a silicon wafer produced by slicing a silicon single crystal to at least a pn junction forming step and an electrode forming step. A method of manufacturing a solar battery cell comprising: removing the processing strain on the front surface side of the silicon wafer, performing at least a pn junction forming process on the front surface side, and then removing the processing strain on the back surface side. Item 1).
[0013]
In this way, if a process involving a heat treatment such as pn junction formation is performed in a state where the processing distortion only on the front surface side of the wafer in which the processing distortion due to slicing remains is removed, heavy metal impurities and the like generated in the process are Heavy metal impurities and the like of the silicon wafer can be removed by gettering to the processing strain layer and then removing the processing strain on the back surface side.
[0014]
Further, if the processing strain on the front side is removed by spin etching (claim 2), the processing strain only on the front side can be easily removed without protecting the back side with a protective film or the like.
[0015]
Moreover, it is preferable to use a silicon single crystal having Ga as a dopant produced by the CZ method as the silicon single crystal for producing the silicon wafer (Claim 3), and in particular, the concentration of Ga in the silicon single crystal. Is preferably 3 × 10 ¹⁵ atoms / cm ^{3 to} 5 × 10 ¹⁷ atoms / cm ³ (Claim 4).
Thus, if a crystal using Ga as a dopant by the CZ method is used, it is easy to produce a large-diameter silicon single crystal ingot, and if a silicon single crystal wafer obtained by slicing this is used as a solar cell substrate, The lifetime can be effectively prevented from decreasing due to light deterioration, and a solar cell having high conversion efficiency can be manufactured.
[0016]
The present inventor has a possibility that heavy metal impurities are generated during the heat treatment in the manufacturing process of the solar cell, thereby contaminating the silicon wafer and reducing the lifetime, resulting in a decrease in the conversion efficiency of the solar cell. I thought it was. This is especially true in the manufacture of solar cells because low quality wafers may be used as raw material wafers. Accordingly, the inventors have conceived that if the silicon wafer is provided with a gettering capability during the manufacturing process of the solar battery cell, the lifetime can be prevented from being reduced by heavy metal impurities.
[0017]
It is known that there are IG (intrinsic gettering) and EG (extrinsic gettering) as methods for providing a silicon wafer with gettering capability. However, since IG is a method for forming oxygen precipitates in the bulk of a silicon wafer, there is a risk that the lifetime will be reduced, which is not appropriate. Moreover, as EG, there is known a method for forming a strain by forming a polysilicon film on the back surface of a wafer or by damaging by sandblasting or the like, but both require an additional process. Therefore, it is not preferable in the manufacturing process of a solar cell in which a reduction in price is particularly required.
[0018]
Therefore, as a result of diligent research, the present inventor has paid attention to processing strain generated on the surface of a wafer sliced from a silicon single crystal, and at least a pn junction forming step and an electrode on a silicon wafer produced by slicing the silicon single crystal. When manufacturing the solar battery cell by performing the forming step, after removing the processing strain only on the surface side of the sliced silicon wafer and performing at least pn junction forming processing on the surface side, processing strain on the back surface side It has been found that heavy metal impurities can be removed by removing, and the present invention has been completed.
[0019]
That is, according to the above method, a silicon wafer is configured by gettering heavy metal impurities using processing distortion on the back side of the wafer caused by slicing and further removing the processing distortion. Therefore, substantially no additional treatment is required, the lifetime can be prevented from being lowered, and a high-performance solar cell can be manufactured at low cost.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described more specifically with reference to the drawings. However, 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 CZ method (including MCZ method (including pulling method under magnetic field)) or FZ method.
[0021]
Currently, p-type is the main conductivity type of wafers used as solar cells, and boron is usually added to this p-type wafer as a dopant. According to the CZ method, a large-diameter single crystal doped with boron can be produced at a relatively low cost. However, there is a high concentration of oxygen in the crystal produced by the CZ method, and when strong light is applied to a solar cell made using such a p-type CZ method silicon single crystal, boron in the crystal Oxygen may affect lifetime characteristics and cause photodegradation.
[0022]
Therefore, the silicon single crystal prepared in the present invention may be a p-type silicon single crystal doped with boron, which is usually used, but was produced by the CZ method in order to effectively prevent photodegradation of solar cells. It is preferable to use a silicon single crystal having Ga as a dopant.
[0023]
Thus, if a silicon single crystal is produced by the CZ method using Ga as a p-type dopant, it is not necessary to consider the adverse effect on lifetime characteristics due to boron and interstitial oxygen, so it is necessary to use a silicon single crystal with a low oxygen concentration Therefore, it becomes easy to produce a silicon single crystal having a large diameter of 8 inches or more. Moreover, when such a Ga-doped silicon single crystal is used, it is difficult to cause photodegradation, and a solar battery cell having high conversion efficiency can be manufactured.
[0024]
The concentration of Ga to be doped is preferably 3 × 10 ^{15 to} 5 × 10 ¹⁷ atoms / cm ³ (resistivity is 5 to 0.1 Ω · cm).
When the Ga concentration is lower than 3 × 10 ¹⁵ atoms / cm ³ , the resistivity of the silicon wafer to be manufactured becomes higher than necessary, and power is consumed by the internal resistance of the solar cell, which may reduce conversion efficiency. There is. In addition, when the Ga concentration is larger than 5 × 10 ¹⁷ atoms / cm ³ , the resistivity of the wafer is extremely lowered, so that the lifetime of minority carriers due to Auger recombination may be reduced inside the wafer. The Ga concentration in the silicon single crystal is preferably within the above range.
[0025]
In order to produce a silicon single crystal to which Ga is added by the CZ method, after adding Ga to the silicon melt in the crucible, the seed crystal is brought into contact with the silicon melt, and it is pulled up while rotating it. . In this case, the addition of Ga to the melt in the crucible is performed by appropriately calculating a dopant prepared by growing a silicon crystal rod to which a high concentration of Ga has been added in advance and crushing the high concentration Ga-doped silicon crystal rod. It is preferable to add an appropriate amount to the silicon melt.
[0026]
FIG. 1 shows an example of a manufacturing process of a solar battery cell according to the present invention.
The silicon single crystal ingot manufactured by the CZ method or the like as described above is sliced by a normal method using a disk blade or a wire saw to produce a silicon wafer. FIG. 1A shows a silicon wafer 2 immediately after being sliced, and processing strains 1a and 1b due to mechanical impact at the time of cutting exist on both the front and back surfaces of the wafer 2. FIG.
[0027]
FIG. 1B shows a process of removing only the processing strain 1a on the front surface side of the wafer 2 which becomes the light receiving surface of the solar battery cell, and the processing strain 1b on the back surface side is left. The wafer immediately after slicing usually does not distinguish between the front surface side and the back surface side, but in the present invention, the surface serving as the light receiving surface is referred to as the front surface side, and the opposite surface is referred to as the back surface side.
[0028]
The method for removing the processing strain only on the front side is not particularly limited. For example, the back side is protected with an etching-resistant protective film or a protective tape, and mixed acid (for example, hydrofluoric acid, nitric acid, acetic acid, water) Liquid) or a method of immersing in an etching solution such as an alkaline aqueous solution containing KOH, NaOH or the like, or a method of surface grinding or mechanochemical polishing only on one side. Preferably, only processing distortion on the surface side can be removed by so-called spin etching.
[0029]
For example, as shown in FIG. 3, the spin etching is performed by fixing the wafer 11 on the wafer support 10 so that the surface side thereof faces the etching solution supply nozzle 12 and rotating the wafer as described above. Supply. According to such spin etching, since the etching solution contacts and is swung away only on the front surface side, only the processing distortion on the front surface side can be easily removed without the need for protecting the back surface.
Note that the etching margin for removing the processing strain cannot be generally described because the processing strain depth varies depending on the slicing conditions, but is usually about 10 μm to several tens of μm.
[0030]
A treatment (usually alkali etching) for reducing reflection of light called texture treatment is performed on the etched surface as necessary. At this time, similarly to the etching of FIG. 1B, the back side may be protected by a protective film or the like, or the etching may be performed. It is also possible to perform both processing distortion removal on the front side and texture processing.
[0031]
FIG. 1C shows a pn junction formation step, and a diffusion layer having a conductivity type (n-type) opposite to the conductivity type (for example, p-type) of the wafer 2 on the surface side of the wafer 2 from which processing strain has been removed. 3 is formed. In the introduction of impurities at this time, a gas diffusion method, a solid phase diffusion method, an ion implantation method, or the like is used as in the conventional case, and a heat treatment is performed at a temperature of several hundred to 1000 ° C. or higher. Normally, this process is the hottest process in the manufacturing process of solar cells, and in the present invention, the generated heavy metal impurities are gettered to the processed strain layer 1b on the back side of the wafer.
[0032]
In the present invention, at least after the pn junction forming step, a process for removing the processing strain 1b on the back surface side may be performed. However, if the processing strain 1b on the back surface side is left, the heat treatment in each subsequent process is also performed. Heavy metal impurities can be gettered. Therefore, it is preferable to perform the process of removing the processing strain 1b on the back side after performing other steps as much as possible as in the following example.
[0033]
FIG. 1D shows an electrode forming process on the surface side (light-receiving surface side). After forming a very thin oxide film 5 for passivation on the surface of the diffusion layer 3, a vapor deposition method, a plating method, and a printing method are shown. In this step, the electrode 4 made of metal is formed. As an electrode material, Ni, Au, Ag, Ti, Pd, Al, or the like can be used.
Although the processing strain 1b on the back surface side may be removed thereafter, it is preferable to perform the post-process as much as possible as described above.
[0034]
FIG. 1E shows an antireflection film forming process, in which a deposited film 6 as an antireflection film is formed by a CVD method, a PVD method, or the like, and a heat treatment of about several hundred to 800 ° C. is also performed at that time. Added. As the antireflection film 6, for example, SiO ₂ having a refractive index of 1.8 to 1.9 can be used. In addition, CeO ₂ , Al ₂ O ₃ , Si ₃ N ₄ , SnO ₂ , SiO ₂ − TiO ₂ or the like is used. When the antireflection film 6 has two layers, a film having a large refractive index such as TiO ₂ or Ta ₂ O ₅ is usually used.
[0035]
FIG. 1F shows a process of removing the processing strain 1b left on the back side of the wafer. As a method for removing only the processing strain 1b on the back surface side, the same method as in the step of FIG. 1B can be used. The antireflection film is protected by a protective tape or the like, and etching of mixed acid or alkaline aqueous solution is performed. Only the processing strain 1b on the back surface side can be easily removed by a method of dipping in a liquid, a method of mechanochemical polishing of only the back surface, or spin etching.
[0036]
FIG. 1G shows an electrode forming process on the back surface side, in which the electrode metal 7 is formed on the back surface side of the wafer from which processing strain is removed together with the gettered heavy metal impurities. The specific type and formation method of the electrode metal are the same as those for forming the electrode on the surface side (light receiving surface side) in FIG.
[0037]
As described above, in the manufacturing process of the solar cell, after removing only the processing strain on the surface side of the silicon wafer obtained by slicing the silicon single crystal ingot and performing at least the pn bonding step, preferably, heat treatment as much as possible If the processing strain on the back side is removed after many other processes involving the process are performed, heavy metal impurities generated during these steps should be gettered and removed by the processing strain layer on the back side of the wafer. Can do. If gettering is performed by utilizing the processing strain generated by slicing in this way, there is no need for substantially additional steps such as sandblasting, and it is not necessary to form oxygen precipitates for gettering. A decrease in the lifetime of the solar battery cell can be prevented without increasing the manufacturing cost.
[0038]
The present invention is not limited to the above embodiment. The above embodiment is merely an example, and the present invention has the same configuration as that of the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.
[0039]
For example, in the above embodiment, the case where the processing distortion on the back surface side is removed after the antireflection film forming step (FIG. 1E) has been described, but the steps from (A) to (G) in FIG. In addition, after the pn junction forming step (FIG. 1C) or the front surface side electrode forming step (FIG. 1D), the processing strain on the back side can be removed.
For example, when processing distortion on the back surface side is removed after the electrode forming step (FIG. 1D), the antireflection film forming step can be performed after forming the electrode on the back surface side.
In addition, in the manufacturing process of the solar battery cell, the process may be replaced, added, or omitted, such as omitting the addition of texture processing or the formation of an antireflection film.
[0040]
【The invention's effect】
According to the method for manufacturing a solar cell of the present invention, processing distortion of a sliced silicon wafer is left only on the back surface side, and heavy metal impurities generated in the pn junction forming process, the electrode forming process, or the antireflection film forming process are gettered. Then, since the back side processing distortion is removed, an additional process is substantially unnecessary, and the lifetime of the silicon wafer can be prevented from being lowered. Therefore, it is possible to suppress an increase in manufacturing cost and to achieve an improvement in the performance of the solar battery cell. As a result, a solar battery cell with high conversion efficiency can be provided at a low cost.
[Brief description of the drawings]
FIG. 1 is a process diagram showing an example of a manufacturing process of a solar battery cell according to the present invention.
FIG. 2 is a process diagram showing a manufacturing process of a conventional solar battery cell.
FIG. 3 is a schematic view showing spin etching for removing processing strain on only one surface of a wafer.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1a, 1b ... Work distortion, 2 ... Silicon wafer, 3 ... Diffusion layer, 4 ... Surface side electrode, 5 ... Oxide film, 6 ... Deposition film (antireflection film), 7 ... Back side electrode, 10 ... Wafer support stand 11 ... Silicon wafer, 12 ... Etch solution supply nozzle.

Claims (4)

In a method of manufacturing a solar cell by subjecting a silicon wafer produced by slicing a silicon single crystal to at least a pn junction forming step and an electrode forming step, processing strain on the surface side of the sliced silicon wafer is removed. Then, after performing at least a pn junction forming process on the front surface side, a processing strain on the back surface side is removed. The method for manufacturing a solar cell according to claim 1, wherein the processing strain on the surface side is removed by spin etching. The method for producing a solar battery cell according to claim 1 or 2, wherein a silicon single crystal having Ga as a dopant produced by a CZ method is used as the silicon single crystal. 4. The method for manufacturing a solar cell according to claim 3, wherein a concentration of Ga in the silicon single crystal is 3 × 10 ¹⁵ atoms / cm ^{3 to} 5 × 10 ¹⁷ atoms / cm ³ .

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