JP2012169420A - Solar cell wafer manufacturing method, solar cell manufacturing method and solar cell module manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solar cell wafer manufacturing method which can reduce reflection loss of light at a surface of a semiconductor by performing poring of the surface including a silicon wafer simply and at low cost.SOLUTION: In the present invention, pore processing is performed on at least one surface of a semiconductor wafer by using a gas evaporated from a mixed liquor containing hydrofluoric acid, nitric acid and sulfuric acid to obtain a solar cell wafer.

Description

  The present invention relates to a method for manufacturing a solar cell wafer, a method for manufacturing a solar cell, and a method for manufacturing a solar cell module. The present invention particularly relates to a method for manufacturing a solar cell wafer capable of reducing the reflection loss of light on the surface by making the surface of a semiconductor wafer including a silicon wafer porous in a simple and inexpensive manner.

  Generally, a solar cell is formed using a semiconductor wafer including a silicon wafer. In order to increase the energy conversion efficiency of the solar battery cell, it is necessary to reduce the light reflected by the light receiving surface of the solar battery cell and the light transmitted through the solar battery cell. Here, for example, when producing a crystalline solar cell using a silicon wafer, the silicon wafer has a low transmittance of visible light that contributes to photoelectric conversion. The reflection loss of visible light on the surface should be kept low, and the incident light can be effectively confined in the solar cell.

  Technologies for reducing the reflection loss of incident light on the surface of a silicon wafer include a technology for forming an antireflection film on the surface and a technology for forming a concavo-convex structure such as a micro pyramid type concavo-convex called a texture structure on the surface. is there. Among the latter techniques, a method of forming a texture structure on the surface is a method suitable for single crystal silicon, and a method of (100) etching a single crystal silicon surface with an alkaline solution is representative. This utilizes the fact that the etching rate of the (111) plane is slower than the etching rates of the (100) plane and the (110) plane in the etching using alkali. Furthermore, as the latter technique, in recent years, a technique has been proposed in which the silicon surface is made porous so that a concavo-convex structure is formed on the surface and the reflection loss of incident light is reduced.

  For example, Patent Document 1 describes a method of forming a large number of fine holes on the surface of a single crystal silicon substrate by anodizing treatment in which a single crystal silicon substrate is used as an anode and Pt is used as a cathode, and current is passed in hydrofluoric acid. Has been. Further, in Patent Document 2, in order to form finer submicron-order irregularities on the surface of a silicon substrate on which a micron-sized texture structure is formed, the surface of the substrate is oxidized after electroless plating. A technique for etching in the liquid phase of a mixed aqueous solution of an agent and hydrofluoric acid is described. Specifically, a p-type single crystal silicon substrate subjected to alkali texture treatment is immersed in an aqueous solution containing silver perchlorate and sodium hydroxide to form silver fine particles on the surface. Thereafter, the substrate is immersed in a mixed solution of hydrogen peroxide, hydrofluoric acid and water to form submicron-order irregularities.

Japanese Patent Laid-Open No. 6-169097 JP 2007-194485 A

  However, the method of anodizing treatment in Patent Document 1 requires a large and expensive DC power source. Moreover, since it is a porous treatment in a liquid phase of a mixed solution of an organic solvent and hydrofluoric acid, a problem remains in operability during the rinsing treatment. Also in the case of the method described in Patent Document 2, there is a problem in operability during the rinsing process as in Patent Document 1 because of the method using the etching action in the liquid phase (wet etching). In addition, wet etching requires a large amount of reagent as compared with a method using the etching action of reactive gas, and there is a problem that the reagent cost is increased.

  The technique of reducing the reflection loss of incident light by making the silicon surface porous is expected to be able to reduce the reflection loss further than forming a normal texture structure, so it is easier to solve the cost problem Therefore, there is a need for a method for creating a porous structure.

  Therefore, in view of the above problems, the present invention manufactures a solar cell wafer capable of reducing the reflection loss of light on the surface by making the surface of a semiconductor wafer including a silicon wafer porous in a simple and inexpensive manner. It is an object of the present invention to provide a method for manufacturing a solar battery cell and a method for manufacturing a solar battery module including the method.

  In order to achieve the above-mentioned object, the present inventor diligently studied and repeated trial and error with various porous processing methods. In addition, the inventors have found that the reflection loss of light on the surface can be reduced, and have completed the present invention. The gist of the present invention is as follows.

  (1) Manufacturing of a solar cell wafer, wherein at least one surface of a semiconductor wafer is made porous by a gas vaporized from a mixed solution containing hydrofluoric acid, nitric acid and sulfuric acid to obtain a solar cell wafer. Method.

  (2) The solar cell according to (1), wherein the porous processing is performed by leaving the gas phase containing air in a sealed space, containing the mixed solution, and introducing the semiconductor wafer into the gas phase. Wafer manufacturing method.

  (3) The said liquid mixture is described in said (1) or (2) whose density | concentration of hydrofluoric acid, nitric acid, and a sulfuric acid is 10-20 mass%, 10-20 mass%, and 20-50 mass%, respectively. Method for manufacturing a solar cell wafer.

  (4) The method for producing a solar cell wafer according to (3), wherein the time for the porous treatment is 60 minutes or less.

  (5) In addition to the steps in the method for producing a solar cell wafer according to any one of (1) to (4), the solar cell further includes a step of producing solar cells with the solar cell wafer. Cell manufacturing method.

  (6) In addition to the process in the manufacturing method of the photovoltaic cell as described in said (5), the manufacturing method of the solar cell module which further has the process of creating a solar cell module from this photovoltaic cell.

  In the present invention, the surface of a semiconductor wafer is made porous by utilizing the etching action of a gas evaporated from a mixed liquid containing hydrofluoric acid, nitric acid and sulfuric acid. Therefore, since the amount of chemicals required is smaller than that of wet etching, the chemical cost is low, and the rinsing process is simple. And a large-scale device such as a DC power supply is not required. Therefore, it has become possible to obtain a solar cell wafer in which the surface of a semiconductor wafer including a silicon wafer is made porous and the reflection loss of light on the surface is reduced more simply and inexpensively.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows the manufacturing method of the typical wafer for solar cells according to this invention, (a) is the semiconductor wafer 10, (b) is the porous processing process by a batch processing system, (c) is porous. The solar cell wafer 20 obtained as a result of the chemical conversion treatment is shown. It is a schematic diagram which shows the manufacturing method of another wafer for solar cells according to this invention, and shows the porous processing process by the continuous processing system suitable for mass production. (A) is an external appearance image of the silicon wafer surface before the porous-izing process by this invention. (B) is the external appearance image of the silicon wafer surface which performed the porous processing by this invention for 10 minutes. (A) is the surface image by SEM of the silicon wafer surface before the porous-ized process by this invention. (B) is the surface image by SEM of the silicon wafer which performed the porous processing by this invention for 10 minutes. (C) is the surface image by SEM of the silicon wafer which performed the porous processing by this invention for 30 minutes. (D) is the surface image by SEM of the silicon wafer which performed the porous processing by this invention for 60 minutes. In an experiment example, it is a graph which shows the relationship between the porosity treatment time by this invention, and the relative reflectance in wavelength 600nm.

  Hereinafter, the present invention will be described in more detail with reference to the drawings.

(Embodiment 1: Porous treatment by batch processing method)
First, the semiconductor wafer 10 used in the present invention shown in FIG. 1A is not particularly limited. Hereinafter, as an embodiment of the present invention, a single crystal or polycrystalline silicon wafer (hereinafter simply referred to as “wafer”) is used. A method for producing a single crystal or polycrystalline silicon solar cell wafer by subjecting them to a porous treatment will be described.

  As the single crystal silicon wafer, one obtained by slicing a single crystal silicon ingot grown by the Czochralski method (CZ method) or the like with a wire saw or the like can be used. Also, the plane orientation of the wafer surface can be selected as required, such as (100), (001) and (111). A polycrystalline silicon wafer can be obtained from a polycrystalline silicon ingot by slicing.

In both cases of a single crystal silicon wafer and a polycrystalline silicon wafer, the surface of the wafer cut out from the ingot is damaged by cracks and crystal distortion introduced into the silicon layer by slicing. For this reason, after slicing, it is preferable to clean the wafer, and to etch the surface of the wafer with acid or alkali to remove the damaged surface. The penetration depth of the damage derived from the slicing process is a factor determined by the slicing process conditions, but is approximately 10 μm or less. Therefore, it is possible to cope with an etching process that is generally performed with an alkali such as KOH or a hydrofluoric acid (HF) / nitric acid (HNO ₃ ) mixed acid.

  The present invention is a method of making a solar cell wafer 20 by making at least one surface 10A of a wafer 10 porous. That is, in the present specification, the “solar cell wafer” means a wafer in a state where at least one surface of the wafer is made porous. This one surface is a surface that serves as a light receiving surface in the solar battery cell. And the characteristic process of this invention is making the wafer for solar cells by carrying out the porous process of at least one surface of a semiconductor wafer with the gas evaporated from the liquid mixture containing a hydrofluoric acid, nitric acid, and a sulfuric acid.

  Hereinafter, the technical significance of adopting the above characteristic steps of the present invention will be described with specific examples together with the effects.

  FIG.1 (b) is a schematic diagram which shows the porous processing process by the batch processing system which is one Embodiment of this invention. As shown in FIG. 1, a mixed liquid 13 containing hydrofluoric acid, nitric acid and sulfuric acid is accommodated in a container 11 that forms a sealed space inside, leaving a gas phase 12 containing air. The container 11 can be a plastic box such as polypropylene or polytetrafluoroethylene (PTFE trade name: Teflon) having a thickness of 1 to 3 mm. A support base 15 can be disposed in the container 11. The support base 15 only needs to support the wafer 10 so as not to come into contact with the liquid mixture 13. For example, the support base 15 is made of PTFE. The support base 15 is placed on the bottom surface of the container 11 and protrudes above the liquid surface of the mixed solution 13. Here, after the liquid mixture 13 is accommodated in the container 11, the container 11 is covered to make the inside of the container 11 a sealed space, and waits until the gas 14 vaporized from the liquid mixture 13 is filled in the container 11. Then, when the wafer 10 is introduced into the portion of the gas phase 12 and then covered again, the surface of the wafer 10 is made porous by the etching action of the gas 14 vaporized from the liquid mixture 13.

In the porous treatment step in the present invention, by using a mixed solution 13 in which sulfuric acid is added to a mixed acid of hydrofluoric acid and nitric acid, sulfuric acid in the mixed solution 13 is each of hydrofluoric acid and nitric acid in the mixed solution. While absorbing moisture, it absorbs moisture in the gas phase 12 in the container 11 to lower the humidity of the sealed space. Thus, vaporization of the liquid mixture 13 is promoted without heating the liquid mixture 13 or pressurizing the container 11, and the vaporized high-concentration HF-HNO ₃ gas 14 is transferred to the wafer on the support 15. 10, the (at least) surface 10 </ b> A of the wafer 10 is made porous. The present inventor can make the wafer surface porous by utilizing this reaction, and as a result, has a light confinement effect due to the formed fine irregularities and is suitable for the production of solar cells (see FIG. It discovered that the solar cell wafer 20) in 1 (c) was obtained, and came to complete this invention.

  The present invention is a method for making a porous structure by utilizing the etching action of the reactive gas 14 vaporized from the liquid mixture 13. Therefore, a smaller amount of reagent is required than in the porous processing method by wet etching, and the cost of the chemical solution can be reduced. Further, according to the present invention, the rinsing process can be greatly shortened as compared with the method of making the porous material by wet etching. And a large-scale device such as a DC power supply is not required. Therefore, it is possible to obtain a solar cell wafer 20 in which the surface of a semiconductor wafer such as a silicon wafer is made more porous in a simpler and cheaper manner and the reflection loss of light on the surface is reduced.

  As mentioned above, although the manufacturing method of the wafer for solar cells of this invention was demonstrated also including the effect, the following 2 points | pieces are mentioned as an additional effect of the manufacturing method by this invention. First, the present invention is applicable not only to single crystal silicon wafers but also to polycrystalline silicon wafers. As described above, the method for forming a texture structure is a method suitable for single crystal silicon, and for polycrystalline silicon in which various plane orientations appear on the surface, a uniform texture structure is formed on the entire wafer surface. It was difficult. However, by the porous treatment of the present invention, the reflectance of the surface of the polycrystalline silicon wafer can be sufficiently suppressed by a simple and inexpensive method. Second, in the present invention, it is not necessary to use an oxidizing agent such as hydrogen peroxide as the treatment liquid. Thereby, the complexity of waste water treatment can be avoided.

Specifically, the mixed solution is preferably a mixture of hydrofluoric acid having a concentration of 50% by mass, nitric acid having a concentration of 70% by mass, and sulfuric acid having a concentration of 98% by mass. The final concentrations of hydrofluoric acid, nitric acid and sulfuric acid are preferably 10 to 20% by mass, 10 to 20% by mass and 20 to 50% by mass, respectively, and 16 to 20% by mass, 13 to 15% by mass and It is more preferable to set it as 20-30 mass%. When hydrofluoric acid and nitric acid exceed the respective upper limit values, the concentration of sulfuric acid is relatively lowered, so that the moisture absorption effect of the system due to sulfuric acid cannot be obtained. May take time. Further, when the sulfuric acid exceeds the upper limit, the concentrations of hydrofluoric acid and nitric acid are relatively lowered, so that it may take a very long time to make the wafer porous. On the other hand, when hydrofluoric acid and nitric acid are below the respective lower limit values, the supply amount of HF—HNO ₃ gas required for the reaction is insufficient, and it takes a very long time to make the wafer porous. . If the sulfuric acid is lower than the lower limit, the sulfuric acid concentration of the system is low, so that the water absorption effect of the system by sulfuric acid cannot be obtained, and there is a possibility that it will take a very long time for the porous processing of the wafer.

  In order to make the porosity more efficient, when the volume of the container 11 is 100%, it is preferable to store the mixed liquid 13 at a ratio of 5 to 40% of the container 11.

  The temperature in the container 11 during the porous treatment is preferably 10 ° C to 50 ° C. If the temperature is less than 10 ° C., it is difficult for the porous material to advance. If the temperature exceeds 50 ° C., the internal pressure of the container increases, and the container may need to be pressurized.

  The gas component constituting the gas phase 12 portion in the container 11 is not particularly limited, and the reaction proceeds without problems even in the atmosphere (mainly nitrogen / oxygen). However, in order to perform the reaction more efficiently, it is desirable to replace the gas component with an inert gas (nitrogen, argon gas, etc.). This is because if a reactive gas (oxygen or the like) is present, a wafer oxidation reaction or the like may proceed separately from the porosification reaction, which may be an impediment to the porosification reaction.

  Since the reaction can proceed until the wafer is completely dissolved when the porous treatment is started, it is preferable to set a treatment time suitable for the porous treatment. When the above mixed solution is used, the porous treatment time is preferably 60 minutes or less. Further, even if a treatment for 60 minutes or more is performed, the effect of reducing the surface reflectance is not improved, and only the etching of the wafer proceeds, which is undesirable from the viewpoint of material loss. From the viewpoint of shortening the process time as much as possible and obtaining the maximum effect of reducing the reflectance, the porous treatment time is more preferably about 2 to 10 minutes. In the present specification, the “porosification time” means that the gas 14 generated from the mixed liquid 13 is filled in the sealed space and then the wafer is introduced into the sealed space, and then the wafer is removed from the sealed space. This is the period until the time of discharge.

(Embodiment 2: Porous treatment by a continuous treatment method)
FIG. 2 is a schematic diagram for explaining a method for manufacturing a solar cell wafer by a continuous processing method, which is another embodiment of the present invention and is suitable for mass production of solar cell wafers. The present invention can also be carried out by a belt conveyor type continuous processing system that passes through a porous processing unit 16, a water rinsing unit 17 and a drying unit 18 which are sealed spaces as shown in FIG. Specifically, the semiconductor wafer 10 is placed on the belt conveyor 19 and first passed through the porous processing unit 16 to perform the porous processing in the present invention. The solar cell wafer 20 completed by the processing in the unit is passed through the water rinse unit 17. In this unit, the wafer 20 is cleaned with pure water, and the chemicals are washed off from the wafer. Thereafter, the wafer 20 passes through the drying unit 18 and is dried.

  In this specification, the “sealed space” is defined as a state where a gas component existing in the container region is not mixed with a gas component existing outside the container region. As in the first embodiment, not only when the sealed space is formed by the shielding container, for example, the pressure of the gas component in the container is higher than the gas pressure outside the container (positive pressure), or substantially the same pressure. The sealed space may be formed by a method of setting and forming a state where no external gas is physically mixed. Alternatively, the closed space state may be formed by a method of installing a forced airflow blocking method such as an air curtain at the container inlet. In the case of this embodiment corresponding to the continuous processing method, it is desirable to install an air curtain at the container entrance and exit to suppress gas movement inside and outside the container area from the viewpoint of preventing the reaction gas from flowing out of the container area.

  Also in the second embodiment, as in the first embodiment, at least one surface 10A of the wafer 10 is made to be porous by the gas 14 vaporized from the mixed solution 13 in the porous processing unit 16 that is a sealed space. Is possible. Moreover, Embodiment 2 can obtain the same effect by the same structure as Embodiment 1 except performing by a continuous processing system.

  As mentioned above, although the manufacturing method of the wafer for solar cells by this invention was demonstrated, these showed the example of typical embodiment, Comprising: This invention is performed by the apparatus structure shown by these embodiment. Without being limited thereto, at least one surface of the semiconductor wafer is made porous by a gas vaporized from a mixed solution containing hydrofluoric acid, nitric acid and sulfuric acid to obtain a solar cell wafer. Various changes are possible.

  In addition, the production of a solar cell wafer for use in a crystalline silicon solar cell from a single crystal or polycrystalline silicon wafer has been described so far, but the present invention is not limited to crystalline silicon, and amorphous silicon. Of course, it is applicable also to the wafer for solar cells for using for a solar cell and a thin film type solar cell.

(Solar cell manufacturing method)
The method for manufacturing a solar cell according to the present invention further includes a step of manufacturing a solar cell with this solar cell wafer, in addition to the steps in the method for manufacturing a solar cell wafer according to the present invention described so far. The cell manufacturing step includes at least a step of forming a pn junction by a dopant diffusion heat treatment and a step of forming an electrode. In the dopant diffusion heat treatment, phosphorus is thermally diffused in the p substrate.

  In addition, you may perform a pn junction formation process before the porous treatment process in this invention. That is, after the etching process for removing damage by slicing, the porous process in the present invention is performed in the state of the wafer in which the pn junction is formed by the dopant thermal diffusion process. An electrode may be formed on the solar cell wafer thus obtained to form a solar cell.

  According to the method for manufacturing a solar cell according to the present invention, it is possible to suppress a reflection loss of incident light on the light receiving surface of the cell and obtain a solar cell with high conversion efficiency.

(Method for manufacturing solar cell module)
The manufacturing method of the solar cell module according to the present invention further includes a step of producing a solar cell module from the solar cell in addition to the steps in the solar cell manufacturing method. The module manufacturing process includes arranging a plurality of solar cells, wiring the electrodes, arranging the solar cells wired on the tempered glass substrate, sealing with a resin and a protective film, and an aluminum frame. Assembling and electrically connecting the terminal cable with the wiring.

  According to the method for manufacturing a solar cell module according to the present invention, it is possible to suppress a reflection loss of incident light on the light receiving surface of the solar cell and obtain a solar cell module with high conversion efficiency.

  In order to further clarify the effects of the present invention, experimental examples will be described below.

  First, a 156 mm square polycrystalline silicon wafer was prepared, and an acidic solution prepared at 50% by mass hydrofluoric acid / 70% by mass nitric acid / water = 1: 4: 5 (volume ratio) was used at room temperature. Etching was performed for 1 minute, and then the wafer was dried. All subsequent steps were performed at room temperature. In a 2 mm thick container made of polypropylene having a length of 250 mm × width 250 mm × height 150 mm, 50 g of hydrofluoric acid 360 g and 68 g of nitric acid 206 g are placed and mixed. This mixed acid is mixed with 98 g of sulfuric acid 255 g. Was gently added to prepare a mixed solution. In this mixed solution, the concentrations of hydrofluoric acid, nitric acid and sulfuric acid are 18% by mass, 14% by mass and 25% by mass, respectively. The liquid level of the mixed liquid was about 1 cm from the bottom of the container. After preparing the mixed solution in the container, the container was covered to make the inside of the container a sealed space, and the container was filled with gas for 30 minutes. The wafer was introduced into the gas phase portion of the container and then sealed again. After a predetermined time, the wafer was taken out of the container, rinsed with pure water, and dried. Seven types of samples having the predetermined times (porosification treatment time) of 1 minute, 2 minutes, 3 minutes, 5 minutes, 10 minutes, 30 minutes, and 50 minutes were prepared.

<Evaluation 1: Appearance photo>
The wafer surface was observed by appearance image. The surface of the silicon wafer subjected to the porous treatment according to the present invention shown in FIG. 3 (b) for 10 minutes is more appearance than the surface of the silicon wafer before the porous treatment according to the present invention shown in FIG. 3 (a). It is black and the reflectance of visible light is expected to decrease. From this result, it was found that the loss of light reflection on the surface of the silicon wafer can be reduced by the porous treatment according to the present invention.

<Evaluation 2: SEM>
The surface to be processed of the wafer was observed with a scanning electron microscope (SEM). As shown in FIG. 4A, irregularities of about several μm existed on the surface of the silicon wafer before the porous treatment according to the present invention. This is a concavo-convex shape formed by an acid etching process. On the other hand, as shown in FIG. 4 (b), the surface of the silicon wafer subjected to the porous treatment according to the present invention for 10 minutes, 30 minutes and 60 minutes has finer unevenness on the uneven surface of about several μm. It was seen. From these results, it was found that fine irregularities can be formed on the surface of the silicon wafer by the porous treatment according to the present invention.

<Evaluation 3: Reflectance measurement>
With a reflectance measuring instrument (Shimadzu Corporation: SolidSpec3700), the reflection spectrum of the wafer surface to be processed was measured in the range of 300 to 1200 nm for seven types of samples. Since sunlight contains a large amount of light having a wavelength of 500 to 700 nm, it is desirable that the reflectance be low in this wavelength region. Therefore, the relative reflectance at a wavelength of 600 nm for seven types of samples is shown in FIG. Further, the relative reflectance of the wafer not subjected to the porous treatment of the present invention was also measured and plotted at “0 minutes”.

  From FIG. 5, the reflectance could be sufficiently reduced by the porous treatment for 1 minute as compared to before the treatment. In addition, the porosity could be reduced more significantly by the porous treatment for 2 minutes or more than before the treatment. This is due to the porous surface. However, it has been found that the effect of reducing the reflectance is saturated when the porous treatment is performed for more than 10 minutes. From these results, it has been found that when the porosification treatment is performed by this method which is simpler and less expensive than the conventional method, the reflection loss of incident light can be reduced and a solar cell with high conversion efficiency can be produced.

  ADVANTAGE OF THE INVENTION According to this invention, it became possible to obtain the solar cell wafer which reduced the reflection loss of the light in the surface by making the surface of semiconductor wafers including a silicon wafer porous easily and cheaply.

DESCRIPTION OF SYMBOLS 10 Semiconductor wafer 10A One side of semiconductor wafer 11 Container 12 Gas phase 13 Mixture 14 Gas 15 Support stand 16 Porous processing unit 17 Water rinse unit 18 Drying unit 19 Belt conveyor 20 Solar cell wafer

Claims (6)

  A method for producing a solar cell wafer, wherein at least one surface of a semiconductor wafer is made porous by a gas vaporized from a mixed liquid containing hydrofluoric acid, nitric acid and sulfuric acid to obtain a solar cell wafer. The porous treatment is
The method for producing a solar cell wafer according to claim 1, wherein the mixed liquid is accommodated while leaving a gas phase containing air in a sealed space, and the semiconductor wafer is introduced into the gas phase.   The concentration of hydrofluoric acid, nitric acid, and sulfuric acid in the mixed solution is 10 to 20% by mass, 10 to 20% by mass, and 20 to 50% by mass, respectively. Production method.   The manufacturing method of the wafer for solar cells of Claim 3 whose time of the said porosification process is 60 minutes or less.   The manufacturing method of the cell for solar cells which further has the process of producing a photovoltaic cell with this wafer for solar cells in addition to the process in the manufacturing method of the wafer for solar cells of any one of Claims 1 thru | or 4.   The manufacturing method of the solar cell module which further has the process of producing a solar cell module from this photovoltaic cell in addition to the process in the manufacturing method of the photovoltaic cell of Claim 5.