JP5880055B2 - Method for producing solar cell wafer, method for producing solar cell, and method for producing solar cell module - Google Patents

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 producing a solar cell wafer for making a surface of a semiconductor wafer porous for the purpose of producing a solar cell with high conversion efficiency.

  Generally, a solar cell is formed using a semiconductor wafer including a silicon wafer. In order to increase the 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 micropores on the surface 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. Patent Document 2 describes a method of making a silicon substrate porous by reactive ion etching.

  Furthermore, in Patent Document 3, in order to form finer submicron-order irregularities on the surface of a silicon substrate having a micron-size texture structure, the substrate is oxidized after electroless plating with metal particles. A technique for etching with 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-T-2004-506330 JP 2007-194485 A

  However, the method using anodizing treatment in Patent Document 1 requires a large and expensive direct current power source, and uses a mixed solution of an organic solvent and hydrofluoric acid. The problem remains. In the method using reactive ion etching of Patent Document 2, it is necessary to secure an ion implantation apparatus dedicated to the process, which is problematic in terms of cost. In the case of the method described in Patent Document 3, it is essential to contain an oxidizing agent in the mixed aqueous solution. However, the oxidizing agent is generally expensive and has a problem that the reagent cost is increased. In particular, when hydrogen peroxide is used as an oxidant, special waste liquid treatment that takes into account the impact on the ecosystem is indispensable due to waste liquid regulations. There is also a problem. For this reason, the method of making the silicon surface porous in order to reduce the reflection loss of incident light has not been put to practical use until now.

  By reducing the reflection loss of incident light by making the silicon surface porous, it is expected that the reflection loss can be further reduced compared to the formation of a normal texture structure, thus eliminating safety and cost issues. There is also a need for a new method for making a porous structure.

  In addition, many of the known porosification techniques have been studied mainly from the viewpoint of reducing the reflectance of the silicon wafer surface as much as possible, and the actual conversion efficiency of solar cells manufactured from these wafers. There are many things that have not been studied.

  Therefore, in view of the above problems, the present invention provides a method for producing a solar cell wafer capable of producing a solar cell having a high conversion efficiency by making the surface of a semiconductor wafer including a silicon wafer porous safely and inexpensively. It aims at providing the manufacturing method of the photovoltaic cell containing the method, and the manufacturing method of a solar cell module.

  In order to achieve the above object, the present inventor has intensively studied and repeated trial and error with various porosification treatment methods. As a result, according to the following method, submicron-order irregularities are formed on the semiconductor wafer surface. And it discovered that the conversion efficiency of the solar cell manufactured from this wafer can be made high, reducing the reflection loss of the light in the surface effectively, and came to complete this invention. This invention is based on said knowledge and examination, The summary structure is as follows.

(1) a first step of bringing a lower alcohol into contact with at least one surface of a semiconductor wafer;
After the first step, a second step of bringing hydrofluoric acid containing metal ions into contact with at least one surface of the semiconductor wafer;
After the second step, a third step of bringing an acid solution containing hydrofluoric acid and nitric acid into contact with at least one surface of the semiconductor wafer;
A method for producing a solar cell wafer, wherein at least one surface of the semiconductor wafer is made porous to form a solar cell wafer.
(2) The semiconductor wafer is a silicon wafer,
The method for producing a solar cell wafer according to (1), wherein the metal ions are ions of a metal nobler than Si.
(3) The step of bringing the solution for removing metal deposited from the metal ions from at least one side of the semiconductor wafer into contact with at least one side of the semiconductor wafer after the second step and before the third step (1) ) Or the method for producing a solar cell wafer according to (2).
(4) The method for producing a solar cell wafer according to any one of (1) to (3), wherein a thickness of the porous layer produced on the one surface is 50 to 400 nm.
(5) Any of the above (1) to (4), wherein the concentration of the hydrofluoric acid in the acid solution is 0.1 to 5.0% by mass and the concentration of the nitric acid is 20 to 50% by mass. The manufacturing method of the wafer for solar cells of 1.
(6) The method for producing a solar cell wafer according to any one of (1) to (5), wherein the time for contacting the acid solution in the third step is 10 minutes or less.
(7) In addition to the process in the manufacturing method of the solar cell wafer of any one of said (1)-(6), the photovoltaic cell which further has the process of producing a photovoltaic cell with this solar cell wafer Manufacturing method.
(8) In addition to the process in the manufacturing method of the photovoltaic cell as described in said (7), the manufacturing method of the solar cell module which further has the process of producing a solar cell module from this photovoltaic cell.

  According to the present invention, it is not necessary to use a large-scale device such as a DC power source or an ion implantation device, and it is not necessary to mix hydrofluoric acid with an organic solvent or use an oxidizing agent. Therefore, it is 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 safely and cheaply than in the prior art. Furthermore, since this invention performs the 3rd process of the process by the acid solution containing hydrofluoric acid and nitric acid following a 1st and 2nd process, conversion efficiency is high using the obtained wafer for solar cells. A solar cell can be produced.

It is a flowchart of the manufacturing method of the typical wafer for solar cells according to this invention.

  Hereinafter, the present invention will be described in more detail with reference to the drawings. FIG. 1 is a flowchart of a method for manufacturing a typical solar cell wafer according to the present invention. First, the semiconductor wafer used in the present invention is not particularly limited. In the following, as an embodiment of the present invention, a single crystal or polycrystalline silicon wafer (hereinafter, also simply referred to as “wafer”) is used. A method for producing a single crystal or polycrystalline silicon solar cell wafer by subjecting it 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 for making a wafer for solar cells by making at least one surface of a wafer 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. The characteristic process of the present invention includes a first process (step S1) in which a lower alcohol solution is brought into contact with at least one surface of a wafer, and a fluoride containing metal ions on at least one surface of the wafer after the first process. A second step (step S2) in which hydroacid is brought into contact; and a third step (step S5) in which an acid solution containing hydrofluoric acid and nitric acid is brought into contact with at least one surface of the wafer after the second step. It is to have.

  Hereinafter, the technical significance of adopting the above characteristic steps of the present invention will be described with specific examples together with the effects. Although the detailed process will be described later in Examples, the present inventor, after acid etching treatment, air-dried p-type (100) single crystal silicon wafer in a lower alcohol such as 2-propanol (isopropyl alcohol; IPA) for a predetermined time. It was found that when immersed in hydrofluoric acid in which copper (Cu) was dissolved for a predetermined time, the wafer surface became black in appearance and the reflectivity was lowered at wavelengths in the entire visible light range. Further, when a scanning electron microscope (SEM) image of the wafer surface was confirmed, many finer irregularities were formed on the irregular surface of about several μm.

  Furthermore, in the conventional porous technology, studies have been made with a focus on reducing the reflectance, and it has been considered that the conversion efficiency of the solar cell is higher as the reflectance is lower. However, according to the study of the present inventor, the greater the degree of porosity on the wafer surface, the better the solar cell surface reflectance is reduced. We found that the efficiency was rather low.

  The reason why the high conversion efficiency as expected from the low reflectivity cannot be obtained is that, in the power generation of the solar cell, electrons must flow through the conductive wire wired on the surface of the solar cell wafer. It was thought that the surface recombination phenomenon in which electrons are trapped frequently occurs in many porous structures, and the phenomenon that the electrons do not reach the conducting wire has occurred. Therefore, a predetermined acid solution treatment step was added to the porous step including the first and second steps, and the porous structure was slightly etched by slightly etching the porous structure. Then, it turned out that conversion efficiency improves. Based on the above findings, the present inventors have completed the present invention.

Although the reason why the wafer surface can be made porous by the surface treatment in the first and second steps is not necessarily clear, the present inventor presumes that the wafer surface has been made porous by the following reaction mechanism. In the second step, when the wafer is immersed in hydrofluoric acid in which Cu is dissolved, a large amount of Cu is precipitated as fine particles starting from some nucleus on the wafer surface. This reaction is a reduction reaction of Cu ²⁺ + 2e ⁻ → Cu. With the charge transfer at this time, electrons are deprived from Si on the wafer surface, and dissolution of Si occurs at the Cu fine particle deposition site. Here, as for the role of hydrofluoric acid, a model (Omimi model) that creates a porous structure by instantaneously dissolving SiO ₂ produced by the reaction of Si with water at the Si dissolution site. ) And a model in which fluorine ions directly oxidize Si (Chemla model). Details of such reactions are described in J. Electrochem. Soc. 144, 3275 (1997) “The Role of Metal Induced Oxidation for Copper Deposition on Silicon Surface” and J. Electrochem. Soc. 144, 4175 (1997) “Electrochemical and Radiochemical study of copper contamination mechanism from HF solution onto silicon substrates ”. In the present invention, the surface potential of the wafer surface is controlled by treating with a lower alcohol, which is a non-polar solvent, in the first step, so that metal deposition is likely to proceed during immersion in hydrofluoric acid. It is considered that the Si dissolution reaction in the second step can be promoted uniformly. Further, it is considered that the treatment with a lower alcohol has an effect of removing organic substances on the wafer surface so that the reaction proceeds.

  In the present invention, a large-scale apparatus such as a DC power source or an ion implantation apparatus is not required, and it is not necessary to mix hydrofluoric acid with an organic solvent or use an oxidizing agent. Therefore, it is 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 safely and cheaply than in the prior art. In addition, since the degree of porosity is somewhat suppressed by the third step, it is possible to suppress surface recombination and manufacture a solar cell with high conversion efficiency.

  In this embodiment, it is preferable to further perform a step (step S3) of bringing hydrofluoric acid not containing metal ions into contact with at least one surface of the wafer after the second step and before the third step. Specifically, the wafer after the second step can be immersed in hydrofluoric acid not containing metal ions for a predetermined time. By this step, the depth of the surface irregularities formed in the second step can be controlled to some extent.

  Moreover, it is preferable to perform 2nd process and / or step S3 in a light irradiation environment. This is because, by performing the second step in a light irradiation environment, the surface becomes porous due to the above reaction mechanism. Further, it is because the wafer surface can be brought into a desired surface state by performing step S3 in a light irradiation environment and controlling the irradiation conditions. Specifically, the light irradiation environment is created by irradiating the reaction surface with fluorescent light or halogen light.

  Further, after at least the second step, and in the present embodiment, before the third step, a step of bringing a solution for removing metal (fine particles) precipitated from the metal ions from at least one side of the wafer into contact with the at least one side of the wafer ( It is preferable to further perform step S4). For example, when using hydrofluoric acid in which Cu is dissolved in the second step, a nitric acid solution is brought into contact with the one surface in order to remove Cu fine particles remaining on the one surface. By this step, the metal particles remaining on and adhered to the wafer surface can be removed.

  And after the above process, the process (step S5) which makes the acid solution containing hydrofluoric acid and nitric acid contact at least the said one surface of this wafer is further performed. Specifically, the wafer after the second step, step S3 or step S4 can be immersed in an acid solution containing hydrofluoric acid and nitric acid for a predetermined time. By this step, the porous layer can be slightly shaved.

  In this embodiment, a solar cell wafer having a porous wafer surface is obtained through such steps. Hereinafter, preferred embodiments of each step will be described. In this specification, the lower alcohol treatment in the first step and the metal ion-containing hydrofluoric acid treatment in the second step, preferably the metal ion-free hydrofluoric acid treatment in Step 3, This is referred to as “process”.

(First step: lower alcohol treatment)
In the present specification, the “lower alcohol” means any linear or branched alcohol having 10 or less carbon atoms. If the carbon number exceeds 10, the viscosity of the alcohol becomes high, and the wafer surface is coated with alcohol. If the number of carbon atoms is 10 or less, the wafer surface can be made nonpolar as a nonpolar solvent having low viscosity. Typically, methanol, ethanol, 2-propanol, N-methylpyrrolidone, ethylene glycol, glycerin, benzyl alcohol, phenyl alcohol and the like can be mentioned. In view of toxicity and price, ethanol and 2-propanol (isopropyl Alcohol; IPA) is preferably used. The processing time, that is, the time during which the lower alcohol liquid is brought into contact with the wafer (hereinafter referred to as the “processing time” in each process is referred to as “processing time”) is not particularly limited, but should be 0.5 minutes to 10 minutes. Is preferable, and it is more preferable to set it to 3 minutes or less. This is because the effect of reducing the reflectivity of the present invention can be sufficiently obtained if the time is 0.5 minutes or more, and the effect of reducing the reflectivity is saturated even if the treatment is performed for more than 10 minutes. The temperature of the lower alcohol solution may be room temperature without any problem as long as the alcohol does not evaporate or solidify.

(Second step: Metal ion-containing hydrofluoric acid treatment)
In the present embodiment, the metal ion is preferably a metal nobler than Si, for example, an ion such as Cu, Ag, Pt, or Au. Thereby, in the second step, metal fine particles are deposited on the wafer surface and Si is eluted efficiently. Considering the price, it is preferable to use Cu ions. Preferred conditions from the viewpoint of sufficiently obtaining the reflectance reduction effect of the present invention are listed below. In the hydrofluoric acid in which the metal is dissolved, the metal concentration is preferably 10 ppm or more and 1000 ppm or less, and more preferably 100 ppm or more and 400 ppm or less. The concentration of hydrofluoric acid is preferably 2% by mass or more and 50% by mass or less, more preferably 10% by mass or more and 40% by mass or less, and further preferably 20% by mass or more and 30% by mass or less. Furthermore, processing time becomes like this. Preferably it is 0.5 minute or more and 30 minutes or less, More preferably, it is 1 minute or more and 10 minutes or less, More preferably, it is 3 minutes or less. The temperature of the metal-containing hydrofluoric acid may be appropriately selected in consideration of processing time, evaporation loss, etc., and is preferably room temperature to 100 ° C.

(Metal ion-free hydrofluoric acid treatment)
In this embodiment, in order to extend the depth of the porous layer formed on the wafer surface to an appropriate depth, the concentration of hydrofluoric acid is preferably 2% by mass or more and 50% by mass or less, more preferably 20% by mass. % To 30% by mass. Further, in this specification, “hydrofluoric acid not containing metal ions” includes not only the case where the content of metal ions is strictly zero but also the case where metals less than 10 ppm are contained as impurities. Shall be. For example, if the number of ions of a metal nobler than Si, such as Cu, Ag, Pt, or Au, is less than 10 ppm, the metal fine particles are newly precipitated and the second unevenness is formed on the wafer surface. The reaction of deepening the unevenness already formed in the process becomes dominant. What is necessary is just to set processing time according to process tact time, Preferably it is 0.5 to 60 minutes. The temperature of the metal-free hydrofluoric acid may be appropriately selected in consideration of processing time, evaporation loss, etc., and is preferably room temperature to 100 ° C.

(Metal removal solution treatment)
In the present embodiment, when Cu is used as the metal in the second step, the Cu fine particles can be removed with a nitric acid (HNO ₃ ) solution. At this time, the nitric acid concentration is preferably in the range of 0.001 to 70% by mass, and more preferably in the range of 0.01 to 0.1% by mass. What is necessary is just to set processing time according to process tact time, Preferably it is 0.5 to 10 minutes, More preferably, it is 1 to 3 minutes. The temperature of the nitric acid solution may be appropriately selected in consideration of the processing time and evaporation loss, and is preferably room temperature to 100 ° C. The treatment liquid used in this step is not limited to nitric acid, and a solution capable of dissolving it may be selected according to the metal to be removed. For example, in the case of Ag, Pt, or Au, aqua regia (HCl / HNO ₃ ), potassium iodide solution (KI), or the like can be used. Suitable concentrations and processing times are the same as for Cu.

(Third step: acid solution treatment)
In this embodiment, the acid solution used in the third step may contain hydrofluoric acid and nitric acid. The concentration in the acid solution is preferably 0.1 to 5.0% by mass for hydrofluoric acid, 20 to 50% by mass for nitric acid, and the rest water. If hydrofluoric acid is 0.1% by mass or more, a sufficient reaction rate can be obtained, and if it is 5.0% by mass or less, the etching rate can be easily controlled so that the reaction rate can be easily controlled. Nitric acid is preferably 20% by mass or more because it can be controlled to a reaction rate that allows easy adjustment of the etching amount, and is preferably 50% by mass or less from the viewpoint of reducing the chemical cost. The temperature of the acid solution is preferably 10 to 40 ° C, more preferably 15 to 30 ° C. A temperature of 10 ° C. or higher is preferable because a sufficient reaction rate can be obtained, and a temperature of 40 ° C. or lower is preferable because the etching rate can be easily controlled. The time for contacting the acid solution is preferably 10 minutes or less, more preferably 3 minutes or less. For example, when a porous treatment is performed for 3 minutes using a solution having a Cu concentration of 170 ppm and a hydrofluoric acid concentration of 25 mass%, an acid solution containing 1 mass% hydrofluoric acid and 35 mass% nitric acid is used. By using this for 1 minute, the thickness of the porous layer can be about 202 nm. However, the mixing conditions and the contact time conditions of hydrofluoric acid and nitric acid may be any other conditions that allow the thickness of the porous layer formed on one side to be 50 to 400 nm. That is, the conditions for the porous treatment step and the acid treatment for the third step may be arbitrarily set so that the thickness of the porous layer falls within the range. By setting the thickness of the porous layer to 50 to 400 nm, it is possible to sufficiently achieve both the conversion efficiency improvement effect and the surface reflectance reduction effect. Regarding the porous treatment, a porous layer having a thickness exceeding 400 nm is formed under the normal conditions as described above, and it is difficult to control the thickness to 50 to 400 nm. Therefore, the third step is necessary.

  The acid solution containing hydrofluoric acid and nitric acid greatly hinders the etching action of the porous layer such as acetic acid, phosphoric acid, sulfuric acid, and surfactants (anionic surfactants, neutral surfactants, etc.) Ingredients may be added.

  In each step, examples of the method of bringing the treatment liquid into contact with the wafer surface include an immersion method and a spray method. Moreover, you may use the casting method which dripping a process liquid to the single side | surface of the wafer used as a light-receiving surface.

  Further, in the porous treatment process, a washing process with water may be performed after at least one of the first to step S3.

  As mentioned above, although the manufacturing method of the solar cell wafer of this invention was demonstrated including the effect, the following 2 points | pieces are mentioned as an additional effect of this manufacturing method. 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, since the porous structure of the present invention can form unevenness on the wafer surface that is finer than the conventional texture structure, the reflectance of the surface of the polycrystalline silicon wafer can be sufficiently suppressed by a safe and inexpensive method. Can do. 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.

  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 battery cell according to the present invention, it is possible to obtain a solar battery cell with high conversion efficiency with little reflection loss of incident light on the light receiving surface of the cell.

(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.

  Although the present invention has been described above, these are examples of typical embodiments, and the present invention is not limited to these embodiments, and various modifications can be made within the scope of the invention. It can be changed.

  In order to further clarify the effects of the present invention, comparative evaluations in which experiments of Examples and Comparative Examples described below were conducted will be described.

<Preparation of sample>
(Examples 1 to 15)
First, a 20 mm square p-type (100) single crystal silicon wafer (thickness: 4.25 mm) was prepared, and 50% by mass hydrofluoric acid / 70% by mass nitric acid / water = 1: 4: 5 (volume ratio). Etching treatment was performed for 3 minutes at room temperature using the acidic solution prepared in (3), and then the wafer was dried. All subsequent steps were performed at room temperature. This wafer was immersed in a 99% by mass ethanol solution for 1 minute. Then, this wafer was immersed in a mixed solution of 5 mL of a copper nitrate solution containing 1000 ppm of Cu, 15 mL of 50% by mass hydrofluoric acid, and 10 mL of water for 3 minutes. In addition, this process was performed in a normal indoor environment, that is, a light irradiation environment. Thereafter, this wafer was immersed in 0.1% by mass nitric acid for 5 minutes and dried in a nitrogen atmosphere. Then, as the acid solution treatment, hydrofluoric acid and nitric acid are contained at the concentrations shown in Table 1, and the rest is immersed in an acid solution, which is water, for the treatment time shown in Table 1, and then dried in a nitrogen atmosphere for solar cells. A wafer was manufactured.

(Comparative Example 1)
Of the manufacturing steps of Example 1, Comparative Example 1 was performed by immersing in 0.1% by mass nitric acid for 5 minutes and drying in a nitrogen atmosphere. That is, Comparative Example 1 does not perform only the acid solution treatment in the present invention (treatment time 0 minutes).

(Comparative Example 2)
Etching treatment is performed for 3 minutes at room temperature using an acidic solution prepared at 50% by mass hydrofluoric acid / 70% by mass nitric acid / water = 1: 4: 5 (volume ratio) in the manufacturing process of Example 1. The process up to the application step was performed, and the wafer was dried to obtain Comparative Example 2. That is, Comparative Example 2 does not perform the porous treatment and acid solution treatment in the present invention.

(Comparative Example 3)
Etching treatment is performed for 3 minutes at room temperature using an acidic solution prepared at 50% by mass hydrofluoric acid / 70% by mass nitric acid / water = 1: 4: 5 (volume ratio) in the manufacturing process of Example 1. The wafer is dried, and then immersed in an acid solution containing 1% by mass hydrofluoric acid and 35% by mass nitric acid as the acid solution treatment, and the rest being water for 1 minute, and in a nitrogen atmosphere. A drying process was performed to obtain Comparative Example 3. That is, Comparative Example 3 does not perform only the porous treatment in the present invention.

<Evaluation 1: Reflectance measurement>
The reflection spectrum on the surface to be processed of the wafer was measured in the range of 300 to 1200 nm with a reflectance measuring device (manufactured by Shimadzu Corporation: SolidSpec 3700). 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, Table 1 shows the relative reflectance at wavelengths of 600 nm and 700 nm. When Examples 1 to 15 and Comparative Examples 2 and 3 were compared, it was found that the reflectance on the surface to be processed of the wafer was lowered by performing the porous treatment of the present invention. Moreover, it turned out by the comparison with Examples 1-15 and the comparative example 1 that a reflectance will raise a little when the acid solution process of this invention is performed.

<Evaluation 2: Conversion efficiency measurement>
P-OCD (manufactured by Tokyo Ohka Kogyo Co., Ltd., model number P-110211) is applied to the wafers of the examples and comparative examples by spin coating, and diffusion heat treatment is performed to form a pn junction. The surface phosphorus glass was removed. Thereafter, an ITO film was formed by sputtering as an antireflection film on the phosphorus diffusion surface of the wafer surface. Also, an Ag paste for Ag electrode was applied to the front surface, an Al paste for Al electrode was applied to the back surface, and then heat treatment was performed to form electrodes on the front and back surfaces of the wafer, thereby producing solar cells. And the result of having measured the energy conversion efficiency with the conversion efficiency measuring device (the Izumi tech company make: YQ-250BX) is shown in Table 2. Each example had higher conversion efficiency than each comparative example. In particular, when Examples 1 to 15 and Comparative Example 1 were compared, Comparative Example 1 was inferior in conversion efficiency despite the lower reflectance than Examples 1 to 15. Thus, it has been found that by performing the porous treatment and acid solution treatment of the present invention, higher conversion efficiency can be obtained by an action other than the action of reducing the reflection loss of the surface. Furthermore, in Examples 1, 2, and 5-15 in which the thickness of the porous layer is 50 to 400 nm, it was found that the conversion efficiency was higher than that in Examples 3 and 4. In this example, since an experiment was performed using a test piece having a thickness of 425 μm, the numerical value was lower than that obtained under the general test conditions. The conversion efficiency when applied to this wafer is expected to increase several percent over the results in Table 2.

  According to the present invention, a surface of a semiconductor wafer such as a silicon wafer is made porous in a safe and inexpensive manner, and a solar cell wafer capable of producing a solar cell with low reflection loss of light on the surface and high conversion efficiency is obtained. It became possible.

Claims (8)

A first step of bringing a lower alcohol into contact with at least one surface of a semiconductor wafer;
After the first step, a second step of bringing hydrofluoric acid containing metal ions into contact with at least one surface of the semiconductor wafer;
After the second step, a third step of bringing an acid solution containing hydrofluoric acid and nitric acid into contact with at least one surface of the semiconductor wafer;
A method for producing a solar cell wafer, wherein at least one surface of the semiconductor wafer is made porous to form a solar cell wafer. The semiconductor wafer is a silicon wafer;
The method for producing a solar cell wafer according to claim 1, wherein the metal ions are ions of a metal nobler than Si.   The method according to claim 1, further comprising a step of bringing a solution for removing metal deposited from the metal ions from at least one side of the semiconductor wafer into contact with at least one side of the semiconductor wafer after the second step and before the third step. The manufacturing method of the wafer for solar cells as described.   The manufacturing method of the wafer for solar cells of any one of Claims 1-3 from which the thickness of the porous layer produced on the said one side becomes 50-400 nm.   The sun according to any one of claims 1 to 4, wherein a concentration of the hydrofluoric acid in the acid solution is 0.1 to 5.0 mass% and a concentration of the nitric acid is 20 to 50 mass%. Manufacturing method of battery wafer.   The manufacturing method of the wafer for solar cells of any one of Claims 1-5 whose time to contact the said acid solution of the said 3rd process is 10 minutes or less.   The manufacturing method of the photovoltaic cell 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 1-6. 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 7.

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