WO2017221710A1 - Solar cell manufacturing method - Google Patents

Abstract

Provided is a solar cell manufacturing method by which an optimally textured structure can be formed on a surface of a silicon substrate. With respect to the silicon substrate, an attach step, a first etching step, and a second etching step are performed in sequence. In the attach step, the silicon substrate is dipped in a first aqueous solution in which hydrofluoric acid and silver nitrate are contained in water, to cause silver to attach to the surface of the silicon substrate. The first aqueous solution is prepared such that the silver nitrate has a molarity in a range of not less than 9 × 10^-5 (mol/L) and not more than 1 × 10^-3 (mol/L). In the first etching step, the silicon substrate is dipped in a second aqueous solution in which hydrofluoric acid and hydrogen peroxide are contained in water, the silicon substrate is etched using precipitated silver as a catalyst, and a number of pores are formed in the surface of the silicon substrate, making the surface porous. In the second etching step, the silicon substrate is dipped in a third aqueous solution in which hydrofluoric acid and nitric acid are contained in water and etched, thereby increasing the depth and opening size of the pores.

Description

Manufacturing method of solar cell

The present invention relates to a method for manufacturing a solar cell, and more particularly to a method for forming fine irregularities on the surface of a silicon substrate.

As an alternative energy source such as coal and oil, sunlight is attracting attention as a clean and inexhaustible energy source, and the spread of solar cells that convert the light energy of the sunlight into electrical energy is further expected.

The surface of the solar cell is formed with innumerable fine irregularities, that is, a texture structure, which has a role of effectively reflecting sunlight by reducing reflection on the surface. In the case of single crystal silicon, a pyramid-shaped texture structure can be easily obtained by etching the (100) surface of silicon (Si) using an alkaline solution. On the other hand, in the case of polycrystalline silicon, since various crystal orientations appear on the surface of the silicon substrate, it is difficult to form a uniform texture structure on the entire surface of the silicon substrate like single crystal silicon.

As a method for forming a texture structure on the surface of a silicon substrate made of polycrystalline silicon, a silicon substrate is added to a mixed aqueous solution of an oxidizing agent (for example, hydrogen peroxide (H ₂ O ₂ )) and hydrofluoric acid containing metal ions. A method of making the surface of a silicon substrate porous by immersing the substrate (for example, Patent Document 1) is disclosed. In this method, a metal is deposited on the surface of a silicon substrate immersed in a mixed aqueous solution, and the metal acts as a reducing catalyst for the oxidant to promote oxidative dissolution of silicon. A large number of pores are formed by digging holes in the substrate. Also, a first step of making the silicon substrate surface porous by immersing the silicon substrate in a mixed aqueous solution of oxidant and hydrofluoric acid containing metal ions, and fluoridating the silicon substrate surface that has undergone the first step A method (for example, Patent Document 2) is disclosed that includes a second step of forming a textured structure by dipping in a mixed acid mainly composed of hydrogen acid and nitric acid and etching.

Japanese Patent No. 3925867 Japanese Patent No. 4610669

By the way, in the above-mentioned patent documents 1 and 2, by depositing a silicon substrate in a mixed aqueous solution of an oxidant and hydrofluoric acid, both containing metal ions, metal deposition and deposition on the surface of the silicon substrate The formation of pores using a metal as a catalyst proceeds simultaneously. For this reason, the variation in the depth and diameter of the formed pores increases, and as a result, the variation in the depth and size (opening size and inner diameter) of the concave portion of the texture structure increases, and reflection on the surface of the silicon substrate is sufficient. It was not possible to reduce it.

An object of the present invention is to provide a solar cell manufacturing method capable of forming an optimal texture structure on the surface of a silicon substrate.

The present invention relates to a method for manufacturing a solar cell in which fine irregularities are formed on the surface of a silicon substrate, containing hydrofluoric acid and silver nitrate, wherein the molar concentration of silver nitrate is 9 × 10 ⁻⁵ (mol / L) or more. A step of immersing the silicon substrate in a first aqueous solution within a range of × 10 ⁻³ (mol / L) or less to attach silver to the surface of the silicon substrate, and containing hydrofluoric acid and hydrogen peroxide A first etching step of immersing the silicon substrate that has undergone the adhesion step in a second aqueous solution and etching the surface of the silicon substrate by a catalytic reaction of silver.

According to the present invention, the silicon substrate is immersed in the first aqueous solution prepared so that the molar concentration of silver nitrate is in the range of 9 × 10 ⁻⁵ (mol / L) to 1 × 10 ⁻³ (mol / L). Since the silicon substrate is immersed in the second aqueous solution containing the oxidizing agent after the silver is attached to the surface of the silicon substrate by the silicon substrate, the silicon substrate in the first aqueous solution in which silver ions are uniformly dispersed by hydrofluoric acid without containing the oxidizing agent. Silver uniformly adheres to the surface of the silicon substrate, and the surface of the silicon substrate is etched by the catalytic reaction of silver in the second aqueous solution containing the oxidizing agent. This makes it possible to form an optimal texture structure on the surface of the silicon substrate that has a small variation in the depth and size (opening size and inner diameter) of the recess and that can sufficiently reduce the reflection on the surface of the silicon substrate. .

It is an end view which shows typically the principal part structure of the solar cell which concerns on this embodiment. It is a flowchart which shows the formation procedure of a texture structure. It is explanatory drawing explaining the difference in the texture structure formed by the difference in the molar concentration of the silver nitrate of 1st aqueous solution. It is a graph which shows the relationship between the reflectance of the surface of a silicon substrate, and the molar concentration of the silver nitrate of 1st aqueous solution. It is a graph which shows the relationship between the depth of the recessed part of a texture structure, and the molar concentration of the silver nitrate of 1st aqueous solution. It is a graph which shows the reflectance for every wavelength.

In FIG. 1, a solar cell 10 includes a silicon substrate 12, a diffusion layer 13 formed on the surface of the silicon substrate 12 on the light receiving surface 10a side, and an antireflection film 14 formed so as to cover the diffusion layer 13. is doing. For example, the silicon substrate 12 is a p-type semiconductor, and the diffusion layer 13 is an n-type semiconductor layer formed by diffusing impurities on the surface of the silicon substrate 12. A PN junction is formed by the silicon substrate 12 and the diffusion layer 13. The antireflection film 14 is a thin film having a single layer structure of titanium oxide (TiO ₂ ) or silicon nitride (SiN) formed by, for example, a chemical vapor deposition (CVD) method, and suppresses reflection of light. To do. Furthermore, although illustration is omitted, the solar cell 10 has a grid electrode provided on the light receiving surface 10a, a back surface field layer and a back electrode laminated on the back surface side of the silicon substrate 12.

The light receiving surface 10a of the solar cell 10 has a texture structure composed of fine irregularities. As is well known, the texture structure is intended to reduce surface reflection loss and increase light absorption by the light confinement effect. The incident light repeatedly transmits and reflects, and as a result, compared to a flat light receiving surface, More light is directed to the PN junction. It is generally known that the texture structure can efficiently confine incident light when the depth and density of the concave portions are uniform, rather than nonuniform. Note that the depth of the concave portion of the texture structure is a difference in level of the unevenness of the texture structure.

The texture structure of the light receiving surface 10a reflects the texture structure of the surface of the silicon substrate 12, and the texture structure of the light receiving surface 10a and the texture structure of the surface of the silicon substrate 12 are substantially the same. A method for forming the texture structure of the silicon substrate 12 will be described with reference to FIG. Note that the silicon substrate 12 is washed with water between the steps described below.

First, the damaged layer is removed in the damaged layer removing step (step SP1). In this damaged layer removing step, the damaged layer of the silicon substrate 12 is removed by immersing the silicon substrate 12 in an alkaline solution. As the alkaline solution used for removing the damaged layer, for example, a sodium hydroxide (NaOH) aqueous solution is used.

After the damaged layer removing step, an attaching step for attaching silver to the surface of the silicon substrate 12 is performed by immersing the silicon substrate 12 in the first aqueous solution (step SP2). The first aqueous solution is an aqueous solution containing hydrofluoric acid (HF) and silver nitrate (AgNO ₃ ). The first aqueous solution is acidic with hydrofluoric acid, and silver nitrate is present in the form of silver ions. When the silicon substrate 12 is immersed in the first aqueous solution, silver is deposited on the surface of the silicon substrate 12. The precipitation of silver is a reduction reaction in which silver ions in the first aqueous solution obtain electrons, and the corresponding electrons are extracted from the silicon (Si) of the silicon substrate 12, so that silicon is dissolved. For this reason, silver is deposited so as to enter the pits which are fine depressions formed on the surface of the silicon substrate 12.

Each silver ion in the first aqueous solution does not agglomerate in the first aqueous solution due to the repulsive force of the mutual electric charge and exists with high dispersibility. Therefore, silver is uniformly dispersed on the surface of the silicon substrate 12 and precipitated. There is also little variation in the size (particle size) of precipitated silver. Moreover, the natural oxide film of silicon on the surface of the silicon substrate 12 is removed by hydrofluoric acid in the first aqueous solution, and adhesion of silver ions to the surface of the silicon substrate 12 is promoted.

The attaching step is a step for attaching silver (depositing silver) to the surface of the silicon substrate 12, and does not make the surface of the silicon substrate 12 porous by etching using silver as a catalyst. Therefore, the first aqueous solution does not contain an oxidizing agent such as hydrogen peroxide (H ₂ O ₂ ) for proceeding etching using silver as a catalyst.

In this attaching step, prior to making the surface of the silicon substrate 12 porous by etching, the silicon substrate 12 is immersed in a first aqueous solution containing silver ions, and silver is attached to the surface of the silicon substrate 12. That is, the adhesion of silver to the surface of the silicon substrate 12 and the porous formation by etching are not performed simultaneously. Thereby, compared with the conventional method which performs adhesion of silver to the surface of the silicon substrate 12 and porous by etching at the same time, silver can be uniformly adhered to the surface of the silicon substrate 12.

In order to make the reflection of light sufficiently small by the texture structure, the molar concentration of silver nitrate in the first aqueous solution (the amount of silver nitrate in the first aqueous solution in unit volume) is 9 × 10 ⁻⁵ (mol / L) or more 1 It is preferably within a range of 10 × ^{3 −3} (mol / L) or less, more preferably within a range of 1 × 10 ⁻⁴ (mol / L) or more and 8 × 10 ⁻⁴ (mol / L) or less. Particularly preferably, it is in the range of 2 × 10 ⁻⁴ (mol / L) to 3 × 10 ⁻⁴ (mol / L). Thus, by adjusting the molar concentration of silver nitrate in the first aqueous solution, the depth and size (opening size and inner diameter) of the recesses of the texture structure are small, and the recesses having a large depth are formed with high density. The reflection of light can be made sufficiently small by the texture structure.

The molar concentration of hydrogen fluoride in the first aqueous solution is preferably in the range of 0.1 (mol / L) to 10 (mol / L). By adjusting the molar concentration of hydrogen fluoride in the first aqueous solution to such a numerical value, it is possible to optimize the dispersibility of silver ions and the ease with which silver adheres to the surface of the silicon substrate 12.

The temperature of the first aqueous solution is preferably in the range of 20 ° C. or higher and 30 ° C. or lower, and the immersion time in the first aqueous solution is within the range of 30 to 120 seconds within this temperature range. preferable. If the immersion time is within this range, the amount of silver adhering to the surface of the silicon substrate 12 can be within an appropriate range without excess or deficiency.

A circulation device (not shown) is connected to the treatment tank (not shown) in which the first aqueous solution is stored. Before the attaching step, the circulating device is operated to flow the first aqueous solution in the treatment tank (flowing step) in order to make the temperature of the first aqueous solution uniform. The apparatus is stopped (flow stop process). In this way, in the attaching step, the flow of the first aqueous solution by the circulation device is eliminated during the immersion of the silicon substrate 12 in the first aqueous solution. This prevents the formation of streak-like traces that cause the reflectance to deteriorate on the surface of the silicon substrate 12. The trace is formed by the silver adhering to the surface of the silicon substrate 12 moving on the surface of the silicon substrate 12 by the flow of the first aqueous solution.

The first etching process is performed after the adhesion process (step SP3). In the first etching step, the silicon substrate 12 is immersed in a second aqueous solution containing hydrofluoric acid and hydrogen peroxide as an oxidizing agent, and etching is performed by silver catalytic action, so that the surface of the silicon substrate 12 is porous. Qualify. That is, the catalytic action of silver deposited on the surface of the silicon substrate 12 advances the reduction reaction of the hydrogen peroxide solution to form pores as if the bits are dug with silver. In this reduction reaction, electrons move from the portion of the silicon substrate 12 in contact with silver to the oxidizing agent, so that holes are generated in the portion of the silicon substrate 12 in contact with silver and oxidative dissolution occurs. As a result, the portion of the silicon substrate 12 on which the silver is not attached is formed so as to dig the pores by the catalytic action of the silver on the portion to which the silver is attached with little progress of etching. To do.

A large number of pores are formed by a large number of silver deposited on the surface of the silicon substrate 12, and the surface of the silicon substrate 12 is made porous. Since etching is performed using the catalytic action of silver deposited as described above on the silicon substrate 12 in a state where silver is deposited in the attaching step, each pore on the surface of the porous silicon substrate 12 is formed. Variations in depth and hole diameter are small.

The molar concentration of hydrogen fluoride in the second aqueous solution is preferably in the range of 0.5 (mol / L) to 50 (mol / L).

By controlling the immersion time of the silicon substrate 12 and the ratio of hydrogen fluoride and hydrogen peroxide in the first etching step, the depth and diameter of the pores formed by the catalytic reaction can be controlled. The hydrogen peroxide solution has a stronger ability to take electrons from the silicon substrate 12 than silver, and when the molar concentration of hydrogen peroxide increases, the ratio of the pore diameter to the depth tends to increase. In order to form the porous layer more reliably, the ratio of the molar concentration of hydrogen peroxide to the molar concentration of hydrogen fluoride in the second aqueous solution is preferably 10% or more and 25% or less. If the ratio of the molar concentration of hydrogen peroxide to the molar concentration of hydrogen fluoride is 25% or less, the etching rate by the hydrogen peroxide solution can be made slower than the etching rate by silver catalysis, and the entire surface of the silicon substrate Can form pores without being flattened. On the other hand, if the ratio of the molar concentration of hydrogen peroxide to the molar concentration of hydrogen fluoride is 10% or more, the pores can be dug while increasing the diameter.

In addition, the temperature of the second aqueous solution is preferably in the range of 20 ° C. or higher and 30 ° C. or lower, and the immersion time in the second aqueous solution is within the range of 30 seconds to 120 seconds within this temperature range. preferable. If the immersion time in the second aqueous solution is 30 seconds or more, pores having a sufficient depth can be formed, and if it is 120 seconds or less, a satisfactory depth is obtained for obtaining a texture structure having a high light confinement effect. Can be formed. The second aqueous solution is intended only to make the surface of the silicon substrate 12 porous by etching, and is not intended to adhere silver to the surface of the silicon substrate 12, and therefore contains the silver and other metals. do not do.

Note that the second aqueous solution is replaced when the concentration change (composition change) of hydrofluoric acid or hydrogen peroxide water occurs as the number of treatments of the silicon substrate 12 increases. Unlike the first aqueous solution, this second aqueous solution does not contain silver ions, and therefore does not require replacement and disposal as in the conventional aqueous solution containing silver ions.

After the first etching process, a second etching process is performed (step SP4). In this second etching step, the silicon substrate 12 is immersed and etched in a third aqueous solution containing hydrofluoric acid and nitric acid. Thereby, the silicon substrate including the inner surface of the pore is etched, the diameter of each pore is enlarged, and the silicon substrate 12 is made macroporous. At the same time, the silver adhering to the silicon substrate 12 is dissolved and removed in the third aqueous solution. Since the dissolved silver may reattach to the surface of the silicon substrate, it is finally removed by a metal removal step described later.

The pores formed in the silicon substrate 12 are etched with the third aqueous solution, so that the depth and size thereof are increased, and the textured structure becomes a recess. Since the pores of the silicon substrate 12 made porous in the first etching step have small variations in depth and size, the depth and size variations of the concave portions of the texture structure are also small. Therefore, a texture structure with small variations in the depth and size of the concave portion of the texture structure is formed.

The molar concentration of hydrogen fluoride in the third aqueous solution is preferably in the range of 0.1 (mol / L) to 10 (mol / L). The molar concentration of nitric acid in the third aqueous solution is preferably in the range of 0.5 (mol / L) to 50 (mol / L). Further, the temperature of the third aqueous solution is preferably in the range of 20 ° C. or higher and 30 ° C. or lower, and within this temperature range, the immersion time in the third aqueous solution is 40 seconds or longer and 120 seconds or shorter. preferable. If the immersion time in the third aqueous solution is 40 seconds or more, a texture structure having a sufficient depth and size can be formed, and if it is 120 seconds or less, the depth and size of the recess are necessary. A texture structure with a good light confinement effect can be formed without enlarging the above.

After the second etching process, a stain removal process is performed (step SP5). Here, the stain is a by-product such as silicon residue or hexafluorosilicic acid (H ₂ SiF ₆ ) generated in the first and second etching steps. In this stain removal step, the stain on the surface of the silicon substrate 12 is removed by immersing the silicon substrate 12 in an alkaline fourth aqueous solution. For example, an aqueous potassium hydroxide solution can be used as the alkaline chemical solution. When the molar concentration of potassium hydroxide in the aqueous potassium hydroxide solution is 8.6 (mol / L), the immersion time may be, for example, 10 seconds.

Following the stain removal process, a metal removal process is performed (step SP6). In this metal removal step, the metal adhering to the silicon substrate 12 is removed by immersing the silicon substrate 12 in the fifth aqueous solution. The metal to be removed is mainly silver reattached in the second etching step, but there are other metals (for example, copper (Cu) and aluminum (Al)) attached to the silicon substrate 12. These are also removed. As the fifth aqueous solution, an aqueous solution of hydrofluoric acid and hydrochloric acid or an aqueous solution of hydrochloric acid and hydrogen peroxide can be used. In the case of an aqueous solution of hydrofluoric acid and hydrochloric acid, the liquid temperature is preferably in the range of 20 ° C. to 30 ° C., and in the case of an aqueous solution of hydrochloric acid and hydrogen peroxide, the liquid temperature is preferably 40 ° C. or higher. Moreover, what is necessary is just to set immersion time, for example in any aqueous solution, to 3 minutes.

As described above, a texture structure is formed on the surface of the silicon substrate 12. This formed texture structure has small variations in the depth and size of the recesses in the texture structure. Moreover, by adjusting the molar concentration of silver nitrate in the first aqueous solution as described above, concave portions having a large depth are formed at a high density, so that a texture structure with sufficiently small light reflection can be obtained.

After the formation of the texture structure, the diffusion layer 13 is formed by diffusing impurities on the surface of the silicon substrate 12, and the antireflection film 14 is formed on the diffusion layer 13. Furthermore, a grid electrode, a back surface electric field layer, and a back electrode are formed on the silicon substrate 12 to form a solar cell.

As described above, the difference in the molar concentration of silver nitrate in the first aqueous solution in the attaching step affects the reflectance of the surface of the silicon substrate 12, that is, the reflectance of the light receiving surface 10a of the solar cell 10. As schematically shown in FIG. 3, the difference in the density of silver on the surface of the silicon substrate 12 due to the difference in the molar concentration of silver nitrate in the first aqueous solution is only the density of the recesses in the texture structure of the silicon substrate 12. It is presumed that this also affects the depth of the recess.

FIG. 3A shows the formation process of the texture structure when the molar concentration of silver nitrate in the first aqueous solution is within the above-described range and is appropriate. The silver ions 21 in the first aqueous solution are dispersed at a density corresponding to the molar concentration of silver nitrate in the first aqueous solution. For this reason, when the silicon substrate 12 is immersed in the first aqueous solution, the silver 22 is dispersed and deposited on the surface of the silicon substrate 12 at a density corresponding to the molar concentration of silver nitrate. Reference numerals 23a to 23c are pits formed by the dissolution of silicon at the silver 22 precipitation portion.

In the first etching step, as described above, the etching of the portion where the silver 22 is not attached on the surface of the silicon substrate 12 hardly progresses, but the portion where the silver 22 is attached becomes the catalyst for the silver 22 attached. Etching proceeds. For this reason, a pore 24a having a deep bit 23a is formed. In the second etching step, a texture structure having a recess 25a in which the pore diameter of the pore 24a is enlarged is formed. And the texture structure formed in this way is a state where the density of the recessed part 25a is high, and the variation of the depth and size of many recessed parts 25a becomes a small structure.

On the other hand, when the molar concentration of silver nitrate in the first aqueous solution is a reference, if the molar concentration of silver nitrate is low, the density of silver ions 21 in the first aqueous solution is low, as shown in FIG. In the process, the density of silver 22 deposited on the surface of the silicon substrate 12 is low. Even in this case, in the first etching step, the etching hardly progresses in the portion where the silver 22 is not adhered, but in the portion where the silver 22 is adhered, the etching proceeds using the silver 22 as a catalyst, and the silicon substrate 12 In the second etching step, a texture structure having a recess 25b in which the diameter of the pore 24b is increased is formed.

By the way, since the number of silvers 22 entering the bit 23b is reduced as compared with the case where the molar concentration of silver nitrate is appropriate, the etching due to the catalytic action of the silver 22 is delayed. Therefore, the pore 24b and the recess 25b formed from the pore 24b become shallow. In addition, since the density of the silver 22 on the surface of the silicon substrate 12 is low, the density of the recesses 25b is also low. As a result, since the shallow concave portion 25b has a texture structure formed with a small density, the reflectance on the surface of the silicon substrate 12 is not sufficiently lowered.

Further, when the molar concentration of silver nitrate in the first aqueous solution is appropriate, the density of silver ions 21 in the first aqueous solution is high as shown in FIG. Therefore, the number of silver 22 deposited on the surface of the silicon substrate 12 in the adhesion process increases, and the density on the surface is high. In this case, for example, silver 22 other than the silver 22 forming the bit 23c may be deposited on the inner surface of the bit 23c. In the first etching step, silver 22 is deposited at a high density on the surface of the silicon substrate 12, so that the surface of the silicon substrate 12 is entirely etched although the pores 24c are formed. Accordingly, the pores 24c become shallower by that amount, or adjacent pores 24c are connected to become one. Then, the concave portion 25c of the texture structure formed by etching the surface of the silicon substrate 12 including the inner surface of the pore 24c in the second etching step is considerably shallow and is quite small. Therefore, the reflectance at the surface of the silicon substrate 12 is not sufficiently lowered.

On the other hand, the texture structure when the molar concentration of silver nitrate is appropriate is a structure in which the density of the recesses 25a is high as described above, and the depth and size variations of the many recesses 25a are small. In addition, the pores 24a are formed deeper than when the molar concentration of silver nitrate is lower than the appropriate value, and the concave portions are not shallow as in the case where the molar concentration is high. As a result, the concave portions 25a are sufficiently deep. have. Therefore, the reflectance of the surface of the silicon substrate 12 can be sufficiently lowered.

(Example)
Next, Examples 1 to 4 in which a texture structure is formed on the silicon substrate 12 by changing the molar concentration of silver nitrate in the first aqueous solution will be described. In Examples 1 to 4, a texture structure was formed on the surface of the silicon substrate 12 according to the above procedure. In Examples 1 to 4, the conditions were the same except that the molar concentration of silver nitrate in the first aqueous solution was different.

The first aqueous solution used in the adhering step was prepared by mixing hydrofluoric acid and an aqueous solution of silver nitrate with water, and preparing each molar concentration of hydrogen fluoride and silver nitrate as shown in Table 1. The liquid temperature of the first aqueous solution was room temperature (25 ° C. ± 5), and the p-type silicon substrate 12 was immersed. The immersion time of the silicon substrate 12 in the first aqueous solution was 60 seconds.

After the adhesion process, the silicon substrate 12 was washed with water and then immersed in the second aqueous solution to perform the first etching process. The second aqueous solution used in the first etching step was prepared by mixing an aqueous solution of hydrofluoric acid and hydrogen peroxide with water and preparing each molar concentration of hydrogen fluoride and hydrogen peroxide as shown in Table 2. The liquid temperature of the second aqueous solution was room temperature (25 ° C. ± 5), and the immersion time of the silicon substrate 12 in the second aqueous solution was 60 seconds. After the first etching step, the silicon substrate 12 was washed with water and then immersed in the third aqueous solution to perform the second etching step. The third aqueous solution used in the second etching step was prepared by mixing each aqueous solution of hydrofluoric acid and nitric acid with water, and the molar concentrations of hydrogen fluoride and nitric acid are as shown in Table 2. The liquid temperature of the third aqueous solution was room temperature (25 ° C. ± 5), and the immersion time of the silicon substrate 12 in the third aqueous solution was 40 seconds.

As Comparative Examples 1 to 4, a texture structure was formed on the surface of the silicon substrate by changing the molar concentration of silver nitrate in the first aqueous solution. The molar concentration of silver nitrate in the first aqueous solutions of Comparative Examples 1 to 4 is as shown in Table 1. The conditions and procedures in Comparative Examples 1 to 4 are the same as those in Examples 1 to 4 except that the molar concentration of silver nitrate is different.

The absolute reflectance of the surface of each silicon substrate of Examples 1 to 4 and Comparative Examples 1 to 4 and the depth of the concave portion of the texture structure were measured. These measurements were performed after the second etching step, after the silicon substrate was washed with water and dried. Table 1 shows the measurement results. The depth of the concave portion of the texture structure is determined by cleaving the silicon substrate, observing the cross section with an SEM (Scanning Electron 、 Microscope), and using the accompanying scale, a plurality of conditions for each condition. It was set as the average value of the depth which measured the recessed part. Since the deepest portion of the concave portion does not always appear in the cross section of the silicon substrate, the depth considered to be the deepest portion was measured from the observed shape inside the concave portion.

FIG. 4 shows a graph of absolute reflectance with respect to the molar concentration of silver nitrate in the first aqueous solution, and FIG. 5 shows a graph of the depth of the concave portion of the texture structure with respect to the molar concentration of silver nitrate in the first aqueous solution. In the graph of FIG. 4, the vertical axis represents the absolute reflectance (%) with respect to light having a wavelength of 700 nm, and in the graph of FIG. 5, the vertical axis represents the depth (nm) of the concave portion of the texture structure. In each of these graphs, the horizontal axis represents the molar concentration (mol / L) of silver nitrate in the first aqueous solution. In FIG. 5, the SEM image of the surface of each silicon substrate of Examples 1, 2, and 4 and Comparative Examples 1 and 3 is shown with the graph. In this SEM image, the deeper the deeper the deeper the recess. Furthermore, the result of having measured the absolute reflectance with respect to the wavelength of the surface of each silicon substrate of Examples 1 and 2 and Comparative Examples 2 and 4 is shown in FIG. In the graph shown in FIG. 6, the molar concentration of silver (silver nitrate) is shown in correspondence with Examples 1 and 2 and Comparative Examples 2 and 4.

From the above results, the depth of the concave portion becomes maximum when the molar concentration of silver in the first aqueous solution, that is, silver nitrate, is 3 × 10 ⁻⁴ (mol / L). The depth of the recess when the molar concentration of the silver nitrate in the first aqueous solution is less than ^{3 × 10 -4 (mol / L} ) decreases rapidly, ^{3 × 10 -4 (mol / L} ) than increases as recesses Depth decreases. An excellent texture structure in which the density of the recesses is high and the depth is large in a range where the concentration of silver nitrate in the first aqueous solution is in the range of 9 × 10 ⁻⁵ (mol / L) to 1 × 10 ⁻³ (mol / L). It can be seen that The reflectance of the surface of the silicon substrate 12 is also minimized when the molar concentration of silver nitrate in the first aqueous solution is 3 × 10 ⁻⁴ (mol / L) corresponding to the depth of the recess, and increases even when the molar concentration decreases. It turns out that even if it increases.

As described above, when the molar concentration of silver nitrate in the first aqueous solution is 3 × 10 ⁻⁴ (mol / L) or more compared to the case where the molar concentration of silver nitrate is smaller than 3 × 10 ⁻⁴ (mol / L), The ratio of the absolute reflectance (%) and the change in the depth of the recess to the change in the molar concentration of silver nitrate is small. This point can be clearly confirmed by converting the horizontal axis (the molar concentration of silver nitrate in the first aqueous solution) in the graphs of FIGS. 4 and 5 to a linear scale. The difference in the absolute reflectance (%) and the ratio of the change in the depth of the recess with respect to the silver nitrate molar concentration of the first aqueous solution is as follows. The molar concentration of silver nitrate in the first aqueous solution is 3 × 10 ⁻⁴ (mol / L If the first aqueous solution is prepared so as to be 3 × 10 ⁻⁴ (mol / L) or more than smaller than), for some reason, the molar concentration of silver nitrate in the first aqueous solution may have some error or This means that even if fluctuations occur, fluctuations in the absolute reflectance (%) of the surface of the silicon substrate 12 and the depth of the recesses can be kept small. Therefore, from the viewpoint of manufacturing the solar cell 10 in which the absolute reflectance (%) of the silicon substrate 12 and the variation in the depth of the concave portion are suppressed to be small, and thus the performance variation is reduced, the molar concentration (mol) of silver nitrate in the first aqueous solution. / L) is preferably 3 × 10 ⁻⁴ (mol / L) or more, and considering the viewpoint of sufficiently reducing the reflectance of the surface of the silicon substrate 12, the molar concentration of silver nitrate in the first aqueous solution ( it is preferable that the mol / L) of 3 × ¹⁰ -4 (mol / L) or 1 × ¹⁰ -3 (mol / L) or less.

10 Solar Cell 12 Silicon Substrate 21 Silver Ion 22 Silver 24a-24c Pore 25a-25c Recess

Claims (5)

In a method for manufacturing a solar cell that forms fine irregularities on the surface of a silicon substrate,
The first aqueous solution containing hydrofluoric acid and silver nitrate, wherein the molar concentration of silver nitrate is in the range of 9 × 10 ⁻⁵ (mol / L) to 1 × 10 ⁻³ (mol / L) An attachment step of immersing the substrate and attaching silver to the silicon substrate surface;
A first etching step of immersing the silicon substrate that has undergone the attaching step in a second aqueous solution containing hydrofluoric acid and hydrogen peroxide, and etching the surface of the silicon substrate by the catalytic reaction of silver. A method for manufacturing a solar cell. 2. The method according to claim 1, further comprising a second etching step in which the silicon substrate that has undergone the first etching step is immersed in a third aqueous solution containing hydrofluoric acid and nitric acid in water to further etch. Solar cell manufacturing method. 3. The solar cell according to claim 1, wherein the first aqueous solution has a molar concentration of hydrogen fluoride within a range of 0.1 (mol / L) to 10 (mol / L). Production method. 4. The solar cell according to claim 1, wherein the second aqueous solution has a molar concentration of hydrogen peroxide of 10% to 25% with respect to a molar concentration of hydrogen fluoride. 5. Production method. The method for manufacturing a solar cell according to claim 2, further comprising a stain removing step of immersing the silicon substrate having undergone the second etching step in an alkaline chemical solution.

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