JP2014090086A - Method for etching silicon substrate, etchant for silicon substrate, and method for manufacturing solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a texture having low reflectance and high flatness on a slope thereof.SOLUTION: In a method for etching, an alkaline aqueous solution is supplied onto the surface of a silicon substrate to perform anisotropic etching. The alkaline aqueous solution contains an alcohol-based additive represented by a chemical formula (1) or a surfactant having an HLB value of 10 or more and a silicate compound represented by a chemical formula (2). A molar ratio between SiOand XO contained in the silicate compound is 1.4-2.1, and the concentration of the silicate compound is 0.2-0.6 wt.% expressed in terms of silicon. X-(OH)n...(1) (where, X represents a saturated or unsaturated hydrocarbon group having a carbon number Cn of 4 or more and 7 or less, and n represents an integer 1 or more and satisfies n<Cn) lXO-mSiO-nHO...(2) (where, X represents an alkaline metal element, l represents the substance quantity (mol) of an alkaline oxide (XO), m represents the substance quantity (mol) of a silicon oxide (SiO)), and n represents the substance quantity (mol) of hydrated water (HO) contained in a silicate compound)

Description

  The present invention relates to a silicon substrate etching method, a silicon substrate etching solution, and a solar cell manufacturing method.

  In a solar power generation device, in order to absorb sunlight efficiently, it is necessary to allow the sunlight irradiated to the silicon substrate to penetrate into the silicon substrate. That is, it is necessary to reduce the light reflectance on the surface of the silicon substrate as much as possible. For this reason, a texture is formed on the surface of the silicon substrate by etching. Here, the texture is a general term for minute irregularities formed on the surface of the silicon substrate. The texture is formed by etching using an alkaline etching solution obtained by adding an additive such as isopropyl alcohol to a hot alkaline aqueous solution. By using such an etchant, anisotropic etching in which etching does not proceed with respect to a specific crystal orientation occurs on the silicon substrate surface, and a minute pyramid structure composed of a stable (111) plane of silicon crystal is formed. Low reflectivity is realized.

  In forming textures with alkaline etching solutions, it is common to use an etching solution in which an additive such as isopropyl alcohol that acts as an etching inhibitor is added to a low-concentration alkaline solution in order to form a texture with a lower reflection. It is. Thus, by adding an appropriate organic substance to the alkaline solution, wet etching on the surface of the silicon substrate is moderately inhibited. Therefore, it is possible to efficiently form a texture and reduce the light reflectance. As a result, it is possible to manufacture a solar power generation device that can absorb more sunlight and has high photoelectric conversion efficiency.

  By the way, in the etching solution, by repeatedly etching the silicon substrate, a silicate compound generated by the reaction between silicon and alkali is accumulated in the solution. In etching solutions containing high concentrations of silicate compounds, the silicate compounds form a gel-like substance over a wide area on the silicon substrate and hinder the texture formation reaction, making it easy to form untextured parts. Become. On the other hand, when the concentration in the alkaline solution is low, the silicate compound acts as an etching inhibitor and is also used as an additive for the alkaline solution, as shown in Patent Document 1, for example.

JP 2012-004528 A

  However, in the concentration range of the silicate compound shown in Patent Document 1 and the range of the molar ratio of alkali components such as sodium and potassium to silicon, the effect of acting as an etching inhibitor and contributing to texture formation is However, the slope of the micropyramid formed on the silicon substrate: The flatness of the (111) plane is not clarified. Rather, in the addition condition of the silicate compound of Patent Document 1, the slope of the micropyramid : There is a concern that the flatness of the (111) plane is deteriorated. The flatness of the slope of the micropyramid: (111) plane affects the growth process of the film formed on the slope: (111) plane of the micropyramid formed on the silicon substrate. When the flatness of the (111) plane of the micropyramid is poor, the quality of the film formed on the silicon substrate is lowered, and the photoelectric conversion efficiency of the solar cell using the silicon substrate is adversely affected. There is a problem of affecting.

  The present invention has been made in view of the above, and in forming a texture on a silicon surface with an alkaline etching solution, it is possible to form a texture having a low reflectance and a high flatness on a textured slope. It aims at obtaining the etching method of a silicon substrate, the etching liquid of a silicon substrate, and the manufacturing method of a solar cell.

In order to solve the above-described problems and achieve the object, the silicon substrate etching method according to the present invention supplies an alkaline aqueous solution to the surface of the silicon substrate and anisotropically etches the surface of the silicon substrate by wet etching. A method for etching a silicon substrate, wherein the alkaline aqueous solution comprises an alcohol-based additive represented by the following chemical formula (1) or a surfactant having an HLB value of 10 or more and a silicic acid represented by the following chemical formula (2): A molar ratio (m / l) of silicon oxide (SiO ₂ ) and alkali oxide (X ₂ O) contained in the silicate compound is 1.4 to 2.1. Yes, the silicon equivalent concentration of the silicate compound is 0.2 wt% to 0.6 wt%.
X- (OH) n (1)
(X is a saturated or unsaturated hydrocarbon group having 4 to 7 carbon atoms, n is an integer of 1 or more, n <Cn)
lX ₂ O · mSiO ₂ · nH ₂ O (2)
(X is an alkali metal element, l is a substance amount (mol) of alkali oxide (X ₂ O), m is a substance amount (mol) of silicon oxide (SiO ₂ ), and n is water contained in the silicate compound. Substance amount of Japanese water (H ₂ O) (mol)

  According to the present invention, in the formation of a texture on the silicon surface with an alkaline etching solution, there is an effect that it is possible to form a texture having low reflectivity and high flatness on the texture slope.

FIG. 1 is a perspective view schematically showing an example of a wet etching process of a silicon substrate for forming a texture on the surface of the silicon substrate in the first embodiment. FIG. 2 shows the SiO ₂ / Na ₂ O ratio in the silicate compound and the light reflectance at a wavelength of 700 nm of the silicon substrate after the etching treatment when the silicon substrate is etched using the etching solution of Example 1. %) Is a characteristic diagram showing the relationship. FIG. 3 shows the SiO ₂ / Na ₂ O ratio of the silicate compound when the silicon substrate is etched using the etching solution according to Example 1, and the slope of the micropyramid in the silicon substrate after the etching process: It is a characteristic view which shows the relationship with Ra (nm) of a (111) plane. FIG. 4 shows the relationship between the SiO ₂ / Na ₂ O ratio of the silicate compound and the etching rate (μm / min) of the silicon substrate when the silicon substrate was etched using the etching solution according to Example 1. FIG. FIG. 5 shows the silicon equivalent concentration (wt%) of the silicate compound and the light reflectance at a wavelength of 700 nm of the silicon substrate after the etching treatment when the silicon substrate is etched using the etching solution of Example 2. %) Is a characteristic diagram showing the relationship. 6 shows the silicon equivalent concentration (wt%) of the silicate compound and the slope of the micropyramid in the silicon substrate after the etching treatment when the silicon substrate is etched using the etching solution according to Example 2. It is a characteristic view which shows the relationship with Ra (nm) of a (111) plane. FIG. 7 shows the relationship between the silicon equivalent concentration (wt%) of the silicate compound and the etching rate (μm / min) of the silicon substrate when the silicon substrate was etched using the etching solution according to Example 2. FIG. FIG. 8-1 is a cross-sectional view of a principal part showing a solar power generation device manufactured using a silicon substrate having a texture formed on the surface by the wet etching method for a silicon substrate according to the first embodiment. FIG. 8-2 is a top view of the solar power generation device manufactured using the silicon substrate having a texture formed on the surface by the wet etching method of the silicon substrate according to the first embodiment.

  Embodiments of a silicon substrate etching method, a silicon substrate etching solution, and a solar cell manufacturing method according to the present invention will be described below in detail with reference to the drawings. In addition, this invention is not limited to the following description, In the range which does not deviate from the summary of this invention, it can change suitably. In the drawings shown below, the scale of each member may be different from the actual scale for easy understanding. The same applies between the drawings.

Embodiment 1 FIG.
In order to form a texture on the silicon substrate surface with a low reflectivity and a high flatness on the textured slope, the present inventors studied a wet etching process using an alkaline etchant to which an additive has been added.

  FIG. 1 is a perspective view schematically showing an example of a wet etching process of a silicon substrate for forming a texture on the surface of the silicon substrate in the first embodiment. In order to form a texture on the surface of the silicon substrate, a wet etching process is performed by immersing the prepared silicon substrate 1 (FIG. 1A) in an alkaline etching solution 3 stored in an etching tank 2 (FIG. 1). 1 (b)). Thereby, the texture 6 formed by the micro pyramid (micro unevenness | corrugation) of the size whose base length is 1-30 micrometers is formed in the surface of the silicon substrate 5 after an etching process at random (FIG.1 (c)).

  The flatness of the (111) plane, which is the slope of the micropyramid on the silicon substrate formed by wet etching using an alkaline etchant, has been evaluated by an atomic force microscope (AFM). In the present embodiment, the flatness of the (111) plane is evaluated by AFM, and the surface average roughness (arithmetic average roughness: Ra (nm)) in the range of 500 nm × 500 nm of the (111) plane is calculated. 111) planes are compared. If Ra is small, the flatness of the (111) plane is high, and if Ra is large, the flatness of the (111) plane is low.

  When the (111) plane, which is the slope of the micropyramid on the silicon substrate, is evaluated by AFM, there is a problem that a part of the sensing lever comes into contact with the apex of the micropyramid and the accuracy of the evaluation deteriorates. It was very difficult. However, the present inventors have succeeded in accurately evaluating the flatness of the (111) plane of the micropyramid by examining a new AFM measurement method suitable for the micropyramid and evaluated by a more accurate method. Comparison is made with Ra on the (111) plane.

  Ra of the (111) plane, which is the slope of the micropyramid formed on the silicon substrate, has a great influence on the characteristics of a solar power generation device manufactured using the silicon substrate. When a silicon film is formed on a textured silicon substrate when manufacturing a solar power generation device, the silicon film is formed on a (111) surface that occupies 90% or more of the surface area of the silicon substrate. . For this reason, Ra on the (111) plane particularly affects the silicon film formed on the silicon substrate.

  Although depending on conditions such as the film quality and film composition of the silicon film, when the Ra of the (111) plane is large, epitaxial growth in the silicon film formed on the (111) plane is promoted, and the silicon film and the silicon substrate The passivation effect at the interface is reduced. Suppressing the epitaxial growth of the silicon film deposited on the (111) plane by reducing the Ra on the (111) plane is particularly a photovoltaic power generation having a structure that requires a silicon film with a thickness of 10 nm or less. In the device, for example, a double-sided heterojunction solar cell, the influence on the power generation characteristics is large. It leads to improvement of F.

  Therefore, the present inventors have developed an alkaline etching solution containing an additive used as an etching inhibitor in a wet etching process for forming a texture structure on the surface of the silicon substrate in order to reduce the light reflectance on the surface of the silicon substrate. The effect of adding a small amount of the silicate compound was examined by the texture surface shape after the etching treatment and the flatness (= Ra) of the (111) plane which is the slope of the micropyramid formed on the silicon substrate.

  In general, silicon (Si) on the silicon substrate 1 reacts with an alkali reagent such as sodium hydroxide (NaOH) in the liquid of the etching liquid 3 and is eluted in the liquid of the etching liquid 3. A texture 6 is formed on the surface. This etching reaction is represented by the following chemical formula (3). At this time, hydrogen gas 4 is generated and a silicate compound is deposited. Further, in order to further activate this generation, it is necessary to carry out under conditions of high alkali or high temperature.

_{^{Si + 2OH - + 4H 2 O}} → Si (OH) 6 2- + 2H 2 ↑
... (3)

The silicate compound deposited by the etching reaction of the chemical formula (3) between alkali and silicon (Si) is specifically defined by the following chemical formula (4), and includes alkali oxide (Na ₂ O) and silicon oxide. (SiO ₂ ) and hydrate (H ₂ O).

lNa ₂ O · mSiO ₂ · nH ₂ O (4)
(1 is the amount of Na ₂ O (mol), m is the amount of SiO ₂ (mol), m is the amount of hydrated water contained in the silicate compound (mol))

  The chemical formula (4) shows a chemical formula of a silicate compound that is precipitated when silicon reacts with NaOH as a representative alkali, and is precipitated when silicon reacts with a hydroxide of a Group 1 element. The silicic acid compound is represented by the same chemical formula and is defined by the following chemical formula (5).

lX ₂ O · mSiO ₂ · nH ₂ O (5)
(X is an alkali metal element, l is a substance amount (mol) of alkali oxide (X ₂ O), m is a substance amount (mol) of silicon oxide (SiO ₂ ), and n is water contained in the silicate compound. Substance amount of Japanese water (H ₂ O) (mol)

Silicate compounds, alkali oxides and in accordance with the silicon oxide and amount of substance, such as lithium orthosilicate _{(2Li 2 O · SiO 2 ·} nH 2 O), lithium metasilicate _{(Li 2 O · SiO 2 ·} nH 2 O), lithium Pirokei acid _{(3Li 2 O · 2SiO 2 ·} nH 2 O), meta lithium disilicate _{(Li 2 O · 2SiO 2 ·} nH 2 O), sodium orthosilicate _{(2Na 2 O · SiO 2 ·} nH 2 O), sodium metasilicate _{(Na 2 O · SiO 2 ·} nH 2 O), sodium Pirokei acid _{(3Na 2 O · 2SiO 2 ·} nH 2 O), meta sodium disilicate _{(Na 2 O · 2SiO 2 ·} nH 2 O), orthosilicate potassium _{(2K 2 O · SiO 2 ·} nH 2 O), potassium metasilicate _{(K 2 O · SiO 2 ·} nH 2 O), Pirokei Potassium _{(3K 2 O · 2SiO 2 ·} nH 2 O), meta dipotassium silicate _{(K 2 O · 2SiO 2 ·} nH 2 O) , and the like.

  The present inventors insert the silicon substrate 1 into the solution of the etching solution 3 containing the additive used as an etching inhibitor, and repeat the wet etching process to thereby form the micro substrate formed on the silicon substrate 1. It has been found that the flatness (= Ra) of the slope of the pyramid (111) plane can be improved. Among them, it was found that the silicate compound precipitated by the reaction between the silicon substrate 1 and the alkali was accumulated in the solution of the etching solution 3 and had a great influence on the flatness of the (111) plane. Therefore, in order to clarify that the silicate compound precipitated by silicon dissolved in the solution of the etching solution 3 affects the quality of the texture 6, silicate is added to the etching solution 3. It was investigated.

  Note that in this specification, the “silicon substrate” includes a single crystal silicon substrate and a polycrystalline silicon substrate, but a single crystal silicon substrate whose principal surface is a (100) plane is particularly preferable.

  Here, the alkaline etching solution 3 is mainly composed of an alkaline aqueous solution in which a strong alkaline reagent such as sodium hydroxide (NaOH), potassium hydroxide (KOH), trimethylammonium hydroxide (TMAH) is dissolved in water. The alkali concentration in the etching solution 3 is, for example, 0.5 wt% to 25 wt%. When the alkali concentration in the etching solution is less than 0.5 wt%, it is not preferable because the etching of silicon is difficult to proceed. When the alkali concentration in the etching solution exceeds 25 wt%, the texture itself is not easily formed, which is not preferable.

  An additive (A) is added to the etching solution 3 in order to moderately inhibit the etching reaction on the surface of the silicon substrate 1 and form a texture. As the additive (A) to be added to the etching solution 3, for example, an alcohol-based additive represented by the following chemical formula (6) is used.

X- (OH) n (6)
(X is a saturated or unsaturated hydrocarbon group having 4 to 7 carbon atoms, n is an integer of 1 or more, n <Cn)

  Examples of such alcohol additives include isopropyl alcohol (IPA), normal propyl alcohol, ethylene glycol (EG), 1-pentanol, 2-pentanol, 1-hexanol, 2-hexanol, 3-hexanol, 1,2-pentanediol, 1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, 1,2-hexanediol, 1,3-hexanediol, 1,4-hexanediol, 1 , 5-hexanediol, 1,6-hexanediol, various hexanetriols and mixtures thereof can be used.

  Further, as the additive (A) added to the etching solution 3, a surfactant can also be used. The surfactant is preferably a nonionic surfactant, and preferably has a HLB (Hydrophile-Lipophile Balance) value of 10 or more. When the HLB value is less than 10, etching is excessively suppressed and it is difficult to form a good texture effective for reducing the light reflectance on the surface of the silicon substrate.

  The presence of the additive (A) in the etching solution 3 inhibits the etching reaction of the chemical formula (3) on the surface of the silicon substrate 1. More specifically, since the surface of the silicon substrate 1 is hydrophobic, the carbon chain portion of the additive (A) that is a hydrophobic group is adsorbed on the surface of the silicon substrate 1, and the etching reaction of the chemical formula (3) is performed. Inhibit. By utilizing this inhibition reaction, the texture 6 can be randomly formed on the surface of the silicon substrate 5. The quality of the texture 6 formed on the surface of the silicon substrate 5 varies depending on the material of the additive (A).

  When any of the above-described additives (A) is used, the amount added to the etching solution 3 is preferably 0.05 g / L to 20 g / L, for example. When the amount added to the etching solution 3 is less than 0.05 g / L, the texture 6 is not formed on the silicon substrate surface. When the amount added to the etching solution 3 exceeds 20 g / L, the etching rate of silicon becomes too slow, which is not preferable.

  The temperature of the etching solution 3 is preferably 40 ° C. to 100 ° C., for example, and more preferably 60 ° C. to 95 ° C. When the temperature of the etching solution 3 is less than 40 ° C., the etching time is too long, which is not preferable. When the temperature of the etching solution 3 exceeds 100 ° C., a pressurized container or the like may be required to store the etching solution 3, and further, the stability of the liquid composition of the etching solution 3 is caused by evaporation of the etching solution 3. It is easy to be damaged.

  The etching time is, for example, 10 minutes to 60 minutes. When the etching time is less than 10 minutes, the etching of silicon does not proceed and the texture is not formed on the silicon substrate surface. When the etching time exceeds 60 minutes, the etching of silicon proceeds too much, which adversely affects texture formation.

The inventors added a silicate compound as the additive (B) to the alkaline etchant 3 containing the additive (A), and performed etching treatment of the silicon substrate 1 with the etchant 3. The silicate compound to be added is preferably a liquid or granular material. The silicate compound to be added is defined by the chemical formulas shown in the chemical formulas (4) and (5), and the molar ratio of silicon oxide (SiO ₂ ) to alkali oxide (Na ₂ O) ( SiO ₂ / Na ₂ O ratio = m / l).

The SiO ₂ / Na ₂ O ratio in the silicate compound varies depending on the additive (A) used as the etching inhibitor, but the optimum value for increasing the flatness of the slope of the micropyramid: (111) plane is: The range is 1.4 to 2.1. When the SiO ₂ / Na ₂ O ratio in the silicate compound is less than 1.4, the flatness of the slope of the micropyramid formed on the silicon substrate: (111) plane cannot be sufficiently improved, and silicon The overall texture size of the substrate surface is miniaturized. When the SiO ₂ / Na ₂ O ratio in the silicate compound is greater than 2.1, the flat surface of the micropyramid: the (111) plane is improved but the texture is not formed (100) plane Is partly formed and becomes a factor for increasing the light reflectance.

Na ₂ O constituting the silicate compound has a smaller molecular weight than an organic additive used as an etching inhibitor in, for example, an etching solution disclosed in International Publication No. 2007/129555. Acts as an etch inhibitor to reduce. On the other hand, SiO ₂ functions as a gel substance having low fluidity to flatten the (111) plane by remaining on the (111) plane during the etching process. However, since the SiO ₂ / Na ₂ O ratio in the silicate compound becomes too high, SiO ₂ is deposited thickly as a gel material on the (100) surface in the process of forming the texture, and the texture is not formed. A (100) plane is formed, which increases the light reflectance.

In addition, when the silicate compound is added as an additive to the etching solution 3 containing only the alkali reagent and not containing the additive (A), the addition amount of the silicate compound and the SiO ₂ / Na ₂ O ratio Regardless, the texture 6 is hardly formed, and the light reflectance reduction effect cannot be sufficiently obtained.

The silicon equivalent concentration of the silicate compound added to the etching solution 3 containing the additive (A) is preferably 0.2 wt% to 0.6 wt%, and more preferably 0.4 wt% to 0 wt. .6 wt%. When the silicon equivalent concentration of the silicate compound is less than 0.2 wt%, the influence of the addition of the silicate compound is small, and the slope of the micropyramid formed on the silicon substrate is improved in the flatness of the (111) plane. The effect cannot be obtained sufficiently. When the silicon equivalent concentration of the silicate compound is larger than 0.6 wt%, the concentration of the silicate compound is increased, so that SiO ₂ is deposited thickly on the silicon substrate as a gel substance and the reaction due to the formation of the texture 6 Is excessively inhibited, and the rate at which the (100) surface on which no texture is formed is formed on the silicon substrate surface increases.

Usually, in the wet etching process of a silicon substrate, SiO ₂ and Na ₂ O constituting the silicate compound are increased by the reaction of the above chemical formula (3) by dissolving silicon in an alkali hydroxide. For this reason, the silicon equivalent amount of the silicate compound in the etching solution increases with time. In particular, if the number of silicon substrates subjected to wet etching at a time increases, the rate of increase in the amount of silicon equivalent to the processing time increases. Therefore, in order to perform the etching process under more preferable conditions, after adding the silicate compound, in order to keep the SiO ₂ / Na ₂ O ratio constant, silicon is added while adding pure water according to the processing time. It is preferable to perform a wet etching process on the substrate.

  On the other hand, in the wet etching process of the silicon substrate, the concentration of alkali hydroxide decreases by the amount of reaction between alkali and silicon. In addition, the addition of pure water relatively decreases the alkali concentration and the concentration of the additive (A) that acts as an etching inhibitor. For this reason, it is necessary to add an alkali reagent and an additive (A) simultaneously with pure water. The above method has the greatest influence when the number of sheets processed simultaneously in one etching process is large.

In addition, in the case where the silicon substrate is etched again with the same condition liquid (same etching liquid 3) after the silicon substrate etching process is performed once, SiO ₂ is contained in the etching liquid 3 after the etching process. And a ratio of Na ₂ O is calculated to leave a certain amount of the etching solution 3. Then, the pure water and alkali hydroxide required for the second etching solution, and the additive (A) used as an etching inhibitor are added to the etching solution 3 remaining in a certain amount at a concentration necessary for the etching process. To do. As a result, the silicon substrate can be etched under the same conditions as the first time. By carrying out such a method, a cost reduction effect is obtained by reducing the amount of alkali hydroxide, additive (A), and silicate compound used in the production of new etching solutions for the second and subsequent times. It is done.

Next, the result of verifying the effect when the silicate compound is added to the etching solution 3 containing the additive (A) with different SiO ₂ / Na ₂ O ratios will be described. FIG. 2 shows an etching treatment of a silicon substrate using the etching solution 3 (Example 1) to which an additive (A) and a silicate compound are added, and changing the SiO ₂ / Na ₂ O ratio in the silicate compound. and when, and is a characteristic diagram showing the relationship between the light reflectance (%) at a wavelength of 700nm of SiO 2 / _Na 2 _O ratio and the silicon substrate 5 after etching process in a silicate compound. Here, for convenience, the etching solution 3 to which the additive (A) and the silicate compound are added is referred to as an example. The same applies to the following.

FIG. 3 shows the SiO ₂ / Na ₂ O ratio of the silicate compound and the slope of the micropyramid in the silicon substrate 5 after the etching process when the silicon substrate is etched using the etching solution 3 according to the example 1. : Is a characteristic diagram showing the relationship with Ra (nm) of the (111) plane. FIG. 4 shows the SiO ₂ / Na ₂ O ratio of the silicate compound and the etching rate (μm / min) of the silicon substrate 5 when the silicon substrate was etched using the etching solution 3 according to Example 1. It is a characteristic view which shows the relationship.

In the etching solution 3 according to Example 1, isopropyl alcohol (IPA) as an additive (A) is added to a 4.0 wt% NaOH aqueous solution at an addition amount of 10 g / L, and SiO ₂ / Na is used as a silicate compound. It is prepared by adding liquid sodium silicate having a different ₂ O ratio. The SiO ₂ / Na ₂ O ratio of sodium silicate is 0.2, 0.5, 0.8, 1.2, 1.4, 1.8, 2.1, 2.3, 2.5, 2 .8, 3.2, 3.6. The SiO ₂ / Na ₂ O ratio was adjusted with the silicon equivalent concentration of sodium silicate being 0.6 wt%. Then, the silicon substrate 1 was etched under the conditions of the etching solution 3 temperature of 75 ° C. and the etching time of 20 minutes.

  The light reflectance is actually measured by measuring the light reflectance of light having a wavelength of 300 to 1200 nm on a silicon substrate, and a wavelength of 700 nm is adopted as a typical wavelength. The lower the light reflectance, the more uniformly the texture 6 is formed on the surface of the silicon substrate 5, and the size of the texture 6 is relatively small. However, when the texture size with a base length of 1 μm or less becomes dominant on the silicon substrate 5, the light reflectance increases.

  The slope of the micropyramid on the silicon substrate 5 after the etching process: Ra of the (111) plane is evaluated by AFM. In the AFM evaluation, a part of a large number of micropyramids formed on the surface of the silicon substrate 5 was extracted, and the slope (111) plane was evaluated in a range of 500 nm × 500 nm.

  The etching rate of the silicon substrate 5 is a value obtained by dividing the etching amount by the etching processing time. The etching amount is calculated from the difference in mass by measuring the weight of the silicon substrate before and after the etching process.

  For comparison, the silicon substrate 1 was etched using an etching solution (Comparative Example 1) prepared in the same manner as in Example 1 except that no silicate compound (sodium silicate) was added. FIG. 2 shows the light reflectance (%) at a wavelength of 700 nm in the silicon substrate 5 after the etching treatment, FIG. 3 shows the slope (111) plane Ra (nm) of the micropyramid, and FIG. 3 shows the etching rate (μm / min). ) Are also shown in FIG.

2, 4, since the light reflectivity and the etching rate is reduced when a low SiO 2 / _Na 2 _O ratio of the silicate compound in Example 1 (sodium silicate), in particular Na ₂ O Is acting as an etching inhibitor in the texture formation reaction. On the other hand, in FIG. 3, in Example 1, when the SiO ₂ / Na ₂ O ratio of the silicate compound (sodium silicate) is high, the flatness of the (111) plane Ra is improved. It can be seen that SiO ₂ contributes to the improvement of the flatness of the (111) plane. However, since SiO ₂ is a gel-like substance with low fluidity, if the SiO ₂ / Na ₂ O ratio is too high, a portion where etching does not progress occurs due to thick deposition in the gaps between the textures. The (100) plane on which no is formed is partially formed, and the light reflectance in the silicon substrate 5 increases.

As can be seen from FIGS. 2 and 3, in order to simultaneously reduce the light reflectance and the (111) plane Ra, the SiO ₂ / Na ₂ O ratio of the silicate compound (sodium silicate) is 1.4 to 2.1 is preferable. When the SiO ₂ / Na ₂ O ratio is less than 1.4, the light reflectance decreases due to the decrease in the texture size of the surface of the silicon substrate 5, but the Ra of the (111) plane increases, and the silicate The effect of improving the flatness of Ra on the (111) plane due to the addition of the compound cannot be sufficiently obtained. When the SiO ₂ / Na ₂ O ratio is larger than 2.1, the Ra of the (111) plane is lowered, but the texture size of the surface of the silicon substrate 5 is increased and the texture is formed on the surface of the silicon substrate 5. The (100) surface that is not formed begins to be formed, the light reflectivity increases rapidly, and the low-reflectance texture formation effect due to the addition of the silicate compound cannot be sufficiently obtained.

Here, the case where sodium silicate is added to the etching solution 3 as the silicate compound is shown. However, even when a silicate compound such as potassium silicate and lithium silicate is added to the etching solution 3, It has been confirmed that the same effect as described above can be obtained in the same SiO ₂ / Na ₂ O ratio range. In addition, it is confirmed that the above-mentioned range of the SiO ₂ / Na ₂ O ratio does not change even when the etching conditions such as the temperature of the etching solution 3, the etching time, the alkali concentration of the etching solution 3, and the additive (A) concentration change. It is done.

  Although the case where NaOH is used as the alkali reagent added to the etching solution 3 is shown here, it has been confirmed that the same effect as described above can be obtained even when the above alkali reagent other than NaOH is used. Yes.

  Although the case where IPA is used as the additive (A) to be added to the etching solution 3 is shown here, the same effect as described above can be obtained when the above additive (A) other than IPA is used. It has been confirmed that Furthermore, even when two or more types of the above-mentioned additive (A) are simultaneously contained in the etching solution 3, it has been confirmed that the same effect as described above can be obtained.

Next, the result of verifying the effect when the silicate compound is added in different amounts to the etching solution 3 containing the additive (A) will be described. FIG. 5 shows the use of an etching solution 3 (Example 2) to which an additive (A) and a silicate compound having a defined SiO ₂ / Na ₂ O ratio are added, and the amount of the silicate compound added is determined in terms of silicon equivalent concentration. The relationship between the silicon equivalent concentration (wt%) of the silicate compound and the light reflectance (%) at a wavelength of 700 nm of the silicon substrate 5 after the etching treatment when the silicon substrate is etched by varying (wt%). FIG.

  6 shows the silicon equivalent concentration (wt%) of the silicate compound and the slope of the micropyramid in the silicon substrate 5 after the etching process when the silicon substrate is etched using the etching solution 3 according to Example 2. FIG. : Is a characteristic diagram showing the relationship with Ra (nm) of the (111) plane. FIG. 7 shows the silicon equivalent concentration (wt%) of the silicate compound and the etching rate (μm / min) of the silicon substrate 5 when the silicon substrate was etched using the etching solution 3 according to Example 2. It is a characteristic view which shows the relationship.

The etching solution 3 according to Example 2 was prepared by adding isopropyl alcohol (IPA) as an additive (A) to a 4.0 wt% NaOH aqueous solution at an addition amount of 10 g / L, and forming a silicon equivalent concentration as a silicate compound. It is prepared by adding different liquid sodium silicate. The silicon equivalent concentration of sodium silicate is 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 1.0, 1 12 conditions of .2, 1.5. The silicon equivalent concentration of sodium silicate was adjusted by setting the SiO ₂ / Na ₂ O ratio of sodium silicate to 2.1. Then, the silicon substrate 1 was etched under the conditions of the etching solution 3 temperature of 75 ° C. and the etching time of 20 minutes.

  For comparison, the silicon substrate 1 was etched using an etching solution (Comparative Example 2) prepared in the same manner as in Example 2 except that no silicate compound (sodium silicate) was added. FIG. 5 shows the light reflectance (%) at a wavelength of 700 nm in the silicon substrate 5 after the etching treatment, FIG. 6 shows the slope (111) plane Ra (nm) of the micropyramid, and the etching rate (μm / min). ) Are also shown in FIG.

  In FIG. 5, in Example 2, the silicon equivalent concentration of sodium silicate up to 0.6 wt% is almost the same as in the case where sodium silicate is not included (Comparative Example 2), and in the silicon substrate 5 after the etching process. Little effect on light reflectance. However, when the silicon equivalent concentration of sodium silicate becomes 0.7 wt% or more, the texture size starts to decrease and the (100) surface where the texture is not formed starts to be partially formed, and the light reflectance increases.

On the other hand, in FIG. 6, in Example 2, when the silicon equivalent concentration of sodium silicate is in the range from 0.1 wt% to 0.4 wt%, Ra on the (111) plane rapidly decreases, and 0.4 wt% to 0. In the range up to 7 wt%, Ra on the (111) plane gradually decreases. And in the range where the silicon conversion density | concentration of a sodium silicate is 0.7 wt% to 1.5 wt%, Ra of (111) plane shows the behavior which does not change a lot. In the process in which the silicon equivalent concentration of sodium silicate changes to 0.7 wt%, the influence of SiO ₂ constituting the sodium silicate begins to increase gradually, and Ra on the (111) plane decreases. At a silicon equivalent concentration of 0.7 wt% or more, the increase in the influence of SiO ₂ is reduced, and the contribution effect to the flatness of the (111) plane of SiO ₂ is converged.

  As can be seen from FIGS. 5 and 6, in order to simultaneously reduce the light reflectance and the (111) plane Ra, the silicon equivalent concentration of the silicate compound (sodium silicate) is 0.2 wt% to 0.6 wt%. %, And more preferably 0.4 wt% to 0.6 wt%. When the silicon equivalent concentration is less than 0.2 wt%, there is little influence on the (111) plane Ra, and the flatness improvement effect of the (111) plane Ra due to the addition of the silicate compound cannot be sufficiently obtained. . When the silicon equivalent concentration is larger than 0.6 wt%, the Ra of the (111) plane is lowered, but the texture size of the surface of the silicon substrate 5 is increased and the texture 6 is not formed on the surface of the silicon substrate 5. The (100) plane begins to be formed, the light reflectivity increases rapidly, and the low-reflectance texture formation effect due to the addition of the silicate compound cannot be sufficiently obtained.

Here, the case where sodium silicate is added to the etching solution 3 as the silicate compound is shown. However, even when a silicate compound such as potassium silicate and lithium silicate is added to the etching solution 3, It has been confirmed that the same effect as described above can be obtained within the same silicon equivalent concentration range. In addition, the range of the SiO ₂ / Na ₂ O ratio according to this embodiment described above varies depending on etching conditions such as the temperature of the etching solution 3, the etching time, the alkali concentration of the etching solution 3, and the additive (A) concentration. Has not been changed.

  Although the case where NaOH is used as the alkali reagent added to the etching solution 3 is shown here, it has been confirmed that the same effect as described above can be obtained even when the above alkali reagent other than NaOH is used. Yes.

  Although the case where IPA is used as the additive (A) to be added to the etching solution 3 is shown here, the same effect as described above can be obtained when the above additive (A) other than IPA is used. It has been confirmed that Furthermore, even when two or more types of the above-mentioned additive (A) are simultaneously contained in the etching solution 3, it has been confirmed that the same effect as described above can be obtained.

  As described above, in Embodiment 1, the molar ratio of silicon oxide to alkali oxide is 1.4 to 2.1 in the etching solution 3 containing the additive (A) that acts as an etching inhibitor. A small amount of a silicate compound is contained. Thereby, the low reflectivity texture 6 can be formed on the surface of the silicon substrate 5 due to the etching inhibition effect of the additive (A) and the silicate compound.

  In the first embodiment, the silicon contained in the silicate compound acts as a low-fluidity gel-like etching inhibitor, and the slope of the micropyramid formed on the surface of the silicon substrate 5: (111) plane Adhere to. Since this silicon oxide is in the form of a gel with low fluidity, it takes longer time to adhere to the micropyramid slope: (111) plane, and the flatness of the micropyramid slope: (111) plane in texture formation is reduced. The effect to improve is acquired.

  As described above, in the first embodiment, the molar ratio of silicon oxide to alkali oxide is 1.4 to 2 in the etching solution 3 containing the additive (A) that acts as an etching inhibitor. By using the etching solution 3 containing 1 silicate compound in the range of 0.2 wt% to 0.6 wt%, more preferably in the range of 0.4 wt% to 0.6 wt% in terms of silicon, The silicon oxide contained in the acid salt compound acts as a low-fluidity gel-like etching inhibitor, and has the effect of improving the flatness of the slope (111) plane of the micropyramid in texture formation.

  Increasing the flatness of the micropyramid slope (111) plane on the surface of the silicon substrate 5 affects the growth process of the film deposited on the (111) plane, that is, the film quality is poor. Thus, the epitaxial growth can be suppressed and the film quality can be improved. Improving the quality of the film formed on the silicon substrate 5 contributes to the open circuit voltage and the fill factor of the solar cell. And by improving the quality of the film | membrane formed into a film on the silicon substrate 5, the electric power generation characteristic of the solar cell produced using this silicon substrate 5 can be improved, and the solar cell excellent in photoelectric conversion efficiency Can be obtained. In particular, in the double-sided heterojunction solar cell in which the film formed on the silicon substrate 5 needs to be thin, the above-described effects can be obtained more remarkably.

Embodiment 2. FIG.
In the second embodiment, a heterojunction solar power generation device (solar cell) is manufactured using a silicon substrate 5 having a texture formed on the substrate surface by wet etching using the etching solution 3 used in the first embodiment. did. Below, the influence which the quality of the texture 6 formed in the surface of the silicon substrate 5 has on the electric power generation characteristic of a solar power generation device is demonstrated.

In addition, the silicon substrate 5 used in the photovoltaic power generation apparatus has a plurality of different conditions in which the SiO ₂ / Na ₂ O ratio of the silicate soda added to the etching solution 3 and the silicon equivalent amount of sodium silicate are adjusted. The silicon substrate 5 of Examples 3 to 11 was produced by wet etching using the etching solution 3 (Examples 3 to 11). In the wet etching process, the n-type silicon substrate 1 having a substrate size of 15 cm × 15 cm and a thickness of 200 μm was processed. And the solar power generation device was each produced using the silicon substrate 5 of Example 3-Example 11 from which these conditions differ.

In the etching solution 3 of Examples 3 to 8, IPA was added as an additive to a 4.0 wt% NaOH aqueous solution at an addition amount of 10 g / L, and the SiO ₂ / Na ₂ O ratio was 0.2 (implementation). Examples 3), 0.5 (Example 4), 1.4 (Example 5), 2.1 (Example 6), 2.5 (Example 7), 3.2 (Example 8) Sodium salt is added so that the Si equivalent concentration is 0.6 wt%. And the silicon substrate 1 of Example 3-Example 8 which etched the silicon substrate 1 on the conditions of the temperature of 75 degreeC of the etching liquid 3, and the etching time for 20 minutes, and formed the texture on the surface was produced.

In the etching solutions 3 of Examples 9 to 11, IPA was added as an additive to a 4.0 wt% NaOH aqueous solution at an addition amount of 10 g / L, and the SiO ₂ / Na ₂ O ratio was 2.1. Sodium carbonate is added so that the Si equivalent concentration is 0.1 wt% (Example 9), 0.8 wt% (Example 10), and 1.2 wt% (Example 11). And the silicon substrate 1 of Example 9-11 which formed the texture on the surface by etching the silicon substrate 1 on the conditions of the temperature of 75 degreeC of the etching liquid 3, and the etching time for 20 minutes was produced.

Table 1 shows the SiO ₂ / Na ₂ O ratio of silicon silicate added to the etching solution 3 used for etching the silicon substrate 5 of each photovoltaic power generation device and the silicon equivalent amount.

  Next, the solar power generation device was produced using the silicon substrate 5 which completed the wet etching process process as mentioned above. 8A and 8B are diagrams illustrating a solar power generation device manufactured using the silicon substrate 5 having a texture formed on the surface by wet etching of the silicon substrate according to the first embodiment. -1 is a cross-sectional view of the main part of the solar power generation device, and FIG. 8-2 is a top view of the solar power generation device. The photovoltaic power generation apparatus shown in FIGS. 8A and 8B includes a laminated film of a silicon-based thin film 11 and a p-type silicon-based thin film 12 and a transparent conductive film 13 on the light-receiving surface side surface of the silicon substrate 5. Are formed in this order, and the laminated film of the silicon-based thin film 15 and the n-type silicon-based thin film 16 and the transparent conductive film 17 are formed in this order on the back surface. Further, on the light receiving surface side surface (front surface) of the silicon substrate 5, the comb-shaped light receiving surface side electrode 14 formed on the transparent conductive film 13 and the surface (back surface) opposite to the light receiving surface of the silicon substrate 5. And a comb-shaped back side electrode 18 formed on the transparent conductive film 17.

  The light receiving surface side electrode 14 includes a grid electrode 14a and a bus electrode 14b. FIG. 8A is a cross-sectional view in a cross section perpendicular to the longitudinal direction of the grid electrode 14a. Further, although the plan view is omitted, the back surface side electrode 18 includes the grid electrode 18 a and the bus electrode similarly to the light receiving surface side electrode 14. In FIG. 8A, only the grid electrode is shown for both the light receiving surface side electrode 14 and the back surface side electrode 18. The silicon substrate 5 is a 15 cm square solar power generation device using a silicon substrate on which the texture 6 is formed by the wet etching process using the etching solution 3 described above.

  Below, the process for manufacturing the solar power generation device shown to FIGS. 8-1 and FIGS. 8-2 using the silicon substrate 5 mentioned above is demonstrated. In addition, since the process demonstrated here is the same as the manufacturing process of the solar power generation device using a general single crystal silicon substrate, it does not show in particular in figure.

  In order to clean the surface of the silicon substrate 5 on which the wet etching process according to the first embodiment is completed, the following first and second steps are performed. In the first step, after the silicon substrate 5 is immersed in a cleaning solution containing concentrated sulfuric acid and hydrogen peroxide solution to form an oxide film on the surface of the silicon substrate 5, the silicon substrate 5 is immersed in a hydrofluoric acid solution. The oxide film on the surface of the silicon substrate 5 is removed in the hydrofluoric acid solution. In the second step, after the silicon substrate 5 is immersed in a cleaning solution containing hydrochloric acid and hydrogen peroxide water to form an oxide film on the surface of the silicon substrate 5, the silicon substrate 5 is immersed in a hydrofluoric acid solution. The oxide film on the surface of the silicon substrate 5 is removed in a hydrofluoric acid solution.

  The first step and the second step are repeated until organic contamination, metal contamination, and contamination by particles on the surface of the silicon substrate 5 are sufficiently reduced. Further, the cleaning of the surface of the silicon substrate 5 is not limited to the above process, and cleaning with functional water such as cleaning with ozone water or cleaning with carbonated water may be used.

After cleaning the surface of the silicon substrate 5, the silicon-based thin film 11 is formed on the surface of the silicon substrate 5, and the silicon-based thin film 15 is formed on the back surface of the silicon substrate 5. As the silicon-based thin film, an intrinsic amorphous silicon thin film (a-Si: H (i)) formed of silicon and hydrogen is formed by plasma CVD, for example. Silane (SiH ₄ ) gas and hydrogen (H ₂ ) gas are used to form this intrinsic amorphous silicon thin film, but gases such as CH ₄ , CO ₂ , NH ₃ , GeH ₄ are mixed. By doing so, you may form into a film, changing the band gap by alloying. This intrinsic amorphous silicon thin film may be a single layer, or silicon thin films having different physical properties such as conductivity, band gap and crystallization rate may be laminated. It is also possible to employ a configuration in which the silicon-based thin film 11 and the silicon-based thin film 15 are not provided.

Next, the p-type silicon thin film 12 and the n-type silicon thin film 16 are formed by, for example, plasma CVD using diborane (B ₂ H ₆ ) or phosphine (PH ₃ ) as a doping gas. The p-type silicon-based thin film 12 is, for example, a p-type amorphous silicon-based thin film (a-Si (p)), and the n-type silicon-based thin film 16 is, for example, an n-type amorphous silicon-based thin film (a-Si). (N)) is formed.

Next, a transparent conductive film 13 is formed on the p-type silicon-based thin film 12, and a transparent conductive film 17 is formed on the n-type silicon-based thin film 16. The transparent conductive film 13 and the transparent conductive film 17 are formed of a conductive oxide containing indium oxide as a main component, for example. As a method for forming the transparent conductive film 13 and the transparent conductive film 17, sputtering film formation, ion plating vapor deposition, electron beam vapor deposition, or the like is mainly used. The process gas used at the time of film formation includes argon (Ar) gas as a main component, oxygen (O ₂ ), hydrogen (H ₂ ), water vapor, nitrogen (N ₂ ), and the like as appropriate.

  Next, on the transparent conductive film 13 on the light receiving surface side of the silicon substrate 5, the electrode paste mixed with silver is printed in a comb shape by screen printing to form the light receiving surface side electrode 14 (before firing). Thereafter, an electrode paste mixed with silver is printed in a comb shape by screen printing on the transparent conductive film 17 on the back surface of the silicon substrate 5 to form a back surface side electrode 18 (before firing). After printing the electrode paste, the electrode paste is solidified by heat treatment (firing). As described above, as the solar power generation device according to the second embodiment, the heterojunction solar power generation device shown in FIGS.

  For comparison, the silicon substrate 1 was etched using an etching solution (Comparative Example 1) prepared in the same manner as in Examples 3 to 11 except that no sodium silicate was added. Thus, a silicon substrate 5 of Comparative Example 3 having a texture formed on the surface was produced. And the solar power generation device was produced using the silicon substrate 5 of the comparative example 3. FIG.

Next, the produced photovoltaic power generation devices of Examples 3 to 11 and Comparative Example 3 were actually operated, and the power generation characteristics were measured and evaluated. As a result, photoelectric conversion efficiency η (%), open circuit voltage Voc (V), short circuit current Isc (mA / cm ² ), fill factor F.V. F. Is shown in Table 1.

As can be seen from Table 1, when Examples 3 to 8 are compared with each other, with respect to photoelectric conversion efficiency η, the preferable SiO ₂ / Na ₂ O ratio of sodium silicate is 1.4 to 2.1, more preferably. Is 2.1. When the SiO ₂ / Na ₂ O ratio of sodium silicate is 0.5 or less, the light reflectivity at the silicon substrate 5 is reduced, so that the short-circuit current Isc is about 0.1 mA / cm as compared with Comparative Example 1. ² but the flatness of the (111) plane of the micropyramid of texture 6 is hardly improved or deteriorated, resulting in the open circuit voltage Voc and the fill factor F.V. F. However, it does not improve or deteriorates as compared with Comparative Example 1.

On the other hand, when the SiO ₂ / Na ₂ O ratio of sodium silicate is 2.5 or more, the light reflectivity at the silicon substrate 5 is rapidly increased, so that the short circuit current Isc is decreased. Therefore, when Example 3 to Example 8 are evaluated comprehensively, the SiO ₂ / Na ₂ O ratio of preferable sodium silicate is 1.4 to 2.1.

Further, as can be seen from Table 1, regarding the silicon equivalent concentration of sodium silicate, a preferable range in the photoelectric conversion efficiency η is 0.6 wt%. When the silicon equivalent concentration of sodium silicate is 0.6 wt% and the SiO ₂ / Na ₂ O ratio is 1.4 to 2.1, compared to the case where no sodium silicate is added (Comparative Example 3), Since the light reflectivity at the silicon substrate 5 does not change greatly, the short circuit current hardly changes, and the Ra of the (111) plane of the micropyramid of the texture 6 is improved, so that the open circuit voltage and the fill factor tend to be improved.

  When the silicon equivalent concentration of sodium silicate is 0.8 wt%, the Ra of the (111) plane of the micropyramid of the texture 6 is improved, so that the open circuit voltage and the fill factor are improved. However, in this case, since the (100) surface where the texture 6 is not formed in part is formed, the light reflectivity at the silicon substrate 5 is increased, and the short circuit current is decreased.

  When the silicon equivalent concentration of sodium silicate is 1.2 wt%, the Ra of the (111) plane of the micropyramid of the texture 6 is improved, so that the open circuit voltage and the fill factor are improved. However, in this case, since the (100) surface where the texture 6 is not formed is formed more than when the silicon equivalent concentration of sodium silicate is 8 wt%, the light reflectance at the silicon substrate 5 is higher. Thus, the short circuit current is further reduced.

  From Table 1, it can be seen that the flatness of the (111) plane of the texture 6 micropyramid has a great influence on the open-circuit voltage and the fill factor in the power generation characteristics when the solar power generation device is manufactured. In particular, the film quality of the silicon film formed on the (111) plane is affected by the flatness of the (111) plane. If the (111) plane is good, the epitaxial growth with poor quality is suppressed. Thus, the quality of the silicon film is improved. Due to the good quality of the silicon film, defects at the interface between the silicon film and the silicon substrate can be reduced, leading to an improvement in open circuit voltage and fill factor. For this reason, the photoelectric conversion efficiency is the highest in Example 6 in which the flatness of the (111) plane is good and the (100) plane where no texture is formed is hardly seen.

  Usually, in the heterojunction solar power generation device as described above, the silicon-based thin film 11 on the light-receiving surface side is formed with a film thickness of 10 nm or less so that sunlight reaches the silicon substrate serving as the power generation layer without light loss. Yes. When the silicon-based thin film 11 is formed with a thickness of 10 nm or less, the influence of the film quality of the silicon film on the open-circuit voltage and the fill factor is significant, and the flatness of the (111) plane of the micropyramid slope is enhanced. Is effective in improving the photoelectric conversion efficiency.

Here, the case where sodium silicate is added to the etching solution 3 as the silicate compound is shown. However, even when a silicate compound such as potassium silicate and lithium silicate is added to the etching solution 3, It has been confirmed that the same effect of improving the power generation characteristics of the solar power generation device as above can be obtained in the same silicon equivalent concentration range. In addition, the preferable ranges of the above-described SiO ₂ / Na ₂ O ratio and silicon equivalent concentration are such that the etching conditions such as the temperature of the etching solution 3, the etching time, the alkali concentration of the etching solution 3, and the additive (A) concentration change. It has been confirmed that it will not change.

  Although the case where NaOH is used as the alkali reagent added to the etching solution 3 is shown here, the power generation characteristics of the photovoltaic power generation apparatus similar to the above are improved also when the above alkali reagent other than NaOH is used. It has been confirmed that an effect can be obtained.

  Although the case where IPA is used as the additive (A) to be added to the etching solution 3 is shown here, the same photovoltaic power generation as described above can be performed when the above additive (A) other than IPA is used. It has been confirmed that an effect of improving the power generation characteristics of the device can be obtained. Furthermore, it has been confirmed that even when two or more of the above-mentioned additives (A) are contained in the etching solution 3 at the same time, the same effect of improving the power generation characteristics of the solar power generation device as described above can be obtained.

  Moreover, although the solar power generation device concerning this Embodiment formed the pn junction by depositing a silicon thin film on the silicon substrate in which the texture structure was formed, the pn junction was formed on the silicon substrate in which the texture structure was formed by a thermal diffusion method. May be formed. For example, an emitter type solar cell in which an impurity having a conductivity opposite to that of the crystalline silicon substrate is doped on the crystalline silicon substrate by a thermal diffusion method may be used. Alternatively, the silicon substrate may be p-type and the layer to which impurities are added may be n-type.

  As described above, the method for etching a silicon substrate according to the present invention is useful for forming a texture having a low reflectivity and a high flatness on the texture slope.

  DESCRIPTION OF SYMBOLS 1 Silicon substrate, 2 Etching tank, 3 Etching liquid, 4 Hydrogen gas, 5 Silicon substrate, 6 Texture, 11 Silicon type thin film, 12 p-type silicon type thin film, 13 Transparent conductive film, 14 Light-receiving surface side electrode, 14a Grid electrode , 14b bus electrode, 15 silicon thin film, 16 n-type silicon thin film, 17 transparent conductive film, 18 back side electrode.

Claims (8)

A method for etching a silicon substrate, comprising supplying an alkaline aqueous solution to the surface of the silicon substrate and anisotropically etching the surface of the silicon substrate by wet etching,
The alkaline aqueous solution is
An alcohol-based additive represented by the following chemical formula (1) or a surfactant having an HLB value of 10 or more;
A silicate compound represented by the following chemical formula (2);
Containing
The molar ratio (m / l) of silicon oxide (SiO ₂ ) and alkali oxide (X ₂ O) contained in the silicate compound is 1.4 to 2.1,
The silicon equivalent concentration of the silicate compound is 0.2 wt% to 0.6 wt%,
A method for etching a silicon substrate.
X- (OH) n (1)
(X is a saturated or unsaturated hydrocarbon group having 4 to 7 carbon atoms Cn, n is an integer of 1 or more, n <Cn)
lX ₂ O · mSiO ₂ · nH ₂ O (2)
(X is an alkali metal element, l is a substance amount (mol) of alkali oxide (X ₂ O), m is a substance amount (mol) of silicon oxide (SiO ₂ ), and n is water contained in the silicate compound. Substance amount of Japanese water (H ₂ O) (mol) The silicon equivalent concentration of the silicate compound is 0.4 wt% to 0.6 wt%,
The method for etching a silicon substrate according to claim 1. The silicon substrate is a single crystal silicon substrate whose main surface is a surface along the (100) plane;
The method for etching a silicon substrate according to claim 1, wherein: An etchant for anisotropically etching the surface of the silicon substrate by supplying to the surface of the silicon substrate by wet etching,
Mainly alkaline aqueous solution,
An alcohol-based additive represented by the following chemical formula (3) or a surfactant having an HLB value of 10 or more;
A silicate compound represented by the following chemical formula (4);
Containing
The molar ratio (m / l) of silicon oxide (SiO ₂ ) and alkali oxide (X ₂ O) contained in the silicate compound is 1.4 to 2.1,
The silicon equivalent concentration of the silicate compound is 0.2 wt% to 0.6 wt%,
Etching solution for silicon substrate characterized by
X- (OH) n (3)
(X is a saturated or unsaturated hydrocarbon group having 4 to 7 carbon atoms, n is an integer of 1 or more, n <Cn)
lX ₂ O · mSiO ₂ · nH ₂ O (4)
(X is an alkali metal element, l is a substance amount (mol) of alkali oxide (X ₂ O), m is a substance amount (mol) of silicon oxide (SiO ₂ ), and n is water contained in the silicate compound. Substance amount of Japanese water (H ₂ O) (mol) The silicon equivalent concentration of the silicate compound is 0.4 wt% to 0.6 wt%,
The silicon substrate etching solution according to claim 4. Forming a semiconductor layer having a conductivity type opposite to that of the silicon substrate on the conductive silicon substrate etched by the etching method according to claim 1;
A method for manufacturing a solar cell. The semiconductor layer includes an amorphous silicon thin film layer having a conductivity type opposite to that of the silicon substrate;
The method for producing a solar cell according to claim 6. The semiconductor layer is a layer in which an intrinsic amorphous silicon thin film layer and an amorphous silicon thin film layer having a conductivity type opposite to the silicon substrate are laminated,
The method for producing a solar cell according to claim 6 or 7, wherein:

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