JP5745598B2 - Surface treatment method for solar cell substrate - Google Patents

Description

  The present invention relates to a surface treatment method for a solar cell substrate. More specifically, the present invention relates to a method for treating the surface of a solar cell substrate, which can be efficiently structured regardless of the type and quality of the lattice structure of the substrate.

  Recently, due to the anticipated depletion of existing energy resources such as oil and coal, there is increasing interest in alternative energy alternatives. Among them, the solar cell is in the spotlight as a next-generation battery using a semiconductor element that directly converts solar energy into electric energy. The solar cell is roughly classified into a silicon solar cell, a compound semiconductor solar cell, and a stacked solar cell.

  On the other hand, silicon solar cells, which are the majority of solar cells currently mass-produced, use silicon as a semiconductor substrate, and silicon uses an indirect interband transition semiconductor as an energy that exceeds the band gap of silicon. Only the light it has can generate electron-hole pairs, and the light absorptance is lower. Accordingly, silicon-based solar cells reflect 30% or more of the light incident on the inside of the solar cell on the surface of the silicon wafer that is the substrate, so that the efficiency of the solar cell is reduced.

  In order to reduce such optical loss, a surface treatment method is used in silicon solar cells. In the surface treatment method, in order to absorb as much light energy as possible inside the wafer substrate, the surface roughness is increased by forming irregularities on the surface of the silicon substrate of the silicon solar cell. When the first light reaches and strikes a tilted pyramid wall, some will be absorbed and some will be reflected back, allowing the returning light to continue to hit other surrounding pyramid walls As a result, the amount of light absorption is increased. In this way, the amount of light absorption is increased by the pyramid structure, and as a result, the cell efficiency can be improved. Therefore, if a silicon solar cell substrate is manufactured by the surface treatment method, the surface reflection of the solar cell can be reduced, the carrier collection effect can be improved, and the light confinement effect by the internal reflection of the solar cell can be realized.

  Surface treatment methods can be broadly divided into two types: wet surface treatment using an acidic or alkaline solution and dry surface treatment using a reactive ion gas. An alkaline solution or an acidic solution is used for the wet surface treatment. The alkaline solution is suitable for a single crystal wafer, and the acidic solution is suitable for a surface treatment of a polycrystalline wafer. The dry surface treatment uses a reactive ion gas and is suitable for a polycrystalline wafer.

  On the other hand, recently, a method has been developed in which a quasi-single crystal substrate that has a simple process, can be produced in a large amount at once, and has high price competitiveness, is used for manufacturing a solar cell.

  As for the pseudo single crystal silicon, for example, US Publication No. 201100193031, European Patent No. 0748884, etc. disclose a method of growing pseudo single crystal silicon by using a seed crystal.

  Since pseudo single crystal silicon includes both single crystal and polycrystalline regions depending on the growth method, it is difficult to uniformly treat all the regions. Therefore, in order to develop pseudo single crystal silicon for solar cells, research on surface treatment techniques to maximize the light absorption of the substrate is required.

US Publication No. 201100193031 European Patent No. 0748888

  In order to solve the above problems, the present invention provides a method for effectively surface-treating a substrate for a solar cell, particularly a pseudo single crystal substrate, regardless of the type and orientation of crystals. Objective.

  To achieve the above object, the present invention includes performing a first wet etching on a solar cell substrate, performing a reactive ion etching, and a second wet etching. A surface treatment method for a solar cell substrate, comprising: performing the method.

  ADVANTAGE OF THE INVENTION According to this invention, a solar cell substrate can be effectively surface-treated, a reflectance can be made low and a light absorption rate can be increased, and a highly efficient solar cell can be manufactured.

  In addition, since a uniform surface treatment can be performed with only one technique regardless of the types of pseudo single crystal substrates that are divided into various types depending on crystallinity, the process can be simplified and the cost can be reduced.

It is the photograph which expanded and photographed the result of carrying out the wet surface treatment with respect to a single crystal substrate and a polycrystalline substrate using a scanning electron microscope at 5,000 times. 2 is a diagram illustrating substrates of various qualities. It is the photograph which expanded and image | photographed the result of having surface-treated with the basic solution and the acidic solution with respect to each pseudo single crystal substrate using a microscope. It is the photograph which enlarged and image | photographed the surface of the board | substrate after performing to the reactive ion etching process in Example 1 70,000 times using the scanning electron microscope. It is the photograph which expanded the surface of the board | substrate after performing to the reactive ion etching process and the 2nd wet etching process in Example 1 by 5,000 times using the scanning electron microscope. It is a graph which shows the reflectivity of the board | substrate before performing surface treatment, and the reflectivity of the board | substrate after performing the surface treatment process in Example 1 and Comparative Example 1. FIG.

The surface treatment method of the present invention comprises:
Performing a first wet etch on a solar cell substrate;
Performing reactive ion etching; and
Performing a second wet etch.

  In the present invention, terms such as first and second are used to describe various components, and the above terms are used only for the purpose of distinguishing one component from other components.

  Also, the terminology used herein is merely used to describe illustrative examples and is not intended to limit the invention. The singular form includes the plural form unless the context clearly dictates otherwise. In this specification, terms such as “comprising”, “comprising” or “having” are intended to indicate that there is a feature, number, step, component, or combination thereof implemented. Thus, it should be understood that the existence or addition of one or more other features or numbers, steps, components, or combinations thereof is not excluded in advance.

  Also, in the present invention, when it is mentioned that each layer or element is formed “on” or “on” each layer or element, each layer or element is directly formed on each layer or element. It means that other layers or elements can additionally be formed on the object, substrate between each layer.

  While the invention is amenable to various modifications and alternative forms, specific embodiments have been illustrated and described in detail below. However, this should not be construed as limiting the invention to the specific forms disclosed, but should be understood to include all modifications, equivalents or alternatives that fall within the spirit and scope of the invention.

  Hereinafter, a surface treatment method for a solar cell substrate of the present invention will be described in detail with reference to the drawings.

  Substrates required for semiconductors for solar cells have many differences in quality and price due to purity, precision of technology used for substrate preparation, and the like. The main requirements are high purity that minimizes the concentration of impurities, high quality that minimizes crystal defects, and low price that enables mass production. These are the most important factors that determine mass production of solar cells. It is an important part. Due to such a tendency, recently developed substrates include a quasi-single crystal substrate (MLM wafer) having both single crystal and polycrystal forms.

  The pseudo single crystal substrate combines the advantages of single crystal and polycrystal, and is produced using the polycrystalline ingot growth method. Usually, a single crystal ingot can be made by using a heat exchange method (HEM / Heat Exchange Method), which is one of polycrystalline ingot growth methods, and can be used to produce a high-efficiency solar cell. The polycrystalline ingot growth method has a characteristic that it is simple in process, can be produced in large quantities at once, and has a high price competitiveness, and has recently been developed for use as a substrate for solar cells.

  On the other hand, in order to enhance the light absorption of the solar cell, a process of texturing (also referred to as texturing, “surface structuring” or “surface treatment”) is performed. If a solar cell substrate is manufactured by texturing, the surface reflection of the solar cell can be reduced, the carrier collection effect can be improved, and the light confinement effect due to the internal reflection of the solar cell can be realized.

  A single crystal substrate refers to a single crystal substrate having a single crystal orientation that is consistent throughout, and in the case of the single crystal substrate, the surface of the substrate is textured with a basic solution such as NaOH or KOH. Anisotropic etching is effective.

  On the other hand, a polycrystalline substrate refers to a crystalline substrate in which a number of randomly oriented crystals are within the body of the substrate. In the case of the polycrystalline substrate, the direction of the crystal is not constant, so an acid solution was used rather than a base. Isotropic etching is more effective.

  FIG. 1 is a photograph obtained by enlarging a result of wet surface treatment on a single crystal substrate and a polycrystalline substrate at 5,000 times using a scanning electron microscope.

  In FIG. 1, the left photograph is a photograph obtained by wet surface treatment with a basic solution on a single crystal substrate, and the right photograph is a photograph obtained by wet surface treatment with an acidic solution on a polycrystalline substrate. Referring to FIG. 1, it can be seen that the single crystal substrate and the polycrystalline substrate have a uniform etching pattern within a single substrate, although their forms differ depending on the crystal characteristics.

  However, the pseudo single crystal substrate is divided into a 100% single crystal substrate, a substrate in which a single crystal and a polycrystalline region are mixed, and a 100% polycrystalline substrate depending on the growth method and conditions. Therefore, it is difficult to uniformly treat the entire surface.

  FIG. 2 is a diagram illustrating substrates of various grades.

  Referring to FIG. 2, a 100% single crystal substrate (Mono wafer) from the left side, a pseudo single crystal substrate (MLM wafer grade A, B) in which a single crystal and a polycrystalline region are mixed, and a 100% polycrystalline substrate. It can be seen that it exists in various qualities such as (Multiwafer).

  In general, according to the ratio of the monocrystalline region, the pseudo single crystal substrate is grade A (quality A) in which the proportion of the single crystal region is 90% or more, and grade B (70% or more). Quality B), which is 25% or more, can be classified into three grades as grade C (quality C).

  In general, a grade A (quality A) pseudo single crystal substrate is almost the same as a single crystal wafer and can be etched using a basic solution. However, a lower substrate, that is, a grade B (quality B) or C substrate contains a plurality of single crystal regions and polycrystalline regions, respectively, so that an optimum texture is realized when a basic or acidic solution is processed. There are difficulties.

  FIG. 3 is a photograph of the result of surface treatment of the pseudo single crystal substrate with a basic solution and an acidic solution, respectively, using a microscope. In FIG. 3, the left photograph is a photograph of the pseudo single crystal substrate surface-treated with a basic solution, and the right photograph is a photograph of the pseudo single crystal substrate surface treated with an acidic solution.

  When performing surface organization with a basic solution on a quasi-single crystal substrate, the single crystal region is etched in a fine pyramid structure with low reflectivity because the crystal direction is the same, but the polycrystalline region Shows a different etching form from that of a single crystal due to various crystal directions. Confirm that the maple-shaped phenomenon like the leaves of autumn leaves occurs due to such non-uniform surface organization of single crystal and polycrystal as shown in the left picture of FIG. Can do. Such a phenomenon is not suitable because it causes a drop in commercial value.

  On the other hand, when surface organization is performed with an acidic solution, a random pyramid is formed at the same etching rate regardless of the crystal direction. -Shape) does not occur. However, a substrate whose surface is structured with an acidic solution exhibits a higher reflectivity than a substrate whose surface is structured with a basic solution. Therefore, surface organization with an acidic solution is not suitable.

  According to the surface treatment method of the present invention, surface organization is possible regardless of a lattice structure such as a single crystal region or a polycrystalline region on the substrate surface. Therefore, it is possible to realize a uniform surface organization and solve the problem of maple shape as a phenomenon in which the surface organization is different depending on the lattice structure, whereby the maple shape (maple shape) can be solved. ) Can prevent the product value from falling. In addition, the process procedure is simple and the process time and cost can be saved.

  In the surface treatment method of the present invention, the solar cell substrate to be subjected to the surface treatment may be a pseudo single crystal substrate. As described above, the pseudo single crystal substrate may be a single crystal substrate, a substrate in which a single crystal and a polycrystalline region are mixed, or a polycrystalline substrate, and may be a substrate of various grades. possible. According to the surface treatment method of the present invention, the process can be applied regardless of the type and quality of the substrate. Therefore, even if a low-quality substrate with a relatively low price is used, a surface treatment result equivalent to that when a high-quality substrate is used can be obtained, and production costs are significantly reduced. be able to.

  According to an embodiment of the present invention, the solar cell substrate may be a substrate made of, for example, p-type conductivity type silicon. When the solar cell substrate has a p-type conductivity type, it contains impurities of trivalent elements such as boron (B), gallium, indium and the like.

  Alternatively, the solar cell substrate may be a substrate made of n-type conductivity type silicon. When the solar cell substrate has an n-type conductivity type, a pentavalent element such as phosphorus (P), arsenic (As), antimony (Sb), or the like can be contained.

  First, a first wet etching is performed on the solar cell substrate.

  The first wet etching process can simultaneously achieve sawing damage removal and first texturing on the substrate surface. The cutting damage removal is a step of removing defects caused by cutting the substrate and removing an oxide film formed on the surface.

  According to an embodiment of the present invention, the first wet etching is performed using an acid solution.

More specifically, the acid solution used in the first wet etching process includes HF, HNO ₃ , and H ₂ O, and HF: HNO ₃ : H ₂ O is about 1: 2 to 4: 1 to 3, preferably It can be included in a volume ratio of about 1: 3: 2.

  The first wet etching can be performed by immersing the substrate in the solution at a temperature of about 5 to 10 ° C. for about 1 to about 5 minutes, preferably about 1 to 2 minutes.

  According to an embodiment of the present invention, the first wet etching process as described above is performed, followed by a primary rinse with distilled water (DI water), in a basic solution such as KOH. A summing step, a second rinsing step with distilled water (DI water), a HF washing step, and a third rinsing step with distilled water (DI water) can be further performed.

  By performing the first wet etching as described above, damage due to the cutting of the substrate is removed, and the substrate surface is primarily etched to have a height of about 2 to 10 μm, preferably about 3 to 5 μm. Can be formed.

  Next, a reactive ion etching (RIE) process is performed on the substrate having undergone the first wet etching process.

The reactive ion etching process can be performed by turning an etching gas into plasma to activate it into a highly reactive state and causing it to collide with the surface of the substrate. As the etching gas, one or more selected from the group consisting of a fluorine-based gas such as F ₂ , SF ₆ , and CF ₄ , a chlorine-based gas such as Cl ₂ , and O ₂ may be used in combination. Can be used.

According to an embodiment of the present invention, the etching gas may include O ₂ in order to adjust an etching pattern in the reactive ion etching process. By containing O ₂ in the etching gas, an oxide film serving as a mask is formed on the side surface where ion bombardment is not performed, and the film is not formed on the lower portion where ion bombardment is performed, so that etching is performed only on the lower portion. The etching mode can be adjusted depending on the phenomenon.

According to an embodiment of the present invention, an etching process may be performed using Cl ₂ , SF ₆ , and O ₂ as the etching gas.

  More specifically, after the substrate is placed in the chamber, an etching gas is injected into the chamber. Next, a certain amount of electric power is applied to the two electrodes provided between the substrates, and plasma based on the etching gas is generated in the space between the two electrodes. The generated plasma has active species (radials) and ions (ions) that are highly reactive. The plasma particles thus generated are accelerated and collide with the surface of the substrate. Accordingly, a plurality of concavo-convex structures can be formed on the surface of the substrate by a combination of physical impact and chemical reaction.

  In the reactive ion etching process, an ion bombardment having a higher energy than that of a general plasma etching is applied to the substrate because a higher negative potential is formed on the electrode as compared with a grounded electrode.

  By performing the second texturing on the substrate surface by the reactive ion etching process as described above, a pyramid structure is formed on the substrate surface.

  The pyramid structure contributes to improving the light absorption efficiency of the solar cell by lowering the light reflectance of the substrate surface. The pyramid structure can be formed in various shapes and sizes by adjusting process conditions of the reactive ion etching process, that is, etching gas, pressure, temperature, electrode height, and power.

  According to an embodiment of the present invention, in the pyramid structure, a ratio of a height to a width (height / width) may be about 0.75 to about 1.1. In addition, it may have a height and width of several nanometers to several hundred nanometers within a range that maintains the ratio. For example, the pyramid may have a width of about 100 to 300 nm and a height of about 150 to 350 nm. According to the present invention, a uniform pyramid structure can be formed on the surface regardless of the crystal structure of the substrate by performing the first wet etching and the reactive ion etching process stepwise as described above. Thereby, the amount of light reflected from the surface is reduced, and low reflectivity can be realized.

  However, surface damage may occur due to plasma bombardment in the reactive ion etching process. If the surface damage is not removed, the current value decreases due to an increase in the surface recombination rate, and it is difficult to expect an increase in cell conversion efficiency. In other words, the reactive ion etching process can reduce the reflectivity and increase the light absorption, but the surface damage that occurs along with this can accelerate the disappearance of the electron-hole pairs, so that the increased light absorption effect can be obtained. become unable. This causes low Voc and FF.

  In addition, if the pyramid structure generated in the reactive ion etching process is excessively maintained, a high leakage current may be generated in an upper layer portion of the pyramid.

  Accordingly, in order to remove electrical surface damage while maintaining low reflectivity, a surface removal removal (DRE) process is performed after the reactive ion etching process.

The surface damage removing process is for removing the surface damage generated in the reactive ion etching process. However, if the surface damage removing process proceeds excessively, the surface is excessively etched and formed by the reactive ion etching process. The removed pyramid is removed and a low reflectivity effect is not obtained. Therefore, the surface damage removal process needs to be performed under the optimum conditions that can maintain the pyramid structure formed by the reactive ion etching process and at the same time remove the oxide film generated by O ₂ and effectively remove the surface damage. There is.

According to the surface treatment method of the present invention, the second wet etching is performed in the surface damage removing step. More specifically, the second wet etching process is performed using an acid solution containing HF, HNO ₃ , and H ₂ O, and HF: HNO ₃ : H ₂ O is about 1:13 to 17:15 to 19 , Preferably with a solution containing about 1:15:17 volume ratio. By performing the surface damage removing process under the above conditions, the pyramid structure formed by the reactive ion etching process is maintained without excessive etching of the surface, and the ratio of the height to the width of the pyramid structure is maintained. However, the upper layer portion of the pyramid structure can be formed smoothly to improve the reflectivity and prevent leakage current. In addition, it is possible to effectively deposit the antireflection film in the subsequent antireflection film forming process, and to bring about an effect that the reflectivity is additionally improved by a synergistic effect therewith. According to an embodiment of the present invention, by performing the surface damage removing process, the reflectivity is further improved by about 1 to 3% before performing the surface damage removing process.

  At this time, the process temperature of the second wet etching process can be normal temperature, for example, about 20 to 30 ° C., and is performed by being immersed for about 10 seconds to about 60 seconds, preferably about 20 seconds to about 50 seconds. it can.

  According to an embodiment of the present invention, an etching amount of the second wet etching process is about 0.009 to 0.035% with respect to a weight of the substrate before the second wet etching process is performed. The second wet etching process can be performed.

  The substrate subjected to the surface treatment using the surface treatment method of the present invention as described above can exhibit a surface reflectance of about 9 to 12%.

  A high-efficiency solar cell can be obtained by forming an emitter layer, an antireflection film, a front electrode, and a rear electrode on a substrate that has undergone a surface treatment process as described above by a general method for manufacturing a solar cell. can do.

  More specifically, an emitter layer is formed on the surface-treated solar cell substrate. A PN junction can be formed in the emitter layer by doping with an impurity opposite to the substrate. According to an embodiment of the present invention, the emitter layer can be applied to a high efficiency solar cell by forming a shallow emitter layer with a thickness of about 100 to 500 nm.

  In addition, according to an embodiment of the present invention, the emitter layer may have a high sheet resistance with high photoelectric conversion efficiency, for example, a sheet resistance of about 85 to 100 Ω / sq.

  Next, an antireflection film is formed on the emitter layer.

The antireflection film is formed by vacuum deposition, chemical vapor deposition, spin coating, screen printing, or spray coating, but is not limited thereto. The antireflection film is, for example, any one selected from the group consisting of a silicon nitride film, a silicon nitride film containing hydrogen, a silicon oxide film, a silicon oxynitride film, MgF ₂ , ZnS, TiO _2, and CeO ₂ . Although it may have a multilayer structure in which a single film or two or more films are combined, it is not limited thereto. According to an embodiment of the present invention, the antireflection film is a silicon nitride film having a refractive index of about 2.0 to about 2.2, and can be formed to a thickness of about 75 to 85 nm.

  Next, a solar cell is manufactured by screen-printing silver (Ag) paste and then heat-treating to form a front electrode, printing aluminum paste on the back surface of the substrate and then heat-treating to form a back-side electrode. Can do. When the aluminum (Al) paste is heat-treated, aluminum diffuses through the back surface of the substrate, thereby forming a back surface field layer on the interface between the back electrode and the substrate. When the rear surface electric field layer is formed, the carrier can be prevented from moving to the back surface of the substrate and recombined. Can be improved.

  Hereinafter, the present invention will be described in more detail with reference to examples according to the present invention. However, such embodiments are merely presented as examples of the invention, and the scope of rights of the invention is not determined thereby.

<Example>
Example 1
A p-type pseudo single crystal substrate (MLM wafer, Grade B) doped with an impurity of a Group 3 element was prepared. The first wet etching is performed by immersing the substrate in a solution containing HF: HNO ₃ : H ₂ O in a 1: 3: 2 volume ratio at a temperature of 7 ° C. for 1 minute and 30 seconds, thereby removing the cutting damage and the first. One texturing was performed simultaneously. Etching was performed at a depth of 3 to 5 μm from the surface by the first texturing.

Next, a reactive ion etching process was performed using Cl ₂ / SF ₆ / O ₂ as an etching gas. The pyramid structure generated by the reactive ion etching process has a distribution in which the ratio H / W of height (height) to width (width) is 0.75 to 1.1.

In the second wet etching, it is immersed in a solution containing HF: HNO ₃ : H ₂ O at a volume ratio of 1:15:17 at a temperature of 25 ° C. for 30 seconds, and the weight is 0.025% compared with that before performing the etching. The surface treatment was completed by etching to reduce.

Phosphorus (P) was doped by a diffusion process using POCL ₃ to form an emitter layer having an 85Ω / sq resistance. A silicon nitride film having a refractive index in the range of 2.0 to 2.2 was formed in two layers on the emitter layer using PECVD equipment, and a total thickness of 85 nm was formed.

  Screen printing is performed on the rear surface using an aluminum (Al) paste, and after a drying process at a temperature of less than 200 ° C., a front electrode is formed with a width of 70 μm using a silver (Ag) paste, After passing through a drying process at a temperature lower than 200 ° C., the front electrode and the rear electrode were formed by sintering in a belt firing furnace at 940 ° C.

Comparative Example 1
A solar cell was manufactured in the same manner as in Example 1 except that the reactive ion etching step was not performed.

<Experimental example>
Surface Treatment Result Evaluation FIG. 4 is a photograph of the surface of the substrate after performing the reactive ion etching process in Example 1 magnified 70,000 times using a scanning electron microscope.

  FIG. 5 is a photograph of the surface of the substrate after performing the reactive ion etching process and the second wet etching process in Example 1 with a scanning electron microscope magnified 5,000 times.

  FIG. 6 is a graph showing the reflectivity of the substrate before performing the surface treatment and the substrate after performing the surface treatment process in Example 1 and Comparative Example 1.

Evaluation of electrical performance of solar cells The electrical performance of the solar cells produced in Example 1 and Comparative Example 1 was measured according to ASTM G-173-03 under the conditions of AM1.5 solar conditions, Hanwha Solarone limited in China. HSOL), solar tester, H. a. l. Measurement was performed using m cetis PV-products, and the results are shown in Table 1 below.

  In Table 1 below, Isc means the maximum current transmitted through the solar cell corresponding to the short circuit condition when the impedance is low, and the current that flows when the voltage across the solar cell is 0, and the current that flows per unit area. Vsc is the voltage that appears at both ends of the solar cell when the current is 0, which means the maximum voltage obtained from the solar cell, and the parallel resistance (Rsh) is a resistance that connects a certain circuit in parallel, and is low The parallel resistor causes a leakage current and reduces the current voltage. The series resistance (Rs) is a resistance acting in series between the upper and lower electrodes of the solar cell, and a current flow through the solar cell emitter and base, that is, a vertical resistance component is defined as Rs. The parameter that is greatly affected is FF (Fill Factor). FF [%] is the most important measure in solar cell quality, Fill Factor is calculated by comparing maximum power with theoretical power that outputs with open circuit voltage and short circuit current, Eta [%] means efficiency and solar It is the most important factor indicating the performance of the battery, and is defined by the ratio of the output energy to the energy incident from the sun.

  From the results in Table 1, it can be seen that in the case of a solar cell manufactured using a substrate that has been subjected to the surface treatment method of the present invention, the electrical performance is improved compared to a substrate that has been subjected to surface treatment only by wet etching.

Claims (6)

Performing a first wet etching using a solution containing HF: HNO ₃ : H ₂ O in a volume ratio of 1: 2 to 4: 1 to ₃ on a solar cell substrate which is a pseudo single crystal substrate;
Performing reactive ion etching; and
Performing a second wet etching using a solution containing HF: HNO ₃ : H ₂ O in a volume ratio of 1:13 to 17:15 to 19 ; and a surface treatment method for a substrate for a solar cell.   The method of claim 1, wherein the first wet etching simultaneously performs saw damage removal and first texturing.   The surface treatment method for a solar cell substrate according to claim 1, wherein a pyramid structure is formed on a surface of the substrate by performing second texturing by the reactive ion etching. The method of claim 1, wherein the reactive ion etching is performed using a gas containing Cl ₂ , SF ₆ , and O ₂ .   4. The surface treatment method for a solar cell substrate according to claim 3, wherein a ratio (height / width) of a height (height) to a width (width) of the pyramid structure is 0.75 to 1.1. 5. .   The method of claim 1, wherein the second wet etching process is performed at a temperature of 20 to 30 ° C. for 10 to 60 seconds.