阅读说明：本技术 一种采用电化学法预处理单晶硅片表面的制绒方法 (Texturing method for pretreating surface of monocrystalline silicon wafer by electrochemical method ) 是由 张宏 李凤 王世栋 于 2021-01-19 设计创作，主要内容包括：一种采用电化学法预处理单晶硅片表面的制绒方法,将去损伤层后的单晶硅片浸渍于电解液溶液中,以单晶硅片为阳极,石墨或不锈钢为阴极,在阳极硅片和阴极之间加直流电压,阳极硅片为电压正极,对电极的阴极为电压负极,对硅片表面进行电化学预处理,然后将单晶硅片放入制绒液中进行制绒,得到金字塔结构的绒面。本发明可以使硅片绒面金字塔生长时间缩短,金字塔大小分布更加均匀,从而得到均匀细密的优质绒面。本发明采用低压电解方法,工艺投资低,能耗小,符合现阶段光伏发电高效低成本化的要求。本发明采用低压电解方法,反应条件温和,常温下即可进行,对人体和环境无危害,结构简单,操作方便,易于控制。(A texturing method for pretreating the surface of a monocrystalline silicon wafer by an electrochemical method comprises the steps of immersing the monocrystalline silicon wafer with a damage layer removed in an electrolyte solution, performing electrochemical pretreatment on the surface of the silicon wafer by taking the monocrystalline silicon wafer as an anode and graphite or stainless steel as a cathode, applying direct-current voltage between the anode silicon wafer and the cathode, taking the anode silicon wafer as a voltage anode, taking the cathode of the electrode as a voltage cathode, and then putting the monocrystalline silicon wafer into texturing solution for texturing to obtain a textured surface with a pyramid structure. The invention can shorten the pyramid growth time of the silicon wafer texture surface, and the pyramid size distribution is more uniform, thereby obtaining uniform and fine high-quality texture surface. The invention adopts a low-voltage electrolysis method, has low process investment and low energy consumption, and meets the requirements of high efficiency and low cost of photovoltaic power generation at the present stage. The invention adopts a low-voltage electrolysis method, has mild reaction conditions, can be carried out at normal temperature, has no harm to human bodies and environment, and has simple structure, convenient operation and easy control.)

1. A texturing method for pretreating the surface of a monocrystalline silicon wafer by an electrochemical method is characterized in that the monocrystalline silicon wafer with a damaged layer removed is immersed in an electrolyte solution, the monocrystalline silicon wafer is taken as an anode, graphite or stainless steel is taken as a cathode, direct-current voltage is applied between the anode silicon wafer and the cathode, the anode silicon wafer is taken as a voltage anode, the cathode of an electrode is taken as a voltage cathode, the surface of the silicon wafer is electrochemically pretreated, and then the silicon wafer is placed in a texturing solution for texturing to obtain a textured surface with a pyramid structure.

2. The method of claim 1, wherein the step of removing the damaged layer comprises immersing the monocrystalline silicon wafer in a 1% -10% NaOH or KOH solution at 75-90 deg.C for 2-10 min.

3. The texturing method for pretreating a surface of a single-crystal silicon wafer by an electrochemical method according to claim 1, wherein the electrolyte is water, a neutral salt solution, an alkaline solution or an acidic solution.

4. A texturing method for pretreating a surface of a single-crystal silicon wafer by an electrochemical method according to claim 3, wherein the concentration of the electrolyte is 0.1 to 1 mol/L; the neutral salt solution is sodium sulfate solution or potassium sulfate solution, the alkaline solution is NaOH solution or KOH solution, and the acidic solution is hydrochloric acid or sulfuric acid.

5. The method of claim 1, wherein the cathode conductive material is stainless steel or graphite, the area of the cathode electrode is greater than or equal to the area of the monocrystalline silicon piece, the cathode electrode is disposed parallel to and coaxial with the monocrystalline silicon piece, and the distance between the cathode electrode and the monocrystalline silicon piece is 5-25 mm.

6. The method of claim 1, wherein the power supply is a DC power supply with constant voltage output or square wave voltage output.

7. A texturing method for pretreating a surface of a single-crystal silicon wafer by an electrochemical method as described in claim 6, wherein the voltage volt of the DC power supply is 1 to 3.5V, and the total time of voltage application of the DC power supply is 30 to 500 s.

8. The texturing method for pretreating a surface of a monocrystalline silicon wafer by an electrochemical method according to claim 1, wherein the monocrystalline texturing solution is an aqueous solution containing alkali and a texturing additive; wherein, the mass fraction of the alkali in the monocrystal texture-making liquid is 1-2.5%, and the volume fraction of the texture-making additive is 1-2%.

9. The method of claim 8, wherein the alkali is sodium hydroxide or potassium hydroxide, and the texturing additive is a monocrystalline silicon texturing additive.

10. The method of claim 1, wherein the texturing temperature is 78-85 ℃ and the texturing time is 3-10 minutes.

Technical Field

The invention belongs to the technical field of solar cell surface treatment, and particularly relates to a texturing method for pretreating the surface of a monocrystalline silicon wafer by an electrochemical method.

Background

Texturing the surface of a solar cell to form an effective antireflection effect is called texturing, and is one of important means for increasing the efficiency of the solar cell. At present, a corrosion method adopting a cheap alkaline solution system is widely applied in industry, a pyramid-like suede structure can be formed on the surface of a monocrystalline silicon wafer through anisotropic corrosion in an alkaline solution, and incident light is reflected on the surface of the silicon wafer for multiple times to improve the absorption of a solar cell to light, so that the reflectivity of the surface of the silicon wafer is reduced. However, the pyramid structure obtained by the process has uneven size, and the reflection reducing effect of the texture is reduced. Researches find that the suede uniformity can be improved by pretreating a silicon wafer with hydrogen peroxide or directly adding hydrogen peroxide into the suede making liquid. Related patents (see patent application nos. CN201410058182.8, CN201610234684.0, CN201410850205.9, etc.) are also described.

However, hydrogen peroxide is a dangerous corrosive chemical, is unstable in chemical properties and is very easily decomposed, and can react with combustible substances to release a large amount of heat and oxygen to cause ignition and explosion. Hydrogen peroxide used for texturing in the actual solar cell manufacturing process is large in consumption, most of hydrogen peroxide in the market is high-concentration products with the concentration of 30% -35%, the hydrogen peroxide is strong in corrosivity and high in risk coefficient, if the hydrogen peroxide is not leaked or improperly treated, the hydrogen peroxide can cause great harm to the environment, the transportation and storage requirements are high, the cell preparation cost is obviously increased, and therefore the hydrogen peroxide is very necessary to be replaced.

Disclosure of Invention

The invention aims to provide an efficient and low-cost texture making method for pretreating the surface of a monocrystalline silicon wafer by an electrochemical method, which can reduce the cost of a battery and environmental pollution and simultaneously prepare a texture with uniform and dense pyramid sizes.

In order to achieve the purpose, the technical scheme adopted by the invention is as follows:

a texturing method for pretreating the surface of a monocrystalline silicon wafer by an electrochemical method comprises the steps of immersing the monocrystalline silicon wafer with a damaged layer removed in an electrolyte solution, performing electrochemical pretreatment on the surface of the silicon wafer by taking the monocrystalline silicon wafer as an anode and graphite or stainless steel as a cathode, applying direct-current voltage between the anode silicon wafer and the cathode, taking the anode silicon wafer as a voltage anode, taking the cathode of the electrode as a voltage cathode, and then putting the silicon wafer into texturing solution for texturing to obtain a textured surface with a pyramid structure.

The further improvement of the invention is that the method for removing the damage layer is to immerse the monocrystalline silicon piece in NaOH or KOH solution with the mass concentration of 1-10% and react for 2-10 minutes at the temperature of 75-90 ℃.

The invention is further improved in that the electrolyte is water, a neutral salt solution, an alkaline solution or an acidic solution.

The further improvement of the invention is that the concentration of the electrolyte is 0.1-1 mol/L; the neutral salt solution is sodium sulfate solution or potassium sulfate solution, the alkaline solution is NaOH solution or KOH solution, and the acidic solution is hydrochloric acid or sulfuric acid.

The invention has the further improvement that the cathode conductive material is stainless steel or graphite, the area of the cathode electrode is larger than or equal to that of the monocrystalline silicon piece, the cathode electrode and the monocrystalline silicon piece are arranged in parallel and coaxially, and the distance between the cathode electrode and the monocrystalline silicon piece is 5-25 mm.

The invention is further improved in that the power supply is a direct current power supply with constant voltage output or square wave voltage output.

The invention is further improved in that the voltage value of the direct current power supply is 1-3.5V, and the total voltage pressurization time of the direct current power supply is 30-500 s.

The invention has the further improvement that the monocrystal texture etching liquid is an aqueous solution containing alkali and texture etching additives; wherein, the mass fraction of the alkali in the monocrystal texture-making liquid is 1-2.5%, and the volume fraction of the texture-making additive is 1-2%.

The invention has the further improvement that the alkali is sodium hydroxide or potassium hydroxide, and the texturing additive is a monocrystalline silicon texturing additive.

The invention has the further improvement that the wool making temperature is 78-85 ℃, and the wool making time is 3-10 minutes.

Compared with the prior art, the invention has the following beneficial effects: the invention provides a method for forming a pyramid texture surface by using a solar monocrystalline silicon wafer as an electrolytic anode and forming an oxide layer on the surface of the silicon wafer under the action of an external voltage, wherein the oxide layer can accelerate the pyramid nucleation speed of the silicon wafer in the initial texture surface making stage, the nucleation quantity is more and the distribution is more uniform, and a pyramid structure with uniform size can be formed on the surface of the silicon wafer after the texture surface making is finished, so that a high-quality texture surface with high pyramid coverage rate and density is obtained. The invention does not relate to the use of dangerous chemicals such as hydrogen peroxide, and the like, avoids the problem of secondary pollution caused by adding other medicaments, and the problems of extremely high consumption of hydrogen peroxide, high transportation and storage cost and increased production cost of the solar cell in the actual production process by adopting a hydrogen peroxide pretreatment method. The invention adopts a low-pressure electrolysis method, has mild reaction conditions, can be carried out at normal temperature, has no harm to human bodies and environment, has simple structure, convenient operation and easy control, and overcomes the problem that the hydrogen peroxide pretreatment method adopted in the prior art needs heating treatment.

Furthermore, the invention can be processed by normal temperature and low pressure electrolysis (1-3.5v), and the cost is reduced because the energy consumption is low.

Drawings

FIG. 1 is an SEM photograph of pyramids on the surface of a single-crystal silicon wafer prepared in example 1.

FIG. 2 is an SEM photograph of pyramids on the surface of the single-crystal silicon wafer obtained in example 2.

FIG. 3 is an SEM photograph of pyramids on the surface of the single-crystal silicon wafer obtained in example 3.

FIG. 4 is an SEM image of pyramids on the surface of the single crystal silicon wafer prepared in comparative example 1.

FIG. 5 is an SEM image of pyramids on the surface of the single crystal silicon wafer prepared in comparative example 2.

Detailed Description

The present invention will be described in detail with reference to the accompanying drawings.

The invention comprises the following steps:

(1) soaking a monocrystalline silicon wafer in an alkaline solution for treatment, and removing cutting line marks, a damaged layer and the like;

in the step (1), the alkaline solution is strong alkaline solution such as NaOH or KOH, the mass concentration of the strong alkaline solution is 1-10%, the temperature of the strong alkaline solution is 75-90 ℃, and the treatment time is 2-10 minutes.

(2) Performing electrochemical surface pretreatment on the monocrystalline silicon wafer treated in the step (1): soaking the processed silicon wafer in an electrolyte solution, taking the silicon wafer as an anode, taking a conductive material which does not react with the electrolyte as a cathode, applying direct-current voltage between the anode silicon wafer and the cathode, taking the anode silicon wafer as an anode and the cathode of a counter electrode as a cathode, and performing constant-voltage electrolysis on the electrolyte and oxidation reaction on the surface of the silicon wafer;

in the step (2), the electrolyte is water, a neutral salt solution, an alkaline solution or an acidic solution. The neutral salt solution is sodium sulfate solution or potassium sulfate solution, the alkaline solution is NaOH solution or KOH solution, and the acidic solution is hydrochloric acid or sulfuric acid. The concentration of the electrolyte is preferably 0.1-1mol/L, and the current of the anode and the cathode can be ensured to be 1-10 mA; the cathode conductive material is stainless steel or graphite, the area of the cathode electrode is larger than or equal to that of the monocrystalline silicon piece, the cathode electrode and the silicon piece are arranged in parallel and coaxially, and the positions of the cathode electrode and the silicon piece are about 5-25 mm; the power supply is a direct current power supply, can be output by constant voltage or square wave voltage, the voltage value of the direct current voltage is 1-3.5V, and the total pressurizing time of the direct current voltage is 30-500 s.

(3) And (3) cleaning the silicon wafer treated in the step (2) by using pure water, and then performing texturing in conventional monocrystalline silicon texturing liquid to obtain the monocrystalline silicon wafer with the pyramid textured structure.

In the step (3), the conventional single crystal texturing solution comprises alkali, texturing additives, water and the like. The mass fraction of the alkali is 1-2.5%, the volume fraction of the texturing additive is 1-2%, the texturing temperature is 78-85 ℃, and the texturing time is 3-10 minutes. The alkali is sodium hydroxide or potassium hydroxide, and the texturing additive is a monocrystalline silicon texturing additive provided by Wuxi crystal sharp energy chemical industry Co.

The following are specific examples.

Example 1

The texturing method for pretreating the surface of the monocrystalline silicon wafer by the electrochemical method comprises the following steps:

(1) immersing a solar monocrystalline silicon wafer into absolute ethyl alcohol, and ultrasonically cleaning for 5 minutes at 45-50 ℃ to remove impurities and organic pollution on the surface of the silicon wafer. Then taking out the silicon wafer, washing with deionized water, drying, immersing into 10 mass percent NaOH aqueous solution, soaking at 85 ℃ for 10 minutes to remove the surface damage layer of the silicon wafer, then taking out the silicon wafer, washing with deionized water, and drying.

(2) Performing electrochemical surface pretreatment on the monocrystalline silicon wafer treated in the step (1): soaking the silicon chip in K of 0.5mol/L at normal temperature₂SO₄In the solution, a silicon wafer is taken as an anode, a graphite sheet with the area larger than or equal to that of the silicon wafer is taken as a cathode, a graphite electrode and the silicon wafer are arranged in parallel and coaxially, and the positions of the graphite electrode and the silicon wafer are about 5 mm. Applying a DC constant voltage of 2V between an anode silicon wafer and a cathode, wherein the anode silicon wafer is a positive electrode, the cathode of a counter electrode is a negative electrode, and the electrolyte (K) is subjected to₂SO₄Solution) is subjected to constant voltage electrolysis and oxidation reaction is carried out on the surface of the silicon wafer, and the pressurizing electrolysis time is 180 s.

(3) And (3) cleaning the silicon wafer treated in the step (2) by using pure water, and then performing texturing in a conventional monocrystalline silicon texturing solution, wherein the conventional monocrystalline silicon texturing solution consists of sodium hydroxide, a texturing additive and water, the mass fraction of the sodium hydroxide in the monocrystalline texturing solution is 1.8%, the volume fraction of the texturing additive is 1%, and the texturing additive is a monocrystalline silicon texturing additive provided by the tin-free crystal sharp energy chemical company Limited. And (3) taking out the silicon wafer, cleaning the silicon wafer by using deionized water and drying the silicon wafer to obtain the monocrystalline silicon wafer with the pyramid textured structure, wherein the texturing temperature is 80 ℃ and the texturing time is 4 minutes, and referring to fig. 1.

Example 2

In this embodiment, a silicon wafer is immersed in a 0.5mol/L NaOH aqueous solution at normal temperature, a DC constant voltage of 2.5V is applied between an anode silicon wafer and a cathode, and other steps are the same as those in embodiment 1, so as to obtain a single crystal silicon wafer with a pyramid textured structure, as shown in FIG. 2.

Example 3

In this embodiment, a silicon wafer is immersed in a hydrochloric acid solution of 0.5mol/L at normal temperature, a direct current constant voltage of 2.5V is applied between an anode silicon wafer and a cathode, and other steps are the same as those in embodiment 1, so as to obtain a single crystal silicon wafer with a pyramid textured structure, as shown in FIG. 3.

Comparative example 1

In the comparative example, the single crystal silicon wafer from which the cutting line mark and the damaged layer were removed was immersed in a 5% hydrogen peroxide aqueous solution at a temperature of 65 ℃ to oxidize for 8 minutes, and after taking out the silicon wafer, the silicon wafer was cleaned with pure water. Texturing is carried out in the monocrystalline silicon texturing solution to obtain a monocrystalline silicon wafer with a pyramid textured structure, and the reference is made to fig. 4.

Comparative example 2

The comparative example specifically included the following steps:

(1) the silicon wafer was treated as in step (1) of example 1.

(2) And (2) texturing the silicon wafer treated in the step (1) in a conventional monocrystalline silicon texturing solution, wherein the conventional monocrystalline silicon texturing solution consists of 1.8% by mass of sodium hydroxide, 1% by volume of a texturing additive and pure water, and the texturing additive is a monocrystalline silicon texturing additive provided by Zhang Hongcong Bo photoelectricity technology limited company. The texturing temperature is 80 ℃, and the texturing time is 4 minutes. And finally, taking out the silicon wafer, cleaning the silicon wafer with deionized water and drying the silicon wafer to obtain the monocrystalline silicon wafer with the pyramid textured structure, and referring to fig. 5.

As can be seen from FIGS. 1 to 5, the sizes of the pyramids formed on the surfaces of the single crystal silicon wafers obtained in examples 1 to 3 and comparative example 1 were uniform and dense, and the texturing effects of examples 1 to 3 after the electrolytic oxidation treatment were close to the texturing effects of comparative example 1 after the hydrogen peroxide treatment. The size of the pyramid formed on the surface of the monocrystalline silicon wafer obtained in the comparative example 2 is not uniform, which shows that the textured surface with uniform and dense pyramid structure size can be obtained by oxidizing the silicon wafer by the electrochemical method.

Example 4

The texturing method for pretreating the surface of the monocrystalline silicon wafer by the electrochemical method comprises the following steps:

(1) immersing a solar monocrystalline silicon wafer into absolute ethyl alcohol, and ultrasonically cleaning for 5 minutes at 45-50 ℃ to remove impurities and organic pollution on the surface of the silicon wafer. And then taking out the silicon wafer, washing with deionized water, drying, immersing into a KOH aqueous solution with the mass fraction of 1%, soaking for 2 minutes at 75 ℃ to remove cutting line marks and damage layers of the silicon wafer, taking out the silicon wafer, washing with deionized water, and drying.

(2) Performing electrochemical surface pretreatment on the monocrystalline silicon wafer treated in the step (1): soaking the silicon chip in 0.5mol/L Na at normal temperature₂SO₄In the solution, a silicon wafer is taken as an anode, stainless steel with the area larger than or equal to that of the silicon wafer is taken as a cathode, a graphite electrode and the silicon wafer are arranged in parallel and coaxially, and the positions of the graphite electrode and the silicon wafer are about 25 mm. Applying a constant DC voltage of 1V between an anode silicon wafer and a cathode, wherein the anode silicon wafer is a positive electrode, the cathode of a counter electrode is a negative electrode, and the electrolyte is 0.3mol/L K₂SO₄Solution) is subjected to constant voltage electrolysis and oxidation reaction is carried out on the surface of the silicon wafer, and the pressurizing electrolysis time is 30 s.

(3) And (3) cleaning the silicon wafer treated in the step (2) by using pure water, and then performing texturing in a conventional monocrystalline silicon texturing solution, wherein the conventional monocrystalline silicon texturing solution consists of sodium hydroxide, a monocrystalline silicon texturing additive and water, the mass fraction of the sodium hydroxide in the monocrystalline texturing solution is 1%, and the volume fraction of the monocrystalline silicon texturing additive is 1%. The texturing temperature is 80 ℃, and the texturing time is 3 minutes. And finally, taking out the silicon wafer, cleaning with deionized water and drying to obtain the monocrystalline silicon wafer with the pyramid textured structure.

Example 5

(1) Immersing a solar monocrystalline silicon wafer into absolute ethyl alcohol, and ultrasonically cleaning for 5 minutes at 45-50 ℃ to remove impurities and organic pollution on the surface of the silicon wafer. And then taking out the silicon wafer, washing with deionized water, drying, immersing into a KOH aqueous solution with the mass fraction of 5%, soaking for 10 minutes at 90 ℃ to remove cutting line marks and damage layers of the silicon wafer, taking out the silicon wafer, washing with deionized water, and drying.

(2) Performing electrochemical surface pretreatment on the monocrystalline silicon wafer treated in the step (1): soaking the silicon chip in K of 0.1mol/L at normal temperature₂SO₄In the solution, a silicon wafer is used as an anode, a graphite flake with the area larger than or equal to that of the silicon wafer is used as a cathode, and a graphite electrode is flat with the silicon waferThe rows were placed coaxially with the graphite electrodes at approximately 15mm from the silicon wafer. Applying a constant DC voltage of 3.5V between the anode silicon wafer and the cathode, the anode silicon wafer being the anode, the cathode being the cathode, and the electrolyte (K)₂SO₄Solution) is subjected to constant voltage electrolysis and oxidation reaction is carried out on the surface of the silicon wafer, and the pressurizing electrolysis time is 500 s.

(3) And (3) cleaning the silicon wafer treated in the step (2) by using pure water, and then performing texturing in a conventional monocrystalline silicon texturing solution, wherein the conventional monocrystalline silicon texturing solution consists of potassium hydroxide, a monocrystalline silicon texturing additive and water, the mass fraction of the potassium hydroxide in the monocrystalline texturing solution is 2%, and the volume fraction of the monocrystalline silicon texturing additive is 2%. The texturing temperature is 78 ℃, and the texturing time is 6 minutes. And finally, taking out the silicon wafer, cleaning with deionized water and drying to obtain the monocrystalline silicon wafer with the pyramid textured structure.

Example 6

(1) Immersing a solar monocrystalline silicon wafer into absolute ethyl alcohol, and ultrasonically cleaning for 5 minutes at 45-50 ℃ to remove impurities and organic pollution on the surface of the silicon wafer. And then taking out the silicon wafer, washing with deionized water, drying, immersing into 2 mass percent KOH aqueous solution, soaking at 75 ℃ for 2 minutes to remove cutting line marks and damage layers of the silicon wafer, taking out the silicon wafer, washing with deionized water, and drying.

(2) Performing electrochemical surface pretreatment on the monocrystalline silicon wafer treated in the step (1): soaking a silicon wafer in water, taking the silicon wafer as an anode, taking a graphite flake with the area larger than or equal to that of the silicon wafer as a cathode, and coaxially placing a graphite electrode and the silicon wafer in parallel, wherein the positions of the graphite electrode and the silicon wafer are about 20 mm. And (3) applying a direct-current constant voltage of 3V between an anode silicon wafer and a cathode, wherein the anode silicon wafer is a positive electrode, the cathode of a counter electrode is a negative electrode, performing constant-voltage electrolysis on an electrolyte (1mol/L sulfuric acid solution) and simultaneously performing oxidation reaction on the surface of the silicon wafer, and the pressurizing electrolysis time is 400 s.

(3) And (3) cleaning the silicon wafer treated in the step (2) by using pure water, and then performing texturing in a conventional monocrystalline silicon texturing solution, wherein the conventional monocrystalline silicon texturing solution consists of sodium hydroxide, a monocrystalline silicon texturing additive and water, the mass fraction of the sodium hydroxide in the monocrystalline texturing solution is 2.5%, the volume fraction of the monocrystalline silicon texturing additive is 1.5%, the texturing temperature is 85 ℃, and the texturing time is 10 minutes. And finally, taking out the silicon wafer, cleaning with deionized water and drying to obtain the monocrystalline silicon wafer with the pyramid textured structure.

The statistical results of the pyramid structures formed on the surfaces of the obtained single crystal silicon wafers of examples 1 to 6 and comparative examples 1 to 2 are shown below, referring to Table 1:

TABLE 1 statistical results

From the statistical results in table 1, the electrochemical method is adopted to pretreat the surface of the monocrystalline silicon wafer, so that the pyramid structure size is reduced and the number of the pyramid structures is increased after the silicon wafer is subjected to texturing, the standard deviation of pyramid size distribution is reduced, and the pyramid distribution is more uniform.

The invention adopts a low-voltage electrolysis method, has low process investment and low energy consumption, and meets the requirements of high efficiency and low cost of photovoltaic power generation at the present stage. The invention adopts a low-voltage electrolysis method, has mild reaction conditions, can be carried out at normal temperature, has no harm to human bodies and environment, has simple structure, convenient operation and easy control, and overcomes the problems of heating, high hydrogen peroxide storage and transportation cost and the like in the prior art adopting a hydrogen peroxide pretreatment method.