Nitrogen-free crystalline silicon solar cell polishing method and polishing liquid

Abstract

The present invention discloses a nitrogen-free polishing method for crystalline silicon solar cell wafers and a polishing liquid thereof, wherein the polishing liquid comprises a fluoride ion source, a chloride-containing ion source, an oxidant, and deionized water. A diffused SE single crystal silicon wafer is placed in the polishing liquid, etched at a temperature of 8°C to 40°C for 30s to 240s, and the velvet surface of the SE single crystal silicon wafer is etched and polished, so that the upper part of the pyramid morphology structure is removed, leaving the pyramid base. Compared with the alkali polishing process, the polishing method provided by the present invention is simple and stable, compatible with the current production line etching process, does not require new polishing equipment, has good etching, polishing, and smoothing effects, can smooth the top and bottom of the pyramid and passivate the tetrahedral plane, and has controllable reflectivity. In addition, the polishing liquid used has a reasonable formula, does not contain nitrogen, has low waste liquid treatment cost, has little environmental pressure, and has low cost and easy availability of raw chemical liquid, which reduces production costs while also reducing pollution to the environment.

Description

Technical Field

[0001] The invention belongs to the technical field of polishing of crystalline silicon solar cells, and in particular relates to a polishing method for nitrogen-free crystalline silicon solar cells and a polishing liquid thereof. Background Art

[0002] Solar cells have been developing in the direction of high efficiency, low cost and environmental protection. At present, there is no significant room for improvement in the efficiency of conventional aluminum back surface field (Al-BSF) solar cells. The passivated emitter and rear surface cell (PERC) process only needs to add the steps of back passivation and laser grooving on the basis of the original process to passivate the surface and improve the long-wave response, thereby significantly improving the cell efficiency. Therefore, PERC cell technology has become the mainstream high-efficiency solar cell technology on the market. Due to the addition of back passivation, higher requirements are placed on the flatness of the polished structure on the back of the cell. The improvement of the back flatness can increase the secondary reflection of the long-wave band of sunlight, further increase the absorption of transmitted light, and improve the short-circuit current of the cell; in addition, the flat surface structure has a lower specific surface area, reduces the recombination of carriers, and improves the passivation effect. The entire process of PERC cells includes: 1. Texturing - 2. Diffusion - 3. Laser SE - 4. Etching (back polishing) - 5. Back coating (aluminum oxide plus silicon nitride) - 6. Front coating (silicon nitride) - 7. Laser grooving - 8. Screen printing - 9. Sintering.

[0003] Currently, there are two common etching and polishing methods in the industry: acid polishing and alkaline polishing. Acid polishing uses a mixture of hydrofluoric acid and nitric acid to polish and etch the back of the diffused silicon wafer. The reflectivity of the back surface after acid polishing is between 25-30%, the polishing effect is average, and the chemical liquid consumption and cost are high. In addition, the nitrogen emissions in nitric acid pose a great pressure on the environment, and the cost of treating the acid waste liquid is very high. To reduce nitrogen emissions, an alkaline polishing process has been developed. However, this process requires the addition of an alkaline polishing tank equipment on top of the chain equipment. In addition, the alkaline polishing process is incompatible with the current SE process. The superimposed SE process requires an additional oxidation process to protect the SE area, which increases the risk of process contamination and makes the process complex and unstable.

[0004] In summary, existing technologies have the following drawbacks: 1. The acid polishing process suffers from suboptimal polishing results, resulting in low reflectivity and ultimately low efficiency. Acid costs are high, and the wastewater contains nitric nitrogen, which is not environmentally friendly, and wastewater treatment costs are high. 2. The alkaline back polishing process requires three additional sets of equipment (an oxidation furnace, a chain-type back-removal phosphosilicate glass, and a tank-type alkaline polishing process). This equipment investment is high, has poor compatibility with the SE process, and the overall process is complex, increasing the risk of process contamination and causing process instability. Furthermore, alkaline polishing requires the purchase and use of additional alkaline polishing additives. Summary of the Invention

[0005] In view of the above-mentioned deficiencies, the present invention aims to provide a method and a polishing liquid for polishing crystalline silicon solar cells without nitrogen, which has a simple process, is easy to implement, effectively reduces the cost of chemical liquid and waste liquid treatment, and has a good polishing effect.

[0006] To achieve the above purpose, the technical solution provided by the present invention is:

[0007] A method for polishing a crystalline silicon solar cell wafer without nitrogen comprises the following steps:

[0008] (1) preparing a diffused single crystal silicon wafer having a velvet surface covered with pyramidal structures, wherein the pyramidal structures have a base length of 1 to 5 microns and a height of 0.7 to 3.5 microns; the diffused single crystal silicon wafer includes P-type and N-type silicon wafers, and is applicable to raw single crystal silicon, polycrystalline silicon, and cast single crystal silicon wafers;

[0009] (2) preparing a polishing solution: sequentially mixing a fluoride ion source, a chloride-containing ion source, an oxidant, and deionized water and stirring them uniformly to obtain a polishing solution; wherein the concentration of the fluoride ion source is 0.1 mol / L to 8 mol / L, the concentration of the chloride-containing ion source is 0.01 mol / L to 13 mol / L, and the concentration of the oxidant is 0.01 mol / L to 3 mol / L;

[0010] (3) The diffused SE single crystal silicon wafer is placed in the polishing liquid and etched at a temperature of 8°C to 40°C for 30s to 240s, preferably at a temperature of 25°C to 35°C for 40s to 90s. The velvet surface of the SE single crystal silicon wafer is etched and polished, so that the upper part of the pyramid morphology structure is removed, leaving the pyramid base. The outer contour of the pyramid base is a quadrilateral with a side length of 1 to 5 microns and a height of 0.1 to 1 micron. The overall flatness is high, comparable to the crystal surface polishing of alkaline polishing, which improves the passivation effect and reflectivity, thereby increasing the battery opening voltage and increasing the short-circuit current.

[0011] As a preferred embodiment of the present invention, the fluoride ion source is selected from hydrofluoric acid.

[0012] As a preferred embodiment of the present invention, the chlorine-containing ion source is selected from hydrochloric acid, sodium hypochlorite, sodium chlorite, chlorine dioxide, chlorine gas, sodium chlorate or sodium perchlorate.

[0013] As a preferred embodiment of the present invention, the chlorine-containing ion source is selected from one or more of chloride ions, hypochlorite ions, chlorate ions, perchlorate ions, and acids or salts containing the above acid ions.

[0014] As a preferred embodiment of the present invention, the chloride ion is selected from one or more of hydrochloric acid, sodium chloride, potassium chloride, and calcium chloride. For example, one or more of salts containing chloride ions, such as hypochlorous acid, sodium hypochlorite, potassium hypochlorite, chlorous acid, sodium chlorite, potassium chlorite, chlorine dioxide, chlorine, chloric acid, sodium chlorate, potassium chlorate, perchloric acid, sodium perchlorate, potassium perchlorate, hydrochloric acid, sodium chloride, potassium chloride, and calcium chloride; specifically, the hypochlorite ion is selected from one or more of hypochlorous acid, sodium hypochlorite, and potassium hypochlorite. The chlorite ion is selected from one or more of chlorous acid, sodium chlorite, chlorine dioxide, and potassium chlorite; the chlorate ion is selected from one or more of chloric acid, sodium chlorate, and potassium chlorate; and the perchlorate ion is selected from one or more of perchloric acid, sodium perchlorate, and potassium perchlorate.

[0015] As a preferred embodiment of the present invention, the oxidant is selected from one or more of sodium persulfate, hydrogen peroxide, acetic acid, potassium permanganate, and concentrated sulfuric acid.

[0016] As a preferred embodiment of the present invention, the polishing liquid in step (2) is further added with an additive in a mass ratio of 100:0.2 to 5. After adding the additive, the rounding modification process can be made more stable and the rounding modification effect can be improved.

[0017] As a preferred embodiment of the present invention, the additive is a mixture of one or more of sodium lauryl sulfate, sodium silicate, benzyltrimethylammonium chloride, sodium citrate, and guar gum.

[0018] As a preferred embodiment of the present invention, the additive is composed of the following components in mass fraction ratio: 0.5-5 parts by mass of sodium lauryl sulfate, 0.2-2 parts by mass of sodium silicate, 0.5-2 parts by mass of benzyltrimethylammonium chloride, 1-3 parts by mass of sodium citrate, and 100 parts by mass of water.

[0019] The invention discloses a nitrogen-free crystalline silicon solar cell polishing liquid used in a nitrogen-free crystalline silicon solar cell polishing method.

[0020] The beneficial effects of the present invention are as follows: compared with the alkali polishing process, the polishing method provided by the present invention is simple and stable, has good polishing effect and high reflectivity, is compatible with the current production line etching process, does not require new polishing equipment, and has a simple process, good etching, polishing and smoothing modification effects, can smooth the top and bottom of the pyramid and passivate the quadrangular pyramid plane, has controllable reflectivity, is easy to deposit the subsequent passivation film, improves reflectivity, ensures product quality, and improves solar cell efficiency, and the polishing liquid used has a reasonable formula, does not contain nitrogen, has low waste liquid treatment cost, has little pressure on the environment, and has low cost and easy to obtain raw chemical liquid, which reduces production costs while also reducing pollution to the environment.

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the SEM surface structure of SE single crystal silicon wafer before polishing.

[0023] Figure 2 This is a SEM surface structure diagram after polishing using the polishing method of the present invention.

[0024] Figure 3 This is a SEM cross-sectional structural diagram after polishing using the polishing method of the present invention.

[0025] Figure 4 This is the SEM surface structure image obtained by conventional acid etching. DETAILED DESCRIPTION

[0026] Example 1: This embodiment provides a nitrogen-free crystalline silicon solar cell polishing method and polishing liquid thereof, the components of which include hydrofluoric acid, sodium hypochlorite and deionized water. In this embodiment, sodium hypochlorite also has the effect of an oxidant, so there is no need to add an additional oxidant. Of course, in other embodiments, an oxidant may also be added. A polishing liquid is obtained by mixing hydrofluoric acid, sodium hypochlorite and deionized water, wherein the concentration of sodium hypochlorite is 0.05 mol / L and the concentration of the hydrofluoric acid is 1.2 mol / L. An additive is added to the nitrogen-free crystalline silicon solar cell polishing liquid, and the mass ratio of the additive to the nitrogen-free crystalline silicon solar cell polishing liquid is 2:100. The additive is composed of 3 parts by mass of sodium lauryl sulfate, 0.5 parts by mass of sodium silicate, 1 part by mass of benzyltrimethylammonium chloride, 2 parts by mass of sodium citrate and 100 parts by mass of water. In this embodiment, the SE single crystal silicon wafer after diffusion is taken as an example. In other embodiments, the SE single crystal silicon wafer can be a P-type or N-type silicon wafer, which can be suitable for polishing original single crystal silicon, polycrystalline silicon and ingot single crystal silicon wafers.

[0027] The diffused SE single crystal silicon wafer was placed in the nitrogen-free crystalline silicon solar cell polishing solution and etched for 60 seconds at a reaction temperature of 35° C. to obtain a polished sample. Testing showed that the reflectivity after polishing was 38%.

[0028] Example 2: This example provides a nitrogen-free polishing method for crystalline silicon solar cells and a polishing liquid thereof, which are basically the same as those in Example 1, except that the polishing liquid comprises hydrofluoric acid, hydrogen peroxide, hydrochloric acid and additives.

[0029] A polishing solution is prepared by mixing hydrofluoric acid, hydrogen peroxide, and hydrochloric acid, wherein the concentration of the hydrofluoric acid is 0.2 mol / L, the concentration of the hydrogen peroxide is 0.5 mol / L, and the concentration of the hydrochloric acid is 10.8 mol / L. An additive is added to the polishing solution at a mass ratio of 1:100. The additive comprises 1 part by mass of sodium lauryl sulfate, 1 part by mass of sodium silicate, 0.5 part by mass of benzyltrimethylammonium chloride, 1 part by mass of sodium citrate, and 100 parts by mass of water.

[0030] The diffused SE single crystal silicon wafer was placed in the nitrogen-free crystalline silicon solar cell polishing solution and etched for 60 seconds at a reaction temperature of 35°C to obtain a polished sample. Figure 2 and Figure 3 , can effectively flatten the pyramid tip, obtain a flat surface, more conducive to the deposition of the passivation film. According to the test, the reflectivity after polishing is 35%.

[0031] Example 3: This example provides a nitrogen-free polishing method for crystalline silicon solar cells and a polishing solution thereof. The method is substantially the same as that described in Example 1, except that the polishing solution comprises hydrofluoric acid, sodium persulfate, and hydrochloric acid. A nitrogen-free polishing solution for crystalline silicon solar cells is obtained by mixing hydrofluoric acid, sodium persulfate, and hydrochloric acid, wherein the concentration of hydrofluoric acid is 0.2 mol / L, the concentration of sodium persulfate is 0.3 mol / L, and the concentration of hydrochloric acid is 8.8 mol / L. An additive is added to the nitrogen-free polishing solution for crystalline silicon solar cells, wherein the mass ratio of the additive to the nitrogen-free polishing solution is 0.5:100. The additive comprises 3 parts by mass of sodium lauryl sulfate, 2 parts by mass of sodium silicate, 2 parts by mass of benzyltrimethylammonium chloride, 3 parts by mass of sodium citrate, and 100 parts by mass of water.

[0032] The diffused SE single crystal silicon wafer was placed in the nitrogen-free crystalline silicon solar cell polishing solution and etched for 60 seconds at a reaction temperature of 30° C. to obtain a polished sample. Testing showed that the reflectivity after polishing was 33%.

[0033] Example 4: This example provides a nitrogen-free polishing method for crystalline silicon solar cells and a polishing solution thereof, which are substantially the same as those in Example 1, except that the polishing solution comprises hydrofluoric acid, sodium hypochlorite, and deionized water. A nitrogen-free polishing solution for crystalline silicon solar cells is obtained by mixing the hydrofluoric acid, sodium hypochlorite, and deionized water, wherein the concentration of the sodium hypochlorite is 0.3 mol / L and the concentration of the hydrofluoric acid is 0.8 mol / L. An additive is added to the nitrogen-free polishing solution for crystalline silicon solar cells, wherein the mass ratio of the additive to the nitrogen-free polishing solution for crystalline silicon solar cells is 5:100. The additive comprises 0.5 parts by mass of sodium lauryl sulfate, 0.5 parts by mass of sodium silicate, 1 part by mass of benzyltrimethylammonium chloride, 1 part by mass of sodium citrate, and 100 parts by mass of water.

[0034] The diffused SE single crystal silicon wafer was placed in the nitrogen-free crystalline silicon solar cell polishing solution and etched for 60 seconds at a reaction temperature of 25° C. to obtain a polished sample. Testing showed that the reflectivity after polishing was 40%.

[0035] Example 5: This example provides a nitrogen-free polishing method for crystalline silicon solar cells and a polishing solution therefor. The method is substantially the same as that described in Example 1, except that the polishing solution comprises hydrofluoric acid, sodium hypochlorite, hydrogen peroxide, and deionized water. A nitrogen-free polishing solution for crystalline silicon solar cells is obtained by mixing the hydrofluoric acid, sodium hypochlorite, hydrogen peroxide, and deionized water. The hydrofluoric acid, sodium hypochlorite, hydrogen peroxide, and deionized water have a concentration of 2 mol / L, 0.5 mol / L, and 0.3 mol / L, respectively.

[0036] An additive is added to a nitrogen-free crystalline silicon solar cell polishing liquid at a mass ratio of 3:100. The additive comprises 2 parts by mass of sodium lauryl sulfate, 1 part by mass of sodium silicate, 1 part by mass of benzyltrimethylammonium chloride, 2 parts by mass of sodium citrate, and 100 parts by mass of water.

[0037] The diffused SE single crystal silicon wafer was placed in the nitrogen-free crystalline silicon solar cell polishing solution and etched for 60 seconds at a reaction temperature of 30° C. to obtain a polished sample. Testing showed that the reflectivity after polishing was 37%.

[0038] Example 6: This example provides a method and a polishing solution for polishing nitrogen-free crystalline silicon solar cells. The method is substantially the same as that described in Example 1, except that the polishing solution comprises hydrofluoric acid, sodium hypochlorite, sulfuric acid, and deionized water. The polishing solution is prepared by mixing hydrofluoric acid, sodium hypochlorite, sulfuric acid, and deionized water. The concentration of the hydrofluoric acid is 1 mol / L; the concentration of the sodium hypochlorite is 0.1 mol / L; and the concentration of the sulfuric acid is 0.1 mol / L.

[0039] An additive is added to the polishing liquid in a mass ratio of 4:100 to the nitrogen-free crystalline silicon solar cell polishing liquid. The additive comprises 4 parts by mass of sodium lauryl sulfate, 1 part by mass of sodium silicate, 2 parts by mass of benzyltrimethylammonium chloride, 3 parts by mass of sodium citrate, and 100 parts by mass of water.

[0040] The diffused SE single crystal silicon wafer was placed in the nitrogen-free crystalline silicon solar cell polishing solution and etched for 90 seconds at a reaction temperature of 30° C. to obtain a polished sample. Testing showed that the reflectivity after polishing was 41%.

[0041] Example 7: This example provides a nitrogen-free polishing method for crystalline silicon solar cell wafers and a polishing liquid thereof, which are basically the same as those in Example 1, except that the polishing liquid comprises hydrofluoric acid, sodium chlorite and deionized water.

[0042] In this embodiment, sodium chlorite also has the function of an oxidant, so there is no need to add an additional oxidant. Of course, in other embodiments, an oxidant may also be added.

[0043] A nitrogen-free crystalline silicon solar cell polishing solution was prepared by mixing hydrofluoric acid, sodium chlorite, and deionized water, wherein the hydrofluoric acid concentration was 2 mol / L and the sodium chlorite concentration was 0.025 mol / L. An additive was added to the nitrogen-free crystalline silicon solar cell polishing solution at a mass ratio of 1:100. The additive consisted of 2 parts by mass of sodium lauryl sulfate, 1 part by mass of sodium silicate, 2 parts by mass of benzyltrimethylammonium chloride, 2 parts by mass of sodium citrate, and 100 parts by mass of water. A diffused SE single crystal silicon wafer was placed in the nitrogen-free crystalline silicon solar cell polishing solution and etched for 60 seconds at a reaction temperature of 35°C to obtain a polished sample. Testing showed a reflectivity of 35% after polishing.

[0044] Comparative Example 1: The diffused SE single crystal silicon wafer was placed in a conventional hydrofluoric acid and nitric acid mixture to polish and etch a silicon wafer with a pit structure. The hydrofluoric acid concentration was 2.7 mol / L; the nitric acid concentration was 9.7 mol / L; the reaction temperature was 12°C, and the reaction time was 60 seconds. The SEM surface structure is shown in Figure 4 .

[0045] Examples 1-7 and Comparative Example 1 were prepared into solar cells according to the PERC process and the solar cell performance was compared and tested. The specific results are shown in Table 1:

[0046] Table 1

[0047] Uoc(V) <![CDATA[Jsc(mA / cm 2 )]]> FF(%) Eff(%) Example 1 0.6886 39.97 81.71 22.49 Example 2 0.6868 39.95 81.66 22.41 Example 3 0.6855 39.94 81.63 22.35 Example 4 0.6894 39.98 81.73 22.53 Example 5 0.6882 39.96 81.70 22.47 Example 6 0.6901 39.99 81.69 22.54 Example 7 0.6873 39.94 81.62 22.41 Comparative Example 1 0.6865 39.91 81.58 22.35

[0048] A comparison in Table 1 shows that silicon wafers treated with the nitrogen-free polishing method for crystalline silicon solar cells according to the present invention, when used to make solar cells using the PERC process, generally outperform those processed using conventional processes in all aspects. Furthermore, the polishing liquid of the present invention is rationally formulated and nitrogen-free, effectively avoiding the emission of nitrogen from acid polishing wastewater. This reduces wastewater treatment costs and environmental impact, thereby lowering production costs while also minimizing environmental pollution.

[0049] The above embodiments are only preferred implementations of the present invention. The present invention cannot list all implementations one by one. Any technical solution that adopts one of the above embodiments, or equivalent changes made based on the above embodiments, are within the scope of protection of the present invention.

[0050] Based on the disclosure and teachings of the above description, those skilled in the art may also make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and modifications and variations of the present invention should also fall within the scope of protection of the claims of the present invention. In addition, although certain specific terms are used in this description, these terms are merely for convenience of description and do not constitute any limitation to the present invention. Other polishing methods and polishing fluids obtained using the same or similar methods and components are also within the scope of protection of the present invention.

Claims

1. A method for polishing a nitrogen-free crystalline silicon solar cell wafer, characterized in that: It includes the following steps: (1) A SE single crystal silicon wafer after preparation and diffusion, wherein the SE single crystal silicon wafer has a velvet surface covered with pyramidal morphology structures, wherein the base length of the pyramidal morphology structures is 1 to 5 microns and the height is 0.7 to 3.5 microns; (2) preparing a polishing liquid: sequentially mixing a fluoride ion source, a chloride-containing ion source, an oxidant and deionized water and stirring them uniformly to obtain a polishing liquid; wherein the concentration of the fluoride ion source is 0.1 mol / L to 8 mol / L, the concentration of the chloride-containing ion source is 0.01 mol / L to 13 mol / L, and the concentration of the oxidant is 0.01 mol / L to 3 mol / L; an additive is added to the polishing liquid, and the mass ratio of the polishing liquid to the additive is 100:0.2 to 5; the additive is composed of the following components in mass fraction ratio: 0.5 to 5 parts by mass of sodium lauryl sulfate, 0.2 to 2 parts by mass of sodium silicate, 0.5 to 2 parts by mass of benzyltrimethylammonium chloride, 1 to 3 parts by mass of sodium citrate, and 100 parts by mass of water; (3) The SE single crystal silicon wafer after diffusion is placed in the polishing liquid, and etched at a temperature of 8°C to 40°C for 30s to 240s, and the velvet surface of the SE single crystal silicon wafer is etched and polished, so that the upper part of the pyramid morphology structure is removed, leaving the pyramid base. The outer contour of the pyramid base is a quadrilateral with a side length of 1 to 5 microns and a height of 0.1 to 1 micron.

2. The method for polishing a nitrogen-free crystalline silicon solar cell according to claim 1, wherein: The fluoride ion source is selected from hydrofluoric acid.

3. The method for polishing a nitrogen-free crystalline silicon solar cell according to claim 1, wherein: The chlorine-containing ion source includes one or more of chloride ions, hypochlorite ions, chlorate ions, perchlorate ions, and acids or salts containing the above acid ions.

4. The method for polishing a nitrogen-free crystalline silicon solar cell according to claim 3, wherein: The hypochlorite ion is selected from one or more of hypochlorous acid, sodium hypochlorite, and potassium hypochlorite; the chlorite ion is selected from one or more of chlorous acid, sodium chlorite, and potassium chlorite; the chlorate ion is selected from one or more of chloric acid, sodium chlorate, and potassium chlorate; the perchlorate ion is selected from one or more of perchloric acid, sodium perchlorate, and potassium perchlorate.

5. The method for polishing a nitrogen-free crystalline silicon solar cell according to claim 3, wherein: The chloride ions include one or more of hydrochloric acid, sodium chloride, potassium chloride, and calcium chloride.

6. The method for polishing a nitrogen-free crystalline silicon solar cell according to claim 1, wherein: The oxidant includes one or more of sodium persulfate, hydrogen peroxide, acetic acid, potassium permanganate, and concentrated sulfuric acid.

7. A nitrogen-free crystalline silicon solar cell polishing liquid used in the nitrogen-free crystalline silicon solar cell polishing method according to any one of claims 1 to 6.

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