Acid polishing method and polishing liquid for low-nitrogen crystalline silicon solar cell

Abstract

The present invention discloses a method for acid polishing of low-nitrogen crystalline silicon solar cell sheets and a polishing liquid thereof. A diffused velvet sheet is placed in an acid polishing liquid, and etching is performed at a temperature of 5°C to 45°C for 30s to 240s. The velvet surface of the velvet sheet is polished to form pits with a diameter of 2 to 8 microns and tower mounds with a bottom side length of 2 to 8 microns and a height of 0.1 to 1.5 microns on the velvet surface, thereby improving the flatness of the velvet surface. Compared with conventional acid polishing processes, the acid polishing method for low-nitrogen crystalline silicon solar cell sheets provided by the present invention has a significantly reduced nitrogen content, low waste liquid treatment cost, low environmental pressure, and is compatible with the etching process of a solar cell production line. No new polishing equipment is required, which reduces costs, and the process is simple, with good polishing effect. Compared with conventional acid polishing structures, the roughness is lower and the flatness is higher, which improves the passivation effect and reflectivity, effectively improves the battery opening voltage and back reflection, increases the short-circuit current, and improves the battery efficiency.

Description

Technical Field

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

[0002] Passivated emitter and rear surface cell (PERC) is the most widely used high-efficiency solar cell on the market. The entire process of PERC cell 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. Among them, the back coating (aluminum oxide plus silicon nitride) process mainly coats a layer of Al2O3 and silicon nitride protective layer on the back surface of the cell, achieving a better back surface passivation effect, thereby achieving the purpose of improving efficiency, which is the core process of PERC cell. The etching (back polishing) process is the previous process of back coating, and its reflectivity and uniformity after polishing also directly affect the effect of back coating. Therefore, the etching (polishing) process also occupies an important position in the entire production process of PERC cell.

[0003] At present, there are two main etching methods in the industry, alkali polishing and acid polishing. The alkali polishing process is complicated, and it is necessary to remove the PSG on the back of the silicon wafer on the chain equipment, and then collect the silicon wafer and transfer it to the tank equipment for alkali polishing. At the same time, the alkali polishing process is poorly compatible with the current SE process, and additional oxidation equipment is required to use the oxidation process to protect the SE area, which increases the equipment cost and greatly increases the risk of process contamination; acid polishing mainly refers to the use of hydrofluoric acid / nitric acid mixed acid to polish the back of the silicon wafer.

[0004] For example, the publication number “CN111057468A”, entitled “A PERC battery acid polishing additive and process”, discloses a PERC battery acid polishing additive and process, wherein the acid components in the acid etching solution include hydrofluoric acid and nitric acid.

[0005] Publication number "CN110524398A", titled "An additive for acid polishing of crystalline silicon and an acid polishing method", discloses an additive for acid polishing of crystalline silicon and an acid polishing method, wherein the acidic solution comprises 2-6% by mass hydrofluoric acid and 30-55% by mass nitric acid.

[0006] The polishing method mentioned above has a general effect, and the reflectivity is about 30% and it is difficult to further improve it. At the same time, the concentration of nitric acid is as high as 9mol / L to 13.7mol / L, the nitrogen emission pressure is huge, and the wastewater treatment is difficult and costly. Summary of the invention

[0007] In view of the above-mentioned deficiencies, the object of the present invention is to provide a low-nitrogen crystalline silicon solar cell acid polishing method and polishing liquid which is simple in method, does not require the addition of new equipment, is compatible with existing production lines, has low nitrogen content, effectively reduces nitrogen emissions and waste liquid treatment costs, and has good polishing effect.

[0008] To achieve the above purpose, the technical solution provided by the present invention is:

[0009] A method for acid polishing a low-nitrogen crystalline silicon solar cell sheet comprises the following steps:

[0010] (1) a prepared diffused velvet sheet having a velvet surface covered with pyramidal structures, wherein the base of the pyramidal structures is 1 to 5 microns long and the height is 0.7 to 3.5 microns;

[0011] (2) preparing an acid polishing solution: sequentially mixing a fluoride ion source, a chloride ion source, and an oxidant and stirring them uniformly to obtain an acid polishing solution, wherein the concentration of the fluoride ion source is 1 mol / L to 20 mol / L, the concentration of the chloride ion source is 0.1 mol / L to 8 mol / L, and the concentration of the oxidant is 0.1 mol / L to 8.5 mol / L;

[0012] (3) Polishing: placing the diffused velvet sheet in an acid polishing solution, etching for 30s to 240s at a temperature of 5°C to 45°C, preferably etching for 40s to 90s at a temperature of 25°C to 35°C; polishing the velvet surface of the velvet sheet to form pits with a diameter of 2 to 8 microns and tower hills with a bottom edge length of 2 to 8 microns and a height of 0.1 to 1.5 microns on the velvet surface, thereby improving the flatness of the velvet surface;

[0013] (4) Cleaning: The polished velvet sheet is cleaned. Specifically, the polished velvet sheet is transported by a conveyor chain to the next tank for cleaning.

[0014] As a preferred embodiment of the present invention, an additive is further added to the acid polishing liquid in step (2) at a mass ratio of 100:0.1 to 5. After adding the additive, the polishing process can be made more stable and the polishing effect can be improved.

[0015] As a preferred embodiment of the present invention, the additive is composed of the following components in mass fraction ratio: 0.2-5 parts by mass of sodium dodecylbenzene sulfonate, 0.2-2 parts by mass of sodium silicate, 0.5-3 parts by mass of sodium citrate and 100 parts by mass of water.

[0016] As a preferred embodiment of the present invention, the additive consists of the following components in mass fraction ratio: 0.5-5 parts by mass of sodium dodecyl 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.

[0017] As a preferred embodiment of the present invention, the fluoride ion source includes one or more of hydrofluoric acid and ammonium fluoride.

[0018] As a preferred embodiment of the present invention, the chloride ion source includes one or more of hypochlorous acid, sodium hypochlorite, potassium hypochlorite, chlorous acid, sodium chlorite, potassium chlorite, chlorine dioxide, chloric acid, sodium chlorate, potassium chlorate, perchloric acid, sodium perchlorate, potassium perchlorate, hydrochloric acid, sodium chloride, potassium chloride and calcium chloride.

[0019] As a preferred embodiment of the present invention, the oxidant includes one or more of hydrogen peroxide, nitric acid, persulfate, acetic acid, peracetic acid and potassium permanganate.

[0020] A low-nitrogen crystalline silicon solar cell acid polishing liquid used in a low-nitrogen crystalline silicon solar cell acid polishing method.

[0021] The beneficial effects of the present invention are as follows: compared with the conventional acid polishing process, the acid polishing method for low-nitrogen crystalline silicon solar cell sheets provided by the present invention has a significantly lower nitrogen content, low waste liquid treatment cost, low environmental pressure, and is compatible with the etching process of the solar cell production line. No new polishing equipment is required, which reduces costs. The process is simple and the polishing effect is good. Compared with the conventional acid polishing process, the roughness of the structure is lower and the flatness is higher, which improves the passivation effect and reflectivity, effectively improves the battery open voltage and back reflection, increases the short-circuit current, and improves the battery efficiency.

[0022] The present invention is further described below in conjunction with the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the SEM surface structure of the velvet sheet before polishing.

[0024] Figure 2 This is a schematic diagram of the SEM surface structure of the velvet sheet in Example 1 after polishing.

[0025] Figure 3 This is a schematic diagram of the SEM cross-sectional structure of the velvet sheet in Example 1 after polishing.

[0026] Figure 4 This is a schematic diagram of the SEM surface structure of the velvet sheet in Example 4 before polishing.

[0027] Figure 5 This is a schematic diagram of the SEM surface structure of the velvet sheet in Example 4 after polishing. DETAILED DESCRIPTION

[0028] In the embodiment of the present invention, a diffused velvet sheet is prepared in advance, and the velvet sheet has a velvet surface covered with pyramid morphology structures, the base of the pyramid morphology structure is 1 to 5 microns long, and the height is The side length, i.e. the height, is about 0.7 to 3.5 microns. Figure 1 .

[0029] Embodiment 1:

[0030] The present embodiment provides a method for acid polishing of low-nitrogen crystalline silicon solar cell wafers, wherein the diffused velvet sheet is placed in the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers, etched at a temperature of 35°C for 60s, and tested after polishing, and the reflectivity after polishing is 38%. The acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers includes hydrofluoric acid, nitric acid and hydrochloric acid. The concentration of hydrofluoric acid is 15 mol / L; the concentration of nitric acid is 3.3 mol / L; the concentration of hydrochloric acid is 3.5 mol / L; the mass ratio of the additive to the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers is 2:100. The additive is composed of 0.8 parts by mass of sodium dodecylbenzene sulfonate, 1 part by mass of sodium silicate, 0.5 parts by mass of sodium citrate and 100 parts by mass of water.

[0031] Embodiment 2:

[0032] The present embodiment provides a method for acid polishing of low-nitrogen crystalline silicon solar cell wafers, wherein the diffused velvet sheet is placed in the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers, etched at a temperature of 35°C for 60s, and tested after polishing, and the reflectivity after polishing is 41%. The acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers includes ammonium fluoride, peracetic acid and hydrochloric acid. The concentration of ammonium fluoride is 7.6 mol / L; the concentration of peracetic acid is 2.6 mol / L; the concentration of hydrochloric acid is 4.3 mol / L; the mass ratio of the additive to the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers is 1:100. The additive is composed of 2 parts by mass of sodium dodecylbenzene sulfonate, 1 part by mass of sodium silicate, 0.3 parts by mass of sodium citrate and 100 parts by mass of water.

[0033] Embodiment 3:

[0034] The present embodiment provides a method for acid polishing of low-nitrogen crystalline silicon solar cell wafers, wherein the diffused velvet sheet is placed in the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers, etched at a temperature of 35°C for 60s, and tested after polishing, and the reflectivity after polishing is 36%. The acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers includes hydrofluoric acid, ammonium persulfate and hydrochloric acid. The concentration of hydrofluoric acid is 9.6mol / L; the concentration of ammonium persulfate is 4.2mol / L; the concentration of hydrochloric acid is 3.8mol / L; the mass ratio of the additive to the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers is 1.5:100. The additive is composed of 1.2 parts by mass of sodium dodecylbenzene sulfonate, 0.8 parts by mass of sodium silicate, 1 part by mass of sodium citrate and 100 parts by mass of water.

[0035] Comparative Example 1:

[0036] The diffused textured sheet was placed in a conventional acid polishing solution, which was composed of hydrofluoric acid and nitric acid. The concentration of hydrofluoric acid was 2.9 mol / L; the concentration of nitric acid was 9.1 mol / L; the reaction temperature was 12°C, and the reaction time was 60 s. The reflectivity after polishing was 33%.

[0037] Embodiment 4:

[0038] The present embodiment provides a method for acid polishing of low-nitrogen crystalline silicon solar cell wafers, wherein the diffused velvet sheet is placed in the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers, and etched at a temperature of 35°C for 60 seconds. After polishing, it is tested that the reflectivity after polishing is 35%. The acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers includes hydrofluoric acid, nitric acid and sodium hypochlorite. The concentration of hydrofluoric acid is 1.6 mol / L, the concentration of nitric acid is 3.3 mol / L, and the concentration of sodium hypochlorite is 3.0 mol / L; the mass ratio of the additive to the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers is 2:100, wherein the additive is composed of 3 parts by mass of sodium dodecyl 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. See. Figure 4 and Figure 5 , it can also flatten the pyramid tip to obtain a flat surface, which is beneficial to improve the uniformity of the back coating.

[0039] Embodiment 5:

[0040] The present embodiment provides a method for acid polishing of low-nitrogen crystalline silicon solar cell wafers, wherein the diffused velvet sheet is placed in the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers, etched at a temperature of 30°C for 60s, and tested after polishing, and the reflectivity after polishing is 37%. The acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers includes hydrofluoric acid, nitric acid and sodium chlorate. The concentration of hydrofluoric acid is 1.6 mol / L, the concentration of nitric acid is 3.0 mol / L, and the concentration of sodium chlorate is 5.0 mol / L; the mass ratio of the additive to the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers is 0.5:100, and the additive is composed of 3 parts by mass of sodium dodecyl 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.

[0041] Embodiment 6:

[0042] The present embodiment provides a method for acid polishing of low-nitrogen crystalline silicon solar cell wafers, wherein the diffused velvet sheet is placed in the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers, etched at a temperature of 35°C for 60s, and tested after polishing, and the reflectivity after polishing is 36%. The acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers includes hydrofluoric acid, nitric acid and sodium perchlorate. The concentration of hydrofluoric acid is 1.8 mol / L; the concentration of nitric acid is 2.0 mol / L; the concentration of sodium perchlorate is 8 mol / L; the mass ratio of the additive to the acid polishing liquid of low-nitrogen crystalline silicon solar cell wafers is 1:100, and the additive is composed of 3 parts by mass of sodium dodecyl 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.

[0043] Comparative Example 2:

[0044] The diffused textured sheet was placed in a conventional acid polishing solution, which consisted of hydrofluoric acid and nitric acid. The concentration of hydrofluoric acid was 2.7 mol / L; the concentration of nitric acid was 9.7 mol / L; the reaction temperature was 12°C, and the reaction time was 60 seconds. The reflectivity after polishing was 33%.

[0045] Examples 1-6 and Comparative Examples 1-2 were prepared into solar cells according to the PERC process flow, and the performance of the solar cells was measured. The specific results are shown in Table 1:

[0046] Table 1

[0047] Uoc(V) <![CDATA[Jsc(mA / cm 2 )]]> FF(%) Eff(%) Example 1 0.6871 39.93 81.68 22.44 Example 2 0.6875 39.95 81.69 22.47 Example 3 0.6865 39.93 81.60 22.38 Example 4 0.6865 39.95 81.66 22.40 Example 5 0.6858 39.98 81.66 22.41 Example 6 0.6855 39.90 81.58 22.34 Comparative Example 1 0.6867 39.91 81.60 22.37 Comparative Example 2 0.6865 39.91 81.58 22.35

[0048] By comparison of Table 1, the various properties of the silicon wafers treated by the acid polishing method of the low-nitrogen crystalline silicon solar cell sheet of the present invention and prepared into solar cells according to the PERC process flow basically reach or exceed the traditional process treatment, and the pyramid top can be well flattened, and pits with a diameter of 2 to 8 microns and a bottom edge length of 2 to 8 microns and a tower mound with a height of 0.1 to 1.5 microns are formed on the velvet surface, and the overall flatness is good. The acid polishing method of the low-nitrogen crystalline silicon solar cell sheet of the present invention is compatible with the current production line velvet making process, and the process is simple, and no new equipment is required. On the basis of ensuring the original polishing effect or even improving it, the polishing process is simplified. Compared with the conventional acid polishing process, the amount of nitric acid used is significantly reduced, the nitric nitrogen emission pressure is small, the pollution to the environment is reduced, and the emission of nitrogen in the acid polishing waste liquid is greatly reduced, and the cost is reduced while reducing the pollution to the environment.

[0049] The above embodiments are only preferred embodiments of the present invention. The present invention cannot list all the embodiments one by one. Any technical solution adopting one of the above embodiments, or equivalent changes made according to the above embodiments, are within the protection scope of the present invention.

[0050] According to the disclosure and teaching of the above description, those skilled in the art to which the present invention belongs 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 some modifications and changes to the present invention should also fall within the protection scope of the claims of the present invention. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation to the present invention. Other polishing methods obtained by the same or similar methods are all within the protection scope of the present invention.

Claims

1. A method for acid polishing of low-nitrogen crystalline silicon solar cell wafers, characterized in that: It includes the following steps: (1) a prepared diffused velvet sheet having a velvet surface covered with pyramidal structures, wherein the base of the pyramidal structures is 1 to 5 microns long and the height is 0.7 to 3.5 microns; (2) preparing an acid polishing solution: sequentially mixing a fluoride ion source, a chloride ion source, and an oxidant and stirring them uniformly to obtain an acid polishing solution, wherein the concentration of the fluoride ion source is 1 mol / L to 20 mol / L, the concentration of the chloride ion source is 0.1 mol / L to 8 mol / L, and the concentration of the oxidant is 0.1 mol / L to 8.5 mol / L; (3) Polishing: placing the diffused velvet sheet in an acid polishing solution, etching at a temperature of 5°C to 45°C for 30s to 240s, and polishing the velvet surface of the velvet sheet to form pits with a diameter of 2 to 8 microns and tower hills with a bottom edge length of 2 to 8 microns and a height of 0.1 to 1.5 microns on the velvet surface, thereby improving the flatness of the velvet surface; (4) Cleaning: The polished velvet sheet is cleaned in the next tank; The acid polishing liquid of step (2) is further added with an additive in a mass ratio of 100:0.1 to 5; The additive is composed of the following components in mass fraction ratio: 0.2 to 5 parts by mass of sodium dodecylbenzene sulfonate, 0.2 to 2 parts by mass of sodium silicate, 0.5 to 3 parts by mass of sodium citrate and 100 parts by mass of water; or the additive is composed of the following components in mass fraction ratio: 0.5 to 5 parts by mass of sodium dodecyl 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.

2. The acid polishing method for low-nitrogen crystalline silicon solar cell sheets according to claim 1, characterized in that: The temperature in step (3) is 25°C to 35°C.

3. The acid polishing method for low-nitrogen crystalline silicon solar cell according to claim 1, characterized in that: The etching time in step (3) is 40s to 90s.

4. The acid polishing method for low-nitrogen crystalline silicon solar cell sheets according to claim 1, characterized in that: The fluoride ion source includes one or more of hydrofluoric acid and ammonium fluoride.

5. The acid polishing method for low-nitrogen crystalline silicon solar cell according to claim 1, characterized in that: The chloride ion source includes one or more of hypochlorous acid, sodium hypochlorite, potassium hypochlorite, chlorous acid, sodium chlorite, potassium chlorite, chlorine dioxide, chloric acid, sodium chlorate, potassium chlorate, perchloric acid, sodium perchlorate, potassium perchlorate, hydrochloric acid, sodium chloride, potassium chloride and calcium chloride.

6. The acid polishing method for low-nitrogen crystalline silicon solar cell according to claim 1, characterized in that: The oxidant includes one or more of hydrogen peroxide, nitric acid, persulfate, acetic acid, peracetic acid and potassium permanganate.

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

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