Preparation method of surface-strengthened silicon wafer substrate of photovoltaic cell

Abstract

The invention relates to the technical field of photovoltaic cells, in particular to a preparation method of a surface-strengthened silicon wafer substrate of a photovoltaic cell, which comprises the following steps of: texturing the silicon wafer substrate, and performing surface strengthening treatment twice to obtain the surface-strengthened silicon wafer substrate; the two times of surface strengthening treatment are specifically as follows: during the first time of surface strengthening treatment, an etching solution A is adopted to smooth the surface appearance defects of the silicon wafer substrate, and during the second time of surface strengthening treatment, an etching solution B is adopted to smooth the surface lattice defects of the silicon wafer substrate, the acidic etching solution as the etching solution B comprises hydrofluoric acid, an oxidizing agent and deionized water, and the oxidizing agent is one or a combination of more of a nitro compound and a nitroso compound. The method has the beneficial effects that through two times of surface strengthening treatment, the bending breaking strength of the silicon wafer substrate is improved, and the bending breaking strength of the crystalline silicon photovoltaic cell is finally improved; the method can be completed without special equipment, and has the advantages of being low in production cost and suitable for large-scale production.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photovoltaic cells, in particular to a preparation method of a surface-strengthened silicon wafer substrate of a photovoltaic cell. BACKGROUND

[0002] At present, most photovoltaic modules are packaged by high-efficiency crystalline silicon photovoltaic cells. Common high-efficiency crystalline silicon photovoltaic cells include PERC cells, TOPCon cells and HJT cells. The HJT cell is usually prepared by using a double-sided pyramid textured silicon wafer substrate. The silicon wafer substrate of the PERC cell and the TOPCon cell usually has a pyramid textured front surface and a polished tower base textured back surface. The pyramid textured surface has tower tips, tower surface corners and tower base junctions. The polished tower base textured surface also has tower platform corners and tower base junctions. These places are prone to stress concentration when the silicon wafer substrate is subjected to bending stress, which may cause the silicon wafer substrate to crack.

[0003] When the photovoltaic module is subjected to impact, vibration and bending during use, the crystalline silicon photovoltaic cell in the photovoltaic module is prone to hidden cracks and fragments due to bending stress, which may reduce the performance and reliability of the module.

[0004] Therefore, measures need to be taken to reduce the probability of hidden cracks and fragments of the photovoltaic module when subjected to bending, vibration and impact.

[0005] The bending fracture strength of the photovoltaic cell is determined by the silicon wafer substrate. In the prior art, in order to solve the technical problem, a technique of smoothing, passivating or polishing the edge of the textured silicon wafer substrate is used to improve the bending fracture strength of the textured silicon wafer substrate. However, this requires additional process steps, which is difficult to implement. SUMMARY

[0006] The technical problem to be solved by the present application is that the surface strength of the existing crystalline silicon photovoltaic cell is low, and the cell is prone to hidden cracks and fragments when subjected to bending, impact and vibration.

[0007] The technical solution adopted by the present application to solve the technical problem is a preparation method of a surface-strengthened silicon wafer substrate of a photovoltaic cell. The silicon wafer substrate is textured and then subjected to two surface strengthening treatments to obtain a surface-strengthened silicon wafer substrate. The two surface strengthening treatments are: in the first surface strengthening treatment, etching solution A is used to smooth the surface morphology defects of the silicon wafer substrate, and in the second surface strengthening treatment, etching solution B is used to smooth the surface lattice defects of the silicon wafer substrate. The etching solution B is an acidic etching solution. The acidic etching solution used as the etching solution B comprises hydrofluoric acid, an oxidizing agent and deionized water. The oxidizing agent is one or a combination of several of nitro compounds and nitroso compounds.

[0008] In some embodiments, the etching solution A is an alkaline etching solution or an acid etching solution.

[0009] In some embodiments, the etching solution A is an alkaline etching solution, and the alkaline etching solution as the etching solution A comprises 0.5-10wt% inorganic base, and the balance is deionized water. The etching temperature in the first surface strengthening treatment is 20-40℃, and the time is 1-10min. The inorganic base is one of NaOH and KOH or a combination of the two.

[0010] In some embodiments, the alkaline etching solution as the etching solution A further comprises 0.1-1vol% additive A1, the additive A1 comprises 0.1-1wt% chelating agent and 0.1-5wt% surfactant, and the balance is deionized water.

[0011] In some embodiments, the etching solution A is an acid etching solution, and the acid etching solution as the etching solution A comprises 0.2-20wt% hydrofluoric acid, 2-30wt% hydrochloric acid and 0.5-10wt% oxidizing agent, and the balance is deionized water. The etching temperature in the first surface strengthening treatment is 20-40℃, and the time is 1-10min. The oxidizing agent is one of persulfate, hypochlorite, chlorite, perchlorate, bromate, iodate, and peroxyacetate or a combination of several thereof.

[0012] In some embodiments, the acid etching solution as the etching solution B comprises 0.5-10wt% hydrofluoric acid and 1-10wt% oxidizing agent, and the balance is deionized water. The etching temperature in the second surface strengthening treatment is 5-30℃, and the time is 0.5-10min.

[0013] In some embodiments, the acid etching solution as the etching solution B further comprises 0.1-5wt% surfactant.

[0014] In some embodiments, the etching solution A is an acid etching solution, and the acid etching solution as the etching solution A is a mixed solution of hydrofluoric acid and nitric acid, or the acid etching solution as the etching solution A is a mixed solution of hydrofluoric acid and ozone.

[0015] In some embodiments, the photovoltaic cell is a TOPCon cell, comprising the following steps: Step S1: texturing the silicon wafer substrate to form a pyramid texture on the surface of the silicon wafer substrate; Step S2: diffusing boron on the textured silicon wafer substrate to form a boron-doped emitter layer on the surface of the silicon wafer substrate; Step S3: single-sided BSG film and alkali polishing treatment are performed on the silicon wafer substrate after boron diffusion, to form a boron-doped textured surface on the front surface of the silicon wafer substrate and an undoped polished tower base textured surface on the back surface of the silicon wafer substrate. Step S4: twice surface strengthening treatment is performed on the silicon wafer substrate after back surface polishing treatment.

[0016] The present application has the beneficial effect that the bending fracture strength of the silicon wafer substrate is improved through twice surface strengthening treatment, and the bending fracture strength of the crystalline silicon photovoltaic cell is ultimately improved. Specifically, in the first surface strengthening treatment, the surface morphology defects of the silicon wafer substrate are smoothed to reduce the stress concentration points on the surface of the textured silicon wafer substrate and improve the bending fracture strength of the silicon wafer substrate. In the second surface strengthening treatment, the surface lattice defects of the silicon wafer substrate are smoothed to reduce the potential micro-crack generation points on the surface of the textured silicon wafer substrate and further improve the bending fracture strength of the silicon wafer substrate.

[0017] Compared with the prior art of smoothing, passivating or polishing the edges of the textured silicon wafer substrate, the present application only needs to make simple adjustments to the existing wet cleaning equipment and process, without the need to add glue coating, PECVD mask or plasma edge etching equipment. The method is simple and easy to implement, has the advantages of low production cost and is suitable for large-scale production.

[0018] The textured surface of the silicon wafer substrate with different morphology characteristics, especially the textured surface of the single crystal silicon wafer substrate, including the positive pyramid textured surface, the inverted pyramid textured surface, the polished tower base textured surface, the black silicon textured surface, etc., can significantly improve the bending fracture strength.

[0019] It is suitable for bending strength improvement of different types of crystalline silicon photovoltaic cells, including HJT, PERC, TOPCon and BC cells. BRIEF DESCRIPTION OF DRAWINGS

[0020] The present application will be further described below in conjunction with the drawings and examples; Figure 1 The preparation flowchart of the surface-strengthened silicon wafer substrate for the photovoltaic cell of the present application. DETAILED DESCRIPTION

[0021] As shown in Figure 1 A preparation method of a surface-strengthened silicon wafer substrate for a photovoltaic cell, twice surface strengthening treatment is performed on the textured silicon wafer substrate to obtain a surface-strengthened silicon wafer substrate. The twice surface strengthening treatment is specifically: in the first surface strengthening treatment, etching solution A is used to smooth the surface morphology defects of the silicon wafer substrate, and in the second surface strengthening treatment, etching solution B is used to smooth the surface lattice defects of the silicon wafer substrate.

[0022] The etching solution A preferentially etches the surface topography defects of the silicon wafer substrate, and realizes smoothing treatment of the surface topography defects of the silicon wafer substrate. The etching solution B uses an etching solution with a low etching rate, which is about 1 / 5-1 / 100 of the etching solution A, and can exhibit preferential etching of the surface lattice defects of the silicon wafer substrate, and realizes smoothing treatment of the surface lattice defects of the silicon wafer substrate.

[0023] The etching solution B uses an acidic etching solution. The acidic etching solution used as the etching solution B comprises hydrofluoric acid, an oxidizing agent and deionized water. The oxidizing agent is used to react with the silicon wafer substrate to form silicon oxide on the surface of the silicon wafer substrate. The hydrofluoric acid is used to react with the silicon oxide formed on the surface of the silicon wafer substrate to etch and remove the silicon oxide. The oxidizing agent used is an oxidizing agent with weak oxidizing property, so that the etching rate of the etching solution B is very slow, and the etching solution B can exhibit preferential etching of the surface lattice defects of the silicon wafer substrate, and realizes smoothing treatment of the surface lattice defects of the silicon wafer substrate.

[0024] The etching solution A is an alkaline etching solution or an acidic etching solution.

[0025] The first scheme of the two surface strengthening treatments after texturing of the silicon wafer substrate is as follows.

[0026] A method for preparing a surface-strengthened silicon wafer substrate of a photovoltaic cell. The silicon wafer substrate is textured, and then subjected to two surface strengthening treatments to obtain a surface-strengthened silicon wafer substrate. The two surface strengthening treatments are as follows: in the first surface strengthening treatment, the etching solution A is used to smooth the surface topography defects of the silicon wafer substrate; and in the second surface strengthening treatment, the etching solution B is used to smooth the surface lattice defects of the silicon wafer substrate.

[0027] The etching solution A used in the first surface strengthening treatment is an alkaline etching solution, which comprises 0.5-10wt% inorganic base and 0.1-1vol% additive A1, and the balance is deionized water. The etching temperature in the first surface strengthening treatment is 20-40℃, and the time is 1-10min. The inorganic base is one of NaOH and KOH or a combination of the two. The additive A1 comprises 0.1-1wt% chelating agent and 0.1-5wt% surfactant, and the balance is deionized water.

[0028] The chelating agent is one or a combination of several of polyphosphates, aminocarboxylic acids, hydroxycarboxylic acids, hydroxylaminocarboxylic acids, organic polybasic phosphonic acids and polycarboxylic acid compounds. The chelating agent can complex the trace metal elements in the etching tank, so as to avoid the reduction deposition of metal ions on the surface of the silicon wafer substrate or the combination of the metal ions with sodium silicate to form insoluble silicate precipitate.

[0029] The surfactant in the additive A1 is one or a combination of several of sodium dodecyl sulfate, fatty alcohol ether sulfate, fatty alcohol ether carboxylate, fatty acid methyl ester sulfonate, phosphate of fatty alcohol, and phosphoric acid ester of fatty alcohol ether.

[0030] The etching solution B used in the second surface strengthening treatment is an acidic etching solution, and the acidic etching solution used as the etching solution B comprises 0.5-10 wt% hydrofluoric acid, 1-10 wt% oxidant, and 0.1-5 wt% surfactant, with the balance being deionized water. The etching temperature in the second surface strengthening treatment is 5-30°C, and the time is 0.5-10 min. The oxidant is one or a combination of several of nitro compound and nitroso compound.

[0031] The oxidant reacts with the silicon wafer substrate to form silicon oxide on the surface of the silicon wafer substrate, and then reacts with the hydrofluoric acid to remove the silicon oxide by etching. The oxidizing property of the oxidant in the acidic etching solution in the present solution is weak, and can exhibit preferential etching to the surface lattice defects of the silicon wafer substrate, so as to realize smoothing treatment of the surface lattice defects of the silicon wafer substrate, such as nanoscale steps, protrusions, or depressions on the surface of the silicon wafer substrate. These places are potential micro-crack generation points, and after the smoothing treatment, the bending and breaking strength of the silicon wafer substrate after texturing can be further improved.

[0032] More specifically, the surfactant in the acidic etching solution used as the etching solution B is one or a combination of several of sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, lauryl alcohol ether phosphate, monolauryl phosphate, and cetyltrimethylammonium bromide.

[0033] The etching solution A used in the first surface strengthening treatment can perform smoothing treatment on the surface morphology defects of the submicron protrusions and depressions of the textured surface of the silicon wafer substrate.

[0034] When the textured surfaces of the front side and the back side of the silicon wafer substrate are both pyramid textured surfaces, or the textured surfaces of the front side and the back side of the silicon wafer substrate are pyramid textured surface and polished base textured surface respectively, the surface morphology defects of the submicron protrusions and depressions of the textured surface of the silicon wafer substrate include the tower tip, tower surface corner, and tower base junction of the pyramid textured surface of the front side of the silicon wafer substrate, the tower tip, tower surface corner, and tower base junction of the pyramid textured surface of the back side of the silicon wafer substrate, or the tower platform corner and tower base junction of the polished base textured surface of the back side of the silicon wafer substrate.

[0035] The etching solution B used in the second surface strengthening treatment performs smoothing treatment on the surface lattice defects of the nanoscale steps, protrusions, or depressions on the surface of the silicon wafer substrate after the smoothing treatment.

[0036] Through the above-mentioned secondary surface strengthening treatment, the stress concentration points and potential micro-crack generation points on the surface of the silicon wafer substrate after texturing can be reduced, and the bending fracture strength of the final photovoltaic cell can be improved.

[0037] The second scheme of the two times of surface strengthening treatment after texturing of the silicon wafer substrate is as follows.

[0038] A method for preparing a surface-strengthened silicon wafer substrate of a photovoltaic cell, which comprises two times of surface strengthening treatment after texturing of the silicon wafer substrate to obtain a surface-strengthened silicon wafer substrate, and the two times of surface strengthening treatment specifically comprises: in the first time of surface strengthening treatment, using etching solution A to round the surface topography defects of the silicon wafer substrate, and in the second time of surface strengthening treatment, using etching solution B to smooth the surface lattice defects of the silicon wafer substrate.

[0039] In the first time of surface strengthening treatment, the etching solution A used is an acidic etching solution, the acidic etching solution used as the etching solution A comprises 0.2-20wt% hydrofluoric acid, 2-30wt% hydrochloric acid, 0.5-10wt% oxidant and 0.1-2wt% surfactant, and the balance is deionized water. The etching temperature in the first time of surface strengthening treatment is 5-30℃, and the time is 0.5-10min. The oxidant is one or a combination of several of peroxysulfate, hypochlorite, chlorite, perchlorate, bromate, iodate and peroxoacetic acid.

[0040] More specifically, the surfactant in the acidic etching solution used as the etching solution A is one or a combination of several of sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, lauryl ether phosphate, monolauryl phosphate and cetyltrimethylammonium bromide.

[0041] The oxidant in the acidic etching solution used in the first time of surface strengthening treatment reacts with the silicon wafer substrate to form silicon oxide on the surface of the silicon wafer substrate, and then reacts with the hydrofluoric acid to etch and remove the silicon oxide. Due to the strong oxidizing property of the oxidant and the catalytic auxiliary effect of the hydrochloric acid, the reaction is sufficient to overcome the potential barriers at the sub-micron level protrusions, depressions and other surface topography defects of the textured surface, such as the potential barriers at the tower top, the tower surface corners and the tower base junctions of the pyramid textured surface, so that the sub-micron level protrusions, depressions and other surface topography defects of the textured surface can be rounded, and the stress concentration points on the surface of the silicon wafer substrate after texturing can be reduced.

[0042] The second time of surface strengthening treatment in the present scheme is the same as the second time of surface strengthening treatment in the first scheme.

[0043] Through the above-mentioned secondary surface strengthening treatment, the stress concentration points and potential micro-crack generation points on the surface of the silicon wafer substrate after texturing can be reduced, and the bending fracture strength of the final photovoltaic cell can be improved.

[0044] In the preparation methods of the surface-strengthened silicon wafer substrate of the two photovoltaic cells, the surfactants are used to increase the contact between the strengthening solution and the silicon wafer substrate, so that the etching reaction can be effectively carried out, and the residence time of bubbles on the surface of the silicon wafer substrate is reduced, thereby solving the problem of uneven etching.

[0045] The preparation methods of the surface-strengthened silicon wafer substrate of the two photovoltaic cells can obviously improve the bending fracture strength of the textured surface of the silicon wafer substrate with different morphological characteristics, in particular the textured surface of the monocrystalline silicon wafer substrate, including the positive pyramid textured surface, the inverted pyramid textured surface, the polished tower base textured surface, the black silicon textured surface, etc.

[0046] The acidic etching solution as the etching solution A can also be a mixed solution of hydrofluoric acid and nitric acid or a mixed solution of hydrofluoric acid and ozone.

[0047] The above two schemes are further described in the following multiple embodiments.

[0048] Embodiment 1, a preparation method of a surface-strengthened silicon wafer substrate of an HJT cell, comprising the following steps: Step S1: performing texturing treatment on the silicon wafer substrate to form a pyramid textured surface on the surface of the silicon wafer substrate; Step S2: performing twice surface strengthening treatment on the silicon wafer substrate after texturing, first smoothing the surface morphological defects of the pyramid textured surface of the silicon wafer substrate after texturing, such as tower tip, tower surface edge, tower base junction, and then smoothing the nanoscale surface lattice defects on the surface of the silicon wafer substrate after smoothing, such as steps, protrusions or depressions; The etching solution A used in the first surface strengthening treatment is an alkaline etching solution, and the alkaline etching solution as the etching solution A contains 2wt% NaOH and 0.5vol% additive A1, and the balance is deionized water; The etching temperature in the first surface strengthening treatment is 20-40℃, and the time is 4-8min; The additive A1 contains 0.1-1wt% chelating agent and 0.1-5wt% surfactant, and the balance is deionized water; The chelating agent is one or a combination of several of polyphosphate, aminocarboxylic acid, hydroxycarboxylic acid, hydroxylaminocarboxylic acid, organic polybasic phosphonic acid, and polycarboxylic acid compound; The surfactant is one or a combination of several of sodium dodecyl sulfate, fatty alcohol ether sulfate, fatty alcohol ether carboxylate, fatty acid methyl ester sulfonate, fatty alcohol phosphate, and fatty alcohol ether phosphate; The etching solution B used in the second surface strengthening treatment is an acidic etching solution, and the acidic etching solution used as the etching solution B comprises 5wt% hydrofluoric acid, 2wt% oxidant and 0.3wt% surfactant, and the balance is deionized water; The etching temperature in the second surface strengthening treatment is 20-30℃, and the time is 3-8min; The oxidant is one or a combination of several of nitro compounds and nitroso compounds; The surfactant is one or a combination of several of sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, lauryl alcohol ether phosphate, monolauryl phosphate and cetyl trimethyl ammonium bromide; Step S3: sequentially cleaning and drying the silicon wafer substrate after the two surface strengthening treatments.

[0049] The silicon wafer substrate after the above two surface strengthening treatments is processed through subsequent processes, including PECVD deposition of amorphous silicon intrinsic layer and doped layer on the front and back surfaces, PVD deposition of transparent conductive layer on the front and back surfaces, screen printing of low-temperature metal electrode, solidification and light injection, and finally prepared into an HJT cell.

[0050] The bending fracture strength of the HJT cell is measured by the three-point bending strength test method, and compared with the reference group without treatment, and the results are as follows:

[0051] It can be seen that the bending fracture strength of the HJT cell is improved by the two surface strengthening treatments.

[0052] Embodiment 2, a preparation method of a surface strengthened silicon wafer substrate of a back polished HJT cell, comprising the following steps: Step S1: performing texturing and single-side polishing treatment on the silicon wafer substrate to form a pyramid textured surface on the front surface and a polished tower base textured surface on the back surface of the silicon wafer substrate; Step S2: performing two surface strengthening treatments on the silicon wafer substrate after the texturing and single-side polishing, first smoothing the surface topography defects of the pyramid textured surface on the front surface of the silicon wafer substrate, such as tower tip, tower surface edge, tower base junction, and the surface topography defects of the polished tower base textured surface on the back surface, such as tower platform edge and tower base junction, and then smoothing the nanoscale surface lattice defects on the surface of the silicon wafer substrate after the smoothing treatment, such as steps, protrusions or depressions; The etching solution A used in the first surface strengthening treatment is an acidic etching solution, and the acidic etching solution used as the etching solution A comprises 5wt% hydrofluoric acid, 5wt% hydrochloric acid, 3wt% oxidant and 0.5wt% surfactant, and the balance is deionized water; The etching temperature in the first surface strengthening treatment is 5-30℃, and the time is 3-6min; The oxidizing agent is one or a combination of several of the following: persulfate, hypochlorite, chlorite, perchlorate, bromate, iodate, peroxoacetic acid.

[0053] The surfactant is one or a combination of several of the following: sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, lauryl alcohol ether phosphate, monolauryl phosphate, cetyltrimethylammonium bromide; The etching solution B used in the second surface strengthening treatment is an acidic etching solution, and the acidic etching solution used as the etching solution B comprises 1wt% hydrofluoric acid, 3wt% oxidizing agent, and 0.5wt% surfactant, and the balance is deionized water; The etching temperature in the second surface strengthening treatment is 5-30℃, and the time is 0.5-10min; The oxidizing agent is one or a combination of several of the following: nitro compound, nitroso compound; The surfactant is one or a combination of several of the following: sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, lauryl alcohol ether phosphate, monolauryl phosphate, cetyltrimethylammonium bromide; Step S3: The silicon wafer substrate after the two surface strengthening treatments is sequentially cleaned and dried.

[0054] The single-side polished and texturized silicon wafer substrate after the above two surface strengthening treatments is processed through subsequent processes, including PECVD deposition of amorphous silicon intrinsic layer and doped layer on the front and back surfaces, PVD deposition of transparent conductive layer on the front and back surfaces, screen printing of low-temperature metal electrode, curing and photo injection, to finally prepare a back-polished HJT cell.

[0055] The bending fracture strength of the back-polished HJT cell is measured by the three-point bending strength test method, and compared with the reference group without surface treatment, and the results are as follows:

[0056] It can be seen that through the two surface strengthening treatments, the bending fracture strengths of the front and back surfaces of the back-polished HJT cell are obviously improved, and the difference in bending strength between the front textured surface and the back polished tower base textured surface is obviously reduced.

[0057] Embodiment 3, a method for preparing a surface-strengthened silicon wafer substrate of a TOPCon cell, comprising the following steps: Step S1: performing texturing treatment on the silicon wafer substrate to form a pyramid textured surface on the surface of the silicon wafer substrate; Step S2: performing boron diffusion on the texturized silicon wafer substrate to form a boron-doped emitter layer on the surface of the silicon wafer substrate; Step S3: performing single-side BSG film removal and alkali polishing treatment on the boron-diffused silicon wafer substrate to form a boron-doped textured surface on the front surface of the silicon wafer substrate and an undoped polished tower base textured surface on the back surface of the silicon wafer substrate; Step S4: The silicon wafer substrate after backside polishing is subjected to two surface strengthening treatments. Firstly, the etching solution A is used to smooth the surface topography defects of the silicon wafer substrate after texturing, such as the tower tip, tower surface corner, tower base junction of the positive pyramid texturing surface, and the tower platform corner and tower base junction of the backside polishing tower base texturing surface. Then, the etching solution B is used to smooth the nanoscale surface lattice defects on the surface of the silicon wafer substrate after smoothing treatment. The etching solution A used in the first surface strengthening treatment is an acidic etching solution, which contains 4wt% hydrofluoric acid, 6wt% hydrochloric acid, 5wt% oxidant and 0.3wt% surfactant, and the balance is deionized water. The etching temperature in the first surface strengthening treatment is 5-30℃, and the time is 3-6min. The oxidant is one or a combination of several of the following: persulfate, hypochlorite, chlorite, perchlorate, bromate, iodate, and peroxyacetate. The surfactant is one or a combination of several of the following: sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, lauryl alcohol ether phosphate, monolauryl phosphate, and cetyltrimethylammonium bromide. The etching solution B used in the second surface strengthening treatment is an acidic etching solution, which contains 2wt% hydrofluoric acid, 4wt% oxidant and 0.2wt% surfactant, and the balance is deionized water. The etching temperature in the second surface strengthening treatment is 5-30℃, and the time is 0.5-10min. The oxidant is one or a combination of several of the following: nitro compound and nitroso compound.

[0058] The surfactant is one or a combination of several of the following: sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, lauryl alcohol ether phosphate, monolauryl phosphate, and cetyltrimethylammonium bromide. Step S5: The silicon wafer substrate after two surface strengthening treatments is sequentially subjected to cleaning and drying.

[0059] The backside polished boron-diffused silicon wafer substrate after the above two surface strengthening treatments is processed through subsequent processes, including LPCVD deposition of tunnel oxide layer and intrinsic amorphous silicon, phosphorus diffusion to form doped polysilicon, poly removal, front side ALD deposition of AlOx, PECVD deposition of front and back SiNx anti-reflection layers, screen printing of metal paste electrodes, sintering and light injection, to finally prepare a TOPCon cell.

[0060] The bending fracture strength of the TOPCon cell is measured by the three-point bending strength test method, and compared with the reference group without surface treatment, and the results are as follows:

[0061] It can be seen that through twice surface strengthening treatment, the front and back bending fracture strengths of the TOPCon cell are obviously improved, and the difference in bending strength between the front surface and the back surface of the polished tower base is obviously reduced.

Claims

1. A method of preparing a surface reinforced silicon wafer substrate for a photovoltaic cell, characterized by: The surface of the silicon wafer substrate is strengthened twice after texturing, and the surface of the silicon wafer substrate is strengthened twice, that is, the surface of the silicon wafer substrate is smoothed by etching solution A in the first surface strengthening treatment, and the surface lattice defects of the silicon wafer substrate are smoothed by etching solution B in the second surface strengthening treatment. The etching solution B is an acidic etching solution, and the acidic etching solution as the etching solution B comprises hydrofluoric acid, an oxidizing agent and deionized water, and the oxidizing agent is one or a combination of several of nitro compounds and nitroso compounds.

2. The method of claim 1, wherein the method further comprises: The etching solution A is an alkaline etching solution or an acidic etching solution. ​ 3. The method of claim 1, wherein the method further comprises: The etching solution A is an alkaline etching solution, and the alkaline etching solution as the etching solution A comprises 0.5-10wt% inorganic base and the balance is deionized water. ​ The etching temperature in the first surface strengthening treatment is 20-40℃, and the time is 1-10min. The inorganic base is one or a combination of two of NaOH and KOH.

4. The method of claim 3, wherein the method further comprises: The alkaline etching solution as the etching solution A further comprises 0.1-1vol% additive A1, the additive A1 comprises 0.1-1wt% chelating agent and 0.1-5wt% surfactant, and the balance is deionized water.

5. The method of claim 1, wherein the method further comprises: The etching solution A is an acidic etching solution, and the acidic etching solution as the etching solution A comprises 0.2-20wt% hydrofluoric acid, 2-30wt% hydrochloric acid and 0.5-10wt% oxidizing agent, and the balance is deionized water. ​ The etching temperature in the first surface strengthening treatment is 5-30℃, and the time is 0.5-10min. The oxidizing agent is one or a combination of several of persulfate, hypochlorite, chlorite, perchlorate, bromate, iodate and peroxyacetate.

6. The method of claim 1, wherein the method further comprises: The acidic etching solution as the etching solution B comprises 0.5-10wt% hydrofluoric acid and 1-10wt% oxidizing agent, and the balance is deionized water. The etching temperature in the second surface strengthening treatment is 5-30℃, and the time is 0.5-10min.

7. The method of claim 6, wherein the method further comprises: The acidic etching solution as the etching solution B further comprises 0.1-5wt% surfactant.

8. The method of claim 1, wherein the method further comprises: The etching solution A is an acidic etching solution, and the acidic etching solution as the etching solution A is a mixed solution of hydrofluoric acid and nitric acid, or the acidic etching solution as the etching solution A is a mixed solution of hydrofluoric acid and ozone.

9. The method of claim 1, wherein the method further comprises: The photovoltaic cell is a TOPCon cell, comprising the following steps: ​ Step S1: texturing the silicon wafer substrate to form a pyramid textured surface on the surface of the silicon wafer substrate; Step S2: boron diffusion on the textured silicon wafer substrate to form a boron-doped emitter layer on the surface of the silicon wafer substrate; Step S3: single-side BSG film removal and alkaline polishing treatment on the boron-diffused silicon wafer substrate to form a boron-doped textured surface on the front surface of the silicon wafer substrate and an undoped polished tower base textured surface on the back surface of the silicon wafer substrate; Step S4: two surface strengthening treatments on the silicon wafer substrate after back surface polishing treatment.

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