A topcon solar cell and a preparation method thereof

Abstract

The application discloses a TOPCon solar cell and a preparation method thereof. Pure silicon nitride is used as a core passivation body, the defects of large interface loss and low power upper limit of a traditional three-layer film are completely avoided, high-power output of a terminal assembly is ensured, a surface layer of silicon oxynitride can optimize short-wave spectral response, light utilization is further improved, and the power of the assembly is secondarily improved on the basis of pure silicon nitride. The application adds a silicon oxynitride optical improvement layer on the surface layer of the pure silicon nitride, constructs a continuous and gradually changing refractive index optical system, effectively disperses a blue light sharp reflection peak, balances the optical reflection characteristics of the center and the edge of the silicon wafer, offsets optical distortion caused by the thermal stress of the adhesive film in the assembly lamination process, solves the mass production common faults of the blue edge of the pure silicon nitride cell and the aggravated blue edge of the assembly after lamination, greatly improves the appearance consistency of the assembly, and reduces the degradation loss of appearance sorting.

Description

Technical Field

[0001] This invention relates to the field of solar photovoltaic cell technology, and in particular to a TOPCon solar cell and its preparation method. Background Technology

[0002] As the photovoltaic industry rapidly iterates towards higher power, higher efficiency, and greater uniformity in appearance, tunneling oxide passivated contact (TOPCon) cells have become the mainstream high-efficiency mass production technology in the industry. Currently, the passivation and antireflection films on the front of mass-produced TOPCon cells mainly fall into two mainstream systems: silicon nitride + silicon oxynitride + silicon oxide (SiN). x The SiON / SiO2 three-layer composite film system and pure silicon nitride (SiN) x For membrane systems, the two processes have long been plagued by an industry contradiction where performance and appearance cannot be simultaneously achieved.

[0003] The industry traditionally uses SiN x The SiON / SiO2 three-layer laminated film process relies on multi-layer refractive index gradient matching to effectively optimize the optical appearance of the cell and avoid issues such as bluing and color difference in the cell and module, resulting in good uniformity of appearance after module lamination. However, this three-layer film structure has inherent performance defects: numerous multi-layer film interfaces lead to significant light reflection and scattering losses, and severe carrier recombination at the interfaces, significantly weakening the cell's photoelectric conversion capability. This directly results in lower output power of the terminal module, making it difficult to increase the power ceiling and meet the current mass production requirements of high-power, high-efficiency modules. Furthermore, the three-layer film coating process is cumbersome, has low process error tolerance, and results in higher overall non-silicon production costs, limiting mass production capacity.

[0004] To overcome the technical bottleneck of insufficient power in three-layer film modules, the industry is gradually promoting the application of pure silicon nitride passivation technology. Compared with the traditional three-layer stacked structure, single-layer pure silicon nitride film has fewer interfaces, extremely low light loss across the entire spectrum, and excellent field passivation capability, which can effectively improve the open-circuit voltage and short-circuit current of the cell. After mass production and encapsulation, the power of the module is significantly higher than that of the three-layer film module. At the same time, the process is simplified, eliminating the need for multi-layer step-by-step coating, resulting in lower mass production costs and higher production capacity. It is the core preferred technology for high-efficiency, high-power TOPCon modules.

[0005] However, the pure silicon nitride process suffers from unavoidable mass production problems: due to the concentrated electric field at the edge of the PECVD coating and the airflow wrapping effect, the silicon wafer has uneven film thickness at the edge, resulting in a slight bluish tint at the edge of the cell. Crucially, in the subsequent high-temperature lamination process, the encapsulant melts and flows, generating interfacial stress, which further amplifies the optical difference between the cell center and edge. This significantly exacerbates the bluish tint defect after lamination, leading to a substantial increase in the defect rate of finished module appearance and the proportion of downgraded components. This severely impacts mass production yield and end-product delivery quality, hindering the large-scale promotion of high-power pure silicon nitride modules.

[0006] In addition, existing methods use a bottom layer of SiON and a top layer of SiN. x The reverse composite film process cannot disperse the blue light interference peak on the surface, cannot improve the blueing problem at the lamination edge, and the passivation capability of the underlying SiON is weak, which will directly sacrifice the cell efficiency and module power, and is not worth mass production application.

[0007] Therefore, it is necessary to develop a TOPCon solar cell and its fabrication method to solve the above problems. Summary of the Invention

[0008] The purpose of this invention is to design a TOPCon solar cell and its preparation method in order to solve the above problems.

[0009] The present invention achieves the above objectives through the following technical solutions: A TOPCon solar cell includes, from front to back, the following layers connected in sequence: a front metal electrode, a front composite passivation antireflection layer, an aluminum oxide passivation layer, a P+ emitter, an N-type crystalline silicon substrate, an ultrathin tunneling oxide layer, an N-type doped polycrystalline silicon layer, a back antireflection layer, and a back metal electrode. The front composite passivation antireflection layer is a two-layer composite structure, including a silicon nitride passivation layer in contact with the aluminum oxide passivation layer at the bottom layer, and a silicon oxynitride optical modification layer on the surface of the silicon nitride passivation layer. The silicon nitride passivation layer and the silicon oxynitride optical modification layer together form a continuously graded refractive index optical system. The front metal electrode is electrically connected inward to the P+ emitter for collecting and exporting hole carriers. The back metal electrode is electrically connected inward to the N-type doped polycrystalline silicon layer for collecting and exporting electron carriers.

[0010] Preferably, the alumina passivation layer is an alumina thin film prepared by atomic layer deposition.

[0011] Preferably, the P+ emitter is a heavily doped layer formed by boron doping, and the front side of the P+ emitter has a pyramidal textured surface.

[0012] Furthermore, the ultrathin tunneling oxide layer is a silicon oxide thin film; the N-type doped polycrystalline silicon layer is a phosphorus-doped polycrystalline silicon layer.

[0013] Preferably, the antireflective layer on the back is a silicon nitride thin film.

[0014] A method for fabricating TOPCon solar cells includes the following steps: S1. Texturing: Alkali texturing is performed on the front side of the N-type crystalline silicon substrate to form a pyramidal textured surface structure. S2, Front-side boron diffusion: Boron diffusion is performed on the front side of the texturized silicon wafer to form a P+ emitter and generate a borosilicate glass layer on the surface; S3. Laser selective emitter fabrication: Laser heavy doping is performed on the gate line contact area on the front side of the silicon wafer to form a selective emitter structure; S4. Borosilicate glass removal and back-side alkaline polishing: Remove the borosilicate glass from the back and edges of the silicon wafer and polish the back of the silicon wafer. S5. Backside passivation contact deposition: PECVD is used to sequentially deposit an ultrathin tunneling oxide layer and a phosphorus-doped amorphous silicon layer on the backside of the silicon wafer. S6. Annealing and crystallization: The silicon wafer is annealed at high temperature to crystallize the phosphorus-doped amorphous silicon layer into an N-type doped polycrystalline silicon layer. S7. Removal of Phosphor Silicon Glass and Cleaning: Remove the phosphor silicon glass from the front and edges of the silicon wafer, and clean and remove the polycrystalline silicon coated on the front and edges. S8. Front Atomic Layer Deposition: An aluminum oxide passivation layer is deposited on the front side of the silicon wafer using atomic layer deposition technology; S9. Front and back film deposition: A front composite passivation and antireflection layer is deposited on the front side of the battery using PECVD process, and a back antireflection layer is deposited on the back side of the battery at the same time; the front composite passivation and antireflection layer adopts a continuous deposition method in the same cavity, first depositing a silicon nitride passivation layer, and then depositing a silicon oxynitride optical modification layer in situ. S10, Electrode preparation: Metal paste is printed on the front and back of the battery respectively, and then sintered at high temperature to form the front metal electrode and the back metal electrode; S11, Light Injection and Testing & Sorting: The solar cells undergo light injection annealing treatment, followed by testing and sorting.

[0015] Furthermore, in step S3, a secondary boron amplification is performed to repair laser damage and optimize the junction depth.

[0016] Preferably, in step S4, hydrofluoric acid is used to remove the borosilicate glass on the back and edges of the silicon wafer, and an alkaline solution is used to polish the back of the silicon wafer.

[0017] Preferably, in step S5, the ultrathin tunneling oxide layer is generated by reacting N2O with the silicon wafer; the phosphorus-doped amorphous silicon layer is obtained by in-situ doping with SiH4, H2 and PH3.

[0018] The beneficial effects of this invention are as follows: This invention uses pure silicon nitride as the core passivation substrate, fully retaining the core advantages of pure silicon nitride, such as low light loss, high field passivation, and low interfacial carrier recombination. It completely avoids the defects of traditional three-layer films, such as high interfacial loss and low power limit, ensuring high power output of the terminal module. At the same time, the surface silicon oxynitride can optimize the short-wavelength spectral response and further improve the light utilization rate, achieving a second power increase of the module on the basis of pure silicon nitride.

[0019] This invention constructs a continuously gradient refractive index optical system by adding a silicon oxynitride optical modification layer to the surface of pure silicon nitride. This effectively disperses the sharp blue light reflection peak, balances the optical reflection characteristics between the center and edge of the silicon wafer, and counteracts the optical distortion caused by the thermal stress of the encapsulant film during module lamination. It fundamentally solves the common mass production problem of blue edges on pure silicon nitride cells and increased blue edges after module lamination, significantly improves the consistency of module appearance, and reduces the loss due to appearance sorting downgrade.

[0020] The high fixed negative charge field passivation of the bottom silicon nitride and the densification effect of the surface silicon oxynitride film work together to further reduce carrier recombination on the silicon wafer surface, which can increase the open circuit voltage of the cell by 1-2mV and the fill factor by 0.2%. At the same time, the double-layer composite film can fill the microscopic pores of the single-layer silicon nitride, improve the film density and water vapor and sodium ion barrier ability, reduce the outdoor PID degradation and long-term damp heat degradation of the cell, and improve the long-term power generation stability and service life of the module.

[0021] The double-layer composite membrane adopts the PECVD co-cavity continuous deposition process, which requires no new equipment or modification of core processes and is fully compatible with the existing TOPCon mass production line. At the same time, compared with the traditional three-layer membrane process, the process is simplified and the cost of special gas is reduced by 10%-15%, continuing the low-cost mass production advantage of pure silicon nitride and adapting to the needs of large-scale high-efficiency module mass production. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of the TOPCon solar cell in this application.

[0023] Among them, 1-front metal electrode; 2-front composite passivation antireflection layer; 3-alumina passivation layer; 4-P+ emitter; 5-N-type crystalline silicon substrate; 6-ultra-thin tunneling oxide layer; 7-N-type doped polycrystalline silicon layer; 8-back antireflection layer; 9-back metal electrode. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0025] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0026] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0027] In the description of this invention, it should be understood that the terms "upper," "lower," "inner," "outer," "left," "right," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use, or the orientation or positional relationship commonly understood by those skilled in the art. They are only used to facilitate the description of this invention and to simplify the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0028] Furthermore, the terms "first," "second," etc., are used only to distinguish descriptions and should not be interpreted as indicating or implying relative importance.

[0029] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, terms such as "set" and "connection" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0030] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0031] like Figure 1 As shown, a TOPCon solar cell includes, from front to back, the following layers connected in sequence: a front metal electrode 1, a front composite passivation antireflection layer 2, an aluminum oxide passivation layer 3, a P+ emitter 4, an N-type crystalline silicon substrate 5, an ultrathin tunneling oxide layer 6, an N-type doped polycrystalline silicon layer 7, a back antireflection layer 8, and a back metal electrode 9. The front composite passivation antireflection layer 2 is a double-layer composite structure, including a silicon nitride passivation layer located at the bottom layer in contact with the aluminum oxide passivation layer 3, and a silicon oxynitride optical modification layer located on the surface of the silicon nitride passivation layer. The silicon nitride passivation layer and the silicon oxynitride optical modification layer together form a continuously graded refractive index optical system. The front metal electrode 1 is electrically connected inward to the P+ emitter 4 for collecting and exporting hole carriers. The back metal electrode 9 is electrically connected inward to the N-type doped polycrystalline silicon layer 7 for collecting and exporting electron carriers.

[0032] The alumina passivation layer 3 is an alumina thin film prepared by atomic layer deposition. The P+ emitter 4 is a heavily doped layer formed by boron doping, and the front side of the P+ emitter 4 has a pyramidal textured structure. The ultrathin tunneling oxide layer 6 is a silicon oxide thin film; the N-type doped polycrystalline silicon layer 7 is a phosphorus-doped polycrystalline silicon layer. The back antireflection layer 8 is a silicon nitride thin film.

[0033] A method for fabricating TOPCon solar cells includes the following steps: S1. Texturing: Process content: Use an alkaline solution (such as KOH) to anisotropically etch the front side of the N-type silicon wafer to form a pyramid structure, thereby reducing surface reflectivity, increasing light absorption, and removing the mechanical damage layer and organic contaminants on the surface of the silicon wafer. S2, Front-side boron diffusion: Process details: Under high temperature environment, a boron source (such as BCl3) is used to diffuse the front side of the silicon wafer to form a P+ emitter 4 (PN junction), separate photogenerated carriers, and form a borosilicate glass (BSG) layer on the front side. S3. Laser Selective Emitter Fabrication: Process Details: Utilizing the localized thermal effect of laser, the gate contact area on the front side of the silicon wafer is heavily doped to reduce contact resistance, while the remaining areas remain lightly doped to form a selective emitter structure; some processes involve secondary boron expansion to repair laser damage and optimize junction depth. S4. Deborosilicate glass and back alkaline polishing: Process content: Use hydrofluoric acid (HF) to remove BSG from the back and edges of the silicon wafer, and use an alkaline solution to polish the back of the silicon wafer to remove the doped layer wrapped around the back, making the back surface flat to improve the back reflectivity. S5. Backside passivation contact deposition: Process details: Using a PECVD equipment, an ultrathin tunneling oxide layer 6 (SiO_x, about 1-2nm) is first deposited on the backside of the silicon wafer by reacting N2O with Si. Then, SiH4, H2 and PH3 are introduced for in-situ doping, depositing an N+ amorphous silicon / microcrystalline silicon layer (phosphorus-doped amorphous silicon layer) to form a backside passivation contact structure. S6. Annealing and Crystallization: Process content: Annealing the silicon wafer at high temperature (800-1000℃) to crystallize the amorphous silicon / microcrystalline silicon into an N-type doped polycrystalline silicon layer 7, activate phosphorus atoms, form good electron tunneling channels, and repair interface defects. S7. Removal of Phosphosilicate Glass and Cleaning: Process content: Use HF to remove the phosphosilicate glass (PSG) on the front and edges of the silicon wafer, and use RCA (wet cleaning) to remove the poly-Si plating on the front and edges. Retain the oxide layer on the back as a mask so that the poly-Si on the edges and front can be etched away, exposing a clean silicon surface. S8. Front Atomic Layer Deposition: Process details: A very thin aluminum oxide (Al2O3) passivation layer is deposited on the front side of the silicon wafer using atomic layer deposition (ALD) to provide surface passivation and field passivation effects, reduce surface minority carrier recombination, and improve open circuit voltage; S9. Front and back film deposition: A front composite passivation and antireflection layer 2 is deposited on the front side of the battery using PECVD process, and a back antireflection layer 8 is deposited on the back side of the battery. The front composite passivation and antireflection layer 2 is deposited in a continuous cavity manner, first depositing a silicon nitride passivation layer (SiNx) (to reduce surface light reflection and protect the passivation film layers on the surface and back of the battery), and then depositing a silicon oxynitride optical modification layer in situ. S10. Electrode preparation: Using a screen printing machine, silver paste (or silver-aluminum paste) is printed on the front and back of the battery to form positive and negative electrode patterns. In a high-temperature sintering furnace, the organic components in the paste are removed by high temperature, so that the metal paste and the silicon wafer can form good ohmic contact. At the same time, the passivation film is repaired to form the front metal electrode 1 and the back metal electrode 9. S11, Light Injection and Testing Sorting: The solar cells are annealed through light injection (LIF) to improve their stability and conversion efficiency; finally, the solar cells are graded, sorted and packaged through electrical performance testing, EL testing and other methods.

[0034] The functional principles of each component in this application are as follows: 1. High-power passivation principle: Traditional SiN x The triple-layer SiON / SiO2 film, with its multiple interfacial layers, suffers from high light loss and carrier recombination losses, directly limiting cell conversion efficiency and module power. This patent uses pure silicon nitride as the bottom core passivation layer, retaining its core advantages of high negative charge, low interfacial recombination, and low light loss. This significantly reduces dark saturation current, ensuring high photoelectric conversion efficiency and resulting in significantly higher power output for the final module compared to traditional triple-layer film modules.

[0035] 2. Principle of Improving Blue Edges After Lamination: Pure silicon nitride films have a single refractive index and fixed optical interference, resulting in an inherent blue edge at the cell edge after coating. Furthermore, during the high-temperature lamination process of the module, the thermal stress and penetration of the encapsulant film amplify the optical color difference, exacerbating the blue edge defect after lamination. This patent incorporates a thin SiON layer on the surface of pure silicon nitride to construct a continuously gradient refractive index optical system. This effectively disperses the sharp blue light reflection peaks, balances the optical reflection characteristics between the center and edges of the silicon wafer, and counteracts the optical distortion caused by the lamination process. This completely solves the core problem of blue edges after module lamination, significantly improving the uniformity of the module's appearance.

[0036] 3. Full-spectrum efficiency enhancement principle: The gradient composite film can optimize the short-wavelength spectral response of 300–500nm, reduce short-wavelength reflection loss, and further improve light utilization compared with pure silicon nitride and three-layer film structure, effectively increasing the short-circuit current of the battery and achieving a steady increase in module power.

[0037] 4. Mass production compatibility and stability principle: The double-layer membrane adopts PECVD continuous deposition in the same cavity, which does not require new equipment or complex processes and is fully compatible with existing TOPCon mass production lines; at the same time, the double-layer membrane structure is more compact and has stronger water vapor and ion barrier capabilities, which can reduce the long-term outdoor degradation of the battery and meet the mass production requirements of high power, high appearance quality and high reliability.

[0038] Example 1: Silicon nitride TOPCon solar cell with surface composite silicon oxynitride according to this application A 183×210mm battery sample was prepared following the complete steps described above. The electrical and appearance properties of three parallel samples were tested, and the results are as follows:

[0039] Comparative Example 1: Pure Silicon Nitride TOPCon Cell On the same substrate, only pure SiN is deposited. x For the positive film, the rest of the manufacturing process is exactly the same, and the average test values ​​are as follows:

[0040] Comparative Example 2: Traditional SiN x / SiON / SiO2 three-layer film TOPCon battery

[0041] Experimental conclusions Compared to traditional SiN in the industry x The SiON / SiO2 three-layer film process, a patented composite film structure, offers lower light loss and superior passivation, significantly addressing the low power output of three-layer film modules. Compared to pure silicon nitride processes, this patent retains the core advantages of high power and low cost while fundamentally solving the mass production challenges of bluish tint at cell edges and increased blue edges after module lamination through surface SiON optical improvements. This greatly enhances module appearance consistency and mass production yield. This solution boasts strong process compatibility, low mass production cost, and combines high power generation performance with high appearance quality, making it highly valuable for industrialization.

[0042] The above are merely preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A TOPCon solar cell, characterized in that, The structure consists of the following components connected sequentially from front to back: front metal electrode, front composite passivation antireflection layer, aluminum oxide passivation layer, P+ emitter, N-type crystalline silicon substrate, ultrathin tunneling oxide layer, N-type doped polycrystalline silicon layer, back antireflection layer, and back metal electrode. The front composite passivation antireflection layer is a two-layer composite structure, including a silicon nitride passivation layer in contact with the aluminum oxide passivation layer at the bottom layer, and a silicon oxynitride optical modification layer on the surface of the silicon nitride passivation layer. The silicon nitride passivation layer and the silicon oxynitride optical modification layer work together to form a continuously gradient refractive index optical system. The front metal electrode is electrically connected to the P+ emitter inward to collect and extract hole carriers. The back metal electrode is electrically connected inward to an N-type doped polycrystalline silicon layer to collect and extract electron carriers.

2. The TOPCon solar cell according to claim 1, characterized in that, The alumina passivation layer is an alumina thin film prepared by atomic layer deposition.

3. A TOPCon solar cell according to claim 1, characterized in that, The P+ emitter is a heavily doped layer formed by boron doping, and the front side of the P+ emitter has a pyramidal textured surface.

4. A TOPCon solar cell according to claim 1, characterized in that, The ultrathin tunneling oxide layer is a silicon oxide thin film; the N-type doped polycrystalline silicon layer is a phosphorus doped polycrystalline silicon layer.

5. A TOPCon solar cell according to claim 1, characterized in that, The antireflective layer on the back is a silicon nitride thin film.

6. A method for preparing the TOPCon solar cell according to any one of claims 1-5, characterized in that, Including the following steps: S1. Texturing: Alkali texturing is performed on the front side of the N-type crystalline silicon substrate to form a pyramidal textured surface structure. S2, Front-side boron diffusion: Boron diffusion is performed on the front side of the texturized silicon wafer to form a P+ emitter and generate a borosilicate glass layer on the surface; S3. Laser selective emitter fabrication: Laser heavy doping is performed on the gate line contact area on the front side of the silicon wafer to form a selective emitter structure; S4. Borosilicate glass removal and back-side alkaline polishing: Remove the borosilicate glass from the back and edges of the silicon wafer and polish the back of the silicon wafer. S5. Backside passivation contact deposition: PECVD is used to sequentially deposit an ultrathin tunneling oxide layer and a phosphorus-doped amorphous silicon layer on the backside of the silicon wafer. S6. Annealing and crystallization: The silicon wafer is annealed at high temperature to crystallize the phosphorus-doped amorphous silicon layer into an N-type doped polycrystalline silicon layer. S7. Removal of Phosphor Silicon Glass and Cleaning: Remove the phosphor silicon glass from the front and edges of the silicon wafer, and clean and remove the polycrystalline silicon coated on the front and edges. S8. Front Atomic Layer Deposition: An aluminum oxide passivation layer is deposited on the front side of the silicon wafer using atomic layer deposition technology; S9. Front and back film deposition: A front composite passivation antireflection layer is deposited on the front side of the battery using PECVD process, while a back antireflection layer is deposited on the back side of the battery. The front composite passivation antireflection layer adopts a continuous deposition method in the same cavity, first depositing a silicon nitride passivation layer, and then depositing a silicon oxynitride optical modification layer in situ. S10, Electrode preparation: Metal paste is printed on the front and back of the battery respectively, and then sintered at high temperature to form the front metal electrode and the back metal electrode; S11, Light Injection and Testing & Sorting: The solar cells undergo light injection annealing treatment, followed by testing and sorting.

7. The preparation method according to claim 6, characterized in that, In step S3, a secondary boron amplification will be performed to repair laser damage and optimize junction depth.

8. The preparation method according to claim 6, characterized in that, In step S4, hydrofluoric acid is used to remove the borosilicate glass on the back and edges of the silicon wafer, and an alkaline solution is used to polish the back of the silicon wafer.

9. The preparation method according to claim 6, characterized in that, In step S5, the ultrathin tunneling oxide layer is generated by reacting N2O with the silicon wafer; the phosphorus-doped amorphous silicon layer is obtained by in-situ doping with SiH4, H2 and PH3.

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