Method for improving mechanical strength of crystalline silicon wafer and crystalline silicon wafer

Abstract

The application discloses a method for improving mechanical strength of a crystalline silicon wafer and the crystalline silicon wafer, wherein the method comprises the following steps: depositing a polycarbonate (PC) film on the surface of the crystalline silicon wafer with a texturing pyramid structure, so that the PC film covers and fills the surface and the gap of the pyramid structure; then performing wet alkaline etching on the crystalline silicon wafer, and controlling etching conditions, so that the PC film located at the top of the pyramid is selectively removed, and the PC film located in the gap between the pyramids is reserved, thereby forming a PC connector between the pyramids. The application also relates to the crystalline silicon wafer prepared by the method. The method constructs a polymer connection network between the micro-pyramid structures of the crystalline silicon wafer, significantly enhances the anti-fracture ability and mechanical strength of the crystalline silicon wafer without affecting the surface photoelectric functional area, and effectively reduces the fragment rate of the crystalline silicon wafer in the manufacturing and subsequent processes.

Description

Technical Field

[0001] This invention relates to the field of solar cell manufacturing technology, and more specifically to a method for improving the mechanical strength of crystalline silicon wafers and the resulting silicon wafers. Background Technology

[0002] Crystalline silicon wafers are one of the key materials for manufacturing solar photovoltaic modules. While they exhibit excellent photoelectric conversion efficiency, their mechanical strength has always been a key concern. The following are some key issues regarding the mechanical strength of crystalline silicon wafers:

[0003] 1. Crystalline silicon wafers are inherently brittle materials, prone to breakage or crack propagation. This makes them susceptible to mechanical damage during manufacturing, transportation, and installation.

[0004] 2. In order to improve photoelectric conversion efficiency and reduce material costs, the thickness of the material is constantly being reduced. However, reducing the thickness also means a decrease in mechanical strength.

[0005] 3. During the production process, especially in high-temperature processes (such as diffusion and coating), internal stresses are generated within the silicon wafer. These stresses may cause cracks or fractures in the silicon wafer during the cooling stage or subsequent processing (such as laser etching and welding).

[0006] 4. During transportation and installation, silicon wafers may be subjected to external impacts and vibrations, which can also lead to mechanical damage.

[0007] The mechanical strength of crystalline silicon wafers is a complex issue involving multiple factors and levels, requiring a comprehensive solution from the perspectives of materials, design, manufacturing, and application. Currently, the thickness of monocrystalline silicon wafers used in solar energy has reached 110µm. With the reduction in silicon thickness, the breakage rate of solar crystalline silicon wafers during manufacturing also increases significantly. This solution provides intrinsic mechanical protection by adding a protective layer to the surface of the crystalline silicon wafer before fabricating it. Summary of the Invention

[0008] Based on the above problems, this invention proposes a method to improve the mechanical strength of crystalline silicon wafers. By adding a protective layer to the surface of the crystalline silicon wafer, the textured pyramid shape will be covered with the protective layer. The pyramids are connected by a PC film. After the crystalline silicon wafer is made, it is not easy for the crystalline silicon wafer to break from the connection point of the pyramid when it is bent, thus providing internal mechanical protection.

[0009] Technical solution:

[0010] A method for improving the mechanical strength of crystalline silicon wafers includes the following steps:

[0011] S1, deposit a polycarbonate film on the surface of a silicon wafer with a textured pyramid structure, so that the polycarbonate film covers and fills the surface and gaps of the pyramid structure;

[0012] S2, wet alkaline etching is performed on the silicon wafer on which the polycarbonate film is deposited, and the etching conditions are controlled to selectively remove the polycarbonate film located at the top of the pyramid, while retaining the polycarbonate film located in the gap between the pyramids, thereby forming a polycarbonate connector between the pyramids.

[0013] Preferably, before step S1, the process further includes annealing and surface activation of the silicon wafer, wherein the annealing is performed at a temperature of 600°C to 900°C, and the surface activation is performed by plasma cleaning with oxygen.

[0014] Preferably, in step S1, the polycarbonate film is deposited using hot-wire chemical vapor deposition.

[0015] Preferably, the process conditions for hot-wire chemical vapor deposition include: a hot-wire temperature of 200°C to 300°C and a chamber vacuum of 3 × 10⁻⁶. -5 mbar~3×10 -6 mbar.

[0016] Preferably, in step S1, the thickness of the polycarbonate film formed by deposition is 10 nm to 50 nm.

[0017] Preferably, after step S1 and before step S2, a second annealing process is performed on the silicon wafer with the polycarbonate film deposited thereon, wherein the temperature of the second annealing process is 100°C to 200°C.

[0018] Preferably, in step S2, the etching solution used in the wet alkaline etching is a potassium hydroxide solution or a sodium hydroxide solution with a concentration of 5N to 8N, and contains the surfactant isopropanol or sodium alkylbenzene sulfonate; the reaction temperature of the wet alkaline etching is 50°C to 70°C.

[0019] Preferably, in step S2, the etching time is determined based on the initial thickness of the polycarbonate film, the concentration of the etching solution, and the temperature. The exposure of the pyramid tip is observed through a microscope, so that while removing the polycarbonate film in the exposed area above the top and sides of the pyramid on the surface of the silicon wafer, the polycarbonate film filling the bottom of the narrow gap between the pyramids is retained.

[0020] Preferably, the surface of the silicon wafer has a textured pyramid structure, and the gaps between the pyramids are filled with polycarbonate connectors, with the top of the pyramid being an exposed surface not covered with carbonate material.

[0021] The present invention also discloses a crystalline silicon wafer having a pyramid structure formed by texturing on its surface, and polycarbonate connectors filling the gaps between the pyramids, wherein the top of the pyramid is an exposed surface not covered with carbonate material.

[0022] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects:

[0023] (1) Significantly improves mechanical strength and reduces breakage rate: This invention constructs a micro-connection network made of polycarbonate (PC) material between the pyramid structures formed by texturing silicon wafers. When the silicon wafer is subjected to bending or impact stress, this network can effectively transfer and disperse the stress, preventing stress concentration at the brittle pyramid connection points, thereby fundamentally enhancing the fracture resistance of the silicon wafer and solving the core problems of decreased mechanical strength and increased breakage rate during manufacturing in the prior art after silicon wafer thinning. At the same time, this invention does not simply cover or thicken the entire structure, but rather fills the entire area first and then selectively removes the reinforcing material, retaining only the reinforcing material at the weakest and most stress-concentrated gaps in the structure. This achieves a significant strengthening effect with minimal increase in additional material and weight.

[0024] (2) Good process compatibility and no impact on battery efficiency: This invention selectively removes the PC film at the top of the pyramid through a precisely controlled alkaline etching process, thus re-exposing the pyramid tip. This ensures that the morphology and characteristics of the main light-harvesting and photoelectric conversion region (pyramid tip) on the surface of the silicon wafer are restored, without affecting subsequent key battery processes such as anti-reflection film deposition and electrode fabrication, thus guaranteeing the photoelectric conversion efficiency of the final solar cell.

[0025] (3) Easy to implement: The core methods of the present invention, such as PC thin film deposition and selective wet etching, can be implemented by adjusting existing semiconductor or photovoltaic production process equipment. There is no need to introduce complex or expensive special equipment, which makes it easy to integrate and promote the application on existing production lines. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below.

[0027] Figure 1 This is a pyramidal surface structure without a deposited PC film according to one embodiment of the present invention;

[0028] Figure 2 This is a pyramidal surface structure after depositing a PC film according to one embodiment of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the following detailed description, in conjunction with the accompanying drawings and embodiments, further illustrates the invention. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention.

[0030] A method for improving the mechanical strength of crystalline silicon wafers includes the following steps:

[0031] 1) Annealing treatment: Prepare silicon of the appropriate size, and after pre-cleaning (removing dirt from the surface of the silicon wafer), perform medium-temperature annealing treatment (600℃~900℃). Medium-temperature annealing can further reduce internal lattice defects in silicon and improve the arrangement of the crystal structure.

[0032] 2) Activation treatment: The surface of the silicon wafer is treated with plasma micro-cleaning using oxygen (O2) to improve its surface activity. The purpose of the activation treatment is to improve the adhesion and affinity of the silicon wafer surface, making it easier to bond with PC materials.

[0033] 3) Prepare PC material precursors: Heat the PC material precursors (such as carbonate monomers) to the evaporation temperature so that they can decompose into polycarbonate during the deposition process.

[0034] 4) PC Thin Film Deposition: The hot-wire CVD filament is heated to 200°C~300°C, and the chamber vacuum is 3E-6mbar to ensure that the quality and thickness of the PC film meet the requirements. Heated PC precursor molecular vapor is introduced, and the PC precursor undergoes pyrolysis through the hot filament, generating active species that are deposited on the surface of the silicon wafer to form a PC film. The PC film adheres to the surface of the silicon wafer to form a protective layer, and the gaps between the pyramids are filled and connected by the PC film, reducing or eliminating defects on the surface of the silicon wafer, thereby improving the mechanical strength of the silicon wafer; the thickness of the PC film is 10nm~50nm.

[0035] 5) Secondary annealing treatment: After the PC film deposition is completed, the supply of PC precursor vapor is stopped, the silicon wafer is taken out and subjected to secondary annealing treatment (100℃~200℃) to improve the adhesion, crystallinity and quality of the PC film to the surface of the silicon wafer.

[0036] Mechanical properties of PC (polycarbonate) film:

[0037] 1. Tensile strength: The tensile strength of PC film is generally between 60 MPa and 80 MPa.

[0038] 2. Elongation at break: The elongation at break of PC film is typically between 80% and 150%.

[0039] 3. Bending strength: The bending strength of PC film is generally between 100 MPa and 140 MPa.

[0040] 4. Tear resistance: The tear resistance of PC film is usually between 10 kN / m and 25 kN / m.

[0041] Finally, the silicon wafers with completed PC deposition undergo wet alkaline etching. Leveraging the characteristic of PC material being resistant to weak alkalis but not strong alkalis, the concentration, temperature, and time of the alkali solution are strictly controlled to etch away the surface PC while preserving the PC film residue between the pyramids, thus achieving microscopic connectivity between the pyramids. This step primarily aims to expose the pyramid tips while retaining the PC film between the pyramids, facilitating subsequent cell deposition processes and inherently improving the mechanical strength of the cell.

[0042] The specific steps of wet alkaline surface etching are as follows:

[0043] An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with a concentration of 5N to 8N (preferably 6.25N in this embodiment) is prepared as the etching solution. To improve the uniformity of etching, an appropriate amount of surfactant can be added to the etching solution. The surfactant is isopropanol (IPA) or sodium alkylbenzene sulfonate, and the amount of the additive added is 0.01% to 0.1% (preferably 0.05% in this embodiment) of the total mass of the etching solution. The addition of surfactant helps to reduce the surface tension of the etching solution, helps hydrogen bubbles to detach from the silicon wafer surface, allowing it to better wet the polycarbonate film surface, and promotes the removal of reaction byproducts, thereby obtaining a more uniform etching effect.

[0044] A silicon wafer with a polycarbonate film deposited on it is immersed in the etching solution, and the temperature of the etching solution is maintained between 50°C and 70°C. Within this temperature range, the etching reaction rate of the alkaline solution on the polycarbonate is moderate, facilitating precise time control. The etching time needs to be determined based on the initial thickness of the polycarbonate film, the concentration of the etching solution, and the temperature. By precisely controlling the etching time, it is possible to completely remove the polycarbonate film from the exposed areas above the top and sides of the pyramids on the silicon wafer surface while retaining the polycarbonate film filling the bottom of the narrow gaps between the pyramids, thereby forming the desired microstructure. The etching time can be determined through preliminary process experiments and the endpoint can be determined in actual production through online or offline monitoring (such as observing the exposure of the pyramid tips under a microscope).

[0045] The principle of this application is as follows:

[0046] Structurally, after texturing, the surface of a silicon wafer forms numerous micron-scale pyramid structures to enhance light-trapping capabilities. However, the "valleys" of these pyramids (i.e., the connection points between pyramids) are structurally weak points, prone to stress concentration and cracking under load. In this embodiment, after depositing a polycarbonate (PC) film, the PC material fills and adheres tightly to the surface of all pyramids and the gaps between them. Subsequently, selective etching removes only the PC film at the top of the pyramids, leaving the PC material filling the gaps. This retained PC material forms robust "connecting bridges" or "reinforcing skeletons" between adjacent pyramids.

[0047] In terms of process, the key principle for achieving the above structure is to selectively remove materials by utilizing differences in their corrosion resistance. The deposition stage employs hot-filament CVD, which decomposes the PC precursor gas and uniformly deposits it across the entire silicon wafer surface (including the tops, sides, and valleys of all pyramids), achieving initial complete filling and coverage. The etching stage utilizes the characteristic of PC material being resistant to weak alkalis but not to strong alkalis. By precisely controlling the concentration, temperature, and reaction time of the alkaline etching solution (such as KOH solution), the etching reaction preferentially and rapidly removes the thinner PC film from the protruding parts (the tops of the pyramids), exposing the silicon surface. For the PC material filling the narrow gaps (between the pyramids), due to the limited penetration of the etching solution and the reaction rate, this portion of PC material remains and is retained, thus precisely forming the desired microscopic interconnection network.

[0048] Example 2

[0049] This embodiment discloses a crystalline silicon wafer prepared by the method of Example 1. The crystalline silicon wafer has a pyramid structure formed by texturing on its surface, and polycarbonate connectors are filled in the gaps between the pyramids. The top of the pyramid is an exposed surface that is not covered with carbonate material.

[0050] like Figure 1 As shown, this is the surface structure of a pyramid without a deposited PC film. The image reveals a relatively clear and independent pyramid structure, with distinct valleys and ridges between the pyramids. Figure 2As shown, the surface structure of the pyramid after PC film deposition is revealed. The image reveals a fundamental change in surface morphology; the pyramid outline becomes blurred and rounded, exhibiting a hemispherical or continuously convex structure. This indicates that the PC film is not simply laid flat on the surface, but rather uniformly covers and fills every facet of the pyramid structure and the gaps between the pyramids. Its mechanical function is as follows: when the silicon wafer bends or is impacted, stress is transferred and redistributed between the pyramid structures through these PC "connecting bridges," thus preventing excessive stress concentration at the brittle silicon material connection points. This effectively suppresses crack initiation and propagation, improving the overall mechanical strength and fracture resistance at the microstructural level.

[0051] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for improving the mechanical strength of crystalline silicon wafers, characterized in that, Includes the following steps: S1, deposit a polycarbonate film on the surface of a silicon wafer with a textured pyramid structure, so that the polycarbonate film covers and fills the surface and gaps of the pyramid structure; S2, wet alkaline etching is performed on the silicon wafer on which the polycarbonate film is deposited, and the etching conditions are controlled to selectively remove the polycarbonate film located at the top of the pyramid, while retaining the polycarbonate film located in the gap between the pyramids, thereby forming a polycarbonate connector between the pyramids.

2. The method according to claim 1, characterized in that, Before step S1, the process further includes annealing and surface activation of the silicon wafer. The annealing is performed at a temperature of 600°C to 900°C, and the surface activation is performed by plasma cleaning with oxygen.

3. The method according to claim 1, characterized in that, In step S1, the polycarbonate film is deposited using hot-wire chemical vapor deposition.

4. The method according to claim 3, characterized in that, The process conditions for the hot-wire chemical vapor deposition method include: a hot-wire temperature of 200°C to 300°C and a chamber vacuum of 3 × 10⁻⁶. -5 mbar~3×10 -6 mbar.

5. The method according to claim 1, characterized in that, In step S1, the thickness of the polycarbonate film formed by deposition is 10 nm to 50 nm.

6. The method according to claim 1, characterized in that, After step S1 and before step S2, a second annealing process is also included for the silicon wafer with the polycarbonate film deposited thereon, wherein the temperature of the second annealing process is 100°C to 200°C.

7. The method according to claim 1, characterized in that, In step S2, the etching solution used in the wet alkaline etching is a potassium hydroxide solution or a sodium hydroxide solution with a concentration of 5N to 8N, and contains the surfactant isopropanol or sodium alkylbenzene sulfonate; the reaction temperature of the wet alkaline etching is 50°C to 70°C.

8. The method according to any one of claims 1-7, characterized in that, In step S2, the etching time is determined based on the initial thickness of the polycarbonate film, the concentration of the etching solution, and the temperature. The exposure of the pyramid tip is observed through a microscope, so that while removing the polycarbonate film in the exposed areas above the top and sides of the pyramid on the surface of the silicon wafer, the polycarbonate film filling the bottom of the narrow gap between the pyramids is retained.

9. The method according to claim 1, characterized in that, The surface of the silicon wafer has a textured pyramid structure, and the gaps between the pyramids are filled with polycarbonate connectors, with the top of the pyramid being an exposed surface not covered with carbonate material.

10. A crystalline silicon wafer, characterized in that, Its surface has a pyramid structure formed by flocking, and the gaps between the pyramids are filled with polycarbonate connectors, with the top of the pyramid being an exposed surface that is not covered with carbonate material.

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