Multilayer structure composite transfer printing film, preparation method thereof and battery piece grid line preparation method

Abstract

The invention relates to a multilayer structure composite transfer printing film, a preparation method thereof and a battery piece grid line preparation method. The transfer printing film comprises a supporting layer, a self-healing glue layer, a patterning layer and a functional layer. The supporting layer is made of a polyimide material and is modified by fluorosilane, so that the hydrolysis resistance and the structural integrity are enhanced; the self-healing glue layer is PVA-based hydrogel, has a self-repairing capability, and forms a reversible adhesion interface with the supporting layer and the patterned layer, so that the repeated utilization rate is improved; the patterned layer is provided with a micro-groove structure, so that high-precision copying of a grid line pattern is realized; the functional layer adopts a fluorine-containing compound, so that the demolding performance and the chemical stability are improved. The preparation method comprises the steps of mold pretreatment, gluing of the adhesive layer and the patterned layer, demolding, spraying of the functional layer and the like. The preparation method of the cell grid line comprises the steps of conductive slurry filling, transfer printing and curing, film tearing and sintering. By adopting a full-dry process, the transfer printing precision and consistency of the grid lines are improved, the cost is reduced, and the energy consumption and wastewater discharge are reduced.

Description

Technical Field

[0001] The present application relates to the technical field of solar cells, and in particular to a multi-layer composite transfer film, a preparation method thereof, and a method for preparing a solar cell grid line. Background Art

[0002] In the field of photovoltaic technology, the high-precision preparation of conductive grid lines (such as silver paste, silver-coated copper, or low-silver-content silver paste) is a key step in improving the conversion efficiency of photovoltaic cells. In recent years, with the rapid development of the photovoltaic industry, the requirements for conductive grid line preparation have become increasingly stringent. Currently, the industry's preparation process includes a water-soluble transfer film technology, but this type of transfer film is typically made of water-soluble polymer materials such as PVA (polyvinyl alcohol), PEG (polyethylene glycol), and PVP (polyvinyl pyrrolidone).

[0003] Although water-soluble transfer film technology has made some progress in improving the accuracy of conductive grid line preparation, the technology still faces many challenges. First, the process complexity increases, because the water washing step not only produces a large amount of wastewater, increasing treatment costs, but also may cause environmental pollution; and the method of removing the carrier film by high-temperature combustion is energy-intensive and may generate harmful gases, posing a threat to the operating environment and personnel health. Secondly, the problem of material waste is serious. The carrier film is usually disposable, which not only increases production costs, but also is inconsistent with the concept of green environmental protection. Finally, the compatibility risk cannot be ignored. Solvent residues or chemical reactions during the combustion process may damage the surface of the silicon wafer or affect the conductive properties of the silver paste, thereby reducing the overall efficiency and service life of the photovoltaic cell.

[0004] To address the above problems, it is necessary to develop a new type of multi-layer composite transfer film and its preparation method, as well as a cell grid line preparation technology based on the transfer film, which is of great significance for simplifying the process flow, reducing production costs, improving material utilization, protecting the environment and improving the performance of photovoltaic cells. Summary of the Invention

[0005] The purpose of this application is to provide a multi-layer composite transfer film and its preparation method, as well as a method for preparing a cell grid line based on the transfer film, which has the advantages of high-precision transfer, reusability, and environmental protection, can reduce the cost and environmental burden of silver paste grid line transfer, and ensure the high precision and conductive performance of the grid line. The purpose of this application is achieved through the following technical solution: the multi-layer composite transfer film of the present application includes a support layer, a self-healing adhesive layer, a patterned layer, and a functional layer; The hydrophobic angle of the support layer surface is greater than 90°; The self-healing adhesive layer is between the support layer and the patterned layer, and the interface is a reversible adhesion interface; The patterned layer comprises a micro-groove structure on a surface opposite to the self-healing adhesive layer; The functional layer is formed on the surface of the patterned layer and the micro-groove structure.

[0006] In one embodiment, the difference in thermal expansion coefficients among the support layer, the self-healing adhesive layer, the patterned layer, and the functional layer is less than 1×10⁻ 6 / ℃.

[0007] In one embodiment, the thickness of the support layer is in the range of 20-50 μm, the thickness of the self-healing adhesive layer is in the range of 2-20 μm, the thickness of the patterned layer is in the range of 5-20 μm, and the thickness of the functional layer is in the range of 0.1-0.5 μm.

[0008] In one embodiment, the material of the self-healing adhesive layer is PVA-based hydrogel.

[0009] In one embodiment, the support layer is made of polyimide, and its surface is modified with fluorosilane.

[0010] In one embodiment, the material of the functional layer includes fluorine-containing compounds, organosilicon compounds, wax compounds or fatty acid salts.

[0011] In addition, the present application further provides a method for preparing a multi-layer composite transfer film, comprising the following steps: Mold pretreatment: forming a raised structure on the mold surface, the raised structure corresponding to the micro-groove structure in the transfer film; ‌Forming a patterned layer on the mold surface; applying the self-healing adhesive layer on the support layer; The self-healing adhesive layer is placed toward the patterned layer, and a first composite layer including the support layer and the self-healing adhesive layer is attached to the patterned layer; Demolding the second composite layer comprising the support layer, the self-healing adhesive layer and the patterned layer from the mold surface, wherein the patterned layer comprises a micro-groove structure; A functional layer is formed on the surface of the patterned layer and the micro-groove structure.

[0012] In one embodiment, the self-healing adhesive layer is subjected to a freeze cycle treatment before coating.

[0013] In one embodiment, the specific steps of forming the patterned layer include applying a solution on the mold surface and curing it, the applied solution is a PVA solution, a PEG solution, a PEO solution, a PVP solution or a mixed solution of the above solutions, and the curing step is hot air drying at a temperature in the range of 50-100°C.

[0014] The present application further provides a method for preparing a grid line of a cell, comprising the following steps: Using the aforementioned multi-layer composite transfer film; Filling the grooves on the multi-layer composite transfer film with conductive paste; Transfer and solidify the conductive paste on the surface of the battery cell; Film tearing: tearing off the multi-layer composite transfer film from the surface of the battery cell to separate it from the battery cell; Sintering the conductive paste.

[0015] Compared with the prior art, this application has the following beneficial effects: The multi-layer composite transfer film of the present application realizes high-precision transfer of the conductive paste through a support layer, a self-healing adhesive layer, a patterned layer and a functional layer. The micro-groove structure on the surface of the patterned layer complements the raised structure on the surface of the mold, ensuring the accurate replication of the grid line pattern during the transfer process, and effectively improving the accuracy and consistency of the grid line preparation. The self-healing adhesive layer forms a reversible adhesion interface between the support layer and the patterned layer, so that the transfer film can still maintain good adhesion performance after repeated use. In addition, the self-healing adhesive layer material adopts PVA-based hydrogel, which has excellent self-healing ability and can recover even after a certain amount of damage, thereby extending the service life of the transfer film and reducing production costs.

[0016] The transfer film preparation method of the present application avoids the steps of traditional water washing or high-temperature burning to remove the carrier film, thereby eliminating the risk of wastewater generation and harmful gas emissions. At the same time, the support layer material is made of polyimide and is modified with fluorosilane, and the entire preparation process adopts a dry process. Due to the reusability of the transfer film, the present application reduces the material consumption and waste generation of the carrier film, thereby reducing production costs and environmental burdens. At the same time, the functional layer adopts a fluorine-containing compound material, which is formed on the surface of the patterned layer and the micro-groove structure by spraying, thereby improving the demolding performance between the transfer film and the conductive slurry, and facilitating the subsequent easy separation from the silicon wafer without affecting the conductive grid line.

[0017] The multi-layered composite transfer film in this application also features an optimized thermal expansion coefficient design. The differences in thermal expansion coefficients between the layers are minimal, effectively reducing film stress caused by temperature fluctuations and ensuring the dimensional stability of the grid lines during the transfer and curing processes. Furthermore, by precisely controlling the thickness of each layer and the manufacturing process parameters, this application further ensures the high precision and excellent conductivity of the grid lines, providing a guarantee for improving the overall efficiency of photovoltaic cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic structural diagram of a multi-layer composite transfer film in one embodiment of the present application; Figure 2 This is a schematic flow chart of a method for preparing a multi-layer composite transfer film in one embodiment of the present application; Figure 3 This is a structural schematic diagram of a method for preparing a multi-layer composite transfer film in one embodiment of the present application; Figure 4 This is a schematic flow chart of a method for preparing a grid line of a cell in one embodiment of the present application; Figure 5 This is a structural diagram of a method for preparing a grid line of a cell in one embodiment of the present application; Explanation of reference numerals: 100, supporting layer; 110, mold; 200, self-healing adhesive layer; 300, patterned layer; 310, micro-groove structure; 400, functional layer; 500, conductive paste; 600, battery cell. DETAILED DESCRIPTION

[0019] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are described in detail below in conjunction with the accompanying drawings. It will be understood that the specific embodiments described herein are only used to explain the present application, rather than to limit the present application. It should also be noted that, for ease of description, only some, rather than all, structures related to the present application are shown in the accompanying drawings. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0020] As used herein, the terms "comprise," "comprising," and "having," and any variations thereof, are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or apparatus comprising a series of steps or elements is not limited to the listed steps or elements but may optionally include steps or elements not listed, or may optionally include other steps or elements inherent to the process, method, product, or apparatus.

[0021] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0022] The preparation process of the conductive grid lines of the cell is one of the core links that affect the photovoltaic conversion efficiency of the cell. The traditional grid line preparation method is often limited by factors such as process complexity and material utilization. In order to meet the photovoltaic industry's demand for further improvement in efficient production. This application proposes an innovative multi-layer composite transfer film and its preparation method, and develops a new type of cell grid line preparation technology based on this transfer film. The multi-layer composite transfer film can achieve high-precision transfer of grid line patterns, reusability of the transfer film, and environmental protection and energy saving in the preparation process through the selection of each layer structure and material. Compared with the existing technology, the technical solution of the present application not only reduces the transfer cost and environmental burden of the conductive paste for forming the grid lines, but also ensures the high precision and excellent conductive performance of the grid lines, providing a new solution for the efficient preparation of photovoltaic cells. The specific structure, preparation method and application of the multi-layer composite transfer film in the preparation of cell grid lines will be elaborated in detail below. Please refer to Figure 1 In a preferred embodiment of the present application, a multilayer composite transfer film includes a support layer 100, a self-healing adhesive layer 200, a patterned layer 300, and a functional layer 400. The hydrophobic angle of the support layer 100 surface is greater than 90°. The self-healing adhesive layer 200 is between the support layer 100 and the patterned layer 300, and the interface is a reversible adhesion interface. The patterned layer 300 includes a micro-groove structure 310 on the surface relative to the self-healing adhesive layer 200. The functional layer 400 is formed on the surface of the patterned layer 300 and the micro-groove structure 310.

[0023] The support layer 100 serves as the basic structure of the transfer film. A high-strength, high-stability polyimide material can be selected and modified with fluorosilane to make its surface hydrophobic angle greater than 90°, showing hydrophobic properties. The high hydrophobic surface not only enhances the hydrolysis resistance of the transfer film and extends its service life, but also improves the stability of the transfer film in complex environments and reduces performance fluctuations caused by changes in environmental humidity. At the same time, the high strength characteristics of the polyimide material ensure the structural integrity of the transfer film during multiple uses. The self-healing adhesive layer 200 is arranged between the support layer 100 and the patterned layer 300 to form a reversible adhesion interface. The adhesive layer can be made of a PVA-based hydrogel material with excellent self-healing ability. The design of the reversible adhesion interface enables the transfer film to fit closely to the surface of the battery cell 600 during the transfer process, ensuring the transfer of the grid line pattern. The self-healing properties of the PVA-based hydrogel further enhance the durability of the adhesive layer. Even if minor damage occurs after multiple uses, it can quickly self-repair and restore its original performance, thereby increasing the reuse rate of the transfer film.

[0024] The patterned layer 300 is a key part of the transfer film to realize the transfer of the grid line pattern. A micro-groove structure 310 is designed on the surface relative to the self-healing adhesive layer 200. These micro-groove structures 310 correspond to the raised structures on the surface of the mold 110. The presence of the micro-groove structure 310 enables the conductive paste 500 to be accurately filled therein to form a grid line structure that is completely consistent with the pattern of the mold 110. This high-precision pattern replication capability ensures the uniform distribution of the grid lines on the battery cell 600 and excellent conductive performance. The functional layer 400 is made of fluorine-containing compound material and is evenly formed on the surface of the patterned layer 300 and the micro-groove structure 310 by spraying. It is used to improve the demolding performance between the transfer film and the silver paste, so that the transferred grid lines can be easily detached from the transfer film and maintain a complete pattern structure. At the same time, the functional layer 400 also has excellent chemical stability and weather resistance, which can protect the grid lines from the influence of external environmental factors and extend the service life of the battery cell 600.

[0025] In the design of multi-layer composite transfer films, a key technical consideration is to ensure that the difference in coefficient of thermal expansion (CTE) between the support layer 100, the self-healing adhesive layer 200, the patterned layer 300, and the functional layer 400 is controlled to be less than 1×10⁻ 6 / ℃ range. The support layer 100 is made of polyimide (PI) material, while the self-healing adhesive layer 200 is made of PVA-based hydrogel. By regulating the chemical composition and microstructure of PI and PVA, the thermal expansion coefficients of the two are matched. In addition, in the interface design, by introducing specific chemical groups, the hydrogen bonding energy between PI and the adhesive layer is enhanced to be greater than 15 kJ / mol, thereby forming a stable interfacial chemical bond. The synergistic effect of this matching and bonding effectively reduces the interfacial stress caused by the mismatch of thermal expansion coefficients, ensuring the structural stability of the transfer film during thermal cycling, so that when the transfer film undergoes temperature changes, each layer can expand and contract synchronously, avoiding interlayer delamination or deformation caused by thermal stress, thereby ensuring the high precision and long-term stability of the grid line transfer.

[0026] A PVA layer is directly formed on the surface of the mold 110 using a spin coating / micro-gravure coating process. The uniformity and controllability of spin coating or micro-gravure coating are utilized to coat the PVA solution on the surface of the mold 110. A subsequent heat treatment or curing step is performed to form a micro-groove structure 310 on the surface of the mold 110 through the PVA layer. This molding method avoids the relative sliding or deformation between the mold 110 and the material that may occur in the traditional embossing process, thereby ensuring high-precision replication of the grid line pattern and enabling the groove depth deviation on the surface of the mold 110 to be less than 0.3 μm, which is much lower than the 2.5 μm deviation of the traditional embossing process. This avoids material deformation and pattern errors caused by uneven pressure distribution in the traditional embossing process, further improves the accuracy and consistency of the grid line transfer, and has the characteristics of high efficiency and speed. It can shorten the preparation cycle of the transfer film, improve production efficiency, and meet the needs of large-scale industrial production.

[0027] This application also regulates the thickness of each layer of the multi-layer composite transfer film. The specific parameters are as follows: the thickness of the support layer 100 is 20-50 μm, the thickness of the self-healing adhesive layer 200 is 2-20 μm, the thickness of the patterned layer 300 is 5-20 μm, and the thickness of the functional layer 400 is 0.1-0.5 μm. Within the aforementioned thickness range, the support layer 100 ensures the tear and bending resistance of the polyimide (PI) substrate, maintaining a deformation rate below 0.3% during high-speed transfer at 0.5 m / s. By increasing the entanglement density of the PI molecular chains, the Young's modulus is raised to above 4.5 GPa, ensuring structural stability under a lamination pressure of 0.5 MPa. The thermal conductivity of the support layer 100 is stabilized at 0.12-0.18 W / (m·K). Combined with the fluorosilane-modified layer, the support layer achieves a deformation of less than 0.5 μm after 1000 hours under the 85°C / 85% RH dual 85 test, ensuring the dimensional stability of the transfer film in hot and humid environments. Within this thickness range, the PI substrate's tolerance to winding tension fluctuations is improved to ±5%, making it compatible with 100 m / min high-speed coating equipment and reducing the film breakage rate by more than 0.3%.

[0028] The self-healing adhesive layer 200 forms a 15-18 kJ / mol hydrogen bond network with the PI support layer 100, achieving a reversible peel force of 0.3 N / cm². The cross-linking density of the PVA molecular chains can be optimized to achieve an interfacial shear strength of 0.8 MPa between the adhesive layer and the patterned layer 300, ensuring adhesion stability after 20 repeated transfers. Within the aforementioned thickness range, the adhesive layer achieves a microcrack (<10 μm) repair rate of >98% at 60°C / 30 minutes. By controlling the PVA side chain hydroxyl density (0.3-0.5 mol / g), a post-repair elastic modulus recovery rate of >95% can be achieved.

[0029] Within the aforementioned thickness, the patterned layer 300 ensures a sidewall perpendicularity of >88° for the micro-grooved structure 310, meeting the requirements for 15μm linewidth gateline replication. Through controlled PVA molecular chain orientation, the groove depth deviation is less than 0.2μm (a 92% improvement compared to traditional imprinting processes), and the gateline cross-sectional area error is less than 3%. This maintains a stable micro-grooved surface energy of 35-40mN / m. Combined with a silver paste thixotropic index (TI) of >3.5, the paste fill rate is >99%, reducing void defects by over 20%. The patterned layer 300 boasts a tensile strength of 18-22MPa and a deformation of <0.8% under a demolding pressure of 0.1MPa, ensuring the integrity of the gateline structure after transfer.

[0030] Through a thickness gradient design (support layer 100 > patterned layer 300 > self-healing adhesive layer 200 > functional layer 400), the interlayer stress of the transfer film during a 0-100°C temperature cycle is reduced to less than 0.5 MPa. This achieves a fill rate of 99.2% at a 100μm grid line spacing, a 2.3 percentage point improvement over conventional processes. The transfer time for a single 600-cell cell is shortened to 8 seconds. By thinning the support layer 100 (reducing PI usage by 15%), optimizing the thickness of the self-healing adhesive layer 200 (extending the service life by three times), and making the functional layer 400 ultra-thin (saving fluorine-containing materials by 40%), the cost per use of the transfer film is reduced.

[0031] This application uses PVA (polyvinyl alcohol) and Al³⁺ ions to form a dynamic network structure through coordination crosslinking, and utilizes the reversible breakage / recombination characteristics of the -O-Al-O- coordination bond to give the adhesive layer self-healing capabilities. The molar ratio of Al³⁺ to PVA hydroxyl groups is strictly controlled at 3:1. Experimental verification shows that at this ratio, the adhesive layer has an adhesion of 6-8 N / cm² to the PVA patterned layer 300. While taking into account the dissociation energy (20-30 kJ / mol) and recombination efficiency of the dynamic bond, it ensures a scratch healing rate of 92% (depth 50μm) within 3 minutes at 25°C. In addition to Al³⁺, Fe³⁺, Zr 4 High-valent metal ions such as ⁺ can also form similar dynamic networks, regulating the coordination bond strength through ionic radius and charge density to achieve a balance between self-healing efficiency (>95%) and adhesion strength (10 N / cm²).

[0032] When subjected to stress, the PVA-Al³⁺ coordination bonds preferentially break, dissipating energy. Once the force is removed, the Al³⁺ re-coordinates with adjacent hydroxyl groups, enabling self-repair at the interface. This ensures adhesion retention of >90% after 50 cycles (compared to traditional adhesive layers, which fail after only 2-3 cycles). By controlling the Al³⁺ concentration gradient, an adhesion gradient (6-10 N / cm²) is created at the interface between the adhesive layer and the PVA patterned layer 300, ensuring both high adhesion during transfer and controlled release during demolding. A PVA solution with a solids content of 10-20% (viscosity of 50-150 mPa·s) is applied to the mold 110 surface via slot coating or micro-gravure coating to form a 5-20μm thick coating with a groove depth deviation of ≤0.3μm (compared to 1.5μm deviation in traditional embossing processes). The thixotropic index (TI) of the PVA solution is greater than 3.5, ensuring that the viscosity is less than 50 mPa·s at low shear rates during coating (facilitating flow and filling) and greater than 500 mPa·s at high shear rates (preventing sagging), enabling precise replication of micron-scale groove structures.

[0033] Through three freeze / thaw cycles at -20°C, the PVA network is squeezed by ice crystal growth, forming through-hole micropores (pore diameter 50-200 nm). After freeze cycling, the tensile strength of the adhesive layer increases to 12-15 MPa (from 8-10 MPa), and the elongation at break exceeds 300%, meeting the dynamic requirements of high-speed transfer (>1 m / s). Adhesion retention exceeds 90% after 50 cycles, attributed to the reversible dissociation and recombination of Al³⁺ coordination bonds under stress, preventing the accumulation of permanent damage. Under cyclic loading at a peel force of 0.5 MPa, the cohesive energy density (G) of the adhesive layer decays by less than 5% per 100 cycles, surpassing the 30% per 10 cycles of conventional adhesives. The adhesive strength decay remains below 15% at 200°C. Thermal stability is enhanced by introducing heat-resistant groups (such as sulfonic acid groups) that form coordinated chelate rings with Al³⁺. Under the conditions of 200°C / 1000 hours, the adhesive layer mass loss rate is less than 2%, and the dynamic bond dissociation temperature (Td) is increased to above 250°C. Furthermore, a 50μm deep scratch heals at a rate of 92% in 3 minutes at 25°C. Micro-CT observations show that the repaired porosity is less than 1%. The adhesion to the PVA patterned layer reaches 10 N / cm², a 300% improvement over traditional adhesive layers, meeting the dynamic requirements of high-speed transfer printing (>5 m / s).

[0034] The self-healing adhesive layer 200 maintains adhesion >90% after 50 cycles, over 20 times higher than traditional adhesive layers. The cost of a single transfer film is reduced to 0.01 yuan per cycle, an 80% decrease compared to traditional solutions. Mold 110's in-situ coating technology reduces groove depth deviation to ≤0.3μm, an 80% improvement over traditional embossing processes. The transfer time for a single cell 600 is reduced to 5 seconds, increasing production capacity by 40%. The adhesive layer exhibits less than 15% bond strength degradation at 200°C. Furthermore, a dynamic coordination bond mechanism achieves scratch self-healing efficiency >95%, reducing maintenance costs.

[0035] The coefficient of thermal expansion (CTE) is 3.5×10⁻ 6 The PI film, which has a temperature of 1000 °C, is designed with molecular chain rigidity to achieve an elastic modulus of 3.2 GPa, a tensile strength of >200 MPa, and a deformation of <0.1% under a lamination pressure of 0.5 MPa, ensuring the dimensional stability of the multilayer structure during the transfer process. The PI surface is activated using O2 plasma (100 W power, 60 s), reducing the contact angle from 90° to 35° and increasing the surface energy to 45 mN / m, enhancing the chemical bonding with the self-healing adhesive layer (hydrogen bond density increased by three times). A 90° peel test shows that the shear strength of the fluorinated PI-adhesive layer interface reaches 8 MPa, a 400% increase compared to the unmodified interface, meeting the requirements of high-speed transfer (>5 m / s).

[0036] Functional layer 400 comprises materials such as fluorinated compounds, organosilicon compounds, wax compounds, or fatty acid salts. During its formation, a 1-2 nm thick CF functional layer is formed by spraying a 0.5 wt% ethanol solution of perfluorooctyltriethoxysilane (FAS-17) onto the patterned layer 300. This layer improves the contact angle from 72° to 115° and reduces the surface energy to 12 mN / m. By controlling the FAS-17 concentration, the contact angle can be adjusted to meet the release requirements of different silver paste systems (for example, low-temperature silver paste requires a contact angle >110°). The fluorinated functional layer 400 reduces the silver paste adhesion from 6.8 N / cm² to 1.2 N / cm² (an 82% reduction), while maintaining a bond strength between the silver paste and the gate structure >15 MPa (verified by shear testing), ensuring uncompromised conductive performance. After immersion in an organic silver paste vehicle (such as ethyl cellulose) for 100 hours, the functional layer 400 exhibits a mass loss of <0.5% and a CF bond coverage >95%, ensuring long-term process stability.

[0037] The synergistic effect of the fluorinated functional layer 400 and the fluorinated PI support layer 100 shortens the demolding time of the single-cell 600, improving production capacity compared to traditional processes. Through PI molecular chain design and fluorosilane self-assembly technology, the mechanical properties, thermal stability and interface desorption properties are synergistically optimized. In addition, a three-level mechanical model of "support layer 100 stiffness - adhesive layer adhesion - functional layer 400 desorption" can be established to quantify the impact of each layer parameter on the transfer yield. In addition, organosilicon compounds, wax compounds or fatty acid salts also use similar methods to form the functional layer 400 on the surface of the patterned layer 300.

[0038] Also, see Figure 2-Figure 3 The present application further provides a method for preparing a multi-layer composite transfer film, comprising the following steps: mold pretreatment: forming a protruding structure on the surface of the mold 110, wherein the protruding structure corresponds to the micro-groove structure 310 in the transfer film, forming a patterned layer 300 on the surface of the mold 110, coating the self-healing adhesive layer 200 on the support layer 100, and orienting the self-healing adhesive layer 200 toward the patterned layer 300, laminating a first composite layer comprising the support layer 100 and the self-healing adhesive layer 200 on the patterned layer 300, demolding a second composite layer comprising the support layer 100, the self-healing adhesive layer 200 and the patterned layer 300 from the surface of the mold 110, wherein the patterned layer 300 includes a micro-groove structure 310, and forming a functional layer 400 on the surface of the patterned layer 300 and the micro-groove structure 310. The patterned layer 300 can be formed by solution casting or transfer printing, where the film material used to form the patterned layer is directly bonded to the self-healing layer and the support layer. The functional layer 400 can be formed by spraying a fluorine-containing compound, an organosilicon compound, a wax compound, or a fatty acid salt onto the surface.

[0039] Mold 110 is made of a high-precision nickel-based alloy (hardness > 50 HRC) with a surface roughness Ra < 0.05μm. The raised structures are formed through an electroforming process with a dimensional accuracy of ±0.1μm and a distribution density of ≥1000 PPI (corresponding to the micro-grooves 310 on the transfer film). The raised surfaces of mold 110 are hydrophobicized to reduce the risk of PVA solution residue. The dimensional error between the raised structures of mold 110 and the micro-grooves on the transfer film is ≤0.2μm, ensuring gate line width consistency of ±0.5μm. The hydrophobicization treatment reduces the demolding force to 0.1 N / cm², compared to the demolding force of 1.2 N / cm² for the untreated mold 110, thus preventing damage to the PVA patterned layer 300.

[0040] A 10-20wt% PVA solution (viscosity 50-150 mPa·s) was used, and 0.01-0.5% cross-linking agent (such as glutaraldehyde) was added. A multi-stage spin coating process was adopted: the first stage (spreading): 500 rpm×10 seconds, centrifugal acceleration 200 g, to evenly spread the solution to the surface of the mold 110 to eliminate edge accumulation. The second stage (film thickness control): 1000-3000 rpm×20 seconds, through the speed gradient (such as 1000→3000 rpm linear deceleration) to control the film thickness to 5-20μm, with a thickness uniformity of ±0.3μm. The curing process: hot air drying at 80℃ for 1-3 minutes, combined with infrared radiation heating (wavelength 800-1000 nm) to form hydrogen bonds between the PVA molecular chains.

[0041] Alternatively, a flat-plate coating solution can be used, with a coating speed of 0.5 m / min. By synergistically controlling the blade pressure (0.1-0.3 MPa) and the drying temperature (60-80°C), a groove structure complementary to the protrusions on mold 110 can be formed, with a depth error of ≤0.3 μm. Combining spin coating with the coating process reduces the groove depth error from 1.5 μm in traditional processes to ≤0.3 μm. Spin coating and other coating methods can be used to directly form a PVA pattern layer on the surface of mold 110, avoiding the stress relaxation issues of traditional stamping and reducing the groove depth error to <5%.

[0042] A PVA-Al³⁺ hydrogel (solid content 15-25wt%) is cast onto the surface of a PI support layer 100 (25μm thickness) to a coating thickness of 10-30μm. A CCD vision positioning system ensures alignment error between the PI adhesive layer and the PVA patterned layer 300 of less than 5μm. Lamination pressure is 0.1-0.3 MPa, with a dwell time of 10-30 seconds, ensuring the adhesive layer fully fills the micro-grooves of the patterned layer 300. A fluorinated modified layer (surface energy 18 mN / m) reduces the demolding force to 0.05 N / cm², preventing damage to the patterned layer 300. A demolding speed of 0.5-2 m / min is used, combined with cryogenic cooling (pre-cooling at -10°C) to reduce residual thermal stress, resulting in a composite structural integrity of over 99% after demolding. The adhesive layer achieves strong adhesion at high temperatures through dynamic coordination between the -OH groups of Al³⁺ and PVA, allowing for multiple uses and extending the life of the film.

[0043] The fluorinated functional layer 400 is deposited using ultrasonic spraying at a frequency of 40 kHz and an amplitude of 50 μm, ensuring a droplet size of less than 10 μm. The spraying distance is 20 cm, the spray angle is 45°, and the flow rate is 0.5 mL / min, resulting in a uniform, nanoscale functional layer 400. The functional layer 400 is made of perfluorodecyltriethoxysilane (FAS-13) at a concentration of 0.5 wt%. Through a hydrolysis-condensation reaction, a C-F bond coating is formed on the PVA surface, resulting in a stable thickness of 50-100 nm. This improves the contact angle to 115° and reduces the peel force from 6.8 N / cm² to 1.2 N / cm², an 82% reduction. Meanwhile, the bond strength between the silver paste and the gate lines remains above 15 MPa. The dimensional matching error between the raised structures on the mold 110 and the PVA grooves is ≤0.2 μm, and the gate line width consistency is ±0.3 μm, meeting the <15 μm line width requirement for TOPCon cells.

[0044] Specifically, the self-healing adhesive layer 200 undergoes a freeze cycle before coating. The process includes a pre-freezing stage, in which the PVA-Al³⁺ hydrogel adhesive (solid content 15-25wt%) is placed in a low-temperature environment of -30°C and cooled at a rate of 2°C / min to prevent structural damage caused by rapid ice crystal growth. The adhesive is kept at -30°C for 12 hours to allow the free water in the adhesive to completely crystallize and form a uniform ice crystal network (ice crystal size <5μm). The temperature is then raised to room temperature (25°C) at a rate of 1°C / min to slowly melt the ice crystals, release internal stress, and prevent pore collapse. The freeze-thaw process is repeated three times to strengthen the crosslinking density of the internal network of the adhesive layer.

[0045] The elastic modulus increased from 0.8 MPa to 1.5 MPa (an 87.5% increase), enhancing the adhesive layer's tolerance to mechanical stress during the transfer process. The elongation at break increased from 200% to 350%, preventing brittle fracture during demolding. The bond strength to the PI support layer 100 was increased from 4 MPa to 6 MPa, ensuring the stability of the composite structure during high-speed transfers (>5 m / s). This treatment enhances self-healing capabilities. At 80°C and 1 MPa pressure, the healing efficiency increased from 70% to 90% within 10 minutes (verified by scratch repair experiments). The number of repeatable healing cycles increased from 5 to 15, extending the service life of the adhesive layer.

[0046] Specifically, the specific steps of forming the patterned layer 300 include applying a solution on the mold surface and curing it. The applied solution is PVA solution, PEG solution, PEO solution, PVP solution or a mixed solution of the above solutions. The curing step is hot air drying at a temperature in the range of 50-100°C.

[0047] See also Figure 4-Figure 5The present application further provides a method for preparing grid lines of a battery cell 600, comprising the following steps: using the aforementioned multi-layer composite transfer film, filling the grooves on the multi-layer composite transfer film with a conductive paste 500, transferring and curing the conductive paste 500 on the surface of the battery cell 600, and tearing the film: tearing the multi-layer composite transfer film off the surface of the battery cell 600 to separate it from the battery cell 600, and sintering the conductive paste 500.

[0048] The technical solution of this application adopts a fully dry process chain, without water from filling the slurry, transferring to tearing the mold, and the process time can be shortened by 40%. The most important point of this solution is that it can be torn out from the surface of the mold 110 when a very thin layer of PVA (8-20um) is applied in the early stage. In addition, after the transfer is completed in the later stage, the multi-layer film will not separate within the layer, and will have a certain adhesion to the battery cell 600, ensuring good adhesion between the filled silver paste and the silicon wafer. Finally, when tearing off the composite transfer film, the silver paste will not be brought out, that is, the groove surface in the film and the silver paste are easy to demold.

[0049] This application has the advantage of microstructure conformality. The direct coating process of mold 110 achieves a groove replication accuracy of 98.7%, which is 12% higher than the embossing process. At the same time, it ensures environmental compatibility. The whole process drying process reduces energy consumption by 65% and reduces wastewater discharge. The technical solution of this application proposes a multi-layer structure transfer film based on the synergistic effect of the self-healing adhesive layer. Through the four-layer functional stacking design, it realizes the dry transfer of silver paste grid lines. Different from the traditional water washing / high temperature process, it uses a dry stripping mechanism. The interface energy of fluorine-containing compounds is used to control the adhesion size with the silicon wafer to achieve solvent-free stripping. By directly coating PVA on the surface of mold 110 to form complementary grooves, the traditional embossing error is eliminated. Specific embodiments Some specific implementation methods will be further introduced below to further explain the technical solution of this application in detail.

[0051] The adhesive layer was prepared using dynamic coordination adhesive layer technology: a 10wt% PVA1799 solution (doped with AlCl3) containing an Al³⁺ / OH⁻ molar ratio of 0.3 was subjected to three -25°C freeze cycles (thawing temperature 25°C, cycle period 2 hours / time), and then evenly coated on the surface of a 25μm thick modified polyimide (PI) support film using a slit coating process to form an adhesive layer with a thickness of 15μm and a cross-linking degree of 88%. The pattern layer was constructed using a mold direct coating process: a 10-20wt% PVA solution was spin-coated on the surface of a silicon mold at 500 rpm (coating time 30 seconds). After thermal curing at 80°C, the deviation between the groove depth and the mold protrusion height was less than 5% (depth 10-15μm, sidewall angle 85-90°). Transfer verification showed that after filling with silver paste, under 1-20 MPa gradient pressure and 80-180°C hot pressing conditions, the interface adhesion strength between the adhesive layer and the silicon wafer reached 12 N / cm². After cooling to room temperature, the composite film can be peeled off without damage (peeling force 1.0 N / cm²). After the silver paste is cured, the aspect ratio of the grid line reaches 0.75-3.0 (line width 3-20μm), and the resistivity is 1.6 μΩ·cm, which is suitable for the high-efficiency grid line preparation requirements of TOPCon / HJT batteries.

[0052] In the comparative example, PLA was used entirely to form the transfer film, and a mold was used to form the grooves.

[0053] By comparing the test data of the traditional PLA film and the specific embodiment, it can be seen that the specific embodiment has achieved improvements in gate line accuracy, material utilization, process stability and environmental protection: in terms of gate line width consistency, the embodiment reaches ±0.3μm (traditional process ±1.5μm), the accuracy is improved by 80%, adapted to the preparation requirements of ultra-fine gate lines below 12μm, and the risk of gate line short circuit is reduced; in terms of silver paste utilization, the silver paste utilization of the embodiment is increased to 98% (traditional process 90%), reducing silver paste waste by about 9%, and reducing the cost of a single cell by 0.012 yuan (calculated based on the unit price of silver paste of 1.2 yuan / mg); and as for the number of times the composite film is repeated, the embodiment can be reused ≥50 times (traditional process only 5 times), and the cost of the support layer is allocated to a single use of only 0.016 yuan (traditional process 0.24 yuan), a reduction of 93%; in terms of the resistivity of the formed gate line, the resistivity of the embodiment is reduced to 1.8 μΩ·cm (2.5μΩ·cm for traditional process), the conductivity is improved by 28%, and the cell fill factor (FF) is improved by 0.4%. In addition, the collapse ratio of the embodiment reaches 0.9 (0.7 for traditional process), the verticality of the gate line sidewall is improved by 29%, the shading area is reduced by 1.2%, the photoelectric conversion efficiency is improved by 0.15%, the transfer yield of the embodiment reaches 98.5% (83% for traditional process), and the single-line production capacity is increased by 18.7% (calculated based on transferring 1,200 wafers per hour). In terms of residue content, the residual silver paste on the surface of the silicon wafer of the embodiment is only 0.02 mg / cm² (0.7 mg / cm² for traditional process), reducing the dark film rate of the EL test to 0.3% (3.2% for traditional process).

[0054] As can be seen from the foregoing, this application provides a multi-layer composite transfer film and its preparation method, as well as a method for preparing solar cell grid lines based on the transfer film. The multi-layer composite transfer film comprises a support layer, a self-healing adhesive layer, a patterned layer, and a functional layer. Each layer is designed through specific material selection and process design to achieve high-precision grid line transfer and efficient reuse.

[0055] The support layer is constructed from high-strength polyimide (PI) material modified with fluorosilane, resulting in a surface hydrophobic angle greater than 90°, exhibiting excellent hydrolysis resistance and environmental stability. The self-healing adhesive layer, located between the support layer and the patterned layer, forms a reversible adhesion interface. This layer utilizes a PVA-based hydrogel material with excellent self-healing capabilities. Through Al³⁺ ion coordination and crosslinking, it forms a dynamic network structure, enabling scratch self-healing and multiple reuse. The patterned layer features a micro-grooved structure that aligns with the mold's raised structure, ensuring precise filling of the conductive paste and forming a high-precision grid line structure. The functional layer, constructed from a fluorine-containing compound material, is applied by spraying onto the patterned layer and micro-grooved surfaces, enhancing demolding performance and protecting the grid lines from environmental influences.

[0056] The thickness of each layer is precisely controlled, with the support layer thickness of 20-50μm, the self-healing adhesive layer of 2-20μm, the patterned layer of 5-20μm, and the functional layer of 0.1-0.5μm, ensuring the structural integrity and dimensional stability of the transfer film during high-speed transfer. By controlling the difference in thermal expansion coefficient of each layer to less than 1×10⁻ 6 / ℃, and enhance interfacial chemical bonding to reduce interlayer delamination or deformation caused by thermal stress.

[0057] The preparation method includes mold pretreatment, patterning layer formation, self-healing adhesive layer coating, composite layer lamination and demolding, and functional layer spraying. Using a high-precision nickel-based alloy mold, the PVA pattern layer is directly formed on the mold surface via spin coating or coating, avoiding the stress relaxation issues associated with traditional stamping and improving the accuracy and consistency of grid line transfer. The self-healing adhesive layer undergoes a freeze cycle before coating to strengthen the internal network crosslink density, increase the elastic modulus and elongation at break, and enhance resistance to mechanical stress.

[0058] The preparation method of the cell grid line adopts a fully dry process chain, without water involved in the entire process from filling the slurry, transferring to tearing the mold, and shortening the process time by 40%. Through the microstructure conformality advantage of the multi-layer composite transfer film, the mold direct coating process achieves a groove replication accuracy of 98.7%, which is 12% higher than the stamping process, while ensuring environmental compatibility. The whole process drying process reduces energy consumption by 65% and reduces wastewater discharge. The technical solution of this application realizes the dry transfer of silver paste grid lines through a four-layer functional stacking design, and uses the interface energy of fluorine-containing compounds to control the adhesion size with the silicon wafer, realizing solvent-free peeling and improving production efficiency and product quality.

[0059] The above is only a specific implementation of the present application. Any other improvements made based on the concept of the present application are considered to be within the scope of protection of the present application.

Claims

1. A multi-layer composite transfer film, characterized in that: It includes a support layer, a self-healing adhesive layer, a patterned layer, and a functional layer; The hydrophobic angle of the support layer surface is greater than 90°; The self-healing adhesive layer is between the support layer and the patterned layer, and the interface is a reversible adhesion interface; The patterned layer comprises a micro-groove structure on a surface opposite to the self-healing adhesive layer; The functional layer is formed on the surface of the patterned layer and the micro-groove structure.

2. The multi-layer composite transfer film according to claim 1, characterized in that: The difference in thermal expansion coefficients among the support layer, the self-healing adhesive layer, the patterned layer, and the functional layer is less than 1×10⁻ 6 / ℃.

3. The multi-layer composite transfer film according to claim 1, characterized in that: The thickness of the support layer is in the range of 20-50 μm, the thickness of the self-healing adhesive layer is in the range of 2-20 μm, the thickness of the patterned layer is in the range of 5-20 μm, and the thickness of the functional layer is in the range of 0.1-0.5 μm.

4. The multi-layer composite transfer film according to claim 1, characterized in that: The material of the self-healing adhesive layer is PVA-based hydrogel.

5. The multi-layer composite transfer film according to claim 4, characterized in that: The material of the support layer is polyimide, and the surface is modified by fluorosilane.

6. The multi-layer composite transfer film according to claim 5, characterized in that: The material of the functional layer includes fluorine-containing compounds, organic silicon compounds, wax compounds or fatty acid salts.

7. A method for preparing a multi-layer composite transfer film, characterized in that: The steps include: Mold pretreatment: forming a raised structure on the mold surface, the raised structure corresponding to the micro-groove structure in the transfer film; ‌Forming a patterned layer on the mold surface; applying the self-healing adhesive layer on the support layer; The self-healing adhesive layer is placed toward the patterned layer, and a first composite layer including the support layer and the self-healing adhesive layer is attached to the patterned layer; Demolding the second composite layer comprising the support layer, the self-healing adhesive layer and the patterned layer from the mold surface, wherein the patterned layer comprises a micro-groove structure; A functional layer is formed on the surface of the patterned layer and the micro-groove structure.

8. The method for preparing a multi-layer composite transfer film according to claim 7, wherein: The self-healing adhesive layer is subjected to a freeze cycle treatment before coating.

9. The method for preparing a multi-layer composite transfer film according to claim 7, wherein: The specific steps of forming the patterned layer include applying a solution on the mold surface and curing it. The applied solution is PVA solution, PEG solution, PEO solution, PVP solution or a mixed solution of the above solutions. The curing step is hot air drying at a temperature in the range of 50-100°C.

10. A method for preparing a grid line of a battery cell, characterized in that: The steps include: Using the multi-layer composite transfer film according to any one of claims 1 to 6; Filling the grooves on the multi-layer composite transfer film with conductive paste; Transfer and solidify the conductive paste on the surface of the battery cell; Film tearing: tearing off the multi-layer composite transfer film from the surface of the battery cell to separate it from the battery cell; Sintering the conductive paste.

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