Shape memory transfer printing film, preparation method thereof and battery piece grid line preparation method

Abstract

The invention relates to a shape memory transfer film, which takes a reusable and biodegradable polylactic acid base film as a core material, realizes automatic separation of a carrier film under specific conditions through a shape memory effect, abandons a traditional demolding mode, adjusts flexibility and processability through a plasticizer, improves mechanical properties and thermal stability through a filler, and reduces cost. And the cross-linking agent improves the shape memory effect and durability. The preparation method comprises the following steps: uniformly mixing the components by adopting a melting processing method or a solution casting method, forming a film, initially shaping at a temperature above a glass-transition temperature, cooling to a temperature below the glass-transition temperature, and forming a hydrophobic layer before initial shaping treatment. The invention further provides a preparation method of the cell grid line, dry transfer printing and automatic separation are achieved through the thermal triggering shape recovery effect based on the shape memory transfer printing film, the process time is shortened, the cell grid line can be recycled and environmentally friendly, the precision and quality of the silver paste grid line can be improved, the resistivity can be reduced, and the performance of a solar cell can be improved.

Description

Technical Field

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

[0002] In the field of photovoltaic cell technology, the high-precision preparation of conductive grid lines (especially grid lines based on silver paste, silver-coated copper, or low-silver-content silver paste) is a key step in achieving high-efficiency cell conversion efficiency. As the photovoltaic industry continues to pursue cell efficiency and cost control, the preparation process of conductive grid lines has become a key focus of technological innovation. Currently, the latest developments in the industry are focusing on the use of water-soluble transfer film technology, such as transfer films based on water-soluble polymer materials such as polyvinyl alcohol (PVA), polyethylene glycol (PEG), and polyvinyl pyrrolidone (PVP), to achieve precise transfer of grid line patterns. However, although this technical approach theoretically shows the potential to simplify the process flow, practical applications still face challenges.

[0003] First, the process complexity increases. The use of traditional water-soluble transfer film requires the subsequent removal of the carrier film by water washing or high-temperature burning. This step not only increases the complexity of the production process, but also the water washing process will produce a large amount of wastewater, which puts a burden on the environment. High-temperature combustion not only consumes a lot of energy, but may also produce harmful gas emissions, which is not in line with the development trend of green manufacturing. Secondly, there is also the problem of material waste. As a disposable material, the carrier film is discarded after completing the transfer task. This not only increases production costs, but also goes against the current concept of circular economy and sustainable development, exacerbating resource consumption and environmental pollution. In addition, there is also a compatibility risk. In the process of removing the carrier film, whether using solvent water washing or high-temperature burning, it is possible to have an adverse effect on the surface of the silicon wafer or the performance of the silver paste. For example, solvent residues may corrode the silicon wafer, and high-temperature treatment may change the microstructure of the silver paste, affecting its conductive properties, and thus affecting the overall efficiency and stability of the battery.

[0004] Therefore, developing a new transfer film technology that can maintain high-precision gridline fabrication capabilities while effectively addressing the aforementioned issues is crucial for further advancements in photovoltaic cell technology. This requires innovation in material selection, fabrication processes, and transfer mechanisms to achieve a more efficient, environmentally friendly, and highly compatible conductive gridline fabrication solution. Summary of the Invention

[0005] The purpose of this application is to provide a shape memory transfer film and a fully dry process that does not require water washing or high-temperature burning. The carrier film is automatically detached through the shape memory effect, and a reusable and biodegradable transfer film is used to reduce costs and environmental burdens, ensuring high-precision transfer of silver paste grid lines (the bottom width of the grid lines can be as low as 3μm) and conductive performance.

[0006] The purpose of the present application is achieved through the following technical solution. The shape memory transfer film of the present application comprises the following components in parts by weight: Polylactic acid 70%-85%; Plasticizer 0.5%-10%; Filler 1%-10%; Cross-linking agent 0.1%-5%; Among them, the thickness of the shape memory transfer film is in the range of 50-500μm, and the glass transition temperature is in the range of 50℃-60℃. During imprinting, the shape memory transfer film will maintain the deformed shape after deformation. When heated again to a temperature higher than the glass transition temperature, the shape memory transfer film will restore its original shape.

[0007] In one embodiment, the invention further comprises a hydrophobic layer formed on the surface of the shape memory transfer film.

[0008] In one embodiment, the filler further includes a light-to-heat conversion material.

[0009] In one embodiment, the filler further includes a conductive material.

[0010] The present application further provides a method for preparing a shape memory transfer film, comprising the following steps: Prepare the following components in parts by weight: Polylactic acid 70%-85%; Plasticizer 0.5%-10%; Filler 1%-10%; Cross-linking agent 0.1%-5%; The components are uniformly mixed and formed into a film by melt processing or solution casting; Initial shaping is performed at a temperature above the glass transition temperature; Cooling to below the glass transition temperature; The glass transition temperature is in the range of 50°C to 60°C.

[0011] In one embodiment, the method further includes forming a hydrophobic layer on the surface before performing the initial shaping.

[0012] In one embodiment, the transfer layer has a thickness in the range of 50-500 μm.

[0013] In one embodiment, the glass transition temperature is maintained for more than 10 minutes.

[0014] The present application further provides a method for preparing a grid line of a cell, comprising the following steps: Using the aforementioned shape memory transfer film, embossing grooves on the shape memory transfer film; filling the groove with metal slurry; Transfer and solidify the metal paste on the surface of the battery cell; Performing heating demolding: heating the shape memory transfer film again to a temperature higher than the glass transition temperature, and removing the shape memory transfer film; Sintered metal slurry.

[0015] In one embodiment, the width of the groove is in the range of 5-100 μm, and the depth of the groove is in the range of 5-50 μm.

[0016] Compared with the prior art, this application has the following beneficial effects: This application utilizes a transfer film with a shape-memory effect, achieving a dry process for producing solar cell grid lines without the need for water washing or high-temperature combustion. During the initial impression, the transfer film deforms and maintains this shape, facilitating the formation of grooves. Above the glass transition temperature, the film automatically returns to its original shape, allowing for automatic detachment from the carrier film. This not only simplifies the production process and reduces energy consumption, but also avoids wastewater pollution from water washing and potentially harmful gas emissions from high-temperature combustion, aligning with the development trend of green manufacturing.

[0017] This application's shape memory transfer film utilizes a reusable, biodegradable polylactic acid (PLA) base film as its primary component, reducing material costs and the environmental burden of waste. Compared to traditional single-use carrier films, this transfer film maintains its performance after multiple uses, further enhancing both economic and environmental benefits.

[0018] By precisely controlling the thickness (30-500μm) and glass transition temperature (50℃-60℃) of the transfer film, and optimizing the width (3-100μm) and depth (5-50μm) of the grooves, this application achieves high-precision transfer of silver paste grid lines, with the bottom width of the grid lines being as low as 3μm, while ensuring the excellent conductive properties of the grid lines, providing support for improving the conversion efficiency of photovoltaic cells.

[0019] Photothermal conversion and conductive materials enable diverse shape recovery methods: The addition of photothermal conversion and conductive materials to the filler further diversifies the transfer film's shape recovery methods. The shape memory effect is induced by localized heating through near-infrared light irradiation or Joule heating after power is applied, providing more options for using the transfer film in different application scenarios. This flexibility not only increases the transfer film's applicability, but also effectively prevents moisture absorption by forming a hydrophobic layer on the transfer film surface, preventing the degradation of shape memory performance caused by moisture absorption. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 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 2 This is a structural diagram of a method for preparing a grid line of a cell in one embodiment of the present application; Figure 3 It is a schematic diagram of the cross-sectional structure during the process of preparing the grid line of a battery cell in one embodiment of the present application.

[0021] Explanation of reference numerals: 100, shape memory transfer film; 110, groove; 200, metal paste; 300, battery cell. DETAILED DESCRIPTION

[0022] 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.

[0023] 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.

[0024] 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.

[0025] As photovoltaic cell technology continues to pursue high efficiency and low cost, the precise preparation of conductive grid lines has become a key link in improving cell performance. Although the use of water-soluble transfer film simplifies the grid line transfer process to a certain extent, it relies on water washing or high-temperature burning to remove the carrier film, which not only increases the process complexity and energy consumption, but also causes environmental problems such as wastewater discharge and harmful gases. At the same time, disposable carrier films also cause material waste and increased costs. In view of this, it is necessary to develop a new transfer film technology that can maintain high-precision grid line preparation capabilities while effectively reducing environmental burdens and costs.

[0026] This application provides a shape memory transfer film, which uses a reusable, biodegradable polylactic acid (PLA) base film as its core material, combined with a specific plasticizer, filler, and crosslinker formulation. This not only improves material cost and environmental performance, but also, through a unique shape memory effect, enables the carrier film to automatically detach under specific conditions, thereby eliminating traditional demolding methods such as water washing or high-temperature burning. The following details the shape memory transfer film and its preparation method, including component selection and processing techniques, and describes its specific application in the preparation of solar cell grid lines. In a preferred embodiment of this application, the shape memory transfer film comprises the following components in parts by weight: Polylactic acid 70%-85%; Plasticizer 0.5%-10%; Filler 1%-10%; Cross-linking agent 0.1%-5%; The thickness of the shape memory transfer film is within the range of 50-500 μm, and the glass transition temperature is within the range of 50°C-60°C. The shape memory transfer film will maintain its deformed shape after deformation during imprinting. When the temperature is higher than the glass transition temperature, the shape memory transfer film 100 returns to its original shape.

[0027] Polylactic acid, as the main base material, accounts for 70% to 85% by weight. With its excellent biocompatibility, degradability and good mechanical properties, it provides a solid foundation for the shape memory transfer film 100 and ensures the basic strength and stability of the material. The plasticizer is added in an amount of 0.5% to 10% to adjust the flexibility and processing properties of polylactic acid, making the transfer film easier to operate during the molding process and improving its adaptability at different temperatures. Fillers account for 1% to 10% by weight. The addition of fillers can improve the mechanical properties, thermal stability and cost of the transfer film. The specific type of filler can be selected according to application requirements. The cross-linking agent is added in trace amounts of 0.1% to 5%. Through chemical reactions, a cross-linking structure is formed between the polylactic acid molecular chains, which significantly improves the shape memory effect and durability of the transfer film, ensuring that it can maintain stable performance in multiple deformation-recovery cycles.

[0028] In terms of physical properties, the thickness of the shape memory transfer film 100 is controlled between 50 and 500 microns (μm), ensuring sufficient mechanical strength while also being easy to handle and apply. Furthermore, its glass transition temperature (Tg) is within the range of 50°C to 60°C, a temperature window critical for the shape memory effect. During imprinting, after being deformed by an external force, the transfer film is able to stably maintain its deformed shape. This temperature can be controlled within a range exceeding the glass transition temperature. After the transfer is complete, when the ambient temperature is raised above Tg again, the transfer film automatically returns to its original shape, enabling removal of the transfer film (transfer template) through its shape memory properties.

[0029] In one specific embodiment, polylactic acid (PLA) serves as the shape memory matrix material, accounting for 80%. By adding plasticizers, fillers, and crosslinkers, the glass transition temperature (Tg) is raised to approximately 60°C. This enables the shape memory effect to occur at a specific temperature, imparting thermal responsiveness and enabling reversible shape changes under varying temperatures. Glycerol, a plasticizer selected at a level of 0.5-2%, interacts with the PLA molecules, effectively lowering the PLA's glass transition temperature to 50-55°C, enhancing the material's flexibility. This allows the material to maintain good bending and deformation capabilities even at lower temperatures, improving its practicality and processing performance while also lowering processing temperature requirements and saving energy costs. Nano-silica (SiO2), a filler selected at a level of 0.5-2%, is evenly dispersed within the PLA matrix, forming a network structure that enhances the material's mechanical properties, such as tensile strength and flexural strength. Furthermore, it effectively prevents deformation and cracking during use, extending the material's service life, improving its reliability and durability, and making it suitable for demanding engineering applications. Dicumyl peroxide (DCP) is added as a crosslinking agent at a level of 0.1%. During processing, it triggers a crosslinking reaction between the polylactic acid molecular chains, forming a three-dimensional network structure. This allows the material to maintain relatively stable performance during multiple thermal deformation and recovery processes, enhancing its recyclability and reducing material loss and cost. The material as a whole possesses excellent shape memory and thermal responsiveness, enabling shape control and rapid response to temperature changes. Polyurethane can also be used as the remaining component, bringing the total content of all components to 100%.

[0030] The structure also includes a hydrophobic layer, which is uniformly and tightly formed across the entire surface of the shape-memory transfer film 100, forming a continuous, dense protective film. This layer effectively blocks moisture intrusion and prevents performance degradation of the shape-memory transfer film 100 due to moisture absorption. It also ensures that the shape-memory transfer film 100 maintains its excellent shape memory properties and transfer accuracy under various complex environmental conditions, thereby extending its service life and improving the quality and stability of the transferred product. The hydrophobic layer has a low surface energy, making it difficult for moisture to spread and penetrate its surface, effectively preventing moisture absorption and blocking the impact of moisture on the internal structure of the shape-memory transfer film 100, ensuring that the accuracy and stability of the shape memory are maintained even after moisture absorption. The hydrophobic layer reduces moisture interference with the material's internal stress, molecular structure, and other microscopic aspects, allowing the shape memory mechanism to function more stably. This has a direct technical effect on improving product yield and reducing defective product rates, reducing sensitivity to changes in ambient humidity and enabling stable operation over a wider temperature and humidity range.

[0031] The filler further includes a photothermal conversion material that efficiently absorbs energy in the near-infrared (NIR) region and converts it into heat. In applications where shape memory transfer film 100 requires shape recovery of a specific area, the target area can be illuminated by a near-infrared light source (such as a laser or near-infrared LED array). The photothermal conversion material quickly absorbs the light energy and generates a localized temperature increase. This localized heating effect triggers the shape memory mechanism within the shape memory transfer film 100, causing it to return to its original shape in a predetermined manner. By controlling the location, intensity, and duration of the near-infrared light, the shape recovery area of the shape memory transfer film 100 can be precisely positioned and controlled. The photothermal conversion material generates heat only in the illuminated area, avoiding unnecessary heating of surrounding, unexposed areas. By heating only the target area, energy waste is significantly reduced compared to traditional heating methods, and the effects of high temperatures on the performance of the cell 300 are minimized. During the shape recovery process of the shape memory transfer film 100 , the heat generated by the photothermal conversion material is uniform and controllable, avoiding material performance degradation or damage caused by local overheating or excessive temperature gradient, thereby improving the stability and reliability of the shape memory transfer film 100 .

[0032] Specifically, the filler may also include a conductive material. The conductive material has excellent electrical conductivity and can form a stable current path when powered. When current is applied to the shape memory transfer film 100, the current passes through the conductive material. Due to the conductive material's inherent resistance, the interaction between the current and resistance generates Joule heat. The Joule heat can evenly and effectively act on the interior of the shape memory transfer film 100, causing its temperature to rise. When the temperature again exceeds the transition temperature of the shape memory material, the shape memory effect is triggered, causing the transfer film to deform and recover according to the preset shape. This internal heating method can more directly act on the shape memory material, greatly shortening the time required to reach the transition temperature, thereby efficiently triggering the shape memory effect. By precisely adjusting the magnitude of the current, the duration of the current flow, and the distribution of the conductive material in the filler, the amount of Joule heat generated and the distribution range can be precisely controlled, making the shape recovery process of the shape memory transfer film 100 highly controllable and capable of precise deformation according to a predetermined shape and degree. Since Joule heat is generated by the electrical resistance of the conductive material itself, the energy conversion efficiency is high, and electrical energy can be more effectively converted into thermal energy, reducing energy consumption. At the same time, the use of conductive materials can also reduce dependence on external heating equipment.

[0033] In a specific technical solution, carbon nanotubes (CNTs) can be added in an amount between 0.1-0.5%. These materials have excellent thermal conductivity and, when added to the polylactic acid matrix, form heat conduction channels, accelerating heat conduction. This enhances the material's thermal response speed, enabling it to respond more quickly to temperature changes, improving its thermal response efficiency and applicability. Additionally, a 1% environmentally friendly masterbatch can be added. Its primary function is to provide color to the material, enabling visual positioning and facilitating accurate alignment of process positions during processing, thereby improving processing accuracy and efficiency. Furthermore, this material meets environmental protection requirements and reduces environmental pollution.

[0034] The present application further provides a method for preparing a shape memory transfer film, comprising the following steps: Prepare the following components in parts by weight: Polylactic acid 70%-85%; Plasticizer 0.5%-10%; Filler 1%-10%; Cross-linking agent 0.1%-5%; The components are uniformly mixed and formed into a film by melt processing or solution casting, initially shaped at a temperature above the glass transition temperature, and cooled to below the glass transition temperature, wherein the glass transition temperature is in the range of 50°C-60°C.

[0035] Weigh the following raw materials: polylactic acid (PLA) as the base material, a plasticizer to improve the flexibility and processing properties of PLA and reduce its brittleness, fillers such as nanoparticles or fibers to enhance the mechanical strength and thermal stability of the transfer film or impart specific functions (such as conductivity and flame retardancy), and a cross-linking agent to form a cross-linked structure between PLA molecular chains, thereby improving the shape fixation and recovery rate of the transfer film and imparting basic shape memory properties and biodegradability to the transfer film.

[0036] Using melt processing or solution casting, the aforementioned components are uniformly mixed under strictly controlled environmental conditions to ensure full dispersion of the ingredients, forming a homogeneous mixture. Through a controlled molding process, the mixture is formed into a film of the desired thickness, providing the physical foundation for the subsequent imparting of shape memory properties. The formed transfer film is placed in an environment above its glass transition temperature (Tg, set between 50°C and 60°C) for initial shaping. This step aims to impart sufficient mobility to the molecular chains within the transfer film through heat treatment, thereby forming a stable initial shape within the external shape constraints. The transfer film is then rapidly cooled to below its Tg, freezing the molecular chains and fixing the initial shape, laying the foundation for subsequent shape memory behavior.

[0037] By controlling the ratio of each component, the overall performance of the transfer film, such as shape memory properties, mechanical strength, flexibility, and biodegradability, can be optimized to meet the needs of different application scenarios. Using melt processing or solution casting, combined with a precisely controlled molding process, ensures the uniformity and consistency of the transfer film, providing guarantees for the stability and repeatability of the shape memory performance. Initial shaping at a temperature above Tg and cooling below Tg effectively fixes the initial shape of the transfer film while retaining its shape memory ability. This heat treatment process not only improves the accuracy and stability of shape memory, but also simplifies the preparation process and reduces production costs.

[0038] By adjusting the raw material ratio and preparation process, the glass transition temperature of the transfer film can be flexibly adjusted to adapt to different usage environments and application requirements. The preparation method of the shape memory transfer film 100 provided in this application achieves an improvement in the shape memory performance of the transfer film through precise raw material ratio, mixing and film-forming technology, optimized heat treatment process and adjustable glass transition temperature.

[0039] The specific processing method can be melt processing or solution casting. In the melt processing method, polylactic acid (PLA) is first blended and granulated. It is used as the base material and mixed with appropriate amounts of plasticizers (such as citrate esters and low molecular weight polyethylene glycol (PEG)) and fillers (such as nano-SiO2). Nanocellulose (CNF-PPG) is a reinforcing filler. Adding 10% can increase the tensile strength to 33.38 MPa and enhance barrier properties. Talc (5–15%) can also be added to improve crystallinity and heat resistance. A cross-linking agent (such as dicumyl peroxide (DCP)) is also added. The mixture is then fed into a twin-screw extruder and melt-blended at a temperature of 170-190°C. During this process, the components are thoroughly mixed under the shear force and heat treatment of the screw, forming uniform blend particles. The melted blend particles are then passed through a casting machine or calendaring roller system and formed into a film with a thickness ranging from 30-100μm at a set temperature and pressure. The casting method is suitable for large-scale, continuous production, while the calendaring method is more suitable for applications requiring higher surface finish and thickness uniformity. The formed film is quickly passed through a cooling device and cooled to room temperature to fix its initial shape and prevent shape change during subsequent processing or use. The melt processing method is suitable for continuous, large-scale production, with high production efficiency and low cost. Since there is almost no material waste throughout the process, the material utilization rate is extremely high.

[0040] In the solution casting method, PLA and a plasticizer are first dissolved in an organic solvent (such as chloroform, tetrahydrofuran, or dichloromethane) at a specific ratio to form a uniform solution. During the dissolution process, parameters such as temperature and stirring speed must be controlled to ensure complete dissolution of the PLA and a stable solution. The dissolved solution is then slowly poured into a pre-prepared mold. The solvent evaporates naturally or by heating to accelerate evaporation, forming a thin film. During the casting process, attention must be paid to the uniformity of the solution and the cleanliness of the mold to avoid film defects. Post-processing is then performed. To completely remove residual solvent from the film, vacuum drying is required. Vacuum drying not only improves the film's purity but also its physical properties and chemical stability. The solution casting method can produce ultrathin films with thicknesses less than 10 μm, meeting the requirements of certain specialized applications. Because the solution spreads evenly across the mold surface during the casting process, the resulting films exhibit excellent uniformity and stable properties. By adjusting parameters such as solution concentration, casting speed, and drying conditions, the film's thickness, structure, and properties can be precisely controlled.

[0041] A hydrophobic layer is formed on the material surface before the initial shaping treatment, which improves the overall performance of the target product. The hydrophobic layer gives the material surface super-hydrophobic properties (contact angle > 150°), effectively reducing the adhesion and penetration of liquids (such as water, oil, etc.) on the surface, reducing cleaning and maintenance costs, and extending the service life of the product. The hydrophobic layer acts as a physical barrier to prevent direct contact between corrosive media (such as acid and alkali solutions, salt spray, etc.) and the substrate. Combined with the material's own chemical stability, a dual protection mechanism is formed to improve the material's durability in harsh environments. In addition, the hydrophobic layer can also serve as a transition layer to evenly disperse the thermal and mechanical stresses during the shaping process, avoiding deformation or cracking of the substrate due to local stress concentration, and ensuring the stability and yield of the initial shaping process.

[0042] Specifically, the film thickness of the transfer layer is within the range of 50-500μm. Controlling the thickness within the above range ensures sufficient mechanical strength while facilitating operation and application. Maintaining the glass transition temperature for more than 10 minutes and above the Tg temperature range for a sufficient time allows the molecular segments of the material to obtain sufficient thermal motion energy, effectively eliminating the internal stress and orientation defects generated during processing, reducing residual strain, and improving the dimensional stability of the material. Long-term Tg temperature treatment can regulate the crystallization rate and crystal nucleus distribution, forming a uniform and fine grain structure, and improving the mechanical properties (such as tensile strength and impact toughness) and optical properties (such as transmittance and haze) of the material. For multi-component composite materials, molecular diffusion and rearrangement at the Tg temperature promote compatibility between components, reduce phase interface defects, enhance interfacial bonding strength, and enhance the synergistic effect of the composite material.

[0043] See also Figures 1 to 2The present application further provides a method for preparing grid lines of a cell 300, wherein a shape memory transfer film 100 based on polylactic acid (PLA) is used to achieve dry transfer and automatic detachment through a heat-triggered shape recovery effect, comprising the following steps: using the aforementioned shape memory transfer film 100, forming grooves 110 on the shape memory transfer film 100 by embossing, and forming grooves 110 structures (width 3-25 μm, depth 5-20 μm) on the surface of the PLA film by template nano-imprinting. μm), in a preferred embodiment, the temperature during the imprinting process can be controlled within a range exceeding the glass transition temperature, the temperature range is selected from 80°C to 120°C, further, the temperature is preferably 100°C, after the imprinting is completed, the PLA film remains in a deformed state, the metal paste 200 is filled in the groove 110, the metal paste 200 is transferred and cured on the surface of the battery cell 300, and hot pressing transfer is performed. In a preferred technical solution, the PLA is heated to the aforementioned imprinting temperature (the temperature range is selected from 80°C to 120°C, preferably 100°C), and pressure (0.1-15 MPa) to transfer the silver paste to the silicon wafer, and at the same time, the silver paste is light-cured and heated for demolding. This process is to heat the shape memory transfer film again, and the heating temperature is higher than the glass transition temperature to remove the shape memory transfer film 100. In this process, the PLA film will automatically return to its original shape after being heated to above Tg, making it easier to separate from the metal paste and silicon wafer and reuse it. The metal paste is sintered 200 and the silicon wafer is directly sintered (500℃-1000℃) without the need to deal with the residual film. Please note that Figure 2 and Figure 3 In the technical solution shown, a technical solution of forming gate lines on the front and back surfaces simultaneously is adopted.

[0044] The width of the groove 110 is formed in the range of 5-100μm, which meets the different silver paste filling requirements, ensures that the silver paste has enough space to be evenly filled, and avoids excessive silver paste usage or loose filling due to excessive width. The depth of the groove 110 is in the range of 5-50μm. The appropriate depth can ensure that the silver paste can be fully embedded in the groove 110 during filling, forming a stable structure, and is also conducive to the smooth progress of subsequent processes such as hot pressing transfer. In addition, the surface roughness of the transfer film is less than 0.1μm. Through the surface treatment process, the surface of the PLA film reaches an extremely high flatness, thereby ensuring uniform silver paste filling, avoiding problems such as uneven filling and bubbles, and improving the quality of the grid line. In a preferred technical solution, the groove 110 formed includes grooves corresponding to the main grid and the fine grid, respectively, wherein the width of the main grid is in the range of 50-100μm and the depth is in the range of 25-50μm, and the width of the fine grid is in the range of 5-20μm and the depth is in the range of 5-20μm. Specific embodiments Some specific implementation methods will be further introduced below to further explain the technical solution of this application in detail.

[0046] In Example 1, polylactic acid, glycerol, nano-silica, and dicumyl peroxide were prepared in proportions of 75%, 10%, 10%, and 5% by weight, respectively. A melt processing method was used, and first, co-blending and granulation were performed to uniformly mix the raw materials. Subsequently, the mixture was fed into a twin-screw extruder and melt-blended at a temperature of 170-190°C to form uniform blend particles. The mixture was then calendered into a film, and the molten blend particles were passed through a calendering roller system and formed into a film with a thickness in the range of 100 μm at a temperature of 70°C, and then cooled to 40°C.

[0047] In Example 2, polylactic acid, glycerol, nano-silica, and dicumyl peroxide were prepared with weight proportions of 85%, 5%, 5%, and 5%, respectively. A solution casting method was used. The raw materials were first dissolved in chloroform. After they were fully dissolved and the solution was stable, the dissolved solution was slowly poured into a pre-prepared mold. A thin film was formed after the solvent evaporated naturally at a temperature of 80°C. The film thickness was selected to be 50 μm, and the film was cooled to 40°C after molding.

[0048] Example 3: Based on Example 1, glycerol is replaced by citrate, and the rest remains the same as Example 1.

[0049] Example 4: Based on Example 2, nano-silicon dioxide is replaced by nano-cellulose and talc, and the rest is consistent with Example 1.

[0050] In the fifth embodiment, based on the first embodiment, the film thickness is adjusted to 500 μm, and the rest remains the same as the first embodiment.

[0051] Example 6: Based on Example 1, the film thickness is adjusted to 50 μm, and the rest remains the same as Example 1.

[0052] Example 7: Based on Example 1, carbon nanotubes are further added in the same amount as nano-silicon dioxide, and the rest remain the same as Example 1.

[0053] Utilizing PLA's thermally triggered shape recovery effect, when heated above its glass transition temperature (Tg), the PLA film transforms from a glassy state to a highly elastic state, restoring its molecular chain orientation and enabling automatic detachment of the carrier film. Compared to traditional detachment methods that require washing or burning, this process eliminates the use of water and waste gas, reduces process time by over 50%, and significantly improves production efficiency. Cross-linking the PLA film enhances its mechanical properties and stability, enabling it to be recycled ≥50 times with >90% mechanical property retention after recycling. This design not only reduces production costs but also waste generation, aligning with the concept of sustainable development. PLA is a fully biodegradable material, with a degradation rate of >95% within 30 days, making it environmentally friendly. Furthermore, the dry process eliminates the risk of solvent contamination, avoiding the environmental issues associated with traditional solvent use, making it a green and environmentally friendly production process. A comparison is shown in Table 1.

[0054] Table 1 Comparison of the effects of PLA film and traditional water-soluble PVA film technology From the above comparison, it can be seen that the PLA film of the present application has advantages in terms of process time, silver paste transfer accuracy, number of film recycling uses, and environmental friendliness. Above Tg, PLA transforms from a glassy state to a highly elastic state, and the molecular chains undergo orientation recovery, allowing the PLA film to automatically return to its original shape and achieve automatic detachment from the carrier film. By adding carbon nanotubes (CNTs), the thermal conductivity of the PLA film is improved, allowing the film to reach Tg within 10 seconds, accelerating the detachment process. The rapid thermal response can shorten the process time, improve production efficiency, and also help reduce energy consumption. In addition, the cross-linking synergy of nano-SiO2 and diisopropylbenzene peroxide (DCP) can further enhance the mechanical properties of the PLA film, and the tensile strength remains ≥20MPa after recycling. This mechanical stability ensures that the PLA film will not show significant performance degradation during multiple recycling processes, extending the service life of the film.

[0055] After transfer, the gridline width error was <1μm, and the resistivity was reduced by 5.2% (compared to the conventional process). This demonstrates that the process described in this application can improve the precision and quality of the silver paste gridlines, reduce resistivity, and thus enhance solar cell performance. After 50 uses, the PLA film exhibited a tensile strength loss of <8% and a groove deformation rate of <20%. These test results validate the reusability and stability of the PLA film, demonstrating that it maintains good performance after multiple cycles.

[0056] Reasonable groove width and depth range and low surface roughness provide good conditions for uniform filling of silver paste, ensuring the quality and performance of the grid lines. In actual production, precise control of these parameters can be achieved through advanced processing equipment and processes. This process not only improves production efficiency, but also avoids environmental pollution problems caused by water washing and burning in traditional processes. At the same time, the automatic detachment method reduces manual operation, reduces production costs and labor intensity. Reusable PLA film greatly reduces production costs and reduces waste generation. This is of great significance for the large-scale production of electronic products such as solar cells and meets the requirements of sustainable development. The biodegradability of PLA and the risk of solvent pollution in the dry process make the process of this application a green and environmentally friendly production method.

[0057] As can be seen from the foregoing, this application provides a shape memory transfer film, a method for preparing it, and its application in the preparation of solar cell grid lines. This shape memory transfer film utilizes a reusable, biodegradable polylactic acid (PLA) base film as its core material, combined with a specific plasticizer, filler, and crosslinker formulation to achieve improvements in material cost and environmental performance. Through its unique shape memory effect, the transfer film can automatically release under specific conditions, eliminating traditional demolding methods such as washing or high-temperature burning. During imprinting, when the ambient temperature is near the glass transition temperature, the transfer film can stably maintain its deformed shape; when the temperature rises above the glass transition temperature, the transfer film automatically returns to its original shape. Furthermore, a hydrophobic layer forms on the surface of the transfer film, effectively blocking the intrusion of external moisture, preventing performance degradation, and extending its service life. The photothermal conversion material efficiently absorbs energy in the near-infrared region and converts it into heat, achieving localized heating and precisely triggering the shape memory effect. The conductive material generates Joule heat by passing electricity, which acts evenly on the inside of the transfer film, efficiently triggering the shape memory effect and reducing dependence on external heating equipment.

[0058] Preparation methods include melt processing and solution casting. The melt processing method achieves uniformity and consistency of the transfer film through steps such as blending and granulation, casting / calendering film formation, and cooling and shaping. The solution casting method, through steps such as dissolution, casting, and post-processing, produces ultra-thin films with a thickness of less than 10μm to meet the needs of special applications. In the preparation of solar cell grid lines, this shape memory transfer film achieves dry transfer and automatic release through a heat-triggered shape recovery effect. The specific steps include: embossing to form grooves near the glass transition temperature, filling with metal slurry, hot pressing and transferring to the silicon wafer, heating and demolding, reheating to automatically restore the transfer film to its original shape and release it from the silicon wafer, and finally sintering the metal slurry. This process does not require the use of water or the generation of exhaust gas, shortens the process time by over 50%, and greatly improves production efficiency. Furthermore, the transfer film can be recycled ≥50 times, and the mechanical property retention rate after recycling is greater than 90%, which is in line with the concept of sustainable development. In summary, the shape memory transfer film of the present application, its preparation method, and its application in the preparation of battery cell grid lines have demonstrated excellent performance and application prospects.

[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 shape memory transfer film, characterized in that: The composition comprises the following components in parts by weight: Polylactic acid 70%-85%; Plasticizer 0.5%-10%; Filler 1%-10%; Cross-linking agent 0.1%-5%; Among them, the thickness of the shape memory transfer film is in the range of 50-500μm, and the glass transition temperature is in the range of 50℃-60℃. During imprinting, the shape memory transfer film will maintain the deformed shape after deformation. When heated again to a temperature higher than the glass transition temperature, the shape memory transfer film will restore its original shape.

2. The shape memory transfer film according to claim 1, characterized in that: The invention also includes a hydrophobic layer formed on the surface of the shape memory transfer film.

3. The shape memory transfer film according to claim 1, wherein: The filler also includes a light-to-heat conversion material.

4. The shape memory transfer film according to claim 1, wherein: The filler also includes conductive material.

5. A method for preparing a shape memory transfer film, characterized in that: The steps include: Prepare the following components in parts by weight: Polylactic acid 70%-85%; Plasticizer 0.5%-10%; Filler 1%-10%; Cross-linking agent 0.1%-5%; The components are uniformly mixed and formed into a film by melt processing or solution casting; Initial shaping is performed at a temperature above the glass transition temperature; Cooling to below the glass transition temperature; The glass transition temperature is in the range of 50°C to 60°C.

6. The method for preparing a shape memory transfer film according to claim 5, wherein: The initial shaping also includes forming a hydrophobic layer on the surface.

7. The method for preparing a shape memory transfer film according to claim 5, wherein: The film thickness of the transfer layer is in the range of 50-500 μm.

8. The method for preparing a shape memory transfer film according to claim 5, wherein: The glass transition temperature is maintained for more than 10 minutes.

9. A method for preparing a grid line of a battery cell, characterized in that: The steps include: Using the shape memory transfer film according to any one of claims 1 to 4, embossing a groove on the shape memory transfer film; filling the groove with metal slurry; Transfer and solidify the metal paste on the surface of the battery cell; Performing heating demolding: heating the shape memory transfer film again to a temperature higher than the glass transition temperature, and removing the shape memory transfer film; Sintered metal slurry.

10. The method for preparing a shape memory transfer film according to claim 9, wherein: The width of the groove is within the range of 5-100 μm, and the depth of the groove is within the range of 5-50 μm.

Images