A battery string and a method of manufacturing the same

Abstract

Embodiments of the present application relate to the field of photovoltaic cells, and provide a cell string and a preparation method thereof, the cell string comprising two cell pieces welded in sequence and a plurality of ribbons connected to the adjacent two cell pieces, the surface of the cell piece being provided with a barrier layer, the barrier layer comprising an interface conductive layer, a gradient transition layer and a welding adaptation layer arranged in sequence from the surface of the cell piece in a direction perpendicular to the cell piece; wherein the thermal expansion coefficients of the interface conductive layer, the gradient transition layer and the welding adaptation layer increase in turn, and the thermal expansion coefficients of the interface conductive layer, the gradient transition layer and the welding adaptation layer are all between the thermal expansion coefficients of the cell piece and the ribbon. The present application sets a multi-layer barrier structure with gradient thermal expansion coefficients on the surface of the cell piece, so that the thermal expansion coefficient is smoothly transitioned from the cell piece to the ribbon, the interface thermal stress is reduced, and the interface shear stress problem caused by the difference in thermal expansion coefficients between the cell piece and the ribbon is effectively solved.

Description

Technical Field

[0001] This application relates to the field of photovoltaic cells, and in particular to a cell string and a method for its fabrication. Background Technology

[0002] Currently, in tandem photovoltaic modules, cells are arranged in an overlapping manner to increase power density. However, a significant difference in the coefficient of thermal expansion between the cell body and the solder ribbon has become a core cause of long-term reliability failures. The cell body is dominated by a silicon substrate, whose coefficient of thermal expansion is approximately 2.6 × 10⁻⁶. -6 / ℃, while the surface of the solder strip is usually made of Sn60Pb40 alloy, with a coefficient of thermal expansion of approximately 24.5×10. -6 / ℃, the difference between the two is as high as 21.9×10 -6 / ℃. Under outdoor temperature cycling conditions, according to the IEC61215 standard, the temperature range is -40℃ to 85℃, a temperature difference of 125℃. Based on thermoelastic theory, the interfacial shear stress can accumulate to 11.26 MPa. This stress level is close to or exceeds the microcrack propagation threshold of the silicon wafer. Under long-term action, it can easily cause microcracks or interface peeling in the solar cells, seriously threatening the 25-year design life of the module.

[0003] In related technologies, attempts have been made to mitigate thermal damage by setting a barrier layer composed of a single material on the gate line. However, this has resulted in failure to match the coefficient of thermal expansion, insufficient interfacial bonding, and only achieves the function of thermal barrier, failing to simultaneously meet the requirements of conductivity optimization and stress buffering. Summary of the Invention

[0004] This application provides a battery string in one or more embodiments to solve or at least partially alleviate the problem of insufficient interfacial bonding caused by the mismatch of the thermal expansion coefficients of battery cells in related technologies.

[0005] The first aspect of this application provides a battery string, which adopts the following technical solution: A battery string includes at least two battery cells welded sequentially and a plurality of solder strips connecting two adjacent battery cells. A barrier layer is provided on the surface of the battery cell. The barrier layer includes an interface conductive layer, a gradient transition layer and a welding adapter layer arranged sequentially from the surface of the battery cell along a direction perpendicular to the battery cell. The solder strips are respectively connected to the welding adapter layer of the first battery cell and the welding adapter layer of the second battery cell in two adjacent battery cells. Among them, the thermal expansion coefficients of the interface conductive layer, the gradient transition layer and the welding adapter layer increase sequentially, and the thermal expansion coefficients of the interface conductive layer, the gradient transition layer and the welding adapter layer are all located between the thermal expansion coefficients of the battery cell and the welding strip.

[0006] By adopting the above technical solution, a multi-layer barrier structure with a gradient coefficient of thermal expansion is set on the surface of the solar cell, allowing the coefficient of thermal expansion to smoothly transition from the solar cell to the solder ribbon. This reduces interfacial thermal stress and effectively solves the problem of interfacial shear stress caused by the difference in the coefficient of thermal expansion between the solar cell and the solder ribbon. Consequently, the risk of microcracks and interfacial delamination in the solar cell under long-term temperature cycling is significantly reduced, while ensuring the reliability of the electrical connection. This solution improves the long-term operational stability and lifespan of photovoltaic modules, meeting the reliability requirements of high-efficiency shingled modules.

[0007] In one embodiment / in some embodiments / in one possible implementation, the barrier layer is in the form of a strip structure corresponding one-to-one with the solder strip, the projection of the solder strip on the cell along the thickness direction of the cell is located within the projection of the corresponding barrier layer on the cell along the thickness direction of the cell, and the width of the projection of the solder strip on the cell along the thickness direction of the cell is 0.05 to 0.10 mm smaller than the width of the projection of the corresponding barrier layer on the cell along the thickness direction of the cell.

[0008] In one embodiment / in some embodiments / in one possible implementation, the coefficient of thermal expansion of the interface conductive layer is 6 × 10⁻⁶. -6 / ℃ to 12×10 -6 / ℃, the thickness of the interface conductive layer is 5 to 8 μm.

[0009] In one embodiment / in some embodiments / in one possible implementation, the interface conductive layer comprises, by weight, 50 to 70 parts of silver powder, 20 to 38 parts of inorganic non-metallic ceramic material, and 5 to 15 parts of resin.

[0010] In one embodiment / in some embodiments / in one possible implementation, the coefficient of thermal expansion of the gradient transition layer is 12 × 10⁻⁶. -6 / ℃ to 18×10 -6 / ℃, with a gradient transition layer thickness of 8 to 15 μm.

[0011] In one embodiment / in some embodiments / in one possible implementation, the gradient transition layer comprises, by weight, 70 to 80 parts silver powder, 5 to 10 parts inorganic non-metallic ceramic material, and 10 to 15 parts silicone-modified epoxy resin.

[0012] In one embodiment / in some embodiments / in one possible implementation, the coefficient of thermal expansion of the welding adapter layer is 18 × 10⁻⁶. -6 / ℃ to 24×10 -6 / ℃, the thickness of the welding adapter layer is 7 to 12 μm.

[0013] In one embodiment / in some embodiments / in one possible implementation, the solder adapter layer comprises, by weight, 80 to 90 parts of solder powder and 10 to 20 parts of silver powder.

[0014] A second aspect of this application provides a method for preparing a battery string, employing the following technical solution: A method for preparing a battery string as described above includes the following steps: S1: On the surface of the battery cells in the battery string, a conductive interface layer, a gradient transition layer, and a welding adapter layer are sequentially screen-printed along the direction perpendicular to the battery cells. S2: Under a nitrogen atmosphere, heat the solar cell to 110 to 130°C and hold for 2 to 4 minutes for preheating, heat to 170 to 190°C and hold for 4 to 6 minutes for curing, and heat to 230 to 250°C and hold for 5 to 7 minutes. S3: Take the solder strips and connect them to the solder adapter layer of the first and second adjacent solar cells respectively.

[0015] By adopting the above technical solution, the problems of discontinuous gradients within the barrier layer, weak interfacial bonding, or welding adaptation failure caused by improper process control during battery string fabrication can be effectively solved. Specifically, step S1 uses screen printing, which can precisely control the deposition sequence and position of the interfacial conductive layer, gradient transition layer, and welding adaptation layer, ensuring a continuous gradient distribution starting from the surface of the battery cell. This maintains a smooth transition of the coefficient of thermal expansion during subsequent use and avoids gradient interruption. Step S2 implements staged temperature-curing under a nitrogen atmosphere. The inert environment prevents material oxidation, and precise temperature gradient control promotes chemical bonding and physical cross-linking between the layers, significantly enhancing interlayer and intralayer bonding and effectively solving the problem of weak interfacial bonding. In addition, this curing process also ensures that each layer of material achieves its expected mechanical and thermal properties, providing a solid foundation for realizing the overall coefficient of thermal expansion gradient. Step S3, by directly connecting the solder ribbon to the welding adapter layer with a high coefficient of thermal expansion, fully utilizes the good thermal expansion matching between the welding adapter layer and the solder ribbon, ensuring a reliable connection between the solder ribbon and the barrier layer, and effectively reducing the impact of welding thermal stress on the gradient structure, thereby avoiding welding adapter failure. Overall, this fabrication method, through the coordinated synergistic effect of each step, ensures the stability, functionality, and structural integrity of the barrier layer during the fabrication stage. This allows the battery string to effectively alleviate thermal stress between the cells and the solder ribbon under long-term temperature cycling, significantly reducing the risk of microcracks and interface delamination in the cells, thereby greatly improving the long-term reliability and service life of the battery string.

[0016] Interface conductive layer, gradient transition layer and welding adapter layer.

[0017] In one embodiment / in some embodiments / in one possible implementation, in S1, the materials of the interface conductive layer, the gradient transition layer and the welding adapter layer are all added to an organic solvent with a solid content of 75% to 80% and a viscosity of 300 ± 50 Pa·s, and then ground and dispersed to a fineness of less than 15 μm. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings of the embodiments will be briefly introduced below. Obviously, the drawings described below only involve some embodiments of this application and are not intended to limit this application.

[0019] Figure 1 This is a schematic diagram of the structure of a battery string with a gap of ≤0 between battery cells according to some embodiments of this application.

[0020] Figure 2 This is a schematic diagram of a battery string with a gap greater than 0 between battery cells according to some embodiments of this application.

[0021] Figure 3 This is a schematic diagram of a battery cell without grid lines on its surface according to some embodiments of this application.

[0022] Figure 4 This is a schematic diagram of a battery cell with grid lines on its surface according to some embodiments of this application.

[0023] Figure 5 This is a schematic diagram of the structure of a battery cell according to some embodiments of this application.

[0024] Figure 6 for Figure 5 Enlarged diagram of point A in the middle.

[0025] Figure 7 This is a schematic flowchart of a method for preparing a battery string according to some embodiments of this application.

[0026] In the figure: 10, solar cell; 11, barrier layer; 111, interface conductive layer; 112, gradient transition layer; 113, welding adapter layer; 20, solder strip; 21, copper substrate; 22, solder coating; 30, grid line. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings showing multiple embodiments according to this application. It should be understood that the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments described in this application without creative effort will fall within the scope of protection of this application.

[0028] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in the description of this application is for the purpose of describing specific embodiments only and is not intended to limit this application; the terms "comprising," "including," "having," "containing," etc., in the description, claims, and accompanying drawings of this application are open-ended terms. Therefore, "comprising," "including," or "having" refers to, for example, a method or apparatus having one or more steps or elements, but is not limited to having only these one or more elements. The terms "first," "second," etc., in the description, claims, or accompanying drawings of this application are used to distinguish different objects, not to describe a specific order or hierarchy. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0029] In the description of this application, it should be understood that the terms "center", "lateral", "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "axial", "radial", "circumferential", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0030] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "attachment" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0031] In this application, the term "implementation" means that a specific feature, structure, or characteristic described in connection with an implementation can be included in at least one implementation of this application. The appearance of this phrase in various places in the specification does not necessarily refer to the same implementation, nor is it a separate or alternative implementation mutually exclusive with other implementations. Those skilled in the art will understand, explicitly and implicitly, that the implementations described in this application can be combined with other implementations.

[0032] Figure 1 and Figure 2 This is a schematic diagram of a battery string according to some embodiments of this application. The battery string is formed by connecting battery cells 10 in series, and the gap between battery cells 10 can be ≤0 to create a negative pitch stack, such as... Figure 1 As shown; it can also be made into a battery string with a normal gap greater than 0, such as Figure 2 As shown.

[0033] One or more embodiments of this application disclose a battery string. (See also...) Figure 1 and Figure 2 The battery string includes at least two sequentially welded battery cells 10 and a plurality of solder strips 20 connecting adjacent battery cells 10, as shown in the figure. Figure 3 The surface of the battery cell 10 is provided with a barrier layer 11. The barrier layer 11 includes an interface conductive layer 111, a gradient transition layer 112 and a welding adapter layer 113 arranged sequentially from the surface of the battery cell 10 along a direction perpendicular to the direction of the battery cell 10. The solder ribbon 20 is respectively connected to the welding adapter layer 113 of the first battery cell 10 and the welding adapter layer 113 of the second battery cell 10. The thermal expansion coefficients of the interface conductive layer 111, the gradient transition layer 112, and the welding adapter layer 113 increase sequentially, and the thermal expansion coefficients of the interface conductive layer 111, the gradient transition layer 112, and the welding adapter layer 113 are all located between the thermal expansion coefficients of the battery cell 10 and the welding strip.

[0034] Specifically, a battery string refers to a component consisting of at least two battery cells 10 connected sequentially by welding, used to achieve series output of electrical energy.

[0035] Solar cell 10: refers to the basic power generation unit in a photovoltaic module, which is usually made of silicon-based materials and has photoelectric conversion function on its surface.

[0036] Solder strip 20: refers to the metal strip used to connect adjacent solar cells 10 and conduct current, typically consisting of a copper substrate 21 and a solder coating 22, see reference. Figure 3 .

[0037] Barrier layer 11: refers to a multifunctional coating structure disposed between the surface of the battery cell 10 and the solder strip 20, used to alleviate thermal stress, provide conductive pathways and enhance interfacial adhesion.

[0038] Interface conductive layer 111: refers to the first layer in the barrier layer 11 that is immediately adjacent to the battery cell 10. Its main function is to provide good electrical contact and initiate the gradient transition of the coefficient of thermal expansion.

[0039] Gradient transition layer 112: refers to the intermediate layer in the barrier layer 11 located between the interface conductive layer 111 and the welding adapter layer 113. Its function is to further smooth the transition of the thermal expansion coefficient and reduce interlayer stress.

[0040] Welding adapter layer 113: refers to the outermost layer of barrier layer 11, which is in direct contact with the solder strip 20. Its main function is to provide an optimized welding interface and ensure a strong connection between the solder strip 20 and barrier layer 11.

[0041] Coefficient of thermal expansion: refers to the proportion by which the dimensions of a material change with temperature, and is a physical parameter for measuring the thermal stability of a material.

[0042] The battery string structure includes at least two sequentially welded battery cells 10 and multiple solder strips 20 connecting adjacent battery cells 10. The battery cells 10 are connected in series to form a battery string with the desired voltage output. The solder strips 20 act as electrical connectors, responsible for transferring electrical energy from one battery cell 10 to the next and ultimately leading it out. The battery cells 10 can be of various sizes and shapes, such as square or rectangular. The materials for the solder strips 20 are also diverse; for example, pure copper or copper alloys can be used as the base material, and surface treatments can be applied as needed.

[0043] By adopting the above technical solution, a multi-layer barrier structure with a gradient coefficient of thermal expansion is set on the surface of the solar cell 10, allowing the coefficient of thermal expansion (CTE) to smoothly transition from the solar cell 10 to the solder ribbon 20. This reduces interfacial thermal stress and effectively solves the problem of interfacial shear stress caused by the difference in the coefficient of thermal expansion between the solar cell 10 and the solder ribbon 20. This significantly reduces the risk of microcracks and interfacial delamination in the solar cell 10 under long-term temperature cycling, while ensuring the reliability of the electrical connection. This solution improves the long-term operational stability and service life of photovoltaic modules, meeting the reliability requirements of high-efficiency shingled modules.

[0044] The implementation principle of a battery string in this embodiment is as follows: 1. Determination of the CTE calculation benchmark for solar cells: The grid lines 30 on the surface of the photovoltaic cell 10 account for approximately 3% to 5% of the total surface area of ​​the cell 10. In the thermal stress analysis, the volume effect and area effect of the grid lines 30 can be ignored. The overall thermal expansion behavior of the cell 10 is dominated by the predominant silicon substrate (volume percentage > 99%, area percentage > 95%), and the CTE of its surface silicon nitride / alumina antireflective film (thickness < 100 nm) is approximately 3.0 × 10⁻⁶. -6 The CTE of the solar cell 10 is close to that of the silicon substrate (°C). Therefore, in this embodiment, the CTE of the solar cell 10 is uniformly set at 2.6 × 10⁻⁶. -6 The CTE difference of the grid line 30 material is no longer considered separately in the / ℃ calculation. This simplification conforms to engineering practice and the mechanical principle of composite materials. 2. Determination of CTE benchmark for solder strip: The substrate of solder strip 20 is copper (CTE=16.5×10). -6 / ℃), the commonly used surface coating is Sn60Pb40 alloy, according to the Sn60Pb40 alloy CTE: 24.5×10 -6 / ℃ (range 20 to 100℃). Since the solder strip 20 mainly contacts the solder coating 22 during welding, the effective CTE is taken as the coating value of 24.5 × 10⁻⁶. -6 / ℃) Core issue: Under an outdoor temperature difference ΔT = 125℃ (IEC 61215 standard, -40℃ to 85℃), the CTE difference between the main body of the solar cell (2.6) and the 20-coated solder strip (24.5) is Δα = 21.9 × 10⁻⁶. -6 / ℃, resulting in significant interfacial shear stress.

[0045] The calculation of interface stress includes: According to the simplified thermoelastic theory, the formula for calculating the interfacial shear stress is: τ = (G×Δα×ΔT) / (2×(1+ν)) Where G is the solder shear modulus, Δα is the CTE difference, ΔT is the temperature change, and ν is Poisson's ratio.

[0046] in: G = 11.1 GPa (Sn60Pb40 solder shear modulus, industry standard value); Δα = 24.5 - 2.6 = 21.9 × 10 -6 / ℃ (CTE difference between solder ribbon and cell); ΔT = 125℃ (temperature variation range); ν=0.35 (Poisson's ratio of solder, a commonly used value in the industry).

[0047] Substitute into the calculation: τ = (11.1×10 9 ×21.9×10 -6 ×125) / (2×(1+0.35)) ≈ 11.26MPa.

[0048] Under long-term temperature cycling, this stress can easily lead to microcracks and interface peeling in the solar cells, affecting the 25-year design life of the module.

[0049] In at least one embodiment, an integrated structure of a "three-layer gradient CTE matching barrier layer 11" is constructed and disposed on the surface of the solar cell 10, located directly below the solder ribbon 20. The overall structure, from bottom to top (i.e., from the side in contact with the surface of the solar cell 10 to the soldering side), consists of: a surface conductive layer, which directly contacts the surface of the solar cell 10 to achieve high conductivity and strong adhesion, with a CTE target value of 6 to 12 × 10⁻⁶. -6 / ℃; Gradient transition layer 112: Intermediate buffer layer, realizing CTE gradient transition and stress dispersion, with a target CTE value of 12 to 18 × 10⁻⁶. -6 / ℃; Welding adapter layer 113: in contact with solder strip 20 to achieve welding adaptation and conductivity optimization, with a CTE target value of 18 to 24 × 10⁻⁶. -6 / ℃; By setting a multi-layer barrier structure with a gradient thermal expansion coefficient on the surface of the battery cell 10, the problem of interface shear stress caused by the difference in thermal expansion coefficient between the battery cell 10 and the solder strip 20 is effectively solved.

[0050] In at least one embodiment, a barrier layer 11 is provided on the surface of the battery cell 10. The barrier layer 11 can be formed on the surface of the battery cell 10 in various ways, such as by spraying, dip coating, or printing. The thickness of the barrier layer 11 can be adjusted according to actual needs to ensure its effective function. The barrier layer 11 forms a coverage area on the surface of the battery cell 10 to accommodate different connection requirements.

[0051] The barrier layer 11 comprises an interface conductive layer 111, a gradient transition layer 112, and a welding adapter layer 113, arranged sequentially from the cell 10 along a direction perpendicular to the cell 10. This multi-layered structure is designed to achieve functional partitioning and a gradual transition in the coefficient of thermal expansion. The interface conductive layer 111 is in direct contact with the surface of the cell 10, and its material selection must balance conductivity and good adhesion to the cell 10. The gradient transition layer 112 is located between the interface conductive layer 111 and the welding adapter layer 113, and its material composition and structure can be designed to provide a smooth physical and chemical transition between the two layers. The welding adapter layer 113, as the outermost layer, should be made of a material with good solderability to form a reliable weld joint with the solder strip 20.

[0052] The solder strip 20 is connected to the welding adapter layer 113 of the first and second adjacent solar cells 10, respectively. The connection between the solder strip 20 and the welding adapter layer 113 can be achieved through various welding processes, such as resistance welding or laser welding. During the connection process, welding parameters, such as welding temperature, pressure, and time, need to be controlled to ensure a strong and electrically conductive joint. The welding adapter layer 113 provides an optimized connection interface for the solder strip 20, contributing to improved welding quality and connection reliability.

[0053] The coefficients of thermal expansion of the interface conductive layer 111, the gradient transition layer 112, and the welding adapter layer 113 increase sequentially. To achieve a gradient change in the coefficient of thermal expansion, materials with different coefficients of thermal expansion can be combined according to the specific function and location of each layer. For example, the interface conductive layer 111 can be made of a conductive material with a relatively low coefficient of thermal expansion, the gradient transition layer 112 can be made of a material with a coefficient of thermal expansion between that of the interface conductive layer 111 and the welding adapter layer 113, and the welding adapter layer 113 can be made of a weldable material with a relatively high coefficient of thermal expansion. This design helps to disperse thermal stress and avoid stress concentration at a certain interface.

[0054] Furthermore, the coefficients of thermal expansion of the interface conductive layer 111, the gradient transition layer 112, and the welding adapter layer 113 are all located between the coefficients of thermal expansion of the solar cell 10 and the solder ribbon. This means that the coefficients of thermal expansion of each layer should be precisely controlled so that they fall within the range of the solar cell 10 (e.g., a silicon-based solar cell, whose coefficient of thermal expansion is approximately 2.6 × 10⁻⁶). -6 / ℃) and solder strip 20 (e.g., Sn60Pb40 solder coating 22, with a coefficient of thermal expansion of approximately 24.5 × 10⁻⁶). -6 The thermal expansion coefficient is within the range of (°C). Therefore, it can be ensured that the entire barrier layer 11 structure, as a whole, can effectively buffer the difference in thermal expansion between the battery cell 10 and the solder ribbon 20 when the temperature changes, thereby significantly reducing the interfacial shear stress.

[0055] In some embodiments, refer to Figure 4 The surface of the solar cell 10 is provided with grid lines 30, which account for about 3% to 5% of the total surface area of ​​the solar cell 10.

[0056] In some embodiments, refer to Figure 5 and Figure 6 The barrier layer 11 has a strip structure that corresponds one-to-one with the solder ribbon 20. The projection of the solder ribbon 20 on the battery cell 10 along the thickness direction is located within the projection of the corresponding barrier layer 11 on the battery cell 10 along the thickness direction. The projection width of the solder ribbon 20 on the battery cell 10 along the thickness direction is 0.05 to 0.10 mm smaller than the projection width of the corresponding barrier layer 11 on the battery cell 10 along the thickness direction.

[0057] Specifically, the technical feature that "the projection of the solder ribbon 20 along the thickness direction of the cell 10 onto the cell 10 lies within the projection of the barrier layer 11 along the thickness direction of the cell 10 onto the cell 10" aims to ensure that the connection area between the solder ribbon 20 and the cell 10 can be completely covered by the barrier layer 11. In practical applications of photovoltaic cell strings, the solder ribbon 20 is connected to the surface of the cell 10 by welding. Due to the difference in the coefficients of thermal expansion between the cell 10 and the solder ribbon 20, stress will be generated at the connection interface during temperature cycling. If the edges of the solder ribbon 20 are directly exposed on the surface of the cell 10, these edge areas will become weak points of stress concentration, which can easily lead to microcracks or interface delamination in the cell 10. To achieve this goal, several methods can be used: one method is to precisely control the printing or coating process of the barrier layer 11 so that it is slightly larger than the expected connection area of ​​the solder ribbon 20 in the width direction. For example, before soldering the solder ribbon 20, a barrier layer 11 with a width slightly wider than the solder ribbon 20 is formed on the surface of the cell 10. Another method is to adjust the width or welding position of the solder ribbon 20 so that it is connected inside the barrier layer 11. For example, a solder ribbon 20 with a width slightly narrower than the barrier layer 11 is used, and the solder ribbon 20 is ensured to be centered and aligned with the barrier layer 11 during the welding process.

[0058] The technical feature that "the projected width of the solder ribbon 20 along the thickness direction of the cell 10 on the cell 10 is 0.05 to 0.10 mm smaller than the projected width of the barrier layer 11 along the thickness direction of the cell 10 on the cell 10" further precisely defines the relative dimensional relationship between the solder ribbon 20 and the barrier layer 11, that is, the barrier layer 11 is 0.05 to 0.10 mm wider than the solder ribbon 20 in the width direction. This small width difference provides a critical buffer area to alleviate stress concentration that may occur at the edges of the solder ribbon 20 and the barrier layer 11 during thermal cycling. Too small a gap may cause the edges of the solder ribbon 20 and the edges of the barrier layer 11 to come into direct contact during thermal expansion and contraction, generating stress peaks; too large a gap may weaken the protective effect of the barrier layer 11 or affect the effective area of ​​the connection. To achieve this precise width difference, the following measures can be taken: One approach is to accurately measure the width of the barrier layer 11 after it has been printed or coated, and then select or cut solder strips 20 of the appropriate width for welding to meet the width difference requirement of 0.05 to 0.10 mm; another approach is to reserve this width difference in the design stage of the barrier layer 11, for example, designing the printing template of the barrier layer 11 according to the width of the standard solder strip 20, so that it is 0.05 to 0.10 mm wider than the solder strip 20 in the width direction.

[0059] By adopting the above technical solution, the projected positional relationship between the solder ribbon 20 and the barrier layer 11 is precisely defined, ensuring that the projection of the solder ribbon 20 in the thickness direction is completely covered by the barrier layer 11, while setting a specific width difference. Specifically, the projection of the solder ribbon 20 on the cell 10 along the thickness direction is located within the projection of the barrier layer 11 on the cell 10 along the thickness direction. This design ensures that the solder ribbon 20 is completely within the protection range of the barrier layer 11, effectively preventing the solder ribbon 20 from being directly exposed to the surface of the cell 10, thereby significantly reducing the edge shear stress caused by the difference in thermal expansion between the cell 10 and the solder ribbon 20. In addition, the projected width of the solder ribbon 20 is 0.05 to 0.10 mm smaller than the projected width of the barrier layer 11. This precise buffer gap allows for a moderate deformation space at the boundary between the solder ribbon 20 and the barrier layer 11 during thermal cycling, effectively preventing stress peaks caused by direct contact, while maintaining sufficient connection area to ensure conductivity and mechanical strength. The barrier layer 11 is composed of an interface conductive layer 111, a gradient transition layer 112, and a welding adapter layer 113 in sequence, with the thermal expansion coefficient of each layer increasing sequentially and located between the cell 10 and the solder ribbon 20. This precise geometric matching and gradient material design work together to further optimize the stress distribution of the entire connection interface, significantly improve the reliability and stability of the battery string under long-term temperature cycling, and effectively avoid the risk of microcracks and interface peeling of the cell 10.

[0060] In at least one embodiment, when the width of the solder strip 20 is 0.20 mm, the width of the barrier layer 11 is set to 0.28 mm, with an extra 0.04 mm on each side, achieving an optimal balance between material usage and reliability.

[0061] In some embodiments, the coefficient of thermal expansion of the interface conductive layer 111 is 6 × 10⁻⁶. -6 / ℃ to 12×10 -6 / ℃, thickness of the interface conductive layer 111 (refer to) Figure 3 and Figure 4 The thickness of h1 is 5 to 8 μm.

[0062] Specifically, the coefficient of thermal expansion (CTE) is a key physical quantity that measures the degree of dimensional change of a material when the temperature changes. In a battery string structure, there is a significant difference in the CTE between the battery cell 10 and the solder ribbon 20, which leads to enormous thermal stress during temperature cycling. As the first layer in the barrier layer 11 adjacent to the battery cell 10, the setting of the CTE of the interface conductive layer 111 is crucial. The CTE of the interface conductive layer 111 is limited to 6 × 10⁻⁶. -6 / ℃ to 12×10 -6 Within a range of / ℃, the aim is to make its coefficient of thermal expansion between that of a solar cell 10 (e.g., the coefficient of thermal expansion of a silicon-based solar cell is approximately 2.6 × 10⁻⁶). -6 / ℃) and subsequent gradient transition layer 112 and welding adapter layer 113 (whose coefficient of thermal expansion gradually increases, eventually approaching the coefficient of thermal expansion of the weld strip 20, for example 24.5×10 ... -6 The coefficient of thermal expansion is between 10°C and 10°C. In this way, the interface conductive layer 111 can act as a first-level buffer layer, gradually transitioning the difference in thermal expansion and avoiding abrupt concentration of thermal stress on the surface of the cell 10. This coefficient of thermal expansion can be achieved by precisely controlling the composition of the interface conductive layer 111 material, for example, by adjusting the ratio of conductive particles, ceramic fillers and polymer matrix to obtain the desired composite material thermal expansion characteristics.

[0063] Meanwhile, the thickness of the interface conductive layer 111 is also a key parameter affecting its performance. This application limits the thickness of the interface conductive layer 111 to 5 to 8 μm. This thickness range is designed to ensure that the interface conductive layer 111 provides sufficient conductive pathways while possessing appropriate mechanical flexibility and stress buffering capacity. If the thickness is too thin, it may lead to insufficient conductivity and is prone to cracking or peeling under thermal stress, failing to effectively protect the battery cell 10; if the thickness is too thick, it may increase material costs, affect the overall thickness of the battery string, and may introduce new internal stresses during the curing process. Therefore, a thickness range of 5 to 8 μm ensures good conductivity while providing sufficient material volume to absorb and disperse thermal strain, thereby effectively reducing interfacial shear stress. This thickness can be achieved through precise printing process (e.g., screen printing) parameter control, including but not limited to adjusting the mesh count of the screen, emulsion thickness, squeegee pressure, and the rheological properties of the paste.

[0064] By adopting the above technical solution, the coefficient of thermal expansion of the interface conductive layer 111 is precisely controlled at 6×10⁻⁶. -6 / ℃ to 12×10 -6 The interface conductive layer 111 is set at a temperature of ℃ and its thickness is limited to 5 to 8 μm, enabling it to serve as an effective buffer medium between the solar cell 10 and subsequent layers. This layer gradually alleviates the difference in thermal expansion coefficients between the solar cell 10 and other parts of the barrier layer 11, thereby significantly reducing the interfacial shear stress generated during temperature cycling. This precise CTE matching and thickness control not only ensures the excellent conductivity of the interface conductive layer 111, but more importantly, it effectively avoids reliability problems such as microcracks and interface peeling in the solar cell 10 caused by thermal stress concentration, thus greatly improving the stability and service life of the battery string under long-term outdoor operation conditions. The interface conductive layer 111, together with the subsequent gradient transition layer 112 and welding adapter layer 113, constitutes a multi-level thermal stress buffer system, fundamentally solving the reliability challenges caused by the huge difference in thermal expansion coefficients between the solar cell 10 and the solder strip 20.

[0065] In some embodiments, the height of the interface conductive layer 111, the gradient transition layer 112, and the welding adapter layer 113 in the barrier layer 11 is defined as the vertical distance measured from the coating surface of the cell 10, without considering the influence of the protrusion of the grid lines 30. During the cell printing process, the leveling property of the paste will automatically fill the depressions between the grid lines 30, forming a composite functional layer with a smooth surface after sintering.

[0066] In some embodiments, the interface conductive layer 111 comprises, by weight, 50 to 70 parts of silver powder, 20 to 38 parts of inorganic non-metallic ceramic material, and 5 to 15 parts of resin.

[0067] Specifically, the silver powder is selected at the nano or micro scale. Nano silver powder has a density of 10.49 g / cm³.3 CTE = 18.9 × 10 -6 / ℃, bulk modulus K=90.3GPa, resistivity= 1.59×10 -8 Ω In the interface conductive layer 111, silver powder, as the main conductive component, has a content ranging from 50 to 70 parts, aiming to ensure that the interface conductive layer 111 possesses excellent conductivity to effectively transmit current. The silver powder can be in various forms, such as spherical silver powder, which has good flowability and filling properties, helping to form a dense conductive network; or flake-shaped silver powder, which can form continuous conductive pathways even with a lower filling amount, thereby further improving conductivity efficiency.

[0068] Inorganic non-metallic ceramic materials include silicon nitride powder and alumina powder. Silicon nitride powder has a density of 3.20 g / cm³. 3 CTE = 3.0 × 10 -6 / ℃, bulk modulus K=300.0GPa, resistivity= 10 14 Ω The content of m ranges from 20 to 35 parts. Its main function is to adjust the coefficient of thermal expansion of the interfacial conductive layer 111, so that it can better match the difference in thermal expansion between the cell 10 and the solder ribbon 20, while enhancing the mechanical strength and stability of the material. Different crystal forms of silicon nitride powder can be selected, such as α-silicon nitride powder, which has high purity and fine particle size, helping to achieve uniform dispersion and precise control of the coefficient of thermal expansion; or β-silicon nitride powder, which may provide different mechanical properties in some applications. In addition, surface-modified silicon nitride powder can also be used to improve its dispersion uniformity in the resin matrix. Alumina powder, CTE = 7.0 × 10⁻⁶ -6 At a temperature of ℃, a content ranging from 28 to 38 parts can maintain the CTE of the interfacial conductive layer 111 at 5.0-7.5 × 10⁻⁶. -6 Within a temperature range of / ℃, it can achieve a smooth transition of CTE gradient with the solar cell 10 and high conductivity.

[0069] Resins include epoxy resin and silicone resin. Epoxy resin has a density of 1.25 g / cm³. 3 CTE = 65.0 × 10 -6 / ℃, bulk modulus K=5.0GPa, resistivity= 10 14 Ω The content of m, ranging from 5 to 15 parts, serves as a binder for the interface conductive layer 111. Its function is to effectively bond the silver powder and silicon nitride powder together, forming a stable composite material structure, and ensuring good adhesion between the interface conductive layer 111 and the surface of the solar cell 10. The epoxy resin can be bisphenol A type epoxy resin, which has good comprehensive performance and a wide range of applications; or bisphenol F type epoxy resin, which typically has lower viscosity, facilitating uniform mixing and coating of the material. The temperature resistance of the silicone resin can be increased to 200℃.

[0070] By adopting the above technical solution, the thermal expansion coefficient, conductivity, and interfacial bonding strength of the interface conductive layer 111 are optimized by precisely defining the material composition and proportion. The appropriate proportion of silver powder ensures that the interface conductive layer 111 has high conductivity, effectively transferring current and reducing resistance loss. The introduction of inorganic non-metallic ceramic materials allows for precise control of the thermal expansion coefficient of the interface conductive layer 111, placing it between the thermal expansion coefficients of the solar cell 10 and the solder ribbon 20, thereby effectively mitigating interfacial shear stress caused by the mismatch in thermal expansion coefficients during temperature cycling. Resin, as a binder, not only ensures the uniform dispersion of each component and the structural integrity of the material but also significantly enhances the bonding strength between the interface conductive layer 111 and the solar cell 10, avoiding potential peeling during long-term use. This multi-component synergistic design allows the interface conductive layer 111 to more effectively buffer thermal stress, reducing the risk of microcracks and interfacial peeling in the solar cell 10, thus significantly improving the long-term reliability and service life of the battery string. The use of commonly used photovoltaic materials lowers the industrialization threshold.

[0071] In this embodiment, silicon nitride powder is used as an example of inorganic non-metallic ceramic material, and epoxy resin is used as an example of resin. The implementation principle of a battery string is as follows: Composition (by weight) of the conductive interfacial layer (in contact with the surface of the solar cell): silver powder (50%-70%, nano or micron grade) + silicon nitride powder (20%-35%) + epoxy resin (5%-15%). Note: All of these are commonly used photovoltaic conductive, insulating, and bonding materials with wide sources. Selected composition (parts by weight): nano silver powder (60%) + silicon nitride powder (30%) + epoxy resin (10%). Material parameters (common standard values ​​in the photovoltaic industry): Nano silver powder: density 10.49 g / cm³ 3 CTE = 18.9 × 10 -6 / ℃, bulk modulus K=90.3 GPa, resistivity=1.59×10 -8 Ω m; Silicon nitride powder: density 3.20 g / cm³3 CTE = 3.0 × 10 -6 / ℃, bulk modulus K=300.0 GPa, resistivity=10 14 Ω m; Epoxy resin: density 1.25 g / cm³ 3 CTE = 65.0 × 10 -6 / ℃, bulk modulus K=5.0 GPa, resistivity=10 14 Ω m.

[0072] Core computational logic: 1. Volume fraction calculation: Volume = Mass / Density, where the properties of the composite material are used as the basis for calculation.

[0073] Assuming a total mass of 100g, the optimal formula is calculated as follows: V_Ag=60g / 10.49g / cm 3 ≈5.719cm 3 ; V_Si3N4=30g / 3.20g / cm 3 ≈9.375cm 3 ; V_resin = 10g / 1.25g / cm 3 =8.000cm 3 ; V_total=5.719+9.375+8.000=23.094cm 3 ; φ_Ag=5.719 / 23.094≈24.76%; φ_Si3N4=9.375 / 23.094≈40.59%; φ_resin=8.000 / 23.094≈34.65%.

[0074] Where V represents the volume of the corresponding material, V_Ag represents the volume of silver powder, V_Si3N4 represents the volume of silicon nitride powder, V_resin represents the volume of resin, and V_total represents the total volume; correspondingly, φ represents the volume fraction of the corresponding material in the total volume.

[0075] 2. CTE Calculation (Improved Turner Model): α_c=(Σφ_i×K_i×α_i) / (Σφ_i×K_i) [Note: Applicable to highly filled composite materials, considering bulk modulus weighting] The numerator is calculated as follows: 0.2476 × 90.3 × 18.9 + 0.4059 × 300.0 × 3.0 + 0.3465 × 5.0 × 65.0 = 899.49. Denominator = 0.2476 × 90.3 + 0.4059 × 300.0 + 0.3465 × 5.0 = 145.86; α_c = 899.49 / 145.86 ≈ 6.17 × 10 -6 / ℃.

[0076] Where α_c represents the coefficient of thermal expansion of the composite material; Σ represents the summation over all different components (i) in the composite material; φ_i represents the volume fraction of the i-th component (where i represents silver powder, silicon nitride powder, or resin); K_i represents the bulk modulus of the i-th component (characterizing the material's compressive strength); and α_i represents the coefficient of thermal expansion of the i-th component.

[0077] 3. Resistivity calculation (parallel model, simplified model dominated by the conductive phase, only the silver phase participates in conduction): 1 / ρ_eff=φ_Ag / ρ_Ag [Note: Only the conductive phase participates in conduction; ceramics and resins are considered insulators] 1 / ρ_eff=0.2476 / (1.59×10 -8 ) = 1.557 × 10 7 ); ρ_eff = 6.42 × 10 -8 Ω m = 6.42 × 10 -6 Ω cm.

[0078] Wherein, ρ_eff: effective resistivity of the composite material; φ_Ag: volume fraction of silver powder; ρ_Ag: resistivity of silver powder.

[0079] Features: High conductivity, strong adhesion, low CTE compatibility, ensuring current transmission and interface stability, and strong material compatibility.

[0080] 4. Process compatibility: 400-mesh stainless steel screen, printing speed 200mm / s, wet film thickness 8 to 10μm (5 to 8μm after drying). The Turner model is applicable under the following conditions: This calculation assumes good interfacial bonding between phases and a porosity of <3% after sintering, making it suitable for high-fill-ratio dense systems.

[0081] Parallel resistance model applicable conditions: This model ignores the tunneling effect and seepage threshold, and is applicable to systems with a conductive phase volume fraction >20% and continuous distribution.

[0082] In at least one embodiment, the interface conductive layer 111 comprises, by weight, 60 parts of nano-sized silver powder, 30 parts of silicon nitride powder, and 10 parts of epoxy resin.

[0083] In some embodiments, the coefficient of thermal expansion of the gradient transition layer 112 is 12 × 10⁻⁶. -6 / ℃ to 18×10 -6 / ℃, the thickness of the gradient transition layer 112 is 8 to 15 μm.

[0084] Specifically, the coefficient of thermal expansion of the gradient transition layer 112 is set at 12 × 10⁻⁶. -6 / ℃ to 18×10 -6 Within a range of 8 °C, the gradient transition layer 112 is designed to ensure a smooth and effective transition of thermal expansion coefficients between the interface conductive layer 111 and the weld adapter layer 113. This range allows the gradient transition layer 112 to effectively bridge the gap between the lower thermal expansion coefficient of the interface conductive layer 111 (close to that of the solar cell 10) and the higher thermal expansion coefficient of the weld adapter layer 113 (close to that of the solder strip 20), thereby avoiding excessive thermal stress concentration between layers. This can be achieved, for example, by precisely controlling the composition of the gradient transition layer 112 material, such as adjusting the ratio of polymer matrix to inorganic fillers (e.g., metal powder, ceramic powder), or by using composite materials with specific thermal expansion characteristics. Another approach is to adjust the overall thermal expansion behavior by introducing microstructures with different thermal expansion coefficients during material preparation, such as through porous structures or fiber reinforcement. Simultaneously, the thickness of the gradient transition layer 112 is set within the range of 8 to 15 μm to provide sufficient material volume to effectively absorb and disperse thermal stress generated during temperature cycling. If the thickness is too small, the gradient transition layer 112 may not be able to adequately buffer stress, causing stress to be directly transferred to the solar cell 10 or the interface, leading to microcracks or delamination. If the thickness is too large, it may increase material costs and may adversely affect the overall electrical conductivity or thermal conductivity of the battery string. For example, the thickness of the gradient transition layer 112 can be precisely controlled using precision coating techniques such as screen printing, spraying, or spin coating. Another approach is to adjust the viscosity and solids content of the coating material, as well as the parameters of the coating equipment, to ensure that the target thickness is achieved after curing.

[0085] By adopting the above technical solution, and specifically limiting the thermal expansion coefficient of the gradient transition layer 112 to 12 × 10⁻⁶, -6 / ℃ to 18×10 -6The range of / ℃ and the thickness of the barrier layer 11, ranging from 8 to 15 μm, optimize the performance of the barrier layer 11 in buffering thermal stress. This range of thermal expansion coefficients ensures that the gradient transition layer 112 forms a continuous and gradual thermal expansion gradient between the interface conductive layer 111 and the weld adapter layer 113, effectively smoothing the large difference in thermal expansion coefficients between the cell 10 and the solder ribbon 20, thereby significantly reducing the accumulation of interfacial shear stress. Simultaneously, this thickness range provides the gradient transition layer 112 with sufficient flexibility and volume, enabling it to effectively absorb and disperse the thermal stress generated under long-term temperature cycling, avoiding microcracks or interface delamination in the cell 10 caused by stress concentration. Through the above technical solutions, the battery string can maintain structural integrity and electrical connection reliability under extreme temperature changes, significantly improving the long-term reliability and service life of the battery string.

[0086] In some embodiments, the gradient transition layer 112 comprises, by weight, 70 to 80 parts silver powder, 5 to 10 parts inorganic non-metallic ceramic material, and 10 to 15 parts silicone-modified epoxy resin.

[0087] Specifically, the gradient transition layer 112 contains silver powder as the main conductive component. The silver powder is selected as micron-sized silver powder with a density of 10.49 g / cm³. 3 CTE = 18.9 × 10 -6 / ℃, bulk modulus K=89.7GPa, resistivity=1.59×10 -8 Ω The content of m, at 70 to 80 parts, is designed to provide excellent conductivity, ensuring efficient current transfer between the cell 10 and the solder ribbon 20, while its own coefficient of thermal expansion helps maintain the overall thermal expansion characteristics of the gradient transition layer 112 within a preset range. In addition to silver powder, other metal powders with good conductivity and suitable coefficients of thermal expansion, such as copper powder, nickel powder, or their alloy powders, can be used to achieve similar conductivity and thermal expansion characteristics.

[0088] Inorganic non-metallic ceramic materials include aluminum nitride powder and silicon carbide powder. Aluminum nitride powder has a density of 3.26 g / cm³. 3 CTE = 4.5 × 10 -6 / ℃, bulk modulus K=275.0GPa, resistivity= 10 14 Ω The aluminum nitride powder, with a content of 5 to 10 parts, primarily functions to precisely adjust the coefficient of thermal expansion of the gradient transition layer 112, enabling it to better match the thermal expansion difference between the solar cell 10 and the solder ribbon 20, thereby effectively buffering thermal stress. Furthermore, aluminum nitride powder enhances the mechanical strength and thermal stability of the material. Besides aluminum nitride powder, other ceramic powders with high thermal conductivity, low coefficient of thermal expansion, and chemical stability, such as silicon nitride powder, alumina powder, or silicon carbide powder, can also be selected to achieve the purpose of adjusting the coefficient of thermal expansion and enhancing mechanical properties. Specifically, silicon carbide powder has a CTE of 4.0 × 10⁻⁶. -6 / ℃, with a content of 6 to 10 parts.

[0089] Organosilicon-modified epoxy resin: density 1.42 g / cm³ 3 CTE = 45.0 × 10 -6 / ℃, bulk modulus K=4.0GPa, resistivity= 10 16 Ω m, in a content of 10 to 15 parts, serves as a binder. It not only provides excellent adhesion and enhances the interfacial bonding between the gradient transition layer 112 and adjacent layers, but the introduction of silicone also imparts better flexibility and thermal cycling resistance to the resin, thereby effectively dispersing stress and reducing the risk of peeling and cracking due to thermal expansion mismatch. Besides silicone-modified epoxy resin, other polymeric materials with good adhesion, flexibility, and thermal cycling resistance, such as polyimide resin, polyurethane resin, or acrylic resin, can also be used to achieve both bonding and stress buffering functions.

[0090] By adopting the above technical solution, the weight ratio of silver powder, inorganic non-metallic ceramic materials, and organosilicon-modified epoxy resin in the gradient transition layer 112 is precisely defined, so that the coefficient of thermal expansion of the gradient transition layer 112 can be precisely controlled at 12 × 10⁻⁶ while maintaining good electrical conductivity. -6 / ℃ to 18×10 -6 The ideal range is / ℃ (e.g., the thermal expansion coefficient of the gradient transition layer 112 mentioned above is 12×10). -6 / ℃ to 18×10 -6 The addition of silicone-modified epoxy resin significantly enhances the interfacial bonding between the gradient transition layer 112, the interface conductive layer 111, and the welding adapter layer 113, thereby improving the material's flexibility, effectively dispersing thermal stress, and preventing microcracks and interface delamination in the cell 10 caused by stress concentration. This optimized material composition works synergistically to ensure the reliability and stability of the battery string under long-term temperature cycling, significantly extending the battery string's lifespan. The use of commonly used photovoltaic materials lowers the industrialization threshold.

[0091] Taking aluminum nitride powder as an example, the implementation principle of a battery string in this embodiment is as follows: Composition (mass range) of the gradient transition layer (intermediate buffer layer): Silver powder (70%-80%, micron-sized) + aluminum nitride powder (5%-10%) + silicone-modified epoxy resin (10%-15%).

[0092] Note: All are commonly used photovoltaic materials; silicone-modified epoxy resin improves weather resistance. Selected composition (parts by weight): micron silver powder (78%) + aluminum nitride powder (8%) + silicone-modified epoxy resin (14%). Material parameters (common standard values ​​in the photovoltaic industry): Micron-sized silver powder: density 10.49 g / cm³ 3 CTE = 18.9 × 10 -6 / ℃, bulk modulus K=89.7 GPa, resistivity=1.59×10 -8 Ω m; Aluminum nitride powder: density 3.26 g / cm³ 3 CTE = 4.5 × 10 -6 / ℃, bulk modulus K=275.0 GPa, resistivity=10 14 Ω m; Organosilicon-modified epoxy resin: density 1.42 g / cm³ 3 CTE = 45.0 × 10 -6 / ℃, bulk modulus K=4.0 GPa, resistivity=10 16 Ω m; Core computational logic: 1. Volume fraction calculation: Assuming a total mass of 100g, the optimal formula is calculated as follows: Silver powder volume fraction increases conductivity, and aluminum nitride adjusts CTE.

[0093] V_Ag=78g / 10.49g / cm 3 ≈7.436cm 3 ; V_AlN=8g / 3.26g / cm 3 ≈2.454cm 3 ; V_resin = 14g / 1.42g / cm 3 ≈9.859cm 3 ; V_total=7.436+2.454+9.859=19.749cm3 ; φ_Ag=7.436 / 19.749≈37.65%; φ_AlN=2.454 / 19.749≈12.43%; φ_resin=9.859 / 19.749≈49.92%.

[0094] Where V represents the volume of the corresponding material, V_Ag represents the volume of silver powder, V_AlN represents the volume of aluminum nitride powder, V_resin represents the volume of resin, and V_total represents the total volume; correspondingly, φ represents the volume fraction of the corresponding material in the total volume.

[0095] 2. CTE Calculation (Improved Turner Model): α_c=(Σφ_i×K_i×α_i) / (Σφ_i×K_i) where the CTE of the intermediate layer is taken as the average value of the cell and the solder strip 20 to achieve a smooth transition.

[0096] The numerator is calculated as follows: 0.3765 × 89.7 × 18.9 + 0.1243 × 275.0 × 4.5 + 0.4992 × 4.0 × 45.0 = 886.11. Denominator = 0.3765 × 89.7 + 0.1243 × 275.0 + 0.4992 × 4.0 = 70.17; α_c = 886.11 / 70.17 ≈ 12.63 × 10 -6 / ℃.

[0097] Where α_c represents the coefficient of thermal expansion of the composite material; ∑ represents the summation of all different components (i) in the composite material; φ_i represents the volume fraction of the i-th component (where i represents silver powder, aluminum nitride powder or resin); K_i represents the bulk modulus of the i-th component; and α_i represents the coefficient of thermal expansion of the i-th component.

[0098] 3. Resistivity calculation (parallel model): 1 / ρ_eff=φ_Ag / ρ_Ag, where only the conductivity of silver powder is considered.

[0099] 1 / ρ_eff=0.3765 / (1.59×10 -8 ) = 2.368 × 10 7 ; ρ_eff = 4.22 × 10 -8 Ω m = 4.22 × 10 -6 Ω cm.

[0100] Where ρ_eff represents the effective resistivity of the composite material; φ_Ag represents the volume fraction of silver powder; and ρ_Ag represents the resistivity of the silver powder.

[0101] Functions: CTE gradient transition, stress buffering, enhanced structural rigidity, and adaptability to the long-term use environment of photovoltaic modules.

[0102] 4. Process compatibility: 350 mesh screen, natural leveling for 2 minutes after printing, wet film thickness 15 to 18 μm (8 to 15 μm after drying).

[0103] In at least one embodiment, the gradient transition layer 112 comprises, by weight, 78 parts of micron-sized silver powder, 8 parts of aluminum nitride powder, and 14 parts of silicone-modified epoxy resin.

[0104] In some embodiments, the coefficient of thermal expansion of the welding adapter layer 113 is 18 × 10⁻⁶. -6 / ℃ to 24×10 -6 / ℃, the thickness of the welding adapter layer 113 is 7 to 12 μm.

[0105] Specifically, the coefficient of thermal expansion of the welding adapter layer 113 is limited to 18 × 10⁻⁶. -6 / ℃ to 24×10 -6 Within a range of / ℃, its function is to provide a transition layer with a coefficient of thermal expansion similar to that of the solder ribbon 20, effectively alleviating the interfacial stress caused by the large difference in coefficients of thermal expansion between the battery cell 10 and the solder ribbon 20. One way to achieve this range of coefficients of thermal expansion is by precisely adjusting the composition of the welding adapter layer 113 material. For example, a metal powder with a high coefficient of thermal expansion (such as tin-based alloy powder or lead-based alloy powder) can be composited with an appropriate proportion of a polymer matrix (such as epoxy resin or polyimide), and the type and content of the metal powder and the type of polymer can be adjusted to ensure that the overall coefficient of thermal expansion falls within the target range. Another approach is to use a multi-layer composite structure, by stacking materials with different coefficients of thermal expansion and controlling the thickness ratio of each layer, so that the overall welding adapter layer 113 exhibits the required equivalent coefficient of thermal expansion. For example, a high CTE metal layer can be combined with a medium CTE ceramic or polymer layer to achieve the thermal expansion characteristics within the target range.

[0106] Meanwhile, the welding adapter layer 113 has a thickness of 7 to 12 μm. Its function is to provide sufficient buffer space to absorb and disperse thermal stress, while ensuring good conductivity and interfacial bonding strength. A layer that is too thin may not effectively buffer stress, causing stress to be directly transmitted to the solar cell 10 or the interface, while a layer that is too thick may increase material costs, affect conductivity efficiency, or introduce new structural instabilities. One way to achieve this thickness is to uniformly coat the welding adapter layer 113 material onto the surface of the gradient transition layer 112 using precision coating techniques, such as blade coating, roller coating, or spray coating, and achieve a thickness of 7 to 12 μm by precisely controlling the coating amount and drying and curing conditions. Another way to achieve this is to use screen printing technology, by selecting an appropriate screen mesh count, squeegee pressure, and number of printing passes, as well as adjusting the viscosity and solids content of the paste, to precisely control the thickness of the printed layer, thereby obtaining the required welding adapter layer 113 thickness of 7 to 12 μm.

[0107] By adopting the above technical solution, the coefficient of thermal expansion of the welding adapter layer 113 is precisely limited to 18×10. -6 / ℃ to 24×10 -6 Within a temperature range of ℃ and with a thickness controlled between 7 and 12 μm, this application ensures a more matched thermal expansion characteristic between the welding adapter layer 113 and the solder ribbon 20, thereby significantly reducing the shear stress at the interface caused by temperature changes. This precise range of thermal expansion coefficients makes the thermal expansion gradient transition smoother, avoiding stress concentration and effectively mitigating the significant difference in thermal expansion coefficients between the cell 10 and the solder ribbon 20. Simultaneously, the 7 to 12 μm thickness provides ample buffer space for thermal stress, allowing the welding adapter layer 113 to effectively absorb and disperse thermal stress, preventing stress from being directly transferred to the cell 10, thus avoiding the risk of microcracks and interface delamination in the cell 10. This optimization not only enhances the structural stability of the cell string but also significantly improves its reliability and durability under long-term temperature cycling, ensuring the 25-year design life of the photovoltaic module.

[0108] In some embodiments, the soldering adapter layer 113 comprises 80 to 90 parts of solder powder and 10 to 20 parts of silver powder by weight.

[0109] Specifically, the solder powder is Sn60Pb40 alloy powder or SAC305 alloy powder (Sn96.5Ag3Cu0.5). Sn60Pb40 alloy powder is a solder alloy powder composed of 60% tin (Sn) and 40% lead (Pb), with a density of 8.56 g / cm³. 3 CTE = 24.5 × 10 -6 / ℃, resistivity = 1.70×10 -7 Ω m; it has a coefficient of thermal expansion similar to that of the solder ribbon 20 (especially its Sn60Pb40 coating). In the solder adapter layer 113, Sn60Pb40 alloy powder is the main component, and its high content (80 to 90 parts) aims to provide thermal expansion characteristics highly matched to the solder ribbon 20, thereby effectively buffering and dispersing thermal stress when the battery string undergoes temperature cycling. The Sn60Pb40 alloy powder can be prepared by atomization or centrifugation to obtain powders with different particle size distributions, thereby optimizing its dispersibility and sintering performance in the slurry. Among them, SAC305 alloy powder (Sn96.5Ag3Cu0.5) is lead-free and environmentally friendly. Furthermore, the silver powder is selected from nano- or micron-sized silver powder, fine particles of highly conductive metallic silver, wherein the nano-sized silver powder has a density of 10.49 g / cm³. 3 CTE = 18.9 × 10 -6 / ℃, resistivity = 1.59×10 -8 Ω m, possessing excellent electrical and thermal conductivity, and a relatively high coefficient of thermal expansion. In the solder adapter layer 113, the addition of silver powder (10 to 20 parts) primarily serves to significantly improve the layer's conductivity, ensuring efficient current transmission. It also helps to fine-tune the overall coefficient of thermal expansion of the solder adapter layer 113, making it more precisely within the target range, and enhances the layer's mechanical strength and fatigue resistance. The silver powder can be in a spherical or sheet-like structure; spherical silver powder helps improve the slurry's fluidity and filling density, while sheet-like silver powder may provide better conductive pathways in specific directions.

[0110] By adopting the above technical solution, using solder powder as the main component of the soldering adapter layer 113, and precisely controlling its content to be between 80 and 90 parts, it is possible to ensure that the overall coefficient of thermal expansion of the soldering adapter layer 113 is 18 × 10⁻⁶, as described in the above solution. -6 / ℃ to 24×10 -6 (°C) and the coefficient of thermal expansion of solder strip 20 (approximately 24.5 × 10⁻⁶). -6The difference between the coefficients of thermal expansion (C / ℃) is minimal. This precise matching of thermal expansion coefficients significantly reduces the shear stress at the interface between the cell 10 and the solder ribbon 20 during long-term temperature cycling of the battery string, effectively preventing microcracks and interface delamination in the cell 10 caused by excessive thermal stress, thus greatly improving the long-term reliability of the battery string. Simultaneously, the addition of 10 to 20 parts of silver powder significantly enhances the conductivity of the solder adapter layer 113, ensuring efficient current transfer from the cell 10 to the solder ribbon 20 even while buffering thermal stress, reducing energy loss and maintaining the electrical performance of the battery string. This specific material combination not only solves the material composition problem of the solder adapter layer 113 itself, but also enables the entire barrier layer 11 system (including the interface conductive layer 111, the gradient transition layer 112, and the solder adapter layer 113) to more effectively mitigate the large difference in thermal expansion between the cell 10 and the solder ribbon 20, thereby comprehensively improving the reliability and performance of the battery string. Furthermore, while ensuring excellent performance, this formulation also considers the compatibility of paste preparation and screen printing processes, which helps to form a uniform and dense welding adapter layer 113, further enhancing the bonding strength and long-term stability of the interface. The use of commonly used photovoltaic materials lowers the industrialization threshold.

[0111] Taking Sn60Pb40 alloy solder powder as an example, the implementation principle of a battery string in this embodiment is as follows: Composition (parts by weight) of the welding adapter layer (in contact with the solder strip): Sn60Pb40 alloy powder (80%-90%) + silver powder (10%-20%, nano or micron grade) Note: Sn60Pb40 is a traditional photovoltaic solder, and silver powder improves its conductivity; Selected composition (parts by weight): Sn60Pb40 alloy powder (85%) + nano silver powder (15%). Material parameters (common standard values ​​in the photovoltaic industry): Sn60Pb40 alloy powder: density 8.56 g / cm³ 3 CTE = 24.5 × 10 -6 / ℃, resistivity = 1.70×10 -7 Ω m; Nano silver powder: density 10.49 g / cm³ 3 CTE = 18.9 × 10 -6 / ℃, resistivity = 1.59×10 -8 Ω m; Core computational logic: 1. Volume fraction calculation: Assuming a total mass of 100g, the preferred formula is calculated to ensure good wetting of the solder ribbon by including a high tin-lead alloy content.

[0112] V_SnPb=85g / 8.56g / cm 3 ≈9.929cm 3 ; V_Ag = 15g / 10.49g / cm³ 3 ≈1.430cm 3 ; V_total = 9.929 + 1.430 = 11.359 cm 3 ; φ_SnPb=9.929 / 11.359≈87.41%; φ_Ag=1.430 / 11.359≈12.59%).

[0113] Where V represents the volume of the corresponding material, V_SnPb represents the volume of Sn60Pb40 alloy powder, V_Ag represents the volume of silver powder, and V_total represents the total volume; correspondingly, φ represents the volume fraction of the corresponding material in the total volume.

[0114] 2. CTE Calculation (Mixing Rule): α_c = φ_SnPb × α_SnPb + φ_Ag × α_Ag, which is applicable to isotropic materials that are physically mixed.

[0115] α_c=0.8741×24.5 + 0.1259×18.9≈23.79×10 -6 / ℃ 3. Resistivity calculation (parallel model): 1 / ρ_eff = φ_SnPb / ρ_SnPb + φ_Ag / ρ_Ag The two conductive phases work together.

[0116] 1 / ρ_eff=0.8741 / (1.70×10 -7 ) + 0.1259 / (1.59×10 -8 ) = 5.142 × 10 6 + 7.918×10 6 =1.306×10 7 ; ρ_eff = 7.66 × 10 -8 Ω m = 7.66 × 10 -6 Ω cm.

[0117] Wherein, ρ_eff represents the effective resistivity of the composite material; φ_SnPb represents the volume fraction of Sn60Pb40 alloy powder; ρ_SnPb represents the resistivity of Sn60Pb40 alloy powder; φ_Ag represents the volume fraction of silver powder; and ρ_Ag represents the resistivity of silver powder. Function: Suitable for welding, highly conductive, and highly compatible with commonly used Sn60Pb40 coated solder ribbon in photovoltaics, ensuring welding reliability.

[0118] 4. Process compatibility: 380 mesh screen, wet film thickness 10 to 12 μm (7 to 12 μm after drying), alloy powder melts to form a metallurgical bond at sintering peak temperature of 250℃.

[0119] In at least one embodiment, the welding adapter layer 113 comprises, by weight, 85 parts of Sn60Pb40 alloy powder and 15 parts of nano-sized silver powder.

[0120] Specifically, the interface stress gradient analysis and theoretical estimation are as follows: 1. Traditional structural stress benchmark: In the traditional scheme, the battery cell 10 is directly welded to the solder strip 20, and the single CTE jump Δα = 21.9 × 10 -6 / ℃, the theoretical interfacial shear stress τ0≈11 MPa, which is the benchmark value for stress analysis.

[0121] 2. The stress decomposition of the gradient structure in this application decomposes a single jump into four small steps through a three-layer design, and the CTE difference of each interface is shown in Table 1.

[0122] Table 1. Decomposition of interfacial stress in a solar cell with a three-layer gradient barrier layer.

[0123] 3. Based on simplified thermoelastic analysis, the interfacial shear stress is mainly affected by the CTE jump amplitude. This scheme reduces the maximum CTE jump from 21.9 × 10⁻⁶. -6 / ℃ decreased to 11.16×10 -6 At / ℃, the theoretically estimated stress concentration factor can be reduced by approximately 49%. The actual stress distribution is affected by multiple factors such as layer thickness, roughness, and interfacial bonding strength. Specific parameters need to be determined through a limited number of experiments. This specification does not limit the protection range based on theoretical calculation values.

[0124] One or more embodiments of this application also disclose a method for preparing a battery string as described above. (Refer to...) Figure 7 This includes the following steps: S1: On the surface of the battery cell 10 of the battery string, a conductive interface layer 111, a gradient transition layer 112 and a welding adapter layer 113 are sequentially screen-printed along a direction perpendicular to the battery cell 10. S2: Under a nitrogen atmosphere, heat the battery cell 10 to 110 to 130°C and hold for 2 to 4 minutes for preheating, heat to 170 to 190°C and hold for 4 to 6 minutes for curing, and heat to 230 to 250°C and hold for 5 to 7 minutes. S3: Take the solder strip 20 and connect it to the welding adapter layer 113 of the first battery cell 10 and the welding adapter layer 113 of the second battery cell 10 respectively.

[0125] Specifically, the step of "screen printing an interface conductive layer 111, a gradient transition layer 112, and a welding adapter layer 113 sequentially on the surface of the battery cells 10 of the battery string along a direction perpendicular to the battery cells 10" aims to precisely control the deposition position, thickness, and sequence of each functional layer. Screen printing, as a mature thick-film preparation technology, can achieve precise patterning and thickness control of the printing paste. Specifically, a high-precision automatic screen printing machine can be used, and by adjusting the mesh count of the screen (e.g., 200 to 300 mesh) and the squeegee pressure, it can be ensured that each layer of paste is uniformly coated with a preset thickness. For example, the interface conductive layer 111 can be printed using a paste composed of silver powder, silicon nitride powder, and epoxy resin; the gradient transition layer 112 can be printed using a paste composed of silver powder, aluminum nitride powder, and silicone-modified epoxy resin; and the welding adapter layer 113 can be printed using a paste composed of Sn60Pb40 alloy powder and silver powder. Each paste requires precise control of its solid content (e.g., 75% to 80%) and viscosity (e.g., 300 ± 50 Pa·s) based on its composition and printing requirements, and is ground and dispersed to a fineness of less than 15 μm to ensure uniformity and precision in printing. This sequential printing method ensures a continuous and smooth transition between material layers starting from the surface of the solar cell 10, laying the foundation for the subsequent formation of a gradient in the coefficient of thermal expansion.

[0126] The step of "preheating the battery cell 10 to 110-130°C for 2-4 minutes under a nitrogen atmosphere, then heating it to 170-190°C for 4-6 minutes for curing, and finally heating it to 230-250°C for 5-7 minutes" is crucial for achieving the structural stability and functionality of the barrier layer 11. First, a nitrogen atmosphere is used to provide an inert environment, requiring an oxygen content of <20ppm. This effectively prevents oxidation reactions of the metal components (such as silver and tin-lead alloys) and organic binders (such as epoxy resin) in each layer during high-temperature processing, thus ensuring the intrinsic properties and interfacial bonding strength of the materials. Second, the staged heating process has a clear purpose: the preheating stage (110-130°C for 2-4 minutes) is mainly used to gently remove volatile solvents from the printing paste, avoiding intralayer defects (such as bubbles and cracks) or interlayer delamination caused by rapid solvent evaporation, thus preparing for the subsequent curing process. During the curing stage (170 to 190°C, held for 4 to 6 minutes), the polymer binders (such as epoxy resin and silicone-modified epoxy resin) in each layer undergo a full cross-linking reaction within this temperature range, forming a dense polymer network. This not only endows each layer with the necessary mechanical strength, but more importantly, promotes chemical bonding and physical entanglement between layers, significantly enhancing interfacial adhesion. The high-temperature holding stage (230 to 250°C, held for 5 to 7 minutes) further promotes the densification of each layer, especially for layers containing metal powder, facilitating the sintering and fusion of metal particles and optimizing their electrical conductivity and thermal stability. Simultaneously, this temperature range ensures the full melting and recrystallization of the Sn60Pb40 alloy powder in the welding adapter layer 113, enabling it to form a welding interface well-matched with the welding strip 20. The entire process can be achieved using a multi-temperature zone conveyor belt sintering furnace, precisely controlling the temperature and residence time of each zone, and continuously introducing high-purity nitrogen to maintain an inert atmosphere.

[0127] The step of "attaching the solder ribbon 20 to the welding adapter layer 113 of the first and second adjacent solar cells 10" aims to establish a reliable electrical and mechanical connection between the solar cells 10. The solder ribbon 20 is typically pre-coated with solder (e.g., Sn60Pb40 alloy). By heating (e.g., using hot-press welding, infrared welding, or laser welding), the solder on the solder ribbon 20 melts and forms a metallurgical bond with the pre-prepared welding adapter layer 113 on the surface of the solar cell 10. The welding adapter layer 113, due to its high coefficient of thermal expansion (e.g., 18 × 10⁻⁶), provides a strong bond. -6 / ℃ to 24×10 -6 / ℃), and the coefficient of thermal expansion of the solder strip 20 (e.g., 24.5 × 10⁻⁶). -6The welding strip 20 and the barrier layer 11 are highly matched (at a specific temperature range), effectively mitigating thermal stress between them during welding and subsequent temperature cycling, ensuring long-term reliability of the connection. Furthermore, the projected width of the welding strip 20 along the thickness direction of the solar cell 10 is 0.05 to 0.10 mm smaller than the projected width of the barrier layer 11 along the thickness direction of the solar cell 10. This dimensional design further optimizes stress distribution and avoids stress concentration. Using mature technology, no new equipment or modification of existing equipment is required; industrialization can be achieved simply by adjusting the screen parameters and sintering curve, lowering the industrialization threshold.

[0128] By adopting the above technical solution, the problems of discontinuous gradients, weak interfacial bonding, or welding adapter failures caused by improper process control during battery string fabrication can be effectively solved. Specifically, step S1 uses screen printing, which can precisely control the deposition sequence and position of the interface conductive layer 111, gradient transition layer 112, and welding adapter layer 113, ensuring a continuous gradient distribution starting from the surface of the battery cell 10. This maintains a smooth transition of the coefficient of thermal expansion during subsequent use and avoids gradient interruption. Step S2 involves staged temperature-curing under a nitrogen atmosphere. The inert environment prevents material oxidation, and precise temperature gradient control promotes chemical bonding and physical cross-linking between the layers, significantly enhancing interlayer and intralayer bonding and effectively solving the problem of weak interfacial bonding. In addition, this curing process ensures that each layer achieves its expected mechanical and thermal properties, providing a solid foundation for realizing the overall gradient of the coefficient of thermal expansion. Step S3 directly connects the solder ribbon 20 to the welding adapter layer 113, which has a high coefficient of thermal expansion. This fully utilizes the good thermal expansion matching between the welding adapter layer 113 and the solder ribbon 20, ensuring a reliable connection between the solder ribbon 20 and the barrier layer 11. It also effectively reduces the impact of welding thermal stress on the gradient structure, thereby preventing welding adapter failure. Overall, this fabrication method, through the coordinated synergistic effect of each step, ensures the stability, functionality, and structural integrity of the barrier layer 11 during the fabrication stage. This allows the battery string to effectively alleviate thermal stress between the battery cell 10 and the solder ribbon 20 under long-term temperature cycling, significantly reducing the risk of microcracks and interface delamination in the battery cell 10, thus greatly improving the long-term reliability and service life of the battery string.

[0129] Interface conductive layer 111, gradient transition layer 112 and welding adapter layer 113.

[0130] In at least one embodiment, step S2 involves gradient sintering under a nitrogen atmosphere. The battery cell 10 is preheated by heating it to 120°C and holding it at that temperature for 3 minutes to remove the solvent used to dissolve the materials of the interface conductive layer 111, the gradient transition layer 112, and the welding adapter layer 113. The temperature is then raised to 180°C and held for 5 minutes for resin curing. Finally, the temperature is raised to 240°C and held for 6 minutes to ensure that the alloy powder melts and metallurgically bonds.

[0131] In some embodiments, in S1, the materials of the interface conductive layer 111, the gradient transition layer 112 and the welding adapter layer 113 are all added to an organic solvent to a solid content of 75% to 80% and a viscosity of 300±50 Pa·s, and then ground and dispersed to a fineness of less than 15 μm.

[0132] Specifically, the addition of organic solvents to the materials aims to adjust the rheological properties of the interfacial conductive layer 111, the gradient transition layer 112, and the welding adapter layer 113 to meet the requirements of the screen printing process. The selection of organic solvents is typically based on their solubility in the polymers or resins in the material, their evaporation rate, and their compatibility with various powder components. For example, high-boiling-point solvents such as terpineol or butyl carbitol acetate can be used to ensure sufficient handling time during printing and reduce viscosity changes caused by rapid solvent evaporation. Alternatively, a mixed solvent system can be used, such as mixing a faster-evaporating solvent with a slower-evaporating solvent in a certain proportion, to achieve precise control over drying speed and printing performance.

[0133] A solid content of 75% to 80% defines the mass percentage of solid components (such as metal powders, ceramic powders, etc.) in the material and is a key parameter affecting the thickness, density, and drying shrinkage of the printed layer. Achieving this solid content range typically involves accurately weighing the solid powder and organic solvent, and monitoring and adjusting them in real time during mixing. For example, the solid content can be measured by periodically sampling during mixing and then fine-tuned by adding appropriate amounts of solvent or solid components based on the measurement results. Another approach is to pre-calculate the theoretical solid content based on the density and desired volume percentage of each component, then mix them in precise proportions, and further optimize the results during subsequent viscosity adjustments.

[0134] A viscosity of 300 ± 50 Pa·s specifies the flow resistance of the material at a particular shear rate, which is crucial for the uniformity and pattern accuracy of screen printing. A suitable viscosity ensures that the material passes smoothly through the mesh under the action of the squeegee and maintains the pattern shape after printing without flowing. Viscosity is typically measured using a rotational viscometer at constant temperature and shear rate. If the measured value exceeds the target range, the viscosity can be reduced by adding a small amount of organic solvent, or increased by adding a small amount of thickener or solid powder. Furthermore, the viscosity of the material can be indirectly affected by adjusting the mixing speed, mixing time, and mixing temperature of the mixing equipment to achieve the target range.

[0135] Grinding and dispersing to a fineness of less than 15 μm aims to ensure that the solid particles in the material have a sufficiently small size and uniform dispersion to avoid clogging the screen mesh, improve the surface smoothness of the printed layer, and enhance the adhesion between layers. Achieving this fineness requirement typically involves using grinding equipment such as a three-roll mill, ball mill, or sand mill. During grinding, the degree of particle refinement is controlled by adjusting the grinding pressure, the number of grinding passes, or the size and filler volume of the grinding media. Fineness can be detected using a scraper fineness gauge or a laser particle size analyzer. The grinding process should continue until the test results show that the particle fineness meets the requirement of less than 15 μm, and there are no obvious coarse particles or agglomerations.

[0136] By employing the above technical solutions, the materials of the interface conductive layer 111, gradient transition layer 112, and welding adapter layer 113 are refined, ensuring the stability and printing quality of the screen printing process. Specifically, the solid content of the material is controlled within the range of 75% to 80%, resulting in a suitable thickness and density of the printed coating. This avoids printing difficulties caused by excessively high solid content or thin coatings and insufficient performance caused by excessively low solid content. Simultaneously, the material viscosity is precisely controlled at 300±50 Pa·s, ensuring good rheological properties during screen printing. This allows the material to pass uniformly through the mesh, forming a smooth, defect-free coating and effectively preventing problems such as uneven printing, dripping, or accumulation. Furthermore, by grinding and dispersing the material to a fineness of less than 15 μm, potentially large particles are eliminated, significantly improving the surface smoothness and uniformity of the printed layer. This enhances the interfacial bonding between layers and optimizes conductivity. These measures work together to effectively solve the problems of printing defects, insufficient interlayer bonding and impaired gradient transition caused by improper material processing, thereby significantly improving the overall reliability of the barrier layer 11, ensuring that the battery string can effectively buffer thermal stress during long-term operation and extending the service life of the battery string.

[0137] In at least one embodiment, the organic solvent is composed of terpineol and butylcarbidol in a mass ratio of 7:3.

[0138] In at least one embodiment, the grinding process is carried out by rolling a three-roll mill more than three times to disperse the particles to a fineness of less than 15 μm.

[0139] Example 1 Table 2 Standard Formulation Scheme

[0140] In Table 2, "-" indicates that the substance was not detected or was not found.

[0141] Example 2 Table 3 Cost Optimization Scheme

[0142] In Table 3, "-" indicates that the substance was not detected or was not found.

[0143] As can be seen from Table 2 of Example 1 and Table 3 of Example 2, the gradient CTE design of the barrier layer 11 on the surface of the cell 10 reduces the interface stress to within the safety threshold, and the measured reliability meets the 25-year service life of the photovoltaic module.

[0144] The basic principles, main features, and advantages of this application have been described above. Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely the principles of this application. Various changes and modifications can be made to this application without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claims. The scope of protection claimed by this application is defined by the appended claims and their equivalents.

Claims

1. A battery string comprising at least two battery sheets (10) which are sequentially welded and a plurality of welding ribbons (20) connected to adjacent two of the battery sheets (10), characterized in that: The surface of the battery cell (10) is provided with a barrier layer (11). The barrier layer (11) includes an interface conductive layer (111), a gradient transition layer (112) and a welding adapter layer (113) arranged sequentially from the surface of the battery cell (10) along a direction perpendicular to the direction of the battery cell (10). The solder strip (20) is respectively connected to the welding adapter layer (113) of the first battery cell (10) and the welding adapter layer (113) of the second battery cell (10). The thermal expansion coefficients of the interface conductive layer (111), the gradient transition layer (112), and the welding adapter layer (113) increase sequentially, and the thermal expansion coefficients of the interface conductive layer (111), the gradient transition layer (112), and the welding adapter layer (113) are all located between the thermal expansion coefficients of the battery cell (10) and the solder strip (20).

2. A battery string as claimed in claim 1, characterized in that: The barrier layer (11) has a strip structure that corresponds one-to-one with the solder strip (20). The projection of the solder strip (20) on the battery cell (10) along the thickness direction is located within the projection of the corresponding barrier layer (11) on the battery cell (10) along the thickness direction. The projection width of the solder strip (20) on the battery cell (10) along the thickness direction is 0.05 to 0.10 mm smaller than the projection width of the corresponding barrier layer (11) on the battery cell (10) along the thickness direction.

3. A battery string as claimed in claim 1, characterized in that: The coefficient of thermal expansion of the interface conductive layer (111) is 6 x 10 -6 / °C to 12 x 10 -6 / °C, and the thickness of the interface conductive layer (111) is 5 to 8 μm.

4. A battery string as claimed in claim 3, characterised in that: By weight, the interface conductive layer (111) comprises 50 to 70 parts of silver powder, 20 to 38 parts of inorganic non-metallic ceramic material, and 5 to 15 parts of resin.

5. A battery string as described in claim 1, characterized in that: The coefficient of thermal expansion of the gradient transition layer (112) is 12 x 10 -6 / °C to 18 x 10 -6 / °C, and the thickness of the gradient transition layer (112) is 8 to 15 μm.

6. A battery string as described in claim 5, characterized in that: The gradient transition layer (112) comprises, by weight, 70 to 80 parts silver powder, 5 to 10 parts inorganic non-metallic ceramic material, and 10 to 15 parts organosilicon-modified epoxy resin.

7. A battery string as claimed in claim 1, characterized in that: The coefficient of thermal expansion of the solder adaptation layer (113) is 18 x 10 -6 / °C to 24 x 10 -6 / °C, and the thickness of the solder adaptation layer (113) is 7 to 12 μm.

8. A battery string as claimed in claim 7, characterised in that: The welding adapter layer (113) comprises 80 to 90 parts of solder powder and 10 to 20 parts of silver powder by weight.

9. The method of producing a battery string according to any one of claims 1 to 8, characterized in that: Includes the following steps: S1: On the surface of the battery cell (10) of the battery string, a conductive interface layer (111), a gradient transition layer (112) and a welding adapter layer (113) are sequentially screen-printed along a direction perpendicular to the battery cell (10). S2: Under a nitrogen atmosphere, the battery cell (10) is heated to 110 to 130°C and kept at that temperature for 2 to 4 minutes for preheating, heated to 170 to 190°C and kept at that temperature for 4 to 6 minutes for curing, and heated to 230 to 250°C and kept at that temperature for 5 to 7 minutes. S3: Take the solder strip (20) and connect it to the welding adapter layer (113) of the first battery cell (10) and the welding adapter layer (113) of the second battery cell (10) respectively.

10. The method of claim 9, wherein: In S1, the materials of the interface conductive layer (111), the gradient transition layer (112) and the welding adapter layer (113) are all mixed with organic solvents to a solid content of 75% to 80% and a viscosity of 300±50 Pa·s, and then ground and dispersed to a fineness of less than 15 μm.

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