Screen printing screen for photovoltaic cells and improved method for its manufacture

Abstract

The application relates to the technical field of photovoltaic cells, and discloses a screen printing screen plate for a photovoltaic cell and an improved method thereof. The improved method comprises the following steps: dividing a screen cloth into a plurality of printing areas with equal intervals along an x axis; analyzing the stress on the left and right sides of each printing area to obtain the stress of each printing area; performing three-dimensional space stress analysis on the stress, and obtaining stress components corresponding to x, y and z according to stress balance and in combination with a secondary projection method of the stress; ignoring the z direction to obtain the shear stress corresponding to each printing area; in order to keep the shear stress of a reference area (a printing area in the middle of the screen cloth) consistent with the shear stress corresponding to the rest of the printing areas of the screen cloth, the number of longitudinal wires corresponding to the rest of the printing areas is increased to balance the corresponding shear stress, the stress of each printing area is offset, the stress of each printing area of the screen cloth is consistent, the difference between the line widths of metal grid lines is greatly reduced, the poor busbar connection and the fragment rate are reduced, the cell efficiency is improved, and the metal paste feeding is facilitated.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic cell technology, and specifically to a screen printing stencil for photovoltaic cells and an improved method thereof. Background Technology

[0002] Photovoltaic cells are products that directly convert solar energy into electrical energy. When exposed to sunlight, photovoltaic cells generate photogenerated carriers, which need to be collected and guided. The common practice is to fabricate metal grid lines on the surface of the photovoltaic cell as metal electrodes to collect and guide these photogenerated carriers. In existing technologies, the mainstream method for fabricating these metal electrodes is screen printing. The specific process of this screen printing process is as follows: first, openings 23 (e.g., in the shape of metal electrodes 41) are made on the screen printing stencil. Figure 1-2 The screen includes a screen frame 10 and a mesh fabric 20 woven from multiple threads 21, with polyimide 22 filling the non-opening portions of the mesh fabric 20. During screen printing, under the force of a squeegee 30 (such as a squeegee), the metal paste 40 is extruded from the openings 23 of the mesh fabric 20, and thus printed onto the battery cell 50 (e.g., ...). Figure 2 The metal electrode pattern is printed by sintering the metal paste 40 to form the metal electrode 41 on the battery cell 50.

[0003] However, in order for the metal paste 40 to be extruded, a downward force needs to be applied to the metal paste 40. Therefore, in actual screen printing, a certain gap d2 must be left between the mesh 20 and the battery cell 50 (e.g., ...). Figure 2 As shown in the diagram, when the squeegee 30 pushes the metal paste 40 forward, it will inevitably generate a large downward stress on the mesh 20. This stress produces different strains on the cross wires (i.e., the wires 21 distributed along the x-axis) and longitudinal wires (i.e., the wires 21 distributed along the y-axis) of the mesh 20, and the strain produced by this stress is also different in different printing areas of the mesh 20. Moreover, even if the mesh 20 uses a material with a high modulus of elasticity, such as stainless steel or tungsten steel, the mesh 20 will still generate stress and corresponding strain under the downward force of the squeegee 30. This stress will first cause a slight expansion or deformation of the pattern actually screen-printed on the battery cell, thus causing a difference between the actual screen-printed pattern and the mesh pattern (e.g., ...). Figure 3Secondly, due to the different strains in each printing area of ​​the screen printing mesh 20, the linewidth of the printed metal grid lines in each printing area of ​​the screen printing mesh 20 will be uneven. Moreover, in order to reduce the light-blocking area of ​​the metal grid lines on the surface of the solar cell, the current trend in the fabrication of metal grid lines is to make them thinner (i.e., the linewidth of the fabricated metal grid lines is smaller). Under this trend, it is also easy to cause grid breakage in the metal grid lines printed on the solar cell. Therefore, inconsistencies in the screen printing pattern, uneven linewidth of the metal grid lines, and even grid breakage will cause poor current collection, affect the ability of the metal electrodes to collect photogenerated carriers, and ultimately affect the cell efficiency.

[0004] Regarding the aforementioned defects of screen-printed metal electrodes, see [link to relevant documentation]. Figure 4 The current solution is to use a double-layer roller (consisting of an upper roller and a lower roller) for screen printing. See also Figure 4 In section 4b, the surface of the rolling mill 60 (such as the upper rolling mill) has an opening 23 similar to a mesh plate, and the interior of the rolling mill 60 carries metal slurry 40; the rotating upper and lower rolling mills together drive the solar cell 50 to move (e.g. Figure 4 As shown in 4a, the direction indicated by the arrow on the battery cell 50 indicates the direction in which the double-layer rollers drive the battery cell 50 to move. Under the action of the scraper 30 inside the roller 60 (such as the upper roller), the metal paste 40 is squeezed onto the battery cell 50 through the opening 23 to complete the printing of the metal electrode pattern. In this double-layer roller, both the upper and lower rollers are cylindrical structures. The upper roller serves as the printing carrier (i.e., the interior of the upper roller carries the metal paste 40), while the lower roller provides support to reduce stress. In this way, although the screen-printed pattern will still be deformed, the deformation is consistent for each point on the cylindrical double-layer roller. Therefore, the actual screen-printed metal electrode pattern can be the same as or similar to the mesh pattern, thereby improving the defect of poor metal electrode convergence caused by existing screen printing.

[0005] However, the double-layer roller design still has many problems: 1. The filling of the metal paste 40 is cumbersome: Screen printing is an open system, which allows for easy replenishment of the metal paste; while the roller 60 is a semi-closed device, making the replenishment of the metal paste 40 cumbersome, thus resulting in wasted labor and low output. 2. The double-layer roller reduces the contact area between the printing device and the solar cell 50, which can easily lead to an increase in the breakage rate of the solar cell 50 during the printing process. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a screen printing stencil for photovoltaic cells and an improved method thereof.

[0007] Based on this, the present invention discloses an improved method for screen printing stencils for photovoltaic cells, comprising the following improved steps:

[0008] S1. Divide the screen printing stencil into several printing areas at equal intervals along the x-axis, with the number of printing areas being a base number greater than or equal to 3; wherein, the screen fabric includes multiple horizontal fibers distributed along the x-axis and multiple vertical fibers distributed along the y-axis.

[0009] S2. When the squeegee of the screen printing pushes the metal paste forward to different printing areas of the screen, the stress on the left and right sides of the different printing areas of the screen is analyzed based on the constant downward pressure of the squeegee on the screen, the screen distance between the screen and the battery cell, the length of the screen, and the distance the squeegee moves along the x-axis, so as to obtain the stress on the different printing areas.

[0010] S3. Perform stress analysis on the different printing areas in three-dimensional space. Based on the stress balance and combined with the second projection method of stress, obtain the stress components corresponding to the stress in the x, y, and z directions respectively.

[0011] S4. For planar mesh fabric, only the x and y directions are considered, while the z direction is ignored. Based on the relationship between the strain corresponding to the stress component and the shear strain corresponding to the shear stress, as well as the relationship between stress and strain, the shear stress of different printing areas of the mesh fabric is obtained. The printing area in the middle of the x-axis of the mesh fabric is taken as the reference area.

[0012] S5. Based on the mesh count of the screen printing stencil and considering the interaction between transverse stress and shear stress, in order to ensure that the shear stress corresponding to the remaining printing areas of the screen fabric is consistent with the shear stress of the reference area, the number of longitudinal wires corresponding to the remaining printing areas of the screen fabric is increased without changing the number of transverse wires. This balances the shear stress corresponding to the remaining printing areas and counteracts the stress on different printing areas of the screen fabric along the x-axis, so that the stress on each printing area of ​​the screen fabric is consistent.

[0013] Preferably, in step S2, obtaining the stress F experienced by different printing areas of the mesh fabric includes the following steps:

[0014] S21. Based on the constant downward pressure F0 of the squeegee on the mesh, the mesh distance D between the mesh and the battery cell, the mesh length L, and the distance e that the squeegee moves along the x-axis, the force analysis is performed on the left and right sides of different printing areas of the mesh, and the middle of the printing area is taken as the force analysis point to obtain the following force relationship (4) in the middle of different printing areas:

[0015] F0 / Sin(180°-α-β)=F L / Sinβ=F R / Sinα (4),

[0016] (4) In the formula, F LF is the tensile force exerted on the left side of the printed area. R α represents the tension force on the right side of the printed area, and α represents the downward force F0 and the left-side tension F. L The angle between them, β is the downward force F0 and the right-side tension F R The angle between them;

[0017] S22, the pulling force F on the left side of the same printing area L With the right-side tension F R Compare the numerical values ​​and assign the larger value to the left-hand tension F. L Or the right-side tension F R The stress F exerted by the scraper on the printed area.

[0018] More preferably, step S21 specifically includes:

[0019] S211. When the squeegee of the screen printing pushes the metal paste forward to a printing area of ​​the screen, based on the constant downward pressure F0 of the squeegee on the screen, combined with the screen distance D between the screen and the battery cell, the screen length L, and the distance e that the squeegee moves along the x-axis, the force analysis is performed on the left and right sides of the middle of the printing area, and the expressions shown in (1) and (2) are obtained respectively:

[0020] α=arctan(e / D) (1),

[0021] (1) In the formula, α is the downward force F0 and the left-side tension F L The angle between them;

[0022] β=arctan((Le) / D) (2),

[0023] (2) In the formula, β is the downward force F0 and the right-side tension F R The angle between them;

[0024] S212. According to the trigonometric function sine theorem, in any triangle, we obtain the following equation (3):

[0025] a / SinA=b / SinB=c / SinC (3),

[0026] (3) In the formula, a, b, and c are the three sides of the triangle, and A, B, and C are the opposite angles of the three sides of the triangle.

[0027] S213. Based on the downward pressure F0 of the mesh and equations (1) to (3), the force relationship (4) in the middle of the printing area is obtained: F0 / Sin(180°-α-β)=F L / Sinβ=F R / Sinα (4).

[0028] Preferably, in step S3, the stress components σ corresponding to the stress F in the x, y, and z directions are obtained respectively. x σ y σ z It includes the following steps:

[0029] S31. In a three-dimensional rectangular coordinate system, the projected components F of stress F in the x, y, and z directions are obtained by the second projection method. x F y F z ;

[0030] S32. The stress F on different printing areas of the mesh is analyzed in three-dimensional space. When the stress balance is reached, the following partial differential equation (6) is obtained:

[0031]

[0032]

[0033]

[0034] (6) In the formula, F x F y F z Let σ be the projected components of stress F in the x, y, and z directions after applying the second projection method; x Let τ be the normal stress component of stress F along the x-axis. xy τ xz Let F be the shear stress components along the y-axis and z-axis, respectively; σ y Let τ be the normal stress component of stress F along the y-axis. yx τ yz Let F be the shear stress components along the x-axis and z-axis, respectively; σ z Let τ be the normal stress component of stress F along the z-axis. zx τ zy Let F be the shear stress components of stress F in the x-axis and y-axis directions, respectively;

[0035] Based on the partial differential equation (6), and combined with the projected component force F x F y F z Obtain the normal stress components σ of stress F in x, y, and z. x σ y σ z .

[0036] More preferably, in step S3, in the three-dimensional rectangular coordinate system, the stress F is decomposed into projected component forces F in the x, y, and z directions by the second projection method.x F y F z The expression (5) is as follows:

[0037]

[0038] (5) In the formula, F is the stress on different printing areas of the mesh fabric, F xy Let F be the projected component of stress F in the Oxy plane, and λ be the angle between the line of action of stress F and the coordinate axis z. z Let F be the projected component of stress F on the z-axis. x For F xy The projected component of the force on the x-axis, F y For F xy The projected component of the force on the y-axis; θ is the angle between the line of action of stress F and the coordinate axis z; λ, stress F and the z-axis, these three factors determine the angle between the plane and the coordinate axis x.

[0039] More preferably, step S4 specifically includes:

[0040] Step S41: For planar mesh, only the x and y directions are considered, while the z direction is ignored. Combined with the following equations (7) and (8), the shear strain γ corresponding to stress F is obtained. xy Expression (10):

[0041]

[0042] When the forces are in equilibrium, according to the partial differential equation (6), the following partial differential equation (7) is obtained:

[0043]

[0044]

[0045]

[0046]

[0047] (7) In equation (7), ε is the strain corresponding to stress F, where ε x Normal stress component σ x The corresponding strain, ε y Normal stress component σ y The corresponding strain, ε z Normal stress component σ z The corresponding strain; the strain components ε corresponding to stress F in the x, y, and z directions are u, v, and w; γ is the shear strain corresponding to shear stress, where γ xy For the shear stress component τ xyThe corresponding shear strain, γ xz For the shear stress component τ xz The corresponding shear strain, γ yz For the shear stress component τ yz The corresponding shear strain;

[0048] Based on strain = stress / elastic modulus and equations (6) to (7), the constitutive equation (8) is obtained as follows:

[0049]

[0050]

[0051]

[0052]

[0053] (8) In the formula, E is the tensile and compressive elastic modulus, G is the shear elastic modulus, and μ is Poisson's ratio;

[0054] S42. Obtain the shear strain γ of different printing areas on the mesh fabric. xy The corresponding shear stress component τ xy The shear stress component of the reference region is denoted as shear stress τ. m The shear stress components corresponding to the remaining printing areas of the mesh fabric are denoted as shear stress τ. n .

[0055] More preferably, step S42 specifically includes:

[0056] Based on constitutive equations (8) and (10), and combined with the following relationship (9) between the tensile and compressive elastic modulus E, the shear elastic modulus G, and Poisson's ratio μ, the shear stress components τ of different printing areas of the mesh fabric are obtained. xy ;

[0057] G = E / 2(1+μ) (9).

[0058] More preferably, step S5 specifically includes:

[0059] S51. Based on the mesh count K of the screen printing stencil and the shear stress τ of the reference area... m The force f borne by each longitudinal wire in the reference region is obtained as shown in equation (11):

[0060] f = τ m / (K / 25.4mm) (11);

[0061] S52. To ensure that the shear stress τn corresponding to the remaining printing areas is consistent with the shear stress τm of the reference area, M longitudinal wires can be added to the remaining printing areas of the mesh, as shown in equation (12):

[0062] M=(τ n -τ m ) / f (12).

[0063] Preferably, in step S1, the number of printing areas divided into equally spaced sections along the x-axis of the mesh fabric is 9, of which printing area 5 is the reference area.

[0064] The present invention also discloses a screen printing stencil for photovoltaic cells, which is prepared by the improved method of the screen printing stencil for photovoltaic cells described above in the present invention.

[0065] Compared with the prior art, the present invention has at least the following beneficial effects:

[0066] (1) The screen printing stencil obtained by the improved method of the present invention can effectively avoid the defect that "the different stresses on different printing areas of the screen fabric caused by the downward pressure of the squeegee can lead to inconsistent strains corresponding to different stresses, resulting in differences in the line width of the metal grid lines printed in different printing areas, or even grid breakage". This makes the line width of the metal grid lines printed by the improved screen printing stencil of the present invention more uniform, reduces production defects, and makes the actual screen-printed metal electrode pattern the same as the improved screen fabric pattern. This improves the defect of poor metal electrode current convergence caused by the existing screen printing, thereby improving battery efficiency.

[0067] (2) Moreover, the screen printing stencil obtained by the improved method of the present invention is still an open printing method. Therefore, compared with the semi-closed printing method of double-layer rollers, it is more convenient to replace the metal paste, which can effectively avoid the defects of labor waste and low output. It can also increase the contact between the battery cell (or silicon wafer) and the printing device during screen printing, reduce the force per unit area on the surface of the battery cell, and effectively reduce the breakage rate of the battery cell.

[0068] (3) The improvement method of the present invention is specifically designed for the mesh fabric of the screen printing stencil. It does not require additional modifications to the battery production equipment. Therefore, the promotion of the improvement method of the present invention in mass production will be easier. Attached Figure Description

[0069] Figure 1 This is a schematic diagram of the mesh structure of an existing screen printing stencil.

[0070] Figure 2 A screen printing process diagram for preparing metal electrodes using existing screen printing stencils.

[0071] Figure 3 This is a schematic diagram showing the expansion and deformation of a metal electrode pattern actually printed using an existing screen printing stencil.

[0072] Figure 4 This is a schematic diagram of an existing double-layer roller screen-printed metal electrode; in which, Figure 4 4a in the diagram is a schematic diagram of the rotating upper and lower rolling mills driving the movement of the battery cells; Figure 4 4b in the diagram is a schematic diagram of the structure of the roller for screen printing.

[0073] Figure 5 This is a schematic diagram of the mesh fabric in Example 1 being divided into 9 printing areas at equal intervals along the x-axis. Figure 5 In the diagram, the numbers 1, 2, 3, 4, 5, 6, 7, 8, and 9 represent printing area 1, printing area 2, printing area 3, printing area 4, printing area 5, printing area 6, printing area 7, printing area 8, and printing area 9, respectively.

[0074] Figure 6 This is a schematic diagram of the force analysis of the mesh fabric under the downward pressure F0 of the scraper on the mesh fabric in Example 1.

[0075] Figure 7 This is a schematic diagram of the secondary projection of stress F in a three-dimensional rectangular coordinate system in Example 1.

[0076] Figure 8 This is a schematic diagram of the force analysis of the three-dimensional object in Example 1.

[0077] Figure 9 This is a comparison chart showing the width of the metal grid lines obtained by screen printing on a 520-mesh screen printing stencil obtained by the improved method in Example 1 and a 520-mesh screen printing stencil obtained by the existing standard process.

[0078] Reference numerals: 10 for screen frame; 20 for mesh fabric; 21 for thread; 22 for polyimide; 23 for opening; 30 for scraper; 40 for metal paste; 41 for metal electrode; 50 for battery cell; 60 for roller. Detailed Implementation

[0079] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0080] Example 1

[0081] This embodiment provides an improved method for screen printing stencils for photovoltaic cells. See [link to relevant documentation]. Figure 5-8 The improvement steps include the following:

[0082] Step S1: Divide the screen printing stencil 20 into several printing areas along the x-axis direction. The number of printing areas is a base number greater than or equal to 3. Take the printing area in the middle of the x-axis of the screen printing stencil 20 as the reference area m, and the remaining printing areas in the x-axis direction of the screen printing stencil 20 as the printing area n.

[0083] Among them, see Figure 5-6 The screen printing stencil includes a stencil frame 10 and a mesh fabric 20 woven from multiple threads 21; see also Figure 5 The mesh fabric 20 includes multiple horizontal and multiple vertical threads. The horizontal threads are the threads 21 distributed along the x-axis in the mesh fabric 20, and the vertical threads are the threads 21 distributed along the y-axis in the mesh fabric 20.

[0084] In one example of this embodiment, such as Figure 5 As shown, the mesh fabric 20 is divided into 9 printing areas at equal intervals along the x-axis. These 9 printing areas are denoted as printing area 1, printing area 2, printing area 3, printing area 4, printing area 5, printing area 6, printing area 7, printing area 8, and printing area 9, respectively. Moreover, printing area 1 and printing area 9 are symmetrically distributed, printing area 2 and printing area 8 are symmetrically distributed, printing area 3 and printing area 7 are symmetrically distributed, and printing area 4 and printing area 6 are symmetrically distributed. Printing area 5 is the printing area in the middle of the x-axis of the mesh fabric 20, that is, printing area 5 is the reference area.

[0085] S2. When the squeegee of the screen printing pushes the metal paste forward to different printing areas of the screen 20, the stress on the left and right sides of the different printing areas of the screen 20 is analyzed based on the constant downward pressure F0 of the squeegee on the screen 20, the screen distance D between the screen 20 and the battery cell 50, the screen length L, and the distance e that the squeegee moves along the x-axis, so as to obtain the stress F on the different printing areas of the screen 20.

[0086] In practice, when using screen printing to fabricate metal electrodes for collecting photogenerated carriers (such as photocurrent) generated by photovoltaic cells, the linewidth and pattern of the metal grid lines are determined by parameters such as the mesh count and aperture width of the screen printing stencil. The mesh count of the screen printing stencil refers to the number of threads 21 contained in one square inch of mesh fabric 20.

[0087] When the squeegee in screen printing pushes the metal paste forward to different printing areas of the screen 20, the downward pressure exerted by the squeegee on the screen 20 is denoted as F0 (F0 is a constant value). The stress generated by this downward pressure F0 in different printing areas of the screen 20 is different. Therefore, Figure 2 Simplified to Figure 6Since forces are mutual, the tensile force on a certain printing area (such as printing area 1) of the mesh fabric 20 can be determined according to the downward force F0. Figure 6 Force analysis shown:

[0088] The thickness of the mesh fabric 20 is denoted as d1; the gap between the mesh fabric 20 and the battery cell 50 during screen printing is denoted as d2; the mesh spacing D between the mesh fabric 20 and the battery cell 50 during screen printing is denoted as d1 + d2; the length of the mesh fabric is denoted as L; the distance the squeegee moves along the x-axis of the mesh fabric 20 (referred to as the squeegee movement distance) is denoted as e; then the distance between the squeegee and the other side of the mesh fabric 20 along the x-axis is Le; at this time, see Figure 6 The tensile force on the left side of the printed area is denoted as F. L The tensile force on the right side of the printed area is denoted as F. R .

[0089] See Figure 6 For any moment in the operation of the scraper, the expressions shown in (1) and (2) can be obtained as follows:

[0090] α=arctan(e / D) (1);

[0091] (1) In the formula, α is the downward force F0 and the left-side tension F L The angle between them.

[0092] β=arctan((Le) / D) (2);

[0093] (2) In the formula, β is the downward force F0 and the right-side tension F R The angle between them.

[0094] According to the trigonometric sine theorem, in any triangle, there exists the following equation (3):

[0095] a / SinA=b / SinB=c / SinC (3);

[0096] (3) In the formula, a, b, and c are the three sides of the triangle, and A, B, and C are the opposite angles of the three sides of the triangle.

[0097] Based on the downward pressure F0 and equations (1) to (3), the following force relationship equation (4) for the printed area is obtained:

[0098] F0 / Sin(180°-α-β)=F L / Sinβ=F R / Sinα (4).

[0099] Further, see Figure 5-6It can be seen that, taking the middle of each printing area as the force analysis point, when the squeegee moves to the middle of the mesh 20 (such as the printing area 5 at the middle of the x-axis of the mesh 20), the left and right tensions on the printing area 5 are symmetrical, and the left tension F on the middle of the printing area 5 is... L Equal to the tension F on the right side R Therefore, in one example of this embodiment, see... Figure 5-6 ,Will Figure 5 Printing area 5 as Figure 6 The analysis of the reference area of ​​the stress is used to balance the shear stress corresponding to the other printing areas, and it is only necessary to analyze the left and right tension of half of the printing areas in the mesh 20 (such as analyzing printing areas 1 to 4 and printing area 5).

[0100] Therefore, in summary, step S2, obtaining the stress F in different printing areas of the mesh fabric 20, specifically includes the following steps:

[0101] Step S21: When the squeegee of the screen printing pushes the metal paste forward to a certain printing area of ​​the screen 20, based on the screen distance D between the screen 20 and the battery cell 50, the screen length L, and the distance e that the squeegee moves along the x-axis, the printing process is as follows: Figure 6 From the force analysis shown, equations (1) and (2) can be obtained; then, based on the downward pressure F0 and equations (1) to (3), the left-side tension F of the printed area of ​​the mesh fabric 20 can be obtained. L and the right-side tension F R .

[0102] Step S22, then apply a pulling force F to the left side of the same printing area. L With the right-side tension F R Compare the numerical values ​​and assign the larger value (i.e., the greater impact on the printed area) to the left-hand tension F. L Or the right-side tension F R The stress F is the pressure exerted by the squeegee on the printed area.

[0103] In one example of this embodiment, a 520-mesh screen printing stencil suitable for M10 photovoltaic cells is used as an example. See [link to example]. Figure 5 The following example illustrates the calculation process of the stress F experienced by the printing area 1 when the squeegee reaches the middle of the printing area 1 of the 520-mesh screen printing stencil:

[0104] The parameters of this 520-mesh screen printing stencil are as follows: See Figure 2The thickness d1 of the screen printing stencil is 3.5±1.5um; the gap d2 between the mesh 20 and the battery cell 50 during screen printing is 1.5±0.5mm; the length L of the mesh is 182mm; and the squeegee pressure F0 during screen printing is 50±30N. The mesh count K of the screen printing stencil is 520 meshes; that is, in this screen printing stencil, every 1 inch of mesh 20 has 520 horizontal wires in the x-axis direction and 520 vertical wires in the y-axis direction. When the squeegee reaches the middle of printing area 1, the squeegee's movement distance e1 = 10.11 mm can be measured based on the mesh length L = 182 mm and the screen printing mesh count K = 520 mesh. When the squeegee reaches the middle of printing area 5, the squeegee's movement distance e5 = 91 mm can be measured based on the mesh length L = 182 mm and the screen printing mesh count K = 520 mesh. Therefore, when the squeegee's downward pressure F0 is a known constant, the force F on the left side of the middle of printing area 1 can be calculated based on the above parameters and equations (1) to (4). L1 and the force F on the right side R1 .

[0105] Next, compare the force F on the left side of the middle of printing area 1. L1 Force F on the right side R1 The magnitude; when the left side is subjected to force F L1 Greater than the force F on the right side R1 At that time, the force F on the left side will be... L1 The stress F in printing area 1 (or, when the right side is subjected to force F) R1 Greater than the force F on the left side L1 At that time, the lateral force F will be applied. R1 Stress F in printing area 1).

[0106] S3. Perform a three-dimensional stress analysis on the stress F in different printing areas. Based on the force balance and using the second projection method of stress F, obtain the normal stress components σ of stress F in the x, y, and z directions. x σ y σ z .

[0107] The stress F experienced by different printing areas of the planar mesh 20 is specifically determined by the second projection method as follows:

[0108] like Figure 7 As shown, in a three-dimensional Cartesian coordinate system, the stress F can be decomposed into Fx, Fy, Fz in the x, y, and z directions respectively. x F y F z These are the three component forces. The magnitudes of these three component forces can be calculated using the method of second projection.

[0109] From trigonometric relationships, we know that if the magnitude of stress F is known, the angle λ between the line of action of F and the coordinate axis z, and the plane determined by stress F and the z-axis (i.e., ...) Figure 7 If the angle between the shaded area (shown) and the coordinate axis x is θ, then the stress F can be projected onto the z-axis and the coordinate plane Oxy, respectively, thus obtaining the projected component F on the z-axis. z and the projected component F on the Oxy plane xy Then, F xy Projected onto the x-axis and y-axis respectively, the projected force F on these two coordinate axes is obtained. x and F y This method requires the stress F to be projected twice to obtain F. z F x and F y Therefore, it is called the double projection method. The stress F is obtained by the double projection method. z F x and F y The expression (5) is as follows:

[0110]

[0111] (5) In the formula, F is the stress on different printing areas of the mesh fabric 20; F xy Let F be the projected component of stress F in the Oxy plane; λ be the angle between the line of action of stress F and the coordinate axis z; F z Let F be the projected component of stress F on the z-axis; x For F xy Projected component of force on the x-axis; F y For F xy The projected component of the force on the y-axis; θ is the angle λ between the line of action of F and the coordinate axis z; the plane determined by the stress F and the z-axis (i.e., Figure 7 The angle between the shaded area shown and the x-axis.

[0112] Furthermore, in practice, the mesh fabric 20 is not subjected to a single force. The mesh fabric 20 is woven from intersecting horizontal and vertical fibers; therefore, the stress on the mesh fabric 20 must also consider shear stress in other directions. The calculation principle for the stress and strain of a three-dimensional object is as follows: Figure 8 As shown, in three-dimensional space (such as...) Figure 8 If a point in a cube in a three-dimensional Cartesian coordinate system is subjected to stress along one coordinate axis (such as the z-axis), shear stress will be generated along the other two coordinate axes (such as the x-axis and y-axis). Figure 8 In this context, x, y, and z represent the coordinate axis directions; σ represents the normal stress component of stress F along one coordinate axis direction (for example, the normal stress component of stress F along its z-axis direction is σ). z); τ represents shear stress; decompose the shear stress τ into two shear stress components parallel to the other two coordinate axes, as shown in the partial differential equation (6) below. In the two subscript letters of the shear stress component, the first subscript letter indicates the coordinate axis of the normal direction of the plane under consideration, and the second subscript letter indicates the coordinate axis of the decomposition direction of the shear stress component (for example, when studying the edge perpendicular to the y-axis, the normal stress component of stress F in the y-axis direction is σ). y The shear stress component of stress F in the x-axis direction is represented by τ. yx The shear stress component of stress F along the z-axis is represented by τ. yz express).

[0113] according to Figure 8 It can be seen that when an object reaches equilibrium under forces, the following partial differential equation holds (6):

[0114]

[0115]

[0116]

[0117] (6) In the formula, F x F y F z This indicates that the stress F in different printing areas, after being projected twice using the above formula (5), is the projected component of the stress in the x, y, and z directions, respectively; σ x Let τ be the normal stress component of stress F along the x-axis. xy τ xz Let F be the shear stress components along the y-axis and z-axis, respectively; σ y Let τ be the normal stress component of stress F along the y-axis. yx τ yz Let F be the shear stress components along the x-axis and z-axis, respectively; σ z Let τ be the normal stress component of stress F along the z-axis. zx τ zy Let F be the shear stress components of stress F in the x-axis and y-axis directions, respectively.

[0118] Therefore, in summary, in step S3, the stress components σ corresponding to the stress F in the x-axis, y-axis, and z-axis directions are obtained respectively. x σ y σ z Specifically, it includes the following steps:

[0119] Step S31: In a three-dimensional rectangular coordinate system, using the second projection method of equation (5), obtain the projected component forces F of stress F in the three directions of x-axis, y-axis, and z-axis.x F y F z .

[0120] Step S32: Based on the partial differential equation (6), and combined with the projected component force F x F y F z Obtain the normal stress components σ of stress F on the x-axis, y-axis, and z-axis. x σ y σ z .

[0121] S4. For the planar mesh fabric 20, only the x and y directions are considered, while the z direction is ignored. Based on the relationship between the strain corresponding to the stress components and the shear strain corresponding to the shear stress, as well as the relationship between stress and strain, the shear stress of different printing areas of the mesh fabric 20 is obtained. The shear stress of the reference region m is denoted as τ. m The shear stress corresponding to the remaining printing areas of the mesh fabric 20 is denoted as τ. n .

[0122] In reality, when an object is subjected to stress, it will produce a corresponding strain, ε representing the strain corresponding to stress F (where ε... x Normal stress component σ x The corresponding strain, ε y Normal stress component σ y The corresponding strain, ε z Normal stress component σ z The strain corresponding to stress F is ε, and the strain components of the strain ε in the x, y, and z coordinate axes are u, v, and w, respectively. γ represents the shear strain corresponding to the shear stress (specifically, γ...). xy For the shear stress component τ xy The corresponding shear strain, γ xz For the shear stress component τ xz The corresponding shear strain, γ yz For the shear stress component τ yz (Corresponding shear strain). When the force is in equilibrium, according to the partial differential equation (6), the following partial differential equation (7) is obtained:

[0123]

[0124]

[0125]

[0126]

[0127] According to Hooke's Law (strain = stress / elastic modulus) and equations (6) to (7) above, the constitutive equation (8) can be obtained as follows:

[0128]

[0129]

[0130]

[0131]

[0132] Among them, the following relation (9) is known:

[0133] G=E / 2(1+μ) (9;

[0134] In equations (8) and (9), E is the tensile and compressive elastic modulus, G is the shear elastic modulus, and μ is Poisson's ratio (Poisson's ratio is the ratio of transverse normal strain to axial normal strain when a material is subjected to uniaxial tension or compression, also called the transverse deformation coefficient, which is an elastic constant reflecting the transverse deformation of the material).

[0135] In one example of this embodiment, the material of the wire 21 in the mesh 20 of the 520-mesh screen printing stencil is generally stainless steel or tungsten steel with higher strength. Here, stainless steel is used, therefore the tensile and compressive modulus E of the wire 21 is 200 GPa, and the Poisson's ratio μ is 0.3. And as... Figure 1-2 As shown, apart from the stainless steel wire, the rest of the mesh fabric 20 is typically made of polyimide 22. The elastic modulus of polyimide 22 is 3-4 GPa (e.g., 3 GPa), which is only 1.5% of the tensile and compressive elastic modulus E of the wire 21. Therefore, in the mesh fabric 20 of the screen printing stencil, the main component providing the elastic modulus is the stainless steel wire. Thus, for ease of calculation, the contribution of PI to the elastic modulus is ignored.

[0136] In summary, step S4 specifically includes:

[0137] Step S41: For the planar mesh 20, only the x and y directions are considered, while the z direction is ignored. Combining equations (7) and (8), the shear strain γ corresponding to the stress F can be obtained. xy :

[0138]

[0139] Step S42: Combining constitutive equations (8), (9) and (10), the shear stress components τ of different printing areas of the mesh fabric 20 can be obtained. xy Among them, the shear stress component τ of the printing areas 1, 2, 3, 4, and 5 of the mesh fabric 20 is... xyThese are respectively referred to as shear stresses τ1, τ2, τ3, τ4, and τ5 (for example, the shear stress component τ of printing area 1 of mesh fabric 20). xy This is called shear stress τ1, and so on.

[0140] S5. Based on the mesh count of the screen printing stencil and considering the interaction between transverse stress and shear stress, in order to ensure that the shear stress corresponding to the remaining printing areas of the screen 20 is consistent with the shear stress of the reference area, the number of longitudinal wires corresponding to the remaining printing areas of the screen 20 is increased without changing the number of transverse wires. This is to balance the shear stress corresponding to the remaining printing areas and to offset the stress on different printing areas of the screen 20 along the x-axis, so that the stress on each printing area of ​​the screen 20 is consistent.

[0141] It should be noted that, since the forces are mutual (e.g., transverse stress and shear stress influence each other), it is only necessary to apply the force in the longitudinal direction of each printing area of ​​the mesh fabric 20 (i.e., ...). Figure 5 By increasing the number of longitudinal wires in each printing area along the y-axis (as shown), the shear stress in each printing area of ​​the mesh 20 can be balanced, thereby correspondingly offsetting the stress (i.e., transverse stress) in different printing areas of the mesh 20 caused by the downward pressure F0 of the squeegee on the mesh 20.

[0142] Since changing the number of both horizontal and vertical wires in the mesh 20 simultaneously alters the mesh count of the screen printing stencil, significantly changing its parameters, this embodiment can adjust the number of vertical wires corresponding to different printing areas of the mesh 20 based on shear stress without changing the number of horizontal wires. This ensures that under a constant pressure F0 from the squeegee on the mesh 20, the stress on each printing area is consistent, ultimately reducing the linewidth difference of the metal grid lines printed by the improved method of this embodiment. This reduces production defects, ensuring that the actual screen-printed metal electrode pattern matches the improved mesh pattern, thus improving the defect of poor metal electrode current convergence caused by existing screen printing and ultimately enhancing battery efficiency.

[0143] Ignoring edge effects, if the stress is shared by each of the filaments 21, then the strain will also be shared by each of the filaments 21.

[0144] Based on this, step S5 specifically includes:

[0145] Step S51: Based on the mesh count K of the screen printing stencil and the shear stress τ of the reference area m... m The force f borne by each longitudinal wire in the reference region m is obtained as follows (11):

[0146] f = τ m / (K / 25.4mm) (11).

[0147] Step S52: To ensure that the shear stress τ corresponding to the remaining printing areas is... n Shear stress τ in reference region m m To maintain consistency, M longitudinal wires can be added to the remaining printing areas of the mesh fabric 20, as shown in formula (12):

[0148] M=(τ n -τ m ) / f (12);

[0149] In equations (11) and (12), M represents the number of longitudinal filaments added to each printing area of ​​the mesh fabric 20, excluding the reference area m; τ n τ represents the shear stress corresponding to the remaining printing areas. m denoted as , where is the shear stress in the reference region m; and f is the force borne by each longitudinal wire in the reference region m.

[0150] In one example of this embodiment, the mesh count K of the screen printing stencil is 520 mesh (that is, in this screen printing stencil, every 1 inch of mesh has 520 horizontal wires in the x-axis direction and 520 vertical wires in the y-axis direction); and 1 inch equals 25.4 mm. Therefore, after conversion, we get 20.5 wires per mm (i.e., 520 divided by 25.4 mm equals 20.5 wires per mm). See also Figure 5 Taking the printing area with balanced forces on both sides as the reference area m, the mesh count K of the screen printing plate is converted to 20.5 threads 21 / mm; therefore, the force f borne by each longitudinal thread in the reference area m is τ5 / 20.5. And if the shear stress τ of the remaining printing areas n of the mesh fabric 20 needs to be reduced... n If the shear stress τ5 experienced by the reference region 5 is consistent with that of the reference region 5, then the number of longitudinal wires required to be increased in the remaining printing regions is M = (τ5 + τ5) / ( ... n -τ5) / f.

[0151] In one example of this embodiment, a 520-mesh screen printing stencil suitable for M10 photovoltaic cells is used as an example, and as follows: Figure 5As shown, the mesh fabric 20 is divided into 9 printing areas at equal intervals along the x-axis. After calculation, with the number of horizontal wires remaining unchanged, the number of vertical wires in different printing areas of the mesh fabric 20 is as follows: the number of vertical wires in printing area 5 remains unchanged at 20.5 wires / mm; the number of vertical wires in printing areas 1 and 9 increases to 31.5 wires / mm; the number of vertical wires in printing areas 2 and 8 increases to 28.1 wires / mm; the number of vertical wires in printing areas 3 and 7 increases to 24.7 wires / mm; and the number of vertical wires in printing areas 4 and 6 increases to 22.4 wires / mm. Thus, when the screen printing stencil is subjected to the downward pressure F0 of the squeegee on the mesh 20, the increase in the number of longitudinal filaments in the y-direction of the mesh 20 causes the transverse strain corresponding to the transverse stress of the transverse filaments in the x-direction to be distributed. Moreover, in the x-direction of the mesh 20, from the printing area in the middle of the mesh 20 to the printing areas on both sides of the mesh 20, the number of longitudinal filaments increases in a stepped manner, which offsets the different stresses (i.e., transverse stress) experienced by each printing area. This makes the width of the metal grid lines prepared by the screen printing stencil obtained by the improved method of this embodiment tend to be uniform. Therefore, metal grid lines with uniform width can improve battery efficiency and reduce the production defects and current-carrying defects of metal electrodes.

[0152] This embodiment also provides a screen printing stencil for photovoltaic cells, which is manufactured using the improved method for a screen printing stencil for photovoltaic cells described above in this embodiment.

[0153] See Figure 9 By statistically analyzing the widths of the metal grid lines produced by screen printing using a 520-mesh screen printing stencil obtained from the existing standard process and the improved 520-mesh screen printing stencil described in the example above, it can be seen that the variance of the width of the metal grid lines obtained using the 520-mesh screen printing stencil obtained from the existing standard process is 0.011; while the variance of the width of the metal grid lines obtained using the improved 520-mesh screen printing stencil described in the example above is only 0.008. Clearly, the variance of the width of the metal grid lines printed using the improved 520-mesh screen printing stencil described in the example above is more convergent, indicating that screen printing using the improved stencil obtained in this embodiment can produce a more uniform width of the metal grid lines.

[0154] Furthermore, the screen printing stencil obtained by the improved method of the present invention is still an open printing method. Therefore, compared with the semi-closed printing method of double-layer rollers, it is more convenient to change the metal paste, which can effectively avoid the defects of labor waste and low output. It can also increase the contact between the battery cell 50 (or silicon wafer) and the printing device during screen printing, reduce the force per unit area on the surface of the battery cell 50, and effectively reduce the breakage rate of the battery cell 50.

[0155] Although preferred embodiments of the present invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of the present invention.

[0156] The technical solution provided by the present invention has been described in detail above. Specific examples have been used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation and application scope based on the idea of ​​the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. An improved method of screen printing a screen for a photovoltaic cell, characterized by, The method comprises the following improved steps: S1, the screen cloth of the screen printing screen plate is divided into a plurality of printing areas along the x-axis direction in sequence and at equal intervals, and the number of the printing areas is a base number greater than or equal to 3; wherein the screen cloth comprises a plurality of horizontal wires distributed along the x-axis direction and a plurality of vertical wires distributed along the y-axis direction; S2, when the squeegee of the screen printing pushes the metal paste to the different printing areas of the screen cloth, the left and right sides of the different printing areas of the screen cloth are analyzed according to the constant pressure of the squeegee on the screen cloth, the screen distance between the screen cloth and the battery piece, the length of the screen cloth and the distance of the squeegee moving along the x-axis forward direction, and the stress of the different printing areas of the screen cloth is obtained; S3, the stress of the different printing areas is analyzed in the three-dimensional space, the corresponding stress components in the x, y and z directions are obtained according to the force balance and the secondary projection method of the stress; S4, for the planar screen cloth, only the x and y directions are considered, and the z direction is ignored, and the shear stress of the different printing areas of the screen cloth is obtained according to the relationship between the stress component corresponding strain and the shear strain corresponding to the shear stress, and the relationship between the stress and the strain, and the x-axis middle printing area of the screen cloth is taken as the reference area; S5, according to the mesh number of the screen printing screen plate, the mutual influence of the transverse stress and the shear stress is combined, so that the shear stress of the remaining printing areas of the screen cloth is consistent with the shear stress of the reference area, the number of the vertical wires corresponding to the remaining printing areas of the screen cloth is increased without changing the number of the horizontal wires, so as to balance the shear stress of the remaining printing areas and offset the stress of the different printing areas of the screen cloth along the x-axis transverse direction, so that the stress of each printing area of the screen cloth is consistent.

2. The improved method of screen printing a photovoltaic cell with a screen as claimed in claim 1, wherein, In step S2, the stress F of the different printing areas of the screen cloth is obtained, comprising the following steps: S21, according to the constant pressure F0 of the squeegee on the screen cloth, the screen distance D between the screen cloth and the battery piece, the length L of the screen cloth and the distance e of the squeegee moving along the x-axis forward direction, the left and right sides of the different printing areas of the screen cloth are analyzed, and the middle part of the printing area is taken as the force analysis point, and the following force relationship formula (4) of the middle part of the different printing areas is obtained: F0 / Sin(180°-α-β) = F L F0 / Sinβ = F R F0 / Sinα (4), (4) where F L is the tension on the left side of the printed area, F R is the tension on the right side of the printed area, a is the angle between the down force F0 and the left side tension F L , and β is the angle between the down force F0 and the right side tension F R . S22, the left side tension F of the same printing area L is compared with the right side tension F R in value, and the left side tension F L or the right side tension F R with the larger value is taken as the stress F of the squeegee on the printing area.

3. The improved method of screen printing a photovoltaic cell with a screen as defined in claim 2, wherein, Step S21 specifically comprises: S211, when the squeegee of the screen printing pushes the metal paste to a printing area of the screen cloth, according to the constant pressure F0 of the squeegee on the screen cloth, the screen distance D between the screen cloth and the battery piece, the length L of the screen cloth and the distance e of the squeegee moving along the x-axis forward direction, the left and right sides of the middle part of the printing area are analyzed, and the following expressions (1) and (2) are obtained respectively: α = arctan (e / D) (1), (1) where a is the angle between the downward force F0 and the left side tension F L ​ β = arctan ((L-e) / D) (2), (2) where β is the angle between the downward force F0 and the right side tension F R ; S212, according to the sine theorem of trigonometric function, in any triangle, the following formula (3) is obtained: a / SinA = b / SinB = c / SinC (3), In formula (3), a, b and c are three sides of the triangle, and A, B and C are angles opposite to the three sides of the triangle. S213、According to the pressing force F0 of the screen cloth and (1) to (3), the stress relationship formula (4) of the middle part of the printing area is obtained: F0 / Sin(180°-α-β)=F L / Sinβ=F R / Sinα (4).

4. The improved method of screen printing a photovoltaic cell with a screen as defined in claim 1, wherein, In step S3, the stress components σ corresponding to the stress F in the x, y, and z directions are obtained respectively. x σ y σ z It includes the following steps: S31, in a three-dimensional rectangular coordinate system, by the secondary projection method, the projection component force F corresponding to the decomposition of the stress F in x, y, z directions is obtained x y z ;​​ S32, stress F of different printing area of the screen cloth is analyzed in three-dimensional space, when stress balance is reached, the following partial differential equation (6) is obtained: (6) where F x , F y , F z are the projection components of the stress F in the x, y, and z directions respectively after the second projection method; σ x is the normal stress component of the stress F in the x-axis direction, τ xy , τ xz are the shear stress components of the stress F in the y-axis and z-axis directions respectively; σ y is the normal stress component of the stress F in the y-axis direction, τ yx , τ yz are the shear stress components of the stress F in the x-axis and z-axis directions respectively; σ z is the normal stress component of the stress F in the z-axis direction, τ zx , τ zy are the shear stress components of the stress F in the x-axis and y-axis directions respectively; According to the partial differential equation (6), and in combination with the projection force F x , F y , F z , the corresponding normal stress components σ x , σ y , σ z of the stress F in x, y, z are obtained.

5. The improved method of screen printing a web for a photovoltaic cell according to claim 4, wherein, In step S3, in the three-dimensional rectangular coordinate system, the stress F is correspondingly decomposed into the projection components F x , F y , F z in the x, y, z directions by the quadratic projection method. The expression (5) of F x , F y , F z is as follows: (5) In the formula, F is the stress on different printing areas of the mesh fabric, F xy Let F be the projected component of stress F in the Oxy plane, and λ be the angle between the line of action of stress F and the coordinate axis z. z Let F be the projected component of stress F on the z-axis. x For F xy The projected component of the force on the x-axis, F y For F xy The projected component of the force on the y-axis; θ is the angle between the line of action of stress F and the coordinate axis z; λ, stress F and the z-axis, these three factors determine the angle between the plane and the coordinate axis x.

6. The improved method of screen printing a photovoltaic cell with a screen as defined in claim 4, wherein, Step S4 specifically comprises: Step S41, for the planar mesh cloth, only the x and y directions are considered, and the z direction is ignored, and the shear strain γ corresponding to the stress F is obtained in combination with the following formulas (7) and (8) xy Expression (10): Wherein, when stress balance is reached, according to partial differential equation (6), the following partial differential equation (7) is obtained: (7) where ε is the strain corresponding to the stress F, where ε x is the normal stress component σ x corresponding to the strain ε y is the normal stress component σ y corresponding to the strain ε z is the normal stress component σ z corresponding to the strain ε; the strain components u, v, w corresponding to the strain ε in the x, y, z directions corresponding to the stress F; γ is the shear strain corresponding to the shear stress, where γ xy is the shear stress component τ xy corresponding to the shear strain γ xz is the shear stress component τ xz corresponding to the shear strain γ yz is the shear stress component τ yz corresponding to the shear strain γ; According to strain = stress / elastic modulus, and (6) to (7) formula, the following constitutive equation (8) is obtained: In formula (8), E is tensile and compressive elastic modulus, G is shear elastic modulus, and μ is Poisson's ratio; S42, the shear strain γ of the different printing areas of the screen cloth is obtained again xy the corresponding shear stress component τ xy wherein the shear stress component of the reference area is denoted as shear stress τ m and the shear stress components of the remaining printing areas of the screen cloth are denoted as shear stress τ n .

7. The improved method of screen printing a web for a photovoltaic cell according to claim 6, wherein, Step S42 specifically is: According to the constitutive equations (8) and (10) and in combination with the following relational expression (9) of the tensile and compressive elastic modulus E, the shear elastic modulus G and the Poisson's ratio μ, the shear stress component τ xy ; G = E / 2 (1 + μ) (9).

8. The improved method of screen printing a photovoltaic cell with a screen according to claim 6 or 7, wherein Step S5 specifically comprises: S51. Based on the mesh count K of the screen printing stencil and the shear stress τ of the reference area... m The force f borne by each longitudinal wire in the reference region is obtained as shown in equation (11): f = τ m (K / 25.4 mm) (11); S52, in order to keep the shear stress τn of the rest of each printing area consistent with the shear stress τm of the reference area, M longitudinal wires are added in the rest of each printing area of the screen cloth, as follows (12): M = (τ n -τ m ) / f (12).

9. The improved method of screen printing a web for a photovoltaic cell according to claim 1, wherein, In step S1, the number of printing areas of the screen cloth divided along the x-axis direction is 9, wherein the printing area 5 is the reference area.

10. A screen printing screen for photovoltaic cells, characterized by It is prepared by the improved method of claim 1-9.

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