Optimization method for improving reliability of laser enhanced contact optimization technology in solar cell

Abstract

The invention discloses an optimization method for improving the reliability of a laser enhanced contact optimization technology in a solar cell, and solves the technical problems of poor battery electrical performance and the like caused by incapability of effectively burning through a non-uniform thin film due to the use of low-lead aluminum-free pure silver paste in an LECO technology. The thickness of a SiNx film in each area after a film coating process is tested on a polished plane of a silicon battery piece, and the sizes of different openings in different areas of a silk-screen printing plate are correspondingly designed, so that different amounts of metal slurry can be coated in different areas during silk-screen printing; in this way, the number of sites in the edge area with the low metal-semiconductor contact site density is kept consistent with the number of sites in the center area, and then the problems of poor electrical performance such as EL blackening and cloud and mist caused by uneven film thickness are reduced.

Description

Technical Field

[0001] The invention relates to the technical field of solar cells, and in particular to an optimization method for improving the reliability of laser enhanced contact optimization technology in solar cells. Background Art

[0002] With the continuous development of solar cell technology, crystalline silicon solar cells have obvious cost advantages due to the abundant reserves of their base material - silicon (Si) in the earth's crust, and currently occupy the mainstream of the photovoltaic market.

[0003] The essence of a solar cell is a large-area PN junction. The front of a cell using an n-type silicon wafer is generally the positive electrode, and its majority carriers are positively charged holes; the back is the negative electrode, and its majority carriers are negatively charged electrons. Solar cells need to collect and export the photogenerated carriers generated by light, and generally use screen printing to prepare metal electrodes on the surface of the cell. However, due to the different properties between metals and semiconductors, metal composite loss will occur when metal-semiconductor contacts.

[0004] With the development of solar cell theory, photovoltaic practitioners have proposed a variety of solutions, including laser selective emitters, silver aluminum paste and other solutions.

[0005] The metal electrode prepared by screen printing needs to go through a melting-recrystallization process. The metal material in the molten state exists in the form of ions. At this time, the metallic silver is in the silver ion state (Ag + ), and reducing it to metallic silver requires negatively charged electrons (e - ) participates, and since most of the carriers on the front side of the n-type solar cell are positively charged holes, Ag + The reduction of ions is hindered, resulting in poor metal-semiconductor contact.

[0006] This gave rise to the development of silver-aluminum paste. Aluminum is ranked 13th in the periodic table, while silicon is the 14th element. Similar physical and chemical properties make aluminum much more soluble in silicon than silver, and the melting point of aluminum is much lower than that of silver. A certain proportion of aluminum is added to the silver metal paste. During sintering, as the temperature increases, aluminum melts first and contacts the silicon substrate. Since aluminum has a higher solubility in silicon, aluminum and silicon are in contact first. Then, the silver powder begins to melt. As metal materials, the solubility between silver and aluminum is higher than that between silver and silicon. Finally, the metallized area formed is no longer a simple silver-silicon alloy, but a complex silver-aluminum-silicon alloy, with aluminum serving as an auxiliary contact material between silver and silicon.

[0007] In order to open the surface silicon nitride passivation film, lead compounds, namely glass frit, need to be added to the metal slurry. The molten glass frit is in a high-temperature viscous state, which can open the insulating passivation film on the surface of the battery.

[0008] The current solution to improve the basic capabilities of metal-semiconductors and reduce contact damage is to use LECO laser to optimize contact resistance technology, which uses laser irradiation to generate a large number of photogenerated carriers on the surface, and then connects a reverse bias voltage to force electrons / holes to move in the opposite direction, so that more electrons and Ag + Contact, Ag + Better reduction to silver element can reduce the use of aluminum, and a large number of carriers are gathered at the gate line position with lower resistance, generating local high temperature, causing the metal to be heated again, reducing the energy required to open the passivation film, reducing the proportion of metal lead, and ultimately increasing the silver content in the metal slurry.

[0009] The addition of lead and aluminum in the slurry leads to hidden dangers in the battery at the component end. The packaging of the component requires the use of adhesive film, which is generally EVA (ethylene-vinyl acetate copolymer). Under light and water vapor conditions, EVA will hydrolyze to produce acetic acid. The metal activity of lead and aluminum is much higher than that of silver, and they will be corroded by acid, causing the performance of the component to deteriorate. In addition, the electrical conductivity of metallic silver is much higher than that of metallic aluminum and lead.

[0010] By using LECO technology, the battery contact resistance is reduced, the open circuit voltage is increased, the conversion efficiency is increased, and at the same time the proportion of active metals in the metal slurry is reduced, reducing the hidden dangers of component use.

[0011] However, as mentioned above, the glass frit in the paste is used to increase the paste viscosity and open the insulating passivation film. The reduction of glass frit content will lead to a decrease in paste viscosity and a weakening of corrosion ability. Figure 1 As shown in the scanning electron microscope (SEM) images of conventional silver-aluminum paste grid lines and LECO's special low-lead aluminum-free pure silver paste grid lines, it can be seen that the sintering depth and viscosity of the LECO technology grid lines are lower than those of conventional silver-aluminum paste grid lines.

[0012] Currently, the manufacturing process of TOPCon cells deposits silicon nitride thin films (SiNx) on the front and back sides of the cell wafers as passivation films, antireflection films, and cell protection films. SiNx has high hardness, excellent chemical stability, oxidation resistance, extremely high temperature resistance, and chemical corrosion resistance. At the same time, it is a high-performance insulating material. Therefore, if the ablation is insufficient and SiNx cannot be fully opened, it will cause an increase in the series resistance of the cell and a decrease in electrical performance. However, during the preparation process, SiNx thin films are prepared on the front and back sides of the cell once each. When preparing the film on the front side, "wrap-around plating" will occur on the back side, that is, when depositing the film on the front side of the cell, a part of the SiNx film will also be deposited on the edge of the back side, and its thickness gradually decreases from the edge to the center; at the same time, when preparing the film on the back side, "wrap-around plating" will also occur on the front side, that is, a certain thickness of SiNx will continue to grow on the already prepared front film, and its thickness also gradually decreases from the edge to the center. The thickness of the front film of currently industrially mass-produced TOPCon cells is about 75 nm. Due to the above-mentioned wrap-around plating problem, in fact, the thickness of the front film of the cell wafer will thicken to about 85 nm in the edge area, with an increase of about 13%.

[0013] When using LECO technology and the corresponding pure silver paste during the first high-temperature sintering, due to the reduction of the glass frit and the uneven thickness of SiNx caused by the front-side silicon nitride wrap-around plating, there is a situation of insufficient corrosion in the edge area of the front side of the cell, which easily leads to blackening of the edge EL, as Figure 2 (a) shown. This situation will cause a decrease in the density of metal-semiconductor contact sites formed on the edge of the front side of the cell, and then during LECO, it is easy to have a reduction in the carrier path and an excessive current (I = Q / t), as Figure 2 (b) shown, resulting in excessive ablation in the edge area of the front side of the cell, thus affecting the improvement of the final solar cell efficiency.

[0014] In the existing publicly available technologies, a specific laser is used to selectively scan the SiNx thin film in the front-side wrap-around plating area to etch the wrap-around plated SiNx thin film and perform local thickness shaping on it, so that the SiNx thin film thickness of the cell wafer is balanced before sintering, thereby avoiding the above-mentioned over-burning problem. However, this method will increase the use of laser equipment and the alignment error, that is: the screen printing area needs to be consistent with the laser thinning area, reducing the process window. Summary of the Invention

[0015] In view of this, the purpose of the present invention is to provide an optimization method for improving the reliability of laser-enhanced contact optimization technology in solar cells, and solve technical problems such as poor electrical performance of the cell caused by the ineffective burning through of uneven films due to the use of lead-free and aluminum-free pure silver paste in LECO technology.

[0016] To achieve the above purpose, the present invention adopts the following content:

[0017] The present invention provides an optimization method for improving the reliability of laser enhanced contact optimization technology in solar cells, comprising the following steps:

[0018] The silicon cell is subjected to conventional process steps to complete the deposition of the passivation layer. The two sides of the silicon cell are marked as side A and side B respectively. Side A undergoes the positive film process, and side B undergoes the back film process.

[0019] Take an N×N area on the A surface of the silicon cell and test the thickness of the anti-reflection film in the N×N area. It is found that the thickness of the anti-reflection film in the N×N area gradually increases from the inside to the outside.

[0020] A screen printing screen is manufactured according to the thickness of the anti-reflection film in the N×N area, wherein the screen printing screen is composed of a mesh cloth formed by metal wire weaving and polyimide coated on the mesh cloth on a mesh frame;

[0021] A corresponding opening area is formed on the mesh cloth of the screen printing screen, and the mesh size on the opening area gradually increases from the inside to the outside, and the mesh size distribution conforms to the thickness of the anti-reflection film in the N×N area;

[0022] Lay the screen printing plate on the A side of the silicon cell, and ensure that the opening area on the screen printing plate coincides with the designed position of the metal grid line on the A side of the silicon cell;

[0023] When printing, pour metal slurry on the upper part of the screen printing screen, apply a certain pressure on the mesh with a scraper, and move toward the other end of the screen printing screen at the same time, so that the slurry is squeezed from the mesh holes in the opening area by the scraper during movement and adheres to the surface of the silicon cell to complete the printing.

[0024] In one example, the operation of testing the thickness of the anti-reflection film in an N×N area includes: accurately calculating the thickness of the silicon nitride film prepared in the N×N area of ​​the A surface of the silicon cell before printing by using an ellipsometer.

[0025] The mesh of the screen printing screen is woven from transverse and longitudinal metal wires that cross each other horizontally and vertically. The mesh count of the mesh is changed by changing the number of transverse or longitudinal metal wires, thereby adjusting the size of the mesh holes in the opening area on the mesh.

[0026] Among them, according to the film thickness data of the N×N area of ​​the silicon cell tested by the ellipsometer, the number of horizontal metal wires or vertical metal wires in the opening area of ​​the screen cloth on the screen printing screen is adjusted accordingly to make the mesh size match the thickness of the silicon nitride film. The thicker the film area, the larger the mesh.

[0027] In one example, several silicon solar cells are taken at three positions, namely the furnace mouth, the middle of the furnace, and the furnace tail, inside the reaction furnace. The thicknesses of the antireflection films within the N×N regions of these silicon solar cells are measured respectively, and finally, all the measurement results are averaged to ensure the design scheme of the aperture sizes on the screen printing stencil.

[0028] In one example, according to the design scheme of the metal grid lines on the A surface of the silicon solar cell, the polyimide in the corresponding area on the mesh cloth is removed to form a corresponding opening area on the mesh cloth.

[0029] In one example, during the operation of applying a certain pressure on the mesh cloth with a squeegee, the squeegee is always kept in a slightly inclined posture.

[0030] The present application adopting the above at least one technical solution can achieve the following beneficial effects:

[0031] By testing the thicknesses of the SiNx thin films in each area after the coating process on the polished plane of the silicon solar cell, and corresponding to design different opening sizes in different areas of the screen printing stencil, it is thus possible to apply different amounts of metal paste in different areas during screen printing. That is, in the edge area where the metal-semiconductor contact site density is relatively low, the same number of sites as in the central area is maintained (more paste × low contact site density = less paste × high contact site density), thereby reducing the proportion problem of poor electrical properties such as EL blackening and cloudiness caused by uneven film thickness. Description of the Drawings

[0032] Figure 1 are the scanning electron microscope (SEM) images of the conventional silver-aluminum paste grid lines and the LECO special low-lead and aluminum-free pure silver paste grid lines;

[0033] Figure 2 is the image of the blackening and excessive ablation at the edge of the solar cell;

[0034] Figure 3 is the schematic diagram of the selection of the N×N area on the surface of the silicon solar cell and the thickness distribution of the silicon nitride thin film within this area;

[0035] Figure 4 is a simple schematic diagram of the screen printing stencil designed in the embodiment of the present invention;

[0036] Figure 5 is the grid line test diagram of the silicon solar cell prepared according to the screen printing stencil designed in the embodiment of the present application by 3D microscope testing. Detailed Embodiments

[0037] To more clearly illustrate the present invention, the present invention will be further described below in conjunction with the preferred embodiments. Those skilled in the art should understand that the specific content described below is illustrative rather than restrictive, and should not be used to limit the protection scope of the present invention.

[0038] The following will describe in detail some embodiments of the present application with reference to the accompanying drawings.

[0039] This embodiment provides an optimization method for improving the reliability of laser-enhanced contact optimization technology in solar cells, specifically including the following steps:

[0040] I. Take a silicon cell product. The silicon cell has completed the deposition of the passivation layer through conventional process steps. The surface of the silicon cell is a silicon surface. Denote the two sides of the silicon cell as side A and side B respectively. Side A has undergone the front film process, and side B has undergone the back film process, that is, an antireflection film (SiNx) is deposited on both sides of the silicon cell.

[0041] II. Take an N×N area on side A of the silicon cell. The area selection method is as Figure 3 shown. For example, select a 6×6 rectangular area, and this rectangular area presents a multi-layered zigzag structure.

[0042] Use an ellipsometer to accurately calculate the thickness of the silicon nitride film in the selected 6×6 area. From the film thickness test results of each area, it can be seen that the thickness of the silicon nitride film in the central area is thinner, while the thickness of the silicon nitride film in the edge area is thicker, and the overall shows a progressive change from the inside to the outside.

[0043] III. Make a screen printing stencil according to the thickness distribution of the silicon nitride film in the above-mentioned measured 6×6 area. Generally speaking, a screen printing stencil is usually composed of a mesh cloth woven by metal wires and polyimide coated on the mesh cloth on a mesh frame.

[0044] Remove the polyimide in the corresponding area on the mesh cloth according to the design scheme of the metal grid lines on side A of the silicon cell, so as to form corresponding opening areas on the mesh cloth, and the metal paste can contact the surface of the silicon cell through the opening areas.

[0045] The mesh cloth of the screen printing stencil is woven by many horizontal and vertical metal wires intersecting each other. The number of horizontal or vertical metal wires can be changed to change the mesh count (the distribution density of horizontal and vertical metal wires) of the mesh cloth, and then adjust the size of the mesh holes in the opening areas on the mesh cloth.

[0046] Specifically, according to the film thickness data of the 6×6 area of the silicon cell measured by the ellipsometer, correspondingly adjust the number of horizontal or vertical metal wires in the opening area of the mesh cloth of the screen printing stencil to make the mesh hole size match the thickness of the silicon nitride film. The thicker the film area, the larger the mesh hole.

[0047] Such as Figure 4As shown, the screen printing stencil design is finally divided into multiple zigzag regions. The larger the region is towards the outside, the larger the mesh holes in the opening area of the screen printing stencil, so that wider grid lines can be prepared and more metal paste can be coated.

[0048] The width of the metal grid lines on the silicon solar cell is determined by the size of the opening area of the screen printing stencil. Currently, the width of the metal grid lines of industrial mass-produced solar cells is generally about 16 ± 3 um.

[0049] In order to ensure that the finally produced screen printing stencil can adapt to as many silicon solar cells as possible, it is necessary to make the data of the silicon nitride thin film in the measured 6×6 region more accurate. Specifically, several silicon solar cells can be taken at three positions: the furnace mouth, the middle of the furnace, and the furnace tail in the reaction furnace, and the thickness of the antireflection film in the 6×6 region of these silicon solar cells is measured respectively. Then, the average calculation is performed on all the measurement results, so as to ensure that the finally designed screen printing stencil can be applied to as many solar cells as possible.

[0050] IV. Fit the screen printing stencil to the A side of the silicon solar cell, and ensure that the opening area on the screen printing stencil coincides with the designed position of the metal grid lines on the A side of the silicon solar cell.

[0051] V. Pour the metal paste onto the upper part of the screen printing stencil during printing, apply a certain pressure on the screen cloth with a slightly inclined squeegee, and move it towards the other end of the screen printing stencil at the same time, so that the paste is extruded from the mesh holes in the opening area by the squeegee and adheres to the surface of the silicon solar cell during the movement, thus completing the printing.

[0052] As Figure 5 shown, the grid line width of the silicon solar cell prepared according to the screen printing stencil designed in the embodiment of the present application is tested by a 3D microscope. On the same grid line, the grid line width is tested from the inside, the middle, and the edge, and multiple places are randomly selected, and the results are tabulated in Table 1.

[0053]

[0054] Table 1

[0055] It can be seen that the grid line width basically meets the range of 16 ± 3 um of the metal grid lines of current industrial mass-produced solar cells.

[0056] Through the screen printing stencil structure designed in the example of the present application, the adhesion of the paste in the edge region of the surface of the silicon solar cell is increased. Without adding additional equipment and additional processes, the contact sites consistent with the central region can be maintained, thereby reducing the defective ratio.

[0057] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or variations can be made based on the above description. It is impossible to list all the implementation manners here. Any obvious changes or variations derived from the technical solutions of the present invention still fall within the protection scope of the present invention.

Claims

1. An optimization method for improving the reliability of laser enhanced contact optimization technology in solar cells, characterized in that: The specific steps include: The silicon cell is subjected to conventional process steps to complete the deposition of the passivation layer. The two sides of the silicon cell are marked as side A and side B respectively. Side A undergoes the positive film process, and side B undergoes the back film process. Take an N×N area on the A surface of the silicon cell and test the thickness of the anti-reflection film in the N×N area. It is found that the thickness of the anti-reflection film in the N×N area gradually increases from the inside to the outside. A screen printing screen is manufactured according to the thickness of the anti-reflection film in the N×N area, wherein the screen printing screen is composed of a mesh cloth formed by metal wire weaving and polyimide coated on the mesh cloth on a mesh frame; A corresponding opening area is formed on the mesh cloth of the screen printing screen, and the mesh size on the opening area gradually increases from the inside to the outside, and the mesh size distribution conforms to the thickness of the anti-reflection film in the N×N area; Lay the screen printing plate on the A side of the silicon cell, and ensure that the opening area on the screen printing plate coincides with the designed position of the metal grid line on the A side of the silicon cell; When printing, pour metal slurry on the upper part of the screen printing screen, apply a certain pressure on the mesh with a scraper, and move toward the other end of the screen printing screen at the same time, so that the slurry is squeezed from the mesh holes in the opening area by the scraper during movement and adheres to the surface of the silicon cell to complete the printing.

2. The optimization method for improving the reliability of laser enhanced contact optimization technology in solar cells according to claim 1, characterized in that: The operation of testing the thickness of the anti-reflection film in the N×N area includes: accurately calculating the thickness of the silicon nitride film prepared in the N×N area of ​​the A surface of the silicon cell before printing by using an ellipsometer.

3. The optimization method for improving the reliability of laser enhanced contact optimization technology in solar cells according to claim 2, characterized in that: The mesh of the screen printing screen is woven from transverse and longitudinal metal wires that cross each other horizontally and vertically. The mesh count of the mesh is changed by changing the number of transverse or longitudinal metal wires, thereby adjusting the size of the mesh holes in the opening area on the mesh.

4. The optimization method for improving the reliability of laser enhanced contact optimization technology in solar cells according to claim 3, characterized in that: According to the film thickness data of the N×N area of ​​the silicon cell tested by the ellipsometer, the number of horizontal or vertical metal wires in the opening area of ​​the screen cloth on the screen printing screen is adjusted accordingly to make the mesh size match the thickness of the silicon nitride film. The thicker the film area, the larger the mesh.

5. The optimization method for improving the reliability of laser enhanced contact optimization technology in solar cells according to claim 1, characterized in that: Several silicon cells are taken from the furnace mouth, furnace center and furnace tail in the reactor, and the thickness of the anti-reflection film in the N×N area of ​​these silicon cells is measured. Finally, all the measurement results are averaged to ensure the design of the mesh size on the screen printing screen.

6. The optimization method for improving the reliability of laser enhanced contact optimization technology in solar cells according to claim 1, characterized in that: According to the design of the metal grid lines on the surface A of the silicon cell, the polyimide in the corresponding area on the mesh is removed to form a corresponding opening area on the mesh.

7. The optimization method for improving the reliability of laser enhanced contact optimization technology in solar cells according to claim 1, characterized in that: When applying a certain pressure on the mesh with a scraper, keep the scraper in a slightly inclined position at all times.

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