Solar cell defect testing method

Abstract

The invention discloses a solar cell defect testing method, which aims at solving the problem that the existing area array PL detection cannot identify the related defects of broken grid, airflow printing and cell surface resistance abnormity, and adopts a line scanning PL bidirectional scanning combination scheme: firstly, scanning along a direction parallel to a fine grid with a laser power density greater than 1sun; scanning along the direction vertical to the fine grid at the laser power density of 1-10 sun; and collecting fluorescence signals through a line scanning camera, synthesizing a complete image, setting a + / -10% threshold value based on an average gray value G0 of a normal region, and judging the defect by combining the shape of a bright and dark region and the physical form matching of the defect. According to the method, the traditional defect detection capability is reserved, the broken gate and airflow mark detection function is added, and the detection comprehensiveness and accuracy are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of solar cell testing technology, specifically a method for testing defects in solar cells. Background Technology

[0002] The principle of photoluminescence is based on the absorption and transfer of photon energy and the radiation of charge carrier transitions. Electrons relax their energy and momentum towards the minimum value of the valence band, eventually recombine with holes, and radiate outward in the form of photons. In existing technologies, defect detection in solar cells mostly adopts the area array PL detection method. This method excites the solar cell by illuminating it from all sides, causing the cell to produce fluorescence. The defects are identified by utilizing the characteristic that the minority carrier density is lower and the fluorescence intensity is weaker at defective areas than at defect-free areas.

[0003] However, array PL detection has significant shortcomings: it cannot reflect solar cell defects caused by photoelectron flow due to abnormal cell surface resistance. Specifically, it cannot detect grid breakage defects and gas flow mark defects caused by process problems, resulting in incomplete detection results and affecting the accuracy of solar cell quality assessment. Summary of the Invention

[0004] The purpose of this invention is to address the problems mentioned in the background section by providing a solar cell defect testing method that solves the problem that existing technologies cannot detect broken grids and gas flow marks, thereby improving the comprehensiveness and accuracy of the testing.

[0005] To achieve the above objectives, the present invention specifically adopts the following technical solution:

[0006] A method for testing defects in solar cells includes the following steps:

[0007] S1: Linear scanning (PL) is used to perform linear scanning on the solar cell, with the scanning direction parallel to the grid direction of the solar cell;

[0008] S2: The fluorescence generated by photoluminescence excitation is captured by a line scan camera, and the local images are synthesized into a complete battery image;

[0009] S3: Determine the defect area based on the gray value of the complete battery image. Set the average gray value of the normal area of ​​the solar cell to G0. If the gray value of a certain area G1≥1.1G0, it is determined to be a bright area. If the gray value of a certain area G2≤0.9G0, it is determined to be a dark area. Combine the imaging bright and dark shape with the physical shape of the defect to determine the corresponding defect.

[0010] S4: Linear scanning PL is used to perform linear scanning of the solar cell, with the scanning direction perpendicular to the grid direction of the solar cell;

[0011] S5: The fluorescence generated by photoluminescence excitation is captured by a line scan camera, and the local images are synthesized into a complete battery image;

[0012] S6: Determine the defect area based on the gray value of the complete battery image. Set the average gray value of the normal area of ​​the solar cell to G0. If the gray value of a certain area G1≥1.1G0, it is determined to be a bright area. If the gray value of a certain area G2≤0.9G0, it is determined to be a dark area. Combine the imaging bright and dark shape with the physical shape of the defect to determine the corresponding defect.

[0013] S7: Based on the combined results of steps S3 and S6, determine the defect type of the solar cell.

[0014] Furthermore, in step S1, the laser power density is greater than 1 sun, and the line scan PL excites local areas on the surface of the solar cell to generate photoelectrons. The photoelectrons are guided away from the non-broken grid area due to the low resistance, resulting in a dark area in the image. The photoelectrons are radiated in the form of photons at the broken grid area due to the high resistance, forming a sheet-like bright area.

[0015] Furthermore, in step S4, the laser power density is between 1 sun and 10 sun. Due to process issues, the resistance is uneven and irregular bright bands are formed in the fine gate isolation area, which are determined to be airflow print defects.

[0016] Furthermore, in steps S3 and S6, the defects corresponding to the dark area include at least one of scratches, suction cup scratches, suction pen scratches, belt scratches, boat scratches, black spots, contamination, and microcracks. The defects present a dark area shape due to the low minority carrier lifetime.

[0017] Furthermore, in steps S2 and S5, the line scan camera acquires local fluorescence images by scanning point by point, and uses image stitching technology to synthesize a complete solar cell image.

[0018] Furthermore, the grayscale value G0 is the average grayscale value of the defect-free area of ​​the solar cell under the same scanning conditions, using the grayscale image standard quantity.

[0019] Beneficial effects:

[0020] More comprehensive detection range: This method, through the bidirectional scanning combination of line scan PL, not only retains the existing area array PL's ability to detect traditional defects such as scratches, suction cup scratches, suction pen scratches, belt scratches, boat scratches, black spots, contamination, and hidden cracks, but also adds the detection function of broken grid and airflow mark defects, thus solving the detection blind spots of the existing technology.

[0021] High detection accuracy: By setting a clear range of laser power density and gray value judgment criteria (10% threshold), the identification of defect areas has a quantitative basis, avoiding subjective judgment errors and improving the accuracy of defect judgment;

[0022] High technical feasibility: It adopts line scanning PL scanning method, combined with mature image synthesis and grayscale analysis technology, without the need for complicated equipment modification. Those skilled in the art can implement it based on existing equipment, and it is easy to promote and apply. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a flowchart of the solar cell defect testing method of the present invention;

[0025] Figure 2 This is a PL image of the broken grid defect in the middle of the product under test in this invention;

[0026] Figure 3 This is a PL image of the edge grid breakage defect of the tested product in this invention;

[0027] Figure 4 This is a PL imaging image of the airflow print defect of the present invention;

[0028] Figure 5 This is a PL imaging image of the scratch defect of the present invention;

[0029] Figure 6 This is a PL imaging image of the scratch defect of the suction cup in this invention;

[0030] Figure 7 This is a PL imaging image of the black spot defect of the present invention;

[0031] Figure 8 This is a PL imaging image of the contamination defects of the present invention;

[0032] Figure 9 This is a PL imaging image of the microcrack defect of the present invention. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0034] Figure 5 In PL imaging, scratch defects appear as dark areas or lines;

[0035] Figure 6 PL imaging of scratches on the suction cup shows dark, patchy areas;

[0036] Figure 7 In PL imaging, the mid-black spot defect appears as a dark area with dot-like aggregation;

[0037] Figure 8 In PL imaging, contamination defects appear as dark areas with irregular patchy shapes.

[0038] Figure 9 In PL imaging, the microcrack defect appears as a dark area extending in a linear pattern;

[0039] Figure 2 and Figure 3 The interrupted gate defect appears as a bright area in PL imaging, in a sheet-like pattern;

[0040] Figure 4 In PL imaging, defects in the gas flow mark appear as bright areas with irregular bright bands.

[0041] This embodiment provides a method for testing defects in solar cells, and the specific implementation steps are as follows:

[0042] 1. Scanning parameter settings and first direction scanning

[0043] Step S1: Select the solar cell sample to be tested, adjust the scanning direction of the line scan PL detection device to make it parallel to the fine grid direction on the surface of the solar cell, adjust the laser power density to 2sun (to meet the setting requirement of greater than 1sun), and start the line scan PL to perform linear scanning on the solar cell.

[0044] A laser beam from a linear scanning laser (PL) irradiates the surface of a solar cell, causing localized areas to be illuminated and excited by photoluminescence, which in turn generates photoelectrons. Because the fine grid on the solar cell surface is made of metal with low resistance, the fine grid in non-broken areas quickly conducts photoelectrons away, reducing the conversion of photoelectrons into photons, thus appearing darker in the image. However, in broken-grid areas, due to the discontinuity of the fine grid, the resistance is significantly higher than in areas with normal fine grids, preventing photoelectrons from being conducted away in time. A large number of photoelectrons are converted into photons and radiated outwards, forming characteristic patchy bright areas.

[0045] 2. First-direction image synthesis and preliminary defect assessment

[0046] Step S2: Start the line scan camera, synchronously acquire the fluorescence signals generated during the scanning process, record the fluorescence intensity information of the local area point by point, and synthesize all local images into a complete solar cell PL image through the image stitching algorithm.

[0047] Step S3: Perform grayscale processing on the synthesized complete image. According to the grayscale image standard, the lowest PL signal (no fluorescence) is quantized as black (grayscale value 0), and the highest PL signal (strong fluorescence) is quantized as white (grayscale value 255). Select an area on the surface of the solar cell without any defects and calculate the average grayscale value G0 = 120 for this area.

[0048] Determine the defect area based on the established standards:

[0049] Bright area determination: If the gray value G1 of a certain area is ≥1.1×120=132, it is determined to be a bright area. Combined with shape characteristics, the patchy bright area is determined to be a broken grid defect.

[0050] Dark area determination: If the gray value G2 of a certain area is ≤0.9×120=108, it is determined to be a dark area. According to the shape of the dark area (such as line, dot, sheet, etc.), it corresponds to defects such as scratch, suction cup scratch, suction pen scratch, belt scratch, boat scratch, black spot, pollution, and hidden crack.

[0051] 3. Second-direction scanning and parameter setting

[0052] Step S4: Adjust the scanning direction of the line scan PL detection device to be perpendicular to the fine grid direction on the surface of the solar cell, adjust the laser power density to 5sun (to meet the setting requirement of 1sun < laser power density < 10sun), and start the line scan PL to perform a linear scan on the solar cell.

[0053] In this scanning direction, the laser beam is perpendicular to the extension direction of the fine grid, which can effectively detect areas of uneven resistance and fine grid isolation caused by airflow during the process: the resistance of such areas is different from that of normal areas, the flow of photoelectrons is blocked, and characteristic irregular bright bands are formed.

[0054] 4. Second-direction imaging synthesis and preliminary defect assessment

[0055] Step S5: The line scan camera synchronously acquires fluorescence signals, and synthesizes a complete solar cell PL image using the same image stitching technology as in step S2.

[0056] Step S6: Perform grayscale processing on the image, using the average grayscale value G0 = 120 for the normal area determined in step S3, and applying the same grayscale value judgment criteria:

[0057] Bright area determination: If the gray value G1 of a certain area is ≥132, it is determined to be a bright area. Bright areas with irregular bright band shapes are determined to be airflow defects.

[0058] Dark area determination: If the gray value G2 of a certain area is less than or equal to 108, it is determined to be a dark area, which corresponds to traditional defects such as scratches and suction cup scratches, and is consistent with the dark area defect type in step S3.

[0059] 5. Comprehensive Defect Assessment

[0060] Step S7: Integrate the judgment results of Step S3 and Step S6, eliminate duplicate traditional defects, and summarize the complete defect information of the solar cell, including broken grid (from the bright area of ​​Step S3), airflow marks (from the bright area of ​​Step S6), and traditional defects such as scratches, suction cup scratches, suction pen scratches, belt scratches, boat scratches, black spots, contamination, and microcracks (from the dark areas of Step S3 and S6).

[0061] In this embodiment, one broken grid defect, two gas flow mark defects, and three scratch defects in the solar cell sample were successfully detected using the above method. After dissection and verification, the location and type of the defects were completely consistent with the detection results, proving the accuracy and reliability of the detection method.

[0062] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for testing defects in solar cells, characterized in that, The method comprises the following steps: S1: linearly scanning the solar cell by using a line-scan PL, and the line-scan direction is parallel to the fine grid direction of the solar cell; S2: capturing the fluorescence generated by photoluminescence excitation by using a line-scan camera, and synthesizing a complete cell image from local images; S3: determining a defect area based on the gray value of the complete cell image, setting the average gray value of a normal area of the solar cell as G0, determining a bright area if the gray value G1 of a certain area is greater than or equal to 1.1G0, determining a dark area if the gray value G2 of a certain area is less than or equal to 0.9G0, and determining a corresponding defect by matching the bright and dark shapes in the image with the physical form of the defect; S4: linearly scanning the solar cell by using a line-scan PL, and the line-scan direction is perpendicular to the fine grid direction of the solar cell; S5: capturing the fluorescence generated by photoluminescence excitation by using a line-scan camera, and synthesizing a complete cell image from local images; S6: determining a defect area based on the gray value of the complete cell image, setting the average gray value of a normal area of the solar cell as G0, determining a bright area if the gray value G1 of a certain area is greater than or equal to 1.1G0, determining a dark area if the gray value G2 of a certain area is less than or equal to 0.9G0, and determining a corresponding defect by matching the bright and dark shapes in the image with the physical form of the defect; S7: determining the defect type of the solar cell by comprehensively determining the results of steps S3 and S6.

2. The method of claim 1, wherein the method further comprises: In step S1, the laser power density is greater than 1 sun, the line-scan PL excites the local area of the surface of the solar cell to generate photoelectrons, the area where the fine grid is not broken is dark in the image because the low resistance leads to the photoelectrons being conducted away, and the area where the fine grid is broken is bright in the image because the high resistance causes the photoelectrons to be radiated in the form of photons to form a bright area.

3. The method of claim 1, wherein the method further comprises: In step S4, the laser power density is between 1 sun and 10 sun, and the resistance is uneven due to process problems and the isolation area of the fine grid forms an irregular bright band, which is determined as an air flow printing defect.

4. The method of claim 1, wherein the method further comprises: In steps S3 and S6, the dark area corresponds to at least one of a scratch, a suction cup scratch, a suction pen scratch, a belt scratch, a boat scratch, a black spot, contamination and a hidden crack, and the defect appears as a dark area because the minority carrier lifetime is low.

5. The method of claim 1, wherein the method further comprises: In steps S2 and S5, the line-scan camera collects local fluorescence images by point-by-point scanning, and the complete solar cell image is synthesized by using an image stitching technique.

6. The method of claim 1, wherein the method further comprises: The gray value G0 is the average gray value of a defect-free area of the solar cell under the same scanning conditions, and a gray image standard is used.

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