After the solar cells inside the module undergo welding, lamination, lamination, framing and other operations and flow processes, certain fragments, tiny cracks or broken grids will inevitably occur, and the solar cells themselves may also appear dark flakes and “” Black Heart”. With the exception of fragmentation, other defects that cannot be directly observed with the naked eye can have a significant impact on the long-term reliability of components and therefore need to be detected and controlled during the manufacturing process. The crack tester can detect whether the solar cell has some appearance defects, such as tiny cracks, dark flakes, etc. At present, there are two types of crack testers in the photovoltaic industry, one is the Photoluminescence (PL) type, and the other is the Electroluminescence (EL) type.
PL is a luminescence phenomenon of semiconductor materials. The electrons in the semiconductor are excited after absorbing external photons. The electrons in the excited state are unstable and will be released in the form of light radiation in the process of transitioning to a lower energy level. energy. The PL. tester uses an excitation light source to illuminate the module, so that the electrons inside the battery radiate light, and then capture the light through a CCD camera, take an image of the battery inside the module, and compare the captured image with the image of the standard film, so as to find the inside of the module microscopic defects. The schematic diagram of the PL. tester is shown in Figure 1.
EL injects minority carriers into the p-region or n-region by applying a forward bias to the battery, and these injected minority carriers will recombine with the majority carriers through direct or indirect pathways, resulting in spontaneous emission. The EL. tester transmits these radiation rays to its CCD camera, takes an image of the battery inside the module, and finds the microscopic defects of the battery inside the module according to the image. The schematic diagram of the EL. tester is shown in Figure 2.
At present, the EL tester is mainly used for defect detection in the production process of components. The EL tester usually requires the CCD camera to have more than 5 million pixels, the test time is less than 25 seconds, and it has the functions of bar code watermark images and automatic continuous testing. At the same time, the test current is required to be adjustable. In order to test the EL images of components and batteries under high and low current conditions, and then automatically identify the EL level of the battery according to the EL judgment standard in the computer. The camera system includes a CCD camera with cooling function, a PC with a communication card and a spare hard disk, a controllable multi-output constant current source with a communication interface, and a communication module. The software composition of the camera system mainly includes the camera software and image processing software of the CCD camera (for adjusting the exposure time and gain under different current conditions), the PC operating system, and the control software.
The operation process of the equipment: The components are firstly corrected and positioned, transported to the top of the test lens, connected to the EL tester, powered on for testing, and the CCD camera acquires the image of the entire component. After software processing, the image is displayed on the PC. The image of the battery judges whether there is a defect or not, so as to judge the level of the component.
The module EL industry standard and the Semi international standard divide the battery defects that can be found through inspection into three categories: shape, position, and brightness. Scratches, concentric circles, etc.; positional defects mainly include broken grid lines of the battery, black edges around, black corners, etc.; brightness defects mainly refer to uneven battery brightness after different batteries are connected in series and mismatched, and due to battery technology Or the uneven brightness of a certain battery caused by virtual welding, over-soldering, etc., or black pieces caused by short circuit, etc.
- Shape class
(1) Penetrating microcracks Such microcracks run parallel to the ribbon and extend from one edge of the cell to the other (Figure 3).
(2) Non-penetrating microcracks refer to cracks that start from the edge of the battery or inside the battery, extend and end inside the battery (Figure 4).
(3) The local area on the split cell has been separated from the whole cell (Fig. 5).
(4) Black spots are irregular black spot-like areas distributed on the battery. In severe cases, the entire battery is black, commonly known as “black heart pieces” (Figure 6).
- Location class
(1) A black area appears on the edge of the black edged battery (Figure 7).
(2) Black areas appear in one or more corners of the black-cornered cells (Figure 8).
(1) Mismatch between cells Different cells in the same assembly exhibit different brightness (Figure 9). Generally, because cells of different efficiency levels are connected in series in a module, the electrical performance parameters between cells are different, resulting in series mismatch. For this type of component, if the light and dark levels are not significantly different when applying high current (usually the short-circuit current of the component, about 8~9A, simulating strong light irradiation), you can simulate weak light irradiation by applying a low current (such as 1A). ) can easily distinguish the differences between the individual cells inside the module, see Figure 10.
(2) The bright spots are distributed in the bright areas on both sides of the welding strip caused by local over-soldering of the battery, which is a manifestation of uneven current distribution (Figure 11).