One of the most critical steps in the production of crystalline silicon solar cells is to create very fine circuits on the front and back of the silicon wafers, leading the photo-generated electrons out of the cell. This metal coating process is usually completed by screen printing technology-the conductive paste containing metal is printed on the silicon wafer through the screen mesh to form a circuit or electrode. A typical crystalline silicon solar cell requires multiple screen printing steps throughout the entire production process. Generally, there are two different processes for screen printing on the front side (contact line and bus bar) and back side (electrode / passivation and bus bar) of the battery. Solar cells are an iconic product for the development and utilization of new energy, low carbon and environmental protection. Screen printing plays a very important role in the production process of the positive, back electrodes and back electric fields of solar cells. The process of screen printing solar electrodes is now quite mature and has become its mainstream process. 1. Basic solar screen printing The printing process begins when the silicon wafer is placed on the printing table. A very fine screen printing plate is fixed on the screen frame and placed above the silicon wafer. The screen closes the area other than the grid lines so that the conductive paste can pass through. The distance between the silicon wafer and the screen should be strictly controlled (called printing gap, or screen pitch). Since the front side requires thinner metal wires, the mesh used for front side printing is usually much finer than the back side printing. Put an appropriate amount of slurry on the wire mesh, apply the slurry with a scraper to make it evenly fill the mesh, and the scraper squeezes the slurry onto the silicon wafer through the wire mesh during the movement. The temperature, pressure, speed and other variables of this process must be strictly controlled. After each printing, the silicon wafer is placed in a drying oven to solidify the conductive paste. Then, the silicon wafer is sent to another different printing machine, printing more lines on its front or back. After all the printing steps are completed, the silicon wafer is sintered in a high-temperature furnace. 2. Printing on the front and back of the silicon wafer Each solar cell has screen printed wires on the front and back, and their functions are different. The lines on the front are thinner than those on the back; some manufacturers print the conductive lines on the back first, and then turn the silicon wafer over to print the lines on the front, thus minimizing possible damage during processing. On the front side (the side facing the sun), most crystalline silicon solar cells are designed with very fine circuits ("finger wires") to transfer the photo-generated electrons collected in the effective area to a larger collection wire-the "busbar" On, and then passed to the component's circuit system. The finger lines on the front are much thinner than the lines on the back (narrow to 80 μm). Because of this, the printing steps on the front require higher precision and accuracy. The printing requirements of the back and front of the silicon wafer are different, and the technology is not so strict. The first step in back printing is to print a layer of aluminum-based conductive material instead of a very thin conductive grid. At the same time, it can reflect the uncaptured light back to the battery. This layer can also "passivate" the solar cell, closing off excess molecular paths and preventing flowing electrons from being caught by these voids. The second step of the back printing is to make the bus bar and connect it with the external circuit system. 3. Related process parameters In order to improve the conversion rate of the solar cell and minimize the shielding of the silver wire to the battery panel, the width of the grid line on the screen should be as narrow as possible, and the grid line too thin may cause the thickness of the conductive silver paste to be too thin It may break. Therefore, the grid line width on the screen version is generally between 80 and 120 μm. Since the width of the conductive silver paste after printing will increase compared to the screen, the width of the printed silver paste after sintering is between 110-150 μm. In order to reduce its series resistance, the total weight of the silver wire printed on the battery board should be as low as possible, basically 0.01 to 0.02g. The printing pressure is between 75 and 80 Newtons. The latest research within the industry shows that if a special photoresist such as Teflon multilayer film screen is used, the conversion rate and stability of solar cells have better performance. Its screen version specifications and related process parameters are: 4. Other conditions The tension of the wire mesh is generally 28 ± 2N. The life of the back electrode wire mesh should be greater than 15000 times, the life of the back electric field wire mesh should be greater than 15000 times, and the life of the positive electrode wire mesh should be greater than 10000 times. The wire mesh should be stored in a constant temperature and constant humidity condition. The rapid change in temperature will cause a decrease in tension, and excessive humidity will cause the denaturation of the latex, which will affect the use effect and even be scrapped. The specific storage conditions are: temperature 22 ± 3 ℃, humidity 50% ± 10%. Before use, the production workshop should take out the wire mesh in advance and leave it to stand in the workshop for more than 24 hours. When it is urgently needed, it should be guaranteed to stand in the plant for more than 2 hours. The wire mesh is always stored in a clean environment before leaving the factory, so it should be kept clean before use. The quality inspection process should be carried out in a relatively clean environment. After the quality inspection, it should be sealed in the original plastic bag and stored in the warehouse. To avoid being contaminated by dust before use. The wire mesh is very sensitive to vibration, and any collision will cause the tension to drop or even collapse the net, so it should be handled gently during movement and use to avoid any collision. The surface of the wire mesh is very fragile, and any blunt object may cause damage to the mesh yarn part, resulting in decreased tension, damaged graphics, shortened life, and even scrapped. Therefore, in the process of moving and using, the surface of the net yarn should avoid contact with any blunt objects. The best storage method is to use a special shelf for vertical storage, if it is laid flat, it should not exceed 5 layers, and the layers should be separated by soft cushions at the net frame. The latex of the wire mesh is not a very stable substance, and the thickness is very small. Some complicated physical and chemical changes will occur during storage, so the wire mesh should be used within one month after entering the factory.
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