Screen printing in solar cells
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
One of the most critical steps in the production of crystalline silicon solar cells is to make very fine circuits on the front and back of the wafer, which are derived from the battery. This metallization process is typically accomplished by screen printing techniques in which a conductive paste containing a metal is embossed through a screen mesh onto a silicon wafer to form a circuit or electrode. A typical crystalline silicon solar cell requires multiple screen printing steps throughout the production process from start to finish. Typically, there are two different processes for screen printing of the front side of the cell (contact line and busbar) and the back side (electrode/passivation and busbar). Solar cells are the iconic products for the development and utilization of new energy sources and low carbon environmental protection. Screen printing plays an important role in the fabrication of the positive and negative electrodes and the back electric field of solar cells. The screen printing solar electrode process is now quite mature and has become its mainstream process.
1. Basic solar screen printing
The printing process begins with the placement of the wafer onto the printing table. A very fine screen plate is attached to the frame and placed over the wafer. The screen closes the area other than the grid lines so that the conductive paste can pass. The distance between the silicon wafer and the screen is strictly controlled (called the print gap, or the mesh distance). Since the front side requires a thinner wire, the mesh used for front printing has a mesh that is typically much smaller than that used for back printing. Place an appropriate amount of slurry on the screen, apply the slurry with a spatula, and evenly fill it in the mesh. The scraper squeezes the slurry through the mesh mesh to the silicon wafer during the moving process.
The temperature, pressure, speed and other variables of this process must be strictly controlled. After each printing is completed, the silicon wafer is placed in a drying oven to solidify the conductive paste. The wafer is then fed into a different press to print more lines on the front or back. After all the printing steps are completed, the wafer is placed in a high temperature furnace for sintering.
2. Printing on the front and back of the wafer
Each solar cell has screen-printed wires on the front and back, and their functions are different. The front lines are thinner than the back side; some manufacturers print the back side conductive lines and then flip the silicon sheets over and print the front lines to minimize damage that can occur during processing. On the front side (the side facing the sun), most crystalline silicon solar cells are designed with very fine circuits ("finger lines") that transfer the photogenerated electrons collected in the active area to the larger acquisition line - "busbar" It is then passed to the component's circuitry. The front finger line is much thinner than the back line (narrow to 80 μm), which is why the front side printing step requires greater precision and accuracy. The printing requirements for the back and front of the wafer are different and technically less stringent. 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, light that is not captured can be reflected back to the battery. This layer also "passivates" solar cells, enclosing excess molecular pathways, and avoiding the flow of electrons trapped by these voids. The second step in back printing is to make the busbars that are connected to the external circuitry.
3. Relevant process parameters
In order to improve the conversion rate of the solar cell, try to reduce the shielding of the silver wire pair and the panel. The width of the grid line on the screen plate should be as narrow as possible, and the grid line is too thin, which may cause the thickness of the conductive silver paste to be too thin, even It is possible to break. Therefore, the grid line width on the screen plate is generally between 80 and 120 μm. Since the conductive silver paste after printing is increased in width compared to the screen plate, the width of the printed silver paste after sintering is between 110 and 150 μm. In order to reduce the series resistance, the total weight of the silver wires printed on the battery board should be as low as possible, basically 0.01 to 0.02 g. The printing pressure is between 75 and 80 Newtons.
The latest research in the industry shows that if a special photosensitive adhesive such as Teflon multilayer film screen is used, the conversion rate and stability of solar cells are better. Its screen version specifications and related process parameters are:
4. Other conditions
The stretch tension of the wire mesh is generally 28 ± 2N. The life of the back electrode wire mesh should be more than 15000 times, the life of the back electric wire mesh should be more than 15000 times, and the life of the positive electrode wire mesh should be more than 10,000 times.
The screen should be stored in a constant temperature, constant humidity condition, a sharp change in temperature will lead to a decrease in tension, and excessive humidity will lead to denaturation of the latex, thereby affecting the use effect or even scrapping.
The specific storage conditions are: temperature 22 ± 3 ° C, humidity 50% ± 10%.
The production workshop should be led out in advance before use, and it should be allowed to stand in the workshop for more than 24 hours. When it is urgently needed, it should be kept in the factory for more than 2 hours.
The screen 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 contamination by dust before use.
The screen is very sensitive to vibration. Any collision will cause the tension to drop or even collapse. Therefore, it should be handled gently during movement and use to avoid any collision.
The surface of the screen is very fragile, and any blunt object can cause damage to the mesh portion, resulting in reduced tension, graphic damage, reduced life or even scrapped. Therefore, the surface of the mesh should be avoided from contact with any blunt objects during moving and using.
The best way to store it is to store it in a vertical shelf. If it is flat, do not exceed 5 layers, and the layers should be separated by a cushion at the frame.
The latex of the screen is not a very stable substance, and the thickness is very small. Some complicated physical and chemical changes will occur during storage, so the screen should be used within one month after entering the factory.