First, solar cells
1. Information identification on solar cells
Since the production line for producing solar cells can produce about 20,000 pieces per day, for the same batch, the products on the same production line are directly printed in the production process, which facilitates the management of future product quality problems, so that it can be found out. Which production line, which day of production and which group of solar cells produced have problems. In view of the above reasons, it is urgent to find a printing technique in which such information is marked on the solar cell during the production process. If this information is randomly tagged on the production line, it is currently only done by inkjet printing because:
1 Since the solar cell obtains energy by surface lighting, it is necessary to retain the light receiving area as much as possible. Therefore, in the process of marking information on the solar cell, it is required that the area occupied by the information occupied by the solar cell is as small as possible, and about 4 digital information such as date, production batch, etc. are marked in a distance of about 2 to 3 mm.
2 The information required for these annotations can be constantly changed as the information to be recorded changes, so that it can be directly controlled by the computer system.
3 In addition to the above two requirements, it is also required that the speed of labeling information should be coordinated with the production speed of the solar cell to achieve production on the assembly line.
4 For the printed logo, it is also required that the solar cell is sintered at a high temperature of 800 ° C, and the marking is easily discerned by the instrument.
5 The color material used for the identification information on the solar cell is preferably a silver paste for printing the electrode wire in the production process, and if the particle size of the silver paste is appropriate, it can be used.
2. New printing method for electrode lines of solar cells
The screen printing currently used is contact printing, which requires a certain pressure from the Shanghai printing house to print the electrode lines we need. As solar cells continue to increase in technology, their thickness continues to decrease. If this traditional screen printing method is still used, there is a possibility of crushing the solar cells in the production process, so that the quality of the product is Not guaranteed. Therefore, we are required to find a new printing method, without printing pressure, without the need to touch to meet the requirements of the electrode line of the solar cell.
Electrode wire requirements: In a square area of ​​15cm × 15cm, a large number of electrode lines are ejected, and these electrode lines are required to have a fineness of 90 μm and a height of 20 μm, and have a certain cross-sectional area to ensure the flow of current. In addition, it is required to complete the printing of a solar cell electrode line in one second.
Second, inkjet printing technology
1. The way of inkjet printing
There are more than 20 inkjet printing methods. The basic principle is to generate small ink droplets and then guide them to the set position, which can be roughly summarized into continuous and intermittent. The so-called continuous inkjet is to produce ink droplets in a continuous manner regardless of the printing or non-printing, and then to recover or disperse the non-printing ink droplets; and the intermittent inkjet produces ink droplets only in the printing portion. .
1 continuous inkjet printing
The offset ink droplets are sprayed, vibrated, and decomposed into small ink droplets, and then passed through the electric field. Due to the electrostatic action, the small ink droplets fly straight ahead after flying over the electric field. When the electromagnetic field is deviated, the ink droplets with a large charge will be strongly attracted and thus have a large amplitude; on the contrary, the deflection will be small. Uncharged ink droplets will be collected in the ink collection groove.
Printing with ink droplets that do not deviate is very similar to the above-mentioned ones, the only difference being that the deviated charges are recovered, and the non-offset charges instead go straight to form a print.
The undesired ink droplet charge splitting ink flow is still pressurized by the nozzle, but the tube hole is more slender and has a diameter of about 10-15 μm. The ink droplets whose pores are so small that the ink droplets are ejected are automatically decomposed into a small droplet of ink, and these fine ink droplets are passed through the charging ring of the same electrode. Since these ink droplets are quite small, the ink droplets of these charges are again split into a mist due to the repelling of the same charge, and at this time, it loses its directivity and cannot be printed. Conversely, an uncharged ink does not split to form a print and can be used as a continuous tone print.
2 batch inkjet printing.
Pull with static electricity. The ink is ejected by the electrostatic pulling force, and the ink at the nozzle hole forms a convex half moon shape, and then an electrode plate is juxtaposed to protrude the surface tension of the ink, which is damaged by the high voltage on the parallel electrode plate. Therefore, the ink droplets are pulled out by the electrostatic force. These ink droplets are electrostatically charged and can be deflected vertically or horizontally, either at a set position or on a shield.
Thermal bubble inkjet. The ink is instantly heated to expand the gas near the resistor, and a small amount of ink will become vapor, which will squeeze the ink out of the nozzle and fly it onto the paper to form a print. After the ink droplets are ejected, the temperature is lowered, so that the temperature inside the ink cartridge is also rapidly lowered, and the protruding ink is pulled back into the ink cartridge by the capillary principle.
2. Application of inkjet printing
Because inkjet printing is a non-contact, pressure-free, non-printing digital printing method, it has the unparalleled superiority of traditional printing. It has nothing to do with the material and shape of the substrate. In addition to paper and printing plates, it can also use metal, ceramic, glass, silk, textiles, etc., and has strong adaptability. At the same time, inkjet printing does not require processes such as film, baking, imposition, and printing, and has been widely used in the printing field.
3. Ink control in inkjet printing
In the case of inkjet printing, in order to ensure the result, it is necessary to perform appropriate control on the parameters of the printing ink, and the conditions for controlling at the time of printing are as follows.
1 In order not to block the ink jet head, it is necessary to pass a 0.2 μm filter.
2 Sodium chloride content must be less than 100ppm. Sodium chloride causes the dye to settle, and sodium chloride is corrosive, especially in bubble jet systems where it can easily corrode the nozzle. Although the nozzles are made of titanium, they are still corroded by sodium chloride at high temperatures.
3 Viscosity control 1 ~ 5cp (1cp = 1 × 10-3Pa · S), micro-piezoelectric inkjet system has higher viscosity requirements, bubble inkjet system viscosity requirements are lower.
4 Surface tension 30~60dyne/cm (1dyne=1×10-5N), the surface tension requirement of the micro piezoelectric inkjet system is low, and the surface tension requirement of the bubble jet system is high.
5 Drying speed should be just too fast, it is easy to plug the inkjet head or break the ink too quickly, it is too slow to spread easily, and the dot overlap is serious.
6 Stability, the dye used in the bubble jet system is better in thermal stability because the ink of the bubble jet system is heated to a high temperature of 400 ° C, and if the dye cannot withstand high temperatures, it will decompose or discolor.
In order to reduce costs, solar cell manufacturers are demanding that the silicon wafers selected for solar cells become thinner and thinner. If conventional screen printing is used, the silicon wafers will be crushed under pressure. The inkjet printing technology is a pressureless printing, which can increase the speed of production by increasing the inkjet head. In the near future, inkjet printing technology will certainly develop better in this field.