Screen printing process
Screen printing is the process of attaching the pattern you want to print to the screen and using the paste to print through the screen. In solar cell printing, the cell is placed under the screen and a layer of paste is applied to the screen. In solar cell printing, the cell is placed under the screen, a layer of paste is applied to the screen, and the paste is squeezed by a squeegee to pass through the hole in the middle of the screen, and the pattern on the screen is printed on the surface of the cell. The pattern on the screen is printed on the surface of the cell. When the screen is designed, the holes of the screen can be sealed by the corresponding technology. The part of the screen without the pattern will be sealed so that the paste cannot pass through, while the part with the pattern can pass through the paste smoothly. This will ensure that the paste will be printed on the substrate in a regular manner through the extrusion of the squeegee, so that there will be only the desired pattern on the cell. This way, only the desired pattern will be printed on the cell. Wire is commonly used as the screen material, silk, nylon polyester, cotton, and other materials. Nylon polyester fiber, cotton fabric, and copper wire can be used as screen materials.
Key points of industrial screen printing process
Several aspects of screen printing are important: the parameters of the screen, the printing paste, the pattern of the screen, the substrate and the squeegee. The width of the printed pattern is determined by the mesh and diameter of the screen. In the selection of the screen, it is required that the screen’s aperture is about 2.5~5 times the particle size of the paste; the mesh of the screen is an important parameter of the screen, the lower the mesh number, the The lower the mesh number, the thinner the screen; the larger the mesh, the better the slurry will pass, and vice versa. Back silver and back aluminum two process The main consideration is the thickness of the printing, the choice of 250 ~ 280 mesh can be. In The silver paste printing process on the positive surface of the silicon wafer is more demanding, mainly to ensure the width of the grid line and the thickness of the grid line Requirements, the general choice of 300 ~ 330 mesh, the width of the cell grid line value depends on the wire diameter of the screen and the width of the mesh
The printing paste is a mixed fluid composed of functional components, adhesive components and organic carriers, the paste is divided into The slurry is divided into conductor slurry, electropositive slurry, dielectric slurry and encapsulating slurry, etc., which are used in the printing of solar cells. All are conductor pastes. A total of three pastes are used in printing, namely the first silver paste, the second aluminum paste and In the industrial production, the third process is particularly important, to the silver content, organic The third process is particularly important in the industrial production, to dig the silver content, organic carriers and inorganic additives. The design of the pattern takes into account the need to save area and at the same time The design of the pattern takes into account the need to save area while maximizing the efficiency of collecting the carriers on the wafer surface, and to match the composition of the paste to achieve the desired effect.
The substrate and the squeegee are mainly considered to be the machinery itself, i.e. the squeegee should be very flat and cannot have The substrate and the squeegee are mainly considered the problem of the machinery itself, that is, the squeegee should be very flat and not have the slightest nick and trace to ensure that the printed pattern is flat and flawless.
The industrial screen printing process is divided into the following steps
1.Back electrode printing (positive) on the back of the battery printed on several silver electrodes, the main role is to make the aluminum back field well connected with the tin strip. Because the contact between aluminum and tin is not good, so the electrode contact point will be replaced with silver paste to
So that it has good solderability, form good ohmic contact, strong tensile strength and reduce the cell’s electric Yang, reduce the current loss.
2.back electric field printing. The carriers generated on the back of the cell are gathered at the back electrode and output to the load.The formation of a layer of silicon aluminum alloy on the back of the silicon wafer will form a P-layer heavy doping area, this heavy doping area can reduce This redoped area can reduce the lesser compound at the contact between the aluminum backfield and the silicon wafer, which in turn increases the open circuit voltage and increases the short circuit current. Backfield The paste used is aluminum paste.
3. front electrode printing (negative), the role is to collect the front generated carriers, from the gate line to the main gate at the gate line and external The height and width of the grid line are specified, because the grid line is too wide will affect the surface area of the silicon.The height and width of the grid line have to be specified, because the grid line is too wide will affect the surface area of the silicon receiving light, so people have been trying to make the grid line narrower and higher, so as not to affect the cell
This will not affect the light area of the cell, but also improve the collection rate of carriers. The paste used for the front electrode is silver paste.It is very important to control the thickness of the aluminum paste on the back side of the screen printing. In the sintering process of silicon wafers, the temperature will reach.If the printing prize material is large, it will be consumed during the sintering process by forming a molten area with silicon, and the alloy area is not suitable for the back metal contact regardless of the cell’s lateral conductivity or the solderability of the tin strip.trigger, however, if the aluminum backfield large thick, not only wasted paste, but also lead to uneven drying, in more serious cases can not volatilize all the organic matter in it, resulting in a good sintered aluminum layer contains other organic substances, can not produce a pure aluminum backfield, and the purpose will also produce aluminum Pod.
The electrode paste printed on the diffusion sheet by screen printing has not yet formed an alloy with the silicon, which means that the electrode paste has not yet been cured into the silicon. The electrode paste printed on the diffusion sheet by screen printing has not yet formed an alloy with silicon, that is to say, the electrode paste has not been cured into silicon, so it cannot form a good electrical contact, and must be sintered. The paste printed on the substrate needs to be sintered to form a thick film conductor, and the printed film needs to be dried before sintering to allow the organic carrier to fully evaporate. The printed film needs to be dried before sintering to allow the organic carrier to evaporate fully, and then sintered. The printed electrode paste can be placed on a conveyor belt and With the rotation of the conveyor belt into the high temperature zone sintering, and finally the conveyor belt will transport the sintered solar cell out. The firing curve of the sintering process should be determined according to the specific conditions of the paste, solar cell surface passivation and anti-reflection media and The firing curve should be determined according to the specific conditions of the paste, solar cell surface passivation and anti-reflection medium and through the process experiments. The temperature should be selected to ensure good ohmic contact and to avoid excessive The high temperature should not only ensure a good ohmic contact, but also avoid too much harmful electrode metal to enter the silicon to obtain a good photoelectric conversion efficiency.
For polycrystalline silicon solar cells, passivation of defects on the material surface and in the body is particularly important due to the presence of high grain boundaries, point defects (vacancies, gap-filling atoms, metal impurities, oxygen, swords and their complexes). The main way of surface passivation is to saturate the suspended bonds on the semiconductor surface and reduce the surface activity as a way to reduce the surface complexation rate of carriers. The main methods of such passivation are silica, alumina, silicon nitride, phosphor silica glass, and a combination of multilayer passivation films. In addition to the aforementioned absorption techniques, there are various methods of passivation processes. Saturating the silicon suspension bond by thermal oxidation is one of the more common methods, which can make the Si-SiO, interface compounding rate greatly reduced, and the passivation effect depends on the surface concentration of the emitting region, the density of interfacial states and the electron and hole floating gain cross section. Annealing in a hydrogen atmosphere can make the passivation effect even more pronounced.
The process principle is as follows: sintering is to co-fire the slurry printed on the silicon wafer at high temperature in a sintering furnace to form a good ohmic contact between the slurry and the silicon material surface, thus reducing the contact resistance between the slurry and the silicon for better collection of photocurrent; and the sintering control must be precise enough for the metal electrode to penetrate the minus reflection film but not the P-N junction.
The purpose of sintering: First, the first purpose of sintering is to make the electrode pass through the reflectance-reducing film so that the electrode will be connected to the tablet and form a good ohmic contact with the solar cell; second, the PECVD process produces hydrogen suspension bonds, which will diffuse into the body of the silicon during the sintering process and play a passivating role. For the sintering process control requirements, a physical diagram of the sintering furnace is shown in Figure 6.10(a). There will be many temperature zones in the sintering furnace, and different temperature zones serve different purposes. The first part of the temperature zone is mainly to dry the paste printed on the wafer surface, and only the last two zones are sintering zones. The temperature control of the sintering zone is crucial. Too low a sintering temperature will make the sintering incomplete and make the series resistance of the cell too large; too high a temperature will cause the P-N junction to burn through and the parallel resistance to be too small. As shown in Figure 6.10 (b) is a photo of the lower material port after the completion of sintering, where the overall color of the wafer is to be tested for uniformity and no local discoloration of the wafer: the slurry is not contaminated on the front side of the cell, and the cell surface is free of aluminum beads and aluminum spurs, in line with the maximum bending degree of the cell. Bending degree requirements will be placed naturally on the wafer on the gold stone table, without changing the wafer itself bending degree, choose the appropriate standard will plug the ruler from one side into the wafer bending edge edge of the arc, measuring the height of the highest point of the curved arc from the horizontal surface, the actual production bending degree requirements to The actual production curvature requirement is below 1.87mm, appearance requirement: the printing appearance requirement should be strictly in accordance with the “crystalline silicon solar cell quality class The appearance requirements: the printing appearance requirements should be strictly in accordance with the “crystalline silicon solar cell quality class classification methods” A grade product inspection standards.