1 INTRODUCTION
With the growing tensions global energy, solar energy to clean the market a large space, such as unique advantage by the extensive attention of all countries in the world, many large international companies in solar cell research and development and production industry. Access to electricity from solar energy required by solar cells to achieve a photoelectric transformation, silicon solar cells is an effective absorption of solar radiation into electricity and semiconductor electronic devices, widely used in various lighting and power systems In.
2 silicon solar cell production processes
Solar cells on the main principles of semiconductor materials for silicon substrate, the proliferation of the use of silicon in the incorporation of impurities: When the incorporation of boron, phosphorus and other impurities, the silicon crystal will be the existence of a hole, the formation of n-type semiconductor; Similarly, the incorporation of phosphorus atom, the silicon crystal there will be an electronic form p-type semiconductor, p-type semiconductor and n-type semiconductor combination of a pn junction, when the sunlight after the silicon crystal, pn junction of the n-type semiconductor The hole to move p-type area, and p-type zone in the e-n-type area to the mobile, thus forming from the n-type area to the current p-type zone in the pn junction in the formation of potential difference, which formed a power supply, see fig. 1.
Figure 2 for silicon solar cell production of the major processes, and can be seen from the screen printing is the production of solar cells of the important processes, and its print quality (thickness, width, thickness consistency) impact of the film battery technology indicators.
3 processes the request of the printing electrode
3.1 back of silver electrode printing (back silver)
In the battery-cathode surface (p district) with silver aluminum paste printing two electrode wires (a width of 3 ~ 4 mm) as a battery-electrode (Figure 3).
3.2 back of aluminum printing (back aluminum)
In the cathode surface cells are used aluminum paste printing the entire surface (except for the back-electrode).
3.3 positive silver printing (now Bank)
The face of the battery (reflective film by spraying the surface) at the same time by printing a row of silver paste evenly between the gate line and the two electrodes (Figure 4), on the request of the gate line spacing of about 3 mm, width of about O.10 ~ 0.12mm:
Principle 4 printing
Figure 5 for the Principle of screen printing, screen printing posed by the five major elements, namely screen, scraper, slurry, workstations and the substrate. Screen printing is a basic principle: the use of the mesh screen graphics through slurry, by some non-mesh size not thoroughly carry out the basic principles of printing. Screen printing at one end into the slurry, using screen-scraper in the slurry must exert pressure on the site, while the other side in the mobile screen. Ink on the move was part of the scraper from the graphics mesh to squeeze in-on-chip. As the viscous slurry of solid imprinted with a certain range, the process of printing screen scraper always with the printing plates and a line of contact, contact with the scraper mobile lines and mobile, because of the screen and printing Maintaining a certain gap between, making the screen printing through its own tension arising from the reaction force of the scraper, this reaction force known as resilience. As the role of resilience, so that the substrate and the screen showed only mobile line contact, screen printing and other parts of the detachment to ensure the accuracy and to avoid printing size printing of Ceng dirty. When printing the entire region scraper scratch after the lift, while also screen out of the substrate, on the expected return to the table position, this is a printed itinerary.
Blade 4.1
Figure 5 from the printing can be seen in principle indicate, Blade is the role of a certain size to the speed and angle will be pressure to the screen size of the leak, the scraper in the printing of the screen to maintain a certain pressure on the blade I pressure in the 10 ~ 15 N / cm between the scraper to make too much pressure on the screen in deformation, the graphics and printing screen graphics inconsistencies, but also aggravate the screen scraper and wear, scraper pressure will be too small After the silk printing online presence residual pulp.
Blade material for the general Juan Zhi rubber or rubber fluoride, hardness range of Shaw A60 ° ~ A90 °, the hardness of the scraper, the lower the greater the thickness of printing graphics, scraper must wear-resistant materials, have a good straight edge , And maintain the screen Keys; scraper general optional diamond scraper, it has four edge can be used individually, a high utilization rate, see Figure 6.
Blade speed: scraper speed decision is the biggest factor efficiency, semi-automatic printing press as an example, the printing of the time normally is the cycle of 2 / 3; printing speed settings from the printing graphics and printing the slurry viscosity decision, the more speed , Scraper driven into the screen size leak the shorter the time, the size of the filling will be worse, a phenomenon shown in Figure 7, if the fine printing lines, the speed should be lower, as shown in Figure 4 are in the process - The gate line in the linewidth 0.1 ~ 0.12 nun, the general rate set at 200 ~ 250 mm / s, as shown in Figure 3 aluminum and back-back-line printing processes because of the speed set in 300 mm / s; printing Pulp Because of different materials and different processes, the corresponding viscosity different, but overall is relatively low viscosity, print faster in the actual printing of the same constant speed is very important, if in the process of printing speed fluctuations, will lead to inconsistent thickness of the graphics .
Blade angle: scraper angle settings and suspensions related to size the higher the viscosity of the mobility of the poorer, the size of the need to scraper downward pressure on the larger perspective of small scraper; scraper angle adjustment range of 45 ° ~ 75 °. In the printing process play a key role in the Blade 2 ~ 3 mm edge of the area, in the printing and screen-scraper under the pressure of friction, the approximate time at the beginning of the printing line, scraper edge of the partial pressure of the big screen, see Figure 5 Show, with the scraper blade of wear and tear, curved edge shape Chengyuan, and its screen size in the direction of the dramatic increase in help, screen role in the screen unit area significantly reduced the pressure, the blade scraper Screen and the practical point of view is much smaller than 45 °, after screen printing surface will be residual pulp, prone to leakage, while fuzzy edge of the printing line. Figure 7, then need to be replaced scraper.
4.2 Screen
Screen material commonly used in stainless steel and nylon are two kinds. Stainless steel wire mesh is characterized by small diameter, the number of heads, wear resistance, and high strength, size, stability, stretching small, the fine wire diameter, through the good performance of the ink, dimensional accuracy and stability, suitable for solar cells The film printing. Nylon mesh is formed by the chemical synthetic fiber production, is of high strength, wear resistance, chemical resistance, water resistance, flexibility relatively well, because wire diameter uniform, smooth surface, so the adoption of ink have excellent. Is its lack of nylon stretch of the larger screen. Stretching after the screen in this period of time, the tension has decreased, the screen printed version of relaxation, decreased accuracy in the printing solar cells are used in stainless steel mesh. Figure 8 for the screen appearance.
Screen production: production by a professional graphic screen manufacturers build, according to the accuracy of printed graphics Mesh choice of high or low, the wire diameter size, open area of the screen size, screen size of the rate of expansion, Table 1 Japan's special fabric will be part of the screen's technical specifications.
Head of the screen and wire diameter decision printable graphics width; silver and back to back these two processes aluminum printing the actual printing complex graphics, so the screen Yaoqiubugao, the main consideration thickness can be printed, the general choice 250 to 280 heads to meet requirements for silver is printed on the printing requirements of a maximum printing processes, and the main gate line is to ensure that the width requirements, and the printing of uniform thickness, the general selected 300 to 330 head, the printing line after the gate Value depends on the width of the screen mesh diameter and the width of the following formula:
K=2s+R， K = 2s + R,
Where: K for the width of lines, S for the screen width of wire diameter, R mesh that the width, such as the choice of 330 head screen, look-up table 1: s = 30 μ m, K = 44 μ m, K = 2 × 30 + 44 = 102μ m, to meet the requirements of the width of grid lines.
The tension settings screen: Screen and the tension of the screen and the heads of relevant materials, different materials, different wire diameter of the screen under different tension, head of the lower wire diameter the rough, under the screen tension The greater screen manufacturers in the technical indicators in a screen of the recommendations of the greatest tension, table 1. If the tension is too low or screen printing screen tension instability in the scraper under the pressure of outlets will expand outlets and lost, affecting the printing accuracy, and back to back Al-general selection process 30 N / cm, are silver processes select 27N/cm (see Figure 9).
Printing width: Photographic film screen and the thickness of a decision after printing graphics that the thickness of the thickness of lines, as shown in Figure 10, in normal circumstances, Mesh, the lower wire, as the crude, after the printing of the slurry Higher by Mesh high, after the print size on the lower layer. Photographic film thickness and Mesh and the width of the lines: the higher the number of heads, the finer wire diameter, photographic film and the contact area the smaller the screen, reduce the adhesion of the two, if the printing line narrow, Photographic film thickness of the increase caused by falling easily, so thin photographic film, photographic film is about the thickness of the screen thickness of 15% to 25% for aluminum and back-back process, select 250 heads screen, the thickness of 58 um , Photographic film thickness of 10 ~ 15 um, while India thickness of 68 ~ 73 μ m, arrived here at the thickness of the wet slurry thickness (wet thickness). After drying required (dry) and sintering (fire) is the ultimate thickness (dry thickness), the thickness of a wet dry thickness of 30% to 40% for silver are processes, select 330 heads screen, the thickness of 44 um, photoreceptor Film thickness of 5 ~ 10 μ m, while India wet thickness of about 49 ~ 54 μ m.
Select screen wire diameter and a few heads when asked mesh size of the hole for a long powder particle size of 2.5 to five times; mesh screen, the lower the more sparse, the mesh is, through the ink of the better; Mesh is, the worse the ink through sexual, as shown in Figure 9.
Network Box: Most network hardware box aluminium and aluminium alloy Stretching to withstand the force generated by connecting the bottom of the screen need a higher plane, about 0.04 mm × 150 mm × 150 mm; general specifications for network printing box Of 2 times: to 150 mm film batteries as an example, the printing of an area of 150 mm × 150 mm, net box, I should be the size of 300 mm × 300 mm, Figure 11.
4.3 Size
Size is by functional group in, group bonding and organic carrier in the composition of a fluid, a conductor pulp slurry, resistance size, medium size, such as pulp and encapsulation. Back in silver, aluminum and back-processes are used in the pulp slurry for the conductor. The conductor size, functional group in general for a mixture of precious metals or precious metals. Polymer carrier is in organic solvents in the solution. Functional group in the film after the decision, and mechanical properties. Vector decision of the thick-film technology, a printing film and the film temporary dry binder. Functional components and general bonding for the group in powder form, in full vector mixing and dispersed after the formation of the thick film paste slurry. Sintering the thick film conductors are bonded by the metal and composition of the group.
Size of the technical performance indicators in the slurry is functional ingredients (back Silver compound of silver aluminum components, Silver is a silver compound ingredients, back in the slurry of aluminum components) and after drying and battery-sintered The Ou Mute, and its impact on the battery-electric performance indicators such as the open circuit voltage, short-circuit current, parallel resistance, the series resistance, conversion rates, and other technical indicators; slurry of the process is the guarantee for achieving the above target, the size manufacturers for 3 A kind of printing process recommended by the technical parameters such as size of the particle size, viscosity, solid content, the mesh screen; mentioned earlier mesh size of the hole for a long powder particle size of 2.5 to 5 times the size Scraper affect the viscosity of the printing speed; solids in the printing after the decision by the thickness of wet and dry sintered after the final thickness. Back aluminum and silver are three different processes of the size, which determines their screen and printing parameters are different, Table 2 for the United States FERRO-related companies have processes in the printing of the slurry.
5 device
The equipment requirements of the following three points:
(1) table of the plane. When the cells are printed by adsorption in the table surface, such as uneven surface, the negative pressure cells are easily broken, with 150 mm film batteries as an example, the table of the flatness of not more than 0.02 mm;
(2) table repeat positioning accuracy. According to the film solar cells precision, table repeat positioning accuracy is 0.01 mm to meet the technological requirements;
(3), screen printing and decided to table the parallel printing of the film thickness consistency, in accordance with the requirement to 150 mm film batteries as an example of two parallel to 0.04 mm.
Cells are the flatness of not more than 0.02 mm, surface roughness of less than 1.6.
6 Conclusion
Solar cells are printing production line is an important process, the quality of the cells are playing an important role in solar cell printing technology is an organic whole, is a combination of various technologies, the need to process engineers and equipment engineers to work together: It is necessary to understand the characteristics of the various parameters, but also understand the constraints of their mutual relations; three kinds of printing processes have in common but differentiated, the need for different processes were optimized to the specific requirements of the process parameters, the development of the implementation of different programmes, Order to meet the technology of printing products.