Screen printing technology of the hottest crystall

Wire printing technology of crystalline silicon solar cells

introduction

one of the most critical steps in the production of crystalline silicon solar cells is to manufacture very fine circuits on the front and back of silicon wafers to export photogenerated electrons out of the battery. This metal coating process is usually completed by wire printing technology - the conductive paste containing metal is stamped on the silicon chip through the wire hole to form a circuit or electrode. Typical crystalline silicon solar cells require multiple silk printing steps throughout the production process. Generally, there are two different processes for wire printing of the front (contact wire and bus) and back (electrode/passivation and bus) of the battery respectively. [figure 1]

Figure 1: the manufacturing of crystalline silicon solar cells requires multiple silk printing steps

over the years, solar silk printing equipment has made great progress in accuracy and automation, and has the ability to repeat multiple printing in micron size. This development has created new advanced applications, such as double printing and selective emitter metal coating. Baccini developed silk printing technology in the field of microelectronics in the 1970s, and extended this technology to the field of solar metal coating in the 1980s. Today, baccini company has become an applied materials company baccini group, leading the development of the industry with a number of advanced technologies

basic solar wire printing

the printing process starts from placing the silicon wafer on the printing table. The fine printing wire is fixed on the frame and placed above the silicon wafer; The wire closes some areas while others remain open so that the conductive paste can pass through [Fig. 2]. The distance between silicon wafer and wire should be strictly controlled (called printing gap). Because the front needs more slender metal wires, the silk lattice used for front printing is usually much smaller than that used for back printing

Figure 2: the printing wire contains open and closed areas, through which the conductive paste can be printed on the silicon wafer

put an appropriate amount of slurry on the wire, and smear the slurry with a scraper to make it evenly fill the hole. In the process of moving, the scraper squeezes the slurry onto the silicon wafer through the wire hole [Fig. 3]. The temperature, pressure, speed and other variables of this process must be strictly controlled

Figure 3: place conductive paste at one end of the wire, apply the paste to the wire with a scraper,

and squeeze it onto the silicon wafer from the hole

after each printing step, the silicon wafer is put into the drying oven to solidify the conductive slurry. Then, the silicon wafer is fed into a different printer and more circuits are printed on its front or back. After all printing steps are completed, the silicon wafer is sintered in a high-temperature furnace

printing on the front and back of silicon

there are wires deposited through silk printing on the front and back of each solar cell [figure 4], and their functions are different. The lines on the front are thinner than those on the back; Some manufacturers will first print the conducting wire on the back, then turn the silicon chip over and then print the wire on the front, so as to minimize the possible damage during processing. On the front side (the solar side of the face enhanced polyethylene foaming solution - the smart packaging solution customized for e-commerce logistics), most crystalline silicon solar cells are designed with very fine circuits ("finger wires") to transfer the photogenerated electrons collected in the effective area to a larger acquisition wire - "bus", and then to the circuit system of the component. The front finger line is much thinner than the back line (as narrow as 80 μ m）。 Because of this, the printing step of the front needs higher precision and accuracy

Figure 4: after printing, there will be wires of different sizes on the front of the silicon wafer to collect electric energy from the effective area

the printing requirements of the back and front of silicon wafer are different, and the technology is not so strict. The first step of back printing is to deposit a layer of aluminum based conductive material instead of a very thin conductive grid. At the same time, it can reflect the light that is not captured back to the battery. This layer can also "passivate" the solar cell, close the redundant molecular path, and prevent flowing electrons from being captured by these gaps. The second step of back printing is to manufacture the busbar and connect it with the external circuit system [figure 5]

Figure 5: the bus on the back can be connected to the outside through welding

Application of a new generation of silk printing

nowadays, the average conversion efficiency of crystalline silicon solar cells is 15%. The development goal of the industry is to increase the conversion efficiency to more than 20%. Silk printing equipment can provide a variety of methods to help achieve this goal. To achieve higher conversion efficiency, we can start from the following two aspects: Battery Technology (creating an effective area that can convert light energy into electrical energy) and metal coating (forming conductive metal wires)

double printing

a negative effect of the conductive line on the front of the battery is shadow: the wire blocks a small amount of sunlight, making it unable to enter the effective area of the battery, thereby reducing the conversion efficiency [figure 6]. In order to minimize this shadow effect, the wire must be as narrow as possible. However, in order to maintain sufficient conductivity, the height of the lines must be increased in order to maintain the same cross-sectional area. The solution to achieve thinner and higher wire cross-section is to superimpose multiple wires. This means that the silk printing machine must be able to print very small lines with high accuracy and high repeatability - the current standard lines are as small as 80 μ M - equivalent to the average thickness of a human hair

Figure 6: the wire blocks the light so that it cannot reach the effective area of the battery

now the size of most wires after sintering is μ M wide, μ M high. The conversion efficiency of lines of this size due to the shadow effect is damaged. The supply of waste plastic particles is in short supply, and the loss is about 1.29%. To reduce this loss, the conductor width must be reduced; At the same time, it is necessary to increase the height of the cross-section of the conductor to optimize the conductivity. [figure 7]. The cross-section size of conductor is from 110 μ M width/12 μ M height changes to 80 μ M width/30 μ After m high, the absolute gain of potential conversion efficiency is 0.5%

Figure 7: reducing the line width reduces the shadow of the effective area, thereby improving the potential conversion efficiency

baccini, an applied materials company, uses two different printing machines to superimpose two materials. This latest technology has achieved 80% in the actual production environment μ M wide, average 30 μ M high conductor cross-section size. This method reduces the shadow loss by about 20%, and correspondingly reduces the resistance coefficient. By adding an additional screen printing brush and drying oven to the existing production line, it is very convenient to realize the multiple printing process in a cost-effective way

the key point of wire double printing (and other advanced printing applications) is the alignment accuracy, because the second layer of printing must be placed on the first layer very accurately. The latest research and development achievements of baccini, an applied materials company, have made the alignment accuracy of the second layer of printed matter reach +/- 15 μ m。 This technology adopts a new high-resolution camera and a new software algorithm, has an automatic adjustment program, and can be controlled additionally in the initial stage of printing. In addition, the sizing formula and silk design must be carefully optimized together, so as to maximize the hardware and process efficiency of silk printing

selective emitter

another emerging application is selective emitter technology - accurately manufacturing a heavily doped n+ region under the wire printed metal wire, so as to further reduce the contact resistance and improve the conversion efficiency. [figure 8]

figure 8: the selective emitter is a heavily doped region directly below the metal line

there are several techniques for making these emitter regions. Each requires multiple printing steps with high accuracy and repeatability. In addition, the emission of three curved solid polar regions must be slightly wider than the upper metal line: for 100 μ For a metal wire with a width of M, the optimal width of the emitter region is 150 μ M or so. The key point is that the subsequent metal wires must be placed directly on the emitter region with great accuracy, otherwise, it will lose its efficiency advantage. The silk printing technology of Applied Materials Company baccini has advantages in maturity, alignment accuracy, low cost and high speed, and is an ideal choice to realize this battery process

productivity of silk printing

with the increasing production scale of the solar photovoltaic industry and the process steps, 13 railways will be started one by one in the next few months, and many problems - including high production and the ability to process thinner silicon wafers - are becoming more and more important

at present, the output of crystalline silicon solar cell factory is about 1500 silicon wafers/hour (each production line), and the industry's goal is to achieve at least 3000 silicon wafers/hour in the near future. This requires the use of very advanced mechanical automation technology to process silicon wafers at a high speed with minimum fragment rate

this means that in the silk printing process, such as silk placement, slurry coating and scraper movement need to be carried out at a faster speed. At the same time, the width and alignment of lines must maintain the original accuracy or even be more accurate

the trend of silicon wafer becoming thinner (and therefore more fragile) has promoted the development of "soft" processing technology, so as to maintain low fragment rate and high yield. Baccini, an applied materials company, has become a renowned leader in the industry with its high-speed soft processing technology and the lowest fragment rate. The engineer team with decades of experience is committed to developing a number of technological innovations, so as to maintain the leading position of baccini silk printing equipment in the field of ultra-thin silicon wafer processing

conclusion

crystalline silicon solar cell wire printing is a technology for depositing metal wires and other applications. It is cost-effective and can be expanded. The latest silk printing system has a high degree of automation, high output and the ability to process ultra-thin silicon wafers. With its excellent alignment accuracy and fine wire production capacity, applied materials' advanced baccini wire printing machine helps the industry realize emerging multiple printing applications, such as dual printing and selective emitter technology, so as to improve battery efficiency and reduce the cost per watt of solar power. (end)