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). ã€Table 1】
Table 1: Fabrication of crystalline silicon solar cells requires multiple screen printing steps. Applied Materials' Baccini products can help achieve the steps in the green box.
Over the years, solar screen printing equipment has made great advances in precision and automation, with the ability to repeat multiple prints on micron-sized sizes. This development has opened up new and advanced applications such as dual printing and selective emitter metal coating. Baccini developed screen printing technology in the field of microelectronics in the 1970s and extended it to the field of solar metal coating in the 1980s. Today, Baccini has become the Applied Materials Baccini Group, leading the industry with a number of advanced technologies.
Basic solar screen printing
The printing process begins with the placement of the wafer onto the printing table. A very fine printed screen is attached to the frame and placed over the wafer; the screen closes certain areas while the other areas remain open for the conductive paste to pass [Fig. 2]. The distance between the silicon wafer and the screen is strictly controlled (called the print gap). 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.
Table 2: The printed screen contains open and closed areas through which conductive paste can be printed onto the wafer.
Place an appropriate amount of slurry on the screen and apply the slurry with a spatula to evenly fill the mesh. The scraper squeezes the slurry through the mesh mesh onto the silicon wafer during the movement [Fig. 3]. The temperature, pressure, speed and other variables of this process must be strictly controlled.
Table 3: A conductive paste was placed at one end of the screen, and the slurry was applied to the screen with a doctor blade and extruded from the mesh onto the silicon wafer.
After each printing step, 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.
Printing on the front and back of the wafer
Each of the solar cells has a wire deposited by screen printing on the front and back sides (Fig. 4), 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 circuitry ("finger lines") to transfer the photogenerated electrons collected in the active area to the larger acquisition conductor - the "busbar" And then passed to the component's circuitry. The front finger line is much thinner than the back line (narrow to 80μm). Because of this, the front printing step requires greater precision and accuracy.
Figure 4: There are different sizes of wires on the front side of the wafer after printing to collect power from the active area. .
The printing requirements for the back and front of the wafer are different and technically less stringent. The first step in backside printing is to deposit 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 connected to external circuitry [Figure 5].
Figure 5: The busbar on the back side can be connected to the outside by soldering.
A new generation of screen printing applications
Today's crystalline silicon solar cells have an average conversion efficiency of 15%, and the industry's development goal is to increase conversion efficiency to more than 20%. Screen printing equipment can provide a variety of methods to help achieve this goal. Achieving higher conversion efficiencies can be achieved in two ways: the battery process (creating an effective area that converts light energy into electrical energy) and the metallization (forming conductive metal lines).
Double printing
One of the negative effects of the conductive lines on the front of the battery is the shadow: the wires block a small amount of sunlight, making it inaccessible to the active area of ​​the battery, reducing conversion efficiency [Figure 6]. In order to minimize this shadowing effect, the wires must be as narrow as possible. However, in order to maintain sufficient conductivity, the height of the lines must be increased to maintain the same cross-sectional area. A solution to achieve a finer, higher wire cross section is to overlay multiple wires. This means that screen printers must be able to print very small lines with high accuracy and repeatability - the current standard line is as small as 80 μm - equivalent to the average thickness of a human hair.
Figure 6: The wire blocks the light from reaching the battery active area
The size of most of the wires after sintering is now 110-120 μm wide and 12-15 μm high. The conversion efficiency loss of such a size line due to the shadow effect is approximately 1.29%. To reduce this loss, the wire width must be reduced; at the same time, the height of the wire cross-section needs to be increased to optimize the conductivity. [Figure 7]. After the wire cross-sectional dimension was changed from 110 μm wide / 12 μm high to 80 μm wide / 30 μm high, the potential conversion efficiency absolute gain was 0.5%.
Figure 7: Lowering the line width reduces shadows in the active area, increasing potential conversion efficiency
Applied Materials' Baccini's method uses two different presses to overlay two materials. This latest process achieves a wire cross-sectional dimension of 80 μm wide and 30 μm high on average in an actual production environment. This method reduces shadow loss by approximately 20% and correspondingly reduces the resistivity. By adding an additional screen printer and drying oven to an existing production line, it is very convenient to implement multiple printing processes in a cost-effective manner.
The most critical aspect of wire double printing (and other advanced printing applications) is alignment accuracy because the second layer of print must be placed very accurately on top of the first layer. The latest developments in Applied Materials' Baccini have resulted in alignment accuracy of the second layer of print at +/-15 μm. This technology uses a new high-resolution camera and new software algorithms with automatic adjustment procedures and additional control during the initial stages of printing. In addition, slurry formulation and screen design must be carefully optimized to maximize the hardware and process performance of screen printing. Globepv.com
Selective emitter
Another emerging application is the selective emitter technology—precisely fabricating a heavily doped n+ region under screen-printed metal lines to further reduce contact resistance for improved conversion efficiency. [Figure 8]
Figure 8: Selective emitter is a heavily doped region directly under the metal line
There are several techniques for making these emitter regions. Each requires multiple printing steps with high precision and high repeatability. In addition, the emitter region must be slightly wider than the upper metal line: for a 100 μm wide metal line, the optimized emitter region width is about 150 μm. The key point is that the subsequent metal wire must be placed directly above the emitter region very accurately, otherwise it loses its efficiency advantage. Applied Materials' Baccini's screen printing technology has advantages in terms of maturity, alignment accuracy, low cost and high speed, making it an ideal choice for this battery process.
Screen printing productivity
As the solar PV industry grows in size and processes (to achieve greater efficiency), many issues – including high throughput and the ability to handle thinner wafers – are becoming more and more important. Copyright Global Photovoltaic Network
Currently, the production of crystalline silicon solar cell plants is about 1500 wafers per hour (per production line), and the industry's goal is to achieve at least 3000 wafers per hour in the near future. This requires the use of very advanced mechanical automation techniques to process wafers at high speeds with minimal fragmentation rates.
This means that in the screen printing process such as screen placement, slurry coating and blade movement need to be performed at a faster speed, while the width and alignment of the lines must maintain the original accuracy or even more accurate.
The trend toward thinner (and therefore more fragile) silicon wafers has driven the development of "soft" processing technology to maintain low fragmentation rates and high yields. Applied Materials Baccini has become a world-renowned leader with its high-speed soft processing technology and lowest fragmentation rate. A team of engineers with decades of experience is working to develop a number of technological innovations to maintain Baccini's leading position in ultra-thin wafer processing. Content from Global Photovoltaic Network
in conclusion
Screen printing of crystalline silicon solar cells is a technology for depositing metal lines and other applications that is cost effective and scalable. The latest screen printing systems are highly automated, with extremely high throughput and the ability to handle ultra-thin wafers. Applied Materials' advanced Baccini screen printers help the industry achieve emerging multi-printing applications such as dual printing and selective emitter technology with its excellent alignment accuracy and fine wire throughput to improve battery efficiency and reduce solar power. Cost per watt.
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