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The Multi-Busbar solar panel refers to a thick electrode strip on the surface of a crystalline silicon solar cell, which acts to direct the photo-generated current generated from the inside of the cell to the outside. The busbar will block some of the sun’s light into the cell,so it is desirable that the busbar be as fine as possible to improve the conversion efficiency of the cell; however, the more the busbar resistance is, the larger the resistance loss is, and the lower the fill factor is. Therefore, the design of the busbar requires balanced shading and conduction. Relationship. At the beginning of the 20th century, Kyocera found that increasing the number of busbar not only reduced the distance traveled by the current in the fine grid, but also reduced the current carried by each busbar, resulting in smaller resistance losses and higher conversion efficiency. Products with multi-busbar have emerged and become one of the major development trends in the future of photovoltaics.

 

Multi-busbar solar panel usually refer to products with more than 10 busbar. Compared with other high-efficiency cell technologies, multi-busbar technology is more popular because of the following advantages:

(1)The length between the busbar of the cell is shortened, and the busbar resistance is effectively reduced, so that the width of the fine busbar can be narrower. The number of busbar increases reduce the path of photo-generated current transmission to the busbar significantly(as shown in Figure 1). The maximum effective transmission path length of the 5BB cell current of the general specification of 156*156mm is about 15.6mm,and the 12BB cell is about 3.5mm, which is more than 75%. The shorter the path of the current on the fine grid is, the smaller the power consumption is, the higher the overall power output.At the same time, it can effectively reduce the working temperature of the module, improve the noct performance of the photovoltaic module, and the long-term generation performance of the module is good. In addition, the width and number of the fine busbar can be further reduced considering the efficiency and cost, and the effect of reducing the consumption of the silver paste while reducing the shielding area of the fine busbar is also achieved.

Figure 1.12 BB and 5BB battery fine gate current effective transmission path

(2) Increasing the number of busbar of the cell drives the resistance and current distribution on the cell more uniform (as shown in Figure 2). The brighter the color, the higher the resistance value, and the darker the color, the lower the resistance. It can be seen that the more the number of busbar, the lower the distribution of resistanc. The lower the current, the lower the impedance loss on the ribbon, while the busbar width design can be narrower. At the same time, the 12BB solar panel can also reduce the total amount of solder ribbon compared to the traditional 5BB solar panel flat ribbon usage at the solar panel end, thereby further reducing silver paste, ribbon consumption and cell occlusion.

Figure 2. Series resistance distribution on the cell.

(3) The reduction in the consumption of silver paste can significantly reduce the cost. Due to the optimization of the busbar, the overall silver paste consumption is reduced, and the 12BB can save at least 30% compared with the 5BB silver paste consumption.The silver paste cost of the cell accounts for more than 50% of the non-silicon cost, which reduces the production cost of the solar cell and the components.The savings in silver paste for multi-busbar solar cell assemblies is even more pronounced for double-sided solar cell.

(4) Component reliability improvement.The area of the multi-busbar solar cell is smaller than that of the 5BB cell in the same case,so the multi-busbar solar panel is more tolerant to the problems of the cell crack, broken gate, crack caused by the continuous operation of the component(as shown in Figure 3). At the same time, the distribution of the solder ribbon on the cell after welding is more uniform, in other words, the force distribution of the cell is more uniform, and the packaging stress of the cell is dispersed, thereby improving the mechanical properties of the cell and the reliability of the module.

Figure 3. Effect of cell rupture zone

(5)Since the single solder ribbon is thinned, a circular soldering strip is generally used for the cell connection in the design of multi-busbar solar panels, and the circular solder ribbon is more suitable for the optical advantage than the flat conventional solder ribbon. Round wire strips have three important areas (shown in Figure 4):

Figure 4. Schematic diagram of the light path of the ribbon structure

In the area (a), the light beam can be directly reflected to the surface of the cell, so that the effective shadow area is reduced to about 70% of the actual band gap area;

In the area (b), the light reflected from the busbar is reflected to the interface between the glass and the air. The refractive index of the air to the glass is 1.5, after forming a total reflection on the interface, it returns to the surface of the cell, and the effective sunshade area of welding tape is reduced to 36%;

In the area (c), the reflected light returns to the glass surface again, and the incident angle of the light is smaller than the total reflection angle, so that the light beam is divided into a reflective portion and a transmissive portion, and the reflected portion forms a secondary absorption, which further reduces the effective shadow of the wire. The effective occlusion area is reduced, and the short-circuit current of the cell pack is increased, whereas the conventional flat ribbon has no such advantage.

Combining the advantages of the above multi-busbar technology, the number of busbar of the cell increases, and the series resistance is reduced. At the same time, the design of the finer and narrower fine busbar effectively reduces the metal shielding area, so that the efficiency of the cell can be increased by 0.3% to 0.5%. The use of the circular soldering strip on the component end reduces the effective shielding area of the cell while increasing the secondary absorption of the incident light. The component power can be increased by 5~8W combined with the advantages of the multi-busbar solar cell.

At present, multi-busbar solar panel still occupy only a small part of the market share, the 3BB are gradually withdrawing from the market, and the 4BB and 5BB are gradually becoming mainstream products. Multi-busbar solar panel will start to increase in 2019, and will occupy most of the market share and become mainstream technology.