36th EU PVSEC, 09 - 13 September 2019
Marseille Chanot Convention and Exhibition Centre, Marseille, France

06 March 2018

Multi-busbar Technology for Record Solar Cells

One reason for solar power’s success in reducing cost is improvements in cell efficiency. While world record efficiency solar cells are light towers that are many years away from commercialization – if at all, they do show potential for optimization and innovation of a technology and inspire other researchers to work even harder on their scientific developments in that or a competing field.

A recent record efficiency of LONGi Solar has again put Passivated Emitter and Rear Cell (PERC) architecture into the spotlight. The vertically-integrated monocrystalline solar module manufacturer announced on 28 February that it attained a world record conversion efficiency of 23.6% for a commercial-size 6-inch monocrystalline PERC cell. In less than half a year, LONGi has improved its peak PERC cell efficiency three times – it reached a 23.26% efficiency level on 27 October 2017, beating its 22.71% record it announced only 10 days earlier.

Although LONGi’s new world record cell comes at a commercial size and uses new but available production technologies, it will need some time to reach such efficiency levels in mass production. However, a 23.6% p-type commercial-size mono PERC cell is likely to back the recent trend towards monocrystalline silicon cells. It is also raising the bar for advanced n-type cell concepts, such as PERT and heterojunction.

The advantages of multi-busbar cells
Adapting its PERC cell to a multi-busbar layout is one of the reasons for LONGi’s record efficiency boost. The bulk of module manufacturers has successfully moved from 3-busbar layout to 4 busbars, and many suppliers are already offering solar panels using 5-busbar cells, a few even offer products with 6 busbars – although experts argue if the move to 6 busbars makes economic sense. Multi-busbar technology is nothing but taking the ‘increasing number of busbars’ idea to the next level – which is employing over 10 thin copper wires instead of flat ribbons. The fundamental benefit of the multi-busbar concept is the same as moving to more busbars, but at higher magnitude.

Switching to wire based interconnection reduces the gap between the busbars, which also shortens the finger length considerably compared to today’s standard 4- or 5-busbars layout. Thus the current load carried by the fingers’ parts between the busbars lessens, which reduces the internal electrical resistance of a solar cell to a great extent as the fingers have the highest impact on resistance losses. At this point, this method to optimize the cell can take two different paths: Keeping the finger width at the same level reduces resistance losses, thereby improving the cell’s electrical characteristics. Alternately, the finger width can be reduced to a level that equates to the resistance of the typical busbar layout, which not only minimizes shading losses, it also decreases cell metallization paste consumption. An additional benefit of multi-busbar approach is that all commercially available technical solutions for this cell interconnection type are designed for busbar-less cells, which means further savings in pastes consumption and an even higher impact on lowering cell manufacturing costs.

Commercial multi-busbar production equipment available
However, the multi-busbar's approach has major implications on module manufacturing, especially the interconnection process. Two leading European production equipment makers, Schmid Group and Meyer Burger, have pioneered the technology, although the methods employed by each of these tool suppliers are fairly different. Swiss company Meyer Burger has re-defined the interconnection process with its SWCT process, which uses an interconnection means - a polymer film embedded with thin copper wires . The process accomplishes the interconnection during the lamination step without causing any mechanical or thermal stress on the cell. German company Schmid’s solution is built on a soldering platform that stays closer to traditional interconnection. Like a typical interconnection tool, it routes the connection media from top of the first cell to the back of the next cell, with the only difference that thin wires are employed instead of flat copper strips. Following the lead of Schmid, which supplied its technology to Korean cell manufacturer LG, several Chinese stringing tool suppliers have started building tools for wire based interconnection, but these are still in their infancies.

 

Only 3 more days: EU PVSEC abstracts deadline on March 9
The world’s leading PV technology conference EU PVSEC, which will take place in Brussels, Belgium in September, is looking forward to discussing the multi-busbar method and many other interesting and innovative technologies to improve the performance of solar wafers, cells, modules and systems. Be part of that group and submit your abstract by 9 March 2018.