Connecting The HJT Contacts

This article is on HJT module making, after we covered  metallization as the last part of solar cell processing in previous articles on our HJT solar manufacturing series (see Silver-Free Contacts Via Plating For HJT)

Coming to panel processing, HJT also needs some optimization at the module level. This is applicable for both the important steps of module making — interconnection and lamination. As for lamination, it is not about processing, but rather an additional care with respect to the bill of material (BOM), especially encapsulation. A suitable backsheet is also preferred in case of the monofacial module. Interconnection on the other hand requires significant optimization for HJT module making.

While the textbooks might disagree, soldering is still the state of the art for interconnection of HJT cells, at least in China, which is still the major manufacturing hub even for HJT. The main change required with soldering here is a reduced temperature of less than 200 °C, and almost every leading stringing tool supplier offers low-temperature soldering support. However, temperature adjustment is just the tip of the iceberg. Modification of soldering material, composition of the ribbon, and the soldering method are just a few examples that require detailed readaptation to HJT, according to Risen. The company has spent a lot of time and effort on optimizing the HJT module making process, not the least of which is soldering. Risen is fully convinced that optimized soldering can be used for interconnection of HJT cells, and the company has a fully developed know-how on the subject. Huasun is also a firm believer of soldering and confirms that the process is already optimized. While not going into details, Huasun’s CTO Wenjing Wang emphasizes that increasing the number of busbars is the way to go. The company has successfully implemented the 9 and 12-busbar layout on M6 cells, and with its current offering of G12 half format, it is offering a 15-busbar based design. Huasun is further evaluating options to increase the busbar count to 18 and beyond. in fact, recent results show the 18-busbar for G12 can further reduce CTM losses (see The Status Quo Of HJT Processing).

Another, but proprietary, variant of MBB is Smart Wire Connection Technology (SWCT). Owned by Meyer Burger, SWCT has already demonstrated the 18-busbar design with the potential to go above 30. In this method, thin wires coated with a low melting-point alloy embedded into a polymer foil are used as the interconnection media instead of solar ribbons. This foil is then placed onto the cell and the stack is laminated, during which the wires are bonded to the cell’s metallization, completing the interconnection process. This low-temperature process makes it a perfect match for HJT. However, this solution, which was part of its HJT technology offering, has now been withdrawn from the open market, as Meyer Burger aims to limit this solution to captive usage. While the primary patents for the technology are about to expire, the Chinese HJT makers view the concept as rather expensive for commercial production.

ECA based interconnection:

Electrical conductive adhesive (ECA) based interconnection is another means of accomplishing interconnection of HJT cells. ECAs are metal fillers loaded into a thermosetting epoxy base, along with some additives, curing and diluting agents. The final form is a glue-like substance, which is applied to fix the ribbon to the solar cell and establish a permanent bonding after curing at about 120°C. This approach is quite popular in Europe. A few companies have also put this solution to use in commercial production.

Currently, two companies offer ECA based interconnection for HJT. Mondragon is the recent one. The company, which is also building some module production equipment for Meyer Burger, has recently launched a tool called MTS-ECA. The solution typically consists of two parts – ECA application, and tabber and stringer finetuned to work with ECA as the connecting media. The specialty of MTS-ECA is that it employs stencil printing instead of screen printing to apply the ECA, according to Xabier Otaño, solar business unit sales head at Mondragon. The company uses separate stencils to apply the ECA on the front and rear, which allows it to control the width, height and consumption of the ECA layer independently on each side. This also means that the thickness of the ECA can be controlled on the front and back separately. While the opening determines the width, the thickness of the stencil governs the height. For the tabber and stringer part, the machine has a +/- 0.2 mm alignment accuracy that helps further optimize the ECA consumption; the alignment accuracy determines the width of the ECA applied on to the cell. For example, when the accuracy drops to 0.3 mm, the ECA costs increase by €100,000 per a 100 MW module line. Flexibility to handle up to 15 busbar wires and the ability to handle wafer sizes up to G12 are the key features highlighted by the company. The MTS-ECA supports a throughput of 2,600 wafers per hour.

Teamtechnik was an early adopter of the ECA based interconnection technology. The German company has also installed tools at a few early developers of the technology, especially in Europe. The latest solution of the company is called STRINGER TT4200 i8 ECA that uses screen printing for the application of ECA. Like any other equipment for solar, this interconnection system is also capable of handling different sizes up to G12. While the spec for cell thickness is given as 180 μm, the machine is also capable of handling thinner cells of 160 μm.

The additional costs that come with an ECA-based solution, driven by silver, are a major concern. The latest generation tools come with the promise of reduced ECA consumption through optimization; for example, the high alignment accuracy of the Mondragon tool as explained above. However, HJT makers, especially in China, don’t find it attractive. On the other hand, there is an indirect way to make the technology more appealing. While yet to be commercially explored, ECA in principle can eliminate the busbar from the cells, i.e., the interconnects are directly glued to the cell. In such a case, the savings in busbar paste costs can compensate for the ECA costs.

The text is an excerpt from 3^rd edition of TaiyangNews’ Heterojunction Technology 2022 report, which provides an overview on the most recent HJT developments as the technology is entering the GW scale production level and can be accessed free of charge here.