Metallization is one of the key processing steps in making HJT cells, next to deposition of the core and AR/conductive electrode. The metallization process, along side of the performance of finished cells, significantly influences manufacturing costs. In fact, it takes the lion's share of the non-silicon costs of HJT cells. The metallization process for HJT is also quite different to mainstream solar cells. The paste systems used for other cell architectures are typically fired at temperatures higher than 700 °C. In case of HJT, the amorphous thin films employed are hydrogenated. And given that hydrogen has the tendency to escape at higher temperatures, the paste system must work at low temperatures of about 200 °C in order to avoid poor passivation. This requires the use of special silver pastes that can be cured at low temperatures.
Such silver pastes are loaded with silver into thermoplastic polymer systems and come with a few inherent limitations, like long processing times and higher resistance. The latter leads to a high paste laydown to keep the conductivity at the same level. These limitations encouraged the development of alternative metallization approaches, such as laser transfer printing (LTP), pattern transfer printing (PTP) and plating. However, screen printing is the predominant means for solar cell metallization of HJT. In addition to the fact that screen printing technology is an old friend of PV manufacturers, there has been tremendous progress in the field of low-temperature cured silver pastes for HJT.
However, what needs to be understood is that the pastes for HJT are fairly different and the metallization costs clearly high, contributing about 25% to the cell manufacturing costs, given the fact that paste consumption is about three times higher than PERC. Of course, PERC uses silver paste only on one side. Silver paste consumption on the back is always higher compared to the front, as the number of fingers is higher to reduce the power loss in the backside TCO to optimize the cell fill factor (see TCO Deposition: First Foreign, Increasingly Local).
Moving to industrial practices, dual printing is the state of the art, according to Huasun's CTO Wenjing Wang, as the paste quality is good enough with dual printing and double printing is not necessary. Dual printing also favors a reduction in paste consumption. Last year, it was about 180 mg for M6 cells realized by printing through a 28μm opening and finger width of 35 μm, according to Wang, which was referring to 9-busbar layout. Adapting to 12-busbar offers the potential to reduce it further. In fact, the company has now already archived 140 mg level on M6 wafer. Huasun has shifted to the 15-busbar layout with its new production line processing G12 half wafers. However, Risen shares its simulation results that 24-busbar would strike the right balance between the electrical and optical properties of the cell and also from a module reliability point of view (see The Status Quo Of HJT).
In terms of the printing technology, currently HJT is mainly applying dual printing that decouples the printing of fingers and busbars. This approach enables the use of different pastes for fingers and busbars. it is not easy to get a good adhesion / solderability and electrical performance with single polymeric type silver paste required for fingers and busbars, according to DKEM's Vice President of Technology and Marketing Kevin Nan. Thus, it makes more sense to use separate pastes. For the fingers, the aim is to realize better aspect ratio and conductivity for performance purpose. Also fine-line printability is critical to further reduce paste consumption. When the busbar paste is separated from it, improving solderability becomes much easier. Another benefit of using a separate busbar paste is that it may have some more room to reduce the silver content.
On the equipment side, leading screen-printing equipment suppliers such as Maxwell and Applied Materials are offering printing solutions designed for HJT. Applied is using the same Tempo Presto tools platform also for HJT that not only allows using thinner wafers, but also allows processing at high speeds as that of the PERC tools. The HJT printing line, from a printing standpoint, is the same but with a few limiting factors. On the printing side, the pastes cannot be printed as fast. The main differences are in the dry and curing section.
The text is an excerpt from 3rd 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.