Technically, a silicon wafer is a solar cell once a p-n junction is formed, but it becomes functional only after the completion of metallization. The process of metallization involves depositing the metallic contacts onto both sides of the wafer and sintering them into silicon, enabling them to collect the generated charge carriers. The step also lays the platform for a series connection of cells in module-making through busbars. Metallization is one of the most important processing steps in cell manufacturing that not only governs the cell's output power but also the manufacturing costs. Developments in metallization are a collective effort from 3 different streams – metallization pastes, screen printers, and screens.
Screen printers have been the workhorses for the metallization process since the beginning of PV development, and screen printing remains the state of the art to this day. In a typical cell manufacturing line, screen printers are integrated with inter-tool transportation systems, driers, buffers, various inspection systems, firing furnaces, IV testers, and sorters. This completely integrated setup is often referred to as the backend line. All the 4 companies that are part of this survey – ASYS, Maxwell, Baccini Cell Systems (a division of Applied Materials (AMAT)), and S.C New Energy – have integrated printing lines and backend lines on offer. However, only ASYS offered to provide some insights.
The points of focus in the screen printers' section in general are high throughput, increasing production capacity per line, and reducing silver paste consumption. Silver consumption can be reduced by narrowing the screen opening that facilitates the application of thin contacts. The screen, an important and integral part of screen printers, plays a vital role here. Screens can be optimized to tweak important parameters of the printing process, for example, the thickness of the emulsion and mesh. Several approaches have been evaluated in this context.
The metallization side of PV saw several screen technologies being introduced. Standard mesh screens were the state of the art from the beginning, while stencils were considered to be effective alternatives to traditional mesh screens back then. However, stencils never made it to the mainstream.
Then, knotless screens were much talked about during 2017; they were very popular in 2019 and 2020. Also, in 2019, polyimide (PI) based screens were introduced. We reported in our previous survey that all these approaches were short-lived with the industry reverting to standard screens with 11 μm wire, and the 430 mesh screen was the mainstream in 2022. However, Sebastian Tepner, product manager at ASYS, has noticed the technology progress in different time frames. Knotless screens with 440 mesh and 13 μm were the most used during 2019-2020, while it has been the 520 mesh and 11 μm wire over the past 1.5 years, which is based on PI instead of emulsion. These screens support laser opening, which allows the openings to be narrower. Taiwan's Brave C&H Supply Co. Ltd. was the inventor of this screen. The details of a PI screen were discussed in TaiyangNews Market Survey on Metallization Pastes 2019/2020. These screens, according to Tepner, support finger openings of 20 or 21 μm, which is the current state of the art for PERC, while the openings at different cell makers may vary by +/- 2 to 3 μm. With these narrow screen openings, cell makers are able to reduce the finger widths down to a range of 23 to 25 μm, while a few have even achieved 20 μm, according to Tepner. As for TOPCon, according to Keven Nan, VP Technology and Marketing at DKEM, the mainstream finger opening for TOPCon is less than equal to 15 or 20 μm, leading to fired finger width of 25 μm or 30 to 35 μm, respectively.
In addition to supporting narrow finger openings, the PI screen has twice the durability of an emulsion based one, almost halving the screen costs. This shift from emulsion to PI-based screens gave a big boost to screen printing technology, which a few years ago was thought to have reached its optimization limit, according to Tepner. As a result, screen printing once again raised the bar for all other alternative metallization approaches. "Even though there are promising upcoming technologies it would be very difficult for any technology to compete with screen printing for the time being." says Tepner.
This bright side of the story is only applicable to PERC, because the technology comes with an undeniable advantage of using silver on only one side. TOPCon is a little different in that it has silver on both sides, and so the silver consumption is simply double. Not just the costs, but supply constraints could also be a factor, according to Tepner. "It won't be an issue as long as TOPCon production remains at its current scale of 100 GW. If TOPCon is to achieve PERC's level, then there simply is not enough silver, which is why the industry is evaluating alternative approaches," says Tepner. He lists a few, one of which is to replace the silver with aluminum partially or completely, at least on one side. Approaches using copper are also being evaluated, where a silver layer is printed first, and the copper is applied on top. A few startups are also working on high-temperature copper pastes suitable for TOPCon metallization, according to Tepner. However, these approaches are all still in the R&D phase.
TaiyangNews Talked To Weili SHI, CEO of Leading Silver Paste Supplier DKEM On Key Developments In Metallization Solutions For TOPCon Solar Cells (see Metallization For TOPCon)
The article is an excerpt from the TaiyangNews Market Survey on Solar Cell Production Equipment 2023, which also includes detailed product specs and can be downloaded for free here.