The metallization process and role of screen printing technology are being discussed in this article of our series on HJT solar technology. Discussing the latest in HJT cell processing, we briefly look at the main approaches required to reduce silver consumption and various alternative screen printing methods involved. This feature follows on an article about TCO deposition in HJT solar cell processing (see TCO Deposition In HJT).
Metallization has attracted utmost attention in terms of developments among all the HJT process stations. That's understandable, as one of the major bottlenecks for HJT currently is its much higher cost, both in terms of CapEx as well as OpEx. Also, metallization paste governs the major chunk of processing costs for HJT. The process for HJT is very different to the mainstream. While the method of metallization is still screen-printing, the paste systems used for other cell architectures are typically fired at temperatures above 700 °C. In case of HJT, the amorphous thin films employed are hydrogenated. Given hydrogen's tendency to escape at higher temperatures, the paste system must work at low temperatures of about 200 °C to avoid poor passivation. This calls for the use of special silver pastes that can be cured at low temperatures.
Screen-printing 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 consistent. Kaien Chang, vice president of technology at Solamet presented the company's low temperature paste solutions for HJT at TaiyangNews conference. Solamet is a Chinese paste supplier named after the former metallization paste brand of DuPont that acquired all metallization assets from DuPont, including intellectual property and workforce. In his talk, Novel Low Temperature Paste Development for HJT and Perovskite Tandem Cells (see presentation here).
Solamet's latest paste product has been designed without the typical larger flakes of silver used in epoxy pastes. This composition improves paste transfer, enabling fine line printability and ultimately resulting in low laydown. The absence of larger silver flakes also improves contact to TCO. Chang presented results of an in-house designed printability test with a single screen having multiple openings ranging from 10 to 20 μm, which showed that at a printing speed of 350 mm/s, the PV42B matched the printability of PERC paste with no disruptions to line quality. The paste also supports low laydown. Solamet also offers another low temperature paste, PV43B, for perovskites tandem cells, which can be processed at even lower temperatures down to 120°C.
When it comes to printing technology, dual printing is the state of the art as it favors low paste laydown. Increasing the number of busbars has proved to be an effective means of reducing paste consumption. While the 12-busbar layout has already been implemented, a few products based on 15 busbars are available, and a few are planning to go beyond 18 or even 24. Silver consumption has been significantly reduced in the industry, and more so in 2022, from 180 mg per M6 wafer to 120 mg. This has been achieved by using SMBB (super MBB, which is nothing but more busbars) and a metal screen, according to Huasun's R&D Director of Solar Cells Su Zhou. Forschungszentrum Jülich's Head of R&D Department of Research Center Dr. Kaining Ding puts it at 18 mg/W for G12 wafers with dual printing and more busbars. Huasun's silver consumption was at 14.6 mg/W at the end of 2022 and is expected to decrease to 10 mg/W during 2023.
Saving on silver: At TaiyangNews' HJT event, Huasun presented the developments in metallization to reduce silver consumption. (source: Huasun)
A noteworthy development with regard to reducing silver usage is the use of paste systems consisting of copper particles that are coated with silver, instead of using pure silver suspension. This directly reduces silver consumption, and thereby costs, albeit not proportionately as the process of coating copper with silver is not cheap.
Zhou shared Huasun's experiences with silver coated copper. The company was using such pastes only on the rear side and just for the fingers, at least end of 2022, because the majority of these pastes are not fully optimized for the front side. Using silver coated copper on the rear side only for fingers, a saving of 24 mg of silver per cell has been achieved, down from 126 mg per M6 wafer to 102 mg. As to the performance, this novel paste material has led to a slight efficiency loss of 0.03% absolute, which is negligible. However, employing these pastes on the front side leads to a considerable dip in efficiency, according to Zhou. The next step for the company is to try silver coated copper paste for busbars and also for frontside fingers with paste formulations optimized for these applications. "Our target is to reach a silver consumption of 70 mg/wafer with silver coated copper," said Zhou.
Reliability is the next area of concern with silver coated copper. "Copper leakage and oxidation of the copper" are the 2 examples quoted by Ding in this regard. However, Huasun's Zhou said that his company has evaluated the reliability of silver coated copper to conclude that it would not affect the longterm stability of the module.
There are several other approaches being pursued to reduce silver consumption. Eliminating busbars is another example. Typically, 25% to 30% of the paste applied on the cells is used for busbars. For example, out of a total laydown of 126 mg of paste per an M6 cell, close to 40 mg is used for busbar formation. The busbar-less cell design first relates to the SmartWire Connection Technology (SWCT), a proprietary interconnection method that enables interconnects to be directly attached to the fingers, eliminating the need for busbars. The technology is protected with IP, and Meyer Burger has withdrawn the solution from the open market. However, MBB also supports a busbar-less cell design. Risen says that its cells that support 24 busbars are based on a no-busbar design, which comes with its own set of challenges in module making. On top, rumor has it that an indigenous technology is also under development in China, while no further details are available.
Day-2 of TaiyangNews virtual conference om High Efficiency Solar Technology focussed on HJT, for more presentations click here.
For more details on HJT, download the TaiyangNews report on Heterojunction Solar Technology 2023 for free here.