TOPCon is a buzzword in today’s PV manufacturing and within the TOPCon, Laser Enhanced Firing (LEF) is a key development trend. The process, while employing lasers, adds a little fraction to the CapEx, resulting in significant efficiency gain. Nearly every TOPCon manufacturer is adopting the technology. Metallization pastes are the key enabler for this new metallization technology. TaiyangNews spoke to Weili SHI, CEO Of leading silver paste supplier DKEM, about the details of this process in addition to the metallization solutions from the company for another hot topic of the industry – tandem structures.
General
TaiyangNews: DKEM has now emerged as a leading paste supplier in the PV industry. How has the journey been so far?
Weili Shi: DKEM’s journey began in 2010. At that time, the PV terminal markets, materials and equipment suppliers were all overseas. The silver paste market was also dominated by overseas manufacturers such as DuPont, Heraeus, Samsung, and Giga. Our journey, first of all, witnessed the rapid development of the global PV industry, especially the rise of the Chinese PV market. Secondly, it witnessed the rise of China’s local PV materials and equipment supply chain and its great contribution to the growth of the global PV industry. Along the way, DKEM has also been fortunate to become the main promoter of the localization of silver paste and then the global leader in photovoltaic metallization pastes. Now DKEM is the world’s largest supplier of n-TOPCon silver pastes.
Looking back on the past 14 years, PV cell technology has rapidly evolved from p-BSF to p-PERC cells, and now to n-TOPCon cells, accompanied by the transition from multi-crystalline to monocrystalline, large-size wafer adoption, and many cell process innovations. Silver paste has not only become an enabler of the cell technology revolution but also an enabler of cell process innovation. The most profound keywords we have learned in the journey include market-oriented R&D strategy, rapid product iteration, and rapid service response. These are the key factors that have enabled DKEM to maintain and strengthen its market-leading position through different cell technology cycles.
TaiyangNews: What are the most important milestones for DKEM?
Weili Shi: As a silver paste supplier, our milestones are closely related to the development of the PV market and cell technologies. Firstly, since 2016, DKEM has started and led the localization of silver paste, making important contributions to improving cell efficiency and reducing costs in the PV industry. Secondly, in the era of p-PERC cells, DKEM was the first to launch specific silver paste products adapted to double-sided AlOx PERC cells, directly or indirectly accelerating the large-scale mass production of p-PERC cells equipped with local machines. Thirdly, in 2020, the company became the first PV silver paste company in China to be successfully listed, which increased the capital market’s recognition and attention to key PV materials. The latest milestones also include a full set of n-TOPCon metallization paste solutions based on our strong technological innovation capabilities, which have led to the rapid industrialization and technological iteration of n-TOPCon cell technology, greatly improving the affordability and inclusiveness of solar power.
TaiyangNews: The solar manufacturing landscape has transformed significantly over the last decade, with 3 technologies now in mass production. What has been the role of metallization pastes in this transformation?
Weili SHI: Silver paste, as a PV material, has a unique charm. We think the industry has yet to realize its importance fully. It is one of the few—if not the only—materials related to, or mutually influenced from, the upstream silicon materials to the downstream solar modules. Metallization through screen printing paste and firing is the last process in solar cell manufacturing and plays the gatekeeper’s role. It needs to be able to cover for and solve the process deviations and uncertainties accumulated in the previous dozen process steps, and finally put out high-efficiency and high-quality solar cells. In other words, whether it is the structural cell technology innovation from p-PERC to n-TOPCon and n-HJT, or cell process innovation such as texturing, diffusion, and passivation, it must be realized through silver paste metallization to be converted into real efficiency and cost benefits. All this means that metallization paste plays the role of the industrialization gatekeeper in the abovementioned technological transformations. Without the innovation and support of metallization paste, the above cell-level innovations cannot really achieve large-scale industrialization.
TaiyangNews: Coming to TOPCon, why is Laser Enhanced Firing (LEF) considered a game-changer in TOPCon?
Weili Shi: The traditional firing process requires adding a certain amount of aluminum powder to the TOPCon p+ emitter silver paste to improve the ohmic contact by forming silver-aluminum spikes, but it will cause severe metallization-induced recombination, which becomes an important bottleneck in improving Voc and efficiency for TOPCon solar cells. After the introduction of the laser enhanced firing (LEF) process, the ohmic contact model of the p+ emitter has changed dramatically. We can significantly reduce the aluminum powder content in the silver paste or even remove it. Not only can we achieve better ohmic contact but, more importantly, it also significantly reduces metallization-induced recombination and opens more windows for cell process optimization. The LEF process combined with special silver paste not only greatly improves the efficiency of TOPCon cells, but also significantly improves the cell yield and module reliability. It triggered a series of process optimization in TOPCon cells and modules, and related materials and equipment.
TaiyangNews: What is the core of the LEF paste?
Weili Shi: In addition to the changes in aluminum powder mentioned above, the most important thing for LEF paste is the change in the design concept of glass frits. In the past, glass frits were needed to balance the seesaw of metallization-induced recombination and ohmic contact while etching the passivation layer heavily and over large areas during firing. With the LEF process, the glass frits just need to moderately and more locally etch the passivation layer to build the basis for subsequent LEF process to achieve high-quality ohmic contact, which greatly reduces the metallization-induced recombination caused by over-etching in the traditional process. Of course, with the changes in glass frits and formula design, silver powder and organic vehicle have also been modified accordingly to achieve better fine-line printability and efficiency.
TaiyangNews: What are the typical finger widths and silver paste laydowns in the LEF process?
Weili Shi: The typical fired width for most customers is about 20 µm. Some customers are aggressively promoting a 10~11 µm finger-opening screen, so the fired width can go to 18 µm. Laydown is highly related to wafer size and pattern design. For 182 x 182, the total laydown is 90 to 100 mg, some aggressive customers can even go to an 85 mg level.
TaiyangNews: What impact does the LEF process have on the silver reduction roadmap?
Weili Shi: There are usually 3 major constraints to the laydown reduction of silver paste: contact window, line resistance, and fine-line printing. With the LEF process, there is a greater opportunity to reduce contact resistance and improve EL yield on both sides, which reduces the sensitivity of cell efficiency and yield under lower laydown. For the p+ emitter, compared with the previous silver aluminum paste, the volume resistivity and printability of LEF paste have been significantly improved due to the reduction in the amount of aluminum powder and glass frits. Therefore, cell manufacturers have further room to reduce laydown through fine-line printing. However, LEF has now become the standard for the production of TOPCon cells; line resistance will help further reduce laydown on both sides.
TaiyangNews: Does the LEF process alter the paste composition, such as doping with aluminum?
Weili Shi: Yes, the content of aluminum powder is greatly reduced or even totally removed. Especially, the glass frits system was restructured and redesigned. The organic vehicle was also modified accordingly.
TaiyangNews: How does LEF influence rear metallization?
Weili Shi: Even though the LEF process was only treated on the front side, laser-induced carrier can also migrate to the rear side, under the bias voltage applied. The rear side metallization also sees some effect, which can improve contact resistance/window to some extent on the rear side. At present, more academic and industrial research is still needed to better understand the impact mechanism of LEF on the rear side n-Poly metallization.
TaiyangNews: Can LEF also be implemented on the rear side, and are there any benefits to doing so?
Weili Shi: Actually, with the standard LEF process where only the front side is laser-treated, there is also some effect on the rear side, which can improve contact and EL yield to some extent on the rear side. However, the SiOx/n-Poly passivation contact structure has already formed very low metallization-induced recombination on the rear side. The opportunity to further increase Voc on the rear side using LEF becomes very limited. But through appropriate paste design, there is a larger window on the rear side to achieve much thinner n-Poly thickness and lower laydown.
TaiyangNews: What are the implications of LEF on other cell processing steps?
Weili Shi: One of the typical implications on other cell processing steps is to redesign the p+ emitter. Because the contact window with LEF is much wider and less sensitive to junction depth than before, there is a great chance to design a much higher Rsheet and shallower junction, even to remove the SE process. There are also chances to optimize the texturing and n-Poly deposition processes on both sides.
TaiyangNews: What are the minimum, peak, and average efficiency benefits of LEF?
Weili Shi: An efficiency benefit of 0.2~0.5% has been successfully achieved in the first stage of LEF adoption.
TaiyangNews: How are these benefits reflected at the module level?
Weili Shi: On the one hand, the cell efficiency benefit boosted by LEF is very obvious, which has significantly improved the power of TOPCon solar modules. At the same time, the LEF benefit is mainly on Voc and FF, which is more friendly to the CTM of modules. On the other hand, it is very important to note that the LEF process, combined with special silver paste, significantly improves the reliability of TOPCon modules compared to the traditional firing combined with silver-aluminum paste in the past, paving the way for the full-scenario application of TOPCon single-glass and double-glass modules.
TaiyangNews: Do you see the potential for further improvement in LEF technology?
Weili Shi: Now it’s just the beginning of LEF technology. There is still great potential to further optimize emitter, screen design and paste formulation for higher efficiency and lower laydown, with given LEF settings. Our view is that we must realize that LEF is not a simple post-processing equipment like other processing approaches of the past. It is actually an important part of metallization. Just like we often optimize the firing temperature in production, the laser power and bias of LEF also need to be revisited and optimized from time to time as the front-end process changes. However, most customers are not aware of this.
TaiyangNews: Can you explain DKEM’s product offerings specific to LEF technology?
Weili Shi: DKEM is one of the earliest silver paste suppliers to promote LEF technology and special silver paste in the market. We have a set of silver paste solutions with leading performance for the TOPCon LEF process.
DK72E front side finger paste: Innovative glass chemistry achieves very low metallization-induced recombination and robust contact performance on high Rsheet SE and non-SE processes. Specially developed organic systems and silver powder combinations enable fine-line printing at ~11 µm screen opening. In addition, it also has excellent production yield and module reliability.
DK95E rear side finger paste: Innovative application strategy of LEF effect significantly broadens the contact window and improves cell efficiency. Controllable glass etching provides leading compatibility and performance with double-sided AlOx and thin n-Poly processes. Robust low solid content or low laydown strategies help customers reduce metallization costs.
DK82E front/rear side busbar paste: Specially upgraded floating glass frits and silver powder packages widen the soldering window and increase adhesion. Meanwhile, it can further boost cell efficiency and significantly improve EL yield in mass production.
TaiyangNews: Are there any challenges associated with LEF?
Weili Shi: One challenge is when a customer aggressively pursues higher and higher Rsheet, the extremely lower surface doping concentration and shallower junction will continuously challenge the contact and recombination limit of silver paste. This requires systematic optimization of emitter, paste, firing and laser treatment to generate more stable and robust contact structures.
Another is that after the industry realized the improved module reliability by LEF, some began to use less reliable encapsulating materials or packages in modules to reduce BOM costs, which might introduce new reliability risks, especially under lower laydown. This requires silver paste to sacrifice the opportunity to improve efficiency to compensate for the reliability risks from less reliable encapsulation on the module side. Although the silver paste formulation has a certain space to modify, we still highly question and do not encourage such behavior. Module encapsulation must assume the responsibility of protection.
TaiyangNews: What is the current status of intellectual property related to the LEF process?
Weili Shi: As a silver paste vendor, DKEM doesn’t have such issues and concerns.
TaiyangNews: What are the next generations of TOPCon that the industry is evaluating, and what does this mean for pastes?
Weili Shi: Based on passivating contact, one upgrade is bifacial poly TOPCon, the other is TBC. Both need quite different silver pastes. The key is how to handle the contact and recombination with n-Poly and p-Poly layers deposited on either textured or polished wafer surface, especially how to get a breakthrough to contact p-Poly layer, which has different doping characteristics from n-Poly and is much thinner than the p+ emitter. We also need to study how to better apply LEF technology on the 2 upgraded cell structures. DKEM already has leading silver paste packages for both bifacial poly TOPCon and TBC technologies, closely cooperating with the top players at the pilot line and mass production level.
TaiyangNews: Are there any innovations in busbar pastes?
Weili Shi: LEF process also has an impact on busbar adhesion or efficiency and EL performance. We are offering a new busbar product, DK82E, for the TOPCon LEF process, with good adhesion and efficiency, and also help cell makers solve the dark EL issue around busbar positions. For TBC cells, we are offering DK82B busbar paste, which can further improve cell efficiency, obviously with a special floating design.
TaiyangNews: Zero-busbar is gaining traction; do you offer any specific solutions for this type of interconnection?
Weili Shi: There are several 0BB designs in practice, with different requirements for busbar and finger pastes. DKEM has already developed specific 0BB busbar/finger packages with good compatibility to standard or lower temperature interconnections for different 0BB designs, and are already in mass production now.
Perovskites and Tandem Structures:
TaiyangNews: The PV industry is exploring next-generation perovskite-based tandem structures. Are you seeing an increased demand for such pastes?
Weili Shi: We are seeing increasingly clear technical requirements, such as ultra-low curing temperatures, and to avoid the interactions between organic systems and TCO layer/perovskite materials. So far, perovskite tandem cells are still mostly in the early stages of research and development, and more use of evaporation metallization for prototype device development. But as a leading paste supplier, we have begun to develop specific silver pastes to meet this new technical challenge and market opportunity.
TaiyangNews: Which concept—cell level or module level tandem—do you think will reach the commercial market first, and why?
Weili Shi: Too early to comment.
TaiyangNews: What are the specific requirements for pastes in these advanced structures?
Weili Shi: Considering the thermal degradation request of perovskite materials, all such pastes must first have an ultra-low curing temperature below 130°C. Secondly, they have to form good contact resistance on TCO surfaces with different texture levels. Thirdly, the organic ingredients in silver pastes must avoid interacting with the TCO layer or perovskite materials to ensure the stability of the structural and functional materials in perovskite tandem cells.
TaiyangNews: How do paste compositions for perovskites differ?
Weili Shi: For the perovskite tandem application, the silver paste needs to use more active metal powders and organic ingredients to achieve good curing performance below 130°C in terms of acceptable low volume/contact resistivity.
TaiyangNews: Between thermosetting and thermoplastic pastes, which do you find more effective, and why?
Weili Shi: Too early to comment.
TaiyangNews: Could you provide details about DKEM’s paste offerings for perovskite-based solar cell structures?
Weili Shi: We are continuously developing and improving our silver paste system for perovskite tandem cells under the commercial name of DK65 by using much smaller metal powders and more active organic ingredients to achieve good volume and contact resistivity below 130°C as the first step, followed by printability optimization to make it formally commercial-ready. Therefore, DK65 is continuously improving and evolving with the development of perovskite tandem cell technology.
TaiyangNews: What are the key areas of improvement for perovskite pastes?
Weili Shi: Metal powders and chemistry system.
TaiyangNews: Finally, the metallization section, particularly the paste wing of the value chain, has consistently surprised the industry by pushing boundaries. However, everything has its limits. What is your outlook on the future role of pastes in the metallization process?
Weili Shi: As mentioned before, metallization paste is the gatekeeper of efficiency and yield in solar cell production. In addition to continuously improving efficiency with different cell technologies, pastes will also be forced to help improve module reliability in the future since more and more aggressive low-cost encapsulation strategies come out. More importantly, as global installed capacity continues to grow and silver prices remain high, paste metallization needs to find a more cost-competitive solution for the future.
TaiyangNews: Thank you for the interview.
