LEF Is Key To TOPCon

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 end 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 domestic 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 at 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 innovations in cell process technologies. Silver paste has not only become an enabler of 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 PV market and cell technologies. First, since 2016, DKEM has started and led the localization of silver paste, making important contributions to improving cells efficiency and reducing costs for the PV industry. Second, in the era of p-PERC cells, DKEM was the first to launch specific silver paste products tailored to double-sided AlOx PERC cells, directly or indirectly accelerated the large-scale mass production of p-PERC cells equipped with domestic processing equipment. Third, 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 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 is a type of PV materials with unique charm. We think that its importance has not been fully recognized by the industry. It is one of the few or only materials that are related or mutually influenced from the upstream silicon materials to the downstream solar modules. Metallization through screen printing paste and firing is the last step in solar cell manufacturing and plays the role of a gatekeeper.   The metallization step needs to be able to cover and solve the process deviations and uncertainties accumulated in the previous dozen process steps, and finally output high-efficiency and high-quality solar cells. In other words, whether it is cell technology innovation from p-PERC to n-TOPCon and n-HJT, or cell process innovation such as texturing, diffusion, and passivation, all of these innovations must be realized through silver paste metallization to be converted into real efficiency and cost benefits. That means, metallization paste plays the role of gatekeeper in the industrialization of above-mentioned technological transformations. Without the innovation and support of metallization paste, above cell-level innovations cannot really achieve large-scale industrialization.

TOPCon

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 causes severe metallization-induced recombination, which becomes an important bottleneck in improving Voc and efficiency for TOPCon solar cells. After the introduction of laser enhanced firing (LEF) process, the ohmic contact model of p+ emitter has changed dramatically. We can significantly reduce the aluminum powder content in the silver paste or even remove it completely. Not only can we achieve better ohmic contact, but more importantly, 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, and 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 the glass frits. In the past, glass frits needed to balance the seesaw of metallization-induced recombination and ohmic contact while etching the passivation layer heavily 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 the subsequent LEF process to achieve high-quality ohmic contact, which greatly reduces the metallization-induced recombination caused by over-etching as in the traditional process. Of course, with the changes in glass frits 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 front-side fired width for most customers is about 20 mm. Some customers are aggressively promoting 10~11um finger-opening screen, so the fired width can go to 18um. Laydown is highly related to wafer size and pattern design. For 182 X182 mm, the total laydown including both the front and rear side is 90 to 100 mg, and some aggressive customers can reach 85 mg level.

TaiyangNews: What impact does the LEF process have on the silver reduction roadmap?

Weili Shi: There are usually three major constraints to 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 more room to further reduce laydown through fine line printing. As LEF has now become the standard for TOPCon cells production, line resistance will impact more on further laydown reduction 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, glass frits system was redesigned. The organic vehicle was also modified accordingly.

TaiyangNews: How does LEF influence rear metallization?

Weili Shi: Even though the LEF process is only treated on the front side, but laser-induced carrier can also migrate to the rear side, under the applied bias voltage. Therefore, some effect also happens at the rear side metallization, which can improve the 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 and 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 standard LEF process where only the front side is treated, there is also some effect on the rear side, which can improve rear side contact and EL yield to some extent. 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 great chance to design much higher Rsheet and shallower junction, even to remove the SE process. There are also opportunities 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: 0.2~0.5% efficiency benefit has been successfully achieved in the first stage of LEF adoption.

TaiyangNews: How are these benefits reflected at the module level?

Weili Shi: On 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 as 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 the 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 in the past. It is actually an important part of the metallization process. 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 yet.

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 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 mm 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 cells 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 to help customers reduce metallization cost.

DK82E front/rear side busbar paste: specially upgraded floating glass frits and silver powder packages widens the soldering window and increases adhesion. Meanwhile, it can further boost cells efficiency and significantly improve EL yield in mass production.

TaiyangNews: Are there any challenges associated with LEF?

Weili Shi: One challenge is when customer aggressively goes to 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 at 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 issue and concern.

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 bi-facial poly TOPCon, the other is TBC. Both needs 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 breakthrough to contact p-Poly layer, which has different doping characteristics from n-Poly and much thinner thickness than p+ emitter. We also need to study how to better apply LEF technology on the two upgraded cell structures. DKEM already has very leading silver paste packages for both bi-facial poly TOPCon and TBC technologies, closely cooperating with the top players at pilot line and mass production level.

TaiyangNews: Are there any innovations in busbar pastes?

Weili Shi: LEF process also has impact on busbar adhesion or efficiency and EL performance. We are offering a new busbar product DK82E for TOPCon LEF process, with good adhesion and efficiency, and also help cell makers solve dark EL issue around busbar positions. For TBC cells, we are offering DK82B busbar paste with special floating design, which can further improve cell efficiency significantly.

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 paste 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: First, considering the thermal stability of perovskite materials and cell structures, all such paste must first have an ultra-low curing temperature below 130oC. Second, they have to form good ohmic contact to  TCO surfaces with different texture levels. Third, the organic ingredients in the silver paste must avoid interacting with the TCO layer or the underlying 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 perovskite tandem application, silver paste needs to use more active metal powders and organic ingredients to achieve good curing performance below 130oC 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 130oC as the first step, following by printability optimization to make it formally commercial ready. Therefore, DK65 is continuously improving and evolving with the development of perovskite tandem cells 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, paste 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.