High Efficiency & High Reliability

TaiyangNews: DuPont has been a leading metallization paste supplier for many, many years – that’s a long time…

Ted Zhi: For over 40 years, DuPont innovations have
led the photovoltaics industry forward, and helped our clients transform the power of the sun into power for us
all. We are a pioneer in metallization pastes and serve the industry on a global basis. This poses unique opportunities for us not least in meeting customer requirements across all solar cell architectures and the growing regions of the world where solar cell development and production are taking place.

As a reliable innovation partner for the industry, DuPont continues to set the pace of innovation in the solar industry by continuously introducing leading performance pastes that work to deliver greater solar panel efficiency and power.

To give you one nice example: Recently, our commitment has been recognized by the American Chemistry Society. This is an annual award for the entire chemistry society. Scientists from DuPont Photovoltaic and Advanced Materials (PVAM) were awarded the American Chemical Society’s (ACS) Heroes of Chemistry Award for their invention of DuPont SolametPV17x, the game-changing metallization paste that greatly improved the energy efficiency of solar cells. DuPont scientists made advances in lead-tellurite frit chemistry and commercialized it as multigenerational Solamet metallization pastes creating a step-change in performance with increasingly higher cell efficiency and performance advantages. This is not only an honor for DuPont scientists, but also a recognition to every stakeholder in the PV industry because the solar energy developments have benefits to all humankind.

TaiyangNews: What has changed in the paste segment over time?

Ted Zhi: The move to higher efficiencies continues across the PV industry. There is still an important focus on mainstream p-type cells with LDE (Lightly Doped Emitter) that still have an important role to play for improving efficiencies and lowering cost of ownership.

P-type LDE is also benefiting from fine line and ultra
fine line technology that can also be used on different
cell technologies to boost efficiencies. But for higher efficiencies this will be PERC (Passivated Emitter Rear Cell). PERC has become one of the most important technologies over the coming years and keeping up with these developments is therefore important. PERC allows the industry to capture the higher efficiencies at lower cost and risk.

Although n-type mono-based technologies have costs significantly higher than [p-type] LDE, programs such as ‘Top Runner’ in China are pushing companies to innovate in higher cell efficiencies and provides opportunities.

In terms of market development, Asia definitely is where the growth come from. We put resources in the right places to support our customers. We expand our presence accordingly to help customers get the most out of their

cell efficiencies. This means that we develop long-
term partnerships with our customers and the long-term commitment to R&D investments serve the PV industry so that we can response to market needs simultaneously.

TaiyangNews: When people in the solar world hear the name DuPont, they may think about Tedlar. How important is the metallization paste segment for you?

Ted Zhi: As the world’s leading PV materials supplier,
we offer a portfolio of solutions that deliver proven power and lasting value over the long term. If you look at LCOE equation, higher efficiency and longer lifetime are two
key factors that enable lower LCOE to reach grid parity faster. In DuPont, Solamet metallization pastes and Tedlar backsheet materials are two pillars that advance solar industry development.

TaiyangNews: From the metallization pastes perspective, what are the major topics of interest or developments with standard Al-BSF technology?

Ben Jin: Regarding standard Al-BSF technology, DuPont has always been focusing on further optimizing fine line printing performance for our frontside metallization paste products. On the other hand, we also keep improving contact resistance for the front side emitter. These are the two major areas we are focusing in our development work in this segment.

Ted Zhi: We see that multicrystalline will probably stay much longer in Al-BSF. So, a lot of development would be around how we can work on a DWS multi base and black silicon surfaces.

TaiyangNews: That means, you don’t see multi is going to stop anytime soon?

Ted Zhi: If you look at PERC technology, multi is moving slower compared to mono. However, we believe both multi and mono have their own advantages.

Ben Jin: Multi will keep existing, especially after the technology has fully transferred into Diamond Wire Sawing (DWS) technology. It still has its merits as its lowest cost of manufacturing. Mono has better wafer quality and it
is easier to achieve higher efficiencies with introducing new processes or structures. That is why mono quickly has been retrofitted for PERC, and it could be the case
for even more advanced technologies in the future. We think both segment will last in the solar market, but it will be clearly differentiated between performance and cost perspectives.

TaiyangNews: The standard cell technology is clearly differentiating between mono and multicrystalline: On which segment is DuPont focusing – and why?

Ted Zhi: We work on both. DuPont continues to be an industry pacesetter for innovation in the solar industry by introducing leading performance metallization pastes. Take Solamet PV21x, the latest front side silver for example, this new generation front side silver is designed to enhance most mainstream cell technologies including lower [P] emitter PERC mono, Bifacial AlOx PERC mono and MCCE multi. SolametPV21x delivers better contact performance and high aspect ratios that enable cell efficiency enhancement >0.1% and maintains high throughput in mass production.

While multi is really leveraging its costs position as it adapts to DWS, we have worked a lot to study paste chemistries that can deliver efficiency gain and achieve adhesion and printing requirements of new multi cell structures.

TaiyangNews: DuPont was one of the first to launch a complete paste system for PERC. What was the rationale behind for this offer?

Ted Zhi: When DuPont launched the complete paste system for PERC back to 2014, PERC technology was still an emerging technology which is like present n-type technology – it requires scale up manufacturing equipment, technology and materials support to push PERC to be commercialized largely. As an industry innovative partners, we layout our technology roadmap and continuously develop advanced materials to enable next generation technology. PERC is one example, LDE is another. You can also see this at DuPont’s booth at this year SNEC, that we collaborate with SERIS in developing a HJT cell, as well as a commercial IBC module from Sharp.

TaiyangNews: Now peak efficiencies are going up very quickly, almost every month there is a record. What is the potential for PERC before moving on to the next cell technology?

Ben Jin: The record efficiency for PERC mono cells
is indeed almost 24%, with introducing several new technologies beyond only PERC, which cannot be realized in mass production yet. But it clearly indicates tremendous potential for p-type PERC mono cell structure to further increase the efficiency in mass production. Based on internal investigations and simulations, we believe that at least 22.5% can be achieved in mass production by further optimizing PERC technology through process of cell as well as material innovations in the next few years.

TaiyangNews: What are the key attributes that you are introducing with your new PV21x paste to make high efficiencies possible?

Ben Jin: DuPont was pioneering and always believes
in fine line printing and lightly doped emitter (LDE) to further enhance solar cell efficiency. Solamet PV21x,
the new generation front side silver, is developed based on innovation on ultra-finer line printing as well as further LDE contacting capability. Solamet PV21x delivers better contact performance and high aspect ratios that enable cell efficiency enhancement >0.1% and maintains high throughput in mass production. Simultaneously, we’ve also upgraded our inorganic formulation, which allows decent ohmic contact on even more lightly doped emitters.

TaiyangNews: When we look at high efficiencies, you already mentioned that companies are using new techniques. One is selective emitters, which did not work in the past, but now seems to be coming back strong. What does it mean for pastes when looking at the combination of selective emitters and PERC?

Ben Jin: Technology-wise selective emitters is not
new to us. Going back to the Solamet PV17x, when everybody was doing BSF structured cells, this product was successful in overcoming the bottleneck at a time
the whole industry was not able to contact lightly doped emitter. This product had similar efficiency performance benefits compared to selective emitter technology at that time. It’s true that selective emitter was not really providing extra efficiency gains in the past, especially on BSF cell structure. But that was limited by the enormous surface recombination at the rear surface and poor reflection of long wavelength photons which will significantly increase the total amount of minority carriers eventually captured by the p-n junction close to the front surface.

In fact, selective emitter has shown significant potential
to increase cell efficiency for PERC structured solarcellsin mass production with effectively reducing the recombination of those additional minority carriers close to the front surface, which generated due to the rear surface passivation and better internal reflection.

TaiyangNews: Do you think that everyone will go with selective emitters as an evolutionary step in PERC?

Ben Jin: Yes, a lot of key players have already started or are seriously evaluating the technology to implement it. We are working closely with our key customers to demonstrate the efficiency gain.

TaiyangNews: Does this development make your job easier? Or are there any new challenges?

Ben Jin: There were different methods for selective emitters. Currently, the majority of PV industry is relying on laser annealing on PSG film as precursor to adopt selective emitters in mass production, which does differentiate the front side silver paste from aconventional product. The emitter is lightly doped before laser processing. DuPont is further improving our front side silver for this specific application through innovation as well as customer cooperation.

TaiyangNews: What are the advanced cell technologies you are focusing on beyond PERC? And why are you focusing on these ones, and not others?

Ben Jin: We have been continuously putting our technical efforts as well as development resources in every new and existing technology beyond PERC for our pastes product development.

TaiyangNews: Performance is key, but costs are also crucial for cell producers when selecting a paste product as pastes are one of the most expensive process consumables in cell processing lines. What are the most important ways to further reduce metallization cost – in particular with regard to the paste supply side?

Ben Jin: Whenever we talk about cost, we should not only look at one single material, but at COO or even further – the overall LCOE. The contribution of pastes
to efficiency enhancements is definitely helping each certain cell or module technology in lowering LCOE. The industry is benefited from continuously paste consumption reduction, which is surely contributed by the development in paste chemistry. On the other hand, cell technology developments, like 5 or 6 busbars or even multi busbars, also helps to reduce paste consumption on each cell due to fewer requirements in grid line resistance.

TaiyangNews: In advanced metallization there is double and dual printing, and standard fineline printing? Which one you think will play the major role in the coming 5 years – and how do you support that with your products?

Ben Jin: No matter it’s single, dual or double printing
– there is one mutual target to deliver fine line printing capabilities. This will always be the path to be followed as it will be helpful to increase efficiency and further reducing cost of manufacturing.

It is difficult to predict exactly which technology will be winning in the next 5 years as it will depend on the very dynamic market development. However, we do have to take into account two important aspects. First, what will be the main cell technologies in the future which will behighly related to a specific screen or printing techniques that will be more suitable. The second is what will be the future module technology, to be more specific how many busbars will be used. This is also highly influencing the suitable printing techniques or structures that are chosen.

The majority of PV industry is now doing 5 busbars and we are already seeing some differentiation in the technologies chosen for screen printing. For example, in case of mono PERC, we think double printing would dominate, as it helps cell makers to improve EL pass yield and helps in utilizing its potential to attain higher efficiencies. In case of DWS multi cells, manufactures who are more sensitive to manufacturing cost more likely choosesingle printing, while some are using dual printing to meet the specific technical needs from the demand side.

TaiyangNews: There are also advanced screen concepts – knotless screens, high mesh screens and stencil screens. Do you have any preference – and again, how do you support that development with your products?

Ben Jin: We don’t have any preference on specific screen concepts or meshes used in screen printing. We are supporting development for each of these depending on our customers’ needs. And if there is a special request, we can help with our customized paste products as we have very strong R&D capabilities.

TaiyangNews: Do you see any trends, or even a best combination of advanced screens and advanced metallization concepts?

Ted Zhi: I would say it’s more about the combination of efficiency benefits and manufacturing readiness with a reasonable cost of ownership. For example, there was a lot of discussion that dual print might replace double print. There are customers that have been trying the one and there are cell makers coming back to double print. This actually says a lot about the dynamic of these advanced screen and metallization technologies. Then there are knotless and high mesh screens, which help single print and dual print to improve efficiency to achieve better aspect ratio and line control. On the other hand, this brings challenges in yield. There could be also more defects in the cell if the combination of screen, paste and printers does not work out perfectly. The PV industry is still trying to improve every aspect.

TaiyangNews: When we look at details of the metallization process, how low do you think can we go for finger screen opening, finger width reduction and paste laydown in the next 5 years?

Ben Jin: In terms of making screens, it is possible to go tovery narrow openings, but it is not easy to keep acceptable production yield in mass production. We can’t see the exact limit, but when the screen is ready and mature, we can develop the corresponding paste products.

TaiyangNews: There was a time when silver prices were very high and people started discussing the era beyond screen printing – namely copper plating. Copper plating is nowhere, while silver pastes and screen printing continues to rule the market. Do you think this will go on forever?

Ben Jin: Screen printing technology was invented in China and has existed for over thousands of years. It is
so robust and flexible to be optimized. It also supports a variety of printing media, including pastes. We still believe that screen printing has many years ahead, at least for
the solar industry, as cost reduction and increasing cell efficiencies is still possible in the near future. It is definitely not yet touching the rooftop. On the other hand, we hear from customers that they are looking at new metallization technologies, but not in the short term. Either way, we are open to collaborate with them in developing paste products for all kinds of technologies.

TaiyangNews: What are the aspects printer and screen suppliers should focus on to improve their tools so that the optimal results can be produced using top quality pastes?

Ted Zhi: Quality consistency and reliability of printers and screen suppliers would be helpful to any material suppliers they are working with. Furthermore, open collaboration within materials suppliers, printers, and screens will help to advance metallization performance.

TaiyangNews: There is continued competition from the other non-Chinese incumbents. But like in any other solar segment, in the pastes business we see increasing competition from local Chinese suppliers: What’s your strategy to stay in the top group of paste suppliers?

Ted Zhi: We call ourselves a science company and if you look at DuPont versus any other suppliers, we are more devoted to basic research and application development.
I remember that one of our customers commented that only DuPont will be looking at future game changing technologies. So, we need to focus on both future game changing technologies and also in the near-term focus on solving today’s customers’problems. Being a global company thatserves in the PV industry, we are able to understand customer requirements from the value chain and every part of the world.

With completion of the merger between Dow and DuPont, DuPont Photovoltaic Solutions is proud to broaden our PV offerings by integrating the Dow Corning portfolio of solar silicone solutions including sealants, adhesives, potting agents, encapsulants and electrically conductive adhesives. These state-of-the-art solar silicone solutions have enabled reliable solar systems with an increase in durability and efficiency. We are proud to introduce
at SNEC an expanded portfolio of products including a new generation of Solamet metallization pastes, new Tedlar backsheet materials as well as the addition of Dow Corning PV silicone products to our PV offerings.

TaiyangNews: Thank you for the interview.

The interview was originally published in the TaiyangNews PERC Solar Cell Technology Report 2018, which can be downloaded for free here.