Process Equipment For High Efficiency TOPCon Solar Cells

Leadmicro offers production equipment solutions for mass production of TOPCon solar cells, while Tube-type PECVD attracts the center stage. Tube-type PECVD is currently one of the two main approaches used for applying the rear passivation stack in TOPCon processing. This technology enables all the necessary steps to be accomplished in a single process, including tunneling oxide application, polysilicon deposition, and in-situ doping. Leadmicro, a company that not only supplies PECVD tools for TOPCon applications but also has the capability to construct other deposition and thermal processing equipment, has successfully sold turnkey projects in China by partnering with experts in areas where it lacks expertise. TaiyangNews had the opportunity of speaking with Baochen Liao, Chief Scientist at Leadmicro, to learn more about the various production equipment platforms that the company is promoting for TOPCon.

TaiyangNews: Leadmicro just got into the PERC business when there was no effective alternate to inline PECVD for aluminum oxide, and then took the market by storm. You were also involved with HJT, but were quick to move to TOPCon, as the market did. I should say, you have learned the art of looking into the crystal ball. What was it that made you bet on TOPCon?

Baochen Liao: The efficiency and Cost. Firstly, from the efficiency point of view, the efficiency potential for TOPCon is as high as HJT. According to Richter's work, the efficiency potential for TOPCon and HJT are 28.7% and 27.5%, respectively. Secondly, from the cost point of view, the Photovoltaic industry is very sensitive to the cost. The GW level investment for HJT is still far higher than TOPCon, at least two times higher at this moment. Thirdly, from the technology maturity point of view, the TOPCon technology is compatible with the existing mainstream PERC cell, just need the breakthrough for several additional processes, namely, Boron diffusion, tunnel oxide and poly-Si. However, HJT is a low-temperature process, completely different from the current mainstream cell technology. Hence, it needs breakthroughs in the whole supply chain, which is much more challenging.

TaiyangNews: What would you regard as your most important innovations?

Baochen Liao: I will leave out PERC from this, though we played a key role in the transition to PERC. PERC is anyway history now, at least in China. Let me give you a summary of our latest innovations and let me also start with core of the TOPCon, which is nothing but the tunneling oxide and polysilicon applications. Our first achievement was that we have resolved the equipment and process bottlenecks with a tube-type PECVD based solution for TOPCon. You may be aware that we have completed the proof-of-concept pilot line with PECVD/ PEALD based TOPCon technology with 24.65% cell efficiency in 2021. Then in 2022, we made quite a bit of progress. We have introduced our 3rd generation ZR5000 PECVD/PEALD system. We actually have transferred the technology from pilot line to mass production; resolved the critical yield issues for mass production. We have also delivered 2 GW turnkey TOPCon mass production projects with greater than 25% efficiency and above 97% yield. In parallel, we have also resolved the efficiency bottleneck of tube-type ALD for aluminum oxide deposition, especially for TOPCon, but the system is increasingly finding an application in XBC solar cells due to its unique capability of double-side coating. It is becoming mainstream technology for high-efficiency solar cells. Again, in 2022, we brought out our latest 6th generation KF18000S. The market has also received it well due to the high conformality of the deposited films and very high throughput. This system alone can support a GW production line with a throughput of 18,000 wafers per hour.

TaiyangNews: It is interesting to see that "turnkey" still has relevance, especially in China, especially in these days; can you talk about some of the technicalities of your turkey project?

Baochen Liao: The core of our technology is the PECVD enabled TOPCon process. It is public knowledge and we have also presented at several events including at your TaiyangNews equipment conference that we finished the delivery of 2 turnkey projects, each with 1 GW for mass production. The guaranteed mass production efficiency is 24.5%, and we exceeded this level by reaching 25%. These 2 projects have one difference though. One of them is completely based on PECVD, i.e., both the tunneling oxide and poly are deposited with tube-type PECVD, whereas the other one uses PEALD for depositing tunneling oxide and PECVD for the poly process. The advantage with the PEALD based process is that it is the only technology that can precisely control the thickness of the tunneling oxide in atomic scale.

TaiyangNews: Which technology is better? Or which one is your favorite?

Baochen Liao: We don't have a priority. We have developed both the technologies and have them on offer. Both the technologies are running in production.

TaiyangNews: Is there any difference in throughput between these 2 technologies?

Baochen Liao: There is no difference. The PEALD process is completed during the temperature ramp up, thus it is a little faster, but the difference is 1 or 2 minutes over a cycle time of 35 minutes. So, it is almost the same, and I will not credit PEALD on the account of throughput. Performance-wise, according to our own data, PEALD has an efficiency gain of 0.05%, which is not attractive enough at this point to compensate for the additional costs for the PEALD technology. Costs weigh a lot in today's mass production, right? Since we have a cheaper alternative with PECVD for silicon oxide, cell makers are not willing to invest into the technology with additional costs. We respect the market choice, but we at least proved and demonstrated that the technology actually works and provided a future upgrade option for the market when 0.05% efficiency gain matters over the cost.

TaiyangNews: What are the additional costs involved with the PEALD process?

Baochen Liao: The main cost for PEALD technology is that in order to grow the silicon oxide in PEALD, the process requires a silicon precursor, which is currently way too expensive. It is similar to TMA during the inception days of PERC. In contrast, PECVD just requires N2O, as silicon coming from the wafer surface, as just 2 nm of tunneling oxide is sufficient.
TaiyangNews: What is the market response to the PEALD based technology?

Baochen Liao: As I just mentioned, costs are crucial and governing factors for making a choice in technologies, especially for mass production, while it's different with R&D or pilot. TOPCon is now in mass production. Due to the cost reason, tube-type PECVD method is the mainstream chosen by the market currently.

TaiyangNews: Since Leadmicro is also focusing on mass production, what is the reason for developing a technology that has less relevance in mass production?

Baochen Liao: When you opt for our TOPCon solution, our PEALD route is the only production method that can control the ultrathin tunnel oxide (< 3 nm) precisely at atomic level, it fills a blank in this area. As a result, it also opens the option of future upgrade, especially when this 0.05% efficiency also matters. When a cell maker is willing to adopt the technology, it would be an easy upgrade both at the hardware and process levels. We have everything ready, in fact, future ready.

TaiyangNews: Stepping back to turnkey solutions, the turnkey projects, was a one-time business case or do you expect this to become a new stream of business?

Baochen Liao: We have several new turnkey projects in hand already. Since the technology is new, a few cell makers opting for the complete solutions, but I cannot give you the number. On the other hand, I personally feel that it will not be a long-term business case. My guess is that the scope for turnkey projects is about 1-2 years, for the new players

TaiyangNews: Please share your experiences with the 2 projects and building the solutions?

Baochen Liao: Not just the 2 projects, we have quite a few in the pipeline. In fact, we are shipping tools as we speak. Nothing specific to turnkey, but as one can imagine, bringing any technology into the mass production is not easy. We had several process or yield related issues, but we have successfully addressed them. For example, at the beginning of 2022, we started working on transferring the technology from the pilot line to mass production. The yield was less than 30% to start with, and very quickly, in half a year, we were able to bring the yield to above 97%. We have also debottlenecked several issues related to equipment stability and process. However, an ongoing issue is the shortage of manpower in China for TOPCon. There is a sudden boost in TOPCon capacity, and the majority of the workforce knows how to do PERC, but not TOPCon. Even if you consider the hypothetical situation of transferring all PERC manpower to do TOPCon, it will still fall short. That's because, PERC has a capacity of around 250 GW, while TOPCon is all set to go to 400 GW and beyond. It was really difficult to scale up the manpower for TOPCon, thus the new projects were not easy to complete. As the cell technology getting more and more complicated, it also put higher requirements for the manpower team.

TaiyangNews: Coming to the technology part, it is very clear that PECVD and LPCVD are the mainstream for TOPCon; what are the advantages of
PECVD?

Baochen Liao: In today's context, every technology has its own set of benefits and shortcomings, there is no perfect technology. As you know, wraparound is a problem with LPCVD. Right now, the workaround for LPCVD, to my knowledge, is additives. However, using additives narrows the process window, but a wide process window is essential for mass production. With PECVD as well, conducting films were an issue, and blistering was yet another problem. We have resolved these issues. Yield was the biggest concern when transferring the new technology from pilot-line to mass production, but it has reached a 98% average with our solution.

TaiyangNews: Are there still further challenges with PECVD?

Baochen Liao: Consumables come first here. The only new material we are introducing with PECVD is phosphene (PH3), which can be considered expensive. Although it was also used in HJT, since it is now also used in TOPCon, the demand has spiked up. Thus, reducing phosphene consumption is one part of the cost reduction roadmap.

TaiyangNews: Apart from the core, which is related to tunneling oxide and poly layer, what are the other important steps that require special care in optimization?

Baochen Liao: Passivation of the front textured surface, not just for TOPCon but for all n-type cells, is also critical. ALD is becoming inevitable here due to its unique feature of conformality, which provides a perfect step-coverage for textured surfaces. The typical 2-in-1 PECVD systems used for aluminum oxide deposition in PERC lines do not work here. For ALD as well, the choice is just between inline and tube type. The inline systems suffer from uptime and the costs are high. After we addressed the efficiency bottleneck with our tube type furnaces, it has now become the mainstream. The next one is boron diffusion. It is not a simple replacement of the precursor. Since boron diffusion takes place at high temperature, all the components used in the furnace building are relatively more expensive than those used for POCl. The chamber as well as heating elementhas to be different.

TaiyangNews: Like HJT, the progress with TOPCon is also labelled as different generations; so what is PETOPCon 2.0?

Baochen Liao: For 2.0, the efficiency is greater than 25.5%. So from where is this 0.5% coming from? It is coming from adding boron selective emitter. It doesn't matter whether you go to the LP or the PE method, having selective emitter is one major improvement. The next one is the rear side where we make the polysilicon thinner. Then we have also improved the quality of the tunneling oxide. All these together give us the 0.5% boost in efficiency.

TaiyangNews: Is there PE-TOPCon 3.0 as well? If yes, what are the key characteristics?

Baochen Liao: Yes, with PE-TOPCon 3.0, the first target is to breach the 26% barrier with efficiency, and it requires several improvements. Based on our PECVD method, we can do poly fingers. This is facilitated by two aspects. First, we can do in-situ doping. The second benefit is we can do the masking very easily, although people don't like to hear the word "mask" as it often relates to lab scale. The difference is in how to realize it. With our PECVD method, masking just takes a minute and can be accomplished simultaneously in the tube. This is not the case with other methods such as LPCVD or PVD, which when used for core TOPCon processing, requires a separate tool to grow mask. The TOPCon 3.0 is about realizing the passivated contact structure in the finger pattern on the front side. This is the efficiency path.

TaiyangNews: What about reducing costs?

Baochen Liao: Yes, the second part of TOPCon 3.0 is all about cost reduction. We are using a high concentration of phosphene that enables us to reduce the gas flow to 1/3rd, which also reduces the consumption and costs proportionately. Next, due to the in-situ doping, we can reduce the polysilicon thickness dramatically. With our technology, we are already using 110 nm. Based on the data we have, we are confident that we can reduce this to 60 to 70 nm without much hassle even with the current generation of pastes available today, although pastes need further improvement. Even at this moment we can go down to 60 to 70 nm; the efficiency is at par with 110 nm. That means we can already reduce the thickness to close to half, which means the phosphene gas usage is half, thus the costs of the phosphene gas. Furthermore, reducing the thickness also brings down the equipment requirement to half or doubles the capacity of the installed tools. All this put together, there is a huge potential for cost reduction. This again brings us to the discussion of LPCVD vs. PECVD, and we are not talking about efficiency; the equipment of choice won't make much difference in efficiency as of today. It is all about future potential for efficiency increase and the costs reduction.

TaiyangNews: Looking at the recent announcements, PECVD is definitely gaining popularity, while LPCVD is also improving quite a bit with respect to wraparound. Which technology has the highest market share?

Baochen Liao: According to PV Infolink 2023 April's report, PECVD's current market share is 53%, and 42% for LPCVD. That means PECVD has 10% more market share than LPCVD at this moment.

TaiyangNews: Any insights into your order pipeline?

Baochen Liao: By the end of year 2022, our company has received about 1.967 billion RMB PV equipment orders in hand. For the first quarter of 2023, we have received another 2.016 billion RMB PV equipment orders. This also indicates how fast is expansion of the new cell technologies in the market.

TaiyangNews: What is your estimation for the installed capacity for TOPCon, and where do you expect it to reach by the end of 2023?

Baochen Liao: If you take any 3rd party report, for example PV Infolink, about 400 GW. My personal estimate is around 200 GW for the installed capacity of TOPCon by the end of 2023.

TaiyangNews: Thank you for the interview.

The interview is an excerpt of TaiyangNews' Cell Production Equipment Survey 2023,  which is available for free download here