Inside Meyer Burger's new HJT pilot line

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pv magazine: What is the significance of this pilot line?

Christophe Ballif: This facility is a further step at the pilot scale to demonstrate that we can make excellent [HJT] devices. Here we will be able to process a lot of samples, make statistics and further validate all the fantastic lab results that we have. The pilot line is also here to show that we can master processes on a larger area. Typically up to 56 wafers can processed at the same time. Indeed several people continue to think that it’s not possible to coat uniformly a few nanometers of silicon. Roth & Rau & Meyer Burger have developed production reactors both for the thin silicon layers and for the transparent conductive oxide, which allows reaching a remarkable uniformity and and a perfect surface passivation. With the pilot tools we can make more samples, and we’ll be able to push the efficiency higher.

The nice thing about this HJT technology is the use of plasma process, because when you are using plasma, you have a lot of possibilities to play with the plasma composition and properties. So you have endless possibilities to design layers which can improve the solar cell parameters.

For me, today is a very special day because I started this project here in 2004-2005 and now, thanks to our industrial collaboration, we have brought the full process from laboratory cells to full production lines, including sawing and module technology. I see a lot of advantages in the HJT technologies.

However, I notice that currently most companies prefer to upgrade existing lines (e.g. going to PERC solar cells), rather than making the change to a new technology. Clearly with HJT, you need to invest in new equipment, which should be an option when a producer targets to increase volume by a few hundred MWs. Indeed the market for high efficiency products is excellent. In countries such as here in Switzerland, consumers buy premium products for a much more per Watt than standard one. There is clearly a premium for high efficiency, even though it applies mostly to space constrained systems.

pv magazine: So for you personally this is the end of a 10-year journey?

Ballif: Yes because now there are nice, production size, tools, which are installed where we can complete projects together and test hundreds of wafers. Of course it is a kind of dream that becomes true. The next step would be to have a full production lines and maybe somewhere in Europe –I hope that this dream comes true one day also. I am convinced that advanced production concepts can be implemented successfully in Europe.

pv magazine: So CSEM was developing HJT for some time?

Ballif: CSEM PV-center as been involved in this technology since its creation in January 2013. I started the activity at my academic laboratory at IMT [Institute of Microengineering], part of the University of Neuchatel, which became integrated into EPFL [École polytechnique fédérale de Lausanne] in 2009. We developed it from 2005 onwards. At the beginning it was one PhD student trying to passivate wafers with our homemade reactors, which were highly inhomogeneous. But quickly we could achieve small solar cells with over 700 mV open-circuit voltage, a remarkable result in 2006.

At the same time we were upscaling the technology for thin film silicon solar cell (a-Si and nc-Si) technology. Upscaling means here mastering plasma processes on surfaces greater than one square meter. So from the thin film silicon technology we knew that upscaling was possible. It meant for heterojunction that if you can make something on a small size, you can make it on a full wafer, and that you can also make it on 50 wafers at the same time. This is basically what we have been developing with Roth & Rau [part of the Meyer Burger Group], high throughput, high-quality reactors for thin a-Si layer deposition.

pv magazine: And the a-Si layers are very thin, aren’t they?

Ballif: Usually, we apply first an intrinsic layer on both sides of the wafers, without doping, which is 5 to 10 nanometer thick. Then we apply the doped p and n layers, which are typically 10-20 nanometers thick. So typically it is around 20 nanometers of a-Si.

pv magazine: So is the deposition of the a-Si layers therefore quite a quick process?

Ballif: Yes you can have a process time of around one to two minutes, because the layers are so thin.

What we are also seeing here is the bringing together of this technology with things like SmartWire. Meyer Burger also has wafers here that have been cut with its Diamond Wire.

pv magazine: How important do you think it is that the whole chain of production develops along with the cell?

Ballif: Of course you can make HJT modules without SmartWire, and you can have two, three, four or five busbars. When you work with HJT cells the processing temperature is only around 200 degrees Celsius. When you work with silver paste [at the lower temperatures] it is not as conductive as when it is used for high temperature solar cells.

pv magazine: Which can typically be around 800 degrees Celsius …

Ballif: That’s right. So you either need more silver [for the same resistive losses] or you need to find a way to save silver. This is why Meyer Burger (Somont) has introduced quite quickly a stringer with five tabs, because if you have five tabs you can already save quite a lot of silver for the fingers. And the logical evolution of that is to go to SmatWire, because then you need a ridiculously small amount of silver. So we turned a weakness of the technology into a strength, because with that [SmartWire] we now use perhaps 20 milligrams of silver per face of the cell, which is a factor of five to eight less than what you do with even a standard solar cell.

Of course you need the foils, including the wires, but in the end you gain in cost and you have other additional benefits of SmartWire: For instance, if a cell breaks, you have so many contact points that you do not see any fill factor losses. It is good technology originally developed by Day4, and then taken over by Meyer Burger and readapted to make it compatible with HTJ, but also with standard cells architecture, such as multicrysalline or monocrystalline fired cells.

Meyer Burger has decided to integrate across the full chain and it is a good idea, because sometimes you might indeed lose the real focus if you only develop a single process. For instance, there can be strong interaction between the wafer sawing and the subsequent saw damage etch. In the Swiss InnoHJT we look right across the full production chain and see how we can save on cost. So I strongly believe that integration of processes can allow you to be in the end more cost-effective.

pv magazine: If you are so convinced of HJT, how do you account for Sanyo’s HIT technology having taken such a long time to be employed more widely in production? Of course it is now Panasonic technology, but we still don’t see HIT being produced in large volumes.

Ballif: Well first I’d say that Panasonic has likely capacity close to 1 GW, and they sell it at high price. Then I think the production technology of Sanyo/Panasonic is different from what we developed (in terms of processes). Also I am not sure that it has pushed towards the most cost effective production technologies.

Indeed, my perception is that Japanese companies are doing fantastic R&D but they can be rather conservative in the way they increase their production capacity. Thy will tend to stick to what is fully proven rather than risking, e.g., going to larger size, more cost competitive tools and production processes.

For instance Panasonic has stayed for a very long time on small size 4-inch wafers. They use a huge amount of silver on the solar cell. Old Sanyo are silver mines!

Globally in our collaboration with Roth and Rau and then Meyer Burger group, we have built-up the technology from scratch, trying to target from scratch a cost effective production. I think what we have now developed a streamlined production technology, with dedicated tools, dedicated materials and material usage. What is remarkable is also the very few steps required to achieve high efficiency. When we make the cost of ownership calculation we can see that it can really be competitive with standard technologies, but with the advantages of higher efficiencies and higher energy yield, thanks to the low temperature coefficient.

In conclusion, I think now we have put in place everything that is necessary to make the process cost effective. As I mentioned, the penetration of new technologies in the market is however difficult. On one side, many companies do not want to consider or even be informed, such as this wrong perception of mastering passivation on a large area. It is disappointing because some tend to ignore all the progress that has been made with coating technologies in general and with thin film technologies. PV thin film coatings was a driving technology as was the flat panel display industry.

pv magazine: I was just about to ask how much the growth of the flat panel display industry has boosted the development of deposition technology.

Ballif: Basically there was a strong synergy between flat panel display (FDP) equipment and the technology for thin film silicon (a-Si). The FDP equipment had to be adapted, redesigned for thin film silicon, and some of the key players in thin film silicon comes from the FDP business sector. In terms of plasma reactor technologies, HJT might also be in a way linked to such plasma reactor technologies. However, the throughput and handling is very demanding, because of the much shorter cycle times.

pv magazine: And is it a misconception that you hope to change with the Swiss-Inno HJT line here?

Ballif: Yes, we have full carriers with [HJT] cells that regularly have 21.5% and 22.5% [conversion efficiency] with remarkable homogeneity: So these processes are mastered. It is only a question as to whether people want to hear the message.

pv magazine: And the PECVD production tool can carry around 50 cells, is that right?

Ballif: The production tool that Meyer Burger/Roth & Rau are selling carries 56 cells. With a tact time of around 90 seconds, you could typically produce 80 MW a year.

pv magazine: Looking at the market once again, we have seen a growth in the application of higher efficiency technologies this year, while only a couple of years ago there was simply no interest in these kinds of technology – the focus was entirely on cost reductions. Do you think that some producers are willing to take the bold step of establishing an entirely new line like HJT?

Ballif: I strongly believe that now is a good time to invest in high efficiency production lines because high efficiencies can be sold at much higher prices. It is also a differentiator. What I notice is that there are a lot of companies that have invested in standard technologies and they want to depreciate their technology over 10 years. They will rather try to make upgrades to existing production lines and will go e.g. from 250W to 260 W, or maybe 270W to 280 W monocrystalline [modules].

pv magazine: And these are largely PERC upgrades, would you say?

Ballif: Yes, but now it is more a question about which company is going to be the next big player: Which one is going to be the company daring to take the next step. Silevo is a good example as it is perusing a HJT like technology.

pv magazine: And that is at gigawatt scale. Do you know who Silevo’s tool partner is?

Ballif: I cannot say anything about this. But what it does show is that high efficiency technologies are available, it is just a case of having producers that are ready to adopt them.

pv magazine: So the HJT technology pitch to firms is that you can produce 22% or maybe 23% efficient cells that can be assembled into a 20% efficient module. Is that right?

Ballif: Lately a 60-cell module with HJT and smartwire interconnection was presented, with a power of 327 W.

pv magazine: But that is a "hero" module isn’t it?

Ballif: Yes that’s right, but it is a 20% module, which is even better than Panasonic commercial HIT, and this with 6-inch cells. There are many companies that assemble modules with the cells very far apart, with a white backsheet and can, with 60 cells, get more power because of internal light reflection. But the efficiency is reduced. Many companies do this and this can be misleading. Interestingly quite often they do not mention the module efficiency, so that you can’t track back to the size of modules. If you are constrained to the normal module space, then with HJT you can attain an output in the range of 300 W to 330 W at the module level, (and we hope of course for more) and then of course you can go to bifacial – which can give you an additional 10% to 20% boost.

pv magazine: Although we don’t have a standard to measure bifacial efficiency or output quite yet.

Ballif: Several propositions for standards are out. But of course the actual energy boost is the highest when you take care to space the module, and when you have a white background, so that you can benefit from the ground reflection. Bificial HJT modules a few meters above fresh snow are terrific.

pv magazine: High efficiency and output is good, but what about cost-per-watt with this HJT technology?

Ballif: By my calculations you can get to the 500 MW level in the typical range of €0.38-€0.45 per watt. Of course the final numbers will depends on the location and a number of choice of consumables (glass, polymers). It also depends on the production value, with 22% being rather at the top end of production efficiency. In my view a producer could start near 21% in average and bring that up to 21.5%-22% after going through line optimization. With excellent wafer quality there is even space for 1 to 1.5% more. There are some companies that provide very high quality wafers while some other companies need to improve to some extent. In general all wafer makers should find way to further improve the material quality. It is a must for the success of PV in general.

I think we have done a fantastic effort here, with Roth & Rau in Germany, and the Meyer Burger Group. But I think we need to bring this new technology to the market (and they are other beautiful technologies). Otherwise we are simply doing more of the same. For PV the next steps are important. Why produce 250 W modules when you can possibly do a 340 W module? You can also consider it a waste of time and money, and somehow linked to the incentives problems. There should be mechanisms to promote new promising technologies, because renewable energies are so important for the future of mankind. Of course the market is what it is, but I am sure that many are now willing to make the jump! The good news is though that a first customer is ramping up production, and that many companies are also planning HJT production (or similar ones).