l'Actualité du Solaire

The P PERC technology for the production of cells and thus panels has reached its technical limits. A replacement must be found. But which one? There is the one that is in line with current production processes; there is the one that presents a process breakthrough, which requires changing production machines, but which could take the conversion rate further. The debate is raging in the industry

the summary

The transition from P PERC to N technology (heterojunction, TOPCon, ...)The P PERC cell has reached its theoretical limit and needs to be replaced. There are two candidates with approximately the same conversion rate. In order to avoid changing equipment, manufacturers are relying on TOPCon

Higher costsPERC technology is cheaper and of course better controlled than the new ones. Researchers have already succeeded in raising the conversion rate of the new technology above the previous one. They need to increase the industrial efficiency, reliability and standardisation of the new products. This explains why the quantity of TOPCon or heterojunction products is almost marginal

Cell and panel manufacturers are divided into two campsThe industry is hesitant about the future. Should we rely on continuity or breakthrough technology?

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the text

The transition from PERC to N-type technology (heterojunction, TOPCon, etc.)

The design offices of cell manufacturers have been working for several years on improving the P-type PERC (passivated emitter rear contact) technology. They have gradually improved the efficiency. Some manufacturers have exceeded the conversion rate of 23% and have almost reached 24%. This brings them closer to the theoretical PERC limit of 24.5%. This makes them look for something else. For a long time, N-type technology has been on the cards, but with several variants, IBC, heterojunction, TOPCon. It seems that IBC is being abandoned as a future technology.

Heterojunction has already been used for about twenty years by some companies (Panasonic, LG Electronics, ...), but it requires a complete change of production equipment. It has a satisfactory energy conversion rate. If this scrapping of machines makes installed producers hesitate, this technology is adopted by new entrants on the market such as REC Solar, Meyer Burger, Hevel, Jinergy, Risen Energy, ... because the first performances in laboratory reach 25,8 % (at LONGi). It remains marginal for the moment but could develop. The heterojunction is considered to be the generation that will follow TOPCon.

The TOPCon (tunnel oxide passivated contacts) technology has the advantage of using the same equipment as for production using PERC technology. It uses N-type and only requires two additional steps in the production process: the formation of the tunnel oxide and the deposition of intrinsic or doped silicon. Obviously, installed manufacturers are exploring this route as a priority. However, they are still in the early stages, as N-type technology (TOPCon + heterojunction) is expected to account for only 7-8% of cell shipments by the end of 2022, compared to over 90% for PERC production, according to PV InfoLink. JinkoSolar has achieved 25.4% in the laboratory with TOPCon.

Thus, all three technologies are currently delivering around 25% in the lab and are in competition. One is well known and inexpensive; the other two need their production process to be improved and mastered.

It is therefore interesting to see the advantages and disadvantages of this TOPCon technology, which is considered to be the one to replace the P-type PERC. (Fraunhofer ISE has written an interesting article from which this text is taken)

Having reached a limit of improvement in the conversion rate of the P-type PERC, the N-type TOPCon must be further developed to surpass the previous technology. As late as 2021, PERC panels were more efficient and cheaper than TOPCon panels! Researchers have made progress in reversing the conversion rate. The efficiency gap in favour of TOPCon is now constantly increasing. However, the industrialisation still needs to be perfected.

Production is not stable 24 hours a day, 7 days a week. It has not yet reached the availability rates equivalent to those of current PERC cell manufacturing facilities.

Higher costs:

A TOPCon cell using an N-type substrate, is more expensive to produce than its P-type counterpart, due to the higher costs of the boron emitter diffusion process compared to the phosphorus diffusion process typically used in PERC cell manufacturing. The technique used is the low pressure chemical vapour deposition (LPCVD) process, which is currently the industry standard for TOPCon products. It has several advantages: it ensures good thickness distribution on the wafer; it produces layers, without pinholes, with good step coverage; it requires a relatively low deposition temperature, close to 600°C; and it has a very low thermal conductivity.It requires a relatively low deposition temperature, close to 600 degrees Celsius; it can process a large number of wafers per batch; and it has the option of in-situ doping with a constant doping profile. In contrast, the LPCVD tool suffers from reduced throughput due to a much longer process time than that used in PERC cell manufacture. This inevitably leads to higher production costs. Hence the search for higher throughput or other alternatives.

TOPCon technology requires silver metal (Ag) contacts on both sides of the cell. The cost of this material requires a reduction in its use as it is expensive and will be a major constraint on the medium to long term development of this process.

The difference between P PERC and TOPCon technology

Fraunhofer ISE compared the costs of the two technologies on 5 MW of ground-mounted panels. At present, the cost of TOPCon cells is higher than PERC cells by 13.5% to 18.6%. The cost of the panels is also higher by 3.6% to 5.5%.

"The additional cost of the TOPCon panel is mainly due to higher capital expenditure and costs related to the production of the cell (consumables). The additional cost of the TOPCon panel is mainly related to higher capital expenditure and costs related to the production of the cell (consumables are significantly more expensive to process the cell due to additional processing steps compared to a PERC cell; higher price of the n-type wafer substrate compared to the p-type PERC substrate). Regarding the average cost of electricity, all TOPCon products evaluated under a single-face illumination have a cost of electricity (i.e., the cost of electricity for a single-face illumination).In terms of average electricity cost, all TOPCon products evaluated under mono-facial illumination have a slightly lower cost of electricity (LCOE) than the p-PERC bifacial technology. They are 0.5% more efficient than PERC, 23.5% versus 23.0%.

What does it take to achieve parity?

According to Fraunhofer ISE, to reach the cost of PERC, the TOPCon process would need to gain at least 0.55% efficiency. "For TOPCon technology to be adopted, a cell efficiency gain of more than 0.40% compared to bifacial p-PERC would already allow cost-effective high-volume manufacturing of TOPCon solar cells."

So TOPCon technology is well on its way to becoming a mainstream technology and to advancing cell and therefore panel conversion rates. But it needs to reduce costs and improve quality. This technology is a transition before being eliminated, probably in favour of the heterojunction, which is simpler to produce.

In September 2021, Taiwanese EnergyTrend estimated the cost per watt of TOPCon cells to be 62.5% wafer, 15.8% silver paste, and 3.7% depreciation. At present, the cost of producing heterojunction cells is higher and is 50% from the cost of a-Si (amorphous silicon). No unit costs are provided for either.

Cell and panel manufacturers are divided into two camps:

Manufacturers split between the two technologies: LONGi, Hanwha, JinkoSolar and more generally the integrated manufacturers have chosen TOPCon

Already, some major manufacturers such as Trina Solar and Canadian Solar are focusing on HJT panels, as the conversion rate obtained in the laboratory is higher than that of TOPCon

Some manufacturers, such as JA Solar, are using both technologies and installing production lines, waiting to see which one will prevail

The importance and diversity of the research work for TOPCon (and also heterojunction) technology make it possible to announce spectacular advances in the conversion rate within a few quarters.

For further information:

https://www.ise.fraunhofer.de/en/research-projects/20plus.html

https://www.pv-magazine.com/2022/01/20/topcon-vs-perc/

https://www.actu-solaire.fr/a-14552-la-cellule-de-type-n-devient-elle-competitive.html