Boron for TOPCon Cells

Boron Diffusion And Annealing Are The Other Important Additional Process Steps Involved In TOPCon Processing

Boron for TOPCon Cells

Better and cleaner: Semco has been a strong advocate of boron trichloride based diffusion process, which according to the company not only delivers better results, but also keeps the process as well as quartz boats clean. (Source: Semco)

  • Emitter formation with boron diffusion is an important step in the TOPCon process
  • Boron diffusion is typically carried out in tube furnaces at low pressure, and almost every leading furnace supplier has such a product platform on offer
  • While boron tribromide has been the most widely used precursor for boronb diffusion, boron trichloride is gaining popularity
  • Annealing accomplished in RTP is required to activate the deposited polysilicon and the step is even more compelling in case of in-situ doping to activate the dopant

The TOPCon cell manufacturing process involves a few additional steps compared to PERC. While emitter formation is not completely new, it is fairly different from what is done in PERC. The difference originate from the fact that today’s TOPCon cells are mainly adapted on n-type wafers, thus boron diffusion is employed instead of phosphorus and there are several equipment vendors offering tools for this process. The process, however, is quite important in that the emitter profile also influences the TOPCon cell performance.

Boron diffusion for TOPCon

Boron diffusion is typically carried out in tube furnaces at low pressure, and almost every leading furnace supplier has such a product platform on offer. The tool setup for boron diffusion, while more or less the same as for phosphorus diffusion, requires considerable optimization. It is typically accomplished at a higher temperature – above 1,000°C – and requires higher cycle times of 150 minutes compared to 102 minutes with phosphorus diffusion that results in lower throughput, as indicated by Chinese cell production equipment maker S.C New Energy.

There is also a difference among the boron diffusion furnaces, i.e., the choice of precursor, and boron tribromide is the most extensively used. This precursor traditionally has one problem — the byproduct of the process acts as a glue to quartz, which may reduce uptime. However, with process optimization and improvements in reactor design, precursor consumption can be reduced considerably to a level where downtime is not a big concern anymore.

French based Semco, on the other hand, uses boron trichloride as it believes that it delivers better results, as the presence of chlorine keeps the tube cleaner. It also helps in gettering. Boron trichloride is supplied in a gaseous form in bottles, eradicating the need for bubblers. BSG, with a chlorine-based precursor, is easy to remove compared to its counterpart. The flip side of using boron trichloride is its corrosive nature and the safety concerns associated with it. Despite concerns, the approach seems to have earned a few more followers.

Laplace’s philosophy is akin to that of Semco’s for boron diffusion. Its tools are also designed to be compatible with horizontal wafer processing, using boron trichloride as the precursor. Similar to the LPCVD tools, the boron diffusion furnaces of the Chinese company are provided with paddle supports at the time of processing so as to avoid tube deformations.

One among those, S.C New Energy, is not only offering boron trichloride as an option, but it also strongly recommends its use as the precursor. According to ITRPV, doping using boron trichloride is expected to garner a market share of 20% by end of 2021 and would increase gradually to 30% in the coming 10 years.

Annealing: Something that is not usually discussed as deeply, but quite important, is annealing. Although the crystallinity of deposited polysilicon varies according to the technology, more towards crystalline with LPCVD and close to amorphous with low-temperature deposition technologies such as PECVD, annealing is inevitable irrespective of the deposition technology. This step is accomplished using simple RTP tools, while the thermal treatment time varies with the technology. Using in-situ doping makes the need for annealing even more compelling — despite the dopant being evenly distributed during the process, it has to be activated. Given the low prices of the thermal processing tools and improved processing yield with annealing, the current practice is to accomplish it in separate furnaces. However, this step can be integrated into the firing process as part of future optimization (see No Wraparound With PECVD).

Emitter passivation: Since TOPCon is typically employed on n-type wafers, it has reverse polarity on the emitter side in contrast to PERC. Therefore, rear side passivation of the PERC structure, typically using a stack of aluminum oxide and silicon nitride, is applied on the emitter side of TOPCon cells. Nearly every aluminum oxide tool supplier offers a tool for this application, while a few have also evaluated silicon oxide and BSG in the past (see Several Paths to TOPCon Cells).

This brief article is taken from our recent TaiyangNews report on TOPCon Solar Technology, which is available for free download here.

An overview of the report was presented during TaiyangNews High Efficiency Solar Technologies Conference, where Day 3 of the conference focussed mainly on TOPCon. To learn more about the conference and view the presentations click here.

About The Author

Shravan Chunduri

Shravan Chunduri is Head of Technology at TaiyangNews.

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