Several Paths To TOPCon Cells

TaiyangNews Report On TOPCon Solar Technologies Discusses Different Process Steps For Making These Cells

Several Paths To TOPCon Cells

Typical process flow: While there are several variants of the TOPCon process flow, applying a stack of silicon oxide and doped polysilicon layers are the crux of the cell process. (Source: Jolywood, graphic: TaiyangNews 2021)

  • TOPCon approach requires 2 important steps – growth of interfacial oxide and applying doped polysilicon layer
  • There are several variants of TOPCon process flow – the process sequence and the steps involved vary based on the technology selected
  • While 3 additional steps are required when upgrading from n-PERT to TOPCon, for PERC one more additional step of boron diffusion and a few more cleaning steps are needed
  • While thermally grown silicon oxide is common for tunnelling oxide, several deposition technologies such as – LPCVD, PECVD, PVD, PEALD – are promoted for polysilicon application

The success of the high efficiency TOPCon process at any cell maker lies in its successful manufacturing. This article shows different variants of the TOPCon cell manufacturing process.

The basic principle of TOPCon is to apply metal contacts without patterning. In order to realize this in the real world, two steps are necessary – growth of interfacial silicon oxide and deposition of intrinsic polycrystalline silicon layers, which is subsequently doped. In principle, 3 additional steps are required when upgrading from n-PERT to passivated contacts. When compared to PERC, since TOPCon is typically employed on n-type, it requires boron diffusion for emitter formation in subsequent cleaning steps. While there are 2 integrated cleaning steps with PERC, TOPCon requires at least 3 steps. Even BSF followers, a rarity these days, can bypass PERC and directly upgrade to TOPCon.

There are several variants of the TOPCon process flow (see info graph).

Given that the technology is in its nascent stages and that there is more than one way that each step can be accomplished, the process sequence and the steps involved are also varied based on the technology selected. And these options present themselves right after the most common first step of any cell processing — saw damage removal and texturing — accomplished with wet-benches.

The majority of the TOPCon manufacturers seem to have chosen the path of emitter formation. This method enables manufacturers to deal with the high thermal budget involving the boron diffusion step right up front, sparing the rest of the cell structure, especially the very thin tunneling oxide, from high temperature processing. Boron diffusion can also be achieved through ion implantation, in which case the above restriction does not apply. However, the industry practice is to start with emitter formation, followed by removing the respective glass with single-side etch. Then the rear surface is polished, again using wet-benches.

Formation of thin tunneling oxide is the next step. There is also a wide range of tunneling oxides to choose from. A few early industrial adopters used a wet-chemical method to generate an ultra-thin oxide. Subsequently, other methods were developed — plasma-assisted oxidation, wet-chemical hydrochloric acid oxidation, wet-chemical nitric acid oxidation, thermal oxidation, and UV/O3 anodization. The thickness of the tunneling oxide, however, remains an important attribute. While there is no standard as of now, according to literature, tunneling becomes less likely in the case of thicker oxides since tunneling is assumed to take place when the oxide is thin, say less than 2 nm. Thermally grown silicon oxide is not only known to provide a superior chemical passivation quality, but even the setup and the process are easy to accomplish. It can also be integrated into the subsequent polysilicon deposition. With cell makers focusing more on simplifying the process flow, integrated application of oxide and polycrystalline silicon in one go is preferred, if the tool platform permits. However, Germany’s Centrotherm believes in decoupling the steps, as it not only reduces costs, but also improves the film quality.

For polycrystalline deposition, the core of the TOPCon process, almost every deposition technology used in PV is promoted here — LPCVD, PECVD, APCVD, ALD and PVD — in varying degrees of availability. The deposited polysilicon has to be doped, and there are options here as well. Commercial TOPCon module pioneer Jolywood, for example, has been known to employ ion-implantation for realizing n+ doping of the polysilicon, which can also be accomplished in tube diffusion furnaces using a POCl process. While the ion-implantation avoids PSG etch in comparison, it requires activation of the dopant as an additional step, which is also the case with another option of in-situ doping as facilitated by certain machine platforms. It doesn’t come as a surprise that in-situ doping is attracting a lot of attention with the simplification that it brings to processing steps.

The TOPCon structure applied on the rear side also results in a parasitic deposition on the front side, which needs to be removed through wet processing, though the degree of effort for stripping the polysilicon layer depends on the deposition technology of choice.

The front surface is passivated with a stack of aluminum oxide and silicon nitride, deposited using either PECVD alone or a combination of ALD and PECVD. The passivated contact stack is also capped with silicon nitride using PECVD. Silver contacts are applied on both sides followed by firing to finish the last leg of the cell manufacturing process, preparing the cells for IV testing and sorting.

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

 As today the work horses of the solar industry are still PERC cells, TaiyangNews  will organize a virtual conference on Pushing PERC Cells to Its Limits on March 22, 2022. To register for free, please click here.

About The Author

Shravan Chunduri

Shravan Chunduri is Head of Technology at TaiyangNews.

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