As with standard cell making, the HJT process also starts with saw damage etching (SDE), followed by texturization and cleaning. All these steps are individually optimized for HJT. The typical PERC process flow avoids SDE, while it is highly relevant for HJT, emphasized Damian Brunner, senior R&D manager solar & battery at RENA in his talk on Wet Chemical Innovations in SHJ & TOPCon Cell Processing (see presentation here). RENA is offering an organic cleaning option before the SDE step, which can be either ozone based or hydrogen peroxide, while the latter is recommended when a high degree of cleaning is required. SDE is alkali based, and the process intensity strongly depends on the quality of the incoming wafer.
The preconditioning process before texturing facilitates homogeneity in pyramid distribution and helps adapt to the height of the pyramids formed in texturing. This is very important as, after texturing, the surface morphology strongly influences the quality of the passivation. In the post-texturing step, rinsing and cleaning are crucial.
Overall, cleaning is the most important change required for HJT at the wet-chemical station. HJT demands a specific cleaning regime to remove organic and metallic impurities. The quality of the deposition – and thus the ultimate passivation – strongly depends on the cleanliness of the wafers, especially since the HJT process is carried out at low temperatures. There are 2 cleaning methods that are largely followed – an ozone-based process and a so-called RCA process using hydrogen peroxide. While RCA is effective as well as robust, it is also expensive, not only due to the high price of the chemicals of which hydrogen peroxide is at the top, the effluent treatment costs are also high. For this reason, ozone-based cleaning is attracting a lot of attention. As a matter of fact, none of this is new, except that ozone-based cleaning is becoming the mainstream.
RENA, for example, uses ozone-based preconditioning and cleaning before SDE, as mentioned above, followed by pre-cleaning before texturing. In post-texturing, the first cleaning step is also based on ozone to get rid of organic residues, and the process ends with ozone-based etch-back and HF-based metal cleaning. At the tool level, most of the leading wet-chemical bench suppliers are offering production tools that can support high throughput of around 7,000 wafers per hour integrating all the above discussed functionalities. The equipment is also designed to support half-wafer processing (see Half Wafer Processing For HJT Cells).
For more details on HJT, download the TaiyangNews report on Heterojunction Solar Technology 2023 for free here.