- While there are many ways to achieve passivated contacts, making the concept work in high temperature processing environment is what is coined as TOPCon in today’s context
- Employing the TOPCon structure on rear side of n-type is a common practice, while the structure can also be implemented on front side as well in principle
- Applying a thin tunneling oxide topped with n+polycrystalline silicon on the rear side is the key for TOPCon structure
TOPCon is a structure formed at the surface of a semiconductor, which combines the functions of a passivation layer and a contact. However, making the concept work in standard crystalline silicon cell manufacturing, especially making it compatible with high-temperature processing, is what is tagged today as the TOPCon approach.
The structure in principle can be applied on either side of the wafer and is also compatible with p- and n-type wafers. However, since polysilicon has a similar band gap as crystalline silicon, it results in larger absorption losses when doped polycrystalline is used on the front side. While there are methods to circumvent losses from unwanted absorption, they are not production ready yet as it involves sophisticated masking and etching steps. Thus, ‘passivated contacts’ is a term used mostly in reference to the rear side engineering in the industry. As to the question of which base wafer should be used, the effort of adding a TOPCon structure to a PERC structure doesn’t produce enough ‘bang for the buck’ per se. As a result, the majority of the industry continues to use passivated contacts on the rear side of n-type wafers, even though researchers also work on p-type passivated contact cells, like German solar research institute ISFH, which presented their approach at the recent TaiyangNews news focus webinar: The Time is Ripe for TOPCon Solar Modules.
Here is the schematic of a TOPCon solar cell from China’s Jolywood, a pioneer of this segment that has led the technology in high-volume manufacturing. The n-type base wafer features p+ boron emitter passivated with a stack of aluminum oxide and silicon nitride dielectric. On the rear, a very thin 1 nm tunneling oxide is topped with n+ polycrystalline silicon and silicon nitride stack, and both the surfaces are metalized with silver paste.
This brief article is taken from our recent TaiyangNews report on TOPCon Solar Technology, which is available for free download and read here.