Passivation is a critical step in solar cell processing, particularly when considering performance. There are 2 complementary methods of passivation. One method is to strongly reduce the charge carriers of one polarity reaching the surface, called field effect passivation. The other is known as chemical passivation, which is attained by saturating the dangling bonds on the surface and impurities to an extent in the bulk. The details of these 2 mechanisms were discussed in depth in several of our previous publications, including the TaiyangNews Report on TOPCon Solar Technology 2021.
Interestingly, passivation has been the key differentiator between different cell architectures. While BSF only provides field effect passivation on the rear of the cells, PERC adds chemical passivation on the top. With TOPCon, the passivation scheme does change, not because of the cell structure, but due to the change of the base wafer polarity from p-type to n-type. The passivation scheme for TOPCon cells is the inverse of that for PERC. Not just that, the layer configurations are also optimized as the requirements for these dielectrics are fairly different for emitters and the rear side of the cells, beyond the chemistry.
The crux of TOPCon also lies in the passivation, not just surface or bulk, but passivating the metal contacts. Unlike previous-generation BSF or PERx technologies, the TOPCon structure mainly addresses metallization-induced recombination losses. This is achieved by electronically separating the contacts from the absorber by insertion of a wider bandgap layer. Today’s TOPCon technology involves implanting this passivated contact structure to the rear of the cell. In a way, TOPCon processing is an extended passivation technique (see From Throughput To Advanced Process Optimization).
At the same time, HJT is yet another variant of passivated contacts. While TOPCon supports high-temperature processing using silicon oxide as a tunneling layer and doped polycrystalline silicon, HJT attains the passivated contacts at low temperatures using intrinsic amorphous silicon and doped multicrystalline silicon. Even the latest, or perhaps the ultimate, single-junction cell structure, the BC architecture, can be based on either TOPCon, HJT, or a combination of both (using one approach for one polarity of the contact).
The text is an edited excerpt from TaiyangNews’ latest Market Survey on Solar Cell Production Equipment 2025, which can be downloaded for free here.
