The Influence of Soldering Flux on Stability of Heterojunction and TOPCon Solar Cells

ABSTRACT Silicon heterojunction technology (HJT) and tunnel oxide passivated contact (TOPCon) solar cell technologies are expected to dominate the photovoltaic market in the coming years. However, there are still some concerns about the long‐term stability of these technologies. This work examines the effects of two widely used commercial soldering fluxes (Flux A and Flux B) on the stability of commercial silicon HJT and TOPCon solar cells. The soldering flux was applied to the solar cells, and the solar cells were annealed at 85°C under a relative humidity (RH) of 0%. The investigated TOPCon solar cells were found to be stable; however, significant degradation was observed in the HJT solar cells after only 50 h. The efficiency of the HJT cells decreased by ~61% rel with Flux A and ~55% rel with Flux B, respectively. We attribute part of the observed degradation to pores present in the HJT cell metallisation after printing, which allow the soldering flux to easily penetrate the contact and subsequently react with the paste constituents. In addition, we find that the indium tin oxide (ITO) layer is very sensitive to soldering flux, showing major cracks and significant peeling after 50 h of annealing. Consequently, this work shows that some soldering flux can react with the ITO layer, without requiring the presence of water. This suggests that certain types of soldering flux can harm HJT solar cells even after encapsulation without the need for moisture ingress. Therefore, paying more attention to the choice of soldering flux is essential, especially when working with HJT cells. It is strongly recommended that users perform comprehensive component analysis testing on soldering fluxes before their official use rather than solely relying on datasheets provided by suppliers.