Assessing the impact of solder flux-induced corrosion on TOPCon solar cells

Abstract

N-type tunnel oxide passivated contact (TOPCon) cells currently dominate the photovoltaic market. However, reliability issues remain, particularly in high-temperature and high-humidity environments, which can result in notable power losses. This study investigates the impact of two industrial fluxes on the reliability under damp heat (DH) conditions of n-type TOPCon solar cells referred to as Sample Group A, B, and C. Two industrially used soldering fluxes, designated as Flux A and Flux B, were applied to unencapsulated cells, followed by DH testing at 100 °C and 95 % relative humidity for 122 h or at 85 °C and 85 % relative humidity for 10 h. Sample A exhibit minimal corrosion regardless of flux type, attributed to their denser Ag structure and lower Al content, which reduced susceptibility to flux infiltration. In contrast, Sample B and C show significant degradation, particularly in the front exposure groups, with the sample C demonstrating severe delamination of the metal contacts. Flux A causes more pronounced corrosion than Flux B, likely due to its higher activator content, greater acid value, and the presence of halogens (undisclosed in the flux specifications). However, it should be noted that the corrosion by Flux B is still substantial. Cross-sectional and top-view analyses revealed extensive corrosion and the formation of Pb crystals, highlighting the corrosive effects of flux residues under DH conditions. The study emphasizes the value of unencapsulated cell-level testing as a rapid and cost-effective method to assess the adverse effects of soldering flux on cell technology, thereby supporting informed decision-making to ensure photovoltaic module reliability.