Unveiling Mechanistic Differences in the Curing of Copper and Silver Pastes for Heterojunction Solar Cells

Reducing silver consumption in silicon heterojunction (SHJ) solar-cell metallization motivates the use of conductive pastes based on mixed silver–copper (AgCu) fillers, but the curing mechanisms and resulting microstructures of such systems remain insufficiently understood. This work investigates an epoxy-based paste platform with AgCu or Ag fillers, formulated either with a linear amine hardener (HD1) or an anhydride curing system (HD2/CAT), to elucidate how curing chemistry and filler composition govern rheology, cure behavior, and conductivity. SEM/EDX is employed to assess AgCu particle morphology and copper exposure, rotational rheometry to track curing kinetics, and four-point probe measurements to evaluate the electrical conductivity of printed SHJ front-side fingers. The results show that incomplete silver shells on AgCu particles expose copper to Cu–amine complexation, inducing strong agglomeration and nonprintable rheology in epoxy–amine systems, whereas the anhydride-based formulation suppresses this interaction and yields stable, printable AgCu pastes, albeit with slower curing than a commercial AgCu reference paste. Using the selected epoxy resin (ER-2) in combination with the anhydride curing system provides suitable shear-thinning behavior, specific line resistance within the commercial range (6.4 μΩ cm), and reasonable adhesion on SHJ device stacks. These findings establish mechanistic links between curing chemistry, metal–polymer interactions, and functional properties, and thereby support the targeted design of low-temperature AgCu metallization pastes that balance material cost, conductivity, and reliability.