Four Failure Modes in Silicon Heterojunction Glass-Backsheet Modules

Silicon heterojunction technology (HJT) is expected to gain a significant market share in the near future. For HJT to deliver a low levelized cost of electricity (LCOE), it needs to have a high initial efficiency and degrade less than 0.5% relative per year. This work investigates damp heat-induced failure modes in silicon HJT glass-backsheet modules. Four unique failure modes are identified after damp heat (DH) testing: point failure (Type-1); failure around the interconnected regions of the busbars and ribbon wires (Type-2); failure between the busbars (Type -3); and failure at/on the interconnected regions of busbars and ribbon wires (Type-4). The Type-1 failure mode is likely caused by a chemical reaction between surface contaminants (introduced to the cells during handling or characterization before encapsulation) and moisture that increase charge carrier recombination and lead to a loss in maximum power (Pmax) of up to 40%rel in this study. Type-2 and Type-3 failure modes cause Pmax losses of -5%rel and 50%rel, respectively, in this study and can appear due to exposure to soldering flux used for con-necting the ribbon wires and busbars. Finally, the Type-4 failure mode causes a Pmax loss of-16%rel in this study after the DH test. The evidence suggests that this failure mode is likely due to the interaction of acetic acid, generated from a reaction between the encapsulation material and moisture, ribbon wires, and silver paste (busbars), resulting in recombination loss. We believe these failure modes must be well understood and mitigated at preferably the solar cell level to ensure that HJT can meet its LCOE potential.