Copper plating can provide significant cost savings over screen printed Ag for industrially produced Si photovoltaic modules, however concerns exist with regard to the durability of Cu-plated modules. This study investigated the effect of different capping layers and their method of application on the durability of Cu-plated PERC one-cell modules, with a focus on durability after damp heat (DH) testing. It is shown that the adhesion of Cu-plated fingers capped with immersion-plated Ag and Sn was poorer than that of fingers capped with light-induced plated (LIP) Ag due to the under-etching of Cu finger by corrosive additives that are present in the immersion plating formulations. It is proposed that the weak finger adhesion resulted in fingers dislodging during DH testing of the modules, thereby opening a pathway for Cu to in-diffuse into Si resulting initially in an increased non-ideal recombination and then increased series resistance due to encapsulant flowing under the dislodged finger. Increased recombination due to the Cu in-diffusion was evident in cells with poor finger adhesion through an increase of the non-ideal recombination current density, J02, of 166%, 253%, 266% for LIP Ag, immersion Ag and immersion Sn, respectively, during the first 1000 h of DH. In comparison, with stronger finger adhesion, J02 increases were <60% in the same DH period. This result highlights the importance of strong finger contact adhesion, and not just busbar adhesion, for durable Cu-plated modules and suggests that both finger and busbar adhesion are routinely monitored in the processing of Cu-plated cells.
- Damp heat