Four Failure Modes in Silicon Heterojunction Glass-Backsheet Modules

Abstract

Silicon heterojunction technology (HJT) is expected to gain a significant market share shortly. For HJT to deliver a low levelized cost of electricity (LCOE), it needs to have a high initial efficiency which only decreases by up to 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 mode), failure around the interconnected regions of the busbars and ribbon wires (Type-2 failure mode), failure between the busbars (Type-3 failure mode) and failure at/on the interconnected regions of busbars and ribbon wires (Type-4 failure mode). The Type-1 failure mode is likely caused by the chemical reaction between surface contaminants and moisture, increasing charge carrier recombination, leading to a loss in maximum power (Pmax) of up to 40%rel. Type-2 and Type-3 failure modes cause Pmax losses of ~5%rel and 50%rel, respectively, and we will present substantial evidence of the involvement of soldering flux used for connecting the ribbon wires and busbars causes these failure modes. Finally, the Type-4 failure mode causes a Pmax loss of ~16%rel 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, 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.

Keywords: Silicon heterojunction solar cells, damp heat, failure mode, humidity-induced degradation, ethylene vinyl acetate, glass-backsheet modules