A research team at the University of New South Wales (UNSW) has investigated failure modes in heterojunction (HJT) solar modules with a glass back structure.
“We have identified four failure modes in the silicon heterojunction glass backplane module that can lead to a power loss of up to 50% after wet heat testing,” researcher Chandany Sen said. pv magazine. “We tried to understand the possible underlying causes of each failure mode and how to quickly detect them at the cellular level.”
The researchers performed their experiments on bifacial half-cut n-type silicon HJT solar cells obtained from industrial production lines of unknown manufacturers. The products were divided into three groups: modules with encapsulated cells; modules with encapsulated cell precursors; and unencapsulated cells. In the first two groups, the researchers used ethylene vinyl acetate (EVA) encapsulant.
“All samples had an n-type wafer, internal hydrogenated amorphous silicon (ia-Si:H) passivation layers on both sides, and phosphorus-doped (na-Si:H) and boron-doped (pa-Si:H) hydrogenated amorphous silicon layers on the front and back sides. , respectively, and then an indium-doped tin oxide (ITO) layer deposited on both sides, they said.
All devices were subjected to a wet heat test at 85°C and 85% relative humidity (RH) between 500 hours and 4,000 hours.
Through this testing, the researchers identified four failure modes for the encapsulated cells, resulting in power losses between 5 and 50 percent. The first type of failure consisted of darkening of the cells at local points and the second darkening around the connection point of the rails and ribbon conductors. The third failure mode contained heavy darkening between the busbars and the interconnected areas of the ribbon conductors, while the fourth showed darkening in the area where the busbars and ribbon conductors were interconnected.
According to the team’s analysis, the first type of failure was due to surface contamination that may have occurred during processing or characterization before the module was encapsulated. As for the second and third groups, the researchers attributed the failures to solder flux involvement.
“The direct effect of flux and lead (Pb) solder on contact degradation after DH testing was also observed in other works,” they said. “It is important to emphasize that in some cases the use of a different Ag paste also led to a type 3 failure.”
For the fourth group, the researchers reported that the defects were likely caused by a byproduct of the EVA used for encapsulation.
“Although the experimental design of this work cannot determine exactly how each failure mode occurred after DH testing, it demonstrates plausible situations that could occur in an industrial environment and lead to the actual failure modes observed,” they concluded.
Sen said that understanding and mitigating these failure modes, preferably at the solar cell level, is critical to realizing HJT’s low cost of electricity (LCOE) potential. He said that while glass-on-glass modules are often used in HJT solar cells because of their reduced susceptibility to moisture intrusion, similar failure modes are likely to occur in these modules over a longer period of time.
The research team presented their findings in the study “Four failure modes in silicon heterojunction glass backplane modules”, published in Solar energy materials and solar cells.