| Work Detail |
Researchers at Australias University of New South Wales have identified four modes of failure caused by moist heat in heterojunction solar panels with a glass backsheet configuration.
A team at the University of New South Wales (UNSW) in Australia has investigated failure modes in heterojunction solar modules (HJTs) with glass backsheet configurations.
“We have identified four failure modes in the glass-sheet silicon heterojunction module, which can cause power loss of up to 50% after wet heat tests,” researcher Chandany Sen explains to pv magazine . "We are trying to understand the possible underlying causes of each failure mode and how to quickly detect them at the cell level."
The scientists conducted their experiment with half-cut n-type silicon bifacial HJT solar cells sourced from industrial production lines from unnamed manufacturers. The products were divided into three groups: modules with encapsulated cells; modules with encapsulated cell precursors; and non-encapsulated cells. In the first two groups, the researchers used an ethyl vinyl acetate (EVA) encapsulant.
“All samples had an n-type wafer, intrinsic hydrogenated amorphous silicon (ia-Si:H) passivation layers on both faces, and phosphorus-doped (na-Si:H) and boron-doped (na-Si:H) hydrogenated amorphous silicon ( pa-Si:H) on the front and back faces, respectively, followed by an indium-doped tin oxide (ITO) layer deposited on both faces.
All devices were subjected to a humid heat test at 85°C and 85% relative humidity for periods of between 500 and 4,000 hours.
Through these tests, the academics identified four failure modes in encapsulated cells, causing power losses ranging from 5% to 50%. The first type of failure consisted of the obscuration of the cells in localized points, and the second was represented by the obscuration around the interconnection of busbars and ribbon cables. The third type of failure consisted of intense darkening between the interconnected regions of the busbars and ribbon cables, while the fourth showed darkening in the interconnected region of the busbars and ribbon cables.
According to the groups analysis, the first type of failure was attributable to surface contamination, which could have occurred during handling or characterization prior to module potting. As for the second and third groups, the scientists attributed the failures to the involvement of the solder flux.
“The direct impact of flux and lead (Pb) solder on contact degradation after DH testing was also observed in other work,” they noted. "It is essential to note that in some cases, the use of a different Ag paste also caused a type 3 failure."
As for the fourth group, the researchers indicated that the failures were likely caused by the EVA by-product used for encapsulation.
“Although the experimental design of this work cannot determine exactly how each failure mode occurred after the DH tests, it demonstrates plausible scenarios that could occur in the industrial environment and lead to the actual observed failure modes,” they concluded.
Sen said that understanding and mitigating these failure modes, preferably at the solar cell level, will be crucial to realizing the levelized low cost of electricity (LCOE) potential of HJT. He said that although glass modules are often used for HJT solar cells due to their lower susceptibility to moisture ingress, similar failures are likely to occur in these modules over longer periods.
The research team presented their findings in the study " Four failure modes in silicon heterojunction glass-backsheet modules ," published in Solar Energy Materials and Solar Cells. |
