Researchers at UNSW studied TOPCon solar modules to find that they degrade under prolonged damp heat conditions
The modules were tested under damp heat at 85°C and 85% relative humidity for up to 2,000 hours
Magnesium additives in white EVA encapsulants were identified as the key driver, triggering corrosion of the SiNx passivation layer
As TOPCon technology continues to gain market share as the leading high-efficiency silicon cell architecture, attention is increasingly shifting from efficiency gains to long-term reliability under field-relevant stress conditions. While earlier studies linked damp-heat degradation in TOPCon modules primarily to metallization corrosion and rising series resistance, new evidence suggests that additional, previously unreported failure pathways may become critical as cell architectures evolve.
Researchers from the University of New South Wales (UNSW) investigated the impact of prolonged damp-heat exposure on bifacial n-type TOPCon modules fabricated using laser-assisted fired contacts. The study is titled "A novel damp heat-induced failure mechanism in PV modules" (with case study in TOPCon)", with Bram Hoex as the corresponding author.
The research focused on glass-backsheet modules with different encapsulation materials, including EVA, POE, and EVA-POE-EVA (EPE) stacks. Bifacial n-type TOPCon modules based on 182 mm wafers were used for the study. The cells used a boron-doped front emitter and passivating contact structures on both sides. Silver grid metallization was applied by screen printing. The modules were subjected to damp-heat testing at 85°C and 85% relative humidity for up to 2,000 hours.
As per the results, maximum power losses ranged from approximately 6% to 16%, depending on the bill of materials. The modules encapsulated with POE on both sides showed the lowest degradation, while configurations with white EVA on the rear side experienced significantly higher power losses.
The analysis showed that degradation was dominated by a reduction in Voc, rather than by an increase in series resistance. Short-circuit current was largely unaffected, suggesting intact light absorption. Further measurements indicated increased recombination, primarily at the rear side of the cell.
To identify the root cause, the researchers used TOF-SIMS, TEM-EDS, and ICP-MS analyses. These showed magnesium-rich species in the rear-side SiNx passivation layer, particularly in modules using white EVA encapsulants. The analysis indicated that magnesium oxide in white EVA hydrates under damp heat, creating an alkaline local environment that accelerates corrosion of the SiNx layer.
This corrosion compromises rear-side passivation, allowing moisture and oxygen to penetrate deeper into the contact stack. The resulting depassivation and formation of localized pinhole-like defects increase rear-side saturation current and drive the observed Voc losses. In contrast, POE-based encapsulants showed significantly lower magnesium content and better stability under the same test conditions.
The study reports this Voc-driven degradation mechanism at the module level for the first time in TOPCon technology, a departure from earlier damp heat failure models that emphasized metallization corrosion alone.