A novel damp heat-induced failure mechanism in PV modules (with case study in TOPCon)

Silicon solar technology continues to dominate the market, with Tunnel Oxide Passivated Contact (TOPCon) technology leading in efficiency. However, as devices approach fundamental performance limits, new failure modes may emerge or existing ones may become more critical, and their long-term reliability remains insufficiently understood. This study investigates the effect of damp heat (DH) exposure on bifacial n-type TOPCon modules with laser-assisted fired contacts, utilising different encapsulants: EVA, POE, and EVA-POE-EVA (EPE). After 2000 h of DH testing, modules showed P max losses ranging from ∼6% rel to ∼16% rel , primarily due to reduced V oc caused by increased rear-side recombination. Modules encapsulated with POE on both sides degraded least (∼8% rel ), while those using white EVA on the rear side suffered higher losses, especially when combined with EPE on the front (∼16% rel ). Material analyses revealed a degradation pathway driven by magnesium (Mg) additives in the white EVA. Under DH exposure, Mg hydrates and generates an alkaline micro-environment that corrodes the SiN x :H layer, facilitating moisture ingress in the poly-Si and SiO x layers. This enhances interfacial hydrogen concentration, leading to depassivation and Mg-rich shunting defects, thereby increasing J 0, rear and reducing V oc . These findings underscore the need to control encapsulant composition by limiting Mg in white EVA and improving cell passivation. The minimodules studied here were specifically fabricated R&D purposes to probe humidity-induced degradation pathways. Through an in-depth understanding of this mechanism and thorough optimisation of cell and encapsulant design, effective mitigation strategies have been integrated upstream of module production, substantially eliminating the risk in commercial modules. • The maximum power (P max ) losses of TOPCon modules ranged from ∼6% rel to ∼16% rel after 2000 h of damp heat testing. • The primary degradation mechanism is a reduction in open-circuit voltage (V oc ) due to increased rear-sided recombination. • Magnesium (Mg)-driven corrosion of SiN x :H was identified as a newly recognised degradation pathway. • Mg hydration formed alkaline micro-environments, causing SiN x :H corrosion and depassivation. • Findings highlight the need for Mg control in white EVA and optimised cell passivation.