Hidden traces: Insights into how solar cell handling drives damp-heat failures in HJT and TOPCon modules

Tunnel oxide passivated contact (TOPCon) is now the dominant photovoltaic technology and heterojunction (HJT) solar cells are expected to rapidly gain market share due to their superior efficiencies. However, both TOPCon and HJT cells remain vulnerable to humidity-induced degradation, especially when encapsulated in front-glass/rear-backsheet configurations. This study investigates the role of “hidden contaminants” in damp heat (DH)-induced degradation in HJT and TOPCon glass-backsheet modules while observing no such effects in passivated emitter and rear cell (PERC) modules that are produced under comparable conditions. “Hidden contaminants” are not deliberately introduced, and their presence is typically only evident after accelerated testing. After 1000 h of DH testing, both TOPCon and HJT cells displayed a high significant sensitivity to “hidden contaminants”, resulting in relative power losses of 10–16 %. Elemental analysis identified sodium (Na), calcium (Ca), magnesium (Mg), chlorine (Cl), and sulphur (S) as likely contributors. These contaminants are hypothesised to originate from (improper) handling during cell or module processing, such as contact with contaminated gloves, cassettes, packaging, or vacuum grippers. Under damp heat testing conditions, ions such as Na⁺, Ca²⁺, Mg²⁺, Cl⁻, and S²⁻ are likely to chemically react with moisture as well as with the passivation layers and/or metal contacts of solar cells. These reactions promote carrier recombination and induce corrosion of the metal contacts, ultimately degrading device performance. To minimise these effects, thorough deionised water cleaning, strict adherence to handling protocols using uncontaminated gloves, cassettes, packaging material, or vacuum grippers, and the exclusive use of clean stages prior to encapsulation are strongly recommended. These findings underscore the relatively high sensitivity of TOPCon and HJT solar cells and thus place critical importance on stringent contamination control to prevent unexpected failure in the field, in particular for module manufacturers with a range of solar cell suppliers.