The research introduces a nitrate-based aging method that better reflects the mildly acidic environment inside EVA-based modules
Degradation is strongly pH-dependent, with fill factor loss driven by increased contact resistance
Low-Al (LAF) contacts show improved stability under realistic conditions, while Ag/Al remains prone to aluminum-driven degradation
TOPCon technology continues to dominate industrial PV manufacturing. However, ensuring long-term reliability under damp-heat conditions remains a challenge, especially as manufacturers move toward lower-cost bill-of-materials (BOM) designs.
A study led by the University of New South Wales in collaboration with Jolywood examines a key limitation in current reliability testing approaches. The paper, titled Bridging accelerated cell-level degradation to module-relevant failure mechanisms in TOPCon solar cells and modules, examines the limitations of conventional accelerated aging methods. These methods, such as acetic acid soaking, impose chemically unrealistic conditions and often fail to reproduce degradation patterns observed at the module level.
To overcome this, the researchers developed a nitrate-based aging approach. The method applies controlled-pH nitrate solutions selectively to the front surface of TOPCon cells, followed by damp-heat exposure at 85°C and 85% relative humidity. This setup aims to better replicate the mildly acidic environment inside EVA-encapsulated modules, where the internal pH has been reported to be close to about 5.
The study compares 2 industrially relevant front-side metallization schemes. One uses conventional Ag/Al paste, while the other employs a low-Al Ag paste with laser-assisted firing (LAF), often referred to as LECO. These 2 approaches differ significantly in their contact structure and degradation pathways.
Results show a clear dependence of degradation on solution acidity. As the pH decreases, both cell types experience increasing series resistance and contact resistivity. This leads to a pronounced reduction in fill factor (FF), which emerges as the dominant contributor to power loss. Under highly acidic nitrate conditions, degradation becomes severe, especially for LAF-based contacts.
Under mildly acidic conditions, closer to real module environments, the degradation trends are similar to those seen in module-level damp-heat tests. The research shows that the nitrate-based method gives a more realistic view of degradation than traditional soaking tests.
The mechanisms differ between the 2 contact types. In Ag/Al cells, degradation happens due to glass-frit corrosion and aluminum oxidation at the contact. Aluminum is reactive and makes degradation faster, even in mild environments. LAF-based contacts contain minimal aluminum and rely on localized Ag-Si contact formation. These contacts show higher sensitivity to acidic environments. Corrosion of the thin glass-frit layer weakens contact adhesion and increases resistance, especially under stronger acidic conditions.
The study also highlights the role of different ionic species. While nitrate-based solutions primarily influence degradation through pH control, chloride-based contaminants, such as NaCl, introduce more aggressive degradation. Chloride ions interact directly with silver, forming Ag-Cl complexes and disrupting conductive pathways. This leads to significantly greater contact degradation than nitrate exposure.
By bridging cell-level testing with module-relevant degradation behavior, the proposed approach provides a more physically meaningful framework for reliability assessment. It enables targeted evaluation of front-side metallization stability while avoiding the limitations of conventional bulk soaking methods.
The TaiyangNews Market Survey on Solar Cell Production Equipment 2025 highlights the growing role of process conditions and equipment in determining cell performance and reliability. It notes that metallization and process integration remain critical, especially for TOPCon, where contact stability directly impacts module-level behavior.