Researchers at Germany’s Forschungszentrum Jülich GmbH have found that heterojunction (HJT) solar cells using aluminum-doped zinc oxide (AZO) as an alternative to indium tin oxide (ITO) for the transparent conductive oxide (TCO) layer are more vulnerable to damp-heat conditions, potentially raising concerns about their long-term stability and reliability in humid environments.
“AZO grows in a polycrystalline structure that favors the rapid penetration of water molecules into the crystal boundaries, which leads to the degradation of the material’s electrical properties,” the scientists said, noting that moisture can reduce conductivity and optical transparency. “Therefore, in this study, we thoroughly investigated the changes in the optoelectronic properties and surface morphology of AZO-incorporated HJT solar cells under damp heat environmental conditions, comparing them with ITO references.”
The researchers fabricated bifacial HJT solar cells with either 70-nm ITO or AZO layers on both the front and rear sides. They also deposited standalone ITO and AZO films on glass substrates to investigate changes in the transparent conductive oxides independently from the cells.
The researchers integrated then the cells into 210 mm × 210 mm single-cell mini-modules. They investigated two module designs: a conventional glass/glass configuration and a lightweight front-sheet/back-sheet structure featuring an ethylene tetrafluoroethylene (ETFE) front sheet. The lightweight modules paired the ETFE front sheet with an aluminum-containing polyolefin back sheet, whereas the conventional modules featured 3.2-mm glass sheets on both sides.
Both module designs were fabricated with either ITO- or AZO-based SHJ cells, with two samples prepared for each configuration. All samples underwent accelerated damp-heat aging at 85 C and 85% relative humidity for 1,000 hours, in accordance with the IEC 61215 standard. Electrical performance and electroluminescence images were assessed every 200 hours, alongside external quantum efficiency and reflectance measurements.
The test results showed that, after 1,000 hours of damp-heat exposure, the bare AZO-based HJT cell showed a 16.38% relative decline in efficiency, driven primarily by a 9.84% relative reduction in fill factor, which was closely associated with rising series resistance (Rs). By comparison, the ITO-based cell proved considerably more stable, recording an efficiency loss of just 2.80% relative over the same testing period.
Electroluminescence (EL) imaging revealed pronounced defects in the AZO-based lightweight modules, attributed mainly to moisture penetration and corrosion of the AZO layer. By comparison, no equivalent defects appeared in AZO glass/glass modules or in the ITO-based devices, indicating better moisture protection from glass/glass encapsulation and greater intrinsic moisture resistance from ITO.
Chemical analysis also found increased hydroxyl-related species in AZO following damp-heat exposure, supporting the hypothesis of moisture-induced chemical modification. Scanning electron microscopy (SEM) imaging provided further evidence, revealing corrosion-related grooves on AZO surfaces after aging, while ITO maintained a uniform and dense structure.
To address this vulnerability, the researchers deposited a 110-nm magnesium fluoride (MgF₂) capping layer on the front surface of AZO-based cells.
With the MgF₂ layer, efficiency degradation was limited to about 7% after 1,000 hours, and electroluminescence defects were substantially reduced compared with uncapped AZO cells.
The researchers concluded that MgF₂ can retard moisture penetration and corrosion, although it does not provide a complete moisture barrier and AZO-based cells still remained less stable than the ITO reference.
“This comprehensive investigation provides substantial insight into the degradation mechanism of AZO-incorporated SHJ solar cells and modules under damp heat conditions, offering practical strategies to improve their durability and performance,” they concluded.
Their findings are available in “Unveiling the damp-heat-induced degradation mechanism of AZO-incorporated silicon heterojunction solar cells and modules,” published in Solar Energy Materials and Solar Cells.
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