Researchers at Forschungszentrum Jülich and RWTH Aachen University investigated the damp-heat degradation of AZO-based HJT cells and mini-modules
Unencapsulated AZO-based HJT cells lost 16.38% of their initial efficiency after 1,000 hours, compared with 2.80% for ITO-based cells
Applying a 110 nm MgF₂ capping layer limited the relative efficiency loss of AZO-based cells to approximately 7%
Indium is a key process consumable in HJT cell production. Its limited availability could drive up costs if demand increases significantly as HJT manufacturing expands. Reducing or eliminating indium from transparent conductive oxide (TCO) layers is therefore an important focus area for the industry. Aluminum-doped zinc oxide (AZO) is being evaluated as an alternative to indium tin oxide (ITO), but its sensitivity to moisture remains a concern.
Researchers at Forschungszentrum Jülich and RWTH Aachen University investigated the damp-heat degradation of HJT cells and mini-modules incorporating AZO. They compared the results with reference devices using ITO. They also tested a magnesium fluoride (MgF₂) capping layer to protect AZO against moisture.
The study used HJT cells with either 70 nm ITO or 70 nm AZO layers on the front and rear. The cells were also integrated into 210 × 210 mm single-cell mini-modules produced in glass/glass and lightweight front sheet/backsheet configurations. The cells and mini-modules underwent damp-heat testing at 85°C and 85% relative humidity for 1,000 hours, following IEC 61215 test conditions.
After 1,000 hours of damp-heat exposure, the unencapsulated AZO-based HJT cell recorded a 16.38% relative efficiency loss, compared with 2.80% for the ITO reference. The AZO cell’s fill factor declined by 9.84% relative, mainly because of increased series resistance. At the module level, both glass/glass mini-modules retained more than 95% of their initial efficiency. However, the AZO-based lightweight front sheet/backsheet mini-module lost 52.36% of its initial efficiency, considerably more than the equivalent ITO-based module. The researchers attributed this severe degradation to moisture-induced deterioration of AZO together with interconnection-foil delamination and structural damage to the AZO layer.
Tests on standalone TCO films showed that damp-heat exposure reduced AZO’s electrical conductivity by lowering the number and mobility of charge carriers. The researchers also found moisture-related chemical changes and grooves on the AZO surface, indicating corrosion. The ITO films showed no comparable changes.
In a separate cell-level experiment, applying a 110 nm MgF₂ capping layer reduced the efficiency loss of the AZO-based cell to approximately 7%. The capped cell also developed fewer electroluminescence defects than the uncapped AZO cell.
The MgF₂ layer increased the initial current of the unencapsulated cell through its antireflection function. However, this optical benefit would be lower after encapsulation. The researchers therefore identified MgF₂ primarily as a layer that slows moisture diffusion rather than a complete moisture barrier.
The findings were published in the research paper titled Unveiling the Damp-Heat-Induced Degradation Mechanism of AZO-Incorporated Silicon Heterojunction Solar Cells and Modules in Solar Energy Materials and Solar Cells.
