Researchers from the East China University of Science and Technology have investigated degradation mechanisms in low-silver electrodes used in heterojunction (HJT) solar cells, with the aim of developing design guidelines for manufacturing cost-competitive and high-efficiency photovoltaic modules.
“Our study systematically investigates the thermal aging behavior of silver (Ag)-coated copper (CU) electrodes in heter solar cells, revealing significant increases in contact resistance due to interdiffusion between the Ag and Cu layers,” corresponding author Xiaojun Ye told pv magazine. “It clarifies the underlying degradation mechanisms and provides important guidance for the design of cost-effective and reliable metallization strategies for HJT solar modules.”
The research was based on the premise that the thermal aging behavior of silver-coated copper electrodes is still not fully understood, particularly for thin Ag shells used in commercial pastes. With this in mind, the scientists investigated, in particular, degradation under accelerated aging, linking microstructural evolution and interdiffusion to electrical performance and long-term reliability.
For their experiments, they used a silver-coated copper paste composed of core–shell particles, submicron silver powder, and an epoxy resin matrix, with particles sized 2–4 μm and an around 70 nm Ag shell. It was screen-printed onto n-type monocrystalline silicon wafers, followed by drying at 150 C and curing at 195 C.
The electrical performance of silver-coated copper electrodes was assessed through the transmission line method (TLM), which is a standard technique used to measure electrical properties of semiconductor contacts. In a typical TLM measurement, a set of metal contacts is fabricated on the surface of a sample with varying distances between them. By measuring the total resistance between different contact pairs and analyzing how this resistance changes with spacing, it becomes possible to separate and quantify key parameters such as contact resistivity and sheet resistance.
This approach is especially valuable in solar cell research because it provides a direct way to assess how contact quality changes under different processing conditions or thermal aging, helping researchers understand degradation mechanisms at the interface. It is primarily used to determine line resistance (Rline) – the electrical resistance along a printed or deposited metal line – and contact resistivity (ρc) – how easily current flows across the metal–semiconductor interface
The analysis showed that both Rline and ρc increase with aging time, with a strong dependence on temperature. Moreover, contact resistivity was found to be significantly more sensitive to temperature than Rline, indicating that interfacial degradation is the dominant factor in electrical failure.
Through energy-dispersive X-ray spectroscopy (EDS), focused ion deam–scanning electron microscopy (FIB-SEM), and X-ray Diffraction(XRD) the researchers confirmed that degradation is driven primarily by Ag–Cu interdiffusion and defect formation.
Overall, the research team concluded that the electrical behavior is governed by two competing processes: on one hand, sintering improves particle-to-particle contact and temporarily enhances electrical connectivity; on the other hand, interdiffusion between Ag and Cu, along with defect formation, progressively damages the internal structure.
As aging progresses, the second process overtakes the first, with the originally continuous conductive network breaking apart into isolated and poorly connected pathways, thus forcing electrons to travel through a more tortuous, discontinuous structure. This transition from a well-connected network to a fragmented transport regime is what ultimately drives severe long-term electrical degradation.
“Theoretical analysis indicates that, under practical operating conditions, interfacial diffusion and void evolution are the primary factors governing long-term reliability. Therefore, enhancing interfacial stability is critical for improving device durability,” the academics emphasized.
The research work was presented in the paper “Thermal aging-induced interdiffusion and reliability degradation in low-silver electrodes for SHJ solar cells,” published in Solar Energy Materials and Solar Cells. “Our findings offer critical insights for balancing silver reduction with long-term module reliability in HJT technology.”
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