Rethinking silver paste design for TOPCon, LECO solar cells

As manufacturing processes such as conventional TOPCon and laser-enhanced contact optimization (LECO) continue to push cell efficiencies beyond 26%, the demands placed on front-side metallization pastes are becoming increasingly complex. Fine-line silver pastes must now reconcile competing requirements in rheology, etching chemistry, and sintering behavior, often with technology-specific constraints that are difficult to capture using conventional evaluation methods.

Against this backdrop, researchers from Northwest University in China have proposed a differentiated material design strategy combined with an objective evaluation framework that directly links laboratory characterization to production-line performance.

“To address current limitations of conventional evaluation approaches, we introduced three novel rheological parameters designed to more accurately capture paste behavior under realistic processing conditions,” the research’s lead author, Lin Bao, told pv magazine. “Building on this, a hybrid evaluation framework combining the analytic hierarchy arocess (AHP) and the entropy weight method (EWM) was developed to overcome the inherent shortcomings of single-method weighting strategies.”

The AHP determines indicator weights based on expert-driven pairwise comparisons, translating qualitative judgments into a structured hierarchical scoring system. The EWM, in contrast, assigns weights objectively according to the degree of data variation, reducing subjectivity by emphasizing information contained in the measurements.

“In addition, our work establishes a clear link between laboratory-scale characterization and production-line performance, enabling more reliable translation of experimental results into industrial outcomes,” Bao added. “Finally, by covering both mainstream TOPCon and emerging LECO photovoltaic technologies, the proposed evaluation system is extended to a broader range of applications, enhancing its general applicability in advanced solar cell metallization development.”

In the study “Based on silver/glass frit screening & AHP-EWM evaluation: Performance and production verification of TOPCon/LECO photovoltaic fine-grid silver pastes,” published in Solar Energy Materials and Solar Cells, the research team explained that TOPCon and LECO architectures require fundamentally different glass-frit behaviors.

For TOPCon, low-softening-point glass frits are essential to enable early softening and chemical etching of the silicon nitride (SiNx) layer, ensuring low-contact resistance. In contrast, LECO relies on high-softening-point frits, where laser-assisted SiNx ablation defines the contact interface, meaning the glass phase primarily serves adhesion and wetting functions rather than aggressive etching.

Through multi-technique screening like scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and contact-angle measurements the researchers selected suitable materials for further formulation.

This screening led to the identification of high-activity silver powders, namely Dowa4-8F, H1-2, and YZ17N1, as well as matched glass systems, namely TG1 for TOPCon applications and LG1 for LECO applications. Together, these selections enabled the development of process-specific silver paste formulations optimized for the distinct requirements of each cell architecture.

The researchers explained that the conventional assessment of silver pastes relies heavily on the three-interval thixotropy test (3ITT), which does not fully capture real production-line behavior.

To address this, they introduced three dynamic rheology descriptors – relative viscosity, relative recovery rate, and T50 – to better describe structural recovery during the critical 10–20 s transition between printing and sintering. These parameters revealed clear formulation-dependent differences, with one type of LECO paste showing rapid early recovery, while a TOPCon paste exhibited slower but more gradual rebuilding that 3ITT alone could not fully interpret.

In practical printing tests, the results partially diverged from rheology-based predictions. Although 3ITT analysis indicated an optimal formulation, the TOPCon pastes showed better line uniformity in practice, with a specific TOPCon formulation exhibiting lower aspect ratio and higher surface roughness. White-light interferometry confirmed roughness variations ranging from below 0.4 μm to over 1.0 μm, highlighting that rheology alone is insufficient to accurately predict printability.

Sintering and electrical characterization further revealed that pastes formulated with Dowa4-8F silver powder achieved the most compact microstructure, whereas higher porosity was associated with increased resistivity. Overall, LECO pastes demonstrated lower resistivity due to improved particle connectivity.

The AHP–EWM model integrated these multi-domain results and showed that one TOPCon formulation achieved the highest score among its group (0.617), while the LECO formulation ranked highest overall (0.908). These rankings were consistent with production-line verification, where the best-performing TOPCon paste improved open-circuit voltage to 10.47 V and the LECO paste achieved 26.7% cell efficiency with stable electroluminescence imaging.

Overall, the study demonstrates that reliable silver paste development requires moving beyond single-method evaluation toward integrated, data-driven frameworks that effectively link laboratory characterization with industrial performance.

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