Researchers in China have developed an evaluation framework to better assess fine silver pastes for TOPCon and LECO solar cells. The approach directly links laboratory characterization to production line performance, addressing the limitations of conventional evaluation methods.
While manufacturing processes such as conventional TOPCon and laser-enhanced contact optimization (LECO) continue to push cell efficiency above 26%, the demands placed on front-side metallization pastes are becoming increasingly complex. Fine line silver pastes must now reconcile competing demands in rheology, etching chemistry, and sintering behavior, often with technology-specific limitations 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 the current limitations of conventional evaluation approaches, we introduced three new rheological parameters designed to more accurately capture the paste behavior under realistic processing conditions,” said the study’s lead author, Lin Bao. pv magazine. “Building on this, a hybrid evaluation framework was developed that combines the analytical hierarchy arocess (AHP) and the entropy weight method (EWM) 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, on the other hand, objectively assigns weights based on the degree of data variation, reducing subjectivity by emphasizing the information in the measurements.
“Additionally, our work establishes a clear link between laboratory-scale characterization and production line performance, enabling more reliable translation of experimental results to industrial results,” Bao added. “Finally, by covering both mainstream TOPCon and emerging LECO photovoltaic technologies, the proposed evaluation system is extended to a wider range of applications, thereby increasing its general applicability in the advanced development of solar cell metallization.”
In the study “Based on Silver/Glass Frit Screening and AHP-EWM Evaluation: Performance and Production Verification of TOPCon/LECO Fine Grid Photovoltaic Silver Pastes”, published in Solar energy materials and solar cellsthe research team explained TOPCon and LECO architectures require fundamentally different glass frit behavior.
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 serves primarily for adhesion and wetting functions rather than aggressive etching.
Through multi-technical screening such as 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 highly active silver powders, namely Dowa4-8F, H1-2 and YZ17N1, as well as matching 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 different 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 reflect the real behavior of the production line.
To address this, they introduced three dynamic rheology descriptors – relative viscosity, relative recovery rate and T50 – to better describe the structural recovery during the critical 10- to 20-second 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 showed a slower but more gradual rebuild that 3ITT alone could not fully interpret.
In practical pressure tests, results partially deviated from rheology-based predictions. Although 3ITT analysis indicated an optimal formulation, in practice the TOPCon pastes showed better line uniformity, with a specific TOPCon formulation showing a lower aspect ratio and higher surface roughness. White light interferometry confirmed roughness variations ranging from less than 0.4 μm to more than 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, while higher porosity was associated with increased resistance. Overall, LECO pastes showed lower resistivity due to improved particle connectivity.
The AHP-EWM model integrated these multidomain results and showed that one TOPCon formulation achieved the highest score within its group (0.617), while the LECO formulation achieved the highest score overall (0.908). These rankings were consistent with the on-line verification, with the best performing TOPCon paste improving open-circuit voltage to 10.47 V and the LECO paste achieving 26.7% cell efficiency with stable electroluminescence imaging.
Overall, the study demonstrates that reliable silver paste development requires moving beyond single-method evaluation to integrated, data-driven frameworks that effectively link laboratory characterization to industrial performance.
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