Front side metallization of silicon solar cells – A high-speed video imaging analysis of the screen printing process

Abstract

Aiming at higher cell efficiency and lower Ag consumption, research focuses on achieving smaller finger lines for silicon solar cell front side metallization. In-depth understanding of the printing process is mandatory to achieve uniform, defect free lines with high aspect ratio. An innovative high-speed video imaging setup allows for tracking of the mechanisms happening within ≈10–50 ms during the screen printing process. Two pastes differing significantly with respect to their rheological as well as wetting properties are used in this study. A paste spreading mechanism consisting of different consecutive process steps determines the final line shape. During nip contact the paste is squeezed between screen emulsion and substrate. Paste spreading further proceeds as long as the screen clings to the substrate, it occurs in a regular pattern and is most pronounced in the regions were mesh wires approach the emulsion surface. Then sudden retraction takes place during snap-off and the final line width is essentially fixed. Spreading is more pronounced for the paste exhibiting low viscosity at high shear rates and strong wetting of screen emulsion and substrate. The overall paste transfer is promoted by poor screen wetting and strong paste cohesion. These insights will stimulate further improvement of Ag paste formulation for solar cell metallization.