Rheology and fine-line screen printing of solar cell front-side metallization pastes – What really matters

Abstract

Current solar research focuses on reducing silver consumption in order to enable the large-scale increase in PV installations needed to address climate change and the growing global energy demand. This study explores how rheological properties of metallization pastes, such as yield stress, high shear viscosity, slip stress, and slip velocity, affect fine-line screen printing outcomes, including line width, height, uniformity, and interruptions. A model system based on capillary suspensions using Texanol as bulk liquid and employing four secondary liquids with varying interfacial tension was developed for systematic paste property variations. The resulting capillary suspensions exhibited a yield stress and pronounced wall slip below the yield stress, with the slip layer consisting of the bulk liquid. Particle volume fraction, secondary liquid to particle ratio, and type of secondary liquid were varied to systematically modify yield stress, high shear viscosity, and slip velocity of the pastes. Printing experiments revealed that higher yield stress reduced spreading but maintained similar paste laydown. High shear viscosity was identified as critical for controlling paste transfer and achieving narrow line widths. Excessive slip caused inhomogeneous line morphology and reduced paste transfer, indicating an optimal slip range for high-quality printing. These findings were applied to develop a metallization paste achieving a 24 μm line width and 0.48 aspect ratio, achieving an efficiency of 23.53 % for PERC cells with 16 % lower laydown than commercial pastes. This study highlights the importance of yield stress, viscosity, and slip in screen printing, aiding future paste development to reduce silver consumption in photovoltaics.