One of the goals of photovoltaic technology development is to continuously improve the efficiency of solar cells and reduce costs. Among them, one of the requirements of the front metalization process of solar cells is the quality of fine line printing, that is, reducing the width of the grid line from 30μm to 25μm, 20μm, 15μm and increasing the aspect ratio of the grid line, thereby reducing the electrode shading area and Reduce the electrode wire resistance, and ultimately achieve the result of reducing slurry consumption and improving battery efficiency.

Therefore, only the perfect cooperation between the printing process, the screen and the paste can promote high-quality fine-line printing. The adjustment of the positive silver paste formulation needs to consider the requirements of the printing process on the rheology of the paste.

Paste changes during printing

During the printing process, the squeegee pushes the paste forward in the direction parallel to the grid line at a certain angle (45-60 degrees) on the screen, and at the same time exerts downward pressure on the paste. The slurry is squeezed by the squeegee at the mesh holes through the wire mesh and filled into the linear grooves of the glue film layer, while being squeezed onto the surface of the silicon wafer. The slurry has a certain adhesion to the silicon wafer, and when it is separated from the plate, the screen (including the screen and the side wall of the glue film) breaks and deposits on the surface of the silicon wafer. If calculated according to the net distance of 1.6mm and the printing speed of 240mm/s, the printing process at each position is less than 0.05 seconds from filling to leaving the plate.
Schematic diagram of printing process

Slurry formulation design

The formulation design of the slurry should consider how to fill the grooves of the screen, break when leaving the plate, and maintain the shape after leaving the plate.

One of the ideal slurry formulation design ideas is to make the slurry have viscoplasticity, which is specifically expressed as the rheological properties of pseudoplastic fluid that makes the slurry have obvious yield stress.

Different forms of slurry

When the external force exceeds the yield value, the slurry exhibits shear thinning fluidity. Therefore, under the force of the squeegee, the slurry realizes forward tumbling movement, the viscosity is reduced and uniformly mixed, and then is squeezed by the squeegee to fill the screen film layer.

When the external force is lower than the yield value, the slurry only exhibits solid elastic deformation. After the slurry enters the screen film layer, the external force disappears, and the force between the particles in the slurry and the carrier is re-established to restore part of the yield stress. The slurry is only affected by gravity and surface tension on the silicon wafer, and sufficient yield stress causes the slurry to lose fluidity and maintain its linearity. As for the off-board, the high-viscosity slurry is mainly dominated by the boundary slip effect. The boundary slip effect is a characteristic of most high solid content dispersion systems. The combination of special additives in the carrier can further promote the boundary slip effect to obtain a more regular linearity and increase the printing weight.

Rheological adjustment points: Yield value adjustment

Yield stress

The spherical silver powder and glass powder in the positive silver paste account for more than 90% of the total weight and are dispersed in an organic vehicle. It is a multi-phase multi-component suspension dispersion system. The yield stress comes from the contact accumulation between solid particles, between the particles and the carrier, and the forces generated between the organic components such as hydrogen bonds, polar bonds, van der Waals forces and so on. The presence of these forces makes the silver paste components form a stable three-dimensional network structure when standing still. This is also the key to the non-settling of the slurry during storage.

When the external force is greater than the force between the components, the three-dimensional structure is gradually destroyed, and the slurry presents the viscosity of a non-Newtonian fluid. In this state, the slurry will be pressed into the mesh by the mechanical force of the squeegee. In the case of high yield stress, the slurry stops moving, the component structure is rearranged, and part of the yield stress quickly returns to a relatively high value, so that the linear shape is easy to maintain. However, when the yield stress is too high, although the mechanical force of the squeegee is much higher than the yield value, the slurry cannot form a uniform rolling in front of the squeegee.

Influencing factors of yield value

Although the slurry as a whole shows fluidity, the fluidity is uneven locally, and the silver powder and other components that have gathered at the places with poor fluidity are not easy to quickly redisperse. This causes the silver paste to not evenly fill the mesh, which is also one of the main reasons for wire breakage.

The diameter of the positive silver paste spherical silver powder is 1-3um, but the surface morphology and surface area are very different. In a system with a high solid content, many factors will affect the adjustment of the yield value. For example: solid components contact with each other cannot be avoided, silver powders with rough surfaces are hindered from re-dispersion after contact with each other, silver powders with a large specific surface area increase the contact area between components, and the hydrogen bond structure in the organic carrier increases, which will significantly increase the system The yield value.

In short, high-quality printing requires that the components in the paste can be re-dispersed quickly at any time, the relative movement resistance is small, and the partial yield stress can be quickly restored when the external force is lost. The boundary slip effect interferes with the rheological test of the slurry, especially the measurement under high shear rate cannot get real data. Therefore, the relationship between the thixotropic viscosity value and printability such as linearity cannot be accurately established.

The measurement of the yield value is done at a very low shear rate, and proper testing hardware settings are required to eliminate the interference of the boundary slip effect. Therefore, the adjustment of the paste can take the yield value as one of the references to establish the relationship with printability.