A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the manifold requirements during processing and application of such complex fluids. In this investigation, measuring protocols for a conclusive assessment of different process relevant rheological parameters have been evaluated. This includes the determination of yield stress, viscosity, wall slip velocity, structural recovery after large deformation and elongation at break as well as tensile force during filament stretching.The importance of concomitant video recordings during parallel-plate rotational rheometry for a significant determination of rheological quantities is demonstrated. The deformation profile and flow field at the sample edge can be determined using appropriate markers. Thus, measurement parameter settings and plate roughness values can be identified for which yield stress and viscosity measurements are possible. Slip velocity can be measured directly and measuring conditions at which plug flow, shear banding or sample spillover occur can be identified clearly. Video recordings further confirm that the change in shear moduli observed during three stage oscillatory shear tests with small deformation amplitude in stage I and III but large oscillation amplitude in stage II can be directly attributed to structural break down and recovery. For the pastes investigated here, the degree of irreversible, shear-induced structural change increases with increasing deformation amplitude in stage II until a saturation is reached at deformations corresponding to the crossover of G' and G'', but the irreversible damage is independent of the duration of large amplitude shear. A capillary breakup elongational rheometer and a tensile tester have been used to characterize deformation and breakup behavior of highly filled pastes in uniaxial elongation. Significant differences were observed in all experiments described above for two commercial screen-printing silver pastes used for front side metallization of Si-solar cells.

Further strong growth of solar energy conversion based on PV (photovoltaic) technology requires constant improvement to increase solar cell efficiency. The challenge in front-side metallization of Si-solar cells is to print uniform fine lines with a high aspect ratio to achieve higher efficiencies simultaneously with a reduced consumption of raw materials. An in-depth understanding of the relationship between paste composition, rheology and screen-printed line morphology is essential. Three model pastes with similar silver content and corresponding vehicles differing in their thixotropic agent content were investigated. Rheological properties (yield stress, viscosity, wall slip velocity, structural recovery, and fracture strain) were determined using steady and oscillatory shear, as well as elongational flow rheometry. Pastes were screen-printed at various speeds through a layout screen including line widths between 20 and 55 µm. Dried fingers were analyzed with respect to line width, aspect ratio (AR) and cross-sectional area. Our investigations reveal that minor changes of thixotropic agent result in substantial variations of the paste’s flow properties. However, this only weakly affects the line morphology. Irrespective of printing speed or finger opening, AR is slightly increasing; i.e., the screen-printing process is robust against changes in paste rheology.

We present a versatile, cost-effective formulation platform for highly conductive silver pastes used in front-side metallization of silicon (Si) solar cells. Pastes based on the capillary suspension concept include silver particles, glass frit and two immiscible fluids. Capillary forces inferred from the second fluid added only in small fractions induce the formation of a percolating particle network. This provides extended shelf-life and distinct flow properties adjustable in a wide range as demanded by the respective printing process, thus yielding residual-free sintered electrodes. Si-wafers are successfully metallized with such pastes using conventional screen-printing, knotless screen and Pattern Transfer Printing™. Paste spreading is studied via high-speed imaging during screenprinting on glass plates. Morphology of printed lines is analyzed using laser scanning microscopy. Electrical properties of the cells are characterized employing a solar simulator and electroluminescence spectroscopy. Results are compared to those obtained using commercial pastes including the same silver particles and glass frits. Paste performance strongly depends on the selected secondary fluid. Aspect ratios ≈0.4-0.5 can be reached and cell efficiencies η eff ≈ 21% on Cz-and 18.6% on mc Si-wafers are obtained. Additional investigations are necessary to further reduce paste spreading and line interruptions thus improving cell performance.

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