Paste Rheology Correlating With Dispensed Finger Geometry

Dispensing technology in crystalline silicon solar cell metallization offers the possibility to improve contact finger geometries compared with the dominating screen-printing approach. However, an optimum contact formation strongly depends on paste rheology and process parameters. In this study, a method is introduced to predetermine dispensed contact geometries by extracting specific rheological parameters from oscillating strain sweeps, allowing for paste optimization prior to printing tests. For this reason, four different dispensing pastes were processed and compared with a commercially available screen-printing paste. Oscillating strain sweeps were conducted with all pastes, in order to determine characteristic values for complex shear modulus G* and yield stress tau(y). In the following, p-type Cz-Si solar cells were processed, which were equipped with dispensed contact fingers using the four rheologically investigated dispensing pastes and compared with samples with an all screen-printed front-side grid. Effects like paste spreading or necking could be isolated and referred to the different pastes and printing processes. Various form parameters that describe specific characteristics of resulting finger geometries were introduced. The elastic part G' of the complex shear modulus G* was found to have the major impact on resulting finger widths, aspect ratios, and a macroscopic contact angle between the finger and the wafer surface.