A parallel dispensing system for an improved front surface metallization of silicon solar cells

Within this thesis, the dispensing technology was enhanced to a high throughput alternative for industrial front side metallization in silicon (Si) Photovoltaic (PV) production lines. For both, the development of the multi-nozzle print heads by means of computational fluid dynamics (CFD), as well as for the optimization of the geometrical shape of dispensed contact geometries the rheology of the involved printing pastes turned out to be of major importance for the progress of this thesis. With the pendant thread approach, a new method was developed that allows for the extraction of a dynamic extensional yield stress as well as the corresponding surface tension of highly filled pastes (Ca >> 1). For the development of the multi-nozzle print heads by means of CFD, a Herschel-Bulkley (HB)-model of the respective pastes was set up, using data at process relevant high shear rates obtained from capillary rheometry and taking wall slip behaviour into account. The resulting ten nozzle print head allowed for dispensing of contact fingers with a record width of w_f=27µm at high aspect ratios of AR_el=A_f/(w_f2)=0.7 and was further applied for processing of commercially available silver (Ag) sinter pastes on industrial solar cells. Finally, a detailed optical contact characterization on cell level and in the encapsulated state was conducted and several electrical characterization methods applied.