The nature of screen printed front side silver contacts - results of the Project MikroSol

The joint research project MikroSol focuses on electrical and microstructural properties of screen printed front side silver contacts. Special attention is given to the contact resistance of the contacts as well as metallization- induced recombination losses in the space charge region of the solar cell. Differently processed industrial solar cells were characterized by electrical measurements as well as analytical electron microscopy including quantitative chemical analyses. The microstructure of these contacts was studied over a large range of length scales from several tens of microns down to a few nanometers. Distinct differences were found for the specific contact resistance depending on the screen printing paste. Microstructural analyses revealed a continuous wetting layer for pastes yielding lower contact resistances. Focused ion beam tomography confirmed the presence of a continuous wetting layer over distances of several microns and revealed new insights about the morphology of front side silver contacts on alkaline textured surfaces. Furthermore, varying densities of nano silver colloids were identified in the continuous glass layer and these colloids were studied by (high-resolution) electron microscopy.