Contact formation with silver (Ag) thick film pastes on boron emitters of n-type crystalline silicon (Si) solar cells is a nontrivial technological task. Low contact resistances are up to present only achieved with the addition of aluminium (Al) to the paste. During contact formation, Al assisted spiking from the paste into the silicon emitter and bulk occurs, thus leading to a low contact resistance but also to a deterioration of other cell parameters. Both effects are coupled and can be adjusted by choosing proper Al contents of the paste and temperatures for contact formation. In this work the microscopic electric properties of single spikes are presented. These microscopic results, i.e. alterations of the local emitter doping density, the pronounced local recombination activity at the interface between spikes and Si and its influence on the charge collection efficiency, are used to explain the observed dependencies of global cell parameters on the Al content of contact pastes. (C) 2014 Elsevier B.V. All rights reserved.