Picosecond laser sources can ablate Si nearly without melting and hence are a promising alternative for pn-junction isolation. In this work the shunt resistance and microstructure obtained by a new high-power picosecond laser source are compared with that of a standard nanosecond laser. As a result, the picosecond laser creates a threefold higher median shunt resistance at a high number of passes. The observed microstructure is a strong function of the laser fluence. In the thermal regime, i. e. at laser fluences above 1.1 J/cm², the n+-diffused layer directly evaporates and the newly created surface is relatively smooth. In the non-thermal regime at laser fluences below 1.1 J/cm² a very rough microstructure arises and porous SiOx is formed. It is assumed that the SiOx decreases the conductivity of the topmost Si layer in the laser groove. In both regimes high shunt resistances can be obtained as confirmed by infrared imaging under reverse bias. In contrast, for cells edge-isolated by means of the nanosecond-laser parasitic shunting is observed.