Recently, the charge carrier transport mechanism of passivating contacts, which feature an ultra-thin oxide layer, has been investigated by studying temperature-dependent current-voltage (I-V) characteristics. The measurement revealed that tunneling is the dominant transport path for tunnel oxide passivated contact (TOPCon) with wet chemically grown oxide layer. Furthermore, higher annealing temperatures led to the deterioration of the surface passivation most likely because of excessive pinhole formation. In this contribution, we are going to extend the previous study by analyzing other interfacial oxides as well. We will show that extremely low recombination current densities and low contact resistivity values can be achieved by differently processed TOPCon structures, which are characterized by a predominant tunnel transport as well as one where current flow via pinholes likely predominates. Furthermore, an I-V(T) study on solar cells with passivating rear contact reveals that fill factor transitions from a nonlinear to linear behavior when the Si layer turns partially crystalline.