The effect of injection and extraction barriers on flat heterojunction (FHJ) and bulk heterojunction (BHJ) organic solar cells is analyzed. The barriers are realized by a combination of p-type materials with HOMOs varying between -5.0 and -5.6 eV as hole-transport layer (HTL) and as donor in vacuum-evaporated multilayer p-i-metal small-molecule solar cells. The HTL/donor interface can be seen as a model for the influence of contacts in organic solar cells in general. Using drift-diffusion simulations we are well able to reproduce and explain the experimental I-V curves qualitatively. In FHJ solar cells the open-circuit voltage (V-oc) is determined by the donor and is independent of the HTL. In BHJ solar cells, however, V-oc decreases if injection barriers are present. This different behavior is caused by a blocking of the charge carriers at a spatially localized donor/acceptor heterojunction, which is only present in the FHJ solar cells. The forward current is dominated by the choice of HTL. An energy mismatch in the HOMOs leads to kinks in the I-V curves in the cases for which V-oc is independent of the HTL.