Interfaces play an important role in emerging organic electronic applications. In order to optimize and control the performance in organic devices such as organic solar cells, a comprehensive understanding of the contacts is essential. However, despite the vast progress made, a fundamental theory of the physical processes taking place at the contacts is still lacking. In this work, a numerical device model is used to clarify the effect of imperfect contacts in organic bulk heterojunction solar cells. The effect of increased injection barriers, reduced surface recombination, interfacial minority carrier doping, and traps for majority carriers at the electrodes causing reduced efficiencies is simulated. Two distinctly different underlying mechanisms leading to different S-shaped features are found, both leading to an effective shift of the built-in voltage. In the case of an extraction barrier to majority carriers at the contact, such as reduced surface recombination, the S kink is due to an induced diffusion potential. In the case of interfacial doping or traps, the S kink results from band bending caused by the fixed or trapped space charge. We derive analytical expressions describing the effective reduction of the built-in voltage and the (effective) open-circuit voltage providing means to quantify and distinguish the mechanisms. We show how to experimentally differentiate between these effects and provide tools to extract the relevant physical parameters.