By using electrical characterization and classical solid state semiconductor device theory, we demonstrate that the open circuit voltage (V_oc) in organic solar cells based on non-intentional doped semiconductors is fundamentally limited by the built-in potential (V_bi) originated at a donor-acceptor abrupt (p-n⁺⁺) heterojunction in case of selective contacts. Our analysis is validated using P3HT:PCBM devices fabricated in our research group. We also demonstrate that such a result can be generalized using data already reported in literature for fullerene-based solar cells. Finally, we show that the dependence of V_oc on the device contacts can be understood in terms of the potential barriers formed by the Fermi level alignment of semiconductors at the heterojunction and at the Schottky junctions.