By exploring the origin of the saturation current (J₀), we show that the polymer/cathode interface determines the open circuit voltage (V_oc) in polymer:fullerene bulk heterojunction solar cells. We fabricate and characterize P3HT:PCBM solar cells and compare their electrical characteristics using as reference P3HT/cathode Schottky diodes. We demonstrate with thermionic emission theory that J₀ has the same origin in both devices. Thus, a general barrier-dependent expression for V_oc, valid for non-ohmic cathode contacts is proposed and discussed.