Rubrene single crystals can serve as a model material platform for studying the intrinsic photophysical processes in organic semiconductors and advance our understanding of material functionality in organic photovoltaic applications. The high degrees of structural order and material purity of organic single crystals enable a level of study that is unattainable in materials of current practical importance. Here, the photovoltaic effect at the Schottky interface of rubrene single crystal–aluminum electrode is demonstrated in a lateral ITO–rubrene–Al device geometry. The mechanism of the effect formation is explained based on the reconstructed energy band diagram of the ITO–rubrene–Al heterostructure. In particular, the open circuit voltage (VOC) of the devices shows a strong dependency on the interfacial band bending and corresponding built-in potential at the rubrene–Al Schottky interface. Initially, the photovoltage is found to be equal to the built-in potential at the Schottky interface defined by the work function difference between the bulk of rubrene and the Al electrode, that is, following the Schottky–Mott model. A good agreement is found between the systematically varied built-in potential and the resulting photovoltage magnitude upon insertion of an ultrathin LiF interlayer between the rubrene and Al electrode.