The electronic structure of rubrene doped with various concentrations of Na was studied by synchrotronradiation photoemission. Three stages of development were found with increasing Na concentration; Na penetrating deep into the organic film, followed by development of gap states, and ended with a metallic Na film. The charge transfer from Na to rubrene resulted in a vacuum-level shift. By doping Na into rubrene, we could control the IP of the organic molecule, which is favorable for application in organic semiconductor devices. (C) 2015 Elsevier B.V. All rights reserved.