Vacuum level alignment in organic guest-host systems

Using a combination of ultraviolet and x-ray photoelectron spectroscopies, we have studied the relative energy level alignment of two phosphorescent guest molecules, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (PtOEP) and tris(2-phenylpyridine)iridium (Ir(ppy)3), doped in an electron transport host, tris(8-hydroxyquinolinato) aluminum (III) (Alq3), and in a hole transport host, 4,4′-bis(carbazol-9-yl)biphenyl. In each of the guest–host systems, we find that the vacuum levels of the guest and the host molecules align, and that the position of the highest occupied molecular orbital of the phosphorescent guest is independent of the guest molecule concentration (0.8%–56% by mass). The vacuum level alignment implies free movement of the Fermi level within the gaps of the guest and the host molecules manifesting the intrinsic nature of these organic semiconductors. These results give insights into the role that the phosphorescent molecules play in trapping and transporting charges within the emissive layer of an organic light-emitting device.