Chemical structure of Al/LiF/Alq interfaces in organic light-emitting diodes

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Abstract
Al/LiF cathode/organic is known to form an excellent interface for electron injection into the organic active layer, resulting in excellent performing organic light-emitting diodes(OLEDs). Here, the chemical structure of the interface between the Al/LiF bilayer cathode and tris (8-hydroxyquinoline) aluminum (Alq) of working OLED devices was investigated by using x-ray photoelectron spectroscopy(XPS). Using a in situ peel-off technique, we are able to characterize the buried interfacestructure without disturbing the chemical states of each element probed. The data show that there are two types of F at the interface; one is attributed to LiF and the other to F attached to the Alq. This F-doped Alq layer could induce a downshift in molecular orbital levels and thus leads to a reduced electron injection barrier. XPS depth profile results show significant O diffusion through Al layer to the interface, and the diffusion of O ends abruptly at the Al/LiF interface.