Chemical and electronic properties of Ba/bis(2-methyl-8-quinolinolato)(4- phenylphenolato)aluminum(III) interfaces for organic light-emitting diodes

The chemistry, electronic structure, and electron-injecting characteristics at the interfaces that were formed between bis(2-methyl-8-quinolinolato)(4- phenylphenolato)aluminum(III) (BAlq) and barium (Ba) were investigated using ultraviolet photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy, X-ray photoemission spectroscopy, and current-voltage-luminance measurements. The device performance of organic light-emitting diodes (OLEDs), which have a glass/ITO/MoO ₃/2-TNATA/ NPB/BAlq/Ba/Au structure, was significantly improved by inserting a Ba coverage (Θ _Ba) of 0.2 nm between BAlq and the cathode. For Θ _Ba's that were thicker than 0.2 nm, however, even though the electron-injecting barrier heights at the Ba-on-BAlq interfaces were all 0.1 eV, the device performance of the OLEDs with Ba at the interface was degraded with increasing Θ _Ba. This result indicates that the device performance is largely dependent on the interfacial chemical degradation of the BAlq molecule itself, rather than the electron-injecting barrier height that is determined by the width and chemical structure of the interface, and the formation of barium-induced new gap states at the Ba-on-BAlq interface.