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/MoO3/2-TNATA/NPB/BAlq/Ba/Au structure, was significantly improved by inserting a Ba coverage (ΘBa) of 0.2 nm between BAlq and the cathode. For ΘBa'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 ΘBa. 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.
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