The electronic structures of the interfaces between Al and tris-(8-hydroxyquinoline)aluminum (Alq3) with and without a LiF interlayer were studied by using UV and X-ray photoelectron spec-troscopy. In both cases, formation of gap states in the valence region and a satellite peak in the N 1s core level were found, indicating charge transfer to the N atoms in Alq3. The gap states were at a binding energy (BE) 1.8 eV lower than the highest occupied molecular orbital of Alq3, and the satellite peak in the N 1s core level was at a BE 1.6 eV lower than the main peak. The BE of the gap states was 1.2 eV higher in Al/LiF/Alq3 than in Al/Alq3, and the intensity was also higher, but the O atoms chemically reacted only in Al/Alq3. We discuss the role of the LiF layer in the differences between the two systems, as well as its possible correlation with the electroluminescence efficiency difference reported in the literature. I.
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Kim et al. (2000) studied this question.