The chemical properties and energy levels of Al/ LiF / Alq 3 were investigated via high-resolution synchrotron-radiation photoemission spectroscopy. No clear chemical reaction was found with LiF deposited on Alq 3 . The core-level spectra show that Li + ion and Alq 3 anion are created only after Al is deposited on LiF / Alq 3 surfaces. Combined with the increase of the electron concentrations indicated by the Fermi-level position in valence-band spectra, the results provide direct evidence of the proposed chemical reaction, 3 LiF + Al+ 3 Alq 3 → AlF 3 +3 Li + Alq 3 − , which leads to the excellent electron injection efficiency in Al/ LiF / Alq 3 .

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A highly efficient hole injection material, boron subphthalocyanine chloride (SubPc), was incorporated in organic light-emitting diodes. Device performance is greatly enhanced by inserting an ultrathin layer of SubPc between anodes and N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidene (NPB). Electronic structures and chemical reaction at the interface between NPB and SubPc are also investigated by photoemission spectroscopy with synchrotron radiation sources. Extra states are observed at the forbidden gap of SubPc with deposition of NPB, resulting from the broken bonds between boron and chlorine on SubPc with presence of NPB. These gap states are attributed to the improvement of device performance.

The influences of substitutions on the electronic structures of oligofluorenes were systematically investigated. Electronic states of oligofluorenes with various types of C9 substitutions were studied by both ultraviolet photoemission spectroscopy and theoretical calculation. It is found that the substituents could have significant interaction with the backbone and substantially modify the electronic structures of oligofluorenes, depending on the types of substitutions. Consequently, the electronic properties that would affect the operations of electronic devices also considerably depend on the substitutions of the oligofluorenes.

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