A simple method of measuring the external quantum efficiency of organic electroluminescent (EL) devices has been investigated. Assuming a perfectly diffusive EL emission surface, this method enables the external quantum efficiency to be calculated from the conventionally measured parameters: luminance, EL spectrum, and current. Its use is limited to EL emission in the visible wavelength range.

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We have investigated the compositional and structural properties of blue emitting BaAl 2 S 4 :Eu electroluminescent thin films fabricated by switching electron-beam evaporation using two targets. The use of X-ray photoelectron spectroscopy (XPS) revealed that the ratio of barium to sulfur in thin films is 1.0 : 2.0, which is identical to the stoichiometry of BaAl 2 S 4 . The X-ray diffraction (XRD) peaks from the thin film were assigned to those of a BaAl 2 S 4 crystal. The photoluminescence (PL) spectrum shows emission from the 4f 6 5d → 4f 7 transition of the activated Eu 2+ ions in BaAl 2 S 4 crystals. These results indicate that the crystalline phase in thin films is only BaAl 2 S 4 . The thin films also contain a large amount of oxygen impurities, which result in the formation of an Al 2 O 3 layer and amorphous barium aluminate.

SrGa2S4:Ce blue electroluminescent thin films with a (440) single orientation have been prepared by a multisource deposition method using a molecular beam epitaxy (MBE) apparatus. The elements' chemical states and the doped oxygen effects on them have been analyzed by X-ray photoelectron spectroscopy (XPS). A small variation in the elements' chemical state with depth indicates that the growth of the thin film is stable. Sr ions exhibit no chemical shift due to a Sr–O bond, but Ga ions exhibit a chemical shift due to a Ga–S bond and a larger shift due to a Ga–O bond, indicating that the oxygen ion does not substitute for a S site in which the S ion bonds with the Sr ion. No Ce4+ ions have been detected and no oxidation of Ce3+ ions occurs even if some of the S ligands of Ce3+ are replaced by doped oxygen ions. The trivalent state of Ce is stable in the SrGa2S4 crystal.

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