The energy level alignment for both Mg/8-hydroxyquinoline aluminum (Alq) and Au/Alq interfaces has been determined by the ultraviolet photoemission measurements. For both interfaces, the difference between the Fermi level and the low-energy edge of the highest occupied molecular orbital (HOMO) is around 1.7 eV. This implies that the Fermi level with respect to the HOMO edge of Alq is independent of the work function of Mg and Au despite a large difference in the metal work function. A Fermi level alignment model is proposed, invoking a charge transfer between the metal and Alq and the formation of a dipolar layer at the metal/Alq interface.

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The chelate phosphine oxide ligand bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO) was used as a unit neutral ligand to prepare the complex Eu(TTA)(3)(DPEPO) 1 (TTA = 2-thenoyltrifluoroacetonate). Compound 1 has a photoluminescence (PL) quantum yield of 55.3%, which is more than the twice of the PL quantum yield of Eu(TTA)(3)(TPPO)(2) (TPPO = triphenylphosphine oxide). Investigation indicated that DPEPO in 1 has the mezzo first triplet excited energy level (T(1)) between the first singlet excited energy level (S(1)) and T(1) of TTA, which may support one more additional energy transfer routines from the T(1) energy level of DPEPO to that of TTA, and consequently results in the improvement of energy transfer in the Eu(III) complex. DPEPO forms a complex with a more rigid and compact structure that can improve energy transfer between ligands and the center Eu(III) ion, support the higher saturation level by the coordinating ability of the oxygen atom in the ether moiety, and consequently enhance the PL intensity and efficiency of the corresponding Eu(III) complex. The multilayered electroluminescent (EL) device of 1 used as the red dopant exhibited an impressive brightness of 632 cd m(-2) at 25 V. The device had the excellent voltage-independent spectral stability with an emission peak at 615 nm. To the best of our knowledge, this luminescence is the brightest emission among Eu complexes with phosphine oxide ligands. The maximum external quantum yield (eta(ext)) of 2.89% and the maximum current and power efficiency of 4.58 cd A(-1) and 2.05 lm W(-1) were achieved at a low turn-on voltage of 7 V and current density of 0.021 mA cm(-2). These properties demonstrate that the chelate phosphine oxides ligand DPEPO can not only be favorable to form the rigid and compact complex structure and increase the efficiency of devices, but also reduce the ability of the formation of exciplex. DPEPO shows much better performance compared with the ordinary phosphine oxide ligand triphenylphosphine oxide.

Based on the WKB approximation of the tunneling model, we calculate the J–V characteristics of organic light-emitting devices (OLEDs) having buffer layers of different thickness. The results show how the insertion of a buffer layer with proper thickness lowers the OLED turn-on voltage. Further calculation suggests some parameters, such as the resistivity ratio and the position of the conduction band minimum of the buffer layer relative to the lowest unoccupied molecular orbital of the organic layer, are important in selecting a buffer material. A quantitative estimation of the optimal buffer layer thickness is also presented to serve as a guide to device design. The model is validated by comparison of its predictions to experimental results.

Bubbles in organic light-emitting diodes can be formed from gas release due to Joule heating effect at localized electrical shorts during operation, which could be simulated by a rapid thermal annealing. The gases in the bubbles consist of not only adsorbed moistures but also the decomposed organic species, which are detected in situ in an ultrahigh vacuum chamber. In the device of Al/tris-͑8-hydroxyquinoline͒ aluminum ͑Alq/N,NЈ-diphenyl-N.NЈ-bis-͕3-methylphenyl͖-͕1,1Јbiphenyl͖-4,4Ј-diamine/indium tin oxide ͑ITO͒, the gases released from ITO surface were mainly of adsorbed moistures, while those released from the organic layers were of both the decomposed products from Alq and the trapped moistures. The decomposition of Alq could not be easily avoided if there were severe localized electrical shorts in the devices.