Organic electroluminescent devices with saturated red emission were developed using 6,13-diphenylpentacene (DPP) doped into tris(8-hydroxyquinolinato) aluminum III (Alq3). DPP exhibits a narrow emission spectrum giving rise to a saturated red peak, centered around 625 nm, with excellent chromaticity coordinates (x=0.63 and y=0.34) in accordance with the Commission Internationale de l’Eclairage. An absolute photoluminescence (PL) quantum yield (φPL) of ∼30% was measured for a composite film of 0.55 mol % of DPP doped into Alq3. An electroluminescence (EL) quantum efficiency of 1.3% at 100 A/m2, close to the estimated theoretical limit (1.5%), was measured for an unoptimized device structure that consists of an active emissive layer sandwiched between hole- and electron-transport layers. In addition, the EL quantum efficiency is constant or stable over a wide range of current densities (1–1000 A/m2) or luminance values (1–1000 cd/m2).

The electroluminescence ͑EL͒ quantum yield ͑QY͒ of double-and triple-layer organic electroluminescent diodes based on a N,NЈ-diphenyl-N,NЈbis͑3-methylphenyl͒-1,1Ј-biphenyl-4,4Ј-diamine /tris ͑8-hydroxyquinolinato͒ aluminum III (Alq 3 ) junction has been measured as a function of the electric field and the emitting guest ͑6,13-diphenylpentacene͒ concentration in the host Alq 3 . The well-resolved maxima of the QY plots versus applied field for neat and low dopant concentration emitter layers ͑EMLs͒ shift strongly toward high fields and disappear at high dopant concentrations. Based on the EL QY data and the measured absolute photoluminescence quantum efficiency of the emitter, the recombination zone width is determined and shown to be a decreasing function of electric field for all of the diodes. The dopant reduces the width of the recombination zone at low dopant concentrations and increases at high dopant concentrations ͑Ͼ0.5 mol %͒. The results are discussed in terms of a two-step recombination mechanism, assuming disorder-controlled charge carrier mobilities. The dopant concentration effect on the recombination zone width and EL QY can be explained using the disorder formalism that predicts low dopant concentrations create a high degree of positional ͑off-diagonal͒ disorder whereas energetic ͑diagonal͒ disorder dominates at higher doping levels in the EMLs. The latter makes the recombination zone width as well as EL QY practically field independent.

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We determined the orbital lineup of the tris ͑8-hydroxyquinolinato͒ gallium (Gaq 3 )/Mg interface using combined x-ray and ultraviolet photoemission spectroscopy ͑XPS and UPS͒ measurements. The Gaq 3 /Mg system is a prototypical model structure for organic electron/low work function electrode transporting materials interfaces found in organic light emitting diodes ͑OLED͒. A Gaq 3 thin film was grown in 15 steps on a previously sputter-cleaned Mg substrate starting at a 1 Å nominal thickness up to a final thickness of 512 Å. Before, and in between the growth steps, the sample surface was characterized by XPS and UPS. The results indicate the formation of a reaction layer of about 12 Å thickness at the Mg interface, which resulted in a 0.96 V interface dipole potential. At Gaq 3 coverages higher than 256 Å, a strong charging shift occurred in the overlayer related UPS-emission lines, which was identified by measuring the high binding energy cutoff ͑secondary edge͒ of both the XP and UP spectra. The several magnitudes different x-ray and ultraviolet source photon intensities allow pinpointing charging shifts with high sensitivity. Due to the low work function of the reacted interface layer, the Gaq 3 electronic states are aligned at a binding energy below the substrate Fermi edge that exceeds the magnitude of the optical gap between the highest occupied and lowest unoccupied molecular orbitals ͑HOMO and LUMO͒. This allowed the conclusion that the ground state exciton binding energy of Gaq 3 needs to be larger than 0.43 eV. Based on these considerations, the lowest possible electron injection barrier matching the experimental data was estimated to be 0.15 eV.