A numerical study of space charge effects in multilayer organic light-emitting diodes ͑OLEDs͒ is presented. The method of solving the coupled Poisson and continuity equations, previously established for single-layer polymer LEDs, has been extended to treat internal organic interfaces. In addition, we consider the transient current and electroluminescence response. We discuss the accumulation of charges at internal interfaces and their signature in the transient response as well as the electric field distribution. Comparison to experimental transient data of a typical bilayer LED based on tris͑8-hydroxyquinolinato͒aluminum (Alq 3) is provided and good agreement is found. Our results are consistent with commonly assumed operating principles of bilayer LEDs. In particular, the assumptions that the electric field is predominantly dropped across the Alq 3 layer and that the electroluminescence delay time is determined by electrons passing through Alq 3 to the internal interface are self-consistently supported by the results of the simulation. Moreover, the creation of emissive singlet excitons is found to be strongly confined to the Alq 3 side of the internal interface and the emission zone width is dictated by the exciton diffusion length. Design principles for trilayer LEDs with improved power efficiency are also discussed.
Transient electroluminescence ͑EL͒ from single-and multilayer organic light-emitting diodes ͑OLEDs͒ was investigated by driving the devices with short, rectangular voltage pulses. The single-layer devices consist of indium-tin oxide ͑ITO͒/tris͑8-hydroxy-quinoline͒aluminum ͑Alq 3 ͒/magnesium ͑Mg͒:silver ͑Ag͒, whereas the structure of the multilayer OLEDs are ITO/copper phthalocyanine ͑CuPc͒/N,NЈ-di͑naphthalene-1-yl͒-N,NЈ-diphenyl-benzidine ͑NPB͒/Alq 3 /Mg:Ag. Apparent model-dependent values of the electron mobility ( e ) in Alq 3 have been calculated from the onset of EL for both device structures upon invoking different internal electric field distributions. For the single-layer OLEDs, transient experiments with different dc bias voltages indicated that the EL delay time is determined by the accumulation of charge carriers inside the device rather than by transport of the latter. This interpretation is supported by the observation of delayed EL after the voltage pulse is turned off. In the multilayer OLED the EL onset-dependent on the electric field-is governed by accumulated charges ͑holes͒ at the internal organic-organic interface (NPB/Alq 3 ) or is transport limited. Time-of-flight measurements on 150-nm-thin Alq 3 layers yield weak field-dependent e values of the order of 1ϫ10 Ϫ5 cm 2 /Vs at electrical fields between 3.9ϫ10 5 and 1.3ϫ10 6 V/cm.
We have performed electric-field and temperature-dependent electron injection studies in an aluminum/ tris͑8-hydroxy-quinolinolato͒aluminum/magnesium:silver single-layer organic light-emitting diode. Analysis of the observed injection currents in terms of the classic Fowler-Nordheim ͑FN͒ tunneling or RichardsonSchottky ͑RS͒ thermionic emission proved to be inadequate. Whereas, the FN-type behavior at high-electric fields must be considered accidental, the injection currents qualitatively resemble those of the RS concept. However, quantitative differences are observed concerning the RS coefficient, the prefactor current, and the temperature dependence. On the other hand, the experimental data are in excellent agreement with a recently presented Monte Carlo simulation ͓U. Wolf et al., Phys. Rev. B 59, 7507 ͑1999͔͒ of carrier injection from a metal to an organic dielectric with random hopping sites. ͓S0163-1829͑99͒14535-1͔
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