PACS. 73.30.+y Surface double layers, Schottky barriers, and work functions, 73.61.Ph Polymers; organic compounds,

An extensive numerical model recently developed for the multilayer organic light-emitting diode is described and applied to a set of real devices. The model contains a detailed description of electrical contacts including dipolar layer formation, thermionic and tunneling injection, space charge effects, field dependent mobilities and recombination processes. The model is applied to simulate several single layer devices and the family of bilayer devices made in our group. It provides insight into the energy level shifts, internal electric fields and charge distribution (and consequently recombination) throughout the device. Finally, the analysis is extended to the optimization of bilayer device.

By using pyran‐containing donor–acceptor dyes as doping molecules in organic light‐emitting devices (OLEDs), we scrutinize the effects of charge trapping and polarization induced by the guest molecules in the electro‐active host material. Laser dyes 4‐(dicyanomethylene)‐2‐methyl‐6‐[2‐(julolidin‐9‐yl)phenyl]ethenyl]‐4H‐pyran (DCM2) and the novel 4‐(dicyanomethylene)‐2‐methyl‐6‐{2‐[(4‐diphenylamino)phenyl]ethenyl}‐4H‐pyran (DCM‐TPA) are used as model compounds. The emission color of these polar dyes depends strongly on doping concentration, which we have attributed to polarization effects induced by the doping molecules themselves. Their frontier orbital energy levels are situated within the bandgap of the tris(8‐hydroxyquinoline)aluminum (Alq3) host matrix and allow the investigation of either electron trapping or both electron and hole trapping. In the case of DCM‐TPA doping, we were able to show that electron trapping leads to a partial shift of the recombination zone out of the doped Alq3 region. To impede charge‐recombination processes taking place in the undoped host matrix, a charge‐blocking layer efficiently confines the recombination zone inside the doped zone and gives rise to increased luminous efficiency. For a doping concentration of 1 wt.‐% we obtain a maximum luminous efficiency of 10.4 cd A–1. At this doping concentration, the yellow emission spectrum shows excellent color saturation with CIE chromaticity coordinates x, y of 0.49 and 0.50, respectively. In the case of DCM2 the recombination zone is much less affected for the same doping concentrations, which is ascribed to the fact that both electrons and holes are being trapped. The experimental findings are corroborated with a numerical simulation of the doped multilayer devices.

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In this work we propose a phenomenological microscopic approach to deal with pseudoinductive charge-relaxation processes (named also as negative capacitance phenomena) in organic molecules (tris-8-hydroxyquinoline-aluminum) and polymeric [poly(2-metoxy-5-(2′-etil-hexiloxy)-1,4-phenylene vinylene)] light-emitting diodes (OLEDs and PLEDs, respectively). The approach is based mainly on the fact that the recombination rate is higher than the slower carrier transit time to reach the recombination zone. The approach is supported by the fact that in both PLEDs and OLEDs, the strong pseudoinductive relaxation process was mainly observed when electron-hole recombination takes place, suggesting this is a recombination dependent phenomenon. Besides, the negative branch, in the real part of the complex capacitance representation as a function of the frequency, was not observed in PLED homopolar devices.