Electroluminescence from both a light-emitting layer and hole transport layer: spectral evidence for charge carrier tunneling injection

We investigate the electroluminescence spectra of a series of double- and t riple-layered devices based on a rare-earth complex PTT,tris-(1-phenyl-3-me thyl-4-isobutyryl-5-pyrozolone)-bis(triphenyl phosphine oxide) terbium. In triple-layer devices, besides green electroluminescence from the light-emit ting layer PTT, blue emission from the hole transport layer TPD (N,N'-bis ( 3-methylphenyl)-N,N'-diphenyl-benzidine) occurs at high electric field stre ngth (EFS) and its peak intensity increases with EFS, This indicates a char ge carrier tunneling injection mechanism. At high EFS, the Fowler-Nordheim (FN) plot is close to Linear, supporting the tunneling mechanism. The large barriers at the cathode and anode interfaces are responsible for this FN b ehavior. In double-layer devices where the 8-hydroxyquinoline-aluminum laye r is absent, TPD emission dominates the electroluminescence, proving that P TT possesses high electron mobility. (C) 1998 Elsevier Science B.V. All rig hts reserved.