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 triple-layered devices based on a rare-earth complex PTT, tris-(1-phenyl-3-methyl-4-isobutyryl-5-pyrozolone)-bis(triphenyl phosphine oxide) terbium. In triple-layer devices, besides green electroluminescence from the light-emitting layer PTT, blue emission from the hole transport layer TPD (N,N′-bis (3-methylphenyl)-N,N′-diphenyl-benzidine) occurs at high electric field strength (EFS) and its peak intensity increases with EFS. This indicates a charge 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 behavior. In double-layer devices where the 8-hydroxyquinoline–aluminum layer is absent, TPD emission dominates the electroluminescence, proving that PTT possesses high electron mobility.