A study on the simple structural phosphorescent organic light-emitting diodes

The recombination dynamic of hole and electron and carrier transport mechanism were studied. We demonstrated a efficient green phosphorescent organic light-emitting diodes (OLEDs) as the doping concentration are varied without hole blocking layer (HBL) and electron transport layer (ETL). In spite of device without HBL and ETL, it is possible to inject the electron from the cathode into the emission layer (EML). It is practicable under the high doping ratio of the dopant in host. The high concentrated doping leads to the direct injection of hole and electron from both electrodes into iridium(III) tris(2-phenylpyridine) [Ir(ppy)₃] used as dopant because the workfunction of Al and the highest occupied molecular orbital (HOMO) state of 4,4´-bis[N-(naphthyl)-N-phenyl-amino]biphenyl 4,4´-bis[N-(naphthyl)-N-phenyl-amino]biphenyl (NPB) are nearly aligned with the lowest unoccupied molecular orbital (LUMO) and HOMO energy state of Ir(ppy)₃, respectively.