| Abstract: | We demonstrate a novel organic light‐emitting diode (LED) heterolayer structure that contains a conjugated dendrimer as the light‐emitting molecule. The LED was prepared by spin‐coating two dendrimer layers from the same solvent. The device consists of a graded bilayer structure formed from a neat dendrimer film covered with a film consisting of the same dendrimer but doped with the electron‐transporting material 2‐(4‐biphenylyl)‐5‐phenyl‐1,3,4‐oxadiazole (PBD). In this device, the heterojunction interface present in conventional bilayer organic light‐emitting diodes is eliminated, and is replaced by a graded interlayer. By optimizing the concentration of PBD in the dendrimer, a peak electroluminescence (EL) external quantum efficiency of 0.16 % at 600 cd m–2 was obtained. The EL quantum efficiency is significantly enhanced in comparison with devices based on a single layer, a conventional bilayer, and a single‐layer doped with PBD. The EL quantum efficiency is a factor of eight larger than that of a conventional bilayer LED made with the conjugated dendrimer as the emissive layer and poly(methylmethacrylate) (PMMA) doped with PBD as the electron‐transporting layer. The best blended device exhibited only one third of the efficiency of the graded device. The improvement in the operating characteristics of the graded device is attributed to the efficient device structure, in which exciton formation is improved by a graded doping profile of electron‐ and hole‐transporting components. |
