The Effect of Charge Blocking Layers in Electroluminescent Devices Made of Molecularly-Doped Polymer Films

Abstract

Several single layer electroluminescent (EL) devices made of molecularly-doped polymer films in which component dyes and their compositions were systematically varied were fabricated. Their EL emission characteristics were examined. Polyvinylcarbazole (PVK), which possesses hole transporting capability, was used as polymer binder and device performances were compared with corresponding devices with inert polymer binder, and little difference was found between two types of devices. Double-layer and triple-layer type EL devices were fabricated using PVK and an oxadi azole derivative, OXD-7 for electron and hole blocking layers, respectively, and the effect of the introduction of charge-blocking layers has been investigated. Though reduction of hole injection and increase of operat ing voltage was observed, current quantum efficiency was increased with the introduction of PVK as the electron-blocking layer. The introduction of charge-blocking layers at both sides of electrodes resulted in the increase of quantum efficiency by about one order of magnitude. By the introduction of chargeblocking layers, the amount of holes and electrons which go through emissive layers without recombination were drastically reduced. This effect was found to contribute for marked increases of current quantum efficiencies.