Multilayer Electroluminescence Research Articles

Electroluminescence from multilayer organic light-emitting diodes using poly(methylphenylsilane) as hole transporting material

We have investigated the optical and electrical properties of polysilane-based multilayer electroluminescent (EL) devices, utilizing poly(methylphenylsilane) (PMPS) as the hole transporting material, in order to elucidate the mechanism of EL emission in these devices. The EL devices which we fabricated have two or three functional organic layers. These layers are composed of a PMPS layer as well as a 3-(2′-Benzothiazolyl)-7-diethylaminocoumarin (Coumarin 6) doped polystyrene (PS) layer and/or a tris-(8-hydroxyquinoline) aluminum (Alq3) layer. An indium-tin-oxide-coated glass and an Al electrode were used as the hole and the electron injecting electrode, respectively. On the basis of a combined analysis of the basic characteristics of these devices, photoexcited fluorescence spectra and decay curves as well as the band diagram of these devices, we concluded that the recombination of charge carriers and the EL emission in the three-layer device occur both in the Coumarin 6:PS and the evaporated Alq3 layers.

Strongly-directed emission from microcavity structure in electroluminescent diodes with europium complex as an emitter

Multilayer electroluminescent (EL) diodes with sharp red emission were fabricated using an europium complex as an emitter. Microcavity structures were introduced into the EL diodes with sharp red emission. The device structures were composed of pairs of TiO 2/SiO 2 dielectric reflector, indium-tin-oxide electrode, hole transport layer (HTL), europium complex as an emission layer (EML), electron transport layer (ETL) and MgAg electrode. The dielectric reflector and the MgAg metal electrode constituted a planar microcavity. Sharply directed emission from europium complex was observed when operated under dc drive voltage at room temperature. In fabricating such EL diodes with optical microcavity made of dielectric reflector and a metal mirror, the best total thickness of the organic layers was found to depend on the stack structure of dielectric reflector.

Electroluminescent device using two dimensional semiconductor (C 6H 5C 2H 4NH 3) 2PbI 4 as an emitter

Layered Perovskite compounds expressed by the formula (C 6H 5C 2H 4NH 3) 2PbX 4 (X: halogen) naturally formed a dielectric quantum well structure in their spin-coated films. Strong exciton absorption and sharp photoluminescence from the exciton were observed in the spin-coated films even at room temperature; exciton absorption and emission bands of the layered Perovskite compounds were located at 3.65 eV and 3.57 eV for X=Cl, 3.08 eV and 3.05 eV for X = Br, and 2.41 eV and 2.38 eV for X = I, respectively, Using the combination of (C 6H 5C 2H 4NH 3) 2PbI 4 (PhEPbI 4), one of the luminescent layered Perovskite compounds, and an electron-transporting oxadiazole derivative (OXD7), a multilayered electroluminescent (EL) device consisting of indium-tin-oxide (ITO)/PhEPbI 4/OXD7/MgAg was prepared. Intense EL corresponding well to the exciton emission of PhEPbI 4 was observed in the device. The EL intensity was reached to luminance of about 12,000 cd m −2 at current density of 2 A cm −2 at liquid nitrogen temperature.

Electroluminescent Behaviors in Multilayer Thin-Film Electroluminescent Devices Using 9,10-Bisstyrylanthracene Derivatives

Using nine 9,10-bisstyrylanthracene derivatives (BSA's) with different substituents as emission layer materials, multilayer electroluminescent (EL) devices were fabricated. Among nine BSA's, three BSA's were found to exhibit high EL performance. Four types of devices, a single-layer device with a BSA emission layer, two types of two-layer devices in which BSA emission layers were combined with a triphenylamine dimer as a hole transport layer or an oxadiazole derivative as an electron transport layer, and a three-layer device, were fabricated using the three BSA's. The relationships between the device structures and EL performances of these devices were studied. Ionization potential values in vacuum-deposited films of BSA's were measured. It was found that the introduction of an electron withdrawing group increased electron injection/transport capability, and that of electron donating groups increased hole injection/transport capability. The relative EL efficiencies of various devices were discussed in terms of the electronic nature of BSA's.

Electroluminescence of doped organic thin films

Electroluminescent (EL) devices are constructed using multilayer organic thin films. The basic structure consists of a hole-transport layer and a luminescent layer. The hole-transport layer is an amorphous diamine film in which the only mobile carrier is the hole. The luminescent layer consists of a host material, 8-hydroxyquinoline aluminum (Alq), which predominantly transports electrons. High radiance has been achieved at an operating voltage of less than 10 V. By doping the Alq layer with highly fluorescent molecules, the EL efficiency has been improved by about a factor of 2 in comparison with the undoped cell. Representative dopants are coumarins and DCMs. The EL quantum efficiency of the doped system is about 2.5%, photon/electron. The EL colors can be readily tuned from the blue-green to orange-red by a suitable choice of dopants as well as by changing the concentration of the dopant. In the doped system the electron-hole recombination and emission zones can be confined to about 50 Å near the hole-transport interface. In the undoped Alq, the EL emission zone is considerably larger due to exciton diffusion. The multilayer doped EL structure offers a simple means for the direct determination of exciton diffusion length.