Electroluminescent (EL) devices with multilayer structures which are composed of two or three vacuum-sublimed dye films have been shown to exhibit high device performances. The working mechanism for charge-injection-type EL devices is reviewed and then the design of dye molecules used for hole transport, electron transport and emissive layers for multilayer EL devices are summarized. It is also demonstrated that the molecular design concept of dye films is applicable for the cases of polymerdispersed dye systems. The possibility of extension of the molecular design concept to polymers with chromophores in skeletons is discussed. In addition, the importance of the adoption of multilayer structures for obtaining high EL efficiency is discussed.

Effect of layered structures on the location of emissive regions was studied in four types of organic electroluminescent (EL) devices: a single-layered (SL) device consisting only of an emissive layer (EML), two types of double-layered (DL-H and DL-E) devices in which a hole-transport layer (HTL) or an electron-transport layer (ETL) is attached to an EML, and a triple-layered (TL) device in which an EML is sandwiched between a HTL and an ETL. As EML, HTL and ETL material, 9, 10-bis[4-(diphenylamino)styryl]anthracene, 4,4′-bis[(3-methylphenyl)phenylamino]biphenyl and 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene, respectively, were used. Within EML layers, a thin sensing layer doped with a squarilium dye, 2,4-bis[4-diethylamino)-2- hydroxyphenyl]cycrobutenediylium-1,3-dioxide was inserted. The change in emission intensity from the dopant, when the location of the sensing layer was systematically varied, gave information on emissive regions in each type of EL device. The emissive region in the SL device extended through the EML, and that in the DL-H device resided near the HTL/EML boundary. On the contrary, those in the DL-E and TL devices were located within a 10-nm-wide region adjacent to the EML/ETL boundary. Moreover, the emission efficiencies of the DL-E and TL devices were found to be higher than those of the SL and DL-H devices. It was experimentally demonstrated that the carrier recombination within the narrow region adjacent to the EML/carrier transport layer boundary gave high emission efficiency.

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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.

Electroluminescent (EL) characteristics in EL devices made of vacuum-sublimed dye films and spin-coated polymer films were compared. Low-molar-mass dye, 9,10-bis[4-(N,N-diphenylamino)styryl]anthracene (dye-BSA), for the preparation of vacuum-sublimed films, and polymer with 9,10-bis[4-(N,N-diphenylamino)-styryl]anthracene chromophore linked with alkylether groups (polymer-BSA) were employed. Single-layer devices, indium-tin oxide (ITO)/dye-BSA/MgAg and ITO/polymer-BSA/MgAg were prepared, and EL performances were compared. Double-layer devices which have an oxadiazole derivative (OXD-7) electron transport layer, ITO/dye-BSA/OXD-7/MgAg and ITO/polymer-BSA/OXD-7/MgAg, were also prepared. The current density-voltage relationships between dye-BSA devices and polymer-BSA devices were considerably different mainly due to the poor film quality of polymer-BSA. The comparisons of the luminance-current density relationships of the devices with two classes of BSA films showed that the polymer-BSA devices exhibited similar EL characteristics as the dye-BSA devices in the region of current density higher than 10 mA/cm2. The possibility of the use of common material design concept for low-molar-mass materials and polymers was discussed.