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Electroluminescent (EL) devices based on organic thin layers are one of the most promising next-generation flat panel display systems. These devices can be made into large-area, extremely thin full color displays that can be operated by batteries. The structure of the devices is simple, having organic thin layers sandwiched between two electrodes. Because organic layers can be formed by vacuum evaporation or solution casting, the fabrication cost could be less than that of the liquid crystal displays, which are today the most widely used flat panel displays.
In organic EL devices, the generation of light is the consequence of the recombination of holes and electrons injected from the electrodes. Such carrier recombination in the organic emitter layer excites the emitting centers. In a chemical sense, the reaction of radical cations (holes) and radical anions (electrons) provides excited molecules that emit light as one of the decay processes. A variety of materials have been investigated as active materials (1-9), and a number of device structures (10-22) have been proposed because it is important to find an appropriate device structure to maximize the carrier recombination efficiency.
The use of multilayer structures was first demonstrated by Tang and VanSlyke (1). They used a. hole-transport layer for hole injection from the electrode into the electron-transporting emitter layer, which significantly improved the EL efficiency to 1.5 lm/W. Bright electroluminescence of over 1000 cd/m sup 2 , which is high enough for practical applications, with a voltage below 10 V was achieved first. Their device consisted of two active layers with an organic hole-transport layer and an electron-transporting luminescent metal complex layer. The hole-transport layer plays an important role not only in transporting holes but also in blocking electrons, thus preventing electrons from moving into the opposite electrode without recombining with holes. The recombination, therefore, mainly takes place in the emitter layer. Since this first device, various types of EL devices with a multilayer structure have been reported. For instance, a luminescent hole-transporting material can also be an emitter layer when combined with an appropriate electron-transport layer (4, 9, 13).
In the above examples, various emitting colors have been obtained, including the three primary colors of blue, green, and red. However, there are few white light-emitting devices owing to the lack...