Bright single-layer white light-emitting organic electroluminescent devices were developed by using dye-dispersed poly(N-vinylcarbazole) (PVK). The active layer consists only of one polymer layer that is simply sandwiched between two electrodes, indium-tin oxide, and Mg:Ag. In order to achieve bipolarity in the single polymer emitter layer, PVK was molecularly dispersed with electron-transporting additives such as 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole. In addition, several fluorescent dyes, having different emission colors, were dispersed as emitting centers. By adjusting the concentration of the fluorescent dyes, white light with a maximum luminescence of 4100 cd/m2 was obtained, which is the brightest white light ever observed for organic electroluminescent devices.

We investigated the suitability of aromatic amine-containing polymers as active layers in organic electroluminescent devices. Polymers used in this study include poly(N-vinylcarbazole) (PVK), poly(N-substituted methacrylamide)s, poly(methacrylate) and poly(arylene ether)s. The device structures are a double-layer-type and a single-layer type. The double-layer-type devices consist of a polymer hole transport layer and an electron-transporting emitter layer. High luminance of over 14,000 cd/m 2 was observed for some double-layer devices using the polymer hole transport layer. Single-layer devices with dye-dispersed PVK emitted white light with a luminance of over 4000 cd/m 2 . This demonstrates that dye-dispersed polymer systems are quite useful to obtain white light.Organic electroluminescent (EL) devices are the subject of study by many researchers because of their potential application as light-emitting devices which operate at low drive voltages. These devices are injection-type, in which carriers, such as electrons (radical anions) and holes (radical cations), are injected into the organic emitter layer where they recombine. It is, therefore, necessary for the component organic materials to possess carrier-transporting properties as well as fluorescence properties.Although electroluminescence in organic molecules have been known since the 1960's (7), a major breakthrough was made in the 80's by Tang and VanSlyke who developed a device exhibiting practical brightness (2). Their device consisted of a double layer structure with an organic hole transport layer and a luminescent metal complex layer. The hole transport layer plays an important role in transporting holes and blocking electrons, thus preventing electrons from moving into the electrode without recombining with holes. Aromatic diamine derivatives, such as yV^V-diphenyl-,^V-bis(3-methylphenyl)-l,r-biphenyl-4,4'-diamine (TPD), have been widely used as a hole transport layer because of their high hole mobilities (10" 3 cm 2 /Vs) and good

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