Studies have been actively performed on an organic electroluminescent device having a structure containing a cathode, an anode, and a light emitting layer containing a light emitting material that is formed between the anode and the cathode. Most of the studies are to develop a monochrome light emitting organic electroluminescent device by adding one light emitting material having a particular light emission wavelength to the light emitting layer, and some of them are to develop a multiple wavelength light emitting organic electroluminescent device by using plural light emitting materials that have different light emission wavelengths. For example, such one has been studied that a light emitting layer containing a blue light emitting material, a light emitting layer containing a green light emitting material and a light emitting layer containing a red light emitting material are formed, and white light emission is to be achieved by mixing light emission from the light emitting materials.
Patent Document 1 describes an organic electroluminescent device having a light emitting layer containing a blue or blue-green fluorescent light emitting material and a light emitting layer containing a phosphorescent light emitting material other than blue, in which the lowest excitation triplet energy level of the fluorescent light emitting material is larger than the lowest excitation triplet energy level of the phosphorescent light emitting material. In the examples thereof, such an organic electroluminescent device is described that contains an anode, a hole transporting layer, a hole-side intermediate layer, an orange-red light emitting layer, a blue light emitting layer, a green light emitting layer, an electron-side intermediate layer, an electron transporting layer, and a cathode, which are formed in this order. The blue light emitting layer is formed of α-NPD, the orange-red light emitting layer is doped with iridium(III) bis(2-methyldibenzo[f,h]quinoxaline (acetylacetonate) as a light emitting material, and the green light emitting layer is doped with fac-tris(2-phenylpyridine) iridium ((Ir(ppy)3)). However, Patent Document 1 does not describe the use of a delayed fluorescent material.
Patent Document 2 describes an organic electroluminescent device having a light emitting layer containing a green light emitting layer containing a green delayed fluorescent material formed in contact with a hole transporting layer, a red phosphorescent light emitting layer containing a red light emitting material formed in contact with the green light emitting layer, and a blue light emitting layer containing a blue light emitting material formed in contact with the red light emitting layer. The organic electroluminescent device is designed in such a manner that the HOMO of the green delayed fluorescent material is deeper than the HOMO of the material of the hole transporting layer and is shallower than the HOMO of the red phosphorescent light emitting material. Patent Document 2 described that according to the design, the barrier between the HOMOs of the hole transporting layer and the green light emitting layer is reduced, and thus red phosphorescent light may be emitted with a high efficiency.