By applying voltage to an organic electroluminescence device (also referred to as “organic EL device”), holes from an anode and electrons from a cathode are injected into a light emitting layer. The holes and electrons injected into the light emitting layer recombine to form excitons. Singlet excitons and triplet excitons are formed in a ratio of 25%:75% according to spin-statistics theorem. Since the fluorescence utilizes the emission from singlet excitons, it has been known that the internal quantum efficiency of a fluorescent organic EL device is limited to 25% at most. In contrast, since the phosphorescence utilizes the emission from triplet excitons, it has been known that the internal quantum efficiency of a phosphorescent organic EL device can be increased to 100% if the intersystem crossing occurs efficiently.
In the development of known organic EL devices, an optimum device design has been made depending upon the emission mechanism such as fluorescence and phosphorescence. It has been known in the art that a high-performance phosphorescent organic EL device cannot be obtained by a mere application of the fluorescent technique to the phosphorescent device, because the emission mechanisms are different from each other. This may be generally because the following reasons.
Since the phosphorescence utilizes the emission from triplet excitons, a compound with larger energy gap is required to be used in the light emitting layer. This is because that the singlet energy (energy difference between the lowest excited singlet state and the ground state) of a compound is generally larger than its triplet energy (energy difference between the lowest excited triplet state and the ground state).
Therefore, to effectively confine the triplet energy of a phosphorescent dopant material within a device, a host material having triplet energy larger than that of the phosphorescent dopant material should be used in the light emitting layer. In addition, if an electron transporting layer and a hole transporting layer are formed adjacent to the light emitting layer, a compound having triplet energy larger than that of the phosphorescent dopant material should be used also in the electron transporting layer and the hole transporting layer. Thus, the device design conventionally employed for developing a phosphorescent organic EL device has been directed to the use of a compound having an energy gap larger than that of a compound for use in a fluorescent organic EL device, thereby increasing the voltage for driving an organic EL device.
A hydrocarbon compound highly resistant to oxidation and reduction, which has been known as a useful compound for a fluorescent device, has a small energy gap because of a broad distribution of π-electron cloud. Therefore, such a hydrocarbon compound is not suitable for use in a phosphorescent organic EL device and, instead, an organic compound having a heteroatom, such as oxygen and nitrogen, has been selected. However, a phosphorescent organic EL device employing such an organic compound having a heteroatom has a shorter lifetime as compared with a fluorescent organic EL device.
In addition, the relaxation time of triplet excitons of a phosphorescent dopant material is extremely longer than that of singlet excitons, this largely affecting the device performance. Namely, in the emission from singlet excitons, since the relaxation speed which leads to emission is high, the diffusion of excitons into a layer adjacent to the light emitting layer (for example, a hole transporting layer and an electron transporting layer) is difficult to occur and efficient emission is expected. In contrast, the emission from triplet excitons is a spin-forbidden transition and the relaxation speed is low. Therefore, the diffusion of excitons into adjacent layers occurs easily and the thermal energy deactivation occurs in most compounds other than the specific phosphorescent compound. Thus, as compared with a fluorescent organic EL device, it is more important for a phosphorescent organic EL device to control the region where electrons and holes are recombined.
For the above reasons, the development of a high performance phosphorescent organic EL device requires the selection of materials and the consideration of device design which are different from those for a fluorescent organic EL device.
Patent Document 1 discloses the combined use of a phosphorescent host material wherein a carbazole and an azine are connected to each other and a hole transporting material having a carbazole-containing amine structure with a large triplet energy. Although the monoamine material which has been used successfully as the hole transporting material is used, the durability against charges is poor because of its structure. In addition, the proposed host material has a large ionization potential because carbazoles are not directly bonded to each other. Therefore, holes are accumulated in the interface between the transporting material and the host material to adversely affect the performance of device.
Patent Document 2 discloses the combined use of a phosphorescent host material having a biscarbazole structure wherein carbazoles are boned to each other and a hole transporting material having a carbazole-containing amine structure with a large triplet energy. Since the material having a small ionization potential is used as the host material, the hole injecting ability from the hole transporting material is improved. However, since the conventional monoamine material is used as the hole transporting material, the triplet energy is likely to easily diffuse.