When voltage is applied to an organic electroluminescence device (hereinafter, occasionally referred to as an organic EL device), holes are injected from an anode to an emitting layer while electrons are injected from a cathode to the emitting layer. The injected holes and electrons are recombined in the emitting layer to form excitons. At this time, singlet excitons and triplet excitons are generated at a ratio of 25%:75% according to statistics of electron spin. In the classification according to the emission principle, in a fluorescent EL device which uses emission caused by singlet excitons, an internal quantum efficiency of the organic EL device is believed to be limited to 25%. On the other hand, it has been known that the internal quantum efficiency can be improved up to 100% under efficient intersystem crossing from the singlet excitons in a phosphorescent EL device which uses emission caused by triplet excitons.
A technology for extending a lifetime of a fluorescent organic EL device has recently been improved and applied to a full-color display of a mobile phone, TV and the like. However, an efficiency of a fluorescent EL device is required to be improved.
Based on such a background, a highly efficient fluorescent organic EL device using delayed fluorescence has been proposed and developed. For instance, an organic EL device using TTF (Triplet-Triplet Fusion) mechanism that is one of mechanisms for delayed fluorescence has been proposed. The TTF mechanism utilizes a phenomenon in which singlet excitons are generated by collision between two triplet excitons.
By using delayed fluorescence by the TTF mechanism, it is considered that an internal quantum efficiency can be theoretically raised up to 40% even in fluorescent emission. However, as compared with phosphorescent emission, the fluorescent emission is still problematic on improving efficiency. Accordingly, in order to enhance the internal quantum efficiency, an organic EL device using another delayed fluorescence mechanism has been studied.
For instance, TADF (Thermally Activated Delayed Fluorescence) mechanism is used. The TADF mechanism utilizes a phenomenon in which inverse intersystem crossing from triplet excitons to singlet excitons is generated by using a material having a small energy gap (ΔST) between the singlet level and the triplet level. An organic EL device with use of the TADF mechanism is disclosed, for instance, in non-Patent Literature 1.
Non-Patent Literature 1 describes carbazolyl dicyanobenzene (CDCB) as a luminescent material for TADF. Non-Patent Literature 1 describes that CDCB includes carbazole as a donor and dicyanobenzene as an electron acceptor and emits light in a range from a blue color (473 nm) to an orange color (577 nm).