Light-emitting element, light-emitting device, electronic device, lighting device, and organic compound
摘要
說明書
When the light-emitting substance is a fluorescent compound, singlet excitation energy of the exciplex is transferred to the singlet excited state of the light-emitting substance, and light emission from the singlet excited state (i.e., fluorescence) is achieved. On the other hand, triplet excitation energy of the exciplex is transferred to the triplet excited state of the light-emitting substance and thermally deactivated; thus, it seems that a higher efficiency cannot be achieved. However, the exciplex that is an energy donor has a small difference between singlet excitation energy and triplet excitation energy and thus emits thermally activated delayed fluorescence from itself. In other words, in the exciplex, reverse intersystem crossing from part of or the entire triplet excited state to the singlet excited state occurs, so that the proportion of singlet excitons is higher than that in the normal situation. The proportion of singlet excitons in the exciplex that is an energy donor is high, and singlet excitation energy of the exciplex is transferred to the singlet excited state of the light-emitting substance, whereby the emission efficiency is high even in the case of using a fluorescent compound as a light-emitting substance. This phenomenon is also one feature of the present invention.
As described above, a light-emitting element in which an exciplex serves as an energy donor in a light-emitting layer is effective in all the cases where a phosphorescent compound, a thermally activated delayed fluorescent compound, and a fluorescent compound are used as a light-emitting substance; however, there is a problem in controlling a light-emitting region.
