In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By applying a voltage between the pair of electrodes of this element, light emission from the light-emitting substance can be obtained.
Since the above light-emitting element is of a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, low power consumption, and the like. Furthermore, the display device also has advantages that it can be formed to be thin and lightweight, and has high response speed.
In a light-emitting element (e.g., an organic EL element) whose EL layer contains an organic compound as a light-emitting substance and is provided between a pair of electrodes, application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the EL layer having a light-emitting property and thus a current flows. By recombination of the injected electrons and holes, the organic compound having a light-emitting property is brought into an excited state to provide light emission.
Note that an excited state formed by an organic compound can be a singlet excited state (S*) or a triplet excited state (T*). Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence. The formation ratio of S* to T* in the light-emitting element is 1:3. Thus, a light-emitting element containing a compound emitting phosphorescence (phosphorescent compound) has higher light emission efficiency than a light-emitting element containing a compound emitting fluorescence (fluorescent compound). Therefore, light-emitting elements containing phosphorescent compounds capable of converting energy of a triplet excited state into light emission has been actively developed in recent years (e.g., see Patent Document 1).
Energy for exciting an organic compound depends on an energy difference between the LUMO level and the HOMO level of the organic compound. The energy difference approximately corresponds to singlet excitation energy. In a light-emitting element including a phosphorescent compound, triplet excitation energy is converted into light emission energy. Accordingly, when the organic compound has a large difference between the singlet excitation energy and the triplet excitation energy, the energy for exciting the organic compound is higher than the light emission energy by the energy difference. The difference between the energy for exciting the organic compound and the light emission energy affects element characteristics of a light-emitting element: the driving voltage of the light-emitting element increases. For this reason, a method for reducing driving voltage has been searched (see Patent Document 2).
Among light-emitting elements including phosphorescent compounds, a light-emitting element that emits blue light has not been put into practical use yet because it is difficult to develop a stable compound having a high triplet excitation energy level. Accordingly, development of a highly reliable light-emitting element that is formed using a phosphorescent compound and has high emission efficiency is required.