1. Field of the Invention
The present invention relates to a bipyridine compound that can be used for a light-emitting element material. The present invention also relates to a light-emitting element material and an organic semiconductor material each using the bipyridine compound. The present invention further relates to a light-emitting element, a light-emitting module, a light-emitting device, a lighting device, a display device, and an electronic device each using the bipyridine compound.
2. Description of the Related Art
As next generation lighting devices or display devices, display devices using light-emitting elements (organic EL elements) in which organic compounds are used as light-emitting substances have been developed rapidly because of their advantages of thinness, lightweightness, high-speed response to input signals, low power consumption, and the like.
In an organic EL element, voltage application between electrodes between which a light-emitting layer is provided causes recombination of electrons and holes injected from the electrodes, which brings a light-emitting substance into an excited state, and the return from the excited state to the ground state is accompanied by light emission. Since the wavelength of light emitted from a light-emitting substance is peculiar to the light-emitting substance, use of different types of organic compounds for light-emitting substances makes it possible to provide light-emitting elements which exhibit various wavelengths, i.e., various colors.
In the case of display devices which are expected to display images, such as displays, at least three-color light, i.e., red light, green light, and blue light are necessary for reproduction of full-color images. Furthermore, in application to lighting devices, light having wavelength components evenly spreading in the visible light region is ideal for obtaining a high color rendering property, but actually, light obtained by mixing two or more kinds of light having different wavelengths is often used for lighting application. Note that it is known that mixing light of three colors of red, green, and blue allows generation of white light having a high color rendering property.
Light emitted from a light-emitting substance is peculiar to the substance as described above. However, important performances as a light-emitting element, such as lifetime, power consumption, and even emission efficiency, are not only dependent on the light-emitting substance but also greatly dependent on layers other than the light-emitting layer, an element structure, properties of an emission center substance and a host material, compatibility between them, carrier balance, and the like. Therefore, there is no doubt that many kinds of light-emitting element materials are necessary for a growth in this field. For the above-described reasons, light-emitting element materials with a variety of molecular structures have been proposed (e.g., see Patent Document 1).
As a substance of a host material in a host-guest type light-emitting layer or a substance contained in each transport layer in contact with a light-emitting layer, a substance having a wider band gap or a higher triplet level (a larger energy difference between a triplet excited state and a singlet ground state) than an emission center substance is used for efficient conversion of excitation energy into light emission from the emission center substance.
Therefore, a host material and a carrier-transport material each having a further wider band gap are necessary in order that light emission having a shorter wavelength such as blue fluorescence is efficiently obtained. It is difficult to develop a substance to be a light-emitting element material which has a wide band gap while enabling a balance between important characteristics of a light-emitting element, such as low driving voltage and high emission efficiency.