Light-emitting elements using an organic compound as a light emitter, which have features such as thinness, lightweight, high-speed response, and direct current low voltage driving, have been considered to be applied to next-generation flat panel displays or next-generation lights. In particular, a display device in which light-emitting elements are arranged in matrix is considered to have an advantage in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
It is said that light-emitting elements have a mechanism of light emission as follows: by applying voltage between a pair of electrodes with an electroluminescent (EL) layer interposed therebetween, electrons injected from a cathode and holes injected from an anode recombine with each other in an emission center of the EL layer to form molecular excitons, and energy is released when the molecular excitons relax to the ground state; accordingly light is emitted. Singlet excitation and triplet excitation are known as excited states, and it is thought that light emission can be obtained through either of the excited states.
An EL layer included in a light-emitting element includes at least a light-emitting layer. In addition, the EL layer can have a stacked structure including a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, and/or the like, in addition to the light-emitting layer.
In addition, metal oxides attract attention as materials having semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor (also referred to as a TFT) in which a channel formation region is formed using such metal oxides having semiconductor characteristics is known (Patent Documents 1 and 2).
Further, TFTs including oxide semiconductors have high field-effect mobility. Therefore, driver circuits in display devices or the like can be formed using the TFTs.