In recent years, research and development of light-emitting elements using electroluminescence (EL) have been extensively conducted. In the basic structure of such a light-emitting element, a light-emitting substance is interposed between a pair of electrodes. By applying a voltage to this element, light emission can be obtained from the light-emitting substance.
Since this type of light-emitting element is a self-luminous type, it has advantages over a liquid crystal display in that visibility of a pixel is high and that no backlight is needed. Therefore, light-emitting elements are thought to be suitable as flat panel display elements. Further, such a light-emitting element also has advantages in that the element can be fabricated to be thin and lightweight and that response speed is very high.
Further, since this type of a light-emitting element can be formed to have a film shape, surface light emission can be easily obtained. This feature is difficult to realize with point light sources typified by a filament lamp and an LED or with linear light sources typified by a fluorescent light. Therefore, such light-emitting elements also have a high utility value as surface light source that can be applied to lighting apparatuses or the like.
Light-emitting elements using electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance.
When an organic compound is used as a light-emitting substance, by application of a voltage to a light-emitting element, electrons and holes are injected into a layer containing the light-emitting organic compound from a pair of electrodes, whereby a current flows. Then, the carriers (i.e., electrons and holes) recombine to place the light-emitting organic compound into an excited state. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Such a light-emitting element using a light-emitting organic compound is generally referred to as an organic EL element.
Note that an excited state of an organic compound can be of two types: a singlet excited state and a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
Proposal of a heterostructure in which layers of different organic compounds are stacked has brought about significant development of such light-emitting elements (see Non-Patent Document 1). That is because adopting a heterostructure increases carrier recombination efficiency and then improves emission efficiency. Then, a double heterostructure in which a light-emitting layer is formed between a hole-transport layer and an electron-transport layer has been proposed (see Non-Patent Document 2). An element structure based on the double heterostructure is now mainstream.
Further, by increasing the number of stacked layers, an attempt to improve lifetime has been made lately. For example, in Patent Document 1, an increase in lifetime is attempted by providing a layer to decelerate transport of electrons in addition to an electron-transport layer.    [Patent Document 1] Japanese Published Patent Application No. 2006-66890.    [Non-Patent Document 1] C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Vol. 51, No. 12, 913-915, 1987.    [Non-Patent Document 2] C. Adachi, et al., Japanese Journal of Applied Physics, Vol. 27, L269-L271, 1988.