Light-emitting device

Details Light-emitting device Light-emitting device

2001-11-15

2004-11-16

Yamnitzky, Marie (Department: 1774)

Stock material or miscellaneous articles

Composite

Of inorganic material

C428S917000, C313S504000, C313S506000

Reexamination Certificate

active

06818325

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a material for use in an organic electroluminescent light-emitting device (hereinafter referred to as “light-emitting device”) and a light-emitting device, which can preferably be utilized as a display element or as a plane light source for a full-color display, a back light or plane illumination, or as a light source array in a printer.
BACKGROUND OF THE INVENTION
Light-emitting devices seem to have a bright prospect as solid, light-emitting inexpensive devices for large-area full color displays or as light source arrays for recording, and many developments have been made on them. The light-emitting devices are generally constituted by a light-emitting layer interleaved between a pair of opposed electrodes. Light emission is a phenomenon which occurs when electrons and holes respectively injected from the cathode and the anode upon application of an electric field across the electrodes are recombined in the light-emitting layer and their energy level returns from conduction band to valence band with emitting the energy as light.
Many organic light-emitting devices having formed therein an organic thin film have so far been reported (see references described in Macromolecular Symposium, vol.125, p.1, 1997). These light-emitting devices, however, involve the problem that their light-emitting efficiency is extremely lower than that of inorganic LED devices or fluorescent tubes. It is known that conventional light-emitting devices have generally required a high driving voltage, while with providing a low light-emitting luminance and a low light-emitting efficiency. In recent years, however, various techniques have been reported to solve the problems. For example, there has been proposed an organic light-emitting device having an organic thin film formed by vapor deposition of an organic compound (Applied Physics Letters, vol.51, p.913, 1987). The organic light-emitting device described there has a laminated two-layered structure of an electron-transporting layer comprising an electron-transporting material and a hole-transporting layer comprising a hole-transporting material, and shows a markedly improved light-emitting performance in comparison with conventional single-layered devices. This light-emitting device uses a low molecular amine compound as a hole-transporting material and an aluminum (Al) complex of 8-quinolinol (hereinafter abbreviated as “Alq”) as an electron-transporting and light-emitting material, with the color of emitted light being green.
Most of the at present proposed light-emitting devices contain fluorescence-emitting compound as a light-emitting material in a light-emitting layer and utilize fluorescent light emission obtained from singlet excitons of the compound. According to the knowledge of quantum chemistry, the ratio of singlet excitons generating fluorescence to triplet excitons generating phosphorescence in number is 1:3. Hence, as long as fluoresence is utilized, only 25% of the generated excitons can be effectively utilized, thus light-emitting efficiency is unavoidably low. On the other hand, if phosphorescence obtained from the triplet excitons can be utilized, it serves to improve light-emitting efficiency.
Thus, a phosphorescent device using an iridium-phenylpyridine complex has recently been reported (Applied Physics Letter, vol.75, p.4, 1999; Japanese Journal of Applied Physics, vol.38, p.L1502, 1999). In these reports, the light-emitting devices are reported to show light-emitting efficiency 2 to 3 times as much as that of conventional fluorescence-utilizing light-emitting devices. However, their light-emitting efficiency is still lower than theoretical limit of light-emitting efficiency, and much more improvement of light-emitting efficiency has been demanded.
In addition to the above-described problems, conventional processes for producing light-emitting devices involve serious problems of deterioration due to crystallization of a low molecular compound, high production cost, and poor productivity, since they are dry filming processes by deposition. Thus, in order to reduce the production cost or to apply to production of large-area devices, there have been reported light-emitting devices formed by filming a high-molecular compound according to a wet filming process. As such high-molecular compound, there are illustrated poly-p-phenylenevinylene capable of emitting green light (Nature, vol.347, p.539, 1990), poly(3-alkylthiophene) capable of emitting reddish orange light (Japanese Journal of Applied Physics, vol.30, p.L1938, 1991), and polyalkylfluorene as blue light-emitting device (Japanese Journal of Applied Physics, vol.30, p.L1941, 1991). In addition, in Japanese Patent Laid-Open No. 223188/1990, it is reported to disperse a low molecular compound in a binder resin and coat the dispersion in a wet manner to form a film. However, all of these techniques utilize fluorescence obtained from singlet exitons, and hence involve the essential problem of low light-emitting efficiency.
Light-emitting devices utilizing triplet exitons and formed by a wet filming process have not been reported at all. Organic light-emitting devices which show a high light-emitting efficiency and a high light-emitting luminance, which can be produced at a low production cost, and which can be made into a large-area device have not so far been developed, and have been eagerly desired.
SUMMARY OF THE INVENTION
The present invention has been made for solving the problem with the related art and attaining the following objects. That is, the invention provides a light-emitting device, which has a laminated two- or three-organic layer structure, which has an electron-transporting material having a specified minimum excitation triplet energy level, ionization potential and electron mobility, which shows an excellent light-emitting efficiency and light-emitting luminance utilizing a phosphorescent compound, in which at least one of the organic compound layers is formed by a wet filming method, and which permits to produce large-area displays at a reduced production cost.
The above-described object of the invention can be solved by the following light-emitting devices of the invention.
(1) A light-emitting device comprising: a pair of electrodes formed on a substrate; and organic compound layers provided in between the electrodes, wherein the organic compound layers comprise a light-emitting layer comprising a hole-transporting material and a phosphorescent compound and an electron-transporting layer comprising an electron-transporting material, and an ionization potential of the electron-transporting material is 5.9 eV or more.
(2) The light-emitting device as described in (1), wherein a minimum excitation triplet energy level of the electron-transporting material is from 60 kcal/mol to 90 kcal/mol.
(3) The light-emitting device as described in (1) or (2), wherein an electron mobility of the electron-transporting material is 1×10
−4
cm
2
·V
−1
·s
−1
or more in an electric field of 1×10
5
V·cm
−1
.
(4) The light-emitting device as described in one of (1) to (3), wherein the electron-transporting material is an aromatic heterocyclic compound comprising a hetero atom.
(5) The light-emitting device as described in one of (1) to (4), wherein the electron-transporting material is an aromatic heterocyclic compound which has an azole skelton.
(6) The light-emitting device as described in one of (1) to (5), wherein the electron-transporting material is at least one of an aromatic heterocyclic compound which has a condensed azole skelton and an aromatic heterocyclic compound which has a triazine skelton.
(7) The light-emitting device as described in one of (1) to (6), wherein the electron-transporting material is an aromatic heterocyclic compound which has an condensed imidazopyridine.
(8) The light-emitting device as described in one of (1) to (7), wherein the content of the electron-transporting material is from 20 to 100% by weight based on the total content of the electron-transporting l

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