Red phosphorescent compound and organic electroluminescent device using the same

This application claims the benefit of Korean Patent Application No. 10-2006-0026630 filed on Mar. 23, 2006, which is hereby incorporated by reference as if fully set forth herein.

1. Field of the Invention

The present invention relates to a red light-emitting phosphorescent compound (hereinafter, referred to simply to as a ‘red phosphorescent compound’) and an organic electroluminescent (EL) device using the same. More particularly, the present invention relates to a red phosphorescent compound and an organic electroluminescent device realizing an improvement in color purity by using the red phosphorescent compound as a dopant of a light-emitting layer.

2. Discussion of the Related Art

Since electroluminescent devices are self-luminous display devices, they have the advantages of a wide viewing angle, a large contrast ratio, and a high response speed. Electroluminescent devices are divided into inorganic electroluminescent devices and organic electroluminescent devices depending upon the kind of the material for a light-emitting layer. Organic electroluminescent devices have advantages of high luminescence, low operation voltage, high response speed, and superior multi-color representation, as compared to inorganic electroluminescent devices.

When charge carriers are injected into an organic light-emitting layer formed between an electron injecting electrode (cathode) and a hole injecting electrode (anode) of an organic electroluminescent device, electrons combine with holes to create electron-hole pairs, which then decay to emit light. Organic electroluminescent devices have advantages in that they can be fabricated on flexible transparent substrates (e.g., plastic substrates) and can be operated at a voltage (e.g., 10V or below) lower than voltages required to operate plasma display panels (PDPs) and inorganic electroluminescent devices. Other advantages of organic electroluminescent devices are relatively low power consumption and excellent color representation. Further, since organic electroluminescent (EL) devices can emit light of three colors (i.e., green, blue and red), they have been the focus of intense interest lately as next-generation display devices capable of producing images of various colors. A general method for fabricating organic EL devices will be briefly explained below.

First, a transparent substrate is covered with an anode material. Indium tin oxide (ITO) is generally used as the anode material. A hole injecting layer (HIL) is laminated to a thickness of 10 to 30 nm on the anode. Copper (II) phthalocyanine (CuPc) is mainly used as a material of the hole injecting layer. Then, a hole transport layer is formed by depositing 4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (NPB) to a thickness of about 30 to about 60 nm on the hole injecting layer.

An organic light-emitting layer is formed on the hole transport layer. If necessary, a dopant may be added to a material for the organic light-emitting layer. For green light emission, tris(8-hydroxyquinoline)aluminum (Alq 3 ) as a material for the organic light-emitting layer is deposited to a thickness of about 30 to about 60 nm on the hole transport layer, and N-methylquinacridone (MQD) is mainly used as the dopant.

An electron transport layer (ETL) and an electron injecting layer (EIL) are sequentially laminated on the organic light-emitting layer. Alternatively, an electron injecting/transport layer is laminated on the organic light-emitting layer. In the case of green light emission, since Alq 3 has excellent electron-transport ability, the formation of the electron injecting/transport layer may be unnecessary. A cathode is deposited on the electron injecting layer, and finally a passivation film is covered thereon.

The type of the organic electroluminescent devices (i.e. blue, green and red light-emitting devices) will be determined depending on the kind of materials for the light-emitting layer.

In the light-emitting layer, holes injected from the anode are recombined with electrons injected from the cathode to form excitons. Singlet excitons and triplet excitons are involved in the fluorescence and phosphorescence processes, respectively. Fluorescent materials using triplet excitons, which are involved in the phosphorescence process, whose probability of formation is 75%, exhibit high luminescence efficiency, as compared to fluorescent materials using singlet excitons whose probability of formation is 25%. In particular, the luminescence efficiency of red phosphorescent materials is considerably high, compared to that of fluorescent materials. Accordingly, a number of studies associated with the use of red phosphorescent materials in organic electroluminescent devices are being made to enhance the luminescence efficiency of the organic electroluminescent devices.

Phosphorescent materials for use in organic EL devices must satisfy the requirements of high luminescence efficiency, high color purity and long luminescence lifetime. As shown in FIG. 1 , as the color purity of an organic EL device using a red phosphorescent material becomes higher (i.e. as the x-values on CIE chromaticity coordinates increase), the spectral luminous efficacy of the organic EL device decreases, making it difficult to achieve high luminescence efficiency of the organic EL device.

Accordingly, the present invention is directed to a red phosphorescent compound and an organic electroluminescent (EL) device using the same that substantially obviate one or more problems due to limitations and disadvantages of the related art.

An object of the present invention is to provide a red phosphorescent compound that exhibits desirable chromaticity coordinate characteristics (CIE color purity X>0.65), high luminescence efficiency, and long luminescence lifetime.

Another object of the present invention is to provide an organic electroluminescent (EL) device using the red phosphorescent compound.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a red phosphorescent compound of Formula 1:

wherein

is

R1 is selected from the group consisting of C 1 -C 4 alkyl and C 1 -C 4 alkoxy; R2, R3, R4 and R5 are independently selected from the group consisting of hydrogen, C 1 -C 4 alkyl and C 1 -C 4 alkoxy; and

is selected from the group consisting of 2,4-pentanedione, 2,2,6,6,-tetramethylheptane-3,5-dione, 1,3-propanedione, 1,3-butanedione, 3,5-heptanedione, 1,1,1-trifluoro-2,4-pentanedione, 1,1,1,5,5,5-hexafluoro-2,4-pentanedione, and 2,2-dimethyl-3,5-hexanedione.

In another aspect of the present invention, there is provided an organic electroluminescent (EL) device comprising a light-emitting layer wherein the red phosphorescent compound of Formula 1 is used as a dopant for the light-emitting layer.

In yet another aspect of the present invention, there is provided an organic electroluminescent (EL) device comprising a first electrode, a light-emitting layer using a red phosphorescent compound as a dopant, and a second electrode, and exhibiting a luminescence of 1,300 to 1,450 cd/m 2 at an electric current of 0.8 to 1 mA and an applied voltage of 5.7 to 6.1 V.

It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:

FIG. 1 shows a graph showing a phenomenon wherein the color purity of an organic EL device becomes higher (i.e. as the x-values on CIE chromaticity coordinates increase), the relative spectral sensitivity of the organic EL device decreases; and

FIG. 2 shows the structural formulas of NPB, copper (II) phthalocyanine (CuPc), (btp) 2 Ir(acac), Alq 3 , BAlq, and CBP used in one embodiment of the red phosphorescent compound according to the present invention.

Reference will now be made in detail to the preferred embodiments of the present invention associated with a red phosphorescent compound and an organic electroluminescent (EL) device using the red phosphorescent compound according to the present invention, examples of which are illustrated in the annexed drawings.

The present invention provides a red phosphorescent compound of Formula 1:

wherein

is

R1 is selected from the group consisting of C 1 -C 4 alkyl and C 1 -C 4 alkoxy; R2, R3, R4 and R5 are independently selected from the group consisting of hydrogen, C 1 -C 4 alkyl and C 1 -C 4 alkoxy; and

is selected from the group consisting of 2,4-pentanedione

2,2,6,6,-tetramethylheptane-3,5-dione

1,3

propanedione

1,3-butanedione

3,5-heptanedione

1,1,1-trifluoro-2,4-pentanedione

1,1,1,5,5,5-hexafluoro-2,4-pentanedione

and 2,2-dimethyl-3,5-hexanedione

in Formula 1 is selected from the following compounds:

Examples of preferred compounds that can be represented by Formula 1 include the following compounds:

Further, the present invention provides an organic electroluminescent (EL) device comprising an organic light-emitting layer wherein the red phosphorescent compound of Formula 1 is used as a dopant for the organic light-emitting layer.

A host used in the organic light-emitting layer of the organic EL device according to the present invention may be selected from Al complexes, Zn complexes, and carbazole derivatives. The red phosphorescent compound of Formula 1 may be used as the dopant in an amount of 0.5 to 20% by weight. When the dopant is used within this range, improvements in chromaticity coordinate characteristics (CIE color purity X≧0.65), high luminescence efficiency, and long luminescence lifetime of the organic EL device can be realized. The Al and Zn complexes may have at least one ligand selected from the group consisting of quinol, biphenyl, isoquinol, phenyl, methylquinol, dimethylquinol and dimethylisoquinol. The carbazole derivatives may be preferably 4,4′-N,N′ dicarbazole biphenyl (CBP).

Hereinafter, a method for synthesizing iridium (III) (3-(3,5-dimethylphenyl)-4-methylisoquinolinato-N,C 2′ )(2,4-pentanedionate-O,O) (“A-7”), which is a red phosphorescent compound represented by Formula 1 according to the present invention.

Synthesis Example

1. Synthesis of 3-(3,5-dimethylphenyl)-4-methylisoquinoline

Propane-2-yl-benzene (100 mmol), acetic acid palladium (2.7 mmol), triphenylphosphine (5 mmol), and sodium carbonate (50 mmol) were put in a dried two-neck round-bottom flask and dissolved in DMF (100 ml). Subsequently, tert-butyl-(2-iodo-benzylidene)amine (50 mmol) was added thereto. The resulting solution was stirred in a bath at 100° C. for 24 hours. After completion of the reaction, the temperature was allowed to cool to room temperature. The reaction mixture was extracted with ether and saturated NH 4 Cl. The extract was distilled under reduced pressure. The resulting residue was purified by silica gel column chromatography (hexane: EtOAc=5:1) to yield 3-(3,5-dimethylphenyl)-4-methylisoquinoline.

2. Synthesis of Dichloro-Crosslinked Dimer Complex

Iridium (III) chloride hydrate (1 mmol), 3-(3,5-dimethylphenyl)-4-methylisoquinoline (2.5 mmol), and a mixed solvent (3:1) of ethoxyethanol and distilled water were put in a dried two-neck round-bottom flask. After the mixture was refluxed for 24 hours, water was added thereto. The resulting solid was filtered, followed by washing with methanol and petroleum ether to yield the dichloro-crosslinked dimer complex.

3. Synthesis of iridium (III) 3-(3,5-dimethylphenyl)-4-methylisoquinato-N,C 2′ )(2,4-pentanedionate-O,O)

The dichloro-crosslinked dimer complex (1 mmol), 2,4-pentanedione (3 mmol), Na 2 CO 3 (6 mmol) and 2-ethoxyethanol (30 mL) were put in a dried two-neck round-bottom flask. Then, the mixture was refluxed for 24 hours. After the temperature was allowed to room temperature, distilled water was added thereto to obtain a solid. The solid was filtered and dissolved in dichloromethane. The solution was filtered through silica gel. The solvent was distilled off under reduced pressure and the residue was washed with methanol and petroleum ether to yield the compound.

Hereinafter, a detailed description will be made of preferred examples associated with the organic electroluminescent (EL) device according to the present invention. More specifically, the examples relate to an organic electroluminescent (EL) device comprising an organic light-emitting layer which uses the red phosphorescent compound of Formula 1 as a dopant. The invention is not to be construed as being limited to the examples.