WO1999063023A1

WO1999063023A1 - Organic electroluminescent device and high melting point organometallic complex therefor

Info

Publication number: WO1999063023A1
Application number: PCT/KR1998/000309
Authority: WO
Inventors: Se Hwan Son; Kong Keum Kim; Ok Hee Kim; Jae Soon Bae; Seok Hee Yoon; Hyo Seok Kim; Youn Gu Lee
Original assignee: Lg Chemical, Ltd.
Priority date: 1998-05-30
Filing date: 1998-10-09
Publication date: 1999-12-09
Also published as: KR19990086810A

Abstract

Disclosed is an organic electroluminescent device including a pair of counter electrodes, an organic luminescent layer located between the counter electrodes, and at least one of a hole injecting/transporting layer and electron injecting/transporting layer. Also disclosed is an organometallic complex, which has a light emitting property and a high melting point. The organometallic complexes are used as a material for an electron injecting/transporting layer of the electroluminescent device.

Description

ORGANIC ELECTROLUMINESCENT DEVICE AND HIGH MELTING POINT ORGANOMETALLIC COMPLEX THEREFOR

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to an electroluminescent device, more particularly, to an organic electroluminescent device having an organometallic material layer.

Description of Related Technology An organic electroluminescent device (hereinafter referred to as an organic EL device) is basically composed of an anode, an electroluminescent layer, and a cathode. Early attempts for fabricating an efficient organic EL device have not been successful due to difficulties of injecting carriers from opposite electrodes onto the electroluminescent layer, which containes organic dye molecules. In order to fabricate an efficient organic EL device, it is desirable that the organic dye molecules have a capability of accepting carriers from the electrodes and that mobility of the carriers be high. In addition, the carrier recombination zone should be away from the electrodes to prevent exciton quenching by the metallic electrodes.

The problem has been solved by a two-layer organic EL device including an anode, a hole transporting layer, a electron transporting layer and a cathode, which is disclosed in U.S. Patent No. 4,359,507. The two-layer organic EL device uses a triphenylamine-containing compound and an aluminum complex of 8- hydroxyquinoline as a hole transporting material and an electron transporting material respectively, in which the latter compound acts as a light emitting material of the device as well. Facile injection of carriers and high mobility of the carriers in the two-layer EL device lower the driving voltage of the device. Further, formation of the recombination zone near the interface of the transporting layers, but away from the cathode caused by low mobility of the holes in the electron transporting layer compared to that of electrons, reduces the probability of the exciton quenching by the metallic cathode. As a result, the power conversion efficiency of the organic EL device has been improved up to 1.51m/W, and the brightness is hundreds cd/m² at a voltage of less than 10 V.

Further improvement of an organic EL device in its durability and efficiency has been made by introducing a hole injecting layer containing a copper phthalocyanine (hereinafter referred to as CuPc) between the anode and hole transporting layer, which is disclosed in U.S. Patent No. 4,539,507. Enhancing the carrier injection has been considered as an important factor for improving the properties of EL devices.

More recently, improvements in uniformity of light emission and emission efficiency have also been made. An organic EL device having an adhesive layer in contact with the cathode and the light emitting layer is disclosed in U.S Patent No. 5,516,577. In the above device, the adhesive layer is a metal complex of 8- hydroxyquinoline or a derivative thereof. The adhesive layer has an energy gap and a thickness smaller than those of the light emitting layer, respectively, and enhances the uniformity of light emission and the durability of the device. In general, the ease of electron injection into the electron transporting layer from the cathode strongly influences the power conversion efficiency of the organic EL device. Therefore, intensive investigation has been also made to reduce the injection barrier of electrons by lowering the work function of the cathode to match the LUMO (lowest unoccupied molecular orbital) level of the electron transporting layer. Alloys of Mg:Ag and Al:Li are most popular cathode materials for this purpose. More recently, a cathod layer including LiF/Al bilayer structure, which is highly efficient and more stable in environmental perspective, has been developed. According to IEEE transactions on electron devices 44, 1222, 1997, thermal breakdown of an organic EL device during operation could be a major problem for the high information displays. Thus, thermally more stable materials are required for a reliable operation of the organic EL devices. In this perspective, one of the most popular EL device material is chelated oxinoid compounds which satisfy the following structural formula:

Wherein, "Me" represents a metal; "n" is an integer of from 1 to 3; and "Z" independently in each occurrence represents atoms which complete a nucleus having at least two fused aromatic rings. Tris (8-hydroxyquinoline) aluminum complex is a widely investigated material, which satisfies the above general structure. The material provides highly efficient and stable organic EL device with a proper selection of a hole transporting material, a fluorescent dopant and a cathodic material. Organic EL devices emitting green, yellow and red can be fabricated by using the tris (8-hydroxyquinoline) aluminum complex as an electron transporting host material and a fluorescent dopant molecule, which allows the host-guest energy transfer process.

Also, metal chelates of benzazoles have been developed by different groups as an emitting material with the electron transporting property. Such materials are disclosed in Europe Patent No. 0,652,273 Al; Japanese Patent Application Laid-Open No. 6-336586/1994; Europe Patent No. 0,700,917 A2; U.S. Patent No. 5,779,937; and U.S. Patent No. 5,486,406; and their general structures are as shown below:

SUMMARY OF THE INVENTION The present invention provides an organometallic complex which has a high melting point for use in organic EL devices. The present invention provides an organometallic complex which lowers the drive voltage of organic EL devices and enhances the durability of organic EL devices. The present invention further provides a synthetic method for the organometallic complexe for use in organic EL devices. Still further, the present invention provides an organic EL device containing the organometallic complex.

BRIEF DESCRIPTION OF THE DP AWTNGS Fig. 1 is a schematic diagram showing an organic EL device with three-layer structure. Fig. 2 is a schematic diagram showing an organic EL device with two-layer structure.

Fig. 3 is a schematic diagram showing an organic EL device with another two- layer structure.

Fig. 4 is a graph showing changes of luminescence with respect to voltage applied to the organic EL devices of Example 2-1, Example 2-2, Comparative Example 2-1 and Comparative Example 2-2. Fig. 5 is a graph showing changes of luminescence with respect to time of organic EL devices of Example 2-1, Example 2-2, Comparative Example 2-1 and Comparative Example 2-2.

Fig. 6 is a graph showing changes of driving voltage with respect to time of organic EL devices of Example 2-1, Example 2-2, Comparative Example 2-1 and Comparative Example 2-2.

Fig. 7 is a graph showing changes of luminescence with respect to voltage applied to the organic EL devices of Example 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with the present invention, an organic EL device comprises a layer which contains an organometallic complex of the following general formula (I):

( I ) In the formula (I), Ri to R₇ represent substitution possibilities at each position, and each represents hydrogen atom, hydrocarbons, halogen atoms, aromatic groups, heterocyclic rings or any other functional groups. "X" indicates sulfur or oxygen atom. "M" is a divalent metal such as beryllium, zinc, calcium and the like.

Following chemical formulas are examples of the organometallic complex which has the structure represented by the general formula (I) in accordance with the present invention.

Chemical Formula 1

Chemical Formula 2

Chemical Formula 3

Organometallic complexes having high melting points are obtained by satisfying the general formula (I). Previously, only a few methods have been known to make high melting point organometallic complexes. The most popular method to achieve it is done by using the dimerized structure of an aluminum complex, which is disclosed in U.S. Patent No. 5,466,392. In accordance with the present invention, the goal is achieved by even a simpler way, in which neither the molecular weight nor length of the molecule is significantly increased.

Melting points of the molecules satisfying the general formula (I) are unexpectedly high, in comparison with the conventional organometallic complex molecules, which are suggested in Europe Patent No. 0,652,273 Al, Japanese Patent Application Laid-Open No. 6-336586/1994, Europe Patent No. 0,700,917 A2, U.S. Patent No. 5,779,937 and U.S. Patent No. 5,486,406. Also the presence of pyridine- like ring in the ligand molecule forming the organometallic complex results in good adhesion to the cathode layer. Comparing with a organic EL device in which Alq3 is in contact with the cathode layer, a device using a molecule having a chemical formula 3 satisfying the general formula (I) but having lower melting point than that of the Alq3 is more durable. The organometallic complex molecules having high melting point provide organic EL devices with significant improvement in the lifetime which will be shown in the experimental section. The driving voltage of an organic EL device can also be lowered when a molecule having a structure of the chemical formula 1 which satisfies the general structure (I) is used as an electron injecting/transporting layer. Also the ratio of voltage increase during operation can be minimized for both molecules having chemical formula 1 and 3 satisfying the general formula (1) which will be shown in the experimental section.

In accordance with the present invention, the organic EL device includes a pair of counter electrodes, an organic luminescent layer located between the counter electrodes, and at least one of a hole injecting/transporting layer and electron injecting/transporting layer. For example, as illustrated in Fig. 1, the device includes a substrate 1, an anode 2, a hole injecting/transporting layer 3, a light emitting layer 4, an electron injecting and transporting layer 5, and a cathode 6. Alternatively, as illustrated in Fig. 2, the device may include a substrate 1, an anode 2, a light emitting layer 4, an electron injecting/transporting layer 5, and a cathode 6. Further, as illustrateed in Fig. 3, the device may include a substrate 1, an anode 2, a hole injecting/transporting layer 3, a light emitting layer 4, and a cathode 6. In these devices, the luminescent layer, the hole injecting/transporting layer, and the electron injecting/ transporting layer can be provided more than once.

The organometallic complex structure represented by the general formula (I) is advantageously applied to the luminescent layer or the electron injecting/transporting layer. The substrate material is not particularly limited, and conventionally used materials, such as glass, transparent plastic, quarts or the like can also be used.

The anode preferably has a work function of greater than 4 eV. The anode is, advantageously, made of carbon, aluminum, vanadium, chromium, copper, zinc, silver, gold and similar metals and alloys of these metals, and zinc oxide, indium oxide, ITO, and similar tin oxide or tin oxide indium based complex compounds, such as copper iodide, mixtures of oxides and metals, such as ZnO:Al, SnO₂:Sb, and conductive polymers, such as poly (3-methylthiophene), poly[3,4-(ethylene-l,2- dioxy)thiophene], polypyrrole, and polyaniline. Film thickness of the anode is in the range of about 10 nm to about 1000 nm, and more preferably, it is from about 10 nm to about 500 nm although it is varied depending on the kind of the materials.

The cathode preferably has a work function of smaller than 4 eV. It is advantageously made of the magnesium, calcium sodium, potassium, titanium, indium, yttrium, lithium, gadolinium, aluminum, silver, tin, lead, and similar metals and alloys thereof. Also a cathode having a bilayer structure, such as LiF/Al and Li₂O/Al can be used. Film thickness of the cathode is preferably in the range of about 10 nm to about 1000 nm, and more preferably, from about 10 nm to about 500 nm.

The hole injecting/transporting layer contains a hole transfer compound and has the function of transferring holes injected from the anode to the light emitting layer. By the presence of this hole injecting/transporting layer between the anode and the light emitting layer, a large amount of hole is injected into the light emitting layer at a low electric field. If necessary, the hole injecting/transporting layer may be divided into two sub-layers, one for hole injection and the other for hole transfer. In this structure, the hole injection layer is advantageously located between the anode and the hole transporting layer. Accordingly, the hole transporting layer is advantageously located between the hole injecting layer and the light emitting layer.

Preferably, the HOMO level of the hole injecting layer is located between that of the hole transporting layer and the work function of the anode. Hole injecting compounds are, for example, metal porphyrine, as disclosed in U.S. Patent No. 4,356,429, and derivatives of quinacridone, as disclosed in U.S. Patent No. 5,616,427 and conjugated polymers such as polythiophene having high hole mobility. Examples of the hole transporting compound are oxadiazole derivatives, triazole derivatives, phenylene derivatives, arylamine derivatives, conjugated poylmers, block co-polymers with conjugated and non-conjugated repeating units, and the like. The hole transporting compound can be also used as a hole injecting/transporting material without the hole injecting layer. Advantagously, a derivative of the arylamine, 4,4'- bis[N-(l-naphthyl)-N-phenylamino]biphenyl(hereinafter referred to as NPB) is used as the hole injecting/ transporting material for the fabrication of organic EL devices.

Examples of the arylamine compound are disclosed in U.S. Pat. Nos. 5,554,.450; 5,358,788; 5,536,949; 5,256,945; 5,374,489 and 5,707,747. The hole injecting/transporting layer or the hole transporting layer may contain one or more of these hole injecting/transporting materials. Also, the hole transporting layer comprises other materials with high fluorescent quantum efficiency and capability of accepting electrons from the electron transporting layer to form an emission center at the hole transporting layer. The compounds satisfying the general formula (I) are preferably used as the light emitting material of the light emitting layer. The light emitting layer containing the organometallic complexes described above may be doped with a fluorescent substance, if desired. Advantageously, the band gap of the fluorescent substance is close to or smaller than that of the organometallic complexes. By a proper selection of the fluorescent substance and its concentration, quantum efficiency and the longevity of the device can be enhanced. Also, by a proper selection of the fluorescent substance and its concentration, the emission color of the organic EL device can be tuned, and a narrow emission spectrum can be obtained. When the compound satisfying the general formula (I) is used in a layer other than the light emitting layer, the material of the light emitting layer is not particularly limited to but selected for use from generally known materials.

Light emitting materials which does not satisfy the general formula (I) may also be used in the light emitting layer if they have suitable properties to form a thin film. Especially, a compound having high fluorescent quantum efficiency is preferable. Examples of such compounds include 8-hydroxyquinoline metal complex, dimerized styryl compound (U.S Patent No. 5,366,811), BAlq (U.S. Patent No 5,150,006), bis(10-hydroxybenzo[hlquinolinato)bery Ilium (U.S. Patent No. 5,529,853), 2-(2' -hydroxy-5'methylphenyl)benzotriazole metal complex (U.S. Patent No. 5,486,406), benzoxazole, benzthiazole, benzimidazole and derivatives thereof (U.S. Patent No. 5,645,948), poly(p-phenylene vinylene) and derivatives thereof (Synthetic Metals 91, 35, 1997 and Synthetic Metals 91, 109, 1997), polyfluorene or the like, with or without doping.

The present invention will be illusrtated by the following examples, which should not be taken to limit the scope of the invention.

Synthesis Example 1 : Chemical Formula 1 (2-benzothiazole-2-yl-pyridine-3-ol beryllium complex)

To a solution of 40 mg of beryllium chloride dissolved in 5 mL of distilled water, another solution containing 0.23 g of 2-benzothiazole-2-yl-pyridine-3 -ol and

40 mg of sodium hydroxide dissolved in 50 mL of ethanol and 10 mL of water was added. The mixture was then stirred for 3 hours 60 ^°C . After cooling the solution, the powder precipitated out was separated by vacuum filtration, washed with ethanol and dried to obtain 0.22 g of a yellow powder. Yield of the object product was 99 %. The product was further purified using a train sublimation method.

As the result of elemental analysis, the following values were obtained (values in the parentheses are theoretical values): C: 62.1 (62.1) %; H: 3.0 (3.0) %; N: 11.0 (12.0) %; Be: 1.6 (1.9) %.

The luminescent peak wave length was 460 nm.

Synthesis Example 2: Chemical Formula 2 (2-benzooxazole-2-yl-pyridine-3-ol beryllium complex) To a solution of 40 mg of beryllium chloride dissolved in 5 mL of distilled water, another solution containing 0.21 g of 2-benzooxazole-2-yl-pyridine-3-ol and 40 mg of sodium hydroxide dissolved in 50 mL of ethanol and 10 mL of water was added. The mixture was then stirred for 3 hours at 60 ^°C . After cooling the solution, the powder precipitated out was separated by vacuum filtration, washed with ethanol and dried to obtain 0.20 g of a white powder. Yield of the object product was 94 %. Yield of the object product was 99 %. The product was further purified using a train sublimation method.

As the result of elemental analysis, the following values were obtained(values in the parentheses are theoretical values): C: 66.7 (66.8) %; H: 3.1 (3.3) %; N: 12.7 (12.9) %: Be: 2.0 (2.1) %.

The luminescent peak wave length was 432 nm.

Synthesis Example 3: Chemical Formula 4 (2-benzothiazole-2-yl-pyridine-3-ol zinc complex) To a solution of 110 mg of zinc acetate dihydrate dissolved in 50 mL of absolute ethanol, 230 mg of 2-benzothiazole-2-yl-pyridine-3-ol was added. The mixture was then stirred for 6 hours at 60 ^°C . After cooling the solution, the powder precipitated out was separated by vacuum filtration, washed with ethanol and dried to obtain 0.21 g of a pale yellow powder. Yield of the object product was 84 %. The product was further purified using a train sublimation method.

As the result of elemental analysis, the following values were obtained (values in the parentheses are theoretical values): C: 55.4 (55.4) %; H: 2.5 (2 7) %; N: 10.5 (10.7) %; Zn: 12.1 (12.6) %.

The luminescent peak wave length was 500 nm.

Example 1

Melting points of some materials satisfying general formula (I) of the present invention are summarized in Table 1. Peak maximum values of photoluminescent spectra of the materials prepared as thin films on a glass plate by thermal evaporation are also summarized in Table 1. Also, to make the melting point improvement obvious, a comparative example is provided below.

Table 1

Entry number Chemical Melting PL peak Max Structure Point

Chemical Formula 1 426°C 460 nm

Chemical Formula 2 330°C 432 nm

Chemical Formula 3 384°C 500 nm

Comparative Example 1

Melting points of most representative materials of the prior arts having structures most similar to those of the present invention are summarized in Table 2. Peak maximum values of photoluminescent spectra of the materials prepared as thin films on a glass plate by thermal evaporation are also summarized in Table 2.

Iahle 2

Entry number Chemical Melting PL peak Max Structure Point

Chemical Formula 4 324°C 465 nm

Chemical Formula 5 265°C 430 nm

Chemical Formula 6 300°C 491 nm

Example 2-1

A glass plate (a product of General vacuum) on which an ITO transparent electrode was formed was used as the transparent substrate. The substrate was subjected to ultrasonic cleaning with methanol, acetone, isopropyl alcohol, acetone and methanol in sequence for 5 minutes in each solvent, and dried in a vacuum oven for 1 hour at 110^°C . The substrate was further cleaned by an oxygen plasma under reduced pressure in a vacuum chamber. The substrate was transferred into a thermal vacuum deposition chamber without breaking the vacuum and fixed by a substrate holder. An organic EL device, which comprises laminating layers in the order of ITO/CuPc/NPB/Alq3/Chemical Formula 1/LiF/Al with a film thickness of 20 nm(CuPc), 40 nm(NPB), 35 nm(Alq3), 15 nm(Chemical Formula 1), 0.5 nm(LiF) and 150 nm(Al) respectively, was fabricated by thermal vacuum evaporation. During the process, the deposition rates are maintained in the range of 0.1 ~ 0.3 nm/sec for the organic compounds, 0.02 nm/sec for the LiF and 0.3 ~ 0.7 nm for the aluminum in a high vacuum of about 10^"6 torr with substrate temperature set at a room temperature. A shadow mask was used for the patterning of the cathode electrode to make active device area of 0.08 cm².

When the device was forward biased, green light from the Alq3 layer was observed. The light intensities at various voltages are illustrated in Fig. 4. The luminescence decay of the device under constant current driving condition (60 mA/cm2 of DC) under a nitrogen atmosphere (8 % relative humidity at room temperature) is illustrated in Fig. 5. The driving voltage change of the device during the lifetime test is illustrated in Fig. 6. Also, to make the capability of low voltage driving and the stability enhancement obvious, a comparative example is provided below. Example 2-2

An organic EL device, which comprises laminating layers in the order of ITO/CuPc/NPB/Alq3/Chemical Formula 3/LiF/Al with a film thickness of 20 nm(CuPc), 40 nm(NPB), 35 nm(Alq3), 15 nm(Chemical Formula 3), 0.5 nm(LiF) and 150 nm(Al) respectively, was fabricated by thermal vacuum evaporation in the same manner as in Example 2-1, except that the Chemical Formula 3 was used instaed of Chemical Formula 1 as an electron injecting/transporting material.

When the device was forward biased, green light from the Alq3 layer was observed. The light intensities at various voltages are illustrated in Fig. 4. The luminescence decay of the device under constant current driving condition (60 mA/cm2 of DC) under a nitrogen atmosphere (8 % relative humidity at room temperature) is illustrated in Fig. 5. The driving voltage change of the device during the lifetime test is illustrated in Fig. 6.

Comparative Example 2-1

An organic EL device, which comprises laminating layers in the order of ITO/CuPc/NPB/Alq3/Chemical Formula 3/LiF/Al with a film thickness of 20 nm(CuPc), 40 nm(NPB), 50 nm(Alq3), 0.5 nm(LiF) and 150 nm(Al) respectively, was fabricated by thermal vacuum evaporation in the same manner as in Example 2- 1 , except that the Chemical Formula 1 was not used.

When the device was forward biased, green light from the Alq3 layer was observed. The light intensities at various voltages are illustrated in Fig. 4. The luminescence decay of the device under constant current driving condition (60 mA/cm2 of DC) under a nitrogen atmosphere (8 % relative humidity at room temperature) is illustrated in Fig. 5. The driving voltage change of the device during the lifetime test is illustrated in Fig. 6. Comparative Example 2-2

An organic EL device, which comprises laminating layers in the order of ITO/CuPc/NPB/Alq3/Chemical Formula 6/LiF/Al with a film thickness of 20 nm(CuPc), 40 nm(NPB), 35 nm(Alq3), 15 nm(Chemical Formula 6), 0.5 nm(LiF) and 150 nm(Al) respectively, was fabricated by thermal vacuum evaporation in the same manner as in Example 2-1, except that the Chemical Formula 6 was used instaed of Chemical Formula 1 as an electron injecting/transporting material.

Example 3

An organic EL device, which comprises laminating layers in the order of ITO/NPB/Chemical Formula hRubrene (20:1)/Chemical Formula 1/LiF/Al with a film thickness of 60 nm (NPB), 30 nm (Chemical Formula 1 :Rubrene), 30 nm (Chemical Formula 1), 0.5 nm (LiF) and 150 nm (Al) respectively, was fabricated by thermal vacuum evaporation in the same manner as in Example 2-1, except that the Rubrene doped Chemical Formula 1 was used as a material forming the light emitting layer.

When the device was forward biased, yellow light emission from the rubrene was observed. The light intensities at various voltages are illustrated in Fig. 7 and the emission spectrum from the device is illustrated in Fig.8.

The above results show that the compounds satisfying the general formula (I) have high melting points. Further, when a compound satisfying the general formula (I) is used in a device, the device exhibits the excellent properties with respect to the low voltage driving capability, slow voltage increase during device operation and longer device lifetime.

Claims

WHAT TS CLAIMED TS:

1. An organic electroluminescent device comprising a layer containing a compound having a structure satisfying the following general formula (I):

( I ) wherein Ri to R₇ represent substitution possibilities at each position, and each represents hydrogen atom, hydrocarbons, halogen atoms, aromatic groups, heterocyclic rings or any other functional groups. "X" indicates sulfur or oxygen atom. "M" is a divalent metal such as beryllium, zinc, calcium and the like.

2. An organic electroluminescent device as defined in claim 1, wherein

"X" represents sulfur atom..

3. An organic electroluminescent device as defined in claim 1 wherein "M" represents beryllium atom.

4. An organic electroluminescent device as defined in claim 1 wherein "M" represents zinc atom.

5. An organic electroluminescent device as defined in claim 1, wherein the layer containing the compound satisfying the general formula (I) comprises a light emitting layer.

6. An organic electroluminescent device as defined in claim 1, wherein the layer containing the compound satisfying the general formula (I) comprises an electron injecting/transporting layer.

7. An organic electroluminescent device comprising in layered order of an anode, a hole injecting/transporting layer, a light emitting layer, an electron injecting/transporting layer and a cathode and the molecule satisfying the general formula (I) comprises the electron injecting/transporting layer.

8. An organic electroluminescent device as defined in claim 7, wherein the layer containing the Chemical Formula 1 comprises the electron injecting/transporting layer.

9. An organic electroluminescent device as defined in claim 7, wherein the layer containing the Chemical Formula 3 comprises the electron injecting/transporting layer.

10. An organic electroluminescent device comprising in layered order of an anode, a light emitting layer, an electron injecting/transporting layer and a cathode and the molecule satisfying the general formula (I) comprises the electron injecting/transporting layer.

11. An organic electroluminescent device as defined in claim 10, wherein the layer containing the Chemical Formula 1 comprises the electron injecting/transporting layer.

12. An organic electroluminescent device as defined in claim 10, wherein the layer containing the Chemical Formula 3 comprises the electron injecting/transporting layer.

13. An organic electroluminescent device as defined in claim 10, wherein the layer containing a luminescent polymer comprises the light emitting layer.