JPWO2008120355A1 - Organic EL device - Google Patents

Abstract

The organic EL device includes a light emitting layer disposed between a pair of a cathode and an anode, a hole transport layer disposed between the light emitting layer and the anode, and an electron transport layer disposed between the light emitting layer and the cathode. In the organic EL element, the light-emitting layer includes a first phosphorescent material, a second phosphorescent material, and a host material having a nitrogen-containing aromatic heterocycle, and the excited triplet level of the second phosphorescent material is It is higher than the excited triplet level of one phosphorescent material, and the host material has an ionization potential energy of at least 0.5 eV or more higher than that of the first phosphorescent material and the second phosphorescent material.

Description

  The present invention utilizes an organic compound electroluminescence (hereinafter also referred to as EL) that emits light by current injection, and includes an organic EL element (hereinafter also referred to as an organic EL element) having a light emitting layer in which such a substance is formed in a layer shape. Say).

  In general, each organic EL element constituting a display panel using an organic material has a glass substrate as a display surface, an anode as a transparent electrode, a plurality of organic material layers including an organic light emitting layer, and a cathode made of a metal electrode. In this way, it has a structure in which thin films are sequentially stacked. In addition to the organic light-emitting layer, the organic material layer has a layer made of a material having a hole transport ability such as a hole injection layer and a hole transport layer, and an electron transport ability such as an electron transport layer and an electron injection layer. An organic EL element including a layer made of a material and provided with these layers has also been proposed. The electron injection layer includes an inorganic compound.

  When an electric field is applied to the organic EL element of the organic light emitting layer and the laminate of the electron or hole transport layer, holes are injected from the anode and electrons are injected from the cathode. In the organic EL element, the electrons and holes are recombined in the organic light emitting layer, excitons are formed, and light emission emitted when it returns to the ground state is used. In order to increase the efficiency of light emission and to stably drive the device, the light emitting layer may be doped with a dye as a guest material.

  In recent years, it has been proposed to use a phosphorescent material in addition to a fluorescent material for the light emitting layer.

  Patent Document 1 is characterized in that the light emitting layer contains at least two impurities including a host material, a first phosphorescent impurity and a second phosphorescent impurity, and the first phosphorescent impurity and the second phosphorescent impurity each contain iridium or platinum. An organic electroluminescent display device is disclosed. This suggests that the use of the first phosphorescent impurity having excellent lifetime characteristics improves the lifetime of the second phosphorescent impurity. The first phosphorescent impurity is used at a concentration of 0.1% to 30%, and the second phosphorescent impurity is used at a concentration of 0.1% to 20%. Since energy transfer from the host material is primarily transferred to the first phosphorescent impurity, the light emission efficiency and the light emitting region are determined by the first phosphorescent impurity.

  Patent Document 2 is an organic electroluminescence element including a light emitting layer between a first electrode and a second electrode, wherein the light emitting layer includes two or more different kinds of light emitting materials, and the two different kinds of light emitting materials. An organic electroluminescent element is disclosed in which at least one of the above light emitting materials is a phosphorescent light emitting material. That is, a long wavelength light emitting layer and a short wavelength light emitting layer are sequentially formed between an anode and a cathode, and the long wavelength light emitting layer includes a phosphorescent material and an auxiliary dopant having a hole transport capability. The HOMO level H (h) of the light emitting layer host material, the HOMO level H (p) of the phosphorescent material, and the HOMO level H (a) of the auxiliary dopant are | H (h) −H (a) | <0.4 eV, | The relationship of H (a) −H (p) | <0.4 eV is satisfied. The auxiliary dopant is made of an amine-based material, an anthracene derivative, or an iridium complex.

Patent Document 2 includes a pair of electrodes and an organic layer disposed between the pair of electrodes, wherein the organic layer includes at least a light emitting layer, and the light emitting layer includes a host material, a first dopant, An organic electroluminescent device comprising at least a dopant having at least a second dopant, wherein the triplet lowest excitation level of the first dopant is higher than the triplet lowest excitation level of the host material; and An organic electroluminescent device is disclosed in which the triplet lowest excitation level of the second dopant is lower than the triplet lowest excitation level of the host material. This suggests that the first dopant concentration is preferably 0.1% by weight or more and 40% by weight or less.
JP2004-281386 JP 2004-311420 A JP 2006-128632 A

  When CBP is used as a light emitting layer host material as in the prior art, CBP has a high hole transporting property. Therefore, in order to increase the efficiency of the device, a hole blocking layer adjacent to the light emitting layer on the cathode side. It is essential to insert The hole blocking layer is generally composed of a material having a high ionization potential energy and an extremely high electron transporting property. It is considered that the material itself is deteriorated when holes cannot be allowed to exist and holes enter.

  On the other hand, a phosphorescent material typified by Ir (ppy) 3 has a hole transporting property. Therefore, when the light emitting layer has such a phosphorescent material at a high concentration, a CBP hole transporting layer which is a light emitting layer host material. In addition to this, holes are easily injected into the hole blocking layer through the phosphorescent material. Conventionally, in such a state, although the host material can be prevented from oxidative deterioration, the hole blocking material is remarkably oxidized and deteriorated by excessive holes, and as a result, the driving life of the organic EL element is shortened.

  Thus, the problem to be solved by the invention is to provide an organic EL element capable of extending the life as an example.

  The organic EL device of the present invention is a light emitting layer disposed between a pair of cathode and anode, a hole transport layer disposed between the light emitting layer and the anode, and disposed between the light emitting layer and the cathode. An organic EL element including an electron transport layer, wherein the light-emitting layer includes a first phosphorescent material, a second phosphorescent material, and a host material having a nitrogen-containing aromatic heterocycle, and the second phosphorescent material The excited triplet level of the first phosphorescent material is higher than the excited triplet level of the first phosphorescent material, and the host material has an ionization potential that is at least 0.5 eV higher than the first phosphorescent material and the second phosphorescent material. It has energy.

  In the present invention, the host material has a larger ionization potential energy than the first and second phosphorescent materials by at least 0.5 eV or more so that positive holes can be reliably obtained. The host material can be prevented from being oxidized and deteriorated due to the excessive holes trapped by the first and second phosphorescent materials. In the present invention, when a hole blocking layer is used, excessive supply of holes to the electron transport layer is prevented.

  In the embodiment of the present invention, BAlq can be used as the light emitting layer host material. In the above prior arts of Patent Documents 1 and 2, only CBP is disclosed as the host material. In Patent Document 3, a fluorene multimer is disclosed. Is only disclosed. In the above prior art of Patent Document 2, the difference in the HOMO level between the light emitting layer host material and the auxiliary dopant is less than 0.4 eV. In the present invention, the light emitting layer host material and the auxiliary dopant correspond to the first. The difference in ionization potential energy between the two phosphorescent materials needs to be 0.5 eV or more.

1 is a structural diagram showing an organic EL device according to the present invention. 1 is a structural diagram showing an organic EL device according to the present invention. 1 is a structural diagram showing an organic EL device according to the present invention.

Explanation of symbols

1 Glass substrate 2 Transparent electrode (anode)
3 Organic hole transport layer 3a Hole injection layer 4 Organic light emitting layer 6 Electron transport layer 7 Metal electrode (cathode)
7a Electron injection layer

BEST MODE FOR CARRYING OUT THE INVENTION

  Embodiments of the present invention will be described below with reference to the drawings.

  As shown in FIG. 1, the organic EL device of the present invention is composed of at least an anode 2, a hole transport layer 3, a light emitting layer 4, an electron transport layer 6 and a cathode 7, for example, on a transparent substrate 1 such as glass. Thus, a transparent anode 2, a hole transport layer 3 made of an organic compound, a light emitting layer 4 made of an organic compound, an electron transport layer 6 made of an organic compound, and a metal cathode 7 of a low work function material, for example, are obtained. .

In addition to the above structure, other organic EL element structures include those in which an electron injection layer 7a such as Li ₂ O is laminated as a thin film between the electron transport layer 6 and the cathode 7 as shown in FIG. included. Further, in addition to the above structure, other organic EL element structures include a hole injection layer such as a porphyrin compound such as copper phthalocyanine (CuPc) between the anode 2 and the hole transport layer 3 as shown in FIG. Also included are those obtained by laminating and forming 3a as a thin film.

  The cathode 1 may be made of a metal having a small work function such as aluminum, magnesium, indium, silver, or an alloy of each of which has a thickness of about 100 to 5000 angstroms. Further, for example, the anode 2 is made of a conductive material having a large work function such as indium tin oxide (hereinafter referred to as ITO) and has a thickness of about 1000 to 3000 angstroms or gold and a thickness of about 800 to 1500 angstroms. Things can be used. In addition, when gold is used as an electrode material, the electrode is in a translucent state. One of the cathode and the anode may be transparent or translucent.

  In the organic EL device of this embodiment, the light emitting layer 4 is doped with an organic material having an electron transporting ability as a host material and a phosphorescent material as a guest material, and the ionization potential energy (Ip) of the organic host material is It has an ionization potential energy that is at least 0.5 eV or more greater than that of the first and second phosphorescent materials. In the light emitting layer 4, the film thickness is 5 nm to 3000 nm, and the total content of the first and second phosphorescent materials is preferably 50% by weight or less doped with respect to all kinds of materials. This is for inhibiting the passage of holes from the light emitting layer to the electron transporting layer through the phosphorescent material.

  As a host material of the light emitting layer 4, any of the aluminum chelate complexes containing the quinoline ring shown by the following (A1) may be used, for example.

上記（Ａ１）中、Ｒ^１はアルキル基、オキシ基、アミノ基、又は少なくとも１個の炭素原子を有する炭化水素基が置換基に含まれ、いずれの炭化水素部分においても、炭素原子数が１〜１０個であり、Ｒ^２〜Ｒ^６は独立に、水素原子、アルキル基、オキシ基、アミノ基、又は少なくとも１個の炭素原子を有する炭化水素基が置換基に含まれ、いずれの炭化水素部分においても、炭素原子数が１〜１０個であり、また、Ｒ^４、Ｒ^５、及びＲ^６は、独立に、シアノ、ハロゲン、並びに１０個以下の炭素原子を含有するα−ハロアルキル、α−ハロアルコキシ、アミド、スルホニル、カルボニル、カルボニルオキシ、及びオキシカルボニル置換基、から選択され得、Ｌは、下記（Ａ２）の何れかである。(A1)

In the above (A1), R ¹ is an alkyl group, an oxy group, an amino group, or a hydrocarbon group having at least one carbon atom in the substituent, and any hydrocarbon moiety has 1 carbon atom. R ^{2 to} R ⁶ are each independently a hydrogen atom, an alkyl group, an oxy group, an amino group, or a hydrocarbon group having at least one carbon atom, and any hydrocarbon. The moiety also has 1 to 10 carbon atoms, and R ⁴ , R ⁵ , and R ⁶ are independently cyano, halogen, and α-haloalkyl containing 10 or less carbon atoms, α -Haloalkoxy, amide, sulfonyl, carbonyl, carbonyloxy, and oxycarbonyl substituents, wherein L is any of (A2) below.

上記（Ａ２）中、Ｒ^７〜Ｒ^１１は、独立して、水素又は炭素原子数１〜１２個の炭化水素基を表し、また、Ｒ^７及びＲ^８は一緒に、あるいはＲ^８及びＲ^９は一緒に縮合ベンゾ環を形成され得、Ｒ^１２〜Ｒ^２６は、独立して、水素又は炭素原子数１〜１２個の炭化水素基を表し、また、Ｒ^１２及びＲ^１３若しくはＲ^１３及びＲ^１４は一緒に、Ｒ^１７及びＲ^１８若しくはＲ^１８及びＲ^１９は一緒に、並びにＲ^２２及びＲ^２３若しくはＲ^２３及びＲ^２４は一緒に、縮合ベンゾ環を形成され得る。(A2)

In the above (A2), R ^{7 to} R ¹¹ independently represent hydrogen or a hydrocarbon group having 1 to 12 carbon atoms, and R ⁷ and R ⁸ together, or R ⁸ and R ⁹ Can form a fused benzo ring together, R ^{12 to} R ²⁶ independently represent hydrogen or a hydrocarbon group having 1 to 12 carbon atoms, and R ¹² and R ¹³ or R ¹³ and R ¹⁴ together, R ¹⁷ and R ¹⁸ or R ¹⁸ and R ¹⁹ together, and R ²² and R ²³ or R ²³ and R ²⁴ together can form a fused benzo ring.

  Although there exist some which are shown by following formula (44)-(73) in this aluminum chelate complex, it is not limited to these.

(44)

(45)

(46)

(47)

(48)

(49)

(50)

(51)

(52)

(53)

(54)

(55)

(56)

(57)

(58)

(59)

(60)

(61)

(62)

(63)

(64)

(65)

(66)

(67)

(68)

(69)

(70)

(71)

(72)

なお、上記式中、Ｂｕはブチル基を示し、ｔ−Ｂｕは第３級ブチル基を示す。(73)

In the above formula, Bu represents a butyl group, and t-Bu represents a tertiary butyl group.

  Moreover, the electron transporting host material of the light emitting layer 4 of the organic EL element of this embodiment is an electron transporting nitrogen-containing aromatic complex containing a pyridine ring and a carbazolyl group as shown in the following formulas (B1) to (B3). It may be a ring compound.

上記式（B１）中、Ｚは、直接結合、あるいは、カルバゾール環の窒素原子同士を共役可能とする任意の連結基を表す。Ｑは、Ｇにつながる直接結合を表す。Ｂは、ヘテロ原子としてＮ原子をｎ個有する六員環の芳香族複素環である。ｎは、１〜３の整数である。Ｇは、環ＢのＮ原子のオルト位及びパラ位にあるＣ原子に結合する。Ｇは、Ｑにつながる場合は、Ｑにつながる直接結合または任意の連結基を表す。Ｇは、Ｑにつながらない場合は、芳香族炭化水素基を表す。ｍは、３〜５の整数である。一分子中に存在する複数個のＧは、同一であっても異なっていてもよい。環Ｂは、Ｇ以外にも置換基を有していてもよい。(B1)

In the above formula (B1), Z represents a direct bond or an arbitrary linking group capable of conjugating nitrogen atoms of the carbazole ring. Q represents a direct bond leading to G. B is a 6-membered aromatic heterocycle having n N atoms as heteroatoms. n is an integer of 1 to 3. G is bonded to C atoms at the ortho and para positions of the N atom of ring B. When G is connected to Q, it represents a direct bond or any linking group connected to Q. When G does not lead to Q, it represents an aromatic hydrocarbon group. m is an integer of 3-5. A plurality of G present in one molecule may be the same or different. Ring B may have a substituent other than G.

上記式（B２）中、Ｚ１及びＺ２は、直接結合または任意の連結基を表す。Ｚ１、Ｚ２及び環Ａは、置換基を有していてもよい。Ｚ１及びＺ２は、同一であっても異なっていてもよい。Ｑは、Ｇにつながる直接結合を表す。Ｂは、ヘテロ原子としてＮ原子をｎ個有する六員環の芳香族複素環である。Ｇは、環ＢのＮ原子のオルト位及びパラ位にあるＣ原子に結合する。Ｇは、Ｑにつながる場合は、Ｑにつながる直接結合または任意の連結基を表す。Ｇは、Ｑにつながらない場合は、芳香族炭化水素基を表す。ｍは、３〜５の整数である。一分子中に存在する複数個のＧは、同一であっても異なっていてもよい。環Ｂは、Ｇ以外にも置換基を有していてもよい。(B2)

In said formula (B2), Z1 and Z2 represent a direct bond or arbitrary coupling groups. Z1, Z2 and ring A may have a substituent. Z1 and Z2 may be the same or different. Q represents a direct bond leading to G. B is a 6-membered aromatic heterocycle having n N atoms as heteroatoms. G is bonded to C atoms at the ortho and para positions of the N atom of ring B. When G is connected to Q, it represents a direct bond or any linking group connected to Q. When G does not lead to Q, it represents an aromatic hydrocarbon group. m is an integer of 3-5. A plurality of G present in one molecule may be the same or different. Ring B may have a substituent other than G.

上記式（B３）中、Ｚ１及びＺ２は、直接結合又は任意の連結基を表す。Ｚ１及びＺ２は同一であっても異なっていても良い。環Ｂ１及び環Ｂ２は、ピリジン環である。Ｚ１、Ｚ２、環Ｂ１及び環Ｂ２は、それぞれ置換基を有していても良い。(B3)

In said formula (B3), Z1 and Z2 represent a direct bond or arbitrary coupling groups. Z1 and Z2 may be the same or different. Ring B1 and ring B2 are pyridine rings. Z1, Z2, ring B1 and ring B2 may each have a substituent.

  Specific examples of such nitrogen-containing aromatic heterocyclic compounds include compounds represented by the following formulas (74) to (79).

(74)

(75)

(76)

(77)

(78)

以上のように、高い電子輸送性を期待できる材料、つまり、キノリン環やピリジン環を有する含窒素複素環化合物を発光層ホスト材料とする。(79)

As described above, a material that can be expected to have a high electron transport property, that is, a nitrogen-containing heterocyclic compound having a quinoline ring or a pyridine ring is used as the light emitting layer host material.

  Furthermore, the first and second phosphorescent materials in the light emitting layer 4 of the organic EL element of this embodiment may be any organometallic complex represented by the following general formula (C1).

式（Ｃ１）中、ＭはＯｓ、Ｉｒ、Ｐｔ、Ａｕのいずれかの金属であり、ｍは該金属の価数を表し、Ｒ^１〜Ｒ^１１は独立に、水素原子、アルキル基、オキシ基、アミノ基、又は少なくとも１個の炭素原子を有する炭化水素基が置換基に含まれ、いずれの炭化水素部分においても、炭素原子数が１〜１０個であり、また、Ｒ^１〜Ｒ^１１は、独立に、シアノ、ハロゲン、並びに１０個以下の炭素原子を含有するα−ハロアルキル、α−ハロアルコキシ、アミド、スルホニル、カルボニル、カルボニルオキシ、及びオキシカルボニル置換基、から選択され得、また、Ｒ^１及びＲ^２は、一緒に、あるいはＲ^２及びＲ^３は一緒に、あるいはＲ^３及びＲ^４は一緒に、あるいはＲ^４及びＲ^５は一緒に、あるいはＲ^５及びＲ^６は一緒に、あるいはＲ^６及びＲ^７は一緒に、あるいはＲ^７及びＲ^８は一緒に、縮合ベンゾ環を形成され得る。
イオン化ポテンシャルエネルギーは、光電子分光法により求めることができる。上記式（４４）乃至（７９）の化合物のイオン化ポテンシャルエネルギーを、光電子分光法（理研計器ＡＣ−１）により測定した結果を表１にまとめる。(C1)

In the formula (C1), M is a metal selected from Os, Ir, Pt, and Au, m represents the valence of the metal, and R ^{1 to} R ¹¹ are independently a hydrogen atom, an alkyl group, or an oxy group. , An amino group, or a hydrocarbon group having at least one carbon atom is included in the substituent, and in any hydrocarbon portion, the number of carbon atoms is 1 to 10, and R ^{1 to} R ¹¹ are , Independently, cyano, halogen, and α-haloalkyl, α-haloalkoxy, amide, sulfonyl, carbonyl, carbonyloxy, and oxycarbonyl substituents containing up to 10 carbon atoms, and R ¹ and R ² together, R ² and R ³ together, R ³ and R ⁴ together, R ⁴ and R ⁵ together, R ⁵ and R ⁶ together, or R ⁶ Fine R ⁷ together, or R ⁷ and R ⁸ together may form a fused benzo ring.
The ionization potential energy can be obtained by photoelectron spectroscopy. Table 1 summarizes the results of measuring the ionization potential energy of the compounds of the above formulas (44) to (79) by photoelectron spectroscopy (RIKEN KEIKI AC-1).

実施形態において、発光中心となる第１燐光材料に加えて、第２燐光材料を発光層に添加する。

In the embodiment, a second phosphorescent material is added to the light emitting layer in addition to the first phosphorescent material that becomes the emission center.

  The ionization potential energy of the light emitting layer host material is selected to be 0.5 eV or greater than the ionization potential energy of the first and second phosphorescent materials.

  Some of the first and second phosphorescent materials are represented by the following formulas (80) to (91), but are not limited thereto.

(80)

(81)

(82)

(83)

(84)

(85)

(86)

(87)

(88)

(89)

(90)

燐光材料の励起三重項準位は、燐光発光スペクトルから求めることができる。燐光発光スペクトルの最大強度波長から求めた励起三重項準位と、前記発光層ホスト材料と同様に測定したイオン化ポテンシャルエネルギーを表２にまとめる。(91)

The excited triplet level of the phosphorescent material can be determined from the phosphorescence emission spectrum. Table 3 summarizes the excited triplet level obtained from the maximum intensity wavelength of the phosphorescence emission spectrum and the ionization potential energy measured in the same manner as the light emitting layer host material.

さらにまた、実施形態において、正孔輸送層３に含まれる成分は、例えば、下記式（７）〜（２８）に示される正孔輸送能力を有する物質である。

Furthermore, in the embodiment, the component contained in the hole transport layer 3 is a substance having a hole transport ability represented by the following formulas (7) to (28), for example.

(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22)

(23)

(24)

(25)

(26)

(27)

正孔輸送層の膜厚は５ｎｍ〜３０００ｎｍであり、単層に限られず、複数の異なる材料から構成されていてもよい。複数の層から構成される場合において、発光層と隣接する正孔輸送層を第一の正孔輸送層とし、それ以外の正孔輸送層の構成層よりも第一の正孔輸送層を第一還元電位の小さいワイドバンドギャップの正孔輸送性材料から構成すると、発光層内で生成した励起子の発光層内への束縛がより促進され、効率が向上する場合がある。(28)

The thickness of the hole transport layer is 5 nm to 3000 nm, and is not limited to a single layer, and may be composed of a plurality of different materials. In the case of being composed of a plurality of layers, the hole transporting layer adjacent to the light emitting layer is the first hole transporting layer, and the first hole transporting layer is more than the other constituent layers of the hole transporting layer. When a hole transporting material having a wide band gap with a small one reduction potential is used, the binding of excitons generated in the light emitting layer into the light emitting layer is further promoted, and the efficiency may be improved.

  Furthermore, in the embodiment, in the embodiment, the component included in the electron transport layer 6 may be selected from, for example, substances represented by the following formulas (29) to (43).

(29)

(30)

(31)

(32)

(33)

(34)

(35)

(36)

(37)

(38)

(39)

(40)

(41)

(42)

なお、上記式中、Ｂｕはブチル基を示し、ｔ−Ｂｕは第３級ブチル基を示す。(43)

In the above formula, Bu represents a butyl group, and t-Bu represents a tertiary butyl group.

  In the embodiment, the component contained in the electron transport layer 6 can be selected from the organic materials having the electron transport capability mentioned as the organic host material.

  The film thickness of the electron transport layer is 5 nm to 3000 nm, and is not limited to a single layer, and may be composed of a plurality of different materials. In the case of being composed of a plurality of layers, the electron transport layer adjacent to the light-emitting layer is the first electron transport layer, and the first electron transport layer has a first oxidation potential more than the constituent layers of the other electron transport layers. When the electron transporting material has a large wide band gap, the binding of excitons generated in the light emitting layer into the light emitting layer is further promoted, and the efficiency may be improved.

  On the ITO anode (thickness 110 nm) on the glass substrate, materials were sequentially formed as follows to produce an organic EL element.

  Copper phthalocyanine was deposited by vacuum deposition to form a hole injection layer having a thickness of 25 nm.

  Next, similarly, NPB (4,4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl) of the above formula (22) is formed to form a 55 nm-thick hole transport layer. did.

  Further, on the hole transport layer, Ir (piq) 2acac (bis (phenylisoquinoline) iridium acetylacetonate) of the above formula (89) as the first phosphorescent material and Ir of the above formula (86) as the second phosphorescent material. (ppy) 2acac (bis (2-phenylpyridine) iridium acetylacetonate), and BAlq (bis (2-methyl-8-quinolinolato-N1, O8)-(1, 1′-biphenyl-4-olato) aluminum) was used to form a 40 nm light emitting layer by co-vacuum deposition. At this time, the content of Ir (piq) 2acac as the first phosphorescent material in the light emitting layer was adjusted to 5% by weight. Further, the content of Ir (ppy) 2acac as the second phosphorescent material in the light emitting layer was adjusted to 0 wt%, 19 wt%, 32 wt%, and 48 wt% as shown in Table 3 below.

  Next, Alq3 of the above formula (29) was vapor-deposited to a thickness of 37.5 nm on this mixed light emitting layer to form an electron transport layer.

  Furthermore, LiF was vapor-deposited with a film thickness of 1 nm as an electron injection layer on the light emitting layer.

  Furthermore, 100 nm-thickness of aluminum (Al) was laminated as a cathode on the electron injection layer, and organic EL element samples 1 to 4 were produced.

These organic EL element samples were continuously driven at a current density of 20 mA / cm ² , and as shown in Table 3, the initial luminance, current efficiency, and the time during which the luminance was reduced by 50% were measured.

表３の素子サンプル１が比較例、素子サンプル２が実施例に相当する。発光層中に第２燐光材料として、Ir(ppy)2acacを１９重量％含む素子サンプル２の電流効率は、比較例である素子サンプル１の０．６倍となったが、輝度半減時間は２．３倍と大幅に改善している。

The element sample 1 in Table 3 corresponds to a comparative example, and the element sample 2 corresponds to an example. The current efficiency of the device sample 2 containing 19% by weight of Ir (ppy) 2acac as the second phosphorescent material in the light emitting layer was 0.6 times that of the device sample 1 of the comparative example, but the luminance half time was 2 It has improved significantly by 3 times.

As the detachment of the holes trapped in the phosphorescent material, detachment due to thermal excitation and detachment due to the tunnel effect when an electric field is applied can be considered. When driving the organic EL element, a high electric field of 10 ⁶ V / m is applied. Even in such a state, by setting the difference in ionization potential energy between the light emitting layer host material and the phosphorescent material to be 0.5 eV or more, holes are surely captured by the first and second phosphorescent materials, and an excessive positive polarity is obtained. The holes can prevent the host material from being deteriorated by oxidation, and the driving life is considered to be improved.

Further, when the mobility of electrons and holes in BAlq was measured by the time-of-flight method, the electron mobility was 1.0 × 10 ⁻⁵ cm ² / Vs, the hole mobility was 2.7 × 10 ⁻⁷ cm ² / Vs. BAlq is higher in electron mobility than hole mobility and can be said to be an electron transporting material.

  As described in the above example, Ir (ppy) 2acac is added as a second phosphorescent material in addition to BAlq, which is an electron transporting host material, and Ir (piq) 2acac, which is a first phosphorescent material. The driving life is improved. This is considered to be a result of suppressing the oxidative degradation of BAlq, which is an electron transporting host material, by the second phosphorescent material. That is, the holes injected into the light emitting layer are surely captured by the first and second phosphorescent materials, and excess holes can be prevented from remaining in the host material. Further, since the total content of the first and second phosphorescent materials in the light emitting layer reaches 24%, it is considered that holes are directly injected into these phosphorescent light emitting materials.

  Thus, from the viewpoint of reliably capturing holes on the phosphorescent material, the host material needs to have an ionization potential energy that is at least 0.5 eV or more larger than that of the first and second phosphorescent materials. is there. However, since the first phosphorescent material is considered as the emission center, it is essential that the second phosphorescent material has a higher excited triplet level than the first phosphorescent material. The total content of the first and second phosphorescent materials in the light emitting layer is preferably 50% by weight or less. If the content exceeds this, phosphorescent materials such as Ir (piq) 2acac and Ir (ppy) 2acac have hole transporting properties, so that holes do not stay in the light-emitting layer, and electrons are transported through the phosphorescent material. This is because the efficiency is remarkably reduced by passing through the layers.

  The same driving life improvement effect can be obtained even when the light emitting layer is composed of only the electron transporting host material and the first phosphorescent material contained in the light emitting layer at a high concentration. The so-called concentration quenching significantly reduces the efficiency.

  In the above embodiment, the light emitting layer is composed of the host material BAlq, the first phosphorescent material Ir (piq) 2acac, and the second phosphorescent light emitting material Ir (ppy) 2acac, but is not limited thereto.

Claims (5)

  An organic layer comprising: a light emitting layer disposed between a pair of cathodes and an anode; a hole transport layer disposed between the light emitting layer and the anode; and an electron transport layer disposed between the light emitting layer and the cathode. In the EL element, the light-emitting layer includes a first phosphorescent material, a second phosphorescent material, and a host material having a nitrogen-containing aromatic heterocycle, and the excited triplet level of the second phosphorescent material is: It is higher than the excited triplet level of the first phosphorescent material, and the host material has an ionization potential energy of at least 0.5 eV or more higher than that of the first phosphorescent material and the second phosphorescent material. Organic EL element. The host material is an aluminum chelate complex, and the aluminum chelate complex has the following general formula

However, in the formula (A1), R ¹ is an alkyl group, an oxy group, an amino group, or a hydrocarbon group having at least one carbon atom in the substituent, and in any hydrocarbon part, the number of carbon atoms 1 to 10 and R ^{2 to} R ⁶ independently include a hydrogen atom, an alkyl group, an oxy group, an amino group, or a hydrocarbon group having at least one carbon atom in the substituent. The hydrocarbon moiety also has 1 to 10 carbon atoms, and R ⁴ , R ⁵ , and R ⁶ are independently cyano, halogen, and α-haloalkyl containing 10 or fewer carbon atoms. , Α-haloalkoxy, amide, sulfonyl, carbonyl, carbonyloxy, and oxycarbonyl substituents, wherein L is any of the following formula (A2):

However, in formula (A2), R ^{7 to} R ¹¹ independently represent hydrogen or a hydrocarbon group having 1 to 12 carbon atoms, and R ⁷ and R ⁸ together, or R ⁸ and R ⁹ together can form a condensed benzo ring, R ^{12 to} R ²⁶ independently represent hydrogen or a hydrocarbon group having 1 to 12 carbon atoms, and R ¹² and R ¹³ or R ¹³ And R ¹⁴ together, R ¹⁷ and R ¹⁸ or R ¹⁸ and R ¹⁹ together, and R ²² and R ²³ or R ²³ and R ²⁴ together can form a fused benzo ring. It consists of a compound, The organic EL element of Claim 1 characterized by the above-mentioned.   The organic EL device according to claim 1, wherein the host material is a compound having a carbazolyl group and a pyridine ring. The first phosphorescent material and the second phosphorescent material are organometallic complexes, and the organometallic complex is represented by the following general formula (C1)

However, in Formula (C1), M is any metal of Os, Ir, Pt, Au, m represents the valence of the metal, R ^{1 to} R ¹¹ are independently a hydrogen atom, an alkyl group, The substituent includes an oxy group, an amino group, or a hydrocarbon group having at least one carbon atom, and any hydrocarbon moiety has 1 to 10 carbon atoms, and R ^{1 to} R ¹¹ can be independently selected from cyano, halogen, and α-haloalkyl, α-haloalkoxy, amide, sulfonyl, carbonyl, carbonyloxy, and oxycarbonyl substituents containing up to 10 carbon atoms, and R ¹ and R ² together, or R ² and R ³ together, R ³ and R ⁴ together, R ⁴ and R ⁵ together, or R ⁶ and R ⁷ together. ,is there The organic EL device according to any one of R ⁷ and R ⁸ together claims 1 to 3, characterized in that it consists of fused benzo rings may be formed, a compound represented by the. 5. The organic EL element according to claim 1, wherein the total content of the first phosphorescent material and the second phosphorescent material in the light emitting layer is 50% by weight or less.

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