JP6691848B2 - Light emitting element, display device and electronic device - Google Patents

Description

  The present invention relates to a light emitting element, a display device, and an electronic device.

  Researches for improving the light emission efficiency of a light emitting element (organic EL element) utilizing electroluminescence have been actively conducted. In recent years, in an attempt to increase the luminous efficiency of the device, a device utilizing light emission from a triplet excited state using a phosphorescent substance has been developed. On the other hand, most of the phosphorescent emitters are complexes containing a rare metal such as iridium as a central metal, and there is concern about the cost and the stability of supply.

  Therefore, as a material capable of converting a triplet excited state into light emission without using a rare metal, a material that exhibits thermally activated delayed fluorescence (TADF) (hereinafter, also referred to as TADF material). ) Has been conducted (see Non-Patent Document 1). In the TADF material, the singlet excited state (S1) is generated from the triplet excited state (T1) by the inverse intersystem crossing, and the singlet excited state is converted into emission.

  In an organic EL device using such a TADF material for the light emitting layer, it has been reported that high light emitting efficiency can be obtained by using a hole blocking material having a high triplet excitation level T1 for the hole blocking layer ( Non-Patent Document 2). This feature is basically the same as that of the organic EL element using the phosphorescent material in the light emitting layer.

Appl. Phys. Let. , 98, 083302 (2011) Nakanotani et al. Sci. Rep. , 3, 2127 (2013)

  However, the organic EL device including the hole blocking layer described in Non-Patent Document 2 has low heat resistance. Considering the application of the organic EL element to an in-vehicle application, a vehicle or the like has a high temperature in the sunshine, and thus high heat resistance is required. In addition, even when the organic EL element is used outdoors, it is required to have high heat resistance even if it is not used for a vehicle.

  In view of the above points, the present invention has an object to improve heat resistance in a light emitting device using a hole blocking material having a high triplet excitation level T1 for a hole blocking layer.

In order to achieve the above object, in the invention according to claim 1, the anode (11), the cathode (17), and the first layer (13) provided between the anode and the cathode and having a function of emitting light. And a second layer (14) provided between the cathode and the first layer. The second layer is composed of a first organic compound in which at least a main π bond in the molecule exists on the same plane and a second organic compound having steric hindrance , and the first organic compound is T2T or T2T. -3tBu, T2T-4tBu, or T2T-tert-Ph, and the second organic compound is 2CzPN, 2CzBN, 2CzBN3,6Me, 3CzBN, or 3CzBN3,6Me .

  According to the present invention, the first organic compound having high planarity and the second organic compound having steric hindrance are mixed to form the second layer. Crystallization can be inhibited by the second organic compound having steric hindrance, and heat resistance of the light emitting device can be improved. As a result, even when the light emitting element is used in a high temperature environment, it is possible to suppress deterioration of functions such as light emission efficiency.

  Note that the reference numerals in parentheses of the above-mentioned means indicate the correspondence with the specific means described in the embodiments described later.

It is a schematic sectional drawing of the organic EL element which concerns on embodiment of this invention. It is a chart which shows the energy level etc. of a 1st organic compound. It is a chart which shows the energy level etc. of a 2nd organic compound. It is a figure which shows the energy band of a light emitting layer, a hole blocking layer, and an electron carrying layer. It is a figure which shows the change of luminous efficiency when baking treatment is performed to an organic EL element. It is a figure which shows the change of the light emission spectrum when baking treatment is performed to an organic EL element.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. As shown in FIG. 1, the organic EL device 1 includes a positive electrode 11 formed in advance on a glass substrate 10, a hole injection / transport layer 12, a light emitting layer 13, a hole blocking layer 14, and an electron transport layer 15. The electron injection layer 16 and the cathode 17 are laminated in this order. The light emitting layer 13 corresponds to the first layer of the present invention, and the hole blocking layer 14 corresponds to the second layer of the present invention.

  In the present embodiment, the hole injecting / transporting layer 12 and the light emitting layer 13 are configured as coating film forming layers formed by coating. Further, the hole blocking layer 14, the electron transport layer 15, the electron injection layer 16, and the cathode 17 are configured as a vapor deposition film forming layer formed by vacuum vapor deposition. In this embodiment, the glass substrate 10 has a thickness of 1 mm, the anode 11 has a thickness of 100 nm, the hole injection / transport layer 12 has a thickness of 40 nm, the light emitting layer 13 has a thickness of 30 nm, and the hole blocking layer 14 has a thickness of 30 nm. The thickness is 10 nm, the thickness of the electron transport layer 15 is 40 nm, the thickness of the electron injection layer 16 is 1 nm, and the thickness of the cathode 17 is 100 nm. The thickness of each of these layers 10, 11, 12, 13, 14, 15, 16, 17 is described as an example, and is not limited to these numerical values.

  An anode 11 made of a transparent conductive material is formed on the transparent glass substrate 10. The anode 11 has a function of injecting holes toward the light emitting layer 13. As the anode 11, a material having a high work function is preferably used, and for example, a material having 4 eV or more is preferably used. In this embodiment, non-alkali glass is used as the glass substrate 10, and ITO, which is an indium-tin oxide, is used as the anode 11.

  A hole injecting / transporting layer 12 made of a hole injecting material or a hole transporting material is formed on the anode 20. In this embodiment, PEDOT: PSS shown below is used as the hole injection / transport layer 12.

発光層１３は、陽極１１から注入された正孔および陰極１７から注入された電子が再結合することにより励起子が生成した後、発光する層である。本実施形態の発光層１３は、ホスト材料にゲスト材料を１５％ドープしたものを用いている。

The light emitting layer 13 is a layer that emits light after excitons are generated by recombination of holes injected from the anode 11 and electrons injected from the cathode 17. The light emitting layer 13 of this embodiment uses a host material doped with 15% of a guest material.

  In this embodiment, CBP shown below is used as the host material, and 4CzIPN shown below is used as the guest material. 4CzIPN is a TADF material that expresses thermally activated delayed fluorescence (TADF) and exhibits green emission.

  The TADF material is a fluorescent dopant material having a small energy difference ΔEst between the singlet excited state and the triplet excited state. Since the TADF material has a small ΔEst, it can cause a state transition (that is, an inverse intersystem crossing) due to thermal energy from the triplet excited state (T1) to the singlet excited state (S1).

正孔阻止層１４は、発光層１３を経由した正孔が陰極１７側へ移動するのを防げる機能を有している。正孔阻止層１４については、後で詳細に説明する。

The hole blocking layer 14 has a function of preventing holes that have passed through the light emitting layer 13 from moving to the cathode 17 side. The hole blocking layer 14 will be described in detail later.

  The electron transport layer 15 is a layer for transporting electrons injected from the cathode 17 toward the light emitting layer 13. In the present embodiment, Bpy-TP2 shown below is used as the electron transport layer 15.

電子注入層１６は、陰極１７から電子輸送層１５へ電子の注入障壁を小さくするための層である。本実施形態では、電子注入層１６としてＬｉＦ（フッ化リチウム）を用いている。

The electron injection layer 16 is a layer for reducing an electron injection barrier from the cathode 17 to the electron transport layer 15. In this embodiment, LiF (lithium fluoride) is used as the electron injection layer 16.

  The cathode 17 has a function of injecting electrons toward the light emitting layer 13. As such a cathode 17, it is preferable to use a material having a low work function, for example, 4 eV or less. In this embodiment, a bottom emission structure is adopted, and Al (aluminum) is used as the cathode 17.

  Next, the hole blocking layer 14 will be described. The hole blocking layer 14 is provided between the light emitting layer 13 and the cathode 17. In this embodiment, the hole blocking layer 14 is formed so as to be in contact with the light emitting layer 13.

  The hole blocking layer 14 also has a function of an electron transport layer. That is, the hole blocking layer 14 has a role of blocking the arrival of the holes in the electron transport layer 15 while transporting electrons, thereby improving the recombination probability of electrons and holes in the light emitting layer 13. Can be made

  The hole blocking layer 14 of this embodiment is a mixed material in which a plurality of kinds of organic compounds are mixed. The hole blocking layer 14 contains a first organic compound and a second organic compound. The first organic compound and the second organic compound do not contain a metal element and are composed of elements other than the metal element. In this embodiment, the hole blocking layer 14 is formed by co-evaporating a mixed material obtained by mixing 50% of the first organic compound and 50% of the second organic compound.

  Examples of the first organic compound include at least any of triazine derivatives of T2T, T2T-3tBu, T2T-4tBu, and T2T-tert-Ph shown below.

図２は、第１有機化合物のΔＥｓｔ（一重項励起状態と三重項励起状態のエネルギー差）、ＨＯＭＯ準位、ＬＵＭＯ準位、バンドギャップ（ＨＯＭＯ準位とＬＵＭＯ準位のエネルギー差）、Ｔ１準位（三重項励起準位）、Ｓ１準位（一重項励起準位）、振動子強度ｆ、分子量を示している。なお、図２に示す数値は、Ｇａｕｓｉａｎ０９ Ｒｅｖ Ｃ．でＢ３ＬＹＰ／６−３１Ｇ（ｄ）を用いて求めた計算値である。

FIG. 2 shows ΔEst (energy difference between singlet excited state and triplet excited state), HOMO level, LUMO level, band gap (energy difference between HOMO level and LUMO level), T1 level of the first organic compound. Position (triplet excitation level), S1 level (singlet excitation level), oscillator strength f, and molecular weight. The numerical values shown in FIG. 2 are based on Gaussian 09 Rev C. Is a calculated value obtained by using B3LYP / 6-31G (d).

  Examples of the second organic compound include at least one of 2CzPN, 2CzBN, 2CzBN3,6Me, 3CzBN, 3CzBN3,6Me shown below.

図３は、第２有機化合物のΔＥｓｔ、ＨＯＭＯ準位、ＬＵＭＯ準位、バンドギャップ、Ｔ１準位、振動子強度ｆ、分子量を示している。なお、図３に示す数値は、Ｇａｕｓｉａｎ０９ Ｒｅｖ Ｃ．でＢ３ＬＹＰ／６−３１Ｇ（ｄ）を用いて求めた計算値である。

FIG. 3 shows ΔEst, HOMO level, LUMO level, band gap, T1 level, oscillator strength f, and molecular weight of the second organic compound. The numerical values shown in FIG. 3 are based on Gaussian 09 Rev C. Is a calculated value obtained by using B3LYP / 6-31G (d).

  The hole blocking layer 14 also functions as an exciton blocking layer. The exciton blocking layer is a layer for preventing excitons generated by recombination of holes and electrons in the light emitting layer 13 from diffusing into the electron transport layer 15. By inserting this layer, excitons can be efficiently confined in the light emitting layer 13, and the light emitting efficiency of the organic EL element 1 can be improved.

  In order for the hole blocking layer 14 to function as an exciton blocking layer, the T1 level of the first organic compound and the second organic compound is lower than the T1 level of the organic compound having the lowest T1 level contained in the light emitting layer 13. Is also getting higher. In the present embodiment, the organic compound having the lowest T1 level among the organic compounds contained in the light emitting layer 13 is 4CzIPN. Therefore, the T1 level of the first organic compound and the second organic compound is higher than the T1 level of 4CzIPN, which is 2.4 eV.

  When the organic EL element 1 emits green light, it is desirable that the T1 level of the first organic compound and the second organic compound be 2.6 eV or more. That is, 1240/530 nm (green wavelength) = 2.3 eV, and in order to inhibit exciton transfer from the light emitting layer 13, the T1 level of the first organic compound and the second organic compound is 2.6 eV or more. Is desirable. Further, among the second organic compounds illustrated in FIG. 3, from the viewpoint of the T1 level, it is desirable to use 2CzBN or 2CzBN3,6Me as the second organic compound.

  The first organic compound is a hole block material having a deep HOMO level. In general, a hole block material having a high T1 level needs to have a short π conjugation and is realized by a small molecule, a structure in which benzene rings are connected at a meta position, or both. In the case of the first organic compound in this example, it has a structure in which a small molecule is connected to the benzene ring at the meta position. The first organic compound is a material having a large HOMO-LUMO overlap and a large ΔEst. The first organic compound is preferably ΔEst ≧ 0.5 eV, and more preferably ΔEst ≧ 0.6 eV.

  The first organic compound having such a structure is a material having high planarity without steric hindrance. In other words, in the first organic compound, the main π bond in the molecule exists in the same plane. Therefore, in the first organic compound, the molecules are easily packed with each other, and the first organic compound is easily crystallized. As a result, when the hole blocking layer 14 is composed of only the first organic compound, the heat resistance becomes low. In addition, the glass transition temperature Tg of the first organic compound having the above characteristics is often low. The first organic compound in the present embodiment has a glass transition temperature Tg <100 ° C.

  The second organic compound is a material having steric hindrance. By mixing the first organic compound having high planarity and the second organic compound having steric hindrance to form the hole blocking layer 14, packing between molecules is hindered and crystallization is hindered. As a result, the heat resistance of the organic EL element 1 can be improved.

  The second organic compound is designed to have a twisted molecular structure in order to provide steric hindrance in the molecule. HOMO-LUMO is separated by twisting the molecular structure, and as a result, ΔEst of the second organic compound is reduced. In the second organic compound, ΔEst <0.5 eV is desirable, and ΔEst ≦ 0.4 eV is more desirable.

  The second organic compound is a TADF material that has a small ΔEst and a vibrator strength f> 0 and that exhibits thermally activated delayed fluorescence (TADF).

  In addition, the glass transition temperature Tg of the second organic compound having the above characteristics is often high. It is desirable that the glass transition temperature Tg of the second organic compound ≧ 100 ° C.

  FIG. 4 shows energy bands of the light emitting layer 13, the hole blocking layer 14, and the electron transport layer 15 of the organic EL element 1 of this embodiment. As shown in FIG. 4, in this embodiment, T2T is used as the first organic compound, and 2CzBN3,6Me is used as the second organic compound. T2T has a glass transition temperature Tg <70 ° C., and 2CzBN3,6Me has a glass transition temperature Tg = 125 ° C.

  FIG. 5 shows the luminous efficiency before and after the baking process when the organic EL device 1 of the present embodiment is baked. In FIG. 5, the external quantum efficiency is shown as the luminous efficiency. In FIG. 5, as a comparative example, the light emission efficiency of the organic EL element including the hole blocking layer made of only T2T is shown. The baking treatment was performed at 90 ° C. for 30 minutes.

  As shown in FIG. 5, in the comparative example, the luminous efficiency after the baking treatment is reduced by about 87% as compared with before the baking treatment. On the other hand, in the present embodiment, the luminous efficiency after the baking treatment is reduced by about 20% as compared with that before the baking treatment. That is, in the present embodiment, it is shown that even after the baking treatment, the decrease in luminous efficiency is suppressed and the heat resistance is improved.

  FIG. 6 shows emission spectra before and after the baking process when the organic EL device 1 of the present embodiment is baked. In FIG. 6 also, as a comparative example, the emission spectrum of the organic EL element including the hole blocking layer made of only T2T is shown.

  As shown in FIG. 6, in both the present embodiment and the comparative example, the emission intensity around 530 nm corresponding to green is large before the baking treatment.

  In the comparative example, the emission intensity around 450 nm (that is, the wavelength corresponding to blue) after the bake treatment is higher than that before the bake treatment. This can be presumed to be because in the comparative example, the hole blocking layer is thermally deteriorated by the baking treatment, so that the hole blocking property is deteriorated, holes are removed from the light emitting layer, and light is emitted in a layer other than the light emitting layer.

  On the other hand, in the present embodiment, the emission spectrum after the bake treatment hardly changes from that before the bake treatment. That is, in the present embodiment, the hole blocking property is maintained and the heat resistance is improved even after the baking treatment by suppressing the thermal deterioration of the hole blocking layer 14.

  According to the present embodiment described above, by mixing the first organic compound having high planarity and the second organic compound having steric hindrance to form the hole blocking layer 14, the second organic compound is crystallized. Inhibit. As a result, the heat resistance of the organic EL element 1 can be improved. As a result, even when the organic EL element 1 is used in a high temperature environment, it is possible to suppress deterioration in functions such as light emission efficiency.

(Other embodiments)
The present invention is not limited to the above-described embodiments, but can be variously modified as follows without departing from the spirit of the present invention. Further, the means disclosed in each of the above embodiments may be appropriately combined within a practicable range.

  (1) The organic EL element 1 shown in the above embodiment can be suitably used as a display device for displaying various information. Further, the display device including the organic EL element 1 can be preferably used as a display unit of an electronic device.

  As an electronic device including a display unit including the organic EL element 1, a vehicle-mounted audio device, a vehicle instrument, a vehicle-mounted display such as a car navigation device can also be mentioned. Further, the electronic device provided with the display unit including the organic EL element 1 is not limited to the in-vehicle device, but includes, for example, a telephone including a mobile phone, a digital camera, a personal computer, a television, a portable television, a viewfinder type / monitor direct view. Type video tape recorders, PDAs, portable game machines, pagers, electronic organizers, calculators, watches, word processors, workstations, videophones, POS terminals, devices equipped with a touch panel, and the like.

  (2) In the above embodiment, the TADF material is used as the guest material of the light emitting layer 13, but the present invention is a light emitting device having the light emitting layer 13 that requires a high T1 level in the hole blocking layer 14. Applicable. Therefore, the guest material that can be used for the light emitting layer 13 of the present invention is not limited to the TADF material, and may be a phosphorescent material such as Ir (ppy) 3.

  (3) In the present embodiment described above, a material that emits green light is used as the guest material of the light emitting layer 13, but the guest material that can be used in the light emitting layer 13 of the present invention is not limited to a material that emits green light, and for example, 2CzPN. Alternatively, a blue light emitting material such as FIrpic or a red light emitting material such as HAP-3TPA or Ir (piq) 3 may be used.

  (4) In the above embodiment, an example in which the present invention is applied to a light emitting element having a bottom emission structure has been described, but the present invention is not limited to the bottom emission structure and can be applied to a top emission structure and the like. In the case of a top emission structure, a semi-transmissive film of an alloy such as MgAg (magnesium silver) or AgAl (silver aluminum) can be used as the cathode 17.

  (5) In the above embodiment, the hole blocking layer 14 is configured by mixing the first organic compound and the second organic compound by 50%, but the hole blocking layer 14 is not limited to this. The mixing ratio can be set arbitrarily.

  (6) In the above embodiment, the light emitting layer 13 and the hole blocking layer 14 are configured to be in contact with each other, but the present invention is not limited to this, and even if a thin film exists between the light emitting layer 13 and the hole blocking layer 14. Good.

  (7) In the above embodiment, the T1 level of the first organic compound and the second organic compound contained in the hole blocking layer 14 is higher than the T1 level of the organic compound having the lowest T1 level contained in the light emitting layer 13. Also configured to be higher. Not limited to this, the T1 level of the first organic compound and the second organic compound may be the same as or slightly lower than the T1 level of the organic compound having the lowest T1 level included in the light emitting layer 13. .. As the same material as the T1 level of the organic compound having the lowest T1 level contained in the light emitting layer 13, for example, 4CzIPN which is a guest material of the light emitting layer 13 can be used.

  As a result, excitons generated in the light emitting layer 13 can move to the hole blocking layer 14. As a result, extra excitons generated in the light emitting layer 13 can be consumed, the life of the organic EL element 1 can be extended, and the durability can be improved.

1 Organic EL Element 11 Anode 13 Light Emitting Layer (First Layer)
14 Hole blocking layer (second layer)
17 cathode

Claims (13)

An anode (11),
The cathode (17),
A first layer (13) provided between the anode and the cathode and having a function of emitting light;
A second layer (14) provided between the cathode and the first layer,
The second layer is composed of at least a first organic compound having a main π bond in the molecule in the same plane and a second organic compound having steric hindrance ,
The first organic compound is at least one of T2T, T2T-3tBu, T2T-4tBu, and T2T-tert-Ph,
The light emitting device, wherein the second organic compound is at least one of 2CzPN, 2CzBN, 2CzBN3,6Me, 3CzBN, and 3CzBN3,6Me .   The light emitting device according to claim 1, wherein the first layer contains an organic compound that exhibits thermally activated delayed fluorescence. The first organic compound has ΔEst ≧ 0.5 eV, and the second organic compound has ΔEst <0.5 eV.
The light emitting device according to claim 1, which has a voltage of 0.5 eV. The first organic compound has ΔEst ≧ 0.6 eV, and the second organic compound has ΔEst ≦ 0.6 eV.
The light emitting device according to claim 3, which has a voltage of 0.4 eV.   The triplet excitation level of the first organic compound and the second organic compound is higher than the triplet excitation level of the organic compound having the lowest triplet excitation level contained in the first layer. 5. The light emitting device according to any one of items 4 to 4.   The light emitting device according to claim 1, wherein the first organic compound is a triazine derivative.   The light emitting device according to claim 1, wherein the second organic compound is composed of an element other than a metal element.   8. The light emitting device according to claim 1, wherein the second organic compound is a material that exhibits thermally activated delayed fluorescence.   The light emitting device according to claim 1, wherein the glass transition temperature of the second organic compound is 100 ° C. or higher.   The light emitting device according to claim 1, wherein the glass transition temperature of the first organic compound is lower than 100 ° C. 10.   The light emitting device according to claim 1, wherein the first layer and the second layer are in contact with each other.   A display device comprising the light-emitting element according to claim 1.   An electronic device comprising the display device according to claim 12.

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