A stacked organic electroluminescent device

Abstract

The present invention discloses a stacked organic electroluminescent device, comprising an anode layer, at least two light-emitting units, a cathode layer, and a connecting layer for connecting two adjacent light-emitting units, which are arranged in sequence from bottom to top. The connecting layer comprises a first connecting layer, a second connecting layer, a third connecting layer, and a fourth connecting layer. The first connecting layer and the fourth connecting layer are directly connected to the two adjacent light-emitting units, respectively, and the first connecting layer and the fourth connecting layer are gradient-doped connecting layers, which are composed of a host and a doped guest. In the first connecting layer, the mass percentage of the doped guest is lower on the side contacting the light-emitting unit and increases toward the side not contacting the light-emitting unit. In the fourth connecting layer, the mass percentage of the doped guest is higher on the side contacting the light-emitting unit and decreases toward the side not contacting the light-emitting unit. The stacked organic electroluminescent device of the present invention has the characteristics of low driving voltage, high efficiency, and long life.

Description

Technical Field

[0001] The present invention relates to the technical field of organic photoelectric materials and devices, and in particular to a stacked organic electroluminescent device. Background Art

[0002] Organic light-emitting diode (OLED) technology involves thin organic semiconductor films that emit light under an applied voltage. Its advantages include flexibility, self-luminescence, thinness, and low power consumption, and it has been widely used in smartphones, wearable devices, and automotive displays. The structure and fabrication process of OLED devices are crucial for the luminescence performance of OLED materials and are a key component of OLED display and lighting technology. Therefore, the exploration of new OLED device structures and fabrication processes with low driving voltages, high luminous efficiency, and long lifespans has become a research hotspot in the field of OLED technology.

[0003] In order to improve the luminous brightness and life stability of OLED devices, a plurality of light-emitting units can be stacked together through a connecting layer (Charge Generation Layer, CGL) structure to obtain a light-emitting element with a stacked structure. A light-emitting unit usually includes at least one light-emitting layer, a hole transport layer and an electron transport layer. Compared with single-light-emitting layer OLED devices, the luminous brightness and device life of stacked devices driven at the same current density will be multiplied. That is, at the same brightness, the current density required for the stacked OLED is smaller than that of the conventional single-layer OLED, thereby achieving the effect of extending the life; however, at a constant current density, the brightness of the stacked OLED is higher than that of the conventional single-layer OLED, and the voltage also increases accordingly. Therefore, for stacked OLED devices, reducing the driving voltage drop of the connecting layer between the light-emitting units and improving the stability of the connecting layer are the key to obtaining low driving voltage, high efficiency and long life stacked OLED devices.

[0004] In terms of improving the performance of the connection layer, researchers and panel manufacturers have carried out systematic optimization work. For example, in patent application CN114520301A, a stacked organic light-emitting device is disclosed. A spacer layer structure is introduced between the n-type doped layer and the p-type doped layer to prevent the n-type dopant from easily diffusing into the p-type doped layer and the adjacent light-emitting layer, thereby causing the device driving voltage to increase. On the other hand, by optimizing the combination of the charge generation layer and the adjacent compounds, the injection and balance of carriers are improved, thereby achieving the purpose of improving the overall performance of the device; for example, in patent application KR20170062938, by optimizing the combination of the energy level and hole mobility of the p-type charge generation layer and the hole transport layer in contact with it, the purpose of reducing voltage, improving efficiency and life is achieved.

[0005] However, existing studies on the performance of the connecting layer rarely involve gradient doping, and are unable to further reduce the energy level barrier between the connecting layer and the light-emitting unit transmission layer, improve the energy level matching between the connecting layer and the light-emitting unit, thereby reducing the driving voltage drop of the connecting layer in the stacked OLED device and improving the efficiency and life stability of the device. Summary of the Invention

[0006] In view of the above-mentioned deficiencies in the prior art, the present invention aims to provide a novel stacked organic light-emitting device. By providing a gradient doping process for the connecting layer, the energy barrier between the connecting layer and the light-emitting unit transmission layer is effectively reduced, and the energy level matching between the connecting layer and the light-emitting unit is improved. The resulting device has the characteristics of low driving voltage, high efficiency and long life.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] A stacked organic electroluminescent device comprises, arranged in order from bottom to top, an anode layer, at least two light-emitting units, a cathode layer, and a connecting layer for connecting two adjacent light-emitting units; the connecting layer comprises a first connecting layer, a second connecting layer, a third connecting layer, and a fourth connecting layer;

[0009] The first connecting layer and the fourth connecting layer are directly connected to two adjacent light-emitting units respectively, and the first connecting layer and the fourth connecting layer are gradient-doped connecting layers; the gradient-doped connecting layers are composed of a host and a doped guest;

[0010] The doping guest of the first connecting layer is an n-type dopant material selected from at least one of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, gold, silver, copper, iron, nickel, platinum, palladium, ruthenium, ytterbium, molybdenum trioxide, vanadium pentoxide, tungsten trioxide, cesium fluoride, cesium carbonate, lithium fluoride, lithium carbonate, lithium 8-hydroxyquinolinate, sodium chloride, ferric chloride and ferrosoferric oxide;

[0011] The doping guest of the fourth connecting layer is a p-type dopant material selected from at least one of MoO3, WO3, V2O5, MoO2, Co3O4 and the following compounds (1-1) to (1-20):

[0012]

[0013]

[0014] The main body of the first connecting layer is an organic material having electron transport properties, selected from at least one of the following compounds (2-1) to (2-42):

[0015]

[0016]

[0017]

[0018] The second connection layer is an n-type dopant material selected from at least one of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, gold, silver, copper, iron, nickel, platinum, palladium, ruthenium, ytterbium, molybdenum trioxide, vanadium pentoxide, tungsten trioxide, cesium carbonate, lithium carbonate, sodium chloride, ferric chloride and ferroferric oxide.

[0019] The third connecting layer is a p-type dopant material selected from at least one of MoO3, WO3, V2O5, MoO2, Co3O4 and the following compounds (4-1) to (4-20):

[0020]

[0021]

[0022] In the first connecting layer, the mass percentage of the doped guest is lower on the side contacting the light-emitting unit and increases toward the side not contacting the light-emitting unit; in the fourth connecting layer, the mass percentage of the doped guest is higher on the side contacting the light-emitting unit and decreases toward the side not contacting the light-emitting unit.

[0023] Furthermore, the doped guests in the first connecting layer and the fourth connecting layer are gradient doped at a constant mass percentage change rate; wherein, in the first connecting layer, the mass percentage of the doped guest is lower on the side contacting the light-emitting unit, and the mass percentage of the doped guest is greater than 0, and increases toward the side not contacting the light-emitting unit at a constant doping mass percentage change rate; in the fourth connecting layer, the mass percentage of the doped guest is higher on the side contacting the light-emitting unit, and decreases toward the side not contacting the light-emitting unit at a constant doping mass percentage change rate, and the mass percentage of the doped guest is greater than 0.

[0024] The light-emitting unit includes any one or a combination of a hole injection layer, a hole transport layer, an electron blocking layer, a light-emitting layer, a hole blocking layer, an electron transport layer and an electron injection layer.

[0025] The thickness of the first connection layer is 10-50 nm, the thickness of the second connection layer and the third connection layer are both 1-20 nm, and the thickness of the fourth connection layer is 5-50 nm.

[0026] In the first connection layer, the mass percentage of the doped guest is 1 to 70 wt %; in the fourth connection layer, the mass percentage of the doped guest is 0.5 to 50 wt %.

[0027] The number of the light-emitting units is 2 to 5.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) The present invention uses an n-type dopant in the first connecting layer and a p-type dopant in the fourth connecting layer, and both the first connecting layer and the fourth connecting layer adopt a gradient doping process, so that the LUMO energy level of the first connecting layer is more closely matched with the LUMO energy level of the electron transport layer of the adjacent light-emitting unit, and the HOMO energy level of the fourth connecting layer is more closely matched with the HOMO energy level of the hole transport layer of the adjacent light-emitting unit, thereby reducing the injection barrier of the carrier injection transport layer, reducing the driving voltage of the stacked device, and improving the power efficiency;

[0030] (2) The doping percentage mass concentration of the n-type dopant in the first connecting layer is gradually reduced, which is beneficial to inhibiting the diffusion of the n-type dopant to the light-emitting layer of the adjacent light-emitting unit, thereby avoiding the problem of quenching of the light-emitting excitons and shortening the device life. On the other hand, the first connecting layer is arranged between the electron transport layer of the adjacent light-emitting unit and the second connecting layer, which can also prevent the high preparation temperature of the n-type dopant material in the second connecting layer from damaging the electron transport layer of the light-emitting unit.

[0031] (3) A second connection layer and a third connection layer are provided between the first connection layer and the fourth connection layer, so as to realize rapid separation of electrons and holes under the action of an external electric field, and realize injection of electrons into the first connection layer and injection of holes into the fourth connection layer, thereby obtaining efficient carrier generation. At the same time, because the first connection layer is doped with n-type dopants and the fourth connection layer is doped with p-type dopants, the injection barriers of electrons injected into the first connection layer and holes injected into the fourth connection layer can be reduced, which is beneficial to reducing the driving voltage of the stacked device and improving the power efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0033] Figure 1 A schematic structural diagram of a stacked organic electroluminescent device according to the present invention is shown;

[0034] Figure 2 The current density versus voltage characteristic curves of Example 1 of the present invention and Comparative Example 20 are shown;

[0035] Figure 3The device life characteristic curves of Example 1 of the present invention and Comparative Example 21 are shown.

[0036] In the figure: 100, anode layer; 101, hole injection layer; 102, first hole transport layer; 103, second hole transport layer; 104, first light-emitting layer; 105, second electron transport layer; 106, first electron transport layer; 107, first connecting layer; 108, second connecting layer; 109, third connecting layer; 110, fourth connecting layer; 111, third hole transport layer; 112, fourth hole transport layer; 113, second light-emitting layer; 114, fourth electron transport layer; 115, third electron transport layer; 116, electron injection layer; 117, cathode layer. DETAILED DESCRIPTION

[0037] To make the purpose, technical solutions, and advantages of the embodiments of the present disclosure more clear, the technical solutions of the embodiments of the present disclosure will be clearly and completely described below in conjunction with the drawings of the embodiments of the present disclosure. Obviously, the described embodiments are part of the embodiments of the present disclosure, not all of the embodiments. Based on the described embodiments of the present disclosure, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] It should be understood that, under the premise of no conflict, any and all embodiments of the present invention can be combined with the technical features in any other embodiment or multiple other embodiments to obtain additional embodiments. The present invention includes such combinations to obtain additional embodiments.

[0039] In this specification, groups and substituents thereof can be selected by those skilled in the art to provide stable structural moieties and compounds. When substituents are described by conventional chemical formulas written from left to right, the substituents also include chemically equivalent substituents obtained when the structural formula is written from right to left.

[0040] The section headings used in this specification are for organizational purposes only and should not be construed as limitations on the subject matter described. All documents or portions of documents cited in this specification, including but not limited to patents, patent applications, articles, books, manuals, and papers, are incorporated herein by reference in their entirety.

[0041] Unless otherwise specified, all technical and scientific terms used herein have the standard meanings in the art to which the claimed subject matter belongs. If there are multiple definitions for a term, the definition herein shall prevail.

[0042] It should be understood that the singular forms used in the present invention, such as "a", include plural references unless otherwise specified. In addition, the term "comprising" is an open limitation rather than a closed limitation, that is, it includes the content specified in the present invention, but does not exclude other aspects of the content.

[0043] The present invention provides a stacked organic electroluminescent device, which is composed of a substrate, an anode layer, at least two light-emitting units, a cathode layer, and a connecting layer for connecting two adjacent light-emitting units, wherein the light-emitting unit includes any one or a combination of a hole injection layer, a hole transport layer, an electron blocking layer, a light-emitting layer, a hole blocking layer, an electron transport layer and an electron injection layer.

[0044] The main function of the light-emitting layer of an organic electroluminescent device is to recombine holes and electrons injected into the light-emitting layer to produce excitons, which complete the electro-optical conversion process through radiative transition. The material constituting the light-emitting layer can be an undoped light-emitting compound composed only of the first compound, a binary light-emitting composition composed of the first compound and the third compound, or a ternary light-emitting composition composed of the first compound, the second compound, and the third compound. For the first compound being a light-emitting material that produces a radiative transition, a fluorescent material, a phosphorescent material, a thermally activated delayed fluorescent material, etc. can be selected according to the luminescence mechanism. Preferably, phosphorescent materials containing coordination metals such as iridium and platinum, thermally activated delayed fluorescent materials containing boron nitrogen derivatives, boron oxide derivatives, indole and carbazole derivatives, and boron fluorine derivatives are used as the first compound. Fluorescent materials containing fluoranthene derivatives, pyrene derivatives, and imidazole derivatives are used as the second compound. The second compound preferably contains a benzonitrile derivative, a triazine derivative, a pyrimidine derivative, a pyridine derivative, a pyrazine derivative, an imidazole derivative, a derivative of benzothiophene oxide, a phenanthroline derivative, a benzonitrile derivative, and a thermally activated delayed fluorescent material of a phosphorus oxide derivative as the second compound.

[0045] The main function of the anode layer of the organic electroluminescent device is to inject holes into the hole transport layer or the light-emitting layer. It is preferred to use an anode layer material with a work function of 4.5eV or more. The anode layer material is preferably selected from indium tin oxide alloy (ITO), tin oxide (NESA), indium gallium zinc oxide (IGZO), silver, etc. The anode layer can be formed into an anode layer thin film by thermal evaporation, sputtering, etc. It is preferred that the light transmittance of the visible area of ​​the anode is greater than 80%. In addition, the anode layer sheet resistance is preferably 500Ω / cm -1 Hereinafter, the film thickness is preferably selected within the range of 10 to 200 nm.

[0046] The main function of the cathode layer of the organic electroluminescent device is to inject electrons into the electron injection layer, electron transport layer or light-emitting layer, and a material with a small work function is preferably used. The cathode material is not particularly limited, and is preferably selected from aluminum, magnesium, silver, magnesium-silver alloy, magnesium-aluminum alloy, aluminum-lithium alloy, etc. The cathode layer can be formed into a cathode layer thin film by thermal evaporation, sputtering, etc., similar to the anode layer. The cathode layer film thickness is preferably selected in the range of 10 to 200 nm. In addition, light emission can also be extracted from the cathode side as needed.

[0047] The organic electroluminescent element of the present invention preferably has an electron injection layer in the interface region between the cathode layer and the electron transport layer or the light-emitting layer. The main function of the electron injection layer is to promote the injection of electrons from the cathode layer to the electron transport layer or the light-emitting layer, thereby improving the luminous brightness and device life of the organic electroluminescent device. Here, the electron injection layer material refers to a material having a work function of 3.8 eV or less, which can be preferably selected from lithium, cesium, barium, ytterbium, cesium fluoride, cesium carbonate, lithium fluoride, lithium carbonate, 8-hydroxyquinoline lithium, barium oxide, etc. The electron injection layer can be formed into an electron injection layer thin film by thermal evaporation, and the evaporation rate is preferably The thickness of the cathode layer thus produced is preferably selected within the range of 0.1 to 15 nm.

[0048] The electron transport layer of the organic electroluminescent device is an organic layer formed between the light-emitting layer and the cathode layer (or electron injection layer), and its main function is to transport electrons from the cathode to the light-emitting layer. The electron transport layer may be composed of a layer of organic layer material, which is defined as the first electron transport layer; or it may be composed of two layers of organic layer materials, with the organic layer close to the cathode layer being defined as the first electron transport layer, and the organic layer close to the light-emitting layer being defined as the second electron transport layer. The electron transport material used for the electron transport layer is preferably an aromatic heterocyclic compound containing one or more heteroatoms in the molecule, and a nitrogen-containing ring derivative is particularly preferred. In addition, as a nitrogen-containing ring derivative, an aromatic ring having a nitrogen-containing six-membered ring or five-membered ring skeleton, or a condensed aromatic ring compound having a nitrogen-containing six-membered ring or five-membered ring skeleton is preferred.

[0049] The electron transport layer of the organic electroluminescent device of the present invention is preferably selected from the following compounds but is not limited to the following structures:

[0050]

[0051]

[0052]

[0053]

[0054] The thickness of the electron transport layer is not particularly limited, but is preferably 10 to 100 nm. Specifically, when the electron transport layer of the organic electroluminescent device consists of a first electron transport layer, the thickness of the first electron transport layer is preferably 10 to 100 nm; when the electron transport layer of the organic electroluminescent device consists of a first electron transport layer and a second electron transport layer, the thickness of the first electron transport layer is preferably 9 to 70 nm, and the thickness of the second electron transport layer is preferably 1 to 30 nm.

[0055] The hole transport layer of an organic electroluminescent device is an organic layer formed between the light-emitting layer and the anode layer (or hole injection layer). Its main function is to transport holes from the anode to the light-emitting layer. The hole transport layer may be composed of a single layer of organic layer material, defined as the first hole transport layer; or it may be composed of two layers of organic layer material, with the organic layer closer to the anode layer defined as the first hole transport layer and the organic layer closer to the light-emitting layer defined as the second hole transport layer. As the hole transport material for the hole transport layer, an aromatic amine compound is preferably used, such as an aromatic amine derivative represented by the following formula (70).

[0056]

[0057] In the above formula (70), Ar1 to Ar4 represent a substituted or unsubstituted aromatic hydrocarbon group having 6 to 50 (preferably 6 to 30, more preferably 6 to 20, and further preferably 6 to 12) ring carbon atoms, or a fused aromatic hydrocarbon group having 6 to 50 (preferably 6 to 30, more preferably 6 to 20, and further preferably 6 to 12) ring carbon atoms which may have a substituent, a substituted or unsubstituted aromatic heterocyclic group having 5 to 50 (preferably 5 to 30, more preferably 5 to 20, and further preferably 5 to 12) ring atoms, or a substituted or unsubstituted fused aromatic heterocyclic group having 5 to 50 (preferably 5 to 30, more preferably 5 to 20, and further preferably 5 to 12) ring atoms, or a group in which these aromatic hydrocarbon groups or fused aromatic hydrocarbon groups are bonded to an aromatic heterocyclic group or a fused aromatic heterocyclic group.

[0058] A ring may be formed between Ar1 and Ar2, and between Ar3 and Ar4. In the above formula (70), L represents a substituted or unsubstituted aromatic hydrocarbon group having 6 to 50 ring carbon atoms (preferably 6 to 30, more preferably 6 to 20, and even more preferably 6 to 12), or a fused aromatic hydrocarbon group having 6 to 50 ring carbon atoms (preferably 6 to 30, more preferably 6 to 20, and even more preferably 6 to 12) which may have a substituent, or a substituted or unsubstituted aromatic heterocyclic group having 5 to 50 ring atoms (preferably 5 to 30, more preferably 5 to 20, and even more preferably 5 to 12), or a substituted or unsubstituted fused aromatic heterocyclic group having 5 to 50 ring atoms (preferably 5 to 30, more preferably 5 to 20, and even more preferably 5 to 12).

[0059] As the hole transport material used in the hole transport layer, another aromatic amine compound, for example, an aromatic amine derivative represented by the following formula (71) is preferably used.

[0060]

[0061] In the above formula (71), the definitions of Ar1 to Ar3 are the same as those of Ar1 to Ar4 in formula (70).

[0062] The hole transport layer of the organic electroluminescent device of the present invention, the compound described in formula (70) and (71) is preferably selected from the following compounds but is not limited to the following structures:

[0063]

[0064]

[0065]

[0066]

[0067]

[0068]

[0069] The thickness of the hole transport layer is not particularly limited, but is preferably 20 to 200 nm. Specifically, when the hole transport layer of the organic electroluminescent device consists of a first hole transport layer, the thickness of the first hole transport layer is preferably 20 to 200 nm; when the hole transport layer of the organic electroluminescent device consists of a first hole transport layer and a second hole transport layer, the thickness of the first hole transport layer is preferably 19 to 150 nm, and the thickness of the second hole transport layer is preferably 1 to 50 nm.

[0070] The organic electroluminescent element of the present invention is preferably doped with an n-type dopant in the electron transport layer and a p-type dopant in the hole transport layer. The main functions of the n-type dopant and the p-type dopant are to enhance the transport properties of the electron transport layer and the hole transport layer, respectively, and to reduce the driving voltage of the organic electroluminescent device. Here, the n-type dopant is preferably lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, gold, silver, copper, iron, nickel, platinum, palladium, ruthenium, ytterbium, molybdenum trioxide, vanadium pentoxide, tungsten trioxide, cesium fluoride, cesium carbonate, lithium fluoride, lithium carbonate, lithium 8-hydroxyquinolinate, sodium chloride, ferric chloride, ferrosoferric oxide, etc.; the p-type dopant is preferably MoO3, WO3, V2O5, MoO2, Co3O4 and at least one of the following compounds (HI-1) to (HI-20):

[0071]

[0072]

[0073] When the hole transport layer contains the p-type dopant and the hole transport material, the doping concentration of the p-type dopant is preferably 0.1-50.0 wt %; when the hole transport layer contains the n-type dopant and the electron transport material, the doping concentration of the n-type dopant is preferably 1.0-90.0 wt %.

[0074] Specifically, if Figure 1 As shown, the organic electroluminescent device includes a substrate, an anode layer 100, a first light-emitting unit, a connecting layer, a second light-emitting unit and a cathode layer 117 stacked in sequence from bottom to top, the first light-emitting unit includes in sequence any one or more combinations of a hole injection layer 101, a first hole transport layer 102, a second hole transport layer 103, a first light-emitting layer 104, a second electron transport layer 105 and a first electron transport layer 106, the second light-emitting unit includes in sequence any one or more combinations of a third hole transport layer 111, a fourth hole transport layer 112, a second light-emitting layer 113, a fourth electron transport layer 114, a third electron transport layer 115 and an electron injection layer 116, and the connecting layer includes in sequence a first connecting layer 107, a second connecting layer 108, a third connecting layer 109 and a fourth connecting layer 110.

[0075] The specific preparation process of the stacked organic electroluminescent device of the present invention is as follows:

[0076] Examples 1 to 6

[0077] (1) A 30 mm × 30 mm × 0.7 mm thick glass substrate with an ITO transparent electrode (anode layer 10, ITO film thickness set to 95 nm) was ultrasonically cleaned in acetone, detergent, ultrapure water (3 times), and isopropyl alcohol, with each ultrasonic cleaning time of 10 minutes. The cleaned ITO glass substrate was placed in an oven at 80°C for 3 hours.

[0078] (2) performing vacuum plasma cleaning on the baked ITO glass substrate for 10 minutes;

[0079] (3) The plasma-treated glass substrate was mounted on a substrate holder of a vacuum evaporation apparatus, and the compound HATCN was first deposited on the surface on which the transparent electrode lines were formed, so as to cover the transparent electrode, thereby forming a hole injection layer 101 with a thickness of 10 nm.

[0080] (4) Compound HT-10 was evaporated on the hole injection layer 101 to form a first hole transport layer 102 with a thickness of 30 nm;

[0081] (5) evaporating compound HT-48 on the first hole transport layer 102 to form a second hole transport layer 103 with a thickness of 10 nm;

[0082] (6) A third compound RH and a first compound RD were co-evaporated on the second hole transport layer 103 to form a first light-emitting layer 104 with a thickness of 25 nm. The concentration of the first compound RD in the first light-emitting layer 104 was set to 4 wt %. The structures of the third compound RH and the first compound RD used are shown below:

[0083]

[0084] (7) ET-15 was evaporated on the first light-emitting layer 104 to form a second electron transport layer 105 with a thickness of 10 nm;

[0085] (8) ET-4 was evaporated on the second electron transport layer 105 to form a first electron transport layer 106 with a thickness of 10 nm;

[0086] (9) Co-evaporating an n-type dopant guest and an electron transport compound on the first electron transport layer 106 in a gradient doping manner to form a first connection layer 107 with a thickness of 20 nm;

[0087] (10) evaporating an n-type dopant material, ytterbium, on the first connection layer 107 to form a second connection layer 108 with a thickness of 3 nm;

[0088] (11) vapor-depositing a p-type dopant material 4-7 on the second connection layer 108 to form a third connection layer 109 with a thickness of 3 nm;

[0089] (12) Co-evaporating a p-type dopant guest and a hole transport compound on the third connection layer 109 in a gradient doping manner to form a fourth connection layer 110 with a film thickness of 20 nm;

[0090] (13) Compound HT-10 was evaporated on the fourth connecting layer 110 to form a third hole transport layer 111 with a thickness of 20 nm;

[0091] (14) Compound HT-48 was evaporated on the third hole transport layer 111 to form a fourth hole transport layer 112 with a thickness of 10 nm;

[0092] (15) The third compound RH and the first compound RD were co-evaporated on the fourth hole transport layer 112 to form a second light-emitting layer 113 with a thickness of 25 nm. The concentration of the first compound RD in the second light-emitting layer 113 was set to 4 wt %;

[0093] (16) ET-15 was evaporated on the second light-emitting layer 113 to form a fourth electron transport layer 114 with a thickness of 10 nm;

[0094] (17) ET-4 was evaporated on the fourth electron transport layer 114 to form a third electron transport layer 115 with a thickness of 30 nm;

[0095] (18) Liq was evaporated on the third electron transport layer 115 to form an electron injection layer 116 with a thickness of 3 nm;

[0096] (19) Metal Al was evaporated on the electron injection layer 116 to form a cathode layer 117 with a film thickness of 100 nm.

[0097] The compound combination and gradient doping process used in forming the first connection layer 107 and the fourth connection layer 110 in step (9) and step (12) are shown in Examples 1-6 in Table 1 below.

[0098] Table 1

[0099]

[0100] Examples 7 to 12

[0101] The combinations of n-type dopant materials used in forming the second connecting layer 108 and the p-type dopant materials used in forming the third connecting layer 109 in steps (10) and (11) of the stacked organic electroluminescent devices prepared in Examples 7 to 12 are shown in Table 2. Otherwise, the same procedures as in Example 1 were followed to prepare organic electroluminescent devices.

[0102] Table 2

[0103]

[0104]

[0105] Comparative Examples 20 to 24

[0106] In the stacked organic electroluminescent devices prepared in Comparative Examples 20 to 24, the connecting layer structure was changed to the structure shown in Table 3. Otherwise, the same operation as in Example 1 was carried out to prepare organic electroluminescent devices.

[0107] Table 3

[0108]

[0109] Performance evaluation of organic electroluminescent devices

[0110] The organic electroluminescent devices prepared in Examples 1 to 12 and Comparative Examples 20 to 24 were measured using a spectroradiometer CS-2000 (Konica Minolta) and a digital source meter 2420 (Keithley) at a current density of 10 mA / cm 2The driving voltage, external quantum efficiency (EQE) and CIE1931 chromaticity coordinates (x, y) of the prepared organic electroluminescent device were measured using a device life test with a current density of 50 mA / cm 2 When the prepared organic electroluminescent device is driven, the device brightness decays to 95% of the initial brightness (T95). The device performance evaluation results of Examples 1 to 12 and Comparative Examples 20 to 24 are shown in Table 4 below.

[0111] Table 4

[0112]

[0113]

[0114] Comparing the device performance results of Examples 1 and 3 with Comparative Examples 20 and 22 in Table 4, it can be seen that the use of a gradient doping process for the dopant guest in the first and fourth connecting layers is beneficial for reducing the driving voltage and improving the device lifespan, with the lifespan improvement exceeding 30%. This is because, on the one hand, the gradient doping effect allows for a better energy level match between the first connecting layer and the electron transport layer of the adjacent light-emitting unit, as well as between the fourth connecting layer and the hole transport layer of the adjacent light-emitting unit, reducing the carrier injection barrier and thus helping to reduce the device driving voltage. On the other hand, this is because the gradient doping process results in a lower concentration of n-type dopant material on the side adjacent to the light-emitting unit, preventing the dopant guest from entering the light-emitting layer and causing accelerated device aging. In addition, comparing the device performance results of Examples 1 and 3 with Comparative Examples 21, 23, and 24 in Table 4, it can be seen that the establishment of a second and third connecting layer between the first and fourth connecting layers can effectively reduce the device driving voltage. This is due to the good charge generation effect of the second and third connecting layers, which reduces the driving voltage drop of the device at the connecting layers, thereby achieving the effect of reducing the device driving voltage. By comparing the device performance of Examples 1 to 12 and Comparative Examples 20 to 24 in Table 4, it can be seen that after further replacing the compound combinations used in the first connecting layer, the second connecting layer, the third connecting layer and the fourth connecting layer, the device driving voltage and device life of Examples 1 to 12 are better than those of Comparative Examples 20 to 24, indicating that the connecting layer structure described in the present application is generally better than the connecting layer structure in the comparative examples in improving the performance of the stacked organic electroluminescent device.

[0115] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A stacked organic electroluminescent device, characterized in that: The device comprises an anode layer, at least two light-emitting units, a cathode layer, and a connection layer for connecting two adjacent light-emitting units, which are arranged in sequence from bottom to top; The connecting layer includes a first connecting layer, a second connecting layer, a third connecting layer and a fourth connecting layer; the first connecting layer and the fourth connecting layer are directly connected to two adjacent light-emitting units respectively, and the first connecting layer and the fourth connecting layer are gradient-doped connecting layers, and the gradient-doped connecting layers are composed of a host and a doped guest; The doping guest of the first connecting layer is an n-type dopant material selected from at least one of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, gold, silver, copper, iron, nickel, platinum, palladium, ruthenium, ytterbium, molybdenum trioxide, vanadium pentoxide, tungsten trioxide, cesium fluoride, cesium carbonate, lithium fluoride, lithium carbonate, lithium 8-hydroxyquinolinate, sodium chloride, ferric chloride and ferrosoferric oxide; The doping guest of the fourth connecting layer is a p-type dopant material selected from at least one of MoO3, WO3, V2O5, MoO2, Co3O4 and the following compounds (1-1) to (1-20): In the first connecting layer, the mass percentage of the doped guest is lower on the side contacting the light-emitting unit, and the mass percentage of the doped guest is greater than 0 and increases toward the side not contacting the light-emitting unit; In the fourth connecting layer, the mass percentage of the doped guest is higher on the side contacting the light-emitting unit and decreases toward the side not contacting the light-emitting unit, and the mass percentage of the doped guest is greater than 0.

2. The stacked organic electroluminescent device according to claim 1, characterized in that: The main body of the first connecting layer is an organic material having electron transport properties, selected from at least one of the following compounds (2-1) to (2-42):

3. The stacked organic electroluminescent device according to claim 1, characterized in that: The second connection layer is an n-type dopant material selected from at least one of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, gold, silver, copper, iron, nickel, platinum, palladium, ruthenium, ytterbium, molybdenum trioxide, vanadium pentoxide, tungsten trioxide, cesium carbonate, lithium carbonate, sodium chloride, ferric chloride and ferroferric oxide.

4. The stacked organic electroluminescent device according to claim 1, characterized in that: The third connecting layer is a p-type dopant material selected from at least one of MoO3, WO3, V2O5, MoO2, Co3O4 and the following compounds (4-1) to (4-20):

5. The stacked organic electroluminescent device according to claim 1, characterized in that: The doped guests in the first connecting layer and the fourth connecting layer are gradient doped at a constant mass percentage change rate; wherein, in the first connecting layer, the mass percentage of the doped guest is lower on the side contacting the light-emitting unit, and increases toward the side not contacting the light-emitting unit at a constant doping mass percentage change rate; in the fourth connecting layer, the mass percentage of the doped guest is higher on the side contacting the light-emitting unit, and decreases toward the side not contacting the light-emitting unit at a constant doping mass percentage change rate.

6. The stacked organic electroluminescent device according to claim 1, characterized in that: The light-emitting unit includes any one or a combination of a hole injection layer, a hole transport layer, an electron blocking layer, a light-emitting layer, a hole blocking layer, an electron transport layer and an electron injection layer.

7. The stacked organic electroluminescent device according to claim 1, characterized in that: The thickness of the first connection layer is 10-50 nm, the thickness of the second connection layer and the third connection layer are both 1-20 nm, and the thickness of the fourth connection layer is 5-50 nm.

8. The stacked organic electroluminescent device according to claim 1, characterized in that: In the first connection layer, the mass percentage of the doped guest is 1 to 70 wt %; in the fourth connection layer, the mass percentage of the doped guest is 0.5 to 50 wt %.

9. The stacked organic electroluminescent device according to claim 1, characterized in that: The number of the light-emitting units is 2 to 5.

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