Organic electroluminescent device and display apparatus

Abstract

Provided in the present application are an organic electroluminescent device and a display apparatus. The organic electroluminescent device comprises a plurality of light-emitting units, which are arranged in a stacked manner, wherein each light-emitting unit comprises a hole transport region, a light-emitting layer and an electron transport region, which are sequentially arranged in a stacked manner, the hole transport region of at least one light-emitting unit comprises a first low-refractive layer, and the first low-refractive layer comprises a hole transport material having a refractive index of less than 1.8. The present application can improve the performance of the current efficiency, etc., of a stacking device.

Description

Organic electroluminescent device and display device

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on June 28, 2023, with application number 202310778705.5 and application name “Organic electroluminescent device and display device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of display technology, and in particular to an organic electroluminescent device and a display apparatus. Background Art

[0003] Organic light-emitting diodes (OLEDs) are self-luminous devices with advantages such as light weight, flexibility, high contrast, and a wide color gamut. Laminated devices, which consist of multiple stacked light-emitting units, are gaining widespread attention and application. However, the current efficiency and other performance of existing laminated devices need to be further improved.

[0004] Summary of the Invention

[0005] The present application provides an organic electroluminescent device and a display device, which can improve the current efficiency and other performance of the stacked device and effectively overcome the defects of the existing technology.

[0006] In one aspect of the present application, an organic electroluminescent device is provided, comprising a plurality of light-emitting units stacked in layers, each of the light-emitting units comprising a hole transport region, a light-emitting layer, and an electron transport region stacked in sequence, the hole transport region of at least one of the light-emitting units comprising a first low-refractive layer, the first low-refractive layer comprising a hole transport material having a refractive index less than 1.8.

[0007] In another aspect of the present application, a display device is provided, comprising the above-mentioned organic electroluminescent device.

[0008] In the organic electroluminescent device provided in the present application, the hole transport region of at least one light-emitting unit includes a first low-refractive layer, and the first low-refractive layer includes a hole transport material with a refractive index less than 1.8, which can effectively improve the current efficiency of the stacked device and at the same time enable the stacked device to have both low driving voltage and long life and other performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG1 is a schematic structural diagram of an organic electroluminescent device according to an embodiment of the present application;

[0010] FIG2 is a schematic structural diagram of an organic electroluminescent device according to another embodiment of the present application. DETAILED DESCRIPTION

[0011] To enable those skilled in the art to better understand the solutions of the present application, the present application is further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present application. The examples cited are only used to explain the present application and do not limit the scope of the present application. Based on the embodiments of the present application, all other implementation methods obtained by those of ordinary skill in the art without making creative efforts are within the scope of protection of this application.

[0012] Tandem OLEDs (tandem OLEDs) feature multiple stacked light-emitting units (LEs). A charge generation layer (CGL) is typically placed between each adjacent LE to efficiently distribute charge to each LE. The CGL features a PN junction, consisting of an N-type charge generation layer (N-CGL) and a P-type charge generation layer (P-CGL) stacked in sequence. Due to limitations in the tandem device's structure and the charge-supply capabilities of layers like the CGL, tandem devices often exhibit relatively low current efficiency and other performance characteristics.

[0013] For example, compared with single-layer OLED devices, the light-emitting layer in the stacked device is closer to the electrode, which makes the surface plasmon polaritons (SPP) quenching effect of the electrode greater, resulting in higher optical loss and lower current efficiency in the stacked device.

[0014] In view of this, an embodiment of the present application provides an organic electroluminescent device, as shown in Figures 1 and 2, the organic electroluminescent device includes a plurality of light-emitting units stacked together, each light-emitting unit includes a hole transport region, a light-emitting layer and an electron transport region stacked in sequence, the hole transport region of at least one light-emitting unit includes a first low-refractive layer, and the first low-refractive layer includes a hole transport material with a refractive index less than 1.8.

[0015] The organic electroluminescent device of the embodiment of the present application is a stacked device. A hole transport material having a refractive index less than 1.8 is introduced into the first low-refractive layer to enable it to perform a hole transport function. At the same time, by making at least one light-emitting unit include the first low-refractive layer, the current efficiency of the stacked device can be effectively improved, and the stacked device can also have the performance of a lower driving voltage and a longer life.

[0016] According to the inventors' research and analysis, by making the hole transport region of at least one light-emitting unit include a first low-refractive layer, that is, introducing a hole transport material with a refractive index less than 1.8 into the hole transport region of at least one light-emitting unit, the distance between the light-emitting layer and the electrode can be increased while maintaining the thickness of the microcavity, thereby weakening the SPP effect (or SPP quenching effect) of the electrode, thereby reducing the optical loss of the stacked device and improving its current efficiency and other performance.

[0017] Specifically, the above-mentioned light-emitting layer can be a red light-emitting layer, a green light-emitting layer or a blue light-emitting layer. In the embodiment of the present application, by setting the first low-refractive layer, the current efficiency and other performance of the red light-emitting stacked device (the light-emitting layer is a red light-emitting layer), the green light-emitting stacked device (the light-emitting layer is a green light-emitting layer) and the blue light-emitting stacked device (the light-emitting layer is a blue light-emitting layer) can be improved. Among them, relatively speaking, the green light-emitting layer and the blue light-emitting layer are more seriously affected by the SPP quenching effect of the electrode. Therefore, the current efficiency of the green light-emitting stacked device and the blue light-emitting stacked device is worse, and the embodiment of the present application can significantly improve the current efficiency and other performance of the green light-emitting stacked device and the blue light-emitting stacked device.

[0018] Specifically, the refractive index of the hole transport material with a refractive index less than 1.8 may be greater than 1.2. For example, the refractive index of the hole transport material with a refractive index less than 1.8 is, for example, 1.25, 1.3, 1.4, 1.5, 1.6, 1.7, 1.75 or a range consisting of any two thereof.

[0019] Generally, in the first low-refractive layer, the mass percentage of the hole transport material having a refractive index less than 1.8 is greater than or equal to 95%, for example, 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, 99.5%, 100% (such as the hole transport layer and electron blocking layer described below) or a range consisting of any two thereof. The refractive index of the first low-refractive layer is substantially equal to the refractive index of the hole transport material therein, that is, the refractive index of the first low-refractive layer is less than 1.8, which is beneficial to further improve the current efficiency and other performance of the stacked device.

[0020] In the embodiments of the present application, the refractive index refers to the refractive index for light with a wavelength of 450nm to 630nm.

[0021] Specifically, as shown in Figures 1 and 2, the above-mentioned organic electroluminescent device also includes a first electrode and a second electrode stacked with the light-emitting unit. The above-mentioned multiple light-emitting units are located between the first electrode and the second electrode and are stacked in sequence from the first electrode to the second electrode. The polarity of the second electrode is opposite to that of the first electrode. For example, the first electrode is an anode and the second electrode is a cathode. In each light-emitting unit, its hole transport region is closer to the first electrode, and the electron transport region is closer to the second electrode.

[0022] For example, the number of the light-emitting units can be two (the stacked device has a two-order microcavity structure), three (the stacked device has a three-order microcavity structure), four (the stacked device has a four-order microcavity structure), etc. Taking the stacked device shown in Figure 1 or Figure 2 as an example, the number of light-emitting units is two, namely, a first light-emitting unit and a second light-emitting unit stacked in sequence along the direction from the first electrode to the second electrode, the first light-emitting unit including a light-emitting layer EML-1, and the second light-emitting unit including a light-emitting layer EML-2.

[0023] In addition, as shown in Figures 1 and 2, the above-mentioned organic electroluminescent device may further include a charge generation layer located between each two adjacent light-emitting units, and the charge generation layer includes an n-type charge generation layer n-CGL and a p-type charge generation layer p-CGL stacked in the direction from the first electrode to the second electrode, so that the charges can be effectively distributed to each light-emitting unit, thereby improving the current efficiency of each light-emitting unit.

[0024] Generally, the closer the light-emitting unit is to the first electrode, the closer the distance between its light-emitting layer and the first electrode is, and the stronger the SPP quenching effect of the first electrode on the light-emitting unit. Therefore, it is generally preferred that the hole transport region of the light-emitting unit closest to the first electrode (the first light-emitting unit as shown in Figures 1 and 2) includes a first low-refractive layer, which can more significantly reduce the SPP quenching effect of the first electrode and further improve the current efficiency and other performance of the device. The hole transport regions of the remaining light-emitting units (the second light-emitting unit as shown in Figures 1 and 2) may include or not include the first low-refractive layer (that is, the refractive index of the hole transport material in the hole transport region is not less than 1.8). Preferably, the hole transport regions of these light-emitting units each include the first low-refractive layer (that is, the hole transport regions of the above-mentioned multiple light-emitting units (the first light-emitting unit and the second light-emitting unit as shown in Figures 1 and 2) each include the first low-refractive layer), which can further improve the current efficiency and other performance of the device.

[0025] In general, the hole transport region may include one or more of a hole injection layer, a hole transport layer, and an electron blocking layer. Among the multiple light-emitting units, the hole transport region of one or at least two light-emitting units may include a hole injection layer, and generally the hole transport region of the light-emitting unit closest to the first electrode (the first light-emitting unit as shown in Figures 1 and 2) includes a hole injection layer; the hole transport region of one or at least two light-emitting units may include a hole transport layer, and preferably the hole transport region of each light-emitting unit (the first light-emitting unit and the second light-emitting unit as shown in Figures 1 and 2) includes a hole transport layer respectively; the hole transport region of one or at least two light-emitting units may include an electron blocking layer, and preferably the hole transport region of each light-emitting unit (the first light-emitting unit and the second light-emitting unit as shown in Figures 1 and 2) includes an electron blocking layer respectively.

[0026] For example, as shown in Figures 1 and 2, the hole transport region of the first light-emitting unit includes a hole injection layer HIL, a hole transport layer HTL-1 and an electron blocking layer EBL-1 stacked in sequence along the direction from the first electrode to the second electrode, and the hole transport region of the second light-emitting unit includes a hole transport layer HTL-2 and an electron blocking layer EBL-2 stacked in the direction from the first electrode to the second electrode.

[0027] The above-mentioned hole injection layer, hole transport layer and electron blocking layer each include a hole transport material, and the first low-refractive layer can be at least one of the hole injection layer, the hole transport layer and the electron blocking layer, that is, the hole transport region of at least one light-emitting unit of the above-mentioned stacked device includes at least one of a hole injection layer having a refractive index of less than 1.8 of a hole transport material, a hole transport layer having a refractive index of less than 1.8 of a hole transport material, and an electron blocking layer having a refractive index of less than 1.8 of a hole transport material.

[0028] In some embodiments, the hole transport region of the light-emitting unit closest to the first electrode (the first light-emitting unit shown in Figures 1 and 2) (i.e., the hole transport region closest to the first electrode) includes a hole injection layer, which is a first low-refractive layer, i.e., the refractive index n1 of the hole transport material in the hole injection layer is less than 1.8; and / or, the hole transport region closest to the first electrode includes a hole transport layer, which is a first low-refractive layer, i.e., the refractive index n2 of the hole transport material in the hole transport layer is less than 1.8; and / or, the hole transport region closest to the first electrode includes an electron blocking layer, which is a first low-refractive layer, i.e., the refractive index n3 of the hole transport material in the electron blocking layer is less than 1.8. Thus, the hole transport region of the light-emitting unit closest to the first electrode includes the first low-refractive layer.

[0029] In some specific embodiments, in the stacked device shown in Figures 1 and 2, at least one of the hole injection layer HIL, the hole transport layer HTL-1 and the electron blocking layer EBL-1 in the hole transport region of the first light-emitting unit is a first low-refractive layer, so that the hole transport region of the light-emitting unit closest to the first electrode includes the first low-refractive layer.

[0030] In some preferred embodiments, the hole transport layer in the hole transport region of each light-emitting unit is a first low-refractive layer, that is, the refractive index n2 of the hole transport material in the hole transport layer of each light-emitting unit is less than 1.8, which helps further improve the current efficiency and other performance of the stacked device. The refractive indices of the hole transport materials in these hole transport layers can be equal or unequal (in the stacked devices shown in Figures 1 and 2, the refractive index of the hole transport material in the hole transport layer HTL-1 in the first light-emitting unit can be equal to or unequal to the refractive index of the hole transport material in the hole transport layer HTL-2 in the second light-emitting unit).

[0031] In some preferred embodiments, the electron blocking layer in the hole transport region of each light-emitting unit is a first low-refractive layer, i.e., the refractive index n3 of the hole transport material in the electron blocking layer of each light-emitting unit is less than 1.8. This further improves the luminous efficiency and other performance of the stacked device. Specifically, the refractive indices of the hole transport materials in these electron blocking layers can be equal or unequal (in the stacked devices shown in Figures 1 and 2, the refractive index of the hole transport material in the electron blocking layer EBL-1 in the first light-emitting unit can be equal to or unequal to the refractive index of the hole transport material in the electron blocking layer EBL-2 in the second light-emitting unit).

[0032] In some embodiments, the first low-refractive layer is a hole injection layer. In the first low-refractive layer (a hole injection layer having a refractive index of less than 1.8), the refractive index of the hole transport material having a refractive index less than 1.8 is n1 (i.e., n1 < 1.8), and the thickness of the first low-refractive layer is H1, 6nm < n1 × H1 < 54nm. That is, when the hole transport region of at least one light-emitting unit of the stacked device includes a hole injection layer having a refractive index of less than 1.8, the thickness H1 of the hole injection layer and the refractive index n1 of the hole transport material satisfy 6nm < n1 × H1 < 54nm, which is beneficial to further improve the efficiency and other performance of the stacked device. According to the research and analysis of the inventors, controlling the layer product of n1 and H1 (i.e., n1 × H1) within the above range can make the hole injection layer have more suitable optical thickness and other properties, thereby further optimizing the performance of the stacked device.

[0033] Illustratively, n1×H1 may be 7 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 53 nm, or a range consisting of any two thereof.

[0034] For example, in the hole injection layer where the refractive index of the hole transport material is less than 1.8, the refractive index n1 of the hole transport material is, for example, 1.25, 1.3, 1.4, 1.5, 1.6, 1.7, 1.75 or a range consisting of any two thereof.

[0035] In addition, the thickness H1 of the hole injection layer having a refractive index of less than 1.8 of the hole transport material may be 5 nm to 30 nm, for example, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, or any two thereof.

[0036] Taking the stacked device shown in Figure 1 or Figure 2 as an example, when the hole injection layer HIL in its first light-emitting unit is the first low-refractive layer, the refractive index n1 of the hole transport material therein is less than 1.8, and the thickness H1 of the hole injection layer HIL can be 5nm to 30nm, and can satisfy 6nm<n1×H1<54nm.

[0037] In some embodiments, the first low-refractive layer is a hole transport layer. In the first low-refractive layer (a hole transport layer having a refractive index of less than 1.8), the refractive index of the hole transport material having a refractive index less than 1.8 is n2 (i.e., n2 < 1.8), and the thickness of the first low-refractive layer is H2, 6nm < n2 × H2 < 360nm. That is, when the hole transport region of at least one light-emitting unit of the stacked device includes a hole transport layer having a refractive index of less than 1.8, the thickness H2 of the hole transport layer and the refractive index n2 of the hole transport material satisfy 6nm < n2 × H2 < 360nm, which is beneficial to further improve the efficiency and other performance of the stacked device. According to the research and analysis of the inventors, controlling the layer product of n2 and H2 (i.e., n2 × H2) within the above range can make the hole transport layer have more suitable optical thickness and other properties, thereby further optimizing the performance of the stacked device.

[0038] Illustratively, n2×H2 can be 7 nm, 10 nm, 30 nm, 50 nm, 70 nm, 100 nm, 130 nm, 150 nm, 180 nm, 200 nm, 230 nm, 250 nm, 280 nm, 300 nm, 330 nm, 355 nm, or a range consisting of any two thereof.

[0039] Illustratively, in the hole transport layer having a refractive index of less than 1.8, the refractive index n2 of the hole transport material is, for example, 1.25, 1.3, 1.4, 1.5, 1.6, 1.7, 1.75 or a range consisting of any two thereof.

[0040] In addition, the thickness H2 of the hole transport layer having a refractive index of less than 1.8 can be 5 nm to 200 nm, for example, 5 nm, 30 nm, 50 nm, 80 nm, 100 nm, 130 nm, 150 nm, 180 nm, 200 nm or a range consisting of any two thereof.

[0041] Taking the stacked device shown in Figure 1 or Figure 2 as an example, the hole transport layer HTL-1 of the first light-emitting unit may be the first low-refractive layer. In this case, the refractive index n2 of the hole transport material of the hole transport layer HTL-1 of the first light-emitting unit is less than 1.8, and the thickness H2 of the hole transport layer HTL-1 may be 5nm to 200nm, and may satisfy 6nm < n2 × H2 < 360nm; or the hole transport layer HTL-2 of the second light-emitting unit may be the first low-refractive layer. In this case, the refractive index n2 of the hole transport material of the hole transport layer HTL-2 of the second light-emitting unit is less than 1.8. The thickness H2 of the hole transport layer HTL-2 can be 5nm~200nm, and can satisfy 6nm<n2×H2<360nm; or the hole transport layer HTL-1 of the first light-emitting unit and the hole transport layer HTL-2 of the second light-emitting unit are both first low-refractive layers. At this time, the hole transport layer HTL-1 of the first light-emitting unit and the hole transport layer HTL-2 of the second light-emitting unit can both satisfy: the refractive index n2 of the hole transport material is less than 1.8, the thickness H2 of the hole transport layer can be 5nm~200nm, 6nm<n2×H2<360nm.

[0042] In some embodiments, the first low-refractive layer is an electron blocking layer. In the first low-refractive layer (the electron blocking layer whose hole transport material has a refractive index less than 1.8), the refractive index of the hole transport material with a refractive index less than 1.8 is n3 (i.e., n3 < 1.8), and the thickness of the first low-refractive layer is H3, 6nm < n3 × H3 < 54nm. That is, when the hole transport region of at least one light-emitting unit of the stacked device includes an electron blocking layer whose hole transport material has a refractive index less than 1.8, the thickness H3 of the electron blocking layer and the refractive index n3 of the hole transport material satisfy 6nm < n3 × H3 < 54nm, which is beneficial to further improve the efficiency and other performance of the stacked device. According to the research and analysis of the inventors, controlling the layer product of n3 and H3 (i.e., n3 × H3) within the above range can make the electron blocking layer have more suitable optical thickness and other properties, thereby further optimizing the performance of the stacked device.

[0043] Illustratively, n3×H3 may be 7 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 53 nm, or a range consisting of any two thereof.

[0044] Illustratively, in the electron blocking layer having a hole transport material with a refractive index less than 1.8, the refractive index n3 of the hole transport material is, for example, 1.25, 1.3, 1.4, 1.5, 1.6, 1.7, 1.75 or a range consisting of any two thereof.

[0045] In addition, the thickness H3 of the electron blocking layer of the hole transport material with a refractive index less than 1.8 can be 3nm to 50nm, for example, 3nm, 5nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm or a range consisting of any two thereof.

[0046] Taking the stacked device shown in FIG. 1 or FIG. 2 as an example, the electron blocking layer EBL-1 of the first light-emitting unit may be the first low-refractive layer. In this case, the refractive index n3 of the hole transport material of the electron blocking layer EBL-1 of the first light-emitting unit is less than 1.8, and the thickness H3 of the electron blocking layer EBL-1 may be 3 nm to 50 nm, and may satisfy 6 nm < n3 × H3 < 54 nm; or the electron blocking layer EBL-2 of the second light-emitting unit may be the first low-refractive layer. In this case, the refractive index n2 of the hole transport material of the electron blocking layer EBL-2 of the second light-emitting unit is less than 1.8. The thickness H2 of the electron blocking layer EBL-2 can be 3nm~50nm, and can satisfy 6nm<n3×H3<54nm; or the electron blocking layer EBL-1 of the first light-emitting unit and the electron blocking layer EBL-2 of the second light-emitting unit are both first low-refractive layers. At this time, the electron blocking layer EBL-1 of the first light-emitting unit and the electron blocking layer EBL-2 of the second light-emitting unit can both satisfy: the refractive index n3 of the hole transport material is less than 1.8, the thickness H3 of the electron blocking layer can be 3nm~50nm, and 6nm<n3×H3<54nm.

[0047] Specifically, the mass percentage of the hole transport material in the hole injection layer, the hole transport layer and the electron blocking layer can each independently be greater than or equal to 95%. For example, the mass percentage of the hole transport material in the hole transport layer is 100% (i.e., the hole transport layer is entirely formed of the hole transport material), and the mass percentage of the hole transport material in the electron blocking layer is 100% (i.e., the electron blocking layer is entirely formed of the hole transport material). The refractive index or hole mobility of these layers is basically consistent with the hole transport material included therein.

[0048] In some embodiments, the hole injection layer may include a first p-type doping material. In the hole injection layer, the mass percentage of the first p-type doping material may be 0.5% to 5%, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or any two thereof. This can improve the hole transport capacity of the hole injection layer and further optimize the performance of the stacked device.

[0049] In addition, when the first low-refractive layer is a hole injection layer, the content of the first p-type doping material is within the above-mentioned range (0.5% to 5%), which basically does not affect the refractive index of the hole injection layer (the hole injection layer whose refractive index of the hole transport material is less than 1.8), that is, the refractive index of the hole injection layer is basically equal to the refractive index of the hole transport material therein, that is, the refractive index of the hole injection layer is less than 1.8, which is conducive to further improving the current efficiency and other performance of the stacked device.

[0050] In addition, the above-mentioned organic electroluminescent device further includes a first adjacent layer adjacent to the first low-refractive layer. The first adjacent layer adjacent to the first low-refractive layer is in direct contact with the first low-refractive layer, and there is no other film layer between the two. The refractive index of the first low-refractive layer is greater than or less than the refractive index of the first adjacent layer. The difference between the refractive index of the first low-refractive layer and the refractive index of the first adjacent layer can be specifically less than or equal to 0.5, for example, less than or equal to 0.4, less than or equal to 0.3, less than or equal to 0.2, less than or equal to 0.1, etc. In this way, adverse effects such as interface reflection caused by excessive refractive indices of the first low-refractive layer and the first adjacent layer can be prevented, thereby further optimizing the microcavity effect and improving the current efficiency and other performance of the device.

[0051] Illustratively, the difference between the refractive index of the first low-refractive layer and the refractive index of the first adjacent layer is 0, 0.01, 0.03, 0.05, 0.07, 0.08, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or a range consisting of any two thereof.

[0052] The first adjacent layer may be located on one side of the first low-refractive layer (e.g., located on the side of the first low-refractive layer facing the first electrode, or located on the side of the first low-refractive layer facing the second electrode), or may be located on two opposite sides of the first low-refractive layer (i.e., the refractive index of the film layers on the two opposite sides of the first low-refractive layer is different from the refractive index of the first low-refractive layer, for example, the difference between the refractive index of the film layers on the two opposite sides of the first low-refractive layer and the refractive index of the first low-refractive layer is less than or equal to 0.5, and the two opposite sides are respectively the side of the first low-refractive layer facing the first electrode and the side of the first low-refractive layer facing the second electrode).

[0053] In some embodiments, the first low-refractive layer is a hole injection layer, and the first adjacent layer may be a hole transport layer. For example, in the hole transport region of the first light-emitting unit shown in Figures 1 and 2, the hole injection layer HIL is the first low-refractive layer. In this case, the first adjacent layer of the hole injection layer HIL is the hole transport layer HTL-1 (i.e., the refractive index of the hole injection layer HIL is greater than or less than the refractive index of the hole transport layer HTL-1).

[0054] In some embodiments, the first low-refractive layer is a hole transport layer, and the first adjacent layer includes a hole injection layer adjacent to one side of the first low-refractive layer (the side facing the first electrode), and an electron blocking layer or a light-emitting layer adjacent to the other side of the first low-refractive layer (the side facing the second electrode).

[0055] When the hole transport region includes a hole injection layer, a hole transport layer, and an electron blocking layer stacked in sequence, and the hole transport layer is a first low-refractive layer, the first adjacent layers on opposite sides of the hole transport layer are respectively the hole injection layer and the electron blocking layer (i.e., the refractive index of the hole transport layer is greater than or less than the refractive index of the hole injection layer, and greater than or equal to the refractive index of the electron blocking layer). Taking the hole transport region of the first light-emitting unit shown in Figures 1 and 2 as an example, when its hole transport layer HTL-1 is the first low-refractive layer, the first adjacent layers include the hole injection layer HIL and the electron blocking layer EBL-1.

[0056] When the hole transport region is composed of a stacked hole injection layer and a hole transport layer (no electron blocking layer is provided), and the hole transport layer is the first low-refractive layer, the hole transport layer is located between the hole injection layer and the light-emitting layer, and the first adjacent layers on its opposite sides are the hole injection layer and the light-emitting layer, respectively (that is, the refractive index of the hole transport layer is greater than or less than the refractive index of the hole injection layer, and greater than or equal to the refractive index of the light-emitting layer adjacent to it).

[0057] In other embodiments, the hole transport region of one or more light-emitting units in the stacked device may not include a hole injection layer (such as the hole transport region of the second light-emitting unit shown in Figures 1 and 2). These hole transport regions, for example, may only be provided with a hole transport layer. When the hole transport layer is a first low-refractive index layer, its first adjacent layer is the light-emitting layer adjacent thereto. Alternatively, these hole transport regions may include a hole transport layer and an electron blocking layer. When the hole transport layer is a first low-refractive index layer, its first adjacent layer includes a p-type charge generation layer adjacent to one side thereof and an electron blocking layer adjacent to the other side thereof, with the p-type charge generation layer and the electron blocking layer being located on opposite sides of the hole transport layer. Taking the hole transport region of the second light-emitting unit shown in Figures 1 and 2 as an example, when its hole transport layer HTL-2 is a first low-refractive index layer, the first adjacent layer includes a p-type charge generation layer p-CGL and an electron blocking layer EBL-2.

[0058] In some embodiments, the first low-refractive layer is an electron blocking layer, and the first adjacent layer includes a hole transport layer adjacent to one side of the first low-refractive layer and a light-emitting layer adjacent to the other side of the first low-refractive layer. For example, as shown in Figures 1 and 2, in the first light-emitting unit, the electron blocking layer EBL-1 is located between the hole transport layer HTL-1 and the light-emitting layer EML-1. When the electron blocking layer EBL-1 is the first low-refractive layer, the first adjacent layers on the opposite sides are the hole transport layer HTL-1 and the light-emitting layer EML-1 (that is, the refractive index of the electron blocking layer EBL-1 is greater than or less than the refractive index of the hole transport layer HTL-1, and is greater than or equal to the refractive index of the light-emitting layer EML-1); in the second light-emitting unit, the electron blocking layer EBL-2 is located between the hole transport layer HTL-2 and the light-emitting layer EML-2. When the electron blocking layer EBL-2 is the first low-refractive layer, the first adjacent layers on the opposite sides are the hole transport layer HTL-2 and the light-emitting layer EML-2 (that is, the refractive index of the electron blocking layer EBL-2 is greater than or less than the refractive index of the hole transport layer HTL-2, and is greater than or equal to the refractive index of the light-emitting layer EML-2).

[0059] In addition, when one or some hole transport layers are not the first low refractive index layer (that is, the refractive index of the hole transport material in the hole transport layer is not less than 1.8), the refractive index of the adjacent film layer (such as the hole injection layer, the electron blocking layer or the light-emitting layer) may also be greater than or equal to the refractive index of the hole transport layer, and the difference between the two may be specifically less than or equal to 0.5, for example, less than or equal to 0.4, less than or equal to 0.3, less than or equal to 0.2, less than or equal to 0.1, etc. For example, the difference between the two can be 0, 0.01, 0.03, 0.05, 0.07, 0.08, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5 or a range consisting of any two of them.

[0060] In addition, when one or some electron blocking layers are not the first low-refractive layer (that is, the refractive index of the hole transport material in the hole transport layer is not less than 1.8), the refractive index of the adjacent film layer (such as the hole transport layer or the light-emitting layer) may also be greater than or equal to the refractive index of the hole transport layer, and the difference between the two may specifically be less than or equal to 0.5, for example, less than or equal to 0.4, less than or equal to 0.3, less than or equal to 0.2, less than or equal to 0.1, etc. For example, the difference between the two can be 0, 0.01, 0.03, 0.05, 0.07, 0.08, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5 or a range consisting of any two of them.

[0061] In some specific embodiments, in the stacked device as shown in Figure 1, in the first light-emitting unit, the refractive index of each adjacent two of the hole injection layer HIL, the hole transport layer HTL-1, the electron blocking layer EBL-1 and the light-emitting layer EML-1 is different, for example, the difference in the refractive index of each adjacent two is less than or equal to 0.5; in the second light-emitting unit, the hole transport layer HTL-2 is the first low-refractive layer, and the refractive index of each adjacent two of the hole transport layer HTL-2, the electron blocking layer EBL-2 and the light-emitting layer EML-2 is different, for example, the difference in the refractive index of each adjacent two is less than or equal to 0.5.

[0062] Furthermore, in the stacked device, the hole mobility of the hole transport layer closer to the first electrode decreases, and the ratio of the hole mobility of two adjacent hole transport layers can be specifically 1.1 to 2, such as 1.1, 1.3, 1.5, 1.8, 2, or any combination thereof. This can compensate for the insufficient electron generation capacity of the charge generation layer adjacent to the light-emitting unit closest to the first electrode, thereby improving carrier balance and further enhancing the efficiency and other performance of the stacked device. The two adjacent hole transport layers can both be first low-refractive layers.

[0063] For example, as shown in Figures 1 and 2, the hole mobility of the hole transport layer HTL-1 of the first light-emitting unit is less than the hole mobility of the hole transport layer HTL-2 in the second light-emitting unit, and the difference between the two can be 1.1 to 2, for example, 1.1, 1.3, 1.5, 1.8, 2 or a range consisting of any two of them.

[0064] Furthermore, the hole mobility of the electron blocking layer closer to the first electrode decreases. The ratio of the hole mobility of two adjacent electron blocking layers can be specifically 1.1-2, for example, 1.1, 1.3, 1.5, 1.8, 2, or a range consisting of any two thereof. This can compensate for the insufficient electron generation capacity of the charge generation layer adjacent to the light-emitting unit closest to the first electrode, thereby improving carrier balance and further enhancing the efficiency and other performance of the stacked device. The two adjacent electron blocking layers can both be first low-refractive layers.

[0065] For example, as shown in Figure 1, the hole mobility of the electron blocking layer EBL-1 in the first light-emitting unit is less than the hole mobility of the electron blocking layer EBL-2 in the second light-emitting unit, and the difference between the two can be 1.1 to 2, for example, 1.1, 1.3, 1.5, 1.8, 2 or a range consisting of any two of them.

[0066] Generally, the hole mobility is the space charge limited current hole mobility, which can be measured by conventional methods in the art.

[0067] Specifically, the p-type charge generation layer includes a second host material and a second p-type doping material.

[0068] In some embodiments, the at least one p-type charge generation layer is a second low-refractive layer, and the second low-refractive layer includes a second host material having a refractive index less than 1.8.

[0069] Specifically, the refractive index n4 of the second main material with a refractive index less than 1.8 may be greater than 1.2. Exemplarily, the refractive index n4 of the second main material with a refractive index less than 1.8 is, for example, 1.25, 1.3, 1.4, 1.5, 1.6, 1.7, 1.75 or a range consisting of any two of them.

[0070] Generally, in the second low-refractive layer, the mass percentage of the second main material having a refractive index less than 1.8 is greater than or equal to 90%, for example, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, or a range consisting of any two thereof. The refractive index of the second low-refractive layer is substantially equal to the refractive index n4 of the second main material, that is, the refractive index of the second low-refractive layer is less than 1.8, which is conducive to further improving the current efficiency and other performance of the stacked device.

[0071] Specifically, the above-mentioned p-type charge generation layer may also include a second p-type doping material. In the p-type charge generation layer, the mass percentage of the second p-type doping material is 3% to 10%, for example, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range composed of any two of them, which can further improve the performance of the stacked device.

[0072] In addition, when the p-type charge generation layer is the second low-refractive layer, the content of its second p-type doping material is within the above-mentioned range (3% to 10%), which basically does not affect the refractive index of the p-type charge generation layer, that is, the refractive index of the p-type charge generation layer is basically equal to the refractive index of the second main material therein, that is, the refractive index of the p-type charge generation layer is less than 1.8, which is conducive to further improving the current efficiency and other performance of the stacked device.

[0073] In some embodiments, the refractive index of the second host material having a refractive index less than 1.8 is n4, the thickness of the second low-refractive layer is H4, and 6nm<n4×H4<36nm. This is conducive to further improving the efficiency and other performance of the stacked device. According to the research and analysis of the inventors, controlling the layer product of n4 and H4 (i.e., n4×H4) within the above range can make the p-type charge generation layer have more suitable optical thickness and other properties, thereby further optimizing the performance of the stacked device.

[0074] Illustratively, n4×H4 may be a range consisting of any two of 6 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, and 36 nm.

[0075] In addition, the thickness H2 of the second low-refractive layer may be 5 nm to 20 nm, for example, 5 nm, 8 nm, 10 nm, 13 nm, 15 nm, 18 nm, 20 nm, or a range consisting of any two thereof.

[0076] In some embodiments, the organic electroluminescent device includes a second adjacent layer adjacent to the second low-refractive layer, the second adjacent layer adjacent to the second low-refractive layer is in direct contact with the second low-refractive layer, there is no other film layer between the two, the refractive index of the second low-refractive layer is greater than or less than the refractive index of the second adjacent layer, and the difference between the refractive index of the second low-refractive layer and the refractive index of the second adjacent layer can be specifically less than or equal to 0.5, for example, less than or equal to 0.4, less than or equal to 0.3, less than or equal to 0.2, etc., so as to prevent adverse effects such as interface reflection caused by excessive refractive index between the second low-refractive layer and the second adjacent layer, thereby further optimizing the microcavity effect and improving the current efficiency and other performance of the device.

[0077] Illustratively, the difference between the refractive index of the second low-refractive layer and the refractive index of the second adjacent layer is 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or a range consisting of any two thereof.

[0078] Specifically, the second adjacent layer may include a hole transport layer and / or an n-type charge generation layer adjacent to the second low-refractive layer, that is, the second adjacent layer may include a hole transport layer located on one side of the second low-refractive layer and / or an n-type charge generation layer located on the other side of the second low-refractive layer, the n-type charge generation layer is located on the side of the second low-refractive layer facing the first electrode, and the hole transport layer is located on the side of the second low-refractive layer facing the second electrode.

[0079] In addition, in some embodiments, as shown in FIG2 , at least one charge generation layer further includes a metal layer located between the n-type charge generation layer and the p-type charge generation layer. This can reduce the driving voltage of the stacked device and improve the stability of the device, which can be specifically manifested in that the voltage variation during the operation of the device is smaller.

[0080] Specifically, the metal layer may include metals from Group IIIA of the periodic table and / or transition metals, for example, one or more of aluminum, silver, gold, molybdenum, and titanium.

[0081] In some embodiments, the hole transport region adjacent to the p-type charge generation layer of the charge generation layer including the metal layer (such as the hole transport region of the second light-emitting unit in Figure 2) includes a first low-refractive layer (that is, for the charge generation layer including the metal layer, the hole transport region adjacent to its p-type charge generation layer includes the first low-refractive layer), and the first low-refractive layer can be a hole transport layer and / or an electron blocking layer, etc. (taking the stacked device shown in Figure 2 as an example, the hole transport layer HTL-2 and / or the electron blocking layer EBL-2 in its second light-emitting unit is the first low-refractive layer), which can further improve the current efficiency and other performance of the stacked device.

[0082] In the above devices, the first electrode, hole injection layer, hole transport layer, electron blocking layer, light-emitting layer, charge generation layer, second electrode and other film layers can be made of conventional materials in the field that meet the preset refractive index and other property requirements, without special restrictions.

[0083] Exemplarily, the anode includes transparent conductive oxide materials such as indium tin oxide (or indium tin oxide, ie, ITO), indium zinc oxide (or indium zinc oxide, ie, IZO), tin dioxide (SnO2), zinc oxide (ZnO), and any combination thereof.

[0084] For example, the cathode material can be metals or alloys such as magnesium (Mg), silver (Ag), aluminum (Al), aluminum-lithium (Al-Li), calcium (Ca), magnesium-indium (Mg-In), magnesium-silver (Mg-Ag), and any combination thereof.

[0085] For example, in the hole injection layer, the hole transport material may include an aromatic amine compound, specifically a triarylamine compound, such as HTM1 or NPB; and the first p-type doping material may include NDP-9.

[0086] Exemplarily, the hole transport material of the hole transport layer may include an aromatic amine compound, specifically a triarylamine compound, such as one or more of HTM1, HTM2, NPB, the hole transport material in patent document CN104718636B, the aromatic amine derivative as shown in general formula (1-d) used in the hole transport layer in patent document US8394510B2 or CN101535256B (the specific structure of the aromatic amine derivative is detailed in US8394510B2 or CN101535256B, which will not be repeated here), etc., but is not limited to these.

[0087] Exemplarily, the hole transport material of the electron blocking layer may include an aromatic amine compound, specifically a triarylamine compound, such as BCzPh, an aromatic amine derivative as shown in the general formula (1-d) in patent document US8394510B2 or CN101535256B (for the specific structure of the aromatic amine derivative, please refer to US8394510B2 or CN101535256B, which will not be repeated here), etc., but is not limited to this.

[0088] In addition, the second main material in the p-type charge generation layer is a hole transport material, which may include an aromatic amine compound, specifically a triarylamine compound. For example, the second main material may include one or more of HTM1, NPB, the hole transport material in patent document CN104718636B, etc., but is not limited to this.

[0089] Exemplarily, the second p-type doping material in the p-type charge generation layer may include NDP-9, but is not limited thereto.

[0090] In addition, the n-type charge generation layer includes a third host material and an n-type dopant material. The third host material includes, for example, B-Phen, and the n-type dopant material includes, for example, Li, but is not limited thereto.

[0091] In some embodiments, the mass percentage of the n-type dopant material in the n-type charge generation layer can be 1% to 10%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or any two thereof.

[0092] In addition, the light-emitting layer may include a first host material and a guest material. The light-emitting layer may be formed using conventional host materials and guest materials in the art. For example, the above-mentioned stacked device is a blue light device, and its light-emitting layer is a blue light-emitting layer. The first host material may include α, β-AND, and the guest material may include t-DABNA, but is not limited thereto.

[0093] Specifically, in the above-mentioned light-emitting layer, the mass percentage of the guest material can be 0.5% to 15%, for example, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15% or the range of any two thereof.

[0094] Generally, when the light-emitting layer is a blue light-emitting layer or a red light-emitting layer, the weight percentage of the guest material therein may be 0.5% to 5%, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a range consisting of any two thereof. When the light-emitting layer is a green light-emitting layer, the weight percentage of the guest material therein may be 1% to 15%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or a range consisting of any two thereof.

[0095] In addition, the electron transport region may include at least one of an electron injection layer, an electron transport layer, and a hole blocking layer. Among the above-mentioned multiple light-emitting units, the electron transport region of one or at least two light-emitting units may include an electron injection layer, and generally the electron transport region of the light-emitting unit closest to the second electrode (the second light-emitting unit as shown in Figures 1 and 2) includes an electron injection layer; the electron transport region of one or at least two light-emitting units may include an electron transport layer, and preferably the electron transport region of each light-emitting unit (the first light-emitting unit and the second light-emitting unit as shown in Figures 1 and 2) includes an electron transport layer respectively; the electron transport region of one or at least two light-emitting units may include a hole blocking layer, and preferably the electron transport region of each light-emitting unit (the first light-emitting unit and the second light-emitting unit as shown in Figures 1 and 2) includes a hole blocking layer respectively.

[0096] For example, as shown in Figures 1 and 2, the electron transport region of the first light-emitting unit includes a hole blocking layer HBL-1 and an electron transport layer EIL-1 stacked in sequence from the first electrode to the second electrode, and the electron transport region of the second light-emitting unit includes a hole blocking layer HBL-2, an electron transport layer EIL-2 and an electron injection layer HIL stacked in sequence from the first electrode to the second electrode.

[0097] For example, the electron injection layer may include one or more of LiQ, LiF, NaCl, CsF, Li2O, Cs2CO3, BaO, Na, Li, Ca, Mg, and Yb, but is not limited thereto.

[0098] In addition, the electron transport layer may include an electron transport material and a doping material. The electron transport material may include, for example, DppyA, and the doping material may include, for example, one or more of LiQ, LiF, NaCl, CsF, Li2O, Cs2CO3, BaO, Na, Li, Ca, Mg, Ag, and Yb, but is not limited thereto.

[0099] In some embodiments, the mass percentage of the doping material in the electron transport layer can be 30% to 70%, for example, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70% or a range consisting of any two thereof.

[0100] Exemplarily, the electron blocking layer may include CzPhPy, but is not limited thereto.

[0101] Specifically, the organic electroluminescent device may be a top-emitting device (top-emitting stacked device).

[0102] In some embodiments, as shown in Figures 1 and 2 , the organic electroluminescent device may further include a light extraction layer (CPL) disposed on the side of the second electrode facing away from the first electrode. This can further optimize the device's performance, such as light color and efficiency. The light extraction layer (CPL) may be made of conventional materials for such layers in the art, but is not limited thereto.

[0103] Typically, the organic electroluminescent device comprises a substrate. The anode can be formed on the substrate by sputtering or depositing an anode material. The remaining layers can be formed by conventional methods in the art, such as vacuum thermal evaporation, spin coating, and printing, and will not be described in detail. The substrate can be made of glass or a polymer material with excellent mechanical strength, thermal stability, water resistance, and transparency. Furthermore, the substrate used for a display can also include thin-film transistors (TFTs).

[0104] The display device of the embodiments of the present application includes the aforementioned organic electroluminescent device. Specifically, the display device may be a display device such as an OLED display, as well as any product or component with a display function, such as a television, digital camera, mobile phone, or tablet computer, that includes the display device. The advantages of the display device and the aforementioned organic electroluminescent device over the prior art are the same and will not be further elaborated here.

[0105] The organic electroluminescent device of the present application is further described below through specific examples. In the following examples and comparative examples, the refractive index is the refractive index of light with a wavelength of about 460 nm.

[0106] Example 1

[0107] The organic electroluminescent device of Example 1 is a top-emitting stacked device, the structure of which is shown in FIG1 . The properties of each film layer, such as material and thickness, are shown in Table 1.

[0108] Table 1

[0109] Example 2: The difference from Example 1 is that the refractive index of the HIL and the p-CGL is less than 1.8. The other conditions are the same as those in Example 1, as shown in Table 2 for details.

[0110] Table 2

[0111] Example 3 and Example 4: The difference from Example 1 is that the hole mobility of HTL-1 and HTL-2 is different, as shown in Table 3. The other conditions are the same as those of Example 1.

[0112] Table 3

[0113] Example 5: The difference from Example 1 is that a metal layer is provided between the p-CGL and the n-CGL (the structure of the stacked device is shown in FIG2 ). The material of the metal layer is Ag and the thickness is 1 nm. The other conditions are the same as those of Example 1.

[0114] Comparative Example 1: The difference from Example 1 is that the refractive indexes of HIL, HTL-1, EBL-1, HTL-2, and EBL-2 are all greater than 1.8, as shown in Table 4. The other conditions are the same as those in Example 1.

[0115] Table 4

[0116] The voltage (driving voltage when lit), current efficiency (current efficiency when the device reaches a brightness of 1050 nits), and LT95 life of the devices of each embodiment and comparative example were measured. The results are shown in Table 5.

[0117] The LT95 life test process is as follows: Use a luminance meter at an initial 1000cd / m 2 Under the same brightness, the current is kept constant and the brightness of the organic electroluminescent device is measured to be reduced to 950cd / m 2 The time is in hours (h).

[0118] Table 5

[0119] It can be seen that, compared with Comparative Example 1, the introduction of the first low-refractive layer into the hole transport region of the stacked devices of Examples 1 to 5 can improve the current efficiency and other performance of the device, while maintaining a lower driving voltage (not higher than 6.6) and a longer life (LT95 not lower than 900h) of the device.

[0120] Specifically, compared with Comparative Example 1, HTL-1 and HTL-2 of Example 1 use a hole transport material with a refractive index less than 1.8, which can improve the current efficiency and life of the device and reduce the driving voltage.

[0121] Specifically, compared with Example 1, HTL-1, HTL-2, HIL and p-CGL of Example 2 each use a hole transport material with a refractive index less than 1.8, which can further improve the current efficiency of the device.

[0122] Specifically, compared to Example 1, the hole mobility of HTL-1 in Example 4 was greater than that of HTL-2, and the device's performance, such as efficiency and lifespan, was inferior to that of Example 1. Compared to Example 1, the hole mobility of HTL-1 in Example 3 was less than that of HTL-2, and the device's current efficiency was further improved. This indicates that the closer the hole transport layer is to the anode, the lower the hole mobility, which can further improve device performance, such as current efficiency.

[0123] Specifically, compared with Example 1, Example 5 introduces a metal layer between the n-type charge generation layer and the p-type charge generation layer, which can further improve the current efficiency of the device and reduce the driving voltage.

[0124] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application 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 or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

Claims

1. An organic electroluminescent device, wherein: It comprises a plurality of light-emitting units arranged in a stacked manner, each of the light-emitting units comprises a hole transport region, a light-emitting layer and an electron transport region which are stacked in sequence, the hole transport region of at least one of the light-emitting units comprises a first low-refractive layer, and the first low-refractive layer comprises a hole transport material having a refractive index less than 1.

8.

2. The organic electroluminescent device according to claim 1, wherein: The hole transport material having a refractive index less than 1.8 has a refractive index greater than 1.2; And / or, in the first low-refractive layer, the mass percentage of the hole transport material having a refractive index less than 1.8 is greater than or equal to 95%.

3. The organic electroluminescent device according to claim 1, wherein: It also includes a first electrode stacked with the light-emitting unit, and among the plurality of light-emitting units, the hole transport region of the light-emitting unit closest to the first electrode includes the first low-refractive layer.

4. The organic electroluminescent device according to claim 3, wherein: The first electrode is an anode; And / or, the hole transport regions of the plurality of light-emitting units respectively include the first low-refractive layer.

5. The organic electroluminescent device according to claim 1, wherein: The light-emitting layer is a red light-emitting layer, a green light-emitting layer or a blue light-emitting layer.

6. The organic electroluminescent device according to claim 1, wherein: The hole transport region includes one or more of a hole injection layer, a hole transport layer and an electron blocking layer, and the first low refractive layer is at least one of the hole injection layer, the hole transport layer and the electron blocking layer.

7. The organic electroluminescent device according to claim 6, wherein: The hole injection layer further comprises a first p-type doping material, and the mass percentage of the first p-type doping material in the hole injection layer is 0.5% to 5%; and / or, the first low-refractive layer is the hole injection layer, wherein the thickness of the first low-refractive layer is 5 nm to 30 nm, and / or, the refractive index of the hole transport material having a refractive index less than 1.8 is n1, the thickness of the first low-refractive layer is H1, 6 nm < n1 × H1 < 54 nm; and / or, the first low-refractive layer is the hole transport layer, wherein the thickness of the first low-refractive layer is 5 nm to 200 nm, and / or, the refractive index of the hole transport material having a refractive index less than 1.8 is n2, the thickness of the first low-refractive layer is H2, 6 nm < n2 × H2 < 360 nm; And / or, the first low-refractive layer is the electron blocking layer, wherein the thickness of the first low-refractive layer is 3nm to 50nm, and / or, the refractive index of the hole transport material with a refractive index less than 1.8 is n3, the thickness of the first low-refractive layer is H3, 6nm<n3×H3<54nm.

8. The organic electroluminescent device according to claim 6, wherein: Also includes a first electrode, wherein The first electrode is an anode; and / or, the hole transport region closest to the first electrode includes the hole injection layer, and the hole injection layer is the first low-refractive layer; and / or, the hole transport region closest to the first electrode comprises a hole transport layer, and the hole transport layer is the first low-refractive layer; And / or, the hole transport region closest to the first electrode includes an electron blocking layer, and the electron blocking layer is the first low-refractive layer.

9. The organic electroluminescent device according to claim 8, wherein: The hole transport region of each of the light-emitting units includes the hole transport layer, and the hole transport layer of the hole transport region of each of the light-emitting units is the first low-refractive layer; And / or, the hole transport region of each of the light-emitting units includes the electron blocking layer, and the electron blocking layer of the hole transport region of each of the light-emitting units is the first low-refractive layer.

10. The organic electroluminescent device according to claim 6, wherein: The hole transport regions of at least two of the light-emitting units include the hole transport layer; And / or, the hole transport region of at least two of the light-emitting units includes the electron blocking layer.

11. The organic electroluminescent device according to claim 10, wherein: The hole transport region of each of the light-emitting units includes the hole transport layer; And / or, the hole transport region of each of the light-emitting units includes the electron blocking layer respectively.

12. The organic electroluminescent device according to claim 10, wherein: The organic electroluminescent device further comprises a first electrode stacked with the light-emitting unit, wherein: The first electrode is an anode; and / or, the hole mobility of the hole transport layer closer to the first electrode is smaller; and / or, the ratio of the hole mobilities of two adjacent hole transport layers is 1.1 to 2; and / or, the hole mobility of the electron blocking layer closer to the first electrode is smaller; And / or, the ratio of hole mobilities of two adjacent electron blocking layers is 1.1-2.

13. The organic electroluminescent device according to claim 6, wherein: It also includes a first adjacent layer adjacent to the first low-refractive layer, wherein: The refractive index of the first low-refractive layer is greater than or less than the refractive index of the first adjacent layer; and / or, the difference between the refractive index of the first low-refractive layer and the refractive index of the first adjacent layer is less than or equal to 0.5; And / or, when the first low-refractive layer is a hole injection layer, the first adjacent layer is a hole transport layer; when the first low-refractive layer is a hole transport layer, the first adjacent layer includes a hole injection layer adjacent to one side of the first low-refractive layer, and an electron blocking layer or the light-emitting layer adjacent to the other side of the first low-refractive layer, or the first adjacent layer includes a p-type charge generation layer adjacent to one side of the first low-refractive layer, and an electron blocking layer adjacent to the other side of the first low-refractive layer; when the first low-refractive layer is an electron blocking layer, the first adjacent layer includes a hole transport layer adjacent to one side of the first low-refractive layer, and the light-emitting layer adjacent to the other side of the first low-refractive layer.

14. The organic electroluminescent device according to any one of claims 1 to 13, wherein: It also includes a charge generation layer located between every two adjacent light-emitting units, wherein the charge generation layer includes an n-type charge generation layer and a p-type charge generation layer; wherein at least one of the p-type charge generation layers is a second low-refractive layer, and the second low-refractive layer includes a second host material with a refractive index less than 1.8; And / or, at least one of the charge generation layers further includes a metal layer located between the n-type charge generation layer and the p-type charge generation layer.

15. The organic electroluminescent device according to claim 14, wherein: For the charge generation layer including the metal layer, the hole transport region adjacent to the p-type charge generation layer thereof includes the first low-refractive layer; and / or the refractive index of the second host material having a refractive index less than 1.8 is greater than 1.2; And / or, the refractive index of the second main material with a refractive index less than 1.8 is n4, the thickness of the second low-refractive layer is H4, 6nm<n4×H4<36nm; And / or, the thickness H2 of the second low-refractive layer is 5 nm to 20 nm.

16. The organic electroluminescent device according to claim 14, wherein: The metal layer includes a metal of Group IIIA of the Periodic Table and / or a transition metal.

17. The organic electroluminescent device according to claim 16, wherein: The metal layer includes one or more of aluminum, silver, gold, molybdenum, and titanium.

18. The organic electroluminescent device according to claim 14, wherein: The organic electroluminescent device comprises a second adjacent layer adjacent to the second low-refractive layer, wherein: The refractive index of the second low-refractive layer is greater than or less than the refractive index of the second adjacent layer; or, the difference between the refractive index of the second low-refractive layer and the refractive index of the second adjacent layer is less than or equal to 0.5; And / or, the second adjacent layer includes a hole transport layer located on one side of the second low-refractive layer, and / or, the second adjacent layer includes an n-type charge generation layer located on the other side of the second low-refractive layer.

19. The organic electroluminescent device according to claim 14, wherein: The p-type charge generation layer includes a second p-type doping material. In the p-type charge generation layer, the mass percentage of the second p-type doping material is 3% to 10%.

20. A display device, wherein: An organic electroluminescent device comprising any one of claims 1 to 19.

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