Organic electroluminescent device and display device

Abstract

An organic electroluminescent device comprises an anode (2), a cathode (11), a light-emitting layer (7) disposed between the anode (2) and the cathode (11), and an auxiliary light-emitting layer (6) located on the side of the light-emitting layer (7) facing the anode (2); the auxiliary light-emitting layer (6) comprises a first host material and a first fluorescent guest material; the light-emitting layer (7) comprises a second host material, a TADF material, and a second fluorescent guest material; the first host material and the first fluorescent guest material satisfy the following conditions: S1(B)-T1(B)>0.2eV; S1(B)> S1(C); T1(B)>T1(C); ||HOMO(B)‑LUMO(C)|‑S1(C)|≤0.2eV; wherein, the lowest singlet energy of the first host material is S1(B), and the lowest triplet energy is T1(B); the lowest singlet energy of the first fluorescent guest material is S1(C), and the lowest triplet energy is T1(C); the highest occupied molecular orbital energy level of the first host material is HOMO(B); and the lowest unoccupied molecular orbital energy level of the first fluorescent guest material is LUMO(C).

Description

Technical Field

[0001] The embodiments of the present disclosure relate to, but are not limited to, the field of display technology, and specifically to an organic electroluminescent device and a display apparatus. Background Art

[0002] Thermally activated delayed fluorescence (TADF) materials are the third generation of organic light-emitting materials, following organic fluorescent and organic phosphorescent materials. They have experienced rapid development in recent years and hold great potential for application in organic light-emitting diode (OLED) technology. Superfluorescence technology based on TADF sensitizers is considered a promising solution for implementing TADF materials in OLED technology. However, OLED devices using TADF materials currently face numerous challenges, such as high operating voltages, low efficiency, and short lifespans. Summary of the Invention

[0003] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0004] The present disclosure provides an organic electroluminescent device, comprising an anode, a cathode, a light-emitting layer disposed between the anode and the cathode, and an auxiliary light-emitting layer disposed on a side of the light-emitting layer facing the anode; the auxiliary light-emitting layer comprises a first host material and a first fluorescent guest material; the light-emitting layer comprises a second host material, a TADF material, and a second fluorescent guest material;

[0005] The first host material and the first fluorescent guest material satisfy:

[0006] S1(B)-T1(B)>0.2eV;

[0007] S1(B)>S1(C);

[0008] T1(B)>T1(C);

[0009] ||HOMO(B)-LUMO(C)|-S1(C)|≤0.2eV;

[0010] Among them, the lowest singlet energy of the first host material is S1(B), and the lowest triplet energy is T1(B); the lowest singlet energy of the first fluorescent guest material is S1(C), and the lowest triplet energy is T1(C); the highest occupied molecular orbital energy level of the first host material is HOMO(B); and the lowest unoccupied molecular orbital energy level of the first fluorescent guest material is LUMO(C).

[0011] An embodiment of the present disclosure further provides a display device, comprising the organic electroluminescent device described in any embodiment.

[0012] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings are intended to provide a further understanding of the technical solutions of the present disclosure and constitute a part of the specification. Together with the embodiments of the present disclosure, they are used to explain the technical solutions of the present disclosure and do not constitute a limitation of the technical solutions of the present disclosure. The shapes and sizes of the components in the accompanying drawings do not reflect the actual scale and are intended only to illustrate the contents of the present disclosure.

[0014] Figure 1 Spectra of TADF materials and fluorescent guest materials in the light-emitting layer of organic electroluminescent devices according to some exemplary embodiments;

[0015] Figure 2 Schematic diagrams of the structures of organic electroluminescent devices according to some exemplary embodiments. DETAILED DESCRIPTION

[0016] Those skilled in the art should understand that the technical solutions of the embodiments of the present disclosure may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the embodiments of the present disclosure, and all should be included in the scope of the claims of the present disclosure.

[0017] In the drawings, the sizes of components, layer thicknesses, or regions are sometimes exaggerated for clarity. Therefore, the embodiments of the present disclosure are not necessarily limited to these dimensions, and the shapes and sizes of each component in the drawings do not reflect true proportions. Furthermore, the drawings schematically illustrate some examples, and the embodiments of the present disclosure are not limited to the shapes or values ​​shown in the drawings.

[0018] TADF materials are the third generation of organic light-emitting materials developed after organic fluorescent materials and organic phosphorescent materials. They have achieved rapid development in recent years and have good application potential in OLED technology. TADF materials have a small energy difference between the lowest singlet state and the lowest triplet state (△E ST), the lowest triplet state (T1) exciton can be converted into the lowest singlet state (S1) exciton through reverse intersystem crossing (RISC), and further fluorescence emission can be achieved through the radiative transition of the lowest singlet state (S1) exciton to the ground state (S0), thereby achieving a theoretical internal quantum efficiency of 100%. Superfluorescence technology based on TADF sensitizers is considered to be a valuable implementation solution for TADF materials in OLED technology. However, at present, OLED devices using TADF materials face many problems that hinder their practical application, such as: (1) In order to prevent the diffusion of triplet excitons in the light-emitting layer, hole blocking layer materials and electron blocking layer materials with high T1 are used on both sides of the light-emitting layer, which limits the selection of corresponding film materials and reduces the flexibility of device design; (2) There are charge traps in the light-emitting layer, which leads to a high operating voltage and high power consumption of the device.

[0019] An embodiment of the present disclosure provides an organic electroluminescent device, comprising an anode, a cathode, a light-emitting layer arranged between the anode and the cathode, and an auxiliary light-emitting layer located on the side of the light-emitting layer facing the anode; the auxiliary light-emitting layer comprises a first host material and a first fluorescent guest material; the light-emitting layer comprises a second host material, a TADF material, and a second fluorescent guest material.

[0020] The organic electroluminescent device of the embodiment of the present disclosure is provided with an auxiliary light-emitting layer on the side of the light-emitting layer facing the anode. Thus, in some possible embodiments, the auxiliary light-emitting layer can be provided between the electron blocking layer and the light-emitting layer, and the first host material in the auxiliary light-emitting layer can be provided with a higher hole transport capability and the ability to block leakage of triplet excitons in the light-emitting layer. In this way, it is not necessary to require the electron blocking layer material to have a higher triplet energy to block leakage of triplet excitons in the light-emitting layer, which can improve the flexibility of the selection of the electron blocking layer material. In addition, since the light-emitting layer is a superfluorescent system, the hole transport capability of the second host material in the light-emitting layer is stronger than the hole transport capability of the TADF material. After providing the auxiliary light-emitting layer, the auxiliary light-emitting layer can be provided without containing a material having TADF properties, or the doping concentration (which can be a mass percentage) of the material having TADF properties contained in the auxiliary light-emitting layer is less than the doping concentration of the TADF material in the light-emitting layer. Thus, the hole transport impedance between the electron blocking layer and the light-emitting layer can be reduced, and the holes can be better transported to the light-emitting layer, thereby reducing the operating voltage of the device.

[0021] Herein, for ease of description, the first host material may be referred to as material B, the first fluorescent guest material may be referred to as material C, the second host material may be referred to as material D, the TADF material may be referred to as material E, and the second fluorescent guest material may be referred to as material F.

[0022] The highest occupied molecular orbital energy level of a material is referred to as the HOMO energy level, and the lowest unoccupied molecular orbital energy level is referred to as the LUMO energy level. The lowest singlet energy of a material is referred to as the S1 energy, and the lowest triplet energy is referred to as the T1 energy.

[0023] The lowest singlet energy of the first host material is S1(B), and the lowest triplet energy is T1(B); the lowest singlet energy of the first fluorescent guest material is S1(C), and the lowest triplet energy is T1(C); the lowest singlet energy of the second host material is S1(D), and the lowest triplet energy is T1(D); the lowest singlet energy of the TADF material is S1(E), and the lowest triplet energy is T1(E); the lowest singlet energy of the second fluorescent guest material is S1(F), and the lowest triplet energy is T1(F).

[0024] The HOMO energy level of the first host material is HOMO(B), and the LUMO energy level is LUMO(B); the HOMO energy level of the first fluorescent guest material is HOMO(C), and the LUMO energy level is LUMO(C); the HOMO energy level of the second host material is HOMO(D), and the LUMO energy level is LUMO(D); the HOMO energy level of the TADF material is HOMO(E), and the LUMO energy level is LUMO(E); the HOMO energy level of the second fluorescent guest material is HOMO(F), and the LUMO energy level is LUMO(F).

[0025] Herein, the electroemission spectrum of a material refers to the luminescence spectrum obtained when a power-on state is applied to an OLED device fabricated by doping the material at a certain ratio (e.g., 20%) into a host material. The descriptions of the S1 and T1 of the host material, if the host material is in the form of an exciplex containing two components, refer to the S1 and T1 of the exciplex. The descriptions of the HOMO and LUMO of the host material, if the host material is in the form of an exciplex, refer to the smaller HOMO and the larger LUMO, respectively, of the two components forming the exciplex.

[0026] In some exemplary embodiments, the first host material in the auxiliary light-emitting layer may be a hole-forming material, and the hole mobility of the first host material may be at least ten times higher than the electron mobility. For example, the first host material may be a carbazole-based material. The first host material does not exhibit TADF properties, that is, the first host material satisfies the following conditions: S1(B)-T1(B)>0.2 eV.

[0027] The first fluorescent guest material can be a common fluorescent material, such as anthracene, fluorene, pyrene, pyrrole, or a boron-containing material with a multi-resonance effect. The doping concentration of the first fluorescent guest material can be 0.5% to 10%, that is, the weight percentage of the first fluorescent guest material in the auxiliary light-emitting layer is 0.5% to 10%.

[0028] The first host material and the first fluorescent guest material satisfy:

[0029] S1(B)>S1(C);

[0030] T1(B)>T1(C);

[0031] ||HOMO(B)-LUMO(C)|-S1(C)|≤0.2eV.

[0032] In this embodiment, the auxiliary light-emitting layer meets the above conditions, and the first host material in the auxiliary light-emitting layer can play the role of dispersing the first fluorescent guest material, the role of hole transport, and the role of blocking triplet excitons in the light-emitting layer. In this way, when the device also includes an electron blocking layer, it is not necessary to require the electron blocking layer material to have a high triplet energy to block the triplet excitons in the light-emitting layer, which can improve the flexibility of the selection of electron blocking layer materials. In addition, in the embodiment of the present disclosure, excitons are mainly generated from the TADF material of the light-emitting layer, and the first host material does not have TADF properties. In this way, the first host material will not compete with the light-emitting layer for excitons and will not lose the function of blocking triplet excitons. In addition, the first host material and the first fluorescent guest material will not form an exciton complex, so that the charge in the light-emitting layer will not be consumed in large quantities, thereby not affecting the device efficiency and not causing the device emission color to redshift.

[0033] In some exemplary embodiments, the auxiliary light-emitting layer may be composed of two components: the first host material and the first fluorescent guest material. The auxiliary light-emitting layer may not contain a material having TADF properties; or the auxiliary light-emitting layer may contain a material having TADF properties, and the doping concentration (which may be a mass percentage) of the material having TADF properties in the auxiliary light-emitting layer is less than the doping concentration of the TADF material in the light-emitting layer. The auxiliary light-emitting layer may be formed by dual-source co-evaporation. The thickness of the auxiliary light-emitting layer may be 1 nm to 10 nm, for example, the thickness may be 8 nm.

[0034] In some exemplary embodiments, in the light-emitting layer, the second host material may be a single-component material (such as carbazole-based), or a hybrid material with exciplex properties (such as "carbazole + triazine", "carbazole + pyridine", or "carbazole + oxadiazole", etc.). When the second host material is a single-component material, the second host material may satisfy: S1(D)-T1(D)>0.2 eV, and the hole mobility of the second host material may be at least ten times higher than the electron mobility.

[0035] The TADF material satisfies: △E ST (E)=S1(E)-T1(E)<0.2 eV. The materials that can be selected for the TADF material include but are not limited to those with a relatively small △E ST such as triazine-based, pyridine-based, ketone-based, quinone-based, etc. The proportion of the TADF material in the light-emitting layer (which can be the mass percentage) is X, 5% < X < 50%, for example, 15% < X < 40%.

[0036] The second fluorescent guest material can be a common fluorescent material, such as anthracene-based, fluorene-based, pyrene-based, or pyrrole-based. Alternatively, the second fluorescent guest material is a boron-containing material with multiple resonance TADF properties, S1(F)-T1(F)<0.2 eV. The doping concentration of the second fluorescent guest material can be 0.5% to 5%, that is, the mass percentage of the second fluorescent guest material in the light-emitting layer is 0.5% to 5%. In this way, it is beneficial to reduce the probability of the second fluorescent guest material capturing excitons and beneficial to the formation of excitons mainly in the TADF material.

[0037] The second host material, the TADF material, and the second fluorescent guest material may satisfy:

[0038] S1(D)>S1(E)>S1(F);

[0039] T1(D)>T1(E)>T1(F);

[0040] |HOMO(D)-HOMO(E)|<0.2 eV;

[0041] |LUMO(D)-LUMO(E)|>0.3 eV;

[0042] ||HOMO(D)-LUMO(E)|-S1(E)|≤0.2 eV;

[0043] ||HOMO(D)-LUMO(E)|-T1(E)|≤0.2 eV.

[0044] The absorption spectrum of the second fluorescent guest material and the emission spectrum of the TADF material (which may be an electroemission spectrum) can have a significant overlap. Under normalized conditions, the overlapping area of ​​the absorption spectrum of the second fluorescent guest material and the emission spectrum of the TADF material can be no less than 70% of the area of ​​the absorption spectrum of the second fluorescent guest material. The peak position difference between the lowest energy absorption peak of the absorption spectrum of the second fluorescent guest material and the highest energy emission peak of the emission spectrum of the TADF material can be no more than 20 nm. Thus, the greater the overlapping area between the absorption spectrum of the second fluorescent guest material and the emission spectrum of the TADF material, the more conducive it is for the TADF material to transfer energy to the second fluorescent guest material via the Forster energy transfer mechanism, causing the second fluorescent guest material to emit light.

[0045] In this embodiment, the light-emitting layer meets the above conditions, and the holes in the second host material can be better transferred to the TADF material. The TADF material acts as an electron trap for the second host material, which helps excitons to be mainly formed in the TADF material. It can also ensure that the TADF material can transfer energy to the second fluorescent guest material after generating excitons, causing the second fluorescent guest material to emit light. It can be understood that the TADF material is a sensitizer for the second fluorescent guest material. In addition, the second host material and the TADF material do not form an exciplex.

[0046] In some exemplary embodiments, in the light-emitting layer, the second host material, the TADF material, and the second fluorescent guest material may further satisfy the following relationship: λ(D)>λ(E)>λ(F), where λ(D) represents the strongest emission peak wavelength of the second host material, λ(E) represents the strongest emission peak wavelength of the TADF material, and λ(F) represents the strongest emission peak wavelength of the second fluorescent guest material. This facilitates energy transfer in the light-emitting layer toward the second fluorescent guest material, thereby improving device efficiency.

[0047] In some exemplary embodiments, the first host material in the auxiliary light-emitting layer and the second host material in the light-emitting layer may be the same or different, and the first fluorescent guest material in the auxiliary light-emitting layer and the second fluorescent guest material in the light-emitting layer may be the same or different.

[0048] When the first host material is different from the second host material, the following conditions are met:

[0049] |HOMO(B)-HOMO(D)|<0.2eV;

[0050] |HOMO(B)-HOMO(E)|<0.2eV;

[0051] T1(B)>T1(E), for example, T1(B)-T1(E)>0.2eV;

[0052] ||HOMO(B)-LUMO(E)|-S1(E)|≤0.2eV;

[0053] ||HOMO(B)-LUMO(E)|-T1(E)|≤0.2eV.

[0054] In this embodiment, the first host material and the second host material are different and meet the above-mentioned conditions. This ensures that holes can more efficiently enter the light-emitting layer from the first host material and that the first host material effectively blocks triplet excitons. Furthermore, the first host material in the auxiliary light-emitting layer does not form exciplexes with the TADF material in the light-emitting layer, preventing competition with the light-emitting layer for excitons.

[0055] When the first fluorescent guest material and the second fluorescent guest material are different, the following conditions can be met: the emission spectrum of the first fluorescent guest material is close to that of the second fluorescent guest material, with a peak position difference of less than 5 nm; under normalized conditions, the overlap area between the emission spectrum of the first fluorescent guest material and the emission spectrum of the second fluorescent guest material can be greater than 90% of the area of ​​the emission spectrum of the first fluorescent guest material and greater than 90% of the area of ​​the emission spectrum of the second fluorescent guest material. The absorption spectrum of the first fluorescent guest material can have a significant overlap with the emission spectrum (which may be an electroemission spectrum) of the TADF material in the light-emitting layer; under normalized conditions, the spectral overlap area can be no less than 70% of the absorption spectrum area of ​​the first fluorescent guest material. In this way, the TADF material in the light-emitting layer can better transfer energy to the first and second fluorescent guest materials via the Forster energy transfer mechanism, causing the first and second fluorescent guest materials to emit light. This can be understood as the TADF material acting as a sensitizer for the second fluorescent guest material and a sensitizer for the first fluorescent guest material. The emission spectra of the first and second fluorescent guest materials have a high degree of overlap, ensuring the purity of the device's luminescent color. Furthermore, the first fluorescent guest material participates in luminescence and is located in a different layer from the TADF material, physically separating the exciton recombination center from the luminescent center, which improves device performance. The luminescent colors of the first and second fluorescent guest materials can be any color, such as red, green, or blue.

[0056] In some exemplary embodiments, the light-emitting layer may be composed of the second host material, the TADF material, and the second fluorescent guest material, and may be formed by multi-source co-evaporation.

[0057] In some exemplary embodiments, the thickness of the light-emitting layer may be 10 nm to 30 nm. The thickness of the light-emitting layer is at least twice that of the auxiliary light-emitting layer, that is, the ratio of the thickness of the light-emitting layer to the thickness of the auxiliary light-emitting layer is greater than 2:1.

[0058] In some exemplary embodiments, the organic electroluminescent device may further include an electron blocking layer disposed on a side of the auxiliary light-emitting layer facing the anode.

[0059] In one example of this embodiment, the material of the electron blocking layer and the first host material can satisfy the following conditions: |HOMO(A)-HOMO(B)|≤0.2eV, |LUMO(B)|-|LUMO(A)|>0.2eV, where HOMO(A) represents the highest occupied molecular orbital energy level of the electron blocking layer material, and LUMO(A) represents the lowest unoccupied molecular orbital energy level of the electron blocking layer material. This allows holes to be better transferred from the electron blocking layer to the auxiliary light-emitting layer, while suppressing electron transfer.

[0060] In some exemplary embodiments, the organic electroluminescent device may further include a hole blocking layer disposed on a side of the light-emitting layer facing the cathode.

[0061] In one example of this embodiment, the T1 energy of the material of the hole-blocking layer can be greater than the T1 energy of the TADF material in the light-emitting layer, and the difference between the two can be greater than 0.2 eV. In this way, the hole-blocking layer can prevent leakage of triplet excitons from the light-emitting layer. The absolute value of the HOMO energy level of the material of the hole-blocking layer can be greater than the absolute value of the HOMO energy level of the second host material in the light-emitting layer by more than 0.2 eV. The thickness of the hole-blocking layer can be 5 nm to 30 nm, and the hole-blocking layer can be formed by an evaporation process.

[0062] In some exemplary embodiments, the organic electroluminescent device may further include a hole injection layer and a hole transport layer disposed between the anode and the electron blocking layer. The hole injection layer, hole transport layer, and electron blocking layer may be stacked sequentially on the anode.

[0063] In one example of this embodiment, the hole transport layer can be formed of a material with good hole transport capability, such as an aromatic amine or carbazole material. The absolute value of the HOMO energy level difference between the material of the hole transport layer and the material of the electron blocking layer can be no greater than 0.2 eV. The thickness of the hole transport layer can be 1 nm to 200 nm, and the hole transport layer can be formed by an evaporation process.

[0064] The hole injection layer can be formed using materials such as CuPc (copper phthalocyanine), HATCN (2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene), or MnO3 (manganese anhydride), or by p-doping a hole transport material. The hole injection layer can have a thickness of 1 nm to 30 nm and can be formed by evaporation.

[0065] In some exemplary embodiments, the organic electroluminescent device may further include an electron injection layer and an electron transport layer disposed between the cathode and the hole blocking layer. The electron transport layer and the electron injection layer may be sequentially stacked on the hole blocking layer.

[0066] In one example of this embodiment, the electron transport layer can be formed by evaporating an electron transport material having good electron transport capability, or by doping the electron transport material with materials such as LIQ3, Li (lithium), and Ca (calcium). The thickness of the electron transport layer can be 10 nm to 70 nm.

[0067] The electron injection layer can be formed by evaporating a metal salt material such as LiF (lithium fluoride) or LiQ3 using a low work function metal such as Li, Ca or Yb, and can have a thickness of 0.5 nm to 2 nm.

[0068] In some exemplary embodiments, the cathode may be formed of a metal with a relatively low work function, such as Al, Ag, or Mg, or an alloy containing a low work function metal material. When the organic electroluminescent device is a bottom-emitting device, the thickness of the cathode may be greater than 80 nm to ensure good reflectivity (for example, the reflectivity for light with a wavelength of 550 nm may be greater than 85%). When the organic electroluminescent device is a top-emitting device, the thickness of the cathode may be 10 nm to 20 nm to ensure a certain transmittance (for example, the transmittance for light with a wavelength of 550 nm may be greater than 45%).

[0069] In some exemplary embodiments, the organic electroluminescent device may further include an optical cover layer (CPL) disposed on the surface of the cathode facing away from the light-emitting layer to enhance optical output. The CPL may be formed by evaporation of an organic small molecule material with a refractive index greater than 1.8, and may have a thickness of 50 nm to 100 nm.

[0070] In some exemplary embodiments, the anode may be a material having a high work function. For example, for a bottom-emitting device, the anode may be made of a transparent oxide material, such as indium tin oxide (ITO) or indium zinc oxide (IZO), and the thickness may be 80 nm to 200 nm. Alternatively, for a top-emitting device, the anode may be a composite structure of metal and transparent oxide, such as Ag / ITO, Ag / IZO, Al / ITO, Al / IZO, or ITO / Ag / ITO, which can ensure good reflectivity. The thickness of the metal layer in the anode may be 10 nm to 100 nm, and the thickness of the oxide layer may be 5 nm to 20 nm.

[0071] In some exemplary embodiments, Figure 2As shown, the organic electroluminescent device may include an anode 2, a hole injection layer (HIL) 3, a hole transport layer (HTL) 4, an electron blocking layer (EBL) 5, an auxiliary luminescent layer (Assistant Luminescent Layer, referred to as ALL) 6, an emitting layer (EML) 7, a hole blocking layer (HBL) 8, an electron transport layer (ETL) 9, an electron injection layer (EIL) 10, and a cathode 11, which are sequentially stacked on a substrate 1. The organic electroluminescent device may further include an encapsulation layer 12 provided on a side of the cathode 11 facing away from the substrate 1, and the encapsulation layer 12 plays a protective role.

[0072] In some exemplary embodiments, the organic electroluminescent device may be a top-emitting device. The thickness of the organic layer between the cathode and anode can be designed to meet the optical path requirements of the optical microresonator, thereby achieving optimal light output intensity and a desired color. The internal optical path of the top-emitting device can be adjusted by varying the thickness of the hole transport layer or the electron blocking layer.

[0073] The performances of the organic electroluminescent devices according to some exemplary embodiments of the present disclosure are tested and compared below.

[0074] Device 1 (comparative example): Ag (100nm) / ITO (8nm) / HIL / HTL / EBL-1 / TM:TH:RD (25nm, 70%:29.3%:0.7%) / HBL / ETL / EIL (1nm) / Mg:Ag (15nm) / CPL

[0075] Device 2 (comparative example): Ag (100nm) / ITO (8nm) / HIL / HTL / EBL-2 / TM:TH:RD (25nm, 70%:29.3%:0.7%) / HBL / ETL / EIL (1nm) / Mg:Ag (15nm) / CPL

[0076] Device 3 (Example): Ag (100 nm) / ITO (8 nm) / HIL / HTL / EBL-2 / TM:RD (3 nm, 1%) / TM:TH:RD (22 nm, 70%:29.3%:0.7%) / HBL / ETL / EIL (1 nm) / Mg:Ag (15 nm) / CPL

[0077] Device 4 (Example): Ag (100 nm) / ITO (8 nm) / HIL / HTL / EBL-1 / TM:RD (3 nm, 1%) / TM:TH:RD (22 nm, 70%:29.3%:0.7%) / HBL / ETL / EIL (1 nm) / Mg:Ag (15 nm) / CPL

[0078] Device 5 (Example): Ag (100 nm) / ITO (8 nm) / HIL / HTL / EBL-1 / TM:RD (6 nm, 1%) / TM:TH:RD (19 nm, 70%:29.3%:0.7%) / HBL / ETL / EIL (1 nm) / Mg:Ag (15 nm) / CPL

[0079] Among the five devices mentioned above, device 1 and device 2 are devices of comparative examples, and device 3, device 4 and device 5 are devices of three exemplary embodiments of the present disclosure. Among them, TM is the main material in the auxiliary light-emitting layer and the light-emitting layer (that is, the first main material of the auxiliary light-emitting layer and the second main material of the light-emitting layer are both TM); TH is two different materials with TADF properties (both materials can have TADF properties); RD is an ordinary red fluorescent guest material; EBL-1 and EBL-2 are two different hole transport materials. Taking device 3 as an example, the thickness of the auxiliary light-emitting layer is 3nm, and the doping concentration of RD in the auxiliary light-emitting layer is 1%; the thickness of the light-emitting layer is 22nm, and the proportions of TM, TH and RD in the light-emitting layer are 70%, 29.3% and 0.7% respectively. The molecular energy orbital and excited state information of the above materials are shown in Table 1. The emission spectrum of TH, and the absorption spectrum and emission spectrum of RD are shown in Table 1. Figure 1 , Figure 1 In the figure, curve a represents the electroemission spectrum of TH, curve b represents the absorption spectrum of RD, and curve c represents the electroemission spectrum of RD. The performance of the five devices described above is shown in Table 2. In Table 2, the LT95 lifetime (the duration required for the brightness of the light emitted by the device to decay to 95% of the initial brightness) of devices 2 through 5 is based on the LT95 lifetime of device 1. The LT95 lifetime of each device from devices 2 through 5 is the ratio of the LT95 lifetime of device 1 to the LT95 lifetime of each device.

[0080] Table 1 Molecular energy orbitals and excited states of materials

[0081] HOMO(eV) LUMO(eV) T1(eV) S1(eV) TM -5.8 -2.6 2.70 3.48 TH -5.9 -3.6 2.35 2.43 RD -5.6 -3.6 Not detected 2.03 EBL-1 -5.7 -2.5 2.61 / EBL-2 -5.6 -2.3 2.53 /

[0082] Table 2 Device performance

[0083]

[0084] Regarding the comparative examples, Devices 1 and 2, both devices have conventional superfluorescent bottom-emitting devices, differing only in the materials used for the electron-blocking layer. The remaining structures are identical. Tables 1 and 2 show that EBL-1 and EBL-2 have similar HOMO and LUMO energy levels, but EBL-2 has a smaller T1, which can easily lead to leakage of triplet excitons in the light-emitting layer. Consequently, the efficiency of Device 2 is significantly reduced relative to that of Device 1. The improved lifetime of Device 2 relative to Device 1 is primarily due to the shallower HOMO energy level of EBL-2 relative to EBL-1. This creates a potential barrier near the interface between the electron-blocking layer and the light-emitting layer, which helps slow hole injection into the light-emitting layer and results in a better carrier balance.

[0085] Regarding the devices 3, 4 and 5 of the embodiments of the present disclosure, the devices 3, 4 and 5 all include an auxiliary light-emitting layer and a light-emitting layer. As can be seen from Table 1 and Table 2:

[0086] Compared with device 1, device 3 includes an auxiliary light-emitting layer. Since the auxiliary light-emitting layer can block triplet excitons in the light-emitting layer, the efficiency of device 3 is not sensitive to the T1 of the electron blocking layer. Although device 3 uses EBL-2 with a lower T1, it still maintains good efficiency and life. In addition, since device 3 is provided with an auxiliary light-emitting layer, the hole transmission impedance between the electron blocking layer and the light-emitting layer can be reduced, which can better transmit holes to the light-emitting layer. Therefore, the voltage of device 3 is significantly reduced.

[0087] Compared to device 1, devices 4 and 5 both incorporate an auxiliary light-emitting layer, which reduces the hole transfer impedance between the electron-blocking layer and the light-emitting layer, allowing holes to be better transferred to the light-emitting layer. Consequently, devices 4 and 5 have lower operating voltages while maintaining good efficiency. Furthermore, when the thickness of the auxiliary light-emitting layer increases from 3 nm to 6 nm, the device lifetime decreases.

[0088] In summary, the organic electroluminescent device of the exemplary embodiment of the present disclosure, by providing an auxiliary light-emitting layer on the side of the light-emitting layer facing the anode, and reasonably selecting materials and matching material energy levels for the auxiliary light-emitting layer and the light-emitting layer, can achieve a lower operating voltage while ensuring better efficiency and lifespan compared to devices with a single light-emitting layer structure, and has greater flexibility in the selection of electron blocking layer materials.

[0089] The embodiment of the present disclosure further provides a display substrate, comprising the organic electroluminescent device described in any of the above embodiments.

[0090] In some exemplary embodiments, the display substrate includes a display area comprising a plurality of organic electroluminescent devices arranged in an array. In a direction perpendicular to the display substrate, the display substrate may include a drive circuit layer disposed on a substrate, a light-emitting structure layer disposed on a side of the drive circuit layer distal from the substrate, and an encapsulation structure layer disposed on a side of the light-emitting structure layer distal from the substrate. The drive circuit layer includes a pixel drive circuit comprising a thin-film transistor and a storage capacitor. The light-emitting structure layer includes a plurality of organic electroluminescent devices, with the anode of each organic electroluminescent device connected to the drain electrode of the thin-film transistor of the corresponding pixel drive circuit. The encapsulation structure layer may be encapsulated using UV adhesive or thin-film encapsulation. For example, the encapsulation structure layer may include a first encapsulation layer, a second encapsulation layer, and a third encapsulation layer stacked in sequence. The first and third encapsulation layers may be made of inorganic materials, while the second encapsulation layer may be made of organic materials. The encapsulation structure layer may prevent external moisture from entering the display area. The substrate may be a flexible or rigid substrate, such as polyimide, glass, sapphire, or silicon wafer. If a bottom-emitting device is designed, the substrate may have a transmittance greater than 85% for light with a wavelength of 550 nm. The display substrate may further include other film layers, such as spacers and columns, which are not limited in the present disclosure.

[0091] The present disclosure also provides a display device comprising the organic electroluminescent device described in any of the preceding embodiments. The display device can be any product or component with a display function, such as a mobile phone, tablet computer, television, monitor, laptop computer, digital photo frame, navigation system, car display, smartwatch, or smart bracelet.

[0092] Although the embodiments disclosed in this disclosure are as described above, the contents described are merely embodiments adopted to facilitate understanding of the disclosure and are not intended to limit the disclosure. Any person skilled in the art may make any modifications and changes in the form and details of the implementation without departing from the spirit and scope of the disclosure. However, the scope of patent protection of this application shall still be based on the scope defined by the attached claims.

Claims

1. An organic electroluminescent device comprising an anode, a cathode, a light-emitting layer disposed between the anode and the cathode, and an auxiliary light-emitting layer disposed on a side of the light-emitting layer facing the anode; the auxiliary light-emitting layer comprising a first host material and a first fluorescent guest material; and the light-emitting layer comprising a second host material, a TADF material, and a second fluorescent guest material. The first host material and the first fluorescent guest material satisfy: S1(B)-T1(B)>0.2eV; S1(B)>S1(C); T1(B)>T1(C); ||HOMO(B)-LUMO(C)|-S1(C)|≤0.2eV; in, The lowest singlet energy of the first host material is S1(B), and the lowest triplet energy is T1(B); the lowest singlet energy of the first fluorescent guest material is S1(C), and the lowest triplet energy is T1(C); the highest occupied molecular orbital energy level of the first host material is HOMO(B); and the lowest unoccupied molecular orbital energy level of the first fluorescent guest material is LUMO(C).

2. The organic electroluminescent device according to claim 1, wherein The first host material is a hole-type material, and the hole mobility of the first host material is at least ten times higher than the electron mobility.

3. The organic electroluminescent device according to claim 1, wherein: The auxiliary light-emitting layer does not contain any material having TADF properties.

4. The organic electroluminescent device according to claim 1, wherein The second host material, the TADF material and the second fluorescent guest material satisfy: S1(E)-T1(E)<0.2eV; S1(D)>S1(E)>S1(F); T1(D)>T1(E)>T1(F); |HOMO(D)-HOMO(E)|<0.2eV; |LUMO(D)-LUMO(E)|>0.3eV; ||HOMO(D)-LUMO(E)|-S1(E)|≤0.2eV; ||HOMO(D)-LUMO(E)|-T1(E)|≤0.2eV; Among them, the lowest singlet energy of the second host material is S1(D), and the lowest triplet energy is T1(D); the lowest singlet energy of the TADF material is S1(E), and the lowest triplet energy is T1(E); the lowest singlet energy of the second fluorescent guest material is S1(F), and the lowest triplet energy is T1(F); the highest occupied molecular orbital energy level of the second host material is HOMO(D), and the lowest unoccupied molecular orbital energy level is LUMO(D); the highest occupied molecular orbital energy level of the TADF material is HOMO(E), and the lowest unoccupied molecular orbital energy level is LUMO(E).

5. The organic electroluminescent device according to claim 4, wherein: The overlapping area of ​​the absorption spectrum of the second fluorescent guest material and the emission spectrum of the TADF material is not less than 70% of the absorption spectrum area of ​​the second fluorescent guest material; The peak position difference between the lowest energy absorption peak of the absorption spectrum of the second fluorescent guest material and the highest energy emission peak of the emission spectrum of the TADF material is no more than 20 nm. The organic electroluminescent device according to claim 4 , wherein: The second host material is a single component material, or a mixed material having exciplex properties; When the second host material is a single-component material, the second host material satisfies: S1(D)-T1(D)>0.2 eV, and the hole mobility of the second host material is at least ten times higher than the electron mobility.

7. The organic electroluminescent device according to claim 4, wherein: The second host material, the TADF material and the second fluorescent guest material further satisfy: λ(D)>λ(E)>λ(F); Wherein, λ(D) represents the strongest emission peak wavelength of the second host material, λ(E) represents the strongest emission peak wavelength of the TADF material, and λ(F) represents the strongest emission peak wavelength of the second fluorescent guest material.

8. The organic electroluminescent device according to claim 4, wherein: The light-emitting layer is composed of the second host material, the TADF material and the second fluorescent guest material.

9. The organic electroluminescent device according to claim 1, wherein: The first host material is the same as or different from the second host material; When the first host material is different from the second host material, the following conditions are met: |HOMO(B)-HOMO(D)|<0.2eV; |HOMO(B)-HOMO(E)|<0.2eV; T1(B)>T1(E); ||HOMO(B)-LUMO(E)|-S1(E)|≤0.2eV; ||HOMO(B)-LUMO(E)|-T1(E)|≤0.2eV; Among them, the highest occupied molecular orbital energy level of the first host material is HOMO (B); the highest occupied molecular orbital energy level of the second host material is HOMO (D); the highest occupied molecular orbital energy level of the TADF material is HOMO (E), and the lowest unoccupied molecular orbital energy level is LUMO (E); the lowest triplet energy of the first host material is T1 (B); the lowest singlet energy of the TADF material is S1 (E), and the lowest triplet energy is T1 (E).

10. The organic electroluminescent device according to claim 1, wherein The first fluorescent guest material and the second fluorescent guest material are the same as or different from each other; When the first fluorescent guest material and the second fluorescent guest material are different, the following conditions are met: the overlapping area of ​​the emission spectrum of the first fluorescent guest material and the emission spectrum of the second fluorescent guest material is greater than 90% of the area of ​​the emission spectrum of the first fluorescent guest material, and greater than 90% of the area of ​​the emission spectrum of the second fluorescent guest material; the overlapping area of ​​the absorption spectrum of the first fluorescent guest material and the emission spectrum of the TADF material is not less than 70% of the absorption spectrum area of ​​the first fluorescent guest material.

11. The organic electroluminescent device according to claim 1 , further comprising an electron blocking layer disposed on a side of the auxiliary light-emitting layer facing the anode; The material of the electron blocking layer and the first host material meet the following conditions: |HOMO(A)-HOMO(B)|≤0.2eV,|LUMO(B)|-|LUMO(A)|>0.2eV,including, HOMO(A) represents the highest occupied molecular orbital energy level of the material of the electron blocking layer, and LUMO(A) represents the lowest unoccupied molecular orbital energy level of the material of the electron blocking layer. 12 . The organic electroluminescent device according to claim 11 , further comprising a hole injection layer and a hole transport layer disposed between the anode and the electron blocking layer.

13. The organic electroluminescent device according to claim 1, further comprising a hole blocking layer provided on a side of the light-emitting layer facing the cathode; The lowest triplet energy of the hole blocking layer material is greater than the lowest triplet energy of the TADF material, and the absolute value of the highest occupied molecular orbital energy level of the hole blocking layer material is greater than the absolute value of the highest occupied molecular orbital energy level of the second host material by more than 0.2 eV. 14 . The organic electroluminescent device according to claim 13 , further comprising an electron injection layer and an electron transport layer disposed between the cathode and the hole blocking layer.

15. The organic electroluminescent device according to claim 1, wherein: The mass percentage of the TADF material in the light-emitting layer is X, 5% <X<50%。 16. The organic electroluminescent device according to claim 1, wherein: The mass percentage of the second fluorescent guest material in the light-emitting layer is 0.5% to 5%.

17. The organic electroluminescent device according to claim 1, wherein: The thickness of the light-emitting layer is at least twice the thickness of the auxiliary light-emitting layer.

18. The organic electroluminescent device according to claim 1, wherein: The thickness of the light-emitting layer is 10 nm to 30 nm.

19. The organic electroluminescent device according to claim 1, wherein: The auxiliary light-emitting layer has a thickness of 1 nm to 10 nm.

20. A display device comprising the organic electroluminescent device according to any one of claims 1 to 19.

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