D-f transition ce(iii) complex sensitized fluorescent small molecule devices

Abstract

The application discloses an electroluminescent device, which comprises a cathode, an anode and a light-emitting layer between the cathode and the anode, wherein the light-emitting layer comprises a Ce(III) complex and an organic fluorescent molecule, and the emission spectrum of the Ce(III) complex and the absorption spectrum of the organic fluorescent molecule overlap. The Ce(III) complex used in the application has a nanosecond-level excited state lifetime, which is beneficial to improving the stability of the device. In addition, the price of the Ce raw material of the same quality is only one thousandth of the price of Ir, and for red and green light materials, the electroluminescent device of the application can greatly reduce the cost compared with a device using a noble metal phosphorescent iridium (III) complex. Finally, the electroluminescent device of the application can improve the efficiency of the current blue fluorescent device.

Description

Technical Field

[0001] This invention belongs to the field of organic electroluminescence. In particular, this invention relates to an electroluminescent device that utilizes a Ce(III) complex material with df transition to sensitize fluorescent small molecules. Background Technology

[0002] Compared to traditional display technologies, organic light-emitting diodes (OLEDs) are considered a new generation of green display technology due to their unique advantages such as high theoretical efficiency, flexibility, and ultra-thinness. Data shows that in 2020, the OLED market in China alone exceeded US$35 billion.

[0003] Luminescent materials are one of the core materials of OLEDs, and their luminescent properties directly affect the performance of OLEDs. Under electric field excitation, 25% singlet excitons and 75% triplet excitons are formed in the device. The utilization efficiency of the luminescent material for excitons is directly related to the luminescent efficiency of the device. Currently, OLED luminescent materials are mainly divided into three categories:

[0004] 1. Traditional fluorescent materials: can only utilize singlet excitons, resulting in low theoretical efficiency; short excited state lifetime, but long device operating lifetime.

[0005] 2. Transition metal phosphorescent materials: By utilizing the strong spin-orbit coupling effect of transition metals, triplet excitons can be utilized, achieving 100% exciton utilization; microsecond-level excited state lifetime. Red / green phosphorescent materials with lower excited state energy can meet commercial needs, but blue phosphorescent materials have high excited state energy and short device operating lifetime.

[0006] 3. Thermally activated delayed fluorescence materials: By utilizing a special molecular structure design, the energy difference between singlet and triplet states is greatly reduced. Triplet excitons can be converted into singlet excitons by antisystem crossing, achieving 100% exciton utilization. They also have microsecond-level excited state lifetimes, resulting in short operating lifetimes for blue light devices.

[0007] Currently, the green and red light materials used in commercial OLEDs are phosphorescent iridium(III) complexes with a theoretical exciton utilization rate of 100%. However, the blue light material is a pure organic fluorescent material with an exciton utilization rate of only 25%. This is mainly because high-efficiency blue phosphorescent and thermally activated delayed fluorescence (TADF) materials have long excited-state lifetimes, leading to device instability.

[0008] To address the low theoretical exciton utilization of fluorescent materials, sensitization with TADF materials is a common solution in the literature. TADF materials can convert triplet excitons generated during electro-excitation to singlet states via anti-system crossing, and then transfer them to the fluorescent material through a resonant energy transfer process, thus enabling the fluorescent material to also possess 100% exciton utilization. However, TADF materials themselves have a long excited-state lifetime (typically greater than 1 microsecond), which is not conducive to obtaining blue light devices with long operating lifetimes. Currently, there is a lack of blue OLEDs on the market that can simultaneously achieve high efficiency and long operating lifetimes. Therefore, finding new blue light materials and OLED devices that can potentially combine high efficiency and long operating lifetimes is of great significance for comprehensively improving the performance of OLEDs. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention aims to provide an electroluminescent device, comprising a cathode, an anode, and a light-emitting layer located between the cathode and the anode, wherein the light-emitting layer comprises a Ce(III) complex and an organic fluorescent molecule, and the emission spectrum of the Ce(III) complex and the absorption spectrum of the organic fluorescent molecule overlap.

[0010] According to one embodiment of the present invention, for example, the Ce(III) complex has one or more Ce... 3+ Ions as central luminescent ions and with Ce 3+ The ions form ligands with coordinate bonds, and these Ce 3+ The first coordination layer surrounding the ion has 3-15 coordinating atoms, which include one or more of C, N, O, F, Cl, Br, and I.

[0011] Preferably, the formula weight of the ligands in the Ce(III) complex is less than 2000, more preferably less than 1000, wherein it does not contain a conjugated system or contains only a small conjugated system. The ligands include C and H atoms, and one or more of B, N, O, P, S, F, Cl, Br, and I atoms. The small conjugated system refers to: a single pentagonal or hexagram conjugated ring, or several pentagonal or hexagram conjugated rings connected to each other in a non-conjugated manner, or two rings directly connected in the same plane or ring-coupling.

[0012] According to one embodiment of the present invention, for example, the Ce(III) complex has any of the following structures:

[0013]

[0014] Each of R is independently selected from H, C1-C8 alkyl groups, halogen-substituted C1-C8 alkyl groups, or halogen atoms; the halogen is selected from at least one of fluorine, chlorine, bromine, or iodine;

[0015] Preferably, each of R is independently selected from H, a C1-C4 alkyl group or a halogen-substituted C1-C4 alkyl group, or a halogen atom; the halogen is selected from at least one of fluorine, chlorine, and bromine;

[0016] More preferably, the Ce(III) complex has any of the following structures:

[0017]

[0018] According to one embodiment of the present invention, for example, the molecular weight of the fluorescent molecule is less than 1000; preferably, the ultraviolet absorption edge wavelength of the fluorescent molecule is greater than the absorption edge wavelength of the Ce(III) complex;

[0019] Preferably, the fluorescent molecule has any of the following structures:

[0020]

[0021] According to one embodiment of the present invention, for example, the overlap area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 5% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex, preferably more than 10%, more than 15%, more than 20%, more than 30%, more than 40%, more than 50%, more than 60%, more than 70%, more than 80%, or more than 90%.

[0022] Preferably, the emission spectrum of the Ce(III) complex is in the range of 350nm-750nm, or 380nm-750nm, or 400nm-750nm, or 410nm-750nm; and the absorption spectrum of the fluorescent molecule is in the range of 200nm-650nm, or 310nm-460nm, or 450nm-490nm, or 450nm-550nm.

[0023] According to one embodiment of the present invention, for example, the mass ratio of the organic fluorescent molecule to the Ce(III) complex is 1:1-99, preferably 1:1-70, more preferably 1:1-50, more preferably 1:2-40, more preferably 1:4-35, and more preferably 1:10-30.

[0024] According to one embodiment of the present invention, for example, the light-emitting layer further includes a host material;

[0025] Preferably, the HOMO-LUMO energy level difference of the host material is greater than that of the Ce(III) complex;

[0026] Preferably, the HOMO-LUMO energy level difference of the host material is greater than the HOMO-LUMO energy level difference of the fluorescent small molecule;

[0027] Preferably, the electron mobility of the host material is >1×10⁻⁶. -8 cm 2 V -1 s -1 and / or hole mobility > 1×10 - 6 cm 2 V -1 s -1 .

[0028] According to one embodiment of the present invention, for example, the main material includes mCP (N,N-dicarbazolyl-3,5-benzene), mCBP (3,3-di(9H-carbazol-9-yl)biphenyl), CzSi (9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole), PCzAc (9,9-dimethyl-10-(9-phenyl-9H-carbazolyl-3-yl)-9,10-dihydroacridine). l-3-yl)-9,10-dihydroacridine, CBP(4,4′-bis(9-carbazolyl)-1,10-biphenyl, 4,4′-bis(9-carbazolyl)-1,10-biphenyl), TCTA(tris(4-(9-carbazolyl)phenyl)amine, 4′,4″,4″′-tris-(N-carbazolyl)-triphenylamine), TAPC(4,4′-cyclohexyldi[N,N-di(4-methylphenyl)aniline], 1,1-bis[4-[N,N'-di(ptolyl)amino]phenyl]cyclohexane), DPEPO(bis[2-((oxo)diphenylphosphino)phenyl]ether, bis-(2-(diphenylphosphino)phenyl)ether oxide), mCPCN(9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-carboxynitrile, 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazole-3-carbonitrile), BCPO(bis-4-(N-carbazolyl)phenyl)phenylphosphine oxide);

[0029] Preferably, the mass ratio of the fluorescent molecule, Ce(III) complex to the host material is 1:0.1-20:1-200, more preferably 1:0.2-10:5-100, more preferably 1:0.5-8:8-40, more preferably 1:1-5:10-30, and even more preferably 1:2:17.

[0030] According to one embodiment of the present invention, for example, the electroluminescent device further includes an electron transport layer located between the cathode and the light-emitting layer, the electron transport layer comprising TmPyPB (1,3,5-tris[(3-pyridyl)-3-phenyl]benzene, 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene), DPEPO (bis-(2-(diphenylphosphino)phenyl)ether oxide), Bphen (4,7-diphenyl-1,10-bathophenanthroline) and / or TPBi (1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene, 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene);

[0031] Preferably, the electroluminescent device further includes a hole transport layer located between the anode and the light-emitting layer; preferably, the hole transport layer includes PCzAc(9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-dihydroacridine), mCP(N,N-diazolyl-3,5-benzene), m-MTDATA(4,4',4”-trimethylamine ... [phenyl(m-tolyl)amino]triphenylamine, 4,4',4”-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine), TCTA (tris(4-(9-carbazolyl)phenyl)amine, 4′,4″,4″′-tris-(N-carbazolyl)-triphenylamine) and / or TAPC (4,4′-cyclohexyldi[N,N-di(4-methylphenyl)aniline], 1,1-bis[4-[N,N'-di(ptolyl)amino]phenyl]cyclohexane);

[0032] Preferably, the electroluminescent device further includes an electron transport layer located between the cathode and the light-emitting layer, and a hole transport layer located between the anode and the light-emitting layer;

[0033] Preferably, the hole transport layer includes mCP, and the electron transport layer includes TmPyPB.

[0034] According to one embodiment of the present invention, for example, the thickness of the light-emitting layer is 1-100nm, preferably 5-80nm, preferably 10-40nm, preferably 15-30nm, preferably 20-25nm, and most preferably 20nm. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the chemical structure of the rare earth complex and fluorescent material used in the embodiments of the present invention.

[0036] Figure 2 These are the UV-Vis absorption spectra (top left) of the fluorescent small molecules and the thin film emission spectra (top right, bottom left, bottom right) of the Ce(III) complex in the embodiments of the present invention.

[0037] Figure 3 This is a graph showing the relationship between the power efficiency, EQE, and brightness of devices T8 and R1 in this embodiment of the invention.

[0038] Figure 4 These are the emission spectra of devices T8 and R1 in this embodiment of the invention. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. However, those skilled in the art will understand that this invention is not limited to the accompanying drawings and the following embodiments.

[0040] In the process of developing OLED materials and devices, the inventors of this application recognized that improving the utilization efficiency of excitons and reducing the lifetime of excited states are important prerequisites for obtaining stable and efficient devices. Fluorescent materials have short excited-state lifetimes and high device stability, but the inability to utilize all excitons is the main reason limiting their efficiency improvement.

[0041] Most organic compounds and metal complexes have an even number of electrons. In their ground state, all electrons are paired, resulting in an electron spin multiplicity of 1, exhibiting singlet characteristics. When electro-excited in OLED devices, these molecules form 25% singlet and 75% triplet excitons. Traditional research on electroluminescent materials has focused on how to efficiently utilize triplet excitons for luminescence.

[0042] Therefore, not limited to existing theoretical understanding, the inventors of this application have creatively proposed that using rare-earth complexes with df transitions to sensitize fluorescent materials can overcome this difficulty. Unlike most organic and metal complexes, Ce(III) complexes have an odd number of electrons, thus both their ground and excited states are doublet states. Under electro-excitation, Ce(III) complexes can directly trap electrons and holes, generating 100% doublet excitons. Then, Ce(III) complexes can emit light themselves or transfer excited-state energy to the singlet state of the fluorescent material, thereby avoiding the waste of triplet excitons in the fluorescent material and improving the exciton utilization rate of the fluorescent material.

[0043] Compared to TADF materials with excited-state lifetimes in the microsecond range, Ce(III) complexes have excited-state lifetimes in the nanosecond range, which theoretically helps improve device stability.

[0044] The price of Ce raw materials of the same mass is only one-thousandth that of Ir. Therefore, for red and green light materials, the combination of df transition rare earth complexes and fluorescent materials holds promise for replacing noble metal phosphorescent iridium(III) complexes, which is beneficial for reducing device costs. For blue light materials, the addition of df transition rare earth complexes can also improve the efficiency of current blue fluorescent devices.

[0045] To address the shortcomings of existing technologies, this invention aims to provide an electroluminescent device fabricated based on a combination of rare-earth electroluminescent materials and small-molecule fluorescent materials. These materials include rare-earth Ce(III) complexes, which can simultaneously and effectively utilize exciton energy in OLEDs, with excited-state lifetimes as short as nanoseconds. This invention provides the following technical solution.

[0046] An electroluminescent device includes a cathode, an anode, and a light-emitting layer located between the cathode and the anode. The light-emitting layer includes a Ce(III) complex and an organic fluorescent molecule. To improve energy transfer efficiency, the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex are required to overlap to a certain extent. For example, after normalizing the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex (i.e., adjusting their maximum values ​​to be the same), the overlap area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 5% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex, preferably more than 10%, more than 15%, more than 20%, more than 30%, more than 40%, more than 50%, more than 60%, more than 70%, more than 80%, or more than 90%.

[0047] More specifically, for example, preferably, the emission spectrum of the Ce(III) complex is in the range of 350nm-750nm, or 380nm-750nm, or 400nm-750nm, or 410nm-750nm; and the absorption spectrum of the fluorescent molecule is in the range of 200nm-650nm, or 310nm-460nm, or 450nm-490nm, or 450nm-550nm.

[0048] from Figure 2 It can be seen that the blue fluorescent molecule TBPe has strong absorption in the range of 310nm-460nm. The blue delayed fluorescent material BCzBN has an absorption peak near 450nm-490nm. The orange-red molecule TBRb has a broad absorption peak in the range of 450nm-550nm.

[0049] For Ce(III) sensitized materials, Ce-1 exhibits the bluest emission peak, covering the range of 400 nm to 600 nm. 3-Me shows a relative redshift, with its emission peak located in the range of 425 nm to 625 nm. Ce-2 shows the reddest emission peak, covering the range of 430 nm to 650 nm.

[0050] Ce(III) complexes 1, 2-Me, 2-Br, 4-Pz, 4- i Pr, 4- n Bu possesses a similar chemical structure and the same df transition luminescence mechanism as Ce-1, Ce-2, and 3-Me. The emission spectrum of complex 1 ranges from 350 nm to 600 nm. The emission spectrum of complex 2-Me ranges from 350 nm to 560 nm, complex 2-Br ranges from 360 nm to 620 nm, and complex 4-Pz ranges from 400 nm to 600 nm. i The emission spectrum of Pr is in the range of 410 nm-600 nm, and the complex 4- n The emission spectrum of Bu ranges from 380 nm to 600 nm. The emission spectrum coverage of these complexes is also similar to that of Ce-1, Ce-2, and 3-Me. Therefore, these six complexes should have a similar ability to sensitize fluorescent molecules in electroluminescent devices as Ce-1, Ce-2, and 3-Me.

[0051] In an embodiment of the present invention, the rare earth complex Ce-2 was first used as both the host material and the sensitizing material of the device to prepare a sensitized device with the following structure:

[0052] D1:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / Ce-2:TBPe (mass ratio 0.4:0.01, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0053] D2:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / Ce-2:TBRb (mass ratio 0.4:0.01, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0054] The device initially uses MoO3 as the hole injection material and mCP as the hole transport material. Introducing MoO3-doped mCP into the injection and transport layers reduces the interfacial barrier and improves hole transport capability. TmPyPB is used as the electron transport material. LiF is the electron injection layer, and Al is used as the cathode. Electrical energy first excites the Ce(III) complex and then transfers it to the fluorescent molecules, mainly for two reasons: 1) the Ce(III) complex center's Ce... 3+ Ions readily lose electrons and possess a strong ability to trap holes, which can induce charge carriers to first appear in Ce. 3+ 2) Compared to Ce(III) complexes, the lower doping concentration of fluorescent molecules also reduces the probability of carrier recombination on fluorescent molecules.

[0055] First, binary doped devices were fabricated using Ce-2 complexes as both the host and sensitizing materials. Blue fluorescent molecule TBPe and orange-red fluorescent material TBRb were then incorporated, respectively. The devices exhibited characteristic emission colors, but their efficiency and brightness were low.

[0056] D3:ITO / MoO3 (2nm) / PCzAc (40nm) / Ce-2:TBPe (mass ratio 0.4:0.01, 2nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0057] D4:ITO / MoO3 (2nm) / PCzAc (40nm) / Ce-2:TBRb (mass ratio 0.4:0.01, 2nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0058] Changing the hole transport layer to PCzAc effectively improves the hole injection and transport capabilities of the device. Compared to D1 and D2, D3 and D4 show improvements in both device efficiency and maximum luminous intensity.

[0059] D5:ITO / MoO3 (2nm) / PCzAc (40nm) / Ce-2:BCz-BN (mass ratio 0.35:0.02, 2nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0060] D6:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / Ce-2:BCz-BN (mass ratio 0.35:0.02, 2nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0061] In addition, embodiments of the present invention also attempted to introduce the blue delayed fluorescence small molecule material BCz-BN. Results confirmed that Ce-2 can also transfer energy to the BCz-BN molecule. Due to the dual carrier transport capability of BCz-BN, the turn-on voltage of device D5 is reduced, and the efficiency and maximum brightness are significantly improved compared to the blue fluorescent small molecule TBPe device D3.

[0062] D5 and D6 compare two hole transport materials. The mCP device D6 has a higher maximum brightness, while the PCzAc device D5 has higher efficiency.

[0063] Table 1 Summary of Device Performance (D1-D6)

[0064]

[0065] a Device start-up voltage, device brightness reaches 1 cd m -2 The driving voltage at that time; b Maximum external quantum efficiency of the device; c Maximum current efficiency of the device; d Maximum brightness of the device; e The device brightness is 100 cd m. -2 Time-color coordinates.

[0066] Since Ce(III) complexes have characteristic bimodal emission, while fluorescent molecules emit a single peak and each fluorescent molecule has a specific peak shape and a specific maximum emission wavelength, it can be determined from the emission spectrum of the above devices that the emitted light is ultimately emitted by fluorescent molecules, rather than by Ce(III) complexes.

[0067] In summary, the low carrier mobility of the complex Ce-2 may be a significant factor affecting the performance of binary doped devices.

[0068] Therefore, in embodiments of the present invention, a host material is introduced into the device, and the rare earth complex is used only as a sensitizing material, thus fabricating a ternary doped device, the structure of which is shown below:

[0069] T1:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-2:TBPe (mass ratio 0.8:0.2:0.01, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0070] T2:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-2:TBRb (mass ratio 0.8:0.2:0.01, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0071] Ce-2 sensitized TBRb and TBPe. Compared with binary doped devices D1 and D2, efficiency and brightness were significantly improved.

[0072] T3:ITO / MoO3 (2nm) / PCzAc (40nm) / mCP:Ce-2:BCz-BN (mass ratio 0.6:0.35:0.05, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0073] T4:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-2:BCz-BN (mass ratio 0.6:0.35:0.05, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0074] For the blue delayed-fluorescence molecule BCz-BN, ternary doping significantly improves efficiency and brightness compared to binary doping. Optimized hole transport layer results in p-mCP performing better than PCzAc, with higher efficiency and brightness.

[0075] T5:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-2:BCz-BN (mass ratio 0.90:0.05:0.05, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0076] T6:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-2:BCz-BN (mass ratio 0.85:0.10:0.05, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0077] T7:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-2:BCz-BN (mass ratio 0.75:0.20:0.05, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0078] Optimization of Ce-2 doping concentration fixed the ratio of host material to phosphor material: increasing the concentration of the complex had little effect on device efficiency and brightness. Efficiency decreased after Ce-2 concentration increased to 20%, because: too low a Ce(III) complex concentration would result in incomplete carrier capture, while too high a concentration would affect carrier transport.

[0079] T8:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-2:BCz-BN (mass ratio 0.85:0.10:0.05, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0080] T9:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-2:BCz-BN (mass ratio 0.85:0.10:0.08, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0081] T10:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-2:BCz-BN (mass ratio 0.80:0.10:0.10, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0082] T11:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-2:BCz-BN (mass ratio 0.80:0.10:0.13, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0083] Optimization of BCz-BN doping concentration for delayed fluorescence molecules. Overall, as the concentration of fluorescent small molecules increases, the luminous efficiency and maximum brightness tend to decrease.

[0084] T12:ITO / MoO3 (2nm) / TAPC (50nm) / TCTA (5nm) / mCBP:Ce-2:BCz-BN (mass ratio 0.9:0.10:0.01, 30nm) / TmPyPB (30nm) / LiF (0.7nm) / Al

[0085] T13:ITO / MoO3 (2nm) / TAPC (50nm) / TCTA (5nm) / mCBP:Ce-2:BCz-BN (mass ratio 0.85:0.10:0.05, 30nm) / TmPyPB (30nm) / LiF (0.7nm) / Al

[0086] We tried replacing the hole transport materials TAPC and TCTA, using mCBP as the host material. The efficiency was relatively high, but the brightness was low. The efficiency decreased as the doping concentration of the fluorescent material increased.

[0087] T14:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:3-Me:BCz-BN (mass ratio 0.85:0.10:0.05, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0088] T15:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:3-Me:BCz-BN (mass ratio 0.90:0.05:0.05, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0089] The sensitizer was changed from Ce-2 to 3-Me, and mCP was used as the host material and hole transport layer. The device performance was acceptable, indicating that 3-Me can sensitize BCz-BN luminescence. However, the overall performance was not as good as the device sensitized with Ce-2.

[0090] T16:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-1:BCz-BN (mass ratio 0.85:0.10:0.05, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0091] T17:ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:Ce-1:BCz-BN (mass ratio 0.90:0.05:0.05, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al

[0092] The sensitizer was changed from Ce-2 to Ce-1, the host material was mCP, and the hole transport layer was mCP. The brightness was relatively low.

[0093] Table 2 Summary of T1-T17 Device Performance

[0094]

[0095] a Device start-up voltage, device brightness reaches 1 cd m -2 The driving voltage at that time; b Maximum external quantum efficiency of the device; c Maximum current efficiency of the device; d Maximum brightness of the device; e The device brightness is 100 cd m. -2 Time-color coordinates.

[0096] The best device is T8, which achieves a maximum external quantum efficiency of 10.3% and a maximum current efficiency of 19.1 cd A. -1 The maximum brightness is 42100 cd / m². -2 Device R1, with the same structure as T8 but without Ce-2 as a sensitizer, has a maximum EQE of 7.6% and a maximum luminance of only 1666 cd / m². -2 This result demonstrates that Ce(III) complexes can facilitate carrier recombination in the luminescent layer and effectively transfer energy to fluorescent molecules, thereby improving the luminous efficiency and maximum brightness of the device. It showcases the performance advantages and potential of df transition rare-earth complex-sensitized fluorescent materials.

[0097] Theoretically, the superior performance of T8 devices may be attributed to the following factors: The Ce(III) complex in the T8 device exhibits excellent carrier transport capabilities within the host material; the triplet energy level of the host material is higher than the frontier orbital energy level of the Ce(III) complex, making energy transfer from the doublet state of the complex to the triplet state of the host material less likely (thus avoiding energy waste). The frontier orbital energy level of the fluorescent molecule is the lowest among the three, ensuring energy transfer from the Ce(III) complex to the fluorescent molecule. Furthermore, the df transition of the Ce(III) complex is a single-electron doublet transition, which can directly convert all electrons and holes injected by the electrode into doublet excitons, further transferring them to the singlet state of the fluorescent material. This bypasses the problem of non-luminescence or low luminescence efficiency of triplet states generated under traditional material electroexcitation. This is one of the main reasons for the high efficiency of df transition-sensitized luminescence.

[0098]

[0099] For comparison, device R1 was fabricated. The device structure of R1 is: ITO / MoO3 (2nm) / mCP:MoO3 (20wt%, 30nm) / mCP (10nm) / mCP:BCz-BN (mass ratio 0.95:0.05, 20nm) / TmPyPB (40nm) / LiF (0.7nm) / Al.

[0100] In a ternary doped system (host material + rare earth complex + fluorescent molecule), the host material only serves as a carrier transport medium. The rare earth (Ce(III)) complex has a strong carrier trapping ability; after trapping a pair of electrons and holes, it is excited and then transfers energy to the fluorescent molecule. Figure 3 The power efficiency, EQE, and brightness relationship of devices T8 and R1 are shown. Figure 4 The emission spectra of devices T8 and R1 are shown. Taking devices T8 and R1 as examples, the device structure of R1 is a binary system, namely a combination of a traditional host material and fluorescent small molecules. In R1, because the carrier-trapping ability of the fluorescent small molecules is limited, some carriers recombine on the host material. At the same time, due to the incomplete energy transfer process between the host material and the fluorescent small molecules, ultraviolet light from the host material is present in its emission spectrum.

[0101] When Ce(III) complexes are introduced as sensitizing materials in the T8 device, the disappearance of the ultraviolet portion of the emitted light from the host material indicates that the Ce(III) complexes completely capture the charge carriers. Simultaneously, the emission spectrum of the T8 device shows a superposition state between the Ce(III) complexes and fluorescent small molecules, indicating energy transfer from the Ce(III) complexes to the small molecules.

[0102] Meanwhile, compared to the two devices, the EQE of T8 was improved to some extent. In particular, the maximum brightness of the device increased by an order of magnitude, indicating that the ternary doping strategy plays a very important role in improving the device's performance.

[0103] The following principles apply to the selection of the main material: 1) Good carrier transport capability (electron mobility > 1×10⁻⁶) -8 cm 2 V -1 s -1 Or hole mobility > 1×10 -6 cm 2 V -1 s -1 2) The HOMO-LUMO energy level difference is greater than that of Ce(III) complexes and fluorescent small molecules. 3) The photoluminescence quantum yield of the host material and Ce(III) complex-doped films is high. 4) The photoluminescence quantum yield of the host material and fluorescent molecule-doped films is high.

Claims

1. An electroluminescent device, comprising a cathode, an anode, and a light-emitting layer located between the cathode and the anode, characterized in that, The luminescent layer comprises a Ce(III) complex and an organic fluorescent molecule, and the emission spectrum of the Ce(III) complex and the absorption spectrum of the organic fluorescent molecule overlap; the Ce(III) complex has one or more Ce... 3+ Ions as central luminescent ions and with Ce 3+ The ions form ligands with coordinate bonds, and these Ce 3+ The first coordination layer surrounding the ion has 3-15 coordinating atoms, which include one or more of C, N, O, F, Cl, Br, and I. The formula weight of the ligands in the Ce(III) complex is less than 1000, and it contains no conjugated system or only a small conjugated system. The ligands include C and H atoms, and one or more of B, N, O, P, S, F, Cl, Br, and I atoms. The small conjugated system refers to: a single pentagonal or hexagram conjugated ring, or several pentagonal or hexagram conjugated rings connected non-conjugated to each other, or two rings directly connected in the same plane or ring-fused.

2. The electroluminescent device according to claim 1, characterized in that, The Ce(III) complex has any of the following structures: Each of R is independently selected from H, C1-C8 alkyl groups, halogen-substituted C1-C8 alkyl groups, or halogen atoms; the halogen is selected from at least one of fluorine, chlorine, bromine, or iodine.

3. The electroluminescent device according to claim 2, characterized in that, R is independently selected from H, C1-C4 alkyl or halogen-substituted C1-C4 alkyl, or halogen atom; the halogen is selected from at least one of fluorine, chlorine, and bromine.

4. The electroluminescent device according to claim 3, characterized in that, The Ce(III) complex has any one of the following nine structures: 。 5. The electroluminescent device according to claim 1, characterized in that, The molecular weight of the fluorescent molecule is less than 1000; the ultraviolet absorption edge wavelength of the fluorescent molecule is greater than the absorption edge wavelength of the Ce(III) complex.

6. The electroluminescent device according to claim 5, characterized in that, The fluorescent molecule has any of the following structures: 。 7. The electroluminescent device according to claim 1, characterized in that, The overlap area between the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 5% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex.

8. The electroluminescent device according to claim 7, characterized in that, The overlap area between the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 10% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex.

9. The electroluminescent device according to claim 7, characterized in that, The overlap area between the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 15% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex.

10. The electroluminescent device according to claim 7, characterized in that, The overlap area between the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 20% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex.

11. The electroluminescent device according to claim 7, characterized in that, The overlap area between the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 30% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex.

12. The electroluminescent device according to claim 7, characterized in that, The overlap area between the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 40% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex.

13. The electroluminescent device according to claim 7, characterized in that, The overlap area between the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 50% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex.

14. The electroluminescent device according to claim 7, characterized in that, The overlap area between the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 60% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex.

15. The electroluminescent device according to claim 7, characterized in that, The overlap area between the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 70% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex.

16. The electroluminescent device according to claim 7, characterized in that, The overlap area between the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 80% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex.

17. The electroluminescent device according to claim 7, characterized in that, The overlap area between the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex accounts for more than 90% of the total area of ​​the absorption spectrum of the fluorescent molecule and the emission spectrum of the Ce(III) complex.

18. The electroluminescent device according to claim 7, characterized in that, The emission spectrum of Ce(III) complexes ranges from 350 nm to 750 nm.

19. The electroluminescent device according to claim 7, characterized in that, The emission spectrum of Ce(III) complexes ranges from 380 nm to 750 nm.

20. The electroluminescent device according to claim 7, characterized in that, The emission spectrum of Ce(III) complexes ranges from 400 nm to 750 nm.

21. The electroluminescent device according to claim 7, characterized in that, The emission spectrum of Ce(III) complexes ranges from 410 nm to 750 nm.

22. The electroluminescent device according to claim 7, characterized in that, The absorption spectrum of the fluorescent molecule ranges from 200 nm to 650 nm.

23. The electroluminescent device according to claim 7, characterized in that, The absorption spectrum of the fluorescent molecule is in the range of 310 nm to 460 nm.

24. The electroluminescent device according to claim 7, characterized in that, The absorption spectrum of the fluorescent molecule is in the range of 450 nm to 490 nm.

25. The electroluminescent device according to claim 7, characterized in that, The absorption spectrum of the fluorescent molecule is in the range of 450 nm to 550 nm.

26. The electroluminescent device according to claim 1, characterized in that, The mass ratio of the organic fluorescent molecule to the Ce(III) complex is 1:1-99.

27. The electroluminescent device according to claim 26, characterized in that, The mass ratio of the organic fluorescent molecule to the Ce(III) complex is 1:1-70.

28. The electroluminescent device according to claim 26, characterized in that, The mass ratio of the organic fluorescent molecule to the Ce(III) complex is 1:1-50.

29. The electroluminescent device according to claim 26, characterized in that, The mass ratio of the organic fluorescent molecule to the Ce(III) complex is 1:2-40.

30. The electroluminescent device according to claim 26, characterized in that, The mass ratio of the organic fluorescent molecule to the Ce(III) complex is 1:4-35.

31. The electroluminescent device according to claim 26, characterized in that, The mass ratio of the organic fluorescent molecule to the Ce(III) complex is 1:10-30.

32. The electroluminescent device according to claim 1, characterized in that, The light-emitting layer also includes a host material.

33. The electroluminescent device according to claim 32, characterized in that, The HOMO-LUMO energy level difference of the host material is greater than that of the Ce(III) complex.

34. The electroluminescent device according to claim 32, characterized in that, The HOMO-LUMO energy level difference of the host material is greater than that of the fluorescent molecule.

35. The electroluminescent device according to claim 32, characterized in that, The electron mobility of the host material is >1×10⁻⁶. -8 cm 2 V -1 s -1 and / or hole mobility > 1×10 -6 cm 2 V -1 s -1 .

36. The electroluminescent device according to claim 32, characterized in that, The main material includes mCP (N,N-dicarbazo-3,5-benzene), N,N -dicarbazolyl-3,5-benzene), mCBP (3,3-di(9H-carbazole-9-yl)biphenyl, 3,3-di(9H-carbazole-9-yl)biphenyl, 3,3-di(9H-carbazole-9-yl)biphenyl H -carbazol-9-yl)biphenyl), CzSi (9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole, 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole), PCzAc (9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-dihydroacridine, 9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-dihydroacridine), CBP (4,4′-bis(9-carbazolyl)-1,1′-biphenyl, 4,4′-bis(9-carbazolyl)-1,1′-biphenyl), TCTA (tris(4-(9-carbazolyl)phenyl)amine, 4′,4′′,4′′′-tris-( N -carbazolyl)-triphenylamine), TAPC (4,4′-cyclohexylbis[N,N-di(4-methylphenyl)aniline], 1,1-bis[4-[ N,N' - di(ptolyl) amino]phenyl]cyclohexane), DPEPO (bis[2-((oxo)diphenylphosphino)phenyl)ether oxide), mCPCN (9-(3-(9H-carbazole-9-yl)phenyl)-9H-carbazole-3-carboxynitrile), 9-(3-(9H-carbazole-9-yl)phenyl)-9H-carbazole-3-carboxynitrile), 9-(3-(9H-carbazole-9-yl)phenyl)-9H-carbazole-3-carboxynitrile, ...)-9-(3-(9H-carbazole-9-yl)phenyl)-9H-carbazole H -carbazol-9-yl)phenyl)-9 H -carbazole-3-carbonitrile), BCPO (bis-4-(N-carbazole)phenyl)phenylphosphine oxide, bis-4-( N -carbazolyl)phenyl)phenylphosphine oxide).

37. The electroluminescent device according to claim 36, characterized in that, The mass ratio of the fluorescent molecule, Ce(III) complex to the host material is 1:0.1-20:1-200.

38. The electroluminescent device according to claim 36, characterized in that, The mass ratio of the fluorescent molecule, Ce(III) complex to the host material is 1:0.2-10:5-100.

39. The electroluminescent device according to claim 36, characterized in that, The mass ratio of the fluorescent molecule, Ce(III) complex to the host material is 1:0.5-8:8-40.

40. The electroluminescent device according to claim 36, characterized in that, The mass ratio of the fluorescent molecule, Ce(III) complex to the host material is 1:1-5:10-30.

41. The electroluminescent device according to claim 36, characterized in that, The mass ratio of the fluorescent molecule, Ce(III) complex, and host material is 1:2:

17.

42. The electroluminescent device according to claim 36, characterized in that, The electroluminescent device further includes an electron transport layer located between the cathode and the light-emitting layer, the electron transport layer comprising TmPyPB (1,3,5-tris[(3-pyridyl)-3-phenyl]benzene, 1,3,5-tri( m -pyrid-3-yl-phenyl)benzene), DPEPO (bis[2-((oxo)diphenylphosphino)phenyl)ether, bis-(2-(diphenylphosphino)phenyl)ether oxide), Bphen (4,7-diphenyl-1,10-bathophenanthroline) and / or TPBi (1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene, 1,3,5-tris( N -phenylbenzimidazol-2-yl)-benzene).

43. The electroluminescent device according to claim 42, characterized in that, The electroluminescent device further includes a hole transport layer located between the anode and the light-emitting layer.

44. The electroluminescent device according to claim 43, characterized in that, The hole transport layer comprises PCzAc(9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-dihydroacridine), mCP(N,N-diazolyl-3,5-benzene), N,N -dicarbazolyl-3,5-benzene), m-MTDATA (4,4',4''-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine), TCTA (tris(4-(9-carbazolyl)phenyl)amine, 4′,4′′,4′′′-tris-( N -carbazolyl)-triphenylamine) and / or TAPC (4,4′-cyclohexylbis[N,N-di(4-methylphenyl)aniline], 1,1-bis[4-[ N,N' -di(ptolyl)amino]phenyl]cyclohexane).

45. The electroluminescent device according to claim 44, characterized in that, The electroluminescent device further includes an electron transport layer located between the cathode and the light-emitting layer, and a hole transport layer located between the anode and the light-emitting layer.

46. ​​The electroluminescent device according to claim 45, characterized in that, The hole transport layer includes mCP, and the electron transport layer includes TmPyPB.

47. The electroluminescent device according to claim 1, characterized in that, The thickness of the light-emitting layer is 1-100 nm.

48. The electroluminescent device according to claim 47, characterized in that, The thickness of the light-emitting layer is 5-80 nm.

49. The electroluminescent device according to claim 47, characterized in that, The thickness of the light-emitting layer is 10-40 nm.

50. The electroluminescent device according to claim 47, characterized in that, The thickness of the light-emitting layer is 15-30 nm.

51. The electroluminescent device according to claim 47, characterized in that, The thickness of the light-emitting layer is 20-25 nm.

52. The electroluminescent device according to claim 47, characterized in that, The thickness of the light-emitting layer is 20 nm.

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