Organic electroluminescence device

Abstract

To provide an organic electroluminescence device capable of inducing circularly polarized light emission from existing luminescent materials without requiring a special device structure.SOLUTION: An organic electroluminescence device according to the present invention is an organic electroluminescence device including a light-emitting layer, the light-emitting layer containing a liquid crystal material. The liquid crystal material is one of a single compound having a single molecular weight and a composition obtained by mixing a plurality of such compounds. The liquid crystal material satisfies the following condition at 25°C that: (i) the material exhibits a chiral nematic liquid crystal phase; or (ii) the material has molecular orientation reflecting a chiral nematic liquid crystal phase and exhibiting a glassy state.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to organic electroluminescent devices. [Background technology]

[0002] Organic electroluminescence technology has been put to practical use in a wide range of applications, including smartphones, televisions, head-mounted displays, and lighting. In many organic electroluminescence displays, phase conversion to polarized or circularly polarized light is performed using polarizers and quarter-wave retarders to improve visibility and for optical control.

[0003] However, phase conversion using polarizers theoretically results in a 50% attenuation of light intensity, which reduces the energy efficiency of the entire device and poses a major challenge in the development of display devices.

[0004] To solve this problem, polarizer-less device structures and polarized organic electroluminescent devices have been proposed. The former are cost-inefficient because they require the benefits of polarized light, such as improved viewing angle and anti-reflection, to be implemented in separate components. The latter has been actively explored for polymer organic electroluminescent materials, but it is difficult to impart polarization properties to materials that utilize the energy-efficient triplet excited state, and they have not yet been put to practical use.

[0005] Furthermore, circularly polarized light emission using polymer organic electroluminescent light-emitting materials has not yet been achieved in a manner that combines circularly polarized light emission characteristics with electrical properties and external quantum yield. Therefore, the development of light-emitting materials that are endowed with polarized and circularly polarized light emission characteristics is expected to contribute to the further development of organic electroluminescence technology.

[0006] Utilizing the helical structure of chiral nematic liquid crystals to impart circular polarization properties to light-emitting materials is a promising technology due to its high circular polarization purity. Chiral nematic liquid crystals are characterized by the presence of optically active molecules in nematic liquid crystals with an orderly orientation, which causes a twist between adjacent molecules, resulting in a helical periodic structure perpendicular to the molecular axis. This periodic structure exhibits long-range correlation, and the period can persist over several centimeters. The pitch of the periodic structure (i.e., the helical pitch p) ranges from several hundred nanometers to infinity.

[0007] One of the distinctive properties of chiral nematic liquid crystals is selective reflection due to the Bragg reflection of the helical periodic structure. The wavelength λ of selectively reflected light is defined as λ=1 / np, where p is the helical pitch and n is the refractive index of the liquid crystal molecules. In addition, only circularly polarized light in one direction, corresponding to the helical structure of the chiral nematic liquid crystal, is selectively reflected, while the other circularly polarized light is transmitted.

[0008] In order to use circularly polarized light-emitting materials as the light-emitting layer of organic electroluminescence devices, they must be made into thin films to keep the driving voltage low, and the film thickness must be kept on the order of nanometers. It is generally known that most circularly polarized light-emitting materials using chiral nematic liquid crystals lose their circular polarization properties when the film thickness is on the order of nanometers or less.

[0009] As an example of a device that exhibits circular polarization properties on the order of nanometers or less, Non-Patent Document 1 describes a liquid crystalline conjugated polymer, F8BT, which is combined with helicene as a chiral inducer to induce a special chiral structure, resulting in circularly polarized light emission in a thin film. This chiral structure is only generated in a limited combination, and a more versatile method of circularly polarized light emission is desired. Meanwhile, Non-Patent Document 2 describes a device in which a chiral nematic liquid crystal is partially oriented, thereby inducing an ideal phase change and achieving circularly polarized light emission. However, this method requires the formation of an alignment film on the device, which deteriorates the electrical characteristics of the organic electroluminescence device when it is operated, so a method that does not use an alignment film is desired.

[0010] Generally, there are no circularly polarized luminescent materials that can produce pure circularly polarized light; only those that produce elliptically polarized light, a mixture of right- and left-handed circularly polarized light, have been reported. The g value, which indicates the degree of polarization of left-handed circularly polarized light, is widely used as an index of the purity of circular polarization. The g value is expressed as g = 2(IL - IR) / (IL + IR), where IL is the left-handed circularly polarized light intensity and IR is the right-handed circularly polarized light intensity. The g value ranges from -2 to 2, with g = 0 indicating non-circularly polarized light and |g| = 2 indicating pure circular polarization. [Prior art documents] [Non-patent literature]

[0011] [Non-Patent Document 1] Nature Communications, 2020, 11, 6137 [Non-patent document 2] Adv. Optical. Mater., 2021, 2101674 Summary of the Invention [Problem to be solved by the invention]

[0012] An object of the present invention is to provide an organic electroluminescence device that induces circularly polarized light emission from existing light-emitting materials without requiring a special device structure. [Means for solving the problem]

[0013] Specific means for solving the above problems include the following embodiments. <1> An organic electroluminescent device comprising a light-emitting layer, the light-emitting layer comprises a liquid crystal material; the liquid crystal material is a compound of one kind having a single molecular weight or a composition in which a plurality of kinds of such compounds are mixed, The liquid crystal material has the following (i) or (ii) at 25°C: (i) exhibit a chiral nematic liquid crystal phase; (ii) It has molecular orientation reflecting the chiral nematic liquid crystal phase and exhibits a glassy state; An organic electroluminescent device that satisfies the above conditions.

[0014] <2> The thickness of the light-emitting layer is 10 nm or more and 500 nm or less. <1> 10. The organic electroluminescent device according to claim 19.

[0015] <3> The liquid crystal material contains a compound or composition exhibiting nematic liquid crystal properties and a chiral compound. <1> or <2> 10. The organic electroluminescent device according to claim 19.

[0016] <4> the light-emitting layer is composed only of a compound having a molecular weight of 200 or more and 5000 or less; <1> ~ <3> 10. The organic electroluminescent device according to any one of claims 1 to 9.

[0017] <5> the light-emitting layer further comprises a light-emitting material; the maximum emission wavelength of the light-emitting material is longer than the maximum emission wavelength of the liquid crystal material; <1> ~ <4> 10. The organic electroluminescent device according to any one of claims 1 to 9.

[0018] <6> The light-emitting material is a phosphorescent light-emitting material or a light-emitting material having TADF (thermally activated delayed fluorescence) properties; <5> 10. The organic electroluminescent device according to claim 19.

[0019] <7> No alignment film is formed on the surface adjacent to the light-emitting layer. <1> ~ <6> 10. The organic electroluminescent device according to any one of claims 1 to 9.

[0020] <8> The absolute value of the circular polarization asymmetry factor g is 0.1 or more. <1> ~ <7> 10. The organic electroluminescent device according to any one of claims 1 to 9. [Effects of the Invention]

[0021] According to the organic electroluminescent device of the present invention, it is possible to reduce the loss of light when light passes through a circular polarizing plate. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a diagram illustrating an example of the configuration of an organic electroluminescence element. DETAILED DESCRIPTION OF THE INVENTION

[0023] The organic electroluminescent device according to this embodiment includes a light-emitting layer, as shown in Fig. 1. The light-emitting layer contains a liquid crystal material, which is a compound having a single molecular weight or a mixture of multiple compounds, and exhibits the following (i) or (ii) at 25°C: (i) exhibit a chiral nematic liquid crystal phase; (ii) It has molecular orientation reflecting the chiral nematic liquid crystal phase and exhibits a glassy state; A liquid crystal material that satisfies the above condition (ii) is one that exhibits a chiral nematic liquid crystal phase at high temperatures and exhibits a glassy state while maintaining the orientation state when cooled. By exhibiting the glassy state, the orientation state of the chiral nematic liquid crystal is less likely to change due to thermal or electrical stimuli, and it is also easier to form a thin film with a uniform thickness.

[0024] (liquid crystal materials) The liquid crystal molecules used as the liquid crystal material (i.e., molecules exhibiting a nematic or chiral nematic liquid crystal phase) are not limited in chemical structure, but are preferably a single compound having a single molecular weight or a mixture of multiple compounds. A compound or mixture having a single molecular weight is preferable to a polymer compound having a molecular weight distribution, because it facilitates the formation of a long-range ordered structure of chiral nematic liquid crystal in a thin film. Furthermore, the molecular weight of the liquid crystal molecules is preferably 200 to 5000, more preferably 500 to 2000. A molecular weight of 5000 or less can suppress a decrease in fluidity in a thin film, facilitate the formation of a long-range ordered structure of chiral nematic liquid crystal, and prevent the loss of circularly polarized luminescence. A molecular weight of 200 or more can prevent aggregation during film formation and subsequent processes, ensuring uniformity in film thickness.

[0025] The liquid crystal material preferably contains a compound having the structure of the following formula (1).

[0026] [ka] [In the formula, R 1 Ha-(CH2) n R represents a group represented by CH3, and n represents an integer of 0 or more. 2 represents a group represented by the following formula (4) or (5).

[0027] [ka] [In the formula, m1 represents an integer of 2 or more and 7 or less, m2 represents an integer of 0 or more and 4 or less, and m3 represents an integer of 0 or more and 2 or less.]

[0028] The compound having the structure of the above formula (1) has a tricarbazole structure bonded at the 2- and 7-positions, an ether structure at the terminal, and further has R 1 and R 2The compound has an alkyl structure at the position of 1.5. Due to the tricarbazole structure and the alkyl structure of an appropriate chain length, the compound tends to exhibit a nematic liquid crystal phase both alone and in a mixture, and tends to exhibit a glassy state at room temperature (25°C). The compound also exhibits fluorescent or organic electroluminescent properties.

[0029] Examples of compounds having the structure of the above formula (1) include compounds represented by the following formulas (1-1) to (1-8).

[0030] [ka]

[0031] [ka]

[0032] [ka]

[0033] [ka]

[0034] [ka]

[0035] [ka]

[0036] [ka]

[0037] [ka]

[0038] R in the above formula (1) 1 The alkyl chain length in the formula (1) significantly affects the phase transition temperature. When n is 5 or more and 9 or less, the compound alone having the structure of formula (1) exhibits a stable nematic liquid crystal phase and a glassy state at room temperature. By setting n to 5 or more, the crystallinity is prevented from becoming too high, and it becomes possible to exhibit a glassy state at room temperature. By setting n to 9 or less, it becomes possible to prevent the glass transition temperature from becoming too low, and it becomes possible to exhibit a glassy state at room temperature.

[0039] R in the above formula (1) 2 When m1 represents a group represented by the above formula (4), it is preferable that m1 is 2 or more and 7 or less, since the glassy state tends to exist stably. By making m1 2 or more, it is possible to increase the number of factors that inhibit the intermolecular interactions of the tricarbazole moiety, thereby preventing the promotion of crystallization and suppressing the instability of the glassy state. By making m1 7 or less, it is possible to prevent the orientation of the tricarbazole moiety in the main chain direction from increasing, and suppress the instability of the glassy state that accompanies crystallization.

[0040] R in the above formula (1) 2 When R represents a group represented by the above formula (5), if m2 is 0 or more and 4 or less and m3 is 0 or more and 2 or less, the crystallinity of the tricarbazole moiety is inhibited, and the crystallinity and glass transition temperature are reduced, so that a stable nematic liquid crystal phase and a glass state at room temperature can be exhibited. 2 may have chirality.

[0041] Among the compounds having the structure of the above formula (1), the compounds represented by the above formula (1-3), (1-7), or (1-8) are preferred because they exhibit a stable nematic liquid crystal phase and a glassy state at room temperature when used alone.

[0042] When two or more compounds having the structure of the above formula (1) are mixed, the mixing ratio can be set arbitrarily as long as the resulting mixture exhibits a nematic liquid crystal phase and a glassy state at room temperature. 1 and R 2 is not particularly limited.

[0043] When two or more compounds having the structure of formula (1) are mixed, it is preferable that the mixture contains a compound that exhibits a stable nematic liquid crystal phase and a glassy state at room temperature by itself. For example, it is preferable that the mixture contains a compound represented by formula (1-3), (1-7), or (1-8). The proportion of the compound represented by formula (1-3), (1-7), or (1-8) in the mixture is preferably 10 mol% or more, more preferably 25 mol% or more, even more preferably 35 mol% or more, and particularly preferably 45 mol% or more.

[0044] In addition, R in the above formula (1) 2 is a group represented by the above formula (4), and R 2 A mixture obtained by mixing a compound in which R in the above formula (1) is a group represented by the above formula (5) in a molar ratio that shows an appropriate phase transition temperature is preferred because it exhibits a stable nematic liquid crystal phase and a glassy state at room temperature. 2 is a group represented by the above formula (4), and R 2 The molar ratio of the compound represented by formula (5) to the compound represented by formula (5) is, for example, preferably 10:90 to 90:10, more preferably 25:75 to 75:25, further preferably 35:65 to 65:35, and particularly preferably 45:55 to 55:45.

[0045] In this embodiment, it is preferable that the liquid crystal molecules used as the host material of the light-emitting layer have good charge transport properties. Examples of such liquid crystal molecules include compounds represented by the above formula (1-1), (1-3), or (1-7) having a tricarbazole structure. Also included are compounds represented by the following formula (1-9) or (1-10), which are partial structures of conjugated polymers that have been used in the light-emitting layer of organic electroluminescent devices. These liquid crystal molecules have appropriate molecular weights and phase transition temperatures, making them suitable as host materials in the light-emitting layer of organic electroluminescent devices.

[0046] [ka]

[0047] [ka]

[0048] (Chiral compounds) In this embodiment, the liquid crystal material may be either a chiral nematic liquid crystal obtained by adding a chiral compound to a compound or mixture exhibiting a nematic liquid crystal phase, or a chiral nematic liquid crystal exhibited by liquid crystal molecules having a chiral moiety. The former is preferred because it does not require the incorporation of expensive chiral moieties into the liquid crystal molecules and the chiral structure can be easily controlled by the amount of chiral compound added.

[0049] The chiral compound may or may not exhibit a liquid crystal phase. The amount of chiral compound added may be determined arbitrarily as long as it exhibits a chiral nematic liquid crystal phase and has an appropriate chiral structure. Furthermore, these nematic liquid crystals and chiral nematic liquid crystals may be single compositions using one type of liquid crystal molecule, or mixtures using multiple liquid crystal molecules. A mixture is preferred because it tends to inhibit crystallization. Examples of chiral compounds to be added include compounds represented by the following formula (2-1) or (2-2) having a binaphthyl structure; compounds represented by the following formula (2-3) having an asymmetric center structure; and the like.

[0050] [ka]

[0051] [ka]

[0052] [ka]

[0053] The chiral nematic liquid crystal in this embodiment preferably has a helical pitch of 0.1 μm or more and 20 μm or less. A helical pitch of 20 μm or less allows for a stable helical structure and prevents the inclusion of nematic liquid crystal portions. This prevents a decrease in the absolute value of the g value. To obtain a higher g value, the helical pitch is more preferably 10 μm or less, and even more preferably 5 μm or less. Furthermore, a helical pitch of 0.1 μm or more makes the liquid crystal molecules more susceptible to phase changes due to birefringence, allowing for the development of circular polarization. To obtain purer circularly polarized light, the absolute value of the g value is preferably closer to the upper limit, |g| = 2. When |g| ≥ 0.1, the intensity of left-handed circularly polarized light and the intensity of right-handed circularly polarized light differ by 10% or more. This is preferable because it makes it easier to detect the difference in intensity when the resulting light passes through left-handed and right-handed circular polarizers.

[0054] (luminescent material) In this embodiment, the liquid crystal material primarily functions as a host material in the emissive layer of the organic electroluminescent device, and the emissive layer contains an emissive material. In this case, the liquid crystal molecules constituting the liquid crystal material may function as the emissive material, or a separately added compound may function as the emissive material. The structure of the latter emissive material (separately added emissive material) is not particularly limited. However, it is preferable that the emissive material have a longer emission wavelength than the emissive wavelength of the liquid crystal material, since this facilitates charge or energy transfer. Furthermore, the latter emissive material is preferably an emissive material having phosphorescence or TADF properties. Examples of the emissive material to be added include a compound represented by the following formula (3-1) having fluorescence properties; a compound represented by the following formula (3-2) having TADF properties; and the like. TADF properties refer to the property of being thermally excited from the lowest triplet excited state (T1) to the lowest singlet excited state (S1) at room temperature. Having phosphorescence or TADF properties, when used in the emissive layer of an organic electroluminescent device, the emissive material can achieve highly efficient light emission utilizing the triplet excited state.

[0055] [ka]

[0056] [ka]

[0057] The light-emitting layer containing the liquid crystal material in this embodiment is formed into a thin film for application to an organic electroluminescence device. The method for forming the thin film is not limited, and spin coating, slit coating, vacuum deposition, inkjet technology, etc. can be used. When forming the thin film, a solution in which the liquid crystal composition is dissolved in any organic solvent may be used. The organic solvent is not particularly limited as long as it can dissolve the liquid crystal composition. For example, chloroform, tetrahydrofuran, dimethylformamide, 1,4-dioxane, diethylene glycol monomethyl ether, diethylene glycol dimethyl ether, N-methylpyrrolidone, etc. can be used. However, after applying this solution, a drying process is performed by heating or degassing to remove the organic solvent, and the liquid crystal material must exhibit a chiral nematic liquid crystal phase or a glassy state that exhibits an alignment state reflecting the chiral nematic liquid crystal phase.

[0058] The thickness of the coating film (thin film after drying) is preferably 10 nm or more and 500 nm or less. The thickness is more preferably 200 nm or less, and even more preferably 100 nm or less. If the thickness is 500 nm or less, it is not possible to form a long-range order of chiral nematic liquid crystals exceeding the visible light wavelength in the film thickness direction, and therefore the selective reflection characteristics due to Bragg reflection tend not to be exhibited. On the other hand, if the thickness is 500 nm or less, the driving voltage of the organic electroluminescent device can be suppressed from increasing, making it applicable to the device. Furthermore, if the thickness is 10 nm or more, it is possible to avoid the influence of the disorder of the liquid crystal alignment occurring near the interface, and to exhibit a stable chiral nematic phase.

[0059] In the organic electroluminescent device according to this embodiment, it is preferable not to form an alignment film adjacent to the light-emitting layer in order to maintain a low driving voltage of the organic electroluminescent device.

[0060] In this embodiment, an annealing step is preferably performed after thin film formation to facilitate the formation of long-range order in the chiral nematic liquid crystal. This annealing step can significantly improve the g value. Annealing at a temperature equal to or higher than the isotropic phase transition temperature is particularly preferable.

[0061] The present invention is not limited to the present embodiment and examples, and various modifications are possible without departing from the spirit of the present invention. [Example]

[0062] The chemical structures of the liquid crystal compounds, chiral compounds, and light-emitting materials exemplified in the Examples and Comparative Examples are shown below.

[0063] [Liquid crystal compound] [ka]

[0064] [ka]

[0065] [ka]

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[0070] [ka]

[0071] [Chiral compounds] [ka]

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[0074] [Luminescent materials] [ka]

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[0076] The glass transition temperature Tg and nematic-isotropic phase transition temperature TN-I shown in each example and comparative example were determined by observation using a polarizing microscope (AXIO IMAGER M1M, manufactured by Carl Zeiss) equipped with a temperature-variable HP and measurement using a differential scanning calorimeter (DSC-8000, manufactured by Kitahama Seisakusho Co., Ltd.). The molecular weight and molecular weight distribution (PDI) were determined from the polystyrene-equivalent molecular weight measured by gel permeation chromatography (GPC). The specific measurement method for GPC is as follows: (Various GPC conditions) Manufacturer: JASCO Corporation Column: Agilent PLgel MIXED-D (PL1110-6504) Flow rate: 1.0mL / min Column temperature: 40℃ Solvent used: THF (tetrahydrofuran) Detector: RI (differential refractive index detector)

[0077] <Examples 1 to 8, Comparative Example 2> The liquid crystal compound, chiral compound, and luminescent material were mixed at the molar ratio shown in Table 1 and dissolved in chloroform to a concentration of 13 mg / mL to prepare a homogeneous solution. The resulting solution was spin-coated onto an ITO-evaporated quartz glass substrate using a spin coater (Opticoat MS-A150, manufactured by Mikasa Co., Ltd.) at 1000 rpm for 60 seconds. The substrate was then annealed on a hot plate at 130°C for 30 minutes. After cooling to room temperature, the circularly polarized fluorescence emission characteristics were evaluated using a circularly polarized luminescence measurement system (CPL-2000, manufactured by JASCO Corporation), and the g value was calculated.

[0078] Furthermore, an evaluation cell with the configuration shown in Figure 1 was fabricated in a glove box, and organic electroluminescence (EL) emission was evaluated according to the following evaluation criteria. In the organic electroluminescence device 10 shown in Figure 1, the PEDOT / PSS layer 2 as the hole injection transport layer, the BPOPB layer 4 as the charge transport layer, and the light-emitting layer 3 were formed by spin coating, while the ITO layer 1 as the anode, the CsF layer 5 as the charge injection layer, and the Al layer 6 as the cathode were formed by vapor deposition. The thicknesses of the layers were as follows: ITO layer: 150 nm, PEDOT / PSS layer: 40 nm, light-emitting layer: 60 nm, BPOPB layer: 40 nm, CsF layer: 0.5 nm, and Al layer: 250 nm. -Evaluation criteria- ◯: EL light emission is stable. ×: No EL luminescence.

[0079] <Comparative Examples 1 and 3> The g value of the light-emitting layer was calculated in the same manner as above, except that the annealing temperature was set to 180° C., and the possibility of organic electroluminescence (EL emission) was evaluated.

[0080] [Table 1]

[0081] Examples 1 to 4, 7, and 8 satisfy the requirements of the present invention, and the liquid crystal compound itself functions as a light-emitting material. It was confirmed that the obtained devices are circularly polarized organic electroluminescent light-emitting materials that have both a high g value and organic electroluminescent light-emitting properties.

[0082] Examples 5 and 6 satisfy the requirements of the present invention, and are obtained by mixing a light-emitting material having a maximum emission wavelength longer than that of the liquid crystal compound into the composition. The obtained device was confirmed to be a circularly polarized organic electroluminescent light-emitting material that combines a high g value and organic electroluminescence light-emitting properties. Furthermore, the light emitted from the device exhibited an emission wavelength derived from the added light-emitting material.

[0083] In Comparative Examples 1 to 3, polymer compounds were used as the liquid crystal compounds. In all cases, the absolute value of the g value was 0.01 or less, and the purity of the circularly polarized light was insufficient. It is clear that when a polymer compound is used as the liquid crystal compound, the requirements of the present invention cannot be met. [Explanation of symbols]

[0084] 1 ITO layer 2 PEDOT / PSS layer 3. Light-emitting layer 4 BPOPB layer 5 CsF layer 6 Al layer 10 Organic electroluminescence element

Claims

1. An organic electroluminescent device comprising a light-emitting layer, the light-emitting layer comprises a liquid crystal material; the liquid crystal material is a compound of one kind having a single molecular weight or a composition in which a plurality of kinds of such compounds are mixed, The liquid crystal material has the following (i) or (ii) at 25° C.: (i) exhibits a chiral nematic liquid crystal phase; (ii) It has molecular orientation reflecting a chiral nematic liquid crystal phase and exhibits a glassy state; An organic electroluminescent device that satisfies the above conditions.

2. 2. The organic electroluminescent device according to claim 1, wherein the thickness of the light-emitting layer is 10 nm or more and 500 nm or less.

3. 3. The organic electroluminescent device according to claim 1, wherein the liquid crystal material comprises a compound or composition exhibiting nematic liquid crystal properties and a chiral compound.

4. 3. The organic electroluminescent device according to claim 1, wherein the light-emitting layer is composed solely of a compound having a molecular weight of 200 or more and 5,000 or less.

5. the light-emitting layer further comprises a light-emitting material; 3. The organic electroluminescent device according to claim 1, wherein the maximum emission wavelength of the light-emitting material is longer than the maximum emission wavelength of the liquid crystal material.

6. 6. The organic electroluminescent device according to claim 5, wherein the light-emitting material is a phosphorescent light-emitting material or a light-emitting material having TADF (thermally activated delayed fluorescence) properties.

7. 3. The organic electroluminescent device according to claim 1, wherein no alignment film is formed on a surface adjacent to the light-emitting layer.

8. 3. The organic electroluminescent device according to claim 1, wherein the absolute value of the circularly polarized light asymmetry factor g is 0.1 or more.

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