Ink composition for light-emitting layer and electroluminescent device using the same

Abstract

The present invention provides an ink composition for an electroluminescent light-emitting layer applicable to an inkjet printing method, and a light-emitting device having a light-emitting layer formed from the composition. In the present invention, a small amount of a dispersant is mixed into a quantum dot solution to form an ink, which enables the formation of a light-emitting layer by an inkjet printing method, and the addition of a dispersant makes it possible to improve the coffee ring phenomenon.

Description

[Technical Field]

[0001] The present invention relates to an ink composition for a light-emitting layer of an electroluminescent device that can be applied to an inkjet printing method, and a light-emitting device that includes a light-emitting layer formed from the composition. [Background technology]

[0002] To use quantum dot materials in electronic devices, a technology is required to form quantum dot patterns inside the electronic device. The most common method for forming quantum dot patterns is to directly pattern quantum dots on a substrate using vacuum deposition. However, unlike light-emitting organic molecules, quantum dot materials are heavy, making them difficult to vacuum-deposit. They are also vulnerable to heat and moisture, meaning that they cannot be deposited using the deposition method used for self-emitting OLEDs.

[0003] To solve the above problems, methods have been proposed, such as transferring quantum dot material onto a substrate like pressing a stamp, or printing using a principle similar to that of an inkjet printer. However, these transfer methods have limitations on how large the stamp can be, making them difficult to apply to large-area substrates. For this reason, active research is being conducted on quantum dot light-emitting devices (QLEDs) using inkjet printing methods, which involve turning quantum dot materials into ink.

[0004] In recent years, much research has been done on quantum dot solutions using inkjet printing methods, but there has been relatively little research on the emission layer (EML) for inkjet printing. Therefore, there is currently a need for research on the composition of the emission layer (EML) that can be inkjet printed, especially in order to form a uniform emission layer. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been devised to solve the above-mentioned problems, and an object of the present invention is to provide an ink composition for a light-emitting layer, which enables the formation of a light-emitting layer by inkjet printing by adding a small amount of dispersant to a quantum dot solution containing quantum dots and a solvent, and which enables the formation of a uniform film by improving the coffee-ring phenomenon caused by the addition of a dispersant. Another object of the present invention is to provide a light-emitting device having a light-emitting layer formed by an ink-jet printing method using the ink composition as described above. Other objects and advantages of the present invention will be more clearly explained in the detailed description of the invention and claims that follow. [Means for solving the problem]

[0006] In order to solve the above-mentioned technical problems, the present invention provides an ink composition for an emissive layer, which comprises quantum dots, an acrylic dispersant, and a solvent, and the (meth)acrylic dispersant is contained in an amount of 30% by volume or less relative to 100% by volume of the solvent.

[0007] In one embodiment of the present invention, the solvent may have a vapor pressure of 0.001 mmHg or greater. In one embodiment of the present invention, the solvent may include at least two or more solvents having different vapor pressures.

[0008] In one embodiment of the present invention, the acrylic dispersant may be a di(meth)acrylic compound. In one embodiment of the present invention, the quantum dots are contained in an amount ranging from 1 to 30% by weight relative to the total weight of the composition.

[0009] In one embodiment of the present invention, the quantum dots may include at least one of red-emitting quantum dots, green-emitting quantum dots, and blue-emitting quantum dots.

[0010] In one embodiment of the present invention, the composition may have a viscosity at 20°C of 1.0 to 5.0 cps, a vapor pressure at 20°C of 0.1 to 10 mmHg, a contact angle of 10 to 30°, and a solid content of 30 wt% or less.

[0011] In one embodiment of the present invention, the printed pattern formed after jetting can contain up to 10% by volume of solvent and dispersant due to removal of volatile components.

[0012] In one embodiment of the present invention, the height (H1) of the printed pattern after jetting is 500 to 2,000 nm, and the height (H F ) can be 5 to 60 nm.

[0013] The present invention also provides a light-emitting element including: a first electrode; a second electrode disposed opposite the first electrode; a light-emitting layer disposed between the first electrode and the second electrode and formed from the ink composition for a light-emitting layer described above; a hole transport layer disposed between the first electrode and the light-emitting layer; and an electron transport layer disposed between the light-emitting layer and the second electrode.

[0014] In one embodiment of the present invention, the light-emitting layer may be formed by an inkjet printing method. In one embodiment of the present invention, the light emitting device may further include at least one of a hole injection layer and an electron injection layer. [Effects of the Invention]

[0015] According to one embodiment of the present invention, by mixing a small amount of dispersant into a quantum dot solution containing quantum dots and a solvent to form an ink, it is possible to provide an ink composition for a light-emitting layer of an electroluminescent device, which can easily be ejected uniformly by an inkjet method and can form a uniform film of a light-emitting layer using the ejected ink.

[0016] As a result, the ink composition for a light-emitting layer of the present invention is not only useful for producing a light-emitting element, specifically a self-luminous display, by an inkjet printing process, but also has advantageous effects for commercialization and large-scale production by utilizing a simple and inexpensive inkjet process. The effects of the present invention are not limited to the above-mentioned contents, and more various effects are included in the present specification. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 2 is an image diagram of the ink composition for a light-emitting layer produced in Example 1. [Figure 2] FIG. 2 is an image diagram of ejected ink using the ink composition for a light-emitting layer produced in Example 1. [Figure 3] 1 is an image diagram of ejected ink using the ink composition for a light-emitting layer produced in Comparative Example 1. FIG. [Figure 4] 1 is an image diagram of ejected ink using the ink composition for a light-emitting layer produced in Comparative Example 2. FIG. [Figure 5] 1 is an image showing the evaluation of the pattern thickness immediately after jetting the ink composition for a red light-emitting layer prepared in Example 1. FIG. [Figure 6] 1 is an image diagram illustrating an analysis of the patterns for R, G, and B immediately after jetting the ink composition for a light-emitting layer in Example 1. FIG. [Figure 7] 10 is an image diagram illustrating an analysis of the patterns for R, G, and B immediately after jetting the ink composition for a light-emitting layer in Comparative Example 2. FIG. [Figure 8] 1 is a diagram showing a cross-sectional structure of an electroluminescent element. [Figure 9] 1 is a graph showing current density characteristics of red light emitting devices fabricated using ink compositions for light emitting layers of Example 1 and Comparative Example 2. [Figure 10] 1 is a graph showing the luminous efficiency characteristics of red light emitting devices fabricated using the ink compositions for a light emitting layer of Example 1 and Comparative Example 2. [Figure 11]1 is a graph showing quantum efficiency characteristics of red light emitting devices fabricated using ink compositions for light emitting layers of Example 1 and Comparative Example 2. [Figure 12] 1 is a graph showing current density characteristics of green light emitting devices fabricated using ink compositions for light emitting layers of Example 1 and Comparative Example 2. [Figure 13] 1 is a graph showing the luminous efficiency characteristics of green light emitting devices fabricated using the ink compositions for a light emitting layer of Example 1 and Comparative Example 2. [Figure 14] 1 is a graph showing quantum efficiency characteristics of green light-emitting devices fabricated using ink compositions for light-emitting layers of Example 1 and Comparative Example 2. [Figure 15] 1 is a graph showing current density characteristics of blue light-emitting devices fabricated using ink compositions for light-emitting layers of Example 1 and Comparative Example 2. [Figure 16] 1 is a graph showing the luminous efficiency characteristics of blue light emitting devices fabricated using the ink compositions for luminescent layers of Example 1 and Comparative Example 2. [Figure 17] 1 is a graph showing quantum efficiency characteristics of blue light-emitting devices fabricated using ink compositions for light-emitting layers of Example 1 and Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described in detail below. Unless otherwise specified, all terms (including technical and scientific terms) used in this specification have the meanings that are commonly understood by those having ordinary skill in the art to which the present invention belongs. Furthermore, terms defined in commonly used dictionaries are not interpreted ideally or excessively unless otherwise defined. Furthermore, throughout this specification, when a part "comprises" a certain component, unless otherwise specified, it should be understood as an open-ended term that does not exclude other components but includes the possibility of further including other components. Furthermore, throughout this specification, "on" or "above" means not only being located above or below the target part, but also including cases where another part exists between them, and does not necessarily mean being located above with respect to the direction of gravity.

[0019] In addition, in this specification, "(meth)acrylate" means acrylate and methacrylate, "(meth)acrylic" means acrylic and methacrylic, and "(meth)acryloyl" means acryloyl and methacryloyl.

[0020] In this specification, the terms "monomer" and "monomer" have the same meaning. In the present invention, a monomer is distinguished from an oligomer and a polymer and refers to a compound having a weight-average molecular weight of 1,000 or less.

[0021] The present invention provides an ink composition for a light-emitting layer of an electroluminescent device that can be ejected by inkjet printing and form a uniform film, and that can achieve the device's characteristics. Therefore, in the present invention, by adding and mixing a small amount of dispersant into a solvent in which red, green, and / or blue quantum dots are dispersed, the coffee ring effect (CRF) during inkjet ejection can be significantly improved and film uniformity can be ensured. That is, with conventional ink compositions, the cause of the coffee ring phenomenon, namely, the droplet edges do not move when the solvent evaporates. In contrast, with the ink composition of the present invention to which a specific dispersant has been added, the droplet edges gradually and regularly shrink in diameter toward the center as evaporation progresses, thereby improving the coffee ring phenomenon.

[0022] In addition, the present invention allows for the formation of a light-emitting layer by inkjet printing by selecting a solvent that can be ejected in consideration of the appropriate viscosity and vapor pressure in an inkjet device and using the selected solvent as the solvent for the ink composition. Furthermore, since the solvent is easily volatilized and removed under typical device manufacturing conditions, a high-quality light-emitting layer composed of quantum dots can be ensured.

[0023] As a result, the present invention can provide a light-emitting layer produced by an inkjet printing process and a self-emissive display including the same, specifically a quantum dot QLED.

[0024] <Ink composition for light-emitting layer> The ink composition for an emissive layer according to one embodiment of the present invention is a quantum dot ink composition that can be ejected by conventional inkjet printing to form a uniform film of an emissive layer (EML) for an electric field device. This ink composition is distinguished from conventional ink compositions in that it does not contain resins, inorganic particles, scattering agents, and / or additional dispersants, and the final emissive layer is composed of quantum dots.

[0025] In one embodiment, the composition contains quantum dots, an acrylic dispersant, and a solvent, and the dispersant is contained in a predetermined amount. If necessary, the composition may further contain at least one additive commonly used in the art.

[0026] The composition of the ink composition for the light-emitting layer will be described in detail below.

[0027] quantum dots The ink composition for a light-emitting layer according to the present invention can use ordinary quantum dots well known in the art without any restrictions.

[0028] Quantum dots (QD) can also refer to nano-sized semiconductor materials. Atoms form molecules, and molecules form clusters, which are molecular aggregates of a small number of molecules, to form nanoparticles. When such nanoparticles exhibit semiconductor properties, they are called quantum dots. When quantum dots receive external energy and become floating, they release energy according to the corresponding energy band gap.

[0029] Such quantum dots may have a homogeneous single-layer structure, a multi-layer structure such as a core-shell type or a gradient structure, or a mixture of these. When the shell has multiple layers, each layer may contain a different component, for example, a metal oxide (sub)layer.

[0030] Quantum dots (QDs) can be freely selected from II-VI compounds, III-V compounds, IV-VI compounds, Group IV elements, Group IV compounds, and combinations thereof. When the quantum dots are core-shell type, the core and at least one shell component are selected from II-VI compounds, III-V compounds, IV-VI compounds, Group IV elements, Group IV compounds, and combinations thereof, respectively, as described below. More specifically, they can be freely composed of the components exemplified below.

[0031] In one example, the II-VI compound may be a binary compound selected from the group consisting of CdO, CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, MgSe, MgS, and mixtures thereof, CdSeS, CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe, CdZnTe, CdHgS, CdH and ternary compounds selected from the group consisting of CdZnSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, MgZnSe, MgZnS, and mixtures thereof, and quaternary compounds selected from the group consisting of CdZnSeS, CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe, HgZnSTe, and mixtures thereof.

[0032] In another example, the III-V compound may be selected from the group consisting of binary compounds selected from the group consisting of GaN, GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, and mixtures thereof; ternary compounds selected from the group consisting of GaNP, GaNAs, GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs, InNSb, InPAs, InPSb, and mixtures thereof; and quaternary compounds selected from the group consisting of GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb, GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb, InAlPAs, InAlPSb, and mixtures thereof.

[0033] In another example, the IV-VI compound may be selected from the group consisting of binary compounds selected from the group consisting of SnS, SnSe, SnTe, PbS, PbSe, PbTe, and mixtures thereof; ternary compounds selected from the group consisting of SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe, SnPbTe, and mixtures thereof; and quaternary compounds selected from the group consisting of SnPbSSe, SnPbSeTe, SnPbSTe, and mixtures thereof.

[0034] In another example, the group IV element may be selected from the group consisting of Si, Ge, and mixtures thereof, and the group IV compound may be a binary compound selected from the group consisting of SiC, SiGe, and mixtures thereof.

[0035] The two-element compound, three-element compound, or four-element compound described above may be present in a particle at a uniform concentration, or may be present in the same particle with partially different concentrations. Furthermore, the quantum dot may have a core-shell structure in which one quantum dot envelops another. The interface between the core and shell may have a concentration gradient in which the concentration of the element present in the shell decreases toward the center.

[0036] The quantum dots may have a part of their surface substituted with an organic ligand. Such an organic ligand can be bound to the surface of the quantum dots to stabilize the quantum dots. Usable organic ligands include, for example, C5 to C 20Examples of suitable organic ligands include, but are not limited to, alkyl carboxylic acids, alkenyl carboxylic acids, or alkynyl carboxylic acids, pyridine, mercaptoalcohol, thiol, phosphine, phosphine oxide, primary amines, secondary amines, and combinations thereof. In the present invention, the method for substituting a portion of the surface of a quantum dot with an organic ligand is not particularly limited, and can be performed by a conventional method in the art.

[0037] The shape of the quantum dots is not particularly limited as long as it is a shape commonly used in the art, and may be, for example, spherical, rod-shaped, pyramidal, disc-shaped, multi-armed, or cubic nanoparticles, nanotubes, nanowires, nanofibers, or plate-shaped nanoparticles.

[0038] The size of the quantum dots is not particularly limited and can be adjusted appropriately within a normal range known in the art. 50) can be 1 to 20 nm, specifically 2 to 15 nm. When the particle size of the quantum dots is controlled within the range of approximately 1 to 20 nm, light of a desired color can be emitted. For example, when the particle size of the core of a quantum dot containing CdSe is approximately 2.5 to 3 nm, light with a wavelength of approximately 500 to 550 nm can be emitted, and when the particle size of the core of a quantum dot containing CdSe is approximately 3.5 to 4 nm, light with a wavelength of approximately 580 to 650 nm can be emitted. Specifically, in the present invention, quantum dots (QDs) that emit light in the range of 370 to 440 nm when absorbing light with a wavelength of 365 nm can be used. For example, blue-emitting quantum dots (QDs) can be Cd-based II-VI QDs (e.g., CdZnS, CdZnSSe, CdZnSe, CdS, CdSe), non-Cd-based II-VI QDs (e.g., ZnSe, ZnTe, ZnS, HgS), or non-Cd-based III-V QDs (e.g., InP, InGaP, InZnP, GaN, GaAs, GaP).

[0039] Furthermore, quantum dots have a full width at half maximum (FWHM) of the emission wavelength spectrum of about 45 nm or less, preferably about 40 nm or less, and more preferably about 30 nm or less, which can improve color purity and color reproducibility. Furthermore, light emitted through such quantum dots is emitted in all directions, improving the light viewing angle.

[0040] In the present invention, at least one of conventional red-emitting quantum dots, green-emitting quantum dots, and blue-emitting quantum dots known in the art may be included, and specifically, all of these may be included.

[0041] The content of the quantum dots is not particularly limited and can be adjusted appropriately within a range known in the art. For example, the quantum dots are contained in an amount of 30% by weight or less, specifically 1 to 30% by weight, more specifically 2 to 15% by weight, based on the total weight (e.g., 100% by weight) of the ink composition for the light-emitting layer.

[0042] Dispersants The ink composition for a light-emitting layer according to the present invention contains a dispersant. The dispersant is not particularly limited as long as it can uniformly disperse the quantum dots and / or other components. As an example, a (meth)acrylic monomer can be used, and specifically, it is preferable to use a di(meth)acrylic monomer containing two (meth)acrylate functional groups in one molecule.

[0043] Generally, the key factors that determine the effective jetting characteristics of an ink include the viscosity (μ), density (ρ), surface tension (σ) of the solution, and the nozzle diameter (L) according to the following mathematical formula (1): These variables are related to the Z value (Z -1 ), which allows us to numerically predict the behavior of ejected droplets depending on the physical properties of the liquid.

[0044] [Mathematical formula 1] For example, if the viscosity of an inkjet solution without dispersant is 2 cps, the density is 0.95 g / ml, the surface tension is 36 mN / m, and the nozzle diameter is 21.5 μm, the Z value (Z -1 ) will show a value of about 13.5. In this case, if a dispersant with a viscosity of about 8 to 9 cps is included, the viscosity of the existing inkjet solution (for example, 2 cps) will increase to about 2.8 to 3.5 cps, and as a result, the Z value (Z -1 ) will be reduced to 10 or less (for example, density is 0.96 g / ml, surface tension is 38 mN / m, and nozzle diameter is the same). If the viscosity value of the dispersant used is too high, the Z value (Z -1 ) may actually become lower, making it difficult for ink to be ejected.

[0045] In the present invention, the Z value (Z -1In consideration of these factors, an acrylic monomer is used as a dispersant, and this acrylic monomer dispersant is optimal for the composition of currently used inkjet solvents in terms of viscosity characteristics. Furthermore, the surface tension characteristics of the acrylic monomer dispersant are relatively high compared to existing inkjet solvents, but the change in the denominator value to which the surface tension (σ) is applied, as shown in Equation 1 above, is not so great. Thus, in the present invention, by using an acrylic monomer dispersant and adjusting the amount used within a predetermined range, stable droplets can be formed, thereby simultaneously achieving an improved pattern shape.

[0046] Examples of dispersants of acrylic monomers that can be used include ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polyolefin glycol di(meth)acrylate, ethoxylated polypropylene glycol di(meth)acrylate, 2-hydroxy-3-acryloyloxypropyl methacrylate, 2-hydroxy-1,3-dimethacryloxypropane, dioxane glycol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, glycerin di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and 1,9-nonane. Examples of the diol di(meth)acrylate include, but are not limited to, diol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 2-methyl-1,8-octanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, butyl ethyl propanediol di(meth)acrylate, 3-methyl-1,5-pentanediol di(meth)acrylate, and di(meth)acrylates having an aromatic ring, such as ethoxylated bisphenol A di(meth)acrylate, propoxylated ethoxylated bisphenol A di(meth)acrylate, and ethoxylated bisphenol F di(meth)acrylate. The above-mentioned components may be used alone or in combination of two or more.

[0047] In one embodiment, the dispersant may have a weight average molecular weight (Mw) of 70 g / mol or more, more specifically 150 to 1,200 g / mol, and a viscosity of 2 to 10 cps (based on 25° C.).

[0048] In the present invention, the content of the dispersant can be adjusted appropriately within a range known in the art and is not particularly limited. For example, the content of the dispersant can be 30% by volume or less, specifically 5 to 30% by volume, relative to 100% by volume of the solvent described below. The content of the dispersant can be expressed as a volume ratio. For example, the content ratio of the ink containing dissolved quantum dots to the dispersant can be 1:10 to 100 by volume. In one embodiment, 1 to 10 μl of dispersant can be added to 1 ml of ink containing dissolved quantum dots.

[0049] In another example, the dispersant may be present in an amount of 0.1 to 10 wt % or less, specifically 0.1 to 4 wt %, based on 100 wt % of the quantum dot solution containing quantum dots and a solvent. When the dispersant content is within the above range, the components are well mixed, resulting in excellent workability and processability, reducing the coffee ring phenomenon and enabling uniform film formation. Furthermore, the surface roughness of the light-emitting layer is improved compared to a control group that does not contain an acrylic monomer dispersant.

[0050] solvent The ink composition for a light-emitting layer according to the present invention can contain any ordinary solvent known in the art without any restrictions, but is characterized in that the solvent composition is configured so that the vapor pressure is 0.001 mmHg or more.

[0051] In other words, in a self-luminous electroluminescent device, electrons and holes injected from the outside through the electrodes meet in the light-emitting layer (EML) and emit light of a specific wavelength possessed by the quantum dots. If a large number of materials with dielectric properties are present in the light-emitting layer, the transport of electrons and holes is hindered, making it difficult for the device to operate normally.

[0052] Therefore, in the present invention, the solvent and / or dispersion medium contained in the ink composition for the luminescent layer is almost completely volatilized under typical conditions for fabricating a device, so that no other materials than quantum dots remain in the final luminescent layer. When a solvent is used alone to improve volatility, it is preferable to use a solvent with a vapor pressure of 0.001 mmHg or more, or to mix a solvent with a relatively low vapor pressure and a solvent with a relatively high vapor pressure in a predetermined ratio, thereby adjusting the vapor pressure to satisfy the above-mentioned numerical range.

[0053] The ink composition for a light-emitting layer of the present invention is not particularly limited in terms of the specific components and / or the content and composition of the solvent constituting the composition, as long as it satisfies the vapor pressure characteristics described above. Usable solvents include, but are not limited to, hexane, octane, decane, dodecane, styrene, cyclohexylbenzene, chlorobenzene, dichlorobenzene, cyclohexanone, hexadecane, etc. The above-mentioned components may be used alone or in combination of two or more.

[0054] In the present invention, the content of the solvent is not particularly limited and can be appropriately adjusted within a range known in the art. For example, the content of the solvent is the remainder for satisfying 100 parts by weight of the ink composition for the light-emitting layer, specifically, 70 to 95 parts by weight.

[0055] In addition to the components described above, the ink composition for a light-emitting layer of the present invention may further contain at least one additive known in the art, within a range that does not impair the effects of the present invention. The ink composition for the light-emitting layer according to the present invention contains quantum dots (QDs), an acrylic dispersant, at least one solvent with an adjusted vapor pressure, and other additives that are blended as needed, and can be produced by mixing and stirring them using a conventional method well known in the art.

[0056] The mixing method is not particularly limited, and examples thereof include ordinary mixers known in the art, such as homodispersers, homomixers, universal mixers, planetary mixers, kneaders, and three-roll mixers.

[0057] The ink composition for a light-emitting layer of the present invention produced as described above may contain, relative to the total weight of the composition, 1 to 30 parts by weight of quantum dots, 0.1 to 10 parts by weight of an acrylic dispersant, and the remainder being a solvent, more specifically, 2 to 15 parts by weight of quantum dots, 0.1 to 4 parts by weight of an acrylic dispersant, and the remainder being a solvent. However, the composition is not limited thereto.

[0058] Meanwhile, the ejection conditions for inkjet devices can be broadly divided into viscosity and vapor pressure. If the viscosity is too high or too low, a uniform film cannot be obtained, and the degree of ejection is determined by the vapor pressure. In the present invention, a solvent is selected in consideration of the viscosity and vapor pressure suitable for inkjet ejection, and the content of the solvent is controlled to prepare an ink composition for an emissive layer. The ink composition for an emissive layer of the present invention has optimized properties such as viscosity, vapor pressure, and contact angle, thereby achieving excellent workability and processability. In particular, uniformity and stability are ensured in all aspects of inkjet ejection, the shape of the ejected ink, and the shape of the final pattern formed, thereby achieving device characteristics useful for inkjet printing methods.

[0059] In one embodiment, the composition may have a viscosity of 1.0 to 5.0 cps at 20°C, a vapor pressure of 0.1 to 10 mmHg at 20°C, a contact angle of 10 to 30°C, and a solids content of 30 wt% or less. More specifically, the composition may have a viscosity of 2.0 to 4.0 cps at 20°C, a vapor pressure of 1.0 to 5.0 mmHg at 20°C, a contact angle of 15 to 25°C, and a solids content of 5 to 30 wt%.

[0060] In another embodiment, the Z value (Z -1 ) can be 1 to 10. The Z value makes it possible to predict whether or not the droplets can be discharged, the shape of the discharged droplets, and the like.

[0061] Furthermore, the ink composition for a light-emitting layer of the present invention, which contains a solvent having a predetermined vapor pressure, produces a relatively high pattern height immediately after jetting due to the presence of the solvent. However, after a predetermined time has passed, the solvent is volatilized and removed by drying without the need for a separate drying process, thereby forming a uniform, thin, high-quality light-emitting layer made of quantum dots.

[0062] In another embodiment, the ink pattern (e.g., light-emitting layer) formed after jetting the composition may contain up to 10% by volume of solvent and dispersant due to removal of volatile components. In another embodiment, the height (H1) of the ink pattern (e.g., the light-emitting layer) formed after jetting is 500 to 2,000 nm, and the height (H F ) can be 5 to 60 nm.

[0063] <Light-emitting element> A light emitting device according to an embodiment of the present invention includes a light emitting layer formed from the ink composition for a light emitting layer described above.

[0064] In one embodiment, the light-emitting element includes a first electrode, a second electrode facing the first electrode, a light-emitting layer between the first electrode and the second electrode and formed from the ink composition for the light-emitting layer, a hole transport layer between the first electrode and the light-emitting layer, and an electron transport layer between the light-emitting layer and the second electrode. Optionally, the light-emitting element may further include at least one of a hole injection layer and an electron injection layer.

[0065] Hereinafter, the present invention will be described using a quantum dot light emitting device as an example, but the present invention is not limited thereto, and the light emitting device may be applied to various light emitting devices such as an organic light emitting device.

[0066] The first electrode is disposed on a substrate. Such a substrate can be a transparent, flat glass substrate or a transparent plastic substrate. The substrate can be used after ultrasonic cleaning with a solvent such as isopropyl alcohol, acetone, or methanol to remove contaminants, and UV ozone treatment.

[0067] The first electrode can function as an anode. For example, the anode may be made of a metal, a metal oxide that meets the respective transparent / opaque conditions, or other non-oxide inorganic material. For bottom emission, the first electrode may be made of a transparent conductive metal such as transparent ITO, IZO, ITZO, or AZO.

[0068] The hole injection layer and the hole transport layer are located on the first electrode. These hole injection layer and hole transport layer facilitate hole injection from the first electrode and transport holes to the light-emitting layer. The hole transport layer can be made of either organic or inorganic materials. For organic materials, the layer can be made of CBP (4,4'-N,N'-dicarbazole-biphenyl), α-NPD (N,N'-diphenyl-N,N'-bis(1=naphthyl)-1,1'-biphenyl-4,4"-diamine), TCTA (4,4',4"-tris(N-carbazolyl)-triphenylamine), TFB, or DNTPD (N,N'-di(4-(N,N'-diphenyl-amino)phenyl)-N,N'-diphenylbenzidine). For inorganic materials, the layer can be made of NiO or MoO3 oxide. As an example, the hole injection layer may be poly(ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), and the hole transport layer may be TFB or poly(9-vinylcarbazole) (PVK), etc.

[0069] The light-emitting layer is located on the hole transport layer, and the quantum dots are provided in the light-emitting layer. For example, the light-emitting layer can be formed by inkjet printing the ink composition for the light-emitting layer described above, followed by volatilizing the solvent.

[0070] The electron transport layer facilitates electron injection from the second electrode and transports electrons to the light-emitting layer. Such an electron transport layer can be formed using any conventional electron transport material known in the art, including, for example, ZnO or Zn-containing metal oxide nanoparticles alloyed with a metal that can increase the bandgap of ZnO. For example, the electron transport layer can be formed by coating a dispersion of a metal oxide in a solvent on the light-emitting layer using a solution process, followed by evaporating the solvent. Examples of the coating method include drop casting, spin coating, dip coating, spray coating, flow coating, screen printing, and inkjet printing, which can be used alone or in combination. The electron transport layer of the present invention may be provided as a single layer structure that also functions as an electron injection layer, or it may be formed as a laminate structure with a separate electron injection layer.

[0071] The second electrode is positioned on the electron injection / transport layer and serves as a cathode. The second electrode may be made of a metal, a metal oxide, or other non-oxide inorganic material that meets the requirements for transparency or opacity. In particular, the second electrode may be made of a metal having a low work function and excellent internal reflectivity to facilitate electron injection at the LUMO level of the emissive layer. Specifically, metals with a low work function, such as I, Ca, Ba, Ca / Al, LiF / Ca, LiF / Al, BaF / Al, BaF / Ca / Al, Al, Mg, or Ag:Mg alloys, may be used to facilitate electron injection.

[0072] The above description concerns a case where the light-emitting device according to the present embodiment is a quantum dot light-emitting device. However, unlike the above, the light-emitting device may be various light-emitting devices. For example, the light-emitting device may be an organic light-emitting device. Furthermore, although the present embodiment describes the electron injection / transport layer as being made of a single material, the electron injection layer and the electron transport layer may be provided separately. The present invention will be described in detail below with reference to examples. However, the examples described below are merely illustrative of the present invention, and the present invention is not limited to these examples. [Example]

[0073] [Example 1] Preparation of ink composition for light-emitting layer for inkjet printing The quantum dots used were colloidally dispersed quantum dot solutions in toluene. The red quantum dots were quantum dots with a core-shell structure composed of indium phosphide (InP) / zinc selenide (ZnSe), and the green quantum dots were indium phosphide (InP) / zinc sulfide (ZnS). The blue quantum dots were core-multiple shell structures composed of selenium zinc telluride (ZnSeTe) / zinc selenide (ZnSe) / zinc sulfide (ZnS). The ligand for the red, green, and blue quantum dots was oleic acid.

[0074] The solutions containing the red, green, and blue quantum dots were centrifuged to obtain the quantum dots. The quantum dots were then dispersed in a solvent consisting of cyclohexylbenzene (vapor pressure: 1 mmHg) and cyclohexanone (vapor pressure: 3 mmHg) in an 8:2 volume ratio. The concentrations of the red, green, and blue quantum dots were 45 mg / ml, 80 mg / ml, and 35 mg / ml, respectively. An acrylic dispersant (diethylene glycol dimethacrylate) was added at 2 wt. % to each of the dispersed quantum dot solutions to prepare ink compositions for ink-jet printable emissive layers.

[0075] The image of the ink composition for the light-emitting layer of Example 1, in which the acrylic dispersant is mixed as described above, is shown in the attached Figure 1. In addition, the Z value (Z -1 ) was 8.75.

[0076] Comparative Example 1: Preparation of ink composition for light-emitting layer for inkjet printing After forming the red, green, and blue quantum dots, an ink composition for an emissive layer for inkjet printing of Comparative Example 1 was prepared in the same manner as in Example 1, except that the red, green, and blue quantum dots were dispersed at 18 mg / ml in octane solvent (vapor pressure: 11 mmHg) instead of the mixed solvent of cyclohexylbenzene and cyclohexanone. At this time, the Z value (Z -1 The viscosity of the ink was 35.49 (density 0.703 g / ml, surface tension 21.61 mN / m, nozzle diameter was the same, and viscosity was 0.509 cps). The ink thus produced was evaluated for jetting and patterning in the same manner as in Example 1.

[0077] Comparative Example 2: Preparation of ink composition for light-emitting layer for inkjet printing An ink composition for an emissive layer for inkjet printing of Comparative Example 2 was prepared in the same manner as in Example 1, except that no acrylic dispersant was used. At this time, the Z value (Z -1 The ink thus prepared was subjected to evaluation of jetting and pattern in the same manner as in Example 1.

[0078] [Experimental Example 1] Evaluation of inkjet ejection and shape The ink compositions for light-emitting layers prepared in Example 1 and Comparative Examples 1 and 2 were analyzed for ejection and shape during inkjet printing according to the following method, and the results are shown in Table 1 below and FIGS.

[0079] (1) Jetting performance evaluation Each of the obtained ink compositions for the light-emitting layer for inkjet printing was filled into a cartridge head (Fujifilm Dimatix 10pL, DMC-11610), and then ejected in a one-drop pattern using an inkjet printing device (Omnjet200) to evaluate whether inkjet printing could be performed.

[0080] (2) Analysis of quantum dot patterns for inkjet printing A fully automatic non-contact 3D surface texture measuring instrument (NV9000, resolution: 0.06 nm) was used to analyze the quantum dot pattern formed on the substrate. To quantify the degree of the coffee ring effect (CRF), the following [Equation 2] was introduced, and the results are shown in Table 1.

[0081] [Mathematical formula 2] JPEG0007777676000002.jpg14166 (in the formula, H Max is the maximum thickness of the pattern, H Min indicates the thinnest thickness of the pattern, and the CRF value indicates the degree of the coffee ring effect. That is, CRF=1 indicates that the coffee ring is completely removed.

[0082] [Table 1]

[0083] As shown in Table 1 above, ejection and pattern formation by the inkjet method were impossible in Comparative Example 1, and Comparative Example 2 had relatively poor properties compared to Example 1. In contrast, the ink composition for a light-emitting layer of the present invention, which contains a dispersant, was easily ejected from a general inkjet printing device, and the shapes of the ejected ink and the ink formed on the substrate were uniform and close to 1, confirming its usefulness in inkjet printing (see Figures 2 to 7).

[0084] On the other hand, the Z value of the Ohnesorge number (Z -1) can be used to predict whether ink can be ejected and the shape characteristics of the ejected droplets. As shown in Figures 2 to 4, Comparative Example 1 has a very high Z value (35.49), which means that there is a problem of other droplets being present in addition to the main droplets, while Comparative Example 2 has additional droplets formed at the beginning of ejection, resulting in an uneven shape of the final patterned droplets.

[0085] In contrast, Example 1 has a Z value (8.75) that allows stable droplet formation, and the ejected ink is ejected stably without forming tails or additional droplets. This means that the Z value of the ink composition can also be used to numerically predict the droplet ejection characteristics before jetting.

[0086] [Experimental Example 2] Evaluation of ink volatilization level and pattern height Using the ink compositions for light-emitting layers prepared in Example 1 and Comparative Examples 1 and 2, the degree of ink volatilization during inkjet printing and the height of the formed patterns were evaluated as follows, and the results are shown in Table 2 below.

[0087] Specifically, the height (H1) of the pattern immediately after jetting each ink composition for the light-emitting layer onto the substrate and the height (H F ) and were measured to confirm the degree of volatilization of the solvent and dispersant used in the quantum dot ink.

[0088] [Table 2]

[0089] As shown in Table 2 above, in Comparative Example 1, it was impossible to form a pattern by the inkjet method, and the height of the pattern formed in Comparative Example 2 tended to be relatively high. In contrast, with the ink composition for a light-emitting layer of the present invention containing a dispersant, the height of the patterns formed after jetting and drying was all uniform, and relatively low and thin, confirming that it was possible to form a uniform light-emitting layer.

[0090] [Experimental Example 3] Evaluation of quantum dot electroluminescent devices for inkjet printing Electroluminescent devices were fabricated using the ink compositions for light-emitting layers prepared in Example 1 and Comparative Examples 1 and 2, and then their physical properties were evaluated.

[0091] Specifically, an indium tin oxide (ITO) substrate was cleaned with isopropyl alcohol and acetone for 15 minutes each, and then dried in an oven at 60°C for 30 minutes. After the substrate was dried, it was subjected to UV ozone treatment for 20 minutes, and then spin-coated with PEDOT:PSS to form a hole injection layer (HIL). The spin-coating conditions were 4,500 rpm for 60 seconds and heat treatment conditions were 150°C for 20 minutes.

[0092] Next, under a nitrogen gas (N2) atmosphere, a film was formed from Poly-TPD material dissolved in chlorobenzene at 6 mg / ml at 4,500 rpm for 30 seconds, and then heat-treated at 150°C for 30 minutes to form a hole-transporting layer (HTL).

[0093] Thereafter, each of the ink compositions prepared in Example 1 and Comparative Examples 1 and 2 was inkjet printed on the hole transport layer (HTL) to form an emissive layer (EML). Next, zinc oxide nanoparticles were dispersed in an ethanol solvent and spin-coated at 1,500 rpm for 30 seconds to form an electron transport layer (ETL), after which electrodes were formed by vacuum deposition, completing the fabrication of the electroluminescent device shown in Figure 6.

[0094] The efficiency of the light-emitting devices of Example 1 and Comparative Example 2 manufactured by the above-described method was evaluated using an IVL measurement device, and the results are shown in the following Table 3 and FIGS. 9 to 17, respectively.

[0095] [Table 3]

[0096] As shown in Table 3 above, the light emitting device of Comparative Example 1 was unable to be driven, and the light emitting device of Comparative Example 2 had poorer device performance than that of Example 1. In contrast, it was confirmed that the light emitting device of Example 1, which had an emitting layer formed using the ink composition for an emitting layer of the present invention, simultaneously had high brightness for each of R, G, and B, and excellent luminous efficiency and external quantum efficiency (EQE) (see FIGS. 9 to 17).

Claims

1. An ink composition for forming a light-emitting layer of an electroluminescent device by inkjet printing, comprising: quantum dots, (meth)acrylic dispersants, and a solvent having a vapor pressure of 0.001 to 10 mmHg at 20°C; The ink composition for a light-emitting layer, wherein the (meth)acrylic dispersant is contained in an amount of 0.1 to 4% by weight relative to 100% by weight of a quantum dot solvent containing the quantum dots and the solvent.

2. The ink composition for a light-emitting layer according to claim 1 , wherein the solvent comprises at least two or more solvents having different vapor pressures.

3. 2. The ink composition for a light-emitting layer according to claim 1, wherein the (meth)acrylic dispersant is a di(meth)acrylic compound.

4. The quantum dots are contained in an amount ranging from 1 to 30% by weight based on the total weight of the ink composition for the light-emitting layer. The ink composition for a light-emitting layer according to claim 1 , wherein

5. The ink composition for a light-emitting layer according to claim 1 , wherein the quantum dots include at least one of red-emitting quantum dots, green-emitting quantum dots, and blue-emitting quantum dots.

6. The ink composition for a light-emitting layer is The viscosity at 20°C is 1.0 to 5.0 cps, The vapor pressure at 20°C is 0.1 to 10 mmHg, The contact angle is 10 to 30°, 2. The ink composition for a light-emitting layer according to claim 1, wherein the solid content is 30% by weight or less.

7. The height of the ink pattern after jetting (H １ ) is 500 to 2,000 nm, The height of the printed pattern after drying (H Ｆ 2. The light-emitting layer according to claim 1, wherein the thickness of the first electrode is 5 to 60 nm. Ink composition.

8. a first electrode; a second electrode disposed opposite the first electrode; a light-emitting layer disposed between the first electrode and the second electrode and formed from the ink composition for a light-emitting layer according to any one of claims 1 to 7; a hole transport layer disposed between the first electrode and the light-emitting layer; and an electron transport layer disposed between the light-emitting layer and the second electrode; A light-emitting element comprising:

9. The light-emitting device of claim 8 , wherein the light-emitting layer is formed by inkjet printing.

10. The light-emitting device of claim 8 , further comprising at least one of a hole injection layer and an electron injection layer.

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