Multi-layered quantum dot color conversion layer and method for manufacturing same

Abstract

The present invention relates to a multi-layered quantum dot color conversion layer and a method for manufacturing same. The method for manufacturing a multi-layered quantum dot color conversion layer comprises the steps of: preparing a green quantum dot solution, a yellow quantum dot solution, and a red quantum dot solution via a colloidal synthesis method; forming a green quantum dot film, a yellow quantum dot film, and a red quantum dot film, respectively by spin-coating the green quantum dot solution, the yellow quantum dot solution, and the red quantum dot solution onto an ODTS-treated donor substrate; and forming a stacked multi-layered quantum dot color conversion layer by sequentially transferring each of the green quantum dot film, the yellow quantum dot film, and the red quantum dot film from the donor substrate to a target substrate by using a thermal release tape.

Description

Multilayer quantum dot color conversion layer and method for fabricating the same

[0001] The present invention relates to a multilayer quantum dot color conversion layer and a method for producing the same, and more specifically, to a technology for producing a transfer-printed multilayer quantum dot color conversion layer capable of providing a white light-emitting diode having high efficiency, a high color rendering index, and balanced color.

[0002] This invention is derived from research conducted with funding from the government (Ministry of Science and ICT) and supported by the National Research Foundation of Korea's Nano and Materials Technology Development Project.

[0003] The information regarding the national research and development project related to this patent is as follows.

[0004] Project ID: 2710006167

[0005] Project No.: RS-2024-00402972

[0006] Ministry Name: Ministry of Science and ICT

[0007] Project Management (Specialized) Agency Name: National Research Foundation of Korea

[0008] Research Project Name: Materials Global Young Connect

[0009] Research Project Title: Development of Molten Phase Transition-Based Precision Shape Control Substrate Materials for Freeform Displays

[0010] Project Performing Organization Name: Daegu Gyeongbuk Institute of Science and Technology

[0011] Research Period: April 1, 2024 – December 31, 2025

[0012]

[0013] Furthermore, the present invention is derived from research conducted with the support of the Individual Basic Research Project funded by the government (Ministry of Science and ICT).

[0014] The information regarding the national research and development project related to this patent is as follows.

[0015] Project ID: 1711191749

[0016] Project No.: 2021R1C1C1007997

[0017] Ministry Name: Ministry of Science and ICT

[0018] Project Management (Specialized) Agency Name: National Research Foundation of Korea

[0019] Research Project Name: Individual Basic Research (Ministry of Science and ICT)

[0020] Research Project Title: Development of a Hemispherical Artificial Eye Based on a Self-Healing Curved Neuromorphic Image Sensor

[0021] Project Performing Organization Name: Ulsan National Institute of Science and Technology

[0022] Research Period: 2024.03.01 ~ 2025.02.28

[0023] White light sources are indispensable elements in lighting and display applications; they must be designed to meet diverse environments and human needs while possessing high luminous efficiency and a high color rendering index (CRI). An ideal white light source should be able to accurately reproduce the true colors of objects through a spectral distribution similar to natural light, while simultaneously providing high energy efficiency. Furthermore, the correlated color temperature (CCT) of a white light source significantly influences human circadian rhythms and physiological processes; cool white light with a high color temperature enhances concentration and productivity, while warm white light with a low color temperature induces psychological stability and relaxation. Therefore, in lighting applications, an ideal white light source must be able to provide a wide range of color temperatures.

[0024] However, existing white light source technologies have limitations in meeting these demands. Incandescent bulbs provide a high Color Rendering Index (CRI) close to natural light, but they have low energy efficiency as most of the energy is lost as heat. Fluorescent bulbs are energy efficient, but their low CRI makes them unsuitable for applications requiring high-quality lighting. AC electroluminescent (ACEL) sources have limited brightness and color control capabilities. Due to these limitations, White Light-Emitting Diodes (White LEDs) are attracting attention as the next-generation white light source, offering higher efficiency and durability compared to conventional light sources.

[0025] A common method for manufacturing white LEDs is to produce white light by coating a yellow phosphor onto a blue LED. While this method offers the advantages of a simple structure and manufacturing process, it is unsuitable for high-quality lighting applications due to a low Color Rendering Index (CRI) resulting from the lack of red components. To address this, using a mixture of red, green, and yellow phosphors can yield a more balanced spectrum and a higher CRI; however, this mixing method suffers from reduced light conversion efficiency caused by energy transfer between phosphor particles.

[0026] Quantum dots (QDs) are attracting attention as next-generation light-emitting materials for lighting and display applications. Quantum dots allow for precise control of emission wavelengths by adjusting their size and composition, and provide excellent optical properties with high luminous efficiency and a narrow spectral width. Furthermore, quantum dots exhibit high absorption coefficients in the blue light and near-ultraviolet regions and are utilized as high-efficiency light conversion materials suitable for color conversion layer (CCL) applications. In particular, quantum dots composed of inorganic materials maintain high stability even at high temperatures above 80°C and operate effectively under high-power backlight lighting conditions.

[0027] However, there are several technical challenges in fabricating quantum dot-based color conversion layers. Conventional technology primarily utilized a drop-coating method of quantum dot solutions onto blue LEDs; however, this process induces a coffee ring effect due to solution evaporation, which degrades film uniformity. Furthermore, interparticle energy transfer between mixed quantum dots or phosphors reduces luminous efficiency, consequently leading to a degradation in device performance.

[0028] To solve the problems of the prior art described above, the present invention provides a method for fabricating a multilayer quantum dot color conversion layer. The multilayer quantum dot color conversion layer of the present invention suppresses energy transfer between particles by sequentially stacking monochromatic quantum dot films, and can precisely control the color temperature of the white light of a white light-emitting diode containing the multilayer quantum dot color conversion layer by adjusting the thickness of the yellow quantum dot film. Through this, the present invention can provide a white light-emitting diode capable of realizing high-efficiency, high-quality white light.

[0029] A method for fabricating a multilayer quantum dot color conversion layer according to one embodiment of the present invention may include the steps of: preparing a green quantum dot (QD) solution, a yellow quantum dot solution, and a red quantum dot solution by a colloidal synthesis method, respectively; spin-coating the green quantum dot solution, the yellow quantum dot solution, and the red quantum dot solution onto a donor substrate treated with ODTS (Octadecyltrichlorosilane) to form a green quantum dot film, a yellow quantum dot film, and a red quantum dot film, respectively; and sequentially transferring the green quantum dot film, the yellow quantum dot film, and the red quantum dot film, respectively, from the donor substrate to a target substrate using a heat-releasing tape to form a stacked multilayer quantum dot color conversion layer.

[0030] In a method for fabricating a multilayer quantum dot color conversion layer according to one embodiment of the present invention, the step of forming each of a green quantum dot film, a yellow quantum dot film, and a red quantum dot film may further include the step of controlling the speed of spin coating to control the thickness of each quantum dot film.

[0031] A method for fabricating a multilayer quantum dot color conversion layer according to one embodiment of the present invention further includes the step of setting the color temperature of a white light-emitting diode, and the step of forming each of a green quantum dot film, a yellow quantum dot film, and a red quantum dot film may further include the step of forming the thickness of the yellow quantum dot film to a thickness of the yellow quantum dot film capable of expressing a preset color temperature of the white light-emitting diode including the multilayer quantum dot color conversion layer.

[0032] In a method for fabricating a multilayer quantum dot color conversion layer according to an embodiment of the present invention, the step of forming a stacked multilayer quantum dot color conversion layer by sequentially transferring a green quantum dot film, a yellow quantum dot film, and a red quantum dot film from a donor substrate to a target substrate using a heat-emitting tape may further include the step of forming a first green quantum dot layer by transferring a green quantum dot film onto a target substrate, the step of forming a yellow quantum dot layer by transferring a yellow quantum dot film onto the first green quantum dot layer, the step of forming a second green quantum dot layer by transferring a green quantum dot film onto the yellow quantum dot layer, and the step of forming a red quantum dot layer by transferring a red quantum dot film onto the second green quantum dot layer.

[0033] A method for fabricating a multilayer quantum dot color conversion layer according to one embodiment of the present invention may further include the step of forming a polymethyl methacrylate (PMMA) protective layer on each of the green quantum dot film, yellow quantum dot film, and red quantum dot film to maintain separation between each quantum dot film in the multilayer quantum dot color conversion layer before transferring each of the green quantum dot film, yellow quantum dot film, and red quantum dot film from a donor substrate to a target substrate.

[0034] A multilayer quantum dot color conversion layer according to one embodiment of the present invention includes a green quantum dot layer, a yellow quantum dot layer, and a red quantum dot layer, and the green quantum dot layer, the yellow quantum dot layer, and the red quantum dot layer can be sequentially stacked through a transfer printing process.

[0035] A multilayer quantum dot color conversion layer according to one embodiment of the present invention may include a first green quantum dot layer disposed on a blue LED, a yellow quantum dot layer disposed on the first green quantum dot layer, a second green quantum dot layer disposed on the yellow quantum dot layer, and a red quantum dot layer disposed on the second green quantum dot layer.

[0036] A yellow quantum dot layer of a multilayer quantum dot color conversion layer according to one embodiment of the present invention may be formed with a thickness of the yellow quantum dot layer according to a preset color temperature of a white light-emitting diode including a multilayer quantum dot color conversion layer.

[0037] A multilayer quantum dot color conversion layer according to one embodiment of the present invention may further include a PMMA protective layer formed on each of the green quantum dot layer, the yellow quantum dot layer, and the red quantum dot layer to maintain separation between each quantum dot layer.

[0038] The multilayer quantum dot color conversion layer (QD CCL) of the present invention can be laminated through a transfer printing process, thereby effectively solving the problem of reduced efficiency caused by non-uniform thickness and energy transfer between particles that occurs in conventional technology. The multilayer quantum dot color conversion layer comprising red, yellow, and green quantum dot layers of the present invention can generate high-quality white light that approximates the International Commission on Illumination (CIE) color coordinates (0.33, 0.33) by combining with a blue LED to realize a balanced spectrum, and can provide excellent color quality with a color rendering index (CRI) of 90 or higher.

[0039] Furthermore, by adjusting the thickness of the yellow quantum dot color conversion layer of the present invention, the color temperature (CCT) of the white light of a white light-emitting diode including the multilayer quantum dot color conversion layer of the present invention can be adjusted from 4260K to 6660K, thereby providing white light suitable for various application environments such as household lighting, commercial lighting, and industrial lighting.

[0040] FIG. 1 is a flowchart of a method for fabricating a multilayer quantum dot color conversion layer according to one embodiment of the present invention.

[0041] FIG. 2 is a side view of a white light-emitting diode including a multilayer quantum dot color conversion layer according to one embodiment of the present invention.

[0042] FIG. 3 is a diagram illustrating the absorption and emission spectra and TEM image of a quantum dot (QD) according to one embodiment of the present invention.

[0043] Figure 4 is a graph showing the particle size distribution of red quantum dots, yellow quantum dots, and green quantum dots of a multilayer quantum dot color conversion layer according to one embodiment of the present invention.

[0044] Figure 5 is a graph of the X-ray Diffraction (XRD) results of red quantum dots, yellow quantum dots, and green quantum dots of a multilayer quantum dot color conversion layer according to one embodiment of the present invention.

[0045] FIG. 6 is the FT-IR (Fourier transform infrared) spectrum of red quantum dots, yellow quantum dots, and green quantum dots of a multilayer quantum dot color conversion layer according to one embodiment of the present invention.

[0046] FIG. 7 is a diagram illustrating the process of transfer printing a single-layer quantum dot layer according to one embodiment of the present invention.

[0047] FIG. 8 is an AFM image of a yellow quantum dot layer according to one embodiment of the present invention.

[0048] FIG. 9 is a diagram comparing the characteristics of a white light-emitting diode using a multilayer quantum dot color conversion layer according to one embodiment of the present invention and a conventional white light-emitting diode using a mixed color conversion layer.

[0049] FIG. 10 is a diagram illustrating the light emission quality of a white light-emitting diode according to the configuration of a multilayer quantum dot color conversion layer according to one embodiment of the present invention.

[0050] Figure 11 is a graph of the thickness of each quantum dot layer in (a), (d), and (g) of Figure 10.

[0051] Figure 12 is a graph comparing several stacked examples of tricolor quantum dot layers.

[0052] FIG. 13 is a diagram illustrating the color temperature of a white light-emitting diode according to a change in the thickness of a yellow quantum dot layer among a multilayer quantum dot color conversion layer according to one embodiment of the present invention.

[0053] Specific structural or functional descriptions of embodiments according to the concept of the present invention disclosed herein are provided merely for the purpose of explaining embodiments according to the concept of the present invention, and embodiments according to the concept of the present invention may be implemented in various forms and are not limited to the embodiments described herein.

[0054] Embodiments according to the concept of the present invention may be subject to various modifications and may take various forms; therefore, embodiments are illustrated in the drawings and described in detail in this specification. However, this is not intended to limit the embodiments according to the concept of the present invention to specific disclosed forms, and includes modifications, equivalents, or substitutions that fall within the spirit and scope of the present invention.

[0055] Terms such as "first" or "second" may be used to describe various components, but said components should not be limited by said terms. For the sole purpose of distinguishing one component from another, for example, without departing from the scope of rights according to the concept of the present invention, the first component may be named the second component, and similarly, the second component may be named the first component.

[0056] When it is stated that one component is "connected" or "connected" to another component, it should be understood that while it may be directly connected or connected to that other component, there may also be other components in between. Conversely, when it is stated that one component is "directly connected" or "directly connected" to another component, it should be understood that there are no other components in between. Expressions describing the relationships between components, such as "between," "exactly between," or "directly adjacent to," should be interpreted in the same way.

[0057] The terms used herein are used merely to describe specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, terms such as “comprising” or “having” are intended to specify the existence of the described features, numbers, steps, actions, components, parts, or combinations thereof, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0058] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in this specification.

[0059]

[0060] Hereinafter, embodiments will be described in detail with reference to the attached drawings. However, the scope of the patent application is not limited or restricted by these embodiments. Identical reference numerals in each drawing indicate identical components.

[0061]

[0062] FIG. 1 is a flowchart of a method for fabricating a multilayer quantum dot color conversion layer according to an embodiment of the present invention. Referring to FIG. 1, a method for fabricating a white light-emitting diode according to an embodiment of the present invention comprises the steps of: preparing a quantum dot solution (S110); spin-coating the quantum dot solution onto a donor substrate treated with ODTS (Octadecyltrichlorosilane) to form a single-color quantum dot film of each color (S120); and transferring the single-color quantum dot film from the donor substrate to a target substrate to form a stacked multilayer quantum dot color conversion layer (S130).

[0063] Specifically, the step of preparing the quantum dot solution (S110) can prepare a green quantum dot (QD) solution, a yellow quantum dot solution, and a red quantum dot solution, respectively, by colloidal synthesis.

[0064] The quantum dots (QDs) used in this invention are quantum dots having red, yellow, and green luminescence characteristics, and are used as the core material of the color conversion layer (CCL) described in this invention. The process of preparing quantum dots using a colloidal synthesis method in this invention is as follows.

[0065] First, CdO, Zn(OAc)2, OA, and ODE are mixed to form the core of the quantum dot. This mixture is heated under vacuum at approximately 120°C to form a Cd(oleate)2 and Zn(oleate)2 complex. Subsequently, the temperature is raised to 310°C while injecting TOP-Se (Trioctylphosphine-Selenium) to form CdSe nuclei. During this process, reaction time and temperature conditions act as important variables determining the size of the CdSe nuclei and the emission wavelength.

[0066] Next, a multilayer shell structure of ZnSe and ZnS is grown to form a shell surrounding the CdSe nucleus. First, Zn(oleate)2 and TOP-Se are added to form an intermediate ZnSe shell, and then Zn(oleate)2 and TOP-S (trioctylphosphine sulfide) are added to grow an outer ZnS shell. This multilayer shell structure serves to stabilize the CdSe nucleus, protect it from the external environment (e.g., moisture, heat), and improve luminescence efficiency.

[0067] After the synthesis of quantum dots is completed, cyclohexane and acetone are used to remove impurities and reaction byproducts in order to purify the generated quantum dots to a high purity, and finally, the quantum dots are redispersed in cyclohexane to prepare a homogeneous solution. Through this purification and redispersion process, the purity of the quantum dots is increased, and stable processing is enabled for subsequent applications (spin coating, film fabrication, etc.).

[0068] The quantum dots prepared in the present invention have a core / shell structure with emission wavelengths tuned to red (619 nm), yellow (580 nm), and green (525 nm), and can provide high efficiency and high stability characteristics. These quantum dots serve as a key component of the multilayer quantum dot color conversion layer (CCL) used in the present invention and contribute to realizing high-quality white light by converting the light source of a blue LED.

[0069] As such, the quantum dots prepared in the present invention can be designed to be suitable for generating high-efficiency, high-quality white light through a multilayer structure and a precise synthesis process that differentiates them from the mixed-type quantum dots used in the prior art.

[0070] In the method for manufacturing a white light-emitting diode according to the present invention, the step (S120) of forming a single-color quantum dot film of each color by spin-coating a quantum dot solution onto a donor substrate treated with ODTS (Octadecyltrichlorosilane) can form a green quantum dot film, a yellow quantum dot film, and a red quantum dot film, respectively, by spin-coating a green quantum dot solution, a yellow quantum dot solution, and a red quantum dot solution onto a donor substrate treated with ODTS (Octadecyltrichlorosilane).

[0071] And the step (S130) of transferring a single-color quantum dot film from a donor substrate to a target substrate to form a stacked multilayer quantum dot color conversion layer can be formed by sequentially transferring a green quantum dot film, a yellow quantum dot film, and a red quantum dot film from a donor substrate to a target substrate using a thermal emission tape.

[0072] In other words, the white light-emitting diode of the present invention utilizes a process for forming a multilayer structure by sequentially stacking monochromatic quantum dot films (red, yellow, and green) using a transfer printing process. This process ensures high uniformity and stability and can solve the problem of inefficient phosphor mixing methods that occur in conventional technology.

[0073] First, to fabricate a single-color quantum dot film, single-color quantum dot solutions (red, yellow, and green) are prepared. The prepared quantum dot solutions are coated onto a glass donor substrate surface-treated with ODTS (Octadecyltrichlorosilane) using a spin coating method. ODTS is a hydrophobic surface treatment agent that weakens the adhesion between the donor substrate and the quantum dot film, allowing the film to be easily separated during the transfer step. The spin coating process allows for precise control of the thickness and uniformity of the quantum dot film.

[0074] In a method for fabricating a multilayer quantum dot color conversion layer according to one embodiment of the present invention, the thickness of each of the green quantum dot film, the yellow quantum dot film, and the red quantum dot film can be controlled by controlling the speed of spin coating.

[0075] A method for fabricating a multilayer quantum dot color conversion layer according to one embodiment of the present invention can set the color temperature of a white light-emitting diode and control the speed of spin coating to form the thickness of a yellow quantum dot film to a thickness of a yellow quantum dot film capable of expressing the set color temperature of the white light-emitting diode.

[0076] In a method for fabricating a multilayer quantum dot color conversion layer according to one embodiment of the present invention, when the thickness of the yellow quantum dot film is thinner than the critical thickness, the color temperature of the white light-emitting diode containing the multilayer quantum dot color conversion layer is greater than 5850K, and when the thickness of the yellow quantum dot film is thicker than the critical thickness, the color temperature of the white light-emitting diode containing the multilayer quantum dot color conversion layer may be less than 5850K.

[0077] In one embodiment, after a monochromatic quantum dot film is formed, a polymethyl methacrylate (PMMA) protective layer may be additionally coated to ensure the physical protection and stability of the film. The PMMA solution is dynamically cast onto the quantum dot film to form the protective layer. The PMMA protects the film from damage during the transfer and stacking process and can serve to suppress interparticle energy transfer by maintaining physical separation between each quantum dot layer.

[0078] Afterward, the quantum dot film formed on the donor substrate is separated using a thermal release tape. The thermal release tape has the characteristic of weakening its adhesive strength when heat is applied. The separated quantum dot film is sequentially transferred to a target substrate to form a multilayer quantum dot color conversion layer. The stacking sequence proceeds in the order of a green quantum dot layer, a yellow quantum dot layer, a green quantum dot layer, and a red quantum dot layer, thereby enabling the generation of high-quality white light.

[0079] The transfer printing process used in the present invention can precisely control the thickness and stacking order of the multilayer quantum dot color conversion layer, and maximizes the structural stability and light efficiency of the multilayer quantum dot color conversion layer, thereby enabling the realization of high-quality white light.

[0080] FIG. 2 is a side view of a white light-emitting diode including a multilayer quantum dot color-changing layer according to an embodiment of the present invention. Referring to FIG. 2, the structure of a white light-emitting diode including a multilayer quantum dot color-changing layer fabricated using the method of fabricating a multilayer quantum dot color-changing layer of FIG. 1 can be understood. A white light-emitting diode including a multilayer quantum dot color-changing layer according to an embodiment of the present invention may include a green quantum dot layer (220, 240), a yellow quantum dot layer (230), and a red quantum dot layer (250), wherein the green quantum dot layer (220, 240), the yellow quantum dot layer (230), and the red quantum dot layer (250) are sequentially stacked through a transfer printing process and may include a blue LED (Light-Emitting Diode) (210) disposed below the multilayer quantum dot color-changing layer and emitting blue light with a peak emission wavelength of 450 nm toward the multilayer quantum dot color-changing layer.

[0081] A multilayer quantum dot color conversion layer according to one embodiment of the present invention may include a first green quantum dot layer (220) disposed on a blue LED, a yellow quantum dot layer (230) disposed on the first green quantum dot layer (220), a second green quantum dot layer (240) disposed on the yellow quantum dot layer (230), and a red quantum dot layer (250) disposed on the second green quantum dot layer (240).

[0082] In a white light-emitting diode including a multilayer quantum dot color conversion layer according to one embodiment of the present invention, the color temperature of the white light-emitting diode can be determined according to the thickness of the yellow quantum dot layer (230). For example, if the thickness of the yellow quantum dot film (230) is thinner than the critical thickness, the color temperature may be greater than 5850K, and if the thickness of the yellow quantum dot film (230) is thicker than the critical thickness, the color temperature may be less than 5850K.

[0083] A multilayer quantum dot color conversion layer according to one embodiment of the present invention may further include a PMMA protective layer formed on each of the green quantum dot layer (220, 240), yellow quantum dot layer (230), and red quantum dot layer (250) to maintain separation between each quantum dot layer.

[0084] FIG. 3 illustrates the absorption and emission spectra and TEM images of quantum dots (QDs) according to an embodiment of the present invention. Quantum dots fabricated according to an embodiment of the present invention are applied to the color conversion layer of a white light-emitting diode and may have red, yellow, and green emission characteristics. Each quantum dot was fabricated using a colloidal synthesis method, and the emission wavelengths were adjusted to 619 nm for red quantum dots, 580 nm for yellow quantum dots, and 525 nm for green quantum dots. These emission characteristics were confirmed through the measurement of absorption spectra and photoluminescence (PL) spectra for each quantum dot in FIG. 3 (a) to (c).

[0085] The size and structure of the quantum dots were analyzed using transmission electron microscopy (TEM) as illustrated in Figures 3 (d) to (f). Specifically, Figure 4 is a graph showing the particle size distribution of red, yellow, and green quantum dots in a multilayer quantum dot color conversion layer according to an embodiment of the present invention. Referring to Figures 3 and 4, the average particle size of the red quantum dots was measured to be 18.5 ± 1.78 nm, the yellow quantum dots 13.6 ± 1.62 nm, and the green quantum dots 14.5 ± 1.51 nm, and they have a thick shell structure. This thick shell structure improves the chemical stability of the quantum dots and serves to protect the core from external environments such as oxidation and moisture.

[0086] The crystal structure of quantum dots can be analyzed through X-ray diffraction (XRD). Figure 5 shows the X-ray diffraction (XRD) results graph of red, yellow, and green quantum dots of a multilayer quantum dot color conversion layer according to one embodiment of the present invention. As a result of the XRD analysis, strong reflection peaks appeared between the positions of bulk CdSe and ZnS crystals in all three types of quantum dots, indicating that successful alloying occurred at the core-shell interface. For comparison, XRD data of bulk wurtzite-type structured CdSe (wurtzite CdSe) and ZnS are shown in Figure 5 for reference.

[0087] Furthermore, FIG. 6 illustrates the Fourier Transform Infrared Spectroscopy (FT-IR) spectra of red, yellow, and green quantum dots of a multilayer quantum dot color conversion layer according to an embodiment of the present invention. The surface stability of the quantum dots can be analyzed through Fourier Transform Infrared Spectroscopy (FT-IR). In the FT-IR analysis of FIG. 6, the functional groups of the oleic acid (OA) ligand used in the synthesis process were identified. In particular, the C=O bonds detected at 1700–1725 cm¹ and the CH bonds detected at 2840–3000 cm¹ indicate ligands successfully bound to the quantum dot surface.

[0088] FIG. 7 is a diagram illustrating the process of transfer printing a single-layer quantum dot layer according to an embodiment of the present invention. With reference to FIG. 7, the process of fabricating a multilayer quantum dot color conversion layer through a transfer printing process to suppress energy transfer between particles and generate uniform white light is described in detail.

[0089] First, a glass substrate (710) used as a donor substrate can be surface-treated with ODTS (Octadecyltrichlorosilane). ODTS weakens the adhesion between the donor substrate and the quantum dot film, allowing the quantum dot film to be easily separated during the subsequent transfer process. A single-color quantum dot film can be formed by coating a single-color quantum dot solution (red, yellow, green) onto the prepared donor substrate using a spin coating method. At this time, the thickness of the quantum dot film can be precisely controlled by adjusting the spin coating speed. FIG. 8 is an AFM image of a yellow quantum dot layer according to an embodiment of the present invention. The surface characteristics of the spin-coated quantum dot layer can be confirmed through the AFM image. Referring to FIG. 8, the surface roughness of the yellow quantum dot layer was measured to be 2.22 nm. This surface roughness is consistent with the characteristics of a high-performance QD-LED.

[0090] In one embodiment, a protective layer of PMMA (Polymethyl Methacrylate) may be additionally coated to ensure the physical stability and protection of the quantum dot film. PMMA protects the film from the external environment and prevents damage during the transfer process. The prepared single-color quantum dot film is then separated from the donor substrate using a thermal release tape (730). The thermal release tape has the property that its adhesive strength weakens when heat is applied, thereby allowing the film to be easily separated.

[0091] The separated quantum dot film is transferred and stacked onto a target substrate. This process is repeated in the order of green (720), yellow (750), green (760), and red (770) to form a multilayer structure, and this multilayer structure can contribute to suppressing energy transfer between particles by physically separating the quantum dot layers of each color. The formed multilayer quantum dot color conversion layer structure can finally be aligned on a blue LED with a peak emission wavelength of 450 nm.

[0092] FIG. 9 is a diagram comparing the characteristics of a white light-emitting diode using a multilayer quantum dot color conversion layer according to an embodiment of the present invention and a conventional white light-emitting diode using a mixed color conversion layer. Referring to FIG. 9(a), compared to a mixed quantum dot color conversion layer, the multilayer quantum dot color conversion layer of the present invention allows quantum dots of each color to emit light independently and provides a uniform spectrum. In a mixed quantum dot color conversion layer, an imbalance in light emission intensity occurs as quantum dots of a specific color transfer energy to quantum dots of another color, but in a multilayer structure, such energy transfer is suppressed so that the light emission characteristics of each layer can be maintained.

[0093] FIG. 9(b) illustrates the emission spectra of a white LED using a mixed quantum dot color conversion layer and a multilayer quantum dot color conversion layer of the present invention, respectively. The LED using the multilayer quantum dot color conversion layer exhibits emission peaks corresponding to red, yellow, and green emitting quantum dots. On the other hand, the LED using the mixed quantum dot color conversion layer shows that emission in the red wavelength region is more dominant than in the green and yellow regions.

[0094] As shown in Figures 9(c) and (d), a comparison of the characteristics of the multilayer quantum dot color conversion layer and the mixed quantum dot color conversion layer through time-resolved photoluminescence (TRPL) analysis revealed that the radiative recombination characteristics in the multilayer structure were similar to those of a monochromatic quantum dot film. This indicates that physical separation between quantum dots occurred in the multilayer structure. On the other hand, in the mixed structure, as shown in the lifetime analysis results of the TRPL data in Figures 9(e) and (f), the recombination lifetime of the green quantum dots decreased and the lifetime of the red quantum dots increased. In other words, an energy transition phenomenon was observed from quantum dots with a large band gap to quantum dots with a small band gap.

[0095] The method for fabricating a multilayer quantum dot color conversion layer according to the present invention enables the production of white LEDs that provide a high color rendering index (CRI) and high luminous efficiency by suppressing energy transfer and realizing uniform luminescence characteristics. Furthermore, by utilizing spin coating and transfer printing processes, the film thickness and stacking order can be precisely controlled, which is advantageous for adjusting color temperature (CCT) and optical properties. Through this, the present invention enables the realization of high-quality lighting and displays in white LED applications.

[0096] FIG. 10 illustrates the light emission quality of a white light-emitting diode according to the configuration of a multilayer quantum dot color conversion layer according to an embodiment of the present invention. FIG. 6 analyzes the effect of the configuration of the multilayer quantum dot color conversion layer on the light emission quality and color rendering index (CRI) of the white light-emitting diode. Here, the Color Rendering Index (CRI) is an indicator representing how accurately a light source can reproduce the color of an object relative to natural light; the closer it is to 100, the better the color reproduction, and generally, a value of 80 or higher is considered high quality. FIG. 10 (a), (d), and (g) illustrate a multilayer color conversion layer composed of a monochromatic quantum dot layer, a dichromatic quantum dot layer, and a trichromatic quantum dot layer. FIG. 10 (b), (c), (e), (f), (h), and (i) evaluate in detail the emission spectrum, color coordinates, color rendering index, and optical characteristics according to each configuration. Furthermore, FIG. 11 is a graph of the thickness of each quantum dot layer of FIG. 10 (a), (d), and (g). Specifically, FIG. 11(a) shows the thickness of the yellow quantum dot layer, which is the monochromatic quantum dot layer of FIG. 10(a). FIG. 11(b) shows the thickness of the green quantum dot layer and the red quantum dot layer of the dichromatic quantum dot layer of FIG. 10(d). FIG. 11(c) shows the thickness of the red quantum dot layer, the yellow quantum dot layer, and the green quantum dot layer of the trichromatic quantum dot layer of FIG. 10(g).

[0097] First, the color conversion layer, which consists of a single-color quantum dot layer, is composed of a yellow quantum dot layer and generates white light by converting some of the light from the blue LED into yellow light. While this method has the advantage of a simple manufacturing process, the yellow quantum dot layer alone cannot sufficiently cover the red and green spectra. Consequently, the generated white light appears as CIE color coordinates (0.33, 0.28), deviating from the ideal white light target value (0.33, 0.33). Here, CIE color coordinates are a color coordinate system defined by the International Commission on Illumination (CIE), which is a system for numerically expressing the color of a specific light source, and (0.33, 0.33) represents the color coordinates of ideal white light. Furthermore, the Color Rendering Index (CRI) is low at 57, indicating a limitation in terms of color accuracy and vibrancy. In other words, a single-color quantum dot layer alone cannot cover the entire visible light spectrum in a balanced manner.

[0098] The color conversion layer, composed of a dichromatic quantum dot layer, consists of a green quantum dot layer and a red quantum dot layer, which are stacked sequentially to generate white light. This method provides wider spectral coverage than a monochromatic color conversion layer, and the CIE color coordinates of the generated white light are (0.34, 0.36), which is an improvement over the monochromatic color conversion layer. However, the Color Rendering Index (CRI) is 60, which still falls short of the industry standard of 80, and the reproduction of intermediate colors such as cyan and yellow is limited due to a lack of spectrum between red and green. Consequently, it exhibits limitations in that the vividness of color representation is reduced.

[0099] The color conversion layer, composed of three-color quantum dot layers, includes red, yellow, and green quantum dot layers and can be fabricated by stacking them in the order of green -> yellow -> green -> red. This method provides broad spectral coverage, and the physical separation of each color quantum dot layer suppresses energy transitions (Fφrster resonance energy transition, FRET). Here, FRET is a non-radiative energy transition phenomenon between quantum dots with different band gaps, which can cause a decrease in luminous efficiency and distortion of the luminescence intensity of specific colors. Additionally, the PMMA protective layer improves the yield of the transfer process and provides an additional energy transition suppression effect by maintaining physical separation between each quantum dot layer. The three-color color conversion layer achieves ideal white light color coordinates (0.33, 0.33) and has a Color Rendering Index (CRI) of 91, enabling natural and vivid color reproduction.

[0100] FIG. 12 is a graph comparing various stacked embodiments of tricolor quantum dot layers. FIG. 12(a) illustrates a white LED emission spectrum including a tricolor quantum dot color conversion layer having a quantum dot layer structure of 6.0 μm red, 5.0 μm green, 10.1 μm yellow, and 4.9 μm, as in one embodiment of the present invention, and a quantum dot layer structure of 6.0 μm red, 10.1 μm yellow, and 4.9 μm green. FIG. 12(b) illustrates a white LED emission spectrum including a tricolor quantum dot color conversion layer having a quantum dot layer structure of 6.0 μm red, 5.0 μm green, 10.1 μm yellow, and 4.9 μm green, as in one embodiment of the present invention, and a quantum dot layer structure of 6.0 μm red, 10.1 μm yellow, 4.9 μm green, and 5.0 μm green. FIG. 12(c) shows CIE color coordinates measured in a white LED containing a tricolor quantum dot color conversion layer having different structures. Referring to FIG. 12(a) to (c), it can be seen that the aforementioned red, green, yellow, and green quantum dot layer structure of the present invention improves conversion efficiency and spectral balance compared to a tricolor quantum dot layer structure stacked in a different order.

[0101] FIG. 13 is a diagram illustrating the color temperature of a white light-emitting diode according to a change in the thickness of the yellow quantum dot layer among a multilayer quantum dot color conversion layer according to an embodiment of the present invention. Correlated Color Temperature (CCT) is an indicator representing the color of a light source, measured in Kelvin (K), where a low value indicates a warm hue (yellow / red) and a high value indicates a cool hue (blue). Color temperature is an important factor in lighting applications and has a significant impact on the atmosphere of a space and the psychological comfort of the user. In the present invention, by adjusting the thickness of the yellow quantum dot layer, the color temperature can be precisely controlled, and high-quality white light suitable for various lighting environments can be realized.

[0102] The tricolor multilayer quantum dot color conversion layer according to the present invention is composed of red, yellow, and green quantum dot layers and is stacked in the order of green -> yellow -> green -> red. This structure maintains color coordinates within the Planck locus and generates ideal white light. The Planck locus is a path representing the color of light emitted by a black body radiator according to temperature, and ideal white light is located on this locus. In particular, by adjusting the thickness of the yellow quantum dot layer, the relative ratio of blue light to yellow light can be changed, thereby allowing for precise control of the color temperature. This method is very useful in lighting and display applications as it allows for color temperature control without additional complex processes.

[0103] Specifically, the color temperature depends on the thickness of the yellow quantum dot layer. As the thickness of the yellow quantum dot layer increases, more blue light is converted into yellow light, resulting in a lower color temperature and the production of warm white light. Conversely, as the thickness of the yellow quantum dot layer decreases, the proportion of blue light increases, the color temperature rises, and cool white light is produced.

[0104] When the yellow quantum dot layer is at the standard thickness, the CIE color coordinates of the white light are (0.33, 0.33), and the color temperature is 5850 K. This state represents neutral white light and is suitable for various applications. When the thickness of the yellow quantum dot layer is reduced, the CIE color coordinates of the white light shift to (0.31, 0.32), and the color temperature reaches 6660 K. This cool white light is suitable for high-contrast lighting environments, such as offices or commercial spaces. When the thickness of the yellow quantum dot layer is increased, the CIE color coordinates of the white light shift to (0.37, 0.38), and the color temperature decreases to 4260 K. This warm white light provides a comfortable and cozy atmosphere and is suitable for applications in residential or relaxation spaces.

[0105] Accordingly, the method for fabricating a multilayer quantum dot color conversion layer according to the present invention can precisely control the color temperature of a white LED containing a multilayer quantum dot color conversion layer without additional process changes by adjusting the thickness of the yellow quantum dot layer. In addition, the color temperature range can be extended from 4260 K (warm white) to 6660 K (cool white) to meet various lighting requirements. This color temperature control function enables adaptive lighting in various application environments, such as offices, commercial spaces, and residential spaces. Furthermore, the tricolor multilayer quantum dot color conversion layer maintains color coordinates within the Planck locus and can provide natural and vivid white light.

[0106] The multilayer quantum dot color conversion layer of the present invention is laminated through a transfer printing process, which can effectively solve the problem of efficiency degradation caused by non-uniform thickness and energy transfer between particles that occurs in conventional technology. By combining blue LEDs to realize a balanced spectrum, the multilayer quantum dot color conversion layer of the present invention can generate high-quality white light that approaches the International Commission on Illumination (CIE) color coordinates (0.33, 0.33) and can provide excellent color quality with a Color Rendering Index (CRI) of 90 or higher.

[0107] Furthermore, the multilayer quantum dot color conversion layer of the present invention can adjust the color temperature (CCT) of the white light containing the multilayer quantum dot color conversion layer from 4260K to 6660K by adjusting the thickness of the yellow quantum dot layer, thereby providing white light suitable for various application environments such as household lighting, commercial lighting, and industrial lighting.

[0108]

[0109] Although the embodiments have been described above with reference to the limited drawings, those skilled in the art can make various modifications and variations from the description above. For example, suitable results can be achieved even if the described techniques are performed in a different order than described, and / or the components of the described system, structure, device, circuit, etc. are combined or assembled in a form different from described, or replaced or substituted by other components or equivalents.

[0110] Therefore, other implementations, other embodiments, and equivalents to the claims also fall within the scope of the claims set forth below.

Claims

As a method for fabricating a multilayer quantum dot color conversion layer, A step of preparing a green quantum dot (QD) solution, a yellow quantum dot solution, and a red quantum dot solution, respectively, using a colloidal synthesis method; A step of spin-coating the green quantum dot solution, the yellow quantum dot solution, and the red quantum dot solution onto a donor substrate treated with ODTS (Octadecyltrichlorosilane) to form a green quantum dot film, a yellow quantum dot film, and a red quantum dot film, respectively; and A step of sequentially transferring the green quantum dot film, the yellow quantum dot film, and the red quantum dot film, respectively, from the donor substrate to the target substrate using a heat dissipation tape to form a stacked multilayer quantum dot color conversion layer. A method for fabricating a multilayer quantum dot color conversion layer including In paragraph 1, The step of forming each of the above green quantum dot film, the above yellow quantum dot film, and the above red quantum dot film is A step of controlling the speed of the spin coating to control the thickness of each quantum dot film A method for fabricating a multilayer quantum dot color conversion layer, further comprising In paragraph 1, The step of forming each of the above green quantum dot film, the above yellow quantum dot film, and the above red quantum dot film is Step of forming the thickness of the yellow quantum dot film to a thickness of the yellow quantum dot film capable of expressing a preset color temperature of a white light-emitting diode including the multilayer quantum dot color conversion layer A method for fabricating a multilayer quantum dot color conversion layer, further comprising In paragraph 1, The step of forming a stacked multilayer quantum dot color conversion layer by sequentially transferring the green quantum dot film, the yellow quantum dot film, and the red quantum dot film, respectively, from the donor substrate to the target substrate using the heat-emitting tape, is A step of transferring the green quantum dot film onto the target substrate to form a first green quantum dot layer; A step of forming a yellow quantum dot layer by transferring a yellow quantum dot film onto the first green quantum dot layer; A step of forming a second green quantum dot layer by transferring the green quantum dot film onto the yellow quantum dot layer; and Step of forming a red quantum dot layer by transferring a red quantum dot film onto the second green quantum dot layer. A method for fabricating a multilayer quantum dot color conversion layer, further comprising In paragraph 1, The above method for fabricating a multilayer quantum dot color conversion layer is, Before transferring each of the green quantum dot film, the yellow quantum dot film, and the red quantum dot film from the donor substrate to the target substrate, a step of forming a PMMA (polymethyl methacrylate) protective layer on each of the green quantum dot film, the yellow quantum dot film, and the red quantum dot film to maintain separation between each quantum dot film in the multilayer quantum dot color conversion layer. A method for fabricating a multilayer quantum dot color conversion layer including further It includes a green quantum dot layer, a yellow quantum dot layer, and a red quantum dot layer, A multilayer quantum dot color conversion layer characterized in that the green quantum dot layer, the yellow quantum dot layer, and the red quantum dot layer are sequentially stacked through a transfer printing process. In paragraph 6, The above-mentioned multilayer quantum dot color conversion layer is, A first green quantum dot layer placed on the above blue LED; A yellow quantum dot layer disposed on the first green quantum dot layer above; A second green quantum dot layer disposed on the above yellow quantum dot layer; and A red quantum dot layer disposed on top of the second green quantum dot layer. A multilayer quantum dot color conversion layer comprising In paragraph 6, The above yellow quantum dot layer is, A multilayer quantum dot color conversion layer formed with the thickness of the yellow quantum dot layer according to a preset color temperature of a white light-emitting diode including the multilayer quantum dot color conversion layer. In paragraph 6, The above-mentioned multilayer quantum dot color conversion layer is, PMMA protective layers formed respectively on the green quantum dot layer, the yellow quantum dot layer, and the red quantum dot layer to maintain separation between each quantum dot layer. A multilayer quantum dot color conversion layer further comprising

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