Patent Application
20220149338
The present invention relates to a light-emitting device that is provided with a light-emitting element including quantum dots, and a method for manufacturing the light-emitting device.
PTL 1 discloses a semiconductor nanoparticle (quantum dot) having a core/shell structure and a ligand that coordinates with the semiconductor nanoparticle.
The inventors have discovered an approach for further improving luminous efficiency of a light-emitting device in comparison with the light-emitting device provided with a quantum dot layer in which conventional quantum dots as disclosed in PTL 1 are simply layered.
In order to solve the above problem, a method for manufacturing a light-emitting device of the present invention is a method for manufacturing a light-emitting device that is provided with, on a substrate, a light-emitting element including a first electrode, a second electrode, and a quantum dot layer interposed between the first electrode and the second electrode, the method including forming the quantum dot layer. The forming the quantum dot layer includes: performing first application that involves applying a first solution on a position overlapping with the substrate; performing first heating, subsequent to the performing first application, that involves raising an atmospheric temperature around the substrate to a temperature equal to or higher than a first temperature; and performing second heating, subsequent to the performing first heating, that involves raising the atmospheric temperature to a second temperature. The first solution contains a first solvent, a plurality of quantum dots, a ligand to coordinate with each of the plurality of quantum dots, and a first inorganic precursor. The quantum dot includes a core and a first shell with which the core is coated. The first temperature is the higher temperature of a melting point of the ligand and a boiling point of the first solvent. The second temperature is a temperature which is higher than the first temperature, and at which the first inorganic precursor epitaxially grows around the first shell to form a second shell with which the first shell is coated. At least one set of the quantum dots adjacent to each other is connected to each other via the second shell in the performing second heating.
In order to solve the problem described above, a method for manufacturing a light-emitting device of the present invention is a light-emitting device that is provided with, on a substrate, a light-emitting element including a first electrode, a second electrode, and a quantum dot layer interposed between the first electrode and the second electrode. The quantum dot layer includes a quantum dot structure. The quantum dot structure includes a quantum dot including a core and a first shell with which the core is coated, and a second shell with which the first shell is coated. The first shell and the second shell have a crystal structure. At least one set of the quantum dots adjacent to each other is connected to each other via the crystal structure of the second shell.
According to the configurations described above, luminous efficiency may be further improved in a light-emitting device provided with quantum dots.
(a) of
As illustrated in (a) of
At a position overlapping with the light-emitting face DS in plan view, as illustrated in (b) of
The light-emitting element layer 2 includes, on a first electrode 4, a first charge transport layer 6, a light-emitting layer 8 as a quantum dot layer, a second charge transport layer 10, and a second electrode 12, which are sequentially layered from the lower layer. The first electrode 4 of the light-emitting element layer 2 formed in the upper layer of the array substrate 3 is electrically connected to the TFT of the array substrate 3. In the present embodiment, the first electrode 4 is an anode electrode and the second electrode 12 is a cathode electrode, for example.
In the present embodiment, the light-emitting element layer 2 includes a first light-emitting element 2R, a second light-emitting element 2G, and a third light-emitting element 2B. The first light-emitting element 2R, the second light-emitting element 2G, and the third light-emitting element 2B are QLED elements in which the light-emitting layer 8 includes a semiconductor nanoparticle material, that is, a quantum dot material.
Each of the first electrode 4, the first charge transport layer 6, and the light-emitting layer 8 is separated by edge covers 14. In particular, in the present embodiment, the first electrode 4 is, by the edge covers 14, separated into a first electrode 4R for the first light-emitting element 2R, a first electrode 4G for the second light-emitting element 2G, and a first electrode 4B for the third light-emitting element 2B. The first charge transport layer 6 is, by the edge covers 14, separated into a first charge transport layer 6R for the first light-emitting element 2R, a first charge transport layer 6G for the second light-emitting element 2G, and a first charge transport layer 6B for the third light-emitting element 2B. Further, the light-emitting layer 8 is, by the edge covers 14, separated into a first light-emitting layer 8R, the second light-emitting layer 8G, and a third light-emitting layer 8B.
The second charge transport layer 10 and the second electrode 12 are not separated by the edge covers 14, and are each formed in a shared manner. As illustrated in (b) of
In the present embodiment, the first light-emitting element 2R includes the first electrode 4R, the first charge transport layer 6R, the first light-emitting layer 8R, the second charge transport layer 10, and the second electrode 12. The second light-emitting element 2G includes the first electrode 4G, the first charge transport layer 6G, the second light-emitting layer 8G, the second charge transport layer 10, and the second electrode 12. Furthermore, the third light-emitting element 2B includes the first electrode 4B, the first charge transport layer 6B, the third light-emitting layer 8B, the second charge transport layer 10, and the second electrode 12.
In the present embodiment, the first light-emitting layer 8R, the second light-emitting layer 8G, and the third light-emitting layer 8B emit red light that is light of a first color, green light that is light of a second color, and blue light that is light of a third color, respectively. In other words, the first light-emitting element 2R, the second light-emitting element 2G, and the third light-emitting element 2B are light-emitting elements that emit the red light, the green light, and the blue light, respectively, which are different colors from each other.
Here, the blue light is, for example, light having a light emission center wavelength in a wavelength band of 400 nm or more and 500 nm or less. Further, the green light is, for example, light having a light emission center wavelength in a wavelength band of greater than 500 nm and 600 nm or less. Further, the red light is, for example, light having a light emission center wavelength in a wavelength band of greater than 600 nm and 780 nm or less.
The first electrode 4 and the second electrode 12 include conductive materials and are electrically connected to the first charge transport layer 6 and the second charge transport layer 10, respectively. Of the first electrode 4 and the second electrode 12, the electrode closer to the light-emitting face DS is a transparent electrode.
In particular, in the present embodiment, the array substrate 3 is a transparent substrate, and the first electrode 4 is a transparent electrode. The second electrode 12 may be a reflective electrode. Therefore, light from the light-emitting layer 8 passes through the first charge transport layer 6, the first electrode 4, and the array substrate 3, and is emitted from the light-emitting face DS to the outside of the light-emitting device 1. Due to this, the light-emitting device 1 is configured as a bottom-emitting type light-emitting device. Since both the light emitted in the upward direction from the light-emitting layer 8 and the light emitted in the downward direction from the light-emitting layer 8 are available as light emission from the light-emitting device 1, the light-emitting device 1 can improve the usage efficiency of the light emitted from the light-emitting layer 8.
Note that the configuration of the first electrode 4 and the second electrode 12 described above is an example, and may be configured with other materials.
The first charge transport layer 6 is a layer that transports charges from the first electrode 4 to the light-emitting layer 8. The first charge transport layer 6 may have a function of inhibiting the transport of charges from the second electrode 12. In the present embodiment, the first charge transport layer 6 may be a hole transport layer that transports positive holes from the first electrode 4, which is an anode electrode, to the light-emitting layer 8.
The second charge transport layer 10 is a layer that transports the charge from the second electrode 12 to the light-emitting layer 8. The second charge transport layer 10 may have a function of inhibiting the transport of the charges from the first electrode 4. In the present embodiment, the second charge transport layer 10 may be an electron transport layer that transports electrons from the second electrode 12, which is a cathode electrode, to the light-emitting layer 8.
Next, the configuration of the light-emitting layer 8 will be described in detail with reference to (c) of
In the present embodiment, the light-emitting layer 8 includes a quantum dot structure 16 and a ligand 18. The quantum dot structure 16 includes a plurality of quantum dots 20. The quantum dot 20 has a core/shell structure including a core 22 and a first shell 24, with which the periphery of the core 22 is coated. The quantum dot structure 16 also includes a second shell 26. The periphery of the first shell 24, which is an outer shell of each quantum dot 20, is coated with the second shell 26.
The quantum dot 20 may have a multi-shell structure in which a plurality of shells are provided around the core 22. In this case, the first shell 24 refers to a shell corresponding to the outermost layer among the plurality of shells.
The ligand 18 may coordinate with the quantum dot structure 16 on an outer surface of the second shell 26 to fill a void in the quantum dot structure 16. The ligand 18 may be, for example, trioctylphosphine oxide (TOPO).
Among the quantum dots 20, at least one set of quantum dots 20 adjacent to each other is connected to each other via the second shell 26. The first shell 24 and the second shell 26 have a crystal structure, and in particular, in the present embodiment, the second shell 26 has a crystal structure formed by epitaxial growth on the first shell 24. Thus, the adjacent quantum dots 20 described above are connected to each other by the crystal structure of the second shell 26. In the present embodiment, all the quantum dots 20 within an identical light-emitting element may be connected to each other by the crystal structure of the second shell 26 to form the integral quantum dot structure 16. The first shell 24 and the second shell 26 may be polycrystalline.
The core 22 and first shell 24 of the quantum dot 20 may include an inorganic material used for the quantum dots of a known core/shell structure. In other words, the first light-emitting layer 8R, the second light-emitting layer 8G, and the third light-emitting layer 8B may include known quantum dot materials used for light-emitting layers of red, green, and blue QLED elements, respectively.
In addition, similar to the first shell 24, the second shell 26 may include an inorganic shell material used for the quantum dots of a known core/shell structure. The first shell 24 and the second shell 26 may be made of the same material. The specific resistance of the second shell 26 is preferably greater than or equal to the specific resistance of the first shell 24. The magnitude of the band gap of the second shell 26 is preferably greater than or equal to the magnitude of the band gap of the first shell 24. With this configuration, the efficiency of charge injection from the second shell 26 to the first shell 24 is improved.
Examples of specific materials for the core 22 include group II-VI semiconductors such as CdSe (band gap 1.73 eV), CdTe (band gap 1.44 eV), ZnTe (band gap 2.25 eV), and CdS (band gap 2.42 eV). Examples of other specific materials for the core 22 include group III-V semiconductors such as InP (band gap 1.35 eV) and InGaP (band gap 1.88 eV).
In general, the wavelength emitted by the quantum dot is determined by the particle diameter of the core. Therefore, it is preferable to employ a semiconductor material having an appropriate band gap as a material of the core 22 in order to control the light emitted from the core 22 to be any of red, green, and blue colors, by controlling the particle diameter of the core 22.
The band gap of the material of the core 22 included in the first light-emitting layer 8R is preferably equal to or lower than 1.97 eV in order for the first light-emitting layer 8R serving as a red light-emitting layer to emit red light having a wavelength of 630 nm. In order for the second light-emitting layer 8G serving as a green light-emitting layer to emit green light having a wavelength of 532 nm, the band gap of the material of the core 22 included in the second light-emitting layer 8G is preferably equal to or lower than 2.33 eV. Furthermore, in order for the third light-emitting layer 8B serving as a blue light-emitting layer to emit blue light having a wavelength of 630 nm, the band gap of the material of the core 22 included in the third light-emitting layer 8B is preferably equal to or lower than 2.66 eV. The light-emitting device 1 provided with the first light-emitting layer 8R, the second light-emitting layer 8G, and the third light-emitting layer 8B is preferable from the perspective of satisfying the color space criteria in the International Standard BT 2020 of UHDTV.
Examples of specific materials for the first shell 24 and the second shell 26 include group II-VI semiconductors such as ZnSe (band gap 2.7 eV) and ZnS (band gap 3.6 eV). Further, examples of specific materials for the first shell 24 and the second shell 26 include group III-V semiconductors such as GaP (band gap 2.26 eV).
The material of the core 22 preferably has low specific resistance and a small band gap compared to the material of the first shell 24 and the second shell 26. With this configuration, the efficiency of charge injection from the first shell 24 and second shell 26 to the core 22 is improved.
In the present embodiment, the average film thickness of the first shell 24 from the outer surface of the core 22 is less than a minimum film thickness of the second shell 26. The minimum film thickness of the second shell 26 refers to the smallest film thickness of the film thickness of the second shell 26 between two quantum dots 20 connected to each other via the second shell 26 and the film thickness from the first shell 24 to the outer surface of the second shell 26.
As illustrated in (c) of
Next, a method for manufacturing the light-emitting device 1 according to the present embodiment will be described with reference to
First, an array substrate is formed (step S1). Formation of the array substrate may be performed by forming a plurality of TFTs on the substrate to match positions of the subpixels.
Next, the first electrode 4 is formed (step S2). In step S2, for example, after a transparent electrode material having electrical conductivity, such as ITO, is film-formed by sputtering, the first electrode 4 may be formed for each subpixel by patterning while matching a shape of the subpixel. Alternatively, the first electrode may be formed for each subpixel by vapor-depositing a transparent electrode material by using a vapor deposition mask.
Next, the edge covers 14 are formed (step S3). The edge covers 14, after being applied on the array substrate 3 and the first electrode 4, may be obtained by patterning while leaving the positions covering the side surfaces and peripheral end portions of the first electrodes 4 between the adjacent first electrodes 4. The patterning of the edge covers 14 may be performed by photolithography.
Next, the first charge transport layer 6 is formed (step S4). The first charge transport layer 6 may be formed for each subpixel by separately patterning with an ink-jet method, vapor deposition using a mask, or patterning using photolithography.
Next, the light-emitting layer 8 is formed (step S5). The step of forming the light-emitting layer 8 will be described in more detail with reference to
As illustrated in (a) of
The first solution 28 is a solution in which the plurality of quantum dots 20 with the ligand 18 being coordinated and a first inorganic precursor 30 are dispersed in a first solvent 32, as illustrated in (b) of
The step of the first application is performed at an atmospheric temperature of a temperature T0, as illustrated in
Next, the array substrate 3, on which the first solution 28 is applied, is set in a furnace or the like, and heating of the atmosphere is started. Here, a step of first heating is performed by heating the atmosphere until the atmospheric temperature becomes equal to or higher than a first temperature T1 indicated in
The first temperature T1 is the higher temperature of the melting point of the ligand 18 and the boiling point of the first solvent 32. A temperature TA indicated in
The melting point of TOPO is in a range from 50 degrees Celsius to 54 degrees Celsius, and the boiling point of hexane is in a range from 68.5 degrees Celsius to 69.1 degrees Celsius. Accordingly, in a case where the ligand 18 is TOPO and the first solvent is hexane, the temperature TA is the melting point of the TOPO, and the first temperature T1 is the boiling point of the hexane.
The ambient temperature of the quantum dots 20 follows a rise of the atmospheric temperature, as depicted in
By further carrying on the heating of the atmosphere, one of the melting of the ligand 18 and the evaporation of the first solvent 32 ends, and the ambient temperature of the quantum dots 20 begins to rise again. Then, when the ambient temperature of the quantum dots 20 rises up to the first temperature T1 and the other one of the melting of the ligand 18 and the evaporation of the first solvent 32 begins, the ambient temperature of the quantum dots 20 maintains the first temperature T1 for a while.
Thus, the melting of the ligand 18 and the evaporation of the first solvent 32 are completed by the step of the first heating. When the first temperature T1 is the boiling point of the first solvent 32, in the step of the first heating, the first solvent 32 vaporizes after the ligand 18 has melted. On the other hand, when the first temperature T1 is the melting point of the ligand 18, in the step of the first heating, the ligand 18 melts after the first solvent 32 has vaporized.
In a case where the melting of the ligand 18 is earlier than the vaporization of the first solvent 32, immediately after the vaporization of the first solvent 32, aggregate of the quantum dots 20, around which the solid ligand 18 is attached, is formed in an upper layer relative to the first charge transport layer 6. The aggregate is unstable as a film, which makes it difficult for the inorganic precursor 30 to be present in some case. Accordingly, in the step of the first heating, from the perspective of forming a stable film including the quantum dots 20 and the inorganic precursor 30, it is preferable that the first solvent 32 vaporize after the melting of the ligand 18.
After the completion of the step of the first heating, as illustrated in (a) of
Subsequently, the heating of the atmosphere is continued until the atmospheric temperature reaches a second temperature T2 indicated in
After the completion of the melting of the ligand 18 and the evaporation of the first solvent 32, the ambient temperature of the quantum dots 20 rises from the first temperature T1 and reaches the second temperature T2. Since the atmospheric temperature is maintained at the second temperature T2, the ambient temperature of the quantum dots 20 having reached the second temperature T2 is also maintained at the second temperature T2.
The second temperature T2 is higher than the first temperature T1, and is a temperature at which the first inorganic precursor 30 epitaxially grows around the first shell 24 by thermochemical reaction. Thus, while the ambient temperature of the quantum dots 20 is maintained at the second temperature T2, the first inorganic precursor 30 gradually grows epitaxially around the first shell 24. With this, the second shell 26 is formed around the first shell 24 of each quantum dot 20, as illustrated in (b) of
The second shell 26 is formed in such a manner that the film thickness gradually increases from the outer surface of the first shell 24 around each quantum dot 20. At a position where the sum of the film thicknesses of the second shells 26 formed around two quantum dots 20 adjacent to each other is greater than a distance between the first shells 24 of the quantum dots 20 adjacent to each other, the two quantum dots 20 are connected via the second shells 26. In the present embodiment, the step of the second heating is performed until at least one set of adjacent quantum dots 20 is connected to each other via the second shells 26.
As described above, as illustrated in (b) of
Note that in the present embodiment, the step of forming the light-emitting layer 8 is described with reference to the enlarged cross-sectional view of the periphery of the second light-emitting layer 8G. However, a difference in the forming method of each of the first light-emitting layer 8R, second light-emitting layer 8G, and third light-emitting layer 8B is only a difference in the materials contained in the first solution 28. That is, regardless of luminescent colors of the light-emitting layer 8 to be formed, the steps of the first application, the first heating, and the second heating may be implemented by the same method.
In the step of the first application, the material contained in the first solution 28 may be changed for each luminescent color of the corresponding light-emitting element, the first solution 28 may be subjected to separately patterning by an ink-jet method, and then the steps of the first heating and second heating described above may be performed. As a result, the light-emitting elements having mutually different luminescent colors can be formed by continuous single heating.
Next, the second charge transport layer 10 is formed (step S6). The second charge transport layer 10 may be applied and formed in common to all of the subpixels by a spin coat technique or the like.
Finally, the second electrode 12 is formed (step S7). The second electrode 12 may be film-formed in common to all of the subpixels by vapor deposition or the like. As described above, the light-emitting element layer 2 is formed on the array substrate 3, and the light-emitting device 1 illustrated in
In the method for manufacturing the light-emitting device 1 in the present embodiment, the quantum dots 20 having a core/shell structure are applied, and thereafter the second shell 26 is epitaxially grown around the first shell 24 of each quantum dot 20. Because of this, the film thickness of the shell of each quantum dot 20 can be increased compared to the case where the quantum dots 20 having a core/shell structure are simply layered.
For example, in the quantum dot having a core/shell structure, it is considered to thicken the film thickness of the shell in order to reduce the exudation of electrons injected into the core of the quantum dot. However, when the quantum dots are formed by layering quantum dots having a thick film thickness of the shell, the packing ratio of the quantum dots with respect to the volume of the light-emitting layer becomes low. This makes it difficult to achieve a sufficient density of the quantum dots in the light-emitting layer, leading to a reduction in luminous efficiency of the light-emitting element.
In the method for manufacturing the light-emitting device 1 in the present embodiment, the quantum dots 20 each provided with the thin first shell 24 are applied, and thereafter the second shell 26 is formed on each quantum dot 20. In the light-emitting layer 8 of the present embodiment, the film thickness of the shell formed around the core 22 can be considered to be the total film thickness of the first shell 24 and the second shell 26.
As a result, the density of the quantum dots 20 in the light-emitting layer 8 can be enhanced compared to the case of simply layering the quantum dots provided with the shells having the same film thickness. Thus, the density of the quantum dots 20 in the light-emitting layer 8 is enhanced while reducing the electron exudation from the quantum dots 20, which leads to an improvement in luminous efficiency of the light-emitting device 1.
In the present embodiment, since at least one set of quantum dots 20 is connected via the second shell 26, an area of the outer surface of the second shell 26 is smaller in the above one set of quantum dots 20 than that in the case of not being connected. That is, in the present embodiment, an area of the outer surface of the quantum dot structure 16 can be reduced compared to the case where the quantum dots are simply layered.
By reducing the area of the outer surface of the quantum dot structure 16, the area of the surface of the second shell 26, through which moisture may infiltrate from the outside, can be reduced. Accordingly, this configuration may reduce damage to the second shell 26 due to the moisture infiltration, and may consequently suppress degradation in a surface protection function of the quantum dot 20 of the second shell 26 due to the damage described above.
When the ligand 18 coordinates on the outer surface of the quantum dot structure 16, the reduction of the area of the outer surface makes it possible to reduce the ligand 18 possible to be damaged by the moisture infiltration. Accordingly, it is possible to reduce the damage to the second shell 26 due to the loss of the protection function by the ligand 18 for the second shell 26.
By reducing the area of the outer surface of the quantum dot structure 16, it is possible to reduce the surface area of the second shell 26 possible to be damaged when the light-emitting device 1 is driven. Thus, the above-discussed configuration may reduce damage to the second shell 26 accompanying the drive of the light-emitting device 1, and may consequently reduce the formation of defects in the second shell 26 due to the damage. As a result, by reducing the area of the outer surface of the quantum dot structure 16, the occurrence of a non-emitting process caused by recombination of electrons and holes in the defects is suppressed, and consequently a decrease in luminous efficiency of the light-emitting device 1 is suppressed.
As described above, because of the outer surface of the quantum dot structure 16 being small, it is possible to reduce the area of the outer surface of the quantum dot structure 16 possible to be damaged, and reduce deactivation of the quantum dots 20 due to damage to the quantum dot structure 16.
According to NPL 1, the average value of a random close packing ratio in the packing of rigid spheres is approximately 63.66 percent. Accordingly, in the present embodiment, the proportion of the volume of the quantum dot structure 16 in the light-emitting layer 8 is preferably greater than or equal to 63.7 percent. With the above configuration, the density of the quantum dots 20 in the light-emitting layer 8 can be enhanced compared to the case of randomly layering quantum dots each provided with a shell whose film thickness is equal to the total film thickness of the first shell 24 and second shell 26. Furthermore, with the configuration described above, it is possible to efficiently decrease the area of the outer surface of the quantum dot structure 16 compared to the case of randomly layering quantum dots.
Conditions required to connect all of the quantum dots 20 in the quantum dot structure 16 via the second shells 26 will now be described.
It is assumed that the quantum dots 20 are arranged in an array of m rows and n columns on a plane. Positions where adjacent quantum dots 20 can be connected, that is, the number of positions between lattice points arranged in the array of m rows and n columns is obtained by an equation of m×(n−1)+n×(m−1)=2 mn−m−n.
It is also assumed that, in a case where all of the quantum dots 20 on the same plane are connected via the second shells 26, the number of sets of mutually connected quantum dots 20 is assumed to be minimal. As one example of this case, an example is cited in which all the sets of adjacent quantum dots are connected in all the in-between positions of the rows, and any one set of adjacent quantum dots is connected to each other in each of all the in-between positions of the columns. In this case, the number of positions where the adjacent quantum dots 20 are connected is obtained by an equation of m×(n−1)+1×(m−1)=mn−1.
Accordingly, in the case of the above-discussed conditions, the proportion of the positions where the quantum dots 20 are actually connected to each other via the second shells 26 to the positions where all the quantum dots 20 can be connected via the second shells 26 is expressed by a relation of (mn−1)/(2 mn−m−n).
The number of quantum dots 20 included in the light-emitting layer 8 of the actual light-emitting device 1 is significantly large, and therefore it is possible to consider that both m and n are sufficiently large. Thus, when m and n positively diverge, the above-mentioned proportion can be derived to be approximately 0.5.
Therefore, in the case where all of the quantum dots 20 on the same plane are connected via the second shells 26, and among all of the sets of adjacent quantum dots 20, the number of sets thereof being connected via the second shells 26 is minimal, these sets may be considered to be approximately 50 percent of all of the sets. Accordingly, in the case where, among all of the sets of adjacent quantum dots 20, the sets thereof connected via the second shells 26 exceeds 50 percent, it can be said that there is a high probability that all the quantum dots 20 in each of the layers being layered are connected via the second shells 26.
In the case where all the quantum dots 20 are connected via the second shells 26, when the quantum dot 20 is assumed to be one atom, it can be assumed that the quantum dot structure 16 forms a crystal structure in which the quantum dots 20 are connected to each other by the second shells 26. The above configuration may more efficiently decrease the area of the outer surface of the quantum dot structure 16. Therefore, in the quantum dot structure 16, it is preferable for the ratio of the adjacent quantum dots 20 being connected to each other by the crystal structure of the second shells 26 to be greater than 50 percent and less than or equal to 100 percent.
In the present embodiment, the average film thickness of the first shell 24 from the outer surface of the core 22 is smaller than the minimum film thickness of the second shell 26. Due to this, the quantum dots 20 may be more densely layered between the step of the first heating and the step of the second heating, and the second shell 26 having a relatively thick film thickness may be formed in the step of the second heating performed later.
Thus, in the above-mentioned heating steps, the first shell 24 and the second shell 26 may be formed having a film thickness able to sufficiently reduce the electron exudation from the core 22, derived from the electron wave function, in a state in which the quantum dots 20 are densely layered. Accordingly, with the configuration described above, it is possible to increase the density of the quantum dots 20 in the quantum dot structure 16 while sufficiently securing the film thicknesses of the first shell 24 and the second shell 26.
In the present embodiment, the light-emitting layer 8 is formed after the formation of the array substrate 3, the first electrode 4, the edge covers 14, and the first charge transport layer 6. Therefore, it is preferable that the array substrate 3, the first electrode 4, the edge covers 14, and the first charge transport layer 6 contain a material having heat resistance with respect to heating in the above-mentioned heating steps.
The array substrate 3 may be, for example, a glass substrate containing alkali glass or the like having a sufficiently high strain point. Further, the array substrate 3 may be an organic substrate containing an organic material having a high glass-transition temperature, such as polyimide.
For example, when the light-emitting element layer 2 forms a bottom-emitting type light-emitting element and the first electrode 4 is an anode electrode, ITO is commonly used for the first electrode 4. However, in order to suppress an increase in specific resistance due to heating in the above-mentioned heating steps, the first electrode 4 preferably includes a material having high heat resistance such as a composite material of FTO and ITO. When the first charge transport layer 6 is a hole transport layer, it is preferable to contain an inorganic material having higher heat resistance than an organic material, such as NiO, MgNiO, Cr2O3, Cu2O, or LiNbO3.
In order to achieve a shape having a certain level of height and inclination, an organic material is generally used for the edge cover 14. In the present embodiment, from the perspective of reducing damage caused by heating in the above-mentioned heating steps, the edge cover 14 preferably contains an organic material having a high glass-transition temperature, such as polyimide.
The second charge transport layer 10 and the second electrode 12 are formed after the light-emitting layer 8 is formed. Accordingly, it is possible to employ a material not having heat resistance against the heating in the above-mentioned heating steps for the material of the second charge transport layer 10 and the second electrode 12. For example, the second charge transport layer 10 may contain a material used for a conventionally known electron transport layer, and the second electrode 12 may contain a material used for a conventionally known cathode electrode.
(a) of
The light-emitting device 1 according to the present embodiment may have the same configuration as that of the light-emitting device 1 according to the previous embodiment except that the layering order of each of the layers in a light-emitting element layer 2 is reversed. In other words, the light-emitting element layer 2 according to the present embodiment includes a second charge transport layer 10, a light-emitting layer 8, a first charge transport layer 6, and a first electrode 4, which are sequentially layered from the lower layer on a second electrode 12.
In comparison with the light-emitting device 1 according to the previous embodiment, each of the second electrode 12 and the second charge transport layer 10 is separated by edge covers 14. In particular, in the present embodiment, the second electrode 12 is, by the edge covers 14, separated into a second electrode 12R for a first light-emitting element 2R, a second electrode 12G for a second light-emitting element 2G, and a second electrode 12B for a third light-emitting element 2B. Further, the second charge transport layer 10 is, by the edge covers 14, separated into a second charge transport layer 10R for the first light-emitting element 2R, a second charge transport layer 10G for the second light-emitting element 2G, and a second charge transport layer 10B for the third light-emitting element 2B.
In comparison with the light-emitting element 1 according to the previous embodiment, the first charge transport layer 6 and the first electrode 4 are not separated by the edge covers 14, and are each formed in a shared manner.
In the present embodiment, the first electrode 4 may be a transparent electrode and the second electrode 12 may be a reflective electrode. Therefore, light from the light-emitting layer 8 passes through the first charge transport layer 6 and the first electrode 4, and is emitted from a light-emitting face DS to the outside of the light-emitting device 1. Due to this, the light-emitting device 1 is configured as a top-emitting type light-emitting device. Because of this, in the present embodiment, an array substrate 3 need not necessarily be a transparent substrate.
The light-emitting device 1 according to the present embodiment can be manufactured by performing each of the steps illustrated in
For example, when the light-emitting element layer 2 forms a top-emitting type light-emitting element and the second electrode 12 is a cathode electrode, the second electrode 12 preferably contains a metal material with a high melting point from the perspective of enhancing heat resistance with respect to heating in the heating step described above. For example, it is preferable for the second electrode 12 to contain a metal such as Al or Ag, or an intermetallic compound such as AgMg. When the second charge transport layer 10 is an electron transport layer, it is preferable to contain an inorganic material having higher heat resistance than an organic material, such as MgO. The materials described above are also materials used as a cathode electrode material and an electron transport layer material in general.
The first charge transport layer 6 and the first electrode 4 are formed after the light-emitting layer 8 is formed. Accordingly, it is possible to employ a material not having heat resistance against the heating in the above-mentioned heating step for the material of the first charge transport layer 6 and the first electrode 4. For example, the first charge transport layer 6 may contain a material used for a conventionally known hole transport layer, and the first electrode 4 may contain a transparent conductive material used for a conventionally known anode electrode, such as ITO.
The light-emitting device 1 according to the present embodiment has a low level of necessity to change the materials of each layer in the light-emitting element layer 2 in comparison with the light-emitting device 1 according to the previous embodiment. Accordingly, it is possible for the light-emitting device 1 according to the present embodiment to improve the degree of freedom in material selection in comparison with the light-emitting device 1 according to the previous embodiment.
(a) of
The light-emitting device 1 according to the present embodiment may have the same configuration as that of the light-emitting device 1 of the first embodiment except that a light-emitting layer 8 does not include a ligand 18. As illustrated in (c) of
The light-emitting device 1 according to the present embodiment is manufactured by the same method except for step S5, that is, a step of forming the light-emitting layer among the steps illustrated in
In the step of forming the light-emitting layer according to the present embodiment, the same method as that described in the first embodiment is performed from step S10 to step S12. At the point in time of the completion of step S12, the quantum dot structure 16 and a ligand 18 are formed in an upper layer relative to a first charge transport layer 6, as illustrated in (a) of
In the present embodiment, subsequent to step S12, a step of third heating is performed in which the atmosphere is heated so that the atmospheric temperature rises to reach a third temperature T3 or higher (step S13). The third temperature T3 is higher than the second temperature T2, and is equivalent to a boiling point of the ligand 18. For example, in the case where the ligand 18 is the aforementioned TOPO, the third temperature T3 is 411.2 degrees Celsius.
When the ambient temperature of the quantum dots 20 reaches the third temperature T3 by heating the atmosphere in the step of the third heating, evaporation of the ligand 18 begins and the ambient temperature of the quantum dots 20 maintains the third temperature T3 for a while. Thus, in the step of the third heating, the ligand 18 vaporizes, resulting in the light-emitting layer 8 without the ligand 18 as illustrated in (b) of
The light-emitting device 1 according to the present embodiment does not include the ligand 18 in the light-emitting layer 8. In general, a ligand coordinating with quantum dots includes an organic material in many cases. Because of this, the light-emitting layer 8 without the ligand 18 in the present embodiment has a low organic material content with respect to the inorganic material, and is resistant to deterioration due to moisture infiltration or the like. Accordingly, the light-emitting device 1 according to the present embodiment is able to further improve the reliability.
From the description of NPL 1 described earlier, the average value of the proportion of the voids not occupied by the rigid spheres in the space where the rigid spheres are randomly closely-packed is approximately 36.34 volume percent. Therefore, for example, in the light-emitting layer 8, a volume ratio of organic matter to inorganic matter is preferably equal to or smaller than 36.3 volume percent. In this case, the proportion of organic matter in the light-emitting layer 8 may be reduced in comparison with a light-emitting layer in which conventional quantum dots are randomly closely-packed and voids between the quantum dots are filled with an organic ligand. Accordingly, with the configuration described above, it is possible to more efficiently improve the reliability of the light-emitting layer 8.
The expression “without the ligand” or “not include the ligand” herein refers to a situation in which substantially no ligand is included. For example, in the light-emitting layer 8 of the present embodiment, a residue of impurities or ligands may remain to such an extent that the residue does not significantly degrade the reliability of the light-emitting layer 8. Specifically, the light-emitting layer 8 of the present embodiment may contain the residue of impurities or ligands mentioned above in an amount of approximately three volume percent with respect to the total volume of the light-emitting layer 8.
In the present embodiment as well, as in the previous embodiments, the area of the outer surface of the quantum dot structure 16 can be reduced. As a result, in the step of the third heating in the present embodiment, the surface area of a second shell 26 possible to be damaged by heating may be reduced. Thus, with the above-discussed configuration, as described earlier, it is possible to suppress the formation of defects in the second shell 26 due to the damage to the second shell 26, and consequently suppress a reduction in luminous efficiency of the light-emitting device 1 due to the above defects.
(a) of
The light-emitting device 1 according to the present embodiment may have the same configuration as that of the light-emitting device 1 according to the previous embodiment except that a light-emitting layer 8 includes a quantum dot structure 36 in place of the quantum dot structure 16.
The quantum dot structure 16 includes a third shell 38 in addition to a quantum dot 20 and a second shell 26, as illustrated in (c) of
The third shell 38 fills at least part of the voids in the periphery of the second shell 26. The third shell 38 may contain the same material as the second shell 26, and may contain an inorganic shell material used for known quantum dots of a core/shell structure.
The light-emitting device 1 according to the present embodiment is manufactured by the same method except for step S5, that is, a step of forming the light-emitting layer among the steps illustrated in
In the step of forming the light-emitting layer according to the present embodiment, the same method as that described in the previous embodiment is performed from step S10 to step S13. In the present embodiment, subsequent to step S13, a step of cooling is performed in which the atmospheric temperature is lowered to a temperature lower than the third temperature T3 (step S14).
In the present embodiment, with the step of cooling, cooling is performed until the atmospheric temperature becomes a temperature TB, which is lower than the temperature TA. The temperature TB may be higher than the temperature T0 or may be equal to the temperature T0. When the atmospheric temperature drops, the temperature around the quantum dots 20 also drops following the atmospheric temperature. At the point in time of the completion of the cooling step, a quantum dot structure 16 is formed in an upper layer relative to a first charge transport layer 6, as illustrated in (a) of
After the atmospheric temperature has reached the temperature TB by the cooling step, a step of second application is performed in which a second solution 40 is applied on a position overlapping with the array substrate 3 as illustrated in (b) of
The second solution 40 contains a second solvent 42, an organic material 44, and a second inorganic precursor 46. The second solvent 42 may be the same as the first solvent 32, and may be hexane. The organic material 44 may be an organic material used for the ligand of conventionally known quantum dots, or may be the same as the material of the ligand 18. The second inorganic precursor 46 contains the same material as the third shell 38 described above. In the case where the material of the third shell 38 is the same as the material of the second shell 26, the second inorganic precursor 46 is the same as the first inorganic precursor 30.
Subsequently, the heating of the array substrate 3, on which the second solution 40 is applied, is restarted. Here, a step of fourth heating is performed by heating the atmosphere to a temperature equal to or higher than a fourth temperature T4 indicated in
The fourth temperature T4 is the higher temperature of a melting point of the organic material 44 and a boiling point of the second solvent 42. A temperature TC indicated in
The ambient temperature of the quantum dots 20 follows a rise of the atmospheric temperature, as depicted in
By further carrying on the heating of the atmosphere, one of the melting of the organic material 44 and the evaporation of the second solvent 42 ends, and the ambient temperature of the quantum dots 20 begins to rise again. Then, when the ambient temperature of the quantum dots 20 rises up to the fourth temperature T4 and the other one of the melting of the organic material 44 and the evaporation of the second solvent 42 begins, the ambient temperature of the quantum dots 20 maintains the fourth temperature T4 for a while.
Thus, by the step of the fourth heating, the melting of the organic material 44 and the evaporation of the second solvent 42 are completed. After the completion of the step of the first heating, as illustrated in (a) of
Subsequently, the heating of the array substrate 3 is continued until the atmospheric temperature reaches a fifth temperature T5 indicated in
After the completion of the melting of the organic material 44 and the evaporation of the second solvent 42, the ambient temperature of the quantum dots 20 rises from the fourth temperature T4 and reaches the fifth temperature T5. Since the atmospheric temperature is maintained at the fifth temperature T5, the ambient temperature of the quantum dots 20 having reached the fifth temperature T5 is also maintained at the fifth temperature T5.
The fifth temperature T5 is higher than the fourth temperature T4, and is a temperature at which the second inorganic precursor 46 epitaxially grows around the second shell 26 by thermochemical reaction. Thus, while the ambient temperature of the quantum dots 20 is maintained at the fifth temperature T5, the second inorganic precursor 46 gradually grows epitaxially around the second shell 26. With this, the third shell 38 is formed around the second shell 26 of each quantum dot structure 16, as illustrated in (b) of
As described above, as illustrated in (b) of
Subsequently, a step of sixth heating is performed in which the atmosphere is heated so that the atmospheric temperature further rises to reach a sixth temperature T6 or higher (step S18). The sixth temperature T6 is higher than the fifth temperature T5, and is equivalent to the boiling point of the organic material 44.
When the ambient temperature of the quantum dots 20 reaches the sixth temperature T6 by heating the atmosphere in the step of the sixth heating, evaporation of the organic material 44 begins and the ambient temperature of the quantum dots 20 maintains the sixth temperature T6 for a while. With this, in the step of the sixth heating, the organic material 44 vaporizes, and as illustrated in
In the light-emitting device 1 according to the present embodiment, the third shell 38 is formed around the second shell 26. The third shell 38 is so formed as to fill the voids 34 in the periphery of the quantum dot structure 16.
Therefore, the proportion of the volume of the quantum dot structure 36 is high with respect to the total volume of the light-emitting layer 8, as compared to the quantum dot structure 16 of the previous embodiment. That is, in the light-emitting layer 8 of the present embodiment, the filling ratio of the shells formed around the cores 22 of the quantum dots 20 is further improved in the light-emitting layer 8. In other words, after performing the fifth heating step, the density of inorganic matter with respect to the total volume of the light-emitting layer 8 is higher than that before the fifth heating step is performed. Accordingly, with the configuration described above, it is possible for the light-emitting device 1 according to the present embodiment to further improve the reliability of the light-emitting layer 8.
In the present embodiment as well, after performing the step of the second heating, the step of the third heating may be omitted, and the cooling step and subsequent steps may be performed sequentially. That is, the vaporization of the ligand 18 and the vaporization of the organic material 44 may be performed collectively in the step of the sixth heating. With this, the number of heating steps is decreased, which leads to a decrease in tact time and a reduction in manufacturing cost.
In the present embodiment, although not limited thereto, the step of forming the light-emitting layer is described in which the fourth temperature T4 is equal to the first temperature T1, the fifth temperature T5 is equal to the second temperature T2, and the sixth temperature T6 is equal to the third temperature T3. Such a configuration can be simply and easily achieved, as discussed above, by making the first solvent 32 and the second solvent 42 be the same, making the material of the ligand 18 and the organic material 44 be the same, and making the first inorganic precursor 30 and the second inorganic precursor 46 be the same.
With this, the temperature as the heating reference for each heating step may be adjusted between the steps from the first heating to third heating and the steps from the fourth heating to sixth heating. Accordingly, the configuration described above leads to a simplification of the entire step of forming the light-emitting layer.
In each of the embodiments described above, a case has been described in which the quantum dot layer including the quantum dots 20 is the light-emitting layer 8. However, no such limitation is intended, and the first charge transport layer 6 or the second charge transport layer 10 may be the quantum dot layer including the quantum dots 20, for example. In this manner, in the case where each charge transport layer includes the quantum dots 20, these quantum dots 20 may be provided with a function to transport carriers. In this case, in comparison with a charge transport layer including conventional quantum dots, the stability of the quantum dots 20 in each charge transport layer is improved, so that the efficiency of carrier transport in each of the charge transport layers is improved, leading to an improvement in the luminous efficiency of the light-emitting device 1. Each of the charge transport layers including the quantum dots 20 described above may also be formed by the same technique as the step of forming the quantum dot layer in each of the embodiments.
In each of the above-described embodiments, a display device including a plurality of light-emitting elements and having a display face DS is exemplified to describe the configuration of the light-emitting device 1. However, the present invention is not limited thereto, and the light-emitting device 1 in each of the embodiments described above may be a light-emitting device including a single light-emitting element.
The disclosure is not limited to each of the embodiments described above, and various modifications may be made within the scope of the claims. Embodiments obtained by appropriately combining technical approaches disclosed in each of the different embodiments also fall within the technical scope of the disclosure. Furthermore, novel technical features can be formed by combining the technical approaches disclosed in each of the embodiments.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/006367 | 2/20/2019 | WO | 00 |
