The disclosure relates to a light-emitting element, a display device, and a method for manufacturing a light-emitting element. This application claims priority based on Japanese Patent Application No. 2019-189065 filed in Japan on Oct. 16, 2019, the contents of which are incorporated herein by reference.
A light-emitting element described in PTL 1 includes a light-emitting layer containing quantum dots. In the light-emitting element described in PTL 1, a density of the quantum dots in a thickness direction of the light-emitting layer decreases from an anode side toward a cathode side.
In the light-emitting element of PTL 1, a coverage rate of quantum dots on the cathode side (electron transport layer side) of the light-emitting layer is about 10%, and thus the quantum dots are considered not necessary. Accordingly, in PTL 1, the cathode side of the light-emitting layer has a large percentage of vacancy, the vacancy being a region in which the quantum dots are not present, compared to the anode side. However, as in the light-emitting element described in PTL 1, because the coverage rate of quantum dots on the cathode side of the light-emitting layer is about 10%, when the percentage of the vacancy is increased to the extent that the quantum dots are no longer present, the barrier to electrons becomes extremely large in the light-emitting layer, causing the number of electrons in the light-emitting layer to be insufficient with respect to the positive holes. As a result, the recombination rate between electrons and positive holes in the light-emitting layer decreases, and luminous efficiency (external quantum efficiency) of the light-emitting element decreases.
An object of a light-emitting element according to one aspect of the disclosure is to improve luminous efficiency.
A light-emitting element according to one aspect of the disclosure includes an anode, a cathode, and a quantum dot layer provided between the anode and the cathode and including a plurality of quantum dots and a vacancy, the vacancy being a region between the plurality of quantum dots. The quantum dot layer includes both the plurality of quantum dots and the vacancy in the quantum dot layer in an entire region of the quantum dot layer in all cross sections having a normal line in a direction from the cathode toward the anode.
According to one aspect of the disclosure, it is possible to realize a light-emitting element having improved luminous efficiency.
Hereinafter, exemplary embodiments and examples of the disclosure will be described with reference to the drawings. Note that description of duplicate items in each of the embodiments and the examples will be omitted as appropriate. Further, in the following, a “same layer” means that the layer is formed through the same process (film formation process), a “lower layer” means that the layer is formed in a process before the layer being compared, and an “upper layer” means that the layer is formed in a process after the layer being compared.
The display device 10 has a structure in which the resin layer 12, a barrier layer 13, a thin film transistor (hereinafter. TFT) layer 14 including TFTs, a light-emitting element layer 15 including the light-emitting elements 20 and cover films 151, a sealing layer 16, and a second film 17 are layered in this order on an upper layer of the first film 11.
The first film 11 is a support member in the display device 10 having flexibility. The first film 11 can be formed of a material having flexibility such as polyethylene terephthalate (PET), for example. Note that, in a case in which the display device 10 does not require flexibility, a substrate formed of a hard material such as glass may be used as the support member instead of the first film 11.
The resin layer 12 is provided between the first film 11 and the barrier layer 13. The resin layer 12 is a layer used for peeling a support substrate (not illustrated) used in a manufacturing process of the display device 10 from the barrier layer 13 and bonding the first film 11 having flexibility to the barrier layer 13. The resin layer 12 is partially removed when the support substrate is peeled from the barrier layer 13. The resin layer 12 may have a multilayer structure in which a plurality of resin films are layered, or may have a multilayer structure in which an inorganic film is interposed between a plurality of resin films. Note that, in a case in which the display device 10 does not require flexibility, the resin layer 12 may be omitted.
The barrier layer 13 is a layer for preventing foreign matters such as water and oxygen from entering the TFT layer 14 and the light-emitting element layer 15. The barrier layer 13 is a single layer or a multilayer insulating film, and can be formed of an insulating material, such as silicon oxide, silicon nitride, or silicon oxynitride, for example.
The TFT layer 14 includes a semiconductor film 141, a gate insulating film 142 in an upper layer above the semiconductor film 141, and a gate electrode GE and a gate wiring line (not illustrated) in an upper layer above the gate insulating film 142. Further, the TFT layer 14 includes a first insulating film 143 in an upper layer above the gate electrode GE and the gate wiring line, a capacitance electrode CE in an upper layer above the first insulating film 143, and a second insulating film 144 in an upper layer above the capacitance electrode CE. The TFT layer 14 includes a source wiring line SW and a drain wiring line DW (not illustrated) in an upper layer above the second insulating film 144, and a flattening film 145 in an upper layer above the source wiring line SW and the drain wiring line DW.
The TFT includes the semiconductor film 141, the gate insulating film 142, the gate electrode GE, the first insulating film 143, and the second insulating film 144. A source region and a drain region (not illustrated) of the semiconductor film 141 are regions in which high concentration doping is performed on an upper face of the semiconductor film 141, and function as a source electrode and a drain electrode. The source wiring line SW and the drain wiring line DW are respectively connected to the source region and the drain region via contact holes passing through the gate insulating film 142, the first insulating film 143, and the second insulating film 144. The gate electrode GE is connected to a gate wiring line (not illustrated), and the gate wiring line is connected to a driver integrated circuit (IC; not illustrated). The source wiring line SW is connected to a driver IC (not illustrated). The drain wiring line DW is connected to a pixel electrode (not illustrated).
The semiconductor film 141 can be formed of, for example, a semiconductor material such as low-temperature polysilicon (LTPS) or an oxide semiconductor (for example, an In—Ga—Zn—O based semiconductor). The gate electrode GE, the gate wiring line, the capacitance electrode CE, the drain wiring line DW, and the source wiring line SW are single layer or multilayer conductive films.
The first insulating film 143 and the second insulating film 144 are single layer or multilayer insulating films, and can be formed of an insulating material, such as silicon oxide or silicon nitride, for example.
The flattening film 145 is a film layered on the TFT for flattening irregularities formed by the TFT. The flattening film 145 can facilitate layering the light-emitting element layer 15 thereon.
Note that although a structure of the TFT included in the TFT layer 14 is illustrated as a top gate type in
The light-emitting element layer 15 includes a plurality of the light-emitting elements 20 and the cover films 151. The plurality of light-emitting elements 20 are provided in a matrix shape in a display region of an image in the display device 10.
The cover films 151 are provided between the plurality of light-emitting elements 20, covering a side surface of each light-emitting element 20 and an end portion of each anode 21. The cover film 151 is provided in a lattice pattern in the display region. The cover film 151 is an insulating film, and can be formed of an organic material, for example.
The sealing layer 16 is a layer for preventing foreign matters such as water and oxygen from entering the TFT layer 14 and the light-emitting element layer 15 by sealing the light-emitting element layer 15.
For example, the first sealing film 161 and the third sealing film 163 are single layer or multilayer inorganic insulating films, and can be formed of an inorganic material such as a silicon oxide film, a silicon nitride film, and a silicon oxynitride film. The second sealing film 162 is, for example, a light-transmissive organic film, and can be formed of a light-transmissive organic material such as acrylic.
The second film 17 can be formed of, for example, a PET film. As a result, the display device 10 having flexibility can be realized. Note that in a case in which the display device 10 does not require flexibility, a hard substrate of glass or the like may be used instead of the second film 17.
Of the first film 11 and the second film 17, the film provided on a light-outputting side of the light-emitting element 20 is the display region side of the display device 10. As the film on the display region side, a function film having an optical compensation function, a touch sensor function, a protection function, or the like can be used, for example.
The light-emitting element 20 of the present embodiment illustrates a configuration in which light is output from the anode 21 side to the outside of the display device 10. Thus, the first film 11, the resin layer 12, the barrier layer 13, and the TFT layer 14 on the light-outputting side are preferably formed of highly light-transmissive material. Further, in the configuration of the display device 10, of the anode 21 and the cathode 26, at least one of the sealing layer 16 and the second film 17 provided on the cathode 26 side, which is the opposite side, preferably has a reflecting function.
Note that the light-emitting element 20 may be configured to output light from the cathode 26 side to the outside of the display device 10. In this case, in the configuration of the display device 10, the sealing layer 16 and the second film 17 provided in a direction in which light is output are preferably formed of highly light-transmissive material. Further, in the configuration of the display device 10, of the anode 21 and the cathode 26, at least one of the first film 11, the resin layer 12, the barrier layer 13, and the TFT layer 14 provided on the anode 21 side, which is the opposite side, preferably has a reflecting function.
Further, although not illustrated, an electronic circuit board and a power circuit board (for example, an IC chip, a driver IC, or a flexible printed circuit (FPC)) are disposed outside the display region of the display device 10. A plurality of the TFTs and the light-emitting elements 20 described above are arrayed on a plane to constitute the display region of the display device 10. Power is supplied from each of the circuits described above to the plurality of TFTs and light-emitting elements 20 arranged on the plane, and each operation is controlled by each of the circuits. Thus, a screen display of the display device 10 is performed.
As the process for preparing the display device 10 of the present embodiment, first, the resin layer 12 is formed in an upper layer above the support substrate (resin layer 12 forming process). Next, the barrier layer 13 is formed in an upper layer above the resin layer 12. Next, the TFT layer 14 including the TFTs is formed in an upper layer above the barrier layer 13. Next, the light-emitting element layer 15 including the bottom-emitting type light-emitting elements 20 is formed in an upper layer above the TFT layer 14. Next, the sealing layer 16 is formed in an upper layer above the light-emitting element layer 15. Next, the second film 17 is bonded to an upper layer above the sealing layer 16.
Laser light is transmitted through the support substrate and irradiated onto the resin layer 12. As a result, the resin layer 12 is partially removed, and the support substrate is peeled from the resin layer 12 (support substrate peeling process). Next, the first film 11 is bonded to a lower face of the resin layer 12 from which the support substrate was peeled (bonding process). Next, the layered body including the first film 11, the resin layer 12, the barrier layer 13, the TFT layer 14, the light-emitting element layer 15, the sealing layer 16, and the second film 17 is divided to obtain a plurality of individual pieces. Next, the electronic circuit board is disposed on a portion of a non-display region outside the display region. Note that these processes are performed by a manufacturing apparatus of the display device 10.
In a case of manufacture of the display device 10 that does not require flexibility, the resin layer 12 forming process, the support substrate peeling process, and the first film 11 bonding process are not necessary. In this case, for example, the first film 11 need only be replaced with a glass substrate or the like, and the barrier layer 13 forming process and subsequent processes need only be performed. Further, a method corresponding to the material of each layer, such as an application method, sputtering, a photolithography method, or chemical vapor deposition (CVD) can be used as appropriate as a method for layering each layer in the processes described above.
The anode 21 is an electrode for injecting positive holes into the quantum dot layer 24. The cathode 26 is an electrode for injecting electrons into the quantum dot layer 24. The anode 21 and the cathode 26 can be formed of a conductive material. The anode 21 is in contact with the hole injection layer 22. The cathode 26 is in contact with the electron transport layer 25.
For example, one of the anode 21 and the cathode 26 is a light-transmissive electrode and the other is a non-light-transmissive electrode. The light-transmissive electrode can be formed of a conductive material such as ITO, IZO, ZnO, AZO, BZO, or FTO, for example. The non-light-transmissive electrode can be formed of a metal material having high light reflectivity such as Al, Cu, Au, Ag, Mg, or alloys thereof, for example. With use of a material having high light reflectivity as the non-light-transmissive electrode, the light emitted by the quantum dot layer 24 can be reflected in the direction in which light is output from the light-emitting element 20. In the present embodiment, the light emitted by the quantum dot layer 24 is reflected by the cathode 26, transmitted through the anode 21, and output from the light-emitting element 20 to the outside of the display device 10.
The hole injection layer 22 is a layer for injecting positive holes from the anode 21 into the hole transport layer 23. The hole transport layer 23 is a layer for transporting positive holes injected from the hole injection layer 22 to the quantum dot layer 24. Note that, of the hole injection layer 22 and the hole transport layer 23, only the hole injection layer 22 may be provided between the anode 21 and the quantum dot layer 24, or the hole injection layer 22 and the hole transport layer 23 may be omitted and the anode 21 and the quantum dot layer 24 may be directly in contact with each other.
The hole injection layer 22 and the hole transport layer 23 can be formed of an organic material containing a conductive compound such as PEDOT-PSS, TFB, and PVK, or an inorganic material containing a metal oxide such as NiO, Cr2O3, MgO, MgZnO, LaNiO3, MoO3, and WO3, for example.
The electron transport layer 25 is provided between the quantum dot layer 24 and the cathode 26. One surface of the electron transport layer 25 is in contact with the quantum dot layer 24, and the other surface is in contact with the cathode 26. The electron transport layer 25 is a layer for transporting electrons from the cathode 26 to the quantum dot layer 24.
The electron transport layer 25 can be formed of a metal oxide film such as TiO2, ZnO, ZAO, ZnMgO, ITO, or an In—Ga—Zn—O based semiconductor, for example. Further, the electron transport layer 25 can be formed of a conductive polymer material such as Alq3, BCP, or t-Bu-PBD.
The material of the electron transport layer 25 is desirably a material having a small electron affinity or work function to facilitate the injection of electrons from the cathode 26. Further, the material of the electron transport layer 25 is desirably a stable material having high physical durability to prevent foreign matters such as water and oxygen from entering the quantum dot layer 24. Accordingly, an inorganic material is suitable for the material of the electron transport layer 25. Inorganic materials typically have high electron mobility, and have high carrier density of electrons. Thus, the injection density of electrons into the quantum dot layer 24 can be increased by using an inorganic material for the electron transport layer 25.
The quantum dot layer 24 emits light as a result of an occurrence of recombination between positive holes 57 injected from the anode 21 and electrons 56 injected from the cathode 26. The quantum dot layer 24 is provided between the anode 21 and the cathode 26. The quantum dot layer 24 includes quantum dots 27 that are nano-sized semiconductor particles and a vacancy 28 that is a region in which the quantum dots 27 are not included. The quantum dot layer 24 is layered so as to include both a plurality of the quantum dots 27 and the vacancy 28 in all cross sections 29 orthogonal to a normal line NL, the normal line NL being a direction from the anode 21 toward the cathode 26. In the present embodiment, the quantum dot layer 24 is provided in contact with a contact surface 231, which is the surface of the hole transport layer 23.
A particle size of the quantum dots 27 is, for example, from about 2 to 15 nm. The smaller the particle size of the quantum dots 27, the shorter the light emission wavelength, changing the luminescent color from red to green and from green to blue. Thus, the light emission wavelength of the light-emitting element 20 can be controlled by changing the particle size of the quantum dots 27. When the display device 10 of the present embodiment is configured, a red light-emitting element 20, a green light-emitting element 20, and a blue light-emitting element 20 are arrayed as one set in the light-emitting element layer 15.
The cross section 29 is a virtual plane when, with respect to a plane on the cathode 26 side and a plane on the anode 21 side of the quantum dot layer 24, the quantum dot layer 24 is cut in a direction parallel to both of these planes (left-right direction in
An outer shape of the quantum dots 27 is a spherical shape, and thus the entire region of the quantum dot layer 24 cannot be filled with the quantum dots 27, in principle. Thus, the vacancy 28 always exists in the quantum dot layer 24. Accordingly, “both the vacancy 28 and the quantum dots 27 are always included” in the description above specifically means that, in the cross section 29 of the quantum dot layer 24, at any position in the thickness direction, there is no cross section 29 with only the vacancy 28, and the quantum dots 27 are always included in the cross section 29.
In the present embodiment, the vacancy 28 is the space between the plurality of quantum dots 27, and a gas such as air, nitrogen, or hydrogen, for example, may be present. The vacancy 28 may be space having a level close to a vacuum level with respect to electron transport. Further, in the vacancy 28, an insulating solvent may be present as a liquid or an insulating solid may be present. Furthermore, in the vacancy 28, a solvent and a material that is different from the quantum dots 27 and having much lower conductivity than the quantum dots 27, or the like, may be present.
According to the above-described configuration of the light-emitting element 20, when a potential difference is applied between the anode 21 and the cathode 26, the positive holes 57 are injected from the anode 21 and the electrons 56 are injected from the cathode 26, toward the quantum dot layer 24, as illustrated in
When the density of electrons becomes exceedingly higher than that of positive holes in the interior of the quantum dot layer, there are many excessive electrons that cannot recombine with positive holes in the quantum dot layer, and the density of the positive holes in the quantum dot layer decreases. As a result, the recombination rate of the quantum dot layer decreases, and the luminous efficiency of the light-emitting element decreases. Further, when the percentage of the interior of the quantum dot layer occupied by the vacancy becomes too large, conversely there are not enough electrons for the positive holes, and the recombination rate in the quantum dot layer decreases.
In the light-emitting element 20 of the present embodiment, the quantum dot layer 24 includes both the plurality of quantum dots 27 and the vacancy 28 in the quantum dot layer 24 in all cross sections 29 orthogonal to the normal line NL, the normal line NL being the direction from the cathode 26 toward the anode 21. With this configuration, it is possible to keep the influence of the vacancy 28 from becoming large (described below with reference to
In a case in which the light-emitting element 20 outputs light from the cathode 26 side to the outside, the electron transport layer 25 and the cathode 26 are each formed of a light-transmissive material, and are preferably formed of a material having a light transmittance of 95% or greater, for example. As a result, attenuation of light emitted from the quantum dot layer 24 to the outside due to the electron transport layer 25 and the cathode 26 can be suppressed.
Next, the flow of electrons in the quantum dot layer 24 will be described using
In
In
As illustrated in
The first ligand 51 and the second ligand 52 each include an organic molecular group. The barrier 53 is a barrier formed by, among the organic molecular groups respectively contained in the first ligand 51 and the second ligand 52, organic molecular groups aggregated without chemical bonding. The barrier 54 is a barrier formed by, among the organic molecular groups respectively contained in the first ligand 51 and the second ligand 52, organic molecular groups chemically bonded. The aggregated organic molecular groups do not have a bond that serves as an electron transport path, making it more difficult for the electrons 56 to move than in the organic molecular groups chemically bonded. Thus, the barrier 53 is larger than the barrier 54. Note that, when the aggregated organic molecular groups are in close contact and an electrical field of a certain degree is applied, the electrons 56 can easily cross the barrier 53. Thus, of the electrons 56, electrons 561 that cross the barrier 53 can be easily moved in the first ligand 51 and the second ligand 52.
The vacancy 28 is a region having extremely low electrical conductivity or a region having insulating properties. Thus, the barrier 55 formed by the vacancy 28 is very large between the first ligand 51 and the second ligand 52. Therefore, the barrier 55 is much larger than the barrier 53 and the barrier 54 formed in the first ligand 51 and the second ligand 52 that include the organic molecular groups having electrical conductivity. Therefore, among the electrons 56 that cross the barrier 53 and the barrier 54 formed in the first ligand 51, only a portion of electrons 562 can cross the barrier 55 and move from the first ligand 51 side to the second ligand 52 side. Among the electrons 56 that cross the barrier 53 and the barrier 54 in the first ligand 51, remaining electrons 563 that cannot cross the barrier 55 remain on the first ligand 51 side. Thus, the movement of the electrons 56 between the plurality of quantum dots 27 can be suppressed by the barrier 55 formed by the vacancy 28.
The number of the electrons 56 in the quantum dot layer 24 is large in the position of the electron transport layer 25 side, and small in the position of the hole transport layer 23 side. Further, in the quantum dot layer 24, the electrons 563 are retained at or near an interface with the electron transport layer 25, and thus the electrons 56 are less likely to be transported from the electron transport layer 25 toward the quantum dot layer 24. Accordingly, the density of the electrons 56 in the quantum dot layer 24 can be suppressed by the vacancy 28.
If, as in the light-emitting element described in PTL 1, a portion of the region of the quantum dot layer in the layering direction includes a region in which the percentage of the vacancy is too large, the height of the barrier formed by the vacancy increases excessively and the electrons can no longer cross the barrier formed by the vacancy. As a result, the luminous efficiency in the quantum dot layer is lowered. On the other hand, as described using
The quantum dot layer 24 preferably has an area filling rate from 40% to 80%, the area filling rate being a percentage of all cross sections 29 orthogonal to the normal line NL occupied by the plurality of quantum dots 27 in the quantum dot layer 24. Specifically, the area filling rate is the percentage of the cross section 29 occupied by the quantum dots 27. Thus, the height and the width of the barrier 55 formed by the vacancy 28 illustrated in
The method for manufacturing the light-emitting element 20 of the present embodiment will be more specifically described below. First, the anode 21 is formed on the TFT layer 14 with the support substrate on which the TFT layer 14 is formed as a base (step S31). The anode 21 can be formed by layering conductive material on the TFT layer 14 by sputtering, vapor deposition, or metal CVD, for example.
Next, the hole injection layer 22 is formed on the anode 21 (step S32). The hole injection layer 22 can be formed by layering an inorganic material by sputtering, vapor deposition, or metal CVD, for example. Further, the hole injection layer 22 can be formed by layering an organic material by a method of applying a liquid organic material or the like.
Next, the hole transport layer 23 is formed on the hole injection layer 22 (step S33). The hole transport layer 23 can be formed using similar materials by a method similar to that of the hole injection layer 22 described above. Note that either one of step S32 and step S33 may be omitted.
Next, the quantum dot layer 24 is formed on the hole transport layer 23 (step S34). The quantum dot layer 24 can be formed by applying a dispersion in which the quantum dots 27 are dispersed in an organic solvent onto the hole transport layer 23 by a spin coating method or an ink-jet method, for example.
In step S34, examples of parameters affecting the array of the plurality of quantum dots 27 include the particle size of the quantum dots 27, the length of the ligand attached to the surface of the quantum dots 27, the density of the quantum dots 27 in the solvent, temperature, electrostatic force, and solvent viscosity. The array of quantum dots 27 in the quantum dot layer 24 can be adjusted by adjusting these parameters as appropriate. Thus, the quantum dots 27 can be layered so that both the plurality of quantum dots 27 and the vacancy 28 in the quantum dot layer 24 are included across the entire region of the quantum dot layer 24, in all cross sections 29 of the quantum dot layer 24.
For example, the quantum dot layer 24 is formed by a spin coater method. In this case, a colloidal solution in which the quantum dots 27 are dispersed in a solvent, or a resist in which the quantum dots 27 are dispersed is dripped onto a rotating formation surface, spreading and forming the quantum dot layer 24 over the entire formation surface. At this time, the lower the viscosity of the solution and the higher the rotational speed, the greater the number of random minute vortices generated when the solution spreads and the more the solution spreads non-uniformly. With the solution spreading non-uniformly, the vacancy 28 can be formed between the plurality of quantum dots 27 in the quantum dot layer 24. Then, the viscosity and the rotational speed of the solvent can be adjusted as appropriate, and the distribution of the vacancy 28 in the quantum dot layer 24 can be adjusted. For example, as the solvent, a material having a viscosity of less than 0.5 mPa·s, such as toluene, hexane, or pentane, is preferably used. Further, the rotational speed when the solution is applied is preferably less than 3000 rpm, for example.
Further, in the spin coater method, the distribution of the vacancy 28 in the quantum dot layer 24 can be adjusted by dividing the application and formation of the quantum dot layer 24 into two applications, and using solvents having different liquid repellencies in the first application and the second application. For example, in the first application, dodecanethiol having a relatively high liquid repellency is used as a solvent for dispersing the quantum dots 27. Then, the distribution of the quantum dots 27 is non-uniform due to the liquid repellency of the solvent. As a result, the quantum dots 27 are coarsely and densely distributed in the surface of the layer formed by the first application. In the second application, hexadecylamine, oleylamine, or the like having a relatively low liquid repellency is used as the solvent. The coarse and dense distribution on the surface of the layer formed by the first application affects the distribution of the quantum dots 27 applied the second time, forming the vacancy 28.
Next, the electron transport layer 25 is formed on the quantum dot layer 24 (step S35). The electron transport layer 25 can be formed by, for example, sputtering, vapor deposition, and an application method.
Next, the cathode 26 is formed on the electron transport layer 25 (step S36). Similar to the anode 21 described above, the cathode 26 can be formed by layering a conductive material by, for example, sputtering, vapor deposition, or metal CVD.
The following describes specific examples of the light-emitting element 20 described in the first embodiment on the basis of examples 1-1 to 1-5 and comparative examples 1 and 2.
The electrical characteristics of the light-emitting elements 20 in a case in which the value of the area filling rate, which is the percentage of the cross section 29 of the quantum dot layer 24 according to the light-emitting element 20 described above in which the plurality of quantum dots 27 are included, is changed will now be described with reference to
The area filling rate is expressed as a percentage of a value obtained by dividing the total area of the cross section 29 occupied by the quantum dots 27 by the entire area of the cross section 29 of the quantum dot layer 24. Note that the area filling rate does not need to satisfy the numerical range described above in the cross section 29 at every position of the quantum dot layer 24. The area filling rate in the cross section 29 cut at a position on any normal line NL of the quantum dot layer 24 need only be averaged, and the average value need only satisfy the numerical range of the area filling rate described above. Note that, in a case in which a ligand is present on the surface of the quantum dots 27, the area filling rate of the quantum dots 27 need only be calculated by the area obtained by adding the area occupied by the ligand and the area occupied by the quantum dots 27.
As described above, in the light-emitting element 20 including the quantum dot layer 24 as the light-emitting layer, injection of the positive holes 57 is naturally difficult. Therefore, when the voltage applied to the light-emitting element 20 is increased, the electrons 56 start to be injected into the quantum dot layer 24 first and then, once the voltage rises a certain extent, the positive holes 57 start to be injected into the quantum dot layer 24. The threshold voltage Vth of the current is the voltage value when either the electrons 56 or the positive holes 57 start to be injected into the quantum dot layer 24. The luminance threshold voltage V1 is the voltage value when both the electrons 56 and the positive holes 57 are injected into the quantum dot layer 24 and the light-emitting recombination starts. In
As shown in
As shown in
The light-emitting element 20 of example 1-2 has an area filling rate of 70%, resulting in Vth=3.43 V, Lmax=59000 cd/m2, and EQEmax=14.3%. Next, the light-emitting element 20 of example 1-3 has an area filling rate of 60%, resulting in Vth=3.45 V, Lmax=60000 cd/m2, and EQEmax=13.3%. Furthermore, the light-emitting element 20 of example 1-4 has an area filling rate of 50%, resulting in Vth=3.46 V, Lmax=60000 cd/m2, and EQEmax=14%. As described above. EQEmax is maximized and the luminous efficiency of the light-emitting element 20 is the highest in the light-emitting element 20 of example 1-3.
The light-emitting element 20 of example 1-5 has an area filling rate 40%, resulting in Vth=3.48 V, Lmax=58000 cd/m2, and EQEmax=13%. Furthermore, the light-emitting element 20 of comparative example 2 has an area filling rate of 30%, resulting in Vth=3.48 V, Lmax=52000 cd/m2, and EQEmax=10%. Thus, the luminous efficiency is further reduced in the light-emitting element 20 of comparative example 2 than in the light-emitting element 20 of comparative example 1. This is because lowering the area filling rate to 30% excessively suppresses the density of the electrons 56 in the quantum dot layer 24, disrupting the balance between the positive holes 57 and the electrons 56 and reducing the recombination rate in the quantum dot layer 24 to a greater extent than in comparative example 1.
From the results described above, in the light-emitting element 20, the area filling rate of the quantum dot layer 24 is preferably in a range from 40% to 80%, which is that of examples 1-1 to 1-5. With the area filling rate of the quantum dot layer 24 being in the range from 40% to 80%, it is possible to increase the luminous efficiency of the light-emitting element 20 to a greater extent than in the case of the 90% area filling rate in comparative example 1 and the 30% area filling rate in comparative example 2. Further, when the area filling rate of the quantum dot layer 24 is within a range from 50% to 70% as in examples 1-2 to 1-4, the luminous efficiency of the light-emitting element 20 is further increased, and thus such a range is more preferable.
The percentage of the quantum dots 27 included in the quantum dot layer 24 may be smaller on the cathode 26 side than on the anode 21 side. For example, in the thickness direction of the quantum dot layer 24, the area filling rate of the quantum dots 27 in the half region on the anode 21 side may be set to 80%, and the area filling rate of the quantum dots 27 in the half region on the cathode 26 side of the quantum dot layer 24 may be set to 40%. Accordingly, it is possible to suppress the injection of excess electrons 56 from the electron transport layer 25 to the quantum dot layer 24, and suppress the movement of the electrons 56 in the quantum dot layer 24 toward the anode 21 side and outflow to the hole transport layer 23 side. As a result, the recombination rate between the positive holes 57 and the electrons 56 in the quantum dot layer 24 can be increased, and the luminous efficiency of the light-emitting element 20 can be further improved.
Below, the light-emitting element 20 according to a second embodiment will be described. The layered structure of the light-emitting element 20 according to the present embodiment is the same as the layered structure of the light-emitting element 20 according to the first embodiment described with reference to
The protruding portion is a structural portion in which a portion of the intermediate layer protrudes from the contact surface 231 toward the quantum dot layer 24 side or a structure disposed on the contact surface 231, and the recessed portion is a structural portion in which a portion of the intermediate layer is recessed from the contact surface 231 toward the side opposite to the quantum dot layer 24. Note that the intermediate layer is, for example, a layer intermediately disposed between the anode 21 and the quantum dot layer 24, such as the hole injection layer 22 and the hole transport layer 23, and is a function layer with the contact surface 231 on which the quantum dot layer 24 is layered serving as an upper face thereof.
In a case in which a surface roughness of the hole transport layer 23 formed in step S73 is smaller than the particle size of the quantum dots 27 of the quantum dot layer 24, the array of the quantum dots 27 is not affected. For example, if an average surface roughness of the surface of the hole transport layer 23 is in the order of 0.1 nm, the size is about one-tenth of the particle size of the typical quantum dot 27 and does not affect the array of the quantum dots 27.
In step S74, in the hole transport layer 23 that is the intermediate layer, a plurality of recessed portions 81 or a plurality of protruding portions 82 are formed in the contact surface 231, which is the surface with which the quantum dot layer 24 comes into contact.
As another method, after formation of the hole transport layer 23, as step S74, it is possible to form the recessed portion 81 in the contact surface 231 by applying an organic solvent or a permeable liquid to the contact surface 231 of the hole transport layer 23 and subsequently applying a heat treatment of about 100 degrees, thereby causing a portion of the hole transport layer 23 to contract.
Further, step S73 and step S74 can also be performed as a series of steps. For example, when the material of the hole transport layer 23 is applied and subsequently heat treated and fixed, it is possible to form the recessed portions 81 in the contact surface 231 by rapidly increasing the temperature at the end of the heat treatment, thereby causing the solvent to volatilize at high speed and thus partially aggregate the hole transport layer 23.
For example, while the viscosity of a typical colloidal solution is about 2 mPa·s, when the concentration of the solvent used for the material of the hole transport layer 23 is changed to increase the viscosity to about 4 mPa·s, the protruding portions 82 are distributed on the contact surface 231 of the hole transport layer 23. When the quantum dots 27 applied in step S75 are on the protruding portions 82, the quantum dots 27 move, falling off of the protruding portions 82. The distance between the moved quantum dots 27 and the other adjacent quantum dots 27 increases, making it possible to control the size of the vacancy 28 generated between these quantum dots 27.
The average surface roughness of the surface of the hole transport layer 23 increases by the recessed portions 81 or the protruding portions 82 described with reference to
In the present embodiment, in step S74, the plurality of recessed portions 81 or the plurality of protruding portions 82 are provided on the contact surface 231 of the hole transport layer 23. The average surface roughness of the contact surface 231 of the hole transport layer 23 increases by these recessed portions 81 or protruding portions 82. The quantum dots 27 applied to the contact surface 231 of the hole transport layer 23 move by hitting the recessed portions 81 or the protruding portions 82, thereby adjusting the spacing of the plurality of quantum dots 27 and generating the vacancy 28. Accordingly, the spacing of the plurality of quantum dots 27 adjacent to each other can be adjusted as desired and the vacancy 28 of any desired size can be provided. As a result, the respective percentages of the cross-sectional area occupied by the quantum dots 27 and the vacancy 28 at a lowermost portion of the quantum dot layer 24 can be determined.
The spacing between the plurality of quantum dots 27 layered in a layer further above the lowermost portion of the quantum dot layer 24 is affected by the array of the plurality of quantum dots 27 of the layer below. Thus, according to the array of the quantum dots 27 in the layer below, the respective percentages of the cross-sectional area occupied by the quantum dots 27 and the vacancy 28 can be determined in positions above the lowermost portion of the quantum dot layer 24 as well. Accordingly, by the plurality of recessed portions 81 or the plurality of protruding portions 82 on the contact surface 231 of the hole transport layer 23, the respective percentages of the cross-sectional area occupied by the quantum dots 27 and the vacancy 28 across an entire thickness direction of the quantum dot layer 24 can be determined. As a result, layering can be performed so that both the plurality of quantum dots 27 and the vacancy 28 in the quantum dot layer 24 are included across the entire region of the quantum dot layer 24, in all cross sections 29 of the quantum dot layer 24.
For example, the plurality of recessed portions 81 or the plurality of protruding portions 82 need only have a size in a height direction or a width direction thereof that is equal to or greater than the particle size of the quantum dots 27. For example, in a case in which the particle size of the quantum dots 27 is from about 2 to 15 nm, the height and the width of the recessed portions 81 or the protruding portions 82 need only be from about 2 to 15 nm on average. Accordingly, the recessed portions 81 or the protruding portions 82 readily affect the array of the quantum dots 27, making it possible to easily adjust the area filling rate of the quantum dot layer 24.
An in-plane density of the recessed portions 81 or the protruding portions 82 on the contact surface 231 of the intermediate layer may be adjusted so that the spacing between the plurality of recessed portions 81 or the plurality of protruding portions 82 is within 10 times the particle size of the quantum dots 27 on average across the entire contact surface 231 of the hole transport layer 23 on which the quantum dot layer 24 is layered. Accordingly, the distribution in the formation surface of the plurality of recessed portions 81 or the plurality of protruding portions 82 can be made more uniform. As a result, the plurality of quantum dots 27 and the vacancy 28 can be more uniformly distributed in all cross sections 29 of the quantum dot layer 24. Note that “across the entire contact surface 231” refers to the entire region in which the contact surface 231 is in contact with the quantum dot layer 24.
As described above, the area filling rate of the quantum dot layer 24 can be adjusted to a desired value by appropriately adjusting the size of the plurality of recessed portions 81 or the plurality of protruding portions 82, the in-plane density of the plurality of recessed portions 81 or the plurality of protruding portions 82 across the entire contact surface 231 of the hole transport layer 23 on which the quantum dot layer 24 is layered, or the like.
Although the present embodiment illustrates a case in which the hole transport layer 23 is the intermediate layer, the disclosure is not limited to this configuration. For example, in a case in which the hole transport layer 23 does not exist, the hole injection layer 22 may be used as the intermediate layer. Further, a transparent conductive film may be formed in advance on a surface that forms the quantum dot layer 24, and the transparent conductive film may be the intermediate layer.
The present invention is not limited to the embodiments described above. Embodiments obtained by modifying the embodiments described above and embodiments obtained by appropriately combining technical approaches disclosed in the embodiments described above also fall within the technical scope of the present invention.
Using
The second cross section 329 is a virtual plane when, with respect to a plane of the quantum dot layer 24 on the cathode 26 side and a plane of the quantum dot layer 24 on the anode 21 side, the quantum dot layer 24 is cut in a direction parallel to both of these planes (left-right direction in
The outer shape of the quantum dots 27 is a spherical shape, and thus the entire region of the quantum dot layer 24 cannot be filled with the quantum dots 27, in principle. Thus, the vacancy 28 always exists in the quantum dot layer 24. Accordingly. “always cross both the vacancy 28 and the quantum dots 27” in the description above specifically means that, in the first cross section in the quantum dot layer 24, at any position in the thickness direction, there is no intersecting line 429 that crosses only the vacancy 28, and the intersecting line 429 always crosses the quantum dots 27.
As is understood from the description of
For example, the light-emitting element 20 includes the anode 21, the cathode 26, and the quantum dot layer 24 provided between the anode 21 and the cathode 26 and including the plurality of quantum dots 27 and the vacancy 28, the vacancy 28 being a region between the plurality of quantum dots 27. Given the first cross section as at least one cross section among cross sections of the quantum dot layer 24 parallel to the direction 3NL from the anode 21 toward the cathode 26, and the second cross section 329 as all cross sections orthogonal to the direction, all intersecting lines of the first cross section and the second cross section 329 cross both the plurality of quantum dots 27 and the vacancy 28 in the first cross section.
Further, for example, the light-emitting element 20 includes the anode 21, the cathode 26, and the quantum dot layer 24 provided between the anode 21 and the cathode 26 and including the plurality of quantum dots 27 and the vacancy 28, the vacancy 28 being a region between the plurality of quantum dots 27. Given the first cross section as at least one cross section among the cross sections of the quantum dot layer 24 parallel to the direction 3NL from the anode 21 toward the cathode 26, and the second cross section 329 as all cross sections orthogonal to the direction, all intersecting lines of the first cross section and the second cross section 329 cross the plurality of quantum dots 27 in the first cross section.
The percentage of the quantum dots 27 included in the quantum dot layer 24 may be smaller on the cathode 26 side than on the anode 21 side. Accordingly, it is possible to suppress the injection of excess electrons 56 from the electron transport layer 25 to the quantum dot layer 24, and suppress the movement of the electrons 56 in the quantum dot layer 24 toward the anode 21 side and outflow to the hole transport layer 23 side. As a result, the recombination rate between the positive holes 57 and the electrons 56 in the quantum dot layer 24 can be increased, and the luminous efficiency of the light-emitting element 20 can be further improved.
Number | Date | Country | Kind |
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2019-189065 | Oct 2019 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2020/032115 | 8/26/2020 | WO |