Light emitting diode

A light-emitting diode according to an exemplary embodiment includes: a first electrode; a second electrode overlapping the first electrode; an emission layer positioned between the first electrode and the second electrode; and a first capping layer positioned on the first electrode, wherein the first capping layer includes at least one among LiF, MgF2, AlF3, NaF, and AlOx, and a thickness of the first capping layer is 30 nm to 40 nm.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2017-0099542 filed in the Korean Intellectual Property Office on Aug. 7, 2017, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

(a) Technical Field

The present disclosure relates to a light-emitting diode, and in particular, relates to a light-emitting diode including a capping layer of an optimized thickness.

(b) Description of the Related Art

A light-emitting diode (LED) is a self-emissive element that has advantages of a wide viewing angle, a superior contrast ratio, a fast response speed, excellent luminance, a driving voltage, and response speed characteristics, and a multi-coloring characteristic.

The light-emitting diode may have a structure in which a first electrode is disposed on a substrate, and a hole transport region, an emission layer, an electron transport region, and a second electrode are sequentially formed on the first electrode. Holes injected from the first electrode move to the emission layer via the hole transport region, and electrons injected from the second electrode move to the emission layer via the electron transport region.

Carriers such as the holes and the electrons are recombined in the emission layer region, thereby generating excitons. Light is generated while the excitons is changed from an excited state to a ground state.

A plurality of light-emitting diodes included in a display device may have different stacked structures from each other by considering a wavelength of the emitted light or a material of the emission layer. In detail, the plurality of light-emitting diodes may have a thickness and a structure considering a micro-cavity distance between two electrodes depending on a wavelength of the light emitted from each emission layer pattern.

However, as intensity of the light emitted from each light-emitting diode by the micro-cavity effect increases, viewing angle characteristics may deteriorate. In detail, if the intensity of the light emitted from each light-emitting diode by the micro-cavity effect increases, a color shift depending on a viewing angle of the organic light emitting diode display increases, thereby causing a change of the color shown to the user depending on the viewing angle. Particularly, in a case of realizing a white of the emissive diode display by mixing light of different colors emitted from the plurality of light-emitting diodes, since a color deviation of the white light depending on the viewing angle further increases by each color shift of the light emitted from the plurality of light-emitting diodes, display quality of the emissive diode display may also be largely deteriorated.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure, and therefore it may contain information that does not form a prior art that is already known to a person of ordinary skill in the art.

SUMMARY

Exemplary embodiments of the present disclosure provide a light-emitting diode with improved color distribution.

A light-emitting diode according to an exemplary embodiment of the present disclosure includes: a first electrode; a second electrode overlapping the first electrode; an emission layer positioned between the first electrode and the second electrode; and a first capping layer positioned on the first electrode, wherein the first capping layer includes at least one among LiF, MgF2, AlF3, NaF, and AlOx, and a thickness of the first capping layer is 30 nm to 40 nm.

A second capping layer positioned between the first electrode and the first capping layer may be further included.

The thickness of the second capping layer may be less than 65 nm.

Reflectance for a 550 nm wavelength may be less than 0.1 at an interface between the first electrode and the first capping layer.

A refractive index of the first capping layer may be 1.1 to 1.4.

A light-emitting diode according to another exemplary embodiment of the present disclosure includes: a first electrode; a second electrode overlapping the first electrode; an emission layer positioned between the first electrode and the second electrode; a first capping layer positioned on the first electrode; and a second capping layer positioned between the first capping layer and the first electrode, wherein the first capping layer includes at least one among LiF, MgF2, AlF3, NaF, and AlOx, and a thickness of the second capping layer is less than 65 nm.

The first capping layer may include LiF alone, and the thickness of the first capping layer may be 30 nm to 40 nm.

Reflectance for a 550 nm wavelength may be less than 0.1 at an interface between the first electrode and the second capping layer.

The second capping layer may be in direct contact with the first electrode.

A light-emitting diode according to another exemplary embodiment of the present disclosure includes: a first electrode; a second electrode overlapping the first electrode; an emission layer positioned between the first electrode and the second electrode; and a first capping layer positioned on the first electrode, wherein a refractive index of the first capping layer is 1.1 to 1.4, and a thickness of the first capping layer is 30 nm to 40 nm.

A second capping layer positioned between the first electrode and the first capping layer may be further included.

A thickness of the second capping layer may be less than 65 nm.

The first capping layer may include at least one among LiF, MgF2, AlF3, NaF, and AlOx.

According to the exemplary embodiments, the light-emitting diode with improved color distribution is provided.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In order to clearly explain the present disclosure, a portion that is not directly related to the present disclosure may be omitted, and the same reference numerals are attached to the same or similar constituent elements through the entire specification.

In addition, the size and thickness of each configuration shown in the drawings are arbitrarily shown for better understanding and ease of description, but the present disclosure is not limited thereto. In the drawings, thickness of layers, films, panels, regions, etc., may be exaggerated for clarity. In the drawings, for better understanding and ease of description, thicknesses of some layers and areas may be exaggerated.

In addition, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising” will be understood to imply inclusion of stated elements but not exclusion of any other elements.

Further, throughout the specification, the phrase “on a plane” means viewing a target portion from the top, and the phrase “on a cross-section” means viewing a cross-section formed by vertically cutting a target portion from a side.

Now, a light-emitting diode according to an exemplary embodiment of the present disclosure will be described with reference to accompanying drawings.FIG. 1is a cross-sectional view of a light-emitting diode according to an exemplary embodiment. Referring toFIG. 1, the light-emitting diode according to an exemplary embodiment includes a first electrode10, a second electrode20overlapping the first electrode10, and an emission layer30positioned between the first electrode10and the second electrode20. In addition, a hole transport layer40may be positioned between the second electrode20and the emission layer30, and an electron transport layer50may be positioned between the first electrode10and the emission layer30.

A first capping layer70is positioned on an upper surface of the first electrode10. Further, a second capping layer60is positioned between the first capping layer70and the first electrode10. In the present exemplary embodiment, the light-emitting diode is described by including the first capping layer70and the second capping layer60on the surface of the first electrode10. In the present exemplary embodiment, the second capping layer60is in direct contact with the first electrode10, and is a constituent element that is distinguished from an encapsulation layer positioned on the first capping layer70.

The first capping layer70may include at least one among LiF, MgF2, AlF3, NaF, and AlOx, wherein x may be 1 to 5. The refractive index of the first capping layer70may be 1.1 to 1.4. For example, the first capping layer70may be an LiF layer. In this case, the first capping layer70may include only LiF.

In the present exemplary embodiment, the thickness of the first capping layer70may be about 30 nm to about 40 nm. In addition, the thickness of the second capping layer60may be less than about 65 nm. These ranges provide optimized thicknesses of the first capping layer70and the second capping layer60to improve a color distribution, for example, the White Angular Dependency (WAD) distribution, of the light-emitting diode. That is, the light-emitting diode having the first capping layer70and the second capping layer60of the above-thickness range decreases green reflectance and red reflectance, thereby inducing the WAD distribution to be in a preferable direction.

Herein, the WAD distribution represents a displayed color coordinate on a 1976 color coordinate system when the light-emitting diode displays white. Repeated experiments are performed by diversifying an angle of viewing the light-emitting diode, and the color displayed for each experiment is displayed on the 1976 color coordinate system. A distribution of colors in which points are collected on the color coordinate system is referred to as the WAD distribution.

A color characteristic of the light-emitting diode is changed depending on a shape in which the WAD distribution appears on the color coordinate system. For example, if the WAD distribution is distributed in a green or red region, red may appears even when the display device displays white. Particularly, when the WAD distribution is positioned in the red region, this is well recognized to a user.

However, when the WAD distribution is positioned in a blue region, this is not well recognized by the user. Accordingly, to increase the display quality of the light-emitting diode, it is preferable to position the WAD distribution in the blue region.

On the color coordinate system representing the WAD distribution, when the points representing the WAD distribution are distributed in the direction from a left upper end to a right lower end, the WAD distribution is distributed in the green and red regions. When the points representing the WAD distribution on the color coordinate system are distributed in the direction from the left lower end to the right upper, the WAD distribution is distributed in the blue region.

The light-emitting diode according to an exemplary embodiment of the present disclosure reduces the reflectance for green and red and distributes the WAD distribution in the blue region by appropriately controlling the thickness of the first capping layer70and the second capping layer60.

FIG. 2is a view showing a WAD distribution of a light-emitting diode according to an exemplary embodiment of the present disclosure. InFIG. 2, the thickness of the LiF layer (first capping layer) is 40 nm, and the thickness of the second capping layer is 60 nm.FIG. 3is a view showing a WAD distribution of a light-emitting diode according to a comparative example of the present disclosure. In the comparative example ofFIG. 3, the thickness of the LiF layer (first capping layer) is 20 nm, and the thickness of the second capping layer is 82 nm.

When comparingFIG. 2andFIG. 3, the points representing the WAD distribution are distributed in a direction parallel to a horizontal axis and a direction from the left bottom toward the right top in the case ofFIG. 2, however the points representing the WAD distribution are distributed in a direction from the left top toward the right bottom in the case ofFIG. 3.

That is, in the case ofFIG. 3, the points representing the WAD distribution are distributed in the green and red regions on the color coordinate system. This indicates that a user may recognize an image of a white color to include a red color when the light-emitting diode is applied to the display device.

However, in the case ofFIG. 2, the points representing the WAD distribution are distributed in the blue region on the color coordinate system. In this case, when the points representing the WAD distribution are distributed in the blue region, compared with the case ofFIG. 3that the points are distributed in the red region or the green region, since the points are not well recognized, the display quality of the light-emitting diode may be improved. That is, the image closer to the white may be provided to the user.

This is because the reflectance of green and red decreases by controlling the thickness of the first capping layer70and the second capping layer60in the light-emitting diode according to an exemplary embodiment of the present disclosure, thereby adjusting the WAD distribution. In the light-emitting diode, the thicknesses of the first capping layer70and the second capping layer60positioned on the light-emitting diode affect the reflectance of the light-emitting diode, and the present disclosure derives the optimized thicknesses capable of reducing the green reflectance.

FIG. 4shows the reflectance on the interface of the first electrode10for Exemplary Embodiment 1 in which the thickness of the LiF layer is 40 nm and the thickness of the second capping layer60is 60 nm, and Comparative Example 1 in which that the thickness of the LiF layer is 20 nm and the thickness of the second capping layer is 82 nm.

Referring toFIG. 4, Exemplary Embodiment 1 shows in comparison with Comparative Example 1 that the reflectance is reduced in the green region. In detail, for the 550 nm (green) wavelength, the reflectance of Comparative Example 1 is about 0.15, whereas the reflectance of Exemplary Embodiment 1 is decreased at less than 0.1 at an interface between the first electrode10and the second capping layer60. It is noted that the same level of decrease in the reflectance for the green wavelength is obtained in a case only one capping layer is included in the light-emitting diode as shown inFIGS. 5 and 6.

That is, in the light-emitting diode according to the present exemplary embodiment, the reflectance is controlled in the green region through the thickness control of the first capping layer70and the second capping layer60, and the WAD distribution is positioned in the blue region.

In the present exemplary embodiment, the second capping layer60may be at least one among SiO2, SiNx, ZnS, PA, PI, TeO2, WO3, V2O5, AlOy, ZnSe, a triamine derivative, an arylene diamine derivative, CBP, and tris(8-hydroxy-quinolinato)aluminum (Alq3), wherein the x may be 1 to 5, however the present disclosure is not limited thereto. The second capping layer60may include a material having a higher refractive index than the first capping layer70.

In the present exemplary embodiment, the first electrode10may include an alloy made of two or more materials selected from a group including Ag, Mg, Al, and Yb. For example, the first electrode10may include AgMg, in which the content of Ag may be larger than the content of Mg in the first electrode10. For example, the content of Mg may be about 10 volume % of AgMg. In another example, the first electrode10may include AgYb, and the content of Yb may be about 10 volume % of AgYb. However, these are only exemplary, and the present disclosure is not limited thereto. The first electrode10may be a transflective electrode acting as a cathode.

The second electrode20may include a conductive oxide such as indium tin oxide (ITO), indium zinc oxide (IZO), zinc tin oxide (ZTO), copper indium oxide (CIO), copper zinc oxide (CZO), gallium zinc oxide (GZO), aluminum zinc oxide (AZO), tin oxide (SnO2), zinc oxide (ZnO), or combinations thereof, calcium (Ca), ytterbium (Yb), aluminum (Al), silver (Ag), magnesium (Mg), samarium (Sm), titanium (Ti), gold (Au), or alloys thereof, graphene, carbon nanotubes, or a conductive polymer such as PEDOT:PSS. However, the present disclosure is not limited thereto, and the second electrode20may be formed of the stacked structure of two or more layers. For example, the second electrode20may be a reflecting electrode acting as an anode and having a structure of ITO/Ag/ITO.

The light generated from the emission layer30is reflected from the second electrode20as the reflecting electrode, and is resonated and amplified between the first electrode10as the transflective electrode and the second electrode20. The resonated light is reflected from the second electrode20and is emitted onto the upper surface of the first electrode10.

The emission layer30may include a host and a light emitting dopant. In this case, the content of the dopant is varied depending on a composition of the emission layer30, for example 3 to 10 weight % with reference to a total weight (100 weight %) of the host and the dopant combined.

The dopant included in the emission layer30may include 8-hydroxyquinoline and complexes of similar derivatives, benzazole derivatives, and the like, however the present disclosure is not limited thereto.

In some embodiments, the emission layer30may include a quantum dot.

An electron transport layer50may be positioned between the emission layer30and the first electrode10. The electron transport layer50may include at least one of the electron transport layer and an electron injection layer. In addition, a hole transport layer40may be positioned between the emission layer30and the second electrode20. The hole transport layer40may include at least one of the hole transport layer and the hole injection layer.

In the above description, a case in which the thicknesses of both the first capping layer70and the second capping layer60are within an optimized range has been described as an example, however a similar effect may be obtained in another case in which one of the thicknesses of the first capping layer70or second capping layer60is within an optimized range as another exemplary embodiment of the present disclosure. In addition, a similar effect may also be obtained in a case of a single capping layer structure including either the first capping layer70or the second capping layer60. That is, a case in which the light-emitting diode includes one of the first capping layer70or the second capping layer60is possible.

FIG. 5is a cross-sectional view of a light-emitting diode according to another exemplary embodiment of the present disclosure. Referring toFIG. 5, the light-emitting diode according to an exemplary embodiment ofFIG. 5is the same as the exemplary embodiment ofFIG. 1except for the second capping layer60that is omitted. The detailed description for the same constituent elements is omitted.

Referring toFIG. 5, in the light-emitting diode according to the present exemplary embodiment, the thickness of the first capping layer70is about 30 nm to about 40 nm. Referring to the previous example in which the light-emitting diode includes two capping layers as shown inFIG. 1, the reduction of the green reflectance of the light-emitting diode is obtained due to the thickness increase of the first capping layer70and the thickness reduction of the second capping layer60. While the effect of reducing the green reflectance increases in the light-emitting diode including two capping layers, the reduction of the green reflectance can be obtained in a case in which a single capping layer70is within an optimized thickness range. Accordingly, in the exemplary embodiment ofFIG. 5, similar to the previous exemplary embodiment, the WAD distribution on the color coordinate may be positioned in the blue region.

FIG. 6is a cross-sectional view of a light-emitting diode according to another exemplary embodiment. Referring toFIG. 6, the light-emitting diode according to the present exemplary embodiment is the same as the exemplary embodiment ofFIG. 1except for the first capping layer70that is omitted. The detailed description for the same constituent elements is omitted.

Referring toFIG. 6, in the light-emitting diode according to the present exemplary embodiment, the thickness of the second capping layer60is less than about 65 nm. The reduction of the green reflectance of the light-emitting diode is obtained due to the thickness increase of the first capping layer70and the thickness reduction of the second capping layer60in the previous example in which the light-emitting diode includes two capping layers as shown inFIG. 1, however the reduction of the green reflectance can be obtained in a case in which a single capping layer60is within an optimized thickness range. Accordingly, in the exemplary embodiment ofFIG. 6, similar to the previous exemplary embodiment, the WAD distribution on the color coordinate system may be positioned in the blue region.

That is, as above-described, as the light-emitting diode according to an exemplary embodiment of the present disclosure satisfies at least one of the conditions that the thickness of the first capping layer is about 30 nm to about 40 nm or the thickness of the second capping layer is less than about 65 nm, the reflectance of the green wavelength is reduced in the light-emitting diode and the WAD color distribution is positioned toward the blue region. By placing the WAD color distribution toward the blue region, it is possible to mitigate or prevent a phenomenon that an image of a white color looks red to a user, and the display quality of a display device may be improved.

Next, the display device according to an exemplary embodiment will be described with reference toFIG. 7.FIG. 7is a cross-sectional view of a light-emitting diode according to another exemplary embodiment.

Referring toFIG. 7, a buffer layer111made of a silicon oxide or a silicon nitride is positioned on a substrate110.

A semiconductor layer151is positioned on the buffer layer111. The semiconductor layer151includes a source region153and a drain region155that are doped with a p-type impurity, and further includes a channel region154positioned between the source region153and the drain region155.

A gate insulating layer140is positioned on the semiconductor layer151and the buffer layer111, and may include a silicon oxide or a silicon nitride. A control electrode124overlaps the channel region154of the semiconductor layer151and is positioned on the gate insulating layer140.

An interlayer insulating layer160is positioned on the control electrode124and the gate insulating layer140. The interlayer insulating layer160has a first contact hole165and a second contact hole163.

A data conductor including a data line171, an input electrode173, and an output electrode175is positioned on the interlayer insulating layer160.

The output electrode175is connected to the drain region155through the first contact hole165. In addition, the input electrode173is connected to the source region153through the second contact hole163.

A passivation layer180is positioned on the data conductor (171,173, and175) and the interlayer insulating layer160, and the passivation layer180has a contact hole185.

A pixel electrode190is positioned on the passivation layer180. The pixel electrode190is connected to the output electrode175through the contact hole185. A partition361is positioned on the passivation layer180. A light-emitting diode layer370is positioned on the pixel electrode190, and a common electrode270is positioned to overlap the light-emitting diode layer370. The light-emitting diode includes the pixel electrode190, the light-emitting diode layer370, and the common electrode270.

In this case, the pixel electrode190may be an anode of a hole injection electrode and the common electrode270may be a cathode of the electron injection electrode. However, the present disclosure is not limited thereto, and the pixel electrode190may be the cathode and the common electrode270may be the anode depending on the driving method of the display device.

The second capping layer60and the first capping layer70overlapping the common electrode270are formed. The description for the second capping layer60and the first capping layer70is the same as described above such that the detailed description for the same constituent elements is omitted. That is, the thickness of the first capping layer70may be about 30 nm to about 40 nm, and the thickness of the second capping layer60may be less than about 65 nm, thereby reducing the reflectance of the green wavelength in the light-emitting diode and positioning the WAD color distribution in the blue region.

An encapsulation layer390overlapping the second capping layer60and the first capping layer70is formed. The encapsulation layer390may include an organic material or an inorganic material, or the organic material and the inorganic material may be alternately stacked. The encapsulation layer390may protect the display device from external moisture, heat, and other pollutants.

The structure of the above-described display device is an example, and the light-emitting diode according to an embodiment of the present disclosure may be applied to a display device having another structure, which is self-evident.

DESCRIPTION OF SYMBOLS