Electroluminescent display device

An electroluminescent display device comprises a substrate; a thin film transistor disposed on the substrate; an overcoat layer disposed on the thin film transistor; and a light-emitting diode electrically connected to the thin film transistor through the overcoat layer, wherein the light-emitting diode includes a first electrode, a light-emitting layer on the first electrode and a second electrode on the light-emitting layer, and an emissive area is an area in which the light-emitting layer emits light by the first electrode or the second electrode, wherein the overcoat layer includes a micro lens at a position corresponding to the emissive area, and the light-emitting diode conforms to a morphology of the micro lens, and wherein the first electrode includes a first region and a second region, the first region comprises an electrode layer, and the second region includes the electrode layer and an electrode pattern disposed under the electrode layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 2017-0163169, filed Nov. 30, 2017, in the Korean Intellectual Property Office, which is incorporated herein by reference in its entirety.

BACKGROUND

Technical Field

The present disclosure relates to an electroluminescent display device, and more particularly, to an electroluminescent display device capable of improving light extraction efficiency and reliability.

Description of the Related Art

In recent years, flat panel displays having excellent characteristics such as being thin, lightweight, and having low power consumption have been widely developed and applied to various fields.

Among the flat panel displays, an electroluminescent display device is a device in which electrical charge carriers are injected into a light-emitting layer formed between a cathode, which is an electron-injecting electrode, and an anode, which is a hole-injecting electrode, such that excitons are formed, and then radiative recombination of the excitons occurs, thereby emitting light.

The electroluminescent display device can be formed using a flexible substrate such as plastic because it is self-luminous, and has excellent contrast ratios. Further the electroluminescent display device has a response time of several micro seconds, and there are advantages in displaying moving images. The electroluminescent display device also has wide viewing angles and is stable under low temperatures. Since the electroluminescent display device is driven by a low voltage of direct current DC 5V to 15V, it is easy to design and manufacture driving circuits.

FIG. 1is a schematic cross-sectional view of a related art electroluminescent display device.

As illustrated inFIG. 1, an electroluminescent display device1includes a substrate10, a thin film transistor Tr disposed on the substrate10, a light-emitting diode D disposed on the substrate10and connected to the thin film transistor Tr, and a color filter pattern50under the light-emitting diode D. An encapsulation layer (not shown) may be disposed on the light-emitting diode D.

The light-emitting diode D includes a first electrode41, a light-emitting layer42, and a second electrode43, wherein light from the light-emitting layer42is output to the outside through the first electrode41.

The light emitted from the light-emitting layer42passes through various configurations of the electroluminescent display device1and exits the electroluminescent display device1.

However, an optical waveguide mode which is configured by a surface plasmon component generated at a boundary between a metal and the light-emitting layer42and the light-emitting layer42inserted between reflective layers at both sides accounts for about 60 to 70% of emitted light.

Accordingly, among light emitted from the light-emitting layer42, rays of light that are trapped in the electroluminescent display device1instead of exiting the electroluminescent display device1are present. Thus, there is a problem in that light extraction efficiency of the electroluminescent display device1is degraded.

BRIEF SUMMARY

Accordingly, embodiments of the present disclosure are directed to an electroluminescent display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

It is an object of the present disclosure to provide an electroluminescent display device with improved light extraction efficiency and reliability through an overcoat layer having a micro lens and a first electrode including a first thickness and a second thickness which is greater than the first thickness.

To achieve the above-described object, the present disclosure provides an electroluminescent display device comprises; a thin film transistor disposed on the substrate; an overcoat layer disposed on the thin film transistor; and a light-emitting diode electrically connected to the thin film transistor through the overcoat layer, wherein the light-emitting diode includes a first electrode, a light-emitting layer on the first electrode and a second electrode on the light-emitting layer, and an emissive area is an area in which the light-emitting layer emits light by the first electrode or the second electrode, wherein the overcoat layer includes a micro lens at a position corresponding to the emissive area, and the light-emitting diode conforms to a morphology of the micro lens, and wherein the first electrode includes a first region and a second region, the first region comprises an electrode layer, and the second region includes the electrode layer and an electrode pattern disposed under the electrode layer.

According to one embodiment, an electroluminescent display device is disclosed. The electroluminescent display device comprises a substrate; a thin film transistor disposed on the substrate; an overcoat layer disposed on the thin film transistor; and a light-emitting diode electrically connected to the thin film transistor through the overcoat layer, wherein the light-emitting diode includes a first electrode, a light-emitting layer on the first electrode and a second electrode on the light-emitting layer, and an emissive area is an area in which the light-emitting layer emits light by the first electrode or the second electrode, wherein the overcoat layer includes a micro lens at a position corresponding to the emissive area, and the light-emitting diode conforms to a morphology of the micro lens, wherein the first electrode includes a first region having a first thickness and a second region having a second thickness larger than the first thickness, and wherein the micro lens includes a plurality of protruding portions and a plurality of depressed portions, a range from a central point of one depressed portion to a central point of another depressed portion adjacent thereto is one individual pattern of the micro lens, and the first region and the second region are alternately disposed for each or several individual patterns.

It is to be understood that both the foregoing general description and the following detailed description are by example and explanatory, and are intended to provide further explanation of the present disclosure as claimed.

DETAILED DESCRIPTION

First Embodiment

FIG. 2is a circuit diagram illustrating a subpixel area of an electroluminescent display device according to an embodiment of the present disclosure.

As illustrated inFIG. 2, the electroluminescent display device according to the embodiment of the present disclosure includes a gate line GL, a data line DL, a switching thin film transistor Ts, a driving thin film transistor Td, a storage capacitor Cst and a light-emitting diode D. The gate line GL and the data line DL cross each other to define a subpixel area SP. The switching thin film transistor Ts, the driving thin film transistor Td, the storage capacitor Cst and the light-emitting diode D are formed in the subpixel area SP.

More specifically, a gate electrode of the switching thin film transistor Ts is connected to the gate line GL and a source electrode of the switching thin film transistor Ts is connected to the data line DL. A gate electrode of the driving thin film transistor Td is connected to a drain electrode of the switching thin film transistor Ts, and a source electrode of the driving thin film transistor Td is connected to a high voltage supply VDD. An anode of the light-emitting diode D is connected to a drain electrode of the driving thin film transistor Td, and a cathode of the light-emitting diode D is connected to a low voltage supply VSS. The storage capacitor Cst is connected to the gate electrode and the drain electrode of the driving thin film transistor Td.

The electroluminescent display device is driven to display an image. For example, when the switching thin film transistor Ts is turned on by a gate signal applied through the gate line GL, a data signal from the data line DL is applied to the gate electrode of the driving thin film transistor Td and an electrode of the storage capacitor Cst through the switching thin film transistor Ts.

When the driving thin film transistor Td is turned on by the data signal, an electric current flowing through the light-emitting diode D is controlled, thereby displaying an image. The light-emitting diode D emits light due to the current supplied through the driving thin film transistor Td from the high voltage supply VDD.

That is, the amount of the current flowing through the light-emitting diode D is proportional to the magnitude of the data signal, and the intensity of light emitted by the light-emitting diode D is proportional to the amount of the current flowing through the light-emitting diode D. Thus, subpixel areas SP show different gray levels depending on the magnitude of the data signal, and as a result, the electroluminescent display device displays an image.

The storage capacitor Cst maintains charges corresponding to the data signal for a frame when the switching thin film transistor Ts is turned off. Accordingly, even if the switching thin film transistor Ts is turned off, the storage capacitor Cst allows the amount of the current flowing through the light-emitting diode D to be constant and the gray level shown by the light-emitting diode D to be maintained until a next frame.

A transistor and/or a capacitor other than the switching and driving thin film transistors Ts and Td and the storage capacitor Cst may be further added in the subpixel area SP.

FIG. 3is a cross-sectional view schematically illustrating an electroluminescent display device according to a first embodiment of the present disclosure, andFIG. 4is an enlarged view of portion A ofFIG. 3.

As illustrated inFIG. 3, an electroluminescent display device100according to the first embodiment of the present disclosure includes a substrate110, a thin film transistor120, a color filter pattern150, an overcoat layer160, and a light-emitting diode D electrically connected to the thin film transistor120.

The electroluminescent display device100according to the first embodiment of the present disclosure is illustrated as being a bottom emission type in which light from a light-emitting layer142is output to the outside through a first electrode141, but embodiments are not limited thereto.

That is, the electroluminescent display device100according to the first embodiment of the present disclosure may also be a top emission type in which the color filter pattern150is disposed opposite the substrate110(above the light-emitting diode D), and light from the light-emitting layer142is output to the outside through a second electrode143.

The electroluminescent display device100according to the first embodiment of the present disclosure may include, on the substrate110, a thin film transistor120which includes a gate electrode121, an active layer122, a source electrode123, and a drain electrode124.

Specifically, the gate electrode121of the thin film transistor120and a gate insulating layer131may be disposed on the substrate110.

The active layer122which overlaps the gate electrode121may be disposed on the gate insulating layer131.

An etch stopper132for protecting a channel region of the active layer122may be disposed on the active layer122.

The source electrode123and the drain electrode124may be disposed on the active layer122and contact the active layer122.

The electroluminescent display device100to which the first embodiment of the present disclosure is applicable is not limited to that illustrated inFIG. 3. The electroluminescent display device100may further include a buffer layer disposed between the substrate110and the active layer122, and the etch stopper132may not be disposed thereon.

For convenience of description, only the driving thin film transistor has been illustrated from among various thin film transistors that may be included in the electroluminescent display device100. In the figure, although the thin film transistors120has an inverted staggered structure or bottom gate structure in which the gate electrode121is disposed at an opposite side of the source electrode123and the drain electrode124with respect to the active layer122, this is merely an example, and a thin film transistor, which has a coplanar structure or top gate structure in which the gate electrode121is disposed at the same side as the source electrode123and the drain electrode124with respect to the active layer122, may also be used.

A passivation layer133may be disposed on the drain electrode124and the source electrode123, and the color filter pattern150may be disposed on the passivation layer133.

In this case, although the passivation layer133acts as a planarizing layer over an upper portion of the thin film transistor120, the passivation layer133may also be disposed to conform to the shapes of surfaces of elements located below the passivation layer133instead of acting as a planarizing layer over the upper portion of the thin film transistor120.

The color filter pattern150is configured to change a color of light emitted from the light-emitting layer142and may be one of a red color filter pattern, a green color filter pattern, and a blue color filter pattern.

The color filter pattern150may be disposed at positions which correspond to an emissive area EA on the passivation layer133and may be disposed only in some portions of the emissive area EA.

Emissive area EA refers to an area in which the light-emitting layer142emits light by the first electrode141and the second electrode143, and the color filter pattern150being disposed on a position corresponding to the emissive area EA means that the color filter pattern150is disposed to prevent a blurring phenomenon and a ghost phenomenon which occur due to mixing of light emitted from adjacent emissive areas EA.

For example, the color filter pattern150may be disposed to overlap the emissive area EA and have a size smaller than or equal to that of the emissive area EA.

However, the arrangement position and size of the color filter pattern150may be determined by various factors such as a distance between the color filter pattern150and the first electrode141, a distance between the color filter pattern150and a protruding portion PP and a depressed portion DP of a micro lens ML included in the overcoat layer160, and a distance between an emissive area EA and another emissive area EA, as well as the size and position of the emissive area EA.

A pixel of the present disclosure may include one or more subpixels. For example, a single pixel may include two to four subpixels.

Subpixel refers to a unit in which a specific type of color filter pattern150is formed or in which a single light-emitting diode D is capable of emitting a particular color without the color filter pattern150. A pixel generally includes two or more subpixels, each of a different color.

Colors defined in a subpixel may include red R, green G, blue B, and optionally white W, but embodiments are not limited thereto. A pixel will generally include at least one of a R, G and B subpixel, and optionally also a W subpixel, but embodiments are not limited thereto.

The overcoat layer160may be disposed on the color filter pattern150and the passivation layer133.

The passivation layer133may be omitted. That is, the overcoat layer160may be disposed on the thin film transistor120.

In the figure, the color filter pattern150is disposed on the passivation layer133, but embodiments are not limited thereto. The color filter pattern150may be disposed on any position between the overcoat layer160and the substrate110.

Particularly, in order to improve light extraction efficiency in the electroluminescent display device100according to the first embodiment of the present disclosure, the micro lens ML may be included in the overcoat layer160corresponding to the emissive area EA.

The micro lens ML may include a plurality of depressed portions DP and a plurality of protruding portions PP, but embodiments are not limited thereto, and the micro lens ML may have various other forms.

For example, a micro lens ML including protruding portions PP and connecting portions connecting adjacent protruding portions PP may also be formed in the overcoat layer160. As a further example, the top surface of the overcoat layer160may be corrugated. The corrugated shape may be uniform with equally spacing and equal height in each protrusion along its entire length. Alternatively, it can be an uneven, asymmetrical, non-uniform or other type of corrugated shape.

The overcoat layer160serves as a planarizing layer in a region in which the plurality of depressed portions DP and the plurality of protruding portions PP are not disposed.

Each of the plurality of depressed portions DP may have various shapes in plan view, such as a hexagonal shape, a semicircular shape, a semielliptical shape, and a quadrilateral shape. In the embodiment in which the top surface is a corrugated shape, it can be fluted, which is preferred in one embodiment. It can be a non-uniform fluted shape, or it can be a diamond, square, triangle, curved, non-uniform or other corrugation.

The light-emitting diode D including the first electrode141, the light-emitting layer142, and the second electrode143may be disposed on the overcoat layer160.

To block the spread of outgassing from the overcoat layer160to the light-emitting diode D, a second passivation layer (not illustrated) which has an insulating property may be disposed between the overcoat layer160and the first electrode141.

That is, the second passivation layer which conforms to the morphology of the plurality of depressed portions DP and the plurality of protruding portions PP exactly may be disposed between the overcoat layer160and the first electrode141.

Meanwhile, the first electrode141may be disposed on the overcoat layer160.

In this case, the first electrode141may be an anode or cathode for supplying one of electrons or holes to the light-emitting layer142.

A case in which the first electrode141of the electroluminescent display device100according to the first embodiment of the present disclosure is an anode will be described as an example.

The first electrode141may be formed of a conductive material having relatively high work function. For example, the first electrode141may be formed of a transparent conductive material such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO).

The first electrode141may be connected to the source electrode123of the thin film transistor120through a contact hole formed in the overcoat layer160and may be separately formed for each subpixel area.

Although the electroluminescent display device according to the first embodiment of the present disclosure has been described as an example in which the thin film transistor120is an N-type thin film transistor and the first electrode141is connected to the source electrode123, embodiments are not limited thereto. When the thin film transistor120is a P-type thin film transistor, the first electrode141may be connected to the drain electrode124.

The first electrode141may also be electrically connected to the light-emitting layer142by being in contact with the light-emitting layer142with a conductive material therebetween.

The first electrode141is disposed in a shape which follows, namely, conforms to the morphology of a surface of the overcoat layer160.

That is, the first electrode141may be disposed in a form which conforms to the morphology of the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer160exactly.

A fully conformal layer has a uniform thickness at all places, regardless of the surface on which it is deposited, with the top surface of the fully conformal layer having the exact same shape as the top surface of the layer on which it is deposited. A partially conformal layer generally has a uniform thickness and its top surface generally has the same shape as the top surface on which it is deposited, but it may have slight variations in thickness at bends, corners, edges and depressions, steep slopes or step changes in the underlying surface on which it is deposited. Accordingly, in one embodiment, the first electrode is deposited as a fully conformal layer. In other embodiments, it might also be deposited as partially conformal layer, being slightly thicker in depressions and at the top of protrusions and somewhat thinner on steep slope surfaces between the depressions and protrusions.

A bank layer136may be disposed on the overcoat layer160and the first electrode141.

The bank layer136may include an opening136aexposing the first electrode141.

The bank layer136may be disposed between adjacent pixel or subpixel areas and serve to differentiate the adjacent pixel or subpixel areas.

In this case, the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer160may be disposed in the opening136aof the bank layer136.

That is, since the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer160are disposed to overlap the color filter pattern150, the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer160may overlap the color filter pattern150, which is disposed thereunder, and overlap the opening136aof the bank layer136, which is disposed thereover.

The light-emitting layer142may be disposed on the exposed first electrode141.

The light-emitting layer142may have a tandem white structure in which a plurality of light-emitting layers are stacked to emit white light.

For example, the light-emitting layer142may include a first light-emitting layer configured to emit blue light and a second light-emitting layer disposed on the first light-emitting layer and configured to emit light having a color which turns white when mixed with blue.

The second light-emitting layer may be a light-emitting layer configured to emit yellow-green light.

The light-emitting layer142may include only light-emitting layers that emit one of blue light, red light, and green light. In this case, the electroluminescent display device100may not include the color filter pattern150.

Here, a luminescent material of the light-emitting layer142may be an organic luminescent material or an inorganic luminescent material such as a quantum dot.

Also, the light-emitting layer142may have a shape which conforms to the morphology of the overcoat layer160.

The second electrode143for supplying one of electrons or holes to the light-emitting layer142may be disposed on the light-emitting layer142.

In this case, the second electrode143may be an anode or a cathode.

A case in which the second electrode143of the electroluminescent display device100according to the first embodiment of the present disclosure is a cathode will be described as an example.

The second electrode143may be formed of a conductive material having relatively low work function and may be located substantially all over a display area. For example, the second electrode143may be formed of aluminum (Al), magnesium (Mg), silver (Ag), or an alloy thereof, but embodiments are not limited thereto.

The second electrode143may have a shape which conforms to the morphology of the overcoat layer160. It may be a fully conformal layer or a partially conformal layer.

The first electrode141, the light-emitting layer142, and the second electrode143form the light-emitting diode D, and the light-emitting diode D conforms to the morphology of the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer160.

The shape of the light-emitting diode D may be realized using the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer160.

Accordingly, some of light emitted from the light-emitting layer142, which has not been extracted to the outside due to being totally reflected inside the first electrode141and the light-emitting layer142, may be made to travel at an angle smaller than a total reflection critical angle. In this way, external quantum efficiency may be improved through multiple reflections.

FIGS. 5A and 5Bare views schematically illustrating an optical path in accordance with a thickness of a first electrode of the electroluminescent display device according to the first embodiment of the present disclosure. Description will be given with reference toFIGS. 5A and 5Balong withFIG. 4.

FIG. 5Aillustrates an optical path in a case in which a thickness d1of the first electrode141is 250 Å, andFIG. 5Billustrates an optical path in a case in which a thickness d2of the first electrode141is 500 Å.

ComparingFIGS. 5A and 5B, it can be seen that light emitted from an inclined surface of the light-emitting layer142is reflected at the second electrode143, passes through the first electrode141, and is extracted to the outside in the case ofFIG. 5Ain which the thickness d1of the first electrode141is 250 Å. In the case ofFIG. 5Bin which the thickness d2of the first electrode141is 500 Å, some of the light emitted from the inclined surface of the light-emitting layer142is totally reflected at the first electrode141and the second electrode143and thus not output to the outside.

That is, some of the light emitted from the light-emitting layer142, which has not been able to be output to the outside due to being totally reflected inside the light-emitting diode D, may be output to the outside through the first electrode141in the case in which the first electrode141has the relatively thin thickness d1.

FIG. 6is a table showing a driving voltage, a current-luminance efficiency, an external quantum efficiency (EQE), and a color temperature in accordance with a thickness of the first electrode of the electroluminescent display device according to the first embodiment of the present disclosure. Description will be given with reference toFIG. 6along withFIGS. 5A and 5B.

First, in terms of a driving voltage V, it can be seen that the driving voltage V is increased because the resistance increases with a decrease of the thickness of the first electrode141.

That is, the driving voltage is 10.6 V when the thickness of the first electrode141is 400 Å, the driving voltage is 10.7 V when the thickness of the first electrode141is 300 Å, and the driving voltage is 10.8 V when the thickness of the first electrode141is 250 Å. From this, it can be seen that the driving voltage will be increased as the thickness of the first electrode141decreases to achieve the same effect on the light emitting layer142. Next, in terms of current-luminance efficiency (cd/A) which indicates brightness for unit current, it can be seen that the current-luminance efficiency (cd/A) increases with a decrease of the thickness of the first electrode141.

That is, the current-luminance efficiency (cd/A) is 109.8 cd/A when the thickness of the first electrode141is 400 Å, the current-luminance efficiency is 114.9 cd/A when the thickness of the first electrode141is 300 Å, and the current-luminance efficiency is 116.3 cd/A when the thickness of the first electrode141is 250 Å. From this, it can be seen that the current-luminance efficiency increases as the thickness of the first electrode141decreases.

In terms of external quantum efficiency (%), it can be seen that the external quantum efficiency (%) increases with a decrease of the thickness of the first electrode141.

That is, the external quantum efficiency (%) is 45.2% when the thickness of the first electrode141is 400 Å, the external quantum efficiency is 47.2% when the thickness of the first electrode141is 300 Å, and the external quantum efficiency is 48% when the thickness of the first electrode141is 250 Å. From this, it can be seen that the external quantum efficiency increases as the thickness of the first electrode141decreases.

Lastly, in terms of a color temperature or correlated color temperature (CCT (K)) which expresses chromaticity of a light source or a reference white color with a temperature of the closest area on a radial curve instead of coordinates on the two-dimensional chromaticity diagram, it can be seen that the color temperature decreases with a decrease of the thickness of the first electrode141.

That is, the correlated color temperature (CCT(K)) is 7018 K when the thickness of the first electrode141is 400 Å, the color temperature is 6965 K when the thickness of the first electrode141is 300 Å, and the color temperature is 6782 K when the thickness of the first electrode141is 250 Å. From this, it can be seen that the color temperature decreases as the thickness of the first electrode141decreases.

As described above, the external quantum efficiency (%) and current-luminance efficiency (cd/A) of the electroluminescent display device100ofFIG. 3according to the first embodiment of the present disclosure may be improved by forming the thickness of the first electrode141to be in a range of 250 Å to 300 Å.

However, since the driving voltage V is increased with an increase of resistance when the thickness of the first electrode141is formed to be in the range of 250 Å to 300 Å. Accordingly, there is some chance that the reliability of the electroluminescent display device100ofFIG. 3might be degraded.

Hereinafter, an electroluminescent display device100ofFIG. 3capable of preventing an increase in resistance while improving external quantum efficiency (%) and current-luminance efficiency (cd/A) will be described according to a second embodiment.

Second Embodiment

Hereinafter, detailed description of configurations identical or similar to those of the first embodiment may be omitted.

FIG. 7is a cross-sectional view schematically illustrating an electroluminescent display device according to a second embodiment of the present disclosure.

As illustrated inFIG. 7, an electroluminescent display device200according to a second embodiment of the present disclosure includes a substrate210, a thin film transistor220, a color filter pattern250, an overcoat layer260, and a light-emitting diode D electrically connected to the thin film transistor220.

The thin film transistor220may include a gate electrode221, an active layer222, a source electrode223, and a drain electrode224.

Specifically, the gate electrode221of the thin film transistor220and a gate insulating layer231may be disposed on the substrate210.

The active layer222which overlaps the gate electrode221may be disposed on the gate insulating layer231.

An etch stopper232for protecting a channel region of the active layer222may be disposed on the active layer222.

The source electrode223and the drain electrode224may be disposed on the active layer222and contact the active layer122.

A passivation layer233may be disposed on the drain electrode224and the source electrode223, and the color filter pattern250may be disposed on the passivation layer233.

The overcoat layer260may be disposed on the color filter pattern250and the passivation layer233.

In order to improve light extraction efficiency in the electroluminescent display device200according to the second embodiment of the present disclosure, the micro lens ML may be included in the overcoat layer260corresponding to the emissive area EA.

The micro lens ML may include a plurality of depressed portions DP and a plurality of protruding portions PP, but embodiments are not limited thereto, and the micro lens ML may have various other forms.

For example, a micro lens ML including protruding portions PP and connecting portions connecting adjacent protruding portions PP may also be formed in the overcoat layer260.

The overcoat layer260serves as a planarizing layer in an area in which the plurality of depressed portions DP and the plurality of protruding portions PP are not disposed. For example, the overcoat layer260in a non-emissive area may have a flat top surface.

Each of the plurality of depressed portions DP may have various shapes in plan view, such as a hexagonal shape, a semicircular shape, a semielliptical shape, and a quadrilateral shape.

The micro lens ML including the depressed portions DP and the protruding portions PP may be formed through a photolithography process using a mask including a light-blocking portion and a light-transmitting portion. The light-transmitting portion may correspond to the depressed portions DP and the light-blocking portion may correspond to the protruding portions PP, but embodiments are not limited thereto. Alternatively, the light-transmitting portion may correspond to the protruding portions PP and the light-blocking portion may correspond to the depressed portions DP.

The overcoat layer260may be formed of an organic material having a refractive index in a range of about 1.5 to 1.55, but embodiments are not limited thereto.

The light-emitting diode D including a first electrode241, a light-emitting layer242, and a second electrode243may be disposed on the overcoat layer260.

In this case, the first electrode241may be an anode or cathode for supplying one of electrons or holes to the light-emitting layer242.

A case in which the first electrode241of the electroluminescent display device200according to the second embodiment of the present disclosure is an anode will be described as an example.

The first electrode241may be formed of a conductive material having relatively high work function. For example, the first electrode241may be formed of a transparent conductive material such as ITO and IZO.

Particularly, the first electrode241of the electroluminescent display device200according to the second embodiment of the present disclosure may be disposed to not fully conform to a shape of a top surface of the overcoat layer260and may be formed to have different thicknesses in each region of an emissive area EA.

That is, the first electrode241may include a first region A1having a first thickness and a second region A2having a second thickness which is greater than the first thickness.

For example, the first region A1of the first electrode241may be formed of an electrode layer241b, and the second region A2of the first electrode241may include an electrode layer241band an electrode pattern241adisposed thereunder.

Accordingly, the first electrode241may be formed to have different thicknesses in each of the first and second regions A1and A2, and through such a structure, an increase in resistance may be prevented while the external quantum efficiency (%) and current-luminance efficiency (cd/A) are improved. In a preferred embodiment, each region A1has the same thickness as all regions A1in the subpixel and each region A2has the same thickness as all regions A2in the subpixel.

Although the first electrode241has been described as being formed to have different thicknesses in each of the respective first and second regions A1and A2, embodiments are not limited thereto. The first electrode241may be formed to have different thicknesses in each of a plurality of regions, which includes more than two regions.

The thickness of the first electrode241in each of the first and second regions A1and A2will be described in more detail below.

The first electrode241may be connected to the source electrode223of the thin film transistor220through a contact hole formed in the overcoat layer260and may be separately formed for each pixel area.

The first electrode241is disposed to follow the morphology of a surface of the overcoat layer260.

That is, the first electrode241may have different thicknesses in each of the first and second region A1and A2and be disposed in a form which follows the morphology of the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer260exactly.

In addition, the first electrode241may have a refractive index of about 1.8 or higher, but embodiments are not limited thereto.

A bank layer236may be disposed on the overcoat layer260and the first electrode241.

The bank layer236may include an opening236aexposing the first electrode241.

The bank layer236may be disposed between adjacent pixel or subpixel areas and serve to differentiate the adjacent pixel or subpixel areas.

In this case, the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer260may be disposed in the opening236aof the bank layer236.

That is, since the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer260are disposed to overlap the color filter pattern250, the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer260may overlap the color filter pattern250, which is disposed thereunder, and overlap the opening236aof the bank layer236, which is disposed there over.

The bank layer236may be formed of a photo acrylic organic material having a refractive index of 1.6 or lower, but embodiments are not limited thereto.

The light-emitting layer242may be disposed on the first electrode241.

The light-emitting layer242may have a shape that is fully conformal to the morphology of the first electrode241.

The second electrode243for supplying one or electrons or holes to the light-emitting layer242may be disposed on the light-emitting layer242.

Here, the second electrode243may be an anode or a cathode.

A case in which the second electrode243of the electroluminescent display device200according to the second embodiment of the present disclosure is a cathode will be described as an example.

The second electrode243may be formed of a conductive material having relatively low work function and may be located substantially all over a display area. For example, the second electrode243may be formed of Al, Mg, Ag, or an alloy thereof, but embodiments are not limited thereto.

The second electrode243may have a shape which is fully conformal to the morphology of the light-emitting layer242.

As described above, the first electrode241having different thicknesses for each region, the light-emitting layer242, and the second electrode243form the light-emitting diode D, and the light-emitting diode D is conformal to the morphology of the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer260.

FIG. 8is an enlarged view of portion B ofFIG. 7.

As illustrated inFIG. 8, the electroluminescent display device200ofFIG. 7according to the second embodiment of the present disclosure includes the passivation layer233, the color filter pattern250disposed on the passivation layer233, the overcoat layer260disposed on the color filter pattern250, and the light-emitting diode D disposed on the overcoat layer260.

Here, a micro lens ML may be included in the overcoat layer260.

The micro lens ML may include a plurality of depressed portions DP and a plurality of protruding portions PP.

Particularly, the first electrode241of the electroluminescent display device200according to the second embodiment of the present disclosure may be disposed along a top surface of the overcoat layer260in the emissive area EA ofFIG. 7and may be formed to have different thicknesses in each region of the emissive area EA.

That is, the first electrode241may have different thicknesses in each region and not be fully conformal to the morphology of the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer260exactly. For example, the first electrode241may include a first region A1having a first thickness D1and a second region A2having a second thickness D2which is greater than the first thickness D1.

In this case, the first region A1of the first electrode241may be formed of a single layer, and the second region A2of the first electrode241may be formed of multiple layers.

For example, the first region A1of the first electrode241may be formed of an electrode layer241b, and the second region A2of the first electrode241may include an electrode layer241band an electrode pattern241adisposed thereunder.

That is, by forming the electrode pattern241ain a region corresponding to the second region A2on the overcoat layer260and forming the electrode layer241bwhich covers the overcoat layer260and the electrode pattern241a, the second region A2, in which the electrode pattern241aand the electrode layer241boverlap each other on the overcoat layer260, and the first region A1, in which only the electrode layer241bis disposed on the overcoat layer260, may be formed.

For example, when a range from a central point CP of a depressed portion DP to a central point CP of another depressed portion DP adjacent thereto is defined as an individual pattern IP of the micro lens ML, the first region A1and the second region A2may be alternately disposed for each individual pattern IP. However, this is merely an example, and the first region A1and the second region A2may also be alternately disposed for every two individual patterns IP.

Also, the second region A2may be disposed in two individual patterns IP, and the first region A1may be disposed in a single individual pattern IP such that the first region A1and the second region A2are alternately disposed. Alternatively, the first region A1may be disposed in two individual patterns IP, and the second region A2may be disposed in a single individual pattern IP such that the first region A1and the second region A2are alternately disposed.

The second region A2may also be disposed in a portion of an individual pattern IP. For example, the second region A2may be disposed in a depressed portion DP or protruding portion PP of the micro lens ML, or the second region A2may also be disposed in an region configured to connect the depressed portion DP and the protruding portion PP.

That is, positions at which the first region A1and the second region A2of the first electrode241are disposed may be changed in various ways.

In one embodiment ofFIGS. 7 and 8, the thickness d1of the electrode layer241bof the first electrode241may be in a range of 50 Å to 200 Å, and the thickness d2of the electrode pattern241aof the first electrode241may be in a range of 200 Å to 350 Å but embodiments are not limited thereto.

As a result, a thickness D1of the first region A1of the first electrode241may be in a range of 50 Å to 200 Å, and a thickness D2of the second region A2of the first electrode241may be in a range of 250 Å to 550 Å, but embodiments are not limited thereto.

Although the electrode pattern241ais illustrated inFIG. 8as being formed as a single layer below the electrode layer241b, embodiments are not limited thereto, and the electrode pattern241amay also be formed as multiple layers.

The electrode layer241band the electrode pattern241adisposed below the electrode layer241bmay be formed of the same material. For example, the electrode layer241band the electrode pattern241adisposed below the electrode layer241bmay be formed of a transparent conductive material such as ITO and IZO.

It can be considered that electrode pattern241ais a first sublayer of the first electrode and that electrode layer241bis the first electrode layer itself since it is continuous. Thus, viewingFIGS. 7 and 8, it can be considered that layer241ais a first electrode sublayer disposed at selected locations below the first electrode layer. In one embodiment, the first electrode sublayer241ais positioned only on each protrusion of the overcoat layer and is not positioned in some portions of bottom region of the depressions in one embodiment. In another embodiment, as can be seen inFIGS. 7 and 8, the first electrode sublayer is positioned only the top region of selected ones of the protrusions of the overcoat layer and is not positioned on the non-selected ones of the protrusions of the overcoat layer. In this embodiment, the first electrode sublayer241ais maintained only on every other protrusion. In the embodiment ofFIG. 9, the first electrode sublayer241ais positioned only in the depressed portions of the overcoat layer and is not positioned on the protrusions.

In a method of forming the first electrode241, a transparent conductive material such as ITO and IZO is deposited on the overcoat layer260through sputtering or the like.

Then, the transparent conductive material in a region corresponding to the first region A1is removed by a pattern and etch sequence or other technique to thereby form the electrode pattern241aat some locations and exposing the overcoat layer260at other locations which corresponds to the first region A1.

Next, a transparent conductive material such as ITO and IZO is deposited as a conformal layer on the overcoat layer260and the electrode pattern241athrough sputtering or the like so as to form the electrode layer241b.

Accordingly, the second region A2, in which the electrode pattern241aand the electrode layer241boverlap each other on the overcoat layer260, and the first region A1, in which only the electrode layer241bis disposed on the overcoat layer260, may be formed.

As described above, the first electrode241having different thicknesses D1and D2for the first and second regions A1and A2may be formed by the electrode layer241band the electrode pattern241ain the electroluminescent display device200ofFIG. 7according to the second embodiment of the present disclosure.

Through such a structure, reliability of the electroluminescent display device200ofFIG. 7may be improved by preventing an increase in resistance through the second region A2while improving the external quantum efficiency (%) and the current-luminance efficiency (cd/A) through the first region A1.

Further, the correlated color temperature may be adjusted depending on the thicknesses D1and D2of the first region A1and the second region A2.

FIG. 9is a cross-sectional view schematically illustrating a first modified example of the electroluminescent display device according to the second embodiment of the present disclosure. Detailed description of configurations identical or similar to those of the second embodiment will be omitted.

As illustrated inFIG. 9, the first modified example of the electroluminescent display device200ofFIG. 7according to the second embodiment of the present disclosure may include a passivation layer333, a color filter pattern350disposed on the passivation layer333, an overcoat layer360disposed on the color filter pattern350, and a light-emitting diode D disposed on the overcoat layer360.

In this case, a micro lens ML may be included in the overcoat layer360.

The micro lens ML may include a plurality of depressed portions DP and a plurality of protruding portions PP.

Particularly, a first electrode341of the first modified example of the electroluminescent display device200ofFIG. 7according to the second embodiment of the present disclosure may be disposed to conform to a shape of a top surface of the overcoat layer360in an emissive area EA and may be formed to have different thicknesses in each region of the emissive area EA ofFIG. 7.

That is, the first electrode341may have different thicknesses in first and second regions A1and A2, and the first electrode341may be disposed in a conformal layer which follows the morphology of the plurality of depressed portions DP and the plurality of protruding portions PP of the overcoat layer360exactly.

The first electrode341may include the first region A1having a first thickness D1and the second region A2having a second thickness D2which is greater than the first thickness D1.

In this case, the first region A1of the first electrode341may be formed of a single layer, and the second region A2of the first electrode341may be formed of multiple layers.

For example, the first region A1of the first electrode341may be formed of an electrode layer341b, and the second region A2of the first electrode341may include an electrode layer341band an electrode pattern341adisposed below the electrode layer341b.

Particularly, in the first electrode341of the first modified example of the electroluminescent display device200ofFIG. 7according to the second embodiment of the present disclosure, the first region A1may be disposed corresponding to the protruding portions PP of the micro lens ML of the overcoat layer360, and the second region A2may be disposed corresponding to the depressed portions DP.

In this case, a thickness d1of the electrode layer341bof the first electrode341may be in a range of 50 Å to 200 Å, and a thickness d2of the electrode pattern341aof the first electrode341may be in a range of 200 Å to 350 Å, but embodiments are not limited thereto.

The thickness D1of the first region A1of the first electrode341may be in a range of 50 Å to 200 Å, and the thickness D2of the second region A2of the first electrode341may be in a range of 250 Å to 550 Å, but embodiments are not limited thereto.

Although the electrode pattern341ais illustrated inFIG. 9as being formed as a single layer below the electrode layer341b, embodiments are not limited thereto, and the electrode pattern341amay also be formed as multiple layers.

The electrode layer341band the electrode pattern341adisposed below the electrode layer341bmay be formed of the same material. For example, the electrode layer341band the electrode pattern341adisposed below the electrode layer341bmay be formed of a transparent conductive material such as ITO and IZO.

Accordingly, the second region A2, in which the electrode pattern341aand the electrode layer341boverlap each other on the overcoat layer360, and the first region A1, in which only the electrode layer341bis disposed on the overcoat layer360, may be formed.

Particularly, in the first electrode341of the first modified example of the electroluminescent display device200ofFIG. 7according to the second embodiment, the first region A1having a small thickness may be disposed corresponding to the protruding portions PP of the micro lens ML of the overcoat layer360, and the second region A2having a large thickness may be disposed corresponding to the depressed portions DP.

Through such a structure, an increase in resistance may be effectively prevented by disposing the second region A2of the first electrode341in a region other than a main emissive area of the light-emitting diode D while further improving the external quantum efficiency (%) and the current-luminance efficiency (cd/A) by disposing the first region A1of the first electrode341in an region which corresponds to the main emissive area of the light-emitting diode D. This can be accomplished using the same layer deposit then pattern and etch steps described with respect toFIGS. 7 and 8.

FIG. 10is a cross-sectional view schematically illustrating a second modified example of the electroluminescent display device according to the second embodiment of the present disclosure. Detailed description of configurations identical or similar to those of the second embodiment will be omitted.

As illustrated inFIG. 10, the second modified example of the electroluminescent display device200ofFIG. 7according to the second embodiment of the present disclosure may include a passivation layer433, a color filter pattern450disposed on the passivation layer433, an overcoat layer460disposed on the color filter pattern450, and a light-emitting diode D disposed on the overcoat layer460.

In this case, a micro lens ML may be included in the overcoat layer460.

The micro lens ML may include a plurality of protruding portions PP.

In this case, the plurality of protruding portions PP may be disposed to be spaced apart from each other.

Particularly, a first electrode441of the second modified example of the electroluminescent display device200ofFIG. 7according to the second embodiment of the present disclosure may include a first electrode layer441aand a second electrode layer441b.

In this case, the first electrode layer441aof the first electrode441may be disposed along a shape of a top surface of the overcoat layer460in the emissive area EA ofFIG. 7.

That is, the first electrode layer441amay include round portions RP which correspond to the plurality of protruding portions PP of the overcoat layer460and flat portions FP which correspond to regions in which the plurality of protruding portions PP are spaced apart from each other.

An insulating pattern462may be formed on the flat portions FP of the first electrode layer441a.

That is, the insulating pattern462may be formed corresponding to the regions in which the plurality of protruding portions PP of the micro lens are spaced apart from each other.

For example, in plan view, the first electrode layer441amay be disposed substantially on an entire surface of the top of the micro lens in which the plurality of protruding portions PP are formed to be spaced apart from each other, the insulating pattern462may be disposed in the form of an island corresponding to the regions in which the plurality of protruding portions PP of the micro lens ML are spaced apart from each other on the first electrode layer441a, and the second electrode layer441bconfigured to cover the insulating pattern462and the first electrode layer441amay be disposed thereon.

Here, although the insulating pattern462may be formed to have the same size as that of the plurality of protruding portions PP of the micro lens, embodiments are not limited thereto. The insulating pattern462may be formed to have a size smaller than that of the plurality of protruding portions PP of the micro lens or formed to have a size greater than that of the plurality of protruding portions PP of the micro lens.

In this case, the insulating pattern462may be formed of the same material as that of the bank layer236ofFIG. 7.

That is, since the insulating pattern462may be formed on the flat portions FP of the first electrode441by using a process of forming the bank layer236ofFIG. 7without going through a separate process, a separate process is not required. For example, the bank layer236ofFIG. 7and the insulating pattern462may be formed using a transflective mask.

The second electrode layer441bmay be disposed to cover the round portions RP of the first electrode layer441aof the first electrode441and the top of the insulating pattern462.

Accordingly, the first electrode441which is in contact with the light-emitting layer442may include a first region A1having a first thickness D1and a second region A2having a second thickness D2which is greater than the first thickness D1.

In this case, the first region A1of the first electrode441may be formed of the second electrode layer441b, and the second region A2of the first electrode441may include the second electrode layer441band the round portions RP of the first electrode layer441awhich is in contact with the second electrode layer441b.

The thickness d1of the second electrode layer441bof the first electrode441may be in a range of 50 Å to 200 Å, and the thickness d2of the first electrode layer441aof the first electrode441may be in a range of 200 Å to 350 Å, but embodiments are not limited thereto.

The first thickness D1of the first region A1of the first electrode441may be in a range of 50 Å to 200 Å, and the thickness D2of the second region A2of the first electrode441may be in a range of 250 Å to 550 Å, but embodiments are not limited thereto.

In this case, the first electrode layer441aand the second electrode layer441bmay be formed of the same material. For example, the first electrode layer441aand the second electrode layer441bmay be formed of a transparent conductive material such as ITO and IZO.

Through such a structure, reliability of the electroluminescent display device200ofFIG. 7may be improved by preventing an increase in resistance through the second region A2while improving the external quantum efficiency (%) and the current-luminance efficiency (cd/A) through the first region A1.

Further, the correlated color temperature may be adjusted through the thicknesses D1and D2of the first region A1and the second region A2.

Particularly, by disposing the insulating pattern462, which is formed of the same material as that of the bank layer236ofFIG. 7, between the first electrode layer441aand the second electrode layer441bin the first electrode441of the second modified example of the electroluminescent display device200ofFIG. 7according to the second embodiment of the present disclosure, the first electrode441having different thicknesses D1and D2for the regions A1and A2may be realized without going through a separate process. In this way, a process may be simplified.

Further, since a service life of the light-emitting diode D decreases with an increase of an amount of the overcoat layer460due to outgassing of the light-emitting diode D, the insulating pattern462is disposed between the plurality of protruding portions PP of the overcoat layer460in the second modified example of the second embodiment of the present disclosure. In this way, since the absolute amount of the overcoat layer460may be decreased, the service life of the light-emitting diode D may be increased. In the embodiment ofFIG. 10, the first electrode441has the combined thickness of d1and d2for its entire length, providing low resistance, however on top of some protrusions462, only layer441bis present and layer441ais flat while on the top of other protrusions in overcoat layer460, both layer441aand441bare present, thus this structure provides different properties for light reflection at different locations in the ML as has been discussed. In the present disclosure, an overcoat layer having a micro lens is disposed so that light extraction efficiency can be improved.

Further, a first electrode, which has a first thickness and a second thickness, is disposed on the overcoat layer so that the light extraction efficiency can be further improved without an increase in resistance.

The present disclosure has been described above with reference to exemplary embodiments thereof. However, those of ordinary skill in the art should understand that various modifications and changes may be made to the present disclosure within the scope not departing from the technical spirit and scope of the present disclosure described in the claims below.