KR101717075B1 - organic electroluminescent display device - Google Patents

Abstract

The present invention relates to an organic electroluminescent device and a method of manufacturing the same.
An organic electroluminescent device of the present invention includes: a substrate having a plurality of pixel regions defined therein; A thin film transistor formed on each of the pixel regions on the substrate; A first electrode corresponding to the plurality of pixel regions, respectively, and electrically connected to the thin film transistor; A bank formed between the pixel regions adjacent to the first electrode; (-CN, -NC), a hydroxyl group (-OH), a halide group (-I, -Br, -F), and the like which are in contact with the first electrode and the bank and have a thickness of 50 Å or less. An electron withdrawing layer formed of a material containing an electron withdrawing functional group; A hole transporting layer formed on the electron-withdrawing layer; A light emitting material layer formed on the hole transport layer and including first to third emission patterns spaced apart from each other at an upper portion of the bank, the first to third emission patterns corresponding to the plurality of pixel regions, respectively; An electron transport layer on the light emitting material layer; And a second electrode formed on the electron transport layer. Therefore, the path of the leakage current is cut off, and the occurrence of the luminance deviation can be prevented.

Description

[0001] The present invention relates to an organic electroluminescent display device,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device having improved lifetime and efficiency, a low power consumption, and a method of manufacturing the same.

2. Description of the Related Art Flat panel displays having excellent characteristics such as thinning, lightening, and low power consumption have been widely developed and applied to various fields.

An organic electroluminescent device or an organic electroluminescent display, which is also referred to as an organic light emitting diode (OLED), is an organic electroluminescent display device in which charges are injected into a light emitting layer formed between a cathode serving as an electron injecting electrode and a cathode serving as a hole injecting electrode It is a device that emits light while the electron and hole are paired and then disappears. Such organic electroluminescent devices can be formed not only on a flexible substrate such as a plastic but also because they are excellent in color due to self-luminescence and are low in plasma display panel (Plasma Display Panel) and inorganic electroluminescence It has the advantage of being able to drive (less than 10V) voltage and relatively low power consumption.

Organic electroluminescent devices can be classified into a passive matrix type and an active matrix type. Active organic electroluminescent devices capable of low power consumption, high definition, and large size are widely used in various display devices.

Hereinafter, the basic structure and operating characteristics of the active organic electroluminescent device will be described in detail with reference to the drawings.

1 is a circuit diagram of one pixel region of a general active organic electroluminescent device.

1, one pixel region P of the active organic electroluminescent device includes a switching thin film transistor STr, a driving thin film transistor DTr, a storage capacitor StgC, and an organic electroluminescent diode E, .

A gate line GL is formed in a first direction and a data line DL arranged in a second direction intersecting the first direction and defining the pixel region P together with the gate line GL is formed And a power supply line PL for applying a power supply voltage, which are spaced apart from the data line DL, are formed.

A switching thin film transistor STr is formed at the intersection of the gate line GL and the data line DL and a driving thin film transistor STr electrically connected to the switching thin film transistor STr is formed in each pixel region P (DTr) is formed.

At this time, the driving thin film transistor DTr is electrically connected to the light emitting diode E. That is, the first electrode, which is one terminal of the light emitting diode E, is connected to the drain electrode of the driving thin film transistor DTr, and the second electrode, which is the other terminal, is grounded.

On the other hand, the power supply line PL is connected to the source electrode of the driving thin film transistor DTr to transmit the power supply voltage to the light emitting diode E. A storage capacitor StgC is formed between the gate electrode and the source electrode of the driving thin film transistor DTr.

Therefore, when a signal is applied through the gate line GL, the switching thin film transistor STr is turned on, and the signal of the data line DL is transmitted to the gate electrode of the driving thin film transistor DTr, DTr are turned on so that light is output through the light emitting diode E. At this time, when the driving thin film transistor DTr is turned on, the level of the current flowing from the power supply line PL to the organic electroluminescent diode E is determined, A gray scale can be realized. The storage capacitor StgC serves to keep the gate voltage of the driving thin film transistor DTr constant when the switching thin film transistor STr is turned off so that the switching thin film transistor STr is turned off The level of the current flowing through the light emitting diode E can be kept constant until the next frame.

Here, the light emitting diode E includes an emitting material layer (EML) located between an anode and a cathode. A hole transporting layer (HTL) is formed between the anode and the light emitting material layer, and an electron transporting layer (HTL) is formed between the cathode and the light emitting material layer to inject electrons from the anode and the cathode into the light emitting material layer. ETL) is located. In order to more efficiently inject holes and electrons, a hole injecting layer (HIL) is formed between the anode and the hole transporting layer, and an electron injecting layer (EIL) is provided between the electron transporting layer and the cathode .

However, when a current is applied to the light-emitting diode E having such a structure, the interface between the anode and the hole injection layer is unstable and the current injection is not properly performed. As a result, the device is deteriorated, . In order to compensate the interface instability, a buffer layer may be formed between the anode and the hole injection layer to improve the lifetime of the device. In this case, the buffer layer becomes a barrier against the flow of charges. Therefore, a larger driving voltage is required to obtain the same efficiency of the light emitting layer, and the power consumption increases.

Further, in the organic electroluminescent device, in order to obtain the maximum efficiency, the injection of electrons and holes must be balanced. In general, however, there is a problem that the hole transport speed is higher than the electron transport speed and the holes are not trapped in the light emitting layer and are moved to the electron transport layer, resulting in a decrease in efficiency of the light emitting layer.

It is an object of the present invention to provide an organic electroluminescent device capable of improving the lifetime and luminous efficiency of a device and a method of manufacturing the same.

It is another object of the present invention to provide an organic electroluminescent device capable of preventing leakage current caused by uneven brightness by blocking leakage current and a method of manufacturing the same.

An organic electroluminescent device of the present invention includes: a substrate having a plurality of pixel regions defined therein; A thin film transistor formed on each of the pixel regions on the substrate; A first electrode corresponding to the plurality of pixel regions, respectively, and electrically connected to the thin film transistor; A bank formed between the pixel regions adjacent to the first electrode; (-CN, -NC), a hydroxyl group (-OH), a halide group (-I, -Br, -F), and the like which are in contact with the first electrode and the bank and have a thickness of 50 Å or less. An electron withdrawing layer formed of a material containing an electron withdrawing functional group; A hole transporting layer formed on the electron-withdrawing layer; A light emitting material layer formed on the hole transport layer and including first to third emission patterns spaced apart from each other at an upper portion of the bank, the first to third emission patterns corresponding to the plurality of pixel regions, respectively; An electron transport layer on the light emitting material layer; And a second electrode formed on the electron transport layer.

And a hole injection layer between the electron-withdrawing layer and the hole transporting layer.

A method of manufacturing an organic electroluminescent device of the present invention includes the steps of: forming a thin film transistor on a substrate corresponding to each of a plurality of pixel regions; Forming a first electrode corresponding to the plurality of pixel regions, respectively, and electrically connected to the thin film transistor; Forming, on the first electrode, a corresponding bank between adjacent pixel regions; (-CN, -NC), a hydroxyl group (-OH), a halide group (-I, -Br, -F), and the like which are in contact with the first electrode and the bank and have a thickness of 50 Å or less. Forming an electron-withdrawing layer with a substance containing an electron-withdrawing functional group; Forming a hole transport layer on the electron-withdrawing layer; Forming a light emitting material layer on the upper portion of the hole transport layer, the first light emitting layer including first to third light emitting patterns spaced apart from each other on the upper portion of the bank, Forming an electron transport layer on the light emitting material layer; And forming a second electrode on the electron transport layer.

Here, the method may further include forming a hole injection layer between the electron-withdrawing layer and the hole transporting layer.

The electron-withdrawing layer has a thickness of 20 A or less.

The bank has a thickness of about 1.5 micrometers and has a side inclined from 40 degrees to 50 degrees.

The electron-withdrawing layer is formed of a material represented by the following formula 1-1 or 1-2.

(1-1)

1-2

In the present invention, in order to facilitate injection and transport of holes, a driving voltage can be lowered by further forming an electron-withdrawing layer between the anode and the light-emitting material layer. At this time, the electron withdrawing layer is formed directly on the bank and the anode having a certain thickness and inclined side face, and the inclined side surface of the bank breaks the connection between the portion located above the anode and the portion located above the bank. Therefore, the path of the leakage current through the electron withdrawing layer is shut off, the luminance deviation does not occur, and the lighting failure is improved.

1 is a circuit diagram of one pixel region of a general active organic electroluminescent device.
2 is a cross-sectional view of an organic electroluminescent device according to an embodiment of the present invention.
3 is an enlarged view of the area A in Fig.
4A and 4B are views showing an image of an organic electroluminescent device according to an embodiment of the present invention, observed with a microscope.
FIG. 5 is a view showing a current leakage path in an organic electroluminescence device according to an embodiment of the present invention. FIG.
6 is a schematic view illustrating a structure of an organic electroluminescence device according to another embodiment of the present invention.
7 is an enlarged view of a region B in Fig.
8 is a view showing an image of an organic electroluminescent device according to another embodiment of the present invention, observed with a microscope.

Hereinafter, the organic electroluminescent device according to the present invention will be described in detail with reference to the accompanying drawings.

2 is a cross-sectional view of an organic electroluminescent device according to an embodiment of the present invention. Here, the switching thin film transistor of each pixel region P is omitted, and only the driving thin film transistor DTr corresponding to one pixel region P is shown. The switching thin film transistor has the same structure as the driving thin film transistor DTr.

2, the organic electroluminescent device according to the embodiment of the present invention includes a substrate 110 on which a display area for displaying an image and a non-display area around a display area to which a driving integrated circuit is connected are defined, , And the display region of the substrate 110 includes a plurality of pixel regions (P).

Here, the substrate 110 may be made of a transparent glass material or a transparent plastic or a polymer film having excellent flexibility.

Gate wirings (not shown) and data wirings 130 are formed on the substrate 110 to define the pixel regions P so as to cross the gate wirings (not shown) or the data wirings 130, (Not shown) is formed. A switching thin film transistor (not shown) and a driving thin film transistor DTr are formed in each of the plurality of pixel regions P and connected to the drain electrode 136 of the driving thin film transistor DTr to form a first electrode 147 Is formed. A light emitting material layer 155 is formed on the first electrode 147. The light emitting material layer 155 is formed of red, green, blue, And the first to third light emission patterns. A second electrode 158 is formed on the entire surface of the display region on the light emitting material layer 155. At this time, the first electrode 147, the light emitting material layer 155, and the second electrode 158 constitute a light emitting diode E.

Although not shown, an encapsulation substrate is provided in correspondence with the substrate 110 having the above-described structure, and a transparent organic insulating material or a polymer material having a bonding property and having a frit or adhesive property of a transparent material, A seal is formed between these two substrates by applying or attaching to the whole surface. By forming such a seal 180, the substrate 110 and the encapsulation substrate (not shown) remain in a cemented state.

The structure of the organic electroluminescent device according to the embodiment of the present invention will be described in more detail.

As shown in the figure, in the substrate 110, a pixel region P in the display region, a first region 113a made of polysilicon and constituting a channel are formed at a position where the thin film transistor is to be formed, And a second region 113b doped with a high concentration of impurities are formed on both sides of the semiconductor layer 113a. An insulating layer (not shown) made of silicon oxide (SiO ₂ ) or silicon nitride (SiN x), which is an inorganic insulating material, is formed between the semiconductor layer 113 and the substrate 110, As shown in FIG. The reason why such an insulating layer is provided below the semiconductor layer is to prevent deterioration of the characteristics of the semiconductor layer 113 due to the release of alkali ions from the inside of the substrate 110 when the semiconductor layer 113 is crystallized.

A gate insulating layer 116 is formed on the entire surface of the substrate 110 so as to cover the semiconductor layer 113. A portion of the gate insulating layer 116 corresponding to the first region 113a of the semiconductor layer 113 A gate electrode 120 is formed.

A gate wiring (not shown) extending in one direction is formed on the gate insulating layer 116 in connection with a gate electrode (not shown) of the switching thin film transistor. At this time, the gate electrode 120 and the gate wiring (not shown) may be formed of a first metal material having low resistance characteristics such as aluminum (Al), aluminum alloy (AlNd), copper (Cu), copper alloy, molybdenum (Mo), and moly titanium (MoTi), or may have a double layer structure or a triple layer structure by stacking two or more of the first metal materials mentioned above. In the drawing, the gate electrode 120 and the gate wiring (not shown) have a single layer structure.

An interlayer insulating film 123 (for example, silicon oxide (SiO ₂ ) or silicon nitride (SiNx)), which is an inorganic insulating material, is formed on the entire surface of the display region over the gate electrode 120 and the gate wiring Is formed. The semiconductor layer contact hole 125 exposing the second regions 113b of the respective semiconductor layers is formed in the interlayer insulating layer 123 and the gate insulating layer 116 below the interlayer insulating layer 123.

In addition, the pixel region P is defined on the interlayer insulating film 123 including the semiconductor layer contact hole 125 to cross the gate wiring (not shown), and a second metal material, for example, A data wire made of any one or more of aluminum (Al), aluminum alloy (AlNd), copper (Cu), copper alloy, molybdenum (Mo), molybdenum (Mo), chrome (Cr) And a power supply wiring (not shown) is formed therebetween. At this time, the power supply wiring (not shown) may be formed so as to be spaced apart from the gate wiring (not shown) on the layer on which the gate wiring (not shown) is formed, that is, on the gate insulating film.

The source and drain electrodes 123 and 123 are formed on the interlayer insulating layer 123 and are in contact with the second region 113b exposed through the semiconductor layer contact hole 125, And drain electrodes 133 and 136 are formed.

At this time, the semiconductor layer 113, the gate insulating film 116, the gate electrode 120, and the interlayer insulating film 123, which are sequentially stacked, together with the source and drain electrodes 133 and 136 spaced apart from each other, , And more specifically, a driving thin film transistor DTr.

In the drawing, the data line 130 and the source and drain electrodes 133 and 136 all have a single layer structure. However, these elements may have a double layer structure or a triple layer structure.

As described above, a switching thin film transistor having the same lamination structure as the driving thin film transistor DTr is formed on the substrate 110 as well. At this time, the switching thin film transistor (not shown) is electrically connected to the driving thin film transistor DTr, the gate wiring (not shown) and the data wiring 130. That is, the gate wiring (not shown) and the data wiring 130 are respectively connected to a gate electrode (not shown) and a source electrode (not shown) of the switching thin film transistor (not shown) A drain electrode (not shown) of a switching thin film transistor (not shown) is electrically connected to the gate electrode 120 of the driving thin film transistor DTr.

In the substrate 110 for an organic EL device according to an embodiment of the present invention, the driving thin film transistor DTr and the switching thin film transistor (not shown) have a polysilicon semiconductor layer 113, Type TFT transistor and a switching thin film transistor (not shown) may be a bottom gate type having a semiconductor layer of amorphous silicon It is self-evident.

When the driving and switching thin film transistors are constructed as a bottom gate type, the laminated structure includes a gate electrode, a gate insulating film, an active layer of pure amorphous silicon, a semiconductor layer formed of an ohmic contact layer of impurity amorphous silicon spaced apart from each other, And a source electrode and a drain electrode. At this time, a gate wiring is formed on the same layer as the gate electrode and is connected to the gate electrode of the switching thin film transistor, the data wiring is formed on the same layer as the source electrode of the switching thin film transistor, and is connected to the source electrode . A protective layer 140 having a drain contact hole 143 exposing the drain electrode 136 of the driving thin film transistor DTr is formed on the driving thin film transistor DTr and the switching thin film transistor .

The protective layer 140 is in contact with the drain electrode 136 of the driving thin film transistor DTr through the drain contact hole 143, An electrode 147 is formed. The first electrode 147 may be formed of a transparent conductive material such as indium tin oxide (ITO).

Next, a bank 150 made of an insulating material, for example, an organic insulating material such as benzocyclobutene (BCB) or photo acryl, is formed on the boundary of each pixel region P above the first electrode 147 . At this time, the bank 150 is formed so as to overlap with the edge of the first electrode 147 in the form of surrounding each pixel region P, and has a grid shape having a plurality of openings as a whole in the display region.

A light emitting material layer 155 is formed on the first electrode 147 in each pixel region P surrounded by the bank 150. The light emitting material layer 155 is composed of first to third light emitting patterns (not shown) that emit red, green, and blue light, respectively. A hole injecting layer and a hole transporting layer are further formed under the light emitting material layer 155 and an electron transporting layer and an electron injecting layer (electron injecting layer) may be further formed. This will be described below again.

A second electrode 158 is formed on the light emitting material layer 155 and the bank 150 over the entire surface of the display region. The second electrode 158 may be made of a material such as opaque metal. The light emitting material layer 155 interposed between the first electrode 147 and the second electrode 158 and between the two electrodes 147 and 158 constitutes a light emitting diode E.

Meanwhile, the first electrode 147 may be formed of an opaque material, and the second electrode 158 may be formed of a transparent material.

Fig. 3 is an enlarged view of the region A in Fig. 2, and shows a laminated structure of the light emitting diodes E at the boundary of the pixel region P. Fig.

3, a first electrode 147 is formed on the protective layer 140, and a bank 150 is formed on the first electrode 147 in correspondence with the boundary of the pixel region. A buffer layer 151 is formed on the bank 150 and a first hole injection layer 152a, an electron attraction layer 153 and a second hole injection layer 152b are sequentially formed on the buffer layer 151. [ A hole transport layer 154 is formed on the second hole injection layer 152b and a light emitting material layer including the first emission pattern 155a and the second emission pattern 155b is formed thereon. The first and second light emission patterns 155a and 155b correspond to the respective pixel regions and emit light of different colors. On the other hand, the light emitting material layer further includes a third light emitting pattern (not shown).

An electron transport layer 156 is formed on the light emitting material layers 155a and 155b, and a second electrode 158 is formed thereon.

Here, the first electrode 147 serves as an anode and the second electrode 158 serves as a cathode.

In the organic electroluminescent device having such a structure, the buffer layer 151 is formed to compensate the interface instability between the first electrode 147 and the first hole injection layer 152a adjacent thereto. Since the buffer layer 151 functions as a barrier to increase the driving voltage, the buffer layer 151 may be formed between the buffer layer 151 and the light emitting material layers 155a and 155b The electron-withdrawing layer 153 is further formed. At this time, the electron attracting layer 153 is formed between the first and second hole injecting layers 152a and 152b, and its thickness is about 55 ANGSTROM.

The electron withdrawing layer 153 is formed of an organic material containing electron withdrawing functional groups such as a cyanating group (-CN, -NC), a hydroxyl group (-OH), and a halide group (-I, -Br, -F). For example, the electron withdrawing layer 153 may be formed of LGC101 represented by the following formula 1 or F4TCNQ represented by the following formula (2).

Formula 1

(2)

Accordingly, in the organic electroluminescence device according to the embodiment of the present invention, the driving voltage can be lowered, and the luminous efficiency and lifetime can be improved.

However, when such an organic electroluminescent device is turned on, defects in brightness in the longitudinal direction are caused due to variations in luminance between data wirings.

4A and 4B are views showing an image of an organic electroluminescent device according to an embodiment of the present invention, observed with a microscope. Fig. 4A shows a plurality of pixel regions, and Fig. 4B shows one pixel region in which defects appear.

As shown in FIGS. 4A and 4B, it can be seen that a pixel region having a lower luminance than that of a normal pixel region is observed.

5, a first path R1 in which a lateral current flows through the electron-withdrawing layer 153 having an excellent electrical conductivity from the first electrode 147, This is because a second path R2 is generated in which a vertical current flows through a region where no light emitting material layer is formed between the light emission patterns 155a and 155b. As a result, a leakage path is formed, resulting in a luminance difference between pixel regions.

In another embodiment of the present invention, an electron-withdrawing layer of a certain thickness is directly formed on the first electrode and the bank in order to prevent a luminance deviation due to such a leakage current.

FIG. 6 is a schematic view of a structure of an organic electroluminescent device according to another embodiment of the present invention, showing a laminated structure of light emitting diodes at a boundary of a pixel region.

6, a first electrode 247 is formed corresponding to each pixel region on the protective layer 240, and a bank 250 is formed on the first electrode 247 in correspondence with the boundary of the pixel region . The bank 250 has a thickness of about 1.5 micrometers and is formed with a side sloping structure. The inclination angle of the bank 250 is about 40 degrees to about 50 degrees with respect to the upper surface of the first electrode 247. [

An electron withdrawing layer 252 is formed on the bank 250 to a thickness of about 50 A or less. A hole transporting layer 253 is formed on the electron withdrawing layer 252 and a light emitting material layer including one emission pattern 255a and the second emission pattern 255b is formed on the hole transporting layer 253. The first and second emission patterns 255a and 255b correspond to the respective pixel regions and emit light of different colors. On the other hand, the light emitting material layer further includes a third light emission pattern (not shown) corresponding to another pixel region.

An electron transport layer 256 is formed on the light emitting material layers 255a and 255b, and a second electrode 258 is formed thereon.

A hole injecting layer (not shown) may be further provided between the electron attracting layer 252 and the hole transporting layer 253, and an electron injecting layer (not shown) may be interposed between the electron transporting layer 256 and the second electrode 258. (Not shown).

7 is an enlarged view of a region B in Fig.

7, the electron attracting layer 252 is directly formed on the first electrode 247 and the bank 250. The electron attracting layer 252 is formed on the first electrode 247 by the inclination of the bank 250, And the portion located above the bank 250 are disconnected without being connected. As a result, the leakage path is cut off at the inclined portion of the bank 250, thereby preventing the leakage current from being generated.

The thickness of the electron-withdrawing layer 252 is about 50 angstroms or less, more preferably about 20 angstroms or less.

8 is a view showing an image of an organic electroluminescent device according to another embodiment of the present invention, observed with a microscope.

As shown in Fig. 8, it can be seen that the luminance of the organic electroluminescent device is uniform. That is, the path of the leakage current is cut off, the luminance deviation does not occur, and the lighting failure is improved.

The present invention is not limited to the embodiment, and various changes and modifications are possible without departing from the spirit of the present invention.

240: protective layer 247: first electrode
252: electron withdrawing layer 253: positive hole transporting layer
255a, 225b: emission pattern 256: electron transport layer
258: second electrode

Claims (10)

A substrate on which a plurality of pixel regions are defined;
A thin film transistor formed on each of the pixel regions on the substrate;
A first electrode corresponding to the plurality of pixel regions, respectively, and electrically connected to the thin film transistor;
A bank formed between the pixel regions adjacent to the first electrode;
A first electrode and a portion directly above the bank, the first electrode being in contact with the first electrode and the bank and being disconnected between the upper portion of the first electrode and the upper portion of the bank, An electron withdrawing layer formed of a material containing an electron withdrawing functional group selected from a hydroxyl group (-OH) or a halide group (-I, -Br or -F);
A hole transporting layer formed on the electron-withdrawing layer;
A light emitting material layer formed on the hole transport layer and including first to third emission patterns spaced apart from each other at an upper portion of the bank, the first to third emission patterns corresponding to the plurality of pixel regions, respectively;
An electron transport layer on the light emitting material layer;
A second electrode formed on the electron transport layer,
Lt; / RTI >
The method according to claim 1,
Wherein the electron withdrawing layer has a thickness of 50 ANGSTROM or less.
The method according to claim 1,
Wherein the bank has a thickness of 1.5 micrometers and has a side inclined by 40 to 50 degrees.
The method according to claim 1,
Wherein the electron withdrawing layer is formed of a material represented by the following formula (1) or (2).
Formula 1

(2)

The method according to claim 1,
And a hole injection layer between the electron-withdrawing layer and the hole transporting layer.
Forming thin film transistors corresponding to each of a plurality of pixel regions on a substrate;
Forming a first electrode corresponding to the plurality of pixel regions, respectively, and electrically connected to the thin film transistor;
Forming, on the first electrode, a corresponding bank between adjacent pixel regions;
Wherein the first electrode and the bank are connected to the first electrode and the bank, and the first electrode is directly connected to the first electrode and the bank, Forming an electron-withdrawing layer with a material containing electron-withdrawing functional groups selected from a group (-OH) or a halide group (-I, -Br or -F);
Forming a hole transport layer on the electron-withdrawing layer;
Forming a light emitting material layer on the upper portion of the hole transport layer, the first light emitting layer including first to third light emitting patterns spaced apart from each other on the upper portion of the bank,
Forming an electron transport layer on the light emitting material layer;
Forming a second electrode on the electron transport layer
Emitting layer.
The method of claim 6,
Wherein the electron withdrawing layer has a thickness of 50 ANGSTROM or less.
The method of claim 6,
Wherein the bank has a thickness of 1.5 micrometers and has a side inclined by 40 to 50 degrees.
The method of claim 6,
Wherein the electron withdrawing layer is formed of a material represented by the following formula (1) or (2).
Formula 1

(2)

The method of claim 6,
And forming a hole injection layer between the electron-withdrawing layer and the hole transporting layer.

Images