KR20040094078A - Organic elcetroluminescence display emitting white color - Google Patents

Abstract

PURPOSE: A white organic electroluminescence device is provided to achieve improved luminous efficiency, and reduce the weight of the device. CONSTITUTION: A white organic electroluminescence device comprises an anode, a hole transport layer, an emitting layer, an electron transport layer, and a cathode. The anode is formed by depositing an organic layer on an indium-tin-oxide through a vacuum deposition/sputtering process. An anode buffer layer is used as a hole injection layer which contains TPD, NPD, and star-burst based compounds. The emitting layer formed between the electron transport layer and the hole transport layer is constituted by two or more organic substances for emitting light through the control of change of wavelength.

Description

White organic light emitting device {Organic elcetroluminescence display emitting white color}

The present invention relates to a white organic light emitting device, and more particularly, to a proposal of a new configuration of an organic light emitting device that can be used as a backlight as a white light emitting device.

Among the various types of display devices, the proportion is gradually shifting to the flat panel display (FPD), among which the liquid crystal display (LCD), the light emitting plasma (PDP), and the field emission display (FED) Increasingly, thin-film and high-performance FPDs such as light emitting diodes (LEDs), vaccum fluorescent displays (VFDs), and organic light emitting devices (OLEDs) are increasingly being used.

LCD, which is widely used at present, is expected to remain strong in terms of demand for a long time due to its small volume, light weight, and low power consumption compared to CRT.However, since LCD is a light-receiving device rather than a self-light emitting device, brightness, contrast, viewing angle, and facing There are technical limitations in redundancy, and efforts to overcome these shortcomings are being made globally.

The organic electroluminescence display, which emits light more than LCD using backlight, has several advantages such as low voltage driving, light weight, high luminous efficiency, wide viewing angle and fast response speed. It will be able to solve the shortcomings, and it will be commercialized as a next-generation information display for mobile phones and personal digital assistants (PDAs).

The present invention relates to an anode electrode formation technique, a light emitting layer formation technique, a cathode electrode formation technique, a buffer layer formation technique, and the like, which are the basic techniques for manufacturing a white light emitting organic backlight having high luminous efficiency and luminance. An object of the present invention is to provide an organic light emitting device that emits white light that can be used as a backlight of various devices.

In addition, the technical feature to be provided through the present invention is to develop an organic light emitting device capable of emitting white light with high luminance while having a light and flexible flash.

In other words, the thickness of the light emitting device is 5 cm × 5 cm and the light emitting device has a brightness of 100 cd / m ² or more, and the thickness of the light emitting device is about several millimeters. It is.

1 is a perspective view showing a configuration of a conventional organic light emitting device.

FIG. 2 is an explanatory diagram illustrating a light emission process by recombination by injecting holes and electrons into an organic light emitting diode; FIG.

3 is an AFM photograph of the surface of two ITO thin films having different surface resistances.

4 is an AFM photograph showing the change in surface roughness according to the buffer layer coated on ITO.

5 is a graph showing the reduction of surface roughness of a substrate to which a buffer layer is applied.

Fig. 6 is a chart showing the relationship between light emitting life according to various kinds of hole transport layer materials and kinds of materials.

7 is a schematic configuration diagram of each structure of an OLED device of the present invention.

8 is a spectral diagram according to the applied voltage of each of the manufactured OLED devices of FIG.

9 is an alignment diagram of HOMO-LUMO energy levels for structures I and III of FIG.

10 is an explanatory diagram showing a change according to a driving voltage of a light emission wavelength.

The white organic light emitting diode of the present invention will be described in more detail together with the accompanying drawings.

Hereinafter, the organic light emitting device of the present invention will be described in more detail based on the basic description of the conventional organic light emitting device of FIG. 1.

The organic light emitting device has a laminated structure that realizes the highest luminous efficiency possible through recombination of holes and electrons. In general, the substrate uses glass, but in some cases, a plastic or film type that can be flexibly bent may be applied.

The anode electrode on the substrate is formed of ITO (Indium-Tin-Oxide) formed by vacuum deposition or sputtering, and the organic layer is vacuum deposited for low molecular weight compounds, spin coating for high molecular weight compounds, or spin coating or printing. Apply.

Magnesium or lithium, which has a small work function, is applied to the cathode. Magnesium is formed by simultaneously depositing a small amount (1-5%) of silver with relatively good adhesion to an organic layer. At this time, the concentration of lithium is very low, 0.5-1% level.

In order to operate the device at low voltage, the total thickness of the organic thin film layer is very thin, such as 100-200 nm, and the surface is uniform and it is very important to maintain the stability of the device. The balance of the hole and electron density when the emission center is excited by the injection of the carrier can be said to be a key factor in the high efficiency of the device.

For example, when an electron transport layer (ETL) is positioned between an organic emitting layer (EML) and a cathode, most of the electrons injected into the emission layer at the cathode are anodes to recombine with holes. Will move toward

As shown in FIG. 1, when a hole transport layer (HTL) is inserted between an anode and an organic light emitting layer, electrons injected into the light emitting layer are blocked at an interface with the hole transport layer and are no longer moved and trapped in the organic light emitting layer. The recombination efficiency is improved.

In addition, the recombination region may be separated from the electrode interface, thereby improving the luminous efficiency. Therefore, an efficient organic EL device structure includes an anode-hole transport layer-organic light emitting layer in which a light emitting region by recombination, in which a hole and an electron transport layer is inserted between each electrode and the light emitting layer, is limited to the light emitting layer. It can be referred to as a five-layer structure of the emitting layer (electron transport layer)-cathode (cathode).

In addition, when the device is composed of a bipolar light emitting layer capable of simultaneously transporting holes and electrons and a light emitting layer having low carrier transport, it is necessary to completely separate the carrier transport and the light emitting functions.

The simplest device structure is a three-layer structure in which an organic light emitting layer is simply inserted between an anode and a cathode. In this case, the light emitting layer has the same function of transporting holes and electrons, but it is difficult to obtain high efficiency light emission.

On the other hand, a buffer layer, that is, a hole injection layer (HIL), is additionally inserted between the anode and the hole transport layer, which is a work function (4.7-5.0 eV) of the ITO anode electrode and the ionization energy (IP: ionization) of the hole transport layer. This is because the organic material having a work function of 5.0-5.2 eV is selected in consideration of the potential, and the role of lowering the energy barrier when injecting holes from the anode to the hole transport layer enables more effective hole injection.

In order to improve the efficiency of the device between the cathode electrode and the electron transport layer, an additional buffer layer is further laminated.In this case, unlike the hole injection layer concept, a highly reducible metal such as lithium is doped near the cathode interface by co-deposition to form an electron injection barrier. By lowering the effect of reducing the driving voltage is obtained.

The basic configuration of the two-layer stacked device includes a hole transport layer that injects and transports holes at an anode, that is, a HTL (Hole Transport Layer) and an electron transport layer that injects and transports electrons at a cathode, that is, an ETL (Electron Transport Layer). .

In such a two-layer laminated structure, it is better to think of the case where the ETL or HTL is responsible for the light emission function, and the case where the portion where holes and electrons are recombined is on the ETL side and on the HTL side.

A light emitting mechanism that emits light in a two layer stack of a typical OLED is shown in FIG.

The light emitting layer, that is, the EML (Emitting Layer) is arranged between the two layers independently from the HTL and the ETL, and has a three-layer lamination structure. The reason why luminous efficiency is improved by using a laminated structure rather than a single layer structure using a polymer is because it is possible to first obtain a proper balance between electrons and holes injected from an electrode.

Second, the hole injection / transport function and the electron injection / transport function at the electrode can be divided into different compounds so that the optimum design of the materials used for both layers is possible, and the electric field required for carrier injection / transport is possible. It is possible to lower it. In other words, the required injection current density can be realized with a low applied voltage.

At the anode, with the help of the applied voltage, holes are injected into the HTL and move towards the HTL / ETL interface. Holes are injected into the ETL again because of the small difference in ionization potential between the two layers at the HTL / ETL interface. Since the mobility of holes in the ETL is low, holes injected into the ETL cannot penetrate deep into the ETL.

On the other hand, electrons injected into the ETL in the cathode using a metal electrode having a small work function reach the HTL / ETL interface, but are not injected into the HTL because the difference in electron affinity at the interface where the electron affinity of HTL is small is too large. . In other words, it can be said that HTL plays a role of blocking electrons at the interface. Therefore, the density of holes and electrons is higher than the ETL layer near the HTL / ETL interface, where carrier recombination occurs.

The organic EL is similar to the inorganic EL compound in the concept of light emission by electric energy of a fluorescent material, but there are some differences in terms of excitation, which is the center of the light emitting mechanism.

Inorganic EL emits light by the energy generated by the collision of electrons accelerated by high voltage, while organic EL emits light by recombination of holes and electrons injected from the anode and the cathode.

In other words, as the frequency of the AC voltage increases, the number of emission times increases proportionally as the number of times of excitation within the unit time increases. However, in the organic EL, electrons and holes are injected from the outside, and the recombination energy therebetween has a direct current operating mechanism that excites the emission center.

Step by step in this excitation process ① carrier (electron and hole) injection from the anode and cathode electrodes to the organic layer; ② movement of the injected carrier to the positive and negative poles; ③ exciton generation by recombination of carriers; ④ movement and spread of the exclusion; ⑤ It can be said that it consists of light emission from here.

The organic materials used should be semi-insulated and capable of chemically injecting radical anions (electrons) and radicals (holes) into the organic layers. Therefore, radical anions for reducing electrons at the cathode and organic layer interfaces and radical oxidizing electrons should be formed at the anode and organic layer interfaces, which inject electrons into the LUMO (Lowest Unoccupied Molecular Orbital) of radical anions or HOMO of radical applications. It is possible by leaving electrons from (Highest Occupied Molecular Orbital). The excitons exist in the form of singlet and triplet, and organic materials generally do not emit light in the triplet state at room temperature.

In the case of excitons, luminescence is observed. Since the ratio of singlet and triplet excitons is statistically 1: 3, the internal quantum efficiency of the organic EL device is theoretically up to 25%. Recently, efforts have been made to overcome the theoretical limitations of internal quantum efficiency by enabling the emission of triplet excitons using rare earth organometallic complexes.

FIG. 2 conceptually shows a process in which holes and electrons are injected into the device to emit light to the outside by recombination, and may be used to design materials in consideration of fluorescence quantum efficiency and fluorescence spectrum characteristics to maximize the inherent advantages of the organic light emitting body. It can be seen that.

Hereinafter, the structural features of the organic light emitting device of the present invention will be described in more detail with reference to FIG. 3.

(1) influence by an ITO substrate;

The surface roughness and surface resistance of ITO, a transparent electrode used as a substrate, have a great influence on the characteristics of the organic light emitting device fabricated thereon.

Since the organic material is very thin in the organic light emitting device, if the surface of the ITO is rough, the thickness may vary depending on the part of the device, and this problem greatly affects the characteristics of the device.

3 shows AFM images of the surfaces of two ITO thin films having different surface resistances. ITO in FIG. 3 (a) has a surface resistance of 30 to 40 Ω and ITO shown in (b) has a resistance of 5 to 10 Ω.

As can be seen from the AFM image, the surface of the low resistance ITO is more uniform, and the higher resistance has a rough surface.

In addition, the surface roughness difference is about 10 times higher in luminance at the same voltage when the device is made. Therefore, the characteristics of the organic light emitting device show a big difference according to the selection of ITO, which is influenced by the electrical resistance and roughness of the surface of the ITO.

The ratio of oxygen in ITO obtained as a result of XPS analysis of ITO (1) and ITO (2) substrates is based on the ratio of [O] / (1.5 [In] +2.0 [Sn]) to 1 for pure ITO. As low as 0.88.

This low oxygen ratio reduces the electrical conductivity of the ITO and thus causes a decrease in the performance of the light emitting device, so it is necessary to increase the oxygen ratio. The methods mainly used to increase the oxygen concentration in the ITO substrate include O ₂ plasma treatment or treatment in O ₃ gas.

By changing the oxygen ratio in the ITO substrate close to 1 through this process, the HOMO level of ITO can be further lowered, thereby lowering the energy barrier for hole injection, thereby improving the performance of the device.

(2) Effect by the anode wood buffer layer:

It is known to use a buffer layer to facilitate the injection of holes into the hole transport layer on the ITO 2 substrate having lower surface resistance and lower surface roughness.

The buffer layers mainly used are PEDOT, m-MTDATA, CuPc, and the like. In the experimental process of the present invention, devices were fabricated using all of the buffer layers and analyzed.

After the three buffer layers were coated on the ITO (2), the change in the roughness of the surface was analyzed by AFM. This result is shown in FIG.

(A) ITO (2), (b) ITO (2) / CuPc, (c) ITO (2) / PEDOT 4000 (rpm) (d) ITO (2) / m-MTDATA will be.

Although not shown in the figure, the root mean squre (rms) roughness of the surface after application of the respective buffer layers is CuPc 2.591 (nm), PEDOT 1.118 (nm), and m-MTDATA 1.0966 (nm), respectively. It can be seen that the roughness of the significantly reduced than 2.737 (nm).

This result shows that the influence of the roughness of the ITO substrate on the light emitting device can be significantly reduced by using the buffer layer.

The results are summarized in FIG. 5.

However, the influence on the performance of the organic light emitting device by the ITO substrate is not only related to the roughness of the surface, but also affected by the easy hole injection by the buffer layer used.

The buffer layer used is known to facilitate the hole injection because it reduces the energy barrier (barrier) at the interface formed between the ITO and the hole transport layer.

Therefore, the effect of the buffer layer can be said to have a two-sided effect of reducing the surface roughness and at the same time lowering the energy barrier at the interface to facilitate hole injection.

(3) Effect of hole transport layer and electron transport layer:

As the hole transport layer material, TPD or NPD, and starburst organic materials are mainly used.

As described above, the thickness of the hole transport layer has been found to have a very important effect on the light emission characteristics of the organic light emitting device, and the hole transport layer material is important because the hole transport layer material plays a large role in the life of the organic light emitting device. The selection of is very important for the fabrication of the organic light emitting device.

6 shows light emitting lifespans according to various kinds of hole transport layer materials and types of materials.

TPD is the most widely used material among various kinds of hole transport layers, but TPD is the most easily used due to the heat generated during operation of the device because of the lowest glass transition temperature (T _g ) among other hole transport layer materials. There is a disadvantage that the life of the device is shortened by crystallization.

Therefore, α-NPD having a similar structure but having a high glass transition temperature of about 30 ^° C. is currently used the most. In fact, the glass transition temperature of Alq ₃ , which is the most widely used electron transport layer material, is much higher than that of the hole transport layer material. Thus, the material that has the greatest influence on the lifetime of the device is the hole transport layer.

Currently, the use of other types of materials is being considered as shown in FIG. 6 to develop a hole transport layer material having a higher glass transition temperature.

These materials are known as starburst materials because they have more benzene rings, their molecular weight is large, and their structure is quite complicated.

The hole transport layer with low glass transition temperature, such as TPD, can also extend the life of the device when the dopant such as rubrene is used, because the glass transition temperature of the material containing impurities is higher than the glass transition temperature of pure material. .

Alq ₃ is mainly used for the electron transport layer material. Alq ₃ is used as a light emitting layer (green) material itself, but is used as the most stable electron transport layer.

(4) Realization of white color by controlling emission wavelength

7 is a schematic view of an OLED device. 7 (a), (b) and (c) show structures I, II and III, respectively.

Structures I and II have the same structure except for the difference in concentration of DCM2 of the red light emitting dopant. The light emitting layer is in the order of blue, red, and green from the hole transport layer. In structure III, the order of the light emitting layers was the same as that of structures I and II, but α-NPD was inserted to limit the carrier movement between the red light emitting layer and the green light emitting layer.

The light emitting layer had a total thickness of 45 nm, and in the case of structure III, α-NPD was inserted at a thickness of 5 nm to limit the movement of the carrier.

8 is a spectrum according to the applied voltage of each manufactured OLED device. Structures I and II started emitting red light from the Alq ₃ layer doped with DCM2 at about 7V.

This is because electrons move at about twice the speed of holes in the light emitting layer. In other words, if electrons were injected simultaneously from the ETL and holes from the HTL, light emission starts in front of the red light emitting layer, which is the middle layer among the three light emitting layers, that is, about 15 nm away from the HTL-ETL interface.

As shown in FIG. 8A, when the applied voltage is increased to 9V, it can be seen that the blue light emitting layer has almost the same intensity as the red light emitting layer.

This is thought to be because electrons, which have relatively high speeds in the light emitting layer, move toward the hole transport layer as the applied voltage increases, and the recombination region is expanded from red to blue. Therefore, the green light emitting phenomenon, the third light emitting layer, could not be seen.

When the applied voltage is increased a little, the recombination region returns from the blue emission region to the red emission region and the blue emission is significantly reduced.

This phenomenon can be explained for two reasons as follows. The first reason is that electrons accumulated in the HTL-ETL boundary layer at high voltage do not pass through the HTL to affect the emission. The second reason is that the electrons accumulated in the HTL-ETL boundary layer are diffused into the red light emitting layer with low concentration. But just for this reason can not explain. Among the light emitting materials used in the present invention, a material exhibiting the lowest glass transition temperature (T _g ) is DPVBi.

Therefore, it is thought that DPVBi is thermally damaged by heat generated when a high voltage of 11 V or higher is applied, thereby degrading luminous efficiency. The emission spectrum of increasing the concentration of DCM2, a red luminescent dopant, to 10 wt% is shown in FIG.

As shown in the figure, as a result of doubling the concentration of the dopant, it was observed that only the red wavelength (about 650 nm) region doubled without changing the intensity of the other wavelength region. It can be seen that.

As mentioned above, green luminescence could not be observed with structures I and II. This is considered to be a phenomenon due to electrons having a relatively fast moving speed in the light emitting layer, and thus an experiment was performed to limit the movement of electrons by forming a structure as shown in FIG.

As shown in Fig. 8C, in the case of the structure III, the light emission phenomenon is slightly different from the structures I and II.

The emission of light at about 7 V is no different from that of structure I, but unlike the structure of the light emitting region in which red color is emitted in structure I, green emits light in structure III first.

When the applied voltage was increased to 9V, green and red light emitted together, and when the voltage was increased to 11V, the light emission intensity of green and red increased and a slight blue light emission phenomenon began to be observed. To explain this phenomenon, HOMO-LUMO energy levels for structures I and III are shown in FIGS. 9A and 9B.

As shown in the figure, in the case of FIG. 9 (a) in which α-NPD is not inserted, since there is no LUMO that can limit the movement of electrons, the movement of electrons is not hindered and moves toward HTL. Therefore, recombination is not easy to occur in the green light emitting layer.

However, when the α-NPD having a relatively high LUMO is inserted between the red and green light emitting layers, as shown in FIG. 9 (b), when the applied voltage is low, the recombination is green because the movement of electrons is restricted compared to the movement of holes. It happens in the light emitting layer.

However, when the applied voltage is increased, electrons can cross the energy barrier by the strong electric field, and the electrons start emitting light in the red region. At higher voltages, relatively fast electrons can cross the energy barrier of the blue and red boundary layer, and eventually emit blue light.

As shown in region (a) of FIG. 10, the CIE chromaticity coordinate of Structure I also changed as the applied voltage changed. As the voltage increases, red light is emitted and blue light is emitted, and when a higher voltage is applied, red light is emitted again.

This is not only due to the movement of the recombination region with the applied voltage. In fact, when the voltage is applied at 11V, the blue light emitting layer DPVBi is thermally damaged, and when the voltage is raised above 13V, it no longer emits light.

10 (b) also describes the same phenomenon as the region (a), but since the concentration of DCM2, which is only a red dopant, is high, the CIE value may be shifted toward a more red color.

In the region (c) of FIG. 10, green light emission is observed by inserting α-NPD between the red light emitting layer and the green light emitting layer, and when a higher voltage is applied, the recombination region moves to the blue and red regions. .

The organic light emitting device of the present invention having the configuration as described above; It provides a relatively high brightness white light emitting organic device and has utility in that it can be applied to various uses in a flexible form.

Claims (3)

In an organic light emitting device having a basic configuration of an anode (hole transport layer)-organic light emitting layer (emitting layer)-electron transport layer (electron transport layer)-cathode, The anode is composed of a thin film coated on the indium tin oxide (ITO) by vacuum deposition / sputtering by vacuum deposition, spin coating and printing; An anode buffer layer as a hole injection layer on the ITO, The hole transport layer includes a compound of TPD, NPD, starburst series, The organic light emitting layer between the electron transport layer and the hole transport layer is a white organic light emitting device, characterized in that the white light emission as the emission of the organic matter of the two paper forms of mixed light emission by controlling the change of the emission wavelength by the change of the drive voltage. The method of claim 1, The ITO is treated with an oxygen plasma treatment or O ₃ gas on the substrate surface to increase the oxygen concentration. The method of claim 1, The organic light emitting layer is a white organic light emitting device, characterized in that the laminate formed by stacking Alq ₃ , Alq ₃ DCM2, DPVBi for white light emission by the control of the emission wavelength.