KR100492850B1 - Organic electroluminescent device comprising a luminescent layer having a dopant with high bandgap - Google Patents

Abstract

The present invention relates to an organic electroluminescent device comprising a dopant having a large bandgap in a light emitting layer, and comprising an organic light emitting layer containing a dopant having a larger bandgap than a host. Electroluminescent devices have improved color purity, efficiency and lifetime.

Description

ORGANIC ELECTROLUMINESCENT DEVICE COMPRISING A LUMINESCENT LAYER HAVING A DOPANT WITH HIGH BANDGAP}

The present invention relates to an organic electroluminescent device comprising a light emitting layer having a dopant having a large bandgap, and the organic electroluminescent device according to the present invention has improved color purity and efficiency, and also has a long lifespan.

Conventionally, when dopants are doped into the light emitting layer to improve the performance of the organic electroluminescent device, a dopant with a small bandgap is usually doped by a host with a large bandgap to a low energy dopant. Excitons have been used to transmit and emit light.

For example, patents related to doping, such as US Pat. Nos. 4,769,292 (Eastman Kodak), 6,303,238 (Princeton University), and 6,310,360 (Princeton University), all have light emitting layers in which the bandgap of the host is greater than the bandgap of the dopant. An organic electroluminescent device comprising is disclosed, in which case the final luminous color of the device depends on the color of the dopant.

However, based on this conventional doping concept, not only is the synthesis of a host with a large bandgap practically easy, but also hole injection at the anode and electron injection at the cathode become difficult due to the large bandgap of the light emitting layer host, and naturally The on voltage is high and the efficiency is low. For this reason, the color purity of blue and green organic light emitting diodes has not yet met the NTSC standard.

Accordingly, it is an object of the present invention to provide an organic electroluminescent device having improved efficiency and lifespan, including color purity, by doping a light emitting layer with a dopant having a different characteristic than ever before.

In order to achieve the above object, in the present invention, in the organic electroluminescent device comprising a positive electrode, an organic layer and a negative electrode, the organic layer includes a light emitting layer containing a dopant having a bandgap larger than the host (host) It provides an organic electroluminescent device characterized in that.

Hereinafter, the present invention will be described in detail.

The structure of the organic electroluminescent device according to one embodiment of the present invention is shown in FIG. 1. As shown in FIG. 1, the organic electroluminescent device of the present invention may be composed of a substrate, a positive electrode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a negative electrode.

Here, the positive electrode and the negative electrode can be changed according to the configuration of the device, the light transmission can also be adjusted. The substrate generally uses glass, and may be variously applied to a silicon wafer, a polymer sheet, a metal, an inorganic material, etc. according to a special purpose. The hole injection layer and the hole transport layer are conventional ones used in the device, and may be made of a single molecule, a polymer, or a mixture of a single molecule and a polymer. The light emitting layer is mainly composed of a mixed thin film of a monomolecular host and a dopant, and a polymer mixed thin film is also possible. The electron transport layer and the electron injection layer are also conventionally used in the device, the electron transport layer can be both a single molecule and a polymer, the electron injection layer is an inorganic thin film such as LiF, an organic thin film such as Alq ₃ and other polymers with electron injection ability, etc. This is all possible. Generally, stable aluminum is used as the negative electrode, but lithium-aluminum alloy, magnesium-silver alloy, calcium, strontium, lithium, etc. may be used depending on the structure of the device.

The technical feature of the present invention lies in the light-emitting layer. According to the present invention, since the light-emitting layer includes a dopant having a larger band gap than the host, unlike the conventional doped thin film, the emission center is in the host, not the dopant, and the dopant is It stabilizes the light emitting layer and prevents a decrease in efficiency due to self-absorption between hosts. In addition, a host having a smaller bandgap can facilitate hole injection at the anode and electron injection at the cathode, thereby lowering the ON voltage and suppressing the decrease in efficiency.

In the present invention, any compound having a bandgap smaller than the dopant can be used as the host, and any compound having a bandgap larger than the host as the dopant can be used. For example, when using 4,4'-bis (2,2-diphenyl-vinyl) -biphenyl (DPVBi) (bandgap = 2.9 eV) of the formula (1) as a host as a dopant 2- (2-hydroxyphenyl) benzoxazola lithium (LiPBO) (bandgap = 3.1 eV) can be used.

According to the present invention, the dopant may be doped in an amount of 0.1 to 50% by weight with respect to the light emitting layer. When the amount of the dopant is less than 0.1% by weight, there is little effect of doping. The composition of reversed is difficult to obtain the desired effect.

The organic electroluminescent device according to the present invention including the light emitting layer having such characteristics can be used in all of blue, green and red, and is particularly advantageous in controlling color purity of blue and green required for commercializing full-color displays, and It has excellent lifespan and has a lifespan exceeding that of conventional TFT-LCDs such as showing an operating life of 10,000 hours or more, and thus can be usefully applied to devices using organic semiconductors such as organic photodiodes and organic photovoltaic batteries.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the scope of the present invention is not limited only to the following Examples.

Example 1: Fabrication of an organic electroluminescent device comprising a light emitting layer doped with a bandgap dopant 5%

(A) Cleaning the substrate

First, an indium tin oxide (ITO) coated glass substrate was immersed in deionized water containing 5% of a non-phosphorus cleaner, sonicated for 5 minutes, and then added 2-3 times in 5 minutes in deionized water. It was sonicated with. Subsequently, the glass substrates were sequentially sonicated in acetone and ethanol for 5 minutes each, followed by spin drying, and then placed in a glove box to perform UV-ozone cleaning.

(B) Formation of Organic Thin Film

The cleaned, ITO-coated glass substrate was placed in a deposition chamber, and as a hole injection layer, 4,4 ', 4 "-tris (N- (2-naphthyl) -N-phenylamino) -triphenyl After the amine (2TNATA) was deposited to a final thickness of 600 kPa at a rate of 0.4 kW / s, N, N'-di (naphthalen-1-yl) -N, N'-diphenyl represented by the following formula (4) as a hole transport layer -Benzidine (NPB) was deposited to a final thickness of 120 kPa at a rate of 1 kW / s, followed by 4,4'-bis (2,2-diphenyl-vinyl) -biphenyl (DPVBi) ( 2- (2-hydroxyphenyl) benzoxazolato lithium (LiPBO) (compound of formula 2, bandgap = 3.1eV) as a dopant at a rate of 1 dl / s Was deposited to a final thickness of 400 kPa at a rate of 0.2 kW / s As the electron transporting layer, tris (8-hydroxyquinolinolato) aluminum (Alq ₃ ) of Chemical Formula 5 was added at a rate of 1 kW / s. Deposition to 200Å It was.

(C) Formation of electron injection layer and negative electrode

LiF was deposited as an electron injection layer at a rate of 0.1 mW / s to have a final thickness of 22 mW, and then aluminum was continuously deposited at a speed of 7 mW / s to have a final thickness of 2500 mW to finish device fabrication.

(D) encapsulation of the device

After mounting a prepared stainless steel can in a holder and dispensing the photocurable resin, the fabricated device is appropriately placed thereon, and the device is encapsulated by irradiating with ultraviolet (wavelength 365nm, energy 6000mJ) for 120 seconds. It was.

Example 2 Fabrication of Organic Electroluminescent Device Comprising Light-Emitting Layer Doped with 10% Dopant with Large Band Gap

An encapsulated organic electroluminescent device was manufactured in the same manner as in Example 1, except that LiPBO, a dopant of the light emitting layer, was doped at a rate of 0.4 μs / s.

Comparative Example 1: Fabrication of an organic electroluminescent device comprising an undoped light emitting layer

An encapsulated organic electroluminescent device was manufactured in the same manner as in Example 1, except that doping was not performed.

Comparative Example 2: Fabrication of an organic electroluminescent device comprising a light emitting layer doped with a 2.5% dopant having a small band gap

As a dopant of the light emitting layer, 4,4'-bis (2-N-ethylcarbazolyl-vinyl) -biphenyl (DSA) (Band gap = 2.7 eV) of the following Chemical Formula 6 was doped at a rate of 0.1 μs / s. Then, an organic electroluminescent device encapsulated was manufactured in the same manner as in Example 1.

Figure 1 shows the current density-voltage characteristics, luminance-voltage characteristics, luminance-current density characteristics, power efficiency-luminance characteristics, color coordinates-luminance characteristics, and electroluminescence spectra of the organic electroluminescent devices fabricated in Examples 1, 2 and Comparative Example 1. FIG. It is shown to 2-7 respectively.

It can be seen from the current density-voltage and luminance-voltage graphs of FIGS. 2 and 3 that the current density and luminance decrease as the doping amount of LiPBO increases. However, since the organic electroluminescent devices fabricated in Examples and Comparative Examples are not voltage-dependent but current-dependent, the luminance of the organic electroluminescent device is changed according to the change of the current density. As a result, it can be seen from FIG. 4 that the luminance increases as the doping amount of LiPBO increases at the same current density, and the difference becomes larger as the current density increases.

Further, it can be seen from the power efficiency-luminance graph of FIG. 5 that the power efficiency also increases as the doping amount of LiPBO increases, and the difference also increases as the luminance increases.

From the color coordinate-luminance graph of FIG. 6, the color coordinates of LiPBO-doped devices (Examples 1 and 2) are basically better than those without dope (Comparative Example 1) and their width remains almost constant regardless of luminance. It can be seen. That is, in the case of blue, the NTSC standard color coordinates are (x = 0.14, y = 0.08), and it can be seen that the color coordinates of the device including the light emitting layer doped with LiPBO are closer to the standard than those without dope.

As can be seen from the electroluminescence spectrum of FIG. 7, the main peaks of the devices obtained in Examples 1, 2 and Comparative Example 1 are all about 450 nm, but there is no significant difference, but the spectral intensity is different in the long wavelength region between 500 nm and 700 nm. . In general, the good and bad color purity is mainly influenced by the tail portion of the spectrum, especially the long wavelength region in blue and green, and the short wavelength region in red. Therefore, as the content of LiPBO, which is a dopant with a large band gap, increases, the intensity of the long wavelength region decreases, and thus the color coordinate is improved. In particular, the efficiency of the device decreases toward the blue and red centering on the green. As can be seen from the above results, the efficiency of the device is increased by more than 15% compared to Comparative Example 1 in spite of the good color coordinate in Example 2 It clearly shows that the performance of.

The electroluminescence spectrum of the organic electroluminescent device manufactured in Comparative Example 2 is shown in FIG. 7 as shown in FIG. 8. 8 shows that in Comparative Example 2 using a dopant having a small band gap, it is not easy to obtain excellent color purity because the main peak of the spectrum shifts. In order to minimize the shift of the spectrum, even if a dopant having a minimum difference in the bandgap is used, the bandgap is basically smaller than that of the host.

Experimental Example: Measurement of Life of Organic Electroluminescent Device by Temperature Accelerated Life Measurement

The temperature acceleration life measurement method is a method of measuring the decrease in initial luminance over time after raising the temperature around the device to be measured above room temperature. The life at room temperature can also be predicted based on a known Arennius equation. . The lifespan of the organic electroluminescent device manufactured in Example 2 was measured according to the temperature acceleration life measurement method under the following conditions.

Initial luminance: 30 cd / m ²

-Current application: DC constant current mode

-Temperature conditions: 4 kinds of temperature of 50 ℃, 65 ℃, 75 ℃ and 85 ℃

Humidity condition: 45% RH

The measured decrease in temperature (50 ° C., 65 ° C., 75 ° C. and 85 ° C.) luminance-time and life-temperature characteristics are shown in FIGS. 9 and 10, respectively. From Fig. 9, the time at which the luminance of the device according to the present invention is halved at 75 ° C, that is, the half-life is about 650 hours, far surpassing 240 hours at 70 ° C, which is generally guaranteed by a TFT-LCD for a mobile terminal It can be seen that it has. In addition, the graph predicting the room temperature life of FIG. 10 shows that the time when the brightness of the device according to the present invention is reduced by 65% is estimated to be about 9,500 hours, and the half-life is close to about 20,000 hours.

The organic electroluminescent device according to the present invention can be used for both blue, green and red, and is particularly advantageous for controlling the color purity of blue and green required for commercializing full-color displays, and has excellent efficiency and shows an operating life of 10,000 hours or more. Since the product has a lifespan exceeding that of TFT-LCD, it can be usefully applied to devices using organic semiconductors such as organic photodiodes and organic photovoltaic batteries.

1 is a view showing the structure of an organic electroluminescent device according to an embodiment of the present invention,

2 to 7 show current density-voltage graphs, luminance-voltage graphs, luminance-current density graphs, power efficiency-luminance graphs, and color coordinates-luminances of the organic electroluminescent devices fabricated in Examples 1, 2 and Comparative Example 1, respectively. Graph and electroluminescence spectrum,

FIG. 8 shows the electroluminescence spectrum of the organic electroluminescent device manufactured in Comparative Example 2 together with the electroluminescence spectrum of FIG.

9 and 10 show a luminance reduction rate-time graph and a lifetime-temperature graph of the organic electroluminescent device manufactured in Example 2, respectively.

Claims (3)

An organic electroluminescent device comprising a positive electrode, an organic layer and a negative electrode, wherein the organic layer comprises an organic electroluminescent device comprising a light emitting layer containing a blue light emitting host and a blue light emitting dopant having a larger bandgap. device. The method of claim 1, Wherein the host is 4,4′-bis (2,2-diphenyl-vinyl) -biphenyl (DPVBi) and the dopant is 2- (2-hydroxyphenyl) benzoxazolato lithium (LiPBO) Organic electroluminescent device. The method of claim 1, An organic electroluminescent device, characterized in that the light emitting layer comprises 0.1 to 50% by weight of dopant.