KR100809937B1 - Light emitting device - Google Patents

Abstract

A light emitting device is provided to emit white light with high color purity by supplying holes to a second light emitting layer by interposing an electron accepting layer between first and second light emitting layers. A light emitting device includes a substrate(200), a first electrode(210), a first light emitting layer(241), a second light emitting layer(243), and a second electrode(260B). The first electrode is placed on the substrate. The first light emitting layer is placed on the first electrode and includes a host(241A), a dopant(241B), and a hole transport material(241C). The second light emitting layer is placed on the first light emitting layer and includes a host(243A) and a dopant(243B). The second electrode is placed on the second light emitting layer. A third light emitting layer is positioned between the second light emitting layer and the second electrode. The second light emitting layer further includes a hole transport material.

Description

Light Emitting Device

1 is a cross-sectional view showing a conventional electroluminescent device.

2 is a graph showing an emission spectrum of a conventional electroluminescent device.

3 is a cross-sectional view showing an electroluminescent device according to an embodiment of the present invention.

4 is an energy diagram of an electroluminescent device according to an embodiment of the present invention.

5 is a graph showing an emission spectrum of an electroluminescent device according to an embodiment of the present invention.

The present invention relates to an electroluminescent device.

Among flat panel display devices, the electroluminescent device is a self-luminous display device, which does not require a backlight, which enables light weight and thinness, simplifies the process, enables low temperature production, and has a response speed of 1 ms or less. It has a fast response speed and exhibits characteristics such as low power consumption, wide viewing angle, and high contrast.

In particular, the organic light emitting device includes a light emitting layer including an organic material between the anode and the cathode so that holes supplied from the anode and electrons received from the cathode combine in the organic light emitting layer to form an exciton, a hole-electron pair. The excitons emit light by the energy generated when they return to the ground state.

The electroluminescent device may emit white light by mixing light emitted from subpixels having a red, green, or blue light emitting layer, respectively, or a structure in which the red and blue light emitting layers are stacked or a red, green, and blue light emitting layer White may be emitted by forming subpixels in a stacked structure.

1 is a cross-sectional view showing a conventional white electroluminescent device.

Referring to FIG. 1, a conventional white electroluminescent device includes a first electrode 110, a second electrode 160, and a first electrode 110 and a second electrode 160 positioned on a substrate 100. And a resumed light emitting layer 140.

The first electrode 110 may be an anode for supplying holes to the emission layer (EML) 140, and the second electrode 160B may be a cathode for supplying electrons to the emission layer 140. Can be. In addition, a hole injection layer (HIL) 120 and / or a hole transport layer (HTL) 130 may be formed between the first electrode 110 and the light emitting layer 140 to facilitate hole injection and transport. An electron injection layer (EIL; 160A) or an electron transport layer (ETL; 150) may be interposed between the second electrode 160 and the light emitting layer 140 to facilitate electron injection and transport. This may be intervened.

The light emitting layer 140 may include a blue light emitting layer 141 emitting blue light, specifically, a sky blue, and a red light emitting layer emitting red light, specifically orange red, to emit white light. 143). Accordingly, the electroluminescent device emits white light by combining the light emitted from the two light emitting layers 141 and 143.

Alternatively, the light emitting layer 140 may emit white light by combining the light emitted from the three light emitting layers, including the red, green, and blue light emitting layers.

When a voltage equal to or greater than a driving voltage is applied between the first electrode 110 and the second electrode 160B, the first electrode 110 makes holes in the light emitting layer 140 through the hole injection layer 120 and the hole transport layer 130. The second electrode 160B supplies electrons to the light emitting layer through the electron injection layer 160A and the electron transport layer 150. Electrons and holes recombine in the emission layer 140 to generate excitons, and the excitons fall to the ground to emit light.

2 is a graph illustrating an emission spectrum of a conventional electroluminescent device.

Referring to FIG. 2, it can be seen that a peak is detected in the blue wavelength region A of 450 to 525 nm. As described above, in the spectrum of an electroluminescent device having a structure in which two or more light emitting layers 141 and 143 are stacked, peaks should be detected in the blue wavelength region A and the red wavelength region B, respectively. In the spectrum, a peak with strong intensity is detected only in the blue wavelength region A, whereby it can be seen that the recombination of holes and electrons is biased in the blue light emitting layer.

This is because the host of the light emitting layer 140 has excellent electron transport ability, which tends to prevent the injection of holes from the hole transport layer 130 to the light emitting layer 140. Therefore, in particular, holes are not sufficiently supplied from the first electrode 110 to the second light emitting layer 143 away from the hole transport layer 130. As a result, since the excitons are not sufficiently generated in the second emission layer 143, the electroluminescent device emits more blue light than red light.

Therefore, the white light emitted from the conventional electroluminescent device has a problem in that the color coordinates are shifted toward the blue color and thus the color purity is lowered.

Accordingly, an object of the present invention is to provide an electroluminescent device capable of realizing white light having improved color purity.

In order to achieve the above object, the present invention provides a substrate, a first electrode located on the substrate, a first light emitting layer on the first electrode, the first light emitting layer comprising a host, a dopant and a hole transport material, Provided is an electroluminescent device comprising a second light emitting layer including a host and a dopant and a second electrode positioned on the second light emitting layer.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

3 is a cross-sectional view illustrating the structure of an electroluminescent device according to an embodiment of the present invention, and FIG. 4 is an energy diagram of the electroluminescent device according to an embodiment of the present invention.

3 and 4, the first electrode 210, the hole injection layer 220, the hole transport layer 230, the light emitting layer 240, the electron transport layer 250, and the electron injection layer on the substrate 200 ( 260A and the second electrode 260B are sequentially positioned.

In more detail, the first electrode 210 is positioned on the substrate 200. The first electrode 210 may be an anode supplying holes to the emission layer 240, and may be formed of a metal having a high work function such as indium tin oxide (ITO).

The hole injection layer 220 and the hole transport layer 230 are sequentially positioned on the first electrode 210. The hole injection layer 220 and the hole transport layer 230 are used to transfer the holes supplied from the first electrode 210 to the light emitting layer 240. The hole injection layer 220 and the hole transport layer 230 lower the driving voltage by increasing the quantum efficiency. Therefore, the hole injection layer 220 and the hole transport layer 230 preferably lowers the levels of the highest occupied molecular orbital (HOMO) sequentially.

In general, the hole injection layer may be formed of copper phthalocyanine (CuPC), and the hole transport layer may be formed of α-NPD (4,4´-bis [N- (1-naphtil) -N-phenylamibi] biphenyl). can do.

The hole injection layer 220 and the hole transport layer 230 as described above may form a hole injection and transport layer as one layer, it may be selectively formed.

The emission layer 240 is positioned on the hole transport layer 230. In order to emit white light, the light emitting layer 240 may be formed by stacking the blue light emitting layer 241 and the red light emitting layer 234. In this case, in order to prevent the light emitted from the two light emitting layers from being absorbed and extinguished, the light emitting layer adjacent to the light emitting direction may be a light emitting layer that emits light having a short wavelength.

Therefore, the electroluminescent device of an embodiment of the present invention is a bottom emission type organic electroluminescent device in which light is emitted to the back of the substrate, so that the first light emitting layer 241 may be a blue light emitting layer, and the second light emitting layer 243 is red. It may be a light emitting layer.

The light emitting layer 240 may be formed of only the hosts 241A and 243A or the dopants 241B and 243B, but the efficiency and brightness are very low and the self-packing between the molecules may occur. In addition to the unique properties of each molecule, excimer properties occur at the same time, which is undesirable. Therefore, the light emitting layer 240 is mainly formed by doping the dopant to the host, in which the excitons are generated in the host and transferred to the dopant, and emits light while falling to the bottom in the dopant.

Accordingly, the first and second light emitting layers 241 and 243 may include a host and a dopant, and the first light emitting layer 241 may include a hole transport material (HTM) 243C.

Here, the first emission layer 241 may use an anthracene-based material as the host 241A. Typical examples include MADN (binaphtyl-methylantracene), TBDN (binaphthyl- (t-butylantracene), and the like. As the dopant 241B, perylene-based, ananthrene-based and stilbene-based Substances can be used.

The red light emitting layer that is the second light emitting layer 243 may be a carbazole-based organic material, that is, CBP (carbazole biphenyl) or the like as the host 243A. The dopant 243B forming the red light emitting layer may be a phosphorescent dopant, a fluorescent dopant, or a mixed dopant thereof.

For example, the phosphorescent dopant may be one selected from the group consisting of PIQIr (acac), PQIr (acac), PQIr (tris (1-phenylquinoline) iridium) and PtOEP. The fluorescent dopant may be one selected from the group consisting of DCJTB, DCDDC, AAAP, DPP, and BSN.

Since the host 241A has a property of transporting electrons, the host 241A tends to prevent the transfer from the hole transport layer 230 to the light emitting layer 240. In particular, the second light emitting layer 243 located far from the hole transport layer 230 is prevented from receiving a sufficient amount of holes necessary for emitting light by the hosts 241A and 243A of the first and second light emitting layers 243 as described above. do.

Therefore, the first light emitting layer 241 of the electroluminescent device according to an embodiment of the present invention may efficiently transport holes to the second light emitting layer 243 including the hole transport material 241C.

The hole transport material 241C may include a tri aryl amine-based material. That is, it may include α-NPD, β-NPD, TPD, CBP, BMA-nT, TBA, o-TTA, p-TTA, m-TTA, spiro-TPD, or TPTE. These materials are used in the formation of the hole transport layer 230, and have excellent hole transport capability.

The first emission layer 241 may be formed by co-depositing a host and a hole transport material and then doping the dopant.

In this case, the host 241A and the hole transport material 241C may be co-deposited at a 5: 1 to 1: 1 ratio. Here, when the ratio of the host and the hole transporting material is 5: 1 or more, the hole transporting capacity of the first light emitting layer 241 can be sufficiently secured. When the ratio of the host and the hole transporting material is 1: 1 or less, the first emitting layer The luminance of 241 can be sufficiently secured.

Here, the thickness of the first emission layer 241 may be 10 to 35 nm. If the thickness of the first light emitting layer 241 is 10 nm or more, the luminance and lifespan of the first light emitting layer 241 may be ensured. If the thickness of the first light emitting layer 241 is 35 nm or less, holes may be sufficiently filled in the second light emitting layer 243. Can transport.

The electron transport layer 250 and the electron injection layer 260A may be positioned on the emission layer 240. The electron transport layer 250 and the electron injection layer 260A lower the energy barrier of electron injection to facilitate electron injection and transport. In order to make it, it can form selectively.

As the electron transport layer 250, a material having excellent electron transport ability such as aluminum quinolate (AlQ 3) is mainly used, and a metal compound such as lithium fluoride (LiF) may be used as the electron injection layer 260A.

The second electrode 260B is positioned on the electron injection layer 260A. The second electrode 260B may be a cathode for supplying electrons to the light emitting layer 140, and may be formed of magnesium (Mg), silver (Ag), aluminum (Al), calcium (Ca), and alloys thereof having a low work function. It may be formed of any one or more selected from the group.

Hereinafter, the light emitting mechanism of the electroluminescent device according to an embodiment of the present invention will be described with reference to FIG. 4. The upper part of the diagram shown in the figure represents LUMO, the Lowest Unoccupied Molecular Orbital, and the lower part, HOMO, the Highest Occupied Molecular Orbital.

Referring to FIG. 4, when a voltage higher than the driving voltage is applied between the first electrode 210 and the second electrode 260B, holes from the first electrode 210 occupy the highest occupied molecular track of the hole injection layer 220. Electrons are injected from the second electrode 260B into the lowest unoccupied molecular orbit of the electron transport layer 250.

Since the highest occupied molecular orbital levels of the hole injection layer 220 and the hole transport layer 230 are sequentially lowered, holes are supplied to the first light emitting layer 241 via the hole injection layer 220 and the hole transport layer 230. Electrons are transferred to the second emission layer 243 through the electron injection layer 260A and the electron transport layer 250.

Electrons transferred to the second emission layer 243 are transferred to the first emission layer 241, and the first emission layer 241 supplied with holes and electrons generates excitons in the host 241A and transfers them to the dopant 241B. do. The excitons transferred to the dopant 241B fall into the ground state and emit blue light. The hole transport material 241C included in the first light emitting layer 241 serves to easily move holes present in the first light emitting layer 241 to the second light emitting layer 243.

Meanwhile, the second light emitting layer 243 receives holes from the first light emitting layer 243, and holes and electrons are combined in the host 243A of the second light emitting layer 243 to generate excitons and transfer them to the dopant 243B. As a result, red light is emitted.

5 is a graph illustrating light emission spectra of an electroluminescent device according to an exemplary embodiment of the present invention.

Referring to FIG. 5, it can be seen that a peak is detected in the blue region A corresponding to 450 to 525 nm, and a peak is also detected in the red region B corresponding to 570 to 630 nm.

That is, as described above, the electroluminescent device according to the embodiment of the present invention has an electron accepting layer interposed between the first light emitting layer and the second light emitting layer, it is possible to effectively supply holes to the second light emitting layer. Accordingly, by generating excitons in both the first emission layer and the second emission layer to emit light, white light having excellent color purity can be realized.

In the exemplary embodiment of the present invention, the light emitting layer is described as a stack of red and blue light emitting layers, but the present invention is not limited thereto, and the red, green, and blue light emitting layers may be stacked. When the red, green, and blue light emitting layers are stacked, the hole transport material may be deposited on the green light emitting layer, thereby effectively supplying holes to the red, green, and blue light emitting layers, thereby emitting white light having high color purity.

While the invention has been shown and described with reference to specific embodiments thereof, the invention is not so limited, and the invention may be varied without departing from the spirit or scope of the invention as set forth in the claims below. It will be readily apparent to those skilled in the art that the present invention may be modified and changed.

The present invention can provide an electroluminescent device capable of realizing white light with improved color purity.

Claims (12)

Board; A first electrode on the substrate; A first light emitting layer on the first electrode and including a host, a dopant and a hole transport material; A second light emitting layer on the first light emitting layer and including a host and a dopant; And An electroluminescent device comprising a second electrode located on the second light emitting layer. The method of claim 1, Further comprising a third light emitting layer between the second light emitting layer and the second electrode, The second light emitting layer further comprises a hole transport material. The method according to claim 1 or 2, The hole transport material includes a tri aryl amine-based material electroluminescent device. The method of claim 3, wherein The hole transport material comprises at least one selected from the group consisting of α-NPD, β-NPD, TPD, CBP, BMA-nT, TBA, o-TTA, p-TTA, m-TTA, spiro-TPD and TPTE Electroluminescent element. The method according to claim 1 or 2, And the first light emitting layer or the second light emitting layer is formed by doping a dopant after co-depositing a host and a hole transport material. The method of claim 5, wherein The co-deposition ratio of the host and the hole transport material is 5: 1 to 1: 1. The method according to claim 1 or 2, The thickness of the first light emitting layer or the second light emitting layer is 10 to 35 nm electroluminescent device. The method of claim 1, The first light emitting layer is a blue light emitting layer, the second light emitting layer is a red light emitting layer. The method of claim 2, Wherein the first light emitting layer is a blue light emitting layer, the second light emitting layer is a green light emitting layer, and the third light emitting layer is a red light emitting layer. The method according to claim 1 or 2, An electroluminescent device having at least one of a hole injection layer and a hole transport layer interposed between the first electrode and the first light emitting layer. The method of claim 1, An electroluminescent device having at least one of an electron injection layer and an electron transporting layer interposed between the second light emitting layer and the second electrode. The method of claim 2, An electroluminescent device having at least one of an electron injection layer and an electron transporting layer interposed between the third light emitting layer and the second electrode.

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