KR20080058142A - Organic light-emitting diode - Google Patents

Abstract

An organic light emitting diode is provided to ensure improved life characteristic and luminous efficiency. An organic light emitting diode includes a light emitting layer comprising (a) a first triplet emitter which absorbs energy to convert the ground state into the triplet exciton; and (b) a second triplet emitter which is coupled with the triplet exciton of the first triplet emitter to convert the ground state into the triplet state through a dexter process, and emits a light when the triplet state returns to the ground state. The first triplet emitter is an organometallic complex of a transition metal. The second triplet emitter is an organometallic complex of a transition metal.

Description

Organic light-emitting diode

The present invention relates to an organic light emitting diode (OLED).

OLEDs are light emitting diodes made of organic semiconductor polymers or small molecules, which display specific graphics.

An OLED is a sandwich structure in which two electrodes and one or more organic material layers are disposed between the electrodes. In general, the anode is made of indium tin oxide (ITO) and disposed on a glass or polymer substrate. Low work function materials, such as base metals such as aluminum, calcium, magnesium, and the like, provide the cathode. In order to cause luminescence, positive or negative charge carriers must first be injected into the organic material disposed between the electrodes. Therefore, the organic material forms radical anions on one side of the cathode when a sufficiently high voltage is applied in the conductive direction to inject electrons into the LUMO of the organic material. On the other hand, the anode generates radical cations to inject positive charge carriers (holes) into the HOMO. In contrast to inorganic semiconductors, there is a very weak intermolecular interaction between molecules in organic solids such that the orbital overlap of each molecule is limited or even no band structure formation occurs. Thus, in organic materials, charge transport occurs in a narrow region between the conduction band and the valence band, the charge transport being independent of energy and location, and the distance of charge transport is the respective molecule. You can think of it as a unit. Charge transport in such a narrow region can occur through hopping and tunneling processes. Each of the various materials for hole and electron transport is already known.

On the other hand, in the light emitting layer, recombination of holes and electrons occurs to form bi-radical excitons that do not charge. The excitons may be naturally decayed to emit photons, and the emission wavelength is determined by the energy of the excitons. There are two ways of emitting light from the excited singlet state-by spin flipping-or from the triplet state. The first emission is called fluorescence and the second emission is called phosphorescence. Light emission from the singlet state is lower in quantum efficiency than light emission through the triplet state.

In order to provide OLEDs with high quantum efficiency, the oligomeric or polymeric material (corresponding to the parent) of the light emitting layer can be doped with phosphorescent guest molecules (corresponding to triplet emitters). Phosphorescent guest molecules include platinum or iridium organometallic complexes such as PTOEP and Ir (ppy) 3. It is called a polymer light-emitting diode (PLED) by including a polymer as a matrix. At this time, the triplet and singlet excitons generated in the parent accumulate at the low energy level of the triplet emitter. Singlet excitons are capable of moving to triplet emitters over large intermolecular distances (40 to 100 μs) (Perster action), while triplet excitons are only moved by diffusion of neighboring molecules (10 to 15 μs) (dexter action ).

Despite these actions, the lifetime of OLEDs is still limited for applications in various technical fields. In particular, it is limited to apply to various OLEDs / PLEDs while reducing the number of organic layers, which is more limited from a manufacturing point of view, and in such a limited environment, the OLEDs / PLEDs perform some operations already mentioned (single layer concept). ).

The present invention is to solve the above-mentioned problems and other various problems, to provide an organic light emitting device that can improve the life characteristics and luminous efficiency.

The present invention provides an organic light emitting device having a light emitting layer including at least two triplet emitters.

The light emitting layer converts from a ground state to a triplet state through a dexter action by coupling with a first triplet emitter that converts from a ground state into a triplet exciter by energy absorption with a triplet exciter of the first triplet emitter. And a second triplet emitter emitting light while returning from the triplet state to the ground state.

A series of mechanisms of the triplet emitters have been found to be effective despite the short distances at which the underlying actions of energy transfer take place. Since the Ferster action plays a minor role, high concentrations of triplet emitters are required, while high efficiencies are derived. In addition, the combined lifetime of the triplet emitters is extended compared to the separate use of triplet emitters alone.

Triplet excitons of the first triplet emitter may be formed by the following methods.

(1) recombination of charge carriers to an excited state, followed by spin inversion of the excited electrons to transport to the triplet level (inter system crossing (ISC), which is a triplet capable of accepting positive and negative charge carriers). It requires a radiator.

(2) When the two triplet emitters are doped into the mother, one of the photons emitted by the mother is taken to the excited state and then subjected to ISC. And / or

(3) When the two triplet emitters are doped into the parent, they are coupled to the parent triplet excitons via dexter action.

By coupling the second triplet emitter using triplet excitons of the first triplet emitter via dexter action, the second triplet emitter is converted into triplet excitons. According to the invention, the formation of triplet excitons of the second triplet emitter via dexter action is a dominating action from the energy exchange to the second triplet emitter. In addition, other forms of energy exchange from the excitons of the parent and first triplet emitters also occur, but have a low efficiency compared to the dexter action mentioned above. Preferably, at least 50%, more preferably at least 70% of the second triplet excitons occur through the dexter process mentioned above.

In order to design the dexter action mentioned above as efficient as possible, the energy level involvement of the two triplet emitters must be matched. Referring to the Jablonski diagram of the triplet state of the two emitters, the selection indicates that the energy position of the lowest energy activated triplet level of the first triplet emitter is at the high energy triplet level of the second triplet emitter. In a corresponding way. From the end, the exchange to the lowest energy triplet level of the second triplet emitter is caused by vibrational relaxation. Alternatively or in addition, the selection may be made based on the maximum position of the phosphorescence emission spectrum of the two triplet emitters. The maximum emission value of the first triplet emitter may be selected to a wavelength as short as about 50 to 200 nm. Preferably, the maximum emission value of the first triplet emitter is 520-565 nm, and the maximum emission value of the second triplet emitter has a wavelength range of 625-740 nm. That is, the first triplet emitter serving as a donor phosphor exhibits green light, and the second triplet emitter exhibits red light as an acceptor.

The first triplet emitter is an organometallic complex of a transition metal, preferably iridium. Phosphorescent iridium complexes are known in the art and may preferably be porphyrin and 2-phenylpyridine complexes. More preferably, tac-tri (2-phenylpyridine) iridium (III), bis (2- (9,9-dihexylfluorenyl) -1-pyridine)-(acetylacetonate) iridium (III), tri -[4- (4-methyl) -phenylpyridine] iridium (III).

The second triplet emitter is also an organometallic complex of transition metal, preferably iridium or platinum. More preferably, bis (3- (2- (2-pyridyl) benzo-denoyl)-(mono-acetylacetonate) iridium (III), tri (1-phenylisoquinoline) iridium (III), bis ( 1-4'-tert-butyl-phenyl) isoquinoline) -1,3-pentadionate iridium (III), (2,3,7,8,12,13,17,18-octaethyl-21H, 23H Porphine) platinum (III).

The molar ratio of the first triplet emitter to the second triplet emitter is preferably 5: 1 to 1: 5.

The light emitting layer further includes a matrix of oligomers or polymers containing the triplet emitters. Preferably, the parent oligomer or polymeric material is selected from the group consisting of oligo- or polycarbazoles, or oligo- or polyarylketones. More preferably, the oligomeric or polymeric material may be fluorine oligo- or polyarylketones. Despite the recombination of the charge carriers, the parent converts from the ground state to the excited state (parent exciter). The material for the parent / first triplet emitter can be selected and transformed into a triplet state either directly or indirectly, starting from the parent exciter and through Perster action, dexter action and / or charge exchange.

According to the light emitting layer variant of the above-mentioned embodiment, the cumulative weight ratio of the first and second triplet emitters to the total weight of the light emitting layer may be 5 to 40%, preferably 15 to 25%.

The organic light emitting device according to the present invention further includes a first electrode and a second electrode, and the light emitting layer mentioned above is disposed between the first electrode and the second electrode.

As described above, the present invention can provide an organic light emitting device having improved luminous efficiency and lifespan.

Hereinafter, the present invention can be described in detail by the embodiments and the two figures.

Experimental Example -According to the invention Emitting layer  Equipped OLED

The pre-cleaned glass substrate with an anode contact made of indium tin oxide (ITO) has four test regions each separated from each other. The glass substrate was first cleaned in an ultraviolet bath. And washed with isopropyl alcohol. Then dried at 150 ° C. Testing of substrates with 50 nm thickness of poly (3,4-ethylene dioxythiophene) poly (styrene sulfonate (PEDT / PSS) (available in aqueous solution from Leverkusen, HC Starck Gmbh, Germany) Spin coated onto the area and then dried in air exposed for 10 minutes at 180 ° C. The spin coated layer acts as a hole transport layer (HTL).

To prepare the light emitting layer, triplet emitter solution was prepared from the following composition.

(a) PVK (poly (9-vinylcarbazole), secondary standard) available from Aldrich, Germany,

(b) PBD (2- (4-biphenylyl) _5 (4-tert-butyl-phenyl) -1,3,4- under the trade name ST 1563.S, available from the company Wolfen, Sensient inagin Technologies GmbH, Germany. Oxadiazole),

(c) Ir (bpi) (bis- (1- (4'-tert.-butyl-phenyl)-under the trade name ST 2347.S, available from the company Sensient Imaging Technologies Gmbh, Wolfen, Germany, as a second triplet emitter; Isoquinoline) -1,4-pentanedionate iridium (III),

(d) TBD (N, N'-bis (3-methylphenyl) -N, N'-diphenylbenzidine) under the trade name ST 2356.S, available from Wolfen, Sensient Imaging Technologies Gmbh, Germany,

(e) TBD (N, N'-bis (3-methylphenyl) -N, N'-diphenylbenzidine) under the trade name ST 16 / 1.S, available from Wolfen, Sensient Imaging Technologies Gmbh, Germany,

Compositions (a) and (b) were dissolved in 5% and 3% by weight chlorobenzene, respectively. (c) The composition was also dissolved in chlorobenzene (1% by weight). (d) The composition was also dissolved in chlorobenzene (1% by weight). The obtained solutions were mixed so that (a) the composition contained 63% by weight of the total solid components, (b) the composition 24% by weight, and (c) the composition 4% by weight. The cumulative weight ratio of composition (d) in the solution is about 9% by weight. The solution mixed with the emitter was spin coated to 70 nm thickness on the HTL layer and then heated at 80 ° C. for 30 minutes under inert conditions. A cathode (6 nm barium capped with 500 nm aluminum) was prepared by depositing through a mask on the light emitting layer under about 2 × 10 −6 mbar pressure. Finally, the substrates were sealed under inert conditions.

Comparative example  1 and 2

 (1) The OLED was manufactured in the same manner as described in the above Experimental Example, except that the hole transporting material TBD (e) was added to the light emitting layer in an amount of 9% by weight based on the total solid components instead of Ir (mppy) (c). . TBD (e) was dissolved in chlorobenzene at 1% by weight.

(2) An OLED was manufactured in the same manner as described in the above Experimental Example, except that neither Ir (mppy) (c) nor hole transporting material TBD (e) was added to the light emitting layer. Therefore, the composition contains (a) 69% by weight, (b) 26% by weight, and (c) 5% by weight relative to the total solid components.

The efficiency according to the voltage was measured as follows. An increasing voltage was applied to the OLED with a 0.2V / s slope from -6V to + 8V. And the current and brightness flowing through the OLED was detected by V- lambda-adaptad photodiode. Since the photodiode current is proportional to the luminance, the specific current corresponds to the specific luminance. A scaling factor was used to allow the light emitted from the OLED to be measured. From the OLED current, the active light emitting area and the brightness of the OLED, the efficiency was calculated.

1 is a graph showing the relationship between the voltage and the luminous efficiency applied to the OLEDs of the Examples and Comparative Examples of the present invention. Although the luminous efficiency was increased, it can be seen that the color values are not different.

The time dependent shape determines: OLEDs work by supplying a specific current. The OLED emits light in an amount corresponding to the current. The value was recorded by the photodiode and set to 100%. Under constant current flowing, the amount of light emitted from the OLED was measured, which was reduced and plotted over time. When the 50% point of the initial light emission amount was reached, the lifetime was determined.

2 is a graph showing a relationship between time and luminance of OLEDs of Examples and Comparative Examples of the present invention. The lifetime of the experimental example of the present invention was obviously improved (the lifetime was reduced by 50% at the initial luminance of 800 cd / m 2). The color values of the experimental examples and the comparative examples of the present invention were not different.

1 and 2, it can be seen that the combination of the materials described in the experimental example of the present invention compared with the comparative examples is improved efficiency and life.

As described above, the present invention can provide an organic light emitting device having improved luminous efficiency and lifespan.

Although the present invention has been described with reference to the embodiments shown in the drawings, these are merely exemplary, and those skilled in the art will understand that various modifications and equivalent other embodiments are possible. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

1 is a graph showing a relationship between voltage and luminous efficiency applied to an OLED and two comparative examples according to the present invention.

2 is a graph showing the relationship between the time of OLED and the formation of the OLED according to the present invention and two comparative examples.

Claims (12)

(a) a first triplet emitter that absorbs energy and converts from the ground state into triplet excitons, (b) a second triple coupled to the triplet exciton of the first triplet emitter, converting from the ground state to the triplet state through a dexter process and emitting light from the triplet state back to the ground state; An organic light emitting device comprising a light emitting layer comprising an anti-radiator. The organic light emitting device of claim 1, wherein the first triplet emitter is an organometallic complex of a transition metal. The organic light emitting device of claim 2, wherein the transition metal is iridium. The organic light emitting device according to any one of claims 1 to 3, wherein the second triplet emitter is an organometallic complex of a transition metal. The organic light emitting device of claim 4, wherein the transition metal is iridium (Ir) or platinum (Pt). The organic light emitting device of claim 1, wherein the first triplet emitter has a maximum emission value at a wavelength of 520-565 nm, and the second triplet emitter has a maximum emission value at a wavelength of 625-740 nm. . The organic light emitting device of claim 1, wherein a molar ratio of the first triplet emitter and the second triplet emitter is 5: 1 to 1: 5. The method of claim 1, wherein the light emitting layer, (c) An organic light-emitting device further comprising a matrix composed of oligomers or polymers. The organic light emitting device of claim 8, wherein the oligomer or polymer is at least one selected from the group consisting of oligo- or polycarbazole and oligo- or polyarylketone. 10. The organic light emitting device of claim 9, wherein the parent oligomer or polymer is a fluorine oligo- or polyaryl ketone. The organic light emitting device of any one of claims 8 to 10, wherein a weight ratio of the first and second triplet emitters to the total weight of the light emitting layer is 1 to 40% by weight. The method of claim 1, further comprising a first electrode and a second electrode, The light emitting layer is disposed between the first electrode and the second electrode.

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