JP4214482B2 - Organic electroluminescence device - Google Patents

Description

  The present invention relates to an organic electroluminescence element.

  With the rapid development of the information society, there is an increasing need for a flat panel display (FDP) that is lightweight and consumes little power. Various display methods are known as candidates for the FDP, and among them, an organic electroluminescence element (hereinafter sometimes referred to as “organic EL element”) has attracted attention. In an organic EL element, various emission colors can be obtained by selecting a light emitting material. In addition, since it can be easily made into a thin film and can be manufactured on a flexible substrate such as a plastic substrate, it has an advantageous feature in realizing a full-color flexible display.

  In an organic EL element, electrons and holes injected into the element recombine inside the light emitting layer or at the interface between the light emitting layer and another organic layer to emit light. Therefore, the light emitting layer is an important part that determines the luminous efficiency of the device. Specifically, holes and electrons injected into the element from the hole injection electrode and the electron injection electrode are transported to the light emitting layer through the hole transport layer and the electron transport layer and recombined there. A part of the light emitting layer material forms an excited state by this recombination. At this time, an excited singlet state and an excited triplet state are generated at a ratio of 25%: 75%, respectively. In order to efficiently obtain light emission from the generated excited state, a fluorescent dye or phosphorescent dye having a high fluorescence quantum yield is used as the material of the light emitting layer. Since fluorescence is emitted from an excited singlet state, only 25% of the excited state generated by recombination can be used. On the other hand, if phosphorescence, which is light emission from an excited triplet state, is used, 75% of the generated excited state can be used, so that very high emission efficiency can be realized.

As a material exhibiting strong phosphorescence at room temperature, an organometallic complex having iridium or platinum as a central metal is known (see Non-Patent Document 1). These materials are used as a light emitting material of the organic EL element and exhibit high efficiency light emission. However, since emission from the excited triplet state is essentially a forbidden transition, materials that exhibit strong phosphorescence at room temperature are limited to some special and expensive compounds such as iridium complexes (Ir (ppy) ₃ ) and platinum porphyrin complexes. There was a problem.

Conventionally, organic EL elements that emit light from a phosphor material using energy transfer from a phosphorescent material to the phosphor material have also been proposed (Non-Patent Document 2 and Patent Document 1).
JP 2002-50483 A MA Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence”, Applied Physics Letters, USA, 1999, Vol. 75, No. 1, p. 4-6 MA Baldo et al., "High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer", Letters to Nature, USA, 2000, Vol. 403, p.750-753

  However, the phosphorescent material that has been studied in the above prior art has been an expensive metal complex. Further, in the above prior art, an organic compound suitable for energy transfer from a phosphorescent material to a fluorescent material has not been sufficiently studied.

  In view of such a situation, an object of the present invention is to provide an organic electroluminescence element that can emit light with high luminance by using energy transfer and can be manufactured at low cost.

In order to achieve the above object, an organic electroluminescent device of the present invention is an organic electroluminescent device including a light emitting layer that emits light by carrier injection, and the light emitting layer includes a first organic compound and a second organic compound. And the proportion of the first organic compound in the organic compound constituting the light emitting layer is 50 mol% or more, and the first organic compound is 4-phenylbenzophenone or 4,4′-dibromobenzophenone And the energy level of the lowest excited singlet state of the second organic compound is lower than the energy level of the lowest excited triplet state of the first organic compound, and the excited singlet state of the second organic compound. Of the first organic compound in a level higher than the energy level of the lowest excited singlet state. There are levels in the range of ± 0.3 eV relative to the energy level of the electromotive triplet state, wherein the second organic compound by injection of carriers into the light emitting layer emits fluorescence.
Further, another organic electroluminescence element of the present invention is an organic electroluminescence element comprising a light emitting layer that emits light by carrier injection, wherein the light emitting layer includes a first organic compound and a second organic compound, The proportion of the first organic compound in the organic compound constituting the light emitting layer is 50 mol% or more, and the first organic compound is 4-phenylbenzophenone or 4,4′-dibromobenzophenone, The second organic compound is rubrene, coumarin 540, coumarin 6, DCM, rhodamine B, or Nile red, and the second organic compound emits fluorescence when carriers are injected into the light emitting layer.

  In the organic electroluminescence device of the present invention, it is preferable that the first organic compound includes a benzophenone skeleton or a ketocoumarin skeleton. That is, from another viewpoint, the organic electroluminescent element of the present invention is an organic electroluminescent element including a light emitting layer that emits light by carrier injection, and the light emitting layer includes a first organic compound and a second organic compound. And the ratio of the first organic compound to the organic compound constituting the light emitting layer is 50 mol% or more, the first organic compound contains a benzophenone skeleton or a ketocoumarin skeleton, and the carrier to the light emitting layer The second organic compound emits fluorescence by the injection of.

  According to the present invention, light emission with high brightness can be obtained by using energy transfer from the first organic compound (host material) to the second organic compound (guest material). In the organic EL device of the present invention, since the intensity of phosphorescence emission at room temperature of the first organic compound may be low, an inexpensive organic compound can be used from various organic materials and can be manufactured at low cost. It is.

  Embodiments of the present invention will be described below. The organic EL of the present invention is an organic EL element including a light emitting layer that emits light by carrier injection, and the light emitting layer includes a first organic compound and a second organic compound, and the organic compound constituting the light emitting layer The proportion of the first organic compound is 50 mol% or more, and the first organic compound is an organic compound that forms a stable excited triplet state at room temperature (25 ° C.), and carriers are injected into the light emitting layer. As a result, the second organic compound emits fluorescence.

  The organic EL device of the present invention can be formed in the same manner as a known organic EL device except for the light emitting layer. The organic EL device of the present invention usually includes a hole injection electrode and an electron injection electrode, and the light emitting layer is disposed between the hole injection electrode and the electron injection electrode. At least one of the two electrodes is formed of a conductive film that transmits light emitted from the light emitting layer. For the hole injection electrode, for example, gold, platinum, indium-tin oxide, aluminum-zinc oxide or the like having a large work function is used. For example, a magnesium-silver alloy or a metal having a small work function such as lithium, cesium, sodium, calcium and aluminum is used for the electron injection electrode.

  Other layers such as a hole transport layer may be formed between the hole injection electrode and the light emitting layer. In addition, another layer such as an electron transport layer may be formed between the electron injection electrode and the light emitting layer. For the hole transport layer and the electron transport layer, known organic materials having excellent hole transport characteristics and electron transport characteristics can be used, respectively. There is no particular limitation on the thickness of each layer. The thickness of the light emitting layer may be selected from the range of 5 nm to 1000 nm, for example.

  The ratio of the 1st organic compound (host material) to the organic compound which comprises a light emitting layer is 50 mol% or more, for example, 80-99.99 mol%.

  The first organic compound is an organic compound that forms an excited triplet state that is stable at room temperature by carrier recombination. As a result of studying various materials, the inventor of the present invention uses the organic compound containing a benzophenone skeleton or a ketocoumarin skeleton as the first organic compound, so that the second organic compound is removed from the triplet excited state of the first organic compound. It has been found that energy transfer to a singlet excited state can be efficiently generated. That is, it is preferable to use benzophenone, a benzophenone derivative, a ketocoumarin, or a ketocoumarin derivative as the first organic compound. The benzophenone skeleton is represented by the following formula (1). The ketocoumarin skeleton is represented by the following formula (2), and the compound having the ketocoumarin skeleton may be, for example, a compound represented by the following formula (3) or a derivative thereof. In addition, the ring of the following formulas (1) to (3) may be further condensed with another ring.

  Examples of the benzophenone derivative and ketocoumarin derivative include compounds represented by the following formulas (4) to (6).

In formulas (4) to (6), R ¹ , R ² , R ³ , R ⁵ and R ⁶ are each independently selected from the group consisting of halogen atoms, alkyl groups, aryl groups, aromatic rings and heteroaromatic rings. It may be a group. R ⁴ is an arbitrary organic group. R ¹ , R ² , R ³ , R ⁵ and R ⁶ may be bonded to any position of the benzene ring as long as the benzene ring is maintained. For example, R ¹ and R ² are independently bonded to any position in the ortho, meta, or para position relative to the carbonyl group.

  An organic compound containing a benzophenone skeleton or a ketocoumarin skeleton is preferable because it has characteristics required for the first organic compound and is very inexpensive. The inventor has found that an organic compound having a benzophenone skeleton is particularly preferable as the first organic compound. An organic compound having a benzophenone skeleton is inexpensive and has an excited triplet state having a high energy level, and thus causes energy transfer with high efficiency. However, the first organic compound of the present invention may be an organic compound having another skeleton.

  Specific examples of the compound having a benzophenone skeleton include 4-phenylbenzophenone and 4,4′-dibromobenzophenone.

  The phosphorescence lifetime of the first organic compound is, for example, 1 microsecond or more, preferably 1 millisecond or more, and more preferably 100 milliseconds or more. The longer the phosphorescence lifetime, that is, the longer the lifetime of the excited triplet state, the higher the rate of energy transfer from the excited triplet state of the first organic compound to the excited singlet state of the second organic compound, and the emission intensity. Can be increased.

  The first organic compound does not need to be a compound having high phosphorescence intensity when photoexcited at room temperature, and may be a compound that does not substantially generate phosphorescence at room temperature. For example, the phosphorescent quantum yield of the first organic compound may be 1% or less. Such compounds are available at low cost.

  The second organic compound is an organic compound that is excited by energy transfer from the first organic compound, and is an organic compound that emits fluorescence. The second organic compound can be selected from various fluorescent materials, and an inexpensive fluorescent material exhibiting highly efficient light emission characteristics can be used as the second organic compound.

  When a molecule is excited by recombination of carriers, an excited singlet state and an excited triplet state are generated at a ratio of 1: 3. Therefore, it is important to effectively use the excited triplet state in order to obtain high emission intensity. However, since light emission (phosphorescence) from a normal excited triplet state is a forbidden transition, materials that exhibit strong emission intensity at room temperature are limited. Even in the case of a substance that emits phosphorescence, since the lifetime of the excited state is long, the ratio of deactivation without light emission due to triplet-triplet annihilation increases as the current density flowing through the element increases.

  In contrast, in the light emitting layer of the organic EL device of the present invention, energy transfer from the excited triplet state of the first organic compound (host material) to the excited singlet state of the second organic compound (guest material). And emission (fluorescence) from the excited singlet state of the second organic compound occurs. Since this fluorescence emission is an allowable transition, it has the advantages that it emits light efficiently and has a short lifetime in the excited singlet state. Further, the first organic compound does not necessarily have high phosphorescence emission intensity, and thus various materials can be used.

  In the organic EL device of the present invention, the lowest excited singlet state energy level of the second organic compound is lower than the lowest excited triplet state energy level of the first organic compound, and the second organic compound is excited. It is preferable that an energy level that causes absorption exists in the energy level of the singlet state at a position substantially equal to the energy level of the lowest excited triplet state of the first organic compound. An example of such an energy level is schematically shown in FIG.

As shown in the energy level diagram of FIG. 1, the first organic compound has at least the lowest excited singlet state energy level E1 _S1 and the lowest excited triplet state energy level E1 _T1 . The second organic compound includes at least the energy level E2 _{S1 in the} lowest excited singlet state and the energy level E2 _Sn in the excited singlet state higher than E2 _S1 (n is an integer greater than 1). There is). As shown in FIG. 1, energy level E2 _Sn excited singlet state of the second organic compound is a substantially equal energy levels and the energy level E1 _T1 of the lowest excited triplet state of the first organic compound . Specifically, E2 _Sn is preferably in the energy level in the range of ± 0.3 eV relative to E1 _T1. By selecting such an organic compound, energy transfer from the energy level E1 _T1 of the first organic compound to the energy level E2 _{Sn of} the second organic compound is more likely to occur. In the organic EL device of the invention, the energy level E2 _Sn energy transfer from the energy level E1 _T1 occurs may also be in the energetically higher than the energy level E1 _T1. Such energy transfer is caused by thermal excitation.

Note that the energy transfer from the excited triplet state energy level of the first organic compound to the excited singlet state energy level of the second organic compound is from the energy level E1 _T1 to the energy level E2 _Sn . It is not limited to energy transfer, and may occur between other levels.

  In the light emitting layer of the present invention, it is considered that so-called Forster energy transfer occurs. Since the Forster-type energy transfer is caused by dipole-dipole interaction, the distance between the molecules that transfer energy may be relatively long. Therefore, in the light emitting layer of the present invention, energy transfer is performed smoothly even if the amount of the second organic compound is small.

  In the organic EL device of the present invention, it is preferable that the peak of the phosphorescence spectrum of the first organic compound and the peak of the absorption spectrum of the second organic compound overlap at least partially. This causes energy transfer from the excited triplet state of the first organic compound to the second organic compound.

  In the organic EL device of the present invention, the ratio of the second organic compound to the organic compound constituting the light emitting layer is preferably in the range of 0.01 mol% to 20 mol%. By setting the ratio of the second organic compound in a relatively low range, the occurrence of concentration quenching can be suppressed. The molar ratio of the first organic compound to the second organic compound in the light emitting layer is [number of moles of the first organic compound]: [number of moles of the second organic compound] = 80: 20 to 99.99: A range of 0.01 is preferable, for example, a range of 90:10 to 99.9: 0.1.

  In the organic EL device of the present invention, the second organic compound is selected according to the first organic compound, and for example, a polycyclic aromatic compound can be used. Examples of the polycyclic aromatic compound include rubrene, coumarin 540, coumarin 6, DCM, rhodamine B, and Nile red.

  As a preferable combination of the first organic compound and the second organic compound, for example, a combination of 4-phenylbenzophenone and rubrene shown in the examples can be mentioned.

  The light emitting layer of the organic EL device of the present invention is typically composed of a mixture of the first organic compound and the second organic compound. As long as the effects of the present invention can be obtained, the first and first Other substances may be included in addition to the two organic compounds.

  The organic EL device of the present invention can be produced by the same method as a general organic EL device. Specifically, it can be produced by laminating an electrode layer and an organic material layer on a substrate using a known method such as an evaporation method, a sputtering method, or a spin coating method.

  Hereinafter, an example of the organic EL element of the present invention will be described. In this example, the organic EL element 10 shown in FIG. 2 was produced.

<Example 1>
First, a glass substrate 11 on which a transparent electrode (hole injection electrode 12) made of indium-tin oxide was formed was prepared. Then, N, N′-di-1-naphthyl-N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine (N, N′-di-1) is formed on the hole injection electrode 12. A hole transport layer 13 (thickness 50 nm) made of -naphthyl-N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPD)) was formed.

  Next, the light emitting layer 14 (thickness 50 nm) was formed on the hole transport layer 13. The light-emitting layer 14 includes 99.5 mol% 4-phenylbenzophenone (4PBP) as a host material and 0.5 mol% rubrene (5, 6, 11, 11) as a light-emitting dopant. 12-tetraphenylnaphthacene). 4PBP and rubrene were manufactured by Aldrich. The reason for combining 4PBP and rubrene is that the phosphorescence spectrum of 4PBP and the absorption spectrum of rubrene show good overlap as shown in FIG.

Next, an electron transport layer 15 (thickness: 15 nm) made of Alq3 (tris (8-quinolinol) aluminum) was formed on the light emitting layer. Next, an electron injection electrode 16 was formed on the electron transport layer 15. The electron injection electrode 16 was formed by sequentially laminating a LiF layer (thickness 0.5 nm) and an aluminum layer (thickness 80 nm). All organic layers and electrodes were formed by resistance heating type vacuum deposition. The pressure during vapor deposition was 1.3 × 10 ⁻⁴ Pa (1 × 10 ⁻⁶ Torr). The substrate temperature during vapor deposition was room temperature.

Light emission was observed by applying a voltage to the device thus fabricated. Specifically, when a positive voltage is applied to the hole injection electrode and a negative voltage is applied to the electron injection electrode, a current density of 10 mA / cm ² is obtained when a voltage of 4.1 V is applied, and a yellow color of 188 cd / m ² is obtained. A current density of 100 mA / cm ² was obtained at a voltage of 5.7 V, and yellow light emission with a high luminance of 1609 cd / m ² was observed. When the emission spectrum of this element was measured, a spectrum having a peak at 560 nm was shown as shown in FIG. This spectrum almost completely coincided with the fluorescence spectrum of rubrene shown in FIG. From this, it was confirmed that this luminescence was from rubrene.

<Comparative example>
A device was prepared in the same manner as in Example 1 except that 0.5 mol% of rubrene was not added. When a positive voltage was applied to the hole injection electrode and a negative voltage was applied to the electron injection electrode, a current density of 10 mA / cm ² was obtained when a voltage of 3.6 V was applied, and a blue-green light emission of 15 cd / m ² was obtained. Was observed, and a current density of 100 mA / cm ² was obtained at a voltage of 5.0 V, and blue-green light emission of 249 cd / m ² was observed. This was a light emission luminance of about 1/10 compared to Example 1. When the emission spectrum of this element was measured, a spectrum having a peak at 500 nm was shown as shown in FIG. This spectrum almost completely matched the phosphorescence spectrum of 4PBP. From this, it was confirmed that this luminescence was phosphorescence from 4PBP.

  The embodiments of the present invention have been described above by way of examples, but the present invention is not limited to the above-described embodiments, and can be applied to other embodiments based on the technical idea of the present invention.

  The present invention can be applied to an organic EL element and various electronic devices using the organic EL element.

It is a figure which shows typically an example of the mechanism of the typical light emission in the organic EL element of this invention. It is sectional drawing which shows typically the structure of the organic EL element produced in the Example. It is a figure which shows the emission spectrum and absorption spectrum of the organic compound used in the Example. It is a figure which shows the emission spectrum of the organic EL element produced in the Example. It is a figure which shows the fluorescence spectrum of the 2nd organic compound used in the Example. It is a figure which shows the emission spectrum of the organic EL element produced by the comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Organic electroluminescent element 11 Glass substrate 12 Hole injection electrode 13 Hole transport layer 14 Light emitting layer 15 Electron transport layer 16 Electron injection electrode

Claims (7)

An organic electroluminescence device comprising a light emitting layer that emits light by carrier injection,
The light-emitting layer includes a first organic compound and a second organic compound;
The proportion of the first organic compound in the organic compound constituting the light emitting layer is 50 mol% or more,
The first organic compound is 4-phenylbenzophenone or 4,4′-dibromobenzophenone ;
The energy level of the lowest excited singlet state of the second organic compound is lower than the energy level of the lowest excited triplet state of the first organic compound,
In the energy level of the excited singlet state of the second organic compound that is higher than the energy level of the lowest excited singlet state, the lowest excited triplet state of the first organic compound There is a level in the range of ± 0.3 eV with respect to the energy level,
An organic electroluminescence device in which the second organic compound emits fluorescence when carriers are injected into the light emitting layer. The organic electroluminescence device according to claim 1, wherein the peak of the phosphorescence spectrum of the first organic compound and the peak of the absorption spectrum of the second organic compound overlap at least partially. 2. The organic electroluminescence device according to claim 1, wherein a ratio of the second organic compound to an organic compound constituting the light emitting layer is in a range of 0.01 mol% to 20 mol%. The organic electroluminescence device according to claim 1, wherein the second organic compound is a polycyclic aromatic compound. An organic electroluminescence device comprising a light emitting layer that emits light by carrier injection,
The light-emitting layer includes a first organic compound and a second organic compound;
The proportion of the first organic compound in the organic compound constituting the light emitting layer is 50 mol% or more,
The first organic compound is 4-phenylbenzophenone or 4,4′-dibromobenzophenone;
The second organic compound is rubrene, coumarin 540, coumarin 6, DCM, rhodamine B, or Nile Red;
An organic electroluminescence device in which the second organic compound emits fluorescence when carriers are injected into the light emitting layer. The first organic compound is 4-phenylbenzophenone;
The organic electroluminescence device according to claim 5, wherein the second organic compound is rubrene. The organic electroluminescence device according to claim 5, wherein a ratio of the second organic compound to an organic compound constituting the light emitting layer is in a range of 0.01 mol% to 20 mol%.