JP2011199019A - Organic light emitting element and display device having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase a carrier injection barrier because a host material of a phosphorescent light emitting layer has a high lowest singlet (S1). Therefore, the phosphorescent light emitting element has a higher driving voltage than the fluorescent light emitting element.
A light emitting layer disposed between a pair of electrodes and the pair of electrodes, wherein the light emitting layer includes a guest material and a host material, and the guest material is a phosphorescent light emitting material, An organic light-emitting element, wherein an energy difference between a lowest excited singlet level and a lowest excited triplet level of a host material is 0.03 eV or less.
[Selection] Figure 1

Description

  The present invention relates to an organic light emitting element and a display device having the same.

  An organic light emitting element is an element having a pair of electrodes and a light emitting layer disposed between the pair of electrodes. By supplying carriers from these electrodes, the organic compound in the light emitting layer is excited and emits light when returning from the excited state to the ground state.

  Fluorescence emitted from the lowest excited singlet (S1) and phosphorescence emitted from the lowest excited triplet (T1) are known for light emission of the organic light emitting device. Moreover, it is known that the S1 level is high when S1 and T1 of the same compound are compared.

  In the phosphorescent light-emitting element, T1 of the host material that is the main component of the light-emitting layer has a higher level than T1 of the guest material that is the subcomponent. This is because energy is transferred from T1 of the host material to T1 of the guest material. On the other hand, in the case of a fluorescent light emitting element, energy is transferred from S1 of the host material to S1 of the guest material. When there is a phosphorescent light emitting element and a fluorescent light emitting element that emit the same color, the guest material T1 of the phosphorescent light emitting element and the guest material S1 of the fluorescent light emitting element are at the same level. When attention is paid to the host material of each element, S1 of the host material of the phosphorescent light emitting element is generally higher than S1 of the host material of the fluorescent light emitting element, and phosphorescence emission of S1 lower than S1 of the fluorescent light emitting element. The element has not been found.

  The high level of S1 means that the band gap, that is, the HOMO-LUMO energy gap is also large. Since the range of selection of materials that can be used as the adjacent layer adjacent to the light emitting layer is limited, it is difficult to increase the energy gap of the adjacent layer in accordance with the energy gap of the light emitting layer. Therefore, in the phosphorescent light-emitting device, the barrier between HOMOs or the barrier between LUMOs between the light-emitting layer and the adjacent layer adjacent to the light-emitting layer increases, and the energy barrier tends to be large. From this, it can be said that the phosphorescent light emitting element has a high driving voltage.

  On the other hand, even if the driving voltage is lowered, the luminous efficiency is lowered and the performance as an element is low. Therefore, it is required to maintain or improve the luminous efficiency even when the driving voltage is lowered.

JP 2009-267255 A

  As described above, the phosphorescent light emitting element generally has a high driving voltage. For example, the phosphorescent light emitting element described in Patent Document 1 has a high driving voltage of 8.1V. This is because the S1 of the host material is high and the hole and electron injection barrier to the host material becomes large.

  An object of the present invention is to provide a phosphorescent light emitting device having a low driving voltage. More specifically, in an organic light emitting device in which the organic compound layer has a three-layer structure of a hole transport layer, a light emitting layer, and an electron transport layer, phosphorescence having a driving voltage of 5.0 V or less and a light emission efficiency of 13 cd / A or more. An object is to provide a light-emitting element.

  Therefore, the present invention includes a pair of electrodes and a light emitting layer disposed between the pair of electrodes, the light emitting layer includes a guest material and a host material, and the guest material is a phosphorescent material. Provided is an organic light-emitting element, wherein an energy difference between the lowest excited singlet level and the lowest excited triplet level of the host material is 0.03 eV or less.

  According to the present invention, since the S1 of the host material of the light emitting layer is low, a phosphorescent light emitting device having a low carrier injection barrier to the host material and a low driving voltage can be provided.

It is a schematic diagram which shows the lowest excitation singlet and the lowest excitation triplet. It is a cross-sectional schematic diagram which shows an organic light emitting element and the switching element connected to an organic light emitting element.

  The present invention includes a pair of electrodes and a light emitting layer disposed between the pair of electrodes, the light emitting layer includes a guest material and a host material, the guest material is a phosphorescent material, The organic light-emitting element is characterized in that the energy difference between the lowest excited singlet level and the lowest excited triplet level of the host material is 0.03 eV or less.

  The present inventor has conceived that, in the phosphorescent light emitting device, the driving voltage can be lowered by reducing the S1 of the host material while maintaining the T1 height of the host material. For that purpose, the smaller the energy difference between S1 of the host material and T1 of the host material, the better. Alternatively, reverse intersystem crossing of the host material can be used to increase the light emission efficiency of the device. For that purpose, it was found that 0.03 eV or less is preferable.

  Below this value, the phosphorescent light emitting element can achieve the target of a driving voltage of 5.0 V or lower and a luminous efficiency of 13 cd / A or higher regardless of the emission color. Note that 0.03 eV is a value empirically obtained from experiments, and is reliable enough up to the second decimal place in the measurement conditions.

  This value of 0.03 eV means that at room temperature, the energy difference is so small that the reverse term can cross from T1 of the host material to S1 of the host material. Since the reverse term crossing from T1 of the host material to S1 of the host material can contribute to the improvement of the light emission efficiency since the energy that should have been heat-inactivated from the T1 of the host material can be converted into energy that can be emitted again. It is done.

  Here, the host material is a compound having the largest weight ratio in the light emitting layer, and the guest material has a weight ratio smaller than that of the host material in the light emitting layer, specifically 0.1 wt% with respect to the host material. It is a compound that mainly emits light with a content of 30 wt% or less.

  Here, phosphorescence is emission from T1, and is characterized by a longer emission lifetime than fluorescence. Ordinary fluorescent light emission has a light emission lifetime at the nanosecond level due to light emission from S1, whereas light emission life from the T1 has a light emission lifetime of the microsecond level.

  Phosphorescence increases in the non-radiation deactivation rate with increasing temperature, so that the emission intensity decreases with increasing temperature.

  In the phosphorescent light emitting device, it is preferable to select a small difference between T1 of the host material and T1 of the guest material so as not to increase S1 of the host material.

  The present invention will be described with reference to FIG. FIG. 1 is an energy diagram showing the energy level of each level.

  Reference numeral 2 in FIG. 1 is T1 of the guest material, reference numeral 3 is energy of phosphorescence, and reference numeral 4 is ground state energy, which is 0 eV. The energy of 3 determines the emission wavelength of phosphorescence. It is an essential condition for phosphorescence emission that T1 of the host material of reference numeral 1 is higher than T1 of the guest material of reference numeral 2. T1 of the host material of the reference numeral 1 and S1 of the host material of the reference numeral 5 perform energy transfer at the intersection of the reference numeral 6. The magnitude of energy required to supply energy to S1 of the host material with reference numeral 5 is represented by reference numeral 8, and as a result, it affects the driving voltage of the light emitting element.

  This means that when the energy 8, that is, S1 is high, the carrier injection barrier to the host material increases, and thus the drive voltage increases.

  Here, reference numeral 7 indicates S1 of the host material of the phosphorescent light emitting element as a reference, and energy 9 necessary for supplying energy to S1. Compared with S1 of the host material as a reference example, S1 of the host material according to the present invention indicated by reference numeral 5 is low.

  The smaller the difference between S1 of the host material and T1 of the host material, the better. This is because it can be considered that the smaller this difference, the lower the S1 of the host material, and the lower the drive voltage.

  Since the phosphorescent light emitting device according to the present invention has a low S1 of the host material, the driving voltage can be lowered. Specifically, since the difference between S1 and T1 is 0.03 eV or less, a phosphorescent light-emitting element having a driving voltage of 5.0 V or less and a light emission efficiency of 13 cd / A can be provided.

  The host material according to the present invention may be any compound as long as it satisfies the condition of 0.03 eV or less, but a transition metal complex is preferable.

  Among the transition metal complexes, copper and platinum are preferable, and the transition metal in the compound can exist stably if it is one kind.

  Specific examples of the host material according to this embodiment are shown below.

  Exemplary compounds H1 to H3 are represented by the general formula [1], and H4 to H6 are represented by the general formula [2].

  General formula [1]

(X represents a halogen atom.)

  General formula [2]

(R _{1 to} R ₄ are each independently selected from a hydrogen atom, a fluorine atom, and an alkyl group.
Examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, and a tert-butyl group. )

  H5 and H6 can be regarded as compounds in which the compound of H4 has a substituent. H5 has a fluorine atom as a substituent, and H6 has a methyl group as a substituent. The effect is considered to be the same regardless of the position of the substituent.

  Table 1 shows actual measurements of S1 and T1 of the host material of the present embodiment and the host material of the reference example.

In addition, S1 measured the absorption edge wavelength of the absorption spectrum in the toluene solution prepared to about 10 ^<-5 > M molar concentration, and computed it using the following formula | equation 1. FIG.

  T1 was obtained by measuring the emission spectrum of the compound in the powder state, measuring the emission edge (rising edge of the spectrum) wavelength on the high energy side, and calculating from the following formula 1.

  E is energy, h is Planck's constant, c is the speed of light, e is elementary charge, and λ is the measured wavelength.

  Reference example RH-1 is a blue phosphorescent element, RH-2 is a green and red phosphorescent element, and RH-3 is a compound used as a host of the red phosphorescent element. The S1 value of the compound of the present embodiment is a very small value with respect to these S1 values. These host materials of the present embodiment are materials having a small S1 with respect to blue, green, and red phosphorescent light emitting materials, that is, a small energy gap between HOMO and LUMO. Therefore, the energy difference between the work function of the electrodes used for the anode and the cathode and the host material of this embodiment is reduced, and the hole injection barrier and the electron injection barrier are reduced, so that the device can be driven at a low voltage.

The organic light-emitting device according to this embodiment is a device having an anode and a cathode as a pair of opposed electrodes, and a light-emitting layer disposed between these electrodes. This organic light emitting element has a light emitting layer having a host material and a guest material, and the energy difference between S1 of the host material and T1 of the host material is 0.03 eV or less. Therefore, when the organic compound layer has a three-layer structure of a hole transport layer, a light emitting layer, and an electron transport layer, this organic light emitting device satisfies a drive voltage of 5.0 V or less and a light emission efficiency of 13 cd / A or more. In the embodiment, the drive voltage is 4.9 V, so
The inventor thinks that the effect of lowering the driving voltage is not limited to the organic light emitting device having the organic compound layer having a three-layer structure, and the driving voltage is similarly lowered and high luminous efficiency can be obtained regardless of the number of layers.

  This organic light emitting device may have a plurality of layers in addition to the above three-layer structure. Examples of the multiple layers include a hole injection layer, a hole blocking layer, an exciton blocking layer, and an electron injection layer, and these layers can be used in appropriate combination.

  As the anode material, a material having a large work function is used. Examples thereof include, but are not limited to, single metals such as Au, Pt, Ag, Cu, Ni, Pd, Co, Se, V, and W, alloys thereof, and metal oxides such as ITO and IZO. Further, the anode may have a single layer structure or a multilayer structure.

  The compounds constituting the hole transport layer or hole injection layer provided between the light emitting layer and the anode are triarylamine derivatives, phenylenediamine derivatives, triazole derivatives, oxadiazole derivatives, imidazole derivatives, pyrazoline derivatives, pyrazolone derivatives, oxazoles. Examples include, but are not limited to, derivatives, fluorenone derivatives, hydrazone derivatives, stilbene derivatives, phthalocyanine derivatives, porphyrin derivatives, poly (vinyl carbazole), poly (silylene), poly (thiophene), and other conductive polymers.

  The light emitting layer according to this embodiment includes a host material and a guest material, and a known phosphorescent material can be used as the guest material. The light emitting layer may further include an assist material. The assist material is a material having a weight ratio smaller than that of the host material and assisting the guest material without contributing to main light emission.

  The compounds constituting the electron injection layer or electron transport layer provided between the light emitting layer and the cathode are oxadiazole derivatives, oxazole derivatives, pyrazine derivatives, triazole derivatives, triazine derivatives, quinoline derivatives, quinoxaline derivatives, phenanthroline derivatives, organic Although aluminum complex etc. are mentioned, it is not limited to these.

  A cathode material having a small work function is used. Examples thereof include alkali metals such as Li, alkaline earth metals such as Ca, simple metals such as Al, Ti, Mn, Ag, Pb and Cr, alloys of these simple metals, and metal oxides such as ITO. It is not limited to.

  In the organic light emitting device according to the present invention, the organic compound layer can be formed by, for example, the following method.

  The layer can be formed by vacuum deposition or a solution coating method in which it is dissolved in an appropriate solvent and applied at a predetermined position, and then the solvent is dried.

  Hereinafter, the use of the organic light emitting device according to the present invention will be described.

  The organic light emitting device according to the present invention can be used in a display device or a lighting device. In addition, there are an exposure light source of an electrophotographic image forming apparatus and a backlight of a liquid crystal display device.

The display device includes the organic light emitting element according to the present invention in a display portion. The display unit includes a plurality of pixels. The pixel includes the organic light emitting device and the TFT device according to the present invention. The anode or cathode of the organic light emitting device, the drain electrode or the source electrode of the TFT device, Is connected. The display device can be used as an image display device such as a PC.
The display device may be an image input device further including an image input unit.

  The image input device includes an image input unit that inputs information from an area CCD, a linear CCD, a memory card, and the like, and a display unit that displays the input information. If an image pickup optical system is further provided, an image pickup apparatus such as a digital camera is obtained. In addition, the display unit of the imaging apparatus or the inkjet printer has both an image output function for displaying an image based on image information input from the outside and an input function for inputting processing information to the image as an operation panel. It may be. The display device may be used for a display unit of a multifunction printer.

  Next, a display device using the organic light emitting device according to the present invention will be described.

  FIG. 2 is a schematic cross-sectional view of a display device having an organic light emitting element according to the present invention and a TFT element which is an example of a switching element for switching light emission / non-light emission of the organic light emitting element. In this figure, two sets of organic light emitting elements and TFT elements are shown. Although not shown, a transistor for controlling light emission luminance may be further included. The display device displays the information by turning on or off the organic light emitting element by driving the switching element according to the information, and transmits the information. Details of the structure will be described below.

  In the display device of FIG. 2, a substrate 10 such as glass and a moisture-proof film 11 for protecting the TFT element or the organic compound layer are provided on the substrate 10. Reference numeral 12 denotes a metal gate electrode 12. Reference numeral 13 denotes a gate insulating film 13, and reference numeral 14 denotes a semiconductor layer 14.

  The TFT element 17 has a semiconductor layer 14, a drain electrode 15, and a source electrode 16. An insulating film 18 is provided on the TFT element 17. The anode 20 and the source electrode 16 of the organic light emitting element are connected via the contact hole 19. The display device is not limited to this configuration, and any one of the anode and the cathode may be connected to either the source electrode or the drain electrode of the TFT element.

  In this figure, the organic compound layer 21 is illustrated as a single layer of multiple organic compound layers. A first protective layer 23 and a second protective layer 24 are provided on the cathode 22 to suppress deterioration of the organic light emitting device.

  The display device having the organic light emitting element of this embodiment can perform stable display even for a long time display.

  Hereinafter, the present invention will be described in detail with reference to examples.

[Example 1]
An organic light emitting device having the following configuration was produced.
ITO / PF01 (40 nm) / H1 (host) + D1 (dopant) (20 nm) / Bphen (50 nm) / KF (1 nm) / Al (100 nm)
An ITO film (120 nm) was formed on a non-alkali glass substrate having a thickness of 1.1 mm by sputtering, and used as an anode-side transparent electrode. On top of this, PF01 was formed as a hole transport layer with a layer thickness of 40 nm by vacuum deposition under the condition of a degree of vacuum of 3.0 × 10 ⁻⁵ Pa. Next, as a light emitting layer, compound H1 was used as a host material, and D1 shown below was used as a dopant material. The concentration of the dopant material with respect to the host material was set to ⁵ vol%, and a light emitting layer was formed to a thickness of 20 nm by a co-evaporation method under a vacuum degree of 3.0 × 10 ⁻⁵ Pa.

Next, Bphen (vasophenanthroline) was formed as an electron transport layer with a layer thickness of 50 nm by a vacuum deposition method under a vacuum degree of 3.0 × 10 ⁻⁵ Pa. Next, 1 nm of potassium fluoride (KF) was formed as an electron injection layer by a vacuum deposition method under a vacuum degree of 2.0 × 10 ⁻⁴ Pa. Finally, Al was formed as a cathode material to a thickness of 100 nm by vacuum deposition under a vacuum degree of 2.0 × 10 ⁻⁴ Pa to obtain an organic light emitting device.

The organic light emitting device was evaluated by the following method. A DC constant current power supply (manufactured by ADC Corporation, trade name: R6243) was used as the drive power supply. The luminance was a luminance meter (trade name: BM-7FAST, manufactured by Topcon Corporation). The evaluation of the organic light emitting device manufactured in this example was performed from the values of driving voltage and light emission efficiency at a luminance of 100 cd / m ² .

The organic light emitting device manufactured in this example showed light emission with a peak wavelength of light emission of 550 nm. The driving voltage at a luminance of 100 cd / m ² was 4.5 V, and the light emission efficiency was 20 cd / A.

[Example 2]
An organic light emitting device was fabricated in the same manner as in Example 1 except that the host material in Example 1 was changed to H4 and the dopant material was changed to D2.

The organic light emitting device manufactured in this example showed light emission with a peak light emission wavelength of 620 nm. The driving voltage at a luminance of 100 cd / m ² was 4.3 V, and the light emission efficiency was 13 cd / A.

[Example 3]
An organic light emitting device was fabricated in the same manner as in Example 1 except that the host material in Example 1 was changed to H6 and the dopant material was changed to D3.

The organic light emitting device manufactured in this example emitted light having a peak light emission wavelength of 510 nm. The driving voltage at a luminance of 100 cd / m ² was 4.9 V, and the light emission efficiency was 21 cd / A.

[Comparative Example 1]
An organic light emitting device was produced in the same manner as in Example 1 except that the host material in Example 1 was changed to RH-2. The organic light emitting device manufactured in this example showed light emission with a peak wavelength of light emission of 550 nm. The driving voltage at a luminance of 100 cd / m ² was 5.4 V, and the light emission efficiency was 13 cd / A.

[Comparative Example 2]
An organic light emitting device was produced in the same manner as in Example 2 except that the host material in Example 2 was changed to RH-2. The organic light emitting device manufactured in this example showed light emission with a peak light emission wavelength of 620 nm. The driving voltage at a luminance of 100 cd / m ² was 4.7 V, and the light emission efficiency was 11 cd / A.

[Comparative Example 3]
An organic light emitting device was produced in the same manner as in Example 3 except that the host material in Example 3 was changed to RH-2. The organic light emitting device manufactured in this example emitted light having a peak light emission wavelength of 510 nm. The driving voltage at a luminance of 100 cd / m ² was 6.5 V, and the light emission efficiency was 16 cd / A.

1 Lowest excited triplet level of host material 2 Lowest excited triplet level of guest material

Claims (4)

  A light emitting layer disposed between the pair of electrodes, the light emitting layer includes a guest material and a host material, the guest material is a phosphorescent light emitting material, and the minimum of the host material An organic light-emitting element, wherein an energy difference between an excited singlet level and a lowest excited triplet level is 0.03 eV or less. The organic light emitting device according to claim 1, wherein the host material is represented by the following general formula [1], general formula [2], or structural formula [3].
General formula [1]

(X represents a halogen atom.)
General formula [2]

(R _{1 to} R ₄ are each independently selected from a hydrogen atom, a fluorine atom, and an alkyl group.)
Structural formula [3]

  A display device comprising a plurality of pixels, wherein the pixels include the organic light-emitting element according to claim 1 or 2 and a switching element connected to the organic light-emitting element.   An image input unit for reading an image and a display unit for outputting an image, wherein the display unit includes a plurality of pixels, and the pixels include the organic light-emitting element according to claim 1 or 2 and the organic An image input device comprising: a switching element connected to the light emitting element.

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