JP2012146764A - Display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a display device including a low voltage driven red organic EL element and a blue organic EL element having high luminous efficiency while suppressing the manufacturing cost by minimizing an amount of a film adhered to a patterning mask.SOLUTION: Each organic EL element has a first electron transport layer 19 which is disposed for each organic EL element between a cathode 21 and a light emitting layer 16 and which contacts with a red light emitting layer 16R, and a blue organic EL element has a second electron transport layer 18 which is disposed between the first electron transport layer 19 and a blue light emitting layer 16B and which contacts with the blue light emitting layer 16B. A band gap of a material forming the second electron transport layer 18 is larger than a band gap of a material forming the first electron transport layer 19.

Description

  The present invention relates to a display device including organic EL (electroluminescence) elements of three colors of red, green, and blue.

  The organic EL element has a laminated structure in which an organic compound layer including at least a light emitting layer is disposed between a pair of electrodes composed of an anode and a cathode, and the organic compound layer has a purpose of improving durability characteristics and light emission efficiency. Further, a hole transport layer, an electron transport layer, an electron injection layer, and the like are inserted.

  In addition, layers constituting the organic EL element such as a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer are generally formed by depositing an organic material on a substrate by vacuum deposition. Therefore, in a full color display organic EL display device using organic EL elements of three colors of red, green and blue, each of the red, green and blue light emitting layers is provided with a metal mask for patterning that matches the pixel shape of each color. Used for vacuum evaporation. In recent years, the pixel size of each color has been miniaturized as the organic EL display device has become higher definition, and the metal mask for patterning that matches the pixel shape has also been used for high definition. Manufacturing and maintenance costs are very large.

  Patent Document 1 describes a technique for forming an electron transport layer in contact with a light emitting layer using a patterning mask of the light emitting layer as it is after forming each light emitting layer of red, blue, and green. . That is, after forming at least one light emitting layer of red, blue, and green, the electron transport layer is formed using the patterning mask, thereby forming the light emitting layer and the electron transport layer in the same vacuum chamber. It is possible to prevent impurities from entering the light emitting layer and improve durability. However, at this time, the formation of the electron transport layer using the patterning mask of the light emitting layer increases the amount of deposition of the patterning mask of the light emitting layer, so that the cleaning frequency increases and the manufacturing cost increases. For this reason, the electron transport layer is thinly formed using a patterning mask in contact with the light-emitting layer, and red, green, and blue are shared on the remaining electron transport layer without using the patterning mask. It is shown.

US Pat. No. 7,535,167

  As described above, the patterning mask for the light emitting layer is expensive, and it is required to reduce the deposition amount as much as possible.

  On the other hand, when a hole transport layer, an electron transport layer, an electron injection layer, etc. are inserted into the organic compound layer, the hole transport layer, the electron transport layer, and the electron injection for each color according to the red, green, and blue light emitting layers. It is easy to improve luminous efficiency by coating different layers such as layers. In particular, a red light-emitting layer with a narrow band gap and a blue light-emitting layer with a wide band gap have different optimum electron transport layers. For example, when an electron transport layer having an optimum band gap for the red light emitting layer is used for blue, the electrons cannot be sufficiently injected, and a phenomenon that holes leak from the light emitting layer occurs, resulting in poor light emission efficiency. In addition, for example, when an electron transport layer having an optimum band gap for a blue light-emitting layer is used for red, electron injection is good and leakage of holes can be prevented, but compared with the case of using an electron transport layer optimal for red. As a result, there is a problem that the barrier for injecting electrons into the electron transport layer becomes large and the voltage increases.

  An object of the present invention is to solve the above-mentioned problems and to suppress the manufacturing cost by minimizing the deposition amount of the patterning mask, and to reduce the red organic EL element driven at a low voltage and the blue organic EL element having good light emission efficiency. It is providing the display apparatus provided with these.

The present invention includes an organic EL element that emits red, an organic EL element that emits green, and an organic EL element that emits blue. Each organic EL element includes an anode, a cathode, the anode, and the cathode. A light emitting layer between the display device and the display device,
Each of the organic EL elements has a first electron transport layer that is disposed in common between the cathode and the light emitting layer and is in contact with the light emitting layer of the organic EL element that emits red light.
The organic EL element that emits blue light is disposed between the first electron transport layer and the light emitting layer of the organic EL element that emits blue, and is in contact with the light emitting layer of the organic EL element that emits blue light. Having an electron transport layer,
The band gap of the material forming the second electron transport layer is larger than the band gap of the material forming the first electron transport layer.

  In the present invention, since the suitable electron transport layers are in contact with the light emitting layer of the blue organic EL element and the light emitting layer of the red organic EL element, respectively, the blue organic EL element does not increase the driving voltage of the red organic EL element. High luminous efficiency can be obtained. In the present invention, since the patterning mask is used only during the formation of the second electron transport layer in the step of forming the electron transport layer, the deposition amount of the patterning mask is larger than the patterning of the electron transport layer with all colors. In addition, the cleaning frequency can be reduced, and the manufacturing cost can be suppressed.

It is a cross-sectional schematic diagram for 1 pixel which shows the structure of one Embodiment of the display apparatus of this invention. It is a typical energy band figure of the blue organic EL element of one Embodiment of the display apparatus of this invention.

  The present invention is an organic EL display device having an organic EL element that emits red, an organic EL element that emits green, and an organic EL element that emits blue. Each organic EL element includes an anode and a cathode. A light emitting layer is disposed between the pair of electrodes. Each organic EL element has a first electron transport layer disposed in common between the cathode and the light emitting layer. The first electron transport layer is in contact with the light emitting layer of the red organic EL element (hereinafter referred to as a red light emitting layer. The light emitting layers of the green and blue organic EL elements are also described in the same manner). The blue organic EL element further includes a second electron transport layer between the first electron transport layer and the blue light emitting layer. The second electron transport layer is in contact with the blue light emitting layer. In the following description, red, green, and blue organic EL elements are referred to as a red element, a green element, and a blue element, respectively.

  Hereinafter, a description will be given with reference to FIG. FIG. 1 is a schematic cross-sectional view showing one pixel of one embodiment of the display device of the present invention. This example is an example in which an organic EL element having a top emission type and having a reflective electrode as an anode is used. The present invention includes not only the top emission type but also the bottom emission type and the case where the reflective electrode is a cathode.

  In FIG. 1, 11 is a substrate, 12 is an anode, 13 is a metal layer, 14 is a transparent conductive layer, 15 is a hole transport layer, 16 is a light emitting layer, 17 is an electron transport layer, 20 is an electron injection layer, and 21 is a cathode. It is. The light emitting layer 16 includes a red light emitting layer 16R, a green light emitting layer 16G, and a blue light emitting layer 16B corresponding to red, green, and blue light emission colors. The electron transport layer 17 is formed of a first electron transport layer 19 and a second electron transport layer 18, and the second electron transport layer 18 is formed in the same shape as the blue light emitting layer 16B using a patterning mask, and the first electron transport layer 17 is formed. Layer 19 is formed without using a patterning mask.

  In the present invention, as described above, the second electron transport layer 18 is formed using a patterning mask, and the first electron transport layer 19 is formed without using a patterning mask, thereby patterning the electron transport layer in all colors. Rather than doing so, the use of the patterning mask can be reduced.

  Although the anode 12 of this example is formed on the substrate 11 and has a laminated structure of the metal layer 13 and the transparent conductive layer 14, it may be formed of only a metal having high reflectivity and conductivity.

  As the material used for the hole transport layer 15, conventionally known materials such as arylamines can be used, and different materials may be laminated, and red, green may be used to increase the light extraction efficiency. The optical interference may be adjusted by changing the film thickness for each blue.

  A known material can be used for the light emitting layer 16 as the light emitting layer of the organic EL display device. In the present invention, the red light emitting layer 16R, the green light emitting layer 16G, and the blue light emitting layer 16B are individually formed, and each light emitting layer 16 is formed by vapor deposition using a patterning mask.

  In the present invention, the blue light emitting layer 16B preferably has a main light emitting position on the second electron transport layer side. In that case, the blue light emitting layer 16B further prevents the holes from leaking from the light emitting layer 16B. Luminous efficiency can be improved.

  Then, the first electron transport layer 18 is formed on the blue light emitting layer 16B by a vapor deposition method using a patterning mask. In the present invention, the second electron transport layer 18 may also be formed on the green light emitting layer 16G.

  The first electron transport layer 19 is also in contact with the red light emitting layer 16R, and is formed in common for each color of red, green, and blue without using a patterning mask. At this time, the band gap of the material forming the second electron transport layer 18 is larger than the band gap of the material forming the first electron transport layer 19. The band gap of a material is the difference between the energy of the HOMO (highest occupied orbit) level and the energy of the LUMO (lowest empty orbit) level of the material. The first electron transport layer 19 may be formed in contact with the second electron transport layer 18.

  In the present invention, the relationship between the HOMO level energy and the LUMO level energy of the first electron transport layer 19 and the second electron transport layer 18 preferably satisfies the relationship of the following formula (A).

LUMO ₂ <LUMO ₁ <HOMO ₁ <HOMO ₂ (A)

In the formula (A), LUMO ₁ and LUMO ₂ are the absolute values of the LUMO level energy of the first electron transport layer 19 and the second electron transport layer 18, respectively. HOMO ₁ and HOMO ₂ are the first electron transport layer, respectively. 19 shows the absolute value of the energy of the HOMO level of the second electron transport layer 18.

  FIG. 2 shows a representative energy band diagram of a blue element of one embodiment of the display device of the present invention. As shown in FIG. 2, by forming the second electron transport layer 18 having a large band gap in contact with the blue light emitting layer 16B with this configuration, electrons can be sufficiently injected and holes can leak from the light emitting layer 16B. Therefore, the light emission efficiency of the blue element can be improved.

  Further, since electrons are easily injected into the LUMO level of the first electron transport layer 19, the first electron transport layer 19 having a narrow band gap is formed in contact with the red light emitting layer 16R, thereby reducing the voltage of the red element. I can do it. In the present invention, the first electron transport layer 19 may be formed by stacking different materials. In this case, the material used in contact with the second electron transport layer 18 also satisfies the relationship of the formula (A).

  In the present invention, it is preferable that the energy of the HOMO level of the host material of the blue light emitting layer 16B and the second electron transport layer 18 satisfies the relationship of the following formula (B).

HOMO _bh <HOMO ₂ (B)

In the formula (B), HOMO _bh and HOMO ₁ represent the absolute values of the energy of the HOMO level of the host material of the blue light emitting layer 16B and the second electron transport layer 18, respectively.

  In this case, as shown in FIG. 2, since the HOMO level of the second electron transport layer 18 is deeper than the HOMO level of the host material of the blue light emitting layer 16B, it functions as a hole blocking layer and holes leak from the light emitting layer 16B. And the luminous efficiency of the blue element can be further improved.

  The blue light emitting layer 16B includes at least a host material and a light emitting dopant material, and the energy of the HOMO level and the energy of the LUMO level of the host material and the light emitting dopant material of the blue light emitting layer satisfy the relationship of the following formula (C). Is more preferable.

LUMO _bh <LUMO _bd <HOMO _bh <HOMO _bd (C)

In the formula (C), LUMO _bh and LUMO _bd represent the absolute values of the LUMO level energy of the host material and the light emitting dopant material of the blue light emitting layer 16B, respectively. HOMO _bh and HOMO _bd represent the absolute values of the energy of the HOMO level of the host material and the light emitting dopant material of the blue light emitting layer 16B.

  In this case, as shown in FIG. 2, since the blue light emitting layer 16B easily traps electrons, the main light emitting position of the blue light emitting layer 16B is closer to the electron transport layer 17 side (second electron transport layer 18 side). , Holes can be prevented from leaking from the light emitting layer 16B. Therefore, the luminous efficiency of the blue element can be further improved. Furthermore, it can suppress that a hole leaks from the blue light emitting layer 16B by satisfy | filling Formula (B).

  In the present invention, conventionally known electron transport materials can be used for the first electron transport layer 19 and the second electron transport layer 18. Examples include aluminum quinolinol complexes, naphthalene compounds, anthracene compounds, phenanthrene compounds, chrysene compounds, pyrene compounds, fluoranthene compounds, phenanthroline compounds, aza compounds, and the like.

  An electron injection layer 20 may be formed on the first electron transport layer 19, and a light-transmitting cathode 21 is formed thereon.

  With the above-described configuration, the present invention provides an organic EL display device in which the deposition amount of the patterning mask and the cleaning frequency are minimized, and the blue element has high luminous efficiency without increasing the voltage of the red element. it can.

  The band gap defined in the present invention is obtained from the absorption edge of the absorption spectrum, and HOMO and LUMO are shown as absolute values of energy levels. HOMO was measured using atmospheric photoelectron spectroscopy (AC-2). LUMO was calculated by subtracting the band gap obtained from the absorption edge of the absorption spectrum from the HOMO value measured by the above method.

  Examples of the present invention will be described below. The materials and device configurations used in the following examples are particularly preferred examples, but are not limited thereto.

(Example 1, Comparative Example 2)
A blue element of the organic EL display device having the cross-sectional structure shown in FIG. 1 was produced.

  Glass is used as the substrate 11, a substrate with an electrode on which an anode 12 in which silver (film thickness 200 nm) is laminated as the metal layer 13 and IZO (film thickness 20 nm) is laminated as the transparent conductive layer 14 is formed, and UV / ozone cleaning is performed. gave.

Then, it attached to the vacuum evaporation system (made by ULVAC), and exhausted to 1.33 * 10 <^-4> Pa (1 * 10 < ^-6 > Torr). Thereafter, an N, N′-α-dinaphthylbenzidine film having a thickness of 20 nm was formed on the anode 12 without using a patterning mask to form a hole transport layer 15.

  Next, a fluoranthene compound (2% by mass) represented by the following structural formula (1) is used as the light-emitting dopant material, and a pyrene compound represented by the following structural formula (2) is used as the host material to form a blue light-emitting layer 16B having a thickness of 20 nm. did.

  Next, an aza compound (band gap 3.41 eV) represented by the following structural formula (3) was formed to a thickness of 10 nm as the second electron transport layer 18. Further, a phenanthroline compound (band gap 3.18 eV) represented by the structural formula (4) was formed as a first electron transport layer 19 on the second electron transport layer 18 with a film thickness of 10 nm.

  Next, a co-deposited film of cesium carbonate (3% by volume) and a phenanthroline compound represented by the structural formula (4) was formed to a thickness of 40 nm on the first electron transport layer 19, thereby forming an electron injection layer 20. Subsequently, the substrate formed up to the electron injection layer 20 is transported in a vacuum to a sputtering apparatus (manufactured by ULVAC), and indium tin oxide (IZO) is formed on the electron injection layer 20 by sputtering to a thickness of 30 nm. Then, the cathode 21 was formed. Thereafter, the substrate was transferred to a glove box and sealed with a glass cap containing a desiccant in a nitrogen atmosphere.

  The LUMO level energy of the fluoranthene compound of the structural formula (1) used as the luminescent dopant was 3.06 eV, and the HOMO level energy was 5.85 eV. In addition, the LUMO level energy of the pyrene compound of the structural formula (2) used as the host material was 2.71 eV, and the HOMO level energy was 5.70 eV.

  The LUMO level energy of the aza compound represented by the structural formula (3) used as the second electron transport layer 18 was 2.59 eV, and the HOMO level energy was 6.00 eV. The LUMO level energy of the phenanthroline compound of the structural formula (4) used as the first electron transport layer 19 was 2.65 eV, and the HOMO level was 5.83 eV.

  Therefore, the relationship between the energy of the HOMO level and the energy of the LUMO level of the first electron transport layer 19 and the second electron transport layer 18 satisfies the relationship of the above formula (A). As a result, the first electron transport layer 19 having a narrow band gap can be formed in contact with the red light emitting layer 16R described in Example 2 described later, and thus the voltage of the red element can be lowered.

  Further, the host materials of the second electron transport layer 18 and the blue light emitting layer 16B satisfy the relationship of the above formula (B), the second electron transport layer 18 functions as a hole blocking layer, and holes leak from the blue light emitting layer 16B. I was able to prevent more.

  Furthermore, the blue light emitting layer 16B satisfy | filled the relationship of the said Formula (C), and the light emission position was the electron carrying layer 17 side.

  Further, as Comparative Example 1, a blue device was produced in the same manner as in Example 1 except that the electron transport layer 17 was formed only from the phenanthroline compound represented by the structural formula (4).

  When the blue elements of Example 1 and Comparative Example 1 were compared, the blue element of Comparative Example 1 had a current efficiency of 3.4 cd / A, whereas the blue element of Example 1 had a high current efficiency of 4.0 cd / A. It was good.

(Example 2, comparative example 2)
Instead of the blue light emitting layer 16B, a red light emitting layer 16R having a thickness of 70 nm is formed, and the electron transport layer 17 (the red element is only the first electron transport layer 19) is formed of the phenanthroline compound represented by the structural formula (4). A red element was produced in the same manner as in Example 1 except that.

  For the red light emitting layer 16, an Ir complex (18% by volume) known to emit red light as a light emitting dopant material and 4,4'-N, N'-dicarbazole-biphenyl as a host material were used.

  In this example, since the first electron transport layer 19 is formed of the phenanthroline compound represented by the structural formula (4) having a narrow band gap, the voltage of the red element can be lowered.

  Further, as Comparative Example 2, a red element was produced in the same manner as in Example 2 except that the electron transport layer 17 was formed of an aza compound represented by the structural formula (3).

When the red elements of Example 2 and Comparative Example 2 were compared, the voltage of the red element of Comparative Example 2 was about 0.5 V higher than the red element of Example 2 when a current of 20 mA / cm ² was passed. . This is probably because the voltage was increased because an electron transport material suitable for a blue element was used as the electron transport layer 17.

  By using the blue and red elements obtained by the procedures of Examples 1 and 2 above, the patterning mask is used only once for the second electron transport layer 18 except for the light emitting layer 16. An organic EL display device having high luminous efficiency and low driving voltage of a red element is provided.

  12: anode, 16: light emitting layer, 16R: red light emitting layer, 16G: green light emitting layer, 16B: blue light emitting layer, 17: electron transport layer, 18: second electron transport layer, 19: first electron transport layer, 21 :cathode

Claims (5)

It has an organic EL element that emits red, an organic EL element that emits green, and an organic EL element that emits blue, and each organic EL element is between an anode, a cathode, and the anode and the cathode. A display device having a light emitting layer,
Each of the organic EL elements has a first electron transport layer that is disposed in common between the cathode and the light emitting layer and is in contact with the light emitting layer of the organic EL element that emits red light.
The organic EL element that emits blue light is disposed between the first electron transport layer and the light emitting layer of the organic EL element that emits blue, and is in contact with the light emitting layer of the organic EL element that emits blue light. Having an electron transport layer,
A display device, wherein a band gap of a material forming the second electron transport layer is larger than a band gap of a material forming the first electron transport layer. 2. The display device according to claim 1, wherein the energy of the HOMO level and the energy of the LUMO level of the first electron transport layer and the second electron transport layer satisfy the relationship of the following formula (A).
LUMO ₂ <LUMO ₁ <HOMO ₁ <HOMO ₂ (A)
(In Formula (A), LUMO ₁ and LUMO ₂ are the absolute values of the LUMO level energy of the first electron transport layer and the second electron transport layer, respectively, and HOMO ₁ and HOMO ₂ are the first electrons, respectively. (The absolute value of the energy of the HOMO level of the transport layer and the second electron transport layer is shown.) The light emitting layer of the organic EL element emitting blue light contains a host material and a light emitting dopant material, and the energy of the HOMO level of the host material and the second electron transport layer satisfies the relationship of the following formula (B): The display device according to claim 1 or 2.
HOMO _bh <HOMO ₂ (B)
(In Formula (B), HOMO _bh and HOMO ₂ represent the absolute values of the HOMO level energies of the host material and the second electron transport layer, respectively.)   4. The display device according to claim 1, wherein a light emitting position of a light emitting layer of the organic EL element that emits blue light is on the second electron transport layer side. 5. The light emitting layer of the organic EL element emitting blue light includes a host material and a light emitting dopant material, and the energy of the HOMO level and the energy of the LUMO level of the host material and the light emitting dopant material satisfy the relationship of the following formula (C). The display device according to claim 1, wherein the display device is a display device.
LUMO _bh <LUMO _bd <HOMO _bh <HOMO _bd (C)
(In Formula (C), LUMO _bh and LUMO _bd are the absolute values of the LUMO level energy of the host material and the light-emitting dopant material, respectively, and HOMO _bh and HOMO _bd are the host material and the light-emitting dopant material, respectively. (The absolute value of the energy of the HOMO level is shown.)

Images