JP2009147065A - Multicolor organic el element array - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multicolor organic EL element array which suppresses deterioration in durability characteristic while reducing cost. <P>SOLUTION: The multicolor organic EL element array includes: a positive electrode and a negative electrode; an organic light-emitting layer sandwiched between the positive electrode and negative electrode and colored in a plurality of colors by pixels; and a common electron transport layer formed over the plurality of pixels. The EL element array is characterized in that a voltage adjusting layer is provided closer to the negative electrode than the organic light-emitting layer of a pixel having the lowest driving voltage. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an organic EL element that emits light by energizing an organic layer including an organic light-emitting layer sandwiched between electrodes, and more particularly, a multicolor formed by forming a plurality of organic EL elements on a substrate. The present invention relates to an organic EL element array.

  In recent years, self-luminous devices compatible with flat panels have attracted attention. Examples of the self-luminous device include a plasma light-emitting display element, a field emission element, and an electroluminescence (EL) element.

  In particular, organic EL elements are being researched and developed vigorously, and an array of area color types that add a single color such as green, blue, red, etc. has been commercialized and is now full color. Development is becoming more active.

  In the case of producing a full-color light emitting array, a method is known in which a light emitting layer is separately applied to each pixel (element). For example, when the organic layer is formed by vapor deposition, it is generally performed to form an organic light emitting layer only on necessary pixels using a shadow mask.

  In this case, in order to reduce costs by reducing the number of processes, layers other than the organic light emitting layer may be shared by each pixel. In Patent Document 1, an anode, an organic light emitting layer, and a thin film transistor are formed for each pixel, a planarization insulating film is formed on the thin film transistor (hereinafter referred to as TFT), and the hole transport layer, the electron transport layer, and the cathode are a plurality of pixels. The present invention is characterized by being formed over a wide area.

  In addition, when an active drive using TFTs is adopted as a drive method for a full-color light emitting array, several methods have been proposed. Among these, there is a voltage driving method in which gradation display is performed by controlling a light emission period while keeping a driving voltage supplied to an organic EL element constant.

  Patent Document 2 discloses an invention characterized in that, in the time-division gray scale method, the voltage applied to each color pixel is changed depending on the color to be displayed.

  Further, Patent Document 3 includes a circuit that performs gradation display based on a light emission period control from an analog input signal and corrects variations in TFT threshold (Vth), and further includes a lighting period and a non-lighting period in one frame. An invention characterized by being provided is disclosed. The invention described in Patent Document 3 performs gradation display by correcting variations in Vth and controlling the light emission period. Therefore, if the transconductance, which is the current driving capability of the OLED driving TFT, is sufficiently large, it is possible to provide a display device that is not affected by variations in TFT characteristics and suppresses in-plane luminance variations.

JP 2004-6362 A JP 2001-159878 A JP 2003-122301 A

  In the case of the organic EL element described in Patent Document 1, since the hole transport layer and the electron transport layer are a common layer formed over a plurality of pixels, the drive voltages differ between pixels displaying different colors. Can happen naturally.

  At that time, for example, if the drive voltage is fixed and the gradation display method based on the light emission time is used and the power supply line and the power supply source are forcibly shared, a pixel with a low drive voltage is applied more than necessary. Light is emitted for a short time with higher brightness. As a result, there is a concern that the load on the durability characteristics of the element increases.

  In the case of using the voltage driving method described in Patent Document 2, when the driving voltage is greatly different between pixels of different colors, different power supply lines and power supply circuits are provided for each pixel in order to supply different voltages for each pixel. Must have. This complicates the drive circuit and increases costs. Further, when the drive circuit is formed around the substrate in the same manner as the TFT on the substrate, the drive circuit area around the substrate is enlarged, and the frame of the panel is enlarged.

  In the drive circuit described in Patent Document 3, when the drive voltage of a pixel of a certain display color is extremely low, variation compensation may be difficult. In the pixel, the TFT is used in a saturation region, not in a linear region used in other pixels. In this case, although the shift of Vth is compensated, it becomes difficult to compensate for variations in mobility, and the ability to suppress in-plane luminance variations is reduced.

  The present invention has been made in view of the above circumstances, and provides a multicolor organic EL element array in which deterioration of durability characteristics is suppressed while reducing costs.

As means for solving the above problems, the present invention provides:
An anode and a cathode, an organic light emitting layer sandwiched between the anode and the cathode and coated in a plurality of colors at least for each pixel, and a common electron transport layer formed over the plurality of pixels In at least a multicolor organic EL element array,
A voltage adjustment layer is provided on the cathode side of the organic light emitting layer of the pixel having the lowest driving voltage.

  ADVANTAGE OF THE INVENTION According to this invention, the multicolor organic EL element array which can reduce cost can be provided, suppressing deterioration of a durable characteristic.

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

  FIG. 1 is a schematic sectional view showing an embodiment of the present invention. On the substrate 1, a wiring and a switching element (not shown) are provided, an anode 2 divided for each pixel, a hole transport layer 3, an organic light emitting layer 4 divided for each pixel, and a voltage provided only for a specific pixel An adjustment layer 6, an electron transport layer 5, an electron injection layer 7, and a cathode 8 are provided. Since the anode is divided for each pixel, a desired light emission pattern can be obtained by flowing a desired current for each pixel via a wiring and a switching element (not shown).

  In the present invention, the voltage adjusting layer 6 is provided on a cathode side of the organic light emitting layer 4 in a specific pixel, that is, a pixel having the lowest driving voltage. The voltage adjustment layer 6 is a layer in which the drive voltage increases by being provided. By providing the voltage adjustment layer 6, the drive voltage can be adjusted between pixels of different colors, so that the drive wiring can be shared for each pixel, and the cost can be reduced.

  Usually, in an organic EL element, in order to reduce power consumption, a configuration in which the voltage is increased as in the present invention is not employed. Further, according to the study by the present inventors, it has been found that durability characteristics often decrease with increasing drive voltage. For this reason, the present inventors have intensively studied a method for increasing the voltage without deteriorating the durability characteristics, and have reached the present invention in which the voltage adjusting layer 6 is provided on the cathode side with respect to the organic light emitting layer 4.

  The reason why the voltage adjusting layer 6 is provided on the cathode side with respect to the organic light emitting layer 4 is that the durability characteristics are excellent as compared with the case where the voltage adjusting layer 6 is provided on the anode side with respect to the organic light emitting layer 4. The reason for this is not clear, but is presumed as follows.

  The green organic EL element is hole-limited, and it is assumed that the light emitting region is biased toward the hole transport layer. In this case, if the voltage adjustment layer 6 is provided on the anode side of the organic light emitting layer 4, holes that are rate-determining carriers are further reduced. For this reason, it is considered that a change in carrier balance and electron leakage are caused, and the durability characteristics are adversely affected. From the above, it is preferable that the light emitting region is located closer to the hole transport layer than the center of the organic light emitting layer 4 in the thickness direction.

  The material of the voltage adjustment layer 6 is not particularly limited as long as it increases the driving voltage of the organic EL element. For example, an electron transport material can be used, and an insulator or a semiconductor can be used. A material having a smaller electron mobility than the electron transport layer 5 may be used.

  Further, a material having a large electron injection barrier from the electron transport layer 5 to the voltage adjustment layer 6 or a material having a large injection barrier from the voltage adjustment layer 6 to the organic light emitting layer 4 may be used. As an example of the former, a material in which the electron affinity of the voltage adjustment layer 6 is smaller than the largest electron affinity among the materials included in the organic light emitting layer 4 and smaller than the electron affinity of the electron transport layer 5 is used. Can be mentioned. As an example of the latter, a material in which the electron affinity of the voltage adjustment layer 6 is larger than the largest electron affinity among the materials included in the organic light emitting layer 4 and larger than the electron affinity of the electron transport layer 5 is used. Can be mentioned.

  Moreover, as the said voltage adjustment layer 6, an inorganic substance may be used, an organic substance may be used, and these mixtures may be used. The film forming method is not particularly limited. In addition to wet processes such as printing and inkjet methods, dry processes such as vapor deposition and sputtering can be cited as examples. Among these, it is more preferable to provide an organic material by vapor deposition from the viewpoint of reducing damage and impurities to the lower layer.

  The film thickness of the voltage adjustment layer 6 is not particularly limited as long as the voltage between pixels can be adjusted. However, if the voltage adjustment layer 6 is too thin, sufficient voltage adjustment capability cannot be exhibited, and if it is too thick, restrictions on optical adjustment become severe. A range of 0.1 nm to 100 nm is preferable, and a range of 0.5 nm to 20 nm is more preferable.

  The voltage adjustment layer 6 is provided in a pixel having the lowest driving voltage in consideration of a light emission voltage difference between pixels of each color. Since a voltage higher than necessary is not applied to a pixel having a low driving voltage, deterioration of durability characteristics of the organic EL element can be suppressed.

  There is no restriction | limiting in the color of the pixel which provides the said voltage adjustment layer 6, It is preferable to provide in a green pixel, although it changes also with materials to be used. This is because the drive voltage tends to be lower in the green pixel than in the other color pixels. Green is highly visible due to its high visibility, and the film thickness of the organic EL element is thinner when compared to red, which has a longer wavelength, when adjusting the film thickness of the organic EL element to obtain the desired color and brightness. This is the reason why the drive voltage is lowered.

  On the other hand, blue has low visibility and large energy of radiated light, or it is difficult to obtain high efficiency as a real problem. Therefore, in order to obtain desired luminance, the drive voltage tends to be high.

  The voltage adjustment layer 6 may be provided for one type of pixel or two or more types of pixels depending on the voltage difference between different pixels. In the case where the pixel is provided for one type of pixel, it is preferable because an increase in cost due to painting can be minimized. In addition, it is preferable to provide two or more types of pixels because the light emission voltage difference between the pixels can be adjusted more finely.

  The arrangement of the voltage adjustment layer 6 is provided between the organic light emitting layer 4 and the electron transport layer 5 in this embodiment, but is not limited to this as long as the voltage can be increased. It may be provided close to the cathode 8, or may be provided between other layers between the organic light emitting layer 4 and the cathode 8, for example, between the electron transport layer 5 and the electron injection layer 7. When the organic light emitting layer 4 is provided close to the organic light emitting layer 4, the organic light emitting layer 4 is originally applied separately, so that the voltage adjustment layer 6 can be continuously formed with the shadow mask applied when forming the organic light emitting layer 4. In this case, it is not necessary to newly prepare a shadow mask for the voltage adjustment layer, and the process is not extended due to the time required for alignment, so that an increase in cost for providing the voltage adjustment layer 6 can be minimized. More preferable.

  Among the TFTs provided in the pixels of the respective colors when determining the pixel on which the voltage adjustment layer 6 is provided, the drain characteristics of the driving TFT that plays the role of driving the organic EL element, and the voltage-current characteristics of the organic EL element Compare As a result, it is preferable to provide the voltage adjustment layer 6 in a pixel in which the voltage-current characteristics of the organic EL element are in the saturation region of the drain characteristics. By doing so, the driving capability of the driving TFT becomes relatively sufficient, and the capability of correcting variation is expanded.

  Hereinafter, other components will be described.

  As the substrate 1, any material such as quartz, glass, resin or metal may be used. On the substrate 1, switching elements such as thin film transistors (not shown) and wirings are provided. At this time, it is preferable to use a method in which gradation is controlled by controlling a light emission period using a switching element. In the above-described method, a predetermined voltage is applied to the pixel, but the light emission period is controlled by controlling the application period, and gradation control is performed. Therefore, different voltages are generally applied to pixels having different emission colors, and it is necessary to provide wirings having different driving voltages for each color. In the present invention, by providing the voltage adjustment layer, it is possible to adjust the driving voltage of the pixels having different emission colors, to share the wiring, and to reduce the cost. Further, by sharing the voltage, pixels of different colors can be connected to the same wiring, and thus there is an advantage that the degree of freedom of arrangement of the pixels on the substrate is increased.

  As the anode 2, for example, chromium, aluminum, silver, or an alloy thereof can be used when used as a reflective electrode. Moreover, when using as a transparent electrode, metal oxides, such as ITO, can be used, However, It is not limited to these.

  As the hole transport layer 3, a known material can be suitably used. For example, a triphenyldiamine derivative, an oxodiazole derivative, a polyphylyl derivative, a stilbene derivative, and the like can be used, but are not limited thereto. The organic light emitting layer 4 may also serve as a hole transport layer, or a stacked body of a hole injection layer and a hole transport layer, that is, a plurality of layers may serve as the hole transport layer 3. Further, the thickness of the hole transport layer 3 may be common to each pixel, or the thickness may be changed according to the color in order to match interference.

  As the organic light emitting layer 4, any known light emitting material can be suitably used. As the light emitting material, a material that functions as an organic light emitting layer by itself may be used, or a material that functions as a mixed system of a host material and a light emitting dopant, a charge transport dopant, a light emission assisting dopant, or the like may be used. The organic light emitting layer 4 uses a different material depending on the color displayed in each pixel.

  A known material can be suitably used for the electron transport layer 5. Examples include, but are not limited to, aluminum quinolinol derivatives, oxadiazole derivatives, triazole derivatives, phenylquinoxaline derivatives, silole derivatives, phenanthroline derivatives, and the like.

  Any material may be used as the electron injection layer 7. For example, an alkali metal or alkaline earth metal oxide, fluoride or carbonate may be used in a single layer, or the electron transport material may be doped with the alkali metal or alkaline earth metal compound. . Incidentally, an alkali metal or alkaline earth metal contained in the electron injection layer 7 or a compound thereof having a diffusivity in the voltage adjustment layer smaller than that of the common electron transport layer may be used. . When these alkali metal or alkaline earth metal or a compound thereof diffuses into the electron transport layer, the conductivity of the electron transport layer may increase and the voltage may decrease. By providing a voltage adjustment layer having a lower diffusibility between the electron transport layer and the electron injection layer, the diffusion to the electron transport layer is suppressed, the conductivity of the electron transport layer is suppressed, and the voltage is increased. Can do.

  In the present embodiment, the electron transport layer 5 and the electron injection layer 7 are provided. However, the present invention is not limited to this. As another embodiment, one layer may serve both functions, and a plurality of layers may be provided. It may be responsible for that function. Further, when the cathode material contains a low work function substance such as an alkali metal and electrons are injected from the cathode 8 without any trouble, or when the material of the organic light emitting layer 4 is suitable for electron injection, Separately, the electron transport layer 5 and the electron injection layer 7 may not be provided. In this case, it is assumed that the cathode 8 is an electron injection layer common to each pixel.

  The cathode 8 may be a top emission element using a transparent electrode such as ITO as illustrated in FIG. 1, or may be a bottom emission element using a reflective electrode such as aluminum (Al), and is particularly limited. Not.

  Further, in the present embodiment, the description has been given with the configuration in which the anode is provided on the substrate side and the cathode is provided on the opposite side, but the present invention is not limited to this, and the configuration in which the cathode is provided on the substrate side and the anode is provided on the opposite side. Good.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

<Example 1>
A multicolor organic EL element array having three colors of organic EL elements (pixels) of R (red), G (green), and B (blue) having the structure shown in FIG. 1 is produced by the following method. Chemical formulas of organic compounds used in this example are shown in Chemical Formula 1, Chemical Formula 2, and Chemical Formula 3.

  Switching elements and wirings (not shown) are formed on the substrate 1 using a known photolithography technique. On top of that, a laminated film of 100 nm thick aluminum and 20 nm thick ITO is formed and patterned by sputtering, so that it is divided for each pixel, and for each pixel via a switching element (not shown). The anode 2 that is driven independently is formed.

  Further, an element isolation film (not shown) is formed from polyimide resin. This is subjected to ultrasonic cleaning with isopropyl alcohol (IPA) and then dried after boiling and drying. Further, after UV / ozone cleaning, an organic compound is deposited by vacuum deposition.

First, using a shadow mask, HT-1 is deposited to a thickness of 20 nm as a B hole transport layer 3c by vacuum deposition. The degree of vacuum during vapor deposition is 1 × 10 ⁻⁴ Pa and the film formation rate is 0.3 nm / sec. Subsequently, as the organic light emitting layer 4c of B, EM-1 as a host and EM-2 as a dopant are co-evaporated so as to have a volume ratio of 95: 5, and a film having a thickness of 35 nm is formed.

  The shadow mask is replaced, and HT-1 is deposited to a thickness of 60 nm by vacuum deposition as the R hole transport layer 3a. Subsequently, as R organic light emitting layer 4a, EM-5 as a host, EM-6 and EM-7 as dopants are co-deposited so that the volume ratio is 80: 4: 16, and a film having a thickness of 50 nm is formed.

  Using another shadow mask, HT-1 is formed as a G hole transport layer 3b to a thickness of 30 nm by vacuum deposition. Subsequently, as a G organic light emitting layer 4b, EM-3 as a host and EM-4 as a dopant are co-deposited so as to have a volume ratio of 90:10 to form a film having a thickness of 30 nm.

Further, using the same shadow mask, a known Alq3 film having a thickness of 10 nm is formed as the voltage adjustment layer 6 only on the G organic light emitting layer 4b. The degree of vacuum during vapor deposition is 1 × 10 ⁻⁴ Pa and the film formation rate is 0.3 nm / sec.

As the common electron transport layer 5, bathophenanthroline (Bphen: ET-1) is formed to a thickness of 10 nm by vacuum deposition. The degree of vacuum during vapor deposition is 1 × 10 ⁻⁴ Pa and the film formation rate is 0.3 nm / sec.

A common electron injection layer 7 is formed. ET-1 as a host and cesium carbonate as a dopant of an alkali metal compound are formed into a film thickness of 40 nm by co-evaporation. The degree of vacuum during vapor deposition is 1 × 10 ⁻⁴ Pa, the film formation rate is 0.009 nm / sec for cesium carbonate, and 0.3 nm / sec for ET-1.

  The cathode 8 is formed by moving to another chamber and depositing ITO with a film thickness of 40 nm by sputtering. Finally, a glass cap provided with a desiccant is attached to the substrate 1 in a nitrogen atmosphere and sealed to obtain a multicolor organic EL element array.

<Comparative Example 1>
A multicolor organic EL element array of Comparative Example 1 is obtained in substantially the same manner as in Example 1. That is, in Comparative Example 1, the voltage adjustment layer is not provided on the G organic light emitting layer, and HT-1 is formed to a thickness of 10 nm as the G hole transport layer, and the hole is formed thereon. N, N′-α-dinaphthylbenzidine (α-NPD), which is conventionally known as a transport layer, is formed to a thickness of 20 nm. Other configurations are the same as those of the first embodiment.

When the multicolor organic EL element array of Example 1 is made to emit light, the voltage necessary for emitting white light of 250 cd / m ² is 4.1 V for all of R, G, and B, and is driven by the same power supply line. The cost of the multicolor organic EL element array can be reduced. In addition, no deterioration in display quality due to in-plane luminance variations is observed.

Further, when the durability characteristic was measured by continuously driving the organic EL element of G at 10 mA / cm ² , the half-life was 4000 hours, which is superior to the multicolor organic EL element array of Comparative Example 1. .

When the multicolor organic EL element array of Comparative Example 1 is caused to emit light, the voltage required for emitting white light of 250 cd / m ² is 4.1 V for all of R, G, and B. When the organic EL element of G is continuously driven at 10 mA / cm ² and the durability characteristic is measured, the half time is 2000 hours, and the durability characteristic is greatly deteriorated.

  The multicolor organic EL element array of the present invention can be suitably used for various displays. A display is an image display device such as a display unit of a television or a personal computer or a display unit mounted on an electronic device. The display unit mounted on the electronic device is preferably a display unit in a car, an image display unit of a digital camera, or an operation panel of office equipment such as a copying machine or a laser beam printer.

It is a schematic sectional drawing showing one Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Anode 3 Hole transport layer 4 Light emitting layer 5 Electron transport layer 6 Voltage adjustment layer 7 Electron injection layer 8 Cathode

Claims (10)

An anode and a cathode, an organic light emitting layer sandwiched between the anode and the cathode and coated in a plurality of colors at least for each pixel, and a common electron transport layer formed over the plurality of pixels In at least a multicolor organic EL element array,
A multicolor organic EL element array, wherein a voltage adjustment layer is provided on the cathode side of the organic light emitting layer of the pixel having the lowest driving voltage.   2. The multicolor organic EL element array according to claim 1, wherein the pixels of the plurality of colors are red, green, and blue, and the voltage adjustment layer is provided in a green pixel. 3.   3. The multicolor organic EL device according to claim 1, wherein a light emitting region of the pixel provided with the voltage adjustment layer is closer to a hole transport layer than a center of the organic light emitting layer. array.   4. The multicolor organic material according to claim 1, wherein the voltage adjustment layer is provided between the common electron transport layer and the organic light emitting layer. 5. EL element array.   4. The multicolor organic EL element array according to claim 1, wherein the voltage adjustment layer is provided on a cathode side with respect to the common electron transport layer. 5.   The electron affinity of the voltage adjustment layer is smaller than the largest electron affinity among the materials contained in the organic light emitting layer and smaller than the electron affinity of the electron transport layer. Item 6. The multicolor organic EL element array according to any one of Items 5.   The electron affinity of the voltage adjustment layer is larger than the largest electron affinity among the materials contained in the organic light emitting layer and larger than the electron affinity of the electron transport layer. Item 6. The multicolor organic EL element array according to any one of Items 5.   The multicolor organic EL element array according to claim 1, further comprising an electron injection layer containing an alkali metal, an alkaline earth metal, or a compound thereof.   9. The multicolor organic material according to claim 8, wherein the voltage adjusting layer has a diffusivity of the alkali metal or alkaline earth metal or a compound thereof smaller than that of the common electron transport layer. EL element array.   2. The multicolor organic EL element array according to claim 1, wherein each pixel includes a switching element, and a driving method of each pixel is a method of controlling gradation by controlling a light emission period. .

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