JP6663142B2 - Organic electroluminescent device - Google Patents

Description

  The present invention relates to an organic electroluminescence device, and more particularly, to an organic electroluminescence device using a graphene film for an anode electrode.

BACKGROUND ART An organic electroluminescence (EL) element utilizes a phenomenon in which a current is injected into an organic material to emit light (organic electroluminescence), and is composed of a transparent conductive film provided on a transparent substrate. It has a basic structure in which an anode, an organic layer including a light emitting layer made of an organic EL material, and a cathode made of a metal film are stacked.
In a general organic EL element, a conductive polymer such as a mixture of polyethylene dioxythiophene and polysulfonic acid (PEDOT: PSS) is provided between a transparent electrode (anode) and an organic EL material. It has been performed to improve the hole injection efficiency.

Until now, indium tin oxide (ITO) has been used for the transparent electrode (anode) used in the organic EL element. However, it has been replaced with ITO from the viewpoint of depletion of rare metal resources and flexibility. New electrode materials are needed.
Under these circumstances, graphene having a honeycomb structure composed of only carbon is expected to be a material having high mobility, high flexibility, and high thermal conductivity as a material for solving the above problems.

  The transparent conductive film used for the organic EL element is required to have a low resistance and a high transmittance, and the present inventors have responded to this demand by conducting research aimed at improving the quality of the graphene film. An organic EL device is formed using one or two layers of graphene films having low D band and high crystallinity, and having a basic structure of a cathode / organic light emitting layer / hole injection layer (PEDOT: PSS) / graphene film (anode). Along with (Non-Patent Document 1), a method has been proposed in which one to six graphene films are formed by a plasma CVD method without using a carbon-containing gas such as methane gas, and the number of layers is controlled (Patent) Reference 1).

  In addition, a light emitting device using an organic EL element has attracted attention as a next-generation display because of its flexibility (flexibility) (see Non-Patent Document 2), and a flexible substrate (flexible). (Substrate) is being considered.

  For example, prototypes announced at the 2013 SID (Society for Information Display) include an organic EL display device (OLED) using a PEN substrate based on polyethylene naphthalate (PEN) and a polyimide (PI) plate. OLEDs and the like used are listed. However, in these OLEDs, the transparent electrode is made of a transparent conductive film (ITO: Indium Tin Oxide).

  In the case of using a graphene film for the transparent electrode, it was reported that a flexible OLED was manufactured by modifying the graphene anode to increase the work function and lower the sheet resistance. A terephthalate (PET) substrate is used (Non-Patent Document 3).

  Furthermore, in an OLED using a mixture of graphene oxide and PEDOT: PSS instead of a graphene film for the anode, polyethylene naphthalate (PEN) is used for the substrate (Non-Patent Document 4).

When a graphene film is formed on a resin substrate, a graphene film is formed on a catalyst metal by a CVD method, and then the graphene film formed on the catalyst metal is formed on a flexible base material such as PET by various methods. Is adopted. For example, in Patent Literature 2, a film-like intermediate medium having an adhesive surface is used, and the adhesive surface of the intermediate medium is pressure-bonded to the surface of a graphene film formed on a catalytic metal to be temporarily fixed. Patent Document 3 discloses that a graphene roll film deposited by a roll-to-roll method on a metal thin film having a catalytic ability, which is a first base material, is transferred to a second base material. And various resins are described as the second base material.
However, these patent documents only describe an example of transfer to a PET substrate, but do not mention a specific device, particularly an organic EL element.

JP-A-2013-013797 International Publication No. 2012/153674 JP 2014-24700 A

2014 61st JSAP Spring Meeting 20a-2E-12 Research and development trend of flexible display, NHK Giken R & D, No. 145, pp. 4-17 (May 2014) Nature Photonics 6,105-110 (2012) Journal of Materials Chemistry C, 2014,2,4044-4050

The present inventors have further studied the organic EL device using the graphene film reported in Non-Patent Document 1, Patent Document 1, etc., and found that the presence of the leak current has a bad influence on the device performance. It turned out to be possible. As one example, it is considered that the influence of Joule heat generated by the leak current causes the deterioration of the organic EL material around it.
From such a background, it is considered that suppression of a leak current is indispensable as one technique for increasing the luminance of an organic EL element using graphene.
An object of the present invention is based on such knowledge, and it is an object of the present invention to reduce leakage current and realize high luminance in an organic EL device using a graphene film as a transparent conductive film.

  One of the causes of the leak current is presumed to be the aggregation of the chemical material that is doped into the graphene film. In other words, in organic EL devices using graphene, chemical doping materials have been mainly used so far in order to increase the work function and lower the resistance of graphene in driving high luminance and low voltage driving. However, chemical doping materials have poor atmospheric stability and, depending on the material, the doping material may aggregate into particles, which can be a source of leakage current. If the thickness of the aggregate is equal to or greater than the thickness of the EL material or the conductive polymer molecule, the aggregate causes an electrical short between the anode and the cathode, resulting in leakage.

For this reason, the present inventors have sought to increase the luminance and lower the voltage of the organic EL element without using a chemical doping material (see Japanese Patent Application No. 2014-188104). One is that the flatness of the transparent substrate itself is not so high.
The present inventors have conducted intensive studies to achieve the above object, and as a result, by using a highly flat polyethylene naphthalate (PEN) substrate for the transparent resin substrate, the leakage current of the organic EL element was reduced. And found that high brightness can be realized.

The present invention has been completed based on these findings, and according to the present invention, the following inventions are provided.
[1] A method for manufacturing an organic EL device in which two graphene films, a hole injection layer, an organic EL layer, and a cathode are stacked on a transparent resin substrate made of polyethylene naphthalate and having a surface roughness (Ra) of 2 nm or less. Wherein the two-layer graphene film is formed on a metal having catalytic activity by a plasma CVD method using a gas containing hydrogen gas as a main component, and then transferred to the transparent resin substrate. Of manufacturing an organic EL device.

  According to the present invention, since the flatness of the PEN substrate used is very high, each layer constituting the organic EL element can be kept uniform in thickness, the leak current can be reduced, and as a result, the electroluminescence EL The effect of increasing the luminance in the element is obtained. In addition, since the heat resistance of the PEN substrate used is high (up to 150 ° C.), an effect is obtained that the surface structure of the PEN substrate hardly changes even after the device fabrication process.

FIG. 2 is a cross-sectional view schematically illustrating the structure of an organic light-emitting device manufactured according to the present invention. FIG. 4 is a diagram illustrating a typical Raman spectrum of a synthesized graphene film. The figure which shows typically the method of transferring a graphene film to a transparent substrate. It is a figure which shows a luminance-voltage measurement result, (a) is a PEN substrate and (b) is a PET substrate. It is a LOG plot of a luminance-voltage measurement result, (a) is a PEN substrate, (b) is a PET substrate. It is a figure which shows a current-voltage measurement result, (a) is a PEN board and (b) is a PET board. It is a device light emission observation photograph using a CCD camera, (a) is a PEN substrate, (b) is a PET substrate. It is a figure which shows the flatness evaluation of the transparent substrate using AFM, (a) is a PEN substrate (with a hard coat), (b) is a PEN substrate (without a hard coat), (c) is a PET substrate, (d) Is a quartz substrate.

The organic light emitting device of the present invention is an organic light emitting device in which a transparent resin substrate, an anode made of a graphene film on the transparent resin substrate, a hole injection layer, an organic light emitting layer, and a cathode are laminated in this order. A highly flat polyethylene terephthalate is used for the resin substrate.
The surface roughness of the transparent resin substrate is 3 nm or less, preferably 2 nm or less.

FIG. 1 shows a cross-sectional view of the organic light emitting device of the present invention.
As shown in FIG. 1, the organic light-emitting device of the present invention comprises, in order from the bottom, a transparent resin substrate (101), a graphene film (anode) (102), a hole injection layer (103), an organic light-emitting material (104), and a cathode. (105). By applying a voltage between the anode (102) and the cathode (105), holes are injected from the anode and electrons are injected into the organic EL material from the cathode to emit light.

Hereinafter, preparation of a graphene film in the present invention will be described.
The method for producing a graphene film in the present invention is not particularly limited. However, as a method capable of forming one to several layers of graphene films by controlling the number of graphene films, the following gas containing hydrogen gas as a main component is used. The method by the used plasma CVD method is preferably used (see Patent Document 1).
That is, while heating the substrate, the carbon component in the substrate is activated by the energy of charged particles and electrons generated by the plasma, and graphene is generated using the carbon source contained in the substrate. As the carbon source, a carbon component contained in a metal base material made of any of carbon, hardly soluble metal, copper, iridium or platinum, or a carbon alloy with any of these metals, And / or a trace amount of carbon component contained in a gas used for plasma treatment. According to this method, graphene can be formed in a shorter time as compared with the conventional thermal CVD method or resin carbonization method. The metal content of the metal substrate is desirably 4 to 10,000 ppm or less, and the surface roughness Ra of the substrate is desirably 200 to 0.095 nm. Further, as the substrate heating condition, it is desirable that the substrate temperature be 850 ° C. or less.

As an example, FIG. 2 shows a Raman spectrum of a graphene film obtained in an example described later. The laser wavelength used was 638 nm.
In Figure 2, a small D intensity of the bands (~1350cm ^-1) _{(I D)} is compared to the intensity _{(I G)} of G-band _{^{(~1585cm -1) (I D <}} I G), also strong 2D peak Is characterized by the appearance. D intensity ratio of the intensity of the band intensity _(I D) and G band _(I G) is rather smaller than 1, the intensity of the 2D peak _{(I 2D)} and the intensity of G-band _{(I G)} intensity ratio, of about 1 is there.

It is thought that the origin of the D band reflects the number of lattice defects, the domain size, and the edge of graphene. In particular, I _D / I _G is used to estimate the domain size of graphene. The calculation formula for estimating the domain size is given by La (nm) = (2.4 × 10 ⁻¹⁰ ) λ ⁴ ( _ID / _IG ) ⁻¹ , where λ is the Raman laser wavelength (APL 88, 163106 (2006)). When I _D / I _G = 1, the domain size is about 40 nm. From these estimates, the produced graphene film can be rephrased as a film in which flakes having an average domain size of 40 nm or more are collected.

  In the present invention, the reason why the organic EL element can have high luminance by using the PEN substrate is that the flatness of the PEN substrate is very high. If the flatness of the transparent substrate is not very good, the thickness of the PEDOT: PSS or the EL layer becomes uneven, and there is a possibility that a leak current is likely to be generated from a thin layer. In addition, as a factor for increasing the luminance of the device, the heat shrinkage of PEN is small. It is presumed that the rate at which the substrate shrinks due to the heat applied during the device process is small, and it is possible to reduce graphene cracks (cracks) due to the shrinkage of the substrate.

Hereinafter, the present invention will be described based on examples, but the present invention is not limited to these examples.
(Manufacture of graphene film)
Specifically, it is as follows. First, a copper foil having a thickness of about 18 μm was prepared as a catalyst metal and set in a surface wave plasma CVD chamber. Plasma was irradiated for 120 seconds while the temperature of the copper foil was raised by energizing heating. The gas was hydrogen (500 sccm) and methane (5 sccm), and the synthesis was performed at a synthesis temperature of about 900 ° C.
FIG. 2 shows a Raman spectrum of the obtained high-quality graphene film.

(Manufacture of organic light-emitting device using PEN substrate)
In this example, two types of 40 mm × 40 mm PEN films (Teonex (registered trademark) manufactured by DuPont Film Co., Ltd.) were used as transparent resin substrates. One is a PEN film with a hard coat (PQDA5), and the other is a PEN film without a hard coat (Q65F).
The graphene film was peeled and transferred on each substrate as follows.
FIG. 3 is a diagram schematically showing a method of peeling a graphene film from a copper foil and transferring the graphene film to a transparent substrate.
First, the graphene film (102) formed on the copper foil (106) was bonded to a heat release sheet (Riba Alpha) (107) manufactured by Nitto Denko Corporation. Thereafter, the copper foil is etched using ammonium persulfate (0.5 mol / l), and the substrate is washed with running water.

  After attaching the heat-peelable sheet / graphene film obtained by etching the copper foil to the transparent substrate (101), the heat-peelable sheet was peeled off by heating to form a graphene film on the transparent substrate. Finally, the heat release sheet was removed by heating with a hot plate, and a transparent conductive film in which a graphene film was stacked on a transparent substrate was produced.

(Measurement of sheet resistance of transparent conductive film)
Four-terminal measurement was performed on the sheet resistance of the obtained PEN with graphene. The average value of the sheet resistance was about 810Ω and 830Ω for the PEN substrate with and without the hard coat, respectively.

  In order to improve the wettability of the hole injection layer (PEDOT: PSS), the graphene film was subjected to UV ozone treatment. An apparatus manufactured by Filgen was used. The irradiation time was 20 minutes. In another experiment, changes in sheet resistance before and after irradiation with UV ozone for 20 minutes were evaluated, and it was confirmed that there was almost no change in sheet resistance.

  After performing the above-described processing, materials related to the organic EL element were laminated. First, a hole injection layer was applied by spin coating and hardened by pre-baking. In this example, PEDOT: PSS was used for the hole injection layer. Next, an organic EL material was applied, spin-coated, and prebaked. The organic EL material used was SUPER YELLOW manufactured by MERCK, and toluene was used as the solvent. For the cathode as the last layer, a LiF / Al electrode was deposited by a vacuum deposition apparatus.

  A sealing operation was performed so that the organic EL material did not deteriorate as much as possible. Specifically, first, a desiccant (DryPaste) manufactured by SAES Getters is placed on the cathode. Thereafter, the device was covered with Teflon (registered trademark) tape so that moisture did not enter from around.

(Current-voltage measurement and luminance measurement)
A source meter (2400) manufactured by Keithley KK was used for the current-voltage measurement, and a luminance meter (BM9) manufactured by TOPCON was used for the luminance measurement.

FIG. 4A shows the luminance-voltage measurement results. Here, the measurement results of four devices are shown. When a voltage of 12 V was applied, the luminance showed 2240 to 6400 cd / m ² . The average value was 4340 cd / m ² . In addition, among the two types of PEN substrates, it was found that the brightness of the PEN substrate without the hard coat was higher than that of the PEN substrate.

FIG. 5A is a LOG plot of the vertical axis of FIG. 4A to check the threshold voltage from the luminance-voltage measurement. Note that the threshold voltage is defined as a voltage exceeding 1 cd / m ² . The threshold value of the device using the PEN substrate was from 2.4 V to 2.7 V.

  FIG. 6A shows the current-voltage measurement results measured simultaneously with the luminance measurement. It can be confirmed that a current of 0.1 mA or more starts flowing at approximately 2 V to 3 V. When 12 V is applied, a current of 10 mA to 17 mA flows.

  FIG. 7 shows a typical example of a device element in which light emission is photographed by a CCD camera. It was confirmed that light was emitted uniformly on the surface.

(Comparative example)
An organic EL device using a PET substrate was manufactured as a comparative object for confirming the usefulness of the organic EL device using a PEN substrate.
The manufacturing procedure and evaluation of the organic EL element are almost the same as those in Example 1. The difference is that the material of the transparent substrate used as the organic EL element is different. Note that the sheet resistance is about 500Ω, which is lower than that of the PEN substrate.

FIG. 4B shows the luminance-voltage measurement results. Here, the measurement results of six devices are shown. When a voltage of 12 V was applied, the luminance showed 322 to 1620 cd / m ² . The average value was 740 cd / m ² .

  FIG. 5B is a LOG plot of the vertical axis of FIG. 4B for checking the threshold voltage from the luminance-voltage measurement. The threshold value of the device using the PET substrate was 2.7V to 4.2V. From these results, when PEN and PET were compared, it was found that the threshold voltage of the PEN substrate tended to be lower.

  FIG. 6B shows the current-voltage measurement results measured simultaneously with the luminance measurement. It can be confirmed that there are many devices in which a current of 0.1 mA or more has already started flowing when 1 V is applied. When 12 V is applied, a current of 28 mA to 38 mA flows. From these results, it was found that the PEN substrate had a smaller leak current and a smaller current flowing as a whole than the PET substrate.

FIG. 7B shows a typical example of a device element in which light emission is photographed by a CCD camera. There was a non-light emitting region (black particles).
From these results and the results of the above examples, it was shown that the brightness of the organic EL element could be increased several times by using the PEN substrate.

(Measurement of flatness)
The flatness of the substrate was compared between the two types of PEN substrate and PET substrate. Note that a quartz substrate was prepared as a reference. The flatness was measured using an atomic force microscope (AFM) and represented by an arithmetic average value (Ra) obtained in a scan area of 5 μm square. FIG. 7 shows the measured results.
The indexes (Ra) representing the flatness of the PEN substrate with hard coat, the PEN substrate without hard coat, the PET substrate, and the quartz substrate were 0.76, 1.46, 5.67, and 0.23 nm, respectively. .
These results revealed that the PEN substrate had higher flatness than the PET substrate.

  According to the present invention, by using a highly flat PEN substrate as the transparent resin substrate, it is possible to suppress a leak current and realize high luminance.

101: transparent resin substrate 102: graphene film (anode)
103: hole injection layer 104: organic light emitting material 105: cathode 106: copper foil 107: heat release sheet

Claims (1)

In the method for manufacturing an organic EL device, two graphene films, a hole injection layer, an organic EL layer, and a cathode are stacked on a transparent resin substrate made of polyethylene naphthalate and having a surface roughness (Ra) of 2 nm or less. An organic EL device comprising: forming a two-layer graphene film on a metal having catalytic activity by a plasma CVD method using a gas containing hydrogen gas as a main component ; and transferring the formed graphene film onto the transparent resin substrate. Device manufacturing method.

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