JP2010218984A - Organic el element and its manufacturing method - Google Patents

Abstract

An organic EL element having high luminous efficiency and long performance and sufficient performance in practical use is provided.
An organic electroluminescent device comprising a pair of electrodes and a surface graft polymer layer including a surface graft polymer bonded to at least one of these electrodes.
[Selection] Figure 1

Description

  The present invention relates to an organic electroluminescence element (hereinafter, electroluminescence is abbreviated as “EL”) and a method for producing the same, and more particularly to an organic EL element that can emit light with high efficiency and a long lifetime. .

  EL elements using electroluminescence are highly visible due to self-emission and are completely solid elements, so they have features such as excellent impact resistance, so they are attracting attention as light emitting elements in various display devices. Has been.

  This EL element includes an inorganic EL element using an inorganic compound as a light emitting material and an organic EL element using an organic compound. Among these, the organic EL element can significantly reduce the applied voltage. Since it is easy to increase in size, consumes little power, can emit surface light, and easily emits light in three primary colors, research on its practical application as a next-generation light-emitting element has been actively conducted.

  The structure of the organic EL element is basically the structure of anode / organic light emitting layer / cathode, which is appropriately provided with a hole transport layer or electron transport layer, for example, anode / hole transport layer / organic light emitting layer / electron. The thing of the structure of a transport layer / cathode etc. is known (patent document 1).

  In many cases, organic EL elements have a multilayer structure, and while such structures contribute to the improvement of element characteristics, there are reports that peeling at the heterointerface and diffusion of molecules and atoms cause element deterioration. .

JP2009-043612

  An object of this invention is to provide the organic EL element which suppressed peeling of the interface and diffusion of an organic molecule which cause deterioration, and its manufacturing method.

  As a result of intensive studies to achieve the above object, the present inventors have grown a polymer on a transparent electrode by surface graft polymerization using a living radical method, and have developed all charge transporting and light emitting functionalities. It has been found that a new polymer organic EL device in which the group is covalently bonded to the transparent electrode can be obtained.

The present invention provides the following organic EL device and manufacturing method thereof.
Item 1. An organic electroluminescence device comprising a pair of electrodes and a surface graft polymer layer including a surface graft polymer bonded to at least one of these electrodes.
Item 2. Item 2. The organic electroluminescence device according to Item 1, wherein one or both of the pair of electrodes is a transparent electrode.
Item 3. Item 2. The organic electroluminescence device according to Item 1, wherein the surface graft polymer is bonded to the anode.
Item 4. Item 4. The organic electroluminescence device according to any one of Items 1 to 3, wherein the surface graft polymer has a hole transport block, a light emission block, and an electron transport block in order from the anode.
Item 5. Item 5. The organic electroluminescence device according to Item 4, wherein the hole transport block and the light emission block and / or the light emission block and the electron transport block have a gradient composition.
Item 6. The surface graft polymer is obtained by growing the polymer from the anode in the order of the hole transport block, the light emission block, and the electron transport block by living radical polymerization, or from the cathode substrate in the order of the electron transport block, the light emission block, and the hole transport block. A method for producing an organic electroluminescence device having an anode / surface graft polymer / cathode structure, characterized in that:

The organic EL device according to the present invention can have the following characteristics.
[1] Since the polymerization initiator is always on the surface, changing the monomer charged in the middle of polymerization to a hole transport material, a light emitting material, and an electron transport material, even in polymer systems, “stacked” organic EL devices and “compositions” "Tilt-type" organic EL elements can be realized.
[2] Charge-transporting, luminescent functional groups, including the transparent electrode-organic layer interface and the organic layer-organic layer interface, all of which are connected by covalent bonds, causing the deterioration of the device. It is possible to solve the problems of delamination between layers, material diffusion in each organic layer and between layers, and associated interface disturbance.
[3] Since the stereoregularity is atactic, it does not crystallize and can have a stable amorphous structure.
[4] Since the polymer chain can be grafted on the substrate at a high density, the charge transporting and luminescent functional groups can be highly oriented while in an amorphous state.
[5] In addition, the distance between the charge transporting functional groups can be shortened accordingly, and the charge transport property can be improved.
[6] A pinhole-free film can be formed.

  These characteristics make it possible to realize high-molecular organic EL devices that have excellent light emission and charge transport capabilities and are resistant to deterioration.

  The “polymer organic EL device by surface graft polymerization” in the present invention does not simply provide a laminated type or a composition gradient type. Regarding the lifetime, various factors such as interfacial debonding, material diffusion, and interface turbulence based on it, changes in intra- and intermolecular structures such as crystallization, and the occurrence of dark spots can be considered. EL elements can drastically improve the lifetime of the element from the viewpoints such as interface peeling, diffusion, and pinholes. In addition, since all organic materials are connected by covalent bonds, they have a stable structure, a high glass transition temperature, high orientation, and excellent charge transport properties due to short inter-functional group distances. In this way, a single concept of “polymer organic EL device by surface graft polymerization” brings about various advantages and can drastically improve various problems that are currently problematic for organic EL. Is.

1 shows a surface graft polymer EL device of the present invention. All organic materials are covalently bonded to the transparent electrode. The characteristics of the novel surface graft organic EL device of the present invention will be described. Energy level of the material used for the graft polymer (a) Current density vs voltage, (b) Luminance vs voltage, (c) Current efficiency vs current density for device F-I. Voltage for current density vs. device X-Z

  As shown in FIGS. 1 and 2, the organic EL device of the present invention is a device having a structure having a pair of electrodes and a light emitting layer (surface graft polymer layer) sandwiched between these electrodes. The surface graft polymer constituting the light emitting layer can be stretched by living radical polymerization. The surface graft polymer may include a light-emitting block and may be a polymer composed of only the light-emitting block. In addition to the light-emitting block, the surface transport polymer further includes a hole transport block (anode side of the light-emitting block) and an electron transport block (cathode side of the light-emitting block). ) Are preferred. Most preferably, it has a structure of (anode)-(hole transport block)-(light emission block)-(electron transport block)-(cathode). In some cases, a hole injection block, an electron injection block, a hole blocking block, and an electron blocking block may be included.

  In the surface graft polymer of the present invention, in the hole transport block / light-emitting block, light-emitting block / electron transport block, hole transport block / light-emitting block / electron transport block, the composition of adjacent blocks may be clearly separated, Near the boundary between adjacent blocks, the repeating units constituting both blocks may be mixed and have a gradient composition. In such a gradient composition, the monomer constituting the adjacent block may be supplied to the substrate with the gradient composition. Monomer supply with such a gradient composition is performed by, for example, a method of living radical polymerization by sequentially immersing an electrode substrate in a plurality of monomer solutions having a gradient composition, for example, Adv. Polym. Sci. 197, 1-45 (2006). It can be realized by the method described.

  The organic EL device of the present invention includes an EL light emitting element and a transistor for driving the EL light emitting element. The organic EL device of the present invention may be either a bottom emission system or a top emission system. The bottom emission method is a method for extracting light from the light emitting layer through the substrate. The top emission method is a method in which light from the light emitting layer is extracted through a sealing film on the side opposite to the substrate.

  The thickness of each of the hole transport block, the light emission block, and the electron transport block is not particularly limited, but may be, for example, about 1 nm to 400 nm, preferably about 2 nm to 200 nm. The thickness of the entire surface graft polymer layer is about 5 to 1000 nm, more preferably about 10 to 800 nm, and still more preferably about 20 to 400 nm.

  The material of the electrode of the organic EL light emitting element differs depending on whether the organic EL device is a bottom emission method or a top emission method.

Both of the pair of electrodes used in the present invention may be transparent electrodes, one may be a transparent electrode, and the other may be a reflective electrode. Examples of the material of the transparent electrode include ITO and IZO. The reflective electrode is preferably formed from a plurality of partial electrodes, using a highly reflective metal, amorphous alloy, or microcrystalline alloy. High reflectivity metals include Al, Ag, Mo, W, Ni, Cr, and the like. High reflectivity amorphous alloys include NiP, NiB, CrP, CrB, and the like. The highly reflective microcrystalline alloy includes NiAl and the like. The reflective electrode may be used as an anode or a cathode. When the reflective electrode is used as an anode, a conductive metal oxide such as SnO ₂ , In ₂ O ₃ , ITO, IZO, ZnO: Al is laminated on the above-described high reflectivity material, and the positive electrode with respect to the organic EL layer is formed. The hole injection efficiency may be improved. When using a reflective electrode as a cathode, you may improve the efficiency of the electron injection with respect to an organic EL layer by using the structure layer of the organic EL layer which contacts a reflective electrode as an electron injection layer.

  The living radical polymerization of the present invention is performed using a polymerization initiator. As the polymerization initiator, those that generate radicals by heat are preferable, and examples thereof include TEMPO, ATRP, RAFT, and RTCP systems, and TEMPO, RAFT, and RTCP systems are more preferable. The RTCP system was developed by the present inventors [Goto et al., J. Am. Chem. Soc. 129, 13347 (2007)]. Among TEMPO systems, DEPN system is particularly preferable. The polymerization initiator has a radical generating group that provides a polymerization point for living radical polymerization and a group that can bind to the electrode.

  As radical generating groups for living radical polymerization, those having the following capping agents are known:

  As the capping agent, a capping agent similar to the above can be widely used.

  Specific polymerization initiators used in the living radical polymerization of the present invention are exemplified below:

The polymerization initiator I (a) was synthesized by Beyou et al. [Macromolecules 36, 7946 (2003)]. This polymerization initiator is composed of 10 methylene chain CH _2, and the polymerization initiator I As shown in (b), the polymerization initiator having four methylene chains shortened is expected to improve the hole injection efficiency. It is possible to further improve the hole injection efficiency by further shortening the methylene chain.

  The materials for the polymerization initiators II (a) and (b) are described in the literature (Yan, Marks et al., J. Am. Chem. Soc. 127, 3172 (2005); Ganzorig, Fujihira et al., Appl. Phys. Lett., 79, 272 (2001)] is a material obtained by modifying a living radical polymerization initiator DEPN. In the above-mentioned document, by modifying these materials on the ITO surface, the emission threshold voltage and the luminance of the low molecular weight organic EL can be improved. For this reason, the use of these initiators makes it possible to realize a surface graft device having a lower voltage than the usual and excellent in luminance.

  In the present invention, it has been clarified that the current density is increased by two orders of magnitude by performing ultrasonic cleaning after bonding a polymerization initiator to an electrode such as ITO. In the production method of the present invention, by performing ultrasonic polymerization and then performing graft polymerization to produce an element, more excellent element characteristics can be obtained.

As described above, the polymerization initiator of the present invention is a group (for example, SiCl ₃ , Si (CH ₃ ) Cl ₂ , Si (CH ₃ ) ₂ Cl, Si (OR) ₃ (wherein R Represents methyl, ethyl, propyl or butyl)) and radical generating groups (eg DEPN: (CH ₃ ) ₃ C—N (O ·) —CH (PO (OEt) ₂ ) —C ((CH ₃ ) ₃ ) Containing two types of groups such as TEMPO, ATRP, RAFT, RTCP, and the like} including polymerization initiators other than those described above. The reaction between the group capable of binding to the electrode of the polymerization initiator and the electrode is carried out using water or alcohol (methanol, ethanol, propanol, etc.) or an organic solvent as a solvent in the presence of an acid or base catalyst as necessary. The polymerization initiator can be bound to the electrode. For example, THF added with an aqueous ammonia solution can be used as a solvent.

  Four mechanisms have been proposed as main mechanisms for activating radical generating groups of the polymerization initiator. The first is the thermal dissociation or photodissociation mechanism of PX, the second is the exchange chain transfer mechanism in which P · pulls out X of PX, and the third is the transition metal, etc., that acts as activator A for PX The fourth is the mechanism of reversible chain transfer to non-transition metal compounds. The fourth is a mechanism in which the non-transition metal radical A. acts as an activator of PX.

  Nitroxyl systems such as DEPN include (a) thermal dissociation as the only important activation mechanism. Polymerization is induced by heat. In tellurium and bismuth systems, (a) thermal dissociation and (b) exchange chain transfer coexist, and (b) exchange chain transfer is the main mechanism. Polymerization is induced by heat or a radical initiator (azo compound, peroxide) that supplies P ·. Dithioester (RAFT), iodine, and antimony systems include (b) exchange chain transfer as the only important activation mechanism. The polymerization is induced with a radical initiator (azo compound, peroxide). The bromine / copper complex system (ATRP) contains (c) atom transfer as the only important activation mechanism. Polymerization is induced by the addition of a catalyst. In iodine / germanium, phosphorus, nitrogen or oxygen compound systems (RTCP), (b) exchange chain transfer and (d) reversible chain transfer coexist, and (d) reversible chain transfer is the main mechanism. Polymerization is induced by the addition of a radical initiator (azo compound, peroxide) and a catalyst.

  The amount of the polymerization initiator is preferably such that it is about 0.00001 to 0.1 equivalent to one monomer molecule. Thereby, while being able to fully act a polymerization initiator, use of an excessive polymerization initiator can be controlled. The polymerization temperature is usually about 60 to 180 ° C, preferably about 70 to 150 ° C, more preferably about 80 to 140 ° C, although it depends on the type of solvent. The solvent is not particularly limited as long as it can dissolve the monomer. Aromatic hydrocarbons such as toluene, xylene and anisole, chlorinated hydrocarbons such as chloroform, carbon tetrachloride and 1,2-dichloroethane, DMF , DMSO, N-methylpyrrolidone and the like, which can be appropriately selected depending on the reaction temperature, the solubility of the monomer, and the like.

  A surface graft polymer of the present invention can be obtained by sequentially reacting a monomer for a hole transport block, a monomer for a light-emitting block, a monomer for an electron transport block, and the like with the polymerization initiator bonded to the electrode.

Examples of the monomer that can be used in the hole transport block include a group represented by (hole transport material) -CH = CH ₂ , where (hole transport material) is a hole transport material in an organic EL element. Anything that can be used as is included. Similarly, the monomer that can be used for the light-emitting block and the monomer that can be used for the electron-transport block are (light-emitting material) -mon represented by CH = CH ₂ and (electron-transport material) -CH = CH ₂ Here, the (light emitting material) and the (electron transport material) include all of those that can be used as the light emitting material and the electron transport material in the organic EL device, respectively.

Introduction of a vinyl group (—CH═CH ₂ ) into a hole transport material, a light emitting material, or an electron transport material can be carried out by those skilled in the art according to a conventional method. For example, according to the description of Schemes 1 to 3 in the Examples. It can be carried out.

  For the (hole transport material), (electron transport material), and (light emitting material) used in the present invention, all materials used in low molecular organic EL can be used. An example is shown below. A material obtained by bonding a vinyl group to the following materials is used as a monomer. The position at which the vinyl group is bonded is arbitrary.

(X represents an N, B, or P atom. R ₁ and R ₂ are monovalent or divalent organic groups (eg, alkyl, alkenyl, alkynyl, aryl, aralkyl, R ₁ and R ₂ together) Represents a group having a fluorene skeleton.)
Examples of the alkyl group include linear or branched alkyl groups having 1 to 6 carbon atoms such as methyl, ethyl, and isopropyl. Examples of the alkenyl group include alkenyl groups having 2 to 6 carbon atoms such as vinyl and allyl. Examples of the alkynyl group include alkynyl groups having 2 to 6 carbon atoms such as ethynyl group and propynyl group. Examples of the aryl group include phenyl group and naphthyl group. Examples of the aralkyl group include benzyl group and phenethyl group. Is mentioned. When R1 and R2 together represent a group having a fluorene skeleton, the compound (B) is a compound having a symmetric structure having two fluorene skeletons.

For other light emitting materials, hole transport materials, and electron transport materials, for example,
Organic EL device physics / material chemistry / device application, CM Publishing (2007).
Development of organic electronics Latest development status and issues for practical application, Information Organization (2007).
Organic EL display, Ohmsha (2004).
Story of organic EL, Nikkan Kogyo Shimbun (2003).
Organic EL materials and displays, CM Publishing (2001).
Chemical industry, Chemical industry, 59, (2008).
Are described in their entirety and are hereby incorporated by reference in their entirety.

  Preferable examples of the monomer that can be used for the hole transport block, the monomer that can be used for the light emission block, and the monomer that can be used for the electron transport block are shown below.

  In the hole transport material described above, TPA has a smaller rearrangement energy than TPD and can be expected to have good hole transport properties. In general, TPA is not used in organic EL devices because it is easily crystallized and a uniform vapor deposition film cannot be formed. However, in this study, it is introduced into the side chain of an atactic polymer, so it does not crystallize even in TPA. A simple film can be formed.

  In addition to the above monomers, the following monomers with improved solubility in solvents can also be used:

  As described above, the solubility in a solvent can be improved by reducing the number of benzene rings or introducing a substituent into the benzene ring. By increasing the solubility of the monomer and polymer, a higher degree of polymerization or molecular weight of the polymer can be achieved. Thus, the surface graft polymerization layer can be thickened by increasing the polymerization degree or molecular weight of the polymer.

  Various monomers such as a hole transport block monomer, a light emission block monomer, and an electron transport block monomer used in the present invention may be used alone or in combination of two or more. .

  When living radical polymerization is performed using the above monomers, a surface graft polymer in which a hole transport block, a light emission block, and an electron transport block are bonded can be obtained.

  In the present invention, the preferred hole transport material to electron transport material molar ratio of the light-emitting block is hole transport material: electron transport material = 10: 1 to 1:10, preferably 8: 1 to 1: 8, more preferably. 6: 1 to 1: 6, more preferably 4: 1 to 1: 4. In addition, it is possible to use a bipolar material. The light emitting material is preferably 0.5 to 50 mol%, preferably about 1 to 40 mol%, more preferably about 2 to 30 mol%, and still more preferably 3 to the total amount of the hole transport material and the electron transport material. It is about ~ 20 mol%.

  The surface graft polymer of the present invention has a structure in which a polymer of a vinyl group is used as a skeleton, and a charge transporting functional group and a light emitting functional group are grafted thereon. Since the main chain is linked by a methylene chain, it is arranged with respect to the substrate while maintaining an amorphous state, and therefore, the functional groups of the side chain are expected to be oriented parallel to the substrate on average (Figure 1). . In addition to the effects of covalent bonds, the amorphous structure has a high degree of orientation, so the distance between functional groups is lower than that of low molecular weight systems or polymer films prepared by ordinary spin coating methods. Short and functional groups are expected to have a more planar structure. Although not wishing to be bound by theory, the present inventors believe that the polymer of the present invention has excellent properties as an organic EL device due to these structural features.

  The surface graft polymer of the present invention has a molecular weight of 10,000 or more, preferably about 10,000 to 10,000,000, more preferably about 20,000 to 5,000,000. Further, the polydispersity (Mw / Mn) of the surface graft polymer is 1.8 or less, preferably 1.6 or less, more preferably 1.4 or less, and particularly preferably 1.2 or less.

  In the present invention, not only a device having a clear interface between a hole transport material, a light emitting material, and an electron transport material, but also a device having an inclined composition can be created by gradually changing the composition of the monomer to be charged. By making this composition gradient element, the HOMO and LUMO of the element can be changed smoothly to improve the characteristics.

  For the polymer of the present invention, a bipolar material that can transport both holes and electrons can be used. By using such a material, the properties of the polymer can be further improved.

    Various iridium complexes can be used for the light emitting material of the present invention. In particular, by introducing various functional groups into an iridium complex, a light emitting material monomer having red, blue, and green colors can be obtained. By arranging such a polymer containing a light emitting material on an electrode substrate such as ITO, an organic EL element in which RGB is separately applied can be obtained.

  When the surface graft element according to the present invention is developed on a display, RGB coating is required. In order to coat red-green-blue (RGB), not only the TEMPO (nitroxide) system using the conventional polymerization initiator DEPN but also two other polymerization systems are required. For example, nitroxyl (polymerization induced by heat), RAFT (polymerization induced by radical initiator), ATRP (polymerization induced by transition metal catalyst), RTCP (combination of radical initiator and nonmetal catalyst) And the like). Specifically, the application of RGB can be performed by patterning the above-described TEMPO-based, RAFT-based, and RTCP-based initiators on the surface of the substrate using an inkjet technique. More specifically, three kinds of polymerization initiators that can be activated under different conditions are separately applied by the printing technology of the ink jet tower, and each polymerization initiator corresponds to three colors of red, blue, and green as light emitting material monomers. By extending the monomers by living radical polymerization, polymers of three colors corresponding to the three painted patterns are generated. In addition to the above, a color display can be formed by applying a color filter to an organic EL element.

EXAMPLES Next, although an Example demonstrates this invention still in detail, this invention is not limited by these Examples.
Example 1
2. Method
2.1. Monomer synthesis

Molecule 1 (v-TPA), a hole transport material, was synthesized by Ullmann reaction of 4-bromostyrene and diphenylamine (Scheme 1). Molecule 2 [Ir (ppy) ₂ vppy], a phosphorescent material, is an Ir dimer [Ir (ppy) ₂ Cl] ₂ cross-linked with Cl from IrCl ₃ • 3H ₂ O and 2-phenylpyridine. Synthesized and then synthesized by orthometalation reaction of 2- (4-vinylphenyl) pyridine and [Ir (ppy) ₂ Cl] ₂ (Scheme 2) ^{4, 5} . Molecule 3 (v-PBD), which is an electron transporting material, is obtained by acylating 2- (4-biphenyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole by Friedel-Crafts reaction. Synthesized by reduction, dehydration (Scheme 3) ⁶ . The DEPN shown in FIG. 2 was synthesized by adding metachloromethane to 2,2-dimethyl-1- (1,1-dimethylethylamino) propyldiethylphosphonate and oxidizing it. The radical initiator 4 was synthesized by an atom transfer radical addition reaction in which copper and monovalent copper bromide were added to DEPN and pentamethyldiethylenetriamine. Initiating group 5 was synthesized by hydrosilylation reaction using triethoxysilane after esterification with 2-bromo-2-phenylethanol and 4-pentenoyl chloride, followed by addition of DEPN in the same manner as the synthesis of radical initiator 4. did.
2.2. Surface initiated living radical polymerization (surface initiated LRP)

The ITO substrate treated with UV-ozone was immersed in a THF solution containing the initiating group 5 and NH ₃ aq. For 24 hours to bind the initiating group 5 to the ITO substrate surface. The obtained substrate was rinsed with chloroform, and this substrate was placed in a Pyrex (registered trademark) tube together with molecules 1-3, radical initiator 4, and anisole. After repeating freezing and degassing three times, the reduced pressure state was changed. Sealed while keeping. The molar ratio of molecules 1-3 is shown in Table 1. The radical initiator 4 is 1/500 equivalent to these molecules, and the amount of anisole is 800 wt% with respect to 1-4. Surface-initiated LRP was performed by reacting at 100 ^° C. for 24 hours under shaking.
2.3. Fabrication of organic EL device The polymerized substrate was ultrasonically washed with chloroform for 30 minutes, and then heat-treated at 100 ^° C. under reduced pressure for 1 hour to remove the solvent, and then gradually cooled to room temperature. Under vacuum (2.0-3.0 × 10 ⁻⁴ Pa), BCP and Alq ₃ were deposited on the graft polymer layer at vapor deposition rates of 0.15 to 0.25 nm · s ⁻¹ and 0.25 to 0.35 nm · s ⁻¹ , respectively. Subsequently, after once exposed to the atmosphere, the substrate was transferred to a metal chamber, and Cs ₂ CO ₃ (electron injection layer) was formed at 0.01 nm · s ⁻¹ at 1.0-3.0 × 10 ⁻³ Pa, and then used as a cathode. Al was deposited at 0.02-0.2 nm · s ^-1 . The fabricated device structure is ITO / graft copolymer / BCP 25 nm / Alq ₃ 40 nm / Cs ₂ CO ₃ 1.5 nm / Al. The thus obtained organic EL device (light emitting surface: 1 × 2 mm) was evaluated for current density-voltage-luminance characteristics. Also, ITO / a-NPD (Figure 3) 100 nm / Al, ITO / initiating group 5 (after immersion, rinsed) / a-NPD 100 nm / Al, and ITO / initiating group 5 (after immersion, ultrasonic cleaning ) / a-NPD100 nm / Al 3 devices (light emitting surface: 2 × 2 mm) were fabricated and the current density-voltage characteristics were evaluated.

3. Results and Discussion Table 1 shows the results of GPC measurement of polymers obtained by surface-initiated LRP. For this measurement, free polymer dissolved in anisole and not directly bonded to the ITO substrate was used. The surface-initiated LRP studied this time gave a polymer with a weight average molecular weight of several tens of thousands with a molecular weight distribution index (PDI) of 1.30-1.69. For polymer J, which is a homopolymer of v-TPA, a slightly smaller PDI value was obtained, but for polymer AI, there was no clear correlation between monomer molar ratio, molecular weight, and PDI. .

Table 2 shows the characteristics of the devices fabricated using the substrate with surface initiated LRP. Each bold alphabet corresponds to a polymer in Table 1. Device characteristics B and G, and D and I are compared, and the molar ratio of the hole transport material v-TPA to the electron transport material v-PBD is either 1: 1 or 1: 4. It is apparent that the brightness is increased by increasing the dopant Ir (ppy) ₂ vppy from 1 mol% to 3 mol%. This is because the emission sites increased due to an increase in Ir (ppy) ₂ vppy, which is a phosphorescent material, and it is considered that concentration quenching does not occur in this concentration range.

  Among all the devices shown in Table 2, light emission starts when the molar ratio of v-TPA to v-PBD is 1: 1 or the ratio of v-TPA is larger, such as B, F, and G. The voltage is low. This is presumably because the hole injection property was improved by increasing the proportion of the hole transport material covalently bonded to ITO. Figure 3 shows the HOMO and LUMO levels of these materials. The difference between the work function of ITO and HOMO of TPA is 0.5 eV, whereas the difference between the work function of ITO and HOMO of PBD is 1.3 eV, which is the most efficient hole injection efficiency when TPA binds to ITO. I understand that is good.

FIG. 4 (a) shows the measurement results of current density-voltage characteristics for the element FI in which the molar ratio of Ir (ppy) ₂ vppy, which is a phosphorescent material, is 3 mol%. From this figure, it can be seen that the current density shows a similar behavior above the emission start voltage. Fig. 4 (b) shows the luminance-voltage characteristics. At a molar ratio of Ir (ppy) ₂ vppy of 3 mol%, the element G having a v-TPA: v-PBD monomer ratio of 1: 1 shows the highest luminance. These results suggest that the carrier balance in the graft layer is the main factor determining the device characteristics. When the monomer ratio of v-TPA: v-PBD is 1: 1, the carrier balance is the best, and it is considered that holes and electrons are efficiently recombined in the graft layer. Figure 4 (c) shows the current efficiency vs. current density characteristics. Reflecting the carrier balance, device G having a v-TPA: v-PBD monomer ratio of 1: 1 showed a maximum current efficiency of 3.2 cd / A. However, the current density of these elements is lower than that of conventional low molecular weight organic EL elements. Therefore, in order to examine the influence of the starting group 5 on current injection, the current density-voltage characteristics were compared with the element XZ shown below. The results are shown in FIG. In Figure 5:
Element X: ITO / a-NPD 100 nm / Al
Element Y: ITO / initiating group 5 / a-NPD 100 nm / Al. In this element, after the initiating group 5 was bonded, rinsing was performed.
Element Z: ITO / initiating group 5 / a-NPD 100 nm / Al. In this element, after the initiating group 5 was bonded, ultrasonic cleaning was performed.

It can be seen that the element Y has a considerably lower current density than the element X, and hole injection is greatly suppressed. However, the element Z that was subjected to ultrasonic cleaning had a current density comparable to that of element Y under low voltage, but it rose significantly at 6-8 V, and it was comparable to element X at 9 V and above. showed that. In order to further examine the device Z, a voltage was applied again to the device to which a voltage had been applied once. As a result, it was found that the value was comparable to that of the device X from a low voltage of 2 V. These results suggest that the initiating group 5 may not essentially affect the hole injection property. In the future, we will attempt ultrasonic cleaning of the substrate to which the starting group 5 is bound, and improve the conditions of drying and heat treatment performed on the substrate after the surface initiation LRP, so that the presence of solvent and impurities will affect the device characteristics. Consider the possibility of giving. In addition, Ir (ppy) ₂ vppy was examined at a concentration of 3 mol% or less, but in the case of the device using the surface-initiated LRP advanced in this study, the problem of concentration quenching can be greatly improved. There is sex. In the future, we intend to further improve the device characteristics by increasing the concentration of Ir (ppy) ₂ vppy.

Claims (6)

An organic electroluminescence device comprising a pair of electrodes and a surface graft polymer layer including a surface graft polymer bonded to at least one of these electrodes. The organic electroluminescent element according to claim 1, wherein one or both of the pair of electrodes is a transparent electrode. The organic electroluminescent device according to claim 1, wherein the surface graft polymer is bonded to the anode. The organic electroluminescent element in any one of Claims 1-3 in which a surface graft polymer has a positive hole transport block, a light emission block, and an electron transport block in an order from an anode. The organic electroluminescence device according to claim 4, wherein the organic EL device has a gradient composition between the hole transport block and the light-emitting block and / or between the light-emitting block and the electron transport block. The surface graft polymer is obtained by growing the polymer from the anode in the order of the hole transport block, the light emission block, and the electron transport block by living radical polymerization, or from the cathode substrate in the order of the electron transport block, the light emission block, and the hole transport block. A method for producing an organic electroluminescence device having an anode / surface graft polymer / cathode structure, characterized in that:

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