JP2009009894A - Electroluminescent element, its manufacturing method, and display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electroluminescent element of good light emitting efficiency. <P>SOLUTION: An electroluminescent element 1 includes: a positive electrode (a lower electrode on a substrate 11) and a negative electrode (an upper electrode 31); at least a charge-transporting layer 12 and a light-emitting layer 21 sandwiched between the positive electrode and the negative electrode, wherein the charge transporting layer 12 includes at least one of π conjugated compounds, the light-emitting layer 21 includes a semiconductor nano-particle light-emitting material, the semiconductor nano-particle light-emitting material is chemically bonded with the positive electrode or the negative electrod via the π conjugated compounds. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electroluminescent device, a manufacturing method thereof, and a display.

  An organic light emitting diode (OLED) is a light emitting element having a structure in which a fluorescent or phosphorescent organic compound, a charge transporting organic compound, or the like is formed in a thin film and sandwiched between electrodes. When a voltage is applied between the electrodes, electrons and holes are injected into the thin film made of the organic compound and recombined to generate excitons of the light-emitting organic compound, which return to the ground state. In turn, the OLED emits light.

  OLEDs have characteristics such as low voltage driving, high luminous efficiency, high-speed response, self-emission, no viewing angle limitation, various emission wavelengths, and light weight. For this reason, OLED is expected as a next-generation light-emitting device in a wide range of fields from thin displays to illumination.

  On the other hand, semiconductor nanoparticles having a particle size of several nanometers to 20 nm are expected to be used as constituent materials for electroluminescent devices, and various studies have been reported. For example, CdSe nanoparticles have characteristics such as higher durability, narrow spectral width, and control of luminescent color depending on particle size compared to organic luminescent materials. Furthermore, the CdSe nanoparticle has a core-shell structure such as CdSe / ZnS, thereby obtaining effects such as inactivation of surface defects and increase in internal quantum confinement. As a result, a high internal quantum efficiency exceeding 50% is realized. By using such nanoparticles, physical and chemical properties that cannot be seen in the bulk are expressed, and realization of devices having characteristics higher than those of conventional devices is expected.

  Further, since the outermost surface of the nanoparticles is usually covered with a ligand made of an organic compound, it can be dispersed in an organic solvent without agglomeration. Therefore, coating film formation is possible in the manufacturing process of the electroluminescent element, and it can be expected to correspond to a low-cost and large-area light-emitting device.

  Until now, several electroluminescent devices using nanoparticles as a light emitting material have been reported. For example, in Non-Patent Document 1, an electroluminescent element having a very narrow spectral width of about 30 nm is realized by a simple process using a coating method using CdSe / ZnS nanoparticles. However, all of the electroluminescent devices using nanoparticles as a light emitting material reported so far have a problem that the luminous efficiency is one digit or more lower than that of the current OLED.

  Here, in order to improve the light emission efficiency of the nanoparticle electroluminescent device, it is considered necessary to improve the structure control of the nanoparticle layer, which is the light emitting layer, and the efficiency of charge injection into the semiconductor nanoparticles.

  By the way, since the nanoparticles themselves are difficult to conduct electricity, the driving voltage of the electroluminescent device can be expected to decrease by making the light emitting layer as thin as a single particle layer. However, if the nanoparticles are not present in the light emitting surface and there are many portions where the hole transport layer and the electron transport layer directly form an interface, the light emission efficiency derived from the nanoparticles decreases. For this reason, it is necessary to control the structure of the nanoparticle light-emitting layer so that the nanoparticle layer as the light-emitting layer is as thin as possible and the film density of the nanoparticle layer on the light-emitting surface is as high as possible. When forming the light emitting layer which consists of nanoparticles by the apply | coating method, the method currently disclosed by the nonpatent literature 1 is mentioned, for example. Specifically, it is a method of applying a mixed solution of nanoparticles and a hole transport material. As a result, the nanoparticles are phase-separated on the surface of the hole transport layer to form a nanoparticle layer. In addition, as disclosed in Patent Document 1, there is an example in which a nanoparticle single particle thin film layer is formed by arranging inorganic compound nanoparticles on a substrate using a self-assembled film.

US Pat. No. 5,751,018 Nature, 2002, Vol. 420, p. 800

  However, in the method of forming a nanoparticle layer using phase separation disclosed in Non-Patent Document 1, the structure of the nanoparticle layer is greatly affected by the concentration of nanoparticles in the coating solution and the handling of the film after coating. Therefore, it is difficult to produce an electroluminescent element that is reproducible and has high luminous efficiency. In addition, in the method of arranging nanoparticles on a substrate using the self-assembled film disclosed in Patent Document 1, when an electric charge is injected from the electrode substrate to the nanoparticles, the alkane site in the self-assembled film is used. Becomes a charge injection barrier, and it is difficult to improve the light emission efficiency of the electroluminescent device.

  Moreover, in order to improve the light emission efficiency of the nanoparticle electroluminescent device, it is necessary to satisfy both the structure control of the nanoparticle layer which is a light emitting layer and the improvement of the efficiency of charge injection into the nanoparticle.

  An object of the present invention is to provide an electroluminescent device having good luminous efficiency.

  The electroluminescent device of the present invention comprises an anode, a cathode, and at least a charge transport layer layer and a light emitting layer sandwiched between the anode and the cathode, wherein the charge transport layer comprises a compound having π conjugate. Including at least one kind, wherein the light-emitting layer is made of a semiconductor nanoparticle light-emitting material, and the semiconductor nanoparticle light-emitting material is chemically bonded to the anode or the cathode via the compound having the π conjugate. And

  ADVANTAGE OF THE INVENTION According to this invention, an electroluminescent element with sufficient luminous efficiency can be provided.

  The electroluminescent element of the present invention is characterized in that a light emitting layer is sandwiched between an anode, a cathode, a layer containing at least one compound having π conjugation, and the anode and the cathode.

  Hereinafter, the electroluminescent element of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a cross-sectional view showing an embodiment of the electroluminescent device of the present invention. In the electroluminescent device 1 of FIG. 1, a charge transport layer 12, a light emitting layer 21, an electron transport layer 31, and an upper electrode 32 are sequentially provided on an 11 substrate having a lower electrode (not shown). In the electroluminescent device 1 of FIG. 1, the lower electrode provided on the substrate 11 functions as an anode. The charge transport layer 12 functions as a hole injection (transport) layer. The light emitting layer 21 is made of a semiconductor nanoparticle light emitting material. The upper electrode 32 functions as a cathode.

  Next, constituent members of the respective layers constituting the electroluminescent element of FIG. 1 will be described.

The charge transport layer 12 sandwiched between the substrate 11 and the light emitting layer 21 has two functions of controlling the structure of the nanoparticle thin film forming the light emitting layer 21 and injecting charge into the nanoparticle. In the charge transport layer 12, a compound having π conjugation contained in at least one kind in the charge transport layer 12 is generally represented by a structural formula represented by the following formula (1).
X—R ₁ —C—R ₂ —Y (1)

  In the formula (1), X represents a site that forms a bond with the electrode surface. Specific examples include a thiol group, a carboxyl group, and an alkoxysilyl group.

  In Formula (1), Y represents the site | part which forms a coupling | bonding with a semiconductor nanoparticle. Specific examples include a thiol group, a carboxyl group, and an alkoxysilyl group.

In the formula (1), C represents a group having a divalent to tetravalent π conjugate. When C is trivalent or more, the plurality of R ₂ and the plurality of Y may be the same or different.

  The group having π conjugation represented by C refers to a substituent having a function of increasing the charge injection efficiency from the electrode to the semiconductor nanoparticles when the element is driven. Here, examples of the π conjugate include aromatic rings, arylamines, condensed polycycles, and heterocycles. Specific examples of the structure represented by the group C having π conjugation are shown below. In the following formula, R represents an arbitrary organic group. The present invention is not limited to the following structure.

  By forming a layer containing a compound having π conjugation between the semiconductor nanoparticle layer that is the light emitting layer and at least one of the electrodes, the efficiency of charge injection into the semiconductor nanoparticles is improved.

  In general, in an electroluminescent device using semiconductor nanoparticles as a light emitting material, the efficiency of hole injection into semiconductor nanoparticles is poor. For this reason, it is desirable that the group C having π conjugation has a hole transport function. By doing so, the efficiency of hole injection into the semiconductor nanoparticles is improved, and as a result, the luminous efficiency of the device is improved.

In Formula (1), R ₁ and R ₂ represent a hydrocarbon group such as a single bond or an alkyl chain. Examples of the hydrocarbon group represented by R ₁ and R ₂ include alkylene groups such as a methylene group, an ethylene group, and a propylene group.

  As described above, the compound having π conjugation constituting the charge transport layer 12 has a site X that binds to the electrode surface and a site Y that binds to the nanoparticles, as represented by the formula (1). Thereby, the compound having π conjugation has an orientation in the direction perpendicular to the charge transport layer 12.

  In the electroluminescent device of the present invention, a charge transport layer 12 is formed on an electrode formed on a substrate, and then a light emitting layer 21 composed of semiconductor nanoparticles is formed on the charge transport layer 12. For this purpose, the molecules constituting the layer containing a compound having π conjugation form a chemical bond with the semiconductor nanoparticle and the site (X) that forms a bond with the electrode surface, as shown in Formula (1). Has both sites (Y). The chemical bond here includes a covalent bond, a coordination bond, an electrostatic interaction, an ionic bond, a hydrogen bond, and the like. Among these, a covalent bond is desirable because of excellent structural stability. Thus, the charge transport layer 12 forms a chemical bond, particularly a covalent bond, with the electrode and the semiconductor nanoparticles. This makes it possible to form an element having excellent structural stability during and after formation.

  The semiconductor nanoparticle light-emitting material constituting the light-emitting layer 21 is a nanoparticle that emits light by relaxation from an excited state due to energy absorption or charge recombination in the nanoparticle. By the way, the semiconductor nanoparticle light-emitting material refers to fine particles having a particle diameter of 0.1 nm to 1 μm, preferably 1 nm to 20 nm. The materials constituting the semiconductor nanoparticle light-emitting material are specifically II-VI group semiconductors such as ZnS, ZnO, ZnSe, ZnTe, CdS, CdSe, and CdTe, and III-V group semiconductors such as InP, GaN, and GaAs. And IV group semiconductors such as Si and Ge. These semiconductor nanoparticle light emitting materials may have a core-shell structure composed of two or more kinds of compounds. Further, a small amount of another chemical species may be doped in the semiconductor nanoparticle light emitting material. The semiconductor nanoparticle light-emitting material used in the electroluminescent device of the present invention is preferably CdSe nanoparticles because the half-value width of the emission spectrum is narrow and the emission color can be controlled by the particle size. In particular, CdSe / ZnS core-shell nanoparticles having a structure in which the surface of CdSe is coated with ZnS are preferable because they exhibit excellent emission quantum yield. The surface of these semiconductor nanoparticles may be covered with an organic ligand such as trioctylphosphine oxide (TOPO).

  The substrate used in the electroluminescent element of the present invention is not particularly limited as long as an electrode is formed on the substrate surface. Specific examples of the electrode include a simple metal such as Al, Au, Ag, Pt, and Mg, an alloy in which a plurality of these metal simple substances are combined, a metal oxide such as ITO, ZnO, and CuO. In order to increase the charge injection efficiency from the electrode, a conductive metal oxide layer may be formed on the electrode surface.

  Next, the manufacturing method of the electroluminescent element of this invention is demonstrated.

The method for producing an electroluminescent element of the present invention includes the following steps (i) and (ii).
(I) Step of forming a charge transport layer on a substrate on which an electrode is formed (ii) Step of forming a light emitting layer composed of a semiconductor nanoparticle light emitting material on the charge transport layer

  Specific methods for performing the step (i) include a coating method, a dipping method, a chemical vapor deposition method, and the like. Specifically, it is known to form an alignment film made of alkanethiol on a gold substrate, or to form an alignment film made of alkylsilane on a silica substrate. In the electroluminescent element of the present invention, the type of the bonding site between the compound having π conjugate and the substrate may be determined according to the type of the substrate.

  Further, specific methods for performing the above step (ii) include a coating method and a dipping method.

Hereinafter, the manufacturing method of the electroluminescent element of the present invention will be described in detail with reference to the drawings. Specific examples include a glass substrate having an ITO electrode and a compound represented by the following formula (2). The compound represented by the following formula (2) is a compound in which X is an alkoxysilyl group, Y is a thiol group, and C is triphenylamine having a hole transport function in the formula (1). , R ₁ and R ₂ are propylene.

  First, step (i) is performed. Specifically, a solution in which the compound of formula (2) is dissolved in an organic solvent is prepared, and a substrate with an ITO electrode is immersed in this solution. Then, a silanol bond is formed between the OH group remaining in a small amount on the ITO substrate and the alkoxysilyl group of the compound of formula (2). Silanol bonds are also formed between the alkoxysilyl groups of the compound of formula (2). As a result, as shown in FIG. 2, a molecular layer (charge transport layer 12) having a hole transport function composed of the compound of the formula (2) is formed on the surface of the substrate 11 with ITO electrode. As shown in FIG. 2, if a silanol bond is formed between the surface of the substrate with ITO electrode 11 and the compound constituting the charge transport layer 12, the top surface of the charge transport layer 12 has a semiconductor nanostructure. The thiol groups for forming bonds with the particles are arranged.

  Depending on the molecular structure of the compound constituting the charge transport layer 12, steric hindrance between molecules and electrostatic repulsion caused by a group having a π conjugate represented by C in the formula (1) may be represented by the formula (2). ) Will occur between compounds. As a result, the charge transport layer 12 may not be a uniform film. In such a case, for example, the charge transporting layer 12 is formed by mixing a compound of the formula (2) with another compound having little steric hindrance and electrostatic repulsion such as hexyltrimethoxysilane and the like. . By doing so, the charge transport layer 12 is formed on the surface of the substrate with ITO electrode 11 in the form shown in FIG. When the form shown in FIG. 3 is adopted, intermolecular steric hindrance and electrostatic repulsion, which are considered to occur between the compounds of formula (2), are alleviated, and the charge transport layer 12 is made of a homogeneous film having a high film density. Become.

  Next, after cleaning the surface of the substrate on which the charge transport layer 12 is formed, a step of providing the light emitting layer 21 made of semiconductor nanoparticles on the charge transport layer 12 (step (ii)) is performed. This step is performed by, for example, contacting and reacting a solution containing CdSe / ZnS nanoparticles by coating / immersing or the like, and then removing excess nanoparticles by a cleaning process or the like. By performing this step, a light emitting layer 21 made of semiconductor nanoparticles covalently bonded to the charge transport layer 12 as shown in FIG. 4 is formed. At this time, since only the semiconductor nanoparticles having a covalent bond with the charge transport layer 12 remain on the upper surface of the charge transport layer 12, the film thickness of the light emitting layer 21 is almost similar to the single particle layer. Further, since the charge transport layer 12 has a high film density, the thiol group exposed on the upper surface of the charge transport layer 12 also has a higher density in the upper surface. Therefore, the light emitting layer 21 made of semiconductor nanoparticles formed on the charge transport layer 12 also has a high film density.

  By forming on the electrode by the step (i), it is possible to form a charge transport layer which is uniform in the surface and whose thickness is strictly controlled. In addition, a light emitting layer that is uniform in the plane and controlled to a thickness of about a single particle can be formed by the step (ii).

  For example, the electroluminescent element shown in FIG. 1 is formed by forming an electron transport layer, an electron injection layer, an electrode, and the like on the light emitting layer 21 formed as described above by vacuum deposition or the like. Here, for each layer formed on the light emitting layer 21, an organic compound generally used as a constituent material of the electroluminescent element can be used.

  For example, examples of the electron transport material include an aluminum quinoline complex and a derivative thereof, an oxadiazole derivative, a triazole derivative, a phenanthroline and a derivative thereof. Examples of the electron injection material include lithium fluoride, cesium carbonate, and calcium. Moreover, you may use an inorganic material for an electron carrying layer.

  By using the electroluminescent element obtained in this way, a display or display element with high luminous efficiency utilizing various characteristics of the semiconductor nanoparticle luminescent material can be realized.

Example 1
The electroluminescent element shown in FIG. 1 was produced.

  First, the surface of the substrate 11 having an ITO electrode on the surface of the glass substrate was sequentially washed with acetone and ethanol, and then irradiated with UV ozone for 20 minutes to clean the substrate 11 and remove deposits on the electrode surface. .

  On the other hand, the compound represented by the following formula (2) was hydrolyzed using methanol, water and an acid catalyst to form a sol.

  This sol solution was dissolved in methanol at a concentration of 1% by weight to prepare a reaction solution A. Next, the washed substrate was immersed in 10 ml of the reaction solution A, sealed, and left at room temperature for 24 hours. Thereafter, the substrate was taken out from the reaction solution A, washed successively with methanol and toluene, and then heat-treated at 110 ° C. for 30 minutes in a nitrogen atmosphere.

  As a result, a layer (charge transport layer 12) in which the compound represented by the above formula (2) was chemically bonded to the surface of the substrate with ITO electrode 11 as shown in FIG. 2 was formed.

  Next, 10 ml of a toluene dispersion of CdSe / ZnS (manufactured by Evident Technologies, average core particle size: about 4 nm, nanoparticle concentration: 1 mg / ml), which is a core-shell type semiconductor nanoparticle light emitting material, is prepared. It was set as the reaction solution B. Next, the substrate in which the charge transport layer 12 was formed on the surface of the substrate 11 was immersed in the reaction solution B and sealed, and left at room temperature for 24 hours. Thereafter, the substrate was taken out from the reaction solution B and washed with toluene.

  As a result, a laminate in which the light emitting layer 21 formed of a single particle film of a semiconductor nanoparticle light emitting material was formed on the charge transport layer 12 shown in FIG. 4 was obtained.

Thereafter, Alq ₃ as the electron transport layer 31 and Al as the upper electrode 32 were sequentially formed on the semiconductor nanoparticle layer by a vacuum deposition method, thereby obtaining the electroluminescent device shown in FIG. 1 was subjected to nitrogen sealing treatment after the upper electrode 32 was formed.

  When voltage was applied to the obtained electroluminescent device, red light emission derived from nanoparticles was shown. The voltage at the start of light emission was 3.5 V, the external quantum efficiency was 1.5%, and the light emission peak wavelength was 620 nm.

(Example 2)
In Example 1, as a constituent material of the charge transport layer 12, an electric field was obtained in the same manner as in Example 1 except that a compound represented by the following formula (3) was used instead of the compound represented by the formula (2). A light emitting device was obtained.

  When voltage was applied to the obtained electroluminescent device, red light emission derived from nanoparticles was shown. The voltage at the start of light emission was 4.0 V, the external quantum efficiency was 1.0%, and the light emission peak wavelength was 620 nm.

(Example 3)
In Example 1, as a constituent material of the charge transport layer 12, an electric field was obtained in the same manner as in Example 1 except that a compound represented by the following formula (4) was used instead of the compound represented by the formula (2). A light emitting device was obtained.

  When voltage was applied to the obtained electroluminescent device, red light emission derived from nanoparticles was shown. The voltage at the start of light emission was 3.5 V, the external quantum efficiency was 1.0%, and the emission peak wavelength was 620 nm.

(Comparative Example 1)
In Example 1, as a constituent material of the charge transport layer 12, the same as Example 1 except that mercaptopropyltrimethoxysilane represented by the following formula (5) was used instead of the compound represented by the formula (2). Thus, an electroluminescent device was obtained.

  When voltage was applied to the obtained electroluminescent device, red light emission derived from nanoparticles was shown. The voltage at the start of light emission was 8.0 V, the external quantum efficiency was 0.1%, and the light emission peak wavelength was 620 nm.

Example 4
In Example 1, the constituent material of the charge transport layer 12 is a mixture of the compound represented by the formula (2) and hexyltrimethoxysilane (weight mixing ratio = 2) instead of the compound represented by the formula (2). / 1) was used. Except this, an electroluminescent element was obtained in the same manner as in Example 1. In addition, as shown in FIG. 3, the layer containing the compound having π conjugation includes a unit derived from hexyltrimethoxysilane and a unit derived from the compound represented by the formula (2) on the ITO electrode surface. It is the structure which is chemically bonded alternately.

  When voltage was applied to the obtained electroluminescent device, red light emission derived from nanoparticles was shown. The voltage at the start of light emission was 3.5 V, the external quantum efficiency was 1.5%, and the light emission peak wavelength was 620 nm.

(Example 5)
In Example 1, CdSe / ZnS which is three kinds of core-shell type semiconductor nanoparticle light emitting materials having different emission peak wavelengths was used as a constituent material of the layer 21 made of the semiconductor nanoparticle light emitting material. These three types of nanoparticle light-emitting materials are commercially available products (manufactured by Evident Technologies), and the emission peak wavelengths of the particles themselves are 490 nm, 540 nm, and 620 nm in order from the smallest. Except for this, electroluminescent elements that output three types of light emission of blue light emission, green light emission, and red light emission were formed in a matrix by the same method as in Example 1. Next, the display for image display was produced by forming the drive circuit which controls the drive of each electroluminescent element.

It is sectional drawing which shows one Embodiment in the electroluminescent element of this invention. It is sectional drawing which shows a mode when the layer containing the compound which has (pi) conjugation is formed on the board | substrate. It is sectional drawing which shows the modification of FIG. It is sectional drawing which shows a mode when the layer containing the compound which has (pi) conjugation and the light emitting layer are formed on the board | substrate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electroluminescent element 11 Substrate 12 Charge transport layer 21 Light emitting layer 31 Electron transport layer 32 Back electrode

Claims (5)

An anode and a cathode;
At least a charge transport layer and a light emitting layer are sandwiched between the anode and the cathode,
The charge transport layer contains at least one compound having π conjugation,
The light emitting layer is made of a semiconductor nanoparticle light emitting material, and the semiconductor nanoparticle light emitting material is chemically bonded to the anode or the cathode through the compound having the π conjugate. element.   The electroluminescent device according to claim 1, wherein the π conjugate is an aromatic ring, an arylamine, a condensed polycyclic ring, or a heterocyclic ring.   The electroluminescent device according to claim 1, wherein the charge transport layer is a layer having a hole transport function. The manufacturing method of the electroluminescent element characterized by including the following process (i) and (ii).
(I) Step of forming a charge transport layer on a substrate on which an electrode is formed (ii) Step of forming a light emitting layer composed of a semiconductor nanoparticle light emitting material on the charge transport layer   A display device comprising the electroluminescent element according to claim 1.

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