JP2008004950A - Material for electroluminescent device, and electroluminescent device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a material for an electroluminescent device with which a buffer layer is formed without using water as its solvent, unlike a conventional polymeric material used for the buffer layer; and to provide an electroluminecsent device using the same. <P>SOLUTION: As shown in Fig. 1(A), the electroluminescent device includes a first electrode 101, a buffer layer 102, an electroluminescent film 103, and a second electrode. A material for forming the buffer layer 102 disposed on the first electrode 101 includes a polymer compound having a conjugate in the main chain or in a side chain as a polymeric material soluble in an organic solvent, and a conductive material soluble in an organic solvent containing a compound having donor property against the polymeric material above. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electroluminescent element in which fluorescence or phosphorescence can be obtained by applying an electric field to an element provided with a film containing an organic compound (hereinafter referred to as “electroluminescent film”) between a pair of electrodes. In particular, the present invention relates to an electroluminescent element using a conductive polymer material (electroluminescent element material) as a part thereof.

  An electroluminescent element using a material having characteristics such as self-light-emitting, thin and light, high-speed response, and direct current low-voltage driving as a light emitter is expected to be applied to next-generation flat panel displays, particularly portable devices. Note that a light-emitting device in which electroluminescent elements are arranged in a matrix is considered to be superior to a conventional liquid crystal display device in that it has a wide viewing angle and excellent visibility.

  The light emitting mechanism of the electroluminescent element is such that electrons injected from the cathode and holes injected from the anode are regenerated at the emission center in the electroluminescent film by applying a voltage with the electroluminescent film sandwiched between a pair of electrodes. It is said that it combines to form molecular excitons, and emits energy by emitting energy when the molecular excitons return to the ground state. Singlet excitation and triplet excitation are known as excited states, and light emission is considered to be possible through either excited state.

  In the case where such a light emitting device is applied to a portable device, low power consumption is required. Therefore, reducing the driving voltage of the electroluminescent element is one of important issues.

  To date, attempts have been made to provide a buffer layer at the interface between the electrode and the electroluminescent film as a technique for reducing the drive voltage. For the buffer layer, there are a case where a low molecular weight material is used and a case where a high molecular weight (polymer based) material is used. Specifically, in the case of a low molecular weight material, copper phthalocyanine (Cu—Pc) is formed at the interface with the anode. ), A high molecular weight arylamine called starburst amine represented by m-MTDATA (Reference 1: Y.Shirota, Y.Kuwabara, H.Inada, T.Wakimoto, H.Nakada, Y.Yonemoto, S.Kawami and K. imai .: Appl. Phys. Lett., 65, 807 (1994)) has been reported to provide a buffer layer. Note that these materials have high HOMO levels and have values close to the work function of the electrode material forming the anode, so that the hole injection barrier can be reduced.

  In the polymer system, polyethylene dioxythiophene (PEDOT) (Reference 2: JMBharathan and Y. Yang: Appl. Phys. Lett., 72, 2660 (1998)) was used as a buffer layer at the interface with the anode. Examples have been reported. Note that PEDOT is usually doped with polystyrene sulfonic acid (PSS) and has conductivity that functions as a conductive polymer.

  In the case of a polymer system, by providing a buffer layer made of a conductive polymer having a large bonding area with the electrode, the adhesion with the light emitting layer formed on the electrode through the buffer layer is increased, Since the hole injection efficiency can be improved, the driving voltage can be lowered as a result.

  Recently, a method has been reported in which an inorganic material that functions as a Lewis acid is allowed to act on a triphenylamine derivative, which is a polymer material, to form a radical cation and use a layer with increased conductivity at the interface with the electrode. (Reference 3: A. Yamamori, C. Adachi, T. Koyama and Y. Taniguchi: Appl. Phys. Lett., 72, 2147 (1998)).

  A polymer material is a preferable material in forming a buffer layer because it is easier to handle and has higher heat resistance than a low-molecular material, but when PEDOT is used as a polymer material, conductivity is obtained. In order to use organic sulfonic acid as a dopant for this, it is an essential condition to use water as a solvent.

  However, since it is generally known that electroluminescent devices are significantly degraded by water, in order to improve the reliability of the device, a buffer layer made of a polymer material is used without using water as a solvent. Is required.

  In order to impart conductivity to the polymer material, there is a method using an inorganic material as a dopant as described above. In this case, a substance harmful to the environment such as antimony (Sb) is used. There is a need, and this is not preferable in the industry.

  Therefore, the present invention provides an environment-friendly electroluminescent element material that can form a buffer layer without using water as a solvent, unlike a polymer material used for a conventional buffer layer. An object of the present invention is to provide an electroluminescent device capable of improving the injectability of carriers from an electrode by using such a material for an electroluminescent device, reducing the drive voltage of the device, and improving the reliability.

  Therefore, in order to solve the above problems, the present inventor, in an electroluminescent element having a first electrode 101, a buffer layer 102, an electroluminescent film 103, and a second electrode as shown in FIG. As a material for forming the buffer layer 102 formed on the first electrode 101, a polymer compound (so-called conjugated polymer) that is soluble in an organic solvent and includes a conjugate in the main chain or side chain, and an organic solvent It was considered to use a conductive material that is soluble and contains an acceptor property or a donor property with respect to the polymer compound.

  Note that in the preparation of the buffer layer 102 in the present invention, as the compound having an acceptor property or a donor property that is soluble in an organic solvent, an aprotic or neutral compound is used. The conjugated polymer compound is not particularly limited as long as it is dissolved in an organic solvent. In particular, by injecting holes from the anode by doping either an acceptor compound or a donor compound. Alternatively, it is preferable to use a redox polymer (a redox polymer) capable of forming both a buffer layer with a high electron injection property from the cathode.

  The polymer compound that is soluble in the organic solvent and contains conjugation in the main chain or side chain (conjugated polymer) has a low number of repeating structural units (degree of polymerization) of about 2 to 20. A polymer (oligomer) is included.

  Here, the reaction occurring in the buffer layer 102 of the present invention is shown in FIG. Note that in the case where the buffer layer 102 includes a conjugated polymer and an acceptor compound (abbreviated as an acceptor in the drawing), electrons in the conjugated polymer are extracted by the acceptor compound, and as a result, conjugation occurs. The polymer is present as a carrier (hole). That is, in this case, the electrode formed in contact with the buffer layer 102 is an anode. On the other hand, when the buffer layer 102 is composed of a conjugated polymer and a donor compound (abbreviated as a donor in the figure), electrons are given to the conjugated polymer by the donor compound, and as a result, The conjugated polymer exists as a carrier (electron). That is, in this case, the electrode formed in contact with the buffer layer 102 is a cathode.

  FIG. 1C shows a conceptual diagram in the case where the buffer layer 102 is formed from a conjugated polymer and an acceptor compound. In this case, the first electrode (anode) 101 extracts electrons from the acceptor level present in the conjugated polymer, and simultaneously injects holes into the buffer layer at the acceptor level. Note that the injected holes move to the HOMO level in the buffer layer, and further move to the HOMO level of the electroluminescent film 103. Note that in this case, the movement from the first electrode 101 to the buffer layer is easily performed because it is performed where there is almost no energy difference. In addition, when the injected holes move from the acceptor level to the HOMO level of the electroluminescent film 103, the energy difference is reduced compared to the case where the injected holes are directly injected from the first electrode 101. The hole injectability from the electrode can be improved.

  FIG. 1D shows a conceptual diagram in the case where the buffer layer 102 is formed of a conjugated polymer and a donor compound. In this case, electrons are injected from the first electrode (cathode) 101 to the donor level existing in the conjugated polymer. The injected electrons move to the LUMO level in the buffer layer 102 and further move to the LUMO level of the electroluminescent film 103. Note that in this case, the movement from the first electrode 101 to the buffer layer is easily performed because it is performed where there is almost no energy difference. Further, when the injected electrons move from the LUMO level in the buffer layer 102 to the LUMO level of the electroluminescent film 103, the energy difference is reduced compared to the case where the injected electrons are directly injected from the first electrode 101. The injectability of electrons from the first electrode can be improved.

一般式〔２〕

一般式〔３〕

一般式〔４〕

一般式〔５〕

一般式〔６〕

一般式〔７〕

The constitution of the present invention is a combination of a polymer compound containing conjugation in the main chain or side chain and any one of the compounds having acceptor properties represented by the following general formulas [1] to [7]. It is a material for an electroluminescent element.
General formula [1]

General formula [2]

General formula [3]

General formula [4]

General formula [5]

General formula [6]

General formula [7]

一般式〔９〕

一般式〔１０〕

一般式〔１１〕

Another structure of the present invention is that any one of a polymer compound including conjugation in a main chain or a side chain, and a compound having a donor property represented by the following general formulas [8] to [11] It is the material for electroluminescent elements which combines these.
General formula [8]

General formula [9]

General formula [10]

General formula [11]

  Furthermore, another configuration of the present invention is an electroluminescent device having an anode, a buffer layer, an electroluminescent layer, and a cathode, wherein the buffer layer formed in contact with the anode is conjugated to a main chain or a side chain. Electroluminescence characterized by using an electroluminescent element material comprising a combination of a polymer compound containing the compound and any one of the compounds having an acceptor property represented by the above general formulas [1] to [7] It is an element.

  Furthermore, another configuration of the present invention is an electroluminescent device having an anode, a buffer layer, an electroluminescent layer, and a cathode, wherein the buffer layer formed in contact with the cathode is conjugated to a main chain or a side chain. Electroluminescence characterized by using an electroluminescent element material comprising a combination of a polymer compound containing the compound and any one of the compounds having donor properties represented by the general formulas [8] to [11]. It is an element.

  By using the electroluminescent element material of the present invention, the buffer layer can be formed without using water as a solvent, unlike the case where the buffer layer is formed using a conventional polymer material. Note that in an electroluminescent element formed using the electroluminescent element material of the present invention, the carrier injectability from the electrode can be improved, the driving voltage of the element can be reduced, and the reliability can be increased.

Embodiments of the present invention will be described below.
(Embodiment 1)
In the first embodiment, as shown in FIG. 2, the buffer layer 202 is formed on the first electrode, and the electroluminescent film 203 and the second electrode 204 are formed on the buffer layer 202, respectively. In this case, the buffer layer 202 includes a compound containing conjugation in the main chain or side chain (hereinafter referred to as a conjugated polymer) and an acceptor as shown in the means for solving the problems in this specification. A parabenzoquinone derivative represented by the general formula [1], a naphthoquinone derivative represented by the general formula [2], a tetracyanoquinodimethane derivative or a dicyanoquinodiimine derivative represented by the general formula [3], 4], a compound represented by the general formula [5], a compound represented by the general formula [6], or a compound represented by the general formula [7]. And forming together look.

Specific examples of the compounds having acceptor properties represented by the general formulas [1] to [7] are shown in the following chemical formulas (A1) to (A8), respectively.

  Note that in the case of Embodiment Mode 1, since the buffer layer 202 is formed using an acceptor material, the first electrode 201 functions as an anode. In addition, since the first electrode 201 is an electrode that functions as an anode, the first electrode 201 is preferably formed using an anode material having a high work function. However, formation of the buffer layer 202 improves hole injectability. Therefore, it is not always necessary to use a material having a high work function.

  However, a transparent conductive film made of indium tin oxide (ITO) is used here as an anode material for forming the first electrode 201 in order to further improve element characteristics (FIG. 2B )).

  Next, the buffer layer 202 is formed over the first electrode 201. Note that as the material for forming the buffer layer 202, any of the materials described above can be used in combination, but here, as shown in FIG. 2B, emeraldine-based polyaniline (as a conjugated polymer) Hereinafter, it is formed with a film thickness of 20 to 50 nm (preferably 30 nm) using tetracyanoquinodimethane (hereinafter TCNQ) as an acceptor molecule using EB-PAni.

  Next, an electroluminescent film 203 is formed on the buffer layer 202. The electroluminescent film 203 may be formed of a single material, but may have a stacked structure formed of a plurality of materials.

  In the case of a laminated structure, at least a layer having each function such as a hole injection layer, a hole transport layer, a light emitting layer, a hole blocking layer (blocking layer), an electron transport layer, an electron injection layer is combined. A structure including a layer having a light-emitting property may be used.

In Embodiment Mode 1, the electroluminescent film 203 is formed to have a stacked structure of a hole transport layer 211 and an electron transport layer 212 as shown in FIG. Specifically, the hole transporting layer 211 is 4,4′-bis [N- (1-naphthyl) -N-phenyl-amino] -biphenyl (hereinafter referred to as α-NPD) which is a hole transporting material. ) With a thickness of 30 nm, and the electron transport layer 212 is formed with a thickness of 50 nm of tris (8-quinolinolato) aluminum (hereinafter referred to as Alq ₃ ), which is an electron transporting material. In the case of this stacked structure, Alq ₃ forming the electron transport layer 212 has a light emitting property.

  Next, the second electrode 204 is formed over the electroluminescent film 203. Note that the second electrode 204 is an electrode that functions as a cathode, and thus is formed using a cathode material having a low work function (specifically, a material having a work function of 3.5 eV or less). Note that the second electrode 204 may have a single-layer structure formed of a single material, but may have a stacked structure of a plurality of materials. In the first embodiment, as shown in FIG. 2B, the cathode 204 is formed by stacking lithium fluoride (LiF) with a thickness of 2 nm and aluminum (Al) with a thickness of 100 nm. Show. In this case, it is possible to form an electrode having both functions of reducing the work function of the cathode 204 by using LiF and increasing the conductivity of the cathode 204 by using Al. As the cathode material, known materials having a small work function can be used in any combination.

  As described above, a compound containing conjugation in the main chain or side chain (hereinafter referred to as a conjugated polymer), a parabenzoquinone derivative represented by the general formula [1] having acceptor properties, A naphthoquinone derivative represented by the formula [2], a tetracyanoquinodimethane derivative or a dicyanoquinodiimine derivative represented by the general formula [3], a compound represented by the general formula [4], a compound represented by the general formula [5] A buffer that does not use water as a solvent by using a material (electroluminescent device material) in combination with any one of the compound represented by the general formula [6] or the compound represented by the general formula [7] A layer can be formed. Note that by forming this buffer layer, the carrier (hole) injection property from the electrode (in the first embodiment, the anode) is improved, so that the driving voltage of the element is reduced and the electroluminescence is highly reliable. An element is formed.

(Embodiment 2)
In the second embodiment, as shown in FIG. 3, a buffer layer 302 is formed on the first electrode 301, and an electroluminescent film 303 and a second electrode 304 are formed on the buffer layer 302, respectively. In this case, the buffer layer 302 includes a compound containing conjugation in the main chain or side chain (hereinafter referred to as a conjugated polymer), a compound represented by the general formula [8] having a donor property, the general formula [ And 9], a compound represented by the general formula [10], and a compound represented by the general formula [11].

Specific examples of the compound having donor properties represented by the general formulas [8] to [11] are shown in the following chemical formulas (D1) to (D4), respectively.

  Note that in the case of Embodiment Mode 2, since the buffer layer 302 is formed using a material having a donor property, the first electrode 301 functions as a cathode. In addition, since the first electrode 301 is an electrode that functions as a cathode, the first electrode 301 is preferably formed using a cathode material having a low work function. However, the formation of the buffer layer 302 improves the electron injecting property, so the first electrode 301 is not necessarily formed. It is not necessary to use a material with a low work function.

  Note that here, as the cathode material for forming the first electrode 301, Al formed with a thickness of about 120 nm is used (FIG. 3B).

  Next, the buffer layer 302 is formed over the first electrode 301. Note that as a material for forming the buffer layer 302, any of the materials described above can be used in combination, but here, EB-PAni is used as a conjugated polymer as shown in FIG. Tetrathiofulvalene (hereinafter referred to as TTF) is used as the conductive polymer, and the film thickness is 20 to 50 nm (preferably 30 nm).

  Next, an electroluminescent film 303 is formed on the buffer layer 302. The electroluminescent film 303 may be formed of a single material, but may have a stacked structure formed of a plurality of materials.

  In the case of a laminated structure, at least a layer having each function such as a hole injection layer, a hole transport layer, a light emitting layer, a hole blocking layer (blocking layer), an electron transport layer, an electron injection layer is combined. A structure including a layer having a light-emitting property may be used.

In Embodiment Mode 2, as shown in FIG. 3B, the electroluminescent film 303 is formed to have a stacked structure of an electron transport layer 311, a hole transport layer 312, and a hole injection layer 313. Specifically, the electron transport layer 311 has an electron transport material of Alq ₃ having a film thickness of 50 nm, the hole transport layer 312 has a hole transport material of α-NPD of 30 nm, The hole injection layer 313 is formed by setting copper phthalocyanine (hereinafter referred to as Cu-Pc), which is a hole transporting material, to a thickness of 20 nm.
In the case of this stacked structure, Alq ₃ forming the electron transport layer 311 has a light emitting property.

  Next, the second electrode 304 is formed over the electroluminescent film 303. Note that since the second electrode 304 is an electrode functioning as an anode, the second electrode 304 is formed using an anode material having a high work function (specifically, a material having a work function of 4.0 eV or more). Note that the second electrode 304 may have a single-layer structure formed using a single material, but may also be formed using a stacked structure including a plurality of materials. In Embodiment Mode 2, as shown in FIG. 3B, a case where the second electrode 304 is formed by stacking gold (Au) films having a thickness of 20 nm is described. Note that as the anode material used for the second electrode 304, a known material having a high work function can be used in any combination.

  From the above, in the buffer layer of the electroluminescent device, the compound containing conjugation in the main chain or side chain (hereinafter referred to as conjugated polymer), the compound represented by the general formula [8] having donor properties, the general formula [ By using a material (electroluminescent device material) in combination with any one of the compound represented by 9], the compound represented by the general formula [10], and the compound represented by the general formula [11], A buffer layer that does not use water as a solvent can be formed. Note that the formation of this buffer layer improves the injectability of carriers (electrons) from the electrode (in the second embodiment, the cathode), so that the driving voltage of the element is reduced and the electroluminescent element is highly reliable. Is formed.

(Embodiment 3)
In this Embodiment 3, the result of having measured the electrical property about the electroluminescent element of this invention is shown. Note that the structure of the electroluminescent element used for measurement is a structure in which the buffer layer is formed in contact with the anode as described in the first embodiment.

Moreover, in order to compare the effect by providing the buffer layer formed using the material of this invention, (1) When there is no buffer layer, (2) When Cu-Pc is used for a buffer layer, (3) Buffer layer of the present invention (EB-PAni + TCNQ)
Three types of electroluminescent elements were prepared, and the characteristics were measured.

As the above three elements, (1) without a buffer layer, ITO (120 nm) (anode) / α-NPD (50 nm) / Alq ₃ (50 nm) / CaF (2 nm) / Al (100 nm) ( In the case of using an element laminated in the order of (cathode) and (2) using Cu-Pc for the buffer layer, ITO (120 nm) (anode) / Cu-Pc (20 nm) (buffer layer) / α-NPD In the case of using an element laminated in the order of (30 nm) / Alq ₃ (50 nm) / CaF (2 nm) / Al (100 nm) (cathode), and (3) having the buffer layer (EB-PAni + TCNQ) of the present invention , ITO (120nm) (anode) / (EB-PAni + TCNQ ) ( about 30 nm) (buffer layer) / α-NPD (30nm) / Alq 3 (50nm) / CaF Used 2nm) / Al (100nm) (the elements are stacked in this order cathode), respectively.

  The measurement results are as shown in FIG. 4, and the result (3) that the electroluminescent device having the buffer layer of the present invention has the lowest driving voltage was obtained. The driving voltage is lower than that in the case of using Cu—Pc shown in (2) because the buffer layer in (3) has conductivity (due to acceptor doping) and is formed of a polymer film. This is probably due to the flatness of the film.

FIG. 6 illustrates an electroluminescent element of the present invention. The figure explaining the electroluminescent element which has a buffer layer in the anode side. 3A and 3B illustrate an electroluminescent element having a buffer layer on the cathode side. The figure which shows the result measured about the electrical property of an electroluminescent element.

Explanation of symbols

101 First electrode 102 Buffer layer 103 Electroluminescent film 104 Second electrode

Claims (10)

A material for an electroluminescent device comprising a polymer compound soluble in an organic solvent and containing a conjugate in the main chain or side chain, and a compound represented by the following general formula [8].
General formula [8]

A material for an electroluminescent device comprising a polymer compound soluble in an organic solvent and containing a conjugate in the main chain or side chain, and a compound represented by the following general formula [9].
General formula [9]

A material for an electroluminescent device comprising a polymer compound soluble in an organic solvent and containing a conjugate in the main chain or side chain, and a compound represented by the following general formula [10].
General formula [10]

A material for an electroluminescent device comprising a polymer compound soluble in an organic solvent and containing a conjugate in the main chain or side chain, and a compound represented by the following general formula [11].
General formula [11]

  5. The material for an electroluminescent device according to claim 1, wherein the polymer compound that is soluble in the organic solvent and includes a conjugate in the main chain or side chain is polyaniline in an emeraldine state. . An electroluminescent device having an anode, an electroluminescent layer, a buffer layer, and a cathode,
As the buffer layer formed in contact with the cathode, for an electroluminescent device comprising a polymer compound soluble in an organic solvent and containing a conjugate in the main chain or side chain, and a compound represented by the following general formula [8] An electroluminescent element using a material.
General formula [8]

An electroluminescent device having an anode, an electroluminescent layer, a buffer layer, and a cathode,
As the buffer layer formed in contact with the cathode, for an electroluminescent device comprising a polymer compound soluble in an organic solvent and containing a conjugate in the main chain or side chain, and a compound represented by the following general formula [9] An electroluminescent element using a material.
General formula [9]

An electroluminescent device having an anode, an electroluminescent layer, a buffer layer, and a cathode,
As the buffer layer formed in contact with the cathode, for an electroluminescent device comprising a polymer compound soluble in an organic solvent and containing a conjugate in the main chain or side chain, and a compound represented by the following general formula [10] An electroluminescent element using a material.
General formula [10]

An electroluminescent device having an anode, an electroluminescent layer, a buffer layer, and a cathode,
As the buffer layer formed in contact with the cathode, for an electroluminescent device comprising a polymer compound soluble in an organic solvent and containing a conjugate in the main chain or side chain, and a compound represented by the following general formula [11] An electroluminescent element using a material.
General formula [11]

  10. The electroluminescent device according to claim 6, wherein the high molecular compound that is soluble in the organic solvent and includes conjugation in the main chain or side chain is polyaniline in an emeraldine state. 10.

Images