JP2006114731A - Display device and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a display device which is driven in a wide range from low luminance to high luminances, such as a light source use, etc., operates stably over a range of wide luminance and excellent in lifetime characteristics, using an organic EL element. <P>SOLUTION: This display device includes at least one set of electrodes 2 and 5. A plurality of functional layers are configured between the electrodes 2 and 5. The functional layer has a layer 4 made of a polymer substance having at least one kind of tree-shape multiple branch structures and having a light emitting function, and an organic electroluminescent element including a charge injection layer 3 made of at least one kind of inorganic substance. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a display device and a manufacturing method thereof, and more particularly to a display device using an organic electroluminescent element (hereinafter referred to as an organic EL element) in which a light emitting layer is formed of a polymer material.

An electroluminescent element is a light emitting device using electroluminescence of a solid fluorescent substance. At present, an inorganic electroluminescent element using an inorganic material as a light emitter has been put into practical use, and it is applied to a backlight or a flat display of a liquid crystal display. Application development has been made. However, inorganic electroluminescent elements have a high driving voltage of several tens of volts and must be driven by alternating current, which complicates the driving circuit and consumes a large amount of power, making it difficult to apply to portable devices. is there.

  On the other hand, research on electroluminescent elements using organic materials has been attracting attention for a long time, and various studies have been conducted, and some have been put into practical use in recent years.

  Organic EL elements can be classified into several groups based on the difference in materials used for the light emitting layer. One of the typical ones is an element using a low molecular weight organic compound for the light emitting layer, which is mainly produced by vacuum deposition. The other is a polymer organic EL device using a polymer compound in the light emitting layer, to which the present invention relates.

  Polymer organic EL devices can be made into thin films by spin coating, ink jet, printing, etc. by using a solution in which the material constituting each functional layer is dissolved. It is attracting attention as a technology that can be expected.

  A typical polymer organic EL device is produced by laminating a plurality of functional layers such as a charge injection layer and a light emitting layer between an anode and a cathode. The structure of a typical polymer organic EL element and its production procedure will be described below.

  First, a PEDOT: PSS (mixture of polythiophene and polystyrenesulfonic acid: hereinafter referred to as PEDOT) thin film as a charge injection layer is formed by spin coating or the like on a glass substrate on which ITO (indium tin oxide) as an anode is formed. To do. PEDOT is a material that has become a de facto standard as a charge injection layer, and functions as a hole injection layer by being disposed on the anode side.

  On the PEDOT layer, polyphenylene vinylene (hereinafter referred to as PPV) and a derivative thereof, or polyfluorene and a derivative thereof are formed as a light emitting layer by a spin coating method or the like. Then, a metal electrode as a cathode is formed on these light emitting layers by vacuum deposition to complete the device.

  As described above, the polymer organic EL has an excellent feature that it can be produced by a simple process, and is expected to be applied to various uses, but it cannot obtain a sufficiently large emission intensity. These two points are problems to be improved, that is, the point that the life is not sufficient when driving for a long time.

The decrease in the emission intensity of the polymer organic EL device, that is, the deterioration proceeds in proportion to the product of the energization time and the current flowing through the device, but the details have not been clarified yet and intensive research is ongoing. By the way.

  Various speculations have been made about the cause of the decrease in the emission intensity, but the deterioration of PEDOT is considered as one of the main ones. As described above, PEDOT is a mixture of two polymer substances, polystyrene sulfonic acid and polythiophene, the former being ionic and the latter having local polarity in the polymer chain. Due to the Coulomb interaction resulting from such charge anisotropy, the two are loosely coupled, thereby exhibiting excellent charge injection characteristics.

  In order for PEDOT to exhibit excellent properties, close interaction between the two is indispensable. In general, however, a mixture of polymer substances tends to cause phase separation due to a subtle difference in solubility in a solvent. This is no exception for PEDOT (Non-Patent Document 1). The occurrence of phase separation means that the loose bond between the two polymers can be easily removed and can be unstable when the PEDOT is driven in an organic EL device. This indicates that components that have not contributed to bonding as a result of phase separation, particularly ionic components, may be diffused by an electric field accompanying energization and have an undesirable effect on other functional layers. Thus, although PEDOT has excellent charge injection characteristics, it cannot be said that it is a stable substance.

In response to such concerns related to PEDOT, a proposal has been made to eliminate PEDOT itself (Non-Patent Document 1). In this Non-Patent Document 1, a silicon oxide (SiO ₂ ) layer having an electron blocking action is used instead of the PEDOT layer, which is certainly more efficient than the case where there is nothing between the ITO electrode and the light emitting layer. Although it is improved, the characteristics are deteriorated as compared with an element having a PEDOT layer.

In addition, a proposal has been made to insert a buffer layer having an electronic blocking function between the PEDOT layer and the light emitting layer while the PEDOT layer is provided (Non-patent Document 2). When the electron blocking layer is inserted, the carrier density in the vicinity of the interface between the light emitting layer and the electron blocking layer is increased and the light emission efficiency is improved. An increase in luminous efficiency means an increase in emission intensity with respect to input power, so that the amount of current passing through the element is reduced in order to obtain the same amount of light, and as a result, the degradation of the PEDOT layer is reduced and the life characteristics are improved. That's it. However, when the current density is further increased, even if the current density is further increased, the light emission intensity cannot be increased. For this reason, there is a limit to the luminance that can be obtained, and it cannot be said that high luminance can be obtained any more and the luminance level is sufficiently satisfactory, and the lifetime is not sufficient.
In particular, when used in a large outdoor display panel or the like, high luminance characteristics are required, and intensive research has been conducted to further increase the luminance.

  As described above, examples of polymer organic EL have been described. As described above, there is a group of so-called low molecular organic EL elements in which a low molecular material is used for a light emitting layer in organic EL elements. Various proposals have been made.

For example, in Patent Document 1 and Non-Patent Document 3, the driving voltage of an element is obtained by laminating oxide thin films such as vanadium (V), molybdenum (Mo), ruthenium (Ru) instead of the ITO electrode or on the ITO electrode. We are trying to reduce it. In this example, the reason why the durability of the device is poor is that the barrier between the electrode and the hole transport layer or the light emitting layer is high, and an excessive voltage is applied to this barrier, and the work function is that of the conventional anode material. By using a metal oxide thin film larger than a certain ITO, the barrier between the electrode and the hole transport layer or the light emitting layer is lowered to reduce the driving voltage and improve the durability (Patent Document 1 and Non-Patent Document). 3).

JP-A-9-63771 Applied Physics Letters Vol. 79 pp 1193-1195 Applied Physics Letters Vol. 80 pp 2436-2438 Journal of Physics D: Applied Physics Vol. 29 pp 2750-2753

  The present invention has been made in view of the above circumstances, and can be driven in a wide range from a low luminance to a high luminance such as a light source, and can operate stably over a wide luminance range and has excellent life characteristics. It is an object to provide a display device using an EL element.

  The organic electroluminescent element used in the display device of the present invention includes at least one pair of electrodes and a plurality of functional layers formed between the electrodes, and the functional layer is at least one kind of tree-like shape. It includes a layer having a light emitting function made of a polymer material having a multi-branched structure, at least one buffer layer, and a charge injection layer made of at least one inorganic substance.

  According to this configuration, by using an inorganic substance as the charge injection layer, a display device with extremely high emission intensity and stable characteristics can be obtained. It is possible to maintain stable characteristics without becoming unstable even when the current density is increased, such as PEDOT in which loose coupling due to Coulomb interaction between two kinds of polymer materials is easily released. This is because the emission intensity can be increased. By providing the charge injection layer made of at least one inorganic material in this way, the light emission intensity and light emission efficiency of the device can be maintained at a high level over a wide range of current densities, and the lifetime is also improved. Therefore, it is possible to realize a display device including an organic electroluminescent element that operates stably over a wide luminance range up to high luminance and has excellent lifetime characteristics. Here, the light emitting layer is preferably a conjugated polymer.

  An organic compound having a tree-like multi-branched structure used as a layer having a light-emitting function is a tree-like multi-branched polymer structure or a tree-like multi-branch structure in which a plurality of external structural units surround a light-emitting structural unit three-dimensionally. Since it has a branched low molecular structure, the luminescent structural unit is three-dimensionally isolated, and the organic compound itself takes the form of fine particles. For this reason, when the organic compound aggregate is formed in a thin film shape, the presence of the external structural unit hinders adjacent light emitting structural units from coming close to each other, and the light emitting structural unit is uniform in the thin film. It is possible to maintain light emission with high intensity and long life.

  In the display device of the present invention, the layer having a light emitting function includes a layer having a dendritic multi-branched polymer structure mainly having a light emitting structural unit.

  In the display device of the invention, the layer having a light emitting function includes a layer having a dendritic multi-branched low molecular structure mainly having a light emitting structural unit.

  The organic compound having a dendritic multi-branched polymer structure or a dendritic multi-branched low molecular structure has a light emitting property even when a voltage is applied or heat is continuously applied due to the above structure. The uniform distribution state of the structural units does not change, and a stable structure can be maintained over time.

Therefore, when a light-emitting layer is formed using such an organic compound as a light-emitting material, the light-emitting structural units are uniformly dispersed in the light-emitting layer regardless of the formation method, and each structural unit interacts. As a whole, light is emitted, and a light emitting layer having high light emission efficiency and long life can be obtained.

  In the display device of the present invention, the electrodes are formed of striped electrodes formed at a predetermined interval, and an image display array is formed by scanning through at least one switching element. Including

  In the display device of the present invention, each electrode constituting the electrode pair has a striped anode in which the electrodes are arranged at a predetermined interval, and a stripe shape in which the electrodes are arranged in a direction orthogonal to the stripe of the anode. Including the cathode of the above.

  In the display device of the present invention, any one of the electrodes constituting the electrode pair is electrically separated for each pixel, and the separated electrode is scanned via at least one switching element. In this way, an image display array is formed.

  In the display device of the present invention, the functional layer includes at least one buffer layer.

  With this configuration, by using at least one type of buffer layer, for example, the escape of electrons to the anode can be prevented, and current can be prevented from flowing without contributing to light emission.

  The display device of the present invention includes one in which the buffer layer is composed of a polymer layer.

  In the display device of the present invention, the buffer layer contains an organic solvent.

  With this configuration, since the buffer layer can be formed by a coating method, it can be formed without going through a vacuum process.

  Moreover, the organic electroluminescent element of the display apparatus of this invention contains the high molecular compound in which the layer which has the said light emission function contains a fluorene ring. Here, the polymer compound containing a fluorene ring refers to a compound in which a desired group is bonded to the fluorene ring to form a polymer. Although high molecular compounds having various groups bonded thereto are commercially available, the details are not described here because many details are not known.

Further, in the organic electroluminescent element of the display device of the present invention, the layer having the light emitting function is polyfluorene represented by the following general formula (I) and derivatives thereof (R1 and R2 each represent a substituent) including.

  In the organic electroluminescent element of the display device of the present invention, the layer having the light emitting function contains a phenylene vinylene group.

  In the organic electroluminescent element of the display device of the present invention, the layer having the light emitting function includes polyphenylene vinylene represented by the following general formula (II) and derivatives thereof (R3 and R4 each represent a substituent). .

In the organic electroluminescent element of the display device of the present invention, the buffer layer has an absolute value of energy value representing the electron affinity of the buffer layer (hereinafter referred to as electron affinity) from the electron affinity of the layer having the light emitting function. Including those using small materials.
With this configuration, the loss of charge can be blocked, and the charge can effectively contribute to light emission in the light emitting layer.

Moreover, the organic electroluminescent element of the display apparatus of this invention contains the charge injection layer containing an oxide.
As oxides used here, chromium (Cr), tungsten (W), vanadium (V), niobium (Nb), tantalum (Ta), titanium (Ti), zirconium (Zr), hafnium (Hf) , Scandium (Sc), yttrium (Y), thorium (Tr), manganese (Mn), iron (Fe), ruthenium (Ru), osmium (Os), cobalt (Co), nickel (Ni
), Copper (Cu), zinc (Zn), cadmium (Cd), aluminum (Al), gallium (Ga), indium (In), silicon (Si), germanium (Ge), tin (Sn), lead (Pb) ), Antimony (Sb), bismuth (Bi), or oxides of so-called rare earth elements from lanthanum (La) to lutetium (Lu). Above all, aluminum oxide (AlO)
Copper oxide (CuO) and silicon oxide (SiO) are particularly effective for extending the life.

  In the organic electroluminescent element of the display device of the present invention, the charge injection layer contains an oxide of a transition metal.

  The organic electroluminescent element of the display device of the present invention includes one in which the charge injection layer is made of an oxide of molybdenum or vanadium.

In particular, the charge injection layer can be selected and used from oxides or nitrides of transition metals such as molybdenum and vanadium.
For example, since a transition metal compound has a plurality of oxidation numbers, it can take a plurality of potential levels, facilitate charge injection, and reduce driving voltage.

  In the organic electroluminescent device of the present invention, the charge injection layer contains a nitride.

  In the organic electroluminescent element of the display device of the present invention, the charge injection layer contains a transition metal nitride.

In addition, there are very many types of nitrides, and many of them are used as functional materials. Films can be formed mainly by sputtering or CVD. Various compounds are known, ranging from those used as semiconductors to those with very high insulation properties. However, as a result of various experiments, the film thickness of highly insulating compounds is about 5 nm during film formation. It was found that carrier injection becomes possible by making the following. Specific examples of the compound include the following, and titanium nitride (TiN) is preferable. TiN is known as a very robust material and is stable to heat.

In addition, gallium nitride (GaN), indium nitride (InN), aluminum nitride (AlN),
Boron nitride (BN), silicon nitride (SiN), magnesium nitride (MgN), molybdenum nitride (MoN)
), Calcium nitride (CaN), niobium nitride (NbN), tantalum nitride (TaN), vanadium nitride (BaN), zinc nitride (ZnN), zirconium nitride (ZrN), iron nitride (FeN), copper nitride (CuN)
), Barium nitride (BaN), lanthanum nitride (LaN), chromium nitride (CrN), yttrium nitride (YN), lithium nitride (LiN), titanium nitride (TiN), and composite nitrides thereof are also applicable. .

  In the organic electroluminescent element of the display device of the present invention, the charge injection layer contains an oxynitride.

In the organic electroluminescent element of the display device of the present invention, the charge injection layer includes an oxynitride of a transition metal.
For example, ruthenium (Ru) oxynitride crystal Ru ₄ Si ₂ O ₇ N ₂ etc. is a highly stable material with high heat resistance (1500 ° C), so it can be applied as a charge injection layer by forming a thin film. It is. In this case, the film can be formed by performing a heat treatment after forming the film by a sol-gel method.

In addition, barium sialon (BaSiAlON), calcium sialon (CaSiAlON), cerium sialon (CeSiAlON), lithium sialon (LiSiAlON), magnesium sialon (MgSiAlON), scandium sialon (ScSiAlON), yttrium sialon (YSiA)
lON), erbium sialon (ErSiAlON), neodymium sialon (NdSiAlON), etc.
Oxynitrides such as IIA, IIIB sialon, or multi-element sialon are applicable. These can be formed by CVD, sputtering, or the like. In addition, lanthanum silicon nitride (LaSiON), lanthanum europium silicon nitride (LaEuSi ₂ O ₂ N ₃ ), silicon oxynitride (SiON ₃ ), and the like are also applicable. Since these are mostly insulators, it is necessary to reduce the film thickness to about 1 nm to 5 nm. These compounds have a large exciton confinement effect and may be formed on the side where electrons are injected.

  In the organic electroluminescent element of the display device of the present invention, the charge injection layer includes a composite oxide containing a transition metal.

The reason is not clear, but when a transition metal composite oxide was used for the charge injection layer, the emission intensity could be greatly improved.
In addition, there are a great many types of complex oxides, and many of them have electronically interesting properties. Specific examples include the following compounds, but these are merely examples.

For example, barium titanate (BaTiO ₃ ), strontium titanate (SrTiO ₃ ), calcium titanate (CaTiO ₃ ), potassium niobate (KNbO ₃ ), bismuth iron oxide (BiFeO ₃ ),
Lithium niobate (LiNbO _3), sodium vanadate (Na ₃ VO _4), vanadium iron (FeVO _3), titanate vanadium (TiVO _3), chromic acid vanadium (CrVO _3), vanadium, nickel (NiVO _3), magnesium vanadate (MgVO _3), calcium vanadate (CaVO _3), vanadium lanthanum (LaVO _3), molybdate vanadium (VMoO _5), molybdate vanadium (V ₂ MoO _8), lithium vanadate (LiV ₂ O ₅ ), Magnesium silicate (Mg ₂ SiO ₄ ), magnesium silicate (MgSiO ₃ ), zirconium titanate (ZrTiO ₄ ), strontium titanate (SrTiO ₃ ), lead magnesium acid (PbMgO ₃ ), lead niobate (PbNbO ₃ ), Barium borate (BaB ₂ O ₄ ), lanthanum chromate (LaCrO ₃ ), lithium titanate (LiTi ₂ O ₄ ), lanthanum cuprate (LaCuO ₄ ), zinc titanate (ZnTiO ₃ ), calcium tungstate (CaWO ₄ ) and the like are possible.

Although any of these can be used to carry out the present invention, barium titanate (BaTiO ₃ ) is preferably used. BaTiO ₃ is a typical dielectric and is a complex oxide with high insulating properties, but it is possible to inject carriers when used in thin films based on the results of various experiments. I understood. BaTiO ₃ and strontium titanate (SrTiO ₃ ) are stable as compounds and have a very large dielectric constant, so that efficient carrier injection can be performed. For film formation, a sputtering method, a sol-gel method, a CVD method, or the like can be selected as appropriate.

Moreover, the organic electroluminescent element of the display device of the present invention includes one in which the buffer layer is disposed between the charge injection layer disposed on the hole injection side and the layer having a light emitting function.
With this configuration, the escape of electrons can be blocked, and the electrons can effectively contribute to light emission in the layer having a light emitting function.

The organic electroluminescent element of the display device of the present invention includes an anode formed on a light-transmitting substrate, a hole injection layer formed on the anode, and a buffer layer formed on the hole injection layer. And an electron injecting layer formed on the layer having the light emitting function so as to face the hole injecting layer through the layer having the light emitting function, and a cathode.
With this configuration, a buffer layer such as an electron blocking layer is formed on the hole injection layer side where electrons are likely to escape, and a layer having a light emitting function is formed on these layers. It is possible to prevent the layer having damage from being damaged during the formation of the hole injection layer.
Here, the cathode is preferably formed as a multilayer structure in which a layer having a low work function such as a calcium (Ca) layer or a barium (Ba) layer for facilitating electron injection is arranged on the light emitting layer side.

Moreover, the manufacturing method of the display device of the present invention is characterized in that the layer having the light emitting function of the organic electroluminescent element is formed by a wet method.
Thereby, since it can form, without requiring a vacuum process, an installation is simple and formation to a large area board | substrate becomes easy.

  In addition, in the said compound, the compound from which a valence differs easily exists, and what takes the form of the compound from which a valence differs other than what was illustrated is included.

  Further, the layer having a light emitting function is not limited to a layer having only a light emitting function, but includes a layer having other functions such as a charge transport function. In the following embodiments, the light emitting layer is simplified.

  According to the organic electroluminescent element of the display device of the present invention, since it operates stably over a wide luminance range up to high luminance, which could not be achieved conventionally, and has excellent life characteristics, it is mild for display applications. It is possible to realize stable charge injection and maintenance of light emission efficiency from a driving condition region to a severe driving condition of a strong electric field, a large current, and a high luminance.

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

(Embodiment 1)
FIG. 1 is a schematic perspective view of a display device using a polymer organic EL element according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view of an essential part of the organic EL element.

  In the present embodiment, the anode 2 and the cathode 5 patterned in a stripe shape are arranged in directions orthogonal to each other, and this intersection region constitutes each pixel region. By applying a direct current voltage or direct current to the display device, the light emitting layer 4 in the orthogonal region emits light, and a simple matrix type display device is provided.

  As shown in FIG. 2, the organic electroluminescent element of the present embodiment includes a substrate 1 made of a light-transmitting glass material, and ITO (indium tin oxide) as an anode 2 formed on the substrate 1. ), A metal oxide thin film as the charge injection layer 3 formed thereon, an electron block layer made of a polymer material as the buffer layer B, a light emitting layer 4 made of a polymer material, and a metal material. And the cathode 5.

When a direct current voltage or direct current is applied with the anode 2 of the organic electroluminescent element as a positive electrode and the cathode 5 as a negative electrode, the light emitting layer 4 is passed through the charge injection layer 3 and the buffer layer B from the anode 2. Then, holes are injected and electrons are injected from the cathode 5. In the light emitting layer 4, the holes and electrons injected in this way are recombined, and a light emission phenomenon occurs when excitons generated along with the recombination shift from the excited state to the ground state. This light emission is applied to a display device.

According to the organic electroluminescent element of the present embodiment, the charge injection layer 3 is composed of a metal oxide thin film, and holes can be easily injected, and the buffer layer B blocks the escape of electrons. Thus, electrons can effectively contribute to light emission in the layer having a light emitting function. Accordingly, it is possible to obtain an organic electroluminescent element having a long lifetime and a highly reliable light emission characteristic.

Next, the manufacturing process of the display device of the present invention will be described.
First, an ITO thin film is formed on a glass substrate 1 by a sputtering method, and then a metal oxide thin film is formed by a vacuum deposition method, and these are patterned by photolithography to form an anode 2 and a charge injection layer 3.
Thereafter, the buffer layer B and the light emitting layer 4 made of a polymer material are applied and formed by a coating method.
Finally, the cathode 5 is formed.
As described above, according to the method of the present invention, since the buffer layer B and the light emitting layer 4 are formed by applying the polymer material, the manufacturing is easy and the area can be increased.

Next, examples of the present invention will be described.
The structure is the same as that shown in FIGS. 1 and 2, and will be described with reference to FIGS.
The organic electroluminescent element of Example 1 includes a glass substrate 1 designated as Corning 7029 # having a thickness of 1 mm, an anode 2 made of an ITO thin film having a thickness of 20 nm formed on the upper layer, and A charge injection layer 3 made of a 20 nm thick molybdenum oxide thin film formed on the upper layer of the anode 2, and a polycrystal as a buffer layer B made of a 20 nm thick polyfluorene compound formed on the charge injection layer 3. [(9,9-Dioctylfluorenyl-2,7-diyl) -alt-co- (N, N'-diphenyl) -N, N'di (p-butyl-oxyphenyl) -1,4-diamino Benzene)], ie Poly [(9,9-dioctylfluorenyl-2,7-diyl) -alt-co- (N, N'-diphenyl) -N, N'di (p-butyl-oxyphenyl) -1,4 -diaminobenzene)] and Ir (ppy) 3 structural formula with a thickness of 80 nm: Tris [2- (2-pyridinyl) phenyl-C, N] -iridium, ie Tris [2- (2-pyridinyl) phenyl-C, N] -iridium as the core And a cathode 5 comprising a calcium (Ca) layer 5a having a thickness of 20 nm and an aluminum (Al) layer 5b having a thickness of 100 nm formed on the light emitting layer 4. Has been. Here, the anode 2 and the charge injection layer 3 are patterned in a stripe shape, and the cathode 5 is patterned in a stripe shape so as to be orthogonal thereto.

  Here, as the light emitting layer, a tree-like multibranched polymer or a tree-like multibranched low molecule having a basic structure represented by the following formula is also applicable. Here, the number of branches n is preferably 2 or more. In the following formula, the number of branches of X in parentheses, which is one branch, is exemplified as two (two stages). However, the number of branches is not particularly limited as long as it is within the range where synthesis is easy. X may have 0 branching, that is, linear.

  In addition, this organic compound has a tree-like multibranched polymer structure or a tree-like multibranched low molecule in which other chemical structures surround the periphery three-dimensionally with a luminescent structural unit composed of a phosphorescent component as a center (core). Have. Here, in the case of a dendritic multi-branched polymer, the organic compound preferably has a molecular weight of 800 or more.

It should be noted that other chemical structures surrounding the light-emitting structural unit (phosphorescent light-emitting unit), in other words, a branched branch unit (hereinafter referred to as an external structural unit) contains a charge carrier transport component in the structure. Have. Furthermore, -O as external structural units will be described later between the structural units of the luminescent _{-, - CH 2 -, -} CH 2 -CH 2 - or -O-CH ₂ - that are connected by such More preferable.
In the organic compound having this structure, the charge carrier transport component of the luminescent structural unit and the external structural unit, and the charge carrier transport components are each non-conjugated, that is, linked by one or more single bonds, It is preferred that the charge carrier transport component has an optically larger optical gap (energy difference between the lowest unoccupied molecular orbital and the highest occupied molecular orbital) and a higher triplet energy level than the luminescent structural unit.

  The organic compound has a dendritic multi-branched polymer structure or a dendritic multi-branched low molecular structure in which a plurality of external structural units are three-dimensionally surrounded by a light-emitting structural unit. It becomes a three-dimensionally isolated state, and the organic compound itself takes the form of fine particles. For this reason, for example, when an organic compound aggregate is formed into a thin film solid, adjacent light emitting structural units are prevented from adjoining due to the presence of external structural units, and the light emitting structural units are contained in the thin film solid. Evenly distributed.

  In addition, because of the above structure, this organic compound has a structure in which the uniform distribution state of the light-emitting structural units does not change even when voltage is applied continuously or heat is applied, and the structure is stable over time. Can be maintained.

  Therefore, in an organic EL device having a molybdenum oxide as a charge injection layer, when such an organic compound is used as a light emitting material to form a light emitting layer, the light emitting structural units are uniformly dispersed in the light emitting layer. Each structural unit interacts to emit light as a whole, and a light emitting layer having high luminous efficiency and long life can be obtained.

An organic electroluminescent device (FIGS. 1 and 2) for a display device using molybdenum oxide as a charge injection layer formed in this manner (FIGS. 1 and 2) (hereinafter referred to as “molybdenum oxide device”) is connected to a DC power source. The relationship between the applied voltage and the value of the current flowing at that time converted to a value per unit area was plotted, and the current density-applied voltage curve at this time was measured. The vertical axis represents emission intensity (cd / m ² ), and the horizontal axis represents current density (mA / cm ² ). At this time, the relationship between the current density of the element and the intensity of light extracted from the molybdenum oxide element was measured to obtain a light emission intensity-current density curve. For comparison, an element of Comparative Example 1 (referred to as a PEDOT element) in which the vanadium molybdate oxide thin film of the element of FIG. 1 was replaced with PEDOT was plotted simultaneously. Further, for comparison, in addition to the PEDOT element of Comparative Example 1, the element of Comparative Example 2 in which the buffer layer thin film was removed from the element of FIG. 1 (referred to as an element having no buffer layer) was plotted simultaneously.

As a result, in the case of the molybdenum oxide element of Example 1 of the present invention as compared with Comparative Example 1, the rise is quicker and the slope is steep, and the IV characteristics are almost equal even when the current density exceeds 800 (mA / cm ² ). It was found that linearity can be maintained.

Surprisingly, it can be seen that good linearity is maintained in an extremely wide range where the emission intensity exceeds one digit to 40000 (cd / m ² ). On the other hand, in the PEDOT element of Comparative Example 1, the emission intensity-current density curve showed linearity up to 40000 (cd / m ² ) at the most, and the emission intensity higher than that could not be obtained. On the other hand, in the element having no buffer layer of Comparative Example 2, the light emission intensity obtained with respect to the current density is small, and it can be seen that the current does not contribute to the light emission effectively.
From these results, it can be seen that the molybdenum oxide element, which is the organic electroluminescent element of Example 1 of the present invention, operates stably over a wide range of emission intensity and can obtain high-efficiency emission intensity.

In addition, the time variation of the light emission luminance when the molybdenum oxide element of Example 1 was driven at a constant current value (140 mA / cm ² ) and the time variation of the applied voltage necessary to keep the current value constant were measured. did. Again, the values of the PEDOT elements of Comparative Example 1 are plotted at the same time for comparison. As a result, it was found that according to the molybdenum oxide element of Example 1, it was possible to realize highly reliable driving without increasing the applied voltage for a long time. In addition, as a result of measuring the relationship between the elapsed time and the emission intensity of the molybdenum oxide element of Example 1 and the PEDOT element of Comparative Example 1, the molybdenum oxide element of Example 1 showed a further decrease in emission intensity over a long period of time. It can be seen that it requires less and has a longer life.

  As described above, although the current starts to flow from a relatively low voltage in the PEDOT element of Comparative Example 1, the current density does not increase much even if the voltage is increased because the resistance of the PEDOT film is high. In addition, a saturation tendency is observed in a high voltage region, that is, a region with a large current density, and it can be understood that the current density cannot be further increased. On the other hand, in the molybdenum oxide element of Example 1 of the present invention, the current flows from a low current value, and the resistance of the molybdenum oxide thin film is low, so the increase with respect to the voltage is quick and linear. This indicates that the molybdenum oxide thin film has very good characteristics over PEDOT as a charge injection layer. Further, the saturation of the molybdenum oxide element of Example 1 of the present invention as in the PEDOT element is not observed even in the large current region. Such characteristics are ideal as voltage-current density characteristics of the organic EL element. Such excellent characteristics of the molybdenum oxide element have the same tendency in the emission intensity described below.

Thus, in the PEDOT element, the emission intensity increases in proportion to the current in the region where the current density is low, but the tendency quickly reaches a peak, and the luminance rapidly increases in the large current region where the saturation tendency is observed. It is falling. This is thought to be due to the rapid progress of deterioration due to the high current density. The surface of the element after the evaluation is discolored to such an extent that it can be seen by visual observation, and has undergone an irreversible change. Actually, even if the element after evaluation is evaluated again, the initial characteristics are not reproduced at all. Moreover, although the element which does not have a buffer layer does not show the tendency of a peak like a PEDOT element, light emission intensity is lower than a PEDOT element over most areas. This is because an element that does not have a buffer layer lacks an electron blocking function by the buffer layer, so that some of the electrons injected from the cathode into the light emitting layer escape to the molybdenum oxide thin film without contributing to light emission. This is probably because of this.

  On the other hand, the superiority of the molybdenum oxide element is clear, and even in a low current density region, it is equivalent to the PEDOT element and shows an increase in emission intensity proportional to the current up to a high current density region. The increase in emission intensity is proportional to the current density, indicating that the emission efficiency is constant over a wide current range. In addition, the element after the evaluation hardly deteriorates even though a very large current flows, and the same characteristics can be obtained even if repeated evaluation is performed.

This fact is amazing. The area of the light emitting portion of the element used for the evaluation is 10 mm ² , and the thickness is at most several hundred nm. For example, if the applied voltage is 7 V, a current flows through the element at a current density of 50 mA, that is, 500 mA / cm ^2. At this time, energy of 300 mW or more is actually applied to this extremely small and thin element. . The organic EL device has high luminous efficiency, which is the conversion ratio of input electric power to light, but heat generation is still unavoidable, and the molybdenum oxide device is too hot to be touched with bare hands after evaluation. It has become. Moreover, this is through a glass substrate, and it can be easily estimated that the temperature of the element itself during light emission is extremely high.

As described above, the reason why the molybdenum oxide thin film exhibits excellent characteristics by using the thin film as a charge injection layer is not clearly understood, but is presumed and interpreted based on a model of a general charge injection mechanism. Then it becomes as follows. The following explanation is only a guess, and the actual phenomenon has not been clarified yet.
FIG. 3 is a schematic explanatory diagram for explaining the energy state of the electric charge of the simplest organic EL element. 4 is a simplified explanatory diagram for explaining the energy state of the charge of an element obtained by adding a PEDOT layer as a charge injection layer to the element configuration of FIG. 3, and FIG. It is a schematic explanatory drawing for demonstrating the energy state of the electric charge of the element which added the molybdenum oxide layer.

  In FIG. 3, 120 is a line indicating the energy level of the anode, 121 is a line indicating the energy level of the cathode, 122 is a line indicating the interface between the anode and a functional layer that controls light emission (hereinafter referred to as a light emitting layer), and 123 is a cathode. A line indicating the interface between the light emitting layer and the light emitting layer, 124 is the highest occupied orbit (hereinafter referred to as HOMO) of the light emitting layer, 125 is the lowest unoccupied orbit (hereinafter referred to as LUMO), 126 is a hole on the anode, and 127 is a hole injected into the light emitting layer. , 128 are electrons on the cathode, 129 is an electron injected into the light emitting layer, and 30 is a line representing the recombination of holes 127 injected into the light emitting layer and electrons 129 injected into the light emitting layer.

  In FIG. 4, 160 represents a portion of PEDOT as the charge injection layer, 161 represents a line indicating the interface between the anode and the PEDOT layer, 162 represents a line indicating the interface between the PEDOT layer and the light emitting layer, and 163 represents a position in the PEDOT layer. A hole 164 is a line indicating the energy level of the level in PEDOT. In FIG. 5, 150 represents a portion of the molybdenum oxide thin film as the charge injection layer, 151 represents a line indicating the interface between the anode and the molybdenum oxide thin film, and 152 represents a line indicating the interface between the molybdenum oxide thin film and the light emitting layer. , 153 are holes in the molybdenum oxide thin film, and 154 is a line indicating the energy level of the level in the molybdenum oxide thin film.

  Prior to the description, it should be noted that FIGS. 3, 4, and 5 are simplified and modeled views. These show the concepts necessary for explaining the phenomenon, and it goes without saying that the actual device operation process is more complicated.

First, the simplest operation of the organic EL element will be described with reference to FIG.
The light emission of the organic EL element is generated when the recombination energy of holes and electrons is released in the form of light as in the case of inorganic LEDs. First, as shown in FIG. 3, holes 126 on the anode are injected into the HOMO 124 of the light emitting layer, and electrons 128 on the cathode are injected into the LUMO of the light emitting layer. The holes 127 and the electrons 129 injected into the light emitting layer move in opposite directions to each other on the counter electrode side in the light emitting layer in accordance with the applied electric field. Each charge encounters a counter charge and generates a hole-electron pair, so-called exciton, with a certain probability while moving through the light emitting layer. Exciton is a mass of energy, and when this energy is released in the form of light, the device emits light.

Next, charge injection in the present invention will be described in detail.
In FIG. 3, attention is paid to a line 120 indicating the energy level of the anode and a line 127 indicating the HOMO of the light emitting layer. The positions of these lines in FIG. 4 indicate the electric field energy as they are, and the difference in height between the lines 120 and 127 indicates the difference in energy level between them. Different energy levels mean that holes at each energy level have different energies. Generally, in the expression as shown in FIG. Is defined as having higher energy in the upper part. Therefore, the hole 127 in the light emitting layer has higher energy than the hole 126 on the anode. At this time, in order to inject the hole 126 on the anode with lower energy into the light emitting layer, it is necessary to apply energy corresponding to the difference between the hole 126 and the hole 127 from the outside, and a part of the voltage applied to the element. Will be applied to this.

  From the above, it seems that it is only necessary to apply a voltage corresponding to the difference in energy level in order to inject carriers, but in practice, by using a charge injection layer, it is practically practical. Carrier injection is possible at a lower voltage. This is the same for both the anode and the cathode, but here, further explanation will be given focusing on the phenomenon on the anode side which is related to the present invention.

  FIG. 4 is a diagram for explaining the energy level of an element provided with PEDOT 60 as a charge injection layer on the anode side of the simplest organic EL element shown in FIG. PEDOT may be considered to have substantially one energy level, which is generally configured to be located between the energy levels of the anode and the light emitting layer.

  Now, when holes are injected, the holes transition from the anode to the light emitting layer over the energy level, and the transition probability depends on the difference between the average energy of the holes and the transition energy level. The larger the average energy of the holes and the smaller the difference in energy level, the more holes are injected into the light emitting layer. In this case, the PEDOT energy level 164 is intermediate between the energy level 120 of the anode and the energy level 124 of the light emitting layer. Because. 3 and 4, when the same applied voltage, that is, the same energy is applied to the holes, in the device of FIG. 4, the energy level difference is smaller than the energy level 127 of the light emitting layer. Because there is a PEDOT energy level 164, a transition occurs very easily here. Further, holes that have reached the energy level 164 of PEDOT easily transition to the energy level 124 of the light emitting layer for the same reason.

  This situation makes it easier to obtain a more intuitive understanding when anthropomorphic. That is, consider the situation when a human climbs a high step. At this time, it is easier to climb in two stages, with a one-step landing in the middle than to climb a large step at once. The same can be said for the injected holes. The potential energy finally obtained by the act of climbing the steps is the same regardless of the presence or absence of the landing, but the process up to that point is made easier.

  The molybdenum oxide thin film described in this embodiment can realize charge injection more easily than PEDOT. To quote the analogy that anthropomorphizes the reason again, it can be said that PEDOT has a dance hall, but the molybdenum oxide thin film is equivalent to a staircase, making it easier to climb the steps.

  In FIG. 5, there are a plurality of energy levels 154 in the molybdenum oxide thin film 150. This is the energy level expressed as a staircase. As the energy level from the anode 120 to the light emitting layer 124 is further subdivided, it is very easy for the hole 153 moving in the molybdenum oxide thin film to transition between a plurality of energy levels 153 having a small difference. .

  So why are there multiple energy levels 154 expressed as steps in the molybdenum oxide thin film? This can be interpreted as originating from the composition of the molybdenum oxide thin film.

The molybdenum oxide thin film in this embodiment is an amorphous thin film manufactured by vacuum deposition. The environment during vacuum deposition is a reducing atmosphere, and molybdenum oxide undergoes reduction in the process of being heated and sublimated and deposited on the substrate. The reduced molybdenum oxide yields several oxides with smaller oxidation numbers in addition to hexavalent MoO ₃ . They are, for example, tetravalent MoO ₂ and trivalent Mo ₂ O ₃ . Since receiving is equivalent to receiving electrons, an oxide having a reduced valence is more likely to release electrons than an oxide having a higher valence, that is, more likely to receive holes. This is equivalent to having a higher energy level as represented in FIGS.

  As a result, a step-like structure having a plurality of energy levels as shown as the energy level 154 of the molybdenum oxide in FIG. 5 is produced. The energy level 154 of the molybdenum oxide in FIG. 5 corresponds to the lowest energy level having a valence of 6 and can be interpreted as the valence decreasing upward.

  In this way, it is considered that a plurality of energy levels 154 expressed as the steps described above are generated. More precisely, diversification of energy levels caused by amorphous films should be considered in addition to oxides having different valences. The energy levels normally discussed in oxides and nitrides are based on the crystalline state, and in complex structures with many dangling bonds, such as amorphous films, all compounds are more or less here. It is often a thin film with multiple energy levels as described.

As described above, the drive voltage can be reduced and, as will be described later, it is possible to achieve high efficiency by preventing the escape of electrons on the anode side and reducing the reactive current, but the emission intensity is obtained. Therefore, a large current is required. The rapid deterioration of the PEDOT element in the large current region is considered to be caused by the remarkable acceleration of the deterioration caused by the generated heat. On the other hand, molybdenum oxide is an inorganic substance and is inherently very robust to heat. Therefore, it is considered that the result of maintaining stable characteristics over a wide current density range is obtained. It is done.

Such a characteristic of molybdenum oxide that is robust against heat exhibits its superiority to the maximum when applied to a polymer organic EL element. In general, a low molecular organic EL element is vulnerable to a high temperature environment due to a structure in which a low molecular organic compound group constituting a functional layer is formed by vacuum deposition and is an amorphous thin film on a substrate. This is because when the functional layer is exposed to a high temperature, the crystallization of the low molecular weight organic compound proceeds and the device characteristics deteriorate.
The crystallization temperature is at most 100 ° C., and how to raise the crystallization temperature while maintaining the sublimation characteristics necessary for vapor deposition is one of the problems of the low molecular organic EL material. That is, since a functional layer made of an organic material such as a light emitting layer is weak against heat, even if molybdenum oxide as a charge injection layer is robust against heat, the characteristics cannot be fully utilized.

  In contrast, polymer organic EL materials are resistant to high temperature environments. This is because a polymer material forms a thin film by intertwining long molecular chains in a complicated manner. There is no clear crystallization temperature in the polymer material, and there is only an index that can be called a softening start temperature called a glass transition point. Furthermore, even a clear glass transition point may not be observed in many polymer organic EL materials. That is, in other words, the polymer organic EL material cannot move freely and crystallize due to its intertwined structure even when exposed to high temperatures. A general characteristic of such a polymer material appears as a great advantage of heat resistance when the polymer material is applied to an organic EL element.

  As described above, the display device using the molybdenum oxide element of this example is a combination of a heat-resistant molybdenum thin film and a polymer organic EL material, so that it can still be used under extremely severe conditions under a large current density. A stable element can be constructed, and a great effect more than the function of good charge injection characteristics originally expected for molybdenum oxide is realized.

  Here, although PEDOT is also a polymer, a question arises as to why the PEDOT element deteriorates rapidly. This is not inconsistent with the stability of the polymer to heat.

  As described above, the good charge injection characteristics of the PEDOT element are obtained by the delicate interaction of two kinds of polymers. The rapid deterioration of the PEDOT element under a large current density is caused not by the deterioration of the polymer material itself constituting PEDOT but by the change in the physical structure thereof. That is, the change is that the loose coupling due to the Coulomb interaction between the two types of polymer materials is broken, or the state of phase separation that occurs is changed. The characteristics of PEDOT depend largely on the structure. When the structure changes, the characteristics are lost.

  As described above, the molybdenum oxide element has excellent characteristics. However, the driving of the organic EL element in the large current region as described above is not yet general, and at first glance, it is practical. The meaning does not seem great. However, when used as a light source for a large-sized display device such as a display device used outdoors, a high-brightness one is required. In addition, it is a fact that deterioration gradually proceeds even under a low-brightness mild driving condition such as that used in a small display device, and it has been revealed that this is promoted by heat. In other words, the deterioration of the organic EL element is governed by the reaction kinetics based on the activation energy as well as many other chemical phenomena. This means that deterioration under mild driving conditions is nothing but a slow observation of a phenomenon that proceeds under severe conditions. Therefore, like the molybdenum oxide element shown in the present embodiment, an element that is stable and superior to the PEDOT element even under severe conditions of high current density has excellent characteristics even under mild driving conditions. It is clear.

  It is clear that the molybdenum oxide element greatly exceeds the PEDOT element in its life characteristics. In addition, the time change of the applied voltage, which is an index of the state change of the element, is slower than that of the PEDOT element, and it can be said that the stability of the element is well represented. The relationship between the two is not reversed even under milder conditions as required for displays and the like.

Next, a second embodiment of the present invention will be described.
In the first embodiment, a dendrimer having Ir (ppy) ₃ as the core is used as the light emitting layer. In this example, a dendritic multi-branch structure having diphenylanthracene as the core is used as the light emitting layer 4.
Other structures were the same as in Example 1.
In this case, it is possible to form an organic electroluminescent element with higher emission intensity than in the case of Example 1, which is effective as an outdoor display device.

  In Example 1 and Example 2, a glass substrate was used as the substrate 1, but the substrate is not limited to glass. Generally, glass is used. In this embodiment, a glass substrate is also used. As the substrate material, many materials such as plastic films in addition to glass have been proposed, and any of them can be used as the substrate 1 in the present invention. Further, if the light emission direction is the surface direction opposite to the substrate, a light-shielding substrate such as a ceramic substrate or a metal substrate may be used.

Examples of the substrate 1 include inorganic oxides such as translucent or semi-translucent soda lime glass, barium / strontium-containing glass, lead glass, aluminosilicate glass, borosilicate glass, barium borosilicate glass, and quartz glass. Inorganic glass such as glass and inorganic fluoride glass, or translucent or translucent polyethylene terephthalate, polycarbonate, polymethyl methacrylate, polyether sulfone, polyvinyl fluoride, polypropylene, polyethylene, polyacrylate, amorphous Polymer films such as polyolefin and fluororesin, or chalcogenoid glasses such as translucent or semi-translucent As ₂ S ₃ , As ₄₀ S ₁₀ , S ₄₀ Ge ₁₀ , ZnO, Nb ₂ O, _{ta 2 O 5, SiO, Si} 3 N 4, HfO 2, TiO 2 , etc. of a metal oxide Insulating treatment is applied to materials such as nitrides, semiconductor materials such as light-shielding silicon, germanium, silicon carbide, gallium arsenide, and gallium nitride, or the above-described translucent substrate materials containing pigments, etc. It is possible to use an appropriate metal material selected from the above-described metal materials and the like, and it is also possible to use a laminated substrate in which a plurality of substrate materials are laminated.

As the anode 2, an electrode made of ITO is used. ITO is often used as an electrode on the light extraction side because it has high conductivity and good light transmission. The ITO in the above embodiment is formed on a substrate by sputtering and then appropriately patterned using a photolithography technique. Further, in the case where light is not extracted to the anode side, it can be made of a light shielding metal material. Electrode materials include transparent conductive films such as ITO, tin oxide (SnO ₂ ), and zinc oxide (ZnO), as well as chromium (Cr), nickel (Ni), copper (Cu), tin (Sn
), Tungsten (W), gold (Au), or other metals having a high work function, alloys thereof, oxides, or the like can be used. In addition, since a stable and highly reliable charge injection layer is used, the electrode may be made of a material having a low resistance and necessary physical properties, and the degree of freedom of selection is high. Thereby, deterioration of the electrode itself can be prevented.

In the embodiment, the molybdenum oxide thin film functioning as the charge injection layer 3 is formed by vacuum deposition on the substrate 1 patterned with ITO as the anode 2. In the above embodiment, the molybdenum oxide thin film has a thickness of 20 nm. The film thickness of the molybdenum oxide thin film is not particularly limited, but an extremely thin film is also effective if uniform film quality can be obtained. In general, an extremely thin film or a thick film is often difficult to obtain a uniform film quality. In addition, the molybdenum oxide thin film is slightly colored, so if it is too thick, the light extraction efficiency decreases. However, depending on the application, some coloring may have the effect of increasing the contrast ratio of light emission / non-light emission, which is advantageous. There is also. The range of the thickness of the molybdenum oxide thin film is generally in the range of 1 nm to 200 nm for the practice of the present invention.

As described above, the charge injection layer 3 made of an inorganic material shown in FIG. 1 is made of vanadium (V), copper (Cu), nickel (Ni), ruthenium (Ru), titanium (Ti) in addition to molybdenum oxide. Many transition metal oxides such as zirconium (Zr), tungsten (W), yttrium (Y), and lanthanum (La) exhibit similar characteristics. It is also effective in many transition metal nitrides including the above metals, although the injection characteristics are somewhat inferior.

  In order to obtain a good electron blocking function, it is desirable to use a buffer layer B having an electron affinity smaller than that of the light emitting layer in the present invention. As an example, poly [9,98-dioctylfluorene-co-N- (4-butylphenyl) -diphenylamine)] (poly [9,98-dioctylfluorene-co-N- (4-butylphenyl) -diphenylamine]) Examples include fluorene compounds, but the buffer layer material is limited by the light emitting layer material. The buffer layer used in this example has a sufficiently small electron affinity for the light emitting material and satisfies the requirements. In this embodiment, the buffer layer has a thickness of 10 nm and is stacked on the molybdenum oxide thin film by a spin coating method. The buffer layer thin film can serve as an electronic block at 10 nm. Increasing the thickness of the buffer layer thin film results in an increase in the driving voltage of the device, so that it is not preferable to be too thick. A thickness of about 50 nm or less is desirable. The buffer layer may be made of a material having not only an electronic blocking function but also improved adhesion or an energy ranking between the charge injection layer and the light emitting layer. Furthermore, when the charge injection layer is formed by a dry process such as vacuum deposition as well as the one formed by spin coating, the buffer layer may also be made of an inorganic material and sequentially formed into a film. .

  Furthermore, it is also possible to use a material generally called a plastic that does not have special electrical characteristics such as a polymer organic EL material, that is, a polystyrene compound, a polycarbonate compound, an acrylic compound, or the like. It is. These materials have high insulating properties, but high insulating properties mean that the band gap is large, and generally the electron affinity of these plastics is small. Of course, since the insulating property is high, increasing the film thickness increases the drive voltage of the element, which is not desirable, but the present invention can be realized by setting the film thickness appropriately.

As described above, the buffer layer B may be an inorganic substance in addition to the polymer compound, and may not have an electronic block function as in the above embodiment. For example, a material that can improve the adhesion between the light emitting layer and the charge injection layer, or a material that can lower the hole injection barrier to the extent that electron leakage can be suppressed may be used. If the dendritic structure of the light emitting layer is a material with high electron transport properties such as polyfluorene, it is essential to have a buffer layer with an electron blocking function, but it is a material with low electron transport properties such as PPV. In some cases, the electronic block function may be omitted.

  Further, the buffer layer B needs to be appropriately selected according to the polymer organic EL material constituting the light emitting layer 4 as described above. In the said Example, since the requirements of the buffer layer B are having an electron affinity smaller than the electron affinity of the light emitting layer 4, it is also possible to use the compound which can be utilized originally as a light emitting layer, for example. Accordingly, many substances can be selected, including compounds such as those shown in Formula (I) and Formula (II) and their derivatives.

In Examples 1 and 2, as the light emitting layer 4, a dendrimer formed by spin coating was used, and the film thickness was about 80 nm. The film thickness of the light emitting layer 4 should be appropriately changed according to the use conditions of the element. A film thickness in the range of 50 to 200 nm is suitable for the practice of the present invention.
As in the case of other thin films, it is difficult to obtain a uniform film if the film thickness is too thin. On the other hand, if the film thickness is large, the voltage required for driving becomes too high.

  The light emitting layer 4 is not only a dendrimer, but also a dendrimer, a polyfluorene compound, a PPV compound and derivatives thereof, or a so-called pendant type polymer in which a low molecular weight light emitting material is chemically bonded to a polymer skeleton. A compound, a high-molecular organic EL material, a mixture with a low-molecular organic EL material, or a blend of them can be used as appropriate.

  The structures of polyfluorene and PPV are shown in chemical formula (I) and chemical formula (II), respectively, and a large number of derivatives having these as the basic skeleton have been proposed. For example, examples of PPV and polyfluorene derivatives and detailed explanations are given in (WO9813408) or (WO0228983). Any of these compound groups is a substance group called a conjugated polymer compound, and can be used as a light-emitting layer in combination with an appropriate buffer material, thereby realizing the effects of the present invention.

  Here, when a material having a high hole transport property is used as the light emitting layer, the light emission efficiency can be further increased by disposing a buffer layer having a hole blocking function on the cathode side.

In addition, since the light emitting layer is made of a polymer material (polymer material) containing a dendrimer, a film can be formed with a uniform film thickness even in a large area, so that a large area organic EL element can be produced.
In addition, the stability of the light-emitting layer to heat is increased, and defects and pinholes at the interface between layers can be suppressed, so that a highly stable organic EL element can be formed.

  When these functional layers (light emitting layer, or hole injection layer or electron injection layer formed as necessary) are formed of a polymer material, spin coating, casting, dipping, bar A wet film forming method such as a code method or a roll coating method may be used. This eliminates the need for a large-scale vacuum apparatus, so that it is possible to form a film with inexpensive equipment, and it is possible to easily create a large-area organic EL element and to adhere between each layer of the organic EL element. Therefore, short circuit in the element can be suppressed, and a highly stable organic EL element can be formed.

  In addition, when used in a color display device or the like, it is necessary to separately coat the light emitting layer that realizes light emission of each color of RGB. However, by using an ink jet method or the like, it is possible to easily realize the coloring.

Further, as the cathode 5 of the organic electroluminescence element, a metal or alloy having a low work function is used. In addition to a Ca—Al two-layer structure, a Ba—Al two-layer structure, or Ca, Ba, In, Mg, Metals such as Ti, Mg alloys such as Mg-Ag alloy and Mg-In alloy, Al alloys such as Al-Li alloy, Al-Sr alloy and Al-Ba alloy are used. Alternatively, a laminated structure such as LiO ₂ / Al or LiF / Al is suitable as the cathode material.

A transparent cathode can also be formed by forming an ultrathin film with high light transmittance using a metal having a small work function and laminating a transparent electrode on the ultrathin film. By using this transparent cathode, an element configuration called a so-called top emission can be taken.

  Thus, the polymer organic EL device having the inorganic compound as the charge injection layer and the polymer material as the buffer layer is maintained at a high level of light emission intensity and light emission efficiency of the device over a wide range of current density, In addition, it exhibits good life characteristics. Therefore, it is possible to realize an organic electroluminescent element that operates stably over a wide range of luminance and has excellent lifetime characteristics.

(Embodiment 2)
The simple matrix display device has been described according to the first embodiment of the present invention.
The display device using the organic EL element of the present invention can also be applied to an active matrix display device in which a drive circuit is formed for each pixel. FIG. 6 is an equivalent circuit diagram of the active matrix display device, FIG. 7 is a layout explanatory diagram, and FIG. 8 is a perspective view.

  As shown in the equivalent circuit diagram of FIG. 6 and the layout explanatory diagram of FIG. 7, this display device includes an organic EL element (EL) forming a pixel, two TFTs (T1, T2), and a drive circuit including a capacitor C. A plurality of electrodes are arranged vertically and horizontally, and the gate electrodes of the first TFTs (T1) of the respective drive circuits arranged in the left-right direction are connected to the scanning lines to give scanning signals. A drain electrode of one TFT is connected to a data line, and a light emission signal is supplied. A driving power supply (not shown) is connected to one end of the EL element (EL), and one end of the capacitor C is grounded.

FIG. 8 is a cutaway perspective view of the main part of the display device. On the glass substrate 11 on which a driving thin film transistor (not shown) is formed, an anode (ITO) 12, a charge injection layer 13, a buffer layer B, A light emitting layer 14 and a cathode 15 are formed to form an organic EL element. As a structure, the anode and the charge injection layer are individually formed, the light emitting layer is integrally formed from the buffer layer, and the cathode is formed in a stripe shape. In this driving thin film transistor, for example, an organic semiconductor layer (polymer layer) is formed on a glass substrate 11, and this is covered with a gate insulating film, and a gate electrode is formed on the organic semiconductor layer (polymer layer). Source / drain electrodes are formed through holes. Then, an insulating layer (flat layer) is formed by applying a polyimide film or the like thereon, and an anode (ITO) 12, an organic semiconductor layer 14 and a cathode 15 are formed thereon to form an organic EL element. Have. In FIG. 11, although capacitors and wirings are omitted, they are also formed on the same glass substrate. A plurality of pixels including such TFTs and organic EL elements are formed in a matrix on the same substrate to constitute an active matrix display device.

  According to this configuration, a display device that can be driven at high speed and has high reliability can be provided.

  Note that the thin film transistor may be an organic transistor. A structure in which an organic EL element is stacked on a thin film transistor or a structure in which a thin film transistor is stacked on an organic EL element is also effective.

  In addition, in order to obtain a high-quality EL display device, the EL substrate electrode and the TFT substrate electrode are connected to an EL substrate on which an organic EL element is formed and a TFT substrate on which a TFT, capacitor, wiring, etc. are formed. You may make it stick together so that it may connect using a bank.

(Embodiment 3)
FIG. 9 is a cross-sectional view showing the main part of the organic EL element used in the display device according to Embodiment 3 of the present invention. In the present embodiment, the device configuration of the display device is the same as that used in the first embodiment.

  The illustrated organic EL element is different from the first embodiment in that it does not have the buffer layer B.

  In the above description, the organic EL element is driven by DC, but may be driven by AC voltage, AC current, or pulse wave.

  The light emitted from the organic EL element is taken out from the substrate side, but may be taken out from the side opposite to the substrate (here, the cathode) or from the side.

The film formation of each layer constituting the organic electroluminescent device of the present invention is not limited to the above method, but vacuum deposition method, electron beam deposition method, molecular beam epitaxy method, sputtering method, reactive sputtering. Method, ion plating method, laser ablation method, thermal CVD method, plasma CVD method, MOCVD method and other vacuum methods, or sol-gel method, Langmuir-Blodget method (LB method), layer-by-layer method, spin coating method, inkjet Needless to say, any method may be used as long as it can be appropriately selected from a wet method such as a method, a dip coating method, and a spray method, and can be formed so that the effect of the present invention can be obtained as a result. Absent.

  A display device using an organic EL element according to the present invention can increase brightness, operate stably over a wide range of brightness, and has excellent lifetime characteristics, and thus includes a wide range including flat panel displays and display elements. Useful in application.

It is a perspective view which shows the display apparatus in Embodiment 1 of this invention. It is principal part sectional drawing of the display apparatus. It is a figure which shows the relationship between the applied voltage with respect to the molybdenum oxide element in Example 1 of this invention, and the current density which flows at that time. It is a figure which shows the relationship between the current density in Example 1 of this invention, and emitted light intensity. It is a figure which shows the time-dependent change of the brightness | luminance at the time of the constant current drive of the molybdenum oxide element in Example 1 of this invention, and an applied voltage. It is a perspective view which shows the display apparatus in Embodiment 2 of this invention. FIG. 2 is a layout diagram of the display device. It is a principal part fracture perspective view of the display apparatus. It is sectional drawing which shows the organic EL element of the display apparatus in Embodiment 3 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Anode 3 Charge injection layer B Buffer layer 4 Light emitting layer 5 Cathode

Claims (25)

A pair of electrodes at least one of which is electrically divided and arranged to have opposing regions;
A plurality of functional layers formed between the electrodes constituting the electrode pair,
The functional layer includes a layer having a light emitting function made of at least one polymer substance having a tree-like multi-branched structure, and a charge injection layer made of at least one kind of inorganic substance,
A display device in which organic electroluminescent elements having the opposing regions as unit pixels form an image display array. The display device according to claim 1,
The layer having a light emitting function is a display device having a tree-like multi-branched polymer structure centered on a light emitting structural unit. The display device according to claim 1,
The layer having a light emitting function is a display device having a dendritic multi-branched low molecular structure having a light emitting structural unit as a center. The display device according to any one of claims 1 to 3,
The display device is configured such that the electrodes are formed of stripe-like electrodes formed at a predetermined interval, and an image display array is formed by scanning through at least one switching element. The display device according to claim 4,
Each of the electrodes constituting the electrode pair is a display device comprising a striped anode arranged at a predetermined interval and a striped cathode arranged in a direction perpendicular to the stripe of the anode. The display device according to any one of claims 1 to 3,
Any one of the electrodes constituting the electrode pair is electrically separated for each pixel, and the separated electrodes are scanned through at least one switching element to form an image display array. Display device. The display device according to any one of claims 1 to 6,
The display device, wherein the functional layer includes at least one buffer layer. The display device according to claim 7,
A display device in which the buffer layer is formed of a polymer layer. The display device according to claim 8,
A display device in which the buffer layer contains an organic solvent. A display device according to any one of claims 1 to 9,
The display device in which the layer having a light emitting function includes a polymer compound including a fluorene ring. The display device according to claim 10,
The display device, wherein the layer having a light emitting function includes polyfluorene represented by the following general formula (I) and derivatives thereof (R1 and R2 each represent a substituent).

A display device according to any one of claims 1 to 10,
The display device, wherein the layer having a light emitting function includes a phenylene vinylene group. A display device according to claim 12,
The display device, wherein the layer having a light emitting function includes polyphenylene vinylene represented by the following general formula (II) and a derivative thereof (R3 and R4 each represent a substituent).

The display device according to any one of claims 7 to 11,
A display device in which an absolute value of an energy value representing an electron affinity of the buffer layer is smaller than an absolute value of an energy value representing an electron affinity of the layer having the light emitting function. The display device according to claim 1,
A display device in which the charge injection layer contains an oxide. A display device according to claim 15,
The display device, wherein the charge injection layer includes an oxide of a transition metal. The display device according to claim 16, wherein
The display device, wherein the charge injection layer contains molybdenum or vanadium oxide. The display device according to claim 1,
A display device in which the charge injection layer contains nitride. The display device according to claim 18, wherein
The display device, wherein the charge injection layer includes a transition metal nitride. The display device according to claim 1,
The display device, wherein the charge injection layer includes oxynitride. The display device according to claim 20, wherein
The display device, wherein the charge injection layer includes a transition metal oxynitride. The display device according to claim 1,
The display device, wherein the charge injection layer includes a composite oxide containing a transition metal. The display device according to any one of claims 7 to 22,
The buffer layer is a display device disposed between a charge injection layer disposed on a hole injection side and a layer having a light emitting function. A display device according to any one of claims 1 to 23,
An anode formed on a translucent substrate;
A hole injection layer formed on the anode;
A buffer layer formed on the hole injection layer;
A layer having a light emitting function formed on the buffer layer; an electron injection layer formed on the layer having the light emitting function;
A display device including a cathode formed on the electron injection layer. A method of manufacturing a display device according to any one of claims 1 to 24,
The layer having the light emitting function of the organic electroluminescent element is a method for manufacturing a display device formed by a wet method.

Images