Patent Application
20080129195
This application claims priority to Japanese application number 2006-326742, filed on Dec. 4, 2006, which is incorporated by reference.
1. Field of the Invention
The present invention relates to an organic EL display, which is one kind of flat-panel displays.
2. Description of the Related Art
As flat-panel displays, liquid crystal displays and plasma displays are commercialized. However, liquid crystal displays have such problems as a narrow viewing angle and insufficient response to high-speed pixel signals, and plasma displays have such problems as a large amount of power consumption and many technical subjects to be solved for increasing to a larger size than current one.
What is drawing attention recently in contrast to these is an organic electroluminescence display (organic EL display) that uses organic light-emitting materials. By using organic compounds as light-emitting materials, it is expected as a display that can realize a flat-panel display that is self light-emitting, high in response speed, and further free from viewing angle dependency with low power consumption.
Organic EL displays include a simple matrix type one that does not use a thin film transistor (TFT), and an active matrix type one that uses TFT. The simple matrix type display is composed of parallel plural electrodes, an organic EL layer, and plural electrodes perpendicular to the parallel plural electrodes, which are formed on a glass substrate. In a simple matrix type display, the organic EL layer is required to perform very high-brightness emission in a selected moment, thus there are such problems that the organic EL layer degrades severely and wiring resistance results in large voltage drop. On the other hand, in an active matrix type display, on a glass substrate, a TFT circuit is formed, on which an organic EL layer exists, and further a counter electrode is formed. Usually, for a semiconductor of the TFT circuit, amorphous silicon or polysilicon is used. In this case, generally, the counter electrode is used as a cathode to take out light from the substrate side.
However, in active matrix type organic EL displays based on selective coating of R, G and B light-emitting layers, there is such a problem that color purity is insufficient, that is, green is whitish and red is yellowish. In addition, there is such a problem that a “white EL+color filter” system and a “blue EL+color conversion layer” system can be applied only to simple matrix type displays. For the purpose of solving these problems, there is such a method that color filters are formed on TFT on a substrate, on which an organic EL layer is formed (JP-A-2001-249627, JP-A-2002-184575, JP-A-2004-207065, JP-A-2004-303609). However, in the method, it is necessary to connect the TFT and the organic EL by means of a via hole, to result in a complex construction and low reliability.
In addition, since the amount of light that can be taken out from the substrate side is limited to that from a portion given by subtracting the area of nontransparent TFT, an aperture ratio (=light-emitting area/pixel area) is small, and then light with high brightness must be emitted (although not so small for simple matrix types).
A purpose of the present invention is to create active matrix type organic EL displays with a simple structure having color filters, and to provide organic EL displays of high quality having good color purity. In addition, another purpose thereof is to improve the aperture ratio of active matrix type organic EL displays, thereby reducing load imposed on the organic EL layer. One aspect of the invention is a color EL display characterized in that at least color filters, a thin film transistor circuit, an organic EL layer, and a common electrode are built in this order on a transparent substrate.
a)-(d) are cross-sectional and top views showing an example of the method for producing a color EL display of the invention.
e)-(h) are cross-sectional and top views showing an example of the method for producing a color EL display of the invention.
i)-(l) are cross-sectional and top views showing an example of the method for producing a color EL display of the invention.
a)-(d) are cross-sectional and top views showing another example of the method for producing a color EL display of the invention.
e)-(h) are cross-sectional and top views showing another example of the method for producing a color EL display of the invention.
i)-(l) are cross-sectional and top views showing another example of the method for producing a color EL display of the invention.
1: transparent substrate, 2: color filters, 2R: red transmissive color filter, 2G: green transmissive color filter, 2B: blue transmissive color filter, 2W: white transmissive color filter, 2RR: red conversion type color filter, 2GG: green conversion type color filter, 2OC: over coating, 3: TFT circuit, 31: first electrode layer, 32: first insulating layer, 33: first semiconductor layer, 34: second electrode layer, 35: second insulating layer, 36: second semiconductor layer, 37: third electrode layer, 38: third insulating layer, 39: fourth electrode layer, G1 to G3: gate electrode, S1 to S3: source electrode, D1 to D3: drain electrode, C: capacitor electrode, 4: organic EL layer, 41: hole transport layer, 42: light-emitting layer, 42R: red light-emitting layer, 42G: green light-emitting layer, 42B: blue light-emitting layer, 42W: white light-emitting layer, 5: common electrode, 6: seal, 101: ink-replenishing apparatus, 102: doctor apparatus, 103: inking apparatus, 104: flexoplate, 105: cylinder, 106: body to be transferred, 107: stage, 108: ink, 108a: inkpattern
Hereinafter, embodiments of the present invention are described on the basis of attached drawings. In the invention, color filters 2, a TFT circuit 3, an organic EL layer 4, and a common electrode 5 are built in this order on a transparent substrate 1, as shown in
As the transparent substrate 1, glass may be employed, and, as described later, plastic may also be employed. For example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethersulfone (PES), polyimide (PI) and the like may be employed. When the transparent substrate 1 is made of plastic, a barrier film formed of SiO2, SiON, Al2O3, Y2O3 or the like is desirably provided thereon to prevent the permeation of moisture and oxygen.
The color filters 2 include at least R, G and B. In addition to R, G and B, as shown in
When the organic EL layer 4 is blue, it is sufficient in the color filters 2, that R and G are of a color conversion type, and B is of a transmitting band-restricting type (
The TFT circuit 3 has at least two TFTs and one or more capacitors, for example, as shown in
In the invention, as a semiconductor layer of TFT, an oxide semiconductor is used. Since an oxide semiconductor is transparent, the use of a transparent material for the electrode and insulating layer enables a substantially transparent TFT to be formed, to prevent degradation of an aperture ratio (here, the phrase “substantially transparent” means that the transmittance is 70% or greater in the wavelength range from 400 nm to 700 nm, which constitute visible light). Further, for forming a conventional amorphous silicon film or polysilicon film, a higher temperature process than 200° C. is necessary. In contrast, many oxide semiconductors operate well even when they are formed into a film at a low temperature such as from room temperature to 200° C., and all the other subsequent processes (photolithography, formation of the organic EL layer 4 and common electrode 5) can be performed at 200° C. or less, thus there is no danger of damaging the color filters 2 (which are usually formed at a temperature of slightly higher than 200° C.). Furthermore, when the formation of the color filters 2 is performed at 200° C. or less, it becomes possible to use a plastic film as the substrate 1, and also possible to produce flexible EL displays.
In addition, conventionally the semiconductor layer of all TFTs of the TFT circuit 3 was formed in a single plane. This is due to high cost for forming amorphous silicon and polysilicon. However, by using an oxide semiconductor, an inexpensive sputtering method can be employed and two layers or more of oxide semiconductors can be utilized. On this occasion, it is possible to use oxide semiconductors having different conditions for respective layers, to result in a larger degree of freedom in circuit design. For example, in
For the oxide semiconductor, an oxide including at least anyone kind of In, Ga, Zn, Sn and Mg can be employed. Specifically, there can be mentioned indium oxide, zinc oxide, tin oxide, ZnMg oxide, InGaZn oxide, InxZn1-x oxide, InxSn1-x oxide, Inx(Zn,Sn)1-x oxide, GaSn oxide, InGaSn oxide, InGaZnMg oxide and the like. These can be formed into a film by sputtering, laser ablation, vacuum evaporation and the like. In particular, it is possible to obtain easily with sufficient reproducibility InGaZn oxide having a mobility of 5 cm2/Vs or greater even when it is subjected to sputtering film formation at any temperature from room temperature to 200° C., and is a suitable raw material. Further, InGaZnMg oxide has such characteristics as having a mobility equivalent to that of InGaZn oxide, and further being resistant to ultraviolet rays (hardly malfunctions) since it has a large band gap. Here, InGaZn oxide has a composition ratio close to In:Ga:Zn:O=1:1:1:4. However, actually there exist some amount of oxygen vacancies, and properties of the oxide do not change even when some amount of deviation of metal composition exists. Therefore, allowable composition ratios are In:Ga:Zn:O=(0.7 to 1.3):(0.7 to 1.3):(0.7 to 1.3):(3 to 4). Basically it is in an amorphous state, but may partially contain a microcrystalline structure. InGaZnMg oxide is formed by substituting a part (50% or less) of Zn in InGaZn oxide with Mg. For sputtering, RF or DC reactive sputtering is suitable.
In addition, high molecular weight organic semiconductor materials such as polythiophene, polyallylamine, fluorene-bithiophene copolymer, and derivatives thereof, and low molecular weight organic semiconductor materials such as pentacene, tetracene, cupper phthalocyanine, perylene, and derivatives thereof can be employed. In view of lowering cost, imparting flexibility and enlarging area, the use of organic semiconductors, to which a printing method can be applied, is desirable. Further, carbon compounds such as carbon nanotube and fullerene, and a semiconductor nano-particle dispersion liquid can also be employed as a semiconductor material. As a printing method of an organic semiconductor, publicly known methods such as gravure printing, offset printing, screen printing and ink jet method can be employed. The above-described organic semiconductor has, generally, low solubility in a solvent, therefore the use of flexographic printing, reverse offset printing, ink jet method, or dispenser, which are suitable for printing a low viscosity solution, is desirable.
For the electrode, indium tin oxide (ITO), indium zinc oxide (IZO) and the like are suitably used. For the insulating layer, SiO2, SiON, Al2O3, Y2O3 and the like are suitably used. These can also be formed into a film at a temperature from room temperature to 200° C. by sputtering, laser ablation, vacuum evaporation and the like. In particular, reactive sputtering is suitable. Post-annealing may also be performed. Temperature for post-annealing may also be 200° C. or less. Or, the use of a transparent organic insulating layer is also possible. For example, fluorine-containing resin, polyvinyl alcohol, epoxy resin, acrylic resin and the like can be used. The use of a photosensitive resin makes patterning easy. Furthermore, insulating layers of a heterogeneous type may be overlapped.
When a transparent material is used for all of the electrode, semiconductor and insulating layer, as described above, the resulting whole TFT circuit becomes transparent, thereby making it possible to enhance the aperture ratio. Incidentally, for the patterning of an electrode, semiconductor and insulating layer, photolithography+etching are used, wherein the photolithographic process is usually performed at 120° C. or less, and etching is also performed at several tens degree centigrade or less.
Next, on a pixel electrode (a portion where either a source electrode or drain electrode of a driving TFT is formed in a large area, or an electrode having a large area connected to either a source electrode or drain electrode, which is to be contacted with the organic EL layer 4), the organic EL layer 4 is formed. As the organic EL layer 4, usually, a multilayer structure of a hole transport layer 41, light-emitting layer 42 and the like is used.
As a material to form the hole transport layer 41, there can be mentioned electroconductive polymer materials such as polyaniline derivatives, polythiophene derivatives, polyvinyl carbazole derivatives, a mixture of poly(3,4-ethylenedioxythiophene) and polystyrene sulfonic acid (PEDOT:PSS), and the like. These hole-transporting materials can be dissolved or dispersed in a single or mixed solvent of toluene, xylene, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, methanol, ethanol, isopropyl alcohol, ethyl acetate, butyl acetate, water and the like, to be coated by such a coating method as spin coating, bar coating, wire coating or slit coating. Patterning may be performed according to need. Further, to the hole transport layer 41, a surfactant, oxidation inhibitor, viscosity modifier, ultraviolet absorber and/or the like may be added according to need. The thickness of the hole transport layer 41 is preferably in the range from 10 nm to 200 nm. Or, low molecular weight materials such as TPD (triphenyl diamine) and α-NPD(bis[N-naphthyl-N-phenyl]benzidine) may be employed.
When employing an inorganic material as a hole-transporting material, an inorganic material such as Cu2O, Cr2O3, Mn2O3, FeOx (x: around 0.1), NiO, CoO, Pr2O3, Ag2O, MoO2, Bi2O3, ZnO, TiO2, SnO2, ThO2, V2O5, Nb2O5, Ta2O5, MoO3, WO3 and MnO2 may be formed into a film using a vacuum evaporation method or a sputtering method. A mask may be used according to need. Further, patterning may be performed after film formation according to need.
Next, on the hole transport layer 41, the light-emitting layer 42 is built (laminated). The light-emitting layer 42 may have not only a monolayer structure, but also a multilayer structure in which a charge transport layer and the like are additionally provided. As the light-emitting layer 42, for example, there may be employed an organic light-emitting material that is soluble in an organic solvent, such as coumarin-based, perylene-based, pyran-based, anthrone-based, porphyrin-based, quinacridone-based, N,N′-dialkyl-substituted quinacridone-based, naphthalimide-based and N,N′-diaryl-substituted pyrrolopyrrole-based materials, and iridium complexes; a dispersion formed by dispersing the organic light-emitting material in a polymer such as polystyrene, polymethyl methacrylate and polyvinyl carbazole; and polymer fluorescent materials such as polyarylene-based, polyarylene vinylene-based and polyfluorene-based materials. These polymer fluorescent materials can be dissolved in a single or mixed solvent of toluene, xylene, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, methanol, ethanol, isopropyl alcohol, ethyl acetate, butyl acetate, water and the like, to be coated by such a coating method as a spin coating method, curtain coating method, bar coating method, wire coating method and slit coating method. In addition, a printing method can also be employed (described later). To a polymer fluorescent material layer, a surfactant, oxidation inhibitor, viscosity modifier, ultraviolet absorber and/or the like may be added, according to need. In each case of a mono- or multi-layer structure, the total thickness of the light-emitting layer 42 is preferably 1000 nm or less, more preferably in the range of from 50 nm to 150 nm. Alternatively, low molecular weight fluorescent materials prepared by doping quinacridone, a coumarin derivative, rubrene, a DCM(4-(Dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4 H-pyran) derivative, perylene, an iridium complex and/or the like to an aluminum-quinoline complex, distyryl derivative or the like can be employed.
For low molecular weight fluorescent materials, color to be emitted is determined by the material itself and/or a dopant. There are employed materials prepared by doping a styrylarylene derivative or styrylamine derivative to a distylarylene derivative for blue emitting light; an aluminum-quinoline complex and the like for green emitting light; amaterial prepared by doping DCM to an aluminum-quinoline complex, and the like for red emitting light; and a multilayer structure of a blue light-emitting material and a yellow to orange light-emitting material, and the like for white emitting light. On the other hand, for polymer fluorescent materials, the color to be emitted can be adjusted by the replacement of a side chain, and a polymer having the same basic skeleton can be used for R, G and B emitting light. By mixing these, white emitting light can be obtained.
When an organic EL layer is formed by an R, G and B selective coating system, it is performed through masked vacuum evaporation in the case of a low molecular light-emitting layer, but performing uniform selective coating for a large area is difficult. In the case of a high molecular light-emitting layer, a printing method can be used, to enable uniform selective coating for a large area to be performed. As the printing method, an ink jet, reverse printing and flexographic printing methods, and the like can be employed. In particular, a flexographic printing method can perform uniform printing for a large area in a short period of time, and is most preferable (described later). Incidentally, for masked vacuum evaporation, and also for printing methods such as ink jet, reverse printing and flexographic printing, room temperature is usable as substrate temperature.
As the common electrode 5, one corresponding to light-emitting properties of an organic EL layer can be used, including, for example, single metal of lithium, magnesium, calcium, ytterbium or aluminum, or an alloy thereof, or an alloy of these and a stable metal such as gold or silver. These materials can be provided by a vacuum evaporation method using common resistance heating, EB heating or the like. The thickness thereof is not particularly limited, but it is preferably in the range from 1 nm to 500 nm. A thin film of lithium fluoride or the like may be arranged between a cathode layer and a light-emitting layer. Further, on the cathode layer, a protective layer formed of an insulating inorganic material or resin may be provided. In these processes also, room temperature suffices as substrate temperature.
Next, a method for producing a color EL display of the invention is described in detail.
Next, as a first electrode layer 31 including a gate electrode G1 and capacitor electrode C, ITO is formed into a film by reactive sputtering or the like, which is patterned through photoresist coating, exposure, development, etching, resist strip and the like (
Then, the organic EL layer 4 is formed. Firstly, a hole transport layer 41 is coated on the whole surface (
Further, a common electrode 5 is formed on the whole surface by vacuum evaporation (
The aforementioned process of forming a TFT circuit and all the subsequent processes (
Here, flexographic printing is described in detail.
After an excess ink has been removed with the doctor apparatus 102, inking is performed to a cylinder 105, at least the surface of which is constituted as a flexo plate 104 for producing an organic EL element. The inking to the flexo plate 104 for producing an organic EL element is sufficient if it is performed at least to an image-forming portion of the plate. An ink that has been inked to the flexo plate 104 for producing an organic EL element is printed to a body to be transferred 106. The body to be transferred 106 is a sample having the color filters 2 and TFT circuit 3 formed on a transparent substrate 1. An ink 108a containing an organic light-emitting material that has been printed on the body to be transferred 106 forms the light-emitting layer 42 of an organic EL element by drying.
As described above, an ink containing an organic light-emitting material is supplied to convex portions of the flexo plate 104, and then printed on the aforementioned substrate to be transferred 106. Examples of the solvent used for inks, which dissolves or disperses a light-emitting material, include a single or mixed solvent of toluene, xylene, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, methanol, ethanol, isopropyl alcohol, ethyl acetate, butyl acetate, chloroform, methylene chloride, dichloroethane, trichloroethane, tetrachloroethylene, water and the like. In particular, aromatic solvents and halogen-containing solvents excel in dissolving an organic light-emitting material. To an ink containing an organic light-emitting material, a surfactant, oxidation inhibitor, viscosity modifier, ultraviolet absorber and/or drying agent may be added, according to need.
Next, another example of the method for producing a color EL display of the invention is described in detail.
Next, ITO is formed into a film as the first electrode layer 31 containing the gate electrodes G1 and G2 by reactive sputtering or the like, which is patterned through photoresist coating, exposure, development, etching, resist strip, and the like (
Then, the organic EL layer 4 is formed. Firstly, the hole transport layer 41 is formed on the whole surface (
As a method for forming a light-emitting layer on the whole surface, a vacuum evaporation method, coating method and the like are mentioned, and, in order to form effectively a large area, a coating method can suitably used. As a coating method, a publicly known method such as a die coat method, gravure method, spin coating method, flexographic printing method, and offset printing method can suitably employed.
Furthermore, the common electrode 5 is formed on the whole surface by vacuum evaporation (
The aforementioned process of forming a TFT circuit and all the subsequent processes (
According to the invention, since TFT is formed on color filters, the TFT and an organic EL layer can easily be connected, and it is possible to result in a simple and highly reliable structure. Further, by using an oxide semiconductor as a semiconductor of TFT, and using a transparent electroconductive film as an electrode, it is possible to make a TFT circuit be transparent and/or have a multi-built structure, and to improve the aperture ratio.
Furthermore, according to the production method of the invention, color EL displays can be produced easily with high accuracy.
Hereinafter, specific embodiments of the present invention are described in detail while using Examples.
As a transparent substrate 1, glass was used, on which color filters 2 were formed through pigment-dispersed resist coating, exposure, development and burning (
Next, ITO was formed into a film as the first electrode layer 31 including the gate electrode G1 and capacitor electrode C, by reactive sputtering (room temperature, DC sputtering) under an Ar+O2 gas using ITO as a target, which was patterned through photoresist coating, exposure, development, wet etching with hydrochloric acid, and resist strip (
Then, the organic EL layer 4 was formed. Firstly, a PEDOT:PSS solution was spin-coated on the whole surface as the hole transport layer 41, which was burned at 100° C. (
Further each of calcium 10 nm and silver 300 nm was formed into a film on the whole surface as a common electrode 5 by vacuum evaporation (
The aforementioned process of forming the TFT circuit and all the subsequent processes (
As the transparent substrate 1, glass was used, on which color filters 2 were formed through pigment-dispersed resist coating, exposure, development, burning, fluorescent coloring material-containing resist coating, exposure, development and burning (
Next, ITO was formed into a film as a first electrode layer 31 including the gate electrodes G1 and G2 by reactive sputtering (room temperature, DC sputtering) under an Ar+O2 gas using ITO as a target, which was patterned through photoresist coating, exposure, development, wet etching with hydrochloric acid, and resist strip (
Then, the organic EL layer 4 was formed. Firstly, a PEDOT:PSS aqueous solution was spin-coated on the whole surface to form the hole transport layer 41, which was burned at 110° C. (
Further each of calcium 10 nm and silver 300 nm was formed into a film on the whole surface as a common electrode 5 by vacuum evaporation (
The aforementioned process of forming the TFT circuit and all the subsequent processes (
A color EL display was produced in the same process as in Example 2, except that, as the light-emitting layer 42, a polyolefin-based material was used as a blue light-emitting layer 42B in place of a white light-emitting layer 42W. The process of forming a TFT circuit and all the subsequent processes were performed at 200° C. or less. The color EL display thus produced could perform display having good color purity and brightness due to a large aperture ratio, when being viewed from the transparent substrate 1 side.
A color EL display was produced in the same process as in Example 2, except that a band-limiting type alone was used as the color filters 2. The process of forming a TFT circuit and all the subsequent processes were performed at 200° C. or less. The color EL display thus produced could perform display having good color purity and brightness due to a large aperture ratio, when being viewed from the transparent substrate 1 side.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-326742 | Dec 2006 | JP | national |
