Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B33/00—Electroluminescent light sources
  • H05B33/12—Light sources with substantially two-dimensional radiating surfaces
  • H05B33/26—Light sources with substantially two-dimensional radiating surfaces characterised by the composition or arrangement of the conductive material used as an electrode
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K50/00—Organic light-emitting devices
  • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
  • H10K50/11—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers
  • H10K50/125—OLEDs or polymer light-emitting diodes [PLED] characterised by the electroluminescent [EL] layers specially adapted for multicolour light emission, e.g. for emitting white light
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B33/00—Electroluminescent light sources
  • H05B33/12—Light sources with substantially two-dimensional radiating surfaces
  • H05B33/20—Light sources with substantially two-dimensional radiating surfaces characterised by the chemical or physical composition or the arrangement of the material in which the electroluminescent material is embedded
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B33/00—Electroluminescent light sources
  • H05B33/12—Light sources with substantially two-dimensional radiating surfaces
  • H05B33/22—Light sources with substantially two-dimensional radiating surfaces characterised by the chemical or physical composition or the arrangement of auxiliary dielectric or reflective layers
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K50/00—Organic light-emitting devices
  • H10K50/10—OLEDs or polymer light-emitting diodes [PLED]
  • H10K50/14—Carrier transporting layers
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K50/00—Organic light-emitting devices
  • H10K50/80—Constructional details
  • H10K50/85—Arrangements for extracting light from the devices
  • H10K50/852—Arrangements for extracting light from the devices comprising a resonant cavity structure, e.g. Bragg reflector pair
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
  • H10K59/00—Integrated devices, or assemblies of multiple devices, comprising at least one organic light-emitting element covered by group H10K50/00
  • H10K59/30—Devices specially adapted for multicolour light emission
  • H10K59/38—Devices specially adapted for multicolour light emission comprising colour filters or colour changing media [CCM]
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S428/00—Stock material or miscellaneous articles
  • Y10S428/917—Electroluminescent

Definitions

• the present invention relates to organic electroluminescent (EL) devices. More specifically, the present invention relates to an organic EL device with improved stability, efficiency, and color purity. BACKGROUND OF THE INVENTION
• Organic electroluminescent (EL) devices or organic light-emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. Tang et al. in Applied Physics Letters 51, p913, 1987; Journal of Applied Physics, 65, p3610, 1989; and commonly assigned U.S. Patent 4,769,292 demonstrated highly efficient OLEDs. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
• Color conversion OLED has been described by Tokailin et al. in commonly assigned U.S. Patent 5,126,214.
• the color conversion OLED device is provided with a color conversion layer that includes a fluorescent material responsive to the color of light emitted by the light- emitting layer and is capable of changing the wavelength of the light and thereby re-emitting different colored light. It can be particularly useful to form an OLED device that generates light of a single hue (e.g., blue light) and include color conversion layers that convert the generated light into light of one or more different hues (e.g., green, red) that will be perceived by a viewer. Thus, it is possible to construct a full color OLED device with a light-emitting layer that produces only a single hue of light.
• a single hue e.g., blue light
• color conversion layers that convert the generated light into light of one or more different hues (e.g., green, red) that will be perceived by a viewer.
• the OLED display is designed to emit blue light.
• the subpixels designed to emit light of other hues are provided with florescent materials that can absorb the blue light emitted from the OLED and re-emit the other desired colors.
• full color displays have many advantages, however, one of the key problems is that blue emitting OLEDs are generally the least stable and least efficient of OLED devices. There is therefore a need to provide stable OLED devices and, in particular, blue light-emitting OLED devices without color contamination. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an OLED device with improved stability and color quality.
• a stabilized OLED device for emitting light of a specific color, comprising: a) a metallic anode and a metallic cathode spaced from the anode; b) a light-emitting layer including a host and a dopant disposed between the anode and the cathode, the dopant being selected to produce light having a spectrum containing light of the specific color; c) a stabilizer provided in one of the device layers which improves the useful lifetime of the OLED device, wherein the stabilizer has an emission spectrum different from that of the light-emitting layer, and d) wherein one of the electrode layers is semitransparent and the other one is substantially opaque and reflective such that the stabilized OLED device forms a microcavity that emits a narrow band light with the specific color.
• the preferred material for the semitransparent electrode layer includes Ag or Au, or alloys thereof, and the material for the opaque and reflective electrode layer preferably includes Ag, Au, Al, Mg, or Ca, or alloys thereof.
• ADVANTAGES It is an advantage of the present invention that it provides a monocolor OLED device with improved stability and performance. It is a further advantage of the present invention that it permits the use in an OLED device of stabilizing materials that have emissions at unwanted wavelengths without such unwanted emissions affecting the desired emissions. It is a further advantage of the present invention that it provides a light source for a color-conversion type OLED display with improved efficiency and stability.
• FIG. 1 is a cross-sectional view of a pixel of an OLED device according to a first embodiment of the present invention, and also schematically showing the effect of light emission in a resulting microcavity;
• FIG. 2 shows the effect of a stabilizing dopant on the emission spectrum of a conventional (non-microcavity) OLED device;
• FIG. 3 shows the effect of stabilizing dopant on the stability of an OLED device;
• FIG. 4 shows the effect of a microcavity structure according to an embodiment of the present invention in eliminating the contaminating emission from a stabilizing dopant. Since device feature dimensions such as layer thicknesses are frequently in sub-micrometer ranges, the drawings are scaled for ease of visualization rather than dimensional accuracy.
• display or "display panel” is employed to designate a screen capable of electronically displaying video images or text.
• pixel is employed in its art-recognized usage to designate an area of a display panel that can be stimulated to emit light independently of other areas.
• OLED device is used in its art-recognized meaning of a display device comprising organic light-emitting diodes as pixels.
• a colored OLED device emits light of at least one color.
• multicolor is employed to describe a display panel that is capable of emitting light of a different hue in different areas. In particular, it is employed to describe a display panel that is capable of displaying images of different colors. These areas are not necessarily contiguous.
• full color is employed to describe multicolor display panels that are capable of emitting in the red, green, and blue regions of the visible spectrum and displaying images in any hue or combination of hues.
• the red, green, and blue colors constitute the three primary colors from which all other colors can be generated by appropriately mixing these three primaries.
• the term “hue” refers to the intensity profile of light emission within the visible spectrum, with different hues exhibiting visually discernible differences in color.
• the pixel or subpixel is generally used to designate the smallest addressable unit in a display panel.
• pixel or subpixel For a monochrome display, there is no distinction between pixel or subpixel.
• the term "subpixel" is used in multicolor display panels and is employed to designate any portion of a pixel, which can be independently addressable to emit light of a specific color.
• a blue subpixel is that portion of a pixel, which can be addressed to emit blue light.
• a pixel In a full-color display, a pixel generally comprises three primary-color subpixels, namely blue, green, and red.
• the terms "pixel” and “subpixel” will be used interchangeably.
• the term “pitch” is used to designate the distance separating two pixels or subpixels in a display panel. Thus, a subpixel pitch means the separation between two subpixels.
• microcavity OLED device is used to designate an OLED device that comprises an organic EL element disposed between two reflecting mirrors having over 30% reflectivity. In most cases one of the reflecting mirror is essentially opaque and the other one is semitransparent having an optical density less than 1.0.
• the light-emitting element can comprise one or more organic layers that emits light under applied voltage during the operation of the OLED device.
• the two reflecting mirrors form a Fabry-Perot microcavity that strongly affects the emission characteristics of the OLED device.
• Emission near the wavelength corresponding to the resonance wavelength of the cavity is enhanced and those with other wavelengths are suppressed.
• the net result is a significant narrowing of the bandwidth of the emitted light and a significant enhancement of its intensity.
• Most microcavity OLED devices in the prior art used a quarter- wave stack (QWS) as the semitransparent mirror.
• QWS quarter- wave stack
• the QWS is complicated in structure and expensive. It was discovered quite unexpectedly through extensive modeling and experimental efforts that high performance microcavity OLED devices that enhance the light emission output efficiency and color quality can actually be fabricated using all metallic mirrors. It was discovered that the material selection for both the reflective and the semitransparent metallic electrodes is important and the thickness of the semitransparent metallic electrode is also important.
• the reflective electrode Only a small number of metals, including Ag, Au, Al, Mg, or Ca, or alloys thereof, defined as alloys having at least 50 atomic percent of at least one of these metals, are preferably used as the reflective electrode. When other metals are used, the benefits of luminance output increase and color quality improvement due to microcavity effect are much reduced. Similarly, for the semitransparent electrode only a small number of materials including Ag or Au, or alloys thereof are preferably used. The thickness range of the semitransparent electrode is also limited. Too thin a layer does not provide a significant microcavity effect and too thick a layer reduces the luminance output. In addition, the location of the light-emitting layer within the microcavity also strongly affects the luminance output and needs to be optimized.
• microcavity OLED device with emission output efficiency and color quality significantly improved over that of corresponding non- cavity OLED devices be achieved. It was further discovered that an absorption- reducing layer disposed next to the light transmissive electrode layer outside the microcavity further improves the luminance performance of a microcavity device.
• Metallic mirrors are simpler in structure and easier to fabricate than a QWS. The use of two metallic mirrors that also function as electrodes eliminates the need for a separate transparent conductive electrode.
• the sheet conductivity of the semitransparent metallic electrode can be much higher than the transparent conductive electrodes used in the prior art. The increased conductivity reduces Ohmic loss in an OLED device, especially if the device area is large.
• the emission bandwidths using appropriately designed metallic mirrors are broader than those obtained using QWS and hence the luminance output is increased. On the other hand, the emission bandwidth is still narrow enough to provide excellent color selectivity and color quality, also referred to as chromaticity.
• Many approaches have been attempted to improve the operation stability of OLEDs. One of the most successful approaches is to dope one or more of the organic layers in the OLED structure with stabilizers.
• OLED devices with high luminance efficiency and operational stability can be obtained by doping yellow rubrene dopant, or super rubrene derivative dopants 6,1 l-diphenyl-5,12-bis(4-(6-methyl-benzofhiazol-2- yl)phenyl)naphthacene (DBzR), or 5,6,11,12-tetra(2-naphthyl)naphfhacene (NR) in the hole-transporting layer and distyrylamine derivatives blue dopant in the host light-emitting layer.
• the yellow emission from the doped hole-transporting layer and the blue emission from the host blue light-emitting layer combine to form the white emission from the OLED device.
• an OLED device with an added stabilizer is constructed to have a microcavity structure as described above.
• the microcavity structure is tuned to have a resonance wavelength corresponding to the specific color desired for the application.
• the wavelength-selecting and bandwidth-narrowing effect of the microcavity structure effectively suppresses the emission of the undesirable contaminating light from the stabilizing dopant and enables the emission of light of the desired specific color.
• the microcavity structure can further improve the emission efficiency of the OLED device at the desirable emission wavelength as well as the chromaticity of the emitted color. All these benefits are achieved while maintaining the stabilizing effect of the stabilizer.
• the present invention is thus effective in providing a stabilized OLED device for emitting light of a specific color, that is, a monochrome OLED device with improved stability, emission efficiency, and chromaticity.
• the present invention is also effective in providing improved color conversion OLED display devices.
• the light-emitting layer can be designed to emit, for example, blue light, and the color conversion layer can be provided to absorb the emitted blue light and re-emit light of a different color.
• a stabilizing dopant is added to improve the stability of the device.
• microcavity structure improves the emission efficiency and chromaticity of the blue emission and, as a result, provides an improved color conversion OLED display device.
• Microcavity OLED devices have been reported in the prior art to achieve improved chromaticity and emission efficiency. These microcavity OLED devices, however, have increased viewing angle dependence. Because the resonance of the microcavity changes with viewing angle, the emitted light changes color and intensity with viewing angle, which is undesirable in many applications.
• a color conversion display device is constructed using a blue emitting OLED in a microcavity structure. The color conversion layer absorbs the blue emission from the microcavity OLED and re-emits isotropically.
• OLED device 10 can be a subpixel as defined above. Although OLED device 10 is shown as emitting light from the bottom (that is, a bottom-emitting device), it will be understood that in some embodiments OLED device 10 can be a top-emitting device.
• the pixel includes at a minimum a substrate 20, an anode 30, a cathode 65 spaced from anode 30, and a light-emitting layer 50.
• the pixel can also include one or more of a color conversion layer 25, a hole-injecting layer 40, a hole-transporting layer 45, an electron-transporting layer 55, and an electron-injecting layer 60. Some embodiments can also include a transparent conductive spacer layer 35. These components will be described in more detail.
• Substrate 20 can be an organic solid, an inorganic solid, or a combination of organic and inorganic solids. Substrate 20 can be rigid or flexible and can be processed as separate individual pieces, such as sheets or wafers, or as a continuous roll. Typical substrate materials include glass, plastic, metal, ceramic, semiconductor, metal oxide, semiconductor oxide, semiconductor nitride, or combinations thereof.
• Substrate 20 can be a homogeneous mixture of materials, a composite of materials, or multiple layers of materials.
• Substrate 20 can be an OLED substrate, that is a substrate commonly used for preparing OLED devices, e.g., active-matrix low-temperature polysilicon or amorphous-silicon TFT substrate.
• the substrate 20 can either be light transmissive or opaque, depending on the intended direction of light emission. The light transmissive property is desirable for viewing the EL emission through the substrate. Transparent glass or plastic are commonly employed in such cases.
• the transmissive characteristic of the bottom support is immaterial, and therefore can be light transmissive, light absorbing or light reflective.
• OLED device 10 can be a color conversion OLED device and include a color conversion layer 25.
• Color conversion layer 25 has been described by Tokailin et al. in commonly assigned U.S. Patent 5,126,214.
• Color conversion layer 25 includes a fluorescent material responsive to the color light emitted by light-emitting layer 50 and is capable of changing the wavelength of light and thereby re-emitting different colored light.
• an OLED device that generates light of a single hue (e.g., blue light) and include color conversion layers 25 that convert the generated light into light of one or more different hues (e.g., green, red) that will be perceived by a viewer.
• color conversion layers 25 that convert the generated light into light of one or more different hues (e.g., green, red) that will be perceived by a viewer.
• the fluorescent material to be used in color conversion layer 25 is not critical as long as it has a strong fluorescent in the solid state, including a dispersion state in a resin.
• the fluorescent material can include coumarin dyes such as 2,3,5,6-lH,4H- tetrahydro-8-trichloromethylquinolizino(9,9a, 1 gh)coumarin, cyanine-based dyes such as 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyrylene)-4H-pyran, pyridine-based dyes such as l-ethyl-2-(4-(p-dimethylaminophenyl)-l,3- butadienyl)-pyridium perchlorate, xanthene-based dyes such as rhodamine B, and oxazine-based dyes.
• coumarin dyes such as 2,3,5,6-lH,4H- tetrahydro-8-trichloromethylquinolizino(9,9a, 1 gh)coumarin
• cyanine-based dyes such as 4-dicyanomethylene-2-methyl-6-(p-
• the fluorescent material can also include inorganic phosphors.
• the fluorescent material can be in the form of a film formed by vacuum deposition, by sputtering, or by spin coating. In other embodiments, the fluorescent material can be dispersed in a resin such as a bonding resin.
• the thickness of color conversion layer 25 is not critical as long as it sufficiently absorbs light from light-emitting layer 50. The positioning of color conversion layer 25 will depend on the properties of OLED device 10. For some embodiments such as a top-emitting device, it will be preferable that color conversion layer 25 will be over cathode 65.
• An electrode is formed over substrate 20 and is most commonly configured as an anode 30.
• anode 30 When EL emission is viewed through the substrate 20, anode 30 should be made of a reflective metal and should be thin enough so that it has a finite transmittance at the wavelength of the emitted light, hereafter referred to as being semitransparent. Only a small number of metals, including Ag or Au, or alloys thereof, defined as alloys having at least 50 atomic percent of at least one of these metals, are preferable materials for semitransparent anode 30. The thickness range of anode 30 is limited and is selected to optimize the luminance light output at a predetermined wavelength from OLED device 10, as will be further described. In certain circumstances, it is possible to also include a transparent conductive oxide layer in combination with the thin reflective metal layer in anode 30.
• anode 30 is preferably a reflective metal with a thickness that gives an optical density of 1.5 or greater so that it is substantially opaque and reflective. The emission efficiency of the OLED device increases with increasing reflectivity of anode 30.
• the material for an opaque and reflective anode 30 is preferably selected from a list including Ag, Au, Al, Mg, or Ca, or alloys thereof.
• a hole-injecting layer 40 be formed over anode 30 in an organic light-emitting display.
• the hole- injecting material can serve to improve the film formation property of subsequent organic layers and to facilitate injection of holes into the hole-transporting layer.
• Suitable materials for use in hole-injecting layer 40 include, but are not limited to, porphyrinic compounds as described in U.S. Patent 4,720,432, plasma-deposited fluorocarbon polymers as described in U.S. Patent 6,208,075, and inorganic oxides including vanadium oxide (NOx), molybdenum oxide (MoOx), nickel oxide ( ⁇ iOx), etc.
• hole-injecting materials reportedly useful in organic EL devices are described in EP 0 891 121 Al and EP 1 029 909 Al. While not always necessary, it is often useful that a hole-transporting layer 45 be formed and disposed between anode 30 and cathode 65. Desired hole-transporting materials can be deposited by any suitable means such as evaporation, sputtering, chemical vapor deposition, electrochemical means, thermal transfer, or laser thermal transfer from a donor material.
• Hole-transporting materials useful in hole-transporting layer 45 are well known to include compounds such as an aromatic tertiary amine, where the latter is understood to be a compound containing at least one trivalent nitrogen atom that is bonded only to carbon atoms, at least one of which is a member of an aromatic ring.
• the aromatic tertiary amine can be an arylamine, such as a monoarylamine, diarylamine, triarylamine, or a polymeric arylamine. Exemplary monomeric triarylamines are illustrated by Klupfel et al. in U.S. Patent 3,180,730.
• Suitable triarylamines substituted with one or more vinyl radicals and/or comprising at least one active hydrogen-containing group are disclosed by Brantley et al. in U.S. Patents 3,567,450 and 3,658,520.
• a more preferred class of aromatic tertiary amines are those which include at least two aromatic tertiary amine moieties as described in U.S. Patents 4,720,432 and 5,061,569.
• Such compounds include those represented by structural Formula A. wherein: Qi and Q 2 are independently selected aromatic tertiary amine moieties; and G is a linking group such as an arylene, cycloalkylene, or alkylene group of a carbon to carbon bond.
• At least one of Qi or Q 2 contains a polycyclic fused ring structure, e.g., a naphthalene.
• G is an aryl group, it is conveniently a phenylene, biphenylene, or naphthalene moiety.
• a useful class of triarylamines satisfying structural Formula A and containing two triarylamine moieties is represented by structural Formula B.
• Ri and R 2 each independently represent a hydrogen atom, an aryl group, or an alkyl group or R] and R 2 together represent the atoms completing a cycloalkyl group; and R 3 and R 4 each independently represent an aryl group, which is in turn substituted with a diaryl substituted amino group, as indicated by structural
• R 5 and R 6 are independently selected aryl groups.
• at least one of R 5 or R 6 contains a polycyclic fused ring structure, e.g., a naphthalene.
• Another class of aromatic tertiary amines are the tetraaryldiamines.
• Desirable tetraaryldiamines include two diarylamino groups, such as indicated by Formula C, linked through an arylene group.
• Useful tetraaryldiamines include those represented by Formula D. wherein: each Are is an independently selected arylene group, such as a phenylene or anthracene moiety; n is an integer of from 1 to 4; and Ar, R , R 8 , and R 9 are independently selected aryl groups.
• at least one of Ar, R 7 , R 8 , and R is a polycyclic fused ring structure, e.g., a naphthalene.
• the various alkyl, alkylene, aryl, and arylene moieties of the foregoing structural Formulae A, B, C, D, can each in turn be substituted.
• Typical substituents include alkyl groups, alkoxy groups, aryl groups, aryloxy groups, and halogens such as fluoride, chloride, and bromide.
• the various alkyl and alkylene moieties typically contain from 1 to about 6 carbon atoms.
• the cycloalkyl moieties can contain from 3 to about 10 carbon atoms, but typically contain five, six, or seven carbon atoms ⁇ e.g., cyclopentyl, cyclohexyl, and cycloheptyl ring structures.
• the aryl and arylene moieties are usually phenyl and phenylene moieties.
• the hole-transporting layer in an OLED device can be formed of a single or a mixture of aromatic tertiary amine compounds. Specifically, one can employ a triarylamine, such as a triarylamine satisfying the Formula B, in combination with a tefraaryldiamine, such as indicated by Formula D. When a triarylamine is employed in combination with a tefraaryldiamine, the latter is positioned as a layer interposed between the triarylamine and the electron- injecting and transporting layer.
• Illustrative of useful aromatic tertiary amines are the following: 1 , 1 -Bis(4-di- ⁇ -tolylaminophenyl)cyclohexane 1,1 -Bis(4-di- -tolylaminophenyl)-4-phenylcyclohexane 4,4'-Bis(diphenylamino)quadriphenyl Bis(4-dimethylamino-2-methylphenyl)-phenylmethane N,N,N-Tri(p-tolyl)amine 4-(di-p-tolylamino)-4'-[4(di-p-tolylamino)-styryl]stilbene N,N,N',N'-Tetra-p-tolyl-4-4'-diaminobiphenyl N,N,N',N'-Tetraphenyl-4,4'-diaminobiphenyl N-Phenylcarbazole Poly(N
• polymeric hole-transporting materials can be used such as poly(N-vinylcarbazole) (PNK), polythiophenes, polypyrrole, polyaniline, and copolymers such as poly(3 ,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) also called PEDOT/PSS.
• PNK poly(N-vinylcarbazole)
• Polythiophenes polypyrrole
• polyaniline polyaniline
• copolymers such as poly(3 ,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) also called PEDOT/PSS.
• Light-emitting layer 50 produces light in response to hole-electron recombination.
• Light-emitting layer 50 is formed over anode 30 and over any other layers formed, such as hole-transporting layer 45.
• Desired organic light- emitting materials can be deposited by any suitable means such as evaporation, sputtering, chemical vapor deposition, electrochemical means, or radiation thermal transfer from a donor material.
• Useful organic light-emitting materials are well known.
• the light-emitting layers of the organic EL element comprise a luminescent or fluorescent material where electroluminescence is produced as a result of electron- hole pair recombination in this region.
• the light-emitting layers can be comprised of a single material, but more commonly include a host material doped with a guest compound or dopant where light emission comes primarily from the dopant and can be of any color.
• the dopant is selected to produce color light having a particular spectrum. For a color conversion OLED device, the dopant is frequently selected to produce blue light.
• the host materials in the light-emitting layers can be an electron-transporting material, as defined below, a hole-transporting material, as defined above, or another material that supports hole-electron recombination.
• the dopant is usually chosen from highly fluorescent dyes, but phosphorescent compounds, e.g., transition metal complexes as described in WO 98/55561, WO 00/18851, WO 00/57676, and WO 00/70655 are also useful. Dopants are typically coated as 0.01 to 10 % by weight into the host material.
• Metal complexes of 8-hydroxyquinoline and similar derivatives constitute one class of useful host materials capable of supporting electroluminescence, and are particularly suitable for light emission of wavelengths longer than 500 nm, e.g., green, yellow, orange, and red.
• M represents a metal
• n is an integer of from 1 to 3
• Z independently in each occurrence represents the atoms completing a nucleus having at least two fused aromatic rings.
• the metal can be a monovalent, divalent, or trivalent metal.
• the metal can, for example, be an alkali metal, such as lithium, sodium, or potassium; an alkaline earth metal, such as magnesium or calcium; or an earth metal, such as boron or aluminum.
• any monovalent, divalent, or trivalent metal known to be a useful chelating metal can be employed.
• Z completes a heterocyclic nucleus containing at least two fused aromatic rings, at least one of which is an azole or azine ring.
• CO-1 Aluminum trisoxine [alias, tris(8-quinolinolato)aluminum(III)]
• CO-2 Magnesium bisoxine [alias, bis(8-quinolinolato)magnesium(II)]
• CO-3 Bis[benzo ⁇ f ⁇ -8-quinolinolato]zinc (II)
• CO-4 Bis(2-methyl-8-quinolinolato)aluminum(III)- ⁇ -oxo-bis(2-methyl-8- quinolinolato) aluminum(III)
• CO-6 Aluminum tris(5-methyloxine)
• R 1 , R 2 , R 3 , R , R 5 , and R 6 represent one or more substituents on each ring where each substituent is individually selected from the following groups: Group 1: hydrogen, or alkyl of from 1 to 24 carbon atoms; Group 2: aryl or substituted aryl of from 5 to 20 carbon atoms; Group 3: carbon atoms from 4 to 24 necessary to complete a fused aromatic ring of anthracenyl, pyrenyl, or perylenyl; Group 4: heteroaryl or substituted heteroaryl of from 5 to 24 carbon atoms as necessary to complete a fused heteroaromatic ring of furyl, thienyl, pyridyl, quinolinyl or other heterocyclic systems; Group 5: alkoxylamino, alkylamino, or arylamino of from 1 to 24 carbon atoms; and Group 6: fluorine, chlorine, bromine or cyano.
• Benzazole derivatives (Formula G
• n is an integer of 3 to 8;
• Z is O, NR or S;
• R' is hydrogen; alkyl of from 1 to 24 carbon atoms, for example, propyl, t-butyl, heptyl, and the like; aryl or heteroatom substituted aryl of from 5 to 20 carbon atoms for example phenyl and naphthyl, furyl, thienyl, pyridyl, quinolinyl and other heterocyclic systems; or halo such as chloro, fluoro; or atoms necessary to complete a fused aromatic ring; and
• L is a linkage unit including alkyl, aryl, substituted alkyl, or substituted aryl, which conjugately or unconjugately connects the multiple benzazoles together.
• auseful benzazole is 2, 2', 2"-(l,3,5- phenylene)tris [ 1 -phenyl- 1 H-benzimidazole] .
• Desirable fluorescent dopants include derivatives of anthracene, tetracene, xanthene, perylene, rubrene, coumarin, rhodamine, quinacridone, dicyanomethylenepyran compounds, thiopyran compounds, polymethine compounds, pyrilium and thiapyrilium compounds, and carbostyryl compounds.
• useful dopants include, but are not limited to, the following:
• organic emissive materials can be polymeric substances, e.g., polyphenylenevinylene derivatives, dialkoxy-polyphenylenevinylenes, poly-para- phenylene derivatives, and polyfluorene derivatives, as taught by Wolk et al. in commonly assigned U.S. Patent 6,194,119 Bl and references cited therein.
• One advantageous embodiment of the present invention uses a light-emitting layer 50 wherein a dopant is selected to produce light having a spectrum including light of a specific color, and in particular blue light.
• the use of an array of color conversion layers 25 with fluorescent materials responsive to blue light can convert portions of the blue light into different color light, e.g., red and green.
• the device can additionally comprise two or more emissive layers, if such is desired for proper emissive properties of the resulting OLED device.
• the device can also be in a stacked structure, as disclosed in U.S. Patents 6,107,734; 6,337,492; and 6,274,980 While not always necessary, it is often useful that OLED device 10 includes an electron-transporting layer 55 formed over light-emitting layer 50.
• Desired electron-transporting materials can be deposited by any suitable means such as evaporation, sputtering, chemical vapor deposition, electrochemical means, thermal transfer, or laser thermal transfer from a donor material.
• Preferred electron-transporting materials for use in electron-transporting layer 55 are metal chelated oxinoid compounds, including chelates of oxine itself (also commonly referred to as 8-quinolinol or 8-hydroxyquinoline). Such compounds help to inject and transport electrons and exhibit both high levels of performance and are readily fabricated in the form of thin films.
• Exemplary of contemplated oxinoid compounds are those satisfying structural Formula E, previously described.
• Other electron-transporting materials include various butadiene derivatives as disclosed in U.S. Patent 4,356,429 and various heterocyclic optical brighteners as described in U.S. Patent 4,539,507. Benzazoles satisfying structural Formula E, previously described.
• Formula G are also useful electron-transporting materials.
• Other electron-transporting materials can be polymeric substances, e.g., polyphenylenevinylene derivatives, poly-para-phenylene derivatives, polyfluorene derivatives, polythiophenes, polyacetylenes, and other conductive polymeric organic materials such as those listed in Handbook of Conductive
• light- emitting layer 50 can have hole-transporting properties or electron-transporting properties as desired for performance of the OLED device.
• Hole-transporting layer 45 or electron-transporting layer 55, or both, can have emitting properties. In such a case, fewer layers can be sufficient for the desired emissive properties.
• An electron-injecting layer 60 can also be present between the cathode and the electron-transporting layer. Examples of electron-injecting materials include alkaline or alkaline earth metals, alkali halide salts, such as LiF mentioned above, or alkaline or alkaline earth metal doped organic layers.
• Cathode 65 is an electrode formed over the electron-transporting layer 55, or over light-emitting layers 50 if an electron-transporting layer is not used.
• the cathode 65 material is preferably a reflective metal with a thickness that gives an optical density of 1.5 or higher so that it is essentially opaque and reflective.
• the emission efficiency of the OLED device increases with increasing reflectivity of cathode 65.
• the material for an opaque and reflective cathode 65 is preferably selected from a list including Ag, Au, Al, Mg, or Ca, or alloys thereof.
• cathode 65 needs to include a reflective metal that is thin enough such that it is semitransparent to the emitted light.
• the material for a semitransparent cathode 65 is preferably selected from a list including Ag or Au, or alloys thereof.
• the thickness range of cathode 65 is limited and is selected to optimize the luminance light output at a predetermined wavelength from OLED device 10, as will be further described.
• Suitable materials include indium oxide (InO), tin oxide (SnO), zinc oxide (ZnO), molybdenum oxide, vanadium oxide, antimony oxide (SbO), or mixtures thereof.
• Cathode materials can be deposited by evaporation, sputtering, or chemical vapor deposition. When needed, patterning can be achieved through many well known methods including, but not limited to, through-mask deposition, integral shadow masking as described in U.S. Patent 5,276,380 and EP 0 732 868, laser ablation, and selective chemical vapor deposition.
• Cathode 65 is vertically spaced apart from anode 30.
• Cathode 65 can be part of an active matrix device and in that case is a single electrode for the entire display. Alternatively, cathode 65 can be part of a passive matrix device, in which each cathode 65 can activate a column of pixels, and cathodes 65 are arranged orthogonal to anodes 30.
• Anode 30 and cathode 65 form a Fabry-Perot microcavity that effectively limits the bandwidth of the emitted light spectrum to produce light of a specific color. Emission near the wavelength corresponding to the resonance wavelength of the cavity is enhanced through the semitransparent electrode and those with other wavelengths are suppressed.
• FIG. 1 also shows schematically the effect of light emission in a microcavity. For simplicity, hole-injecting layer 40, hole-transporting layer 45, light-emitting layer 50, electron-transporting layer 55, and electron-injecting layer
• organic EL element 95 60 will be referred to collectively as organic EL element 95.
• the thickness of the organic EL element 95 can be varied in order to adjust the microcavity resonance wavelength.
• a transparent conductive spacer layer 35 can be used as an additional means to adjust the microcavity resonance wavelength.
• Transparent conductive spacer layer 35 can be disposed between one of the metallic electrodes and organic
• EL element 95 It needs to be transparent to the emitted light and it needs to be conductive to carry the charge carriers between the metallic electrode and the organic EL element 95. Since only through-film conductance is important, a bulk resistivity of less than about 10 8 ohm-cm is adequate.
• metal oxides such as, but not limited to, indium-tin oxide (ITO), zinc-tin oxide (ZTO), tin-oxide
• TinOx antimony oxide
• ZnOx zinc oxide
• Light 105 is emitted in the direction of reflective cathode 65 and is reflected as reflected light 110.
• Light 115 is emitted in the direction of semitransparent reflective anode 30 and is partially reflected as partially reflected light 120, and partially transmitted as partially transmitted light 125.
• Partially transmitted light 125 can be absorbed by color conversion layer 25 and re-emitted as emitted light 130 of a different hue.
• the combined thickness of the organic EL element 95 and transparent conductive spacer layer 35 is selected to tune the microcavity OLED device 10 to have the resonance at the predetermined wavelength to be emitted from the device.
• n is the refractive index and Li is the thickness of the nth sublayer in organic EL element 95
• n s is the refractive index
• L s is the thickness, which can be zero, of the transparent conductive spacer layer 35
• Q ml and Q n ⁇ are the phase shifts in radians at the two organic EL element-metal electrode interfaces, respectively
• ⁇ is the predetermined wavelength to be emitted from the device
• m is a non-negative integer. It is preferred to have m as small as practical, typically less than 2.
• a blue-light-emitting layer stabilized by the presence of a yellow-emitting dopant such as a rubrene derivative in hole- transporting layer 45 one can select the microcavity effect to enhance the emission of blue light (as transmitted light 125) and reduce the emission of undesirable yellow wavelengths.
• the total thickness between the metal electrodes is the most important factor in determining the microcavity resonance wavelength.
• the resonance wavelength and more particularly the strength of the resonance also depend on the distance between the emitting layer and each of the two electrodes.
• the thickness of the transparent conductive spacer layer 35 should be included if it is disposed between light-emitting layer 50 and reflective cathode 65.
• n A and L A are the refractive index and the thickness of the absorption- reduction layer, respectively; m-and Lp are the real part of the refractive index and the thickness of the semitransparent metal bottom anode, respectively; and m is a non-negative integer. It is preferred to have m A as small as practical, usually 0 and typically less than 2.
• the semitransparent electrode can be the cathode and the metallic reflective electrode can be the anode.
• the organic EL element 95 is appropriately oriented so that the hole-injecting and hole-transporting layers are closer to the anode and the electron-injecting and electron-transporting layers are closer to the cathode.
• Example 1 OLED devices #1, #2, and #3 were constructed in the following manner. Substrates coated with 80 nm ITO were sequentially ultrasonicated in a commercial detergent, rinsed in deionized water, and degreased in toluene vapor. These substrates were treated with oxygen plasma for about one minute and coated with 1 nm fluorocarbon layer by plasma assisted deposition of CHF 3 . These substrates were loaded into a deposition chamber for organic layers and cathode depositions.
• the devices of Example 1 were prepared by sequential deposition of 150 nm NPB hole-transport layer (HTL) doped with various amounts of rubrene, 20 nm blue emission layer (EML) comprising AND host with 1.5% TBP blue dopant, 35 nm Alq electron-transport layer (ETL), and 100 nm Mg:10%Ag alloy as the cathode.
• the above sequence completed the deposition of the OLED device.
• Device #1 had no rubrene doping into the HTL; device #2 had 0.5% of rubrene doped into the HTL; and device #3 had 2% rubrene doped into the HTL.
• the OLED devices were then hermetically packaged in a dry glove box filled with nitrogen for protection against ambient environment.
• the ITO patterned substrates used for preparing these OLED devices contained several test patterns. Each of the devices was tested for current voltage characteristics and the electroluminescence yield. The spectral output of the devices operating at 20 mA/cm 2 current density are plotted in FIG. 2.
• Device #1 showed two emission peaks at 464 nm and 492 nm, respectively, and CIE color coordinates of (0.166,0.253), typical of what normally seen for a blue-emitting OLED device using a TBP doped light emitting layer.
• Device #2 showed a much different spectrum. In addition to the two peaks at 464 nm and 492 nm, a new peak at 560 nm due to rubrene emission was clearly visible.
• Device #3 showed a spectrum that was dominated by the rubrene emission at 560 nm and the color coordinates shifted to (0.383, 0.421). The emission had an orange-white appearance and therefore the blue emission from TBP was greatly contaminated.
• the operation stability was tested by running the devices continuously using 50% duty cycle square waveform AC current at an average current density of 20 mA/cm 2 . The luminance output was continuously monitored and the output data are plotted in FIG. 3.
• Example 2-a (comparative): OLED devices #2-a was constructed in a manner similar to the samples in Example 1 A substrate coated with 40 nm ITO was sequentially ultrasonicated in a commercial detergent, rinsed in deionized water, and degreased in toluene vapor.
• This substrate was treated with oxygen plasma for about one minute and coated with 1 nm fluorocarbon layer by plasma assisted deposition of CHF 3 .
• the substrate was then loaded into a vacuum evaporation deposition chamber for sequential deposition of 87 nm of undoped NPB hole-transport layer (HTL), 20 nm of NPB hole-transport layer doped with 2.5% of rubrene, 30 nm blue emission layer (EML) comprising TBADN host with 1.5% TBP blue dopant, 32 nm Alq electron-transport layer (ETL), 0.5 nm Li electron injection layer, and 50 nm Ag alloy as the cathode.
• EML blue emission layer
• ETL Alq electron-transport layer
• 0.5 nm Li electron injection layer and 50 nm Ag alloy as the cathode.
• the above sequence completed the deposition of the OLED device.
• the OLED device was then hermetically packaged in a dry glove box filled with nitrogen for protection against ambient environment.
• the ITO patterned substrate used for preparing these OLED devices contained several test patterns. Each of the devices was tested for current voltage characteristics and the electroluminescence yield. The spectral output of the device operating at 20 mA/cm 2 current density is plotted in FIG. 4.
• Device #2-a showed a spectrum that was dominated by the rubrene emission at 560 nm and the color coordinates are at (0.376, 0.461). The emission had a strong orange-white appearance and therefore the blue emission from TBP was greatly contaminated.
• Sample 2-b (inventive) was prepared with a microcavity structure.
• a glass substrate was sequentially ultrasonicated in a commercial detergent, rinsed in deionized water, and degreased in toluene vapor. This substrate was then coated with a DC-sputtered layer of 93 nm thick Ag film through a metal mask to create an anode pattern.
• the substrate was then placed in a vacuum evaporation deposition chamber for sequential deposition of 3 nm of MoO 3 hole injection layer, 139 nm of undoped NPB hole-transport layer (HTL), 20 nm of NPB hole- transport layer doped with 2.5% of rubrene, 20 nm blue emission layer (EML) comprising TBADN host with 1.5% TBP blue dopant, 20 nm Alq electron- transport layer (ETL), 0.5 nm Li electron injection layer, and 22.5 nm Ag alloy as the semitransparent cathode, and 85 nm Alq as the absorption-reduction layer.
• the above sequence completed the deposition of the OLED device.
• the OLED device was then hermetically packaged in a dry glove box filled with nitrogen for protection against ambient environment.
• the OLED structure formed a microcavity with the Ag anode and the Ag cathode layers as the reflecting mirrors.
• the thickness of the layers was selected so that the resonance wavelength of the microcavity was in the blue region and the emission had a good efficiency.
• the spectral output of the device is also shown in FIG. 4. It included a single narrow peak at 460 nm with a color coordinate of (0.145, 0.079).
• the radiance at 460 nm is almost ten times higher than the non-cavity sample 2-a at this wavelength.
• the OLED device constructed in accordance with the present invention thus showed much improved color and emission efficiency.
• the device is expected to maintain the stabilization effect of rubrene doped into HTL but the color contamination due to rubrene doping is completely eliminated.
• the invention has been described in detail with particular reference to stabilizing a blue emitting OLED but it will be understood that the invention can be applied to other colored OLED devices as well. PARTS LIST
• OLED device substrate color conversion layer anode transparent conductive spacer layer hole-injecting layer hole-transporting layer light-emitting layer electron-transporting layer electron-injecting layer cathode organic EL element light reflected light light partially reflected light partially transmitted light emitted light

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