WO2000079616A1 - Ecran plat a contraste ameliore - Google Patents

Ecran plat a contraste ameliore Download PDF

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Publication number
WO2000079616A1
WO2000079616A1 PCT/GB2000/002377 GB0002377W WO0079616A1 WO 2000079616 A1 WO2000079616 A1 WO 2000079616A1 GB 0002377 W GB0002377 W GB 0002377W WO 0079616 A1 WO0079616 A1 WO 0079616A1
Authority
WO
WIPO (PCT)
Prior art keywords
layer
light emitting
intermediate layer
back electrode
transparent
Prior art date
Application number
PCT/GB2000/002377
Other languages
English (en)
Inventor
Oleg Victorovich Salata
Olivier Renault
Victor Christou
Original Assignee
Isis Innovation Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from GBGB9914372.9A external-priority patent/GB9914372D0/en
Priority claimed from GBGB9927116.5A external-priority patent/GB9927116D0/en
Application filed by Isis Innovation Limited filed Critical Isis Innovation Limited
Priority to AU55485/00A priority Critical patent/AU5548500A/en
Publication of WO2000079616A1 publication Critical patent/WO2000079616A1/fr

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Classifications

    • 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
    • H10K59/80—Constructional details
    • H10K59/805—Electrodes
    • H10K59/8052—Cathodes
    • H10K59/80523—Multilayers, e.g. opaque multilayers

Definitions

  • This invention relates to a non-reflective electrode which can be used in a flat panel display.
  • Flat panel displays are the critical enabling technology for many current applications including laptop computers, portable displays and "head up" displays.
  • the flat panel display market is dominated by liquid crystal technology, but these materials are non- emissive requiring the light source to be filtered which reduces the efficiency of the device.
  • Much work is currently being undertaken on developing alternative emissive technologies including organic electroluminescent (EL) displays.
  • EL organic electroluminescent
  • Contrast is the difference in brightness between when a pixel is on and when it is off.
  • a proportion of the ambient light that is incident on a device is reflected, so that the total light observed is the sum of the emitted light and the reflected ambient light. If the proportion of reflected ambient light is relatively large, then the contrast is poor. This is frequently the case with the current display devices. There is therefore a need to find a way of increasing contrast.
  • a light emitting device which comprises sequentially a transparent substrate layer, a transparent electrode layer, a light emitting layer and a back electrode which is in the form of a layer of carbon and an at least semi- transparent intermediate layer between the light emitting layer and the back electrode, said intermediate layer having its lowest empty energy level or levels, or lowest empty energy band, positioned above the conduction band of the carbon layer when the two layers are brought into contact.
  • the back electrode is a metal such as aluminium which is highly reflective and consequently the contrast is poor. By replacing this metal by a carbon layer which is substantially black contrast can be improved significantly.
  • the intermediate layer is at least semi- transparent ie. it should be at least 50% transmissive, preferably at least 70% transmissive and more preferably at least 90% transmissive in the visible region (380 to 780 nm) .
  • This intermediate layer serves to improve electron injection into the device, possibly by lowering the barrier to injection and/or improving the conductivity of the electrode. Without such a layer it is generally found that it is necessary to operate at a higher voltage since the carbon layer does not function so effectively as a source of electrons.
  • This semi- transparent intermediate layer may be defined as the layer having its lowest empty energy level or levels, or lowest empty energy band, positioned above the conduction band of the carbon layer when the two layers are brought into contact.
  • the intermediate layer is generally made of a semi- transparent metal or alloy or is a dielectric layer.
  • Metals with a lower work function than carbon generally have empty energy levels above the conduction band of carbon.
  • Dielectric materials generally have a large energy separation between the full HOMO (or valence band) and empty LUMO (or conduction band) and to be effective the LUMO would have to be positioned above the carbon conduction band. It will be appreciated that the values for most metals and dielectrics are known or can, if necessary, be determined.
  • the intermediate layer is advantageously made of a low work function metal or alloy such as magnesium, which is preferred, aluminium, silver, calcium, lithium and indium or an alloy such as Mg/Ag or Mg/Al and Li/Al.
  • the semi-transparent layer may be of a dielectric material generally of a solid ionic metal or metalloid salt, typically an oxide or halide, for example a chloride or fluoride, of a metal, typically of Group I, II or III, especially
  • the dielectric has a band gap of at least 4.5 electron volts .
  • metal films are normally highly reflective, a sufficiently thin metal layer is semi-transparent, transmitting rather than reflecting a large proportion of the incident visible light.
  • a carbon layer is deposited behind such a thin metal layer, a significant proportion of the incident light is ultimately absorbed by the carbon film rather than reflected.
  • the thickness of the intermediate layer will depend to some extent on its nature. Generally, a metal or alloy layer will have a thickness from 500 to 0.5 nm, typically 50 to 1 nm, and especially from 25 to 2 nm. Generally if the intermediate layer is made of a dielectric material, lower thicknesses are desirable, a general range being from 100 to 0.1 nm, typically 10 to 0.5 nm and preferably from 5 to 1 nm; it is preferably less than 10 nm.
  • the source of carbon used for the carbon layer is not particularly critical and includes isotropic graphite, anisotropic graphite, vitreous carbon and carbon fibre although isotropic graphite of high purity is generally considered to be the best.
  • the carbon layer will be thicker than the intermediate layer.
  • the general thickness will range from 5 ⁇ m to 5 nm, typically from 5 ⁇ m to 500 nm if the layer is produced by, for example, screen printing or spraying.
  • the layer is produced by, for example, sputtering, evaporation or low temperature chemical deposition thinner layers will generally be more suitable, for example, from 200 to 5 nm and especially from 60 to 10 nm.
  • the precise nature of the deposition is unimportant provided that the layer is as uniform as possible and continuous and that the carbon layer absorbs generally at least 10% of the visible light and preferably at least 25%, especially at least 50%, of the visible light.
  • a metal or alloy layer is placed behind the back electrode. This serves several purposes. It acts to protect the back electrode and it generally will have high electrical conductivity. Also the presence of the metal layer can assist production. This is because carbon deposition is relatively slow. Once the carbon layer has built up to a sufficient thickness to enhance contrast it is then beneficial to switch to the more rapid metal deposition. Apart from encapsulating the carbon layer it also serves to conduct heat away from the device which may increase the stability and life of the device.
  • the metal layer which can be of the same metals or alloys as discussed above in relation to the intermediate layer, for example aluminium, will be significantly thicker than the immediate layer, typically from 10 to 2000 nm.
  • the metal layer is generally of a comparable thickness to the carbon layer.
  • Thicker layers can be attached to the back electrode in the form of foils. Any good heat conducting metal can be used for this purpose although, naturally, a relatively light metal is preferred in order to reduce the overall weight of the device.
  • aluminium foil can be used for this purpose.
  • Such a foil acts as a heat sink which facilitates high brightness where higher operating powers increase the heat dissipated in the device.
  • Such layers are typically attached to the back electrode by adhesion, for example using an epoxy resin.
  • the epoxy resin is preferably one which is curable at room temperature. Also it is preferred that the resin should not release byproducts on curing which could adversely affect the other components of the device. Accordingly, it is desirable that the epoxy resin should be one which is catalytically curable while, of course, it should be thermally conducting.
  • suitable epoxy resins which can be used include Loctite 315 and the thermally conductive 199-1402 epoxy resins from RS components.
  • the transparent electrode is the anode and the rear electrode forms the cathode.
  • An electron injecting and/or transporting layer is often advantageously inserted between the light emitting layer and the cathode.
  • a hole injecting and/or transporting layer may often advantageously be inserted between the light emitting layer and the anode. The nature of such layers is known in the art.
  • Typical materials which can be used for this purpose include polyphilic compounds, aromatic tertiary amines including compounds in which the tertiary nitrogen atom is attached to 3 phenyl rings such as N,N'-diphenyl-N,N'-bis- (3-methyl) -1,1 ' -biphenyl-4 , 4'- diamine (TPD) , stilbenes, triazoles, oxadiazoles such as BBO (2- (4-phenylyl) -5- (4- '-butylphenyl) -1,3,4- oxadiazole, imidazoles.
  • Other suitable materials can be found in US Patent No. 5,756,224 and Macromol. Symp. 125, 1 -48 (1997)
  • the nature of the light emitting layer is not particularly critical although, as indicated above, it should be selected such that there is a suitable energy gap between it and the carbon layer.
  • Suitable materials include organic and organo metallic molecular species including organolanthanide complexes such as di-or trivalent lanthanide metal ions complexed with one or more polydentate ligands containing one or more pyrazolyl-derived groups e.g. trispyrazolyl borates anions (see GB Application No. 9820805.1 and WO 98/55561) , light emitting polymers for example carbazoles such as PVK (poly- (9-vinyl carbazole) and quinolate complexes such as ALQ (tris- (8-hydroxy-quinoline) aluminium. Further details can be found in EP - 0120673 and US Patent Nos . 5,756,224 and 5,792,567.
  • the total thickness will be from 10 nm to 1 micron and especially from 60 to 150 nm with the transport layers generally having a thickness from 20 to 80 nm.
  • the transparent electrode which typically forms the anode of the device is preferably made from indium tin oxide (ITO) although other similar materials including indium oxide/tin oxide, tin oxide/antimony and zinc oxide/aluminium can also be used.
  • ITO indium tin oxide
  • Conducting polymers such as PANI (polyaniline) and PEDT may also be used.
  • the present invention also provides a process for preparing the device which comprises depositing a transparent electrode on a transparent substrate, sequentially applying over the transparent electrode, the light emitting layer and the back electrode.
  • Figure 1 shows, schematically, a typical preferred light emitting device of the present invention.
  • Figure 2 shows the current-voltage characteristics of a device of the present invention and of comparative devices.
  • Figure 3 shows an electroluminescent spectrum of a device of the present invention
  • Figure 4 shows the cathode reflectivity of a device of the present invention compared with a comparative device .
  • a light emitting device which comprises a transparent substrate 2, a transparent front electrode 4, a light emitting layer 6, an intermediate layer 8, and a carbon back layer 10.
  • An eye 1 indicates the front (viewing side of the device) .
  • the transparent substrate 2 is typically made of glass although other transparent dimensionally stable materials such as polyesters including PET, acrylic resins and polyamides such as nylon can also be used.
  • the devices are typically made pixelated or patterned.
  • the transparent electrode can be formed using standard lithography to provide electrodes as little as, for example, 10 x 10 microns.
  • the carbon back electrode is deposited, for example, through a shadow mask, to give an array of contacts which may be as little as, say, 20 nm.
  • both electrodes are made pixelated.
  • the electrode which is not pixelated can be as large as desired, typically for a laptop computer with a diagonal dimension of 17 inches.
  • Low resistance ITO coated glass (20 ohms/D) was cut into 1" x 1" (2.5 x 2.5cm) plates and patterned by standard lithography (HC1 etchant ) to give continuous electrodes (0.2mm x 0.8mm).
  • the patterned ITO plates were ultrasonically cleaned for 30 min in a boiling ammonia and hydrogen peroxide aqueous solution (RSA process), and then dried in an oven at 100°C. After cleaning and drying, the substrates were placed inside a vacuum evaporator with a base pressure better than 1 x 10 "6 Torr and the organic layers were sequentially evaporated from Mo boats at a deposition rate of 0.1-0.4 nm/s as measured by a calibrated crystal thickness monitor.
  • the hole transporting layer was a 40 nm thick layer of TPD
  • the light emitting layer was a 34 nm thick layer of the METb013
  • the electron transporting layer was a 60 nm thick layer of TAZ .
  • the rear electrode (s) was deposited. The thickness and details of the contacts is explained in the individual Examples.
  • EL measurements were made under forward bias (ITO positive) and the emission output was viewed in the forward direction through the transparent ITO electrode.
  • the current-voltage (I-V) characteristics were measured with a Thurlby-Thandar TSX3510P programmable DC power supply and a Keithley 617 programmable electrometer both controlled by IBM compatible PC via IEEE488 interface.
  • EL characteristics of the devices including the spectral and power dependencies of light output were measured with a LOT- Oriel Instaspec IV charge-coupled device (CCD) detector attached to an Oriel Multispec 1/8 M spectrograph with 400 lines/mm ruled grating.
  • CCD charge-coupled device
  • a fibre lined sighting optic was used to focus the image of the device to the entrance of the spectrograph.
  • the system was calibrated with an Oriel 200 W QTH calibrated lamp connected to an Oriel 300 W radiometric power supply. Values of radiance and luminance were also measured with an International Light IL1700 Research Radiometer equipped with a calibrated Si photodetector . Transmission and reflection was measured with a Perkin-Elmer Lambda 19 spectrometer.
  • an organic electroluminescent device was produced with a conventional Al/Mg metal cathode and a patterned ITO electrode.
  • the organic layers were deposited according to the description above and then Mg and Al were sequentially evaporated (10-20 nm Mg and 100-120 nm Al) to form the contact.
  • the IV curve is shown in fig 2 and the transmission curve is shown in fig 4.
  • the efficiency was 4.8cd/A or ⁇ 2% external quantum efficiency.
  • the maximum brightness of the device was 77 cd/m 2 at 22V, and 2.7 mA/cm 2 which corresponds to an efficiency of 2.8 cd/A.
  • An organic electroluminescent device was prepared with a "black" electrode instead of an Al/Mg cathode.
  • a 2 nm intermediate layer of Mg (evaporation rate 0.1-0.2 nm/s) followed by a 20 nm thick layer of carbon, followed by 20 nm of Mg, followed by -100 nm of Al were sequentially evaporated on top of the organic layers.
  • the carbon was deposited by evaporation at a rate of 1 nm/s and a pressure of ⁇ 1 x 10 "6 Torr.
  • the carbon source was carbon fibre .
  • the IV curve is shown in Figure 2, while Figure 3 shows the EL spectrum, and Figure 4 shows the transmission curve of the device.
  • Table 1 lists the best efficiency and best brightness of each device.
  • a 5 nm carbon film was directly deposited on to the organic layer followed by a 100 nm Al/Mg layer.
  • This Example illustrates the significance of the thin metal layer under the carbon film. When thicker carbon films were deposited directly on the organic layers the device performance was poor.

Landscapes

  • Electroluminescent Light Sources (AREA)

Abstract

L'invention concerne un dispositif d'émission de lumière qui comporte une couche de substrat transparente (2), une couche d'électrode transparente (4), une couche d'émission de lumière (6) et une électrode arrière (10) qui est composée d'une couche de carbone.
PCT/GB2000/002377 1999-06-18 2000-06-19 Ecran plat a contraste ameliore WO2000079616A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU55485/00A AU5548500A (en) 1999-06-18 2000-06-19 Flat panel display with improved contrast

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
GBGB9914372.9A GB9914372D0 (en) 1999-06-18 1999-06-18 Flat panel display with improved contrast
GB9914372.9 1999-06-18
GBGB9927116.5A GB9927116D0 (en) 1999-11-16 1999-11-16 Flat panel display with improved contrast
GB9927116.5 1999-11-16

Publications (1)

Publication Number Publication Date
WO2000079616A1 true WO2000079616A1 (fr) 2000-12-28

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Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB2000/002377 WO2000079616A1 (fr) 1999-06-18 2000-06-19 Ecran plat a contraste ameliore

Country Status (2)

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AU (1) AU5548500A (fr)
WO (1) WO2000079616A1 (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2003094255A2 (fr) * 2002-05-03 2003-11-13 Luxell Technologies Inc. Diodes electroluminescentes organiques a contraste ameliore
WO2004050793A1 (fr) * 2002-12-05 2004-06-17 Elam-T Limited Dispositifs et materiaux electroluminescents

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH065367A (ja) * 1992-06-18 1994-01-14 Pioneer Electron Corp エレクトロルミネッセンス素子
JPH088065A (ja) * 1994-06-25 1996-01-12 Toppan Printing Co Ltd 薄膜型ｅｌ素子
WO2000035028A1 (fr) * 1998-12-08 2000-06-15 Cambridge Display Technology Ltd. Dispositifs d'affichage

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH065367A (ja) * 1992-06-18 1994-01-14 Pioneer Electron Corp エレクトロルミネッセンス素子
JPH088065A (ja) * 1994-06-25 1996-01-12 Toppan Printing Co Ltd 薄膜型ｅｌ素子
WO2000035028A1 (fr) * 1998-12-08 2000-06-15 Cambridge Display Technology Ltd. Dispositifs d'affichage

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
GYOUTOKU A ET AL: "AN ORGANIC ELECTROLUMINESCENT DOT-MATRIX DISPLAY USING CARBON UNDERLAYER", SYNTHETIC METALS,CH,LAUSANNE, vol. 91, no. 1/03, 21 May 1997 (1997-05-21), pages 73 - 75, XP000890057, ISSN: 0379-6779 *
PATENT ABSTRACTS OF JAPAN vol. 018, no. 195 (E - 1533) 5 April 1994 (1994-04-05) *
PATENT ABSTRACTS OF JAPAN vol. 1996, no. 05 31 May 1996 (1996-05-31) *

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2003094255A2 (fr) * 2002-05-03 2003-11-13 Luxell Technologies Inc. Diodes electroluminescentes organiques a contraste ameliore
WO2003094255A3 (fr) * 2002-05-03 2004-02-05 Luxell Technologies Inc Diodes electroluminescentes organiques a contraste ameliore
WO2004050793A1 (fr) * 2002-12-05 2004-06-17 Elam-T Limited Dispositifs et materiaux electroluminescents
JP2006509008A (ja) * 2002-12-05 2006-03-16 エラム−ティー リミテッド エレクトロルミネッセンス物質および装置
US7718275B2 (en) 2002-12-05 2010-05-18 Merck Patent Gmbh Electroluminescent materials and devices

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Publication number Publication date
AU5548500A (en) 2001-01-09

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