WO2006097711A1

WO2006097711A1 - Light emissive device

Info

Publication number: WO2006097711A1
Application number: PCT/GB2006/000905
Authority: WO
Inventors: Matthew Roberts; David Lidzey; Ali Adawi
Original assignee: Cambridge Display Technology Limited; The University Of Sheffield
Priority date: 2005-03-15
Filing date: 2006-03-14
Publication date: 2006-09-21
Also published as: TW200642525A; GB0505298D0

Abstract

An organic light emissive device comprising: a cathode; a transparent anode (22), and an organic light emissive region (23) which comprises subpixels of red, green and blue light emitting materials and is positioned between the cathode (24, 28) and the anode so that the cathode injects electrons into each subpixel, wherein the cathode comprises a reflective silver layer (28) and a transparent barium layer, in which the barium layer (24) is positioned between the silver layer and the organic light emissive region.

Description

Light Emissive Device

The present invention relates to organic light emissive devices, to methods of making such devices and the use of cathodes therein.

Organic light emissive devices (OLEDs) generally comprise a cathode, an anode and an organic light emissive region between the cathode and the anode. Light emissive organic materials may comprise small molecular materials such as described in US4539507 or polymeric materials such as those described in PCT/WO90/13148. The cathode injects electrons into the light emissive region and the anode injects holes. The electrons and holes combine to generate photons.

Figure 1 shows a typical cross-sectional structure of an OLED. The OLED is typically fabricated on a glass or plastics substrate 1 coated with a transparent anode 2 such as an indium-tin-oxide (ITO) layer. The ITO coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material 3 and cathode material 4 of low workfunction metal such as calcium is applied, optionally with a capping layer of aluminium (not shown). Other layers may be added to the device, for example to improve charge transport between the electrodes and the electroluminescent material.

There has been a growing interest in the use of OLEDs in display applications because of their potential advantages over conventional displays. OLEDs have relatively low operating voltage and power consumption and can be easily processed to produce large area displays. On a practical level, there is a need to produce OLEDs which are bright and operate efficiently but which are also reliable to produce and stable in use.

The structure of the cathode in OLEDs is one aspect under consideration in this art. In the case of a monochrome OLED, the cathode may be selected for optimal performance with the single electroluminescent organic material. A variety of cathode configurations have been proposed, each of which involves an additional layer to improve electron injection. For example, it is known from Applied Phys.Let.70, 150, 1997 that a layer of metal fluoride located between the organic emissive layer and the metal cathode can result in an improvement in device efficiency. LiF/Al cathodes are proposed in Applied Phys. Lett. 91 (5), 563-565, 2001. Other arrangements are found in Synth. Metals 2000, 111-112, pl25-128 and WO03/019696. A light absorbent cathode may be formed of LiF optionally codeposited with Al for use as an electron- injecting layer according to WO00/35028. US6278236 also provides a multilayer organic electroluminescent device with an electron-injecting layer. In this arrangement, the electron-injecting layer includes aluminium and at least one alkali metal halide or at least one alkaline earth metal halide. A composite electron- injecting layer comprising lithium fluoride and aluminium is exemplified. Another composite cathode is described in Jabbour et al in Applied Phys. Letts. 73 (9), 1185- 1187 (1998). US2001/0051284A also describes a composite electron-injection layer in a multilayer organic electroluminescent device. Ultrathin layer alkaline earth metals are described as stable electron-injecting electrodes for polymer light emitting diodes in J. Applied Physics 88(6), 3618-3623 (2000). Longer operating life is purportedly achieved through the use of ultrathin layers of calcium, strontium and barium as the cathode. Aluminium is used in the reported experiments as a capping layer on top of the alkaline earth metal layer. A reflecting cathode using a layer of aluminium or silver is described in a monochrome device in Applied Phys. Lett 85(13), 2469-2471 (2004). This arrangement avoids the need for a reactive low workfunction metal by using an electron-injecting trilayer OfAIq₃-LiF-Al.

Unlike a monochrome OLED, a full colour OLED comprises red, green and blue light organic emissive materials. Such a device requires a cathode capable of injecting electrons into all three emissive materials, i.e. a "common cathode". WO2004/023574 describes methods for producing a full-colour organic electroluminescent device. A device is described using a common cathode which comprises layers of aluminium and barium or layers of aluminium and calcium. In Nature 421_, 829-832 (2003) a method for solution processing is described for the production of multi-colour organic light emitting displays. This paper is concerned primarily with a class of electroluminescent polymers that can be patterned in a way similar to standard photoresist materials by providing soluble polymers which can be cured photochemically to yield an insoluble form. The general device structure used to demonstrate these polymers is a bilayer of calcium and silver as a cathode. Generally speaking, in device applications where drive circuitry and other structures are situated adjacent to the cathode, a transparent anode is used. Conveniently, the cathode uses a capping layer of aluminium as the material of choice because this is both conductive and reflective. In arrangements of the type described in WO2004/023574, the aluminium layer can be made reflective so as to increase the efficiency of light output from the device by reflecting the light emitted from the emissive layer through the anode.

Improving the external efficiency of organic LEDs is an important device engineering issue as it presents a ready method to improve the operational lifetime of devices. This is because the operational lifetime of organic LEDs is approximately proportional to the square of their efficiency, and thus modest improvements in efficiency will lead to significant improvements in lifetime. Such improvements in device lifetime are in general very significant, as in many cases, this is the most severe technological hurdle that limits their commercial application.

A problem exists in relation to multicolour displays. The materials available for use as the red, green and blue light emitting materials vary in their efficiency to produce light. This is particularly the case with red light emitting materials whose efficiency within a device is often lower than that of the green and blue light emitting materials. The present invention aims to address this problem and aims to provide improvements in full colour organic light emissive devices.

In a first aspect, the present invention provides an organic light emissive device comprising: a cathode; a transparent anode; and an organic light emissive region which comprises subpixels of red, green and blue light emitting materials and is positioned between the cathode and the anode so that the cathode injects electrons into each subpixel; wherein the cathode comprises a reflective silver layer and a transparent barium layer, in which the barium layer is positioned between the silver layer and the organic light emissive region.

It has surprisingly been found that a combination of a reflective silver layer and a transparent barium layer in a cathode used in a full-colour organic light emissive device provides advantages over prior art arrangements. In particular, the efficiency of light emission from red pixels in the devices of the present invention is found to be increased. This gives rise to an improved balance between the colours of the device and allows the device to be driven at a lower voltage. The lifetime of the device may thereby be increased and the power consumption reduced.

The present inventors have found that absorption in reflective Ba/ Al occurs mainly within the thick layer of aluminum and that replacing the Al layer with Ag results in the most significant absorption being within the thin layer of barium. This is attributed to the higher reflectivity of silver compared to aluminum, which (at 650 nm) is 0.98 and 0.91 respectively.

In one embodiment, the cathode comprises a bilayer of silver and barium. In this embodiment, the thickness of silver in the reflective silver layer is typically at least 50nm, preferably in the range 50nm to 1000 nm more preferably in the range 50nm to 500 nm most preferably around 250 nm. In this embodiment the silver must be of sufficient thickness to be reflective. A reflective layer according to the invention is one which typically reflects at least 95 % of incident light at a wavelength of 650 nm. In an alternative embodiment, the cathode comprises a multilayer arrangement in which the reflective silver layer comprises a layer of silver adjacent to the barium layer and a layer of a reflective metal. In this embodiment, the thickness of the silver in combination with the layer of the reflective metal must be sufficient to be reflective. The thickness of the silver in the reflective silver layer is typically less than 50nm and is preferably in the range of from 20nm to 50nm. In this embodiment, the reflective metal preferably comprises aluminium. One of the functions of the barium layer is to provide electron-injection into the organic light emissive region when the device is in use. The thickness of the barium layer is typically in the range of from 2nm to lOnm, preferably around 5nm. However, the thickness in a finished device may vary due to differences in deposition rate calibration for reactive metals. The thickness of the barium layer may therefore be in the range 25 to 35nm, preferably 30nm. The organic light emissive region comprises discrete sub-pixels of red, green and blue light emitting materials. The cathode injects electrons into each sub-pixel. In this way there is no need for separate cathodes to inject electrons into each sub-pixel separately. This greatly simplifies construction of multicolour organic light emissive devices. The construction of multicolour and full colour displays with a common cathode will be apparent to the skilled person. For example, an inkjet printed full colour display is disclosed in Synth. Metals 2000, 111-112, p.125-128.

Optionally, the cathode further comprises an encapsulant layer of SiO₂ or ZnS in electrical contact with the side of the cathode furthest from the emissive region. In this arrangement, the device may be encapsulated to prevent ingress of moisture and oxygen. Other suitable encapsulants include a sheet of glass, films having suitable barrier properties such as alternating stacks of polymer and dielectric as disclosed in, for example, WO 01/81649 or an airtight container as disclosed in, for example, WO 01/19142. A getter material for absorption of any atmospheric moisture and / or oxygen that may permeate through the substrate or encapsulant may be disposed between the substrate and the encapsulant. The anode may be constructed of any conventional material and typically has a workfunction greater than 4.3eV, usually around 4.8 eV. Conventional anode materials include tin oxide, high workfunction metals such as gold or platinum and indium tin oxide (ITO). Indium tin oxide is preferred. Other materials include chromium and alloys of chromium and nickel.

The organic light emissive region may comprise any suitable organic light emitting material such as an electroluminescent polymer, an electroluminescent dendrimer, an electroluminescent small molecule, or any combination thereof which is electroluminescent. The organic light emitting material is typically applied or formed as a layer thereof. Small molecule electroluminescent materials include 8-hydroxy quinoline aluminium (alq3 as described in US4539507). These materials are typically deposited as an organic thin film in OLEDs. Other small molecule emitters may be deposited in a host material which is usually polymeric, as part of a host-dopant system as disclosed in, for example, J. Appl. Phys. 1989, 65(9), 3610-3616.

Electroluminescent polymers include those described in PCT/WO90/13148 such as polyarylene vinylenes, including poly(para-phenylene vinylene) (PPV). Other materials include poly(2-methoxy-5(2'-ethyl)hexyloxyphenylene-vinylene) ("MEH- PPV"), one or more PPV-derivatives (e.g. di-alkoxy or di-alkyl derivatives), polyfluorenes and/or co-polymers incorporating polyfluorene segments, PPVs and related co-polymers, poly(2,7-(9,9-di-n-octylfluorene) ("F8"), ρoly(2,7-(9,9-di-n- octylfluorene)-(l,4-phenylene-((4-secbutylphenyl)imino)-l,4-phenylene)) ("F8-

TFB"), poly(2,7-(9,9-di-n-octylfluorene)-( 1 ,4-phenylene-((4-methylphenyl)imino)— 1 ,4-phenylene-((4-methylphenyl)imino)-l ,4-phenylene)) ("F8-PFM"), poly(2,7-(9,9- di-n-octylfluorene)-( 1 ,4-phenylene-((4-methoxyphenyl)imino)- 1 ,4-phenylene-((4- methoxyphenyl)imino)-l,4-phenylene)) ("F8-PFMO"), or (2,7-(9,9-di-n- octylfluorene)-3,6-Benzothiadiazole) ("F8BT").

Methods of forming layers of these polymers in OLEDs are well known in this art.

Electroluminescent dendrimers are also known for use in organic emissive layers in OLEDs. Such dendrimers preferably have the formula:

CORE - [DENDRITE]_n

in which CORE represents a metal cation or a group containing a metal ion, n represents an integer of 1 or more, each DENDRITE, which may be the same or different represents an inherently at least partially conjugated dendritic structure comprising aryl and/or heteroaryl groups or nitrogen and, optionally, vinyl or acetylenyl groups connected via sp or sp hybridised carbon atoms of said (hetero)aryl vinyl and acetylenyl groups or via single bonds between N and (hetero)aryl groups, CORE terminating in the single bond which is connected to an sp hybridised (ring) carbon atom of the first (hetero)aryl group or single bond to nitrogen to which more than one at least partly conjugated dendritic branch is attached, said ring carbon or nitrogen atom forming part of said DENDRITE.

Emission is typically provided by the dendrimer core; emission may be fluorescent as disclosed in WO 99/21935 or phosphorescent as disclosed in WO 02/066552.

Optionally the organic light emissive device may include a hole transporting layer between the anode and the organic light emissive region. Such a layer may assist hole injection from the anode into the emissive region. Examples of organic hole injection materials include PEDTVPSS as disclosed in EP0901176 and EP0947123, or polyarylene as disclosed in US5723873 and US5798170. PEDTVPSS is polystyrene sulphonic acid doped polyethylene dioxythiophene. Other hole transporting materials include PPV and poly(2,7-(9,9-di-n-oxtylfluorene)-(l,4-phenylene-(4-imino(benzoic acid))-l,4-phenylene-(4-imino(benzoic acid))-l,4-phenylene)) (BFA) and polyaniline.

Optionally, the reflective silver layer and a half-mirror layer provided on the opposite side of the organic light emissive region together form a resonant cavity LED (RCLED) for the red light emitting material to further improve the external electroluminescence efficiency of the LED. RCLEDs of this type are disclosed in, for example, Applied Physics Letters 63(5), 1993, 594-595.

The half-mirror reflects a proportion and transmits a proportion of red light incident on it, and is preferably a distributed Bragg reflector (DBR) comprising n pairs of first and- second dielectric materials of differing refractive index wherein n is at least 1, preferably 2. Preferably the first dielectric material is silicon dioxide and the second dielectric material is silicon nitride.

Preferably, the half-mirror layer is provided such that the anode is located between the mirror layer and the organic light emissive region, in which case the mirror layer may be in direct contact with the anode or spaced from the anode. Preferably, the RCLED for enhancing emission of red light defines a resonant cavity for the red subpixels only. Further RCLEDs for having the appropriate optical lengths for enhancing one or both of green and blue light emission may be formed with the green and blue subpixels respectively as described in, for example, US 6791261 or WO 01/15246.

The RCLED can comprise a high finesse structure or a low finesse structure. Preferably the RCLED comprises a low finesse structure.

In a further aspect, the present invention provides a process for the manufacture of an organic light emissive device, comprising: providing a portion of the device, which portion comprises an anode and an organic light emissive region which comprises subpixels of red, green and blue light emitting materials; depositing a barium layer; and depositing a reflective silver layer to form a cathode on the organic light emissive region.

The present invention will now be described in further detail, by way of example only, with reference to the accompanying drawings in which:

FIGURE 1 shows in diagrammatic form a typical cross-sectional structure of an

OLED;

FIGURE 2 shows in diagrammatic form a typical cross-sectional structure of an

OLED according to the invention.

Examples

Figure 2 shows in diagrammatic form a cross-sectional structure of an OLED according to the present invention. An anode material 22 such as ITO may be situated on a transparent glass or plastics substrate 25. Hole transporting material 26 is PEDT/PSS and is situated between anode 22 (ITO) and emissive layer 23. Optionally, a further intermediate layer 27 may be applied between the electron- injecting layer and the light emitting layer. A barium layer 24 is deposited over the light emitting layer 23 by electron beam evaporation or thermal evaporation. Over this layer is deposited a silver layer 28 also by electron beam evaporation or thermal evaporation. Optionally SiO₂ or ZnS is deposited over the silver layer to form a transparent encapsulation layer so as to protect the device from ingress of oxygen and moisture. The encapsulation layer is generally a dielectric or polymer-dielectric composition.

Device fabrication

Example 1 - Blue device

Poly(ethylene dioxythiophene) / poly(styrene sulfonate) (PEDT / PSS), available from H C Starck of Leverkusen, Germany as Baytron P ® is deposited over an indium tin oxide anode supported on a glass substrate (available from Applied Films, Colorado, USA) by spin coating. A hole transporting layer of F8-TFB (shown below) is deposited over the PEDT / PSS layer by spin coating from xylene solution to a thickness of about 10 nm and heated at 18O⁰C for 1 hour. A blue electroluminescent polymer as disclosed in WO 03/095586 is deposited over the layer of F8-TFB by spin-coating from xylene solution to form an electroluminescent layer having a thickness of around 65 nm. A 5nm thick layer of Ba is formed over the electroluminescent layer by evaporation of Ba until the desired thickness is reached. A 50 nm thick layer of Ag is then similarly formed over the Ba layer. Finally, the device is sealed from the atmosphere by placing a glass plate over the device such that the device is located within a cavity formed within the centre of the glass plate and gluing the glass plate to the substrate.

"F8-TFB" Example 2 - Red device

Devices may be prepared in accordance with the process of Example 1, except that the electroluminescent layer is formed from a red electroluminescent polymer comprising 50 mol% 9,9-di-n-octylfluorene-2,7-diyl, 17 mol% "TFB" repeat units (illustrated below), 30 mol% l,3,2-benzothiadiazole-4,7-diyl, and 3 mol% 4,7-bis(2-thiophen-5- yl)-l,3,2-benzothiadiazole. Materials of this type are disclosed in WO 00/46321 and WO 00/55927.

"TFB"

Example 3 - Green electroluminescent device

Devices may be prepared in accordance with the process of Example 1, except that the electroluminescent layer is formed from a green electroluminescent polymer as disclosed in, for example, WO 00/55927 and WO 00/46321.

Example 4 - Full colour device

A full colour device may be prepared according to the method of Example 1 except that the PEDT / PSS and F8-TFB layers are deposited by inkjet printing into inkjet wells formed by photolithography defining red, green and blue subpixel areas followed by inkjet printing the aforementioned red, green and blue electroluminescent polymers.

Claims

Claims:

1. An organic light emissive device comprising: a cathode; a transparent anode; and an organic light emissive region which comprises subpixels of red, green and blue light emitting materials and is positioned between the cathode and the anode so that the cathode injects electrons into each subpixel; wherein the cathode comprises a reflective silver layer and a transparent barium layer, in which the barium layer is positioned between the silver layer and the organic light emissive region.

2. An organic light emissive device according to claim 1, wherein the thickness of silver in the reflective silver layer is at least 50nm.

3. An organic light emissive device according to claim 1, wherein the reflective silver layer comprises a layer of silver adjacent to the barium layer and a layer of a reflective metal.

4. An organic light emissive device according to claim 3, wherein the thickness of silver in the reflective silver layer is in the range of from 20nm to 50nm.

5. An organic light emissive device according to claim 3 or claim 4, wherein the reflective metal comprises aluminium.

6. An organic light emissive device according to any preceding claim, wherein the thickness of the barium layer is in the range of from 2nm to IOnm.

7. An organic light emissive device according to any preceding claim, wherein the organic light emissive region comprises a light emitting polymer or dendrimer.

8. An organic light emissive device according to any preceding claim, which further comprises a half-mirror layer positioned on the side of the organic light emissive region opposite the reflective silver layer so that a resonant cavity LED is formed for the red light emitting material.

9. An organic light emissive device according to claim 8, wherein the half-mirror layer comprises a distributed Bragg reflector comprising at least one pair of first and second dielectric materials.

10. An organic light emissive device according to claim 9, wherein the first dielectric material is silicon dioxide and the second dielectric material is silicon nitride.

11. An organic light emissive device according to any one of claims 8 to 10 wherein the anode is positioned between the half-mirror layer and the organic light emissive region.

12. A process for the manufacture of an organic light emissive device according to any preceding claim, comprising: providing a portion of the device, which portion comprises an anode and an organic light emissive region which comprises subpixels of red, green and blue light emitting materials; depositing a barium layer; and depositing a reflective silver layer to form a cathode on the organic light emissive region.