WO2005036665A1

WO2005036665A1 - Method for improving the uniformity of emission of an electroluminescent device by irradyating it with ligh

Info

Publication number: WO2005036665A1
Application number: PCT/GB2004/004331
Authority: WO
Inventors: Nalinkumar Patel; Matthew Roberts
Original assignee: Cambridge Display Technology Limited
Priority date: 2003-10-09
Filing date: 2004-10-08
Publication date: 2005-04-21
Also published as: GB2421855B; GB0323720D0; GB2421855A; GB0606262D0

Abstract

A method of improving the uniformity of emission of an organic light emitting device (OLED), wherein the OLED comprises an organic electroluminescent material located between a first electrode for injection of positive charge carriers and a second electrode for injection of negative charge carriers; the method comprising the step of exposing the organic electroluminescent material to a treatment light having a wavelength shorter than the light emitted from said organic electroluminescent material when the OLED is driven, wherein the intensity of the treatment light incident on the organic electroluminescent material is greater than the intensity of ambient light incident on the organic electroluminescent material.

Description

METHOD FOR IMPROVING THE UNIFORMITY OF EMISSION OF AN ELECTROLUMINESCENT DEVICE BY IRRADYATING IT WITH LIGH

Field of the Invention

The invention relates to a method of treatment for organic electroluminescent devices.

Background of the Invention

One class of opto-electrical devices is that using an organic material for light emission (an organic light emitting device or "OLED") or as the active component of a photocell or photodetector (a "photovoltaic" device). The basic structure of these devices is a semiconducting organic layer sandwiched between a cathode for injecting or accepting negative charge carriers (electrons) and an anode for injecting or accepting positive charge carriers (holes) into the organic layer. In a practical device, at least one of the electrodes is transparent to allow photons to enter or escape the device.

In an organic electroluminescent device, electrons and holes are injected into the semiconducting organic layer where they combine in to generate excitons that undergo radiative decay. In WO 90/13148 the organic light-emissive material is a polymer, namely poly (p-phenylenevinylene) ("PPV"). Other light emitting polymers known in the art include polyfluorenes and polyphenylenes. In US 4,539,507 the organic light- emissive material is of the class known as small molecule materials, such as (8- hydroxyquinoline) aluminium ("Alq3"). WO 99/21935 discloses the class of materials known as dendrimers. The specific materials mentioned above emit light predominantly by fluorescence (i.e. by radiative decay of singlet excitons), however heavy metal complexes may be used to induce phosphorescent emission (i.e. by radiative decay of triplet excitons) as disclosed in, for example, Chem. Phys. Lett, 1993, 210, 61, Nature (London), 2001 , 409, 494, Synth. Met, 2002, 125, 55 and references therein.

OLEDs may comprise a single organic light-emissive material, in which case the OLED is a monochrome device. Alternatively, multicolour devices may be formed by use of two or more organic light emissive materials having different wavelengths of emission. In particular, a full colour OLED may be formed by appropriate combination of red, green and blue (RGB) organic light emissive materials. The luminance of an OLED decays gradually with time except when it is first driven, when the initial luminance drops sharply before stabilising and decaying at a slower rate. This initial sharp drop in luminance is a phenomenon known as "burn-in" and is observed across the range of colours and material classes. In a segmented or pixellated display, i.e. a display comprising at least two independently addressable emissive regions, burn- in is problematic because different segments or pixels will be driven for different lengths of time and at different intensities. As a result, different pixels burn-in at different times, resulting in non-uniform brightness for both monochrome and multicolour (particularly full colour) OLEDs. Furthermore, it is likely that a significant proportion of pixels in a pixellated OLED display will be driven infrequently (depending on the information displayed by the OLED) and as a result the non-uniformities in the display caused by those pixels not being bumed-in may persist for a lengthy period. In addition to this non- uniformity, it is clearly undesirable for the luminance of an OLED to fall dramatically in the early part of its operation.

A solution to the problem of burn-in is described in Proceedings of SPIE-The International Society for Optical Engineering (2003), 5004 (Poly-Silicon Thin Film Transistor Technology and Applications in Displays and Other Novel Technology Areas), 127-139. This describes an arrangement wherein non-uniformities resulting from burn-in are compensated for by drive circuitry responding to signals from an optical feedback system. This arrangement has the drawback of increasing device complexity by inclusion of detectors and the appropriate drive circuitry. Therefore, a need exists for a simpler means of reducing the effect of burn-in.

WO 02/07235 discloses exposure of an OLED to ultraviolet light by selectively exposing areas of the device to ultraviolet light to degrade the electroluminescent material in those areas and thereby reduce the emissivity of those areas, such that when the device is energised a pattern consisting of variations in luminescence is created. This disclosure is solely concerned with forming a pattern of emissive areas from an unpatterned layer of electroluminescent material, and is not concerned with the problem of burn-in.

It is therefore an object of the invention to provide a method for reducing the burn-in effect in an OLED, but without significantly affecting the luminance of the OLED following burn-in. Summary of the Invention

The present inventors have surprisingly found that burn-in can be reduced, without significantly effecting luminance after burn-in, by exposing an OLED to light of a suitable wavelength.

Accordingly, in a first aspect the invention provides a method of improving the uniformity of emission of an organic light emitting device (OLED) wherein the OLED comprises an organic electroluminescent material located between a first electrode for injection of positive charge carriers and a second electrode for injection of negative charge carriers; the method comprising the step of exposing the organic electroluminescent material to a treatment light having a wavelength shorter than the light emitted from said organic electroluminescent material when the OLED is driven, wherein the intensity of the treatment light incident on the organic electroluminescent material is greater than the intensity of ambient light incident on the organic electroluminescent material.

By "ambient light" is meant internal lighting which is typically of the order of 0.05 mW/cm². The intensity of the treatment light incident on the emissive regions is preferably at least one order of magnitude, more preferably at least two orders of magnitude, more intense than the intensity of ambient light incident on the emissive regions. Preferably, the treatment light is an artificial light source having an intensity in the range of 0.5 - 100 mW / cm², more preferably 20-40 mW / cm².

Preferably, the OLED has a plurality of independently addressable emissive regions. More preferably, the OLED comprises at least two different organic electroluminescent materials that emit light of different wavelengths. Most preferably, the OLED comprises red, green and blue electroluminescent materials.

In one preferred embodiment, each different organic electroluminescent material is exposed to the same treatment light. In another preferred embodiment, each different organic electroluminescent material is separately exposed to the treatment light.

Preferably, the treatment light has a wavelength in the range 300-600 nm.

Preferably, the OLED is exposed to a dosage of 200 J/cm² from the treatment light.

Preferably, the OLED is exposed to the treatment light for less than 2 hours. Preferably, the organic electroluminescent material is a conjugated polymer.

In a second aspect, the invention provides an OLED obtainable according to the method of the first aspect of the invention.

In a third aspect, the invention provides a method of improving the uniformity of emission of a plurality of organic light emitting devices (OLEDs) wherein each OLED comprises an organic electroluminescent material located between a first electrode for injection of positive charge carriers and a second electrode for injection of negative charge carriers; the method comprising the step of exposing the organic electroluminescent material of each of the plurality of OLEDs to a treatment light having a wavelength shorter than the light emitted from the OLED when the OLED is driven, wherein each of the plurality of devices is exposed to substantially the same dosage of light.

Preferably, each of the plurality of OLEDs are substantially the same.

Preferred features of each of the plurality of OLEDs and treatment light according to the third aspect of the invention are as defined in the first aspect of the invention.

Brief Description of the Drawings

The invention will now be described in detail with reference to the drawings wherein:

Figure 1 shows a device made in accordance with the method of the invention; and

Figure 2 shows a plot of luminance vs. time for a device treated in accordance with the method of the invention.

Detailed Description of the Invention

With reference to figure 1 , an OLED preparable according to the method of the invention comprises a substrate 1, an anode 2 of indium tin oxide, a layer 3 of organic hole injection material, a layer 4 of organic semiconducting material and a cathode 5.

Optical devices tend to be sensitive to moisture and oxygen. Accordingly, the substrate preferably has good barrier properties for prevention of ingress of moisture and oxygen into the device. The substrate is commonly glass, however alternative substrates may be used, in particular where flexibility of the device is desirable. For example, the substrate may comprise a plastic as in US 6268695 which discloses a substrate of alternating plastic and barrier layers or a laminate of thin glass and plastic as disclosed in EP 0949850.

Although not essential, the presence of layer 3 of organic hole injection material is desirable as it assists hole injection from the anode into the layer or layers of semiconducting polymer. Examples of organic hole injection materials include polyethylene dioxythiophene) (PEDT / PSS) as disclosed in EP 0901176 and EP 0947123, or polyaniline as disclosed in US 5723873 and US 5798170.

Cathode 5 comprises a layer containing barium. This layer may consist solely of barium or it may comprise barium and another material, for example an alloy comprising barium. As an alternative to elemental barium, cathode 5 may comprise a layer containing a dielectric barium salt, in particular barium fluoride as disclosed in Appl. Phys. Lett. 2001, 79(5), 2001.

Cathode 6 may also comprise a capping layer of a relatively inert material over the layer containing barium. Suitable inert materials include silver, gold and aluminium.

The device is preferably encapsulated with an encapsulant (not shown) to prevent ingress of moisture and oxygen. 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.

At least one of the electrodes is semi-transparent in order that light may be emitted from the OLED when it is driven and furthermore in order that the treatment light may be absorbed by the organic light emitting material. Where the anode is transparent, it typically comprises indium tin oxide. Examples of transparent cathodes are disclosed in, for example, GB 2348316. An example of a transparent, barium containing cathode comprises a bilayer of barium and gold.

The organic light emitting material used to form layer 4 may be a fluorescent small molecule, dendrimer or polymer. The layer 4 may alternatively comprise a fluorescent or phosphorescent material doped into a host material. Suitable polymers for use as layer 4 include poly(p-phenylene vinylenes), polyphenylenes and polyfluorenes as disclosed in Adv. Mater. 2000 12(23) 1737-1750 and references therein. A single polymer or a plurality of polymers may be deposited from solution to form layer 4. Where a plurality of polymers are deposited, they may comprise a blend of at least two of a hole transporting polymer, an electron transporting polymer and, where the device is a PLED, an emissive polymer as disclosed in WO 99/48160. Alternatively, layer 4 may be formed from a single polymer that comprises regions selected from two or more of hole transporting regions, electron transporting regions and emissive regions as disclosed in, for example, WO 00/55927 and US 6353083. Each of the functions of hole transport, electron transport and emission may be provided by separate polymers or separate regions of a single polymer. Alternatively, more than one function may be performed by a single region or polymer. In particular, a single polymer or region may be capable of both charge transport and emission. Each region may comprise a single repeat unit, e.g. a triarylamine repeat unit may be a hole transporting region. Alternatively, each region may be a chain of repeat units, such as a chain of polyfluorene units as an electron transporting region. The different regions within such a polymer may be provided along the polymer backbone, as per US 6353083, or as groups pendant from the polymer backbone as per WO 01/62869.

The OLED may be monochrome, multicolour or full colour. Processes for production of monochrome displays include spin coating and dip-coating. Processes for production of full colour displays include inkjet printing as described in, for example, EP 0880303 and laser induced thermal imaging as disclosed in, for example, EP 1003354.

The OLED may be exposed to the treatment light across the whole of its emissive area in a one step treatment or in separate emissive regions only as part of a multi-step treatment. Multicolour OLEDs comprising two or more electroluminescent materials that emit light at different wavelengths may be exposed to a single treatment light having a wavelength shorter than the wavelength of light emitted from the electroluminescent material having the shortest wavelength. Alternatively, such multicolour OLEDs may be exposed to a plurality of treatment lights wherein each electroluminescent material is selectively exposed to a treatment light having a wavelength shorter than the light emitted by that electroluminescent material. The step of exposing the OLED to a treatment light may take place at any stage following manufacture of the essential components of the OLED, namely the anode, cathode, the electroluminescent layer and any further layers located between the anode and cathode. Conveniently, such exposure takes place after encapsulation of the OLED such that it does not need to take place in a controlled environment such as an inert atmosphere. It will be appreciated that exposure to treatment light should ideally take place before any incident light filtering layers, such as UV filtering layers, are applied to the OLED.

The method of the invention is amenable to a large scale manufacturing process involving the production of a plurality of OLEDs that are substantially the same, e.g. as formed during a batch manufacturing process. As part of the manufacturing process, this plurality of OLEDs may be exposed together to the same dosage of light to provide a uniform reduction in burn-in for each discrete OLED within the batch.

The plurality of discrete OLEDs may be formed on a single substrate and treated with light in accordance with the invention before the substrate is scribed and broken to physically separate each of the discrete OLEDs. Alternatively, the plurality of discrete OLEDs may be physically separate at the time of treatment in accordance with the invention.

Examples

Two identical devices were made according to the process set out below.

A layer of PEDT / PSS, available from H C Starck of Leverkusen, Germany as Baytron P ®, was deposited by spin coating onto indium tin oxide supported on a glass substrate (available from Applied Films, Colorado, USA). Onto this layer was deposited by spin coating from xylene solution a film of blue electroluminescent polymer as disclosed in WO 02/92723 comprising the repeat units 9,9-dioctylfluorene-2,7-diyl, 9,9- diphenylfluorene-2,7-diyl, "TFB" and "PFB" as illustrated below. A cathode comprising a first layer of lithium fluoride, a second layer of calcium and a capping layer of aluminium, as disclosed in WO 00/48258 was formed over the polymer by vacuum evaporation, and the device was sealed using an airtight container available from Saes Getters SpA.

TFB¹ "PFB"

One device was treated for a time of 30 minutes by irradiation using a lamp producing light of wavelength 330-380nm with a power density of 30 mW/cm².

As can be seen from Figure 2, the "burn-in" of the treated device is dramatically reduced as compared to the untreated device, without any significant effect on luminance of the device following burn-in.

Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications, alterations and / or combinations of features disclosed herein will be apparent to those skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims.

Claims

1 ) A method of improving the uniformity of emission of an organic light emitting device (OLED), wherein the OLED comprises an organic electroluminescent material located between a first electrode for injection of positive charge carriers and a second electrode for injection of negative charge carriers; the method comprising the step of exposing the organic electroluminescent material to a treatment light having a wavelength shorter than the light emitted from said organic electroluminescent material when the OLED is driven, wherein the intensity of the treatment light incident on the organic electroluminescent material is greater than the intensity of ambient light incident on the organic electroluminescent material.

2) A method according to claim 1 wherein the OLED has a plurality of independently addressable emissive regions.

3) A method according to claim 2 wherein the OLED comprises at least two different organic electroluminescent materials that emit light of different wavelengths.

4) A method according to claim 3 wherein the OLED comprises red, green and blue electroluminescent materials.

5) A method according to claim 2, 3 or 4 wherein each different organic electroluminescent material is separately exposed to the treatment light.

6) A method according to any preceding claim wherein the treatment light incident on the organic electroluminescent material has a wavelength in the range 300-600 nm.

7) A method according to any preceding claim wherein the OLED is exposed to a dosage of 200 J/cm² from the treatment light.

8) A method according to any preceding claim wherein the OLED is exposed to the treatment light for less than 2 hours. 9) A method according to any preceding claim wherein the organic electroluminescent material is a conjugated polymer.

10) An OLED obtainable by the method according to any preceding claim.

11) A method of improving the uniformity of emission of a plurality of organic light emitting devices (OLEDs) wherein each OLED comprises an organic electroluminescent material located between a first electrode for injection of positive charge carriers and a second electrode for injection of negative charge carriers; the method comprising the step of exposing the organic electroluminescent material of each of the plurality of OLEDs to a treatment light having a wavelength shorter than the light emitted from the OLED when the OLED is driven, wherein each of the plurality of devices is exposed to substantially the same dosage of light.

12) A method according to claim 11 wherein each of the plurality of OLEDs are substantially the same.