WO2005011017A1

WO2005011017A1 - Organic elettroluminescent device with low oxygen content

Info

Publication number: WO2005011017A1
Application number: PCT/IB2004/051203
Authority: WO
Inventors: Michael Buechel; Peter Van De Weijer; Marcel P. H. Ligter
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2003-07-24
Filing date: 2004-07-13
Publication date: 2005-02-03
Anticipated expiration: 2006-01-24
Also published as: TW200518617A; WO2005011017A8

Abstract

An organic electroluminescent device comprising a cathode and an ode and, disposed therebetween, an organic layer has oxygen atoms accumulated near the int rface of the organic layer and the cathode in a maximum concentration of less than about 6 t%, the maximum concentration being expressed in terms of the peak maximum of a sputte profile of the Ois concentration near the interface of the organic layer and the cathode meas red by means of X-ray photo-electron spectroscopy. Having such an unprecedented low m imum oxygen concentration, the EL device has an extended operational lifetime.

Description

ORGANIC ELETTROLUMINESCENT DEVICE WITH LOW OXYGEN CONTENT

The invention relates to an organic electroluminescent device comprising a cathode and an anode and, disposed therebetween, an organic layer.

An electroluminescent (EL) device is a device which emits light when a suitable voltage is impressed on its electrodes. If the electroluminescent device has an organic material facilitating charge transport and/or light emission it is generally referred to as an organic electroluminescent device. In such EL device, the electrodes are typically referred to as cathode and anode. Organic electroluminescent devices can be made, by suitable choice of emissive material, to produce any color at low voltages. Being emissive, thin, light weight, flexible and/or of large area such devices are particularly suitable for display, signage and lighting applications. An organic electroluminescent device may comprise organic compounds of relatively low molecular weight, also referred hereinafter as small molecule electroluminescent devices, or compounds of high molecular weight, hereinafter also referred to as polymer electroluminescent devices. In a device as mentioned in the opening paragraph, operational lifetime is a concern. A factor limiting operational lifetime is ingress of oxygen and water. It is generally known in the art that organic electroluminescent devices are sensitive to oxygen and, in particular, water. To avoid ingress of water and oxygen during the operational lifetime, an organic electroluminescent device typically comprises an air and water-tight housing, while manufacture of the electroluminescent device typically takes place in an inert atmosphere such as nitrogen or, e.g. when depositing a metal cathode, vacuum. Although these measures provide a dramatic improvement in lifetime there is still a need to improve operational lifetime further.

It is an object of the invention to, inter alia, provide an electroluminescent device having an improved operational lifetime. These and other objects are achieved by means of an electroluminescent device as mentioned in the opening paragraph which, in accordance with the invention, has oxygen atoms accumulated near the interface of the organic layer and the cathode in a maximum concentration of less than 6 at%, the maximum concentration being expressed in terms of the peak maximum of a sputter profile of the Oι_s concentration near the interface of the organic layer and the cathode as measured by means of X-ray photo-electron spectroscopy. Preferably, the maximum concentration is less than 4 at%. The inventors have found to their surprise that, despite having taking all conventional measures to prevent ingress of water and oxygen during manufacture and operation, oxygen still accumulates in the device in amounts which are so high as to adversely affect operational lifetime. By taking measures which go beyond the conventional ones, the inventors have been able to bring down oxygen concentration down to unprecedented low levels of less than 6 at % or even 4 at %. Not desiring to be bound by any theory, the inventors believe the oxygen accumulated near the interface to be a result of ingress of water during manufacture and to reduce the ingress of water additional measures are required. The Oi_s signal associated with the interface of cathode and organic layer may be conveniently identified by measuring the Cι_s peak. The organic layer and thus the Oι_s peak associated with the interface, begins where the Cι_s peak begins to rise. The Oj_s signal may a single peak maximum or several local peak maxima. The latter case would occur when the signal gets noisy because it is near the detection limit. In either case, the maximum concentration of accumulated oxygen is equated to the largest value of the Oι_s signal which is attributable to oxygen accumulated oxygen near the interface, that is the overall peak maximum. The maximum concentration of accumulated oxygen can be lowered to unprecedented levels of less than about 6 at % or even 4 at % by means of a rigorous heat treatment using, for example, an infra-red heater or preferably a refractory metal heater. An example of a rigorous heat treatment is heating in a vacuum for example in a dedicated vacuum chamber or in the vacuum chamber of the cathode deposition apparatus. Compared to other infra-red heaters, a refractory metal heater has the advantage of radiating substantially only infra-red light no blue visible or UV is emitted. Thus, damage to the organic layer is prevented. A heat treatment used to remove solvent from a wet-deposited EL layer in a dry inert atmosphere is typically not rigorous enough. From the viewpoint of optimizing operational lifetime it is preferred that the maximum concentration of oxygen is as low as possible, preferably zero. However, practical considerations, such as imperfections of the method of manufacture, may set a lower limit on the maximum concentration attainable. So in a particular embodiment a lower limit is the detection limit of the 0[_S signal of the X-ray photo-electron spectrometer. On an absolute scale, a suitable lower limit is about 0.1 at % or more specific about 0.5 at % may be used. The invention is particularly useful if in the manufacture of the electroluminescent device use is made of a wet-deposition method, in particular if the formulation used for such deposition is water-based. A preferred embodiment of the EL device in accordance with the invention is one wherein the organic layer comprises a sublayer obtained by wet-depositing a water-based formulation. If a wet-deposition method is used, in particular a water-based one, contamination with water is a problem of particular concern and hence the solution provided by the present invention of particular advantage. The deposition of hole-transporting layer is often water-based. Examples include poly-aniline hole-transport sub-layers and hole-transport sub-layers comprising an alkoxy-substituted poly-thiophene. Poly-diethyleneoxythiophene, optionally used in combination with polystyrenesulphonic acid, is a very suitable hole-transport material in the context of the present invention. Water contamination during manufacture is particularly a problem for polymer electroluminescent devices. A preferred embodiment of the invention, the organic electroluminescent device comprises an electroluminescent polymer. The EL device is, apart form the low levels of accumulated oxygen, of a conventional construction. The anode is typically formed of a high work function material, such as gold, aluminum, platinum or, for reasons of optical transparency preferably, an inorganic (semi)conducting oxide such as indium tin oxide. The cathode is typically formed of a low work function material, for example a group I or II metal such as Na, Li, K, Cs, Rb, Mg, Ba or Ca, or Sc or Yb, or salts derived therefrom such as LiF, or alloys of such metals with other metals, such as Al or Ag. Preferably, to reduce the risk of oxidation, the low work function material is covered with a less reactive material such as Al. A Ba/Al or Al/LiF cathode is preferred. The organic layer of the electroluminescent device may be formed of a single electroluminescent layer or may be a stack of a number of distinct sub-layers. Typical examples of such stacks of sub-layers include HIE HTL/LEL/EIE, HIE/LEL ETL/EIE, HIE/HTL/LEL/ETL/EIE, HIE/LEL HBL/EIE, HIE/EBL/LEL/EIE, HIE/HTL/LEL/HBL/EIE, HIE/HTL/EBL/LEL/EIE, HIE/LEL/HBL/ETL/EIE, HIE HTL/LEL/HBL/ETL/EIE, HIE/HTL/HBL/LEL/ETL/EIE or HIE/HTL/HBL/LEL/EIE, HIE/HTL/LEL/XBL/EIE wherein HIE means hole-injecting electrode, EIE electron-injecting electrode, HTL hole- transport and/or hole-injecting layer, ETL electron-transport and/or injecting layer, LEL light-emission layer, HBL hole-blocking layer, EBL electron-blocking layer and XBL exciton blocking layer. Such stacks and sub-layers are known in the art as such and may be suitably used in the electroluminescent device in accordance with the invention. The light-emission layer may be small molecule but preferably is polymeric. It may be a host-guest system, in particular a host-guest system where the host is a polymer or the guest is a triplet emitter, such systems being known in the art as such. Typically, the EL device includes a substrate which may be formed of glass, metal, ceramic, a silicon wafer or synthetic resin or combination thereof. The EL device may be arranged to emit light through the substrate, also known as bottom-emissive, or through in the opposite direction (top-emissive) or both. The EL device may be used as a lighting, a signage or a display device.

Display devices include single-pixel, segmented and matrix devices, both active and passive. The display may be monochrome, multi-color or even full-color. These and other aspects of the invention will be apparent from and elucidated with reference to the examples described hereinafter.

In the drawings: Fig. 1 shows, schematically, in a cross-sectional view, an organic electroluminescent device; Fig. 2 shows a graph of sputter time t (in minutes) versus concentration c (in at

%) of an electroluminescent device not in accordance with the invention, where the curve labeled Oι_s represents the oxygen Oj_s concentration and the curve labeled C_ls represents the carbon Cι_s concentration both as measured by X-ray photo-electron spectroscopy; Fig. 3 shows a graph of sputter time t (in minutes) versus concentration c (in at %) of an electroluminescent device in accordance with the invention, where the curve labeled Ois represents the oxygen Oι_s concentration and the curve labeled Cι_s represents the carbon Cis concentration both as measured by X-ray photo-electron spectroscopy; Fig. 4 shows a graph of sputter time t (in minutes) versus concentration c (in at %) of another electroluminescent device not in accordance with the invention, where the curve labeled Oι_s represents the oxygen Oι_s concentration and the curve labeled Cι_s represents the carbon Cι_s concentration both as measured by X-ray photo-electron spectroscopy; and Fig. 5 shows a graph of sputter time t (in minutes) versus concentration c (in at i %) of another electroluminescent device in accordance with the invention, where the curve labeled Oι_s represents the oxygen Oι_s concentration and the curve labeled Cι_s represents the carbon Cι_s concentration both as measured by X-ray photo-electron spectroscopy.

Example 1 (not in accordance with the invention) Device structure Fig. 1 shows, schematically, in a cross-sectional view, an organic electroluminescent device 1. The organic EL device 1 has an organic layer 3 disposed between an anode 5 and a cathode 7. The organic layer 3 is in the form of a stack of sub- layers, the stack including an electroluminescent sub-layer 9 and a hole-transporting sub-layer 11. The EL device 1 further comprises a transparent substrate 13. In order to avoid ingress of oxygen and water, the EL device 1 is enclosed in a housing. In the present example, the housing is formed of the substrate 13 and a lid 15 glued together by means of an epoxy perimeter seal 17. Commonly, a getter (not shown) is arranged in the housing to absorb any water inadvertently entering the housing. Device manufacture A typical method of manufacturing the electroluminescent device 1, using typical representatives of the various parts shown in Fig. 1, is as follows: Referring to Fig. 1, a substrate 13 of soda lime glass is coated with indium tin oxide in a sputter process via shadow mask (ITO, 170 run thick, executed by Balzers) resulting in a structured anode layer 5. The ITO-coated substrate is washed with water under ultrasonic treatment, dried in a centrifuge, and UV /ozone cleaned for 15 min. The ITO side of the substrate 13 is provided with an aqueous formulation of PEDOT (poly-3,4- ethylenedioxythiophene) and polystyrenesulfonic acid (PSS) in the ratio 1 :20 (commercially available from HC Starck (fonnerly Bayer) as BAYTRON^® P VP CH 8000 by means of a spin coating process. The spin-coated layer is dried for 2 min at 200°C in air on a hotplate resulting in a 200 nm thick hole-transporting sub-layer 11 of a poly-ethylenedioxythiophene. The electroluminescent sub-layer 9 is also provided by means of spin-coating. It is formed of a green-emitting electroluminescent polyfluroene-based material (Lumation 1300 series, available from Dow). The EL device 1 is thus a polymer electroluminescent device. The solvent used is volatile and is removed by the spinning process. The EL sub-layer 9 is then covered, in succession, with a Ba and an Al layer of 5 run and 100 nm thickness, respectively, by means of deposition of metal vapor in vacuo via a shadow mask, resulting in the cathode 7. The evaporation is carried out in a conventional vapor deposition apparatus at room temperature. The vacuum has a pressure of less than 1 E-6 mbar. During evaporation the samples heat up to maximally 50 °C. In a glove-box wherein a dry nitrogen atmosphere is maintained, a perimeter seal 17 of epoxy is provided on the substrate 13 and a metal lid 15 having a recession for accommodating getter material (not shown) adhered to the seal 17 to close the housing which completes the manufacture of the EL device 1. X-ray photo-electron spectrum The EL device is transported to a Phi Quantum 2000 X-ray photo-electron spectrometer (XPS). The Quantum 2000 is equipped with a monochromatic AlKα radiation source and an Ar⁺ sputter source. The monochromatic AlKα radiation source is for generating photo-electrons and the sputter source is for removing matter from the EL device. It is arranged that the sample (EL device) can be moved back and forth between the sputter source and the radiation source thus allowing a sputter profile to be recorded. To allow the radiation and sputter access to the cathode surface, the lid 15 is removed in a glove-box (N2 atmosphere; H20 en 02 < 1 ppm) which is coupled to the spectrometer via a load lock. While maintaining a nitrogen atmosphere, the opened EL device is positioned in the XPS apparatus and a sputter profile of the Oι_s and Cι_s signal is recorded. Specifically, when measuring, the angle between the analyzer and the substrate surface of the EL device is set to 45°, in which case the penetration depth of the X- ray radiation is about 5 nm. Measurement is in high power mode (100 W, measurement spot 100 μm, raster area 1400 x 500 μm²). A sputter profile of the Oι_s signal is obtained by alternatively sputtering (with Ar ions at Vi = 4 kV; raster 3 x 3 mm) for typically one minute (these conditions provide a sputtering speed of 6.2 nm/min on a Siθ₂ substrate) and measuring the XPS signal. To convert raw measurement data to concentrations standard sensitivity factors are used which may introduce an error, the error being usually less than 20 % and when mutually comparing samples less than 5 %. The resulting sputter profile of the Oι_s XPS signal is shown in Fig. 2. Fig. 2 shows a graph of sputter time t (in minutes) versus concentration c (in at

%) of the electroluminescent device not in accordance with the invention, where the curve labeled Oι_s represents the oxygen Oι_s concentration and the curve labeled Cι_s represents the carbon Cι_s concentration both as measured by X-ray photo-electron spectroscopy. At about time t = 0, a large Oι_s signal is detected. This corresponds to oxygen accumulated at the surface of the cathode because this is the side at which sputtering began. The rise of the Cι_s marks the beginning of the organic layer and it is this location where the Ois signal corresponds to oxygen atoms accumulated near the interface of the organic layer and the cathode. This Oι_s peak rises and reaches a maximum at 12 min. The peak maximum of the sputter profile of the Oι_s concentration is equated to the maximum concentration of oxygen atoms accumulated near the interface of the organic layer and the cathode and amounts to 6.5 at%. Operational lifetime Following the method of manufacture above, a second EL device is manufactured which is identical to the EL device used for obtaining the sputter profile. The EL device thus obtained is subjected to an operational lifetime test. The

EL device is stressed at a constant current density of about 6.25 mA/cm² at ambient conditions of 80 °C and 50 % relative humidity. The operational lifetime, that is the time within which the luminance drops to half its initial value, of the EL device of this Example 1 is 59 hours. Example 2 (in accordance with the invention) Device structure The device structure is identical to that used in Example 1. Device manufacture The method of manufacturing of the EL device is identical to that of Example

1 except that after deposition of the electroluminescent sub-layer, but before cathode deposition, the EL device being manufactured is subjected to a heat treatment in vacuum. To effect the heat treatment in vacuum, the cathode deposition chamber is fitted with a heat source, in particular a refractory metal infra-red .heater which when heated to about 900 °C emits substantially exclusively infra-red light and no blue or UV light. Such heaters are known in the art as such. The IR radiation heats the EL device being manufactured to about 110 to 120 °C. This temperature is maintained for 5 minutes (excluding warm up and cool down). X-ray photo-electron spectrum The EL device manufactured in accordance with the method of this Example 2 is subjected to the X-ray photo-electron measurement described in Example 1. The resulting sputter profile is shown in Fig. 3. Fig. 3 shows a graph of sputter time t (in minutes) versus concentration c (in at %) of the electroluminescent device in accordance with the invention, where the curve labeled Oι_s represents the oxygen Oι_s concentration and the curve labeled C_ls represents the carbon Cι_s concentration both as measured by X-ray photo-electron spectroscopy. The peak maximum of the sputter profile of the Oι_s concentration is equated to the maximum concentration of oxygen atoms accumulated near the interface of the organic layer and the cathode and amounts to 3.2%. Operational lifetime The operational lifetime of the EL device is measured to be 144 hours. Comparing the results of the Example 1 and Example 2 shows that a maximum concentration of oxygen, expressed in terms of the peak maximum of a sputter profile of the Oι_s concentration near the interface of the organic layer and the cathode as measured by means of X-ray photo-electron spectroscopy, of less than 4 % results in a significant improvement in operational lifetime, the improvement being more than a factor of two.

Example 3 (not in accordance with the invention) Example 1 is repeated with the difference that as electroluminescent sub-layer, a layer of a yellow light-emitting phenyl-substituted poly-phenylenevinylene commercially available from Covion GmbH and a different type of PEDOT/PSS, viz. Baytron P CH8000, is used. Fig. 4 shows a graph of sputter time t (in minutes) versus concentration c (in at

%) of this electroluminescent device not in accordance with the invention, where the curve labeled Oι_s represents the oxygen Oι_s concentration and the curve labeled Cι_s represents the carbon Cι_s concentration both as measured by X-ray photo-electron spectroscopy. The sputter profile of the Oι_s concentration near the interface of the organic layer and the cathode has a peak maximum slightly over 7 at%. The operational lifetime is measured to be 108 hours at a current density of 6.25 mA/cm². Example 4 £in accordance with the invention) Example 2 is repeated with the difference that as electroluminescent layer, a layer of a yellow light-emitting phenyl-substituted poly-phenylenevinylene commercially available form Covion GmbH and a different type of PEDOT/PSS, viz. Baytron P CH8000, is used. The EL device thus has been subjected to the heat treatment with the refractory metal heater. Fig. 5 shows a graph of sputter time t (in minutes) versus concentration c (in at %) of another electroluminescent device in accordance with the invention, where the curve labeled Oι_s represents the oxygen Oι_s concentration and the curve labeled Cι_s represents the carbon Ct_s concentration both as measured by X-ray photo-electron spectroscopy. The sputter profile of the Oι_s concentration near the interface of the organic layer and the cathode has a peak maximum of about 5.5 at%. The operational lifetime of the EL device is measured to be 565 hours at a current density of 6.25 mA/cm². Comparing the results of the Example 3 and Example 4 shows that a maximum concentration of oxygen, expressed in terms of the peak maximum of a sputter profile of the Oι_s concentration near the interface of the organic layer and the cathode measured by means of X-ray photo-electron spectroscopy, of less than about 5.5 at % results in a five-fold improvement in operational lifetime. Similar results are obtained when the heat treatment is a separate dedicated vacuum chamber, placed in a room with normal ambient atmosphere, after which the device is transferred to the cathode deposition apparatus without exposing the device to ambient atmosphere.

Claims

CLAIMS:

1. An organic electroluminescent device comprising a cathode and an anode and, disposed therebetween, an organic layer, the electroluminescent device having oxygen atoms accumulated near the interface of the organic layer and the cathode in a maximum concentration of less than about 6 at%, the maximum concentration being expressed in terms of the peak maximum of a sputter profile of the Oι_s concentration near the interface of the organic layer and the cathode as measured by means of X-ray photo-electron spectroscopy.

2. An organic electroluminescent device as claimed in claim 1 wherein the maximum oxygen concentration is less than about 4 at%.

3. An organic electroluminescent device as claimed in claim 1 or 2 wherein the organic layer comprises a sub -layer obtained by wet-depositing a water-based formulation.

4. An organic electroluminescent device as claimed in claim 3 comprising a water •based hole-transporting sub-layer.

5. An organic electroluminescent device as claimed in claim 1, 2, 3 or 4 wherein the water-based hole-transporting sub-layer comprises an alkoxy-substituted poly-thiophene.

6. A organic electroluminescent device as claimed in claim 1, 2, 3, 4 or 5 wherein the electroluminescent device comprises an electroluminescent polymer.