WO2001095403A1

WO2001095403A1 - An electroluminescent device

Info

Publication number: WO2001095403A1
Application number: PCT/GB2001/002473
Authority: WO
Inventors: Craig Murphy; David Lacey
Original assignee: Cambridge Display Technology Ltd
Current assignee: Cambridge Display Technology Ltd
Priority date: 2000-06-05
Filing date: 2001-06-05
Publication date: 2001-12-13
Anticipated expiration: 2002-12-05
Also published as: AU2001260508A1; GB0013681D0

Abstract

An electroluminescent device comprising a first charge carrier injecting layer for injecting positive charge carriers, a second charge carrier injecting layer for injecting negative charge carriers and a light emissive layer located between the charge carrier injecting layers and comprising a first conjugated, light emissive polymer for accepting and combining charge carriers from the first and second charge carrier injecting layers to generate light and having a first band gap and a second conjugated, light emissive polymer for accepting and combining charge carriers from the first and second charge carrier injecting layers to generate light and having a second band gap, wherein a property of at least a part of at least one of the charge carrier injecting layers is selected so that the device is capable of emitting light selectively from either the first or second conjugated, light emissive polymer in at least a part of the light emissive layer.

Description

AN ELECTROLUMINESCENT DEVICE

The present invention relates to an electroluminescent device.

Electroluminescent devices that employ an organic material for light emission are described in PCT/WO90/13148 and US 4,539,507. The basic structure of these devices is a light-emissive organic layer, for instance a film of poly (p-phenylene vinylene) "ppv" sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes) . The electrons and holes combine in the organic layer generating photons. In PCT/WO90/13148 the organic light- emissive material is a polymer. In US 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinolino) aluminium "Alq". In a practical device, one of the electrodes is typically transparent, to allow the photons to escape the device.

Other organic light-emissive materials, having different bandgaps, can take the place of ppv in order to generate light of other colours. The bandgap is defined as the difference in energy between the "highest occupied molecular orbital" (HOMO) and "lowest unoccupied molecular orbital" (LUMO) energy levels of the light-emissive materials. These energy levels can be estimated from measurements of photo emission and particularly measurements of the electrochemical potentials for oxidation and reduction. It is well understood in the field that such energies are affected by a number of factors, such as the local environment near an interface, and the point on the curve from which the value is determined. Accordingly, the use of such values is indicative rather than quantitative. A helpful but not essential feature of an electroluminescent device is a hole transport layer. As disclosed in EP 0686662, this may be made from doped polyethylene dioxythiophene ("PEDOT") . The hole transport layer provides an intermediate energy level between the anode and the HOMO level in the light- emissive material which helps the holes injected from the anode to reach the HOMO level in the light-emissive materials.

International application number O99/48160 discloses an electroluminescent device comprising a first charge carrier injecting layer for injecting positive charge carriers, a second charge carrier injecting layer for injecting negative charge carriers and a light-emissive layer located between the charge carrier injecting layers and comprising a mixture of a first component for accepting positive charge carriers from the first charge carrier injecting layer, a second component for accepting negative charge carriers from the second charge carrier injecting layer and a third, organic light-emissive component for generating light as a result of combination of charge carriers from the first and second components at least one of the first, second and third components forming a type 2 semiconductor interface with another of the first, second and third components.

The above-described devices have great potential for display. However, each of these devices comprises a single light-emissive material or component for combining holes and electrons to generate light.

Light-emitting devices are known which use several layers of conjugated polymers with differing bandgaps which provide a range of different colour light-emitting layers. Emission from more than one layer is produced simultaneously. This allows control of the colour of device emission. Chem. Phys. Lett. (1992) , 200 (1-2), 46-54 discloses ppv' s and derivatives thereof as the light-emitting material . The ordering of the polymer layers in the device is disclosed as a further way of controlling the colour of device emission. The device is capable only of emitting light of one particular colour.

Adv. ater . (1997) , 9(1), 33-36 realises bright-red, green and blue emission colours based on an all-organic colour transformation technique using an electroluminescence device of the organic semiconductor parahexaphenyl "PHP". The deep blue PHP-emitted light can be converted into other colours by using dielectric mirrors or filters. The light-emissive material i.e. the "PHP" is capable of emitting light of only one particular colour .

In light of the above, known electroluminescence devices, the present applicants have identified a need for an electroluminescent device which is capable of selectively emitting light of one colour or another. This simplifies the device structure and, consequently, device manufacturing costs because dielectric mirrors or filters are not required. In addition, such devices have a more flexible range of uses as a consequence of being able to selectively emit light of one colour or another rather than a single colour only.

It is an aim of the present invention to overcome the deficiencies of prior art electroluminescent devices.

Accordingly, the present invention provides an electroluminescent device comprising a first charge carrier injecting layer for injecting positive charge carriers, a second charge carrier injecting layer for injecting negative charge carriers and a light emissive layer located between the charge carrier injecting layers and comprising a first conjugated, light emissive polymer for accepting and combining charge carriers from the first and second charge carrier injecting layers to generate light and having a first band gap and a second conjugated, light emissive polymer for accepting and combining charge carriers from the first and second charge carrier injecting layers to generate light and having a second band gap, wherein a property of at least a part of at least one of the charge carrier injecting layers is selected so that the device is capable of emitting light selectively from either the first or second conjugated, light emissive polymer in at least a part of the light emissive layer.

It is envisaged that the first bandgap will be different from the second bandgap. When the first bandgap is different from the second bandgap, light which is selectively emitted from the first conjugated, light emissive polymer will be a different colour to light emitted selectively from the second conjugated, light emissive polymer.

Selective emission from either the first or second conjugated, light emissive polymer is achieved by controlling charge injection from at least one of the charge carrier, injecting layers. Preferably, this control is achieved by at least a part of at least one of the charge carrier injecting layers comprising one or more discrete regions . The property of a discrete region is selected so that the device is capable of emitting light selectively from either the first or second conjugated, light emissive polymer in a part of the light emissive layer. Particularly, the charge injection properties of a region are so selected. In order to optimise the usefulness of a device according to the present invention, it is envisaged that a discrete region will have a surface area of approximately 10 microns to 1mm, measured horizontally across the device. Advantageously, a discrete region may be provided by photolithography and chemical etching techniques or by physical vapour deposition techniques, specifically by evaporation or sputtering deposition techniques. More specifically, a shadow mask sputtering deposition technique may be used. Such techniques will be well known to a person skilled in this art.

Preferably, the charge injection property of the charge injection layer which is selected so that the device is capable of emitting light selectively from either the first or second conjugated, light emissive polymer in a part of the light emissive layer is the workfunction of the charge carrier injecting layer. The workfunction would be selected so that charge injection is possible only from the discrete region into either the first or second conjugated polymer light emissive polymers in a part of the light-emissive layer.

Preferably, the workfunction of at least a part of the first charge carrier injecting layer is such that positive charge carriers are capable of being injected from the first charge carrier injecting layer into the HOMO level of either the first or second conjugated, light emissive polymers in at least a part of the light-emissive layer. More preferably, the workfunction of at least a part of the first charge carrier injection layer is such that positive charge carriers are capable of being injected from the first charge carrier injecting layer into the HOMO level of only the first conjugated, light emissive polymer or only the second conjugated, light emissive polymer in at least a part of the light-emissive layer.

Also preferably, the workfunction of at least a part of the second charge carrier injecting layer is such that negative charge carriers are capable of being injected from the second charge carrier injecting layer into the LUMO level of either the first or second conjugated, light emissive polymer in at least a part of the light-emissive layer. More preferably, the workfunction of at least a part of the second charge carrier injecting layer is such that negative charge carriers are capable of being injected from the second charge carrier injecting layer into the LUMO level of only the first conjugated, light-emissive polymer or only the second conjugated, light-emissive polymer in at least a part of the light-emissive layer.

The workfunction of at least a part of the first or second charge carrier injecting layers may be selected by treating the surface of the at least a part of the first or second charge carrier injecting layers.

The surface of the first or second charge carrier injecting layer may be treated with 0₂ plasma to oxidise the surface and raise the workfunction or CF₄ plasma to reduce the surface and lower the workfunction. Accordingly, by oxidising or reducing the surface a larger or smaller bandgap of the charge carrier injecting layer surface may be selected. Thus, the colour of emission may be selected. The extent of oxidation or reduction will be affected by a number of parameters including plasma type, treatment time, electric power supplied and chamber pressure.

Multiple-layer devices have the problem that, where layers are deposited from solution, it is difficult to avoid one layer being disrupted when the next is deposited. Problems can arise also with voids or material trapped between the increased number of inter-layer boundaries. Accordingly, the light-emissive layer advantageously may comprise a blend of the first and second conjugated, light emissive polymers in a device according to the present invention. It is preferable that, in such a blend, the first and second conjugated, light emissive polymers are at least partially phase-separated in the light-emissive layer .

A device according to the present invention has been realised wherein the blend comprises (i) a polymer comprising:

"pDsCΦr"

where n is from about 5 to about 500 and (ii) a poly-fluorene . Such a polymer is known from "Pulse excitation of low mobility LEDs : Implication for organic lasers", N. Tessler, D.J. Pinner, V. Cleave, D.S. Thomas, G. Yahiohlu, P. Le Barny, R.H. Friend, Appl. Phys. Lett. 74, 2766 (1999) where poly[4-(N-4 vinylbenzyloxyethyl, N-methylamino) -N- (2, 5-di-tertbutylphenyl- naphthali ide) ] is disclosed.

It is envisaged that other polymers may be used provided that a property of at least a part of at least one of the charge carrier injecting layers may be selected so that the device is capable of emitting light selectively from either the first or second conjugated, light emissive polymer in a part of the light-emissive layer.

A blend comprising the first and second conjugated, light emissive polymers may be a 1:99 to 99:1, typically 5-20:95-80 by volume blend. The ratio of first conjugated, light emissive polymer to second conjugated, light emissive polymer in the blend may be selected by the skilled person according to his needs. A preferred blend compositions is 50:50. The ratios quoted here are by volume.

In a further aspect according to the present invention, the light-emissive layer may further comprise a third conjugated, light emissive polymer for accepting and combining charge carriers from the first and second charge carrier injecting layers. The third conjugated, light emissive polymer should have a third bandgap. Preferably, the third bandgap is different from both the first and second bandgaps . Accordingly, the device will be capable of selectively emitting light from either the first, second or third conjugated, light emissive polymers in at least a part of the light-emissive layer.

Preferably, in the further aspect according to the present invention, the first, second and third conjugated, light emissive polymers are at least partially phase-separate in the light-emissive layer.

It is preferred that the workfunction of at least a part of the first charge carrier injecting layer is selected so that the positive charge carriers are capable of being injected from the first charge carrier injecting layer into the HOMO level of one, preferably one only, of the first, second or third conjugated, light emissive polymers in at least a part of the light-emissive layer.

Also preferably, the workfunction of at least a part of the second charge carrier injecting layer is selected so that negative charge carriers are capable of being injected from the second charge carrier injecting layer into the LUMO level of one, preferably one only, the first, second or third conjugated, light emissive polymers in at least a part of the light emissive layer .

Advantageously, the present device may comprise a positive charge carrier transport layer which helps hole injection from an anode electrode layer to the HOMO level in one of the first, second or third conjugated, light emissive polymers in the light emissive-layer. Accordingly, the first charge carrier injecting layer may be a positive charge carrier transport layer which is located between the light emissive layer and an anode electrode layer. Suitable materials for the positive charge carrier transport layer include polyphenylene vinylene, "NPD" 4,4-bis[N- (1-naphthyl) -N-phenyl-amino] biphenyl, polyvinylcarbazole or doped "PEDOT" polyethylene dioxythiophene . It is thought that, where a green light-emissive polymer is used as the first conjugated, light-emissive polymer and a blue light-emissive polymer is used as the second conjugated, light-emissive polymer, the use of PEDOT as a positive charge carrier transport material in the present device will promote green-type emission and reduce blue-type emission. In contrast to this, it is thought that where polyvinylcarbazole (PVK) is used instead of PEDOT as a positive charge carrier transport material in the same device, blue-type emission will be promoted and green-type emission will be reduced.

Where the device does not have a positive charge carrier transport layer, the first charge carrier injecting layer is an anode electrode layer.

In any device according to the present invention, the anode electrode layer suitably may comprise indium tin oxide, tin oxide, indium zinc oxide or a mixture thereof. Usually, the second charge carrier injecting layer is a cathode electrode layer. The cathode electrode layer suitably may be a calcium layer.

Advantageously, the device further comprises an encapsulation layer on the cathode electrode layer.

When making a device according to the present invention, it is preferred that the light-emissive layer is provided by a large area coating technique. Such techniques will be known to a person skilled in this art.

Examples of such a technique include spin coating, laid coating, dip coating, spray coating, and printing such as screen, flexographic and gravure printing.

The present invention now will be described in more detail with reference to the following Figures in which:

Figure 1 shows a general device structure according to the present invention. An approximately 200 nm encapsulation layer 1 made from aluminium is laid down on an approximately 10 nm calcium cathode layer 2. An approximately 45 nm polymer emissive layer 3 comprises a blend of a first and second or first, second and third conjugated, light emissive polymers. An approximately 50 nm hole transport layer 4 is included to help hole injection from the anode 5 to the light-emissive layer 3. The hole transport layer 4 may be a PEDOT or polyvinylcarbazole. The anode is a glass and InSnO_x layer which has been cleaned and plasma treated with Ar/0₂.

Figure 2 shows a schematic diagram of the device shown in Figure 1, exemplifying the first charge carrier injecting layer have discrete regions (A,B,C), wherein a property of each region is selected so that the device is capable of emitting light selectively from either the first, second or third conjugated, light emissive polymer. Spatially-discrete emission of different coloured light (a,b,c) thus is obtained.

Figure 3 shows a graph of electroluminescent intensity versus wavelength for polymers prepared as described in Example 1. The electroluminescence spectra show dominant emission from the PST material, when PEDOT is the hole transport layer. This is evidenced by the broad peak, centred at a wavelength ^~520nm. Multiple peaks are seen when polyvinylcarbazole (PVK) is used as the hole transport layer, with PST:F8 1:1 blend as the emitter. F8 is poly (9, 9 dioctylfluorene) . The dominant peaks at short wavelengths (420nm and 440nm) are due to the F8 polymer. The contribution from the PST is greatly diminished.

Figure 4 (a) -(e) show spatially-discrete emission colours obtainable from a device according to the present invention. The images show electroluminescence from LED pixels which are 8mm in length. For PST:F8 1:1 emitter layer on PEDOT hole transport layer, green light is observed, whereas, for the same emitter blend on PVK hole transport layer, blue light is seen.

Figure 4 (a) shows green emission having a wavelength of 520nm.

Figure 4 (b) shows dominant blue emission having a wavelength of 420nm with a small contribution from green emission having a wavelength at 520nm.

Figure 4 (c) shows green emission having a wavelength of 520nm.

Figure 4 (d) shows dominant green emission having a wavelength of 520nm with a small contribution from blue emission having a wavelength of 420nm. Figure 4 (e) shows green emission having a wavelength of 550nm.

Examples

Example 1: Two colour LED device with patterned hole transport layer

A two colour LED device may be made by using PEDOT and PVK as the two hole transport layers and an indium tin oxide anode layer. These layers will be spatially patterned. The hole transport materials are deposited by spin-coating. The first layer is deposited, then photoresist, a light sensitive organic material, is coated on top. The resist is patterned by UV exposure and etching. The exposed hole transport layer is then etched away. The second layer is then deposited and patterned by photolithography to leave material where the first layer is not present. A blend emitter, consisting of F8 polymer and PST polymer in the ratio 1:1 by weight, is then spin-coated onto the hole transport layer pattern. A cathode layer is deposited by metal evaporation and the whole device is encapsulated using two component epoxy and a glass sheet. This process may be extended to a three colour LED, using three hole transport materials and a three component emitter polymer blend.

Example 2 : Two colour LED device with surface treated anode

In an alternative embodiment to that described in Example 1, the indium tin oxide anode layer may be surface treated using a radio frequency gas plasma to alter the workfunction of the anode layer. An oxygen plasma will produce a workfunction which lies deeper than the untreated material (a larger voltage) , by up to 0.5eV. The surface properties of the indium tin oxide layer may be patterned by depositing photoresist onto the anode, etching away the resist in selected areas, then exposing the sample to the oxygen plasma in a barrel etch for a time of 5mins at a pressure of 1.5torr, a flow rate of 400sccm oxygen and a power of 400W. The resist is then completely removed from the sample. The emitter polymer blend (F8:PST 1:1) is then spin- coated onto the device, a cathode is deposited and the sample is encapsulated to complete the fabrication.

Claims

Cl aims :

1. An electroluminescent device comprising: a first charge carrier injecting layer for injecting positive charge carriers; a second charge carrier injecting layer for injecting negative charge carriers; and a light emissive layer located between the charge carrier injecting layers and comprising: a first conjugated, light emissive polymer for accepting and combining charge carriers from the first and second charge carrier injecting layers to generate light and having a first band gap; and a second conjugated, light emissive polymer for accepting and combining charge carriers from the first and second charge carrier injecting layers to generate light and having a second band gap, wherein a property of at least a part of at least one of the charge carrier injecting layers is selected so that the device is capable of emitting light selectively from either the first or second conjugated, light emissive polymer in at least a part of the light emissive layer.

2. An electroluminescent device according to claim 1, wherein the first band gap is different from the second band gap.

3. An electroluminescent device according to claim 1 or claim 2, wherein the at least a part of at least one of the charge carrier injecting layers comprises one or more discrete regions, wherein a property of each region is selected so that the device is capable of emitting light selectively from either the first or second conjugated, light emissive polymer in a part of the light emissive layer.

4. An electroluminescent device according to claim 3, wherein each discrete region is approximately 10 microns to 1mm in size.

5. An electroluminescent device according to claim 3 or claim 4, wherein the discrete regions are provided by sputtering deposition techniques.

6. An electroluminescent device according to anyone of claims 1 to 5, wherein the selected property is the workfunction.

7. An electroluminescent device according to claim 6, wherein the workfunction of at least a part of the first charge carrier injecting layer is such that positive charge carriers are capable of being injected from the first charge carrier injecting layer into the HOMO level of either the first or second conjugated, light emissive polymer in at least a part of the light emissive layer.

8. An electroluminescent device according to claim 7, wherein the at least a part of the first charge carrier injecting layer comprises at least two discrete regions such that the workfunction of a first discrete region is such that positive charge carriers are capable of being injected from the first discrete region into only the HOMO level of the first conjugated, light-emissive polymer and the workfunction of a second discrete region is such that positive charge carriers are capable of being injected from the second discrete region into only the HOMO level of the second conjugated, light-emissive polymer .

9. An electroluminescent device according to any one of claims 6 to 8, wherein the workfunction of at least a part of the second charge carrier injecting layer is such that negative charge carriers are capable of being injected from the second charge carrier injecting layer into the LUMO level of either the first or second conjugated, light emissive polymer in at least a part of the light emissive layer.

10. An electroluminescent device according to any one of claims 6 to 9, wherein the workfunction is selected by treating the surface of the at least a part of at least one of the charge carrier injecting layers.

11. An electroluminescent device according to claim 10, wherein the surface is treated with 0₂ plasma or CF₄ plasma.

12. An electroluminescent device according to any one of the preceding claims, wherein the light emissive layer comprises a blend of the first and second conjugated, light emissive polymers .

13. An electroluminescent device according to claim 12, wherein the first and second conjugated, light emissive polymers are at least partially phase-separated in the light emissive layer.

14. An electroluminescent device according to claim 12 or claim 13, wherein the blend comprises: (i) a polymer comprising:

where n is from 5 to 500 and (ii) a poly-fluorene .

15. An electroluminescent device according to any one of claims 12 to 14, wherein the blend is a 1:99 to 99:1 by volume blend.

16. An electroluminescent device according to any one of claims 1 to 15, wherein the light emissive layer further comprises a third conjugated, light emissive polymer for accepting and combining charge carriers from the first and second charge carrier injecting layers and having a third band gap.

17. An electroluminescent device according to claim 16, wherein the third band gap is different from both the first and second band gaps .

18. An electroluminescent device according to claim 16 or claim 17, wherein the first, second and third conjugated, light emissive polymers are at least partially phase-separated in the light emissive layer.

19. An electroluminescent device according to any one of claims 16 to 18, wherein the workfunction of at least a part of the first charge carrier injecting layer is such that positive charge carriers are capable of being injected from the first charge carrier injecting layer into the HOMO level of one of the first, second or third conjugated, light emissive polymers in at least a part of the light emissive layer.

20. An electroluminescent device according to any one of claims 16 to 19, wherein the workfunction of at least a part of the second charge carrier injecting layer is such that negative charge carriers are capable of being injected from the second charge carrier injecting layer into the LUMO level of one of the first, second or third conjugated, light emissive polymers in at least a part of the light emissive layer.

21. An electroluminescent device according to any one of claims 1 to 20, wherein the first charge carrier injecting layer is a positive charge carrier transport layer which is located between the light emissive layer and an anode electrode layer.

22. An electroluminescent device according to claim 21, wherein the positive charge carrier transport layer comprises polyphenylene vinylene, 4 , 4-bis [N- (1-naphthyl) -N-phenyl-amino] biphenyl, polyvinylcarbazole or doped PEDOT.

23. An electroluminescent device according to any one of claims 1 to 20, wherein the first charge carrier injecting layer is an anode electrode layer.

24. An electroluminescent device according to claim 23, wherein the anode electrode layer comprises indium tin oxide, tin oxide, indium zinc oxide or a mixture thereof.

25. An electroluminescent device according to any one of the preceding claims, wherein the second charge carrier injecting layer is a cathode electrode layer.

26. An electroluminescent device according to claim 25, wherein the device further comprises an encapsulation layer on the cathode electrode layer.

27. An electroluminescent device according to any one of the preceding claims, wherein the light emissive layer is capable of being provided by a large area coating technique.