GB2439356A

GB2439356A - Organic electroluminescent devices

Info

Publication number: GB2439356A
Application number: GB0510719A
Authority: GB
Inventors: Jan Jongman; Faisal Qureshi
Original assignee: Cambridge Display Technology Ltd
Current assignee: Cambridge Display Technology Ltd
Priority date: 2005-05-25
Filing date: 2005-05-25
Publication date: 2007-12-27
Also published as: GB0510719D0

Abstract

The device comprises a glass substrate 20, a thin silver intermediate layer 22, an ITO anode 24, a layer of PEDOT 26, a light emitting polymer layer 28 and a barium/aluminium cathode 30. Limiting the separation distance between the upper surface of the intermediate layer and the upper surface of the first electrode to 110nm (1/4 x 440nm) ensures that only one effective microcavity is formed over the red, green and blue parts of the visible spectrum. Alternatively, limiting the separation distance to 150nm (1/4 x 600nm) ensures only one microcavity is formed over the red part of the visible spectrum.

Description

Organic Electroluminescent Device

Field of Invention

The present invention relates to an organic electroluminescent device. The present invention also relates to a substrate for an organic electroluminescent device.

Background of the Invention

Organic electroluminescent devices are known, for example from PCTIWO/13148 and US4539507. Such devices generally comprise a substrate 2, a first electrode 4 disposed over the substrate 2 for injecting charge of a first polarity; a second electrode 6 disposed over the first electrode 4 for injecting charge of a second polarity opposite to said first polarity; an organic light-emitting layer 8 disposed between the first and the second electrodes; and an encapsulant 10 disposed over the second electrode 6. In one arrangement shown in Figure 1, the substrate 2 and the first electrode 4 are transparent to allow light emitted by the organic light-emitting layer 8 to pass therethrough.

Such an arrangement is known as a bottom-emitting device. In another arrangement shown in Figure 2, the second electrode 6 and the encapsulant 10 are transparent so as to allow light emitted from the organic light-emitting layer 8 to pass therethrough. Such an arrangement is known as a top-emitting device.

Variations of the above-described structures are known. The first electrode may be the anode and the second electrode may be the cathode. Alternatively, the first electrode may be the cathode and the second electrode may be the anode. Further layers may be provided between the electrodes and the organic light-emitting layer in order to aid charge injection and transport. The organic material in the light-emitting layer may comprise a small molecule, a dendrimer or a polymer and may comprise phosphorescent moieties and/or fluorescent moieties. The light-emitting layer may comprise a blend of materials including light emitting moieties, electron transport moieties and hole transport moieties.

These may be provided in a single molecule or on separate molecules.

By providing an array of devices of the type described above, a display may be formed comprising a plurality of emitting pixels. The pixels may be of the same type to form a monochrome display or they may be different colours to form a multicolour display.

A problem with organic electroluminescent devices is that much of the light emitted by organic light-emitting material in the organic light-emitting layer does not escape from the device. The light may be lost within the device by scattering, internal reflection, waveguiding, absorption and the like. This results in a reduction in the efficiency of the device. Furthermore, these optical effects can lead to low image intensity, low image contrast, ghosting and the like resulting in poor image quality.

A further problem with organic electroluminescent devices is that of achieving intense, narrow band-width emission so as to improve the colour purity of emission.

One way of solving the aforementioned problems is to utilize microcavity effects within a device.

A microcavity is formed when the organic light-emitting layer is disposed between two reflecting mirrors, one of which is semitransparent. The photon density of states is modified such that only certain wavelengths, which correspond to allowed cavity modes, are emitted with emission intensity being enhanced in a direction perpendicular to the layers of the device. Thus emission near the wavelength corresponding to the resonance wavelength of the cavity is enhanced through the semitransparent mirror and emission at wavelengths away from the resonance is suppressed.

A weak microcavity is achievable using a standard device structure of the type described above. For example, in a boftom-emitting device such as that illustrated in Figure 1, a metal cathode 6 is generally utilized along with an ITO anode 4. The metal cathode is highly reflective while the ITO is substantially transparent but is weakly reflective. This weak reflectivity of ITO can result in a weak cavity effect. However, ITO provides a poor cavity not only because it is weak, but also because its refractive index is very variable over the visible spectrum. This results in variable performance with difference wavelengths and viewing angles.

In light of the above, it is known to alter the structure of organic electroluminescent devices in order to provide an improved microcavity effect.

US 6,861,800 discloses several modified arrangements. In one arrangement, illustrated in Figure 3a of US 6,861,800, the ITO anode is replaced with a semitransparent silver anode. A stronger microcavity is thus formed between the semitransparent silver anode and a reflective silver cathode when compared with an arrangement using an ITO anode as illustrated in Figure 3b of US 6,861,800. In fact, the microcavity effect of ITO is so low that in US 6,861,800 such an arrangement is described as having no microcavity.

One problem with replacing the anode electrode with a stronger mirror is that the electrical properties of the device will be altered.

In US 6,861,800 an alternative arrangement shown in Figure 3c has been proposed in which a Quarter Wave Stack (alternatively known as a Distributed Bragg Reflector) is disposed between the ITO anode and transparent substrate.

A QWS is a multi-layer stack of alternating high and low index dielectric layers which may be tuned so as to have very high reflectance, very low transmittance and practically zero absorbance over a given range of wavelengths. Such an arrangement provides a very strong microcavity.

WO 00/76010 also discloses the use of QWS between a substrate and an anode of a bottom-emitting device. As in US 6,861,800, ITO is not considered to contribute a cavity effect in the device and is described as transmissive.

In "A. Dodabalapur et a!., Physics and applications of organic microcavity light emitting diodes, J. AppI. Phys. 80 (12), 1996, 6954-6964" an arrangement is disclosed in which a QWS and a filler layer are provided between a transparent substrate and an ITO anode. The filler layer is of variable thickness so as to provide a number of cavities tuned to different colours. As in the aforementioned documents, the ITO anode is not considered to contribute a cavity effect in the device and is described as transmissive.

One problem the present applicant has found with arrangements which utilize a QWS is that the microcavity can be too strong. Although the absorbance of the QWS is practically zero, the layers of material between the QWS and the cathode do absorb light. Thus, because light is trapped in the cavity until it enters a mode which can pass through the low transmittance QWS, absorption of light by layers of material within the cavity becomes significant. Furthermore, the QWS results in a very narrow emission resulting in a narrow viewing angle and the colour of the emission changes with viewing angle. Additionally, a QWS is complicated and expensive to manufacture requiring the deposition of a number of additional layers and increases the thickness of the finished device.

Another problem the present applicant has found with arrangements which utilize a QWS is that the microcavity can be too large. Utilizing a QWS, the net effect of reflection from the stack of layers occurs approximately in the middle of the stack. As such, the distance between the net reflection from the QWS and the reflecting electrode forming the other side of the microcavity is large. The mode spacing is thus small as the mode spacing of a microcavity is inversely proportional to the size of the microcavity. With a large microcavity, while some parts of the spectrum are enhanced, other parts of the spectrum are reduced in intensity due to the small mode spacing allowing many modes to be accessed.

As a result, although spectral narrowing with an increase in intensity of certain wavelengths can be achieved with a QWS, the overall enhancement of light output may be minimal due to reduction in intensity in other parts of the spectrum.

In "T Shiga, Design of multiwavelength resonance cavities for white organic light-emitting diodes, J. AppI. Phys. 93 (1), 2003, 19-22" an organic electroluminescent device is disclosed in which two microcavities are provided.

Such an arrangement is illustrated in Figure 2 of this document. A first cavity is formed between an ITO anode and a cathode. A second cavity is formed between the cathode and a layer of high refractive index dielectric material spaced apart from the ITO anode by a spacer layer of a low refractive index dielectric material. Such an arrangement is designed to produce two emission peaks in different areas of the visible spectrum which mix to form a white emission.

One problem with this arrangement is that it is not suitable for improving the colour purity of emission as it is specifically directed to producing a white emission. Another problem with this arrangement is that, as stated previously, the ITO layer does not form a good cavity.

Summary of the Invention

It is an aim of the present invention to solve one or more of the problems described above.

According to a first aspect of the present invention there is provided an organic electroluminescent device comprising: a transparent substrate; a first electrode disposed over the substrate for injecting charge of a first polarity; a second electrode disposed over the first electrode for injecting charge of a second polarity opposite to said first polarity; an organic light-emitting layer disposed between the first and the second electrode, wherein the second electrode is reflective, the first electrode is transparent or semi-transparent, and one or more intermediate layers of dielectric material with a refractive index greater than 1.8 or a metal material disposed between the substrate and the first electrode forming a semi-transparent mirror whereby a microcavity is provided between the reflective second electrode and the semi-transparent mirror, all the intermediate layers disposed between the substrate and the first electrode having a surface nearest the organic light-emitting layer not more than 1 5Onm from a surface of the first electrode nearest the organic light-emitting layer.

The present inventors have found that the provision of intermediate layers disposed between the substrate and the first electrode all of which have a surface nearest the organic light-emitting layer not more than l5Onm from a surface of the first electrode nearest the organic light-emitting layer results in an increase in out-coupling of light from the device and also improves colour purity.

Furthermore, no change in colour with viewing angle is observed.

The provision of intermediate layers having surfaces within I 5Onm of the upper surface of the first electrode of material avoids the aforementioned problems associated with the use of a QWS as the resultant microcavity is not as strong.

Unlike the QWS, the intermediate layers do absorb some light. However, as the microcavity formed is weaker than that formed by a QWS, light emitted from the light-emitting layer is not trapped so strongly within the cavity. As a result, absorption from the other layers of material within the cavity is reduced.

Surprisingly, it has been found that any increase in absorbance by the intermediate layers when compared to a QWS can be off-set by a reduction in absorbance by the other layers within the cavity while still achieving the advantages of colour enhancement provided by a microcavity. Furthermore, the intermediate layers form a smaller microcavity when compared with a QWS as the net reflection from the intermediate layers is closer to the second reflecting electrode than the net reflection achieved from a QWS. As such, the mode spacing is larger for the intermediate layers compared to a QWS.

Accordingly, less modes are accessible which results in a decrease in the intensity of certain parts of the spectrum resulting in an overall enhancement of the spectrum which is superior to a QWS.

As the intermediate layers are not disposed between the two electrodes, the electrical properties of the device are not altered and the electrode materials do not need to be changed. The electrodes are generally selected for their charge injection properties which depend on the work function of the materials used.

For example, it has been found that ITO has a work function which can be tuned so as to achieve good hole injection. Furthermore, organic materials for emission and charge transport have been optimised by the present applicant for use with ITO anodes. Accordingly, the present applicant would preferably like to avoid changing the electrodes of the electroluminescent device based on their optical properties.

Additionally, unlike in "1 Shiga, Design of multiwavelength resonance cavities for white organic light-emitting diodes, J. Appl. Phys. 93 (1), 2003, 19-22", the intermediate layers are placed such that they have a surface not more than I 5Onm from a surface of the first electrode nearest the organic light-emitting layer. With the presently proposed arrangement, a single cavity effect is observed rather than two separate cavity effects from the ITO and the high index dielectric material as in the prior art document. The presently proposed arrangement allows for colour enhancement and also avoids the aforementioned problems with using ITO alone to form a microcavity. The microcavity of embodiments of the present invention is stronger than that which can be achieved with a standard ITO electrode. In fact, the microcavity formed by embodiments of the present invention is of a strength intermediate between that achieved using ITO alone and that achieved by using a QWS.

Thus, the present invention enhances the small reflectivity of the first electrode by the provision of intermediate layers placed such that they have a surface not more than I 5Onm from a surface of the first electrode nearest the organic light-emitting layer in order that the electrode and the intermediate layers together form a single microcavity with the reflective second electrode. The resultant microcavity is of intermediate strength compared with the prior art microcavities.

The present applicant has found that such a microcavity provides good properties for displays, e.g. high colour purity, low change in colour with viewing angle, increased out-coupling of light, low absorption within the device and simple device manufacture.

While not being bound by theory, to form one effective microcavity providing a single enhanced, phase shifted optical peak, the upper surface of the intermediate layers and the upper surface of the first electrode layer should be less than a quarter of the wavelength of the light emitted by the light-emitting layer.

Alternatively, again not being bound by theory, to form one effective microcavity the distance between the upper surface of the intermediate layers and the upper surface of the first electrode layer should be small enough whereby a supported mode has a width (which is inversely proportional to this distance) which is greater than the emission band width from the light-emitting layer. With such an arrangement any additional modes supported by the intermediate layers do not lead to spectral narrowing and accordingly only one microcavity effect is observed.

Preferably, the distance between the upper surface of the or each intermediate layer and the upper surface of the first electrode is less than l5Onm (1/4 X 600nm). This arrangement ensures that only one effective microcavity is formed over the red part of the visible spectrum. More preferably still, the distance between the upper surface of the or each intermediate layer and the upper surface of the first electrode is less than 125nm (1/4 X 500nm). This arrangement ensures that only one effective microcavity is formed over the green and red parts of the visible spectrum. Most preferably, the distance between the upper surface of the or each intermediate layer and the upper surface of the first electrode is less than llOnm (1/4 X 440nm). This arrangement ensures that only one effective microcavity is formed over the blue, green and red parts of the visible spectrum.

Preferably, the or each intermediate layer of material is transparent to visible light having a wavelength over 600nm. More preferably, the or each intermediate layer of material is transparent to visible light having a wavelength over 500nm. Most preferably, the or each intermediate layer of material is transparent to visible light having a wavelength over 400nm. The particular transparency required will depend on the emission frequency of the light-emitting layer. In multicoloured display devices, a common layer transparent to all visible frequencies is preferred.

Preferably, the or each intermediate layer of material comprises independently one of titanium dioxide, silicon oxynitride, silicon nitride, zinc sulphide, silver and silver alloys.

For metallic intermediate layers, advantageously the or each intermediate layer is 1-2Onm thick, more preferably 1-lOnm, more preferably still 3-7nm thick and -10 -most preferably around 5nm thick. A thin layer will be cheaper and quicker to deposit and will minimize the increase in thickness of the layered structure.

This may be advantageous in that no additional changes to a manufacturing method may be needed. For example, encapsulant cavities may not need to be increased in size to compensate for the additional layer or layers.

For a high refractive index dielectric intermediate layer, advantageously the or each intermediate layer is 1O-lOOnm thick, more preferably 20-7Onm, more preferably still 30-5Onm thick and most preferably around 4Onm thick.

Preferably, the first electrode is transparent to visible light having a wavelength over 600nm. More preferably, the first electrode is transparent to visible light having a wavelength over 500nm. Most preferably, the first electrode is transparent to visible light having a wavelength over 400nm whereby the first electrode is transparent over substantially all the visible spectrum.

Preferably, the first electrode has a work function over 4.0eV. Preferably the first electrode comprises ITO.

Preferably the transparent substrate has a refractive index of between 1.4 and 1.7. Glass sheets are preferable for rigid devices due to their inertness and their impermeability to air and moisture. Plastic sheets are useful for flexible devices.

Preferably the substrate comprises a colour filter. In the off state, light reflected by a microcavity is not the same colour as that emitted by the microcavity in the on-state. Accordingly, a colour filter may be added so as to transmit the colour of light emitted by the micro-cavity in the on state whilst absorbing light of the colour reflected by the microcavity in the off state.

The colour filter may be provided on an opposite surface of the transparent substrate to the intermediate layer. However, preferably the colour filter is provided on the same side of the transparent substrate as the or each intermediate layer. With this arrangement, the colour filter is closer to the emitting layer and potential parallax problems are avoided.

Preferably, the colour filter is the same colour as that emitted by the microcavity for increasing contrast of the electoluminescent device. For example, in one embodiment of the invention a microcavity optimised for emitting red light reflects green light in the off state. By adding a red colour filter, the off state becomes black enhancing the contrast, while in the on state the filter is transmissive to red light emitted by the microcavity. With this arrangement contrast is enhanced without significant luminous loss as is the case when a circular polarizer is used. The colour of reflected light in the off state will depend on the material used for the intermediate layer and also the structure of the microcavity. For arrangements in which, for example, a green emitting polymer is used, the microcavity is optimised for the green colour, and a green coloured filter may be used. Similarly, for a blue emitter, a blue coloured filter may be used.

The present applicant recognises that the advantageous effect achieved by providing an organic light-emissive device comprising a microcavity and a colour filter is not limit to the particular arrangement described above. Thus, in a more general form, there may be provided an organic electroluminescent device comprising: a first electrode for injecting charge of a first polarity; a second electrode for injecting charge of a second polarity opposite to said first polarity; and an organic light-emitting layer disposed between the first and the second electrode, wherein the second electrode is reflective and the first electrode is semi-transparent whereby a microcavity is provided between the first and second electrodes, wherein a colour filter is provided at an opposite side of the first electrode to the organic light-emitting material. The electrodes -12 -may comprise a number of layers providing a suitable reflectance to form the microcavity (for example, the intermediate layer(s) previously described). The colour filter is preferably the same colour as that emitted by the microcavity.

This will be a different colour from that emitted by the light-emissive material in the light-emitting layer as the microcavity will shift the wavelength of light.

Preferably, a single intermediate layer is provided, disposed between, and in contact with, the substrate and the first electrode. The provision of a single intermediate layer of material ensures that the aforementioned problems associated with the use of a QWS are avoided. Furthermore, a single intermediate layer of material is cheaper and easier to provide compared to a QWS.

Preferably, the first electrode is either: metallic and has a thickness of between and 3Onm; or is an inorganic oxide with a thickness of between 50 and I 5Onm. This is because, as the thickness of the first electrode becomes too small the conductivity of the film and/or the films structural integrity decreases.

Preferably, electric contacts are provided directly to the first electrode. If a conductive material is utilized for the intermediate layers then electrical contacts may be provided to the intermediate layers. However, as the intermediate layers are selected for their optical properties rather than their electrical properties, for many intermediate layer materials it is advantageous that the electrical contacts are provided directly to the first electrode so as to avoid the intermediate layers detrimentally affecting the electrical properties of the device.

In particular, if the intermediate layers are provided by a high refractive index dielectric material which does not have a high conductivity then it is advantageous to apply the electrical contacts directly to the first electrode.

An advantage of using a dielectric material as opposed to a metal for the intermediate layer is that the intermediate layer does not need to be patterned due to its lower conductivity. If a highly conductive material is used for the -13 -intermediate layer then the layer may require patterning so as to avoid shorting pathways in the device.

In one embodiment of the present invention, the organic electroluminescent device comprises a plurality of pixels forming a display, each pixel having its own microcavity. The substrate of such a display may be common to the plurality of pixels. Furthermore, the or each intermediate layer may be common to the plurality of pixels.

For an active matrix display, the substrate comprises a plurality of thin film transistors forming an active matrix back plane. In one such an arrangement a plurality of first electrodes are provided and a single second electrode. In contrast, for a passive matrix display a plurality of first electrodes and a plurality of second electrodes may be provided.

Optionally, the display comprises pixels which emit different colours. In such a multicolour display it is preferred that the size of the pixel's microcavities are different for emitting the different colours. That is, the cavities are tuned for a particular wavelength. The cavity size may be varied by, for example, varying the thickness of the first electrode (or one of the other layers of materials within the light emitting structures). Alternatively, the display may be monochrome in which case the cavity size of all the light-emitting structures is preferably the same.

According to a second aspect of the present invention there is provided a transparent substrate for an organic electroluminescent device, the transparent substrate comprising a layer of transparent material, a layer of transparent or semitransparent conductive material disposed over the layer of transparent material, and one or more intermediate layers of a dielectric material having a refractive index greater than 1.8 or a metal material disposed between the layer of transparent material and the layer of conductive material forming a semi- -14 -transparent mirror, all the intermediate layers disposed between the layer of transparent material and the layer of conductive material having a surface furthest from the layer of transparent material which is not more than I 5Onm from a surface of the conductive material furthest from the layer of transparent material.

According to a third aspect of the present invention there is provided a use of the substrate described herein in a method of manufacturing an organic electroluminescent device.

According to a fourth aspect of the present invention there is provided a method of manufacturing an organic electroluminescent device as described herein, the method comprising the steps: providing a prefabricated substrate; and depositing the other layers of the organic electroluminescent device thereon.

According to a fifth aspect of the present invention there is provided an organic electroluminescent device comprising: a transparent substrate; a first electrode disposed over the substrate for injecting charge of a first polarity; a second electrode disposed over the first electrode for injecting charge of a second polarity opposite to said first polarity; an organic light-emitting layer disposed between the first and the second electrode, wherein the second electrode is reflective, the first electrode is transparent or semi-transparent, and an intermediate layer of dielectric material with a refractive index greater than 1.8 or a metal material is disposed between, and in contact with, the substrate and the first electrode forming a semi-transparent mirror whereby a microcavity is provided between the reflective second electrode and the semi-transparent mirror.

According to a sixth aspect of the present invention there is provided a transparent substrate for an organic electroluminescent device, the transparent substrate comprising a layer of transparent material, a layer of transparent or -15 -semitransparent conductive material disposed over the layer of transparent material, and a layer of dielectric material having a refractive index greater than 1.8 or a metal material disposed between, and in contact with, the layer of transparent material and the layer of conductive material forming a semi-transparent mirror.

Thus, embodiments of the present invention may be provided with a single intermediate layer. The provision of a single intermediate layer avoids the aforementioned problems associated with the use of a QWS as the resultant microcavity is not as strong. Unlike the QWS, the singleintermediate layer does absorb some light. However, as the microcavity formed is weaker than that formed by a QWS, light emitted from the light-emitting layer is not trapped so strongly within the cavity. As a result, absorption from the other layers of material within the cavity is reduced. Surprisingly, it has been found that any increase in absorbance by the single intermediate layer when compared to a QWS can be off-set by a reduction in absorbance by the other layers within the cavity while still achieving the advantages of colour enhancement provided by a microcavity. A single layer of material is cheaper and easier to provide compared to a QWS. Furthermore, the single intermediate layer forms a smaller microcavity when compared with a QWS as the net reflection from the single intermediate layer is closer to the second reflecting electrode than the net reflection achieved from a QWS. As such, the mode spacing is larger for the single intermediate layer compared to a QWS. Accordingly, less modes are accessible which results in a decrease in the intensity of certain parts of the spectrum resulting in an overall enhancement of the spectrum which is superior to a QWS.

If the reflectance from the first electrode is small, then this reflectance could be ignored. In this case, it would not be necessary to locate the other reflecting surfaces within a certain distance of the electrode surface. However, it would require that all the reflective surfaces are themselves within a certain distance -16 -of each other so as to avoid providing more than one effective reflection (i.e. all the reflective surfaces of the reflector would need to be within a distance of wavelength). Thus, in accordance with another aspect of the present invention there is provided an organic electroluminescent device comprising: a transparent substrate; a first electrode disposed over the substrate for injecting charge of a first polarity; a second electrode disposed over the first electrode for injecting charge of a second polarity opposite to said first polarity; an organic light-emitting layer disposed between the first and the second electrode comprising an organic light-emissive material having a peak emission wavelength, wherein the second electrode is reflective, the first electrode is transparent or semi-transparent, and one or more intermediate layers of a dielectric material with a refractive index greater than 1.8 or a metal material are disposed between the substrate and the first electrode forming a semi-transparent mirror whereby a microcavity is provided between the reflective second electrode and the semi-transparent mirror, all the intermediate layers disposed between the substrate and the first electrode having a surface nearest the organic light-emitting layer within a distance of less than the peak emission wavelength. That is, all the reflective surfaces of the intermediate layers are located within a distance of less than 1/4 the peak emission wavelength. This ensures that the reflective surfaces of the intermediate layers effectively act as a single reflective surface in contrast to a QWS which has multiple inphase reflections.

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried in both effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: -17 -Figure 1 shows a known bottom emitting organic electroluminescent device; Figure 2 shows a known top emitting organic electroluminescent device; Figure 3 shows an organic electroluminescent device according to a first embodiment of the present invention; Figure 4 shows a graph indicating the spectral shift achieved using a green emitting device of the type shown in Figure 3; Figure 5 shows the shift that is evident with respect to a CIE chart and PAL green using the green emitting device; Figure 6 shows an increase in efficiency against voltage for the green emitting device; Figure 7 shows an increase in efficiency against voltage measured for a red emitting device of the type illustrated in Figure 3; Figure 8 shows a CIE plot for blue emitting devices of the type illustrated in Figure 3; Figure 9 illustrates modelling results showing an increase in emission for blue emitting devices of the type illustrated in Figure 3; Figure 10 shows spectra shifts with angle for a device of the type illustrated in Figure 3; Figure 11 shows an organic electroluminescent device according to another embodiment of the present invention.

-18 -Detailed Description of the Preferred Embodiments of the Invention Figure 3 shows a bottom-emitting organic electroluminescent device comprising a glass substrate 20, a thin intermediate layer of silver 22 disposed on the glass substrate 20, an anode 24 comprising ITO disposed on the thin silver layer 22, a layer of PEDOT 26 disposed on the ITO 24, a light emitting polymer layer 28 disposed on the PEDOT 26 and a barium/aluminium cathode 30 disposed on the light-emitting polymer layer 28.

Optical modelling has shown that modifying the anode structure of the polymer light emitting diode structure by including a thin layer of semi-transparent metal improves optical out-coupling and also provides colour tuning of the emission towards the PAL region.

Modelling has shown that the best materials for the intermediate layer are either high refractive index dielectrics (e.g. titanium dioxide, silicon oxynitride and silicon nitride) or metals that are transparent in the RGB region (e.g. silver or alloys such as Mg: Ag).

Figure 4 shows a graph indicating the spectral shift achieved using the device shown in Figure 3 when compared with a device of comparable structure but without the intermediate silver layer. The device comprises a standard green emitting polymer.

In terms of CIE co-ordinates, the device without the intermediate silver layer was measured to be CIE x=0.41 y=0.57. The shifted spectrum of the device comprising the intermediate silver layer was measured to be CIE x=0.35 y0.62.

Figure 5 shows the shift that is evident with respect to the CIE chart and the PAL green.

-19 -It has thus been shown that a thin silver layer may be used to improve colour saturation. Furthermore, no colour change was observed with changing viewing angle.

In terms of optical out-coupling, Figure 6 shows an increase in efficiency against voltage for the device having the silver layer compared with the control. The voltage required to achieve a particular luminance value is lower compared with the control device and thus a greater lmIW value for the silver layer device is achieved. A contributing factor is the increase in anode conductivity due to the metal layer.

The present results are for devices which have not been fully optimised.

Accordingly, by optimising the device structure greater Cd/A values will be achieved.

Figure 7 shows an increase in efficiency against voltage for a red emitting device of the type illustrated in Figure 3 in comparison with a control device in which the silver layer is absent. These devices utilize a standard red emitting polymer. Again, these devices have not been optimised as yet.

Figure 8 shows a CIE plot for blue devices fabricated with and without the silver layer. The blue emission has been tuned from CIE x=O. 17 y=O.20 to CIE x=O.16 y=O.16 by the provision of the silver layer.

Figure 8 shows that the addition of a silver layer shifts the emission colour to a better blue. In the particular embodiment illustrated, the PEDT layer is 5Onm thick. The other points on the graph are for a control without any silver at the anode side (but otherwise having the same device structure). The control showed no colour shift towards the blue.

-20 -For best results, the intermediate layer should have good reflectivity and also good transmission. The layer must also be depositable on the substrate to form an optical coating.

Figure 9 illustrates modelling results showing an increase in emission for a blue light-emitting device using ZnS in between a glass substrate and an ITO anode.

Figure 10 shows spectra shifts with angle for the blue light emitting device using ZnS in between a glass substrate and an ITO anode. The spectra show angle change from 0 to 70 degrees in air. A colour shift is observed from CIE x=0.144 y=0.l39 to x=0.16 y=0.156.

Figure 11 shows another embodiment of the present invention. This embodiment is similar in structure to the previously described embodiment shown in Figure 3 but with a colour filter 32 on an outer surface of the glass substrate. The other layers of the device are the same.

While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined in the appendant claims.

Claims

-21 -Claims 1. An organic electroluminescent device comprising: a

transparent substrate; a first electrode disposed over the substrate for injecting charge of a first polarity; a second electrode disposed over the first electrode for injecting charge of a second polarity opposite to said first polarity; an organic light-emitting layer disposed between the first and the second electrode, wherein the second electrode is reflective, the first electrode is transparent or semi-transparent, and one or more intermediate layers of dielectric material with a refractive index greater than 1.8 or a metal material is disposed between the substrate and the first electrode forming a semi-transparent mirror whereby a microcavity is provided between the reflective second electrode and the semi-transparent mirror, all the intermediate layers disposed between the substrate and the first electrode having a surface nearest the organic light-emitting layer not more than l5Onm from a surface of the first electrode nearest the organic light-emitting layer.

2. An organic electroluminescent device according to claim 1, wherein all the intermediate layers disposed between the substrate and the first electrode have the surface nearest the organic light-emitting layer less than I 5Onm, less than I 25nm, or less than 11 Onm from the surface of the first electrode nearest the organic light-emitting layer.

3. An organic electroluminescent device according to claim I or 2, wherein the or each intermediate layer is transparent to visible light having a wavelength over 600nm, over 500nm, or over 400nm.

-22 - 4. An organic electroluminescent device according any preceding claim, wherein the or each intermediate layer comprises independently one of titanium dioxide, silicon oxynitride, silicon nitride, zinc sulphide, silver and silver alloys.

5. An organic electroluminescent device according any preceding claim, wherein the intermediate layer, or at least one of the intermediate layers, comprises a metal material and has a thickness of 1-2Onm, 1-lOnm, 3-7nm or around 5nm.

6. An organic electroluminescent device according any preceding claim, wherein the intermediate layer, or at least one of the intermediate layers, comprises a dielectric material with a refractive index greater than 1.8 and has a thickness of 1O-lOOnm, 20-7Onm, 30-5Onm or around 4Onm.

7. An organic electroluminescent device according any preceding claim, wherein the first electrode is transparent to visible light having a wavelength over 600nm, over 500nm or over 400nm.

8. An organic electroluminescent device according any preceding claim, wherein the first electrode has a work function over 4.0eV.

9. An organic electroluminescent device according any preceding claim, wherein the first electrode comprises ITO.

10. An organic electroluminescent device according any preceding claim, wherein the transparent substrate has a refractive index of between 1.4 and 1.7.

11. An organic electroluminescent device according claim 10, wherein the transparent substrate comprises glass or plastic.

-23 - 12. An organic electroluminescent device according any preceding claim, wherein the substrate comprises a colour filter.

13. An organic electroluminescent device according to claim 12, wherein the colour filter is disposed on the same side of the substrate as the or each intermediate layer.

14. An organic electroluminescent device according to claim 12 or 13, wherein the colour filter is the same colour as that emifted by the microcavity.

15. An organic electroluminescent device according any preceding claim, wherein a single intermediate layer is provided, disposed between, and in contact with, the substrate and the first electrode.

16. An organic electroluminescent device according to any preceding claim, wherein the first electrode is either: metallic and has a thickness of between 5 and 3Onm; or is an inorganic oxide with a thickness of between 50 and I 5Onm.

17. An organic electroluminescent device according to any preceding claim, wherein electric contacts are provided directly to the first electrode.

18. An organic electroluminescent device according any preceding claim, comprising a plurality of pixels forming a display, each pixel having its own microcavity.

19. An organic electroluminescent device according to claim 18, wherein the substrate is common to the plurality of pixels.

20. An organic electroluminescent device according to claim 18 or 19, wherein the or each intermediate layer is common to the plurality of pixels.

-24 - 21. An organic electroluminescent device according to any one of claims 18 to 20, wherein the display comprises a plurality of first electrodes.

22. An organic electroluminescent device according to any one of claims 18 to 21, wherein the substrate comprises an active matrix back plane.

23. An organic electroluminescent device according to claim 22, wherein the display comprises a single second electrode common to the plurality of pixels.

24. An organic electroluminescent device according to claim 21, wherein the display comprises a plurality of second electrodes.

25. An organic electroluminescent device according to any one of claims 18 to 24, wherein the pixels emit different colours, the size of the pixel's microcavities being different for emitting the different colours.

26. An organic electroluminescent device according to claim 25, wherein the thickness of the first electrode is variable for forming cavities of differing sizes.

27. An organic electroluminescent device according to any one of claims 18 to 24, wherein the pixels emit the same colour forming a monochrome display, the size of the pixel's microcavities being the same.

28. A substrate for an organic electroluminescent device, the substrate comprising a layer of transparent material, a layer of transparent or semitransparent conductive material disposed over the layer of transparent material, and one or more intermediate layers of a dielectric material having a refractive index greater than 1.8 or a metal material disposed between the layer of transparent material and the layer of conductive material forming a semi-transparent mirror, all the intermediate layers disposed between the layer of transparent material and the layer of conductive material having a surface -25 -furthest from the layer of transparent material which is not more than l5Onm from a surface of the conductive material furthest from the layer of transparent material.

29. A substrate according to claim 28, wherein all the intermediate layers disposed between the layer of transparent material and the layer of conductive material have the surface furthest from the layer of transparent material less than l5Onm, less than 125nm, or less than llOnm from the surface of the conductive material furthest from the layer of transparent material.

30. A substrate according to claim 28 or 29, wherein the or each intermediate layer is transparent to visible light having a wavelength over 600nm, over 500nm, or over 400nm.

31. A substrate according to any one of claims 28 to 30, wherein the or each intermediate layer comprises independently one of titanium dioxide, silicon oxynitride, silicon nitride, zinc suiphide, silver and silver alloys.

32. A substate according to any one of claims 28 to 31, wherein the intermediate layer, or at least one of the intermediate layers, comprises a metal material and has a thickness of 1-2Onm, 1-lOnm, 3-7nm or around 5nm.

33. A substate according to any one of claims 28 to 32, wherein the intermediate layer, or at least one of the intermediate layers, comprises a dielectric material with a refractive index greater than 1.8 and has a thickness of 10-lOOnm, 20-7Onm, 30-5Onm or around 4Onm.

34. A substrate according to any one of claims 28 to 33, wherein the layer of conductive material is transparent to visible light having a wavelength over 600nm, over 500nm or over 400nm.

-26 - 35. A substrate according to any one of claims 28 to 34, wherein the layer of conductive material has a work function over 4.0eV.

36. A substrate according to any one of claims 28 to 35, wherein the layer of conductive material comprises ITO.

37. A substrate according to any one of claims 28 to 36, wherein the transparent material comprises a colour filter.

38. A substrate according to claim 37, wherein the colour filter is disposed on the same side of the transparent material as the or each intermediate layer.

39. A substrate according to any one of claims 28 to 38, wherein the transparent material has a refractive index of between 1.4 and 1.7.

40. A substrate according to claim 39, wherein the transparent material comprises glass or plastic.

41. A substrate according to any one of claims 28 to 40, wherein a single intermediate layer is provide, disposed between, and in contact with, the layer of transparent material and the layer of conductive material.

42. A substrate according to any one of claims 28 to 41, wherein the layer of conductive material is either: metallic and has a thickness of between 5 and 3Onm; or is an inorganic oxide with a thickness of between 50 and I SOnm.

43. Use of a substrate according to any one of claims 28 to 42 in a method of manufacturing an organic electroluminescent device according to any one of claims I to 27.

-27 - 44. A method of manufacturing an organic electroluminescent device according to any one of claims I to 27, the method comprising the steps: providing a prefabricated substrate according to any one of claims 28 to 42; and depositing the other layers of the organic electroluminescent device thereon.

45. An organic electroluminescent device comprising: a transparent substrate; a first electrode disposed over the substrate for injecting charge of a first polarity; a second electrode disposed over the first electrode for injecting charge of a second polarity opposite to said first polarity; an organic light-emitting layer disposed between the first and the second electrode, wherein the second electrode is reflective, the first electrode is transparent or semi-transparent, and an intermediate layer of dielectric material with a refractive index greater than 1.8 or a metal material is disposed between, and in contact with, the substrate and the first electrode forming a semi-transparent mirror whereby a microcavity is provided between the reflective second electrode and the semi-transparent mirror.

46. A transparent substrate for an organic electroluminescent device, the transparent substrate comprising a layer of transparent material, a layer of transparent or semitransparent conductive material disposed over the layer of transparent material, and a layer of dielectric material having a refractive index greater than 1.8 or a metal material disposed between, and in contact with, the layer of transparent material and the layer of conductive material forming a semi-transparent mirror.