WO2005064695A2

WO2005064695A2 - Low refractive index gap for enhanced light extraction from a display or lighting element

Info

Publication number: WO2005064695A2
Application number: PCT/BE2004/000183
Authority: WO
Inventors: Kristiaan Neyts
Original assignee: Universiteit Gent
Priority date: 2003-12-29
Filing date: 2004-12-24
Publication date: 2005-07-14
Also published as: TW200524469A; WO2005064695A3

Abstract

A display or lighting element (20) is described allowing an enhanced emission intensity to be coupled out. The display or lighting element comprises a first substrate (22) whereon in sequence a bottom electrode (24), a light emitting structure (26) and a top electrode (30) is deposited and whereon a second substrate (34) is mounted on top of the top electrode (30), thereby providing a gap (32) between the second substrate (34) and the top electrode (30). By tuning the thickness of the gap (32), which is vacuum or filled with a material having a low refractive index, the emission intensity can be increased based on microcavity properties like wide angle interference and multiple beam interference. The enhanced emission intensity only has a limited wavelength and angle dependency.

Description

LOW REFRACTIVE INDEX GAP FOR ENHANCED LIGHT EXTRACTION FROM A DISPLAY OR LIGHTING ELEMENT

Technical field of the invention The present invention relates to a display or lighting element and its use in displays as well as a method of manufacturing the same. More specifically the invention relates to a display or lighting element with a large emission intensity having only a limited angle dependence as well as a method of manufacturing the same.

Background ofthe invention In light emitting thin film structures consisting of a stack of thin films with a large refractive index, an important fraction of the generated light suffers from total internal reflection and can not be transmitted into air. This is the case in e.g. organic light emitting diodes (OLED's) and inorganic light emitting diodes such as LED's or EL display elements. As a result, in the past several solutions have been proposed to extract the light emitted from thin film structures at larger inclinations. The suggested solutions include embedding the thin film in a geometrical structure, roughening of the substrate interface or including periodic or random diffractive elements in the substrate. For example, an OLED device combined with a structured substrate or cover layer leads to enhanced light extraction because of scattering or diffraction effects. To avoid blooming of the light generation region, the structured interface should be as close to the emitting layer as possible, but this makes the technological process more demanding. Improvement of the emission intensity can also be obtained in structures having two reflective interfaces. These structures usually are called microcavities. The intensity and the spectral distribution of the light emitted from such a rnicrocavity depends on the angie of observation. The microcavity structures have been applied to many technologies: inorganic electroluminescence (EL displays), inorganic light emitting diodes, organic light emitting diodes. The enhancement of the emission intensity in a microcavity is based on provoking well known interference effects. Two types of interference effects can play an important role: wide angie interference shown in Fig. 1a and multiple beam interference shown in Fig. 1 b. Wide angie interference occurs for emission of emitters 10 emitting in all directions. If e.g. light emission occurs by an emitter 10 situated above a mirror 12, light emitted towards the mirror 12 will be reflected to the opposite direction and the light will possibly interfere with light that is directly emitted in the opposite direction. Wide angle interference depends on the emitter 10, the polarisation of the light and the distance between the emitter 10 and the mirror 12. Multiple beam interference occurs when the emitter 10 is situated between two reflecting surfaces 14. Light rays then can be partly reflected between these two surfaces 14, thereby providing the possibility that interference occurs between the x^th and x+2^nd reflection of the light ray. This is shown in Fig. 1b. Multiple beam interference depends on the angle of the emission, the polarisation of the light and the distance between the two reflecting surfaces 14. These two interference effects are well known and their occurrence is e.g. described in more detail in Journal of the Optical Society of America A, Vol 15 (4) p962-971 by K. Neyts, (1998). The importance of reducing the thickness of the microcavity is illustrated by the multiple beam interference factor (MBF) for the emission intensity, which shows the sensitivity of the intensity for angle and wavelength variations.

In this equation, factor n_e, α ^■ ₍e

and d_e are, respectively, the index of refraction, the angle of the emission, and the thickness of the emitting medium between the mirrors. The efficiency of the structure for outcoupling of the light is given by the following equation:

in this equation the integrated useful emission, transmitted through the mirror, in the direction of outcoupling, also called the + direction, is divided by the total amount of light produced in the light generating layer. The K functions depend on the polarisation of the light considered, the orientation of the dipole emitter and the direction of the emission. They are a function of K, the amplitude of the wavevector component in plane of structure. They are of the form:

With f a function of , r the amplitude reflection coefficient for light travelling opposite to the direction of outcoupling, also called the - direction, and φ the phase retardation due to the distance between the emitter and the mirror in the

- direction. MBF is the multiple beam interference factor introduced previously. An example of the use of a microcavity is the use of a semi-transparent reflector as top electrode in an OLED device. US 6,366,017 describes the use of a distributed Bragg reflector in an organic light emitting diode. A distributed Bragg reflector consists of a stack of alternating layers of high and low refractive indices. This approach allows to obtain high spectral densities, for example in the perpendicular direction, but the wavelength and angle dependence of the emission tends to be rather strong. The enhanced extraction being only visible in a narrow solid angle is a disadvantage, e.g. if the structure is to be used in a display. This strong angle dependency is created by the fact that, for the semi-transparent distributed Bragg reflectors, usually solid materials with rather large refractive indices are used. Because the contrast in refractive indices, compared with the OLED layers, is small, more layers are needed to obtain a reasonable reflectivity, thereby increasing the total thickness of the microcavity and thus leading to a larger angle dependence. For display or lighting applications, enhancement of the light over a very wide viewing angle is desired, with a minimal change in the apparent colour. Because these structures contain many layers which should have accurate thickness, they also are rather expensive to fabricate. Another solution is the use of two metallic mirrors, one of which is semitransparent. This is e.g. described in j. Opt. Soc. Am. B 17 (2000) p114 - 119 by Neyts et al. The document describes the application of a microcavity for OLED's having a thick silver mirror and a semitransparent silver mirror. This allows a strong enhancement of the iight intensity. As it furthermore allows to reduce the optical path length to half a wavelength - which is not possible using a distributed Bragg reflector -, the angle and wavelength dependence of the emission is reduced considerably. Thus, in the above described method, the thickness of the cathode metal is adjusted to change the reflectivity, and consequently to tailor the microcavity effects. Nevertheless, making the top electrode metal too thin will result in a decreased electrical performance while making the cathode metal to thick will result in an increased absorption of the light by the metallic mirror. Therefore, the possibility to tune the top electrode thickness is only limited. Another possibility to increase the emission intensity coupled out is described in J. Appl. Phys. 94 (8) p5290-5296. Riel et al. describe the use of a capping layer on top of a metallic semi-transparent cathode of an OLED structure to enhance the emission intensity of the OLED device. The document discusses the influence of a dielectric capping layer ZnSe on the enhancement of the emission intensity and the influence on the spectral dependence of the viewing angle. The obtained advantages of the prior art solutions are somehow limited due to the limited contrast in refractive indices between the capping layer and the organic layers. Furthermore, the above solutions can suffer from additional light trapping in the layer with high refractive index and subsequent absorption in this layer. None of the above documents provides an optimum solution for a display or lighting apparatus and display method obtaining an enhanced emission intensity.

Summary of the invention It is an object of the present invention to provide a display or lighting element and method of manufacture thereof having an enhanced emission intensity with a limited angie dependence and a minimal spectral dependence. It is furthermore an object of the current invention to provide a display or iignting element that is relatively easy to produce and that only has a limited number of thicknesses that need to be tuned. It is another object of the current invention to provide a display or lighting element that furthermore reduces the problem of absorption by the metallic mirror of internally reflected light. One or more of the above objectives is accomplished by a method and device according to the present invention. The present invention relates to a display or lighting element adjusted for emitting at a dominant emission wavelength λ, comprising: a first substrate carrying in sequence a bottom electrode, a light emitting structure and an at least semi-transparent top electrode. The display or lighting element further comprises a second substrate, whereby said display or lighting element is characterised in that said second substrate is located adjacent to the first substrate such that a separator layer or gap is provided between the second substrate and the at least semi-transparent top electrode, said separator layer or gap having a thickness between 0.1 times the dominant emission wavelength λ and 3 times the dominant emission wavelength λ and an optical refractive index less than or equal to 1.3 . The separator layer may have a thickness in a range between 20 nm and 3 micrometer, preferably between 20 nm and 2 micrometer and an optical refractive index less than or equal to 1.3. The thickness of the separator layer may be adjusted, so that the amount of emission coupled out of the display and lighting element is increased by interference effects. This may be an increase of 5%, preferably 10%, more preferably 20% and most preferably 50% compared to structures without separator layer between the second substrate and the at least semi- transparent top electrode. The amount of emission coupled out of the display and lighting element may be increased by microcavity effects. The top electrode may be semi-transparent or completely transparent. The thickness of the separator layer may be determined by separator elements deposited on the first substrate, on the second substrate or on the top electrode before the first and second substrates are located adjacent to each other, e.g. before joining them together, or by moulding the second substrate before- joining it to the first substrate. The thickness of the separator layer may be preferably between 0.2 times λ and 2 times λ, most preferably between 0.2 times λ and λ, with λ the dominant wavelength of the emission created by the display or lighting element or the average wavelength of the emission created by the display or lighting element or the wavelength to be displayed by the display or lighting element. The display or lighting element may be an organic light emitting device. The separator layer between the second substrate and the at least semi-transparent top electrode may comprise a vacuum or it may be filled with a material having an index of refraction between 1 and 1.3, preferably between 1 and 1.2. This material may be a gas such as air. This material also may be a foam or an aerogel. The thickness of the light emitting structure, the top electrode and the separator may be optimised to obtain an enhanced emission intensity coupled out from the display or lighting element by means of the microcavity effect. In the display or lighting element, the bottom electrode may be reflective and the second substrate may be transparent as to couple light emission out of the display or lighting element through the second substrate. In a different embodiment, the second substrate may be a metallic mirror, the bottom electrode may be at least semi-transparent and the first substrate may be transparent as to couple light emission out of the display or lighting element through the first substrate. An additional layer with a refractive index lower than that of the materials of the light emitting structure may be provided between the bottom electrode and the first substrate. The additional layer may have a refractive index between 1 and 1.3. The invention also may relate to a display comprising display or lighting elements having any of the above described features. The invention furthermore may relate to a method for enhancing the emission intensity of a display or lighting element comprising forming a light emitting element on a first substrate for emitting light of a dominant wavelength emitting light λ, providing a second substrate on top of the light emitting eiement allowing forming of a separator layer, which typically is a gap that can be filled or not, between said second substrate and a top layer of the light emitting element by locating the second substrate with respect to said light emitting element such that the thickness of the created separator layer is between 0.1 times the dominant emission wavelength λ and 3 times the dominant emission wavelength λ and the separator layer has an optical refractive index iess than or equal to 1.3 . Preferably, the created separator layer thickness is between 0.1 times the dominant emission wavelength λ and 2 times the dominant emission wavelength λ or most preferably between 0.1 times the dominant emission wavelength λ and the dominant emission wavelength λ Although there has been constant improvement, change and evolution of devices in this field, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient, stable and reliable devices of this nature. The teachings of the present invention permit the design of improved display or lighting elements and methods to design display or lighting elements. These and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.

Brief description of the drawings Fig. 1 a - prior art - is a schematic representation of wide angle interference.

Fig. 1b - prior art - is a schematic representation of multiple beam interference.

Fig. 2 is a schematic representation of a top-emitting display or lighting element according to an embodiment of the present invention. Fig. 3 is a schematic representation of a bottom-emitting display or lighting element according to a specific embodiment of the present Invention.

Fig. 4 is a schematic representation of a top-emitting OLED display or lighting element according to another embodiment of the present invention.

Fig. 5 is a graph of the emission intensity in a glass substrate as a function of emission angle and wavelength for a conventional display or lighting element.

Fig. 6 is a graph of the emission intensity in a glass substrate as a function of emission angle and wavelength for a pixel structure according to an embodiment of the present invention.

Fig. 7 is a schematic representation of a typical quantum well light emitting diode with gap according to an embodiment of the present invention.

Fig. 8 is a graph of the emission intensity as a function of wavelength and emission angle of a standard quantum well light emitting diode.

Fig. 9 is a graph of the emission intensity as a function of wavelength and emission angle of a quantum well light emitting diode having an additional gap in accordance with embodiment of the present invention.

In the different figures, the same reference figures refer to the same or analogous elements.

Description of illustrative embodiments The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. The present invention relates to a device and method for enhancing emission from a display or lighting element by providing a separator in the display or lighting element structure. This separator typically is a gap which can be not filled, i.e. vacuum, or can be filled with a material having a low index of refraction. The separator thus acts as a layer having a low index of l eii ϋouui i. An example of a display or lighting element 20 with enhanced emission intensity according to the current invention is shown in Fig. 2. The display or lighting eiement 20 comprises a first substrate 22, a bottom electrode 24, a light emitting structure 26 wherein the emission only occurs in a limited area 28 of the light emitting structure 26 and a top electrode 30. Typical light emitting structures 26 whereby the created emission originates in a limited area 28 of the light emitting structure 26 are light emitting diodes (LEDs) or organic light emitting diodes (OLEDs) or similar. Some examples of such structures may be but are not limited to double heterostructure LEDs, quantum well LEDs and polymer OLEDs. Such structures are well known by persons skilled in the art. The display or lighting element 20 according to the present invention furthermore comprises a gap 32, created by mounting a second substrate 34 on top of the first substrate 22, thereby spacing the second substrate 34 from the top electrode 30 by separating elements, also called spacers 36. The gap is either vacuum or filled with a material of low optical refractive index, e.g. nitrogen gas, air, or an aerogel. The refractive index of the material used in the gap can be between 1 and 1.3, preferably between 1 and 1.2, more preferably between 1 and 1 ,1. The use of spacers can be omitted if substrate 34 is appropriately structured. The spacers can either be placed on top of the top electrode or directly between the two substrates, on an area of the first substrate 22 carrying no layers. The different layers in the display or lighting

- element 20 and the gap 32 between the substrates can be tuned so that they have an appropriate thickness to enhance the light extraction. This enhancement occurs due to wide angle and multiple beam interference processes. The thicknesses of the layers can be varied in the production process of the light emitting structure, while the thickness of the gap 32 can be determined e.g. by molding the second substrate appropriately or by depositing spacers 36 with the appropriate thickness on one of the substrates or on top of the light emitting element before joining the substrates together. Depending on the transparency and reflectivity of the electrodes 24, 30 and the substrates 22, 34, the display or lighting element 20 can be either top emitting or bottom emitting. Furthermore it is also possible to make a transparent display or lighting eiement 20 according to the present invention, e.g. if semi transparent mirrors are used for both electrodes. Electrodes typically are chosen in agreement with the light emitting structure 26. Typical transparent electrodes that are used in the visible area are, without limiting, e.g. indium tin oxide, tin oxide, zinc oxide, zinc tin oxide. Transparent electrode materials in the infra-red may be silicon, GaAs or other semiconductor materials. Non-transparent highly reflective electrodes typically are metal electrodes, whereby the selection of the metal or combination of metals is such that a high reflectivity and good electrical conductance is obtained. For a top emitting display element, the bottom electrode 24 typically is a reflector, e.g. a metallic reflector. The distance between the emitting layer 28 and the metallic reflecting bottom electrode 24 is tuned to be nearly a quarter wavelength or odd multiples thereof, so that wide angle interference enhances the emission in the z direction. The top electrode 30 then is a transparent electrode. The thickness of the top electrode and the gap 32 is furthermore set so that multiple beam interference is important. The second substrate 34 is transparent and allows extraction of the emission. Optionally, this second substrate may also comprise additional layers on its surface, to increase wide angle interference. In a bottom emitting display or lighting element, the second substrate 34 is a metallic mirror or is covered by a metallic mirror, while the first substrate 22 is transparent for allowing extraction of the emission. Furthermore, the bottom electrode 24 is at least semi-transparent. The different layer thicknesses again can be tuned so as to increase the emission intensity extracted from the display element 20 and furthermore as to reduce the angle dependency of the emission. Optionally, as is shown in Fig. 3, a low refractive index material 40 is used between the transparent bottom electrode 24 and the first substrate 22. The latter allows to trap light between the two materials with low index of refraction having an increased probability to be reabsorbed by the light emitting structure 26, which is a second mechanism of the enhancement of the light extraction. In a specific embodiment of the present invention, enhancement of light extraction for a top-emitting organic iight emitting diode (OLED) is achieved. A typicai OLED structure 50 according to the present invention is shown in Fig. 4. A first substrate 22 (not shown in Fig. 4) is provided for supporting a stack of layers. For a top emitting structure, this substrate 22 does not have to be transparent and may be a semiconductor substrate, e.g. silicon or germanium, or another substrate like e.g. glass or Si0₂. It will be appreciated by a person skilled in the art that other substrates also may be used. Flexible substrates iike polyethylene teraphthalate (PET), polyethersulphone (PES), polycarbonate (PC), Kapton can also be used. In this case, bending the substrates is still possible, provided that the spacers are able to keep the distance between the two substrates fixed. As bending the structures generates additional stress in the spacer balls, evacuation of the gasses between the two substrates may be required to compensate negative tension over the spacer^" balls and to avoid the two substrates from separating. On the substrate a reflective electrode 24, being a cathodic layer, is deposited. Furthermore, a stack of an electron transporting layer 52, an emission layer 54 and a hole transport layer 56 is deposited. On top, a transparent top electrode 30, i.e. the anode, is deposited. Other types of OLED light emitting structures can also be used. On top of the OLED structure a second transparent substrate 34 is applied. In the structure according to the present invention a gap 32 is provided between the second transparent substrate 34 and the OLED structure. The specific structure described above may be altered as required by the skilled person and the present invention is not limited to this specific stack of layers. For example, in other light emitting diodes other layers may be used. The choice of the reflective electrode 24 is determined by its conductivity and reflectivity properties and may be any conductive metallic layer having good reflectivity properties. A non-limiting list of examples is Al, Mg, Ca, Ni, Ag. The deposition technique used for depositing the reflective cathodic layer may be any conventional deposition technique. The reflective cathodic layer acts as a mirror. The thickness of this layer is not critical, but should preferably be such that there is no transmission of light. The electron transporting layer 52 may be any layer that is typically used for this purpose in organic light emitting devices. Some examples are tris(δ-hydroxyquinoline) aluminum Alq₃ or poly (pheπylene vinylene) derivatives or pϋiyfiuorene copoiymerε. The organic emitting layer 54 can be any layer that is typically used for organic iight emitting devices. Some examples are 4,4' - N-N' - dicarbazole - 1 ,1' -hiphenyl (CBP) doped with 6% tris(2- phenylpyridine)iridium (lr(ppy)₃) or tris (8-hydroxyquinoline) aluminum (Aiq₃). The electron transporting layer 52 may also serve as emitting iayer 54. The hole transporting layer 56 may be any layer that is typically used for this purpose in organic light emitting devices. The hole transport layer 56 can be a tertiary amϊne derivative or a hole transporting conducting polymer. The examples of useful materials are only for illustration and are non-limiting for the scope of the invention. It may be appreciated by a person skilled in the art that other materials for the organic layers can be used. The top electrode 30, which is a transparent conductor, may be, but is not limited to, indium tin oxide, tin oxide, zinc tin oxide, or an organic conducting layer. As the structure is top-emitting and therefore the light needs to be extracted through the second substrate 34, the second substrate 34 is a transparent substrate. This can be, but is not limited to, e.g. a glass or Si0₂. The gap 32 can be either empty, i.e. vacuum can be present, or it can be filled with a material having a low index of refraction. Typical materials that can be used are air, a gas, a foam or an aerogel with refractive index close to unity. Every type of suitable gas can be used such as but not limited to Ar or N₂. Depending on the refractive index, foam materials also can be used. Non- limiting examples are polystyrene having a low refractive index of around 1.020 and silica or alumina aerogels having refractive index between 1.03 and 1.2. In general, the refractive index of the material used for filling the gap can be between 1 and 1.3, preferably between 1 and 1.2, more preferably between 1 and 1,1. Other low value materials also may be used, like e.g. MgF₂. It is a preferable feature of the current invention that the gap corresponds with a layer having a low refractive index. The different layers of the OLED structure, i.e. reflective cathode 24, electron transport layer 52, emission layer 54, hole transport layer 56, transparent anode 30 and the gap 32 between the two substrates and possibly additional films on the second substrate, have dimensions of the order of one wavelength, i.e. 30 - 300nm, metal films may be 10 to 1000 nm thick. This thickness can be tuned during the production process of the different layers. The thickness of the gap 32 can be tuned during production using different methods. One example is depositing spherical or cylindrical spacers 36 with the appropriate diameter on either the second substrate 34 or on the first substrate 22 or on top of the transparent anodic layer 30 before joining the two substrates together. The spacers 36 used then typically are made of electrically insulating materials like glass and can be spin-coated in a solution (e.g. isopropano!) onto the substrate. Spacers can also be obtained by depositing or etching certain material in a particular pattern, using evaporation masks or photolithographic means. Examples of typical materials that can be used are BN, polyimide, Si0₂, Al 0₃, ... The spacers 36 can also be placed directly between the first and the second substrate 34, on an area of the first substrate 22 where no structure is present. In this case, the condition of the spacers 36 being electrically insulating is not necessary. Instead of using separate spacers 36, it is also possible to use e.g. a molded second substrate 34. If the first substrate 22 is chosen as molded substrate, the extrusions need to be provided on areas where no light emitting structure will be deposited. The different layers in the OLED structure and the gap 32 between the substrates are tuned to have an appropriate thickness so that enhancement of the light extraction is obtained. Wide angle and multiple beam interference thus lead to an increase of the emission through the second substrate 34 and a decrease the amount of internally reflected light. If the gap 32 with low index of refraction has a thickness of about a quarter wavelength or odd multiples thereof, the reflectivity of the second substrate 34 will be high and the multiple beam interference will be important. If the distance between the emitting layer 54 and the metallic bottom electrode 24 is about a quarter wavelength or odd multiples thereof, the wide angle interference will enhance the emission in the perpendicular direction. The thickness of the other layers can also be tuned in such a way that the light which has reflected on both mirrors is again in phase with the original emitted iight. In addition, the thicknesses can be tuned to keep the angle dependence of the color of the emission minimal: as materials with very low index of refraction are used, the contrast in refractive index is sufficiently large to allow using short cavities, thus reducing the angle dependency. The use of a gap 32 with low refractive index has a further beneficial effect because it tends to confine the light which is subjected to total internal reflection in the neighbourhood of the light generation region. Some of this confined light can be reabsorbed by the organic layers and thus this can iead to the emission of a new photon. In this case, the absorption in the organic layer will be in competition with the absorption by the metallic mirror 24 and with light leakage through the gap 32 with low index of refraction. The thicknesses of the layers have to be adjusted in order to maximize the possibility for reabsorption. It is a specific advantage of the current invention that at least part of the emission enhancement can be obtained without the necessity to change the basic structure of the light emitting structure, e.g. OLED structure. This is advantageous as it allows to enhance at least partly the emission intensity for every iight emitting device such as an LED or OLED device. Furthermore, as at least for part of the enhancement of the emission intensity, no change of the actual light emitting structure is necessary, this allows to avoid possible problems with changed electrical conditions for the light emitting structure. The above advantages are at least partly obtained for the different embodiments of the invention described. Another specific embodiment describes a display or lighting element which has substantially the same structure as the previous embodiment, but wherein the second substrate 34 is a metallic mirror and whereby the first substrate 22 carries a semi-transparent bottom electrode 24. The second substrate 34 may be either a metallic mirror itself or it may comprise a reflective thin film on its surface. Tuning the thickness of the gap 32 and optionally also the thickness of the different layers allows to enhance the emission that can be extracted from the display element and its angle dependency. Optionally, a low refractive index material can be added between the transparent cathode 24 and the bottom substrate 22. The refractive index of the material of the layer used can be between 1 and 1.3, preferably between 1 and 1.2, more preferably between 1 and 1 ,1. Such a layer may consist of e.g. an aerogel or MgF₂. in such a structure the iight is trapped between the two materials with low index of refraction and thereby has an increased probabiiity to be reabsorbed by the layers with high index of refraction, e.g. the emitting iayer 54. This again allows enhancement of the light extraction. Reabsorption is especially useful in lighting applications or in displays with large elements where the distance the light has travelled before reabsorption is not an issue. In conventional OLED structures without an additional gap, light that is subjected to total internal reflection is absorbed by the metallic mirror. It is a specific advantage of the above embodiment that the absorption of light that undergoes total internal reflection is not absorbed by the metallic mirror used by that it can be reabsorbed by the emitting layer 54. Because the organic OLED layers have a larger refractive index (1.9) than the substrate (1 .5) on which they are deposited, an important amount of light is trapped in the organic layers. If the organic layers are in direct contact with a metallic electrode, the trapped light will normally be absorbed in the metallic mirror after a few (5-20) reflections, even for highly reflective mirrors. If an air gap is located between the metallic mirror and the organic layers, the chance of absorption in the metallic mirror can become extremely low and therefore the chances for reabsorption in the emitting layer (creating a new exciton) or scattering of light into the substrate are increased considerably. In this structure, the layer adjacent to the air gap should be a conductor, and current is supplied from the metallic mirror through conductive spacers. The reduction of absorption is advantageous as it increases the efficiency of lighting applications. As an illustration, the emission intensity of a typical top-emitting OLED structure without and a typical OLED structure with an air gap 32 is shown in Fig. 5 and Fig. 6. These figures correspond with calculated values for the emission intensity of an OLED structure based on a rubrene doped Alq₃ emission iayer 54. The emission intensity has its maximum at 565nm. For the transparent conductor 30 and the organic layers, a refractive index of 1.9 has been used. The OLED structure used for the calculations has a thick silver bottom electrode 24, an electron transport iayer 52, a thin emitting layer 54, a hole transport iayer 56, a transparent conductor 30, an air gap 32 - for the device according to the invention - and a thick glass substrate 34 on top. The optimised thickness for the different layers and the optical properties of these layers, in case the gap is filled with air, are given in the table below.

Fig. 5 and Fig. 6 show the emission intensity in the glass substrate 34 as a function of the emission angle and wavelength. Only the light which is emitted at an angle below 41.8 degrees will not be internally reflected at the front glass interface and can reach the observer. The display or lighting elements according to the above described embodiments may be formed into an array, e.g. a fixed format display, in which each display element or each group of display elements may be addressable to thereby display an arbitrary image, e.g. a still image or a video. The display may be monochromatic or may be colored. In another specific embodiment of the present invention, enhancement of light extraction for a bottom-emitting quantum well light emitting diode (QW- LED) is achieved. A typical QW-LED structure 60 according to the present invention is shown in Fig. 7. A first substrate 22, typically also acting as the bottom electrode, is provided for supporting a stack of layers. For a bottom emitting structure, this substrate 22 is transparent for the emitting wavelength and may be a semiconductor substrate, e.g. GaAs or A!₂0₃. On the substrate a material with a lower index of refraction like for example AIO_x is grown which has the role of a semi-transparent reflector 40. The iight emitting structure may be either a p-n junction or an n-p junction. It consists of a p-type (or n-type) semiconductor layer 62, which may e.g. be GaAs or GaN, a thin, single or multiple quantum well 64, which generates the light and an n-type (or p-type) semiconductor iayer of 66 (for example GaAs or GaN). The thin single or multiple quantum well 64 typically is less than 1μm thick and may be e.g. inGaAs. According to this embodiment of the present invention, a gap 32 is provided between the top semiconductor layer 66 and the second substrate 34, the latter covered with a reflective mirror 68, which also is the upper electrode. The specific structure described above is provided as an example 5 and the present invention is not limited to this specific stack of layers. For example, in other light emitting diodes other layers may be used. The choice of the reflective electrode 68 is determined by its conductivity and reflectivity properties and may be any conductive metallic layer having good reflectivity properties. A non-limiting list of examples is Al, 10 Mg, Ca, Ni, Ag. The deposition technique used for depositing the reflective cathodic layer may be any conventional deposition technique. The reflective cathodic layer acts as a mirror. The thickness of this layer is not critical, but should preferably be such that there is no transmission of light. The thickness of the quantum well should be preferably less than 200 15 nm, more preferably less than 50 nm. The two semiconductor layers embedding the quantum well provide the current to the quantum well and should have a thickness between 20 nm and 400 nm. The gap 32 can be either empty, i.e. vacuum can be present, or it can be filled with a material having a low index of refraction. Typical materials that 20 can be used are air, a gas, a foam or an aerogel with refractive index close to unity. Every type of suitable gas can be used such as but not limited to Ar or N₂. Depending on the refractive index, foam materials also can be used. Non- limiting examples are polystyrene having a low refractive index of around 1.020 and silica or alumina aerogels having refractive index between 1.03 and 25 1.2. In general, the refractive index of the material used for filling the gap can be between 1 and 1.3, preferably between 1 and 1.2, more preferably between 1 and 1 ,1. Other low value materials also may be used, like e.g. MgF₂. It is a particular feature of embodiments of the current invention that the gap corresponds with a layer having a low refractive index. - - - - — - ^~ - ^■ on The first substrate 22 may be covered with a material 70 with a curved interface, in order to reduce total internal reflection and to enable more light to exit the component into air. The tuning of the thickness of the gap 32 can be performed in a similar way as described in the previous embodiments. As an illustration, the emission into the bottom material 70 has been optimised by tuning the thicknesses of the different layers in the stack. The emitting structure and the substrate are GaAs (n=3.51), the intermediate layer 40 is AiOx (n=1.55), there can be an airgap (n=1 ) and the electrode 68 is in Al (n complex). This light emitting structure -without gap- also has been used by e.g. Bienstman et al., in IEEE Journal of quantum electronics vol 36 p669, 2000. The spacers are preferably conductive to allow the current to enter the light generating structure from the Al. The simulated results for optimized devices with a half wavelength GaAs emitting layer without and with a gap are shown respectively in figures 8 and 9. The presence of the gap increases the reflectivity of the top mirror. In particular, if the angle of emission in the GaAs is larger than 17°, total internal reflection on the GaAs/air interface occurs and the reflectivity of the top part is close to unity. The integrated emission into GaAs with an angle below 26°, i.e. the part which can be transmitted in medium 70 with refractive index 1.55, is increased by about 15% due to the presence of the gap. An additional benefit of the structure with a gap is that the large amount of light pumped into the guided mode, i.e. at about 75 degrees in GaAs, is now much less absorbed by the Al electrode. This increases the probability of reabsorption in the quantum well and the total emission and therefore contributes to an increased outcoupling efficiency. The magnitude of this effect depends on the importance of reabsorption and thus on the material properties of the quantum well. It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope and spirit of this invention. For example, if both the bottom electrode and the second substrate are semi-transparent mirrors, wide angie interference and multiple beam interference and thus enhancement of iight extraction still will occur, albeit reduced, and a transparent display element can be made. For displays made of these display elements, information behind the display may still be available for viewing by a user of the display.

Claims

1.- A display or lighting element (20,50) adjusted for emitting at a dominant emission wavelength λ, comprising: a first substrate (22) carrying in sequence a bottom electrode (24), a light emitting structure (26) and an at least semi-transparent top electrode (30), said display or lighting element (20,50) further comprising a second substrate (34), whereby said display or lighting element (20,50) is characterised in that said second substrate (34) is located adjacent to the first substrate (22) such that a separator layer (32) is provided between the second substrate (34) and the at least semi-transparent top electrode (30), said separator layer (32) having a thickness between 0.1 times the dominant emission wavelength λ and 3 times the dominant emission wavelength λ and an optical refractive index less than or equal to 1.3 .

2.- A display or lighting element (20,50) according to the previous claim, adjusted for emitting at a dominant emission wavelength λ, said separator layer (32) having a thickness between 0.1 times the dominant emission wavelength λ and 2 times the dominant emission wavelength λ, preferably having a thickness between 0.1 times the dominant emission wavelength λ and the dominant emission wavelength λ.

3.- A display or lighting element (20,50) according to claim 1 or 2, wherein the thickness of said separator layer (32) is determined by separator elements (36) deposited on the first substrate (22), on the second substrate (34) or on the top electrode (30) before the first and the second substrates (22, 34) are located adjacent to each other.

4.- A display or lighting element according (20,50) to claim 1 or 2 wherein the thickness of said separator layer (32) is determined by moulding the second substrate (34) before locating it adjacent to the first substrate (22).

5.- A display or lighting element (20,50) according to any of the previous claims wherein said display or lighting element is an organic iight emitting device.

5 6.- A display or lighting eiement (20,50) according to any of the previous claims wherein said separator iayer (32) is a space under vacuum.

7.- A display or lighting element (20,50) according to any of claims 1 to 5 wherein said separator layer (32) has an index of refraction between 1 and 1.2. 10

8.- A display or lighting element (20,50) according to claim 7 wherein said separator layer (32) is an air gap.

9.- A display or lighting element (20,50) according to claim 7 wherein said 15 separator layer (32) comprises a gas.

10.- A display or lighting element (20,50) according to claim 7 wherein said separator layer (32) comprises a foam.

20 11.- A display or lighting element (20,50) according to any of the previous claims wherein the thickness of the light emitting structure (26), the top electrode (30) and the separator layer (32) are optimised to obtain an enhanced emission intensity coupled out from the display or lighting element (20,50) by means of the microcavity effect.

25 12.- A display or lighting element (20,50) according to any of claims 1 to 11 wherein said bottom electrode (24) is reflective and said second substrate (34) is transparent so as to couple light emission out of the display or lighting element (20,50) through the second substrate (34).

<3U 13.- A display or lighting element according to any of claims 1 to 11 wherein said second substrate (34) is reflective, said bottom electrode (24) is at least semi-transparent and said first substrate (22) is transparent so as to couple iight emission out of the display or fighting element (20,50) through the first substrate (22).

14.- A display or lighting element (20,50) according to claim 13 wherein an additional layer (40) with a refractive index lower than that of the materials of the iight emitting structure (26) is provided between the bottom electrode (24) and the first substrate (22).

15.- A display or lighting element (20,50) according to claim 14 wherein said additional layer (40) has a refractive index between 1 and 1.3.

16.- A display comprising display or lighting elements (20,50) according to any of the previous claims.

17.- A method of manufacture of a display or lighting element for enhancing the emission intensity thereof, comprising forming a light emitting element on a first substrate (22) for emitting light at a dominant wavelength λ, providing a second substrate (34) on top of the light emitting element, locating the second substrate (34) with respect to said light emitting element to thereby form a separator layer (32) between said second substrate (34) and a top layer of the light emitting element, the thickness of the created separator layer (32) being between 0.1 times the dominant emission wavelength λ and 3 times the dominant emission wavelength λ, and the separator layer (32) having an optical refractive index less than or equal to 1.3 .

18. A method according to claim 17, wherein said separator iayer (32) has a thickness between 0.1 times the dominant emission wavelength λ and 2 times the dominant emission wavelength λ, preferably has a thickness between 0.1 times the dominant emission wavelength λ and the dominant emission wavelength λ.

19.- A method according to claim 17 or 18, further comprising depositing separator elements (36) on the first substrate (22), on the second substrate (34) or on the iight emitting element before the first and the second substrates (22, 34) are being located adjacent to each other, wherein the thickness of said separator iayer (32) is determined by the separator elements (36).

20.- A method according to claim 17 or 18 further comprising moulding the second substrate (34) before locating it adjacent to the first substrate (22) to thereby define the thickness of said separator iayer (32).