WO2016009178A1

WO2016009178A1 - Organic lighting device

Info

Publication number: WO2016009178A1
Application number: PCT/GB2015/051991
Authority: WO
Inventors: Natasha Conway; Alexander Doust; Colin Baker; Jeremy Burroughes
Original assignee: Cambridge Display Technology Ltd
Current assignee: Cambridge Display Technology Ltd
Priority date: 2014-07-14
Filing date: 2015-07-09
Publication date: 2016-01-21
Anticipated expiration: 2017-01-14
Also published as: GB201412455D0

Abstract

A method of fabricating an organic lighting device comprises: providing a transparent substrate; providing a light scattering polymer layer on said substrate, wherein said polymer layer comprises a volume light scattering material to scatter light from a light emitting layer of the device; providing a first set of electrodes for said device, wherein said first set of electrodes comprises a plurality of conductive tracks dispersed on or embedded in said polymer layer; providing a set of organic lighting device layers over said first set of electrodes and polymer layer; and providing a second electrode layer over said set of organic lighting device layers.

Description

ORGANIC LIGHTING DEVICE

BACKGROUND OF THE INVENTION This invention relates to improved methods for fabricating organic electronic lighting devices, and to devices fabricated by such methods.

BACKGROUND TO THE INVENTION It is important to be able to reduce the cost and increase the efficiency of organic electronic lighting devices in order to ensure their commercial success. It is known that volume scattering can improve the efficiency with which light is extracted from an OLED (Organic Lighting Emitting Diode) structure, as described for example in 'Light Extraction from Organic Light-EmittingDiodes Enhanced by Spontaneously Formed Buckles', Won Ho Koo Nature Photonics, 4222-226 (April 2010); US7,589,463; US8,022,619; and WO2010/1 12789A.The patent literature describes furtherexampies of such structures, for example in WO20 2/047054; WO2007/041 116; WO2006/ 13319; EP2, 151 ,878A; EP1 ,406,474A; and US2012/0038876. However these prior art approaches suffer from various disadvantages and, in particular, are not well adapted to lighting applications.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is therefore provided a method of fabricating an organic lighting device, the method comprising: providing a transparent substrate; providing a light scattering polymer layer on said substrate, wherein said polymer layer comprises a volume light scattering material to scatter light from a light emitting layer of the device; providing a first set of electrodes for said device, wherein said first set of electrodes comprises a plurality of conductive tracks dispersed on or embedded in said polymer layer; providing a set of organic lighting device layers over said first set of electrodes and polymer layer; and providing a second electrode layer over said set of organic lighting device layers. Broadly speaking, embodiments of the invention employ a combination of a mesh or grid-type electrode structure comprising a plurality of very fine tracks in combination with an underlying or integrated polymer layer arranged to provide volume scattering of the light produced by the light emitting layer(s). This results in a structure which, in optical terms, is significantly different from previous structures providing both enhanced output light coupling and reduced fabrication costs. In preferred embodiments the first set of electrodes comprises set of anode electrodes for the device; and the device is preferably bottom-emitting, that is through the transparent substrate (which is typically fabricated from glass or plastic).

In some preferred fabrication methods the first set of (anode) electrodes is provided by depositing a pre-cursor layer of electrically conducted material to provide a seed layer and then using electroless plating to deposit the conductive electrode tracks on to this seed layer. In principle, however, other techniques such as lithographic techniques may alternatively be employed. A range of materials may be employed for the anode electrode metal including, but not limited to gold, aluminium, nickel, silver (including alloys thereof, in particular alloys that inhibit ion migration such as APC, an Ag alloy comprising Palladium at, for example 0.9 at.% and Copper at, for example 1.7 at. %), and copper (including alloys thereof with up to 5% impurities). In a preferred embodiment the anode electrode metal may comprise a NiP alloy (a mixture of nickel and phosphorous), in particular a copper NiP alloy. In still other arrangements the NiP alloy may be provided as a protective or capping layer on the anode electrode metal, for example copper; this may be deposited by electro-or eiectroiessly plating the NiP alloy on the anode metal (for example copper), it has been found that this helps to increase device lifetime.

In some embodiments the first set of electrodes is disposed onto the light scattering polymer layer, in other approaches the first set of electrodes is embedded into the light scattering polymer layer (for example by lithography). Optionally in the latter case the light scattering polymer layer may act as a planarization layer for the first set of electrodes, though this is not essential. The scattering layer may be applied or deposited between the metal electrodes (grid lines). In some preferred embodiments the light scattering polymer layer comprises a layer of epoxy. In preferred embodiments the volume scattering of light from the light emitting layer(s) may be provided by oxide nanoparticles, for example zirconium nanoparticles, embedded into the epoxy or other polymer layer. However other light scattering materials may be employed and, in principle, light scattering centres may even be provided by, for example, bubbles in the epoxy or polymer layer, in embodiments the scattering centres may have a mean size (maximum dimension) in the range 30nm to 300nm. In some preferred embodiments the out-coupling is enhanced by providing an index- contrast layer of high-refractive index material over the first set of electrodes and light scattering polymer layer - that is a layer of higher refractive index (at a wavelength of operation of the device) than that of the light scattering polymer layer. The light emitting stack is deposited over this index contrast layer, which helps to couple light out through the transparent substrate. Such an index contrast layer may comprise, for example, layer of ITO (indium Tin Oxide or some other transparent conducing oxide). Additionally or alternatively the index contrast layer may comprise metal nanostructures, such as metal nano wires, in particular silver nanowires (see, for example, WO2007/022226).

In some preferred embodiments the organic lighting device comprises an OLED device, preferably a light emitting polymer (LEP) and the set of organic lighting device layers comprises a hole injection layer (HIL) followed by an LEP stack to provide light in one or more colours or white light.

In some preferred embodiments of the method the fabricated device is packaged for use as an organic lighting device such as a lighting tile.

In a related aspect the invention provides an organic lighting device, the device comprising: a transparent substrate; a light scattering polymer layer on said substrate, wherein said polymer layer comprises a volume light scattering material; a first set of electrodes for said device, wherein said first set of electrodes comprises a plurality of conductive tracks disposed on or embedded in said polymer layer; a set of organic lighting device layers over said first set of electrodes and polymer layer; and a second electrode layer over said set of organic lighting device layers. BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying Figures in which:

Figures 1 a and 1 b show, respectively, a cross-section through an OLED lighting tile, and a view of a front, light-emitting face of the tile, and an example of an !TO-free lighting system according to an embodiment of the invention;

Figures 2a to 2c show, respectively, a first example of an OLED structure with improved optical outcoupling, computed ie scattering patterns for zirconium oxide spheres with diameter d embedded in a matrix of MSQ (poly(methyl-siisesquioxane)) and computed attenuation/transmittance curves for varying diameters of zirconium oxide spheres; and

Figures 3a and 3b show, respectively, first and second OLED structures according to embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Organic light emitting structures In this specification references to organic LEDs include organometaliic LEDs, and OLEDs fabricated using either polymers or small molecules. Examples of polymer - based OLEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so called small molecule based devices are described in US 4,539,507. OLED devices (which here includes organometaliic devices and devices including one or more phosphors) may be fabricated using either polymers or small molecules in a range of colours and in multicoloured displays depending upon the materials used. For general background information reference may be made, for example, to WO90/13148, WO95/06400, WO99/48160 and US4,539,570, as well as to Organic Light Emitting Materials and Devices" edited by Zhigang Li and Hong Meng, CRC Press (2007), ISBN 10: 1-57444-574X, which describes a number of materials and devices, both small molecule and polymer.

To aid in understanding embodiments of the invention it is helpful to describe an example structure of an OLED lighting tile. Thus referring to Figure 1a, this shows a vertical cross-section through a portion of an OLED lighting tile 10 comprising a glass or plastic substrate 12 on which metal, for example copper tracks 14 are provided to provide a first electrode connection, in the illustrated example an anode connection. The anode tracks may be deposited by provided, for example, by lithographic patterning or by electroless plating onto a printed or lithographically patterned seed layer,

A hole injection layer 16 is deposited over the anode electrode tracking, for example a conductive transparent polymer such as PEDOT: PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene). This is followed by a light emitting polymer (LEP) stack 18, for example comprising a PPV (poiy(p-phenylenevinylene) - based material: The hole injection layer helps to match the hole energy levels of this layer to the anode metal. This is followed by a cathode layer 20, for example comprising a low work function metal such as calcium or barium with an optional electron injection layer (EIL; not shown) such as lithium fluoride or, more preferably, sodium fluoride, or a charge transporting polymer for energy matching, over which is deposited a reflective back (cathode) electrode 22, for example of aluminium or silver. Where an electron injection layer is employed the low work function metal may be omitted. Preferably the light emitting structure is encapsulated to reduce oxygen/moisture ingress and increase device lifetime.

The example of Figure 1 a is a "bottom emitter" device in which light is emitted through the transparent glass or plastic substrate. However a "top emitter" device may also be fabricated in which an upper electrode of the device is substantially transparent, for example fabricated from indium tin oxide (ITO) or a thin layer of cathode metal (say less than 100 μπΊ thickness).

Referring now to Figure 1 b this shows a view of the light emitting tile 10 of Figure 1a looking towards the LEP stack through the substrate 12, that is looking into the light— emitting face of the device through the "bottom" of the device. This view shows that the anode electrode tracks 14 are, in this example, configured as a hexagonal grid or mesh, in order to avoid obscuring too much light from the LEP stack. The (anode) electrode tracks are connected to a solid copper busbar 30 which runs substantially all the way around the perimeter of the device, optionally with one or more openings, which may be bridged by an electrical conductor) to facilitate that connection to the cathode layer of the device. As described in our earlier patent application, GB2482110A, the grid may be irregular to increase the average in-plane conductivity towards the electrical busbars where greater conductivity is desirable.

Referring again to Figure 1 a, various configurations of the LEP stack 18 are possible. For example this may comprise red green and blue emitting layers to make a white emitter, or alternatively a white emitter may have a single layer incorporating red, green and blue emitting materials/moieties. The LEP stack may incorporate fluorescent and/or phosphorescent layers, optionally with a triplet diffusion protection layer between. Optionally an interlayer (not shown) may be included between the hole injection layer (HIL) and the light emitting layer(s). The interlayer may have a higher hole mobility than electron mobility (by contrast, electron transport is favoured in the light emitting iayer(s)), so that electron and hole charges accumulate at the LEP/IL interface, reducing exciton quenching by the cathode and anode. Optionally the interlayer (IL) may incorporate a light-emitting material/moiety, for example a red- emitting material/moiety and optionally a further layer may then incorporate green and blue emitting materials/moieties. For different, coloured lighting stacks with only one or two different colours of emission may be used.

The anode may comprise any material with a work function suitable for injection of holes into the light emitting layer/stack. The hole injection layer preferably comprises a conducting material; it assists hole injection from the anode into the light emitting layer. Representative examples of materials that may be used to form the hole injection layer include PEDOT:PSS, PANI (polyaniline), poiypyrole, optionally substituted, doped poiy(ethylene dioxythiophene) (PEDT), in particular PEDT doped with a charge- balancing poiyacid such as polystyrene sulfonate (PSS) as disclosed in EP0901 76 and EP0947123, polyacrylic acid or a fluorinated sulfonic acid, for example Nafion (R); polyaniline as disclosed in US 5723873 and US 5798170; and optionally substituted poiythiophene or poly(thienothiophene). Other suitable materials are summarized in the book by Zigang Li and Hong eng, Chapter 3.3 page 303 - 12. Examples of conductive inorganic materials include transition metal oxides such as VO_x, O_x and i

RuO_x as disclosed in Journal of Physics D: Applied Physics (1996), 29(11), 2750-2753. Suitable materials for use as the hole injection layer are commercially available, e.g. from Plextronics Inc/Solvay SA, Where a hole injection layer is employed an auxiliary layer of organic conductive material may optionally be included between the anode electrode tracks and the hole injection layer.

Preferably the hole injection layer is deposited by a solution-based processing method over the anode tracks. Any conventional solution-based processing method may be used. Representative examples of solution-based processing methods include spin coating, gravure printing, fiexographic printing, dip coating, slot die coating, doctor blade coating and ink-jet printing. In preferred methods, however, depositing is by spin coating. The parameters used for spin coating the hole injection layer such as spin coating speed, acceleration and time are selected on the basis of the target thickness for the layer. After deposition, the hole injection layer is preferably annealed by heating, e.g. at 150 to 200 °C for 5 to 30 minutes in air. The thickness of the hole injection layer may be in the range 15 to 200 nm, for example around 130nm. The rest of the LEP stack (including the interlayer where present) may similarly have a thickness of order 100-2Q0nm.

Electrodes containing NiP alloys

The electrode metal may comprise silver (or an alloy thereof such as A PC), aluminium, titanium, tantalum, molybdenum or steel but copper is preferred as it is highly conductive and is cheap. Where the structure is such that light shines through an electrode the electrode may be in the form of a regular or irregular grid and/or thin enough to allow light through. Use of a NiP alloy as part of an electrode, in particular the anode, can help to increase device lifetime, it is speculated by mitigating electromigration as well as in other ways.

The NiP may be present as a layer of an electrode; in embodiments the layer comprising NiP alloy has a thickness of 1 nm to 1000 nm, more preferably 1 nm to 200 nm and still more preferably 5 nm to 100 nm. Preferably the layer comprising NiP alloy has a substantially uniform thickness. In embodiments at least one polymeric layer is deposited onto the electrode, for example a hole injection layer. Preferably the polymeric layer comprises acidic groups. Representative examples of hole injection layers include poly(3,4-ethylenedioxythiophene) (PEDOT), PEDOT: PSS, polythiophene conductive polymer, polyaniline (PANi), polypyrole, polyacrylic acid or a fluorinated sulfonic acid, for example Nafion, Preferably the polymeric layer is solution processed. Advantageously the NiP alloy constitutes a hydrophilic layer on the metal and facilitates deposition of polymeric layers by solution processing from water.

In embodiments an electrode comprising an NiP alloy, for example an NiP alloy layer, comprises 1 to 15 %wt phosphorus, more preferably 5 to 12 %wt and still more preferably 8 to 12 %wt phosphorus. The phosphorus content of the NiP alloy is preferably determined by the test set out in IS04527 Annex D. The amount of phosphorus present in the NiP alloy is important as it can affect the microstructure and performance characteristics of the alloy. Usually NiP alloys comprising greater than about 10 %wt phosphorus are amorphous and those comprising less than about 8 %wt phosphorus are microcrystaliine. Alloys containing an intermediate amount of phosphorus, e.g. 8 to 10 %wt are generally semi-crystalline. An NiP alloy may be present in one or both the electrodes of the present invention and may be semi- crystalline or amorphous, more preferably amorphous. Semi-crystalline and particularly amorphous NiP alloys comprise fewer grain boundaries that may act as sites for intergranular corrosion and therefore tend to provide improved corrosion resistance compared to crystalline NiP alloys.

Some preferred NiP alloys present in an electrode comprise 85-99 %wt nickel, 1 to 15 %wt of phosphorus and 0-2 %wt of impurities, e.g. other metal such as Pd. Still further preferred NiP alloys comprise 90-92 %wt nickel, 8-10 %wt phosphorus, and 0-2 %wt of impurities, e.g. other metal such as Pd. Further preferred NiP alloys present in an electrode may consist essentially of, e.g. consist of, nickel and phosphorus. Low amounts (e.g. 0 to 2 %wt) of impurities such as a second metal used as a catalyst may be present. Particularly preferred NiP alloys consist of 85-99 %wt nickel and 1 to 15 %wt of phosphorus, wherein the total weight of nickel and phosphorus is 100 %. Still further preferred NiP alloys consist of 90-92 %wt and 8-10 %wt of phosphorus, wherein the total weight of nickel and phosphorus is 100 %.

Optical outcoupling

Broadly speaking we will describe structures and fabrication methods which improve the outcoupling of guided modes in an OLED device, by making use of scattering structures deliberately incorporated into an anode or planarization layer. The techniques we describe combine volume scattering with particular anode metallisation technologies in order to provide structures which are straightforward to fabricate, and which result in more efficient devices.

A significant reduction in the efficiency of an OLED device results from light being trapped in optical modes within the device. Embedding high refractive index scattering centres in a matrix within a layer of such a device can improve outcoupiing efficiency. Thus Figure 2a shows a simple OLED structure 200 in which a transparent substrate 202 is provided with a polymer layer 204 comprising volume scattering centres, in embodiment's zirconium oxide (Zr0₂) nanoparticles. A transparent conductive electrode layer 208, here fabricated from indium tin oxide (ITO) overlays layer 204, and this is followed by a hole injection layer 208 and LEP stack 210. A cathode layer 212 (which may be non-transparent) provides a second electrode, with an optional intermediate layer 214 acting as an electron transport layer.

The device illustrated in Figure 2a is provided with a iensiet array 216 comprising cylindrical, spherical or other shaped iensiets, on a light-exiting surface of the substrate to further improve outcoupiing. The skilled person will appreciate that in embodiments of the invention as described later similar structures may also be employed to improve external outcoupiing. Generally the layers within the device are pianarised to better than 100nm so that in general there is no cathode scattering.

Broadly speaking the volume scattering layer 204 provides an internal scattering mechanism to enable photons trapped as guided modes in the device to be extracted into air. Such guided modes can otherwise trap a significant percentage of the light produced by the device.

As previously noted the scattering layer may comprise polymer, such as MSG (poly(methyi-silsesquioxane)), in which are embedded nanoparticles, typically with a maximum dimension in the range 30nm to 3Q0nm. Figure 2b shows computed scattering patterns of zirconium oxide nanoparticles of different diameters. These typically have a relatively high refractive index, for example n=1.95 (dependent on heat treatment at 450°C); the SQ typically has a much lower refractive index, for example n=1.4, providing a high refractive index contrast. The plots shown illustrate scattering for s-polarised (outer curves) and p-polarised (inner curves) light. A typical volume fraction for the particles is in the range 0.25-0.40. Figure 2c shows attenuation by zirconium oxide spheres embedded in MSG at a volume fraction of 0.28, at wavelength of 550nm showing (on the left) a computed film-thickness scattering- attenuation co-efficient σ and (on the right) computed scattering optical-density spectra of 1 ,0μΓη films. As can be seen, there is a larger scattering attenuation co-efficient for larger particles, suggesting that particles with a maximum dimension of greater than 300nm are less preferred. Thus a typical thickness of scattering film may in the range 500nm-100Qnm; a typical volume fraction may be in the range 0.25 or 0.35-0.40; and a preferred maximum dimension of nanoparticles may be less than 300nm.

Referring now to Figure 3a, this shows an OLED structure 300 according to a first embodiment of the invention. The structure comprises a transparent substrate 302 bearing an epoxy layer 304 comprising embedded zirconium oxide nanoparticles 306. An anode metal grid 308 overlays the epoxy layer 304 and, in the illustrated embodiment, comprises a seed or precursor layer 308a onto which is grown anode metal 308b. A hole injection layer (HIL) 310 overlays the anode metal grid, and this is followed by an LEP stack 312, for example as previously described, which is in turn overlayed by a cathode electrode 314.

To fabricate the device of Figure 3a the epoxy layer 304 may be deposited by any suitable technique, for example spin coating or slot die coating. The epoxy material comprises volume scattering centres such as zirconium oxide nanoparticles to provide high refractive index scattering centres. A precursor catalyst 308a is then deposited where the metal grid is required, again using any suitable technique, for example flexographic printing. The substrate may then be immersed in an electroless plating bath to grow the anode metal grid 308b where the precursor catalyst is deposited. The anode electrode metal may comprise, for example, copper, nickel, gold or a combination of these materials. The LEP layers may then be fabricated in the usual manner.

To further enhance the outcoupling a refractive index contrast layer, schematically illustrated by dashed line 316, may be deposited on top of the metal grid. Such a layer may comprise, for example, ITO or a layer comprising silver nanowires (for exampie in a polymer, for exampie deposited as ink). However such an additional layer is not essential. Referring to Figure 3b, this shows a related OLED device structure 350 according to a second embodiment of the invention. In Figure 3b like elements to those of Figure 3a are indicated by like reference numerals. In Figure 3b the anode metal grid is embedded in the epoxy layer 304, which may be arranged to planarise the anode electrode tracks. Again volume scattering nanoparticies are incorporated into the epoxy layer and again an optional transparent, electrically conductive index contrast layer may be provided over the polymer (epoxy)layer 304 to provide a high refractive index transparent anode, in embodiments the metal grid may be formed as previously described, first opening apertures in the epoxy layer 304 down to substrate 302, on which the seed (precursor catalyst) material 308a may be deposited.

In embodiments the structure is arranged so that at least some of the scattering centres lie within the evanescent mode light from an adjacent layer such as adjacent layer 316; this, in embodiments, is a layer of relatively higher refractive index. In this context a scattering centre may be considered to be within an evanescent mode if it is within the 1/e penetration depth of the evanescent mode light. The volume concentration of scattering centres in the light scattering layer may be adjusted to achieve this. Embodiments of such structures provide "anode scatter'' of the type described, and hence particularly efficient extraction of light which would otherwise be trapped in guided modes within the device, rather than just volume scattering within a region of the device.

Although we have described embodiments of the invention in which the techniques we describe are applied to an anode, the skilled person will recognise that for, say, a top- emission OLED device similar techniques may be applied to a cathode. No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art and lying within the scope of the claims appended hereto.

Claims

1. A method of fabricating an organic lighting device, the method comprising: providing a transparent substrate;

providing a light scattering polymer layer on said substrate, wherein said polymer layer comprises a volume light scattering material to scatter light from a light emitting layer of the device;

providing a first set of electrodes for said device, wherein said first set of electrodes comprises a plurality of conductive tracks dispersed on or embedded in said polymer layer;

providing a set of organic lighting device layers over said first set of electrodes and polymer layer; and

providing a second electrode layer over said set of organic lighting device layers.

2. A method as claimed in claim 1 wherein said providing of said first set of electrodes comprises depositing a precursor layer of electrically conductive material followed by eiectroiess plating of said conductive tracks on said precursor layer. 3. A method as claimed in claim 1 or 2 wherein said first set of electrodes is disposed on said light scattering polymer layer.

4. A method as claimed in claim 1 or 2 wherein said first set of electrodes is embedded in said light scattering polymer layer.

5. A method as claimed in claim , 2, 3 or 4 wherein said light scattering polymer layer comprises a plurality of oxide nanoparticles in a layer of epoxy or other polymer material, 6. A method as claimed in any one of claims 1 to 5 further comprising providing an index contrast layer of high refractive index material over said first set of electrodes and said light scattering polymer layer, wherein at a wavelength of operation of said organic lighting device a refractive index of said index contrast layer is greater than a refractive index of the polymer of said light scattering polymer layer.

7. A method as claimed in any one of claims wherein said index contrast layer comprises a layer of ITG,

8. A method as claimed in claim 8 wherein said index contrast layer comprises a layer of metal nanostructures.

9. A method as claimed in any preceding claim wherein said transparent substrate is a glass or plastic substrate; and

wherein said organic lighting device comprises an organic light emitting diode device, and wherein said set of organic lighting device layers comprises at least a hole injection layer and an organic light emitting diode stack of layers, more particularly a light emitting polymer stack.

10. A method as claimed in any preceding claim further comprising packaging said organic lighting device for use.

11. An organic lighting device, the device comprising:

a transparent substrate;

a light scattering polymer layer on said substrate, wherein said polymer layer comprises a volume light scattering material;

a first set of electrodes for said device, wherein said first set of electrodes comprises a plurality of conductive tracks disposed on or embedded in said polymer layer;

a set of organic lighting device layers over said first set of electrodes and polymer layer; and

a second electrode layer over said set of organic lighting device layers.

12. An organic lighting device as claimed in claim 11 wherein said first set of electrodes is disposed on said light scattering polymer layer.

13. An organic lighting device as claimed in claim 11 wherein said first set of electrodes is embedded in said light scattering polymer layer.

14. An organic lighting device as claimed in any one of claims 1 to 13 wherein said light scattering polymer layer comprises a plurality of oxide nanoparticles in a layer of epoxy or other polymer material. 15, An organic lighting device as claimed in any one of claims 1 to 16 further comprising an index contrast layer of high refractive index material over said first set of electrodes and said light scattering polymer layer, wherein at a wavelength of operation of said organic lighting device a refractive index of said index contrast layer is greater than a refractive index of the polymer layer of said light scattering polymer layer.

18. An organic lighting device as claimed in claim 15 wherein said index contrast layer comprises a layer of ITO.

17, An organic lighting device as claimed in claim 5 wherein said index contrast layer comprises a layer of metal nanowires.

18, An organic lighting device as claimed in any one of claims 11 to 17 wherein said transparent substrate is a glass or plastic substrate, and wherein said conductive tracks comprise a metal layer deposited over a precursor layer of electrically conductive material.

19, An organic lighting device as claimed in any one of claims 11 to 18 wherein said organic lighting device comprises an organic light emitting diode device, and wherein said set of organic lighting device layers comprises at least a hole injection layer and an organic light emitting diode stack of layers, more particularly light emitting polymer stack.

20, A lighting tile comprising an organic lighting device as claimed in any one of claims 1 to 19.

21 , A method, organic lighting device, or lighting tile as claimed in any one of claims 1 to 20 wherein said first set of electrodes defines one or more anode electrodes for said organic lighting device.