WO2010026390A1

WO2010026390A1 - Light emitting device with improved light extraction

Info

Publication number: WO2010026390A1
Application number: PCT/GB2009/002134
Authority: WO
Inventors: Nicholas Simon Walker; Rifat Iqbal; Lionel Chaudet
Original assignee: Microsharp Corporation Limited
Priority date: 2008-09-04
Filing date: 2009-09-04
Publication date: 2010-03-11
Also published as: GB0816136D0

Abstract

A light emitting device comprises a light emitting structure in the form of an organic electroluminescent device formed on a transparent substrate (8) through which light from the light emitting structure is emitted. A polarisation-preserving light extraction member with a multiplicity of optical elements (2) over the area of the light emitting structure, is optically coupled to the outside of the transparent substrate to reduce total internal reflection of light within the substrate.

Description

LIGHT EMITTING DEVICE WITH IMPROVED LIGHT EXTRACTION

The present invention relates primarily to a light emitting device with improved light extraction. However aspects of the invention are also applicable to light absorbing devices.

A variety of light emitting devices are known in which a light emitting or generating structure is formed on a substrate. In some of these the light is transmitted for use through the substrate which is in this case, of course, transparent. An example of this type of device is an organic light emitting diode (OLED) which typically consists of a multi layer sandwich of a planar glass or flexible plastic substrate, a layer of indium-tin-oxide (ITO) as a transparent anode, one or more organic layers which may comprise small-molecule, polymeric or dendrimeric materials, and a reflecting cathode. By applying a voltage between the ITO anode and the cathode, the organic layers electroluminescence. The light which is generated, and reflected from the cathode, is emitted through the transparent ITO electrode and through the transparent glass substrate. Other layers, such as a hole transporting layer (HTL) and an electron transporting layer (ETL) may also be present in these structures. Such OLED's may be used as light-emissive panels useful as large area lighting, as backlights, e.g. in LCD's, as elements of pixelated OLED display devices or in segmental displays of information.

However, a significant problem with such devices is that a considerable proportion of the light which is generated within the device is not emitted to the outside from the face of the transparent substrate, but instead is trapped due to total internal reflection (TIR) within the substrate and emitted from the edges - a process which may be termed "wave-guided" in this specification. Total internal reflection is the reflection of light from the interface between a medium with index of refraction ni, within which the light is travelling and a medium with index of refraction n₂, where n₂ < n_1; when the light incident on the interface makes an angle of θ> θc to the normal of the interface, where θc is known as the critical angle and is equal to sin^'1 (n₂ / ni). Thus, as illustrated in Fig. 1 of the accompanying drawings, light generated by organic films and reflected by the cathode enters the transparent glass substrate, but only a proportion of the light is emitted from the substrate in a useful direction, with some light being trapped and wave-guided in the substrate. In fact, the amount of wave-guided light is much greater than the amount of useful light emitted, with perhaps up to 80% of the generated light being lost by wave-guiding and other mechanisms; see T. Tsutsui, E. Aminaka, c.p Lin, D. -U Kim, Phil Trans. R. Soc. London, 1997,355.801. Thus the useful light may only be 20% of the generated light.

Various methods have been proposed to increase the coupling efficiency. They can be classified into four general schemes:

( 1 ) Apply a corrugated microstructure on the substrate to increase the coupling efficiency through Bragg-scattering the light bounded in lateral guided modes.

(2) Modify the substrate surface to reduce the TIR loss at the substrate/air interface, such as incorporation of monolayer of silica microsphere in the substrate, shaping of the device into a mesa structure, patterning a polymer microlens array, or directly placing a large size hemispherical lens directly on top of the substrate surface.

(3) Insert a highly porous medium between ITO layer and supporting substrates to scatter the light out.

(4) Make use of a microcavity structure.

Although these methods can increase the coupling efficiency, they also have drawbacks, such as strong angular dependent emission spectrum, changes in the electrical characteristics, or costly and complex processing. Some optical films are included with LCD backlights which increase the proportion of collimated light output by the backlight. However, these films do not actually increase the proportion of light coupled out of the electroluminescent backlight device; they only redistribute the resulting light. Furthermore, there are examples of films bonded to the front face of LCD's but these are intended to redirect ingoing ambient light and outgoing light.

In accordance with the present invention, it has been appreciated that for an OLED backlighting device there is a potential to increase the forward intensity of the device by a combination of:

1 ) Increasing the total light outcoupled from the device using an appropriate light outcoupling means.

2) Redistributing the light from wide emission angles to low angular values (collimation).

Viewed from one aspect of the present invention there is provided a light emitting device comprising a light emitting structure in the form of an organic electroluminescent device formed on a transparent substrate through which light from the light emitting structure is emitted, wherein a polarisation-preserving light extraction member comprising a multiplicity of optical elements over the area of the light emitting structure, is optically coupled to the outside of the transparent substrate to reduce total internal reflection of light within the substrate. Included in this optical film are microoptical features which also collimate the light. The optical structure proposed comprises two integrated components:

1) A film of microstructures with volumetric microstructures with differing refractive indices, where these structures take the form of circular or elliptical columns or slabs - so called Graded Index (GRIN) diffusers (materials with volumetric properties which generate microGRIN lenses). 2) Microoptical structures directly on top of this film (either formed of the same structured material or of an optically clear material on top of and optically adherent to the GRIN film) which collimate the light emitted.

Because the light extraction member is optically coupled to the light emitting side of the transparent substrate most of the light enters the light extraction member and the amount of total internal reflection within the substrate is reduced, and thus the amount of light that is wave-guided in the substrate, and which would usually effectively be lost, is reduced. The structure of the light extraction member is such that an increased proportion of the light striking the outside surface of the member does so at an angle less than the critical angle, and hence is refracted out of the member, rather than being totally internally reflected. Therefore, to produce the same external light output, a light emitting device including the light extraction member consumes less power than the same device without the light extraction member. Furthermore, the fact that the light extraction member is polarization preserving implies that it would not adversely affect the reflective mode of operation of an LCD display that has a reflective OLED backlight (incorporating such a light extraction member) as described in PCT/GB00/00497 (WO0049453). In such a display it is important in the reflective mode that the polarization of the incoming ambient light is not significantly altered by the OLED structure in the process of being reflected by the metal cathode, such that substantially all of the reflected light can pass back through the rear linear polarizer of the LCD. Also, given that most contrast-enhancement layers for use in OLEDs are based on polarization of the light transmitted through them, the fact that the light extraction member is polarization preserving implies that it would not adversely affect the contrast of a device incorporating, for example, a circular polarizer for the reduction of reflected ambient light from an internal reflective layer.

The light extraction member may be optically coupled to the transparent substrate simply by being formed tightly adherent to that substrate, e.g. as a result of a coating procedure or by using an optical adhesive (i.e. which is transparent and has a refractive index which matches the substrate and member) or a refractive index matching liquid. Where the refractive indices of the substrate and member differ, the adhesive or liquid should have a refractive index which is some value between them. Thus refractive-index matching means the minimization of the difference between the refractive indices of two or more media, in order to maximize the amount of light transmitted from one medium to the next and to minimize the amount of reflected light. As the refractive indices of all materials vary with wavelength (with the largest variation in the absorption region) and, to a lesser extent, temperature, refractive-index matching cannot be perfect across the whole of the visible spectrum and at all temperatures. However, it is possible to optimize the refractive-index matching for a range of wavelengths and a range of temperatures.

In general the term "optically coupled" as used in this specification means that the refractive indices of the two surfaces are closely matched and the adhesive (gel or liquid) used to join them has a refractive index which is equal to, or falls between, the values of the two surfaces. The better the matching of the refracting indices, the better the optical coupling. Preferably the indices should not differ by more than 10%. Optical adhesive is an adhesive that is transparent to visible light, i.e. light in the wavelength range 400 nm to 700 nm, and is used to optically couple optical components in order to minimise optical losses from reflection.

The light extraction member, which is polarization preserving, preferably comprises a multiplicity of optical elements across the area of the light emitting structure.

A preferred combination of properties for this approach to outcoupling has been found to be:

1) There exists a very tight optical coupling between the substrate (e.g. glass) and the "GRIN" film;

2) The "GRIN" film has a wide angle of view diffusion property; 3) On top of the GRIN film, either formed directly on the film or on another film above the GRIN film, are prismatic features, with an apex angle of for example 90 degrees and a pitch of for example 50 microns in one preferred embodiment, to maximise forward gain for display backlighting. Other values of apex angle can be used if a wider diffusion of light is required, e.g. for room lighting. Alternatively, an array of microlens optical structures can be used on top of the GRIN film.

As used herein, prismatic refers to features having a cross-sectional shape including an apex. The angle of the apex is the prism angle.

In general, the prism apex angle is preferably in the range of about 70 to about 110 degrees, and more preferably about 75 or about 80 degrees to about 105 degrees. In some preferred arrangements, the prism apex angle is preferably about 80 to about 100 degrees and more preferably about 85 to about 95 degrees. In one preferred embodiment the prism apex angle is preferably about 90 degrees. The prism is preferably in the form of an isosceles triangle.

In general, the pitch of the prismatic features is preferably in the range of about 25 to about 75 microns, and in some preferred arrangements the pitch of the prismatic features is preferably in the range of about 25 to about 40 microns and in one embodiment is about 30 microns. The pitch may be in the range of about 35 to about 65 microns. In some preferred arrangements the pitch of the prismatic features is preferably in the range of about 40 to about 60 microns, and more preferably in the range of about 45 to about 55 microns. In one preferred embodiment the pitch is about 50 microns.

Some possible arrangements use a prism apex angle of 70 degrees and a pitch of 30 microns ("70/30"). Some use a prism apex angle of 105 degrees and a pitch of 50 microns ("105/50"). A preferred embodiment uses a prism apex angle of 90 degrees and a pitch of 50 microns ("90/50"). In general there be any permutation of the preferred prism angles with the preferred pitches. Accordingly, every prism angle value in the range of about 70 to about 110 degrees; or about 75 to about 105 degrees; or about 80 to 105 degrees; or about 80 to about 100 degrees; or about 85 to about 110 degrees; or about 85 to about 95 degrees; or about 100 degrees to about 110 degrees; or about 70 degrees, or about 90 degrees, or about 105 degrees; may be used with every pitch value in the range of about 25 to about 75 microns; or about 25 to about 40 microns; or about 30 microns; or about 35 to about 65 microns; or about 40 to about 60 microns; or about 45 to about 55 microns; or about 50 microns.

Viewed from another aspect of the present invention, there is provided a light emitting device comprising a light emitting structure formed on one side of a transparent substrate through which light from the light emitting structure is emitted, and provided on the other side of the substrate a volumetric gradient index film, there being provided on the gradient index film a surface relief provided with parallel prismatic features, with the prism apex angle being in the range of about 70 degrees to about 110 degrees, and the pitch of the prismatic features being in the range of about 25 microns to about 75 microns.

In one preferred arrangement the prism apex angle is in the range of about 85 degrees to about 110 degrees, and is preferably in the range of about 85 degrees to about 95 degrees, preferably about 90 degrees, or in the range of about 100 degrees to about 110 degrees, preferably about 105 degrees.

In one preferred arrangement, the pitch of the prismatic features is preferably in the range of about 40 to about 60 microns, or the range of about 45 to about 55 microns; and preferably is about 50 microns.

Viewed from another aspect of the present invention, there is provided a light emitting device comprising a light emitting structure formed on one side of a transparent substrate through which light from the light emitting structure is emitted, and provided on the other side of the substrate a volumetric gradient index film, there being provided on the gradient index film a surface relief provided with an array of microlenses. The microlenses in the array can, for example, be square or hexagonal. Preferably each individual lens is in close abutment with the adjacent lenses in the array.

In one preferred arrangement the array of microlenses comprises a hexagonal array of microlenses with a radius of about 25 to about 50 microns and a height of about 20 to about 45 microns.

Preferably the light emitting structure is an organic electroluminescent device.

In preferred embodiments the gradient index film is closely optically coupled to the substrate. The surface relief may be provided on the gradient index film or may be provided as a separate structure closely optically coupled to the gradient index film. The OLED top surface is generally glass or plastic. It is important that the GRIN diffuser is coated directly on the OLED otherwise the out-coupling can be poor. Between the prismatic structure and the GRIN diffuser there must be no air gap and an adhesive may be used. In some embodiments using glass OLED 's the GRIN diffuser is coated directly on the glass and then an adhesive is put down onto the GRIN diffuser, onto which is attached the prismatic or other structure. If the OLED is plastic then the GRIN can be coated directly to the plastic and then the prismatic structure coated directly on the GRIN diffuser, avoiding an adhesive step and enhancing the forward gain by approximately 2%. In some embodiments using a plastic OLED, an adhesive may be employed.

There are many other aspects of the present invention. For example, viewed from another aspect, the invention provides a light emitting device comprising as a light emitting structure an organic electroluminescent device formed on a transparent substrate through which light from the light emitting structure is emitted, wherein a polarisation-preserving light extraction member comprises: a) a volumetric gradient index film containing multiple circular, elliptical columns or elongated vertical slabs of differing refractive index, oriented either vertically or at a slanted angle with respect to the film surface and with features which are less than about 50 microns in size; and

b) a surface relief film directly adhered to or formed from the gradient index film, with prismatic, lenticular, microlens or micropyramid features (or combinations thereof) forming a multiplicity of optical elements over the area of the light emitting structure,

the light extraction member being optically coupled to the outside of the transparent substrate to reduce total internal reflection of light within the substrate and partially collimate or change the light emission distribution of the device.

The light extraction member may be optically coupled to the transparent substrate by having been formed by directly coating the substrate with material and then forming the light extraction member on the substrate.

The light extraction member may be optically coupled to the transparent substrate using an optical adhesive. The light extraction member may be optically coupled to the transparent substrate using a refractive index matching liquid.

There may be a plurality of light emitting structures.

The light extraction member is micro structured by comprising a multiplicity of surface shape optical elements over the area of the light emitting structure.

There may be different types of GRIN e.g. GRIN with an Angle of View of 20 degrees or more.

There may be different types of light extraction microoptical features, including prismatic features, prism apex angles of between 80 and 105 degrees and so forth. The optical elements may be parallel prismatic ribbed surface-shape features on the light extraction member. The parallel prismatic ribs may have a triangular cross- section. The triangular cross section may be that of an isosceles triangle.

The parallel prismatic ribs may have a prism angle of between 70 and 10 degrees.

The parallel prismatic ribs may have a prism angle of between 60 and 20 degrees.

The parallel prismatic ribs may have a prism angle of between 45 and 25 degrees.

The parallel prismatic ribs may have a prism angle of about 42 degrees.

The optical elements may be parallel cylindrical (i.e. without an apex) ribbed surface-shape features on the light extraction member. The parallel cylindrical ribs may have a semi-circular cross-section.

The optical elements may be square-cross-section ribs.

The optical elements may be micro-lenses, micro-pyramids or micro-cubes.

The light extraction member may have a flat face optically coupled to the transparent substrate and an opposite microstructured face from which light is emitted.

The surface shape optical elements have a pitch of about 30 to about 70 micrometers, preferably about 40 to about 60 micrometers, and preferably about 50 micrometers.

Artefacts in the gradient index film may provide variations in refractive index. The variations in refractive index may form light guides. In either case the light extraction member may have a flat face optically coupled to the transparent substrate and an opposite flat face from which light is emitted. The variations in refractive index occur over a region of about 2 to about 10 micrometers.

Multiple light extraction members may be optically coupled together with the base light extraction member having a flat face optically coupled to the transparent substrate. The light extraction members may comprise a polarization-preserving diffuser film having volume refractive index features and a prismatic film having surface shape optical features. The polarization-preserving diffuser film may have a diffusion angle of between about 15 degrees and about 25 degrees. The prismatic film may have a base prism angle of between about 35 degrees and about 45 degrees.

The optical elements may be periodic or random or a combination of both.

The light extraction member may be a film.

Viewed from another aspect the invention provides a light-emissive panel comprising a light emitting device as described above.

Viewed from another aspect, there is provided a backlit or transflective liquid crystal display module comprising as a backlight a light emitting device as described above.

Viewed from another aspect there is provided an electronic device comprising such a liquid crystal display module.

Whilst the above concerns outcoupling, thin film solar power cells using the organic layers which have photovoltaic properties are being developed. These have very similar structures to the OLEDs described above. In photovoltaic devices exactly the same problem exists as discussed above, but 'in reverse'. External light needed to be incoupled into the device so as to interact with a thin light absorbing layer. The structure of, for example Organic photovoltaic devices, resembles in very close detail that of OLED's, and in some cases many of the materials and chemicals are the same.

The problem in these devices is incoupling of light propagating from the sun and sky into the device so that the light can interact with the absorbing layer. There is a similar, though inverse problem therefore of incoupling light into the devices. The same approaches which work with outcoupling from light emitting devices can be used to improve in-coupling in light absorbing devices.

Therefore precisely the same optical film approaches which work with outcoupling of light from OLED's can be used for in-coupling of light into photovoltaic devices, particularly Organic photovoltaic devices.

Accordingly the features discussed above can be applied to incoupling, subject to any changes that may be necessary.

Thus, the present invention provides an optical film linked to a light emitting or a light absorbing device in which either the light output of the light emitting device is improved by coupling a greater proportion of the light generated in device to the outside or when linked to a light absorbing device (for example a photovoltaic cell) in which the quantity of light which reaches the light absorbing element of the device is improved by a coupling a greater proportion of impinging on the device to the inside of the device.

For example, viewed from another aspect, the invention provides a light absorbing device comprising as a light-absorbing structure an organic photovoltaic device formed on a transparent substrate through which light from outside enters, wherein a polarisation-preserving light collection member comprises:

a) a volumetric gradient index film containing multiple circular, or elliptical columns or elongated vertical slabs of varying refractive index, preferably with features which are less than 50 microns in size; b) a surface relief film directly adhered to or formed from the material of the gradient index film with prismatic, lenticular, microlens or micropyramid features, or combinations thereof, forming a multiplicity of optical elements over the area of the light emitting structure;

the light collection member being optically coupled to the outside of the transparent substrate to increase the quantity of light incoupled into the optical structure.

Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows a known OLED arrangement;

Figure 2 shows an arrangement in accordance with the present invention;

Figures 3, 4 and 5 illustrate some examples of optical gain from the arrangements in accordance with the invention;

Figure 6 shows the Gain vs different combinations of GRIN 2 coated directly onto the glass substrate of a red OLED;

Fig. 7 is a schematic diagram of an organic electroluminescent display;

Fig. 8 shows an OLED pixel with films on it represented in three views - 1/ right side; 2/ front side; and 3/ top view; and Figure 9 is a general schematic of the film set up;

With reference to the figures, Figure 2 shows an OLED structure, a glass or plastic substrate 8, a GRIN diffuser 4, and a prismatic structure 2, with an adhesive layer 6 between the GRIN diffuser 4 and the prismatic structure 2. In some examples, onto a GRIN 2 SY24aF (symmetric 24 degree diffuser, Riffix™ from MCL) is cast a 90 degree apex angle, 50 micron pitch prismatic microoptical structure and the film is placed on top of the glass OLED. The variation in the gain with current has been measured with the brightness for the three colours green, blue and red, respectively in Figures 3, 4 and 5. Figure 3 shows a plot of Gain vs Brightness for a Green OLED with GRIN 2 SY24aF + microoptical structure DT070405-01 90-50. Figure 4 shows a Gain vs Brightness plot for a Blue OLED with GRIN 2 SY24aF + microoptical structure DT070405-01 90-50. Figure 5 shows a Gain vs Brightness plot for Red OLED with GRIN 2 SY24aF + microoptical structure DT070405-01 90-50.

The green OLED has an average gain of 20% with a maximum at 24%, the blue OLED has an average gain of 28% with a maximum at 32% and the red OLED has an average gain of 35% with a maximum at 41%.

Figure 6 shows Gain vs different combinations of GRIN 2 coated directly onto the glass of red OLED.

Here the term GRIN 24 refers to a symmetric 24 degree diffuser Riffix™ from MCL coated directly onto the glass of the red OLED. The terms nl and n2 refer to samples 1 and 2. The term GRIN 24*2 refers to two layers of symmetric 24 degree diffuser Riffix™ coated directly on the glass. The term GRIN 24*2+10*1 refers to the latter plus a symmetric 10 degree diffuser Riffix™ coated on top of the 24 degree symmetric diffuser. The term GRIN 24*3 refers to three layers of symmetric 24 degree diffuser Riffix™ coated directly on the glass. The term GRIN 24* 1 +10* 1 refers to one layer of symmetric 24 degree diffuser Riffix™ coated directly on the glass plus a symmetric 10 degree diffuser Riffix™ coated on top of the 24 degree diffuser. The "//" in Figure 4 refers to GRIN 24 coated on glass. The term BEF DT07 refers to a microoptical structure produced by Microsharp Corporation Limited. The term RBEF refers to the use of a rounded microoptical structure produced by Microsharp Corporation Limited. The optimum angle of view (full width half maximum on an Eldim plot) for the above results is between 20-40 degrees.

Diffusers are optical films that are not transparent but act to spread out light after it strikes them. These materials are used in the displays industry to even out light from slim backlights, organise the correct viewing angle of screens, and act as projection screens for front and rear projection.

Diffusers are used in LCD displays such as LCD televisions, notebook PCs and LCD monitors to enhance contrast. Generally diffusers are either surface diffusers, where a surface microstructure (such as random indentations or coating of microspherical lenses) scatters the lights, or a bulk diffuser, where elements within the material scatter the light. Holographic diffusers are an additional technological approach.

GRIN diffusers potentially offer a superior alternative; since they are polarisation preserving (LCD displays use polarised light) and their properties can be tailored over a wide range to provide diffuser film material with a variety of properties.

GRIN is an optical polymer sheet diffuser material. GRIN stands for GRaded INdex. The material has refractive index variations within it that form micro light guides. Typically a film from Microsharp may be 50 μm thick. Contrast arises from refractive index variations. Here the refractive index variations are predominantly perpendicular to the surface normal vector.

GRIN is a polarisation-preserving optical diffuser material. While maintaining at least 98% polarisation preservation, and 99% transparency, it is able to alter both the mean direction of light propagating through it, and control the degree of light spread around that mean. Polarisation preservation is a very important property in liquid crystal displays, since within LCD displays the light has to be polarised for an image to be created. The member may have a flat face which is optically coupled to the transparent substrate.

The internal refractive index optical elements may be internal thin-film diffusing structures where reduction in wave-guiding is achieved substantially by refractive index variations within the volume of the film rather than surface features. Such films may be created by the selective UV polymerization of photopolymer materials, such as HRF600 type from DuPont, processed as described in, for example, US 5442482 and US 5695895. In general the structure within such materials can be regular or random in geometry with individual features, columns or strips of higher or lower refractive index). In three embodiments the internal features forming the polarization-preserving diffuser consist of parallel columns perpendicular to the planar surfaces of the film (denoted M 1-29, M 1-20 and Ml-10).

GRIN 2 is the second generation of GRIN diffusers developed at Microsharp. The tradename for this product is Riffix™. It comprises a mixture of monomers and oligomers with acrylate multi-functionality capable of undergoing free radical initiated polymerisation, a silicone prepolymer, or monomer, co-monomer, macromonomer or pre-polymer incorporating a silicone monomer or prepolymer capable of undergoing free radical initiated polymerisation, and a photoinitiator(s) capable of generating free radicals on exposure to radiation of the required wavelengths.

The photopolymerisable system polymerises to form a solid, light-transmitting material, in particular a diffuser, having volume refractive index variations and/or surface relief features dependant on the exposure of the system to polymerising radiation. The polymerising radiation is preferably parallel (collimated) or substantially parallel radiation. The direction of light guide columns that form is determined by the direction of incident parallel light. Prior to irradiation the method of manufacture may involve the fluid layer being overlaid with a clear film. The electromagnetic radiation polymerisation is followed by a thermal process to complete polymerisation.

GRIN 2 contrasts with GRIN 1 films in that GRIN 2 is manufactured using a mask- less process, and therefore this material can be produced continuously on a reel-to- reel coating machine. In addition it uses cheaper photosensitive materials. It is therefore expected that GRIN 2 films will be significantly cheaper than GRIN 1 and sell for about double the price of a conventional diffuser film. GRIN 2 has the capability of having its light redirecting properties extensively tailored for specific applications.

GRIN 2 films have until now been at the laboratory production scale. Initial production trials have demonstrated that commercial production, albeit following a relatively small amount of further development is fully possible.

GRIN 2 is a self organising polymer that forms small domains of differing refractive index, which harden into a film when correctly exposed to ultraviolet light. GRIN 1 requires that the domains are created by using a microstructured light mask. GRIN 2 production only specifically requires a particular type of field illumination unit. The manufacturing approach for GRIN 2 is therefore compatible with a modified coating unit and reel-to-reel production.

Several aspects of the GRIN 2 material can be tailored by modifications in:

The formulation of the initial polymer material;

The thickness of the film;

The precise way the illumination unit is used in its production;

The tailoring alters three main variables: The mean angle of light direction - from zero to +-30 degree (possibly up to +-40 degrees if suitable light source and material arrangements are used)

The angle of spread around that mean - from 14 degrees (down to 10 degrees with further development effort) up to 40 degrees

The degree of light spread asymmetry in the orthogonal directions across the film and along the film.

Dual direction GRIN materials enable the material to have a different angle of view function in the direction across the film as compared to the direction along the film. Thus one can have a narrow angle of view along the film and a wide angle of view across the film. Asymmetries of approximately 2.5 have been achieved.

Light can be trapped and guided by such structures analogous to the light-guiding properties of graded refractive index optical fibres.

It is possible to form on top of these types of optical films surface relief optical films in a variety of structures such as prism, cylindrical lenses (lenticulars) microlens and micropyramids.. This is disclosed in Microsharp US patent 6724535 "Stepped Surface Diffuser". The surface relief films can be formed of the GRIN structure material itself or they can be cast or laminated on top of the GRIN structured material from, for example, UV cured transparent acrylic resins cast using a roll to roll process and a microstructured casting drum.

These light emitting structures - composed of GRIN films with a microoptical structure on top - may be periodic or random or a combination of both. Where there are a plurality of light emitting structures (such as an array of OLEDs) the light extraction member should comprise a multiplicity of the optical elements across each structure. The optical elements may not necessarily be simply prismatic structures, but may for example be other 2D or 3D surface relief features such as parallel prismatic ribs of triangular, square or semicircular (termed micro cylinders) shape, or micro-lenses, micro-pyramids or micro-cubes. Where parallel prismatic ribs are used they may have a prism angle (angle between the base and side) of between 10 and 70 degrees, preferably between 20 and 60 degrees, more preferably between 25 and 45 degrees, or in one embodiment of about 42 degrees.

The microoptical structures may be formed directly on the GRIN film using a film coating approach or by bonding such light extraction members to the transparent substrate of the light emitting device using an optical adhesive.

The surface relief optical elements may have a pitch of from 4 to 100 μm. The internal volume refractive index features may have a pitch of about 2-10 μm. Both the surface and the internal feature light extraction members may conveniently be in the form of a film, which can easily be bonded to the backside of the transparent substrate.

Because of the small size of the optical elements, the light extraction member does not need to be positioned accurately in relation to the light emitting structure, thus making manufacture easy.

The light emitting structure may be an organic electroluminescent device. The device may be used as a backlight in a backlit liquid crystal display of the type which is used in many electronic devices such as mobile telephones, personal digital assistants, computers etc. The device may be used in other lighting and display applications.

Fig. 7 shows a schematic diagram of an organic electroluminescent display comprising an OLED of a type known per se in the art and having, a cathode 12, an electron injector layer 14, an electron transport layer 16, an emitter layer (white) 18, a hole transport layer 20, a hole injector layer 22, and an anode (transparent electrode) 24. The display also comprises, in accordance with the invention, a polymer base film 26, and an optical film 28 for improved outcoupling and light collimation.

In examples it has been found that refractive index matching fluids used between the optical films and the glass OLED improved the head on brightness up to 5% gain. The best film configuration showing the highest forward (luminance) and overall (illuminance) gains was identified as one layer of GRIN24 coated directly on the glass OLED, with an adhesive, ARclear™ 8154 (an optically clear, unsupported transfer adhesive available from Adhesives Research, Inc.) and BEF 90/50.

Investigations into light collimation out-coupling using OLED on glass prototypes and multiple initial film designs have been conducted. The most promising film combination was a symmetrical micro-optical structure with a prism apex angle of 90 degrees and a prism pitch of 50 microns and a GRaded INdex (GRIN) diffuser, type being 24 degree symmetric Riffix™ (GRIN24).

Further light out-coupling experiments have shown the behaviour of the gain with an increase in the brightness and illuminance for red, green and blue OLEDs. The red OLED had a forward gain of 35%, blue OLED was 30% and the green OLED was 15%. The overall gain was positive, for the red (around 25%) and blue (around 11%), but negative for the green (around -11%).

The surface area of the prismatic film and source have an impact on the gain. The gain increase is high for small surfaces but it becomes less and less important with the increase in size of the surface. The BEF has always to cover the entire source surface no less, no more, to optimise the coupling and therefore the gain.

Optical modelling to optimise micro-optical structures to maximise collimated (less than 10 degrees) light output has been developed. The results confirm that a symmetrical micro-optical structure with prism apex angle of about 90 degrees and a prism pitch of about 50 microns provides the best solution. According to Table 1 the overall gain changes from 1.2% (OLED + GRIN24 without PET) to 3.2% (OLED + GRIN24 coated directly on glass substrate). The use of the adhesive, ARclear 8154, leads to a 2.4% loss in the overall gain to a value of 0.9%. When the vertical AoV in Table 1 is higher, the distribution seems more homogeneous and the overall gain is better.

AoY refers to Angle of View

Table 2 Data for Red OLED wrh and Miώoiir a GRIS24fiιm

Results have shown that the angle of view increases in the presence of the GRIN film, which is helping to diffuse the light. The configuration in Table 1 with the highest diffusion is the OLED with GRIN24 coated on the glass giving an 8° horizontal and 4° vertical increase. When the diffusion is higher, the out-coupling is improved.

The prismatic film with the best forward gains to date has been identified as the 90/50. Other prismatic films tested had lower angles (70°) and performance was less good. A tambour with profile 105/50 was used to produce a prismatic film. The simulations conducted on TRACEPRO outlined that the BEF 90/50 forward gain would be better than the 105/50 profile and the BEF 70/30 would be the poorest one.

G_9O/5θ (%) = 1.57 x G₇₀/30 (%) = 1-25 x G_m/50 (%)

Where G is the gain measured during the simulation by comparison between the source OLED with and without a BEF.

The BEF DT07 (90/50) + GRIN24 had a lower vertical viewing angle (see Table 2) of 135 as compared to 139° for the BEF 105/50 + GRIN24 and the OLED was 141° (see Table 1). The smaller the angle the better the forward and overall gain. The angle which changes more is the vertical one because the prisms are in the horizontal direction.

AoV refers to Angle of View

Table 2. Resu its for the same configuration bin different BEF

It has been established that the 90/50 profile forward gain is 1.5 times higher than the 105/50 profile and the overall gain is 1.4 times higher for 90/50. The BEF DT07 (90/50) is more beneficial with respect to forward and overall gain as compared to the 105/50 profile.

The BEF film has several lines of prisms, that were in the horizontal direction in the experiments. The aim of this film is to redirect the light in the forward direction using the prisms. However, with reference to Figure 8, the prismatic shape is present only in one direction, the vertical (see Figure 8 1/ ). In the horizontal direction, the surface is flat (Figure 8 2/ ). Therefore, the effect would be in one direction (vertical in this experimentation) and the distribution in the other direction would remain unchanged. Figure 8 3/ shows the top view.

Figure 9 is a general schematic for the film set-up. The actual film set-ups are as outlined in Table 3. The GRIN24 is either placed on top of the Red OLED or is coated directly on the glass of the Red OLED as indicated in Table 3. This experimentation outlines different configurations with the prismatic film BEF DT07 90/50. The best configuration will have the highest forward and overall gains and the lowest angle of view. Such a configuration will show the best out-coupling. Table 3 show that when the GRIN24 film is added between the OLED and the prismatic film, the characteristics are improved, i.e. the forward gain increases, as well as the overall gain. The angle of view is smaller with the 90/50 profile, therefore, the effect of the BEF film is improved. The light is redirected and concentrated to the front. The best configuration being OLED + GRIN24 coated on glass + BEF DT07, but a set-up for commercialisation on glass could be with the adhesive ARclear 8154 between the GRIN24 coating and the BEF film.

The best configuration in Table 3 with OLED + GRIN24 coated on glass + BEF DT07 has a vertical viewing angle 8° smaller than OLED + ARclear 8154 + BEF DT07 and 3° smaller for the vertical viewing angle when the GRIN24 film is not coated on the glass. The prisms are horizontal. It explains why the vertical AoV decreases (from 143 to 135° for the best one) and the horizontal one seems to be steadier (from 137 to 134°). When an adhesive is added between the GRIN24 and the BEF, there is a loss of 1 % on the forward gain, but the impact on the overall gain is more significant. The addition of GRIN 24 has a smoothing effect on the overall distribution.

AoV refers to Angle of View

Table 3. Data for different optical configurations ln/ή ihepπsmaϋcβhn BEF DTOl 90/50

In further experiments, the results shown in Table 4 indicate the best forward improvement would be with the prismatic film BEF 90/50. The overall gain for BEF 90/50 is lower than the other BEF films. The difference in the overall gain for each of the profiles was not significant. The BEF prismatic films do not improve the quantity of light going through but modify its distribution, hence mainly the forward results change.

ae ' in cate< r e g -

Table 4

Claims

1. A light emitting device comprising a light emitting structure in the form of an organic electroluminescent device formed on a transparent substrate through which light from the light emitting structure is emitted, wherein a polarisation- preserving light extraction member comprising a multiplicity of optical elements over the area of the light emitting structure, is optically coupled to the outside of the transparent substrate to reduce total internal reflection of light within the substrate.

2. A light emitting device comprising as a light emitting structure an organic electroluminescent device formed on a transparent substrate through which light from the light emitting structure is emitted, wherein a polarisation-preserving light extraction member comprises: a) a volumetric gradient index film containing multiple circular, elliptical columns or elongated vertical slabs of differing refractive index, oriented either vertically or at a slanted angle with respect to the film surface and with features which are less than about 50 microns in size; and b) a surface relief film directly adhered to or formed from the gradient index film, with prismatic, lenticular, microlens or micropyramid features (or combinations thereof) forming a multiplicity of optical elements over the area of the light emitting structure, the light extraction member being optically coupled to the outside of the transparent substrate to reduce total internal reflection of light within the substrate and partially collimate or change the light emission distribution of the device.

3. A device as claimed in claim 1 or 2 wherein the optical elements comprise parallel prismatic ribbed surface-shape features on the light extraction member.

4. A device as claimed in claim 3 wherein the parallel prismatic ribs have a triangular cross-section.

5. A device as claimed in claim 3 or 4 wherein the parallel prismatic ribs have a prism angle of between 70 and 10 degrees, preferably between 60 and 20 degrees, preferably between 45 and 25 degrees, and preferably about 42 degrees.

6. A device as claimed in claim 1 or 2 wherein the optical elements comprise parallel cylindrical ribbed surface-shape features on the light extraction member.

7. A device as claimed in claim 6 wherein the parallel cylindrical ribs have a semi-circular cross-section.

8. A device as claimed in any of claims 1, 2 or 6 wherein the optical elements comprise square-cross-section ribs.

9. A device as claimed in claim 1 or 2 wherein the optical elements comprise micro-lenses, micro-pyramids or micro-cubes.

10. A device as claimed in any preceding claim wherein the optical elements have a pitch of about 30 to about 70 micrometers, preferably about 40 to about 60 micrometers, and more preferably about 50 micrometers.

11. A device as claimed in any preceding claim comprising a plurality of said light extraction members, wherein said light extraction members are optically coupled together with the base light extraction member having a flat face optically coupled to the transparent substrate.

12. A device as claimed in claim 11 wherein the plurality of light extraction members each comprises a polarization-preserving diffuser film having volume refractive index features and a prismatic film having said multiplicity of optical elements.

13. A device as claimed in claim 12 wherein the polarization-preserving diffuser film has a diffusion angle of between about 15 degrees and about 25 degrees.

14. A device as claimed in claim 12 or 13 wherein the prismatic film has a base prism angle of between about 35 degrees and about 45 degrees.

15. A device as claimed in any preceding claim wherein the light extraction member is a film.

16. A device as claimed in any preceding claim wherein the light extraction member is optically coupled to the transparent substrate by having been formed by directly coating the substrate with material and then forming the light extraction member on the substrate.

17. A device as claimed in any preceding claim wherein the light extraction member is optically coupled to the transparent substrate using an optical adhesive.

18. A device as claimed in preceding claim wherein the light extraction member is optically coupled to the transparent substrate using a refractive index matching liquid.

19. A device as claimed in any preceding claim wherein the light extraction member has a flat face optically coupled to the transparent substrate and an opposite micro-structured face from which light is emitted.

20. A device as claimed in claim 1 or any of claims 15 to 19 when dependent on claim 1, wherein the light emitting structure comprises a gradient index film of microstructures with volumetric microstructures with differing refractive indices.

21. A device as claimed in claim 20 wherein the microstructures take the form of circular or elliptical columns or slabs.

22. A device as claimed in claim 2, 20 or 21 wherein the light emitting structure further comprises microoptical structures, directly on top of said gradient index film of microstructures, wherein the microoptical structures collimate the light emitted from the light emitting structure.

23. A device as claimed in any of claims 2, 20, 21 and 22 wherein the device comprises prismatic features on top of the gradient index film of microstructures, either formed directly on the gradient index film of microstructures or on another film above the gradient index film of microstructures.

24. A light emitting device comprising a light emitting structure formed on one side of a transparent substrate through which light from the light emitting structure is emitted, and provided on the other side of the substrate a volumetric gradient index film, there being provided on the gradient index film a surface relief provided with parallel prismatic features, with the prism apex angle being in the range of about 70 degrees to about 110 degrees, and the pitch of the prismatic features being in the range of about 25 microns to about 75 microns.

25. A device as claimed in claim 23 or 24 wherein the prism apex angle is in the range of about 70 to about 110 degrees, preferably about 75 or about 80 degrees to about 105 degrees, preferably about 80 to about 100 degrees, preferably about 85 to about 95 degrees, and preferably is about 90 degrees, or in the range of about 100 degrees to about 110 degrees, preferably about 105 degrees.

26. A device as claimed in claim 23, 24 or 25 wherein the prismatic features are in the form of isosceles triangles.

27. A device as claimed in any of claims 23 to 26 wherein the pitch of the prismatic features is in the range of about 25 to about 75 microns, preferably in the range of about 25 to about 40 microns, and preferably is about 30 microns, or in the range of about 35 to about 65 microns, preferably in the range of about 40 to about 60 microns, preferably in the range of about 45 to about 55 microns, and preferably is about 50 microns.

28. A device as claimed in any of claims 2, 20, 21 and 22 wherein the device comprises an array of microlens optical structures on top of the gradient index film of microstructures, either formed directly on the gradient index film of microstructures or on another film above the gradient index film of microstructures.

29. A light emitting device comprising a light emitting structure formed on one side of a transparent substrate through which light from the light emitting structure is emitted, and provided on the other side of the substrate a volumetric gradient index film, there being provided on the gradient index film a surface relief provided with an array of microlenses.

30. A device as claimed in claim 29 wherein the array of microlenses comprises a hexagonal array of microlenses with a radius of about 25 to about 50 microns and a height of about 20 to about 45 microns.

31. A device as claimed in any of claims 2 and 20 to 30 wherein artefacts in the gradient index film provide variations in refractive index.

32. A device as claimed in claim 31 wherein the variations in refractive index form light guides.

33. A device as claimed in claim 31 or 32 wherein the variations in refractive index occur over a region of about 2 to about 10 micrometers.

34. A device as claimed in any of claims 2 and 20 to 33 wherein the gradient index film is optically coupled to the substrate.

35. A device as claimed in any of claims 2 and 20 to 34 wherein the gradient index film comprises an angle of view of 20 degrees or more.

36. A device as claimed in any preceding claim comprising a plurality of light emitting structures.

37. A device as claimed in any preceding claim wherein the light emitting structure comprises an organic electroluminescent device.

38. A light-emissive panel comprising a light emitting device as claimed in any preceding claim.

39. A backlit or transflective liquid crystal display module comprising as a backlight a light emitting device as claimed in any of claims 1 to 37.

40. An electronic device comprising a liquid crystal display module as claimed in claim 39.

41. A light absorbing device comprising as a light-absorbing structure an organic photovoltaic device formed on a transparent substrate through which light from outside enters, wherein a polarisation-preserving light collection member comprises: a) a volumetric gradient index film containing multiple circular, or elliptical columns or elongated vertical slabs of varying refractive index, preferably with features which are less than 50 microns in size; b) a surface relief film directly adhered to or formed from the material of the gradient index film with prismatic, lenticular, microlens or micropyramid features, or combinations thereof, forming a multiplicity of optical elements over the area of the light emitting structure; the light collection member being optically coupled to the outside of the transparent substrate to increase the quantity of light incoupled into the optical structure.