GB2482193A

GB2482193A - Microlens array and mould for fabricating the array

Info

Publication number: GB2482193A
Application number: GB1012394.1A
Authority: GB
Inventors: Euan Christopher Smith; Victor Musoko
Original assignee: Cambridge Display Technology Ltd
Current assignee: Cambridge Display Technology Ltd
Priority date: 2010-07-23
Filing date: 2010-07-23
Publication date: 2012-01-25
Also published as: GB201012394D0

Abstract

A microlens array 400 comprises a substrate bearing an array of microlenses 402 arranged asymmetrically such that along a first lateral axis 1 of the substrate adjacent microlenses are spaced apart 402a from one another and along a second lateral axis 2 orthogonal to the first axis surfaces of adjacent microlenses overlap, intersect or merge together at regions 402b. The angle between the surfaces of adjacent microlenses at regions 402b is at least 30 degrees, 45 degrees or 60 degrees. The microlenses may be domed or hemispherical and the ratio of pitch to diameter measures along axis 2 is preferably no more than 0.9:1, 0.8:1 or 0.7:1. The microlens array may be used in lighting and display systems, in particular based on organic LEDs (OLEDs). The array is made by forming the microlenses on a flexible substrate using a mould and peeling it away from a surface of the mould, e.g. peeling away a thin film substrate (604) from a rotating drum (602). The mutually intersecting regions 402b form a â Vâ that reduces â openingâ and the risk of substrate/microlens material being trapped in sharp corners of the mould.

Description

Lighting and Display Systems

FIELD OF THE INVENTION

This invention relates to lighting and display systems, in particular based on light emitting diodes (LEDs), more particularly based on organic LEDs (OLEDs). We describe techniques for using microleses to improve such systems.

BACKGROUND TO THE INVENTION

It is known to apply a microlens array to a liquid crystal display panel in order to collect additional light. Background prior art can be found in: WO2008/155878A1; US2006/0066775A1; J P2005/259361 A; KR20081007001 A; J P2005/259361 A; H uang, Yi-Pai; Ko, Fu-Jen; Shieh, Han-Ping David; Chen, Juie-Jun; Wu, Shin-Tson, Society for Information Display International Symposium (2002), 33, 870-873; Huang, Yi-Pai; Shieh, Han-Ping David; Wu, Shin-Tson, Applied Optics (2004), 43 (18), 3656-3663; Huang, Yi-Pai; Chen, Juie-Jun; Ko, Fu-Jen; Shieh, Han-Ping David, Japanese Journal of Applied Physics, Part 1: Regular Papers, Short Notes & Review Papers (2002), 41(2A), 646-651; Ta-Wei Lin (Dept. of Mech. Eng., Nat. Taiwan Univ., Taipei, Taiwan), Chi-Feng Chen; Jauh-Jung Yang; Yunn-Shiuan Liao, Journal of Micromechanics and Microengineering (Sept. 2008), vol.18, no.9, p. 095029 (10 pp.), 25 refs., ISSN: 0960- 1317, lOP Publishing Ltd., UK.

A microlens may be defined as a lens having a maximum lateral dimension or diameter of less than 1000 pm, 500 pm, 200 pm or 100 pm. In a microlens array an aspect ratio may be defined as a ratio of the pitch between adjacent microlenses to the diameter of a microlens. A microlens array may be applied to a display device to increase the out-coupling of light from the display. To provide significant enhancement in the light from the display it is desirable to use high aspect ratio lenses. For example a hemispherical lens -in which the surface of the lens meets the surface of a substrate on which the lens is formed at substantially 90° -is much better at capturing and out-coupling light than a microlens which sits lower" on the substrate so that the internal angle between the surface of the lens and the substrate plane is less than 900.

A microlens array may be manufactured using a mould or master, for example by moulding a plastic substrate or by forming a microlens array on a mould. However the 900 corners of a hemispherical microlens array tend not to release well, causing difficulties in the manufacture of such an array.

There is therefore a need for improved approaches.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is therefore provided a microlens array comprising a substrate bearing an array of microlenses, wherein said rnicrolenses arranged such that along a first lateral axis of said substrate adjacent said rnicrolenses are spaced apart from one another and along a second lateral axis orthogonal to said first axis surfaces of adjacent said microlenses intersect one another above a surface of said substrate.

Broadly speaking, the inventors have recognised that by asymetrically packing the microlenses in the array, the array may be configured such that it has different gaps/intersections between the microlenses in different directions within the lateral plane of the array. This allows the surfaces of the microlenses to meet the substrate at substantially 90° in one direction in the plane of the array, thus providing a high aspect ratio, whilst enabling the microlens surfaces to intersect along a different line (cross-section) in the lateral plane of the array. This latter feature effectively enables a "peel direction" to be defined, that is a direction in which the substrate bearing the array of microlenses can be peeled away from a mould with a much reduced risk of the substrate/microlens material becoming trapped in sharp corners of the mould (the skilled person will appreciate that the microlenses are generally integrally formed with the substrate, for example by moulding a plastic-like material).

In embodiments the peel direction is along (within) the surface of a microlens at a point where it meets the substrate at substantially 90°, so that peeling in this direction effectively opens this corner thus facilitating release of the microlens array from the mould. In the second, orthogonal direction the microlens surfaces intersect above the surface of the substrate so that they do not provide a very narrow "V" in which material could otherwise be trapped, since peeling in the peel direction will not significantly open these "V"s. As described later, either the substrate bearing the array of microlenses may be peeled away from the mould or the mould may be peeled away from the substrate, for example where the mould is carried on the surface of a rotating drum.

In embodiments along the second lateral axis the surfaces of adjacent microlenses intersect to define an (included) angle between the intersecting surfaces of at least 300, 450 or 60°. (The second axis may be defined to lie along a line defined by points at which the surfaces intersect with a greatest included angle of intersection). In embodiments along the first lateral axis the surfaces of the microlens meet the substrate at an angle of greater than 750, 80°, 85°, or in some preferred embodiments at an angle of substantially 90°. Thus in embodiments the microlenses comprise domed or substantially hemispherical microlenses. In embodiments when measured along the second lateral axis the microlenses have a ratio of pitch to diameter (aspect ratio) of no more than 0.9:1, 0.8:1 or 0.7:1. In embodiments the microlenses are arranged to define a substantially hexagonal array of microlenses (putting to one side the aforementioned deviations of strict symmetrical packing.

In embodiments the microlenses themselves may be asymmetrical. Thus they may have a different lens power in a plane defined by the first axis as compared with a plane defined by the second axis. Thus in embodiments a microlens may have one or a pair of flats on the domed or hemispherical surface of the lens, more particularly on one or both sides of a line parallel to the second lateral axis running through a centre of the microlens. Thus in embodiments these flats may lay across or straddle a line parallel to the second axis. Such an asymmetrical arrangement can be used to preferentially direct light into a horizontal plane as compared with a vertical plane, that is such an arrangement can be employed to direct light generally forward and sideways rather than up and down, which generally matches human viewing conditions.

Embodiments of a microlens array as described above may be applied to a pixellated OLED display (they are particularly suitable for an OLED display, for reasons given later), and in this case preferably a microlens is smaller than a pixel (or sub-pixel) of the display. One might imagine that the lateral dimension of a microlens should be matched to that of a pixel so that microlenses and pixels can be aligned, but in practice it is difficult to maintain such an alignment over the area of a large display. It is therefore preferable to make the size of a microlens smaller, if possible much smaller than that of a pixel. In embodiments the pitch of the microlenses (in a direction in the plane of the microlens array) is an integer fraction of the pitch of the pixels (in the same direction) so that there is an integer number of microlenses per pixel. The microlenses are then preferably displaced by half a microlens pitch with respect to a pixel to reduce moire effects. The currently smallest pixel pitch is of order 100 pm whereas microlenses can be fabricated with a diameter well below this.

Although a microlens as described above can be applied to a pixellated display, it has been found in practice that tight scattering issues reduce the black level so that even if the overall brightness is increased the perceived brightness may be reduced. Thus some particularly significant applications for the above described techniques are believed to be for (0) LED lighting elements such as OLED lighting tiles. In such a configuration the microlens array is arranged so that it is over a light-emitting face of the OLED light emitting structure; this may comprise a top-emitting or bottom-emitting OLED structure.

Experiments by the applicant have established that in embodiments there is no significant improvement provided by the inclusion of a microlens array for an OLED structure with a device reflectivity of below around 70% at a wavelength of an emission maximum of the device. Broadly speaking this is because a microlens array provides benefit by coupling out light which would otherwise be (totally) internally reflected within the OLED device structure -the microlens has a refractive index air and thus facilitates coupling this light out of the OLED structure. Thus in preferred embodiments a microlens array is applied as a thin film onto a photon recycling" OLED structure, in general a structure with a strongly absorbing/reflecting cavity. In the example of a bottom-emitting OLED structure the device may have a more-like cathode at the rear, light being emitted through the glass or plastic substrate through a transparent (for example ITO) anode or passed thin anode tracks.

In a related aspect the invention provides an OLED lighting or display device comprising an OLED and a microlens array to couple light out of said device; wherein microlenses of said microlens array are one or both of: i) asymmetric; ii) packed into said array asymmetrically such that they do not substantially overlap a first direction and do overlap a second, different direction.

Again preferably the OLED lighting or display device has a device stack which is reflective at a peak emission wavelength with a reflectivity of at least 70%.

In a further related aspect the invention provides an optoelectric lighting or display device, the lighting element or display device comprising: a substrate; a plurality of electroluminescent elements on said substrate; and a microlens array over said electroluminescent element to couple light from said electroluminescent element out of said device; wherein a said microlens has a domed surface; wherein said microlens array comprises microlenses located at intervals along two different axes in a lateral plane of said device, and wherein along a first of said axes domed surfaces of said microlenses are substantially non-overlapping and along a second of said axes adjacent said domed surfaces intersect one another such that there is a point on said intersection at which said intersecting domed surfaces define an included angle with respect to one another of at least 30 degrees.

The invention also provides a method of manufacturing a microlens array on a flexible substrate, the method comprising: forming said microlens array on said substrate using a mould defining said microlens array; and peeling said microlens array away from a surface of said mould; and wherein said mould defining said microlens array defines microlenses which do not substantially overlap in a first direction within said array and which do overlap in a second different direction within said array.

In embodiments the mould is carried by a rotatable drum and the peeling comprising rotating the drum, the mould being orientated such that the second direction is parallel to an axis of rotation of the drum, the first, orthogonal axis being circumferential, or more precisely tangential. As mentioned, the substrate microlens in film material is preferably a plastic "lacquer", that is a resin or polymer which is embossed by the mould.

The invention also provides a microlens array manufactured by a method as described above.

The invention further provides a mould for fabricating a microlens array, wherein said mould defines microlenses which do not substantially overlap in a first direction within said array and which do overlap in a second different direction within said array.

BRIEF DESCRIPTION FO 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 Ia and lb show, respectively, a cross-section through an OLED lighting tile, and a view of a front, light-emitting face tile; Figures 2a to 2c show, respectively, 3D perspective views and schematic views from above of example configurations of microstructures of a microlens array, and a perspective view of a faceted microlens array; Figures 3a and 3b show, respectively, a view from above and 3d perspective view of a portion of a microlens array comprising substantially hemispherical, hexagonally packed microlenses; Figures 4a to 4c show, respectively, a 3D perspective view, a view from above, and cross-sectional views of a microlens array according to an embodiment of the invention; Figure 5 shows a graph of percentage light out-coupled against OLED device reflectivity for a range of out-coupling microstructures; and Figure 6 schematically illustrates peeling a thin film micro array from a mould on a rotating drum.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Organic light emitting diodes (OLED5) are particularly useful for lighting because they can relatively easily and cheaply be fabricated to cover a large area on a variety of substrates. They are also bright and may be coloured or white (red, green and blue) as desired. In this specification references to organic LEDs include organometallic LEDs, and OLEDs fabricated using either polymers or small molecules. Examples of polymer -based OLEDs are described in Wa 90/13148, WO 95/06400 and WO 99/48160; examples of so called small molecule based devices are described in US 4,539,507.

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 Ia, this shows a vertical cross-section through a portion of an OLED lighting tile 10 comprising a glass substrate 12 on which metal, for example copper tracks 14 are deposited to provide a first electrode connection, in the illustrated example an anode connection. 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 (poly(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 (not shown) such as lithium fluoride for energy matching, over which is deposited a reflective back (cathode) electrode 22, for example of aluminium or silver.

The example of Figure Ia 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 pm thickness). Referring now to Figure lb this shows a view of the light emitting tile 10 of Figure Ia 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.

Figure 2a illustrates some example microstructures which may be incorporated into a microlens array, a domed microlens 202, a pyramidal structure 204, and a flat top pyramid ("hip-roof") 206. Figure 2b illustrates a view from above of the microlens 202, of the pyramidal structure 204, and of a variant 208 of the microlens 202 incorporating flats on the top and bottom edges of the hemisphere so that the microlens broadly speaking acts as a prism in one direction and a lens in an orthogonal direction. The microlens variant 208 may be arranged so that it is lens-like in a horizontal axis and prism-like for light coupled out on a vertical axis.

Simulation results indicate that hemispherical microlenses of the general types 202, 208 out-couple light best, but these are difficult to manufacture.

Figure 2c shows a perspective view of the faceted microlens 208 in a hexagonally packed array, which is beneficial because it provides improved output coupling along one axis as compared with a second, orthogonal axis and can thus be used to concentrate light more in the horizontal than in the vertical plane, and can thus achieve better enhancement.

Referring to Figure 3a, this shows a hexagonally packed array 300 of microlenses, which is useful because it decreases the area of gaps between the microlenses.

Simulation results indicate that more light is out-coupled if the domed surfaces of the microlenses intersect the substrate at a steep angle, for example close to 90°. This example 90° angle is illustrated in the perspective view of the array 300 of Figure 3b.

Although it is convenient to refer to the microlens and substrate, the skilled person will appreciate that in practice these will generally be integrally formed from a single thin film.

A domed microlens is superior to the other microstructures we have described, in particular if a substantially hemispherical microlens can be fabricated. Also as previously described it is beneficial to employ an asymmetric microlens, and it is useful to be able to provide a high aspect ratio (which is a similar aim to that of providing an approximately hemispherical microlens). However release of a microlens array film from the master becomes difficult the steeper the sides of the hemispheric lens.

Nonetheless the release is facilitated in the direction of the peel-off as the peeling action causes the features to move out of position.

We therefore describe an asymmetrical microlens array which improves the manufactureability of the microlens structures as less stress is created on the side of the microlenses when the optical film is being released from the master. This results in minimal distortion of the lens profile and a high degree of uniformity in the microlens array. Another advantage of the approach we describe is that there is a higher percentage of gapless area between the microlenses, enabling more light from the substrate to be collected and thus facilitating an increase in the out-coupled light efficiency.

Thus referring now to Figures 4a and 4b, this shows a perspective view and a view from above of an asymmetric microlens array 400, in particular in which substantially hemispherical microlenses are packed asymmetrically so that they overlap horizontally along a second axis 2, and are spaced apart along a first axis 1. An arrangement of this type is more efficient than a shallow microlens in which only a proportion of a hemisphere forms part of a lens, but is nonetheless manufacturable.

In the example of figure 4 microlenses 402 have a first part of a surface 402a which meets the substrate vertically, at substantially 90°, and a second part of the lens surface 402b which intersects with a complementary portion of an adjacent microlens along axis 2 Figure 2 above the surface of the substrate and effectively at the point of intersection defining an angle of less than 900 between an internal surface of the lens and the substrate, for example of approximately 60°.

Referring to Figure 4c, this illustrates schematically pairs of adjacent microlenses of the array shown in Figures 4a and 4b, illustrating different aspect ratios. The views shown in Figure 4c may be taken to be views from above, in which case lines 410 lie along axis 2. However because the hemispherical microlenses 402 are symmetrical in the schematic illustration line 410 may also be considered as defining the plane of the substrate, in which case it can be seen from Figure 4c that for an aspect ratio of less than 1.0 the surfaces of the microlenses intersect above the plane of the substrate. It can also be seen from Figure 4c that for an aspect ratio of 1.0 (when the microlens diameter is equal to the niicrolens pitch) the included angle between the adjacent microlens surfaces tends to zero, but that as the aspect ratio is decreased the included angle is increased and the depth of the "V" between the adjacent surfaces is also reduced, thus facilitating peeling off the thin film from the master or mould.

Referring now to Figure 5, this shows the effect of the reflectivity of the OLED device stack on the out-coupling benefit provided by the microstructure array. Referring again to Figure Ia, the cathode structure is generally highly reflective, and total internal reflection of the light emitted by the LEP stack 18 can occur within substrate 12, an effect which can be reduced by using a microlens array. Figure 5 shows the benefit provided by a microstructure array as compared with a planar surface illustrated by line 500. Thus it can be seen that a microlens array 502 provides significant benefit compared with the pyramidal structure 204 or hip-roof" structure 206 of Figure 2a as shown by lines 504 and 506 respectively. The graph of Figure 5 is specifically for OLED-type emission which, for a structure of the type shown in Figure Ia, is different to Lambertian emission. For Lambertian emission microlenses as opposed to pyramids are also better, but they are better for all reflectivities -there is no minimum device reflectivity which emerges for Lambertian emission. Close inspection of Figure reveals that the enhancement factor provided by inclusion of a microlens array becomes positive at around 50% reflectivity, is around 5% for 60% reflectivity, and around 10% for around 70% reflectivity. Thus taking a 10% enhancement factor as being worthwhile, it is suggested that a microlens array be employed for devices of a reflectivity of at least around 70% (although this could be extended down to 60% or 50%). The reflectivity should be measured at an emission wavelength, more particularly an emission peak of the device structure.

Referring now to Figure 6 this shows, schematically, apparatus 600 for manufacturing a microlens array of the type shown in Figure 4. Thus a drum 602 rotates and a thin film 604 bearing the microlens array is pulled away from the drum, thus pulling apart the microlenses 402 where their surfaces intersect the thin film substrate at substantially 900. Along the axis 2 the microlens structures mutually intersect so that they can be peeled away from the drum 602 without being "opened" by the peeling process. A practical embodiment of apparatus 600 has a master or mould 606 carried on the drum 602, a means for applying the thin film to be embossed to the mould, and means for removing the embossed thin film from the mould/drum.

The skilled person will appreciate that although embodiments of a microlens array have been described with hemispherical microlenses, the techniques we describe are also applicable to other domed surfaces, in particular aspherical surfaces, and to compound microstructures comprising a domed lens in combination with a prism, for example as provided by one or a pair of flats on a domed microlens.

Broadly speaking we have described the use of asymmetric tenses of optical enhancement films to improve the manufactureabitity of high aspect ratio microlens structures. Asymmetric microlenses increase the light efficiency of an OLED device by out-coupling light which is totally internally reflected inside the glass substrate.

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 lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS: I. A microlens array comprising a substrate bearing an array of microlenses, wherein said microlenses arranged such that along a first lateral axis of said substrate adjacent said microlenses are spaced apart from one another and along a second lateral axis orthogonal to said first axis surfaces of adjacent said microJenses intersect one another above a surface of said substrate.
2. A microlens array as claimed in claim I wherein along said second lateral axis said surfaces of adjacent said microlenses intersect to define an angle between the intersecting surfaces of at least 30 degrees, 45 degrees or 60 degrees.
3. A microlens array as claimed in claim I or 2 wherein along said first lateral axis said surfaces of said microlenses meet said substrate at an angle of greater than 80 degrees preferably substantially 90 degrees.
4. A microlens array as claimed in claim 3 wherein said microlenses comprise domed or substantially hemispherical microlenses.
5. A microlens array as claimed in claim 3 or 4 wherein measured along said second lateral axis said microlenses have a ratio of pitch: diameter of no more than 0.9:1, 0.8:1 or 0.7:1.
6. A microlens array as claimed in any preceding claims wherein said microlenses are arranged to define a substantially hexagonal array of microlenses.
7. A microlens array as claimed in any preceding claim wherein a said microlens has one or more flats on a said surface of the microlens.
8. An OLED lighting element comprising an OLED light emitting structure and a microlens array as claimed in any one of claims I to 7 over a light-emitting face of said light emitting structure.
9. An OLED light emitting element as claimed in claim 8 wherein said OLED structure has a reflectivity at an emission wavelength of said light emitting element of at least 70%.
10. A pixellated OLED display having a microlens array as claimed in any one of claims I to 7, wherein a pitch of said microlenses is substantially an integer fraction of a pixel pitch of said display.
11. An OLED lighting or display device comprising an OLED and a microlens array to couple light out of said device; wherein microlenses of said microlens array are one or both of: i) asymmetric; ii) packed into said array asymmetrically such that they do not substantially overlap a first direction and do overlap a second, different direction.
12. An OLED lighting or display device as claimed in claim 11 wherein said OLED structure has a reflectivity at an emission wavelength of said device of at least 70%.
13. An optoelectric lighting or display device, the lighting element or display device comprising: a substrate; a plurality of electroluminescent elements on said substrate; and a microlens array over said electroluminescent element to couple light from said electroluminescent element out of said device; wherein a said microlens has a domed surface; wherein said microlens array comprises microlenses located at intervals along two different axes in a lateral plane of said device, and wherein along a first of said axes domed surfaces of said microlenses are substantially non-overlapping and along a second of said axes adjacent said domed surfaces intersect one another such that there is a point on said intersection at which said intersecting domed surfaces define an included angle with respect to one another of at least 30 degrees.
14. A method of manufacturing a microlens array on a flexible substrate, the method comprising: forming said microlens array on said substrate using a mould defining said microlens array; and peeling said microlens array away from a surface of said mould; and wherein said mould defining said microlens array defines microlenses which do not substantially overlap in a first direction within said array and which do overlap in a second different direction within said array.
15. A method as claimed in claim 14 wherein said mould is carried by a rotatable drum, wherein said peeling comprises rotating said drum, and wherein said second direction is parallel to an axis of rotation of said drum.
16. A microlens array manufactured by the method of claims 14 or 15.
17. A mould for fabricating a microlens array, wherein said mould defines microlenses which do not substantially overlap in a first direction within said array and which do overlap in a second different direction within said array.