Method for making a light emitting device and light emitting device
FIELD OF THE INVENTION
The invention relates to a method for making a light emitting device.
Furthermore, the invention relates to a light emitting device for displaying a static picture on the basis of light emitting polymer material, wherein light emitting polymer material is arranged above a substrate and between an electrode and a counter electrode,
BACKGROUND OF THE INVENTION
For example, PLED or OLED displays based on light emitting polymers are known. A display for displaying dynamic pictures in general consists of an array of individually addressable pixels. In such a case a display cell usually contains an addressing electrode, the light emitting polymer, a hole transporting layer and a transparent (ITO) top electrode to enable light coming out of ihc addressed polymer. The OLED polymers arc typically deposited via evaporation, while PLED polymers are typically deposited via Inkjet printing techniques. If all pixels are addressed simultaneously, a light emitting panel is obtained, that can be used for plane illuimination. If a selected group of pixels is addressed continuously, a static picture can be displayed. A disadvantage of such a light source used for displaying static pictures is, of course, that the manufacturing costs of such an active matrix 'display' arc very high.
To provide a cheaper light emitting device for displaying a static picture on the basis of light emitting polymer material, it is for example known from WO 2005/053057 A! to provide the desired picture between the electrode and the counter electrode by applying the OLED solution via a printing process, preferably by "ink" jetting the OLED solution onto the cathode (counter electrode). If a multi color picture is to be created, OLED solutions emitting different colors can be used in the printing process. However, particularly "ink" jetting processes arc time-consnming and therefore still expensive. In addition, the quality of the image is determined by the resolution of the printing device. High-quality and high-resolution printing techniques are therefore required to obtain a high-quality image.
It is therefore the object of the invention to provide a cheaper light emitting device for displaying a static picture, as well as a method for making such a device.
SUMMARY 01" THE INVENTION
This object is solved by the features of the independent claims. Further developments and preferred embodiments of the invention arc outlined in the dependent claims.
!n accordance with a first aspect of the invention, the method for making u light emitting device for displaying a static picture on the basis of light emitting polymer material comprises the following steps:
a) providing a substrate;
b) providing a relief structure (pit/bump) on the substrate;
c) providing a counter electrode layer above the relief structurc;
d) providing light emitting polymer material above the counter electrode layer such that in cavities of the relief structurc there is provided more light emitting polymcr material than in other areas of the relief structure; and
c) providing an electrode layer above the light emitting polymer material.
Providing light emitting polymer materia! at least in the cavities of the relief structure can be performed easily and much faster than using, for example, ''ink" jetting processes, as will be outlined below in greater detail. The light emitting polymer matcrial(s) is (arc) preferably of the OLED or PLED type. For example, homopolymers, copolymers, oligomers, and-'or electro-fluorescent materials can be used as light emitting polymer materials. A monochromic device is obtained, if only one polymer is used, for example red, green, blue, or a mi.xture of these. The picture is then visualized via the difference in optical contrast (or amount of emitted light) between the structured and unstructured areas of the substrate. Structured areas may emit light while un.structured areas (sort of mirrors) may not emit light. It is also possible that oprical contrast is generated by differences in the applied structures. For example, areas with prc-grooves may emit more light than areas with pits. .Another po.ssibility is to generate optical contrast from different groove/pit depths. For example, an area with deep grooves emits more light than an area with shallow grooves. Another possibility is to make areas with broad grooves and areas with narrow grooves. For example, an area with narrow grooves emits less light than an area with broad grooves. Combinations of groove/pit depth and width are also possible. It is also possible to deposit the device partially with the light emitting polymer material. The uncovered areas are not emitting light when the device is addressed. This also generates contrast. Patterning the substrate with different colors (for example red, blue, green) will give color to the device. The patterning can be done in the form of an image, picture etc. Printing techniques, such as micro-contact printing, inkjct printing etc. can additionally be used to deposit different "inks", i.e. light emitting polymer materials on the substrate. Color is, tor example, generated via the combination
rcd-green-b!ue, as in conventional PLED or other displays. While in organic displays the color is usually determined by the inteasity of the individual basiic colors (pixels), it is also possible to generate color by varying the ratio of red, green and blue. In this way, a predetermined color is obtained when operated at nominal conditions. Therefore, coloring each panel pixel will add color to the light emitting device. The color is, according to the pre-recorded information, present in the light emitting device. Color may also be generated by mixing the ba,sic PLED or OLED polymers. The image or picture is then applied according to the requested color. In all cases, a pre-recorded image/ picture is obtained that emits light when the entire device is addressed. The relief structure of the substrate may contain pits or grooves (like in the conventional pre-groovcd recordable and rewritable and read only memory optical discs). It is also possible to use a substrate with protruding bumps or dams (such as obtained with certain phase-transition materials). The counter electrode can be formed by a metallic (under) layer which is, for example, made of Ag or Al and is at lea,st mainly reflective for the hght emitted by the light emitting polymer material. In general, the counter electrode can be a rather thick reflecting metal (under) layer that acts as addressing electrode for the entire substrate. To guarantee optical contrast from the relief structure, it is important that the counter electrode does completely fill the relief structure. A general rule of thumb is that t)ie thickness does not exceed the (icpth of the relief structure. The electrode is prcfcrabl\ transnarcnt or at least semi-transparent for the light emitted by fne light emitting polymer material and is, for example, madcof ITO, PEDOT or any other conducting but transparent material. Optionally, at least one additional layer for improving the performance of the device (hole transport) can be provided. For example, a PEDOT layer and.'Or a Calcium or Barium layer can be arranged between the light emitting polymer material and the electrode. Furthermore, a cover layer can be provided for protection of the light emitting device. Such a cover layer should provide sufficient resistance to scratches and should be transparent in the visible wavelength. Organic layers arc very suitable. The device can be provided with a cover layer via a spin coat or other type of deposition process or via bonding. The cover layer may be structured as well to influence the performance of the emitted light (fresnel structure, diffraction structures, etc).
In accordance with a first preferred embodiment of the invention, the above step b) is performed on the basis of a photoresist mastering process. In such a process, for example, a thin layer of photoresist is spin coated on top of a glass substrate. After illumination of the photosensitive layer, the exposed areas are being developed in a so-called development process. Under influence of photons, acids are formed thai dissolve in the base development liquid. This dissolution rcsult,s in physical holes inside the photo-rcsist layer. The layer thickness is a natural barrier for the created holes since the used glass is insensitive to laser light exposure
and the subsequent development liquid. Possible photoresists are, for example, Chemically-Amplified ('CA) resist, Clariant, Ultra, and Shipley.
In this context it is further possible that a stamper made on the basis of the photoresist mastering process is used for providing the relief structure. The substrate with patterned layer can be u.sed in different way.s to Finally end up with a stamper for mass-replication of the patterned ,substTates. By way of example four of these ways our outlined in the following. In accordance with a first possibility a Ni layer is directly sputter-deposited on top of the patterned resist (photoresist or inorganic resist) layer. This Ni layer is galvanically grown such that a rather thick stamp with bump or pit structure remains. A family can be grown from the stamper to obtain multiple stampers or to invert the polarity of the relief structure. In accordance with a second possibility the patterned layer serves as mask layer for an etch step. The two-dimensional information contained by the mask layer is transferred to the underlying substrate via wet or dry etching. After etching, the mask layer is removed and a patterned substrate remains (example: a mask layer may be directly deposited on a Si substrate. After patterning, the Si substrate may be etched with 02 or Fluorine plasma). A stamper can be grown from, the patterned surface. In accordance with a third possibility the patterned layer may serve as mask layer for a further illumination of the substrate. Furthermore, in accordance wdth a fourth possibility, the patterned substrate serves as stamp. .An additional thin coating can be applied to improve or reduce the wetting properties of the information side of the stamp.
In accordance with a second preferred embodiment of the invention, the above step b) is performed on the basis of a thermal mastering process. For e.xam.ple. phase transition mastering (PTM) is a relatively new method to make high-density relief structures and stampers. PTM is known in the field of mass-fabrication of optical discs, but it is also suitable in the context of the present invention Phase-transition materials can be transformed from the initial unwritten state to a different state via laser-induced heating. Heating of the recording stack can, for example, cause mixing, melting, amorphisation. phase-separation, decomposition, etc. One of the two phases, the initial or the written state, dissolves fa.stcr m acids or alkaline development liquids than the other phase does. In this way, a written data pattern can be transformed to a high-density relief structure with protruding bumps or pits.
Also with the second preferred embodiment it is possible that a stamper made on the basis of the thermal mastering process is used for providing the relief structure. Depending on the material actually used for the substrate, the stamper can be made as outlined above or at least similar thereto.
In accordance with a further development of the method in accordance with the invention, the above step b) coinprises using a Laser Beam Recorder for providing the rciicf structure. A Laser Beam Recorder fLBR) can, for example, be used for both photoresist and thermal mastering processes. Patterning with a laser beam recorder is standard practice for optical disc mastering. A LBR comprises a rotation labic, on which the substrate is mounted, and a translation sledge on which the optical components arc is mounted. The optical components arc used to shape and modulate the laser beam and focus the beam through an objective lens onto the substrate. When the substrate is rotated and the sledge is gradually pulled outside (or inside) a spiral remains. 'The laser beam can be modulated (a pit pattern remains) or not (the recording layer is continuously illuminated, grooves remain). Conventionally, a laser beam is modulated with a certain frequency to obtain masters with a data pattern for pre-recorded (ROM) media, or operated continuously to obtain masters with a prc-groovc for recordable and rewritable optical media. In addition, a deflector is utilized to deviate the focused laser beam with respect to its nominal position such that the pre-groovc contains a wobble for data recovery. Some LBRs are equipped with a second laser beam (dual-beam LBR) to master, for example, pro-pits in the adjacent tracks of a pre-groovcd disc (for example for DVD-r-R/'+RW and MO ibmiats). In most optical formats, the mastering is done with a constant linear velocity. This implies that the modulation frequency can be kept constant throughout mastering the entire disc. The physical pit lengths remain then constant from the inner to the outer part of the disc. A constant linear velocity requires that the rotation frequency is continuously adapted to the actual position of the writing stylus. Synchronization of data tracks can be used to write any two-dimensional ,stnjcturc. The basic idea is to perform a coordinate transformation. The position of the ia.ser .spot on the substrate can be derived from the total elapsed time, the angle and the radius. It is for example possible to map the spiral on a preferred two-dimensional pattern. The resolution that can be achieved depends vciy much on the optica! spot size and the track pitch of writing. For some two-dimensional patterns, a much .smaller track pitch is required to write the pattern with the required spatial accuracy.
An other possibility is that the above step b) comprises using an E-Bcam patterning device. In such a case the focused E-Beam deteriorates the organic layer, for instance. The written areas arc dissolved in a wet chemical etching process. The resulting pit pattern is treated via conventional electrochemical plating techniques. A mass-replication process such as injection molding and UV lacquer replication is subsequently used to make
patterned .substrates from the stamper. The light emitting devices can have all shapes, triangles, stxir-shapcs, circles, squares, etc.
As regards the above step d), a non-uniform coverage process can be used. Thereby, a difference in coverage of the light cmttting polymer material on the feature and initial areas of the device can be obtained. Such a non-unifonn coverage processes can be, for example, a spin coating process. In such a case the prc-pits/groovcs get a much higher layer coverage than the intermediate land areas.
Alternatively, in the above step d) a uniform coverage process followed by a leveling process can be used. The uniform coverage process can be a deposition process and the subsequent leveling process can, for example, be an anisotropic patterning (wet etching) process, to obtain more land than prc-pit erosion.
With some embodiments of the method in accordance with the invention all layers arranged above the light emitting polymer material at least in sections arc at least semi-transparent for Hght emitted by the light emitting polymer material. Suitable materials for semi-transparent electrodes or counter electrodes arc, for example, ITO or PEDOT. In cases where the piclure.''image is intended to be visible only from one side of the device, electrodes or coimter electrodes that arc at least mainlv reflective for light emitted by the light emitting polymer material are preferred, for example, Ag or .Al electrodes or counter electrodes.
Alternatively or additionally, all layers arranged below the ligjit emitting polymer material at least in sections arc at lea.st semi-transparent for light emitted by the light emitting polymer material. Thereby, it is possible to make devices showing the picture-image only from the top side, only from the bottom side, or from both sides.
In accordance with a second aspect of the invention, there is provided a light emitting device for displaying a static picture on the basis of light emitting polymer material, particularly a light emitting device made by the above method according to the invention, wherein light emitting polymer material is airangcd above a substrate and between an electrode and a counter electrode, wherein the substrate comprises a relief structure, and wherein in cavities of the relief structure there is provided more light emitting polymer material than in other areas of the relief structure. As mentioned above, providing light emitting polymer material at least in the cavitics of the relief structure can be performed easily and much faster than using, for example, "ink" jetting processes. Therefore, the light emitting device in accordance with the invention can be manufactured in a very cost-effective way. In general, the characteristics and advantageous discussed above in connection with the method for making a light emitting device do also apply to the device in accordance with the
invention, and vice versa. Therefore, lo avoid repetitions, reference is made to the above discussion, i.e. only some of the relevant points will be mentioned again in the following.
Also in connection with the device in accordance with the invention it is preferred that the counter electrode is formed by a layer covering both, cavities and other areas of the relief structure. Such an electrode layer for driving the whole light emitting polymer material can be easily formed by methods well known to the person skilled in the art.
Also with the device in accordance with the invention it is possible that all layers arranged above the light emitting polymer material at least in sections arc at least semi-transparent for light emitted by the light emitting polymer material. As mentioned above, ITO or PEDOT can be used for transparent electrodes and-^or counter electrodes, while Ag or Al arc suitable materials for electrodes or counter electrodes that are at least mainly reflective.
Alternatively or additionally it is possible all layers an-anged below the light emitting polymer material at least in sections are at least semi-transparent for light emitted by the light emitting polymer material.
It is also possible that at least one additional layer for improving the performance of the device (hole transport) is provided between the electrode and the light emitting polymer material. Such an additional layer can, for example, be a PEDOT layer and'or a Calcium or Barium layer.
.A,lso for the device in accordance with the invention it is preferred that a cover layer is provided which is at least mainly transparent for light emitted by the light emitting polymer material. The covering layer may be provided via a spin coat or an other deposition process, or via bonding.
In this connection it is also possible that the cover layer at least in sections comprises a structured surface. Such a structure may be used to influence the performance of the emitted light.
In summar\' the invention results in a preferably plane light device based on light emitting polymers and a method for making the same. The device may consist of a ■structured substrate, a light emitting stack, and a protecting cover layer. The light emitting stack can comprise a metallic layer, a light emitting polymer layer, an additional PEDOT layer and a transparent electrode, The substrate comprises grooves and/or pits, i.e. a relief structure, such that an image or typical pattern is generated. The metallic under layer can, for example, be made of Ag or Al and is mainly reflective. The semi-transparent electrode can be
made via mass replication, such as injection molding and 2P replication (UV-curablc photo-lacquer). The stamper which can be used for mass-replication is prcferabl made via conventional techni(]ucs (E-Bcam patteningg, lithography, laser beam recorders, etc.). Monochromic and colored devices arc possible. The light emitting device in accordance with the invention contains a pre-recorded monochromic or colored image or picture. The above and further aspects and advantages of the invention will be apparent and elucidated with reference to the embodiments described hereinafter and shown in the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures la to Ig illustrate different steps of the making of a light emitting device in accordance with the invention by applymg the method in accordance with the invention; and
Figure 2 illustrates a schematic of the trajectory of an optical stylus and an overlapping two-dimensional pattern.
DESCRIPTION OF A PREFERRED EMBODIMENT
With the embodiment described in the following, the light emitting polymer material, i.e. the picture/image is visible only from the top side of the device. However, as mentioned above the invention is not limited to such embodiments and it is also within the scope of the invention to make devices showing the picture/image only from the bottom side (through the substrate) or from both the top arsd the bottom side.
As shown in Figure la substrate 10 is provided, in case of a mass fabrication this substrate can, for example, be a polycarbonate substrate which is still moldable by applying a stamper. However, it is also within the scope of the invention that the substrate is treatable by any suitable mastering process known to the person skilled in the art, for example photoresist or thermal mastering, to provide a desired relief structure thereon.
Figure lb schematically shows a relief structure 12 provided on the substrate 10 by using a stamper 24. The stamper 24 can be made by any suitable process known to the person skilled in the art, for example by one of the processes discussed above in the section "Summary of the Invention". .However, as mentioned above the invention is not limited to the use of a stamper to provide the relief structure.
Figure Ic illustrates the substraic !() after a counter electrode layer 14 has been deposited over the entire relief structure 12. The thickness of the rather thick counter electrode layer 14 is relative uniform. Furthermore, the counter electrode layer 14, which acts as addressing electrode for the entire substrate, is made of an mainly reflective and conductive material like Ag or Ai.
In accordance with Figure 1d a light emitting polymer layer 16 has been provided above the counter electrode layer 14. in the example shown the light emitting polymer materia! of the OLED or PLED has been applied by a non-uniform coverage process like spin coating. Therefore, the pre-pits/grooves gct a much higher layer coverage than the intermediate land areas. However, even if not shown, it is also possible to completely remove the light emitting polymer rhatcrial present on the land areas, for example to subsequently apply at least one further light emitting polymer layer emitting light of a different color, if a muiti color device is desired. For example red, green and blue polymers arc commonly used in displays and can be used in the device in accordance with the invention as well.
As shown in Figure le an optional additional layer 18 for improving the performance of the device. In the illustrated example the additional layer IK is an PEDOT layer 18 which has been provided on the light emitting polymer material 16 to improve the performance of the device (hole transport). Howcver, other materials are also suitable for this reason, for example Calcium or Barium. Furthermore, it is possible to use more than one additional layer. For example an additional Calcium or Barium layer (not shown) can be provided between the additional PEDOT layer 18 and the light emitting polymer material 16. The PEDOT layer 18 should be as transparent as possible for the light emitted by the light emitting polymer material 16.
Figure If shows the device after an electrode layer 20 has been deposited on the PEDOT layer 18. Also the electrode layer 20 is at least semi-transparent for the light emitted by the light emitting polymer material 16, For example, the electrode layer 20 can be an ITO layer.
Finally, Figure le illustrates the finished light emitting device in accordance with the invention. Compared to Figure If a cover layer 22 has been provided on the electrode layer 20 to protect the device, for example against scratches. It is clear that also the cover layer 22 ha.s to be made from a material that is as transparent as possible in the visible wavelength. Organic layers arc very suitable for this purpose. Optionally, the cover layer 22 or its surface are can be structured to influence the performance of the emitted light. For example, a frcsnel structure or a diffraction structure can be provided.
Figure 2 shows a schematic of ihc trcijcctoiry of an optical stylus and an overlapping two-dimensional pattem. The solid lines indicate the nominal trajectory; the dotted lines are the outer bonds of the stylus when the laser beam is deflected. The basic idea is to perform a coordinate transformation. The position of the laser spot on the substrate can be derived nrom the total elapsed time, the angle and the radius. If the spiral is mapped on a preferred two-dimensional pattern, the sketch given in Figure 2 is obtained. The resolution that can be achieved depends very much on the optical spot size and the track pitch of writing. For the two-dimensional pattem m Figure 2, a much smaller track pitch is required to write the pattern with the required spatial accuracy. ,A high spatial resolution and short writing times can be obtained by making use of the deflector and dual-beam capabilities of conventional Laser Bcam Recorders.
Finally, it is to be noted that equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

CLAIMS;
I ■ A method for making a light emitting device for displaying a static picture on
the basis of light emitting polymer materia!, the method comprising the following steps:
a} providing a substrate (10);
b) providing a relief structure (12) on the substrate (10);
c) providing a counter electrode layer (14) above the relief structure (12);
d) providing light emitting polymer material (16) above the counter electrode layer (14) such that in cavities of the relief structure (12) there is provided more light emitting polymer material (16) than in other ureas of the relief structure (12); and
c) providing an electrode layer (20) above the light emitting pol>aner material
(16).
2. The method according to claim 1, wherein step b) is performed on the basis of a photoresist mastering process,
3. The method according to claim 2, wherein a stamper (24) made on the basis of the photoresist mastering process is used for providing the relief structure (12).
4. The method according to claim 1, wherein step b) is performed on the basis of a thermal mastering process.
5. The method according to claim 4, wherein a stamper (24) made on the basis of the thermal mastering process is used for providing ihc relief structure (12).
6. The method according to claim 1, wherein step b) compiiscs using a Laser Beam Recorder for providing the rchef structure (12).
7. The method according to claim 6, wherein step b) comprises using an E-Beam patterning device.
^- the method according to claim 1, wherein in step d) a non-uniform coverage
process is used,
9. Ttie method according to claim 1, wherein in step d) a uniform coverage process followed by a leveling process is used.
10. The method according to claim 1. wherein all layers aiTanged above the light emitting polymer material (16) at least in sections arc at least semi-transparcni for light emitted by the light emitting polymer material (16).
11. The method according to claim I or 10, wherein all layers arranged below the light emitting polymer material (16) at least in sections are at least semi-transparent For light emitted by the light emitting polymer material (16),
12. A light emitting device for displaying a static picture on the basis of light emitting polymer material, particularly a iighi emitting device made by a method according to any of claims 1 to 9, wherein light emitting polymer miatcrial (16) is arranged above a substrate (10) and between an electrode (20) and a counter electrode (14), characterized in that the substrate (10) comprises a relief structure (12), wherein in cavities of the relief .structure (12) there is provided more light emitting polymer material (16) than in other areas of the relief structure (12).
13. The light emitting device according to claim 12, wherein the counter electrode (14) is formed by a layer covering both, cavities and other areas of the relief structure (12).
14. The light emitting device according to claim 12, wherein all layers arranged above the light emitting polymer material (16) at least in sections arc at least semi-transparent for light emitted by the light emitting polymer material (16).
15. The light emitting device according to claim 12, wherein all layers arranged below the light emitting polymer material (16) at least in sections arc at least semi-transparent for light emitted by the light emitting polymer material (16).
16. The light emitting device according to claim !2, wherein at least one additional layer (18) for improving the performance of the device is provided between the electrode (20) and the light emitting polymer material f 16).
17. The light emitting device according to claim 12, wherein a cover layer (22) is provided which is at least mainly transparent for light emitted by the light emitting polymer material (16).
18. The light emitting device according to claim 17, wherein the cover layer (22) at least in sections comprises a structured surface.