FIELD OF THE INVENTION

The invention describes an OLED device and a method of manufacturing such an OLED device.

BACKGROUND OF THE INVENTION

An organic light-emitting diode (OLED) device is manufactured by building up a series of layers, usually comprising an active or organic layer sandwiched between an anode and a cathode. The organic or functional layer, which may in fact comprise several layers, is often collectively referred to as the 'active layer', since it is in this active layer that light is generated when a current flows from the anode to the cathode. For OLEDs used in flat displays or illumination devices, the anode and cathode are generally accessed from the same side of the device, for example from the light-emitting side. Therefore, a first stage comprises creating electrically separated patterns of a conductive coating on a carrier such as glass. The required pattern, which comprises both anode and cathode contact areas, can be created by using subtractive methods like photo lithography, laser ablation, etc. where parts of the conductive coating are removed in certain areas. In a next stage, the functional or active layers - usually comprising small-molecule organic material - are deposited by thermal evaporation in vacuum. After completion of the organic and cathode deposition processes, the device is encapsulated to protect the organic layer from moisture. This encapsulation can be carried out in various ways. For example, a glass cover lid can be applied to the device using glue with low water permeability. In another approach, the outer surface of the device can be coated with a suitable film applied in a further deposition step. To allow electrical access to the cathode and anode, these must both extend beyond the sealing line to give contact areas, usually on different sides of the device.

In a top-emitting OLED, the 'outer' electrode (usually the cathode) is transparent to allow light to pass through. However, since the cathode should be transparent, only a very thin layer of metal can be used, but even a thin layer detracts from the overall transparency. Alternatively, a transparent conductive oxide (TCO) can be used for the outer electrode, having better optical properties. Unfortunately, since the conductivity of such a TCO is low, there is a significant voltage drop across this layer leading to inhomogeneous light emission. Such an OLED may exhibit darker areas, particularly in the centre, owing to the voltage drop between the contact areas at the outer edge of the device and the centre. Evidently, a larger surface area of an OLED will be associated with a correspondingly large voltage drop and a correspondingly 'darker' central area. This unsatisfactory property limits the maximum size of such a device. The largest practicable OLED device area which can be achieved is limited to about 5 cm x 5 cm.

It is therefore an object of the invention to provide an improved OLED device which avoids the problems described above.

SUMMARY OF THE INVENTION

This object is achieved by the OLED device of claim 1, and by the method of claim 10 of manufacturing an OLED device.

According to the invention, the OLED device comprises a carrier, a first electrode applied onto the carrier, an active layer applied onto the first electrode, and a second electrode applied onto the active layer. The OLED device further comprises a conductivity augmentation means for essentially homogeneously distributing an electric potential over the area of the second electrode during operation of the OLED device, which conductivity augmentation means comprises at least one printed line of electrically conductive material on an outer surface of the second electrode, and which conductivity augmentation means is arranged to project outward from the second electrode. Here, the term 'outer surface' in the context of the second electrode is used to refer to that face of the second electrode which is effectively on the 'outside' of the OLED device, even though this second electrode may in turn be covered by an encapsulating layer or 'cover lid' in a later manufacturing step.

An obvious advantage of the OLED device according to the invention is that the conductivity augmentation means increases the overall surface conductivity of the second electrode. Since the voltage drop between an outer edge of the second electrode and an inner or central area is therefore advantageously reduced, the light output of the OLED device is favourably homogenous. The advantages of printing the conductive lines onto the second electrode are that extremely narrow lines can be achieved in a rapid and uncomplicated manner. Established printing technologies such as ink-jet printing or screen-printing can be used to print or dispense tiny droplets to give one or more lines of conductive material. Developing technologies such as nano-ink printing can be used to dispense even smaller droplets of conductive material so that essentially invisible narrow line structures can be printed on the second electrode. Furthermore, by using such a conductivity augmentation means to ensure a homogenous current distribution between the first and second electrodes through the active layer, the OLED device according to the invention can have an emitting area much greater than that which can be achieved using prior art manufacturing techniques. Such a large OLED device with homogenous light quality may be used in a wide variety of applications for which prior art OLED devices are unsuitable.

The method of manufacturing an OLED device comprises the steps of applying a first electrode to a carrier, applying an active layer onto the first electrode, and applying a second electrode onto the active layer. Subsequently, the method comprises augmenting the electrical conductivity of the second electrode by printing at least one line of electrically conductive material on the outer surface of the second electrode to essentially homogeneously distribute an electric potential over the area of the second electrode during operation of the OLED device, which printed line is realised to project outward from the second electrode.

The dependent claims and the subsequent description disclose particularly advantageous embodiments and features of the invention.

In the following, for the sake of simplicity but without restricting the invention in any way, the 'first electrode' (applied to the carrier) can be assumed to be the anode, and the 'second electrode' can be assumed to be the cathode. Evidently, in a top emitting OLED, the alternative arrangement - in which the top layer is the anode - is equally possible. Also, even though reference is made in the following to a single OLED device with a conductivity augmentation means, an OLED device may in fact comprise several individual such OLED devices arranged in a suitable manner, for example by tiling, to give a larger overall emitting area.

After the basic layers of the OLED device have been deposited, the conductivity augmentation means can be printed to the second electrode using any suitable technology or technique. For example, a line can be printed onto the cathode using standard printing technologies like ink jet printing, screen printing or dispensing.

Since the conductivity augmentation means is preferably essentially invisible to the naked eye during operation of the OLED device, a printed line of the conductivity augmentation means therefore preferably has a width of at most 50 μιη, more preferably at most 30 μιη, most preferably at most 10 um.

The conductivity of a line deposited in this manner is directly related to only to the electrical properties of the material but also to the physical dimensions of the line, i.e. its height and width. Obviously, a wide thick line would conduct very favourably, but would be optically unsatisfactory. A very thin, flat line may be to all intents and purposes invisible, but will conduct less. Therefore, to obtain a thin line which also has a favourably high conductivity, in a preferred embodiment of the invention, the conductivity augmentation means preferably extends outward from the second electrode to a height of at least 1 μιη, more preferably at least 3 um, most preferably at least 5 um.

However, properties of the electrically conductive material used, such as its viscosity, may not allow the printing of such a 'high' line in a single process step. In a particularly preferred embodiment of the invention, therefore, the conductivity augmentation means preferably comprises a first printed line on the outer surface of the second electrode and at least one further printed line superimposed onto the first line in the form of a 'stack'. In this way, favourably narrow congruent lines can be printed consecutively, so that a minimum width is achieved and a favourable height. A 'stack' is then effectively an electrically conductive band or strip arranged essentially perpendicularly to the outer surface of the second electrode. A stack of printed lines preferably comprises at least 3 congruent printed lines, more preferably at least 5 congruent printed lines, and most preferably at least 10 congruent printed lines. By depositing congruent lines in this manner, the aspect ratio between height and width can be improved, i.e. narrow high lines can be created which are essentially invisible but have high conductivity. In a particularly preferred embodiment of the invention, the heightwidth aspect ratio of a printed line or stack of the conductivity augmentation means comprises at least 1 : 10, more preferably at least 1;5, most preferably at least 1 :3. Of course, with advances in printing techniques, better aspect ratios for more narrow lines could be achieved. This heightwidth aspect ratio can apply to a line printed in a one-step process, or to a stack of congruent printed lines. A combination of printed lines and stacks can be used. For example, for lines that pass over a central area of the cathode, a stack realisation with multiple congruent printed lines and a corresponding large heightwidth aspect ratio may be preferred. For lines that are located further away from the centre of the cathode, for example, a single printed line with correspondingly low heightwidth aspect ratio may suffice.

For the OLED device according to the invention, the lines of the conductivity augmentation means can be printed such that the lines do not extend to the outer edges and therefore to the contact pads of the second electrode. Even so, electrical current flowing laterally through the second electrode during operation of the device can be transported partially by these printed lines, so that the overall electrical conductivity of the second electrode is favourably boosted. However, in a preferred embodiment of the invention, the lines are printed such that they make electrical contact to the contact pads of the second electrode arranged around one or more outer edges of the device. In this way, the electrical conductivity of the second electrode is optimally improved.

Evidently, the conductivity across the cathode will be improved by printing a plurality of conductive lines onto the second electrode. In a simple realisation, a printed line or stack can simply extend from one cathode contact pad to another cathode contact pad. The contact pads can be located on the same side, on adjacent sides, or on opposite sides of the OLED, depending on the device realisation. For example, a plurality of parallel lines can be printed onto the cathode to connect pairs of contact pads on opposite sides. In a particularly preferred embodiment of the invention, however, the conductivity augmentation means comprises an arrangement of printed lines and/or stacks wherein the thickness of a printed line or stack and the distance between neighbouring printed lines and/or stacks are chosen according to their placement on the second electrode. For example, thinner and/or narrower lines can be printed in an outer region of the second electrode, where the electric potential is highest. Thicker and/or higher stacks can be printed in an inner region of the second electrode to compensate for the drop in electric potential caused by the poor conductivity of the second electrode. In this way, the current distribution across the OLED surface can be optimised.

The lines or stacks of the conductivity augmentation means can be printed in a random manner, or to follow any suitable pattern. In a preferred embodiment of the invention, however, the conductivity augmentation means preferably comprises a regular grid arrangement and/or a concentric arrangement of printed lines and/or stacks, preferably such that the greater proportion of these printed lines and/or stacks is located in or near the central area of the cathode.

Any suitable material can be used to realise the conductivity augmentation means. For example, a conductive liquid such as an electrically conductive ink or glue could be used. Effectively any material can be used that is conductive and that lends itself favourably to a printing process, particularly in the miniscule amounts desirable for achieving the very narrow printed lines. In a further preferred embodiment of the invention, the material of the conductivity augmentation means comprises electrically conductive silver paste. This material has a favourable electrical conductivity, is suitable for printing in small amounts, and has a favourable viscosity satisfying the requirements of ink-jet printing and screen printing. Since the printed lines of the conductivity augmentation means realised in the manner described above are exceedingly narrow and therefore to all intents and purposes invisible to the naked eye, the OLED device according to the invention is particularly suited for use as a transparent OLED, in which the carrier as well as both first and second electrodes are transparent. Therefore, in a further preferred embodiment of the invention, the second electrode comprises an essentially transparent layer so that practically all the light generated in the active layer of the OLED is also emitted through the second electrode layer.

In another preferred embodiment of the invention, the OLED comprises a number of metal shunt lines electrically connected to the first electrode for augmenting the electrical conductivity of the first electrode. Theses shunt lines can be applied on the previously deposited first electrode in any suitable manner, for example in a photolithography process. Alternatively, to avoid an uneven surface caused by such raised shunt lines, these can be embedded in the carrier in an embossing step before the first electrode is applied.

The OLED device can be manufactured using the usual materials, which will be known to the skilled person. Preferably, the OLED device according to the invention is constructed using essentially transparent materials. For example, the carrier can comprise a transparent glass substrate; the first electrode can act as anode and be made of a transparent conducting oxide such as aluminium doped zinc oxide (ZnO) or tin-doped indium oxide (ITO) or a highly ductile transparent conductive polymer such as Poly(3,4-ethylene- dioxythiophene) poly(styrenesulfonate) PEDOT:PSS. The active layer can comprise one or more organic semiconductor materials chosen for their light-emitting properties. The second electrode can act as cathode and can be realised to be essentially transparent, for example by using a transparent material such as thin silver, thin aluminium or a transparent conductive oxide like ITO. To seal the OLED layers and protect them from moisture, a transparent cover lid can then be applied over the second electrode to encapsulate the entire OLED. Such a transparent OLED can be used favourably in applications in which a transparent region is desired when the transparent OLED is inactive or 'off, and which is illuminated when the transparent OLED is active or On', for example windows which are transparent during the day, and which can be used as light sources at night. Since the conductivity augmentation means allows a homogenous current distribution over the entire second electrode, device sizes with emitting areas exceeding 30 x 30 cm can easily be obtained, making the OLED according to the invention very attractive for many applications requiring larger emitting areas than can be realised for prior art OLEDs. A electrically conductive ink or metal 'paste' of the type mentioned above, for printing or dispensing, comprises small particles or globules of metal, usually encapsulated in a polymer shell and suspended in a suitable solvent. In order to improve the contact between the first printed line and the second electrode, therefore the method according to the invention preferably comprises a step of annealing a printed line. During annealing, the metal paste is effectively 'baked' at a high temperature so that the encapsulation melts or dissolves to some extent, allowing any solvent to evaporate and causing the metal globules to join together forming a conductive 'network'. This improves the contact between the first printed line and a subsequent congruent printed line, so that the function of the conductivity augmentation means - namely to improve the conductivity of the second electrode - is enhanced. The temperature necessary for a successful annealing process depends on the metal paste used. However, a high temperature may be detrimental to the functionality of the active layer. An annealing step is usually carried out in an oven, where the entire OLED device is exposed to these temperatures. When using silver nano inks or silver metal precursor inks, a favourably low temperature of 130°C or even 90°C can be sufficient. Heat treatment of OLEDs between 90°C and 130°C for brief periods are possible without affecting the device performance.

To allow successful annealing at higher temperatures without damaging the OLED device (for example when using a different metal paste), the step of annealing a printed line in a further preferred embodiment of the invention comprises a local deposition of energy in the material of the printed line, so that effectively only the printed line is heated and the remainder of the OLED is not directly exposed to the heat. Any suitable source of thermal energy can be used, for example a UV or IR light source. In a preferred embodiment of the invention, however, the annealing step comprises directing a laser light beam at the printed line to heat essentially only the material of that printed line. The laser parameters can be chosen such that the laser light energy is absorbed essentially only in the printed line and not in the body of the second electrode or even the active layer. In this way, the heat exposure is limited to the area in the immediate vicinity of the metal line itself.

For a stack of congruent printed lines, a sufficient electrical contact between the individual subsequently printed congruent lines can be achieved without the requirement of an additional annealing step for these congruent lines. Therefore, in a further preferred embodiment of the invention, only a first printed line is annealed in an annealing step, and one or more subsequent congruent lines of electrically conductive material are simply printed over the annealed first line to give an electrically conductive stack. If it is not intended to carry out an annealing step, the reduced conductivity can be compensated for by using a correspondingly greater line width for at least the first printed line.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.

Fig. 1 shows a plan view and a cross-section of a prior art OLED device;

Fig. 2 shows a cutaway view of an OLED device according to an embodiment of the invention;

Fig. 3 shows cross-sections through various realisations of a conductivity

augmentation means according to the invention;

Fig. 4 shows elevation views of conductivity augmentation means on a second

electrode of an OLED according to various embodiments of the invention; Fig. 5 shows a cross-section through an OLED device according to the invention, during operation of the device;

Fig. 6 schematically illustrates printing and annealing steps in the construction of a conductivity augmentation means for an OLED according to the invention.

In the diagrams, like numbers refer to like objects throughout. Elements of the diagrams are not necessarily drawn to scale, particularly the OLED device layer thicknesses and the printed lines of the conductivity augmentation means.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 shows a simplified rendering of a prior art OLED device. Basically, such a device comprises a carrier 10, which can be glass, plastic or any other suitable and preferably transparent material, onto which a first electrode layer 11 is applied, for example a transparent conducting oxide (TCO) such as zinc oxide, indium tin oxide, etc. An active layer

12 comprising any suitable organic light-emitting material is applied onto the first electrode layer 11. Finally, a second electrode 13 is applied onto the active layer 12. The layers 11, 12,

13 can be applied using the known techniques of masking, vapour deposition, etc. For a transparent OLED, the second electrode 13 must also be a transparent conductor, and electrical contacts 11a, 13a for applying a voltage across the electrodes must be arranged outside of the emitting area. However, the transparent materials available are generally only poor conductors. As a result, a voltage applied across the electrodes 11, 13 (by means of the contact areas 11a, 13 a) results in an uneven distribution of the electric potential across the second electrode, as indicated in the upper part of the diagram. In an outer area 41, which is closest to the contact points 11a, 13 a, the electric potential is highest. However, because of the poor conductivity of the second electrode material and the associated voltage drop, the electric potential is lower in an intermediate area 42 and lowest in an inner area 43. This results in correspondingly lower levels of brightness in the intermediate and inner areas, which effectively appear dark. In practice, therefore, such devices are not suitable for decorative lighting applications in which a homogenous emitting area is desired or required. The areas 41, 42, 43 of different potential shown here are only exemplary. The current distribution and therefore also the distribution of electric potential would of course be different for OLEDs with a different arrangement of the contact areas around the edges of the device.

Fig. 2 shows a cutaway view of an OLED device 1 according to an

embodiment of the invention. Here, the OLED 1 is constructed in essentially the same manner as the prior art OLED 4 described in Fig. 1, with a carrier 10, first electrode 11, active layer 12, and second electrode 13. An additional shunt line 14 is indicated on the first electrode 11. The next step in manufacture, however, ensures a homogenous distribution of electric potential over the second electrode 13 and therefore also a homogenous light output: after applying the second electrode 13, its conductivity is augmented by a conductivity augmentation means 2 comprising one or more lines of electrically conductive material printed onto the second electrode 13. This diagram only shows a cross-section through a small region of the OLED 1, showing that the conductivity augmentation means 2 projects outward or away from the second electrode 13 and is a thin, visually unobtrusive structure compared to the rest of the OLED.

Fig. 3 shows four cross-sections A, B, C and D of various possible realisations of conductivity augmentation means 2 (the height and width of the lines in each realisation are not drawn to scale). The first embodiment (A) of a conductivity augmentation means 2 comprises two printed lines 20, 21 of conductive ink. Here, the material of the second printed line 21 has spread to cover the first printed line 20 so that it is essentially the same width as the first printed line 20. The next realisation (B) shows a first printed line 20, upon which successive lines 21 are printed in order to achieve a relatively 'high' or 'tall' electrically conductive band 23 or stack 23 arranged essentially perpendicularly to the outer surface of the second electrode. A third realisation (C) shows a similar construction in which the successive printed lines 21 are narrower, giving a favourable height: width aspect ratio. Each band 23 of the conductivity augmentation means 2 is essentially a stack 23 of at least two congruent printed lines 20, 21. In each of these three realisations, the first printed line 20 can be annealed in a thermal annealing process to improve the conductivity of the printed line 20. The overall height of the stack 23 in each case is governed by the number of additional printed lines 21. The last realisation (D) on the right shows a conductivity augmentation means 2 comprising a printed line in the form of a narrow band or ribbon 22 applied in a single step. Again, this narrow band 22 can be annealed.

Fig. 4 shows plan views of a number of possible embodiments (I, II and III) for a conductivity augmentation means 2 printed onto the second electrode 13 of an OLED 1. The printed lines of the conductivity augmentation means 2 could be applied in any of the realisations described in Fig. 3, for example. In the upper part of the diagram, a first embodiment (I) shows a conductivity augmentation means 2 in the form of a grid of printed lines, some of which extend between the contact areas 13a of the second electrode 13, and where the printed lines intersect at essentially right angles. The lines can be printed such that the line density is greater in a central area 44 of the second electrode 13 than in the outer areas. In a second embodiment (II), the lines are printed as irregular or wavy lines, again arranged to give a higher density in the central region 44 of the second electrode 13, and connecting the contact areas 13 a. In a third embodiment (III), lines are printed in the manner of a number of concentric circles. In these examples, the contact areas 13a of the second electrode 13 are shown to be on adjacent sides of the device 1, but could equally well be on the same side or on opposite sides, depending on the device construction. Of course, these are only exemplary embodiments, and the arrangement of printed lines on the outside of the second electrode can follow any suitable pattern, depending to some extent on the

capabilities of the devices used to print the conductive material.

Fig. 5 shows a cross-section through a transparent OLED device 1 according to the invention, during operation of the device 1. A voltage is applied across the first and second electrodes 11, 13 in the usual manner to cause the active layer 12 to emit light L as indicated by the wide arrows leaving the device on either side of the active layer. Here, the voltage supply 3 is only indicated schematically. Usually, the voltage is applied at several contact areas along the outer edges of the device 1 , as will be known to the skilled person. The conductivity augmentation means 2 is also connected at several points to the lateral contact areas as described in Fig. 4, but, for the sake of clarity, this is not shown in the diagram. Shunt lines 14 of the first electrode 11 are indicated. As the diagram demonstrates, the light L emitted by the OLED 1 during operation is essentially unaffected by the width of the printed lines of the conductivity augmentation means 2 on the second electrode 13. When the device is turned off, the transparent OLED is essentially transparent since the printed lines of the conductivity augmentation means 2 are effectively too small to be noticeable.

Fig. 6 illustrates the printing and annealing steps in the construction of a conductivity augmentation means. Beginning at the left of the diagram, a droplet of electrically conductive silver paste 50 is dropped from a suitable applicator 5, for example a printing head for nano-ink. While printing, the applicator 5 can be moved with respect to the second electrode 13 (or vice versa) to give a first printed line 20 across the entire width of the second electrode, or only over a certain length, as appropriate, depending on the pattern that is to be applied. A beam 6 of laser light is then directed at the first printed line 20 to locally deposit thermal energy into the conductive paste and improve the conductivity. Once the first printed line 20 is annealed and cooled, the printing head 5 can be re-positioned to dispense another series of droplets 50 onto the first printed line 20, where they can spread to give a congruent printed line 21 of essentially the same width as the first printed line 20. In this way, one or more further lines 21 can be printed congruently onto the first line 21 to give a stack 23 or band 23 of electrically conductive material projecting outward from the second electrode 13.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art from a study of the drawings, the disclosure, and the appended claims. For the sake of clarity, it is to be understood that the use of "a" or "an" throughout this application does not exclude a plurality, and "comprising" does not exclude other steps or elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.