WO2008078979A1

WO2008078979A1 - Oled display, and method for operating and method for manufacturing such oled display

Info

Publication number: WO2008078979A1
Application number: PCT/NL2006/000663
Authority: WO
Inventors: Nikolay Iosad; Franciscus Cornelius Dings
Original assignee: Otb Group B.V.
Priority date: 2006-12-22
Filing date: 2006-12-22
Publication date: 2008-07-03

Abstract

An OLED display provided with a matrix of pixels, wherein the pixels each contain at least an anode and a cathode with a light-emitting layer between them, which light-emitting layer emits light when a voltage difference is applied between the anode and the cathode of the respective pixel, wherein, per pixel, an individually controllable control element is present, wherein the control element comprises a layer of material with a programmable resistance. The invention further provides a method for operating and manufacturing such an OLED display.

Description

Title: OLED display, and method for operating and method for manufacturing such OLED display

The invention relates to an OLED display provided with a matrix of pixels, where the pixels each contain at least an anode and a cathode with a light-emitting layer between them, which light -emitting layer emits light when a voltage difference is applied between the anode and the cathode of the respective pixel, while, per pixel, an individually controllable control element is present.

Such an OLED is described in WO 2004084168. With that known OLED display, per pixel, at least two thin -film transistors (TFT) are provided. In order to prevent problems caused by variation in transistor properties and for solving the problem of optical degradation as a result of optical feedback, in general, much more complex TFT circuits are used. Forming such transistors is done by complex etching and masking techniques, utilizing semiconductors, such as for instance silicon. Since a particularly high accuracy is required for forming the semiconductor thin-film transistors and the forming of such transistors, in addition, requires a large number of etching steps and a large number of times of applying masks and depositing inorganic substances, forming such semiconductor thin-film transistors is expensive. In addition, the method for manufacturing such transistors is very sensitive to errors and when only a very limited number of pixels do not function, the OLED display already needs to be rejected. The yield of the known process for manufacturing an active matrix OLED is therefore low. In addition, with OLEDs, the TFT structures take up a relatively large surface of the pixels in that these TFTs need to switch a relatively great voltage/current. This results in light output loss, which is highly undesired. The invention contemplates an OLED display where the above - described problems are considerably reduced. To this end, the invention provides an OLED pixel of the type described in the introduction which is characterized in that the control element comprises a layer of material with a programmable resistance.

By now, materials are known whose resistance can be varied by subjecting them to a voltage or current programming pulse with a particular amplitude and a particular duration. After subjecting the material to a programming pulse, it has a lower or higher resistance than in an initial state. The initial state can then be achieved in different manners, which will be returned to in the following.

What is particularly favorable of the proposal according to the invention is that only a single layer of the material with the programmable resistance needs to be applied in the pixel or a part of the pixel. The complex process to be repeated several times of applying masks, depositing semiconductor or conductor material and then etching material away can then be omitted completely. The programmable resistance material may, for instance, simply be applied with an inkjet printer. Such an inkjet process is excellently controllable and, in addition, an inkjet process may also be used for applying other layers, such as for instance the light -emitting layer.

Alternative deposition techniques are spin coating and vacuum evaporation. In these techniques, with masks or etching away or ablation, it needs to be brought about that the excess material is removed again, so that it is finally only present in the OLED pixels. Another important advantage of the invention is that an active matrix OLED display can be made with the simpler techniques of a passive matrix OLED display. Even if a transistor or diodes are built in to reduce the crosstalk, the structure will be simpler with less stringent requirements. The programmable resistance can be arranged on the side of the light-emitting layer facing away from the light-emitting side, so that the complete surface of the pixel remains available for emitting light. When the programmable resistance is arranged on the light -emitting side of the light-emitting layer, it needs to take up less than 20%, in particular less than 10% of the light-emitting surface. A further advantage of the OLED according to the invention is that the opening angle of the pixel, and accordingly the angle at which the image of the display is still well visible, is particularly large.

According to a further elaboration of the invention, the material with programmable resistance can be included between, on the one side, above- mentioned cathode or anode and, on the other side, a control electrode, while the cathode or anode and the control electrode, as well as the programmable resistance layer located therebetween are connected to a programming circuit which can bring the programmable resistance to at least two different resistance states. With such an embodiment, the programming pulse for bringing the programmable resistance to the at least two different resistance states can be applied without this resulting in the light -emitting layer lighting up. In addition, with the aid of a programming circuit, the value of the resistance layer can be brought from a relatively low value to an initial value by applying a reset pulse within a frame period.

In one embodiment, a programming circuit is provided in which the programmable resistance is included, while the programming circuit is designed for generating a programming pulse with a particular duration and a particular voltage, while the duration and the voltage determine the resistance value of the programmable resistance.

When the programming pulse has a high voltage and/or a long duration, this will result in a relatively large change in the resistance in the resistance material compared to the original value of the resistance. When the programming pulse has a relatively low voltage and/or a short duration, this will result in a smaller change . The reset pulse of different polarity than the programming pulse brings the resistance material to the original state. In the embodiments without optical feedback, the original value of controllable resistance can be high or low. In the embodiments with. optical feedback, the original value of controllable resistance needs to be high, so that OLED does not emit light when the controllable resistance is in the original state. In one embodiment, the programming circuit is arranged for generating a reset pulse, for bringing the programmable resistance to a high initial value. A reset pulse will generally involve a voltage with a negative value with respect to the programming pulse. The programmable resistance may be manufactured from a material which is bringable to a state with a relatively low resistance under the influence of a programming pulse with a particular duration and amplitude, while the material is light-sensitive in the sense that, after the above- mentioned programming pulse, the resistance gradually increases under the influence of light, while the programmable resistance is arranged such that it is operatively lighted by the light emitted by the light -emitting layer of the respective pixel.

With such an embodiment, optical feedback is provided, which has the advantage that pixels which emit very much light are dimmed in the course of a frame period, while obsolete pixels which emit little light are dimmed less in the course of the same frame period, so that the average light emission per frame period is approximately equal despite the fact that the quality of the light -emitting capacity of the light -emitting layer is different for the two pixels. Optionally, use may even be made of the optical feedback to completely reduce the emission of light within a frame period to zero. According to a further elaboration, to this end, the increase of the value of the resistance under the influence of the lighting is such that, within a frame period, the programmable resistance has obtained the high initial value prior to the programming pulse, so that no or hardly any light is emitted by the light -emitting layer. With such an embodiment, then no reset pulse needs to be applied and a single circuit is sufficient which serves both for the power supply of the light-emitting layer and for applying programming pulses. Therefore it is then not necessary to provide a separate control electrode because the programming pulse is also applied via the anode and cathode already present anyway. Such an embodiment with complete optical feedback can therefore be controlled and designed relatively simply.

According to a further embodiment of the invention, the programmable resistance can assume different resistances within a particular range of resistances, such that a respective pixel generates much light with a small resistance, less light with a larger resistance and no light or at least hardly any light with a large resistance. Thus, per pixel, not only two resistance values (on/off) but intermediate values for creating, for instance, shades of gray can be realized.

The shades of gray may have an "analogue" and "discrete" character. The programmable resistance layer consists of a large number of particles having a threshold switching field. In an "analogue" regime, a partial switching of the programmable resistance layer is caused by a current or voltage pulse of a special form, which, for instance, produces less energy than is necessary to completely switch the programmable resistance layer. Another option for implementing such a regime is to use a programmable resistance layer with a variable distance between electrodes. In this case, electrodes may have a gradually varying distance, for instance in that the thickness of the programmable resistance layer within a pixel gradually increases. The switching will only take place in those areas where the electrical field is larger than the threshold field.

The "discrete" regime can be implemented by the use of a mixture of different materials, which have different switching thresholds . Another option for implementing such a regime is the use of electrodes between which the distance varies stepwise across the surface of the pixel. In that case, the programmable resistance layer comprises areas with different distances between parallel electrodes. This can be realized by stepwise varying the thickness of the programmable resistance layer across the surface of the pixel.

According to a further elaboration of the invention, the physical principle in the resistance material may be based on charge transfer complexes. To this end, the resistance material may contain tetracyanoquinodimethane (TCNQ) metal complex, such as for instance CuTCNQ, AgTCNQ or LiTCNQ.

In an alternative further elaboration of the invention, the physical principle in the resistance material may be based on electro reduction and conformational change of the molecules, which change causes conjugation modification. To this end, the resistance material may contain Rose Bengal. Rose Bengal increases in resistance under the influence of light and is therefore suitable for optical feedback.

In still another alternative embodiment, the physical principle in the resistance material may be based on doping polymers with a low conductivity with nanoparticles of metal, semiconductor material or organic material, encapsulated in organic or inorganic material. With such an embodiment, the encapsulation can create a potential threshold which can intercept the charge. Depending of the amount of intercepted charge, the resistance value of the resistance material varies.

When, with that alternative embodiment, the polymers are provided with a doping with nanoparticles from a II-VT semiconductor or metal, a resistance material is created which is suitable for optical feedback.

According to another elaboration of the invention, the resistance material may comprise a mixture of a number of the above -described resistance materials, allowing the realization of different resistance levels. These different resistance levels can be created within a pixel, so that the resistance does not only have a high and a low resistance but, for instance, a high resistance, a medium-sized resistance, a still lower resistance and a lowest resistance. This can be important for creating different light intensities per pixel. According to a further elaboration of the invention, per pixel, a crosstalk eliminating element, such as for instance two diodes or a single transistor, may be provided to eliminate cross-talk between the different pixels. The term cross-talk is understood to refer to the phenomenon that, upon programming of the resistance material of the pixel, the resistance of the resistance material of nearby pixels changes as well. A diode or a single transistor can simply be provided per pixel. In any case more simply than the conventional TFT structures which need to be provided with conventional active matrix displays.

The invention further provides a method for operating an OLED display according to the invention or a further elaboration thereof, where such an OLED is provided, while, per pixel, a programmable resistance present in that pixel is brought from an initial state with initial resistance value to a required state by a programming pulse with a particular amplitude and a particular duration, while then a voltage is applied over the pixel, so that the light-emitting layer emits more or less light depending on the programmed resistance.

According to a further elaboration of the invention, within a frame period after applying the programming pulse, the programmable resistance can be brought to the original resistance value with the aid of a reset pulse. For an OLED provided with a programmable resistance manufactured from a material which gradually changes from the state with the lower resistance to a state with higher resistance under the influence of light and where the programmable resistance reaches the initial state under the influence of light before a next programming pulse is provided, according to a further elaboration of the method, only programming pulses can be applied. With such a method there is complete optical feedback and no reset pulses need to be applied.

For an OLED provided with a programmable resistance manufactured from a material which gradually changes from the state with the lower resistance to a state with higher resistance under the influence of light and where the programmable resistance does not reach the initial state under the influence of light within a frame period, according to a further elaboration of the method, within a frame period a programming pulse and a reset pulse can be applied. The invention further provides a method for manufacturing an

OLED display provided with a number of OLED pixels according to the invention, while a substrate is provided to which at least a number of active layers are applied, for forming inter alia an anode, a cathode and a light- emitting layer assembly located therebetween, while further a material with programmable resistance is applied.

An OLED display manufactured with such a method has the advantages which have already been indicated in the description of the OLED display according to the invention.

According to a further elaboration, the material with programmable resistance can be applied with the aid of an inkjet printing process. Often, an inkjet process is also already used for applying, for instance, the light-emitting substances. From that point of view, it is particularly favorable when the material with the programmable resistance is also applied with such a process. In addition, the inkjet printing process is accurate and relatively inexpensive. Further, this printing process is excellently suitable for applying the material with programmable resistance per OLED pixel. Incidentally, the programmable resistance material may also be applied with full-layer application techniques, such as spin coating and evaporation techniques. Of course, then masks or etching techniques will need to be used to ensure that finally the resistance material is on!j^r present in the pixels.

The invention will now be explained in more detail on the basis of three exemplary embodiments, with reference to the drawing, in which: Fig. 1 schematically shows a cross section of a first exemplary embodiment of a pixel of an OLED display with a programmable resistance and with upward light emission;

Fig. 2 shows a similar cross -sectional view of a second exemplary embodiment of an OLED pixel with cross-talk elimination and downward light emission;

Fig. 3 shows a similar cross-sectional view of a third exemplary embodiment of an OLED pixel with partial or complete optical feedback and upward light emission;

Fig. 4 shows a similar cross-sectional view of a fourth exemplary embodiment of an OLED pixel with partial or complete optical feedback and downward light emission;

Fig. 5 shows the circuit without optical feedback corresponding with the exemplary embodiment of Fig. 1;

Fig. 6 shows the circuit with cross-talk elimination corresponding with Fig. 2;

Fig. 7 shows the circuit with partial optical feedback and crosstalk elimination corresponding with Figs. 3 and 4;

Fig. 8 shows the circuit with complete optical feedback and crosstalk elimination corresponding with Figs. 3 and 4; Fig. 9 shows the variation of the programming pulse, the operating voltage and the light emitted by the pixel associated with an embodiment without optical feedback;

Fig. 10 shows the variation of the programming pulse and the operating voltage, and the light emitted by the pixel associated with an embodiment with partial optical feedback; Fig. 11 shows the variation of the programming pulse and ths operating voltage, and the light emitted by the pixel associated with an embodiment with complete optical feedback;

Fig. 12 shows the current/voltage diagram of a cross-talk eliminating element;

Fig. 13 shows a schematic cross section of the programmable resistance layer of a pixel, where the pixel has an analogue character; and

Fig. 14 shows a schematic cross section of the programmable resistance layer of a pixel, where the pixel has a discrete character. The exemplary embodiment of a pixel P of an OLED shown in

Fig. 1 is provided with a substrate 1, a conductive layer forming a first electrode 2, a layer with programmable resistance 3, a conductive layer forming a second electrode 4, one or a number of active layers 5 which are light-emitting and a transparent layer forming a third electrode 6. In general, an OLED display is provided with a matrix of such pixels P. The transparent, first electrode 6 is, for instance, formed by ITO. The light-emitting layer assembly 5 generally comprises a number of layers whose structure is known to a skilled person. With the OLED pixel, a layer 4 with programmable resistance is included between the transparent, third electrode 6 and the first electrode 2. In order to be able to program the layer 3 from programmable resistance material, a second electrode 4 is provided which is applied to the side of the layer 3 from programmable resistance material facing away from the first electrode 2. The programmable resistance 3 with the first electrode 2 and the second electrode 4 can be included in a programming circuit 7 provided with a programming generator 8. To the first electrode 2 and the third electrode 6, a supply circuit 9 is connectable, provided with a power supply 10 for operating the pixel, such that the light-emitting layer 5 emits light. In the exemplary embodiment shown, the light L is emitted upwards. Fig. 2 shows a similar cross-sectional view as shown in Fig. 1. In this exemplary embodiment, the light L is emitted downwards and a crosstalk eliminating element 11 is added. From bottom to top, the pixel comprises a substrate 1, a transparent third electrode 6, a light -emitting layer assembly 5, a second electrode 4, a layer of material with programmable resistance 3 and a first electrode 2. The cross-talk eliminating element 11 prevents the resistance value of the resistance layer of the pixel shown from also being influenced by programming and resetting the resistance layer 3 of a nearby pixel. The cross-talk eliminating element 11 may, for instance, comprise a diode or a single transistor.

Fig. 3 shows a similar cross -sectional view as shown in Figs. 1 and 2. In this exemplary embodiment, the light L is emitted upwards and there is a complete or partial optical feedback by the light generated by the light-emitting layer assembly 5. From bottom to top, the pixel comprises a substrate 1, a first electrode 2, a material layer with programmable resistance 3 extending only over a part of the surface of the pixel, a second electrode 4, a light-emitting layer assembly 5, a third transparent electrode 6. On top of the third electrode 6, a non-light-transmitting material layer 12 is applied which screens the programmable resistance material 3 from ambient light. It goes without saying that the non-light- transmitting material layer 12 is only applied on top of the resistance material 3, so that it only covers a part of the light -emitting surface of the pixel, in particular no more than 20% an preferably no more than 10% of the surface of the pixel. In the remaining part of the surface, a transparent leveling layer may be provided. Further, a programming circuit 7 is shown with a programming generator 8 and a cross-talk eliminating element 11 therein. Finally, further a supply circuit 9 with a power supply 10 therein is shown. The resistance value of the programmable resistance 5 will partly increase due to optical feedback after a programming pulse is applied thereto. With a separate programming circuit 7, the programmable resistance 3 can be reset to a relatively high initial value and again be brought to a desired relatively low value. For that matter, the Figure can also represent an embodiment with total optical feedback. That is to say an embodiment where the resistance value of the programmable resistance material 3 is brought back to the original high value within a frame period due to the light of the light-emitting layer assembly 5 incident thereon. In the embodiment with complete optical feedback, the programming circuit 7 , at least the programming generator 8, may be omitted therefrom. This is because applying a pulse to bring the programmable resistance layer to the low resistance value can take place with the supply circuit 9 and the power supply 10 present therein.

Fig. 4 shows a similar cross-sectional view as shown in the preceding Figures, where there is a partial or complete optical feedback and where the light L is emitted downwards. From the bottom upwards, the pixel is provided with a substrate 1, a transparent first electrode 2, a non-light-transmitting layer 12, a layer of material 3 with programmable resistance, a second transparent electrode 4, a light-emitting layer assembly 5 and a third electrode 6. The non-light-transmitting layer 12 and the layer of material 3 with programmable resistance extend only over a part of the surface of the pixel. Preferably, this part is as small as possible — smaller than 20% and preferably smaller than 10% - to keep the light output of the pixel as large as possible. This Fig. 4 again shows a programming circuit 7 with a programming generator 8 and a cross-talk eliminating element 11 therein. Further, the Figure shows a supply circuit 9 with a power supply 10 therein for operating the light -emitting layer 5. Just like in the embodiment of Fig. 3, the Figure may represent both an embodiment with partial and with complete optical feedback.

The exemplary embodiment of Figs. 3 and 4 may be based on complete optical feedback in the sense that, when the light -emitting layer 5 emits light, the light is incident on the programmable resistance 3 and the programmable resistance 3 proceeds from a programmed, relatively low resistance value to a high initial value within a frame period. When the programmable resistance 3 again has a relatively high initial value, again a programming pulse is applied by the power supply 10, so that the programmable resistance 3 assumes a relatively low value again after which the light-emitting layer 5 will again emit relatively much light.

Fig. 5 schematically shows a diagram of an OLED pixel of the exemplary embodiment of Fig. 1. In the diagram, the diode represents the light-emitting layer 5 and the controllable resistance represents the programmable resistance 3. The lower connecting point corresponds with the first electrode 2, the upper connecting point corresponds with the third electrode 6 and the middle connecting point corresponds with the second electrode 4 which is connected to a programming circuit 7.

Fig. 6 schematically shows the diagram of an OLED pixel of the exemplary embodiment of Fig. 2. This diagram relates to a pixel with a cross-talk elimination and without optical feedback. In the diagram, the diode represents the light-emitting layer 5 and the controllable resistance represents the programmable resistance layer 3. The lower connecting point corresponds with the first electrode 2, the upper connecting point corresponds with the third electrode 6. The arrow L represents the light emitted by the light-emitting layer 5. The cross-talk eliminating element 11 is clearly indicated.

Fig. 7 shows a schematic diagram of an OLED pixel with partial optical feedback, as for instance shown in Figs. 3 and 4. The arrow L indicates that the light emitted by the light -emitting layer assembly 5 represented as a diode is incident on the programmable resistance material 3. Just like in Figs. 5 and 6, the lower connecting point corresponds with the first electrode 2, the middle connecting point with the second electrode 4 and the upper connecting point with the third electrode 6. Pig. 8 shows a schematic diagram of an OLED pixel with complete optical feedback. With such an embodiment, the programming pulse can take place via the same circuit as for the excitation for the light -emitting layer assembly 5. However, this is not necessary, as for instance appears from Figs. 3 and 4 which can likewise represent a pixel with complete optical feedback. The light L emitted is incident on the layer from the programmable resistance material 3 and resets this material to the high resistance value within a frame period.

Fig. 9 shows the variation in time of the voltage in the programming circuit 7 (upper Figure part), the voltage in the supply circuit 9 (middle Figure part) and the light emitted by the light -emitting layer 5 (lower Figure part), respectively, of an OLED pixel without optical feedback, for instance as shown in Figs. 1, 2, 5 and 6. The distance between the left starting point of the horizontal axis and the vertical line on the horizontal axis represents one frame period. The programming pulse 15 shown in the upper Figure part shows that first a pulse with a positive voltage is generated, which results in a relatively low resistance value in the programmable resistance 3. Then the supply circuit 9 is excited and a constant voltage is applied over the light-emitting layer 5. This constant supply voltage results in a constant emission of light until a reset pulse 16 is applied to the programmable resistance 3 to bring the resistance value to the original high initial value. The reset pulse 16 has a negative voltage with respect to the programming pulse 15. Directly after this, a new programming pulse 15' follows which has, in this example, a lower value than the preceding programming pulse 15. As a result thereof, the programmable resistance 3 will get a less low resistance value, as a result whereof the light-emitting layer 5 will emit less light which is clearly visible in the lower part of Fig. 9.

Fig. 10 shows the programming/operating voltage variation and the light-emitting variation of an exemplary embodiment of an OLED pixel with complete optical feedback. The upper part of the Figure shows the voltage variation in the supply circuit 9. The lower part of the Figure shows the variation of the emitted light. It is clearly visible that the intensity of the emitted light decreases to a value 0 during a frame period, so that a new programming pulse 15' is needed to make the respective pixel emit light again.

In a similar manner as shown in Figs. 9 and 10, Fig. 11 shows the variation of the voltage in the programming circuit 7 (an upper Figure part), the supply circuit 9 (middle Figure part) and the variation of the emitted light (lower Figure part) of the light -emitting layer 5 of an OLED pixel with partial optical feedback. It is clearly visible in the lower Figure part that, due to the optical feedback, the amount of light emitted gradually decreases within a frame period and, at the end of the frame period, the reset pulse 16 brings the resistance to the original state. It is also clear that a programming pulse 15' with a lower value results in a less low resistance in the programmable resistance 3 which results in less light being emitted.

Fig. 12 shows the current-voltage characteristic of the cross-talk eliminating element 11. When the cross-talk eliminating element 11 has such a I/V characteristic, the cross-talk phenomenon, i.e. programming voltages or supply voltages through nearby pixels influencing nearby pixels, is prevented. The most important parameter of the I/V characteristic is the threshold voltage.

Fig. 13 shows a schematic cross section of a programmable resistance layer 3 and two electrodes 2, 4 of a pixel located on both sides thereof, while the pixel has an analogue character. This is realized in this embodiment in that the programmable resistance layer 3 has a decreasing thickness tapering substantially continuously. As is known, the strength of an electrical field between two electrodes depends on the potential difference between the electrodes and the distance between the electrodes. Because the programmable resistance layer 3 has a decreasing thickness tapering substantially continuously, the distance between the electrodes 2, 4 also decreases substantially continuously. As a result thereof, with a particular voltage difference between the electrodes, the field strength varies, in the sense that, in those places where the electrodes are close to each other, the electrical field is stronger than in the places where the electrodes 2, 4 are further apart. Since the programmable resistance layer is only brought from the initial state, with for instance high resistance, to the required state, with for instance lower resistance, above a particular threshold value field strength, the resistance layer 3 will be brought to the required state only over a part of the total surface of that layer. The total resistance of the programmable resistance layer in the pixel depends on the part of the surface of the programmable resistance layer which has been brought to the required state and the part which is still in the initial state. So, depending on the programming pulse applied between the electrodes, for instance programming voltage, a larger or less large surface of the resistance layer can be brought to the required state. It will be clear that thus a continuous variation of the strength of the light emission can be brought about.

Fig. 14 shows a similar cross section as shown in Fig. 13. In this cross section, the thickness of the resistance layer 3 does not decrease continuously but stepwise, i.e. in a number of discrete steps. Depending on the strength of the programming pulse between the electrodes 2, 4, in the present exemplary embodiment either 1/3 or 2/3 or the complete surface of the programmable resistance layer 3 can be brought to the required state. Thus, a stepwise or discrete variation of the strength of the light emission can be brought about.

For reasons of clarity, in Figs. 13 and 14, the light -emitting layer assembly, the substrate, the third electrode, if any, and other parts not essential for the explanation of an analogously and discretely programmable resistance layer are not shown. In the tεibles below, the successive steps for controlling a pixel are listed in the left column. The next column describes, per physical principle of the programmable resistance, to which state the respective step brings the programmable resistance 3. The next column describes the resistance value of the programmable resistance. Tables 1, 3 and 5 relate to a so-called analogue programmable resistance 3, which means that the programmable resistance can assume a continuum of resistance levels, depending on the amplitude and duration of the programming pulse applied thereto. Tables 2, 4 and 6 relate to a so-called discrete programmable resistance, which means that the programmable resistance can assume only a few, in the most simple form only two, resistance levels.

Table 1: Sequence of steps of pixel operation with no optical feedback for analogue regime.

Table 2: Sequence of steps of pixel operation with no optical feedback for discrete regime.

discrete elements and/or parts of the programmable resistance are in the initial state.

Programming of required Almost all donors and/or acceptors Required resistance value of a belonging to the fractions with activation resistance value programmable resistance; threshold voltage lower than the is programmed. a voltage pulse of required difference between the programming form is applied to the pulse voltage and the voltage drop on programmable resistance via OLED or cross-talk eliminating element programming electrode. are activated; and/or almost all CT complexes belonging to the fractions with the threshold voltage lower than the difference between the programming pulse voltage and the voltage drop on OLED or cross-talk eliminating element are activated (formed); and/or almost all supramolecular structures and/or molecules belonging to the fractions with activation threshold voltage lower than the difference between the programming pulse voltage and the voltage drop on OLED or cross-talk eliminating element have almost completely changed their conformational state; and/or all elements or parts of the programmable resistance with the activation threshold voltage lower than the difference between the programming pulse voltage and the voltage drop on OLED or cross-talk eliminating element are switched.

Luminance; a constant The system remains in its previous state. Resistance voltage is applied to the value of is circuit consisting of OLED constant. and the programmable resistance.

Table 3: Sequence of steps of pixel operation with partial optical feedback for analogue regime.

Table 4: Sequence of steps of pixel operation with partial optical feedback for discrete regime.

Table 5: Sequence of steps of pixel operation with complete optical feedback for analogue regime:

Table 6: Sequence of steps of pixel operation with complete optical feedback for discrete regime:

It is clear that the invention is not limited to the exemplary embodiments described but that various modifications are possible within the framework of the invention as defined by the claims.

Claims

1. An OLED display provided with a matrix of pixels, wherein the pixels each contain at least an anode and a cathode with a light-emitting layer between them, which light-emitting layer emits light when a voltage difference is applied between the anode and the cathode of the respective pixel, wherein, per pixel, an individually controllable control element is present, wherein the control element comprises a layer of material with a programmable resistance.

2. An OLED display according to claim 1, wherein the material with programmable resistance is included between, on the one side, said cathode or anode and, on the other side, a control electrode, wherein the cathode or anode and the control electrode as well as the programmable resistance layer located therebetween are connected to a programming circuit which can bring the programmable resistance to at least two different resistance states.

3. An OLED display according to claim 1 or 2, wherein a programming circuit is provided in which the programmable resistance is included, wherein the programming circuit is arranged for generating a programming pulse with a particular duration and a particular voltage, wherein the duration and the voltage determine the resistance value of the programmable resistance.

4. An OLED display according to claim 2 or 3, wherein the programming circuit is arranged for generating a reset pulse, for bringing the programmable resistance to a high resistance state.

5. An OLED display according to any one of the preceding claims, wherein the programmable resistance is manufactured from a material which is bringable to a state with a relatively low resistance under the influence of a programming pulse with a particular duration and amplitude, wherein the material is light-sensitive in the sense that, after the said programming pulse, the resistance gradually increases under the influence of light, wherein the programmable resistance is arranged such that it is operatively lighted by the light emitted by the light-emitting layer of the respective pixel.

6. An OLED display according to claim 5, wherein the increase of the value of the resistance under the influence of the lighting is such that, within a frame period, the programmable resistance has obtained the high initial value prior to the programming pulse, so that no or hardly any light is emitted by the light-emitting layer.

7. An OLED display according to any one of the preceding claims, wherein the programmable resistance can assume different resistances within a particular range of resistances, such that a respective pixel generates much light with a small resistance, less light with a larger resistance and no light or at least hardly any light with a large resistance.

8. An OLED display according to any one of the preceding claims, wherein the physical principle in the resistance material is based on charge transfer complexes.

9. An OLED display according to any one of claims 1-7, wherein the physical principle in the resistance material is based on electro reduction and conformational change of the molecules which cause conjugation modification.

10. An OLED display according to any one of claims 1-7, wherein the physical principle in the resistance material is based on doping polymers with a low conductivity with nanoparticles of metal, semiconductor material or organic material, encapsulated in organic or inorganic material.

11. An OLED display according to claim 8, wherein the resistance material contains tetracyanoquinodimethane (TCNQ) metal complex, such as for instance CuTCNQ, AgTCNQ or LiTCNQ.

12. An OLBD display according to claim 9, wherein the resistance material contains Rose Bengal.

13. An OLED display according to claim 10, wherein the polymers are provided with a doping with nanoparticles from a II-VI semiconductor or metal.

14. An OLED display according to any one of the preceding claims, wherein the resistance material comprises a mixture of a number of the resistance materials as described in at least one of claims 7-10 or a combination of resistance materials from at least two of claims 7-10.

15. An OLED display according to any one of the preceding claims, wherein, per pixel, a cross-talk eliminating element, such as for instance one or more diodes or a single transistor, is provided to eliminate cross-talk between the different pixels.

16. A method for operating an OLED display according to any one of the preceding claims, wherein an OLED display according to any one of the preceding claims is provided, wherein, per pixel, a programmable resistance present in that pixel is brought from an initial state with initial resistance value to a state with a required resistance by a programming pulse with a particular amplitude and particular duration, wherein then a voltage is applied over the pixel, so that the light -emitting layer emits more or less light depending on the programmed resistance.

17. A method according to claim 16, wherein, within a frame period after applying the programming pulse, the programmable resistance is brought to the initial state with initial resistance value with the aid of a reset pulse.

18. A method according to claim 16, wherein, for the purpose of an OLED provided, per pixel, with programmable resistance manufactured from a material which gradually changes from the state with the lower required resistance to a state with higher resistance under the influence of light and where the programmable resistance reaches the initial state under the influence of light before a next programming pulse is provided, only programming pulses are applied.

19. A method according to claim 16, wherein, for the purpose of an OLED display provided, per pixel, with programmable resistance manufactured from a material which gradually changes from the state with the required lower resistance to a state with higher resistance under the influence of light and where the programmable resistance does not reach the initial state under the influence of light within a frame period, within a frame period a programming pulse and a reset pulse are applied.

20. A method for manufacturing an OLED display according to any one of claims 1-15 provided with a number of OLED pixels, wherein a substrate is provided to which at least a number of active layers are applied, for forming inter alia an anode, a cathode and a light-emitting layer assembly located therebetween, wherein further a material with programmable resistance is applied.

21. A method for manufacturing an OLED display according to claim 20, wherein the material with programmable resistance is applied with the aid of an inkjet printing process.

22. A method according to claim 20 or 21, wherein the material with programmable resistance is applied per OLED pixel.

23. A method according to claim 20, wherein the material with programmable resistance is applied with the aid of a spin coating process.

24. A method according to claim 20, wherein the material with programmable resistance is applied with the aid of an evaporation process.