Abstract:
In this display, an upper electrical insulation layer covers the active matrix; each electrooptic element comprises a lower supply electrode that is applied to this insulation layer. According to the invention, this lower electrode includes an organic conducting layer applied directly to said insulation layer and a metal layer covering the organic conducting layer. Such a structure makes it possible to optimize both planarization and light extraction by an optical cavity effect thereby improving the performance of the display.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to an active-matrix display comprising a substrate that supports: 
        an array of circuits for supplying and driving electrooptic elements, and arrays of electrodes for driving each of the circuits and for supplying electrooptic elements through these circuits, these circuits and electrodes therefore forming an active matrix;     an upper electrical insulation layer covering the arrays of circuits and electrodes; and     an array of electrooptic elements placed on the insulation layer.        
 
         [0005]     In such a display, each circuit corresponds to an electrooptic element and is intended to drive this element. Each circuit includes a supply input terminal connected to a supply electrode of one of the electrode arrays and a supply output terminal. Each circuit includes, for example, the current modulator of the TFT transistor type comprising two current electrodes, called in this case the drain and source electrodes, one serving as input terminal and the other as output terminal.  
         [0006]     Each electrooptic element comprises an upper supply electrode and a lower supply electrode that is applied to the insulation layer and is connected to the supply output terminal of the circuit corresponding to this element through a through-hole that passes through said insulation layer. Such through-holes are generally called “vias”.  
       DESCRIPTION OF THE PRIOR ART  
       [0007]     Document U.S. Pat. No. 6,780,688 describes a display of the aforementioned type, in which the electrooptic elements are organic electroluminescent diodes and in which, consequently, an organic electroluminescent layer is inserted between the lower electrode and the upper electrode of each diode.  
         [0008]     Referring to the attached  FIG. 1 , the active-matrix electroluminescent display described in that document comprises a substrate  1  and, at each diode, a TFT transistor  12  comprising two current electrodes and a storage capacitor  13  that are components of a circuit for driving this diode, an upper electrical insulation layer  14  covering this circuit, which, as the figure illustrates, has the function of planarizing the features formed by the components  12 ,  13  of the circuit, a lower electrode  16 , serving here as anode, applied to the insulation layer  14 , an electroluminescent layer  17 , an upper electrode  18 , common to all the diodes, serving here as a cathode, and a protective layer  19 . The substrate  1  and the drive/supply circuits form an active matrix  11  for the array of diodes. The upper electrode  18  and the protective layer  19  are transparent to the light emitted by the diodes. The lower supply electrode  16  for each diode is connected to a current electrode of the TFT transistor of the circuit that corresponds to it by means of a via  20 , which in particular passes through the insulation layer  14 .  
         [0009]     Such a structure has several drawbacks: 
        the insulation layer must be relatively thick so that the volume of material applied in order to form this layer may completely fill the features formed by the components of the circuits and the electrodes of the active matrix. Furthermore, this insulating material must be suitable for being applied so as to fill the features, for example by spin coating, which may require the use of a solvent;     when the lower electrode itself provides, as illustrated in  FIG. 1 , contact with the TFT transistor  12  through vias  20 , a replica  21  of this via  20  is transferred to the upper face of this lower electrode and forms a depression relative to the plane surface of the electrode. On the contours of these hollow replicas  21 , the abrupt breaks in slope induce parasitic electrical contacts between thin films, especially those of the diodes, thereby generating leakage currents and degrading the performance of the display.        
 
       SUMMARY OF THE INVENTION  
       [0012]     It is an object of the invention to avoid one and/or other of the aforementioned drawbacks, especially by better planarizing the upper face of the lower electrodes.  
         [0013]     In particular for this purpose, the invention proposes the addition to the lower electrode itself of a planarization function which may even, according to one advantageous embodiment, replace that provided by the insulation layer.  
         [0014]     For this purpose, the subject of the invention is a display comprising a substrate that supports: 
        an active matrix;     an upper electrical insulation layer covering said active matrix; and     an array of electrooptic elements placed on said insulation layer, each including a lower supply electrode applied to said insulation layer ( 14 ;  14 ′), characterized in that said lower supply electrode ( 16 ;  16 ″) of each electrooptic element includes an organic conducting layer ( 161 ;  161 ′) applied directly to said insulation layer ( 14 ;  14 ′) and a metal layer ( 162 ) covering the organic conducting layer.        
 
         [0018]     The active matrix generally comprises an array of circuits for supplying and driving the electrooptic elements, and arrays of electrodes for driving each of said circuits and for supplying these electrooptic elements via said circuits.  
         [0019]     The insulation layer is applied so as to be in contact with the circuits and with the electrodes of this active layer, especially so as to electrically isolate the components of these circuits and these electrodes.  
         [0020]     The insulation layer, which may be structured in the form of several sublayers, is therefore inserted between the active matrix, especially the circuits and the electrodes that it comprises, and the lower electrodes of the electrooptic elements.  
         [0021]     The lower electrodes of the electrooptic elements are in direct contact with the insulation layer.  
         [0022]     Each electrooptic element generally includes an upper electrode. The electrooptic elements are for example electroluminescent diodes or liquid-crystal cells. Each electrooptic element therefore preferably has a layer of electrooptic material such as electroluminescent or liquid-crystal material, which is inserted between its lower electrode and its upper electrode. Preferably, the upper electrodes form one and the same common electrode for supplying all of the electrooptic elements.  
         [0023]     Since the insulation layer directly covers the active matrix, the components of its circuits and its electrodes form features on the substrate that are generally transposed to the top of the insulation layer, especially when this layer has a small thickness compared with the magnitude of these features, and/or especially when this layer has a uniform thickness as a result of the application method used.  
         [0024]     Thanks to the organic conducting layer of the lower electrodes, the features of the active matrix are planarized. This results in lower electrodes of greater planarity than in the prior art and contributes to improving the performance of the electrooptic elements provided with these electrodes.  
         [0025]     The following documents of the prior art U.S. Pat. No. 6,586,764, U.S. Pat. No. 5,723,873 and U.S. Pat. No. 5,719,467 also propose a two-layer lower electrode, one layer of which is also made of an organic conducting material but is inserted, unlike the invention, between the other layer, for example made of ITO, in contact with the substrate, and the electroluminescent layer of the diodes. To fulfil a planarization function, it is important for the thickness of such an organic conducting layer to be greater than the magnitude of the features to be planarized. Such a thickness runs the risk of no longer being compatible with the constructive, especially first-order, interference criteria in the optical resonant cavities that are formed within each diode and make it possible to optimize the level of light extraction. The improvement in planarity of the lower electrodes therefore runs the risk of being at the expense of light extraction, and the overall performance of the diodes will not be improved.  
         [0026]     Preferably, each electrooptic element also includes an upper electrode, and said electrooptic elements are electroluminescent diodes each having at least one organic electroluminescent layer inserted between the lower electrode and this upper electrode. Preferably, for each diode, the lower electrode is reflecting towards the light emitted by this diode and the upper electrode is semireflecting towards the same light and the distance between these electrodes is suitable for forming, between these electrodes, a resonant optical cavity for said emitted light.  
         [0027]     Preferably, said distance is equal to 80 nm or less. This condition expresses a first-order resonance for the emitted light. Since, according to the invention, the organic conducting layer of the lower electrodes is placed outside the resonant optical cavities formed in each diode, construction benefits from the entire desired latitude for obtaining the required planarization effect, without the risk of impairing the tuning of the optical cavity and the light extraction performance. The means for obtaining an optimum optical cavity and the means for obtaining optimum planarization are completely separate. It should be noted that document US2002/025391 makes no mention of this problem of the prior art making the difficult compromise between planarity and light extraction, because the electrooptic elements it deals with are liquid-crystal cells.  
         [0028]     Preferably, each electrooptic element also includes an upper electrode that is transparent or partially transparent to visible light, and the lower electrode of this element is reflecting or partially reflecting for visible light through said element.  
         [0029]     The metal layer covering the organic conducting layer of the lower electrode for supplying each electrooptic element therefore serves for reflecting visible light, something which is not generally possible by using lower electrodes formed from a single organic conducting layer as in documents U.S. Pat. No. 5,705,888 and U.S. Pat. No. 5,719,467 (a variant).  
         [0030]     If the electrooptic elements are liquid-crystal cells, these cells therefore operate by reflection. For example, the display may be of the LCOS (liquid crystal on silicon) type. If the electrooptic elements are electroluminescent diodes, the display is then of the “top-emitting” type, that is to say the light is emitted by the diodes on the opposite side from the substrate.  
         [0031]     The bilayer structure of the lower electrode for supplying each electrooptic element therefore has the following advantages, 
        an organic conducting layer for the planarization;     a metal layer, the metal of which is suitable, especially as regards its work function, for optimizing, in a manner known per se, the electrical properties of the junction with the organic electroluminescent layer, in general via a carrier injection or transport layer that is generally in contact with this metal layer.        
 
         [0034]     This carrier injection or transport layer is therefore inserted between the lower electrode and the electroluminescent layer of each diode. The carriers are electrons if the lower electrode is a cathode, or they are holes if the lower electrode is an anode.  
         [0035]     As a variant, for each diode, the upper electrode is a cathode and the lower electrode is an anode. This is therefore a conventional structure.  
         [0036]     In another variant, for each diode, the upper electrode is an anode and the lower electrode is a cathode. It is therefore an inverted structure.  
         [0037]     Preferably, said display comprises only a single substrate ( 1 ) supporting both said active matrix and the diode array. Thus, the diode array is attached to the single substrate and not, at least in part, also to another “upper” substrate which would be placed on the opposite side from the substrate supporting the active layer. This means that the organic electroluminescent layers and, where appropriate, the carrier transport and/or injection layers are deposited, during fabrication of the display, on the metal layers of the lower electrodes, in which case the organic materials of these layers have not penetrated significantly into the metal of the metal layers. Conversely, if there is an “upper” substrate, it is on this substrate that the various layers of the diodes, including the electrodes, would have been deposited, and the organic materials of the organic layers would then have penetrated into the metal of the metal layers of the lower electrodes.  
         [0038]     The display according to the invention nevertheless may be provided with a protective encapsulation layer, which then covers the upper electrodes, this layer therefore being transparent to the light emitted by the diodes. The diodes are then inserted between the single substrate and this layer. This protective encapsulation layer may be a rigid or flexible sheet.  
         [0039]     Preferably, away from any vias passing through the insulation layer, the insulation layer has a constant thickness that varies by no more than 10% over its area. This insulation layer may be subdivided into several sublayers, which comprise the set of insulating sublayers lying between the upper parts of the circuits or electrode arrays of the active matrix and the lower electrodes of the electrooptic elements. The overall thickness of this layer is then in particular identical at the location of the aforementioned features, and alongside these features. Consequently, the insulation layer therefore provides virtually no planarization function. It appears that the organic conducting layer of the lower electrodes therefore provides almost entirely the planarization function that enhances the performance of the electrooptic elements.  
         [0040]     Preferably, the thickness of said insulation layer is equal to 100 nm or less. Such a low overall thickness of this insulation layer generally no longer allows this layer to provide any planarization function, thereby further enhancing the benefit of the organic conducting layer of the lower electrode of each electrooptic element for obtaining sufficient planarity of the lower electrodes of the electrooptic elements.  
         [0041]     Preferably, said insulation layer is made of inorganic material. As inorganic material, it is possible to use for example silica, silicon nitride, tantalum oxide (Ta 2 O 5 ), or other materials, preferably having a high dielectric permittivity. Inorganic materials are advantageously much more stable than organic materials. Advantageously, an insulating material suitable for providing, as described in documents U.S. Pat. No. 6,597,111 and U.S. Pat. No. 6,765,351 (column 6, lines 20-27), a diffusion barrier function, which prevents the risk of mutual contamination between the drive/supply circuits for the active matrix and the electrooptic elements, is chosen. Given their usual mode of application for forming layers, such inorganic materials are ill-suited for providing a planarization function, thereby further enhancing the benefit of the organic conducting layer of the lower electrode of each electrooptic element for obtaining sufficient planarity of the lower electrodes of the electrooptic elements.  
         [0042]     Preferably, the active matrix comprises an array of circuits for supplying and driving the electrooptic elements, each circuit having a supply output terminal, and the organic conducting layer of the lower supply electrode of each electrooptic element passes through, by means of a via, said insulation layer so as to electrically connect said lower electrode of this electrooptic element to the supply output terminal of one of the circuits for supplying and driving the active matrix. The organic conducting layer of the lower electrodes thus penetrates into each via until coming into contact with a supply output terminal of a circuit. This layer then has two functions—planarization and electrical connection of the output terminals of the circuits to the lower electrodes. Advantageously, this thus dispenses with a specific step of depositing connectors (often called vias) in each of the holes before deposition of the organic conducting layer.  
         [0043]     Each supply/drive circuit generally also includes a supply input terminal connected to a supply electrode of one of the arrays of electrodes for the active matrix. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0044]     The invention will be more clearly understood on reading the description that follows, given by way of non-limiting example, and with reference to the appended drawings in which:  
         [0045]      FIG. 1 , already described, illustrates, in cross section, an active-matrix display of the prior art;  
         [0046]      FIG. 2  illustrates, in cross section, a first embodiment of the invention in which the insulation layer serves for planarizing, as in the prior art; and  
         [0047]      FIG. 3  illustrates, in cross section, a second embodiment of the invention in which the insulation layer is much thinner than in the prior art and no longer serves for planarizing. 
     
    
       [0048]     To simplify the description and to bring out the differences and advantages that the invention affords over the prior art, identical references are used for the elements that fulfil the same functions.  
       DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0049]     The fabrication of a display according to a first embodiment of the invention will now be described with reference to  FIG. 2 .  
         [0050]     An active matrix  11  produced in an identical way to that described in document U.S. Pat. No. 6,780,688 already mentioned, on which an upper insulation layer  14  is formed by applying, as in the prior art and as illustrated in the figure, a sufficiently large volume of insulating material for planarizing the features formed by the circuits and the electrodes (not shown) integrated into the matrix. As in the prior art, at a current electrode for each TFT transistor for modulating the current in each circuit, vias  20  are formed through the upper insulation layer  14  and through other possible subjacent insulation layers. Again as in the prior art, an array of separating barriers  15  defining the locations of the diodes is formed.  
         [0051]     At the location of each diode, a layer of organic conducting material  161  is applied so that this material penetrates to the bottom of the vias  20  and ensures electrical contact of the lower electrodes with a current electrode of the TFT  12  of each circuit for driving the active matrix  11 . An organic material having a suitable conductivity for giving the layer sufficient conductance for the lower electrode function is chosen in a manner known per se.  
         [0052]     The organic conducting material of this layer is preferably chosen from the group formed by the following polymers and their blends: polythiophene, polypyrrole, polyamine, polyaniline and polyacetylene. Where appropriate, these polymers are doped and/or blended with other compounds in a manner known per se in order to obtain the desired level of conductivity.  
         [0053]     Thanks to the use according to the invention of an organic material as lower layer of the lower electrodes of the electrooptic elements of the display, which here are diodes, very much better planarization of these electrodes to that in the prior art is obtained. In particular, it is found that the hollow replicas  21  of the vias  20  have virtually disappeared.  
         [0054]     A metal layer  162  is applied to the organic conducting layer  161  thus deposited, which metal layer therefore is very well planarized.  
         [0055]     Deposited on this metal layer  162  are, as in the prior art, an organic electroluminescent layer  17 , an upper layer  18 , here common to all the diodes, and a protective layer  19 ′.  
         [0056]     At each diode of the display thus obtained, the organic conducting layer  161  and the metal layer  162  form the lower electrode  16 ′ of this diode.  
         [0057]     The organic electroluminescent layer  17  generally comprises several sublayers, including a carrier transport and/or injection sublayer, electrons on the cathode side and holes on the anode side, on either side of a specifically emissive organic sublayer.  
         [0058]     Thus, one of the carrier transport and/or injection sublayers is in direct contact with the metal layer  162  of the lower electrode  16 .  
         [0059]     The metal of the metal layer is chosen, in a manner known per se, so as to obtain the best properties for injecting carriers into the electroluminescent layer  17 , especially into the carrier transport and/or injection sublayer with which this metal is in direct contact. The work function of the metal is a well-known criterion for choosing it.  
         [0060]     Preferably, the thickness of this metal layer  162  is sufficient for obtaining a reflecting effect for the light emitted by the electroluminescent layer  17 . There is therefore an optical cavity between this metal layer  162  and the upper electrode  18 , especially when this upper electrode is semireflecting.  
         [0061]     To improve extraction of the light emitted by the diodes, it is known to adjust the height of this optical cavity and the position of the emissive sublayer in this cavity, as described for example in the documents U.S. Pat. No. 6,505,901, EP 1 154 676, US 2003/122481, WO 2004/034750, EP 1 406 315, EP 1 439 589, EP 1 443 572. Resonant optical cavities with a height of generally around 70 to 80 nm are therefore obtained. This adjustment results in the thickness of the various sublayers lying between the electrodes being fixed so that the sum of these thicknesses therefore does not exceed 70 to 80 nm. This total thickness of 70 to 80 nm does not leave much room for an effective planarization layer. This is why, according the invention, the planarization layer is placed outside the gap between the electrodes. Because, according to the invention, the organic conducting layer  161  of the lower electrode lies outside the optical cavity, unlike the organic layers of the electrodes described in U.S. Pat. No. 6,586,764, U.S. Pat. No. 5,723,873 and U.S. Pat. No. 5,719,467 mentioned above, this organic conducting layer  161  is not involved in tuning the optical cavity. Consequently, the structure benefits from the entire desired latitude for obtaining the required planarization effect, without any risk of impairing the tuning of the optical cavity and the light extraction performance. The means for obtaining an optimum optical cavity and the means for obtaining optimum planarization are completely separate.  
         [0062]     The function of planarizing the organic conducting layer of the lower electrode may even substitute that of the insulation layer  14 , as illustrated by the second embodiment of the invention, which will now be described with reference to  FIG. 3 .  
         [0063]     As illustrated in this figure, this embodiment differs from the first embodiment only in that: 
        the thickness of the upper insulation layer  14 ′, deposited on the active matrix  11 , is much smaller than previously, being in particular 100 nm or less, and/or has an approximately constant value, especially to within 10%, over its entire area. This thickness means in practice the overall thickness of insulating material lying between, on the one hand, the upper electrodes  121 ,  131  integrated into the active matrix  11  of the components  12 ,  13  of the supply/drive circuits and, on the other hand, the organic conducting layer  161 ″ of the lower electrode  16  of the diodes. According to a variant, this insulation layer  14 ′ may be distributed as several insulating sublayers; and     the thickness of the organic conducting layer  161 ′ of the lower electrode  16 ″ of the diodes is however higher than previously, preferably equal to 100 nm or more, and has a sufficient volume and a thickness that varies so as to planarize the features formed, especially by the components  12 ,  13  of the drive/supply circuits for the active matrix  11 .        
 
         [0066]     As organic conducting material for this layer  161 ′, a conducting polymer suitable for planarization is chosen in a manner known per se. Preferably, PEDOT-PSS, that is to say a blend of poly(3,4-ethylenedioxythiothene) and polystyrenesulphonate, is used. This conducting polymer is known for being applicable for liquid processing—the layer of organic conducting material is therefore applied by liquid processing, thereby making it possible to achieve the desired filling of the features easily.  
         [0067]     In a first variant of the first or second embodiment, the step of producing the separating barriers  15  is eliminated. In this case, on the insulation layer  14 ,  14 ′, a continuous organic layer with no holes (that is to say no “apertures”—called a “full layer”) is deposited over the entire active surface of the display, and produced in this layer are conducting polymer features electrically isolated from one another so as to end up with the same features of organic conducting layers  161 ,  161 ′ as previously. For this purpose, one or other of the methods described in documents U.S. Pat. No. 5,705,888 and EP 0 727 100 is used. As explained in those documents, it does not matter whether the starting material is an insulating continuous organic layer with the zones corresponding to the lower electrodes of the diodes made conducting, or, conversely, a conducting continuous organic layer with the inter-diode zones lying between the zones corresponding to the lower electrodes of the diodes made insulating.  
         [0068]     A metal layer  162  is applied to the organic conducting layer  161  thus deposited, said metal layer being applied for example by vacuum deposition with interposition of a suitable mask positioned so as to deposit metal only at the location of the diodes. It is a known method for depositing electrodes.  
         [0069]     The procedure then continues as described above.  
         [0070]     A display in which the organic conducting layer  161 ′ is continuous is therefore obtained. Despite the continuity of this layer, it is possible to maintain sufficient electrical insulation between the lower electrodes using an organic material exhibiting anisotropic conductivity, being substantially more conducting transversely than along its surface, and/or for maintaining a sufficient distance between the elements of the metal layer  162  that are specific to each electrode.  
         [0071]     One advantageous means for obtaining good electrical insulation between the lower electrodes consists in eliminating those parts of the organic conducting layer that lie between the elements of the metal layer  162 : after the elements of the metal layer  162  have been deposited, a plasma etching treatment is applied for example, which degrades and eliminates the organic conducting layer parts lying between the elements of the metal layer  162  without significantly affecting the elements of the metal layer  162  and the subjacent elements of the organic conducting layer  161 ′.  
         [0072]     This variant, with its subvariants, has the advantage of eliminating the step of producing separating barriers and of also resulting in better planarity of the upper surface of the display.  
         [0073]     The present invention has been described with reference to a top-emitting active-matrix electroluminescent display. It is obvious to a person skilled in the art that it may apply to other types of display without departing from the scope of the claims appended hereto.