Transparent supported electrode for OLED

Abstract

A supported transparent electrode for an OLED, includes, in succession: (i) a transparent substrate made of mineral glass; (ii) a scattering layer formed from a high-index enamel containing at least 30% by weight Bi.sub.2O.sub.3; (iii) a barrier layer of at least one dielectric metal oxide chosen from the group consisting of Al.sub.2O.sub.3, TiO.sub.2, ZrO.sub.2 and HfO.sub.2, deposited by ALD; and (iv) a layer of a transparent conductive oxide (TCO).

Claims

1. Supported transparent electrode for an OLED, comprising, in succession: (i) a transparent substrate made of mineral glass; (ii) a scattering layer formed from a high-index enamel containing at least 30% by weight Bi2O3; (iii) a barrier layer of at least one dielectric metal oxide chosen from the group consisting of Al2O3, TiO2, ZrO2 and HfO2, deposited by ALD; and (iv) a layer of a transparent conductive oxide (TCO), wherein the barrier layer deposited by ALD comprises a plurality of Al2O3 layers in alternation with layers of oxides of higher indices (n>2) chosen from TiO2, ZrO2, and HfO2.

2. The electrode according to claim 1, further comprising a metal grid under or over the TCO layer and making direct contact therewith.

3. The electrode according to claim 1, wherein the barrier layer deposited by ALD is comprised between 5 and 200 nm in thickness.

4. The electrode according to claim 3, wherein the barrier layer deposited by ALD is comprised between 10 and 100 nm in thickness.

5. The electrode according to claim 1, wherein the high-index enamel forming the scattering layer contains elements that scatter light, dispersed through the thickness of the layer.

6. The electrode according to claim 1, wherein the interface between the high-index enamel and the underlying transparent substrate made of mineral glass has a roughness profile with an arithmetic mean deviation R.sub.a at least equal to 0.1 m.

7. The electrode according to claim 6, wherein the arithmetic mean deviation R.sub.a is comprised between 0.2 and 5 m.

8. The electrode according to claim 7, wherein the arithmetic mean deviation R.sub.a is comprised between 0.3 and 3 m.

9. OLED comprising an electrode according to claim 1.

10. Process for manufacturing a supported transparent electrode for an OLED, comprising the following successive steps: (a) providing a transparent substrate made of mineral glass bearing, on one of its faces, a scattering layer formed from a high-index enamel containing at least 30% by weight Bi2O3; (b) forming, by atomic layer deposition (ALD), a dielectric metal oxide barrier layer that comprises a plurality of Al2O3 layers in alternation with layers of oxides of higher indices (n>2) chosen from TiO2, ZrO2, and HfO2 on the high-index enamel layer, said dielectric metal oxide barrier layer making direct contact with the high-index enamel layer; and (c) forming a layer of a transparent conductive oxide (TCO) on the dielectric metal oxide barrier layer.

11. The process according to claim 9, further comprising a step (d) of forming a metal grid making direct contact with the transparent conductive oxide layer, this the step (d) comprising at least one step of acid etching.

12. The process according to claim 8, wherein the step (d) is carried out after step (b) and before step (c) so that the metal grid makes contact both with the dielectric metal oxide barrier layer and with the TCO layer.

13. The process according to claim 8, wherein the step (d) is carried out after step (c) so that the metal grid makes contact with the TCO layer but not with the dielectric metal oxide barrier layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows two scanning electron microscope (SEM) micrographs of a partially open bubble (bubble partially open) and a completely open bubble (open bubble) on the surface of an enamel;

(2) FIG. 2 shows, on the left, SEM micrographs of two surface defects (air bubbles having solidified as they burst) observed on the enamels protected by an ALD layer and, on the right, SEM micrographs of the same surface defects after the etching step;

(3) FIG. 3 shows two SEM micrographs of surface defects observed after acid etching, and

(4) FIG. 4 shows pinholes in the form of hollowed-out craters.

(5) The substrate made of mineral glass may be any thickness compatible with the envisaged use, Conventionally, glass sheets having a thickness comprised between 0.3 and 5 mm and in particular between 0.7 and 3 mm are used. However, the use of ultra-thin glass sheets having smaller thicknesses, typically comprised between 50 nm and 300 nm, is also envisageable, providing that the mechanical problems involved with forming an enamel layer, in this case the high-index enamel layer, on glass sheets of such small thicknesses are solved.

(6) The substrate is surmounted with a scattering layer formed from a high-index enamel containing at least 30% by weight Bi.sub.2O.sub.3. The expression high-index is here understood to mean an enamel having a refractive index (at =550 nm) at least equal to 1.7 and preferably comprised between 1.8 and 2.2.

(7) The scattering layer (ii) plays the role of an internal light-extraction layer (IEL).

(8) For a long time, in the field of OLEDs, it has been known that only a small fraction of the light produced by the light-emitting layer is emitted towards the exterior, through the transparent anode and the glass substrate. Specifically, as the optical index of the glass substrate (n.sub.glass=1.5) is lower than that of the organic layers (n=1.7-1.8) and of the transparent anode (n=1.9 to 2.1), most (about 50%) of the light is trapped in these high-index layers as in a waveguide and absorbed after a certain number of reflections. An analogous effect occurs at the interface between the glass of the substrate (n.sub.glass=1.5) and the surrounding air (n.sub.air=1.0), trapping about 20% of the light emitted by the light-emitting layer.

(9) It is known to decrease this trapping of the light (total internal reflection) in the high-index layers of an OLED by inserting, between the glass substrate and the transparent anode, a means for extracting light, for example formed by a high-index enamel containing scattering particles or by an interface that is rough enough to be scattering, which interface is planarized by a high-index enamel layer.

(10) The expression scattering layer therefore encompasses in the present invention: an enamel layer of high refractive index in which scattering elements are dispersed; and a rough interface between two media of different indices, typically the surface of the glass having a relief of a certain roughness covered with a high-index enamel layer.

(11) In one embodiment, the high-index enamel forming the scattering layer therefore contains elements that scatter light, dispersed through the thickness of the layer. These scattering elements have a higher or lower refractive index than the index of the enamel. In order to scatter light, these elements must be non-negligible in size relative to the wavelength of the light to be extracted, and are for example between 0.1 and 5 m and preferably between 0.4 and 3 m in size. These scattering elements may for example be solid particles added to the glass frit before it is melted, crystals formed when the frit is melted or even air bubbles formed during the step of melting the frit and trapped in the solidified enamel.

(12) In another embodiment, the scattering effect results from the roughness of the interface between the high-index enamel (n1.7) and the underlying medium of lower index (glass substrate or low-index layer formed on the surface of the glass). The interface between the high-index enamel and the underlying medium of lower index (substrate) preferably has a roughness profile with an arithmetic mean deviation R.sub.a at least equal to 0.1 m, preferably comprised between 0.2 and 5 m and in particular between 0.3 and 3 m.

(13) In the case where an intermediate layer of low refractive index (n<1.6) is provided between the glass substrate and the enamel, for example a barrier layer protecting the high-index enamel from diffusion of alkali-metal ions originating from the substrate, it is the interface between this low-index layer and the high-index enamel that has the relief with this roughness profile.

(14) Of course, it is possible to combine these two embodiments of the scattering layer, for example by introducing scattering elements, such as air bubbles, into a high-index enamel deposited on a rough glass surface, the essential point being that the upper face of the IEL must coincide with the upper face of the high-index enamel.

(15) There are a number of glass compositions allowing high-index enamels to be obtained. The present invention particularly focuses on enamels having high bismuth contents, which have a quite low chemical resistance to acids, resulting in leakage currents and pinholes as explained in the introduction.

(16) The high-index enamel of the present invention contains at least 30% by weight, preferably at least 50% by weight and in particular at least 65% by weight Bi.sub.2O.sub.3. These enamels are known and for example described in the international patent application WO2013/187736, and in the patent applications PCT/FR 2014/050370 and FR 1 360 522, in the name of the Applicant, which were still unpublished at the time of filing of the present application.

(17) The high-index enamel for example contains from 55 to 84% by weight Bi.sub.2O.sub.3, as much as about 20% by weight BaO, from 5 to 20% by weight ZnO, from 1 to 7% by weight Al.sub.2O.sub.3, from 5 to 15% by weight SiO.sub.2, from 5 to 20% by weight B.sub.2O.sub.3 and as much as 0.3% by weight CeO.sub.2.

(18) In the present invention a dielectric metal oxide layer (layer (iii)) is deposited by ALD (atomic layer deposition) on the high-index enamel described above. This deposition is preferably carried out directly on the surface of the high-index enamel. Atomic layer deposition is a well-known method of allowing extremely thin, uniform and impermeable layers to be formed.

(19) A gaseous precursor, brought into contact with a surface, adsorbs thereon in the form of a monolayer by chemisorption or physisorption. After the precursor gas has been purged, a second gaseous component, capable of reacting with the adsorbed precursor, is admitted into the chamber. After the reaction, the chamber is purged again and the adsorption-purge-reaction-purge cycle may restart.

(20) The table below gives a few examples of precursors and reactants allowing the dielectric metal oxides of the layer (iii) of the present invention to be formed.

(21) TABLE-US-00001 Dielectric Gaseous metal oxide Gaseous precursor reactant Al.sub.2O.sub.3 Al(CH.sub.3).sub.3 H.sub.2O ZrO.sub.2 Tetrakis(ethylmethylamino)zirconium H.sub.2O Zr[(N(CH.sub.3)(C.sub.2H.sub.5)].sub.4 TiO.sub.2 Tetrakis(dimethylamino)titanium H.sub.2O Ti[N(CH.sub.3).sub.2].sub.4

(22) The reader may also refer to review articles such as the article by Markku Leskel et al. Atomic layer deposition (ALD): from precursors to thin film structures, Thin Solid Films, 409 (2002) 138-146 and the article by Steven M. George entitled Atomic Layer Deposition: An Overview, Chem. Rev. 2010, 110, 111-131, which give many examples of precursor/reactant systems.

(23) The barrier layer may be a simple layer consisting of a single metal oxide, or indeed a complex layer formed from a plurality of successive sublayers of various metal oxides, all deposited by ALD.

(24) In one preferred embodiment of the present invention, the ALD barrier layer comprises a plurality of Al.sub.2O.sub.3 layers (n1.7) in alternation with layers of oxides of higher indices (n>2) preferably chosen from TiO.sub.2, ZrO.sub.2, and HfO.sub.2. Specifically, aluminium oxide has the advantage of being very resistant to the strong acids, such as aqua regia, used to etch metals. However, its relatively low refractive index relative to that of the organic layers of the OLED and the optical loss that results therefrom prohibits the use of thick Al.sub.2O.sub.3 monolayers. By alternating Al.sub.2O.sub.3 layers with layers of TiO.sub.2, ZrO.sub.2 or HfO.sub.2 it is possible to increase the overall thickness of the ALD layer without increasing optical losses.

(25) The overall thickness of the ALD layer, whether it is simple or complex, is preferably comprised between 5 and 200 nm and in particular between 10 and 100 nm. When it is a question of a complex layer comprising in alternation sublayers of Al.sub.2O.sub.3 and sublayers of higher index, such as sublayers of TiO.sub.2, ZrO.sub.2 and HfO.sub.2, the thickness of each of the sublayers is preferably comprised between 1 and 50 nm and in particular between 2 and 10 nm. The number of sublayers may be comprised between 2 and 200, preferably between 3 and 100 and in particular between 5 and 10. The number of Al.sub.2O.sub.3 sublayers is preferably comprised between 2 and 5 and it is in particular equal to 2 or 3.

(26) In one preferred embodiment, the two external layers of the stack of sublayers are Al.sub.2O.sub.3 layers that ensure a good contact with adjacent materials.

(27) Under an electron microscope, a layer of a dielectric metal oxide deposited by ALD may be easily differentiated from a layer deposited by cathode sputtering. It is characterized, as is known, by an extremely uniform thickness; a perfect continuity, even for small thicknesses; and by a high conformity to the relief of the underlying substrate, even over surfaces with a very pronounced relief.

(28) The actual transparent electrode is located above the layer formed by ALD. This electrode is composed of a TCO layer, generally deposited by cathode sputtering, and a metal grid, these two structures making contact with each other. As explained above, the metal grid may be under the TCO layerbetween the TCO layer and the ALD layeror on the TCO layer.

(29) The present invention is not particularly limited to certain grid structures or grid dimensions. The nature of the metal forming the grid is also not critical. It is however essential for the grid to be formed using a process comprising a step of acid etching a metal layer, typically through a mask. Specifically, as explained in the introduction, the Applicant has observed that it is this acid etching step that seems to be the root cause of the defects (leakage currents, pinholes) observed in the finished product. Processes for forming such grids by photolithography and acid etching are known.

(30) The Applicant has, rarely, observed pinholes giving rise to leakage currents even when the OLED comprises no metal grid. Examination under electron microscope of the aspect of these pinholes has shown that they also evidently correspond to hollowed-out craters (see FIG. 4). The Applicant assumes that chemical etching of these surface defects takes place during the acid etching of the TCO layer in regions where the surface relief is too pronounced to be suitably protected by a thin (1 to 2 m) layer of photoresist. Using a barrier layer deposited by ALD between the enamel of the IEL layer and the TCO effectively prevents this type of pinhole.

(31) The TCO layer is deposited on the high-index enamel protected by the dielectric metal oxide layer by conventional deposition processes such as magnetron cathode sputtering, sol-gel processes or pyrolysis (CVD).

(32) In principle, any transparent or translucent conductive oxide having a sufficiently high refractive index, close to the average index of the organic multilayer of an OLED (HTL/LE/ITL), may be used for this electrode layer. Mention may be made, by way of example of such materials, of transparent conductive oxides such as aluminium-doped zinc oxide (AZO), indium-doped tin oxide (ITO), tin zinc oxide (SnZnO) or tin dioxide (SnO.sub.2). These materials advantageously have an absorption coefficient very much lower than that of the organic materials forming the HTL/LE/ITL multilayer, preferably an absorption coefficient lower than 0.005 and in particular lower than 0.0005. ITO will preferably be used. The thickness of the transparent conductive oxide layer is typically comprised between 50 and 200 nm.

(33) The process for manufacturing a transparent supported electrode for an OLED, of the present invention, comprises at least the three following successive steps: (a) providing a transparent substrate bearing, on one of its faces, a scattering layer formed from a high-index enamel containing at least 30% by weight Bi.sub.2O.sub.3; (b) forming, by atomic layer deposition (ALD), on the high-index enamel and making direct contact therewith, a layer (barrier layer) of at least one dielectric metal oxide chosen from the group consisting of Al.sub.2O.sub.3, TiO.sub.2, ZrO.sub.2 and HfO.sub.2; and (c) forming a TCO layer on the dielectric metal oxide layer (b).

(34) When the process according to the invention only comprises these three steps, an intermediate product (substrate/high-index enamel/ALD layer/TCO layer) is obtained that is intended to subsequently receive the metal grid.

(35) The process for manufacturing a complete supported transparent electrode according to the invention will of course furthermore comprise an additional step (step (d)) of forming a metal grid making direct contact with the transparent conductive oxide layer, this step (d) comprising at least one step of acid etching.

(36) This step of acid etching is carried out on a continuous metal layer covered with a mask created for example by screen printing or photolithography, the acid serving to remove the metal in certain zones not covered by the mask, so as to form the apertures of the grid.

(37) The thickness of the metal layer, and therefore the height of the resulting grid, its about a few hundred manometers, typically from 0.5 to 1 m and preferably from 0.6 to 0.8 m. The width of the strands of the grid is generally comprised between 10 m and about 100 m.

(38) In a first embodiment of the process according to the invention, step (d) is carried out after step (c) so that the metal grid makes contact with the TCO layer but not with the metal oxide barrier layer.

(39) In a second embodiment, step (d) is carried out after step (b) and before step (c) so that the metal grid makes contact both with the dielectric metal oxide barrier layer and with the TCO layer.

(40) The metal grid always forms a relief, because even when the TCO layer is deposited on the metal grid, as in the second embodiment, it is obvious, on account of the respective thicknesses of these two structures (0.05 to 0.2 m for the TCO layer and 0.5 to 1 m for the grid), that the TCO layer will not cover and planarize this relief.

(41) In any case, the metal grid must therefore be covered with a passivating layer that of course leaves the apertures etched by the acid, which form the illuminated zones of the final OLED, unobstructed. The passivation of electrode grids with a passivating layer also forms part of the general knowledge of those skilled in the art of OLED manufacture.

(42) Before the light-emitting layers are applied, the OLED substrate is advantageously covered in a known way with an organic hole injection material such as PEDOT/PSS (polyethylenedioxythiophene/poly(styrene sulfonate)) that allows the relief of the substrate described above to be planarized.

EXAMPLE

(43) On a 0.7 mm-thick sheet of mineral glass, a high-index enamel layer was deposited by melting a glass frit having the following composition (in % by weight): 65% Bi.sub.2O.sub.3, 12.6% ZnO, 12.9% SiO.sub.2, 2.6% Al.sub.2O.sub.3, and 6.9% B.sub.2O.sub.3.

(44) A paste of glass frit in an organic medium (75% by weight frit, 22% by weight volatile organic solvent and 3% ethyl cellulose) was deposited by screen printing and dried (about 20 minutes at 130 C.); the ethyl cellulose was removed with a 20 minute-long heat treatment at 430 C. and then the frit was heated to 540 C. for 10 minutes. This melting step was carried out at atmospheric pressure which led many air bubbles to form in the enamel layer. The high-index enamel layer thus formed contained surface defects due to air bubbles solidifying as they burst.

(45) FIG. 1 shows two scanning electron microscope (SEM) micrographs of a partially open bubble (bubble partially open) and a completely open bubble (open bubble) on the surface of the enamel.

(46) Next, on two substrate samples bearing this high-index enamel, a layer of Al.sub.2O.sub.3 was deposited with a thickness of 10 nm and 50 nm, respectively. FIG. 2 shows, on the left, SEM micrographs of two surface defects (air bubbles having solidified as they burst) observed on these enamels protected by the ALD layer.

(47) Next, the same substrates were subjected to a step of acid etching in a phosphoric acid solution of pH<1 for 100 seconds at a temperature of 45 C.

(48) FIG. 2 shows, on the right, SEM micrographs of the same surface defects after the etching step. It may be seen that their aspect is strictly identical to that before the etching.

(49) By way of comparison, FIG. 3 shows two SEM micrographs of surface defects observed after acid etching (in a phosphoric acid solution of pH<1 for 100 seconds at a temperature of 45 C.) in a metal layer deposited on an ITO anode (about 150 m-thick), itself deposited on a 100 nm-thick SiON barrier layer on an enamel of identical composition to above, the ITO anode and the barrier layer being deposited by magnetron cathode sputtering.

(50) These defects (same magnification as FIG. 2) are considerably larger than the original air bubbles.