Printed electroluminescent device

CROSS-RELATION TO OTHER APPLICATIONS

[0001] None

FIELD OF THE INVENTION

[0002] The present disclosure relates to an electroluminescent device, produced by a printing method. More specifically, the electroluminescent phosphor particles of the electroluminescent phosphor layer are covalently bonded with the polymer chain of the polymer conducting layer.

BACKGROUND OF THE INVENTION

[0003] Electroluminescent devices are known for a long time. In a typical electroluminescent device, electroluminescent phosphors are sandwiched between two conducting layers. Supplying AC current to the electroluminescent device generates light. The light come out of the device through one of the two conducting layers, which is transparent to the visible light as mentioned in US Patent number US748274 B2, US 7148623 B2, US 6926972 B2 and in European Patent number EP 1374644 Bl. A prior- art electroluminescent device is shown in Figure 1. A transparent conductor layer 130 is provided on top of a transparent substrate 120. The transparent conductor layer 130 can be indium-tin oxide, fluorine doped tinoxide or aluminum doped zincoxide. The transparent conductor layer 130 can also be a conjugated polymer e.g. poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), polyaniline (PANI) etc. An electroluminescent phosphor layer 140 is provided on top of the transparent conductor layer 130. On top of the electroluminescent phosphor layer 140 is provided another conductor layer 150, which can be transparent or non-transparent. The light come out of the device through the transparent substrate 120. Transparent glass and plastic have been used regularly as a transparent substrate 120.

[0004] In prior-art, in order to deposit electroluminescent device on a non-transparent substrate e.g. paper, a different device structure is used, as shown in Figure 2. Here, a non- transparent conductor layer 250 is provided on top of a non-transparent substrate. An example of non-transparent conductor layer 250 is a silver layer. An electroluminescent phosphor layer 240 is provided on top of the non-transparent conductor layer 250. On top of the electroluminescent phosphor layer 240 is provided another conductor layer 230, which is transparent or semi-transparent to visible light. The preferred transparent conductor layer 230 is PEDOT:PSS, as it is available in the form of ink and it can easily be printed. In the electroluminescent device 200 of Figure 2, the light come out of the device through the transparent conductor layer 230. The problem in this device structure is delamination of PEDOT:PSS layer 230 from the electroluminescent phosphor layer 240 over time. The electroluminescent phosphor layer 240 is deposited on top of the non-transparent conductor layer 250 by using a solvent based electroluminescent phosphor ink. The electroluminescent phosphor ink comprises a hydrophobic polymeric binder, electroluminescent phosphor particles and at least one organic solvent. Therefore, after drying out the organic solvent, the top surface of the electroluminescent phosphor layer 240 becomes hydrophobic in nature. The PEDOT:PSS layer 230 is provided on top of the electroluminescent phosphor layer 240 by printing a water based PEDOT:PSS ink. The water based PEDOT:PSS ink is hydrophilic in nature. Therefore, the adhesion of PEDOT:PSS layer 230 on top of the electroluminescent phosphor layer 240 layer becomes weaker over time. This leads to a decrease in the performance of the electroluminescent device.

SUMMARY OF THE INVENTION

[0005] The present invention relates to an electroluminescent device produced by printing method. A bottom conducting layer is provided on a substrate. An electroluminescent phosphor layer is provided on top of the bottom conducting layer. The electroluminescent phosphor layer comprises at least one phosphor particle and at least one anchoring molecule. The anchoring molecule comprises at least one first functional group and at least one second functional group. The at least one first functional group is covalently bonded to a surface of the at least one phosphor particle. A polymeric conducting layer is provided on top of the electroluminescent phosphor layer. The polymeric conducting layer comprises at least one polymer chain. The at least one polymer chain comprises at least one third functional group. Some of the at least one third functional group are covalently bonded to some of the at least one second functional group. The anchoring molecule starts reacting with the surface of the phosphor particles and with the polymer chain just after the printing step of the polymeric conducting layer i.e. during the drying process of the polymeric conducting layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 is a cross sectional view of a prior art electroluminescent device.

[0007] FIG. 2 is a cross sectional view of another prior art electroluminescent device.

[0008] FIG. 3 is an electroluminescent device on a substrate, in accordance with an aspect of the present invention.

[0009] FIG. 4 is a flow chart showing a manufacturing process to produce an electroluminescent device according to one exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[00010] The invention will now be described in detail. Drawings and examples are provided for better illustration of the invention. It will be understood that the embodiments and aspects of the invention described herein are only examples and do not limit the protector's scope of the claims in any way. The invention is defined by the claims and their equivalents. It will be understood that features of one aspect or embodiment of the invention can be combined with the feature of a different aspect or aspects and/or embodiments of the invention.

[00011] The terms "print," "printability," "printing," "printable" and "printed", as used in this disclosure, refer to production methods using functional inks. More specifically, these production methods include, but are not limited to, screen-printing, stenciling, flexography, gravure, off-set, thermal transfer and ink-jet printing. These printing methods can be roll-to-roll or sheet-fed or manual. The term "ink" as used in this disclosure refers to a material that is in liquid or semi- solid or, paste form. It will be understood that, after printing of an ink on a surface, a drying or curing process may be required to convert the ink into a solid or a gel form. Typically, heat and/or radiation are used for the drying or curing processes. The drying or curing processes can also be self-activated. The term "functional group" as used in this disclosure refers to a specific group of atoms or bonds within a molecule or a polymer that are responsible for a specific chemical reaction. For example, in a conjugated polymer double bonds between carbon atoms are called functional groups, which may participate in a radical reaction. A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms e.g. the bond between hydrogen atom and the oxygen atom in a water molecule is a covalent bond.

[00012] The present invention is about an electroluminescent device, produced by a printing method. More specifically, the electroluminescent phosphor particles of the electroluminescent phosphor layer are covalently bonded with the polymer chain of the polymer conducting layer.

[00013] Figure 3 illustrates a cross-sectional view of an electroluminescent device 300. The electroluminescent device 300 includes a substrate 320. The substrate 320 can be made of, but not limited to, plastic, paper, wood, textile, glass or a laminate of different material. The substrate 320 can be transparent, semi-transparent or opaque to visible light. It can be mechanically flexible or mechanically rigid. On top of the substrate 320 is printed a bottom conducting layer 350. In one non-limiting aspect, the bottom conducting layer 350 is produced by printing and drying a silver particles based ink. An example of the silver particles based ink is DuPont 5028 silver conductor. On top of the bottom conducting layer 350 is printed an electroluminescent phosphor layer 340. The electroluminescent phosphor layer 340 is produced by printing an electroluminescent phosphor ink. The electroluminescent phosphor ink comprises at least one phosphor particle 333, at least one polymeric binder, at least one organic solvent and at least one anchoring molecule 335. After printing the electroluminescent phosphor ink, the printed layer is dried at a temperature > 70 °C. This drying process removed organic solvent. Therefore, the dried electroluminescent phosphor layer 340 comprises at least one phosphor particle 333, at least one polymeric binder and at least one anchoring molecule 335. The phosphor particle 333 is generally doped oxide particles in a size range of 5 micrometers to 40 micrometers. In some special cases the phosphor particle 333 is coated with aluminum oxide layer, in order to increase working life of the electroluminescent device. In both cases - coated or uncoated phosphor particle - the surface 334 of the phosphor particle 333 comprises some hydroxy functional groups. In a non-limiting example, the polymeric binder can be polystyrene, poly(methyl methacrylate) PMMA, ethyl cellulose, polyvinylidene fluoride etc. The organic solvent can be a solvent, but not water, that dissolves the polymeric binder.

[00014] The anchoring molecule 335 comprises at least one first functional group 336 and at least one second functional group 337. In a non-limiting example, the anchoring molecule 335 can be selected from the group of 3-glycidyloxypropyl trimethoxysilane, 3-glycidyloxypropyl triethoxysilane, 3-(trimethoxysilyl)propyl methacrylate, 3-(trimethoxysilyl)-l-propanethiol or a combination thereof. In all these examples of the anchoring molecule 335, alkoxysilyl works as the first functional group 336. For example, in 3-glycidyloxypropyltriethoxysilane, the three ethoxysilyl functional groups work as the first functional group 336 and one epoxy functional group works as the second functional group 337. In 3-(trimethoxysilyl)propyl methacrylate, the three methoxysilyl functional groups work as the first functional group 336 and one methacrylate functional group works as the second functional group 337. In 3- (trimethoxysilyl)-l-propanethiol, the three methoxysilyl functional groups work as the first functional group and one thiol functional group works as the second functional group. The anchoring molecule 335 is selected in such a way the at least one second functional group is acrylate, thiol, methacrylate, vinyl, allyl, epoxy or a combination thereof. In a non-limiting example, the weight ratio of the phosphor particle 333 to anchoring molecule 335 is greater than 20. In the dried electroluminescent phosphor layer 340, most of the anchoring molecules 335 are unreacted till the top polymeric conducting layer 330 is printed on top of the electroluminescent phosphor layer 340. The polymeric conducting layer 330 is produced by printed a polymeric conducting ink. The polymeric conducting ink comprises at least one polymer chain 331 wherein the at least one polymer chain 331 comprises at least one third functional group 332. The polymer chain 331 is a conjugated polymer, which is used to conduct electricity. The polymeric conducting ink is water based. In a non-limiting example, the polymeric conducting ink is water based dispersion of PEDOT:PSS. The polymer chain 331 in PEDOT:PSS are PEDOT and PSS. PSS polymer chain comprises some unreacted sulfonate and/or sulfonic acid function group, which work here as the third functional group 332. The third functional group 332 can be selected from a group of thiol, sulfonate, carboxylate, sulfonic acid, carboxylic acid, primary amine, secondary amine or a combination thereof.

[00015] If 3-glycidyloxypropyltriethoxysilane is used as the anchoring molecule in the electroluminescent phosphor layer 340, printing of the water passed PEDOT:PSS ink on top of the electroluminescent phosphor layer 340 causes the conversion of three ethoxysilyl functional groups into three silanol functional groups (Si-OH). The generated silanol functional groups are very reactive and some of these silanol functional groups react with the hydroxy groups of the surface 334 of the phosphor particle 333. The water based PEDOT:PSS ink causes another reaction - some unreacted sulfonate and/or sulfonic acid function group of PSS polymer chain 331 react with some of the epoxy functional group of 3-glycidyloxypropyltriethoxysilane. These two reactions take place during the drying step of the printed PEDOT:PSS ink. The drying step of the PEDOT:PSS ink removes water from the printed ink layer. This way 3- glycidyloxypropyltriethoxysilane servers here as an anchoring agent to strongly connect the electroluminescent phosphor layer 340 and the polymeric conducting layer 330.

[00016] If 3-(trimethoxysilyl)-l-propanethiol is used as the anchoring molecule 335 in the electroluminescent phosphor layer 340, printing of the water based PEDOT:PSS ink on top of the electroluminescent phosphor layer 340 causes the conversion of three methoxysilyl functional groups into three silanol functional groups (Si-OH). The generated silanol functional group are very reactive and some of these silanol functional group react with the hydroxy groups of the surface 334 of the phosphor particle 333. Some of the double bonds of PEDOT polymer chain 331 react with some of the thiol group of 3-(trimethoxysilyl)-l-propanethiol through 'click' reaction mechanism.

[00017] If 3-glycidyloxypropyltriethoxysilane is used as the anchoring molecule 335 in the electroluminescent phosphor layer 340, printing of the water based doped polyaniline (PANI) ink on top of the electroluminescent phosphor layer 340 causes the conversion of three ethoxysilyl functional groups into three silanol functional groups (Si-OH). The generated silanol functional group are very reactive and some of these silanol functional group reacts with the hydroxy groups of the surface 334 of the phosphor particle 333. The water based PANI ink causes another reaction - some amine function group of polyaniline polymer chain 331 react with some of the epoxy functional group of 3-glycidyloxypropyltriethoxysilane.

[00018] In a non-limiting example, a dielectric layer is also provided between the bottom conducting layer 350 and the electroluminescent phosphor layer 340. The light from the electroluminescent device 300 comes out through the polymeric conducting layer 330.

[00019] The printed electroluminescent device 300 be produced according to the following exemplary process, as illustrated in Figure 4:

[00020] In the step 410, a bottom conducting ink is printed on a substrate 320.

[00021] In the step 420, the bottom conducting ink is solidified into a bottom conducting layer 350 by drying out the solvent present in the printed layer and/or by curing the printed layer with ultraviolet radiation. [00022] In the step 430, an electroluminescent phosphor ink is printed on top of the bottom conducting layer 350, wherein the electroluminescent phosphor ink comprises at least one phosphor particle 333 and at least one anchoring molecule 335, wherein the anchoring molecule 335 comprises at least one first functional group 336 and at least one second functional group 337;

[00023] In the step 440, the electroluminescent phosphor ink is solidified into an electroluminescent phosphor layer 340 by drying out the solvent present in the printed layer and/or by curing the printed layer with ultraviolet radiation.

[00024] In the step 450, a polymeric conducting ink is printed on top of the electroluminescent phosphor layer 340, wherein the polymeric conducting ink comprises at least one polymer chain 331 wherein the at least one polymer chain 331 comprises at least one third functional group 332.

[00025] In the step 460, the polymeric conducting ink is solidified into a polymeric conducting layer 330 by drying out water present in the polymeric conducting ink. During the solidifying process of the polymeric conducting ink some of the at least one first functional group 336 covalently bind with a surface 334 of the at least one phosphor particle 333 and some of the at least one second functional group 337 covalently bind with some of the at least one third functional group 332.

[00026] In a non-limiting example, a dielectric ink is printed on top of the bottom conducting layer 350 and the dielectric ink is solidified. The electroluminescent phosphor ink is then printed on top of the solidified dielectric layer.