Method of producing PEDOT:PSS based electrodes

CROSS-RELATION TO OTHER APPLICATIONS

[0001] None

FIELD OF THE INVENTION

[0002] The present disclosure relates to a production method of a strongly adhered PEDOT:PSS layer on a surface of a substrate.

BACKGROUND OF THE INVENTION

[0003] Printed electronics is a field in which electronic devices are produced by printing methods and by using inks. One of the most commonly used ink in printed electronics is an aqueous dispersion of Poiy(3,4-ethylenedioxythiophene:poly(styrenesulfonate) (hereinafter referred to as PEDOT:PSS). PEDOTrPSS is used as a transparent and conducting electrode. It is also used, but not limited to, in electrochromic dev ices. Water is most commonly used solvent for PEDOT:PSS ink and the content of water in the ink is more than 95% by w eight. Water based PEDOTrPSS ink is deposited on a surface by using a printing method or by using a coating method. After that w ater is removed by a drying process. A dried layer of PEDOT:PSS is left on the surface. Di fferent additives are used in PEDOTrPSS ink to lower its surface ten ion in order to make PEDOT:PSS adhesion stronger on the surface, as mentioned in US 20 160056397 A l and EP 20 15 135 B 1 . In order to further increase the adhesion of PEDOT:PSS on the surface, the surface is plasma or corona treated before depositing the PEDOT:PSS ink. Corona and/or plasma treatment of a plastic substrate generate various reactive functional groups hydroxy 1. carboxyi, carbonyl, hydropero ide etc. - on the surface of the plastic.

Plasma are/or corona treatment of plastic substrate are very common techniques to increase adhesion of ink on a surface. Despite al l these efforts, the adhesion of PEDOTrPSS on a plastic or on a glass surface is still not strong enough. A dried layer of PEDOTrPSS deposited on a surface can easily be removed from the surface by a scotch-tape test. The reason behind this weak adhesion is that there is no or negligible covalent bonds between polymeric chains of PEDOT:PSS and the surface. A covalent bond is a chemical bond that involves 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. Covalent bonds make adhesion stronger. [0004] Figure I shows a cross section view of a printed electronic device 100. A PEDOT:PSS dried layer 320 is provided on a surface 1 1 1 of a substrate 110. The PEDOT:PSS dried layer 120 is produced by depositing a PEDOT:PSS w et layer on the surface 1 1 1 and then drying out water from the wet layer. On top of the PEDOT:PSS dried layer 120 are deposited other electronically functional layers e.g. semiconducting layer, conducting layer, electron blocking layer, hole blocking layer, light emitting particles, electrolyte layer, electronically insulating layer etc. The printed electronic dev ice 100 can be, but not limited to, a solar cell, a light emitting diode, a diode, an inorganic particle based electroluminescent device, electrochemical light emitting cel l, electrochromic device, transistor, sensor etc. In a non-limiting example, the printed electronic device 100 can be an OLED which comprises a semiconducting light emitting polymer layer 130 ( e.g. Super Yellow from Merck ), an electron transporting layer 140 ( e.g. zinc ox ide ), a top conducting silver layer 150 and a barrier layer 160. In another non-limiting example, the printed electronic device 100 can be an alternating current driven e lec t t o 1 u m i ne see n t device which comprises a zinc oxide particle based phosphor layer 130, a dielectric layer 140 (e.g. BaTi03), a top conducting silver layer 150 and a barrier layer 160. The problem with all these printed electronic dev ices is that with time the performance of these devices decrease by mechanical bending of the device. This is due to the delamination of PEDOTrPSS dried layer 120 from the substrate 1 10.

[0005] In some applications printed electronic device 200 is used as a sticker, as shown in Figure 2 A. A PEDOT:PSS dried layer 220 is provided on a surface 2 1 1 of a substrate 210. The PEDOT:PSS dried layer 220 is produced by depositing a PEDOT:PSS wet layer on the surface 2 1 1 and then drying out water from the wet layer. On top of the PEDOT:PSS dried layer 220 are provided different electronically functional layers (e.g. 230, 240, 250). A barrier layer 260 can be provided too. A pressure sensitive adhesive layer 270 is provided on top of the barrier layer 260. A release paper 280 ( e.g. sil icone coated paper) is provided on top of the pressure sensitive adhesive layer 270. Before the application of the dev ice 200, the release paper 280 is removed and the device 200 is fi ed on another surface through the pressure sensitive adhesive layer 270. In a desired case, after removing the release paper 280 the device 200 should look like as shown in Figure 2B i.e. no damage in any layer. But in some cases, some part of the device 200 is also removed w ith the release paper 280, as shown in Figure 2C. This happened because of the weak adhesion of PEDOT:PSS dried layer 220 on the substrate 210. [0006] Tnalkoxysilane molecules are known for their adhesion promotion properties. Some example of tnalkoxysilane molecules are, but not-limited to (3- Glycidyloxypropyl)trimethoxysilane, (3-Glycidyloxypropyl)triethoxysilane, [2-(3,4- Epoxycyclohexyl)ethyl]trimethoxysilane, 3-[2-(2- Aminoethylamino)ethylamino]propyltrimethoxysilane, (3-Aminopropyl)triethoxysilane, [3- (2-Aminoethylamino)propyl]trimethoxysilane, (3-Mercaptopropyl)triethoxysilane and (3- Aminopropyl)trimethoxysilane. Trialkoxysilane molecule comprises three alkoxysilane functional groups and a carbon chain with an anchoring functional group. For example, in (3- Glycidyloxypropyl)trimethoxysilane, the anchoring functional group is epoxy and alkoxysilane functional group is methoxysilane. In (3-Aminopropyl)triethoxysilane the anchoring functional group is a primary amine and the alkoxysilane functional group is ethoxysilane. Trialkoxysilane molecules are mixed into an ink before printing the ink. After printing, the ink is dried by a drying or a curing step. Trialkoxysilane molecules are also present in the dried layer of the ink. Over time the three alkoxysilane functional groups of a trialkoxysilane molecule are hydrolyzed into three silanol functional groups in presence of water (water can come into the dried layer of the ink from moisture of the surrounding or some other source). Silanol functional groups are very reactive and they form covalent bonds with surface functional groups present on the surface of a substrate. They also react with each other and create a Si-O-Si covalent bond. They can also react with the material present in the ink e.g. surface hydroxyl functional group of an oxide particle or with hydroxyl function group present on polymeric chains. This way the adhesion between the dried ink layer and the surface of the substrate is enhanced. But the problem with PEDOT:PSS ink is that it comprises more than 95% water and the ink is acidic in nature. Therefore, after mixing trialkoxysilane molecules into the PEDOT:PSS ink, all the alkoxysilane functional groups immediately react with water and are converted into silanol functional groups. After that silanol functional groups react to each other in the PEDOT:PSS ink itself. Hence, trialkoxysilane molecule does not work as a good adhesion promotor for water based PEDOT:PSS ink.

[0007] Pre-treatment of a surface of a substrate with trialkoxysilane molecules is known in the literature to increase adhesion of PEDOT:PSS on the surface. But this too does not create strong adhesion. After treating a surface with trialkoxysilane molecules, alkoxysilane are hydrolyzed into silanol functional groups. The silanol functional groups covalently binds with the surface functional group or to each other. The unreacted anchoring functional group of the trialkoxysilane molecules are used to reacts with inks. When an aqueous ink of PEDOT:PSS is printed on top of the treated surface, the H+ ions present in the PEDOT:PSS ink react with the anchoring functional group and make them useless for further covalent bond formation. For example, water and H+ ions react with an epoxy anchoring functional group and creates two hydroxyl group. There are very less chance that sulfonic acid or sulfonate of polymeric chain of PEDOT:PSS make nucleophilic addition to the epoxy anchoring functional group. This nucleophilic addition can make the adhesion of PEDOT:PSS on the surface stronger. But sulfonic acid and/or sulfonate functional groups are very less in number in a PEDOT:PSS ink and hence their chances to react with anchoring functional group is very low.

SUMMARY OF THE INVENTION

[0008] The present invention relates to a method of producing a PEDOT:PSS based electrode. A surface is provided which comprises at least one surface functional group. A PEDOT:PSS dried layer is provided on the surface. PEDOT:PSS polymeric chains, present in the PEDOT:PSS dried layer, comprises at least one hanging functional group. An adhesion promotor layer is provided on top of the PEDOT:PSS dried layer. The adhesion promotor layer comprises at least one organic solvent and at least one trialkoxysilane molecule. The at least one trialkoxysilane molecule comprises an anchoring functional group and three alkoxysilane functional groups. The at least one trialkoxysilane molecule is less than 5% of the weight of the adhesion promotor layer. The at least one organic solvent is evaporated. After that covalent bonds formation between some of the at least one surface functional group and some of the three alkoxysilane functional groups and between some of the anchoring functional group and some of the at least one hanging functional group is allowed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a cross sectional view of a printed electronic device, comprising PEDOT:PSS.

[00010] FIG. 2A is a cross sectional view of a printed electronic device, comprising a release liner. [00011 ] FIG. 2B is a cross sectional view of a printed electronic device after perfectly removed release liner.

[00012] FIG. 2C is a cross sectional view of a damaged printed electronic device after removed release liner.

[00013] FIG. 3 A is a cross sectional view of a substrate and a PEDOT:PSS wet layer.

[00014] FIG. 3B is a cross sectional view of a substrate and a PEDOT:PSS dried layer.

[00015] FIG. 3C is a cross sectional view of a substrate, a PEDOT:PSS dried layer and a wet adhesion promotor layer.

[00016] FIG. 3D is a cross sectional view of a substrate, a PEDOT:PSS dried layer and a dried adhesion promotor layer.

[00017] FIG. 3E is a cross sectional view of a substrate and a strongly adhered PEDOT:PSS dried layer.

[00018] FIG. 4 is a flow chart showing a manufacturing process to produce a strongly adhered PEDOT:PSS dried layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[00019] 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.

[00020] The terms "print," "printability," "printing," "printable" and "printed", as used here, 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 here 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.

[00021] The present invention is about a method of production of a strongly adhered PEDOT:PSS layer on a surface of a substrate. Figure 3A shows the first step of the production method in which a PEDOT:PSS wet layer 319 is deposited on the surface 311 of a substrate 310. The substrate can be, but not limited to, glass, plastic, PET, PEN, PP, PE, HDPE etc. The surface 311 can be corona treated or plasma treated to increase number of surface functional groups 312. In one non-limiting aspect, the surface functional group can be hydroxyl, hydroperoxide, carbonyl, carboxyl and a combination thereof. The PEDOT:PSS wet layer 319 comprises a plurality of PEDOT:PSS polymeric chains 321 dispersed in water. The PEDOT:PSS PEDOT:PSS polymeric chains 321 comprises some hanging functional group 322. These hanging functional group 322 are sulfonic acid and/or sulfonate hanging on the chain of PSS polymer. The deposition of PEDOT:PSS wet layer 319 can be done by using a printing method or by using a coating method. After the deposition of PEDOT:PSS wet layer 319, a drying process is used to evaporate water. The removal of water creates a solid film of a PEDOT:PSS dried layer 320, as shown in Figure 3B. In the next step as shown in Figure 3C, an adhesion promotor layer 329 is printed on top of the PEDOT:PSS dried layer 320. The adhesion promotor layer 329 comprises at least one organic solvent 335 and at least one trialkoxysilane molecule 331. The trialkoxysilane molecule 331 comprises an anchoring functional group 333 and three alkoxysilane functional groups 332. Non-limiting examples of trialkoxysilane molecule are (3-Glycidyloxypropyl)trimethoxysilane, (3- Glycidyloxypropyl)triethoxysilane, [2-(3,4-Epoxycyclohexyl)ethyl]trimethoxysilane, 3-[2-(2- Aminoethylamino)ethylamino]propyltrimethoxysilane, (3-Aminopropyl)triethoxysilane, [3- (2-Aminoethylamino)propyl]trimethoxysilane, (3-Aminopropyl)trimethoxysilane. In a non- limiting aspect, the alkoxysilane functional group 332 can be methoxysilane or ethoxysilane. In a non-limiting aspect, the anchoring functional group 333 can be epoxy or primary amine. In a non-limiting aspect, the adhesion promotor layer 329 can further comprise a semiconducting polymer 334, as used in organic light emitting diode or in organic photovoltaic or in organic diode. In another non-limiting aspect, the adhesion promotor layer 329 can further comprise doped zinc oxide particle based phosphors 334. Doped zinc oxide particles are used as light emitting layer in an alternating current driven electroluminescent device. The at least one organic solvent 335 can be any organic liquid or a mixture of organic solvents that can dissolve the trialkoxysilane molecule.

[00022] In the next production step, as shown in Figure 3D, the organic solvent 335 is evaporated. And a dried adhesion promotor layer 330 is left on top of the PEDOT:PSS dried layer 320. The trialkoxysilane molecules 331 are very small in size compared to PEDOT:PSS polymeric chains 321, they can easily penertae into the PEDOT:PSS dried layer 320. Most of the bigger particle or polymer 334, if present in the adhesion promotor layer 330 remains on the top surface of the PEDOT:PSS dried layer 320. In the next production step, the PEDOT:PSS based electrode is allowed to stay in a humid environment, with a relative humidity level of more than 40%. In presence of water molecules, the three alkoxysilane functional groups 332 are hydrolyzed into three reactive silanol function groups. The reactive silanol group, if present close to the surface 311, can covalently bind with the surface functional groups 312 i.e. hydroxyl, hydroperoxide, carboxyl. Some of the silanol functional group will also react to each other and will create a network of Si-O-Si bonds. This network of Si-O-Si can also help to keep the PEDOT:PSS polymeric chains 321 locked to the surface 311. Some silanol functional group can also react with the hanging functional group 322 e.g. sulfonic acid and/or sulfonate of PEDOT:PSS polymeric chain. As the PEDOT:PSS dried layer 320 is closely packed and there is no liquid water present, it is very likely that the hanging functional group 322 will come close to the anchoring functional group 333 and can make covalent bonds. For example, epoxy functional group of (3-Glycidyloxypropyl)triethoxysilane can react with the sulfonate group by nucleophilic addition and can create covalent bond. In another example, amine functional group of (3-Aminopropyl)triethoxysilane can react with sulfonic acid and create N-0 bonds.

[00023] The at least one trialkoxysilane molecule 331 is less than 5% of the weight of the adhesion promotor layer 329. More specifically, weight percentage of trialkoxysilane molecule 331 is less than 2 %.

[00024] The method of producing a PEDOT:PSS based electrode is illustrated in Figure 4.