Multi-ink layered printed electrode for wrap around electrodes

Abstract

A method of manufacturing an electrode includes: printing a metal precursor ink onto a contact pad on a substrate; drying the metal precursor ink; printing a nanoparticle ink onto the metal precursor ink; and bonding the metal precursor ink and the nanoparticle ink such that the metal from the precursor ink and the metal from the nanoparticle ink are fused together. An electrode includes: a metal precursor ink electrically contacting a contact pad; and a nanoparticle ink electrically contacting the metal precursor ink on the contact pad. An array substrate may include the electrode.

Description

[Technical Field]

[0001] 1. Cross-reference to related applications

[0002] This application claims priority to U.S. Provisional Application Serial No. 63 / 222215, filed July 15, 2021, the contents of which form the basis of this document and are incorporated herein by reference in their entirety. 2. Field of Disclosure

[0003] This disclosure relates to technology for edge electrodes used in micro LED electronic displays. More specifically, this disclosure relates to borderless, bezel-less, or tiled displays. [Previous Technology]

[0004] 3. Related Technical Description

[0005] A micro-LED display consists of an array of microscopic light-emitting diodes (LEDs) that define individual pixel elements. Top-emitting micro-LED displays require a method for electrically interconnecting the micro-LEDs on the top surface of a substrate with a driver board located behind the substrate. Typically, this is achieved by using flexible connectors attached to the edge of the top surface of the substrate and bending around the substrate. In the case of borderless, bezel-less, or tiled displays, using flexible connectors attached to the top surface of the substrate is undesirable. In this configuration, the flexible connector is visible to the observer, thus requiring a bezel for concealment, or the flexible connector occupies too much space between the tiles and prevents seamless splicing. In both cases, typical flexible connectors are too large to meet the requirement of providing a tight assembly package while remaining invisible to the observer. One solution for electrically connecting the top surface of the micro-LED display substrate to the driver board behind it is to use wound electrodes. An example is shown in Figure 1.

[0006] Figure 1 shows an edge portion of a substrate 10, which includes a micro-LED array 20 located on the front surface of the substrate 10. Contact pads for micro-LED electrodes 30 extend from the micro-LED array 20 and are connected to contact pads for rear surface electrodes 50 via wraparound electrodes (WAEs) 40. As shown, the WAEs are manufactured to wrap around the edge of the micro-LED display substrate 10. This allows the electrodes to occupy less physical space and be less noticeable. WAEs have been used in borderless, frameless, and tiled displays. These electrodes have been constructed using various electrode materials and manufacturing methods. Such technologies include: printed lines, vacuum deposition / patterned metal lines, flexible connectors, and electroplated lines. Although success has been achieved in demonstrating functional micro-LED displays, the illustrated micro-LED displays have limitations in brightness and lifespan. This limited operation is attributed to the electrical performance and reliability of currently used WAEs. Methods to improve the electrical performance and reliability of WAEs are being developed through material and process improvements.

[0007] Electrode printing can be performed by depositing conductive metal nanoparticle ink in a predetermined pattern onto a substrate, and then sintering the metal nanoparticles at a mild temperature to provide a conductive path. Sintering removes the ink solvent and breaks down the organic material that isolates the nanoparticles, allowing the metal of adjacent nanoparticles to contact and bond together.

[0008] Compared to existing sputtering technologies, electrode printing offers a low-cost 3D solution due to its relatively high thickness and the 3D patterning required to create wound electrodes. In an exemplary printing process, nanoparticle ink is delivered to a nozzle, which guides the ink onto a substrate in the desired pattern. The deposited ink is then dried and solidified at a relatively low temperature (i.e., ≤ 250°C). A protective coating (typically a polymer) is then applied to the substrate to provide mechanical protection and electrical isolation for the electrodes and to optically conceal the electrodes at the edges of the substrate. Figure 2 shows an example of two patch substrates including a microLED display with this type of configuration.

[0009] Figure 2 shows two adjacent microLED displays similar to the microLED display shown in Figure 1. As shown, each microLED display includes a substrate 10, which includes a microLED array 20 located on the front surface of the substrate 10. MicroLED electrodes 30 extend from the microLED array 20 and are connected to a rear surface electrode 50 via a WAE 40. The sides, front surface, and rear surface of the substrate 10 include a protective coating 60 to cover and protect the WAE 40.

[0010] The resistance of printed electrodes can be as low as 2.5 times the bulk resistivity of the conductor material, depending on the size of the nanoparticles used, the bonding conditions, and the organic composition of the ink. Specifically, the resistivity of the printed electrode decreases as the average size of the nanoparticles decreases. Therefore, smaller nanoparticles are preferred. However, to prevent nanoparticles from agglomerating in the ink suspension, an organic coating surrounds the nanoparticles. This organic coating helps the nanoparticles repel each other, thereby reducing unwanted precipitation.

[0011] If the organic coating on the nanoparticles is not removed during the consolidation process, such organic coatings may cause several problems to the printed electrodes. These problems may include: (a) affecting electrical performance due to increased line resistance and contact resistance caused by the resulting porosity; (b) increasing the contact resistance between the printed electrodes and the contact pads if not completely removed; (c) reducing the reliability of the printed lines due to trapped organic matter or moisture trapped in the pore spaces of the printed lines over time; and (d) reducing the rate and extent of consolidation because sintering requires breaking down such coatings so that the metal of adjacent nanoparticles can be fused together.

[0012] Although the contact resistance of the conductive contacts of the nanoparticle ink to the electrodes is not very high if the contact area is large, the contact area decreases as the size of the display pixels shrinks. Therefore, for micro LED displays, the total resistance of the WAE is usually dominated by the contact resistance. [Summary of the Invention]

[0013] In order to overcome the above problems, the preferred embodiment of this disclosure provides a method comprising several steps including separating individual micro LED display substrate assemblies (patches) from sheets, setting patch size / shape, edge processing, electrode printing, and applying a protective coating.

[0014] This disclosure addresses the contact resistance limitations of printed electrodes using nanoparticle inks. The reduction in contact resistance is achieved by incorporating at least one additional layer specifically designed to increase the electrical contact between the printed WAE and the electrode contact pad. Using a single printed layer alone does not meet the electrical requirements of the device. However, providing a printed layer integrated into a multilayer structure satisfies the requirements.

[0015] A novel method and composition for printing electrodes to reduce contact resistance are disclosed. These methods involve the deposition of two or more conductive inks. At least one layer is designed to establish good electrical contact and low contact resistance between the device electrode and the printing electrode. This layer may include a metal precursor ink composed of a metal compound that improves the contact between the printing electrode and the nanoparticle metallic ink. An additional layer is designed to establish low line resistance of the printing electrode. This layer may be composed of a variety of conventional nanoparticle conductive inks, the thickness of which can be deposited is increased compared to the thickness that can be deposited by a single metal precursor ink, thereby increasing printing throughput.

[0016] When bonded together, the combination of multiple layers uniquely produces a film with low contact resistance to the contact pad, low metal wire resistance, and high adhesion to the deposited substrate and contact pad material.

[0017] This disclosure includes several innovations in printing conductive electrodes using various compositions, processes, and methods. These innovations simultaneously achieve low contact resistance and low line resistance in wound electrodes. These innovations include: 1) multilayer printed electrodes; 2) process flow for multi-component ink systems; 3) utilization of metal precursor inks that are metallized after deposition; 4) processes for nanoparticle ink deposition and methods for bonding multilayer printed electrodes; 5) explanations of how multi-ink solutions are superior to printed electrodes consisting only of nanoparticles and printed electrodes consisting only of metal compounds, including: a) increased contact with contact pads; b) maximizing the thickness of the printed electrode stack, thereby reducing costs; c) mitigating oxidation of printed metal electrode or contact pad materials.

[0018] The currently disclosed embodiments include a method of manufacturing an electrode, comprising: printing a metal precursor ink onto a contact pad of a substrate; drying the precursor ink; printing nanoparticle ink onto the metal precursor ink; and bonding the metal precursor ink and the nanoparticle ink such that the metal from the precursor ink and the metal from the nanoparticle ink are fused together.

[0019] In this embodiment, the steps of printing the metal precursor ink and printing the nanoparticle ink can be performed by one of an aerosol printer, a pad printer, an inkjet printer and a spray printer.

[0020] In this embodiment, the step of drying the metal precursor ink can be performed under ambient conditions.

[0021] In this embodiment, the step of consolidating the metal precursor ink and the nanoparticle ink can be performed at a temperature of less than or equal to 250°C.

[0022] In this embodiment, the contact pad may include two contact pads, one being a display contact pad extending from the micro-LED array on the substrate, and the other being a rear surface contact pad located on the side of the substrate opposite to the micro-LED array.

[0023] The method may further include the step of providing a wound electrode that contacts both the display contact pad and the rear surface contact pad.

[0024] In this embodiment, the substrate may be glass.

[0025] In this embodiment, the metal in the metal precursor ink and the nanoparticle ink may be silver or copper.

[0026] In this embodiment, the metal in the metal precursor ink and the metal in the nanoparticle ink may be the same.

[0027] In this embodiment, the metal in the metal precursor ink and the metal in the nanoparticle ink may be different.

[0028] In this embodiment, the step of consolidating the metal precursor ink and the nanoparticle ink can be performed using a laser.

[0029] In this embodiment, the step of drying the metal precursor ink can be carried out at a temperature of up to 150°C.

[0030] In another embodiment disclosed herein, an array substrate includes: a substrate; a micro-LED array located on a front surface of the substrate and including a display contact pad; a metal precursor ink located directly on and in electrical contact with the display contact pad; and a nanoparticle ink located directly on and in electrical contact with the metal precursor ink on the display contact pad.

[0031] The array substrate may further include: a rear surface contact pad located on a rear surface of the substrate opposite to the front surface; the metal precursor ink electrically contacting the rear surface contact pad; and the nanoparticle ink electrically contacting the metal precursor ink on the rear surface contact pad.

[0032] The array substrate may further include: a wound electrode that contacts the display contact pad and the nanoparticle ink on the rear surface contact pad.

[0033] In another embodiment disclosed herein, an electrode includes: a metal precursor ink that electrically contacts a contact pad; and a nanoparticle ink that electrically contacts the metal precursor ink on the contact pad.

[0034] The above and other features, elements, characteristics, steps and advantages of this disclosure will become more apparent from the following detailed description of the preferred embodiments of this disclosure with reference to the accompanying drawings.

Implementation Method

[0039] Figure 3 illustrates a process for a multilayer ink printing system according to an embodiment of the present disclosure. In this embodiment: 1) at least one layer is optimized to establish a strong electrical contact between the contact pad and the printed electrode, and 2) at least one layer is optimized to establish a low line resistance. Although more than two layers are possible, the following specific examples focus on electrode systems with at least two layers.

[0040] According to the process shown in Figure 3, the spliced ​​substrate is first placed on the platform of the printer and aligned. The printer can use aerosol jetting, inkjet, spraying, screen printing, rollers, troughs, die-cutting, pads, blades, or any other suitable technology to deposit the desired ink onto the substrate. Additionally, different solution coating or printing methods can be used to deposit different ink layers to suit specific ink properties and desired layer structures and dimensions. Next, the metal precursor ink is printed in the desired pattern to establish electrodes. The metal precursor ink consists of metal compounds that are soluble in solution and may contain cationic metals (such as silver, copper, gold, aluminum, nickel, or other metals) and anionic types (such as acetates, nitrates, sulfates, or any other type that readily forms salts in solution). The salt can be dispersed in an aqueous solvent or an organic solvent. A solvent system consisting of binary, ternary, or even higher-order solvent mixtures is desired to produce the physical and chemical properties required for printing.

[0041] Next, the metal precursor ink is dried. Depending on the metal precursor ink used, drying can be carried out under ambient conditions or accelerated by heating. Heating the substrate and / or ink to a maximum of 150°C can be used to modify the solvent evaporation rate to control the printed electrode dimensions, such as line width and thickness. Once the metal precursor lines are sufficiently dry, additional layers can be printed with the same ink or different inks to achieve the desired electrical and physical properties.

[0042] When printing metal precursor ink, the solvent evaporates, thereby increasing the concentration of the metal salt. This leads to a metallization reaction, in which the metal is reduced to form metal nuclei, which increase in size and precipitate from the solution to form a solid metal film. The byproducts of the reaction are generally gaseous or volatile. Heat can be used to initiate or accelerate the metallization process and remove the byproducts of the redox reaction from the printed electrodes. Depending on the metal selected to minimize oxidation, this can be carried out in air, an inert, or a reducing atmosphere (a reducing atmosphere is one that includes a reducing agent such as hydrogen to prevent oxidation of the contact pads and / or the printed electrodes). The metal precursor ink may consist of acidic and / or alkaline components that react to reduce surface oxides on the contact pads of the micro-LED display electrodes and the back surface electrodes, thereby further helping to achieve low contact resistance between conductive contacts. The resulting film is a solid metal electrode composed of a particle-like, mesh-like, or stranded structure of metal fused within the precursor. The deposited metal electrodes conform to the substrate and the underlying electrical structure, resulting in improved electrical contact between the printed electrodes and the front and rear contact pads. It should be noted that although a micro-LED display is specifically described, this multi-ink electrode, which simultaneously achieves low contact resistance and low line resistance, is generally applicable to other electronic and optoelectronic applications. Devices in these applications can utilize this multi-ink electrode as a surface conductor or the aforementioned WAE. These applications may include display technology (LCD and OLED), photovoltaics, lighting, sensors, and flexible electronics.

[0043] After the "contact resistance layer" of the metal precursor is formed, an additional layer that produces low line resistance is defined. After drying the metal precursor ink, nanoparticle metal ink is deposited on top of the partially or fully metallized precursor ink to achieve the desired printed electrode size. Depending on the process conditions, the metal precursor ink may not be completely sintered to ensure that all metals are fused.

[0044] The two printed electrode layers are then bonded at a temperature of ≤ 250°C using heat, laser, UV, IR, or any other suitable mechanism. Bonding can be performed in various atmospheres, depending on the choice of metal for the ink used for printing to prevent oxidation. For example, bonding can be performed at any suitable combination of temperature, pressure, and gas. The bonding step provides several functions. In nanoparticle inks, bonding removes the solvent, breaks down the binder surrounding the nanoparticles that inhibits sintering, and fully sinters the nanoparticles to create electrical continuity within the nanoparticle layer. In metallized precursor layers, the bonding process can complete any additional metallization of the metal precursor ink. Bonding also achieves metal sintering and fusion between the metal precursor layer and the nanoparticle layer. Although the precursor ink and nanoparticle ink can be separate layers prior to bonding, during bonding, these layers can be combined into a single conductive structure. Depending on the bonding conditions and ink composition, the resulting multiple layers can have different porosities and microstructures, or multiple layers can be combined to form a single conductive structure without clear boundaries. While two printed electrodes are preferred, any number of layers can be provided to achieve the desired performance characteristics. Furthermore, the ink layers do not need to be continuous within the patterned area. For example, a "contact resistance layer" or other layers can be discontinuous islands above the device electrode contact pads.

[0045] (Metal precursors and nanoparticles) The metal composition of the ink can be the same, such as gold, silver, copper, nickel, tin, etc., but can be different to further expand the performance properties (i.e., electrical performance, adhesion, reliability, etc.) of the printed multilayer ink electrodes. The metal selection and bonding process must be fully compatible to produce the desired performance.

[0046] Figure 4 shows the resulting dual-ink electrode configuration. Figure 4 shows a micro-LED array 420 including electrode 430 on the front surface of substrate 410. Substrate 410 may be glass, ceramic, glass-ceramic, polymer, composite material, metal, alloy, any combination thereof, or any other suitable material. Rear surface electrode 450 may be located on the rear surface of substrate 410. A bonding layer defining the dual ink electrodes 480 and 490 of the metal precursor ink and nanoparticle ink produced according to the process described above with respect to Figure 3 may be respectively disposed on micro-LED electrode 430 and rear surface electrode 450. As shown, a conventional WAE 440 is used to connect the dual ink electrodes 480 and 490. Alternatively, the nanoparticle ink layer may be extended to define a continuous WAE 440.

[0047] Dual-ink printing systems offer additional advantages for wound electrode applications. While electrode printing systems based on single-metal compound precursors can be used to produce electrodes with similar geometries and low contact resistance, metal compound inks are limited by the thickness that can be deposited in a single pass. This is because the metallic portion of the ink is limited by the solubility limit of the silver compound in the solvent system. Although commercially available metal nanoparticle inks can contain 50% to 70% metal, most commercially available metal compound inks contain less than 20% metal and are highly fluid, resulting in wide and short printed patterns. Therefore, it may be necessary to print up to 20 layers of metal compound ink to achieve the same thickness achievable with a single pass of nanoparticle ink. Furthermore, deposited metal compound inks are more textured than their nanoparticle counterparts, and therefore often exhibit worse conductivity than their nanoparticle counterparts, even if the nanoparticle inks contain higher porosity. Therefore, combining two types of metal inks produces superior performance compared to their individual components. Table 1 shows the performance of different electrode materials.

[0048] metal electrode Contact materials Contact resistivity (Ω•μm) 2 ) Only Ag NP Ti / Cu / ITO 100,000 - 500,000 Ag precursor + Ag NP Ti / Cu / ITO 14,000

[0049] Table 1 shows that the resistivity of silver-based nanoparticle ink (NP) on Ti / Cu / ITO contact materials is 100,000 - 500,000 Ω•μm2. On the other hand, the resistivity of silver-based nanoparticle ink on top of a silver-based precursor is significantly lower at 14,000 Ω•μm2.

[0050] The multi-ink layer printed electrode method allows different metallic inks to be printed with different electrode patterns to address local performance requirements as needed. Metallic precursor inks can be printed only on the contact pads or over the entire printed electrode pattern. Similarly, metallic precursor inks can be printed partially on top of the metallic precursor ink or over the entire printed electrode pattern. The choice of the printed pattern for the metallic compound ink depends on the exact size to be patterned, the capabilities of the printing equipment, and other considerations such as the step height of the contact pad material. For example, the metallic precursor ink pattern (or other ink layer) can be completely covered by the metallic compound ink pattern (or other layer). Alternatively, the metallic precursor ink or any other underlying layer can extend from beneath the metallic compound ink or the upper layer without being covered. In this way, any of the individual layers can have different widths, lengths, and thicknesses, as long as they overlap and are electrically connected.

[0051] It should be understood that the foregoing description is merely illustrative of this disclosure. Various alternatives and modifications will be conceived by those skilled in the art without departing from the invention. Therefore, this disclosure is intended to cover all such alternatives, modifications, and variations falling within the scope of the appended claims. [Simplified Explanation of the Diagram]

[0035] Figure 1 is a cross-sectional view of a micro LED display according to the related technology.

[0036] Figure 2 is a cross-sectional view of two adjacent micro LED displays according to the relevant technology.

[0037] Figure 3 illustrates a process flow according to an embodiment of the present disclosure.

[0038] Figure 4 is a cross-sectional view of the micro LED display according to the present disclosure. [Biomaterial Storage]

[0053] Domestic storage information (please note in order of storage institution, date, and number): None. International storage information (please note in order of storage country, institution, date, and number): None.

Claims

1. A method for manufacturing an electrode, the method comprising the steps of: printing a metal precursor ink onto a contact pad on a substrate; drying the metal precursor ink; printing a nanoparticle ink onto the metal precursor ink; and bonding the metal precursor ink and the nanoparticle ink such that the metal from the precursor ink and the metal from the nanoparticle ink are fused together.

2. The method as described in claim 1, wherein the steps of printing the metal precursor ink and printing the nanoparticle ink are performed by one of an aerosol printer, a pad printer, an inkjet printer, and a spray printer.

3. The method as described in claim 1, wherein the step of drying the metal precursor ink is performed under ambient conditions.

4. The method as described in claim 1, wherein the step of drying the metal precursor ink is carried out at a temperature of up to 150°C.

5. The method as described in claim 1, wherein the step of consolidating the metal precursor ink and the nanoparticle ink is performed at a temperature less than or equal to 250°C.

6. The method as described in claim 1, wherein the step of consolidating the metal precursor ink and the nanoparticle ink is performed using a laser.

7. The method as claimed in claim 1, wherein the contact pad comprises two contact pads, one being a display contact pad extending from a micro-LED array on the substrate, and the other being a rear surface contact pad on the side of the substrate opposite to the micro-LED array.

8. The method as described in claim 7 further comprises the step of: providing a wound electrode that contacts both the display contact pad and the rear surface contact pad.

9. The method as described in claim 1, wherein the substrate is glass.

10. The method as described in claim 1, wherein the metal in the metal precursor ink and the nanoparticle ink is silver.

11. The method as described in claim 1, wherein the metal in the metal precursor ink and the nanoparticle ink is copper.

12. The method as described in claim 1, wherein the metal in the metal precursor ink and the metal in the nanoparticle ink are the same.

13. The method as described in claim 1, wherein the metal in the metal precursor ink and the metal in the nanoparticle ink are different.

14. An array substrate comprising: a substrate; a micro-LED array located on a front surface of the substrate and including a display contact pad; a metal precursor ink electrically contacting the display contact pad; and a nanoparticle ink electrically contacting the metal precursor ink on the display contact pad.

15. The array substrate as claimed in claim 14, further comprising: a rear surface contact pad located on a rear surface of the substrate opposite to the front surface; a metal precursor ink electrically contacting the rear surface contact pad; and a nanoparticle ink electrically contacting the metal precursor ink on the rear surface contact pad.

16. The array substrate as claimed in claim 15 further comprises: a wound electrode that contacts the nanoparticle ink on the display contact pad and the rear surface contact pad.

17. The array substrate as claimed in claim 14, wherein the metal in the metal precursor ink and the nanoparticle ink is silver.

18. The array substrate as claimed in claim 14, wherein the metal in the metal precursor ink and the nanoparticle ink is copper.

19. The array substrate as described in claim 14, wherein the substrate is glass.

20. The array substrate as claimed in claim 14, wherein the metal in the metal precursor ink and the metal in the nanoparticle ink are the same.

21. The array substrate as claimed in claim 14, wherein the metal in the metal precursor ink and the metal in the nanoparticle ink are different.

22. An electrode comprising: a metal precursor ink that electrically contacts a contact pad; and a nanoparticle ink that electrically contacts the metal precursor ink on the contact pad.

23. The electrode as claimed in claim 22, wherein the metal in the metal precursor ink and the nanoparticle ink is silver.

24. The electrode as claimed in claim 22, wherein the metal in the metal precursor ink and the nanoparticle ink is copper.

25. The electrode as claimed in claim 22, wherein the metal in the metal precursor ink and the metal in the nanoparticle ink are the same.

26. The electrode as claimed in claim 22, wherein the metal in the metal precursor ink and the metal in the nanoparticle ink are different.

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