CN105658742B - The composition for ink for including polythiophene for ink jet printing - Google Patents

Abstract

An ink composition comprising polythiophene and polymethylsiloxane formulated for inkjet printing of a hole injection layer (HIL) of an organic light emitting diode (OLED) is provided. Also provided is a method of inkjet printing a HIL using the ink composition. Ink compositions for inkjet printing of layers in organic light emitting diodes (OLEDs) have been proposed. However, problems associated with insufficient wetting properties of ink compositions have inhibited the development of printable inks, since improper wetting leads to non-uniform film formation and thus organic luminescence from incorporation of printed films. Uneven light emission of diode pixels.

Description

Ink composition comprising polythiophene for inkjet printing

Cross References to Related Applications

This application claims priority to US Provisional Patent Application No. 61/898,343, filed October 31, 2013, entitled Ink Compositions Containing Polythiophene for Ink Jet Printing, the entire contents of which are incorporated herein by reference. This application was filed September 14, 2012, and claims priority to U.S. Provisional Patent Application No. 61/535,413, filed September 16, 2011, entitled U.S. Patent Application No. entitled Film-Forming Formulations for Substrate Printing 13/618,157, a continuation-in-part, which is hereby incorporated by reference in its entirety.

background

Ink compositions for inkjet printing layers in organic light emitting diodes (OLEDs) have been proposed. However, problems associated with insufficient wetting properties of ink compositions have stifled the development of printable inks because improper wetting leads to uneven film Non-uniform light emission of organic light-emitting diode pixels. Another challenge that has hampered the development of inkjet printable compositions for OLED applications is the inability to incorporate high concentrations of reactive polymers into the ink while maintaining a jettable ink formulation.

summary

An ink composition comprising polythiophene formulated for inkjet printing a hole injection layer (HIL) of an OLED is provided. Some embodiments of the ink composition are characterized by the inclusion of polymethicone as a pinning agent. Other features include the inclusion of aprotic solvents capable of incorporating high concentrations of polythiophene into the ink. Also provided is a method of inkjet printing a HIL using the ink composition.

One embodiment of a method of forming a HIL for an organic light emitting diode comprises the step of inkjet printing a droplet (i.e., at least one droplet) of an ink composition on an electrode layer in a pixel unit of an organic light emitting diode, the pixel The cells are defined by pixel banks; and the volatile components of the ink composition are allowed to evaporate, thereby forming a hole injection layer. One embodiment of the ink composition that can be used in the method comprises: conductive polythiophene; water; at least one organic solvent; and polymethylsiloxane, wherein polymethylsiloxane is used to provide The amount of contact line pinning of the droplet exists.

Some embodiments of the ink composition comprise: poly(3,4-ethylenedioxythiophene); water; at least one organic solvent having a surface tension of no greater than 55 dynes/cm at 25°C, at 25 a viscosity of no greater than 15 cPs at °C and a boiling point of at least 200 °C; and polymethylsiloxane. The at least one organic solvent may be, for example, sulfolane.

Other principal features and advantages of the present invention will become apparent to those skilled in the art upon examination of the following drawings, detailed description and appended claims.

Brief description of the drawings

Exemplary embodiments of the present invention will hereinafter be described with reference to the accompanying drawings, wherein like numerals denote like elements.

Figure 1 is a block diagram illustrating an OLED inkjet printing system.

FIG. 2 is a schematic diagram of a gas enclosure system that may house the printing system shown in FIG. 1 .

3 is a schematic diagram of a flat panel display including a plurality of OLEDs arranged in a matrix of pixel cells, each pixel cell defined by a pixel bank.

Figure 4A is a photomicrograph image of an ink composition containing 0.08 wt.% polymethylsiloxane pinned in an OLED pixel cell.

Figure 4B is a black and white line drawing of the micrograph in Figure 4A.

Figure 5A is a photomicrograph image of an ink composition overflowing an OLED pixel cell without methicone.

Figure 5B is a black and white line drawing of the micrograph in Figure 5A.

Figure 6A is a photomicrograph image of an ink composition for a dewetting OLED pixel cell without methicone.

Figure 6B is a black and white line drawing of the photomicrograph in Figure 6A.

Figure 7A is a photomicrograph of the luminescence emitted by an OLED pixel with a HIL printed with an ink composition comprising polymethylsiloxane as a pinning agent.

Figure 7B is a black and white line drawing of the micrograph in Figure 7A.

Figure 8A is a photomicrograph of the luminescence emitted by an OLED pixel in which a HIL was printed with an ink composition comprising polymethylsiloxane as a pinning agent and sulfolane as an organic solvent.

Figure 8B is a black and white line drawing of the photomicrograph in Figure 8A.

Figure 9A is a photomicrograph of the luminescence emitted by an OLED pixel in which a HIL was printed with an ink composition comprising polymethylsiloxane as a pinning agent and 1,3-propanediol as an organic solvent.

Figure 9B is a black and white line drawing of the micrograph in Figure 9A.

10 is a graph of drop volume of ink compositions versus time, as described in Example 2, before and after a 30 minute idle of the inkjet printing nozzles.

11 is a graph of drop velocity versus time for ink compositions, as described in Example 2, before and after a 30 minute idle of the inkjet printing nozzles.

12 is a graph of drop angle versus time for an ink composition, as described in Example 2, before and after a 30 minute idle of the inkjet printing nozzle.

Detailed Description

Ink compositions comprising polythiophenes formulated for inkjet printing HILs of OLEDs are provided. Also provided is a method of inkjet printing a HIL using the ink composition.

The ink compositions are characterized by a high concentration of conductive polythiophenes such as poly(3,4-ethylenedioxythiophene) (PEDOT), which also provides features that make them well suited for inkjet printing onto pixelated substrates such as OLEDs. Wetting, jetting, and retardation characteristics on pixel units). Furthermore, the ink composition provided printed HILs with highly uniform thickness and homogenous composition. Thus, the printed HILs contribute to a highly uniform light emission profile for OLEDs into which they are incorporated. The enhanced printability provided by the ink composition can be attributed, at least in part, to the realization that, at appropriate concentrations, polymethylsiloxane can act as a contact line puller for droplets of the ink composition in a pixel cell . By providing contact line pinning, the polymethylsiloxane ensures that the footprint of a droplet of ink composition deposited into a pixel cell remains unchanged from its original form during the drying process.

A basic embodiment of the ink composition is an aqueous solution comprising conductive polythiophene, polymethylsiloxane, at least one organic solvent and water. A basic embodiment of a method of forming an HIL of an OLED using an ink composition comprises depositing droplets of the ink composition on a layer of conductive material (i.e., an anode) in a pixel cell of an organic light emitting diode array, and allowing the ink to combine A step in which the volatile components of the compound evaporate, leaving behind solid HIL. The step of allowing volatile components (such as water and organic solvents) to evaporate can be accomplished by subjecting the printed ink composition to reduced pressure (ie, exposing it to a vacuum), by exposing the printed ink composition to elevated temperature, or a combination of both. Promote.

Polymethylsiloxane is a silicone oil polymerized from siloxane. They are also known as methylhydrogensiloxanes or methylsiloxanes. Polymethicones are commercially available and sold as surfactants under the trade name Botanisil® by Botanigenics (Northridge, CA). These include Botanisil® AD-13, AM-14, ATC-21, BPD-100, CD-80, CD-90, CE-35, CM-12, CM-13, CM-70, CP-33, CPM- 10. CS-50, CTS-45, DM-60M, DM-85, DM-90, DM-91, DM-92, DM-93, DM-94, DM-95, DM-96, DM-97, DTS-13, DTS-35, GB-19, GB-20, GB-23, GB-25, GB-35, L-23, ME-10, ME-12, PSS-150, PT-100, S- 18, S-19, S-20, TSA-16 and TSS-1. Methicones are also available from Lubrizol Corporation (Wickliffe, Ohio) under the tradename SilSense®. These include SilSense® Copolyol-1 Silicone (PEG-33 (and) PEG-8 Dimethicone (and) PEG 14), SilSense® DW-18 Silicone (Dimethicone PEG-7 Iso Stearate), SilSense® SW-12 Silicone (Dimethicone PEG-7 Cocoate), SilSense® IWS (Dimethiconol Dimethiconol Stearate), SilSense® A-21 Silicone (PEG-7 Amodimethicone), SilSense® PE-100 Silicone (Dimethicone PEG-8 Phosphate), and Ultrabee™ WD Silicone (Dimethicone PEG-8 Beeswax).

In the ink compositions of the present invention, the amount of methicone is carefully controlled so that the methicone acts as a contact line pinning agent. This is important because it prevents pinned ink composition droplets from being pulled away from part of the bank of the pixel cell (dewetting), which sometimes accompanies overflow in other parts of the pixel cell. It also prevents the ink composition from building up on the sides of the pixel cell or spreading beyond the pixel cell, as would occur with more complete wetting.

The ink composition can be used to form HIL on various OLED electrode materials. Most commonly, the electrode substrate will comprise a transparent conducting material such as a transparent conducting oxide (TCO) or silicon. The appropriate concentration range for the polymethylsiloxane in the ink composition will depend on the nature of the underlying substrate. However, for a given substrate, the range of concentrations of methicone that provide contact line pinning can be determined by observing the wetting behavior of ink droplets with different methicone concentrations that It has been applied to the surface via a drop casting method. By way of illustration, some embodiments of the ink compositions of the present invention comprise methicone in an amount no greater than 0.15 weight percent (wt.%), no greater than 0.12 wt.%, or no greater than 0.1 wt.%. , based on the total weight of the ink composition. This includes embodiments of the ink composition wherein the methicone is present in an amount of 0.02 to 0.15 wt.%, further includes embodiments wherein the methicone is present in an amount of 0.03 to 0.12 wt.%, And still further included are embodiments wherein the polymethylsiloxane is present in an amount of 0.05 to 0.1 wt.%, based on the total weight of the ink composition. Such a range is suitable when printing the HIL ink composition onto known anode materials used in OLED devices. For example, in the case of OLED devices in which light is emitted through the anode (known as bottom emitting), a transparent or translucent anode material is used. Transparent or translucent anode materials may include indium oxide, zinc oxide, indium tin oxide (ITO), indium zinc oxide (IZO), and the like. In the case of OLED devices in which light is emitted through the cathode (referred to as top emitting), a reflective layer is formed below the transparent anode. Reflective layer materials include silver (Ag), silver-palladium-copper (APC), silver-rubidium-gold (ARA), molybdenum-chromium (MoCr) and the like.

The aqueous ink composition further comprises one or more conductive polythiophenes. For example, PEDOT and mixtures of PEDOT and poly(styrenesulfonate) (PEDOT:PSS) may be included in the ink composition. Notably, polythiophenes can be included in ink compositions at very high concentrations when combined with appropriate solvents (as discussed in more detail below). For example, some embodiments of the ink composition comprise at least 30 wt.% polythiophene, at least 40 wt.% polythiophene, at least 50 wt.% polythiophene, at least 55 wt.% polythiophene, or at least 60 wt.% polythiophene. % polythiophene, based on the total weight of the ink composition. In such embodiments, the polythiophene may be PEDOT.

The aqueous ink composition contains at least one organic solvent. For example, the composition may contain solvents that reduce the surface tension and/or viscosity of the composition, solvents that increase the retardation of the printed ink composition, or combinations of these types of solvents. The at least one organic solvent may be a solvent having a relatively high boiling point, which increases the retardation of the printed ink composition. This is advantageous because it helps prevent the ink composition from drying onto the print nozzles during printing and clogging the print nozzles. Such solvents desirably have a boiling point of at least 200°C. More desirably, they have a boiling point of at least 230°C, at least 250°C or even at least 280°C. Glycols and glycols such as propylene glycol, pentylene glycol, diethylene glycol, and triethylene glycol are examples of organic solvents that can be used to increase retardation. Unfortunately, diols and glycols tend to have relatively high viscosities and surface tensions, which can reduce the jettability of ink compositions containing them. Accordingly, some embodiments of the ink compositions of the present invention are free of glycols and glycol-based solvents. In these embodiments, an aprotic solvent having a boiling point of at least 240°C, a viscosity of no greater than 15 cPs, and a surface tension of no greater than 55 dynes/cm can be used in place of the diol or glycols. This includes aprotic solvents having viscosities no greater than 12 cPs, and further includes those having viscosities no greater than 10 cPs. For the purposes of this disclosure, recited boiling points refer to boiling points at atmospheric pressure. The listed viscosities and surface tensions refer to the viscosities and surface tensions at the printing temperature. For example, if printing occurs at room temperature, the viscosity and surface tension will be those at about 25°C.

Sulfolane, 2,3,4,5-tetrahydrothiophene-1,1-dioxide, also known as tetramethylene sulfone, is an example of a relatively high-boiling, relatively low-viscosity aprotic solvent, It provides good lag without sacrificing jettability. Furthermore, ink compositions comprising sulfolane as an organic solvent can incorporate high concentrations of both solvent and polythiophene while maintaining good jettability. For example, the ink composition may comprise sulfolane in an amount of at least 5 wt.%, at least 10 wt.%, or at least 12 wt.%. Suitable concentration ranges for sulfolane in the ink composition include about 3 wt.% to about 15 wt.%. At these sulfolane concentrations, ink compositions can incorporate high concentrations of PEDOT (eg, 35 to 70 wt.%). In some ink compositions, sulfolane is the main solvent, ie it accounts for more than 50 wt.% of the total organic solvent content of the ink composition. Other suitable solvents include propylene carbonate and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, also known as dimethylpropylene urea.

In order to enhance the jettability of the composition, the ink composition may further comprise a co-solvent which acts as a surface tension reducing agent. For example, ink compositions containing glycols, glycols, sulfolane, or other high boiling point solvents may contain additional solvents that have lower surface tensions and typically lower boiling points than those solvents. Propylene glycol methyl ether or other similar ethers can be used for this purpose.

In general, for an ink composition to be useful in inkjet printing applications, the surface tension, viscosity, retardation and wetting properties of the ink composition should be tailored to allow the composition to be stabilized at the temperature used for printing (e.g., room temperature; ~25°C). °C) without drying onto the nozzle or clogging the nozzle through the inkjet printing nozzle. Therefore, the optimum characteristics will vary according to such factors as nozzle size, printing speed and printing temperature. Generally, acceptable viscosities will include those of about 1 to about 20 cPs, and acceptable surface tensions will include those of less than about 50 dynes/cm. In order to eliminate or minimize nozzle clogging, a delay (at room temperature and not under vacuum) of 20 minutes or longer (e.g., 30 minutes or longer) is desirable, where delay refers to the presence of a significant decrease in performance ( The nozzles may be left uncovered and idle for a period of time before eg a reduction in droplet velocity which would significantly affect image quality.

Inkjet printers suitable for printing ink compositions are commercially available and include drop-on-demand printheads available from, for example, Fujifilm Dimatix (Lebanon, N.H.), Trident International (Brookfield, Conn.), Epson (Torrance, Calif.), Hitachi Data systems Corporation (Santa Clara, Calif.), Xaar PLC (Cambridge, United Kingdom), and Idanit Technologies, Limited (Rishon Le Zion, Israel) and Ricoh Printing Systems America, Inc. (Simi Valley, CA). For example, Dimatix Materials Printer DMP-3000 can be used.

As depicted in the block diagram of FIG. 1 , various embodiments of an OLED inkjet printing system 100 may consist of a number of devices, devices, systems, etc., that allow ink droplets to be reliably placed at specific locations on a substrate. According to various embodiments of the systems and methods, a printing system may include, for example and without limitation, a substrate transport system 110, a substrate support device 120, a motion system 130, a printhead assembly 140, an ink delivery system 150, and a control system 160.

The OLED substrate may be inserted into and removed from the printing system 100 using the substrate delivery system 110 . Depending on the various implementations of printing system 100, substrate transport system 110 may be a mechanical conveyor, a substrate floatation station with a gripper assembly, a robot with an end effector, and combinations thereof. Additionally, during the printing process, the substrate may be supported by a support device 120, which may be, for example but not limited to, a chuck or a floating table. Because printing requires relative motion between the print head and the substrate, various embodiments of the printing system 100 may have a motion system 130, which may be, for example but not limited to, a gantry or splitaxis XYZ system .

Printhead assembly 140 may include at least one printhead device that may be mounted to motion system 130 . At least one printhead device included in printhead assembly 140 may have at least one inkjet printhead capable of ejecting droplets of an ink composition through at least one orifice at a controlled rate, velocity, and size. Various embodiments of printing system 100 according to the present teachings can have from about 1 to about 60 printhead devices. Furthermore, various embodiments of the printhead apparatus can have from about 1 to about 30 inkjet printheads in each printhead apparatus, where each inkjet printhead can have from about 16 to about 2048 nozzles. According to various embodiments of printhead assembly 140, each nozzle of each inkjet printhead may expel a drop volume of from about 0.1 pL to about 200 pL. A printhead assembly 140 having at least one inkjet printhead can be in fluid communication with an ink composition delivery system 150 that can supply an ink composition to one or more jets of the printhead assembly 140. ink print head.

With regard to various embodiments of the motion system 130, the printhead assembly 140 may move over a fixed substrate (gantry type) during the printing process, or in the case of a split-axis configuration, the printhead assembly 140 and Both substrates can be moved. For the various embodiments of the split axis configuration, Z-axis control can be provided by moving the printhead assembly 140 relative to the substrate. In yet another embodiment of the motion system, the printhead assembly 140 can be fixed and the substrate can move relative to the printhead assembly 140 in the X and Y axes, through the Z-axis of the printhead assembly 140 or through the substrate. The Z-axis movement of the material provides the Z-axis motion. During the printing process, as the printhead assembly 140 moves relative to the substrate, droplets of the ink composition are ejected at the correct time at the desired location to be deposited on the substrate.

For various embodiments of printing system 100, control system 160 may be used to control the functions of the printing process. Various embodiments of the control system 160 may be accessed by end users through a user interface. Control system 180 may be used to control, send data to, and receive data from substrate transport system 110, substrate support device 120, motion system 130, printhead assembly 140, and ink delivery system 150. Control system 160 may be a computer system, microcontroller, application specific integrated circuit (ASIC), field programmable gate array (FPGA), electronic circuitry capable of sending and receiving control and data information and capable of executing instructions, and combinations thereof. Control system 160 may include an electronic circuit or control system between substrate transport system 110, substrate support device 120, motion system 130, printhead assembly 140, and ink composition delivery system 150 for the purpose of providing communication between components, for example. Multiple electronic circuits distributed in .

Additionally, various embodiments of control system 160 of printing system 100 may provide data processing, display, and report preparation functions. All such instrument control functions may be dedicated locally to printing system 100, or control system 160 may provide remote control of some or all of the control, analysis and reporting functions. Finally, various embodiments of printing device 100 may be housed in closed system 200 of FIG. 2 .

Figure 2 is a schematic diagram of a gas enclosure system 200 that may house the printing system 100 of Figure 1, according to various embodiments. Various embodiments of gas enclosure system 200 may include gas enclosure assembly 250 , gas purge circuit 230 in fluid communication with gas enclosure assembly 250 , and at least one thermal regulation system 240 in accordance with the present teachings. Additionally, various embodiments of the gas enclosure system can have a pressurized inert gas recirculation system 260 that can supply inert gas for operating various equipment, such as the substrate floating stage of an OLED printing system. Various Embodiments of Pressurized Inert Gas Recirculation System 260 Various embodiments of the inert gas recirculation system 260 may use compressors, blowers, and combinations of both as sources. Additionally, the gas enclosure system 200 can have a filtration and circulation system (not shown) inside the gas enclosure system 200 that, along with other components such as a floating stage, can provide a substantially low particle printing environment.

As depicted in FIG. 2, for various embodiments of gas enclosure assembly 200 according to the present teachings, gas purge loop 230 may include outlet line 231 from gas enclosure assembly 250 to solvent removal assembly 232, and then to Gas cleaning system 234. The inert gas, purged of solvent and other reactive gaseous species such as oxygen and water vapor, is then returned to gas enclosure assembly 250 through inlet line 233 . The gas purge circuit 230 may also include appropriate conduits and connections, and sensors, such as oxygen, water vapor, and solvent vapor sensors. A gas circulation unit such as a fan, blower or motor etc. may be provided separately or integrated eg in the gas cleaning system 234 to circulate the gas through the gas cleaning circuit 230 . According to various embodiments of the gas enclosure assembly, although the solvent removal system 232 and the gas purification system 234 are shown as separate units in the schematic diagram shown in FIG. Units are housed together. Thermal regulation system 240 may include, for example and without limitation, at least one cooler 241 which may have a fluid outlet line 243 for circulating the coolant into the gas enclosure assembly, and a fluid outlet line 243 for returning the coolant to the cooler. Inlet line 245 .

For various embodiments of the gas enclosure assembly 200, the gas source may be an inert gas, such as nitrogen, any noble gas, and any combination thereof. For various embodiments of the gas enclosure assembly 200, the gas source may be a gas source such as clean dry air (CDA). For various embodiments of the gas enclosure assembly 200, the gas source may be a source that supplies a combination of an inert gas and a gas such as CDA.

The gas enclosure system 200 can maintain levels of each of the various reactive gaseous species at 100 ppm or less, such as 10 ppm or less, 1.0 ppm or less, or 0.1 ppm or less Includes various reactive atmospheric gases such as water vapor and oxygen, as well as organic solvent vapors. Furthermore, various embodiments of the gas enclosure assembly can provide a low particulate environment that meets the specification range for atmospheric particulate matter according to ISO 14644 Class 1 to Class 5 cleanroom standards.

Although an exemplary OLED inkjet printing system and gas enclosure system are given above, it will be appreciated by those skilled in the art that any combination of one or more of the devices and devices of FIGS. 1 and 2 and additional devices can be used and devices to build such a system.

The final inkjet printed product is a HIL with highly uniform thickness and composition. For example, layers having a thickness variation of no more than 10% across the entire width of the layer are possible. Thickness across layers can be measured using a metrology tool such as a stylus contact profilometer or an interferometric microscope. Suitable interferometers for optical interferometry are commercially available from Zygo instrumentation.

The ink composition can be used to print HILs directly in multilayer OLED architectures. A typical OLED includes a supporting substrate, an anode, a cathode, a HIL disposed on the anode, and an emissive layer (EML) disposed between the HIL and the cathode. Other layers that may be present in the device include a hole transport layer provided between the HIL and the emissive layer to assist in the transport of holes to the emissive layer, and an electron transport layer (ETL) disposed between the EML and the cathode. The substrate is usually a transparent glass or plastic substrate.

In these multi-layer architectures, one or more layers other than the HIL can be formed via inkjet printing, while other layers can be deposited using other film-forming techniques. Typically, various layers will be formed within one or more pixel cells. Each pixel cell includes a base and is defined by a bank that defines the perimeter of the cell. Optionally, surfaces within the unit may be coated with surface modifying coatings such as surfactants. However, in some embodiments, such surfactants are absent because they can quench the emission of the emissive layer.

3 is a schematic diagram of a flat panel display including a plurality of OLEDs arranged in a matrix of pixel cells. FIG. 3 depicts an enlarged view 320 of an area of panel 300 showing an arrangement 330 of pixel cells, including red-emitting pixel cells 332 , green-emitting pixel cells 334 and blue-emitting pixel cells 336 . Additionally, integrated circuits 338 may be formed on the flat panel display substrate such that circuitry is adjacent to each pixel cell for the purpose of applying voltage to each pixel in a controlled manner during use. The size, shape and aspect ratio of the pixel elements may vary according to, for example, but not limited to, the desired resolution. For example, a pixel cell density of 100 ppi may be sufficient for a panel for a computer display, wherein for a high resolution of, for example, about 300 ppi to about 450 ppi, it may result in a higher efficiency subject to effective packaging on the surface of the substrate. Various pixel cell designs for pixel density.

While the above disclosure has focused on aqueous ink compositions formulated for inkjet printing of polythiophene-based HILs, another aspect of the present technology provides non-aqueous, Ink compositions based on organic solvents. This organic HIL/HTL ink composition contains components conventionally considered wetting agents, but incorporated into the HTL ink in carefully controlled amounts such that it virtually prevents uncontrolled wetting that may occur as a result of wetting. The spread and pixel-unit overflow of . In some embodiments, the organic ink comprises: (1) a hole injection material or a hole transport material; (2) one or more organic solvents that dissolve the hole injection or hole transport material; and (3) Fluorosurfactant. The hole-injecting or hole-transporting material is typically not greater than about 5 wt.%, more typically not greater than 2 wt.% and still more typically not greater than about 1 wt.% (e.g., about 0.1 to about 1 wt. .%) is present in an amount based on the total weight of the ink composition. Organic solvents typically comprise from about 95 to about 99.8 wt.% of the ink composition. Fluorinated surfactants are typically present in an amount not greater than about 0.15 wt.%. For example, in some embodiments of the organic solvent-based ink composition, the fluorinated surfactant is present in an amount of about 0.03 wt.% to about 0.1 wt.%.

As noted above, suitable hole injecting materials for organic solvent based ink compositions include polythiophenes. Suitable hole transport materials include polyvinylcarbazole or its derivatives, polysilane or its derivatives, polysiloxane derivatives with aromatic amines in the side chain or main chain, pyrazoline derivatives, aryl Amine derivatives, stilbene derivatives, triphenyldiamine derivatives, polyaniline or its derivatives, polythiophene or its derivatives, polyarylamine or its derivatives, polypyrrole or its derivatives, poly(p- Phenylvinylene) (poly(p-phenylenevinylene)) or its derivatives, or poly(2,5-thienylene vinylene) (poly(2,5-thienylene vinylene)) or its derivatives.

Suitable organic solvents for use in HIL/HTL ink compositions include alkoxy alcohols, alkyl alcohols, alkyl benzenes, alkyl benzoates, alkyl naphthalenes, amyl octanoate, anisole, aryl alcohols, benzyl Alcohol, butylbenzene, propyl phenyl ketone, cis-decalin, dipropylene glycol methyl ether, dodecylbenzene, mesitylene, methoxypropanol, methyl benzoate, methylnaphthalene, methyl Pyrrolidone, phenoxyethanol, 1,3-propanediol, pyrrolidone, trans-decalin, valerophenon, and mixtures thereof.

Fluorosurfactants are surfactants that contain fluorinated alkyl chains. E. I. du Pont de Nemours and Company (Wilmington, Delaware) sells fluorinated surfactants under the tradenames Capstone and Zonyl. The fluorosurfactant may be, for example, a fluorotelomer (eg telomere B monoether with polyethylene glycol or 2-perfluoroalkylethanol). Commercially available fluorosurfactants include Zonyl® FS 1033D, Zonyl® FS 1176, Zonyl® FSG, Zonyl® FS-300, Zonyl® FSN, Zonyl® FSH, Zonyl® FSN, Zonyl® FSO, Zonyl®FSN- 100, Zonyl® FSO-100, Zonyl® FSH, Zonyl® FSN, Zonyl® FSO, Zonyl® FSH, Zonyl® FSN, Zonyl® FSO, Zonyl® FS 500, Zonyl® FS 510, Zonyl® FSJ, Zonyl® FS- 610, Zonyl® 9361, Zonyl® FSA, FSP, FSE, FSJ, Zonyl® FSP, Zonyl® 9361, Zonyl® FSE, Zonyl® FSA, Zonyl® UR, Zonyl® 8867L, Zonyl® FSG, Zonyl® 8857A, Foraperle® 225, Forafac® 1268, Forafac® 1157, Forafac® 1183, Zonyl® 8929B, Zonyl® 9155, Zonyl® 9815, Zonyl® 9933LX, Zonyl® 9938, Zonyl® PFBI, Zonyl® PFBEI, Zonyl® PFBE, Zonyl® PFHI, Zonyl® BA, -8- Zonyl® PFHEI, Zonyl® TM, Zonyl® 8932, Zonyl® 7910, Zonyl® 7040, Foraperle® 321/325, Zonyl® 9464, Zonyl® NF, Zonyl® RP, Zonyl® 321, Zonyl® ® 8740, Zonyl® 225, Zonyl® 227, Zonyl® 9977, Zonyl® 9027, Zonyl® 9671, Zonyl® 9338 and Zonyl® 9582, Capstone® ST-500, Capstone® ST-300, Capstone® ST-200, Capstone ® ST-110, Capstone® P-640, Capstone® P-623, Capstone® P-620, Capstone® P-600, Capstone® FS-10, Capstone® FS-17, Capstone® FS-22, Capstone® FS -30, Capstone® FS-31, Caps tone® FS-3100, Capstone® FS-34, Capstone® FS-35, Capstone® FS-50, Capstone® FS-51, Capstone® FS-60, Capstone® FS-61, Capstone® FS-63, Capstone® FS-64, Capstone® FS-64, Capstone® FS-65, Capstone® FS-66, Capstone® FS-81, Capstone® FS-83, Capstone® LPA, Capstone® 1460, Capstone® 1157, Capstone® 1157D, Capstone® 1183, Capstone® CPS, Capstone® E, Capstone® LMC, Capstone® CP, Capstone® PSB, Capstone® 4-I, Capstone® 42-I, Capstone® 42-U, Capstone® 6-I, Capstone® 62-AL, Capstone® 62-I, Capstone® 62-MA, Capstone® TC, Capstone® TR, and Capstone® TS.

Example

Example 1: Effect of polymethylsiloxane on intra-pixel uniformity

The following examples illustrate the contact line containment provided by polymethylsiloxanes in HIL inkjet ink compositions and the resulting improvement in uniformity of light emission.

Materials and methods.

Preparation of HIL ink composition:

HIL ink compositions A and B were prepared with the components and concentrations shown in Table 1. Compositions A and B both contained the indicated concentrations of methicone. As a comparative example, an ink composition containing the ingredients listed in Table 2 but lacking methicone (comparative composition) was prepared.

Table 1

.

Table 2

.

The ink composition was formulated by placing a clean vial on a balance and using a Pasteur pipette to transfer the required amount of Botanisil S-18 into the vial. Tare the balance and pipette sequentially 1,3-propanediol, water and DPGME into vials. The vial was then removed from the balance, capped and swirled to mix the resulting aqueous solution. The vial was then returned to the balance and the required amount of PEDOT dispersion (Haraeus Clevios™ PVP Al 4083) was pipetted into the vial. The vial was then removed from the balance, capped and swirled to mix the PEDOT with the other components of the mixture. The resulting PEDOT ink composition was then filtered with a polytetrafluoroethylene (PTFE) filter membrane (2.0 μm), and the filtered composition was collected in an amber bottle. Finally, the bottle was sonicated for 15 minutes before use.

A comparative ink composition was prepared using the same procedure without Botanisil S-18.

Viscosity and Surface Tension Measurements:

Viscosity measurements were performed using a DV-I Prime Brookfield Rheometer. Surface tension was measured using a SITA bubble pressure tensiometer. In Tables 1 and 2, measurements are provided for ink compositions A and B and a comparative ink composition (Comparative Composition) comprising methicone.

HIL inkjet printing and OLED manufacturing:

The HIL ink composition was printed onto the ITO anode in the OLED architecture. The substrate of the OLED is glass with a thickness of 0.5 mm, on which an anode of 60 nm ITO (Indium Tin Oxide) is patterned. The bank material (also known as the pixel defining layer) is then patterned on the ITO, forming cells in which the inkjet printed layer is deposited. The bank material is a negative working photoresist designed for inkjet printing. The resulting cells have banks with a height of about 0.5 to 2 µm that are angled at 45° relative to the bottom of the cell such that the opening of each cell is wider than its bottom. A 45° angle is representative of typical bank angles ranging from about 5° to about 70°. The width and length dimensions of the cells are approximately 60 x 175 µm. The HIL layer was then inkjet printed into the cell using the ink compositions of Tables 1 and 2, dried under vacuum and baked at high temperature to remove water and solvent from the layer.

The HIL ink composition was printed at room temperature using the inkjet printing system described in PCT Application Publication No. WO 2013/158310, the entire disclosure of which is incorporated herein by reference. Inkjet printing into pixel cells was performed by filling a bulk ink reservoir with the HIL ink composition. The bulk ink reservoir is in fluid communication with the main distribution reservoir and provides a continuous supply of HIL ink composition to the main distribution reservoir during printing. The HIL ink composition is then fed into a printhead comprising a plurality of nozzles through which the HIL ink composition is ejected into the pixel cells. Typical drop volumes during printing are about 10 pl, and about 3 to 10 drops are printed into each cell to form drops of the ink composition in the cell.

OLEDs incorporating HIL printed with ink composition A were fabricated as follows. The HTL layer was inkjet printed onto the HIL layer, then dried under vacuum and baked at high temperature to remove solvent and induce crosslinking in the crosslinkable polymer. The EML layer was then inkjet printed onto the HTL layer, followed by drying under vacuum and baking at high temperature to remove the solvent. The HTL and EML layers were inkjet printed using the printer described above. The HTL ink composition consists of a hole transport polymer material in an ester based solvent system consisting of a mixture of distilled and degassed diethyl octanoate and octyl octanoate in a 1:1 weight ratio . The EML ink composition consists of an organic electroluminescent material in diethyl sebacate.

The ETL layer is then applied by vacuum thermal evaporation followed by the cathode layer. The ETL material includes lithium quinolate (LiQ) as an emission material, and the cathode layer consists of 100 nm of aluminum.

result.

Droplets of Compositions A and B printed into the pixel cells were pinned to the pixel banks and experienced neither overflow nor detachment. An image of a HIL layer prepared using Composition A (0.1 wt.% polymethylsiloxane) printed in and pinned to the pixel unit is shown in FIG. 4A . In contrast, the images of the comparative composition printed to the pixel cell (FIGS. 5 and 6) show that in the absence of methicone, the ink composition spreads uncontrollably and overflows the side 500 of the pixel cell (FIGS. 5A) or de-wetting from the bank of the pixel cell (dewetting), resulting in a dewetting area 602 on the bottom of the cell combined with some pixel cell overflow 600 (FIG. 6A).

For each photomicrograph shown in Figures 4A-6A above and Figures 7A-9A below, a black and white line drawing is provided and labeled with the corresponding "B" picture.

The electroluminescent properties of OLED pixels comprising inkjet printed HILs prepared with composition A were also investigated. Once the OLED was fabricated, its electroluminescence uniformity was studied by applying a current across the diode and imaging the light emission. The resulting luminescence is shown in the photomicrograph of Figure 7A. As can be seen in this figure, the HIL layer printed with Ink Composition A contributes to the uniform light emission of the OLED pixels into which the HIL layer is incorporated.

Embodiment 2: the influence of sulfolane on printing characteristics

The following examples illustrate the improved printing properties imparted by sulfolane to HIL ink compositions.

Materials and methods.

Preparation of HIL ink composition:

HIL ink compositions containing the methicone, sulfolane, and other ingredients listed in Table 3 were prepared.

table 3

Element Wt.% PEDOT 59.9 DPGME 5 Sulfolane 10 H ₂ O 25 Surfactant (S18) 0.1 Viscosity [cP] 5.9 ST [dynes/cm] 45.0

Ink compositions were formulated as described in Example 1, except that sulfolane was used instead of 1,3-propanediol.

Viscosity and Surface Tension Measurements:

Viscosity and surface tension measurements were performed as in Example 1.

HIL inkjet printing and OLED manufacturing:

As described in Example 1, the HIL ink composition was printed and OLED pixels were formed for electroluminescence testing.

Latency measurement:

Ink retardation measurements were performed using the inkjet printing system described in PCT Application Publication No. WO 2013/158310. Measurements were made by firing a nozzle and measuring 300 data points for volume, velocity and directionality. The nozzle was then left to idle for 30 minutes. The nozzle was restarted after 30 minutes and more than 300 data points were recorded.

Data sets were plotted and compared at the beginning of the second data set (after a 30-minute idle) compared to steady-state injection (end of first data set, before 30-minute idle) to look for any priming effects (usually speed drop and volume change).

Ink retardation measurements were also performed using a Dimatix Fujifilm DMP-2831 printer. In the droplet-watching setup, turn on all 16 nozzles and verify that all nozzles are firing. Spraying was then stopped for 5 minutes. Restart spraying and check that all nozzles are still working. Then, continuous spraying was carried out for 15 and 30 minutes. Lag time was measured as the time between the end of jetting and the beginning of drying of the ink in the uncapped nozzle, which resulted in improper droplet ejection. To determine when the ink compositions were dry, they were examined under a microscope in white light and fluorescent modes.

result.

Once the OLEDs were fabricated, their electroluminescence uniformity was studied by applying a current across each diode and imaging the light emission. Electroluminescence was measured for OLEDs with HILs printed with the ink compositions of Table 3 and for OLEDs with HILs printed with the ink compositions of Table 1 . A comparison of the micrographs of Figures 8 and 9 shows that sulfolane in the HIL ink composition provided more uniform pixel luminescence (Figure 8A) than propylene glycol (Figure 9A).

Furthermore, the maximum stable jetting frequency (1000 Hz) of the ink composition comprising sulfolane was higher than that of the ink composition comprising diol. Finally, the delay time for the ink composition containing sulfolane was over 30 minutes compared to only 15 minutes for the ink composition containing diol. The results of the retardation tests measured using the inkjet printing system described in PCT Application Publication No. WO 2013/158310 are shown in FIGS. 10 to 12 . In these figures, the ink containing sulfolane is named P113. Figure 10 is a graph of the drop volume of an ink composition over 14 minutes before idle and after a 30 minute idle. Figure 11 is a graph of drop velocity over 14 minutes for an ink composition before idle and after a 30 minute idle. As can be seen in this figure, the droplet velocity at restart is only 4% lower than the droplet velocity before idle. Figure 12 is a graph of drop angles for ink compositions over 14 minutes before idle and after a 30 minute idle. No significant difference in drop angle was observed before and after idling.

The word "exemplary" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other aspects or designs. In addition, for the purposes of this disclosure, the use of "and" or "or" is intended to include "and/or" unless expressly stated otherwise.

The foregoing description of exemplary embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teaching or may be acquired from practice of the invention. The embodiment was chosen and described in order to explain the principles of the invention and as a practical application of the invention to enable others skilled in the art to use the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the appended claims and their equivalents.

Claims (19)

1. A method of forming a hole injection layer for an organic light emitting diode, the method comprising: Inkjet printing droplets of an ink composition on an electrode layer in pixel cells of an organic light emitting diode, the pixel cells being defined by pixel banks, the ink composition comprising: Conductive polythiophene; water: at least one organic solvent; and a polymethylsiloxane, wherein the polymethylsiloxane is present in an amount to provide contact line containment of the droplets in the pixel cell; and The volatile components of the ink composition are allowed to evaporate, thereby forming a hole injection layer. 2. The method of claim 1, wherein the conductive polythiophene is PEDOT. 3. The method of claim 2, wherein the PEDOT is present in an amount of at least 50 wt%. 4. The method of claim 2, wherein said electrode layer comprises a transparent conductive material. 5. The method of claim 4, wherein the transparent conductive material is indium tin oxide and polymethylsiloxane is present in an amount of 0.03 wt% to 0.12 wt%. 6. The method of claim 1, wherein the at least one organic solvent is an aprotic solvent having a surface tension of no greater than 55 dynes/cm and a viscosity of no greater than 15 cPs at 25°C. 7. The method of claim 6, wherein the at least one organic solvent has a boiling point of 240°C or higher at atmospheric pressure. 8. The method of claim 1, wherein said at least one organic solvent is sulfolane. 9. The method of claim 8, wherein the sulfolane is the main organic solvent in the ink composition. 10. The method of claim 8, wherein the sulfolane is present in an amount of at least 5 wt%. 11. The method of claim 3, wherein said electrode layer comprises indium tin oxide, polymethylsiloxane is present in an amount of 0.05 wt% to 0.1 wt%, and said at least one organic solvent is sulfolane at 5 present in amounts of up to 12 wt%. 12. The method of claim 8, wherein the ink composition further comprises a second organic solvent having a lower surface tension and a lower boiling point than sulfolane. 13. The method of claim 12, wherein the second organic solvent is propylene glycol methyl ether. 14. The method of claim 1, wherein the ink formulation has a surface tension of no greater than 47 dynes/cm at 25°C, a viscosity of no greater than 15 cPs at 25°C, and a delay time of at least 20 minutes at 25°C. 15. An ink composition comprising: PEDOT: water: at least one organic solvent having a surface tension of no greater than 55 dynes/cm at 25°C, a viscosity of no greater than 15 cPs at 25°C, and a boiling point of at least 200°C; and A polymethylsiloxane, wherein the polymethylsiloxane is present in an amount to provide contact line containment of the droplets in the pixel cell. 16. The ink composition of claim 15, wherein the at least one organic solvent is sulfolane. 17. The ink composition of claim 16, further comprising a second organic solvent having a lower surface tension and a lower boiling point than sulfolane. 18. The ink composition of claim 17, wherein the second organic solvent is propylene glycol methyl ether. 19. An ink composition comprising: 50 to 70 wt% of PEDOT; 3 wt% to 10 wt% sulfolane; and 0.03 to 0.12 wt% of polymethicone.