1. Field of the Invention
Generally, this disclosure relates to light-emitting devices (LEDs). More specifically, it relates to a method and apparatus for improved printed cathodes for LEDs.
2. Description of the Related Art
In recent years, light-emitting polymer devices (LEPDs) have been developed for use as indicators, back-lighting in liquid crystal displays and instrument panels, and to replace vacuum fluorescent and liquid crystal displays. There are several patents that teach how different LEPD layers enable the efficient production of electrically stimulated light emission. For instance, U.S. Pat. No. 6,284,435 to Cao discloses electrically active polymer compositions and their use in efficient, low operating voltage, polymer light-emitting diodes with air-stable cathodes. Additionally, U.S. Pat. No. 5,399,502 to Friend et al. shows a method of manufacturing electroluminescent devices. Finally, U.S. Pat. No. 5,869,350 to Heeger et al. demonstrates the fabrication of visible light emitting diodes from soluble semiconducting polymers.
Screen printing, gravure printing, flexographic printing and ink-jet printing can be cost-effective fabrication techniques that can be used to deposit some or all of the layers of LEPDs. As an example, U.S. Patent Application Publication No. 2002/0013013 to Victor et al., which is fully incorporated herein by reference for all purposes, describes a novel screen printing technique for LEPDs. Printing techniques, as opposed to conventional vacuum deposition, photolithography and subtractive patterning, are particularly attractive for LEPD fabrication because they can have high throughput, patterning can be easily customized and they are capable of processing flexible substrates. Furthermore, printing operations that can be carried out in ambient conditions, or printed layers that are stable in ambient conditions at all or some intermediate stages of the fabrication and encapsulation process, can make processing considerably less expensive and complex.
In conventional organic and polymer light emitting device processing, even those typical approaches that utilize printing to deposit the emitting and transport layers, the top electrode, i.e., the cathode, is not directly printed (i.e. via liquid processes under atmospheric conditions). Instead, techniques such as vacuum evaporation of low work function metals have been used for this top electrode, which can greatly increase the complexity and cost of fabricating the LEPDs. In addition to the costs of the deposition process itself, the low work function and/or reactive electrode or electrode interlayers (e.g., such as Calcium (Ca), Barium (Ba), Lithium Fluoride (LiF) or others) usually dictate that all subsequent processing be performed in oxygen and/or water free environments to prevent degradation.
In U.S. Patent Application Publication Nos. 2003/0151700 and 2003/0153141, both to Carter et al., which are fully incorporated by reference for all purposes, ink formulations, compositions, and structures for printed LEPD and printed cathode layers which can be stable in air are described. These approaches outline a path toward low cost, high-volume, web-printable LEPDs on flexible and rigid substrates through the high throughput, reduced cost and reduced complexity inherent to printing. However, printed LEP and/or printed electrode devices can require higher voltages and/or have lower efficiencies than their vacuum or area-coated counterparts (such as spin-coated). It would therefore be helpful to reduce this voltage and improve the efficiency to allow for lower cost drivers, easier battery integration, lower power consumption, and so on. Furthermore, high voltages, low efficiencies and high current densities (either for the whole device or at localized areas) can lead to low operational-lifetimes and shelf-lifetimes which can be detrimental for many applications. Printed LEPD devices can have effective resistances ranging from 1.5 k ohms-cm2 to 20 k ohms-cm2 at bias voltages ranging from 3 volts to >30 volts for typical drive conditions in the range of 0.5 mA/cm2 to 5 mA/cm2 current density and luminances ranging from 30 Cd/m to 250 Cd/m2. Efficiencies for ideal spin coated and evaporated LEPD devices can exceed 10 candelas per ampere (Cd/A). However, all-printed devices can range from 1-7 cd/A maximum efficiency.
One initial approach to printable cathode materials is to adapt conventional conducting inks, such as silver-flake (Ag-flake) ink, used for inorganic electroluminescent devices, flex circuit, membrane switch, and other applications. These inks are commercially available form a number of sources including Dupont, Acheson, Cookson, Conductive Compounds, Sumitomo MM, Englehard, Dow-Corning and others. These inks can be thermoplastic and thermoset inks, including binders and metallic particle and flake particles. Typically, thermal treatments are required to achieve mechanical properties, adhesion, high conductivities and efficient injection electrodes and interconnects for light-emitting devices. (See, “Understanding and measuring electrical resistivity in conductive inks and adhesives”, Banfield, D., SGIA Journal, June 2000.
This thermal treatment can perform several functions including: removal of solvents, removal or decomposition of additives or byproducts, melting of binders to allow particle settling, reaction of thermosetting binders, film shrinkage, better particle to particle contact, and nestling of particles for higher density and increased electrode-to-LEP contact area, particularly for flattened flakes or other non-spherical particles. For some lower temperature-curing inks suitable for use with flexible substrates, cure temperatures of 90° C. and above may be required for maximum cathode and interconnect performance. However, exposure to these temperatures for extended time periods can cause deterioration or variation of the properties of the underlying substrate, environmental barrier layers, anode materials, LEP materials or the interfaces of these materials, particularly if processing is carried out in air or relatively high oxygen and water containing atmosphere for the sake of process simplicity and minimized cost and time. Furthermore, exposure of the LEP-containing layer to liquid and/or solvent containing cathode ink can lead to degradation of the LEP layer and/or the LEP/cathode interface.
Therefore, what are needed are methods and systems concerning the printing and processing of inks to fabricate high performance, printed cathode LEPDs that produce low voltage, high brightness and/or high efficiency printed LEPDs by accelerating cathode curing and solvent removal while still achieving high conductivity and effective cathode to LEP-layer contact.
Rapid thermal processing of printed cathodes for light-emitting polymer devices (LEPDs) to prevent detrimental cathode ink/LEP layer interactions is described herein. The ink layer printed cathode can be thinned during fabrication using high mesh count screens, calendered mesh screens, high squeegee pressures, high hardness squeegees, high squeegee angles and combinations thereof. Alone, or in combination with a thinned ink layer, the printed cathode can be cured using reduced time hot plate processing, infrared processing, heated gas flow processing, or combinations thereof. Where it may be advantageous to have an overall thicker cathode layer for improved conductivity, particle-to-particle contact and/or nestling once the cathode has dried, the cathode layer can be advantageously deposited as a series of thinner wet layers as opposed to a single wet thicker layer.
Aspects and features of the present invention will become apparent to those ordinarily skilled in the art from the following detailed description of certain embodiments in conjunction with the accompanying drawings, wherein:
a) illustrates a simplified cross-sectional diagram of a printed LEPD on flexible substrate as is known in the art today;
b)(1)-(3) illustrates a sequence of cross-sectional diagram of printing an LEPD on flexible substrate according to the present invention,
a) illustrates an example of actual effects of using different cathode ink screen mesh configurations on printed cathode LEPD devices according to certain embodiments;
b) shows the impact of different cathode mesh sizes on performance.
a)-(d) illustrate printing of a cathode using multiple thin cathode layers according to the present invention.
Embodiments will now be described in detail with reference to the drawings, which are provided as illustrative examples so as to enable those skilled in the art to practice the embodiments and are not meant to limit the scope of the disclosure. Where aspects of certain embodiments can be partially or fully implemented using known components or steps, only those portions of such known components or steps that are necessary for an understanding of the embodiments will be described, and detailed description of other portions of such known components or steps will be omitted so as not to make the disclosure overly lengthy or unclear. Further, certain embodiments are intended to encompass presently known and future equivalents to the components referred to herein by way of illustration.
Rapid thermal processing of printed cathodes for light-emitting polymer devices (LEPDs) to substantially prevent detrimental cathode ink/LEP layer interactions is described herein. Some novel innovations that can be included in certain embodiments include fast solvent removal, which can help prevent detrimental Ag ink solvent/LEP interface interactions and can also help prevent softening or flow of underlying LEP. Softening and flow of the underlying LEP can lead to Ag penetration into LEP layer and shorting. Preventing softening of the underlying LEP can also help prevent partial dissolution and/or redistribution of the LEP layer, which can lead to thickness and EL variation. Higher temperatures, provided in more detail hereinafter, can help enable good Ag ink particle settling/nestling for high lateral conductivity and Ag/LEP contact. Further, short heating times, also described further hereinafter, can help limit detrimental heating effects on LEP, LEP/cathode interface and low T substrate, and can help limit deformation, oxidation, and/or other reactions; and rapid transfer between the printing station and first stage drying operation.
Through techniques described herein, it is possible to minimize cathode solvent dissolution, LEP layer softening, or other effects that may lead to shorting or detrimental changes of underlying layer morphology, composition or chemistry can be minimized. Chemical or compositional changes can include leaching of components from LEP or transport layers, solvent degradation of materials, and the introduction of harmful solvent residues or other harmful surfactants, cosolvents, impurities or other species from the cathode ink in to the active layer of the device.
In a particular, embodiment, the rapid thermal processing includes disposing the conductive ink, and thereby the solvent, over the organic light emitting layer for a period of time that is less than about 1 minute before a majority of the solvent, and preferably more than 70% of the solvent, evaporates. Evaporation that occurs even more quickly is desirable.
In another embodiment, maximizing the volume percentage of solid, such that there is less than 40% solvent by volume and/or less than 25% solvent by weight, and thereby minimizing the amount of solvent, in the conductive ink also assists with the rapid thermal processing and a resulting device having better characteristics. Having even percentages where the amount of solvent is even less is further preferred, such as less than 20% solvent by weight.
In a particular embodiment, the solvent amount limit is in the lower parameter range for a typical printed Ag conductor in general applications. This is in order to minimize solvent deposited in the cathode ink layers. In such an embodiment the cathode ink layer deposit includes less than 10 g solvent/m2 of printed area, which is an estimate of the largest amount of solvent deposited in a cathode print pass based on the following Tables I and II that show a range of typical solvents, the range of ink deposits from the different screens described herein, and the upper range of solvent content/m2 of printed cathode ink area (40% solvent mass fraction).
In another embodiment, the solvent containing ink deposit is a solvent containing layer of less than 12 microns in thickness, which can be used in forming a cathode from multiple layers where each layer is less than 12 microns thick.
b)(1)-(3) illustrates a sequence of cross-sectional diagram of printing an LEPD on flexible substrate according to the present invention. Layer thicknesses are not to scale. As shown, a typical substrate 210 thickness may be between 100 and 200 microns and may be, for example, a plastic substrate composed of polyethylene teraphthalate (PET), poly ethylene napthalate (PEN), poly carbonate (PC) or similar. As with the conventional devices, in some cases, to improve shelf life and product lifetime, the substrate includes a barrier film, composed of inorganic and/or organic materials, which restrict the ingress of water, oxygen and other species into the active areas of the device. Disposed on substrate 210 can be a transparent anode 220, such as indium-tin-oxide (ITO) layer, which can have a thickness between 50-300 nm. A light-emitting polymer 230, with a thickness between 200 nm and 1 micron can be disposed on transparent anode 220. Finally, a cathode 240, which is deposited as a wet printed ink layer and typically results in a dry thickness of between 100 nm and 10 microns, depending on fabrication approach, can be disposed on light-emitting polymer 230.
The cathode ink affected zone (at the interface of polymer 230 and cathode 240) should be reduced below 300 nm, and preferably below 100 nm. The teachings herein describe how to do that, in order to minimize any adverse affects of the wet ink that is used to form the cathode 240 on the polymer 230 below.
In certain embodiments, the printed cathode ink layer thickness can be reduced to a wet thickness as shown in Table III below, although it should be understood that the volume or weight percentage of the solvent within the cathode ink will also have an affect. For Table 1, the characteristics of the conductive ink are preferably >70% solid by weight. In this manner, a thin printed cathode ink layer contains less solvent, resulting in less material to interact with the LEP layer on which it is disposed and less solvent and/or other byproducts to remove in the curing and drying steps. Secondly, solvent and/or other byproduct removal is faster from a thin film, as the transit distance for these components to the free surface, from which these components can escape the sample, is shorter in a thin film. Table 1, below, shows various exemplary printed cathode screen configurations (e.g., plain weave). Of particular interest are the 380 mesh count and higher designs that produce small theoretical ink deposits (and therefore thinner ink films). The mesh opening size can also be considered, as a small mesh opening, as compared to the ink particle size, can lead to clogging.
a) illustrates an example of actual effects 200 of using different cathode ink screen mesh configurations on printed cathode LEPD devices according to certain embodiments. This figure shows averaged experimental data for printed cathode LEPDs where different screen mesh configuration were used to print the same Ag flake-based cathode ink. Varying the cathode screen mesh count varies the ink deposit volume/area and therefore the ink film thickness. In experimental devices, the 380 mesh screen, which has a 45% smaller theoretical ink deposit than the 230 mesh screen (e.g., 12 cm3/m2 vs. 22 cm3/m2, as presented in Table 1), results in a higher LEPD brightness during continuous, constant current finished display driving (in N2) and at a lower voltage (voltage not pictured here). The 380 mesh screen produces a brightness of about 76.5 cd/A, while the 230 mesh only achieves about 67.5 cd/A. In a further example, a 508 thread/inch calendered mesh screen shows improved voltage and brightness over a device with a thicker cathode printed from a 460 mesh screen, as also shown in Table 1.
b) shows the impact of different cathode mesh sizes on performance. In the case of the higher 460 mesh count (and reduced ink deposit), the maximum luminance and the persistence of luminance levels under bias stress over time is greater than for the device with a cathode printed from a lower 380 mesh count. Furthermore, the voltage under bias stress over time is also reduced which is an additional benefit of the higher mesh count, reduced ink deposit screen.
It is also noted that the above description discussed printing in the preferred embodiment as screen printing. It is noted that the aspects of this invention relating to cathode layer thicknesses, thermal treatments and drying conditions can also be applied to electrodes printed from inks by gravure, ink jet, coating, offset, spray coating, stencil printing, etc. can also be used within the scope of the invention.
In certain embodiments, the use of a higher mesh count screen for printing of cathode inks can help reduce the thickness of the cathode ink, which may help reduce the amount of ink solvent available to detrimentally interact with the LEP. Having a thinner ink layer can also assist in speeding up solvent removal, since there is less of an overlying ink layer to prevent solvent from escaping from interior regions of the film, particularly those regions closest to the LEP interface. Also, in certain embodiments, the use of calendered mesh screens for cathode deposition can be used to help reduce the deposited ink film thickness. The calendering of screens is a process by which the woven mesh is flattened, resulting in deformation of the threads and reduction of the theoretical ink volume of the mesh by compressing the ink holding volume in the screen. These aspects can be used (or not) alone or in combination with each other, as well as other aspects and embodiments presented herein.
According to certain embodiments, the use of high squeegee pressures and harder squeegees can also be used to minimize printed film thickness.
In certain embodiments, hot plate curing can facilitate rapid heating of LEPDs and/or organic light-emitting devices (OLEDs) on flexile substrates through direct heat transfer from plate to substrate. This can provide very fast cathode ink curing and solvent removal as the sample can be transferred directly from the cathode ink print station to the hot plate in a rapid operation leading to uncured cathode ink residence times of less than 30, or even less than 10, seconds depending on cure temperature. In certain embodiments, during hot plate curing, samples can be heated through the underlying substrate and films, resulting in heating of the bottom of the printed ink film first (i.e., closest to the LEP in a bottom anode/LEP/top cathode printed configuration), which can further result in a higher bottom temperature due to the temperature gradient that normally forms between the heated bottom surface and the cooler, top, free surface. This heating profile through the thickness of the film favors loss of solvent from the bottom surface of cathode ink layer first, which is generally the most critical area of the film as it is in direct contact with the LEP layer. This heating profile can also reduce detrimental skin effects, which can result from curing of the top layer of the printed ink first (i.e., curing the top layer can produce a cured ‘skin’ which can slow solvent and/or curing byproduct removal from the interior of the film).
In certain embodiments, rapid printed cathode ink cure can also be facilitated using hot plate curing and a process-specific heating and temperature profile that is induced in the film. For example, a temperature regulated hot plate with a mechanism for good thermal contact and heating uniformity can be used. Further, a nitrogen flow/environment, potentially heated using convection, which typically operates at temperatures of 80-120 degrees Celsius, can be used to help reduce possible oxidation during thermal processing of the cathode ink. A patterned, metal weight frame can be used to press flexible samples against the hot plate to help increase thermal contact, which might increase heating rate and efficiency (i.e., even in a vacuum environment). A vacuum hold down apparatus can also be used to hold down flexible sample and help improve thermal contact. Ambient lighting with no significant spectral components above the LEP absorption edge can be used to help reduce photo-degradation of the LEP layer(s), particularly at high temperatures. A controlled atmosphere (e.g., N2 purge) can be used to help reduce detrimental oxidation at higher temperatures. These aspects can be used (or not) alone or in combination with each other, as well as other aspects and embodiments presented herein.
In certain embodiments, rapid selective heating of a cathode can be used. This step can include either (or both) irradiation from the cathode side or irradiation through infrared (IR) transparent or partially transparent substrates and LEP layers to the IR opaque metal cathode. This form of heating can be easily performed on flexible substrates in sheet or roll-form using a separate heating unit or using an in-line processor in a web. Irradiating through the substrate/LEP side also heats the bottom surface of the cathode ink layer first. This bottom-first heating can lead to solvent removal and curing of the LEP/cathode interface first, which can otherwise be detrimentally effected from prolonged contact to some cathode ink solvent components. Bottom-first heating can also be a more efficient mode of solvent removal as opposed to heating of the top surface, which can lead to skin formation and trapping of detrimental solvents and cure byproducts within the film. Furthermore, the composition of the device layers and/or the spectrum of the IR lamp can be adjusted to reduce thermal absorption and heating in the non-cathode layers (e.g., such as the substrate, LEP, anode, etc.) to reduce degradation of these other layers during the cathode curing process.
In certain embodiments, fast heating can be achieved by directing a heated gas stream at the LEPD, for example, the substrate. This can quickly deliver heat to the LEP/cathode ink interface, as well as accelerate drying by removing ink solvent and byproducts from the ink film surface and near surface area, while maintaining a high concentration gradient between the film, film surface and the adjacent ambient atmosphere. A further embodiment includes the use of an inert gas to limit oxidation. In a further embodiment, the heated inert gas process is preceded by an inert gas wash, or purge, that clears the ink area of oxygen and water prior to the application of heat to prevent unwanted oxidation of the cathode material and/or the underlying LEP-containing layers. This could be achieved in a gas stream apparatus in which the heating element could be activated after some purge period.
In certain embodiments, it can be advantageous to deposit the cathode or interconnects as multiple cathode and/or interconnect layers, as opposed to depositing in a single layer. This can serve to increase conductivity, ensure that all metal particles are electrically connected to the cathode, improve nestling and particle contact and other effects. For a cathode or interconnect, it is more advantageous to deposit this as series of thinner layers as opposed to a single thick layer or a multitude of thick layers as the thin layer approach can promote more rapid drying and expose the LEP to less detrimental solvents or other materials that adversely affect the LEP, particularly when the film is wet or incompletely dried. These thin layers can be achieved by the screen variations discussed above based on low ink deposit configurations.
a)-(d) illustrate printing of a cathode using multiple thin cathode layers according to the present invention as described above using a sequence of cross-sectional diagrams of printing an LEPD on flexible substrate. Layer thicknesses are not to scale. As shown, a typical substrate 310 thickness may be between 50 and 200 microns and may be, for example, a plastic substrate composed of polyethylene teraphthalate (PET), poly ethylene napthalate (PEN), poly carbonate (PC) or similar. As with the conventional devices and the embodiment described above, in some cases, to improve shelf life and product lifetime, the substrate includes a barrier film, composed of inorganic and/or organic materials, which restrict the ingress of water, oxygen and other species into the active areas of the device. Disposed on substrate 310 can be a transparent anode 320, such as indium-tin-oxide (ITO) layer, which can have a thickness between 50-300 nm. A light-emitting polymer 330, with a thickness between 200 nm and 1 micron can be disposed on transparent anode 320. Finally, a cathode 340, which is deposited as a multiple wet printed ink layers 340(a) and 34(b) as shown in
Multilayer printing allows for the benefits of thick films: reduced resistance, less conducting particle isolation, and better thermal conductivity away from active layer while minimizing wet cathode ink interactions with active layer. Multi-step printing reduces solvent interaction by minimizing wet ink on active layer surface. Also, the 1st printed layer can provide a barrier to 2nd layer interaction with substrate. This technique also enables use of functional multilayer printing of high stability and or high injection efficiency cathode interface materials, such as carbon, gold, etc. but with enhanced conductivity and/or reduced cost by use of a high conductivity silver layer for the top ‘interconnect layer’ which provide low resistance electrical connectivity.
Although the present invention has been particularly described with reference to embodiments thereof, it should be readily apparent to those of ordinary skill in the art that various changes, modifications, substitutes and deletions are intended within the form and details thereof, without departing from the spirit and scope of the invention. For example, in the claims set forth below, various different combinations of dependent claims not specifically recited are intended as being within the scope of the invention. Accordingly, it will be appreciated that in numerous instances some features of the invention will be employed without a corresponding use of other features. Further, those skilled in the art will understand that variations can be made in the number and arrangement of inventive elements illustrated and described in the above figures. It is intended that the scope of the appended claims include such changes and modifications.