Patent Application
20050079278
Not Applicable
As a result of the broad use of thin films across a wide variety of industrial applications, a tremendous amount of research has been conducted towards the development of various types of thin films and methods for manufacturing them in a cost effective manner. One such type of thin film is the small molecule organic semiconductors, which are currently under development for a number of applications, including displays, transistors and memories. One particular area of interest for these materials is the application of organic light emitting devices (“OLEDs”) for interior room lighting. Proof-of principle experiments have shown that OLEDs can operate at as high as 60 lm/W. Although this record is at low brightness and for a green device, it stands as evidence that large area lighting at this efficiency is scientifically feasible. Indeed, this result was obtained in a planar device geometry from which only ˜20% of the photons generated in the device escape to an observer, thus demonstrating that the theoretical upper limit for device efficiency is at least 300 lm/W in the green. Other areas of application are in large area, very low cost electronics based on organic thin films transistors and low cost, large area photovoltaics.
Despite the promise of energy efficient lighting presented by these materials, a high quality, low cost, high throughput deposition system for these materials does not currently exist. Conventional physical vapor deposition techniques or spin coating, although effective for small area, high value-added applications, are too slow to be viable for the production of low cost lighting. Organic vapor phase deposition using low vacuum and shower-head type geometries derived from the chemical vapor deposition industry have not proven capable of the high deposition rates required for roll-to-roll fabrication. Printing techniques are also too slow and generally restricted to batch manufacturing. A high throughput, continuous feed, roll-to-roll deposition technique for small molecule semiconductors is thus likely the only viable route to high volume production of OLED lighting panels with a cost of ownership competitive with conventional lighting solutions.
Polymer multilayer deposition (PML) is a well-known technique for the high speed deposition of extremely uniform thin films of acrylate-based polymers. In general, the PML process has two forms—evaporative and non-evaporative. Each begins by degassing the working monomer, which is a reactive organic liquid. In the evaporative process, the monomer is metered through an ultrasonic atomizer into a hot tube where it flash evaporates and exits through a nozzle as a monomer gas. The monomer gas then condenses on the substrate as a liquid film that is subsequently cross-linked to a solid polymer by exposure to UV radiation or an electron beam. In the non-evaporative process, the degassed liquid monomer is extruded through a slotted die orifice onto the substrate. It is then cross-linked in the same fashion as in the evaporative process. Salts, graphite or oxide powders, and other nonvolatile materials can be deposited in a homogeneous mixture with the monomer. Such mixtures cannot be flash evaporated, but are required for electrolyte, anode, cathode, and capacitor film layers. The evaporative process has been shown to produce thicknesses up to approximately 10 microns at speeds as great as 1000 feet per minute. The non-evaporative process have been shown to deposit thicknesses from 10 microns to about 50 mils at substrate speeds approaching several hundred feet per minute. Various aspects of the PML processes are described in greater detail in the following U.S. patents, the entire contents of each of which are hereby incorporated herein by this reference: U.S. Pat. No. 6,613,395 Method of making molecularly doped composite polymer material, U.S. Pat. No. 6,570,325 Environmental barrier material for organic light emitting device and method of making, U.S. Pat. No. 6,544,600 Plasma enhanced chemical deposition of conjugated polymer, U.S. Pat. No. 6,522,067 Environmental barrier material for organic light emitting device and method of making, U.S. Pat. No. 6,509,065 Plasma enhanced chemical deposition of conjugated polymer, U.S. Pat. No. 6,506,461 Methods for making polyurethanes as thin films, U.S. Pat. No. 6,497,924 Method of making non-linear optical polymer, U.S. Pat. No. 6,497,598 Environmental barrier material for organic light emitting device and method of making, U.S. Pat. No. 6,358,570 Vacuum deposition and curing of oligomers and resins, U.S. Pat. No. 6,274,204 Method of making non-linear optical polymer, U.S. Pat. No. 6,268,695 Environmental barrier material for organic light emitting device and method of making, U.S. Pat. No. 6,228,436 Method of making light emitting polymer composite material, U.S. Pat. No. 6,228,434 Method of making a conformal coating of a microtextured surface, U.S. Pat. No. 6,224,948 Plasma enhanced chemical deposition with low vapor pressure compounds, U.S. Pat. No. 6,217,947 Plasma enhanced polymer deposition onto fixtures, U.S. Pat. No. 6,207,239 Plasma enhanced chemical deposition of conjugated polymer, U.S. Pat. No. 6,207,238 Plasma enhanced chemical deposition for high and/or low index of refraction polymers, U.S. Pat. No. 5,902,641 Flash evaporation of liquid monomer particle mixture, U.S. Pat. No. 5,681,615 Vacuum flash evaporated polymer composites, U.S. Pat. No. 5,547,508 Vacuum deposition and curing of liquid monomers apparatus, U.S. Pat. No. 5,395,644 Vacuum deposition and curing of liquid monomers, U.S. Pat. No. 5,260,095 Vacuum deposition and curing of liquid monomers.
Unfortunately, the polymeric materials amenable to PML deposition are electrically inert, and although it is possible to incorporate guest molecules into the PML flux, it is difficult to achieve a high enough loading of active material to realize an electrically active device such as an efficient, low voltage OLED. Furthermore, the evaporative mode of PML which is generally used to make films of an appropriate thickness for electronics devices, the guest molecules tend to fractionate out into the flash evaporation box where they accumulate, rather than being deposited on the target substrate. One similar approach is described in U.S. Pat. No. 6,471,327, the entire contents of which are incorporated herein by this reference. As described by the '327 patent, an apparatus and method of focusing a functional material includes a pressurized source of fluid in a thermodynamically stable mixture with a functional material. A discharge device having an inlet and an outlet is connected to the pressurized source at the inlet. The discharge device is shaped to produce a collimated beam of functional material, where the fluid is in a gaseous state at a location before or beyond the outlet of the discharge device. The fluid can be one of a compressed liquid and a supercritical fluid. The thermodynamically stable mixture includes one of the functional material being dispersed in the fluid and the functional material being dissolved in the fluid. Drawbacks associated with the approach of the '327 patent include issues related to handling a highly pressurized fluids, such as a supercritical fluids. Accordingly, to solve the manufacturing problems associated with low cost or large area electronics, a need exists for a technique with similar characteristics to PML, but which is useful for organic semiconductors and which avoids the problems associated with gasses under high pressure.
Accordingly, it is an object of the present invention to provide a method for coating a thin film of a non-polymeric compound on a substrate. The method of the present invention is different from PML in several key ways. One such key difference is related to the difference between PML coatings, which are generally made up of monomer or oligomer materials which are readily delivered in a liquid form to an ultrasonic nozzle directed into a flash evaporation box, and the non-polymeric compounds which ultimately form the coatings of the present invention, such as, by way of example, organic semiconductors. The coatings of the present invention are mostly solids at room temperature and many sublime without passing through a liquid phase and are therefore not readily evaporated in the manner PML coatings are evaporated. A second key difference is that the PML coatings use a monomeric or oligomeric starting material but the deposited film is typically polymeric or rendered polymeric by treatment on the substrate shortly after deposition, whereas the coatings of the present invention are chemically substantially similar to the starting materials. To overcome these differences, the present invention provides a mixture of the non-polymeric compound and a fluid carrier. The mixture typically consists of a slurry of the non-polymeric compound in the fluid carrier. However, the mixture may have all or a portion of the non-polymeric compound in solution in the fluid carrier, or as a colloidal suspension in the carrier, or combinations thereof. As used here, the term “mixture” should be broadly construed to contemplate all of these possibilities. This mixture is then pumped into the interior of a heated evaporation box having an internal temperature sufficient to convert substantially all of the non-polymeric compound and fluid carrier to a gaseous form. The non-polymeric compound and fluid carrier are then removed from the evaporation box via an exit slit in the evaporation box. Adjacent to the exit slit, and maintained in a vacuum, is a substrate upon which the non-polymeric compound condenses. The substrate is in motion relative to the evaporation box, for example on a web roller, thereby allowing a continuous coating of the non-polymeric compound to be applied to the substrate.
Typically, the substrate is maintained at a temperature sufficiently high so that the fluid carrier does not condense on the substrate, thus allowing the formation of a coating of the non-polymeric coating free of any of the fluid carrier. However, this objective can also be accomplished by maintaining the substrate at a temperature sufficiently high so that any fluid carrier that might initially condense upon contact with the substrate quickly evaporates. Alternatively, by maintaining the substrate at a temperature sufficiently low to allow both the fluid carrier and the non-polymeric compound to condense on the substrate at the exit slit of the evaporation box, and subsequently increasing the temperature of the substrate to a temperature sufficient to cause the fluid carrier to evaporate, a coating of the non-polymeric compound free of any of the fluid carrier is likewise formed. The preferred mode of operation is likely to depend on the organic compound being evaporated and/or the desired morphology of the deposited film (i.e. crystalline or amorphous).
Under either approach, the goal is to produce a coating of the non-polymeric material substantially free of any of the fluid carrier. In this manner, the fluid carrier may be captured, allowing the subsequent use of the fluid carrier to provide additional mixture of the non-polymeric compound with the fluid carrier. Capturing the fluid carrier is easily accomplished by providing a cold trap to condense the fluid carrier.
A suitable apparatus for the process is shown in
To fully evaporate the non-polymeric compound and the fluid carrier, the evaporation box temperature is preferably provided as greater than 100° C. The important criterion is that the box be hot enough to evaporate the entire fluid flux entering and be provided with sufficient energy that such temperature is maintained at the desired rate of fluid input. In this mode, the deposition rate on the substrate depends not on the temperature of the evaporation box but only on the fluid pump speed and the condensation efficiency on the substrate. Also, mixtures can be employed, for example, and not meant to be limiting, a doped light emitting layer consisting of 4%fac-tris(2-phenylpyridine)iridium in 4,4′-N,N′-dicarbazole-biphenyl, the emissive layer for a green phosphorescent OLED, could be deposited by starting from an intimately ground mixture of the component materials in a fluid carrier with the evaporation box maintained at such a temperature so as to evaporate both components and the fluid. This improves on the most commonly used technique to produce doped films of small molecule organic semiconductors where two spatially separated sources are individually heated in a high vacuum chamber with the rate of deposition of each material being controlled by the temperature of its respective source. The problem is that the evaporation rate is to first order exponentially dependent on the source temperature so very accurate temperature control is required. In the present invention, the evaporation box is only required to be maintained above a minimum temperature and the composition of the deposited film is substantially determined by the composition of the starting mixture.
The non-polymeric compound may be selected as an organic material, including but not limited to OLED materials such as a metal (8-hydroxyquinoline) chelate, or inorganic materials, or mixtures thereof. It is important to note that as used herein the term “non-polymeric” simply means that the compound that is provided in the mixture is substantially the same as the compound that is ultimately applied to the substrate; ie. it has not been polymerized, as is typical in the PML process. Notably, some monomers which are capable of polymerization, but which are nevertheless NOT polymerized during the deposition process, would therefore qualify as “non-polymeric” compounds as that term is used herein. Oligomers also would be included, as per IUPAC definitions. Suitable OLED materials, together with OLED fabrication techniques and suitable structures for multi-layer materials utilizing OLEDs, have been described in great detail in the patent literature. Suitable OLEDs are described in the following U.S. Patents, the entire contents of each of which are hereby incorporated herein by this reference: U.S. Pat. No. 6,613,395 “Method of Making Molecularly Doped Composite Polymer Material.” (2003), U.S. Pat. No. 6,602,540 “Fabrication of non-polymeric flexible organic light emitting devices.” (2003), U.S. Pat. No. 6,596,134 “Method of Fabricating Transparent Contacts for Organic Devices.” (2003), U.S. Pat. No. 6,582,838 “Red-emitting organic light emitting devices (OLED's).” (2003), U.S. Pat. No. 6,579,632 “OLEDs doped with phosphorescent compounds.” (2003), U.S. Pat. No. 6,570,325 “Environmental Barrier Material for Organic Light Emitting Device and Method of Making.” (2003), U.S. Pat. No. 6,558,736 “Low pressure vapor phase deposition of organic thin films.” (2003), U.S. Pat. No. 6,548,956 “Transparent contacts for organic devices.” (2003), U.S. Pat. No. 6,497,924 “Method of Making a Nonlinear Optical Polymer.” (2002), U.S. Pat. No. 6,469,437 “Highly Transparent Organic Light Emitting Devices Employing a Non-Metallic Cathode” (2002), U.S. Pat. No. 6,468,819 “Method for Patterning Organic Thin Film Devices Using a Die (2002), U.S. Pat. No. 6,451,455 “Metal Complexes Bearing Both Electron Transporting and Hole Transporting Moieties” (2002), U.S. Pat. No. 6,420,031 “Highly Transparent Non-Metallic Cathodes” (2002), U.S. Pat. No. 6,403,392 “Method for Patterning Devices” (2002), U.S. Pat. No. 6,396,860 “Organic Semiconductor Laser.” (2002), U.S. Pat. No. 6,365,270 “Organic Light Emitting Devices” (2002), U.S. Pat. No. 6,358,631 “Mixed Vapor Deposited Films for Electroluminescent Devices”(2002), U.S. Pat. No. 6,337,102 “Low Pressure Vapor Phase Deposition of Organic Thin Films.” (2002), U.S. Pat. No. 6,329,085 “Red-Emitting Organic Light Emitting Devices (OLEDs)” (2001), U.S. Pat. No. 6,330,262 “Organic Semiconductor Lasers” (2001), U.S. Pat. No. 6,303,238 “OLEDs doped with phosphorescent compounds” (2001), U.S. Pat. No. 6,297,516 “Method for Deposition and Patterning of Organic Thin Film.” (2001), U.S. Pat. No. 6,294,398 “Method for Patterning Devices.” (2001), U.S. Pat. No. 6,274,980 “Single Color Stacked Organic Light Emitting Device.” (2001), U.S. Pat. No. 6,264,805 “Method of Fabricating Transparent Contacts for Organic Devices.” (2001), U.S. Pat. No. 6,232,714 “Saturated Full Color Stacked Organic Light Emitting Devices.” (2001), U.S. Pat. No. 6,214,631 “Method for Patterning Light Emitting Devices Incorporating a Movable Mask.” (2001), U.S. Pat. No. 6,160,828 “Organic Vertical-Cavity Surface-Emitting Laser.” (2000), U.S. Pat. No. 6,125,226 “Light Emitting Devices Having High Brightness.” (2000), U.S. Pat. No. 6,111,902 “Organic Semiconductor Laser.” (2000), U.S. Pat. No. 6,097,147 “Structure for High Efficiency Electroluminescent Device.”(2000), U.S. Pat. No. 6,091,195 “Displays Having Mesa Pixel Configuration.” (2000), U.S. Pat. No. 6,048,630 “Red-Emitting Organic Light Emitting Devices (OLEDs) (2000), U.S. Pat. No. 6,046,543 “High Reliability, High Efficiency, Integratable Organic Light Emitting Devices and Methods of Producing Same” (2000), U.S. Pat. No. 6,045,930 “Materials for Multicolor Light Emitting Diodes” (2000), U.S. Pat. No. 6,030,715 “Azlactone-Related Dopants in the Emissive Layer of an OLED” (2000), U.S. Pat. No. 6,030,700 “Organic Light Emitting Devices” (2000), U.S. Pat. No. 6,013,538 “Method of Fabricating and Patterning OLEDs” (1999), U.S. Pat. No. 6,005,252 “Method and Apparatus for Measuring Film Spectral Properties” (1999), U.S. Pat. No. 5,998,803 “An Organic Light Emitting Device Containing a Hole Injection Enhancement Layer” (1999), U.S. Pat. No. 5,986,401 “High Contrast Transparent Organic Light Emitting Device Display” (1999), U.S. Pat. No. 5,981,306 “Method for Depositing Indium Tin Oxide Layers in Organic Light Emitting Devices” (1999), U.S. Pat. No. 5,986,268 “Organic Luminescent Coating for Light Detectors” (1999), 5,953,587 “Method for Deposition and Patterning of Organic Thin Film” (1999), U.S. Pat. No. 5,917,280 “Stacked Organic Light Emitting Devices” (1999), U.S. Pat. No. 5,932,895 “Saturated Full Color Stacked Organic Light Emitting Devices” (1999), U.S. Pat. No. 5,874,803 “Light Emitting Device with Stack of OLEDs and Phosphor Downconverter” (1999), U.S. Pat. No. 5,861,219 “Organic Light Emitting Devices Containing a Metal Complex of 5-Hydroxy-Quinoxaline As a Host Material” (1999), U.S. Pat. No. 5,844,363 “Vacuum Deposited Non-Polymeric Flexible Organic Light Emitting Devices” (1998), U.S. Pat. No. 5,834,893 “High Efficiency Organic Light Emitting Devices with Light Directing Structures” (1998), U.S. Pat. No. 5,757,139 “Driving Circuit for Stacked Organic Light Emitting Devices” (1998), U.S. Pat. No. 5,757,026 “Multicolor OLED” (1998), U.S. Pat. No. 5,721,160 “Multicolor Organic Light Emitting Devices” (1998), U.S. Pat. No. 5,707,745 “Multicolor Organic Light Emitting Devices” (1998), U.S. Pat. No. 5,703,436 “Transparent Contacts for Organic Devices” (1997), U.S. Pat. No. 5,554,220 “Method and Apparatus Using Organic Vapor Phase Deposition for the Growth of Organic Thin Films with Large Optical Non-Linearities” (1996). As will be recognized by those having skill in the art, in addition to the manufacture of light emitting devices, the present invention is readily applicable to the manufacture of thin film transistors, photovoltaic devices and other devices and products requiring a thin coating of an organic material.
The fluid carrier is preferably a fluid that will readily evaporate at the preferred temperature of the evaporation box, and which condenses at a temperature higher than the condensation temperature of the non-polymeric compound. Suitable fluid carriers include, but are not limited to straight chain and branched alcohols and diols, amides, dimethylsulfoxide, N-methylpyrrolidinone, toluene, ketones, esters, halogenated solvents, and combinations thereof.
An experiment was conducted to demonstrate one preferred embodiment of the present invention. An apparatus as described above and shown in
In manufacturing versions of the process, the solvent could be recovered for recycling. The solvent used in this demonstration was 1-hexanol, and the non-polymeric compound was Alq3, but it should be noted that 1-hexanol is a poor solvent for Alq3 and the mixture is mostly a fine slurry (made by grinding and ultrasonic agitation) rather than a clear solution. This is acceptable for the process as long as the slurry does not clog the feed system; the role of the solvent is only as a rechargeable, continuous feed fluid source and it is designed so as not to be incorporated in the deposited thin film. Desirable characteristics of the solvent are a sufficiently high vapor pressure to cause minimal incorporation into the deposited films and the correct viscosity to transport well through the feed system under positive pressure (i.e. flow-controlled rather than temperature-controlled).
Alternative embodiments of the present invention would include, but are not limited to, a continuous feed system based on the above design where two or more fluid source reservoirs were switched by means of a multi-way valve. The temperature of the evaporation box is controlled so that everything entering as a fluid exits as a vapor, therefore film thickness is flow-controlled, not temperature-controlled. Doped films are also possible, with very accurate control over the doping ratio by premixing the dopant in the fluid reservoir rather than the current method used in thermal evaporation systems of independently controlling two or more thermal evaporation sources. Since the evaporation rate in thermal sublimation systems is exponentially dependent on temperature, accurate doping control is difficult. The present invention therefore overcomes this drawback.
If premixing is for some reason proscribed, more than one metered source can feed the same evaporation box at independently controlled rates either via separate atomizers, or by routing both sources into the same atomizer.
The distance from the slit to the substrate can be adjusted within the limits set by the geometry of the deposition system, but that with suitable optimization of the slit size, flow rate, temperature (and hence pressure in the box) good uniformity across a very long exit slit can be obtained even at close slit to substrate distances (i.e. 1 cm or less). This leads to the embodiment shown in
While a preferred embodiment of the present invention has been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.
This invention was made with U.S. Government support under Contract DE-AC0676RL01830 awarded by the U.S. Department of Energy. The U.S. Government has certain rights in the invention.
