Patent Grant
7883583
The present invention relates to metering of powdered materials, over a large range of feed rates, into a vaporization apparatus.
There is a need to be able to accurately and precisely continuously meter small quantities of powdered materials, for example 1 to 9 micrograms per second. The electronics industry has a need to meter small quantities of powdered materials to a vaporization zone for direct vapor deposition or for precursors in chemical vapor deposition (CVD). There is also a need to be able to accurately and precisely meter material amounts three orders of magnitude higher, for example 1000 micrograms per second. In many systems, it would be advantageous to be able to meter powdered materials over the range of 1 to 1000 micrograms with the same equipment. Organic light emitting diode devices (OLEDs), for instance have a light emitting layer that often contains a host and a dopant that are deposited in amounts differing by two to three orders of magnitude. There would be a great advantage in OLED manufacturing to be able to independently, and continuously, meter powdered organic materials to a vaporization region using a common transport design for host, co-host and dopant materials.
It is well known that precisely metering small amounts of powdered materials is difficult. There are numerous examples of systems that make use additional materials as carriers and additives to facilitate the transport of powdered materials. Carriers that have been used include inert gases, liquids, and solids. The use of any sort of additive increases the material transport complexity, for the carrier or additive needs to be added, removed and handled separately from the actual material of interest. The use of carriers also increases the risk of contamination, which is particularly detrimental in the pharmaceutical and electronics manufacturing industries where there is a particular need to meter materials.
In U.S. Pat. No. 3,754,529, Fleischner describes an auger device for transporting powdered material that has been mixed with an inert carrier, preferably sand. The ratio of active material to sand is reported to be 1:9. Transporting a mixture that is mostly inert carrier adds costs and complexities to the system, and increases the potential for the introduction of contaminates into the material feed.
Commonly assigned U.S. Patent Application Publication Nos. 2006/0062918 and No. 2006/0177576 use a traditional auger design to meter powders, where there is a patterned screw within a smooth barrel.
The metering device of this disclosure can also be used as one part of a larger vapor deposition system. Vapor deposition systems of particular interest are those designed for manufacturing organic light emitting diode (OLED) devices. An OLED device includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
Physical vapor deposition in a vacuum environment is the principal way of depositing thin organic material films as used in small molecule OLED devices. Such methods are well known, for example Barr in U.S. Pat. No. 2,447,789 and Tanabe et al. in EP 0 982 411. The organic materials used in the manufacture of OLED devices are often subject to degradation when maintained at or near the desired rate-dependent vaporization temperature for extended periods of time. Exposure of sensitive organic materials to higher temperatures can cause changes in the structure of the molecules and associated changes in material properties.
To overcome the thermal sensitivity of these materials, only small quantities of organic materials have been loaded in sources and they are heated as little as possible. In this manner, the material is consumed before it has reached the temperature exposure threshold to cause significant degradation. The limitations with this practice are that the available vaporization rate is very low due to the limitation on heater temperature, and the operation time of the source is very short due to the small quantity of material present in the source. In the prior art, it has been necessary to vent the deposition chamber, disassemble and clean the vapor source, refill the source, reestablish vacuum in the deposition chamber and degas the just-introduced organic material over several hours before resuming operation. The low deposition rate and the frequent and time-consuming process associated with recharging a source has placed substantial limitations on the throughput of OLED manufacturing facilities.
A secondary consequence of heating the entire organic material charge to roughly the same temperature is that it is impractical to mix additional organic materials, such as dopants, with a host material unless the vaporization behavior and vapor pressure of the dopant is very close to that of the host material. Additionally, the standard use of separate sources creates a gradient effect in the deposited film where the material in the source closest to the advancing substrate is over-represented in the initial film immediately adjacent the substrate while the material in the last source is over represented in the final film surface. This gradient co-deposition is unavoidable in prior art sources where a single material is vaporized from each of multiple sources directly onto a substrate. The gradient in the deposited film is especially evident when the contribution of either of the end sources is more than a few percent of the central source, such as when a co-host is used.
Commonly assigned U.S. Patent Application Publication Nos. 2006/0062918 and No. 2006/0062919 overcome many of the shortcomings of the use of separate point sources by metering materials to a flash vaporization zone. U.S. Patent Application Publication No. 2006/0062918 teaches the metering of host and dopant mixtures in a single powder transport mechanism, and using a manifold to distribute the vapor to the substrate. U.S. Patent Application Publication No.2006/062919 discloses the ability to mix organic vapors in the manifold and deliver a mixture of materials to the substrate surface. However, none of these earlier teachings anticipate the need to have independent metering control for the host and dopant materials. The transport mechanisms are therefore unable, by virtue of design, to meter at the low rates, 1-10 micrograms/second, required for an independent dopant feed.
U.S. Patent Application Publication Nos. 2007/0084700 and 2006/0157322, U.S. Pat. Nos. 6,832,887 and 7,044,288 disclose powder feeding pumps for moving powders from an entry port to a discharge port using parallel spaced disks that rotate within a housing having an internal cavity that defines a volume having an increasing volume from the input port to the discharge port. These powder feeding pumps are intended for use with much larger particle size powders and are not adapted to metering powder on a milligram or microgram basis.
There continues to be a need to precisely control the metering of milligram to microgram quantities of powdered material into a vaporization apparatus.
It is therefore an object of the present invention to precisely control the metering and delivery of milligram to microgram quantities of powder to a vaporization device.
This object is achieved by an apparatus for vaporizing a particulate material, comprising:
(a) a metering apparatus including:
(b) a flash evaporator that receives and vaporizes the metered material.
It is an advantage of this invention that it can provide adjustable controlled metering and vaporization of small quantities of powdered material more uniformly than has heretofore been possible. The particulate material transport apparatus of the present invention has a unique feature in that it can deliver small amounts of powdered material such as 1 microgram per second, as well as much larger amounts, such as up to 1000 micrograms per second. It is a further advantage of the present invention that it can meter powders uniformly without the use of a carrier, such as an inert gas, liquid or solid. It is a further advantage of the present invention that it can maintain a steady vaporization rate with a continuously replenished charge of organic material and with no heater temperature change required as the source material is consumed. It is a further advantage of the present invention that the particulate material is maintained at room temperature in the material reservoir and the transport apparatus and is heated only as it is discharged into the associated vaporization apparatus. The device permits extended operation of the source with substantially higher vaporization rates than in prior art devices with substantially reduced risk of degrading even very temperature-sensitive organic materials. It is a further advantage of the present invention that it can be used in a vaporization system for independently controlling dopant and host feed rates. It is a further advantage of this invention that it permits rapid starting and stopping of vaporization. It is a further advantage of this invention that it can deliver controlled volumes of vapor and thereby control the deposited film thickness in area deposition processes. It is a further advantage of the present invention that it can provide a vapor source in any orientation, which is frequently not possible with prior-art devices.
Turning now to
Housing 140 also includes an internal volume 150. Rotatable shaft 170 has a smooth surface and a shape corresponding to that of internal volume 150, e.g. cylindrical in this embodiment, and is disposed in internal volume 150. Rotatable shaft 170 also has a circumferential groove, which shall become apparent in further drawings. Rotatable shaft 170 is preferably constructed of thermally conductive material such as nickel that can be actively cooled and serves to maintain the particulate material in the circumferential groove at a temperature well below the effective vaporization temperature of the particulate material. Hard coatings such as titanium nitride or diamond-like carbon are advantageously applied to the internal volume 150 and the rotatable shaft 170. A motor 180 rotates rotatable shaft 170 at a predetermined rate. Motor 180 can also be used to rotate agitator 190a. Housing 140 also includes first and second openings whose nature and function will become apparent. Vaporizing apparatus 100 also includes a flash evaporator 120a within an evaporator enclosure 210. Vaporizing apparatus 100 can further optionally include pressure sensor 230, which can be used to monitor the rate of material vaporization.
Turning now to
The residence time of the material at elevated temperature, that is, at the rate-dependent vaporization temperature, is orders of magnitude less than many prior art devices and methods (seconds vs. hours or days in the prior art), which permits heating material to higher temperatures than in the prior art. Thus, the current device and method can achieve substantially higher vaporization rates, without causing appreciable degradation of the organic material components because the product of exposure time and temperature is substantially lower than in prior art devices.
Turning now to
It is important for the purposes of this invention to limit the transfer of particulate material by rotatable shaft 170 to the region of circumferential groove 175. Rotatable shaft 170 and internal volume 150 cooperate such that the particulate material will be substantially transported by circumferential groove 175 and not along the remainder of rotatable shaft 170. By this it is meant that the spacing between rotatable shaft 170 and housing 140 is chosen to be smaller than the average particle size of the particulate material so as to substantially exclude particulate material transfer from all parts of shaft 170 outside of circumferential groove 175.
Second opening 160 is sized to have substantially the same width as circumferential groove 175 in rotatable shaft 170, and the opening has an increasing cross sectional area as it penetrates into evaporator enclosure 210 to encourage the material dislodged from the groove by scraper 185 to fall into evaporator enclosure 210 without clinging to the walls of the opening. The immediate proximity or light contact between rotatable shaft 170, scraper 185, and second opening 160 of both housing 140 and evaporator enclosure 210 creates a positive displacement metering configuration that forces the material out of the groove and into evaporator enclosure 210 without reliance on gravity.
The material feeding and vaporizing device of this disclosure is effective at transporting particulate material when the grooved shaft is horizontal as shown in
Experimentally, it has been observed that fine powder is considerably more difficult to meter in a partial vacuum below half an atmosphere. The powder agglomerates as residual air molecules are removed, and behaves more like a solid than a pourable powder. Despite this tendency, the material feeding and vaporizing device of this disclosure has proven capable of dispensing powders with a particle size dispersion below 50 microns as well as powders prepared to have particle size dispersions between 50 and 100 microns and between 100 and 200 microns.
Managing condensation in powder-fed vaporization systems is extremely important, as this is a primary cause of material feed problems and material degradation or fractionalization. Second opening 160 to evaporator enclosure 210 is made sufficiently large to permit the material to pass into the evaporator enclosure, but at less than 1 mm diameter, the opening is purposely made to have a low conductance to the backflow of vapor. At second opening 160, the material in circumferential groove 175 is in the form of a consolidated powder where it serves as a vapor seal, preventing vaporized material in the evaporator enclosure that has a pressure greater than the ambient vacuum level from flowing back along circumferential groove 175 to the particulate material reservoir. Scraper 185 is maintained at a temperature sufficient to prevent condensation, but a small amount of vapor will condense on the cold face of the particulate material still in the groove. Material vapor will additionally condense on the cooled shaft immediately adjacent the groove. All of the points of condensation in this device, however, are self limiting and confined to very small areas. After an initial vaporization period, the material feed per unit revolution will stabilize. The vapor that condenses on the cold face of the particulate material in the groove will be vaporized as soon as the shaft rotates farther to dislodge the material from the groove. Any vapor that condenses in the groove can accumulate to the point where it contacts the heated scraper and is mechanically removed from the groove as the shaft rotates. The condensate effectively reduces the groove dimensions until they perfectly conform to the scraper dimensions. The condensate accumulation is therefore self-limiting. Similarly, the vapor that condenses on the shaft will accumulate until it mechanically interferes with and is removed by the sharp edges of the first and second openings and likewise reaches a stable, self-limiting dimension.
This configuration substantially increases the attainable temperature gradient in the particulate material between the temperature of circumferential groove 175 and that of evaporator enclosure 210 with scraper 185. This gradient prevents the usual leaching of more volatile constituents from bulk volumes of mixed-component materials and enables a single source to co-deposit multiple-particulate materials. This large gradient is further instrumental in maintaining the particulate material in a consolidated powder form upon reaching second opening 160, even when employing materials that liquefy at temperatures as low as 100° C. After the desired quantity of material has been metered, rotatable shaft 170 can be rotated a few degrees in the reverse direction to separate the particulate material in circumferential groove 175 from contact with scraper 185 and shield the material within the cooled housing 140 from radiant heat emitted by evaporator enclosure 210. This action is further instrumental in maintaining the large thermal gradient for all of the particulate material. The small quantity of vapor that condenses in the empty portion of the exposed groove will be removed by scraper 185 when rotatable shaft 170 is again rotated in the feed direction.
In practice, vaporization apparatus 100 is used as follows. Particulate material is received in reservoir 130a. Rotatable shaft 170 as described above is rotated in internal volume 150, which is formed in housing 140 as described above, whereby circumferential groove 175 receives particulate material from reservoir 130a through first opening 155 and discharges it from second opening 160. Scraper 185 scrapes the particulate material from circumferential groove 175 to deliver metered amounts of particulate material through second opening 160 to evaporator enclosure 210 and flash evaporator 120a, wherein the metered particulate material is flash evaporated. In some embodiments, the position of scraper 185 can be adjusted to control the amount of metered material delivered through second opening 160.
Turning now to
Turning now to
Turning now to
Turning now to
In many applications, this precise metering ability can simplify the control system such that the motor speed or number of motor revolutions alone provides a sufficiently accurate measure of the material feed rate without the need to actually weigh the metered amount of material.
Turning now to
Turning now to
Other embodiments of vaporizing with multiple metering apparatus are also possible. For example, one can employ two side-by-side vaporizing apparatus, such as vaporizing apparatus 100 as shown in
Turning now to
Turning now to
Substrate 320 can be an organic solid, an inorganic solid, or a combination of organic and inorganic solids. Substrate 320 can be rigid or flexible and can be processed as separate individual pieces, such as sheets or wafers, or as a continuous roll. Typical substrate materials include glass, plastic, metal, ceramic, semiconductor, metal oxide, semiconductor oxide, semiconductor nitride, or combinations thereof. Substrate 320 can be a homogeneous mixture of materials, a composite of materials, or multiple layers of materials. Substrate 320 can be an OLED substrate, that is a substrate commonly used for preparing OLED devices, e.g. active-matrix low-temperature polysilicon or amorphous-silicon TFT substrate. The substrate 320 can either be light transmissive or opaque, depending on the intended direction of light emission. The light transmissive property is desirable for viewing the EL emission through the substrate. Transparent glass or plastic are commonly employed in such cases. For applications where the EL emission is viewed through the top electrode, the transmissive characteristic of the bottom support is immaterial, and therefore can be light transmissive, light absorbing or light reflective. Substrates for use in this case include, but are not limited to, glass, plastic, semiconductor materials, ceramics, and circuit board materials, or any others commonly used in the formation of OLED devices, which can be either passive-matrix devices or active-matrix devices.
An electrode is formed over substrate 320 and is most commonly configured as an anode 330. When EL emission is viewed through the substrate 320, anode 330 should be transparent or substantially transparent to the emission of interest. Common transparent anode materials useful in this invention are indium-tin oxide and tin oxide, but other metal oxides can work including, but not limited to, aluminum- or indium-doped zinc oxide, magnesium-indium oxide, and nickel-tungsten oxide. In addition to these oxides, metal nitrides such as gallium nitride, metal selenides such as zinc selenide, and metal sulfides such as zinc sulfide, can be used as an anode material. For applications where EL emission is viewed through the top electrode, the transmissive characteristics of the anode material are immaterial and any conductive material can be used, transparent, opaque or reflective. Example conductors for this application include, but are not limited to, gold, iridium, molybdenum, palladium, and platinum. The preferred anode materials, transmissive or otherwise, have a work function of 4.1 eV or greater. Desired anode materials can be deposited by any suitable way such as evaporation, sputtering, chemical vapor deposition, or electrochemical process. Anode materials can be patterned using well known photolithographic processes.
While not always necessary, it is often useful that a hole-injecting layer 335 be formed over anode 330 in an organic light-emitting display. The hole-injecting material can serve to improve the film formation property of subsequent organic layers and to facilitate injection of holes into the hole-transporting layer. Suitable materials for use in hole-injecting layer 335 include, but are not limited to, porphyrinic compounds as described in U.S. Pat. No. 4,720,432, plasma-deposited fluorocarbon polymers as described in U.S. Pat. No. 6,208,075, and inorganic oxides including vanadium oxide (VOx), molybdenum oxide (MoOx), and nickel oxide (NiOx). Alternative hole-injecting materials reportedly useful in organic EL devices are described in EP 0 891 121 A1 and EP 1 029 909 A1.
While not always necessary, it is often useful that a hole-transporting layer 340 be formed and disposed over anode 330. Desired hole-transporting materials can be deposited by any suitable way such as evaporation, sputtering, chemical vapor deposition, electrochemical process, thermal transfer, or laser thermal transfer from a donor material, and can be deposited by the device and method described herein. Hole-transporting materials useful in hole-transporting layer 340 are well known to include compounds such as an aromatic tertiary amine, where the latter is understood to be a compound containing at least one trivalent nitrogen atom that is bonded only to carbon atoms, at least one of which is a member of an aromatic ring. In one form the aromatic tertiary amine can be an arylamine, such as a monoarylamine, diarylamine, triarylamine, or a polymeric arylamine. Exemplary monomeric triarylamines are illustrated by Klupfel et al. in U.S. Pat. No. 3,180,730. Other suitable triarylamines substituted with one or more vinyl radicals or including at least one active hydrogen-containing group are disclosed by Brantley et al. in U.S. Pat. Nos. 3,567,450 and 3,658,520.
A more preferred class of aromatic tertiary amines are those which include at least two aromatic tertiary amine moieties as described in U.S. Pat. Nos. 4,720,432 and 5,061,569. Such compounds include those represented by structural Formula A.
wherein:
In one embodiment, at least one of Q1 or Q2 contains a polycyclic fused ring structure, e.g., a naphthalene. When G is an aryl group, it is conveniently a phenylene, biphenylene, or naphthalene moiety.
A useful class of triarylamines satisfying structural Formula A and containing two triarylamine moieties is represented by structural Formula B.
where:
wherein R5 and R6 are independently selected aryl groups. In one embodiment, at least one of R5 or R6 contains a polycyclic fused ring structure, e.g., a naphthalene.
Another class of aromatic tertiary amines are the tetraaryldiamines. Desirable tetraaryldiamines include two diarylamino groups, such as indicated by Formula C, linked through an arylene group. Useful tetraaryldiamines include those represented by Formula D.
wherein:
In a typical embodiment, at least one of Ar, R7, R8, and R9 is a polycyclic fused ring structure, e.g., a naphthalene.
The various alkyl, alkylene, aryl, and arylene moieties of the foregoing structural Formulae A, B, C, D, can each in turn be substituted. Typical substituents include alkyl groups, alkoxy groups, aryl groups, aryloxy groups, and halogens such as fluoride, chloride, and bromide. The various alkyl and alkylene moieties typically contain from 1 to about 6 carbon atoms. The cycloalkyl moieties can contain from 3 to about 10 carbon atoms, but typically contain five, six, or seven carbon atoms—e.g., cyclopentyl, cyclohexyl, and cycloheptyl ring structures. The aryl and arylene moieties are usually phenyl and phenylene moieties.
The hole-transporting layer in an OLED device can be formed of a single or a mixture of aromatic tertiary amine compounds. Specifically, one can employ a triarylamine, such as a triarylamine satisfying the Formula B, in combination with a tetraaryldiamine, such as indicated by Formula D. When a triarylamine is employed in combination with a tetraaryldiamine, the latter is positioned as a layer interposed between the triarylamine and the electron-injecting and transporting layer. The device and method described herein can be used to deposit single- or multi-component layers, and can be used to sequentially deposit multiple layers.
Another class of useful hole-transporting materials includes polycyclic aromatic compounds as described in EP 1 009 041. In addition, polymeric hole-transporting materials can be used such as poly(N-vinylcarbazole) (PVK), polythiophenes, polypyrrole, polyaniline, and copolymers such as poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) also called PEDOT/PSS.
Light-emitting layer 350 produces light in response to hole-electron recombination. Light-emitting layer 350 is commonly disposed over hole-transporting layer 340. Desired organic light-emitting materials can be deposited by any suitable way such as evaporation, sputtering, chemical vapor deposition, electrochemical process, or radiation thermal transfer from a donor material, and can be deposited by the device and method described herein. Useful organic light-emitting materials are well known. As more fully described in U.S. Pat. Nos. 4,769,292 and 5,935,721, the light-emitting layers of the organic EL element include a luminescent or fluorescent material where electroluminescence is produced as a result of electron-hole pair recombination in this region. The light-emitting layers can include a single material, but more commonly include a host material doped with a guest compound or dopant where light emission comes primarily from the dopant. The dopant is selected to produce color light having a particular spectrum. The host materials in the light-emitting layers can be an electron-transporting material, as defined below, a hole-transporting material, as defined above, or another material that supports hole-electron recombination. The dopant is usually chosen from highly fluorescent dyes, but phosphorescent compounds, e.g., transition metal complexes as described in WO 98/55561, WO 00/18851, WO 00/57676, and WO 00/70655 are also useful. Dopants are typically coated as 0.01 to 10% by weight into the host material. The device and method described herein can be used to coat multi-component guest/host layers without the need for multiple vaporization sources.
Host and emitting molecules known to be of use include, but are not limited to, those disclosed in U.S. Pat. Nos. 4,768,292; 5,141,671; 5,150,006; 5,151,629; 5,294,870; 5,405,709; 5,484,922; 5,593,788; 5,645,948; 5,683,823; 5,755,999; 5,928,802; 5,935,720; 5,935,721; and 6,020,078.
Metal complexes of 8-hydroxyquinoline and similar derivatives (Formula E) constitute one class of useful host materials capable of supporting electroluminescence, and are particularly suitable for light emission of wavelengths longer than 500 nm, e.g., green, yellow, orange, and red.
wherein:
From the foregoing it is apparent that the metal can be a monovalent, divalent, or trivalent metal. The metal can, for example, be an alkali metal, such as lithium, sodium, or potassium; an alkaline earth metal, such as magnesium or calcium; or an earth metal, such as boron or aluminum. Generally any monovalent, divalent, or trivalent metal known to be a useful chelating metal can be employed. Z completes a heterocyclic nucleus containing at least two fused aromatic rings, at least one of which is an azole or azine ring. Additional rings, including both aliphatic and aromatic rings, can be fused with the two required rings, if required. To avoid adding molecular bulk without improving on function the number of ring atoms is usually maintained at 18 or less.
The host material in light-emitting layer 350 can be an anthracene derivative having hydrocarbon or substituted hydrocarbon substituents at the 9 and 10 positions. For example, derivatives of 9,10-di-(2-naphthyl)anthracene constitute one class of useful host materials capable of supporting electroluminescence, and are particularly suitable for light emission of wavelengths longer than 400 nm, e.g., blue, green, yellow, orange or red.
Benzazole derivatives constitute another class of useful host materials capable of supporting electroluminescence, and are particularly suitable for light emission of wavelengths longer than 400 nm, e.g., blue, green, yellow, orange or red. An example of a useful benzazole is 2,2′,2″-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole].
Desirable fluorescent dopants include perylene or derivatives of perylene, derivatives of anthracene, tetracene, xanthene, rubrene, coumarin, rhodamine, quinacridone, dicyanomethylenepyran compounds, thiopyran compounds, polymethine compounds, pyrilium and thiapyrilium compounds, derivatives of distryrylbenzene or distyrylbiphenyl, bis(azinyl)methane boron complex compounds, and carbostyryl compounds.
Other organic emissive materials can be polymeric substances, e.g. polyphenylenevinylene derivatives, dialkoxy-polyphenylenevinylenes, poly-para-phenylene derivatives, and polyfluorene derivatives, as taught by Wolk et al. in commonly assigned U.S. Pat. No. 6,194,119 B1 and references cited therein.
While not always necessary, it is often useful that OLED device 310 includes an electron-transporting layer 355 disposed over light-emitting layer 350. Desired electron-transporting materials can be deposited by any suitable way such as evaporation, sputtering, chemical vapor deposition, electrochemical process, thermal transfer, or laser thermal transfer from a donor material, and can be deposited by the device and method described herein. Preferred electron-transporting materials for use in electron-transporting layer 355 are metal chelated oxinoid compounds, including chelates of oxine itself (also commonly referred to as 8-quinolinol or 8-hydroxyquinoline). Such compounds help to inject and transport electrons and exhibit both high levels of performance and are readily fabricated in the form of thin films. Exemplary of contemplated oxinoid compounds are those satisfying structural Formula E, previously described.
Other electron-transporting materials include various butadiene derivatives as disclosed in U.S. Pat. No. 4,356,429 and various heterocyclic optical brighteners as described in U.S. Pat. No. 4,539,507. Benzazoles satisfying structural Formula G are also useful electron-transporting materials.
Other electron-transporting materials can be polymeric substances, e.g. polyphenylenevinylene derivatives, poly-para-phenylene derivatives, polyfluorene derivatives, polythiophenes, polyacetylenes, and other conductive polymeric organic materials such as those listed in Handbook of Conductive Molecules and Polymers, Vols. 1-4, H. S. Nalwa, ed., John Wiley and Sons, Chichester (1997).
An electron-injecting layer 360 can also be present between the cathode and the electron-transporting layer. Examples of electron-injecting materials include alkaline or alkaline earth metals, alkali halide salts, such as LiF mentioned above, or alkaline or alkaline earth metal doped organic layers.
Cathode 390 is formed over the electron-transporting layer 355 or over light-emitting layer 350 if an electron-transporting layer is not used. When light emission is through the anode 330, the cathode material can include nearly any conductive material. Desirable materials have good film-forming properties to ensure good contact with the underlying organic layer, promote electron injection at low voltage, and have good stability. Useful cathode materials often contain a low work function metal (<3.0 eV) or metal alloy. One preferred cathode material includes a Mg:Ag alloy wherein the percentage of silver is in the range of 1 to 20%, as described in U.S. Pat. No. 4,885,221. Another suitable class of cathode materials includes bilayers including a thin layer of a low work function metal or metal salt capped with a thicker layer of conductive metal. One such cathode includes a thin layer of LiF followed by a thicker layer of Al as described in U.S. Pat. No. 5,677,572. Other useful cathode materials include, but are not limited to, those disclosed in U.S. Pat. Nos. 5,059,861; 5,059,862; and 6,140,763.
When light emission is viewed through cathode 390, it must be transparent or nearly transparent. For such applications, metals must be thin or one must use transparent conductive oxides, or a combination of these materials. Optically transparent cathodes have been described in more detail in U.S. Pat. No. 5,776,623. Cathode materials can be deposited by evaporation, sputtering, or chemical vapor deposition. When needed, patterning can be achieved through many well known methods including, but not limited to, through-mask deposition, integral shadow masking as described in U.S. Pat. No. 5,276,380 and EP 0 732 868, laser ablation, and selective chemical vapor deposition.
Cathode materials can be deposited by evaporation, sputtering, or chemical vapor deposition. When needed, patterning can be achieved through many well known methods including, but not limited to, through-mask deposition, integral shadow masking as described in U.S. Pat. No. 5,276,380 and EP 0 732 868, laser ablation, and selective chemical vapor deposition.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
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