Patent Publication Number: US-6703180-B1

Title: Forming an improved stability emissive layer from a donor element in an OLED device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned U.S. patent application Ser. No. 10/060,837 filed Jan. 30, 2002 by Burberry et al., entitled “Using Spacer Elements to Make Electroluminescent Display Devices”, and to commonly assigned U.S. patent application Ser. No. 10/055,579, now U.S. Pat. No. 6,582,875, filed Jan. 23, 2002 by Kay et al., entitled “Using a Multichannel Linear Laser Light Beam in Making OLED Devices by Thermal Transfer”; the disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to organic electroluminescent (EL) devices, also known as organic light-emitting diodes (OLED) that emit color light. 
     BACKGROUND OF THE INVENTION 
     In color or full-color organic electroluminescent (EL) displays (also known as organic light-emitting diode devices, or OLED devices) having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color-producing organic EL media are required to produce the RGB pixels. The basic OLED device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium can consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the emissive layer or light-emitting layer of the organic EL medium. Other organic layers present in the organic EL medium can provide electronic transport functions primarily and are referred to as either the hole-transporting layer (for hole transport) or electron-transporting layer (for electron transport). In forming the RGB pixels in a full-color OLED display panel, it is necessary to devise a method to precisely pattern the emissive layer of the organic EL medium or the~entire organic EL medium. 
     In commonly assigned U.S. Pat. No. 5,937,272, Tang has taught a method of patterning multicolor pixels (e.g. red, green, blue subpixels) onto a thin-film-transistor (TFT) array substrate by vapor deposition of an EL material. Such EL material is deposited on a substrate in a selected pattern via the use of a donor coating on a support and an aperture mask. 
     Using an unpatterned donor sheet and a precision light source, such as a laser, is another method of radiation transfer. Such a method is disclosed by Littman in U.S. Pat. No. 5,688,551, and in a series of patents by Wolk et al. (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553). While this is a useful technique for manufacturing, EL devices that include emissive layers prepared this way often suffer from decreased stability relative to El devices with emissive layers prepared in other ways, e.g. vapor deposition. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a way to form an emissive layer using a radiation transfer process from a donor element and to improve the stability of the thus formed emissive layer. 
     This object is achieved by a method of forming an organic light-emitting device with improved stability, comprising: 
     a) forming an anode over a substrate; 
     b) providing a cathode spaced from the anode; 
     c) providing a donor element including light-emitting material and positioning such donor element in a material-transferring relationship with the substrate; 
     d) illuminating the donor element with radiation to cause the transfer of light-emitting material to deposit the light-emitting material to form an emissive layer over the anode; and 
     e) forming an organic layer including an organic compound doped with a low work function metal or metal compound capable of acting as a donor dopant between the emissive layer and the cathode to lower the electron-injecting barrier from the organic layer into the emissive layer thereby improving the stability of the organic light-emitting device. 
     ADVANTAGES 
     It is an advantage of this invention that an OLED device manufactured by radiation transfer of a light-emitting material has improved operational stability. It is a further advantage of this invention that the operating voltage at a given current density is reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-sectional view of an OLED device that can be used in the practice of this invention; 
     FIG. 2 shows a cross-sectional representation of the process of selectively transferring light-emitting material from a donor element to a substrate by radiation transfer; and 
     FIG. 3 is a block diagram showing the steps involved in a method according to the present invention. 
    
    
     Since device feature dimensions such as layer thicknesses are frequently in sub-micrometer ranges, the drawings are scaled for ease of visualization rather than dimensional accuracy. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The term “pixel” is employed in its art-recognized usage to designate an area of a display panel that can be stimulated to emit light independently of other areas. The term “OLED device” is used in its art-recognized meaning of a display device comprising organic light-emitting diodes as pixels and will also be referred to as an organic light-emitting device. A color OLED device emits light of at least one color. The term “multicolor” is employed to describe a display panel that is capable of emitting light of a different hue in different areas. In particular, it is employed to describe a display panel that is capable of displaying images of different colors. These areas are not necessarily contiguous. The term “full-color” is employed to describe multicolor display panels that are capable of producing light in the red, green, and blue regions of the visible spectrum and displaying images in any combination of hues. The red, green, and blue colors constitute the three primary colors from which all other colors can be generated by appropriately mixing these three primaries. The term “hue” refers to the intensity profile of light emission within the visible spectrum, with different hues exhibiting visually discernible differences in color. The pixel or subpixel is generally used to designate the smallest addressable unit in a display panel. For a monochrome display, there is no distinction between pixel or subpixel. The term “subpixel” is used in multicolor display panels and is employed to designate any portion of a pixel, which can be independently addressable to emit a specific color. For example, a blue subpixel is that portion of a pixel, which can be addressed to produce blue light. In a full-color display, a pixel generally comprises three primary-color subpixels, namely blue, green, and red. The term “pitch” is used to designate the distance separating two pixels or subpixels in a display panel. Thus, a subpixel pitch means the separation between two subpixels. 
     Turning now to FIG. 1, there is shown a cross-sectional view of an OLED device that can be used in the practice of this invention. OLED device  14  includes substrate  10 . Substrate  10  can be an organic solid, an inorganic solid, or a combination of organic and inorganic solids that provides a surface for receiving the emissive material from a donor. Substrate  10  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  10  can be a homogeneous mixture of materials, a composite of materials, or multiple layers of materials. Substrate  10  can be an OLED substrate, that is a substrate commonly used for preparing OLED devices, e.g. active-matrix low-temperature polysilicon TFT substrate. The substrate  10  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 is 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. 
     Anode  20  is formed over substrate  10 . When EL emission is viewed through the substrate  10 , anode 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 means such as evaporation, sputtering, chemical vapor deposition, or electrochemical means. Anode materials can be patterned using well known photolithographic processes. 
     While not always necessary, it is often useful that a hole-injecting layer  22  be formed over anode  20  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 the hole-injecting layer  22  include, but are not limited to, porphyrinic compounds as described in U.S. Pat. No. 4,720,432, and plasma-deposited fluorocarbon polymers as described in U.S. Pat. No. 6,208,075. 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  24  be formed over hole-injecting layer  22 , or over anode  20  if no hole-injecting layer is used. Desired hole-transporting materials can be deposited by any suitable means such as evaporation, sputtering, chemical vapor deposition, electrochemical means, thermal transfer, or laser thermal transfer from a donor material. Hole-transporting materials useful in hole-transporting layer  24  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 and/or comprising 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: 
     Q 1  and Q 2  are independently selected aromatic tertiary amine moieties; and 
     G is a linking group such as an arylene, cycloalkylene, or alkylene group of a carbon to carbon bond. 
     In one embodiment, at least one of Q 1  or Q 2  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: 
     R 1  and R 2  each independently represent a hydrogen atom, an aryl group, or an alkyl group or R 1  and R 2  together represent the atoms completing a cycloalkyl group; and 
     R 3  and R 4  each independently represent an aryl group, which is in turn substituted with a diaryl substituted amino group, as indicated by structural Formula C.                    
     wherein R 5  and R 6  are independently selected aryl groups. In one embodiment, at least one of R 5  or R 6  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: 
     each Are is an independently selected arylene group, such as a phenylene or anthracene moiety; 
     n is an integer of from 1 to 4; and 
     Ar, R 7 , R 8 , and R 9  are independently selected aryl groups. 
     In a typical embodiment, at least one of Ar, R 7 , R 8 , and R 9  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 satisfyng 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. Illustrative of useful aromatic tertiary amines are the following: 
     1,1-Bis(4-di-p-tolylaminophenyl)cyclohexane 
     1,1-Bis(4-di-p-tolylaminophenyl)-4-phenylcyclohexane 
     4,4′-Bis(diphenylamino)quadriphenyl 
     Bis(4-dimethylamino-2-methylphenyl)-phenylmethane 
     N,N,N-Tri(p-tolyl)amine 
     4-(di-p-tolylamino)-4′-[4(di-p-tolylamino)-styryl]stilbene 
     N,N,N′,N′-Tetra-p-tolyl-4-4′-diaminobiphenyl 
     N,N,N′,N′-Tetraphenyl-4,4′-diaminobiphenyl 
     N-Phenylcarbazole 
     Poly(N-vinylcarbazole) 
     N,N′-di-1-naphthalenyl-N,N′-diphenyl-4,4′-diaminobiphenyl 
     4,4′-Bis[N-(1-naphthyl)-N-phenylamino]biphenyl 
     4,4″-Bis[N-(1-naphthyl)-N-phenylamino]p-terphenyl 
     4,4′-Bis[N-(2-naphthyl)-N-phenylamino]biphenyl 
     4,4′-Bis[N-(3-acenaphthenyl)-N-phenylamino]biphenyl 
     1,5-Bis[N-(1-naphthyl)-N-phenylamino]naphthalene 
     4,4′-Bis[N-(9-anthryl)-N-phenylamino]biphenyl 
     4,4″-Bis[N-(1-anthryl)-N-phenylamino]-p-terphenyl 
     4,4′-Bis[N-(2-phenanthryl)-N-phenylamino]biphenyl 
     4,4′-Bis[N-(8-fluoranthenyl)-N-phenylamino]biphenyl 
     4,4′-Bis[N-(2-pyrenyl)-N-phenylamino]biphenyl 
     4,4′-Bis[N-(2-naphthacenyl)-N-phenylamino]biphenyl 
     4,4′-Bis[N-(2-perylenyl)-N-phenylamino]biphenyl 
     4,4′-Bis[N-(1-coronenyl)-N-phenylamino]biphenyl 
     2,6-Bis(di-p-tolylamino)naphthalene 
     2,6-Bis[di-(1-naphthyl)amino]naphthalene 
     2,6-Bis[N-(1-naphthyl)-N-(2-naphthyl)amino]naphthalene 
     N,N,N′,N′-Tetra(2-naphthyl)-4,4″-diamino-p-terphenyl 
     4,4′-Bis{N-phenyl-N-[4-(1-naphthyl)-phenyl]amino}biphenyl 
     4,4′-Bis[N-phenyl-N-(2-pyrenyl)amino]biphenyl 
     2,6-Bis[N,N-di(2-naphthyl)amine]fluorene 
     1,5-Bis[N-(1-naphthyl)-N-phenylamino]naphthalene 
     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. 
     An emissive layer  26  producing light in response to hole-electron recombination is formed over anode  20  and over any other layers formed, such as hole-transporting layer  24 . Desired organic light-emitting materials can be deposited by any suitable means such as evaporation, sputtering, chemical vapor deposition, electrochemical means, or radiation transfer from a donor material. However, for the purposes of this invention, the deposition method shall be limited to radiation transfer from a donor material. 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 emissive layer  26  of the organic EL element comprises a luminescent or fluorescent material where electroluminescence is produced as a result of electron-hole pair recombination in this region. The emissive layer  26  can be comprised of a single material, but more commonly includes a host material doped with a guest compound or dopant where light emission comes primarily from the dopant and can be of any color. The host materials in the emissive layer  26  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. 
     An important relationship for choosing a dye as a dopant is a comparison of the bandgap potential which is defined as the energy difference between the highest occupied molecular orbital and the lowest unoccupied molecular orbital of the molecule. For efficient energy transfer from the host material to the dopant molecule, a necessary condition is that the band gap of the dopant is smaller than that of the host material. 
     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: 
     M represents a metal; 
     n is an integer of from 1 to 3; and 
     Z independently in each occurrence represents the atoms completing a nucleus having at least two fused aromatic rings. 
     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. 
     Illustrative of useful chelated oxinoid compounds are the following: 
     CO-1: Aluminum trisoxine [alias, tris(8-quinolinolato)aluminum(III)] 
     CO-2: Magnesium bisoxine [alias, bis(8-quinolinolato)magnesium(II)] 
     CO-3: Bis[benzo {f}-8-quinolinolato]zinc (II) 
     CO-4: Bis(2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-8-quinolinolato) aluminum(III) 
     CO-5: Indium trisoxine [alias, tris(8-quinolinolato)indium] 
     CO-6: Aluminum tris(5-methyloxine) [alias, tris(5-methyl-8-quinolinolato)aluminum(III)] 
     CO-7: Lithium oxine [alias, (8-quinolinolato)lithium(I)] 
     Derivatives of 9,10-di-(2-naphthyl)anthracene (Formula F) 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.                    
     wherein R 1 , R 2 , R 3 , R 4 , R 5 , and R 6  represent one or more substituents on each ring where each substituent is individually selected from the following groups: 
     Group 1: hydrogen, or alkyl of from 1 to 24 carbon atoms; 
     Group 2: aryl or substituted aryl of from 5 to 20 carbon atoms; 
     Group 3: carbon atoms from 4 to 24 necessary to complete a fused aromatic ring of anthracenyl, pyrenyl, or perylenyl; 
     Group 4: heteroaryl or substituted heteroaryl of from 5 to 24 carbon atoms as necessary to complete a fused heteroaromatic ring of furyl, thienyl, pyridyl, quinolinyl or other heterocyclic systems; 
     Group 5: alkoxylamino, alkylamino, or arylamino of from 1 to 24 carbon atoms; and 
     Group 6: fluorine, chlorine, bromine or cyano. 
     Benzazole derivatives (Formula G) 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.                    
     where: 
     n is an integer of 3 to 8; 
     Z is O, NR or S; 
     R′ is hydrogen; alkyl of from 1 to 24 carbon atoms, for example, propyl, t-butyl, heptyl, and the like; aryl or heteroatom substituted aryl of from 5 to 20 carbon atoms for example phenyl and naphthyl, furyl, thienyl, pyridyl, quinolinyl and other heterocyclic systems; or halo such as chloro, fluoro; or atoms necessary to complete a fused aromatic ring; and 
     L is a linkage unit consisting of alkyl, aryl, substituted alkyl, or substituted aryl, which conjugately or unconjugately connects the multiple benzazoles together. 
     An example of a useful benzazole is 2, 2′, 2″-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole]. 
     Desirable fluorescent dopants include derivatives of anthracene, tetracene, xanthene, perylene, rubrene, coumarin, rhodamine, quinacridone, dicyanomethylenepyran compounds, thiopyran compounds, polymethine compounds, pyrilium and thiapyrilium compounds, and carbostyryl compounds. Illustrative examples of useful dopants include, but are not limited to, the following: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 X 
                 R1 
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                 L9  
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                 L11 
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                 L13 
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                 L14 
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                 L15 
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                 L16 
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                 L18 
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                 L19 
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                 L21 
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                 L22 
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                 L27 
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                 L28 
                 O 
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                 L29 
                 O 
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                 L30 
                 S 
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                 L31 
                 S 
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                 L32 
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                 L33 
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                 L34 
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                 L36 
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                 phenyl 
               
               
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                 methyl 
               
               
                 L43 
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     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. 
     Although not shown, light-emitting layer  26  can additionally comprise two or more emissive layers, if such is desired for proper emissive properties of the resulting OLED device. 
     Organic layer  28  is formed over emissive layer  26 . Organic layer  28  is commonly an electron-transporting layer. Desired electron-transporting materials can be deposited by any suitable means such as evaporation, sputtering, chemical vapor deposition, electrochemical means, thermal transfer, or laser thermal transfer from a donor material. Preferred electron-transporting materials known in the art for use in organic layer  28  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 known in the art 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 known in the art 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). 
     Cathode  30  is formed over organic layer  28  and is spaced from anode  20 . When light emission is through the anode, the cathode material can be comprised of 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 (&lt;3.0 eV) or metal alloy. One preferred cathode material is comprised of 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 comprised of a thin layer of a low work function metal or metal salt capped with a thicker layer of conductive metal. One such cathode is comprised of 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 the cathode, the cathode 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. 
     Other layers not shown in this embodiment are sometimes useful in OLED devices. For example, an electron-injecting layer can be deposited between the cathode  30  and the electron-transporting layer, that is, the electron-injecting layer is formed on organic layer  28 , which is also electron-transporting layer  28 , and then cathode  30  is formed on the electron-injecting layer. Examples of electron-injecting materials include alkali halide salts, such as LiF. 
     For the purposes of this invention, emissive layer  26  is formed by radiation transfer from a donor material. FIG. 2 shows a cross-sectional representation of the process of selectively transferring light-emitting material  50  from donor element  40  to portions of substrate  10  by radiation transfer from a donor material in one method of treatment with light. Radiation transfer is herein defined as any mechanism such as sublimation, ablation, vaporization or other process whereby material can be transferred upon initiation by radiation such as light or heat. Donor element  40  has been prepared with light-absorbing layer  48  over donor support element  46 . Donor support element  46  has been described by Tang et al. in commonly assigned U.S. Pat. No. 5,904,961. Light-absorbing layer  48  comprises radiation-absorbing material capable of absorbing light in a predetermined portion of the spectrum to produce heat. Light-absorbing layer  48  can include a dye such as the dyes specified in U.S. Pat. No. 5,578,416, a pigment such as carbon, or a metal such as nickel, chromium, titanium etc. Donor element  40  includes light-emitting material  50 , also called an emissive layer, coated on the donor element. 
     Donor element  40  is positioned in a material-transferring relationship with substrate  10 , which can be an OLED substrate. By material-transferring relationship is meant that donor element  40  is positioned in contact with substrate  10  or is held with a controlled separation from substrate  10 . If substrate  10  has been coated with other layers as described above, contact with substrate  10  means that donor element  40  actually is in contact with the top coated layer over substrate  10 . In this embodiment, donor element  40  is in contact with substrate  10  and gap  56  is maintained by the structure of anodes  20  of thin-film transistors and intervening raised surface portions  58 . Gap  56  has previously been described by commonly assigned U.S. patent application Ser. No. 10/060,837 filed Jan. 30, 2002 by Burberry et al., entitled “Using Spacer Elements to Make Electroluminescent Display Devices”, the disclosure of which is incorporated herein by reference. It will be understood that embodiments are possible in which there is no gap. Laser source  44  selectively illuminates non-transfer surface  52  of donor element  40  with radiation e.g. laser light  42 . Laser source  44  can be e.g. an infrared laser of a power which is sufficient to cause enough heat  54  to be formed to effect the radiation transfer described herein. Light-absorbing layer  48  of donor element  40  absorbs laser light  42  and produces heat  54 . Some or all of the heated portion of material coated on donor element  40  is sublimed, vaporized, or ablated and thus deposited to receiving surface  38  of substrate  10  in a patterned transfer. In this manner, light-emitting material  50  can be transferred to selectively form emissive layer  26  over anode  20  and any intervening layers. Thus, the vaporization transfer of host materials and dopant materials, which comprise the various layers of light-emitting material  50 , is effected. 
     In a series of patents and applications (U.S. Pat. No. 6,396,202, U.S. Pat. No. 6,013,384, EP 1 011 155, and U.S. Ser. No. 2002/0004146), Kido et al. disclose that an organic layer which functions as an electron-transporting layer in contact with a cathode and comprises an organic compound mixed with either a metal or a metal compound can lower the driving voltage of an OLED device, and provide increased efficiency and luminescence. However, the teachings do not mention any improvement in stability. Stability is defined as the time to reach half the initial luminance at a given current density. 
     It has been found that the use of an organic layer functioning as an electron-transporting layer and comprising an organic compound mixed with a low work function metal or metal compound can provide a remarkable and unexpected improvement in the stability of an OLED device in which the emissive layer has been formed by radiation transfer from a donor element. 
     In the practice of this invention, organic layer  28  can be an electron-transporting layer including an organic compound doped with a low work function metal or metal compound, or mixture thereof, capable of acting as a donor dopant between emissive layer  26  and cathode  30 , so as to lower the electron-injecting barrier from organic layer  28  into emissive layer  26 , thereby improving the stability of OLED device  16 . By low work function is meant a work function of less than  4 . 0  eV. The organic compound can be any electron-transporting material as described above for an electron-transporting layer such as organic layer  28 . The low work function metal can be an alkali metal such as Li, Na, K, Rb, Cs, an alkaline earth metal such as Ca, Sr, Ba, or a rare-earth metal such as La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu, or Y, or mixtures thereof. Particularly preferred are alkali metals, such as Li or Cs. The preferred ratio of dopant/host should be in the stoichiometric range from 0.2:1 to 3:1. 
     In the case of a metal compound dopant, the metal compound used is not restricted to a specific one, insofar as it contains, as a low work function metal ion thereof, at least one metal ion of the alkali metal ions, alkali earth metal ions and rare earth metal ions, or mixtures thereof The anionic part of the metal compound can be a halide or an organic ligand, or mixtures thereof. For halides, fluoride is preferably used, and alkali fluorides, e.g. LiF, NaF, KF, RbF, and CsF, or mixtures thereof, are particularly preferred as metal halides. As the organic ligand for the metal compound, although they are not restricted to the below-described, quinolinol, benzoquinolinol, acrydinol, phenanethridinol, hydroxyphenyloxazole, hydroxyphenylthiazole, hydroxydiaryloxadiazole, hydroxydiaryldiazole, hydroxyphenylpyridine, hydroxyphenylbenzoimidazole, hydroxybenzotriazole, hydroxyfurborane, bipyridyl, phenanethroline, phthalocyanine, porphyrin, cyclopentadiene, β-diketones, azomethines and derivatives thereof, or mixtures thereof, can be preferably used. 
     Turning now to FIG. 3, and referring also to FIG.  1  and FIG. 2, there is shown a block diagram comprising the steps in one embodiment of a method for forming an organic light-emitting device according to the present invention. At the start (Step  60 ) of the process, anodes or patterns of anodes  20  are formed over the substrate  10  (Step  62 ). Alternatively, the anodes  20  can be part of the substrate  10 , e.g. an OLED substrate. Then a hole-injecting layer  22  is optionally uniformly formed on the entire surface over anodes  20  (Step  64 ). Then a hole-transporting layer  24  is optionally formed uniformly on the entire surface over the hole-injecting layer  22  (Step  66 ). Then a donor element  40  is positioned in a material-transferring relationship as described above with substrate  10  (Step  68 ). Donor element  40  is then illuminated with radiation, such as laser light  42 , to heat donor element  40 , deposit light-emitting material  50 , and form emissive layer  26  over anode  20  and any intervening layers (Step  70 ). An organic layer  28  as described above is formed uniformly over emissive layer  26  (Step 74 ). Then a cathode layer or a series of cathodes  30  is deposited over the organic layer  28  (Step  76 ). There can be further steps, for example depositing a protective layer, before the end of the process (Step  78 ). 
     The invention and its advantages can be better appreciated by the following inventive and comparative examples. 
     EXAMPLE 1 
     Inventive Example 
     An OLED device with an organic layer doped with a low work function metal satisfying the requirements of the invention was constructed in the following manner: 
     1. A clean glass substrate was vacuum-deposited with indium tin oxide (ITO) to form a transparent electrode of 34 nm thickness. 
     2. The above-prepared ITO surface was treated with a plasma oxygen etch, followed by plasma deposition of a 1.0 nm layer of a fluorocarbon polymer (CFx) as described in U.S. Pat. No. 6,208,075. 
     3. The above-prepared substrate was further treated by vacuum-depositing under a vacuum of approximately 10 −6  Torr a 170 nm hole-transporting layer of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) from a heated boat source. 
     4. The above-prepared substrate was removed from the vacuum and exposed to air for about five minutes before being placed in a nitrogen drybox. 
     5. A donor substrate was prepared by vacuum coating 40 nm of chromium onto a 75 micrometer sheet of polysulfone. 
     6. The above-prepared donor substrate was further vacuum-coated with 20 nm of 2-(1,1-dimethyethyl)-9,10-bis(2-naphthalenyl)anthracene (TBADN) doped with 1.25% 2,5,8,11-tetra-t-butylperylene (TBP), then coated with an additional 0.8 nm of NPB to create a donor element. 
     7. The above-prepared donor element was removed from the vacuum and exposed to air for about five minutes before being placed in a nitrogen drybox. 
     8. The above donor element was placed in a material-transferring relationship with the substrate from step  3  above using a spacer to maintain a 75 micrometer gap between the donor element and the substrate. 
     9. A multichannel laser as described in commonly assigned U.S. patent application Ser. No. 10/055,579, now U.S. Pat. No. 6,582,875, filed Jan. 23, 2002 by Kay et al., entitled “Using a Multichannel Linear Laser Light Beam in Making OLED Devices by Thermal Transfer”, the disclosure of which is incorporated herein by reference, was used to illuminate the donor element with 755 mJ/cm 2  of energy to form the light-emitting layer over the hole-transport layer or the substrate. 
     10. The above substrate was exposed to the air for about 10 minutes, then returned to a vacuum. 
     11. A 35 nm electron-transporting layer of tris(8-quinolinolato)aluminum (III) (Alq) with 1.2% lithium by volume (Alq:Li) was vacuum-deposited onto the substrate at a coating station that included two heated boat sources, one each for Alq and lithium. 
     12. A 220 nm cathode layer was deposited onto the receiver element at a coating station with separate tantalum boats, one of which contained silver and one of which contained magnesium. The cathode layer was a 10:1 volume ratio of magnesium and silver. 
     13. The OLED device was then transferred to a dry box for encapsulation. 
     EXAMPLE 2 
     Comparative Example 
     An OLED device was constructed in the manner described in Example 1, except that step  11  (the deposition of an electron-transporting layer) was as follows: 
     11. A 35 nm electron-transporting layer of tris(8-quinolinolato)aluminum (III) (ALQ) was vacuum-deposited onto the substrate at a coating station that included a heated boat source. 
     Results 
     The devices in Examples 1 and 2 were tested by applying a constant current across the electrodes of 80 mA/cm 2  at room temperature and measuring the initial voltage and the time for the luminance to decrease to 50% of the initial luminance (80 mA/cm 2  half life). The relative voltage and relative half-life for each example was then calculated relative to the comparative control of Example 2. The following table shows the results. 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                 Electron- 
                 Relative 
                   
               
               
                   
                   
                 transport 
                 Voltage @ 
                 Relative 
               
               
                   
                   
                 layer 
                 80 
                 Half-Life @ 
               
               
                   
                 Example (Type) 
                 (35 nm) 
                 mA/cm 2   
                 80 mA/cm 2   
               
               
                   
                   
               
             
            
               
                   
                 1 (Inventive) 
                 Alq:Li 
                 0.9 
                 2.7 
               
               
                   
                 2 (Comparative) 
                 Alq 
                 1   
                 1   
               
               
                   
                   
               
            
           
         
       
     
     It is evident that the device made without the organic layer doped with a low work function metal (Example 2) has a shorter half-life and a higher voltage than the device with the organic layer doped with a low work function metal (Example 1). The addition of the lithium dopant to the electron transport layer increases the half-life by more than 2 times (Example 2 vs. Example 1), thus giving improved stability of the device. 
     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. 
     
       
         
           
               
             
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 10 
                 substrate 
               
               
                 14 
                 OLED device 
               
               
                 16 
                 OLED device 
               
               
                 20 
                 anode 
               
               
                 22 
                 hole-injecting layer 
               
               
                 24 
                 hole-transporting layer 
               
               
                 26 
                 emissive layer 
               
               
                 28 
                 organic layer 
               
               
                 30 
                 cathode 
               
               
                 38 
                 receiving surface 
               
               
                 40 
                 donor element 
               
               
                 42 
                 laser light 
               
               
                 44 
                 laser source 
               
               
                 46 
                 donor support element 
               
               
                 48 
                 light-absorbing layer 
               
               
                 50 
                 light-emitting material 
               
               
                 52 
                 non-transfer surface 
               
               
                 54 
                 heat 
               
               
                 56 
                 gap 
               
               
                 58 
                 raised surface portion 
               
               
                 60 
                 block 
               
               
                 62 
                 block 
               
               
                 64 
                 block 
               
               
                 66 
                 block 
               
               
                 68 
                 block 
               
               
                 70 
                 block 
               
               
                 74 
                 block 
               
               
                 76 
                 block 
               
               
                 78 
                 block