Abstract:
An organic light-emitting diode (OLED) device which produces substantially white light includes an anode; a hole-transporting layer disposed over the anode; and a blue light-emitting layer having a host doped with a blue light-emitting compound disposed directly on the hole-transporting layer and the blue light-emitting layer being doped with an electron-transporting or a hole-transporting material or both selected to improve efficiency and operational stability. The device also includes an electron-transporting layer disposed over the blue light-emitting layer; a cathode disposed over the electron-transporting layer; and the hole-transporting layer or electron-transporting layer, or both the hole-transporting layer and electron-transporting layer, being selectively doped with a compound which emits light in the yellow region of the spectrum which corresponds to an entire layer or a partial portion of a layer in contact with the blue light-emitting layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of application Ser. No. 10/391,727, filed Mar. 19, 2003 now abandoned entitled “White Light-Emitting OLED Device Having a Blue Light-Emitting Layer Doped With an Electron-Transporting or a Hole-Transporting Material or Both” by Tukaram K. Hatwar et al. 
     Reference is made to commonly assigned U.S. patent application Ser. No. 09/651,624 filed Aug. 30, 2000, now U.S. Pat. No. 6,699,177 issued Feb. 24, 2004, by Tukaram K. Hatwar, entitled “White Organic Electroluminescent Devices with Improved Stability and Efficiency”; Ser. No. 10/191,251 filed Jul. 8, 2002, now U.S. Pat. No. 6,720,092 issued Apr. 13, 2004, by Tukaram K. Hatwar, entitled “White Organic Light-Emitting Devices Using Rubrene Layer”; Ser. No. 10/183,242 filed Jun. 27, 2002, now U.S. Pat. No. 6,661,013 issued Dec. 19, 2003, by Benjamin P. Hoag et al., entitled “Organic Element for Electroluminescent Devices”; Ser. No. 10/086,067 filed Feb. 28, 2002, now U.S. Pat. No. 6,824,893 issued Feb. 28, 2002, by Benjamin P. Hoag et al., entitled “Organic Element for Electroluminescent Devices”; and Ser. No. 10/184,356 filed Jun. 27, 2002 (now abandoned) by Lelia Cosimbescu, entitled “Device Containing Green Organic Light-Emitting Diode”, the disclosures of which are incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to organic light-emitting OLED devices, which produce white light with an enhanced blue light component. 
     BACKGROUND OF THE INVENTION 
     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 U.S. Pat. Nos. 4,769,292 and 4,885,211. 
     Efficient white light producing OLED devices are considered a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have Commission International d&#39;Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color. 
     The following patents and publications disclose the preparation of organic OLED devices capable of emitting white light, comprising a hole-transporting layer and an organic luminescent layer, and interposed between a pair of electrodes. 
     White light producing OLED devices have been reported by J. Shi (U.S. Pat. No. 5,683,823) wherein the luminescent layer includes red and blue light-emitting materials uniformly dispersed in a host emitting material. This device has good electroluminescent characteristics, but the concentration of the red and blue dopants are very small, such as 0.12% and 0.25% of the host material. These concentrations are difficult to control during large-scale manufacturing. Sato et al. in JP 7,142,169 disclose an OLED device, capable of emitting white light, made by placing a blue light-emitting layer next to the hole-transporting layer and followed by a green light-emitting layer having a region containing a red fluorescent layer. Kido et al., in Science, Vol. 267, p. 1332 (1995) and in APL Vol. 64, p. 815 (1994), report a white light producing OLED device. In this device three emitter layers with different carrier transport properties, each emitting blue, green or red light, are used to generate white light. Littman et al. in U.S. Pat. No. 5,405,709 disclose another white emitting device, which is capable of emitting white light in response to hole-electron recombination, and comprises a fluorescent in a visible light range from bluish green to red. Recently, Deshpande et al., in Applied Physics Letters, Vol. 75, p. 888 (1999), published a white OLED device using red, blue, and green luminescent layers separated by a hole blocking layer. 
     However, these OLED devices require very small amounts of dopant concentrations, making the process difficult to control for large-scale manufacturing. Also, emission color varies due to small changes in the dopant concentration. Full-color devices are made by combining white OLEDs with color filters. However, the color filter transmits only about 30% of the original light. Thus, when the white light is passed through the blue color filter, the blue component is very low in luminance intensity. Due to its low intensity, the blue channel of the R, G, B full-color display is required to operate at much higher current density. This reduces the lifetime of the blue color. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to produce an effective white light-emitting organic device with improved efficiency and operational stability of blue light emission. 
     This object is achieved by an organic light-emitting diode (OLED) device which produces substantially white light, comprising: 
     a) an anode; 
     b) a hole-transporting layer disposed over the anode; 
     c) a blue light-emitting layer having a host doped with a blue light-emitting compound disposed directly on the hole-transporting layer and the blue light-emitting layer being doped with an electron-transporting or a hole-transporting material or both selected to improve efficiency and operational stability; 
     d) an electron-transporting layer disposed over the blue light-emitting layer; 
     e) a cathode disposed over the electron-transporting layer; and 
     f) the hole-transporting layer or electron-transporting layer, or both the hole-transporting layer and electron-transporting layer, being selectively doped with a compound which emits light in the yellow region of the spectrum which corresponds to an entire layer or a partial portion of a layer in contact with the blue light-emitting layer. 
     ADVANTAGES 
     The following are features and advantages of the present invention. 
     White light OLED devices, in accordance with the present invention, have significantly improved device efficiency and operational stability. More particularly, by adding a hole-transporting or electron-transporting material as co-dopants in a small amount along with the blue emitting dopant to the blue light-emitting layer, significant improvements can be achieved. 
     High efficiency white OLEDs can be used to fabricate full-color devices using the substrate with the on chip color filters (OCCF) and integrated thin film transistors. 
     OLED devices made in accordance with the present invention eliminate the need for using a shadow mask for making light-emitting layers in full-color OLED devices. 
     OLED devices made in accordance with the present invention can be produced with high reproducibility and consistency to provide high light efficiency. 
     These devices have high operational stability and also require low drive voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a prior art organic light-emitting device; 
         FIG. 2  depicts another prior art organic light-emitting device; 
         FIG. 3  depicts a white light producing OLED device wherein the hole-transporting layer is doped with the super rubrene yellow dopant; 
         FIG. 4  depicts another structure of white light producing OLED device wherein hole-transporting layer is doped with super rubrene yellow dopant and has two sublayers; 
         FIG. 5  depicts a white light producing OLED device wherein the electron-transporting layer is doped with yellow dopant; 
         FIG. 6  depicts another structure of white light producing OLED device wherein both the hole-transporting layer and the electron-transporting layer are doped with yellow dopant; 
         FIG. 7  depicts another structure of white light producing OLED device wherein both the hole-transporting layer and the electron-transporting layer are doped with yellow dopant and has two sublayers; 
         FIG. 8  depicts a white light producing OLED device wherein the hole-transporting layer is doped with the yellow dopant and has an additional green-emitting layer; 
         FIG. 9  depicts another structure of white light producing OLED device wherein the hole-transporting layer is doped with yellow dopant and has two sublayers and has an additional green-emitting layer; 
         FIG. 10  depicts a white light producing OLED device wherein the electron-transporting layer is doped with yellow dopant and has an additional green-emitting layer; 
         FIG. 11  depicts another structure of white light producing OLED device wherein both the hole-transporting layer and the electron-transporting layer are doped with yellow dopant and has an additional green-emitting layer; and 
         FIG. 12  depicts another structure of white light producing OLED device wherein both the hole-transporting layer and the electron-transporting layer are doped with yellow dopant and has two sublayers, and has an additional green-emitting layer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A conventional light-emitting layer of the organic OLED device comprises a luminescent or fluorescent material where electroluminescence is produced as a result of electron-hole pair recombination in this region. In the simplest construction, the device  100  as shown in  FIG. 1  has a substrate  110  and a light-emitting layer  140  sandwiched between anode  120  and cathode  170 . The light-emitting layer  140  is a pure material with a high luminescent efficiency. A well known material is tris(8-quinolinato) aluminum (Alq) which produces excellent green electroluminescence. 
     The simple structure can be modified to a three-layer structure (device  200 ) as shown in  FIG. 2 , in which an additional electroluminescent layer is introduced between the hole- and electron-transporting layers to function primarily as the site for hole-electron recombination and thus electroluminescence. In this respect, the functions of the individual organic layers are distinct and can therefore be optimized independently. Thus, the electroluminescent or recombination layer can be chosen to have a desirable OLED color as well as high luminance efficiency. Likewise, the electron and hole-transporting layers can be optimized primarily for the carrier transport property. It will be understood to those skilled in the art that the electron-transporting layer and the cathode can be made to be transparent, facilitating illumination of the device through its top layer and not through the substrate. 
     Turning to  FIG. 2 , an organic light-emitting device  200  has a light-transmissive substrate  210  on which is disposed a light-transmissive anode  220 . An organic light-emitting structure is formed between the anode  220  and a cathode  270 . The organic light-emitting structure is comprised of, in sequence, an organic hole-transporting layer  240  (HTL), an organic light-emitting layer  250 , and an organic electron-transporting layer (ETL)  260 . Layer  230  is a hole-injecting layer (HIL). When an electrical potential difference (not shown) is applied between the anode  220  and the cathode  270 , the cathode will inject electrons into the electron-transporting layer  260  and the electrons will migrate across layer  260  to the light-emitting layer  250 . At the same time, holes will be injected from the anode  220  into the hole-transporting layer  240 . The holes will migrate across layer  240  and recombine with electrons at or near a junction formed between the hole-transporting layer  240  and the light-emitting layer  250 . When a migrating electron drops from its conduction band to a valance band in filling a hole, energy is released as light, and which is emitted through the light-transmissive anode  220  and substrate  210 . 
     The organic OLED devices can be viewed as a diode, which is forward biased when the anode is at a higher potential than the cathode. The anode and cathode of the organic OLED device can each take any convenient conventional form, such as any of the various forms disclosed by Tang et al. in U.S. Pat. No. 4,885,211. Operating voltage can be substantially reduced when using a low-work function cathode and a high-work function anode. The preferred cathodes are those constructed of a combination of a metal having a work function less than 4.0 eV and one other metal, preferably a metal having a work function greater than 4.0 eV. The Mg:Ag of Tang et al. U.S. Pat. No. 4,885,211 constitutes one preferred cathode construction. The Al:Mg cathodes of Van Slyke et al. U.S. Pat. No. 5,059,062 is another preferred cathode construction. Hung et al. in U.S. Pat. No. 5,776,622 has disclosed the use of a LiF/Al bilayer to enhanced electron injection in organic OLED devices. Cathodes made of either Mg:Ag, Al:Mg or LiF/Al are opaque and displays cannot be viewed through the cathode. Recently, a series of publications by Gu et al. in APL 68, 2606 (1996); Burrows et al., J. Appl. Phys. 87, 3080 (2000); Parthasarathy et al. APL 72, 2138 9198); Parthasarathy et al. APL 76, 2128 (2000); and Hung et al. APL, 3209 (1999) have disclosed transparent cathodes. These transparent cathodes are based on the combination of a thin semitransparent metal (˜10 nm) and indium-tin-oxide (ITO) on top of the metal. An organic layer of copper phthalocyanine (CuPc) also replaced thin metal. 
     Conventionally, anode  220  is formed of a conductive and transparent oxide. Indium tin oxide has been widely used as the anode contact because of its transparency, good conductivity, and high-work function. 
     In a preferred embodiment, an anode  220  can be modified with a hole-injecting layer  230 . 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 include, but are not limited to, porphyrinic compounds such as CuPC 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. and some aromatic amines, for example, m-MTDATA (4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine). Alternative hole-injecting materials reportedly useful in organic EL devices are described in EP 0 891 121 A1 and EP 1 029 909 A1. An example of material in such a hole-injecting layer are the fluorocarbons disclosed by Hung et al. in U.S. Pat. No. 6,208,075. 
     The OLED device of this invention is typically provided over a supporting substrate  210  where either the cathode or anode can be in contact with the substrate. The electrode in contact with the substrate is conveniently referred to as the bottom electrode. Conventionally, the bottom electrode is the anode, but this invention is not limited to that configuration. The substrate 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, silicon, ceramics, circuit board materials, and polished metal surface. Of course, it is necessary to provide in these device configurations a light-transparent top electrode. 
     The white OLED emission can be used to prepare a full-color device using red, green, and blue (R, G, B) color filters. The R, G, B filters may be deposited on the substrate (when light transmission is through the substrate), incorporated into the substrate, or deposited over the top electrode (when light transmission is through the top electrode). When depositing a R, G, B filter array over the top electrode, a buffer layer may be used to protect the top electrode. The buffer layer may comprise inorganic materials, for example, silicon oxides and nitrides, or organic materials, for example, polymers, or multiple layers of inorganic and organic materials. Methods for providing R, G, B filter arrays are well known in the art. Lithographic means, inkjet printing, and laser thermal transfer are just a few of the methods by which R, G, B filters may be provided. 
     This technique of producing a full-color display using white light plus R, G, B filters has several advantages over the precision shadow masking technology used for producing full-color displays. This technique does not require precision alignment, is low cost and easy to manufacture. The substrate itself contains thin film transistors to address the individual pixels. U.S. Pat. Nos. 5,550,066 and 5,684,365 to Ching et al. describe the addressing methods of the TFT substrates. 
     The hole-transporting layer contains at least one hole-transporting compound 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. 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. 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. The hole-transporting layer can be formed of a single or a mixture of aromatic tertiary amine compounds set forth in Table 1. Illustrative of useful aromatic tertiary amines is the following list. In accordance with the present invention, these materials can also be used as dopants in the blue light-emitting layer and, for the purpose of this disclosure, will be called blue stabilizing hole-transporting materials. 
     
       
         
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 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,N,N′,N′-tetra-1-naphthyl-4,4′-diaminobiphenyl 
               
               
                 N,N,N′,N′-tetra-2-naphthyl-4,4′-diaminobiphenyl 
               
               
                 N-Phenylcarbazole 
               
               
                 4,4′-Bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) 
               
               
                 4,4′-Bis[N-(1-naphthyl)-N-(2-naphthyl)amino]biphenyl (TNB) 
               
               
                 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 
               
               
                 4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine (MTDATA) 
               
               
                 4,4′-Bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (TPD) 
               
               
                   
               
             
          
         
       
     
     Another class of useful hole-transporting materials includes polycyclic aromatic compounds as described in EP 1 009 041. Tertiary aromatic amines with more than two amine groups may be used including oligomeric materials. 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. 
     Preferred materials for use in forming the electron-transporting layer of the organic OLED devices of this invention are metal chelated oxinoid compounds, including chelates of oxine itself (also commonly referred to as 8-quinolinol or 8-hydroxyquinoline) as disclosed in U.S. Pat. No. 4,885,211. Tris(8-quinolinolato)aluminum(III) also commonly known as Alq is one of the commonly used electron-transporting materials. Such compounds exhibit high levels of performance and are readily fabricated in the form of thin layers. Some examples of useful electron-transporting materials are:
     Aluminum trisoxine [alias, tris(8-quinolinolato)aluminum(III)]   Magnesium bisoxine [alias, bis(8-quinolinolato)magnesium(II)]   Bis[benzo{f}-8-quinolinolato]zinc (II)   Bis(2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-8-quinolinolato)aluminum(III)   Indium trisoxine [alias, tris(8-quinolinolato)indium]   Aluminum tris(5-methyloxine) [alias, tris(5-methyl-8-quinolinolato) aluminum(III)]   Lithium oxine [alias, (8-quinolinolato)lithium(I)]   Gallium oxine [alias, tris(8-quinolinolato)gallium(III)]   Zirconium oxine [alias, tetra(8-quinolinolato)zirconium(IV)]   

     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 and triazines are also useful electron-transporting materials. 
     Another material of the series, BAlq, has been used as an electron-transporting material. U.S. Pat. No. 5,141,671 issued to Bryan et al. discusses such materials. The BAlq is comprised of a mixed ligand aluminum chelate, specifically a bis(Rs-8-quinolinolato)(phenolato)aluminum(II) chelate, where Rs is a ring substituent of the 8-quinolinolato ring nucleus. These compounds are represented by the formula Rs-Q2-Al—O-L where Q in each occurrence represents a substituted 8-quinolinolato ligand, Rs represents an 8-quinolinolato ring substituent to block sterically the attachment of more than two substituted 8-quinolinolato ligand to the aluminum atom, O-L is phenolatoligand, and L is a hydrocarbon of from 6 to 24 carbon atoms comprised of phenyl moiety. One such compound, particularly ((1,1′-biphenyl)-4-olato)bis(2-methyl-8-quinolinoato N1,O8)aluminum, has been used as a hole blocking material by T. Watanabe et al., Proceedings of SPIE Vol. 4105 (2001), p. 175-182. 
     A preferred embodiment of the luminescent layer consists of a host material doped with fluorescent dyes. Using this method, highly efficient EL devices can be constructed. Simultaneously, the color of the EL devices can be tuned by using fluorescent dyes of different emission wavelengths in a common host material. Tang et al. in commonly assigned U.S. Pat. No. 4,769,292 has described this dopant scheme in considerable detail for EL devices using Alq as the host material. 
     Shi et al. in commonly assigned U.S. Pat. No. 5,935,721 have described this dopant scheme in considerable detail for the blue emitting OLED devices using 9,10-di-(2-naphthyl)anthracene (ADN) derivatives as the host material. 
     Derivatives of 9,10-di-(2-naphthyl)anthracene (Formula 1) constitute one class of useful hosts 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 , 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 naphthyl, anthracenyl; phenanthryl, pyrenyl, or perylenyl; 
     Group 4: heteroaryl or substituted heteroaryl of from 5 to 24 carbon atoms such as thiazolyl, furyl, thienyl, pyridyl, quinolinyl or other heterocyclic systems, which may be bonded via a single bond, or may complete a fused heteroaromatic ring system; 
     Group 5: alkoxylamino, alkylamino, or arylamino of from 1 to 24 carbon atoms; or 
     Group 6: fluorine, chlorine, bromine or cyano. 
     Illustrative examples include 9,10-di-(2-naphthyl)anthracene (ADN) and 2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN). Other anthracene derivatives can be useful as a host in the LEL, such as diphenylanthracene and its derivatives, as described in U.S. Pat. No. 5,927,247. Styrylarylene derivatives as described in U.S. Pat. No. 5,121,029 and JP 08333569 are also useful hosts for blue emission. For example, 9,10-bis[4-(2,2-diphenylethenyl)phenyl]anthracene and 4,4′-Bis(2,2-diphenylethenyl)-1,1′-biphenyl (DPVBi) are useful hosts for blue emission. 
     Many blue fluorescent dopants are known in the art, and are contemplated for use in the practice of this invention. Particularly useful classes of blue-emitting dopants include perylene and its derivatives such as 2,5,8,11-tetra-tert-butyl perylene (TBP), and distyrylamine derivatives as described in U.S. Pat. No. 5,121,029, such as B1 (structure shown below) 
                        
 
     Another useful class of blue-emitting dopants is represented by Formula 2 and is described in commonly assigned U.S. Pat. No. 6,661,023, the disclosure of which is incorporated herein. 
                        
 
wherein:
 
     A and A′ represent independent azine ring systems corresponding to 6-membered aromatic ring systems containing at least one nitrogen; 
     each X a  and X b  is an independently selected substituent, two of which may join to form a fused ring to A or A′; 
     m and n are independently 0 to 4; 
     Z a  and Z b  are independently selected substituents; and 
     1, 2, 3, 4, 1′, 2′, 3′, and 4′ are independently selected as either carbon or nitrogen atoms. 
     Desirably, the azine rings are either quinolinyl or isoquinolinyl rings such that 1, 2, 3, 4, 1′, 2′, 3′, and 4′ are all carbon; m and n are equal to or greater than 2; and X a  and X b  represent at least two carbon substituents which join to form an aromatic ring. Desirably, Z a  and Z b  are fluorine atoms. 
     Preferred embodiments further include devices where the two fused ring systems are quinoline or isoquinoline systems; the aryl or heteroaryl substituent is a phenyl group; there are present at least two X a  groups and two X b  groups which join to form a 6—6 fused ring, the fused ring systems are fused at the 1-2, 3-4, 1′-2′, or 3′-4′ positions, respectively; one or both of the fused rings is substituted by a phenyl group; and where the dopant is depicted in Formula 3, 4, or 5. 
                        
 
wherein each X c , X d , X e , X f , X g , and X h  is hydrogen or an independently selected substituent, one of which must be an aryl or heteroaryl group.
 
     Desirably, the azine rings are either quinolinyl or isoquinolinyl rings such that 1, 2, 3, 4, 1′, 2′, 3′, and 4′ are all carbon; m and n are equal to or greater than 2; and X a  and X b  represent at least two carbon substituents which join to form an aromatic ring, and one is an aryl or substituted aryl group. Desirably, Z a  and Z b  are fluorine atoms. 
     Illustrative, non-limiting examples of boron compounds complexed by two ring nitrogens of a deprotonated bis(azinyl)amine ligand, wherein the two ring nitrogens are members of different 6,6 fused ring systems in which at least one of the systems contains an aryl or heteroaryl substituent, useful in the present invention are the following: 
                        
                         
 
     Preferred materials for uses as a yellow-emitting dopant in the hole-transporting or electron-transporting layers are those represented by Formula 6. 
                        
 
wherein R 1 , R 2 , R 3 , and R 4  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 phenyl, naphthyl, anthracenyl; phenanthryl, pyrenyl, or perylenyl; 
     Group 4: heteroaryl or substituted heteroaryl of from 5 to 24 carbon atoms such as thiazolyl, furyl, thienyl, pyridyl, quinolinyl or other heterocyclic systems, which may be bonded via a single bond, or may complete a fused heteroaromatic ring system; 
     Group 5: alkoxylamino, alkylamino, or arylamino of from 1 to 24 carbon atoms; or 
     Group 6: fluorine, chlorine, bromine or cyano. 
     R 5  and R 6  are defined in the same way as R 1 -R 4  except that they do not form a fused ring. 
     Further, at least one of R 1 -R 4  must be substituted with a group. Preferred groups for substitution on R 1 -R 4  are Groups 3 and 4. 
     Examples of particularly useful yellow dopants include 5,6,11,12-tetraphenylnaphthacene (rubrene); 6,11-diphenyl-5,12-bis(4-(6-methyl-benzothiazol-2-yl)phenyl)naphthacene (DBzR); and 5,6,11,12-tetra(2-naphthyl)naphthacene (NR), the formulas of which are shown below: 
                        
 
     Coumarins represent a useful class of green-emitting dopants as described by Tang et al. in U.S. Pat. Nos. 4,769,292 and 6,020,078. Examples of useful green-emitting coumarins include C545T and C545TB. Quinacridones represent another useful class of green-emitting dopants. Useful quinacridones are described in U.S. Pat. No. 5,593,788, and Publication JP 09-13026A, the disclosure of which is incorporated herein. 
     Examples of particularly useful green-emitting quinacridones are shown below: 
                        
 
     Another useful class of green-emitting dopants is represented by Formula 7 below. 
     Compounds useful in the invention are suitably represented by Formula 7: 
                        
 
wherein:
 
     A and A′ represent independent azine ring systems corresponding to 6-membered aromatic ring systems containing at least one nitrogen; 
     each X a  and X b  is an independently selected substituent, two of which may join to form a fused ring to A or A′; 
     m and n are independently 0 to 4; 
     Y is H or a substituent; 
     Z a  and Z b  are independently selected substituents; and 
     1, 2, 3, 4, 1′, 2′, 3′, and 4′ are independently selected as either carbon or nitrogen atoms. 
     In the device, 1, 2, 3, 4, 1′, 2′, 3′, and 4′ are conveniently all carbon atoms. The device may desirably contain at least one or both of ring A or A′ that contains substituents joined to form a fused ring. In one useful embodiment, there is present at least one X a  or X b  group selected from the group consisting of halide and alkyl, aryl, alkoxy, and aryloxy groups. In another embodiment, there is present a Z a  and Z b  group independently selected from the group consisting of fluorine and alkyl, aryl, alkoxy and aryloxy groups. A desirable embodiment is where Z a  and Z b  are F. Y is suitably hydrogen or a substituent such as an alkyl, aryl, or heterocyclic group. 
     The emission wavelength of these compounds may be adjusted to some extent by appropriate substitution around the central bis(azinyl)methene boron group to meet a color aim, namely green. Some examples of useful formulas follow: 
                        
 
     The invention and its advantages are further illustrated by the specific examples that follow. The term “percentage” indicates the volume percentage (or a thickness ratio as measured on the thin film thickness monitor) of a particular dopant with respect to the host material. 
       FIGS. 3-14  show schematics of the white light producing OLED device structure that have been made in accordance with the present invention and graphs of various parameters of their operations. The invention and its advantages are further illustrated by the specific examples that follow. 
     Turning to  FIG. 3 , an organic white light-emitting device  300  has a light-transmissive substrate  310  on which is disposed a light-transmissive anode  320 . An organic white light-emitting structure  300  is formed between the anode  320  and a cathode  370 . The organic light-emitting structure is comprised of, in sequence, a hole-injecting layer  330 , and an organic hole-transporting layer  340 , which is doped with yellow-emitting dopants. An organic light-emitting layer  350  is a blue light-emitting layer comprising TBADN host, B-1 dopant, and co-dopants selected from a group of NPB, Alq, and BAlq. An organic electron-transporting layer  360  is made of Alq. 
       FIG. 4  depicts an organic white light-emitting device  400  which is similar to that shown in  FIG. 3 , except that the organic hole-transporting layer comprises two sublayers, layers  441  and layer  442 . Layer  442  is made of undoped NPB and the layer  441 , which is adjacent to the blue light-emitting layer  450 , is doped with yellow-emitting dopant. Other layers of the structure  400  are substrate  410 , anode  420 , hole-injecting layer  430 , electron-transporting layer  460 , and cathode  470 . 
       FIG. 5  depicts an organic white light-emitting device  500 . The electron-transporting layer comprises two sublayers,  561  and  562 . Electron-transporting sublayer  561  is doped with the yellow-emitting dopant. Electron-transporting sublayer  562  is not doped with a light-emitting dopant. The blue light-emitting layer  550  comprises TBADN host, B-1 dopant, and co-dopants selected from a group of NPB, Alq, and BAlq. Other layers of the structure  500  are substrate  510 , anode  520 , hole-injecting layer  530 , hole transport layer  540  and cathode  570 . 
       FIG. 6  depicts an organic white light-emitting device  600 , which is a combination of structure  300  and structure  500 . The hole-transporting layer  640  is doped with a yellow-emitting dopant. The electron-transporting layer comprises two electron-transporting sublayers,  661  and  662 , and sublayer  661  is doped with a yellow-emitting dopant. The blue light-emitting layer  650  is made of TBADN host, B-1 dopant, and co-dopants selected from a group of NPB, Alq, and BAlq. Other layers of structure  600  are substrate  610 , anode  620 , hole-injecting layer  630 , electron-transporting layer  662 , and cathode  670 . 
       FIG. 7  depicts an organic white light-emitting device  700  which is similar to that shown in  FIG. 6 , except that the organic hole-transporting layer consists of two sublayers, sublayers  741  and  742 . Sublayer  742  is made of undoped NPB, and sublayer  741 , adjacent to the blue light-emitting layer  750 , is doped with a yellow-emitting dopant. The electron-transporting layer comprises two sublayers, sublayers  761  and  762 . Electron-transporting sublayer  761  is adjacent to the blue light-emitting layer  750 , and is also doped with yellow-emitting dopant. Electron-transporting sublayer  762  is not doped with a light-emitting dopant. Other layers of structure  700  are substrate  710 , anode  720 , hole-injecting layer  730 , and cathode  770 . 
       FIG. 8  depicts an organic white light-emitting device  800  that is similar to that shown in  FIG. 3 , except that the electron-transporting layer comprises two sublayers,  861  and  862 . Electron-transporting sublayer  861  comprises a green-emitting dopant such as C545T, CFDMQA, and DPQA, and sublayer  861  is adjacent to the blue light-emitting layer  850 . Electron-transporting sublayer  862  is not doped with a light-emitting dopant. The blue light-emitting layer is  850  and consists of TBADN host, B-1 dopant and co-dopants selected from a group of NPB, Alq, and BAlq. The hole-transporting layer  840  is doped with a yellow-emitting dopant. Other layers of the structure  800  are substrate  810 , anode  820 , hole-injecting layer  830 , and cathode  870 . 
       FIG. 9  depicts an organic white light-emitting device  900  which is similar to that shown in  FIG. 8 , except that the organic hole-transporting layer comprises two sublayers,  941  and  942 . Hole-transporting sublayer  942  is made of undoped NPB, and sublayer  941  adjacent to the blue light-emitting layer  950  is doped with a yellow-emitting dopant. The electron-transporting layer comprises two sublayers,  961  and  962 . The electron-transporting sublayer  961  is adjacent to the blue light-emitting layer  950 , and comprises Alq doped with green dopants such as C545T, CFDMQA, and DPQA. Electron-transporting sublayer  962  is not doped with a light-emitting dopant. The blue light-emitting layer i 950  consists of TBADN host, B-1 dopant and co-dopants selected from a group of NPB, Alq, and BAlq. Other layers of the structure  900  are substrate  910 , anode  920 , hole-injecting layer  930 , and cathode  970 . 
       FIG. 10  depicts an organic white light-emitting device  1000 . Here, the electron-transporting layer comprises three sublayers  1061 ,  1062 , and  1063 . The electron-transporting sublayer  1061  is doped with a yellow-emitting dopant, and this layer is adjacent to the blue light-emitting layer  1050 . Electron-transporting sublayer  1062  comprises a green-emitting dopant such as C545T, CFDMQA, or DPQA. Electron-transporting sublayer  1063  is not doped with a light-emitting dopant. The blue light-emitting layer  1050  can comprise TBADN host, B-1 dopant, and co-dopants selected from a group of NPB, Alq, and BAlq. Other layers of the structure  1000  are substrate  1010 , anode  1020 , hole-injecting layer  1030 , hole-transporting layer  1040 , and cathode  1070 . 
       FIG. 11  depicts an organic white light-emitting device  1100 . Here, the electron-transporting layer comprises three sublayers  1161 ,  1162 , and  1163 . The electron-transporting sublayer  1161  is doped with a yellow-emitting dopant, and this layer is adjacent to the blue light-emitting layer  1150 . Electron-transporting sublayer  1162  comprises a green-emitting dopant such as C545T, CFDMQA, or DPQA. Electron-transporting sublayer  1163  is not doped with a light-emitting dopant. The blue light-emitting layer  1150  can comprise TBADN host, B-1 dopant, and co-dopants selected from a group of NPB, Alq, and BAlq. The hole-transporting layer  1140  is doped with a super rubrene yellow dopant. This device shows very high stability, high luminance efficiency, and good spectral radiance for all colors after the R, G, B color filters. Other layers of the structure  1100  are substrate  1110 , anode  1120 , hole-injecting layer  1130 , and cathode  1170 . 
       FIG. 12  depicts an organic white light-emitting device  1200 . Here, the electron-transporting layer comprises three sublayers  1261 ,  1262 , and  1263 . The electron-transporting sublayer  1261  is doped with the yellow-emitting dopant, and this layer is adjacent to the blue light-emitting layer  1250 . Electron-transporting sublayer  1262  comprises a green-emitting dopant such as C545T, CFDMQA, or DPQA. Electron-transporting sublayer  1263  is not doped with a light-emitting dopant. The blue light-emitting layer  1250  can comprise TBADN host, B-1 dopant, and co-dopants selected from a group of NPB, Alq, and BAlq. The hole-transporting layer comprises two sublayers,  1241  and  1242 . Hole-transporting sublayer  1241  is undoped NPB. Hole-transporting sublayer  1242  is adjacent to blue light-emitting layer  1250 , and is doped with a yellow-emitting dopant. Other layers of the structure  1200  are substrate  1210 , anode  1220 , hole-injecting layer  1230 , and cathode  1170 . 
     The invention and its advantages are further illustrated by the specific following examples. 
     Device Examples 1 to 6 given in Table 2 indicate the improvement in the luminance and stability performance of the white devices when the blue emitting layer is doped with an electron-transporting material such as Alq. 
     An OLED device was constructed in the following manner. 
     Substrates coated with 80 nm ITO were sequentially ultrasonicated in a commercial detergent, rinsed in deionized water, and degreased in toluene vapor. These substrates were treated with an oxygen plasma for about one minute and coated with one nm fluorocarbon layer by plasma assisted deposition of CHF 3 . The same procedure was used for preparing all other devices described in this invention. 
     These substrates were loaded into a deposition chamber for organic layers and cathode depositions. 
     The device of Example 1 was prepared by following the structure of OLED  300  as shown in  FIG. 3  by sequential deposition of 150 nm NPB hole-transporting layer (HTL) doped with 2% DBzR yellow dopant, 20 nm blue light-emitting layer (LEL) comprising TBADN host with 2% TBP blue dopant, 35 nm Alq electron-transporting layer (ETL), and then 200 nm MgAg cathode. The above sequence completed the deposition of the OLED device. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 White emitting OLED device performance with Alq doping into the blue emission layer 
               
             
          
           
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Operational 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 stability, 
               
               
                   
                   
                   
                   
                   
                 Electron 
                 Lumi- 
                   
                   
                   
                 T70 (Hours 
               
               
                   
                 Hole Transport Layer 
                 Blue 
                 Blue layer 
                 Blue layer 
                 transport 
                 nance 
                   
                   
                 Drive 
                 for 30% 
               
               
                 Device 
                 doped with yellow 
                 emission layer 
                 dopant 
                 dopant 
                 layer 
                 Yield 
                   
                   
                 Voltage 
                 decrease in 
               
               
                 Number 
                 dopant 
                 Host (TBADN) 
                 (TBP) 
                 (Alq) 
                 thickness 
                 (cd/A) 
                 CIE_x 
                 CIE_y 
                 (volts) 
                 luminance) 
               
               
                   
               
               
                 1 
                 150 nm + 2.0% DBzR 
                 20 nm TBADN 
                 2% TBP 
                   0% Alq 
                 35 nm 
                 5.44 
                 0.34 
                 0.34 
                 8.4 
                 620 
               
               
                 2 
                 150 nm + 2.0% DBzR 
                 20 nm TBADN 
                 2% TBP 
                   1% Alq 
                 35 nm 
                 5.50 
                 0.39 
                 0.41 
                 8.3 
                 720 
               
               
                 3 
                 150 nm + 2.0% DBzR 
                 20 nm TBADN 
                 2% TBP 
                 2.5% Alq 
                 35 nm 
                 5.60 
                 0.41 
                 0.43 
                 8.3 
                 800 
               
               
                 4 
                 150 nm + 2.0% DBzR 
                 20 nm TBADN 
                 2% TBP 
                   5% Alq 
                 35 nm 
                 5.60 
                 0.45 
                 0.45 
                 8.5 
                 850 
               
               
                 5 
                 150 nm + 2.0% DBzR 
                 20 nm TBADN 
                 2% TBP 
                  10% Alq 
                 35 nm 
                 5.60 
                 0.45 
                 0.46 
                 8.4 
                 900 
               
               
                 6 
                 150 nm + 2.0% DBzR 
                 20 nm TBADN 
                 2% TBP 
                  25% Alq 
                 35 nm 
                 5.80 
                 0.48 
                 0.49 
                 8.4 
                 980 
               
               
                   
               
             
          
         
       
     
     The OLED device was then “hermetically” packaged in a dry glove box filled with nitrogen for protection against ambient environment. The ITO patterned substrates used for preparing these OLED devices contained several test patterns. Each of the devices was tested for current voltage characteristics and the electroluminescence yield. 
     Devices of Examples 2 to 6 were prepared following structure of OLED  300  as shown in  FIG. 3 , and all the layers were similar except that the 20 nm (TBADN+2% TBP) blue emitting layer was doped with varying amounts of Alq concentrations varying from 1% to 25%. It was found that the devices of Examples 2 to 6 show an increase in luminance efficiency and an increase in the operational stability of the devices. However, the original white color of the device was shifted to the higher wavelength (and was more on the orange side). Device Example 1 shows the CIEx,y color coordinates of (0.34, 0.34), whereas device Example 3 with 2.5% Alq in the blue emitting layer has CIEx,y equal to (0.41, 0.43). 
     It was found that co-doping NPB and Alq could reduce this shift in the color of the spectra into the blue emitting layer along with the blue-emitting dopant. Simultaneously, the device luminance efficiency and the operational stability were improved. The operational stability of the encapsulated OLED devices in ambient environments was found by measuring the changes in the drive voltage and the luminance as a function of time when OLED devices were operated at a constant current density of 20 mA/cm 2 . 
     Devices of Examples 7 to 10 given in Table 3 show the improvement from co-doping Alq and NPB dopants in blue OLED devices. Device Example 7 was prepared with the layer structure: 150 nm NPB HTL/20 nm A_DN host+2% TBP dopant as the blue emitting layer/25 nm Alq ETL/200 nm MgAg cathode. It has luminance efficiency of 3.35 cd/A, drive voltage 6.3 volts and CIEx,y=0.16, 0.23 respectively. Device Example 8 was prepared similar to device Example 7, except that the blue emitting layer has 10% NPB as a co-dopant with TBP blue emitting dopant. It has luminance efficiency of 4.18 cd/A, drive voltage 6.2 volts, and CIEx,y=0.16, 0.23, respectively. Thus, the efficiency of the device in Example 8 is higher than that of the device of Example 7. Device Example 9 was prepared similar to device Example 7, except that the blue emitting layer has 10% Alq as a co-dopant with TBP blue emitting dopant. It has luminance efficiency of 3.6 cd/A and CIEx,y=0.23, 0.36, respectively. This efficiency is higher than that of device Example 7, however the color is shifted toward green. Device Example 10 was prepared similar to device Example 7, except that the blue emitting layer contained 10% NPB and 10% Alq as co-dopants with TBP blue emitting dopant. It has luminance efficiency of 4.8 cd/A and CIEx,y=0.20, 0.25, respectively. The luminance efficiency of the device in Example 10 is higher than that of the devices of Examples 7, 8, and 9, and the color is similar to that of the device Example 7. Thus, higher luminance efficiency and good color was obtained when both Alq and NPB co-dopants were doped in the blue emitting layer along with blue emitting dopant TBP. This blue light-emitting layer doped with blue dopant and the blue stabilizing dopant materials of device Example 10 can be used to make white emitting OLED devices using the structure shown in FIG.  3 . 
                                                                                                                 TABLE 3                   EL properties of Blue emitting OLEDs wherein the blue emitting layer is doped with dopants NPB and BAlq                Hole Transport                   Electron                           Layer (undoped   Blue               transport               Drive       Device   NPB layer   emission layer   Blue layer   Blue layer   Blue layer   layer   Luminance           Voltage       Number   thickness)   Host (ADN)   dopant 1   dopant 2   dopant 3   thickness   Yield (cd/A)   CIE_x   CIE_y   (volts)                    7   150 nm   20 nm ADN   2% TBP    0% NPB    0% Alq   25 nm   3.35   0.16   0.23   6.3       8   150 nm   20 nm ADN   2% TBP   10% NPB    0% Alq   25 nm   4.18   0.16   0.23   6.2       9   150 nm   20 nm ADN   2% TBP    0% NPB   10% Alq   25 nm   3.60   0.23   0.36   6.0       10   150 nm   20 nm ADN   2% TBP   10% NPB   10% Alq   25 nm   4.80   0.20   0.25   6.4                    
Device Examples 11 to 15 (Table 4): Table 4 describes the use of other blue stabilizing co-dopants, such as NPB and BAlq, in the blue light-emitting layer of the white light-emitting devices. NPB is the hole-transporting blue stabilizing dopant, and BAlq is the electron-transporting blue stabilizing dopant in the blue light-emitting layer.
 
     The device of Example 11 was prepared by following the structure of OLED  300  as shown in FIG.  3 . By sequential deposition of 130 nm undoped NPB hole-transporting layer (HTL), 20 nm NPB HTL doped with 2% rubrene yellow dopant, 15 nm blue light-emitting layer (LEL) comprising TBADN host with 5% OP31 blue dopant (blue dopant formula B-1), and 10% NPB co-dopant, 35 nm Alq electron-transporting layer (ETL), and then 0.5 nm LiF/200 nm aluminum as the cathode. The above sequence completed the deposition of the OLED device. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 EL properties of White OLEDs wherein the blue emitting layer is doped with blue dopant and other dopants NPB or BAlq 
               
             
          
           
               
                   
                 Hole 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 Transport 
               
               
                   
                 sublayer 1 
                 Hole Transport 
                 Blue 
                 Blue 
                 Blue 
                 Blue 
               
               
                   
                 (undoped 
                 sublayer 2 
                 emission 
                 emission 
                 emission 
                 emission 
                 Electron 
                 Lum 
               
               
                 Device 
                 NPB layer 
                 doped with 
                 layer Host 
                 layer 
                 layer 
                 layer 
                 transport 
                 Yield 
                   
                   
                 Drive 
                 Operational 
               
               
                 Number 
                 thickness) 
                 yellow dopant 
                 (TBADN) 
                 dopant 1 
                 dopant 2 
                 dopant 3 
                 layer 
                 (cd/A) 
                 CIEx 
                 CIEy 
                 Voltage 
                 stability 
               
               
                   
               
               
                 11 
                 130 nm 
                 20 nm NPB + 3.5% 
                 15 nm 
                 5% OP31 
                 NPB 
                  0% 
                 25 nm 
                 7.8 
                 0.26 
                 0.37 
                 5.3 
                 132 
               
               
                   
                   
                 rubrene 
                 TBADN 
                   
                 10% 
                   
                 Alq 
               
               
                 12 
                 130 nm 
                 20 nm NPB + 3.5% 
                 15 nm 
                 5% OP31 
                 NPB 
                  1% BAlq 
                 25 nm 
                 8.2 
                 0.31 
                 0.40 
                 5.5 
                 N.A. 
               
               
                   
                   
                 rubrene 
                 TBADN 
                   
                 10% 
                   
                 Alq 
               
               
                 13 
                 130 nm 
                 20 nm NPB + 3.5% 
                 15 nm 
                 5% OP31 
                 NPB 
                  3% BAlq 
                 25 nm 
                 8.3 
                 0.31 
                 0.41 
                 5.5 
                 139 
               
               
                   
                   
                 rubrene 
                 TBADN 
                   
                 10% 
                   
                 Alq 
               
               
                 14 
                 130 nm 
                 20 nm NPB + 3.5% 
                 15 nm 
                 5% OP31 
                 NPB 
                  5% BAlq 
                 25 nm 
                 8.4 
                 0.32 
                 0.41 
                 5.5 
                 N.A. 
               
               
                   
                   
                 rubrene 
                 TBADN 
                   
                 10% 
                   
                 Alq 
               
               
                 15 
                 130 nm 
                 20 nm NPB + 3.5% 
                 15 nm 
                 5% OP31 
                 NPB 
                 10% BAlq 
                 25 nm 
                 8.7 
                 0.33 
                 0.42 
                 5.6 
                 164 
               
               
                   
                   
                 rubrene 
                 TBADN 
                   
                 10% 
                   
                 Alq 
               
               
                   
               
             
          
         
       
     
     Devices of Examples 12 to 15 were prepared following the structure of OLED  300  as shown in FIG.  3 . All the layers were similar to the device in Example 11 except that the 15 nm (TBADN+5% OP31) blue emitting layer was co-doped with 10% NPB and varying amounts of BAlq concentrations varying from 1% to 10%. It was found that the devices of Example 12 to 15 show increased luminance efficiency and increased operational stability of the devices. The color of the white OLED was not significantly affected. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 R, G, B characteristics of the White OLEDs after the color filter wherein the blue 
               
               
                 emitting layer is doped with blue dopants and other dopants NPB or Balq 
               
             
          
           
               
                   
                   
                   
                   
                 Predicted power 
               
               
                   
                   
                   
                   
                 (Watts) (Panel 
               
               
                   
                   
                   
                   
                 luminance 80 cd/m2 
               
               
                   
                   
                   
                   
                 for 2.2″ display, 
               
               
                   
                   
                   
                   
                 0.44 polarizing 
               
               
                 Device 
                 Red color after color filter 
                 Green color after color filter 
                 Blue color after color filter 
                 transmission and 
               
             
          
           
               
                 Number 
                 Lum Yield (cd/A) 
                 CIEx 
                 CIEy 
                 Lum Yield (cd/A) 
                 CIEx 
                 CIEy 
                 Lum Yield (cd/A) 
                 CIEx 
                 CIEy 
                 0.42 aperature ratio) 
               
               
                   
               
               
                 11 
                 1.20 
                 0.57 
                 0.36 
                 5.16 
                 0.25 
                 0.54 
                 1.96 
                 0.11 
                 0.22 
                 1.95 
               
               
                 12 
                 1.55 
                 0.59 
                 0.36 
                 5.31 
                 0.29 
                 0.55 
                 1.63 
                 0.12 
                 0.23 
                 1.78 
               
               
                 13 
                 1.55 
                 0.59 
                 0.36 
                 5.43 
                 0.29 
                 0.55 
                 1.68 
                 0.12 
                 0.24 
                 1.78 
               
               
                 14 
                 1.61 
                 0.59 
                 0.36 
                 5.60 
                 0.29 
                 0.55 
                 1.71 
                 0.11 
                 0.24 
                 1.75 
               
               
                 15 
                 1.75 
                 0.60 
                 0.36 
                 5.72 
                 0.31 
                 0.55 
                 1.63 
                 0.12 
                 0.25 
                 1.75 
               
               
                   
               
             
          
         
       
     
     The luminance and the color data of the devices in Examples 11 to 15 given in Table 4 were used to predict the R, G, B color efficiency and the color when white light is passed through the R, G, B color filters. The power consumption on a 2.2″ diagonal distance display was predicted at starting luminance of 80 cd/m2. It was found that the power consumption decreased from 1.95 watts to 1.75 watts. The stability of the device was simultaneously improved. This shows that the improvement in the luminance efficiency, reduction in power consumption, and improved lifetime was achieved by using NPB and BAlq co-dopants in the blue emitting layer along with a blue emitting dopant. Thus, white OLED devices can be prepared by following the different structures of this invention to have high performance and high operational stability. 
     Devices of Examples 16 to 21 were prepared following the structure of OLED  300  as shown in FIG.  3 . Device Example 16 is a control. It has a glass substrate, 85 nm ITO anode, and 0.5 nm CF X  hole injection layer. Thereafter, a 130 nm NPB layer was deposited as the hole transport layer followed by 20 nm NPB layer doped with 2% DBzR. Then was deposited a 20 nm blue EML consisting of a TBADN host and 2.5% blue dopant B1, followed by 25 nm Alq and cathode layers. This completed the device fabrication. The device was then encapsulated to protect it from moisture and environment. This device emitted white light. Device Examples 17 to 21 were prepared following the same procedure as control device Example 16, except that the blue emission layer had additional combinations of dopants as shown in Table 6. All of the layers for Devices 16 to 21 are the same except the blue emission layer. Device Examples 17 and 18 have a blue emission layer containing 5% and 10% Alq dopants along with the host and blue dopant B1. Device Example 19 has the blue emission layer containing 10% NPB dopant along with the host and blue dopant B 1. Device Examples 20 and 21 have the emission layers, which contain both Alq and NPB co-dopants along with the host and blue dopant B1. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 EL properties of White OLEDs wherein the blue emitting layer is doped with blue dopant and other dopants NPB or/and Alq 
               
             
          
           
               
                   
                   
                 Hole 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Oper- 
               
               
                   
                 Hole 
                 Transport 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 ational 
               
               
                   
                 Transport 
                 sublayer 2 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 stability 
               
               
                   
                 sublayer 1 
                 doped 
                 Blue 
                   
                 Blue 
                 Blue 
                   
                   
                   
                   
                   
                 (Half-life 
               
               
                   
                 (undoped 
                 with 
                 emission 
                 Blue emission 
                 emission 
                 emission 
                 Electron 
                 Lum 
                   
                   
                   
                 at 70 
               
               
                 Device 
                 NPB layer 
                 yellow 
                 layer Host 
                 layer dopant 1 
                 layer 
                 layer 
                 transport 
                 Yield 
                   
                   
                 Drive 
                 degree C.) 
               
               
                 Number 
                 thickness) 
                 dopant 
                 (TBADN) 
                 (Dopant B1) 
                 dopant 2 
                 dopant 3 
                 layer 
                 (cd/A) 
                 CIEx 
                 CIEy 
                 Voltage 
                 (hours) 
               
               
                   
               
               
                 16 
                 130 nm 
                 20 nm NPB + 
                 20 nm 
                 2.5% Dopant B1 
                  0% 
                  0% 
                 25 nm 
                 5.5 
                 0.33 
                 0.38 
                 7.5 
                 400 
               
               
                   
                   
                 2% DBzR 
                 TBADN 
                   
                   
                   
                 Alq 
               
               
                 17 
                 130 nm 
                 20 nm NPB + 
                 20 nm 
                 2.5% Dopant B1 
                  0% 
                  5% Alq 
                 25 nm 
                 5.5 
                 0.44 
                 0.47 
                 8.0 
                 700 
               
               
                   
                   
                 2% DBzR 
                 TBADN 
                   
                   
                   
                 Alq 
               
               
                 18 
                 130 nm 
                 20 nm NPB + 
                 20 nm 
                 2.5% Dopant B1 
                  0% 
                 10% Alq 
                 25 nm 
                 5.9 
                 0.46 
                 0.48 
                 7.7 
                 750 
               
               
                   
                   
                 2% DBzR 
                 TBADN 
                   
                   
                   
                 Alq 
               
               
                 19 
                 130 nm 
                 20 nm NPB + 
                 20 nm 
                 2.5% Dopant B1 
                 10% 
                  0% 
                 25 nm 
                 5.1 
                 0.29 
                 0.33 
                 7.8 
                 350 
               
               
                   
                   
                 2% DBzR 
                 TBADN 
                   
                 NPB 
                   
                 Alq 
               
               
                 20 
                 130 nm 
                 20 nm NPB + 
                 20 nm 
                 2.5% Dopant B1 
                 NPB 
                  5% Alq 
                 25 nm 
                 6.6 
                 0.38 
                 0.47 
                 7.8 
                 950 
               
               
                   
                   
                 2% DBzR 
                 TBADN 
                   
                 10% 
                   
                 Alq 
               
               
                 21 
                 130 nm 
                 20 nm NPB + 
                 20 nm 
                 2.5% Dopant B1 
                 NPB 
                 10% Alq 
                 25 nm 
                 6.8 
                 0.39 
                 0.48 
                 7.6 
                 1100  
               
               
                   
                   
                 2% DBzR 
                 TBADN 
                   
                 10% 
                   
                 Alq 
               
               
                   
               
             
          
         
       
     
     The luminance properties of the devices of Examples 16 to 21 are given in Table 6. The fade stability of these devices was measured at 70 degree centigrade temperature and at a constant average alternating (50% duty cycle) current density of 20 mA/cm 2 . The fade stability of these devices is also included in Table 6. The data in Table 6 shows that the device Examples 20 and 21 have the highest luminance efficiency and the highest stability. This luminance level and the stability could not be obtained if either of the dopant Alq or NPB was co-doped along with the host and blue dopant B1 such as Example 17,18, or 19. Thus, the highest performing devices were prepared with the emission layer containing both the dopants and having hole transporting properties such as NPB and the dopant with electron transporting properties such as Alq provided in the blue emission layer containing the host and the blue dopant. 
     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. For example, multiple dopants can be used in any of the hole-transporting, electron-transporting, or light-emitting layers. 
     PARTS LIST 
     
         
           100  OLED with a simple structure 
           110  substrate 
           120  anode 
           140  light-emitting layer 
           170  cathode 
           200  OLED with a multilayer structure 
           210  light-transmissive substrate 
           220  light-transmissive anode 
           230  hole-injecting layer (HIL) 
           240  hole-transporting layer (HTL) 
           250  light-emitting layer (LEL) 
           260  electron-transporting layer (ETL) 
           270  cathode 
           300  OLED 
           310  substrate 
           320  light-transmissive anode 
           330  hole-injecting layer 
           340  hole-transporting layer 
           350  light-emitting layer 
           360  electron-transporting layer 
           370  cathode 
           400  OLED 
           410  substrate 
           420  anode 
           430  hole-injecting layer 
           441  hole-transporting sublayer 
           442  hole-transporting sublayer 
           450  light-emitting layer 
           460  electron-transporting layer 
           470  cathode 
           500  OLED 
           510  substrate 
           520  anode 
           530  hole-injecting layer 
           540  hole-transporting layer 
           550  blue light-emitting layer 
           561  electron-transport sublayer 
           562  electron-transport sublayer 
           570  cathode 
           600  OLED 
           610  substrate 
           620  anode 
           630  hole-injecting layer 
           640  hole-transporting layer 
           650  blue light-emitting layer 
           661  electron-transporting sublayer 
           662  electron-transporting sublayer 
           670  cathode 
           700  OLED 
           710  substrate 
           720  anode 
           730  hole-injecting layer 
           741  hole-transporting layer sublayer 
           742  hole-transporting layer sublayer 
           750  blue light-emitting layer 
           761  electron-transport sublayer 
           762  electron-transport sublayer 
           770  cathode 
           800  OLED 
           810  substrate 
           820  anode 
           830  hole-injecting layer 
           840  hole-transporting layer 
           850  light-emitting layer 
           861  electron-transport sublayer 
           862  electron-transport sublayer 
           870  cathode 
           900  OLED 
           910  substrate 
           920  anode 
           930  hole-injecting layer 
           941  hole-transport sublayer 
           942  hole-transport sublayer 
           950  blue light-emitting layer 
           961  electron-transport sublayer 
           962  electron-transport sublayer 
           970  cathode 
           1000  OLED 
           1010  substrate 
           1020  anode 
           1030  hole-injecting layer 
           1040  hole-transporting layer 
           1050  blue light-emitting layer 
           1061  electron-transporting sublayer 
           1062  electron-transporting sublayer 
           1063  electron-transporting sublayer 
           1070  cathode 
           1100  OLED 
           1110  substrate 
           1120  anode 
           1130  hole-injecting layer 
           1140  hole-transporting layer 
           1150  blue light-emitting layer 
           1161  electron-transport sublayer 
           1162  electron-transport sublayer 
           1163  electron-transport sublayer 
           1170  cathode 
           1200  OLED 
           1210  substrate 
           1220  anode 
           1230  hole-injecting layer 
           1241  hole-transporting layer sublayer 
           1242  hole-transporting layer sublayer 
           1250  blue light-emitting layer 
           1261  electron-transport sublayer  1   
           1262  electron-transport sublayer  2   
           1263  electron-transport sublayer  3