Abstract:
Multilayer polymer light-emitting diodes (PLEDs) are demonstrated using semiconducting polymers blended with organometallic emitters as the emissive layer and one or both of an electron transport layer and a hole transparent layer on the appropriate electron injection and hole injection sides of the emissive layer. The transport layers reduce energy potential gaps between the hole injection electrode and the emissive polymer and between the electron injection electrode and the emissive polymer. A solvent-processing based procedure for preparing these devices is also disclosed It uses nonpolar solvent-based solutions of emissive polymers to form the emissive layer and polar solvent-based solutions to form the transport layers to minimize etching and other undesirable interactions as the multiple layers are being laid down. Illumination quality white light can be obtained with stable Commission Internationale de l&#39;Eclairage coordinates, stable color temperatures, and stable color rendering indices, all close to those of “pure” white light. These multilayer white light-emitting PLEDs are useful as backlights for liquid crystal displays and for solid state lighting applications.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention is in the field of organic polymer-based light-emitting diodes. More particularly this invention relates to multilayer polymer light-emitting diodes (PLEDs) that, in certain embodiments, emit white light that is useful for solid state lighting applications. This invention also pertains to methods for preparing these diodes.  
       BACKGROUND OF THE INVENTION  
       [0002]     Polymer light-emitting diodes (PLEDs) which employ semiconducting polymers as emitting layers have been demonstrated. A wide range of colors of emission can be achieved by varying the materials present in the emitting layers. Blends of emitting polymers alone and together with organometallic emitters can be used to achieve additional color shades of emitted light including white light.  
         [0003]     LEDs that emit white light are of interest and potential importance for use as back lights in active matrix displays (with color filters) and because they can be used for solid state lighting [A. J. Heeger, Solid State Commu., 1998. 107,673 &amp; Rev. Modem Phys., 2001,73, 681; B. W. D&#39;Andrade, S. R. Forrest, Adv. Mater., 2004, 16, 1585; R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. J. Ögdlund, and. W. R. Salaneck, Nature, 1999, 397, 121]. In such applications, the fabrication of large-area devices and the use of low-cost manufacturing technology will be the major issues. The fabrication of PLEDs by processing the active materials from solution (e.g. by use of ink-jet printing or other printing technologies) promises to be less expensive than that of OLEDs (organic light-emitting diodes based on small molecules) where deposition of the active layers requires the use of vacuum technology [B. W. D&#39;Andrade, S. R. Forrest, Adv. Mater., 2004, 16, 1585] Several approaches have been used to generate white light and light of other colors from OLEDs and PLEDs [J. Kido, H, Shionoya, K, Nagai, Appl. Phys. Lett., 1995, 67, 2281-2283; C. Zhang, A. J. Heeger, J. Appl. Phys., 1998, 84, 1579; Z. Shen, P. E. Burrows, V. Bulvić, S. R. Forrest, M. E. Thompson, Science, 1997, 276, 2009; Y. Hamada, T. Sano, H. Fujii, Y. Nishio, Jpn. J. Appl. Phys., 1996, 35, L1339; Y. Z. Wang, R. G. Sun, F. Meghdadi, G. Leising, A. J. Epstein, Appl. Phys. Lett., 1999, 74, 3613; M. Strukelj, R. H. Jordan, A. Dodabalapur, A.; J. Am. Chem. Soc., 1996, 118, 1213; B. W. D&#39;Andrade, R. J. Holmes, and S. R. Forrest, Adv. Mater., 2004, 16, 624]. In the approaches in the articles listed above, the efficiency was modest and the lifetime was limited by that of the blue emitters [J. Kido, H, Shionoya, K, Nagai, Appl. Phys. Lett., 1995, 67, 2281; Y. Hamada, T. Sano, H. Fujii, Y. Nishio, Jpn. J. Appl. Phys., 1996, 35, L1339; Y. Z. Wang, R. G. Sun, F. Meghdadi, G. Leising, A. J. Epstein, Appl. Phys. Lett., 1999, 74, 3613; M. Strukelj, R. H. Jordan, A. Dodabalapur, A.; J. Am. Chem. Soc., 1996, 118, 1213; U. Scherf, E. J. W. List, Adv. Mater. 2002, 14, 477; S. Setayesh, D. Marsitzky, K. Müllen, Macromolecules, 2000, 33, 2016; X. Gong, P. Iyer, D. Moses, G. C. Bazan, A. J. Heeger, Adv. Func. Mater., 2003, 13, 325].  
         [0004]     PLEDs fabricated with semiconducting polymers doped with organometallic emitters offer the additional promise of “plastic” electronics. Representative examples of such devices are described in U.S. application Ser. No. 10/680,084 filed Oct. 3, 2003. The emissive layers of PLEDs can be fabricated by casting polymers and blends from solution, thereby enabling relatively simple and low cost manufacturing processes [G. D. Müller, A. Falcou, N. Reckefuss, M. Roijhn, V. Wiederhim, P. Rudati, H. Frohne, O. Nuyken, H. Becker, K. Meerholz, Nature, 2003, 421, 829].  
         [0005]     The fabrication techniques most favored for producing multilayer PLEDs include the use of sputtering and various vapor deposition methods to lay down inorganic layers such as high work function metal-metal oxide contacts (electrodes) and protective metallic overlayers. Solvent deposition methods such as spin-casting or printing successive layers from solution can be used to lay down organic polymer emissive layers as well as other layers in the devices. When multiple organic layers are present there can be problems with successive layers interacting. The solvent of a later layer can dissolve or disfigure (etch) a prior layer. It is often desirable to have each layer be smooth and coherent, thus this interaction can be destructive.  
         [0006]     Light may be characterized by three quantities: the CIE (Commission Internationale de l&#39;Eclairage) coordinates, the color temperature (CT) and the color rendering index (CRI). “Pure” white light has CIE coordinates of (0.333, 0.333), and is obtained by balancing the emission of the colors employed. For illumination applications, the CT needs to be equivalent to that of a blackbody source between 3000° K. and 7500° K. Average daylight has CT=6500° K., while a fluorescent lamp (warm white) has CT=3000° K. [R. W. G. Hunt, Measuring Color, 2nd Ed. Ellis Horwood, 1991]. The CRI is a numerical measure of how “true” colors look when viewed with the light source. CRI values range from 0 to 100, with 100 representing true color reproduction. Fluorescent lamps have CRI ratings between 60 and 99. Though a CIU value of at least 70 may be acceptable for certain applications, a preferred white light source will have a CRI of about 80 or higher. The demonstration of PLEDs which emit illumination quality white light with high brightness, high efficiency, suitable CT, high CRI and stable CIE coordinates is of importance to the future of solid state lighting.  
       STATEMENT OF THE INVENTION  
       [0007]     We have now discovered an improvement in multilayer PLEDs that enhances their efficiency and facilitates their fabrication.  
         [0008]     We have found that the addition of an electron transport layer and/or a hole transport layer adjacent to the emissive layer of a PLED improves the PLED&#39;s performance. We have further found that these additional layers can be incorporated into PLEDs by solvent processing techniques if the materials employed in these additional layers are differentially soluble in solvents which differ in polarity from the solvents used to dissolve and solvent process the emissive layer.  
         [0009]     Thus, this invention can provide multilayer polymer light-emitting diodes (PLEDs) that in certain embodiment emit white light and are useful for solid state lighting applications. More specifically, the present invention can provide multilayer white PLEDs comprising semiconducting polymers blended with organometallic emitters as a relatively nonpolar solvent-soluble emissive layer, and relatively polar solvent-soluble organic materials (polymers or small molecules) as a hole injection/transport layer (HIL/HTL) and/or as an electron injection/transport layer (EIL/ETL); all layers preferably being cast from the corresponding solutions. The white emission of these preferred materials of the present invention can be used for backlights in liquid crystal displays (LCDs) and for solid state lighting applications. The white light is emitted from the polymer blend in a single emissive layer. The strategy developed in this invention enables the fabrication of multilayer white emitting PLEDs by casting the emissive polymer blends, HIL/HTL, and EIL/ETL from the corresponding solutions. This invention also enables the relatively simple fabrication of multilayer PLEDs which emit illumination quality light in all colors from blue to red and including white light. The methodology presented in this invention enables the relatively simple fabrication of multilayer PLEDs which emit illumination quality white light with high brightness, high efficiency, suitable color temperature, high color rendering index, and stable CIE (Commission Internationale de l&#39;Eclairage) coordinates. The method for fabrication of multilayer PLEDs presented in this invention can be used for large-area multilayer displays and other large-area multilayer opto-electronic devices fabricated by casting the various layers from solution.  
         [0010]     The devices of the present invention employ an emissive layer and at least one of a hole injection/transport layer (HIL/HTL) and an electron injection/transport layer (EIL/ETL) adjacent to the emissive layer. The benefits of the transport layers can be observed in devices which employ a single layer hole injection anode, in which case the hole transport layer may at times be referred to as a “hole injection/transport layer” or “HIL/HTL.” and also in devices which employ a bilayer anode with the second layer of the bilayer itself being a “hole injection layer.” In this second case, to avoid confusion, the transport layer is referred to simply as a “hole transport layer” or “HTL” and the art-known “hole injection layer” or “HIL” retains its usual name. The devices of the present invention employ relatively nonpolar solvent-soluble semiconducting polymers blended with organometallic emitters as their emissive layers and polar solvent (for example water and/or lower alkanol)-soluble polymers and small molecules as both HIL/HTL and EIL/ETL layers. The devices of the present invention can employ two or three luminescent emitters (represented in the Examples as Type I and Type II devices), in a single emissive region, rather than red, green and blue emission in different regions that appear white when averaged by the observer. More than three emitters can be used as well. The luminescent emitters can emit white light via fluorescence (from singlet states) or a combination of fluorescence (from singlet states) and phosphorescence (from triplet states). White light can be achieved from two or three luminescent emitters blended into a single material that forms a single emissive thin film layer through the combined emission from the host polymer (such as a conjugated polymer) and from the additional emitters such as green and red-emitting components blended into the host polymer. A single emissive layer comprising two or three or more emissive centers allows the fabrication of emitting PLEDs and especially white light-emitting PLEDs by solution processing.  
         [0011]     The HIL/HTL and/or EIL/ETL layers provide a means to balance the electron and hole currents and increase the efficiency of the devices. Importantly, using polar solvent-soluble materials as both the HTL and ETL and less polar solvent-soluble materials as the emissive layer allows the fabrication of multilayer PLEDs that emit light with different colors within the visible spectrum, from blue to red and especially white by processing the various layers from solution. The strategy of the present invention enables the relatively simple fabrication of bright and efficient multilayer PLEDs, including white-emitting PLEDs that are characterized by a high color rendering index, suitable color temperature and desired CIE coordinates. Moreover, the color rendering index, color temperature and CIE coordinates from these multilayer electrophosphorescent PLEDs are insensitive to brightness, insensitive to the applied voltage and insensitive to the current density. Furthermore, the method for fabrication of multilayer PLEDs presented in this invention can be used for development of large-area displays comprising multilayer light-emitting diodes and other large-area multilayer opto-electronic devices processed from solution by printing technology.  
         [0012]     One object of the present invention is to provide a method to produce multilayer PLEDs and especially white light-emitting PLEDs that exhibit high luminous efficiency, high external quantum efficiency and brightness adequate for applications in solid state lighting and as backlights for liquid crystal displays (LCDs).  
         [0013]     Another object of the present invention is to produce high-performance multilayer PLEDs by using polar solvent-soluble polymers and small molecules as a hole injection/transporting layer and/or as an electron injection/transporting layer.  
         [0014]     A third object of the present invention is to provide a technology which can be used for development of multilayer displays comprising light-emitting diodes and other multilayer opto-electronic devices processed from solution by printing technology.  
         [0015]     Yet another object of the present invention is to produce multilayer white PLEDs that exhibit white light with high color rendering index, suitable color temperature and desired CIE coordinates.  
         [0016]     A further object of the present invention is to utilize polar solvent-soluble polymers and small molecules as hole injection/transport layers and/or as electron injection/transport layers in PLEDs.  
         [0017]     An additional object of the present invention is to produce multilayer white emitting PLEDs with stable color rendering index, stable color temperature and stable CIE coordinate all of which are insensitive to brightness, applied voltage and current density.  
         [0018]     Yet another object of the present invention is to produce multilayer white light-emitting PLEDs that produce white emissions having CIE x, y-chromaticity coordinates close to the CIE coordinates of pure white light (0.333, 0.333).  
         [0019]     An additional object of the present invention is to produce multilayer white light-emitting PLEDs that produce white emissions having color temperature close to the 6400° K. value characteristic of average daylight or close to the 4500° K. value characteristic of sunlight at solar altitude 20°.  
         [0020]     A further object of the present invention is to produce multilayer white light-emitting PLEDs that produce white emission having color rendering indices in excess of 80.  
         [0021]     Yet another object of the present invention is to produce multilayer white light-emitting PLEDs that produce white emission having color rendering indices in excess of 90.  
       DETAILED DESCRIPTION OF THE INVENTION 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     This invention will be further described with reference being made to the drawings in which:  
         [0023]      FIG. 1  shows the molecular structures of representative materials employed in the fabrication of devices of this invention including: poly(9,9-dioctylfluorene) (FFO); poly(9,9-dioctylfluorene) end-capped with 5-biphenyl-1,3,4-oxadiazole (PFO-ETM) (the electron-transport-moiety); poly(9,9-dioctylfluorene-co-fluorenone) with 1% fluorenone (PFO-F(1%)); tris (2,5-bis-2′-(9′,9′-dihexylfluorene) pyridine) iridium (III), Ir(HFP) 3 ; poly(vinylcarbazole) sulfonic lithium (PVK-S0 3 Li); and 4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazole-2-yl)-biphenyl-4′-yl sulfonic sodium (t-Bu-PBD-S0 3 Na);  
         [0024]      FIG. 2  shows several representative device configurations in schematic cross-section;  
         [0025]      FIG. 3  shows the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of PFO-ETM, PVK-S03Li, t-Bu-PBD-S03Na and the work functions of PEDOT:PSS and Ba;  
         [0026]      FIG. 4  shows the HOMO and LUMO energy levels of PFO-ETM, PFO-F (1%) and Ir (HFP) 3 ;  
         [0027]      FIG. 5  shows the electroluminescent spectra of the devices made with pure PFO-ETM, PFO-F (1%) and Ir(HFP) 3  doped into PFO-ETM (Ir(HFP) 3 :PFO-ETM=1 wt.-%), as emissive layers;  
         [0028]      FIG. 6  shows the electroluminescent spectra obtained from Type I electrophosphorescent PLEDs at different applied voltages;  
         [0029]      FIG. 7  shows the electroluminescent spectra obtained from Type II electrophosphorescent PLEDs at different applied voltages;  
         [0030]      FIG. 8  shows the CIE (193 1) chromaticity diagram, with coordinates corresponding to the emission from Type I devices (□□□) and Type II devices (∘∘∘) biased at different applied voltages. Also shown are the equi-energy point (E) for pure white light (0.333, 0.333) (▪) and the coordinates corresponding to color temperatures of 4000° K.(▴), 5000° K.(▾) and 6500° K.(●). The dotted line indicates different color temperatures; the dotted oval indicates the approximate area where the human eye perceives the color as white;  
         [0031]      FIG. 9  shows the luminance versus applied voltage and current density versus applied voltage for Type I devices;  
         [0032]      FIG. 10  shows the luminance versus applied voltage and current density versus applied voltage for Type II devices;  
         [0033]      FIG. 11  shows the forward viewing external luminous efficiency (LE ext ) versus current density, J (mA/cm 2 ) for devices with poly(3,4-ethylenedioxythophene):styrene sulfonic acid PEDOT:PSS as the HIL/HTL and t-Bu-PBD-SO 3 Na as the ETL, and for devices with PVK-SO 3 Li as the HTL and t-Bu-PBD-SO 3 Na as the ETL, respectively; and the forward viewing external power efficiency (PE ext , 1 m/W) versus J (mA/cm 2 ) for Type I devices;  
         [0034]      FIG. 12  shows the forward viewing external luminous efficiency (LE ext ) versus current density, J (mA/cm 2 ) for the devices with PEDOT:PSS as the HIL/HTL and t-Bu-PBD-SO 3 Na as the EIL/ETL, and for devices with PVK-SO 3 Li as the HIL/HTL and t-Bu-PBD-SO 3 Na as the EIL/ETL, respectively; and the forward viewing external power efficiency (PE ext , 1 m/W) versus J (mA/cm 2 ) for Type II devices;  
         [0035]      FIG. 13  shows the total external luminous efficiency (LE total ) and the total external power efficiency (PE total , 1 m/W) versus current density, J (mA/cm 2 ) for Type I devices with PEDOT:PSS as the HIL, PVK-SO 3  as the HTL and t-Bu-PBD-SO 3 Na as the EIL/ETL; and  
         [0036]      FIG. 14  shows the total external luminous efficiency (LE total ) and the total external power efficiency (PE total , 1 m/W) versus current density, J (mA/cm 2 ) for Type II devices with PEDOT:PSS as the HIL and PVK-SO 3 Li as the HTL and t-Bu-PBD-SO 3 Na as the EIL/ETL.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0000]     Nomenclature and Abbreviations  
         [0037]     In this description of this invention a variety of chemical compounds will be referred to. Some of the compounds being named are depicted in  FIG. 1 .  
         [0038]     In addition, the following abbreviations will be employed.  
         [0039]     CIE Commission Internationale de l&#39;Eclairage  
         [0040]     CT color-rendering index  
         [0041]     EIL/ETL electron injection-transport layer  
         [0042]     ETM electron-transport layer  
         [0043]     HIL/HTL hole injection-transport layer  
         [0044]     HIL hole injection layer  
         [0045]     HTL hole transport later  
         [0046]     HFP 9,9-dihexylfluorene pyridine  
         [0047]     HOMO highest occupied molecular orbital  
         [0048]     ITO indium tin oxide  
         [0049]     Ir(HFP) 3  tris 2,5-bis-2′(9′,9′ dihexylfluorene) pyridine iridium (III)  
         [0050]     LCD liquid crystal display  
         [0051]     LUMO lowest unoccupied molecular orbital  
         [0052]     MEH-PPV poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene  
         [0053]     MBL-PPV poly[5-methoxy-2-(4-sulfobutoxy)-1,4-phenylenevinylene  
         [0054]     OLED organic-light-emitting diode  
         [0055]     PLED polymer light emitting diode  
         [0056]     PPV poly(phenylenevinylene)  
         [0057]     PFO poly(9,9-dioctylfluorene)  
         [0058]     PFO-ETM poly(9,9-dioctylfluorene) endcapped with electro-transport-moiety for example, poly(9,9-dioctylfluorene) endcapped with 5-biphenyl-1,3,4 oxadiazole  
         [0059]     PFO-F poly(9,9-dioctyfluorene)-fluorenone  
         [0060]     t-Bu PBD-SO 3 Na 4-(5-(4-tert-butylphenyl)-1,3,4-oxasiazole-2-yl)biphenyl-4′-yl sulfonic sodium  
         [0061]     PVK poly(vinylcarbazole)  
         [0062]     PVK-SO 3 Na poly(vinylcarbazole) sulfonic sodium  
         [0063]     PVK-SO 3 Li poly(vinylcarbazole) sulfonic lithium  
         [0064]     PEDOT: SSS poly(3,4-ethylenedioxythophene): styrene sulfonic acid  
         [0065]     poly(BT[PD-Si-PFCB) poly(bis)tetraphenyldiamino)biphenyl-perfluorocyclobutane  
         [0066]     THF tetrahydrofuran  
         [0067]     DMSO dimethyl sulfoxide  
         [0068]     DMF dimethylformamide  
         [0000]     Definitions  
         [0069]     In this description of this invention and in the claims, at times reference is made to solvents as being “polar” or “nonpolar” and reference is made to a material being “differentially soluble” or having “differential solubility” in “polar” or “nonpolar” solvents.  
         [0070]     Solvent polarity is defined herein in accord with the teachings of Christian Reichart, Solvents and Solvent Effects in Organic Chemistry, VCH Publishers 2nd ed., 1988. Reichart provides values for relative polarity ranging from a high of 1.000 for water to a low of 0.006 for cyclohexane. Using these relative polarity values, a “polar solvent” is defined to be a solvent having a relative polarity of from 0.400 to 1.000. Such solvents include, for example, water, glycerin, ethylene glycol, methanol, diethylene glycol, ethanol; the propanols, acetonitrile, dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) and mixtures thereof. Water, lower alkanols (C1-C3) and mixtures thereof are preferred polar solvents.  
         [0071]     A “nonpolar solvent” is defined to be a solvent having a relative polarity of from 0.006 to about 0.300. Such solvents include, for example, cyclohexane, pentane, hexane, heptane; carbon tetrachloride, xylene toluene, benzene, diethyl ether, methyl-tert-butyl-ether (MTBE), dioxane, tetrahydrofuran (THF), ethyl acetate, glyme, diglyme and chloroform. C6-C8 hydrocarbons and especially benzene and toluene are preferred nonpolar solvents.  
         [0072]     In the processing of this invention various materials are said to be “differentially soluble” in a “polar” solvent or a “nonpolar” solvent. This means, at its most general, that the material is more soluble in one family of solvents than the other. Preferably “differential solubility” implies in that a material is at least 150% and more preferably at least 200% and most preferably at least 300% as soluble in one family as in the other.  
         [0000]     Device Configurations  
         [0073]     As shown as “A” in  FIG. 2 , PLEDs of the prior art have been made up of a semiconducting luminescent emitter layer  115  contacted with a low work function electron injection electrode  113  and a high work function hole injection electrode  118 . As shown in “A,” PLEDs are supported on a substrate  119  which provides mechanical strength and commonly contain a passivation layer  112  to mechanically and chemically protect the electrode on the side away from the support. The positions of support  119  and passivation layer  112 , relative to the hole injection electrode  118  and electron injection electrode  113  are most commonly as depicted in  FIG. 2 . One could reverse these positions if desired and put the electron injection electrode on the support without departing from the spirit of the invention. Similarly, in describing the invention and its advantages, at times references to these “support  119 ” and “protective outer layer  112 ” layers may be omitted for simplicity.  
         [0074]     As also shown in  FIG. 2  at “B,” the prior art further contemplated PLEDs which included a bilayer anode made up of an organic hole injection layer  117  located between the hole injection electrode  118  and the emissive layer  115 . This bilayer anode hole injection configuration is known [D. Braun and A. Heeger, Appln. Phys. Lett., 119, 58, 1982].  
         [0075]      FIG. 2 , at  10 ,  11  and  12 , depicts three configurations for devices of this invention in which one or two additional “transport” layers  114  and  116  are present. When two “transport” layers are present they are on opposite sides of the emissive layer  115  (see  10 ). When one “transport” layer is present it can be on the electron injection side of layer  115  as EIL/ETL  114  (see  11 ) or on the hole injection side of layer  115  as HTL  116  (see  12 ). We have obtained best results when both of EIL/ETL and HTL are present.  
         [0076]     It should be kept in mind that the bilayer anode hole injection electrode of prior art B in  FIG. 2  includes an organic hole injection layer  117 . In embodiment 13, hole injection layer is combined with the hole transport layer as a single HIL/HTL organic layer  120 .  
         [0077]     These individual layers will next be described.  
         [0000]     The Emissive Layer ( 115 )  
         [0078]     The emissive layer  115  shown in  FIG. 2  comprises a blend (mixture) of one or more emitting polymers (or copolymers) with one or more organometallic emitters. Preferred emitting polymers are generally conjugated. Preferred examples include devices made from PFO or poly(9,9-dioctylfluorene) end-capped with 5-biphenyl-1,3,4-oxadiazol (PFO-ETM) blended with tris (2,5-bis-2′-(9′,9′-dihexylfluorene)pyridine) iridium (III), (Ir(HFP) 3 ) and devices made from blends of PFO-ETM with poly(9,9-dioctylfluorene-co-fluorenone) with 1% fluorenone (PFO-F(1%)) and Ir(HFP) 3 .  FIG. 1  shows the molecular structures of PFO-ETM, PFO-F(1%) and Ir(HFP) 3 . The synthesis of PFO-ETM has been reported in the scientific literature (X. Gong, W. L. Ma, J. C. Ostrowski, K. Bechgaad, G. C. Bazan, D. Moses, A. J. Heeger, S. Xiao, Adv. Func. Mater., 2004, 14,393]. Other emitting polymers and especially blue-emitting polymers can also be used in the practice of the invention. The synthesis of Ir(HFP) 3  has been reported in the scientific literature [J. C. Ostrowski, M. R. Robinson, A. J. Heeger and G. C. Bazan, Chem. Commun., 2002, 7, 784]. The synthesis of PFO-F(1%) was also reported [X. Gong, D. Moses and A. J. Heeger, Synth. Met. 2004, 141, 17]. Ir(HFP) 3  is representative of the useful organometallic emitters which are complexes and compounds having Ir, Pr, Os, Ru or Au or the like as a center atom.  
         [0079]     High-performance PLEDs based on PFO-ETM as host and organometallic emitters as guests have been previously demonstrated. [X. Gong, W. L. Ma, J. C. Ostrowski, K. Bechgaad, G. C. Bazan, D. Moses, A. J. Heeger, S. Xiao, Adv. Func. Mater., 2004, 14, 393; X. Gong, J. C. Ostrowski, D. Moses, G. C. Bazan, A. J. Heeger, M. S. Liu, A. K-Y. Jen, Adv. Mat. 2003,15,45; X. Gong, J. C. Ostrowski, D. Moses, G. C. Bazan, A. J. Heeger, J. Poly. Sci. Poly. Phys., 2003, 41, 2691].  
         [0080]     The polymer materials used in these emissive layers commonly show differential solubility in nonpolar solvents, such as hydrocarbon solvents.  
         [0081]     The materials described herein as preferred are preferred in settings where white light emission is desired. The polymers such as PFO-ETM themselves are blue-emitting materials. The organometallic emitter and the other host polymers can produce emissions which result in a white overall emission.  
         [0000]     The Electron Injection/Transport Layer ( 114 )  
         [0082]     The electron injection/transport layer (EIL/ETL), typically 20 to 30 nm thick, is cast from solution onto the top surface of emissive layer  115  as shown in  FIG. 2 . The electron injection/transport layer is fabricated from a semiconducting organic polymer material with a relatively large electron affinity; i.e. with a lowest unoccupied molecular orbital (LUMO) close in energy to that of the bottom of the π*-band of the luminescent polymer in the emissive layer, for example within about 1 eV. Preferably the EIL/ETL is fabricated from a material having a LUMO closer to the LUMO of the emissive layer than the work function of the low work function electron injection electrode. Examples include t-Bu-PBD SO 3 Na [T. J. Boyd, R. R. Schrock, Macromolecules, 1999, 32, 6608]. This layer is cast from a polar solvent-based solution such as an aqueous and/or lower alkanol solution.  
         [0000]     The Hole Transport Layer ( 116 )  
         [0083]     The hole transport layer (HTL), typically 20 to 30 nm thick, is cast from solution onto the top surface of hole injection layer  117 . If the hole injection electrode is a single layer anode  118  which does not have a layer  117 , then the layer  116  will be deposited directly in electrode  118  as an HIL/HTL as will explained in paragraph 0054. The hole injection/transport layer is fabricated from a semiconducting organic polymer material with a relatively small ionization potential; i.e., with highest occupied molecular orbital (HOMO) close in energy to that of the top of the n-band of the luminescent polymer in the emissive layer, for example within about 1 eV. Preferably the HTL is fabricated from a material having a HOMO closer to the HOMO of the emissive layer than the work function of the hole injection electrode. Examples include PVK-S03Li [S. Wang, Z. Zeng, S. Yang, L.-T. Weng, P. C. L. Wong, K. Ho, Macromolecules, 2000, 33, 3232. This layer is cast from a polar solvent-based solution such as an aqueous and/or lower alkanol solution.  
         [0084]     The devices of the invention may include a bilayer anode. One layer of a bilayer anode is generally referred to as a “Hole Injection Layer” or “HIL.” If such a layer is present, then this layer  116  will be referred to as a “Hole Transport Layer” or “HTL.” If a separate Hole Injection Layer is not present then layer  116  can serve both functions and can be referred to as a “Hole Injection Transport Layer” or “HIL/HTL.” 
         [0000]     The Optional Hole Injection Layer ( 117 )  
         [0085]     When a hole injection layer  117  is present to provide a bilayer anode, it is typically 20 to 30 nm thick and is cast from solution onto the electrode  118 . Examples of materials used in layer  117  include semiconducting organic polymers such as PEDOT:PSS cast from a polar (aqueous) solution or the precursor of poly(BTPD-Si-PFCB) [S. Liu, X. Z. Jiang, H. Ma, M. S. Liu, A. K.-Y. Jen, Macro., 2000, 33, 3514; X. Gong, D. Moses, A. J. Heeger, S. Liu and A. K.-Y. Jen, Appl. Phys. Lett., 2003, 83, 183]. PEDOT:PSS is preferred. On the other hand, by using poly(BTPD-Si-PFCB) as hole injection layer, many processing issues existing in PLEDs, brought about by the use of PEDOT:PSS, such as the undesirable etching of emissive polymers, undesirable etching of ITO electrodes, and the formation of micro-shorts can be avoided [G. Greczynski, Th. Kugler and W. R. Salaneck, Thin Solid Films, 1999, 354, 129; M. P. de Jong, L. J. van Ijzendoorn, M. J. A. de Voigt, Appl. Phys. Lett. 2000, 77, 2255].  
         [0086]     It will be noted that the advantages of using an emissive layer made of polymers differentially soluble in nonpolar solvents with transfer layers made of materials differentially soluble in polar solvents also are achieved when the optional hole injection layer is made of materials differentially soluble in polar solvents. This means that in embodiment 13 of  FIG. 2  where a single hole injection/transport layer  120  is employed, it is advantageously differentially soluble in a polar solvent to achieve the desired processing advantages. The materials in this layer  120  can essentially duplicate the materials in layer  117  and layer  116 , if desired.  
         [0000]     The High Work Function Electrode ( 118 )  
         [0087]     The high work function hole injection electrode is typically a transparent conductive metal-metal oxide or sulfide material such as indium-tin oxide (ITO) with resistivity of 20 ohm/square or less and transmission of 89% or greater @ 550 nm. Other materials are available such as thin, transparent layers of gold or silver. A “high work function” in this context is generally considered to be a work function of about 4.5 eV or greater. This electrode is commonly deposited on the solid support  112  by thermal vapor deposition, electron beam evaporation, RF or Magnetron sputtering, chemical deposition or the like. These same processes can be used to deposit the low work-function electrode  113  as well. The principal requirement of the high work function electrode is the combination of a suitable work function, low resistivity and high transparency.  
         [0000]     The Low Work Function Electrode ( 113 )  
         [0088]     The low work function electrode  113  serves as an electron injection contact. It is typically made of a low work function metal or alloy placed on the opposite side of the active emissive polymeric layer  115  from electrode  118 . Low work function metals in the context of the present invention include materials with a work function of about 4.3 eV or less and are known in the art to include, for example Ba, Ca, Mg, In and Th. They are often accompanied by a layer of stable metal such as Ag, Au, Al or the like. This serves as a protection layer on top of reactive materials such as Ba, Ca, Tb. Other low work function (low ionization potential) conducting materials can be used in place of a conventional metal as the electron injection contact. The thickness of the electron injection electrode film is not critical and can be adjusted to achieve the desired surface resistance (surface resistance or sheet resistance is defined as the resistivity divided by the thickness) and can typically vary in the range of from significantly less than 100 Å to about 2000 Å or more. These materials are generally laid down as thin films with the techniques set out in the description of electrode  118 .  
         [0000]     The Support ( 119 )  
         [0089]     The various active layers  113 - 118  and passivation layer  112  are usually supported by a solid substrate  119 . This can be a rigid material such as plastic, glass, silicon, ceramic or the like or a flexible material such as a flexible plastic as well. This support may be transparent (as is the support shown in  FIG. 2 ) in which case the light can be emitted through it and through the transparent electrode  118 . Alternatively, the support can be non-transparent, in which case the transparent electrode  118 , through which light is emitted, is on the surface of the emissive layer away from the support.  
         [0000]     The Passivation Layer ( 112 )  
         [0090]     The passivation (protection) layer on the cathode is commonly made up of a stable metal that is typically thermally deposited in vacuum onto the top surface of the low work function metal cathode. Useful metals for the passivation layer are known in the art and include, for example, Ag and Al and the like. The thickness of the passivation layer is not critical and can be adjusted to achieve the desired surface resistance (surface resistance or sheet resistance is defined as the resistivity divided by the thickness) and can vary in the range of from few hundred Angstroms to more than one thousand Angstroms.  
         [0000]     Fabrication Methods  
         [0091]     The PLEDs of this invention may be fabricated using techniques known in the art, such as solution casting, screen printing, contact printing, precursor polymer processing, melt-processing, and the like to lay down the emissive polymer blend layer  115 , hole injection layer  117  and the one or two transport layers  114  and  116 . Sputtering, evaporation and the like may be used to lay down the electrode materials in layers  113  and  118  and the passivation materials in layer  112 .  
         [0092]     In a preferred embodiment, the present invention provides a method for obtaining efficient electrophosphorescent PLEDs by solution processing. The PLED is built up with successive layers as described above. In a most typical embodiment the first of the organic layers, hole injection layer  117  of the bilayer electrode is deposited on a transparent metal/metal oxide electrode  118  itself present on the substrate  119 . Layer  117  is cast or printed onto the electrode as a solution. The solvent is removed by evaporation and the next layer in the sequence, hole transport layer  116 , is cast onto the previously-deposited layer  117 , again as a solution and again with the solvent being removed by evaporation. Next the emissive layer  115  is cast from solution. This solution contains the luminescent polymers and the organometallic emitters that make up the emissive layer. Solvent is removed and the next layer, electron transport layer  114  is deposited as a solution which is dried and overcoated by vacuum depositing electron injection electrode  113  followed by passivation layer  112 . In this embodiment it is advantageous if the layers which bound the emissive layers are differentially soluble in a more polar solvent such as water or a relatively polar organic liquid such as a 1 to 3 carbon alkanol, that is methanol, ethanol, propanol or isopropanol or a blend of water and such alkanol and if the solution of luminescent polymers blended with organometallic emitters from which layer  115  is formed is formed in a suitable relatively nonpolar solvent such as a relatively nonpolar organic solvent, especially a relatively nonpolar hydrocarbon or the like. This prevents these successive layers from disrupting, etching and dissolving one another.  
         [0093]     By processing the emissive layer and the one or two transport layers from solutions, and particularly solutions in a less polar solvent for the emissive layer and in more polar solvents for the transport layers, the emitted light can be tuned by varying the concentrations. Thus, by processing from solution, efficient white light with stable CIE coordinates, high CRI values and stable color temperature can be achieved from electrophosphorescent PLEDs.  
         [0094]     This invention provides white electrophosphorescent PLEDs which have high brightness, stable CIE coordinates close to CIE coordinates (0.333, 0.333) of pure white light, high CRI values and stable color temperature. And more importantly, the white electrophosphorescent PLEDs described herein have CIE coordinates, CIU values and color temperatures that are insensitive to brightness, applied voltages and applied current density.  
         [0095]     High brightness, stable CIE coordinates close to those of pure white light (0.333, 0.333) high CRI values, and stable color temperature are critical parameters for light sources that are useful for solid state lighting applications [D. B. Judd and G. Wyszelki, Color in Business, Science and Industry, 3th ed. (John Wiley &amp; Sons) 1975, pp. 91-388; G. Wyszelki and W. S. Stiles, Color Science, 2nd ed. (Wiley, New York) 1982, pp. 117-2321. Thus by processing all active layers from the solutions, high brightness, stable CIE coordinates close to (0.333, 0.333), high CRI values and stable color temperature are obtainable from multilayer white emitting PLEDs. Therefore, this invention discloses a method for obtaining high performance multilayer white PLEDs; a method which is useful for solid state lighting applications.  
         [0000]     Mechanism for Generating White Light  
         [0096]     The mechanism for achieving white light from the Type I PLEDs of this invention can be described with reference to the representative emissive layer made up of an Ir(HFP) 3 : PFO-ETM blend. In this case the mechanism involves hole trapping on the Ir(HFP) 3  followed by electron trapping on the Ir(HFP) 3   +  cation [X. Gong, J.C. Ostrowski, D. Moses, G. C. Bazan, and A. J. Heeger, Appl. Phys. Lett., 2002, 81, 3711]. In this representative white emitting PLEDs, a portion of the injected holes (from ITO/PEDOT:PSS or poly(BTPD-Si-PFCB) and electrons (from the Ca/Ag or Ba/Al) recombine on the PFO-ETM main chain to produce blue and/or green light, [X. Gong, P. Iyer, D. Moses, G. C. Bazan, A. J. Heeger, Adv. Func. Mater., 2003, 13, 325]. Another portion of injected holes and electrons are trapped by Ir(HFP) 3  with subsequent emission of red light from the triplet of Ir(HFP) 3  [X. Gong, J. C. Ostrowski, D. Moses, G. C. Bazan, A. J. Heeger, M. S. Liu, A. K-Y. Jen, Adv. Mat. 2003, 15, 45; X. Gong, J. C. Ostrowski, D. Moses, G. C. Bazan, A. J. Heeger, J. Poly. Sci. Poly. Phys. 2003, 41, 2691; X. Gong, J. C. Ostrowski, D. Moses, G. C. Bazan, and A. J. Heeger, Appl. Phys. Lett., 2002, 81,3711].  
         [0097]     In the Type II PLEDs, such as those made from the blends of Ir(HFP) 3 :PFO-F(1%):PFO-ETM, injected holes and electrons recombine by two processes; direct recombination on the main chain (PFO-ETM) to produce blue and/or green emission in parallel with electron and hole trapping on the fluorenone units and on the Ir(HFP) 3  followed by radiative recombination, with green light from PFO-F (1%) [X. Gong, D. Moses and A. J. Heeger, Synthe. Met., 2004, 141, 171 and red light from the triplet excited state of Ir(HFP) 3  [X. Gong, J. C. Ostrowski, D. Moses, G. C. Bazan, A. J. Heeger, M. S. Liu, A. K-Y. Jen, Adv. Mat. 2003, 15, 45; X. Gong, J. C. Ostrowski, D. Moses, G. C. Bazan, A. J. Heeger, J. Poly. Sci. Poly. Phys. 2003, 41, 2691; X. Gong, J. C. Ostrowski, M. R. Robinson, D. Moses, G. C. Bazan, and A. J. Heeger, Adv. Mat., 2002, 14, 581; X. Gong, J. C. Ostrowski, D. Moses, G. C. Bazan, and A. J. Heeger, Appl. Phys. Lett., 2002, 81, 3711].  
         [0000]     Approaching Balanced Charge Injection and Transport in White PLEDs  
         [0098]     The performance improvements attained by the addition of the hole transport layer and the electron transport layer can be explained by reference to  FIGS. 3 and 4 .  FIG. 3  presents the energy levels of the top of the π-band (highest occupied molecular orbital, HOMO) and the bottom of the π*-band (lowest unoccupied molecular orbital, LUMO) of poly(9,9-dioctylfluorene) end-capped with 5-biphenyl-1,3,4-oxadiazol(PFO-ETM), poly(vinylcarbazole) sulfonic lithium (PVK-SO 3 Li) and 4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazole-2-yl)-biphenyl-4′-yl sulfonic sodium (t-Bu-PBD-SO 3 Na) and the work functions of barium (Ba) and poly(3,4-ethylene dioxythiophene): poly(styrene sulfonic acid) (PED0T:PSS).  FIG. 4  shows the corresponding energy levels of PFO-ETM, tris (2,5-bis-2′-(9′,9′-dihexylfluorene) pyridine) iridium (111), Ir(HFP)3, and fluorenone.  
         [0099]     The HOMO energy level of PVK-SO 3 Li, at −5.75 eV, is well aligned with the HOMO energy level of PFO-ETM, at −5.80 eV, implying a nearly ohmic contact for hole injection from PVK-So 3 Li to PFO-ETM. The LUMO of t-Bu-PBD-SO 3 Na, at −2.60 eV, is −0.10 eV higher than the work function of barium, at −2.70 eV. However, even this small electron injection barrier will be reduced by the formation of an interface dipole layer at the Ba/t-Bu-PBD-SO 3 Na interface [X. Gong, P. Iyer, D. Moses, G. C. Bazan, A. J. Heeger, Adv. Func. Mater., 2003, 13, 325; A. Rajagopal, C. I. Wu, A. Kahn, J. Appl. Phys., 1998, 83, 2649; S. T. Lee, X. Y. Hou, M. G. Mason, C. W. Tang, Appl. Phys. Lett., 1998, 72, 1593]. Therefore, the use of PVK-SO 3 Li as the hole injection/transport layer from the anode to the emissive polymer layer and t-Bu-PBD-SO 3 Na as the electron injection/transport layer from the cathode to the emissive polymer layer results in improved transport and high performance white light-emitting PLEDs (see  FIGS. 9-14  for results demonstrating this).  
         [0100]     In addition, the HTL and ETL block the transport of the electrons and holes, respectively, at the interface between the semiconducting emissive polymer layer and the HTL and/or ETL, thereby enhancing the probability of radiative recombination within the emissive layer. As a result, higher values of luminous efficiency, power efficiency and luminance are achieved (see  FIGS. 11 and 12 ).  
         [0000]     Solid State Lighting  
         [0101]     For solid state lighting applications, one should include the light emitted through the surface and edge of the glass/ITO substrate when calculating the total efficiency [H. A. E. Keitz, “Light Calculations and Measurements,” 2nd Edition, Macmillan and Co Ltd, 1971; A. D. Ryer, “Light Measurement Handbook.” International Light Inc., 1998]. Assuming typical values for the refractive indices of the glass (n=1.5), IT0 (n=1.8-2.0) and polymer (n=1.6-1.8), the critical angle, θ, between the direction of the light emitted in the polymer layer and the substrate surface normal is ˜36° at the air-polymer interface and ˜62° at the glass-polymer interfaces [B. W. D&#39;Andrade, R. J. Holmes, and S. R. Forrest, Adv. Mater., 2004, 16, 624; M. H. Lu, J. C. Sturm, J. Appl. Phys., 2002, 91, 595; J. Kido, Y. Lizurni, Appl. Phys. Lett., 1998, 73, 2721; N.C. Greenham, R. H. Friend, and D. D. C. Bradley, Adv. Mater., 1994, 6, 491]. Light incident on the interface at an angle greater than the critical angle will be totally internally reflected within the glass/ITO and then waveguided within the device. Although some of the guided light escapes by scattering, the remainder is either partially absorbed within the device or coupled out at the edges of the glass/ITO substrate. Theoretically, the fraction of light emitted in the forward direction is 1/(2n2) of the total where n is the index of refraction of emitter layer [N. C. Greenham, R. H. Friend, and D. D. C. Bradley, Adv. Mater., 1994, 6, 491]. More detailed optical modeling predicted (¾n 2 ) as the fraction emitted in the forward direction [J. S. Kim, P. H. Ho, N. C. Greenham, and R. H. Friend, J. Appl. Phys., 2000, 88, 1073]. Through a series of experiments using an integrating sphere, Cao et al. demonstrated that the measured reduction factor is approximately a factor of 2-2.5 less than the theoretical value, (2n 2 )≈6 (assuming n= 1 . 7  for emitted layer); i.e. closer to 4n 2 /3≈3.85 [Y. Cao, I. D. Parker, G. Yu, C. Zhang, and A. J. Heeger, Nature, 1999, 397, 414]. Forrest and colleagues have obtained similar results; in the small device approximation, they found that the total LE is larger by a factor of 1.7˜-2.4 than observed in the forward viewing direction [B. W. D&#39;Andrade, S. R. Forrest, Adv. Mater., 2004, 16, 1585; B. W. D&#39;Andrade, R. J. Holmes, and S. R. Forrest, Adv. Mater., 2004, 16,624].  
       EXAMPLES  
     Example 1  
       [0102]     Synthesis of PVK-SO 3 Li: The sulfonation of PVK was performed by a procedure described in the literature [S. Wang, Z. Zeng, S. Yang, L.-T. Weng, P. C. L. Wong, K. Ho, Macromolecules, 2000, 33, 3232]. The degree of sulfonation of PVK was about 28%. The sulfonated PVK was dissolved in a minimum amount of hot ethanol, and the resulting solution was cooled down to room temperature. To this solution was added excess EtOLi solution in ethanol and white precipitate was formed. The precipitate was collected by filtration, washed with cooled ethanol and dried under vacuum to obtain the lithium salt PVK-SO 3 Li.  
       Example 2  
       [0103]     Synthesis of t-Bu-PBD-SO 3 Na: t-Bu-PBD-SO 3 H was synthesized by a procedure described in the literature [T. J. Boyd, R. R. Schrock, Macromolecules, 1999, 32, 6608]. A concentrated solution of t-Bu-PBD-SO 3 H in water/THF(v/v 6:1) was added into a salt brine to afford a white precipitate. The precipitate was extracted into ethanol and the ethanol was removed to obtain the desired sodium salt t-Bu-PBD-SO 3 Na.  
       Example 3  
       [0104]     Three stock solutions, PFO-ETM, PFO-F (1%) and Ir(HFP) 3  were prepared by dissolving 50 mg PFO-ETM, 20 mg PFO-F (1%) and 5 mg Ir(HFP) 3  into 1 ml toluene, respectively. The resulting 0.5 wt.-% Ir(HFP) 3  solution was diluted to 0.05 wt.-% Ir(HFP) 3 . The mixtures were stirred overnight at 65° C. and then cooled to room temperature.  
         [0105]     This example demonstrates that the emissive materials used in the practice of this invention are soluble in common nonpolar organic solvents.  
       Example 4  
       [0106]     Preparation of Type I solution: 2.4 μl of a solution of 0.05 wt.-% Ir(HFP) 3  in toluene and 400 μl of a solution of 5 wt.-% PFO-ETM in toluene were added into 197.6 μl of pure toluene.  
         [0107]     Preparation of Type II solution: 19.2 μl of a solution of 0.05 wt.-% Ir(HFP) 3  in toluene and 400 μl of a solution of wet.-% PFO-ETM in toluene were added into 180.8 μl of pure toluene.  
         [0108]     This example demonstrates that solutions of the emissive materials can be made in nonpolar organic solvent at desired concentrations by blending conjugated polymers with organometallic emitters.  
       Example 5  
       [0109]     A solution of 0.5 wt.-% PVK-S0 3 3Li in ethanol was prepared.  
         [0110]     A solution of 0.5 wt.-% t-Bu-PBD-S0 3 Na in ethanol was prepared.  
         [0111]     This example demonstrates that solutions of PVK-S0 3 Li and t-Bu-PBD-SO 3 Na can be made at desired concentrations in polar solvents.  
       Example 6  
       [0112]     A PVK-S03Li solution prepared according to Example 5 was spin-cast at 5000 rpm in nitrogen atmosphere onto a preformed hole injection layer of PEDOT:PSS and thereafter baked at about 85° C. in a vacuum oven for 24 hours to yield a hole transport layer  116  on top of a hole injection layer  117 . Alternative hole transport layers  116  can be used such as, for example, poly(BTPD-Si-PFCB).  
       Example 7  
       [0113]     Type I and Type II solutions prepared according to Example 4 were spin-cast at 2000 rpm in nitrogen atmosphere onto PVK-S0 3 Li layers  116  prepared according to Example 6, and thereafter baked at 65° C. in a nitrogen atmosphere for 20 minutes to yield a variety of emissive layers  115  on hole transport layers  116 .  
       Example 8  
       [0114]     t-Bu-PBD-SO 3 Na solution prepared according to Example 5 was spin-cast at 5000 rpm in nitrogen atmosphere onto the emissive layer  115  and thereafter baked at about 95° C. in vacuum oven for 24 hours to yield a representative electron transport layer  114  on emissive layer  115 .  
       Example 9  
       [0115]     A Ba electrode  113  (for electron injection) was formed with a thickness of approximate 100 angstroms onto the t-Bu-PBD-S03Na layer  114  and then a protective Al over layer  112  was deposited with a thickness of approximate 2000 angstroms by vapor deposition at 10 −6  Torr. [X. Gong, J. C. Ostrowski, M. R. Robinson, D. Moses, G. C. Bazan, and A. J. Heeger, Adv. Mat. 2002, 14, 581; X. Gong, J. C. Ostrowski, D. Moses, G. C. Bazan, A. J. Heeger, M. S. Liu, A. K-Y. Jen, Adv. Mat. 2003, 15, 45].  
         [0116]     Ca or other low work function metals (and their alloys) can be used for the electron injection layer  113 .  
         [0117]     The overlayer  112  can be made using any inert metal, for example, silver or gold.  
         [0118]     Examples 6-9 together demonstrate that multilayer PLEDs can be fabricated by solution processing the organic layers.  
       Example 10  
       [0119]      FIG. 5  shows the electroluminescent spectra obtained from devices made from pure PFO-ETM, PFO-F (1%) and Ir(HFP) 3  doped into PFO-ETM (at a concentration of Ir(HFP) 3 :/PFO-ETM=1 wt. %). The strong green emission from “blue-emitting” PFO-ETM results from fluorenone defects generated during device fabrication/operation [x. Gong, P. Iyer, D. Moses, G. C. Bazan, A. J. Heeger, Adv. Func. Mater., 2003, 13, 325]. The broad green emission from PFO-F (1%) originates from excitation energy transfer in the copolymer from the PFO-ETM majority component to the fluorenone minority component. [X. Gong, D. Moses and A. J. Heeger, Synthe. Met. 2004, 141, 17]. The red emission with maximum at 600 nm and a shoulder at 620 is the Ir(HFP) 3  triplet emission. [X. Gong, J. C. Ostrowski, D. Moses, G. C. Bazan, A. J. Heeger, M. S. Liu, A. K-Y. Jen, Adv. Mat. 2003, 15, 45; J. C. Ostrowski, M. R. Robinson, A. J. Heeger and G. C. Bazan, Chem. Commun., 2002, 7, 784].  
       Example 11  
       [0120]      FIG. 6  shows the electroluminescent spectra obtained from Type I devices at different applied voltages. White light was generated from two components, PFO-ETM and Ir(HFP) 3 ; both blue and green from PFO-ETM [X. Gong, P. Iyer, D. Moses, G. C. Bazan, A. J. Heeger, Adv. Func. Mater., 2003, 13, 325] and red from Ir(HFP) 3 .  
       Example 12  
       [0121]      FIG. 7  shows the electroluminescent spectra obtained from Type II devices at different applied voltages. In this white light PLED, PFO-F (1%) was added into the PFO-ETM:Ir(HFP) 3  blends to fine-tune the color distribution. Therefore, white light was generated by Type II devices from three components, PFO-ETM, PFO-F (1%) and Ir(HFP)3; blue and green from PFO-ETM [X. Gong, P. Iyer, D. Moses, G. C. Bazan, A. J. Heeger, Adv. Func. Mater., 2003, 13, 325] and red from Ir(HFP) 3 , green from PFO-F (1%) [X. Gong, D. Moses, and A. J. Heeger, Synthe. Met. 2004, 141, 171 and red from Ir(HFP) 3 .  
       Example 13  
       [0122]     The CIE coordinates, CT and CRI were quantitatively evaluated from the electroluminescence spectra obtained in Example 12 [G. Wyszelki and W. S. Stiles, Color Science, 2nd ed. (Wiley, New York) 1982; D. B. Judd and G. Wyszecki, Color in Business, Science and Industry, 3rd ed. (John Wiley &amp; Sons) 1975].  
       Example 14  
       [0123]      FIG. 8  shows the 1931 CIE chromaticity diagram, with coordinates corresponding to the emission from electrophosphorescent PLEDs: data points are shown for both Type I devices (open squares) and Type II devices (open circles) biased at different applied voltages. For Type I devices, the CIE coordinates shift from (0.328, 0.334) at J=0.10 mA/cm 2  to (0.296, 0.290) at J=33 mA/cm 2 ; For Type II devices, the CIE coordinates shift from (0.380, 0.400) at J=0.2 mA/cm 2 ; to (0.346, 0.368) at J=115 mA/cm 2 . All are very close to the CIE coordinates for pure white light, (0.333, 0.333). The stability of the CIE coordinates as a function of applied voltage is much better than reported previously for white PLEDs/OLEDs [J. Kido, H, Shionoya, K, Nagai, Appl. Phys. Lett., 1995,67,2281; Y. Hamada, T. Sano, H. Fujii, Y. Nishio, Jpn. J. Appl. Phys., 1996, 35, L1339; M. Strukelj, R. H. Jordan, A. Dodabalapur, A.; J. Am. Chem. Soc., 1996,118, 1213; B. W. D&#39;Andrade, R. J. Holmes, and S. R. Forrest, Adv. Mater., 2004, 16, 624; Kido, H. Hongawa, K. Okuyama and K. Nagai, Appl. Phys. Lett. 1994, 64, 815; Y. Z. Wang, R. G. Sun, F. Meghdadi, G. Leising, A. J. Epstein, Appl. Phys. Lett., 1999, 74, 3613].  
       Example 15  
       [0124]     Type I devices have CT ˜6400° K. (see  FIG. 8 ), very close to the CT of average daylight (6500° K.), [R. W. G. Hunt, Measuring Color, 2 nd  Ed. Ellis Horwood, 1991] and CRI=92. Type II devices have CT ˜4500° K. (see  FIG. 4 ), very close to the CT of sunlight at solar altitude 20° (4700° K.) [R. W. G. Hunt, Measuring Color, 2nd Ed. Ellis Horwood, 1991], and CRI=86 (all values insensitive to J). The specific concentrations used in the Type I and Type II devices were chosen for example only; the CIE coordinates can be changed continuously by changing the composition of the blends. In  FIG. 8 , the dotted line indicates different color temperatures; the dotted oval indicates the approximate area where the human eye perceives the color as white.  
       Example 16  
       [0125]      FIG. 9  shows the luminance (L) versus voltage (V) and current-density (J) versus voltage (V) characteristics for Type I devices. All devices turn on at approximately 6 V, which is ˜1 V higher than the devices without PVK-SO 3 Li, due to the larger film thickness. Type I devices have L max ≈2.4 10 4  cd/m 2  at 25 V.  
       Example 17  
       [0126]      FIG. 10  shows the luminance (L) versus voltage (V) and current-density (J) versus voltage (V) characteristics for Type II devices. All devices turn on at approximately 6 V, which is ˜1 V higher than the devices without PVK-SO 3 Li, due to the larger film thickness. Type II devices have L max ≈2.4×10 4  cd/m 2  at 25 V.  
       Example 18  
       [0127]      FIG. 11  shows the forward viewing external luminous efficiency (LE ext ) versus current density, J (mA/cm 2 ) for Type I devices with PEDOT:PSS as the HIL/HTL, t-Bu-PBD-S0 3 Na as the ETL, and PVK-S0 3 Li as the HTL and t-Bu-PBD-S0 3 Na as the ETL, respectively; and the forward viewing external power efficiency (PE ext , lm/W) versus J (mA/cm 2 ).  
         [0128]     For display applications, a Lambertian intensity profile was assumed; the forward viewing efficiencies, LE ext  and PE ext  shown in  FIG. 11 , were measured with the following results: [K. Müllen, Editor, Electroluminescence-from Synthesis to Devices, Wiley-VCH, 2005 (in press). Type I devices have LE ext =10.4 cd/A, L=2391 cd/m 2  and PE ext =3 lm/W at J=23 mA/cm 2  (V=11 V); Note that even at J=200 mA/cm 2 , the Type I devices have L=19500 cd/m 2 , LE ext =9.5 cd/A and PE ext =2 lm/W. The LE ext  and PE ext  at 200 mA/cm 2  are significantly higher than any reported previously for white OLEDs and PLEDs [J. Kido, H, Shionoya, K, Nagai, Appl. Phys. Lett., 1995, 67, 2281; C. Zhang, A. J. Heeger, J. Appl. Phys., 1998, 84, 1579; Z. Shen, P. E. Burrows, V. Bulvić, S. R. Forrest, M. E. Thompson, Science, 1997, 276, 2009; Y. Hamada, T. Sano, H. Fujii, Y. Nishio, Jpn. J. Appl. Phys., 1996, 35, L1339; Y. Z. Wang, R. G. Sun, F. Meghdadi, G. Leising, A. J. Epstein, Appl. Phys. Lett., 1999, 74, 3613; M. Strukelj, R. H. Jordan, A. Dodabalapur, A.; J. Am. Chem. Soc., 1996, 118, 1213; B. W. D&#39;Andrade, R. J. Holmes, and S. R. Forrest, Adv. Mater., 2004, 16, 6241.  
         [0129]     As shown in  FIG. 11 , the LE ext  from white PLEDs with t-Bu-PBD-SO 3 Na as is higher than that without t-Bu-PBD-SO 3 Na. Similarly, the LE ext  from white PLEDs with both PVK-SO 3 Na as HTL and t-Bu-PBD-SO 3 Na as ETL are higher than that with only t-Bu-PBD-SO 3 Na as ETL. Therefore, these results demonstrate that white PLEDs comprising HTL and ETL which reduce the energy barriers for hole and electron injection have the highest LE ext  and PE ext  and, correspondingly, the highest L at a given J.  
       Example 19  
       [0130]      FIG. 12  shows the forward viewing external luminous efficiency (LE ext ) versus current density, J (mA/cm 2 ) for Type II devices with PED0T:PSS as the HIL/HTL, t-Bu-PBD-SO 3 Na as the ETL, and PVK-SO 3 Li as the HTL and t-Bu-PBD-SO 3 Na as the ETL, respectively; and the forward viewing external power efficiency (PE ext /lm/W) versus J (mA/cm 2 ).  
         [0131]     The measurement approach of Example 18 was repeated LE ext =7.2 cd/A, L=882 cd/m 2  and PE ext =1.5 lm/W at J=12 mA/cm 2  (V=15 V). Note that even at J=200 mA/cm 2 , Type II devices have L=9600 cd/m 2 , LE ext =4.8 cd/A and PE ext =0.65 lm/W. Again, the LE ext  and PE ext  at 200 mA/cm 2  are significantly higher than any reported previously for white OLEDs and PLEDs.  
         [0132]     Moreover, as shown in  FIG. 12 , the LE ext  from white Type II PLEDs with t-Bu-PBD-SO 3 Na as ETL is higher than that without t-Bu-PBD-SO 3 Na. Similarly, the LE ext  from white PLEDs with both PVK-SO 3 Na as HTL and t-Bu-PBD-SO 3 Na as ETL are higher than that only with t-Bu-PBD-SO 3 Na as ETL.  
       Example 20  
       [0133]     For solid state lighting applications, a Lambertian intensity profile was assumed; the total external luminous efficiency (LE total ) and power efficiency (PE total ) were measured with the results shown in  FIGS. 13 and 14 : [N/. C. Greenham, R. H. Friend, and D. D. C. Bradley, Adv. Mater., 1994, 6, 491-494; J. S. Kim, P. H. Ho, N. C. Greenham, and R. H. Friend, J. Appl. Phys., 2000, 88, 1073; Y. Cao, I. D. Parker, G. Yu, C. Zhang, and A. J. Heeger, Nature, 1999, 397, 414; Commission International de l&#39;Éclairage: Measurement of LEDs, CIE publication 127; B. W. D&#39;Andrade, S. R. Forrest, Adv. Mater., 2004, 16, 1585; B. W. D&#39;Andrade, R. J. Holmes, and S. R. Forrest, Adv. Mater., 2004, 16, 624; K. Mullen, Edited,  Electroluminescence - from Synthesis to Devices , Wiley-VCH, 2005 (in press); M. H. Lu, J. C. Sturm, J. Appl. Phys., 2002, 91, 5951.  
         [0134]      FIG. 13  shows the total external luminous efficiency (LE total ,) and the total external power efficiency (PE total , lm/W) versus current density J (mA/cM 2 ) for Type I devices with PEDOT:PSS as the HIL, PVK-SO 3 Li as the HTL and t-Bu-PBD-SO 3 Na as the EIL/ETL.  
         [0135]      FIG. 14  shows the total external luminous efficiency (LE total ) and the total external power efficiency (PE total , MW) versus current density J (mA/cm 2 ) for Type II devices with PEDOT:PSS as the HIL and PVK-SO 3 Li as the HTL and t-Bu-PBD-SO 3 Na as the EIL/ETL. As reflected in these Figures, Type I devices have LE total =21 cd/A and PE total =6 lm/W at J=23 mA/cm 2  and Type 11 devices have LE total =16 cd/A and PE tota =3 lm/W at J=12 mA/cm 2 .