Patent Publication Number: US-6210814-B1

Title: Color-tunable organic light emitting devices

Description:
This is a continuation-in-part application of U.S. application Ser. No. 09/058,142, filed Apr. 10, 1998, now U.S. Pat. No. 5,333,766. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to organic light emitting devices (OLEDs) comprising an emissive layer. In particular, the wavelength of light emitted by the emissive layer may be tuned by varying the concentrations of polar dopant molecules in the emissive layer. 
     BACKGROUND OF THE INVENTION 
     Organic light emitting devices (OLEDs) are comprised of several organic layers in which one of the layers is comprised of an organic material that can be made to electroluminesce, by applying a voltage across the device. C. W. Tang et al.,  Appl. Phys. Lett.  51, 913 (1987). Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays. S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995. Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor. 
     A transparent OLED (TOLED), which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was reported in U.S. Pat. No. 5,703,436, Forrest et al. This TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg—Ag—ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mg—Ag—ITO electrode layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color, red or blue. This colored emission could be transmitted through the adjacently stacked transparent, independently addressable, organic layer, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the red and blue color-emitting layers. 
     U.S. Pat. No. 5,703,745, Forrest et al, disclosed an integrated SOLED for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. U.S. Pat. No. 5,703,745, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices. 
     Such devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, OLEDs are comprised of at least two thin organic layers between an anode and a cathode. The material of one of these layers is specifically chosen based on the material&#39;s ability to transport holes, a “hole transporting layer” (HTL), and the material of the other layer is specifically selected according to its ability to transport electrons, an “electron transporting layer” (ETL). With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the HTL, while the cathode injects electrons into the ETL. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. These excitons are trapped in the material which has the lowest energy. Recombination of the short-lived excitons may be visualized as an electron dropping from a lowest unoccupied molecular orbital (LUMO) to a highest occupied molecular orbital (HOMO), with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism. 
     The materials that function as the ETL or HTL of an OLED may also serve as the medium in which exciton formation and electroluminescent emission occur. Such OLEDs are referred to as having a “single heterostructure” (SH). Alternatively, the electroluminescent material may be present in a separate emissive layer between the HTL and the ETL in what is referred to as a “double heterostructure” (DH). 
     In a single heterostructure OLED, either holes are injected from the HTL into the ETL where they combine with electrons to form excitons, or electrons are injected from the ETL into the HTL where they combine with holes to form excitons. Because excitons are trapped in the material having the lowest energy gap, and commonly used ETL materials generally have smaller energy gaps than commonly used HTL materials, the emissive layer of a single heterostructure device is typically the ETL. In such an OLED, the materials used for the ETL and HTL should be chosen such that holes can be injected efficiently from the HTL into the ETL. Also, the best OLEDs are believed to have good energy level alignment between the HOMO levels of the HTL and ETL materials. 
     In a double heterostructure OLED, holes are injected from the HTL and electrons are injected from the ETL into the separate emissive layer, where the holes and electrons combine to form excitons. 
     Various compounds have been used as HTL materials or ETL materials. HTL materials mostly consist of triaryl amines in various forms which show high hole mobilities (˜10 −3  cm 2 /Vs). One common HTL material is 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD), having the structure:                    
     There is somewhat more variety in the ETLs used in OLEDs. A common ETL material is aluminum tris(8-hydroxyquinolate) (Alq 3 ), having the structure:                    
     Other common ETL materials include oxidiazol, triazol, and triazine. 
     A number of technological challenges related to OLEDs need further attention, including increasing device lifetime and developing OLEDs that emit bright, saturated colors. Typically, the broad electroluminescence (EL) spectrum of OLEDs results in unsaturated emission colors which can be narrowed using, for example, an absorption filter or microcavity. Unfortunately, these methods can lead to a reduction in the OLED quantum efficiency or a strong angular dependence of the emitted color. It is therefore desirable to develop an OLED which emits a saturated color without the assistance of such external filters. Achievement of saturated and bright red OLEDs has proven to be particularly difficult. 
     Dyes have been used as dopants in the emissive layers of OLEDs to affect the wavelength and increase the efficiency of light emission. Molecules of the host transfer excitons to molecules of the dye through a non-radiative process. The exciton then recombines on the dye, and emits a photon having a wavelength characteristic of the dye, as opposed to the host. Several of these dyes are polar molecules, i.e., are molecules having a significant dipole moment, such as DCM1 and DCM2. DCM1 has a molecular structure represented by the formula:                    
     DCM2 has a structure represented by the formula:                    
     DCM2 has been described as a red emitting chromophore useful in OLED applications. C. W. Tang et al.,  Electroluminescence of doped organic thin films,  J. Appl. Phys. 65, 3610 (1989). Indeed, OLEDs based on Alq 3  doped with DCM2 are shown to exhibit very high brightness. Tang et al. also observed that the emission of Alq 3  doped with DCM1 undergoes a spectral shift towards higher wavelengths as the concentration of DCM1is increased, and attributed this spectral shift to excimer formation. Id. Tang et al. also observed that, with increasing concentration of DCM1, the efficiency of the emission at first increases, and then decreases. 
     DCM2 has also been described as showing promise as a laser material. M. Berggren et al.,  Light amplification in organic thin films using cascade energy transfer,  Nature 389, 466 (1997). 
     In studies of solutions of quinacridone in polar solvents, J. Kalinowski et al.,  Electroabsorption study of excited states in hydrogen-bonding solids: epindolidone and linear trans-quinacridone,  Chem. Phys. 182, 341(1994), spectral shifting has been attributed to hydrogen bonding in solution. 
     A red shift in the emission of DCM1 has been observed for DCM1 in polar solvents. M. Martin et al.,  Ultrafast intramolecular charge transfer in the merocyanine dye DCM,  Chem. Phys. 192, 367 (1995). 
     SUMMARY OF THE INVENTION 
     A method is provided for tuning the wavelength of light emitted by an organic light emitting device. An emissive dopant molecule is selected, adapted to emit light when present as a dopant in an emissive layer of the organic light emitting device. The wavelength of light emitted by said emissive dopant molecule depends on the local dipole moment of the emissive layer. The local dipole moment of the emissive layer that would result in said emissive dopant molecule emitting a desired wavelength of light is determined, and an emissive layer is fabricated, doped with the emissive dopant molecule and having the local dipole moment that results in the emissive dopant molecule emitting the desired wavelength of light. 
     In an organic light emitting device comprising a heterostructure for producing luminescence, an emissive layer is provided, having a host material, an emissive molecule, present as a dopant in said host material, adapted to luminesce when a voltage is applied across the heterostructure, and a polarization molecule, present as a dopant in said host material and having a dipole moment, that affects the wavelength of light emitted when said emissive dopant molecule luminesces, wherein said wavelength depends on the concentrations of said polarization dopant molecule in said host material. 
     Alq 3 , known in the art as a host material, can be used in the invention as a host material. Previously unknown in the art as host materials, quinoline-based materials for use in the invention are Mq 4  and Mq 2 R 2 , illustrated below.                    
     Mq 4 , M=group 4or 14 metal 
     Mq 2 R 2 , M=group 4 or 14 metal 
     R=akyl, aryl, alkoxy, halide, pseudohalide, etc. 
     B=alkyl, aryl, alkoxy, halide, pseudohalide, etc. 
     A group 4 metal is a metal selected from group 4 of the periodic table, which group includes elements under titanium. A group 14 metal is a metal selected from group 14 of the periodic table, which group includes elements under carbon. R can be the same or different from B. 
     The polarization dopant molecule can be selected from the group consisting of yellow stilbene, N,N-dimethylparanitroaniline (“PNA”), polar-1, polar-2, polar-3 and polar-4 as defined below:                    
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE  
               
               
                   
               
               
                 type 
                 D 
                 A 
                 n 
                 μ(D) 
                 λ max  (nm) 
                 reference 
               
               
                   
               
             
            
               
                 polar-1 
                 (CH 3 ) 2 N 
                 CN 
                 1 
                 5.6 
                 290 
                 a 
               
               
                 polar-1 
                 (CH 3 ) 2 N 
                 NO 2   
                 1 
                 6.4 
                 376 
                 a 
               
               
                 polar-1 
                 (CH 3 ) 2 N 
                 CHO 
                 1 
                 5.1 
                 326 
                 a 
               
               
                 polar-1 
                 (CH 3 ) 2 N 
                 COCF 3   
                 1 
                 5.9 
                 356 
                 a 
               
               
                 polar-1 
                 CH 3 O 
                 CN 
                 1 
                 4.8 
                 248 
                 a 
               
               
                 polar-1 
                 CH 3 O 
                 NO 2   
                 1 
                 4.6 
                 303 
                 a 
               
               
                 polar-2 
                 (CH 3 ) 2 N 
                 CN 
                 — 
                 5.7 
                 382 
                 a 
               
               
                 polar-2 
                 (CH 3 ) 2 N 
                 NO 2   
                 — 
                 6.6 
                 427 
                 a 
               
               
                 polar-2 
                 CH 3 O 
                 CN 
                 — 
                 3.8 
                 340 
                 a 
               
               
                 polar-2 
                 CH 3 O 
                 SO 2 C 6 F 13   
                 — 
                 7.8 
                 347 
                 a 
               
               
                 polar-2 
                 CH 3 O 
                 NO 2   
                 — 
                 4.5 
                 364 
                 a 
               
               
                 polar-3 
                 CH 3 O 
                 NO 2   
                 — 
                 4.7 
                 356 
                 b 
               
               
                 polar-3 
                 (CH 3 ) 2 N 
                 NO 2   
                 — 
                 6.0 
                 417 
                 b 
               
               
                 polar-3 
                 H 
                 CN 
                 — 
                 3.9 
                 284 
                 b 
               
               
                 polar-4 
                 (CH 3 ) 2 N 
                 CN 
                 — 
                 6.1 
                 372 
                 b 
               
               
                 polar-4 
                 CH 3 S 
                 CN 
                 — 
                 4.0 
                 333 
                 b 
               
               
                 polar-4 
                 CH 3 O 
                 NO 2   
                 — 
                 4.4 
                 356 
                 b 
               
               
                 polar-4 
                 CH 3 S 
                 NO 2   
                 — 
                 4.0 
                 362 
                 b 
               
               
                 polar-4 
                 (CH 3 ) 2 N 
                 NO 2   
                 — 
                 6.1 
                 415 
                 b 
               
               
                   
               
            
           
         
       
     
     Reference a is L. T. Cheng, W. Tam, S. H. Stevenson, G. R. Meredith, G. Rikken, S. R. Marder, Journal of Physical Chemistry, 1991, 95, 10631-10643. Reference b is L. T. Cheng, T. Tam, S. R. Marder, A. E. Steigman, G. Rikken, C. W. Spangler, Journal of Physical Chemistry, 1991, 95, 10643-10652. 
     The aromatic molecule at positions other than those occupied by D and A may be substituted by either hydrogen or alkyl. The depictions of polar-1, polar-2, polar-3 and polar 4 include geometric isomers created by different substitution patterns of D and A, which isomers appropriately effect the wavelength of light emitted when the emissive dopant luminesces. 
     Thus, representative examples of suitable light emitting diodes comprise the following: the host material may be Alq 3 , Mq 4 , or Mq 2 R 2  or other suitable host materials well-known to those skilled in the art, the emissive dopant molecule may be selected from the group consisting of DCM1 and DCM2 or other suitable emissive dopant molecule known to one of ordinary skill in the art, and the polarization dopant molecule may be yellow stilbene, N,N-dimethylparanitroaniline (“PNA”), polar-1, polar-2, polar-3, or polar-4 as defined above. One can use Mq 4  or Mq 2 R 2  in organic light emitting devices with other suitable polarization dopant molecules. One can make organic light emitting diodes by using a method based on selecting a suitable host material and polarization dopant molecule chosen from the compounds disclosed herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a double heterostructure organic light emitting device. 
     FIG. 2 shows electroluminescent (EL) spectra for the OLEDs of FIG. 1, having an Alq 3 :DCM2 separate emissive layer, for various concentrations of DCM2. The inset shows plots of power output v. current for the OLEDs. 
     FIG. 3 shows photoluminescent (PL) spectra for DCM2 in various polar and non-polar solvents, as well as a PL spectra for a neat thin film of DCM2. 
     FIG. 4 shows current v. voltage (I-V) plots for OLEDs having an Alq 3 :DCM2 separate emissive layer, for various concentrations of DCM2. 
     FIG. 5 shows the color of emission of the OLEDs, plotted on a CIE chromaticity diagram. 
     FIG. 6 shows emission spectra of DCM2 doped into polystyrene with varying amounts of N,N-dimethylparanitroaniline added. 
     FIG. 7 shows the dependence of luminescence of λmax on percentage of N,N-dimethylparanitroaniline in the polystyrene. 
     FIG. 8 shows the absorption spectra of doped polystyrene thin films. 
     FIG. 9 shows solution spectra of DCM2 in benzene, chloroform, ethanol and dimethylsulfoxide (DMSO). 
     FIG. 10 is a proton NMR spectrum of a Zr-quinaldine complex. 
     FIG. 11 is a proton NMR spectrum of a Hf-quinaldine complex. 
    
    
     DETAILED DESCRIPTION 
     A method is provided for tuning the wavelength of light emitted by an organic light emitting device. An emissive dopant molecule is selected, adapted to emit light when present as a dopant in an emissive layer of the organic light emitting device. The wavelength of light emitted by said emissive dopant molecule depends on the local dipole moment of the emissive layer. The local dipole moment of the emissive layer that would result in said emissive dopant molecule emitting a desired wavelength of light is determined, and an emissive layer is fabricated, doped with the emissive dopant molecule and having the local dipole moment that results in the emissive dopant molecule emitting the desired wavelength of light. 
     While not intending to be limited by any particular theory or mechanism for explaining exactly how or why the fields generated by neighboring polar molecules, referred to herein as the “local dipole moment,” causes a shift in the wavelength of light emitted by an emissive dopant molecule, it is believed that the local dipole moment affects the orbital structure of an emissive molecule to the extent that the energy levels of the molecule may shift. The inventors&#39; analysis shows that polar molecules within about 50-100 Å would provide significant contributions to the local dipole moment at the emissive molecule. In particular, it is believed that the gap between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of a polar molecule is decreased as the local dipole moment increases, because the local dipole moment affects the distribution of charge on the polar molecule, which will in turn affect the energy level positions. When an excited electron falls from the LUMO to the HOMO, a photon is emitted having an energy determined by the gap between the LUMO and HOMO. It is therefore possible to control the wavelength of light emitted by an emissive molecule in an OLED by controlling the gap between the LUMO and HOMO. 
     Prior explanations for the red-shifting of the emission of certain dopants are not consistent with the inventors&#39; observations. For example, excimer formation would not result in the continuous, rigid shift of the EL spectrum shown in FIG. 2, while hydrogen bonding in solution is not possible for DCM2 molecules. 
     A local dipole moment is expected to affect the LUMO-HOMO gap of both non-polar and polar molecules (a molecule having a high dipole moment). However, it is expected that a local dipole moment will have a greater effect on the LUMO-HOMO gap of a polar molecule, relative to a less polar or non-polar molecule. As a result, the red-shift in the emission of the emissive dopant molecule due to a local dipole moment is expected to be greater if the emissive dopant molecule is a polar molecule. It is therefore expected a lower local dipole moment would be required to achieve a particular spectral shift for emissive dopant molecules having a high dipole moment. It is therefore preferable, that the emissive dopant molecule has a dipole moment above about 5 debyes, more preferably above about 7 debyes, and most preferably above about 10 debyes. 
     The inventors have demonstrated a shift in the spectral emission of OLEDs having an emissive layer of aluminum tris(8-hydroxyquinoline) (Alq 3 ) doped with DCM2, by changing the concentration of DCM2. As the concentration of DCM2 in Alq 3  was increased, the spectral emission changed from green to red, while the spectral width remained constant. This effect may be due to an increase in the local dipole moment of the Alq 3 :DCM2 layer with increasing concentration of DCM2, a polar molecule. In particular, increasing the concentration of DCM2 increases the local dipole moment, which affects the LUMO-HOMO gap of DCM2, resulting in an emission that is red-shifted. However, the increase in DCM2 concentration also results in increase in formation of DCM2 aggregates. Aggregate formation at high DCM2 concentrations provides sites for non-radiative recombination (e.g. excimers), which reduce the possibility for radiative emission and decreases quantum efficiency. 
     A “polarization dopant molecule,” i.e., a molecule that contributes to the local dipole moment, separate from the emissive dopant molecule, may be used as a dopant to increase the local dipole moment of the host without increasing the concentration of the emissive dopant molecule, thereby avoiding the formation of aggregates of the emissive dopant molecule and the resultant quenching and decrease in efficiency. Preferably, the polarization dopant molecule has a high dipole moment, more preferably has a dipole moment above about 5 debyes, more preferably above about 7 debyes, and most preferably above about 10 debyes. A high dipole moment allows the local dipole moment of the host to be changed using only small concentrations of the polarization dopant molecule. 
     The polarization dopant molecule is preferably transmissive in the spectral range corresponding to the OLED emission, and does not significantly interfere with the light emitted by the emissive dopant molecule. For example, a polarization dopant molecule having a LUMO-HOMO gap that is greater than the energy of a photon emitted by the emissive dopant molecule is unlikely to absorb such photons. Preferably, the polarization dopant molecule does not act as a carrier trap in Alq 3 , and does not affect the conduction properties or current-voltage characteristics of the OLED. Preferably, the polarization dopant molecule does not provide radiative or non-radiative recombination sites for excitons. For example, a polarization dopant molecule having a LUMO-HOMO gap that is greater than the energy of an exciton on a molecule of the host is unlikely to accept the transfer of such an exciton. 
     The polarization dopant molecule may be doped into the emissive layer of an OLED at a high concentration, up to about 50%, without significantly adversely affecting OLED performance. However, at very high concentrations of dopant, above about 50%, OLED performance may be adversely affected by the decrease in the amount of the host and the emissive dopant molecule. The polarization dopant molecule is therefore preferably present in a concentration less than about 50%, and more preferably less than about 20%. Where the polarization dopant molecule has a dipole moment within the preferred ranges, it should be possible to attain the local dipole moment required to achieve emission of a particular wavelength from the emissive dopant molecule at concentrations of the polarization dopant molecule that are significantly less than about 20%. 
     One molecule that could be used as the polarization dopant molecule is yellow stilbene, having the structure:                    
     The local dipole moment in the emissive layer of the OLED may therefore be controlled by changing the concentration of the polarization dopant molecule, which in turn will affect the wavelength of light emitted by the emissive dopant molecule. Because the color tuning of the OLED is achieved by changing the concentration of the polarization dopant molecule, different colors of emission may be achieved while holding constant the concentration of the emissive dopant molecule. Preferably, the concentration of the emissive dopant molecule is low, just sufficient to maximize energy transfer from the host to the emissive dopant molecule, and the efficiency of the OLED. Because aggregates tend to form at higher concentrations of the emissive dopant molecule, keeping the concentration low keeps the density of aggregates low, and the OLED efficiency is kept high. For example, if the host is Alq 3  and the emissive dopant molecule is DCM2, the concentration of DCM2 is preferably between about 0.3% and 20%, more preferably between about 1% and 10%, and most preferably about 1%. If the emissive dopant molecule is also polar, the local dipole moment would depend on the concentrations of both the emissive dopant molecule and the polarization dopant molecule. 
     One or more emissive dopant molecules may be used, for example to achieve an OLED that emits white light. One or more polarization dopant molecules may be used, for example to achieve a higher local dipole moment than would be possible using only one polarization dopant molecule. 
     Over a large volume, the net dipole moment caused by dopants such as DCM2 averages to zero, because the molecules in the solid solution are randomly distributed. However, because the dipole field decreases as 1/r 3 , where r is the distance between dipoles, there is a net local dipole moment contribution from neighboring DCM2 molecules which, on average, influences the spectral emission of radiating DCM2 molecules. This local dipole moment can also be inferred from the dielectric constant for DCM2, which is somewhat higher than is typically observed for nonpolar molecular solids. As will be shown below, these local variations in the distribution of dipoles is also reflected in the luminescent efficiency of lightly DCM2 doped Alq 3  films. 
     The luminescence of DCM2 molecules is preceded by non-radiative Förster transfer of energy from Alq 3  to DCM2. The rate of energy transfer depends on the distance R between the host (Alq 3 ) and the guest (DCM2) molecules, and is given by Equation (1):                  K     H   -   G            (   R   )       =       (     1   τ     )            (       R   0     R     )     2               (   1   )                         
     where R 0  is the Förster radius, i.e., the average distance between host and dopant molecules, and τ is the average host exciton lifetime for recombination in the absence of energy transfer, corresponding to rate K π =1/τ. When R=R 0  then K H-G =K H , and the probability that an exciton will recombine at the host is equal to its transfer probability. Equation (1) describes the interaction of a single host molecule with a single guest molecule. In a doped thin film, however, the exciton on the host can interact with a distribution of dopant molecules, and hence the total rate of transfer is obtained by integrating over the film volume:                K     H   -   G     TOT     =       4      π          ∫     R   =     2      a         R   =   ∞                (     1   τ     )     6          ρ   a          QR   2           =       (     1   τ     )                         R   0   6        Q       8        a   6                     (   2   )                         
     Here, a is the radius of an Alq 3  molecule, and ρ α =3/(4πα 3 ). The probability for exciton transfer from a host to a dopant molecule is then:                P     H   -   G       =       K     H   -   G     TOT         K   H     +     K     H   -   G     TOT                 (   3   )                         
     From Equations (2) and (3), and using data measured from Alq 3 :DCM2 OLEDs, R 0  for those OLEDs can be estimated by determining the percentage of EL from Alq 3  and DCM2 as a function of the doping fraction, Q. A linear fit of the spectrum of a Alq 3 :1% DCM2 OLED shows that 15% of the EL is due to direct radiative recombination of excitons on Alq 3 , resulting in a small shoulder in the spectrum of such an OLED at λ=520 nm (See FIG.  2 ). Using an internal luminescence quantum efficiency of η int   Alq =0.32 for Alq 3  thin films (as reported by D. Z. Garbuzov et al.,  Chem. Phys. Lett,  249, 433 (1996)), and assuming an efficiency of η int   DCM2 =1.0 for DCM2, it is estimated that about 35% of the excitons recombine on Alq 3  molecules without transferring to DCM2, or equivalently P H-G =65%, from which it can be inferred that R 0 =16 Å. However, it is likely that η int   DCM2  is less than unity since DCM2 molecules can interact with the Alq 3  host, resulting in non-radiative decay, in which case an even smaller fraction of excitons recombine on the Alq 3  molecules. For example, if it is assumed that η int   DCM2 =0.33, then P H-G =85% and R 0 =19 Å. This distance is within factor of two of R 0 =39 Å calculated using the Alq 3  photoluminescent (PL) and DCM2 absorption spectral overlap in the Förster integral. 
     In considering the differences between these two estimates of R 0 , it is noted that there are local variations in the distribution of DCM2 in Alq 3 , as already inferred from analysis of the spectral shift. This inhomogeneity, however, is contrary to the assumptions used in Equation 2. To estimate the magnitude of the effects of these inhomogeneities, it is noted that, for a Alq 3 :1% DCM2 OLED, the average separation between neighboring DCM2 monomers is about 5.3 Alq 3  molecules. With an Alq 3  molecular weight of 460 g/mol, and a thin film density of 1.3 g/cm 3  (as reported by H. Schmidbaur et al.,  Z. Naturforsch,  B 46, 901 (1991)), the average spacing between Alq 3  molecules is about 9.4 Å, from which an average DCM2 spacing of d DCM2 =50 Å can be estimated. In fact, some pairs of DCM2 molecules will be farther apart than d DCM2 , and some will be closer together. The net effect results in a higher probability for recombination at an Alq 3  molecule prior to transfer. The net effect of the inhomogeneous dopant distribution is therefore to increase the host and decrease the dopant luminescence, or equivalently P H-G  (homogeneous)&gt;P H-G  (inhomogeneous). The inhomogeneous distribution of distances between DCM2 molecules in a Alq 3 :1% DCM2 OLED has been modeled as a Gaussian distribution centered at d DCM2 =50 Å, with a full-width at half-maximum of w. With R 0 =39 Å, calculated via the Förster integral, and w=12 Å to 15 Å, P H-G  is 85% to 75%, in agreement with P H-G  obtained from the EL spectral measurement. Local variations in the spacing between DCM2 dopant molecules may therefore account for the observed enhancement in Alq 3  luminescence. 
     In summary, it has been demonstrated that molecular polarization effects in small-molecular-weight organic thin films can significantly influence the emission spectra of the luminescent molecules. This effect was demonstrated by tuning the emission color of Alq 3 :DCM2 OLEDs by varying the concentration of DCM2, shifting the peak of the EL spectrum by as much as 50 nm. The inventors&#39; experimental results show that a Alq 3 :1% DCM2 OLED has particularly high luminescence and external quantum efficiency when compared to known OLEDs that emit in the yellow to red region of the visible spectrum. However, luminescence and external quantum efficiency drop off at higher concentrations of DCM2. 
     The inventors analysis shows that it would be possible to shift the emission of an Alq3:DCM2 OLED by doping the emissive layer with a polarization dopant molecule, such as yellow stilbene, that increases the local dipole moment without otherwise affecting the OLED properties. This polarization dopant molecule is preferably not emissive, and preferably does not accept excitons from the host material. Such a polarization dopant molecule can be used to red-shift the emission of the DCM2, without increasing the concentration of DCM2, thereby maintaining the high luminescence and external quantum efficiency of the Alq 3 :1% DCM2 OLED. 
     The inventors&#39; analysis also shows that other emissive molecules, such as DCM1, may be used instead of DCM2. 
     The inventors&#39; analysis also shows that mechanisms other than Förster transfer may be used to create excitons on emissive molecules present as a dopant in a host. For example, Dexter transfer, which involves the tunneling of an excited electron from a molecule of the host to a molecule of the dopant, and the simultaneous tunneling of a non-excited electron from the dopant to the host, may produce excitons on the dopant. 
     Another such mechanism is the trapping of charge carriers on molecules of the dopant. For example, the LUMO levels of the host and dopant may be arranged such that the LUMO level of the dopant is lower than the LUMO level of the host, such that the dopant molecule would act as an electron trap. Once an electron is trapped on a particular dopant molecule, a hole would eventually jump the that molecule, producing an exciton. Similarly, the HOMO levels of the host and dopant may be arranged such that the HOMO level of the dopant is higher than the HOMO level of the host, such that the dopant molecule would act as a hole trap. Once a hole is trapped on a particular dopant molecule, an electron would eventually jump the that molecule, producing an exciton. 
     In addition, one or more dopants, referred to herein as “transfer” dopants, may be used to facilitate the transfer of energy from the host to the emissive dopant. For example, cascade doping may be used, which involves the non-radiative transfer of excitons from a molecule of the host through one or more transfer dopants to the emissive dopant. These intermediate transfers may be by Förster transfer, Dexter transfer, hole trapping or electron trapping that eventually leads to the formation of an exciton on the transfer dopant or the emissive dopant, or any other suitable mechanism. 
     One problem that may occur, particularly if dopants are used in high concentrations, is the recrystallization of dopant molecules. This problem may be addressed by doping with one or more additional dopants, referred to herein as “stabilizing” dopants, that prevent such recrystallization. 
     The OLEDs of the present invention are comprised of a heterostructure for producing electroluminescence which may be fabricated as a single heterostructure or as a double heterostructure. As used herein, the term “heterostructure for producing electroluminescence” refers to a heterostructure that includes for a single heterostructure, for example, a substrate, a hole injecting anode layer in contact with the substrate, a HTL in contact with the anode layer, an ETL in contact with the HTL, and an electrode injecting cathode layer in contact with the ETL. If the cathode layer is a metal cathode layer of Mg:Ag, then a metal protective layer, for example, made of a layer of Ag for protecting the Mg:Ag cathode layer from atmospheric oxidation, may also be present. 
     The heterostructure for producing electroluminescence may also include a protection layer and/or an injection enhancement layer between the anode layer and the HTL or the cathode layer and the ETL. The protection layer serves to protect the underlying organic layers from damage during deposition of an ITO layer, for example. An injection enhancement layer serves to enhance injection of holes from the anode into the adjacent HTL, such as disclosed in copending Ser. No. 08/865,491, now U.S. Pat. No. 5,998,803, for example, or to enhance injection of electrons from the cathode into the adjacent ETL, such as disclosed in copending application Ser. No. 08/964,863, and copending application entitled “Highly Transparent Non-Metallic Cathodes,” Ser. No. 09/054,707 (filed Apr. 3, 1998), for example. 
     If a double heterostructure is used to produce electroluminescence, a separate emissive layer is included between the HTL and the ETL. The term “emissive layer” as used herein may refer either to the emissive electron transporting layer or emissive hole transporting layer of a single heterostructure or the separate emissive layer of a double heterostructure. The emissive layer of a double heterostructure is referred to as a “separate” emissive layer so as to distinguish it from the ETL of a single heterostructure, which may also be an emissive layer. The materials, methods and apparatus for preparing the organic thin films of a single or double heterostructure are disclosed, for example, in U.S. Pat. No. 5,554,220, which is incorporated herein in its entirety by reference. 
     Alternatively, the heterostructure for producing electroluminescence may have an inverted (IOLED) structure in which the sequence of layers deposited on the substrate is inverted, that is, an electron injecting cathode layer is in direct contact with the substrate, an electron transporting layer is in contact with the cathode layer, a hole transporting layer is in contact with the electron transporting layer, and a hole injecting anode layer is in contact with the hole transporting layer. 
     If the heterostructure for producing electroluminescence is included as part of a stacked OLED (SOLED), one or both of the electrodes of an individual heterostructure may be in contact with an electrode of an adjacent heterostructure. Alternatively, dependent on the circuitry used to drive the SOLED, an insulating layer may be provided between adjacent electrodes of two of the OLEDs in the stack. 
     The single or double heterostructures as referred to herein are intended solely as examples for showing how an OLED embodying the present invention may be fabricated without in any way intending the invention to be limited to the particular materials or sequence for making the layers shown. For example, a single heterostructure typically includes a substrate which may be opaque or transparent, rigid or flexible, and/or plastic, metal or glass; a first electrode, which is typically a high work function, hole-injecting anode layer, for example, an indium tin oxide (ITO) anode layer; a hole transporting layer; an electron transporting layer; and a second electrode layer, for example, a low work function, electron-injecting, metal cathode layer of a magnesium-silver alloy, (Mg:Ag) or of a lithium-aluminum alloy, (Li:Al). 
     Materials that may be used as the substrate in a representative embodiment of the present invention include, in particular, glass, transparent polymer such as polyester, sapphire or quartz, or substantially any other material that may be used as the substrate of an OLED. 
     Materials that may be used as the hole-injecting anode layer in a representative embodiment of the present invention include, in particular, ITO, Zn—In—SnO 2  or SbO 2 , or substantially any other material that may be used as the hole-injecting anode layer of an OLED. 
     Materials that may be used in the HTL in a representative embodiment of the present invention include, in particular, N,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′biphenyl-4,4′diamine (TPD), 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD) or 4,4′-bis[N-(2-naphthyl)-N-phenyl-amino]biphenyl (β-NPD). Materials that may be used as the ETL include, in particular, aluminum tris(8-hydroxyquinolate) (Alq 3 ), a carbazole, an oxadiazole, a triazole, a thiophene or oligothiophene group. Other materials that may be used as the separate emissive layer, if present, include, in particular, dye-doped Alq 3 , or substantially any other material that may be used as the separate emissive layer of an OLED. 
     Materials that may be used as the electron-injecting, metal cathode layer in a representative embodiment of the present invention include, in particular, Mg—Ag, Li—Ag or Ca, or a non-metallic material such as ITO, such as disclosed in copending Ser. No. 08/964,863, or substantially any other material that may be used as the cathode layer of an OLED. 
     The insulating layer, if present, may be comprised of an insulating material such as SiO 2 , SiN x  or AlO 2 , or substantially any other material that may be used as the insulating material of an OLED, which may be deposited by a variety of processes such as plasma enhanced chemical vapor deposition (PECVD), electron beam, etc. 
     The OLEDs of the present invention have the advantage that they can be fabricated entirely from vacuum-deposited molecular organic materials as distinct, for example, from OLEDs in which some of the layers are comprised of polymeric materials, which cannot be readily deposited using vacuum deposition techniques. A vacuum-deposited material is one which can be deposited in a vacuum typically having a background pressure less than one atmosphere, preferably about 10 −5  to about 10 −11  torr for vacuum deposition, or about 50 torr to about 10 −5  tort for vapor deposition. 
     Although not limited to the thickness ranges recited herein, the substrate may be as thin as 10μ, if present as a flexible plastic or metal foil substrate, such as aluminum foil, or substantially thicker if present as a rigid, transparent or opaque, substrate or if the substrate is comprised of a silicon-based display driver; the ITO anode layer may be from about 500 Å (1 Å=10 −8  cm) to greater than about 4000 Å thick; the hole transporting layer from about 50 Å to greater than about 1000 Å thick; the separate emissive layer of a double heterostructure, if present, from about 50 Å to about 200 Å thick; the electron transporting layer from about 50 Å to about 1000 Å thick; and the metal cathode layer from about 50 Å to greater than about 1000 Å thick, or substantially thicker if the cathode layer includes a protective silver layer and is opaque. 
     Thus, while there may be substantial variation in the type, number, thickness and order of the layers that are present, dependent on whether the device includes a single heterostructure or a double heterostructure, whether the device is a SOLED or a single OLED, whether the device is a TOLED or an IOLED, whether the OLED is intended to produce emission in a preferred spectral region, or whether still other design variations are used, the present invention is directed to those devices in which the wavelength of the emission has been controlled using the local dipole moment of the emissive layer. 
     The subject invention as disclosed herein may be used in conjunction with co-pending applications: “High Reliability, High Efficiency, Integratable Organic Light Emitting Devices and Methods of Producing Same”, Ser. No. 08/774,119 (filed Dec. 23, 1996), now U.S. Pat. No. 6,046,543; “Novel Materials for Multicolor LED&#39;s”, Ser. No. 08/850,264 (filed May 2, 1997), now U.S. Pat. No. 6,045,930; “Electron Transporting and Light Emitting Layers Based on Organic Free Radicals”, Ser. No. 08/774,120 (filed Dec. 23, 1996), now U.S. Pat. No. 5,811,833; “Multicolor Display Devices”, Ser. No. 08/772,333 (filed Dec. 23, 1996), now U.S. Pat. No. 6,013,982; “Red-Emitting Organic Light Emitting Devices (LED&#39;s)”, Ser. No. 08/774,087 (filed Dec. 23, 1996), now U.S. Pat. No. 6,048,630; “Driving Circuit For Stacked Organic Light Emitting Devices”, Ser. No. 08/792,050 (filed Feb. 3, 1997), now U.S. Pat. No. 5,757,139; “High Efficiency Organic Light Emitting Device Structures”, Ser. No. 08/772,332 (filed Dec. 23, 1996), now U.S. Pat. No. 5,834,893; “Vacuum Deposited, Non-Polymeric Flexible Organic Light Emitting Devices”, Ser. No. 08/789,319 (filed Jan. 23, 1997), now U.S. Pat. No. 5,844,363; “Displays Having Mesa Pixel Configuration”, Ser. No. 08/794,595 (filed Feb. 3, 1997); “Stacked Organic Light Emitting Devices”, Ser. No. 08/792,046 (filed Feb. 3, 1997), now U.S. Pat. No. 5,917,280; “High Contrast Transparent Organic Light Emitting Device Display”, Ser. No. 08/821,380 (filed Mar. 20, 1997), now U.S. Pat. No. 6,986,401; “Organic Light Emitting Devices Containing A Metal Complex of 5-Hydroxy-Quinoxaline as A Host Material”, Ser. No. 08/838,099 (filed Apr. 15, 1997), now U.S. Pat. No. 5,861,219; “Light Emitting Devices Having High Brightness”, Ser. No. 08/844,353 (filed Apr. 18, 1997), now U.S. Pat. No. 5,932,895; “Organic Semiconductor Laser”, Ser. No. 60/046,061 (filed May 9, 1997); “Organic Semiconductor Laser”, Ser. No. 08/859,468 (filed May 19, 1997); “Saturated Full Color Stacked Organic Light Emitting Devices”, Ser. No. 08/858,994 (filed May 20, 1997); “An Organic Light Emitting Device Containing a Hole Injection Enhancement Layer”, Ser. No. 08/865,491 (filed May 29, 1997), now U.S. Pat. No. 5,998,803; “Plasma Treatment of Conductive Layers”, Ser. No. PCT/US97/10252; (filed Jun. 12, 1997); Patterning of Thin Films for the Fabrication of Organic Multi-Color Displays”, Ser. No. PCT/US97/10289 (filed Jun. 12, 1997); “Double Heterostructure Infrared and Vertical Cavity Surface Emitting Organic Lasers”, Ser. No. 60/053,176 (filed Jul. 18, 1997), which was converted to non-provisional application Ser. No. 09/010,594; “Oleds Containing Thermally Stable Asymmetric Charge Carrier Materials”, Ser. No 08/925,029, (filed Dep. 8, 1997); “Light Emitting Device with Stack of Oleds and Phosphor Downconverter”, Ser. No. 08/925,403 (filed Sep. 9, 1997), now U.S. Pat. No. 5,874,803; “An Improved Method for Depositing Indium Tin Oxide Layers in Organic Light Emitting Devices”, Ser. No. 08/928,800 (filed Sep. 12, 1997), now U.S. Pat. No. 5,981,306; “Azlactone-Related Dopants in the Emissive Layer of an Oled” (filed Oct. 9, 1997), Ser. No. 08/948,130, now U.S. Pat. No. 6,030,715; “A Highly Transparent Organic Light Emitting Device Employing A Non-Metallic Cathode”, (filed Nov. 3, 1997), Ser. no. 08/064,005, (Provisional), “A Highly Transparent Organic Light Emitting Device Employing a Non-Metallic Cathode”, (filed Nov. 5, 1997), Ser. No. 08/964,863, “Low Pressure Vapor Phase Deposition of Organic Thin Films” (filed Nov. 17, 1997), Ser. No. 08/972,156, “Method of Fabricating and Patterning Oleds”, (filed Nov. 24, 1997), Ser No. 08/977,205, now U.S. Pat. No. 6,013,538; “Method for Deposition and Patterning of Organic Thin Film”, (filed Nov. 24, 1997), Ser. No. 08/976,666, “Oleds Doped with Phosphorescent Compounds”, (filed Dec. 1, 1997), Ser. No. 08/980,986, now U.S. Pat. No. 5,953,587; “Organic Vertical-Cavity Surface-Emitting Laser Confirmation”, (filed Jan. 22, 1998), Ser. No. 09/010,594; “Electron Transporting and Light Emitting Layers Based on Organic Free Radicals”, (filed Feb. 18, 1998), Ser, No. 09/025,660, now U.S. Pat. No. 5,922,396; “Method of Making a Display”, (filed Mar. 30, 1998), Ser. No. 09/050,084; “Aluminum Complexes Bearing Both Electron Transporting and Hole Transporting Moieties” (filed Apr. 1, 1998), Ser. No. 09/053,030, “Highly Transparent Non-Metallic Cathodes” (filed Apr. 3, 1998), Ser. No. 09/054,707; and “OLEDs Containing Thermally Stable Glassy Organic Hole Transporting Materials” (filed Apr. 10, 1998), Ser. No. 09/058,305, each copending application being incorporated herein by reference in its entirety. The subject invention may also be used in conjunction with the subject matter of each of co-pending U.S. patent application Ser. No. 08/354,674, now U.S. Pat. No. 6,030,700, Ser. No. 08/613,207 now U.S. Pat. No. 5,703,436, Ser. No. 08/632,322, now U.S. Pat. No. 5,757,026 and Ser. No. 08/693,359 and provisional patent application Ser. No. 60/010,013, which was converted to a non-provisional application and issued as U.S. Pat. No. 5,986,268, Ser. No. 60/024,001, which was converted to a non-provisional application and issued as U.S. Pat. No. 5,844,363 and Ser. No. 60/025,501, which was converted to non-provisional Ser. No. 08/844,353, each of which is also incorporated herein by reference in its entirety. 
     OLEDs of the present invention may be fabricated using the materials and structures as disclosed in these co-pending applications. 
     The OLED of the present invention may be used in substantially any type of device which is comprised of an OLED, for example, in OLEDs that are incorporated into a larger display, a vehicle, a computer, a television, a printer, a large area wall, theater or stadium screen, a billboard or a sign. 
     This invention will now be described in detail with respect to showing how certain specific representative embodiments thereof can be made, the materials, apparatus and process steps being understood as examples that are intended to be illustrative only. In particular, the invention is not intended to be limited to the methods, materials, conditions, process parameters, apparatus and the like specifically recited herein. 
     Experiments 
     Group I 
     The inventors have demonstrated red, orange, and yellow OLEDs having an emissive layer comprising Alq 3  doped with DCM2. The color of the emission was controlled by varying the concentration of DCM2. 
     Double heterostructure OLEDs, as shown in FIG. 1, were fabricated. OLED 100 has an anode  120 , a hole transporting layer  130 , a seperate emissive layer  140 , an electron transporting layer  150 , a cathode  160 , and a cap  170 , sequentially layered on a substrate  110 . Substrate  110  and anode  120  were made of precleaned, indium-tin-oxide (ITO) coated glass, where substrate  110  is glass and anode  120  is ITO. Hole transporting layer  130 , seperate emissive layer  140 , electron transporting layer  150 , cathode  160 , and cap  170  were grown, in that order, in a high vacuum (10 −6  Torr) by thermal evaporation, at deposition rates that ranged from about 1 to 5 Å/s. Hole transporting layer  130  is a 300 Å thick layer of 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino] biphenyl (α-NPD), seperate emissive layer  140  is a 300 Å thick layer of Alq 3  doped with from 1% to 10% by mass of DCM2, electron transporting layer  150  is a 100 Å thick layer of Alq 3 , cathode  160  is a 500 Å thick layer of 25:1 Mg:Ag alloy, and cap  170  is a 1000 Å thick layer of Ag. Cathode  160  and cap  170  were deposited through a shadow mask to define 1 mm diameter circular cathodes  160  and caps  170 . OLEDs were fabricated having a seperate emissive layer  140  that is Alq 3 :1% DCM2 (Q=0.01), Alq 3 :2% DCM2 (Q=0.02), Alq3:5% DCM2 (Q=0.05), and Alq 3 :10% DCM2 (Q=0.10). 
     As shown in FIG. 2, the concentration of DCM2 in seperate emissive layer  140  strongly influenced the OLED electroluminescence (EL) spectrum and efficiency. As the DCM2 concentration in Alq 3  was increased from 1% to 10% (represented by Q increasing from 0.01 to 0.10 in FIG.  2 ), the output spectrum of OLEDs 100 was red shifted by 50 nm, with a relatively unchanged peak width. This shift is accompanied by a concomitant decrease in the external EL quantum efficiency from η=0.5% to η=0.07%, as shown in the plots of output optical power vs. input current (L-I) in the inset of FIG.  2 . The pronounced red shift on increasing DCM2 concentration can be understood by examining FIG.  3  and Table 2. FIG. 2 also shows the contribution of emission from Alq 3  to the emission of the Alq 3 :1% DCM2 OLED. 
     FIG. 3 shows normalized photoluminescence (PL) spectra for DCM2 in a variety of solutions, as well as the PL spectrum of a neat thin film of DCM2. The solutions are very dilute solutions of DCM2 (˜1/1000 of the saturated solution concentration) in various polar and non-polar solvents, including benzene (C 6 H 6 ), chloroform (CHCl 3 ), ethanol (C 2 H 5 OH), and DMSO ((CH 3 ) 2 S:O). Such dilute solutions ensure that the spectra observed are due to monomer fluorescence, and that the contribution from aggregate luminescence is negligible. The solutions were optically excited using a multi-wavelength Ar ion laser, having a λ (wavelength) between about 460 nm and 514 nm, and the PL emissions were measured to give the results shown in FIG.  3 . There was no significant difference in the quantum yield of any of the solutions. 
     Table 1 shows the dipole moment μ and the dielectric constant E for various liquid and thin film solutions, and the peak emission wavelength λ max  of DCM2 in these solutions: 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Material 
                 μ(debyes) 
                 ε 
                 λ max  (nm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 LIQUIDS 
                   
                   
                   
               
               
                   
                 Benzene (C 6 H 6 ) 
                 0 
                 2.27 
                 565 
               
               
                   
                 Chloroform (CHCl 3 ) 
                 1.15 
                 4.81 
                 595 
               
               
                   
                 ethanol (C 2 H 5 OH) 
                 1.69 
                 24.3 
                 645 
               
               
                   
                 DMSO (CH 3 ) 2 S:O 
                 3.9 
                 46.7 
                 675 
               
               
                   
                 THIN FILMS 
               
               
                   
                 Alq 3   
                 5.5 
                 2.96 
                 520 
               
               
                   
                 Alq 3 :1% DCM2 
                 5.56 
                 2.97 
                 600 
               
               
                   
                 Alq 3 :2% DCM2 
                 5.61 
                 2.98 
                 625 
               
               
                   
                 Alq 3 :5% DCM2 
                 5.78 
                 3.01 
                 640 
               
               
                   
                 Alq 3 :10% DCM2 
                 6.0 
                 3.06 
                 650 
               
               
                   
                 DCM2 
                 11.3 
                 4.0 
                 685 
               
               
                   
                   
               
            
           
         
       
     
     Values for μ and ε for the solvents are from “Solvent Guide,” J. T. Przybytek, Ed., Jackson Laboratories, 2nd Ed. (1982) and “Handbook of Chemistry and Physics,” R. C. Weast, Ed., 56th Ed., CRC Press (1976). For Alq 3  and DCM2, semi-empirical molecular orbital calculations based on an intermediate neglect of differential overlap algorithm, parametrized from solution electronic spectra for the optimized molecular geometrics, were used to obtain μ. For Alq3 and DCM2, ε was as inferred from the refractive index of DCM2, measured ellipsometrically. For Alq 3 :DCM2 thin film solutions, μ and ε were calculated by linear extrapolation from the values for the constituent compounds. 
     Table 1 shows a high correlation between the dipole moment of the solution and the peak emission wavelength λ max  of DCM2, where λ max  is red shifted for solutions having a higher dipole moment. For DCM2 in liquid solutions, λ max  varies from 565 nm for the nonpolar solvent, benzene, with a dielectric constant of ε=2.27, to 675 nm for dimethylsufoxide (DMSO) with a dipole moment of 3.9 D and ε=46.7. For DCM2 in thin film solutions with Alq 3 , λ max  varies from 600 nm for Alq 3 :1% DCM2 to 650 nm for Alq 3 :10% DCM2. Indeed, the PL of the neat DCM2 thin film is shifted to λ max =685 nm, a shift of 120 nm from DCM2 in a non-polar solvent. 
     The broad emission spectrum of the OLEDs yields a saturated color that is dependent on the concentration of DCM2 in Alq 3 , and has an external quantum efficiency of as high as 0.5%. A shift of 50 nm in the peak emission wavelength of the OLEDs was observed as DCM2 concentration was increased from 1% to 10%. For red and yellow-orange OLEDs, a maximum luminance of 1400 cd/m 2  and 15200 cd/m 2  is measured, respectively. The current vs. voltage dependencies of these devices are consistent with trap-limited conduction, unaffected by the presence of DCM2. A luminance of 100 cd/m 2  is attained at 100 mA/cm 2  and 14 V for the red Alq 3 :10% DCM2 OLED, whereas the same luminance intensity is attained at 10 mA/cm 2  and 12 V for the yellow-orange Alq 3 :1% DCM2 OLED. The ten times higher current of the red OLED is due to the lower quantum efficiency of these OLEDs, as well as to the lower photopic response of the human eye to red as compared to yellow light. 
     The similarity of the spectral widths, and the magnitudes of the peak shifts in FIGS. 2 and 3, suggests that DCM2 suspended in a solid matrix of Alq 3  undergoes an energy shift due to self-polarization. That is, as the DCM2 concentration in the relatively non-polar Alq 3  is increased, the distance between nearest neighbor, highly polar, DCM2 molecules decreases, thereby increasing the local polarization field, which tends to red-shift the DCM2 emission spectrum. The effect is increased until the DCM2 concentration is 100% in an undiluted DCM2 film, where ;max is close to that attained for DCM2 in the polar solvent, DMSO. By comparison, the most dilute solution of 1% DCM2 in Alq 3  has λ max =600 nm, which is similar to that for DCM2 in CHCl3, which has a dipole moment of 1.15 D (and ε=4.81). This suggests that even at concentrations as low as 1%, the emission of DCM2 in a Alq 3 :DCM2 OLED is influenced by the presence of other DCM2 molecules, given that a smaller shift than that observed would be expected from interactions only with the less polar Alq 3 . 
     FIG. 4 shows forward biased current-voltage (I-V) characteristics of Alq 3 :DCM2 OLEDs. These I-V characteristics are similar to those previously reported for other OLEDs, where trap-limited conduction (I∝V m+1 ) is observed. See P. E. Burrows et al., J. Applied Physics 79, 7991 (1996). For Alq 3 :DCM2, an m of 11±1 is observed at high applied voltage, independent of the concentration of DCM2. At lower voltages, an m of about 2 is observed, characteristic of space-charge limited transport. Concentrations of DCM2 in Alq 3  do not therefore significantly influence the Alq 3  trap distribution. As previously concluded, FIG. 4 shows that the I-V characteristics of Alq 3 :DCM2 OLEDs are primarily limited by bulk currents in the Alq 3 , at least for DCM2 concentrations of up to 10%. 
     FIG. 5 shows the chromaticity coordinates of Q=0.01, 0.02, 0.05, and 0.10 DCM2:Alq 3  OLEDs, as well as an undoped α-NPD/Alq 3  device. Saturated red emission is observed for an Alq 3 :10% DCM2 OLED, and yellow-orange emission for the Alq 3 :1% DCM2 OLED. At concentrations of DCM2 lower than 1%, yellow and yellow-green EL emission should also be achievable. 
     Table 2 shows the electroluminescence characteristics of selected OLEDs with emission between red and yellow, including maximum reported luminance under DC bias (L MAX ), current and voltage at L MAX , external quantum efficiency of photons per electron (η), CIE coordinates, and perceived color. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 (I, V) 
                   
                   
                   
               
               
                   
                   
                 [mA/ 
               
               
                   
                 L MAX   
                 cm 2 , V] 
                   
                 CIE (x, y) - 
               
               
                 Material/Method 
                 [cd/m 2]   
                 @L MAX   
                 η [%] 
                 color 
                 Ref. 
               
               
                   
               
             
            
               
                 Alq 3 :10% DCM2 
                  1400 
                 (2200, 19) 
                 0.07 
                 (0.64, 0.36) - 
                 THIS 
               
               
                   
                   
                   
                   
                 red 
               
               
                 Alq 3 :1% DCM2 
                 15200 
                 (2000, 17) 
                 0.5  
                 (0.54, 0.44) - 
                 WORK 
               
               
                   
                   
                   
                   
                 yellow 
               
               
                 Eu(DBM) 3 (phen) 
                  460 
                  (300, 16) 
                 — 
                 (0.68, 0.32) - 
                 [1] 
               
               
                 in PBD 
                   
                   
                   
                 red 
               
               
                 Eu(TTA) 3  (phen) 
                  137 
                  (200, —) 
                 0.03 
                 (0.68, 0.32) - 
                 [2] 
               
               
                 in 1AZM-Hex 
                   
                   
                   
                 red 
               
               
                 (Q = 0.05) 
               
               
                 Luminescent Blue 
                  120 
                  (20, 10) 
                 — 
                 (0.65, 0.32) - 
                 [3] 
               
               
                 PL Conversion &amp; 
                   
                   
                   
                 red 
               
               
                 Filtering 
               
               
                 TPP in Alq 3   
                   60 
                  (45, 35) 
                 0.11 
                 (0.68, 0.32) - 
                 [4] 
               
               
                 (Q = 0.03) 
                   
                   
                   
                 red 
               
               
                 Eu (TTA) 3 (phen) 
                   30 
                  (100, 16) 
                 — 
                 (0.68, 0.32) - 
                 [5] 
               
               
                   
                   
                   
                   
                 red 
               
               
                 Eu (ttfa) 3  in 
                 0.3 
                  (22, 18) 
                  0.001 
                 (0.69, 0.13) - 
                 [6] 
               
               
                 PMPS 
                   
                   
                   
                 red 
               
               
                 DCM1 in Alq 3   
                  150 
                  (—, 10) 
                 — 
                 (0.62, 0.36) - 
                 [7] 
               
               
                 (Q = 0.10) 
                   
                   
                   
                 red 
               
               
                 DCM1 in Alq 3   
                 — 
                  (65, —) 
                 2.3  
                 (0.57, 0.43) - 
                 [8] 
               
               
                 (Q = 0.0025) 
                   
                   
                   
                 orange 
               
               
                 DCM1 in Alq 3   
                 — 
                  (—, —) 
                 0.03 
                 (0.57, 0.43) - 
                 [9] 
               
               
                   
                   
                   
                   
                 orange 
               
               
                   
               
            
           
         
       
     
     The Alq 3 :DCM2 OLED data of Table 2 is from the inventor&#39;s own work, while the other data is from the literature. The data from the literature falls into two categories: OLEDs with a guest:host EL layer: [1] J. Kido et al.,  Appl. Phys. Lett.,  65, 2124 (1994); [2] T. Sano et al.,  Japan J. Appl. Phys.,  34, 1883 (1995); [4] Z. Shen et al.,  Science,  276, 2009 (1997); [5] T. Tsutsui et al.,  Appl. Phys. Lett,  65, 1868 (1994); [6] J. Kido et al.,  J. Alloys =and Compounds,  192, 30 (1993); [7] Y. Kijima et al.,  IEEE Transactions on Electron Devices,  44, 1222 (1997); [8] C. W. Tang et al.,  J. Appl. Phys.,  65, 3610 (1989); [9] J. Littman et al.,  J. Appl. Phys.,  72, 1957 (1992); and structures filtered by microcavity band-pass filters: [3], S. Tasch et al.,  Adv. Mater.,  9, 33 (1997). The chemical names and/or structures of the molecules referred to in the table can be determined from these cited references, which are incorporated herein by reference. For OLEDs exhibiting saturated red emission, DCM2-doped Alq 3  devices have the highest maximum brightness and an external quantum efficiency comparable to the highest efficiencies reported elsewhere. The high luminescence and external quantum efficiency of the Alq 3 :DCM2 OLEDs, particularly for Alq 3 :1% DCM2, indicates that Alq3:DCM2 OLEDs have significant potential. 
     Group 2 
     There are other polarization dopant molecules which can be used to shift electroluminescence. This approach for an OLED involves an emissive layer with three different components. For example, in a simple two layer OLED, consisting of tertiary amine HTL and aluminum tris(8-hydroxyquinolate) (“Alq3”) ETL layers, the ETL layer would be composed of three materials. In addition to Alq3, the ETL will have a dye dopant and a polarization dopant. The polarization dopant acts to shift the emission spectrum of the dye by changing the polarity of the medium. This shifting effect can be observed in the photoluminescence spectra. For the experiment depicted in FIGS. 6 through 9, DCM2 was loaded into polystyrene at 0.4 weight %. This D2M2 doped polystyrene was then doped with varying amounts of N,N-dimethylparanitronaniline (PNA), which PNA serves as a polarization dopant molecule. The dipole moment of PNA is 6.5 D. As the amount of PNA in the polystryene in the polystyrene/DCM2 mixture is increased, the photoluminescence spectrum of the DCM2 gradually red shifts. This shift can be seen in the spectra of FIG.  6  and in the graph of FIG.  7 . The wavelength of excitation energy was chosen to be low enough that the PNA is not absorbing (i.e., 500 nm, see FIG.  8 ). The quantum yield for emission from these films is not significantly affected by doping. A similar effect is observed when DCM2 is dissolved in a range of different solvents (see FIG.  9 ). The more polar the solvent is the more red shifted the emission spectrum is. 
     The mechanism of PNA induced shifts is believed to be the same as described above, in Group I. The PNA gradually changes the polarity of the medium, thereby red shifting emission. The highest PNA doping level does not appeal to make sense with the red shift being directly related to the doping level of PNA (i.e., 16.8% PNA is to the red of 25% PNA). The reason for this anomalous result is that PNA crystallizes at the higher doping level, giving relatively less PNA dissolved in polystryene. 
     Not all dyes will red shift on increasing the polarity of the medium. Some dyes blue shift and some dyes do not shift at all (see: J. A. Barltrop and J. D. Coyle, Principles of Photochemistry, John Wiley &amp; Sons, New York, 1978). The materials that give the largest shifts are dyes whose emission comes from a charge transfer transition. This charge transfer will give the excited state a very different dipole moment than that of the ground state. The more polar state (ground or excited) will be stabilized in polar solvents and destabilized in nonpolar solvents, leading to a solvent dependent highest-occupied molecular orbital-lowest unoccupied molecular orbital gap (“HOMO-LUMO gap”). There will thus be a solvent dependent emission energy. Centrosymmetric dyes, such as fused polycyclic aromatic compounds, will show relatively little solvent dependence and will not be significantly affected by solvent or polarity changes induced by adding dopants like PNA. 
     Embodiments of the shifting mechanism for tuning color in OLEDs involve doping one of the transport layers with a luminescent dye at a fairly low level (typically 0.5% to 3% by weight for fluorescent and 1% to 10% for phosphorescent dyes). The polarization dopant molecule is added at a level to produce the correct emission spectrum. The choice of the polarization dopant molecule is made according to several criteria. The polarization dopant molecule should have a high dipole moment, have all of its absorption bands situated so that do no overlap with the emission lines of either the luminescent dopant or the transport material, should be stable against crystallization in the film, must not form an exciplex with either the luminescent dye or the transport material, and must not be a carrier trap in the transporting material. For vacuum deposited devices the polarization dopant molecule must be stable to sublimation. Illustrative polarization dopant molecules are shown below:                    
     Herein, D stands for an electron donor (which donates electron density to the aromatic system) and A stands for an electron acceptor (which accepts electron density from the aromatic system). Specific examples are given in Table 3 wherein D, A, and n are defined Table 3. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 type 
                 D 
                 A 
                 n 
                 μ(D) 
                 λ max  (nm) 
                 reference 
               
               
                   
               
             
            
               
                 polar-1 
                 (CH 3 ) 2 N 
                 CN 
                 1 
                 5.6 
                 290 
                 a 
               
               
                 polar-1 
                 (CH 3 ) 2 N 
                 NO 2   
                 1 
                 6.4 
                 376 
                 a 
               
               
                 polar-1 
                 (CH 3 ) 2 N 
                 CHO 
                 1 
                 5.1 
                 326 
                 a 
               
               
                 polar-1 
                 (CH 3 ) 2 N 
                 COCF 3   
                 1 
                 5.9 
                 356 
                 a 
               
               
                 polar-1 
                 CH 3 O 
                 CN 
                 1 
                 4.8 
                 248 
                 a 
               
               
                 polar-1 
                 CH 3 O 
                 NO 2   
                 1 
                 4.6 
                 303 
                 a 
               
               
                 polar-2 
                 (CH 3 ) 2 N 
                 CN 
                 — 
                 5.7 
                 382 
                 a 
               
               
                 polar-2 
                 (CH 3 ) 2 N 
                 NO 2   
                 — 
                 6.6 
                 427 
                 a 
               
               
                 polar-2 
                 CH 3 O 
                 CN 
                 — 
                 3.8 
                 340 
                 a 
               
               
                 polar-2 
                 CH 3 O 
                 SO 2 C 6 F 13   
                 — 
                 7.8 
                 347 
                 a 
               
               
                 polar-2 
                 CH 3 O 
                 NO 2   
                 — 
                 4.5 
                 364 
                 a 
               
               
                 polar-3 
                 CH 3 O 
                 NO 2   
                 — 
                 4.7 
                 356 
                 b 
               
               
                 polar-3 
                 (CH 3 ) 2 N 
                 NO 2   
                 — 
                 6.0 
                 417 
                 b 
               
               
                 polar-3 
                 H 
                 CN 
                 — 
                 3.9 
                 284 
                 b 
               
               
                 polar-4 
                 (CH 3 ) 2 N 
                 CN 
                 — 
                 6.1 
                 372 
                 b 
               
               
                 polar-4 
                 CH 3 S 
                 CN 
                 — 
                 4.0 
                 333 
                 b 
               
               
                 polar-4 
                 CH 3 O 
                 NO 2   
                 — 
                 4.4 
                 356 
                 b 
               
               
                 polar-4 
                 CH 3 S 
                 NO 2   
                 — 
                 4.0 
                 362 
                 b 
               
               
                 polar-4 
                 (CH 3 ) 2 N 
                 NO 2   
                 — 
                 6.1 
                 415 
                 b 
               
               
                   
               
            
           
         
       
     
     Reference a is L. T. Cheng, W. Tam, S. H. Stevenson, G. R. Meredith, G. Rikken, S. R. Marder, Journal of Physical Chemistry, 1991, 95, 10631-10643. Reference b is L. T. Cheng, T. Tam, S. R. Marder, A. E. Steigman, G. Rikken, C. W. Spangler, Journal of Physical Chemistry, 1991, 95, 10643-10652. 
     Disubstituted benzenes (polar-1, n=1) have relatively high dipole moments and are typically transparent to visible light. Substituted stilbenes (polar-2) also have high dipole moments, but the increase conjugation length makes them absorbing in the visible spectrum. 
     One host material used in OLEDs is Alq 3 . This material has a dipole moment of approximately 5 D (estimated from semiempirical theoretical calculations). In order for a polar dopant to affect the polarity of the medium it must have a volume dipole moment larger than Alq 3 . PNA has a dipole moment of 6.4 D, which is only slightly larger than that of Alq 3 . The volume of PNA is roughly ¼ of that of Alq 3 , so that doping of PNA, or of other small molecules with reasonable dipole moments, will increase the polarity of the matrix. The effect of polar molecule doping would be enhanced at low doping levels if the matrix itself were nonpolar. The advantage of low doping is that there is less of a chance that the doping will decrease the carrier conduction in the matrix material and there will be less of a chance that the dopant will crystallize. Two examples of nonpolar matrix materials that make suitable electron transporters are Mq 4  and Mq 2 R 2 , shown below.                    
     Mq 4 , M=Alp 4 or 14 metal 
     Mq 2 R 2 , M group 4 or 14 metal 
     R=dkyl, aryl, halide, pseudohalide, 
     B=dkyl, aryl, alkoxy, halide, pseudohalide, 
     Pseudohalides have chemistry similar to that of halides and include —CN and —CNS. Mq 4  exists as several structural isomers. They all have approximately cubic coordination about the central metal atom, but have different dispositions of the four quinolates. Two of the nonpolar (centrosymmetric) structures are shown:                    
     In order to favor these isomers, it may be important to increase the steric bulk at the 2 position of the quinolate or prepare the quinolates as pairs with bridges between the “2 positions” of the quinolates. 
     One member of the Mq 4  class, Zrq 4 , has been structurally characterized in D. F. Lewis, R. C. Fay, Chem. Soc. Chem. Commun., 1046-1047 (1974). The structure of this material is similar to that pictured in stereo a, which is the preferred structure for forming a nonpolar molecule. The factors that control which isomer is formed are related to both steric and electronic effects. We have prepared quinaldine complexes in which the 2-carbon (adjacent to the nitrogen) is substituted with methyl. The complex is depicted in the following.                    
     Mq′ 4 , M=group 4 or 14 metal 
     The compounds with the methyl group increase the steric congestion and favor the stereo a isomer. Nuclear magnetic resonance (NMR) spectra of the Zr and Hf complexes in solution are shown in FIGS. 10 and 11. The spectra show four well resolved peaks for the four hydrogens bound to the aromatic system and a single peak for the methyl group. This simple coupling pattern would not be expected for asymmetric isomers (which can be more complicated; see H. Schmidbauer, J. Lettenbauer, D. L. Wilkinson, G. Mueller, O. Kumberger,  Z. Naturforschung,  1991, 46B, 901), but is exactly what is expected for either stereo a or stereo b. Alternately, these isomers could be isomerizing in fluid solution so that only the averaged signals are observed. However, this isomerization is doubtful considering similar processes are not observed for group 3 and 13 triquinolates. We have confirmed that these materials can be sublimed cleanly. 
     The OLED of the present invention may be used in substantially any type of device which is comprised of an OLED, for example, in OLEDs that are incorporated into a larger display, a vehicle, a computer, a television, a printer, a large area wall, theater or stadium screen, a billboard or a sign.