Abstract:
A method of making an electroluminescent device having a substrate, and at least one dopant receiving layer containing a host material which when doped provides an emissive layer, includes providing at least one dopant layer having a dopant disposed over or under the dopant receiving layer; providing an anode and a cathode so that the dopant receiving layer and the dopant layer are disposed between such anode and cathode; and heating the electroluminescent device to cause the dopant to diffuse from the dopant layer into the dopant receiving layer and forming the emissive layer having uniformly dispersed dopant in the host material.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Reference is made to commonly-assigned U.S. patent application Ser. No. 09/589,731 filed Jun. 8, 2000,entitled “Organic Electroluminescent Devices With Improved Stability and Efficiency” by T. K. Hatwar et al, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to methods of making organic light-emitting devices and, more particularly to methods of providing in a light-emitting layer of an organic light-emitting device formed by diffusing a dopant from a dopant layer to a dopant receiving layer. 
     BACKGROUND OF THE INVENTION 
     Organic light-emitting devices, also referred to as organic electroluminescent (EL) devices or as organic internal junction light-emitting devices, contain spaced electrodes separated by an organic light-emitting structure (also referred to as an organic EL medium) which emits light in response to the application of an electrical potential difference across the electrodes. At least one of the electrodes is light-transmissive, and the organic light-emitting structure can have a multi-layer of organic thin films which provide for hole injection and transport from an anode, and for electron injection and transport from a cathode, respectively, with light emission resulting from electron-hole recombination at an internal junction formed at an interface between the hole-transporting and the electron-transporting thin films. As employed herein, the term “thin film” refers to layer thicknesses of less than 1 micrometer with layer thickness of less than about 0.5 micrometer being typical. Examples of organic light-emitting devices containing organic light-emitting structures and cathode constructions formed by thin film deposition techniques are provided by commonly-assigned U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; and 4,769,292. 
     During operation of an organic light-emitting device, the spectral distribution of emitted light (measured in terms of spectral radiance) is related to the electroluminescent properties of the organic thin films used in the device construction. For example, if an organic light-emitting structure includes a layer which contains a light-emitting host material, the emitted light will be dominated by the light emission from the host material. 
     The above-cited commonly-assigned U.S. Pat. No. 4,769,292 recognized that advantageous performance features of an organic light-emitting device could be obtained if the device included a luminescent zone (or light-emitting layer) of less than 1 micrometer in thickness and comprised of an organic host material capable of sustaining hole-electron recombination, and a small amount of fluorescent material capable of emitting light in response to energy released by hole-electron recombination. The introduction of a fluorescent material into a layer of a light-emitting host material will modify the color of the light emission, and can improve the operational stability of an organic light-emitting device. In analogy to terminology used in the semiconductor industry, fluorescent materials dispersed uniformly at relatively low concentration in light-emitting organic host materials are called “dopants.” 
     As currently practiced, the organic thin films of a light-emitting device are formed by vapor deposition (evaporation or sublimation) in successive deposition steps within a vacuum system which employs a deposition rate control. When a fluorescent dopant is to be uniformly incorporated within an organic light-emitting layer, the light-emitting host material and the fluorescent dopant material are co-deposited from two independently controlled deposition sources. It is necessary to control the individual deposition rates of a fluorescent dopant and a host material when a desired dopant concentration in the host material of the organic light-emitting layer is at or near a lower end of a dopant concentration range of 10 −3  to about 10 mole percent. The difficulty of reliably controlling the deposition rates of an organic light-emitting host material and of a fluorescent dopant material has been an obstacle in the process of reproducibly fabricating organic electroluminescent devices containing a fluorescent dopant or fluorescent dopants. 
     Another recent method for fabrication of electroluminescent devices disclosed in WO 99/39373 uses patterning of the components, but does not operate on a completed device, hence requiring breaking of the vacuum to pattern, followed by completion of the device under vacuum or inert atmosphere. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved method of forming a doped emissive layer which overcomes difficulties associated with prior art methods. 
     Accordingly, it is an object of the present invention to provide a method of making an electroluminescent device having a substrate, an anode provided over the substrate, and at least one dopant receiving layer which when doped provides an emissive layer disposed over the anode and a cathode disposed over the emissive layer, and diffusing the dopant form the dopant layer into the dopant receiving layer. 
     These objects are achieved in a method of making an electroluminescent device having a substrate, and at least one dopant receiving layer containing a host material which when doped provides an emissive layer, comprising the steps of: 
     (a) providing at least one dopant layer having a dopant disposed over or under the dopant receiving layer; 
     (b) providing an anode and a cathode so that the dopant receiving layer and the dopant layer are disposed between such anode and cathode; and 
     (c) heating the electroluminescent device to cause the dopant to diffuse from the dopant layer into the dopant receiving layer and forming the emissive layer having uniformly dispersed dopant in the host material. 
     An important feature of the invention is that the dopant layer can be patterned prior to diffusing into the dopant receiver layer, thus producing a patterned, multi-color emissive device. 
     Another feature of the invention is that it separates the deposition of the organic dopant receiver material forming the emissive layer from the deposition of an organic dopant material, and introduces the dopant material into the host material after completion of the processing of the device. 
     By separating the depositing step of an organic light-emitting layer from the depositing step of a fluorescent dopant layer, each of these layers can be formed separately to a desired thickness, thereby obviating the problems of deposition rate control associated with the prior art method of forming a doped light-emitting layer. Thus, the deposition processes are greatly simplified and require equipment of reduced complexity. 
     The light-emitting layer of the organic light-emitting host material can be formed by conventional vapor deposition (evaporation, sublimation) and, alternatively, by other coating of a polymeric organic light-emitting material. 
     The dopant layer can be formed by conventional vapor deposition and, alternatively, by thermally induced transfer from a dopant donor layer formed on a donor support or other printing methods such as ink jet, gravure, offset, screen, flexographic, or xerographic printing. 
     Two or more dopant layers containing different dopants can be formed in a particular pattern, thereby providing a broader range of options for modifying the light emitted from the light-emitting layer. 
     Diffusion of a dopant or of dopants from a dopant layer or from dopant layers into an organic light-emitting layer by a heat treatment process of the completed device requires relatively simple equipment and provides accurate control of processing to achieve uniform dispersion of the dopant or dopants throughout the organic light-emitting layer of the host material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG  1 A depicts an untreated (by heat) device with the dopant layer placed in the dopant receiving layer; 
     FIG. 1B shows a device of FIG. 1A after heating where the dopant is uniformly dispersed throughout an emissive layer, resulting in light emission from the device which is modified by the dopant diffusion; 
     FIG. 2A depicts another preferred embodiment of the present invention having a hole transport layer and an electron transport layer which is unheated or untreated with the dopant layer placed in the dopant receiving layer; 
     FIG. 2B shows the treated device of FIG. 2A wherein dopant is uniformly dispersed throughout an emissive layer, resulting in light emission from the device which is modified by the dopant diffusion; 
     FIGS. 3A and 3B show the sequence of process steps for making an organic light-emitting device to produce a organic light emitting device, where two additional layers are included in the device, namely a hole-transport layer and an electron transport layer and a different initial location for the dopant layer; 
     FIGS. 4A and 4B show the sequence of process steps for making an organic light-emitting device to produce a organic light emitting device, where two additional layers are included in the device, namely a hole-transport layer and an electron transport layer and a still different initial location for the dopant layer; and 
     FIGS. 5A and 5B are similar to FIGS. 3A and 3B except they show a patterned dopant layer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description of the various methods and process sequences used for making an organic light-emitting device in accordance with aspects of the invention, corresponding elements have been designated with corresponding numerals. Exceptions to this correspondence will be discussed in detail as they arise. 
     Turning now to FIGS. 1A and 1B, there are shown devices which illustrate processing steps for making an organic light-emitting device in accordance with the present invention. 
     In FIG. 1A, an organic light-emitting device  100  shows, in sequence, a substrate  102 , an anode  104 , a dopant receiving layer  108  formed over the anode, and a cathode  106  provided over the light-emitting structure. Either, or both, the cathode or the anode can be transparent. Within the dopant receiving layer  108 , is placed a dopant layer  110 . The direction of diffusion of the dopant layer  110  upon heating, is shown by arrow  140 . 
     FIG. 1B schematically illustrates an organic light emitting structure  150  which is formed after heating the organic light-emitting device  100 . Here, the dopant layer  110  has diffused into the dopant receiving layer  108  to form an emissive layer  112 . As above, the anode  104 , is on the support  102 , and a cathode  106  is provided as shown. 
     Heating can be performed by a variety of methods including, but not limited to heating on a hot plate, oven, infrared lamp, flash lamp, and laser. The temperature range is 50° C. to 250° C. with an optimal temperature near the glass transition point tg, of the materials. 
     Turning now to FIGS. 2A and 2B, there are shown devices which illustrate processing steps which, taken together, illustrate another aspect of the present invention for making an organic light-emitting device with additional layers. 
     FIG. 2A shows the untreated organic light-emitting device  200  which differs from the untreated organic light emitting device  100  of FIG. 1A in that it now contains a hole transport layer  216  and an electron transport layer  214  over the anode  204  and substrate  202 . As before, the dopant layer  210  is placed in the dopant receiving layer  208 . A cathode,  206  is provided as before. Upon heating the dopant layer  210  diffuses as shown by arrows  240 . 
     FIG. 2B shows the resultant organic light-emitting device  250  with the anode  204 , the hole-transporting layer  216 , the electron-transporting layer  214 , and the cathode  206  over the substrate  202 , and the formation of the emissive layer  212 . 
     Turning now to FIGS. 3A and 3B, there are shown devices which illustrate processing steps which, taken together, illustrate yet another aspect of the present invention for making an organic light-emitting device with additional layers 
     FIG. 3A shows the untreated organic light-emitting device  300  which differs from the untreated organic light emitting device  200  of FIG. 2A in that now the dopant layer  310  is placed on the anode  304  and over the substrate  302 . As before, the hole transporting layer  316 , the dopant receiving layer  308 , the electron transporting layer  314  and a cathode  306  are provided. Upon heating the dopant layer  310  diffuses as shown by arrows  340 . 
     FIG. 3B shows the resultant organic light-emitting device  350  with the anode  304 , the hole-transporting layer  316 , the electron-transporting layer  314 , and the cathode  306  over the substrate  302 , and the formation of the emissive layer  312 . 
     Turning now to FIGS. 4A and 4B, there are shown devices which illustrate processing steps which, taken together, illustrate yet another aspect of the present invention for making an organic light-emitting device with additional layers. 
     FIG. 4A shows the untreated organic light-emitting device  400  which differs from the device of FIG. 3A in that the dopant layer  410  is now placed on the electron transport layer  410  and over the substrate  402 . As before, the hole transporting layer  416 , the dopant receiving layer  408 , the electron transporting layer  414 .and a cathode  406  are provided. Upon heating the dopant layer  410  diffuses as shown by arrows  440 . 
     FIG. 4B shows the resultant organic light-emitting device  450  with the anode  404 , the hole-transporting layer  416 , the electron-transporting layer  414 , and the cathode  406  over the substrate  402 , and the formation of the emissive layer  412 . 
     Turning now to FIGS. 5A and 5B which are similar to FIGS. 3A and 3B, where parts correspond the same reference numerals will be used. It will be noted that in FIGS. 5A and 5B the dopant layer  310  is deposited and patterned by conventional techniques such as vacuum deposition through an aperture mask, ink jet printing, thermal dye diffusion printing, offset printing, wax transfer printing or other printing techniques to yield patterned dopant in different areas  310   a ,  301   b , and  310   c , as shown. The anode  304  and cathode  306  are provided as shown and the dopant receiving layer  308  and the dopant layer  310  are disposed between such anode  304  and cathode  306 . This structure is now heated at a temperature and time sufficient to cause dopant to diffuse from the dopant layer  310  into the dopant receiving layer  308  to form the emissive layer  312  so that when an electrical potential is applied between the anode  304  and cathode  306  light will be emitted from the emissive layer  312  yielding patterned colored emissive layers  312   a ,  312   b ,  312   c , as shown. 
     EXAMPLES 
     The following examples are presented for a further understanding of the invention. For purposes of clarity, the material and the layers formed therefrom will be abbreviated as given below. 
     ITO: indium tin oxide (anode) 
     NPB: 4,4′-bis-[N-(1-naphthyl)-N-phenylamino]-bi-phenyl (hole-transporting layer) 
     Alq: tris(8-quanolinato-N1,08)-aluminum (light-emitting layer; electron-transporting layer) 
     MgAg: magnesium:silver at a ratio of 10:1 by volume (cathode) 
     DCJTB: 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramthyljulolidyl-9-enyl)-4H-pyran (dopant) 
     Rubrene 9,10,11,12-tetraphenylnapthacene 
     Example 1 
     A device, constructed for measurement of electrophotoluminescence, was constructed as follows: 
     a) a light-transmissive anode of ITO-coated glass was washed with detergent solution then high-pressure deionized water, and dried with a nitrogen stream and a heat lamp; 
     b) cleaned under an oxygen plasma for 30 seconds at a 40% flow, 300 watts, then treated in a CHF 3  plasma for 10 seconds at a 40% flow, 100 watts, both at a pressure less than 40 mTorr; 
     c) a 75 nm thick NPB layer was deposited over the ITO anode by conventional vacuum evaporation; 
     d) a 75 nm thick Alq was formed over the NPB layer by conventional vacuum evaporation; and 
     e) a 400 nm thick MgAg layer was coated over the Alq dopant layer by conventional vacuum evaporation. This device is abbreviated as: 
     ITO/NPB(75)/Alq(75)/MgAg 
     Example 2 
     A device, designated for measurement of photoluminescence, was constructed was constructed in the manner of Example 1, but with an additional dopant layer as follows: 
     a) a light-transmissive anode of ITO-coated glass was washed with detergent solution then high-pressure deionized water, and dried with a nitrogen stream and a heat lamp; 
     b) cleaned under an oxygen plasma for 30 seconds at a 40% flow, 300 watts, then treated in a CHF 3  plasma for 10 seconds at a 40% flow, 100 watts, both at a pressure less than 40 mTorr; 
     c) a 75 nm thick NPB layer was deposited over the ITO anode by conventional vacuum evaporation; 
     d) 4 nm of Rubrene, 5.3%, was co-evaporated with a 75 nm thick Alq over the NPB layer by conventional vacuum evaporation; and 
     e) a 400 nm thick MgAg layer was coated over the Alq dopant layer by conventional vacuum evaporation. This device is abbreviated as: 
     ITO/NPB(75)/Rubrene(4):Alq(75)/MgAg 
     Example 3 
     Another organic light-emitting device was constructed in the following manner: 
     a) a light-transmissive anode of ITO-coated glass was washed with detergent solution then high-pressure deionized water, and dried with a nitrogen stream and a heat lamp; 
     b) cleaned under an oxygen plasma for 30 seconds at a 40% flow, 300 watts, then treated in a CHF 3  plasma for 10 seconds at a 40% flow, 100 watts, both at a pressure less than 40 mTorr; 
     c) a 4 nm thick Rubrene layer was deposited over the ITO anode by conventional vacuum evaporation; 
     d) a 75 nm thick NPB layer was deposited over the Rubrene layer by conventional vacuum evaporation; and 
     e) a 400 nm thick MgAg layer was coated over the Alq dopant layer by conventional vacuum evaporation. This device is abbreviated as: 
     ITO/Rubrene(4)/NPB(75)/Alq(75)/MgAg 
     Example 4 
     A organic light-emitting device was constructed in the same manner as Example 3. 
     Example 5 
     Still another organic light-emitting device was constructed in the following manner: 
     a) a light-transmissive anode of ITO-coated glass was washed with detergent solution then high-pressure deionized water, and dried with a nitrogen stream and a heat lamp; 
     b) cleaned under an oxygen plasma for 30 seconds at a 40% flow, 300 watts, then treated in a CHF 3  plasma for 10 seconds at a 40% flow, 100 watts, both at a pressure less than 40 mTorr; 
     c) a 75 nm thick NPB layer was deposited over the ITO anode by conventional vacuum evaporation; 
     d) a 4 nm thick Rubrene layer was deposited over the NPB layer by conventional vacuum evaporation; and 
     e) a 400 nm thick MgAg layer was coated over the Alq dopant layer by conventional vacuum evaporation. This device is abbreviated as: 
     ITO//NPB(75)/Rubrene(4)/Alq(75)/MgAg 
     Example 6 
     A final organic light-emitting device was constructed in the following manner: 
     a) a light-transmissive anode of ITO-coated glass was washed with detergent solution then high-pressure deionized water, and dried with a nitrogen stream and a heat lamp; 
     b) cleaned under an oxygen plasma for 30 seconds at a 40% flow, 300 watts, then treated in a CHF 3  plasma for 10 seconds at a 40% flow, 100 watts, both at a pressure less than 40 mTorr; 
     c) a 4 nm thick Rubrene layer was deposited over the ITO anode by conventional vacuum evaporation 
     d) a 1 nm thick DCJTB layer was deposited over the Rubrene layer by conventional vacuum evaporation 
     e) a 75 nm thick NPB layer was deposited over the DCJTB layer by conventional vacuum evaporation; and 
     f) a 400 nm thick MgAg layer was coated over the Alq dopant layer by conventional vacuum evaporation. This device is abbreviated as: 
     ITO/Rubrene(4)/DCJTB(1)/NPB(75)/Alq(75)/MgAg 
     The spectral radiance of the emitted light of each of the above examples were measured under a current density of 20 mA/cm2 with equipment available from Photo Research Laboratory (Model PR650) before and after heat treatment. The results are shown in the table below. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Before Treatment 
                 After Treatment 
               
             
          
           
               
                   
                   
                   
                 Wave- 
                   
                 Wave- 
                   
               
               
                   
                   
                   
                 length 
                   
                 length 
                   
               
               
                   
                 Heating 
                   
                 of 
                 Effi- 
                 of 
                 Effi- 
               
               
                   
                 Temper- 
                 Du- 
                 Maximum 
                 ciency 
                 Maximum 
                 ciency 
               
               
                 Ex- 
                 ature 
                 ration 
                 Emission 
                 (Watts/ 
                 Emission 
                 (Watts/ 
               
               
                 ample 
                 (° C.) 
                 (Hours) 
                 (nm) 
                 Amp) 
                 (nm) 
                 Amp) 
               
               
                   
               
             
          
           
               
                 1. 
                 104 
                 3.5 
                 540 
                 0.017 
                 540 
                 0.022 
               
               
                 2. 
                 105 
                 3.5 
                 568 
                 0.040 
                 568 
                 0.048 
               
               
                 3. 
                 104 
                 3.5 
                 540 
                 0.018 
                 564 
                 0.037 
               
               
                 4. 
                 90 
                 3.5 
                 540 
                 0.022 
                 540 
                 0.021 
               
               
                   
                   
                 Addi- 
                   
                   
                 540 
                 0.023 
               
               
                   
                   
                 tional 
               
               
                   
                   
                 32 
               
               
                   
                   
                 hours 
               
               
                 5. 
                 105 
                 3.5 
                 564 
                 0.008 
                 564 
                 0.025 
               
               
                 6. 
                 105 
                 5.5 
                   
                   
                 540 
                 0.023 
               
               
                   
               
             
          
         
       
     
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 100 
                 organic light-emitting device 
               
               
                   
                 102 
                 substrate 
               
               
                   
                 104 
                 anode 
               
               
                   
                 106 
                 cathode 
               
               
                   
                 108 
                 dopant receiving layer 
               
               
                   
                 110 
                 dopant layer 
               
               
                   
                 112 
                 emissive layer 
               
               
                   
                 140 
                 direction of diffusion of the dopant 
               
               
                   
                 150 
                 organic light-emitting device after heat treatment 
               
               
                   
                 200 
                 organic light-emitting device 
               
               
                   
                 202 
                 substrate 
               
               
                   
                 204 
                 anode 
               
               
                   
                 206 
                 cathode 
               
               
                   
                 208 
                 dopant receiving layer 
               
               
                   
                 210 
                 dopant layer 
               
               
                   
                 212 
                 emissive layer 
               
               
                   
                 214 
                 electron-transporting layer 
               
               
                   
                 216 
                 hole-transporting layer 
               
               
                   
                 240 
                 direction of diffusion of the dopant 
               
               
                   
                 250 
                 organic light-emitting device after heat treatment 
               
               
                   
                 300 
                 organic light-emitting device 
               
               
                   
                 302 
                 substrate 
               
               
                   
                 304 
                 anode 
               
               
                   
                 306 
                 cathode 
               
               
                   
                 308 
                 dopant receiving layer 
               
               
                   
                 310 
                 dopant layer 
               
               
                   
                 310a 
                 patterned dopant 
               
               
                   
                 310b 
                 patterned dopant 
               
               
                   
                 310c 
                 patterned dopant 
               
               
                   
                 312 
                 emissive layer 
               
               
                   
                 312a 
                 patterned colored emissive layer 
               
               
                   
                 312b 
                 patterned colored emissive layer 
               
               
                   
                 312c 
                 patterned colored emissive layer 
               
               
                   
                 314 
                 electron-transporting layer 
               
               
                   
                 316 
                 hole-transporting layer 
               
               
                   
                 340 
                 direction of diffusion of the dopant 
               
               
                   
                 350 
                 organic light-emitting device after heat treatment 
               
               
                   
                 400 
                 organic light-emitting device 
               
               
                   
                 402 
                 substrate 
               
               
                   
                 404 
                 anode 
               
               
                   
                 406 
                 cathode 
               
               
                   
                 408 
                 dopant receiving layer 
               
               
                   
                 410 
                 dopant layer 
               
               
                   
                 412 
                 emissive layer 
               
               
                   
                 414 
                 electron-transporting layer 
               
               
                   
                 416 
                 hole-transporting layer 
               
               
                   
                 440 
                 direction of diffusion of the dopant 
               
               
                   
                 450 
                 organic light-emitting device after heat treatment