Abstract:
Systems for displaying images and fabrication method thereof are provided. A representative system incorporates an electroluminescent device that includes a substrate, an anode formed on the substrate, a plurality of electroluminescent layers formed on the anode, an electron injection layer formed on the electroluminescent layers, and a cathode formed directly on the electron injection layer. Particularly, the electron injection layer can be a lanthanide-containing layer or actinide-containing layer.

Description:
BACKGROUND  
       [0001]     The present invention relates to an electroluminescent device and a method for fabricating the same and, more particularly, to an electroluminescent device having an improved efficiency in injecting electrons from a cathode to electroluminescent layers and fabrication method thereof.  
         [0002]     Recently, with the development and wide application of electronic products such as mobile phones, personal digital assistants, and notebook computers, there has been an increased demand for flat display devices which consume less power and occupy less space. Organic electroluminescent devices are self-emitting and highly luminous, have a wider viewing angle, faster response, and a simple fabrication process, making them an industry display of choice.  
         [0003]     As shown in  FIG. 1 , an organic electroluminescent device  10  is basically configured such that an anode  14  is formed on a substrate  12 , and a hole transport layer  16 , an emitter layer  18 , an electron transport layer  20 , and a cathode  22  are sequentially stacked on the anode  14 . Here, the hole transport layer  16 , the emission layer  18  and the electron transport layer  20  are organic layers made of organic materials.  
         [0004]     In organic electroluminescence, electrons are propelled from the cathode and holes from the anode, and the applied electric field induces a potential difference, such that the electrons and holes move and centralize in the emission layer via the electron or hole transport layer respectively, resulting in luminescence through recombination thereof. The recombination takes place within the emission layer at a region near the interface between the emission layer and the hole transport layer (or the electron transport layer) to generate excitons. The generated excitons de-excite from an excited state to a ground state to emit light, thus forming an image.  
         [0005]     In order to improve a low driving voltage characteristic and charge balance between electrons and holes, it is necessary to increase an efficiency in injecting electrons from the cathode into the electron transport layer. Conventional methods for increasing such injection efficiency have been proposed in U.S. Pat. Nos. 5,429,884, 5,059,862 and 4,885,211, describing use of an alkali metal having a low work function, e.g., lithium or magnesium, codeposition of an alkali metal and a metal such as aluminum or silver, and use of alloys of an alkali metal and a metal such as aluminum or silver, respectively. Since the metal that has a low work function is very unstable and highly reactive, use of the metal, however, is disadvantageous in view of the processability and the stability of EL device.  
         [0006]     Other techniques for increasing the electron injection efficiency have been proposed in U.S. Pat. Nos. 5,776,622, 5,776,623, 5,937,272 and 5,739,635, and Appl. Phy Lett. 73 (1998) P. 1185, in which an electron injection layer containing inorganic materials such as LiF, CsF, SrO or Li 2 O, is formed between the cathode and the electron transport layer with a thickness of 5˜20 Å.  
         [0007]     Recently, another method for increasing electron injection efficiency has been proposed in which a metal alkylate or metal arylate, such as CH 3 COOLi or C 6 H 5 COOLi, is formed between the cathode and the electron transport layer. This method is also problematic in that it is difficult to form a thin film having a uniform thickness of 5˜40 Å, which is not suitable for large-area deposition.  
         [0008]     Thus, in order to enhance luminescent efficiency, an active matrix organic electroluminescent device having improved efficiency in injecting electrons from a cathode to electroluminescent layers is called for.  
       SUMMARY  
       [0009]     Systems for displaying images are provided. In this regard, an exemplary embodiment of such as system comprises an organic electroluminescent device, comprising a substrate, an anode formed on the substrate, a plurality of electroluminescent layers formed on the anode, an electron injection layer formed on the electroluminescent layers, and a cathode formed directly on the electron injection layer. The electron injection layer can be a lanthanide-containing layer or actinide-containing layer.  
         [0010]     Methods for fabricating the system for displaying images are also provided, in which a substrate is provided. An anode, electroluminescent layers, an electron injection layer, and a cathode are sequentially formed on the substrate, wherein the electron injection layer comprises a lanthanide-containing layer or actinide-containing layer. The electron injection layer is directly formed on the cathode.  
         [0011]     A detailed description is given in the following with reference to the accompanying drawing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The invention can be more fully understood by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein:  
         [0013]      FIG. 1  shows a cross section of a conventional electroluminescent device.  
         [0014]      FIG. 2  shows a cross section of an electroluminescent device according to embodiments of the invention.  
         [0015]      FIG. 3  shows a graph plotting operating voltage against current density of the electroluminescent device ( 1 ) as disclosed in Example 1.  
         [0016]      FIG. 4  shows a graph plotting operating voltage against brightness of the electroluminescent device ( 1 ) as disclosed in Example 1.  
         [0017]      FIG. 5  shows a graph plotting operating voltage against current density of the electroluminescent devices as disclosed in Examples 2˜4.  
         [0018]      FIG. 6  shows a graph plotting operating voltage against brightness of the electroluminescent devices as disclosed in Examples 2˜4.  
         [0019]      FIG. 7  shows a graph plotting operating voltage against efficiency of the electroluminescent devices as disclosed in Examples 2˜4.  
         [0020]      FIG. 8  schematically shows another embodiment of a system for displaying images  
     
    
     DETAILED DESCRIPTION  
       [0021]     The invention uses an electron injection layer to facilitate injection of electrons into electroluminescent layers from a cathode.  
         [0022]      FIG. 2  shows an embodiment of a system for displaying images that includes an electroluminescent device  100 . According to one embodiment, the electroluminescent device  100  comprises a substrate  110 , an anode  120 , electroluminescent layers  130 , an electron injection layer  140 , and a cathode  150 , as shown in  FIG. 2 . The substrate  110  can be glass or plastic. Suitable material for the anode  120  is transparent metal or metal oxide, such as indium tin oxide (ITO), indium zinc oxide (IZO), aluminum zinc oxide (AZO), or zinc oxide (ZnO), formed by sputtering, electron beam evaporation, thermal evaporation, or chemical vapor deposition.  
         [0023]     The electroluminescent layers  130  may comprise a hole injection layer  131 , a hole transport layer  132 , an emission layer  133 , and an electron transport layer  134 , including organic semiconductor materials, such as small molecule materials, polymer, or organometallic complex, formed by thermal vacuum evaporation, spin coating, dip coating, roll-coating, injection-filling, embossing, stamping, physical vapor deposition, or chemical vapor deposition. The thickness of each layer is not particularly limited, but if too thick, a large applied voltage is required to obtain a fixed light output, thus reducing efficiency. On the other hand, if it is too thin, pin-holes are generated. The thickness of each of the layers  131 ,  132 ,  133 , and  134  is preferably of 1 nm to 1 μm.  
         [0024]     Particularly, the electron injection layer  140  comprises a lanthanide-containing layer or actinide-containing layer, formed between the electroluminescent layers  130  and the cathode  150 , 0.1˜5 nm thick, preferably 0.1˜1 nm thick. The actinide-containing layer may comprise actinide fluoride, actinide chloride, actinide bromide, actinide oxide, actinide nitride, actinide sulfide, actinide carbonate, or combinations thereof, and the lanthanide-containing layer may comprise lanthanide fluoride, lanthanide chloride, lanthanide bromide, lanthanide oxide, lanthanide nitride, lanthanide sulfide, lanthanide carbonate, or combinations thereof. Wherein, the lanthanide or actinide element may be selected from the group of elements consisting of Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and U. For example, in embodiments of the invention, the electron injection layer  140  can be cerium halide (such as CeF 3  or CeF 4 ), cerium nitride, cerium oxide, cerium sulfide, cerium oxyfluoride, cerium carbonate, or combinations thereof.  
         [0025]     The cathode  150  can be capable of injecting electrons into the electroluminescent layer  130  via the electron injection layer  140 , for example, a low work function material such as Ca, Ag, Mg, Al, Li, or alloys thereof, formed by sputtering, electron beam evaporation, thermal evaporation, or chemical vapor deposition.  
         [0026]     The following examples are intended to illustrate the invention more fully without limiting their scope, since numerous modifications and variations will be apparent to those skilled in this art.  
       EXAMPLE 1  
       [0027]     A glass substrate with an indium tin oxide (ITO) film of 100 nm was provided and then washed by a cleaning agent, acetone, and isopropanol with ultrasonic agitation. After drying with nitrogen flow, the ITO film was subjected to uv/ozone treatment. Next, a hole transport layer, light-emitting layer, electron transport layer, electron injection layer, and aluminum electrode were subsequently formed on the ITO film at 10 −5  Pa, obtaining the electroluminescent device ( 1 ). For purposes of clarity, the materials and layers formed therefrom are described in the following.  
         [0028]     The hole transport layer, with a thickness of 150 nm, consisted of NPB (N,N′-di-1-naphthyl-N,N′-diphenyl-1,1′-biphenyl-1,1′-biphenyl-4,4′-diamine). The light-emitting layer  18 , with a thickness of 40 nm, consisted of C545T (10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-11-one) as dopant, and Alq 3  (tris(8-hydroxyquinoline)aluminum) as light-emitting material host, wherein the weight ratio between Alq 3  and dopant was 100:1. The electron transport layer, with a thickness of 10 nm, consisted of BeBq2 (bis(10-hydroxybenzo[h]quinolinato)beryllium). The electron injection layer, with a thickness of 1 nm, consisted of cerium fluoride (CeF 4 ).  
         [0029]     The emissive structure of the electroluminescent device. ( 1 ) can be represented as below:  
         [0030]     ITO 100 nm/NPB 150 nm/Alq 3 :C545T 100:1 40 nm/BeBq 2  30 nm/CeF 4  10 Å/Al 150 nm  
         [0031]     The optical property of electroluminescent device ( 1 ), as described in Example 1, was measured by PR650 (purchased from Photo Research Inc.) and Minolta TS110.  FIG. 3  illustrates a graph plotting operating voltage against current density of the electroluminescent device ( 1 ), and  FIG. 4 a  graph plotting operating voltage against brightness.  
       EXAMPLE 2  
       [0032]     A glass substrate with an indium tin oxide (ITO) film of 100 nm was provided and then washed by a cleaning agent, acetone, and ethanol with ultrasonic agitation. After drying with nitrogen flow, the ITO film was subjected to uv/ozone treatment. Next, a hole injection layer, hole transport layer, light-emitting layer, electron transport layer, electron injection layer, and aluminum electrode were subsequently formed on the ITO film at 10 −5  Pa, obtaining the electroluminescent device ( 2 ). For purposes of clarity, the materials and layers formed therefrom are described in the following.  
         [0033]     The hole injection layer, with a thickness of 5 nm, consisted of LGC101 (purchased by LG Chem, Ltd.). The hole transport layer, with a thickness of 150 nm, consisted of NPB (N,N′-di-1-naphthyl-N,N′-diphenyl-1,1′-biphenyl-1,1′-biphenyl-4,4′-diamine). The light-emitting layer  133 , with a thickness of 40 nm, consisted of C545T (10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-11-one) as dopant, and Alq 3  (tris(8-hydroxyquinoline)aluminum) as light-emitting material host, wherein the weight ratio between Alq 3  and dopant was 100:1. The electron transport layer, with a thickness of 10 nm, consisted of BeBq2 (bis(10-hydroxybenzo[h]quinolinato)beryllium). The electron injection layer, with a thickness of 0.3 nm, consisted of cerium fluoride (CeF 4 ).  
         [0034]     The emissive structure of the electroluminescent device ( 2 ) can be represented as below:  
         [0035]     ITO 100 nm/LG101 5 nm/NPB 150 nm/Alq 3 :C545T 100:1 40 nm/BeBq 2  30 nm/CeF 4  3 Å/Al 150 nm  
       EXAMPLES 3˜4  
       [0036]     Examples 3 and 4 were performed as Example 2 except that the thickness of the cerium flouride was increased to 0.5 nm and 1 nm, respectively.  
         [0037]     FIGS.  5 ˜ 7  also illustrate the differences between properties for the electroluminescent devices as described respectively in Examples 2˜4. In FIGS.  5 ˜ 7 , the electroluminescent device, with a 10 Å thick cerium fluoride layer, disclosed in Example 4 has lower operating voltages and higher efficiency.  
         [0038]      FIG. 8  schematically shows an embodiment of a system for displaying images which, in this case, is implemented as a display device  160  or an electronic device  200 . The described organic electroluminescent device  100  can be incorporated into a display panel that can be an OLED panel. As shown in  FIG. 8 , the display panel  160  comprises an electroluminescent device, such as the electroluminescent device  100  shown in  FIG. 2 . The display panel  160  can form a portion of a variety of electronic devices. Generally, the system for displaying images, such as electronic device  200 , can comprise the display panel  160  and an input unit  180 . Further, the input unit  180  is operatively coupled to the display panel  160  and provides input signals (e.g., an image signal) to the display panel  160  to generate images. The electronic device  200  can be a mobile phone, digital camera, personal digital assistant, notebook computer, desktop computer, television, car display, or portable DVD player, for example.  
         [0039]     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. It is therefore intended that the following claims be interpreted as covering all such alteration and modifications as fall within the true spirit and scope of the invention.