Patent Publication Number: US-2007103067-A1

Title: Organic electroluminescence device and electron transporting layer

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to a luminescence device, and more particularly to, an organic electroluminescence device and an electron transportation layer thereof.  
      2. Description of Related Art  
      Displays are communication interfaces between human and information processors, and have current trends in planar displays. Among displays, an organic electroluminescence display (OLED) is believed to be a main stream of the next generation planar displays because it has advantages of self-luminescence, free-viewing angle, low power consumption, easy fabrication, low cost, low operating temperature range, high response speed and full-colorized display.  
      The OLED mainly utilizes a self-luminescence feature of the organic electroluminescence device to achieve a display effect. In addition, the organic electroluminescence device is comprised of a pair electrodes and an organic material layer. When a current passes through an anode and a cathode, electrons and holes in the organic material layer combine to form excitons that permit the organic material layer to emit light with different colour in accordance with characteristics of the organic material, thereby allowing the OLED to achieve the display effect.  
       FIG. 1  schematically shows a conventional organic electroluminescence device structure. As shown in  FIG. 1 , the conventional organic electroluminescence device  100  comprises a substrate  110 , an anode  120 , a hole transportation layer  130 , a luminescence layer  140 , an electron transportation layer  150  and a cathode  160 . Electrons are injected into the electron transportation layer  150  from the anode  120 , and then transported to the hole transportation layer  130 , when a bias voltage is applied to the anode  120  and the cathode  160 . On the other hand, the holes are injected into the hole transportation layer  130 , and then transported to the luminescence layer  140 . In the meantime, the recombination phenomena of the electrons and holes occurs in the luminescence layer  140 , which further produces excitons for emitting light.  
      Among the convention technologies, material of the electron transportation layer  150  is usually Alq3; however, since an electron mobility in Alq3 is smaller than a hole mobility in the hole transportation layer, there exists an carrier transportation non-equilibrium problem in the conventional organic electroluminescence device  100 , and this problem in turn affects the organic electroluminescence device  100 &#39;s light-emitting efficiency.  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention is directed to provide an organic electroluminescence device that has a higher light-emitting efficiency.  
      The present invention is further directed to provide a electron transportation layer so as to promote an organic electroluminescence device  100 &#39;s light-emitting efficiency.  
      Based on the above and other objectives, an organic electroluminescence device of the present invention comprises a substrate, a first electrode layer, a hole transportation layer, a luminescence layer, an electron transportation layer and a second electrode layer. In addition, on the substrate is disposed the first electrode layer on which the hole transportation layer is disposed. In addition, on the hole transportation is disposed the layer luminescence layer. Moreover, on the luminescence layer is disposed the electron transportation layer on which the second electrode layer is further disposed. Moreover, the electron transportation layer comprises n+1 first sub-transportation layers and n second sub-transportation layers, wherein n is an integer. The n+1 first sub-transportation layers are stacked on the luminescence layer, each of the n second sub-transportation layers is disposed between every two neighbor first sub-transportation layers and a band-gap of the first sub-transportation layer is different from that of second sub-transportation layer.  
      The organic electroluminescence device further comprises a hole-injected layer that is disposed between the first electrode layer and the hole transportation layer.  
      The organic electroluminescence device further comprises an electron-injected layer that is disposed between the second electrode layer and the electron transportation layer.  
      The present invention further provides an electron transportation layer that comprises n+1 first sub-transportation layers and n second sub-transportation layers, wherein n is an integer. The n+1 first sub-transportation layers are stacked one another, and each of the n second sub-transportation layers is disposed between every two neighbor first sub-transportation layers and a band-gap of the first sub-transportation layer is different from that of second sub-transportation layer.  
      In the organic electroluminescence device and the electron transportation layer, a band-gap of the first sub-transportation layer is larger than that of the second sub-transportation layer.  
      In the organic electroluminescence device and the electron transportation layer, a band-gap of the first sub-transportation layer is smaller than that of the second sub-transportation layer.  
      In the organic electroluminescence device and the electron transportation layer, each of the first sub-transportation layers and each of the second sub-transportation layers have thicknesses, for example, between 10˜200 Angstrom.  
      In the organic electroluminescence device and the electron transportation layer, each of the first sub-transportation layers and each of the second sub-transportation layers have thicknesses, for example, between 20˜100 Angstrom.  
      In the organic electroluminescence device and the electron transportation layer, each of the first sub-transportation layers and each of the second sub-transportation layers have thicknesses, for example, between 10˜20 Angstrom.  
      In the organic electroluminescence device and the electron transportation layer, material of the first sub-transportation layers and the second sub-transportation layers, for example, is selected from one of the four compound, each of which has its chemical formulation: 
 
 Chemical Formulation (1):  
                 
 
 Chemical Formulation (2):  
                 
 
 Chemical Formulation (3):  
                 
 
 Chemical Formulation (4):  
                 
 
      Based on the above description, as the electron transportation layer of the present invention is constituted by a stacked super-lattice structure of the first sub-transportation layers and the second sub-transportation layers, the electron mobility of the electron transportation layer can be promoted and thus ameliorates the carrier transportation non-equilibrium problem occurred in the conventional organic electroluminescence device.  
      The objectives, other features and advantages of the invention will become more apparent and easily understood from the following detailed description of the invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
       FIG. 1  schematically shows a conventional organic electroluminescence device structure.  
       FIG. 2  schematically shows an organic electroluminescence device structure of one embodiment of the present invention.  
       FIG. 3  schematically shows an organic electroluminescence device structure of another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.  
       FIG. 2  schematically shows an organic electroluminescence device structure of one embodiment of the present invention. Referring to  FIG. 2 , an organic electroluminescence device  200  of this embodiment comprises a substrate  210 , a first electrode layer  220 , a hole transportation layer  230 , a luminescence layer  240 , an electron transportation layer  250  and a second electrode layer  260 . In addition, on the substrate  210  is disposed the first electrode layer  220  on which the hole transportation layer  230  is disposed. Moreover, on the hole transportation layer  230  is disposed the luminescence layer  240  on which the electron transportation layer  250  is further disposed, and eventually, the second electrode layer  260  is disposed on the electron transportation layer  250 . Moreover, the electron transportation layer  250  comprises n+1 first sub-transportation layers  252  and n second sub-transportation layers  254 , wherein n is an integer. The n+1 first sub-transportation layers  252  are stacked on the luminescence layer  240 , each of the n second sub-transportation layers  254  is disposed between every two neighbor first sub-transportation layers  252  and a band-gap of the first sub-transportation layer  252  is different from that of second sub-transportation layer  254 .  
      In the organic electroluminescence device  200 , the first electrode layer  220  may be, for example, an anode, and the second electrode layer  260  may be, for example, a cathode. The electrons are injected into the electron transportation layer  250  from the second electrode layer  260 , and then transported to the luminescence layer  240 , when a bias voltage is applied to the first electrode layer  220  and the second electrode layer  260 . On the other hand, the holes are injected into the hole transportation layer  230 , and then transported to the luminescence layer  240 . In the meantime, the recombination phenomena of the electrons and holes occurs in the luminescence layer  240 , which further produces excitons for emitting light.  
      In this embodiment, the electron transportation layer  250  has a super-lattice structure that is constituted by the first sub-transportation layer  252  and the second sub-transportation layer  254 , each of which has a highest occupied molecular orbital and the lowest unoccupied molecular orbital. In addition, a junction between the first sub-transportation layer  252  and the second sub-transportation layer  254 , forms a two-dimension quantum well, in which free electrons are generated and wanders around the junction. This electrons generated in the super-lattice structure, is called “two-dimension free electrons.” As the two-dimension free electrons seldom collide one another, their electron mobility is larger than a general electron&#39;s electron mobility.  
      As a result, the electron transportation layer  250  with the super-lattice structure is able to promote electron&#39;s mobility so that the electron mobility of the electron transportation layer  250  approaches, even equals, the hole mobility of the hole transportation layer  230 , thereby ameliorating the carrier transportation non-equilibrium problem and further promoting the organic electroluminescence device  200 &#39;s light-emitting efficiency. In addition, since the super-lattice structure has a feature of low resistance, a better ohmic contact between the second electrode layer  260  and the electron transportation layer  250 , can be formed so as to promote the organic electroluminescence device  200 &#39;s light-emitting efficiency and lower its operating voltage.  
      In one embodiment of the present invention, material of the first sub-transportation layer  252  and the second sub-transportation layer  254  are organic material, which, for example, is selected from one of the four compound, each of which has its chemical formulation: 
 
 Chemical Formulation (1):  
                 
 
 Chemical Formulation (2)  
                 
 
 Chemical Formulation (3):  
                 
 
 Chemical Formulation (4):  
                 
 
      For example, material of the first sub-transportation layer  252  and the second sub-transportation layer  254  are Alq3 with a smaller band-gap and JBEM with a larger band-gap, respectively, or are JBEM and Alq3, respectively. In other words, in this embodiment, the band-gap of the first sub-transportation layer  252  may be larger than that of the second sub-transportation layer  254 , or may be smaller than that of the second sub-transportation layer  254 .  
      In this embodiment, the first sub-transportation layer  252  may have, or have not the same thickness as the second sub-transportation layer  254 , depending on users&#39; need. In addition, each of the first sub-transportation layers  252  may have, or have not the same thickness as each of the second sub-transportation layers  254 , depending on the users&#39; need. Moreover, the first sub-transportation layers and the second sub-transportation layers have thicknesses about between 10-100 Angstrom or between 20-100 Angstrom, and preferably between 10˜20 Angstrom.  
       FIG. 3  schematically shows an organic electroluminescence device structure of another embodiment of the present invention. Referring to  FIG. 3 , it is similar to  FIG. 2  except that the organic electroluminescence device  200 ′ shown in  FIG. 3 , further comprises an electron-injected layer  270  and a hole-injected layer  280 . Moreover, the electron-injected layer  270  is disposed between the second electrode layer  260  and the eelectron transportation layer  250 , while the hole-injected layer  280  is disposed between the hole transportation layer  230  and the first electrode layer  220 , thereby promoting the light-emitting efficiency of the organic electroluminescence device  200 ′.  
      It is noticeable that one of the electron-injected layer  270  and the hole-injected layer  280 , can be chosen to be disposed inside the organic electroluminescence device  200 ′. In addition, although the aforementioned embodiment employs the electron transportation layer  250  comprised of the first sub-transportation layer  252  and the second sub-transportation layer  254 , the present invention is not limited to this embodiment. Actually, the embodiment may increase the number of the first sub-transportation layers  252  and the second sub-transportation layers  254 .  
      In conclusion, the organic electroluminescence device of the present invention at least has the following advantages: 
          1. As the electron transportation layer of the present invention has a higher electron&#39;s mobility because of the electron transportation layer being comprised of the first sub-transportation layer and the second sub-transportation layer with their thickness less than 10-200 Angstrom, and an overlapped energy gap between these two sub-transportation layers, the carrier transportation non-equilibrium occurred in the conventional technology, can be ameliorated, which further promotes the organic electroluminescence device&#39;s light-emitting efficiency.     2. As the electron transportation layer of the present invention has a feature of low resistance, a better ohmic contact between the second electrode layer and the electron transportation layer, can be formed, thereby promoting the organic electroluminescence device&#39;s light-emitting efficiency and lowering its operating voltage.        

      It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.