Abstract:
To provide an inexpensive EL display device of high definition. An anode, a light emitting layer, and a cathode are formed on a substrate, and selective doping using at least one selected from the group consisting of an alkali metal element, an alkaline earth metal element and a halogen element is then performed to form at least ones of electron transmitting regions and hole transmitting regions. In such a structure, only a part of the light emitting layer, where at least ones of the electron transmitting regions and the hole transmitting regions are formed, emits light when a given voltage is applied to the light emitting layer, whereby images are displayed as desired.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention involves techniques relating to a display device comprising an EL (electro luminescence) element (hereinafter referred to as an EL display device) and to a display that uses the EL display device.  
           [0003]    2. Description of the Related Art  
           [0004]    Studies of EL display devices comprising EL elements as self-light emitting elements have flourished in recent years. In particular, organic EL display devices using organic materials for EL materials have held the attention. The organic EL display devices are also called organic EL displays (OELDs) or organic Light Emitting Diodes (OLEDs).  
           [0005]    EL display devices are of self-light emitting type, unlike liquid crystal display devices, and therefore the view angle does not matter in the EL display devices, which forms one of their characteristics. That is, the EL display devices are more suitable for displays used outdoors than the liquid crystal display devices, presenting so many possible ways of their use.  
           [0006]    EL elements have the structure in which one EL layer is sandwiched between a pair of electrodes. An EL layer usually has a layer structure. As a typical example thereof, a layer structure consisting of hole transmitting layer/light emitting layer/electron transmitting layer which has proposed by Tang, et al. from Kodak Eastman company can be named. This is so highly efficient in terms of light emission that most of EL display devices whose research and development is now under way employ that structure.  
           [0007]    Light is emitted by applying a given voltage generated between the pair of electrodes to the EL layer having the above structure to cause re-combination of carriers in the light emitting layer. A method to achieve this is chosen from two options in which one is to form the EL layer between two kinds of stripe-like electrodes arranged perpendicular to each other (simple matrix method) and the other is to form the EL layer between pixel electrodes that are connected to TFTs and are arranged in matrix and opposite electrodes (active matrix method).  
           [0008]    Both methods requires to form the EL layer on the electrodes that have been patterned out. The EL layer, however, is easily influenced and degraded by changes such as level differences, so that various kinds of contrivance are made to improve the flatness. This leads into a problem of complication of manufacturing process and, accordingly, an increase in production cost.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention has been made in view of the problem above, and an object of the present invention is therefore to provide a method of manufacturing an EL display device of high definition by uncostly measures. Another object of the present invention is to provide an inexpensive electronic device having the EL display device as such.  
           [0010]    According to the present invention, a light emitting region is distinguished from a non-light emitting region by a totally novel method which has not been found in prior art. Specifically, the novel method is a technique characterized in that a light emitting layer is selectively doped with a specific impurity element to cause selective light emission from the region doped with the impurity element. In this specification, the term specific impurity element means an impurity element that is capable of making the light emitting layer function as a hole transmitting layer (or a hole injection layer) or an electron transmitting layer (or an electron injection layer) when used to dope the light emitting layer.  
           [0011]    The present invention includes three methods as follows: a first method is to dope into the vicinity of the interface between an anode and a light emitting layer with a specific impurity element, a second method is to dope into the vicinity of the interface between a cathode and a light emitting layer with a specific impurity element, and a third method is to dope into both the vicinity of the interface between the anode and the light emitting layer and the vicinity of the interface between the cathode and the light emitting layer with different specific impurity elements.  
           [0012]    The first method is characterized in that the vicinity of the interface between the anode and the light emitting layer is doped with a halogen element as a specific impurity element, typically, F (fluorine), Cl (chlorine), B (bromine) or I (iodine). The vicinity of the interface between the anode and the light emitting layer refers to an extent 100 nm (50 nm, typically) down the depth of the light emitting layer from the interface between the anode and the light emitting layer. No trouble is caused if the halogen element is contained in the anode.  
           [0013]    The second method is characterized in that the vicinity of the interface between the cathode and the light emitting layer is doped with, as a specific impurity element, an alkali metal element, typically, Li (lithium), Na (sodium), K (potassium) or Cs (cesium), or an alanine earth metal element, typically, Be (beryllium), Mg (magnesium), Ca (calcium) or Ba (barium). The vicinity of the interface between the cathode and the light emitting layer refers to an extent 100 nm (50 nm, typically) down the depth of the light emitting layer from the interface between the cathode and the light emitting layer. No trouble is caused if the alkali metal element or an alkaline earth metal element is contained in the cathode.  
           [0014]    The third method is characterized in that it is a combination of the first method and the second method, and in that the vicinity of the interface between the anode and the light emitting layer is doped with a halogen element as a specific impurity element, while the vicinity of the interface between the cathode and the light emitting layer is doped with, as a specific impurity element, an alkali metal element or an alkaline earth metal element.  
           [0015]    A known doping method suites the doping of the above specific impurity element. Ion doping that does not involve mass separation, ion implantation that involves mass separation, vapor phase doping that utilizes diffusion are preferable. Whichever method is used, it is whether the method allows selective doping of the above specific impurity element that matters.  
           [0016]    According to the present invention, the vicinity of the interface between the anode, or the cathode, and the light emitting layer is selectively doped with a specific impurity element, and only the portion doped with the impurity element emits light when the voltage is applied. In other words, drive voltage of an EL element in the present invention is adjusted such that the light emitting layer emits no light by itself, or emits light of extremely low luminance. Further adjustment is made so that the portion doped with the specific impurity element emits light of satisfiable luminance at the same drive voltage.  
           [0017]    That is, the present invention is characterized by doping the light emitting layer with a specific impurity element to use the doped portion as a hole transmitting layer or an electron transmitting layer in any of the first, second, and third methods. This makes a drive voltage, which is too low to cause substantial light emission of the light emitting layer by itself, sufficient for only the portion doped with the specific impurity element to emit light of satisfiable luminance. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIGS. 1A to  1 C are diagrams showing a process of manufacturing an EL display device of Embodiment 1;  
         [0019]    [0019]FIG. 2 is a diagram showing the sectional structure of an EL display device of Embodiment 1;  
         [0020]    [0020]FIGS. 3A and 3B are diagrams illustrating image display by an EL display device of the present invention;  
         [0021]    [0021]FIGS. 4A and 4B are diagrams showing a process of manufacturing an EL display device of Embodiment 2;  
         [0022]    [0022]FIGS. 5A and 5B are diagrams showing a process of manufacturing an EL display device of Embodiment 3;  
         [0023]    [0023]FIGS. 6A to  6 C are diagrams showing a process of manufacturing an EL display device of Embodiment 4;  
         [0024]    [0024]FIGS. 7A and 7B are diagrams showing a process of manufacturing an EL display device of Embodiment 5; and  
         [0025]    [0025]FIGS. 8A and 8B are diagrams showing a process of manufacturing an EL display device of Embodiment 6. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    Modes for carrying out the present invention will be described in detail with Embodiments shown below.  
         [0027]    [Embodiment 1] 
         [0028]    The constitution of this embodiment will be explained with reference to FIGS. 1A to  1 C. First, a light transmitting substrate  101  is prepared, and an anode  102  made of a transparent conductive film, a light emitting layer  103 , and a cathode  104  are formed sequentially. (FIG. 1A)  
         [0029]    For the anode  102 , any of a compound of indium oxide and tin oxide (hereinafter referred to as ITO or indium tin oxide), a compound of indium oxide and zinc oxide (hereinafter referred to as IZnO or indium zinc oxide), zinc oxide, and tin oxide may be used. The film thickness thereof may range from 80 nm to 200 nm (preferably, 100 to 150 nm).  
         [0030]    The surface of the anode  102  is desirably irradiated with ultraviolet rays in an ozone atmosphere to remove organic or the like from the surface, before the light emitting layer  103  is formed. It is also desirable to completely remove moisture from the surface of the anode  102  by heat treatment.  
         [0031]    If a low molecular-based material is used for the light emitting layer  103 , a known technique such as evaporation, sputtering, and ion plating may be employed. In this case, to form the cathode  104  successively while keeping vacuum is preferable. A metallic film containing an alkali metal element or an alkaline earth metal element (typically, Mg—Ag film, Al—LiF film, etc.) is formed as the cathode to a thickness of 100 to 200 nm.  
         [0032]    If a high molecular-based (polymer-based) material is used for the light emitting layer  103 , a known technique such as spin coating, the ink jet method, and casting may be employed. Preferred in this case is that the light emitting layer is formed in a dry inert atmosphere and then the cathode  104  is formed without exposing the layer to the air (especially, moisture and oxygen).  
         [0033]    The film thickness of the light emitting layer  103  may range from 30 nm to 200 nm (preferably 50 to 100 nm). Shown here is an example in which the light emitting layer  103  is directly formed on the anode  102 , but a hole transmitting layer or a hole injection layer may be formed therebetween.  
         [0034]    Resists  105   a  to  105   e  are then formed, and the vicinity of the interface between the light emitting layer  103  and the cathode  104  is selectively doped with a specific impurity element, in this embodiment, an alkali metal element or an alkaline earth metal element. Used here in Embodiment 1 as the specific impurity element is cesium.  
         [0035]    A minute amount of cesium is sufficient for the doping. The concentration of cesium to be used for doping is typically 1×10 14  to 1×10 18  atoms/cm 3 , preferably 1×10 15  to 1×10 17  atoms/cm 3 . In practice, proper concentration varies depending also upon a material of the light emitting layer  103 . It is therefore required for the operator to find out the optimal concentration in advance.  
         [0036]    In this embodiment, a region doped with cesium extends, utmost, 50 nm down the depth of the light emitting layer from the interface between the light emitting layer  103  and the cathode  104 . That is, regions denoted by  106   a  to  106   d  in FIG. 1B are regions serving as electron transmitting layers (or electron injection layers) (hereinafter referred to as electron transmitting regions).  
         [0037]    Note that cesium is contained also in the cathode  104  though it seems as if only the light emitting layer is doped with cesium in the drawing. However, only the light emitting layer doped with cesium is considered as a region that substantially functions as the electron transmitting region.  
         [0038]    After thus completing the doping of the specific impurity element, the resists  105   a  to  105   e  are removed and an auxiliary electrode  107  is formed on the cathode  104 . A material mainly containing aluminum may be used to form the auxiliary electrode  107 . The auxiliary electrode  107  functions as an assistive electrode that complements the thinness of the cathode  104  so as to lower the resistance. The auxiliary electrode  107  may also be deemed as a protective electrode, for it has a protective effect of preventing moisture or the like from penetrating into the light emitting layer.  
         [0039]    More desirably, a silicon nitride film or a silicon oxide nitride (expressed as SiO x N y ) film is formed as a passivation film on the auxiliary electrode  107 .  
         [0040]    The EL display device is thus completed. In actuality, after the step illustrated in FIG. 1C, a sealing member (also called a housing member)  201  is put over the EL element (indicating here a capacitance element consisting of the anode, the light emitting layer and the cathode) to confine the EL element within a completely closed space. The containment is achieved by bonding the substrate  200  and the sealing member  201  with a sealing agent (or an adhesive)  202 . (FIG. 2)  
         [0041]    Structures in which an EL element is protected by a sealing member have already been known, and any of them may be employed. A drying agent (such as barium oxide) may be provided in a gap  203  formed between the seating member and the EL element.  
         [0042]    The anode  102  and the cathode  104  are led out the sealing member  201  and connected to an FPC (flexible printed circuit)  204 . An external signal is input therefrom. It seems as though only the cathode  104  is led out in FIG. 2, but a look at another section will confirm that the anode  102  is also led out.  
         [0043]    In the thus manufactured EL display device, only a part of the light emitting layer, where the electron transmitting regions  106   a  to  106   d  in FIG. 1B are formed, emits light when a given voltage is applied to the light emitting layer. The given voltage varies depending on a material of the light emitting layer and on the alkali metal element or the alkaline earth metal element used for the doping, and can be chosen within a range of from 3 to 10 V. Preferably, the contrast ratio of the luminance of emitted light is such that the luminance of the region doped with cesium is 10 3  or more (more desirably, 10 4  or more) times that of the not doped region.  
         [0044]    [0044]FIGS. 3A and 3B illustrate an outline of the light emission. FIG. 3A shows the state before the voltage is applied to the light emitting layer. In the drawing, inside a figure drawn by the dotted line is a region doped with an impurity element (cesium, in this embodiment), and a region surrounding the figure is a region that is not doped with the impurity element.  
         [0045]    [0045]FIG. 3B shows the state after the voltage is applied to the light emitting layer. At this point, the region doped with the impurity element in FIG. 3A emits light to be recognized visually as a light emitting region. The region that is not doped with the impurity element in FIG. 3A does not emit light regardless of voltage application.  
         [0046]    As described above, the light emitting layer is selectively doped with a specific impurity element (an alkali metal element or an alkaline earth metal element in this embodiment), and only the region doped with the specific impurity element emits light when the voltage is applied to the light emitting layer. Using the method according to the present invention, the patterning is required only once throughout the entire process. That is, there is no need to pattern the anode nor the cathode, which greatly simplifies the process and presents an advantage of low production cost.  
         [0047]    In addition, the region emitting light (region doped with a specific impurity element) is defined through patterning, making it possible to display with high definition even extremely fine character pattern or figure pattern.  
         [0048]    [Embodiment 2] 
         [0049]    This embodiment gives a description with reference to FIGS. 4A and 4B on an example of separating a light emitting region from a non-light emitting region by measures different from that of Embodiment 1.  
         [0050]    First, resists  105   a  to  105   e  are formed through the same steps as Embodiment 1. An electron transmitting layer or an electron injection layer may optionally be formed on a light emitting layer  103 . This embodiment is characterized in that the vicinity of the interface between an anode  102  and the light emitting layer  103  is doped with a halogen element as a specific impurity element. Fluorine is used for the doping in Embodiment 2 as the halogen element.  
         [0051]    A minute amount of fluorine is sufficient for the doping. The concentration of fluorine to be used for the doping is typically 1×10 14  to 1×10 18  atoms/cm 3 , preferably 1×10 15  to 1×10 17  atoms/cm 3 . In practice, proper concentration varies depending also upon a material of the light emitting layer  103 . It is therefore required for the operator to find out the optimal concentration in advance.  
         [0052]    In this embodiment, a region doped with fluorine extends, utmost, 70 nm down the depth of the light emitting layer from the interface between the light emitting layer  103  and the anode  102 . That is, regions denoted by  401   a  to  401   d  in FIG. 4A are regions serving as hole transmitting layers (or hole injection layers) (hereinafter referred to as hole transmitting regions).  
         [0053]    Note that fluorine is contained also in the anode  102  though it seems as if only the light emitting layer is doped with fluorine in the drawing. However, only the light emitting layer doped with fluorine is considered as a region that substantially functions as the hole transmitting region.  
         [0054]    After thus completing the doping of the specific impurity element, the resists  105   a  to  105   e  are removed and an auxiliary electrode  107  is formed on the cathode  104 . The description given in Embodiment 1 on the auxiliary electrode  107  applies to the auxiliary electrode  107  of this embodiment. As in Embodiment 1, a passivation film may of course be formed.  
         [0055]    Thereafter, a substrate and a sealing member is bonded together with a sealing agent and an FPC is attached, to thereby complete the EL display device having the structure shown in FIG. 2 (however, the structure of the light emitting layer differs). In the thus manufactured EL display device, only a part of the light emitting layer, where the hole transmitting regions  401   a  to  401   d  in FIG. 4A are formed, emits light when a given voltage is applied to the light emitting layer.  
         [0056]    [Embodiment 3] 
         [0057]    This embodiment gives a description with reference to FIGS. 5A and 5B on an example of separating a light emitting region from a non-light emitting region by measures different from that of Embodiment 1.  
         [0058]    First, resists  105   a  to  105   e  are formed through the same steps as Embodiment 1. Hole transmitting regions (regions doped with a halogen element)  401   a  to  401   d  are formed in accordance with the steps of Embodiment 2.  
         [0059]    Using the same resists  105   a  to  105   e , electron transmitting regions (regions doped with an alkali metal element or an alkaline earth metal element)  501   a  to  501   d  are formed through steps similar to those in Embodiment 1. (FIG. 5A)  
         [0060]    Embodiment 2 may be referred to for details of the doping step of the halogen element, while Embodiment 1 may be referred to for details of the doping step of the alkali metal element or the alkaline earth metal element.  
         [0061]    After thus completing the doping of the halogen element and the alkali metal element, or the alkaline earth metal element, the resists  105   a  to  105   e  are removed and an auxiliary electrode  107  is formed on a cathode  104 . The description given in Embodiment 1 on the auxiliary electrode  107  applies to the auxiliary electrode  107  of this embodiment. As in Embodiment 1, a passivation film may of course be formed.  
         [0062]    Thereafter, a substrate and a sealing member is bonded together with a sealing agent and an FPC is attached, to thereby complete the EL display device having the structure shown in FIG. 2 (however, the structure of the light emitting layer differs). In the thus manufactured EL display device, only a part of the light emitting layer, where the hole transmitting regions  401   a  to  401   d  and the electron transmitting regions  501   a  to  501   d  are formed, emits light when a given voltage is applied to the light emitting layer.  
         [0063]    [Embodiment 4] 
         [0064]    The constitution of this embodiment will be described with reference to FIGS. 6A to  6 C. This embodiment relates to an EL display device having a structure in which light emitted from a light emitting layer exits from one side where the light does not transmit a substrate. First, a substrate  601  is prepared to sequentially form thereon an auxiliary electrode  602 , a cathode  603 , a light emitting layer  604 , and anode  605 . Embodiment 1 may be referred to for materials and formation methods of these layers. An electron transmitting layer or an electron injection layer may optionally be formed on the cathode  603  before the light emitting layer  604  is formed. (FIG. 6A)  
         [0065]    Resists  606   a  to  606   e  are formed, and the next step is carried out to selectively dope the vicinity of the interface between the light emitting layer  604  and the anode  605  with a specific impurity element, a halogen element in this embodiment. Used here in Embodiment 4 as the specific impurity element is fluorine.  
         [0066]    A minute amount of fluorine is sufficient for the doping. The concentration of fluorine to be used for the doping is, as in Embodiment 2, typically 1×10 14  to 1×10 18  atoms/cm 3 , preferably 1×10 15  to 1×10 17  atoms/cm 3 . In practice, proper concentration varies depending also upon a material of the light emitting layer  604 . It is therefore required for the operator to find out the optimal concentration in advance.  
         [0067]    In this embodiment, a region doped with fluorine extends, utmost, 70 nm down the depth of the light emitting layer from the interface between the light emitting layer  604  and the anode  605 . That is, regions denoted by  607   a  to  607   d  in FIG. 6B are regions serving as hole transmitting regions.  
         [0068]    Note that fluorine is contained also in the anode  605  though it seems as if only the light emitting layer is doped with fluorine in the drawing. However, only the light emitting layer doped with fluorine is considered as a region that substantially functions as the hole transmitting region.  
         [0069]    After thus completing the doping of the specific impurity element, the resists  606   a  to  606   e  are removed and a passivation film  608  for protecting the anode  605  is formed. A silicon nitride film or a silicon oxide nitride film may be used for the passivation film  608 . A silicon nitride oxide film having high transmittance is preferred, however, for the passivation film is to be arranged in a light path.  
         [0070]    Thereafter, the substrate and a sealing member is bonded together with a sealing agent and an FPC is attached, to thereby complete the EL display device having the structure shown in FIG. 2 (however, the structure of the light emitting layer differs). In the thus manufactured EL display device, only a part of the light emitting layer, where the hole transmitting regions  607   a  to  607   d  are formed, emits light when a given voltage is applied to the light emitting layer.  
         [0071]    [Embodiment 5] 
         [0072]    This embodiment gives a description with reference to FIGS. 7A and 7B on an example of separating a light emitting layer from a non-light emitting layer by measures different from the one described in Embodiment 4.  
         [0073]    First, the state shown in FIG. 6A is obtained through the same steps as Embodiment 4. At this point, a hole transmitting layer or a hole injection layer may optionally be formed between a light emitting layer  604  and an anode  605 . Then resists  606   a  to  606   e  are formed as shown in FIG. 6B.  
         [0074]    The next step is carried out to selectively dope the vicinity of the interface between a cathode  603  and the light emitting layer  604  with a specific impurity element, an alkali metal element or an alkaline earth metal element in this embodiment. Used here in Embodiment 5 as the specific impurity element is potassium. (FIG. 7A) The same amount of potassium as cesium in Embodiment 1 is sufficient for the doping. In this embodiment, a region doped with potassium extends, utmost, 30 nm down the depth of the light emitting layer from the interface between the cathode  603  and the light emitting layer  604 . That is, regions denoted by  701   a  to  701   d  in FIG. 7A are regions serving as electron transmitting regions.  
         [0075]    Note that potassium is contained also in the cathode  603  though it seems as if only the light emitting layer is doped with potassium in the drawing. However, only the light emitting layer doped with potassium is considered as a region that substantially functions as the electron transmitting region.  
         [0076]    After thus completing the doping of the specific impurity element, the resists  606   a  to  606   e  are removed and a passivation film  608  for protecting the anode  605  is formed. As the passivation film  608 , the same materials as Embodiment can be used. (FIG. 7B) Thereafter, a substrate and a sealing member is bonded together with a sealing agent and an FPC is attached, to thereby complete the EL display device having the structure shown in FIG. 2 (however, the structure of the light emitting layer differs). In the thus manufactured EL display device, only a part of the light emitting layer, where the electron transmitting regions  701   a  to  701   d  are formed, emits light when a given voltage is applied to the light emitting layer.  
         [0077]    [Embodiment 6] 
         [0078]    This embodiment gives a description with reference to FIGS. 8A and 8B on an example of separating a light emitting layer from a non-light emitting layer by measures different from the one described in Embodiment 4.  
         [0079]    First, resists  606   a  to  606   e  are formed through the same steps as Embodiment 4. Electron transmitting regions (regions doped with an alkali metal element or an alkaline earth metal element)  701   a  to  701   d  are formed in accordance with the steps of Embodiment 5. Potassium is used in doping in this embodiment.  
         [0080]    Using the same resists  606   a  to  606   e , hole transmitting regions (regions doped with a halogen element)  801   a  to  801   d  are formed through steps similar to those in Embodiment 4. Fluorine is used for doping in this embodiment. (FIG. 8A)  
         [0081]    Embodiment 2 may be referred to for details of the doping step of the halogen element, while Embodiment 1 may be referred to for details of the doping step of the alkali metal element or the alkaline earth metal element.  
         [0082]    After thus completing the doping of the specific impurity elements, the resists  606   a  to  606   e  are removed and a passivation film  608  for protecting the anode  605  is formed. (FIG. 8B)  
         [0083]    Thereafter, a substrate and a sealing member is bonded together with a sealing agent and an FPC is attached, to thereby complete the EL display device having the structure shown in FIG. 2 (however, the structure of the light emitting layer differs). In the thus manufactured EL display device, only a part of the light emitting layer, where the electron transmitting regions  701   a  to  701   d  and the hole transmitting regions  801   a  to  801   d  are formed, emits light when a given voltage is applied to the light emitting layer.  
         [0084]    [Embodiment 7] 
         [0085]    Being a self-light emitting type, an EL display device fabricated by carrying out the present invention is superior to a liquid crystal display device in visibility in a bright place. The EL display device of the present invention thus has a wide variety of uses as a direct view type EL display. One of superior advantages EL displays to liquid crystal displays is wide view angle. Accordingly, EL displays are preferred in a use where a display of 30 inches or more in diagonal is required.  
         [0086]    The EL display using the present invention is suitable as, for example, an electronic signboard (as opposed to painted signboard) for displaying still images or text information, such as a billboard, an emergency exit indicator, a clockface adapted for use in a dim place, a road sign for nighttime, and a display on a keyboard of a personal computer. These displays are better if they cost less as a product, and in that point the present invention suits very well.  
         [0087]    According to the present invention, the patterning step is required only once to make it possible to manufacture an EL display device through a very small number of manufacturing steps. In other words, production cost of the EL display device can be reduced significantly.  
         [0088]    The present invention also makes it possible to manufacture an EL display device of extremely high definition because the image display region can be determined through the patterning step that uses the resist. Therefore, employment of the EL display device of the present invention can realize at a low cost a display capable of displaying precisely text information, or the like, shown in small font.