Abstract:
The present invention has an object to provide a method of raising a re-coupling efficiency of carriers in an EL element to thereby provide a light-emitting device having high emission efficiency. The method is that the electron trap region  106  and the hole trap region  107  are formed in the interior of the emission layer  103 . The electron trap region  106  here is a region that has the action of enclosing within the emission layer an electrons that is transferred at the lowest unoccupied molecular orbit (LUMO) level of the emission layer  103 . In addition, the hole trap region  107  is a region that has the action of enclosing within the emission layer a hole that is transferred at the highest occupied molecular orbit (HOMO) level of the emission layer  103.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a light-emitting device that uses a luminous organic film. Further, the present invention relates to electric equipment using the light-emitting device as a display portion or a light source. It is to be noted that the luminous organic film, which can be used in the present invention, includes all organic films that emit light (fluorescent light and/or phosphorescent light) via either a singlet excitation or a triplet excitation, or via both excitations.  
           [0003]    2. Description of the Related Art  
           [0004]    In recent years, development is proceeding in a light-emitting device (hereinafter referred to as an EL light-emitting device) employing a luminous element (hereinafter referred to as an EL element) that uses a luminous organic film (hereinafter referred to as an organic EL film) that provides EL (Electro Luminescence). The EL light-emitting device has an EL element that is composed of an anode, a cathode, and an organic EL film sandwiched therebetween. The emission of light can be attained by applying a voltage between the anode and the cathode.  
           [0005]    At this point, a hole from the anode is injected into the EL material, and an electron from the cathode is injected therein. Electric charges (carriers) injected from both the electrodes move in the interior of the organic EL film to thereby re-couple. An excitation state is generated by the re-coupling of the carriers, and a portion thereof is converted into photons. Luminescence can be made visible by extracting these photons to the outside.  
           [0006]    Such a conventional light-emitting mechanism of the EL element is shown in FIGS.  2 A and  2 B. Shown in FIG. 2A is the conventional junction structure of the EL element in which reference symbol  201  denotes a cathode, reference symbol  202  denotes an electron transfer layer, reference symbol  203  denotes an emission layer, reference symbol  204  denotes a hole transfer layer, and reference symbol  205  denotes an anode. Further, shown in FIG. 2B is the carrier injection process thereof. A voltage is applied between the cathode  201  and the anode  205  to thereby inject an electron  206  and a hole  207 . The injected electron  206  and hole  207  re-couple, whereby an emission  208  is attained.  
           [0007]    Taking into consideration such a light-emitting mechanism, the efficiency of light emitted from the EL element, that is, the emission efficiency (expressed as η (emission)) is expressed as the following equation.  
           η (emission)=η (injection)×η (re-coupling)×η (excitation)×η (quantum)  
           [0008]    Here in this equation, η (injection) denotes the efficiency when the carrier is injected from the electrode, η (re-coupling) denotes the re-coupling efficiency of the electron and the hole, η (excitation) denotes the efficiency of generating a singlet exciton due to the re-coupling, and η (quantum) denotes the efficiency of converting the singlet exciton to a photon.  
           [0009]    The η (injection) efficiency originates in an electric potential barrier in the interface between the cathode (or the anode) and the EL material, and changes. The lower the electric potential barrier, the higher the η (injection) efficiency is. The η (re-coupling) efficiency changes due to the injection balance of the carrier (balance of the ratio of the injected electron and hole), and is influenced by the carrier transfer characteristic of the emission layer (the organic EL film that will actually emit light). Further, the η (excitation) efficiency is the generating efficiency of the singlet exciton that contributes to the emission of light, and is theoretically set (fixed) at about 0.25. Further, the change of the η (quantum) efficiency depends on whether the emission layer is crystalline or non-crystalline. Generally speaking, a higher value can be attained from a crystalline emission layer than from a non-crystalline one.  
           [0010]    In addition, until the photons, which are generated in the emission layer, are extracted to the outside, most of them are lost (about 80% are lost) due to diffusion and thermal deactivation. Therefore, light that is actually observed includes the loss of the photons. Thus, in the light-emitting mechanism process of the EL element, the emission efficiency is reduced due to various factors. In order to obtain high emission efficiency, the above-mentioned various efficiencies have to be raised to thereby attain a total high emission efficiency.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention has been made in view of the above, and therefore has an object to provide a method of raising a re-coupling efficiency of carriers in an EL element to thereby provide a light-emitting device having high emission efficiency. Further, another object of the present invention is to provide electric equipment using the light-emitting device as a display portion or a light source.  
           [0012]    In the present invention, attention is paid to the band structure of the EL element in order to improve the re-coupling efficiency (expressed as η (re-coupling)) of an electron and a hole, and it is characterized in that the probability of the re-coupling is increased by enclosing the electron and the hole in a specific region to thereby enhance the re-coupling efficiency thereof. Therefore, the EL element having the band structure shown in FIG. 1A is formed.  
           [0013]    In FIG. 1A, reference symbol  101  denotes a cathode, reference symbol  102  denotes an electron transfer layer, reference symbol  103  denotes an emission layer, reference symbol  104  denotes a hole transfer layer, and reference symbol  105  denotes an anode. Further, an electron trap region  106  and a hole trap region  107  are formed in the interior of the emission layer  103 . It is to be noted that the structure of the EL element may be a structure provided with either the electron trap region  106  or the hole trap region  107 .  
           [0014]    The electron trap region  106  here is a region that has the action of enclosing within the emission layer an electron that is transferred at the lowest unoccupied molecular orbit (LUMO) level of the emission layer  103 . In addition, the electron trap region  106  denotes a region that indicates a LUMO level that is lower than the LUMO level of the emission layer  103 . The hole trap region  107  is a region that has the action of enclosing within the emission layer a hole that is transferred at the highest occupied molecular orbit (HOMO) level of the emission layer  103 , and denotes a region that indicates a HOMO level that is higher than the HOMO level of the emission layer  103 .  
           [0015]    The electron trap region  106  can be formed by constructing a structure in which an organic film or a cluster of organic substances, which has the effect of lowering the LUMO level, is sandwiched between the emission layer  103 . Further, the hole trap region  107  can be formed by constructing a structure in which an organic film or a cluster of organic substances, which has the effect of raising the HOMO level, is sandwiched between the emission layer  103 .  
           [0016]    Simultaneously with the provision of the above electron trap region  106  or the hole trap region  107 , a hole prevention layer may be provided between the electron transfer layer  102  and the emission layer  103 , or an electron prevention layer may be provided between the emission layer  103  and the hole transfer layer  104 . Of course, the structure thereof may be a structure that is provided with both the hole prevention layer and the electron prevention layer.  
           [0017]    If the band structure of the EL element shown in FIG. 1A is applied, then the carrier injection process thereof is a process as shown in FIG. 1B. In other words, an electron  108  transferred at the LUMO level is enclosed in the electron trap region  106  that is provided in the interior of the emission layer  103 . On the other hand, a hole  109  transferred at the HOMO level is enclosed in the hole trap region  107 . As a result, the re-coupling of the electron  108  and the hole  109  occurs between the electron trap region  106  and the hole trap region  107 , whereby the emission of light can be obtained.  
           [0018]    At this point, in the present invention, because re-coupling takes place under the, state where either the electron or the hole is enclosed in the trap region, the re-coupling efficiency (η (re-coupling)) can be improved more in comparison with a conventional one. It can be said that the concept of forming a level well in the interior of the emission layer to enclose carriers is an unprecedented concept.  
           [0019]    Thus, among some parameters which contributes to the emission efficiency, the efficiency (η (re-coupling)) that originates from the re-coupling of carriers can be improved by implementing the present invention, whereby the emission efficiency of the EL element is enhanced. Therefore, the driving voltage of the EL element can be set at a low rate even for attaining a luminance equivalent to that of the prior art, so that the consumption power of the light-emitting device can be reduced. In addition, the lowering of the driving voltage leads to the suppression of the deterioration of the organic EL film, resulting in the enhancement of the reliability of the light-emitting device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The above and other objects and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings:  
         [0021]    [0021]FIGS. 1A and 1B are diagrams showing a band structure of an EL element;  
         [0022]    [0022]FIGS. 2A and 2B are diagrams showing a band structure of an EL element;  
         [0023]    [0023]FIGS. 3A and 3B are diagrams showing a pixel of an EL light-emitting device and a cross-sectional structure of an EL element thereof, respectively;  
         [0024]    [0024]FIG. 4 is a diagram showing a cross-sectional structure of an EL element;  
         [0025]    [0025]FIGS. 5A and 5B are diagrams showing a band structure of an EL element;  
         [0026]    [0026]FIGS. 6A and 6B are diagrams showing a band structure of an EL element;  
         [0027]    [0027]FIGS. 7A and 7B are diagrams showing a cross-sectional structure of an EL light-emitting device and a top structure thereof, respectively;  
         [0028]    [0028]FIGS. 8A to  8 F are diagrams showing examples of electric equipments; and  
         [0029]    [0029]FIGS. 9A and 9B are diagrams showing examples of electric equipments. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    In an embodiment mode of the present invention, an explanation will be on a structure of an EL element for obtaining a band structure that is shown in FIG. 1A with reference to FIGS. 3A and 3B. Shown in FIG. 3A is a diagram schematically showing a cross-sectional structure of a pixel of an active matrix light-emitting device.  
         [0031]    In FIG. 3A, reference symbol  301  denotes a substrate, reference symbol  302  denotes a TFT (thin film transistor), and reference symbol  303  denotes a pixel electrode that functions as an anode of the EL element. A glass substrate or a plastic substrate (includes plastic films) may be used as the substrate  301 . Further, any TFT structure may be used to form the TFT  302 , and a planar type TFT or a stagger type TFT can be used. As for the pixel electrode  303 , a transparent conductive film having a large work function, typically a compound film of indium oxide and tin oxide or a compound film of indium oxide and zinc oxide, may be used.  
         [0032]    Next, reference symbol  304  denotes an insulating film formed such that the EL material is not formed in a step of the pixel electrode. An insulating film containing silicon (silicon oxide film, silicon nitride oxide film, or a silicon nitride film) or an organic resin film may be used as the insulating film  304 .  
         [0033]    Further, a single layer or a lamination layer of an organic EL film may be used for an organic EL layer  305 . That is, a single layer of an organic EL film may be used as the emission layer. Alternatively, the emission layer may be a lamination structure in which various organic films are combined, making it possible to make the laminated layers function as the layers which are a hole transfer layer (including a hole injection layer), a hole prevention layer, an emission layer, an electron prevention layer, and an electron transfer layer (including an electron injecting layer), respectively.  
         [0034]    As for a cathode  306 , a material that has a small work function is preferably used, and also, a material containing an element that belongs to Group 1 or Group 2 of the periodic table is used. Typically, an alloy film of aluminum and lithium and an alloy film of magnesium and silver may be used. Of course, other conductive films of any combination may be used.  
         [0035]    An enlarged structure of a region denoted by reference symbol  307  in FIG. 3A is shown in FIG. 3B here. In the embodiment mode of the present invention, the organic EL layer  305  is composed of a hole transfer layer  308 , an emission layer  309 , and an electron transfer layer  310 . Furthermore, the emission layer  309  is a three-layered structure composed of an emission layer  309   a , an emission layer  309   b , and an emission layer  309   c . Of course, the emission layer  309   a , the emission layer  309   b , and the emission layer  309   c  are all made of the same organic EL film. In addition, a cluster of organic substances  311  is formed in the interface of the emission layer  309   a  and the emission layer  309   b , and a cluster of organic substances  312  is formed in the interface of the emission layer  309   b  and the emission layer  309   c.    
         [0036]    Here, the cluster of organic substances  311  forms a hole trap region and the cluster of organic substances  312  forms an electron trap region. However, it is possible that the cluster of organic substances  311  forms the electron trap region and the cluster of organic substances  312  forms the hole trap region by changing the materials thereof.  
         [0037]    The organic EL layer  305  shown here may be formed by evaporation, application (spin coating, dipping, LB method, or knife-edge method), or printing. However, it is desirable that the clusters of organic substances  311  and  312  are formed by evaporation. The cluster of organic substances may be provided regularly or irregularly. Further the shape of the cluster of organic substances may be a granular or a flat board-like shape. The present invention is of course not limited to the above film deposition method, and any film deposition method may be used as long as the structure of FIG. 3B is formed.  
         [0038]    In an EL element that has an organic EL layer composed of a structure as in the embodiment mode, the hole injected from the anode  304  is trapped by the cluster of organic substances  311 , and the electron injected from the cathode  306  is trapped by the cluster of organic substances  312 . The reason for this resides in that the HOMO level has become higher in the cluster of organic substances  311  and that the LUMO level has become lower in the cluster of organic substances  312 .  
         [0039]    Therefore, the electron and the hole enclosed in a locally potential (energy level) well re-couple at a high efficiency, thereby improving the emission efficiency of the entire EL elements. As a result, a light-emitting device having a low consumption power and high reliability can be achieved.  
         [0040]    Note that a fluorescent substance may be doped into the emission layer  309  to thereby move the luminescent center to the fluorescent substance. Further, in the case of emitting color lights, three kinds of emission layers for the colors R (red), G (green), and B (blue) may be formed in line in each of the pixels, or an emission layer luminescing white color in combination with a color filter may be provided. In addition, a color converting layer (CCM) and a color filter may be combined with the emission layer luminescing blue color. A technique called the photo bleaching method (a technique that utilizes a phenomenon in which a fluorescent pigment deteriorates due to light irradiation) may also be used.  
         [0041]    Although an example of an active matrix light-emitting device having the TFTs and the EL elements electrically connected is given in the embodiment mode, the present invention can be implemented in a passive matrix light-emitting device. Further, the present invention may be used a light source serving as a back light of a liquid crystal display or a back light of a fluorescent display lamp.  
         [0042]    Embodiment 1  
         [0043]    In Embodiment 1, an example of a case of using an organic film as a substitute for the clusters of organic substances  311  and  312  shown in FIG. 3B will be illustrated in FIG. 4. It is to be noted that the structure of the light-emitting device is similar to that of FIG. 3A.  
         [0044]    Embodiment 1 is characterized in the structure such that an organic film  401  that has the effect of reducing the LUMO level is provided in order to form an electron trap region or an organic film  402  that has the effect of raising the HOMO level is provided in order to form a hole trap region. Both the organic films  401  and  402  may be provided or either one may be provided. Further, the film thickness of the organic films  401  and  402  may be formed to between 10 and 50 nm (preferably between 20 and 30 nm).  
         [0045]    In the case of adopting the structure of Embodiment 1, the hole is trapped in the organic film  401  and the electron is trapped in the organic film  402 , whereby the re-coupling efficiency can be improved and the driving voltage of the EL elements can be reduced. In other words, a highly reliable light-emitting device having a low consumption power can be attained.  
         [0046]    Embodiment 2  
         [0047]    In the structure shown in FIG. 3B, the cluster of organic substances  311  serving as the hole trap region is provided on the side of the anode  303 , and the cluster of organic substances  312  serving as the electron trap region is provided on the side of the cathode  306 . However, the provision thereof may be inverted. That is, the structure may be such that the cluster of organic substances  311  serving as the hole trap region is provided on the side of the cathode  306 , and the cluster of organic substances  312  serving as the electron trap region is provided on the side of the anode  303 .  
         [0048]    Similarly, in the structure shown in FIG. 4, the structure may have the provision of an organic film  401  that serves as the hole trap region on the side of the cathode  306  and the provision of an organic film  402  that serves as the electron trap region on the side of the anode  303 .  
         [0049]    Embodiment 3  
         [0050]    In Embodiment 3, an explanation will be made on a light-emitting device employing an EL element that has a band structure different from that of FIG. 1A with reference to FIGS.  5 A and  5 B. It is to be noted that an emission layer  500  is the only component in FIG. 5A that is different from FIG. 1A, and therefore the same reference symbols will be used for the other components.  
         [0051]    The structure of Embodiment 3 shown in FIG. 5A is characterized in that an electron trap region  501 , a hole prevention region  502 , a hole trap region  500 , and an electron prevention region  504  are formed inside the emission layer  500 . The hole prevention region  502  in this structure is a region serving as an electric potential barrier for preventing the hole  109  from moving to the side of the cathode  101 . Further, the electron prevention region  504  is a region serving as an electric potential barrier for preventing the electron  105  from moving to the side of the anode  105 .  
         [0052]    In Embodiment 3, the electron trap region  501  and the hole prevention region  502  are made of the same organic material, and the hole trap region  503  and the electron prevention region  504  are made of the same organic material. The same organic material indicates an organic film or a cluster of organic substances. In the present invention, it is necessary to use an organic substance that has the effect of reducing the LUMO level and the HOMO level, and an organic substance that has the effect of raising the LUMO level and the HOMO level. At that point, the electron trap region  501  and the hole prevention region  502  are formed from the organic substance that has the effect of reducing the LUMO level and the HOMO level, and the hole trap region  503  and the electron prevention region  504  are formed from the organic substance that has the effect of raising the LUMO level and the HOMO level.  
         [0053]    Note that although an example of providing the four regions, that is, the electron trap region  501 , the hole prevention region  502 , the hole trap region  503 , and the electron prevention region  504  inside the emission layer  500  is shown in Embodiment 3, the structure may be a structure with the provision of either one, two, or three regions. Of course, in case of two or three regions, the combination of regions is freely selected.  
         [0054]    When adopting the structure of Embodiment 3, the carriers injected from the electrode are blocked by the prevention region and remains in the interior of the emission layer  500 , whereby not only the probability of enclosing the carriers in the trap region but also the re-coupling efficiency (η (re-coupling)) are improved. As a result, the driving voltage of the EL element is reduced. Thus, a reduction in the consumption power and an improvement in the reliability of the light-emitting device can be attained.  
         [0055]    Embodiment 4  
         [0056]    In Embodiment 4, an explanation will be made on a light-emitting device employing an EL element that has a band structure different from that of FIG. 1A with reference to FIGS. 6A and 6B. It is to be noted that an emission layer  600  is the only component in FIG. 6A that is different from FIG. 1A, and therefore the same reference symbols will be used for the other components.  
         [0057]    The structure of Embodiment 4 shown in FIG. 6A is characterized in that an electron trap region  601  and a hole trap region  602  are formed inside the emission layer  600  and that both the regions are formed of the same organic material. The same organic material indicates an organic film or a cluster of organic substances. However, in Embodiment 4, it is necessary to use an organic substance that has the effect of reducing the LUMO level and raising the HOMO level as well.  
         [0058]    When adopting the structure of Embodiment 4, the carriers injected from the electrodes are enclosed in the trap regions formed on the same layer. However, because the band gap between the LUMO level and the HOMO level is extremely small in this case, the re-coupling efficiency (η (re-coupling)) is improved. As a result, the driving voltage of the El element is reduced. Thus, a reduction in the consumption power and an improvement in the reliability of the light-emitting device can be attained.  
         [0059]    Embodiment 5  
         [0060]    An example of a light-emitting device of the present invention will be explained with reference to FIGS. 7A and 7B in Embodiment 5. Note that FIG. 7A is a cross-sectional view of the light-emitting device, and FIG. 7B is a top view thereof.  
         [0061]    In FIG. 7A, reference symbol  701  denotes a substrate having a pixel portion  702  and a driver circuit  703  formed thereon. The pixel portion  702  and the driver circuit  703  can transfer electric signals through wirings  704  to  706 . As long as the substrate  701  is transparent to visible radiation, the substrate may be made of any material.  
         [0062]    In the pixel portion  702 , a plurality of pixels containing TFTs and EL elements (typically pixels having a structure as shown in FIG. 3A) are formed in matrix, and a passivation film  707  is formed thereon. A tantalum oxide film that is formed by sputtering is used as the passivation film  707  in Embodiment 5. However, a silicon nitride film may be provided as long as it is possible to form the silicon nitride film at a temperature that will not cause deterioration of the organic EL film.  
         [0063]    On top of the wirings  704  and  706 , a sealing material  708  made of epoxy-based resin is formed so as to surround the pixel portion  702  and the driver circuit  703 . A printed wiring board (PWB)  709  is bonded thereto by the sealing material  708 . It is to be noted that typically, a piece of glass epoxy, an epoxy-based film, a piece of glass heat resistant epoxy, ceramic, alumina, paper based phenol or paper based epoxy can be used as a material of the printed wiring board  709 . In addition, a glass substrate, a crystallized glass substrate, or a plastic substrate may be used. In Embodiment 5, a piece of glass epoxy-based substrate  711  sandwiching a ceramic  710  as a core material is employed as the printed wiring board.  
         [0064]    At this point, conductors  712  and  713  made from an anisotropic conductive film (typically resin having metallic particles diffused therein) or a metal bump (typically a solder bump, a metal bump, a nickel bump, or a copper bump) are provided in the interior of the sealing material  708 . Either the wiring  704  or the wiring  706  is electrically connected to a group of wiring  714  that is formed on the printed wiring board  709  via the conductors  712  and  713 . Note that the group of wiring mentioned here is a generic term indicating wirings formed on the front surface, the back surface, or the interior of the printed wiring board  709 .  
         [0065]    The group of wiring  714  transmits the electric signals, which are transmitted to an FPC (flexible printed circuit)  715 , to the conductors  712  and  713 , and transmits electric signals between various kinds of ICs (integrated circuits)  715  and  716  that are connected on the printed wiring board  709 . The group of wiring  714  is formed to a thickness of between 1 and 20 μm. A pattern made from typically a copper foil, a gold foil, a silver foil, a nickel foil, or an aluminum foil is used as the group of wiring  714 .  
         [0066]    In the light-emitting device of Embodiment 5 having a structure such as the above, the electric signals transmitted to the FPC  715  are processed by the printed wiring board  709 . The processed electric signals are then transmitted to the pixel portion  702  or the driver circuit  703  via the conductors  712  and  713 . In Embodiment 5 at this point, the printed wiring board  709  is a protection board for protecting the EL elements against external impact. In addition, the printed wiring board  709  may also be used as a substrate for enclosing the EL elements in a closed space.  
         [0067]    Note that in Embodiment 5, although an IC is incorporated in the printed wiring board  709  in order that the printed wiring board  709  has a signal processing function, only the group of wiring may be formed and used as redundant wiring for reducing the resistance of the wirings used in the pixel portion  702  and the driver circuit  703 . Thus, for example, a signal delay in the wiring supplying a current to the EL elements and in the gate wiring of the TFTs can be improved.  
         [0068]    Accordingly, the light-emitting device can be made high-performance and highly reliable by employing the printed wiring board as one of the substrates of the light-emitting device. It is to be noted that the constitution of Embodiment 5 may be implemented by combining it with any one of the constitutions of Embodiments 1 through 4.  
         [0069]    Embodiment 6  
         [0070]    In Embodiment 6, an explanation will be made on an organic EL film that can be used in the present invention. A characteristic of the present invention is in the provision of an organic film or a cluster of organic substances that can locally alter the LUMO level or the HOMO level in the interior of the emission layer. Any organic film that fulfills the above condition may be used. For example, an organic film that emits tight via a triplet excitation can be used as the organic film or the cluster of organic substances in the interior of the emission layer.  
         [0071]    To be more specific, materials such as Alq 3  (tris-6-quinolilite-aluminum complex), DPVBi (distylallylene conductor), BeBq2 (bisbenzoquinolilite berylliumis complex), TPD (triphenylamine conductor), α-NPD, PPV (polyparaphenylene vinylene), and PVK (polyvinyl carbazole) may be used as the emission layer. The molecular structures of the above-mentioned materials that can be used as the emission layer here are given in the following.  
                         
 
         [0072]    Further, specifically, materials such as coumarin 6, DCM1, DCM2, quinacridon, rubrene, and perylene may be used as the fluorescent pigment. The molecular structures of the above-mentioned materials that can be used as the fluorescent pigment here are given in the following.  
                         
 
         [0073]    The band structure of the present invention may be formed by appropriately combining the above-mentioned emission layer and fluorescent substance. It is to be noted that the constitution of Embodiment 6 may be freely combined with any one of the constitutions of Embodiments 1 through 6.  
         [0074]    Embodiment 7  
         [0075]    The light-emitting device manufactured by implementing the present invention can be used as a display portion of various kinds of electric equipments. For instance, when appreciating a television broadcast or the like with a television equipped with a large screen, a display incorporating a light-emitting device of the present invention in a casing may be used as a display of 20 to 60 inch in diagonal. Note that a personal computer display, a television broadcast receiving display, and a display for exhibiting all information such as a display for displaying announcements are included in the displays having the light-emitting device incorporated in a casing.  
         [0076]    Further, the light-emitting device of the present invention may be used as the light source of a back light in a liquid crystal display incorporating a liquid crystal module (liquid crystal panel) in a casing. The light-emitting device of the present invention may also be used as the light source of a fluorescent display lamp or the like at a construction site. The liquid crystal display and the fluorescent display lamp are also electric equipments.  
         [0077]    The light-emitting device of the present invention can further be used as a display portion for various kinds of electric equipments. The following can be given as other electric equipments according to the present invention: a video camera; a digital camera; a goggle type display (head mounted display); a navigation system; an audio playback device (typically, a car audio stereo or an audio component stereo); a notebook type personal computer; a game apparatus; a portable information terminal (typically, a mobile computer, a portable telephone, a portable game machine, or an electronic book); and an image playback device (typically, a device provided with a display portion which plays back images in a recording medium and displays the images). In particular, a wide angle of view is important for a portable information terminal often seen from an oblique angle, and therefore it is preferable to use the light-emitting device. Specific examples of these electric equipments are shown in FIGS. 8A to  9 B.  
         [0078]    [0078]FIG. 8A shows a display having a light-emitting device incorporated in a casing, and the display contains a casing  2001 , a support stand  2002 , a display portion  2003  and the like. The light-emitting device of the present invention can be used as the display portion  2003 . Such a display is a self-emitting type so that a back light is not necessary. Thus, the display portion can be made thinner than that of a liquid crystal display.  
         [0079]    [0079]FIG. 8B shows a video camera, and contains a main body  2001 , a display portion  2102 , a sound input portion  2103 , operation switches  2104 , a batters  2105 , an image receiving portion  2106  and the like. The light-emitting device of the present invention can be used as the display portion  2102 .  
         [0080]    [0080]FIG. 8C is a portion (right side) of a head mounted EL display, and contains a main body  2201 , a signal cable  2202 , a head fixing band  2203 , a display portion  2204 , an optical system  2205 , a light-emitting device  2206  and the like. The present invention can be applied to the light-emitting device  2206 .  
         [0081]    [0081]FIG. 8D is an image playback device equipped with a recording medium (specifically, a DVD playback device), and contains a main body  2301 , a recording medium (such as a DVD)  2302 , operation switches  2303 , a display portion (a)  2304 , a display portion (b)  2305  and the like. The display portion (a)  2304  is mainly used for displaying image information. The display portion (b)  2305  is mainly used for displaying character information. The light-emitting device of the present invention can be used as the display portion (a)  2304  and as the display portion (b)  2305 . Note that the image playback device equipped with the recording medium includes devices such as household game machines.  
         [0082]    [0082]FIG. 8E shows a mobile computer, and contains a main body  2401 , a camera portion  2402 , an image receiving portion  2403 , operation switches  2404 , a display portion  2405  and the like. The light-emitting device of the present invention can be used as the display portion  2405 .  
         [0083]    [0083]FIG. 8F is a personal computer, and contains a main body  2501 , a casing  2502 , a display portion  2503 , a keyboard  2504  and the like. The light-emitting device of the present invention can be used as the display portion  2503 .  
         [0084]    Note that if the luminance increases in the future, then it will become possible to use the light-emitting device of the present invention in a front type or a rear type projector by expanding and projecting light containing output image information with a lens, an optical fiber or the like.  
         [0085]    In addition, since the light-emitting device conserves power in the light emitting portion, it is preferable to display information so as to make the light emitting portion as small as possible. Consequently, when using the light-emitting device in a display portion mainly for character information, such as in a portable information terminal, in particular a portable telephone or an audio playback device, it is preferable to drive the light-emitting device so as to form character information by the light emitting portions while non-light emitting portions are set as background.  
         [0086]    [0086]FIG. 9A shows a portable telephone, and contains a main body  2601 , a sound output portion  2602 , a sound input portion  2603 , a display portion  2604 , operation switches  2605 , and an antenna  2606 . The light-emitting device of the present invention can be used as the display portion  2604 . Note that by displaying white color characters in a black color background, the display portion  2604  can suppress the poster consumption of the portable telephone.  
         [0087]    [0087]FIG. 9B shows an audio playback device, specifically a car audio stereo, and contains a main body  2701 , a display portion  2702 , and operation switches  2703  and  2704 . The light-emitting device of the present invention can be used as the display portion  2702 . Further, a car audio stereo is shown in Embodiment 7, but a portable type or a household audio playback device may also be used. Note that by displaying white color characters in a black color background, the display portion  2704  can suppress the power consumption. This is especially effective in a portable type audio playback device.  
         [0088]    Thus, the application range of the present invention is extremely wide, whereby it may be employed in electric equipments of all fields. Further, the electric equipments of Embodiment 7 may employ the light-emitting device having any of the constitutions of Embodiments 1 through 5.  
         [0089]    Embodiment 8  
         [0090]    In the case of using electric equipment employing the light-emitting device of the present invention as its display portion outdoors, of course there are cases where the display portion is seen in a dark place and a bright place. At this point, even if the luminance thereof is not so high in the dark place, the display can be sufficiently recognized. However, there are cases where the display cannot be recognized if the luminance is not high in a bright place. In the case of the EL light-emitting device, the luminance changes in proportion to the amount of current operating the elements. Therefore, when the luminance is raised, the consumption power is also increased. However, if the emission luminance is adjusted to such a high level, an excessive bright display will be exhibited with only the increased consumption power in a dark place.  
         [0091]    Preparing for such cases, it is effective to provide, in the light-emitting device or the electric equipment of the present invention, a sensor to sense the brightness outdoors such that the light-emitting device or the electric equipment has a function for adjusting the emission luminance in response to the level of the brightness that was sensed. In other words, the emission luminance is made high in a bright place and the emission luminance is made low in a dark place. As a result, together with preventing an increase in the consumption power, an EL light-emitting device that will not impart a feeling of fatigue to the observer can be realized.  
         [0092]    Note that as a sensor for sensing the outdoor brightness, a CMOS sensor and a CCD (charge coupled device) can be used. The CMOS sensor can be formed integrally on the substrate that has the EL elements formed thereon by using a known technique, or an IC may be attached externally. Further, an IC having the CCD formed therein may be attached to the substrate having the EL elements formed thereon, or a structure may be such that the CCD or the CMOS sensor is provided in a part of the electric equipment using the EL light-emitting device as its display portion.  
         [0093]    In response to the signals obtained through the sensor that senses the outdoor brightness, a control circuit for changing the amount of current operating the luminous elements is provided to thereby adjust the emission luminance of the EL elements in response to the outdoor brightness that was sensed. Note that such adjustments may be performed automatically or manually.  
         [0094]    By applying Embodiment 8 to the EL light-emitting device or to the electric equipment of the present invention, it is possible to further reduce the consumption power of the EL light-emitting device. It is to be noted that Embodiment 8 may be implemented in any of the electric equipments shown in Embodiment 7.  
         [0095]    Implementation of the present invention makes it possible to reduce the driving voltage of the EL elements, whereby the reduction in consumption power and the improvement in the reliability (longer life) of the light-emitting device can be attained. In addition, the consumption power of the electric equipment using the light-emitting device of the present invention as its display portion may be reduced.