Abstract:
A cathode structure for inverted OLEDs is provided, which comprise a substrate, a conductive electrode layer, an organic material layer, a dielectric layer, and a metal layer. Wherein, the conductive electrode layer is disposed over the substrate, the organic structure layer is disposed over the conductive electrode layer, the dielectric layer is disposed over the organic material layer, and the metal layer is disposed over the dielectric layer. Such cathode structure can function without using the metals of low work function and high chemical activity so as to benefit the manufacturing of organic light emitting devices and displays, and provide a more stable working conditions.

Description:
1. FIELD OF THE INVENTION  
       [0001]     The present invention relates to a cathode structure for inverted Organic Light Emitting Devices (OLED), in particular to a cathode structure for inverted OLEDs including an electron injection layer.  
       2. BACKGROUND OF THE INVENTION  
       [0002]     For more than decades, researches focused on materials of organic conductive molecules and macromolecules were developing rapidly. By virtue of the maturity of organic conductor, insulator, and semiconductor materials, the application of such organic semiconductor materials in electronic devices and optoelectronic devices, such as organic light-emitting diodes, organic laser, organic memory, solar cells, thin film transistors (TFTs), and so on, have been gradually attracted their potential. Generally, the organic semiconductor-based optoelectronic devices possessing the characteristics of being fabricated as thin film device and by low temperature process can be used in various substrates and large size manufacturing process, that is different from the conventional inorganic semiconductor. 
        The earliest organic light-emitting device (OLED) was disclosed in 1963 by Pope et al., in which a light-emitting phenomenon was observed while a bias voltage of 1000V is being applied on an antrhacene crystal of 1 mm in thickness. However, the operating voltage is too high to be applied in real display devices. Current OLEDs and manufacturing methods of the same were disclosed by C. W. Tang and S. A. VanSlyke of Eastman Kodak Corporation in 1987, that a vacuum evaporation method was used to deposit amorphous organic films sequentially on a glass substrate having a transparent electrode of Indium-Tin-Oxide (ITO), and then the substrate with several layer of organic films deposited thereon is plated with a layer of cathode so as to complete an OLED. Such OLED can largely reduce the operating voltage to below 10V enabling the OLED to be used in real world.        
 
         [0004]     Conventional organic light emitting devices are all forward-stacked structures consisting of bottom anode layer and top cathode layer, as shown in  FIG. 1 . The organic light emitting device  1  comprises a substrate  11 , an anode layer  12 , an organic structure layer  13 , and a cathode layer  14  stacked sequentially. When the OLED is applied in fabricating an active-matrix organic light emitting display (AMOLED), the organic light emitting device  1  is coupling to the transistors of active-matrix driving circuit arranged on the substrate  11  (not shown in the figure) via the anode layer  12 , and it is preferred to employ an equivalent voltage-controlled current source for driving the circuit of the organic light emitting device  1  that the equivalent voltage-controlled current source usually can be a p-channel transistor, as shown in  FIG. 1B . However, in a common transistor, such as a-Si field effect transistor (FET) and poly-Si FET, the electric characteristics of p-channel transistor like carrier mobility are obviously inferior to those of n-channel transistor, and thus there is only n-channel transistor available for the a-Si FET. The inverted OLED including the structure consisting of a bottom cathode layer and a top anode layer is an OLED capable of employing n-channel transistors as an equivalent voltage-controlled current source for driving the circuit of the organic light emitting device. As shown in  FIG. 2A , the inverted OLED  2  comprises a substrate  21 , a cathode layer  22 , an organic structure layer  23 , and an anode layer  24  stacked sequentially, wherein n-channel transistors, as shown in  FIG. 2B , are used as equivalent voltage-controlled current source for driving the circuit of the organic light emitting device, so that not only the design variability of the active-matrix driving circuit can be increased, but also the efficiency of the AMOLED is raised.  
         [0005]     The key factors considered while manufacturing an inverted OLED are the charge transported characteristic and the charge injection capability on the interfaces between device electrode and organic material, and between organic material and another organic material. Since most common organic optoelectronic materials have small electron affinities (EA) (i.e. about or less than 3 ev), some metals of lower work function such as Mg, Ca, Li, and Cs are often chosen to be the cathodes of organic light emitting devices. However, such metals usually have high chemical activity and are easy to deteriorate, that increases the difficulty of process control while the OLED is mass-produced. Besides, the different deposition sequence of metal and organic material will also affect the capability of electron injection on the interface of metal/organic.  
         [0006]     V. Bulovic et al. has disclosed a method of using an alloy of Mg and Ag as the bottom cathode of an inverted OLED in 1997. However, the electron injection capability is not good enough, that cause the operating voltage of the OLED is still too high. In addition, to use metal Mg for the bottom cathode will result in the problem of easy deterioration due to its high chemical activity so that such method will affect the device characteristics, causing some integration problems in the following processes for manufacturing organic light emitting displays.  
         [0007]     In 2002, X. Zhou et al. and S. R. Forrest et al. published an inverted OLED of p-i-n structure, of which the organic material is doped with metals of high activity and low work function, such as Li and Cs, acting as the n-type dopants for promoting the electrons to be injected from the bottom cathode of the inverted OLED into the organic layer. Such method makes it possible to use the conductive materials of low chemical activity but high stability as the bottom cathode of an inverted OLED. However, the electron injection layer of the cathode of the foregoing OLED is formed by doping an organic material layer with metals of high activity and low work function as n-type dopants, that the above mentioned method still suffer the same difficulty of handling metals of high activity and low work function. In addition, atoms of such metals, i.e. Li and Cs, are easily diffused in the organic material, which will affect the operating of device.  
         [0008]     In summary, from the past scientific or technical literatures referring inverted OLEDs, the structures and processes of bottom cathode all include the usage of metals with high chemical activity and low work function, which are easily deteriorated and thus affect the characteristics of the OLED. Moreover, it is still difficult to handle the metals with high chemical activity in the process for manufacturing organic light emitting display.  
       SUMMARY OF THE INVENTION  
       [0009]     It is the primary object of the present invention to provide a cathode structure for an inverted OLED, which includes an electron injection layer containing no metals of high activity and low work function and a cathode layer. The electron injection layer can increase the electron injection capability of the inverted OLED so as to improve the optoelectronic characteristics of the inverted OLED.  
         [0010]     It is another object of the present invention to provide a cathode structure for an inverted OLED, which possesses the excellent electron injection capability without using metals of high activity and low work function so as to prevent the problem of deterioration and be more compatible to the current OLED manufacturing process, and thus enable the inverted OLED to be applied in an active-matrix organic light emitting display (AMOLED).  
         [0011]     In order to achieve the aforesaid objects, the present invention provides a cathode structure for an inverted OLED, comprising: a conductive electrode layer; an organic material layer arranged on top of the conductive electrode layer; a dielectric layer arranged on top of the organic material layer; and a metal layer arranged on top of the dielectric layer; wherein the conductive electrode layer is used as the cathode of the OLED; and the organic material layer, the dielectric layer and the metal layer are used as an electron injection layer of the OLED.  
         [0012]     Further, in order to achieve the aforesaid objects, the present invention provides an inverted OLED, comprising a substrate, a conductive electrode layer, an organic material layer, a dielectric layer, a metal layer, an organic structure layer, and an anode layer. Wherein, the conductive electrode layer is disposed over the substrate, the organic structure layer is disposed over the conductive electrode layer, the dielectric layer is disposed over the organic material layer, the metal layer is disposed over the dielectric layer, the organic structure layer is disposed over the electron injection layer, and the anode layer is disposed over the organic structure.  
         [0013]     The organic layer of the inverted OLED according to the present invention can be a single organic layer possessing the functions of positive/negative charge transporting and light-emitting. It also can be a multi-layer structure, for example, 1) it can be a multi-layer structure consisted of an electron transporting/light-emitting layer and an electron hole transporting g layer sequentially deposited on the cathode structure; 2) it can be a multi-layer structure consisted of an electron transporting layer and an electron hole transporting/light-emitting layer sequentially deposited on the cathode structure; 3) it can be a multi-layer structure consisted of an electron transporting layer, a light-emitting layer, and an electron hole transporting layer sequentially deposited on the cathode structure. The aforesaid stacked structures of organic layers are in an illustrative but not restrictive sense. It is intended that the present invention may not be limited to the particular forms as illustrated.  
         [0014]     Other objects, advantages and novel features of the present invention will become apparent upon study of the remaining portions of the specification and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS FIG.  1 A is the schematic view showing a conventional OLED.  
       [0015]      FIG. 1B  is the schematic view showing the circuit of p-channel transistor.  
         [0016]      FIG. 2A  is the schematic view showing an inverted OLED.  
         [0017]      FIG. 2B  is the schematic view showing the circuit of n-channel transistor.  
         [0018]      FIG. 3  is the schematic view showing an inverted OLED according to the present invention.  
         [0019]      FIG. 4A  is the schematic view showing a bicathode electron/single carrier device according to the first embodiment of the present invention.  
         [0020]      FIG. 4B  is the schematic view showing another bicathode electron/single carrier device according to the first embodiment of the present invention.  
         [0021]      FIG. 4C  is the schematic view showing the relationship of voltage versus current density according to the first embodiment of the present invention.  
         [0022]      FIG. 5A  is the schematic view showing a bicathode electron/single carrier device according to the second embodiment of the present invention.  
         [0023]      FIG. 5B  is the schematic view showing another bicathode electron/single carrier device according to the second embodiment of the present invention.  
         [0024]      FIG. 5C  is the schematic view showing the relationship of voltage versus current density according to the second embodiment of the present invention.  
         [0025]      FIG. 6A  is the schematic view showing the characteristic curves of current density versus voltage of the devices according to the third embodiment of the present invention.  
         [0026]      FIG. 6B  is the schematic view showing the characteristic curves of brightness versus current density of the devices according to the third embodiment of the present invention.  
         [0027]      FIG. 6C  is the schematic view showing the characteristic curves of radiating efficiency versus current density of the devices according to the third embodiment of the present invention.  
         [0028]      FIG. 7A  is the schematic view showing the characteristic curves of current density versus voltage of the devices according to the forth embodiment of the present invention.  
         [0029]      FIG. 7B  is the schematic view showing the characteristic curves of brightness versus current density of the devices according to the forth embodiment of the present invention.  
         [0030]      FIG. 7C  is the schematic view showing the characteristic curves of radiating efficiency versus current density of the devices according to the forth embodiment of the present invention.  
         [0031]      FIG. 8A  is the schematic view showing the characteristic curves of brightness versus voltage of the devices according to the fifth embodiment of the present invention.  
         [0032]      FIG. 8B  is the schematic view showing the characteristic curves of brightness versus current density of the devices according to the fifth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     For your esteemed members of reviewing committee to further understand and recognize the fulfilled functions and structural characteristics of the invention, several preferable embodiments cooperating with detailed description are presented as the follows.  
         [0034]      FIG. 3  is the schematic view showing an inverted OLED according to the present invention. The inverted OLED  3  comprises a substrate  31 , a cathode layer  32 , an electron injection layer  33 , an organic structure layer  34 , and an anode layer  35 . Wherein, the cathode layer  32  is disposed over the substrate  31 , the electron injection layer  33  is disposed over the cathode layer  32 , the organic structure layer  34  is disposed over the electron injection layer  33 , and the anode layer  35  is disposed over the organic structure layer  34 . The electron injection layer  33  further comprises an organic material layer  331  disposed over the cathode layer  32 , a dielectric layer  332  disposed over the organic material layer  331 , and a metal layer  333  disposed over the dielectric layer  332 .  
         [0035]     The organic material layer  331  can be made of Alq 3 . The dielectric layer  332  can be made of a material selected from the group consisting of halogen compounds with alkaline metals, halogen compounds with alkaline-earth metals, alkaline metal oxide, alkaline-earth metal oxide, other alkaline metal and alkaline-earth metal compounds, such as LiF, LiO2, NaF, NaCl, CsF, CaF2, MgF2 etc., or can be a stacked structure consisting of layers of foregoing materials or made of a mixture of foregoing materials. The metal layer  333  can be made of aluminum or the alloy of aluminum. The substrate  31  can be either a transparent substrate, made of a transparent material such as glass, quartz, and plastic, or an opaque substrate, made from silicon wafer and GaAs wafer, etc. The cathode layer  32  can be either transparent electrodes, made of materials such as ITO and IZO, etc. or opaque electrodes, made of materials uch as Au, Ag, Cu, Al, Cr, Mo, Ti, Ni, Pt, Ir, W, and TA, or the stacked or the mixture of the above materials. The anode layer  35  can comprise an electron hole injection layer and a conductive anode (not shown in the figure), wherein the electron hole injection layer can be the layer doped with conductive dopants, such as m-MTDATA:F4-TCNQ and α-NPD:F4-TCNQ, etc. The conductive anode can be either a transparent electrode such as ITO, IZO, AZO, Indium Oxide, Tin Oxide, Zinc Oxide, or the stacked structure or the mixture of these materials, or an opaque electrode such as Au, Ag, Cu, Al, Cr, Mo, Ti, Ni, Pt, Ir, W, and TA, or the stacked structure or the mixture of these materials.  
         [0036]     The following Embodiments 1 through 5 are the preferred embodiments of the cathode structure for an inverted OLED according to the present invention, where the efficiency of the present invention is illustrated by comparing the cathode structures of conventional inverted OLED with those of the present invention.  
       EMBODIMENT 1  
       [0037]     To illustrate that the electron injection layer of the present invention is capable of improving the electron injection capability of the inverted OLED having the cathode made of Ag, we use the bicathode electron/single carrier device for the comparison.  
         [0038]     Please refer to  FIG. 4A , Device A of the present embodiment comprises a substrate  41  made of glass, a bottom cathode layer  42  made of Ag with 80 nm in thickness, an organic electron transporting layer  43  made of Alq 3  with 80 nm in thickness, and a top cathode layer  44  made of a stacked structure of LiF/Al with 0.5 nm and 100 nm in thickness respectively.  
         [0039]     Please refer to  FIG. 4B , Device B comprises a substrate  51  made of glass, a bottom cathode layer  52  made of Ag with 80 nm in thickness, an electron injection layer  53  made of Alq 3 /LiF/Al with 0.2 nm, 0.2 nm and 0.3 nm in thickness respectively, an organic electron transporting layer  54  made of Alq 3  with 80 nm in thickness, and a top cathode layer  55  made of LiF/Al with 0.5 nm and 100 nm in thickness respectively.  
         [0040]     The difference between Device A and B is that Device B has an electron injection layer  53  added between the bottom cathode layer  52  and the organic electron transporting layer  54 .  
         [0041]      FIG. 4C  is the schematic view showing the relationship of voltage versus current according to the first embodiment of the present invention. As shown in  FIG. 4C , while both Device A and B are subjecting to a forward biases, that is, electrons are injected from the top cathode, both Devices A and B shows excellent and almost identical V-I characteristic curves because the both top cathodes possess cathode structures of good electron injection characteristic. However, while both Device A and B are subjecting to a reverse biases, that is, electrons are injected from the bottom cathode, the injection current of Device A is obviously lower than that of Device B at the same operating voltage. In addition, the reverse bias V-I curve of Device B is mirror to the forward bias V-I curve of Device B using the vertical line at V=0 as the symmetry axis, indicating that the electron injection layer of the inverted OLED cathode structure according to the present invention can largely improve the electron injection capability of bottom cathode and such electron injection capability of the bottom cathode is compatible to that of conventional cathode structure.  
       EMBODIMENT 2  
       [0042]     To illustrate that the electron injection layer of the present invention is capable of improving the electron injection capability of the inverted OLED having the cathode made of Al, we use the bicathode electron/single carrier device for the comparison.  
         [0043]     Please refer to  FIG. 5A , Device C of the present embodiment comprises a substrate  61  made of glass, a bottom cathode layer  62  made of Al with 80 nm in thickness, an organic electron transporting layer  63  made of Alq 3  with 80 nm in thickness, and a top cathode layer  64  made of LiF/Al with 0.5 nm and 100 nm in thickness respectively.  
         [0044]     Please refer to  FIG. 5B , Device D comprises a substrate  71  made of glass, a bottom cathode layer  72  made of Al with 80 nm in thickness, an electron injection layer  73  made of Alq 3 /LiF/Al with 0.2 nm, 0.2 nm and 0.3 nm in thickness respectively, an organic electron transporting layer  74  made of Alq 3  with 80 nm in thickness, and a top cathode layer  75  made of LiF/Al with 0.5 nm and 100 nm in thickness respectively.  
         [0045]     The difference between Device C and D is that the Device D has an electron injection layer  73  added between the bottom cathode layer  72  and the organic electron transmitting layer  74 .  
         [0046]      FIG. 5C  is the schematic view showing the relationship of voltage versus current according to the second embodiment of the present invention. As shown in  FIG. 5C , while both Device C and D are subjecting to a forward biases, that is, electrons are injected from the top cathode, both Devices C and D show excellent and almost identical V-I characteristic curves because the both top cathodes possess cathode structures of good electron injection characteristic. However, while both Device C and D are subjecting to a reverse biases, that is, electrons are injected from the bottom cathode, the injection current of Device C is obviously lower than that of Device D at the same operating voltage. In addition, the reverse bias V-I curve of Device D is mirror to the forward bias V-I curve of Device D using the vertical line at V=0 as the symmetry axis, indicating that the electron injection layer of the inverted OLED cathode structure according to the present invention can largely improve the electron injection capability of bottom cathode and such electron injection capability of the bottom cathode is compatible to that of conventional cathode structure.  
       EMBODIMENT 3  
       [0047]     To illustrate the influence of adding an electron injection layer according to the present invention on the electron injection capability of the inverted OLED having the cathode made of Ag, we use the devices described hereinafter for the comparison.  
         [0048]     In this embodiment, the structure of Device E can refer to  FIG. 2A , wherein the substrate  21  is a glass substrate, the cathode layer  22  is made of Ag with 80 nm in thickness, the organic structure layer  23  is a stacked structure consisting of a layer of Alq 3  with 50 nm in thickness, a layer of α-NPD with 40 nm in thickness, and a layer of a mixture of F 4 -TCNQ and 2 wt. % m-MTDATA with 20 nm in thickness. Moreover, the anode layer  24  is made of Ag with 20 nm in thickness, on which a refractive index matched layer made of TeO 2  with 30 nm in thickness is formed.  
         [0049]     The structure of Device F can refer to  FIG. 3 , wherein the substrate  31  is a glass substrate, the cathode layer  32  is made of Ag with 80 nm in thickness, the electron injection layer  33  is a stacked structure consisting of a layer of Alq 3  with 0.2 nm in thickness, a layer of LiF with 0.2 nm in thickness, and a layer of Al with 0.3 nm in thickness, the organic structure layer  34  is a stacked structure consisting of a layer of Alq 3  with 50 nm in thickness, a layer of α-NPD with 40 nm in thickness, and a layer of a mixture of F 4 -TCNQ and 2 wt. % m-MTDATA with 20 nm in thickness. In addition, the anode layer  35  is made of Ag with 20 nm in thickness, on which a refractive index matched layer made of TeO 2  with 30 nm in thickness is formed.  
         [0050]     The difference between Device E and F is that the Device F has an electron injection layer  33  added between the cathode layer  32  and the organic structure layer  34 . In the present embodiment, the layer of Alq 3  with 50 nm in thickness is used as the electron transporting/light-emitting layer, the layer of α-NPD with 40 nm in thickness is used as the electron hole transporting layer, and the layer of the mixture of F 4 -TCNQ and 2 wt. % m-MTDATA with 20 nm in thickness is used as the electron hole injecting layer.  
         [0051]      FIG. 6A  is the schematic view showing the characteristic curves of current density versus voltage of Device E and F. As shown in the figure, Device F including the electron injection layer has more current than Device E in the same operating voltage, indicating that the electron injection layer improves the electron injection characteristic of Device F.  FIG. 6B  is the schematic view showing the characteristic curves of brightness versus current of the two devices, and  FIG. 6C  is the schematic view showing the characteristic curves of radiating efficiency versus current density of the two devices. As shown in  FIG. 6B , the brightness of Device F is much better than Device E in the same injection current so the radiating efficiency of Device F can attain to 5.3 cd/A much higher than that of Device E, as shown in  FIG. 6C , which indicates that the electron injection layer of Device F can efficiently increase the electron injection capability of the device so as to balance the number of electrons and holes in the device thus obtaining the higher radiating efficiency.  
       EMBODIMENT 4  
       [0052]     To illustrate the influence of adding an electron injection layer according to the present invention on the electron injection capability of the inverted OLED having the cathode made of Al, we use the devices described hereinafter for the comparison.  
         [0053]     In this embodiment, the structure of Device G can refer to  FIG. 2A , wherein the substrate  21  is a glass substrate, the cathode layer  22  is made of Al with 80 nm in thickness, the organic structure layer  23  is a stacked structure consisting of a layer of Alq 3  with 50 nm in thickness, a layer of α-NPD with 40 nm in thickness, and a layer of a mixture of F 4 -TCNQ and 2 wt. % m-MTDATA with 20 nm in thickness. Moreover, the anode layer  24  is made of Ag with 20 nm in thickness, on which a refractive index matched layer made of TeO 2  with 30 nm in thickness is formed.  
         [0054]     The structure of Device H can refer to  FIG. 3 , wherein the substrate  31  is a glass substrate, the cathode layer  32  is made of Al with 80 nm in thickness, the electron injection layer  33  is a stacked structure consisting of a layer of Alq 3  with 0.2 nm, a layer of LiF with 0.2 nm, and a layer of Al with 0.3 nm in thickness, the organic structure layer  34  is a stacked structure consisting of a layer of Alq 3  with 50 nm in thickness, a layer of α-NPD with 40 nm in thickness, and a layer of a mixture of F 4 -TCNQ and 2 wt. % m-MTDATA with 20 nm in thickness. In addition, the anode layer  35  is made of Ag with 20 nm in thickness, on which a refractive index matched layer made of TeO 2  with 30 nm in thickness is formed.  
         [0055]     The difference between Device G and H is that Device H has an electron injection layer  33  added between the cathode layer  32  and the organic structure layer  34 . In the present embodiment, the layer of Alq 3  with 50 nm in thickness is used as the electron transporting/light-emitting layer, the layer of α-NPD with 40 nm in thickness is used as the electron hole transporting layer, and the layer of the mixture of F 4 -TCNQ and 2 wt. % m-MTDATA with 20 nm in thickness is used as the electron hole injecting layer.  
         [0056]      FIG. 7A  is the schematic view showing the characteristic curves of current density versus voltage of Device G and H. As shown in the figure, Device H including the electron injection layer has more current than Device G in the same operating voltage, indicating that the electron injection layer improves the electron injection characteristic of Device H.  FIG. 7B  is the schematic view showing the characteristic curves of brightness versus current of the two devices, and  FIG. 7C  is the schematic view showing the characteristic curves of radiating efficiency versus current density of the two devices. As shown in  FIG. 7B , the brightness of Device H is much better than Device G in the same injection current so the radiating efficiency of Device F can attain to 4.5 cd/A much higher than 1.7 cd/A of Device G, as shown in  FIG. 7C , which indicates that the electron injection layer of Device H can efficiently increase the electron injection capability of the device so as to balance the number of electrons and holes in the device thus obtaining the higher radiating efficiency.  
       EMBODIMENT 5  
       [0057]     To illustrate the influence of adding an electron injection layer according to the present invention on the electron injection capability of the inverted OLED having the cathode made of ITO, we use the devices described hereinafter for the comparison.  
         [0058]     In this embodiment, the structure of Device I can refer to  FIG. 2A , wherein the substrate  21  is a glass substrate, the cathode layer  22  is made of ITO with 120 nm in thickness, the organic structure layer  23  is a stacked structure consisting of a layer of Alq 3  with 40 nm in thickness, a layer of α-NPD with 30 nm in thickness, and a layer of a mixture of F 4 -TCNQ and 2 wt. % m-MTDATA with 20 nm in thickness, in addition, the anode layer  24  is made of Ag with 20 nm in thickness.  
         [0059]     The structure of Device J can refer to  FIG. 3 , wherein the substrate  31  is a glass substrate, the cathode layer  32  is made of ITO with 120 nm in thickness, the electron injection layer  33  is a stacked structure consisting of a layer of Alq 3  with 0.2 mm, a layer of LiF with 0.2 nm, and a layer of Al with 0.3 nm in thickness, the organic structure layer  34  is a stacked structure consisting of a layer of Alq 3  with 40 nm in thickness, a layer of α-NPD with 30 nm in thickness, and a layer of a mixture of F 4 -TCNQ and 2 wt. % m-MTDATA with 20 nm in thickness, in addition, the anode layer  35  is made of Ag with 100 nm in thickness.  
         [0060]     The difference between Device I and J is that the Device J has an electron injection layer  33  added between the cathode layer  32  and the organic structure layer  34 . In the present embodiment, the layer of Alq 3  with 40 nm in thickness is used as the electron transporting/light-emitting layer, the layer of α-NPD with 30 nm in thickness is used as the electron hole transporting layer, and the layer of the mixture of F 4 -TCNQ and 2 wt. % m-MTDATA with 20 nm in thickness is used as the electron hole injecting layer.  
         [0061]      FIG. 8A  is the schematic view showing the characteristic curves of brightness versus voltage of the Device I and J, and  FIG. 8B  is the schematic view showing the characteristic curves of brightness versus current density of the two devices. As shown in  FIG. 8A , the brightness of Device J is much better than Device I in the same voltage so the radiating efficiency of Device J can attain to 1.7 cd/A much higher than about 0 cd/A of Device I, as shown in  FIG. 8B , which indicates that the electron injection layer of Device J can efficiently increase the electron injection capability of the device so as to balance the number of electrons and holes in the device thus obtaining the higher radiating efficiency.  
         [0062]     While the preferred embodiment of the invention has been set forth for the purpose of disclosure, modifications of the disclosed embodiment of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.