Abstract:
A white organic light-emitting diode includes two symmetric emission layers and a middle emission layer. The two symmetric emission layers emit a first color light with approximately the same frequency components. The middle emission layer is located between the two symmetric emission layers. The middle emission layer emits a second color light with frequency components different from main frequency components of the first color light. When the voltage applied to the organic light-emitting diode changes and leads to a decrease of luminescent intensity of one of the symmetric emission layers, the other symmetric emission layer automatically increases the luminescent intensity to compensate for the reduced light intensity.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is based on, and claims priority from, Taiwan Application Serial Number 94127032, filed Aug. 9, 2005, the disclosure of which is hereby incorporated by reference herein in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of Invention  
         [0003]     The present invention relates to a light-emitting device. More particularly, the present invention relates to an organic light-emitting diode.  
         [0004]     2. Description of Related Art  
         [0005]     A white organic light-emitting diode (WOLED) can be made by a single luminescent layer or multiple luminescent layers. However, the color of the emitting light of the same WOLED varies when the applied operating voltage varies. That is, the wavelength of the emitted light is shifted by applying different operating voltages to the WOLED.  
         [0006]     The individual layers of a conventional WOLED are a substrate, an anode (ITO), a hole injection layer (PEDOT:PSS, 40 nm), a hole transfer layer (a-NPD, 40 nm), a blue light-emitting layer ((CF 3 ppy) 2 Ir(pic):CDBP, 25 nm)), a hole barrier layer (BAlq, 3 nm), a red light-emitting layer (Ir(btp) 2 (acac):CDBP, 10 nm), an electron transfer layer (BAlq, 45 nm), an electron injection layer (LiF, 0.5 nm), and a cathode (Al, 150 nm). The red light (600-700 nm) emitting intensity becomes greater as the applied voltage of WOLED is increased (Current Applied Physics 2005, Vol. 5, pp. 331-336). Because of this increasing red shift, controlling the emitting light color of a WOLED is difficult.  
       SUMMARY OF THE INVENTION  
       [0007]     It is therefore an aspect of the present invention to provide a WOLED capable of compensating for color shift of its emitting light due to applied voltage variation.  
         [0008]     In another aspect, the present invention provides a WOLED, which can be easily fabricated and can decrease color shift of its emitting light.  
         [0009]     In yet another aspect, the present invention provides a WOLED with a simple structure to decrease color shift of its emitting light.  
         [0010]     In accordance with the foregoing and other aspects of the present invention, a symmetric organic light-emitting device is provided. According to a preferred embodiment of this invention, the symmetric organic light-emitting device comprises two symmetric luminescent layers and a central luminescent layer located between the two symmetric luminescent layers. The two symmetric luminescent layers can generate a first color light having main peaks of about the same frequency. The central luminescent layer generates a second color light having a main peak with a different frequency from the first color light. When a voltage applied to the organic light-emitting device changes to cause decreasing luminescent intensity of one of the two symmetric luminescent layers, the luminescent intensity of the other symmetric luminescent layer automatically increases to compensate for the decreased luminescent intensity.  
         [0011]     In the foregoing, the invention allows two symmetric luminescent layers located on both sides of a central luminescent layer to generate a same color light. Therefore, the problem of color shift cased by applied voltage variation can be resolved.  
         [0012]     It is to be understood that both the foregoing general description and the following detailed description are made by use of examples and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,  
         [0014]      FIG. 1  is a diagram showing variation curves of CIE x values of devices one to four;  
         [0015]      FIG. 2  is a diagram showing variation curves of CIE y values of devices one to four;  
         [0016]      FIG. 3  is an energy diagram of conventional devices one and two, wherein every number shown near the bottom of each bar represents the energy level (in electron volts) of each highest occupied molecular orbital (HOMO), and every number shown near the top of each bar represents the energy level (in electron volts) of lowest unoccupied molecular orbital (LUMO); and  
         [0017]      FIG. 4  is like  FIG. 3  except that devices three and four are diagrammed.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     This invention provides a white organic light-emitting diode (WOLED). Two luminescent layers are arranged on both sides of multiple luminescent layers to resolve the color shift of light emitting from the WOLED when the applied operating voltage varies.  
         [0019]     The chromaticity coordinates (x, y) of standard white defined by Commission International d&#39;Eclairage (CIE) is (0.33, 0.33). However, many other various white colors, such as cold white, warm white, sunlight white, or purple white, are still possible. Therefore, the chromaticity values of white color can vary within a broad range. However for white light generated by an ideal WOLED, the variation of x or y values in CIE coordinates is better when smaller than 0.04.  
         [0020]     In preferred embodiments of this invention, four WOLEDs are produced. All layers are the same in the four WOLEDs except for multiple luminescent layers. The layers of four WOLEDs are sequentially an anode (Al, 1500 Å), an electron injection layer (LiF, 10 Å), an electron transfer layer (BAlq, 300 Å), multiple luminescent layers, a hole transfer layer (NPB, 200 Å), a hole injection layer (IDE 406:F4, 1500 Å) and a cathode (ITO). The multiple luminescent layers of the four WOLEDs above are listed in table 1 wherein device one and device two do not have symmetric red-light-emitting layers but have the same structure as that of a conventional WOLED. In device three and device four, which are designed according to the preferred embodiment of this invention, two symmetric red light-emitting layers are added on both sides of the blue-light-emitting layer.  
                                                           TABLE 1                           Luminescent layers of devices one to four are listed in the       direction from cathode to anode.            Thickness of luminescent layers   Device   Device   Device   Device       (Å)   one   two   three   four                    Red - light-emitting layer   100   100   50   50       (CBP:RD07)       Blue - light-emitting layer   100   200   200   150       (mCP:Ir-pyb)       Red light-emitting layer   —   —   50   50       (CBP:RD07)                  
 
         [0021]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
               
               
                 Luminescent intensities of blue and red light and the CIE 
               
               
                 chromaticity values of devices one to four. 
               
             
          
           
               
                   
                   
                 Variation 
                   
               
               
                   
                 Operating 
                 percentage of 
                 Variation of CIE 
               
               
                 WOLED 
                 voltage (V) 
                 red light 
                 chromaticity 
               
               
                   
               
               
                 Device one 
                 5-10 
                 29.8% 
                 (0.13, 0.02) 
               
               
                 (R100/B100) 
               
               
                 Device two 
                 6-10 
                 57.1% 
                 (0.07, 0.02) 
               
               
                 (R100/B200) 
               
               
                 Device three 
                 7-12 
                 66.3% 
                 (0.05, 0.01) 
               
               
                 (R50/B200/R50) 
               
               
                 Device four 
                 7-12 
                 — 
                 (0.03, &lt;0.01) 
               
               
                 (R50/B150/R50) 
               
               
                   
               
             
          
         
       
     
         [0022]     The experimental results of devices one to four are listed in Table 2, and the CIE chromaticity variations are shown in  FIGS. 1 and 2 .  FIG. 1  is a diagram showing variation curves of CIE x values of devices one to four; and  FIG. 2  is a diagram showing variation curves of CIE y values of devices one to four.  
         [0023]     Referring to Table 2 and  FIGS. 1 and 2 , when applied voltage increases from 5 V to 10 V, the intensity of emitted blue light of device one (R100/B100) remained almost the same, and the relative peak intensity (about 466 nm) in the blue light spectrum was maintained at about 1. However, the intensity of emitted red light of device one (R100/B100) decreased from about 2.35 to about 0.7. That is, the relative peak intensity (about 625 nm) in the red light spectrum at 10 V was about 29.8% of that at 5 V. The CIE chromaticity value of device one varied from (0.47, 0.32) to (0.34, 0.30). The variation of the CIE chromaticity value was up to (0.13, 0.02).  
         [0024]     After increasing the thickness of the blue light-emitting layer to form device two (R100/B200), the luminescence intensity of blue light of the device two remained almost the same, and the relative peak intensity (about 466 nm) in the blue light spectrum was maintained at about 1. However, the intensity of emitted red light of device two still decreased from about 0.35 to about 0.2. That is, the relative peak intensity (about 625 nm) in the red light spectrum at 10 V was about 57.1% of that at 6 V. The CIE chromaticity value of device two varied from (0.30, 0.28) to (0.23, 0.26). The variation of the CIE chromaticity value was up to (0.07, 0.02). Although the variation of CIE y values slightly decreased, the variation was still greater than the ideal variation range, i.e. smaller than 0.04.  
         [0025]     From the experimental results of devices one and two, one can find that the color of the luminescence has a blue shift problem under high operating voltage. Therefore, it was hoped that the problem of unstable red light-emitting intensity could be solved if another red light-emitting layer was arranged on the other side of the blue light-emitting layer to form a structure of one blue light-emitting layer between two red light-emitting layers.  
         [0026]     Reference is made to Table 2 and  FIGS. 1 and 2  again. Although the device three (R50/B200/R50) used two red light-emitting layers, the total thickness of the two red light layers was still maintained at 100 Å. The luminescence intensity of blue light of the device three was also almost the same when the applied operating voltage increased from 7 V to 12 V. The relative peak intensity (about 466 nm) in the blue light spectrum was maintained at about 1. Contrary to devices one and two, the intensity of emitted red light of device three was only decreased from about 0.95 to about 0.63. That is, the relative peak intensity (about 625 nm) in the red light spectrum at 12V was still about 66.3% of that at 7 V. The variation of the CIE chromaticity value of device three varied from (0.43, 0.28) to (0.38, 0.27). The variation amplitude was (0.05, 0.01), which almost satisfied the requirement to be smaller than the ideal CIE variation (0.04).  
         [0027]     The thickness of the blue light-emitting layer of device three was decreased to 150 Å to form device four. The luminescence intensity variation of blue and red light and the CIE chromaticity value variation with the applied voltage variation were also tested. The CIE chromaticity value of device four varied from (0.46, 0.29) to (0.43, &gt;0.28). The variation amplitude was (0.03, &lt;0.01), which satisfied the requirement to be smaller than the ideal CIE variation (0.04).  
         [0028]     From the experimental results of devices three and four, although the total thickness of red light-emitting layer was not increased and was only divided to two layers located on both sides of the blue light-emitting layer, the blue shift problem of the WOLED luminescence color was also greatly improved. If luminescent material and thickness of each luminescent layer in WOLED can be further well designed, the problem of the luminescence color varying with the applied operating voltage can be further improved.  
         [0029]     The recombination region of electron and hole in a conventional WOLED is shifted when the applied operating voltage varies; hence the luminescent color of a conventional WOLED is also varied.  FIG. 3  is an energy diagram of conventional devices one and two. Every number shown near the bottom of each bar represents the energy level (in electron volts) of each highest occupied molecular orbital (HOMO), and every number shown near the top of each bar represents the energy level (in electron volts) of lowest unoccupied molecular orbital (LUMO). In  FIG. 3 , in the direction from anode to cathode, only hole transfer layer  310 , red light-emitting layer  320 , blue light-emitting layer  330 , and electron transfer layer  350  are shown. When devices one and two are operated under a low voltage, electrons generated from the cathode and holes generated from the anode are combined to form excitons at the interface of the red light-emitting layer  320  and the blue light-emitting layer  350 . Then, the excitons release photons going back to ground state. When devices one and two are operated under a high voltage, the exciton-forming region is moved toward the cathode and apart from the red light-emitting layer  320 . Hence, the luminescence intensity of the red light-emitting layer  320  is decreased.  
         [0030]      FIG. 4  is an energy diagram of devices three and four according to a preferred embodiment of this invention. Again, every number shown near the bottom of each bar represents the energy level (in electron volts) of each highest occupied molecular orbital (HOMO), and every number shown near the top of each bar represents the energy level (in electron volts) of lowest unoccupied molecular orbital (LUMO). In  FIG. 4 , in the direction from anode to cathode, only hole transfer layer  410 , red light-emitting layer  420 , blue light-emitting layer  430 , red light-emitting layer  440  and electron transfer layer  450  are shown. Two red light-emitting layers  420  and  440  with the same thickness are arranged to locate on either side of the blue light-emitting layer  430 . When devices three and four are operated under a low voltage, excitons are generated at the central part of the blue light-emitting layer  450 . When devices three and four are operated under a high voltage, the exciton-forming region is still moved toward the cathode to decrease the luminescence intensity of the red light-emitting layer  420 . However, the red light-emitting layer  440  near the cathode can also emit red light to compensate for the red light intensity loss of the red light-emitting layer  420 . Hence, the total red light-emitting intensity of devices three and four can be maintained without large variation when the applied operating voltage varies.  
         [0031]     Although the thickness of the two red light-emitting layers according to the preferred embodiment of this invention are the same, the material and thickness of the two red light-emitting layers can also be different.  
         [0032]     According to the same principle, the multiple luminescent layers of WOLED can also be, for example, red-blue-red, red-green-red, red-green-blue-green-red, red-green-blue-red, red-blue-green-red, blue-red-green-red-blue, or blue-green-red-blue. The two outermost luminescent layers are better when generating the same or similar color light. Therefore, no matter whether the excitons diffuse toward the anode or cathode, the various luminescent intensity of each color light can be maintained at a rather constant value.  
         [0033]     In light of the foregoing, since the two outermost luminescent layers of the WOLED, according to the preferred embodiments of this invention, generate the same or very similar color light, the color shift problem when applied operating voltage varies can be solved.  
         [0034]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.