Patent Publication Number: US-2023141206-A1

Title: Organic light-emitting diode structure and display device

Description:
CROSS REFERENCE TO RELEVANT APPLICATIONS 
     The present application claims the priority and the benefit of the patent application filed before the China National Intellectual Property Administration on Jul. 23, 2020 with the patent application number of 202010728215.0, which is incorporated herein in its entirety by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to the field of chemical design of luminescent materials, and, particularly, the present disclosure relates to an organic light-emitting diode structure and a display device. 
     BACKGROUND 
     In recent years, organic electroluminescent displays (OLED), as a novel flat panel display, are paid increasingly more attention. Because of the characteristics such as active light emission, a high emitted-light brightness, a high resolution, a wide visual angle, a high response speed, a lower energy consumption and a good flexibility, OLEDs have become a very popular mainstream display product currently in the market. Furthermore, with the continuous development of the products, the users are requiring an increasingly higher resolution and an increasingly lower power consumption of the products. Therefore, it is required to develop devices of a high efficiency, a low voltage and a long life. 
     However, Red-green-blue (RGB) single-color OLED devices easily have attenuation in the life, ultimately because of the aging of the interface and the material deterioration caused by the defect of the material. Firstly, the aging of the interface is because too many electric charges accumulate due to an excessively large energy barrier at the interface. However, the blue-light host material (host) in organic luminescent (EL) devices requires a wide band gap, and therefore the energy level of its highest occupied molecular orbital (HOMO) is very deep, which results in a large energy-level difference between the hole-type electron blocking layer (EBL) and the host, whereby the existence of the energy barrier results in too much hole accumulation. Therefore, the interface easily deteriorates and the life of the device attenuates. 
     Secondly, in OLED devices, the material that most easily deteriorates is EBL materials. That is because the EBL materials themselves are generally an electron-rich system, and generally contain an aniline structure. Too many electrons have a repulsive force with the electron-rich structure of the EBL materials themselves. Such a repulsive force results in twisting of the δ bond of the benzene ring in the aniline (as shown in  FIG.  1   ), and the defect caused by the bond breakage caused by the bond twisting is the primary cause of the life attenuation of the material and the device. Therefore, to reduce the electron accumulation in the EBL materials is a method for improving the stability of the device and increasing the life of the device. Furthermore, the overall exciton recombination region of the device is at the EBL/EML interface, and therefore the EBL materials definitely receive the accumulation of the electrons. Therefore, to improve the stability of the luminescent layer (EML) material is to improve the stability and the life of the device. 
     SUMMARY 
     The present disclosure is made on the basis of the following finding of the inventor: 
     The inventor of the present disclosure, in the studying process, takes into consideration the energy-level relation between the host material (host) and the guest material (dophant) of the luminescent layer, the energy-level relation between the host material and the guest material of the luminescent layer and the adjacent electron blocking layer (EBL) and hole blocking layer (HBL), the limitation on the mobility of the host material, and the characteristics of the structures of the host material and the guest material. That may increase the quantity of the holes in the EML, and reduce the electron accumulation at the EBL/EML interface. Moreover, the optimization of the stability of the host material may increase the life of the luminescent layer, and the optimization of the luminous efficiency of the guest material may improve the luminous efficiency of the luminescent layer. 
     In the first aspect of the present disclosure, the present disclosure provides an organic light-emitting diode structure. 
     According to an embodiment of the present disclosure, the organic light-emitting diode structure includes an anode, an electron blocking layer, a luminescent layer, a hole blocking layer and a cathode that are arranged in layer configuration, a material forming the luminescent layer includes a host material and a guest material, and the electron blocking layer, the luminescent layer and the hole blocking layer satisfy the following conditions: 
     HOMO host −HOMO EBL ≤0.3 eV, 
     LUMO host &gt;LUMO HBL , and 
     HOMO HBL −HOMO host ≥0.1 eV, 
     wherein HOMO host  is a highest occupied molecular orbital of the host material, LUMO host  is a lowest unoccupied molecular orbital of the host material, HOMO EBL  is a highest occupied molecular orbital of the electron blocking layer, HOMO HBL  is a highest occupied molecular orbital of the hole blocking layer, and LUMO HBL  is a lowest unoccupied molecular orbital of the hole blocking layer. 
     The inventor has found out by studying that, in the organic light-emitting diode structure according to the embodiment of the present disclosure, the combination of the energy-level relation of the host material and the guest material of the luminescent layer and the adjacent electron blocking layer and hole blocking layer may enable the organic light-emitting diode structure to have a higher stability and a higher luminous efficiency, which may enable the organic light-emitting diode structure to have a longer service life and a lower power consumption. 
     Furthermore, the organic light-emitting diode structure according to the above embodiment of the present disclosure may also have the following additional technical features: 
     According to an embodiment of the present disclosure, the luminescent layer further satisfies the following conditions: 
     HOMO dopant ≤HOMO host , and 
     LUMO dopant &lt;LUMO host , 
     wherein HOMO dopant  is a highest occupied molecular orbital of the guest material, and LUMO dopant  is a lowest unoccupied molecular orbital of the guest material. 
     According to an embodiment of the present disclosure, the host material has a structure I, and the guest material has a structure III, 
     
       
         
         
             
             
         
       
     
     According to an embodiment of the present disclosure, a hole mobility/electron mobility of the host material is ≥0.001. 
     According to an embodiment of the present disclosure, substituents Ar1, Ar2 and Ar3 in the structure I are independently selected from optionally substituted aryl of a ring-carbon-atom quantity of 6-30 or heteroaryl of a ring-atom quantity of 5-20. 
     According to an embodiment of the present disclosure, the substituent Ar2 or Ar3 is an optionally substituted structure II, and a substituent X in the structure II is an oxygen atom, 
     
       
         
         
             
             
         
       
     
     According to an embodiment of the present disclosure, the substituents Ar1, Ar2 and Ar3 in the structure I are independently selected from phenyl, naphthyl, anthryl, phenanthryl, dibenzofuran, dibenzothiophene, benzonaphthofuran and benzonaphthothiophene. 
     According to an embodiment of the present disclosure, the host material is selected from: 1-1, 1-2, 1-3, 1-4, 1-5, 1-6, 1-7, 1-8, 1-9, 1-10 and 1-11. 
     According to an embodiment of the present disclosure, an emission wavelength of the guest material is 400-500 nm. 
     According to an embodiment of the present disclosure, substituents R1, R2, R3, R4 and R5 in the structure III are independently selected from hydrogen, or optionally substituted heteroatom, amino, C1-20 alkyl, and aryl or heteroaryl of a ring-carbon-atom quantity of 6-20, wherein a substituent in the aryl or heteroaryl includes silyl, cyano, halogen or diplogen. 
     According to an embodiment of the present disclosure, R1, R2, R3, R4 and R5 are independently selected from methyl, tert-butyl, benzo, diphenylamino, 3-methyldiphenylamino and trimethylsilyl. 
     According to an embodiment of the present disclosure, the guest material is selected from: 2-1, 2-2, 2-3, 2-4, 2-5 and 2-6. 
     According to an embodiment of the present disclosure, a doping amount of the guest material is 1-10 wt %. 
     In the second aspect of the present disclosure, the present disclosure provides a display device. 
     According to an embodiment of the present disclosure, the display device includes the organic light-emitting diode structure stated above. 
     The inventor has found out by studying that, in the display device according to the embodiment of the present disclosure, its organic light-emitting diode structure has a higher stability and a higher luminous efficiency, which may enable the display device to have a longer service life and a lower power consumption. A person skilled in the art can understand that the features and the advantages described above with respect to the organic light-emitting diode structure are also suitable for the display device, and are not discussed herein further. 
     Some of the additional aspects and advantages of the present disclosure will be given in the following description, and some will become apparent from the following description or be known from the implementation of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-described aspects of the present disclosure will be interpreted with reference to the following description on the embodiments with reference to the drawings, wherein in the drawings: 
         FIG.  1    is a schematic diagram of the principle of the twisting of the δ bond of the benzene ring in the aniline structure; 
         FIG.  2    is a schematic diagram of the energy-level relation of the host and the guest of the luminescent material and the adjacent layers; and 
         FIG.  3    is a schematic cross-sectional structural diagram of the organic light-emitting diode according to an embodiment of the present disclosure. 
     
    
    
     REFERENCE NUMERALS 
       100  anode 
       200  hole injection layer 
       300  hole transporting layer 
       400  electron blocking layer 
       500  luminescent layer 
       600  hole blocking layer 
       700  electron transporting layer 
       800  electron injection layer 
       900  cathode 
     DETAILED DESCRIPTION 
     The embodiments of the present disclosure will be described in detail below. A person skilled in the art can understand that the following embodiments are intended to interpret the present disclosure, and should not be deemed as a limitation on the present disclosure. Unless indicated particularly, where the particular techniques or conditions of an embodiment in the following are not clearly described, a person skilled in the art may implement the embodiment by using commonly used techniques or conditions in the art or according to the specification of the product. 
     In an aspect of the present disclosure, the present disclosure provides an organic light-emitting diode structure. 
     According to an embodiment of the present disclosure, the organic light-emitting diode structure includes an anode, an electron blocking layer, a luminescent layer, a hole blocking layer and a cathode that are arranged in layer configuration. In some embodiments of the present disclosure, referring to  FIG.  3   , the organic light-emitting diode structure may include an anode  100 , a hole injection layer  200 , a hole transporting layer  300 , an electron blocking layer  400 , a luminescent layer  500 , a hole blocking layer  600 , an electron transporting layer  700 , an electron injection layer  800  and a cathode  900  that are arranged in layer configuration. Accordingly, both of the structure and the function of the organic light-emitting diode structure may be further improved. 
     The inventor of the present disclosure has found out in the studying process that, referring to the schematic diagram of the energy-level relation of the host and the guest of the luminescent material and the adjacent layers shown in  FIG.  2   , the energy-level relation of the host material (host) and the guest material (dophant) of the luminescent layer (EML) and the electron blocking layer (EBL) and the hole blocking layer (HBL) adjacent to the luminescent layer (all of the relations of the magnitudes are absolute values, HOMO is the highest occupied molecular orbital, and LUMO is the lowest unoccupied molecular orbital) is required to satisfy the following 3 conditions: (1) HOMO host −HOMO EBL ≤0.3 eV, whereby reducing the energy-level gap may increase the hole transport; (2) HOMO HBL −HOMO host ≥0.1 eV, which facilitates the blocking of the holes; and (3) LUMO host &gt;LUMO HBL , which facilitates the transport of the electrons. Accordingly, the inventor configures the combination of the energy levels of the components, which may facilitate the transport of the charge carriers into the luminescent layer, thereby reducing the accumulation of the charge carriers at the EBL/EML interface. 
     In some embodiments of the present disclosure, (4) HOMO dopant ≤HOMO host , which facilitates the guest material to capture some holes; and (5) LUMO dopant &lt;LUMO host , which may scatter some electrons. Accordingly, the inventor regulates the combination of the energy levels of the host material and the guest material, which may further facilitate the transport of the charge carriers into the luminescent layer, thereby further reducing the accumulation of the charge carriers at the EBL/EML interface. 
     According to an embodiment of the present disclosure, a material forming the luminescent layer includes a host material and a guest material, wherein the host material has a structure I, and the guest material has a structure III. All of the 5 hydrogen atoms in the benzene ring at one end of the host material are replaced by diplogen atoms, which may increase the bond energy between the benzene ring and the substituent Ar1, thereby enabling the host material to have a better stability. Moreover, the structure III is selected by the intermediate structure of the guest material, this fused structure may reduce the wagging vibration and the stretching vibration of the atoms, reduce non-radiative transition, and purify the spectral colors, thereby increasing the front-face luminous efficiency of the luminescent material. 
     
       
         
         
             
             
         
       
     
     By using the AC3 test, the HOMO host  of the host material having the structure I is 5.8-5.9 eV, the LUMO host  is 2.9-3.0 eV; the HOMO dopant  of the guest material having the structure III is 5.1-5.4 eV, and the LUMO dopant  is 2.5-2.7 eV; the HOMO EBL  of the material forming the electron blocking layer (for example, mCBP) is 5.6-5.7 eV; and the HOMO HBL  of the material forming the hole blocking layer (for example, TPBI) is 6.1-6.3 eV, and the LUMO HBL  is 2.3-2.5 eV. Accordingly, the host material forming the luminescent layer has the structure I and the guest material has the structure III, which may satisfy the following formulas: (1) HOMO host −HOMO EBL ≤0.3 eV; (2) HOMO HBL −HOMO host ≥0.1 eV; (3) LUMO host &gt;LUMO HBL ; (4) HOMO dopant ≤HOMO host ; and (5) LUMO dopant &lt;LUMO host . 
     According to an embodiment of the present disclosure, in the structure I of the host material, the substituents Ar1, Ar2 and Ar3 may be independently selected from optionally substituted aryl of a ring-carbon-atom quantity of 6-30 or heteroaryl of a ring-atom quantity of 5-20. Accordingly, by selecting rigid aryl or heteroaryl as the substituents of the host material, the host material may have a better stability. In some embodiments of the present disclosure, the aryl of a ring-carbon-atom quantity of 6-30 may be at least one of phenyl, naphthyl, anthryl and phenanthryl. In some other embodiments of the present disclosure, the heteroaryl of a ring-atom quantity of 5-20 may be at least one of dibenzofuran, dibenzothiophene, benzonaphthofuran and benzonaphthothiophene. Accordingly, by selecting the above substituents of a strong conjugacy, the host material may have a better luminous performance. 
     In some particular embodiments of the present disclosure, the substituent Ar2 or Ar3 may be an optionally substituted structure II, and the substituent X in the structure II may be an oxygen atom (O). Accordingly, by selecting dibenzofuran as the substituent Ar2 or Ar3, that may further facilitate the transport of the holes. 
     
       
         
         
             
             
         
       
     
     In some particular embodiments, the host material may be selected from one of the following 11 compounds. Accordingly, by selecting any one of 1-1 to 1-11, the hole mobility/electron mobility in the mobility of the host material (measured by using the SCLC method) may be ≥0.001, thereby further facilitating the transport of the holes. In some particular embodiments, the host material in the luminescent layer may be 1-2 or 1-7, which may increase the service life of the organic light emitting diode by above 20%. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     According to an embodiment of the present disclosure, in the structure III of the guest material, the substituents R1, R2, R3, R4 and R5 may be independently selected from hydrogen, or optionally substituted heteroatom, amino, C1-20 alkyl, and aryl or heteroaryl of a ring-carbon-atom quantity of 6-20, wherein a substituent in the aryl or heteroaryl includes silyl, cyano, halogen or diplogen. Accordingly, by selecting the above chemical groups as the five substituents in the structure III, the luminescent material may have a higher front-face luminous efficiency. Furthermore, the adjacent substituents R1-R5 may form a ring, or be integrated with the benzene ring to form fused aromatic hydrocarbons. 
     In some embodiments of the present disclosure, R1, R2, R3, R4 and R5 may be independently selected from methyl, tert-butyl, benzo, diphenylamino,  3 -methyldiphenylamino and trimethylsilyl. Accordingly, by selecting the above types of substituents, the rigidity of the guest material may be further increased, whereby the luminescent material may have a higher front-face luminous efficiency. 
     In some particular embodiments, the guest material may be selected from one of the following 6 compounds. Accordingly, by selecting any one of 2-1 to 2-6, the emission wavelength of the guest material may be between 400 nm and 500 nm, which cannot only increase the rigidity of the guest material, but may also enable the light emitted by the luminescent material to have a higher color purity. In some particular embodiments, the guest material may be 2-3 or 2-5, which may increase the luminous efficiency of the organic light emitting diode by above 10%. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     According to an embodiment of the present disclosure, based on the total weight of the luminescent-material composition, the doping amount of the guest material may be 1-10 wt %. Accordingly, by merely adding the guest material 1/99-1/9 of the weight of the host material, the service life of the organic light emitting diode containing the luminescent layer may be increased by above 20% and at the same time the luminous efficiency may be increased by above 10%. 
     According to an embodiment of the present disclosure, the material forming the hole injection layer (HIL)  200  may be an inorganic oxide, which particularly includes, for example, molybdenum oxide, titanium oxide, vanadium oxide, rhenium oxide, ruthenium oxide, chromium oxide, zirconium oxide, hafnium oxide, tantalum oxide, silver oxide, tungsten oxide and manganese oxide. It may also be a P-type dopant of a strong electrophilic system and a dopant of a hole transport material, which particularly includes, for example, hexacyanohexazatriphenylene, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoparaquinonedimethane (F4TCNQ), and 1,2,3-tri[(cyano)(4-cyano-2,3,5,6-tetrafluorophenyl) methylene] cyclopropane. 
     According to an embodiment of the present disclosure, the materials forming the hole transporting layer (HTL)  300  and the electron blocking layer (EBL)  400  may independently be an arylamine-type or carbazole-type material having the characteristic of hole transport, which particularly includes, for example, 4,4′-bis[N-(1-naphthyl)-N-phenylamino] biphenyl (NPB), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (TPD), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (BAFLP), 4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino] biphenyl (DFLDPBi), 4,4′-di(9-carbazolyl) biphenyl (CBP), and 9-phenyl-3-[4-(10-phenyl-9-anthryl) phenyl]-9H-carbazole (PCzPA). 
     According to an embodiment of the present disclosure, the materials forming the hole blocking layer (HBL)  600  and the electron transporting layer (ETL)  700  may independently be an aromatic heterocyclic compound, which includes, for example, a benzimidazole derivative, an imidazopyridine derivative, an imidazole derivative such as a benziimidazophenanthridine derivative, a pyrimidine derivative, an azine derivative such as a triazine derivative, and a compound containing a nitrogen-containing six-membered ring such as a phenanthroline derivative, a quinoline derivative, an iso-quinoline derivative. It also includes a compound in which a heterocyclic ring has a phosphine-oxide-series substituent, which particularly includes, for example, 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl] benzene (OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenyl)-1,2,4-triazole (TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenyl)-1,2,4-triazole (p-EtTAZ), bathophenanthroline (BPhen), bathocuproine (BCP), and 4,4′-bis(5-methylbenzoxazol-2-yl) stilbene (BzOs). 
     According to an embodiment of the present disclosure, the material forming the electron injection layer (EIL)  800  may be an alkali metal or a metal, which particularly includes, for example, LiF, Yb, Mg, Ca and compounds thereof. 
     In conclusion, according to the embodiments of the present disclosure, the present disclosure provides an organic light-emitting diode structure, wherein the combination between the energy-level relation of the host material and the guest material of the luminescent layer and the adjacent electron blocking layer and hole blocking layer may enable the organic light-emitting diode structure to have a higher stability and a higher luminous efficiency, which may enable the organic light-emitting diode structure to have a longer service life and a lower power consumption. 
     In another aspect of the present disclosure, the present disclosure provides a display device. According to an embodiment of the present disclosure, the display device includes the organic light-emitting diode structure stated above. 
     According to an embodiment of the present disclosure, the particular type of the display device is not particularly limited, and particularly includes, for example, a display screen, a television set, a mobile phone, a tablet personal computer and a smart watch, which may be selected by a person skilled in the art correspondingly according to the practical usage requirements of the display device, and are not discussed herein further. It should be noted that, besides the organic light-emitting diode structure, the display device further includes other necessary components and elements, and, taking an OLED display screen as an example, particularly includes, for example, a displaying back plate, a thin-film transistor and a touch component, which may be added by a person skilled in the art correspondingly according to the functions of the display device, and are not discussed herein further. 
     In conclusion, according to the embodiments of the present disclosure, the present disclosure provides a display device, wherein its organic light-emitting diode structure has a higher stability and a higher luminous efficiency, which may enable the display device to have a longer service life and a lower power consumption. A person skilled in the art can understand that the features and the advantages described above with respect to the organic light-emitting diode structure apply to the display device, and are not discussed herein further. 
     The present disclosure will be described below with reference to the particular embodiments. It should be noted that those embodiments are merely illustrative, and do not limit the present disclosure in any way. 
     COMPARATIVE EXAMPLE 1 
     In this comparative example, an organic light-emitting diode is fabricated. The material of the luminescent layer is a comparative host material and a comparative guest material, the material of HIL is F4TCNQ, the material of HTL is m-MTDATA, the material of EBL is mCBP, the material of HBL is TPBI, and the material of ETL is BCP. 
     
       
         
         
             
             
         
       
     
     comparative host material (comparative host) 
     
       
         
         
             
             
         
       
     
     comparative guest material (comparative dopant) 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     COMPARATIVE EXAMPLE 2 
     In this comparative example, an organic light-emitting diode is fabricated. It differs from Comparative Example 1 in that the host material of the luminescent layer is 1-2. 
     COMPARATIVE EXAMPLE 3 
     In this comparative example, an organic light-emitting diode is fabricated. It differs from Comparative Example 1 in that the host material of the luminescent layer is 1-7. 
     COMPARATIVE EXAMPLE 4 
     In this comparative example, an organic light-emitting diode is fabricated. It differs from Comparative Example 1 in that the guest material of the luminescent layer is 2-3. 
     COMPARATIVE EXAMPLE 5 
     In this comparative example, an organic light-emitting diode is fabricated. It differs from Comparative Example 1 in that the guest material of the luminescent layer is 2-5. 
     EXAMPLE 1 
     In this Example, an organic light-emitting diode is fabricated. It differs from Comparative Example 1 in that the host material of the luminescent layer is 1-7 and the guest material is 2-3. 
     EXAMPLE 2 
     In this Example, an organic light-emitting diode is fabricated. It differs from Comparative Example 1 in that the host material of the luminescent layer is 1-7 and the guest material is 2-5. 
     EXAMPLE 3 
     In this Example, an organic light-emitting diode is fabricated. It differs from Comparative Example 1 in that the host material of the luminescent layer is 1-2 and the guest material is 2-3. 
     EXAMPLE 4 
     In this Example, an organic light-emitting diode is fabricated. It differs from Comparative Example 1 in that the host material of the luminescent layer is 1-2 and the guest material is 2-5. 
     SUMMARY 
     Tests on the voltages, the luminous efficiencies and the lives are performed on the organic light-emitting diodes of Comparative Example 1-5 and Examples 1-4, wherein the life tests follow LT95@1000nit. The test results of the organic light emitting diodes are shown in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 The test results of the OLEDs of the 
               
               
                 comparative examples and examples 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 luminous 
                   
               
               
                 material combination 
                 voltage 
                 efficiency 
                 life 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 comparative host 
                 comparative dopant 
                 100%  
                 100% 
                 100% 
               
               
                 host 1-2 
                 comparative dopant 
                 100%  
                 102% 
                 120% 
               
               
                 host 1-7 
                 comparative dopant 
                 98% 
                 105% 
                 129% 
               
               
                 comparative host 
                 dopant 2-3 
                 98% 
                 113% 
                 103% 
               
               
                 comparative host 
                 dopant 2-5 
                 96% 
                 119% 
                 104% 
               
               
                 host 1-7 
                 dopant 2-3 
                 96% 
                 118% 
                 131% 
               
               
                 host 1-7 
                 dopant 2-5 
                 94% 
                 123% 
                 137% 
               
               
                 host 1-2 
                 dopant 2-3 
                 99% 
                 115% 
                 126% 
               
               
                 host 1-2 
                 dopant 2-5 
                 97% 
                 120% 
                 130% 
               
               
                   
               
            
           
         
       
     
     In the description of the present disclosure, the description referring to the terms “an embodiment”, “some embodiments”, “example”, “particular example” or “some examples” and so on means that particular features, structures, materials or characteristics described with reference to the embodiment or example are comprised in at least one of the embodiments or examples of the present disclosure. In the description, the illustrative expressions of the above terms do not necessarily relate to the same embodiment or example. Furthermore, the described particular features, structures, materials or characteristics may be combined in one or more embodiments or examples in a suitable form. Moreover, subject to avoiding contradiction, a person skilled in the art may combine different embodiments or examples described in the description and the features of the different embodiments or examples. 
     Although the embodiments of the present disclosure have already been illustrated and described above, it can be understood that the above embodiments are illustrative, and should not be construed as a limitation on the present disclosure, and a person skilled in the art may make variations, modifications, substitutions and improvements to the above embodiments within the scope of the present disclosure.