Patent Publication Number: US-10312465-B2

Title: Method for making organic light emitting diode

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims all benefits accruing under 35 U.S.C. § 119 from China Patent Application No. 201710764605.1, filed on Aug. 30, 2017, in the China Intellectual Property Office. This application is related to commonly-assigned application entitled, “ORGANIC LIGHT EMITTING DIODE”, concurrently filed Ser. No. 15/851,889; “ORGANIC LIGHT EMITTING DIODE”, concurrently filed Ser. No. 15/848,315; “ORGANIC LIGHT EMITTING DIODE”, concurrently filed Ser. No. 15/848,334; “METHOD FOR MAKING ORGANIC LIGHT EMITTING DIODE”, concurrently filed Ser. No. 15/848,403; “METHOD FOR MAKING ORGANIC LIGHT EMITTING DIODE”, concurrently filed Ser. No. 15/851,896; “METHOD FOR MAKING ORGANIC LIGHT EMITTING DIODE”, concurrently filed Ser. No. 15/851,924. Disclosures of the above-identified applications are incorporated herein by reference. 
     FIELD 
     The present application relates to organic light emitting diodes and methods for making the same. 
     BACKGROUND 
     The organic light emitting diode (OLED) is a light emitting diode including a light emitting layer composed of an organic compound. The OLED has a light weight, thin thickness, multi-color, and low manufacturing cost. Thus, the OLED has been widely used in various fields. 
     The carbon nanotube composite structure formed by carbon nanotubes and polymer can be used as the electron transport layer of the OLED. The carbon nanotube composite structure can be formed by two methods. One method includes dispersing the carbon nanotubes into an organic solvent to form a carbon nanotube dispersion, mixing the carbon nanotube dispersion and a monomer solution, and polymerizing the monomer. However, the carbon nanotubes have poor dispersion in the organic solvent, which affects the uniformity of the carbon nanotubes in the composite structure. Another method include completely melting the polymer, and mixing the melted polymer and the carbon nanotubes. However, the carbon nanotubes have poor dispersion in the melted polymer because the melted polymer has greater viscosity. Thus, the uniformity of the carbon nanotubes in the composite structure is still poor. When the carbon nanotube composite structure is used as the electron transport layer of the OLED, the electron transport layer has poor ability to transmit electrons. 
     What is needed, therefore, is to provide an organic light emitting diode and a method for making the same that can overcome the above-described shortcomings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present technology will now be described, by way of example only, with reference to the attached figures, wherein: 
         FIG. 1  is a schematic process flow of a first embodiment of a method for making a carbon nanotube composite structure. 
         FIG. 2  is a scanning electron microscope (SEM) image of a drawn carbon nanotube film. 
         FIG. 3  is an SEM image of a flocculated carbon nanotube film. 
         FIG. 4  is an SEM image of a pressed carbon nanotube film including a plurality of carbon nanotubes arranged along the same direction. 
         FIG. 5  is an SEM image of a pressed carbon nanotube film including a plurality of carbon nanotubes which is arranged along different directions. 
         FIG. 6  is an SEM image of a first composite structure surface of a CNT/PI composite structure. 
         FIG. 7  is an SEM image of the first composite structure surface of the CNT/PI composite structure coated with a gold film. 
         FIG. 8  is an atomic force microscope (AFM) image of the first composite structure surface of the CNT/PI composite structure. 
         FIG. 9  is an AFM image of the first composite structure surface of the CNT/PI composite structure coated with a gold film. 
         FIG. 10  is a schematic process flow of a second embodiment of a method for making a carbon nanotube composite structure. 
         FIG. 11  is a schematic process flow of a third embodiment of a method for making a carbon nanotube composite structure. 
         FIG. 12  is a schematic process flow of a fourth embodiment of a method for making a carbon nanotube composite structure. 
         FIG. 13  is a schematic view of a fifth embodiment of an organic light emitting diode. 
         FIG. 14  is a schematic process flow of a method for making the organic light emitting diode of  FIG. 13 . 
         FIG. 15  is another schematic process flow of a method for making the organic light emitting diode of  FIG. 13 . 
         FIG. 16  is yet another schematic process flow of a method for making the organic light emitting diode of  FIG. 13 . 
         FIG. 17  is a schematic view of a sixth embodiment of an organic light emitting diode. 
         FIG. 18  is a schematic process flow of a method for making the organic light emitting diode of  FIG. 17 . 
         FIG. 19  is another schematic process flow of a method for making the organic light emitting diode of  FIG. 17 . 
         FIG. 20  is yet another schematic process flow of a method for making the organic light emitting diode of  FIG. 17 . 
         FIG. 21  is a schematic view of a seventh embodiment of an organic light emitting diode. 
         FIG. 22  is another schematic view of the seventh embodiment of the organic light emitting diode. 
         FIG. 23  is a schematic process flow of a method for making the organic light emitting diode of  FIG. 21 . 
         FIG. 24  is another schematic process flow of a method for making the organic light emitting diode of  FIG. 21 . 
         FIG. 25  is a schematic view of an eighth embodiment of an organic light emitting diode. 
         FIG. 26  is a schematic process flow of a method for making the organic light emitting diode of  FIG. 25 . 
         FIG. 27  is another schematic process flow of a method for making the organic light emitting diode of  FIG. 25 . 
         FIG. 28  is a schematic view of a ninth embodiment of an organic light emitting diode. 
         FIG. 29  is a schematic process flow of a method for making the organic light emitting diode of  FIG. 28 . 
         FIG. 30  is another schematic process flow of a method for making the organic light emitting diode of  FIG. 28 . 
         FIG. 31  is a schematic view of a tenth embodiment of an organic light emitting diode. 
         FIG. 32  is a schematic process flow of a method for making the tenth embodiment of treating the carbon nanotube composite structure. 
         FIG. 33  is a schematic process flow of a method for making the organic light emitting diode of  FIG. 31 . 
         FIG. 34  is another schematic process flow of a method for making the organic light emitting diode of  FIG. 31 . 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. The drawings are not necessarily to scale, and the proportions of certain parts may be exaggerated to illustrate details and features better. The description is not to be considered as limiting the scope of the embodiments described herein. 
     Several definitions that apply throughout this disclosure will now be presented. 
     The term “substantially” is defined to be essentially conforming to the particular dimension, shape or other word that substantially modifies, such that the component need not be exact. For example, substantially cylindrical means that the object resembles a cylinder, but can have one or more deviations from a true cylinder. The term “comprising” means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in a so-described combination, group, series and the like. 
     The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
     Referring to  FIG. 1 , a method for making a carbon nanotube composite structure  130  of a first embodiment includes the following steps: 
     S 1 , placing a carbon nanotube structure  110  on a substrate surface  102  of a substrate  100 , wherein the carbon nanotube structure  110  has a first surface  112  and a second surface  114  opposite to the first surface  112 , and the second surface  114  is in direct contact with the substrate surface  102 ; 
     S 2 , coating a monomer solution  140  on the carbon nanotube structure  110 , wherein the monomer solution  140  is formed by dispersing a certain amount of monomers into an organic solvent; 
     S 3 , polymerizing the monomer; and 
     S 4 , removing the substrate  100 . 
     In the step S 1 , the carbon nanotube structure  110  includes a plurality of carbon nanotubes  118  uniformly distributed therein. A gap  116  is defined between adjacent carbon nanotubes  118 . The plurality of carbon nanotubes  118  is parallel to the first surface  112  and the second surface  114 . The plurality of carbon nanotubes  118  is parallel to the substrate surface  102 . The plurality of carbon nanotubes  118  can be combined by van der Waals attractive force. The carbon nanotube structure  110  can be a substantially pure structure of the carbon nanotubes  118 , with few impurities. The plurality of carbon nanotubes  118  may be single-walled, double-walled, multi-walled carbon nanotubes, or their combinations. The carbon nanotubes  118  which are single-walled have a diameter of about 0.5 nanometers (nm) to about 50 nm. The carbon nanotubes  118  which are double-walled have a diameter of about 1.0 nm to about 50 nm. The carbon nanotubes  118  which are multi-walled have a diameter of about 1.5 nm to about 50 nm. 
     The plurality of carbon nanotubes  118  in the carbon nanotube structure  110  can be orderly or disorderly arranged. The term ‘disordered carbon nanotube  118 ’ refers to the carbon nanotube structure  110  where the carbon nanotubes  118  are arranged along many different directions, and the aligning directions of the carbon nanotubes  118  are random. The number of the carbon nanotubes  118  arranged along each different direction can be almost the same (e.g. uniformly disordered). The carbon nanotubes  118  can be entangled with each other. The term ‘ordered carbon nanotube  118 ’ refers to the carbon nanotube structure  110  where the carbon nanotubes  118  are arranged in a consistently systematic manner, e.g., the carbon nanotubes  118  are arranged approximately along a same direction and/or have two or more sections within each of which the carbon nanotubes  118  are arranged approximately along a same direction (different sections can have different directions). The carbon nanotube structure  110  can be a carbon nanotube layer structure including a plurality of drawn carbon nanotube films, a plurality of flocculated carbon nanotube films, or a plurality of pressed carbon nanotube films. 
     Referring to  FIG. 2 , the drawn carbon nanotube film includes a plurality of successive and oriented carbon nanotubes  118  joined end-to-end by van der Waals attractive force therebetween. The carbon nanotubes  118  in the drawn carbon nanotube film substantially extend along the same direction. The carbon nanotubes are parallel to a surface of the drawn carbon nanotube film. The drawn carbon nanotube film is a free-standing film. The drawn carbon nanotube film can bend to desired shapes without breaking. A film can be drawn from a carbon nanotube array to form the drawn carbon nanotube film. 
     If the carbon nanotube structure  110  includes at least two stacked drawn carbon nanotube films, adjacent drawn carbon nanotube films can be combined by only the van der Waals attractive force therebetween. Additionally, when the carbon nanotubes  118  in the drawn carbon nanotube film are aligned along one preferred orientation, an angle can exist between the orientations of carbon nanotubes  118  in adjacent drawn carbon nanotube films, whether stacked or adjacent. An angle between the aligned directions of the carbon nanotubes  118  in two adjacent drawn carbon nanotube films can be in a range from about 0 degree to about 90 degrees. Stacking the drawn carbon nanotube films will improve the mechanical strength of the carbon nanotube structure  110 , further improving the mechanical strength of the carbon nanotube composite structure  130 . In one embodiment, the carbon nanotube structure  110  includes two layers of the drawn carbon nanotube films, and the angle between the aligned directions of the carbon nanotubes  118  in two adjacent drawn carbon nanotube films is about 90 degrees. 
     Referring to  FIG. 3 , the flocculated carbon nanotube film includes a plurality of long, curved, disordered carbon nanotubes  118  entangled with each other. The flocculated carbon nanotube film can be isotropic. The carbon nanotubes  118  can be substantially uniformly dispersed in the flocculated carbon nanotube film. Adjacent carbon nanotubes  118  are acted upon by van der Waals attractive force to obtain an entangled structure. Due to the carbon nanotubes  118  in the flocculated carbon nanotube film being entangled with each other, the flocculated carbon nanotube film has excellent durability, and can be fashioned into desired shapes with a low risk to the integrity of the flocculated carbon nanotube film. Further, the flocculated carbon nanotube film is a free-standing film. 
     Referring to  FIGS. 4 and 5 , the pressed carbon nanotube film includes a plurality of carbon nanotubes  118 . The carbon nanotubes  118  in the pressed carbon nanotube film can be arranged along a same direction, as shown in  FIG. 4 . The carbon nanotubes  118  in the pressed carbon nanotube film can be arranged along different directions, as shown in  FIG. 5 . The carbon nanotubes  118  in the pressed carbon nanotube film can rest upon each other. An angle between a primary alignment direction of the carbon nanotubes  118  and a surface of the pressed carbon nanotube film is about 0 degree to approximately 15 degrees. The greater the pressure applied, the smaller the angle obtained. If the carbon nanotubes  118  in the pressed carbon nanotube film are arranged along different directions, the pressed carbon nanotube film can have properties that are identical in all directions substantially parallel to the surface of the pressed carbon nanotube film. Adjacent carbon nanotubes  118  are attracted to each other and are joined by van der Waals attractive force. Therefore, the pressed carbon nanotube film is easy to bend to desired shapes without breaking. Further, the pressed carbon nanotube film is a free-standing film. 
     The term “free-standing” includes, but not limited to, the drawn carbon nanotube film, the flocculated carbon nanotube film, or the pressed carbon nanotube film that does not have to be supported by a substrate. For example, the free-standing the drawn carbon nanotube film, the flocculated carbon nanotube film, or the pressed carbon nanotube film can sustain the weight of itself when it is hoisted by a portion thereof without any significant damage to its structural integrity. So, if the free-standing the drawn carbon nanotube film, the flocculated carbon nanotube film, or the pressed carbon nanotube film is placed between two separate supporters, a portion of the free-standing the drawn carbon nanotube film, the flocculated carbon nanotube film, or the pressed carbon nanotube film, not in contact with the two supporters, would be suspended between the two supporters and yet maintain film structural integrity. 
     The substrate surface  102  of the substrate  100  is very smooth. The height difference between the highest position of the substrate surface  102  and the lowest position of the substrate surface  102  is nanoscale. The height difference between the highest position of the substrate surface  102  and the lowest position of the substrate surface  102  can be defines as a smoothness of the substrate surface  102 . The smoothness can be greater than or equal to 0 nanometers and less than or equal to 30 nanometers. The smoothness can be greater than or equal to 0 nanometers and less than or equal to 20 nanometers. The smoothness can also be greater than or equal to 0 nanometers and less than or equal to 10 nanometers. The material of the substrate  100  can be sapphire, monocrystalline quartz, gallium nitride, gallium arsenide, silicon, graphene, or polymer. The melting point of the substrate  100  should be greater than the temperature of polymerizing the monomer. The length, width, and thickness of the substrate  100  are not limited. In one embodiment, the substrate  100  is a silicon wafer. 
     Some organic solvent can be dripped on the first surface  112  of the carbon nanotube structure  110 . When the organic solvent is volatilized, the air between the carbon nanotube structure  110  and the substrate surface  102  can be removed under the surface tension of the organic solvent. Thus, the carbon nanotube structure  110  can be tightly bonded to the substrate surface  102  of the substrate  100 . The organic solvent can be ethanol, methanol, acetone, dichloroethane, or chloroform. 
     In the step S 2 , the monomer can be any monomer that can be polymerized to form a polymer  120 . The polymer  120  includes a phenolic resin (PF), an epoxy resin (EP), a polyurethane (PU), a polystyrene (PS), a polymethylmethacrylate (PMMA), a polycarbonate (PC), polyethylene terephthalate (PET), phenylcyclobutene (BCB), polycycloolefin or polyimide (PI), polyvinylidene fluoride (PVDF), and the like. In one embodiment, the monomer is an imide, and the polymer  120  is a polyimide. The organic solvent includes ethanol, methanol, acetone, dichloroethane or chloroform. 
     The monomer solution  140  has a small viscosity and good fluidity. When the monomer solution  140  is coated on the first surface  112  of the substrate  100 , the monomer solution  140  can pass through the gaps  116  and contact with a part of the substrate surface  102 . The first part of the substrate surface  102  is in direct contact with and coated by the monomer solution  140 , the second part of the substrate surface  102  is in direct contact with the carbon nanotubes  118 . The method for coating the monomer solution  140  is not limited and can be spin coating, injection coating, or the like. In one embodiment, the monomer solution  140  is coated on the carbon nanotube structure  110  by spin coating. 
     In the step S 3 , the method for polymerizing the monomer is not limited, such as high temperature treatment. In one embodiment, the substrate  100  and the carbon nanotube structure  110  coated with the monomer solution  140  are placed in a reaction furnace. The reaction furnace is heated to the temperature of polymerizing the monomer, and the monomer is polymerized to form the polymer  120 . The part surface of the carbon nanotube  118 , that is directly contacted with the substrate surface  102  of the substrate  100 , is defined as a contact surface  117 . Because the carbon nanotubes  118  are tubular, the second surface  114  of the carbon nanotube structure  110  is in fact a ups and downs surface. The contact surface  117  is parts of the second surface  114 . Except for the contact surface  117 , the rest of second surface  114  is in direct contact with the monomer solution  140 . The gaps  116  are filled with the monomer solution  140 . When the monomer is polymerized to form the solid polymer  120 , the polymer  120  is combined with the carbon nanotube structure  110  to form the carbon nanotube composite structure  130 . 
     In the step S 4 , the method for removing the carbon nanotube composite structure  130  from the substrate surface  102  of the substrate  100  is not limited. The carbon nanotube composite structure  130  can be peeled off from the substrate surface  102  of the substrate  100  by water immersion, blade, tape, or other tools. 
     The smoothness of the substrate surface  102  is nanoscale, thus the contact surface  117  can be in direct contact with the substrate surface  102  during coating the monomer solution  140  and polymerizing the monomer. Thus, there is no monomer solution  140  between the contact surface  117  and the substrate surface  102  during coating the monomer solution  140  and polymerizing the monomer. Thus, when the carbon nanotube composite structure  130  is peeled from the substrate surface  102 , the contact surface  117  is exposed and not coated by the polymer  120 . A part outer wall of the carbon nanotubes  118 , that is directly contacted with the substrate surface  102 , is exposed and not coated by the polymer  120 . Except for the contact surface  117 , the rest of the outer walls of carbon nanotubes  118  are coated by and in direct contact with the polymer  120 . 
     The carbon nanotube composite structure  130  includes the plurality of carbon nanotubes  118  and the polymer  120 . The plurality of carbon nanotubes  118  are uniformly dispersed in the polymer  120 . The plurality of carbon nanotubes  118  can be joined end-to-end and extend along the same direction. The plurality of carbon nanotubes  118  can also extend along different directions, or entangled with each other to form a network-like structure. The carbon nanotube composite structure  130  has a first composite structure surface  132 . The first composite structure surface  132  is in direct contact with the substrate surface  102  before peeling the carbon nanotube composite structure  130  off from the substrate  100 . The length direction of the plurality of carbon nanotubes  118  is parallel to the first composite structure surface  132 . The surface of the polymer  120  near the substrate  100  is defined as a lower surface  122 . The contact surface  117  and the lower surface  122  together form the first composite structure surface  132 . Thus, the contact surface  117  is a part of the first composite structure surface  132  and exposed from the polymer  120 . The contact surface  117  is an exposed surface and can protrude out of the lower surface  122  of the polymer  120 . The height difference between the exposed surface and the lower surface  122  of the polymer  120  is nanoscale. Since the smoothness of the substrate surface  102  is at nanoscale level, the first composite structure surface  132  is also smooth at nanoscale level. The height difference between the contact surface  117  and the lower surface  122  can be greater than or equal to 0 nanometers and less than or equal to 30 nanometers. The height difference between the contact surface  117  and the lower surface  122  can be greater than or equal to 0 nanometers and less than or equal to 20 nanometers. The height difference between the contact surface  117  and the lower surface  122  can also be greater than or equal to 0 nanometers and less than or equal to 10 nanometers. 
     In one embodiment, the polymer  120  is polyimide, the carbon nanotube structure  110  is two stacked drawn carbon nanotube films, and the angle between the aligned directions of the carbon nanotubes  118  in two adjacent drawn carbon nanotube films is about 90 degrees. 
     In one embodiment, to synthesize poly(amic acid) (PAA) solution, 2.0024 g of ODA (10 mmol) was placed in a three-neck flask containing 30.68 mL of anhydrous DMAc under nitrogen purge at room temperature. After ODA is completed dissolved in DMAc, 2.1812 g of PMDA (10 mmol) is added in one portion. Thus, the solid content of the solution is about 12%. The mixture is stirred at room temperature under nitrogen purge for 12 h to produce a PAA solution. The two stacked drawn carbon nanotube films are located on a silicon wafer, wherein the angle between the aligned directions of the carbon nanotubes  118  in two adjacent drawn carbon nanotube films is about 90 degrees. Then the PAA solution is coated on the two stacked drawn carbon nanotube films, and the PAA solution will gradually penetrate into the two stacked drawn carbon nanotube films to form a preform. The preform is thermal imidized in muffle furnace at 80° C., 120° C., 180° C., 300° C., and 350° C. for 1 h respectively to form a CNT/PI composite structure. Finally, the CNT/PI composite structure is peeled off from the silicon wafer. 
       FIG. 6  is an SEM image of the first composite structure surface of a CNT/PI composite structure. As shown in  FIG. 6 , the carbon nanotubes  118  are uniformly dispersed in the CNT/PI composite structure. 
       FIG. 7  is an SEM image of the first composite structure surface of the CNT/PI composite structure coated with a gold film, and the thickness of the gold film is about 1 nm. As shown in  FIG. 7 , the first composite structure surface  132  is a smooth surface with no ups and downs from the naked eye. The height difference between the highest place of the first composite structure surface  132  and the lowest place of the first composite structure surface  132  is nanoscale.  FIG. 8  is an atomic force microscope (AFM) image of the first composite structure surface of the CNT/PI composite structure.  FIG. 9  is an AFM image of the first composite structure surface of the CNT/PI composite structure coated with a gold film, and the thickness of the gold film is about 3 nm. As shown in  FIG. 8  and  FIG. 9 , it is also find that the first composite structure surface  132  is a smooth surface. 
     Referring to  FIG. 10 , a method for making a carbon nanotube composite structure  160  of a second embodiment includes the following steps: 
     S 21 , placing the carbon nanotube structure  110  on the substrate surface  102  of the substrate  100 , wherein the carbon nanotube structure  110  has the first surface  112  and the second surface  114  opposite to the first surface  112 , and the second surface  114  is in direct contact with the substrate surface  102 ; 
     S 22 , locating a graphene layer  150  on the first surface  112 ; 
     S 23 , coating a monomer solution  140  on the graphene layer  150  and the carbon nanotube structure  110 , wherein the monomer solution  140  is formed by dispersing the monomer into the organic solvent; 
     S 24 , polymerizing the monomer; and 
     S 25 , removing the substrate  100 . 
     In this embodiment, the method for making the carbon nanotube composite structure  160  is similar to the method for making the carbon nanotube composite structure  130  above except that the graphene layer  150  is located on the first surface  112  before coating the monomer solution  140 . 
     The graphene layer  150  is a two dimensional film structure. If the graphene layer  150  includes a plurality of graphene films, the plurality of graphene films can overlap each other to form a large area. The graphene film is a one-atom thick planar sheet composed of a plurality of sp 2 -bonded carbon atoms. The graphene layer  150  can be a free-standing structure. The term “free-standing structure” means that the graphene layer  150  can sustain the weight of itself when it is hoisted by a portion thereof without any significant damage to its structural integrity. So, if the graphene layer  150  is placed between two separate supports, a portion of the graphene layer  150  not in contact with the two supports, would be suspended between the two supports and yet maintain structural integrity. When the plurality of graphene films overlap each other, a gap is formed between adjacent two graphene films. During coating the monomer solution  140 , the monomer solution  140  can pass through the graphene layer  150  and the carbon nanotube structure  110  to arrive at the substrate surface  102 , because both the graphene layer  150  and the carbon nanotube structure  110  have gaps  106 . 
     Referring to  FIG. 11 , a method for making a carbon nanotube composite structure  170  of a third embodiment includes the following steps: 
     S 31 , placing the carbon nanotube structure  110  on the substrate surface  102  of the substrate  100  to form a preform structure  172 , wherein the carbon nanotube structure  110  has the first surface  112  and the second surface  114  opposite to the first surface  112 , and the second surface  114  is in direct contact with the substrate surface  102 ; 
     S 32 , locating two preform structures  172  on a base  174 , wherein the two preform structures  172  are spaced apart from each other, the substrates  100  of the two preform structures  172  and the base  174  form a mold  176  having an opening, and the carbon nanotube structures  110  of the two preform structures  172  are opposite to each other and inside of the mold  176 ; 
     S 33 , injecting the monomer solution  140  into the inside of the mold  176  from the opening of the mold  176 , wherein the monomer solution  140  is formed by dispersing the monomer into the organic solvent; 
     S 34 , polymerizing the monomer; and 
     S 35 , removing the substrates  100  and the base  174 . 
     In this embodiment, the method for making the carbon nanotube composite structure  170  is similar to the method for making the carbon nanotube composite structure  130  above except the steps S 32  and S 33 . 
     In the step S 32 , the method for making the mold  176  is not limited. For example, the two preforms structures  172  and the base  174  are fixed together by sticking or mechanically fastening to form the mold  176 . In one embodiment, the two preforms structures  172  and the base  174  are fixed by a sealant, and the sealant is  706 B vulcanized silicon rubber. The opening is on the top of the mold  176 . The carbon nanotube structure  110  of each of the two preforms structures  172  is located inside of the mold  176 . The substrate  100  of each of the two preforms structures  172  forms the sidewall of the mold  176 . The material of the base  174  is not limited, such as glass, silica, metal or metal oxide. In one embodiment, the material of the substrate  174  is glass. The carbon nanotube structure  110  in the mold  176  would not fall off from the substrate  100  because the carbon nanotube structure  110  itself has viscosity. The organic solvent can be dripped so that the carbon nanotube structure  110  is firmly adhered to the substrate  100 . 
     Furthermore, the length or width of the carbon nanotube structure  110  can be greater than the length or width of the substrate surface  102 . When the carbon nanotube structure  110  is disposed on the substrate surface  102 , the excess carbon nanotube structure  110  can be folded into the back surface of the substrate  100 , and an adhesive can be applied to the back surface of the substrate  100 . Thus, the carbon nanotube structure  110  in the mold  176  is firmly adhered to the substrate  100  and would not fall off from the substrate  100 . The back surface is opposite to the substrate surface  102 , and the substrate surface  102  can be considered the front surface. The melting point of the adhesive needs to be greater than the temperature of polymerizing the monomer. 
     In the step S 33 , the monomer solution  140  is slowly injected into the inside of the mold  176  along the inner wall of the mold  176 . The monomer solution  140  completely submerges the carbon nanotube structure  110 . The monomer solution  140  would not break the integrity of the carbon nanotube structure  110  during injecting the monomer solution  140  because the carbon nanotube structure  110  is supported by the substrate  100 . 
     Referring to  FIG. 12 , a method for making a carbon nanotube composite structure  180  of a fourth embodiment includes the following steps: 
     S 41 , placing the carbon nanotube structure  110  on the substrate surface  102  of the substrate  100 , wherein the carbon nanotube structure  110  has the first surface  112  and the second surface  114  opposite to the first surface  112 , and the second surface  114  is in direct contact with the substrate surface  102 ; 
     S 42 , placing the carbon nanotube structure  110  and the substrate  100  into a container  182 , wherein the container  182  has an opening; 
     S 43 , injecting the monomer solution  140  into the container  182  from the opening of the container  182 , wherein the monomer solution  140  is formed by dispersing the monomer into the organic solvent; 
     S 44 , polymerizing the monomer; and 
     S 45 , removing the substrates  100  and the container  182 . 
     In this embodiment, the method for making the carbon nanotube composite structure  180  is similar to the method for making the carbon nanotube composite structure  130  above except the steps S 42  and S 43 . 
     In the step S 42 , the container  182  has a bottom. When the carbon nanotube structure  110  and the substrate  100  are located in the container  182 , the substrate  100  is located on and in direct contact with the bottom of the container  182 . The carbon nanotube structure  110  spaced apart from the bottom by the substrate  100 . The material of the container  182  is not limited, such as silica, metal, glass, or metal oxide. In one embodiment, the material of the container  182  is glass. 
     In the step S 43 , the monomer solution  140  does not break the integrity of the carbon nanotube structure  110  during injecting the monomer solution  140  because the carbon nanotube structure  110  is supported by the substrate  100 . The amount of the monomer solution  140  can be adjusted so that the monomer solution  140  submerges the entire carbon nanotube structure  110 , or submerges only a part of the carbon nanotube structure  110 . When the monomer solution  140  submerges only a part of the carbon nanotube structure  110 , the thickness of the polymer  120  is less than the thickness of the carbon nanotube structure  110 . Thus, in the carbon nanotube composite structure  180 , some carbon nanotubes  118  are located in and completely coated by the polymer  120 , and some carbon nanotubes  118  are exposed from and extend out of the polymer  120 . In one embodiment, the carbon nanotube structure  110  includes three stacked drawn carbon nanotube films, and the monomer solution  140  submerges only a part of the carbon nanotube structure  110 . In the carbon nanotube composite structure  180 , the first drawn carbon nanotube film, the second drawn carbon nanotube film and the third drawn carbon nanotube film are stacked. The second drawn carbon nanotube film is between the first drawn carbon nanotube film and the third drawn carbon nanotube film. The entire outer walls of the carbon nanotubes  118  in the second drawn carbon nanotube film are coated by the polymer  120 . Partial outer wall of the carbon nanotubes  118  in the first drawn carbon nanotube film are exposed. The contact surfaces  117  of the carbon nanotubes  118  in the third drawn carbon nanotube film are exposed. 
     The monomer solution  140  has a smaller viscosity than the molten polymer, thus after coating the monomer solution  140  and polymerizing the monomer, the carbon nanotubes  118  can uniformly dispersed in the polymer  120 . In above methods, the substrate  100  has a nanoscale smooth substrate surface  102 , thus some carbon nanotubes of the carbon nanotube composites  130 ,  160 ,  170 , and  180  are exposed from the polymer  120 , improving the conductivity of the carbon nanotube composites  130 ,  160 ,  170 , and  180 . 
     The carbon nanotube composite structures  130 ,  170 , and  180  can be used as an electron transport layer in an organic light emitting diode (OLED). The OLEDs and the methods for making the OLEDs are described in detail below. 
     Referring to  FIG. 13 , an OLED  10  of a fifth embodiment includes a support body  11 , an anode electrode  12 , a hole transport layer  13 , an organic light emitting layer  14 , the carbon nanotube composite structure  130 , and a cathode electrode  15 . The support body  11 , the anode electrode  12 , the hole transport layer  13 , the organic light emitting layer  14 , the carbon nanotube composite structure  130 , and the cathode electrode  15  are stacked on each other in that order. The carbon nanotube composite structure  130  includes the plurality of carbon nanotubes  118  and the polymer  120 , and the plurality of carbon nanotubes  118  is dispersed in the polymer  120 . The contact surface  117  of the plurality of carbon nanotubes  118  is in direct contact with the organic light emitting layer  14 . The rest surfaces of the plurality of carbon nanotubes  118  are covered by and in direct contact with the polymer  120 . The carbon nanotube composite structure  130  serves as the electrode transport layer for transporting electrons. In the fifth embodiment, the material of the polymer  120  can transport electrons and is an aromatic compound having a large conjugated plane, such as 8-Hydroxyquinoline aluminum salt (AlQ), 2-(4-tert(4-biphenyl)-1,3,4-oxadiazole (PBD), Beq 2  or 4,4′-Bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi). 
     The support body  11  can be transparent or opaque. The material of the support body  11  can be glass, quartz, transparent plastic or resin. In the fifth embodiment, the support body  11  is a glass plate. The anode electrode  12  is a transparent conductive layer or a porous mesh structure, such as ITO layer, FTO layer, or the like. In the fifth embodiment, the material of the anode electrode  12  is ITO. 
     The hole transport layer  13  has a strong hole transporting ability. The material of the hole transport layer  13  can be N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,4-diamine (NPB), N,N′-diphenyl-N,N′-bis(m-methylphenyl)-1, Biphenyl-4,4′-diamine (TPD), or 4,4′,4″-tris(3-methylphenylaniline) triphenylamine (MTDATA). In the fifth embodiment, the material of the hole transport layer  13  is NPB. 
     The organic light emitting layer  14  is a polymer or a small molecule organic compound having high quantum efficiency, good semiconductivity, and thermal stability. A molecular weight of the polymer can be in a range from about 10000 to about 100000. The polymer can be a conductive conjugated polymer or a semiconductor conjugated polymer. A molecular weight of the small molecule organic compound can be in a range from about 500 to about 20000. The small molecule organic compound can be an organic dye. The organic dye has characteristics of strong chemical modification, wide selection range, easy purification and high quantum efficiency. The small molecule organic compound can be a red, green, or blue material. When the small molecule organic compound is a red material, the red material can be selected from the group consisting of rhodamine dyes, DCM, DCT, DCJT, DCJTB, DCJTI and TPBD. When the small molecule organic compound is a green material, the green material can be selected from coumarin 6, quinacridone (QA), coronene, naphthalimide. When the small molecule organic compound is a blue material, the blue material can be selected from the group consisting of N-arylbenzimidazoles, and 1,2,4-triazole derivatives (TAZ) and distyrylarylene. In the fifth embodiment, the material of the organic light emitting layer  14  is Alq3. The cathode electrode  15  is a transparent layer, an opaque conductive layer, or a porous mesh structure, such as a metal thin film, a metal mesh, an ITO layer, an FTO layer, and the like. In the fifth embodiment, the cathode electrode  15  is an aluminum layer. 
     The OLED  10  can further include a hole injection layer (not shown in figure) and an electron injection layer (not shown in figure). The hole injection layer is between the anode electrode  12  and the hole transport layer  13 . The electron injection layer is between the carbon nanotube composite structure  130  and the cathode electrode  15 . The material of the hole injection layer can be copper phthalocyanine (CuPc) or PEDOT: PSS. The PEDOT is a polymerization of 3,4-ethylenedioxythiophene monomer (EDOT). The PSS is polystyrene sulfonate. The material of the electron injecting layer is an alkali metal or an alkali metal compound having a low work function, such as lithium fluoride (LiF), calcium (Ca), or magnesium (Mg). 
     The carbon nanotube composite structure  130  can conduct and transport electrons, so the cathode electrode  15  can be omitted. When the cathode electrode  15  is omitted, the carbon nanotube composite structure  130  is used as both the electron transport layer and the cathode electrode. In the OLED  10  as shown in  FIG. 13 , the plurality of carbon nanotubes  118  substantially extend along the same direction and are uniformly dispersed in the polymer  120 , thus the transmitting electrons ability of the electron transport layer is enhanced. 
     Referring to  FIG. 14 , a method for making the OLED  10  of the fifth embodiment includes the following steps: 
     S 51 , providing a preform structure  16  including the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14 , wherein the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14  are stacked on each other in that order; 
     S 52 , placing the carbon nanotube structure  110  on the preform structure  16 , wherein the carbon nanotube structure  110  is in direct contact with the surface of the organic light emitting layer  14  away from the hole transport layer  13 ; 
     S 53 , applying the monomer solution  140  to the carbon nanotube structure  110 ; 
     S 54 , polymerizing the monomer to form the polymer  120 ; and 
     S 55 , forming the cathode electrode  15  on the surface of the polymer  120  away from the preform structure  16 . 
     In the step S 51 , the method for sequentially stacking the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14  is not limited, such as sputtering, coating, vapor deposition, mask etching, spraying, or inkjet printing. The steps S 52  to S 54  are similar to the steps S 1  to S 3  of the first embodiment. In order to ensure that the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14  would not be damaged by the temperature of polymerizing the monomer, the melting points of the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14  should be greater than the temperature of polymerizing the monomer. In the step S 55 , the cathode electrode  15  is formed by a conventional method, such as sputtering, coating, vapor deposition or the like. It is to be understood that the step  55  can be omitted when the cathode electrode  15  is omitted from the OLED  10  and the carbon nanotube composite structure  130  is used as both the electron transport layer and the electrode. 
     Referring to  FIG. 15 , another method for making the OLED  10  of the fifth embodiment includes the following steps: 
     S 51 ′, providing the preform structure  16  including the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14 , wherein the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14  are stacked on each other in that order; 
     S 52 ′, placing the carbon nanotube composite structure  130  on the surface of the organic light emitting layer  14  away from the hole transport layer  13 , wherein the first composite structure surface  132  of the carbon nanotube composite structure  130  is in direct contact with the organic light emitting layer  14 ; and 
     S 53 ′, forming the cathode electrode  15  on the surface of the carbon nanotube composite structure  130  away from the organic light emitting layer  14 . 
     In this embodiment, the anode electrode  12 , the hole transport layer  13 , the organic light emitting layer  14 , the carbon nanotube composite structure  130 , and the cathode electrode  15  are stacked on each other in that order on the support body  11 . 
     In the step S 52 ′, in order to make the carbon nanotube composite structure  130  and the organic light emitting layer  14  to be combined firmly, the carbon nanotube composite structure  130  and the organic light emitting layer  14  can be hot pressed or cold pressed before forming the cathode electrode  15 . In one embodiment, the carbon nanotube composite structure  130  is placed on the organic light emitting layer  14  to form a whole structure; the whole structure is located in a hot-press apparatus having a metal roll and a heating element. The heated metal roll presses the whole structure, the polymer  120  and the organic light emitting layer  14  are softened, and the air between the organic light emitting layer  14  and the carbon nanotube composite structure  130  is exhausted, so that the organic light emitting layer  14  and the carbon nanotube composite structure  130  are tightly pressed together. During pressing by the metal roll, the pressure from the metal roll is in a range from about 5 kg to about 20 kg. The temperature of the metal roll should not cause the organic light emitting layer  14  and the carbon nanotube composite structure  130  to melt. It is to be understood that the step  53 ′ can be omitted when the cathode electrode  15  is omitted from the OLED  10  and the carbon nanotube composite structure  130  is used as both the electron transport layer and the electrode. 
     Referring to  FIG. 16 , yet another method for making the OLED  10  of the fifth embodiment includes the following steps: 
     S 51 ″, providing the carbon nanotube composite structure  130 , wherein the carbon nanotube composite structure  130  has the first composite structure surface  132  and a second composite structure surface  138  opposite to the first composite structure surface  132 ; 
     S 52 ″, forming the cathode electrode  15  on the second composite structure surface  138 , wherein the plurality of carbon nanotubes  118  of the carbon nanotube composite structure  130  is spaced apart from the cathode electrode  15 ; and 
     S 53 ″, forming the preform structure  16  on the first composite structure surface  132 , wherein the plurality of carbon nanotubes  118  is in direct contact with the organic light emitting layer  14 . 
     In this embodiment, the method for making the OLED  10  is shown where the cathode electrode  15  is formed on the second composite structure surface  138  of the carbon nanotube composite structure  130 , and the preform structure  16  is formed on the first composite structure surface  132  of the carbon nanotube composite structure  130 . In this embodiment, firstly, the organic light emitting layer  14  is formed on the first composite structure surface  132 , then the hole transport layer  13  is formed on the surface of the organic light emitting layer  14  away from the first composite structure surface  132 ; secondly, the anode electrode  12  is formed on the surface of the hole transport layer  13  away from the organic light emitting layer  14 , then the support body  11  is placed on the surface of the anode electrode  12  away from the hole transport layer  13 . The method for forming the anode electrode  12 , the hole transport layer  13 , or the organic light emitting layer  14  can be sputtering, coating, vapor deposition, mask etching, spraying, or inkjet printing. The step S 2 ″ can be omitted when the cathode electrode  15  is omitted from the OLED  10  and the carbon nanotube composite structure  130  is used as both the electron transport layer and the electrode. The carbon nanotube composite structure  130  is a free-standing structure and can support other elements in the OLED  10 , thus support body  11  of the preform structure  16  can be omitted. 
     Referring to  FIG. 17 , an OLED  20  of a sixth embodiment is shown. The OLED  20  is similar to the OLED  10  above except that the first composite structure surface  132  is in direct contact with the organic light emitting layer  14  in the OLED  10 , and the first composite structure surface  132  is spaced apart from the organic light emitting layer  14  in the OLED  20 . 
     Referring to  FIG. 18 , a method for making the OLED  20  of the sixth embodiment includes the following steps: 
     S 61 , placing the carbon nanotube structure  110  on the cathode electrode  15 ; 
     S 62 , applying the monomer solution  140  to the carbon nanotube structure  110 ; 
     S 63 , polymerizing the monomer to form the polymer  120 ; and 
     S 64 , forming the preform structure  16  on the surface of the polymer  120  away from the cathode electrode  15 , wherein the polymer  120  is in direct contact with the organic light emitting layer  14 , and the carbon nanotube structure  110  is spaced apart from the organic light emitting layer  14 . 
     In order to ensure that the cathode electrode  15  would not be damaged by the temperature of polymerizing the monomer, the melting points of the cathode electrode  15  should be greater than the temperature of polymerizing the monomer. In this embodiment, the steps of applying the monomer solution  140  to the carbon nanotube structure  110  and polymerizing the monomer are similar to the steps S 2  and S 3  of the first embodiment. 
     Referring to  FIG. 19 , another method for making the OLED  20  of the sixth embodiment includes the following steps: 
     S 61 ′, providing the preform structure  16  including the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14 , wherein the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14  are stacked on each other in that order; 
     S 62 ′, placing the carbon nanotube composite structure  130  on the surface of the organic light emitting layer  14  away from the hole transport layer  13 , wherein the first composite structure surface  132  of the carbon nanotube composite structure  130  is spaced apart from the organic light emitting layer  14 ; and 
     S 63 ′, forming the cathode electrode  15  on the first composite structure surface  132 . 
     The method as shown in  FIG. 19  is similar to the method as shown in  FIG. 15  above except that the first composite structure surface  132  is in direct contact with the organic light emitting layer  14  in the method as shown in  FIG. 15 , and the first composite structure surface  132  is spaced apart from the organic light emitting layer  14  in the method as shown in  FIG. 19 . 
     Referring to  FIG. 20 , yet another method for making the OLED  20  of the sixth embodiment includes the following steps: 
     S 61 ″, providing the carbon nanotube composite structure  130 , wherein the carbon nanotube composite structure  130  has the first composite structure surface  132  and the second composite structure surface  138  opposite to the first composite structure surface  132 ; 
     S 62 ″, forming the cathode electrode  15  on the first composite structure surface  132 , wherein the plurality of carbon nanotubes  118  is in direct contact with the cathode electrode  15 ; and 
     S 63 ″, forming the preform structure  16  on the second composite structure surface  138 , wherein organic light emitting layer  14  is in direct contact with the second composite structure surface  138 , and the plurality of carbon nanotubes  118  of the carbon nanotube composite structure  130  is spaced apart from the organic light emitting layer  14 . 
     The method as shown in  FIG. 20  is similar to the method as shown in  FIG. 16  above except that the first composite structure surface  132  is in direct contact with the organic light emitting layer  14  in the method as shown in  FIG. 16 , and the first composite structure surface  132  is spaced apart from the organic light emitting layer  14  in the method as shown in  FIG. 20 . 
     Referring to  FIG. 21 , an OLED  30  of a seventh embodiment is shown. The OLED  30  is similar to the OLED  10  above except that the carbon nanotube composite structure  170  is used as the electron transport layer of the OLED  30 . The carbon nanotube composite structure  170  includes a plurality of first carbon nanotubes  1180  and a plurality of second carbon nanotubes  1182 . The plurality of second carbon nanotubes  1182  and the plurality of first carbon nanotubes  1180  are dispersed in the polymer  120 . Partial surface of each first carbon nanotube  1180  is exposed from the polymer  120  and in direct contact with the organic light emitting layer  14 . Partial surface of each second carbon nanotube  1182  is exposed from the polymer  120  and in direct contact with the cathode electrode  15 . The plurality of second carbon nanotubes  1182  and the plurality of first carbon nanotubes  1180  can be spaced apart from each other, as shown in  FIG. 21 . The plurality of second carbon nanotubes  1182  and the plurality of first carbon nanotubes  1180  can also be in direct contact with each other, as shown in  FIG. 22 . When plurality of second carbon nanotubes  1182  and the plurality of first carbon nanotubes  1180  are in direct contact with each other, the material of the polymer  120  is not limited and can be insulated. 
     Referring to  FIG. 23 , a method for making the OLED  30  of the seventh embodiment includes the following steps: 
     S 71 , placing the carbon nanotube structure  110  on the preform structure  16  to form a first composite structure  173 , wherein the carbon nanotube structure  110  is in direct contact with the surface of the organic light emitting layer  14  away from the hole transport layer  13 ; 
     S 72 , placing the other carbon nanotube structure  110  on the cathode electrode  15  to form a second composite structure  175 ; 
     S 73 , locating the first composite structure  173  and the second composite structure  175  on the base  174 , wherein the first composite structure  173  and the second composite structure  175  are spaced apart from each other; the base  174 , the preform structure  16 , and the cathode electrode  15  form a second mold  177  having an opening; and the carbon nanotube structures  110  of the first composite structure  173  and the second composite structure  175  are opposite to each other and inside of the second mold  177 ; 
     S 74 , injecting the monomer solution  140  into the inside of the second mold  177  from the opening, wherein the monomer solution  140  is formed by dispersing the monomer into the organic solvent; 
     S 75 , polymerizing the monomer; and 
     S 76 , removing the base  174 . 
     The method for making the OLED  30  as shown in  FIG. 23  is similar to the method for making the carbon nanotube composite structure  170  as shown in  FIG. 11  above except that: 1) in  FIG. 11 , the carbon nanotube structure  110  is placed on the substrate  100  to form the preform structure  172 , the substrates  100  of two preform structures  172  and the base  174  form the mold  176 ; in  FIG. 23 , the carbon nanotube structure  110  is placed on the preform structure  16  to form the first composite structure  173 , another carbon nanotube structure  110  is placed on the cathode electrode  15  to form the second composite structure  175 , and the base  174 , the preform structure  16 , and the cathode electrode  15  form the second mold  177 ; 2) in  FIG. 11 , all of the base  174  and the two substrates  100  are removed; in  FIG. 23 , only the base  174  is removed. 
     Referring to  FIG. 24 , another method for making the OLED  30  of the seventh embodiment includes the following steps: 
     S 71 ′, providing the preform structure  16  including the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14 , wherein the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14  are stacked on each other in that order; 
     S 72 ′, placing the carbon nanotube composite structure  170  on the surface of the organic light emitting layer  14  away from the hole transport layer  13 , wherein carbon nanotube composite structure  170  includes the plurality of first carbon nanotubes  1180  and the plurality of second carbon nanotubes  1182 , the part of the surface of each first carbon nanotube  1180  is exposed from the polymer  120  and in direct contact with the organic light emitting layer  14 , and the part of the surface of each second carbon nanotube  1182  is exposed from the polymer  120 ; and 
     S 73 ′, forming the cathode electrode  15  on the surface of the carbon nanotube composite structure  170  away from the organic light emitting layer  14 , wherein the part of the surface of each second carbon nanotube  1182  is exposed from the polymer  120  and in direct contact with the cathode electrode  15 . 
     The method as shown in  FIG. 24  is similar to the method as shown in  FIG. 19  above except that the carbon nanotube composite structure is different in the two methods. 
     Yet another method for making the OLED  30  of the seventh embodiment includes the following steps: 
     S 71 ″, providing the carbon nanotube composite structure  170  including a third surface and a fourth surface opposite to the third surface; 
     S 72 ″, forming the cathode electrode  15  on the third surface; and 
     S 73 ″, placing the preform structure  16  on the fourth surface, wherein the organic light emitting layer  14  is in direct contact with the fourth surface. 
     This method is similar to the method as shown in  FIG. 20  above except that the carbon nanotube composite structure is different in the two methods. This method and the methods shown in  FIGS. 16 and 20  have some advantages below. The parts of surface of the carbon nanotubes  118  are exposed from the polymer  120  before forming the cathode electrode  15  (or the organic light emitting layer  14 ). When the cathode electrode  15  (or the organic light emitting layer  14 ) is formed by sputtering, coating, vapor deposition, the exposed surface of the carbon nanotubes  118  are completely covered by the cathode electrode  15  (or the organic light emitting layer  14 ). Thus, the cathode electrode  15  (or the organic light emitting layer  14 ) has a large contact area with the carbon nanotubes  118 , enhancing the ability of the carbon nanotubes  118  to transmit electrons. 
     Referring to  FIG. 25 , an OLED  40  of an eighth embodiment is shown. The OLED  40  is similar to the OLED  30  above except that the carbon nanotube composite structure  180  is used as the electron transport layer of the OLED  40 . The carbon nanotube composite structure  180  further includes a plurality of third carbon nanotubes  1184  dispersed in the polymer  120 . Entire surface of the plurality of third carbon nanotubes  1184  is in direct contact with the polymer  120 . The plurality of first carbon nanotubes  1180 , the plurality of second carbon nanotubes  1182 , and the plurality of third carbon nanotubes  1184  are in direct contact with each other in the thickness direction of the carbon nanotube composite structure  180 . Thus, the electrons can be transferred from the cathode electrode  15  to the organic light emitting layer  14 . Thus, the carbon nanotube composite structure  180  can be used as the electron transport layer for transporting electrons no matter the material of the polymer  120  can or cannot transmit electrons. Thus, the material of the polymer is not limited. 
     Referring to  FIG. 26 , a method for making the OLED  40  of the eighth embodiment includes the following steps: 
     S 81 , placing the carbon nanotube structure  110  on the preform structure  16 , wherein the carbon nanotube structure  110  is in direct contact with the surface of the organic light emitting layer  14  away from the hole transport layer  13 ; 
     S 82 , placing the carbon nanotube structure  110  and the preform structure  16  into the container  182 , wherein the container  182  has the opening; 
     S 83 , injecting the monomer solution  140  into the container  182  from the opening of the container  182 , wherein the monomer solution  140  is formed by dispersing the monomer into the organic solvent; 
     S 84 , polymerizing the monomer to form the polymer  120 ; 
     S 85 , removing the container  182 ; and 
     S 86 , forming the cathode electrode  15  on the surface of the polymer  120  away from the preform structure  16 . 
     The method for making the OLED  40  as shown in  FIG. 26  is similar to the method for making the carbon nanotube composite structure  180  as shown in  FIG. 12  above except that: 1) in  FIG. 12 , the carbon nanotube structure  110  is placed on the substrate  100 ; in  FIG. 26 , the carbon nanotube structure  110  is placed on the preform structure  16 ; 2) in  FIG. 12 , both the substrate  100  and the container  182  are removed; in  FIG. 26 , only the container  182  is removed, and the cathode electrode  15  is formed on the polymer  120 . 
     Referring to  FIG. 27 , another method for making the OLED  40  of the eighth embodiment includes the following steps: 
     S 81 ′, providing the preform structure  16  including the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14 , wherein the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14  are stacked on each other in that order; 
     S 82 ′, placing the carbon nanotube composite structure  180  on the surface of the organic light emitting layer  14  away from the hole transport layer  13 , wherein carbon nanotube composite structure  180  includes the plurality of first carbon nanotubes  1180 , the plurality of second carbon nanotubes  1182 , and the plurality of carbon nanotubes  1184 ; the part of the surface of each first carbon nanotube  1180  is exposed from the polymer  120  and in direct contact with the organic light emitting layer  14 , and the part of the surface of each second carbon nanotube  1182  is exposed from the polymer  120 ; and entire surface of the plurality of carbon nanotubes is covered by the polymer  120 ; and 
     S 83 ′, forming the cathode electrode  15  on the surface of the carbon nanotube composite structure  180  away from the organic light emitting layer  14 , wherein the part of the surface of each second carbon nanotube  1182  is exposed from the polymer  120  and in direct contact with the cathode electrode  15 . 
     The method as shown in  FIG. 27  is similar to the method as shown in  FIG. 19  above except that the carbon nanotube composite structure is different in the two methods. 
     Yet another method for making the OLED  40  of the eighth embodiment includes the following steps: 
     S 81 ″, providing the carbon nanotube composite structure  180 ; 
     S 82 ″, forming the cathode electrode  15  on the carbon nanotube composite structure  180 ; and 
     S 83 ″, forming the preform structure  16  on the surface of the carbon nanotube composite structure  180  away from the cathode electrode  15 , wherein the organic light emitting layer  14  is in direct contact with the carbon nanotube composite structure  180 . 
     Referring to  FIG. 28 , an OLED  50  of a ninth embodiment is shown. The OLED  50  is similar to the OLED  40  above except that a part of the surface of each first carbon nanotube  1180  is exposed from the polymer  120  and in direct contact with the cathode electrode  15 , a part of the surface of each second carbon nanotube  1182  is exposed from the polymer  120  and in direct contact with the organic light emitting layer  14 , and each second carbon nanotube  1182  is embedded in the organic light emitting layer  14 . 
     Referring to  FIG. 29 , a method for making the OLED  50  of the ninth embodiment includes the following steps: 
     S 91 , placing the carbon nanotube structure  110  on the cathode electrode  15 ; 
     S 92 , placing the carbon nanotube structure  110  and the cathode electrode  15  into the container  182 , wherein the container  182  has the opening; 
     S 93 , injecting the monomer solution  140  into the container  182  from the opening of the container  182 , wherein the monomer solution  140  is formed by dispersing the monomer into the organic solvent; 
     S 94 , polymerizing the monomer to form the polymer  120 ; 
     S 95 , removing the container  182 ; and 
     S 96 , placing the preform structure  16  on the surface of the polymer  120  away from the cathode electrode  15 , wherein the organic light emitting layer  14  is in direct contact with the polymer  120 . 
     The method for making the OLED  50  as shown in  FIG. 29  is similar to the method for making the carbon nanotube composite structure  180  as shown in  FIG. 12  above except that: 1) in  FIG. 12 , the carbon nanotube structure  110  is placed on the substrate  100 ; in  FIG. 29 , the carbon nanotube structure  110  is placed on the cathode electrode  15 ; 2) in  FIG. 12 , both the substrate  100  and the container  182  are removed; in  FIG. 29 , only the container  182  is removed, and the preform structure  16  is placed on the polymer  120 . In this embodiment, the cathode electrode  15  is an aluminum plate. 
     The methods shown in  FIGS. 14, 18, 23, 26, and 29  have some advantages below. The preform structure  16  or the cathode electrode  15  is used as a substrate for supporting the carbon nanotube structure  110 . The monomer solution  140  is coated on the carbon nanotube structure  110 , then the monomer of the monomer solution  140  is polymerized to form the electron transport layer, and then other functional layers are set. The monomer solution  140  has a low viscosity, thus the monomer solution  140  can be uniformly distributed in the carbon nanotube structure  110 . After polymerizing the monomer, the carbon nanotubes  118  have good dispersion in the electron transport layer, thereby the transmitting electrons ability of the electron transport layer is improved. 
     Referring to  FIG. 30 , another method for making the OLED  50  of the ninth embodiment includes the following steps: 
     S 91 ′, placing the carbon nanotube composite structure  180  on the cathode electrode  15 , wherein in the carbon nanotube composite structure  180 , the part of the surface of each first carbon nanotube  1180  is exposed from the polymer  120  and in direct contact with the cathode electrode  15 , the part of the surface of each second carbon nanotube  1182  is exposed from the polymer  120 , and entire surface of the plurality of carbon nanotubes is covered by the polymer  120 ; 
     S 92 ′, forming the organic light emitting layer  14  on the surface of the carbon nanotube composite structure  180  away from the cathode electrode  15 ; and 
     S 93 ′, forming the hole transport layer  13  on the surface of the organic light emitting layer  14  away from the carbon nanotube composite structure  180 ; 
     S 94 ′, forming the anode electrode  12  on the surface of the hole transport layer  13  away from the organic light emitting layer  14 ; and 
     S 95 ′, placing the support body  11  on the surface of the anode electrode  12  away from the hole transport layer  13 . 
     In the step S 91 ′, the cathode electrode  15  can support other element, and the material of the cathode electrode  15  is not limited. In one embodiment, the cathode electrode  15  is the aluminum plate. In the steps S 92 ′ and S 93 ′, the organic light emitting layer  14 , the hole transport layer  13 , and the anode electrode  12  are formed by sputtering, coating, vapor deposition, mask etching, spraying, or inkjet printing. The step S 95 ′ can be omitted. 
     The methods shown in  FIGS. 15, 19, 24, 27, and 30  have some advantages below. The support body  11 , the anode electrode  12 , the hole transport layer  13 , the organic light emitting layer  14 , and the carbon nanotube composite structure  130  ( 170  or  180 ) are successively laminated together. Then, the organic light emitting layer  14  and the carbon nanotube composite structure  130  ( 170  or  180 ) are tightly bonded by pressing. Finally, the cathode electrode  15  is formed on the surface of the carbon nanotube composite structure  130  ( 170  or  180 ) away from the organic light emitting layer  14 . The carbon nanotubes  118  of the carbon nanotube composite structure  130  ( 170  or  180 ) are substantially parallel to each other and extend the same direction. The relatively regular arrangement of the carbon nanotubes  118  can improve the transmitting electrons ability of the electron transport layer. 
     Referring to  FIG. 31 , an OLED  60  of a tenth embodiment is shown. The OLED  60  is similar to the OLED  10 . The difference between the OLED  10  and the OLED  60  is that the electron transport layer of the OLED  10  is the carbon nanotube composite structure  130 , however the electron transport layer of the OLED  60  is a carbon nanotube composite sub-structure  131  which is formed by treating the carbon nanotube composite structure  130 . 
     In the OLED  60 , the carbon nanotube composite sub-structure  131  includes the polymer  120  and a plurality of carbon nanotubes  118  dispersed in the polymer  120 . The length direction of the plurality of carbon nanotubes  118  is from the organic light emitting layer  14  to the cathode electrode  15 . In OLED  60 , each carbon nanotube  118  has a first end and a second end opposite to the first end, the first end is exposed from the polymer  120  and in direct contact with the organic light emitting layer  14 , and the second end is exposed from the polymer  120  and in direct contact with the cathode electrode  15 . The plurality of carbon nanotubes  118  are parallel to each other. The material of the polymer  120  in the carbon nanotube composite sub-structure  131  is not limited. The carbon nanotube composite sub-structure  131  has a first sub-structure surface  137  that is in direct contact with the organic light emitting layer  14 . The length direction of the plurality of carbon nanotubes  118  and the sub-structure surface form an angle, and the angle is grater than 0 degrees and less than or equal to 90 degrees. In one embodiment, the length direction of the plurality of carbon nanotubes  118  is perpendicular to the sub-structure surface. 
     Referring to  FIG. 33 , a method for making the OLED  60  of the tenth embodiment includes the following steps: 
     S 101 , providing the preform structure  16  including the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14 , wherein the support body  11 , the anode electrode  12 , the hole transport layer  13 , and the organic light emitting layer  14  are stacked on each other in that order; 
     S 102 , treating the carbon nanotube composite structure  130  to form the carbon nanotube composite sub-structure  131  having the sub-structure surface and the plurality of carbon nanotubes  118 , wherein the length direction of the plurality of carbon nanotubes  118  is parallel to the thickness direction of the carbon nanotube composite sub-structure  131 ; 
     S 103 , placing the carbon nanotube composite sub-structure  131  on the preform structure  16 , wherein the carbon nanotube composite sub-structure  131  is in direct contact with the organic light emitting layer  14 ; and 
     S 104 , forming the cathode electrode  15  on the surface of the carbon nanotube composite sub-structure  131  away from the preform structure  16 . 
     The step S 102  can be performed before the step S 101 . 
     In the step  102 , the thickness of the carbon nanotube composite structure  130  in this embodiment is greater than the thickness of the carbon nanotube composite structure  130  in the first embodiment. The amount of the monomer solution  140  can be increased so that the thickness of the polymer  120  becomes large, thus the thickness of the carbon nanotube composite structure  130  becomes large. 
     The carbon nanotube composite structure  130  is treated by slicing. The carbon nanotube composite structure  130  is cut by laser along a direction perpendicular to the first composite structure surface  132 , as shown in  FIG. 32 . The carbon nanotube composite structure  130  has a third composite structure surface  134  and a fourth composite structure surface  136  opposite to the third composite structure surface  134 . In one embodiment, the third composite structure surface  134  and the fourth composite structure surface  136  are perpendicular to the first composite structure surface  132 . When the carbon nanotube composite structure  130  is cut, a cut line AB is formed on the carbon nanotube composite structure  130 . The cut line AB can be perpendicular to the length direction of the carbon nanotubes  118  and parallel to the third composite structure surface  134 . The distance between the cut line AB and the third composite structure surface  134  can be adjusted, and the carbon nanotube composite sub-structure  131  is formed after cutting along the cut line AB. The carbon nanotube composite sub-structure  131  is a sheet or a film. The carbon nanotube composite sub-structure  131  has the first sub-structure surface  137  and a second sub-structure surface  138  opposite to the first sub-structure surface  137 . The first sub-structure surface  137  is parallel to the third composite structure surface  134 , and the second sub-structure surface  138  is parallel to the fourth composite structure surface  136 . In one embodiment, the length direction of each carbon nanotube  118  is perpendicular to the third composite structure surface  134 . 
     The carbon nanotube composite sub-structure  131  can conduct and transport electrons, so the cathode electrode  15  can be omitted, and the step S 104  can be omitted. 
     Referring to  FIG. 34 , another method for making the OLED  60  of the tenth embodiment includes the following steps: 
     S 101 ′, treating the carbon nanotube composite structure  130  to form the carbon nanotube composite sub-structure  131  having the first sub-structure surface  137 , the second sub-structure surface  138  and the plurality of carbon nanotubes  118 , wherein the length direction of the plurality of carbon nanotubes  118  is parallel to the thickness direction of the carbon nanotube composite sub-structure  131 ; 
     S 102 ′, placing the carbon nanotube composite sub-structure  131  on the cathode electrode  15 , and the second sub-structure surface  138  is in direct contact with the cathode electrode  15 ; 
     S 103 ′, placing the preform structure  16  on the first sub-structure surface  137 , wherein the carbon nanotube composite sub-structure  131  is in direct contact with the organic light emitting layer  14 . 
     The OLED  60  shown in  FIG. 31  and the methods shown in  FIG. 33  and  FIG. 34  have some advantages below. The electrical conductivity of the carbon nanotube  118  along the longitudinal direction (axial direction) is good, and the electrical conductivity of the carbon nanotube  118  along the radial direction is poor. The length direction of the plurality of carbon nanotubes  118  is from the organic light emitting layer  14  to the cathode electrode  15 , thus transmitting electrons ability of the electron transport layer is improved. In addition, both the polymer having transmitting electrons ability and the polymer without transmitting electrons can be materials of the polymer  120 . 
     The embodiments shown and described above are only examples. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the detail, including in matters of shape, size and arrangement of the parts within the principles of the present disclosure up to, and including, the full extent established by the broad general meaning of the terms used in the claims. 
     Additionally, it is also to be understood that the above description and the claims drawn to a method may comprise some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps.