Patent Application
20120015098
The present subject matter relates generally to carbon nanotube based transparent conductive films and methods for preparing and patterning the same. More particularly, the present subject matter relates to transparent conductive films comprising carbon nanotubes (CNTs) combined with various polymers and methods for preparing and patterning the same.
Transparent conductive films have a wide range of applications, for example they can be used in displays, touch panels, solar cells and other optoelectronic devices. The films typically consist of a transparent substrate upon which a coating or film that is transparent and electrically conductive is disposed. Currently, the dominant materials used for transparent conductive films are indium tin oxide (ITO) based films. However, the ITO based transparent conductors have many limitations. For example, the cost of the ITO based transparent conductor is very high as the ITO coating process requires expensive vacuum sputtering equipment. In addition, ITO is a limited natural resource and the price of ITO has increased significantly in the past few years because of short supply. ITO based transparent conductive films also have poor mechanical durability. That is, the ITO based films are brittle and can break easily if subjected to stress, for example, to bending stress. In addition, ITO based transparent conductors are also yellowish in color, and have a fairly large value of b*. Three CIELAB coordinates L*, a*, and b* represent the lightness of a color. For example, L* yields a position between black and white wherein black has a value of L*=0 and diffuse white has a value of L*=100, although specular white may be higher. A second coordinate, a* indicates a position between red/magenta and green. Negative values of a* indicate green while positive values indicate magenta. The b* coordinate indicates a position between yellow and blue wherein negative values of b* indicate blue and positive values indicate yellow. An ideal transparent conductor should comprise a neutral color. Therefore, ITO based films are not suitable for use with next generation flexible devices such as flexible displays, flexible touch panels and flexible solar cells.
Tremendous efforts have been made in the past few years to develop alternatives to ITO based films at a reduced cost. Up until now, the most promising ITO alternatives comprise conductive polymers, metal nanowires and carbon nanotubes (CNTs). Transparent conductive films formed using such alternatives have demonstrated transparency and conductivity comparable to those formed using ITO based films. In addition, transparent conductive films using these alternatives exhibit superior mechanical durability compared with the ITO based transparent conductors. Compared with conductive polymers and metal nanowires, CNTs have a much higher mechanical strength and chemical stability. Accordingly, CNTs can produce more stable and robust transparent conductive coatings. The performance of transparent conductive films fabricated using CNTs depends greatly on the processes by which the coatings are made. A lower cost and well controlled process for fabricating the CNTs based transparent conductive coatings is in demand. As many applications require patterned transparent conductive films, an efficient patterning process for the CNTs based transparent conductive films is also desired.
Accordingly, it is desirable to provide efficient and economic carbon nanotube based transparent conductive films and methods for preparing and patterning the same. The present subject matter relates to such devices and methods, and it will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and background information.
In accordance with this disclosure, carbon nanotube based transparent conductive films and methods for preparing and patterning the same are disclosed. Such films can comprise a plurality of carbon nanotubes (CNTs). It is, therefore, an object of the present disclosure to provide economic fabrication and patterning methods for creating durable CNTs based transparent conductive films.
This and other objects of the present disclosure as can become apparent from the present disclosure are achieved, at least in whole or in part, by the subject matter described herein.
A full and enabling disclosure of the present subject matter including the best mode thereof to one of ordinary skill in the art is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which:
Reference will now be made in detail to possible embodiments of the present subject matter, one or more examples of which are shown in the figures. Each example is provided to explain the subject matter and not as a limitation. In fact, features illustrated or described as part of one embodiment can be used in another embodiment to yield still a further embodiment. There is no intention to be limited or bound by any theory presented in the preceding background or the following detailed description of the present subject matter. It is intended that the subject matter disclosed and envisioned herein covers any such modifications and variations.
As illustrated in the various figures, some sizes of structures or portions are exaggerated relative to other structures or portions for illustrative purposes and, thus, are provided to illustrate the general structures of the present invention. Furthermore, various aspects of the present subject matter are described with reference to a structure or a portion being formed on other structures, portions, or both. As will be appreciated by those of skill in the art, references to a structure being formed “on” or “above” another structure or portion contemplates that additional structure, portion, or both may intervene. References to a structure or a portion being formed “on” another structure or portion without an intervening structure or portion are described herein as being formed “directly on” the structure or portion.
Furthermore, relative terms such as “on”, “above”, “top”, or “bottom” are used herein to describe one structure's or portion's relationship to another structure or portion as illustrated in the figures. It will be understood that relative terms such as “on”, “above”, “top”, or “bottom” are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, structure or portion described as “above” other structures or portions would now be oriented “below” the other structures or portions. Likewise, if devices in the figures are rotated along an axis, structure or portion described as “above”, other structures or portions would now be oriented “next to” or “left of” the other structures or portions. Like numbers refer to like elements throughout.
Transparent conductive films described herein have transparent conductive coatings comprising carbon nanotubes (CNTs), which under appropriate conditions can be coated with predetermined polymers. Such polymers can comprise for example, surfactants and adhesion promoters. By coating the surface of the CNTs with the proper polymers, improved transmittance and conductivity can be achieved from a CNTs based transparent conductive film. Modifying the surface of the CNTs can greatly enhance the uniformity and stability of a suspension containing CNTs. As a result, improved performance of a transparent conductive film can be achieved. Such improvements can include for example, better conductivity, transmittance, uniformity, stability, environmental stability, and an improved electrical response time of the transparent conductive films.
The transparency of a film can be characterized by its light transmittance (for example, defined by ASTM D1003), that is, the percentage of incident light transmitted through the conductive film and its sheet resistance. In one embodiment of the subject matter disclosed herein, the transparent conductive film can have a total light transmittance of no less than about 88% and a sheet resistance in the range of about 400 Ohms/square. Sheet resistance is applicable to two-dimensional systems where the thin film is considered to be a two-dimensional entity. It is analogous to resistivity as used in three-dimensional systems. When the term sheet resistance is used, the current flows along the plane of the sheet, and not perpendicular to it. In another embodiment, the transparent conductive film can have a sheet resistance in the range of approximately 1 to 1010 Ohms/square. In this regard, the transparent conductive films can be used in various applications such as for example, flat panel displays, solar cells, touch panels, e-papers, anti-static films, and microelectronics.
In an alternative embodiment, transparent, non-conductive substrates 12 and 22 can optionally be pretreated to facilitate the deposition of components of the transparent conductive coating, discussed in more detail below, and/or to facilitate adhesion of the components to the substrate. The pretreatment can comprise, for example a solvent or chemical washing, exposure to controlled levels of atmospheric humidity, heating, or surface treatments such as plasma treatment, UV-ozone treatment, or flame or corona discharge. Alternatively, or in combination, an adhesive (also called a primer or binder) may be deposited onto the surface of the substrate to further improve adhesion of the components to the substrate.
In
Referring to
The CNT inks in steps 34 and 54 can comprise CNTs dispersed in a first solution comprising a solvent and polymer to form a stable suspension. The polymer in the CNT dispersion can comprise an electrically conductive polymer. The second solution which can mix with the CNT dispersion to form the CNT ink can comprise an adhesion promoter as a functional additive thereby forming transparent adhesion promotion layer upon coating the transparent substrate. In one embodiment, the CNT ink comprises a first CNT dispersion comprising at least one solvent, a polymer, and a plurality of CNTs, such as, for example, those CNTs available from Xintek, Inc. and XinNano Materials, Inc. The CNTs used in the ink dispersion can comprise an average thickness or diameter in a range from approximately 2 to 20 nm. The CNTs can comprise an average length in a range from approximately 0.1 μm to 100 μm. The CNTs can comprise approximately 1 ppm to about 4% by weight of the total ink. In a preferred embodiment, the CNTs comprise approximately 0.01 to about 0.6% by weight of the total ink.
The CNT dispersion component of the ink which comprises at least one solvent, a polymer, and a plurality of CNTs, can comprise any suitable solvent known in the art and can comprise any suitable pure fluid or mixture of fluids capable of forming a dispersion with CNTs. The CNT dispersion can be volatilized at a desired temperature, such as a critical temperature. Contemplated solvents can ideally be easily removed within the context of the applications disclosed herein. For example, contemplated solvents can comprise solvents having a relatively low boiling point as compared with boiling points of precursor components. In some embodiments, contemplated solvents comprise a boiling point of less than about 150° C. In other embodiments, contemplated solvents comprise a boiling point in a range from about 50° C. to about 250° C. This can allow the solvent to evaporate from the applied film. Suitable solvents comprise any single or mixture of water, alcohol and other organic, organometallic, or inorganic molecules that may be volatized at a desired temperature.
In other contemplated embodiments of the CNT dispersion component of the CNT ink, the solvent or solvent mixture can comprise aliphatic, cyclic, and aromatic hydrocarbons. Aliphatic hydrocarbon solvents can comprise both straight-chain compounds and compounds that are branched and possibly crosslinked. Cyclic hydrocarbon solvents are those solvents that comprise at least three carbon atoms oriented in a ring structure with properties similar to aliphatic hydrocarbon solvents. Aromatic hydrocarbon solvents comprise generally three or more unsaturated bonds with a single ring or multiple rings attached by a common bond and/or multiple rings fused together. Contemplated hydrocarbon solvents include toluene, xylene, p-xylene, m-xylene, mesitylene, solvent naphtha H, solvent naphtha A, alkanes, such as pentane, hexane, isohexane, heptane, nonane, octane, dodecane, 2-methylbutane, hexadecane, tridecane, pentadecane, cyclopentane, 2,2,4-trimethylpentane, petroleum ethers, halogenated hydrocarbons, such as chlorinated hydrocarbons, nitrated hydrocarbons, benzene, 1,2-dimethylbenzene, 1,2,4-trimethylbenzene, mineral spirits, kerosene, isobutylbenzene, methylnaphthalene, ethyltoluene, and ligroine.
In other contemplated embodiments, the solvent or solvent mixture for the CNT dispersion may comprise those solvents that are not considered part of the hydrocarbon solvent family of compounds, such as ketones (such as acetone, diethylketone, methylethylketone, and the like), alcohols, esters, ethers, amides and amines. Contemplated solvents may also comprise aprotic solvents, for example, cyclic ketones such as cyclopentanone, cyclohexanone, cycloheptanone, and cyclooctanone; cyclic amides such as N-alkylpyrrolidinone, wherein the alkyl has from about 1 to 4 carbon atoms; N-cyclohexylpyrrolidinone and mixtures thereof. Other suitable solvents can comprise methylisobutylketone, dibutyl ether, cyclic dimethylpolysiloxanes, butyrolactone, .gamma.-butyrolactone, 2-heptanone, ethyl 3-ethoxypropionate, 1-methyl-2-pyrrolidinone, propyleneglycol methyletheracetate (PGMEA), hydrocarbon solvents, such as mesitylene, toluene di-n-butyl ether, anisole, 3-pentanone, 2-heptanone, ethyl acetate, n-propyl acetate, n-butyl acetate, ethyl lactate, ethanol, 2-propanol, dimethyl acetamide, and/or combinations thereof. Other organic solvents can be used insofar as they are able to aid dissolution of an adhesion promoter (if used) and at the same time effectively control the viscosity of the resulting dispersion as a coating solution.
Still referring to steps 34 and 54 of
In addition to the CNTs dispersion component of the CNT ink, a second solution contemplated in step 54 can optionally become mixed with the CNT dispersion to form a stable CNT ink. The second solution can comprise a mixture of the electrically conductive polymer and one or more functional additives. Examples of such functional additives can include one or more the following: a high boiling-point solvent known in the art which can improve the conductance of the film made from the CNT ink, dispersants and/or surfactants as known in the art which can help disperse CNTs uniformly, polymerization inhibitors and/or corrosion inhibitors known in the art which can improve the environmental and chemical stability, light stabilizers known in the art which can improve UV stability, wetting agents known in the art which can lower the surface tension of the inks, adhesion promoters known in the art which can be in a solution such as alcohol and/or binders known in the art which can increase the adhesion between the inks and substrates, antioxidants which can be in a solution, such as reducing agents thiols, ascorbic acid, and polyphenols, or other antioxidants known in the art which can prevent the films from oxidation, antifoaming agents known in the can prevent the inks from generating bubbles during coating, detergents, flame retardants, pigments, plasticizers, thickeners, viscosity modifiers, rheology modifiers, and photosensitive and/or photoimageable materials all of which are known in the art can be functional additives. The uniformity and stability of the CNTs suspension, for example, the CNTs ink dispersion, can be further improved by processing the mixed suspension with a centrifuge to remove large particles or aggregates that are not well dispersed in the suspension.
The method for fabricating a CNTs based transparent conductive film can continue as noted in steps 36 and 56 of
As illustrated by
Post processing steps, as previously mentioned can also further include evaporation of the solvent of the CNT dispersion such that the deposited CNTs are no longer mobile on the substrate. In another embodiment, the CNT dispersion may be applied by a conventional rod coating technique and the substrate can be placed in an oven, optionally using controlled air flow, to heat the substrate and dispersion and thus evaporate the solvent. In another example, the solvent can be evaporated at room temperature (15° to 27° C.). In one example, the dispersion can be applied to a heated substrate by spraying the suspension, the ink, onto the substrate at a coating speed that allows for the evaporation of the solvent. If the dispersion comprises a binder, adhesive, or other similar polymeric compound, then the dispersion also can also be subjected to a temperature or UV light that will cure the compound. The post-processing step of curing can be performed before, during, or after the evaporation process. The resultant conductive film can have a surface resistance of less than 2000 Ohms/sq when an optical transmittance of the conductive film excluding the transparent substrate is better than 95%.
Referring to
The method can further comprise step 84 in
As illustrated by
Accordingly, CNT based transparent conductive films having transparent conductive coatings with controlled transmittance and conductivity and methods of preparing and patterning the same are provided. Embodiments of the present disclosure shown in the drawings and described above are exemplary of numerous embodiments that can be made within the scope of the appended claims. It is contemplated that the configurations of CNTs based transparent conductive films and methods of making the same can comprise numerous configurations and processing steps other than those specifically disclosed.
