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25th Anniversary Article: Carbon Nanotube- and Graphene-Based Transparent Conductive Films for Optoelectronic Devices Jinhong Du, Songfeng Pei, Laipeng Ma, and Hui-Ming Cheng* becoming a developing trend and will open new applications and markets. This requires TCF electrodes to be flexible or even wearable and stretchable, cheap, and compatible with large scale manufacturing methods, in addition to being conductive and transparent.[1] Considering the advancement of optoelectronic devices, ITO has several limitations: (i) it is becoming increasingly expensive due to a predicted shortage of indium resources and its ever-rising consumption; (ii) the deposition of ITO requires vacuum procedures and often elevated temperatures; (iii) it lacks flexibility and cracks easily, which limits its use for flexible electronics; (iv) indium is known to diffuse into the active layers of OLEDs or OPV cells, which leads to a degradation of device performance.[2] Therefore, in order to keep the pace of device development, new types of TCFs are urgently needed to replace ITO-TCFs. In recent years, there has a rapid growth in the development of ITO alternatives. TCFs have been produced based on carbon nanotubes (CNTs) and graphene, conducting polymers, the most promising being poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), metal gratings and a random network of metallic nanowires (NWs). Some hybrid TCFs based on CNT/PEDOT:PSS, graphene/PEDOT:PSS and CNT/graphene, etc., are also being developed. Although conducting polymers are extremely flexible, they have the disadvantages of poor stability and a noticeable blue color.[3] The electrical properties and transparency of metallic NWs or gratings can out-perform most ITO electrodes on plastic substrates,[4] but their haze makes them incompatible with various display applications.[3] Their environmental stability and scalable fabrication are still a big problem. Another major limitation is that they are not stretchable, and much less flexible than PEDOT:PSS or CNT- and graphene-based TCFs (CNT- and G-TCFs), although they are more flexible than ITO. Among these potential materials, CNT- and G-TCFs are more competitive and have been extensively investigated.

Carbon nanotube (CNT)- and graphene (G)-based transparent conductive films (TCFs) are two promising alternatives for commonly-used indium tin oxide-based TCFs for future flexible optoelectronic devices. This review comprehensively summarizes recent progress in the fabrication, properties, modification, patterning, and integration of CNT- and G-TCFs into optoelectronic devices. Their potential applications and challenges in optoelectronic devices, such as organic photovoltaic cells, organic light emitting diodes and touch panels, are discussed in detail. More importantly, their key characteristics and advantages for use in these devices are compared. Despite many challenges, CNT- and G-TCFs have demonstrated great potential in various optoelectronic devices and have already been used for some products like touch panels of smartphones. This illustrates the significant opportunities for the industrial use of CNTs and graphene, and hence pushes nanoscience and nanotechnology one step towards practical applications.

1. Introduction Transparent conductive films (TCFs) are optically transparent to visible light and electrically conductive. As transparent electrodes, they are widely used in industry, especially in optoelectronic devices. For example, TCFs are an essential component for liquid crystal displays (LCDs), organic light emitting diodes (OLEDs), organic photovoltaic (OPV) cells and touch panels, etc. A market research report at ReportsnReports.com shows that the touch panel TCF market is $956 million in 2012 and is anticipated to reach $4.8 billion by 2019, indicating that TCFs have a tremendous marketplace. With the rapid development and upgrading of consumer electronics such as e-readers and smart phones, not only is the demand for TCFs growing fast, but also their properties need to be significantly improved. Currently, indium tin oxide (In2O3:Sn, ITO) is the most widely used TCF due to its excellent opto-electrical properties with a sheet resistance (Rs) of 10–25 Ω/sq at about 90% transparency. Current optoelectronic devices are normally assembled on hard substrates such as glass, however, flexible versions are Dr. J. H. Du, Dr. S. F. Pei, Dr. L. P. Ma, Prof. H.-M. Cheng Shenyang National Laboratory for Materials Science Institute of Metal Research Chinese Academy of Sciences, 72 Wenhua Road Shenyang 110016, P. R. China E-mail: [email protected]

DOI: 10.1002/adma.201304135

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1.1. CNT- and G-TCFs Using CNTs and graphene instead of ITO for TCFs will bring multiple advantages: (i) carbon is a cheap, abundant

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Jinhong Du is a professor in the Advanced Carbon Research Division at Shenyang National Laboratory for Materials Science, the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS). She received her Ph.D. in Materials Science at IMR, CAS in 2003. She mainly works on the preparation, properties and applications of nanostructured material (carbon nanotubes and graphene)-based films and polymer composites. Songfeng Pei is an associate professor in the the Advanced Carbon Research Division at Shenyang National Laboratory for Materials Science, the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS). He received his Ph.D. in Materials Science at IMR, CAS in 2010. His main research topic is the chemical modification of carbon nanotubes and graphene for applications.

1.2. Evaluation of CNT- and G-TCFs Visible light transmittance (T) at 550 nm wavelength and Rs are the primary performance metrics for TCFs. To evaluate the quality of a TCF, a figure of merit (FoM) is considered almost mandatory. Several FoMs are used to compare the electrical and optical performance of TCFs.[6,7] Considering the percolation scale law of TCFs with a network structure, the FoM, expressed as the ratio of direct current (DC) electrical conductivity and optical conductivity (σDC/σOP), is commonly used for CNT- and G-TCFs.[8] The Rs and T of a TCF can be linked by Equation (1). It is suggested that a larger value of σDC/σOP corresponds to a TCF with higher T and lower Rs, i.e., better electrical and optical performance. 188.5 σ OP ( λ ) ⎞ ⎛ T (λ ) = ⎜ 1 + ⎝ σ DC ⎟⎠ Rs

Laipeng Ma received his Ph.D. in Materials Science at the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS) in 2009. Currently, he is an associate professor in the Advanced Carbon Research Division at Shenyang National Laboratory for Materials Science, IMR, CAS. His research interests include the CVD synthesis of graphene and the modification of graphenebased transparent conductive films.

−2

(1)

Flexibility is also a very important metric for CNT- and G-TCFs, since it is their distinctive advantage over the commonly used ITO-TCFs and other candidates. The flexibility of a CNT- and G-TCF can be measured by uniaxial tensile strain or by repeated bending. The smaller resistance change and the more times they can be bent with a larger angle mean a better flexibility. The uniformity in T and Rs of large-area TCFs is another important metric that should be evaluated for practical applications. Many examination methods have been developed, including measuring T and Rs at 9 arbitrary locations over a large area,[9] calculating standard deviation in absorbance using a transmission map of the entire film recorded using a commercial scanner,[10] and resistance mapping of large area TCFs.[10] Gupta et al. defined a local FoM based on the diffraction efficiency of a calibrated high-resolution transmission grating overlaid with a given TCF as a measure of the overall quality of the TCF.[11] In addition, many other properties such as surface roughness, work function, and adhesion to the substrate need to be evaluated depending on the different application requirements. As potential alternatives to ITO-TCFs, CNT- and G-TCFs have been investigated and evaluated for nearly ten years. Great advances have been made in their performance and scaleup fabrication by researchers and industrial manufacturers, and they have been used in smart phones. However, their

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material and can meet the increasing demand of devices for transparent electrodes. (ii) CNTs and graphene are much stronger and more flexible than ITO. (iii) CNT- and G-TCFs have shown solution processability and have potential for production at low cost. (iv) CNTs and graphene are chemically stable, which is beneficial for a longer device life. (v) CNT- and G-TCFs have a neutral color and wide transmittance spectrum range, which is significant for most uses including OPV cells, OLEDs and displays.[5] As a result, CNT- and G-TCFs have been considered as ideal transparent electrodes for optoelectronic devices. Therefore, TCFs have been identified to be one of the most promising fields for the industrial application of CNTs and graphene.

Hui-Ming Cheng received his Ph.D. in Materials Science from the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS) in 1992. He worked at AIST-Kyushu and Nagasaki University of Japan, and MIT, USA. He is the Professor and founding Director of the Advanced Carbon Research Division at Shenyang National Laboratory for Materials Science, IMR, CAS since 2001. His research interests focus on the synthesis and applications of carbon nanotubes, graphene and high-performance bulk carbons, and on the development of new energy materials for batteries, supercapacitors, and hydrogen production from water photosplitting.

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transparency and electrical conductivity still lag behind ITOTCFs. In addition, various properties should be considered for specific applications, illustrating that great challenges still remain. Herein, we give a comprehensive overview of CNT- and G-TCFs from their fundamentals to optoelectronic applications including the fabrication, properties, modification, patterning for device assembly, integration into optoelectronic devices, etc. Then, we discuss the challenges faced by these two kinds of TCFs in detail and suggest some possible solutions. Based on their own characteristics and applicability in different devices, we give a comprehensive comparison of these two kinds of TCFs to clarify the questions puzzling scientists and manufacturers, for example, if they are equal or which one is better, and which one will lead in industrialization.

2. Fundamentals of Graphene and CNTs Graphene and CNTs are two carbon allotropes which consist of a meshwork of sp2-hybridized carbon atoms but different structures. Single-walled CNTs (SWCNTs) possess a cylindrical nanostructure formed by rolling up graphene, whereas graphene is a flat two-dimensional sheet. Carbon has an atomic configuration of 1s22s22p2, with four valence electrons in the n = 2 atomic shell. In the graphene structure, three of the four electrons in the outer shell occupy hybridized states. These electrons form three strong ‘σ bonds’ that play the key role in forming the carbon network – each carbon atom forms coplanar σ bonds with three neighboring carbon atoms. That leaves one valence electron (the π electron), which occupies a p orbital. The π electron wavefunctions from different carbon atoms overlap to form delocalized π and π* bands (the lowest unoccupied conduction band and the highest occupied valence band) along the lattice plane. The π and π* bands of graphene degenerate at the corner (K point) of the hexagonal Brillouin zone. The degeneracy occurs at the so-called Dirac crossing energy, which at a normal half-filling condition coincides with the Fermi level, resulting in a pointlike metallic Fermi surface. This feature makes carrier (electron) massless Dirac fermions with a Fermi velocity vF of about 106 m/s,[12] which gives a high mobility (∼2 × 105 cm2/Vs at room temperature[13] in graphene. When a planar graphene sheet is rolled into a SWCNT, curvature-induced pyramidalization and misalignment of the p-obtials of the carbon atoms induce changes in electronic properties. As a result, graphene is a zero-gap semimetal with a small overlap between its valence and conduction bands,[14] while SWCNTs can behave as a metal or semiconductor, with bandgaps inversely proportional to their diameters, depending on their diameter and chirality. Since a long range conjugated structure is inherited from graphene, CNTs show very high carrier mobility (>105 cm2/Vs)[15] though this is only along the CNT axis. These electronic features, combined with the nanoscale dimensions, cause the carriers to mostly distribute on the surface, giving graphene and CNTs three important common characteristics: low carrier concentration and high mobility, anisotropic electrical conductivity, and sensitivity to the environment. These features have a close relationship with the properties of CNT- and G-TCFs.

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2.1. Low Carrier Concentration and High Mobility Undoped graphene is a semimetal, though there is a state crossing at Ed = Ef (Dirac point), the density of states is zero and conduction is possible only with thermally excited electrons at a finite temperature, which results in the low carrier concentration of graphene and CNTs. At room temperature, the free carrier concentration of graphene is ∼1020/cm3 (or ∼1012/cm2 for planar distribution of electrons in sheets[13] and ∼1017/cm3 for a SWCNT.[16] This value is low compared with oxide semiconductors (e.g., ∼1021/cm3 for ITO) or metals (e.g., ∼1022/cm3 for Ag). For SWCNT-TCFs, the simultaneous high transparency and good electrical conductivity can be understood on the basis of three properties: (i) low carrier density; (ii) high electronic mobility; and (iii) the suppression of light absorption and reflection for polarization components perpendicular to the nanotube axis, which reduces the optical density of the disordered SWCNT films for unpolarized incident light.[17] The transmittance of a material is closely related to its electronic structure features. When a light beam is incident on a material, the light is reflected, absorbed and transmitted. For a conductive material, the reflectivity is mainly caused by the interaction of the light with free electrons present in it, which can be considered as a plasma combined with heavy ions in it.[18] The best strategy for optimizing TCFs is to limit carrier concentration and increase carrier mobility.[19] There is another important feature of conductive transparent materials, called the plasma edge wavelength (or cutoff wavelength: λp), which means that incident light (with wavelength λin) will be mostly reflected at the material surface rather than transmitted if λin > λp. λp can be calculated using carrier concentration (n) according to the classical Drude theory as follows: ⎛ ne 2 ⎞ λp = 2π  ⎜ * ⎝ m ε r ε o ⎟⎠

−1/2

(2)

Where m* is the effective mass of carriers and εrε0 is the permittivity of the material.[19] The up-limit of the transparency window (spectral range) is determined by λp. Ellmer summarized opto-electrical properties of three groups of TCF materials including metals, oxide semiconductors and carbon.[19] Metals exhibit very high carrier concentrations, medium mobilities and plasma wavelengths in the deep-ultraviolet range (λp < 200 nm), which is why metals usually show a lustre. Oxide semiconductors possess high carrier concentrations, high mobilities and plasma wavelengths in the near-infrared range (λp ∼ 1 µm). Graphene and SWCNT networks have low carrier concentrations, low mobility and plasma wavelengths in the mid-infrared ringe (λp > 1 µm). Figure 1 shows the transmittance spectra of different transparent and electrically conductive materials including ITO, PEDOT:PSS, Ag grid, and random networks of SWCNTs and graphene sheets.[16] Comparatively, SWCNT and graphene sheets have wider ranges and flatter transmittance spectra, which is significant for OPV cells and display device applications.[20]

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2.2. Anisotropic Electrical Conductivity The anisotropic electrical conductivity of graphene or CNT means that their high conductivity is only limited to within a graphene sheet or along the axis of a SWCNT. As a result, if a conductive pathway passes through several graphene sheets or CNTs, the conductivity of the whole network will be severely decreased by the high contact resistance between the graphene sheets or CNTs. For TCF applications, a large size, defect-free piece of graphene may have ∼97.7% transmittance and ∼60 Ω/ sq;[2] and the theoretical maximum conductivity of a SWCNT network is ∼90000 S/cm,[21] which can give a film a low electrical resistance of 10 Ω/sq and a high transmittance of 92%. Nevertheless, the performance of a graphene network obtained from a solution process is typically ∼2000 Ω/sq at 85% transparency and the highest value reported for SWCNT-TCFs to date is ∼12825 S/cm.[22] The σDC of a single CNT can be up to 200 000 S/cm[23] with a mobility larger than 105 cm2/Vs,[15] while a randomly oriented CNT film can only reach 6600 S/cm[17] and a mobility of ∼1000 cm2/Vs at best.[24] The contact resistance can be investigated through the interlayer or intertube conductivity of graphene sheets or CNTs, but the interlayer conductivity of graphene sheets has not yet been well understood. The c-axis conductivity (σc) of graphite, which can be considered as well stacked and contacting graphene sheets, can give us basic information on such a structure. Three important features of σc in graphite are: (i) the in-plane conductivity (σa) is ∼3000 times higher than σc, which has a typical value of ∼10 S/cm;[25] (ii) the dominant mechanism for c-axis conduction in pristine graphite is band conduction, which is evoked by band overlap on the four π-bands near the Brillouin zone edges, rather than hopping;[25] (iii) stacking faults associated with extended basal plane dislocations rather than the electron-phonon interaction dominate room temperature scattering processes for c-axis conduction.[26] For a network composed of graphene sheets, the long-range electrical conductivity relies on the connection between graphene sheets. The overlapping structure of adjacent graphene sheets produced by a solution process makes it difficult to restore the stacking structure to that present in graphite even after high temperature annealing. As a result, serious stacking faults and basal plane dislocations

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Figure 1. Comparison of spectra for various TCFs.[16] Reproduced with permission. Copyright 2009, American Institute of Physics.

are present and result in a poor interlayer conductivity between graphene sheets in a TCF compared with σc in graphite. The intertube conductivity of CNTs is much more complicated because of their different electronic structures, diameters and wall numbers, and contact means, etc. The individual SWCNTs can be metallic (M) or semiconducting (S) based on their chirality. MM and SS junctions have high conductances, on the order of 0.1 e2/h, while for a MS junction, the semiconducting nanotube is depleted at the junction by the metallic nanotube, forming a rectifying Schottky barrier and results in a high resistance. The magnitude of this intertube resistance is typically on the order of 200 kΩ-20 MΩ,[27] while the resistance along a single CNT one micrometer or longer is ∼10 kΩ + 6 kΩ/µm.[28] Since the bandgap of a semiconducting CNT varies with the tube diameter (d: nm) as 0.7/d eV,[29] larger diameter semiconducting CNTs have smaller bandgaps and correspondingly higher numbers of charge carriers at room temperature. As a result, CNTs with big diameters may have a low intertube resistance and are preferable for applications in TCFs. In addition to the point contact at junctions, CNTs with small diameters, e.g., SWCNTs and double-walled CNTs (DWCNTs), usually form bundles during growth, which may contain several, tens or more nanotubes in one bundle. Furthermore, multi-walled CNTs (MWCNTs) can be considered as being rolled from multilayer graphene or thin graphite sheets. The conductivity between bundled tubes and shells in MWCNTs may have a similar conductance to the σc in graphite. Experimental results have shown that smaller bundles lead to films with higher conductivity and individually dispersed nanotubes are best for TCFs;[30] this is likely due to the presence of noncurrent carrying nanotubes in the middle of CNT bundles. Similarly, films made from MWCNTs show dramatically lower conductivity than those from SWCNTs or DWCNTs,[31,32] as the inner walls again act as non-current carrying voids in the films. Additionally, contact morphology has a significant effect on the contact resistance of SWCNT networks. High contact angle X-type contacts are found to produce larger contact resistance than low contact angle Y-type contacts.[33]

2.3. Sensitivity to Environments The sensitivity of the electric conductivity of graphene and SWCNTs to environments lies in two features: (i) all the carbon atoms in a graphene sheet or a SWCNT are surface atoms, which results in current carriers being directly exposed to environments. The change in electrical conductivity caused by surface adsorbates from the surrounding environment is more severe than that for a material with current carriers inside; (ii) carbon atoms have a moderate electron affinity and a large amount of adsorbates have been detected with the ability of charge transfer to or from them. This results in the shift of the Fermi level of graphene or SWCNTs, which is also called adsorption-induced chemical doping and has been well investigated in graphite intercalation compounds (GIC) and conductive polymers.[34] The first feature has been widely used to fabricate ultra-sensitive sensors based on graphene or SWCNTs, for example, gas sensors made from graphene are capable of detecting individual

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events when a gas molecule attaches to or detaches from the graphene surface.[35] The second feature is more important in the research and application of TCFs because chemical doping is almost an unavoidable requirement to improve the electrical conductivity after film formation of graphene and CNTs.

3. Fabrication of CNT- and G-TCFs 3.1. General Fabrication Methods Research on nano-carbon (NC)-based TCFs started in 2004 when Wu et al.[17] and Saran et al.[36] reported using SWCNTs to make TCFs by filtration-transfer (FT) and dip-coating (DipC), respectively. Since then, hundreds of papers have been published on this topic and TCFs are considered the most common application for both CNTs and graphene. Generally, CNT- and G-TCFs can be fabricated through wet processes or dry processes. An obvious difference between these two routes is whether the graphene or CNTs need to be dispersed in solvents before the assembly of their TCFs. Until now, more than ten methods for the wet route have been proposed. They demonstrate new innovation as well as the inspiration from a highly developed printing industry on film formation using traditional materials. Hu et al. have made a detailed review of the wet methods for the fabrication of CNTTCFs.[37] These methods are also suitable for the fabrication of G-TCFs because the dispersion of graphene and graphene oxide (GO) has very similar characteristics to those of CNTs. Some methods are still considered to be lab-scale only, like FT,[17,38–42] spin-coating (SpinC)[43–45] and Langmuir-Blodgett (LB) coating,[46,47] because they are mainly limited by a low coating efficiency and the nature of the equipment is intrinsically hard to scale up. Some methods have the potential to be scaled up to an industrial scale, like DipC,[36,48] self-assembly coating (SA),[49] spray-coating (SprayC),[32,50–53] electrophoretic deposition (EPD)[54,55] and rod coating (RC).[56,57] The scaling-up of these methods relies on the design of highly automatic equipment with intelligent programming, yet there is no report of industrial application based on these methods. The continuous fabrication of SWCNT-TCFs has now been realized through a roll-to-roll printing process by slot-casting, and printing can be performed at a high speed up to 100 m/min on flexible or rigid substrates several meters wide.[58] Some methods that have been widely used in the printing industry, such as gravure press, reverse roll painting and fountain/curtain coating, are also proposed to realize the industrial production of CNT- and G-TCFs.[3] Although lab-scale fabrication methods are less likely to be used to realize the industrial production of CNT- and G-TCFs, most fundamental research on TCFs such as percolation behavior, temperature- and frequency-dependent transport, and factors that affect their properties are usually based on films fabricated using these methods. This is because the FT method leads to highly uniform and reproducible films, and the network density of CNT- or G-TCFs can be precisely controlled by varying the dispersion concentration. The LB coating method can be used to preciously control the film thickness down to a monolayer of graphene sheets or CNTs, or to control the

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orientation of a CNT network. These features facilitate the fabrication of TCFs with precisely controlled microstructures that may have desired properties. In addition to wet processes, dry methods have been developed to fabricate CNT- and G-TCFs. CNT films can be formed by drawing superaligned CNT arrays[59,60] or collecting a CNT aerosol[9] and transferring to transparent substrates. Graphene films can be grown on metal substrates by CVD, and then transferred to transparent substrates to form G-TCFs.[61] Though the research on dry processes has a relatively shorter history than that of wet processes, both CNT- and G-TCFs fabricated by dry processes are approaching applications. TCFs produced from all the three dry methods mentioned above are now commercially available.

3.2. Fabrication by Wet Processes TCFs fabricated by wet processes rely on the good dispersion of CNTs or graphene sheets in suspensions. Raw CNTs are usually in the form of a black soot, while graphene sheets can come from two distinct routes. One is by exfoliation of graphite powder, and the other is to reduce GO obtained by exfoliation of graphite oxide. The dispersion process of CNTs is very similar to the exfoliation of graphite to get a graphene suspension because they have the similar surface properties as well as interactions between CNTs or graphene sheets. Graphite oxide, also called graphitic acid, is a highly oxidized graphite produced by intercalating and oxidizing with a strong acidic oxidant. During oxidation, the graphene layers of the graphite are decorated with a large amount of oxygen containing groups, e.g., epoxy, hydroxyl and carboxyl, etc., with a typical carbon/oxygen atomic ratio lower than 2, which changes the surface of the graphene sheets from hydrophobic to hydrophilic and enables them to be well dispersed in polar solvents such as water, ethanol and acetone. As a result, graphite oxide is much easier to exfoliate into mono- or few-layer sheets in a solvent (mostly water), and these sheets are named GO. CNT, graphene and GO suspensions can be used to fabricate TCFs by the processes mentioned in Section 3.1. Different from the conductive nature of CNTs and graphene sheets, GO sheets are intrinsically insulating due to the disturbed long-range conjugated structure during oxidation, and reduction treatment is necessary before or after film formation to restore the conductivity of the films. As a result, for the fabrication of TCFs using CNTs and graphene sheets, the dispersion of the CNTs and graphene sheets is the key step, while for those using GO, the reduction plays an important role.

3.2.1. Dispersion of CNTs and Graphene Sheets for TCFs Because of high aspect ratio, large specific surface area and strong van der Waals attraction, CNTs tend to stick together, and even form large bundles, especially those with small diameters such as SWCNTs and DWCNTs. Furthermore, well graphitized CNTs have a surface that is inert to most commonly used solvents, so the interaction between CNTs and solvents is hard to balance against the attraction between CNTs, which results in poor dispersion or even re-aggregation. These features are

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DCE for dipC/sprayC[86] and spinC,[45] and DMF for sprayC[31] have σDC/ σOP values in the range of 5∼10. Natural graphite was exfoliated by sonication in DMF[64] and NMP,[62] to obtain suspensions of graphene sheets. The Rs of the TCFs produced by sprayC of graphene/DMF and by vaccum filtration of graphene/NMP is 5 kΩ/sq at a 90% transparency,[64] and 7.1 kΩ/ sq at ∼42% transparency.[62] The low concentration and high boiling point of these solvents may be two obstacles for high performance TCF fabrication. The maximum concentrations (CMax) of SWCNTs and graphene sheets are usually lower than 0.1 mg/mL, much lower than the requirements for industrial applications (>1 mg/mL).[37] A superacid is a term used to describe a series of acids, including oleum, chlorosulphonic acid (CSA), trifluromethanesulfonic acid, etc., in which the chemical potential of the proton is higher than that in pure sulfuric acid.[89] In 2004, Smalley et al. reported the spontaneous dissolution of SWCNTs in superacids.[90] Different from the colloid nature of SWCNT dispersions in other solvents, SWCNTs in superacids form homogeneous solutions and nematic crystals at high concentrations. Hecht et al. reported the fabrication of SWCNT-TCFs using a SWCNT/CSA solution on PET substrates by the FT method. The Rs and transmittance of the films were 60 Ω/sq and 90.9%,[22] respectively, which corresponds to a DC conductivity of 12 825 S/cm and a σDC/ σOP of 64.1, the second highest DC conductivity reported for CNT-TCFs to date. This is attributed to both the high quality of the CNT and the exfoliation/ doping by the superacid. This process was further developed to fabricate TCFs through a dipC method,[91] which enables the production of large area TCFs. Recently, graphite was also reported to be exfoliated into mono-layer graphene spontaneously in CSA with a concentration as high as ∼2 mg/mL. TCFs were fabricated using this solution with Rs of 1 kΩ/sq at 80% transparency.[73] Hydrazine is a strong reductant currently used in the reduction of GO and is usually considered to be dangerous because of its high toxicity and unstable nature. Recently, anhydrous hydrazine was reported to be a good solvent to directly disperse CNTs and reduced graphene oxide (rGO) sheets.[88] Hybrid suspensions of rGO sheets and CNTs in hydrazine with a concentration higher than 1 mg/mL are stable for months with little aggregation. A hybrid thin film with CNTs and rGO was

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also inherited from graphene. For the dispersion of graphene sheets, more efforts are focused on the exfoliation of graphite to graphene sheets, and simultaneously, to obtain a graphene suspension. In most cases, the strategies used to disperse CNT bundles in solvents have been applied to graphene.[62–65] Generally, there are three major strategies to disperse CNTs and graphene sheets in liquid media: i) directly dispersing in neat solvents;[22,66–75] ii) dispersing non-covalently functionalized CNTs and graphene sheets by adsorption or wrapping of certain dispersing agents such as surfactants, polymers or solubilization agents;[76–78] iii) dispersing covalently functionalized CNTs or graphene sheets.[79,80] Grafting different organic or inorganic functional groups on the CNT surface is a typical feature of the covalent functionalization of CNTs. Research on this topic started almost at the same time as the discovery of CNTs, and many reviews have been published.[80,81] However, very limited reports can be found on the use of covalently functionalized CNTs for the fabrication of TCFs.[82,83] For this dispersion strategy, CNTs are chemically treated to introduce negatively charged carboxylic groups on their surfaces, so that a stable CNT aqueous dispersion can be obtained without any surfactant. Though such a dispersion can be used to make large area films, their optoelectrical performance is rather poor with a typical Rs about 2.5 kΩ/sq at a transparency of 86.5%,[82] because the functionalization procedure disturbs the conjugated structure of pristine CNTs by inducing a large number of defects. Therefore, in this section we will mainly review dispersion strategies (i) and (ii) for TCF fabrication. Dispersion of CNTs and Graphene Sheets in Neat Solvents for TCFs: Because dispersion agents generally can increase the number of processing steps and residual surfactants lead to additional contact resistance of the as-deposited film, directly dispersing CNTs or graphene sheets in neat solvent is a much simpler and more favorable way from the viewpoint of practical application. Coleman has reviewed in detail the exfoliation and dispersion of CNTs and graphene in neat solvents.[84] The commonly used solvents are N,N-dimethylformamide (DMF),[64,85] dichloroethane (DCE)[85,86] and N-methyl-2-pyrrolidone (NMP).[62,87] In Table 1, we summarize the opto-electrical properties of CNT- and G-TCFs assembled with such dispersions. CNT-TCFs assembled by directly dispersing CNTs in

Table 1. Opto-electrical property comparison of TCFs assembled with CNTs or graphene sheets dispersed in neat solvents. CNT/ graphene

SWCNT

Fabrication

Performance

Ref.

Dispersion agent

Method

Substrate

Original Rs [Ω/sq]/T

Doping

Final Rs [Ω/sq]/T

σDC/σOP

DCE

DipC/SprayC

PET

340/80%





4.7

[86]

SWCNT

DCE

SpinC

PET

DWCNT

DMF

SprayC

Glass

SWCNT

acid

85/80%

18.7

[45]

170/80%





9.4

[31]

454/80%

SOCl2

100/80%

15.9

60/90.9%

SWCNT

CSA

FT

PET

64.1

[22]

Graphene

DMF

SprayC

Glass

5000/90%

0.7

[64]

Graphene

NMP

FT

Alumina

7100/42%

0.05

[62]

CSA

FT

Glass

Hydrazine

SpinC

PET

Graphene rGO/SWCNT

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1000/80%

1.6

[73]

240/86%

10

[88]

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deposited onto a variety of substrates by SpinC using this dispersion. After chemical doping by SOCl2 vapor, a TCF with a Rs of 240 Ω/sq at 86% transparency can be achieved. Surfactant Assisted Dispersion of CNTs and Graphene Sheets for TCFs: In contrast to solutions, dispersions of CNTs and graphene sheets show properties as colloids. As a result, surfactants, as interfacial stabilizers in colloidal stabilization, are widely used for the dispersion of CNTs and graphene sheets in liquid media. Thousands of papers and a number of reviews on this topic have been published.[92] Although many surfactants have been investigated, the most common ones used for TCF assembly are sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), sodium cholate (SC) and commercial Triton X-100. There are a number of advantages of using surfactants to disperse CNTs and graphene sheets: (i) CNTs can be dispersed at reasonably high concentrations up to 20 mg/mL.[93] (ii) The original electrical properties of CNTs and graphene sheets are not altered by surfactant dispersion. (iii) More importantly, water can be used as a solvent, which is low cost, environment-friendly and safe. However, surfactants are usually insulating, so a washing-out procedure is necessary to remove them after film formation. Geng et al. reported the dispersion of arc-SWCNTs in a SDS aqueous solution and the fabrication of TCFs using such a dispersion by SprayC.[52] TCFs with a thickness around 50 nm had a Rs ∼ 200 Ω/sq at ∼83% transparency (σDC/σOP ∼ 9.6). After washing in nitric acid, the Rs dramatically decreased to ∼100 Ω/sq. The improvement in conductivity relies on removal of residual SDS among the SWCNTs, which simultaneously densifies the SWCNT network, and chemical doping with HNO3. In Table 2, we list the typical optical and electrical properties of TCFs assembled using a CNT or graphene dispersion with the assistance of different surfactants. A SWCNT-TCF was fabricated by FT with the assistance of Triton X-100 achieved a Rs of 30 Ω/sq at ∼83% transparency.[17] Considering the film thickness was 50 nm, the volume conductivity (σ) of this SWCNTTCF is ∼6700 S/cm, and the calculated σDC/σOP is ∼64.2, which is the highest value reported to date using wet processes. A G-TCF from exfoliated graphene with the assistance of SC[94]

has the best performance of all reported G-TCFs assembled using a graphene dispersion with the assistance of different surfactants. An important treatment in producing graphene suspension is to control the sheet thickness in a narrow distribution by an additional density gradient ultracentrifugation (DGU). The graphene film fabricated by FT using graphene sheets with a mean thickness of around 1.1 nm has Rs of ∼2000 Ω/sq at ∼75% transparency, and the calculated σDC/ σOP is ∼0.6. This value is much higher than that obtained with a graphene suspension using SDBS as surfactant (22.5 kΩ/sq at 62% transparency, σDC/σOP ∼0.03).[63] Though a uniform and small sheet thickness may be important for G-TCFs, the difference in the dispersing ability of the two surfactants may be even more important. For exfoliation of graphite, the exfoliation degree in a SC solution is considered to be much higher than that in a SDBS solution, and the graphene suspension in a SC solution without DGU treatment can fabricate a TCF with Rs of ∼4 kΩ/sq at ∼78% transparency (σDC/σOP ∼0.36).[95] The reason for this may be that the face-like hydrophobic part of SC is more favorable to attach to the graphene surface than the taillike hydrophobic part of SDBS, while the latter was reported to be much preferred to disperse CNTs than SC.[76] The interaction between the hydrophobic region of surfactant molecules and the hydrophobic graphene/CNT surface can be called a hydrophobic interaction, which means that they are forced together by the surface tension of the water since neither of them can be wetted by water. Such an interaction will lose its effect in organic solvents because of their low surface tension. As a result, most surfactants that can disperse CNT/ graphene in water cannot be used to disperse them in organic solvents. Jo et al. reported the dispersion of CNTs in ethanol with assistance of quinquethiophene-terminated poly(ethylene glycol) (5TN-PEG).[43] The oligothiophene part (5TN) in 5TNPEG is strongly physisorbed on the surface of CNTs[43] and graphene sheets[96] by strong π–π interaction, while PEG is soluble in various organic solvents. 5TN-PEG can be washed out by dichloromethane (DCM). CNTs are more easily dispersed in chlorinated solvents in the presence of poly-3-alkylthiophene (P3AT) such as regioregular poly-3-hexylthiophene (rr-P3HT)

Table 2. Opto-electrical property comparison of TCFs assembled by CNTs or graphene dispersion with the assistance of different surfactants. CNT/ graphene

Fabrication Surfactant

1964

Method

Performance Substrate

Original Rs [Ω/sq]/T

Doping

Ref.

Final Rs [Ω/sq]/T

σDC/σOP

SWCNT

Triton X-100

FT

Sapphire

30/83%





64.2

[17]

SWCNT

SDS

FT

Glass/PET

380/87%

SOCl2

160/87%

16.3

[38]

SWCNT

SDS

SpinC

PET

200/85%

HNO3

80/85%

27.8

[52]

SWCNT

5TN-PEG

SpinC

Glass

200/80%

HNO3+SOCl2

100/80%

15.9

[43]

SWCNT

SDS

FT

PET

300/80%

HNO3+SOCl2

115/80%

13.8

[99]

SWCNT

Triton X-100

FT

Quartz

184/77.6%

SOBr2

56/77.6%

24.8

[41] [100]

SWCNT

CBs

FT

Glass

147/82%

HNO3

76/82%

23.7

Graphene

SC

FT

Glass

2000/75%





0.6

[94]

Graphene

SC

FT

Glass

4000/78%





0.36

[95]

Graphene

SDBS

FT

Glass

22500/62%





0.03

[63]

graphene

DSPE-mPEG

LB

Quatz

8000/83%





0.18

[85]

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3.2.2. Reduction of GO for TCFs Different from the covalent functionalization of CNTs, a general method for making graphene materials is through the exfoliation of graphite oxide in water to obtain GO, which can be considered a highly covalently functionalized graphene and is hydrophilic and dispersible in water. Thus, GO can form films by using one of any of the wet fabrication methods reported above and it is easily scaled up. Many more papers using GO for the fabrication of TCFs have been published than those using directly exfoliated graphene sheets as reviewed in Section 3.2.1. However, the as-fabricated GO films are insulating. As a result, the main topic of GO-based TCFs is the reduction of GO

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or poly-3-dodecylthiophene.[97,98] the dispersion of SWCNTs in chloroform with the assistance of rr-P3HT with a concentration of 80–100 µg CNT/mL has been reported.[44] Due to the better conducting nature of these polymers than typical surfactants like SDS, there is no need to wash them out after film formation. After treatment in SOCl2, a Rs of ∼80 Ω/sq at 72% transparency was obtained. Li et al. reported the exfoliation of expanded graphite in DMF with the presence of 1,2-distearoylsn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneg lycol)-5000] (DSPE-mPEG).[85] After centrifugation, the supernatant contains a large amount of graphene sheets with ∼90% single-layer and an average size of ∼250 nm. G-TCFs were fabricated using a graphene suspension in DCE by the LB method. The one-, two- and three-layer LB films afforded Rs of ∼150, 20 and 8 kΩ/sq at room temperature and transmittance of ∼93, 88 and 83% at 1000 nm, respectively. Commercial surfactants are usually organic compounds that are amphiphilic with hydrophobic tails and hydrophilic heads. The hydrophobic tails will attach to CNTs or graphene sheets, while the hydrophilic heads help pull the CNTs or graphene sheets into water. This structure feature can also be found in some carbon materials like GO. Kim et al. demonstrated that GO can act as a two-dimensional molecular surfactant, resulting from the hydrophilic –COOH groups on its edges and hydrophobic π domains interspersed on its basal plane.[101] CNTs were found to disperse well in a GO aqueous solution with a 1:3 mass ratio after sonication and to be stable over a period of 24 h. There is no need to wash out the GO after film formation since it can be reduced to be conductive to form a hybrid TCF. A similar idea was proposed to selectively functionalize carbonaceous byproducts (CBs), e.g., amorphous carbon, carbon nanoparticles, and carbonaceous fragments, that are formed during the synthesis of SWCNTs and are attached to the surface of the SWCNTs. These CBs are usually amorphous or have low crystallinity and consequently, they are more chemically reactive or have lower chemical and thermal stability than SWCNTs with a better structural perfection. These functionalized CBs can act as amphiphilic surfactants owing to their hydrophobic carbon framework and hydrophilic COOH groups and render the SWCNTs spontaneously soluble in solvents (Figure 2). After film formation, these functionalized CBs can be removed by heat treatment. TCFs using such dispersed SWCNTs can achieve Rs of 147 and 76 Ω/sq at 82% transparency, before and after nitric acid treatment, respectively.[100]

Figure 2. (a) Photographs of the spontaneous dissolution of soluble SWCNTs in water, for i) 0 min, ii) 10 min, iii) 1 h, and iv) 2 days. (b) and (c) TEM images of soluble SWCNTs and the SWCNTs from SWCNT-TCFs after heat treatment and HCl washing. Reproduced with permission.[100] Copyright 2011, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

after film assembly. Generally, three methods have been used, chemical reduction (CR), high temperature annealing (HTA) and a combination of both CR and HTA, to reduce GO, and some typical results are listed in Table 3. For details on the methods and mechanism of GO reduction, readers can refer to our recent review on the reduction of GO.[111] Specific to the production of TCFs, hydrazine was the most frequently used reducing reagent. The reduction treatment can remove most of the oxygen-containing groups attached to the carbon plane, and the conjugated structure of graphene can be partly restored to make rGO electrically conductive.[112] However, reduction by hydrazine alone is not sufficient to achieve maximum reduction, and a subsequent annealing can well improve the conductivity of rGOTCFs.[105,109] An rGO-TCF with Rs of 102–103 Ω/sq at a 80% transparency was obtained by a combination of hydrazine reduction and annealing at 1100 °C.[105] High temperature annealing also is found to be efficient to reduce the electrical conductivity of rGO films, and the reduction effect is significantly affected by the heating temperature.[85,105,113] For GO films with the same thickness, the conductivity is 50, 100 and 550 S/cm when the annealing temperature is 500, 700 and 1100 °C, respectively. The rGO-TCF reduced at 1100 °C has a Rs of 1.8 kΩ/sq at 70.7% transparency. Though it is highly effective, high temperature annealing is not suitable for the production of TCFs. One important obstacle is that the most commercial transparent substrate materials, e.g., glass and polymers, cannot stand a temperature higher than 500 °C. As a result, chemical reduction at low temperature is important for rGO-TCFs. Sodium borohydride (NaBH4)[103,114] and hydroiodic acid (HI)[115,116] were reported to be more effective than hydrazine to reduce GO, especially for GO films. A rGO-TCF reduced by NaBH4 has a Rs of 4.4 kΩ/sq

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www.advmat.de Table 3. Opto-electrical property comparison of rGO-TCFs produced by different reduction methods. Reduction methods CR

HTA

CR+HTA

HI acid

Substrate

σ [S/cm]

Rs [kW/sq]

T at 550 nm [%]

σDC/σOP

Ref.

PET

∼1000

1

85

2.2

[102]

NaBH4

PET

Hydrazine

Glass

Hydrazine

Quartz

1100 ºC

4.4

81

0.38

[103]



30

80

0.05

[104]



1.4×105

92

80



[106]

70

0.552

[107]

1100 ºC

Quartz

1100 ºC

Quartz

1786

1000 ºC

Quartz

1425

1100 °C

Quartz

550

Hydrazine+1100 °C

Quartz

550

Hydrazine+200 °C

Glass

Hydrazine+200 ºC

Glass



at 81% transparency,[103] while a rGO-TCF reduced by HI acid has a lower Rs of 1 kΩ/sq at 85% transparency.[102] To summarize the properties of TCFs fabricated by wet processes, an obvious characteristic is that SWCNT-TCFs show much better performance than G-TCFs and rGO-TCFs. The σDC/ σOP of SWCNT-TCFs is usually higher than 10 with the highest value of 64.2. This can satisfy the basic industrial requirement of TCFs (100 Ω/sq at 90% transparency, σDC/σOP = 35).[117] While G-TCFs, no matter whether using directly exfoliated graphene or rGO, mostly have a σDC/σOP value lower than 1, which is far from what is required for applications. The reason for such a difference will be discussed in Section 4.1.

3.3. Fabrication by Dry Processes 3.3.1. CNT -TCFs To date, there are mainly two approaches to fabricate CNT-TCFs by dry processes based on two different ways of making CNTs: (i) floating catalyst chemical vapor deposition (FCCVD) to produce a CNT aerosol and (ii) fixed catalyst chemical vapor deposition to produce a superaligned CNT (SACNT) array. FCCVD was firstly proposed by Cheng et al.[118] in 1998. CNTs grow on vapor-phase flowing catalyst particles and are deposited on the wall of the reactor to form a thin and transparent film, which then can be transferred to a transparent substrate to produce TCFs. By this method, Ma et al. fabricated a SWCNT-TCF with a Rs lower than 50 Ω/sq at 70% transparency.[119] Meanwhile, Nasibulin et al. directly collected floating SWCNTs using a membrane filter at the outlet of a reactor and then transferred them to a transparent substrate to form TCFs.[120] A CNT-TCF up to 70 mm in diameter with a Rs as low as 84 Ω/sq at 90% transparency (σDC/σOP = 41.5) has been fabricated.[9] Due to the simplicity and rapidity of the process, it has been further developed to fabricate a carbon nanobud (CNB, a novel form of carbon combining CNT with fullerene)-TCF by the Canatu Company. This commercial product has a σDC/σOP = 82.1 (150 Ω/sq @ 97% T excluding the substrate), much better than CNT-TCFs.

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1.8

70

0.42

[85]

85

0.64

[108]

100

65

0.022

[109]

11.3

87

0.29

[110]

The growth of a SACNT array was first reported by Jiang et al.[121] in 2002, and the macro- and micro-images of this array are shown in Figure 3a. Then, the drawing sheets from SACNT arrays (Fugure 3b) were reported.[59] The sheets are composed of parallel CNT fibrils and form anisotropic TCFs on a transparent substrate. The formation of these CNT fibrils relies on the intermittently bundled morphology of the CNTs in an array, and individual nanotubes migrate from one bundle of a few nanotubes to another. Bundled nanotubes are simultaneously pulled from different elevations in the array sidewall, so that they join with bundled nanotubes that have reached the top and bottom of the array, thereby minimizing breaks in the resulting fibrils. To realize a continuous fabrication of CNT-TCFs, an automatic roll-to-roll process (Figure 3c) for drawing sheets from SACNT arrays was developed.[60] The performance of the as-drawn TCFs is around 1 kΩ/sq along the drawing direction at 80% transparency, while after suitable laser trimming and deposition of Ni and Au metals, the TCF can have 208 Ω/sq and 24 Ω/sq at 90% and 83.4% transparency (σDC/σOP = 16.7 and 82.4), respectively.

3.3.2. G-TCFs In contrast to the stacking structure of G-TCFs obtained by the wet process, the fabrication of G-TCFs by a dry process is based on the growth of structurally continuous graphene films. Among all the synthesis methods involved in the dry process, CVD has been attracting increased interest for producing G-TCFs as it allows the growth of large-area and high-quality graphene films in a scalable manner, and flexible G-TCFs with the best performance[122] and the largest size[123] have been produced. This process typically involves the CVD growth of a graphene film on a substrate at elevated temperatures and their subsequent transfer to a transparent substrate. To achieve G-TCFs with low Rs, few-layer graphene is generally preferred. This can be clearly seen in Table 4. To this end, two strategies have been developed: layer-by-layer stacking (LBL) of monolayer (or bilayer) graphene films and direct growth

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REVIEW Figure 3. SEM image of (a) macro- (left) and micro- (middle with scale bars: 100 µm; inset, 200 nm and right with scale bars: 500 nm; inset, 100 nm) images of a SACNT array. Reproduced with permission.[121] Copyright 2002, Nature Publishing Group; (b) CNT-TCFs prepared by drawing the SACNT array. Reproduced with permission.[59] Copyright 2005, American Association for the Advancement of Science; (c) CNT-TCFs prepared in a continuous manner by drawing the SACNT array. Reproduced with permission.[60] Copyright 2010,Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

of few-layer films. Monolayer-dominated large-area graphene films are generally synthesized by Cu-based CVD.[124] Relatively uniform few-layer films can then be obtained by direct LBL or the interface coupling LBL of monolayer graphene films. The latter is advantageous in eliminating contamination of other materials during transfer.[125] However, the LBL transfer is not only time-consuming but also significantly increases the cost of production with increasing number of layers transferred. To promote the mass production of G-TCFs, it is desirable to directly grow high-quality few-layer graphene films. Substrates such as Cu,[126–128] Ni[129–131] or Cu-Ni alloys[132,133] allow the direct CVD growth of few-layer graphene films by increasing concentration of carbon source. However, the few-layer films thus obtained suffer from non-uniformity in the number of graphene layers, which reduces conducting channels available for a given transmittance. Compared to optimizing growth parameters, the introduction of hydrogen etching into the growth process is more effective in improving the uniformity of graphene films by reducing the nuclei of thick layers and/ or the carbon content within the metal.[134,135] Nevertheless, as summarized in Table 4, the highest σDC/σOP is only 34.8 for a few-layer G-TCF obtained by metal-based CVD growth, demonstrating that the direct growth of uniform few-layer graphene films still remains a significant challenge. Although CVD growth generates high-quality graphene films, the transfer process generally leads to structural

Adv. Mater. 2014, 26, 1958–1991

damage (e.g., cracks and holes) and structural distortion (e.g., ripples), thus degrading the performance of the G-TCFs. The primary solution to this issue is to improve the transfer process. Extensive investigations have shown that the presence of a supporting layer on the surface of graphene is effective in retaining its structural integrity in a reproducible and scalable manner. Therefore, the choice of supporting layers is the key to the efficient transfer of large-area G-TCFs. Polymethylmethacrylate (PMMA) is a representative and widely used supporting layer for transferring CVD graphene.[136,137] This is because that the PMMA-based process allows the transfer of graphene onto any rigid (e.g., glass and quartz) and flexible (e.g., PET) transparent substrates that can withstand organic solvents (e.g., acetone) as a versatile and reproducible method. It can also be used to transfer few-layer G-TCFs with a LBL process. However, the PMMA layer cannot be completely removed by organic solvents after transfer. The residues are detrimental to both the performance and stability of optoelectronic devices such as OPV cells and OLEDs because they significantly increase the surface roughness of the TCFs. The situation is even worse in the case of few-layer G-TCFs, in which the amount of PMMA residue increases with the number of layers transferred. Meanwhile, the underlying PMMA particles tend to cause cracks and holes during the LBL transfer of fewlayer graphene films. Furthermore, manipulating a large-area PMMA film is difficult because such a thin polymer layer has

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www.advmat.de Table 4. Opto-electrical properties of G-TCFs fabricated by dry processes. Fabrication

Substrate

Number of layers

Original Rs [Ω/sq]/T

Original σDC/σOP

Glass

1

2100/∼97.5%

7

2

∼1000/∼95%

7.2

3

∼600/∼93%

8.5

4

350/∼90%

9.9

Cu-based CVD and PMMA transfer (LBL)

Cu-based CVD and PMMA transfer (LBL)

Glass

Cu-based CVD and PMMA transfer (LBL)

PET

1

980/97.6%

15.7

2

∼540/∼95.3

14.3

3

∼350/∼92.9

14.3

Final Rs [Ω/sq]/T

Final σDC/σOP

[137]

1

725/97.6%

21.2

301/96.6%

35.8

690/92.8%

7.2

111/90.5%

33.1

3

466/87.1%

5.6

93/87%

28.0

58/83.5%

34.4

313/85.1%

7.2

1

725/97.6%

21.2

2

690/92.8%

3 4 P(VDF-TrFE)

Cu-based CVD and PMMA transfer(LBL) Cu-based CVD and PMMA transfer (LBL)

Ref. [136]

2

4

AuCl3

[138]

657/93.3%

8.1

7.2

147/89.2%

21.8

466/87.1%

5.6

120/85.6%

19.4

313/85.1%

7.2

107/83.1%

18.1

1

1440/N/A

N/A

P(VDF-TrFE)

500/∼95%

14.5

Quartz

4

∼200/∼90%

17.4

HCl +HNO3

∼80/∼90%

43.4

[125]

Quartz

8

∼300/80%

5.31

HNO3

90/80%

17.7

[140]

Cu-based CVD and PMMA transfer

Glass

∼6a)

200/85%

11.1

Cu-based CVD and Au transfer (LBL)

Cu-based CVD and PMMA transfer (LBL)

Cu-based CVD and PMMA transfer

PET

HNO3

[138]

[139]

[141]

Glass

4

N/A/N/A

N/A

TCNQ

182/88%

15.6

[142]

Cu-based CVD and TRT transfer (LBL with roll-to-roll)

PET

1

275/97.4%

51.6

HNO3

∼110/∼97%

111.4

[122]

4

∼50/90.1%

70.3

∼30/∼90%

115.8

Cu-based CVD and TRT transfer (LBL with roll-to-roll)

PET

1

∼450/∼97.5%

32.8

AuCl3

∼120/∼97.5%

123.0

4

∼60/∼89%

52.2

∼43/∼89%

72.9

Cu-based CVD and thermal lamination onto EVA coated PET

PET

1

1960/96.7%

5.7

Cu-based CVD and lamination onto epoxy coated PET

PET

1

487/N/A

N/A

AuCl3

197/97.1%

64

Ni-based CVD and PMMA transfer

Quartz

∼5a)

448/89%

7

AuCl3

150/87%

17.4

Ni-based CVD and PDMS transfer

PET

∼10a)

280/76.3%

4.6

[130]

Ni-based CVD and support-free transfer

PET

∼9a)

367/∼80%

4.3

[146]

Ni-based CVD and PMMA transfer

Glass

∼4a)

∼770/∼90%

4.5

[129]

Ni-based CVD and PMMA transfer

PET

∼4a)

100/∼90%

34.8

[134] [133]

[143]

[144] [123] [145]

CuNi-based CVD and PMMA transfer

Glass

∼3a)

287/93%

17.7

Remote catalytic CVD

Quartz

∼1

5800/97%

2.1

[147]

Interfacial CVD

Quartz

∼3a)

2000/94%

3.0

[148]

Interfacial CVD

Quartz

∼2a)

50/95.8%

173.4

[149]

PECVD

Quartz/ Glass

∼6a)

7000/∼85%

0.3

[150]

Glass

5

∼140/88.5%

21.3

GIC a)Few-layer

FeCl3

8.8/84%

234.5

[151]

graphene obtained by direct CVD growth.

a low mechanical strength, limiting the use of the PMMAbased process for continuous transfer of large-area graphene films.

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A polydimethylsiloxane (PDMS) film with low surface energy is another typical supporting layer. The primary difference between PDMS- and PMMA-based transfer is that

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Based on the above summary, the transfer processes currently used inevitably lead to defect formation and/or organic contamination and significantly increases the production cost of G-TCFs. Therefore, transfer-free CVD growth has been proposed to fabricate G-TCFs. Interfacial CVD growth is considered to be a promising strategy with carbon source in the form of both gas[148] and solid phase.[149] A high-performance G-TCF on quartz with a Rs of ∼50 Ω/sq at 95.8% transparency has been synthesized by a single-step interfacial CVD,[149] and yields the highest σDC/σOP of 173.4 for a wafer-size G-TCF. However, the process generally involves the use of high-temperature at ∼1000 °C. Therefore, transparent substrates are limited to those such as quartz with a high melting point. To address this problem, a novel strategy was developed to drive the graphene growth by pressure instead of high temperature in the presence of a Ni catalyst layer.[156] However, the high intensity of D bands and the negligible 2D bands in Raman spectra suggests the presence of a large amount of defects in the graphene film. Alternatively, plasma enhanced CVD (PECVD) was developed, which enables the direct formation of graphene films on quartz and glass in the absence of metal catalysts. Compared with the normal CVD, PECVD is simple and the thickness of graphene films can be tuned by changing the growth duration.[150] More importantly, the impact of metallic residue can be avoided. However, the high Rs (several kΩ/sq) of the resulting graphene films precludes their possible applications in typical optoelectronic devices. Based on the properties of G-TCFs fabricated by a dry process, many G-TCFs show better performance than CNT-TCFs. The highest σDC/σOP is ∼173.4 for a wafer-size CVD G-TCF, while that of a SWCNT-TCF is ∼41.5 and a large area CNBTCF is 82.1. In summary, according to the reported results, the rough sequence for σDC/σOP of TCFs fabricated by dry and wet processes is CVD G-TCFs by a dry process > CVD CNT-TCFs by a dry process > CNT-TCFs by a wet process > G- or rGO-TCFs by a wet process.

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the surface energy difference between PDMS/graphene and the graphene/target substrate is used.[152] PDMS-based process allows the transfer of graphene onto substrates with surface energy larger than PDMS. In contrast to PMMA, PDMS is highly elastic and flexible. Thus, thick PDMS films (typically several millimeters in thickness) can be used as a scalable free-standing supporting layer.[130] Similar to PMMA-based transfer, this method allows the formation of few-layer G-TCFs using a LBL technique. A distinct advantage of PDMS-based transfer is that it allows the patterned transfer of graphene.[153] A detailed discussion of patterning graphene will be presented in Section 6. Moreover, the absence of an organic dissolution process simplifies the transfer and the PDMS support can be recycled. More importantly, it further increases the number of transparent target substrates available since it allows the use of substrate materials that can be dissolved in organic solvents. The highly elastic and flexible PDMS layer also allows the intimate contact of PDMS/graphene with target substrates without post-treatment. However, this is a delicate process and cracks tend to occur as a result of the deformation of the PDMS layer when pressure is used to improve the contact. Therefore, only wafer-size G-TCFs have been achieved using the PDMS-based transfer method. To realize the application of G-TCFs, one critical issue is to develop a transfer method for large area graphene films. In this regard, robust supporting layers compatible with the roll-to-roll technique have shown great promise. A thermal release tape (TRT) has been used as a robust supporting layer for transferring graphene films with a diagonal size of 30 inches onto a PET substrate.[122] The key feature of TRT transfer is that graphene can be released from TRT by removing its adhesive force under thermal treatment rather than dissolving the supporting layer or using substrates with higher surface energy. In principle, TRT transfer can be used to transfer graphene films onto any transparent substrates that can withstand the thermal lamination, and also is compatible with the LBL transfer technique. So far it has generated large area flexible G-TCFs with the best performance with a Rs of ∼50 Ω/sq at ∼90% transparency for four-layer graphene films on PET without doping. However, the use of TRT significantly increases the cost of the transfer process and also suffers from surface contamination of organic residue.[122] To simplify the transfer process and lower the cost of largescale production of G-TCFs, a direct transfer strategy has been developed, in which a target transparent substrate is simultaneously used as the supporting layer. For example, graphene grown on a Cu or Ni foil can be transferred onto a PET film by using thermal rolling lamination directly[154] or by using an adhesive layer.[123,144,155] In the latter case, the performance of G-TCFs is highly dependent on the properties of the adhesive layer. By combining the continuous roll-to-roll graphene growth and transfer technique, Sony Corp. has produced a 100 m long flexible G-TCF with Rs of ∼500 Ω/sq before doping.[123] However, G-TCFs produced by direct transfer generally suffer from optical transmission loss from a relatively high surface roughness because the PET or adhesive layer conforms to the rough surface of the metal foils upon attachment. Such transmission loss can be reduced by smoothing the surface with an additional PET coating.

4. Factors Influencing TCF Properties 4.1. TCFs Based on CNT and Graphene Networks The σDC of CNT- and G-TCFs produced by wet processes relies on the formation of conductive networks. Basically, the electrical conductivity of individual nanotubes and graphene (including rGO) sheets must be high enough to achieve a low Rs of thin films and the lattice perfection is key for both CNTs and graphene sheets. As a result, for a CNT-TCF assembly, covalently functionalized CNTs are seldom used. TCFs based on rGO have a low electrical conductivity. Furthermore, CNT purity may be another critical factor in determining film conductivity. The presence of amorphous or sp3 bonded carbon and surfactants will lower the σDC of resulting TCFs. Typically, films with higher Raman G/D band intensity ratios exhibit higher conductivity.[157] This means that fabrication, purification and post treatment to remove surfactants are critical factors for obtaining highly conductive TCFs. In addition, as has been discussed in Section 2.2, the anisotropic conductivity of individual nanotubes and graphene sheets results in a high contact

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resistance between connected CNTs or graphene sheets. As a result, factors that can influence the contact resistance between CNTs or graphene sheets are also key factors that influence the film conductivity. The factors influencing the performance of CNT-TCFs have been comprehensively reviewed.[3,158] Briefly, the main factors include the type of nanotubes, mean tube/bundle length and size (diameter), and the formation of networks (network density and film thickness). As to the effect of CNT type shown in Figure 4a and b, SWCNT-TCFs have the best performance compared with those assembled from MWCNTs and DWCNTs. And TCFs fabricated with SWCNTs that are produced by arc discharge show obvious superiority than that by laser ablation and CVD with high-pressure carbon monoxide (HiPCO). One explanation is that the former can form a better networks than

the latter two types of tubes. Longer CNT bundles will limit the number of CNT junctions per unit area of film and result in a higher conductivity of the network (Figure 4c).[9] Smaller bundles lead to higher conductivity with the same transparency (Figure 4d)[30] due to the decrease of non-current carrying tubes in the middle of CNT bundles. In addition, orientation of the bundles can affect the efficiency of the conductive network. In this regard, randomly oriented CNTs are highly preferred as they have a lower percolation threshold than networks made from aligned CNTs.[159] To date, research on factors influencing the performance of graphene network films is very limited. As has been mentioned in Section 3, both rGO and exfoliated graphene materials are used to assemble TCFs, but they are very different as raw materials. Structurally, the rGO exfoliated from graphite

Figure 4. Factors influencing the performance of TCFs assembled from (a-d) CNTs and (e, f) graphene sheets. (a) CNT types. Reproduced with permission.[160] Copyright 2008, Korean Physical Society. (b) Synthesis methods of SWCNTs. Reproduced with permission.[161] Copyright 2007, Springer. (c) Mean bundle lengths. Reproduced with permission.[9] Copyright 2010, American Chemical Society. (d) Mean bundle diameters. Reproduced with permission.[30] Copyright 2009, Acta Materialia Inc. (e) rGO sheet areas. Reproduced with permission.[102] Copyright 2010, American Chemical Society. (f) Thickness of exfoliated graphene sheets. Reproduced with permission.[117] Copyright 2010, American Chemical Society.

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decrease the interlayer distance to realize a well stacked structure, which is also known as chemical graphitization.[116] Such structural features improve the TCF conductivity. It is worth noting that, even after reduction, the electrical conductivity of rGO sheets is still much lower than that of exfoliated graphene sheets, while the similar or even better performance of rGOTCFs further confirms that the film conductivity is limited by intersheet stacking for liquid-exfoliated G-TCFs. As has been proposed in Section 3, the performance of SWCNT-TCFs is far superior to that of the G-TCFs produced by wet processes no matter whether using rGO or pristine liquidexfoliated graphene sheets. This difference in performance may mostly result from the difference in the intrinsic conductivity of SWCNTs and graphene sheets and their TCF structure. In general, a SWCNT-TCF shows a porous network structure in which there are void areas for the high transmittance of light, while a stacked graphene film shows a layered structure. For graphene sheets, the transmittance linearly decreases with the number of carbon atomic layers (up to five layers) with each layer absorbing 2.3% over the visible spectrum,[163] which results in decreased transmittance with increasing film thickness. Consequently, to achieve the same transmittance, CNT films can be much thicker than graphene films. De et al. theoretically calculated the upper limit of σDC/σOP values (2.55 and 11) of undoped monolayer graphene and well stacked multilayer graphene (considering multilayer graphene as single crystal graphite).[117] However, TCFs based on graphene networks are mostly multilayers that are neither continuous nor well stacked like graphite, so the resulting high contact resistance definitely degrades the performance of the TCFs. This is especially true for liquid-exfoliated G-TCFs, while for rGO-TCFs, the presence of the remaining oxygen-related defects is the main source of their high intrinsic resistance. Although CNTs also have a large contact resistance due to point contacts and Schottky barriers in the random network of CNT films, the high intrinsic conductivity and ability to achieve TCF conductivity at high thickness

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oxide followed by reduction is mostly monolayer, and large area GO sheets (100∼102 µm) can also be produced through mild treatment. The direct exfoliation of graphite to graphene sheets in a solvent usually needs more violent processing, e.g., long time ultrasonication, which results in the fracture of the graphene sheets. Exfoliated graphene sheets with typical flake sizes ranging from sub-micrometer to several micrometers are mostly multilayer with monolayer content usually lower than 5%. By analogy with nanotube networks, the flake size and degree of exfoliation are two factors influencing the performance of assembled TCFs. As shown in Figure 4e and f, the GO sheets with a large size up to several tens of micrometers have a better conductivity than the small GO sheets with a size distribution from several hundreds of nanometers to several micrometers,[102] and thinner graphene sheets can produce TCFs with better performance using exfoliated graphene.[94] Besides the structural difference of individual graphene sheets, the microstructures of films assembled using different graphene sheets may also have a significant influence on their performance. The microstructures of TCFs assembled with three different graphene sheets are shown in Figure 5. The film assembled with organic solvent-exfoliated graphene sheets (Figure 5a–c) has a very rough surface. Due to the stiff nature of multilayer graphene sheets with a low aspect ratio, graphene sheets contact each other but are not well stacked, which produces a large contact resistance in the network. An improved connecting (stacking) structure can be found in the TCF assembled with CSA-exfoliated graphene (Figure 5d), which has a relatively large area and is mostly monolayer caused by the spontaneous exfoliation of graphite in CSA. The best performance of TCFs assembled with rGO was obtained using big GO sheets with a mean area of ∼7000 µm2 and reduced by HI acid (Figure 5e), which shows a very smooth surface. The negatively charged hydrophilic surface of GO sheets enables them to form a regular layered structure during self-assembly into a film.[162] Moreover, the reduction by HI acid can further

Figure 5. Morphologies (SEM) of graphene films assembled with different graphene (or rGO) sheets. (a-c) organic solvent-exfoliated graphene sheets (σDC/σOP = 0.05). Reproduced with permission.[62] Copyright 2008, Nature Publishing Group. (d) CSA-exfoliated graphene (σDC/σOP = 1.6. The bright horizontal strips are Pd electrodes). Reproduced with permission.[73] Copyright 2010, Nature Publishing Group, and (e) a rGO film assembled using GO sheets with a mean area of ∼7000 µm2 and reduced by HI acid (σDC/σOP = 2.2). Reproduced with permission.[102] Copyright 2010, American Chemical Society.

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cause CNT-TCFs to have a better performance than G-TCFs produced by wet processing. Theoretically, CNT-TCFs produced by wet processing should have a similar performance to those produced by dry processing because they have a similar network structure. However, the results show that dry processing-based SWCNT-TCFs have a higher conductivity than most wet processing-based SWCNTTCFs. The main reason for this is that the dispersion process can inevitably shorten SWCNTs and some surfactant residue is difficult to remove, thus additional contact resistance is introduced into the network. However, experimentally, the record σDC/σOP of 64.2 for SWCNT-TCFs was for films fabricated by a wet process rather than a dry process, which indicates that they should have similar performance if the film assembly conditions are optimized.

4.2. CVD G-TCFs Different from the TCFs based on graphene or CNT networks, G-TCFs obtained by CVD are characterized by a continuous polycrystalline film. Equation (3) below can be used to describe its Rs, which is determined by the two dimensional DC conductivity (σ2D) of individual graphene sheets and the number of layers (N):[164] R s = (σ 2DN)−1 = (nμeN)−1

(3)

where n is the concentration of charge carriers, and µ the carrier mobility. The former can be tuned by doping while the latter is closely related to the structural integrity and defects. Because the CVD-grown graphene usually has a low defect concentration, the transmittance of such G-TCFs linearly decreases with the number of layers (N, up to five layers) with each layer absorbing 2.3% over the visible spectrum.[163] As a result, the performance of a G-TCF is primarily determined by the structural integrity and defects, number of layers, and doping level. As discussed in Section 3.3.2, the structural integrity of a G-TCF is highly dependent on the transfer process while the structural defects mainly originate from the intrinsic defects generated during the CVD growth. The differences in the transfer methods and processing parameters, which lead to different structural integrity (e.g., cracks and holes) of G-TCFs, are the primary reasons for the large variation of Rs of the G-TCFs obtained by using a similar growth process (Table 4). Fortunately, several innovative methods, such as transfer with the assistance of TRT,[122] UV-curable epoxy,[123] and electrochemical exfoliation,[165] have been proposed to achieve well-controlled transfer while maintaining the structural integrity of the graphene films. In addition to the technical factors, some intrinsic properties of as-grown and transferred graphene films such as the number of structural defects and number of layers as well as the effect of chemical doping also influence the performance of G-TCFs. In this section, the effects of structural defects and number of layers will be mainly reviewed and the effect of doping will be discussed in Section 5. All of the large area CVD-grown G-TCFs are characterized by a polycrystalline structure consisting of many single-crystal grains separated by grain boundaries (line defects), which

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may have a significant impact on the Rs. Theoretical calculations indicate that a grain boundary of certain configuration generates a transport gap by increasing the carrier scattering in graphene, which may impede electronic transport.[166] Some experimental measurements[167] also confirm such results by comparing electron transport within and between graphene grains. The inter-grain resistance obtained is always higher than the corresponding intra-grain resistance (R on each side of the grain boundary) and the calculated inter-grain series resistances. As a result, to improve the carrier transport in a graphene film, one promising approach is to reduce the number of grain boundaries by growing large-size grains or even a single crystal film.[165,168,169] On the other hand, some experimental results suggest that well-stitched grain boundaries have little impact on the electrical transport, indicating that uniformly high performance polycrystalline G-TCFs rivaling those of exfoliated samples can be achieved by optimizing growth conditions.[170,171] Compared with the growth of large-grain graphene, fabricating graphene films with a few-layer structure is more efficient in decreasing Rs. According to Equation (3), for a typical few-layer graphene, Rs is inversely proportional to the number of layers. A model of a resistor network has been used to quantitatively describe this relation.[140] It highlights the importance of interlayer conductivity in explaining the performance of macroscopic few-layer G-TCFs. However, an increase in the number of layers simultaneously decreases the transmittance of G-TCFs due to the ∼2.3% optical absorption of monolayer graphene. Therefore, less than 4 layers is generally acceptable considering the typical requirement of >85% transmittance for TCFs in device applications. As we concluded in Section 3, CVD G-TCFs show the best performance in terms of reported σDC/σOP among CNT- and G-TCFs. The reason may be that, in addition to the high conductivity of CVD-grown graphene with a large grain size, the high utilization of graphene sheets and film uniformity are very important. CVD-grown films are quite uniform and additional graphene stacking occurs much less than that in wet processing-based G-TCFs, while bundles are commonly found for CNT-TCFs. This additional stacking or bundling will decrease the optical transmittance instead of increasing the conductivity of the TCFs obtained. In addition, in-plane contact resistance caused by well-stitched grain boundaries for CVD G-TCFs may be smaller than out-of-plane contact resistance by stacking for wet-processing-based G-TCFs and point contact for CNT-TCFs.

4.3. Flexibility of CNT- and G-TCFs Since one possible application of these films is as flexible transparent electrodes for flexible devices, the flexibility of CNT- and G-TCFs is considered as an important property. As shown in Figure 6a, with an increase of bending angle, ITO-TCFs show a rapid increase in Rs due to cracking of the film, while the Rs values of SWCNT- and G-TCFs show almost no significant change. This demonstrates the superiority of SWCNT- and G-TCFs to conventional TCFs.[36,158] When comparing SWCNT- and G-TCFs, the former shows more stable conductivity under bending and stretching than

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REVIEW Figure 6. Comparison of the flexibility of SWCNT- and G-TCFs. (a) Rs of a SWCNT-TCF, a G-TCF and an ITO-TCF with bending angles from 0° to 180°. Reproduced with permission.[158] Copyright 2011, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Rs of PEDOT/SWCNT hybrid TCFs versus radius of curvature for one bend/release cycle measured for films both in compression and in tension. Reproduced with permission.[172] Copyright 2009, American Chemical Society. (c and d) Variation in resistance of a G-TCF at different strains and bending radii. Reproduced with permission.[154] Copyright 2010, American Institute of Physics.

the latter. Typically, as shown in Figure 6b, the Rs of SWCNT/ PEDOT-TCFs versus radius of curvature from 7.5 mm to 2.5 mm varies by 6 and a slight decrease in transparency.[122,138,145] In addition, due to the unique planar morphology, layer-bylayer doping is effective for improving the interaction between dopant and graphene for a few-layer CVD G-TCFs.[138,142] It not only leads to a further decrease of Rs but also increases the stability of the dopant under ambient conditions. Commercial transparent electrodes must undergo strictly accelerated aging tests, and this requires the Rs change of TCFs to be no more than 10% after 250 h at 60 °C/90% relative humidity, or 1 h at 150 °C. TCFs must also be stable in various commonly used solvents, including acids and alkalis. However, most reported doping agents have proven to be transitory. This is because these gaseous or liquid molecules are weakly adsorbed on the surface of CNT- and G-TCFs and are inherently unstable at elevated temperatures where they can desorb and evaporate. Furthermore, moisture or other species in the air can react with the (typical) strongly oxidizing dopants, thus rendering them less effective. For example, the stability of the improvement in the electrical conductivity of p-doped SWCNT-TCFs with HNO3 and SOCl2 were found to be limited in air and under thermal loading.[99] As shown in Figure 7, the Rs of the undoped TCF remains stable over the duration of the experiment, while all doped SWCNT films showed a significant increase of Rs with increasing time. The addition of a capping layer of a material such as PEDOT:PSS to “trap” dopants seems to increase the stability.[99] This poor stability is unacceptable for practical applications. Therefore, the development of stable doping strategies is of particular interest. MoOx was found to be a stable chemical p-type dopant for both CNT- and G-TCFs with excellent thermal stability up to 300 °C.[181] Annealing at 450 °C can substantially activate MoOx and encourage charge transfer from CNTs to MoOx, allowing strong and stable doping. However, the longterm environmental stability (on the scale of years) of MoOx still needs further investigation. The stacking structure of fewlayer graphene with open edges allows the use of heavy doping

observed in GIC. FeCl3-intercalated few-layer graphene flakes show excellent performance with the highest σDC/σOP of 234 (8.8 Ω/sq at 84% transparency) and long-term stability in air for more than a year.[151] However, the obstacle to the practical applications of intercalant doping in TCFs is its compatibility with large area graphene films. In addition, sonochemical modification can cause covalent bonding which may increase doping stability.[183] The use of metal nanoparticles may also be a solution towards increasing the doping stability.[184] Instead of chemical dopants, the use of electrostatic potential to dope graphene may be a promising way to obtain stable doping of TCFs.[185] The key for its application in TCFs is the development of a convenient way for generating a uniform electrostatic potential over a large area film. In this regard, the use of a polymer coating shows great promise. It is found that the use of both a fluoropolymer[186] and a ferroelectric film[139] as the supporting layer decreases the Rs of a G-TCF through the electrostatic doping, and it is stable over one month. A distinct advantage of this method is that the polymer can be used as both a supporting film and dopant, which facilitates the fabrication of large area G-TCFs. The long-term environmental stability of electrostatic doping with polymers also needs further investigation. The changes in carrier concentration produced by doping indicate that the Fermi level (i.e., the work function) of CNTs and graphene has also been changed. Therefore, doping can also be used to tune the work function of TCFs, which is crucial in determining the performance of many optoelectronic devices such as OPV cells and OLEDs. For the anode material in such devices, a work function above 5 eV is generally desirable. The relatively low work function of CNTs and graphene (typically between 4.5∼4.8 eV) suggests that it needs to be increased (i.e., a downshift of the Fermi level) in these applications. Several studies have shown that p-type dopants can increase the work function of CNTs and graphene by hole doping, including HNO3 and SOCl2,[55] metal chlorides,[187,188] tetracyanoquinodimethane[142] and MoO3.[189] In the case of a cathode material, a lower work function is required. The work function of a few-layer graphene film was decreased from 4.4 to 3.77 eV by depositing a sub-nanometer AlOx layer with a low work function.[190] Similarly, the formation of an Al-TiO2 composite layer on a G-TCF surface can lower the work function and improve wettability.[191] The work function of graphene can also be tuned between 4.25 and 3.4 eV by using different alkali metal carbonates.[192] Clearly, doping is very effective to modify CNT- and G-TCFs, but stability is a big issue and should be addressed before application.

5.2. Hybrids

Figure 7. Rs versus time in air of four SWCNT-TCFs. Reproduced with permission.[99] Copyright Year 2008, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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In addition to developing a stable doping strategy, integration with other TCF materials is another promising route to achieve high-performance TCFs, which generally show a higher environmental stability than doped CNT- and G-TCFs. Metal grid and metal NW films are emerging TCFs with a low Rs at high transmittance. However, their applications are limited by their holey structure, high surface roughness and/or low stability in air.[3] CNTs can easily create bridges between AgNWs and, more

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effectively increase the electrode conductivity but also improve hole transport for the anode of many devices. For example, PEDOT/SWCNT hybrid films prepared from a mixed dispersion consisting of a PEDOT:PSS solution and a SWCNT:SDS dispersion using a vacuum-filtration method, could achieve a Rs of 80 Ω/sq at 75% transparency.[172] Graphene/PEDOT hybrid electrodes prepared from a mixed dispersion of PEDOT:PSS with graphene by spin-coating, could reach 80 Ω/sq at 79% transparency. OLEDs based on this hybrid TCF on PET have a higher luminescence efficiency than those built with an ITOTCF due to the better hole-injection capability of the graphene/ PEDOT:PSS hybrid TCF.[201]

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importantly, both CNTs and AgNWs are compatible with respect to solution processing. As a result, CNT/AgNW hybrid TCFs were fabricated by a low-temperature drop coating process. The SWCNTs formed bridges between the AgNWs and filled in the surrounding spaces, resulting in hybrid transparent electrodes with a Rs of 29.2 Ω/sq at 80% transparency.[193] Furthermore, these hybrid electrodes on PET films exhibited excellent stability of Rs under repeated bending tests with a curvature radius of 5 mm. Graphene is characterized as a continuous film with intrinsic low surface roughness and high impermeability. In this regard, developing hybrid films combining the advantages of graphene and a metal grid or NWs is of particular interest. A series of metal grid-graphene hybrid TCFs has been fabricated by transferring monolayer graphene films onto different metal grids deposited on both rigid and flexible substrates.[194] Comparative studies show that the performance is dominated by the metal grid, which is highly dependent on the electrical conducitivity and characteristic sizes of the grid (e.g., the width and spacing of grid lines) and the type of substrates. A low Rs of ∼30 Ω/sq at ∼91% transparency was obtained for a Cu gridCVD grown graphene hybrid flexible TCF. Alternatively, Jeong et al. proposed a percolation-doping strategy to integrate graphene with NWs rather than a simple combination, suggesting that the low Rs of polycrystalline graphene can be achieved by bridging the high resistance grain boundaries with NWs with a slight decrease in transmittance.[195] Based on this strategy, a high-performance (∼24 Ω/sq at ∼91% T) AgNW-CVD grown graphene hybrid TCF was obtained by transferring a graphene film onto a sub-percolation network of AgNWs.[196] This strategy can be further extended to a co-percolation concept by using two films with a percolation network.[197] Experimental results revealed a synergistic effect in the hybrid of continuous AgNW and CVD-grown graphene films,[197,198] which was attributed to the co-percolation mechanism in which the high resistance areas of one film are bridged by the transport channels of the other.[197] In addition, the presence of a graphene coating significantly improves the stability of the underlying AgNWs in air.[197] In all the above work, intimate contact was made in the hybrid films, which is critical to obtaining the bridging effect in these percolation and co-percolation hybrids. However, the flattening effect of graphene films is significantly diminished and the hybrid films show a relatively high surface roughness due to the holey structure of the underlying metal networks. CNTs can also be blended with graphene to prepare high performance CNT/graphene hybrid TCFs, in which individual CNTs bridge the gaps between graphene sheets.[199,200] Large graphene sheets cover the majority of the total surface area, while CNTs act as wires connecting the graphene pads together. The rGO-TCFs (Table 3) generally have a higher Rs, which can be decreased significantly to ∼240 Ω/sq at a transparency of 86% after combining with CNTs, as shown in Table 1.[88] The high performance hybrid TCFs are compatible with flexible substrates and can be fabricated in a simple, inexpensive, scalable way, providing competitive all-carbon TCFs for optoelectronic devices. Incorporating CNTs or graphene with PEDOT:PSS is another effective way to obtain high performance hybrid TCFs.[172,201] As an emerging TCF, PEDOT:PSS can play the role of a hole injection layer (HIL). A hybrid of two components not only can

5.3. Other Modifications There are also many other methods to modify CNT- and G-TCFs for their application in various devices. For touch screen applications, TCFs are frequently subjected to mechanical friction, and a high surface strength is necessary to obtain long-term stability. Yan et al. demonstrated that a transparent polymer coating can protect graphene from damage in repeated sliding by increasing both the friction force and the coefficient of friction of the graphene film.[202] In addition, the polymer coating can improve the ambient stability of doped CNT- and G-TCFs as a passivation layer. However, a strong interaction between polymer and graphene should be avoided, because this leads to a change in the Rs of the G-TCF. HILs such as PEDOT:PSS and metal oxides are widely used in the fabrication of OLED and OPV cells based on CNT- and G-TCFs, which modify the work function of electrodes and lower the surface roughness. A hydrophilic surface is highly desirable as it facilitates the deposition of HIL. However, pristine CNT- and G-TCFs generally suffer from hydrophobicity, and surface modification is required. It has been found that ozone treatment is a simple and highly efficient method to improve the wetting behavior of G-TCFs by attaching oxygencontaining groups on the graphene surface. The contact angle is reduced from 70o for pristine graphene to 58o for a ozonetreated graphene monolithic pattern.[203] A more significant improvement was obtained by depositing Al nanoclusters on the graphene surface, which reduces the contact angle to ∼48o with a slight decrease in optical transmittance.[191] Apparently, these modifications are key to realize the application of CNTand G-TCFs and we will further discuss them in Section 7 together with the use of CNT- and G-TCFs for various optoelectronic devices.

6. Patterning of TCFs Patterning is to locate a film at defined positions on a substrate. To integrate TCFs into devices, high-quality and highresolution patterning is a very important step. It is common practice to pattern an ITO-TCF by means of photolithography and subsequently wet etching. However, due to the chemical inertness, wet etching is not suitable for the patterning of CNT- and G-TCFs. Up to now, many efforts have been made to develop applicable patterning techniques for CNT- and

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G-TCFs including direct growth and transfer printing,[130,204–206] plasma dry etching,[207,208] chemical anchoring,[209] laser photoablation,[210] and photocatalysis,[211] etc. These techniques can be generally classified into bottom-up and top-down patterning strategies. Bottom-up patterning directly deposits a desired pattern on a substrate, while top-down patterning subtracts undesired parts from an entire film to form a pattern on a substrate.

6.1. Bottom-up Patterning The bottom-up patterning strategy generally starts from the fabrication of a pre-patterned catalyst by photolithography,[212] inkjet printing,[204] microcontact printing,[213] or other methods, and the subsequent selective growth of CNT- or G-TCFs on the pre-patterned catalyst area. Because CNTs and graphene are generally grown on a metal or Si substrate, these directly patterned TCFs need to be transferred to a transparent substrate such as glass, PET, polyethylenenaphthalate, etc. Bottom-up patterning can also be achieved by selectively depositing CNTs or graphene from a dispersion onto a transparent substrate which is generally treated to carry some functional groups in a defined area. Direct growth and transfer printing is a typical bottom-up patterning method. Kim et al. developed a growth, etching and transfer process for CVD-grown large-scale graphene patterns.[130] As shown in Figure 8, a pre-patterned Ni layer was first deposited on a SiO2/Si substrate, on which a graphene film was grown by CVD (Figure 8a). Subsequently, two different transfer methods were used to etch the Ni layer and then transfer the isolated graphene film to a target substrate. One is a dry-transfer process by using a soft substrate such as a PDMS stamp (Figure 8b). The other is to use a buffered oxide

etchant (BOE) or hydrogen fluoride solution to remove the SiO2 layer (Figure 8c), so that the patterned graphene and the nickel layer floated together on the solution surface. After transfer to a substrate, further reaction with the BOE or a hydrogen fluoride solution completely removed the remaining nickel layer. A rapid fabrication method for graphene patterns using laserinduced CVD was developed.[214] A focused femtosecond laser beam irradiated a thin nickel foil in a CH4 + H2 environment to induce a local temperature rise, thereby allowing the direct writing of graphene patterns in precisely controlled positions, which then could be transferred to other transparent substrates. Direct growth and transfer printing can also be used for patterning CNT-TCFs.[205] For example, site-selective growth of MWCNTs from a Co nanoparticle catalyst patterned by an inkjet printing technique[204] and site-selective growth of SWCNT films with linear and grid patterns using a SiO2 catalyst[215] have been achieved, respectively. Despite considerable efforts on direct growth and transfer printing, it is difficult to be used in practice because of the limited controllability of the transfer process as discussed in Section 3.3.2. In particular, patterned films can be easily out of shape and even broken during handling so that it is difficult to obtain a precise transfer of the originally grown patterns, which limits their resolution and reproducibility. Moreover, removal of the catalyst is also a big issue, because CNTs and graphene can be contaminated or damaged during this process and then the properties and applicability of their TCFs are degraded. The wet process is low cost and popularly used to prepare TCFs. Therefore, it is expected to directly generate TCF patterns on transparent substrates from a CNT or graphene suspension. Inkjet printing is a digital, non-contact printing method that enables accurate drop placement and can directly generate patterns without any material waste. Due to its high efficiency,

Figure 8. Synthesis, etching and transfer processes of large-scale and patterned graphene films. (a) Synthesis of a patterned graphene film on a thin nickel layers. (b) Etching using FeCl3 (or acids) and transfer of the graphene film using a PDMS stamp. (c) Etching using BOE or a hydrogen fluoride (HF) solution and transfer of the graphene film. RT, room temperature (∼25 °C). Reproduced with permission.[130] Copyright 2009, Nature Publishing Group.

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geometries than photolithography. Another advantage is that it can directly produce patterns and does not waste material because it is an etch-free process. As a result, it is expected to be used for devices, especially for conceptual devices, that require TCF patterns with high resolution and complicated geometries.

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low cost, convenience for mass production, and applicability for flexible substrates, it has become an ideal technology for large-area flexible TCF patterns. CNT-TCFs with different patterns[216,217] were prepared by inkjet printing of CNT dispersions. The hydrophilic treatment of the substrate surface before printing and the proper heat treatment of the substrate during printing are important. GO can be easily dispersed in many solvents for ink preparation and a reasonably good conductivity can be obtained after reduction. Thus, the “GO ink—inkjet printing—rGO film” process is a dominant strategy for inkjet-printed graphene films.[218–220] With a simple standard office inkjet printer, various patterns with high image quality using water-soluble single- and few-layer GO were printed.[221] After thermal reduction, these patterns demonstrated a high electrical conductivity. However, both CNTs and graphene have high aspect ratios and it is difficult to obtain a stable ink, while GO is difficult to completely reduce to a highly conductive graphene material. Furthermore, before inkjet printing, the size of CNTs and graphene or GO must be reduced to smaller than 1/10∼1/50 of the nozzle size (tens of micrometers) to avoid blockage. This reduction in the size of the graphene or CNTs will inevitably decrease the electrical conductivity of the inkjet-printed TCF patterns since a large contact resistance is introduced. As a result, it is difficult for inkjet printing to be widely used for patterning high-performance CNT- and G-TCFs. Self-assembly on a pre-patterned transparent substrate is another effective method to obtain CNT- and G-TCF patterns from their dispersions. A combined technique of chemical anchoring and photolithography has been developed to form a SWCNT pattern on a pre-patterned substrate.[209] As an example, the substrate was first treated with an acid-labile group-protected amine, and an amine pre-pattern was formed using a photolithographic process. A SWCNT monolayer pattern was then formed through the amidation reaction between the carboxylic acid groups of the carboxylated SWCNTs and the pre-patterned amino groups. A high-density multilayer was fabricated by further repeated reaction between the carboxylic acid groups of the carboxylated SWCNTs and the amino groups of the linker with the aid of a condensation agent. Alternatively, one can pre-pattern the hydrophilic and hydrophobic regions on a substrate, and a well-defined SWCNT pattern can be selectively deposited on the hydrophilic region from its dispersion.[222] For self-assembly of graphene patterns, GO is commonly used because it has abundant oxygen-containing functional groups and is negatively charged. As a result, GO is attracted to positively charged substrates. The selective immobilization of GO sheets on ammonized surface patterns with positively charged -NH3+ in an acidic aqueous solution has been achieved and these patterns remain after reduction.[223] However, such self-assembly is dependent on the preparation of single layer patterns, and CNTs and graphene must be treated to contain some functional groups for hydrophilicity or be negatively charged, which results in a decrease of their intrinsic conductivity. In this regard, it is difficult to obtain film patterns with high conductivity. Despite many challenges, the bottom-up strategy has the advantage of obtaining smaller and more complicated

6.2. Top-Down Patterning The top-down approach needs the advance preparation of a TCF over an entire substrate, and subsequent patterning by removing certain areas. Many top-down methods including photolithography or e-beam lithography followed by plasma etching,[224,225] electrochemical patterning,[226] lift-off interlayer,[227] photocatalytic patterning, laser cutting,[210] etc., have been reported. Among these, photolithography with subsequent plasma dry etching is most commonly used. This method has several advantages: (i) it is “borrowed” from silicon-based electronics manufacturing technology and can be scaled up to produce large quantities for commercial use. (ii) It results in high resolution and sharp pattern edges. (iii) It offers good reproducibility and reliability.[207] Figure 9 shows the patterning of a SWCNT-TCF on a PET substrate using standard photolithography and subsequent O2-plasma treatment in a capacitively coupled plasma (CCP) system.[207] During the standard photolithography, a positive photoresist (PR) polymer coating was first deposited onto a homogeneous SWCNT-TCF which had been deposited on a PET substrate. Then, the PR-covered film was exposed to UV light through a designed mask and developed with an AZ400K solution to obtain the SWCNT-TCF pre-patterned with PR polymer. During the plasma treatment, SWCNTs underneath the pre-patterned PR polymer are protected from etching and damage by O2-plasma while the exposed SWCNTs are destroyed. By using this patterning method, various SWCNT patterns with good resolution were obtained (Figure 10). Besides, the patterning of SWCNT-TCFs down to 100 nm in the lateral dimension was obtained by photolithography or e-beam

Figure 9. Schematic of the patterning of a SWCNT film using a standard photolithography method and subsequent O2-plasma treatment in a CCP system. Reproduced with permission.[207] Copyright 2010, American Chemical Society.

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Figure 10. Optical images of patterned SWCNT-TCFs produced using standard photolithography and subsequent O2-plasma treatment. The scale bar represents 300 µm. Reproduced with permission.[207] Copyright 2010, American Chemical Society.

lithography and subsequent O2 plasma etching using an inductively coupled plasma.[208] However, whatever plasma is used, the equipment required is very expensive and the etching process is complex and time-consuming. Distinct from silicon-based electronic materials, CNTs and graphene can be easily insulated by functionalization or by etching in air and other oxidizing gases including a mixture of Ar, O2 and H2O, and mixture of H2S and O2 at ambient pressure.[228] This offers a potential way to pattern CNT- and G-TCFs using a gas phase instead of a vacuum oxygen plasma. Su et al.[203,229] developed a gas exposure method to pattern SWCNT- and G-TCFs on flexible polymeric substrates without using vacuum and high temperature treatment. By simple exposure to ozone for 3 minutes, a high quality SWCNT-TCF (35∼40 nm in thickness) pattern was obtained on a PET substrate. More interestingly, by using a mild ozone flow, it only took 90 s at 120 °C to obtain a well-defined monolithic graphene pattern, in which G-TCF electrodes are separated by insulating GO regions, without the removal of any graphene material. This monolithic graphene pattern has the advantage of a low optical contrast between the graphene and GO regions, which is crucial for high performance graphene-based optoelectronic devices. Meanwhile, ozone exposure is also effective in improving the wetting behavior of a graphene transparent electrode, which is highly desirable as it facilitates the deposition of a hole injection layer when the pattern is used for a transparent electrode in optoelectronic devices such as solar cells and OLEDs, as discussed in Section 5.3. As a result, ozone photolithography is a practical choice for patterning CNT- and G-TCFs on polymeric substrates for the further assembly of flexible devices. Although the above top-down approaches are effective to pattern CNT- and G-TCFs, they are susceptible to PR residue contamination, due to their unique characteristics such as active surface properties of CNTs and graphene, active edge properties of graphene, and a microscopic porous network structure of CNT-TCFs. Such PR contamination consequently degrades the electrical properties and optical transmittance of the TCFs. To address these problems, many efforts have been made to use metals as a protecting or sacrificial layer instead of PR.[230,231] For example, an Al layer was thermally evaporated under vacuum[230] and a Zn coating was sputtered[231] onto a graphene film as a protective layer. Interestingly, Su et al. reported a lift-off Al interlayer method to pattern SWCNTTCFs on a flexible polymer substrate.[232] An Al pattern was fabricated by photolithography and used as an interlayer between a SWCNT-TCF and a PET substrate. Immersion in an acid solution can remove the Al pattern interlayer and the part of the

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SWCNT-TCF adhering to it, leaving a SWCNT-TCF pattern on the bare PET substrate. The resultant SWCNT pattern is free of PR contamination and damage, and preserves the electrical properties of the SWCNT-TCF. Other methods include photocatalytic patterning using a patterned TiO2 photomask,[211] soft lithography by selectively reducing localized GO into rGO through a PDMS mold with an array of microwells filled with a hydrazine solution,[233] the fabrication of patterned graphene structures using versatile photocoupling chemistry,[234] rapid patterning of SWCNT-TCF by PR interlayer lithography,[227] etc. In addition, direct laser cutting without using any PR or other material within a very short time is also effective. The top-down patterning strategy is also relatively easy to be commercialized because it is based on modern electronics manufacturing, although it is more expensive than the wet etching method for traditional ITO-TCFs. Researchers and manufacturers have to choose a suitable patterning method according to the requirements for the properties and quality of the TCF patterns in practical applications.

7. Applications in Optoelectronic Devices Transparent electrodes are an essential component of various optoelectronic devices and ITO-TCFs have been widely used. As new kinds of transparent electrodes, CNT- and G-TCFs with good electrical conductivity, transparency and flexibility have been prepared with large areas, and also can be processed into different patterns. Up to now, more attention has been focused on whether they can replace ITO-TCFs and more importantly whether they can be integrated into flexible devices and commercially used. In this section, we will discuss the feasibility of CNT- and G-TCFs as electrodes in optoelectronic devices by reviewing the development of their applications in OPV cells, OLEDs, and touch panels, etc. The main advances and challenges will be discussed in detail from basic to technical aspects.

7.1. OPV Cells Photovoltaic cells (solar cells) produce electricity directly from sunlight and have attracted significant attention as a promising source of renewable energy. A critical aspect of this kind of optoelectronic devices is their transparent electrode through which light is coupled into the devices. OPV cells,[235–238] dye sensitized solar cells[20,239–241] and silicon solar cells[188,242] based on CNT- and G-TCFs have already been demonstrated. Here we

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Rs [Ω/sq]

T [%]

Device structure

η [%]

Ref.

CNT-TCF using FT method as anode

200

85

PET/CNT/PEDOT:PSS/PCBM:P3HT/Al

2.5

[246]

CNT-TCFs using spinC with DCE as anode

128

90

SprayC with SDS

57

65

2.2

SprayC with SDBS

68

70

1.2

CNT hybrid with PEDOT as anode

350

84

Glass/PEDOT:PSS-SWNTs/P3HT: PCBM/Al

1.3

[248]

99.9% Metallic SWCNT-TCF as anode

188

91.7

Glass/CNT/PEDOT:PSS/P3HT: PCBM/LiF/Al

2.0

[249]

99.9% Semiconducting SWCNT-TCF as anode

411

91.7

HiPCO SWCNT-TCF using FT method as cathode

250

65

PET/CNT/ZnO nanowire/P3HT/Au

0.6

[250]

Free-standing MWCNT-TCF (f-MWCNT) as top cathode

250

38

Glass/ITO/tetrakis (1,3,4,6,7,8-Hexahydro2H-pyrimido[1,2-a]pyrimidinato)ditungsten (II) (W2(hpp)4) dopedC60 /C60 as /mixed (ZnPc):C60 / F6TCNNQ doped BF-DPB/f-MWCNT

1.5

[251]

500∼700

65 ⊥ 85//

Glass/ITO/P3HT:PCBM/PEDOT:PSS/CNT/ PEDOT:PSS/CuPc: C60/C60/BCP/Al

0.31

[252]

rGO-TCF by spinC and hydrazine recuction & thermal annealing reducing as anode

17.9 k

69

Quartz/G/PEDOT:PSS/P3HT:PCBM/LiF/Al

0.13

[253]

rGO-TCF by spinC and reducing hydrazine recuction & thermal annealing reducing as anode

100∼500 k

85∼95

Quartz/G/CuPc/C60/BCP/Ag

0.4

[106]

3.2 k

65

PET/G/PEDOT:PSS/P3HT:PCBM/TiO2 /Al

0.78

[254]

rGO-TCF by FT and thermal annealing after reducing by hydrazine hydrate as anode

1k

80

Quartz/G/PEDOT:PSS/P3HT:PCBM/Al

1.01

[255]

rGO-TCF by spinC and laser beam reducing as anode

1.6k

70

PET/G/PEDOT:PSS/P3HT:PCBM/Al

1.1

[256]

rGO-CNT hybrid TCF by spinC from G-CNT/hydrazine dispersion as anode

240

86

PET/rGO-CNT/PEDOT:PSS/P3HT:PCBM/Ca:Al

0.85

[88]

CVD G-TCF by PDMS transfer as anode

210∼1350

72∼91

Glass/G/PEDOT:PSS/P3HT:PCBM/LiF/Al

1.71

[257]

CVD G-TCF by PMMA transfer as anode

230

72

PET/G/PEDOT:PSS/CuPc/C60 /BCP/Al

1.18

[258]

CVD G-TCF by transfer as anode

606

87@515 nm

Glass/G/PEDOT:PSS/P3HT:PCBM/TiOx /Al

2.58

[259]

CVD G-TCF using LBL stacking as anode

80

90

Quartz/G/MoO3 /PEDOT:PSS/P3HT:PCBM/LiF/Al

2.5

[125]

AuCl3 doped CVD G-TCF by PMMA transfer as anode

300∼500

91∼97

Quaze/G/PEDOT:PSS/CuPc/C60/BCP/Ag

1.63

[260]

CVD G-TCF by transfer as cathode

520∼850

85∼90

Glass/G/WPF-6-oxy-F/P3HT:PCBM/PEDOT:PSS/Al

1.23

[261]

Dry transferred CVD G-TCF hybrid –with AgNW as cathode

34.4

92.8

Glass/G-AgNW/ZnO/ P3HT:PCBM/MoO3/Ag

3.3

[262]

Top laminated CVD G-TCF as top anode

Ten layer graphene

Glass/ITO/ZnO/ P3HT:PCBM/GO/G

2.5

[263]

Glass/ITO PEDOT:PSS/P3HT:PCBM/MoO3/G/MoO3/ ZnPc: C60/LiF/Al

2.9

[264]

Preparation of CNT- or G-TCF

Oriented MWCNT as interlayer

rGO-TCF reduced by thermal annealing and transferring to PET as anode

CVD G-TCF by PDMS stamping as interlayer

500–700



will mainly focus on the use of CNT- and G-TCFs in OPV cells due to the ease of manufacture, light weight, low technological cost, and compatibility with flexible substrates, which can take full use of the advantages of CNT- and G-TCFs. Because power conversion efficiency (η) is the most important merit for a PV cell, we summarize the η of CNT- and G-TCF based OPV cells with different device structures in Table 5 to illustrate their effectiveness. Because several papers have reviewed G-TCFs as the electrode in OPV cells and discussed the effect of the starting materials (GO and CVD-grown graphene), TCF assembly method and performance,[243–245] we will include CNT-TCF results reported and mainly discuss their use as different electrodes for OPV cells.

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PET/CNT/PEDOT:PSS/MDMO-PPV:P3HT/Al

1.2

Glass/CNT/PEDOT:PSS/P3HT: PCBM/LiF/Al

2.3

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Table 5. Summary of the best η for OPV cells based on CNT- and G-TCFs.

[247]

0.27

According to the summary of device structures in Table 5, it is obvious that CNT- and G-TCFs have been widely investigated as the anode of OPV cells. For example, the use of a SWCNTTCF as a transparent anode was demonstrated for efficient, flexible OPV cells with a structure of PET/SWCNT/PEDOT:PSS/ P3HT:PCBM/Al with PEDOT:PSS being used to produce a smooth surface, lower Rs and enhance charge transfer.[246] As shown in Figure 11a, the resulting SWCNT-TCF based OPV cell has a η of 2.5%, close to an ITO/glass-based cell. Additionally, a G-TCF was used as an anode for flexible OPV cells with a structure of CVD G-TCF/PEDOT:PSS/CuPc/C60/BCP/Al, which has a η of 1.18% which is comparable to 1.27% for ITO-based OPV cells with the same structure (Figure 11b and c).[258] As

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Figure 11. (a) Current density vs voltage (J–V) characteristics of P3HT:PCBM devices under AM1.5G conditions using ITO on glass and flexible SWCNTs on PET as anodes. Inset: Schematic of the device and photograph of the highly flexible cell using SWCNTs on PET. Reproduced with permission.[246] Copyright 2006, American Institute of Physics. (b, c) J–V characteristics of CuPc/C60 OPV cells under AM1.5G condition using (b) CVD-grown graphene or (c) ITO as the transparent electrode for different bending angles. Insets show the experimental setup used in the experiments. Reproduced with permission.[258] Copyright 2010, American Chemical Society.

shown in Figure 11a–c insets, both SWCNT- and G-TCF based OPV cells possess good flexibility and demonstrate outstanding ability to operate under large bending angles, while ITO-based devices show cracks and irreversible failure under bending to 60°, indicating the great potential of CNT- and G-TCFs for flexible OPV cells. In addition to serving as transparent anodes, CNT- and G-TCFs have other uses in OPV cells. For instance, they can replace ITO-TCFs as a cathode to prepare OPV cells with an inverted structure, which requires a lower work function of the cathode. A SWCNT-TCF was used as the cathode to fabricate OPV cells with an inverted structure of PET/CNT/ZnO NW/P3HT/Au, which achieved a maximum η of 0.6%.[250] The work function of G-TCF was reduced from 4.58 eV to 4.25 eV by using 0.2 wt% solution of poly(9,9-Bis((6)-(N,N,Ntrimethylammonium) hexyl(-2,7-fluorene)-alt – (9,9-bis)2 – (2-)2-methoxyethoxy(ethoxy) ethyl) – 9-fluorene) dibromide (denoted as WPF-6-oxy-F) in methanol for use as the cathode. The resulting OPV cells with a structure of G/dipole layer/P3HT/PCBM/PEDOT/Al have a η of 1.23%, while no photovoltaic effect was observed for the inverted OPV cells without the WPF-6-oxy-F interlayer.[261] CNT- and G-TCFs can also be used as top electrodes instead of metals. Semi-transparent OPV cells with free-standing

MWCNT (f-MWCNT) and a top laminated G-TCFs as transparent top electrodes, respectively, have been developed.[251,263] The resulting cells showed good long-term stability and comparable η to the devices having a metal electrode. These semitransparent OPV cells consisting of top laminated transparent CNT or graphene electrodes are highly promising in tandem or stacked devices and power-generating windows. Both CNT- and G-TCFs have also been investigated as an interlayer in tandem OPV cells. The short circuit current density (JSC) of the parallel monolithic tandem OPV cell with a transparent MWCNT-TCF as interlayer was higher than the individual JSC of the component cells, and was 63% of the expected sum values.[252] Tong et al. demonstrated a CVD-grown graphene film as an interlayer in tandem solar cells (Figure 12).[264] By using a MoO3-modified graphene interlayer, a high open circuit voltage (Voc) of 1 V and a high Jsc of 11.6 mA/cm2 could be obtained in series and parallel connections, respectively, in contrast to a Voc of 0.58 V and Jsc of 7.6 mA/cm2 in a single PV cell. The value of VOC (JSC) in the tandem cell is very close to the sum of VOC (JSC) obtained from the two single subcells in series (parallel), which confirms good ohmic contact at the photoactive layer/MoO3-modified graphene interface. Based on the above results, CNT- and G-TCFs have shown great potential in OPV cells and not only can replace ITO-TCFs

Figure 12. (a) Schematic of an OPV device structure with G-TCF as interlayer; (b) J–V characteristics of the reference single cell (ITO/PEDOT:PSS/ P3HT:PCBM/LiF/Al (bottom cell), ITO/MoO3/ZnPc:C60/LiF/Al (top cell)) and series connected tandem cell under light illumination. (c) J–V characteristics of the reference single cells (characterized individually from the parallel connected tandem cell) and parallel connected tandem cell under light illumination. The theoretical J–V curve of the tandem cell is also constructed by summing the J–V curves of the single cells (the line with hollow squares). Inset graphs show the optimized thickness of MoO3 in the tandem device. Reproduced with permission.[264] Copyright 2011, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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both CNT and G-TCFs are hydrophobic, they must be treated to be hydrophilic so that an interlayer such as PEDOT:PSS can be coated to tune their work function and allow the uniform deposition of a photoactive layer such as P3HT and PCBM. However, many hydrophilic treatments decrease the conductivity by introducing oxygen-containing groups or defects. Nevertheless, several efficient methods have been developed. For instance, a thin layer (≈ 20 Å) of MoO3 was evaporated on graphene. The presence of hydroxyl groups on MoO3 allows the spreading of PEDOT:PSS on MoO3-coated graphene and the work function of the graphene was also improved.[125] (v) Encapsulation. Device stability in ambient conditions is a crucial issue for their practical application. The commonly used photoactive materials are rather unstable in air, and the barrier properties of polymer substrates such as PET to H2O and O2 are not good enough for their use in OPV cells. Therefore, it is necessary to develop flexible substrates with outstanding barrier properties. Encapsulation/passivation technologies for devices are effective in solving the stability problems.

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as anode and cathode but also can replace metals as the top electrode and interlayer of tandem OPV cells. Except for the fact that rGO-TCF-based OPV cells have a relatively lower η due to their poor electrical conductivity and optical transparency, both CNT- and CVD G-TCF based OPV cells can achieve a high η of ∼2.5%. Especially, the devices using a CVD-G/AgNW hybrid TCF as cathode can achieve a high η of ∼3.3% due to the high opto-electrical performance of the hybrid TCF and efficient electron collection at the interface between the AgNW and ZnO layers.[262] However, these results still cannot compete with ITO-TCFs. There remain many challenges for their practical application for OPV cells: (i) Rs and transparency. The performance of OPV cells is strongly dependent on the quality and properties of CNT- and G-TCFs. As listed in Table 6, the Rs and transparency of CNT- and G-TCFs used in OPV cells are not yet comparable with ITO-TCFs, which only have 10–15 Ω/sq with a high transparency of > 85%. Therefore, it is of primary importance to further decrease the Rs and improve the transparency of CNT- and G-TCFs used in OPV cells. In this case, doping or hybridizing with other materials discussed in Section 5 are considered to be promising. (ii) Surface roughness. Many factors, including the rough surface of the flexible substrates used, shear stress between the TCF and substrate during processing, PMMA residue after transferring a G-TCF to the substrate, wrinkles or folds in G-TCFs formed during their growth and transfer, occasional protruding of CNTs, impurities on the surface, etc., can affect the smoothness of TCFs. A rough surface of TCFs may cause a leakage path between cathode and anode and, as a result, it will largely reduce the performance of the devices and even cause failure. In fact, the success ratio for the assembly of OPV cells based on CNT- and G-TCFs is very low at present due to their high surface roughness. Therefore, much more attention should be paid to keep the surface smooth and clean during the fabrication and processing of CNT- and G-TCFs, especially for CNT-TCFs. (iii) Work function. To achieve high performance OPV cells, the transparent electrode used should have a proper work function to minimize energy barriers for charge injection or electron collection. Normally, CNTs and graphene have a work function of 4.5–4.8 eV, similar to that of an ITO film which has a work function of 4.7–4.9. The work function of TCFs should be tuned for different electrode uses.[259,265,266] According to the summary of device structures in Table 6, PEDOT:PSS is commonly used to improve the work function of CNT- and G-TCFs for charge injection at the anode, while for electron collection at the cathode, their work function can be reduced by a poly(ethylene oxide), Cs2CO3 and WPF-6-oxy-F interlayer. (iv) Hydrophilic character. Because

7.2. OLEDs As a promising electronic display and solid-state lighting technology, OLEDs are a kind of LED in which the emissive electroluminescent layer is an organic compound film which is situated between two electrodes and emits light in response to an electric current. Generally, at least one of the electrodes is transparent so that the emitted light can exit the devices. To fully realize flexible OLEDs, CNT- and G-TCFs with superior flexibility have been widely investigated as the transparent electrodes and have shown comparable results with ITO-TCFs. OLEDs have a similar but opposite light-electricity coupling process compared to OPV cells. Therefore, OLEDs based on CNT- and G-TCF electrodes have similar challenges to OPV cells, i.e., the need for a relatively low Rs and surface roughness, proper work function, hydrophilic character, encapsulation/ passivation technologies. Hu et al. comprehensively discussed some of the above issues and developed approaches to address them for SWCNT-TCFs as an anode in OLEDs.[267] To evaluate the effect of Rs on device performance, they fabricated OLEDs based on 20 Ω/sq ITO on glass and 200 Ω/sq SWCNT on PET with the same active layer and device structure as shown in Figure 13a. Figure 13b and c show images of an operating OLED. Compared with the ITO-OLED, the SWCNT-OLED not only exhibits lower light output intensity, but also poor light emissive uniformity due to the poorer electrical properties of

Figure 13. (a) Structure of a flexible OLED based on a SWCNT or ITO anode; the operating (b) ITO-OLED and (c) SWCNT-OLED. Reproduced with permission.[267] Copyright 2010, IOP Publishing Ltd.

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the SWCNT-TCF anode. Furthermore, they investigated the structural effects of SWCNT-TCFs fabricated by filtration and PDMS stamping for OLEDs and observed SWCNT protrusion by SEM. Such a protrusion can penetrate the active layer to connect with the cathode and result in device failure. As a result, 80% of OLEDs based on the SWCNT-TCF were short circuited. Thus, a fabrication method that avoids tube detachment from the surface and produces a SWCNT-TCF with high conductivity must be chosen. An interfacial/smoothing layer such as PEDOT:PSS is effective in suppressing protrusion by improving the cohesion between the CNTs and the substrate. This interfacial layer can also improve the work function of the electrode used. In this case, the SWCNT-OLED device obtained exhibits much greater current efficiency at high voltages and also good flexibility with no degradation in performance after bending it at a radius of 10 cm. Obviously, an interlayer is very efficient in improving device performance by increasing conductivity and work function and smoothing the surface of TCFs used in OLEDs. For a graphene-OLED, the work function and Rs of a G-TCF was modified to ∼5.95 eV and ∼30 Ω/sq, respectively, by using a selforganized gradient hole injection layer (GraHIL, PEDOT:PSS and a tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulphonic acid copolymer) and doping with p-dopants of HNO3 or AuCl3.[268] OLED devices with a four-layer graphene anode doped with HNO3 or AuCl3 showed an extremely high maximum current efficiency (CE) of 30.2 cd/A and luminous (power) efficiency (LE) of ∼37.2 lm/W or a CE of 27.4 cd/A and LE of 28.1 lm/W, respectively. These values are significantly higher than those of devices with an ITO anode. A bending test with a radius of 0.75 cm showed that the graphene-OLED device retained almost the same current density even after 1000 bending events, while the ITO device completely failed after 800 events, demonstrating the excellent bending stability of the graphene anode. As discussed above, CNT- and G-TCFs have shown strong potential as a transparent electrode in OLEDs. However, OLEDs also have their own problems whatever TCFs are used. Similar to OPV cells, H2O and O2 can easily damage OLEDs so that flexible substrates with outstanding barrier properties and effective encapsulation/passivation are urgently required. The shorter lifetime of blue organic emitters, high manufacturing cost and size limits, etc., are also obstacles to the practical application of OLEDs. Nevertheless, OLEDs with CNT- and G-TCFs can be developed to accelerate the commercialization of flexible OLEDs.

7.3. Touch Panels Touch panels are an effective human/device interface through which users can control devices by single or multiple touches on the panels with fingers or a stylus and can interact directly with devices instead of using a mouse, keyboard, or other intermediates. Touch panels have been widely used in modern optoelectronic devices such as video game consoles, mobile phones, digital cameras, flat panel computers, and so on. With the development of information appliances, especially the popularity of smart phones and other portable electronics, the demands for

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touch panels is rapidly growing. Industrial manufacturers have acknowledged the trend of touch panels as a highly desirable component of modern optoelectronic devices and have begun to attach them to TVs, computers, etc. As a result, we expect a great market demand for transparent electrodes. Contrary to OLEDs and OPV cells, touch panels have no strict requirement for the surface roughness, work function and hydrophilicity of TCFs, and more importantly, the demands of electrical conductivity and transparency of transparent electrodes for touch panels are also inferior to those for OLEDs and OPV cells. For a resistive touch panel, the uniformity of a TCF is very important to precisely recognize the touch position. This has been achieved by controlling the film fabrication. Thus resistive touch panels based on CNT- and G-TCFs have been widely investigated and demonstrate good applicability.[56,60,122] For a capacitive touch panel, patterning is very important. As reviewed in Section 6, many patterning technologies for CNTand G-TCFs have been developed. Therefore, the application of CNT- and G-TCFs in touch panels will be easier to realize, and great attention and investment have been attracted from industry. Recently, it has been disclosed that Foxconn Electronics (Hon Hai Precision Industry) has acquired the technology for CNT touch panel production through cooperation between its subsidiary in Tianjin, China and a R&D team of Tsinghua University in China led by S. S. Fan. Mass producible panel sizes range between 1.52- to 10-inch according to some market watchers, promising the industrial use of CNT-TCFs in touch panels. Unidym Inc. also claimed that their solution processing-based CNT-TCFs can be used for touch panels with the advantages of substantially improved durability/wear resistance, improved daylight readability for outdoor applications (e.g., mobile handsets, POS), significantly higher production capacity with 50–100X faster deposition speed than ITO and a cost effective dry etch patterning option. Product Press Releases in microtips technology notes that capacitive touch panels with CNT technology have been designed for use with LCDs for user input applications. CNT-TCFs offer replacement for ITO in resistive and capacitive touch panels and improve capacitive touch panel design regarding durability, accuracy, sensitivity, flexibility, transmissivity, and resolution. The manufacturing process also lends itself to optimized environmental friendliness and production cost efficiency (http://www.microtipsusa.com/). In October 2013, Canatu opened a CNB-TCF and touch panel manufacturing plant in Helsinki, Finland. The plant manufactures CNB-based projected touch panels in addition to CNB films. Through efforts by these companies, CNTTCF have been successfully used in touch panels of various electronic products. Touch panels using CVD G-TCFs on a polymer substrate were first reported by Bae et al. at Sungkyunkwan University, Korea.[122] After printing silver electrodes and dot spacers, the upper and lower panels were carefully assembled and connected to a controller installed in a laptop computer (Figure 14a–c), and showed extraordinary flexibility. Excited by these results, it is believed that touch panels will be the first use of graphene in commercial terms. In fact, a company in China, The 2D Carbon (Changzhou) TechCo., Ltd., which is devoted to developing G-TCFs for touch panels in smartphones and other

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REVIEW Figure 14. (a) Screen printing process of silver paste electrodes on a graphene/PET film. The inset shows 3.1-inch graphene/PET panels patterned with silver electrodes before assembly. (b) An assembled graphene/PET touch panel showing outstanding flexibility.(c) A graphene-based touch-screen panel connected to a computer with control software. Reproduced with permission.[122] Copyright 2010, Nature Publishing Group.

applications, has already announced the operation of a G-TCF production line of 30000 m2 per year in May 2013. Based on the above information, it is clear that CNT- and G-TCFs have been successfully assembled into touch panels, mainly flexible resistive touch panels because they are more easily created, and will soon be widely used in touch panels for commercial electronic products.

and chemical stability. These multifunctional properties make them promising for a wide range of applications as discussed above. Therefore, TCFs for optoelectronics have become a significant direction for CNT and graphene application, which also opens up possibilities both for basic scientific research and for the development of electronic devices, especially the development of future flexible electronics.

7.4. Other Applications

8. Summary and Prospect

Except for the applications discussed above, CNT- and G-TCFs are also expected to have many other applications in optoelectronics, such as display devices,[269–271] thin film transistors (TFTs),[272] transparent heaters,[143,273] transparent loudspeakers,[120] electromagnetic interference (EMI) shielding,[274] smart windows,[238] etc. Many review papers have reported their application as transparent electrodes in TFTs and display devices.[3,275] High-performance, flexible, transparent heaters based on large-scale G-TCFs with a Rs ∼ 43 Ω/sq at ∼89% transmittance have been demonstrated.[143] Time-dependent temperature profiles and heat distribution analysis show that the performance of the graphene heaters is superior to ITOheaters. In particular, the graphene heaters are mechanically stable against large bending deformation, which allows their application to a curved window surface or as a rollable screen. The transport properties of SWCNT-TCFs in the microwave frequency range from 10 MHz to 30 GHz was measured at temperatures of 20–400 K.[274] Shielding effectiveness values of 43 dB at 10 MHz and 28 dB at 10 GHz were found for films at 90% transparency, suggesting that SWCNT films are promising as a transparent EMI shielding material. Microfluidic devices based on the electrowetting principle[276] and electrochromic devices with polyaniline as an active layer[270] by using SWCNTTCFs as electrodes were also demonstrated. In addition, freestanding SWCNT-TCFs were investigated in many applications, including high efficiency nanoparticle filters with a figure of merit of 147 Pa−1, electrochemical sensors with extremely low detection limits below 100 nM, polymer-free saturable absorbers for ultrafast femtosecond lasers, main components in gas flowmeters, gas heaters, and transparent thermoacoustic loudspeakers, etc.[120] Besides their excellent conductivity, transparency and flexibility, CNT- and G-TCFs also possess many other important properties such as high surface area, high thermal conductivity

Both CNTs and graphene are allotropes of carbon. They have many similar excellent chemical and physical properties such as good transparency, high conductivity, and excellent flexibility, due to the same basic structure unit: a hexagonal honeycomb lattice of carbon. However, they have markedly different geometrical shapes, i.e., CNTs have a quasi-one dimensional tubular structure, while graphene is a two dimensional material. The ways they form TCFs are not the same. CNT-TCFs are formed through interweaving while wet-processed G-TCFs are formed by stacking graphene sheets and CVD G-TCFs by linking boundaries of graphene grains. Therefore, the structure, morphology and properties of CNT- and G-TCFs are different, although both have shown great potential for widespread commercial use in optoelectronic devices. People are always questioning whether CNT-TCFs or G-TCFs are of equal value for applications or which is superior. In this section, based on the above review of their fabrication, properties, modification, patterning and applications, we compare CNT-TCFs with G-TCFs for optoelectronic devices and evaluate their prospects.

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8.1. Comparison between CNT- and G-TCFs Table 6 summarizes the strengths and weaknesses of the key characteristics of CNT- and G-TCFs for optoelectronics applications such as OPV cells, OLEDs and touch panels. As transparent electrodes for optoelectronic devices, the highly conductive and transparent properties of TCFs are of primary importance. As discussed in Sections 4.1 and 4.2, the resistance of a CNT or G-TCF can generally be regarded as a sum of their intrinsic resistance and contact resistance between nanocarbons. As discussed in Section 2, theoretical calculations and experimental results have proved that both graphene and SWCNTs have a high intrinsic electrical conductivity, but the

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www.advmat.de Table 6. Comparison of the key characteristics of CNT- and G-TCFs for optoelectronics. Key characteristics for optoelectronics Electrical conductivity

CNT-TCF Good

G-TCF Good

Remarks Their conductivity is gradually improved. At the same transparency, CVD G-TCFs have lower sheet resistance based on results reported so far.

Transparency

Good

Excellent

To achieve a lower Rs, CNT-TCFs have to be thicker, while CVD G-TCFs are only several atomic layers thick.

Best σDC/σOP

64.2

234.5

The experimental results reported up to now.

Flexibility

Excellent

Good

Network structure makes CNTs interweave with each other and results in TCFs being more flexible and stretchable.

Surface roughness

Poor

Average

CNT-TCFs are intrinsically rough due to their porous morphology, while G-TCFs are intrinsically smooth.

Work function

Good

Good

A similar work function and modification strategy.

Hydrophilicity

Poor

Poor

Both have poor hydrophilicity and need to be improved.

Patterning

Excellent

Good

Patterning CNT-TCFs may be easily achieved by placing aligned CNTs into desired patterns or by printing.

Cost

Good

Average

CVD G-TCFs have high transfer and etching cost at present, while CNT-TCFs can be produced by both wet and dry methods at low cost.

Touch panel

Excellent

good

No requirement for surface roughness and work function. CNT-TCFs may be better due to lower cost and easier patterning.

OPV Cell and OLED

Average

Good

Poorer surface roughness makes CNT-TCFs inferior to G-TCFs.

high conductivity exists only within a graphene sheet or along the axis of a single CNT. The contact resistance plays a more important role in the properties of their TCFs. The types of contact between CNTs and graphene sheets are various. For CVD G-TCFs, the contact resistance mainly comes from graphene grain boundaries, while for wet-processing-based G-TCFs, the contact resistance mainly comes from stacking of the graphene sheets. For CNT-TCFs, the contact resistance arises from the contact/interweaving of CNTs whatever method is used in their manufacture. Based on the results reported so far, CVD G-TCFs have a lower Rs at the same transparency. This may be not only because the resistance of CVD-grown graphene is relatively low, but the in-plane contact resistance from boundaries may also be lower than the out-of-plane one due to stacking, especially when the grain size is large and the boundaries are well-stitched. Moreover, non-current carrying nanotubes in the middle of a bundle may cause the higher resistance of CNTTCFs at the same transparency. The wet-processed G-TCFs with GO as a starting material have a relatively high Rs, mainly due to the presence of the remaining oxygen-related defects, which cause rGO-TCFs to have a higher Rs than CNT-TCFs, despite the prevailing two-dimensional contacts in comparison to the point contacts of CNTs. As a result, the conductivity of CVD G-TCFs may be better than that of CNT-TCFs, while CNT-TCFs are better than rGO-TCFs. In addition, CVD G-TCFs may be superior in transparency. To achieve highly conductive CNTTCFs, the density of CNTs must be higher than the threshold for the formation of a percolation network, which requires that the film is thick enough. It is expected that a 7 nm thick G-TCF has a Rs of 1–10 Ω/sq, while a CNT-TCF needs to be >100 nm thick to obtain a Rs of 10 Ω/sq, which severely affects the transparency of the CNT-TCF. Moreover, as discussed in Section 4.2, CVD G-TCFs have a high utilization efficiency of the graphene sheets and are quite uniform, which is beneficial to the optical transmittance. A large-area G-TCF with a σDC/σOP of 70.2 and 115.8 before and after HNO3 doping, and a G-TCF by interfacial CVD with a σDC/σOP of 173.4 have been demonstrated. These 1984

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results are better than for CNT-TCFs reported to date, since the record σDC/σOP for CNT-TCFs is 64.2 and a CNB-TCF is 82.1, demonstrating that G-TCFs are superior in optical transparency and electrical conductivity to CNT-TCFs. Flexibility and stretchability are a unique advantage of CNTand G-TCFs. However, because of their different modes of contact and the experimental results discussed in Section 4.3, we infer that CNT-TCFs may be better in this respect. CNTs interweave with each other in a CNT-TCF with a network structure, thus the TCF may withstand a large deformation. However, in-plane boundaries in a CVD G-TCF may be destroyed during large deformation, and the contact resistance of wet-processed G-TCFs, which are formed by the stacking of graphene sheets, may increase under deformation as well. Another important property is surface roughness. The high intrinsic surface roughness of CNT-TCFs arises from the dimensionality of CNTs, because their high aspect ratio leads to the production of TCFs with a highly porous surface. More seriously, occasional protrusion of CNTs occurs during film assembly. This particular surface structure is disadvantageous for optoelectronic devices such as OPV cells, OLEDs and LCDs, where very smooth surfaces are required. The intrinsic surface roughness of graphene should be very small, due to its flat plane structure, however, wrinkles or folds in graphene also cause a rough surface. In addition, some residue on TCFs such as PMMA during transfer in a dry fabrication process is another main factor that increases the surface roughness of CVD G-TCFs. Treatments such as plasma etching or annealing are frequently used to suppress the surface roughness of ITO and PEDOT. However, these methods are not applicable to CNT- and G-TCFs, e.g., plasma treatment will decrease their conductivity and even etch away CNTs and graphene. Up to now, one of the most effective ways to improve surface morphology is to fill the pores of the CNT network or smooth the wrinkles of G-TCFs with another material as a buffer layer, such as a PEDOT:PSS layer. But a thick buffer layer will decrease the efficiency of the devices, and the hydrophobic nature of

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Considering the low cost and easiness for patterning, CNTTCFs may precede G-TCFs in this field. Because surface smoothness is very important for OPV cells/OLEDs, G-TCFs may be superior. CNT- and G-TCFs have similarities and differences, each having their own strengths and weaknesses. As a result, we cannot expect one kind of TCFs to meet all applications for optoelectronics and should select an option depending on the device requirements and the key characteristics of the films available.

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CNT- and G-TCFs makes the buffer layer difficult to be uniformly coated. As a result, it is still a big challenge to produce TCFs with the required smooth surface. Comparatively, G-TCFs are superior to CNT-TCFs in terms of surface smoothness because they are intrinsically smooth and the surface roughness can be improved by carefully controlling the growth and transfer of G-TCFs. As mentioned in Section 6, patterning is a key step for TCFs being integrated into optoelectronic devices. Due to the similar chemical and physical properties of CNTs and graphene, they have similar difficulties. However, with respect to a TCF assembled with superaligned CNTs, the patterning may be easier because the aligned CNTs can be easily oriented according to the required patterns. Futhermore, patterned CNT-TCFs can be obtained by printing since CNT-TCFs with high conductivity can be prepared by a wet process while this is not possible for G-TCFs. Despite the above differences, these two kinds of TCFs have many similar characteristics. For example, CNT- and G-TCFs have a similar work function (4.5∼4.8) and hydrophilicity due to their similar basic structure and the nature of the honeycomb carbon lattice. Many methods such as metal or PEDOT:PSS deposition, plasma treatment, and acid doping can be used for both kinds of TCFs to tune these characteristics according to their application. To realize the industrialization of a product, the cost is always important. Graphene was discovered relatively recently, while CNT-TCFs have been more mature and more thoroughly studied, and can be continuously produced by both wet (e.g., Unidym company) and dry methods (e.g., Canatu company and Tsinghua-Foxconn Nanotechnology Research Center) at low cost. Therefore, SWCNT-TCFs are promising to compete with ITO-TCFs on PET (20–30 USD per m2) as far as cost is concerned. Theoretically, it should be possible to make G-TCFs cheap given the low cost of graphene materials exfoliated from natural graphite. However, the opto-electrical performance of TCFs fabricated by the wet method cannot yet meet device requirements, while CVD G-TCFs are at a relatively early production stage and still are expensive due to the laborious transfer and costly etching process required. Several companies including Bluestone (http://bluestonegt.com/ wp-content/uploads/2013/11/BluestoneGlobalTech_GratFilm _PriceSheet_Format2_v2.pdf) and Graphenea (http://www. graphenea.com/collections /graphene-products/products/monolayer-graphene-on-pet-60-mm-x-40-mm#.Uq7KPmRdX3x.) offer CVD G-TCFs, with a price more than 1000 times higher than that of ITO-TCFs on PET. However, there are at least two Chinese companies, for example The 2D Carbon (Changzhou) TechCo., Ltd (http://www.cz2dcarbon.com/product/index.html) and Wuxi graphene film Co., LTD (http://www.graphenefilm. cn), which are producing CVD G-TCFs in a large quantity and have announced the use of these G-TCFs as the touch screen of smartphones with an acceptable price. As shown in Table 6, the key characteristics are not the same for CNT- and G-TCFs, therefore, the advantages of CNTand graphene-TCFs for different devices may be different. For example, for a transparent electrode of touch panels, surface smoothness, work function and hydrophilicity are not strictly required, so that both CNT- and G-TCFs can be used.

8.2. Prospect Based on a comprehensive overview of the fundamentals, fabrication, properties, modification, patterning, assembly into devices and possible applications of CNT- and G-TCFs, we show that both TCFs have excellent properties such as high transparency, high conductivity and good flexibility, indicating their strong potential as transparent electrodes for optoelectronic devices. In particular, they have already been commercially used in touch panels. However, many challenges exist, and a comparison of their key characteristics shows that they are not equal for the same devices. The critical challenges faced by these two kinds of TCFs may be as follows: (i) How to further improve their electrical conductivity and transparency, which are currently not comparable with those of ITO-TCFs. The fabrication of high quality CNTs and graphene, the individual dispersion of CNTs in liquid media, effective etching of catalyst substrates and transfer of G-TCFs to target substrates, etc., are important for improving these two properties. (ii) How to produce TCFs with a large area and at low cost. This is the key and determining factor if these TCFs can achieve commercial use. (iii) How to overcome the issues of dopant stability. Doping is very important to make CNT- and G-TCFs highly conductive, but the poor stability of the doped TCFs limits their practical application. (iv) How to pattern TCFs at low cost without degrading their transparency and conductive properties. For manufacturing, a wet-etching patterning process is preferable. (v) How to tune the work function and increase the hydrophilicity of CNT- and G-TCFs to match device assembly. (vi) How to smooth the surface of TCFs and increase the success ratio of the resulting devices. For the industrialization of these TCFs, it is critical to address the above challenges. CNT- and G-TCFs have some of the above issues in common, but each has its own peculiarity. Therefore, future research should continually tackle these issues on the basis of the device requirements for TCFs. Despite many challenges, properties such as flexibility, a neutral color, and ambient stability of CNT- and G-TCFs are unique and cannot be matched by other current TCF materials. Rapid progress in the large area production and application of CNT- and G-TCFs in recent years has been significantly encouraging. CNT-TCF-based touch panels have been successfully integrated into mobile phones which have already been selling in the market. This is favorable for the sustainable development of CNT- and G-TCFs. It is well known that even an established product also has many challenges to solve and has to keep improving to meet new applications. Therefore, the most likely development way is to first use CNT- and G-TCFs in simple optoelectronic devices such as sensors, smart windows, touch

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panels, etc., because these devices have relatively lower metrics for TCFs. Eventually, CNT- and G-TCFs will be widely used in high performance optoelectronics, such as OPV cells and OLEDs. More interestingly, all-carbon prototype devices based on these two kinds of TCFs including FETs,[277] OPV cells,[278] and other electronics,[279] demonstrate their promising use in transparent electronics with flexibility and environmental stability. In addition, many other alternatives to ITO-TCFs have also been developed, and their hybrids with CNT- or G-TCFs, and even CNT- with G-TCFs may have complementary advantages that will be favorable to overcome some of the above issues. All in all, we believe that CNT- and G-TCFs will become a popular component of optoelectronic devices in a near future. Research on high-performance CNT- and G-TCFs can advance the development of not only industrial applications but also the fundamental science of related nanostructured materials and nanodevices. This is also of significance for promoting the progress and practical applications of nanoscience and nanotechnology.

Acknowledgements This article is part of a series celebrating the 25th anniversary of Advanced Materials. This work was supported by the Key Research Program of Ministry of Science and Technology, China (No. 2011CB932604), the National High-Tech Research and Development Program of China (No. 2012AA030303), Chinese Academy of Sciences (KGZD-EW-303–3), and the National Natural Science Foundation of China (Nos. 51221264, 51172241, 51102243, and 51102241). Received: August 16, 2013 Revised: February 14, 2014 Published online: March 4, 2014 [1] A. Kumar., C. W. Zhou, ACS Nano 2010, 4, 11. [2] J. B. Wu, M. Agrawal, H. A. Becerril, Z. N. Bao, Z. F. Liu, Y. S. Chen, P. Peumans, ACS Nano 2010, 4, 43. [3] D. S. Hecht, L. B. Hu, G. Irvin, Adv. Mater. 2011, 23, 1482. [4] L. Hu, H. S. Kim, J.-Y. Lee, P. Peumans, Y. Cui, ACS Nano 2010, 4, 2955. [5] D. S. Hecht, D. Thomas, L. Hu, C. Ladous, T. Lam, Y. Park, G. Irvin, P. Drzaic, J. Am. Chem. Soc. 2009, 17, 941. [6] R. G. Gordon, MRS Bull. 2000, 25, 52. [7] T. M. Barnes, M. O. Reese, J. D. Bergeson, B. A. Larsen, J. L. Blackburn, M. C. Beard, J. Bult, J. van de Lagemaat, Adv. Energy Mater. 2012, 2, 353. [8] S. De, P. J. King, P. E. Lyons, U. Khan, J. N. Coleman, ACS Nano 2010, 4, 7064. [9] A. Kaskela, A. G. Nasibulin, M. Y. Timmermans, B. Aitchison, A. Papadimitratos, Y. Tian, Z. Zhu, H. Jiang, D. P. Brown, A. Zakhidov, E. I. Kauppinen, Nano Lett. 2010, 10, 4349. [10] E. M. Doherty, S. De, P. E. Lyons, A. Shmeliov, P. N. Nirmalraj, V. Scardaci, J. Joimel, W. J. Blau, J. J. Boland, J. N. Coleman, Carbon 2009, 47, 2466. [11] R. Gupta, G. U. Kulkarni, ACS Appl. Mater. Inter. 2013, 5, 730. [12] C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A. N. Marchenkov, E. H. Conrad, P. N. First, W. A. de Heer, Science 2006, 312, 1191. [13] S. V. Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin, D. C. Elias, J. A. Jaszczak, A. K. Geim, Phys. Rev. Lett. 2008, 100, 016602. [14] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science 2004, 306, 666.

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Hatice Altug Richard Averitt Paul Braun Mark Brongersma Timothy J. Bunning Cornelia Denz Harald Giessen Peter Günter Zhi-Yuan Li David Lidzey Luis Liz-Marzán Cefe López Stefan Maier Holger Moench Dan Oron Albert Polman Ullrich Steiner Jianfang Wang Ralf Wehrspohn Martin Wegener

Free Access until Jan 2015 Advanced Optical Materials is an international, interdisciplinary forum for peer-reviewed papers dedicated to breakthrough discoveries and fundamental research in the field of optical materials. The scope of the journal covers all aspects of light-matter interactions including metamaterials and plasmonics, optical nanostructures, optical devices, photonics and more. Volume 1, 12 issues in 2013. Online ISSN: 2195-1071 Cover picture by Byeong-Kwon Ju, Kyung Cheol Choi and co-workers DOI: 10.1002/adom.201200021

Examples of excellent papers published in Advanced Optical Materials: Magnetoplasmonics: Combining Magnetic and Plasmonic Functionalities Gaspar Armelles, Alfonso Cebollada*, Antonio García-Martín, María Ujué González

http://onlinelibrary.wiley.com/doi/10.1002/adom.201200011/full Optically Reconfigurable Reflective/Scattering States Enabled with Photosensitive Cholesteric Liquid Crystal Cells J. P. Vernon, U. A. Hrozhyk, S. V. Serak, V. P. Tondiglia, T. J. White, N. V. Tabiryan, T. J. Bunning http://onlinelibrary.wiley.com/doi/10.1002/adom.201200014/full Fast and Low-Power All-Optical Tunable Fano Resonance in Plasmonic Microstructures Y. Zhu, X. Hu, Y. Huang, H.Yang, Q. Gong

http://onlinelibrary.wiley.com/doi/10.1002/adom.201200025/full

ZnO p–n Homojunction Random Laser Diode Based on Nitrogen-Doped p-type Nanowires J. Huang, S. Chu, J. Kong, L. Zhang, C. M. Schwarz, G. Wang, L. Chernyak, Z. Chen, J. Liu

http://onlinelibrary.wiley.com/doi/10.1002/adom.201200062/full Broadband and Efficient Diffraction C. Ribot, M.-S. Laure Lee, S. Collin, S. Bansropun, P. Plouhinec, D. Thenot, S. Cassette, B. Loiseaux, P. Lalanne

http://onlinelibrary.wiley.com/doi/10.1002/adom.201300215/full Spoof Plasmon Surfaces: A Novel Platform for THz Sensing Binghao Ng, Jianfeng Wu, Stephen M. Hanham, Antonio I. Fernández-Domínguez, Norbert Klein, Yun Fook Liew, Mark B. H. Breese, Minghui Hong, Stefan A. Maier* http://onlinelibrary.wiley.com/doi/10.1002/adom.201300146/full

25th anniversary article: carbon nanotube- and graphene-based transparent conductive films for optoelectronic devices.

Carbon nanotube (CNT)- and graphene (G)-based transparent conductive films (TCFs) are two promising alternatives for commonly-used indium tin oxide-ba...
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