Review pubs.acs.org/CR
Recent Development of Carbon Nanotube Transparent Conductive Films LePing Yu, Cameron Shearer, and Joseph Shapter* Centre for Nanoscale Science and Technology, School of Chemical and Physical Sciences, Flinders University, Bedford Park, South Australia, Australia 5042 ABSTRACT: Transparent conducting films (TCFs) are a critical component in many personal electronic devices. Transparent and conductive doped metal oxides are widely used in industry due to their excellent optoelectronic properties as well as the mature understanding of their production and handling. However, they are not compatible with future flexible electronics developments where large-scale production will likely involve roll-to-roll manufacturing. Recent studies have shown that carbon nanotubes provide unique chemical, physical, and optoelectronic properties, making them an important alternative to doped metal oxides. This Review provides a comprehensive analysis of carbon nanotube transparent conductive films covering detailed fabrication methods including patterning of the films, chemical doping effects, and hybridization with other materials. There is a focus on optoelectronic properties of the films and potential in applications such as photovoltaics, touch panels, liquid crystal displays, and organic lightemitting diodes in conjunction with a critical analysis of both the merits and shortcomings of carbon nanotube transparent conductive films.
CONTENTS 1. Introduction 2. Properties of CNT TCFs 2.1. Properties of CNTs 2.2. Characterization and Requirements of TCFs 2.2.1. Figure of Merit of TCFs 2.2.2. Practical Requirements of TCFs 2.3. Optical Properties of CNT TCFs 2.4. Transport Properties of CNT TCFs 2.4.1. Effect of Geometry 2.4.2. Effect of Purity and Synthesis Methods 2.4.3. Effect of Electronic Types of CNT 3. Preparation of Transparent CNT Films 3.1. Dry Methods 3.1.1. Floating Catalyst Chemical Vapor Deposition 3.1.2. Spinning CNT Yarns 3.2. Wet Methods 3.2.1. Preparation of CNT Suspension 3.2.2. Coating Processes 3.3. Patterning of CNT Films 3.3.1. Patterning with Surface Modification 3.3.2. Patterning with the Aid of Polymers 3.3.3. Patterning with Post-treatments 3.4. Doping to Improve CNT TCFs 3.5. CNT-Based Hybrid Films 3.5.1. Hybrids with Metallic Nanomaterials 3.5.2. Hybrids with Conductive Polymers 3.5.3. Hybrids with Graphene 4. CNT TCF APPLICATIONS 4.1. Solar Cells © 2016 American Chemical Society
4.1.1. CNT TCFs in OPVs 4.1.2. CNT TCFs in DSSCs 4.1.3. CNT TCFs in CNT/Silicon Heterojunction Solar Cells 4.1.4. CNT TCFs in Perovskite Solar Cells 4.2. Touch Panels 4.3. Liquid Crystal Displays 4.4. Organic Light-Emitting Diodes 5. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References
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1. INTRODUCTION Recently, there has been a proliferation of commercial optoelectronic devices, such as liquid crystal displays (LCDs), touch panels, photovoltaics, and organic light-emitting diodes (OLEDs), which require transparent conducting films (TCFs).1 The LCD industry has been leading the consumption of TCFs for years with sales of approximately USD 1.5 billion in 2014.2 The commercial market in TCFs applied in touch panels was almost USD 1 billion in 2012, and the number is anticipated to be about USD 5 billion by 2019.3 A similar market exists in the thin Received: March 17, 2016 Published: October 5, 2016 13413
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Scheme 1. (a) Unrolled SWCNT Showing Chiral Vector C and How Different Values of the Integers n and m Affect the Electronic Property of the SWCNT; (b) the Direction of the Chiral Vector Affects the Appearance of the Nanotube. Examples of CNTs are shown: (4,4) Armchair Shape, (6,0) Zigzag Shape, and (5, 3) Chiral Shape; and (c) “Ball and Stick” Representation of SingleWalled CNT (SWCNT), Double-Walled CNT (DWCNT), and Multi-walled CNT (MWCNT) (Images Made Using Nanotube Modeller (www.jcrystal.com))
film solar industry, with expected sales of over USD 16.3 billion by 2017.4 The most widespread and well-studied materials for TCFs5−9 in industry are transparent and conductive doped metal oxides (TCOs) that can be both p-type10 or n-type11 semiconductors. Indium tin oxide (ITO), which consists of indium oxide (In2O3) and tin oxide (SnO2), is the best performing and well-known TCO.12 Although ITO has excellent optoelectronic properties with a low sheet resistance (Rs) (ranging from 10 to 100 Ω sq−1) at high transmittance (>85%),13 there are still a series of drawbacks: (1) Scarcity and cost of indium (which accounts for 75% of the ITO mass).14 (2) Brittle/ceramic nature as demonstrated by cracking under relatively low strain,15 and degradation of performance upon cyclic bending.16 Efforts have been made to overcome the inherit brittleness by laminating ITO onto a flexible polyethylene terephthalate (PET) substrate or controlling the ratio of indium and tin, but both of these approaches result in additional production costs and/or poorer optoelectronic properties.17 (3) Limited lifetime: under certain circumstances, performance degradation has been observed as a direct result of the diffusion of indium into the active layer of OLEDs and photovoltaics and corrosion of ITO itself by exposure to small amounts of adhesives and acids in the environment.18−20 (4) High production costs: various approaches have been used to fabricate ITO films, including screen printing,21 molecular beam epitaxy,22 magnetron sputtering,23 sol−gel techniques,24 and pulsed laser deposition,25 but most of these methods either require significant energy consumption due to high-temperature conditions or involve waste of the raw materials through inefficient deposition.26 Alternative doped TCOs, such as fluorine-doped tin oxides (FTOs), also suffer from problems similar to ITO. With the major issues of TCOs relating to the scarcity, redox activity, and brittleness of inorganic materials, researchers have looked for alternative organic-based materials. Conducting polymers have been investigated as TCFs since the 1980s when a range of commonly used polymers, such as polypyrrole and polythiophene, achieved significantly improved electrical conductivities after simply adding chemical dopants, including diamine green black27 and chromotropic acid disodium salts.28 Compared to TCOs, the two most attractive properties of conducting polymer TCFs are their flexibility and easy preparation processes.29−32 However, the electrical instability upon exposure to different stresses including humidity, UV light, and high-temperature conditions is a major concern for
commercial application.33−38 Additionally, a noticeable blue/ green color occurs in thicker films,39 which limits application in display panels. The other option for TCF materials is metallic nanostructures, including metal films, conductive grids, and metallic nanowires, which have been shown to provide better properties than TCOs on plastic substrates.40−57 However, the metallic structures often exhibit a hazy appearance, which is incompatible with display applications but has been shown to be beneficial in various photovoltaics.58 Metallic nanoparticles (NPs) are commonly made from silver, which has a similar cost to indium, meaning there is little financial benefit to using metallic NPs. Furthermore, the chemical, thermal, and aging stability of TCFs from metallic NPs is questionable, with many reports showing fast degradation of properties and leaching.59−61 It is an exciting time for materials scientists because there are a wide range of current materials that are suitable to TCFs, but none of these are currently ideal for large-scale, long-term commercial fabrication. Because of the persistent development of new products reliant on TCFs (such as annually upgrading smart phones), the cost of materials, performance, and cost of manufacture will continue to be a challenge in this burgeoning industry. Thus, research into a novel generation of materials for TCFs is required. Compared to TCOs and other options, carbon nanotubes (CNTs) have a series of advantages including abundance of raw material (carbon), excellent inherent electrical properties with high flexibility, ease of solution-based processing at room temperature, chemical stability, and a wide spectral range of transmittance with a neutral color.62 Here, we first describe the properties of CNTs, follow with a summary of the requirements of TCFs, then present preparation and modification methods of the CNT-based TCFs with a focus on their optoelectronic properties, and finish with a summary of their applications focusing on photovoltaics, touch panels, liquid crystal displays, and organic light-emitting diodes.
2. PROPERTIES OF CNT TCFS 2.1. Properties of CNTs
Known as carbon nanofibers or graphite whiskers during the 1950s, CNTs have been the subject of much research since the discovery of their atomic structure in 1991 by Iijima.63 A singlewalled carbon nanotube (SWCNT) is analogous to a rolled up, single two-dimensional graphene sheet while maintaining the carbon atoms in the sp2 hybridized network.64 As a result, due to 13414
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Table 1. Summary of Properties of CNTs property intrinsic mobility free carriers concentration current carrying capacity electrical conductivity on/off current ratios thermal conductivity Young’s modulus fracture stress surface area
value
remarks
exceeding 105 cm2 V−1 s−1 for individual CNTs at room temperature78,79 ∼1017 cm−3 83,84 exceeding 109 A cm−2 for individual CNT87
almost 100 times higher than that of silicon at 300 K with dopant concentration at 1017 cm−3 80−82 lower than that of graphene (∼1020 cm−3), TCOs (∼1021 cm−3), and most of metals (such as ∼1022 cm−3 for silver)85,86 1000 times higher than copper 88
104−106 S cm−1 89−92
close to that of some metals, such as mercury93,94
higher than 105 95−97 up to 3500 W m−1 K−1 99−101
1000 times higher than a bilayer structure at room temperature98 about 1500 W m−1 K−1 higher than that of diamond102,103
1−2 TPa104−110 50 GPa112−114 1600 m2 g−1 117,118
comparable to that of a single crystal diamond at room temperature111 ∼50 times larger than that of steel wires after density normalization115,116 higher than that of activated carbon (1200 m2 g−1)119,120
the curvature-induced misalignment of p orbitals in the carbon network, the electronic properties change from the zero bandgap semimetal of graphene to a mixture of metallic and semiconducting, depending upon the rolling angle.65 SWCNTs are completely described except for their length by the chiral vector, C, which is created by choosing two atoms on a planar sheet of graphene, where one atom is chosen as the origin (Scheme 1a). The chiral vector is directed from the origin to the next atom and is defined by eq 1, C = na1 + ma 2
Eg =
C π
(1)
(2)
The dependence of the electronic properties of the CNT on n and m are summarized by eq 3: If
|n − m| = 3q
(3)
where if q is an integer, the nanotube will be metallic; otherwise, the nanotubes will be semiconducting.67 The chiral angle, θ, which is specified as the angle between C and the nearest zigzag of C−C bonds (0° ≥ θ ≤ 30°), can be used to describe the pattern of the CNT. The two achiral tube shapes occur either at θ = 30° (n, n) when the tubes are known as “armchair” or if θ = 0° (n, 0) the tubes are “zigzag”. Nanotubes with a chiral angle between 0 and 30° are called chiral (Scheme 1b, zigzag and armchair patterns shown in red). Thus, during a typical nonselective synthesis, two-thirds of the CNTs are semiconducting with one-third metallic formed when all chiral species are growing with the same probability.68 The band gap (Eg in eV) of the semiconducting SWCNT is related to the diameter of the CNT (dt),69 as shown in eqs 4, 5, and 6: vg̃ (mod 1) =
(6)
2.2. Characterization and Requirements of TCFs
2.2.1. Figure of Merit of TCFs. An ideal transparent conducting film would exhibit very high light transmission across the UV−vis-NIR spectrum and a very low sheet resistance (or high conductivity). Practically this has been achieved by measuring UV−vis absorbance of the TCF to obtain the transmission and while Rs is measured with a 4-point probe or a 2-point conductivity probe. In reality there is a tradeoff between these two parameters. Therefore, to directly compare TCFs, the two major properties need to be combined. A few variations have been proposed, but the most common is the ratio of dc electrical conductivity (σdc) and optical conductivity (σOP).121 This was proposed by De et al. and is useful in comparing the overall properties of CNT TCFs from different groups.122 The usual equation is shown in eq 7 and rearranged as eq 8,
[cos(3θ )]1.374 1 × 10−7 cm−1 − 771 cm−1 157.5 + 1066.9d t d t2.272 (4)
vg̃ (mod 2) =
1.6022 × 10−19 J
where h is Planck’s constant, c is the speed of light, and, if the remainder of (n − m)/3 is 1, ṽg̃ (mod 1) is applicable, otherwise ṽg̃ (mod 2) is applicable. Depending on the number of shells (nshell), CNTs can be divided into SWCNTs (nshell = 1), double-walled CNTs (DWCNTs, nshell = 2), and multiwalled CNTs (MWCNTs, nshell ≥ 3), as shown in Scheme 1c. DWCNTs have a higher structural stability than SWCNTs, and the interlayer interaction does not affect its band structure with the potential barrier depending on the chirality pairs.70 Individual MWCNTs of a small diameter (95% of transmittance at 550 nm), Rs is as high as ∼2000 Ω sq−1. In most of the current literature, the transmittances at 550 nm are used to characterize the optical properties of these films.
(7)
1/2
()
ε0 σdc 1 = −1/2 2R s T σOP(λ) −1
(8)
where T is the transmittance, Rs is the sheet resistance, and μ0 (4π × 10−7 N A−2) and ε0 (8.854 × 10−12 C2 N−1 m−2) are the free space permeability and permittivity, respectively. To characterize a film, one can either input the σOP value at 550 nm (150 or 200 S cm−1),96,123 T, and Rs to calculate σdc or calculate the ratio of σdc/ σOP by substituting T and Rs, which can be determined experimentally to obtain the “figure of merit”, and then the value can be used for comparison to TCFs reported in the literature. Alternatively, σdc can be directly calculated from the thickness (t), which is usually estimated by atomic force microscopy (AFM) and the Rs of the film, as shown in eq 9: σdc = (R st )−1
(9)
It is worthwhile noting that the σOP varies with the number of walls of the CNT, and thus if σdc is used to compare the film properties, this equation is only appropriate for films made of the same type of CNTs. In addition, this formula is based on two assumptions: (1) the thickness of the characterized film is much smaller than the wavelength of the incident light, and (2) the reflected light is much less than the absorbed fraction. In the case of CNT films, their thickness is generally 20 kiloton.194,195 To obtain either metallic or semiconducting enriched CNT, a number of selective synthesis methods concerning controlling some vital factors, such as catalyst particles,196−199 carbon source,200−205 growth temperature,206−208 substrates,209−212 and 13419
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Table 3. Comparison of TCOs and Selected CNT TCFs with the Best Optoelectronic Properties Fabricated with Different Wet and Dry Fabrication Methods fabrication methods (materials)
T (%)
Rs (Ω sq−1)
σdc/σOP
advantages
disadvantages
potential application
ref
TCOs
85
83
24
82.7
90
100
34.8
excellent conductivity; no use of additives excellent conductivity; no use of additives; efficient process simple; low cost
scarcity, ceramic nature, short lifetime, and high production cost high process temperature
78
180
7.9
simple; thickness control
little control over thickness, uniformity time consuming
touch panel, LCD screen, and OLED display touch panel and LCD screen touch panel, LCD screen, and OLED display touch panel and LCD screen touch panel
124
90
22.7− 222.7 41.5
excellent optoelectronic properties
FCCVD (SWCNTs) roll to roll-dry (MWCNTs) dip coating (SWCNTs) Langmuir− Blodgett (SWCNTs) brush painting (SWCNTs) Mayer rod (SWCNTs) spin coating (SWCNTs) spray coating (SWCNTs) vacuum filtration (SWCNTs) (di)electrophoresis (SWCNTs)
100− 10 84
79
286
5.1
simplicity
poor uniformity
touch panel
82
75
24
great industrial potential
limited to certain types of substrates
90
128
27.2
fast and simple process
large-scale production limited
89
120
26.2
fast and simple process
not very uniform films
83
30
64.4
uniform films
time consuming
81
220
7.7
fast process
additional etching of metal layer
touch panel and LCD screen touch panel and LCD screen touch panel and LCD screen touch panel and LCD screen touch panel
aligned CNT forest required
233, 234 124, 235, 236 237, 238 239, 240 241 242, 243 244, 245 246−248 249−251 252, 253
Figure 8. Schematic of (a) the FCCVD process to grow CNTs and to directly deposit onto a membrane for transfer, (b) the roll-to-roll process for fabricating CNT TCFs from an aligned CNT forest, and (c) SEM image of CNT TCF prepared from an aligned CNT forest. Reprinted with permission from ref 124. Copyright 2010 John Wiley & Sons, Inc.
strategies have some unique advantages. For example, the films fabricated via dry processes usually have high conductivity while the solution-based films are compatible with some industrial fabrication processes, such as gravure press and reverse roll painting.228−232 Different dry and wet fabrication methods of CNT films with the best optoelectronic properties are compared in Table 3 in terms of the resulting performance as well as a summary of the advantages and disadvantages of each.
other cotreatments,213 during the synthesis have been designed. In addition, the same diversity of nanotubes can also be obtained by post-growth purification, such as density-gradient ultracentrifugation,214−217 gel exclusion of surfactant-wrapped CNT,218 ion-exchange chromatography of DNA-wrapped CNT,219 UV irradiation,220 electrical breakdown,221−223 selective gas-phase plasma etching,224 and alternating current dielectrophoresis.225,226 Recently, a novel separation method was developed to achieve high-purity single-chirality SWCNTs. Specifically, single-chirality SWCNTs were separated from the mixture by a given polymer two-phase system, in which ssDNA was used to wrap the SWCNT to form the hybrids sensitive to the slight differences in two phases. By tuning the type of ssDNA, polymer phases, and partition modulators, 15 species were separated in a cost-effective manner.227 However, the extra effort to obtain single-chirality CNTs is currently not worth the effort for TCFs, especially for semiconducting SWCNTs whose narrow optical absorbance leads to a color to the films. Generally speaking, CNT TCFs fabrication approaches are divided into dry processes and wet processes. Both of these
3.1. Dry Methods
Currently there are two dry approaches to prepare CNT films. One is based on floating catalyst CVD (FCCVD), which was first introduced by Cheng et al. in 1998,254 and the other is based on the concept of drawing CNT yarns from an aligned CNT array.255 3.1.1. Floating Catalyst Chemical Vapor Deposition. FCCVD involves a gas-phase mixture of a carbon source (commonly thiophene or toluene) and an iron source (ferrocene) that react at high temperatures from 700 to 1200 °C to first form iron nanoparticles, which then catalyzes the growth of carbon nanotubes. The produced CNTs are carried 13420
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3.2. Wet Methods
out of the reaction zone by an inert carrier gas and collected on the low-temperature CVD chamber wall.254 By following this method, Ma et al. prepared SWCNT films with large area (5 × 10 cm), and the Rs was 50 Ω sq−1 at 70% transmittance (σdc/σOP = 19.3). 256 The way to produce CNT TCFs with best optoelectronic properties by this method is to collect CNTs by a filter membrane directly at the outlet of the reactor followed by transfer to a transparent substrate, such as PET, as shown in Figure 8a. This method has produced a SWCNT film with Rs of 84 Ω sq−1 at 90% optical transmittance (σdc/σOP = 41.5).233 The high quality of the resulting TCFs is mainly due to the long SWCNT bundles with length about 3 cm prepared by FCCVD, which is beneficial to the charge carrier transport in CNT TCFs (refer to section 2.4.1.3). 3.1.2. Spinning CNT Yarns. The second method comes from the phenomenon that CNTs can be drawn into yarns that are meters in length from a superaligned array, following a process developed by Jiang et al. in 2002.255,257 CNT sheets have been prepared by drawing from an aligned MWCNT forest.258 Due to the bundled nature of the CNTs, individual CNTs may bridge the different bundles by migrating from one to another. A uniform CNT sheet is then formed when interconnected bundles of CNTs are pulled together and the breaks in the films are minimized. In 2010, this method was combined with a roll-to-roll process with the presence of a substrate layer and a release layer to prevent the CNT films from being attached to the roller (which could be peeled off later) (see Figure 8b). The resulting film exhibited Rs of 208 Ω sq−1 at 90% (σdc/σOP = 16.8) and 24 Ω sq−1 at 83.4% (σdc/σOP = 82.7) after laser trimming and metal (Ni and Au) deposition (before these two treatments, the value was 1 kΩ sq−1 at 78% transmittance (σdc/σOP = 1.42)).124 Figure 8c shows an SEM image of the CNT TCF, and the aligned feature of CNTs has been maintained after the roll-to-roll process. Although the as-drawn film has very poor optoelectronic properties, after removing the CNT bundles with large diameter and improving the transmittance by laser trimming, more light could pass through and better charge transport was achieved (refer to section 2.4.1.3). Noticeably, as mentioned before, the aligned feature caused a high percolation threshold for CNT TCFs, but when the film is not extremely thin (1 mg mL−1).291 Although a CNT dispersion of 2 mg mL−1 was prepared with cyclohexylpyrrolidone in 2010,292 the high boiling point (280 °C) of this solvent is not appropriate to allow the application of CNT films onto plastic substrates in some manufacturing techniques. A possible strategy to increase the concentration of CNT in organic solvents involves the reduction of CNT with alkali metals to form polyelectrolyte salts, which are soluble in polar organic solvents with no sonication and dispersants. SWCNT salt can be dissolved spontaneously with a concentration up to 2.0 mg g−1 in dimethyl sulfoxide (DMSO) and 4.2 mg g−1 in sulfolane.163,293 An alternative to a neat organic solvent is the use of a superacid, such as chlorosulfonic acid (CSA), which has been used to disperse a CNT with an ultrahigh concentration (4.5 mg mL−1) at room temperature.294 On the basis of this approach, CNT films have been prepared on PET substrates by filtration transfer (refer to section 3.2.2.4) with Rs of 60 Ω sq−1 at 91% transmittance (σdc/σOP = 65.0), as reported by Hecht et al.295 By the simple dip coating (refer to section 3.2.2.1), the Rs of the film was 471 Ω sq−1 at 86% transmittance (σdc/σOP = 5.2).296 The very good optoelectronic properties of these films are a result of the doping (refer to section 3.4) and exfoliation effect of CSA. However the toxicity and the corrosive nature of CSA may limit its use in industry. 3.2.1.2. Dispersing Agents. The use of dispersing agents, such as polymers, porphyrins, polysaccharides, DNA, cellulose 13421
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derivatives, graphene oxide, and starches, is another promising approach to suspend CNTs in a liquid (water in most cases).297−314 Commonly, the amphiphilic properties of these surfactants, which contain a hydrophobic tail and a hydrophilic head, interact with CNT and solvent, respectively, and enable the suspension of CNTs. A detailed analysis of the role of surfactant was published in 2006.315,316 A few important factors relating to the dispersing ability of these surfactants include the charge on the hydrophilic head, the length of the hydrophobic tail, and the presence of aromatic groups. In detail, the interaction between CNTs and surfactants is mainly determined by three forces:317 (1) Coulombic attraction between the heads with charge and the dipole of the solvent; (2) the hydrophobic and Van der Waals interactions between the surfactant tails and CNT surfaces; and (3) the π−π stacking between the aromatic carbon rings on CNT sidewalls and aromatic rings on the surfactants. A general procedure to prepare the aqueous suspension is mixing CNTs (∼0.1%, mass ratio) with the dispersing agent (about 1%, mass ratio) and sonicating for a period of time (ranging from 10 to 100 min), followed by centrifugation to remove the unwrapped CNTs. By following this approach with spray coating (refer to section 3.2.2.3), a TCF deposited from a CNT solution dispersed using sodium dodecyl sulfate (SDS) was reported with an Rs of ∼100 Ω sq−1 at 83% transmittance (σdc/ σOP = 19.3) after the SDS removal by the acid treatment.318 The dispersing agent-based method has several merits, such as achieving a high concentration, stable dispersion of CNTs (for example, 20 mg mL−1 with sodium dodecylbenzene sulfonate with no aggregation),319 easy processing and environmentally friendly aqueous medium (although CNT can also be dispersed in organic solvents such as ethanol320 and chloroform321 by adding some other dispersants), and nondisrupted intrinsic electronic properties of the CNTs all produced without breaking the CNTs. The main drawback of this method is that the high content of surfactants in the suspension may reduce the conductivity of the as-prepared CNT films, which require some further treatments (such as an acid wash or high temperature) to remove these additives.322 3.2.1.3. Modification of CNTs by Covalent Surface Functionalization. The final solution-based scheme involves covalent functionalization of the CNTs’ outer surface to enhance their interaction with a chosen solvent. Normally, in the preparation of CNT films, CNT powder is first treated with a mixture of concentrated nitric acid (69 wt %) and sulfuric acid (98 wt %) in the ratio of 1:3 to introduce negatively charged carboxylic groups to the CNT surface, and the CNTs can then be dispersed in water without any surfactant.323,324 A film produced on PET by dip coating (refer to section 3.2.2.1) with surface modification by NaOH and poly(diallyldimethylammonium chloride) was reported to have Rs of 2500 Ω sq−1 at 86.5% transmittance (σdc/σOP = 1.0), which is quite poor.325 The main advantage of this method is the aqueous dispersion and additivefree nature, which simplifies processing, while the disadvantage is the decreased conductivity of the TCFs due to the disruption of the conjugated sp2 carbon network during functionalization; the CNTs are also shortened. 3.2.2. Coating Processes. 3.2.2.1. Dip Coating and Langmuir−Blodgett Approach. Compared to the limited number of dry processes, many solution-based coating processes have been developed in the past decade such as filtration, dip coating, and spray coating. Among these methods, the simplest process is dip coating, in which a substrate is dipped into a CNT suspension, withdrawn, and dried,326,327 as shown in Scheme 2a.
Scheme 2. (a) Dip Coating and (b) Langmuir−Blodgett Technique for the Fabrication of CNT TCFs
The main factors influencing the properties of the film are the solution viscosity, the interaction between the solution and the dipped substrates, dipping duration, and drying process, which is dependent on the choice of surfactants.328 The nonionic surfactant Triton X-100 forms more uniform films on PET, compared to some charged dispersing agents, such as SDS.263 As far as we know, the best CNT films produced by following this method were films with Rs of 100 Ω sq−1 at ∼90% transmittance (σdc/σOP = 34.8) by dip-coating glass slides into a solution of CSA-stabilized DWCNTs.237 The reason for the excellent performance is mainly due to the long DWCNTs (10 μm) used, which enhanced the charge transport (refer to section 2.4.1.3). Generally, this method can produce CNT films with Rs of 700 Ω sq−1 at ∼90% transmittance (σdc/σOP = 5). It is a very simple, low-cost, and quick process, but the produced film is often inhomogeneous (although homogeneity can be improved by precise control of the heating conditions and the choice of the surfactant to prepare the dispersion329) and there is little control of thickness. More control over coverage homogeneity can be achieved using the Langmuir−Blodgett (LB) approach in which the substrate is dipped into the liquid medium and withdrawn into the air at a controlled rate, as shown in Scheme 2b. In this case, the CNTs were first treated with H2O2/H2SO4, thionyl chloride, and octadecylamine; dispersed in chloroform (1 mg mL−1); and then spread on the surface of water.330 CNTs are only at the liquid−air interface, which is the major difference from the dipcoating method. By making full use of the hydrophobicity of CNT,331−333 thickness control of the CNT is possible by matching the target substrate surface chemistry.330 By repeated dipping, lifting, and drying, >100 layers can be put on quartz or glass substrates. The best reported SWCNT TCF prepared by this method recorded a Rs of 180 Ω sq−1 at ∼78% transmittance (σdc/σOP = 7.9) without doping.239 Although this is worse than 13422
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wt %)) dispersed HiPco SWCNTs (0.1 wt %) onto glass substrates.335 So far, the best film prepared by this method involved the dispersion of SWCNTs with the aid of a polymer derivative of cellulose (sodium carboxymethyl cellulose) and then the CNTs being coated and dried on a glass slide by Mayer rod and lamp, which sometimes help to evaporate the liquid.242 The resulting film (which is not conductive this stage) was then immersed in HNO3 overnight to remove the nonconductive material and p-dope the CNTs. The final film has a Rs of 75 Ω sq−1 at 82% transmittance (σdc/σOP = 24), and the optoelectronic properties meet the requirement for an LCD screen application (σdc/σOP = 22.5). The main advantage of this approach is its potential to be applied in large-scale production by slot die and forward/reverse gravure in a similar way by feeding the CNT dispersions as the ink during the process.336 It is important to point out, though, that the dispersion needs to meet both electrooptical and rheological properties as a coating fluid on a particular type of substrate. 3.2.2.3. Spin Coating and Spray Coating. Spin coating is a more sophisticated approach than dip coating that involves depositing a small volume of CNT suspension onto a spinning substrate,337,338 as shown in Scheme 4a. The spinning substrate acts to spread the CNT solution and with rapid drying can afford very thin and homogeneous films. Some organic solvents such as dichloroethane can be used to dissolve CNT into liquid medium. This process been used to create CNT TCFs with Rs of 128 Ω sq−1 at ∼90% transmittance (σdc/σOP = 27.2), which is the best reported so far.244 These properties, which meet the requirement
that of CNT TCFs prepared by dip coating, it offers precise control over the thickness of very thin films, especially for the fabrication of monolayers, and allows orientation of CNTs to some degree.330 The major disadvantage of this method is the long repeated dipping and withdrawing process for preparation of films with many layers. 3.2.2.2. Brush Painting. A variation of dip coating is brush painting, which involves painting materials by a conventional brush on a substrate, as shown in Scheme 3a. Shear forces lead to Scheme 3. (a) Brush Painting and (b) Mayer Rod Coating to Produce CNT TCFs
Scheme 4. (a) Spin Coating (b) Spray Coating of CNT TCFs a well-interconnected CNT network, while the optical transmittance and the electrical properties can be tuned by the concentration of the CNT ink (the dispersion of CNTs). Some commercial CNT inks are available. Plasma treatment of the substrate can both remove contaminants and improve CNT adhesion. The first CNT film with Rs of 286 Ω sq−1 at 78.5% transmittance (σdc/σOP = 5.1) was produced by painting an SWCNT ink onto a PET substrate after an atmospheric plasma treatment in 2014.241 The optoelectronic properties are not great but have already met the requirement for a touch panel (σdc/σOP = 4.5). The main advantage of this approach is its simplicity, but it is difficult to prepare a uniform film by this method. The Mayer rod coating process, which has been widely used in the painting industry, involves passing a heated bar (Mayer rod) over a liquid area and drying the resultant film at the same time,334 as shown in Scheme 3b. Specifically, a certain amount of CNT dispersion is placed on a substrate, and the rod passing over the liquid spreads the fluid as a CNT film with the evaporation of solvent at the same time. Since the Mayer rod is a metal rod wrapped with stainless steel wires with different sizes, the thickness of the resulting film is closely related to both the gauges and the wire sizes wrapped on the surface of the rod (as shown in Scheme 3b). The properties and quality of the film are dependent on the surface tension as well as the interaction between the substrate and the CNT suspension. Normally, an aqueous suspension of CNTs is prepared with the help of different surfactants, such as sodium dodecylbenzenesulfonate. By following the process, CNT films with a typical Rs of 100 and 300 Ω sq−1 at 70% and 90% transmittance (σdc/σOP = 9.7 and 11.6), respectively, have been produced by painting surfactant (sodium dodecylbenzenesulfonate (1 wt %) and Triton X-100 (3 13423
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of TCFs for LCD screens, are obtained using doping by HNO3 (see section 3.4). Other approaches apply methanol to the dropping flow of the liquid so the spinning substrate can remove the surfactant and the excessive CNT suspension at the same time, and liquids are confined and mixed on the spinning substrate by the shear flow. The resulting CNTs are aligned to some extent.339 In addition, selective deposition of semiconducting and metallic SWCNTs has been reported by surface modification with either terminating the surface with amine (for semiconducting) or phenyl (for metallic) groups.340 Spin coating is a very quick and simple process but is limited in terms of largescale production due to multiple spinning steps to form a film with specific thickness because the solubilities of CNTs in these solvents are quite low (normally 1 cm2) aligned SWCNT films with a high packing density have been prepared via vacuum filtration. The authors found that several parameters were crucial to achieving large domain alignment, namely, the hydrophobicity of the filter membrane surface, the concentration of the dispersant (below critical micelle concentration), the SWCNT concentration (1−15 μg mL−1), and the filtration flow rate (1−2 mL h−1). Under these conditions, the SWCNTs are expected to self-assemble into a close-packed arrangement and have been successfully shown for a range of SWCNT types and dispersants. With close packing, the films are much flatter and homogeneous than is typical for vacuum filtration, allowing for greater control of film thickness.345 The main advantages of vacuum filtration are the precise control over the transmittance of the CNT film by changing the amount or concentration of CNT stock suspension, the excellent uniformity and reproducibility, the potential for large-scale production, and the inexpensive process.346 At the same time, it has some disadvantages including a time-consuming filtration process, the difficulty in producing a submonolayer film with 13424
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solution (2 mol L−1) to do the p-doping of the CNTs and remove the SDS (refer to section 3.4). The optoelectronic properties are not as good as that prepared by most of the other techniques, which may partly be due to short CNTs (length less than 1 μm), but the films still satisfy the requirement for touch panels. A similar electrochemical process is dielectrophoresis (DEP), where an alternating current electric field is applied and selective deposition of aligned film enriched in metallic CNTs has been achieved at the same time due to their much larger polarizability.349−352 The main advantages of these two electrochemical methods include the short processing times, the ability to design the pattern of the film by controlling the shape of the conductive regions, and the high accuracy due to the electrochemical processes. However, films prepared on nonconductive layers normally require further etching of the metal layer. So far, CNT TCFs can be fabricated via both dry and wet methods on a lab scale. All of these methods have some advantages and disadvantages. The properties of all of these resulting CNT TCFs are still worse than that of TCOs, but they are still able to fulfill some requirements for some industrial applications, including touch panels, LCD screens, and OLED displays, as shown in Table 3. However, none of these films are good enough to be used as a thin-film electrode for photovoltaics. Among the two types of methods, dry methods tend to produce CNT TCFs with high quality while wet methods are more compatible to various substrates, partially due to their lowtemperature processes. Among all of these methods, the roll-toroll process is probably the most promising method in fabricating CNT TCFs due to not only its highly efficient process for industrial manufacturing but also the excellent properties of the resulting films.
high quality, and the requirement for transferring the film from membrane to other substrates. 3.2.2.5. Electrophoretic Deposition. From the point of view of practical application, a potential industrial-scale deposition approach is electrophoretic deposition (EPD) in which an electric field is applied between two electrodes dipped in a CNT suspension with ionic surfactants or covalent functionalization. The charged CNTs are electrostatically attracted to an electrode,347 as shown in Scheme 6. For example, the carboxylic Scheme 6. Electrophoretic Deposition of CNT TCFs
groups of functionalized CNTs are negatively charged and a CNT film is deposited on the positive electrode during the process. It is also possible to deposit CNTs on an insulating substrate, and SWCNT films with Rs of 900 Ω sq−1 at 70% transmittance (σdc/σOP = 1.1) have been prepared in which the SWCNT film was first deposited on a thin layer of conductive metal (titanium or aluminum) on top of the glass substrate and the metal layer was oxidized during EPD and became transparent.348 The CNT film with the best property by this method has a Rs of 220 Ω sq−1 at 81% transmittance (σdc/σOP = 7.7).252 In the preparation SWCNTs were dispersed in 7 × 10−3 mol L−1 of SDS aqueous solution, centrifugation was used to remove the aggregates, and the supernatant after dilution was used as the bath solution in EPD. The SWCNT film was first deposited onto a stainless steel electrode, and the PET film was hot-pressed on the film at 0.5 MPa 200 °C (which is lower than the melting point of PET) for 1 min in order to transfer the SWCNT film onto PET. Finally, it was immersed in a HNO3
3.3. Patterning of CNT Films
To fulfill real industrial manufacturing demand, patterning of CNT films is crucial to improve the compatibility to the final products such as touch panels and video displays. Due to the excellent chemical stability of CNTs, traditional TCF etching approaches based on corrosive chemicals are rarely effective, and some new methods are required to achieve patterned CNT TCFs. 3.3.1. Patterning with Surface Modification. First of all, patterning of the films can be achieved by inkjet printing, which uses a CNT ink that requires good wetting properties to the substrates to deposit CNTs with precise position control.353,354 However, the resolution is largely limited by the substrate, the
Figure 9. (a) AFM and (b, c) SEM images of patterned CNT films prepared by (a) the combination of inkjet printing and surface modification (adapted with permission from ref 357; copyright 2011 AIP Publishing LLC); (b) the combination of surface modification and control over the depletion forces (adapted with permission from ref 361; copyright 2014 John Wiley & Sons, Inc.); and (c) inkjet printing of Co nanoparticle catalysts (adapted with permission from ref 362; Copyright 2003 AIP Publishing LLC). 13425
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Figure 10. (a, b) SEM or (c) optical images of patterned CNT films prepared by (a) microcontact printing with the help of block copolymers.363 Adapted with permission from ref 363. Copyright 2006 American Chemical Society. (b) Aqueous solution of PS beads carriers. Adapted with permission from ref 365. Copyright 2007 Royal Society of Chemistry. (c) Selective vacuum filtration. Reprinted with permission from ref 366. Copyright 2007 AIP Publishing LLC.
attraction to the substrate. The PMMA was lifted off by immersion of the substrate in acetone. Finally, a raftlike patterned SWCNT thin film with large-scale homogeneity over the region was achieved, as shown in Figure 9b. The resulting film is a candidate material for transparent transistor with high on/off ratio above 3 × 105 at 1 V bias.361 The surface modification can also be realized by patterning a catalyst on the substrate. In detail, the designation of patterned nanoparticle catalyst (Co) on a substrate (n-doped silicon) with inkjet printing technique has enabled the formation of patterned MWCNT arrays via the thermal CVD method.362 The patterned MWCNT dot size was 5−30 μm, as shown in Figure 9c. Although dots may not be useful in many applications, such as the electrode of a solar cell, it does have the potential to be used as a display because the highest current density for the MWCNT dot was 83 mA cm−2 with an applied field of 1.9 V μm−1. In this case, stabilization of the nanoparticle catalysts at high concentration without aggregation played an important role in achieving highly homogeneous and aligned CNT patterns. 3.3.2. Patterning with the Aid of Polymers. The nanoparticle catalyst pattern can also be achieved with the help of the copolymer poly(styrene-block-acrylic acid) (PS-b-PAA) via microcontact printing.363 In detail, a PDMS stamp was dipped into an iron-loaded PS-b-PAA micellar solution in toluene, followed by physical transfer to a silicon wafer with aluminum oxide coating via compressive stress. In toluene, the copolymer self-assembled into spherical micelles with PAA domains embedded in the PS matrix after the deposition, and this micellar film acted as the template to create iron oxide nanoclusters. O2 plasma was then used to remove the copolymer and left the patterned iron oxide nanoparticle catalysts on the wafer. The patterned CNTs were then grown from the patterned catalyst particles via CVD. Figure 10a is the example of patterned CNT film by microcontact printing by following the described procedure. The major problem with this method is the poor lateral conductance across the vertical-grown CNT array. There is another polymer-related method for CNT TCFs patterning, in which polymer beads with controlled shape, size, and surface chemistry properties were used as building blocks for preparing template structures on the nanometer scale and to deposit CNT. Specifically, a purified SWCNT aqueous suspension with 1% SDS was mixed with a monodispersed PS bead suspension in the mass ratio of 1:6. In the mixture, SWCNTs were absorbed on the surface of PS beads by π−π interaction from the aromatic rings. Then, a thin PS−SWCNT layer was deposited on silicon substrate via “vertical deposition”.364 Because of the capillary
solvent, and the size of the nozzle. The most important factor is the interaction between the liquid and the substrate, which determines the uniformity of the resulting film. Both plasma treatment and a liquid with a low boiling point, which determines the volatility, are beneficial to the preparation of a uniform film.355,356 Different types of substrates are used such as glass, paper, and plastics. CNTs are usually functionalized with the aid of nitric acid and potassium permanganate solution in order to form the dispersion as well as to improve the adhesion to substrate surfaces. CNT films with Rs of 40 kΩ sq−1 at 85% (σdc/ σOP = 0.1) transmittance by multiple prints have been prepared.353 The properties of this film are poor, which may be due to the limited length of the CNTs (200 μm.353,354 By surface modification, a CNT pattern with a width of 90 μm has been achieved, which was first reported in 2011.357 Specifically, a silicon wafer with a thick silicon dioxide layer was first treated with 1,1,1,3,3,3-hexamethyldisilazane, and a hydrophobic self-assembled monolayer was formed on the surface. A hydrophilic strip region was defined by irradiating UV light through a metal mask. The inkjet printing was then conducted on the strip area, and CNTs dispersed in dimethylformamide (DMF) (a polar hydrophilic solvent) only deposited on the modified hydrophilic area, as shown in Figure 9a. Although the width of the CNT lines can be reduced to 70 μm in a single printing process by changing the nozzle to a smaller diameter,353,354 this high resolution cannot be achieved in multiple prints because of the inaccuracy of the substrate feed.353 In contrast, by surface modification, high-resolution CNT lines can be easily achieved during multiple prints, which can tune the optoelectronic properties. This type of thin CNT TCF has the potential for a transparent transistor application with a tunable on/off ratio from 1.2 to 320, which is not great.357 This process is economical and has a high throughput with the ability to produce flexible films. However, large CNT aggregates may clog the printer nozzle; thus, a high-quality suspension is required.358−360 Another surface-related patterning method by means of depletion force was reported recently.361 In this case, narrow channels in poly(methyl methacrylate) (PMMA) on a silicon wafer with a top layer of silicon dioxide were first defined by ebeam lithography. NH2 functional groups were attached to the exposed oxide layer using 3-aminopropyltriethoxysilane (APTES). Presorted semiconducting SWCNTs dispersed in 1% sodium cholate aqueous solution were then self-assembled onto the surface by tuning the concentration of the surfactant and the SWCNTs in order to control the depletion forces and 13426
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Figure 11. SEM images of patterned CNT films prepared by (a) O2 plasma treatment (the inset shows the area after exposure to the O2 plasma).368 Adapted with permission from ref 368. Copyright 2010 American Chemical Society. (b) Direct laser interference. Adapted with permission from ref 370. Copyright 2008 Elsevier. (c) Laser-assisted procedure.371 Reprinted with permission from ref 371. Copyright 2012 American Chemical Society.
followed by the O2 plasma etching. The SWCNT region under the photoresist was protected while the SWCNTs in the exposed area were damaged from the defects to the whole tubes, and they became the amorphous carbon with the production of volatile CO2, CO, and H2O.369 The photoresist residue could then be removed by organic solvent (ethanol), leaving the patterned SWCNT films on PET substrates. Figure 11a is an example of a patterned CNT film by O2 plasma produced using this method. By controlling the plasma power and the etching period, CNT TCFs with various transmittances and sheet resistances were prepared (normally, higher power and longer period leads to more transparent films with higher sheet resistance). The typical CNT TCFs have Rs of ∼500 Ω sq−1 at 80% transmittance (σdc/ σOP = 3.0), which is close to the requirement for a touch panel application. Noticeably, this method uses photoresist and a patterned mask, which may produce whatever pattern is required but increase the cost for industrial applications. In contrast, direct laser interference patterning, which does not involve photolithography and can form the spatial intensity variation by the interference of more than one light beam, where the variations can be transferred to a light-sensitive material, has been used to produce patterned MWCNT films,370 as shown in Figure 11b. Specifically, a MWCNT film was prepared by spin coating on a borosilicate glass substrate and sintered in air at 300 °C for 20 min. By controlling the laser intensity used, MWCNTs at the interference maximum positions were removed to achieve patterns with different sizes. MWCNT films with both line- and dotlike periodic features were created on borosilicate glasses by using two and three light beams at the same time. This method is very suitable for preparing some regular samples with high resolution. While these two methods involve the preparation of CNT dispersions, which may increase the processing time, fortunately laser-based patterning can also be combined with dry methods to prepare CNT films, including CVD. The transfer and patterning of vertically aligned carbon nanotube (a mixture of SWCNT and DWCNT) arrays onto polycarbonate substrate was achieved at the same time with the help of a laser beam.371 Specifically, uniform, vertically aligned CNTs were first grown on a silicon substrate at 750 °C with the help of e-beam to deposit the iron nanoparticles as the catalyst. The growth conditions was strictly controlled to obtain the vertically aligned CNT forest. The polycarbonate (PC) film with a piece of glass was then applied on top of the forest and pressure was applied. The laser beam is used to irradiate the interface, and the absorbed heat enables the CNTs to adhere to the PC surface. By controlling the movement
forces at the liquid/solid/air interface, the PS beads selfassembled into closely packed stripes in the pulling direction. Precise temperature control was required (60 °C) in order to form highly ordered PS assemblies. The hemispherical network of CNT bundles was formed over a large area after the removal of the PS beads and surfactants by organic solvents and water, as shown in Figure 10b.365 The conductivity of the resulting film is comparable to the bulk materials in SWCNT mats (∼100 S cm−1) with significantly increased transmittance. More importantly, the transmittance of the resulting TCFs can be tuned by controlling the size of the monodispersed PS beads. Another variation of patterned film fabrication is selective vacuum filtration.366 This has been achieved by first patterning anodic aluminum oxide filter membranes (average pore size = 20 nm) with a photoresist via photolithography. During vacuum filtration, CNTs were only deposited on the exposed region, as shown in Figure 10c. Uncured PDMS was then poured onto the filter membrane and cured. By peeling off the elastomer, the patterned CNT networks were transferred from the filter membranes. In addition, this type of design has the potential to be used as a transparent and flexible thin-film transistor. In other work, a similar process was reported except that the patterned filter paper was used to collect CNTs during the CVD process without preparation of a CNT suspension, and highperformance, transparent, conductive CNT films with a microgrid patterned were achieved on a plastic substrate.367 Specifically, CNTs grown by FCCVD (in which CO is the feed gas and ferrocene nanoparticles act as the catalysts) were first collected on the polyvinylidenedifluoride membrane filter with a pore size of 0.45 μm whose microgrid was defined by photolithography, and then were transferred onto the objective polyethylenenaphthalate substrate. The resulting CNT TCF has Rs of 53 Ω sq−1 at 80% transmittance (σdc/σOP = 30.1), which is competitive with some TCOs. 3.3.3. Patterning with Post-treatments. There are some post-processing methods based on etching to define the pattern of the CNTs. For example, because of their low ion energies, O2 plasma based on a capacitively coupled plasma system has been used for patterning SWCNT films on a plastic substrate,368 such as PET. Specifically, the SWCNT film was first prepared on an anodic aluminum oxide membrane by a typical vacuum filtration approach and then transferred onto a flexible PET substrate by dissolving the membrane with a NaOH solution (3 mol L−1). A layer of positive photoresist (AZ4620) was applied by spin coating and patterned on top of SWCNT films by UV light (365 nm) through a mask and development with AZ400K, which was 13427
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Scheme 7. Density of States As a Function of Energy of Semiconducting SWCNTs: (a) No Doping, (b) p-Doping, and (c) nDoping
a
v1, v2, v3, c1, c2, and c3 indicate ith levels of the valence and conduction band. The red dashed lines refer to the Fermi level in each case.
electrons depending on the type of the dopants, p-type or ntype). Furthermore, shifting the Fermi level reduces the size of the Schottky barrier between semiconducting and metallic SWCNTs.372 Therefore, doping improves the overall sheet resistance of CNT TCFs by improving the conductivity of individual CNTs and improving charge percolation within the network. In theory the Fermi Level of semiconducting SWCNTs is at an equipotential position (refer to Scheme 7a) between the conduction and valence band, and thus p-type and n-type doping with electron-withdrawing and -donating effects is achievable (refer to Scheme 7b, c). In practice p-type doping is more commonly studied because the CNTs are slightly p-doped by the oxygen in the air under ambient conditions.373 Whether a dopant causes p- or n-type doping depends on either the electronegativity for atomic dopants or the electrochemical potential for molecules and inorganic materials.372 p-type dopants include NO 2 , 3 7 4 H 2 SO 4 , 3 7 5 , 3 7 6 H 2 SO 4 , 3 7 5 HNO3,318,377−385 SOCl2,375,377,379,386,387 the combination of HNO3 and SOCl2,159 SOBr2,388 HCl,389 Br2,390 Nafion,378 tetrafluorotetracyano-p-quinodimethane (F 4 TCNQ), 391 MoOx,392 oleum,335 iodine,375 bromine,393 triethyloxonium hexachloroantimonate (OA),394 and bis(trifluromethanesulfonyl)imide (TFSI).395 The doping procedure generally involves the immersion of the CNT TCF into a concentrated (often neat) solution of dopant for a period of time (10 s−1 h) or exposure to an atmosphere for gaseous dopants. Table 4 shows performance of CNT films before and after chemical doping. Among these dopants, H2SO4 doping has the highest increase of (σdc/σOP) in percentage (5800%), but this value is probably overexaggerated when considering the low starting value of σdc/σOP (0.1). Although most of the treated films are still below the industrial requirement of the anode for OLED displays, all dopants improved properties, with TFSI being particularly promising. Recently, a systematic study on the doping mechanism was done by Puchades et al.396 In their study, SWCNT films were exposed to >40 dopants and the change in conductivity was measured. They found that the relative potential difference between the redox potential of the chemical dopant and the SWCNT electronic transitions determines the enhanced electrical conductivity due to doping (the greater the redox potential is, the better the dopant is). On the basis of this idea, one can deduce that the effect of the dopants could vary with the chirality of the SWCNTs; in a SWCNT film of mixed chiralities, it is possible to finely tune the conductivity by selectively choosing a chemical dopant with the ideal redox potential. Other
of the laser before separation of the Si−CNT−PC, positive and negative patterns of CNTs were achieved on PC and silicon surface, respectively, as shown in Figure 11c. This method is highly effective and useful because it realizes patterning and transferring at the same time without the use of any CNT dispersions. The main problems in this case are that the control of the CVD conditions are crucial but difficult to deal with and that there is poor lateral conductivity for the aligned film (0.4− 0.6 Ω−1 cm−1). It is possible to increase the CNT film conductivity by collapsing them into different directions. Among all of these methods, both flexible and rigid substrates were used. High-resolution patterns of CNTs with micron resolution were achieved using some simple and rapid approaches with high reproducibility. The capability of patterning CNT TCFs is really attractive for industrial applications, such as touch panels. Most of these industrial products involve a matrix of rows and columns of conductive material. In this case, one can either etch CNT TCFs into a microgrid pattern or perpendicularly combine two CNT TCFs with parallel strips. Normally, the transmittance of the CNT TCFs is increased while the sheet resistance of the CNT TCFs sometimes is increased at the same time due to the formation of a CNT network with lower density. For example, if the power and the period of the O2 plasma increases, the density of the CNT will be below the percolation threshold and thus the conductive channels fail to form. To achieve a higher value of σdc/σOP, both parameters need to be balanced. The other important aspect to take into consideration is the width of the striped pattern, as there is a dependence of the transport properties of the film on the width of the CNT TCFs, especially for randomly aligned CNT films (refer to section 2.4.1.2). However, in most of these cases, the patterning processes of CNTs have only been applied on lab-scale preparations. Some issues such as the use of some high-energy processes and strict chemical reaction conditions remain to be solved. 3.4. Doping to Improve CNT TCFs
To fulfill the practical requirements for CNT-based TCFs, such as the anode of OLED displays (>50 Ω sq−1 at 90% transmittance (σdc/σOP = 69.7)125) and LCD screens (>100 Ω sq−1 at 85% transmittance (σdc/σOP = 22.3)),125 chemical doping is an effective approach to improve the performance of pristine CNT networks. The conductivity of CNT networks is controlled by the conductivity of the individual CNTs and the contact resistance between CNTs (refer to section 2.4). Doping improves both aspects by shifting the Fermi level (as shown in Scheme 7) to increase the density of charge carriers (holes or 13428
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such as HNO3 and SOCl2 slightly increase transmittance somewhere around 1.5 eV (∼830 nm). After the doping treatments, the Rs of films decrease by a factor of 3−8. In a typical p-doping process, by electron-withdrawing from the CNT to the dopants, a downshift of the Fermi level into the valence band happens (corresponding to the work function of the CNT increasing from 4.5−4.8 eV to >5 eV with a higher density of states (DOS)) (as shown in Scheme 7b).348 As a result, more charge carriers are available, leading to a lower Rs, and the charge percolation is reinforced. Oppositely, n-type dopants, such as nitrogen, can raise the Fermi level (as shown in Scheme 7c) by ∼1.21 eV in the graphitic DOS, and the nitrogen states are just above the Fermi level.399 Acid treatments reduce the contact resistance from one individual CNT to another by removing some insulating dispersing agents, such as SDS used during the suspension preparation, to achieve better film conductivity.318,381 In addition, the intercalated dopants can also act as bridges that connect CNTs electrically. Specifically, the dopants, usually alkali metals (such as potassium), can fill the trigonal channels with closely packed ionic chains.400 Therefore, charge carriers can also move along the outside of the tube wall with the help of the overlap of the K 4s related hybridized bands with the Fermi level.401 However, the effect of chemical doping by most of these dopants does not last long because these liquid and gaseous molecules are weakly adsorbed on the surface of CNT and they may desorb. As shown in Figure 12b,379 the stability of the doped films is worse than that of the undoped counterpart, although their Rs is lower than that of the untreated sample. This lack of stability will be an issue for commercial applications. A PEDOT:PSS capping layer has been shown to protect the film from degradation effectively, as shown in Figure 12c.379 A similar protection effect can be achieved via an electrochemical method to dope ClO4− ions on the wall of SWCNTs, and the resulting
Table 4. Influence of the Chemical Doping (p-Type) on the Performances of the Resulting CNT Films with the Best Optoelectronic Properties after Doping; The Values in Brackets Indicate the Increase of σdc/σOP in Percentage after Doping without doping
with doping
T (%)
Rs (Ω sq−1)
σdc/ σOP
T (%)
Rs (Ω sq−1)
H2SO4
93.1
50500
0.1
93.3
1010
HNO3
83
600
3.2
83
48.5
79.6
2800
0.6
79.6
330
SOCl2
87
380
5.3
87
160
HNO3 + SOCl2 SOBr2
80
300
5.3
80
105
77.6
184
7.6
77.6
56
MoOx
85
1000
2.2
85
100
oleum
90
800
4.4
90
300
OA
80
200
8.0
80
90
TFSI
85
600
3.7
85
38.4
dopants
HCl
σdc/σOP
ref
5.3 (5200%) 39.8 (650%) 4.7 (683%) 16.3 (138%) 15.2 (186%) 24.9 (228%) 22.3 (914%) 11.6 (164%) 17.7 (121%) 58.0 (1468%)
375, 376 383 389 386 379 388 392 335 394 395
mechanisms of the most prevalent chemical dopants of CNTs are shown in Table 5 with further explanation. In most cases, these chemical doping treatments do not have a strong influence upon the transmittance of the CNT films, as shown in Figure 12a.159 Through the visible light region, the transmittance is maintained at the same level while some dopants Table 5. Mechanism of the Most Popular p-Type Dopants of CNT chemical dopants H2SO4
HNO3
HCl
SOCl2 SOBr2
MoOx triethyloxonium hexachloroantimonate (OA) bis(trifluromethanesulfonyl) imide (TFSI)
mechanism and effect Intercalated SO3/SO42‑ attracts the electrons from CNT and lowers the Fermi level (by 0.5 eV) with the creation of more holes in the valence band.397 It helps to remove some amphipathic/polymeric dispersant and thus reconstruct the network to reduce the contact junction between CNTs with tighter contacts. Physisorbed HNO3/NO2 molecules on the surface and bundles of SWCNTs enable the transfer of electrons from CNT to NO3− groups, which shifts the Fermi level by 0.2 eV.397 An enhanced contact between individual CNTs is achieved by removal of impurities, including carbon black, metal catalysts, organic solvents (NMP, etc.), and others adsorbed on the walls of CNTs. HCl doping is similar to that of H2SO4 and HNO3 via intercalation. It can shift the Fermi level by 0.1 eV.397 The desorption of HCl is faster than that of H2SO4 and HNO3.397 The hole doping effect is due to the electron-withdrawing nature of the decomposition of SOCl2 (2SOCl2 + 4e− → S + SO2 + 4Cl−). The conduction mechanism changes from thermionic emission to tunneling through the barriers. SOBr2 is larger than SOCl2. Br− anions are chemically bonded to the side walls of CNTs and interact with metallic SWCNT more easily than the semiconducting species. The oxidation of Br− anion occurs during the process, which is the driving force of the electron transfer. It may cause the formation of −C−S−C− bond with the possibility of cross-linking SWCNTs.398 The formation of new transport paths via S atoms between CNTs is possible. Electron transfer from CNT to MoO3 via the following interaction: MoO3 + CNT → CNTδ+ + MoOyδ−.392 The stability of this doping effect can be enhanced by thermal activation. OA is a one-electron oxidant. p-Type doping is achieved via the following scheme with the formation of a charge-transfer complex: SWCNT + OA → SWCNT + [Sb(Cl)6]− + C2H5Cl + SbCl3 + (CcH5)2O.394 Different acidic derivatives of TFSI dopants can adsorb onto defects or the sidewalls of CNTs depending on their chemical structure. They can accept electrons from CNTs and shift the Fermi level down. Among all 3 types of derivatives, bis(trifluoromethanesulfonyl) amine is the most effective dopant.395 13429
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Figure 12. (a) Transmittance comparison of pristine and HNO3 and SOCl2 doped CNT films; the transmittance spectrum does not change much after doping. Reprinted with permission from ref 159. Copyright 2007 AIP Publishing LLC. (b) Performance degradation of the doped/undoped films with exposure to air. Rs of the doped films are not stable, and they increased after a period of time when exposed to the air. (c) Protection effect on the Rs by adding a PEDOT:PSS capping layer. The PEDOT:PSS capping helps to stabilize the Rs by trapping the dopants after doping. Reprinted with permission from ref 379. Copyright 2008 John Wiley & Sons, Inc. (d) Improved stability of the Rs by electrochemical doping of ClO4−. Reprinted with permission from ref 402. Copyright 2015 Royal Society of Chemistry. (e) Response of the MoOx and F4-TCNQ doped CNT films to different conditions. The inset shows the comparative results of other material-doped CNT films.392 Reprinted with permission from ref 392. Copyright 2012 American Chemical Society.
doped film has a stable Rs for >20 days under ambient conditions,402 as shown in Figure 12d. A more promising Rs stability is achieved by the addition of MoOx, which forms a composite material with CNTs (refer to section 3.5). Annealing at high temperature (450 °C) can activate the charge transfer from CNT to MoOx, and the resulting CNT films show great stability under different conditions, as shown in Figure 12e.392 Compared to traditional chemical doping methods, such as (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) (F4TCNQ), Addition of MoOx yields a film with much better stability in Rs, especially when heated in ambient conditions (300 °C). In air, the limit of the thermal stability of Rs for MoOx− CNT is ∼390 °C, which is the oxidization temperature for CNTs. The excellent thermal stability is attributed to the nonvolatility below 390 °C. It will be important to solve the stability issue of the doped CNT films before their practical applications in industry.
3.5. CNT-Based Hybrid Films
Instead of chemical doping, incorporation of other materials with CNTs to form a hybrid is another method to improve the performance of the films by combining the merits of all the components used. The most widely used materials include metal nanoparticles and nanowires, conductive polymers, and graphene. Table 6 shows a summary of the optoelectronic properties of some selected CNT-based hybrid films. 3.5.1. Hybrids with Metallic Nanomaterials. Different metal nanoparticles, such as Pt, Ag, and Au, have been incorporated in CNT networks with both in situ synthesis and ex situ filling methods.407 For example, silver colloids were modified by H2N(CH2)2SH and terminated with amino groups, followed by the surface condensation with DWCNTs, which were functionalized with −COOH groups by concentrated sulfuric acid and nitric acid. Figure 13a shows a transmission electron microscope (TEM) image of the DWCNT reacted with AgNPs, in which AgNPs are spotted all over the DWCNTs. The Ag−DWCNT powder was filtered, purified by rinsing with water, collected, and redispersed in water with SDS. The TCF 13430
DOI: 10.1021/acs.chemrev.6b00179 Chem. Rev. 2016, 116, 13413−13453
Chemical Reviews
Review
transfer processes, respectively. The optoelectronic properties of the hybridized TCFs are much better than those of the pure CNT films. For the films with only AgNWs, the Rs is ∼100 Ω sq−1 at 80% transmittance (σdc/σOP = 16.0) and 500 Ω sq−1 at 84% transmittance (σdc/σOP = 4.1). The improvement is mainly due to the high conductivity of the AgNWs, while CNTs act as both additional charge-transport channels and mechanical support. The issue with metal NPs and NWs is that they involve relatively expensive precursor materials and can produce a hazy TCF.1 3.5.2. Hybrids with Conductive Polymers. All organic TCFs are cheaper, less toxic, and more flexible alternatives to inorganic hybrid TCFs.409 Polyaniline was used with CNT in preparation of transparent hybrid films as early as 1999.410 More recently, PEDOT:PSS was used to form hybrid films with CNT networks.405 The hybrid solution was prepared by mixing the stock solution of HiPco SWCNT and suspension of PEDOT:PSS and CNTs, followed by vacuum filtration and transfer to a flexible PET substrate under heat and pressure. As shown in Figure 13c, both types of CNTs (HiPco and arc discharge) have been merged with PEDOT:PSS uniformly in the resulting films. The hybrid film made with the arc discharge CNTs and PEDOT:PSS has a Rs of 80 Ω sq−1 at 75% transmittance (σdc/σOP = 15.2), which is worse than that of CNT TCFs hybridized with AgNPs and AgNWs. Both of these types of CNT/PEDOT:PSS films had excellent stability under tension (with Rs changing 3 times higher than that of the conventional ITO-based counterpart.460 Another potential OLED anode material is metal grids, which sometimes have better optoelectronic properties than SWCNT networks. However, to maintain the high transmittance, there are many large pore regions between the grids. Holes injected in the anodes cannot reach the center of the spaces when the gap between grid lines is larger than the diffusion length. The recombination of the holes and electrons can only take place near the grid lines, and as a result, luminance only happens in the area close to the lines when the voltage is applied, as shown in Figure 20f.462 By applying P3HT-wrapped SWCNTs with a spin-coater, the resultant SWCNT network covers the spaces between the grid lines that can transport the holes, the recombination process happens uniformly, and homogeneous luminance can be achieved, as shown in the inset in Figure 20f.462 Thus, CNT-based TCFs have already achieved some success in a range of applications on a lab scale. They have been applied in the fabrication of a range of solar cell architectures, touch panels, liquid crystals, and organic light-emitting diodes. However, there are still a few issues to address for each of these applications. First of all, CNT films are usually doped, but the dopants are prone to react with atmosphere or other materials with which they are in contact, increasing Rs and subsequently reducing performance.471 Second, although the optoelectronic properties of CNT TCFs are continually being improved, for application in solar cells, more advances are still required to match and exceed that of traditional TCOs (the σdc/ σOP of TCOs are >200 and normally 29 000 cd m−2, which is much higher than the pure graphene anode-based device (
