ITO Replacements: Carbon Nanotubes … · Carbon Nanotube Networks as Transparent Electronic...

ITO Replacements: Carbon Nanotubes

Axel Schindler*Neuchatel, Switzerland

Abstract

Randomly oriented networks of carbon nanotubes are a promising candidate for ITO replacement. Whilethey do not yet achieve the high conductivity of ITO for an identical optical transmittance, their bigadvantage lies in their high mechanical flexibility, neutral color, and chemical stability. Combined withthe possibility of vacuum-free deposition from suspensions and almost unlimited material resources, theyenable new applications and potentially lower production costs. This chapter shall give an overview of themain aspects of this new material starting with a general description and deposition techniques. In thefollowing, the mechanical, optical, and electrical properties are discussed. The chapter ends withpresented applications in displays, touch devices, and OLEDs, followed by conclusions and futureprospects.

List of Abbreviations

AM Active matrixCNN Carbon nanotube networkCNT Carbon nanotubeCVD Chemical vapor depositionDOS Density of statesdt Nanotube diameterEf Fermi energy levelEg Band gapITO Indium tin oxideLC Liquid crystalLCD Liquid crystal displaym-SWNT Metallic single-walled nanotubePDLC Polymer-dispersed liquid crystalR□ Sheet resistance [O/□]R2R Roll-to-rollRIE Reactive ion etchings-SWNT Semiconducting single-walled nanotubeSWNT Single-walled nanotubeT Optical transmittance [%]TEC Transparent electronic conductorTN Twisted nematicsDC Electrical direct current conductivitysop Optical conductivity

*Email: [email protected]

Handbook of Visual Display TechnologyDOI 10.1007/978-3-642-35947-7_55-2# Springer-Verlag Berlin Heidelberg 2015

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Introduction

For display applications where an optically transparent, but electrically conductive, material is needed,indium tin oxide (ITO) is the predominantly used material. Its electro-optical performance, i.e., theachievable conductivity for a given optical transmittance, is still unmatched, and the processes fordeposition and patterning of ITO are well established. There are however drawbacks besides the limitedindium resources that make the search for alternatives worthwhile. Especially when it comes to applica-tions where increased mechanical flexibility is required, ITO and other brittle metal oxide thin films arenot well-suited candidates. Besides touch modules, where a higher resistivity of the transparent electrodeis acceptable, mechanical stability combined with high conductivity and transparency is mandatory forflexible displays. For proposed applications like rollable or wearable displays, all materials including thetransparent electronic conductor (TEC) need to withstand highly increased mechanical stress compared tocurrent flat panel displays on glass, caused by bending and strain. The remarkable combination ofelectrical, mechanical, and optical properties of thin films created from carbon nanotubes (CNTs)makes them well suited for such applications. In contrast to ITO, they are also very compatible withflexible plastic substrates. The added possibility to deposit these films vacuum-free under room temper-ature might also lead to cost reduction in the fabrication process especially when using fast roll-to-roll(R2R) printing processes. Despite the large improvements in conductivity, the electro-optical perfor-mance of CNT TECs is still inferior compared to ITO. Nevertheless, several prototype displays, fromsimple directly addressed liquid crystal displays (LCDs) to full-color active-matrix (AM) displays, werepresented in the last years demonstrating the applicability of CNT TECs to display applications. Thecommercial applications are so far limited to touch films. To achieve a larger-scale industrialization, thereis still room and need for improvement in material synthesis, processing, and product integration.

Detailed information about carbon nanotubes, including their electronic structure, synthesis, andpurification techniques and production of CNT dispersions and processes to separate metallic fromsemiconducting nanotubes, can be found in the chapter about CNT thin-film transistors. In the followingsections, the details regarding their application as transparent electronic coatings will be discussed,starting with some basic facts about thin films of CNTs and their properties and continuing with depositiontechniques, doping effects, and published applications and ending with future prospects.

Carbon Nanotube Networks as Transparent Electronic Conductor

Single-walled carbon nanotubes (SWNTs) are hollow cylinders of carbon atoms in a hexagonal latticewith diameters from roughly 0.5 to 2 nm and lengths from 100 nm to several centimeters (Popov 2004).Depending on their exact structure, SWNTs can be metallically conducting or semiconducting. Thebandgap Eg of a semiconducting SWNT is inversely scaling with the nanotube diameter dt asEg � 0:7=dt nm½ � eV. With most common synthesis procedures, a mixture of diverse geometries in acertain diameter range is produced. About one third of the produced tubes are metallic while two thirds aresemiconducting. Recently several groups achieved synthesis of a very confined diameter range or evenenrichment in one electronic type.

Carbon nanotubes are synthesized using either a solid or gaseous carbon feedstock under controlled gasatmosphere, high temperatures in a range around 1,000 �C, and metal catalysts that are eitherpre-deposited on a substrate or created in situ from gas phase. When using pre-deposited catalystsCNTs can be directly grown on a substrate using chemical vapor deposition (CVD) processes. Withthis technique, the density and alignment of the CNTs can be closely controlled. The necessary processtemperatures are too high though even for display-grade glass. There are methods to transfer directly


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grown CNNs (Hur et al. 2005; Kaskela et al. 2010) or even aligned CNTarrays (Feng et al. 2010) to plasticsubstrates at room temperature. For the application as TECs, where large areas need to be covered by cost-effective processes, this approach seems however less favorable.

Solution processing on the other hand offers a large variety of deposition methods especially since inthe produced films, an alignment of the tubes is not necessary or even a disadvantage as long as isotropicconductivity is desired. Starting with SWNTs in powder form, the nanotubes can be dispersed in solvents,acids, or ammonia, significantly higher concentrations and longtime stable suspensions can however beachieved by using dispersing agents like polymers, DNA, or surfactants. The still most widely usedapproach is aqueous surfactant solutions. These suspensions can then be deposited using potentiallyinexpensive techniques (see next section) compared to the vacuum processes used for ITO deposition.

In a simplified manner the carbon nanotube network can be imagined as a quasi-2D network ofinterconnected conductive sticks with a very high aspect ratio from 1,000 up to 106. It is easy tounderstand that the conductivity of such a layer is not only determined by the intrinsic conductivity ofthe individual tubes but also by the intertube resistance. When using CNTmaterial with the classic ratio ofone third metallic SWNTs (m-SWNT) and two thirds semiconducting SWNTs (s-SWNT), there are threepossible regimes, depending on the network density or thickness of the layer that can be described bypercolation theory (Hu et al. 2004; Li et al. 2007). (1) If the density is too low and only some nanotubestouch each other, there is no continued conduction. (2) If the density is above the percolation threshold ofthe s-SWNT but due to the 1:2 ratio between metallic and semiconducting nanotubes below thepercolation threshold of the m-SWNT, the CNT layer shows nonlinear I-V characteristics, acting like asemiconductor that can be used for CNT network (CNN) transistors (Snow et al. 2003). (3) For networkdensities above the percolation threshold of m-SWNTs, the CNT layers act like a conductor with linearI-V characteristics (Skákalová et al. 2006). For these kinds of networks, the above-introduced model ofrandomly oriented straight sticks is somewhat misleading. Due to the high aspect ratio of the single tubes,their flexibility, and strong van der Waals interactions, the nanotubes form seamlessly interwovennetworks with no noticeable beginnings and ends of single tubes (see Fig. 1). The tendency to formthicker or thinner bundles depends on processing and deposition technique and results in different

Fig. 1 SEM image at high magnification of an SWNT film on a fused quartz substrate after a dry transfer process (Reprintedfrom (Cui et al. 2014) under Creative Commons Attribution 3.0. Copyright 2014, The Royal Society of Chemistry)


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roughness values of the deposited layers as will be discussed later. Typical thicknesses for high-transmittance layers lie below 100 nm.

Deposition and Patterning Techniques

Being able to disperse CNTs in aqueous solutions enables the use of cost-effective deposition techniquesat low temperatures. The drawback of having to use a dispersant introduces a contamination with aninsulating or even electrically charged material though. In order to receive optimal conductivity, care hasto be taken to completely remove those unwanted species after deposition (Geng et al. 2007). Nowadays,surfactant-free dispersions are also available on the market.

The Kauppinen group presented a dry and room-temperature method where the nanotubes aredeposited directly downstream of a so-called floating catalyst synthesis reactor (Zavodchikovaet al. 2008). The method was demonstrated for high- and low-density networks. A spin-off companycalled Canatu# commercialized the deposition method under the name Direct Dry Printing®. They use itto produce coated plastic films for touch applications in an R2R manner up to 600 mm width. A furtherparticularity is that they do not deposit plane CNTs but a hybrid of SWNTs with a low density ofcovalently bonded fullerenes, called Carbon NanoBud® (Anisimov et al. 2014).

Another dry deposition method was developed at the Tsinghua-Foxconn Nanotechnology ResearchCenter. They grow super-aligned nanotube forests on a wafer that can then be drawn into a film of parallel,aligned bundles and applied to a substrate (Fu et al. 2010).

Dip coating is a very simple setup (Ng et al. 2008). Substrates are treated with an adhesion promoter,dipped into a CNT suspension, and dried. This process can be cycled to achieve the necessary layerthickness. Mirri et al. achieved a sheet resistance of 100 O/□ at 90 % transmittance by using nanotubesdispersed in chlorosulfonic acid and therefore avoiding contamination by dispersing agents (Mirriet al. 2012). While the deposition is relatively simple, homogeneity, scalability, and contamination ofthe substrates’ back surface might be issues. Xiong et al. also demonstrated patterned and aligneddeposition using a controlled pulling process (Xiong et al. 2009).

Lima et al. used electrophoretic deposition for the creation of CNT TECs in thin films (Limaet al. 2008). The substrates are first coated with a thin metal layer that serves as electrode. Immersedinto the surfactant solution, an electric field between substrate and a metal electrode charges the nanotubesand transports them to the substrate surface. After the deposition the metal layer is etched away. While theextra metal layer makes this process less favorable, it has the advantage that the nanotube layer can bepatterned by deposition.

An often used process to create films of CNTs is by vacuum filtration (Wu et al. 2004). For this method,the CNTsuspension is sucked through a filter membrane by vacuum. The nanotubes form a dense layer ontop of the filter. The thickness can be controlled by the volume and concentration of the suspension. Afterthe filter process and sufficient rinsing, the nanotube layer can be transferred by two techniques. In softlithography, an elastomeric stamp is brought in contact with the nanotube network which is thentransferred from the membrane to the desired substrate. In each transfer, the surface energy of the targetmaterial needs to be higher than the surface energy of the source material in order to achieve a sufficienttransfer. By molding of the elastomer, a patterned deposition can be achieved (Meitl et al. 2004; Zhouet al. 2006). In the second method, the nanotube layer is brought in contact with the substrate followed bydissolving of the filter in an appropriate solvent (Chhowalla 2007). While scaling of these methods seemsdifficult, vacuum filtration is a simple method for material testing.

A promising method when it comes to large-area applications is spray coating. CNTsuspensions can beeasily spray coated on large substrates using either a simple air brush (Kämpgen et al. 2005; Schindler


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et al. 2007) or highly automated systems. The substrates are usually moderately heated to accelerate waterevaporation of the striking droplets. Large droplets can increase the layer roughness presumably causedby the coffee stain effect. Layers with very low roughness and high uniformity were reported by Tenentet al. using ultrasonic spraying (Tenent et al. 2009). Combined with the scalability and compatibility withmany substrate materials, including plastic substrates, spray coating seems predestined for displayapplications.

Further different classical printing and coating techniques compatible with sheet or R2R processinglike slot die, ink-jet, or gravure might be used. A general difficulty is to find a well-working combinationof materials and surface chemistry to achieve wetting and homogeneous coatings of a well-definedthickness or network density without having to use additives that would significantly deteriorate theconductivity of the CNN.

Slot die coating combined with an R2R setup allows for high-volume production. Unidym# presentedCNT films on large plastic substrate rolls coated by slot die (Hecht et al. 2009). They have commercializedthe solution deposition methods and sell films for touch, display, and other opto-electrical applications.While giving high-throughput slot die coating does however not have the big advantage of additivepatterning, the layers need to be patterned afterward by a subtractive manner.

Direct patterning during deposition of CNT TECs was presented using the ink-jet method (Songet al. 2008). For good printing results on polymer substrates, a hydrophilic surface treatment wasemployed prior to deposition, and the substrate was heated to 60 �C. A further method close to ink-jetprinting to prepare low-density CNT deposition is using an aerosol jet (Vaillancourt et al. 2008). It alsoenables patterned deposition. While scalability of these methods might not be an issue, high throughputcan only be achieved by cascading multiple printheads. A further trade-off needs to be found betweennanotube length and nozzle size according to the pattern resolution demands.

Since many of the discussed deposition techniques are not capable of producing patterned layers,subtractive patterning processes are necessary. As for other thin films, the patterning by photolithographyand subsequent etching can be applied to nanotube layers as well. Reactive ion etching (RIE) with an O2

plasma can be employed for the effective removal of CNTs (Behnam et al. 2007; Schindler et al. 2008b;Cao et al. 2008). Liftoff techniques were also used. They are however not favorable for productionprocesses and might not work for higher density, interwoven networks. An example for a vacuum-freeremoval of at least low-density CNT networks is the usage of a CO2 snow jet in combination withphotolithography (Sherman et al. 1994; Snow et al. 2003). Avery effective way of patterning without theneed for additional masking steps is laser ablation (Hecht et al. 2011b; Anisimov et al. 2014). It isespecially favorable for touch grids and other applications where TEC patterns need to be electricallyseparated without the need to remove large portions of the layer.

Properties of Thin Carbon Nanotube Films

In order to replace the well-established ITO, the alternatives need to exhibit a certain degree ofpredominance in at least some properties. In the following section, the important mechanical, optical,and electrical properties of carbon nanotube layers to display applications are discussed and comparisonsto ITO are drawn.

Mechanical PropertiesFor reliable processing of CNT thin films, a good adhesion to the substrate is mandatory. Carbonnanotubes show good adhesion to plastic substrates (Li et al. 2006; Schindler et al. 2007). For thedeposition on glass and other inorganic substrates, a self-assembled monolayer of an amine-terminated


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silane is commonly used as adhesion promoter (Meitl et al. 2004; Schindler et al. 2007; Opatkiewiczet al. 2010). Alternatively, organic layers might be used as adhesion promoter. While the achievableadhesion is enough for further processing, mechanical scratching leads to local removal. In areas where adirect electrical contact is not necessary, polymeric capping layers can increase the mechanical stability(Schindler et al. 2008b). Because of the network nature, the polymer soaks into the nanotube layer,increasing also the substrate adhesion.

The high flexibility and mechanical robustness of CNT layers were demonstrated by several groups.Compared to oxide TECs like ITO, CNNs show by far less deterioration of the conductivity bymechanical stress like bending (Ng et al. 2008), folding, abrasion (at least in combination with a binder)(Arthur et al. 2004), or stretching (Hu et al. 2009). While the crystalline phase of the ITO cracks easilyleading to total failure, the CNNs stay intact even when the position of some nanotubes is moved. Huet al. demonstrated electrical conductivity for strains up to 700 %. Table 1 summarizes the differentmechanical stress tests performed by Arthur et al.. They compared a commercial ITO coating with spray-coated CNN with acrylic coating, both on 125 mm thick films. The ITO had a sheet resistance ofR□; = 250 O/□; the CNT/binder coating had a sheet resistance of R□; = 650 O/□ at 90 % transmittance.The results show that the CNT coating clearly outperforms the ITO in all tests.

Another important factor when it comes to active-matrix applications with increased topology is theability to cover edges and bridge step heights up to several hundred nanometers. It was demonstrated thatdue to the fibrous nature, even large step heights above 1 mm can be effectively covered, possibly makingcommon planarization layers dispensable (Park et al. 2008).

Especially for the application in organic light-emitting diodes (OLEDs), a low surface roughness is ofimportance. While planarization can be achieved by organic capping layers like PEDOT:PSS (Williamset al. 2008; Ou et al. 2009), a low roughness after deposition is preferred. Andrade et al. compareddifferent deposition techniques and found that electrophoretic deposition and dip coating lead to lowaverage roughness values of 5 and 11 nm, respectively (Andrade et al. 2007). Values for vacuum-filteredand spray-coated layers were at 42 nm and 55 nm, respectively. The ultrasonically sprayed layers ofTenent et al. have a very low average roughness of 3 nm being close to sputtered ITO (Tenent et al. 2009).It has to be noted though that for today’s OLEDs, vacuum-evaporated ITO is preferred over sputtered ITOdue to its average roughness well below 1 nm.

Optical PropertiesFor display applications, transmission losses in the visible regime between roughly 400 and 800 nmshould be as low as possible. Additionally, an even transmittance over all wavelengths is beneficial forfull-color applications. While for ITO, the transmittance is decreased toward smaller wavelengths leadingto a yellowish color, coatings of unsorted, polydisperse CNTs have a quite even transmittance curve in thevisible spectrum acting almost like a gray filter (Trottier et al. 2005; Schindler et al. 2007) (see Fig. 2).

Table 1 Mechanical stress tests of CNT/acrylic binder and ITO on 125 mm PET film (Data taken from (Arthur et al. 2004))

Test ITO CNT

Uniaxial strain First cracks at 2.5 % strain catastrophicfailure <5 % strain

14 % resistance increase up to 18 % strain

Cyclic rollingd = 19.1 mm, 300�

First cracks <100 cycles catastrophicfailure at � 6,000 cycles

Almost unchanged <27,000 cycle catastrophicfailure at � 32,000 cycles

Folding, 4 kg weightTEC inside

>100 % increase in resistance <5 % increase in resistance

Abrasion 60 cycles �8,000 % increase in resistance <200 % increase in resistance


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For higher film thickness, the lower wavelength spectrum is also absorbed more strongly, however, not asdominant as for ITO. PEDOT:PSS on the other hand has a more bluish color.

The transmission losses in the visible regime of CNT layers are dominated by absorption caused byintraband electronic transitions between van Hove singularities (see section “Structure and ElectronicProperties of Carbon Nanotubes” in ▶CNT-TFTs) of the metallic and semiconducting nanotubes(Wu et al. 2004; Barnes et al. 2007). Since the distance between the van Hove singularities depends onthe CNT diameter, the absorption characteristic of a larger range of CNT diameters as common in mostSWNT feedstock is advantageous for color neutrality. The color could further be tailored by usingnanotube suspensions with sorted diameters which would however strongly increase material costs(Green and Hersam 2008).

Another drawback of ITO is its high refractive index in the order of er� 1.9� 2.0 in the visible regimeleading to increased reflective optical losses because of poor index matching (Yan et al. 2009). Thetransmission losses of CNT layers on the other hand are mainly caused by absorption (Barnes et al. 2007).Park et al. report an extracted refractive index for CNT TECs of er� 1.6, which is very close to plastic andglass substrates (Park et al. 2008). They were however not able to determine the wavelength dependency.Due to the small diameters of nanotubes and bundles, haze is usually below 1 % (Hecht et al. 2009).

Besides the purely optical properties, the highest interest lies in the electro-optical performance, i.e., theoptical transmittance T in relation to the sheet resistance R□ in [O/□]. Both values are, for a givenmaterial, determined by the layer thickness. A thin layer gives a high transmittance but also an increasedsheet resistance and vice versa. In accordance to the application, a compromise has to be found. Layers of

different thickness can be compared by using the figure of merit F, which is defined asF ¼ T10

R□in O�1� �

,

where T is the transmittance in % of the thin film at 550 nm (see section “Generalities About TCOs” inchapter “▶ Indium Tin Oxide (ITO): Sputter Deposition Processes”).

Another common figure of merit to compare the performance of thin TECs is the ratio of the electricalDC conductivity and optical conductivity sDC/sop. As derived in section “Optoelectronic Properties” of▶CNT-TFTs, it can be correlated to T and R□ by the following formula:

100 %

98 %

96 %

94 %Tr

ansm

ittan

ce (

%)

92 %

90 %

88 %

86 %

84 %

82 %

80 %400 450 500 550 600 650

Wavelength (nm)

CNT 450 Ω /

CNT 230 Ω /

700

ITO 200 Ω /

PEDOT 250 Ω /

Fig. 2 Visible light transmittance of ITO, PEDOT, and CNT films (Reprinted with permission from (Trottier et al. 2005).Copyright 2005, Society for Information Display)


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http://link.springer.com/SpringerLink:ChapterTarget



T ¼ 1þ 188OR□

sopsDC

� �

Electrical PropertiesIt was found early on by theory and experiment that individual SWNTs exhibit very high charge carriermobility. In macroscopic CNNs, the conductivity is however limited by the intertube resistance of touchingCNTs and bundles.While between all nanotube types charge carriers are exchanged via tunneling, the transferbetweenmetallic and semiconducting tubes is additionally hindered by a Schottky barrier (Fuhrer et al. 2000).

In recent years, a better understanding of the conduction in CNNs was established and routes forimprovement were introduced. Besides obvious factors like having high-quality and purified raw mate-rial, several factors can increase the conductivity. Since conduction is limited by intertube resistance, longnanotubes are preferred (Hecht et al. 2006). Also a smaller bundle diameter or preferably individualSWNTs give lower junction resistance (Nirmalraj et al. 2009). The complete removal of residualsurfactant by acid treatment can also lead to a better conductivity (Geng et al. 2007). Several groupshave however demonstrated that the main effect of treatment with acids like HNO3 or SOCl2 is not theremoval of unwanted species but electrical p-type doping of the nanotubes (Dettlaff-Weglikowskaet al. 2005; Skákalová et al. 2005; Parekh et al. 2007). The presented doping methods for nanotubeswork however not like in silicon technology where individual atoms in the lattice are replaced by dopantsof another species. Molecules from gas, liquid, or solid phase only bond weakly to the surface of thenanotubes. Such redox dopants were found to increase the delocalized carrier density. But more impor-tantly, the barrier for intertube conduction is lowered by doping (Barnes et al. 2007, 2008; Nirmalrajet al. 2009). Besides intentional doping, CNNs processed under ambient air are p-doped to a certaindegree by oxygen (Collins et al. 2000; Mowbray et al. 2009). As the dopants are only weakly bonded tothe nanotubes, they can be repeatedly desorbed by thermal treatment (Barnes et al. 2008), alternativelyhydrazine (NH2NH2) doping reverses the effect (Blackburn et al. 2008). This makes longtime stable dopingchallenging. Stabilization of the dopants was demonstrated by using capping layers like PEDOT:PSS(Jackson et al. 2008). Strong and stable doping can however also be achieved by evaporated thin filmslikeMoOx onwhich the CNN is deposited afterward (Hellstrom et al. 2012). The as-deposited films show animprovement in conductivity by a factor of 2. A factor of 5–7 was achieved after annealing at 450 �C for 3 h.

While for several years it was propagated that purely metallic CNNs would give an optimal conduc-tivity, the recent availability of highly enriched material led to analyses with unexpected results.Blackburn et al. discovered that redox-doped semiconducting CNNs have a higher conductivity thanmetal-enriched films (Blackburn et al. 2008). The cause can be explained by looking at the density ofstates (DOS) over energy plot in Fig. 3. This plot shows the number of states at each energy level that canbe occupied by a charge carrier for twom-SWNTs and two s-SWNTs. The bandgap of the semiconductingnanotubes is defined by the region where no states are allowed, while metallic nanotubes have states ateach energy level. For a more detailed discussion of the density of states of carbon nanotubes and thecharacteristic peaks called van Hove singularities, please see section “Structure and Electronic Propertiesof Carbon Nanotubes” of chapter “▶Carbon Nanotube TFTs.” In the intrinsic case, i.e., without doping,the Fermi level Ef lies at the energy level in the symmetric center of the plot. Ef is the energy level wherethe probability of a state to be occupied by an electron is 0.5. Above (for higher energies), the probabilitydecreases, while it increases below. This means that in the intrinsic case, m-SWNTs are conducting, whiles-SWNTs are not due to a lack of free electrons in the conduction band or holes in the valence band. This isbecause there are no allowed states or the probability for the existing states to be occupied is too low. Byp-type doping, Ef shifts toward the valence band. With sufficient doping, the Fermi level falls into the van


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Hove singularities outside of the energy gap, leading to a higher density of free charge carriers ins-SWNTs than in m-SWNTs. Additionally, the intertube barrier is lowered more efficiently for junctionsof s-SWNTs (Blackburn et al. 2008). A possible drawback of the p-type doping is the simultaneousincrease in the work function (Jackson et al. 2008). In organic solar cells or organic light-emitting diodes,it can however also be of advantage.

Another important factor for display applications is the nanotube/metal contact resistance. High workfunction metals like Pd, Au, Mo, Ag, and Ti are commonly used to contact carbon nanotubes. Recentlyreported values for the contact resistance to CNNs lie in the order of several 10 mO/cm2 (Schindleret al. 2009; Jackson and Graham 2009). For Ag, e.g., the reported specific contact resistance to ITO is8 mO/cm2 compared to 20 mO/cm2 for undoped CNNs. Lim et al. reported that besides the work function,the wettability of the metal on top of the nanotubes also plays an important factor (Lim et al. 2009).

Performance Overview and Display ApplicationsThe previous sections make clear that there exist many flavors of carbon nanotube processing, even for thelimit field of applications discussed in this chapter. To give an overview over the achievable electro-optical performance, Table 2 lists results of recent publications from research and industry with a variety

of deposition and doping techniques. Figure 4 illustrates the results in plots of R□ versus T andsDCsop

versus T grouped in dry and wet deposition with and without doping.

The figure of meritsDCsop

reaches from roughly 4 to 64 depending on the chosen process and materials.

For comparison sputter-coated ITO on glass can reach values close to 300 (R□ = 12 O/□, T = 90 %,

thickness = 150 nm). ITO on plastic for touch applications stays however well below withsDCsop

�10

(R□ = 250 O/□, T = 87 %, thickness = 15–20nm) (see chapter on ITO). Silver nanowires can achieve

record values ofsDCsop

> 200 after thermal annealing.

Fig. 3 Calculated density of states for (17,0) and (10,5) s-SWNTs and (10,10) and (8,8) m-SWNTs, representative of the1–1.4 nm diameter distribution produced by laser ablation, plotted on an absolute energy scale, versus the normal hydrogenelectrode (NHE) and versus vacuum. Horizontal lines show the approximate Fermi level following intentional (hydrazine andthionyl chloride) chemical treatments and unintentional oxygen adsorption (Reprinted with permission from (Blackburnet al. 2008), DOS for m-SWNTs were slightly corrected. Copyright 2008 American Chemical Society)


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While dry processed CNNs generally show a higher figure of merit, the highest value was achieved bywet processing. Doping is a key factor in achieving a good overall performance. It has to be notedhowever that most of the used doping methods are prone to degradation under severe climatic testing. Thenumber of dopingmethods is however continuously increasing with special attention to longtime stability.

Table 2 CNT TEC performance for different deposition techniques from recent publications

CitationCNTtype

Deposition

Dispersion R□ [O/□] T (%) sDC/sop Dopant SubstrateType Method

(Anisimovet al. 2014)

CanatuCNB™

Dry Direct DryPrinting®

– 100 94 59.8

(Reynaudet al. 2014)

Floatingcatalyst

Dry Dry transferfrom filter

– 224 90 15.5 None Quartz

(Cuiet al. 2014)

Floatingcatalyst

Dry Transfer – 417 89 7.4 None

(Reynaudet al. 2014)

Floatingcatalyst

Dry Dry transferfrom filter

– 86 90 40.4 AuCl3 Quartz

(Kaskelaet al. 2010)

Floatingcatalyst

Dry Transfer – 110 90 31.6 HNO3

(Hechtet al. 2011b)

CVD Wet Spraycoating

Surf. aq. 650 86 3.7 None PC

(Schindleret al. 2009)

HiPCO Wet Spraycoating

Surf. aq. 400 80 4.0 None PES

(Schindleret al. 2009)

HiPCO Wet Spraycoating

Surf. aq. 1,000 90 3.5 None PES

(Nirmalrajet al. 2009)

Arcdischarge

Wet Vacuumfiltration +transfer

Surf. aq. 110 78 12.9 None PET

(Mirriet al. 2012)

HiPCO Wet Dip coating Acid 100 90 34.8 H2SO4

(Hechtet al. 2011a)

CVD Wet Vacuumfiltration +transfer

Acid 60 91 64.1 HSO3Cl PET

(Ostfeldet al. 2014)

CVD Wet Spraycoating

Ammonia 311 93 16.4 HNO3

(Hellstromet al. 2012)

Wet Spraycoating

Solvent 440 87 5.9 MoOx Glass @220 �C

(Hellstromet al. 2012)

Wet Spraycoating

Solvent 100 85 22.2 MoOx +450 �C

Glass @220 �C

(Willeyet al. 2014)

Laserablation

Wet Ultrasonicspray

Solvent 167 77 8.2 HNO3 Glass @200 �C

(Li et al. 2014) CVD Wet Spraycoating

Surf. aq. 240 90 14.5 HNO3 PET

(Gaoet al. 2013)

Arcdischarge

Wet Spraycoating

Surf. aq. 86 80 18.5 HNO3/SOCl2

PET

(Nirmalrajet al. 2009)

Arcdischarge

Wet Vacuumfiltration +transfer

Surf. aq. 37 76 34.5 HNO3 PET

Surf. aq. Aqueous surfactant dispersion


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Although CNT networks were propagated as ITO replacement for display applications in 2003(Glatkowski 2003), it took several years before the first prototypes were presented. The first intermediatestep was the presentation of a single pixel polymer-dispersed liquid crystal (PDLC) cell on plastic byChan-Yu-King et al. (Chan-Yu-King and Roussel 2007). A PDLC display showing real content viadirectly addressed segments was presented shortly after (Schindler et al. 2007). The realization of PDLCdisplays is straightforward since no alignment layers are necessary. They are however not relevant incommercial products due to low contrast ratios and high addressing voltages. These displays only consistof sandwiched glass or plastic substrates with patterned electrodes and the liquid crystal in polymer matrixin between. For the commercially dominant twisted nematic (TN) LCDs, an alignment layer for theorientation of the liquid crystal molecules is necessary. By using a polymer capping layer on the nanotubenetwork that is rubbed after curing, Schindler et al. demonstrated reliable LC orientation even onrelatively rough CNT networks. By increasing the polymer thickness, a complete planarization is possiblethough. In 2008, the group demonstrated the first full-color AMLCD where ITO was completely replacedby CNNs (see Fig. 5a) (Schindler et al. 2008a, b). The fact that the CNNs were integrated as pixelelectrodes into a standard backplane and as common electrode in the color filter process clearlydemonstrates the applicability of this new material into state-of-the-art display processes. In 2009 directlyaddressed flexible TN LCDs on plastic substrates using transparent CNT electrodes were presented (seeFig. 5b) (Schindler et al. 2009). Samsung# in collaboration with Unidym# introduced CNT TECs into

Fig. 4 Scatter plot of data presented in Table 2. Left: R□ versus T. Right: sDC/sop versus T


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electrophoretic displays and demonstrated a 14.3 in. full-color e-paper display at the 8th InternationalMeeting on Information Display (IMID) in 2008.

Carbon nanotubes can however not only be used as ITO replacement in LCDs. They can even serveadditional purposes. Fu et al. used aligned nanotube arrays as TEC and liquid crystal alignment layer at thesame time, superseding the rubbing process (Fu et al. 2010). Lee et al. on the other hand integrated carbonnanotubes into a classically processed polymer alignment layer and achieved a fourfold decrease in LC,switching time and low-power operation due to the locally enhanced electric field around the nanotubes(Lee et al. 2011).

Besides several LCD applications, the use of transparent CNT electrodes in electroluminescentmodules (Yu et al. 2009; Sloma et al. 2014) and in organic light-emitting diodes was also investigatedby several groups. The nanotube networks were used as hole-injecting anodes (Aguirre et al. 2006;Li et al. 2006; Zhang et al. 2006; Williams et al. 2008; Schindler et al. 2008b) or for both anode andcathode (Liu et al. 2009). Brightness levels as high as 9,000 cd/m2 and current efficiencies up to 10 cd/Awere reported on slot die-coated CNT films (Ou et al. 2009). The hole-conducting nature of the nanotubesis not only propagated for hole injection in OLEDs but also as efficient hole collector in organicphotovoltaics (Tenent et al. 2009).

The first commercial applications gain ground in touch modules as the electro-optical performance ofCNT TECs is comparable or even surpassing sputter-coated ITO on plastic in this sector while clearlyoutmatching it in bendability. Due to the high stretchability and thermal stability of the CNNs, they canfurther be directly molded into arbitrarily curved surfaces by injection molding or film insert molding(Anisimov et al. 2014). The lower refractive index further results in higher contrast screens. Wetprocessed, coated substrates were introduced into the market by Unidym# while Canatu# is rampingup its dry process. Foxconn# has also introduced carbon nanotubes into mobile display touch modules injoint collaboration with Tsinghua University in China.

Conclusions and Future Prospects

Much progress in the realization and optimization of transparent conducting films consisting of randomlyoriented carbon nanotube networks was made in the last years. Both dry and wet deposition techniques,capable of homogeneous large-area deposition, were demonstrated, and detailed studies of the networkled to a deeper understanding of the conduction and metal/CNTcontact. This also led to improvements inthe conductivity by doping. The newfound CNT sorting techniques by diameter, length, and electronictype give further possibilities to tailor the material for a certain application.While, interestingly, s-SWNTs

Fig. 5 Twisted nematic LCD prototypes with carbon nanotube pixel electrodes. (a) Full-color 4 in. qVGA AMLCD and(b) flexible directly addressed display on plastic (Schindler et al. 2009)


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are proposed to give the best conductivity after doping, m-SWNT networks can have a lower workfunction.

The applicability of CNT networks as ITO replacement in liquid crystal displays was demonstrated byseveral prototypes, and cooperation with major display panel manufacturers manifests the interest of thedisplay industry. OLEDs were also realized with CNT TECs. The publications are limited to singledevices however. Real displays were not demonstrated so far and due to a generally lower conductivitycompared to ITO CNNs seem so far less favorable for current-driven OLED applications. The firstcommercial products have entered the markets in the form of touch modules for mobile displays. In thissegment CNT TECs can compete with the electro-optical performance of ITOwhile enabling new degreesof freedom in design due to bendability, stretchability, and improved contrast.

Even though much progress in increasing the conductivity of CNT networks was made in recent years,the values of ITO were not achieved so far. This makes CNTs less qualified for high-current applicationslike OLED lighting. In voltage-driven applications like LCDs and especially in active-matrix displayswith only low aspect ratio pixels and common electrodes even with increased resistance, well-workingdisplays can be realized.

The great advantage of CNT TECs over ITO lies in their flexibility and mechanical as well as chemicalrobustness combined with low-temperature vacuum-free coating methods that are compatible with plasticsubstrates. Applications like flexible displays, touch-sensitive devices, and flexible or even stretchableelectronics in general will therefore be the most likely fields where transparent carbon nanotube coatingshave the chance to enter commercialization.

The large variety in material and processes is boon and bane of this evolving technology. This mightlead to diverse well-adapted working flows for different applications, and at the same time it slows downthe efficient optimization. At the current state of research, it is not clear whether dry deposition or wetdeposition techniques will prevail. Both methods seem to be able to coat large volumes of films.While drydeposition intrinsically gives higher material purity and longer CNTs, the wet processing has moreoptions for non-subtractive patterning. With methods like gravure coating, high-volume productionwith low investment and deposition costs would be possible. This would firstly allow for simple lowinformation content but high-volume products like price tags or even decorative displays in clothing.Withevolving manufacturing techniques, also high-resolution displays seem possible.

Besides flexible applications, CNT TECs will have to bring significant cost reduction to capture someof ITO’s market share. Even though wet deposition techniques might lead to savings in investment anddeposition costs, it can so far not be estimated if the entire coating costs per area will be lower than for ITOor even competitive. In today’s display fabrication, more than two thirds of the costs for an ITO coatingare caused by material costs, mainly because of the high indium price (compare chapter “▶ Indium TinOxide (ITO): Sputter Deposition Processes”). Therefore, machine costs only play a secondary role. Thecurrent bottleneck in commercial carbon nanotube production is the growth itself. Many establishedmethods do not allow for high-volume production of high-quality material in terms of purity, defects, anddiameter distribution. The direct production costs today lie in the order of a few hundred dollars per gram.It seems however reasonable to predict that when the demand is growing fast enough to allow forsignificant scale-up, the costs can quickly be reduced by about an order of magnitude.

Post-synthesis processing and the methods of separation by electronic type in particular have evolvedrapidly in recent years (compare “▶ Sorting of Carbon Nanotubes by Electronic Type” of chapter“▶Carbon Nanotube TFTs”). Further improvements will keep the additional costs low so that the overallprice will be dominated by the material. It is worth noting though that as long as the starting material willhave the classic ratio of one third metallic to two thirds semiconducting nanotubes, the cost will at least beincreased by 50 % for s-SWNTs and by 200 % for m-SWNTs.


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Summarizing it can be said that carbon nanotube TECs have a very high potential, especially when itcomes to flexible displays. In this evolving new market, they have a clear edge over the commonly usedITO. The technology is however still very young, and improvements in several areas are still necessary,especially in growth methods and deposition techniques that use the full potential of the wet processingcapabilities. The demonstrated prototype displays have successfully demonstrated the applicability of thisnew material to display applications.

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