**7. Application of CNTTCFs**

substrate interactionis crucial for uniform drying of the liquid. The most useful deposition technique is roll to roll coating of CNT inks onto continuous rolls of plastics. This technique can coat film up to 2 m wide at speeds up to 500 m/min.One such roll-to-roll coating line running continuously would have the equivalent output of 30 traditional sputter coaters, and could produce enough film to satisfy half of the available touch panel market. Examples

**Figure 10.** a) Transparent CNT film pulled from vertically grown CNT forest; b) CNT film transferred to PET using PDMS stamp. c) CNT film spray coated onto large areaplastic; d) Mayer rodcoating schematic. e) Image of CNT film being coated by slot die f) Roll of printed CNTfilm. g) Inkjet printed CNT lines. Reprinted with permissions from Ref. [4] copy‐

During the preparation of CNT water solutions, dispersants are always introduced to assis‐ tant the exfoliation of CNT bundles. Since these dispersants are insulating, their residue decrease the conductance of CNT films significantly. Hence, post-treatments to remove these disper‐ sants are necessary for preparing TCFs with high performance. In addition to remove the dispersants, doping is the other goal of post-treatment. In addition to rinsing with water, acid treatment is the most commonly used method to post-treat CNT films. As reported by Geng, <sup>11</sup> the sheet resistance of CNT films reduced by a factor of 2.5 times after treatment in concen‐ trated nitric acid owing to the removal of surfactants SDS. Except their function on remov‐ ing dispersants, concentrated nitric acid is often used to p-dope CNTs and enhances their conductivity [70]. Although nitric acid was effective to remove dispersants, they induced pdoping of CNTs, which will lead to instability of the films [71]. Besides, PET substrates will turn brittle after long time acid treatment. To solve this problem, Dr Sun's group developed a novel technique combing base treatment and short time acid treatment [72]. In their work, biomolecule RNA was chosen was the dispersant since they are easily degraded by base, acid

of various film fabrication methods were shown in Figure 10.

328 Syntheses and Applications of Carbon Nanotubes and Their Composites

right Wiley

**6. Post-Treatment of CNT Films**

CNTTCFs have found a range of applications, among which we focus on the touch screens, plat panel displays, solar cells and OLEDs.

Touch screen is almost omnipresent in our daily life, such as in cell phones, tablet computers and many other electronics. Transparent electrodes are an essential component in most types of touch screens. High optical transmittance (> 85%) and low sheet resistance Rs (< 500 Ω/sq) are normally needed for touch screens. Meanwhile, extremely excellent durability, flexibility, and mechanical robustness are required given that the touch screen may be under indentation for millions of times. The mechanical robustness demonstrated by CNT touch panels give promises for increasing the lifetime and durability of current touch screens. There are a variety of touchscreen technologies that sense touch in different ways.Figure 11a shows the basic device structure and the transparent conductor arrangement for a 4-wire an‐ alog resistive touchpanel. These panels use two continuous electrodes separated by hemi‐ spheres of polymeric "spacer dots" that are10–100 µm in radius and 1–2.5 mm apart. Only at the edges (where electrode attachment occurs) is the transparent electrode patterned. Sur‐ face capacitive devices share the same type of continuous conductor whereas the projected capacitive deviceuses transparent conductors with specific patterning into predefined geo‐ metries. Resistive touch panels function by current driven measurements andcapacitive de‐ vices depend on capacitive coupling with the input device. Both panel types utilize signal processing controllers todetermine X-Y and sometimes Z position of inputs.

The mechanical durability of the transparent conductors is very important for resistive touch panels, since it involves compressive, sheer, and tensile stress every time it works. Their working process can be summarized as [4]:


Compressive stress is not required to activatethe projected capacitive (ProCap) touch panels (of which theiPhone is a prime example). The ProCap touch panels are activated by a capaci‐ tive coupling with a suitable input device. Thus, there willnot be the mechanical flexing is‐ sues in ProCap devices. Still, the mechanical properties of the conducting layer are important since the conductors may be patterned to a size assmall as 10 µm in width. Metal oxides patterned to such small dimension become susceptible to cracking, fractures,and thermal cycling stress.

DOT:PSS coating dramatically improves the device efficiency from 0.47% to 1.5%. The thin layer of PEDOT:PSS can smooth the CNT surface and enhance the charge transfer according to their investigation. In Hu's work, [75] flexible transparent electrodes were fabricated by printing SWCNT solutions on plastic substrates. The SWCNT films have a sheet resistance of 200 Ω/sq with a transmittance of 85%. The achieved efficiency of 2.5% (AM1.5G) ap‐ proaches that of the controldevice made with ITO/glass (3%). Furthermore, the flexibility is far superior to devices using ITO coated on the same flexible substrate material. However,

Carbon Nanotube Transparent Electrode http://dx.doi.org/10.5772/51783 331

there are several aspects that need to be solved for CNT based electrodes.

**2.** Occasional shorting betweenthe cathode and anode due to protruding CNTs;

Light emitting diodes have an opposite light electricity coupling process as solar cells. Ap‐ plications of nanoscale materials based transparent electrodes are mainly focused on organic light emitting diodes which hold great promise for the future electronics. In Aguirre's work, carbon nanotube anodes were implemented in small molecule OLED devices and achieved performance comparable to ITO-based anodes [76]. Recently, Feng et al [77] proposed a sin‐ gle walled carbon nanotubes-based anodes for organic light-emitting diodes (OLEDs) by spray-coating process without any use of surfactant or acid treatment. A layer of DMSO doped PEDOT:PSS was spray-coated on the SWCNT sheets to not only lessen the surface roughness to an acceptable level, but also improve the conductivity by more than three or‐ ders of magnitude. For the produced SWCNT-based OLEDs, a maximum luminance 4224 cd/m2 and current efficiency 3.12 cd/A were achieved, which is close to the efficiency of ITO-

State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Insti‐

[2] Tyler, T. P., Brock, R. E., Karmel, H. J., Marks, T. J., & Hersam, M. C. (2011). *Adv. En‐*

**1.** Long termelectrical stability;

**3.** Relatively high sheet resistance.

based OLEDs.

**Author details**

and Ranran Wang

\*Address all correspondence to: Jingsun@mail.sic.ac.cn

tute of Ceramics, Chinese Academy of Sciences, China

[1] Niu, C. M. (2011). *MRS Bull.*, 36, 766.

*ergy Mater.*, 1, 785.

Jing Sun\*

**References**

**Figure 11.** a) Schematic of four-wire resistive touch panel operation and functional layers; b) Schematic of the contact resistance experienced at the interface between two rough conductive layers separated by a very thin dielectric; c) Pho‐ tograph of touch panel utilizing CNT film as touch electrode. Reprinted with permission form Ref. [4] copyright Wiley

Display panels are produced at nearly 1.7 billion unitsannually (1.2 billion mobile phones, 200 million televisions,150 million laptops, and 200 million desktop, machine interfaces, monitors etc. There are four common types of displays, which are electrowetting displays (EWD), electrochromic displays (ECD), electrophoretic displays (EPD) and liquid crystal displays (LCD). Currently, LCD devicesare manufactured in the greatest number and will be the mainsubject of this section. A transparent conductor'smajor role in LCD/EPD devices is to serve as pixel and common electrodes. An interesting advantage of using CNT films for LCD is the ability to use them possibly as both the transparent electrode and the alignment layer [73]. Recently, Lee et al demonstrated high performance TN-LC cells with ultra-thin and solution-processible SWNT/PS-*b*-PPP nanocomposite alignmentlayers. At an optimized SWNT density, a nanocompositegave rise to low power operation with a super-fast LC re‐ sponsetime of 3.8 ms, which is more than four times faster than thaton a commercial polyi‐ mide layer due to the locally enhancedelectric field around individually networked SWNTs. Furthermore,TN-LC cells with their SWNT nanocomposite layers exhibited high thermal stability up to 200 o C without capacitance hysteresis.

Transparent electrodes are the essential components forphotovoltaic devices. The traditional electrodes for photovoltaic devices is ITO, which has high transmittance and low sheet re‐ sistance (~10-20 Ω/aq with the transmittance of 90%). However, their application was con‐ strained by the high price of indium. Besides, the brittleness of ITO limited their usage in flexible devices, which will be a developing trend in the future. Therefore, replacing materi‐ als need to be developed. Carbon nanotubes are promising candidates since they have ex‐ tremely high conductivity, high work function of 4.7-5.2 eV, relatively low cost and excellent flexibility. Besides, they are easy to be deposited into film via solution based process. Glat‐ kowskiet al. [74] reported on the application of transparent CNT electrodesand found a PE‐ DOT:PSS coating dramatically improves the device efficiency from 0.47% to 1.5%. The thin layer of PEDOT:PSS can smooth the CNT surface and enhance the charge transfer according to their investigation. In Hu's work, [75] flexible transparent electrodes were fabricated by printing SWCNT solutions on plastic substrates. The SWCNT films have a sheet resistance of 200 Ω/sq with a transmittance of 85%. The achieved efficiency of 2.5% (AM1.5G) ap‐ proaches that of the controldevice made with ITO/glass (3%). Furthermore, the flexibility is far superior to devices using ITO coated on the same flexible substrate material. However, there are several aspects that need to be solved for CNT based electrodes.

**1.** Long termelectrical stability;

sues in ProCap devices. Still, the mechanical properties of the conducting layer are important since the conductors may be patterned to a size assmall as 10 µm in width. Metal oxides patterned to such small dimension become susceptible to cracking, fractures,and

**Figure 11.** a) Schematic of four-wire resistive touch panel operation and functional layers; b) Schematic of the contact resistance experienced at the interface between two rough conductive layers separated by a very thin dielectric; c) Pho‐ tograph of touch panel utilizing CNT film as touch electrode. Reprinted with permission form Ref. [4] copyright Wiley

Display panels are produced at nearly 1.7 billion unitsannually (1.2 billion mobile phones, 200 million televisions,150 million laptops, and 200 million desktop, machine interfaces, monitors etc. There are four common types of displays, which are electrowetting displays (EWD), electrochromic displays (ECD), electrophoretic displays (EPD) and liquid crystal displays (LCD). Currently, LCD devicesare manufactured in the greatest number and will be the mainsubject of this section. A transparent conductor'smajor role in LCD/EPD devices is to serve as pixel and common electrodes. An interesting advantage of using CNT films for LCD is the ability to use them possibly as both the transparent electrode and the alignment layer [73]. Recently, Lee et al demonstrated high performance TN-LC cells with ultra-thin and solution-processible SWNT/PS-*b*-PPP nanocomposite alignmentlayers. At an optimized SWNT density, a nanocompositegave rise to low power operation with a super-fast LC re‐ sponsetime of 3.8 ms, which is more than four times faster than thaton a commercial polyi‐ mide layer due to the locally enhancedelectric field around individually networked SWNTs. Furthermore,TN-LC cells with their SWNT nanocomposite layers exhibited high thermal

Transparent electrodes are the essential components forphotovoltaic devices. The traditional electrodes for photovoltaic devices is ITO, which has high transmittance and low sheet re‐ sistance (~10-20 Ω/aq with the transmittance of 90%). However, their application was con‐ strained by the high price of indium. Besides, the brittleness of ITO limited their usage in flexible devices, which will be a developing trend in the future. Therefore, replacing materi‐ als need to be developed. Carbon nanotubes are promising candidates since they have ex‐ tremely high conductivity, high work function of 4.7-5.2 eV, relatively low cost and excellent flexibility. Besides, they are easy to be deposited into film via solution based process. Glat‐ kowskiet al. [74] reported on the application of transparent CNT electrodesand found a PE‐

C without capacitance hysteresis.

thermal cycling stress.

330 Syntheses and Applications of Carbon Nanotubes and Their Composites

stability up to 200 o


Light emitting diodes have an opposite light electricity coupling process as solar cells. Ap‐ plications of nanoscale materials based transparent electrodes are mainly focused on organic light emitting diodes which hold great promise for the future electronics. In Aguirre's work, carbon nanotube anodes were implemented in small molecule OLED devices and achieved performance comparable to ITO-based anodes [76]. Recently, Feng et al [77] proposed a sin‐ gle walled carbon nanotubes-based anodes for organic light-emitting diodes (OLEDs) by spray-coating process without any use of surfactant or acid treatment. A layer of DMSO doped PEDOT:PSS was spray-coated on the SWCNT sheets to not only lessen the surface roughness to an acceptable level, but also improve the conductivity by more than three or‐ ders of magnitude. For the produced SWCNT-based OLEDs, a maximum luminance 4224 cd/m2 and current efficiency 3.12 cd/A were achieved, which is close to the efficiency of ITObased OLEDs.
