Optical-Electronic Properties of Carbon-Nanotubes Based Transparent Conducting Films ()
1. Introduction
Transparent conductive film or glass is a key component for many optical-electronic devices such as organic light emitting diodes (OLED), organic photovoltaic solar cells (OPV), liquid crystal display (LCD) panels and touch panels, just to name a few. The most critical requirements for transparent conductive film or glass are low sheet resistance and high transparency. Many materials were considered suitable for making transparent conductive film or glass [1-4]. Owing to manufacturing and quality requirements, only indium tin oxide (ITO) film or glass has been commonly used for optical-electronic devices in the current market [5]. Despite its popularity, indium is a rare-earth material, and its price is high [6]. Furthermore, ITO films or glass has to be produced with a vacuum deposition technology. This technology is relatively expensive compared to conventional wet coating processes. Hence, many competitive approaches have been sought to replace ITO film or glass using different materials and coating methods.
Low-cost wet coating processes have been found to be promising in coating nano-scaled conductive media such as silver nanowires [7] or carbon-nanotubes [8-11] on PET film. Silver nanowires can be produced through an efficient chemical approach [12]. However, issues such as how to disperse silver nanowires and reduce their diameter for lower resistance still have to be resolved. Conductive film or glass coated with carbon-nanotubes (CNT) has similar optical-electronic properties, but is also hindered by the dispersion problems. Several effectiveapproaches have been attempted to overcome this issue for multi-walled and single-wall CNT [13-15], but the single-wall CNT (SWCNT) appeared to give better performance [16].
Spin coating is usually applied as a convenient means to prepare samples for laboratory analysis. However, usually over 90% of the coating solution is wasted, and is therefore not suitable for mass production. Recently, several researchers considered different coating methods for CNT solutions. Kim et al. [17] applied spin and spray coating methods for CNT electrode to make organic solar cells. de Andrade et al. [18] compared different technologies for the preparation of CNT networks. They concluded that dip coating and electrophoretic deposition are promising methods for solar cell application. In the present study, three different coating methods were used to make transparent conductive film with a well-dispersed single-wall CNT solution, and the optical and electronic properties of the samples were measured and compared.
The optical requirement for conductive films is that it must be over 85% transparent. The sheet resistance may however vary, depending on special applications. In the present study, it is chosen to be 1000 ohm/sq, which meets the requirement of electrostatic dissipation. The results presented here would be useful for future mass production considerations.
2. Experimental
2.1. Preparation of SWCNT Solution
Single walled carbon nanotubes (SWCNT) were prepared by the floating method in a vertical tube reactor [19] by using alcohol as the carbon source. The alcohol solution with a given composition of ferrocene and thiophene was introduced into the reactor with hydrogen as the carrier gas. The typical reactor temperature was between 1000˚C - 1200˚C. The SWCNT produced were purified by combining two-step processes of thermal annealing in air and acid treatment [20]. The SWCNT produced had average diameters around 2 to 2.5 nm, with purity >90% based on TGA and G/D ratio (Raman) around 35. The aqueous SWCNT dispersion was prepared by ultrasonication using a tip sonicator with sodium dodecyl benzene sulfonate (SDBS) as surfactant. The concentrations of the SWCNT was 0.1% and the ratio of SWCNT to SDBS was 1:1.5.
2.2. Coating Methods
Three coating devices were selected for making samples. The first device was a laboratory blade coater (Zehntner, ZUA 2000), with a minimum coating gap of 5 μm, as shown in Figure 1. The second was a dip coating device shown in Figure 2. A machine arm was attached to grab and lift the sample upward from a solution tank. The coating speed could vary between 0.1 - 2 cm/s. The last was a slot die coater as shown in Figure 3(a). The slot die was attached to the mount of a patch coater as shown in Figure 3(b). The coating solution was delivered by a piston pump (KD scientific, KDS 100) through the slot die, and then coated on the substrate which was fixed on the marble platform of the patch coater.
2.3. Measurements
All the test solutions were coated on the polyethylene terephthalate (PET) films for analysis. A base coat was necessary to prevent the aggregation of SWCNT solution [21]. The PET film was cut into a rectangular shape, 10 cm × 15 cm. All physical properties were measured at fixed positions on the films as marked in Figure 4. The coated samples were placed in an oven and heated at 90˚C for 5 minutes. Two major properties of the ovendried samples were measured. A four-point probe (MCPT600) was used to detect the sheet resistance of the sam-
Figure 3. Photos of (a) the experimental slot die with a shim; (b) the patch coater.
Figure 4. Sample dimensions and marked position for property measurements (10 × 15 cm2).
ples, andan UV-visible spectrophotometer (Varian Cary 50 conc) was used for transparency measurements. The uncoated PET film was used as the reference for comparison. Standard processes were taken to detect the distributions of CNT on the PET films by the scanning electronic microscope (JEOL JSM-5600).
3. Results and Discussion
The SWCNT solutions were coated on the PET substrates by the three coating methods. The transparency and sheet resistance of each coated sample were measured and analyzed.
The results obtained on slot die coating are presented first. In order to produce a very thin wet thickness on the slot die coater, the concentration of the SWCNT solutions must be reduced to 0.1%. This yielded a dry film with thickness as low as 5 nm. The dry film thickness t reported here is an average value which depends on flow rate, coating speed and solid content, t can be evaluated with the following formula:
(1)
here Q is the volumetric flow rate, V is the coating speed, W is the coated width and S% is the solid content. Figure 5 presents the results of transparency and sheet resistance as a function of dry film thickness at the coating speed 10 cm/s. The results indicate that both the sheet resistance and transparency decrease as the dry film thickness increases. The increase in dry thickness is due primarily to the increasing amount of CNT accumulated on the PET substrate. Hence, it is expected that both the sheet resistance and transparency will decrease.
The effects of the coating speed on the sheet resistance and transparency for two different dry film thicknesses, 5 nm and 10 nm, are displayed in Figure 6. It is seen that both transparency and sheet resistance are independent of the coating speed for the thinner film; whereas these two
Figure 5. Sheet resistance and transparency as a function of dry film thickness for samples made by slot die coating (●: sheet resistance (ohm/sq); ○: transparency (%)).
properties decrease slightly as the coating speed increases for the thicker film. The sheet resistance stays around 2000 ohm/sq, and provides a transparency of around 95% for the 5 nm film; where the resistance drops to below 1000 ohm/sq, transparency reduces to about 92% for the 10 nm film when the coating velocity increases from 6 to 10 cm/s. At low coating speed, the coating solution emanating from the slot die exit will expand laterally, but the coating width will contract as the coating speed increases. The lateral movement of coating solution changes the CNT distribution, and affects the two properties. It is noted that the lateral expansion ceases to exist at high coating speed.
Figure 7 shows the results obtained with a blade coater having a coating gap of 20 μm. The wet film thickness for blade coating is usually less than 50% of the blade gap for dilute Newtonian solutions [22]. The average dry film thickness can be evaluated if the solid content is known, which is around 7 - 8 nm in present study. It can be seen that both the sheet resistance and transparency decrease markedly as the coating speed increases. The transparency drops from 95% to 91%, and