Carbon nanotube based transparent electrodes

• Main Author: Sher Mostafa Somik Khan.
• Other Authors: Zhang Qing
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Nanoelectronics
• Online Access: http://hdl.handle.net/10356/54724
• Institution: Nanyang Technological University

The focus for this dissertation project is on carbon nanotube (eNT) based transparent electrodes. Some of the important applications for transparent electrodes includes electrodes for solar cells, touch screens, solid state lighting devices, displays, electrostatic shielding and so on. Commercially indium tin oxide (ITO) is used for transparent conductive material, however it is very expensive as indium is a rare metal and electrodes constructed using such material are brittle. One potential candidate for replacing ITO can be CNT. Minimum optical and electrical requirement for commercial usage of a transparent electrode is transmittance T > 90% and sheet resistance R, < 100Q/sq, however CNT based transparent electrodes has yet to reach that standard. The key to a large scale application of CNT based transparent electrode is to lower the sheet resistance without sacrificing the transmittance of the film that is to be coated. The objective of this dissertation project is to develop and optimize the fabrication process for CNT based transparent electrodes and to study the factors that may affect its performance. The main factors are: type of substrate, substrate surface condition, centrifugation of CNT suspension, tip ultrasonication, gas pressure, duration of spray coating, temperature used for spray coating process, hard baking temperature, functionalization etc. The performance of the CNT based transparent electrodes are characterized by using UV-vis Spectrophotometer, four-point probe and scanning electron microscope (SEM) and the relationship between sheet resistance and transmittance is established. It was found that longer spray coating time resulted in a lower sheet resistance at the price of lower transmittance as denser SWCNT network could form on the surface of the substrate. Tip ultrasonication before centrifugation resulted is a better SWCNT bundle separation and the resulting CNT solution gave lower sheet resistance with comparatively higher transmittance. Optimum spray coating pressure was found to be 1 bar for an even coat. Making the substrate hydrophilic using piranha solution (H2S04 : H202 = 3 : 1 ) resulted in a 36 times sheet resistance with small sacrifice in transmittance. Glass was found to have lower sheet resistance compared to PET plastic as piranha solution could not be used for PET. Optimum spray coating temperature was found to be 250°C with a horizontal heat plate and 30 minutes of hard baking at the same temperature after deposition by spray coating yielded much higher transmittance with a small decrease in conductance. It also improved the performance of the CNT based transparent electrodes. Functionalization of SWCNT using RN03 and H2S04 lowered the sheet resistance to 10% of its original value while transmittance remained virtually unchanged. The final outcome was a sheet resistance of 463 Q/sq with a transmittance of 85.2% at 650nm wavelength was achieved. The main reason behind using spray coating process as opposed to other processes (such as spin coating) is because spray coating process can be easily adapted to industrial manufacturing processes. The final procedure of this dissertation report can be achieved simply by using a heated conveyer belt and multiple spray heads, a common process used by most manufacturing industries.