Flexible, transparent, conducting coatings are of high commercial value in low-cost, flexible photovoltaic cells and large electronic LED displays. Currently, the carbon nanotubes that make up such coatings are highly conducting, but they absorb too much light in the visible region of the spectrum. This impedes their competition with the transparent conducting material indium tin oxide (ITO). Indium is expensive and in limited supply.
Researchers at Oak Ridge National Laboratory (ORNL) have developed a way to improve transparency by controlling the assembly of nanorods. The composites are assembled into macroscopic networks with high optical transmission and electrical conductivity. The high aspect ratio of these nanorods renders flexibility to their assemblies. Buckypaper membranes made of aggregates of single-walled carbon nanotubes (SWNTs), each containing several hundred SWNTs, can be assembled in randomly aligned networks. The optical transparency of these assemblies is still far from optimal to compete with ITO coatings.
The ORNL researchers have overcome these limitations by realigning the nanotubes in composites for optimal transparency and electrical conduction. They begin with SWNTs of higher purity, to increase transparency. The SWNT bundles are grown longer, which gives them better potential conductivity, so fewer are required. The nanotubes are oriented and treated with solvents to straighten them, providing a smaller number of interconnections and therefore better conductivity and higher transparency.
The researchers have also developed methods for controlling the growth of vertically-aligned nanotube arrays to desired lengths, densities, and wall numbers. Processing methods for two and three-dimensional integration of the nanotubes as electrodes and additives for flexible electronics utilize the multifunctional properties of these novel transparent electrodes.