Suitability of Nanotube Films Depends on How They Are Made
Daniel S. Burgess
At the University of Southern California in Los Angeles, scientists have reported that not all carbon nanotubes are equal as replacements for the transparent ITO electrodes in organic LEDs (OLEDs) and other photonic devices, such as organic photovoltaic cells. Comparing the characteristics of films of nanotubes produced using two common commercial methods with the performance of OLEDs incorporating the films, they discovered that films made of one type of nanotube were significantly superior to those made of the other type in terms of surface roughness, conductance and the resulting device lifetime.
Researchers compared the performance of two types of carbon nanotube films as replacements for ITO-based hole-injection electrodes in organic LEDs (OLEDs). As shown in the diagram (top), they constructed OLEDs that comprised films made from commercially available nanotubes, a buffer layer of PEDOT, a hole-transport layer of NPD, an emission layer of Alq3, and top cathodes of LiF and aluminum. The micrograph (bottom) shows a film of nanotubes grown by the arc-discharge method. Films of nanotubes produced by this technique outperformed those produced by the high-pressure catalytic decomposition of CO. Reprinted with permission of Nano Letters.
Chongwu Zhou, an associate professor in the university’s department of electrical engineering, explained that electrically conductive films of carbon nanotubes are of interest as an alternative to ITO for economic reasons as well as for matters of performance, such as their potential to enable the production of large, flexible light-emitting arrays for display and general lighting applications. Although indium is more common in the Earth’s crust than other metals important for industry, such as mercury and silver, not all of the ores in which it appears are suitable for refining, so global production is limited, he noted.
As a result of the increased demand for ITO coatings with the expansion of the flat panel display market, the average price of a kilogram of indium soared from approximately $100 in 2001 and 2002 to more than $800 in 2005. Despite some recent successes in recycling, supplies of indium are expected to remain tight, according to a 2006 report from the US Geological Survey in Reston, Va., and this will continue to affect the expense of manufacturing anything with indium in it.
“The cost of ITO is already a significant proportion of products like organic light-emitting displays and liquid crystal displays,” Zhou said. Given the abundance of carbon on Earth and the expectation that the costs of manufacturing nanotubes will fall dramatically with increased demand, using nanotube films rather than ITO promises to reduce the expense of devices that feature transparent conductive films.
The scientists investigated the properties of films made of base materials generated by the arc-discharge method and by the high-pressure catalytic decomposition of CO, known as HiPCO. Both processes are used to mass produce carbon nanotubes. The researchers employed a vacuum filtration technique to create the films, which they then transferred to glass or flexible polyester substrates by press printing.
They discovered that the films of arc-discharge tubes were much smoother than those made of HiPCO tubes — with average surface roughness of 7 and 11 nm, respectively, for 40-nm-thick films. The latter tended to feature inhomogeneities that the researchers attributed to impurities and to the bundling of the tubes in the source material. At a comparable optical transparency, the arc-discharge films displayed a sheet conductance that was more than an order of magnitude better than the others, which they also attributed to the incidence of impurities as well as to the dimensions of the base nanotubes.
To evaluate the suitability of the arc-discharge films as a replacement for ITO in OLEDs, the investigators first coated the films with PEDOT to further reduce the surface roughness and doped the material with SOCl2 to improve the sheet conductance. After they were spin-coated with the PEDOT buffer layer, the arc-discharge films exhibited an average surface roughness of 3.1 nm, versus 2.4 nm for ITO.
OLEDs incorporating the optimized nanotube film had a lifetime of four to five hours — the limit of the measurements in the experiments — compared with less than 30 seconds for previously demonstrated OLEDs that employed films of HiPCO-grown tubes. But the current density and brightness of the new devices were still one to two orders of magnitude less than those of OLEDs that employ ITO. The scientists are confident, however, that optimization of the materials and the device structure will result in OLEDs using nanotube-based hole-injection electrodes that will perform comparably to those that incorporate ITO.
Zhou said that the degree of disparity in the performance of the nanotube films was surprising and that he hopes factors such as roughness that determine the suitability of the films for use in OLEDs will receive greater attention by the research community. He added that most of the organic molecules used in OLEDs are selected based on their compatibility with ITO and that this work points to a need for studying alternate materials optimized to function with nanotube films.
Nano Letters, online Aug. 12, 2006, doi:10.1021/nl0608543.
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