Uncovering the ultimate performance of single-walled carbon nanotube films as transparent conductors

The ultimate performance - ratio of electrical conductivity to optical absorbance - of single- walled carbon nanotube (SWCNTs) transparent conductive films (TCFs) is an issue of considerable application relevance. Here, we present direct experimental evidence that SWCNT bundling is detrimental for their performance. We combine floating catalyst synthesis of non-bundled, high-quality SWCNTs with an aggregation chamber, in which bundles with mean diameters ranging from 1.38 to 2.90 nm are formed from identical 3 microns long SWCNTs. The as-deposited TCFs from 1.38 nm bundles showed sheet resistances of 310 Ohms/sq. at 90% transparency, while those from larger bundles of 1.80 and 2.90 nm only reached values of 475 and 670 Ohms/sq., respectively. Based on these observations, we elucidate how networks formed by smaller bundles perform better due to their greater interconnectivity at a given optical density. Finally, we present a semi-empirical model for TCF performance as a function of SWCNT mean length and bundle diameter. This gives an estimate for the ultimate performance of non-doped, random network mixed-metallicity SWCNT TCFs at ~80 Ohms/sq. and 90% transparency.