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

Abstract

The ultimate performance -ratio of electrical conductivity to optical absorbance -of singlewalled 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 m long SWCNTs. The as-deposited TCFs from 1.38 nm bundles showed sheet resistances of 310 Ω/☐ at 90% transparency, while those from larger bundles of 1.80 and 2.90 nm only reached values of 475 and 670 Ω/☐, 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 Ω/☐ and 90% transparency.

All‐Carbon Thin‐Film Transistors as a Step Towards Flexible and Transparent Electronics

“…[93,94] The overall performance of CNT TCFs is superior or comparable to that of other TCFs, and it is expected that the ultimate performance of CNT TCFs would be further improved if one used only individual, metallic CNTs. [84] In addition, CNT TCFs have several other advantages, such as lower reflection due to the refractive index of CNT (n for CNT, 1.5-1.6) matching to most plastics (n for polyethylene terephthalate (PET), 1.5), better color neutrality due to equal absorption across the visible spectrum than ITO TCFs; smaller haze value due to relatively smaller surface roughness than silver nanowire TCFs; better environmental/chemical stability due to CNT's chemical inertness than PEDOT:PSS TCFs. [95] …”

“…The sheet resistance obviously increases with an increase of the mean diameters of CNT bundles, that is to say networks formed by smaller bundles have higher transparency due to their greater interconnectivity for a given optical density, for instance, experimental data show that CNT TCFs from single-wall CNTs or bundles with an average diameter of 1.38 nm showed sheet resistances of 310 Ω sq -1 , while the corresponding values from larger CNT bundles of 1.80 and 2.90 nm are respectively 475 and 670 Ω sq -1 at the same transmission of 90%. [84] The large bundles not only lower the transparency of TCFs, but also lead to an increase of junction resistance between CNTs or bundles, since the conductivity of a CNT TCF is dominantly determined by the junction resistance. [85,86] If given individual, i.e., non-bundled, CNTs, the TCFs treated with a strong solution of HNO 3 exhibit a sheet resistance of 63 Ω sq -1 at a transmission of 90%.…”

“…[91] Copyright 2016, Elsevier. d) Approximate distribution of the optoelectrical performance of CNT TCFs prepared by dry-drawing, [57] solution processing, [63,77] gas filtration, [82][83][84] and gas filtration with grids, [90,91] in comparison with that of ITO, [93,94] Ag nanowire [93,94] and PEDOT:PSS TCFs. [94] The transmittance data is given for a wavelength of 550 nm.…”

Phonon contribution to electrical resistance of acceptor-doped single-wall carbon nanotubes assembled into transparent films

“…For practical application, industry specification for transparent conducting films requires transparencies higher than 90% and sheet resistance R sq lower than 90 ohm/sq (see, e.g., [13]). The highly transparent (80-90%) SWCNT films formed from pristine nanotubes with noticed M/S ratio have the values of R sq ranging from unacceptable values (10 3 -10 4 ohm/sq) [2,8] to those (89-310 ohm/sq) [4,[14][15][16] which are in close proximity to required ones. The sheet resistance of pristine SWCNT films depends on the purity of the samples, the length and diameter of the nanotubes, and the size of the SWCNT bundles [13,15].…”

“…The highly transparent (80-90%) SWCNT films formed from pristine nanotubes with noticed M/S ratio have the values of R sq ranging from unacceptable values (10 3 -10 4 ohm/sq) [2,8] to those (89-310 ohm/sq) [4,[14][15][16] which are in close proximity to required ones. The sheet resistance of pristine SWCNT films depends on the purity of the samples, the length and diameter of the nanotubes, and the size of the SWCNT bundles [13,15]. The best value of R sq = 89 ohm/sq has been obtained for 90%-transparent SWCNT films fabricated using aerosol technology, which leads to an almost complete elimination of SWCNT bundling (known for such technique as arc discharge [17]) and a substantial increase in SWCNT lengths via the suppression of bundling-induced growth termination [16] The sheet resistance of SWCNT films can be significantly decreased by acid(HNO 3 ) treatment that results in a high level of p-type doping [9,14,18,19].…”