Very thin transparent, conductive carbon nanotube films on flexible substrates

Abstract

We investigate the morphological, electrical, and optical properties of carbon nanotube thin films, focusing on films with transmittance, T Ͼ 90%. For films with T Ϸ 90% we measure sheet resistance of R s Ͻ 400 ⍀ / ᮀ. However, we show that optoelectrical properties, such as dc and dc / Op , degrade with decreasing film thickness, t, for percolating nanotube networks, i.e., those with t Ͻ 20 nm and T Ͼ 90%. Thus, while reducing t can give T Ͼ 99%, the corresponding R s increases to Ͼ40 k⍀ / ᮀ. Acid treatment improves the conductivity by doping, giving properties such as T Ϸ 98% for R s Ϸ 10 k⍀ / ᮀ. © 2010 American Institute of Physics. ͓doi:10.1063/1.3462317͔ Transparent electrodes are essential components for devices such as displays, organic light emitting diodes, and solar cells. 1 Indium tin oxide ͑ITO͒ is the most widespread material used for such applications. However, ITO has several drawbacks: its price has increased dramatically recently due to increasing demand while its brittleness makes it unsuitable for possible future flexible applications. 2,3 Therefore, much research has been devoted to the development of alternative materials for ITO replacement. The minimum optical and electrical requirements for transparent electrodes are transmittance T Ͼ 90% and sheet resistance R s Ͻ 100 ⍀ / ᮀ. T and R s are related by the following equation: 4,5where Z 0 = 377 ⍀ is the impedance of free space, and Op and dc are the optical and dc conductivities, respectively. Hence, T and R s are effectively controlled by dc / Op . Thus, the minimum industry standard corresponds to dc / Op Ͼ 35. Carbon nanotubes ͑CNTs͒ have been suggested as viable alternatives to ITO. 5-10 At present, the highest dc / Op reported were ϳ13 for pristine CNT films 9 and ϳ15 for composite films. 10 This can be pushed further to 25-35 by post-deposition acid treatment of pristine films. 7,11 However, most of the data available in the literature are reported for relatively low T, usually below 85%. Such films are typically above 40 nm in thickness and generally behave like bulk films. 9 However, a number of applications, such as those in certain displays, require much higher transmittance values. Unfortunately, only limited data are available for highly transparent films. [5][6][7][8][9] In addition, although it is usually assumed that dc ͑and dc / Op ͒ is constant with thickness, recent experiments suggest that this is not the case and that, instead, it decreases for thin films. 7,9 Thus, very thin films, with high T values, are likely to have significantly reduced values of dc / Op and so much higher sheet resistances than expected. This is likely to be a significant problem for high T applications. In this paper, we investigate the morphology and the optical and electrical properties of very thin CNT films, deposited by spray coating, with T ranging from 70% up to 99%. Single wall CNTs, purchased from Iljin Nanotech, were dispersed in sodium dodecylsulphate by 5 min tip-sonication ͑VibraCell CVX, 750 W, 20% 60 kHz͒, then placed in a sonic bat...

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