Carbon nanotube networks for thin film electronic applications

The field of electronic materials and devices, characterized by decades of steady but incremental progress, is now undergoing fundamental transformations. New materials such as organic semiconductors and carbon nanotubes are pushing to replace conventional semiconductors in a wide array of applications including microelectronic circuits, photovoltaic cells, sensors, and displays. In particular, carbon nanotubes have generally been proposed for the fabrication of high performance and novel devices, whereas the use of organic semiconductors is typically envisioned for low-cost consumer products.; This thesis contributes to bringing these two traditionally distinct research fields together by exploiting the unique properties of carbon nanotubes to address major challenges faced by organic electronics. We have focused on the development of materials processing and device fabrication techniques required to engineering the necessary morphologies in order to permit the exploitation of their unique properties, especially for large-scale applications.; In the first part of the thesis, we present the results of an investigation of the electronic properties of carbon nanotube networks made from a vacuum filtration method. Considering the fact that as-prepared single-walled carbon nanotube mixtures consist of a statistical distribution of 1/3 metallic and 2/3 semiconducting species, it is possible to fabricate low-density networks (number of nanotubes per unit area) in which percolation paths between two electrodes have semiconducting behavior. We have first determined the carbon nanotube densities for which the transitions from insulating to semiconducting and semiconducting to metallic behaviors are observed. Thin film transistor (TFT) devices fabricated from semiconducting networks displayed transconductances (values proportional to the effective mobility of the devices) over 0.015 S/m at current outputs of ~ 20 muA for ratios of On state/Off state currents (IOn/IOff) over ~ 105. This last value is sufficiently large for the use of these TFTs in, for example, active displays. The transconductance increases with the carbon nanotube density in the networks. Unfortunately, the appearance of metallic percolation paths between the device electrodes results in a considerable increase of the Off state current thus decreasing IOn/IOff values. We have identified the narrow range of carbon nanotube densities which offers the best compromise between these two properties (IOn/IOff and transconductance) for the fabrication of high performance TFT devices. Furthermore, we have demonstrated that electrical breakdown and the selective functionalization of metallic nanotubes, two often-cited strategies for removing the metallic conduction paths and thus to increase transconductance values, are not efficient solutions. Also, in the context of the use of dense metallic networks as conducting, flexible and transparent electrodes, we have identified that 2,3-dichloro-5,6-dicyano-1,4-benzoquinone is a most effective dopant, allowing us to convert the vast majority of semiconducting nanotubes into degenerately doped semiconductors exhibiting metallic behavior. This treatment resulted in a 7-fold increase in the network conductance. Doped carbon nanotube sheets with optical transparencies (80%) and sheet resistances (58 Ω/square) have been fabricated. These characteristics, obtained for flexible materials prepared at room temperature using a solution-based process, are comparable to those of state-of-the-art inorganic oxides deposited under vacuum at high temperature.; Transport in carbon nanotube networks is governed by hopping conduction. Little being known about the scaling properties of carbon nanotube TFT devices that operate in a percolation transport regime, we have conducted a study of electronic transport as a function of channel length for networks having different nanotube configurations. We first developed a wet chemical fabrication method whi...