The extraordinary mechanical and charge transport properties of semiconducting single-walled carbon nanotubes (SWNTs) make them a promising material for solution-processable, flexible and stretchable electronics. Many of these remarkable features are even obtained in randomly-oriented SWNT networks that are compatible with established large-scale thin-film processes based on printing techniques or optical lithography. Given the enormous progress in the purification of solely semiconducting nanotubes as well as in the preparation of SWNT networks with a uniform and defined morphology in recent years, their widespread application as active layers in field-effect transistors (FETs) has become feasible. Likewise, this progress raised subsequent questions of what key parameters determine the charge transport processes across these networks and how they can further be optimized.
This thesis investigates charge transport and its limitations in polymer-sorted semiconducting SWNT networks with a focus on the precise nanotube network composition. The employed FET geometry enabled a reproducible and undistorted analysis of composition- and temperature- dependent transport parameters such as the charge carrier mobility. A comparison between nanotube networks with various selected or even precisely defined SWNT species distributions and average tube diameters reveals that additional energy barriers created at the junctions of adjacent nanotubes with different diameters result in inferior transport properties. While the network charge transport was formerly considered to be solely limited by the charge transfer across these inter-nanotube junctions, the results of this work imply that also the transport within each individual SWNT is important. The specific diameter dependence of this intra-nanotube transport can rationalize the substantially higher carrier mobilities observed for large-diameter networks with a certain SWNT bandgap distribution compared to monochiral networks that contain only a single small-diameter nanotube species. These findings suggest that composition optimizations for SWNT network FETs with maximum carrier mobilities should aim at monochiral large-diameter nanotubes.
Aside from insights into the underlying transport mechanisms, this work demonstrates a novel approach to intentionally modify charge transport in semiconducting SWNT network FETs by adding photochromic spiropyran compounds to the dielectric layer. The strong impact of the spiropyran and its photoinduced isomerization to merocyanine on the charge carrier mobilities give these transistors the properties of basic optical memory devices. Upon UV illumination the carrier mobilities are severely reduced until their recovery is induced by annealing or illumination with visible light. This implemented light responsiveness illustrates the fundamental suitability of SWNT network FETs for multifunctional applications beyond integrated circuits
