Polymer Sorting of Single-Walled Carbon Nanotubes and Implications for Thin-Film Transistor Fabrication

Abstract

Thin-film transistors (TFTs) are electronic switches and current amplifiers which, through large-scale printing techniques, enable emerging applications such as flexible electronics, sensors, and smart packaging. While polymer-wrapped semiconducting single-walled carbon nanotubes (SWNTs) offer promising mechanical and electrical properties, commercialization of such devices has been hindered as more work is needed to understand the impact of the polymer on device fabrication and performance. Our group fabricated ambipolar TFTs in ambient conditions using highly pure semiconducting SWNTs dispersed with a novel carbazole-based conjugated polymer, yielding TFTs with high mobilities, high on/off ratios, negligible hysteresis, low threshold voltages, and high bias stability in air and under inert atmosphere. Contrary to common knowledge, we demonstrate that removal of excess unbound conjugated polymer prior to device fabrication is not necessary. We also attempt to expand the use of wrapping polymers beyond their use in separation processes by exploiting their interactions with analytes to fabricate chemical and biological sensors. A first example of this work has been demonstrated with the fabrication of CO 2 responsive polymer-SWNT complexes, which demonstrate reversible changes in current and threshold voltage when incorporated in TFT-based sensors. The presence of excess unbound polymer also influences the SWNT network formation when printing. In our first study, it was found to reduce the incidence of bundling in SWNT networks in comparison to films cast using filtered dispersions. In both cases, networks of SWNTs drop-casted onto a surface are prone to a coffee-ring effect, where a substantial number of nanotubes preferentially deposit on the edges of the drop area. This reduces the density of SWNTs within the drop area, leading to inconsistencies in network characteristics when printing. We therefore investigated how varying the droplet size and substrate temperature would affect the network characteristics. Using Atomic Force Microscopy (AFM) and Raman mapping, we demonstrated that substrate temperatures nearing 70°C are conducive to a more even distribution of SWNTs throughout a network on a macroscopic scale, although the networks are more likely to exhibit high degrees of bundling on a microscopic scale. This ultimately worsens the performance of TFTs fabricated using these networks. Finally, our SWNT dispersions have been used in the fabrication of TFTs with a tri-layer dielectric composed of biodegradable polymers – poly(lactic acid), poly(vinyl alcohol) with cellulose nanocrystals and toluene diisocyanate terminated poly(caprolactone). Their incorporation reduced charge trapping between the dielectric and the SWNTs, reduced the threshold voltage, improved film formation, and prolonged n-type air-stability. These TFTs have been thus far demonstrated on rigid and flexible polyimide substrates.