(Invited) Improved Charge and Exciton Transport in Polymer-Removed SWCNT Thin Films: Implications for Photovoltaic and Thermoelectric Energy Harvesting

Abstract

The chemical and physical structure of semiconducting single-walled carbon nanotubes (SWCNTs) results in optical and electronic properties of promise for a wide variety of applications. Until quite recently the presence of metallic SWCNT impurities has hampered efforts to gain a deeper understanding of their true potential, with the additional complication that most commercially available materials contain tens of different chiral species. Significant effort has been devoted to elegant enrichment strategies aimed at extracting tailored semiconducting SWCNT species from the raw soot, from the use of subtly tunable surfactant interactions to the exploitation of specific DNA sequences. However, conjugated polymers, typically based on the fluorene moiety, appear to show the greatest promise with regards to their high selectivity and viability for scalable manufacturing approaches. Unfortunately, the van de Waals forces between the pi-electron systems of the polymer and SWCNT that enable the selective extraction of semiconducting SWCNTs with high purity also make removal of the polymer difficult. Since these polymers typically have a wide bandgap they act as an insulating coating on the surface of the individual SWCNTs within functional networks, inhibiting the transport of energy in the form of excitons and/or charge carriers. Here we demonstrate approaches aimed at replacing the strongly-bound polymers with variants that can be removed using simple solution-based chemical strategies, resulting in networks with vastly improved energy transport properties. We show that removal of the polymer results in a significant enhancement of the charge carrier mobility and electrical conductivity in the SWCNT networks, allowing optimization of the thermoelectric properties for both p-type and n-type transport. Finally, we extend the approach to samples strongly enriched in a single chiral SWCNT species, which allows us to employ transient spectroscopic techniques to probe enhanced exciton transport through the SWCNT network with high spectral fidelity. Our studies highlight a methodology by which high-performance SWCNT thin films can be prepared that could realize their potential for electronic and optoelectronic applications.