Single-walled carbon nanotubes (SWCNTs) have the potential to revolutionize many areas of opto-electronics, particularly thin-film photovoltaics and thermoelectrics due to their high carrier mobility and low-dimensional structure. Much effort has been devoted to increasing the thermoelectric power factor of SWCNTs by controlling the doping levels and doping types for enhanced thermoelectric conversion. Controlling the Fermi energy level within these unique 1D structures is often afforded by charge transfer interactions between host species (SWCNT) and electron or hole accepting species (dopant). Recent advancement of polymer-wrapped SWCNTs allows for thin-films of SWCNTs to be to cast using solution-based processing techniques. Conventional methods to dope SWCNTs rely upon intercalation of dopant molecules within solid-state networks of SWCNTs in a thin film. Here, we discuss methods to dope polymer-wrapped SWCNTs directly in solution by adding a p-type charge transfer dopant, F₄-TCNQ, to SWCNT dispersions prior to thin-film deposition. This approach provides more control and reproducibility of the doping level and allows for SWCNTs inks to directly create doped thin films without the need for post-deposition treatments.

Interestingly, we show that introduction of the dopant at varying stages of polymer wrapping process of SWCNT dispersion impacts the polymer selectivity for semiconducting vs. metallic SWCNTs. By controlling the dopant concentration in solution, we show that the degree of doping is retained upon thin film formation, enabling creation of SWCNT thin films with controllable carrier concentrations for opto-electronic applications. We characterize the thermoelectric properties of the solution-phase doped films, and demonstrate large, tunable power factors that correlate well with the degree of doping in both the solution- and solid-state. Additionally, I will briefly discuss the implications of these results on the chemical interaction of F₄-TCNQ with SWCNTs of various band gaps. This work has implications in the improvement of SWCNT-based thermoelectric devices and capability to create doped SWCNT thin films directly from solution for use in other opto-electronic applications such as thin-film photovoltaics.